Different semiaromatic polyamides (SAPA) have been synthesized by step-grow
th polymerization of an aliphatic diamine, M (the 2-methyl 1,5-pentanediami
ne), and isophthalic acid, I, or terephthalic acid, T, or mixtures of these
two diacids. The influence of the relative amount of randomly distributed
MT units on the viscoelastic properties of the materials was investigated.
It was shown that the glass transition T,, as deduced from DSC thermograms,
and the relevant mechanical relaxation T,raise when the content of MT unit
s increases. In contrast, the broad low-temperature secondary relaxation, c
alled gamma, does not markedly depend on the MT content. Samples systematic
ally studied in the absence of any moisture did not exhibit the intermediat
e-temperature secondary relaxation, called beta, which is characteristic of
the wet polyamides. The study of the plastic behavior was focused on the s
amples MI and I5, which are strictly amorphous, and contain 0% and 50 mol %
of MT units, respectively. Mechanical experiments were carried out in both
the compression and traction modes, at temperatures ranging from -80 degre
es C to T-g. Analysis of the compression data was based on the inspection o
f the temperature dependence of elastic modulus, E(T), yield stress, sigma(
y), plastic flow stress, sigma(pf), and strain softening sigma(y) - sigma(p
f). Whereas the plots of sigma(y) as a function of temperature, T, reveal s
ome differences between MI and I5 behavior, a unique master curve was obtai
ned by plotting sigma(y)/E(T) vs. T - T-g, which means that the plastic beh
avior of these materials is controlled by their chain packing in the glassy
state. The strain softening profile of MI and I5 is similar to that alread
y reported in the case of brittle vinyl polymers. This observation is consi
stent with the traction data, which give evidence for the occurrence of the
tensile yielding of MI and I5 at temperatures rather close to T-g. (C) 199
9 John Wiley & Sons, Inc.