Langmuir 1995,11, 4703-4711
4703
Micellization of Diblock and Triblock Copolymers in Aqueous Solution. New Results for Oxyethylene/ Oxybutylene Copolymers E38B12 and E21BllE21. Comparison of Oxyethylene/Oxybutylene,Oxyethylene/Oxypropylene, and Oxyethylene/Alkyl Systems Yung-Wei Yang, Nan-Jie Deng, Ga-Er Yu, Zu-Kang Zhou,? David Attwood, and Colin Booth* Manchester Polymer Centre, Departments of Chemistry and Pharmacy, University of Manchester, Manchester M13 9PL, U.K. Received March 29, 1995. I n Final Form: September 6, 1995@ Two oxyethyleneloxybutyleneblock copolymers of similar chain length and composition, E38B12 and E21B11E21, were prepared by sequential anionic polymerization and characterized by gel permeation chromatography and NMR spectroscopy. Their association in dilute aqueous solution was investigated by static and dynamic light scattering. Both copolymers associated by closed processes to form micelles, those of the diblock copolymer being the larger. The increment per B unit in the standard Gibbs energy of micellizationof diblock E,B, copolymers was found to be 2.0kJ mol-l. The critical micelle concentrations of oxyethyleneloxybutylene(E,B, and E,B,E,), oxyethylenelalkyl(E,C,), and oxyethyleneloxypropylene (EmPnEm) copolymers are compared, and the effect of chain architecture (diblock and triblock) on the association of diblock and triblock copolymers is discussed.
1. Introduction The micellization of water-soluble block copolymers is a process of academic interest with important industrial and medical applications. Much work has been carried out on oxyethyleneloxypropylene triblock copolymers E,P,E,. Here E represents a n oxyethylene unit and P a n oxypropylene unit. The effect of block structure on micellization and gelation is of obvious interest, but investigation of this aspect has been held back through lack of suitable samples. Triblock P,E,P, copolymers, also available commercially though in limited range, have been much less studied, while diblock E,P, copolymers, which must be specially synthesized, have been largely neglected. The current state of the work on oxyethylenel oxypropylene copolymers can be judged from recent publications and the references contained therein.lT9 Much of our recent work10-16has been concentrated on oxyethyleneloxybutylene diblock and triblock copolymers, i.e., copolymers oftype E,B,, E,B,E,, and B,E,B,, where
B represents a n oxybutylene unit. Limited ranges ofE,B, and E,B,E, copolymers have become available recently from The Dow Chemical C0.17J8 However, the copolymers used in this study were synthesized in our laboratory. The use of oxyethyleneloxybutylene block copolymers avoids serious problems of purity in commercial E P copolymers. Many E,P,E, copolymers examined in this laboratory had two peaks (or a peak and a shoulder) in their gel permeation chromatography (GPO curves: examples of such curves or corresponding chain length distributions have been p u b l i ~ h e d . ~ JThe ~ , ~effect ~ is probably related to chain transfer, a side reaction inherent in the anionic polymerization of propylene oXide21*22 but absent in that of butylene oxide.23 The comparative homogeneity of laboratory-synthesized E/B copolymers has been demonstrated.10-16 Our work has verified the high hydrophobicity of B units (compared with P), a point first made by Lee et aLZ4A study15ofthe effect of B-block length in diblock copolymers E30Bnshowed that micelles are formed in aqueous solutions of these copolymers when n is as small as 4,e.g., micelles of mass-average association numberN,,, x 10for copolymer
EPSRC Visiting Fellow. Permanent address: Department of Chemistry, New York StateUniversity at Stony Brook, Stony Brook, +
NY 11794-3400. Abstract published in Advance ACS Abstracts, November 1, 1995. (1) Wanka, G.; Hoffmann, H.; Ulbricht, W. Macrmolecules 1994,27, 4145. (2)Schillen, K.; Brown, W.; Johnsen, R. M. Macromolecules 1994, 27,4825. (3)Mortensen, K.;Brown, W.; Jorgensen, E. Macromolecules 1994, 27,5654. (4)Zhou, Z.-K.; Chu, B. Macromolecules 1994,27,2025. (5)Alexandridis, P.; Holzwarth,J. F.; Hatton, T. A. Macromolecules 1994,27,2414. (6)Mortenson, K.; Pedersen, J. S. Macromolecules 1993,26, 805. (7)Yang, L.;Bedells, A. D.; Attwood, D.; Booth, C . J . Chem. Soc., Faraday Trans. 1992,88,1447. Wang,Q.-G.;Attwood,D.;Price, (8)Yu,G.-E.;Deng,Y.-L.;Dalton,S.; C.; Booth, C. J . Chem. SOC.,Faraday Trans. 1992,88, 2537. (9)Malmsten, M.; Lindman, B. Macromolecules 1992,25,5440. (10)Luo, Y.-Z.; Nicholas, C. V.; Attwood, D.; Collett, J. H.; Price, C.; Booth, C. Colloid Polym. Sci. 1992,270,1094. (11)Nicholas, C. V.; Luo, Y.-Z.; Deng, N.-J.; Attwood, D.; Collett, J. H.; Price, C.; Booth, C. Polymer 1993,34,138. (12)Luo,Y.-Z.; Nicholas, C. V.; Attwood, D.; Collett, J. H.; Price, C.; Booth, C.; Zhou, Z.-K.; Chu, B. J . Chem. Soc., Faraday Trans. 1993,89, 539. @
0743-746319512411-4703$09.0010
(13)Bedells,A.D.;Arafeh,R. M.;Yang,Z.;Attwood, D.;Heatley,F.; Padget, J. C.; Price, C.; Booth, C. J . Chem. SOC.,Faraday Trans. 1993, 89,1235. (14)Bedells, A. D.; Arafeh, R. M.; Yang, Z.; Attwood, D.; Padget, J. C . ; Price, C.; Booth, C. J . Chem. SOC.,Faraday Trans. 1993,89,1243. (15)Tanodekaew, S.;Deng, N.-J.; Smith, S.; Yang, Y.-W.; Attwood, D., Booth, C. J . Phys. Chem. 1993,97,11847. (16)Yang, Z.; Pickard, S.; Deng, N.-J.; Barlow, R. J.; Attwood, D.; Booth, C. Macromolecules 1994,27,2371. (17)Nace, V.M.; Whitmarsh, R. H.; Edens, M. W. J . A m . Oil Chem. SOC.1994,71,777. (18)Dow Chemical Co., Freeport, TX.B-Series Polyglycols. Butylene Oxide I Ethylene Oxide Block Copolymers; Technical Literature; 1994. (19)Luo, Y.-Z.; Stubbersfield, R. B.; Booth, C. Eur. Polym. J . 1983, 19,107. (20)Wang, Q.-G.; Price, C.; Booth, C. Eur. Polym. J . 1993,29,665. (21)Ding, J.-F.;Heatley,F.;Price, C.;Booth, C.Eur.Polym. J . 1991, 27,895. (22)Yu, G.-E.; Masters, A. J.; Heatley, F.; Booth, C.; Blease, T. G. Macromol. Chem. Phys. 1994,195,1517. (23)Heatley, F.; Yu, G. E.; Sun, W. B.; Pywell, E. J.; Mobbs, R. H.; Booth, C . Eur. Polym. J . 1990,26,583. (24)Lee, J. H.;Kopecek, J.; Andrade, J. Polym. Mater. Sci. Eng. 1987,57,613.
0 1995 American Chemical Society
4704 Langmuir, Vol. 11, No. 12, 1995
Yang et al.
Table 1. Characteristicsof the Block Copolymers precursor block E38 Bii
Mpk/(gmol-') (GPC") 1630 730
Mw/Mn Md(g mol-') (GPCb) (NMR) 1.05 1670 1.02 790
copolymer E38B12 EziBiiEzi
Mpk/(gmol-') (GPO) 2440 2700
Mw/Mn XE~ (GPCb) 1.06 0.760 1.07 0.792
Mn/(gmol-') (NMR) 2540 2640
Mw/(gmol-') (NMR, GPCd) 2690 2820
Molar mass as if the sample is poly(oxyethy1ene). Corrected for spreading. Mole fraction of oxyethylene. Calculated from M, and Mw/Mnof the copolymer.
in solution a t 30 "C. Also, we have reported16 a critical micelle concentration (cmc) of 3.0 g dm-3 for copolymer E21BsE21 in aqueous solution a t 40 "C, which can be compared with similar values (1.5-3.5 g dm-3) reported r e ~ e n t l y ' ,for ~ copolymer P65 (ElgP30E19) in aqueous solution a t 40 "C. A useful approximation sets one B unit equivalent to four P units: see below for further discussion of this point. In our previous study,16 the association of copolymer E21BsE21was compared with that of copolymer E&. The two copolymers were of similar molar mass and composition, i.e., number-average molar mass M , x 2400 g mol-' and mole fraction of E units X E 0.84. Light scattering was used to show that both copolymersin aqueous solution associated by closed processes. Critical micelle concentrations for association at 40 "C, detected by surface tension and light scattering methods, were 0.3 g dm-3 (E41B8)and 3 g dm-3 ( E ~ I B ~ EMass-average ~~). association numbers a t 35 "C were 78 for copolymer E41B8 but only 5 for copolymer E21BsE21. Several theoretical studies of association of block copolymers have been published in recent times, many of which spring from the related problem of microphase separation in bulk and concentrated solution systems. The topic has been r e ~ i e w e d ,and ~ ~ an ' ~ advanced ~ treatment has appeared r e ~ e n t l y . ~Here ' we note that most theories do not address the present interest, being directed toward the simplest problem of diblock copolymers. The general proposition28that the entropy loss associated with looping of the middle block of a triblock copolymer affects its micellization is not in doubt, but the particular importance of the effect for micellization in a nonsolvent for the inner block is not well researched. Linse's recent theoretical examination of m i c e l l i ~ a t i o ndirectly ~~ addresses the problem, including as it does predictions of the effect of the block structure on the association number and critical micelle concentration. His calculations29~30 have been directed toward E/P copolymers. The predictions for copolymers of identical overall chain length and composition (and solutions of given 'I? are29 (i) critical micelle concentration in the order E,P, < E,P,E, and (ii) association number in the order E,P, > E,P,E,. These predictions are broadly consistent with the results of our previous studies16of ED3 copolymers (E41B8and EzlBaEzd described above, but not those7for E/Pcopolymers related to Pluronic L64 (E&g and E13P30E13) which were found to exhibit similar, though limited, extents of association in dilute solution. The present study was undertaken to provide a direct comparison of the micellization of copolymers of similar chain length and composition but different chain architectures (E,B, and E,B,E,, both of which formed welldeveloped micelles (i.e., micelles with high association (25) Brown, R. A.; Masters, A. J.; Price, C.; Yuan, X.-F. In Comprehensive Polymer Science; Booth, C., Price, C., Eds.; Pergamon Press: Oxford, 1989; Vol. 2, Chapter 6. (26) Tuzar, Z.; Kratochvil, P. Insurface and Colloid Science; Matijevic, E., Ed.; Plenum: New York, 1993. (27) Yuan, X.-F.;Masters, A. J.; Price, C. Macromolecules 1992,25, 6876. (28) ten Brinke, G.; Hadziioannou, G.Macromolecules 1987,20,486. (29) Linse, P. Macromolecules 1993,26,4437. (30) Linse, P. J.Phys. Chem. 1993, 97, 13896.
numbers)). As described below in sections 2 and 3, this was achieved by the preparation of copolymers and E21BllE21(blocklengths known to fl).This workenlarged the data base for ED3 copolymers to an extent which made possible (sections 4 and 5) a meaningful comparison of their association behaviors with those of oxyethylene/alkyl (E,C,) and oxyethylene/oxypropylene (E,P,E,) copolymers. Two related interests, stemming from previous work, will be reported in the near future. One concerns the association of triblock copolymers in a nonsolvent for the end blocks, which has been addressed in a recent study31 of the series ofcopolymers B4E40B4, B5E39B5, and B ~ E ~ o B ~ . A second32lies in the thermally-reversible sol gel sol (liquid-crystal) transitions observed in concentrated micellar solutions of the present block copolymers.
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2. Experimental Section 2.1. Preparation of Copolymers. The copolymers E38B12 and EzlBllEzl were prepared by sequential anionic polymerization of ethylene oxide and 1,a-butylene oxide. All reagents were distilled and dried before use, and vacuum line and ampule techniques were used in order to minimize initiation by moisture a t any stage. Initiator solutions were prepared by reacting freshly-cut potassium with either 2-(2-methoxyethoxy)ethanol (for the diblock copolymer) or 1,2-butanediol (for the triblock copolymer). A ratio of [OHl/[OKl = 9 was used to ensure a controlled rate ofpolymerization. The copolymerization methods followed closely those described previously.16 A difference was in the preparation of the diblock copolymer, since in the present work the E block was polymerized first. This difference in preparation meant that the B block of copolymer E38B12 carried a hydroxy end group. 2.2. Characterization of Copolymers. The prepared samples were characterized by gel permeation chromatography (GPC) and nuclear magnetic resonance spectroscopy (NMR).The GPC system consisted of four p-Styragel columns (Waters Associates, nominal porosities from 500 to lo6 A) eluted by tetrahydrofuran (THF) at room temperature and a flow rate of 1cm3min-l. Samples were dissolved in THF at a concentration of 2 g dm-3, and their emergence was detected by differential refractometry (Waters Associates Model 410). Calibration was with a series ofpoly(oxyethy1ene) samples ofknown molar mass, and derived molar masses were determined as if the samples were poly(oxyethy1ene). For 13C NMR spectroscopy, polymer samples were dissolved in CDC13 (0.1 g cm-3) and spectra were obtained by means of a Varian Unity 500 spectrometer operated a t 125.5 MHz. Spectral assignments were taken from previous work.23 The GPC curves directly gave values of the molar mass at the peak (Mpk),and further analysis gave a n estimate of the ratio of mass-average to number-average molar mass (Mw/Mn).The 13CNMR spectra were used primarily to determine the numberaverage molar mass (M, from integrals of end and chain groups) and average composition (i.e., mole fraction of E, XE). The molar masses and molecular formulas ofthe copolymers listed in Table 1were obtained by combining the number-average molar masses of the precursor blocks and the compositions of the copolymers, both determined by NMR. It was noted that all hydroxy end (31)Yang, Y.-W.; Yang, Z.; Zhou, Z.-K.; Attwood, D.; Booth, C. Macromolecules, in press. (32) Yang, Y.-W.;Zhou, Z.-K.; Barlow, R. J.;Mi-Adib, Z.; McKeown, N.; Booth, C. To be published.
Micellization of Diblock a n d Triblock Copolymers groups of the B blocks were secondary, and that no unsaturated end groups were present. 2.3. Purification of Copolymers. The purity of the precursor copolymers was verified by comparing the intensities of NMR resonances from end group carbons with those from initiator carbons. In all cases the intensities were identical within experimental error. Similarly, the purity ofthe block copolymers was checked by comparing the intensities of resonances from end group carbons with those from carbons associated with EB block junctions. In each case a small excess ( 50, and are either (0)uncorrected members: E41B8,EZ4B10, and E50B13. The results are or (W) corrected to a common E block length of m = 40 units plotted as AmicGoagainst n in Figure 11. Scatter in the (using d log(cmc)/dm =Z +0.018;see the text). data caused by the variation in the E block length was example, compared a t constant temperature and compoessentially eliminated by correcting all data to a common sition, the cmc and association number of a diblock E block length of m = 40. This was done by making use copolymer are predicted to be smaller and larger, respecof the result (dAmi,Go/dmx +0.10 kJ mol-') reported47for tively, than those of a triblock copolymer: see Figures 17 E,C, block copolymers, a n advantage in that work being and 18 of ref 29. precise control of uniform C block length while m was varied. The slope of the line through the corrected data is 2.00 i0.02 k J mol-', compared with 2.5 f 0.1 k J mol-l Acknowledgment. We thank Mr. K. Nixon and Dr. obtained from the full data set without correction, i.e., F. Heatley for help in characterizing the copolymers by from the slope of log (cmc) versus n in Figure 9a. GPC and NMR. Financial support came from the Gov5.4. Comparison with Theory. Quantitative comernment of the Republic of China and the Engineering parison with Linse's theoryz9 would require specific and Physical Science Research Council. calculations based on thermodynamic data for the E/B LA950251Z system, which are not available a t the present time. Experiments are in hand to provide this i n f ~ r m a t i o n . ~ ~ Qualitatively the present results are as predicted. For (53) Malmsten, M. Private communication.
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