PPO Block Research Articles

H NMR Relaxation Studies of the Micellization of a Poly(ethylene oxide)−Poly(propylene oxide)−Poly(ethylene oxide) Triblock Copolymer in Aqueous Solution

1H nuclear magnetic resonance (NMR) relaxation studies of the temperature-induced micellization of a poly(ethylene oxide)−poly(propylene oxide)−poly(ethylene oxide), PEO−PPO−PEO, block copolymer in aqueous solution were performed in the range 280−345 K. 1H NMR spectra of the block copolymer were well resolved, thus allowing us to probe specifically the methyl and methylene relaxation processes in the PPO and PEO blocks, respectively. Interpretation of the relaxation data in terms of the Hall−Helfand correlation function leads to four distinct correlation times for the PPO and PEO blocks. The slower correlation time in the PPO block was identified as the Zimm−Rouse first normal mode of the copolymer and served to determine the hydrodynamic radius, RH, of the unimers and the micelles. On a more local scale, the behavior of the correlation time for segmental motions in the PPO block indicates an extension of the PPO chains in micelles relative to the unimers. This conformational change is related to the form...

Non-aqueous high internal phase emulsions. Preparation and stability

Several novel non-aqueous high internal phase emulsions (HIPEs) of petroleum ether dispersed in a number of polar organic solvents [formamide (FA), N,N′-dimethyl formamide (DMF) and dimethyl sulfoxide (DMSO)] as the continuous phase, with internal phase volume ratios (ϕ) of 0.9, have been prepared, employing both polymeric and non-polymeric non-ionic surfactants. A number of factors have been shown to be important in determining HIPE stability. First, the nature of the polar organic phase: the most stable emulsions are formed with formamide. This is attributed to its high polarity and ability to form a water-like hydrogen-bonded network, however, rationalising the relative performance of the other polar solvents is more difficult. Second, the interfacial behaviour of the surfactant employed: interfacial data for aqueous solutions in contact with hydrocarbon do not adequately describe surfactant performance and a much better guide is the hydrophilic–lipophilic balance (HLB) value of the surfactant. Although this may be an empirical guide to surfactant behaviour, in this context it is useful, indeed rather powerful. The third important parameter is the molecular size or complexity of the surfactant. Generally, low-molecular-weight surfactants perform poorly, whereas PEO–PPO–PEO block copolymers of appropriate HLB value allow the formation of stable HIPEs. The PEO block appears to be solubilised in the polar solvent and the PPO block in the hydrocarbon. Presumably, each block attemts to form a (separate) random coil on its own side of the interface. Consequential effects may well include an increase in viscosity and slow desorption kinetics at the interface.

Lamellar Mesophase of Poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) Melts and Water-Swollen Mixtures

Triblock copolymers of poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO) form lamellar mesophases at low temperatures. The order-to-disorder transition is closely related to the melting transition of the PEO subunit. When water is incorporated into the copolymer melt, a single phase is formed at high temperatures, which is possibly a lamellar mesophase, formed as a consequence of the hydrophobic PPO blocks. At low temperatures two phases are present, one of which is similar to the high temperature phase while the other is similar to the lamellar melt, but swollen up to 10%.

Microviscosity in Pluronic and Tetronic Poly(ethylene oxide)-Poly(propylene oxide) Block Copolymer Micelles

The micellar microviscosity afforded by Pluronic and Tetronic poly(ethylene oxide)-poly(propylene oxide) block copolymer aqueous solutions has been investigated by fluorescence and NMR spectroscopy. Comparison is made with bulk poly(propylene oxide) (PPO) samples of different molecular weights. The microviscosity in Pluronic PEO-PPO-PEO copolymer micelles is much larger than that observed in conventional surfactant micelles and depends strongly on the size of the hydrophobic PPO block: the larger this block, the higher the viscosity. Above the critical micellar temperature (CMT), as temperature increases, the microviscosity decreases. However, this decrease is not as important as that observed in bulk PPO. Hence, the relative microviscosity, defined as the ratio of the two observed phenomena, increases. This suggests structural transformation of the micelles resulting in a core becoming more and more compact as temperature increases. Such results have been confirmed by NMR studies that showed broadening of the PPO peak and relatively constant spin-lattice relaxation time, T 1 , with increasing temperature while the PEO signal remained relatively sharp with an exponential increase in T 1 . In addition, solubilization of benzene in Pluronic copolymer micelles as detected by NMR indicated that benzene partitions preferentially in the core of the micelle constituted mainly of PPO

Control of staphylococcal adhesion to polystyrene surfaces by polymer surface modification with surfactants

The adherence of three clinical isolates of Staphylococcus epidermidis to model polystyrene surfaces was studied in vitro using epifluorescent image analysis. A series of 16 Pluronic surfactants (A-B-A block copolymers where A is poly(ethylene oxide) (PEO) and B is poly(propylene oxide) (PPO)) were used as surface modifiers for the model polystyrene surfaces. Substantial reductions (up to 97%) in bacterial adhesion levels were achieved with all copolymers tested, irrespective of the PPO or PEO block lengths. It appears likely that such treatments create a sterically stabilized surface with adsorbed PEO chains, conferring non-specific anti-adhesive properties which can limit bacterial attachment.

High ionic conductivity in PEO. PPO block polymer + salt solutions

The conductivity of LiI dissolved in a block copolymer of polypropylene and polyethylene oxide sections of comparable length has been measured and found greater than for either of the pure polymer solutions. The composition of maximum conductivity occurs at low concentration as predicted from entropy theories in which the salt mainly serves to raise the glass transition temperature. A VTF equation gives a good description of the temperature dependence at high salt concentrations. At low concentrations, however, complications due to redistribution of salt between the distinct microregions of the microscopically segregated block units, causes a complex temperature dependence. At intermediate compositions two distinct glass transitions are observed. These blend into a single transition as the microregions blend at high salt concentrations.