Yaroslav V. Kudryavtsev

Reactions in polymer blends: interchain effects and theoretical problems

Abstract Advances in experimental and theoretical studies of peculiarities of the reactions in polymer blends are reviewed. Experimentalists involve in the studies some relatively simple model systems such as homogeneous and quasi-homogeneous melts, as well as bilayer or trilayer films with well-defined interfaces. Such systems enable one to determine the kinetic parameters of the reaction, to estimate relative contributions of the reactivity and diffusion in the reaction kinetics, to elucidate some peculiarities of the reaction at the interface, etc. Experimental studies of the reactions in the model systems stimulate a formulation of appropriate theoretical problems, an analysis of the interchain interactions being the key approach in a creation of the corresponding theoretical models. The achievements related to different kinds of reactions are considered. For polymer-analogous reaction, the evolution of the blend structure under concerted action of the reaction and interdiffusion has been first described. For end-coupling reaction, the reaction kinetics both in a homogeneous melt and in the interface is described. An influence of the diblock copolymer formed both on the reaction kinetics and on the thermodynamic equilibrium is analyzed. Theoretical models account for the interchain interactions between the block copolymer and homopolymers and analyze such phenomena as segregation of a copolymer to the interface leading to the significant change in the interface properties and the blend stability. For interchain exchange reaction proceeding in a homogeneous melt, an analytical description of the molecular weight and block length transient distributions of the reaction product is developed. Also the reaction in a heterogeneous blend is described using Monte Carlo simulation. Some important unsolved problems are mentioned.

Phase diagrams of block copolymer melts by dissipative particle dynamics simulations

Phase diagrams for monodisperse and polydisperse diblock copolymer melts and a random multiblock copolymer melt are constructed using dissipative particle dynamics simulations. A thorough visual analysis and calculation of the static structure factor in several hundreds of points at each of the diagrams prove the ability of mesoscopic molecular dynamics to predict the phase behavior of polymer systems as effectively as the self-consistent field-theory and Monte Carlo simulations do. It is demonstrated that the order-disorder transition (ODT) curve for monodisperse diblocks can be precisely located by a spike in the dependence of the mean square pressure fluctuation on χN, where χ is the Flory-Huggins parameter and N is the chain length. For two other copolymer types, the continuous ODTs are observed. Large polydispersity of both blocks obeying the Flory distribution in length does not shift the ODT curve but considerably narrows the domains of the cylindrical and lamellar phases partially replacing them with the wormlike micelle and perforated lamellar phases, respectively. Instead of the pure 3d-bicontinuous phase in monodisperse diblocks, which could be identified as the gyroid, a coexistence of the 3d phase and cylindrical micelles is detected in polydisperse diblocks. The lamellar domain spacing D in monodisperse diblocks follows the strong-segregation theory prediction, D∕N(1∕2) ~ (χN)(1∕6), whereas in polydisperse diblocks it is almost independent of χN at χN < 100. Completely random multiblock copolymers cannot form ordered microstructures other than lamellas at any composition.

Phase separation effects and the nematic–isotropic transition in polymer and low molecular weight liquid crystals doped with nanoparticles

Properties of the nematic–isotropic phase transition in polymer and low molecular weight liquid crystals doped with nanoparticles have been studied both experimentally and theoretically in terms of molecular mean-field theory. The variation of the transition temperature and the transition heat with the increasing volume fraction of CdSe quantum dot nanoparticles in copolymer and low molecular weight nematics has been investigated experimentally and the data are interpreted using the results of the molecular theory which accounts for a possibility of phase separation when the system undergoes the nematic–isotropic transition. The theory predicts that the nematic and isotropic phases with different concentrations of nanoparticles may coexist over a broad temperature range, but only if the nanoparticle volume fraction exceeds a certain threshold value which depends on the material parameters. Such unusual phase separation effects are determined by the strong interaction between nanoparticles and mesogenic groups and between nanoparticles themselves.

Synthesis of norbornene–cyclooctene copolymers by the cross-metathesis of polynorbornene with polyoctenamer

Copolymers of norbornene and cyclooctene were synthesized for the first time by the cross-metathesis of polynorbornene with polyoctenamer. This strategy made it possible to use the 1st generation Grubbs catalyst, which exhibits low activity toward copolymerization of those monomers. Statistical multiblock copolymers with average block lengths varying from 200 to 2 units were obtained.

Alkaline hydrolysis of polyacrylonitrile, 2. On the product swelling

The swelling capacities of alkali hydrolyzed polyacrylonitrile in water Q w and in 0.9 wt.-% NaCL aqueous solution Q s were measured. Using these data the fraction of crosslinked units is estimated to vary from 0.8 to 1.6% for different samples. The analysis of Q s dependence on Q w is proposed as a method to estimate the degree of polymer network ionization α. The α value is found to be approximately 0.36 for all samples at the degree of neutralization 0.49-0.70. This indicates that a significant part of counterions does not affect the swelling, being bound by polyions. The influence of the conditions of hydrolysis (polymer/alkali and water/ethanol ratios) on the swelling properties of the product and the nature of crosslinks are discussed. Interchain interaction of an amidine group and an acrylonitrile unit resulting in a formation of β-diketone-like structure is proposed to explain the crosslinking. Kinetic measurements were carried out to estimate the ratio of the initial rate constants for the polyacrylonitrile and polyacrylamide alkaline hydrolysis which was found to be of order 10 -1 .

Phase Diagrams Semicrystalline Polymer–Liquid Revisited: Isotactic Polypropylene-Dibutyl Phthalate and Other Systems

The full phase diagram of an isotactic polypropylene (i-PP)–dibutyl phthalate (DBP) mixture is for the first time constructed by an optical method and discussed within the concept of semicrystalline polymers as microheterogeneous liquids with a three-dimensional network structure. It is demonstrated that the liquidus in this and other polymer–solvent systems is not thermodynamically equivalent to the liquidus in low molecular weight (MW) mixtures. Qualitatively different thermal behavior of those two types of binary systems in the liquidus vicinity is corroborated by differential scanning calorimetry (DSC) experiments. In the former case, a liquid-solid transition resulting in the formation of polymer crystallites does not lead to separating the mixture into crystalline and amorphous phases. On cooling, the system remains macroscopically single phase until the low MW liquid can be fully dissolved in the amorphous regions of the polymer. The correct location of the corresponding borderline is crucially important for the microporous membrane formation via thermally induced phase separation (TIPS). It is also argued that the topology of a phase diagram polymer–low MW liquid does not depend on whether the polymer is amorphous or crystalline.

Simulation of heterogeneous end-coupling reactions in polydisperse polymer blends

The influence of polydispersity on the interfacial kinetics of end-coupling and microstructure formation in the melt of immiscible polymers was studied using dissipative particle dynamics simulations. The irreversible reaction started at a flat interface between two layers, each of which contained polymer chains of two different lengths with functionalized or unreactive end groups. As in the case of fully functionalized monodisperse reactants [A. V. Berezkin and Y. V. Kudryavtsev, Macromolecules 44, 112 (2011)], four kinetic regimes were observed: linear (mean field coupling at the initial interface), saturation (decreasing the reaction rate due to the copolymer brush formation or reactant depletion near the interface), autocatalytic (loss of the initial interface stability and formation of a lamellar microstructure), and terminal (microstructure ripening under diffusion control). The interfacial instability is caused by overcrowding the interface with the reaction product, and it can be kinetically suppressed by increasing chain length of the reactants. Main effects of polydispersity are as follows: (i) the overall end-coupling rate is dominated by the shortest reactive chains; (ii) the copolymer concentration at the interface causing its instability can be not the same as in the lamellas formed afterwards; (iii) mean length of the copolymer product considerably changes with conversion passing through a minimum when a microstructure is just formed.

Effect of Cross-Linking on the Structure and Growth of Polymer Films Prepared by Interfacial Polymerization.

Interfacial polymerization of tri- and bifunctional monomers (A3B2 polymerization) is investigated by dissipative particle dynamics to reveal an effect of cross-linking on the reaction kinetics and structure of the growing polymer film. Regardless of the comonomer reactivity and miscibility, the kinetics in an initially bilayer melt passes from the reaction to diffusion control. Within the crossover period, branched macromolecules undergo gelation, which drastically changes the scenario of the polymerization process. Comparison with the previously studied linear interfacial polymerization (Berezkin, A. V.; Kudryavtsev, Y. V. Linear Interfacial Polymerization: Theory and Simulations with Dissipative Particle Dynamics J. Chem. Phys. 2014, 141, 194906) shows similar conversion rates but very different product characteristics. Cross-linked polymer films are markedly heterogeneous in density, their average polymerization degree grows with the comonomer miscibility, and end groups are mostly trapped deeply in the film core. Products of linear interfacial polymerization demonstrate opposite trends as they are spontaneously homogenized by a convective flow of macromolecules expelled from the reactive zone to the film periphery, which we call the reactive extrusion effect and which is hampered in branched polymerization. Influence of the comonomer architecture on the polymer film characteristics could be used in various practical applications of interfacial polymerization, such as fabrication of membranes, micro- and nanocapsules and 3D printing.

Phase Diagrams of Semicrystalline Polymer–Crystalline Substances: Polyolefins–1,2,4,5-Tetrachlorobenzene

The full phase diagrams of low-density polyethylene (LDPE), high-density polyethylene (HDPE), and isotactic polypropylene (i-PP) mixtures with 1,2,4,5-tetrachlorobenzene (TeCB), including the solubility curve of TeCB in a solid polymer, were constructed by an optical method. The diagrams contain a eutectic point that corresponds to the situation when the crystallization of TeCB out of its solution in a polyolefin is accompanied by the crystallization of monomer units of the macromolecules. As a result, the polymer acquires a gel structure with crystallites as crosslinks and amorphous regions saturated with TeCB. It is demonstrated that the eutectic point position on the phase diagram can be used for ranking polymers with respect to their thermodynamic affinity to a solvent. For the studied systems, the affinity to TeCB was decreased in the order i-PP, HDPE, and LDPE. Direct experimental evidence was obtained that TeCB crystals can be dissolved in a solid polymer via a vapor phase mechanism, which leads to the polymer amorphization.

Thermal fractionation of vinyl acetate-vinyl alcohol copolymers

Thermal fractionation via the method of successive self-nucleation and annealing was used for the first time to study the crystallinity of vinyl acetate-vinyl alcohol copolymers with different random distributions of chain units. The lamella-thickness distribution was calculated through the Gibbs-Thomson equation. It was shown that, for all samples, the minimum lamella thickness is the same and corresponds to a block of no less than 15 vinyl alcohol units. On the basis of these data and with the use of the computer simulation of the polymer-analogous reaction via the Monte Carlo method, the block-length distribution in the crystalline phase was found. It was shown through a comparison of the lamella-thickness and block-length distributions that the maximum lamella thickness increases with the block length and vinyl alcohol content in the copolymer. In crystallites, blocks with lengths exceeding the maximum lamella thickness comprise a significant fraction. Thus, it is probable that these blocks form folds. The dependences of melting temperatures of crystalline lamellas on their thicknesses, as well as the dependences of the melting temperatures of copolymers not subjected to thermal fractionation on the chain-structure parameters, are adequately described by the Flory crystallization theory.