Colin Booth

Effects of block architecture and composition on the association properties of poly(oxyalkylene) copolymers in aqueous solution

A block copolymer of hydrophilic poly-(ethylene oxide) and a hydrophobic poly(alkylene oxide) can associate in dilute aqueous solution to form micelles. The results of recent investigations of the micellisation behaviour and micelle properties of such copolymers are described. Copolymers of ethylene oxide with propylene oxide, 1,2-butylene oxide or styrene oxide are considered, including aspects of their preparation. Experimental methods for determination of critical conditions for micellisation, micelle association number and spherical-micelle radius are summarised. Effects of temperature, composition, block length and block architecture (diblock, triblock and cyclic-diblock) are described and, where possible, related to the predictions of theory. Brief consideration is given to the dynamics of micelle formation/dissociation, to cylindrical micelles, and to effects of added salts.

Micellisation and gelation of triblock copoly(oxyethylene/oxypropylene/oxyethylene), F127

A sample of triblock copoly(oxyethylene/oxypropylene/oxyethylene) Synperonic F127 was purified. The micellisation and gelation properties of aqueous solutions of purified and unpurified copolymers were investigated by surface tension measurement, static and dynamic light scattering, differential scanning calorimetry and NMR spectroscopy. Generally, the results obtained for the two samples were similar: an exception was the surface tension. Endothermic standard enthalpies of micellisation were obtained over a wide concentration range, with corresponding endothermic standard enthalpies of gelation in the high concentration range. Considered on an equivalent basis, i.e. kJ mol–1(chains), gelation was found to be an almost athermal process compared to micellisation. Based on the presented evidence, particularly that from DSC, and considering other recent studies, it was concluded that the thermal gelation of F127 (i.e. gelation on raising the temperature) resulted essentially from the packing of spherical micelles. A small thermal event at the gelation point was ascribed to a disorder–order discontinuity.

Melting of low molecular weight poly(ethylene oxide)

Abstract The melting behaviour of fractions of poly(ethylene oxide) of narrow molecular weight distribution and of molecular weight of 20 000 or less has been studied. Fractions of molecular weight 4000 or less have one melting transition; those with molecular weight of 6000 or greater may have two melting transitions. Consideration of the influence of molecular weight and crystallization temperature on the melting transitions, of the low-angle scattering of X-rays, and of calculations based upon Florys theory of melting enables us to ascribe the transitions to the melting of extended-chain or of variously folded-chain lamellar crystals. The end interfacial free energy ( σ e ) of extended-chain crystals of poly(ethylene oxide) is found to increase within the range 1 to 4 kcal/mol as the molecular weight increases. It is suggested that this increase in σ e is related to an increase in polydispersity of molecular weights in the fractions.

Triad sequence assignment of the 13C-NMR spectra of copolymers of ethylene oxide and 1,2-butylene oxide

Abstract 13C-NMR spectra of homopolymers and block and statistical copolymers of ethylene oxide and 1,2-butylene oxide have been studied in CDCl3 and C6D6 solution at 75.5 MHz with the aid of resolution-enhancement and subspectrum editing techniques. Resonances from triad sequences, endgroups, initiation sites and inverted monomer addition have been identified in C6D6 solution. The assignments have been tested for consistency using necessary statistical relationships, and have been shown to conform to a simple chemical shift additivity scheme.

Use of crown ether in the anionic polymerization of propylene oxide—2. Molecular weight and molecular weight distribution

Abstract The production of unsaturation by hydrogen abstraction from monomer (transfer reaction) during the polymerization of bulk propylene oxide catalysed by potassium alkoxide in the presence of 18-crown-6 ether was assessed by NMR spectroscopy. Chain transfer constants were measured as a function of the ratio of crown ether to potassium ion (r − 0–4.5) over a range of temperatures (T = 20–80°). Conditions were defined for the preparation of monofunctional poly(oxypropylene) of moderate chain length (about 200 P units, M n > 10,000 ) and narrow chain-length distribution ( M w M n ∼ 1.2 ).

Crystallinity and fusion of ethylene oxide/propylene oxide block copolymers: 1. Type PE copolymers

Abstract Lamella spacings, specific volumes and melting points have been determined for a series of well characterized poly(ethylene oxide)/poly(propylene oxide) type PEP block copolymers with E-block length 48 chain units and P-block lengths 0 to 7 chain units. These properties are interpreted in terms of a stacked lamella model with alternating amorphous and crystalline layers. Both extended-chain and once-folded-chain crystalline lamellae are found, the former with thickness about 32 E chain units and the latter with thickness about 21 E chain units. Compared with the specific volume of supercooled melt of the same composition the specific volume of the polymer in the amorphous lamellae is lower in the extended-chain polycrystals and higher in the once-folded-chain polycrystals. The melting points of the copolymers are low compared to that of perfectly crystalline poly(ethylene oxide), i.e. 37 to 55°C compared with T 0 m = 76°C. This is due to the large positive free energy of formation from the melt of the crystalline/amorphous end interface ( σ o ) and the amorphous layer ( σ a ). For extended-chain polycrystals we find σ o ∼ 3 kJ/mol and σ a ∼ 3.5 kJ/mol; for once-folded-chain polycrystals we find σ o ∼ 6 kJ/mol and σ a ∼ 2 kJ/mol. We also find σ o , x = 2.5 kJ/mol for a completely extended-chain end interface and σ o , f = 10 kJ/mol for a completely folded-chain end interface.

Preparation and characterization of poly(ε-caprolactone) polymer blends for the delivery of proteins

A series of ternary blend matrices, based on high- and low-molecular-weight poly(epsilon-caprolactone) (PCL) and a poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) block copolymer (Synperonic L61), has been developed for the delivery of proteins. The inclusion of Synperonic L61 served to enhance the water content of the matrix available for protein diffusion. Blends comprising high-molecular-weight PCL (PCLH) and Synperonic L61 alone were found by scanning electron microscopy to be incompatible over a wide composition range. The addition of a low-molecular-weight PCL (PCLL) enhanced compatibility of the polymeric components. Analysis of the dynamic mechanical and thermal properties of these blends showed a significant shift in the glass transition temperature of the material (-38 to -55 degrees C), as the weight fraction of Synperonic L61 increased to 30 wt%, indicative of limited miscibility of the components in the non-crystalline phase. All ternary blended matrices showed a higher degree of hydration relative to homogeneous PCLH. The maximum water content could be modified by adjusting the weight fraction of Synperonic L61 in the blend. The rate of release of a model protein, bovine serum albumin (BSA), from matrices containing Synperonic L61 was considerably higher than that from PCLH and PCLH/PCLL alone. Analysis of the release mechanisms of BSA from these matrices suggested that, whilst diffusion was the predominant mode of protein transport, the elution of Synperonic L61 from blended matrices during the dissolution caused a notable deviation from Fickian control.

Association and surface properties of block-copoly-(oxyethylene/oxypropylene/oxyethylene) L64

The mode of association of a purified sample of block-copoly(oxyethylene/oxypropylene/oxyethylene) L64 in aqueous solution has been examined as a function of temperature and concentration by surface tension and light-scattering techniques. The association at 34.5 and 40 °C is described by a cooperative association model which assumes aggregate growth by stepwise addition of unimers. Inflections in the curves of surface tension vs. log concentration are shown to be consistent with the concentrations at which enhanced light scattering is first detectable. Only limited association has been detected at 27 °C.

Micelles and gels of oxyethylene–oxybutylene diblock copolymers in aqueous solution: The effect of oxyethylene-block length

Block copolymer E90B10 (E=oxyethylene, B=oxybutylene) was synthesised and characterised by gel permeation chromatography and 13C NMR spectroscopy. Dynamic light scattering (DLS) and static light scattering (SLS) were used to characterise the micelles in solution (both in water and in aqueous 0.2 mol dm-3 K2SO4), yielding the micellar association numbers, the hydrodynamic and thermodynamic radii, and related expansion factors. Micellar parameters were also obtained by small-angle neutron scattering (SANS) for solutions of a similar copolymer, E86B10, in water, i.e., the association number, the hard-sphere radius, the micelle volume fraction and the corresponding expansion factors. A comparison of the appropriate quantities showed good agreement between the two techniques. SANS gave additional information e.g., volume fraction profiles for the micelles and volume fraction of water in the micelle core. Moderately concentrated solutions of copolymer E90B10 were studied in the gel state by small-angle X-ray scattering (SAXS) in tandem with rheology (oscillatory shear). Values for the dynamic elastic modulus (G′) of the gels significantly exceeded 104 Pa across the range of temperature (25–80°C) and frequency (0.1–100 rad s-1) explored. The SAXS patterns for the orientated gels showed them to have a body-centred-cubic structure, as expected for packed, spherical micelles. This structure persisted over a wide concentration range, e.g., until crystallisation of the E blocks occurred at high concentration (70 wt.% copolymer). By combining the present and published results, a comparison was made of the micelle and gel properties of copolymers with the same B-block length but different E-block lengths, i.e., E90B10, E40B10 and E18B10. As would be expected, as the E-block length was increased, the micelle association numbers decreased while the micelle radii and expansion factors increased. The critical gel concentration (cgc) also decreased as the E-block length was increased. Moreover, the cgc correlated quantitatively with the thermodynamic expansion factors obtained by SLS and SANS from the exclusion properties of the micelles.

DETERMINATION OF REACTIVITY RATIOS FOR THE ANIONIC COPOLYMERIZATION OF ETHYLENE OXIDE AND PROPYLENE OXIDE IN BULK

Abstract Reactivity ratios have been determined for the anionic copolymerization of ethylene oxide and propylene oxide in bulk over the temperature range 0–80°C. The ratios were obtained by two methods, the first using the variation of copolymer composition with monomer feed composition according to the Mayo-Lewis equation, and the second using the triad sequence distribution derived from 13 C-NMR spectra. In both methods, the ratios were obtained by a computer search for the values giving the best fit between simulated and experimental data. The program took composition drift into account by numerical simulation of the copolymerization as a series of small steps, the feed composition being adjusted after each step. The reactivity ratios were independent of temperature within experimental error, with average values r E = 2.8 ± 0.6 and r p = 0.25 ± 0.07 from the Lewis-Mayo equation, and r E = 3.1 ± 0.4 and r p = 0.30 ± 0.04 from the triad distribution.