Unique mechanism of the complex liquid crystalline phase behavior of poly (chloro-p-phenylene terephthalamide) in dimethylacetamide/LiCl solvent : revealed by light scattering photometry

Dilute and concentrated solution studies of the molecular and phase behavior of poly(chloro-p-phenylene terephthalamide) (CIPPTA) and poly(p-benzamide) (PBA) in N,N-dimethylacetamide (DMA)/LiCl solvent were performed. Light scattering was the main technique used to study macromolecular and phase behavior as a function of LiCl concentration (C_s), polymer concentration (C_p), molecular weight and temperature.

Light scattering intensities in both dilute and concentrated solution could be corrected for depolarization by separate measurement of the horizontal (H_u) and vertical (V_u) components of the scattered light and application of the equality H_v = V_h = H_h,90° = 4/3^V_v, aniso. CLPPTA is unassociated and semi flexible in very dilute solutions of DMA/4% LiCl while the PBA macromolecules are associated. For both polymers decreasing C_s below 4.0% LiCl leads to increased polymer-polymer association. When more than about 4.5 macromolecules on average from an associated species dramatic increases in intrinsic viscosity [n] are observed due to a more end-to-end type of association. In dilute solutions with DMA/4.0% LiCl solvent reversible increases in association are seen at temperatures greater than 30°C for PBA but only at temperatures greater than 100°C for CIPPTA. Phase diagrams at various c_s levels and a generalized phase diagram were constructed for CIPPTA in DMA/LiC1 from the concentrated solution light scattering studies. These showed the dependence of liquid crystalline phase behavior on C_s, C_pand temperature. Polymers polymer association changes with these variables are also represented.

Unique lower consolute temperature (LCI) to upper consolute temperature (UCT) liquid crystalline phase behavior was observed for CIPPTA in DMA/LiC1 (4 or 6%) solvent. This phenomenon resulted from exothermic polymer solvent specific interactions which decreased at higher temperatures. In the same solutions the level of polymer-polymer association increased with increasing temperature between the LCT and UCT. Polymer axial ratio increased as a result giving a lower value of the athermal C_p at which a liquid crystalline phase would form. PBA in DMA/LiC1 undergoes irreversible polymer-polymer association at elevated temperatures (>110°C) in isotropic concentrated solutions. Therefore precipitation of a crystalline phase at such high temperatures from a liquid crystalline phase is not proof of an inherent instability of the liquid crystalline state as claimed by others.