The effect of structural relaxation on the stress-relaxation
behaviour of polymer blends was studied. Short-term
stress-relaxation measurements were performed on three
different systems at T s - 20°C, Ts - 35°C and T s - 50°C.
The stress-relaxation data were fitted with the WilliamsWatts
equation:
E(t)/Eo = exp[- (t/z)P]
and the effect of physical ageing was analysed in terms
of changes in [3 and z. It was found that:
Values for [3 obtained through optimization fell within
a narrow range centred at 0.41 for all amorphous systems
investigated.
A power-law formula for z, in the form of:
In z = a + Cr ln(Tg - T) + C, In te
with constant Cr and Ct was found for blends of PMMA
with SAN.
Exposure of blends of PMMA and SAN to 150°C for
240 h raised z and Tg, and resulted in a narrower
transition region, but did not affect [3, Crand C,.
For blends of PS with PPO, a universal value for [3
and power-law formula for z held, but the parameters
Cr and C, varied with the blend composition.
The stress-relaxation behaviour of a semicrystalline
blend of PMMA with PVDF (50/50) was consistent with
that predicted from a three-phase morphology.
The stress-relaxation of an amorphous blend of
PMMA with PVDF (of 30 wt% PVDF) was fitted with
626 POLYMER, 1991, Volume 32, Number 4
parameters comparable to those for PMMA/SAN
blends.