M. Ilavský

Effect of electrostatic interactions on phase transition in the swollen polymeric network

Abstract An earlier relation describing swelling equilibria in polyelectrolyte networks is analysed from the viewpoint of the effect of electrostatic interactions on the existence of phase transition in polyelectrolyte gels. While the increase in the number of charges on the chain initiates the transition, both the increasing concentration of the low-molecular weight electrolyte and the decreasing flexibility of the chain (increasing length of the statistical segment) suppress this phenomenon. Swelling experimental data obtained on polyacrylamide (PAAm) gels may be described semiquantitatively assuming the existence of ∼1 mol— charges on the PAAm chain.

The photoelastic behaviour of dry and swollen networks of poly (N,N-diethylacrylamide) and of its copolymer with N-tert.butylacrylamide

Abstract The viscoelastic and equilibrium behaviour of dry and water-swollen networks of poly(N,N-diethylacrylamide) and of its copolymer with N-tert.butylacrylamide with a small amount of methylenebis-acrylamide as the cross-linking agent was investigated. Superimposed curves of the mechanical and optical functions of dry networks have a form typical of amorphous polymers with side-chains: the mechanical and optical shift factors obtained by using the time-temperature superposition exhibit the same temperature dependence. Increasing temperature pronouncedly reduces the degree of swelling of gels in water but in the range 275–325 K the gels are in the rubberlike state, because the rise in temperature compensates for deswelling. The equilibrium mechanical behaviour corresponds to that predicted for Gaussian networks, while the optical behaviour is more complicated. The observed fast increase in the negative stress-optical coefficients (C e ) with swelling and the network density may be qualitatively explained by assuming correlations between the side chains. Cross-links and swelling interfere with these correlations, while the introduction of the tert. butyl group into the side-chain stabilizes them, owing to the steric hindrances to the motion of ethyl groups of diethylacrylamide units.

The structure and elasticity of polyurethane networks: 1. Model networks of poly(oxypropylene) triols and diisocyanate

Abstract The equilibrium mechanical and optical behaviour of networks prepared from poly(oxypropylene) triols (PPT) and 4,4′-diphenylmethane diisocyanate (MDI) at various initial molar ratios of reactive groups, r H = [ OH ] [ NCO ] , in the range 0.6 A = 1 3 (phantom network). (4) The difference between the eperimental and theoretical moduli for A = 1 3 may be adequately described by using Langleys concept of trapped entanglement contribution (for networks of the longer triol quite satisfactorily, for those of shorter PPT with some systematic deviation) with the same proportionality constant. (5) For correlations between the experiment and theory a generalized plot of the reduced modulus of the weight fraction of the gel proved to be useful.

Photomechanical effects in crosslinked photochromic polymers

Abstract The effect of radiation on photochromic crosslinked polymers containing azo group side chains is investigated. After irradiation at constant sample length, the swollen gels of light-sensitive polymers exhibit a reversible increase in the elastic retractive force. The light induced conformational changes are also accompanied by changes in the swelling equilibrium and temperature of the sample due to absorption of radiation and this added complication to the interpretation of the data. Interactions of the polymer with the solvent and the heat effect were eliminated by determining the temperature dependences of the elastic retractive force for the irradiated and unirradiated rubbery dry networks. The photomechanical effect increased with an increase in the content of photochromic groups and for the polymer with 5.4 mol % of azo groups, the photoinduced contraction of the sample amounted to 1%.

Phase transition in swollen gels: 7. Effect of charge concentration on the temperature collapse of poly(N,N-diethylacrylamide) networks in water

The swelling and mechanical behaviour of gels of the copolymer of diethylacrylamide (DEAAm) with a small quantity of sodium methacrylate (mole fraction xMNa = 0–0.067) swollen in water was investigated in the temperature range 1–80°C. For networks in the range xMNa > 0.0095 phase transition was observed; both the critical transition temperature Tc,c and the extent of the collapse Δc increase with increasing xMNa. The formation of associations in the collapsed state contributes to the overall extent of the transition; these structures give rise to stable turbid gels at elevated temperatures. Evidence for the formation of ‘associated’ structures is also supported by the observed independence of cloud temperature of the concentration of DEAAm, c, in the polymerization DEAAm-water mixture without the crosslinking agent in the range c = 0.5–80 vol%. While PDEAAm solutions are formed in the range c < 4.5–6 vol%, physical gels arise at higher concentrations.

Preparation and properties of poly-(N-butylmethacrylamide) networks

Abstract The gel-effect influences the copolymerization of N-butylmethacrylamide with methylene-bis-acrylamide similarly to other vinyl-divinyl copolymerizations. The fraction of crosslinks wasted in cycles is very high at the gel point, reaching 0.84–0.95; it decreases with increasing content of the crosslinking agent and with decreasing content of dimethylsulphoxide (DMSO) in the polymerization mixture. The effect of residual time processes on the mechanical characteristics in the rubberlike region decreases with increasing concentration of elastically active network chains, νe. The equilibrium deformational behaviour obeys the kinetic theory of rubber elasticity. With increasing dilution of the system, the efficiency of the cross-linking reaction decreases and so does the effect of trapped entanglements. A comparison of the gel points and the mechanical characteristics for poly(N-butylmethacrylamide) and poly(N-ethylmethacrylamide) networks swollen in DMSO indicates that the substituent at the amide nitrogen atom has a dilution effect depending on its size. The interaction parameter of the system poly(N-butylmethacrylamide)-DMSO increases linearly with the volume fraction of the polymer in the swollen gel, ν2; χ = 0.498 + 0.362 ν2 for ν2 ϵ (0.1, 0.33) and 25°.

The thermal effect in the photomechanical conversion of a photochromic polymer

SummaryThe effect of radiation on the mechanical behaviour of the crosslinked copolymer of maleic anhydride and styrene containing photochromic azo groups was investigated. The analysis of results of the observed photomechanical conversion proved a considerable influence of the thermal effect in addition to the effect of photoisomerization of the photochrome. The interpretation of some photomechanical experiments described earlier may be obscured by a rise in temperature caused by the absorption of radiation.

Preparation and properties of poly(N-ethylmethacrylamide) networks

Abstract The copolymerization kinetics of N -ethylmethacrylamide with methylene- bis -acrylamide are affected by the gel effect, as for most vinyl-divinyl systems. The concentration of crosslinks wasted in cycles at the gel point and determined from critical conversions is comparatively high, (60–90%) and increases with increasing concentration of the crosslinking agent and with dilution. The overall dependence of stress on time and elongation has been divided into deformational and time parts: the latter part decreases with increasing concentration of the crosslinking agent and decreasing concentration of the diluent, the equilibrium stress-strain dependences being described in terms of the kinetic theory of rubber elasticity. The interaction parameter Z of the system poly( N -ethylmethacrylamide)-water is linearly dependent on the volume fraction of polymer υ 2 : Z = 0.48 + 0.319 υ 2 (25: υ 2 ). The effect of intermolecular interactions (entanglements) on the equilibrium mechanical behaviour disappears with increasing dilution. KCNS and Co(NO 3 ) 2 raise the swelling of gels in water while Mg(C1O 4 ) 2 has a pronounced salting-out effect, contrary to expectation.

Effect of pressure on the cooperative relaxation properties and glass transition temperature of amorphous polymers

Abstract The Adam and Gibbs (AG) theory which describes the cooperative relaxation phenomena in the main transition region in terms of the configurational entropy of the system has been extended to describe also the dependence of both the relaxation times and the glass transition temperature Tg on the external pressure P. The resulting relations contain quantities accessible both by thermo-dynamic (discontinuities in the isobaric heat capacity and in the thermal expansion coefficient) and by viscoelastic (parameters of the temperature dependence of relaxation times) measurements. Theoretical relations adequately describe pressure experimental data on polystyrene, poly(vinyl acetate), and poly(vinyl chloride). The expression dTg /dP derived from the AG theory is at variance with Ehrenfests relation; the connection between the AG theory and the order parameter description of the glass transition is discussed.

Structure and elasticity of polyurethane networks based on poly(butadiene) diol, 4,4′-diphenylmethane diisocyanate and poly(oxypropylene) triol

Abstract Relations between structure and equilibrium mechanical properties of polyurethane networks based on poly(butadiene) diol, 4,4′-diphenylmethane diisocyanate (MDI) and poly(oxypropylene) triol (POPT) prepared at various initial ratios of reactive groups were studied. An inhomogeneous structure of the networks was revealed by small-angle X-ray scattering. The microdomains consist of POPT molecules crosslinked with MDI and they have a diameter of about 4–5 nm. The characteristic distance of microdomains is about 9 nm. The poor mutual miscibility of the reaction components and the existence of hydroxyl group association in the reaction mixture are the probable reasons for microdomain formation. The strain dependence of the equilibrium elastic force obeyed the Mooney-Rivlin equation. The experimental moduli of dry and equilibrium swollen samples were higher than those calculated for the affine limit of the Flory junction fluctuation model on the assumption of a homogeneous network formation.