RELAXATION PROPERTIES OF MOLECULAR CHAINS WITH RESTRICTED CONFORMATIONAL VERSATILITY OF THE BACKBONE

Abstract

This work deals with the study of the dielectric and mechanical behavior of polymers in which the conformational versatility of the backbone is severely restricted. The mechanical spectrum of the model, poly(dicyclohexylmethylene itaconate) (PDCMI), presents two absorptions in the glassy state that in increasing temperature order are called gamma and beta relaxations. Contrary to what occurs in most amorphous systems, the polymer only presents a weak glass-rubber relaxation, named alpha. By using deconvolution techniques to separate the peaks associated with the different absorptions, one finds that the strength of the beta peak is nearly 6 and 3 times larger than the strength of the alpha and gamma relaxations, respectively. The dielectric spectrum also presents two well-developed absorptions in the glassy state, one called gamma, in the lower temperature side of the spectrum, that is much weaker than the beta, located close to the glass-rubber relaxation; in addition, a shoulder at low frequencies in the spectrum is detected that presumably corresponds to a peak (beta') that overlaps with the beta process. Although conductivity processes overlapping with the dipolar contribution to the alpha absorption preclude the possibility of determining the strength of this latter relaxation, the analysis of the electric loss spectra permits us to conclude that both the dipolar alpha and beta relaxations roughly have the same intensity. The values of the relaxation strength for the beta and gamma relaxations at 100 Hz are 0.88 and 0.07, respectively. The fact that both the mechanical and dielectric beta relaxations are located in the same region of the spectrum, and also have the same activation energy (almost-equal-to 42 kcal mol-1), indicates that both relaxations are caused by the same molecular mechanisms. Thermally stimulated discharge current (TSDC) spectra exhibit a well-developed glass-rubber relaxation whose peak is centered at 50-degrees-C, followed by two subglass relaxations, beta and gamma. The activation enthalpy for the peaks obtained by partial depolarization techniques in the beta region is nearly half of that corresponding to the same zone of the spectrum obtained in a variable electric field. The analysis of the dipole moments of the chains suggests that as far as their polarity is concerned they behave as freely jointed chains. The dipolar correlation coefficient for the subglass region is calculated by assuming that the dielectric activity is only produced by reorientations of the side groups with respect to the frozen main chain. The fact that the value of the coefficient thus obtained is somewhat larger than the experimental one suggests that dipolar intermolecular interactions cannot be neglected in the dielectric behavior of these chains.