J Mater Sci(2008)43:6747-6757 753 elastic response of systems with coordinated movement of magnitude greater specific surface area of the true nano- large number of atoms, such as observed in polymer fillers, almost all the polymer chains are in contact with the opposites [23]- The role of the nano-scale"interphase"to control the addition, continuum mechanics has only limited validity at posites(vr < 0.05)and high ve nano-composites(vr>0.85). viscoelastic response of the nano-composite matrix e. rformance and reliability of the FrC parts has to be this length scale and the discrete molecular structure pre- considered from two perspectives: (i)low vr nano-com- vails resulting in strong effect of non-local character of The (i) represents the direction to preparing new nano- The interphases in high vr nano-composites were studied structured advanced matrices while the (ii) leads to using the abalone shells [36]. These shells represent a designing new nano-structured advanced reinforcements. laminated sheet reinforced composite with over 95 vol % o of With few exceptions, most of the published literature on the aligned 500 nm thin aragonite sheets embedded in a protein synthetic nanocomposites deals with the low vr nanocom- matrix in apparently mesh-like fibrillar form(Fig. 5). In the posites [24-31], while, on the other hand, most of the work of Hansma and co-workers [12, 13], the model literature published on high vr nano-composites is related to sacrificial bonds has been proposed to explain the observed the mechanics of bio-composites such as bones, teeth and high-fracture resistance of nacreous composites. It has been shells [10-13 shown by Zidek and Jancar [37 ], that the hypothesis of the Most of the experimental evidence related to the inter- sacrificial bonds can also be used to simulate deformation phase in the low vr nano-composites were obtained at response of lightly cross-linked long flexible chain network temperatures below the polymer Tg using meso-scale test polymer fibril. In order to apply the model [37] to the specimens. Assuming the chain immobilization to be the behavior of an ensamble of chains in the vicinity of rigid primary reinforcing mechanism on the nano-scale, spatial weakly attractive nanometer sized inclusion, the immoil- distribution of the conformation entropy within the poly- ization phenomenon has to be investigated as the source of mer phase is of primary importance. Hence, experimental the drastic change in the viscoelastic behavior of polymers data for nano-composites above the matrix Tg has to be with addition of small amount of nano-scale inclusion considered. Sternstein at al [32] published interpretation of the viscoelastic response of rubbery nanocomposite above the matrix Tg, i.e., the Payne effect. Kalfus and Jancar [33, Chain immobilization on the nanoscale 64] analyzed the viscoelastic response of polyvinylacetate filled with nano-sized hydroxyapatite over the temperature Reducing the size of rigid inclusions from micro-to nano- range from -40 to +120C and observed strain softening scale is accompanied by 2-3 orders of magnitude increase in similar to the Payne effect [35]. The modulus recovery the internal contact area between the chains and the inclu- experiments allowed to determine the terminal relaxation sions. Moreover, above 2 vol % o nano-particle content, the time of reptation motion of bulk and surface immobilized average interparticle distance is reduced below 2 radii of chains, supporting the hypothesis that there is no"inter- gyration, Rg, of the chains. Hence, almost all the chains are hase"per se when nano-scale is considered. In order to in contact with the solid surface, possess reduced segmenta bridge the gap between the continuum interphase on the mobility at temperatures T2 Tg Below Tg, main chain micro-scale and the discrete molecular structure of the segmental mobility is frozen and only secondary low tem- matrix consisting of freely reptating chains in the bulk and perature side chain mobility can be affected. In addition, the retarded reptating chains in contact with the inclusions, conformation statistics of chains near solid surface can be higher-order elasticity combined with a suitable molecular altered from Gaussian random coil to Langevin coil above dynamics model could be utilized Tg and this phenomenon can be transformed into the was demonstrated, that the large specific surface area behavior of immobilized chains also upon solidification of the nanosized filler is capable of immobilizing large below Tg entanglements causing the steep increase of E In order to characterize the reduction in chain mobility addition of nanoparticles. This observation in an entangled melt quantitatively, one can use the char- seemed to confirm the purely entropic character of the acteristic reptation relaxation time, Trep, introduced by reinforcement mechanism on the nanoscale. All the data de Gennes [38. The trep is given for an entangled chain as: published support the dominant role of the chain immobi- L2 NL2 lization as the main reinforcing mechanism. light of the seems that the term "interphase"defined as a continuum where L is the length of the reptation path, N the number of phase of limited extent looses its physical meaning when monomer units in a chain, De and Do are diffusion considering true nano-composites. Due to two orders of constants of a chain and a monomer, respectively. The 2 Springerelastic response of systems with coordinated movement of large number of atoms, such as observed in polymer composites [23]. The role of the nano-scale ‘‘interphase’’ to control the performance and reliability of the FRC parts has to be considered from two perspectives: (i) low vf nano-com￾posites (vf\0.05) and high vf nano-composites (vf[0.85). The (i) represents the direction to preparing new nano￾structured advanced matrices while the (ii) leads to designing new nano-structured advanced reinforcements. With few exceptions, most of the published literature on the synthetic nanocomposites deals with the low vf nanocom￾posites [24–31], while, on the other hand, most of the literature published on high vf nano-composites is related to the mechanics of bio-composites such as bones, teeth and shells [10–13]. Most of the experimental evidence related to the inter￾phase in the low vf nano-composites were obtained at temperatures below the polymer Tg using meso-scale test specimens. Assuming the chain immobilization to be the primary reinforcing mechanism on the nano-scale, spatial distribution of the conformation entropy within the poly￾mer phase is of primary importance. Hence, experimental data for nano-composites above the matrix Tg has to be considered. Sternstein at al [32] published interpretation of the viscoelastic response of rubbery nanocomposite above the matrix Tg, i.e., the Payne effect. Kalfus and Jancar [33, 34] analyzed the viscoelastic response of polyvinylacetate filled with nano-sized hydroxyapatite over the temperature range from -40 to ?120 C and observed strain softening similar to the Payne effect [35]. The modulus recovery experiments allowed to determine the terminal relaxation time of reptation motion of bulk and surface immobilized chains, supporting the hypothesis that there is no ‘‘inter￾phase’’ per se when nano-scale is considered. In order to bridge the gap between the continuum interphase on the micro-scale and the discrete molecular structure of the matrix consisting of freely reptating chains in the bulk and retarded reptating chains in contact with the inclusions, higher-order elasticity combined with a suitable molecular dynamics model could be utilized. It was demonstrated, that the large specific surface area of the nanosized filler is capable of immobilizing large amount of entanglements causing the steep increase of E0 with small addition of nanoparticles. This observation seemed to confirm the purely entropic character of the reinforcement mechanism on the nanoscale. All the data published support the dominant role of the chain immobi￾lization as the main reinforcing mechanism. In the light of the existing experimental evidence, it seems that the term ‘‘interphase’’ defined as a continuum phase of limited extent looses its physical meaning when considering true nano-composites. Due to two orders of magnitude greater specific surface area of the true nano- fillers, almost all the polymer chains are in contact with the surface at very low filler loadings above 2 vol.%. In addition, continuum mechanics has only limited validity at this length scale and the discrete molecular structure pre￾vails resulting in strong effect of non-local character of viscoelastic response of the nano-composite matrix. The interphases in high vf nano-composites were studied using the abalone shells [36]. These shells represent a laminated sheet reinforced composite with over 95 vol.% of aligned 500 nm thin aragonite sheets embedded in a protein matrix in apparently mesh-like fibrillar form (Fig. 5). In the work of Hansma and co-workers [12, 13], the model of sacrificial bonds has been proposed to explain the observed high-fracture resistance of nacreous composites. It has been shown by Zidek and Jancar [37], that the hypothesis of the sacrificial bonds can also be used to simulate deformation response of lightly cross-linked long flexible chain network polymer fibril. In order to apply the model [37] to the behavior of an ensamble of chains in the vicinity of rigid weakly attractive nanometer sized inclusion, the immoil￾ization phenomenon has to be investigated as the source of the drastic change in the viscoelastic behavior of polymers with addition of small amount of nano-scale inclusions. Chain immobilization on the nanoscale Reducing the size of rigid inclusions from micro- to nano￾scale is accompanied by 2–3 orders of magnitude increase in the internal contact area between the chains and the inclu￾sions. Moreover, above 2 vol.% nano-particle content, the average interparticle distance is reduced below 2 radii of gyration, Rg, of the chains. Hence, almost all the chains are in contact with the solid surface, possess reduced segmental mobility at temperatures T C Tg. Below Tg, main chain segmental mobility is frozen and only secondary low tem￾perature side chain mobility can be affected. In addition, the conformation statistics of chains near solid surface can be altered from Gaussian random coil to Langevin coil above Tg and this phenomenon can be transformed into the behavior of immobilized chains also upon solidification below Tg. In order to characterize the reduction in chain mobility in an entangled melt quantitatively, one can use the char￾acteristic reptation relaxation time, srep, introduced by deGennes [38]. The srep is given for an entangled chain as: srep ffi L2 Dc ffi NL2 D0 ; ð1Þ where L is the length of the reptation path, N the number of monomer units in a chain, Dc and D0 are diffusion constants of a chain and a monomer, respectively. The J Mater Sci (2008) 43:6747–6757 6753 123