Structure-dynamics relationships of the ?-relaxation in flexible copolyesters during crystallization as revealed by real-time methods

Abstract

The evolution of the α‐relaxation during an isothermal crystallization process of a series of flexible copolyesters of hydroxybutyrate (HB) and hydroxyvalerate (HV) has been followed in real‐time by wide‐angle X‐ray scattering and dielectric complex permittivity measurements. The change of the dielectric parameters with crystallization time can be phenomenologically described in terms of the Havriliak‐Negami equation. The dielectric strength follows a sigmoidal‐shaped pattern similar to that shown by the crystallinity. A reduction of the overall mobility with crystallization time of the polymeric chains in the amorphous phase has been observed. This slowing down effect depends on the HV molar content. The influence of the chain flexibility on the crystalline‐induced restriction has been discussed in the light of similar studies carried out with more rigid polymers. Dielectric experiments suggest that the progressive immobilization of polymer segments as crystallization proceeds cannot be exclusively associated with the amount of crystalline material. Differences in microstructure, depending on the HV molar content, seem to be responsible for the observed behavior. The progressive broadening and symmetrization of the α‐relaxation with increasing crystallization time has been explained as due to a restriction of the large‐scale motions of the polymeric chains, as the material is being filled in with crystals. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 37–49, 1999

The influence of the industrial processing on the degradation of poly(hidroxybutyrate) - PHB

“…These relaxations also shift to higher frequencies as the temperature rises, due to the higher mobility resulting from heating. The temperature dependence of the β-process follows an Ahrrenius behavior, while the α-process displays a curvature at the higher measured frequencies characteristic of a Vogel-Fulcher-Tamman dependence, as already observed in previous works 19,20 .…”

“…The graphs of dielectric relaxation at constant frequency (isochronal graphs) show a β-relaxation starting at -50 °C, and an α-relaxation starting at 0 °C. It is known that PHB presents these primary α and β relaxations 19,20 , where β-relaxation was attributed to local shifts of ester groups attached to the polymeric chains, while the α-relaxation was ascribed to shifts in the polymer chains in the amorphous phase in the range of the glass transition temperature. These relaxations also shift to higher frequencies as the temperature rises, due to the higher mobility resulting from heating.…”

Study of reorientational dynamics during real-time crystallization of absorbable poly(p-dioxanone) by dielectric relaxation spectroscopy

“…Similar observations during isothermal crystallization have been reported earlier on other polymers, including PET, 13 poly-(vinylidene fluoride), 17 and copolymers of HB and HV. 20 Furthermore, systematic changes in the relaxed part of the dielectric constant [marked by the arrow in Fig. 1(b)] allows one to compute crystallization kinetics at a constant single frequency.…”

“…The dielectric strength was correlated with the degree of crystallinity derived from real-time wide-angle Xray patterns via synchrotron radiation. 19 As a continuation of this work, Nogales et al 20 used dielectric methods to characterize changes occurring in a series of flexible copolymers of hydroxybutyrate (HB) and hydroxyvalerate (HV) during isothermal crystallization in real time. With respect to different microstructure caused by varying HV molar contents, the data suggested that the progressive immobilization of copolymer segments as crystallization proceeds cannot be exclusively associated with the amount of crystalline material.…”

Localized translational motions in semicrystalline poly(ethylene terephthalate) studied by incoherent quasielastic neutron scattering

“…It is a non-Debye relaxation when observed in the frequency domain (stretched exponential decay in the time domain) and the temperature dependence of its relaxation times strongly departs from the common Arrhenius behaviour. The  relaxation dynamics of semicrystalline polymers, for instance as explored by means of dielectric spectroscopy DS, is affected by the presence of crystals, resulting in general in a less intense, broader and slower relaxation [8,[16][17][18][19][20]. In the context of semicrystalline polymers, poly(ethylene terephthalate) PET is one of the most common polymers for industrial necessities and, as a consequence, it has frequently attracted the attention from the scientific community during the last decades.…”