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Surface Characteristics of Comblike Copolymers from Hexadecylacrylamide and Acrylic Acid at the Air/Water Interface Geetha Baskar,*,† L. J. Milton Gaspar,† and A. B. Mandal‡ Industrial Chemistry Laboratory and Inorganic and Physical Chemistry Laboratory, Central Leather Research Institute, Adyar, Chennai 600020, India Received May 8, 2003. In Final Form: July 12, 2003 The interfacial organization characteristics of comblike copolymers from hexadecylacrylamide and acrylic acid at the air/water interface have been investigated using a Langmuir film balance at pH 6.5 and 10.0. The homopolymer and the copolymers (CHXP1, CHXP2) exhibit Ao values of about 0.26 nm2/molecule with a πmax value of about 40 mN/m. The small differences in the surface characteristics of the copolymers suggest that the hexadecylacrylamide polymers undergo little changes in packing characteristics with the introduction of acrylic acid even up to 15 mol %. However, the copolymer consisting of 22 mol % acrylic acid tends to form a more expanded film, as is suggested from the comparatively higher Ao (0.28 nm2/ molecule) and compressibility coefficient, k of 1.07 × 10-2 m/mN at π ) 25 mN/m. The high surface concentration (Γ), 2.0-2.2 mg/m2, and πmax of 40 mN/m of the investigated polymers are in support of the packing of the hexadecyl side chain at the interface. The results of the π-A isotherm and hysteresis experiments at pH 10.0 suggest that an introduction of an optimum level of 15 mol % acrylic acid in the copolymer imparts stabilization of the hexadecylacrylamide polymer film at the interface. The presence of 22 mol % acrylic acid shifts the amphiphilic balance of the copolymer to a more hydrophilic structure and promotes the desorption of the film into an aqueous subphase, especially under alkaline conditions leading to instability.
Introduction The organization of surface-active polymers underlying most of surface-related processes such as spreading, wetting, and adhesion draws significant attention in material and surface science.1-5 The surface-active polymers act as steric stabilizers and provide advantages of stability against high electrolyte concentrations, freezethaw cycles, and steric repulsion.6,7 They are capable of adjusting the lifetime of organized assembled structures, for example, micelles, to be in the range of seconds or minutes. Various copolymer architectures from block, graft, and random polymers perform as polymeric amphiphiles.8 Among them, comblike polymers consisting of long alkyl side chains are significant in tailoring the surface properties of substrate and find potential in the design of adhesives, coatings, and biomembranes.9-11 * Corresponding author. Fax: 91-44-24911589. Telephone: 9144-24911386/24911108. E-mail:
[email protected]. † Industrial Chemistry Laboratory, Central Leather Research Institute. ‡ Inorganic and Physical Chemistry Laboratory, Central Leather Research Institute. (1) Yekta, A.; Duhamel, J.; Brochard, P.; Adiwidjaja, H.; Winnik, M. A. Macromolecules 1993, 26, 1829. (2) Kumacheva, E.; Rharbi, Y.; Winnik, Y. M. A.; Guo, L.; Tam, K. C.; Jenkins, R. D. Langmuir 1997, 13, 182. (3) Zhang, Y. B.; Wu, C.; Fang, O.; Zhang, Y. X. Macromolecules 1996, 29, 2494. (4) McCormick, C. L.; Middleton, C. L. Polym. Mater. Sci. 1987, 57, 700. (5) Chen, J.; Jiang, M.; Zhang, Y.; Zhou, H. Macromolecules 1999, 32, 4861. (6) Kusters, J. M. H.; Napper, D. H.; Gilbert, R. G.; German, A. L. Macromolecules 1982, 25, 7043. (7) Pirma, I. Polymeric Surfactants; Surfactant Science Series No. 42; Marcel Dekker: New York, 1993. (8) Foster, S.; Antonietti, M. Adv. Mater. 1998, 10, 195. (9) Crevoisier, G.; Fabre, P.; Corpat, J. M.; Leibler, L. Science 1999, 285, 1246. (10) Dessipri, E.; Tirrell, D. A.; Atkins, E. D. T. Macromolecules 1996, 29, 3545.
The surface characteristics of the polymers are dependent on the conformation or orientation at the air/ water (a/w) interface, as is influenced by the flexibilities of the polymeric chain and hydrophilic/hydrophobic properties.12 The surface energy serves as one of the simple and sensitive parameters providing information on surface characteristics, and this has been well documented.13,14 In a comblike polymer, the length, packing behavior, and mobility of the alkyl chain play significant roles in controlling the surface-energy characteristics. Comblike polymers consisting of long alkyl or perfluorinated chains are used to generate surfaces with low surface energies. It has been shown that the surface energy of a comblike polymer 15,16 decreases with an increase in the length of the alkyl side chains up to a certain critical volume. Yoon et al.17 have demonstrated that the comblike polymer consisting of (CF2)8F perfluorinated side chains exhibits a lower surface energy that is attributed to crystalline properties, in comparison to the shorter homologue, (CF2)6F. The influence of the side-chain-group interaction on the surface-energy characteristics has been demonstrated in alternating copolymers of N-alkylacrylamide with propene or styrene.18 (11) Hester, J. F.; Banerjee, P.; Mayer, A. M. Macromolecules 1999, 32, 1643. (12) Kawaguchi, M.; Tohayama, M.; Mutoh, Y.; Takahashi, A. Langmuir 1988, 4, 407. (13) Adamson, A. W. Physical Chemistry of Surfaces, 5th ed.; Wiley: New York, 1990. (14) Israelechivili, J. N. Intermolecular and surface forces, 2nd ed.; Academic Press: San Diego, CA, 1992. (15) Mol, E. A. L.; Shindler, J. D.; Shelaginov, A. N.; Dejeck, W. H. Phys. Rev. 1996, 54, 536. (16) Stoeke, T.; Mach, P.; Grantz, S.; Huang, C. Phys. Rev. 1996, 53, 1662. (17) Luning, J.; Stohr, J.; Sorg, K. Y.; Hawker, C. J.; Iodice, P.; Nguyen, C. V.; Yoon, D. Y. Macromolecules 2001, 34, 1128. (18) Grundke, K.; Zschoche, S.; Poschel, K.; Gietzelt, T.; Michel, S.; Friedel, P.; Jehnichen, D. Macromolecules 2001, 34, 6768.
10.1021/la034784g CCC: $25.00 © 2003 American Chemical Society Published on Web 09/11/2003
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The organization and modification in the surface characteristics of comblike polymer composites consisting of additives drawn from polyelectrolytes, low-molecularweight amphiphiles, or proteins are highly interesting and significant in the design of advanced materials. For example, Thunemann et al.19 have demonstrated the scope for lowering the surface energy of a fluorinated surfactant through complexation with a polyelectrolyte. The copolymers from comblike polymers present an interesting and special class of surfactants, wherein the comonomer influences the packing characteristics and interaction of the side chain through the spacer-group effect. The investigations on the organization characteristics of comblike copolymers are essential in adding to the fundamental knowledge that gives an understanding of the molecular forces governing such processes. The comblike copolymers from the perfluorinated acrylateco-n-alkyl acrylate system are reported to exhibit optimum organization leading to a crystalline lamellar structure.20,21 The comblike copolymers from octadecyl methacrylate (ODMA) and acrylic acid have been shown to exhibit different structures varying from a collapsed coil to a random coil in a tetrahydrofuran (THF) solvent.22 The effective adsorption characteristics at the styrene/water interface of these comblike copolymers from ODMA and acrylic acid/methacrylic acid have been exploited in generating polystyrene latex with a particle size of about 100 nm through the miniemulsion polymerization technique.23 Comblike polymers from alkylacrylamides are significant in view of photoresist properties and application in high-resolution lithography.24 The acrylamide copolymers are reported to exhibit interfacial organization characteristics as a function of the copolymer composition and contribute to sensitive changes in the dipole moment.25 This property is exploited in CO2 sensor magnetoelastic polymeric film coatings from alkylacrylamide derivatives. Comblike copolymers consisting of acrylic acid comonomer are significant, especially in view of the hydrophilic acid functional group controlling the charge characteristics and the hydrophilic/lipophilic balance (HLB) of the resulting surfactant. Furthermore, such copolymers with low HLB values as controlled by the copolymer composition are expected to find immense potential in the formation of double emulsions, water/oil/water, and the design of coreshell latex structures.26 The design of comblike copolymers from alkylacrylamide and acrylic acid with controlled surface characteristics demand a detailed investigation on the interfacial characteristics. Several techniques such as X-ray diffraction and infrared and Raman spectroscopies have been applied to investigate the surface characteristics of polymeric films. Among them, investigations on the polymeric monolayer film using the Langmuir film balance (LFB) technique have been found to be useful in exploring the static/ dynamic properties of polymer chains in two-dimensional (19) Antonietti, M.; Wenzel, A.; Thunemann, A. F. Langmuir 1996, 8, 211. (20) Gibbs, J. H.; Smyth, C. P. J. Am. Chem. Soc. 1951, 73, 5115. (21) Foret, L.; Wierger, A. Phys. Rev. Lett. 2001, 86, 5930. (22) Baskar, G.; Ramya, S.; Mandal, A. B. Colloid Polym. Sci. 2002, 280, 886. (23) Baskar, G.; Landfester, K.; Antonietti, M. Macromolecules 2000, 33, 9228. (24) Guo, Y.; Feng, F.; Miyashita, T. Macromolecules 1999, 32, 1115. (25) Cai, Q. Y.; Cammers-Goodwin, A.; Grimes, C. A. J. Environ. Monit. 2000, 2, 556. (26) Kanouni, M.; Rosano, H. L.; Naouli, N. Adv. Colloid Interface. Sci. 2002, 99, 229.
Baskar et al. Table 1. Synthesis of Comblike Copolymers: Copolymer Compositions and GPC Characteristics copolymer mole fraction polymer hexadecylacrylamide HXP CHXP1 CHXP2 CHXP3
1.0 0.92 0.85 0.78
acrylic mol wt acid (×104) polydispersity 0 0.08 0.15 0.22
2.75 2.96 3.02 3.54
2.18 2.69 2.72 2.56
spaces.27 In this study, we have chosen a series of comblike copolymers derived from hexadecylacrylamide and acrylic acid for detailed investigations on interfacial characteristics at the a/w interface. The effect of the acrylic acid comonomer in influencing the surface energy, adsorption characteristics, and packing behavior of the copolymers at the a/w interface has been addressed. Experimental Section Materials. Hexadecylamine and acryloyl chloride (Aldrich) used in synthesis were used as were received. The monomer, hexadecylacrylamide, was synthesized using the procedure reported elsewere.24 Acrylic acid was from Sisco Research Laboratories, India. THF, chloroform, ethyl methyl ketone, and methanol were from S.d.Fine Chemicals, India, and were used as were received. Synthesis of the Copolymers of Hexadecylacrylamide and Acrylic Acid. The homopolymer and statistical copolymers of hexadecylacrylamide and acrylic acid have been synthesized employing a simple solution polymerization technique. In a typical copolymerization reaction, acrylic acid (0.012 mol) and benzoyl peroxide (1% on a monomer-weight basis) were added to a stirred homogeneous solution of hexadecylacrylamide (1.35 mmol) in THF. The reaction was performed by heating the mixture for 8 h to 70 °C. The ultimate polymer was isolated from the reaction medium using methanol as the precipitating solvent. Pure polymer was obtained from repeated washings with methanol. The polymer was dried under a vacuum and further characterized using nuclear magnetic resonance (NMR) and gel permeation chromatography (GPC) techniques. Three sets of copolymers have been prepared, employing different mole fractions of comonomers in the feed mixture. The copolymer composition has been estimated from pH titration using alcoholic potassium hydroxide as the titrant, and the results are presented in Table 1. Characterization of the Copolymers. 1H NMR measurements for the characterization of the copolymers were performed on a JEOL GSX 400 (400 MHz) spectrometer using CDCl3 as the solvent and tetramethylsilane as the internal standard. The molecular-weight estimations were carried out on a GPC440 water chromatograph fitted with Ultrastyragel columns (103105 Å) using a refractive index detector. The apparent molecular weights and polydispersity in THF were estimated using polystyrene calibration. Conditions of Measurements of π-A Isotherms Using the LFB. In all the experiments using the LFB, π-A isotherms were measured after confirmation of the purity of water from the surface tension value (71.5-72 mN/m at 25 °C) and absence of surface-active impurities in the solvent by running a blank run on water using solvent alone. In all these experiments, water and solvent were found to be pure and free from such impurities. For the estimation of the area/molecule, the molecular weight of the copolymer side-chain structure was taken into consideration because these are the groups that are expected to be organized at the surface. The area/molecule, thus, refers to the area/ monomer unit. The monolayers were spread from chloroform (Merck HPLC grade) solution on Milli-Q deionized water from the Millipore system. The surface pressure of the monolayers was measured using a NIMA 611 single barrier trough fitted (27) (a) Gaines, T. G. L. Insoluble monolayers at liquid Gas Interfaces; Intersciences: New York, 1966. (b) Minenos, J.; Yebra-Pimentel, E.; Condes, O.; Iribarnegaray, E.; Cascas, M. Langmuir 1994, 10, 1888. (c) Grainger, D. W.; Sunamoto, J.; Akiyoshi, K.; Goto, M.; Knutson, K. Langmuir 1992, 8, 2479.
Comblike Copolymers Scheme 1. Structural Representation of (a) HXP and (b) CHXP1
with a Wilhelmy balance (accuracy 0.01 mN/m). The monolayers were compressed at a speed of 2 × 10-1 nm2/(mol‚min). Each sample monolayer was checked at least three times for checking the reproducibility of the isotherms.
Results and Discussion Characterization of Comblike Polymers. The structural representation of HXP and a representative copolymer, CHXP1, are presented in Scheme 1(a and b). The 1H NMR spectrum of the copolymer (CHXP1) in CDCl3 suggests the absence of unreacted monomers and also the formation of polymer through addition across the vinyl bond, as is shown by the complete absence of vinyl peaks at δ 5.5-6.5. Peak assignments for the typical copolymer CHXP1 are as follows: δ 0.85-0.90, CH3 end of the hexadecyl chain; δ 1.42-1.50, >CH2 of the side chain; δ 2.20-2.34, >CH2 of the backbone chain; δ 2.73, methine (>CH) of the backbone chain; δ 3.32-3.45, R-CH2 adjacent to the amide group; δ 4.45, -NH. An almost similar 1H NMR spectrum was observed for the polymers HXP, CHXP2, and CHXP3. The weight-average molecular weight of the polymers estimated using GPC are presented in Table 1. The polymers exhibited a molecular weight in the range of 2.75-3.54 × 104 with a polydispersity of 2.182.72. The compositions of the polymers, as was estimated from the titration method, are presented. Under the experimental conditions of the polymerization reaction and by varying the copolymer composition in the feed, the synthesis of a series of copolymers consisting of 0.080.22 mole fraction of acrylic acid has been enabled. It is well-known that various factors such as the reactivity ratio, composition of the monomers in the feed mixtures, and propagating radicals control the copolymer composition. Surface Pressure (π)-Area (A) Isotherm Characteristics of Hexadecylacrylamide Copolymers. The surface pressure (π)-area (A) isotherm characteristics measured on an aqueous subphase for hexadecylacrylamide homopolymer (HXP) and the series of copolymers (CHXP1, CHXP2, and CHXP3) consisting of different mole fractions of acrylic acid are presented in Figure 1A (a-d). All the isotherms have been found to be reproducible under the conditions of the experiment. The π-A isotherm curves (Figure 1) suggest that the polymer remains at the a/w interface after spreading and is compressed by the barrier. This leads to the main conclusion that the polymers are surface-active, that is, amphiphilic having the hydrophobic and hydrophiphilic parts in fair balance for film formation. The shape of the π-A isotherm curves (Figure 1) is suggestive of pressure-dependent characteristic phases.
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The polymers exhibit a liquid expanded (LE) phase in the low-pressure region, which gets transformed into a liquid condensed (LC) phase film upon further compression, resulting in steep π-A isotherm curves. The appearance of a small kink observed in all the π-A isotherm curves probably is related to such phase transitions. The support for this inference is obtained from analysis of the compressibility data, as is discussed in a later section. The shape of the π-A isotherm curves is related to the hydrophilicity or hydrophobicity of the polymers, as was described by Crisp.28 The polymers HXP, CHXP1, CHXP2, and CHXP3 can be considered to form a compressed-type film, as is shown by the first increase in the surface pressure at small areas (Ai, HXP, 0.33 nm2/molecule, Table 2), which becomes steep upon further compression similar to those observed for hydrophobic polymers such as poly(methyl methacrylate) (PMMA) or polyglutamate (PLGA) with bulky side chains.12 The polymer tends to form a more-expanded-type film with an increase in the change in amphiphilic balance toward slightly more hydrophilic character, enabled by incorporation of about 22 mol % acrylic acid. Thus, CHXP3, consisting of 22 mol % acrylic acid, results in a less-compressed-type film, as was observed with poly(ethylene oxide) or polyvinyl acetate12 exhibiting more hydrophilic characteristics. A comparison of the surface characteristics of the polymer films estimated from π-A isotherm curves are presented in Table 2. The values of surface characteristics represent the average of three individual π-A isotherm experiments. The close-packed surface area (Ao)/molecule is obtained by the extrapolation of the linear and steep portion of the upper part of the π-A isotherm. The homopolymer HXP exhibits an Ao value of 0.26 nm2/molecule, in accordance with reported values24 within the limits of experimental error. A much higher Ao of the hexadecyl side chain of the polymer in comparison to the low-molecular-weight hexadecyl compounds of fatty acid or amine suggests that the polymeric backbone unit, or, in other words, main chain, hinders the close-packing of the hexadecyl side chains. The polymer tends to form a less-compressed film at the interface, in comparison to the low-molecular-weight analogues, the most condensed ones of which may be compressed to a solidlike state. Polymers from PMMA or polybutyl acrylate with a similar backbone are reported to exhibit πmax values of about 15-25 mN/m.29 The high maximum surface pressure (πmax) of the homopolymer similar to those observed in low-molecular-weight hexadecyl-chain compounds is suggestive of packing of the hexadecyl chain of the polymer at the interface and, possibly, the amido group providing an anchoring site to the water subphase through hydrogen bonding. The copolymers from CHXP1 and CHXP2 exhibit Ao values of about 0.26 nm2/molecule, which are very close to that of the homopolymer. The surface characteristics of the copolymers suggest that the packing behavior of the hexadecyl side chain at the interface is not significantly altered by the introduction of acrylic acid up to 15 mol %. However, the copolymer CHXP3 (22 mol %) exhibits a slightly more expanded isotherm, and this is also shown in the small increase in Ao to an extent of 8%. Effect of pH on the π-A Isotherm Characteristics of Polymers. The π-A isotherms of polymers measured on an aqueous subphase under alkaline conditions at pH 10.0 are shown in Figure 1B. The homopolymer HXP tends to form a more expanded isotherm with less packing order, as was observed from a significant increase in the close(28) Crisp, D. J. J. Colloid. Sci. 1946, 1, 49. (29) Kawaguchi, M.; Nagata, K. Langmuir 1991, 7, 1478.
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Figure 1. Surface pressure (π)-surface area (A) isotherms of hexadecylacrylamide polymers on an aqueous subphase: (A) pH 6.5 at 25 °C for (a-d) HXP, CHXP1, CHXP2, and CHXP3; (B) pH 10.0 for (a-c) HXP, CHXP1, and CHXP2. Table 2. Surface Pressure (π) and Surface Area (A) Characteristics of Hexadecylacrylamide Polymers on an Aqueous Subphase at pH 6.5 and 10.0 at 25 °C close-packed surface area Ao (nm2/molecule)
initial area of increase of surface pressure Ai (nm2/molecule)
maximum surface pressure (mN/m)
polymer (composition; HXA:AA)
pH 6.5
pH 10.0
pH 6.5
pH 10.0
pH 6.5
pH 10.0
HXP (1:0) CHXP1 (0.92:0.08) CHXP2 (0.85:0.15) CHXP3 (0.78:0.22)
0.26 0.26 0.26 0.28
0.38 0.27 0.29
0.33 0.31 0.32 0.35
0.48 0.33 0.35
43.01 43.56 43.56 44.42
41.38 43.75 43.25
packed area Ao from 0.26 nm2/molecule (Table 2, HXP, pH 6.5) to 0.38 nm2/molecule (Table 2). However, there is a negligible change in πmax. The strong interaction of the
hydroxyl ions, from an aqueous subphase, with an amide group at pH 10.0 might account for the expanded isotherm. A close examination of the π-A isotherm and surface
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Table 3. Compressibility Coefficient (k; ×10-2 m/mN) of Hexadecylacrylamide Polymers on an Aqueous Subphase, pH 6.5 and 10.0, at 25 °C and at Different Pressures (π) π ) 5 mN/m
π ) 10 mN/m
π ) 15 mN/m
π ) 25 mN/m
polymer
pH 6.5
pH 10.0
pH 6.5
pH 10.0
pH 6.5
pH 10.0
pH 6.5
pH 10.0
HXP CHXP1 CHXP2 CHXP3
1.95 1.89 2.07 3.56
3.45 2.96 2.06 -
1.57 1.43 1.88 2.08
2.07 1.78 1.89
1.11 1.26 1.29 1.56
1.45 1.24 1.41
0.94 0.82 0.83 1.07
1.61 0.87 0.99
characteristics of the copolymers CHXP1 and CHXP2 (Table 2) at pH 10.0 indicates a small increase in the surface areas to an extent of about 5 and 10%, respectively, in contrast to the homopolymer. The presence of OH- in the subphase no doubt influences the organization of the copolymers at the interface, and in fact, acrylic acid comonomer controlling the hydrophilicity seems to play a key role in influencing the organization characteristics. The πmax (40 mN/M) and Ao values of CHXP1 and CHXP2 (Table 2) suggest that the acrylic acid spacer group in fact promotes the formation of a film with a higher packing density under alkaline conditions. Furthermore, the copolymer CHXP3 was found to form an unstable film at the interface, as was shown by a large variation in the surface area and π characteristics upon repeated compression and expansion, and it was not possible to obtain reproducible isotherms. It is possible that the formation of sodium acrylate salts under alkaline conditions promotes the desorption of the polymer film into the aqueous subphase. Comparative Packing Characteristics of Polymers. The compressibility coefficient (k) estimated using the extrapolated area (A) and inverse of slope of the π-A isotherm (δA/δπ) applying eq 1 is useful in interpreting information on the packing behavior of polymeric films.
k ) 1/Ao(δA/δπ)
(1)
The k values estimated at different surface pressures are presented in Table 3. The correlation between k and the packing characteristics of the adsorbed film at the a/w interface has been demonstrated in low-molecular-weight long-chain fatty acids.30 Following this, an analysis of k for the polymeric films suggests interesting results. The polymer HXP exhibits a k value of 1.1-1.9 × 10-2 m/mN in the π range of 5-15 mN/m, suggestive of LE phase structures, which upon compression tend to form a more condensed film, at high π regions typical of LC films, resulting in a k value of about 0.94 × 10-2. An almost similar k of about 0.8 × 10-2 of the CHXP1 and CHXP2, especially in higher pressure regions, further demonstrates the formation of the film in LC phase structures, in agreement with the close-packed-area results. The higher k value of CHXP3 is in support of the formation of a less-condensed film with less packing order, and this is in agreement with π-A isotherm characteristics. It is significant to note that the polymers such as polymethacrylate or poly(n-butylacrylate/isobutylacrylate), consisting of a similar backbone, exhibit high k values of 1.7-2.7 × 10-2 m/mN.31 It can be inferred that the C16 side chain of the polymers accounts for a more compressed packing of the polymeric chain at the interface. A comparative higher k at different π values of HXP at pH 10.0 (Table 3) further demonstrates less packing order. On the other hand, the much lower k of CHXP2 (π ) 25, (30) Baskar, G.; Venkatesan, S.; Dhathathreyan, A.; Mandal, A. B. J. Am. Oil Chem. Soc. 1999, 76 (7), 853. (31) Souheng, W.; Huntsberger, J. R. J. Colloid Interface Sci. 1969, 29 (1), 138.
k ) 0.99 × 10-2 m/mN) suggests that the presence of acrylic acid comonomer up to 15 mol % promotes better organization at pH 10.0, in accordance with the close-packed area characteristics. We believe that the appearance of a kink in all the π-A isotherm curves is related to transformation in phase structures from the LE state in the low-π region to a condensed film as a result of compression. The appearance of less pronounced kinks in the π-A isotherm curves of polymers at pH 10.0 is probably indicative of transformation of the LE phase (low-π region) to a less compressed phase. This is well supported by compressibility coefficient data analysis. Surface Concentration (Γ) Characteristics of Polymers. The surface concentration (Γ) characteristics at the a/w interface of the polymer films serve as a direct estimate of the adsorption characteristics. The plots of Γ (1/A) versus the surface pressure for the polymers are shown in Figure 2. The surface concentration (Γ) is related to the packing order, or, in other words, the compactness of the film as demonstrated by Kawaguchi et al. from ellipsometric studies. The high Γ values are generally suggestive of compact packing.12 The homopolymer and the series of copolymers contribute to Γ of 2.2-2.3 mg/m2 at π of 2530 mN/m at pH 6.5 (Figure 2A, Table 4). The introduction of acrylic acid comonomer up to 22 mol % causes small changes in the surface concentration of the polymer. The polymers are found to exhibit higher Γ in comparison to those (e.g., PMMA π ) 10, Γ ) 1.5 mg/m2) with a similar backbone. The polymers PMMA or PLGA established to form compressed monolayers at the a/w interface have been found to show a maximum π of 15-20 mN/m and Γ in the range of 1.2-1.5 mg/m2 (π ) 10 mN/m), as was established from ellipsometric studies. The high Γ of 2.22.3 mg/m2 and max π for the sets of the polymers arise from the compact organization of the hexadecyl side chains at the interface. At high pH, considerable reduction in Γ to an extent of about 25% (Figure 2B, Table 4) is observed in the case of the homopolymer, suggesting less packing order of the film. Interestingly, the copolymers consisting of 8 or 15 mol % acrylic acid exhibit small changes in Γ to an extent of about 12% (Figure 2B, Table 4), further suggesting that the introduction of acrylic acid up to 15 mol % effects small changes in the adsorption characteristics and rather aids in stabilizing the adsorbed films at the interface under alkaline conditions, as was already discussed. Stability Studies. The expanded-type monolayer formation is generally associated with instability. The promotion of a more expanded monolayer formation in respect to the homopolymer, in contrast to the copolymers CHXP1 and CHXP2 at pH 10.0 demands detailed investigations on the stability of polymer films. Hysteresis experiments performed on monolayer films are well-known to throw information on stability characteristics. While performing several cycles of hysteresis experiments, sufficient care was taken to compress the monolayer to a fixed surface pressure and subsequently relax it to its original state so as to ensure that the domains were relaxed and brought back to the same position. The results of
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Figure 2. Surface concentration (Γ)-surface pressure isotherms of the polymers at the a/w interface at 25 °C: (A) pH 6.5 for (a-d) HXP, CHXP1, CHXP2, and CHXP3; (B) pH 10.0 for (a-c) HXP, CHXP1, and CHXP2. Table 4. Surface Concentration (Γ; mg/m2) at Different Surface Pressures (π) of Hexadecylacrylamide Polymers on an Aqueous Subphase, pH 6.50 and 10.0, at 25 °C π ) 10 mN/m
π ) 20 mN/m
π ) 30 mN/m
polymer
pH 6.5
pH 10.0
loss (%)
pH 6.5
pH 10.0
loss (%)
pH 6.5
pH 10.0
loss (%)
HXP CHXP1 CHXP2
1.771 1.774 1.885
1.385 1.500 1.692
21.79 15.30 10.23
2.106 2.083 2.28
1.611 1.793 1.993
23.50 13.92 12.58
2.399 2.343 2.596
1.872 2.069 2.278
21.96 11.69 12.25
typical first and third hysteresis cycles performed on the homopolymer HXP and the copolymer CHXP2 at pH 6.5 and pH 10.0 are shown in Figure 3A-D. It is interesting
to observe that compression and decompression cycles bring about small changes in the surface characteristics, in the cases of the homopolymer at pH 6.5 and the
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Figure 3. Hysteresis π-A isotherm curves measured on an aqueous subphase, at 25 °C, of HXP and CHXP2 at pH 6.5: (A and B) pH 10.0; (C and D) curves a and a′, first cycle, and b and b′, third cycle.
copolymer CHXP2 at pH 6.5 and 10.0. Normally, such changes would be suggestive of instability of the film, although an excellent reproducibility of consecutive hysteresis runs demonstrates the stability of the films. The hysteresis curves indicate viscoelastic and not purely elastic characteristics typical of a polymer monolayer film. A similar trend of reproducibility in consecutive hysteresis experiments was observed with the copolymer CHXP1 at pH 6.5 and 10.0. On the contrary, the homopolymer undergoes significant shifts in mean molecular areas to lower values in subsequent hysteresis cycles, suggesting instability of the film. Generally, instability of the monolayer film is associated with the desorption of the material into the subphase, changes in molecular orientation, or overriding of molecules, which would result in continuous shifts in the mean molecular area toward lower values during consecutive hysteresis experiments, and this has been well documented. In the case of the homopolymer, it is possible that strong interactions of the -OH group at pH 10.0 with the amido group probably promote the reorganization and reorientation of the film at the interface, thus leading to instability. A prolonged interaction of the amido group with the hydroxyl ions (OH-) is expected to bring about hydrolysis, resulting in the formation of amine (long chain) and acid. The poor reproducibility in compression-decompression π-A isotherm experiments of CHXP3 under alkaline conditions no doubt suggests instability of the film. The shift in amphiphilicity to a higher hydrophilic character by the incorporation of acrylic acid (22 mol %) forming sodium acrylate at pH 10.0 probably promotes the desorption of the material into an aqueous subphase under alkaline conditions.
The effect of a controlled modification in the HLB of the comblike polymers afforded through copolymerization with acrylic acid on the interfacial organization characteristics has been shown to give significant results. This work demonstrates that the introduction of acrylic acid up to 15 mol % aids in stabilizing the polymer film under alkaline conditions at the a/w interface, although with small changes in the packing characteristics, whereas the homopolymer and the copolymers consisting of a high acrylic acid fraction (∼22%) form an unstable film tending to desorb into the aqueous phase. The presence of an optimum level up to 15 mol % sodium acrylate comonomer under alkaline conditions in the copolymer, in fact, suppresses the hydrolysis of the amido group, thus stabilizing the polymer film at the interface. It is followed that the presence of excess sodium acrylate component arising from either the hydrolysis product of the homopolymer or the comonomer component of the polymer promotes the desorption of the film into the subphase, effecting the destabilization of the film. Acknowledgment. The authors thank Dr. T. Ramasami, Director, Central Leather Research Institute (CLRI), Chennai, for his encouragement and permission to publish this paper. The support of Dr. A. Dhathathreyan in the LFB measurements is acknowledged. The authors thank Dr. B. S. R. Reddy for his constant support. The financial grant from DST through SP/S1/H38 is gratefully acknowledged. LA034784G
