Article pubs.acs.org/Langmuir
Building Polyzwitterion-Based Multilayers for Responsive Membranes Joris de Grooth, Mo Dong, Wiebe M. de Vos,* and Kitty Nijmeijer Membrane Science and Technology, Mesa+ Institute for Nanotechnology, University of Twente, Enschede, The Netherlands S Supporting Information *
ABSTRACT: We systematically investigate the assembly of multilayers based on a polyzwitterion (PSBMA) and a polycation (PDADMAC) for the development of ionic strength responsive membranes. Although the polyzwitterion is essentially charge neutral, we show that specific electrostatic interactions with the PDADMAC allow for the formation of stable multilayers. The growth of this LbL system is monitored on model surfaces (silica) via optical reflectometry for different pH values and ionic strengths. While no effect of pH on the layer growth is observed, we did observe a strong dependence on the ionic strength. Upon increasing the ionic strength during deposition from 0.005 to 0.5 M NaCl, the adsorbed amount is significantly decreased, a behavior that is opposite to classical LbL systems. Similar results to those obtained on silica are also observed on top of classical LbL systems and on polymeric membranes. This demonstrates that the growth of the polyzwitterion multilayers is independent of the substrate. Coating these polyzwitterion multilayers on hollow fiber membranes via dip-coating yields membranes that are stimuli responsive toward the ionic strength of the filtration solution, with an increase in permeability of up to 108% from 0 to 1.5 M NaCl. We show that the fabrication of the polyzwitterion multilayers is an easy and controlled way to provide surfaces, such as membranes, with the specific functionalities of polyzwitterions.
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INTRODUCTION
A more facile membrane modification method that does not require a harsh chemical reaction is the deposition of polyelectrolyte multilayers (PEMs) via the layer-by-layer (LbL) approach.9,10 In this approach, alternating between the deposition of polyanions and polycations leads to a very welldefined multilayer system. While most of the research on the layer-by-layer buildup has been performed on model surfaces, it has already been proven that these layers can be transformed to functional membranes.10−14 By carefully choosing the number of layers and by changing the coating conditions, the properties of the PEMs can be precisely controlled.15−17 In addition, by choosing the appropriate polyelectrolytes, the PEMs can be made stimuli-responsive.18,19 PEMs from weak polyelectrolytes, such as poly(acrylic acid) (PAA) and poly(allylamine hydrochloride) (PAH), change their thickness upon changes in pH by protonation and deprotonation.20 By incorporation of the well-known temperature-responsive poly(N-isopropylacrylamide) (PNIPAM),1,21,22 the PEMs become responsive to the temperature as well.23 A stimuli that has received far less attention in the case of PEMs is the ionic strength. While the ionic strength is often employed to control the PEM structure and thickness during the multilayer buildup, once PEMs have been prepared, they generally do not show significant changes
Responsive polymeric systems have received wide attention over the years with a multitude of applications including controlled molecular transport,1 fouling control,2,3 (nano)sensors, and drug delivery.4−6 Out of the possible applications, membrane-based separation processes, with separation at a molecular scale, are very promising areas for these polymeric systems.7 A reversible increase in the membrane permeability can for instance be used to increase the convective flow during a backwash of the membrane in order to enhance the effectiveness of the backwash. Several approaches have been taken to render membranes or membrane materials stimuliresponsive. Unfortunately, these methods typically require organic solvents and chemical reactions to graft the responsive polymer to the substrate in a stable manner.6 A more mild approach is photoinitiation or plasma initiation, usually operated in aqueous environments. However, this can be accompanied by substantial substrate degradation,8 which is inherent to the grafting process. Furthermore, this approach is limited to the modification of the outside of the membrane surface and as such necessitates the use of flat sheet membranes or the outside-to-in filtration of hollow fiber membranes. An ideal membrane modification method to make membranes stimuli-responsive should allow for modification of all membrane geometries without the use of any harsh chemicals, organic solvents, or invasive treatments. © 2014 American Chemical Society
Received: March 5, 2014 Revised: April 16, 2014 Published: April 21, 2014 5152
dx.doi.org/10.1021/la500857b | Langmuir 2014, 30, 5152−5161
Langmuir
Article
strong zwitterionic polymer at a less extreme pH, where the polyzwitterion has both positive and negatively charges. In this paper we present the building of multilayers on silicon surfaces of the polysulfobetaine poly(N-(3-sulfopropyl)-N(methacryloxyethyl)-N,N-dimethylammonium betaine) (PSBMA) and poly(diallyldimethylammoium chloride) (PDADMAC), see Figure 1, under various coating conditions.
in thickness as a function of the ionic strengths over a wide concentration range after formation.24 It is however known that the properties of the multilayers can alter after preparation when exposed to very high ionic strengths. Dubas et al. showed that the surface roughness of a PEM can be reduced by exposing the layer to 0.5 M NaCl.25 It was also shown by Han et al. that at even higher salt concentrations (>3 M NaCl), the layers started to decompose.26 Although the loss of (part of the) layer can be used in for instance cleaning procedures for surfaces,27 it is not reversible and can thus not be utilized for stimuli-responsive layers. The building of PEMs at high ionic strengths (3 M) can lead to reversible thickness changes upon switching between low and high salinities.24 Polyzwitterions, charged polymers with both a positive and negative moiety on the same repeating unit, are polymers that are known to have a strong response to changes in ionic strength and are thus expected to be suitable to make ionic strength responsive membranes. Polyzwitterions show an antipolyelectrolyte effect: an increase in solubility and/or swelling with increasing ionic strengths.28 Although they are commonly used in antifouling applications, their antipolyelectrolyte effect also gives potential for ionic strength based responsive behavior. This can for instance be used in chromatography for the separation and isolation of proteins.29 Zhai et al. grafted the zwitterion N,N′-dimethyl(methylmethacryloylethyl)ammonium propanesulfonate (DMAPS) to poly(vinylene fluoride) for electrolyte-responsive microfiltration membranes.30 Because of the antipolyelectrolyte effect, the polyzwitterions swell and block the pores at high NaCl concentrations, leading to a reduced permeability. In this work, we will incorporate a zwitterionic polymer in the layer-by-layer technique to make electrolyte-responsive membranes. Although a polyzwitterion contains ionic charges, its net charge is neutral. This means that the growth of PEMs with polyzwitterions follows a complex mechanism, which may not solely rely on charge−charge interactions. Kharlampieva et al. investigated the assembly of the weak polyzwitterion polycarboxybetaine with polyanions.31 Only below certain critical pH levelsdue to the protonation of the carboxylic acidthe multilayers were formed and found to be stable. Later, Kharlampieva et al. also investigated the layer assembly of the polycarboxybetaine with the neutral poly(N-vinylcaprolactam).32 The nature of the interactions of the subsequent layers is not classical ion−ion based, but based on hydrogen bond acceptors and donors. Again, the layers were only self-assembled and stable at low pH values, where the protonated carboxylic groups of the zwitterionic polymer can act as a hydrogen bond donor. Comparable results were obtained by Gui et al.33,34 Strong zwitterionic polymers, on the other hand, like polysulfobetaine, got less attention. Nevertheless, the growth of multilayers with polysulfobetaine at very low pH values has been investigated.35,36 At extreme pH values (pH
