In:
The Journal of Chemical Physics, AIP Publishing, Vol. 117, No. 5 ( 2002-08-01), p. 2435-2448
Abstract:
Broadband dielectric spectroscopy, heat capacity spectroscopy (3ω method), and viscosimetry have been used to study the dynamic glass transition of two glass-forming epoxy resins, poly [(phenyl glycidyl ether)-co-formaldehyde] and diglycidyl ether of bisphenol-A. In spite of their rather simple molecular structure, the dynamics of these systems is characterized by two well-separated crossover regions where the relaxation times of main transition and the two secondary relaxations β and γ approach each other. The main transition has three parts: The a process at high temperature, the a′ process between the two crossover regions, and the α process at low temperatures. Both the γ-crossover region [around a temperature Tc(γ)∼(1.4–1.5)Tg and a relaxation time τc(γ)≈10−10 s] and the β-crossover region [around Tc(β)∼(1.1–1.2)Tg and τc(β)≈10−6 s] could be studied within the experimentally accessible frequency–temperature window. Different typical crossover properties are observed in the two regions. The γ-crossover region is characterized by onset of the (a′,α) process, with a relaxation time about one decade greater than that of the quasicontinuous (a,γ) trace. The β-crossover region is characterized, besides splitting of main andβ relaxation times, by a change in the temperature dependence of the main-relaxation time as reflected by a bend in the Stickel plot of the continuous (a′,α) trace, the separation of individual temperature dependences of different transport properties such as impurity-ions diffusion coefficient and viscosity, and a temperature-dependent main relaxation time that starts to be in accordance (at lower temperatures) with the Adam–Gibbs model. The cooperativity of the main process between the γ and β crossover seems to be small. Below the β crossover, cooperativity increases up to values of order Nα∼100 near Tg, and configurational entropy seems to correlate with the main relaxation time.
Type of Medium:
Online Resource
ISSN:
0021-9606
,
1089-7690
Language:
English
Publisher:
AIP Publishing
Publication Date:
2002
detail.hit.zdb_id:
3113-6
detail.hit.zdb_id:
1473050-9
Bookmarklink