Gm. Bartenev, The effect of chain microtacticity and degree of crystallinity of poly(methyl methacrylates) on alpha-relaxation and glass transition process, VYSO SOED, 41(6), 1999, pp. 936-944
Citations number
27
Categorie Soggetti
Organic Chemistry/Polymer Science
Journal title
VYSOKOMOLEKULYARNYE SOEDINENIYA SERIYA A & SERIYA B
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.