Cooling-induced crystallization of cross-linked poly(cyclooctene) films under a tensile load results in significant elongation and subsequent heating to melt the network reverses this elongation (contracting), yielding a net two-way shape memory (2W-SM) effect. The influence of cross-linking density on the thermal transitions, mechanical properties, and the related 2W-SM effect was studied by varying the concentration of cross-linking agent dicumyl peroxide (DCP) and using differential scanning calorimetry (DSC), gel fraction measurements, dynamic mechanical analysis (DMA), and customized 2W-SM analysis. The latter showed that there is crystallization-induced elongation on cooling and melting-induced shrinkage on heating (2W-SM), with lower cross-link density leading to higher elongation at the same applied stress. For a given cross-link density, however, increasing the tensile stress applied during cooling resulted in greater stress-induced crystallization. We further observed that the onset temperatures for elongation on cooling (T c ) and contraction on heating (T m ) shifted to higher temperatures with decreasing cross-link density. Similarly, the degree of molecular orientation achieved upon deformation was found to increase with decreasing cross-link density. The impact of stress on the 2W-SM effect was examined using wide-angle X-ray diffraction (WAXD), revealing a transition from bimodal to unimodal orientation. As the crystalline structure evolves from bimodal (low stress) to unimodal (high stress), the crystallization occurs along a single preferred orientation thus inducing greater elongation along the stretching direction. We anticipate that the observed 2W-SM property in a semicrystalline network will enable applications heretofore possible only with costly shape memory alloys and liquid crystalline elastomers.

This paper sets down in detail the observation, first announced as a note (Romo-Uribe, A.; Windle, A. H. Macromolecules 1993, 26, 7100), of a strident and reversible orientation transition in a specially synthesized series of thermotropic random copolyester melts of the Vectra A900 type. The transition is between the flow-aligning regime normally seen in nematics and alignment of the global director at right angles to flow-alignment and parallel to the vorticity axis, which we refer to as the “log-rolling” regime. The orientation transition is observed in a comparatively narrow band of temperature above the crystal melting point, the width of this band decreasing with increasing strain rate and increasing molecular weight. We believe that observation of log-rolling in the main-chain thermotropic random copolymers provides confirmatory evidence for the occurrence of a smectic phase in isolated regions which are the precursors of the non periodic layer (npl) crystals which form on further cooling. The fact that the log-rolling regime is not observed at high molecular weights and higher strain rates suggests that the low density of entanglements, which will occur in liquid crystalline polymers with less than completely rigid chains, will nevertheless compromise the log-rolling regime. Slow strain rates and lower molecular weights would each tend to enhance the relaxation of entanglements during shear and thus reduce the deleterious effect which entanglements would have on the log-rolling regime. Recent rheological measurements on the same set of polymers add support to this hypothesis.

Introduction. This paper reports some preliminary results of a study of flow-induced orientation in a series of liquid crystalline polymers based on hydroxybenzoic acid (HBA) and hydroxynaphthoic acid ( "A) units. Previous, in situ studies of orientation in flowing liquid crystalline polymers of this type have shown the development of significant chain orientation in the direction of shear,' as well as the tendency for shear-induced crystallinity at temperatures close to the melting point.2 A recent series of measurements, covering a range of molecular weights, shear temperatures, and strain rates, have shown a striking orientational transition for samples of lower molecular weight at temperatures only slightly above the crystal melting point.Experimental Section. The polymerwhich forms the main basis of this report is a random copolymer of composition 75/25 HBA/HNA and molecular weight 8600 supplied by Hoechst Celanese Corp. The molecular weight was stabilized a t this value hy the addition of a small amount of terephthalic acid at polymerization and measured by viscometry. The crystallinity of this polymer is 20% a t room temperature? and the crystal melting point is 275 "C by DSC.The X-ray measurements of orientation were carried out in a shear cell designed by Mackley and described elsewhere? The cell permits control of temperature to within *l "C, while the shear conditions can be programmed. In the experiment described in this papei, the diffraction patterns were recorded on film and subsequently digitized for analysis.Results. Figure 1 shows diffraction patterns of the shearing melt of the M, = 8600 polymer at two different shear rates at 285 " C (a) 100 and (b) 5 s-l. While at the higher strain rate the orientation is typical of a flowing thermotropic polymer melt with the chain axes aligning parallel with the shear direction (as shown by the main equatorial reflections lying on the axis normal to the chain axis on the diffraction pattern), at the lower strain rate thechainaxesorient orthogonalto theshear axisalthough they still lie in the shear plane. Subsequent threedimensional analysis of a sample quenched during shear confirms this orientation. Figure 2 plots a series of azimuthal intensity scans (around the equatorial circle) for different strain rates at 285 "C. At strain rates of 10 s-l and below, the equatorial reflections concentrate on the shear axis of the diffraction pattern, indicating the orthogonal orientation of the chains to the shear. At the transition strain rate, which is 80 s-l at this temperature, very little preferred orientation can be seen at all, while at higher strain rates it develops again, but now with the chains parallel to the shear axis.As the temperature is increased, ye, the critical strain rate for the orientation transition, decreases. At 295 OC, for example, it has dropped to around 10 s-l. Figure 3 is a plot of yc against the temperature of shear. Above 300 O C it was not possible to obtain transverse orientation, even at the lowest strain rates available.It is well-...