Peter R. Laity

Observations of water migration during thermoporometry studies of cellulose films

Abstract Thermoporometry is widely used for measuring pore size distribution in porous materials by differential scanning calorimetry based on melting point depression and the Gibbs–Thomson effect shown by a liquid contained in the pores. However, measurements on water-swollen cellophane showed that the shapes of heat flow vs. temperature plots (and, therefore, derived pore size distributions) changed with heating rate. Results obtained in heating were the product of two sequential and overlapping effects: an endothermic melting of the smallest ice crystals within pores in the film, followed by diffusion of the water and an exothermic re-freezing on larger crystals at the film surface. This caused the volume of larger pores to be underestimated or missed completely. In contrast, diffusion out of the smallest pores preceded freezing for measurements on cooling, so only the larger pores were observable. The rate of diffusion was estimated as 1×10 −13 m 2 s −1 from the quantities of ice in pores and at the film surface observed at different heating rates. The value obtained was much slower than expected for water-swollen cellulose, but consistent with water diffusion through dry cellulose, suggesting that the film surface had been ‘freeze-dried’ during the measurements.

High strain rate properties of a polymer-bonded sugar: their dependence on applied and internal constraints

This paper describes research performed on a polymer-bonded sugar (PBS) consisting of 66% caster sugar in a hydroxyl-terminated polybutadiene (HTPB) binder The mechanical response of the PBS and pure HTPB to applied loading at a strain rate of approximately 2000 s−1 at temperatures from −80 to +22°C is presented. The materials were also characterized using dynamic mechanical analysis, X-ray tomography and quasi-static loading. These measurements are required for the development of intermediate strain rate constitutive models of polymer-bonded explosives, for which PBSs are a commonly used mechanical simulant. The current constitutive modelling suffers from a lack of experimental data on well-characterized composites and binders, especially at intermediate strain rates. This is particularly important for understanding the effects of mixing two materials. Applications of such modelling include explosive safety and fundamental understanding of the various deformation mechanisms. In this paper, the dependences of strength and deformation mechanism on temperature, and, in particular, the glass transition temperature of the binder, are shown. Physical damage plays an important role; X-ray tomography scans support debonding as the primary form of damage during room-temperature deformation. These results are in agreement with previous investigations and are discussed in this context.

Magnetic resonance imaging and X-ray microtomography studies of a gel-forming tablet formulation

The capabilities of two methods for investigating tablet swelling are investigated, based on a study of a model gel-forming system. Results from magnetic resonance imaging (MRI) were compared with results from a novel application of X-ray microtomography (XmicroT) to track the movements of embedded glass microsphere tracers as the model tablets swelled. MRI provided information concerning the movement of hydration fronts into the tablets and the composition of the swollen gel layer, which formed at the tablet surface and progressively thickened with time. Conversely, XmicroT revealed significant axial expansion within the tablet core, at short times and ahead of the hydration fronts, where there was insufficient water to be observed by MRI (estimated to be around 15% by weight for the system used here). Thus, MRI and XmicroT may be regarded as complementary methods for studying the hydration and swelling behaviour of tablets.

Monolithic route to efficient dye-sensitized solar cells employing diblock copolymers for mesoporous TiO2

We present a material and device based study on the fabrication of mesoporous TiO2 and its integration into dye-sensitized solar cells. Poly(isoprene-block-ethyleneoxide) (PI-b-PEO) copolymers were used as structure directing agents for the sol–gel based synthesis of nanoporous monolithic TiO2 which was subsequently ground down to small particles and processed into a paste. The TiO2 synthesis and the formation of tens of micrometre thick films from the paste is a scalable approach for the manufacture of dye sensitised solar cells (DSCs). In this study, we followed the self-assembly of the material through the various processing stages of DSC manufacture. Since this approach enables high annealing temperatures while maintaining porosity, excellent crystallinity was achieved. Internal TiO2 structures ranging from the nanometre to micrometre scale combine a high internal surface area with the strong scattering of light, which results in high light absorption and an excellent full-sun power conversion efficiency of up to 6.4% in a robust, 3 μm thick dye-sensitized solar cell.

The influence of agitation sequence and ionic strength on in vitro drug release from hypromellose (E4M and K4M) ER matrices--the use of the USP III apparatus.

Theophylline extended release (ER) matrices containing hypromellose (hydroxypropyl methylcellulose (HPMC) E4M and K4M were evaluated in media with a pH range of 1.2-7.5, using an automated USP type III, Bio-Dis dissolution apparatus. The objectives of this study were to evaluate the effects of systematic agitation, ionic strength and pH on the release of theophylline from the gel forming hydrophilic polymeric matrices with different methoxyl substitution levels. Tribo-electric charging of hypromellose, theophylline and their formulated blends containing E4M and K4M grades has been characterised, along with quantitative observations of flow, compression behaviour and particle morphology. Agitations were studied at 5, 10, 15, 20, 25, 30 dips per minute (dpm) and also in the ascending and descending order in the dissolution vials. The ionic concentration strength of the media was also varied over a range of 0-0.4M to simulate the gastrointestinal fed and fasted states and various physiological pH conditions. To study the effect of ionic strength on the hydrophilic matrices, agitation was set at 20 dpm. The charge results on individual components imply that the positively charged particles have coupled with the negatively charged particles to form a stable ordered mixture which is believed to result in a more homogeneous and stable system. The particle shape analysis showed the HPMC K4M polymer to have a more irregular morphology and a rougher surface texture in comparison to the HPMC E4M polymer, possibly a contributory factor to the gelation process. The results showed gelation occurred quicker for the K4M tablet matrices. Drug release increased with increased agitation. This was more pronounced for the E4M tablet matrices. The ionic strength also had more of an effect on the drug release from the E4M matrices. The experiments highlighted the resilience of the K4M matrices in comparison with the E4M matrices. The results thus show that despite similar viscosities of E4M and K4M, the methoxyl substitution makes a difference to their control of drug release and as such care and consideration should be given to the choice of polymer used for extended release. The use of systematic change of agitation method and ionic strength may indicate potential fed and fasted effects on drug release from hydrophilic matrices.

Mechanical Deformation of Polyurethanes

Abstract The mechanical properties of thermoplastic polyurethanes (TPU) depend upon their composition and the complex two‐phase morphologies, which originate from microphase separation of chains segments. In the present work, poly(ether‐urethanes) were prepared with hard segment contents from 36% to 71% by weight, by systematically varying the length of the soft‐segment macrodiol. Samples were prepared by hot pressing or solvent casting, and the resulting hard‐ and soft‐segment morphologies were characterized by using small‐angle x‐ray scattering (SAXS) and transmission electron microscopy (TEM). Environmental scanning electron microscopy (ESEM) was used to study the fracture surfaces of TPU samples. The deformation behavior of the morphology was studied in real time, by using 2‐dimensional SAXS (2D‐SAXS) at the Daresbury synchrotron radiation source. Two distinct mechanisms were identified, with the dominant mechanism in a given material dependent on the copolymer composition and the extent of microphase separation, which developed during processing.

Composition and phase changes observed by magnetic resonance imaging during non-solvent induced coagulation of cellulose

The coagulation of cellulose from solution in N-methylmorpholine-N-oxide by water was studied by stray field magnetic resonance imaging. Changes in composition and diffusion were readily observable at different depths within the cellulose gel. It was found that the composition changed relatively quickly, due to diffusive exchange of water and solvent, such that cellulose precipitation occurred deep within the bi-phasic region of the phase diagram. The expected spinodal decomposition mechanism was supported by electron microscopic observations of the cellulose morphology. Large increases in diffusion coefficient and relaxation times were attributable to the phase separation and the progressive development of pores within the gel. Moreover, it appeared that the composition changes and evolution of morphology occurred over separate time-scales.

Structural studies and diffusion measurements of water-swollen cellophane by NMR imaging

NMR imaging and spatially resolved diffusometry have been used to study the distribution of water within swollen cellophane and measure its diffusion coefficient. Water concentration and diffusion coefficient were found to be essentially constant across most of the film thickness. However, significantly slower diffusion was indicated for water near the film surface (D = 0.5 × 10−9 m2 s−1) compared with water in the centre of the film (D = 0.88 × 10−9 m2 s−1). This was also reflected in lower T2 values at the edge of the film indicating water with more restricted motion. These observations were interpreted in terms of dense surface regions of cellulose (skin) over a more porous interior (core).

Native Silk Feedstock as a Model Biopolymer: A Rheological Perspective.

Variability in silks rheology is often regarded as an impediment to understanding or successfully copying the natural spinning process. We have previously reported such variability in unspun native silk extracted straight from the gland of the domesticated silkworm Bombyx mori and discounted classical explanations such as differences in molecular weight and concentration. We now report that variability in oscillatory measurements can be reduced onto a simple master-curve through normalizing with respect to the crossover. This remarkable result suggests that differences between silk feedstocks are rheologically simple and not as complex as originally thought. By comparison, solutions of poly(ethylene-oxide) and hydroxypropyl-methyl-cellulose showed similar normalization behavior; however, the resulting curves were broader than for silk, suggesting greater polydispersity in the (semi)synthetic materials. Thus, we conclude Nature may in fact produce polymer feedstocks that are more consistent than typical man-made counterparts as a model for future rheological investigations.

Formulation and evaluation of floating mucoadhesive alginate beads for targeting Helicobacter pylori

There are various obstacles in the eradication of Helicobacter pylori infections, including low antibiotic levels and poor accessibility of the drug at the site of the infection. This study describes the preparation and characterisation of novel floating mucoadhesive alginate beads loaded with clarithromycin for delivery to the gastric mucosa to improve the eradication of this microorganism.