Nonisothermal crystallization kinetics of poly(etheretherketone) (PEEK)

Analysis of the crystallization kinetics of poly(etheretherketone) (PEEK) was achieved with dynamic differential scanning calorimetry results. A new kinetic model for the nonisothermal crystallization was derived and the possibility of its application was investigated. By evaluating the parameters in the model, the crystallization behavior of PEEK was analyzed. The experimental and predicted crystallinity change showed good agreement, which indicated that the model equation was appropriate to describe the nonisothermal crystallization kinetics of PEEK. As the melt temperature was increased the number of heterogeneous nuclei decreased, hence the crystallization was delayed.     

The influence of crystallinity on interfacial properties of carbon and SiC two‐fiber/polyetheretherketone (PEEK) composites

The influence of the degree of crystallinity on interfacial properties in carbon and SiC two‐fiber reinforced poly(etheretherketone) (PEEK) composites was investigated by the two‐fiber fragmentation test. This method provides a direct comparison of the same matrix conditions. The tensile strength of the PEEK matrix and the interfacial shear strength (IFSS) of carbon or SiC fiber/PEEK exhibited the maximum values at around 30% crystallinity, and then showed a decline. The tensile modulus increased continuously with an increase in the degree of crystallinity. Spherulite sizes in the PEEK matrix became larger as the cooling time from the crystallization temperature increased. Transcrystallinity of carbon fiber/PEEK was developed easily and more densely than with SiC fiber/PEEK. This might have occurred because the unit cell dimensions of the crystallite in the fiber axis direction on the carbon surface was more suitable for making nucleation sites. The IFSS of carbon fiber/PEEK was significantly higher than that of SiC fiber/PEEK because it formed transcrystallinity of IFSS more favorably.     

Crystallization and fiber/matrix interaction during the molding of PEEK/carbon composites

The objective of this work was to investigate the effects of molding conditions (molding temperature, residence time at melt temperature, and cooling rate) on the crystallization behavior and the fiber/matrix interaction in PEEK/carbon composites made from both prepreg and commingled forms. In order to investigate the crystallization behavior of the PEEK matrix, the molding process was simulated by differential scanning calorimetric analysis, DSC. The results show that the prepreg and commingled systems do not have the same matrix morphology; prepreg tape was found to be at its maximum of crystallinity, whereas the commingled system was found to be only partially crystalline. The results show that processing must be carried out at a temperature sufficiently high to destroy the previous thermal history of the PEEK matrix; this is an essential requirement to produce efficient fiber/matrix adhesion in the commingled fabric system. Optical microscopic observations also suggest that matrix morphology near the fibers is dependent on the melting conditions; a well-defined transcrystalline structure at the interface is observed only when the melt temperature is sufficiently high. However, the high temperature of molding can easily result in degradation of the PEEK matrix such as chain scission and crosslinking reactions. Thermal degradation of the matrix during processing is found to affect the crystallization behavior of the composites, the fiber/matrix adhesion, and the matrix properties. This effect is more important in the case of a commingled system containing sized carbon fibers because the sizing agent decomposes in the molding temperature range of PEEK/carbon composites. This produces a decrease of the matrix crystallinity and an elimination of the nucleating ability of the carbon fibers. A transition between cohesive and adhesive fracture is observed when the cooling rate increases from 30°C/min to 71°C/min for the composite made from the commingled fabric. This critical cooling rate is found to closely correspond to a change in the mechanism of crystallization of the PEEK matrix.     

Improvement of carbon fiber/PEEK hybrid fabric composites using plasma treatment

A carbon fiber (CF)/polyetheretherketone (PEEK) composite was manufactured using hybrid fabrics composed of CF and PEEK fiber. The fiber/matrix interface was modified by low temperature oxygen plasma treatment. Scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and Fourier transform attenuated total reflection infrared spectroscopy (FTIR-ATR) were used to relate the roughness and the functionality of the CF surface with the interfacial adhesion strength of the CF/PEEK composite. Scanning electron micrographs showed that plasma treatment increased the roughness of the CF surface up to 3 min of plasma treatment time; and prolonged treatment resulted in overall smoothing. XPS results confirmed that increasing treatment time marginally increased surface functionality: treatment for more than 5 min decreased the surface functionality by removing the active site of the CF surface. In addition, flexural strength and interlaminarshear strength (ILSS) of the CF/PEEK composite were measured. Their maximum values were observed at 3 min of plasma treatment time as a result of surface roughening by plasma etching. The SEM results were correlated with mechanical properties of the CF/PEEK composite.     

碳纤维增强聚醚醚酮的非等温结晶行为与力学性能

利用聚酰亚胺(PI)作为碳纤维(CF)界面改性剂,制备了界面改性碳纤维增强聚醚醚酮(MCF/PEEK)复合材料。采用差示扫描量热仪(DSC)讨论了CF及其界面改性对PEEK非等温结晶行为的影响机制与作用规律,并基于莫志深法研究了MCF/PEEK的非等温结晶动力学;借助DSC和小角X射线散射仪(SAXS)表征不同降温速率下PEEK基体的结晶结构,采用万能试验机评价了MCF/PEEK的力学性能。结果发现:CF对PEEK的结晶有较为明显的异相成核促进作用,经过PI界面改性之后成核作用有所下降,但结晶行为仍较纯PEEK更容易发生,整体结晶速率更快;随冷却速率的增大,基体结晶度、片晶厚度与长周期均减小,MCF/PEEK的拉伸强度与模量也显著减小,层间断裂韧性提高。     

Kinetics of strain-induced crystallization during injection molding of short fiber composites of poly(ether ether ketone)

Crystallization kinetics of short glass and carbon fiber composites of poly(ether ether ketone) (PEEK) under melt-strain conditions have been obtained for the first time, using in-situ wide angle X-ray scattering, and have been correlated to a model based on the Avrami equation in order to enable minimization of the processing time for injection molding of these materials. It has been demonstrated that increased flow rate of the melt in the mold and, consequently, increased shear rate accelerates the crystallization process of PEEK composites, analogous to similar trends observed previously in PEEK resin. Short glass fiber composites of PEEK crystallize slower than the resin under identical processing conditions, while short carbon fiber composites crystallize faster than the resin, except at the highest mold temperatures and the lowest flow rates. A model based on the Avrami equation has been proposed to fit the kinetics data obtained experimentally. The Avrami coefficient has been calculated and Arrhenius plots have been used to predict the crystallization kinetics at temperatures lower than those at which experimental data have been obtained here. Fiber orientation, flexural elastic modulus, and flexural fracture toughness of the composites have also been evaluated.     

Effects of sorbed caprolactam on the crystallinity,morphology, and deformation behavior of polyetheretherketone and poly(phenylene sulfide)

Studies on the high temperature sorption of caprolactam by polymer resins and their composites have been conducted. The systems investigated were glass fiber reinforced (GFR) poly(phenylene sulfide) (PPS), polyetheretherketone (PEEK) neat resin, GFR PEEK and carbon fiber reinforced (CFR) PEEK. To measure changes of caprolactam sorption, melting behavior, mechanical properties, and fracture surface morphology were determined. Absorption of caprolactam by the PEEK composites was 30 to 40 percent less than by the neat resin. This is attributed to the fibers, which acted to constrain the matrix and thus limit its swellability. Reductions in melt temperature, percent crystallinity, ultimate tensile strength, and modulus were observed following exposure to the chemical environment. The loss of strength and stiffness was a consequence of the degradation of the matrix/fiber interface by the sorbed caprolactam.     

Crystallization kinetics of poly(phenylene sulfide) (PPS) and PPS/carbon fiber composites

The evolution of crystallinity of neat PPS and of the carbon fiber reinforced polymer under different processing conditions is studied. Crystallization from the amorphous state at low temperatures (cold crystallization), crystallization from the melt during cooling, and crystal melting processes are analyzed using calorimetric techniques under both isothermal and nonisothermal conditions. Cold and melt crystallization kinetics are described using an Avrami equation and an Arrhenius expression for the temperature dependence of the kinetic constant. Also, the melting kinetics of the, reinforced and of the unreinforced polymer are studied in this work. The effect of carbon fibers on the crystallization kinetics of PPS is analyzed, and a comparison of the crystallization behavior of PPS and other semicrystalline thermoplastic matrices, such as poly(etheretherketone) (PEEK), is presented.     

Crystallization of poly(aryl-ether-ether-ketone): Effect of thermal history of the melt on crystallization kinetics

Differential Scanning Calorimetry (DSC) and optical microscopy were used to investigate the effect of the thermal history of the melt on the crystallization of a commercial sample of poly(aryl-ether-ether-ketone) (PEEK). Heating a film of PEEK at a temperature above the melt temperature for various periods of time changes the nucleation and crystal growth rate upon cooling the sample. Destruction of existing nuclei, formation of new nuclei, chain branching, cross linking, and chemical degradation of the macromolecular chains are all believed to take place at different times and to different extents during the thermal melt processing of the polymer. This study suggests that the thermal history of the melt plays an important role in the crystallization of PEEK samples.     

Thermal degradation of poly(aryl-ether–ether-ketone) (PEEK): A differential scanning calorimetry study

Differential scanning calorimetry (DSC) has been used to study the crystallization kinetics and thermal characteristics of poly(aryl-ether–ether-ketone) (PEEK) samples heated under a variety of conditions. Samples were heated in nitrogen and air at temperatures between 380 and 420°C for times up to 120 min. The results indicate that as the holding time and temperature of the melt increased, the amount of recrystallizable material decreased, especially when heated in air. Isothermal crystallization kinetics confirmed the presence of a two-stage crystal nucleation and growth process with Avrami exponents of the order of about 2.4 and 1.5 for the first and second processes, respectively. Analysis of the primary crystallization process using the Avrami equation revealed that PEEK samples heated above the melt temperature in air crystallized at a much slower rate than samples heated in nitrogen.     

聚苯硫醚/热致性液晶聚芳酯的流变性及其纤维的性能研究

将自制热致性液晶聚芳酯(PEE)与聚苯硫醚(PPS)共混,通过熔融纺丝制备了PPS/PEE纤维。采用高压毛细管流变仪研究了PPS/PEE共混物的流变行为,利用扫描电子显微镜观察了PPS/PEE纤维的形态结构,通过差示扫描量热仪、热重分析仪、X射线衍射仪等表征了PEE的含量对PPS/PEE纤维热性能及结晶行为的影响。结果表明:PEE的加入大幅度降低了PPS树脂的表观黏度,减小了其对剪切速率的敏感性,提高了PPS基体的结晶速率,加快了其结晶过程,在一定程度上提高了PPS纤维的热稳定性;PEE在共混纤维中起到异相成核剂的作用,对PPS的晶型没有影响;PEE与PPS相容性较差,PEE以大尺寸微纤的形式分布于基体中。     

Crystallization and chain adsorption of poly(etheretherketone) in discontinuous pitch-derived carbon fiber composites

The crystallization kinetics of poly(etherether ketone) (PEEK) in chopped mesophase pitch-derived carbon fiber/PEEK composites have been studied. Various processing techniques are used in order to obtain controlled fiber length in the composites. Scanning electron microscopy performed on properly etched long fiber composite samples reveals that the nucleating sites density is low at the carbon fiber surface: transcrystalline layers are rarely observed. This is confirmed by differential scanning calorimetry. However, samples processed by mixing carbon fibers and molten polymer in a high temperature mixer have a widely different behavior: the nucleation density and the crystallization rate increase, the glass transition of these samples is displaced towards higher temperatures, and the solubility is dramatically lowered. We ascribe these phenomena to the adsorption of the polymer chains on carbon particles created by attrition during the mixting.     

Thermal behavior of transparent poly(etheretherketone)(PEEK) film

The thermal behavior of poly(etheretherketone)(PEEK) film heated in an open differential scanning calorimetry (DSC) pan at 20°C/min is distorted by relaxation of the strained film. PEEK film in a closed pan or quenched PEEK in open or closed pans shows a glass-transition temperature (T_g) around 144°C, cold crystallization (～22 J/g) at 177°C, melt-temperature (T_m) peaking at 335–340°C, with an enthalpy of fusion of 32–34 J/g, and recrystallization on cooling at 285°C, with a crystallization exotherm of about 40 J/g. The enthalpy of fusion decreases with increasing heating rate from 2–100°C/min and approaches the enthalpy of cold crystallization. With increasing heating rate, further crystallization of PEEK during the DSC scan is suppressed. With increasing cooling rate, PEEK melt crystallizes at larger supercoolings to a lesser extent. Crystallization on cooling the melt was more complete than cold crystallization and annealing on heating.     

Influence of the addition of lubricant on the properties of poly(ether ether ketone) fibers

A high‐temperature lubricant genioplast pellets (GPPS) was used in order to improve the processing behavior of poly(ether ether ketone) (PEEK) resin and high‐performance PEEK fibers were produced by melt‐spinning. The rheological properties of spinning material, morphology, mechanical, and thermal properties of PEEK fibers were characterized by using a polymer capillary rheometer, scanning electron microscopy, single fiber electronic tensile strength tester, wide‐angle X‐ray diffraction and thermal gravimetric analyzer, respectively. The results indicated that the introduction of lubricant GPPS decreased the melting viscosity of PEEK resin and improved spinnability of PEEK resin without sacrificing its thermal properties. The filaments are cylindrical with smooth surface and uniform diameter. The optimized content of GPPS was determined to be 1.0 wt% by balancing the decreased torque and changes of the mechanical properties. The strength and modulus of PEEK fibers were 420 MPa and 3.6 GPa, respectively. This should be due to the improvement in spinnability, followed by the enhancements in the orientation and crystallization of PEEK fibers in the process of drawing and annealing. POLYM. ENG. SCI., 53:2254–2260, 2013. © 2013 Society of Plastics Engineers     

聚醚醚酮纤维的拉伸定形后处理研究

通过熔融纺丝制得聚醚醚酮(PEEK)纤维,并采用差示扫描量热仪(DSC)、声速取向测量仪、热重分析仪、单纱电子强力仪分别研究了干热拉伸及热定形处理对PEEK纤维结晶和取向、热稳定性及力学性能的影响。结果表明:随着热拉伸倍数增大,PEEK纤维取向度、结晶度增加,纤维的断裂强度增加,断裂伸长减小;PEEK纤维的热拉伸温度应选在200～240℃,热定形温度应为220～260℃;PEEK纤维的重结晶主要是在热拉伸过程中完成,热定形则进一步完善纤维的结晶结构;经过后处理,PEEK纤维的断裂强度可达到6.12cN/dtex;且具有优异的热稳定性能,热分解温度高达505℃,后处理几乎不影响PEEK纤维的热稳定性。     

Crystallization behavior of poly(ether ether ketone ketone)

The melting behavior of semicrystalline poly(ether ether ketone ketone) (PEEKK) has been studied by differential scanning calorimetry (DSC). When PEEKK is annealed from the amorphous state, it usually shows two melting peaks. The upper melting peaks arise first, and the lower melting peaks are developed later. The upper melting peaks shown in the DSC thermogram are the combination (addition) of three parts: initial crystal formed before scanning; reorganization; and melting-recrystallization of lower melting peaks in the DSC scanning period. In the study of isothermal crystallization kinetics, the Avrami equation was used to analyze the primary process of the isothermal crystallization; the Avrami constant, n, is about 2 for PEEKK from the melt and 1.5 for PEEKK from the glass state. According to the Lauritzen-Hoffman equation, the kinetic parameter of PEEKK from the melt is 851.5 K; the crystallization kinetic parameter of PEEKK is higher than that of PEEK, and suggests the crystallizability of PEEKK is less than that of PEEK. The study of crystallization on PEEKK under nonisothermal conditions is also reported for cooling rates from 2.5°C/min to 40°C/min, and the nonisothermal condition was studied by Mandelkern analysis. The results show the nonisothermal crystallization is different from the isothermal crystallization. © 1996 John Wiley & Sons, Inc.     

Transcrystallization in fiber-reinforced isotactic polypropylene composites in a temperature gradient

Transcrystallization of isotactic polypropylene (iPP) on different fibers (carbon fiber, glass fiber, and aramid fiber) was conducted in a temperature gradient. The Ultra-High-Module carbon fiber (UHMCF), the High-Module carbon fiber, and the aramid fiber (Twaron) showed sufficient nucleation ability to form transcrystallization of iPP in certain temperature ranges. Among them, the UHMCF showed the best nucleation ability. On the contrary, the Intermediate-Module carbon fiber, the High-Tenacity carbon fiber, and the E-glass fiber showed too low nucleation ability to form transcrystallization of iPP. One efficient way to induce transcrystallization on these fibers was proved by pulling the fibers in supercooled iPP melts. The interface shear between fiber and supercooled matrix melt on crystallization and the interface temperature gradient between fiber and supercooled matrix melt on crystallization are considered to be two very important factors for the formation of transcrystallization. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 65: 67–75, 1997     

Preparation and characterization of high‐performance poly(ether ether ketone) fibers with improved spinnability based on thermotropic liquid crystalline poly(aryl ether ketone) copolymer

High‐performance poly(ether ether ketone) (PEEK) fibers were prepared by melt‐spinning in the presence of thermotropic liquid crystalline poly(aryl ether ketone) copolymer (FPAEKLCP). The rheological and mechanical properties, birefringence, orientation, and crystallization of the resulting PEEK/FPAEKLCP fibers were characterized by using a melt flow indexer, capillary rheometer, single fiber electronic tensile strength tester, polarized light microscopy (PLM), and wide‐angle X‐ray diffraction (WAXD), respectively. The results indicate that the melt viscosity of PEEK significantly reduced by introducing FPAEKLCP, followed by the improvements in the spinnability and the quality of as‐spun fibers. The tensile properties of PEEK/FPAEKLCP fibers mainly depend on the content of FPAEKLCP, drawing temperature, drawing ratio, and annealing processes. Moreover, the tensile strength and modulus of PEEK/FPAEKLCP fibers are obviously higher than those of neat PEEK fibers under the same processing conditions. This should be attributed to an enhancement in the orientation and crystallization of PEEK compounded with FPAEKLCP. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 1406‐1414, 2013