John V. Dawkins

Preparation, characterization and biodegradation studies on cellulose acetates with varying degrees of substitution

Several samples of cellulose acetate polymers with varying degrees of substitution (DS) between 0.7 and 1.7 have been prepared and tested for their biodegradation potential. The original and biodegraded polymers were characterized using chemical techniques as well as nuclear magnetic resonance, Fourier transform infra-red spectroscopy, gel permeation chromatography, solution viscosity and X-ray diffraction. DS, solubility and crystallinity have been investigated as possible factors affecting biodegradability.

Chromatographic Characteristics of polymer-based high-performance liquid chromatography packings

Abstract Rigid macroporous polystyrene-based and polyacrylamide-based packings have been developed for reversed-phase high-performance liquid chromatography. Data on pore size distribution, total pore volume and surface area have been obtained for comparison with silica-based packings. Columns containing these polymer-based packings have good permeability with maximum working pressures of 5000 p.s.i. and 3500 p.s.i. for polystyrene-based packing differs from that for the polyacrylamide-based packing. The dependence of column efficiency on teh flow velocity of the mobile phase as a function of solute type and temperature has been studied. Plate height is reduced as a function of solute type and temperature has been studied. Plate height is reduced considerably at higher temperature. This is tentatively interpreted in terms of a reduction in solute dispersion due to improved mass transfer in the stationary phase.

Nonaqueous poly(methyl methacrylate) dispersions: radical dispersion polymerization in the presence of AB block copolymers of polystyrene and poly(dimethyl siloxane)

Abstract Well defined AB block copolymers of polystyrene (PS) and poly(dimethyl siloxane) (PDMS) have been prepared with PS molecular weights in the range 8 800 to 43 600 and PDMS molecular weights in the range 2 400 to 48 000. Provided the PS and PDMS molecular weights have a ratio within the range 0.5 to 4.0, these block copolymers stabilize particles of poly(methyl methacrylate) in n-alkanes. The particle size over the range 0.1 to 0.5 μm may be varied by performing dispersion polymerizations of methyl methacrylate as a function of monomer content of the seed stage and as a function of the concentration, molecular weight and composition of the block copolymer. From silicon analyses of the poly(methyl methacrylate) particles, values of the surface area stabilized per PDMS chain were established. The results indicate complete surface coverage of the particles.

Comparison of the structural and rheological consequences of micelle formation in solutions of a model di-block copolymer

Abstract In the selective solvent, dodecane, the di-block copolymer polystyrene- b -(ethylene- co -propylene), with a narrow molecular-weight distribution, forms micelles with polystyrene cores. Small-angle neutron scattering experiments were used to investigate both core shape and dimensions and the intercore structure factors as temperature or concentration were varied. Monodisperse spherical structures with radii around 120 A were observed. Above a critical concentration these were arranged in relatively ordered structures in which preferred orientations could easily be induced. The core diameter and the intercore spacing were dependent on sample thermal history. During shear the intercore structure became less ordered. The structural results correlate well with measurements of the dynamic viscosity measured in oscillatory shear, which also show a sharp change from gel-like to liquid behaviour at this critical concentration. Data are compared to model calculations in the regions where the particle form factor or where the interparticle structure factor dominate. In the latter case a hard core potential with a soft tail is found to give reasonable agreement with the data, and to allow changes with shear rate, with concentration or with temperature to be interpreted.

Study of micelle formation by the diblock copolymer polystyrene—b-(ethylene-co-propylene) in dodecane by small-angle neutron scattering

Abstract The diblock copolymer polystyrene—b-[ethylene-co-propylene] forms micelles with polystyrene cores in dodecane at concentrations between 1.4 and 10% solids. Neutron scattering experiments allow determination of the dimensions of the polystyrene cores and show a very highly ordered simple cubic arrangement of the micelles. A preferred orientation of the cubic lattice is found.

High performance g.p.c. with crosslinked polystyrene gels: influence of particle size distribution

Abstract Spherical crosslinked polystyrene gel particles have been separated by air classification into fractions having average particle diameters in the range 10–40 μm. The particle size distributions have been shown to be narrow by Coulter Counter measurements. G.p.c. column performance improves as the average particle diameter decreases and columns packed with gel fractions having a number-average particle diameter below 20 μm give plate counts in excess of 3000 plates per foot. Plate height results as a function of eluent flow rate suggest that chromatogram broadening due to mobile phase dispersion is reduced in columns packed with spherical particles having a narrow size distribution.

Unsaturated polyesters based on terephthalic acid: 4. Sequence distribution of crosslinks in styrene cured resins

Abstract The sequence distribution of the styrene crosslinks in a cured polyester formed from propylene glycol, terephthalic acid and maleic anhydride has been studied by 13 C nuclear magnetic resonance and infrared spectroscopy. The n.m.r. method involved the degradation of the crosslinked polyester into a copolymer of styrene and fumaric acid and the interpretation of the peaks in the 13 C spectrum, which arise from the quaternary phenyl carbon atoms in sequences of styrene units, by considering the 13 C spectra of copolymer standards of styrene and diethyl fumarate. The crosslinking process was monitored directly by infra-red spectroscopy, and the distribution of sequence lengths of styrene units was then estimated by considering that crosslinking may be interpreted by a statistical treatment of copolymerization. Values of mol fractions of styrene units in sequences of monads, diads and triads determined by 13 C n.m.r. and infra-red spectroscopy were in fair agreement.

Flocculation studies of non-aqueous poly(methyl methacrylate) dispersions stabilized by AB block copolymers of polystyrene and poly(dimethyl siloxane)

SummaryWell defined non-aqueous dispersions of poly(methyl methacrylate) stabilized by AB block copolymers of polystyrene and poly(dimethyl siloxane) were prepared with a dispersion medium consisting of a binary mixture of n-heptane and ethanol. Flocculation was induced by cooling, and the lower critical flocculation temperature was studied as a function of the molecular weight and composition of the stabilizing copolymer, the particle size of the dispersion, and the surface coverage of the particles. The theta temperature for poly(dimethyl siloxane) homopolymer in the same binary liquid mixture was determined by two procedures. The lower critical flocculation temperature was found to be close to the theta temperature and was independent of the length of the stabilizing poly(dimethyl siloxane) chains for the molecular weight range 3200-48000.ZusammenfassungGut definierte, durch AB-Block-Copolymere von Polystyrol und Polydimethylsiloxan stabilisierte nichtwässerige Dispersionen von Polymethylmethacrylat wurden mit binären Mischungen von Heptan und Äthylalkohol als Dispersionsmittel hergestellt. Die Flockung wurde durch Abkühlung herbeigeführt und die untere kritische Flockungstemperatur wurde als Funktion des Molekulargewichts und der Zusammensetzung des stabilisierenden Copolymeren, der Größe der dispergierten Teilchen und dem Bedeckungsgrad der Teilchenoberfläche studiert. Die Theta-Temperatur des Polydimethylsiloxan-Homopolymers in derselben binären flüssigen Mischung wurde durch zwei Verfahren bestimmt. Es wurde gefunden, daß die untere kritische Flockungstemperatur nahe an der ThetaTemperatur liegt und im Molekulargewichtsbereich 3200–48000 von der Länge der stabilisierenden Polydimethylsiloxan-Ketten unabhängig ist.

Reversed-phase high-performance liquid chromatography with a C18 polyacrylamide-based packing

Abstract A rigid macroporous polyacrylamide-based packing having an octadecyl (C 18 ) chemically bonded phase has been developed for reversed-phase high-performance liquid chromatography. Separations of non-polar, polar, basic and acidic compounds are demonstrated with this packing and compared with the retention properties of a C 18 silica packing and a polystyrene-based packing. The C 18 polyacrylamide-based packing equilibrates rapidly to changes in the composition of the mobile phase and may be used at pressures up to 25 · 10 6 N/m 2 . The packing retains its performance after prolonged contact with aqueous mobile phases at both low and high pH.

Determination of the molecular weight and compositional heterogeneity of block copolymers using combined gel permeation chromatography and low-angle laser light scattering

Abstract A procedure, using combined gel permeation chromatography and low-angle laser light scattering, has been developed for characterizing block copolymers. A compositional heterogeneity parameter, whose variation with molecular weight can be measured, has been defined. The procedure has been applied to copolymers and blends of polystyrene and poly(dimethylsiloxane).