Theoretical Study of Molecular Association and Thermoreversible Gelation in Polymers

Abstract

ABSTRACT:This paper reviews our recent theoretical studies of molecular association and thermoreversible gelation in polymer solutions and blends. We first classify fundamental types of association, and propose their static and dynamic characterization. We then develope general theory of associating polymers to study phase transitions induced by molecular association. These transitions include macro-and microphase separation, micellization, hydration, thermoreversible gelation and liquid-crystalization. As for the origin of associative forces, we focus on hydrogen bonding and hydrophobic aggregation. Detailed study on thermoreversible gelation with multiple cross-link junctions is presented. Paying special attention to the multiplicity and sequence length of the network junctions, we derive phase diagrams with coexisting gelation and phase separation, and compare them with experimental data. Local and global structures of the gel networks are studied from molecular point of view. The theory is applied to more complex thermoreversible gels such as binary networks (interpenetrating networks, alternating networks and randomely mixed networks), hydrated networks with high-temperature gelation, gelation strongly coupled to polymer conformational transitions such as coil-to-helix transition. To study dynamics of thermoreversible gels, a simple transient network model is introduced, and creation and annihilation of junctions in the networks are theoretically described. Stationary non-linear viscosity and the dynamic mechanical moduli are calculated as functions of the shear rate, frequency and the chain disengagement rate. From the peak of the loss modulus, the lifetime τ × of the junction is estimated, and from the high frequency plateau of the storage modulus, the number of elastically effective chains in the network is found. Transient phenomena such as stress relaxation and stress overshoot are also theoretically studied. Results are compared with the recent experimental reports on the rheological study of hydrophobically modified water-soluble polymers.KEY WORDS Associating Polymers / Thermoreversible Gelation / Junction Multiplicity / Multicriticality / Transient Networks / Shear Thickening / Elastically Effective Chains / This paper reviews our recent theoretical studies of molecular association and thermoreversible gelation in polymer solutions. We introduce new theoretical frameworks to study phase transitions induced by association of polymer chains and dynamics of aggregation/dissociation processes.In most polymer blends or solutions, polymer chains carry active groups interacting with each other via strongly associative forces which are capable of forming temporal bonds. Hence description of the system in terms of van der Waals type contact energy -or χ-parameter in lattice theoretical terminology -is insufficient. These associative interactions include hydrogen bonding, ionic aggregation, electrostatic interaction, micro-crystallite formation, stereocomplex formation, solvent-complex formation, etc. Since ...

Thermoreversible associating polymer networks. I. Interplay of thermodynamics, chemical kinetics, and polymer physics

“…For this reason, APs have been the focus of intense experimental and theoretical study over the past two decades; see Refs. [2,3,5,6,7] for reviews.…”

“…[7,18,19,20,21,22,23,24,25,26,27,28,29,30], have made many useful, experimentally verifiable predictions, including nonlinear behaviors such as shear thickening and strain hardening [30,31]. In the general case, however, the complex interplay of sticky bond thermodynamics and kinetics with the underlying polymer physics in these systems is almost certainly beyond the reach of analytic theory.…”

Phases of polymer systems in solution studied via molecular dynamics

“…Longer alkanes follow similar trends [47], so we assume a similar hydrophobic effect for polymers containing hydrocarbons. In Pluronics R polymers, the oxygen in the PEO groups has a strong hydrogen bonding interaction with water molecules, which is enough to keep PEO soluble at all temperatures [29]. In PPO monomers, however, the additional methyl group (see Figure 4.2) is enough to drive them insoluble at around 10 o C.…”

Nonlinear mechanics of thermoreversibly associating dendrimer glasses

“…The integration of thermoreversibly associating groups into polymers produces a wide variety of complex behavior arising from the finite lifetime of the "sticky", thermoreversible bonds. [1][2][3][4][5][6] Examples of thermoreversible bonds include π-π-stacking, [7][8][9][10] hydrogen bonding, [11][12][13][14][15] and metal ligand bonding. [16][17][18][19][20][21][22][23][24][25] In associating polymer systems (APs), the presence of thermoreversible bonds leads to exquisitely tunable rheological properties, particularly when the product˙ τ sb of the strain rate˙ and sticky bond lifetime τ sb is of order unity.…”

“…Refs. [5,[29][30][31][32][33][34][35], have made many useful, experimentally verifiable predictions, including nonlinear behaviors such as shear thickening and strain hardening. [36][37][38] However, for the sake of tractability, theories have generally neglected one or more features of AP systems that are likely essential to capturing their behavior under certain ambient conditions.…”