S.Ya. Frenkel

The nature of the so-called “liquid-liquid” transition in polymer melts. A review☆☆☆

Abstract The temperature dependence of the most probable relaxation time for the establishment of the segmental dipole polarization, given in log τ −1/ T coordinates, is subject to a change at T > T g within a narrow temperature range. The average of this range, the “U”,-transition temperature of Boyer, the upper validity limit of the Williams-Landell-Ferri formula, and also the temperature at which a number of other properties of polymers change at T > T g , symbolized by T ∗ , is a linear function of T g , i.e. T ∗ ≅ T g +76° C . The analysis of the experimental results led to the conclusion that the described phenomenon is linked with a cooperation change in the thermal vibrations of macromolecular segments in a range close to T ∗ . The liquid-liquid (“ll”) transition in the case of flexible-chain polymers at T > T g is typical for the behaviour which is evident simultaneously at the macromolecular level and on a larger scale, i.e. the melt as a whole.

The effect of stretching on the macromolecular structure of polymer fibres

The increase of the degree of stretching, W, of previously oriented fibres, produced from different crystalline polymers, results in an intensity decrease of the meridianal reflection, I, in low-angle X-ray scattering pictures [1–3]. The same phenomenon was described elsewhere [4] when polyvinyl alcohol (PVA) fibres were stretched at near-melting temperatures. A series of independent measurements of a whole series of structural parameters associated with changes I established that all these changed only slightly [4]. One parameter, not directly measured, which had more effect on I was the density difference between crystalline and amorphous zones, pc−pa; the main part of the reduction in I was therefore attributed to the compensation of this difference [4]. By combining the low-angle X-ray scattering with a fibre shaded with iodine [5], we tried to verify the earlier conclusion that the intrafibrillar amorphous zones become denser, and to evaluate their density. The term “intrafibrillar” is defined as being the zones which directly affect I by their density and dimensions. The remaining unordered zones of the polymer were named interfibrillar.

Correlation of chain configurations, structure and mechanical properties of fibres of polypyromellitimide series☆

Abstract Results are given of investigating the structure and mechanical properties of a polyimide series prepared using the dianhydride of pyromellitic acid and diamines characterized by chemical structure. It was found that mechanical properties of fibres of this series depend on the ability of macromolecules to form ordered structures. Polyimides with low elasticity moduli do not crystallize during heat treatment of fibres. It was shown that to obtain ordered structures in the fibre, it is essential to consider the ratio of the form and cross sectional dimension of diamine and dianhydride components of the chain and bear in mind that the pyromellitimide fragment is the “initiator” of intermolecular packing of macromolecules of this series.

Crystallinity of Poly-2,2′-octamethylene-5,5′-dibenzimidazole☆

Abstract The preparation of heat-resistant polymers containing imidazole rings in the main chain has been described in [1–5]. But, according to [1–6], the polymers of this class—poly-2,2′-tetramethylene-5,5′-dibenzimidazole and poly-2,2′-octamethylene-5,5′-dibenzimidazole— were in the amorphous state. In [6] the mechanical properties were studied in a wide temperature range, together with the heat resistance, for isotropic and oriented films of poly-2,2′-octamethylene-5,5′-dibenzimidazole. The present work uses X-ray diffraction analysis to study the crystalline structure of newly-formed fibres and films of this polymer, and also its variations as a result of heat treatment. Certain mechanical properties of the fibres were studied at the same time.

Selective interactions in polymer chains—I. The hydrodynamic properties and solubility of A 9 : 1 copolymer of methyl methacrylate and methacrylic acid☆

(l) A study has been made of the mechanical degradation of polystyrene and polymethylmethacrylate containing the following acceptors: diphenylamine, ~-naphthylamine, a-naphthol, hydroquinone, p-aminophenol, p-nitrophenol and pentachlorophenol. Degradation was carried out for 120 seconds in vibro-mills cooled with liquid nitrogen. (2) I t was found tha t in the presence of all these acceptors polystyrene forms linear degradation products and that the aeceptors can be arranged in a definite order of efficiency. In contrast to polystyrene, polymethylmethacrylate tends to form non-linear products and the order of the acceptors changes to some extent in conformity with their reactivity toward peroxide radicals.

The degradation of poly (acid) amide based on pyromellitic dianhydride and bis-4-aminophenyl hydroquinone☆

Abstract The results of viscometric studies of the stability of solutions of poly(acid)amide (PAA) based on pyromellitic dianhydride and bis-4-aminophenyl hydroquinone have shown the dry polymer to have the greatest stability, while dilute solutions in dimethylformamide or dimethyl sulphoxide were the least stable. PAA decomposes as a result of first the hydrolytic cleavage of the amide bond, and secondly due to spontaneous imidization.

Theoretical investigation of the formation of condensation polymers-polyamidoacids

Abstract In accordance with a proposed mechanism of formation of polyamidoacids a set of five differential kinetic equations is obtained, to describe the variation during the course of the reaction of the concentration of anhydride and amine functional groups and of water. On the basis of the theory of branching processes expressions are obtained for the number average, weight average and z average molecular weights of polyamidoacids.

Relation between strength and elastic properties of polymers

Formulae of the phonon theory of strength were examined; these formulae relate the activation energy of mechanical breakdown U0 and coefficient ψ of the formula of Zhurkov long-term strength with Youngs modulus and the Gruneisen coefficient. Oriented fibres based on aromatic para-polyamides, polyamidohydrazides, linear polyarimides and polyacrylnitrile were investigated. Experimental evaluation confirmed the validity of the formulae proposed and enabled dimensions of thermal fluctuations causing failure to be established.

Equilibrium melting point of polyethylene oxide

Abstract As a result of using the thermo-kinetic approach to solving the problem of determining the equilibrium melting point of polymers the equilibrium melting point of one of the conformation crystalline modifications of polyethylene oxide was determined by various independent methods.

Thermodynamics of the metastability of aqueous solutions of polyvinyl alcohol

Two approaches have been adopted to solve problems concerning the thermodynamics of the metastability of aqueous polyvinyl alcohol (PVA) solutions: a structural method, such as the method using the turbidity spectrum (determining dimensions of supermolecular particles (SMP) during isothermal formation over a wide range of solution temperature (20–150°) and concentration (1–20%) and the thermodynamic method-determining the second virial coefficient A2 over the range of ∼20–90°. The type of kinetic curves of turbidity τ=τ(t) and dimensions rλ=rλ(t) and the extremal relationship between the rate of formation of supermolecular order and temperature at constant concentration and concentration at constant temperature indicates that SMP are crystalline. The complex structure of supermolecular organization (SMO) has three levels. A system of SMP remaining in the solution after dissolving the main mass of the polymer under given conditions forms the first level. After preparing the solution the second level of SMO is formed, of which the degree of development depends on solution temperature and concentration. SMP formed in solution at high temperature (>80°C) while cooling the solution agregate into anisodiametric structures. These aggregates are very sensitive to temperature variation and determine the third level of SMO. The positive value of A2 in the range studied confirms the stability of relatively amorphous solutions. Consequently, supermolecular order in aqueous solutions of PVA should be related to the instability of the system in relation to crystalline equilibrium.