Arkady D. Litmanovich

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.

Alkaline hydrolysis of polyacrylonitrile. On the reaction mechanism

Some new data were obtained about alkaline hydrolysis of a polyacrylonitrile (PAN) suspension at 75°C in a water/ethanol mixture. At > 95% conversion of nitrile groups, the hydrolyzed polymer contains ≃ 20% of amidines situated in the nearest neighborhood of the amidines, poly[(sodium acrylate)-co-acrylamide] is formed having a Bernoulli distribution of units. During the initial stage of PAN cyclization of 25-97°C, very short sequences of conjugated C=N bonds are formed which are much more reactive in hydrolysis than nitrle groups. These data enable us to reconsider the mechanism of alkaline PAN hydrolysis. A corresponding reaction scheme is suggested.

Alkaline hydrolysis of polyacrylonitrile, 1. Structure of the reaction products

The alkaline bydrolysis of a polyacrylonitrile suspension was carried out in a water/ethanol mixture at 75%. The product structure was studied by means of solid state 13 C NMR spectroscopy. The hydrolyzed samples contain up to 20% of amidines, which are formed in the nearest neighborhood of the COONa groups. Amidines are relatively stable under the given reaction conditions up to almost complete conversion of the nitrile groups, then being hydrolyzed gradually into amides. After complete hydrolysis of the amidines, the resulting poly[(acryl amide)-co-(sodium acrylate)] exhibits a Bernoulli triad distribution of the comonomer units. The data obtained might be useful in elucidating the reaction mechanism.

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 .

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.

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.

Chain statistics in vinyl acetate-vinyl alcohol multiblock copolymers

The kinetics and structure of products of the alkaline hydrolysis of poly(vinyl acetate) in an acetone-water mixture are studied via 1H NMR spectroscopy. The reaction is considerably accelerated and, according to the dynamic light-scattering data, is accompanied by polymer-coil expansion. The model of the effect of neighboring units is shown to be inapplicable to description of the general kinetics of the process; however, it may be successfully used for the quantitative interpretation of the experimental dependences of the triad composition of macromolecules on conversion. Thus, the conformational factors affect the reactivity of vinyl acetate groups, regardless of the nature of their nearest chain neighbors. This circumstance makes it possible to use the neighbor-effect model to describe the unit distribution over chains of the reaction product, vinyl acetate-vinyl alcohol copolymer, and thus to obtain information necessary for studying the relationship between the chain structure and properties of statistical multiblock copolymers.

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%.