J.M.G. Cowie

Physical ageing of poly(methyl methacrylate) from enthalpy relaxation measurements

Abstract The enthalpic ageing behaviour of poly(methyl methacrylate) was studied by differential scanning calorimetry for several ageing temperatures, and ageing times up to 10 4 min. The data are interpreted in terms of both the phenomenological multiparameter model and the Cowie-Ferguson model. The phenomenological multiparameter model was found not to provide an adequate description of the enthalpic ageing process(es) occurring in poly(methyl methacrylate). Problems encountered in defining the equilibrium glassy state by extrapolating from liquid enthalpy data are also discussed.

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 .

Fibres from polypropylene and liquid-crystal polymer blends using compatibilizing agents: 1. Assessment of functional and non-functional polypropylene—acrylic acid compatibilizers

Abstract A graft polymer based upon acrylic acid-functionalized polypropylene (PP-AA) and PP-AA itself were assessed as compatibilizers for polypropylene/liquid-crystal polymer (PP/LCP) polyblend fibres. It was found that the functional compatibilizer improved interfacial adhesion and thus fibre properties, depending on the type of hot-drawing process used, as well as significantly enhancing the thermal stability of the fibre. Incorporation of PP—AA increased fibre crystallinity and orientation, with a slight enhancement of fibre properties observed. This was attributed to the promotion of specific polar interactions between the blend components.

Transition from miscibility to immiscibility in blends of poly(methyl methacrylate) and styrene–acrylonitrile copolymers with varying copolymer composition: a DSC study

Abstract The glass transition and the structural relaxation processes have been studied in blends of poly(methyl methacrylate) (PMMA) and styrene–acrylonitrile (SAN) copolymers with different acrylonitrile (AN) contents. The 50/50 wt.% blend of PMMA with the SAN copolymer containing 30 wt.% of AN is immiscible, while blends with copolymers containing between 13 and 26 wt.% of AN are miscible. Thus the upper limit of miscibility is between 26 and 30 wt.% of AN. The temperature dependence of the relaxation times of the conformational rearrangements of polymer chains around the glass transition have been determined in the blends and pure components by modelling DSC thermograms obtained after different thermal histories in each sample. The slope in the Arrhenius diagram log τ vs 1/ T around the glass transition temperature is significantly smaller in the blend which is closer to the upper limit of miscibility than in the other miscible blends in which SAN copolymer contains less AN. The change of slope can be ascribed to a distribution in the glass transition temperatures of the different rearranging regions, reflecting the appearance of a microheterogeneity in the blend that cannot be detected as a double glass transition in the blend.

Fibres from polypropylene and liquid crystal polymer blends using compatibilizing agents: 3. Assessment of graft side chain liquid crystalline compatibilizers based upon acrylic acid-functionalized polypropylene

Graft side-chain liquid crystalline functional copolymers based upon acrylic acid-functionalized polypropylene (PP-AA) were assessed as compatibilizers for polypropylene/liquid crystalline polymer (LCP) polyblend fibres. The compatibilization effect observed was found to be dependent upon the liquid crystalline phase temperature range of the compatibilizer, with the most favourable being in a liquid crystalline state during fibre melt extrusion and hot drawing, thus contributing a lubricating effect to the blend and giving enhanced physical properties as a result.

Enthalpy relaxation of styrene–maleic anhydride (SMA) copolymers Part 1. Single component systems

Abstract The enthalpy relaxation of styrene–maleic anhydride (SMA) copolymers, at various ageing temperatures ( T a ) below T g and for different ageing times ( t a ), is described. Values of enthalpy lost (Δ H ( t a , T a )) were calculated from experimental data, plotted against log 10 ( t a ) and modelled using the Cowie–Ferguson (CF) semi-empirical approach to give a set of values for three adjustable parameters, Δ H ∞ ( T a ), log 10 ( t c ) and β . These define the relaxation process, which was found to be sensitive to copolymer composition in a non-linear fashion. Increasing MA content initially causes a faster relaxation with a smaller overall enthalpy loss; further increases in MA levels lead to slower changes to a final state of lower equilibrium enthalpy. All experiments indicate that the enthalpy lost by the fully relaxed glass (Δ H ∞ ( T a )) is less than the theoretical amount possible on reaching the state defined by the liquid enthalpy line extrapolated into the glassy region (Δ H max ( T a )), and that the final enthalpy value is dependent on copolymer composition. The implications of this are discussed.

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.

Ionic conductivity in oligo(ethylene oxide) esters of poly(itaconic acid)-salt mixtures: effect of side-chain length

Abstract A series of poly(itaconic acid) esters with oligo(ethylene oxide) side chains have been prepared in which the side chains contain from one to seven ethylene oxide units. These materials can form amorphous solid solutions with LiClO4 and NaClO4. The glass transition temperatures and a.c. ionic conductivities of these polymer-salt mixtures have been examined as a function of side-chain length, salt concentration and temperature. It has been found that the polymer with seven ethylene oxide units per side chain produces a polymer electrolyte with the highest conductivities between 10−3 and 10−6 S cm−1 for the temperature range 400 K to ambient. In these systems the flexibility of the polymer chain seems to be a controlling feature as reflected in the polymer-salt Tg values.