Shao-Min Mai

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.

Aqueous mesophases of block copolymers of ethylene oxide and 1,2-butylene oxide

Recent work on water-soluble block copolymers of ethylene oxide and 1,2-butylene oxide is described. Small-angle X-ray and neutron scattering have been used to probe the structures of aqueous micellar mesophases, with particular attention paid to the effects of steady and oscillatory shear. Effects of temperature and concentration are described, and the relationship of cubic structure (bcc or fcc) to diblock copolymer composition is explained in terms of the nature of the intermicellar potential. The rheology of the complex fluids (so-called soft gels) outside the structured-mesophase (hard gel) regions is discussed.

Crystallization in block copolymer melts: Small soft structures that template larger hard structures

The crystallization of shear oriented oxyethylene/oxybutylene (E/B) diblock copolymers has been studied by simultaneous small and wide angle x-ray scattering. Crystallization of ordered melts can be accompanied by a change in length scale and retention of the melt orientation. Lamellar melts crystallize with an increase in length scale with multiply folded E blocks and the B blocks slightly stretched from their melt conformation. Crystallization from oriented gyroid melts leads to an increase in length scale with preferred melt directions being selected. The retention of layer planes on crystallization from an ordered melt is caused by the local stretching of chains and the locally one-dimensional structure, despite the relative strength of the structural process. We demonstrate that an interfacial preordering effect can cause crystallographic register to jump length scales in a soft matter system showing epitaxial crystallization.

Ordered melts of block copolymers of ethylene oxide and 1,2-butylene oxide

An account is presented of recent work on specially synthesised diblock, triblock and cyclic block copolymers of ethylene oxide and 1,2-butylene oxide. Simultaneous small-angle X-ray scattering and differential scanning calorimetry have been used to investigate the effects of block architecture on the stabilities and structures of microphase-separated melts. Stable lamellar, hexagonal, body-centred cubic and gyroid phases were detected. Phase diagrams are compared, one with another and with those predicted by the exact self-consistent mean-field theory, and centre-block stretching in lamellar phases is confirmed.

The effect of architecture on the morphology and crystallization of oxyethylene/oxybutylene block copolymers from micelles in n-hexane

The effects of polymer architecture on the morphology and crystallization of oxyethylene/oxybutylene diblock copolymers, E76B38, E114B56 and E155B76, in a concentration series of n-hexane solution were investigated with simultaneous synchrotron small-angle X-ray scattering and wide-angle X-ray scattering (SAXS/WAXS). It is observed that all the block copolymers form spherical micelles at higher temperature and the core of the micelles is partially ordered in some cases. On cooling to room temperature, the core becomes more anisotropic and there is an increase in crystallinity. After crystallization the three block copolymers behave quite differently. E76B38 precipitates as plates from the solution very easily, E114B56 only forms plate-like precipitates at high concentration whereas E155B76 remains a micellar solution. Analysis of the SAXS data shows that the B block is highly stretched in the shorter block copolymer. This is interpreted in terms of the aggregation of E76B38 being driven by the need to reduce the total area of the lateral interface because of the unfavorable interaction between the core and the solvent. By contrast, in longer block copolymers the B block is densely packed and an “over-spilling” effect can prevent aggregation by prevention of the interaction between the lateral crystal surfaces and the solvent. The dimensions of precipitated crystals in the presence of solvent are determined and found to be in good agreement with those calculated from self-consistent field theory.

Crystallization behavior of oxyethylene/oxybutylene diblock and triblock copolymers

Abstract The crystallization behavior of poly(oxyethylene)-b-poly(oxybutylene) block copolymers with different compositions, morphologies and architectures (EmBn diblock copolymers and EmBnEm, BnEmBn triblock copolymers) were investigated and the effect of volume fraction and architecture on the crystallization temperature (Tc) in non-isothermal crystallization was determined. It is found that the EmBn diblock copolymers having long E blocks exhibit similar crystallization temperatures, irrespective of volume fraction and morphology, but for the block copolymers with shorter E blocks the crystallization temperature increases with both the volume fraction, φE, and the length, m, of the E block. Some block copolymers with extremely low Tc, which fall into the temperature range normally associated with homogenous nucleation, were chosen for time-resolved small-angle X-ray scattering (SAXS) and isothermal crystallization kinetics experiments. The results show that breakout crystallization occurs in all these block copolymers. Therefore, unlike EmBn/Bh blends, there is no obvious relationship between Tc and crystallization behavior in neat block copolymers and homogeneous nucleation does not definitely lead to confined crystallization. The values of χc/χODT for all the block copolymers with hex and bcc morphology were also calculated. It is found that all the block copolymers have χc/χODT

The effect of hydrophobe chemical structure and chain length on the solubilization of griseofulvin in aqueous micellar solutions of block copoly(oxyalkylene)s

The solubilization capacities of micellar solutions of a series of block copolymers composed of hydrophilic poly(oxyethylene) (E) and hydrophobic poly(oxypropylene) (P), poly(oxybutylene) (B) or poly(oxyphenylethylene) (S, from styrene oxide) have been compared using the poorly water-soluble drug griseofulvin as a model solubilizate. A 1H NMR technique for determining the extent of solubilization has been developed. Considering the optimum result in each case, the amount of griseofulvin solubilized (expressed as moles griseofulvin per mole of hydrophobic units) by ES block copolymers was approximately nine times more than by EP copolymers and three times more than by EB copolymers. The mobility of the S-blocks in the cores of the ES copolymer micelles, as determined by 1H NMR spectroscopy, was sufficient to allow solubilization at ambient temperature.

Crystal thicknesses in semicrystalline oxyethylene/oxybutylene block copolymers by atomic force microscopy and SAXS

Abstract The structure of thin crystallized films of a diblock or a triblock copolymer deposited on silicon has been investigated using atomic force microscopy (AFM). Non-contact mode AFM was used to investigate the topography of crystallites of poly(oxyethylene)/poly(oxybutylene) (E/B) block copolymers at room temperature, where E is crystallized and B is amorphous. The crystal thicknesses determined from AFM were compared to bulk layer spacings determined using small-angle X-ray scattering (SAXS). This technique showed that E 41 B 22 E 41 (the subscript denotes the number of repeat units) largely crystallized in a monolayer with unfolded E blocks at the substrate and folded (looped) B blocks at the polymer-air interface, and with the E blocks tilted at an angle of ca. 60° relative to the substrate plane. Multiple layers with a common step height were observed for the diblock E 27 B 6 crystallites, which were largely comprised of unfolded chains, also with E block tilted at an angle of ca 60° with respect to the substrate plane.

First observation of an ordered microphase in melts of poly(oxyethylene)-poly(oxypropylene) block copolymers

The first observation of ordered microphase structures in poly(oxyethylene)–poly(oxypropylene) diblock copolymers melts is reported. Two diblock copolymers were synthesised by anionic polymerisation, i.e. E130P58 and E107P69 where E represents an oxyethylene unit, OCH2CH2, and P an oxypropylene unit, OCH2CH(CH3). The volume fractions of the two components were in the range 0.5–0.6. Order in the melt phase was confirmed by small angle X-ray scattering, and order-to-disorder transitions (ODT) were located at 106°C (E130P58) and 114°C (E107P69). In a related experiment, a triblock copolymer with perdeuterated P blocks, E33dP42E33, was synthesised and studied in the melt phase by small-angle neutron scattering. This allowed determination of the temperature dependence of the Flory–Huggins interaction parameter for the poly(oxyethylene)–poly(oxypropylene) system, i.e. χ=20.2/T+0.0221.

Lamellar-to-gyroid transition in a poly(oxyethylene)–poly(oxybutylene) diblock copolymer melt

The transition from the lamellar (lam) to the gyroid (gyr) phase in the melt of a poly(oxyethylene)–poly(oxybutylene) (EB) diblock copolymer was shown to proceed via an intermediate perforated lamellar (pl) structure. The transition was studied starting from a shear-oriented lamellar phase using synchrotron small-angle X-ray scattering (SAXS) with simultaneous rheology. The transition occurred with a continuous variation of q*, the position of the principal peak position, although the slope of q* as a function of temperature increased in the region of the transition (pl state). In the intermediate state, the variation of q* with T was shown to be much stronger than that of the radius of gyration of EB diblocks. It was observed that the gyr phase developed from an oriented pl phase precursor as an isotropic distribution of grains. This indicates that growth of this structure did not occur epitaxially on a macroscopic scale. Finally, density images corresponding to lam, hexagonal perforated lamellar and gyr structures were generated using a single wavenumber approximation, based on the SAXS evidence that diffraction patterns are dominated by the first Fourier component.