Polydiarylsiloxanes and polydialkylsiloxanes, with alkyl groups larger than methyl, have liquid-crystalline states, presumably as the result of a chain-rigidifying effect from steric interactions of the bulky aryl or alkyl groups along the chain. In the first paper of this series, we reported on the thermal behaviour of a number of diarylsiloxane polymers with phenyl and/or p-tolyl substituents, all of which showed liquidcrystalline behaviour at high temperatures. The solution properties of several of the 'mixed' poly(phenyl/ptolyl) siloxane polymers have now been investigated in order to characterize their chain stiffness properties and to relate these to the tendency to form a liquid-crystalline state in bulk. The 'mixed' poly(phenyl/ptolyl) siloxanes were chosen for investigation since they have a major advantage in experimental convenience over the 'unmixed parent' polymers, polydiphenylsiloxane and polydi(p-tolyl)siloxane. The former are soluble at room temperature in common solvents, whereas the latter polymers are only soluble at temperatures above 150°C. The various diarylsiloxane polymers were prepared by anionic polymerization of the cyclic trimers, and the preservation of the triad sequences (absence of 'scrambling') was confirmed by 29Si n.m.r. Values of the Mark-Houwink-Sakurada exponent, a, for three different types of poly(phenyl/ptolyl) siloxanes are essentially equal, at ~0.83. This value of the exponent is below the value expected for a rigid chain (a > 1.0) but is above the maximum value expected for a flexible random-coil polymer in a good solvent (a ~< 0.8). The shear viscosity of solutions of the mixed poly(phenyl/p-tolyl)siloxane polymers increases monotonically with increasing polymer concentration up to 60 wt%, and does not show an abrupt drop in viscosity above a critical concentration, as is often observed with rigid-rod polymers. This observation, together with the value of the Mark-Houwink-Sakurada exponent, a < 1, indicates that these polymers do not behave in solution as rigid-rod polymers, but most likely as worm-like chains with a relatively large persistence length.
Посилання на статтю:
Synthesis and properties of diarylsiloxane and (aryl/methyl)siloxane polymers: 2. Solution and rheological properties / Michael K. Lee and Dale J. Meier // Polymer. – 1994. – Vol 35. – P. 3282-3284.
