Structural rigidity and microtacticity of linear polymers

The structural rigidity of polymeric glasses is related to a steep temperat ure dependence of their relaxation properties in the vicinity of the glass transition temperature T-g. The microtacticity of polymer chains is a facto r significantly affecting both T-g and the structural rigidity of high poly mers, which increases in PMMA on the passage from isotactic to syndiotactic chain structure and in PB, in going from a linear (1,4-type units) to bran ched(1,2-type units) chain structure. The structural rigidity parameter D = U-infinity/kT(0) (where U-infinity and T-0 are the structural constants in the Vogel-Fulcher-Tamman equation) is a linear function of the T-g/T-0 rat io. In flexible- chain polymers, the D values fit the same straight line, i rrespective of their chemical structure, microtacticity, molecular mass, an d polarity. The preexponential factor B-alpha, calculated from the slope of this line for the Boltzmann-Arrhenius equation describing the relaxation t ime of the oc-process, is similar to 5 x 10(-12) (which coincides with the value determined using the relaxation spectrometry of polymers). The smalle r the parameter D and the closer T-0 to T-g, the more pronounced is the str uctural rigidity of the polymer. It is suggested that an increase in the st ructural rigidity upon the transition from isotactic to syndiotactic PMMA a nd from 1,4- to 1,2-PB is due to a change in the slope of the temperature d ependence of the free volume caused by the change in microtacticity.