A. A. Askadskii

Synthesis and mechanical behavior of functionally gradient polyisocyanurate materials based on hydroxy-terminated butadiene rubber

Functionally gradient polyisocyanurate-based structural materials in which the modulus of elasticity could be arbitrarily varied over a continuos range from 3 to 2000 MPa were prepared from hydroxy-terminated butadiene rubber and diphenylmethane diisocyanate. The materials are synthetically obtainable both via bulk polymerization (molding) and as composite materials with fillers of any type, including both highly porous compliant fillers that have no effect on the mechanical properties of the polymer matrix and reinforcing fillers, such as carbon and glass clothes. The trends in the main properties were studied; it was found that, over the entire range of elastic moduli relevant to the glass-to-rubber transition, the materials retain the elastic behavior inherent in polymer glasses, not the viscoelastic behavior characteristic of the transitional region between the glassy and rubbery states.

The effect of the conditions of formation of crosslinked polyurethane on the degree of crosslinking and mechanical characteristics

Conditions of synthesis and physicomechanical and thermomechanical characteristics are studied for crosslinked PU based on poly(propylene glycol) with M = 2000 and 2,4-toluene diisocyanate used for the preparation of gradient polymeric materials. The structure of the prepared prepolymer is studied by the methods of IR and 1H NMR spectroscopy. The effect of the content of the amine chain extender on changes in the characteristics of PU is analyzed. To verify the chemical structure of PU, the computer-aided calculation of some physicomechanical and thermomechanical parameters is performed. This analysis shows a fair agreement with the experimental data.

Gradient polymeric materials

Data on gradient polymeric materials that possess broadly varying mechanical, optical, and other properties are generalized. The term “gradient materials of the first type” refers to materials that possess a gradually variable elasticity modulus in the range of 3–2000 MPa for the same material that possesses no layers or interfaces, pasting, heat sealing, etc. They are produced as either transparent molds or composites with various reinforced fillers. The synthesis of these materials is based on the formation of polymer networks containing bulky cross-linked points with short flexible fragments linking them. These networks have been computer simulated using the physical approach developed by the authors and the chosen software, which allows one to calculate polymer properties based on their chemical structure and the implementation of computerized synthesis of polymers with prescribed properties. The term “gradient materials of the second type” refers to materials with abruptly changing elasticity modulus and transition zone between the glassy and the rubbery states.

Development of studies concerning analysis of the porous structure and solubility of polymers

A review of the studies concerning the porous structure and solubility of polymers is given. The studies in these domains undertaken in conjunction with A.A. Tager and her colleagues are briefly outlined; their development at the Laboratory of Polymeric Materials of the Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, is described in more detail. Problems related to the detailed analysis of temperature dependences of the coefficients of molecular packing, the densities, and the molar volumes of polymers are discussed; this makes it possible to make more accurate predictions of these characteristics on the basis of the chemical structure of the polymer and to determine the fractional free volume. The proposed approach to the description of solubility and miscibility of polymers is discussed for a number of specific examples. The dependences of the glass-transition temperatures on the compositions of poly(vinyl methyl ether)-polystyrene and poly(2,6-dimethyl-1,4-phenylene oxide)-polystyrene blends are plotted.

Relaxation properties of gradient polymer materials based on network poly(urethane-isocyanurates) produced via a modified method

The relaxation properties of gradient polymer materials produced via a modified method are studied through dynamic mechanical analysis and through measurement and approximation of stress-relaxation curves in a wide temperature range under static conditions. The physicomechanical properties of polymer materials produced via the new method are substantially improved, and the range of properties is markedly widened. The investigated materials can possess any values of the elastic modulus, including those inherent in the region of transition from the glassy state to the rubbery state; moreover, they demonstrate a quasi-elastic behavior characteristic of glasses and rubbers.

The effect of the degree of polymerization and lyophobic interaction on the condition of solubility of polymers

A modified calculation model that makes it possible to consider the effect of both the degree of polymerization and the lyophobic interaction on the conditions of solubility of polymers is developed. The model is valid for polymers and copolymers consisting of nonpolar and polar parts. In this case, the cohesion energy of a polymer, ΔE*, that appears in the expression for determining the solubility parameter of polymers comprises two parts, namely, the usual cohesion energy of a “dry” polymer and the additional contribution to the cohesion energy from the lyophobic interaction that occurs during the transfer of the nonpolar part of the polymer into a polar solvent and prevents its dissolution. The predictive power of the new criterion is tested with the use of two systems: (i) the polyarylate of isophthalic acid and phenolphthalein and (ii) polysulfone. The analysis of the solubility of these polymers in a wide range of solvents shows that the predictive power of the modified criterion is 93%.

Synthesis and properties of polyisocyanurate networks based on 2,4-toluene diisocyanate and poly(oxytetramethylene) glycol

With the use of the method of bulk polymerization, network polyisocyanurate polymer materials based on poly(oxytetramethylene) glycol and 2,4-toluene diisocyanate have been synthesized for the first time. To analyze their performance and relaxation mechanism, the mechanical characteristics of polyisocy-anurates are studied. The above polymers show a quasielastic mechanical behavior, even though their elastic modulus is characteristic of the temperature interval where all traditional polymers experience the transition from the glassy state to the rubbery state, and demonstrate a well-pronounced viscoelastic behavior. The proposed procedure for the construction of master curves shows that, in a certain temperature interval, shift factor remains invariable; in other words, it does not depend on temperature.

Gradient polymeric materials: Part 2. structure and properties

Data on the synthesis and study of gradient polymeric materials obtained over the last decade are summarized. The review begins with a short description of works that reported the first data on gradient materials. The following chapters deal with a detailed account of principle for preparing gradient materials of new type that provide extremely wide range of elastic modulus continuously varying in the prescribed direction. Gradient materials derived from poly(urethane-isocyanurate) net-works and their use to prepare films with elastic modulus gradient with depth are considered. A separate chapter is dedicated to composite gradient materials processed by hot molding. The one-step synthesis of poly(urethane-isocyanurate) networks and gradient materials based on poly(urethaneisocyanurate)s combined with polyurethane networks is described. Polyurethane networks and opportunities to control and vary elastic modulus in gradient materials are considered. Gradient polymers and composites constructed from poly(epoxy-isocyanurate) matrices reinforced with carbon cloth or aramid fabric are considered in detail.

Kinetic features of the formation of poly(epoxy isocyanurate) networks

A new method is developed for the synthesis of branched and network polymers that give rise to poly(epoxy isocyanurate) matrices during thermal curing. Matrix polymers are prepared on the basis of polyethers (poly(propylene glycol) or poly(tetramethylene glycol)), diisocyanate, and an epoxy oligomer. The chemistry of formation of networks is studied by NMR and IR spectroscopy. Formation of structures with different topologies is analyzed, and optimum conditions for the formation of gradient networks that are complex structural organizations are ascertained. It is shown that, during microphase separation of a three-component system whose components differ appreciably in surface energy and other parameters, the composition of microphases may be estimated. Optimum conditions of the curing process that provide formation of the perfect structure of networks are found, and their chemical structure is investigated.

A calculation scheme for estimation of dielectric loss factors in polymers

For the quantitative estimation of dielectric loss tangent tanδ in linear and network polymers, the calculation scheme based on the Debye theory is proposed. The calculation is performed for both polar and nonpolar dielectrics in a wide frequency interval ranging from 102 to 106 Hz. This calculation requires knowledge of only the chemical structure of a repeating unit in a linear polymer or a repeating fragment in a polymer network. Experiments on the estimation of frequency dependences of tanδ are conducted for polymer networks based on poly(urethanes) and poly(isocyanurates) of different compositions. A fair correlation between calculation and experimental data is obtained. It has been shown that tanδ tends to change with the increasing content of bulky isocyanurate network junctions that are responsible for the specific behavior of the system under the action of alternating mechanical and electric fields.