Available online at www.sciencedirect.com SCIENCE ODIRECT. ELSEVIER Prog.Polm.si.31(200576-602 www.elsevier.com/locate/ppolysi Polymer blends and composites from renewable resources Long Yu"-b.*,Katherine Dean",Lin Lib alth scientific and industrial Research Ora tion.CMIT.Melbour Abstrac This article review grad ble polymers,various oped ov rom t uch as polylactic acid; d (3)polymer rom mic and the technology of sed to improve the adhesion between 2006 Elsevier Ltd.All rights reserved. Keyd Polymer:Blend:Composite:Renewable resource:Biodegradable Contents duction 32 7000 PLA/PHB blends 4. drophilic polymer aut r.C ealth Scientific and Industrial Research Organization,CMIT.Melbourne.Vic.3169.Australia
Prog. Polym. Sci. 31 (2006) 576–602 Polymer blends and composites from renewable resources Long Yua,b,, Katherine Deana , Lin Lib a Commonwealth Scientific and Industrial Research Organization, CMIT, Melbourne, Vic. 3169, Australia b Centre for Polymer from Renewable Resources, School of Food and Light Industrial Engineering, South China University of Technology, Guangzhou, China Received 22 July 2005; received in revised form 17 March 2006; accepted 23 March 2006 Available online 6 June 2006 Abstract This article reviews recent advances in polymer blends and composites from renewable resources, and introduces a number of potential applications for this material class. In order to overcome disadvantages such as poor mechanical properties of polymers from renewable resources, or to offset the high price of synthetic biodegradable polymers, various blends and composites have been developed over the last decade. The progress of blends from three kinds of polymers from renewable resources—(1) natural polymers, such as starch, protein and cellulose; (2) synthetic polymers from natural monomers, such as polylactic acid; and (3) polymers from microbial fermentation, such as polyhydroxybutyrate—are described with an emphasis on potential applications. The hydrophilic character of natural polymers has contributed to the successful development of environmentally friendly composites, as most natural fibers and nanoclays are also hydrophilic in nature. Compatibilizers and the technology of reactive extrusion are used to improve the interfacial adhesion between natural and synthetic polymers. r 2006 Elsevier Ltd. All rights reserved. Keywords: Polymer; Blend; Composite; Renewable resource; Biodegradable Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 577 2. Natural polymer blends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 578 2.1. Melt processed blends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 579 2.2. Aqueous blends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 580 3. Aliphatic polyester blends. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 582 3.1. Blends of PLA family . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 582 3.2. Blends of PHA family . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 584 3.3. PLA/PHB blends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 585 3.4. Other polyester blends. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 586 4. Blends of hydrophobic and hydrophilic polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 586 ARTICLE IN PRESS www.elsevier.com/locate/ppolysci 0079-6700/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.progpolymsci.2006.03.002 Corresponding author. Commonwealth Scientific and Industrial Research Organization, CMIT, Melbourne, Vic. 3169, Australia. Tel.: +61 3 9545 2797; fax: +61 3 9544 1128. E-mail address: long.yu@csiro.au (L. Yu)
L.Yu et al Prog.Polym ScL 31(2006)576-602 4.1.Starch/PLA blends 4.1.1 ued for starch/PLA blends. 4. ch/PHB ble 43 PHB/cellulose derivative blends 9 g 5. Multi Multilayer extrusion 99 6 etwen multi-layers 6.1.Starch reinforced with cellulose fibers 6.1.l Hot press molding and foaming .593 63 oldin 504 6.2 Other starch/cellulose composites 594 Aliphatic polyester reinforced with natural fibers 7. 9 p 96 Protein/nanoclay composites ng rem 9 References 1.Introduction at of oil.Moder vide erful tool tracted an incr nt levels,and last two decades,predominantly due to two major reasons:firstly environmental concerns,and sec and properties.These new levels of understanding ondly the realization that our petroleum resources bring opportunities to develop materials fo are finite. rally,polymers from renewable nev resource: into thre so mean uch as starch.proter onment in los (3) ers from microhial fer nd the polyhydroxybutyrate (PHB).Like numerous other petroleum-based polymers.many properties of limits their application.Another limitation of many PFRR can also be improved through blending and natural polymers is their lower softening tempera ture study and utilization of natural polymers is development o syntheti pol mers using ch as pape natura and ples,st develop th ability of petroleum at rd is PLA. biochemical inertness of petroleum-based products from ag icultural products and is readily biodegrad- have proven disastrous for the natural polymers able.Lactide is a cyclic dimer prepared by the market.It is only after a lapse of almost 50 years controlled depolymerization of lactic acid,which in that the significance of eco-friendly materials has turn can be obtained by the fermentation of corn. been realized once again.These ancient materials sugar cane,sugar beat 1,2.PLA is not a nev
4.1. Starch/PLA blends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 586 4.1.1. Compatiblizers used for starch/PLA blends. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 587 4.1.2. Reactive blending . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 588 4.2. Starch/PHB blends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 589 4.3. PHB/cellulose derivative blends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 589 4.4. Chitosan/PLA blends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 590 4.5. PHB/chitosan and PHB/chitin blends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 590 5. Multilayer composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 591 5.1. Multilayer extrusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 591 5.2. Interfaces between multi-layers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 592 6. Fiber-reinforced composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 592 6.1. Starch reinforced with cellulose fibers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 593 6.1.1. Hot press molding and foaming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 593 6.1.2. Extrusion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 594 6.1.3. Injection molding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 594 6.2. Other starch/cellulose composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 594 6.3. Aliphatic polyester reinforced with natural fibers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 595 7. Novel nanocomposites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 595 7.1. Starch/nanoclay composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 596 7.2. Protein/nanoclay composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 596 7.3. PLA nanocomposites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 597 8. Concluding remarks. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 597 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 598 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 598 1. Introduction Polymers from renewable resources have attracted an increasing amount of attention over the last two decades, predominantly due to two major reasons: firstly environmental concerns, and secondly the realization that our petroleum resources are finite. Generally, polymers from renewable resources (PFRR) can be classified into three groups: (1) natural polymers, such as starch, protein and cellulose; (2) synthetic polymers from natural monomers, such as polylactic acid (PLA); and (3) polymers from microbial fermentation, such as polyhydroxybutyrate (PHB). Like numerous other petroleum-based polymers, many properties of PFRR can also be improved through blending and composite formation. The study and utilization of natural polymers is an ancient science. Typical examples, such as paper, silk, skin and bone arts, can be easily found in museums around the world. However, the availability of petroleum at a lower cost and the biochemical inertness of petroleum-based products have proven disastrous for the natural polymers market. It is only after a lapse of almost 50 years that the significance of eco-friendly materials has been realized once again. These ancient materials have rapidly evolved over the last decade, primarily due to the issue of the environment and the shortage of oil. Modern technologies provide powerful tools to elucidate microstructures at different levels, and to understand the relationships between structures and properties. These new levels of understanding bring opportunities to develop materials for new applications. The inherent biodegradability of natural polymers also means that it is important to control the environment in which the polymers are used, to prevent premature degradation. For example, the water solubility of many natural polymers raises their degradability and the speed of degradation, however, this moisture sensitivity limits their application. Another limitation of many natural polymers is their lower softening temperature. The development of synthetic polymers using monomers from natural resources provides a new direction to develop biodegradable polymers from renewable resources. One of the most promising polymers in this regard is PLA, because it is made from agricultural products and is readily biodegradable. Lactide is a cyclic dimer prepared by the controlled depolymerization of lactic acid, which in turn can be obtained by the fermentation of corn, sugar cane, sugar beat [1,2]. PLA is not a new ARTICLE IN PRESS L. Yu et al. / Prog. Polym. Sci. 31 (2006) 576–602 577
578 L Yu et al.Prog.Polym.Sci 31 (2006)576-602 or o the pactie ve improved ch PLA d r could is at the fo front of the emerging biodegradable This new nanoc ositeshowed dramatic plastics industries In nature.a special group of polyesters is content.The reinforcement with filler is particularly produced by a wide variety of micro-organisms as important for polymers from renewable resources an internal carbon and energy storage,as part of since most of them have the disadvantages of lower their survival mechanism.Poly(B-hydroxybutyrate) and lower modulus (PHB)wa s190 the re, hydrophil a10 of mos nat polymers offers 20 The energy crisis of the 1970s was an incentive to Many natural polymers are hydrophilic and some seek naturally occurring substitutes for synthetic of them are water soluble water solubility raises plastics.which sped up the research and commer degradability and increases the speed of degrada cialization of PHB.The brittleness of PHB was on.however this moisture sensitivity limits their improved through copolymerization of B-hydrox Blends and multilayers of natura polymers wi rn a D(PHBV) s can als new low-cost products v of PHBV is still the n naior ba The ne blends and composites are extending usage the utilization of polymers from renewable resource Like most polymers from petroleum,polymers into new value-added products. from renewable resources are rarely used by themselves.In fact,the history of composites from 2.Natural polymer blends onger Wide range the ark n rom rushe s.p d rdin ve ewabl material.During the them.such as starc opium war more than 1000 vears ago.the Chinese actively used in products today while many other built their castles to defend against invaders using a remain underutilized.Natural polymers can some kind of mineral particle-reinforced composite made times be classified according to their physical from gluten rice,sugar,calcium carbonate and character. For example.starch and cellulose are sand classifie into different groups,but they ers are del enc mate emica mecha pol小ysac prope egetabl polymers surveyec seed-hair fibers.Cellulose ralpolymers perform a diverse set of the maior substance obtained from y functions in their native setting For example and applications for cellulose fiber-reinforced poly polysaccharides function in membranes and intra mers have again come to the forefront with th cellular communication;proteins function as struc- focus on renewable raw materials 7-9.Hydrophilic ural materals and catalysts:and lipids function as cellulose fibers are very compatible with mos energy stores I0.Nature can provide a impressive natural polymers. The rei array ol poryme I to be fibers is a perfect example in fiber coatigs,ge ilms of biodeg on in the ers oba ned fron es is thei polymeric materials.The interest in new nanoscale dominant hydrophilic character.fast degradation
polymer, however, better manufacturing practices have improved the economics of producing monomers from agricultural feedstocks, and as such PLA is at the forefront of the emerging biodegradable plastics industries. In nature, a special group of polyesters is produced by a wide variety of micro-organisms as an internal carbon and energy storage, as part of their survival mechanism. Poly(b-hydroxybutyrate) (PHB) was first mentioned in the scientific literature as early as 1901 and detailed studies begin in 1925 [3,4]. Over the next 30 years, PHB inclusion bodies were studied primarily as an academic curiosity. The energy crisis of the 1970s was an incentive to seek naturally occurring substitutes for synthetic plastics, which sped up the research and commercialization of PHB. The brittleness of PHB was improved through copolymerization of b-hydroxybutyrate with b-hydroxyvalerate [5,6]. This family of materials, known as poly(3-hydroxybutyric acidco-3-hydroxyvaleric acid) (PHBV), was first commercialized in 1990 by ICI. However, the high price of PHBV is still the major barrier to its wide spread usage. Like most polymers from petroleum, polymers from renewable resources are rarely used by themselves. In fact, the history of composites from renewable resources is far longer than conventional polymers. In the biblical Book of Exodus, Moses’s mother built the ark from rushes, pitch and slime— a kind of fiber-reinforced composite, according to the modern classification of material. During the opium war more than 1000 years ago, the Chinese built their castles to defend against invaders using a kind of mineral particle-reinforced composite made from gluten rice, sugar, calcium carbonate and sand. Fibers are widely used in polymeric materials to improve mechanical properties. Vegetable fibers (e.g. cotton, flax, hemp, jute) can generally be classified as bast, leaf or seed-hair fibers. Cellulose is the major substance obtained from vegetable fibers, and applications for cellulose fiber-reinforced polymers have again come to the forefront with the focus on renewable raw materials [7–9]. Hydrophilic cellulose fibers are very compatible with most natural polymers. The reinforcement of starch with cellulose fibers is a perfect example of PFRR composites. The reinforcement of polymers using fillers is common in the production and processing of polymeric materials. The interest in new nanoscale fillers has rapidly grown in the last two decades, since it was discovered that a nanostructure could be built from a polymer and a layered nanoclay. This new nanocomposite showed dramatic improvement in mechanical properties with low filler content. The reinforcement with filler is particularly important for polymers from renewable resources, since most of them have the disadvantages of lower softening temperatures and lower modulus. Furthermore, the hydrophilic behavior of most natural polymers offers a significant advantage, since it provides a compatible interface with the nanoclay. Many natural polymers are hydrophilic and some of them are water soluble. Water solubility raises degradability and increases the speed of degradation, however, this moisture sensitivity limits their application. Blends and multilayers of natural polymers with other kinds of PFRR can be used to improve their properties. Blends can also aid in the development of new low-cost products with better performance. These new blends and composites are extending the utilization of polymers from renewable resource into new value-added products. 2. Natural polymer blends Wide ranges of naturally occurring polymers derived from renewable resources are available for various materials applications [10,11]. Some of them, such as starch, cellulose and rubber, are actively used in products today, while many others remain underutilized. Natural polymers can sometimes be classified according to their physical character. For example, starch and cellulose are classified into different groups, but they are both polysaccharides according to chemical classification. Table 1 lists some natural polymers surveyed by Kaplan [10]. These natural polymers perform a diverse set of functions in their native setting. For example, polysaccharides function in membranes and intracellular communication; proteins function as structural materials and catalysts; and lipids function as energy stores [10]. Nature can provide an impressive array of polymers that have the potential to be used in fibers, adhesives, coatings, gels, foams, films, thermoplastics and thermoset resins. One of the main disadvantages of biodegradable polymers obtained from renewable sources is their dominant hydrophilic character, fast degradation ARTICLE IN PRESS 578 L. Yu et al. / Prog. Polym. Sci. 31 (2006) 576–602
L.Yu et al Prog.Polym Sel.31 (2006)576-602 579 List of natural polymers arch.cellulose.pectin.konjac.alginate, serum,albumin.collagen/gelatine,silks,resilin.polylysine.polyamino acids.poly(y-glutamic acid),elastin. s,surfactants,emulsan overwhelming abundance and its a ewa [131.Ho itself propciple.the erties of natural p rs can be eplac significantly improved by blending with synthetic mostwater soluble.difficult to process and brittle polymers.Polymer blending is a well-used technique when used without the addition of a plasticizer.In whenever modification of properties is required addition,its mechanical properties are very sensitive because it uses conventional technology at low cos to moisture content.Blending two or more chemi The usual objective for preparing a novel blend of not to change the pr ercom components dras ally,but er has blended h starch for on the e 1970s and 1980s.n app with various polyolefins were developed.However. on gelatinized starch nds ann and 1 4trans these blends were not biodegradable and thus the (gutta percha)for food packaging or biomedical advantage of using a biodegradable polysaccharide applications.Components are mixed to an adequate was lost.In this section,polymer blends only from degree of dispersion by thermal pressing.A series o natural raw materials are discussed blends of gutta percha with gelatinized tarch.with are water and without plasticizers or compatibilizers. wa water has s a solvent,di t to pres plas s121 processing ma ty f gutta peren polysaccharides are the main tue The and wate polymers.their interaction with water and with each other in a water medium give the structure-property intermediate values between the two components relationships in these materials.An analysis of the Carvalho et al.I151 studied the blending of starch glass transition temperature and thermal profile with natural rubber.Thermoplastic starch/natura gives one of the best illustrations of the role of water nds were prepared using natural er-reinforced e of ere prepared in intens tch mixer detail in a later section dispersion of the starc matrix was homoge neous because of the presence of 2.1.Melt processed blends the aqueous medium,with rubber particles ranging in size from 2 to &um.The results revealed a Starch is one of the most promising natural reduction in modulus and tensile strength,making polymers because of its inherent biodegradability. the blends less brittle than thermoplastic starch
rate and, in some cases, unsatisfactory mechanical properties, particularly under wet environments. In principle, the properties of natural polymers can be significantly improved by blending with synthetic polymers. Polymer blending is a well-used technique whenever modification of properties is required, because it uses conventional technology at low cost. The usual objective for preparing a novel blend of two or more polymers is not to change the properties of the components drastically, but to capitalize on the maximum possible performance of the blend. In the 1970s and 1980s, numerous blends of starch with various polyolefins were developed. However, these blends were not biodegradable, and thus the advantage of using a biodegradable polysaccharide was lost. In this section, polymer blends only from natural raw materials are discussed. Since the majority of natural polymers are water soluble, water has been used as a solvent, dispersion medium and plasticizer in the processing of many natural polymer blends [12]. Since proteins and polysaccharides are the main constituents of natural polymers, their interaction with water and with each other in a water medium give the structure–property relationships in these materials. An analysis of the glass transition temperature and thermal profile gives one of the best illustrations of the role of water in natural polymers. Natural fiber-reinforced composites are one of the successful examples and will be discussed in detail in a later section. 2.1. Melt processed blends Starch is one of the most promising natural polymers because of its inherent biodegradability, overwhelming abundance and its annual renewal [13]. However, by itself, pure starch is not a good choice to replace petrochemical-based plastics. It is mostly water soluble, difficult to process and brittle when used without the addition of a plasticizer. In addition, its mechanical properties are very sensitive to moisture content. Blending two or more chemically and physically dissimilar natural polymers has shown potential to overcome these difficulties. Natural rubber has been blended with starch for a number of different applications. Arvanitoyannis et al. [14] reported on biodegradable blends based on gelatinized starch and 1,4-transpolyisoprene (gutta percha) for food packaging or biomedical applications. Components are mixed to an adequate degree of dispersion by thermal pressing. A series of blends of gutta percha with gelatinized starch, with and without plasticizers or compatibilizers, was prepared in an attempt to preserve the excellent biocompatibility of gutta percha. A low amount of plasticizer was incorporated into the blends to improve mechanical properties. The gas and water permeability values of the blends were found to be intermediate values between the two components. Carvalho et al. [15] studied the blending of starch with natural rubber. Thermoplastic starch/natural rubber polymer blends were prepared using natural latex and cornstarch. The blends were prepared in an intensive batch mixer at 150 1C, with the natural rubber content varying from 2.5% to 20%. The dispersion of rubber in the thermoplastic starch matrix was homogeneous because of the presence of the aqueous medium, with rubber particles ranging in size from 2 to 8 mm. The results revealed a reduction in modulus and tensile strength, making the blends less brittle than thermoplastic starch ARTICLE IN PRESS Table 1 List of natural polymers Polysaccharides Plant/algal: starch, cellulose, pectin, konjac, alginate, caragreenan, gums Animal: hyluronic acid Fungal: pulluan, elsinan, scleroglucan Bacterial: chitin, chitosan, levan, xanthan, polygalactosamine, curdlan, gellan, dextran Proteins Soy, zein, wheat gluten, casein, serum, albumin, collagen/gelatine, silks, resilin, polylysine, polyamino acids, poly(g-glutamic acid), elastin, polyarginyl–polyaspartic acid Lipids/surfactants Acetoglycerides, waxes, surfactants, emulsan Speciality polymers Lignin, shellac, natural rubber L. Yu et al. / Prog. Polym. Sci. 31 (2006) 576–602 579
580 L Yu et al.Prog.Polym.Sci 31 (2006)576-602 alone.Phase separation in some was depc ntent made e additio 2.2.Aqueous blends of rubber possible.The addition of rubber was. however.limited by phase separation,the appear. Many natural polymers cannot be melt processed. ance of which depended on the glycerol content either because they degrade on or before melting Scanning electron microscopy (SEM)showed good (softening)or because they are designed to incorpo- rate substances that do not stand high temperature h starch ma h drugs,etc.).Fo form withou any kind of nu ion.More are usually biocom atible and the presence of the non-rubher constituents of the non-cytotoxic due to their similarity with living latex was not only responsible for insuring the tissues.Biopolymers are an important source of latex stability,but also for improving the compati- material with a high chemical versatility and with bility between the thermoplastic starch and the high potential to be used in a range of biomedical natural rubbe the hoth the r phases Finally,glycerol seemed to applications [1,19.A great variety of materials contribute derived ces ha of om natural sour al and st3h、 abundant biopolymers on earth.S Since of the variety of biofibe ers such as lig of civilization,the excellent chemical and physical losic natural fibers [20.21].Starch-based polymers properties of these materials have made them a present enormous potential for wide used in the useful component in many applications.However. biomedical field,as these natural polymers are blends of starch and protein have not showi totally biodegradable and inexpensive when com to other bio vailabl he to d egradable polymers av od and rc 20 protein/starch interaction during ties that make them suitable for in established a kinetic model for starch gelatiniza wide array of biomedical applications,ranging from tion and the effect of starch/protein interactions bone replacement to engineering of tissue scaffolds Matveev et al.[12]studied the effect of water on the and drug-delivery systems. glass transition of protein,polysaccharides and Starch-based thermoplastic hydrogels for use as blend considering inter-macromolecular hydrogen one cement ts or drug-delivery carriers hav an et develope ugh b d th bicate 26- Thermoplastic polysaccharides such as cellulose radical 2 5.actetate were polymerization with methyl methacrylate and/or an acrylic acid monomer.The polymerization was simultaneously onto polysaccharides,hydroxy-func- initiated by a redox system consisting of benzoyl tionalized plasticize and also peroxide and 4-dimethlyaminobenzyl alcohol at lov hydroxy-functional fillers.Organosolv lignin,cellu odegradable characte we dified。 polymers poly( ma. sult of me ko ramic con nd h lapatite [32 the excellent compatibility of nolv(. aprolactone)
alone. Phase separation was observed in some compositions, which was dependent on rubber and plasticizer content (glycerol). Increasing the plasticizer content made the addition of higher amounts of rubber possible. The addition of rubber was, however, limited by phase separation, the appearance of which depended on the glycerol content. Scanning electron microscopy (SEM) showed good dispersion of the natural rubber in the continuous phase of the thermoplastic starch matrix. The process employed in this investigation called upon the use of both starch and latex in their natural form, without any kind of purification. Moreover, the presence of the non-rubber constituents of the latex was not only responsible for insuring the latex stability, but also for improving the compatibility between the thermoplastic starch and the natural rubber phases. Finally, glycerol seemed to contribute to both the plasticization of the starch and to the improvement of the starch/rubber interface. Cellulose and starch are some of the most abundant biopolymers on earth. Since the origin of civilization, the excellent chemical and physical properties of these materials have made them a useful component in many applications. However, blends of starch and protein have not shown significant promise in biodegradable materials applications to date. The system of starch/protein, however, has been studied in food field [12,16]. Kokini et al. [16] studied starch conversion and protein/starch interaction during processing, and established a kinetic model for starch gelatinization and the effect of starch/protein interactions. Matveev et al. [12] studied the effect of water on the glass transition of protein, polysaccharides and blends, considering inter-macromolecular hydrogen and dipole–dipole interactions. Warth et al. [17] used the technology of reactive extrusion to develop starch/cellulose acetate blends. Thermoplastic polysaccharides such as cellulose- 2,5-actetate were produced by means of reactive processing technology that grafted cyclic lactones simultaneously onto polysaccharides, hydroxy-functionalized plasticizer, and optionally also onto hydroxy-functional fillers. Organosolv lignin, cellulose, starch and chitin were added for reinforcement of the polymer blends. Compatibility between oligolactone-modified cellulose acetate and fillers were markedly improved when fillers were added during the reactive extrusion process. As a result of the excellent compatibility of poly(e-caprolactone) with numerous polymers, it has been possible to prepare a wide range of new polymer blends. 2.2. Aqueous blends Many natural polymers cannot be melt processed, either because they degrade on or before melting (softening) or because they are designed to incorporate substances that do not stand high temperature (proteins, drugs, etc.). For these examples, aqueous blending is the preferred technology, particularly in biomedical applications. Natural polymers are usually biocompatible and non-cytotoxic due to their similarity with living tissues. Biopolymers are an important source of material with a high chemical versatility and with high potential to be used in a range of biomedical applications [18,19]. A great variety of materials derived from natural sources have been studied and proposed for different biomedical uses, namely polysaccharides (starch, alginate, chitin/chitosan) or protein (soy, collagen, fibrin gel) and, as reinforcement, a variety of biofibers such as lignocellulosic natural fibers [20,21]. Starch-based polymers present enormous potential for wide used in the biomedical field, as these natural polymers are totally biodegradable and inexpensive when compared to other biodegradable polymers available [22–25]. Aqueous blends of soluble starch and cellulose acetate have been studied intensively [26–29] because these blends have a range of properties that make them suitable for use in a wide array of biomedical applications, ranging from bone replacement to engineering of tissue scaffolds and drug-delivery systems. Starch-based thermoplastic hydrogels for use as bone cements or drug-delivery carriers have been developed through blending starch with cellulose acetate [26–28]. Pereira et al. [26] reported on biodegradable hydrogels, based on cornstarch/ cellulose acetate blends, produced by free-radical polymerization with methyl methacrylate and/or an acrylic acid monomer. The polymerization was initiated by a redox system consisting of benzoyl peroxide and 4-dimethlyaminobenzyl alcohol at low temperature. Utilizing the biodegradable character of starch-based blends, with the biostability of the acrylic polymers poly(methyl methylacrylate (PMMA) and poly(acrylic acid) [30,31] used as the matrix of these systems, and the incorporation of the well-known ceramic compound hydroxylapatite [32], Espigares et al. [27] developed partially biodegradable ARTICLE IN PRESS 580 L. Yu et al. / Prog. Polym. Sci. 31 (2006) 576–602
L.Yu et al.Prog.Polym.Sci.31 (2006)576-602 581 acrylic bone cements based on cornstarch/cellulose films.The increase in crystallinity with time resulted acetate blends.Varying amounts of a biocompatible, in decreased gas and water permeability.Peressini osteoconductive and osteophilic mineral component et al.[34]studied the rheological properties of such as hydroxylapatite were incorporated to confer a starch/methylcellulose-based edible films.The flow bone-bonding character to the bone cements in this curves showed shear-thinning behavior.Mechanical type of applications.Arvanitoyannis and Biliaderis spectra and Cox-Merz superposition of steady- [28]reported on aqueous blends of methyl cellulose shear viscosity and dynamic viscosity were consis- and soluble starch,plasticized with glycerol or sugars, tent with polymer solutions containing topological prepared by casting or by extrusion and hot pressing. entanglement interactions of chains.Dispersion The observed Te depression for these polymer blends stability results showed total recovery of the was proportional to the plasticizer content (water, viscoelastic properties of dispersions subject to high glycerol and sugars).Although glycerol had a greater strains,as expected for entangled polymers.MC depressing effect on T than sorbitol,the latter had a was the main factor influencing the apparent greater impact,as a plasticizer,on the mechanical viscosity and viscoelastic properties. properties (higher percentage elongation)of the Demirgoz et al.[35]developed a method to soluble starch/methyl cellulose blends.Generally the control the moisture sensitivity of starch/cellulose tensile strength and flexural moduli of these blends acetate blends through chemical modifications.In were shown to decrease drastically with an increase in their work,starch-based blends with CA were the total plasticizer content. chemically modified by chain crosslinking.This Lepeniotis et al.[29]prepared dry spin fibers by modification was based on the reaction between the blending starch acetate (SA)with a degree of starch hydroxyl groups and tri-sodium tri-meta substitution (ds)of 2.1-3.0 with cellulose acetate phosphate.The resulting compounds were charac- (CA)with a ds of 2.5 to make 25-30 wt%blended terized by Fourier transform infrared (FTIR)and solutions in acetone:water.To develop solubility the respective properties were assessed and com- and phase information,a modified central compo- pared to the original materials by means of the site face/central composite circumscribe (CCF/ hydration degree,the degradation behavior,contact CCC)statistical designed experiment at 5-10 wt% angle measurements and mechanical testing.The solids was completed.This was based on five results showed that the water uptake of these blends factors:(1)acetyl value;(2)acetone versus water could be reduced by up to 15%,and that concentration;(3)ratio of SA to CA;(4)total simultaneously stiffer materials with a less pro- weight of solids in the solution;and (5)mixing nounced degradation rate could be obtained. temperature.The resulting solubility information Levy et al.[36]reported on the design of a could be used to predict the stability of SA:CA polysaccharide crossbridging protein which was blends at the higher concentration and temperature comprised of a cellulose-binding domain from ranges commonly used for fiber spinning.The most Clostridium cellulovorans (CBDelos)and a starch- significant factors influencing phase type was found binding domain from Aspergillus niger B1(SBDAsp). to be SA concentration and the interaction of SA The two genes were fused in-frame via a synthetic percentage and total solids concentration. elastin gene to construct a cellulose/starch cross- Psomiadou et al.[33]developed edible films made bridging protein (CSCP).The CSCP demonstrated from natural resources-microcrystalline cellulose a crossbridging ability in different model systems (MCC),methylcellulose (MC),cornstarch and composed of insoluble or soluble starch and polyols.Aqueous blends of MCC or MC and cellulose.The fact that different carbohydrate- cornstarch with or without polyols were prepared binding modules maintained their binding capacity by extrusion,hot pressed and studied.After over a wide range of conditions,without the need conditioning at different relative humidities,their for chemical reactions,makes them attractive thermal,mechanical,and water and gas permeabil- domains for designing new classes of polysacchar- ity properties were determined.An increase in water ide-binding domains with potential applications in or polyol content showed a considerable increase in the biomaterial field. percentage elongation,but also a decrease in the Other kinds of aqueous blends are made from tensile strength of films.The presence of high chitosan.Kweon et al.[37]reported the preparation cellulose contents increased the tensile strength of aqueous blends of protein with chitosan. and decreased the water vapor transmission of the Antheraea pernyi silk fibroin (SF)/chitosan blend
acrylic bone cements based on cornstarch/cellulose acetate blends. Varying amounts of a biocompatible, osteoconductive and osteophilic mineral component such as hydroxylapatite were incorporated to confer a bone-bonding character to the bone cements in this type of applications. Arvanitoyannis and Biliaderis [28] reported on aqueous blends of methyl cellulose and soluble starch, plasticized with glycerol or sugars, prepared by casting or by extrusion and hot pressing. The observed Tg depression for these polymer blends was proportional to the plasticizer content (water, glycerol and sugars). Although glycerol had a greater depressing effect on Tg than sorbitol, the latter had a greater impact, as a plasticizer, on the mechanical properties (higher percentage elongation) of the soluble starch/methyl cellulose blends. Generally the tensile strength and flexural moduli of these blends were shown to decrease drastically with an increase in the total plasticizer content. Lepeniotis et al. [29] prepared dry spin fibers by blending starch acetate (SA) with a degree of substitution (ds) of 2.1–3.0 with cellulose acetate (CA) with a ds of 2.5 to make 25–30 wt% blended solutions in acetone:water. To develop solubility and phase information, a modified central composite face/central composite circumscribe (CCF/ CCC) statistical designed experiment at 5–10 wt% solids was completed. This was based on five factors: (1) acetyl value; (2) acetone versus water concentration; (3) ratio of SA to CA; (4) total weight of solids in the solution; and (5) mixing temperature. The resulting solubility information could be used to predict the stability of SA:CA blends at the higher concentration and temperature ranges commonly used for fiber spinning. The most significant factors influencing phase type was found to be SA concentration and the interaction of SA percentage and total solids concentration. Psomiadou et al. [33] developed edible films made from natural resources—microcrystalline cellulose (MCC), methylcellulose (MC), cornstarch and polyols. Aqueous blends of MCC or MC and cornstarch with or without polyols were prepared by extrusion, hot pressed and studied. After conditioning at different relative humidities, their thermal, mechanical, and water and gas permeability properties were determined. An increase in water or polyol content showed a considerable increase in percentage elongation, but also a decrease in the tensile strength of films. The presence of high cellulose contents increased the tensile strength and decreased the water vapor transmission of the films. The increase in crystallinity with time resulted in decreased gas and water permeability. Peressini et al. [34] studied the rheological properties of starch/methylcellulose-based edible films. The flow curves showed shear-thinning behavior. Mechanical spectra and Cox–Merz superposition of steadyshear viscosity and dynamic viscosity were consistent with polymer solutions containing topological entanglement interactions of chains. Dispersion stability results showed total recovery of the viscoelastic properties of dispersions subject to high strains, as expected for entangled polymers. MC was the main factor influencing the apparent viscosity and viscoelastic properties. Demirgoz et al. [35] developed a method to control the moisture sensitivity of starch/cellulose acetate blends through chemical modifications. In their work, starch-based blends with CA were chemically modified by chain crosslinking. This modification was based on the reaction between the starch hydroxyl groups and tri-sodium tri-meta phosphate. The resulting compounds were characterized by Fourier transform infrared (FTIR) and the respective properties were assessed and compared to the original materials by means of the hydration degree, the degradation behavior, contact angle measurements and mechanical testing. The results showed that the water uptake of these blends could be reduced by up to 15%, and that simultaneously stiffer materials with a less pronounced degradation rate could be obtained. Levy et al. [36] reported on the design of a polysaccharide crossbridging protein which was comprised of a cellulose-binding domain from Clostridium cellulovorans (CBDclos) and a starchbinding domain from Aspergillus niger B1 (SBDAsp). The two genes were fused in-frame via a synthetic elastin gene to construct a cellulose/starch crossbridging protein (CSCP). The CSCP demonstrated a crossbridging ability in different model systems composed of insoluble or soluble starch and cellulose. The fact that different carbohydratebinding modules maintained their binding capacity over a wide range of conditions, without the need for chemical reactions, makes them attractive domains for designing new classes of polysaccharide-binding domains with potential applications in the biomaterial field. Other kinds of aqueous blends are made from chitosan. Kweon et al. [37] reported the preparation of aqueous blends of protein with chitosan. Antheraea pernyi silk fibroin (SF)/chitosan blend ARTICLE IN PRESS L. Yu et al. / Prog. Polym. Sci. 31 (2006) 576–602 581
582 L.Yu et al.Prog.Polym.Sci.31 (2006)576-602 films were prepared by mixing aqueous solution of et al.[39].Starch/chitosan-blended films were A.pernyi SF and acetic acid solution of chitosan. prepared by irradiation of compression-molded The conformation of A.pernyi SF in blended films starch-based mixtures in physical gel state with an was revealed to be a B-sheet structure,mainly due to electron beam at room temperature.The tensile the effect of using acetic acid as a mixing solvent. strength and the flexibility of the starch film were Blending with A.pernyi SF can enhance the thermal significantly improved after the incorporation of decomposition stability of chitosan.Lazaridou 20%chitosan into the starch film.X-ray diffraction and Biliaderis [38]studied the thermo-mechanical (XRD)and SEM analyses of starch/chitosan blend properties of aqueous solution/cast films of chitosan films indicated that there was an interaction and (C),starch/chitosan (SC)and pullulan/chitosan microphase separation between the starch and (PC)using dynamic mechanical thermal analysis chitosan molecules.Furthermore,in order to (DMTA)and large deformation tensile testing. produce a kind of antibacterial films,the starch/ Incorporation of sorbitol (10%and 30%d.b.) chitosan blend films were irradiated.After irradia- and/or adsorption of moisture by the films resulted tion,there was no obvious change in the structure in substantial depression of the glass transition (T) of the starch/chitosan blend films,but antibac- of the polysaccharide matrix due to plasticization. terial activity was induced even when the content For the composite films there was no clear evidence of chitosan was only 5%,due to the degradation of separate phase transitions of the individual of chitosan in blend films under the action of polymeric constituents or a separate polyol phase; irradiation. a rather broad but single drop of elastic modulus, E,and a single tan 6 peak were observed.Tensile 3.Aliphatic polyester blends testing of films adjusted at various levels of moisture indicated large drops in Young's modulus and Aliphatic polyesters have been recognized for tensile strength (omax)with increasing levels of their biodegradability and susceptibility to hydro- polyol and moisture;the sensitivity of the films to lytic degradation.Examples of this group are PLAs, plasticization was in the order of SC>PC>C. which also have the advantage of controllable Modeling of the modulus data with Fermi's crystallinity and hydrophilicity,and therefore over- equation allowed comparison among samples for all degradation rate [40-45].Another family of the fall in modulus around the glass transition zone polyesters being studied widely are poly(hydrox- as a function of moisture content under isothermal yalkanoate)s(PHAs)that occur in nature.They are conditions.Arvanitoyannis et al.[14]developed produced by a wide variety of micro-organisms as films of chitosan and gelatin by casting their an internal carbon and energy storage,as part of aqueous solutions (pH ~4.0)at 60C and evapor- their survival mechanism [46].Bacterially synthe- ating at 22 or 60C (low-and high-temperature sized PHAs have attracted attention because they methods,respectively).The physical (thermal,me- can be produced from a variety of renewable chanical and gas/water permeation)properties of resources and are truly biodegradable and highly these composite films,plasticized with water or biocompatible thermoplastic materials.Biosynthesis polyols,were studied.An increase in the total and characterization of various copolymers,includ- plasticizer content resulted in a considerable de- ing copolymers of hydroxybutyrate (HB)with crease of elasticity modulus and tensile strength, 3-hydroxyvalerate (3HV)[47,48],3-hydroxypropio- whereas the percentage elongation increased.The nate (3HP)[49],3-hydroxyhexanoate (3HH)[50] low-temperature preparation method led to the and 4-hydroxybutyrate (4HB)[51]have been devel- development of a higher percentage of renaturation oped.Over 90 different types of PHA consisting of (crystallinity)of gelatin,which resulted in a various monomers have been reported and the decrease,by one or two orders of magnitude,of number is still increasing [52]. CO2 and O2 permeability in the chitosan/ gelatin blends.An increase in the total plasticizer 3.1.Blends of PLA family content(water,polyols)of these blends was found to be proportional to an increase in their gas Among the family of biodegradable polyesters, permeability. polylactides(i.e.PLA)have been the focus of much Antibacterial starch/chitosan blend film formed attention because they are produced from renewable under the action of irradiation was reported by Zhai resources such as starch,they are biodegradable and
films were prepared by mixing aqueous solution of A. pernyi SF and acetic acid solution of chitosan. The conformation of A. pernyi SF in blended films was revealed to be a b-sheet structure, mainly due to the effect of using acetic acid as a mixing solvent. Blending with A. pernyi SF can enhance the thermal decomposition stability of chitosan. Lazaridou and Biliaderis [38] studied the thermo-mechanical properties of aqueous solution/cast films of chitosan (C), starch/chitosan (SC) and pullulan/chitosan (PC) using dynamic mechanical thermal analysis (DMTA) and large deformation tensile testing. Incorporation of sorbitol (10% and 30% d.b.) and/or adsorption of moisture by the films resulted in substantial depression of the glass transition (Tg) of the polysaccharide matrix due to plasticization. For the composite films there was no clear evidence of separate phase transitions of the individual polymeric constituents or a separate polyol phase; a rather broad but single drop of elastic modulus, E0 , and a single tan d peak were observed. Tensile testing of films adjusted at various levels of moisture indicated large drops in Young’s modulus and tensile strength (smax) with increasing levels of polyol and moisture; the sensitivity of the films to plasticization was in the order of SC4PC4C. Modeling of the modulus data with Fermi’s equation allowed comparison among samples for the fall in modulus around the glass transition zone as a function of moisture content under isothermal conditions. Arvanitoyannis et al. [14] developed films of chitosan and gelatin by casting their aqueous solutions (pH 4.0) at 60 1C and evaporating at 22 or 60 1C (low- and high-temperature methods, respectively). The physical (thermal, mechanical and gas/water permeation) properties of these composite films, plasticized with water or polyols, were studied. An increase in the total plasticizer content resulted in a considerable decrease of elasticity modulus and tensile strength, whereas the percentage elongation increased. The low-temperature preparation method led to the development of a higher percentage of renaturation (crystallinity) of gelatin, which resulted in a decrease, by one or two orders of magnitude, of CO2 and O2 permeability in the chitosan/ gelatin blends. An increase in the total plasticizer content (water, polyols) of these blends was found to be proportional to an increase in their gas permeability. Antibacterial starch/chitosan blend film formed under the action of irradiation was reported by Zhai et al. [39]. Starch/chitosan-blended films were prepared by irradiation of compression-molded starch-based mixtures in physical gel state with an electron beam at room temperature. The tensile strength and the flexibility of the starch film were significantly improved after the incorporation of 20% chitosan into the starch film. X-ray diffraction (XRD) and SEM analyses of starch/chitosan blend films indicated that there was an interaction and microphase separation between the starch and chitosan molecules. Furthermore, in order to produce a kind of antibacterial films, the starch/ chitosan blend films were irradiated. After irradiation, there was no obvious change in the structure of the starch/chitosan blend films, but antibacterial activity was induced even when the content of chitosan was only 5%, due to the degradation of chitosan in blend films under the action of irradiation. 3. Aliphatic polyester blends Aliphatic polyesters have been recognized for their biodegradability and susceptibility to hydrolytic degradation. Examples of this group are PLAs, which also have the advantage of controllable crystallinity and hydrophilicity, and therefore overall degradation rate [40–45]. Another family of polyesters being studied widely are poly(hydroxyalkanoate)s (PHAs) that occur in nature. They are produced by a wide variety of micro-organisms as an internal carbon and energy storage, as part of their survival mechanism [46]. Bacterially synthesized PHAs have attracted attention because they can be produced from a variety of renewable resources and are truly biodegradable and highly biocompatible thermoplastic materials. Biosynthesis and characterization of various copolymers, including copolymers of hydroxybutyrate (HB) with 3-hydroxyvalerate (3HV) [47,48], 3-hydroxypropionate (3HP) [49], 3-hydroxyhexanoate (3HH) [50] and 4-hydroxybutyrate (4HB) [51] have been developed. Over 90 different types of PHA consisting of various monomers have been reported and the number is still increasing [52]. 3.1. Blends of PLA family Among the family of biodegradable polyesters, polylactides (i.e. PLA) have been the focus of much attention because they are produced from renewable resources such as starch, they are biodegradable and ARTICLE IN PRESS 582 L. Yu et al. / Prog. Polym. Sci. 31 (2006) 576–602
L Yu et al Prog.Polym Sct31(2006)576-602 compostable.and they have very low or no toxicity Controlling the hydrolytic degradability of plas and high mechanical performance,comparable to is of great importance,and numerous studies have those of commercial polymers.However,the been undertaken to elucidate the effects of various thermal stability of PLAs is generally not suffi- components in the systems.Shinoda et al.[58]used ciently high enough for them to be used as an degrad .54 various A copolymer ti ic PLAs po(-lactic acid)(PLLA)ad poly(p-lactide)(ie inher rties of pI A films maintained sufficient transparency The blen olv(p-lactic acid)(PDIA))due to the stro which is interaction between PLLA and PDLA chains [54] one of the most valuable advantages of pure The stereocomplexed PLLA/PDLA blend has a PLA.The increa sed hydrophilicity of the surface melting temperature PLLA C)ca. suggests that PAL and poly(sod m aspartate-c DL may us antistat nd trengt agent a0 up to m [5. PA the d with that of th and PDLA.even when it is amorphous hye PLLA PLA ted hydro unles made,due again to the strong interaction between PLLA and shelf life and be useful for a wide variety of PDLA chains [56,57].X-ray diffractometry and applications.PAL was also found to be effective differen calorimetry (DSC)elucidated as an additive that incre ases the non-enzymati that all the initially amorphous remaine s of both PB cton sis Io mprove A D containing an appreciabl Also the 1。 PLLA is ally hard and brittle,which hinders 220'C for the PLLA/blend film after its usage in medical ap lication s ie orthopedic and the hydrolysis for 24 months were ascribed to those dental surgery.Poly DL-lactic acid (PDLLA)can of homo-and stereocomplex crystallites degrade quickly due to its amorphous structure. tively formed during heating at around 100 and thus the degradation time of PLLA/PDLLA blends 2009 but not during the aut can be controlled through various blending ratio the thes ings enantiome lend the PDL in addi rd com the sur ymer of ethylene and Fukui 53 studied the films a solvent.DSC data ndicate -lactide)(i.e.PLLA)and poly(D-lactide)(i.e that PLLA/PDLLA blends without the surfactan PDLA).and their equimolar enantiomeric blend had two values.With the addition of the (PLLA/PDLA).The films were prepared and the surfactant,there was a linear shift of the single T effects of enantiome c polymer ble nding on the as a function of composition,with lower percen thermal st and degra dation of the films were tages of PLLA produci ng lower glass y an non-isothe emperatures. better nitroger eve bl LA/PI enhance the thermal stability of the without the had high PLLA/PDLA films compared with those of the pure modulus and elongation. and similar results PLLA and PDLA films.The AE value of the L/D were observed after adding 2% surfactant into film was in the range of 205-297 kJ mol-,which was the blends.The 50/50 PLLA/PDLLA/2%surfac higher by 82-110kJmol than the averaged△E tant blend had the highest elastic modulus,yield value of the L and D films. strength and break strength compared with other
compostable, and they have very low or no toxicity and high mechanical performance, comparable to those of commercial polymers. However, the thermal stability of PLAs is generally not suffi- ciently high enough for them to be used as an alternative in many commercial polymers applications [53]. Ikada et al. [54] studied various PLA blends to improve their thermal properties. A stereocomplex is formed from enantiomeric PLAs, poly(L-lactic acid) (PLLA) and poly(D-lactide) (i.e. poly(D-lactic acid) (PDLA)) due to the strong interaction between PLLA and PDLA chains [54]. The stereocomplexed PLLA/PDLA blend has a melting temperature (Tm) (220–230 1C) ca. 50 1C higher than those of pure PLLA and PDLA (170–180 1C), and can retain a non-zero strength in the temperature range up to Tm [55]. Moreover, the PLLA/PDLA blend has a higher hydrolysis resistance compared with that of the pure PLLA and PDLA, even when it is amorphous-made, due again to the strong interaction between PLLA and PDLA chains [56,57]. X-ray diffractometry and differential scanning calorimetry (DSC) elucidated that all the initially amorphous PLA films remained amorphous, even after autocatalytic hydrolysis for 16 (PDLLA film) and 24 (non-blended PLLA and PDLA films, PLLA/PDLA(1/1) blend film) months. Also, the melting peaks observed at around 170 and 220 1C for the PLLA/PDLA(1/1) blend film after the hydrolysis for 24 months were ascribed to those of homo- and stereocomplex crystallites, respectively, formed during heating at around 100 and 200 1C, but not during the autocatalytic hydrolysis. On the basis of these findings, enantiomeric polymer blending is expected to enhance the thermal stability of the PLLA/PDLA blend in the melt compared with those of the pure PLLA and PDLA. Tsuji and Fukui [53] studied the films of poly(L-lactide) (i.e. PLLA) and poly(D-lactide) (i.e. PDLA), and their equimolar enantiomeric blend (PLLA/PDLA). The films were prepared and the effects of enantiomeric polymer blending on the thermal stability and degradation of the films were investigated isothermally and non-isothermally under nitrogen gas using thermogravimetry. The enantiomeric polymer blending was found to successfully enhance the thermal stability of the PLLA/PDLA films compared with those of the pure PLLA and PDLA films. The DEtd value of the L=D film was in the range of 205–297 kJ mol–1, which was higher by 82–110 kJ mol–1 than the averaged DEtd value of the L and D films. Controlling the hydrolytic degradability of PLAs is of great importance, and numerous studies have been undertaken to elucidate the effects of various components in the systems. Shinoda et al. [58] used poly(aspartic acid-co-lactide) (PAL) to accelerate the degradation of PLA. PAL, an amphiphilic copolymer obtained from aspartic acid and lactide, was found to be miscible with PLA and produced homogeneous blend films without impairing the inherent mechanical properties of PLA. The blend films maintained sufficient transparency, which is one of the most valuable advantages of pure PLA. The increased hydrophilicity of the surface suggests that PAL and poly(sodium aspartate-colactide) (PALNa) may be useful as antistatic agents for PLA films. The addition of a small amount of PAL enhanced the degradation rate of PLA in water, soil and compost. The blended PAL resisted hydrolysis unless it contacts water, which ensures that the blend products could have a long shelf life and be useful for a wide variety of applications. PAL was also found to be effective as an additive that increases the non-enzymatic hydrolysis rates of both PBS and polycaprolactone (PCL). PAL also improved the thermal stability of PLA containing an appreciable amount of residual catalyst. PLLA is usually hard and brittle, which hinders its usage in medical applications, i.e. orthopedic and dental surgery. Poly DL-lactic acid (PDLLA) can degrade quickly due to its amorphous structure, thus the degradation time of PLLA/PDLLA blends can be controlled through various blending ratios. Chen et al. [59] prepared blends of biodegradable PLLA and PDLLA, in addition to a third component, the surfactant—a copolymer of ethylene oxide and propylene oxide—at various ratios using dichloromethane as a solvent. DSC data indicated that PLLA/PDLLA blends without the surfactant had two Tg values. With the addition of the surfactant, there was a linear shift of the single Tg as a function of composition, with lower percentages of PLLA producing lower glass transition temperatures, indicating that better miscibility had been achieved. Dynamical mechanical analysis (DMA) data showed that the 40/60 PLLA/PDLLA blends without the surfactant had high elastic modulus and elongation, and similar results were observed after adding 2% surfactant into the blends. The 50/50 PLLA/PDLLA/2% surfactant blend had the highest elastic modulus, yield strength and break strength compared with other ARTICLE IN PRESS L. Yu et al. / Prog. Polym. Sci. 31 (2006) 576–602 583
584 L Yu et al.Prog.Polym.Sci 31 (2006)576-602 ratios of PLLA/PDLLA/2% Th includi wa 3-hv an elast break of 50/50 PLLA/PDLLA2%surfactant hexanoate (phoH t ay have almost 1.2-1.9 times higher than that of 50/50 tions as tissue scaffolds and cardiovascular tissue PLLA/PDLLA and PLLA.Elongation of PLLA Chen and colleagues [61.63,64]have systematically increased with the addition of PCL,but the strength studied various PHB blends to suit biomedica decreased at the same time.In conclusion,adding applications The biocompatibility of microbial PDLLA and surfactant to PLLA via solutio y be an en e way to make PLL. ate (PHE nd' able fo or use in orthopec 161 wa PHB d p wth To achieve a similar outcome.Uravama et al.[60 wth on the films ade g PHB and develoned blends of n PHBHHx showed a dramatic impro ment Bio L-isomeric ratios of the lactate units (PLA99.0 and compatibility was also strongly improved when 77.0,where the numbers correspond to the L-ratios) these polymers were treated with lipases and NaOH. The crystallinity of the blends was similar to that respectively.However,the eflects of treatment were of the blends of PLL A and PDLLA. The glas weakened the PHBHHx content was in transiti r was ind or the crease in the blend was loun hat the lipase the bi tibility more of th NaOH ratio while their th PHB wa s PLA with decreasing compe tion of PLA99.0.Aboy PHBHHx and its dominant blends sho d im the T the sto rage modulus of the blends dropped proved biocompatibility compared to PLA.It was from 2-3 x 10%to 1-3 x 10 Pa and then increased also shown that chondrocytes proliferated better on to a different level depending on the crystalline the PHBHHx/PHB scaffolds than on the PHB one nature of the blends The biodegradability of the 63].Furthermore. the hlms made from blending ing composition of polyes that the ty ca PHBHH del 106% f th Oth PLA 163 samples [601. Detailed studies have shown that th e high deg of crystallization and rapid crystallization rate of 3.2.Blends of PHA family PHB generates pores and protrusions on the PHB film surface.This coralloid surface could prohibit the Bacterially synthesized PHAs attract much atten attachment and growth of mammalian cells [65].The tion because they can be produced from a varietyo presence of PHBHHx in PHB strongly redu ced both resource de PHB. a山 BHHx/PHB ded viron sustainable which allowed cel society.Over 90 different types of PHA consistin and growth.thus strongly of various monomers have been reported and th the biocompatibility of PHB [651.While these results number is increasing.Some PHAs behave similarly explained the improvement in biocompatibility to conventional plastics such as polyethylene and brought about by PHBHHx,they also demons polypropylene,while others are elastomeric [61] of using polymer blends of PHBHHx aing offers much scope for expand PHB as scallo or tissue ing e PHB are u ally ep of this family i dro 661 studied nd b atibility,PHA kinetics of poly(3-hyd serve as a material for tissue engi eering [62].PHA. lerate)(PHB-HV)and PHB/PHB-HV blends.It was
ratios of PLLA/PDLLA/2% surfactant blends. The elongation at break of 50/50 PLLA/PDLLA was similar to that of PLLA. Again, the elongation at break of 50/50 PLLA/PDLLA/2% surfactant was almost 1.2–1.9 times higher than that of 50/50 PLLA/PDLLA and PLLA. Elongation of PLLA increased with the addition of PCL, but the strength decreased at the same time. In conclusion, adding PDLLA and surfactant to PLLA via solution blending may be an effective way to make PLLA tougher and more suitable for use in orthopedic or dental applications. To achieve a similar outcome, Urayama et al. [60] developed blends of polylactides with high and low L-isomeric ratios of the lactate units (PLA99.0 and 77.0, where the numbers correspond to the L-ratios). The crystallinity of the blends was similar to that of the blends of PLLA and PDLLA. The glass transition behavior was indicative of the compatible nature of both polymers. The tensile modulus of the blends was almost identical irrespective of the blend ratio, while their tensile strength decreased with decreasing composition of PLA99.0. Above the Tg, the storage modulus of the blends dropped from 2–3 109 to 1–3 106 Pa and then increased to a different level depending on the crystalline nature of the blends. The biodegradability of the blends increased with decreasing composition of PLA99.0. This difference in degradability can be explained by a random packing model of local helices of the L-sequenced chains for the L-rich PLA samples [60]. 3.2. Blends of PHA family Bacterially synthesized PHAs attract much attention because they can be produced from a variety of renewable resources, and are truly biodegradable and highly biocompatible thermoplastic materials. Therefore, PHAs are expected to contribute to the construction of an environmentally sustainable society. Over 90 different types of PHA consisting of various monomers have been reported and the number is increasing. Some PHAs behave similarly to conventional plastics such as polyethylene and polypropylene, while others are elastomeric [61]. Therefore, blending offers much scope for expanding their range of applications. The most representative member of this family is poly(3-hydroxybutyrate) (PHB). Due to its biodegradability and biocompatibility, PHA may well serve as a material for tissue engineering [62]. PHA, including fragile PHB, a flexible copolymer consisting of PHBV, and an elastomeric copolymer consisting of 3-hydroxyoctanoate and 3-hydroxyhexanoate (PHOH) may have promising applications as tissue scaffolds and cardiovascular tissue. Chen and colleagues [61,63,64] have systematically studied various PHB blends to suit biomedical applications. The biocompatibility of microbial polyesters PHB and poly(hydroxybutyrate-co-hydroxyhexanoate) (PHBHHx) were evaluated in vitro [61]. It was found that the growth of cells L929 was poor on PHB and PLA films, but the growth on the films made by blending PHB and PHBHHx showed a dramatic improvement. Biocompatibility was also strongly improved when these polymers were treated with lipases and NaOH, respectively. However, the effects of treatment were weakened when the PHBHHx content was increased in the blends. It was found that the lipase treatment increased the biocompatibility more than NaOH. After the treatment the biocompatibility of PHB was approximately the same as PLA, while PHBHHx and its dominant blends showed improved biocompatibility compared to PLA. It was also shown that chondrocytes proliferated better on the PHBHHx/PHB scaffolds than on the PHB one [63]. Furthermore, the films made from blending polyesters showed that the elongation to break of the blended PHBHHx/PHB film increased from 15% to 106% when the PHBHHx content in the blend increased from 40% to 60% [63]. Detailed studies have shown that the high degree of crystallization and rapid crystallization rate of PHB generates pores and protrusions on the PHB film surface. This coralloid surface could prohibit the attachment and growth of mammalian cells [65]. The presence of PHBHHx in PHB strongly reduced both the degree of crystallization and the crystallization rate of PHB. The low degree of crystallization of PHBHHx/PHB blends provided films with a fairly regular and smooth surface, which allowed cell attachment and growth, thus strongly improving the biocompatibility of PHB [65]. While these results explained the improvement in biocompatibility brought about by PHBHHx, they also demonstrated the feasibility of using polymer blends of PHBHHx/ PHB as scaffold materials for tissue engineering. Blends of the PHB family are usually compatible and co-crystallization is enhanced. Yoshie et al. [66] studied solid-state structures and crystallization kinetics of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHB-HV) and PHB/PHB-HV blends. It was ARTICLE IN PRESS 584 L. Yu et al. / Prog. Polym. Sci. 31 (2006) 576–602
L Yu et al Prog.Polym Sct31(2006)576-602 585 omd时me he min component.the difference in the solubility para meters 6 and 62 of the blend components in the by high-resolution solid-state C NMR spectro Flory-Huggins equation was estimated to be 0.34 scopy.As co-crystallizable blends,both the blends (Jcm). show complete co-crystallization i.e.the PHE Zhang et al.[7]reported that PHB/PLA blend ame prepared by casting a film from solven On the e over the range PHB-HV blends forr PHB-rich phase had a thicker am lave than that of ate The rystallization of the pHB-Hy conolymers with the same overall Hv PHB in the blends was affected by the level of PLA content.Saad [67]studied the miscibility,melting addition.The thermal history caused a depression and crystallization behavior of poly[(R)-3-hydro- of the melting point and a decrease in the crystal xybutyrate](PHB)and oligo[(RS)-3-hydroxybuty the blends Compared with plair ble sing a DS blend P prop 68s 1g0 HB s "eo 10 racteris the mi ly[(R.S)-3 tion don ndont T and a the hydr atel (atapHB) brium melting te mperature of PHB.The negative weights using DSC and optical micro DSC value of the interaction parameter.determined thermograms for the blends of PLA and ataPHB from the equilibrium melting depression,confirms with Mw=9400 in the range from 0 to 50 wt%of misc bility between blend components. In paralle ataPHB content showed a single glass transition studie transition of differen 20 C,an the valu 0 lig HR Polym temperature after from 59 aPHB the an th aled the iscible ( -relaxation process for blends,indicating ity between the amorphous fractions of PHB and of PLA with high molecular weight ataPHB oligo-HB (M=140.000)showed two glass transition tem- peratures indicating that the binary blend was 3.3.PLA/PHB blends immiscible in the melt.The radial growth rate of e accel PLA/PHB blends have ben stud ied wi weigh edbytheaddnionoi atap o prod -D ts showed th sical PLA of cry sibility【68.Oho5h ct al.1681 and ed hy the ount of Doi [69]studied the miscibility of binary blends of ataPHB component.sugs sting that the addition of bacterial poly[(R)-3-hydroxybutyric acid](P[(R)- ataPHB-3 component facilitated crystallization of 3HBD)with poly[(S)-lactic acid](P[(S)-LAJ)of PLA components in the binary blends.The lamellar various molecular weights using DSC analysis thickness of PLA crystals decreased slightly with an which revealed that the structure of P[(R)-3HB] increase in ataPHB content,suggesting that ataPHB MI(S-LA blen nde on th The PI(R-3HBI of PLA SP e re xation phenon M ses in the melt the dynamic ctra for PLA/ataPHB at 200C.while the blends of P[(R)-3HB]with P[(S) LAl of M values below 18.000 were miscible ir crystallized at 120C,indicating that the partial the melt over the whole composition range.On the phase separation of two components occurs in the basis of the relationship between the miscibility of amorphous phase during the isothermal crystal- blends and the molecular weight of the P[(S)-LA lization process
found that PHB and HV can co-crystallize, and the content in the co-crystalline phase was determined by high-resolution solid-state 13C NMR spectroscopy. As co-crystallizable blends, both the blends show complete co-crystallization, i.e. the PHB content in the crystalline phase is the same as that of the whole blends, and the blends form a PHBrich crystalline phase. On the other hand, the PHB/ PHB-HV blends forming a PHB-rich crystalline phase had a thicker amorphous layer than that of the PHB-HV copolymers with the same overall HV content. Saad [67] studied the miscibility, melting and crystallization behavior of poly[(R)-3-hydroxybutyrate] (PHB) and oligo[(R,S)-3-hydroxybutyrate]-diol (oligo-HB) blends using a DSC. It was found that the thermograms of blends containing up to 60 wt% oligo-HB showed behavior characteristic of single-phase amorphous glasses with a composition-dependent Tg and a depression in the equilibrium melting temperature of PHB. The negative value of the interaction parameter, determined from the equilibrium melting depression, confirms miscibility between blend components. In parallel studies, glass transition relaxations of different melt-crystallized polymer blends containing 0–20 wt% oligo-HB were dielectrically investigated between –70 and 120 1C in the 100 Hz–50 kHz range. The results revealed the existence of a single a-relaxation process for blends, indicating miscibility between the amorphous fractions of PHB and oligo-HB. 3.3. PLA/PHB blends PLA/PHB blends have been studied with the aim of producing PLA-based materials with a wide range of physical properties and improved processibility [68]. Ohkoshi et al. [68] and Koyama and Doi [69] studied the miscibility of binary blends of bacterial poly[(R)-3-hydroxybutyric acid] (P[(R)- 3HB]) with poly[(S)-lactic acid] (P[(S)-LA]) of various molecular weights using DSC analysis, which revealed that the structure of P[(R)-3HB]/ P[(S)-LA] blends was strongly dependent on the molecular weight of the P[(S)-LA] component. The blends of P[(R)-3HB] with P[(S)-LA] of Mw values over 20,000 showed two phases in the melt at 200 1C, while the blends of P[(R)-3HB] with P[(S)- LA] of Mw values below 18,000 were miscible in the melt over the whole composition range. On the basis of the relationship between the miscibility of blends and the molecular weight of the P[(S)-LA] component, the difference in the solubility parameters d1 and d2 of the blend components in the Flory–Huggins equation was estimated to be 0.34 (J cm–3). Zhang et al. [70] reported that PHB/PLA blends prepared by casting a film from a common solvent at room temperature were immiscible over the range of compositions studied, while the melt-blended sample prepared at high temperature showed some evidence of greater miscibility. The crystallization of PHB in the blends was affected by the level of PLA addition. The thermal history caused a depression of the melting point and a decrease in the crystallinity of PHB in the blends. Compared with plain PHB, the blends exhibited an improvement in mechanical properties. Ohkoshi et al. [68] studied the miscibility and phase structure of binary blends of poly[(S)-lactide] (PLA) with atactic poly[(R,S)-3- hydroxybutyrate] (ataPHB) of different molecular weights using DSC and optical microscopy. DSC thermograms for the blends of PLA and ataPHB with Mw ¼ 9400 in the range from 0 to 50 wt% of ataPHB content showed a single glass transition temperature after melting at 200 1C, and the value decreased from 59 to 10 1C with an increase in ataPHB content, indicating that the PLA and low molecular weight ataPHB (Mw ¼ 9400) are miscible in the melt at 200 1C within the ataPHB content up to 50 wt%. In contrast, the binary blends of PLA with high molecular weight ataPHB (Mw ¼ 140,000) showed two glass transition temperatures, indicating that the binary blend was immiscible in the melt. The radial growth rate of PLA spherulites were accelerated by the addition of low molecular weight ataPHB components. X-ray results showed that the level of crystallinity of the PLA components in the melt-crystallized films were increased by the addition of a small amount of ataPHB component, suggesting that the addition of ataPHB-3 component facilitated crystallization of PLA components in the binary blends. The lamellar thickness of PLA crystals decreased slightly with an increase in ataPHB content, suggesting that ataPHB component was incorporated into the interlamellar region of PLA spherulites. The relaxation phenomena were detected at two different temperatures in the dynamic mechanical spectra for PLA/ataPHB blends (containing ataPHB contents over 15 wt%) crystallized at 120 1C, indicating that the partial phase separation of two components occurs in the amorphous phase during the isothermal crystallization process. ARTICLE IN PRESS L. Yu et al. / Prog. Polym. Sci. 31 (2006) 576–602 585
