Aggregation of Hydrophobic Substituents of Poly(acrylate)s and Their

Apr 26, 2011 - School of Chemistry and Physics, University of Adelaide, Adelaide, SA ... Princeton University, Princeton, New Jersey 08544, United Sta...
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Aggregation of Hydrophobic Substituents of Poly(acrylate)s and Their Competitive Complexation by β- and γ-Cyclodextrins and Their Linked Dimers in Aqueous Solution Jie Wang, Li Li, Xuhong Guo,* and Li Zheng State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China

Duc-Truc Pham, Stephen F. Lincoln,* Huy Tien Ngo, Philip Clements, and Bruce L. May School of Chemistry and Physics, University of Adelaide, Adelaide, SA 5005, Australia

Robert K. Prud’homme Department of Chemical Engineering, Princeton University, Princeton, New Jersey 08544, United States

Christopher J. Easton Research School of Chemistry, Australian National University, Canberra, ACT 0020, Australia

bS Supporting Information ABSTRACT: Competition between the aggregation of the octadecyl substituents of 3% randomly substituted poly(acrylate), PAAC18, and of the dodecyl substituents of the PAAC12 analogue, and their complexation by β-cyclodextrin, βCD, the linked dimers, N,N0 -bis(6A-deoxy-6A-β-cyclodextrin)urea, 66βCD2ur, and N,N0 -bis(6A-deoxy-6A-β-cyclodextrin)succinamide, 66βCD2su, and γ-cyclodextrin, γCD, and the analogous dimers 66γCD2ur and 66γCD2su have been studied in aqueous solution. The zero-shear viscosities of 3.3 wt % aqueous solutions of PAAC18, decreases in the presence of βCD, 66βCD2ur, 66βCD2su and γCD, is little changed in the presence of 66γCD2ur, and increases in the presence of 66γCD2su. In contrast, the zero-shear viscosities of aqueous solutions of PAAC12 increase in the presence of βCD, γCD, and their dimers but they are much less than those of the corresponding PAAC18 solutions. These macroscopic variations are due to hostguest complexation of the octadecyl and dodecyl substituents by βCD, γCD, and their dimers (as shown by 2D 1H NOESY NMR spectroscopy) competing with aggregation of the octadecyl and dodecyl substituents where differences in substituent length and cyclodextrin annular size are dominant factors.

1. INTRODUCTION The cross-linking of polymers in aqueous solution to form networks is the basis of the formation of polymeric hydrogels which have attracted considerable interest because of their potential applications in drug delivery, in tissue engineering, and as bioadhesives.18 Accordingly, an understanding of the competing interactions involved in the cross-linking between polymer strands in polymer networks is important in designing hydrogels for such applications.911 Cross-linking may either occur between identical polymers through aggregation of hydrophobic substituents or through such substituents complexing with receptor substituents on a second polymer to form receptor substituenthydrophobic substituent, or hostguest, complexes. The latter type of cross-linking has been reported for polymers where the receptor substituent is a cyclodextrin (CD).916 Alternatively, covalently linked CD dimers may act as receptors for a wide variety of hydrophobic guests1724 and form cross-links between polymers with hydrophobic substituents.2532 In this study the competition between hydrophobic substituent aggregation of randomly substituted poly(acrylate)s and hostguest r 2011 American Chemical Society

complexation by covalently linked CD dimers and the hydrophobic substituents in forming interpolymer strand cross-links is examined in aqueous solution. The formation of aqueous polymer networks may be studied at the macroscopic level through rheological determination of viscosity and at the molecular level by 1H NMR spectroscopy. The effects of competitive interactions are examined in this study of 3% octadecyl and dodecyl randomly substituted poly(acrylate)s, PAAC18 and PAAC12, respectively, alone and in the presence of either βCD or γCD and the linked dimers N,N0 bis(6A-deoxy-6A-β-cyclodextrin)urea, 66βCD2ur, and N,N0 -bis(6A-deoxy-6A-β-cyclodextrin)succinamide, 66βCD2su, and their γCD analogues, 66γCD2ur and 66γCD2su (Figure 1) in aqueous 0.10 mol dm3 NaCl solution at pH 7.0 and 298.2 K. The intrinsic acid dissociation constant of the carboxylic acid groups Received: August 12, 2010 Accepted: April 26, 2011 Revised: April 15, 2011 Published: April 26, 2011 7566

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Figure 2. Viscosity variations with shear rate of 3.3 wt % aqueous solutions of PAAC18 alone and in the presence of βCD, 66βCD2ur, 66βCD2su, γCD, 66γCD2ur, and 66γCD2su at pH 7.0 and [NaCl] = 0.10 mol dm3 at 298.2 K. The concentrations of βCD and γCD and those of the βCD and γCD substituents in the linked dimers are equal to those of the octadecyl poly(acrylate) substituents (8.82  103 mol dm3).

Figure 1. Structures of βCD, γCD, 66βCD2ur, 66βCD2su, their γCD analogues, PAAC12, and PAAC18.

of poly(acrylate), pK0, is 4.1 under these conditions.33 As PAAC12 and PAAC18 are only 3% substituted, a similar situation is likely to apply here also such that all carboxylic acid groups are dissociated and PAAC12 and PAAC18 exist as polyelectrolytes. Substantial variations in zero-shear viscosities occur in the 14 aqueous systems composed of either PAAC18 or PAAC12 alone or in the presence of either βCD or γCD or their linked dimers. This is consistent with competition between octadecyl and dodecyl substituent aggregation and hostguest complexation determining the extent of inter poly(acrylate) strand crosslinking and the viscosity of each system. The variation of these interactions are dominated by the differences in octadecyl and dodecyl substituent lengths and cyclodextrin annular size. This appears to be the first reported study of such complexation by linked γCD dimers.

2. EXPERIMENTAL SECTION Poly(acrylic acid) (Mw = 250 000, Mw/Mn ≈ 2) was purchased (Aldrich) as a 35 wt % aqueous solution and freezedried to constant weight. The 3% dodecyl and octadecyl substituted poly(acrylates), PAAC12 and PAAC18, were prepared and characterized as previously described.34,35 Beta-cyclodextrin and gamma-cyclodextrin, βCD and γCD, were obtained from Nihon Shokuhin Kako Co. The preparations of N,N0 -bis(6Adeoxy-6A-β-cyclodextrin)urea, 66βCD2ur, N,N0 -bis(6A-deoxy-6Aβ-cyclodextrin)succinamide, 66βCD2su, and their γCD analogues, 66γCD2ur and 66γCD2su, were by literature methods.3638 Rheological measurements were carried out with a Physica MCR 501 (Anton Parr GmbH) stress-controlled rheometer with a 25 mm cone-and-plate geometry. Temperature was controlled at 298.2 ( 0.1 K by a Peltier plate. Rheological samples were prepared by dissolution of PAAC18 and PAAC12 in 0.10 mol dm3

aqueous NaCl solution to ensure screening of the electrostatic interactions between the carboxylate groups. The solution pH was adjusted to 7.0 with 0.10 mol dm3 aqueous NaOH solution. The 2D 1H NOESY NMR spectra were recorded on a Varian Inova 600 spectrometer operating at 599.957 MHz, using a standard pulse sequence with a mixing time of 0.3 s. Solutions were prepared in 1 cm3 of D2O, and the pD was adjusted to 7 with 0.10 mol dm3 NaOD in D2O. Each solution contained either PAAC18 or PAAC12 (10 mg) and either βCD or γCD at concentrations equal to those of the dodecyl and octadecyl poly(acrylate) substituents, or either 66βCD2ur, 66βCD2su, 66γCD2ur, or 66γCD2su at concentrations at which their βCD and γCD group concentrations were equal to those of the octadecyl and dodecyl poly(acrylate) substituents. Solutions were allowed to equilibrate for 30 min in 5 mm NMR tubes at 298.2 ( 0.1 K prior to recording of their spectra.

3. RESULTS AND DISCUSSION 3.1. Rheological Determination of Viscosity. Rheological studies show the zero-shear viscosities of the 14 solutions studied vary substantially as solution composition changes. The variations of the viscosities of the seven PAAC18 and the seven PAAC12 solutions with shear rate are shown in Figures 2 and 3, respectively. The zero-shear-viscosity variations, corresponding to the viscosities extrapolated from those observed at the lowest shear rates, are shown graphically and numerically in Figure 4 and its caption. 3.2. Viscosities of PAAC18 and PAAC12 Solutions. As the shear rate increases from 0.002 s1, the PAAC18 solution viscosity profile shows shear thickening at a shear rate of about 0.1 s1 and thereafter decreases sharply as the shear rate increases to 100 s1 (Figure 2). This is consistent with the proportion of intrastrand octadecyl substituent aggregation diminishing as the shear rate increases and the poly(acrylate) strands extend. Consequently, an increase in interstrand octadecyl substituent aggregation occurs and leads to a greater poly(acrylate) network formation and an increase in viscosity, or shear thickening.15,35 As the shear rate further increases, a progressive breaking of the interstrand cross-links and a decrease in viscosity occur. (A visual illustration of PAAC18 hydrogel formation is readily made 7567

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Figure 3. Viscosity variations with shear rate of 3.3 wt % aqueous solutions of PAAC12 alone and in the presence of βCD, 66βCD2ur, 66βCD2su, γCD, 66γCD2ur, and 66γCD2su at pH 7.0 and [NaCl] = 0.10 mol dm3 at 298.2 K. The concentrations of βCD and γCD and those of the βCD and γCD substituents in the linked dimers are equal to those of the dodecyl poly(acrylate) substituents (1.01  102 mol dm3).

Figure 4. Zero-shear viscosities of PAAC18 (clear bar) and PAAC12 (black bar) aqueous 3.3 wt % solutions at pH 7.0 and [NaCl] = 0.10 mol dm3 at 298.2 K alone and in the presence of βCD and γCD, with those of the βCD and γCD components of the linked dimers equal to those of either the octadecyl or dodecyl poly(acrylate) substituents. (A) PAAC18 (645) and PAAC12 (0.016), (B) PAAC18/βCD (0.07) and PAAC12/βCD (0.03), (C) PAAC18/66βCD2ur (339) and PAAC12/66βCD2ur (0.12), (D) PAAC18/66βCD2su (110) and PAAC12/66βCD2su (0.25), (E) PAAC18/γCD (5.8) and PAAC12/ γCD (0.12), (F) PAAC18/66γCD2ur (714) and PAAC12/66γCD2ur (0.14), (G) PAAC18/66γCD2su (2708) and PAAC12/66γCD2su (0.07), where the zero-shear viscosities (Pa s) are shown in parentheses.

through aqueous solutions of PAAC18 forming firm hydrogels in the range 105 wt %, a flowing hydrogel at 2.5 wt %, and a viscous fluid hydrogel at 2.0 wt %.) In contrast, the 40 300-fold less viscous PAAC12 solution (Figure 3) shows little variation with shear rate, consistent with either much less or weaker or both interstrand cross-linking occurring. 3.3. Effects of βCD and γCD on Viscosity. The viscosities of the PAAC18/βCD and PAAC18/γCD solutions (Figure 2) and PAAC12/βCD and PAAC12/γCD solutions (Figure 3) also show little variation with shear rate, consistent with a much less effective interstrand cross-linking occurring by comparison with that occurring in the PAAC18 solution. This is attributable to complexation of the octadecyl substituents by βCD and γCD. (For the 12 solutions containing either βCD, γCD, or their dimers 2D 1H NOESY NMR spectroscopy shows that the

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octadecyl and dodecyl substituents of PAAC18 and PAAC12, respectively, are complexed as is discussed below.) For PAAC18 the sequence of increasing zero-shear viscosities is PAAC18/ βCD (0.07 Pa s) , PAAC18/γCD (5.8 Pa s) , PAAC18 (645 Pa s) (clear bars B, E, and A, Figure 4), consistent with substantial removal of octadecyl substituent aggregation through competitive hostguest complexation of single substituents by βCD and γCD and possibly some interstrand cross-linking through two or more octadecyl substituents being complexed by βCD and γCD. Both annuli have depths of 7.9 Å, and the internal diameters of the narrow and wide ends of the annuli are 6.0 and 6.5 Å for βCD and 7.5 and 8.3 Å for γCD39 such that two octadecyl substituents of a cross-sectional diameter of 3.1 Å fit less readily into the βCD annulus than that of γCD. It has recently been shown that two poly(ethyleneimine) strands which are of a diameter similar to that of the octadecyl strand may be simultaneously complexed in γCD.40 Nevertheless, the overall effect is a substantial lowering of zero-shear viscosity. In contrast, the zero-shear viscosities of the PAAC12 solutions increase in the sequence PAAC12 (0.016 Pa s) < PAAC12/βCD (0.03 Pa s) < PAAC12/γCD (0.12 Pa s) (black bars A, B, and E, Figure 4), consistent with simultaneous complexing of two or more dodecyl substituents forming interstrand cross-links and enhancing the weak interstrand cross-linking in PAA12 alone. [It has previously been shown that βCD lowers the zero-shear viscosity of a 0.5 wt % PAAC18 solution by 50% and γCD slightly more,15 whereas for the 6.6-fold more concentrated and almost 5 orders of magnitude more viscous 3.3 wt % PAAC18 solutions of this study, both βCD and γCD lower zero-shear viscosity by several orders of magnitude. This difference probably arises from the ratio of intra- to interstrand octadecyl and dodecyl substituent aggregation in the 0.5 wt % solutions being substantially greater than in the 3.3 wt % solution,15 as a consequence of the critical overlap concentration at which PAAC18 strand overlap occurs corresponding to a 1.0 wt % solution. However, the relative effectiveness of βCD and γCD in the 3.3 wt % solutions is reversed by comparison with that in the 0.5 wt % solutions. This indicates that at higher PAAC18 concentrations, and the consequently greater proximity of adjacent PAAC18 strands, the disruption of the octadecyl substituent aggregation is compensated for by the ability of the larger γCD to simultaneously complex two octadecyl substituents to form new cross-links more readily than does βCD.] These data indicate that differences in the βCD and γCD annular sizes are important in modification of PAAC18 and PAAC12 solution viscosity and interstrand cross-linking which probably reflects the ability of the larger γCD annulus to complex octadecyl and dodecyl substituents more readily than βCD. 3.4. Effects of the Covalently Linked βCD and γCD Dimers on the Viscosity of PAAC18 Solution. The higher zero-shear viscosities of PAAC18 solutions containing covalently linked βCD and γCD dimers increase in the sequence PAAC18/ 66βCD2su (110 Pa s) < PAAC18/66βCD2ur (339 Pa s) < PAAC18 (645 Pa s) < PAAC18/66γCD2ur (714 Pa s) < PAAC18/ 66γCD2su (2 708 Pa s) (clear bars D, C, A, F, and G, Figure 4), and all show shear thickening profiles similar to that of PAAC18. The 4-fold greater zero-shear viscosity of the PAAC18/ 66γCD2su solution by comparison with that of the PAAC18 solution indicates that complexing of octadecyl substituents by 66γCD2su to form interstrand cross-links reinforces the effect of cross-linking through octadecyl substituent aggregation, 7568

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Figure 5. Representations of single and double hostguest complexation of PAAC18 octadecyl substituents (a) by 66βCD2ur and 66γCD2ur and (b) by 66βCD2su and 66γCD2su, and of PAAC12 dodecyl substituents (c) by 66βCD2ur and 66γCD2ur and (d) by 66βCD2su and 66γCD2su. The simultaneous complexation of two substituents by the βCD and γCD groups may also occur.

although the relative magnitudes of the two effects are uncertain. For the PAAC18/66γCD2ur, PAAC18/66βCD2ur, and PAAC18/ 66βCD2su solutions a simple interpretation of the relative magnitudes of their zero-shear viscosities is that some octadecyl substituents of PAAC18 are singly complexed increasingly effectively in the order 66γCD2ur < 66βCD2su < 66βCD2ur such that interstrand cross-linking through octadecyl substituent aggregation progressively decreases as does viscosity. This may be accompanied by complexing of octadecyl substituents by 66γCD2ur, 66βCD2su, and 66βCD2ur to form interstrand crosslinks as shown schematically in Figure 5a,b, but distinction between these effects is not possible from the data. The overall effect is that the zero-shear viscosities and shear thickening of the PAAC18 and PAAC18/66γCD2ur solutions are similar while those of the PAAC18/66βCD2su and PAAC18/66βCD2ur are decreased by comparison. These observations are also consistent with the linker length and the CD annular size affecting the formation of interstrand cross-links. 3.5. Effects of the Covalently Linked βCD and γCD Dimers on the Viscosity of PAAC12 Solution. The zero-shear-viscosity variations of the much less viscous PAAC12/66βCD2su and PAAC12/66γCD2ur solutions present very shallow shear thickening profiles (Figure 3), which reflect the formation of interstrand cross-links through complexation of dodecyl substituents by 66βCD2su and 66γCD2su as shown in Figure 5c,d. The sequence of the zero-shear viscosities, PAAC12 (0.016 Pa s) < PAAC12/βCD (0.03 Pa s) < PAAC12/66γCD2su (0.07 Pa s) < PAAC12/γCD (0.12 Pa s) ≈ PAAC12/66βCD2ur (0.12 Pa s) < PAAC12/66γCD2ur (0.14 Pa s) < PAAC12/66βCD2su (0.25 Pa s) (black bars A, B, G, E, C, F, and D, Figure 4), only covers a

15.6-fold range consistent with a much weaker PAAC12 dodecyl substituent aggregation forming interstrand cross-links and additional cross-linking occurring through dodecyl substituent complexation by all of the CD species. Again, linker length and the CD annular size affect the formation of interstrand crosslinks, although the sequence of the variation of the zero-shearviscosity magnitudes with the CD species for PAAC12 differs from that for the PAAC18. 3.6. 1H NMR Spectroscopic Determination of HostGuest Complexation. The 2D NOESY 1H NMR spectra of PAAC18 and PAAC12 show strong cross-peaks arising from dipolar interactions between the protons of their octadecyl and dodecyl substituents, respectively, and the H3, H5, and H6 annular protons of either βCD or γCD or their linked dimers, consistent with hostguest complexation (Figure 6). These data do not distinguish between single substituent complexation and the simultaneous complexation of either octadecyl or dodecyl substituents to form intra- and interstrand cross-links. Nevertheless, they add plausibility to the inference drawn from the rheological data that PAAC12 interstrand cross-linking is increased by hostguest complexation while competition between hostguest complexation and octadecyl substituent aggregation decreases the effectiveness of PAAC18 interstrand cross-linking in the presence of βCD, 66βCD2ur, 66βCD2su, and γCD. However, this balance appears to shift so that PAAC18 interstrand cross-links formed through 66γCD2ur and 66γCD2su, as envisaged for the latter species in Figure 7, are of similar and greater stability, respectively, by comparison with those formed through octadecyl substituent aggregation. 7569

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PAAC12 networks are complex. They are consistent with a combination of the lengths of the octadecyl and dodecyl substituents, the sizes of the βCD and γCD annuli, and the length of the linkers in the βCD and γCD dimers controlling the extent and strength of the interstrand cross-links formed through either octadecyl or dodecyl substituent aggregation and hostguest complexation. This provides insight for the design of new aqueous polymer networks and hydrogels with potential for practical application.

’ ASSOCIATED CONTENT

bS

Supporting Information. Additional 2D 1H NOESY NMR spectra and photographs of PAAC18 hydrogel formation. This material is available free of charge via the Internet at http:// pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] (S.F.L.); guoxuhong@ ecust.edu.cn (X.G.).

Figure 6. Two-dimensional 1H NOESY NMR (600 MHz, 0.3 s mixing time) spectrum of 1 wt % solution of PAAC18 and 66γCD2su with equimolar γCD groups and octadecyl substituents in D2O at pD 7.0 with [NaCl] = 0.10 mol dm3 at 298.2 K. The cross-peaks enclosed in the rectangle arise from interactions between the annular H3,5,6 of 66γCD2su and the octadecyl protons.

Figure 7. Schematic illustration of the cross-linking in the polymer network through aggregation and hostguest complexation of the octadecyl substituents (red) attached to the poly(acrylate) backbone (black) of PAAC18 by 66γCD2su (blue). The variation of the number of octadecyl substituents in particular aggregations is unknown, and those shown are illustrative only. It is possible that some octadecyl substituents remain disaggregated and that others are singly complexed by 66γCD2su.

4. CONCLUSIONS The patterns of the relative magnitudes of the effects of βCD and γCD and their linked dimers on the viscosity of PAAC18 and

’ ACKNOWLEDGMENT We gratefully acknowledge the Australian Research Council, NSFC Grants 20774028 and 20774030, 111 Project Grant B08021, Shanghai Shuguang Plan Project 06SG35, Shanghai Pujiang Talent Project 07PJ14022, and the China Scholarship Council for supporting this work. ’ REFERENCES (1) Wichterle, Q.; Lim, D. Hydrophilic gels for biological use. Nature 1960, 185, 117–118. (2) Annaka, M.; Tanaka, T. Multiple phases of polymer gels. Nature 1993, 355, 430–432. (3) Nowak, A. P.; Breedveld, V.; Pakstis, L.; Ozbas, B.; Pine, D. J.; Pochan, D.; Deming, T. J. Rapidly recovering hydrogel scaffolds from self-assembling diblock copolypeptide amphiphiles. Nature 2002, 417, 424–428. (4) Sangeetha, N. M.; Maitra, U. Supramolecular gels: functions and uses. Chem. Soc. Rev. 2005, 34, 821–836. (5) Peppas, N. A.; Hilt, J. Z.; Khademhosseini, A.; Langer, R. Hydrogels in biology and medicine: from molecular principles to bionanotechnology. Adv. Mater. 2006, 18, 1345–1360. (6) Chaterji, S.; Kwon, I. K.; Park, K. Smart polymeric gels: redefining the limits of biomedical devices. Prog. Polym. Sci. 2007, 32, 1083–1122. (7) Guo, X.; Deng, F.; Li, L.; Prud’homme, R. K. Synthesis of biocompatible polymeric hydrogels with tunable adhesion to both hydrophobic and hydrophilic surfaces. Biomacromolecules 2008, 9, 1637–1642. (8) Khan, F.; Tare, R. S.; Oreffo, R. O.; Bradley, M. Versatile biocompatible polymer hydrogels: scaffolds for cell growth. Angew. Chem., Int. Ed. 2009, 48, 978–982. (9) Deng, W.; Yamagushi, H.; Takashima, Y.; Harada, A. A chemicalresponsive supramolecular hydrogel from modified cyclodextrins. Angew. Chem., Int. Ed. 2007, 46, 5144–5147. (10) Ogoshi, T.; Takashima, Y.; Yamaguchi, H.; Harada, A. Chemically-responsive solgel transition of supramolecular single-walled carbon nanotubes (SWNTs) hydrogel made by hybrids of SWNTs and cyclodextrins. J. Am. Chem. Soc. 2007, 129, 4878–4879. (11) Harada, A.; Hashidzume, A.; Takashima, Y. Cyclodextrin-based supramolecular polymers. Adv. Polym. Sci. 2006, 201, 1. 7570

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