58 High Performance Thermoplastic Resins and Their Composites PEEK which has been preheated in the melt for 100 minutes.Correspondingly,as explained later (Foreign Surfaces,section 3.2.2),these composites have shown about twice the transverse tensile strength and strain-to-lailure of those preheated for only 30 minutes. The degrec of erystallinity may also vary through the thickness of a laminate [1.108, 109].Incomplete or non-uniform crystallization may take place if,for example,a thick sheet of APC-2 in the molten state is transfered to a cold metal mold with good thermal transfer from the APC-2 sheet [109].Thin sheets in this case can be totally amorphous.Blundell and Willmouth [109]have presented a simple heat flow model enabling predictions of the temperature profile through the thickness of APC-2 sheets based on time after transfer from the melt state to a moulding tool held at a fixed temperature. 3.2.2 Foreign Surfaces The morphology of a polymer is affected by the presence of foreign surfaces [100.110. 112.1131.PEEK was found to crystallize at higher temperatures with higher nucleation densities as the fibre content of a carbon reinforced composite was increased [100,110l.The fibre surfaces act as nucleating sites.Typical crystalline content pf PEEK in a 50%carbon fibre composite is 35%as measured by DSC [100J.Blundell et al.[106]divide nucleation sites in carbon/PEEK composites into three types:nucleation from contact points between fibres or regions where the fibres are almost in contact(Figure 16).nucleation from within the matrix (Figure 17)and nucleation at a free fibre-polymer interface (Figure 17).The relative abundance of each type of nucleation depends on the circumstances.Crystallization at lower temperatures for example encourages matrix nucleation [106.111]while longer melt holding times favour crystallization on the carbon surfaces [110J.They also observed that the nucleation from a free fibre-matrix surface is relatively rare,at least not common enough to give the appearance of a"trans-crystalline"layer.However,transcrystallinity,described as the columnar growth of crystals perpendicular to the fibre surface has been observed by other researchers [100,110,123.1691.It has been associated with the improvement of the interfacial bond between PEEK and the fibres resulting in better mechanical properties [100.110].But Blundell et al.[106]state that "the reported transcrystallinity effects sometimes seen from fibers embedded in thin PEEK films are related to the fiber-fiber contact sites in APC-2 and occurs where the carbon fiber comes into close contact with the glass slides sandwiching the film".Turner and Cogswell (169]have explored the varying interfacial properties that result from the differing fibre types used in PEEK based composites.Fibre types include E,R and S glass fibres.aramid fibres.and high strength (HS).high modulus (HM).intermediate modulus (IM)and ultra-high modulus (UHM)carbon fibres.All laminates were prepared using the same procedures recommended for APC-2.With all types of fibres,there was a high incidence of spherulitic nuclei associated with two closely adjacent or touching fibres.They believe that this is probably due to the thermal contraction of the resin phase during cooling and the local geometry which together produce a stress concentration at such fibre contact points inducing58 High Performance Thermoplastic Resins and Their Composites PEEK which has been preheated in the melt for 100 minutes. Correspondingly, as explained later (Foreign Surfaces, section 3.2.2). these composites have shown about twice the transverse tensile strength and strain-to-failure of those preheated for only 30 minutes. The degree of crystallinity may also vary through the thickness of a laminate [ 1, 108, 1091. Incomplete or non-uniform crystallization may take place if, for example, a thick sheet of APC-2 in the molten state is transfered to a cold metal mold with good thermal transfer from the APC-2 sheet [ 1091. Thin sheets in this case can be totally amorphous. Blundell and Willmouth [ 1091 have presented a simple heat flow model enabling predictions of the temperature profile through the thickness of AFC-2 sheets based on time after transfer from the melt state to a moulding tool held at a fixed temperature. 3.2.2 Foreign Surfaces The morphology of a polymer is affected by the presence of foreign surfaces (100, 110, 112, 1131. PEEK was found to crystallize at higher temperatures with higher nucleation densities as the fibre content of a carbon reinforced composite was increased [ 100, 1101. The fibre surfaces act as nucleating sites. Typical crystalline content of PEEK in a 50% carbon fibre composite is 35% as measured by DSC [loo]. Blundell et al. ]lOS] divide nucleation sites in carbon/PEEK composites into three types: nucleation from contact points between fibres or regions where the fibres are almost in contact (Figure 16), nucleation from within the matrix (Figure 17) and nucleation at a free fibre-polymer interface (Figure 17). The relative abundance of each type of nucleation depends on the circumstances. Crystallization at lower temperatures for example encourages matrix nucleation [ 106, 11 l] while longer melt holding times favour crystallization on the carbon surfaces [ 1 lo]. They also observed that the nucleation from a free fibre-matrix surface is relatively rare. at least not common enough to give the appearance of a “trans-crystalline” layer. However, transcrystallinity. described as the columnar growth of crystals perpendicular to the fibre surface has been observed by other researchers [ 100, 110. 123, 1691. It has been associated with the improvement of the inter-facial bond between PEEK and the fibres resulting in belter mechanical properties [ 100. 1 lo]. But Blundell et al. [ 1061 state that “the reported transcrystallinity effects sometimes seen from fibers embedded in thin PEEK films are related to the fiber-fiber contact sites in APC-2 and occurs where the carbon fiber comes into close contact with the glass slides sandwiching the film”. Turner and Cogswell [ 1691 have explored the varying interfacial properties that result from the differing fibre types used in PEEK based composites. Fibre types include E,R and S glass fibres, aramid fibres, and high strength (HS), high modulus (HM), intermediate modulus (IM) and ultra-high modulus (UHM) carbon fibres. All laminates were prepared using the same procedures recommended for APC-2. With all types of fibres, there was a high incidence of spherulitic nuclei associated with two closely adjacent or touching fibres. They believe that this is probably due to the thermal contraction of the resin phase during cooling and the local geometry which together produce a stress concentration at such fibre contact points inducing