Comparative dielectric studies of segmental mobility in novel polyurethanes

Abstract

Molecular dynamics in selected novel linear/low-branched polyurethanes (PUs), based on oligo(oxytetramethylene glycol), 4,4'-diphenylmethanediisocyanate (MDI) or 2,6-toluenediisocyanate (TDI), and unsymmetrical dimethylhydrazine (I) and a derivative of that (II) as chain extenders (CE), were studied by dielectric techniques. Special attention was paid to the investigation of the α relaxation, associated to the glass transition, by dielectric relaxation spectroscopy (DRS) and thermally stimulated depolarization currents (TSDC). The TSDC method was used to study the interfacial Maxwell-Wagner-Sillars (MWS) relaxation, related to the accumulation of charges at the interfaces between soft-segment and hardsegment microdomains. The results obtained by DRS and TSDC were in good agreement with each other and in reasonable agreement with results for the microphase separation (MS) obtained by small-angle X-ray scattering and differential scanning calorimetry. TSDC proved to be an attractive complementary technique to DRS for the study of MS in PUs. The results suggest that the position of the MWS band, as well as its separation from the α band, is a good measure of the degree of MS. As regards the PUs studied here, the degree of MS enhances by increasing the mole ratio of CE, and by replacing MDI by TDI or CE I by CE II.

TSDC spectroscopy of relaxational and interfacial phenomena

“…Pissis and colleagues [280][281][282][283][284][285][286][287][288][289][290][291], Tsonos et al [292,293], Diaconu et al [294], Malmonge et al [295], Sakamoto et al [296], and Ledru et al [297] studied various polyurethanes (PUs) by TSDC, DRS, DSC and other methods. Raftopoulos et al [281] investigated PUs characterized by a two-phase microstructure, consisting of hard-segment (HS) microdomains dispersed in a soft-segment (SS) phase.…”

“…The importance of MS on controlling the properties of PUs is a strong motivation for developing methods to calculate the microphase composition and to characterize the DMS [281]. The authors [280,281] presented a comparative DRS and TSDC study of the molecular dynamics in selected linear/low-branched PUs, based on oligooxytetramethylene glycol (OE), 4,4′-diphenylmethane diisocyanate (MDI), or (2,6)-toluene diisocyanate (TDI) and unsymmetrical dimethylhydrazine (I) or a derivative of that (II) as chain extenders (CE). Decomposition of the complex TSDC spectra by the thermal sampling technique revealed a characteristic shape for the E (T m ) dependence of the α mode (Fig.…”

Dielectric Analysis (DEA)

“…Polyurethanes Segmented polyurethanes are typical linear block copolymers of the type (A -B) n , where block A consists of a relatively long and fl exible soft segment (mainly ether, ester, or diene groups) and block B is a highly polar hard segment (mostly aromatic diisocyanates). Thermal sampling analysis of α relaxations in polyurethanes (Hsu et al 1999 ;Roussos et al 2004 ) reveals a typical compensation law behavior (see Section 6.5 ). A representative example of a TSC spectrum with the typical relaxations found in polyurethanes is shown in Fig.…”

Polyurethane networks based on hyperbranched polyesters: Synthesis and molecular relaxations

“…Several dielectric relaxation studies of the linear and branched PU systems have been reported in the literature [12][13][14], in particular on the molecular dynamics 0022 under pressure dependence [15] or the effect of the interfacial interactions [16]. Roussos et al investigated novel linear/low-branched polyurethanes by means of dielectric techniques [17]. They examined the microphase separation in a PU consisting of hard and soft segments.…”