The components of activation entropy of alpha-relaxation in linear polymers

Molecular and thermodynamic analysis of the alpha-relaxation process in lin ear flexible-chain polymers was carried out. This analysis is based on the dual molecular mechanism of segmental mobility and allows one to obtain qua ntitative estimates of the two components of activation entropy Delta S* = S-1 + S-2. The roles of both components in the glass transition process are discussed. It was found that the first component S-1 is negative, and this component is associated with the transition of a segment from a basic to o riented state (the conformational mechanism of segmental mobility). On the contrary, the second component S-2 is positive, and this component is relat ed to the development of the fluctuation free volume in the correlation vol ume, into which a segment is moved (the translational mechanism of segmenta l mobility). The value of S-1 [similar to -10 J/(mol K)] is independent of temperature and provides the contribution to the pre-exponential coefficien t B-alpha of the characteristic relaxation time tau(alpha) of alpha-relaxat ion. As compared with the corresponding coefficient B-beta of beta-relaxati on, B-alpha increases by about 40 times. Component S-2 depends on temperatu re and contributes to the effective activation energy of alpha-relaxation, which equals 5-10% of the activation energy U-alpha. Hence, the activation entropy slightly affects the kinetics of the alpha-relaxation of linear pol ymers.