Effect of Salt on Phosphorylcholine-based Zwitterionic Polymer Brushes.

A quantitative investigation of the responses of surface-grown biocompatible brushes of poly(2-(methacryloyloxy)ethyl phosphorylcholine) (PMPC) to different types of salt has been carried out using ellipsometry, quartz crystal microbalance (QCM) measurements, and friction force microscopy. Both cations and anions of varying valency over a wide range of concentrations were examined. Ellipsometry shows that the height of the brushes is largely independent of the ionic strength, confirming that the degree of swelling of the polymer is independent of the ionic character of the medium. In contrast, QCM measurements reveal significant changes in mass and dissipation to the PMPC brush layer, suggesting that ions bind to phosphorylcholine (PC) groups in PMPC molecules, which results in changes in the stiffness of the brush layer, and the binding affinity varies with salt type. Nanotribological measurements made using friction force microscopy show that the coefficient of friction decreases with increasing ionic strength for a variety of salts, supporting the conclusion drawn from QCM measurements. It is proposed that the binding of ions to the PMPC molecules does not change their hydration state, and hence the height of the surface-grown polymeric brushes. However, the balance of the intra- and intermolecular interactions is strongly dependent upon the ionic character of the medium between the hydrated chains, modulating the interactions between the zwitterionic PC pendant groups and, consequently, the stiffness of the PMPC molecules in the brush layer.

Giant pH-responsive microgel colloidosomes: preparation, interaction dynamics and stability.

The interactions of two oil droplets grown in the presence of swollen, lightly cross-linked cationic poly(tert-butylamino)ethyl methacrylate (PTBAEMA) microgels was monitored using a high-speed video camera. Three oils (n-dodecane, isopropyl myristate and sunflower oil) were investigated, each in the absence and presence of an oil-soluble cross-linker [tolylene 2,4-diisocyanate-terminated poly(propylene glycol), PPG-TDI]. Adsorption of the swollen microgel particles was confirmed by interfacial tension, interfacial elasticity and dilational viscosity measurements on single pendant oil droplets, and assessment of the oscillatory dynamics for coalescing droplet pairs. Like the analogous bulk emulsions, particle adsorption alone did not prevent coalescence of pairs of giant Pickering emulsion droplets. However, prior addition of surface-active PPG-TDI cross-linker to the oil phase results in the formation of highly stable microgel colloidosomes via reaction with the secondary amine groups on the PTBAEMA chains. Colloidosome stability depended on the age of the oil-water interface. This reflects a balance between the adsorption kinetics of the PPG-TDI cross-linker and the microgel particles, each of which must be present at the interface to form a stable colloidosome. Colloidosome formation was virtually instantaneous in n-dodecane, but took up to 120 s in the case of isopropyl myristate. The impact of an acid-induced latex-to-microgel transition on the interaction of giant colloidosomes (originally prepared at pH 10 using isopropyl myristate) was also studied. This acid challenge did not result in coalescence, which is consistent with a closely-related study (A. J. Morse et al., Langmuir, 2014, 30(42), 12509-12519). No evidence was observed for inter-colloidosome cross-linking, which was attributed to retention of an aqueous film between the adjacent pair of colloidosomes.

Microgel colloidosomes based on pH-responsive poly(tert-butylaminoethyl methacrylate) latexes.

Emulsion copolymerization of 2-(tert-butylamino)ethyl methacrylate (TBAEMA) with divinylbenzene (DVB) cross-linker in the presence of monomethoxy-capped poly(ethylene glycol) methacrylate (PEGMA) at 70 °C afforded sterically stabilized poly[2-(tert-butylamino)ethyl methacrylate] (PTBAEMA) latexes at 10% solids at pH 9. Such particles proved to be an effective Pickering emulsifier at pH 10 for both n-dodecane and n-hexane. (1)H NMR spectroscopy was used to follow the model reaction between the secondary amine of the TBAEMA monomer and the isocyanate groups of tolylene 2,4-diisocyanate-terminated poly(propylene glycol) (PPG-TDI). Cross-linking the PTBAEMA latex particles adsorbed at the n-dodecane/water interface using this oil-soluble PPG-TDI cross-linker at around 0 (o)C led to robust colloidosomes that survived an acid challenge. This resistance to demulsification was confirmed via laser diffraction studies following an in situ switch from pH 10 to 3, since no change was observed in either the oil droplet size or concentration (compared to non-cross-linked PTBAEMA-stabilized Pickering emulsions). Such PTBAEMA colloidosomes survived removal of the internal oil phase on washing with excess ethanol. However, because ethanol is a good solvent for the PTBAEMA chains, imaging the ethanol-treated colloidosomes via electron microscopy proved rather problematic due to partial film formation. Therefore, a series of TBAEMA/styrene copolymer latexes (comprising 10, 30, 50, or 60 mol % styrene) were prepared via emulsion copolymerization at 70 °C in the presence of DVB and PEGMA. The higher glass transition temperatures exhibited by these copolymer particles (and their greater resistance to ethanol swelling) enabled better-quality electron microscopy images to be obtained. The presence of nitrogen atoms at the surface of these copolymer latex particles was confirmed via X-ray photoelectron spectroscopy studies; these secondary amine groups allow covalent cross-linking via PPG-TDI when adsorbed at the surface of n-dodecane droplets at TBAEMA comonomer contents as low as 40 mol %. After removal of the n-hexane oil phase by evaporation, fluorescence microscopy studies indicate that these colloidosomes undergo collapse in their latex form at pH 10 but regain their original spherical morphology in their cationic microgel form at pH 3.5.

Arrested coalescence behaviour of giant Pickering droplets and colloidosomes stabilised by poly(tert-butylaminoethyl methacrylate) latexes.

The coalescence of two oil droplets grown at pH 10 in the presence of lightly cross-linked 260 nm diameter charge-stabilised poly(tert-butylamino)ethyl methacrylate (PTBAEMA) latexes was monitored using a high-speed video camera. Three model oils (n-dodecane, isopropyl myristate and sunflower oil) were investigated, each in the absence and presence of an oil-soluble cross-linker [tolylene 2,4-diisocyanate-terminated poly(propylene glycol), PPG-TDI]. In the absence of PPG-TDI, rapid coalescence was observed for giant PTBAEMA-stabilised Pickering oil droplets, which exhibited faster coalescence times compared to bare oil droplets. However, an increase in the damping coefficients for coalescing Pickering droplets (compared to those of bare oil droplets) indicated PTBAEMA latex particle adsorption. Addition of PPG-TDI cross-linker to oil droplets in the absence of latex particles led to a reduction in the interfacial tension confirming its surface-active nature. The oil-soluble PPG-TDI reacts with the secondary amine groups on the PTBAEMA latex, producing giant colloidosomes that remain stable to coalescence when brought into contact. This stability to coalescence was not observed for bare oil droplets in the presence of PPG-TDI, confirming that the cross-linked latex particles at the interface provide the additional stability. Finally, interactions between asymmetric n-dodecane droplets were examined. Adding oil-soluble cross-linker to only one droplet resulted in "arrested coalescence" behaviour in the presence of PTBAEMA latex particles. In this context, the droplet ageing time was found to be critical and is attributed to the relatively slow particle adsorption kinetics. Ageing times of less than 60 s led to catastrophic droplet coalescence, whereas ageing times longer than 60 s indicated cross-linker diffusion from one droplet to the other, which produced inter-cross-linked colloidosomes. Arrested coalescence was only observed for ageing times of approximately 60 s.

Nanoscale contact mechanics of biocompatible polyzwitterionic brushes.

Friction force microscopy has been used to demonstrate that biocompatible, lubricious poly(2-(methacryloyloxy)ethylphosphorylcholine) (PMPC) brushes exhibit different frictional properties depending on the medium (methanol, ethanol, 2-propanol, and water; the latter also with different quantities of added salt). The chemical functionalization of the probe (amine-, carboxylic acid-, and methyl-terminated probes were used) is not as important as the medium in determining the contact mechanics. For solvents such as methanol, where the adhesion between AFM probe and PMPC brushes is negligible, a linear friction-load relationship is observed. In contrast, the friction-load plot is nonlinear in ethanol or water, media in which stronger adhesion is measured. For ethanol, the data indicate Johnson-Kendall-Roberts (JKR) mechanics, whereas the Derjaguin-Muller-Toporov (DMT) model provided a good fit for the data acquired in water. Contact mechanics on zwitterionic PMPC brushes immersed in aqueous solutions of varying ionic strength followed the same trend, with high adhesion energies being correlated with a nonlinear friction-load relationship. These results can be rationalized by treating the friction force as the sum of a load-dependent term, attributed to molecular plowing, and an area-dependent shear term. In a good solvent for PMPC such as methanol, the shear term is negligible and the sliding interaction is dominated by molecular plowing. However, the adhesion energy is significantly larger in water and ethanol and the shear term is no longer negligible.

Effect of brush thickness and solvent composition on the friction force response of poly(2-(methacryloyloxy)ethylphosphorylcholine) brushes.

The frictional properties of poly(2-(methacryloyloxy)ethylphosphorylcholine) (PMPC) brushes grown from planar silicon surfaces by atom transfer radical polymerization (ATRP) have been characterized using in situ friction force microscopy (FFM). The dry thicknesses of the PMPC brushes ranged from 20 to 421 nm. For brush layers with dry thicknesses greater than ca. 100 nm, the coefficient of friction decreased with increasing film thickness. For shorter brushes, the coefficient of friction varied little with brush thickness. We hypothesize that the amount of bound solvent increases as the brush length increases, causing the osmotic pressure to increase and yielding a reduced tendency for the brush layer to deform under applied load. A comparison of the force-displacement plots acquired for various PMPC brushes under water supports this hypothesis, since a greater repulsive force is measured for thicker brushes. FFM was also used to investigate the well-known co-nonsolvency behavior exhibited by PMPC chains. For a PMPC brush layer of 307 nm dry thickness, the friction force was determined as a function of the volume fraction of alcohol in alcohol/water mixtures. Unlike a previous macroscopic study, a significant increase in the coefficient of friction was observed for ethanol/water mixtures at a volume fraction of 90%. This is attributed to brush collapse due to co-nonsolvency, leading to loss of hydration of the brush chains and hence substantially reduced lubrication. Force measurements normal to the surface indicate much greater hysteresis between approaching and retraction curves under co-nonsolvency conditions. However, no such effect was observed for 2-propanol/water and methanol/water mixtures over a wide range of volume fractions, in agreement with recent ellipsometric studies of PMPC brushes.