Block polymers stress-strain curves

Figure 10. Effect of hot-stretching on subsequent room temperature Instron stress strain curves of one BPFC-DMS block polymer. 27% silicone of DPn = 36.

This discovery culminated in the commercial production and the announcement (41) in 1965 of thermoplastic elastomers from block polymers of styrene and butadiene (S-B-S) and of styrene and isoprene (S-I-S). To rubber scientists and technologists the most outstanding property of S-B-S and S-I-S was the unvulcanized tensile strength compared to that of vulcanized NR and vulcanized SBR carbon black stocks. Stress-strain curves, to break, of these latter materials are compared to that of S-B-S in Figure 2. It was pointed out that the high strength of S-B-S must be due to physical crosslinks. 

Figure 4.16 Tensile stress-strain curves for styrene butadiene styrene tri-block copralymers (Huy, T. A et al., Polymer, 44, 1237, 2003) Elsevier.

Effects of Additives. The ratio of endblock phase to midblock phase can be varied by adding materials which associate preferentially with one phase or the other. Coumarone-indene resins, for example, associate with the end-block phase in S-B-S and S-I-S polymers. Figure 7 shows how the initial portion of the stress-strain curve of the neat polymer. Curve B, is shifted upward to Curve A when the end-block phase concentration is increased by adding a resin of that type. Curve C shows how the reverse occurs when, for example, a tacki-fying resin or a plasticizing oil which associates with the rubber phase is present instead. 

FIGURE 16.3 Stress-strain curves for different samples of block copolymers of caprolactone with -butyl acrylate tested at (a) room temperature and (b) 70 °C. Polymer compositions were 0, 20, 39, 50 and 71 wt% n-butyl acrylate from top to bottom in (a). Reprinted with permission from Referenece 1. Copyright 2001 National Academy of Sciences, USA. 

The creep stress was assumed to be shared between the polymer structure yield stress and the cell gas pressure. A finite difference model was used to model the gas loss rate, and thereby predict the creep curves. In this model the gas diffusion direction was assumed to be perpendicular to the line of action of the compressive stress, as the strain is uniform through the thickness, but the gas pressure varies from the side to the centre of the foam block. In a later variant of the model, the diffusion direction was taken to be parallel to the compressive stress axis. Figure 10 compares experimental creep curves with those predicted for an EVA foam of density 270 kg m used in nmning shoes (90), using the parameters ... 

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