Iain J. McEwen

The glass transition and interfacial layer in styrene-butadiene rubber containing silica nanofiller

The mechanical behaviour of a styrene-butadiene rubber, containing 23.5% styrene and filled with up to 55% by weight with silica nanoparticles, has been examined. The spectra of these composites show a second relaxation of relatively low intensity located about 40 °C above the main α relaxation, the glass transition Tg, which occurs at −34.5 °C in the unfilled polymer. This is attributed to an interfacial layer of polymer molecules whose chain relaxation dynamics have been altered by interaction with the filler surface. This is accompanied by a shift of the Tg itself to lower temperature when the filler surface is organophilic, but not when the filler surface is hydrophilic, and possible mechanisms for this are discussed. The amount of interfacial polymer can be quantified by resolving the glass transition and the second relaxation using a curve-fitting approach. A first-order treatment is presented which estimates the layer thickness to be of the order of a few nanometers.

Enthalpy relaxation in glassy polystyrenes: 1

Enthalpy relaxation at temperatures below the glass transition has been measured for three substituted polystyrene samples and for one poly(α-methylstyrene) sample. The overall changes in enthalpy between the experimental glass and the fully relaxed glass (ΔH∞) were estimated by fitting experimental data to the three-parameter Cowie-Ferguson expression. These were compared with enthalpy relaxation in polystyrene itself. There was no experimentally detectable effect from the para substituent on the progress of enthalpy loss in the samples. Poly(4-hydroxystyrene), however, was found to exhibit unexpectedly high values of ΔH∞. The utility of the Cowie-Ferguson approach with respect to multiparameter phenomenological kinetic expressions for enthalpy relaxation is discussed.

The length of cooperativity at the glass transition in poly(vinyl acetate) from the modeling of the structural relaxation process

Abstract The structural relaxation process of poly(vinyl acetate) has been studied by differential scanning calorimetry. The sample was subjected to different thermal treatments including an isothermal annealing at temperature Ta for a time ta. The heat capacity cp(T) was measured during a heating scan. Thus, the experimental results consist of a series of cp(T) curves determined after thermal histories with different values of Ta and ta. The experimental results were compared with the prediction of a multiparameter phenomenological model described in references [15,16]. This model follows the evolution of the configurational entropy of the sample during the whole thermal history. The parameters of the model were calculated by simultaneous fits to five cp(T) curves, corresponding to different thermal histories (the parameters of the model have the character of material parameters, independent of the thermal history). This allows the determination of the temperature dependence of the relaxation times and the β parameter of the Kohlrausch-Williams-Watts equation. From these parameters and an estimate of the internal rotational barrier obtained by molecular mechanics calculations, the size of the smallest cooperative rearranging region in the Adam-Gibbs theory was calculated. This result is compared with the length of cooperativity calculated from the temperature fluctuation theory proposed by Donth.

Novel gel polymer electrolytes based on a cellulose ester with PEO side chains

Abstract A range of gel electrolytes, composed of a hydroxypropylcellulose ester with poly(oxyethylene) side chains, LiCF 3 SO 3 and propylene carbonate (PC), or a mixture of propylene carbonate with ethylene carbonate (EC), were prepared and studied. The conductivities of gels containing 10 wt.% LiCF 3 SO 3 , and having propylene carbonate concentrations varying from 20 to 70 wt.%, exhibit Arrhenius behaviour with activation energies ranging from 16 to 34 kJ mol −1 . Room temperature conductivities of the order of 10 −3 S cm −1 are achieved with propylene carbonate contents above 50 wt.%. Gels containing 70 wt.% propylene carbonate, or a mixtures of propylene carbonate and ethylene carbonate, were cross-linked to form elastomeric films exhibiting good mechanical properties and room temperature conductivities of ∼10 −3 S cm −1 . The conductivities and mechanical strength of these films increased with increasing ethylene carbonate content. An Arrhenius dependence of log conductivity on (1/ T ) is also observed in these films, with activation energies for ion transport around 16 kJ mol −1 .

Prediction of phase-separated regions in ternary blends composed of three miscible binary copolymer pairs

The phase behaviour of ternary copolymer blends poly(styrene-stat-acrylonitrile) (SAN)/poly(methyl methacrylate-stat-acrylonitrile) (MAN)/poly(N-phenylitaconimide-stat-methyl methacrylate) (PIMMMA) was analysed using Flory-Huggins theory. The system studied consists of the combination of three binary pairs SAN/MAN, SAN/PIMMMA and MAN/PIMMMA, and it has been demonstrated that the combination of these three miscible pairings cannot guarantee the formation of a single-phase ternary blend. Phase-separated regions in ternary space can appear at compositions defined by the miscible windows of the binary pairings, provided that the difference in the interactions between them is large. When the molecular weights of the three polymers are unequal, it is suggested that a modified term Δχ′ = (χ′13 − χ′12) is used to replace the normal Δχ, in order to reflect the influence of molecular weight on the asymmetry of the interactions between the different binary polymer pairs. The new term χ′ij = (χijcrit − χij), where χijcrit > χij, is the difference between the critical interaction parameter and the corresponding normal interaction parameter of the i-j pair. It will equal the original expression for Δχ only when the component molecular weights are equal.

Physical ageing in poly(vinyl acetate)—3. Structural relaxation and its effect on the stress relaxation modulus

The extent of physical ageing of a sample of poly(vinyl acetate) (PVAc) has been determined by measuring the shear stress relaxation at three temperatures below the glass transition. The data obtained after various ageing times could be superimposed by shifts along the log time axis using the characteristic mechanical relaxation time as shift factor. A comparison of these ageing time shift factors with enthalpy and volume data already established for the same sample indicates that the mechanical properties of PVAc reach equilibrium before either the volume or enthalpy. These results are compared with those of others and for the case where dynamic mechanical methods are used to obtain shift factors. The possibility of interrelating ageing measurements using different physical properties is discussed.

A study of surface enrichment in polystyrene/poly(vinyl methyl ether) blends using attenuated total reflectance infra-red spectroscopy: 1

Abstract Surface enrichment of one-phase and phase-separated blends of polystyrene (PS) and poly(vinyl methyl ether) (PVME) by the latter component, which has the lower surface free energy, has been examined using attenuated total reflectance infra-red spectroscopy. The apparent surface concentration of PVME, determined by assuming compositional homogeneity within the depth probed, as a function of blend composition, agrees well with data derived by other methods. Although no effect due to variations in the PS molecular weight was evident, star-shaped PS appears to behave differently from linear PS.

The influence of solvent on the apparent reactivity ratios in free radical copolymerisation reactions between itaconic acid and 2-hydroxyethyl acrylate

Abstract The copolymerisation reaction between 2-hydroxyethyl acrylate and itaconic acid has been studied in a range of solvents. In a series of reactions in protic solvents, i.e. water, methanol, ethanol, isopropanol and n -butanol, the reactivity ratios ( r 1 and r 2 ) showed a distinct dependence on the solvent used. Copolymerisations in an aprotic solvent, dimethyl formamide, again yielded different r 1 and r 2 values but the copolymerisation behaviour was very similar to the reactions carried out in water. It was demonstrated that preferential absorption of the monomers by the copolymer occurs and while the systems did not appear to follow the explanation for the phenomenon offered by the “Bootstrap” model, the similarities were sufficient to consider the behaviour observed as a “pseudo bootstrap” effect.

Upper and lower critical solution temperatures in the cosolvent system acetone(1)+ diethyl ether(2)+ polystyrene(3)

Phase diagrams for different molecular weight fractions of polystyrene in the mixed solvent system acetone + diethyl ether have been obtained. The system shows a variation of upper and lower critical solution temperatures with solvent composition which indicates the cosolvent nature of the mixed solvent. No solvent composition is found which can dissolve high molecular weight poly-styrene (M106). The separation of critical solution temperatures can be predicted by the Prigogine theory of polymer solution thermodynamics, but not the absolute values.

Surface enrichment in PS/PVME blends: 2. The effect of specific interactions in the bulk mixture

Abstract The surface composition of blends of styrene copolymers, containing a hydrogen-bonding comonomer, with poly(vinyl methyl ether) has been estimated using infra-red reflectance spectroscopy. Surface enrichment by the polyether, which has the lower surface energy, is markedly less than that observed previously for poly(vinyl methyl ether)/polystyrene homopolymer blends. The extent of this effect depends on the strength of the hydrogen bond involved, and this in turn can be correlated with the degree of blend miscibility as determined by lower critical phase separation temperatures and heats of mixing. The findings confirm that surface enrichment depends not simply on surface energy differences but also on bulk mixing thermodynamics.