Elsevier

Polymer

Volume 214, 1 February 2021, 123287
Polymer

The direction of influence of specific ion effects on a pH and temperature responsive copolymer brush is dependent on polymer charge

https://doi.org/10.1016/j.polymer.2020.123287Get rights and content

Highlights

  • Copolymer brushes of P(MEO2MA-co-DEA) were synthesised via ARGET ATRP.

  • Brush thickness was measured with ellipsometry and neutron reflectometry.

  • The brush responds to temperature, pH and electrolyte identity.

  • Temperature-dependent Hofmeister effects could be changed by altering pH.

  • Overall brush behaviour was PDEA-like at low pH and PMEO2MA-like at high pH.

Abstract

The effect of electrolyte identity on the conformation of homopolymer brushes of poly(2-(2-methoxyethoxy) ethyl methacrylate) (PMEO2MA) and poly(2-(diethylamino)ethyl methacrylate) (PDEA), as well as a P(MEO2MA-co-DEA) copolymer brush of 90:10 mol% composition has been examined. PMEO2MA is a thermoresponsive polymer with a lower critical solution temperature of ~28 °C, PDEA a weak polybase with an apparent pKa of ~7.5, while the copolymer brush exhibits both pH and temperature responsive behaviour. Brushes were synthesised using surface initiated ARGET ATRP. The effect of temperature at low and high pH in the presence of different electrolytes on the thickness of the P(MEO2MA-co-DEA) 90:10 mol% brush was tracked with ellipsometry and the polymer volume fraction profile was elucidated with neutron reflectometry. The effect of each electrolyte on the copolymer brush conformation as the temperature changes could be switched by changing the pH from low (charged DEA segments) to high (uncharged DEA segments). The overall behaviour of the copolymer in each salt was PDEA-like at low pH, and MEO2MA-like at high pH. At low pH a reverse Hofmeister effect was observed, while at high pH a direct Hofmeister effect was observed.

Introduction

Specific ion effects are ubiquitous phenomena which are important in many natural and industrial processes, underpinning the chemistry which allows for all life to exist [[1], [2], [3], [4], [5], [6]]. The Hofmeister series is an empirical ordering of a subset of ions, originally ranking ions with respect to their ability to stabilise (chaotropes) or destabilise (kosmotropes) solutions of egg white proteins [6]. Since their discovery, specific ion effects have been observed in a wide variety of systems influencing properties such as colloid stability [7,8], bubble coalescence [9], protein stability [10], surface tension [11], surfactant behaviour [12], as well as polymer conformation and responsive behaviour [2,5,[13], [14], [15], [16], [17], [18], [19], [20], [21]]. The exact ion properties which produce the wide variety of specific ion effects are still unknown, but behaviour can be superficially explained by considering ion size and polarisability [4,22]. A selection of ions from the Hofmeister series is shown in Fig. 1. The ordering of ions can be perturbed or even reversed by changes in system properties such as: ion concentration, solvent, temperature, surface charge, surface hydrophilicity and pH [[23], [24], [25], [26]].

Ion specificity is particularly relevant for biological systems, where ion selectivity is essential for many intra and intercellular functions [[27], [28], [29], [30], [31], [32], [33], [34], [35]]. Most studies of specific ion effects examine the change in behaviour of a model system with the addition of a single electrolyte in a single solvent (typically water). However, to understand how specific ion effects affect natural systems requires the investigation of more complicated systems. Multi-component systems with mixed solvents [[36], [37], [38], [39]], salts [3,14,18,[40], [41], [42], [43], [44], [45], [46], [47]], and substrates [48,49], have been investigated experimentally. Examining these systems is difficult due to the combinatorial complexity of additional system components. However, ‘mixed’ systems begin to reflect complex natural systems and provide insight into the mechanisms through which different ions influence hydration behaviour [5,14,40].

Polymer brushes are often viewed as an exemplar system with which specific ion effects can be investigated [3]. Polymer brushes are diffuse interfaces formed by end-tethering polymer chains to an underlying substrate at a sufficient grafting density so as to force chains to stretch normal to the surface [50,51]. Polymer brush architectures have been found to exist naturally [[52], [53], [54]], and can be manufactured using grafting-to or grafting-from polymerisation approaches [51,55]. When a surface is functionalised with a responsive polymer brush, interfacial properties, such as the lubricity or wettability, can be modified by the application of a stimulus. The ability to tune surface properties is advantageous for a range of applications including chromatography/separation and biosensing [50,51]. Responsive polymer brushes have also been used in biomedical applications such as cell separation and cell culture supports [56]. The perpendicular structure of the brush, or polymer volume fraction profile (VFP), is a consequence of the chain conformation, which is governed by the intermolecular forces between the substrate, the polymer chains, solvent and any other solutes present [57,58]. For neutral brushes, the polymer VFP is determined by the balance of repulsive excluded volume interactions, which promote swelling of the brush, opposed by the entropy loss resulting from chain stretching [59]. For polyelectrolytes the polymer charge must also be considered [57,[60], [61], [62], [63], [64], [65]]. Neutron reflectometry (NR) can be employed to elucidate polymer brush VFPs [66,67], which allows for the influence of ions on polymer conformation to be assessed [[14], [15], [16],57,[66], [67], [68], [69], [70], [71]].

Here we examine how salt identity and concentration affect the thermoresponsive behaviour and polymer VFP of a pH and temperature responsive poly(2-(2-methoxyethoxy) ethyl methacrylate-co-2-(diethylamino)ethyl methacrylate) (P(MEO2MA-co-DEA)) 90:10 mol% copolymer brush [72]. PMEO2MA is a thermoresponsive homopolymer which exhibits a first-order phase transition as the temperature is raised above its critical solution temperature (CST) of ~28 °C in aqueous solution [[73], [74], [75]]. PMEO2MA is a member of the family of thermoresponsive homopolymers known as the poly(oligo ethylene glycol methyl methacrylates) which vary in the length of their oligo(ethylene glycol) side chain. PDEA is a weak polybase which is soluble in water at pH values below its apparent pKa (~7.5), while at high pH the hydrophobic portions of the polymer determine its solubility [76,77]. At intermediate pH values, PDEA also exhibits thermoresponsive behaviour with a near linear increase in its CST at pH values below its apparent pKa [77]. When grafted in a brush the behaviour of both homopolymers alters due to the loss of translational energy of having one end immobilised and additional steric effects [78]. The chemical structures of both monomers are shown in Fig. 2.

At the chain-water interface, a balance of electrostatic and dispersion interactions, as well as entropic considerations and hydration forces, determine the net effect of specific ions [5]. For polyelectrolyte brushes such as PDEA [17,79] Collins' law of matching water affinities, [80] and Mazzini and Craig's matching effective ion size, [81] as well as considerations of the hydration strengths of the different ions, are generally sufficient to explain observed specific ion effects [5]. At biologically relevant concentrations (≥100 mM), where electrostatic interactions are reduced due to screening effects, other short-range interactions can determine a system's properties. The same ion properties which cause PDEA brushes to salt-in (increased stability) or salt-out (decreased stability) with different ion identities can have the opposite effect on neutral thermoresponsive polymers [3,[13], [14], [15], [16],20,21,69,82], such as PMEO2MA [16].

Herein, the influence of three potassium salts, with anions from different positions along the Hofmeister series (chloride, Cl, nitrate, NO3, and thiocyanate, SCN), on the thermoresponsive behaviour of a P(MEO2MA-co-DEA) 90:10 mol% copolymer at low and high pH is investigated. This composition was chosen as it exhibited significant responses to changes in temperature and pH. The aim was to examine the balance of PDEA and PMEO2MA behaviour at high and low pH as a function of temperature, and whether that balance could be changed with specific ions. By changing the pH, the charge on the PDEA residues changes and is expected to alter the overall balance of the polymer-electrolyte-solvent interaction energy [22]. Changes in brush thickness and VFP were tracked as a function of temperature, pH and ion identity with ellipsometry and neutron reflectometry, respectively. We also examined the behaviour of PMEO2MA and PDEA homopolymer brushes under relevant conditions as a control. Systematically investigating how specific ion effects manifest in copolymer systems compared to their homopolymer equivalents is a novel approach and will help to elucidate the role and importance of the substrate in specific ion effects [5]. Understanding the balance of which monomer in the copolymer brush has a dominant influence on the nature of the specific ion effects will also aid in the design and application of smart polymer brush coatings in myriad applications.

Section snippets

Materials

Native oxide silicon wafers (Silicon Valley Microelectronics, USA) were used for ellipsometry, while native oxide silicon blocks (~10 mm thick, 100 mm diameter) purchased from EL-CAT Inc (USA) were used for NR measurements. (3-Aminopropyl) triethoxysilane (APTES, >99%), triethylamine (Eth3N, 99%) and 2-bromoisobutyryl bromide (BIBB, >99%) were purchased from Sigma-Aldrich and used as received. Tetrahydrofuran (THF, Honeywell Burdick and Jackson, >99%) was dried over 4 Å molecular sieves (ACROS

Results and discussion

This section will examine the influence of different electrolytes (potassium chloride, nitrate and thiocyanate) and concentrations on the thermoresponsive behaviour at low and high pH of a multi-stimulus responsive P(MEO2MA-co-DEA) 90:10 mol% copolymer brush. Ellipsometry is used to gain an overall understanding of the system while neutron reflectometry is used to examine local and global structural changes of the interface. Prior to the investigation of the copolymer brush, specific influences

Conclusions

PMEO2MA, PDEA and P(MEO2MA-co-DEA) 90:10 mol% brushes were synthesised using surface initiated ARGET ATRP. The brush thickness of the PMEO2MA brush as a function of temperature was measured with ellipsometry in the presence of varying concentrations of KCl, KNO3, and KSCN electrolytes. Chloride ions had a significant salting-out effect while thiocyanate ions had a significant salting-in effect on the thermoresponse of the PMEO2MA brush. Nitrate had minimal impact on the thermoresponse of the

Electronic Supporting Information

The electronic supporting information contains a chronology of experimental conditions, a figure illustrating the position of φD and φP as well as distributions of polymer volume fraction profiles, SLD profiles as well as raw and modelled reflectivity data. Notebooks which were used for the analysis of neutron reflectometry data are also available for reproduction.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by ANSTO and the Australian Centre for Neutron Scattering Program Grant (PP6912). ECJ and IJG would like to thank the Australian Government and AINSE Ltd. for providing financial assistance for the period when this work was completed (Research Training Program Scholarships and Post Graduate Research Awards respectively).

References (107)

  • J.A. Loughlin et al.

    A new method for estimating counter-ion selectivity of a cationic association colloid: trapping of interfacial chloride and bromide counter-ions by reaction with micellar bound aryldiazonium salts

    Colloids Surf.

    (1990)
  • W.M. de Vos et al.

    Modeling the structure of a polydisperse polymer brush

    Polymer

    (2009)
  • E.B. Zhulina et al.

    A self-consistent field analysis of the neurofilament brush with amino-acid resolution

    Biophys. J.

    (2007)
  • M. Ballauff et al.

    Polyelectrolyte brushes

    Curr. Opin. Colloid Interface Sci.

    (2006)
  • J. Penfold et al.

    Neutron reflectivity and small angle neutron scattering: an introduction and perspective on recent progress

    Curr. Opin. Colloid Interface Sci.

    (2014)
  • S.M. Kilbey et al.

    Neutron reflectivity as a tool to understand polyelectrolyte brushes

    Curr. Opin. Colloid Interface Sci.

    (2012)
  • V. Bütün et al.

    Synthesis and aqueous solution properties of near-monodisperse tertiary amine methacrylate homopolymers and diblock copolymers

    Polymer

    (2001)
  • F. Liu et al.

    Recent advances and challenges in designing stimuli-responsive polymers

    Prog. Polym. Sci.

    (2010)
  • J.D. Willott et al.

    Physicochemical behaviour of cationic polyelectrolyte brushes

    Prog. Polym. Sci.

    (2017)
  • K.D. Collins et al.

    Ions in water: characterizing the forces that control chemical processes and biological structure

    Biophys. Chem.

    (2007)
  • M. James et al.

    The multipurpose time-of-flight neutron reflectometer “Platypus” at Australia's OPAL reactor

    Nucl. Instrum. Methods Phys. Res. Sect. A Accel. Spectrom. Detect. Assoc. Equip.

    (2011)
  • B.A. Humphreys et al.

    Effect of ionic strength and salt identity on poly(N-isopropylacrylamide) brush modified colloidal silica particles

    J. Colloid Interface Sci.

    (2018)
  • L.-H. Wang et al.

    Anion–dipole interactions regulating the self-assembled nanostructures of polymers

    Polym. Chem.

    (2015)
  • E. Thormann

    On understanding of the Hofmeister effect: how addition of salt alters the stability of temperature responsive polymers in aqueous solutions

    RSC Adv.

    (2012)
  • F. Zhang et al.

    Gold nanoparticles decorated with Oligo(ethylene glycol) Thiols: enhanced Hofmeister effects in colloid−protein mixtures

    J. Phys. Chem. C

    (2009)
  • T. López-León et al.

    Ion-specific aggregation of hydrophobic particles

    ChemPhysChem

    (2012)
  • C.L. Henry et al.

    Ion-specific coalescence of bubbles in mixed electrolyte solutions

    J. Phys. Chem. C

    (2007)
  • A. Dér et al.

    Interfacial water structure controls protein conformation

    J. Phys. Chem. B

    (2007)
  • L.M. Pegram et al.

    Hofmeister salt effects on surface tension arise from partitioning of anions and cations between bulk water and the air−water interface

    J. Phys. Chem. B

    (2007)
  • S.K. Meena et al.

    The role of halide ions in the anisotropic growth of gold nanoparticles: a microscopic, atomistic perspective

    Phys. Chem. Chem. Phys.

    (2016)
  • B.A. Humphreys et al.

    Specific ion modulated thermoresponse of poly(N-isopropylacrylamide) brushes

    Phys. Chem. Chem. Phys.

    (2016)
  • E.C. Johnson et al.

    Temperature dependent specific ion effects in mixed salt environments on a thermoresponsive poly(oligoethylene glycol methacrylate) brush

    Phys. Chem. Chem. Phys.

    (2019)
  • J.D. Willott et al.

    Anion-specific effects on the behavior of pH-sensitive polybasic brushes

    Langmuir

    (2015)
  • E.E. Bruce et al.

    Nonadditive ion effects drive both collapse and swelling of thermoresponsive polymers in water

    J. Am. Chem. Soc.

    (2019)
  • X. Chen et al.

    Specific ion effects on interfacial water structure near macromolecules

    J. Am. Chem. Soc.

    (2007)
  • Y. Zhang et al.

    Chemistry of Hofmeister anions and osmolytes

    Annu. Rev. Phys. Chem.

    (2010)
  • Y. Zhang et al.

    Specific ion effects on the water solubility of macromolecules: PNIPAM and the Hofmeister series

    J. Am. Chem. Soc.

    (2005)
  • K.P. Gregory et al.

    Lewis strength determines specific-ion effects in aqueous and nonaqueous solvents

    J. Phys. Chem.

    (2019)
  • Y. Zhang et al.

    The inverse and direct Hofmeister series for lysozyme

    Proc. Natl. Acad. Sci. Unit. States Am.

    (2009)
  • D.F. Parsons et al.

    Why direct or reversed Hofmeister series? Interplay of hydration, non-electrostatic potentials, and ion size

    Langmuir

    (2010)
  • M. Senske et al.

    The temperature dependence of the Hofmeister series: thermodynamic fingerprints of cosolute-protein interactions

    Phys. Chem. Chem. Phys.

    (2016)
  • P. Lo Nostro et al.

    Hofmeister phenomena: an update on ion specificity in biology

    Chem. Rev.

    (2012)
  • B.W. Ninham et al.

    Molecular forces and self assembly: in colloid, nano sciences and biology

    Phys. Today

    (2011)
  • M.F. Erdoǧan

    Thiocyanate overload and thyroid disease

    Biofactors

    (2003)
  • V. Yeh et al.

    The Hofmeister effect on amyloid formation using yeast prion protein

    Protein Sci.

    (2010)
  • Y.L. Yizhou Wang et al.

    Design and synthesis of multi-responsive copolymers for drug carrier

    Acta Phys. Chim. Sin.

    (2019)
  • P.J. Marek et al.

    Ionic strength effects on amyloid formation by amylin are a complicated interplay among Debye screening, ion selectivity, and Hofmeister effects

    Biochemistry

    (2012)
  • G.B. Irvine et al.

    Protein aggregation in the brain: the molecular basis for Alzheimer’s and Parkinson’s diseases

    Mol. Med.

    (2008)
  • L. Liu et al.

    Mechanistic insights into amplification of specific ion effect in water–nonaqueous solvent mixtures

    J. Phys. Chem. B

    (2013)
  • L. Liu et al.

    Specific anion effect in water–nonaqueous solvent mixtures: interplay of the interactions between anion, solvent, and polymer

    J. Phys. Chem. B

    (2013)
  • Cited by (11)

    • From Hofmeister to hydrotrope: Effect of anion hydrocarbon chain length on a polymer brush

      2023, Journal of Colloid and Interface Science
      Citation Excerpt :

      Neutron reflectometry experiments on polyelectrolyte homo- and copolymer brushes have revealed non-monotonic VF profiles. The presence of these structures has been attributed to: the enrichment of charged monomers within the brush close to the substrate for copolymers [52,65–67] or the modulation of electrostatic interactions between segments within the bulk of the brush and those on the periphery of the brush for homopolymer polyelectrolytes [68–70]. Neither of these mechanisms can explain the structures observed for our PNIPAM neutral homopolymer (Fig. 4) [55].

    • Effect of surfactants on the thermoresponse of PNIPAM investigated in the brush geometry

      2023, Journal of Colloid and Interface Science
      Citation Excerpt :

      In keeping with our previous work [52], we enforce monotonicity (i.e., volume fraction must decrease as distance from the substrate increases) for samples in pure D2O, as no theoretical justification for non-monotonicity in neutral polymer brush profiles exists. However, it has recently been shown that non-monotonic volume fraction profiles can arise in more complicated or non-homogeneous systems [59–61]. As such, non-monotonicity was allowed in our model when a surfactant was present in the system.

    • In-situ formation of fluorophore cross-linked micellar thick films and usage as drug delivery material for Propranolol HCl

      2022, Spectrochimica Acta - Part A: Molecular and Biomolecular Spectroscopy
      Citation Excerpt :

      Another driving force on micellar film formation can also be affected by reducing the contact surface of the PDEAEMA block with alkali subphase. This phenomenon may be supported by diffusing subphase inside the BCP as shown in TEM images captured from sectioning film sample (Fig. 6b,c) and shrinking of PDEAEMA block in the alkali medium [49]. The part of this BCP that makes physically stable in toluene is PDEAEMA block in the case of PGMA block gets hydrophilic after epoxy functional groups reacted with the derivatives located in subphase.

    • PH and thermo dual-sensitive copolymers with fluorescent properties grafted mesoporous silica SBA-15 via metal-free ATRP

      2022, European Polymer Journal
      Citation Excerpt :

      There are many methods to graft polymers on the surface of SBA-15, including traditional radical polymerization, anionic polymerization, cationic polymerization and ring-opening polymerization, but most of the polymerization methods have poor control of the reaction process and the molecular weight distribution of the obtained polymers is wide. Of all kinds of polymers, stimulus-responsive polymers have received the most attention because they undergo conformational changes in response to external stimuli (e.g. temperature, pH, salt concentration, etc.) [14–16]. Poly (N-isopropylacrylamide) (PNIPAM) is one of the most widely studied temperature-responsive polymers with a low critical solution temperature (LCST) of 32 °C [17,18].

    • Ion-specific effect on self-cleaning performances of polyelectrolyte-functionalized membranes and the underlying nanomechanical mechanism

      2021, Journal of Membrane Science
      Citation Excerpt :

      In the applications of polyelectrolyte surfaces, the influence of aqueous ion species, which are practically inevitable, has been conventionally over-simplified [30]. Previous studies have demonstrated that the different ions can adsorb onto polyelectrolyte surface and competitively interact with the charges of the polyelectrolytes, thus significantly affecting the molecular conformation and related performances of polyelectrolytes in aqueous solutions [27,37–43]. However, the interaction mechanisms of water with polyelectrolyte surface across surrounding medium (e.g., hydrophobic oil), especially under ion-specific effect, still remain unrevealed [30,44].

    View all citing articles on Scopus
    View full text