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We promoted order-disorder transformation of poly(3-hexylthiophene) (P3HT) in solution by ultrasonic oscillation which substantially improved crystallinity in its pure film. P3HT with low molecular weight (M(w)) dispersed very well in p-xylene solvent and few aggregates generated in the solution. For P3HT with high M(w), the results suggested the coexistence of two phases: disordered coils in solution and ordered microcrystals in suspension. Upon ultrasonic oscillating, more ordered precursors generated in solution due to increased self-assembly from disordered to ordered configuration, which resulting from decreased extent of chains entanglement existing in high molecular system, and red shift at absorption maximum and growing intensity of the pi-pi* absorption at ca. 604 nm were observed in solution. The films prepared from the oscillated solution then showed increased degree of crystallinity, pi-pi interactions and homogeneously distributed nanofibrils, which should be attributed to the ordered precursors constructed in solution. Furthermore, the best crystallinity of the film was obtained at the oscillating time of 4 min, showing the equilibrium state between the increased content of crystalline molecules and the shortened crystalline length. This simple method paves the way for the decreasing chains entanglement during crystallizing process of conjugated polymer in solution, and it enriches the ways to improve crystalline order in thin films comprising crystallizable polymers.

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Processability of vitrimers strongly relies on the temperature dependence of viscosity. In this study, we analyzed temperature-dependent viscoelasticity of vitrimers based on the dioxaborolane metathesis reaction. A sol-to-gel transition process and a reverse gel-to-sol process are observed in the linear viscoelasticity with increasing content of the cross-linker. The latter gel-to-sol process is owing to a reverse reaction between a two-site interchain cross-linking point with an excess cross-linker, forming two noncross-linking sites. For samples above the gel point, the increasing temperature leads to a weaker acceleration of the decross-linking process than the Rouse-type relaxation, and accordingly, broadening of the plateau region. This trend is easily visualized in samples slightly above the gel point for which the stress relaxation arising from the Rouse-type relaxation and the decross-linking process are not well separated over time. This temperature-dependent behavior reflects a case that the lifetime of the dynamic covalent bond is significantly larger than the Rouse time of the network strands. As a result, the stress borne by a strand relaxes immediately upon decrosslinking, and thus, the low activation energy of the dioxaborolane metathesis reaction governs the strand relaxation.

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Application of ionomers is often disturbed by their brittleness originating from limited stretchability of the network strands physically cross-linked by the ionic sites therein. Thus, an effective method of improving the ductility is to increase the length of network strands (and/or entanglements). Considering this point, this study examined linear viscoelasticity (LVE) and nonlinear elongational rheology of unentangled copolymers of hexyl methacrylate (HMA) and the ionic monomer sodium 4vinylbenzenesulfonate hydrate (SSNa). The ionized SSNa monomer, being randomly distributed along the chain backbone at a concentration ranging from <1 to ∼4 monomers per chain, served as the physical cross-link (or physical branching point). The LVE data showed a sol-to-gel transition, and the ductility of the sample turns out to be strongly related to the degree of gelation. Analysis of those data gave an average length of the network strands, and the ductility of the ionomer samples detected in the nonlinear elongational test was well correlated to this strand length in most cases. An exception was found for the sample slightly above the gel point: the ductility of this sample was much more significant than expected from the strand length, possibly due to the "pseudo-yielding" behavior that reflected exchange of the ionic, physical cross-links, and the resulting motion/displacement of the ionomer chains.

The advances in understanding the dynamics of both the physically and chemically reversible networks are summarized from a rheological perspective. The focus is placed on linear viscoelasticity, which is related to thermodynamics and reversible kinetics at quasi-equilibrium states. We first explain basic assumptions and predictions of the reversible-network theories, including the reversible gelation, sticky-Rouse, and sticky-reptation theories. We then summarize the dynamic features of the state-of-the-art materials that have not been fully incorporated into the current theoretical framework: (1) cooperative motion of the nearby stickers significantly increases the dissociation energy of physically reversible networks, (2) the degree of gelation becomes highly T-dependent when the Gibbs free energy approaches the thermal energy for both the physically and chemically reversible networks, and (3) the exchange rate of dynamic cross-links of chemically reversible networks is reaction-controlled and depends on the chemical species and concentrations of the reactants. Finally, some pathways for future theoretical development are suggested.