Dielectric relaxations in PEEK by combined dynamic dielectric spectroscopy and thermally stimulated current

The molecular dynamics of a quenched poly (ether ether ketone) (PEEK) was studied over a broad frequency range from 10-3 to 106 Hz by combining dynamic dielectric spectroscopy (DDS) and thermo-stimulated current (TSC) analysis. The dielectric relaxation losses e00 KK has been determined from the real part e0 T(x) thanks to Kramers–Kronig transform. In this way, conduction and relaxation processes can be analyzed independently. Two secondary dipolar relaxations, the c and the b modes, corresponding to non-cooperative localized molecular mobility have been pointed out. The main a relaxation appeared close to the glass transition temperature as determined by DSC; it has been attributed to the delocalized cooperative mobility of the free amorphous phase. The relaxation times of dielectric relaxations determined with TSC at low frequency converge with relaxation times extracted from DDS at high frequency. This correlation emphasized continuity of mobility kinetics between vitreous and liquid state. The dielectric spectroscopy exhibits the ac relaxation, near 443 K, which has been associated with the rigid amorphous phase confined by crystallites. This present experiment demonstrates coherence of the dynamics of the PEEK heterogeneous amorphous phase between glassy and liquid state and significantly improve the knowledge of molecular/dynamic structure relationships

Flexibilized styrene-N-substituted maleimide copolymers with enhanced entanglement density

High molecular weight poly(styrene-co-N-substituted-maleimide)-block-poly(tetrahydrofuran) multiblock copolymers with an excellent thermal stability were synthesized using the polymeric iniferter technique. Evidence for the existence of these multiblock copolymers was obtained by submitting them to gradient polymer elution chromatography and by snipping the chemical bonds between the SMI blocks and the PTHF blocks, after which the molecular weight distribution of the remaining SMI blocks was determined. Only one glass transition temperature was observed in a modulated temperature DSC scan of the segmented copolymers, typical for a homogeneous material. The T-g of these flexibilized, single-phase multiblock copolymers is still high enough to qualify them as engineering plastics. The entanglement density, which is closely related to the toughness of materials, of a styrene-N-phenylmaleimide copolymer flexibilized by 12 wt % PTHF was raised by a factor 1.6 as compared to the unflexibilized styrene-N-phenylmaleimide copolymer. These entanglement densities were deduced from the respective rubber plateau moduli, which were corrected for the nonentangled low molar mass fraction by applying the Wasserman-Graessley model

Structure and dynamics in poly(L-lactide) copolymer networks

Poly(L-lactide) networks (PmLA) hydrophilized with different amounts of 2-hydroxyethyl acrylate (HEA) were investigated by dielectric relaxation spectroscopy, thermally stimulated depolarization currents, and differential scanning calorimetry. The incorporation of HEA units in the PmLA network, with the aim of modulating the water sorption capacity of the system, results in a material with a complex behavior. The system consists of phase-separated microdomains richer in one or the other comonomers that constitute the network. Initially, the addition of smalls amount of HEA units in the network gives rise to a one-phase, two-component system; however, when the amount of HEA in the system increases, a new phase (HEA-rich one) is formed containing some mLA chains that modify the main relaxation mode of these domains and the local dynamics of the system. The structure of the system has been analyzed by comparing the relaxational modes in the PmLA and PHEA homonetworks with those in the copolymer networks