Decreasing cross-linking networks

For systems whose particle-particle bonds are permanent (image (b) in Figure 23.1), the film tends to form a secondary layer of wrinkles. The displaced particles do not completely remove themselves from the interface, but remain coimected to the cross-linked network. Contrary to what happens with very breakable bonds, part of the interfacial stress is released — and the area fraction decreases significantly — once the collapse point is passed (see curve (b)). 

Elastomers are classically blended to improve the physical and mechanical properties of the final material. For example, Dubey et aiP incorporated suitable amounts of EPDM in the SBR matrix because it was expected to impart significant heat and ozone resistance to the SBR matrix. The irradiation-induced vulcanization of SBR/EPDM blends was proved to combine desired properties and high mechanical strength. For example, the formation of a cross-linked network led to a decrease in elongation at break. Cross-linking restricts the movement of the polymer chain against the applied force. Moreover, to decrease the radiation dose (and hence, the cost of radiation), multifunctional acrylates (MFAs) and allylic reactive molecules were used. 

As described above for elastin and resilin, the ability of elastomeric proteins to exhibit elasticity relies on the molecular movement, stmctural folding, and conformational freedom of individual components so that they can instantaneously respond to the applied force within a cross-linked network to distribute the stress throughout the system. Stretching initially will interrupt interactions between the loops such as hydrophobic interactions, hydrogen bonding, and electrostatic interactions, while at higher extensions a decrease in conformational entropy will be prevalent. To date, different models are proposed to explain the mechanisms of elasticity for resilin, based on the knowledge from elasticity models that have been proposed for elastin. 

Rolla et ah, used microwave dielectric measurements to monitor the polymerization process of mono functional n-butyl acrylate as well as 50/50 w/w blends with a difunctional hexane-diol diacrylate that gave highly cross-linked networks. In these real time cure experiments the decreasing acrylate monomer concentration was studied via a linear correlation with the dielectric loss index at microwave frequencies. This correlation is a result of the largely different time scales for dipolar polarization in the monomer on one hand and in the polymerized reaction product on the other hand. 

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