Sub-T g annealing studies of rubber-modified and unmodified epoxy systems

Abstract

Epoxy network systems based on DOEBA (bisphenol-A-diglycidyl ether) and NMA (nadic methyl anhydride) systems modified with the low molecular weight CTBN (carboxyl-terminated butudieneacrylonitrile copolymer) rubbers were prepared and studied. It was found that below the glass transition of the epoxy matrix these materials displayed time-dependent changes in their mechanical properties; specifically, the strain to break as well as the rate of stress relaxation were observed to decrease in a nearlinear behavior with the logarithm of time at sub-Tg annealing. Calorimetric methods clearly showed a simultaneous decrease in enthalpy with time that behaved in a similar fashion as the time-dependent mechanical properties. All the calorimetric and mechanical data are qualitatively related. The importance of this phenomena is considered in view of the widespread use of epoxys. Similar behavior is expected for other network glasses thermally quenched into a non equilibrium state.

An overview of the nonequilibrium behavior of polymer glasses

“…and stress relaxation [23,24] experiments have been used in this regard. The small strain creep curves of poly(vinyl chloride) (PVC) are shown in Figure 3 as a function of annealing time below T .It can be seen that the curves are shifted to the right by g* one decade for each tenfold increase in sub-T 9aging time when sub-T annealing temperature is held constant.…”

“…Although it has been previously shown that network epoxies undergo physical aging [3], it was only relatively recently that more complete studies on epoxy network systems and rubber-modified derivatives were completed [24,50,52].Ophir, et al [24] performed stress-strain, stress relaxation, and differential scanning calorimetry studies on a series of unmodified and rubber-modified epoxies. Figs.…”

Physical aging studies of semicrystalline poly(ethylene terephthalate)

“…Almost all experimental work reported in the literature has been conducted on purely glassy linear homopolymers. Although it had been previously established that network glassy epoxies undergo physical aging (2), it was only recently that the first systematic studies of physical aging in these network systems and rubber-modified derivatives were completed (11,12). Since nonequilibrium behavior has been observed to occur in covalently crosslinked systems, it may be asked whether similar behavior occurs in "pseudo" network systems such as semicrystalline polymers, where the crystalline regions serve as physical crosslinks, and in partially glassy domain-forming block copolymers, where the glassy regions may also serve as physical crosslinks.…”

Physical aging in graphite/epoxy composites

“…The long-term properties of amorphous, glassy materials, including the cross-linked thermoset of network epoxiee, have been demonstrated to change as functions of annealing time at temperatures below the material glass transition temperatures, T B 8 (1,2). This sub-T annealing process is typically accompanied by a volume recovery phenomenon known as physical aging (3,4).…”

“…. Specifically, with increased sub-T 9 storage time the material becomes stiffer and less ductile (1,2,8), sorption decreases and molecular mobility is decreased, as noted through the frequency dependence of the dielectric parameters (10,11).…”