The effect of chain microtacticity and degree of crystallinity of poly(methyl methacrylates) on alpha-relaxation and glass transition process

As evidenced by the data of mechanical and dielectric relaxation spectromet ry, microtacticity of macromolecules has a strong effect on a-relaxation an d segmental motion in PMMA. Amorphous PMMA with different degrees of stereo regularity are characterized by glass transition temperatures T-g which ran ge from 31 degrees C (100% isotacticity) to 130 degrees C (100% syndiotacti city). In the case of commercial PMMA, T-g lies within the narrow interval of 100-110 degrees C, and microtacticity primarily involves syndiotactic an d heterotactic components. Analysis of the frequency-temperature dependence s of the alpha-relaxation process showed that, for PMMA with a high degree of isotacticity, the alpha-transitions take place in a low-temperature regi on, whereas, for PMMA with a high degree of syndiotacticity, these transiti ons are observed in a high-temperature region. In the Arrhenius coordinates , these dependences are nonlinear and described with structural constants T -0 and U-infinity by the Vogel-Fulcher-Tamman equation. As T-g is increased , both the Vogel temperature T-0 and the activation energy U-alpha increase , but constant U-infinity decreases. According to the Vogel-Fulcher-Tamman equation, activation energy U-alpha (T) = U-1 + U-2(T) where the first comp onent U-1 = U-infinity is independent of temperature, and the second compon ent U-2 depends on temperature. This behavior agrees: with the speculations about the dual nature of the glass transition process. According to these speculations, U-1 is the energy of the cooperative conformational transitio ns in a segment, and U-2 is the energy of the development of foe volume. Th e first component U-1 = U-infinity decreases as the flexibility of the poly mer chains increases, that is, on transition from isotactic to syndiotactic PMMA. On the contrary, the second component U-2 increases. As a result, U- alpha(T) and, correspondingly, T-g increase. Comparing the processes of the : a-relaxation of isotactic PMMA in the two states (amorphous and amorphous -crystalline), one may conclude that a-relaxation in the amorphous phase of semicrystalline PMMA does not follow the Vogel-Fulcher-Tamman law but is d escribed by the Arrhenius equation with a temperature-independent activatio n energy. This trend implies that the behavior of the amorphous phase in th e crystalline matrix is similar to that of the amorphous polymer at a const ant volume when, under temperature variations, the free volume V-f remains unchanged.