Changes induced chain cross-linking

Chemical changes frequently accompany the shifting of the polymer chains. Cross-links are broken under the combined influence of thermal stress and the induced shear forces and are often reformed in a manner so as to accommodate the newly formed alignment of the polymer chains. When cross-links break and reform to an appreciable extent, polymers will exhibit very poor recovery. 

Hiratsuka, T. (1987) Nucleotide-induced change in the interaction between the 20- and 26-kilodalton heavy-chain segments of myosin adenosine triphosphatase revealed by chemical cross-linking via the reactive thiol SH2. Biochemistry 26, 3168. 

The second example used visible light absorption that increased the temperature locally within the thermosensitive gel [39]. The gel consisted of a covalently cross-linked copolymer network of N-isopropylacrylamide and chloro-phyllin, a combination of a thermo-sensitive gel and a chromophore. In the absence of light, the gel volume changed sharply but continuously as the temperature was varied. Upon illumination the transition temperature was lowered, and beyond a certain irradiation threshold the volume transition became discontinuous. The phase transition was presumably induced by local heating of polymer chains due to the absorption and subsequent thermal dissipation of light energy by the chromophore. The details will be discussed in a later section. 

Both copolymers were cross-linked by copolymerization with (1-2%) ethylene dimethacrylate the effect of irradiation was followed by measuring the change of elastic retractive force at constant elongation. In the first case the contraction upon irradiation of the gel is mainly due to changes of chain conformation and swelling equilibrium induced by trans-cis photoisomerization. In the second case by comparing the retractive force for irradiated and unirradiated samples the heat effect was evaluated, and the photoinduced contraction estimated to 1%. 

The formation of a three dimensional infinite network in a polymer by a chemical or physical process leads to a gel. Polymer gels are classified into two types, irreversible and reversible. The latter are formed by cross-linking due to physical interaction between certain points on different polymer chains, and the sol-gel phase transition is induced by a change in temperature. At temperatures below Tgeb... 

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