Polymer thermal contraction

The uncertainty of the expansion data was evaluated to be less than 5%. The relative thermal expansion AL/L versus T is shown in Fig. 13.3. Curves for other materials are also reported for the sake of comparison. Torlon shows a linear thermal expansion lower than most polymers. As Stycast 2850FT, Torlon thermal contraction closely matches that of some common metals (i.e. aluminium [53] and brass [54]). Smoothed values for the thermal expansion of Torlon relative to 4.2K are reported in Table 13.1. The data... 

Negative thermal expansion coefficients along the chain axes have been observed experimentally on many crystalline polymer lattices7,16 -18). Hence, the thermal contraction along the chain axis seems to be a general phenomenon in crystalline polymers. As a result of this conclusion, we are immediately faced with the question... 

When the substituent groups are large, aryl or branched alkyl, normally no batho-chromic shift takes place. This is observed for (n-alkylSiPh)n polymers, diarylpolysilanes and (c-HexSiMe)n. In some cases a slight hypsochromic shift is observed as the temperature is decreased, perhaps due to thermal contraction of the polymer molecules. 

Frank and co-workers also examined the IR spectrum of this polymer as a function of temperature to understand more clearly the mechanism of thermal contraction (50). However, their investigations were concerned primarily with the intensity and position of the 73 cm band as a function of temperature from 14 K to the melting point. They concluded that the data of the 73 cm absorption correlated well with previous x-ray measurements for expansion along the a axis of the unit cell, and an infiection was observed at ca. 243 K. This temperature would correspond, according to Frank et al., to the temperature at which diffusion of defects into the crystal can occur. 

The resin matrix in a composite absorbs water in service. As discussed in this chapter, this process is kinetically slow, so the concentration of water in the resin will vary with time and location. Thermal cycling will tend to move the water into the strucmre. Furthermore, the absorption of the water causes the polymer to expand however, in a composite, the swelling is constrained in an analogous way to that described for thermal contraction. Thus, the process of conditioning in moist environments leads to a reduction in magnitude of the thermal strain present in the laminate. 

In contrast to Vf, the specific volume V in the glassy state of the polymer exhibits a larger slope than above Tg, which is due to the thermal contraction of Vocc ( occ,g O.Sag). The agreement of the slope dVld vh) fl-om the phenomenological relation Eq. (11.6) with dVf/d vh) from Eq. (11.5) above Tg we consider as evidence that o-Ps detects precisely the free volume calculated fl om S-S hole theory. The larger slope of dVld vh) below Tg supports the conclusion from the S-S equation of state calculation that Vocc shows here, as distinct from above Tg, a certain thermal expansion. 

Raising the temperature to drive contraction by hydrophobic association is the fundamental property of the consilient mechanism as demonstrated in Chapter 5 by means of designed elastic-contractile model proteins. Thermal activation of muscle contraction also correlates with contraction by hydrophobic association, but assisted in this case by the thermal instability of phosphoanhydride bonds associated with ATP, which on breakdown most dramatically drive hydrophobic association. In particular, both muscle and cross-linked elastic protein-based polymer, (GVGVP) contract on raising... 

The pressure referred to here arises from differential thermal contraction during the cooling stage of manufacture, and from solidification shrinkage of the polymer (6.N.6). 

Bucknall, C. B., Ayre, D. S. and Dijkstra, D. J. Detection of rubber particle cavitation in toughened plastics using thermal contraction tests . Polymer, v. 41, issue 15, 2000, p. 5937-5947. 

---