Polyisoprene Linear viscoelastic behavior

The damping function, g(s), in Eq. (6.30) accounts for lack of proportionality between stress and strain. The product, g(e)e, quantifies the nonlinear elasticity (g(e) = 1 for linear viscoelastic behavior). Separability of time and strain is illustrated for 1,4-polyisoprene in Figures 6.4 and 6.5 the time-dependence of the stress relaxation is the same for shear strains of varying amplimde and for different modes of deformation (Fuller, 1988). 

As a tjqiical example of changes of linear viscoelastic behavior of linear polymers with their concentration. Figure 8 shows reduced moduli, Gr = M/CRT G and G," = M/CRT (G"- with and ris being the volume ftaction and viscosity of the solvent, measured for solutions of linear polyisoprene (PI Af = 48.8 X 10 ) in oligobutadiene (oB M = 0.7 x 10 ), a marginal solvent for PI. The numbers indicate the PI mass concentration C (in gem unit) C=0.92gem" corresponds to bulk PI. 

It is not clear why this transition should occur at such a higher level of arm entanglement for polystyrene stars than for other star polymers. This observation is in direct conflict with the standard assumption that through a proper scaling of plateau modulus (Go) and monomeric friction coefficient (0 that rheological behavior should be dependent only on molecular topology and be independent of molecular chemical structure. This standard assumption was demonstrated to hold fairly well for the linear viscoelastic response of well-entangled monodisperse linear polyisoprene, polybutadiene, and polystyrene melts by McLeish and Milner [24]. 

A pseudo solid-like behavior of the T2 relaxation is also observed in i) high Mn fractionated linear polydimethylsiloxanes (PDMS), ii) crosslinked PDMS networks, with a single FID and the line shape follows the Weibull function (p = 1.5)88> and iii) in uncrosslinked c/.s-polyisoprenes with Mn > 30000, when the presence of entanglements produces a transient network structure. Irradiation crosslinking of polyisoprenes having smaller Mn leads to a similar effect91 . The non-Lorentzian free-induction decay can be a consequence of a) anisotropic molecular motion or b) residual dipolar interactions in the viscoelastic state. 

Watanabe, H., S. Ishida, Y. Matsumiya, and T. Inoue. 2004a. Viscoelastic and dielectric behavior of entangled blends of linear polyisoprenes having widely separated molecular weights Test of tube dilation picture. Macromolecules 37 1937-1951. 

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