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POLYVINYLAMINE - A TOOL FOR ENGINEERING INTERFACES Robert Pelton Langmuir, Just Accepted Manuscript • DOI: 10.1021/la5017214 • Publication Date (Web): 25 Jun 2014 Downloaded from http://pubs.acs.org on July 5, 2014
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POLYVINYLAMINE - A TOOL FOR ENGINEERING INTERFACES
Robert Pelton Department of Chemical Engineering JHE-136, McMaster University Hamilton, Ontario, Canada, L8S 4L7 (905) 529 7070 ext. 27045
[email protected] KEYWORDS polyvinylamine, adsorption, review, adhesion, surface functionalization, polyallylamine, polyethyleneimine, chitosan ABSTRACT With the highest content of primary amine functional groups of any polymer, polyvinylamine (PVAm) is a potent tool for modification of macroscopic and nanoparticle surfaces. Based on the free radical polymerization and subsequent hydrolysis of N-vinylformamide, PVAm is prepared as linear polymers (0.8 kDa to >1 MDa), microgels, macrogels, and as copolymers. The amine groups serve as reaction sites for grafting PVAm to surfaces and for preparation of derivatives. Coupling low molecular molecules and oligomers gives PVAm-X, where X includes hydrophobes, carbohydrate oligomers, proteins, TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy), phenylboronic acids, fluorocarbons. This contribution highlights the use of PVAm and PVAm-X to modify solid surface properties. Where possible, the PVAm properties and applications as an interfacial agent are compared with linear polyethyleneimine, polyallylamine, and chitosan.
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Robert Pelton earned his Ph.D. in colloid chemistry at Bristol University in 1977 working with Professor Ronald Ottewill. Pelton has worked for the Pulp and Paper Research Institute of Canada and for Union Carbide, Tarrytown NY. Pelton is a professor of chemical engineering at McMaster University, holds the Tier 1 Canada Research Chair in Interfacial Technologies and is a Fellow of the Royal Society of Canada. He is best known for his invention of polyNIPAM thermosensitive microgels
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INTRODUCTION With the highest content of primary amine functional groups of any polymer, polyvinylamine (PVAm) is a potent tool for modification of macroscopic and nanoparticle surfaces. PVAm adsorbs spontaneously and irreversibly on most surfaces in water, generating cationic interfaces. Adsorbed PVAm is readily modified by bioactive or chemically active agents to promote adsorption/adhesion, to prevent adsorption/adhesion, or to induce specific interactions. Whereas most waterborne polymers in use today were commercially available fifty years ago, PVAm was a challenge to manufacture and has only been widely available for a decade. Herein we summarize the use of PVAm as an agent for modifying interfaces. When possible, the properties and applications of PVAm are compared with those of polyethyleneimine (PEI), polyallylamine (PAH) and chitosan with a view to highlighting the potential advantages and challenges with the use of PVAm. The polyvinylamine technology platform is conveniently divided into four categories: 1. Linear polyvinylamine homopolymer designated PVAm. Many of the academic studies employ PVAm. 2. Copolymers with vinyl monomers designated herein as PVAm-co-X where X is typically acrylamide, N-vinylformamide (NVF), acrylic acid etc. Most commercial products are PVAm-co-NVF. 3. Polyvinylamine derivatives, PVAm-X where X is one or more types of molecules conjugated to some of the pendant primary amine groups. 4. PVAm-containing microgels, core-shell particles, composite particles, etc. - these are not considered herein. Figure 1 shows examples of PVAm-X and PVAm-co-X prepared in the author’s laboratory. This is a small subset of the literature described below. The following sections overview PVAm synthesis, PVAm solution properties, PVAm interactions at interfaces, and applications at interfaces.
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PNVF PVAm-B PVAm-G
NH
O
NH H2 N
O H2 N
HN C O HO CH2 HO C H OH O H C OH OH O C H HO C H OH CH2OH
HN
PVAm-co-NVF
O
NH
NH NH2 NH3+ O
NH3 O
HO B OH OH
PVAm-H NH2 NH2
PVAm NH HN
NH2
NH2 NH
NH NH3
NH3 NH NH2 NH2 NH2 HN
PVAm Microgel
PVAm-T
NH3 O
N O•
Figure 1
The subset of the PVAm landscape investigated in the author’s laboratory.
PVAm SYNTHESIS AND DERIVATIZATION Reynolds and Kenyon first reported PVAm synthesis based on the decomposition of poly(N-vinyl phthalimide). 1 Jones’ paper in this era, entitled “Attempted Preparation of Polyvinylamine”, emphasized the early synthetic challenges. 2 The next major milestone was Katchalsky’s theoretical and experimental analysis of PVAm ionization/pH behavior a decade later. 3 Overberger’s group published a series of papers in 1980s describing many derivatives of PVAm. The modern era of the PVAm story started in the mid 1990s, when patents and publications from Air Products, BASF and Mitsubishi appeared in the literature. By 2000, PVAm and its copolymers were commercially available. Although the current industrial routes to PVAm will be overviewed herein, two recent reviews do an excellent job of describing PVAm synthesis. 4, 5 The title of Pinschmidt’s review, “Polyvinylamine at Last”, is a nice counter point to Jones’ title above. Polyvinylamine has been widely available for about the last 15 years. The goal of this review is to highlight some uses of the PVAm family to influence the properties of surfaces and dispersions. The two main current routes to PVAm and its copolymers are the free radical polymerization of N-vinylformamide (NVF) with subsequent hydrolysis, and the Hofmann rearrangement of polyacrylamide – see Figure 2.
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Figure 2 PVAm from hydrolysis of PNVF (A) and Hofmann rearrangement of polyacrylamide (B). PVAm from N-vinylformamide. N-vinylformamide (NVF) is a water-soluble isomer of acrylamide. Like acrylamide, NVF polymerizes to give high molecular weight linear water-soluble polymers with controlled molecular weights ranging from 0.8 kDa to more than 1 MDa. 6, 7 The review by Kroner et al. is an excellent summary of NVF chemistry. 8 In a second step, the poly(N-vinylformamide), PNVF, is hydrolyzed first to give PVAm-co-NVF. Complete hydrolysis then gives PVAm. 9 McCormick’s group have reported a number of copolymers with NVF, including acrylamide, sodium acrylate, N-butyl acrylate and maleic anhydride.10 Series of PVAm-co-NVF copolymers covering a range of molecular weights and degrees of hydrolysis are commercially available. Side reactions have been reported for PNVF hydrolysis reactions. Reversible amidine formation (Figure 2A) in PNVF-co-VAm copolymers has been reported by a number of authors 5. Recently Witek et al. reported that the amidine groups decompose, giving ammonia, and leaving an alcohol instead of an amine – see Figure 2A. 11 Since they claim up to 42 mole % alcohol, more work is required to clarify this observation. 5 ACS Paragon Plus Environment
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It is generally necessary to purify commercial PVAm samples for scientific investigations. The major side products from alkaline hydrolysis are sodium formate, whereas acid hydrolysis generates formic acid. 12 For small scale, laboratory applications, exhaustive dialysis followed by freeze-drying is effective. However, when reconstituting solutions from freeze dried samples there is an issue in knowing the exact polymer solution composition – the dried mass is not sufficient. Pure PVAm has an equivalent weight of 43.1 Da, whereas the equivalent weight of pure PVAm:HCl is 79.5 Da and of PVAm:H2CO3 is 105.1 Da. Thus, unless the nature and extent of salt formation is known, the mass of freeze-dried dialyzed PVAm does not define the composition. Instead, we use NMR to give PVAm composition, and we use conductometric titration to give the titratable amine concentration of PVAm stock solutions. Hofmann Conversion of Polyacrylamide Since the 1940s 2, 13 there have been reports of PVAm preparation by the Hofmann conversion of polyacrylamide. This is an attractive approach because polyacrylamide is inexpensive and widely available. However, the Hofmann reaction involves sodium hypochlorite treatment at high pH, conditions that lead to side reactions and chain scission. Achari et al. reported high conversions of PVAm from polyacrylamide, with up to 10 mole percent carboxyl and up to 5% urea groups. 14
Post-polymerization Derivatization Much of the utility of polyvinylamine arises from the ease with which primary amine groups can be covalently coupled to surfaces, dramatically changing surface properties, or to small molecules and oligomers, modifying polyvinylamine properties. Herein PVAm-X is a generic designation for post-polymerization PVAm derivatives. Figure 1 presents some examples of PVAm derivatives prepared in our laboratory. Much of the early work employed nonaqueous coupling reactions,1 whereas most of the more recent publications involved carbodiimide and related aqueous phase coupling chemistries. 15 Reported derivatives prepared by coupling small molecules or oligomers include PVAm chains bearing fluorochemical chains, 16 hydrophobes, 17 hydrophobe + peptide, 18 hydrophobe + PEG, 19 hydrophobe + dextran,20 TEMPO (2,2,6,6-tetramethyl-1piperidinyloxy),21 phenylboronic acids, 22 p-nitrophenyl, 23 galactose, 24, maltoseterminated dendrons; 25 streptavidin + fluorescent dye, 26 nucleic acids, 27 and, antibodies. 28 Conjugation reactions forming PVAm derivatives can be performed before or after PVAm has adsorbed onto a surface. PROPERTIES AND INTERACTIONS IN SOLUTION PVAm Ionization. Ethylamine, a low molecular weight analogue of PVAm, has a pKa of 10.7, thus it is essentially completely ionized at pH 7 and below. By contrast, PVAm is partially ionized 6 ACS Paragon Plus Environment
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over most of the pH scale, and PVAm’s ionization behavior cannot be described by a pKa value. This type of behavior is known as the polyelectrolyte effect, and is a consequence of interactions between neighboring amine groups. Since the classic study by Katchalsky et al.,3 at least five subsequent potentiometric PVAm studies have been published – Borkovec’s review is an excellent starting point. 29 Figure 3 compares the ionization vs. pH behavior of PVAm, PAH, linear PEI and chitosan. The branched PEI curve (not shown) is similar to the linear PEI line in Figure 3. Note that these plots are modified Henderson-Hasselbalch fits. The corresponding experimental PVAm titration curves are not symmetric, 29 reflecting the weakness of this 2-parameter model. Although the Henderson-Hasselbalch equation is not the most sophisticated model, these results were chosen because the fits to the three synthetic polymer data sets came from the same laboratory. The PEI and PVAm fits are similar, both showing polyelectrolyte behavior. By contrast, PAH undergoes most of the change in ionization over a couple of pH units, much like isolated amines. It is remarkable that the insertion of one carbon atom between the nitrogen and the main chain, i.e. going from PVAm to PAH, has such a large effect. Finally, chitosan, like PAH, ionizes over a narrow pH range. The differences in the titration curves in Figure 3 can be rationalized by considering the distances between neighboring amine groups that follow the ranking chitosan≫PAH>PVAm~PEI. The shorter the nearest neighbor distances, the stronger the polyelectrolyte effect. Furthermore, electrostatic interactions between neighboring amines are significant over these distance scales. For example, the titrations leading to the curves in Figure 3 were conducted in 1 M KCl or 0.1 M NaCl; the corresponding Debye lengths are 0.3 to 1 nm. When comparing these four polymers as agents for interfacial modification, PVAm and linear PEI present both ionized and free amines over most of the pH range. By contrast, PAH and chitosan have provide significant contents of both amines and ammonium groups over about two pH units. Chitosan has the further challenge of being insoluble when the degree of ionization is less than 0.2.
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1
0.8
Degree of Ioniza on
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0.6
0.4
0.2
0 1
Figure 3
3
5
7 pH
9
11
13
Comparison of the ionization behaviours of PVAm with linear PEI and PAH based on Henderson-Hasselbalch fits to experimental titration curves in 1 M KCl. 30, 31 The chitosan (500 kDa, DS 0.80) fit was for a titration conducted in 0.1 M NaCl. 32
Solution Conformation of PVAm and PVAm Derivatives. In theory, the chain length of PVAm should be close to the chain length of the parent PNVF. The viscosity average molecular weight of PNVF polymers is conveniently determined by intrinsic viscosity measurements. Singley et al. used a large data set to determine the Mark-Houwink parameters for PNVF in 0.1 M NaCl based on light scattering measurements [η]=5.43x10-4 Mw0.75dL/g. 33 The only caveat is that the free radical polymerization on NVF can include crosslink formation, and the crosslinks are cleaved during hydrolysis to PVAm. 34 In this case, the intrinsic viscosity/molecular weight estimate for PNVF would misrepresent the PVAm chain length. Intrinsic viscosity can also be applied to charged polymers if the ionic strength is sufficiently high. Bloys van Tresland reported the corresponding relationship for PVAm based on viscosity and osmotic pressure measurements in 0.01 M NaOH (i.e. pH 12) and 0.1 M NaCl – conditions under which PVAm is uncharged: [η]=6.2x10-3 Mn0.88. 35 For comparison, the Mark-Houwink expression for the hydrochloride salt of PAH in 0.1 M NaCl (pH not specified but should be low for full ionization)is [η]= 7.65x10-3 M0.8 ±0.1. 36 There is little discussion in the literature about the conformation of PVAm in solution. Kobayashi et al. reported ηsp/c versus pH as functions of electrolyte concentration. 37 Up to 0.1 M KCl, the reduced viscosity showed a peak at pH ~4, with the biggest effects at low ionic strength. In 1 M KCl, the reduced viscosity was nearly independent of pH. Kirwan et al. used AFM to image individual PVAm molecules on mica and observed the transitions from coil (pH 3) to pearl-necklace (pH 4) to globule structures (pH 10). 38 Above pH 9, DLS measurements suggest some association of PVAm in solution. 39
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Hydrophobic derivatives of PVAm associate in solution or at the air/water interface (see section below). Hydrophobic associating increases with increasing hydrophobic content and with increasing pH.17 The presence of hydrophobic domains in PVAm-H (pendant octyl groups) solutions was determined by pyrene fluorescence measurements. More exotic phase behaviors were observed when the hydrophobic substituent was phenylboronic acid. Figure 4 shows the solubility of polyvinylamine-g-phenylboronic (PVAm-B) acid, as a function of pH.40 At low pH, the amine groups are protonated and the copolymer is soluble. Above pH 6 the copolymer phase separates, first forming a stable colloid and then a macroscopic precipitate. Under these conditions the copolymer is amphoteric; both hydrophobic and electrostatic interactions contribute to chain collapse. Above pH 9 the copolymer solubility increases because the ionization of the boronic acid groups increases the hydrophilicity of the phenylboronic moieties.
5 Colloid
PVAm-B Conc. /gL-1
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Precipitate
4
Solution
3
2
1
Solution
0 1
3
5
7
9
11
pH
Figure 4
Phase behaviour of PVAm-B. The molecular weight is 150 kDa and measurements were made in 5 mM NaCl at 25 °C. Diagram adapted from Chen et al. 40
PVAm Complexes with Anionic Polyelectrolytes. PVAm, a highly cationic polyelectrolyte over most of the pH range, forms polyelectrolyte complexes with oppositely charged synthetic polymers and biomacromolecules. 41 Arora and Turro used photophysical measurements to probe pyrene-labeled polyacrylic acid with PVAm. 42 They observed complex formation at both low pH, explained by hydrogen bonding and at high pH, explained by coulombic interactions. We reported the properties of PVAm/carboxymethylcellulose polyelectrolyte complexes and the phase diagram of the reaction products obtained from mixing dilute solutions PVAm with carboxymethylcellulose, included regions with soluble complexes, colloidal complexes, and macroscopic precipitates. 43 Colloidal complexes were stable when either polymer was in sufficient excess to give electrosteric stabilization. In summary,
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with respect to polyelectrolyte complex formation, limited published data suggests PVAm shows no unusual properties. Specific PVAm Complexation. For controlled drug release and other “smart” gel applications, pH and temperature sensitive gels are of interest. Mokhtari et al. obtained PVAm gels by mixing phenylboronic acid derivatized PVAm (PVAm-B) with galactose derivatized PVAm (PVAm-G). 24 Reversible condensation reactions between the boronic acid and the galactose moieties gave pH sensitive crosslinks. Figure 5 shows the chemical structures of the crosslinks and a photograph of the gels. Lowering the pH below 4 solubilized the gel as a result of the hydrolysis of the boronate-galactose ester. Addition of a monosaccharide with a higher borate binding constant than galactose also destroyed the crosslinks, causing the gel to dissolve.
CH2
CH
CH2
C
HO
CH
x
NH
NH2
y
O
O
B
CH2 O OH O
HO
O OH HOH2C
C H
OH C H
H C OH
C H
C O
CH2
NH CH
CH2
x
Figure 5
CH NH2
y
Gelation induced in mixtures of PVAm-B with PVAm-G. 24 The gels dissolved when pH was lowered below 4 or when galactose was added. Used with permission from the American Chemical Society.
PVAm Interactions with Surfactants. Many water-soluble polymers bind surfactants giving more surface-active complexes. 44 The properties of both concentrated PVAm/surfactant phases45, 46 and dilute PVAm/surfactant solutions 39 have been reported. Negatively charged surfactants such 10 ACS Paragon Plus Environment
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as SDS interact strongly with PVAm giving colloidal complexes 47 or a precipitate, depending upon the relative concentrations. The PVAm/SDS behaviors seem consistent with the PAH/SDS 48 and PEI (linear and branched)/SDS 49 results, all of which fit the classic description of polymer surfactant interactions – see Figure 2 in Goddard’s classic review. 50 Surface tension measurements suggest that cationic surfactants do not interact with PVAm. 51 On the other hand, there is one report of PVAm interacting with cationic surfactants, presumably driven by hydrophobic interactions. 46 Similarly, PEI interacts with cationic surfactants. 52 For nonionic surfactants, we used surface tension and calorimetric measurements to show that PVAm does not bind a wide range of Pluronics (PEG-bloc-PPG-bloc-PEG), whereas PVAm-H with octyl hydrophobes binds to Pluronic micelles, giving them a cationic PVAm coating. 39 Petkova et al. reported a very slight lowering of surface tension with PVAm/nonionic mixtures compared to nonionic surfactant alone. 51 In summary, PVAm is very hydrophilic – it is not surface active. Based on limited data, PVAm does not appear to interact with nonionic surfactants, whereas coulombic and hydrophobic interactions drive the association of anionic surfactants with PVAm.
Metal Ion Binding to PVAm. The removal of metal ions from solution is an obvious application of PVAm. Metal ions can chelate with free amine groups and will undergo ion-exchange with polymer-bound ammonium ions. One of the earliest publications was in 1965 53 and there have about a dozen papers since then. In a series of papers, Kobayashi et al. compared the metal ion binding characteristics of PVAm with linear PEI and with polyallylamine (PAH). 31 PEI and PVAm showed similar binding constants, whereas PAH binding constants were 1050 times higher. Presumably the increased flexibility provided by the methylene spacer group in PAH facilitates multi-dentate binding. Belfiore and coworkers showed that transition metal binding to PVAm increased the glass transition temperature and compatibilized blends with PEI. 54 This is the only publication on PVAm solid-state properties that I have found.
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INTERACTIONS AT INTERFACES PVAm Adsorption on the Solid/water Interface. Table 1 gives a selection of PVAm adsorption data from the literature. Most reports involve silica, glass or cellulose surfaces. The relatively large number of studies involving cellulose reflects the fact that the use of PVAm as a papermaking additive is the largest commercial application of PVAm. Unlike silica, cellulose is a difficult substrate for fundamental adsorption studies because it is not readily available in smooth, non-porous and well-defined formats. Published studies either involve cellulose fibers where the specific surface area is unknown, 55, 56 or model regenerated cellulose films. 57 Table 1
A selection of PVAm adsorption data from the literature.
% Hydrolysis 94% 32% 100% 90% 100 100 100 100 100 100 100 100 94
kDa
Substrate
pH
520 470 200 40 200 142 12
silica silica glass fiber glass fiber glass fiber silica silica cellulose cellulose cellulose cellulose cellulose polystyrene
4 4 7 7 7 ? ? 10 10
