Biomedical subjects

PF Luckham

Publications and source records attributed to PF Luckham.

5 recordsLinked to original sources

PubMed1999

Effect of Particle Size Distribution on the Rheology of Dispersed Systems.

The rheological properties of aqueous polystyrene latex dispersions from three synthetic batches, with nearly the same z-average particle sizes, 400 nm, but varying degrees of polydispersity, 0.085, 0.301, and 0.485, respectively, were systematically investigated using steady-state shear and oscillatory shear measurements. The particles were sized with photon correlation spectroscopy and transmission electron microscopy and were stabilized sterically with PEO-PPO-PEO triblock copolymer (Synperonic F127). Results from steady-state shear measurements show that the viscosities of the systems exhibit shear-thinning behavior at high solid fractions. However, the degree of shear thinning depends on the breadth of particle size distribution, with the narrowest distribution suspension exhibiting the highest degree of shear thinning. The Herschel-Bulkley relationship best describes the flow curves. The relative viscosities as a function of volume fraction data were compared, and it was found that the broadest distribution suspension had the lowest viscosity for a given volume fraction. In addition, the data were fitted to the Krieger-Dougherty equation for hard spheres. A reasonable agreement of theory with experiment is observed, particularly and surprisingly for the very broad distribution. However, when the contribution to the volume due to the adsorbed polymer layer is considered, the agreement between experiment and theory becomes closer for all the suspensions, although the agreement for the broad distribution suspension is now worse. Fitting the Dougherty-Krieger theory to the experimental data based on our experimental maximum packing fractions gives very good agreement for all the systems studied. From oscillatory shear measurements, the moduli were obtained as a function of frequency at various latex volume fractions. The results show general change of the dispersions from viscous (G" > G') at low volume fractions (0.25-0.30) to moderately elastic (G' > G") at moderately high volume fractions (0.41-0.45). The change at this concentration level is likely due to some compression and interpenetration of the stabilizing polymer chain at the periphery, indicating the dominance of the interparticle forces. Overall, the very broad distribution was found to have the lowest elastic modulus for a given volume fraction. Copyright 1999 Academic Press.

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PubMed1999

The Simultaneous Determination of the Forces and Viscoelastic Properties of Adsorbed Polymer Layers.

The viscoelastic properties of a solvated adsorbed film of gelatin on glass have been determined by modifying a particle forces apparatus which is a derivative of an atomic force microscope (AFM). Here a particle is attached to the end of an AFM lever. Gelatin is allowed to adsorb to the particle and to a flat glass surface. The surface is driven toward the particle using a piezoelectric ceramic and the deflection of the lever monitored using a laser beam and a position-sensitive photodiode. In the experiments reported here a relatively low frequency oscillation (20-1000 Hz) is superimposed on the linear motion. By examining the in-phase and out-of-phase responses of the particle to the applied oscillation, the viscous and elastic properties of the gap may be determined with separation. This information is obtained simultaneously with the forces of interaction between the gelatin-coated glass surfaces. The data show that a viscous response dominates until the gelatin layers interact. After contact the elastic response increases and dominates at high compression of the adsorbed gelatin layers. Problems concerning the quantitative analysis of the data in terms of viscous and elastic moduli are discussed. Copyright 1999 Academic Press.

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PubMed1999

Study of a Solvated Adsorbed Gelatin Layer Using a Modified Force Microscope.

The interaction energies between gelatin-coated surfaces at various electrolyte and pH conditions are reported. The surfaces are of glass and are negatively charged under all conditions used here. Gelatin is a polyampholyte, with an isoelectric pH (IEP) of approximately 4.9. At low pH the gelatin molecules have a net positive charge, and thus the polyampholyte tends to adsorb with a relatively flat conformation. As the pH is increased the strong attractive interaction between the surface and the polyampholyte decreases as more negative charges and then fewer positive charges appear on the polyampholyte, and so the gelatin extends away from the surface. On changing electrolyte concentrations after adsorption no effect was seen at the IEP, but the layer was observed to swell at more alkaline pHs. This is consistent with the net minimum charge situation on the polymer under these conditions. Changing the adsorption conditions was seen to have an effect, and this is attributed to the different affinities of the gelatin chain to the surface depending on the solution chemistry. Results obtained when the gelatin was initially adsorbed on one surface or two were similar, suggesting that the gelatin transfers rapidly from one surface to another. The importance of adsorption conditions as well as current conditions is discussed. Copyright 1999 Academic Press.

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PubMed1997

Addition of (Tri-)Block Copolymers to Phospholipid Vesicles: A Study of the Molecular Morphology and Structure by Using Hydrophobic Dye Molecules as Bilayer Probes

A dispersion of soybean lecithin in water leads to the formation of multilamellar vesicles (MLVs), which on sonication break down into small unilamellar vesicles of approximately 50 nm in diameter. The addition of polymeric molecules in the liposomal system is thought to provide the liposomes with a steric barrier. The molecules used were (tri-) block copolymers (Synperonics) containing a central hydrophobic part (polypropylene oxide) and two hydrophilic chains (polyethylene oxide). The aim of this work was to study whether it was possible to anchor deep inside the lipid bilayer the copolymer hydrophobic block. The exact localization of the copolymer molecules was investigated using a multiprobe technique. The full spectra of two hydrophobic dyes, namely Nile red (NIL) and Pinacyanol chloride (PCYN), were compared while solubilized inside the liposome bilayer. The sensitivity of their spectral characteristics to polarity and self-aggregation produced a monitor of the bilayer microenvironment. The more hydrophobic NIL proved an accurate polarity sensor of the bilayer microenvironment and the formation of PCYN dimers and nonabsorbing aggregates can be directly related to the local (bilayer) concentration of the dye and the volume available to the solubilized dye molecules. Shifts of the maximum absorbance (lambdamax) for both dyes showed that the bilayer environment was becoming more apolar with increasing copolymer concentration. The absorbance peak of PCYN due to dimer/aggregate formation increased at moderate copolymer concentrations, indicating that the polymer is incorporated inside the lipid bilayer.

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PubMed1997

The Effect of Diameter Ratio and Volume Ratio on the Viscosity of Bimodal Suspensions of Polymer Latices

Previously, the rheological properties of different sized monodisperse polymer latices of polystyrene and polymethyl methacrylate (PMMA) were determined. This paper describes procedures in which carefully prepared blends of the same monodisperse latices were mixed together to examine the effect of the differences in size between the latices (diameter ratio) on the rheology of the blend. The relative viscosities of these blends were then measured as a function of the total volume fraction and compared with those of the constituent parts. In a bimodal suspension, theory predicts that a maximum packing fraction and hence a minimum viscosity is achieved with 27% small particles by volume. The experiments revealed that a minimum viscosity was obtained with 25% small particles by volume and with a diameter ratio of 7.83. At this diameter ratio, the small particles are able to pass through the triangular pore between the large particles and this essentially leads to an increased maximum packing fraction of the suspension. Hence the suspension will have a lower viscosity. Other diameter ratios at 25% small particles by volume that led to suspensions with lower viscosities were 4.03, 6.37, and 11.15. On the other hand the suspensions with diameter ratios of 2.81 and 5.67 led to increases in viscosity. This is a result of the small particles being too big to fit in the gaps between the large particles. All the other diameter ratios at all the other compositions (i.e., 50 and 75% small particles by volume) resulted in viscosities higher than that obtained for the monodisperse polystyrene latex. These effects may be explained by changes in the value of the maximum packing fraction. An increased maximum packing fraction of the system leads to lower viscosities and vice versa. The results challenge the conventional view that simply increasing the diameter ratio at a fixed composition leads to reduced viscosities and increased maximum packing fractions. This simple picture does not appear to hold for binary suspensions of colloidal particles at every particle size ratios.

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