Polymer chains stretching/shrinking

The polycarbonate membranes are stretch-oriented during fabrication in order to improve their mechanical properties. If the membrane is subsequently heated above its glass-transition temperature ( 150°C), the polymer chains relax to their unstretched conformation and the membrane shrinks. This shrinking of the membrane around the Au nanowires in the pores causes the junction between the nanowire and the pore wall to be sealed. This is illustrated in Fig. 5, which shows voltammograms for tri-methylaminomethylferrocene (TMAFc+) before (Fig. 5A) and after (Fig. 

Figure 1 Responsiveness in a polymer brush. In a good solvent (a) favourable interactions between polymer segments and solvent molecules lead to the chains being stretched - the loss of configurational entropy attributable to the chain stretching is outweighed by the lowering of energy due to the polymer solvent interaction. If this interaction becomes less favourable (b), due to a change in temperature or pH, for example, the chains reversibly shrink...

This method utilizes pH, chelate formation, oxidation-reduction reaction, or light to deform polymers isothermally and reversibly (see Fig. 5). Unlike the methods described thus far, the main characteristic of this method is reversible shape changes. For example, poly(acrylic acid) fibers that have the shape memory property with changes in pH possess dissociated charges. These charges repel each other and the polymer chains are stretched. On the other hand, in a low pH range, there are no charges and the polymer chains shrink. Another example is partially phosphated poly(vinyl alcohol) film, which responds to chelate formation. If Cu " is included, the film shrinks by formation of crosslinks. 

Oriented materials (see Section 7.2.6) can be caused to shrink by increasing the temperature, even if they are totally amorphous polymers like polystyrene. When orientation occurs, the amorphous regions are stretched into closer packing than is energetically at equilibrium. For partially crystalline polymers, the crystalline regions can be disrupted by orientation forces also. Therefore, when heat is applied and chain motion increases, the chains try to return to their low energy state. 

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