Dielectric and Viscoelastic Behavior of Low-M Linear Polyisoprene Blended in Long Matrix

Abstract

Long polymer chains in concentrated systems deeply overlap/penetrate with each other to mutually constrain their large-scale (global) motion over the end-to-end distance. This constraint, referred to as the entanglement, has been one of central subjects in polymer physics. The tube model has been frequently utilized to describe such entanglement effects on the chain motion/relaxation. In this model, a focused chain is confined in a uncrossable tube that represents the constraint from the surrounding chains. The relaxation of the focused chain occurs through its own motion along the tube axis and also through the motion of the tube. The latter type of relaxation, reflecting the motional correlation of chains in concentrated systems, is referred to as the constraint release (CR) relaxation. The CR process has been modeled as Rouse-like motion of the tube and the chain therein. This Rouse-like motion results in mutual equilibration of the entanglement segments of the chain, and the equilibrated segments as a whole behave as an enlarged stress-sustaining unit (dilated segment). Thus, in a coarse-grained molecular view, the CR process is described as a dynamic tube dilation (DTD) process where a ratio of the effective tube diameter a'(t) (identical to the size of this dilated segment) to the diameter a of the undilated tube increases with increasing time scale t. In fact, most of current tube models adopt this DTD molecular picture to describe rheological behavior of entangled polymers considerably well, although the consistency in the coarse-graining of the length and time scales is to be carefully examined and the basic parameters of the model are desired to be tested with molecular dynamic simulations. In relation to this consistency of coarse-graining, our previous studies focused on dielectric and viscoelastic properties of linear cis-polyisoprene (PI): PI chains have the type-A dipole parallel along the chain backbone, so that their viscoelastic and dielectric properties in long time scales commonly reflect the global chain motion. Nevertheless, the global motion is averaged differently in these properties. Namely, the dielectric relaxation function F(t ) of linear PI, detecting the end-to-end vector fluctuation, is rather insensitive to the DTD process and almost coincides with the survival fraction of the dilated tube, φ '(t ), whereas the viscoelastic relaxation function m(t ) is quite sensitive to the DTD process and is different from φ '(t ) in general. This difference between F(t ) and m(t ) is very useful for testing the molecular picture of full-DTD in which the relaxed portion of the chains is regarded as a solvent and the dilated tube diameter a'(t ) is related to φ '(t ) as a'(t ) = a{φ '(t )} (d @1.3 for PI). In fact, for monodisperse linear PI, experiments showed that the Dielectric and Viscoelastic Behavior of Low-M Linear Polyisoprene Blended in Long Matrix