Dr Joshua Willott

Dr Joshua Willott

Postdoctoral Research Fellow

School of Environmental and Life Sciences

Career Summary

Biography

I am early career colloid and surface scientist currently working as a postdoctoral research fellow in ARC Centre for Excellence for Enabling Eco-Efficient Beneficiation of Minerals at the University of Newcastle, Australia. I apply my expertise to develop, characterise and implement response polymeric reagents for selective flotation of valuable minerals. I also work on novel membrane materials made through non-equilibrium aqueous phase separation and polyelectrolyte complexation approaches. In addition, I study a range of soft-condensed matter systems relevant to lubrication, protective coatings, and separation phenomena. I focus my research on fundamental polymer–molecule and polymer–ion interactions using advanced techniques like neutron reflectometry, spectral ellipsometry and self-consistent field theory. Throughout my career, I have made significant contributions to the understanding of the solid-liquid interface in the presence of pH, salt, temperature, and specific ion-responsive polymers as well as the development of functional membrane materials; evidenced by my strong record of publishing in international peer-reviewed journals of high impact.

Qualifications

  • Doctor of Philosophy in Chemistry, University of Newcastle
  • Bachelor of Science (Chemistry), University of Newcastle
  • Bachelor of Science (Honours), University of Newcastle

Keywords

  • colloid, polymer and interface chemistry
  • functional interfaces & smart coatings
  • membrane science
  • physical chemistry

Fields of Research

Code Description Percentage
340603 Colloid and surface chemistry 100

Professional Experience

UON Appointment

Title Organisation / Department
Postdoctoral Research Fellow University of Newcastle
School of Environmental and Life Sciences
Australia
Casual Academic University of Newcastle
School of Environmental and Life Sciences
Australia

Academic appointment

Dates Title Organisation / Department
9/1/2017 - 1/2/2021 Postdoctoral Research Fellow University of Twente
Faculty of Science and Technology
Netherlands
Edit

Publications

For publications that are currently unpublished or in-press, details are shown in italics.


Journal article (27 outputs)

Year Citation Altmetrics Link
2021 Baig MI, Sari PPI, Li J, Willott JD, de Vos WM, 'Sustainable Aqueous Phase Separation membranes prepared through mild pH shift induced polyelectrolyte complexation of PSS and PEI', Journal of Membrane Science, 625 (2021)

pH shift induced Aqueous phase separation (APS) is a novel and more sustainable water-based approach to create microfiltration, ultrafiltration, and nanofiltration membranes. APS ... [more]

pH shift induced Aqueous phase separation (APS) is a novel and more sustainable water-based approach to create microfiltration, ultrafiltration, and nanofiltration membranes. APS allows for control over membrane pore size and structure in ways analogous to traditional non-solvent induced phase separation (NIPS). Unfortunately, existing APS approaches require extreme pH shifts (from pH 14 to pH 1) to obtain successful membranes, limiting their applicability for large scale production. Here we demonstrate that APS membranes, with tunable pore sizes ranging from ~80 nm to dense nanofiltration type, can be prepared using a mild pH shift (pH 12 to pH 4) based on the complexation of poly(styrene sulfonate) (PSS) and branched polyethyleneimine (PEI) in acetate buffer coagulation baths. The molecular weight of PEI, the concentration and the pH value of the buffer solution, and the concentration of glutaraldehyde cross-linking agent were systematically varied to control and optimize the membrane fabrication conditions. It was found that tight nanofiltration membranes having a molecular weight cut-off of ~200 g mol-1 and excellent salt (97% MgCl2) and micropollutant retentions (~96%) could be prepared alongside ultra/microfiltration type membranes with an average pore size of ~60 nm. These results indicate that APS membranes with tunable pore sizes can be prepared under mild pH conditions with excellent control over separation properties.

DOI 10.1016/j.memsci.2021.119114
Citations Scopus - 3
2021 Baig MI, Willott JD, De Vos WM, 'Enhancing the Separation Performance of Aqueous Phase Separation-Based Membranes through Polyelectrolyte Multilayer Coatings and Interfacial Polymerization', ACS Applied Polymer Materials, 3 3560-3568 (2021)

The aqueous phase separation (APS) technique allows membrane fabrication without use of unsustainable organic solvents, while at the same time, it provides extensive control over ... [more]

The aqueous phase separation (APS) technique allows membrane fabrication without use of unsustainable organic solvents, while at the same time, it provides extensive control over membrane pore size and morphology. Herein, we investigate if polyelectrolyte complexation-induced APS ultrafiltration membranes can be the basis for different types of nanofiltration membranes. We demonstrate that APS membranes can be used as support membranes for functional surface coatings like thin polyelectrolyte multilayer (PEMs) and interfacial polymerization (IP) coatings. Three different PEMs were fabricated on poly(sodium 4-styrene sulfonate) (PSS) poly(allylamine hydrochloride) (PAH) APS ultrafiltration membranes, and only 4.5 bilayers were needed to create nanofiltration membranes with molecular weight cut-off (MWCO) values of 210-390 Da while maintaining a roughly constant water permeability (~1.7 L·m-2·h-1·bar-1). The PEM-coated membranes showed excellent MgCl2 (~98%), NaCl (~70%), and organic micropollutant retention values (>90%). Similarly, fabricating thin polyamide layers on the ultrafiltration PSS-PAH APS membranes by IP resulted in nanofiltration membranes with MWCO values of ~200 Da. This work shows for the first time that APS membranes can indeed be utilized as excellent support membranes for the application of functional coatings without requiring any form of pretreatment.

DOI 10.1021/acsapm.1c00457
2021 Gresham IJ, Humphreys BA, Willott JD, Johnson EC, Murdoch TJ, Webber GB, et al., 'Geometrical Confinement Modulates the Thermoresponse of a Poly(N-isopropylacrylamide) Brush', MACROMOLECULES, 54 2541-2550 (2021) [C1]
DOI 10.1021/acs.macromol.0c02775
Co-authors Grant Webber, Erica Wanless
2021 Nielen WM, Willott JD, Galicia JAR, De Vos WM, 'Effect of solution viscosity on the precipitation of psama in aqueous phase separation-based membrane formation', Polymers, 13 (2021)

Aqueous phase separation (APS) is a recently developed sustainable alternative to the conventional organic solvent based nonsolvent-induced phase separation (NIPS) method to prepa... [more]

Aqueous phase separation (APS) is a recently developed sustainable alternative to the conventional organic solvent based nonsolvent-induced phase separation (NIPS) method to prepare polymeric membranes. In APS, polyelectrolytes are precipitated from aqueous solutions through pH or salinity switches. Although APS differs from NIPS in the polymer and solvents, they share many tuning parameters. In this work, we investigate the APS-based preparation of membranes from poly(styrene-alt-maleic acid) (PSaMA) with a focus on acid concentration in the coagulation bath, and polymer and additive concentration in the casting solution. Nanofiltration membranes are prepared using significantly lower concentrations of acid: 0.3 M HCl compared to the 2 M of either acetic or phosphoric acid used in previous works. It is shown that higher polymer concentrations can be used to prevent defect formation in the top layer. In addition, acetic acid concentration also strongly affects casting solution viscosity and thus can be used to control membrane structure, where lower acetic acid concentrations can prevent the formation of macrovoids in the support structure. The prepared nanofiltration membranes exhibit a very low molecular weight cutoff (210 ± 40 dalton), making these sustainable membranes very relevant for the removal of contaminants of emerging concern. Understanding how the parameters described here affect membrane preparation and performance is essential to optimizing membranes prepared with APS towards this important application.

DOI 10.3390/polym13111775
2020 Durmaz EN, Baig MI, Willott JD, De Vos WM, 'Polyelectrolyte Complex Membranes via Salinity Change Induced Aqueous Phase Separation', ACS Applied Polymer Materials, 2 2612-2621 (2020)

Polymeric membranes are used on very large scales for drinking water production and kidney dialysis, but they are nearly always prepared by using large quantities of unsustainable... [more]

Polymeric membranes are used on very large scales for drinking water production and kidney dialysis, but they are nearly always prepared by using large quantities of unsustainable and toxic aprotic solvents. In this study, a water-based, sustainable, and simple way of making polymeric membranes is presented without the need for harmful solvents or extreme pH conditions. Membranes were prepared from water-insoluble polyelectrolyte complexes (PECs) via aqueous phase separation (APS). Strong polyelectrolytes (PEs), poly(sodium 4-styrenesulfonate) (PSS), and poly(diallyldimethylammonium chloride) (PDADMAC) were mixed in the presence of excess of salt, thereby preventing complexation. Immersing a thin film of this mixture into a low-salinity bath induces complexation and consequently the precipitation of a solid PEC-based membrane. This approach leads to asymmetric nanofiltration membranes, with thin dense top layers and porous, macrovoid-free support layers. While the PSS molecular weight and the total polymer concentrations of the casting mixture did not significantly affect the membrane structure, they did affect the film formation process, the resulting mechanical stability of the films, and the membrane separation properties. The salt concentration of the coagulation bath has a large effect on membrane structure and allows for control over the thickness of the separation layer. The nanofiltration membranes prepared by APS have a low molecular weight cutoff (<300 Da), a high MgSO4 retention (~80%), and good stability even at high pressures (10 bar). PE complexation induced APS is a simple and sustainable way to prepare membranes where membrane structure and performance can be tuned with molecular weight, polymer concentration, and ionic strength.

DOI 10.1021/acsapm.0c00255
Citations Scopus - 9
2020 Durmaz EN, Willott JD, Fatima A, de Vos WM, 'Weak polyanion and strong polycation complex based membranes: Linking aqueous phase separation to traditional membrane fabrication', European Polymer Journal, 139 (2020)

In recent work, Aqueous Phase Separation (APS) based on pH change induced polyelectrolyte complexation has shown great potential for the preparation of sustainable polymeric membr... [more]

In recent work, Aqueous Phase Separation (APS) based on pH change induced polyelectrolyte complexation has shown great potential for the preparation of sustainable polymeric membranes with tunable structures. Unfortunately, thus far this has only been possible with a single polyelectrolyte combination. In this work, we demonstrate that this APS approach extends beyond a single system by preparing sustainable membranes from polyelectrolyte complexes (PECs) of the weak polyanion poly(acrylic acid) (PAA) and the strong polycation poly(diallyldimethylammonium chloride) (PDADMAC). PE solutions are mixed in an acidic medium where PAA is uncharged, and then this mixture is cast and immersed in a coagulation bath at a pH where PAA becomes charged and able to form a PEC with the oppositely charged PDADMAC. Since this process includes both phase separation and PE complexation, it is expected that membrane structure and performance is influenced by a combination of many factors. Casting solution pH, PAA molecular weight, and coagulation bath pH all directly affect the phase separation behavior of PAA/PDADMAC complexes in ways similar to conventional nonsolvent induced phase separation (NIPS). In addition, coagulation bath salinity and PE mixing ratio influence the complexation behavior. Through tuning of all these parameters it is possible to create a wide variety of membrane structures, ranging from nodular symmetrically porous membranes, to asymmetric membranes with cellular pores and in some cases dense top layers. The nodular membranes show good performance as microfiltration membranes with excellent oil retention (>95% for 3¿4 µm droplets) and good water permeances. However for the cellular membranes, filtration led to collapse of the porous structure, emphasizing the importance of PE selection for membrane applications.

DOI 10.1016/j.eurpolymj.2020.110015
Citations Scopus - 4
2020 Nielen WM, Willott JD, Esguerra ZM, de Vos WM, 'Ion specific effects on aqueous phase separation of responsive copolymers for sustainable membranes', Journal of Colloid and Interface Science, 576 186-194 (2020)

Hypothesis: Salt identity and concentration affects the preparation of membranes via the aqueous phase separation approach. The phase inversion process and morphology of the resul... [more]

Hypothesis: Salt identity and concentration affects the preparation of membranes via the aqueous phase separation approach. The phase inversion process and morphology of the resultant membranes is expected to vary as function of these two parameters. Experiments: Polymeric membranes based on the responsive copolymer polystyrene-alt-maleic acid (PSaMA) are prepared using the aqueous phase separation approach and the influence of salt identity (Na2SO4, LiCl, NaCl, NaNO3, NH4Cl, MgCl2, CaCl2) and concentration on resultant membrane morphology and separation performance is investigated. Complementary stability experiments of PSaMA solutions are performed to help understand the intricate aqueous phase separation process. Findings: Specific ion effects are observed during membrane formation by the aqueous phase separation approach. At equal ionic strengths, Na2SO4 and LiCl lead to the formation of more open membrane structures compared to NaCl, NaNO3, NH4Cl, and MgCl2, while CaCl2 results in membranes with dense top layers. These ion-specific effects are likely caused by a combination of ion mobility and interaction potential between the ion and the polyelectrolyte. Overall, from this work it becomes clear that salt identity and concentration are key parameters in the APS process, and they can be optimised to tune membrane structure from open microfiltration to dense nanofiltration membranes.

DOI 10.1016/j.jcis.2020.04.125
Citations Scopus - 4
2020 Gresham IJ, Reurink DM, Prescott SW, Nelson ARJ, De Vos WM, Willott JD, 'Structure and Hydration of Asymmetric Polyelectrolyte Multilayers as Studied by Neutron Reflectometry: Connecting Multilayer Structure to Superior Membrane Performance', Macromolecules, 53 10644-10654 (2020)

Porous membranes coated with so-called asymmetric polyelectrolyte multilayers (PEMs) have recently been shown to outperform commercial membranes for micropollutant removal. They c... [more]

Porous membranes coated with so-called asymmetric polyelectrolyte multilayers (PEMs) have recently been shown to outperform commercial membranes for micropollutant removal. They consist of open support layers of poly(styrene sulfonate) (PSS)/poly(allylamine) (PAH) capped by denser and more selective layers of either PAH/poly(acrylic acid) (PAA) or PAH/Nafion. Unfortunately, the structure of these asymmetric PEMs, and thus their superior membrane performance, is poorly understood. In this work, neutron reflectometry (NR) is employed to elucidate the multilayered structure and hydration of these asymmetric PEMs. NR reveals that the multilayers are indeed asymmetric in structure, with distinct bottom and top multilayers when air-dried and when solvated. The low hydration of the top [PAH/Nafion] multilayer, together with the low water permeance of comparable [PAH/Nafion]-capped PEM membranes, demonstrate that it is a reduction in hydration that makes these separation layers denser and more selective. In contrast, the [PAH/PAA] capping multilayers are more hydrated than the support [PSS/PAH] layers, signifying that, here, densification of the separation layer occurs through a decrease in the mesh size (or effective pore size) of the top layer due to the higher charge density of the PAH/PAA couple compared to the PSS/PAH couple. The [PAH/PAA] and [PAH/Nafion] separation layers are extremely thin (~4.5 and ~7 nm, respectively), confirming that these asymmetric PEM membranes have some of the thinnest separation layers ever achieved.

DOI 10.1021/acs.macromol.0c01909
Citations Scopus - 1
2020 Baig MI, Willott JD, de Vos WM, 'Tuning the structure and performance of polyelectrolyte complexation based aqueous phase separation membranes', Journal of Membrane Science, 615 (2020)

Aqueous Phase Separation (APS) provides a new and sustainable platform to fabricate polymeric membranes entirely in water. Still, little is known on how the casting solution and c... [more]

Aqueous Phase Separation (APS) provides a new and sustainable platform to fabricate polymeric membranes entirely in water. Still, little is known on how the casting solution and coagulation bath compositions can be used to tune membrane structure and performance. This work comprises a detailed investigation on the tuning parameters avaliable to tailor the morphology, pore size distribution, and water permeability of polyelectrolyte complex membranes prepared from poly(sodium 4-styrenesulfonate) (PSS) and polyallylamine hydrochloride (PAH). To avoid complexation of PAH and PSS in the casting solution, an optimum amount of base (NaOH) must be added to deprotonate PAH. In addition, the monomer mixing ratio of PSS to PAH significantly influences membrane morphology by modulating the interactions between the two polyelectrolytes. Coagulation bath pH can be used to control the driving force for complexation. Decreasing bath pH facilitates the formation of denser membranes, allowing ~97% protein retentions, whereas increasing bath pH leads to more open membrane structures. Changing the concentration of crosslinker in the coagulation bath allows tuning of membrane pore size from ~2 nm to ~46 nm, while simultaneously influencing membrane mechanical properties. Overall, this work highlights several key parameters to tune APS membrane morphology, demonstrating the versatility of APS to prepare optimized sustainable membranes for specific applications.

DOI 10.1016/j.memsci.2020.118502
Citations Scopus - 4
2020 Baig MI, Durmaz EN, Willott JD, de Vos WM, 'Sustainable Membrane Production through Polyelectrolyte Complexation Induced Aqueous Phase Separation', Advanced Functional Materials, 30 (2020)

Nonsolvent induced phase separation (NIPS) is the most common approach to produce polymeric membranes. Unfortunately, NIPS relies heavily on aprotic organic solvents like N-methyl... [more]

Nonsolvent induced phase separation (NIPS) is the most common approach to produce polymeric membranes. Unfortunately, NIPS relies heavily on aprotic organic solvents like N-methyl-pyrrolidone. These solvents are unsustainable, repro-toxic for humans and are therefore becoming increasingly restricted within the European Union. A new and sustainable method, aqueous phase separation (APS), is reported that eliminates the use of organic solvents. A homogeneous solution of two polyelectrolytes, the strong polyanion poly(sodium 4-styrenesulfonate) (PSS) and the weak polycation poly(allylamine hydrochloride) (PAH), is prepared at high pH, where PAH is uncharged. Immersing a film of this solution in a low pH bath charges the PAH and results in a controlled precipitation, forming a porous water-insoluble polyelectrolyte complex, a membrane. Pore sizes can be tuned from micrometers to just a few nanometers, and even to dense films, simply by tuning the polyelectrolyte concentrations, molecular weights, and by changing the salinity of the bath. This leads to excellent examples of microfiltration, ultrafiltration, and nanofiltration membranes. Polyelectrolyte complexation induced APS is a viable and sustainable approach to membrane production that provides excellent control over membrane properties and even allows new types of separations.

DOI 10.1002/adfm.201907344
Citations Scopus - 29
2020 Kyriakou N, Merlet RB, Willott JD, Nijmeijer A, Winnubst L, Pizzoccaro-Zilamy MA, 'New Method toward a Robust Covalently Attached Cross-Linked Nanofiltration Membrane', ACS Applied Materials and Interfaces, 12 47948-47956 (2020)

As nanofiltration applications increase in diversity, there is a need for new fabrication methods to prepare chemically and thermally stable membranes with high retention performa... [more]

As nanofiltration applications increase in diversity, there is a need for new fabrication methods to prepare chemically and thermally stable membranes with high retention performance. In this work, thio-bromo "click"chemistry was adapted for the fabrication of a robust covalently attached and ultrathin nanofiltration membrane. The selective layer was formed on a pre-functionalized porous ceramic surface via a novel, liquid-vapor interfacial polymerization method. Compared to the most common conventional interfacial polymerization procedure, no harmful solvents and a minimal amount of reagents were used. The properties of the membrane selective layer and its free-standing equivalent were characterized by complementary physicochemical analysis. The stability of the thin selective layer was established in water, ethanol, non-polar solvents, and up to 150 °C. The potential as a nanofiltration membrane was confirmed through solvent permeability tests (water, ethanol, hexane, and toluene), PEG-in-water molecular weight cut-off measurements (¿700 g mol-1), and dye retention measurements.

DOI 10.1021/acsami.0c13339
Citations Scopus - 2
2020 Reurink DM, Willott JD, Roesink HDW, De Vos WM, 'Role of Polycation and Cross-Linking in Polyelectrolyte Multilayer Membranes', ACS Applied Polymer Materials, 2 5278-5289 (2020)

Alternate deposition of oppositely charged polyelectrolytes is an excellent approach to control the chemistry of interfaces. Membrane technology is one field that benefits from th... [more]

Alternate deposition of oppositely charged polyelectrolytes is an excellent approach to control the chemistry of interfaces. Membrane technology is one field that benefits from the simplicity and tunability of polyelectrolyte multilayers (PEMs). Herein, ultrafiltration support membranes are coated with PEMs to fabricate nanofiltration membranes. Three PEMs, of different polymeric structures, namely, those of poly(4-styrene sulfonate) (PSS)/poly(allylamine hydrochloric acid) (PAH), PSS/poly(ethyleneimine) (PEI, branched), and PSS/poly(4-aminostyrene) (PAS), are prepared and studied from a fundamental perspective in terms of multilayer composition and cross-linking and also from an applied perspective through PEM membrane performance. The low molecular weight cutoff (MWCO) of the PSS/PAH membranes signifies their dense structure (small mesh size), while ion retentions indicate that the dielectric exclusion mechanism is dominant. The PSS/PEI membranes are even denser and have higher selectivities. In contrast, the PSS/PAS membranes are more open, which is likely due to the lower charge density of PAS compared to PEI and PAH. After chemical cross-linking, all of the PEM membranes are denser and therefore more selective and less permeable to water. Micropollutant retention increases for cross-linked PSS/PAH membranes, whereas little to no improvement is seen for cross-linked PSS/PAS and PSS/PEI membranes. Overall, this study shows that completely different membrane properties can be obtained by changing the type of polycation, thus demonstrating the high versatility of PEM-based membranes. In addition, for all PEM membranes, cross-linking acts as an additional tuning parameter that leads to denser and typically more selective layers.

DOI 10.1021/acsapm.0c00992
Citations Scopus - 4
2020 Johnson EC, Willott JD, de Vos WM, Wanless EJ, Webber GB, 'Interplay of Composition, pH, and Temperature on the Conformation of Multi-stimulus-responsive Copolymer Brushes: Comparison of Experiment and Theory.', Langmuir : the ACS journal of surfaces and colloids, 36 5765-5777 (2020) [C1]
DOI 10.1021/acs.langmuir.0c00424
Citations Scopus - 5Web of Science - 5
Co-authors Grant Webber, Erica Wanless
2020 Willott JD, Nielen WM, De Vos WM, 'Stimuli-Responsive Membranes through Sustainable Aqueous Phase Separation', ACS Applied Polymer Materials, 2 659-667 (2020)

Polymeric membranes are used on huge scales for kidney dialysis, wastewater treatment, and drinking water production. However, almost all polymeric membranes are fabricated by a p... [more]

Polymeric membranes are used on huge scales for kidney dialysis, wastewater treatment, and drinking water production. However, almost all polymeric membranes are fabricated by a process reliant on the use of unsustainable, expensive, and reprotoxic dipolar aprotic solvents. In this work, we propose an aqueous phase separation approach for preparing porous membrane films. Poly(4-vinylpyridine) (P4VP), a pH-responsive polymer, is first dissolved at low pH where the polymer is charged and subsequently cast as a thin film. Switching to a high pH where the polymer is uncharged and insoluble results in controlled phase separation and solidification of the polymer into porous membrane structures. This approach gives a large degree of control over membrane structure, leading to symmetric porous microfiltration membranes and asymmetric dense nanofiltration membranes. Moreover, the use of a pH-responsive polymer leads directly to a pH-responsive membrane, where the degree of responsive behavior can be tuned by the degree of cross-linking. Such responsive behavior allows effective cleaning of the membrane, without the use of harsh chemicals. This work outlines an approach toward preparing membranes in a more sustainable fashion - an approach that allows control over the membrane structure and one that naturally leads to advanced membranes with responsive properties.

DOI 10.1021/acsapm.9b01006
Citations Scopus - 15
2020 Johnson EC, Willott JD, Gresham IJ, Murdoch TJ, Humphreys BA, Prescott SW, et al., 'Enrichment of Charged Monomers Explains Non-monotonic Polymer Volume Fraction Profiles of Multi-stimulus Responsive Copolymer Brushes.', Langmuir : the ACS journal of surfaces and colloids, 36 12460-12472 (2020) [C1]
DOI 10.1021/acs.langmuir.0c01502
Citations Scopus - 2Web of Science - 2
Co-authors Erica Wanless, Grant Webber
2019 Willott JD, Humphreys BA, Webber GB, Wanless EJ, De Vos WM, 'Combined Experimental and Theoretical Study of Weak Polyelectrolyte Brushes in Salt Mixtures', Langmuir, 35 2709-2718 (2019) [C1]
DOI 10.1021/acs.langmuir.8b03838
Citations Scopus - 9Web of Science - 8
Co-authors Grant Webber, Erica Wanless
2017 Willott JD, Murdoch TJ, Webber GB, Wanless EJ, 'Physicochemical behaviour of cationic polyelectrolyte brushes', PROGRESS IN POLYMER SCIENCE, 64 52-75 (2017) [C1]
DOI 10.1016/j.progpolymsci.2016.09.010
Citations Scopus - 42Web of Science - 40
Co-authors Erica Wanless, Grant Webber
2017 Murdoch TJ, Humphreys BA, Willott JD, Prescott SW, Nelson A, Webber GB, Wanless EJ, 'Enhanced specific ion effects in ethylene glycol-based thermoresponsive polymer brushes', JOURNAL OF COLLOID AND INTERFACE SCIENCE, 490 869-878 (2017) [C1]
DOI 10.1016/j.jcis.2016.11.044
Citations Scopus - 24Web of Science - 24
Co-authors Grant Webber, Erica Wanless
2016 Murdoch TJ, Willott JD, De Vos WM, Nelson A, Prescott SW, Wanless EJ, Webber GB, 'Influence of Anion Hydrophilicity on the Conformation of a Hydrophobic Weak Polyelectrolyte Brush', Macromolecules, 49 9605-9617 (2016) [C1]

The conformation of a hydrophobic, weak cationic poly(2-diisopropylamino)ethyl methacrylate (PDPA) brush was studied using neutron reflectometry as a function of aqueous solution ... [more]

The conformation of a hydrophobic, weak cationic poly(2-diisopropylamino)ethyl methacrylate (PDPA) brush was studied using neutron reflectometry as a function of aqueous solution pH, ionic strength, and anion identity. In pH 4, 10 mM potassium nitrate the brush is highly charged, resulting in an extended, dilute conformation; at pH 9 the uncharged brush collapses to a single, dense layer. The brush response to added salt at constant pH (4.5) for varying concentrations of the potassium salts of acetate, nitrate, and thiocyanate revealed ion-specific conformations of the brush. At low ionic strength (0.1 mM) the brush was collapsed, independent of salt identity, while at higher ionic strengths (up to 500 mM) the conformation was dependent on counterion identity. The brush exhibited extended conformations in the presence of kosmotropic acetate counterions, while collapsed conformations were retained in the presence of strongly chaotropic thiocyanate counterions. The brush showed a richer set of behaviors in the solutions containing the weakly chaotropic nitrate anion, being similar to acetate (swollen) at intermediate concentrations but similar to thiocyanate (collapsed) at high salt concentrations. Numerical self-consistent field (nSCF) simulations indicate that the response of the brush to pH changes is dominated by the hydrophobicity of the polymer at pH values near the pKa. Furthermore, the simulations reveal that the addition of a single Flory-Huggins interaction parameter analogous to the hydrophilicity of the counterion is sufficient to replicate the observed specific anion response of a hydrophobic weak polyelectrolyte brush.

DOI 10.1021/acs.macromol.6b01897
Citations Scopus - 27Web of Science - 27
Co-authors Grant Webber, Erica Wanless
2016 Murdoch TJ, Humphreys BA, Willott JD, Gregory KP, Prescott SW, Nelson A, et al., 'Specific Anion Effects on the Internal Structure of a Poly(N-isopropylacrylamide) Brush', Macromolecules, 49 6050-6060 (2016) [C1]

The effect of anion identity and temperature on the internal nanostructure of poly(N-isopropylacrylamide) brushes were investigated using neutron reflectometry (NR), atomic force ... [more]

The effect of anion identity and temperature on the internal nanostructure of poly(N-isopropylacrylamide) brushes were investigated using neutron reflectometry (NR), atomic force microscopy (AFM), and quartz crystal microbalance with dissipation monitoring (QCM-D). NR and QCM-D measurements showed that addition of strongly kosmotropic acetate anions shifted the lower critical solution temperature (LCST) to lower temperatures relative to pure D2O/H2O, while strongly chaotropic thiocyanate anions shifted the LCST to higher temperatures. Polymer density profiles derived from NR showed direct evidence of vertical phase separation at temperatures around the LCST in all conditions. Results indicate that the density profiles were not simple modulations of structures observed in D2O to higher or lower temperatures, with both anion identity and ionic strength found to influence the qualitative features of the profiles. In particular, the presence of thiocyanate broadened the LCST transition which is attributed to the ability of the thiocyanate anion to electrosterically stabilize the brush above its LCST. Complementary AFM data showed that the acetate ion induced collapsed structures while a broader transition is observed in the presence of thiocyanate.

DOI 10.1021/acs.macromol.6b01001
Citations Scopus - 32Web of Science - 32
Co-authors Erica Wanless, Grant Webber
2016 Willott JD, Murdoch TJ, Webber GB, Wanless EJ, 'Nature of the Specific Anion Response of a Hydrophobic Weak Polyelectrolyte Brush Revealed by AFM Force Measurements', Macromolecules, 49 2327-2338 (2016) [C1]

Complementary interaction force measurements between an atomic force microscope (AFM) tip or colloid probe and a weak polybasic brush have been shown to yield a number of fundamen... [more]

Complementary interaction force measurements between an atomic force microscope (AFM) tip or colloid probe and a weak polybasic brush have been shown to yield a number of fundamental characteristics of the brush and its response to the presence of specific anions in aqueous solution. Stretching of the poly(2-diisopropylamino)ethyl methacrylate (PDPA) chains physisorbed to the AFM tip and modeling the resultant force curves allowed the persistence and contour lengths, molecular weight, and thus grafting density of the brush to be determined. In kosmotropic acetate, high osmotic forces associated with the swollen PDPA brush repelled the colloid probe during both approach and retraction. For mildly chaotropic nitrate the behavior was similar, but at high ionic strength and during retraction, the interaction was strongly adhesive partly because of decreased brush solvation. For strongly chaotropic thiocyanate, the interaction was adhesive over the entire concentration range studied. Here, physical contact between the poorly solvated brush and the colloid resulted in an attractive force.

DOI 10.1021/acs.macromol.5b02656
Citations Scopus - 32Web of Science - 32
Co-authors Erica Wanless, Grant Webber
2016 Humphreys BA, Willott JD, Murdoch TJ, Webber GB, Wanless EJ, 'Specific ion modulated thermoresponse of poly(N-isopropylacrylamide) brushes', PHYSICAL CHEMISTRY CHEMICAL PHYSICS, 18 6037-6046 (2016) [C1]
DOI 10.1039/c5cp07468a
Citations Scopus - 45Web of Science - 44
Co-authors Grant Webber, Erica Wanless
2015 Willott JD, Humphreys BA, Murdoch TJ, Edmondson S, Webber GB, Wanless EJ, 'Hydrophobic effects within the dynamic pH-response of polybasic tertiary amine methacrylate brushes.', Phys Chem Chem Phys, 17 3880-3890 (2015) [C1]
DOI 10.1039/c4cp05292g
Citations Scopus - 30Web of Science - 29
Co-authors Grant Webber, Erica Wanless
2015 Willott JD, Murdoch TJ, Humphreys BA, Edmondson S, Wanless EJ, Webber GB, 'Anion-specific effects on the behavior of pH-sensitive polybasic brushes.', Langmuir, 31 3707-3717 (2015) [C1]
DOI 10.1021/acs.langmuir.5b00116
Citations Scopus - 44Web of Science - 43
Co-authors Erica Wanless, Grant Webber
2014 Willott JD, Murdoch TJ, Humphreys BA, Edmondson S, Webber GB, Wanless EJ, 'Critical salt effects in the swelling behavior of a weak polybasic brush.', Langmuir, 30 1827-1836 (2014) [C1]
DOI 10.1021/la4047275
Citations Scopus - 52Web of Science - 52
Co-authors Grant Webber, Erica Wanless
2013 Cheesman BT, Neilson AJG, Willott JD, Webber GB, Edmondson S, Wanless EJ, 'Effect of Colloidal Substrate Curvature on pH-Responsive Polyelectrolyte Brush Growth', LANGMUIR, 29 6131-6140 (2013) [C1]
DOI 10.1021/la4004092
Citations Scopus - 27Web of Science - 26
Co-authors Grant Webber, Erica Wanless
2012 Cheesman BT, Willott JD, Webber GB, Edmondson S, Wanless EJ, 'pH-responsive brush-modified silica hybrids synthesized by surface-initiated ARGET ATRP', ACS Macro Letters, 1 1161-1165 (2012) [C1]
DOI 10.1021/mz3003566
Citations Scopus - 45Web of Science - 43
Co-authors Grant Webber, Erica Wanless
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Research Supervision

Number of supervisions

Completed0
Current1

Current Supervision

Commenced Level of Study Research Title Program Supervisor Type
2021 PhD The Adsorption of RAFT Polymer Collectors for Selective Flotation of Specific Minerals PhD (Chemistry), College of Engineering, Science and Environment, The University of Newcastle Co-Supervisor
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Dr Joshua Willott

Positions

Postdoctoral Research Fellow
School of Environmental and Life Sciences
College of Engineering, Science and Environment

Casual Academic
School of Environmental and Life Sciences
College of Engineering, Science and Environment

Contact Details

Email joshua.willott@newcastle.edu.au
Phone 49138622

Office

Room C216b
Building Chemistry
Location Callaghan
University Drive
Callaghan, NSW 2308
Australia
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