Slow Dynamics in Homopolymer Liquids

Abstract

Dynamic physical properties of homopolymer liquids relax through global motion of the polymer chains over their dimension. This motion is affected by a variety of factors. For high molecular weight (M) chains, the entanglement retards the global motion to affect the properties. This entanglement effect changes with the chain architecture as well as the molecular weight distribution. The effect of the chain architecture on the global chain dynamics is found also for non-entangled low-M chains as well. This article gives a very brief summary of the experimental facts and molecular interpretations for these effects.KEY WORDS: Homopolymer Liquid / Relaxation / Slow Dynamics / Entanglement / Viscoelastic Property / Dielectric Property / Type-A Dipole / Needless to say, flexible polymer chains are composed of monomeric units covalently connected in a thread-like structure. In a local length scale, they have their own chemical functionalities such as the polorizability and electron susceptibility to exhibit intrinsic spectroscopic properties, for example, optical and electronic properties. In this sense, chemically different polymers cannot be regarded as the same class of materials. At the same time, however, the threadlike structure of polymer chains allows such chemically different chains to exhibit some universal properties. For example, flexible linear polymer melts exhibit the universal power law dependence of the zero-shear viscosity 0 on the molecular weight M, 0 / M with ¼ $ 1 and ¼ $ 3:5 in low-and high-M regimes, respectively.1 This dependence and related slow viscoelastic relaxation of polymer melts are believed to result from the fundamental thread-like structure of polymer chains irrespective of their chemical details. Scientifically, it is very interesting to explore a relationship(s) between such universal behavior of flexible polymer chains and their slow dynamics and further examine the factors that affect the slow dynamics and properties.This article follows the above viewpoint to give a very brief review of slow viscoelastic and dielectric relaxation behavior of homopolymer liquids. We place our main focus on homopolymer systems where the entanglement resulting from mutual uncrossability of real polymer chains plays the essential role. The entanglement, often modeled as mutual winding/hooking of threads, is equivalent to a topological constraint for thermal motion of the polymer chains that changes its magnitude according to the time and length scales. The linear and nonlinear viscoelastic behavior and related dielectric behavior of homopolymer melts/solutions are summarized on the basis of this molecular understanding. BASICS Molecular Expression of Viscoelastic Relaxation FunctionFor a series of chemically identical linear polymer melts having different molecular weights M, Figure 1 schematically shows the relaxation behavior of shear stress ðtÞ under a small step shear strain :1 The relaxation modulus, GðtÞ ðtÞ=, is double-logarithmically plotted against the time t after imposition of strain. The...

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

“…Extensive viscoelastic measurements have been conducted for PI/PI blends 8,14,15,16,19) to establish an empirical equation for the terminal (second-moment average) viscoelastic CR time t…”

“…2) 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 6,7) 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 [8][9][10] and the basic parameters of the model are desired to be tested with molecular dynamic simulations. [11][12][13] In relation to this consistency of coarse-graining, our previous studies focused on dielectric and viscoelastic properties of linear cis-polyisoprene (PI): 8,[14][15][16] 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.…”

“…This difference between F(t ) and m(t ) is very useful for testing the molecular picture of full-DTD [6][7][8] 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/2 (d @1.3 for PI). In fact,…”

Multiple Representation of Linear Dielectric Response

“…Appendix A). The e* data straightforwardly give t EP and De EP exhibiting these characteristic EP features, while the M* data do not, as noted from comparison of eqs (13) and (14). In particular, the M* data give t' EP being insensitive to L and much smaller than the real t EP for the case of e m << De EP (which is the case for most of ion conductors).…”

“…However, the averaging moment of u n (t) at time t is different for F(t) and G(t); the first and second-order moments for F (t) and G(t), respectively. This difference enables us to examine detailed aspects of chain dynamics (such as the motional correlation of entanglement segments along the chain backbone 11,13,14) ) through comparison between the F(t) and G(t) data, or equivalently, between the e*(w) and G*(w) data. Similar comparison cannot be straightforwardly made for M*.…”