Relaxation transitions in poly(vinyl chloride) revealed by mechanical and dielectric relaxation data

According to the data of dynamic mechanical spectroscopy, a glassy PVC expe riences six short-scale relaxation transitions which are related to the tor sional mobility of chain CH2 groups (beta (CH2) - and beta'(CH2)-transition s) and polar CHCl groups (beta (Cl)- and beta'(Cl)-transitions) and to the dissociation of dipole-dipole CCl . . . CCl bonds between chains which serv e as the pointlike junctions of a molecular network (pi (Cl)- and pi'(Cl)- transitions). As was assumed, the splitting of the transitions into two clo se transitions is provided by the presence of two types of microstructures in the PVC chains, isotactic and syndiotactic fragments which affect both t he dipole moments of CHCl groups and intermolecular interactions. According to the data of dielectric relaxation, one may observe a single well-pronou nced beta (Cl)-transition instead of the six short-scale relaxation transit ions. In this case, the data of mechanical and dielectric relaxation fit a single frequency-temperature curve for the beta (Cl)-relaxation. The other transitions are less pronounced due either to the absence of electric dipol e moments in the groups (a CH2 group) or to a low concentration of the corr esponding polar groups (CHCl groups in the beta'(Cl) -transition) or to a l ow dielectric activity of the junctions of a molecular network (pi (Cl)- an d pi'(Cl) -transitions). For the alpha -relaxation and high-temperature lar ge-scale lambda -transitions, the data of mechanical and dielectric relaxat ion coincide and are well described by similar frequency-temperature depend ences. The dielectric activity of the alpha- and lambda -processes may be e xplained by the fact that the polar CHCl groups are always involved in both free segments participating in the alpha -relaxation and bound segments pa rticipating in the lambda -relaxation. The microtacticity of the polymer ch ains of PVC is shown to exert a marked effect on alpha -relaxation and glas s transition temperature and, probably, on the lambda -relaxation processes .