Ya. V. Kudryavtsev

Simulation of phase separation in melts of regular and random multiblock copolymers

The dissipative particle dynamics method and parallel computing are employed for computer simulation of phase separation in melts of regular and random (Markovian) AB multiblock copolymers of symmetric compositions. For the first time, it is shown that, as the Flory-Huggins parameter χ is increased, lamellar microstructures are formed in all these systems. At preset block length M, the microstructures arise in regular copolymers at lower χ values than those in random copolymers, while the formed lamellas are narrower and have smaller amounts of defects. At χM > 100, a superstrong segregation regime develops in the regular copolymers. In random copolymer melts, bicontinuous structures are observed for a long time; in the long run, they are likewise transformed into lamellar structures whose imperfection substantially increases with M.

Simulation of phase separation in melts of reacting multiblock copolymers

The dissipative particle dynamics method is employed to conduct the computer simulation of the phase separation occurring in a melt of a polymer containing monomer units of two types under the conditions of reversible polycondensation or interchain exchange. The chemical reactions are simulated via the Monte Carlo method. It is shown that growth in the Flory-Huggins parameter leads to macrophase separation, irrespective of the rate and type of the reactions and the spatial structure of an initial system. Relative changes in the probabilities of elementary reactions between units of different types shift the phase-transition point. Different levels of refinement are considered for description of the stationary states being developed. By the example of interchain exchange, it is shown that the structure of the polymer melt in the initial state can substantially affect the dynamics of the phase separation.

Monte Carlo simulation of the interchain exchange reaction in a blend of incompatible polymers

The interchain exchange reaction in a blend composed of two contacting layers of incompatible A and B homopolymers was simulated by means of the dynamic off-lattice Monte Carlo method. The evolution of local molecular-mass and block mass distributions, depending on the effective temperature and the reaction rate, was studied for the first time. It was shown that the components interpenetrate as the copolymer forms in the interphase layer and the average block length decreases below a certain, temperature-dependent value. The state of dynamic equilibrium, whose characteristics are determined mainly by temperature, is established in the system. The time of establishment of equilibrium and the intensity of compatibilization at the early steps of the process are controlled by the rate of the reaction. The results of the study allow the contribution of the reaction to the interchange processes to be evaluated.

Interchain exchange and interdiffusion in blends of poly(ethylene terephthalate) and poly(ethylene naphthalate)

The interchain exchange and interdiffusion in blends of poly(ethylene terephthalate) and poly(ethylene naphthalene-2,6-dicarboxylate) are investigated with reprecipitated commercial samples (M η ∼ 104) and samples containing no polycondensation catalyst (M η ∼ 103) synthesized in the course of this study. The kinetics of multiblock copolymer formation and gradual reduction of the mean block length in quasi-homogeneous blends were shown to fit a simple theoretical model of a second-order reaction. The increase of the reaction-rate constants on the transition from commercial samples to synthesized ones revealed a significant role of chain ends in interchain exchange. The detected activation energy of the interchange in the absence of catalysts (97 kJ/mol) was noticeably less than that previously reported for the polymer pair under study (120–170 kJ/mol). The obtained data were applied for analysing the interdiffusion between melts of the same polymers accompanied by the interchain exchange. By means of the microinterference method, the interdiffusion in the synthesized samples was shown to be much faster than that in the reprecipitated commercial samples, a result that may be due to the better compatibility of the initial polyesters as their molecular mass decreased. In later stages of the process in both systems, the interpenetration of components was slower than that predicted by Fick’s law, owing to formation of copolymer species that diminished the thermodynamical factor of mixing.

Regulation of the degree of blockiness of the norbornene–cyclooctene copolymer synthesized via the cross-metathesis reaction

The kinetics of the polymer cross-metathesis reaction in a mixture of polynorbornene and polycyclooctene catalyzed by the Grubbs first-generation ruthenium catalyst at room temperature is studied. The structure of the reaction product, a multiblock copolymer of norbornene and cyclooctene, is determined by a number of factors typical for a mixture of polymers reacting with each other via the interchange reaction with the participation of terminal groups. The addition of 5 mol % catalyst transforms the mixture into an almost random copolymer over a day. At a lower content of the catalyst, the maximum conversion is reached in a mixture enriched with polynorbornene. The interchain exchange results in an increase in the fraction of trans C=C bonds in polycyclooctene and cyclooctene–norbornene copolymers to 80%.

Interdiffusion in polymer blends: The effect of compressibility

The polymer/polymer interdiffusion in a binary compressible blend consisting of long and short chains is investigated within the framework of the dynamic random phase approximation (RPA). The relative contributions of two (fast and slow) stages into the total relaxation of compositional heterogeneities are explicitly shown to be strongly dependent on the initial conditions and the degree of asymmetry of the blend. Special emphasis is given to the influence of the initial distribution of free volume on the apparent rate of the composition relaxation. The evolution of the dynamic struture factor is described as well. It is shown that most of the peculiarities usually ascribed to the fast-mode behavior can be derived within the RPA approach.

Phase diagram of the high-density polyethylene-1,2,4,5-tetrachlorobenzene mixture

The full phase diagram of a high-density polyethylene-1,2,4,5-tetrachlorobenzene (TeCB) mixture, which includes the solubility curve of TeCB in the polymer, is constructed for the first time by the optical method. It is shown that TeCB crystals are dissolved in polyethylene via vapor phase mechanism on heating, which leads to the polymer amorphization. The eutectic mixture composition corresponds to the situation when the crystallization of TeCB out of its solution in polyethylene is accompanied by the crystallization of elementary units of macromolecules. As a result, the polymer acquires a gel structure with crystallites as crosslinks and amorphous regions saturated with TeCB.

Estimating the melting temperature of macromolecular poly-3,3-bis-(azidomethyl)oxetane

A melting temperature of 90.4°C (complete amorphization) for macromolecular poly-3,3-bis(azidomethyl)oxetane is determined via the hydrostatic-weighing method. This value corresponds to the end of the endothermic peak of the DSC thermogram and is ~10°C higher than the maximum value. The amorphization of the polymer is found to proceed mainly owing to thermomechanical destruction of the crystallites, and only a very small part of them is melted purely thermally at the temperature of complete amorphization. The melting temperature is significantly reduced in the presence of the oligomer fraction in the polymer. The density and equilibrium heat of fusion of the crystalline regions, which coincide for various polymer samples, are found with consideration for the data of X-ray scattering.

Aggregation of nitrile-butadiene copolymers in solutions

The nonuniformity of nitrile-butadiene rubber solutions in toluene and chloroform has been studied. Dynamic-light-scattering measurements show that these solutions contain two kinds of scattering particles: small particles 10–16 nm in radius corresponding to the macromolecular coil and large particles 400–500 nm in radius corresponding to aggregates. The aggregates are stable in solutions for at least several days. Such stability may be interpreted in terms of topological factors supposing that the aggregates are formed via noncovalent interactions of tens of thousands of highly branched entangled chains. The chains become disentangled even during slight shear. Thus, the solution of nitrile-butadiene rubber subjected to soft rolling mostly contains small particles. However, in the absence of external fields, such a structure is very stable in solution and even more so in the rubber block. During storage of nitrile-butadiene rubber, formation of a relatively small quantity of covalent crosslinks suffices for fixing the structure of aggregates and prevents the disentanglement of chains. Thus, the crosslinking of the aggregates may be primarily responsible for the natural aging of nitrile-butadiene rubber during storage.

The role of chain structure in the rheological behavior of vinyl acetate-vinyl alcohol copolymers

A comparative study of the viscoelastic properties of melts of vinyl acetate-vinyl alcohol copolymers with equimolar compositions characterized by different statistical distributions of chain units has been performed. It has been shown that the principle of temperature-frequency superposition is obeyed by copolymers close to a random copolymer, but is violated by copolymers with the block distribution of units. Unlike amorphous random copolymers, a multiblock copolymer is characterized by weak crystallinity, the absence of the relaxation flow state, and a more pronounced tendency to form interchain hydrogen bonds both between two hydroxyl groups and between hydroxyl and ester groups.