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Solvent Effects on the Formation of Nanoparticles and Multilayered Coatings Based on Hydrogen-Bonded Interpolymer Complexes of Poly(acrylic acid) with Homo- and Copolymers of N-Vinyl Pyrrolidone Daulet E. Zhunuspayev,†,‡ Grigoriy A. Mun,‡ Patrick Hole,§ and Vitaliy V. Khutoryanskiy*,† School of Pharmacy, UniVersity of Reading, Whiteknights, P.O. Box 224, Reading RG6 6AD, United Kingdom, Department of Chemical Physics & Macromolecular Chemistry, Kazakh National UniVersity, 95 Karasai Batyr Street, 050012, Almaty, Kazakhstan, and NanoSight, Ltd., 2 Centre One, Lysander Way, Old Sarum Park, Salisbury SP4 6BU, United Kingdom ReceiVed August 31, 2008. ReVised Manuscript ReceiVed September 25, 2008 The formation of hydrogen-bonded interpolymer complexes between poly(acrylic acid) and poly(N-vinyl pyrrolidone) as well as amphiphilic copolymers of N-vinyl pyrrolidone with vinyl propyl ether has been studied in aqueous and organic solutions. It was demonstrated that introduction of vinyl propyl ether units into the macromolecules of the nonionic polymer enhances their ability to form complexes in aqueous solutions due to more significant contribution of hydrophobic effects. The complexation was found to be a multistage process that involves the formation of primary polycomplex particles, which further aggregate to form spherical nanoparticles. Depending on the environmental factors (pH, solvent nature), these nanoparticles may either form stable colloidal solutions or undergo further aggregation, resulting in precipitation of interpolymer complexes. In organic solvents, the intensity of complex formation increases in the following order: methanol < ethanol < isopropanol < dioxane. The multilayered coatings were developed using layer-by-layer deposition of interpolymer complexes on glass surfaces. It was demonstrated that the solvent nature affects the efficiency of coating deposition.
Introduction Association between poly(carboxylic acids) and nonionic proton-accepting polymers in solutions resulting in the formation of insoluble hydrogen-bonded interpolymer complexes (IPCs) was first reported in the 1960s1,2 and initiated a new area of polymer research. Formation of IPCs was extensively studied between different polymeric pairs both in aqueous and organic solutions as well as at interfaces. Studies of hydrogen-bonded IPC have been summarized and analyzed in earlier3-5 and more recent review papers.6-8 A number of potential applications of IPCs have been suggested, including pharmaceutical excipients,8 selective membranes,9 emulsifiers,10,11 thickening reagents,12 and stimuli-responsive hydrogels.13,14 * Corresponding author. † University of Reading. ‡ Kazakh National University. § NanoSight, Ltd. (1) Smith, K. L.; Windslow, A. E.; Petersen, D. E. Ind. Eng. Chem. 1959, 51, 1361–1364. (2) Bailey, F. E.; Lundberg, R. D.; Callard, R. W. J. Polym. Sci., Part A 1964, 2, 845–851. (3) Kabanov, V. A.; Papisov, I. M. Polym. Sci. USSR 1979, 21, 261–307. (4) Bekturov, E. A.; Bimendina, L. A. AdV. Polym. Sci. 1981, 41, 99–147. (5) Tsuchida, E.; Abe, K. AdV. Polym. Sci. 1982, 45, 1–119. (6) Jiang, M.; Li, M.; Xiang, M.; Zhou, H. AdV. Polym. Sci. 1999, 146, 121– 196. (7) Nurkeeva, Z. S.; Mun, G. A.; Khutoryanskiy, V. V. Macromol. Biosci. 2003, 3, 283–295. (8) Khutoryanskiy, V. V. Int. J. Pharm. 2007, 334, 15–26. (9) Yun, Z.; Huang, M.-Y.; Jiang, Y.-Y. Polym. Bull. 1988, 20, 277–284. (10) Mathur, A. M.; Drescher, B.; Scranton, A. B.; Klier, J. Nature 1998, 392, 367–370. (11) Mun, G. A.; Khutoryanskiy, V. V.; Nurkeeva, Z. S.; Urkimbaeva, P. I.; Zhunuspaev, D. J. Polym. Sci., Part B: Polym. Phys. 2004, 42. (12) Sotiropoulou, M.; Bokias, G.; Staikos, G. Macromolecules 2003, 36, 1349–1354. (13) Ilmain, F.; Tanaka, T.; Kokufuta, E. Nature 1991, 349, 400–401. (14) Kwon, I. C.; Bae, Y. H.; Kim, S. W. Nature 1991, 354, 291–293. (15) Decher, G. Science 1997, 277, 1232–1237.
One of the most recent and significant advances in the area of IPCs via hydrogen-bonding is a design of multilayered materials via the layer-by-layer (LBL) approach. This approach was initially developed by Decher15 for fabricating multilayered assemblies formed by oppositely charged polyelectrolytes. It involves an alternating exposure of various solid substrates to solutions of polymers forming insoluble complexes. The LBL approach was successfully employed by Stockton and Rubner,16 Wang et al.,17 and Sukhishvili et al.18,19 for designing multilayered assembly through complexation via hydrogen-bonding. In the past few years, a number of research groups have focused their attention on LBL self-assembly driven by hydrogen-bonding for fabrication of ultrathin films, coatings, and capsules.20-29 The recent progress in this area was also summarized in a review by Kharlampieva and Sukhishvili.30 (16) Stockton, W. B.; Rubner, M. F. Macromolecules 1997, 30, 2717–2725. (17) Wang, L.; Wang, Z.; Zhang, X.; Shen, J.; Chi, L.; Fuchs, H. Macromol. Rapid Commun. 1997, 18, 509–514. (18) Sukhishvili, S. A.; Granick, S. J. Am. Chem. Soc. 2000, 122, 9550–9551. (19) Sukhishvili, S. A.; Granick, S. Macromolecules 2002, 35, 301–310. (20) Kharlampieva, E.; Sukhishvili, S. A. Langmuir 2004, 20, 9677–9685. (21) Quinn, J. F.; Caruso, F. Langmuir 2004, 20, 20–22. (22) Kozlovskaya, V.; Yakovlev, S.; Libera, M.; Sukhishvili, S. A. Macromolecules 2005, 38, 4828–4836. (23) Kozlovskaya, V.; Sukhishvili, S. A. Macromolecules 2006, 39, 5569– 5572. (24) Yang, S.; Zhang, Y.; Guan, Y.; Tan, S.; Xu, J.; Cheng, S.; Zhang, X. Soft Matter 2006, 2, 699–704. (25) Elsner, N.; Kozlovskaya, V.; Sukhishvili, S. A.; Fery, A. Soft Matter 2006, 2, 966–972. (26) Seo, J.; Lutkenhaus, J. L.; Kim, J.; Hammond, P. T.; Char, K. Macromolecules 2007, 40, 4028–4036. (27) Khutoryanskaya, O. V.; Williams, A. C.; Khutoryanskiy, V. V. Macromolecules 2007, 40, 7707–7713. (28) Yang, S.; Zhang, Y.; Zhang, X.; Xu, J. Soft Matter 2007, 3, 463–469. (29) Kim, B.-S.; Park, S. W.; Hammond, P. T. ACS Nano 2008, 2, 386–392. (30) Kharlampieva, E.; Sukhishvili, S. A. J. Macromol. Sci., Part C: Polym. ReV. 2006, 46, 377–395.
10.1021/la802852h CCC: $40.75 2008 American Chemical Society Published on Web 11/04/2008
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Table 1. Composition of the Feed Mixtures and Copolymers
a
feed mixture NVP:VPE, mol %
VNVP, mL
VVPE, mL
VEthanol, mL
composition of NVP:VPE copolymers, mol %
apparent Mw of polymers, kDa
100:0 50:50 40:60 20:80 10:90
6.00 3.25 2.67 1.41 0.72
0 3.41 4.20 5.91 6.83
14.0 14.0 14.0 14.0 14.0
100.0:0.0 74.5:25.5 70.8:29.2 62.4:37.6 50.6:49.4
144 107 81 a a
The apparent Mw of these samples could not be analyzed, as the copolymers were not soluble in water.
Hydrogen-bonded complexes formed by poly(acrylic acid) (PAA) and poly(N-vinyl pyrrolidone) (PVP) are classic examples of IPCs, which are already listed in some textbooks for undergraduate students.31 The complexation between PAA and PVP has been extensively studied in aqueous solutions;32-36 however, fewer studies are available on PAA-PVP complexes formed in organic solutions.4,37,38 Previously, we demonstrated that PAA forms insoluble complexes with nonionic polymers in aqueous media only when solution pH is lower than the critical pH of complexation (pHcrit).34,39-41 The pHcrit found for complexes formed by 0.01 unit-base/mol solutions of PAA and PVP was 4.85 ( 0.05, which is one of the highest pHcrit values known for hydrogen-bonded IPCs.34,39 From a practical point of view, the development of a polymeric system, which forms an insoluble IPC, even at higher pHs, may be quite beneficial. Such a system may open additional opportunities for application, in particular, in pharmaceutics, where pH-dependent solubility of certain polymers is widely used for designing enteric coatings. To increase the pHcrit typical for complexation of PAA and PVP, one of the polymers should be made slightly less polar. This can be achieved through copolymerization of either acrylic acid or N-vinyl pyrrolidone (NVP) with more hydrophobic monomers. Here we studied the effect of solvent nature on the structure and properties of IPCs formed by PAA and PVP both in solutions and at solution-glass interfaces. We also synthesized a series of novel copolymers by copolymerizing NVP with more hydrophobic vinyl propyl ether (VPE) to look at how the introduction of hydoprobic units into PVP affects the complexation.
Materials and Methods Materials. PAA with a weight-average molecular weight of 450 kDa was purchased from Sigma-Aldrich, U.K. (Cat 18,128-5; Lot 04610EI-235) and used without purification. According to the manufacturer, this sample was ∼0.1% cross-linked. NVP and VPE were purchased from Sigma-Aldrich and were purified from inhibitor by vacuum distillation. Deionized water was used in all experiments with aqueous solutions. Methanol, ethanol, and isopropanol were purchased from Fisher Scientific (U.K.), and dioxane was obtained (31) Florence, A. T.; Attwood, D. Physicochemical Principles of Pharmacy, 4th ed.; Pharmaceutical Press: London/Chicago, 2006; p 492. (32) Turro, N. J.; Caminati, G.; Kim, J. Macromolecules 1991, 24, 4054– 4060. (33) Pradip; Maltesh, C.; Somasundaran, P.; Kulkarni, R. A.; Gundiah, S. Langmuir 1991, 7, 2108–2111. (34) Nurkeeva, Z. S.; Mun, G. A.; Khutoryanskiy, V. V.; Bitekenova, A. B.; Dubolazov, A. V.; Esirkegenova, S. Z. Eur. Phys. J. E. 2003, 10, 65–68. (35) Jin, S.; Liu, M.; Chen, S.; Chen, Y. Eur. Polym. J. 2005, 41, 2406–2415. (36) Henke, A.; Kadlubowski, S.; Ulanski, P.; Rosiak, J. M.; Arndt, K.-F. Nucl. Instrum. Method. Phys. Res., Sect. B 2005, 236, 391–398. (37) Ohno, H.; Tsuchida, E. Makromol. Chem. 1978, 179, 755–763. (38) Ohno, H.; Tsuchida, E. Makromol. Chem. 1980, 181, 1227–1235. (39) Khutoryanskiy, V. V.; Mun, G. A.; Nurkeeva, Z. S.; Dubolazov, A. V. Polym. Int. 2004, 53, 1382–1387. (40) Dubolazov, A. V.; Nurkeeva, Z. S.; Mun, G. A.; Khutoryanskiy, V. V. Biomacromolecules 2006, 7, 1637–1643. (41) Khutoryanskiy, V. V.; Dubolazov, A. V.; Nurkeeva, Z. S.; Mun, G. A. Langmuir 2004, 20, 3785–3790.
from Acros Organic (U.K.). All solvents were of HPLC or laboratory/ analytical reagent grade and were used without purification. 2,2Azoisobutyronitrile (AIBN) purchased from Acros (USA) was purified by recrystallization from ethanol. Hydrochloric acid and sodium hydroxide used for adjustment of pH were products of SigmaAldrich and used without purification. Dialysis membranes (molecular weight cutoff 12-14 kDa) were purchased from Medicell International, Ltd. (U.K.). Synthesis, Purification, and Characterization of PVP and NVP-VPE Copolymers. PVP and NVP-VPE copolymers were synthesized by free-radical copolymerization initiated by AIBN (0.01 mol/L) at 60 °C for 26 h in ethanol solutions. Before copolymerization, the monomer mixtures were bubbled with nitrogen for 10 min to remove dissolved oxygen. Polymerization was slowed down by cooling the reaction mixtures with cold water, which resulted in its termination. The polymers were purified by dialysis against 5 L of deionized water (20 changes during 4 days) and then were recovered by freeze-drying. The purity of the copolymers was estimated by 1H NMR spectroscopy and the composition of the copolymers was determined by elemental analysis for content of nitrogen, which is present in NVP only. The weight-average molecular weight of PVP was determined by static light scattering using a Malvern Zetasizer Nano-S (Malvern Instruments, U.K.) in aqueous solution at 25 °C. The value of refractive index increment (dn/dc) of 0.179 used for calculations of the molecular weight was taken from ref 42. Unfortunately, it was not possible to determine the molecular weights of NVP-VPE copolymers accurately as the data on their dn/dc values is not available in literature. However, we assumed that the copolymers containing 25.5 and 29.2 mol % of VPE may have a dn/dc similar to PVP and also attempted an estimation of their molecular weights. As was expected, an increase in VPE content in the copolymers resulted in reduction of their molecular weights due to lower reactivity of vinyl ethers in copolymerization. The composition of the feed mixtures, copolymers, and their apparent molecular weights are summarized in Table 1. The discrepancy between the feed composition and the composition of the copolymers is explained by lower reactivity of vinyl ethers in copolymerization. Preparation of Solutions and Adjustment of pH. Solutions were prepared by dissolving the required amounts of polymers either in deionized water or in organic solvents and leaving them stirring overnight at room temperature. The pH of aqueous solutions was then further adjusted by adding small amounts of 0.2 mol/L HCl or NaOH and was measured using a digital pH-meter (Metrohm, Switzerland). Turbidimetric Measurements. The turbidity of solutions was measured at 400 nm using a V-530PC spectrophotometer (Jasco, U.K.) at room temperature. Turbidity readings were taken immediately after adjusting pH. All experiments were repeated in triplicate, and the turbidity values are reported as mean ( standard deviation of the experimental triplicates. Dynamic Light Scattering (DLS). DLS experiments were performed on a Malvern Zetasizer Nano-S (Malvern Instruments) using a red laser (633 nm) and detection of scattered light at 173°. The results of DLS measurements were processed using multiple narrow mode and are presented as particle size (diameter) distributions. All measurements, performed at 20 °C, were repeated in triplicate, and the values are reported as mean ( standard deviation. The measurements were performed within 5 min after mixing polymer solutions.
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Figure 2. Size distributions of IPCs formed in equimolar mixtures (1:1) of PAA with PVP (1) and copolymers containing 25.5 (2) and 29.2 mol % VPE (3) in aqueous solutions at pH 3.0. Polymer concentration ) 0.01 unit-base mol/L. Inset: images of IPC solutions. Figure 1. Effect of pH on the turbidity of PAA 1:1 mixtures with PVP (1) and NVP:VPE copolymers containing 25.5 (2) and 29.2 mol % VPE (3). Polymer concentrations ) 0.01 unit-base mol/L.
Transmission Electron Microscopy (TEM). TEM images of IPC were acquired using a Philips CM20 analytical TEM at 200 kV. For sample preparation, the copper grids were brought into contact with dispersions of IPC for 30 s and then dried off with a filter paper. Nanoparticle Tracking Analysis (NTA). Nanoparticle tracking and analysis measurements were performed with a NanoSight LM20, using a nitrile “o” ring for solvent resistance. Viscosities of 0.58-0.59 cP were used for methanol at approximately 21 °C. The sample was serially diluted with a maximum single step dilution factor of 100:1. A sample volume of 0.4 mL was injected into the sample chamber. Videos of 90 s length were used, and repeat measurements were performed to verify results. Modal values on a number-concentration basis are reported in all cases. Multilayered Coatings. The multilayered films were developed on the surface of the microscope glass slides using 0.05 and 0.1 unit-base/mol solutions of polymers. Before layering, the glass slides were treated with a 2 mol/L NaOH solution at 60-70 °C for 30 min, then rinsed with deionized water thoroughly and dried in air. An LBL deposition cycle involved immersion of a glass slide into PAA solution for 15 min, with subsequent dipping into deionized water (or organic solvent) for 1 min to wash out excessive unbound macromolecules, followed by immersion into solution of nonionic polymer (PVP or NVP-VPE) for 15 min and then rinsing by dipping into deionized water (or organic solvent) for 1 min. This procedure was repeated the required number of times. All deposition experiments were performed at room temperature. After assembly, the multilayered coatings were dried in air. Thickness of LBL Films. The thickness of multilayered coatings was determined by an electronic micrometer (Fowler IP 54). The area of each glass slide supporting the LBL film was divided into nine zones (3 × 3 grid) with thickness assessed three times in each zone. Data are reported as the mean thickness value from all sites ( standard deviation.
Results and Discussion Formation of Complexes in Aqueous Solutions. It is well documented that the complexes formed by PAA and PVP have stoichiometric 1:1 composition.5 Therefore, in this work we studied the complexation between PAA and PVP or NVP-VPE at 1:1 unit-base molar ratios. The critical pH of complexation (pHcrit) was determined in the aqueous mixtures of polymers by the turbidimetric method. Figure 1 shows the dependence of solution turbidity on pH for mixtures of PAA with PVP and NVP-VPE copolymers containing 25.5 and 29.2 mol % VPE. A sharp increase in solution turbidity at a narrow change of pH is taken as pHcrit. The pHcrit observed for complexes between PAA and PVP is approximately 4.70 ( 0.05, which is close to our previously reported data.34 However, in the case of the IPC
Figure 3. Diagram depicting the multistage formation of IPCs.
involving NVP-VPE copolymers, this value is shifted to a higher pH region. It is observed at 5.10 ( 0.05 and 5.40 ( 0.05 for complexes formed by PAA and copolymers containing 25.5 and 29.2 mol % VPE, respectively. This increase in pHcrit indicates a higher tendency of copolymers to form IPC, which is believed to be due to their more hydrophobic nature compared to PVP and more significant contribution of hydrophobic effects into the stabilization of complexes. A similar shift in pHcrit was reported in our previous study43 of the complexation between PAA and copolymers of 2-hydroxyethyl vinyl ether with hydrophobic vinyl butyl ether (VBE). In this study43 we also demonstrated that the homopolymer of VBE does not form complexes with PAA in organic solvents, which allows us to disregard the role of VPE groups in hydrogen bonding with PAA. To gain insight into the structure of IPCs, we studied the solutions of complexes formed at pH 3.0 by DLS. Figure 2 presents the DLS data for complexes formed by PAA with PVP and copolymers in water at pH 3.0. The IPCs formed by PAA and PVP have bimodal distribution with the presence of particles of two size populations (23 ( 1 and 129 ( 2 nm). It should be noted that, in some of the repetitive experiments on sizing of the IPCs, the particles at 23 ( 1 nm were not observed. Presumably, the smaller particles represent the primary IPCs formed initially, which subsequently transformed into more stable spherical nanoparticles. The formation of primary IPCs and their transformation into the nanoparticles is a fast process. However, the nanoparticles can also slowly aggregate to give larger structures (Figure 3). The described scheme of complexation is in good agreement with the report of Usaitis et al.,44 who studied the aggregation of complexes formed by poly(methacrylic acid) and PVP in aqueous solutions by DLS. They pointed out that the fast (42) Molyneux, P., Water-Soluble Synthetic Polymers: Properties and BehaVior; CRC Press: Boca Raton, FL, 1984; Vol. 1, p 225. (43) Mun, G. A.; Nurkeeva, Z. S.; Khutoryanskiy, V. V.; Bitekenova, A. B. Macromol. Rapid Commun. 2000, 21, 381–384. (44) Usaitis, A.; Maunu, S. L.; Tenhu, H. Eur. Polym. J. 1997, 33, 219–223.
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Figure 4. Size distributions of IPC formed by 0.01 unit-base mol/L solutions of PAA and PVP in methanol (1), ethanol (2), isopropanol (3) and dioxane (4) in equimolar 1:1 mixtures. Inset: images of IPC taken 6 min after mixing solutions of PAA and PVP. Vial with distilled water is shown for comparison.
complexation is followed by the aggregation of the complexes, the rate of which is strongly dependent on environmental conditions. The complexes of PAA with NVP-VPE (74.5:25.5 mol %) show similar bimodal size distribution with both particle populations having larger dimensions (26 ( 1 and 179 ( 1 nm), which is likely due to their higher tendency to aggregate. More hydrophobic copolymer with 29.2 mol % of VPE forms IPCs with even bigger dimensions (243 ( 15 nm), which aggregate to form extra large particles (more than 4 µm) undergoing sedimentation. These DLS results are in a good agreement with visual observations of the IPCs. The solutions of IPCs formed by copolymers are less transparent, and the complexes are more prone to sedimentation. In summary, the incorporation of hydrophobic VPE units into the structure of PVP enhances its ability to form complexes with PAA in aqueous solutions due to more significant contribution of hydrophobic effects into the stabilization of IPCs. Formation of Complexes in Organic Solvents. Complexes of poly(carboxylic acids) with PVP can also be formed in some organic solvents. Ohno and co-workers37,38 studied the complexation of poly(methacrylic acid) and PAA with PVP in water, methanol, ethanol, dimethylformamide, and dimethylsulfoxide. They demonstrated that the intensity of complexation is dependent on the dielectric constant of the solvent used. It was also suggested that the possibility of IPCs in a particular solvent depends on the balance of polymer-polymer and polymer-solvent interactions. IPCs can only be formed if polymer-polymer interactions are stronger than polymer-solvent ones. In the present study we focused on the effect of solvent nature on the size of IPC particles formed and studied the complexation by DLS, TEM, and NTA using NanoSight instrumentation. The size distribution of the IPCs formed by PAA and PVP in methanol determined by DLS (Figure 4) is bimodal and shows the presence of particles of two populations: 21 ( 2 and 87 ( 2 nm. In ethanol, a similar profile of size distribution is observed with particles of 24 ( 2 and 92 ( 1 nm. Similarly to the formation of IPCs in aqueous solutions, the particles of smaller sizes are formed in the beginning of the complexation and then become incorporated into larger aggregates. The IPCs formed in isopropanol are significantly larger: 147 ( 12 and 592 ( 36 nm particle populations. The distribution of IPC particles formed in dioxane looks monomodal, with an average size of 2761 ( 176 nm. However, particles of this size can easily hide smaller objects,
Figure 5. TEM images of IPC formed by equimolar (1:1) mixtures of 0.01 unit-base mol/L PAA and 0.01 unit-base/mol PVP in methanol (a), ethanol (b), and isopropanol (c). Note that magnification of image “a” is higher than those for images “b” and “c”.
resulting in a monomodal distribution. Additionally these particles undergo a rapid sedimentation, which reduces the accuracy of particle sizing by DLS. The different tendency of the IPCs to aggregate in different solvents is seen on photographic images as the solutions become more turbid from methanol to ethanol and isopropanol. Sedimentation of IPC in dioxane can also be observed in the image. The structure of complexes formed by PAA and PVP in methanol, ethanol, and isopropanol was also studied by TEM. Figure 5 shows the TEM images of IPCs formed in methanol, ethanol, and isopropanol. It should be noted that all TEM experiments were performed without staining the samples with heavy metal salts. Individual spherical particles of two size populations can be easily identified in the TEM images of the IPCs formed in methanol. The IPCs formed in ethanol have bigger dimensions and greater tendency to aggregate. In isopronanol, most of the IPC particles are in the aggregated state, forming network-like clusters with dimensions often exceeding 500 nm. The results on particle sizes determined by DLS and TEM may differ because of the different state of the samples used. In DLS experiments, the IPCs are fully solvated and swollen, whereas the complexes probed by TEM may be in a shrunken state because of desolvation caused by sample drying. Interestingly, the tendency of IPCs to aggregation growths in the following order: methanol < ethanol < isopropanol < dioxane, which also coincides with a reduction in solvents dielectric constants: 33, 24.3, 18, and 2.3, respectively. The size distribution of IPCs formed by PAA and NVP-VPE copolymers in methanol and ethanol differs from the situation observed with PAA-PVP complexes. Figure 6a shows a typical result of DLS characterization of these complexes. The majority of IPC particles present have dimensions around 60-70 nm, and only a minor fraction forms significantly larger aggregates, the size of which exceeds 4 µm. The changes of the size of IPC
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Figure 7. Thickness of multilayered coatings formed by PAA and PVP from 0.05 (1) and 0.1 unit-base mol/L (2) aqueous solutions at pH 3.0 as a function of the number of monolayers. Inset: images of multilayered coatings formed by 0.1 unit-base mol/L PAA and PVP solutions. Number of monolayers: 2 (1), 4 (2), 6 (3), and 8 (4).
Figure 6. (a) Size distribution of IPC particles formed by 0.01 unit-base mol/L solutions of PAA and NVP-VPE (70.8:29.2 mol %) in methanol. (b) Effect of VPE content in NVP-VPE copolymer on the size of IPC particles formed in methanol (1) and ethanol (2).
nanoparticles formed in methanol and ethanol with the composition of NVP-VPE copolymers are shown in Figure 6b. The IPCs formed in ethanol still have slightly bigger dimensions compared to the complexes in methanol. The size of nanoparticles decreases with increase in VPE content in the copolymers, which can be related to a better ability of alcohol molecules to solvate IPCs formed by less polar copolymers and reduction in their tendency to aggregate. To gain deeper insight into the nature and properties of the IPC formed by PAA and PVP in methanol, we used a NanoSight LM20 NTA system. This instrument can directly track the Brownian motion of every nanoparticle separately, and, using a CCD camera, it can rapidly obtain a high resolution plot of the particle size distribution, count the absolute number of particles in a sample, and record videos of particle motions. Previously, the NTA technique was used by Tixier et al.45 for studying emulsions. The specific feature of this technique is its extremely high sensitivity compared to DLS; therefore, we had to dilute our 0.01 unit-base/mol IPC solution samples with methanol (1: 100000 dilution). The concentration of IPC particles in 0.01 unit-base mol/L solutions determined by NanoSight was found to be 2 × 1013 particles/mL. However, the IPC average size determined by NanoSight was found to be significantly larger (170 nm) compared to the size determined by DLS (87 ( 2 nm). The results of NanoSight analysis of the IPC along with the videos of nanoparticles motions can be found in the Supporting Information. We believe that this discrepancy in particle sizes arises from the significant difference in the concentrations of solutions being analyzed by these two techniques. A subsequent attempt to analyze a very diluted IPC solution (1:100000 dilution) (45) Tixier, T.; Heppenstall-Butler, M.; Terentjev, E. M. Langmuir 2006, 22, 2365–2370.
by DLS also gave a particle size of 176 nm; however, this result did not meet the Malvern Instruments quality criteria because of the lower sensitivity of this technique. Nevertheless, apparently, a significant dilution of IPC solutions in methanol results in the formation of larger particles, which may be due to either their further aggregation or additional swelling. Further experiments will be necessary to study this transition in greater details. Formation of Complexes via LBL Deposition. The development of multilayered coatings based on PAA and PVP using the LBL deposition approach has previously been reported in several studies.24,28 In most of these studies, the LBL deposition was achieved from aqueous solutions under acidic conditions. For example, Yang et al.24 fabricated multilayered assembly from 10 mg/mL solutions of PAA and PVP in water at pH 2.0 with deposition and rinsing times of 4 and 1 min, respectively. In the present work we used a slightly modified protocol, which was adapted from our previous study on the fabrication of multilayered glass coatings based on PAA and methylcellulose.27 We used two different concentrations of polymers (0.05 and 0.1 unit-base mol/L) to look at the possibility of control over the thickness of coatings as a function of a number of deposited monolayers (Figure 7). A linear growth in the thickness of multilayered coating with the number of deposited monolayers is observed when the LBL was developed from 0.05 unit-base mol/L solutions. At higher polymer concentrations (0.1 unitbase mol/L), this dependence is exponential, indicating more intensive aggregation of IPCs on the surface of glass. The LBL coating developed from 0.1 unit-base mol/L (Figure 7, insert) becomes less transparent with the growth in the number of monolayers; however, its structure is not fully homogeneous with the presence of islands of higher polymer density. When copolymers containing 24.5 and 29.2 mol % VPE were used to develop multilayered coatings, no statistical difference was observed in the film growth compared to PVP (data not shown). It should be noted that the multilayered coatings based on PAA-PVP developed in the present study are significantly thicker compared to the thicknesses of multilayers reported by Kharlampieva and Sukhishvili.30 For example, the thickness of our PAA-PVP coating composed of eight monolayers prepared from 0.05 unit-base/mol solutions is 3.1 ( 1.4 µm, whereas the thickness of the films with the same number of layers described in Kharlampieva and Sukhishvili’s review30 is around 50-70
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as more intensive complexation process, less volatile nature or higher viscosity of isopropanol.
Conclusions
Figure 8. Thickness of multilayered coatings formed by PAA and PVP from 0.1 unit-base mol/L solutions in methanol (1), ethanol (2), and isopropanol (3) as a function of monolayers number. Insert: images of 8 monolayers coating formed in methanol (1), ethanol (2), and isopropanol (3).
nm. It is likely that this difference results from significantly different concentrations of polymers and solution pHs used to assemble the LBL films. Indeed, the concentrations of polymers we used are approximately 5 times higher (0.05 unit-base mol) than the concentrations used by Sukhishvili et al. (1 mg/mL, which is roughly equal to 0.01 unit-base/mol for PAA).19 Yang et al.24 used 10 mg/mL concentrations of PAA and PVP and reported the development of a 16-layered coating on a siliconbased substrate, the thickness of which is 1328 nm. This concentration of polymers is comparable to the one used in our study (10 mg/mL is approximately 0.138 unit-base/mol for PAA). The formation of thicker multilayers in our case can be related to a different layering procedure, i.e. longer deposition times (15 min vs 4 min used in the study of Yang et al.24). Alternatively, the nature of a substrate used (glass vs silicon) can also play a significant role. Hence, there is a strong dependence of multilayer thickness on the concentration of polymers, nature of a substrate, and the deposition time used. The formation of multilayered IPCs on glass slides was also studied in methanol, ethanol and isopropanol (Figure 8). The growth in the film thickness in organic solvents is changing exponentially with the number of monolayers formed and the efficiency of deposition increases in the following order: methanol < ethanol < isopropanol. This order agrees well with the intensity of complexation in solutions, discussed in the previous section. The LBL film formed from isopropanol is more uniform compared to the coatings obtained from methanol and ethanol (Figure 8, insert). This uniformity can be due to a number of factors such
The nature of solvent plays an important role in the formation of hydrogen-bonded complexes between PAA and PVP as well as copolymers of NVP with VPE. In aqueous solutions the formation of complexes is highly dependent on pH. Cloudy solutions of polycomplexes are formed at pH lower than the critical pH of complexation. It was demonstrated that the size of nanoparticles formed by IPCs in water is affected by introduction of hydrophobic VPE into the structure of PVP. The copolymers containing higher levels of VPE have higher complexation ability and form complexes of larger dimensions due to more significant contribution of hydrophobic effects into their aggregation. In organic solvents the ability of the polymers to form complexes is increased in the following order: methanol < ethanol < isopropanol < dioxane. The complexation is believed to be a multistage process, beginning with the formation of primary particles, which undergo further aggregation, resulting either in nanoparticulate dispersions stable to aggregation or larger particles, prone to precipitation. The multilayered coatings on the surface of glass can be developed via LBL deposition of IPCs from aqueous and organic solutions. A linear dependence of the coating thickness on the number of deposited monolayers was observed when 0.05 unitbase/mol aqueous solutions of both polymers were used. At higher polymer concentration (0.1 unit-base/mol) this dependence becomes exponential. The comparison of the results obtained in the present study with the literature data allows concluding about the strong dependence of multilayer thickness on the concentration of polymers, nature of a substrate and the deposition time used. The coatings can also be developed through the complexation in organic solvents with isopropanol being the most optimal and giving better uniformity of the multilayered film. Acknowledgment. D.E.Z. acknowledges the International Association for the promotion of cooperation with scientists from the New Independent States of the former Soviet Union (INTAS) for the Young Scientist Fellowship grant (ref. 06-1000020-6265). The help of Dr. C. Stain and Dr. P. Harris at the Centre for Advanced Microscopy (University of Reading) in transmission electron microscopy experiments is greatly appreciated. Supporting Information Available: Video of IPC nanoparticles in methanol, and size distribution recorded by Nanosight LM20. This information is available free of charge via the Internet at http:// pubs.acs.org. LA802852H
