Molecular relaxation of isotactic polystyrene: Real‐time dielectric spectroscopy and X‐ray scattering studies*

Abstract

The molecular relaxation processes and structure of isotactic polystyrene (iPS) films were investigated with real-time dielectric spectroscopy and simultaneous wide-and small-angle X-ray scattering. The purpose of this work was to explore the restrictions imposed on molecular mobility in the vicinity of the ␣ relaxation (glass transition) for crystallized iPS. Isothermal cold crystallization at temperatures of T c ϭ 140 or 170°C resulted in a sigmoidal increase of crystallinity with crystallization time. The glass-transition temperature (T g ), determined calorimetrically, exhibited almost no increase during the first stage of crystal growth before impingement of spherulites. After impingement, the calorimetric T g increased, suggesting that confinement effects occur in the latter stages of crystallization. For well-crystallized samples, the radius of the cooperativity region decreased substantially as compared with the purely amorphous sample but was always smaller than the layer thickness of the mobile amorphous fraction. Dielectric experiments directly probed changes in the amorphous dipole mobility. The real-time dielectric data were fitted to a Havriliak-Negami model, and the time dependence of the parameters describing the distribution of relaxation times and dielectric strength was obtained. The central dipolar relaxation time showed little variation before spherulite impingement but increased sharply during the second stage of crystal growth as confinement occurred. Vogel-Fulcher-Tammann analysis demonstrated that the dielectric reference temperature, corresponding to the onset of calorimetric T g , did not vary for well-crystallized samples. This observation agreed with a model in which constraints affect primarily the modes having longer relaxation times and thus broaden the glass-transition relaxation process on the higher temperature side.

Dielectric relaxation in vinylidene fluoride–hexafluoropropylene copolymers

“…The authors 56,58 consider it as an indication for the existence of upper glass transition point (T g U ), where RAP relaxation occurs. 12,[43][44][45] The authors 57 do not agree with this. On their opinion, this relaxation process is connected with the transition of ordered regions into the condis-crystal state, and its molecular mechanism is suggested.…”

“…Therefore, the increase in crystallinity usually results in the increase of the relaxation time for the a a -transition [62][63][64] and of the glass transition temperature. 45 Moreover, for the copolymer with higher HFP content the dimensions of the amorphous space between adjacent lamellae packed into stacks increase from 7.6 to 14.6 nm. 47 By the example of isotactic polystyrene it was shown that this must lead to the decrease of relaxation time of the transition under consideration.…”

“…Such problem is discussed with reference to other crystalline polymers. [43][44][45] It is known that…”

“…For example, the transition around 408C (in the temperature region of a c -process) 12 is attributed to the upper glass transition point (T g U ), i.e., to the relaxation in RAP, as it is observed in other crystalline polymers. [43][44][45] EXPERIMENTAL VDF/HFP copolymers of 93/7 and 86/14 ratios have been studied in this paper. Their chain microstructure was earlier characterized by high resolution NMR.…”

Hydrogen bonding, mobility, and structural transitions in aliphatic polyamides

“…Data such as those in Figure 4(c) show that the oriented segments relax at a higher temperature (T g $ 80-90 8C) than the unoriented segments (T g $ 30-50 8C). 62 The higher T g fraction is called the rigid amorphous fraction (RAF), [63][64][65] and the lower T g fraction corresponds to the conventional amorphous fraction. It is sometimes thought that the RAF is at the fold surface of the lamellae.…”