S. Ya. Frenkel

Thermodynamics of crystallization of macromolecules of various degrees of coiling

Abstract The crystallization of flexible-chain molecules under conditions of molecular extension is considered. The molecular orientation was characterized by a parameter β that is the ratio of end-to-end distance to the contour length of macromolecule. Calculations were performed for two types of crystals: folded-chain crystals and extended-chain crystals. Thermodynamical analysis shows that the decrease of conformational entropy owing to the drawing of the molecule into a crystal makes an essential contribution to the free-energy change. This value is of great importance in the definition of the equilibrium state of a system and so it is the reason why it is impossible to get the degree of crystallinity equal to 1. The values of molecular orientation at which each of two mechanisms of crystallization takes place are evaluated. It is shown that there is a critical value β* as a boundary between two ranges of extensions: at β < β*, the crystallization with the formation of folded-chain crystals is thermod...

Concentration redistribution of low-molecular-weight salts of metals in the presence of a strongly swelling polyelectrolyte hydrogel

Abstract This paper deals with the behaviour of a superswelling hydrogel based on sodium polyacrylate crosslinked with the allyl ester of carboxymethyl cellulose in solutions of salts of mono- and polyvalent metals. The effect of cooperative sorption of polyvalent ions by the hydrogel was detected. An expression describing the concentration redistribution in the system hydrogel-univalent salt solution was obtained.

Self-sufficiency of velocity sedimentation for the determination of molecular characteristics of linear polymers☆

Abstract General possibilities of velocity sedimentation in the hydrodynamic study of macromolecules are discussed. It is shown that, using the mass-spectrometric effect of the ultracentrifuge together with the calculation of molecular weights by means of a recently introduced sedimentation parameter βs, allows complete analysis of all the macromolecular parameters of polymers by velocity sedimentation only, without additional procedures. The sedimentation parameter is given by β s =N a [S]k s 1 3 M −2 3 , where [S]=S o η o /(1− ν ϱ o ) is the intrinsic sedimentation constant, ks is Gralens concentration coefficient, M is the molecular weight and Na is Avogadros number. In this way molecular weight distributions etc. can be obtained. The validity of the suggested approach is demonstrated on a cellulose nitrate (CN) sample with a substitution degree of 2.7, in acetone. The sample is characterized by the polydispersity index Mw/Mn=1.66 in the molecular weight range 20⪕M × 10 3 ⪕620 . By means of velocity ultracentrifugation, values of the equilibrium rigidity of CN (Kuhns segment length), A=(360±13) × 10−8 cm, and of hydrodynamic diameter, d=(4.8±0.4) × 10−8 cm, are obtained. All data obtained are in good agreement with published data for real fractions of CN in acetone.

New approach to the description of young's modulus for highly oriented polymers. I. Temperature–time dependences of young's modulus

Abstract A description of the relaxation of Youngs modulus (i.e., a decrease with increasing temperature and time) for highly oriented polymers is suggested and expressed by the equation where T is the absolute temperature, E is Youngs modulus, E0 is its value at T → 0, k is the Boltzmann constant, W0 is the activation energy of the relaxation of Youngs modulus, v is the rate of elastic deformation, and v0 is a constant. It was found that v0 = (1012-1014) Hz and that W0 coincides numerically with the activation energy of mechanical fracture determined from the temperature-time dependences of tensile strength. On the basis of these facts it was concluded that the relaxation of Youngs modulus and the failure strength of drawn polymers are substantially identical in nature.

New approach to the description of young's modulus for highly oriented polymers. ii. Relationship between young's modulus and thermal expansion of polymers over a wide temperature range

Abstract An empirical equation for Youngs modulus is proposed. It originates from thermal fluctuations and relates Youngs modulus for highly oriented polymers, E, to thermal expansion strain of the segments of macromolecular helices, eT, and to measurement time γ: where E 0 is the E value at eT → 0, C 0 is the heat capacity, k is the Boltzmann constant, e* ≅ 0.1, and t 0 = (10−12−10−l4)s. The equation may be used for description of the temperature dependences of Youngs modulus, E(T), at γ = constant over a wide temperature range: from cryogenic to melting temperatures. It was shown that omplicated (nonlinear) E(T) dependences were due to the variability of the γ(T) function because of the substitution of the Boltzmann statistics for the Bose one for both torsional and bending vibrational modes in a polymer solid.

Heterogeneous radical polymerization of ethylene

Abstract In the course of investigating the polymerization of ethylene initiated by azoisobutyronitrile in the presence of small amounts of benzene at 100 to 600 atm and at 70°C several unusual features were found, connected with an unusual polymerization mechanism. The polymerization was carried at a temperature below the melting point of polymer. For this reason the polymer precipitated during the process as a crystalline solid phase containing ‘living’ chains, the presence of which was proved experimentally. A kinetic scheme and equations are given for the dependence of degree and rate of polymerization on the time; they agree with experimental results. A statistical treatment of the molecular weight distribution of the polyethylene obtained under such conditions confirms the kinetic scheme given.

Strength of macromolecules of polyheteroarylenes containing an imide ring in the main chain

Abstract Data on the effective macromolecular tenacity, evaluated by means of IR spectroscopy, are presented for a number of polyheteroarylenes containing an imide ring in the main chain. The numerical values of these chain tenacities are compared to those previously obtained for flexible chain polymers. It is shown that the effective macromolecular tenacity of polyimides with a complex dianhydride fragment in the chain repeating unit is higher by a multiple of 1.5-2.0 than the corresponding value for flexible chain polymers; therefore the former polymer class includes a large reserve of macroscopic strength, which is still open to use.

Pulse n.m.r. study of the dynamics of fluctuations of local anisotropy in the solutions of liquid crystalline R-F polymers

Abstract Temperature dependencies of proton magnetic relaxation times and forms of transversal magnetisation decay in chloroform-d solutions of three liquid crystalline polyesters with fumaroil-bis-oxybenzoate mesogenic units connected by different flexible spacers (CH 2 ) 5 , (CH 2 ) 6 and CH 2 (CH 2 CH 2 O 2 ) 2 CH 2 (poly(R-M5), poly(R-M6) and poly(R-EO2), respectively) are studied in the concentration range 9–20 w/w%. Chemical exchange is shown to influence the form of transversal magnetisation decay and the dependencies of spin—spin relaxation time on the time interval in Curr—Purcell sequence. The rate of exchange is shown to depend on spacer flexibility and evenness, and on molecular mass and concentration. The exchange parameters (life time, chemical shift difference, populations) for poly(R-M6) solutions are estimated, and their dependencies on temperature and concentration are discussed in terms of heterophase fluctuations of local order.

A new approach to the description of the deformation kinetics and relaxation of mechanical properties of oriented polymers

The reduction of Youngs modulus and stress as well as the creep rate of highly oriented polymers with different chemical structure has been investigated. The kinetics of these processes are described by Arrhenius-type equations having the same activation parameters. The deformation and relaxation processes were assumed identical in their physical nature and functions of thermal fluctuation. Evidence for this assumption was obtained by investigating spectroscopically excited extended interatomic bonds of the macromolecules. The generation of excited bonds was found to determine the kinetics of these macroscopic processes in polymers.

Small-angle neutron diffraction study of block copolymer superstructure

Abstract The structural parameters of butadiene-styrene diblock copolymer films prepared by deposition from toluene have been studied with the aid of small-angle neutron scattering (SANS) in the temperature range from −190 up to +210°C. The experimental scattering curves were simulated within the limits of the line-gradient model including the Debye-Waller factor, the static character of which was determined from low-temperature measurements. The structural parameters determined by SANS experiments have been compared with the parameters calculated with the help of the formulae obtained by Skoulios, Helfand and Meier. It was also observed that the system could retain information about the conditions of the superstructure formation process.