Langmuir 2001, 17, 7501-7506
7501
Effects of Grafted Alkyl Groups on Aggregation Behavior of Amphiphilic Poly(aspartic acid) Hyung Seok Kang,†,‡ Seung Rim Yang,† Jong-Duk Kim,*,† Sang-Hoon Han,‡ and Ih-Seop Chang‡ Department of Chemical Engineering, Korea Advanced Institute of Science and Technology, Kusong-dong, Yusong-gu, Taejon 305-701, South Korea, and Nanotechnology Research Laboratory, Pacific Corporation, Yongin-si 449-729, Kyunggi-do, South Korea Received May 29, 2001. In Final Form: August 24, 2001 The aggregation behavior of biodegradable amphiphilic poly(aspartic acid) (PASP) derivatives containing long alkyl chains was characterized by size, interfacial properties, and aggregate formation. The polymers were synthesized by thermal condensation and aminolysis by alkylamine, followed by hydrolysis of the remaining succinimide units in the polymer backbone. The polymers formed self-aggregates by ultrasonication. Strong hydrophobic interaction by a higher amount of grafted alkyl chains induced higher aqueous stability of the self-aggregates. Bending of the stiff PASP backbone and strong association of alkyl chains were considered to be major competitive factors for determining aqueous stability. An aqueous solution of PASP-C18 was not surface-active due to physical cross-linker nature of octadecyl chains, while dodecyl and hexadecyl chains with higher chain flexibility showed surface-active properties. CAC calculated from fluorescence excitation spectra showed logarithmically decreasing behavior as DS of alkyl chains.
Introduction Recently, self-assemblies of polymeric amphiphiles in aqueous media have been investigated for biological importance and pharmaceutical applications.1 Synthetic block copolymers and hydrophobically modified watersoluble polymers are among the most intensively investigated materials for understanding self-assembly formation and their applications.2-5 Polymeric amphiphiles can be synthesized by several methods such as the copolymerization of water-soluble monomer with hydrophobic comonomer,6 attaching hydrophobic moieties such as long alkyl chains or bulky cholesterol derivatives to a water-soluble polymer backbone,7-9 and the block copolymerization on reactive polymer end groups such as PEO-PPO, poly(lactide)PEO, and poly(amino acid)-PEO.1,2,5 Among hydrophobically modified water-soluble polymers, cholesterol-bearing pullulan (CHP) has been intensively investigated. It has been found that CHP selfaggregates provided two domains consisting of the hydrophobic cholesterol moiety and the hydrophilic polysaccharide skeleton and that one self-aggregate was †
Korea Advanced Institute of Science and Technology. Pacific Corporation. * To whom correspondence should be addressed. E-mail kjd@ kaist.ac.kr; telephone 82-42-869-3921; Fax 82-42-869-3910. ‡
(1) Newman, M. J.; Actor, J. K.; Balusubramanian, M.; Jagannath, C. Crit. Rev. Therapeutic Drug Carrier Systems 1998, 15, 89. (2) Yokoyama, M.; Fukushima, S.; Uehara, R.; Okamoto, K.; Kataoka, K.; Sakurai, Y.; Okano, T. J. Controlled Release 1998, 50, 79. (3) Akiyoshi, K.; Deguchi, S.; Moriguchi, N.; Yamaguchi, S.; Sunamoto, J. Macromolecules 1993, 6, 3062. (4) Tanikuchi, I.; Akiyoshi, K.; Sunamoto, J. J. Macromol. Chem. Phys. 1999, 200, 1386. (5) Kwon, G. S.; Okano, T. Adv. Drug Deliv. Rev. 1996, 21, 107. (6) Ezzell, S. A.; Hoyle, C. E.; Greed, D.; McCormick, C. L. Macromolecules 1992, 25, 1887. (7) Akiyoshi, K.; Yamaguchi, S.; Sunamoto, J. J. Chem. Lett. 1991, 1263. (8) Nichifor, M.; Lopes, A.; Carpov, A.; Melo, E. Macromolecules 1999, 32, 7078. (9) Lee, K. Y.; Jo, W. H.; Kwon, I. C.; Kim, Y. H.; Jeong, S. Y. Macromolecules 1998, 31, 378.
composed of several hydrophobic microdomains.3,10 The mean aggregation number of cholesterol moieties in CHP self-aggregates was found to be 3.5-5.7 by the steadystate fluorescence quenching method, while the aggregation numbers of low molecular weight surfactants are reported to be several tens.11 The same result was reported in the self-aggregates of deoxycholic acid-modified chitosan.9 The characterization of self-aggregates can be carried out by size exclusion chromatography, light scattering, fluorescence spectroscopy, electron microscopy, smallangle X-ray scattering, flow field-flow fractionation, and NMR.3,10,12-20 Poly(L-aspartic acid) (PASP) is synthesized by acidcatalyzed thermal polycondensation of L-aspartic acid to give poly(succinimide) (PSI), followed by hydrolysis.21 PASP is an attractive candidate for superabsorbent, ionchelating ability, and delivery carriers due to its completely biodegradable and acid-containing water-soluble properties.22,23 (10) Akiyoshi, K.; Deguchi, S.; Tajima, H.; Nishikawa, T.; Sunamoto, J. Macromolecules 1997, 30, 857. (11) Rosen, M. J. Surfactants and Interfacial Phenomena, 1st ed.; John Wiley & Sons: New York, 1978; Chapter 3. (12) Mizusaki, M.; Morishima, Y.; Winnik, F. M. Macromolecules 1999, 32, 4317. (13) Desjardins, A.; Eisenberg, A. Macromolecules 1991, 24, 5779. (14) Xu, R. L.; Winnik, M. A.; Hallett, F. R.; Riess, G.; Croucher, M. D. Macromolecules 1991, 24, 87. (15) Kriz J, Masar B, Plestil J, Tuzar Z, Pospisil H, Doskocilova, D. Macromolecules 1998, 31, 41. (16) Jada, A.; Hurtrez, G.; Siffert, B.; Riess, G. Macromol. Chem. Phys. 1996, 197, 3697. (17) Massey, J. A.; Temple, K.; Cao, L.; Rharbi, Y.; Raez, J.; Winnik, M. A.; Manners, I. J. Am. Chem. Soc. 2000, 122, 11577. (18) Lakowicz, J. R. Principles of Fluorescence Spectroscopy; Plenum Press: New York, 1983. (19) Wittgren, B.; Wahlund, K. G.; Derand, H.; Wesslen, B. Macromolecules 1996, 29, 268. (20) Mortensen, K.; Talmon, Y. Macromolecules 1995, 28, 8829. (21) Tomida, M.; Nakato, T.; Kuramochi, M.; Shibata, M.; Matsunami, S.; Kakuchi, T. Polymer 1997, 38, 4733. (22) Caldwell, G.; Neuse, N. W.; Perlwitz, A. G. J. Appl. Polym. Sci. 1997, 66, 911. (23) Chang, C. J.; Swift, G. J. Macromol. Sci., Pure 1999, A36, 963.
10.1021/la0107953 CCC: $20.00 © 2001 American Chemical Society Published on Web 11/01/2001
7502
Langmuir, Vol. 17, No. 24, 2001
The acid-catalyzed polycondensation produced a regular structure of high molecular weight and complete biodegradability, while thermal polycondensation was reported to have irregular PASP structure and byproducts and low biodegradability.21 PASP hydrolyzed from PSI is composed of R and β aspartic residue from different ring-opening manners, and R and β units were randomly distributed in the main chain by analyzing the amide carbonyl signals in the 13C NMR.24 The pharmaceutical importance of PASP for delivering poorly soluble drugs has been reported in the literature.2,5 PASP modified with a hydrophobic part is a promising delivery candidate for poorly soluble drugs by entrapping the drugs into hydrophobic domains of aggregates. Therefore, the analysis of aggregation behavior by association of the hydrophobic part can provide useful information about the structure and physicochemical properties of aggregates for developing delivery devices. In our previous work, it was reported that PASP grafted with octadecyl chains formed self-aggregates in aqueous solution and showed pH-sensitive aggregation behavior due to an inherent negative surface charge.25 A similar study of PASP grafted with alkyl groups was reported in earlier work.26 PASP modified with dodecyl groups was characterized by fluorescence and dynamic light scattering, indicating that two kinds of aggregates were observed in the presence of a lower amount of dodecyl group, and unimolecular micelle was formed when the amount of dodecyl groups was increased. In this study, we report the characterization of the selfaggregates of amphiphilic PASP that have dependence on the chain length and the amount of alkyl groups. The structure of the self-aggregates and the hydrophobic effect of grafted alkyl groups on the physicochemical properties of the self-aggregates are discussed. Experimental Section Synthesis. L-Aspartic acid (40 g, 0.30 mol) was suspended in sulfolane (200 mL) in the presence of phosphoric acid (15 mmol) and stirred at 170 °C under a N2 atmosphere. Water generated in condensation was continuously removed by a Dean-Stark trap. After 10 h, the reaction mixture was precipitated in excess methanol and successively washed with water until the pH of the suspension became neutral. The precipitate was washed with methanol and dried at 80 °C in vacuo. The structure of PSI(I) in Figure 1 was confirmed by 1H NMR analysis. 1H NMR (DMSOd6): δ ) 2.69 and 3.18 (R, 2H, s); 5.25 (β, 1H, t). Synthesized PSI (485 mg) was dissolved in dried DMF (5 mL), and the appropriate alkylamine dissolved in DMF (1 mL) was added to the mixture rapidly. The reaction mixture was stirred at 70 °C for 24 h and cooled to room temperature. Insoluble product was filtered out. The solution was added dropwise to 1 N NaOH solution to hydrolyze remaining succinimide units of PSI. After stirring for 3 h at room temperature, the reaction mixture was precipitated in excess methanol twice. The precipitate was filtered and then dried in vacuo at 60 °C. PSI-C18(II): 1H NMR (DMSO-d6): R ) 0.84 (γ, 3H, t); R ) 1.22 (, 30H, s); δ ) 2.69 and 3.18 (R, 2H, s); 5.25 (β, 1H, t). PASP(III): 1H NMR (D2O): δ ) 2.78 and 2.72 and 2.57 (2H, m); R ) 4.49 (βopening, 1H, s); δ ) 4.69 (R-opening, 1H, s). The molecular weight of PASP was determined by GPC analysis eluted by 0.1 M NaNO3 at a flow rate of 0.5 mL/min, with Ultrahydrogel Linear and Ultrahydrogel 120 (Waters) column in series. Mn ) 17 200, Mw ) 22 800, and PD ) 1.32. The yields of each product were 8595%. (24) Wolk, S. K.; Paik, Y. H.; Yokom, K. M.; Smith, R. L.; Simon, E. S. Macromolecules 1994, 27, 7613. (25) Kang, H. S.; Shin, M. S.; Kim, J. D.; Yang, J. W. Polym. Bull. 2000, 45, 39. (26) Suwa, M.; Hashidzume, A.; Morishima, Y.; Nakato, T.; Tomida, M. Macromolecules 2000, 33, 7884.
Kang et al.
Figure 1. Synthesis of PSI and PASP with long alkyl groups. The Greek letters refer to the peak positions in 1H NMR spectra. Sample Preparation. PASP grafted with alkyl groups was suspended in deionized water and mixed by vortexing. These milky suspensions were sonicated for 10 min to give clear solutions using a bath type sonifier (Sonics & Materials) at room temperature. The solutions were purified by passing through a 0.45 µm filter (Whatman) to remove dust. These stock solutions were adjusted to 0.1 M buffer concentration by dilution with phosphate buffer (pH 7.0, 0.2 M) before use. The pH of aggregate solutions is 7.0 if not described otherwise. The solutions of various pH was adjusted by addition of 1.0 N of hydrochloric acid and sodium hydroxide. Measurements. Light scattering measurement was performed for determining the size distribution and second virial coefficient with an apparatus from Brookhaven Instruments Inc. equipped with a diode laser of 523 nm. The scattering angle was fixed at 90° for dynamic light scattering (DLS), the effective diameter was obtained by the Stokes-Einstein relationship, and histograms were calculated with the NNLS routine. The static light scattering (SLS) measurements were performed at 25 °C at angles from 30° to 150°. The second virial coefficient (A2) was obtained by analyzing a Zimm plot obtained from SLS measurement on the basis of eq 1.
(
)(
)
〈rg2〉 16 Kc θ 1 ) 1 + π2n12 2 sin2 + ... + 2A2c + ... ∆Rθ 3 2 Mw λ
()
(1)
where Mw is the weight-average molecular weight, 〈rg2〉 the meansquared radius of gyration, n1 the solvent refractive index, λ the incident light wavelength, and A2 the second virial coefficient for a virial expansion in concentration. K is the constant related to the refractive index, and ∆Rθ is the excess Rayleigh ratio. The critical aggregation concentration (cac) was obtained from fluorescence excitation spectra of pyrene as a fluorescent probe. A known amount of pyrene in acetone was added to each 10 mL vial and dried in 50 °C. The sample solutions were introduced to the vials to give a pyrene concentration in the final solution of 6 × 10-7 M. Steady-state fluorescence spectra were measured using a Perkin-Elmer luminescence spectrometer (LS 50B) with a bandwidth of 2.5 nm for excitation and emission. The excitation spectra were obtained using an emission wavelength of 390 nm. Turbidity was measured at 500 nm using a UV/vis spectrophotometer, and surface tension measurement was carried out at various concentrations with a Kru¨ss K8 tensiometer when the temperature of the sample cell was equilibrated.
Results and Discussion Synthesis. Amphiphilic graft copolymers composed of poly(aspartic acid) as the hydrophilic backbone and octadecyl chain as the hydrophobic segment were successfully synthesized. After hydrolysis of PSI with NaOH, PSI was converted to poly(sodium aspartate). The FTIR analysis showed that the octadecyl group was grafted to the PASP backbone. C-H stretches of grafted octadecyl chain exhibited additional absorption bands in 2852 and
Amphiphilic Poly(aspartic acid)
Langmuir, Vol. 17, No. 24, 2001 7503
Table 1. Degree of Substitution of Octadecyl Groups in PASP-C18
sample
feed mole ratio (succinimide unit/octadecylamine)
PASP C18-D2 C18-D5 C18-D8 C18-D10
100/0 98/2 95/5 92/8 90/10
DS by NMR (%)
av no. of C18 groups per chain
weight fraction of alkyl groups (%)
0.7 2.8 5.1 8.8
0.88 3.5 6.4 11.0
1.6 6.3 11.1 18.3
1H
2925 cm-1 compared with PASP. In the 1H NMR spectra of PASP, two chemical shifts of methine proton appeared in 4.49 and 4.69 ppm by two different ring-opening manners,21,23 and succinimide units were hydrolyzed to give aspartic acid via R- and β-opening. The ratio was 25/75, determined by the integral value of each methine proton in 1H NMR spectra, indicating that β-opening occurred more dominantly in hydrolysis than R-opening. The degree of substitution (DS) is the mole percent of the attached unit with alkyl group per total succinimide unit. Those of the alkyl group of PASP-C18 were determined by 1H NMR as shown in Table 1.For determining DS of PASP-C18, the succinimidyl ring of PSI-C18 was opened by ethanolamine, resulting in poly(2-hydroxyethyl aspartamide)-C18 having solubility in DMSO-d6, because of the insoluble property of PASP-C18 in NMR solvents (DMSO-d6, DMF-d7, CDCl3). Size and Stability of Aggregate. The size distribution of aggregates in aqueous solutions of PASP-C18 with different DS was determined by DLS. Results showed a bimodal size distribution, as presented in Figure 2a. Though the size distribution was not monodisperse due to the presence of small primary and large secondary aggregates, number-average mean diameters were reduced as DS increased, as shown in Figure 2b. Most theories of micelle formation are based on the equilibrium thermodynamic approach. However, block copolymers of hydrophobic segment with high Tg or Tm do not lead to micellar aggregates in a thermodynamic equilibrium state due to the frozen state of the material.17,27 In the same manner, the aggregate formation of PASPC18 is not carried out under equilibrium due to the stiffness of the PASP backbone at room temperature. Hence, the aggregation behavior of PASP-C18 in the aqueous solution should be regarded as a kinetically controlled process. Practically, the incubation of aggregate solution in 60 °C with sonication dramatically reduced the intensity of secondary aggregate by breaking hydrophobic domains formed by intermolecular interaction and reconstitution of secondary into primary aggregate, while the size of primary aggregate was maintained as shown in Figure 3. The secondary aggregates are relatively weak and easily dissociated under small thermal energy. In general, amphiphilic comb-type polymers with hydrophobic side chains can facilitate primary aggregate formation by association with neighboring hydrophobic side chains in the same backbone. Association with hydrophobic side chains in other polymers constitutes secondary aggregate formation that is governed by a kinetically controlled process. In a stiff backbone such as PASP, backbone bending for hydrophobic domain formation is so difficult that secondary aggregate formation by intermolecular interaction is favored over primary aggregate formation by intramolecular interaction. However, (27) Poppe, A.; Willner, J.; Allgaier, J.; Stellbrink, J.; Richter, D. Macromolecules 1997, 30, 7462.
Figure 2. (a) Bimodal size distribution of C18-D10. (b) Plot of number-average mean diameter vs DS of octadecyl group.
hydrophobic domains by intramolecular interaction are preferentially formed because the backbone stiffness lessens on heating, resulting in dominant primary aggregate formation. In general, hydrophobically modified polymers of flexible backbone may easily form primary aggregate by dominant intramolecular interaction between neighboring hydrophobic side chains.3,8 The reasoning indicates that secondary aggregates are formed by the kinetically controlled process and governed by the backbone stiffness. Aggregate systems of PASP-C18 are stabilized by strong hydrophobic interaction acting as noncovalent cross-linker and charge repulsion.25 Zeta-potentials of PASP-C18 series also confirm the stabilization of self-aggregates by the charge repulsion in Figure 4. All aggregate solutions show remarkable negative surface charges due to the anionic aspartic backbone in distilled water. However, zetapotentials of each aggregate solution decrease to around -10 mV at pH 3.2, indicating the stability of each solution becomes low. Ultimately, zeta-potentials show remarkable cationic charges due to the protonation of aspartic backbone at pH 1.0, which results in macroscopic precipitates. After the sonication of solutions, the effective diameter of aggregates of 10% DS remained constant up to 1 month
7504
Langmuir, Vol. 17, No. 24, 2001
Kang et al.
Figure 3. Size distribution of C18-D10 after reconstitution by melting of octadecyl chain. Inset represents the dependence of light scattering intensity on heating, indicating disorganization of secondary aggregate by breaking hydrophobic domains of intermolecular interaction.
Figure 5. Effective diameter change of C18-D10 (O) and C18D2 (b) with time.
Figure 4. Zeta-potentials of self-aggregates of PASP-C18 series in various pH.
Figure 6. Zeta-potential change of self-aggregates of PASPC18 series with time.
while that of 2% DS increased to the equilibrium diameter as shown in Figure 5. Aggregates of 2% DS were loosely formed and reconstituted to a thermodynamically stable conformation with time. The interaction strength of alkyl chains in hydrophobic domains is directly related to DS of alkyl chains.28 Hydrophobic domains by strong intramolecular hydrophobic interaction maintains bending of the stiff PASP backbone, whereas loosely formed hydrophobic domains are disorganized and secondary aggregate formation by intermolecular interaction is facilitated because of inability to maintain bending the stiff PASP backbone. Secondary aggregate formation related to a charged backbone with hydrophobic side chains of low DS has been previously reported.29 Though all PASP-C18 series showed remarkable negative surface charges on measuring zeta-potentials after sonication, the aggregate solutions of lower DS showed lower surface charges than the initial values after 20 days at 4 °C, while that of higher DS maintained the initial
surface charge, as seen in Figure 6. This result means that the aggregate solutions of lower DS have inherent lower stability than that of higher DS. The turbidities of the aggregate solutions of higher DS maintained their initial value, while that of lower DS increased as time went on. In colloidal interaction, DLVO theory predicts the attractive and repulsive potentials between charged particles.30 PASP-C18 with lower DS loses repulsive potentials and migration toward a primary minimum of interaction potential induces interparticle coagulation. However, the colloidal systems of PASP-C18 with higher DS exist in a secondary minimum of interaction potential because the system maintains the balance between attractive and repulsive potentials, maintaining the aqueous stability. Migration of secondary minimum toward primary minimum of interaction potential is induced mainly by predominant hydrophobic interaction over charge repulsion. The migration is directly relevant
(28) Petit-Agnely, F.; Iliopoulos, I.; Zana, R. Langmuir 2000, 16, 9921. (29) Yamamoto, H.; Morishima, Y. Macromolecules 1999, 32, 7469.
(30) Hiemenz, P. C.; Rajagopalan, R. Principles of Colloid and Surface Chemistry, 3rd ed.; Marcel Dekker: New York, 1997; Chapter 13.
Amphiphilic Poly(aspartic acid)
Langmuir, Vol. 17, No. 24, 2001 7505
Figure 8. Fluorescence excitation spectra of C18-D10 with varying concentration.
Figure 7. (a) Surface tension of PASP-Cn as a function of alkyl chain length with varying concentration. (b) Surface tension of PASP-Cn as a function of alkyl chain length with increasing temperature (concentration: 10 mg/mL). Table 2. Second Virial Coefficients of PASP-C18 Aggregates Calculated from SLS sample A2
[10-6
cm3
mol
g-2]
C18-D2
C18-D5
C18-D8
C18-D10
-2.48
-2.21
2.02
3.84
to the relaxation of stiff PASP backbone. In initial sonicated PASP-C18 solutions, the aggregates may be formed under high stiffness on backbone. However, they should be relaxed in time along with the kinetically controlled process to decrease the stiffness of the stiff PASP backbone. Results of SLS support the stability issue of PASP-C18, as shown in Table 2. In the case of 2 and 5% DS, water is a marginally poor solvent, indicating that grafted alkyl groups are loosely associated and contacts water more easily. On the other hand, the graft copolymers of 8% and 10% exist in environments of marginally good solvent because of the difficulty of water contact to the strongly associated hydrophobic domains. Hence, loosely associated aggregates showed low time stability as explained earlier. Extended chain conformation is favorable at lower DS due to backbone stiffness, whereas collapsed conformation is forced at higher DS by stronger hydrophobic interaction.29 Surface Tension. Surface tension of the aqueous aggregate solution is shown in Figure 7a. The surface tension of PASP (concentration ) 10 mg/mL) was 63.1. The surface tensions of C18-D10 showed higher values than that of PASP despite that hydrophobic octadecyl
groups were added. The surface tension was independent of the concentration of C18-D10. Also, the surface tension of other PASP-C18 series showed higher values than that of PASP, irrespective of DS. However, PASP-Cn (n ) 12, 16) showed the typical trend of surface tension of surfactant with concentration. The result means that PASP-Cn (n ) 12, 16) not only exists at the air-water interface to lower surface tension but also forms aggregates, while PASP-C18 exists solely as aggregates. Grafted C12 and C16 have higher flexibility at room temperature and show the typical trend of surfactants, while grafted C18 is in a frozen state under its melting temperature and shows abnormal behavior due to the nature of the physical cross-linker of frozen octadecyl chains. Self-aggregates of cholesterol-bearing pullulan showed the same result that surface tension was kept unchanged even at a higher concentration of the aggregate solution.3 It was suggested that self-aggregates form colloidally stable nanoparticles and the hydrophobic core is stably covered by the hydrophilic shell of the polysaccharide skeleton. This trend is assumed to be caused by the rigidity of the grafted hydrophobic moiety. Figure 7b shows the change of surface tension as temperature changes. As temperature is elevated, the surface tensions of PASP-C18 decrease because the grafted alkyl chain enters a fluidlike phase and loses its physical crosslinker nature and PASP-C18 moves to the air-water interface. Therefore, the aqueous solutions of the aggregates with rigid hydrophobic moiety such as cholesterol and long alkyl chains could exhibit the surface tension of water irrespective of the concentration. Critical Aggregation Concetration. The excitation spectra of pyrene in aggregate solutions were obtained as the concentration of PASP-C18 increased. As shown in Figure 8, the excitation spectra show peaks of the (0,0) band at 333 nm in the low concentration range of PASPC18, indicating that polymer does not form aggregates in dilute solutions. As the concentration increases, the spectra show a shift to 336 nm, which means that grafted alkyl chains start to form hydrophobic domains and polymer forms the aggregates in aqueous solution. The dependence of I336/I333 with the polymer concentration is shown in Figure 9. In the dilute regime, the magnitude of I336/I333 is almost constant. As the concentration increases, I336/I333 ratio rises in a sigmoidal shape. The sigmoidal rising of the ratio means that pyrene binds to the associating amphiphiles prior to aggregate formation, followed by pyrene partitioning into the hydrophobic core.
7506
Langmuir, Vol. 17, No. 24, 2001
Kang et al.
Figure 9. I336/I333 ratio of C18-D10 with varying concentration.
The critical aggregation concentration (cac) was determined as the intersection point of the slopes of a flat region in the dilute concentration and a dramatic rising region in the plot. The cac showed decreasing behavior as the grafting density of alkyl groups increased, like the increasing hydrophobicity of low molecular weight surfactants reduces the cmc values. For most low molecular weight surfactants in aqueous solution, the free energy of micellization is proportional to the length of the alkyl chain as described in following form:31
∆G° ) RT log cmc
(2)
For homologous straight-chain ionic surfactants in an aqueous solution, a relation between cmc and the number of carbon atoms, N, in the alkyl chain was found as the following form:11
log cmc ) A - BN
(3)
where A and B depend on the specific ion type and the temperature. However, the relation can be modified into eq 4 in the case of an anionic copolymer grafted with an alkyl chain.
log cac ) A - B[DS]
(4)
The cac of PASP-C18 series as well as other works showed a linear relationship according to eq 4, as shown in Figure 10. DS of PASP-C18 series can be correlated with the length of alkyl groups in the above relation. (31) Mayers, D. Surfactant Science and Technology; VCH Publisher Inc.: New York, 1988; Chapter 3.
Figure 10. Critical aggregation concentration as a function of DS plotted by eq 4: (a) dextran modified with cholic acid;8 (b) dextran modified with deoxycholic acid;8 (c) present work; (d) chitosan modified with deoxycholic acid.9
Conclusion PASP with long alkyl chains were successfully synthesized from PSI with various DS of alkyl chains. Selfaggregates of PASP-C18 showed bimodal size distribution of primary and secondary aggregates. The factors determining the physicochemical properties of self-aggregates may include DS, the length of grafted alkyl chains, and the backbone stiffness. Bending of stiff PASP backbone and strong association of alkyl chains were competing factors for determining aqueous stability. When stiffness of the PASP backbone was overcome by stronger hydrophobic interaction, the stability of aggregate solution was maintained. Loose hydrophobic domains based on intramolecular interaction were disorganized due to the inability of maintaining PASP bending, and secondary aggregate formation by intermolecular interaction was facilitated. Aqueous solution of PASP-C18 was not surface-active from the physical cross-linker nature of octadecyl chains, while dodecyl and hexadecyl chains with higher chain flexibility showed surface-active properties. Fluorescence excitation spectra showed formation of hydrophobic domains and were used for calculation of cac. The cac showed logarithmically decreasing behavior as DS of alkyl chains. Acknowledgment. This work was supported in part from the Brain Korea 21 Project and from the National Research Laboratory (NRL) program (Project No. 2000-N-NL-01-C-270) by the Ministry of Science and Technology, South Korea. LA0107953
