Viscoelastic properties of physically crosslinked networks. 1. Transient network theory
| Autor: | F. Tanaka, S. F. Edwards |
|---|---|
| Rok vydání: | 1992 |
| Předmět: |
chemistry.chemical_classification
Polymers and Plastics Chemistry Organic Chemistry Ionic bonding Thermal fluctuations Polymer Viscoelasticity Inorganic Chemistry Shear rate symbols.namesake Reptation Classical mechanics Chemical physics Phase (matter) Materials Chemistry symbols van der Waals force |
| Zdroj: | Macromolecules. 25:1516-1523 |
| ISSN: | 1520-5835 0024-9297 |
| DOI: | 10.1021/ma00031a024 |
| Popis: | A simple model is introduced to describe the dynamics of physically cross-linked networks in which junctions are sufficiently weak to break and recombine in thermal fluctuations. The time-evolution equation under arbitrary macrodeformation is derived for the creation and annihilation of the junctions-and hence for the number of elastically effective polymer chains with a fixed end-to-end vector. We focus our attention specifically on the unentangled networks in which the molecular weight M, between neighboring junctions is smaller than the entanglement molecular weight Me, so that each chain obeys Rouse dynamics modified by sticky trapping centers rather than reptation. Stress-strain relation under shear and elonga- tional deformation is detailed for a model network which is made up of polymers of uniform length with associating functional groups at their chain ends. On longer time scales than the junction breakage time, the total number of effective chains decreases with time-resulting in an intranetwork flow. In most polymer blends or solutions of practical interest, polymer chains carry specific groups interacting with each other by associative forces capable of forming bonds, and hence a description in terms of van der Waals type interaction-or x parameter in lattice-theoretical termi- nology-is insufficient. These forces include hydrogen bonding, ionic association, stereocomplex formation, cross- linking by the crystalline segments, and solvent complex- ation. In most common cases the bonding energy is comparable to the thermal energy, so that the bond formation is reversible by the change in temperature or concentration. Thermodynamics of network formation and multiphase competition were studied by the present authors1t2 to describe phase behavior peculiar to the polymer fluids interacting via specific associating forces. The present paper concerns dynamic properties of the thermoreversible networks (or physical gels) to study enhanced viscoelasticity due to the presence of the temporal junctions. There has been an accumulation of evidences that networks with reversible junctions exhibit highly enhanced viscoelastic properties which are quite different from ordinary polymer melts. Typical examples of the exper- imental studies can be taken from biological macro- molecule~~-~ in which networks are formed by hydrogen bonding. It turned out that a mechanical spectrum of dynamical moduli does not show any softening in low frequencies but is rather insensitive over a wide range of frequencies covering from to lo2 rad/s and also that relaxation of stress does not seem to obey a simple exponential decay but is rather well described by a power law.4 It was also found that the complex viscosity is in most cases larger than the stationary viscosity when compared at the frequency which is the same as the stationary shear rate, thus suggesting a failure of the Cox- Merz rule. Another example in current research is that of ther- moplastic elastmers6s7 in which synthetic polymers are cross-linked by specific associating groups. These include, in addition to the hydrogen bonding, charge (electron) transfer, ion pairing, acid-base interaction, dipole |
| Databáze: | OpenAIRE |
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