| Abstract: | The influence of molecular weight on thermal transitions and on the thermodynamic parameters was studied for two polymers based on 4,4′-dihydroxy-α-methylstilbene with either 1,9-dibromononane (HMS-C9 polyethers) or 1,11-dibromoundecane (HMS-C11 polyethers). HMS-C9 polyethers present an enantiotropic nematic mesophase over the entire range of molecular weights and a monotropic smectic mesophase for polymers of number average molecular weights higher than 17,000. The low molecular weight HMS-C11 polyethers are only crystalline. On increasing their molecular weight, the polymers become monotropic nematics, and at higher molecular weights, enantiotropic nematics. Up to a composition containing as little as 20 mol % nonane structural units, the random copolyethers based on 1,9-dibromononane, 1,11-dibromoundecane, and 4,4′-dihydroxy-α-methylstilbene (HMS-C9/11 copolyethers) exhibit on cooling a phase diagram resembling that of HMS-C9 polyether. HMS-C9/11 containing about a 1/1 mole ratio between the two spacers presents both smectic and nematic enantiotropic mesophases. These results suggest that the phase diagram of random liquid crystalline copolymers is controlled by the shorter spacer. The thermodynamic parameters of isotropization for both polyethers and copolyethers are compared and suggest that copolymerization does not significantly decrease the degree of order of the mesogenic units in the mesomorphic phase. |
