Thermomechanical behavior of liquid-crystal polyesters

We have used the thermomechanical method to investigate the processes of self-elongation and shrinkage in uniaxially oriented liquid-crystal (LC) polyesters (CPET - the copolyester of terephthalic acid, phenylhydroquinone, and p-hydroxybenzoic acid; Ultrax - the copolyester of terephthalic acid, isophthalic acid, and bisphenol) and the injection-molded copolyester of hydroxybenzoic and hydroxynaphthoic acids Vectra A900. A universal tensile tester and a thermomechanical tester were used for the investigations. We studied the deformation behavior for the first and second hearing under negligible applied stress, We prepared CPET samples in the form of as-spun and annealed fibers, Vectra samples in the form of cubes cut from the nonoriented part of the molded piece (VI) and from the longitudinally oriented central part (VII), Ultrax samples in the form of strands (noncrystalline UI and crystalline UII). We observed a permanent length increase for the as-spun CPET (Fig. 1) and shrinkage in the glass-transition region for the annealed CPET (Fig. 2) for the first and second heating. Note that the as-spun CPET is a smectic polymer, and the annealed CPET contains smectic and crystalline phases. The thermomechanical curves of VI seem to be typical of any molded polymer (Fig. 3). In contrast, for the oriented VII we see spontaneous elongation upon heating in the direction of orientation, with pronounced transverse shrinkage (Fig. 4). The length increase for the noncrystalline UI is quite considerable for the first heating. We observed both spontaneous elongation below the glass-transition temperature and subsequent shrinkage for crystalline UII for the first heating. All the Ultrax curves are much smoother for the second heating (Fig. 5). Spontaneous elongation upon hearing appears to be a characteristic feature of oriented LC polymers, the polymer being either fully oriented or oriented within some part of the sample. Shrinkage of LC polyesters upon heating is also possible, as a result of both processing factors and crystallinity. There are two types of restructurization processes: orientation and crystallization (possible for LC polymers capable of forming a crystalline phase).