Pes-C blends on carbon fibre

Fur. Polym. J. Vol. 29, No. 12, pp. 1647-1650, 1993 Printed in Great Britain. All rights reserved

0014-3057/93 $6.00 + 0.00 Copyright © 1993 Pergamon Press Ltd

INTERFACIAL CRYSTALLIZATION OF PEEK/PES-C BLENDS ON CARBON FIBRE ZHIYI ZHANG* and HANMIN ZENG Materials Science Institute, Zhongshan University, Guangzhou 510275. People's Republic of China (Received 16 November 1992)

Abstract--The interfacial crystallization of blends of poly(ether ether ketone) (PEEK) with phenolphthalein poly(ether ether sulphone) (PES-C) on carbon fibre (CF) was investigated using optical microscopy. In this system, transcrystallinity (a crystalline interlayer of polymer around fibre) was found to form very easily compared with the CF-PEEK system. The reason for the difference is the fact that amorphous PES-C has a greater influence on the nucleation of PEEK in the bulk than on the fibre. The thickness of the transcrystallinity in which PES-C is interfibrillarlyincorporated varies linearly with time. Its growth rate depends on PES-C content and crystallization temperature. Based on the observation of the morphology, the possible transcrystallinity properties are discussed.

INTRODUCTION

Since blending is an easy way to improve the performance of existing materials, various polymer blends have been used as the matrix in fibre reinforced composites. It has been shown that the properties of composites based on such a hybrid matrix may be better than those of composites made of a single polymeric matrix. For example, excellent thermal properties of composites were obtained by using bismaleimide/polystyrylpyridineblends [1], and better toughness of glass-reinforced composites was obtained by replacing polypropylene (PP) with PP/LDPE blends [2]. In these composites, not only the matrices themselves but also the new fibre-matrix interfaces may affect the composite properties. As shown recently, an interphase favouring mechanical properties of the composites could form in polyurethane/unsaturated polyester blends reinforced with glass fibre [3]. Therefore, it is very important to study the interfaces in the polyblend-based composites.

formance polymer with glass transition temperature (Ts) around 145° [4], with phenolphthalein poly(ether ether sulphone) (PES-C), an amorphous and also high-performance material with Tg at 265 ° [5]. The operating temperature of the blends was found to be higher than that of PEEK [6]. The miscibility, crystallization behaviour and melting behaviour of the blends were reported [7, 8]. Based on the blends, hybrid composites reinforced with short carbon fibre were fabricated. To examine the fibre-matrix interfacial characteristics in the composites, the interfacial crystallization of PEEK/PES-C blends around carbon fibre (CF) was studied in this work. EXPERIMENTAL PROCEDURES

PEEK powder 450P (ICI), PES-C powder with a reduced viscosity of 0.68 ml/g (supplied by Changchun Institute of Applied Chemistry of Academia Sinica) and high-modulus carbon fibre M40 were used. Before use, the fibres were refluxed in acetone for two weeks to remove possible coatings. The chemical structure of PES-C in [5]:

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Previously, we have studied blends of poly(ether ether ketone) (PEEK), a semicrystalline and high-per*To whom all correspondence should be addressed. EPJ 29/12--1

The films in which carbon fibres were embedded in PEEK/PES-C blends were obtained by dissolving PEEK and PES-C at various ratios in ct-chloronaphthalene and casting them together with the fibres on a glass plate. They were melted and pressed and then placed on the heating

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stage of a Leitz optical microscopy. After melting at 390--420° for 10 min, the films were cooled quickly to a particular temperature for isothermal crystallization. The number of nucleating sites on the fibre surface and in the bulk of PEEK/PES-C blends were counted directly during crystallizing under the microscope equipped with a micrometer eyepiece. Their nucleation densities were calculated as the number of nuclei per unit length for those on the fibre and the number of nuclei per unit area for those in the bulk. From the maximum slopes of plots of nucleation densities vs time, the nucleation rates were obtained. The size variations of transcrystallinities and spherulites were determined from photographs taken at various times. RESULTS AND DISCUSSION As a semicrystalline polymer, P E E K may form transcrystallinity, an interfacial crystalline layer around carbon fibre, because of its nucleation on the fibre. Besides the nature of the fibre, thermal treatment is also important in the formation of transcrystallinity. Only under appropriate melting and

Fig. 1. The crystalline morphology of PEEK/PES-C blends in the presence of carbon fibre. The blends were crystallized at 290° after melting at 390° for 10 min. PES-C contents in the blends: (a) 0, (b) 20 wt%, (c) 40 wt%.

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Table 1. Nucleation rates of PEEK/PES-C blends on carbon fibre and in bulk* Nucleation r a t e Nucleation on carbon fibre rate in bulk Rf/R b No. nuclei/min/m No. naclei/min/m2 x l0s PEEK/PES-C (Rt x 10-5) (Rb x 10-9) (m) 100/0 1.28 8.36 1.53 80/20 1.07 3.82 2.80 60/40 0.73 2.24 3.24 *Crystallized at 290° after melting at 390° for 10rain.

crystallization conditions, can the transcrystallinity appear [9-11]. The reason is that the nucleation on the fibre may be intefferred with by the bulk nucleation, and it is unaffected only in these cases [12]. However, when amorphous PES-C is incorporated into PEEK, transcrystallinity is easily obtained, even under conditions such that pure P E E K cannot form transcrystallinity. As shown in Fig. 1, when melted at 390 ° for 10rain and isothermally crystallized at 290 °, transcrystallinities which do not form in the C F - P E E K system [Fig. l(a)] appear in the C F - P E E K / P E S - C system for a wide range of PES-C contents [Fig. l(b, c)]. The PEEK/PES-C transcrystallinities are seen to be different from those of pure P E E K because of the dispersion of PES-C. In PEEK/PES-C blends, it has been found that PES-C is included within the spherulites of P E E K in the form of interfibrillar incorporation as a result of partial miscibility of the blends, and so reduces the spherulitic crystallinity [8]. Since transcrystaUinity is basically a specific type of spherulite growth, similar incorporation of PES-C is also possible in PEEK/ PES-C transcrystallinities. To demonstrate the easy formation of transcrystallinity for PEEK/PES-C blends, the nucleations on the fibre and in the bulk were directly observed. Table 1 lists the nucleation rates for the conditions identical to those indicated in Fig. 1. When PES-C is added to PEEK, both the nucleation rate on the fibre (Rf) and that in the bulk (Rb) decrease, and the ratio R f / R b increases obviously. The increase in the ratio is due to the small change of Rf and large change of R b, and means that the interference from the nucleation in the bulk on that on the fibre becomes weaker. In the study of the crystallization of P E E K on the same kind of fibre, it was suggested that the ratio R f / R b should be >2.17 x 10 -5 (m) for the formation of transcrystallinity [12]. The R f / R b values listed in Table 1 exceed that value after P E E K is mixed with amorphous PES-C. It is considered that the nucleation on the fibre is unhindered for PEEK/PESC blends for such melting and crystallization conditions. Since the nucleation on carbon fibre is heterogeneous [7, 13] whereas that in the bulk includes heterogeneous and homogeneous processes, amorphous PES-C is expected to have less influence on the nucleation on the fibre than on the bulk nucleation. This effect might be one reason for the variation of Rf with PES-C content being distinguishable from that of R b. Further, it is possible that P E E K content on the fibre surface is higher than that in the bulk after melting because of the fibre-polymer attraction. Since we find that the melt viscosity of P E E K is much lower than that of PES-C, P E E K molecules might be

Interfacial crystallization of PEEK/PES-C blends on carbon fibre easier to move towards the fibres in the process of melting. The PEEK/PES-C transcrystalline thickness, i.e. the size in the direction normal to the fibre, varies linearly with time as shown in Fig. 2. With increasing PES-C content, the growth rate falls, as for the radial growth of the spherulites in PEEK/PES-C blends [8]. If the melting temperature and crystallization temperature are relatively low as indicated in Fig. 2, the growth rate of the transcrystalline thickness is almost the same as that of the bulk spherulitic radius. When the temperatures are higher, the transcrystalline growth rate may be higher than that of the spherulites. An obvious effect was seen for higher PES-C content, as shown in Fig. 3. It indicates that there might be a lower PES-C content in the local region near the fibre than in other regions during the crystallization, according to the results that PEEK/ PES-C transcrystalline and spherulitic growth rates increase with decreasing PES-C content. A possible reason is that the movement of PEEK molecules towards the fibre might be easier at higher melting temperatures because of its lower melt viscosity, and the blend segregation which is significant at higher crystallization temperatures [8] might be promoted by the fibre because of the influence of the stress near the fibre on the fibre-polymer attraction. Good fibre-matrix adhesion can be expected in CF reinforced PEEK/PES-C blends because of the easier formation of transcrystallinity. The effect of transcrystallinity in improving the interfacial bond strength and even the transverse tensile strength of composites has been reported previously [9, 14, 15]. However, the lower crystallinity of PEEK/PES-C transcrystallinity as a result of the incorporation of amorphous PES-C may lead to lower Young's modulus of the transcrystallinity according to our study on the interfacial mechanical properties of PEEK reinforced with CF [16]. This effect is not good for the

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Time (min) Fig. 3. The transcrystalline thickness and spherulitic radius of PEEK/PES-C (60/40) as a function of time. Crystallized at 315° after melting at 420° for 10 min. O, transcrystalline thickness; A, spherulitic radius. composite stiffness, but favours its impact property compared with PEEK transcrystallinity because lower interlayer modulus can improve the impact strength of composites [17]. Another change caused by the incorporation of PES-C is that the transcrystallinity-spherulite interface and transcrystallinitytranscrystallinity interface, which appear whenever transcrystallinity forms, become confused as seen in Fig. l(b) and (c). It has been shown that such kinds of interfaces which are present in matrix and look like cracks along the fibre direction are very obvious in the C F - P E E K system and others, They are harmful to the composite properties since cracks may form and grow easily along them [11]. Thus, PEEK/PES-C transcrystallinity can be considered to be an improvement on PEEK transcrystallinity in this sense. Acknowledgement We gratefullyacknowledgethe financial support of the National Natural Science Foundation of China.

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REFERENCES

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Time ( min ) Fig. 2. PEEK/PES-C transcrystalline thickness as a function of time. Crystallized at 290° after melting at 390: for 10min. PES-C contents: O, 20 wt%: A 40 wt%.

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