Limited chain extensibility bimodal networks

Characterization of Limited Chain Extensibility. The molecular origin of the unusual properties of bimodal PDMS networks having been elucidated at least to some extent, it is now possible to utilize these materials in a variety of applications. The first involves the interpretation of limited chain extensibility in terms of the configurational characteristics of the PDMS chains making up the network structure (5,12,13). 

Stress-strain measurements on bimodal PDMS networks exhibited upturns in modulus that were much less pronounced than those in crystal-lizable polymer networks. The upturns are independent of temperature and are not diminished by incorporation of solvent. These characteristics are expected for the case of limited chain extensibility. -... 

More quantitative characterization of limited chain extensibility requires a non-Gaussian distribution function for the end-to-end separation, r, of the short network chains. The Fixman-Alben distribution was used to calculate stress-strain isotherms in elongation for bimodal PDMS networks. Good agreement was found between theory and experiment. Other non-Gaussian distribution functions have also been successfully used. The experimental isotherms can also be interpreted... 

In the case of such noncrystalUzable, unfilled elastomers, the mechanism for network rupture has been elucidated to a great extent by studies of model networks similar to those already described. For example, values of the modulus of bimodal networks formed by end-linking mixtures of very short and relatively long chains as illustrated in Fig. 6.4 were used to test the weakest-link theory [7] in which rupture was thought to be initiated by the shortest chains (because of their very limited extensibility). It was observed that increasing the number of very short chains did not significantly decrease the ultimate properties. The reason [85] is the very nonaffine... 

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