M. Ashraf Alam

Zeolitic imidazolate framework (ZIF-8) based polymer nanocomposite membranes for gas separation

As synthesised ZIF-8 nanoparticles (size ∼ 60 nm and specific surface area ∼ 1300–1600 m2 g−1) were directly incorporated into a model polymer matrix (Matrimid® 5218) by solution mixing. This produces flexible transparent membranes with excellent dispersion of nanoparticles (up to loadings of 30 wt%) with good adhesion within the polymer matrix, as confirmed by scanning electron microscopy, dynamic mechanical thermal analysis and gas sorption studies. Pure gas (H2, CO2, O2, N2 and CH4) permeation tests showed enhanced permeability of the mixed matrix membrane with negligible losses in selectivity. Positron annihilation lifetime spectroscopy (PALS) indicated that an increase in the free volume of the polymer with ZIF-8 loading together with the free diffusion of gas through the cages of ZIF-8 contributed to an increase in gas permeability of the composite membrane. The gas transport properties of the composite membranes were well predicted by a Maxwell model whilst the processing strategy reported can be extended to fabricate other polymer nanocomposite membranes intended for a wide range of emerging energy applications.

Confinement induces both higher free volume and lower molecular mobility in glycerol

We report measurements of the local free volume and mobility of a glass-forming liquid (glycerol) confined in a mesoporous silica glass. The lower molecular mobility in confinement, measured by neutron scattering spectroscopy, is accompanied by a higher mean free volume size between molecules, measured by positron annihilation lifetime spectroscopy. The confined liquid appears to be perturbed to such an extent that the normally observed free volume/mobility relationship is reversed. This study shows that these effects originate locally at a molecular level.

Hydrogen bonding in maltooligomer-glycerol-water matrices: Relation to physical state and molecular free volume.

We use Fourier Transform Infra Red (FTIR) Spectroscopy to explore the effects of water and glycerol on the hydrogen bonding of low water content maltooligomer matrices by monitoring the shifts in the position of the peak associated with the fundamental stretching vibration of the hydroxyl groups, νOH. Changes in hydrogen bonding are investigated in relation to the physical state and the molecular packing of the maltooligomer matrices, which are measured by Positron Annihilation Lifetime Spectroscopy (PALS). In the concentration range studied (0-20 wt.%), glycerol acts as an anti-plasticizer whereby it reduces the average molecular hole size, vh, while modulating the hydrogen bond network of the carbohydrate matrices. Depending on the level of hydration, water can cause anti-plasticization or plasticization of the maltooligomer-glycerol matrices. For water contents below ∼ 5 wt.%, water acts as an anti-plasticizer, whereby it reduces vh and we measure a reduction νOH. At higher water contents, water acts as a plasticizer, causing a systematic increase in vh, while νOH continues to decrease as a function of increasing water content.

Stress Transfer Quantification in Gelatin-Matrix Natural Composites with Tunable Optical Properties

This work reports on the preparation and characterization of natural composite materials prepared from bacterial cellulose (BC) incorporated into a gelatin matrix. Composite morphology was studied using scanning electron microscopy and 2D Raman imaging revealing an inhomogeneous dispersion of BC within the gelatin matrix. The composite materials showed controllable degrees of transparency to visible light and opacity to UV light depending on BC weight fraction. By adding a 10 wt % fraction of BC in gelatin, visible (λ = 550 nm) and UV (λ = 350 nm) transmittances were found to decrease by ∼35 and 40%, respectively. Additionally, stress transfer occurring between the gelatin and BC fibrils was quantified using Raman spectroscopy. This is the first report for a gelatin-matrix composite containing cellulose. As a function of strain, two distinct domains, both showing linear relationships, were observed for which an average initial shift rate with respect to strain of -0.63 ± 0.2 cm(-1)%(-1) was observed, followed by an average shift rate of -0.25 ± 0.03 cm(-1)%(-1). The average initial Raman band shift rate value corresponds to an average effective Youngs modulus of 39 ± 13 GPa and 73 ± 25 GPa, respectively, for either a 2D and 3D network of BC fibrils embedded in the gelatin matrix. As a function of stress, a linear relationship was observed with a Raman band shift rate of -27 ± 3 cm(-1)GPa(-1). The potential use of these composite materials as a UV blocking food coating is discussed.

Phase separation in amorphous hydrophobically modified starch–sucrose blends: Glass transition, matrix dynamics and phase behavior

The phase behavior and matrix dynamics of amorphous blends of octenyl succinic anhydride (OSA) modified starch and sucrose was studied as function of blend composition and water content. Phase separation into two amorphous phases, one enriched in OSA starch and the other in sucrose, was confirmed by differential scanning calorimetry (DSC). DSC and 1H solid-state NMR show that the phase separation is only partial. The glass transition temperature (Tg) of the OSA starch-rich phase was found to be ∼30-100 K higher than the Tg of the sucrose-rich phase, depending on blend composition and water content. A novel type of coupling between changes in physical state of the sucrose-rich phase and plasticizer redistribution is proposed, leading to an unexpected increase of the glass transition temperature of the modified starch-rich phase at higher matrix water contents. A quantitative model for the phase separation of the anhydrous blends into two amorphous phases is presented. The model predicts that, with increasing blend sucrose content, the weight fraction of the sucrose-rich phase decreases, while the sucrose content of both the OSA starch-rich phase and the sucrose-rich phase increases. This novel phenomenon is relevant in the understanding of the stability and performance of multiphase food and pharmaceutical components.

Molecular packing of carbohydrate oligomer encapsulants - a free volume perspective

Positron annihilation lifetime spectroscopy (PALS) is used in conjunction with dilatometry to analyse the effects of water and low molecular weight diluents (maltose and glycerol) on the molecular organisation and density of carbohydrate oligomers commonly used in the pharmaceutical and food industries for the formulation of encapsulation matrices. In the glassy state, both maltose and glycerol act as packing enhancers, causing a non-linear decrease in the average molecular hole size of the carbohydrate matrices. Water exists in a highly non-ideal state in these systems and it alters the molecular organisation of the matrices in a complex manner, whereby it can act either as an anti-plasticiser or a plasticiser, depending on the level of hydration.

Positrons and Free Volume Related Issues in Polymeric Systems: A Sketch of Günter Dlubek's Contributions

Günter was a well respected member of the positron/positronium community of which he has been a part of since the early 1970’s. During his early career, his research concentrated on the studies of metallic systems, particularly of defects and defects related problems in metals and alloys, followed by defects in semiconductors. In the mid nineties, he made a transition to the application of positron annihilation in polymeric materials. Here, he made substantial contributions to the science both within the positron community in terms of the applicability of the technique, aspects of data analysis and correlation between free volume and polymer properties which had impact in in the broader polymer community. Sadly, after suffering from chronic illness for more than 20 years, Günter passed away on the 19th of February 2011 following a medical emergency. This brief note is intended to be a tribute on behalf of the positron community and provides, in the form of a few examples, a sketch (rather than a review) of his contributions to polymeric materials which formed the bulk of the lalest stage of his work. Most of Günter’s positron-polymer work was carried out in collaboration with a number of positron as well as polymer groups. This started from his time as a ‘Benjamin Meaker’ visiting professor with the Bristol Positron Group (UK), in 1994, a collaboration which continued until recent years. Over the years, his positron collaboration expanded to include the Halle positron group and in more recent years the group in Kiel. During this time, Günter also collaborated with a number of Polymer related groups e.g. Departments of Chemistry and Materials Science, University of Halle; Max Planck Institute, Mainz; Leibniz Institute of Polymer Research, Dresden. Günter’s initial work on polymers concentrated with the further understanding of the correlation between ortho-positronium lifetimes / lifetime distributions and the size and size distributions of the corresponding free volume holes. The early collaborative work involved the systematic evaluation of the nature of the free volume as a function of the ‘state/composition’ of polymers e.g. looking at the effects of co-polymerisation, cross linking, branching and partial crystallinity (see for example [1]). Below is an example of the effect of copolymerization of diethylene glycol bis(allyl carbonate), Materials Science Forum Online: 2012-11-29 ISSN: 1662-9752, Vol. 733, pp 1-4 doi:10.4028/www.scientific.net/MSF.733.1