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

This work deals with the study of the dielectric and mechanical behavi or of polymers in which the conformational versatility of the backbone is severely restricted. The mechanical spectrum of the model, poly(di cyclohexylmethylene itaconate) (PDCMI), presents two absorptions in th e glassy state that in increasing temperature order are called gamma a nd beta relaxations. Contrary to what occurs in most amorphous systems , the polymer only presents a weak glass-rubber relaxation, named alph a. By using deconvolution techniques to separate the peaks associated with the different absorptions, one finds that the strength of the bet a peak is nearly 6 and 3 times larger than the strength of the alpha a nd gamma relaxations, respectively. The dielectric spectrum also prese nts two well-developed absorptions in the glassy state, one called gam ma, in the lower temperature side of the spectrum, that is much weaker than the beta, located close to the glass-rubber relaxation; in addit ion, a shoulder at low frequencies in the spectrum is detected that pr esumably corresponds to a peak (beta') that overlaps with the beta pro cess. Although conductivity processes overlapping with the dipolar con tribution to the alpha absorption preclude the possibility of determin ing the strength of this latter relaxation, the analysis of the electr ic loss spectra permits us to conclude that both the dipolar alpha and beta relaxations roughly have the same intensity. The values of the r elaxation strength for the beta and gamma relaxations at 100 Hz are 0. 88 and 0.07, respectively. The fact that both the mechanical and diele ctric 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 exhi bit a well-developed glass-rubber relaxation whose peak is centered at 50-degrees-C, followed by two subglass relaxations, beta and gamma. T he activation enthalpy for the peaks obtained by partial depolarizatio n techniques in the beta region is nearly half of that corresponding t o 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 calcula ted by assuming that the dielectric activity is only produced by reori entations of the side groups with respect to the frozen main chain. Th e fact that the value of the coefficient thus obtained is somewhat lar ger than the experimental one suggests that dipolar intermolecular int eractions cannot be neglected in the dielectric behavior of these chai ns.