Effect of Topological Defects on the Elasticity of Near-Ideal Polymer Networks

In recent years, new types of polymer gels have emerged, which have a well-controlled network structure and few topological defects. These so-called near-ideal polymer networks constitute a good model system to revisit the long-standing problem of structure–property relationships in polymer networks, as well as a promising platform for the development of polymer gels with outstanding mechanical properties. In this study, we investigate the relative contributions of network defects (dangling chains and second-order loops) on the stress–stretch response of near-ideal polymer networks using a computational discrete network model. We identify the average chain prestretch as a key parameter to capture the effect of network topology on the elastic modulus and maximum extensibility. Proper account of the chain prestretch further leads to scaling relations for the elastic properties in terms of topology parameters that differ from classical estimates of rubber elasticity theory. Stress–stretch curves calculated using the discrete network model are also compared to semi-analytical estimates.

Strongylatosis of the gastrointestinal tract and strongyloidosis of horses in the Central area of the Russian Federation

For several years, studies have been conducted to investigation the characteristics of the epizootic process in intestinal nematodoses of horses in the Central region of the Russian Federation. The data were obtained on the extensity and intensity of invasion in case of strongylatosis of the gastrointestinal tract and strongyloidosis of horses in one of the equestrian schools of the Ryazan region. Based on the results of laboratory ovoscopic and larvoscopic examinations of horses, the diagnoses for strongylidoses (delafondiosis and strongylosis), trichonematosis (trichonemosis, cyatostomidoses) and strongyloidosis were confirmed. Clinically expressed and latent forms of intestinal nematodoses of horses Traken, Arab and Yakut breeds were revealed when kept in stalls and levades, without grazing in the pasture. In an experiment on 24 horses, the effectiveness of the drug Alesan® (produced at AVZ S-P LLC, Russia) for horse helminthoses, was studied. Experimental studies have made it possible to establish maximum extensibility indicators (EE = 93–100%) of a parasiticide with a wide spectrum of action of Alezan® for strongylidoses, trichonematidoses and strongyloidosis in horses. The drug Alezan® is recommended for therapeutic and preventive measures in equestrian schools, clubs, for dysfunctional horses for nematodosis, twice a year (May and November).

Development of Some Promising Approaches for The Toughening of High-Temperature Polymers

High-temperature poly(arylene ether) and hydroxypolybenzoxazole polymers were toughened using dispersed rubbery phases. This rubbery phase was a hybrid material which was in-situ generated within the polymer matrix using mixtures of partially-hydrolyzable trialkoxy and dialkoxy organosilanes with a fully hydrolyzable tetraalkoxysilane. The resulting materials were characterized with regard to their structures, mechanical properties (modulus, ultimate strength, maximum extensibility, and toughness), thermal properties, optical properties, and tendencies to absorb water. Some of the results are presented here.

Ultimate Properties of Stereoregular and Stereoirregular 1,4-Polybutadiene Networks and Their Correlation with Theories

The ultimate properties of stereoregular and stereoirregular 1,4-polybutadiene (PB) networks covering a range in micro-chemical structure (cis-trans-vinyl) have been investigated. The dependence of the ultimate properties, namely the ultimate strength and the maximum extensibility, on sterochemical structure, has been demonstrated at different temperatures. Also, dependence of the ultimate properties on temperature has been apparent. Focusing special attention on the physical processes that culminate in rupture of the network, confirmed and extended the primary role played by strain-induced crystallization. More specifically, strain-induced crystallization enhances attainment of greater values of strength and extensibility. Noncrystallizable elastomers were found to have lower ultimate properties due to the absence of rupture-impeding mechanisms and consequently they become incapable of reaching their maximum extensibility. The frequently observed maximum in plots of the ultimate strength fr vs. the molecular mass Mc has been verified as an intrinsic property of the networks. This property was found to depend on the degree of crosslinking, rather than being time dependent. Values of the ultimate strength fr of both crystallizable and noncrystallizable networks were well reproduced by Bueche's theory of rupture. On the other hand, the Griffith criteria were found to have some success with noncrystallizable networks. Calculated values of the threshold surface free energy G0 were in agreement with those reported in the literature for similar noncrystallizable networks.

Reinforcement of Elastomers by The In-Situ Generation of Filler Particles

ABSTRACTThe goal of primary interest in these investigations was the development of novel methods for filling elastomeric networks. The techniques developed employ the in-situ generation of reinforcing fillers such as silica or a glassy polymer such as polystyrene either after, during, or before network formation. The reaction involves decomposition of organometallic compounds, using a variety of catalysts and precipitation conditions, or freeradical polymerization of a suitable monomer. The effectiveness of the technique is gauged by stress-strain measurements carried out on these elastomeric composites to yield values of the maximum extensibility, ultimate strength, and energy of rupture. Also of interest are calorimetric studies of the networks, to determine their crystallizability. Information on the filler particles themselves is obtained from density determinations, electron microscopy, and scattering measurements.

The Structure of Rubber Networks with Multifunctional Junctions

A number of statistical relationships have been derived for calculating the structure of rubber networks with multifunctional junctions. Some of these quantities appear as parameters in the molecular theory of elastic energy in phantom networks. Others relate to Langley's trapping factor for the entanglement contribution to the modulus, to the maximum extensibility, and to the viscoelastic properties of networks.

Effect of Finite Extensibility on the Viscoelastic Properties of a Styrene-Butadiene Rubber Vulcanizate in Simple Tensile Deformations up to Rupture

Stress-strain and rupture data were determined on an unfilled styrene-butadiene vulcanizate at temperatures from −45 to 35° C and at extension rates from 0.0096 to 9.6 min−1. The data were represented by four functions: (1) the well-known temperature function (shift factor) aT; (2) the constant-strain-rate modulus, F (t, T) reduced to temperature T0 and time t/aT, i.e., T0F (t/aT)/T (3) the time-dependent maximum extensibility λm (t/aT); and (4) a function Ω(χ) where χ=(λ−1)λm0/λm, in which λ is the extension ratio and λm0 is the maximum extensibility under equilibrium conditions. The constant-strain-rate modulus characterizes the stress-time response to a constant extension rate at small strains, within the range of linear response; λm is a material parameter needed to represent the response at large λ; and Ω(χ) represents the stress-strain curve of the material in a reference state of unit modulus and λm=λm0. The shift factor aT was found to be sensibly independent of extension. At all values of t/aT for which the maximum extensibility is time-independent, the relaxation rate was also found to be independent of λ. These observations indicate that the monomeric friction coefficient is strain-independent over the ranges of T and λ covered in the present study. It was found that λm0=8.6 and that the largest extension ratio at break (λb)max is 7.3. Thus, rupture always occurs before the network is fully extended.