Browsing by Author "Murdoch, TJ"

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    Examining the structural and mechanical implications of surfactants on neutral polymer brushes through neutron reflectometry
    (Australian Institute of Nuclear Science and Engineering (AINSE), 2018-11-19) Gresham, IJ; Willott, JD; de Vos, WM; Johnson, EC; Humphreys, BA; Murdoch, TJ; Wanless, EJ; Webber, GB; Nelson, A; Prescott, SW
    Layers of densely-tethered polymers (polymer brushes) are of interest due to their potential applications as nano-actuators, biocompatible coatings, and switchable lubricating or antifouling surfaces. These applicable properties are dependant on the structure of the polymer interface, so it is important that the structural effects of common compounds and relevant environmental variables be understood. Neutron Reflectometry (NR) is the only technique capable of providing detailed structural resolution of solvated multi-component polymer brush systems due to its penetrating power and the possibility of isotopic substitution. Here we present a Neutron Reflectometry study on the effects of surface-active molecules (surfactants) on two neutral polymer brushes, poly(ethylene oxide) (PEO) and Poly(N-isopropylacrylamide) (PNIPAM), focusing on small, single tail surfactants. PEO is a widely used biocompatible polymer with a range of medical and commercial applications, whilst PNIPAM is a well known thermoresponsive polymer, undergoing a swollen to collapsed transition over its critical solution temperature (CST) of 32˚C. We show that these two polymers exhibit similar yet distinct interactions with surfactants, with the observed differences having implications for the mechanism of brush-surfactant interaction. The presence of surfactants was found to raise the CST of PNIPAM; we show that this effect is dependent strongly on surfactant identity and concentration. As part of this work we have developed new modelling techniques for the analysis of NR data from polymer brush interfaces. These advancements will be explained in the context of the data at hand, and their applicability to other soft diffuse interfaces will be briefly discussed. © The Authors.
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    Imaging the invisible: resolving polymer brush structure through a freeform bayesian analysis of neutron reflectometry data
    (Australian Institute of Nuclear Science and Engineering (AINSE), 2020-11-11) Gresham, IJ; Murdoch, TJ; Johnson, EC; Grant, W; Wanless, E; Prescott, SW; Nelson, A
    Surfaces covered with densely tethered polymer chains possess desirable properties and are ubiquitous in natural and human-made systems. These properties stem from the diffuse structure of these polymer brush interfaces; consequently, resolving their structure is key to better understanding and designing polymer brush systems. We have been using the PLATYPUS neutron reflectometer at the ACNS to achieve this structural resolution over the past six years, contributing to our understanding of brush structure, as well as fundamental polymer physics. However, the analysis of collected reflectometry data is not without significant challenges; Inflexible models preclude viable structures and the uncertainty around accepted profiles (known as spread) is challenging to quantify. Furthermore, there is no guarantee of profile uniqueness in reflectivity analysis - multiple structures may match the data equally well. Quantifying profile uniqueness and determining the structures that agree with collected data (known as multimodality) has not been previously attempted on brush systems. Historically, data analyses have used least-squares approaches, which do not satisfactorily determine profile spread and bypass the possibility of c. Here we will briefly document our journey in modelling neutron reflectometry data collected from polymer brush systems, culminating in the presentation of our developed methodology. In this methodology, we model our brush with a freeform profile that minimises assumptions regarding polymer conformation while only producing physically reasonable structures. This model is built within refnx‘s Bayesian statistical framework, which enables the characterisation of structural uncertainty and multimodality through Markov Chain Monte Carlo sampling. We demonstrate the rigour of our approach via a round-trip analysis of a simulated system before applying it to real data, examining the well-characterised collapse of a thermoresponsive brush. The method we describe is directly applicable to reflectometry experiments on soft and diffuse systems, but may also be generalised to other instruments where the “inverse problem” hampers data analysis.
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    Specific anion effects on thermoresponsive polymer brushes
    (Australian Institute of Nuclear Science and Engineering, 2016-11-29) Humphreys, BA; Murdoch, TJ; Willott, JD; Nelson, A; Prescott, SW; Webber, GB; Wanless, EJ
    We have recently investigated the influence of anion identity on the physical properties of two different thermoresponsive polymer brushes via neutron reflectometry, atomic force spectroscopy and ellipsometry. [1-3] Polymer brushes of (a) poly(N-isopropylacrylamide) (PNIPAM) and (b) poly(2-2-(methoxyethoxy)ethyl methacrylate) (PMEO2MA) were exposed to potassium salts of acetate and thiocyanate. The temperature response of both polymer brushes was shifted to lower temperatures in the presence of the kosmotropic acetate anions and higher temperatures when exposed to chaotropic thiocyanate anions. The response of the PMEO2MA brush was considerably broader as a function of temperature and the shift in temperature in response to the added salt was greater compared to that displayed by the PNIPAM brush.
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    The specific ion response of polyelectrolyte brushes depends on polymer and anion hydrophilicity
    (Australian Institute of Nuclear Science and Engineering, 2016-11-29) Murdoch, TJ; Willott, JD; de Vos, WM; Nelson, A; Prescott, SW; Wanless, E; Webber, JB
    We have previously shown that the pH and ionic strength response of weak, polybasic brushes is dependent on the location of the counterion within the Hofmeister series and the relative hydrophobicity of the polymer.[1-3] Recent measurements of a hydrophobic poly(2-diisopropylamino) ethyl methacrylate (PDPA) brush on Platypus reflectometer show that a collapsed conformation independent of anion identity is formed at low ionic strength, 0.1 mM. At higher ionic strengths, up to 500 mM, extended conformations are observed in potassium acetate solutions, while potassium thiocyanate solutions collapse the brush. Numerical self consistent field (nSCF) calculations allowed previous hypotheses regarding the accumulation and hydration strength of the counterions to be tested. Addition of a single Flory-Huggins interaction parameter (χ) analogous to the anion hydrophilicity was sufficient to replicate the measured brush behaviour.