High-performance laminates through chemical welding of thin liquid crystalline polymer films

Abstract

A systematic study of the welding (healing) process of polymers with different chemical compositions, structures and properties was carried out. It was shown that welding can be due to a number of physical and/or chemical factors. Diffusion alone led to a relatively weak welding under the selected conditions. A jump was observed in the temperature dependence of the shear stress of polyimides. The abrupt increase in the activation energy of welding can be explained by assuming a change in the nature of the created bonds from physical to chemical ones. For the first time the chemical reactions taking place at the contacting surfaces were directly experimentally proved by electron spin resonance. In some cases, for the first time, the healing process was carried out at temperatures below the glass transition temperature. In this way, the contribution of the diffusion process to the healing may be distinguished from that of the chemical reactions. On the basis of a poly(ethylene terephthalate) copolyester containing 60 mol% p-hydroxybenzoic acid thin liquid crystalline films (160 µm thick) are obtained by melting the polymer at 300°C and chilling at 0°C. The undrawn films have a high degree of orientation as shown by X-ray measurements. These films have excellent mechanical properties because of the molecular orientation. In order to avoid losses in the mechanical strength due to increase in their thickness, laminates were prepared using thin liquid crystalline films. Lamination was carried out by annealing under pressure at 170°C for 6 hr, resulting in samples with excellent mechanical properties regardless of their thickness. A method is proposed that makes it possible to combine the unique mechanical properties of thin films of liquid crystalline polymers with a lamination process in order to obtain thick and very strong materials. Copyright © 1999 John Wiley & Sons, Ltd.