A flow-orientation transition in a thermotropic random copolyester

Abstract

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-...

Why Does an Electric Field Align Structures in Copolymers?

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Rheo-Optical Evidence of a Flow-Induced Isotropic−Nematic Transition in a Thermotropic Liquid-Crystalline Polymer

“…15,16 While providing fertile ground for much intriguing materials physics, the structural complexities associated with the coupling of orientation and flow have proven to be factors limiting the ultimate properties of LCPs. [17][18][19][20][21] Thus, the rheological behavior of both thermotropic liquid-crystalline polymers (TLCPs) [22][23][24][25][26][27][28][29][30][31] and lyotropic LCPs [32][33][34][35][36][37][38][39][40][41][42] has been studied extensively.…”

Direct Evidence for Sequence Ordering of Random Semicrystalline Copolyesters during High-Temperature Annealing

“…12 However, a recent report by Windle shows that when the molecular weight of a 75/25 HBA/HNA copolyester increases from 8600 to 30 000, the increase in melting point is only 12°C (from 283 to 295°C). 27 It is unrealistic then to attribute changes of 40°C or more to an increase in molecular weight.…”