Network polymers glass transition point

We have previously stressed that network formation rapidly increases viscosity and causes gelation. Cross linking after that point increases the apparent glass transition temperature. Figure 11 illustrates the effect of degree of cross linking on the dynamic modulus of a thermoplastic and a cross-linked polymer. The point at which modulus drops off rapidly is approximately the glass transition temperature. 

When the crosslinking reactions of Chapter 6 are driven far beyond the gel point, nearly all species are attached to the gel in a single macroscopic network polymer. Such networks, with either chemical or strong physical bonds, are important soft solids. If the glass transition and melting temperatures are below room temperature, the material is a rubber. Rubbers... 

The simplest self-complementary motif involves two-point donor-acceptor (DA) H-bonding interactions that form cyclic dimers. Carboxylic acids, for example, are ubiquitous and have been investigated for decades. Fourier transform infrared (FTIR) spectroscopy of poly(acrylic acid)s and ethylene-methacrylic acid copolymers show that dimers persist well above the glass transition temperature (Tg) of the materials [12, 13]. Furthermore, carboxylic acids can interact within polymer melts. Instead of forming dimers, they organize into clusters, providing the basis for a supramolecular network. Lillya and colleagues showed that telechelic... 

Vulcanization effectively fixes the ends of flexible polymer chains to form three-dimensional networks. By this way, the stress relaxation of stretched polymer chains can be avoided, and the high entropy elasticity of the rubber can be produced. Moreover, the fixed chain ends also increase both melting points and glass transition temperatures of short flexible chains. 

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