CARBON FIBER REINFORCED PEEK COMPOSITES I. ROLE OF THERMAL HISTORY ON THE CRYSTALLINITY, MORPHOLOGY AND THERMOMECHANICAL BEHAVIOR Tamer Sinmazcelik, Emel Yilgor and Iskender Yilgor Koc University, Cayir Caddesi 5, Istinye 80860, Istanbul, Turkey Introduction There has been a remarkable growth in the industrial applications of fiber reinforced polymer matrix composites in recent years. Because of their superior specific strength, stiffness, good mechanical properties and chemical and corrosion resistance, fiber reinforced polymer composites are excellent materials for a variety of high-performance structural applications especially in aerospace, automotive and chemical industries. There is a wide selection of matrix resins for such high-performance composites, depending on the applications. These include thermoset resins, such as, epoxies, bismaleimides, cyanate esters, or thermoplastic resins, such as poly(phenylene sulfide), polysulfones, polyimides, polyetherimides and polyetheretherketones(1). Polyetheretherketone (PEEK) is a high performance, semicrystalline thermoplastic polymer with excellent chemical resistance and good thermal and mechanical properties. Relatively stiff aromatic backbone of PEEK provides a reasonably high glass transition temperature of about 143C and a high melting temperature range between 300 and 350C. Consequently, PEEK has a relatively high continuous service temperature range with the advantages of ease of processability by injection or compression molding and other techniques commonly used for thermoplastic polymers(2). Being a semicrystalline thermoplastic, the ultimate chemical, surface, mechanical and engineering properties of PEEK based materials are strongly dependent on the overall thermal history of the system. As a direct result of this strong dependence of mechanical performance on microstructure and morphology, considerable efforts have been made to understand the influence of thermal treatments on the crystallization behavior and morphology of PEEK. Optical microscopy studies on PEEK have indicated a dramatic difference in the crystalline morphologies for samples annealed below or above 300 o C. At annealing temperatures below 300 o C, it has been reported that the spherulites consist of narrow lamellae, whereas above 300 o C, the spherulitic morphology also shows branched, bundle-like or stretched lamellae(3). X-ray scattering studies on the isothermal crystallization of PEEK indicate that there may be two processes taking place in the melt, the first one due to the crystallization from unrestrained melt producing thicker lamellar stacks, followed by a secondary recrystallization from the restrained melt leading to thinner lamellae. During the melting of these crystals, one usually observes two endothermic peaks in differential scanning calorimetry. Although there are several hypotheses, this behavior is still not fully understood and there is no general agreement about their origin. Crystallization behavior of PEEK matrix gets even more complicated in fiber reinforced composites, which usually contain fairly high fiber loadings. It is well known that fiber type and its surface structure have dramatic effects on the crystallization kinetics, morphology of the crystals formed and the level of crystallinity achieved in PEEK matrices(1). Another very important factor, which determines the performance of fiber reinforced composites is the nature and structure of fiber/matrix interphase, which is the focus of an increasing number of studies. An interphase (or an interlayer) is a third, relatively thick, intermediary phase between the C-fiber and PEEK matrix. The interphase can also be considered as a region with variable properties between the fiber and the matrix. Clearly, the development, extent and the morphology of such an interphase will show strong dependence on the thermal history of the composite material(4,5). If the elastic and mechanical properties of the interphase can specifically be designed and its morphology can be controlled through thermal treatments, this will have important contributions to the integrity and overall performance of the composite material produced. For example a gradient modulus interphase has been proposed to improve the mechanical properties and the fatigue life by reducing the modulus mismatch between fibers and the matrix. On the other hand a soft, elastic interphase has been recommended for high fracture toughness and for reduced stress concentrations in the matrix around the fibers. In this study, influence of thermal history on the development of matrix crystallinity, bulk morphology and fiber/matrix interphase on C-fiber reinforced PEEK composites will be reported. Influence of bulk morphology on the dynamic mechanical properties and fracture toughness of these composite materials will also be explained. In accompanying paper we will discuss the relationship between thermal history, surface morphology and tribological properties carbon fiber reinforced PEEK composites. Experimental Materials and sample preparation: Carbon fiber reinforced prepregs (Fiberite APC-2) were kindly supplied by ICI, UK. The volume fraction of carbon fiber in the prepreg was 0.68. PEEK used as the matrix resin was Victrex, also a product of ICI. 16-Ply laminates (both cross-ply and unidirectional) were produced by using hot pressing technique. Recommended process consisted of heating the laminates to 380 o C with a heating rate of 10 o C per minute followed by a 10 minute isothermal curing at this temperature. After the curing process, samples were cooled down to room temperature with a cooling rate of 10 o C per minute and kept in sealed polyethylene bags until further investigations. The samples which were thus produced were designated as “cured laminates”. On the average, cured laminates showed 17% crystallinity as determined by wide angle x-ray diffraction (WAXD) studies. Thermal treatments of the laminates: In order to see the effect of thermal history on the development of crystallinity and final morphology of the composite materials produced, 3 different heat treatments were applied to the cured laminates. In two cases, cured laminates were first heated to 380 o C with a heating rate of 10 o C per minute and kept there for 6 minutes in order to melt the crystalline regions completely. The procedures followed afterwards were different. (i) Isothermal crystallization of quenched, amorphous matrix: Molten samples were immediately immersed into an ice water bath in order to produce a completely amorphous morphology in the PEEK matrix. After quenching, the samples were subjected to isothermal crystallization at 150, 200, 250 and 310C for 30, 60, 120 and 240 minutes respectively. (ii) Isothermal crystallization from melt: During this treatment molten samples were cooled down to isothermal crystallization temperatures with a rate of 10 o C per minute. Isothermal crystallizations were conducted at 150, 200, 250 and 310C for 30, 60, 120 and 240 minutes. Then the samples were cooled down to room temperature with a cooling rate of 10 o C per minute. (iii) Annealing of semi-crystalline cured laminates: A set of the cured laminates, which already had 17% crystallinity in the PEEK matrix as determined by WAXD studies, were annealed at 150, 200, 250 and 310C for 30, 60, 120 and 240 minutes. During these processes cured laminates were heated to the annealing temperature with a rate of 10C per minute. Then they were cooled down to room temperature with a rate of 10C per minute. Characterization Techniques Thermal analyses: A Rheometrics Scientific PL-DSC Plus was used to obtain DSC thermograms. Measurements were carried out under nitrogen atmosphere with a heating rate of 10C per minute. Temperature and energy calibration of the instrument was done by using indium, tin and lead standards. Dynamic mechanical thermal analysis (DMTA) of the materials were obtained on a Rheometrics Model Mk III analyzer. Measurements were carried out in bending mode at two different frequencies of 1 and 100 Hz, with a heating rate of 5C per minute. Wide Angle X-Ray Diffraction (WAXD) Studies: WAXD scans were obtained by using a Phillips 1050 vertical goniometer. Unidirectional laminates with dimensions of 20x20 mm were inserted into the sample holder and the diffraction profiles were recorded at a scanning rate of 12 min -1 over an angular range of 12236. WAXD scans for quench cooled (amorphous) and fully crystallized (310C for 240 minutes) PEEK composites are reproduced in Figures 1-a and 1-b respectively. In Fig.1-a the broad peak at 2 = 25.0 is due to the carbon fibers in the system. Figure 1-b on the other hand shows 3 distinct peaks in addition to the carbon fiber peak. These are at 18.9 (110), 20.9 (111) and 23.1 (200), the major one being the (110) reflection, followed by (200) and to a lesser extent (111) reflections. Crystallinity calculations were carried out by using the method described by Blundell and co-workers(6), by measuring the height of the (110) peak (h110), above the interpolated baseline and comparing it with that of the carbon fiber peak (hc). As expected this calculation underestimates the level of crystallinity in the matrix. We also compared (110) and (200) peaks in order to determine the orientation of the crystals in the PEEK matrix, to determine their influence