Langmuir 2003, 19, 5175-5177
Novel pH-Responsive Amphiphilic Diblock Copolymers with Reversible Micellization Properties S. Dai,†,‡ P. Ravi,†,‡ K. C. Tam,*,†,‡ B. W. Mao,§ and L. H. Gan§ Singapore-MIT Alliance, School of Mechanical and Production Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Republic of Singapore, and Natural Sciences, National Institute of Education, Nanyang Technological University, 1 Nanyang Walk, Singapore 637616, Republic of Singapore Received January 15, 2003. In Final Form: March 31, 2003
Stimuli-responsive water-soluble and amphiphilic polymers have attracted much interest because their micelles can be produced or destroyed by changing the temperature, pH, ionic strength, and solvent polarity.1,2 Recently, several research groups have reported on the reversible micellization behavior of block copolymers in aqueous solution.3-10 The research groups of Laschewsky and Armes have successfully synthesized diblock copolymers of poly{N-isopropylacrylamide-block-3-[N-(3-methacrylamidopropyl)-N,N-dimethyl]ammoniopropane sulfonate} [P(NIPAM-b-SPP)] and poly[2-(N-morpholino)ethyl methacrylate-block-2-(sulfobetaine-dimethylamino)ethyl methacrylate] [P(MEMA-b-SBMA)] by the reversible addition fragmentation chain transfer and atom transfer radical polymerization (ATRP), respectively.3,4 By linking segments that possess a lower critical solution temperature and an upper critical solution temperature properties, temperature-responsive block copolymers could be produced. With increasing temperature, the micelles containing SPP or SBMA hydrophobic cores were converted to unimers, which then re-established their micellar structures consisting of NIPAM or MEMA cores, respectively. Armes and co-workers also reported studies on the diblock copolymer of poly[2-(dimethylamino)ethyl methacrylate-block-methacrylic acid] [P(DMA-b-MAA)] synthesized using group transfer polymerization (GTP), which exhibits temperature- and pH-responsive reversible aggregation properties.6,7 Reversible aggregates containing a DMA hydrophobic core and ionized MAA hydrophilic corona were produced at pH 9.5 and 50 °C. The aggregates * To whom correspondence should be addressed. Fax: (65) 67911859. E-mail:
[email protected]. † Singapore-MIT Alliance, Nanyang Technological University. ‡ School of Mechanical and Production Engineering, Nanyang Technological University. § Natural Sciences, Nanyang Technological University. (1) McCormick, C. L. Stimuli-Responsive Water Soluble and Amphiphilic Polymers; American Chemical Society: Washington, DC, 2001. (2) Lowe, A. B.; McCormick, C. L. Chem. Rev. 2002, 102, 4177. (3) Arotcarena, M.; Heise, B.; Ishaya, S.; Laschewsky, A. J. Am. Chem. Soc. 2002, 124, 3837. (4) Weaver, J. V. M.; Armes, S. P.; Butun, V. Chem. Commun. 2002, 18, 2122. (5) Liu, S.; Armes, S. P. Angew. Chem., Int. Ed. 2002, 41, 1413. (6) Lowe, A. B.; Billingham, N. C.; Armes, S. P. Macromolecules 1998, 31, 5991. (7) Lowe, A. B.; Billingham, N. C; Armes, S. P. Chem. Commun. 1997, 13, 1035. (8) Butun, V.; Billingham, N. C.; Armes, S. P. J. Am. Chem. Soc. 1998, 120, 11818. (9) Butun, V.; Armes, S. P.; Billingham, N. C.; Tuzar, Z.; Rankin, A.; Eastoe, J.; Heenan, R. K. Macromolecules 2001, 34, 1503. (10) Liu, S.; Billingham, N. C.; Armes, S. P. Angew. Chem., Int. Ed. 2001, 40, 2328.
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have a large hydrodynamic diameter of ∼400 nm. In addition, the research groups of Hatton and Jerome have also reported on the pH- and temperature-responsive properties of P(DMA-b-MAA).11,12 Depending on the block composition, salt, and pH, the copolymer is either soluble or insoluble or forms large nonmicellar aggregates. Another diblock copolymer with temperature- and pHresponsive reversible micellization properties is the poly[propylene oxide-block-2-(diethylamino)ethyl methacrylate] (PPO-b-PDEA) synthesized by Armes and co-workers using ATRP.10 At high temperature (40 °C) and low pH (5.5), micelles with a hydrophobic PPO core were formed in an aqueous solution, whereas at low temperature (25 °C) and high pH (8.5), micelles containing a hydrophobic DEA core were produced. In addition, the salt-induced pH-responsive block copolymer of P(MEMA-b-DEA) was also synthesized by Armes and co-workers using the GTP method.8,9 At low pH ( 9.2. At the pH values between 5.5 and 9.2, the solution is cloudy, with the maximum turbidity observed at a pH of 7.5. From the potentiometric and conductivity titration results, it is clear that the phase separation occurs after the neutralization of the MAA segment and before the deprotonation of the DEA segment. On the basis of the chemical structure and observed phenomena described above, the pH-dependent reversible micelles are produced at low-pH (below point A) and highpH (above point B) solutions. In the titration, the NaOH first neutralizes the free HCl (between 0 and 0.08 mmol of NaOH), followed by the neutralization of the MAA segment. At low pH (below point A), micelles with a hydrophobic MAA core and protonated DEA corona shell are formed. By increasing the pH beyond point A, the -COOH groups of the MAA segment are progressively
Figure 1. (a) Potentiometric and conductivity titration of 0.04 wt % P(MAA-b-DEA) with NaOH at 25 °C. (b) The UV-visible transmittance of the P(MAA-b-DEA) aqueous solutions at different pH values.
Notes
Figure 2. (a) 1H NMR spectra of P(MAA-b-DEA) at low and high pH values. (b) The hydrodynamic radius distributions and possible structures of P(MAA-b-DEA) at different pH values.
being neutralized, thereby producing more hydrophilic -COO- groups. These -COO- groups interact with the protonated DEA segment through electrostatic attraction, giving rise to the precipitation of the polymer in the solution. As the pH increases beyond the isoelectric point,5 where the total negative charge equals the total positive charge (at electrically neutral conditions), DEA segments are progressively deprotonated and the electrostatic balance is destroyed. The cloudy solution turns clear again as a result of the formation of the reversible micelles containing a deprotonated hydrophobic DEA core and neutralized hydrophilic MAA corona shell. Beyond point B, the DEA segment is completely deprotonated. The 1H NMR spectra of the P(MAA-b-DEA) at low and high pH values are shown in Figure 2a. In the spectrum recorded at low pH, the signals at 1.29 [(-CH3)2], 3.29 [-N(CH2CH3)2], and 3.62 (-NCH2CH2-) ppm correspond to DEA segments. These peaks are clearly detected in the 1 H NMR spectrum, indicating that the DEA block is
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solvated in the solvent (D2O/DCl). At high pH (>9; adjusted using NaOD), all the DEA peaks have disappeared and only the -CH3 protons of methacrylic acid at 0.76-0.89 ppm are observed. This further confirmed that reversible micelles are formed because the nonsolvated DEA blocks are now in the core and the solvated methacrylic acid blocks form the shell of the micelles. The micellization behavior of the block copolymer at different values of the pH was also studied by a laser light scattering technique. At low pH of 3, dynamic light scattering (DLS) data revealed two translational diffusion modes in the decay time distribution functions. On the basis of the Stokes-Einstein relationship, the hydrodynamic radii of the two species determined to be 2.7 and 70 nm were for the unimers and micelles, respectively. The composition of the unimers was estimated to be about 8 wt %. The radius of gyration, Rg, determined from static light scattering (SLS) was found to be 46 nm, giving rise to Rg/Rh ) 0.66, which is smaller than 0.774 for the hard sphere. This suggests the presence of a thicker hydration layer within the core-shell micelle. Because the HLB value of P(MAA-b-DEA) at the prevailing pH value may not be low enough and the pKa of MAA is 5.4, a mixture of unimers and micelles was observed. By increasing the pH from 6 to 9, the solution becomes cloudy as a result of phase separation, as was indicated by the abrupt increase in the scattering intensity. The DLS data indicate the formation of large aggregates with an averaged size of 800 nm. At high pH (>9), the solution becomes clear again and the scattering intensity decreases. Only one narrow q2-dependent peak in the decay time distribution function is evident. Rh and Rg were determined to be 17.5 and 12 nm, respectively, giving rise to Rg/Rh ) 0.69, which indicates again that the micelle possesses a core-shell structure. As a result of the high pKa value of DEA (7.3) associated with the lower HLB values, the critical micelle concentration is extremely small. The aggregation number measured using theSLS technique was found to be ∼50. The hydrodynamic radius distribution and possible microstructure of the reversible pH-responsive micelles of P(MAA-b-DEA) in solution are shown in Figure 2b. To further clarify the pH-dependent micellization properties, steady-state fluorescence spectroscopy and time-resolved fluorescence quenching studies are currently in progress. LA0340652
