On the viscoelastic and plastic behavior of semiaromatic polyamides

Abstract

Different semiaromatic polyamides (SAPA) have been synthesized by step‐growth polymerization of an aliphatic diamine, M (the 2‐methyl 1,5‐pentanediamine), 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 Tg, as deduced from DSC thermograms, and the relevant mechanical relaxation Tα raise when the content of MT units increases. In contrast, the broad low‐temperature secondary relaxation, called γ, does not markedly depend on the MT content. Samples systematically studied in the absence of any moisture did not exhibit the intermediate‐temperature secondary relaxation, called β, which is characteristic of the wet polyamides. The study of the plastic behavior was focused on the samples 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°C to Tg. Analysis of the compression data was based on the inspection of the temperature dependence of elastic modulus, E(T), yield stress, ςy, plastic flow stress, ςpf, and strain softening ςy − ςpf. Whereas the plots of ςy as a function of temperature, T, reveal some differences between MI and I5 behavior, a unique master curve was obtained by plotting ςy/E(T) vs. T − Tg, which means that the plastic behavior 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 already reported in the case of brittle vinyl polymers. This observation is consistent with the traction data, which give evidence for the occurrence of the tensile yielding of MI and I5 at temperatures rather close to Tg. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1131–1139, 1999

Theory of Yielding, Strain Softening, and Steady Plastic Flow in Polymer Glasses under Constant Strain Rate Deformation

“…The predicted nonmonotonic variation of the SSA would seem to contradict some experimental observations that the overshoot either saturates at low temperature or continues to grow. ,− We believe this is a subtle issue due to the high sensitivity of the thermal behavior of the SSA to initial state. Our prediction is based on the “quench-and-wait” protocol.…”

“…A weak upturn (downturn) thus appears in the flow stress and logarithmic relaxation time (density fluctuation amplitude) curves at low strain rate at ∼5À10 K below T g . Some flow stress data in the literature seems to support such trends, 25,26 although accurate measurements are difficult. Other general features are that the density fluctuation amplitude in the plastic flow regime (Figure 6b) monotonically decreases as temperature is lowered, and the steady state alpha relaxation time is very weakly dependent on temperature but strongly deformation rate dependent.…”

“…Monte Carlo simulations of simple tangent bead–spring models of amorphous polymer glasses have found strain softening arises from a drop of interchain contributions to stress . Experiments by Halary, Monnerie, and co-workers reveal a close correlation between strain softening and the issue of whether physical aging effects are, or are not, operative. − For example, the amplitude of the stress overshoot tends to vanish when the highly local beta process motions are the precursors of the alpha relaxation (e.g., unaged poly(methyl methacrylate) (PMMA)) but becomes large when the beta process involves only isolated motions (e.g., aged PMMA). − Phenomenologically, many experiments on diverse polymeric materials ,,− have shown (i) the strain softening amplitude (SSA), defined as the magnitude of the stress overshoot, grows roughly logarithmically with waiting (aging) time and deformation rate, (ii) a rapid thermal quench or predeformation can largely or entirely eliminate the stress overshoot and yield peak, (iii) physical aging restores softening for both thermally and mechanically pretreated glasses, and (iv) annealed and quenched glasses have an identical flow stress and hardening response. These phenomena are believed to be a consequence of so-called “mechanical rejuvenation”, sometimes interpreted as a literal “erasure of aging”, but more generically viewed as the effect of mechanical disordering of one or more key structural variables. , At low strains (“pre-yield” regime), aging, thermal history, and mechanical response are coupled.…”

Structure–Property Relationships of Nanocomposites

“…In this chapter, specific attention will be paid to nanocomposites of polyamide (PA) with HAp. Considering their attractive viscoelastic properties, semiaromatic polyamides (e.g., PA-11T10) have been chosen as the matrix [Choe et al, 1999]. Recently, several publications have been devoted to one-dimensional nanostructures with high-aspect-ratio particles.…”

Molecular Analysis of the Mechanical Behavior of Plasticized Amorphous Polymer

“…Recent studies carried out in our laboratory have provided comprehensive evidence for the relations existing between the yielding behavior and the character of the molecular motions involved in the main mechanical relaxation α and in the sub-Tg secondary relaxations (β, γ). Different families of amorphous polymers have been investigated, including poly(methyl methacrylate) and methyl methacrylate-based copolymers (Tordjeman et al, 1997;Tézé et al, 1999), semi-aromatic polyamides (Choe et al, 1999;Brulé et al, 2001), epoxyresins (Rana, 2002) and blends of polystyrene and poly(phenylene oxide) (Creton et al, 1999).…”