Effect of Chain Structure on the Rheological Properties of Vinyl Acetate-Vinyl Alcohol Copolymers in Solution and Bulk Sergey O. Ilyin, † Alexander Ya. Malkin, † Valery G. Kulichikhin, ‡ Yulia I. Denisova, † Liya B. Krentsel, † Georgiy A. Shandryuk, † Arkadiy D. Litmanovich, † Ekaterina A. Litmanovich, ‡ Galina N. Bondarenko, † and Yaroslav V. Kudryavtsev †, * † Topchiev Institute of Petrochemical Synthesis, Russian Academy of Sciences, Leninsky prosp. 29, 119991 Moscow, Russia ‡ Chemistry Department, Moscow State University, Leninskie gory 1, build. 3, 119991 Moscow, Russia * S Supporting Information ABSTRACT: Bulk and solution studies revealed a strongly pronounced effect of chain structure on the rheological and relaxation behavior of well- characterized vinyl acetate-vinyl alcohol copolymers of similar composi- tion and polymerization degree. The frequency-temperature superposition principle is fully applicable to the random copolymers, which demonstrate all expected relaxation states, whereas a divergence of the reduced dynamic moduli-frequency dependences is observed for the multiblock copoly- mers. In the latter case, the terminal zone is sensitive to the self-assembling of vinyl alcohol blocks into (depending on the copolymer composition) crystalline or amorphous microstructures. The monomer unit distri- bution particularly affects properties of the copolymer solutions in N,N- dimethylformamide (DMF). 5% solutions behave as simple viscoelastic liquids at 20 °C and show viscoplastic behavior at -20 °C, where more blocky chains are characterized by up to 4 orders of magnitude higher yield stress values. The multiblock copolymer solutions demonstrate a pronounced viscosity hysteresis in the heating-cooling cycle, being absent in the random copolymers. 10% solutions of multiblock copolymers are practically gelatinous even at room temperature. The observed effects are explained by examining the peculiarities of hydrogen bonding in vinyl acetate-vinyl alcohol copolymers using FTIR spectroscopy. The multiblock copolymers are characterized by stronger hydroxyl-hydroxyl H-bonds and greater fraction of interchain hydroxyl-acetyloxy H-bonds providing aggregation of chains and high viscosity of the corresponding samples, whereas the random copolymers more strongly interact with the residual solvent. Dynamic light scattering studies prove that the relaxation of concentration fluctuations is completely diffusive, being bimodal in the random copolymers and trimodal in the multiblock ones. The fast mode in the latter case demonstrates anomalous concentration behavior. In the dilute regime, up to very low concentrations, multiblock copolymer chains form stable aggregates, and this fact correlates with an unusual growth of the reduced viscosity in the corresponding rheological experiments. ■ INTRODUCTION Copolymers can be found among all types of polymers, ranging from biological objects, such as DNA, to numerous industrial polyolefins and synthetic rubbers. It is well-known that their chemical and physical properties depend not only on the nature of constituting monomers but also on their distribution along the backbone. 1 Regular block copolymers, especially di- and triblocks, are so far the most thoroughly studied objects, 2,3 as they can be synthesized with precise control of block lengths. The ability of such macromolecules to self-assemble into long- range ordered microstructures with the periodicity of 100- 200 nm is systematically analyzed in theory and experiments and widely used in developing nanomaterials for lithography, membrane technology, transfer of light energy, data storage, and so on. 4-6 Statistical copolymers are more common in chemical engineering due to the ease of their synthesis, though random structure of a polymer chain facilitates averaging rather than combining properties of two or more homopolymers in a single polymer material. Nevertheless, the ability of statistical copoly- mers to form complex (core-shell, vesicular, mushroom-like, etc.) structures in solutions upon temperature or pH varia- tions, which can be enhanced by applying the concept of the conformation-dependent synthesis, 7 makes possible their use for drug delivery or molecular recognition in medicine and biology. 8-10 Theoretical studies, 11-16 computer simulations, 17-26 and a few recent experiments 27,28 demonstrate that statistical (and even completely random) copolymers are capable of self-assembling in the melt state, too. Although microstructures with a long-range Received: February 13, 2014 Revised: June 27, 2014 Published: July 11, 2014 Article pubs.acs.org/Macromolecules © 2014 American Chemical Society 4790 dx.doi.org/10.1021/ma5003326 | Macromolecules 2014, 47, 4790-4804