J. Patrick A. Fairclough

Complex Phase Behavior in Solvent-Free Nonionic Surfactants

Unsolvated block copolymers and surfactant solutions are “soft materials” that share a common set of ordered microstructures. A set of polyethyleneoxide-polyethylethylene (PEO-PEE) block copolymers that are chemically similar to the well-known alkane-oxyethylene (CnEOm) nonionic surfactants was synthesized here. The general phase behavior in these materials resembles that of both higher molecular weight block copolymers and lower molecular weight nonionic surfactant solutions. Two of the block copolymers exhibited thermally induced order-order transitions and were studied in detail by small-angle scattering. The fundamental microstructural spacing was determined to be a crucial parameter in these transitions. Transitions from one ordered state to another occur only when the lattice spacing is nearly matched. These materials highlight the importance of epitaxy and molecular conformation in the phase transformations of soft material.

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.

Unexpected Facile Redistribution of Adsorbed Silica Nanoparticles Between Latexes

Addition of excess sterically stabilized P2VP latex to a colloidal dispersion of P2VP-silica nanocomposite particles (with silica shells at full monolayer coverage) leads to the facile redistribution of the silica nanoparticles such that partial coverage of all the P2VP latex particles is achieved. This silica exchange, which is complete within 1 h at 20 degrees C as judged by small-angle x-ray scattering, is observed for nanocomposite particles prepared by heteroflocculation, but not for nanocomposite particles prepared by in situ copolymerization. These observations are expected to have important implications for the optimization of nanocomposite formulations in the coatings industry.

A scattering study of nucleation phenomena in polymer crystallisation

The mechanism of primary nucleation in polymer crystallisation has been investigated experimentally and theoretically. Two types of experiments have been performed on polypropylene, polyethylene, and poly(ethylene terpthalate). Crystallisations with long induction times, studied by small and wide angle X-ray scattering (SAXS and WAXS), reveal the onset of large scale ordering prior to crystal growth. Rapid crystallisations studied by melt extrusion indicate the development of well resolved oriented SAXS patterns associated with large scale order before the development of crystalline peaks in the WAXS region. The results suggest pre-nucleation density fluctuations play an integral role in polymer crystallisation. A theoretical model has been developed which qualitatively describes the experimental results.

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

Characterization of polymer-silica nanocomposite particles with core-shell morphologies using Monte Carlo simulations and small angle X-ray scattering.

A two-population model based on standard small-angle X-ray scattering (SAXS) equations is verified for the analysis of core-shell structures comprising spherical colloidal particles with particulate shells. First, Monte Carlo simulations of core-shell structures are performed to demonstrate the applicability of the model. Three possible shell packings are considered: ordered silica shells due to either charge-dependent repulsive or size-dependent Lennard-Jones interactions or randomly arranged silica particles. In most cases, the two-population model produces an excellent fit to calculated SAXS patterns for the simulated core-shell structures, together with a good correlation between the fitting parameters and structural parameters used for the simulation. The limits of application are discussed, and then, this two-population model is applied to the analysis of well-defined core-shell vinyl polymer/silica nanocomposite particles, where the shell comprises a monolayer of spherical silica nanoparticles. Comprehensive SAXS analysis of a series of poly(styrene-co-n-butyl acrylate)/silica colloidal nanocomposite particles (prepared by the in situ emulsion copolymerization of styrene and n-butyl acrylate in the presence of a glycerol-functionalized silica sol) allows the overall core-shell particle diameter, the copolymer latex core diameter and polydispersity, the mean silica shell thickness, the mean silica diameter and polydispersity, the volume fractions of the two components, the silica packing density, and the silica shell structure to be obtained. These experimental SAXS results are consistent with electron microscopy, dynamic light scattering, thermogravimetry, helium pycnometry, and BET surface area studies. The high electron density contrast between the (co)polymer and the silica components, together with the relatively low polydispersity of these core-shell nanocomposite particles, makes SAXS ideally suited for the characterization of this system. Moreover, these results can be generalized for other types of core-shell colloidal particles.

Time-Resolved Small-Angle X-ray Scattering Studies of Polymer-Silica Nanocomposite Particles: Initial Formation and Subsequent Silica Redistribution

Small angle X-ray scattering (SAXS) is a powerful characterization technique for the analysis of polymer-silica nanocomposite particles due to their relatively narrow particle size distributions and high electron density contrast between the polymer core and the silica shell. Time-resolved SAXS is used to follow the kinetics of both nanocomposite particle formation (via silica nanoparticle adsorption onto sterically stabilized poly(2-vinylpyridine) (P2VP) latex in dilute aqueous solution) and also the spontaneous redistribution of silica that occurs when such P2VP-silica nanocomposite particles are challenged by the addition of sterically stabilized P2VP latex. Silica adsorption is complete within a few seconds at 20 °C and the rate of adsorption strongly dependent on the extent of silica surface coverage. Similar very short time scales for silica redistribution are consistent with facile silica exchange occurring as a result of rapid interparticle collisions due to Brownian motion; this interpretation is consistent with a zeroth-order Smoluchowski-type calculation.