Duncan Kilburn

Free volume and glass transition in ethylene/1-octene copolymers: positron lifetime studies and dynamic mechanical analysis

Semicrystalline ethylene/1-octene copolymers (P(E-co-O)) with a content of 1-octene comonomer, wO, between 7 and 24 wt% and a crystallinity Xc between 50 and 10% and a high-density polyethylene (HDPE, Xc ¼ 70%) were studied in the temperature range between 115 K and (300– 350) K by positron annihilation lifetime spectroscopy (PALS) and dynamic mechanical analysis (DMA). The samples were characterised by differential scanning calorimetry (DSC) and wide-angle X-ray scattering (WAXS). The Tgs estimated from the thermal expansion of the local free volume estimated from the PALS data decrease linearly with wO between 251 K (extrapolated to wO ¼ 0) and 220 K (wO ¼ 24 wt%). They agree very well with those estimated from DMA (loss modulus E 00 ) and DSC. However, the Tgs obtained from the maximum of tan d do not agree with all of the others, inparticular for higher crystallinities. For HDPE, any indication of a glass transition could not be observed. From this it was concluded that the whole amorphous phase in this highly crystalline polymer is immobile. From the variation of the thermal expansivity of the local free volume the fractions of the mobile and immobile amorphous phase are estimated. It was observed that the o-Ps intensity shows a minimum at Tmin < Tg þ 10 K. q 2002 Elsevier Science Ltd. All rights reserved.

Studies of interdiffusion in polymer blends by PALS

Abstract It is shown that positron annihilation parameters such as the average positron lifetime τav and the ortho-positronium intensity I3 respond to chemical inhomogeneities in disperse blends and their disappearance due to mutual diffusion of molecules of both phases, when Ps formation is inhibited by one of the constituents. This effect can be used to study mixing in initially demixed compatible blends, demixing in initially mixed incompatible blends, and phase diagrams. With a slow positron beam the concentration-distance profile in thin planar double layer films may be scanned systematically.

A nano-scale free volume perspective on the glass transition of supercooled water in confinement

We employ positron annihilation lifetime spectroscopy (PALS) to measure the temperature dependent fluctuations in a local free volume of water confined in a variety of mesoporous materials with different pore sizes and morphologies. We demonstrate that this unconventional approach can be used to probe the dynamics and glass transition temperature, Tg, of water in confinement. In the simplest case of water confined in a 13X molecular sieve (d = 7.4A), the confinement is so severe that it precludes crystallisation altogether, and we measure a Tg = 190 ± 2K. We also show that temperature dependent PALS measurements can be used to probe the glass transition of water molecules confined in matrices with larger pore diameters (SBA-15, KIT-6 and Vycor glass), where the confinement is expected to be less severe.

Structural evolution of the dihydrate to anhydrate crystalline transition of trehalose as measured by wide-angle X-ray scattering.

Wide-angle X-ray scattering measurements were performed to record structural changes during the transition from trehalose dihydrate to crystalline anhydrous alpha-trehalose. The results show that large dihydrate crystals rearrange into smaller sized alpha crystals; from the peak widths we calculate a crystallite size of typically approximately 40 trehalose molecules. We find that the dehydration probably takes place in a two-step process with different time scales for both the water removal step and the molecule rearrangement step. This suggests that there is crystal rearrangement in the dry state some 60 degrees C below the dry glass transition temperature of trehalose, which is unusual for a relatively large and strongly interacting molecule.