Langmuir 1997, 13, 5837-5848
5837
Effect of Block Architecture on the Self-Assembly of Copolymers of Ethylene Oxide and Propylene Oxide in Aqueous Solution Haydar Altinok, Ga-Er Yu, S. Keith Nixon, Peter A. Gorry, David Attwood, and Colin Booth* Manchester Polymer Centre, Department of Chemistry and School of Pharmacy, University of Manchester, Manchester M13 9PL, U.K. Received June 5, 1997. In Final Form: August 8, 1997X Five block copolymers with three linear architectures were prepared by sequential anionic copolymerization, i.e., E102P37, E103P34, E52P34E52, E57P46E57, and P19E113P19 (where E denotes oxyethylene and P denotes oxypropylene), and the properties of their aqueous solutions were studied across a wide range of concentration. The techniques used to study micellization and micellar properties in dilute solution were static and dynamic light scattering, surface tension, and elution gel-permeation chromatography. The results showed, for example, that critical micelle temperatures for solutions of given composition fell in the order EmPn < EmPnEm < PnEmPn, in agreement with the prediction of theory. The gelation of concentrated solutions was also investigated.
1. Introduction The effect of block architecture on the self-assembly of copolymers in solution is a topic of importance to industry, as well as being of academic interest. We have in mind the simpler architectures, i.e., linear diblock, cyclic diblock, and triblock. Considering oxyethylene/oxypropylene copolymers (E/P copolymers), for which water is a selective solvent, these can be represented as EmPn, cyclo-PnEm, EmPnEm, and PnEmPn, where E denotes hydrophilic oxyethylene, OCH2CH2, and P denotes hydrophobic oxypropylene, OCH2CH(CH3). Recent reviews,1 review articles,2 and papers3,4 carry extensive lists of references. Of the various architectures, the commercially available EmPnEm triblock copolymers (e.g., Pluronic, BASF; Synperonic PE, ICI C&P), have been most studied.1a,2-4 There just two reports of substantial work on PnEmPn copolymers,5,6 two on EmPn copolymers,7,8 and none on cyclo-PnEm copolymers. There may be problems of comparability in studies involving commercial products,9 and we see advantage in the use of laboratory synthesized materials. To the best of our knowledge no self-contained study involving synthesis and properties and covering the range of architectures has been published for E/P copolymers. Our interest in the matter stems largely from our work on the self-assembly of oxyethylene/oxybutylene copolymers, i.e. E/B copolymers, B ) oxybutylene, OCH2CHX
Abstract published in Advance ACS Abstracts, October 1, 1997.
(1) (a) Chu, B.; Zhou, Z.-K. In Nonionic Surfactants, Polyoxyalkylene Block Copolymers; Surfactant Science Series Vol. 60; Nace, V. M., Ed.; Marcel Dekker: New York, 1996; p 67. (b) Tuzar, Z; Kratochvil, P. In Surface and Colloid Science; Matijevic, E., Ed.; Plenum Press: New York, 1993; Vol. 15, p 1. (c) Booth, C.; Yu, G.-E.; Nace, V. M. In Amphiphilic Block Copolymers. Self-assembly and Applications; Alexandridis, P., Lindman, B., Ed.; Elsevier: Amsterdam, in press. (2) (a) Almgren, M.; Brown, W; Hvidt, S. Colloid Polym. Sci. 1995, 273, 2. (b) Chu, B. Langmuir 1995, 11, 414. (3) Wanka, G.; Hoffmann, H.; Ulbricht, W. Macromolecules 1994, 27, 4145. (4) Alexandridis, P.; Holzwarth, J. F.; Hatton, T. A. Macromolecules 1994, 27, 2414. (5) Zhou, Z.-K.; Chu, B. Macromolecules 1994, 27, 2025. (6) Mortensen, K.; Brown, W.; Jorgensen, E. Macromolecules 1994, 27, 5654. (7) Yang, L.; Bedells, A. D.; Attwood, D.; Booth, C. J. Chem. Soc., Faraday Trans. 1992, 88, 1447. (8) Nace, V. M. J. Am. Oil. Chem. Soc. 1996, 73, 1. (9) Yu, G.-E.; Altinok, H.; Nixon, S. K.; Booth, C.; Alexandridis, P.; Hatton, P. A. Eur. Polym. J. 1997, 33, 673.
S0743-7463(97)00586-6 CCC: $14.00
(C2H5). Work on a series of copolymers with different architectures based on eight B units has been reported in interconnected papers,10-12 including distributional effects,12,13 while similar studies of copolymers containing 12 B units carried the advantage that their concentrated solutions gelled across the range of architectures.14-16 Considering micelle formation, one B unit is equivalent in hydrophobicity to four P units;14,15,17 hence the detailed effects of block architecture may well differ between the two systems. Additional motivation for the present study arose because the major industrial/commercial effort relates to E/P systems. Results for copolymers with three linear architectures are presented, the work being centered on copolymers with P blocks in the range P34-38 and overall composition 67-70 wt % E, i.e., E102P37, E103P34, E52P34E52, and P19E113P19. A second triblock copolymer was prepared with a different length of P block (E57P46E57), and results for this copolymer are included in the account given below. Copolymer E52P34E52 is similar to the commercial copolymer F77 (nominally E52P35E52). Results for cyclic diblock copolymer cyclo-P34E104 are compared with those for copolymer E52P34E52 in a second paper.18 2. Preparation and Characterization of Copolymers 2.1. Transfer Reaction. The preparation of EmPnEm block copolymers by sequential copolymerization of propylene oxide (PO) followed by ethylene oxide (EO) can be complicated by the so-called transfer reaction, i.e., hydrogen abstraction from the methyl group of propylene (10) Yang, Z.; Pickard, S.; Deng, N.-J.; Barlow, R. J.; Attwood, D.; Booth, C. Macromolecules 1994, 27, 2371. (11) Yu, G.-E.; Yang, Z.; Attwood, D.; Price, C.; Booth, C. Macromolecules 1996, 29, 8479. (12) Yu, G.-E.; Yang, Z.; Ameri, M.; Attwood, D.; Collett, J. H.; Price, C.; Booth, C. J. Phys. Chem. B 1997, 101, 4394. (13) Nace, V. M.; Whitmarsh, R. H.; Edens, M. W. J. Am. Oil Chem. Soc. 1994, 71, 777. (14) Yang, Y.-W.; Deng, N.-J.; Yu, G.-E.; Zhou, Z.-K.; Attwood, D.; Booth, C. Langmuir 1995, 11, 4703. (15) Yang, Y.-W.; Yang, Z.; Zhou, Z.-K.; Attwood, D.; Booth, C. Macromolecules 1996, 29, 670. (16) Yang, Y.-W.; Ali-Adib, Z.; McKeown, N. B.; Ryan, A. J.; Attwood, D.; Booth, C. Langmuir 1997, 13, 1860. (17) Yu, G.-E.; Yang, Y.-W.; Yang, Z.; Attwood, D.; Booth, C.; Nace, V. M. Langmuir 1996, 12, 3404. (18) Yu, G.-E.; Garrett, C. A.; Mai, S.-M., Altinok, H.; Attwood, D.; Price, C.; Booth, C. Submitted for publication, Langmuir.
© 1997 American Chemical Society
5838 Langmuir, Vol. 13, No. 22, 1997
Altinok et al.
Table 1. Initiators for the E/P block copolymers
Table 2. Molecular Characteristics of the E/P Block Copolymersa
copolymer
initiator
E102P37, E103P34
diethylene glycol monomethyl ether CH3(OCH2CH2)2OH PPG2000a propylene glycol HOCH2CH(CH3)OH diethylene glycol H(OCH2CH2)2OH
E52P34E52 E57P46E57 P19E113P19 a
R-Hydro,ω-hydroxypoly(oxypropylene), Mn ) 2000 g mol-1.
oxide (reaction 1) rather than propagation by ring opening (reaction 2):
HPnO-K+ + PO f HPnOH + CH2dCHCH2O-K+ (1) HPnO-K+ + PO f HOPn+1O-K+
(2)
Because of the rapid interchange between OH and O-K+, groups all chains grow at the same rate, but the new chains are monofunctional with respect to OH groups and so, on addition of E, lead to diblock copolymers PnEm in the second stage of copolymerization. In the present work the effect was minimized by working at a high mole ratio of OH to O-K+, as described elsewhere,19 but was not entirely eliminated. Of course, the same reaction occurred when preparing EmPn and PnEmPn copolymers, but in those cases the PO was added last, and the resulting homopoly(oxypropylene) was readily separated from the required product. In order largely to avoid the problem, the preparation of copolymer E52P34E52 was started from a commercial polypropylene glycol (PPG2000), which was known to be highly difunctional. This material (a sample of Emkapyl 2000) was a gift from ICI C&P plc (Wilton) through the good offices of Dr. T. G. Blease. 2.2. Preparation. Liquid initiators (see Table 1 for details) were reacted with freshly cut potassium, the [OH]/ [O-K+] ratio being adjusted to control the reaction rate and the transfer reaction, and an aliquot was transferred to a weighed dried glass ampule which was fitted with a PTFE tap and could be attached to and detached from a vacuum line as needed. The exception was the reaction initiated with PPG2000, for which appropriate amounts of polymer and powdered KOH were added to the ampule, and water was evacuated by holding the mixture at 60 °C under vacuum (10-4 mmHg) for several days. EO and PO were dried (CaH) and freshly distilled before being transferred to the reaction ampule under vacuum. For polymerization, usually over a period of weeks, the ampule was immersed in a water bath and taken to successively higher temperatures in the range 40-80 °C, so ensuring that the safety limit of the glass apparatus was not exceeded while maintaining the reaction mixture in a homogeneous liquid state. Completion of reaction at any stage was checked by cooling a part of the ampule and observing the condensation (if any) of unreacted monomer. At the end of the first stage a small quantity of the homopolymer was removed in order to monitor the process. On completion of the second stage the copolymer was thoroughly evacuated (10-4 mmHg, molten state, 24 h) before storage in a freezer. As required, samples were neutralized by addition of concentrated HCl and evaporation (on the vacuum line) of excess acid and water. 2.3. Characterization. Samples were characterized by gel permeation chromatography (GPC) and 13C NMR spectroscopy. Two GPC systems were used. System A consisted of three µ-Styragel columns (Waters Associates, nominal porosity from 500 to 104 Å) eluted by tetrahy(19) Yu, G.-E.; Masters, A. J.; Heatley, F.; Booth, C.; Blease, T. G. Macromol. Chem. Phys. 1994, 195, 1517.
copolymer
mass% E
Mn/g mol-1
Mw/Mn
Mw/g mol-1
E102P37 E103P34 E52P34E52 E57P46E57 P19E113P19
68 70 70 65 69
6630 6500 6550 7680 7180
1.04 1.03 1.07 1.07 1.03
6900 6700 7000 8200 7400
a M from NMR spectroscopy, M /M from GPC, M from NMR n w n w and GPC.
drofuran (THF) at 20 °C. Sample emergence was detected by differential refractometry (Water Associates Model 410). Sample B consisted of three PL-gel columns (Polymer Laboratories, two mixed B and one 500 Å) eluted by N,N-dimethylacetamide (DMA) at 65 °C, with emerging copolymer detected by a Knauer HT differential refractometer. For each system, the flow rate was 1 cm3 min-1, samples were injected via a 100 mm3 loop at a concentration 2 g dm-3, and calibration was with poly(oxyethylene) samples of known molar mass. The GPC curves gave directly values of the molar mass at the peak (Mpk, as if the copolymers were poly(oxyethylene)), and further analysis gave an estimate of width of the molar mass distribution in the form of the ratio of mass-average to number-average molar mass (Mw/Mn). NMR spectra were recorded by means of a Varian Unity 500 spectrometer operated at 125 MHz for 13C spectra and 500 MHz for 1H spectra. Solutions were ca. 20 wt % in CDCl3. Assignments were taken from previous work.20 The integrals of the resonances from end and chain groups were used to determine average composition (i.e., mole fraction or weight fraction E) and number-average molar mass. 2.4. Purity. NMR showed that the two EmPn copolymers and P19E113P19 had an excess of ends over junctions, including unsaturated ends. Accordingly, they were purified by extraction with warm hexane (liquid state) before cooling to 10 °C and separating the crystalline copolymer. This procedure was repeated up to five times. Copolymer E57P46E57 could not be efficiently purified in this way; however the NMR spectrum showed a very low molar concentration of unsaturated chain ends. Homopoly(oxyethylene) was a possible impurity in the EmPnEm copolymers, this being derived from moisture introduced on addition of EO. Comparison of the intensities of the resonances end and junction group carbons indicated that such material was present (if at all) in very low concentration ( 90 g dm-3). A negative slope of the plot of average diffusion coefficient against concentration gave evidence of micellar linking, much as in the present work (see Figure 6a and associated discussion). Strong evidence for micellar linking has come from work on aqueous solutions of BnEmBn copolymers, where the high hydrophobicity of B blocks enhances the effect. A particularly clear demonstration can be found in a study21 of mixed micelles formed from a diblock B8E41 copolymer, which readily forms micelles in dilute solution (cmc ≈ 1 g dm-3 at 25 °C), and a triblock copolymer B12E76B12, which acts as a linking agent which can be added in different proportions to regulate the effect. Evidence of molecular linking (as well as micellar linking) has also been established for BnEmBn systems.15,48 The present results fit well into this background. 5.6. Gels. Immediate interest in the present project is in the definition of gelling systems for possible pharmaceutical use as vehicles for controlled drug release. For that reason attention was confined to the moderate concentration range (c < 25 wt %) and to copolymers with high cloud points (i.e., not P19E113P19). The general requirements in application are a mobile sol at room temperature and a hard (cubic) gel at body temperature. As can be seen from Figure 15, solutions of copolymer E102P37 of concentration 15 wt % (gel temperature 30 °C) adequately meet these requirements. The lower critical concentration for gelation (at a given temperature) of the diblock copolymer compared to the corresponding triblock copolymer mirrors the results found for E/B systems, e.g., for micellar solutions of E38B12 and E21B11E21.16 The effect has been attributed to the superior exclusion properties of the fringes of micelles formed from diblock copolymers, this being a consequence of the longer E block of a diblock copolymer compared to those of an equivalent triblock copolymer.16 A full discussion of the exclusion effect has been given elsewhere.49 Acknowledgment. The work was supported by the Engineering and Physical Science Research Council (U.K.). Kirikkale University (Turkey) provided funding for H.A. LA970586B (49) Deng, N.-J.; Luo, Y.-Z.; Tanodekaew, S.; Bingham, N.; Attwood, D.; Booth, C. J. Polym. Sci., Part B: Polym. Phys. 1995, 33, 1085.
