Liquid Crystalline Polyurethanes Studied by Positron Annihilation Method

A series of liquid crystalline polyurethanes of different both flexible spacer length and mesogenic group content was studied by means of positron annihilation method. The investigated polymers were targeted on the separation of aromatic hydrocarbons from their mixtures with aliphatic ones. The effect of modification of the liquid crystalline polyurethane chain on the polyurethane free volume size and free volume distribution was determined on the basis of the positron annihilation lifetime spectra. In the positron annihilation lifetime spectra measured for the samples under study two long components of several nanoseconds, characteristic of o-Ps decaying by pick-off, occurred. The correlation of the values of the o-Ps lifetime with the size of the free volume region allowed to recover the free volume distributions with the use of the method employing the numerical Laplace inversion technique. The obtained results were compared with the diffusion data on the mobility of liquid hydrocarbons in the liquid crystalline polyurethanes enabling the correlations between polymer structure and its transport properties to be evaluated.