Alexander M. Jamieson

Studies of protein adsorption on polystyrene latex surfaces

Abstract Adsorption isotherms of bovine serum albumin and y-globulin on polystyrene latices are reported. Under certain conditions, for each protein, the isotherms exhibit bimodal adsorption characteristics in confirmation of earlier reports. The maximal adsorbance values are near expectation for a close-packed monolayer. It is noted that the ratio of the lower to upper plateau adsorbance values are 0.40 for the γ-globulin case and 0.56 for the serum albumin examples. The hydrodynamic thickness of the adsorbed layer as a function of the amount of adsorbed protein has been characterized by photon correlation spectroscopy analysis. No evidence for multilayer adsorption at higher coverages or conformational “flattening” of the adsorbed protein in the lower plateau regime could be established. Based on this information, we propose a mechanism for bimodal adsorption which does not require major conformational changes, and which postulates the existence of a critical supersaturation ratio for homogeneous nucleation of a close-packed two-dimensional surface “crystal” of protein. Below this concentration, the development of the ordered structure is kinetically limited by the finite configurational relaxation time of the protein at the surface. In this regime, a less dense partially organized “glassy” monolayer is found. The lower plateau regime involves random independent adsorption of isolated molecules.

Positron annihilation lifetime studies of changes in free volume on cross-linking cis-polyisoprene, high-vinyl polybutadiene, and their miscible blends

Measurements of average free volume hole sizes, 〈vf〉, and the fractional free volumes, fps, in vulcanized cis-polyisoprene (CPI), high-vinyl polybutadiene (HVBD), and their 50 : 50 blend were made via determination of orthopositronium annihilation lifetimes. The results are compared to corresponding data on the uncured materials. On crosslinking, 〈vf〉 decreases in the rubbery state but remains essentially unchanged in the glass. This is consistent with the expectation that the crosslinks greatly restrict the thermal expansion of the chains above the glass transition temperature (Tg) but have less influence on the packing density in the glass. Scaling relationships between 〈vf〉, fps, the thermal expansion coefficient αf = dfps/dt, and Tg are examined. We find that 〈vf〉g, the hole volume at Tg, and fps,g, the fractional free volume at Tg, each increase significantly with increasing Tg. This behavior is consistent with previous observations reported in the literature and has been interpreted as a manifestation of the kinetic character of the glass transition. High-Tg polymers need a larger free volume to pass into the liquid state. The change in expansion coefficient on passing from the glass to the liquid, Δαf = αf,l − αf,g, increases slowly with Tg, as predicted by free volume theory.

Viscoelastic behaviour at the thermal sol-gel transition of gelatin

Rheological studies of gelatin solutions in the concentration range 4–14% were performed through the sol-gel transition as a function of concentration, pH and ionic strength. At the gel point, a power-law frequency dependence of the viscoelastic functions G′(ω), G″(ω) and η∗(ω) was observed. The power-law exponents n obtained from these dynamic measurements were confirmed by creep experiments and are in agreement with a previous study in a lower concentration range, 1–5%. Under all conditions, the n values fall between 0.64 and 0.72, a range consistent with theoretical treatments of Martin et al. and Muthukumar et al. The critical gel strength, S, increases with concentration approximately as S ∝ C1.33 in the concentration range 4–14%. Within experimental error, S is approximately consistent with an empirical relation suggested in the recent literature, S = G1 − neηno, where Ge and ηo are, respectively, the equilibrium modulus of the fully developed gel and the shear viscosity of the initial sol. The implications of these observations with respect to the structure of the critical gel are discussed.

Conformation of xanthan dissolved in aqueous urea and sodium chloride solutions

Abstract Quasielastic light-scattering and other physical-chemical techniques have been used to compare the conformation and intermolecular interactions of xanthan in water, aqueous sodium chloride, and urea solutions. The results showed that xanthan dissolved in 4 m urea has a disordered conformation after the solution has been maintained for 3 h at 95° and then cooled to room temperature. This conformation is similar to that previously observed only in solutions having low ionic strength at higher temperatures, following disruption of the ordered, low-temperature form. “Anomalous” behavior is seen for xanthan as a function of ionic strength, in that the hydrodynamic radius increases with increase in ionic strength, whereas a decrease is typical for polyelectrolytes. These observations suggest that aggregation of rod-like chains, similar to that seen for other stiff-chain polymers, occurs for xanthan in salt solutions, where the charged groups of the polyelectrolyte are screened by the salt ions. This aggregation may explain some of the high values reported in the literature for the molecular weight.

Self-association of xanthan in aqueous solvent-systems

Abstract Quasi-elastic light-scattering studies of the polysaccharide xanthan in aqueous solution reveal the existence of two different structural forms characterized by different values of the translational diffusion coefficient D . Xanthan dissolved in deionized water yields a value of D  2.40 × 10-8 cm2/s, provided that the solution is centrifuged or filtered immediately prior to data collection. However, if this solution is allowed to stand for one week, D decreases to 1.10 × 10-8 cm2/s. In contrast, D  2.75 × 10-8 cm2/s for solutions of xanthan dissolved in 4 m urea, and this value does not change with time. These results suggest that xanthan undergoes a self-association in aqueous solution and that this association is inhibited in 4 m urea solution, which points to the role of hydrogen bonding in the process. Insertion of our measured values of D and the zero-shear intrinsic viscosity into the Flory-Mandelkern equation yields an estimated molecular weight of M  2.16 × 106 for xanthan in 4 m urea. The aggregation observed in aqueous solutions probably explains the wide variation in molecular weights reported previously for xanthan.

Mutual translational diffusion coefficients in bovine serum albumen solutions measured by quasielastic laser light scattering

Abstract Concentration dependence of the translational diffusion coefficient of monomeric bovine serum albumen has been studied in three particular aqueous solvent systems. Two concentration series at ionic strength I = 0.15 and pH 7.4, and I = 0.10 and pH 4.7, respectively, extended to volume fractions φ ∼ 0.3. These data and literature data are discussed with regard to recent theoretical treatments of hard-sphere diffusion. The importance of including the effect of an appropriate screened coulombic potential in modeling protein diffusion is emphasized. The data are regarded as strong quantitative support for the theoretical treatment by Anderson and Reed [ J. Chem. Phys. 64, 3240 (1976)] of macromolecular diffusion when interparticle interactions are repulsive in nature. A third set of experiments investigated the concentration dependence of D t in salt-free serum albumen solutions at the isoionic point where strong attractive electrodynamic forces are present. These data are in qualitative agreement with theoretical prediction but quantitative discrepancies are found.

Free volume changes in polyvinyl acetate measured by fluorescence spectroscopy

Abstract Fluorescence probes dispersed randomly in polyvinylacetate (PVAc) were used to follow changes in the matrix free volume with temperature and during isothermal physical ageing in the glassy state. The emission intensity of Auramine O shows a change in temperature coefficient at the glass transition temperature of PVAc. During isothermal ageing, after a quench from the equilibrium melt, the emission intensity shows a time-dependent increase, interpreted as due to the relaxation of matrix free volume. The change in Auramine O fluorescence intensity with ageing time is compared with the increase in the characteristic time for momentary stress relaxation of PVAc. These properties are found to be qualitatively self-consistent using theoretical arguments based on free volume concepts.

Temporary droplet-size hysteresis in immiscible polymer blends

The droplet size distribution during steady shearing of model polymer blends is examined in situ by optical microscopy. The volume-average steady-state droplet size during shear is essentially inversely proportional to shear rate, as expected. When the shear rate is increased suddenly, the droplets break up, through a process that involves the transient formation of threads, and rapidly establish a new steady state, comprising ellipsoidal droplets that are extended slightly in the direction of shear. When the shear is stopped, the droplets quickly relax to a spherical shape, but virtually no coalescence is observed, because neither Brownian nor buoyant forces are significant and the volume fraction of the dispersed phase is low. Slow shear, however, induces droplet collisions that lead to coalescence. The coalescence process is much slower than breakup. In contrast to some predictions, however, there is no permanent droplet size hysteresis. The steady-state size produced by breakup of initially larger droplets is eventually produced at large strain by coalescence of initially smaller droplets. The lack of permanent hysteresis has implications concerning appropriate mathematical models of coalescence behavior.

Temperature and time dependence of orthopositronium formation in polystyrene

Abstract Three measurement protocols were performed to investigate the temperature dependence of positron lifetime spectra for polystyrene over the temperature range −70 to 180°C. The polystyrene samples were first subjected to a heating schedule in sequential 10°C increments. This was followed by a cooling schedule, again in 10°C increments. The third schedule involved rejuvenation of the specimen above T g between each measurement. The measured lifetime spectra were resolved into three components. We observed a minimum in the temperature dependence of the o-Ps intensity I 3 at ∼20°C for polystyrene specimens subjected to the heating and cooling schedules, but not for the rejuvenated samples. The influence of the duration of e + -irradiation on the lifetime and intensity of o-Ps annihilation was investigated under isothermal conditions. A decrease in o-Ps intensity I 3 with duration of exposure to e + -irradiation was found whose rate was maximal at 23°C. The results suggest that the time dependence of the o-Ps intensity in the glass can be interpreted within the framework of the spur-model. Specifically, the variation of I 3 is ascribed to the formation at low temperature of free radicals with small electron affinities. After correcting for the e + -irradiation effect, no significant aging effect in the glassy state was detected in the o-Ps intensity and lifetime.

Rheological studies of the interaction of mucins with alginate and polyacrylate

The associative interaction of purified ovine and porcine submaxillary mucins (OSM and PSM) with sodium alginate was evaluated by comparing the rheological properties of mixtures against those of pure alginate and mucin in dilute, semi-dilute, and concentrated solutions. These systems were investigated as models for the interaction of mucin with the extracellular alginate produced by Pseudomonas aeruginosa. In dilute solution, evidence for such interaction cannot be obtained because aggregate species exist both in the OSM-alginate mixtures as well as in pure OSM. However, in the semi-dilute regime, mixtures containing a higher proportion of mucin show systematically higher viscosities than those predicted by simple additivity. In concentrated solutions containing higher proportions of mucin, an enhanced elastic response is observed. These results demonstrate a substantial binding interaction of mucins with alginate. This property is not observed in mixtures containing a high proportion of alginate, suggesting that mucins possess relatively low numbers of interacting sites. Introduction of 3 mM Ca2+ ions to all mucin-alginate mixtures enhances the elasticity due to gelation of alginate. Finally, comparison of the rheological properties of PSM-alginate mixtures with those of PSM-polyacrylate mixtures indicates that the binding strength of alginate to mucin is significantly weaker than that of polyacrylate.