Julia S. Higgins

Studies of cyclic and linear poly(dimethyl siloxanes): 3. Neutron scattering measurements of the dimensions of ring and chain polymers

Abstract The dimensions of both cyclic and linear poly(dimethyl siloxanes) in dilute solution in benzene- d 6 have been measured by small-angle neutron scattering. The mean-square radii of gyration of the linear polymers are consistent with values predicted from published data, including experimental molar cyclization equilibrium constants. The average dimensions of the cyclic poly(dimethyl siloxanes) in fractions containing z -average numbers of bonds n z in the range 130 n z , were found to be considerably smaller than those of the corresponding linear polymers. The neutron scattering results give a value for the ratio of the z -average radii of gyration for linear and ring poly(dimethyl siloxanes) (containing the same number of monomer units) 〈s 2 〉 z,l 2 〉 z,r = 1.9 ± 0.2 . This ratio may be compared with the value of 2.0 predicted theoretically for ‘flexible’ high molecular weight linear and cyclic polymers, unperturbed by excluded volume effects.

Miscibility and kinetics of phase separation in blends of poly(ethylene oxide) and poly(ether sulphone)

Abstract The miscibility and kinetics of phase separation in mixtures of poly(ethylene oxide) (PEO) and poly(ether sulphone) (PES) were studied. The mixture was found to have a lower critical solution temperature which, in contrast to observations by other authors, is higher than the glass transition temperatures of the mixtures. Using a light scattering technique the spinodal decomposition temperatures of the mixture were also obtained. The Cahn-Hillard theory was found to be capable of predicting qualitatively the behaviour of phase separation in one mixture (PES/PEO, 10 90 ). Quantitative description of the spinodal decomposition behaviour would require knowledge of the effects of thermal fluctuation and of non-instantaneous temperature jumps on the experimental results.

Flow-induced mixing and demixing in polymer blends

Abstract The effect of shear flow on the phase behaviour of partially miscible polymer blends exhibiting a lower critical solution temperature behaviour was investigated. Miscibility was detected, with and without the application of flow, as a change from optical clarity to turbidity using light scattering and as a shift in the glass transition temperature. Light scattering data were collected on a rheo-optical device that was designed to monitor phase changes in polymer blends undergoing shear flow between parallel glass plates in a temperature-controlled environment. Glass transition temperatures of quenched sheared blends were measured using a differential scanning calorimeter. It was found that shear-induced demixing and shear-induced mixing may be observed within the same blend depending on the magnitude of the applied flow. The magnitude of the increase or decrease in the lower critical solution temperature varied with the blend composition and also among the three blends used in this study.

Miscibility and kinetics of phase separation in polymer blends of tetramethyl-bisphenol-A polycarbonate and polystyrene

Abstract In this paper we report the phase boundaries of tetramethyl-bisphenol-A polycarbonate with polystyrene of different molecular weights, determined using light scattering techniques as well as microscopic methods. The early stages of spinodal decomposition were observed at several temperatures. Analysis of the data using the Cahn-Hillard theory showed that this theory is not able to quantitatively predict the early stages of spinodal decomposition for this blend. Inclusion of thermal fluctuation effects appears to be necessary.

A simple approach to polymer mixture miscibility

Polymeric mixtures are important materials, but the control and understanding of mixing behaviour poses problems. The original Flory–Huggins theoretical approach, using a lattice model to compute the statistical thermodynamics, provides the basic understanding of the thermodynamic processes involved but is deficient in describing most real systems, and has little or no predictive capability. We have developed an approach using a lattice integral equation theory, and in this paper we demonstrate that this not only describes well the literature data on polymer mixtures but allows new insights into the behaviour of polymers and their mixtures. The characteristic parameters obtained by fitting the data have been successfully shown to be transferable from one dataset to another, to be able to correctly predict behaviour outside the experimental range of the original data and to allow meaningful comparisons to be made between different polymer mixtures.

Thermodynamics of oligomeric binary mixtures of polyethylene glycol and polypropylene glycol methylethers

Abstract Measurements are reported of the heats of mixing and cloud point curves of binary mixtures of methoxylated polyethylene glycol with methoxylated polypropylene glycol. The data are discussed in terms of the Flory equation of state theory and it is shown that both a temperature and concentration dependent X12 parameter and a Q12 term are required.

Ambiguities in the interpretation of small-angle neutron scattering from blends of linear and branched polyethylene

Abstract This paper describes the difficulties of interpretation of small-angle neutron scattering data from blends of linear with lightly branched polyethylenes. The expected ‘phase diagram’ for the blend system was determined using indirect techniques, i.e. differential scanning calorimetry and transmission electron microscopy. Two different blends have been examined by neutron scattering at temperatures well above the melting point of both components. The neutron scattering data from one blend (mixed according to the ‘phase diagram’), containing 50% linear deuterated material and 50% branched polymer, showed it to be demixed. The phase sizes stayed constant at around 23 nm over 36 h. The scattering from the other blend (demixed according to the ‘phase diagram’), containing 10% linear deuterated material and 90% branched polymer, could be interpreted in two different ways. The data are consistent with a homogeneously mixed blend, but can equally well be interpreted as coming from a system which is phase-separated with a large domain size, as seen in the electron micrographs.

Complex miscibility behaviour for polymer blends in flow

Abstract Experimental observations of the effect of shear flow on the miscibility of binary polymer blends are compared to calculations based on a generalized Gibbs energy of mixing Gγ˙. This mixing free energy characterizes the steady state established at shear rateγ˙, as the sum of G z , the equilibrium Gibbs energy and E s , the energy the system stores while flowing.

Comparison of the structural and rheological consequences of micelle formation in solutions of a model di-block copolymer

Abstract In the selective solvent, dodecane, the di-block copolymer polystyrene- b -(ethylene- co -propylene), with a narrow molecular-weight distribution, forms micelles with polystyrene cores. Small-angle neutron scattering experiments were used to investigate both core shape and dimensions and the intercore structure factors as temperature or concentration were varied. Monodisperse spherical structures with radii around 120 A were observed. Above a critical concentration these were arranged in relatively ordered structures in which preferred orientations could easily be induced. The core diameter and the intercore spacing were dependent on sample thermal history. During shear the intercore structure became less ordered. The structural results correlate well with measurements of the dynamic viscosity measured in oscillatory shear, which also show a sharp change from gel-like to liquid behaviour at this critical concentration. Data are compared to model calculations in the regions where the particle form factor or where the interparticle structure factor dominate. In the latter case a hard core potential with a soft tail is found to give reasonable agreement with the data, and to allow changes with shear rate, with concentration or with temperature to be interpreted.

Studies of cyclic and linear poly(dimethyl siloxanes): 12. Observation of diffusion behaviour by quasielastic neutron scattering

Abstract High resolution neutron scattering experiments have been used to observe the diffusive motion of low molecular weight linear and cyclic poly(dimethyl siloxane) molecules in dilute solution in deuterated benzene. Diffusion coefficients (D) and hydrodynamic radii (RH) have been compared with values obtained by light scattering for higher molecular weight samples and with radii of gyration (Rg) obtained by small-angle neutron scattering. While the ratio D ring D chain is close to the predicted value of 0.85, the ratio R g R H falls below the theoretical value for both ring and chain molecules. The scattering curves show effects arising from both centre of mass diffusion and internal molecular motion, and the observed inverse correlation times are compared with calculated behaviour as a function of scattering vector, Q.