Frictional rubbery

EVA polymers have been important for film manufacture. They are not competitive with normal film because of the high surface tack and friction which make them difficult to handle on conventional processing machinery. However, because of their somewhat rubbery nature, gloss, permeability, and good impact... 

Styrene butadiene rubber (SBR) is, quantitatively, the most important synthetic rubber. It is a copolymer of styrene and butadiene in such a ratio that its rubbery nature predominates, vulcanization is carried out with sulphur, reinforcement with carbon black. It is used at a very large scale in tyres for passenger cars, thanks to its excellent combination of abrasion resistance and friction on the road. In large tyres it can not replace natural rubber because of its heat development (hysteresis losses). 

The friction coefficient is customarily obtained from either the relaxation or retardation spectrum, H x) or L x), respectively. At short times, i.e., on the transition from the glassy-like to the rubbery plateau, the viscoelastic processes obey Rouse dynamics, and the relaxation modulus is given by Eq. (11.45). Since H x) = —dG/d nx t, one obtains... 

Friction speed has a small effect on the maximum friction stress. This low dependence on speed can be explained by the very low value of the glass transition temperature PDMS networks are used at ambient temperature, on their rubbery plateau. However, for PDMS 6, a slight decrease in friction stress could be explained by a pseudoplastic behavior of the confined interfacial layer (a shear thinning effect). 

Traditionally it is assumed that the temperature dependences of the retardation times of all viscoelastic modes or mechanisms of polymers are proportional to one and the same monomeric friction coefficient Co (Ferry, 1980 Mark et al., 2004 Plazek and Berry, 1986 Doi and Edwards, 1986). For rubber networks, the viscoelastic modes include those with shorter retardation times responsible for volume change and the glass temperature, and the longer retardation times of polymer strands between crosslinks contributing to rubbery deformation and Jg. Thus, the retardation times X(T) of all the viscoelastic modes contributing to Jp(t) at any temperature Tare related to that at a chosen reference temperature To by the same multiplicative factor given by... 

Glass transition temperatures are characterized by a change from a hard, nonciystal-line, glass-like material to a rubbery sohd. The viscosity at glass transition temperature for all polymers was found to be around lO Pa.s, independent of the chemical stracture of the polymer. In this way friction between molecules (or viscosity) was related to the volume between them, and so to the glass transition temperature, Tg. Consequently, it was accepted that the glass transitions happen at that physical state where all materials exhibit the same fractional free volume . 

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