Steve Edmondson

Polymer brushes via surface-initiated polymerizations

Polymer brushes produced by controlled surface-initiated polymerization provide a route to surfaces coated with well-defined thin polymer films that are covalently bound to the substrate. All of the major controlled polymerization techniques have been applied to the synthesis of polymer brushes and examples of each are presented here. Many examples of brush synthesis in the literature have used the living atom transfer radical polymerization (ATRP) system, and in this tutorial review a particular focus is given to examples of this technique.

Co-Nonsolvency Effects for Surface-Initiated Poly(2-(methacryloyloxy)ethyl phosphorylcholine) Brushes in Alcohol/Water Mixtures

Surface-initiated atom transfer radical polymerization (SI-ATRP) has been used to grow brushes of poly(2-(methacryloyloxy)ethyl phosphorylcholine) (PMPC) from silicon wafers using a polyelectrolytic macroinitiator on planar silicon wafers. Film thicknesses of up to 450 nm were possible within 21 h, and the effect of adding activator and deactivator species on the brush growth rate was studied. The solvation of PMPC brushes in mixed alcohol/water solvents was investigated using in situ ellipsometry. Co-nonsolvency (a re-entrant swelling transition) behavior was observed in water/ethanol binary mixtures; that is, the PMPC brushes were highly swollen in either pure ethanol or water but became deswollen at specific ethanol-rich solvent compositions. A similar effect was obtained with water/2-propanol mixtures, except that in this case pure 2-propanol was not a particularly good solvent for the PMPC chains. However, co-nonsolvency was not observed for water/methanol binary mixtures, since the brushes remained well swollen at all solvent compositions. This is consistent with prior reports of co-nonsolvency effects in both PMPC gels and linear PMPC chains. However, this is the first report of this phenomenon for PMPC brushes and one of the first examples of co-nonsolvency observed for any polymer brush system. A direct comparison of brush and gel swelling reveals an approximate power-law relationship between the equilibrium volumes of these two systems at various solvent compositions, which is interpreted by treating the brush layer as a surface-attached gel. We believe this to be the first quantitative comparison of brush and gel swelling using the same polymer under the same conditions. The kinetics of the PMPC brush response to adjustment of the alcohol/water composition is relatively fast, with the brush volume change occurring on time scales of less than 1 min as judged by in situ ellipsometry.

Direct Measurement of Normal and Shear Forces between Surface-Grown Polyelectrolyte Layers†

This paper presents measurements, using the surface force balance (SFB), of the normal and shear forces in aqueous solutions between polyelectrolyte layers grown directly on mica substrates (grafted-from). The grafting-from was via surface-initiated atom transfer radical polymerization (surface-initiated ATRP) using a positively charged methacrylate monomer. X-ray reflectometry measurements confirm the successful formation of polyelectrolyte layers by this method. Surface-inititated ATRP has the advantages that the polymer chains can be strongly grafted to the substrate, and that high grafting densities should be achievable. Measured normal forces in water showed a long-range repulsion arising from an electrical double layer that extended beyond the polyelectrolyte layers, and a stronger, shorter-range repulsion when the polyelectrolyte brushes were in contact. Swollen layer thicknesses were in the range 15-40 nm. Upon addition of approximately 10(-2)-10(-1) M sodium nitrate, screening effects reduced the electrical double layer force to an undetectable level. Shear force measurements in pure water were performed, and the measured friction may arise from polymer chains bridging between the surfaces.

Synthesis of pH-responsive tertiary amine methacrylate polymer brushes and their response to acidic vapour

Weak polyelectrolyte brushes exhibit pH-responsive swelling behaviour, tuneable surface energy, and some promise as “smart” responsive coatings. In this paper, we demonstrate the growth of two weak polybase brushes by surface-initiated atom transfer radical polymerisation (SI-ATRP) using electrostatically adsorbed polyelectrolyte macro-initiators. Poly[2-(diethylamino)ethyl methacrylate] (PDEA) and poly[2-(diisopropylamino)ethyl methacrylate] (PDPA) brushes of 150 and 170 nm thickness respectively were grown within 22 h at 20 °C. Using in situellipsometry an acid-induced swelling transition was observed at pH 7.4 for PDEA and pH 6.5 for PDPA, similar to the pKa values reported for the corresponding free polymer chains. The kinetics of brush swelling involves an initially fast regime followed by a subsequent slower regime. Reversible surface energy switching with pH modulation was also demonstrated by contact angle goniometry. Finally, it was demonstrated that PDPA brushes respond to the presence of acidic vapours. On exposure to humid HCl vapour, such brushes become hydrophilic, resulting in water uptake and swelling, producing a visible change in the thin film interference colour.

Critical Salt Effects in the Swelling Behavior of a Weak Polybasic Brush

The swelling behavior of poly(2-(diethylamino)ethyl methacrylate) (PDEA) brushes in response to changes in solution pH and ionic strength has been investigated. The brushes were synthesized by ARGET ATRP methodology at the silica-aqueous solution interface via two different surface-bound initiator approaches: electrostatically adsorbed cationic macroinitiator and covalently anchored silane-based ATRP initiator moieties. The pH-response of these brushes is studied as a function of the solvated brush thickness in a constant flow regime that elucidates the intrinsic behavior of polymer brushes. In situ ellipsometry equilibrium measurements show the pH-induced brush swelling and collapse transitions are hysteretic in nature. Furthermore, high temporal resolution kinetic studies demonstrate that protonation and solvent ingress during swelling occur much faster than the brush charge neutralization and solvent expulsion during collapse. This hysteresis is attributed to the formation of a dense outer region or skin during collapse that retards solvent egress. Moreover, at a constant pH below its pKa, the PDEA brush exhibited a critical conformational change in the range 0.5-1 mM electrolyte, a range much narrower than predicted by the theory of the osmotic brush regime. This behavior is attributed to the hydrophobicity of the collapsed brush. The swelling and collapse kinetics for this salt-induced transition are nearly identical. This is in contrast to the asymmetry in the rate of the pH-induced response, suggesting an alternative mechanism for the two processes dependent on the nature of the environmental trigger.

Structure and Collapse of a Surface-Grown Strong Polyelectrolyte Brush on Sapphire

We have used neutron reflectometry to investigate the behavior of a strong polyelectrolyte brush on a sapphire substrate, grown by atom-transfer radical polymerization (ATRP) from a silane-anchored initiator layer. The initiator layer was deposited from vapor, following treatment of the substrate with an Ar/H(2)O plasma to improve surface reactivity. The deposition process was characterized using X-ray reflectometry, indicating the formation of a complete, cross-linked layer. The brush was grown from the monomer [2-(methacryloyloxy)ethyl]trimethylammonium chloride (METAC), which carries a strong positive charge. The neutron reflectivity profile of the swollen brush in pure water (D(2)O) showed that it adopted a two-region structure, consisting of a dense surface region ∼100 Å thick, in combination with a diffuse brush region extending to around 1000 Å from the surface. The existence of the diffuse brush region may be attributed to electrostatic repulsion from the positively charged surface region, while the surface region itself most probably forms due to polyelectrolyte adsorption to the hydrophobic initiator layer. The importance of electrostatic interactions in maintaining the brush region is confirmed by measurements at high (1 M) added 1:1 electrolyte, which show a substantial transfer of polymer from the brush to the surface region, together with a strong reduction in brush height. On addition of 10(-4) M oppositely charged surfactant (sodium dodecyl sulfate), the brush undergoes a dramatic collapse, forming a single dense layer about 200 Å in thickness, which may be attributed to the neutralization of the monomers by adsorbed dodecyl sulfate ions in combination with hydrophobic interactions between these dodecyl chains. Subsequent increases in surfactant concentration result in slow increases in brush height, which may be caused by stiffening of the polyelectrolyte chains due to further dodecyl sulfate adsorption.

Layer-by-layer deposition of polyelectrolyte macroinitiators for enhanced initiator density in surface-initiated ATRP.

The layer-by-layer (L-b-L) deposition of oppositely charged polyelectrolytic macroinitiators has been demonstrated on planar silica substrates. The build-up of the macroinitiator multilayers was monitored by ellipsometry (up to 21 layers) and dual polarization interferometry (up to 17 layers) and good agreement was found between these techniques. The increase in L-b-L thickness was approximately linear, with an average thickness of 2.3 A per layer of deposited macroinitiator. Surface-initiated ATRP of a model nonionic methacrylic monomer, 2-hydroxyethyl methacrylate (HEMA) in a 1:1 methanol/water mixture was conducted at ambient temperature. Increasing the number of macroinitiator layers led to a significant increase in PHEMA brush thickness up to 110 nm, which is attributed to the greater surface grafting density. PHEMA brush thicknesses obtained after 22 h showed a linear dependence on the number of layers of deposited macro-initiator, with all layers exhibiting near-identical growth kinetics. X-ray photoelectron spectroscopy was used to monitor L-b-L assembly and also to confirm PHEMA growth. This technique indicated the loss of small counterions from the multilayers during L-b-L deposition and confirmed an increase in the surface density of bromoester initiator groups as the number of deposited macroinitiator layers was increased. For 17 macroinitiator layers, the bromoester initiator density is estimated to be approximately 4.9 +/- 0.2 nm (-2) from the DPI data. This is comparable to that calculated for ATRP initiator monolayers obtained by either thiol or silane chemistry. Ellipsometry suggested that the macroinitiator multilayers were weakly hydrated prior to the in situ HEMA polymerization. AFM studies indicated that the PHEMA brushes had appreciable surface roughness, but this roughness became negligible compared to the brush thickness with increasing macroinitiator layers.

Neutron Reflectivity Study of the Structure of pH-Responsive Polymer Brushes Grown from a Macroinitiator at the Sapphire-Water Interface

Poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) brushes have been grown by surface-initiated atom transfer radical polymerization (SI-ATRP) from a polyanionic macroinitiator adsorbed at the sapphire-water interface, and neutron reflectivity has been used to characterize the structures and pH response of the brushes. The polymer brushes are well-described by Gaussian density profiles with an additional thin, dense layer close to the solid-liquid interface for the thicker brushes at pH 7 and 9, which produces a spike in the density profile. The spike in the distribution accounts for less than 5% of the polymer and disappears as the brushes swell at pH 3. The observed swelling behavior has been used in combination with the predictions of scaling theory and previous experimental measurements to determine the grafted density of PDMAEMA chains.

Functionalisation of Ti6Al4V components fabricated using selective laser melting with a bioactive compound.

Surface modification of an implant with a biomolecule is used to improve its biocompatibility and to reduce post-implant complications. In this study, a novel approach has been used to functionalise phosphonic acid monolayers with a drug. Ti6Al4V components fabricated using selective laser melting (SLM) were functionalised with Paracetamol (a pharmaceutically relevant biomolecule) using phosphonic acid based self-assembled monolayers (SAMs). The attachment, stability of the monolayers on the SLM fabricated surface and functionalisation of SAMs with Paracetamol were studied using X-ray photoelectron spectroscopy (XPS) and surface wettability measurements. The obtained results confirmed that SAMs were stable on the Ti6Al4V surface for over four weeks and then began to desorb from the surface. The reaction used to functionalise the phosphonic acid monolayers with Paracetamol was noted to be successful. Thus, the proposed method has the potential to immobilise drugs/proteins to SAM coated surfaces and improve their biocompatibility and reduce post-implant complications.

Anion-Specific Effects on the Behavior of pH-Sensitive Polybasic Brushes

The anion-specific solvation and conformational behavior of weakly basic poly(2-dimethylamino)ethyl methacrylate (poly(DMA)), poly(2-diethylamino)ethyl methacrylate (poly(DEA)), and poly(2-diisopropylamino)ethyl methacrylate (poly(DPA)) brushes, with correspondingly increasing inherent hydrophobicity, have been investigated using in situ ellipsometric and quartz crystal microbalance with dissipation (QCM-D) measurements. In the osmotic brush regime, as the initial low concentration of salt is increased, the brushes osmotically swell by the uptake of solvent as they become charged and the attractive hydrophobic inter- and intrachain interactions are overcome. With increased ionic strength, the brushes move into the salted brush regime where they desolvate and collapse as their electrostatic charge is screened. Here, as the brushes collapse, they transition to more uniform and rigid conformations, which dissipate less energy, than similarly solvated brushes at lower ionic strength. Significantly, in these distinct regimes brush behavior is not only ionic strength dependent but is also influenced by the nature of the added salt based on its position in the well-known Hofmeister or lyotropic series, with potassium acetate, nitrate, and thiocyanate investigated. The strongly kosmotropic acetate anions display low affinity for the hydrophobic polymers, and largely unscreened electrosteric repulsions allow the brushes to remain highly solvated at higher acetate concentrations. The mildly chaotropic nitrate and strongly chaotropic thiocyanate anions exhibit a polymer hydrophobicity-dependent affinity for the brushes. Increasing thiocyanate concentration causes the brushes to collapse at lower ionic strength than for the other two anions. This study of weak polybasic brushes demonstrates the importance of all ion, solvent, and polymer interactions.