Synthesis and properties of polyelectrolyte microgel particles

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Abstract

A series of cationic poly(N-isopropylacrylamide/4-vinylpyridine) [poly(NIPAM/4-VP)] polyelectrolyte co-polymer microgels have been prepared by surfactant free emulsion polymerization (SFEP) with varying compositions of 4-VP and NIPAM. The compositions of 4-VP were 15, 25, 35, 45, 55 wt.% relative to NIPAM. The temperature and pH responsive swelling–deswelling properties of these microgels have been investigated using dynamic light scattering (DLS) and electrophoretic mobility measurements. DLS results have shown that the particle diameter of the poly(NIPAM/4-VP) microgels decreases with increasing concentration (wt.%) of 4-VP over the 20–60 °C temperature range due to the increased amount of hydrophobic group. The particle size of all poly(NIPAM/4-VP) microgel series increases with decreasing pH, as the 4-VP units become more protonated at low pH below the pKa (5.39) of the monomer 4-VP. Electrophoretic mobility results have shown that electrophoretic mobility increases as the temperature/pH increases at a constant background ionic strength (1 × 10 4 mol dm 3 NaCl). These results are in good agreement with DLS results. The temperature/pH sensitivity of these microgels depends on the ratio of NIPAM/4-VP concentration in the co-polymer microgel systems. The combined temperature/pH responsiveness of these polyelectrolyte microgels can be used in applications where changes in particle size with small change in pH or temperature is of great consequence.

Introduction

Microgel particles are intra-molecularly cross-linked, supramolecular, polymeric colloidal dispersions [1] which have received significant attention in the recent scientific literature due to their many varied potential applications. These particles are often described as microsponges, as they shrink and swell in response to changes in environmental conditions such as temperature [1], [2], [3]. The extent of swelling (or de-swelling) is an interesting property of microgel particles [4]. These properties make them useful in a variety of fields including the paint industry, in ink jet printing, as drug delivery vehicles, [5], [6], [7] in the synthesis of engineering ceramics [8], as biosensors [9], [10] and in other areas of biotechnology [11].

Polyelectrolytes are polymers that have ionisable groups that can ionise in aqueous solution. Theses polymers are water soluble and can be anionic and/or cationic. As polyelectrolytes are charged their properties will change with pH and ionic strength of the solution [12]. An anionic polymer exhibiting a strong polyelectrolyte effect shows a large change in ionisation on going from a pH value below the pKa to a value above the pKa and the reverse case holds for a cationic polymer [13]. Polyelectrolytes are widely used in various fields of industry, e.g. in paper making and the textile, cosmetics and detergent industries and also in separation processes such as flocculation [14].

Poly(NIPAM), cross-linked with methylene bis-acrylamide, is the most widely investigated microgel system which is temperature-responsive and undergoes volume phase transition (VPTT) at ∼ 34 °C [1], [3], [15] below which the polymer is hydrophilic and above which they become hydrophobic (Fig. 1). It has a hydrophilic amide group and a hydrophobic isopropyl group. It is possible to increase the functionality of microgel particles by finding the right balance of hydrophobic and hydrophilic co-monomers, or tune the VPPT to a desired temperature range by co-polymerisation with a more hydrophilic co-monomer (which raises the VPTT), a more hydrophobic co-monomer (which lowers the VPTT) or an ionic co-monomer (pH-responsive swelling) [16], [17].

The addition of small amount of co-monomer can have a dramatic effect on the overall properties of the resultant microgel particles. Incorporation of an acidic or basic co-monomer into poly(NIPAM) microgels yields particles with temperature and pH responsive swelling. For example, the result of co-polymerising NIPAM with readily available carboxylic acid monomers, such as acrylic acid (AAC) or methacrylic acid (MAA) is a co-polymer microgel that is responsive to both pH and temperature [16], [18], [19]. The microgel particles prepared from ionic monomers/co-monomers that exhibit pH responsive swelling properties are termed polyelectrolyte microgel particles.

Most commonly studied ionic polymers for pH-responsive behaviour include poly(acrylamide) (PAAm), poly(acrylicacid) (PAA), poly(methacrylacid) (PMAA), poly(diethylaminoethyl methacrylate) (PDEAEMA) and poly(dimethylaminoethyl methacrylate) (PDMAEMA) [20]. In aqueous media of appropriate pH and ionic strength, the pendant groups ionise and develop fixed charges on the polymer network, generating electrostatic repulsive forces responsible for the pH-dependent swelling or de-swelling of the microgel [21]. Small changes in pH can result in significant change in the mesh size of the polymeric networks. Pendant groups of anionic microgels are unionised below and ionised above the pKa of the polymeric network, leading to swelling of the microgel at a pH above the polymer pKa because of a large osmotic swelling force by the presence of ions. The reverse is the case for cationic microgels, which swell at lower pH. A schematic representation of the differential swelling of ionic microgels in acidic and alkaline buffers is presented in Fig. 2.

Poly(NIPAM) microgels co-polymerised with AAC have been well studied [22], [23], [24], [25]. Eimer et al. [22] reported swelling behaviour of anionic co-polymer poly(NIPAM/acrylic acid) (poly NIPAM/AAC) microgel particles with varying acrylic acid content. They have shown that poly(NIPAM/AAC) with an AAC concentration of 3.15% (cAAC = 3.15%) shows a slight shift in the transition temperature compared to homo-polymer poly(NIPAM) at neutral pH and for high charge density (cAAC = 12.5%) a second deswelling process appears at higher temperature. It was also reported that the swelling ratio of poly(NIPAM/AAC) with cAAC = 12.5% increases significantly as the pH increases.

Bradley et al. [15] and Snowden et al. [25] investigated the co-polymer microgels of NIPAM and AAC for their swelling/de-swelling on changing their pH, ionic strength and temperature. It was found that microgels containing AAC, increase their size upon increasing AAC content with low amounts of crosslinker. However, the size of the particles decreases both with increasing ionic strength and the temperature in acidic pH. Moreover, Morris et al. [25] reported the use of (poly NIPAM/AAC) microgels for the removal of heavy metals and nitrates from water supplies. The absorption and release of lead from this co-polymer microgel was found to be very sensitive to changes in pH.

In this study, the authors would like to present the synthesis and properties of a series of cationic poly(NIPAM) based polyelectrolyte microgels co-polymerised with 4-vinylpyridine (4-VP). Vinylpyridine (VP) based gels have been studied by a number of research groups [26], [27], [28], [29], [30]. Co-polymer cationic microgels of 2-vinylpyridine (2-VP) and styrene were studied by Loxley et al. [26] The particle morphologies and diameters were characterized by a combination of TEM and photon correlation spectroscopy. At styrene contents less than 40% by weight, the particles were spherical with diameters of approximately 200 nm. The diameter decreased with increasing styrene content and the morphology changed from spherical to irregular. The pH-dependent swelling of the microgel particles was studied in constant ionic strength acetate buffers. Particle diameters increased sharply below pH 4.6 due to ionisation of the 2-VP residues.

Poly(NIPAM/4VP) microgels were investigated by Vincent et al. [30] The morphology and hydrodynamic diameter of the microgel particles were characterized by TEM and photon correlation spectroscopy (PCS). It was reported that the co-polymer microgels show both temperature and pH responsive properties. The hydrodynamic diameters of the co-polymer microgel particles were increased drastically below pH 5. The dependence of pH and temperature on the microgel particles were affected by molar ratio of 4VP/NIPAM.

In this work, The cationic poly(NIPAM/4-vinylpyridine) [poly(NIPAM/4-VP)] has been prepared in varying 4-VP concentration (wt.%) relative to NIPAM. The microgels were synthesized using surfactant free emulsion polymerization (SFEP) [14], [25], [31]. The dual temperature and pH responsive swelling of the synthesized microgel particles have been investigated using dynamic light scattering (DLS) with respect to particle size and also electrophoretic mobility, which are common and extensively studied method for the characterisation of colloidal microgel particles [2], [14], [30], [31].

Section snippets

Materials

The monomers N-isopropylacrylamide (NIPAM), and 4-vinylpyridine (4-VP), the cross-linker N, N′-methylenebisacrylamide (BIS) and cationic initiator 2,2′-azobis(2-methylpropionamidine) dihydrochloride (MPD) were obtained from Aldrich, UK. The silanisation solution II (∼ 2 vol.% dimethylsilane in 1,1,1-trichloroethane) was purchased from Fluka, Switzerland. All chemicals were obtained from commercial and industrial suppliers and used without further purification. Distilled water was used for the

Results and discussion

The temperature/pH responsive swelling properties of cationic poly(NIPAM/4-VP) polyelectrolyte microgel series are outlined in the subsequent sections with respect to DLS and electrophoretic mobility results.

Summary

Poly(NIPAM/4-vinylpyridine) [poly(NIPAM/4-VP)] polyelectrolyte co-polymer microgels of nanometer size have been prepared by SFEP method. The temperature and pH sensitivity of these copolymer microgels have been investigated using DLS and electrophoretic mobility measurements. The effect of temperature showed how the swelling behaviour of these microgel particles is affected by the increased level of 4-VP in the co-polymer microgels. The particle size decreased with an increasing concentration

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