Paul A. FitzGerald

Amphiphilic Self-Assembly of Alkanols in Protic Ionic Liquids

Strong cohesive forces in protic ionic liquids (PILs) can induce a liquid nanostructure consisting of segregated polar and apolar domains. Small-angle X-ray scattering has shown that these forces can also induce medium chain length n-alkanols to self-assemble into micelle- and microemulsion-like structures in ethylammonium (EA(+)) and propylammonium (PA(+)) PILs, in contrast to their immiscibility with both water and ethanolammonium (EtA(+)) PILs. These binary mixtures are structured on two distinct length scales: one associated with the self-assembled n-alkanol aggregates and the other with the underlying liquid nanostructure. This suggests that EA(+) and PA(+) enable n-alkanol aggregation by acting as cosurfactants, which EtA(+) cannot do because its terminating hydroxyl renders the cation nonamphiphilic. The primary determining factor for miscibility and self-assembly is the ratio of alkyl chain lengths of the alkanol and PIL cation, modulated by the anion type. These results show how ILs can support the self-assembly of nontraditional amphiphiles and enable the creation of new forms of soft matter.

Unexpected behavior of polydimethylsiloxane/poly(2-(dimethylamino)ethyl acrylate) (charged) amphiphilic block copolymers in aqueous solution

We report the synthesis and self assembly of amphiphilic block copolymers based on poly(dimethylsiloxane) and poly(2-(dimethylamino)ethyl acrylate) (PDMS-b-PDMAEA). We consider the self-catalyzed hydrolysis of PDMAEA in aqueous solution, and its effect on the self-assembly of the amphiphilic copolymer by investigating poly(dimethylsiloxane)-b-[poly(2-dimethylamino)ethyl acrylate)-co-poly(acrylic acid)] (PDMS-b-(PDMAEA-co-PAA)), obtained from hydrolysis of PDMAEA, and poly(dimethylsiloxane)-b-poly(2-(trimethylammonium iodide)ethyl acrylate) (PDMS-b-PTMAIEA), obtained from quaternization of PDMAEA. We observe that the thermal properties and self-assembly behavior in water of these amphiphilic diblock copolymers depend on the structure of the hydrophilic block, and are strongly influence by hydrolysis of the PDMAEA block. Self-assembly in aqueous solution leads to the formation of spherical micellar structures with diameters between 40 and 70 nm, which were investigated by dynamic light scattering (DLS) and transmission electron microscope (TEM). PDMS-b-(PDMAEA-co-PAA) is shown to be pH responsive, and behaves as a highly negatively charged polymer at high pH values (pH > 10) and as a highly positively charged polymer at low pH values (pH < 2). Critical micelle concentrations (cmc) of the amphiphilic block copolymers were determined by fluorescence measurement with N-phenyl-1-naphthylamine (PNA) as a fluorescence probe. Surprisingly, surface tension measurements suggest that PDMS-b-PTMAIEA is “non-surface active”, forming micelles in bulk solution instead of adsorbing at the air–water interface to form a Gibbs monolayer, even when the polymer concentration was over 100 times above its cmc value. This remarkable behavior is consistent with recent reports that suggest that “non-surface activity” can be observed for both cationic and anionic amphiphilic diblock copolymers due to the effect of image charge repulsion at the air–water interface.

In situ observations of adsorbed microgel particles

The formation and morphological changes of a pH-responsive microgel layer on silica and mica were studied by tapping mode atomic force microscopy. First, lightly cross-linked, sterically-stabilised poly(2-vinylpyridine) (P2VP) particles were adsorbed in their non-solvated latex form at pH 4.8 to produce a structurally-disordered monolayer that covers the entire substrate. Addition of acid to this particulate film induces a latex-to-microgel transition at pH 3.0, causes particle swelling (and also some desorption) and produces a uniform, swollen film with localised hexagonal packing. Returning to pH 4.8 causes partial microgel deswelling to form individual oblate spheroidal P2VP latex particles, which retain the localised order previously induced by swelling. The swelling and collapse of this P2VP film was reversible during subsequent pH cycles, with no further desorption observed. The adsorbed amount of P2VP microgel/latex was quantified at each pH by determining the surface density and dimensions of the adsorbed particles. These measurements allow the microgel surface excess to be calculated for the first time. The initial adsorbed amount is less than that predicted by the standard Random Sequential Adsorption model (RSA) for hard particle adsorption, and is explained by the unexpected deformation of these high Tg particles due to their strong electrostatic attraction to the solid-liquid interface.

Structure of polymerizable surfactant micelles: Insights from neutron scattering

Although polymerization of reactive surfactants (surfmers) in micelles and other self-assembled phases has been studied for at least 30 years, the last decade or so has seen substantial advances in understanding both the structure and dynamics of these systems. In this review we highlight the new insights yielded primarily by small-angle neutron scattering (SANS) using high-flux sources, the perspective this provides for realizing topochemical polymerization in micellar systems, and the prospects and new developments for further exploiting SANS in this field. We present some new neutron contrast variation results exemplifying these elements.

Swelling and Collapse of an Adsorbed pH-Responsive Film-Forming Microgel Measured by Optical Reflectometry and QCM

The swelling and deswelling of a pH-responsive electrosterically stabilized poly[2-(diethylamino)ethyl methacrylate] microgel adsorbed to silica surfaces have been quantified using the techniques of optical reflectometry (OR) and quartz crystal microbalance (QCM). It is shown that by utilizing and comparing OR measurements performed on wafers with differing oxide layer thicknesses the adsorbed amount and film thickness of the adsorbed microgel in both the swollen and deswollen forms can be determined. Also, the kinetics of the transition can be followed, revealing that collapse is a slower process than swelling, and direct support is provided for the formation of a dense outer layer or skin during collapse that slows the deswelling process. It is shown that the adsorption of this low glass transition temperature film-forming microgel latex is robust to changes in pH after an initial swelling event which is responsible for desorption of a large and variable fraction of the initially adsorbed polymer. Subsequent deswelling and swelling of the adsorbed film indicates that adsorption to a surface greatly hinders the volumetric swelling capacity of the microgel film. In its swollen state the film is only 3-4 times thicker than the collapsed film, whereas for particles in bulk the volume increases by a factor of 20 upon protonation of the tertiary amine residues. QCM results show that even in the collapsed form the film contains a considerable amount of water. Further, the viscoelasticity of the deswollen film is similar to that of the swollen film, suggesting that the degree of cross-linking is the primary determinant of viscoelasticity.

Structural and aggregate analyses of (Li salt + glyme) mixtures: the complex nature of solvate ionic liquids.

The structure and interactions of different (Li salt + glyme) mixtures, namely equimolar mixtures of lithium bis(trifluoromethylsulfonyl)imide, nitrate or trifluoroacetate salts combined with either triglyme or tetraglyme molecules, are probed using Molecular Dynamics simulations. structure factor functions, calculated from the MD trajectories, confirmed the presence of different amounts of lithium-glyme solvates in the aforementioned systems. The MD results are corroborated by S(q) functions derived from diffraction and scattering data (HEXRD and SAXS/WAXS). The competition between the glyme molecules and the salt anions for the coordination to the lithium cations is quantified by comprehensive aggregate analyses. Lithium-glyme solvates are dominant in the lithium bis(trifluoromethylsulfonyl)imide systems and much less so in systems based on the other two salts. The aggregation studies also emphasize the existence of complex coordination patterns between the different species (cations, anions, glyme molecules) present in the studied fluid media. The analysis of such complex behavior is extended to the conformational landscape of the anions and glyme molecules and to the dynamics (solvate diffusion) of the bis(trifluoromethylsulfonyl)imide plus triglyme system.

Film-Forming Microgels for pH-Triggered Capture and Release

The pH-responsive behavior for a series of lightly cross-linked, sterically stabilized poly(tertiary amine methacrylate)-based latexes adsorbed onto mica and silica was investigated using in situ tapping mode AFM at room temperature. The adsorbed layer structure was primarily determined by the glass transition temperature, T(g), of the latex: poly[2-(diethylamino)ethyl methacrylate]-based particles coalesced to form relatively featureless uniform thin films, whereas the higher T(g) poly[2-(diisopropylamino)ethyl methacrylate] latexes retained their original particulate character. Adsorption was enhanced by using a cationic poly[2-(dimethylamino)ethyl methacrylate] steric stabilizer, rather than a nonionic poly(ethylene glycol)-based stabilizer, since the former led to stronger electrostatic binding to the oppositely charged substrate. Both types of adsorbed latexes acquired cationic microgel character and swelled appreciably at low pH, even those that had coalesced to form films. Fluorescence spectroscopy was used to study the capture of a model hydrophobic probe, pyrene, by these adsorbed latex layers followed by its subsequent release by lowering the solution pH. The repeated capture and release of pyrene through several pH cycles was also demonstrated. Since these poly(tertiary amine methacrylate) latexes are readily prepared by aqueous emulsion polymerization and adsorption occurs spontaneously from aqueous solution, this may constitute an attractive route for the surface modification of silica, mica and other oxides.

Thermoresponsive behavior of amphiphilic diblock co-oligomers of ethylene glycol and styrene in aqueous solution

We report the thermoresponsive behavior in aqueous solution of amphiphilic diblock co-oligomers of ethylene glycol (EG) and styrene (S) obtained by reversible addition fragmentation chain transfer (RAFT) polymerization (mPEG16-b-PS2, mPEG16-b-PS4 and mPEG16-b-PS6). The block co-oligomers were obtained from the polymerization of styrene mediated by a PEG macro-RAFT agent, mPEG-PBTC. The precise control offered by RAFT polymerization over the length of the short hydrophobic chain permits tuning of the thermoresponsive behavior for all the studied methoxy poly(ethylene glycol)-block-polystyrene copolymers. mPEG16-b-PS2 was shown to self-assemble into micelles at temperatures below the lower critical solution temperature (LCST) of the block co-oligomer, whilst no such critical temperature was observed for solutions of self-assembled mPEG16-b-PS4 and mPEG16-b-PS6. mPEG16-b-PS4 self-assembles into relatively mobile spherical particles which evolve into prolate spheroids with increasing temperature, whilst mPEG16-b-PS6 forms particle clusters, which have lower mobility than the mPEG16-b-PS4 particles. This work is the first study on the aggregation behavior of such short chain lengths of mPEG-b-PS as a function of temperature, and shows how tuning the PS oligomer length permits control of the thermoresponsive behavior of mPEG-b-PS assemblies in water.

Effect of Protic Ionic Liquid and Surfactant Structure on Partitioning of Polyoxyethylene Non-ionic Surfactants

The partitioning constants and Gibbs free energies of transfer of poly(oxyethylene) n-alkyl ethers between dodecane and the protic ionic liquids (ILs) ethylammonium nitrate (EAN) and propylammonium nitrate (PAN) are determined. EAN and PAN have a sponge-like nanostructure that consists of interpenetrating charged and apolar domains. This study reveals that the ILs solvate the hydrophobic and hydrophilic parts of the amphiphiles differently. The ethoxy groups are dissolved in the polar region of both ILs by means of hydrogen bonds. The environment is remarkably water-like and, as in water, the solubility of the ethoxy groups in EAN decreases on warming, which underscores the critical role of the IL hydrogen-bond network for solubility. In contrast, amphiphile alkyl chains are not preferentially solvated by the charged or uncharged regions of the ILs. Rather, they experience an average IL composition and, as a result, partitioning from dodecane into the IL increases as the cation alkyl chain is lengthened from ethyl to propyl, because the IL apolar volume fraction increases. Together, these results show that surfactant dissolution in ILs is related to structural compatibility between the head or tail group and the IL nanostructure. Thus, these partitioning studies reveal parameters for the effective molecular design of surfactants in ILs.

Micellization of monomeric and poly-ω-methacryloyloxyundecyltrimethylammonium surfactants.

We have used small-angle neutron scattering to study how micelle morphology of the tail-polymerizable surfactants MUTAB and MUTAC (ω-methacryloyloxyundecyltrimethylammonium bromide and chloride) is affected by classic self-assembly modifiers such as temperature changes, salt addition, and counterion exchange, as a function of their conversion from monomer into polymer amphiphile in aqueous solution. Contrary to common assumptions about polymerized surfactants, these systems remain in dynamic equilibrium under all conditions examined and at all conversions (except for a small amount of high-molecular-weight precipitation by MUTAC). Counterintuitively, the polymerized methacrylate backbone has little influence on aggregate morphology, except for the formation of rod-like mixed micelles of polymerized and unpolymerized surfactant at intermediate conversions. The addition of salt produces a transition to rod-like micelles at all conversions except in the unpolymerized surfactant, which has some characteristics of an asymmetric bolaform surfactant and retains its spheroidal geometry under almost all conditions.