Stephen Jenkins

Permeability of N2, Ar, He, O2 and CO2 through biaxially oriented polyester films : dependence on free volume

Permeability, diffusion and solubility coefficients are reported for biaxially orientated polyester films based on poly(ethylene terephthalate) [PET], poly(ethylene naphthalate) [PEN] and copolymers containing PET and PEN moieties. Data for cast amorphous sheets and materials produced with different biaxial draw ratios are compared. The crystallinity of the samples was assessed using differential scanning calorimetry and density measurements. The changes in the void structure at a molecular level were investigated using positron annihilation lifetime spectroscopy (PALS). The variation of the gas diffusion behaviour with the gas used (carbon dioxide, nitrogen, argon, helium and oxygen) reflects the effects of change in morphology on the solubility and diffusivity components of the permeability. The diffusivity of the gas is influenced not only by both the changes in the void size and content at a molecular level, but also by the effects of crystallinity on the percolation behaviour of the gas through the matrix. Changes in the extent of chain alignment also have a profound affect on the solubility of the gas in the matrix. The observed behaviour for the gas permeation can be interpreted as being the result of the complex interplay of changes in the crystalline content, the polymer chain alignment and the void structure of the amorphous phase.

Dielectric, mechanical and rheological studies of phase separation and cure of a thermoplastic modified epoxy resin : incorporation of reactively terminated polysulfones

Abstract Application of dielectric, mechanical and rheological measurements to the characterization of cure and phase separation in a thermoplastic modified epoxy resin is reported. These data indicate that the dielectric technique is not only useful for monitoring changes in network dynamics during the process of cure, but can also provide valuable information on the phase separation of the occluded phase. The paper discusses the modelling of the dielectric data, and considers the importance of understanding the relationship between morphology and mechanical properties.

Investigation of thermoplastic-modified thermosets: positron annihilation and related studies of an amine-cured epoxy resin

Abstract Real-time dielectric relaxation and rheological measurements are reported during the course of cure for three thermoplastic-modified thermoset resin blends. Two thermoset resin systems were used, one containing only a difunctional epoxy and the other containing a mixture of tri- and difunctional epoxies. The latter mixture was used to react two reactively terminated polysulfone thermoplastics with slightly different molecular weights. Positron annihilation measurements are reported on one of the thermoplastic-thermoset resin systems as a function of the thermoplastic composition, and indicate that changes in composition lead to variations in the free-volume content. Mechanical properties are presented for all three systems. These data indicate that the phase structure has a significant effect on mechanical performance, but the most dominant effect results from change in the thermoset matrix material.

Physical ageing in poly(ethylene terephthalate) : Its influence on cold crystallisation

The relationship between the crystallisation behaviour and physical ageing history of poly(ethylene terephthalate) is reported. It is shown that short term ageing can enhance the subsequent rate of crystallisation, but longer ageing times can retard the crystallisation process. It is suggested that physical ageing favours the formation of ordered domains.

Enthalpy relaxation in poly(ethylene terephthalate) and related polyesters

Enthalpic relaxation data are presented on poly(ethylene terephthalate), poly(ethylene naphthalate) and their copolymers. Analysis of these data allows the determination of the amount of energy absorbed at the glass transition, Qt, and the location of the enthalpic recovery peak, Tmax, as a function of the time of ageing of the samples. Ageing measurements were carried out for periods of up to 2016 h and at temperatures between 40 °C and 110 °C, depending upon the chemical composition of the system being investigated. The enthalpic relaxation rates are influenced by the chemical structure and reflect the effects of local order pinning the chains and influencing the rate of enthalpic recovery.