Network inhomogeneities, examples

A concomitant of coil shrinkage would seem to be a macroscopic shrinkage of the gel structure, called syneresis, if the crosslinking were carried out in solution. Syneresis is indeed observed at the later stages of crosslinking in highly diluted systems [see Refs. (17) and (257) for examples]. Some evidence of localized network inhomogeneities has also been adduced in polycondensation networks... 

It is the opinion of the present authors that frequently too much emphasis is placed in network studies on the influence of network defects (Chapter II, Section 2), while in reality pre-existing order, inhomogeneous crosslinking, composite network formation, or microsyneresis may play an important role in the mechanical behaviour, as well as in most other network properties. Examples of this will be given in Chapter IV. 

As discussed in Chapter 7, radical polymerization can induce marked inhomogeneities, easily observable by DMTA. For example, in photocured polyurethane-acrylate networks, two a peaks corresponding to distinct phases, separated by about 40-100 K, depending on composition and cure conditions, can be observed in the DMTA dissipation spectrum at 10 Hz (Barbeau et al., 1999). 

The polymer network stmcture and the network properties are dosely rdated to the reaction conditions during gd formation. Cross-linker concentration, initial monomer concentration, temperature, and polymerization method will influence the resulting properties. In particular, for radical aoss-linking it is well known that inhomogeneities are introduced into the network stmcture, for example, spatial heterogendty of the network... 

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