J Mater Sci(2008)43:6747-6757 [14]. Even though the molecular structure of the interphase temperatures of engineering thermoplastics ranging from has been anticipated in many papers, with some exceptions 230 to 350C can exceed the thermal stability of the [15-17]. the main effort has been devoted to the relation- commonly utilized organosilanes ship between the type, thickness and deposition conditions Thickness dependence of the elastic modulus of thin of the fiber coating and the average shear strength of the polycarbonate (PC) layers deposited on a flat E-glass interphase, ta, measured in a simple test employing model substrate was measured over the thickness interval ranging single fiber composite [18]. from 10 to 30 nm [ 9, 21, 22). In all cases investigated Mechanical properties and environmental stability of elastic moduli of the deposited layers, Ei, decreased composites are strongly dependent upon the stability of the constant bulk value for layers thicker than 5 X 10'mr9 both fiber reinforced and particulate filled thermoplastic monotonically with increasing layer thickness reaching interfacial region between the matrix and fibers, especially(Fig. 4). Thermally annealed PC and SiCl4 grafted oligo- when exposed to moist environment. This is of particular PC interphases, exhibited higher elastic moduli than the as importance in glass fiber reinforced thermoplastic compos- received solution deposited PC interphase. No effect of ites since the glass fibers are highly hygroscopic and the thermal annealing on elastic modulus of strongly bonded bond between the fibers and the thermoplastic matrix is oligo-PC interphase was observed. It has been shown that usually weak. Hence, the tailoring of well-bonded, durable the shear strength of the interface, Ta, measured in a sin- interphases between the thermoplastic matrix and glass glefiber fragmentation test exhibited strong dependence on reinforcement has become a critical concern. The use of the interphase Ei. coupling agents, chemically reactive with both matrix and Similarly to the PC interphases, elastic moduli of the reinforcement, and/or chemical modification of the surfaces deposited silane layers decreased monotonically with of one or both constituents have been the most successful increasing layer thickness reaching a bulk value for layers means of providing reasonably well controlled bond thicker than 10 nm [21, 22]. Reactive chlorine containing between matrix and the encapsulated reinforcement [ 19] silane formed always stiffer layers compared to its alkoxy From the published data, it seems clear that a mono- analogues most probably due to stronger interaction molecular interphase layer with engineered molecular between the chlorine and glass surface and, most probably structure specific for the desired combination of resin and due to less defective network structure (Fig. 5). This reinforcement should result in the most favorable mix of hypothesis was further supported by the observed strong properties in thermoplastic matrix composites. Reactive effect of deposition technique, controlling the layer su- end-capped polymers capable of chemically reacting with permolecular structure, on the layer elastic modulus the fiber surface or various methods of grafting matrix Solution deposition technique yielded always layers with molecules onto reinforcement surface are the most prom- lower elastic modulus compared to the layers formed by rf- ising candidates for further investigations [17, 20]. plasma or rf-plasma enhanced CVd deposition of the same Thickness of the interphase can be controlled via modifi- substance. A qualitative explanation of the observe cation of the molecular weight and chain stiffness of the behavior was provided assuming formation of strongly constituent molecules, its mechanical properties can be immobilized layer of constant thickness, t;, and elastic varied by selecting the backbone chain constitution and modulus, Ei, near the bonded interface. Strength of the configuration, and its surface free energy can also be interfacial bond and network density of the polysiloxane controlled by the chain constitution and by the polarity of interphase were proposed to be the factors determining the end-groups. Elastic properties of these layers are con- and Ei for the given external conditions. Experimental data trolled by the attraction forces at the interface as well as the showed that the contribution of this strongly immobilized conformation entropy of the chains forming the layer. layer started to play an important role for interphase Organofunctional silanes are so far the most widely used thickness below 10 nm. This"inner"layer has been coupling agents for improvement of the interfacial adhe- covered with weaker "outer"layer with more defective sion in glass reinforced materials [19]. Upon application of network structure. The thickness of the"outer"layer was a silane from either dilute solution or the vapor phase, a dependent on the concentration of the silane solution it was highly crosslinked multilayer siloxane "interphase"is deposited from. The difference in E; between the outer presumably formed with thickness ranging from 1.5 to inner interphase layer was increasing with strengthening 500 nm. Unlike in thermosetting matrices with extensive the layer-surface interaction interpenetration between organosilane layer and the matrix In order to enhance the performance and reliability of monomer,long chain molecules do not interpenetrate the the FRC structures at the macro-scale, the results obtained organosilane layers significantly. On the other hand, for the micro-scale interphase can be used to control the immobilization phenomena are of a greater importance in stress transfer between the matrix and the reinforcement hermoplastic matrix composites. Moreover, processing Stiff interphases provide very efficient stress transfer, less 2 Springer[14]. Even though the molecular structure of the interphase has been anticipated in many papers, with some exceptions [15–17], the main effort has been devoted to the relation￾ship between the type, thickness and deposition conditions of the fiber coating and the average shear strength of the interphase, sa, measured in a simple test employing model single fiber composite [18]. Mechanical properties and environmental stability of both fiber reinforced and particulate filled thermoplastic composites are strongly dependent upon the stability of the interfacial region between the matrix and fibers, especially when exposed to moist environment. This is of particular importance in glass fiber reinforced thermoplastic compos￾ites since the glass fibers are highly hygroscopic and the bond between the fibers and the thermoplastic matrix is usually weak. Hence, the tailoring of well-bonded, durable interphases between the thermoplastic matrix and glass reinforcement has become a critical concern. The use of coupling agents, chemically reactive with both matrix and reinforcement, and/or chemical modification of the surfaces of one or both constituents have been the most successful means of providing reasonably well controlled bond between matrix and the encapsulated reinforcement [19]. From the published data, it seems clear that a mono￾molecular interphase layer with engineered molecular structure specific for the desired combination of resin and reinforcement should result in the most favorable mix of properties in thermoplastic matrix composites. Reactive end-capped polymers capable of chemically reacting with the fiber surface or various methods of grafting matrix molecules onto reinforcement surface are the most prom￾ising candidates for further investigations [17, 20]. Thickness of the interphase can be controlled via modifi- cation of the molecular weight and chain stiffness of the constituent molecules, its mechanical properties can be varied by selecting the backbone chain constitution and configuration, and its surface free energy can also be controlled by the chain constitution and by the polarity of the end-groups. Elastic properties of these layers are con￾trolled by the attraction forces at the interface as well as the conformation entropy of the chains forming the layer. Organofunctional silanes are so far the most widely used coupling agents for improvement of the interfacial adhe￾sion in glass reinforced materials [19]. Upon application of a silane from either dilute solution or the vapor phase, a highly crosslinked multilayer siloxane ‘‘interphase’’ is presumably formed with thickness ranging from 1.5 to 500 nm. Unlike in thermosetting matrices with extensive interpenetration between organosilane layer and the matrix monomer, long chain molecules do not interpenetrate the organosilane layers significantly. On the other hand, immobilization phenomena are of a greater importance in thermoplastic matrix composites. Moreover, processing temperatures of engineering thermoplastics ranging from 230 to 350 C can exceed the thermal stability of the commonly utilized organosilanes. Thickness dependence of the elastic modulus of thin polycarbonate (PC) layers deposited on a flat E-glass substrate was measured over the thickness interval ranging from 106 to 30 nm [9, 21, 22]. In all cases investigated, elastic moduli of the deposited layers, Ei, decreased monotonically with increasing layer thickness reaching a constant bulk value for layers thicker than 5 9 105 mm (Fig. 4). Thermally annealed PC and SiCl4 grafted oligo￾PC interphases, exhibited higher elastic moduli than the as received solution deposited PC interphase. No effect of thermal annealing on elastic modulus of strongly bonded oligo-PC interphase was observed. It has been shown that the shear strength of the interface, sa, measured in a sin￾glefiber fragmentation test exhibited strong dependence on the interphase Ei. Similarly to the PC interphases, elastic moduli of the deposited silane layers decreased monotonically with increasing layer thickness reaching a bulk value for layers thicker than 105 nm [21, 22]. Reactive chlorine containing silane formed always stiffer layers compared to its alkoxy￾analogues most probably due to stronger interaction between the chlorine and glass surface and, most probably, due to less defective network structure (Fig. 5). This hypothesis was further supported by the observed strong effect of deposition technique, controlling the layer su￾permolecular structure, on the layer elastic modulus. Solution deposition technique yielded always layers with lower elastic modulus compared to the layers formed by rf￾plasma or rf-plasma enhanced CVD deposition of the same substance. A qualitative explanation of the observed behavior was provided assuming formation of strongly immobilized layer of constant thickness, ti, and elastic modulus, Ei, near the bonded interface. Strength of the interfacial bond and network density of the polysiloxane interphase were proposed to be the factors determining ti and Ei for the given external conditions. Experimental data showed that the contribution of this strongly immobilized layer started to play an important role for interphase thickness below 103 nm. This ‘‘inner’’ layer has been covered with weaker ‘‘outer’’ layer with more defective network structure. The thickness of the ‘‘outer’’ layer was dependent on the concentration of the silane solution it was deposited from. The difference in Ei between the outer and inner interphase layer was increasing with strengthening the layer-surface interaction. In order to enhance the performance and reliability of the FRC structures at the macro-scale, the results obtained for the micro-scale interphase can be used to control the stress transfer between the matrix and the reinforcement. Stiff interphases provide very efficient stress transfer, less J Mater Sci (2008) 43:6747–6757 6751 123