Influence of chain extenders and chain end groups on properties of segmented polyurethanes. II. Dielectric study

Comparative dielectric studies of segmental mobility in novel polyurethanes

“…Results to be reported elsewhere give for the glass transition temperature T g of this sample, determined by DSC, 217 K. At low temperatures a broad secondary relaxation is observed as a step in ε'(f) and a peak in ε''(f) (centred at about 10 4 Hz at 173 K). In agreement with previous reports on PUs [11,13], this signal is attributed to the γ relaxation, related to the local motion of (CH 2 ) n sequences with attached dipolar groups [37]. The broadening of the response at the low-frequency side of the spectrum is due to the presence of the slower β relaxation, assigned to the local motion of associations of carbonyl groups and water molecules, as indicated by DRS measurements on samples at various water contents (not presented here), in agreement with the results of TSDC measurements on similar hydrated PUs [37].…”

“…The broadening of the response at the low-frequency side of the spectrum is due to the presence of the slower β relaxation, assigned to the local motion of associations of carbonyl groups and water molecules, as indicated by DRS measurements on samples at various water contents (not presented here), in agreement with the results of TSDC measurements on similar hydrated PUs [37]. The strong loss peak at T > T g , which shifts significantly with temperature, is due to the α relaxation associated to the glass transition, in agreement with previous studies [11,13,37], the DSC result for T g and the results of the analysis to be reported in the following section. Fig.…”

“…In addition to the γ relaxation at about 120 K and the α relaxation at about 220 K, a peak is observed at T > T α , where T α is the peak temperature of the α relaxation. In previous works this peak has been associated with interfacial Maxwell-Wagner-Sillars (MWS) polarization, i.e., mesoscopic transport of charge carriers and trapping at interfaces arising from the microphase-separated structure of PUs [11,13,37]. Thus, analysis of the characteristics of this peak may provide information on the morphology.…”

“…In particular, the primary (segmental) α relaxation associated to the glass transition of the amorphous SS phase has been studied in detail and the results have been interpreted in terms of microphase structure [10][11][12][13][14][15]. Next to the classical dielectric relaxation spectroscopy, which typically consists in measuring the complex dielectric permittivity ε* = ε' -i ε'' in a wide frequency range at constant temperature, thermally stimulated depolarization currents (TSDC) techniques have also been used [11][12][13].…”

Relaxation phenomena in rubber/layered silicate nanocomposites

“…The repeat unit of PUR chain is composed by stiff, rigid blocks and soft rubbery blocks. In that sense IP arises from ionic polarization occurring at the interface of hard and soft regions as well as from variations of the morphology between amorphous and crystalline segments [16][17][18]. The dielectric spectra of the PUR/NR blend include contributions from both polymers, and thus in the low temperature range ( Figure 4) the fast processes of local motions of small parts (β and γ-modes) of the PUR chain as well as the α-mode of NR are present.…”