60 High Performance Thermoplastic Resins and Their Composites nucleation.In all cases involving carbon fibres reinforcement,initiation of spherulitic growth from the fibre surfaces was also apparent in matrix morphology.The on-fibre nucleation indicates that fibre and resin are in close contact but as Turner and Cogswell [169] suggested this does not imply that good bonding is guaranteed.although it is a necessary condition for good bonding. 3.3 Morphology-Property Relationships of Semi-Crystalline Thermoplastics It is well recognized that the mechanical properties of semi-crystalline thermoplastics depend on their morphology [1.58.77.107,108,135.175].The degree of crystallinity,the number and size of spherulites,the crystalline structure and the crystalline orientation affect the properties of semi-crystalline polymers 1135].Orientation can be responsible for anisotropy in the mechanical properties.Larger spherulites are inherently stiffer but less ductile (Table 17).As in cross-linking.crystallization can enhance stiffness and Tg by constraining molecular mobility (75].Flgure 18 shows toughness and tensile strength properties of neat PEEK resin as a function of the degree of crystallinity.In general.a lower level of crystallinity will produce higher elongation and better toughness but with the trade-off of lower strength,thermal stability and chemical resistance.Diffusion of the solvents into a semi-crystalline thermoplastic is more difficult than with an amorphous thermoplastic due to the densely packed spherulites in the crystalline region [1.771.Available results on the effects of morphology on the properties of PEEK and PPS based composites are presented below. PEEK:Talbott et al.[107.108]have evaluated the tensile,compressive and shear properties.as well as fracture toughness of the neat resin PEEK 150P having degrees of crystal- linity ranging from 15 to 40%and having different processing histories.The results which they have obtained are shown in Figures 18 to 22.The strength and stiffness in both tension and shear increase with crystalline content but compression strength remains relatively unaffected.However,due to the highly non-linear behavior of the material during compression,these data must be interpreted with caution.As shown in Figures 18 and 22,the decrease in toughness with increasing degree of crystallinity is significant.Talbott et al.[108] also measured mode I and mode II fracture energies for carbon reinforced PEEK (APC-2)for the crystalline content range of O to 33%.Results are presented in Figure 23 where fracture energies decrease significantly with increasing crystalline content.These authors also concluded that the data they have obtained suggest that the values of the tensile and compres- sive properties of the polymer seem to be sensitive to processing history:e.g.specimens having the same degree of crystallinity but having a different processing history may result in different tensile and compressive properties.However,within the range studied,the shear strengths and moduli of the polymer seem to be insensitive to the thermal history employed during processing and depend mainly on the degree of crystallinity.In the case of fracture energy,cooling only or cooling and reheating resulted in nearly the same fracture energy as long as the crystalline content after processing was the same as before.60 High Performance Thermoplastic Resins and Their Composites nucleation. In all cases Involving carbon Mbres reinforcement, initiation of spherulitic growth from the fibre surfaces was also apparent in matrix morphology. The on-fibre nucleation indicates that fibre and resin are in close contact but as Turner and Cogswell 11691 suggested this does not imply that good bonding is guaranteed, although it is a necessary condition for good bonding. / 3.3 Morphology-Property Relationships of Semi-Crystalline Thermoplastics It is well recognized that the mechanical properties of semi-crystalline thermoplastics depend on their morphology [l. 58. 77. 107. 108, 135, 1751. The degree of crystallinity. the number and size of spherulites, the crystalline structure and the crystalline orientation affect the properties of semi-crystalline polymers [ 1351. Orientation can be responsible for anisotropy in the mechanical properties. Larger spherulites are inherently stiffer but less ductile (Table 17). As in cross-linking, crystallization can enhance stiffness and Tg by constraining molecular mobility [75]. Figure 18 shows toughness and tensile strength properties of neat PEEK resin as a function of the degree of crystallinity. In general, a lower level of crystallinity will produce higher elongation and better toughness but with the trade-off of lower strength, thermal stability and chemical resistance. Diffusion of the solvents into a semi-crystalline thermoplastic is more difficult than with an amorphous thermoplastic due to the densely packed spherulites in the crystalline region [ 1. 771. Available results on the effects of morphology on the properties of PEEK and PPS based composites are presented below. PEEK: Talbott et al. [ 107, 1081 have evaluated the tensile, compressive and shear properties, as well as fracture toughness of the neat resin PEEK 150P having degrees of crystal￾linity ranging from 15 to 40% and having different processing histories. The results which they have obtained are shown in Figures 18 to 22. The strength and stiffness in both tension and shear increase with crystalline content but compression strength remains relatively unaffected. However, due to the highly non-linear behavior of the material during compression, these data must be interpreted with caution. As shown in Figures 18 and 22. the decrease in toughness with increasing degree of crystallinity is significant. Talbott et al. [ 1081 also measured mode I and mode II fracture energies for carbon reinforced PEEK @PC-2) for the crystalline content range of 0 to 33% Results are presented in Figure 23 where fracture energies decrease significantly with increasing crystalline content. These authors also concluded that the data they have obtained suggest that the values of the tensile and compres￾sive properties of the polymer seem to be sensitive to processing history; e.g. specimens having the same degree of crystallinity but having a different processing history may result in different tensile and compressive properties. However, within the range studied, the shear strengths and moduli of the polymer seem to be insensitive to the thermal history employed during processing and depend mainly on the degree of crystallinity. In the case of fracture energy, cooling only or cooling and reheating resulted in nearly the same fracture energy as long as the crystalline content after processing was the same as before