pH-Dependence of the Properties of Hydrophobically Modified

Nov 3, 2005 - either the degree of hydrophobic substitution or the hydrophobic chain length lowered ... copolymers (often called Pluronics) associate ...
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Langmuir 2005, 21, 11673-11677

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pH-Dependence of the Properties of Hydrophobically Modified Polyvinylamine Xiaonong Chen, Yi Wang, and Robert Pelton* McMaster Centre for Pulp and Paper Research, Department of Chemical Engineering, McMaster University, Hamilton, Ontario, Canada, L8S 4L7 Received July 5, 2005. In Final Form: September 30, 2005 A series of N-alkyl or N-benzyl substituted polyvinylamines (PVAm) were prepared and the properties of aqueous solutions were measured as functions of pH. The polymer solutions showed almost no surface activity under acidic conditions whereas surface tension was reduced to 40-50 mN/m around pH 9. Increasing either the degree of hydrophobic substitution or the hydrophobic chain length lowered the pH at which surface tension lowering was observed. Hydrophobic substitution also shifted plots of the degree of ionization versus pH toward lower pH which means lower pH values were required to achieve a given value of polymer charging. The hydrophobically modified PVAm associated in water giving species whose apparent diameter measured by dynamic light scattering decreased with increasing pH, whereas the electrophoretic mobilities of the associated species increased with decreasing pH. Although many hydrophobically modified and pH sensitive polymers have been described in the literature for applications in biomaterials, drug release and as pH sensitive surfactants, the hydrophobically modified PVAms are particularly attractive because they are easily prepared from commercially available polyvinylamines.

Introduction Stimuli-responsive water-borne polymers have received much attention over the last couple of decades1-3 because of potential biomedical applications including protein separation, 4,5 gene transfer,6 tissue engineering, implantable devices, and drug delivery.7,8 Most of the stimuli responsive polymers belong to one of two types: Type (1) responsive polymers are covalently cross-linked into macro or microgels; or, Type (2) responsive polymers associated (self-assemble) to form stimuli responsive gels and other structures. For example, poly(N-isopropylacrylamide)9 is easily covalently cross-linked to give temperature-sensitive gels, and thus is an example of Type 1, whereas poly(ethyleneoxide-b-propyleneoxide-ethyleneoxide) tri-block copolymers (often called Pluronics) associate to form micelles or gels,10 an example of Type 2. In this work, we describe association behavior of derivatives of polyvinyl* To whom correspondence should be addressed. McMaster Centre for Pulp and Paper Research, Department of Chemical Engineering, JHE-136, McMaster University, Hamilton, Ontario, Canada, L8S 4L7. Tel: (905) 529-7070 ext. 27045. Fax: (905) 5285114. E-mail: [email protected]. (1) Osada, Y.; Ross-Murphy, S. B. Sci. Am. 1993, May, 82. (2) de las Heras, C.; Pennadam, A. S.; Alexander, C. Chem. Soc. Rev. 2005, 34, 276-285. (3) Siegel, R. A., Ed. Fundamentals and Applications of Polymer Gels. Macromol. Symp. 2004, 207. (4) Liu, S.; Armes, S. P. Angew. Chem., Int. Ed. 2002, 41, 1413. (5) Ghosh, R. Protein Bioseparation Using Ultrafiltration; Imperial College Press: London, 2003. (6) (a) Liang, E.; Hughes, J. Biochim. Biophys. Acta 1998, 1369, 39. (b) Liang, E.; Rosenblatt, M. N.; Ajmani, P. S.; Hughes, J. A. Eur. J. Pharma. Sci. 2000, 11, 199. (c) Wang, S.; Bui, V.; Hughes, J. A.; King, M. A.; Meyer, E. M. Neurochem. Int. 2000, 37, 1. (7) Martin, T. J.; Prochazka, K.; Munk, P.; Webber, S. E. Macromolecules 1996, 29, 6071. (8) (a) Peppas, N. A. “Hydrogels” in Biomaterials Science; Ratner, B. d., Hoffman, A. S., Schoen, F. J., Lemons, J. E., Eds.; Academic Press: New York, 1996. (b) Heller, J. Drug Delivery Systems; Ratner, B. d., Hoffman, A. S., Schoen, F. J., Lemons, J. E., Eds.; Academic Press: New York, 1996. (9) Pelton, R. H. Temperature Sensitive Aqueous Microgels. Adv. Colloid Interface Sci. 2000, 85, 1. (10) Huang, K.; Lee, B. P.; Ingram, D. R.; Messersmith, P. B. Biomacromolecules 2002, 3, 397-406.

amine (PVAm) which have the potential to be Type 2 stimuli-responsive molecules. Most stimuli responsive polymers are some type of copolymer whose state in solution is a subtle balance of opposing forces. For example, Chen et al. prepared poly(N-isopropyl acrylamide)-g-poly(acrylic acid) graft copolymers which displayed pH- and temperature-sensitive micellization.11 In another example, Scranton et al. reported a comb-type graft copolymer with a poly(methacrylic acid) (PMA) backbone and with poly(ethylene glycol) (PEG) branches. At low pH, the PEG branches complexed with the PMA backbone giving associated structures, whereas at high pH, the copolymer was water soluble showing no association or surface activity. Thus, such a pH-sensitive surfactant allows emulsions and dispersions to be broken and reformed reversibly.12-14 Recent advances in living free radical polymerization strategies has led to a number of papers describing rather exotic copolymers which associate because of electrostatic and/or hydrophobic interactions. For example, Armes’ group synthesized novel hydrophilic-hydrophilic block copolymers based on tertiary amine methacrylates, such as poly[2-(dimethylamino)ethyl methacrylate]-b-poly[2(diethylamino)ethyl methacrylate] copolymers15,16 and their quaternized products.17 The aqueous solution of these diblock copolymers shows pH-induced micellization and very high surface activity under basic conditions due to deprotonation of the amino moieties. Polyvinylamine (PVAm) has recently become commercially available and is an attractive platform for the preparation of stimuli responsive copolymers. The pendant (11) Chen, G.; Hoffman, A. S. Nature 1995, 373, 49. (12) Scranton, A. B.; Klier, J.; Mathur, A. M. PCT Int. Appl. 1997, WO 9,716,463. (13) Mathur, A. M.; Drescher, B.; Scranton, A. B.; Klier, J. Nature 1998, 392, 367. (14) Drescher, B.; Scranton, A. B.; Klier, J. Polymer 2001, 42, 49. (15) Butun, V.; Billingham, N. C.; Armes, S. P. Chem. Com. 1997, 671. (16) Butun, V.; Armes, S. P.; Billingham, N. C. Polymer 2001, 42, 5993. (17) Vamvakaki, M.; Unali, G. F.; Butun, V.; Boucher, S.; Robinson, K. L.; Billingham, N. C.; Armes, S. P. Macromolecules 2001, 34, 6839.

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amine groups are convenient sites for grafting as well as being an ionization site for pH dependent cationic charge. Recognizing this potential Marchant’s group has published a series of papers describing dextran,18,19 PEG,20,21 and hydrophobically derivatized PVAm. Although this work clearly shows that these copolymers associate in solution and are surface active, there remains unanswered questions because neither ionic strength or pH seems to have been controlled. The PVAm charge content is pH sensitive and the unmodified homopolymer is surface active at pH values above 9.22 This paper reports on the surface tension, light scattering and electrophoretic properties of a series of hydrophobically modified PVAms. It turns out that copolymer properties are particularly sensitive to pH, thus providing a route to pH stimulated applications. Experimental Section Materials. 90% hydrolyzed poly(N-vinylformamide, MW 950 kDa (BASF) was completely hydrolyzed in 10% NaOH aqueous solution at 80 °C; the details have been published previously.23 Benzyl bromide (98%), iodoethane (99%), 1-bromobutane (99%), 1-bromohexane (98%), 1-bromooctane (99%), and 1-bromododecane (97%) were purchased from Aldrich and used as received. Milli-Q deionized water was used in the preparation of all aqueous solutions. The pH of polymer solutions were adjusted using 0.1 N HCl and 0.1 N NaOH. Instrumentation. 1H NMR spectra of the PVAm and modified PVAm were recorded in D2O solution (pH 3-4) at room temperature using an AVANCE 200 NMR instrument (Brucker). Surface tension was measured by the pendant drop method using a Kruss V1.50 drop shape analyzer equipped with an environmental chamber maintained at 25 °C and near 100% relative humidity. Measurements were performed using 0.3% (wt) polymer solutions prepared in aqueous 0.001 M KCl at various pH values. Drops were formed with a 1.4 mm o.d. stainless steel syringe and drop volumes varied from 12 to 20 µL. Potentiometric titration was carried out using a PC-Titrate Module (Man-Tech Associates Inc.) at 25 °C under N2 atmosphere. In a typical experiment, 50 mL of 0.004% polymer in 0.001 M KCl was added to the titration vessel fitted with a CORNING “3 in 1” premium GEL Combo combination glass electrode. The initial pH was adjusted to 3 and 0.1 M NaOH was added every 30s with each injection volumes between 0.0001 and 0.04 mL. The degree of ionization versus pH was determined from the difference between the polymer and blank titrations. Hydrophobically modified polyvinylamine (HMPVAm) associated in solution to give species that were characterized by light scattering and electrophoresis. Electrophoretic mobility measurements were made at 25 °C using a ZetaPlus (Brookhaven Instruments Corporation) operating in PALS (phase analysis light scattering) mode (PALS software version 2.5). The reported values were based on 10 measurements with 15 cycles for each measurement. Dynamic light scattering measurements were made at a detector angle of 90° using a Lexel 95 argon ion laser operating at a wavelength of 514 nm and a power of 100mW. Correlation data was analyzed using a BI-9000AT digital autocorrelator, version 6.1 (Brookhaven Instruments Co.) and the CONTIN algorithm was used to calculate the apparent particle size. Viscosities of 0.3 wt % PVAm and HMPVAm in 0.001 M NaCl were determined with a Stresstech HR rheometer fitted with a (18) Holland, N. B.; Qiu, Y.; Ruegsegger, M.; Marchant, R. E. Nature 1998, 392, 799. (19) Qiu, Y.; Zhang, T.; Ruegsegger, M.; Marchant, R. E. Macromolecules 1998, 31, 165. (20) Vacheethasanee, K.; Marchant, R. E. J. Biomed. Mater. Res. 2000, 50, 302. (21) Holland, N. B.; Xu, Z.; Vacheethasanee, K.; Marchant, R. E. Macromolecules 2001, 34, 6424. (22) Hong, J.; Pelton, R. Colloid Polym. Sci. 2001, 280, 203. (23) Gu, L.; Zhu, S.; Hrymak, A. N. J. Appl. Polymer Sci. 2002, 86 (13), 3412.

Figure 1. 1H NMR spectra of PVAm (Top), PVAm-BZ-47 (middle), and PVAm-C12-9.5 (bottom). C40-4, cone type cell with a volume of 1.17 mL. Measurements were performed at various pH values at 25 °C. Preparation of Hydrophobically Modified PVAm. Nalkylated PVAm was prepared by reacting PVAm with an alkyl halide using a method previously reported by Martel et al.24 in which 2% PVAm was dissolved in water/methanol (1/10 weight ratio) containing 0.02% (weight) NaOH and the alkyl halide was slowly added at room temperature. After stirring for 48 h at 60 °C, the product was precipitated using ethanol (pH 2) and purified by dialysis and freeze-drying. The degree of substitution (DS) was calculated based on the peak integrals of the NMR spectra (Figure 1 and Table 1).

Results and Discussion High molecular weight (980 kDa) polyvinylamine (PVAm) bearing pendant hydrophobic groups was prepared by coupling linear alkyl halides or benzyl bromide with PVAm giving a series of polymers summarized in (24) Martel, B.; Pollet, A.; Morcellet, M. Macromolecules 1994, 27, 5258.

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Figure 3. Electrophoretic mobility of hydrophobically modified PVAm at various pH. 100 ppm polymer in 0.001 M KCl aqueous solution, 25 °C. Figure 2. Ionization degree of PVAm and modified PVAm measured at 25 °C, in 0.001 M KCl. Table 1. Polymer Compositions. the Degree of Substitution Is the Number of Hydrophobes Per Nitrogen and the Maximum Possible Value Is 2 Corresponding to Tertiary Amines polymer

alkylating agent

degree of substitution

PVAm-BZ-14 PVAm-BZ-28 PVAm-BZ-47 PVAm-BZ-150 PVAm-C2-105 PVAm-C4-14 PVAm-C4-30 PVAm-C6-9.4 PVAm-C6-16 PVAm-C8-17 PVAm-C8-34 PVAm-C12-9.5 PVAm-C12-18 PVAm-C12-20

benzyl bromide benzyl bromide benzyl bromide benzyl bromide iodoethane 1-bromobutane 1-bromobutane 1-bromohexane 1-bromohexane 1-bromooctane 1-bromooctane 1-bromododecane 1-bromododecane 1-bromododecane

0.14 0.28 0.47 1.5 1.05 0.14 0.30 0.094 0.16 0.17 0.34 0.095 0.18 0.20

Table 1. The degree of substitution, defined as the number alkyl chains per amine, was determined by 1H NMR. Example spectra are shown in Figure 1. The top spectrum was for PVAm before modification, the middle for PVAmBZ, and the bottom was for PVAm-C12. The degree of substitution was determined from the relative peak areas of the backbone and pendant group protons. The charge content of the modified polyvinylamines was determined as a function of pH by potentiometric titration and the results are summarized in Figure 2. All the polymers were highly charged at low pH with charge density decreasing approximately linearly with increasing pH. Closer inspection reveals some structure in the curves. This titration behavior of PVAm has been modeled by a two parameter Ising modelsone parameter is the intrinsic dissociation constant and the other accounts for nearest neighbor interactions.25 Hydrophobic substitution influenced the titration curves in Figure 2. The titration curve for PVAm-BZ-150 (DS ) 1.5), the most hydrophobic polymer, was shifted about two pH units to the left meaning that a much higher acid concentration was required to achieve a given degree of protonation. Presumably, this reflects the increased energy associated with unfolding hydrophobically associated structures. Microelectrophoresis measurements were made to further probe the charge characteristics of the polymers. (25) Katchalsky, A.; Mazur, J.; Spitnik, P. J. Polym. Sci. 1957, 23, 513.

The electrophoretic mobility of modified polyvinylamines is shown as a function of pH in Figure 3. Note that unmodified PVAm could not be measured because it did not scatter sufficient light to be detected. Over most of the pH range the electrophoretic mobility increased with decreasing pH, much like the potentiometric curves. Furthermore, with increased hydrophobization, a lower pH was required for a given mobility which is also similar to the potentiometric curves. From pH 4.5 to 4 the electrophoretic mobility of PVAmC12-9.5 plummeted from 4 to 1 mobility units although the charge content of the polymer increased. Electrophoretic mobility is a balance of electrostatics, driving motion in the electric field, and hydrodynamic drag, opposing motion. An explanation of the decreasing mobility with increasing charge (i.e., decreasing pH, see Figure 3) is that the drag is increasing, due to chain expansion, more quickly than surface charge as the pH is lowered. To support this explanation, the apparent hydrodynamic diameter and the corresponding dynamic light scattering intensities of the PVAm-BZ-150 are shown as a function of pH in Figure 4. At pH less than 8, the scattering intensity was low and the apparent diameter values were very large. We propose that in this regime PVAm-BZ-150 is present as large associated structures. Above pH 8, the scattering intensity is much higher and the apparent hydrodynamic diameters are less than 400 nm suggesting the presence of phase separated polymer particles. PVAm-C12-9.5 behaved similarly with the transition from large associated structures to a particulate suspension occurring at pH 7 compared with pH 8 for PVAm-BZ-150. Further evidence for intermolecular association was obtained from the rheological properties. Figure 5 shows the viscosity of PVAm-C8 as function of pH and the degree of substitution. The maximum viscosity occurred at about pH 4, and the effects increased with the degree of hydrophobic modification. In summary, the charging (protonation) behaviors of the hydrophobically modified polyvinylamines were similar to that of PVAm except for the very hydrophobic PVAm-BZ-1.5 which was significantly less charged. Furthermore, the hydrophobic polymers associated in solution to give species detectable by rheology, dynamic light scattering, and by light scattering-based microelectrophoresis. The surface tension of the modified polyvinylamines was measured by the pendant drop shape technique. Figure 6 shows surface tension versus drop age times for PVAm-C12-9.5 at six pH values. Despite many attempts to control humidity, temperature, and vibrations, we could

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Figure 4. Light scattering intensity and apparent diameter as a function of pH for PVAm-BZ-150 and PVAm-C12-9.5 solutions. The solutions were 100 ppm polymer in 0.001 M KCl at 25 °C.

Figure 7. Break point surface tensions (see Figure 2) as function of pH and degree of hydrophobic substitution (DS) for 0.3% (wt) PVAm-BZ and PVAm-C12 0.001 M KCl at 25 °C.

Figure 5. Viscosity of PVAm-C8 in aqueous solution at a shear rate of 43.6 s-1 and 25 °C. Polymer concentration: 0.3 wt % in 0.001 M NaCl aqueous solution.

Figure 6. Surface tension of 0.3% (wt) PVAm-C12-9.5 as functions of pendant drop age. Measurements were made in 0.001 M KCl aqueous solution, 25 °C.

not obtain steady-state surface tension measurements. The slow decrease in PVAm-C12-9.5 with time suggests a continuous rearrangement of the adsorbed polymer. By contrast, simple surfactants (data not shown) and poly-

(N-isopropylacrylamide), a nonionic surface active polymer, gave constant, steady-state surface tension values. Usually, the steady-state surface tension values are reported which is problematic for the curves in Figure 6. Herein, we report the surface tension values at the break point where the dynamic surface tension curve switches from a rapid to slow changing rate regime (see example in Figure 6). Clearly, the break point surface tension values do not represent equilibrium behavior. However, they are a convenient way to illustrate the effects of pH, ionic strength and polymer composition. For example, Figure 7 shows the effect of pH on break point surface tensions for benzyl (top) and dodecyl (bottom) substituted PVAm. All of the substituted PVAms did not lower surface tension at pH < 6 whereas the breakpoint surface tension decreased substantially with increasing pH above pH 6-7. The behaviors of the dodecyl (C12) derivatives were not very sensitive to the degree of substitution over the range 0.1 to 0.2 (bottom Figure 7). By contrast, a larger range of benzyl substation was probed and, as expected, surface activity increased with the degree of substitution. We propose that at low pH the charge density on the modified polyvinylamines was simply too high to permit the polymer to accumulate at the air/water interface. The slightly hydrophobically modified PVAm-BZ-28 varied only slightly from PVAm, whereas the more hydrophobic PVAm-C12-9.5 and the heavily derivatized PVAm-BZ150 curves were shifted to the left meaning higher acid concentrations were required to produce a given degree

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breakpoint surface tensions values at pH 9 as functions of hydrophobicity index. A wide range of behaviors fell near a single master curve which accounts for both the size and concentration of hydrophobic substituents.

Figure 8. Relationship between the surface tension at break point and the hydrophobicity of modified PVAm (0.3% (wt) of polymer in 0.001 M KCl aqueous solution, pH 9, 25 °C.

of protonation. Presumably, the tendency of the hydrophobic groups to associate opposes the expansion of the chains. In an effort to compare the surface activities of the various copolymers, a hydrophobicity index was defined as DSn, where DS is the number of hydrophobic substituents per nitrogen, and n is the number of hydrophobic carbon atoms in each substituent. Figure 8 shows the

Conclusions (1) Hydrophobically modified polyvinylamine displays pH dependent surface activity in water. The surface tension decreases with increasing pH, with increasing extent of hydrophobic substitution and with increasing size of the hydrophobic substituent. (2) Steady-state surface tensions are rarely observed suggesting a slow, continual rearrangement of the polymer adsorbed at the air/water interface. (3) Most of the hydrophobically modified PVAms displayed some degree of association throughout most of the pH range. At lower pH, the structures were large and weakly scattering whereas at high pH, more compact, highly scattering aggregates formed. (4) The surface tension behaviors of a wide range of copolymers could be collapsed onto a single master curve by plotting surface tension against hydrophobicity defined as the product of the number carbons per hydrophobe times the degree of substitution. Acknowledgment. The authors thank BASF Canada and Science and Engineering Research Canada for funding this research. LA0518039