2034
J. Phys. Chem. 1993,97, 2034-2036
Isothermal Stability of Dilute Aqueous Solutions of Block Copolymers of Poly(0xyethylene)-Poly oxy propylene)-Poly (oxyethylene). A Microcalorimetric Study of Pluronic F87 and Pluronic F88 Joseph J. Irwin, Anthony E. Beezer,’ John C. Mitchell, C. Bucktoqt B. Z. Chowdhry,* D. Ehgland,$ and N. J. Crowthed Chemical Laboratory, The University, Canterbury, Kent, CT2 7NH, U.K. Received: November 17, 1992
Dilute aqueous solutions of block copolymers of poly(oxyethylene)-poly(oxypropylene)-poly(oxyethylene) (pluronic F87 and pluronic F88)have been subject to an isothermal microcalorimetric study at 298 K. Thermal events consistent with a phase transition in solution are observed. Some of the thermodynamic and kinetic detail of these aging phenomena are reported, as are the consequences of the presence in solution of urea and of sodium chloride.
Introduction The ABA block copolymers of poly(oxyethy1ene)-poly(oxypropylene)-poly(oxyethy1ene) are widely used in drug delivery and targeting,’ affect blood rheology,, and are said to affect plant growth.3 They are, therefore of some practical importance and their properties are, of course, of fundamental significance. The physical and chemical properties of these polymers have been well documented-u They have, therefore, been the subject of considerable attention previous to this report and other recent publications from this laborat~ry.~-IO However, almost all of the earlier work has been concerned with studies of the solid state or of relatively concentrated solutions. For example DSC studies have been performed11 on solutions >lo% w/w. Thermally induced volume changes have also been measured, for relatively concentrated solutions, and, in this work these polymers were discussed12 as models for hydrophobic interactions. Gilbert et a1.s used fluorescent probes to study gelation in Pluronic F127 and demonstrated the existence of some critical temperature in dilute aqueous solutions (5-10% concentration w/w) at which diffusioncoefficientsof small molecules through the hydrophobic portions becomes reduced. The effect of temperature on the structural properties of the ABA block copolymers has also been studied” by time-correlated fluorescence probes. Association and surface properties of both diblock and triblock copolymers have likewise received attention.14 Recently it has been shown’s that the distribution of monomers, micelles and aggregates in aqueous solutions of these ABA block copolymers is dependent upon temperature with, unsurprisingly, monomers being the dominant specie at low (298 K) temperature. We have recently reported7-10 the observation of phase transitions in dilute aqueous solutionsof poly(oxyethy1ene)-poly(oxypropylene)-poly (oxyethylene)block copolymers which were occasioned by an increase in temperature. The techniques adopted for these studies were high sensitivity scanning calorimetry (HSDSC)7-9J0and temperature controlled solution phase I3C NMR.8 For the study polymer, pluronic F87, both techniques indicated a phase transition temperature of 307 K. The interpretation of these data was based upon the dehydration of the poly(oxypropy1ene) moiety of the polymer as temperature increased. Indeed the NMR evidence we presented* indicates that it is a change in the relaxation time ( T I )of the methyl group
’
School of Pharmacy, University of London, Brunswick Square, London WClN IAX, UK. I School of Biological and Chemical Sciences, University of Greenwich, Woolwich, London SEl8 6PF, UK. Department of Pharmacy, University of Bradford, Bradford, West Yorkshire, BD7 IDP, UK.
in the poly(oxypropy1ene) moiety which reflects these changes in overall conformation. Subsequent studies by this group on the properties of these polymers have showng~I0that the behavior of the dilute polymer solutions, when subject to HSDSC investigation, relates only to the poly(oxypropy1ene) content of the polymers. There is no relationship that can be discovered between any of the thermodynamic parameters and total molar mass nor with poly(oxyethy1ene) content. These reports are, we believe, the first to report temperature driven phase transitions for these polymers in such dilute solutions. Such data are, of course, important as these polymers find many industrial and pharmaceutical applications (e.g., in cosmetics and in drug delivery systems). Moreover, that it is possible toidentify phase transitions which occur through temperature variation has implications for the stability of creams and emulsions which incorporate these polymers in their formulations. Thus in addition to their fundamental importance they also have practical significance. Recently we have demonstrated? from a thermodynamic development of HSDSC data, that these polymers exhibit both highand low-temperature phase transitions-these are the polymer analogue to dilute protein solution phase transitions.I6.l7 It is well documented18 that polymer solutionsage when stored under constant temperature conditions, i.e., isothermally. The evidence presented for this conclusion arises, for relatively concentrated solutions, from studies of density, surface tension, and dynamic and static light scattering, and from theoretical considerations. There has, to our knowledge, been no report of the direct isothermal, microcalorimetric observationof the aging phenomenon itself nor indeed of the direct observation of the time course of the aging event, and certainly not for the dilute solutions which are the subject of this study. We report here, therefore, what we believe to be the first evidence for the isothermal stability profiles of block copolymers of poly(oxyethylene)-poly(oxypropylene)-poly(oxyethylene) in dilute aqueous solution. These block copolymershave the general formula
H(OCH,CH,),-(OCHMeCH,),-(OCH,CH,),-OH The polymers consist of a relatively hydrophobic moiety-the poly(oxypropy1ene) unit-nd relatively hydrophilic moieties-the poly(oxyethy1enes) unit. As noted above, the earlier publ i c a t i o n ~ on ~ -the ~ ~temperature-controlled conformationsof these blockcopolymers described the observed transitions in dilute (0.1196, w/w) aqueous solution in terms of the dehydration of the polymer that occurred as a result of the increase in temperature and the consequent rearrangement of solvation water resulting from changes in polymer-water interactions. In the studies
0022-3654/93/2097-2034E04.00/0 Q 1993 American Chemical Society
Aqueous Solutions of Block Copolymers
The Journal of Physical Chemistry, Vol. 97, No. 9, 1993 2035
TABLE I
Time Driven Transition Power Curve for F87
A,,*"fl*
polym
Power /(yW/mol) (Thousands)
2o01
F87 F87 F87
\
100
I Ve7
F88
r-- I
-4 \ 0
10
20
30
40
50
time /(hours) Figure 1. Power-time curve for a 0.5% (w/w) aqucoussolution of pluronic F87 at 298 K.
reported here the relaxation in structure is as a result only of time-driven phenomena.
Metbods The poloxamers weredonated by IC1 (Cleveland, UK). Purity was established by the presence of a single elution peak after gel permeation chromatography. All microcalorimetricexperiments were conducted in a Thermal Activity Monitor (Thermometric AB, Jarfalla, Sweden) and this instrument was operated at 298 K in accord with the manufacturer's instructions. All solutions were made up in distilled/deionized water, without significant agitation, at a temperature of 298 K. In a stainless steel ampule was placed 3 cm3 and the ampule was immediately placed in the microcalorimeter. As required (see Results) polymer solutions were made up in urea or NaCl. During the experimental observation period the solutions were unstirred. All experiments were run for a minimum of 64 h. During this time the stability of the microcalorimeter is such that a base line signal does not vary by more than f0.5pW. Calibration of the instrument was by the electrical substitution method. Thedata were manipulated via the software produced by the manufacturer (Digitam 2). All results were reproducible to better than 3%. Temperaturecontrolled light-scattering and density measurements were performed on a Sofica 4200 and a Paar DMA 601 602 respectively. Results and Discussion Figure 1 shows the output from the microcalorimeter upon study of a 0.5% w/w aqueous solution of pluronic F87. This polymer consists of 70% ethylene oxide (EO) and 30% propylene oxide (PO) (F87; av M,= 7700, M,for EO = 5390, M,for PO = 2310). From a much more limited study similar power-time ( p t ) curves to that shown for F87 wereobtainedfor F88. Pluronic F88 consists of 80% ethylene oxide, and 20% propylene oxide (F88; av M, 11 800, M,for EO = 9550, Mrfor P o = 2250). The operation of the microcalorimeter requires that an instrumental equilibration period of ca. 30 min p r d e the measurement period. This therefore means that it is impossible from these results to identify a zero time and hence to construct a baseline from which to make absolute measurements of the enthalpy changes accompanying the transitions. This absence of a prebaseline made calculation of the area under the curve (transformed into enthalpy terms following suitable electrical calibration of the instrument) somewhat arbitrary. However, a consistent approach to the calculation does allow relatioe values of enthalpy to have significance. In this work the following procedure was adopted: the area from the point of t h e p t curve at 5 h to the point at which the p t curve returned to base line
solvent
H20 68NaCl 8moldm-3 urea
H20
W mol-' -9.0 a
3.1
b -8.0
3.2
time to
peak max, h
time to peak end, h
16 10 b
25
12
35
30 b
A significant portion of the transition occurred in the first 3 h, and therefore it was not possible to determine the enthalpy change for the transition. N o transition observed.
value was evaluated via a computer driven graphical digitizing package. Using this procedure the data shown in Table I were calculated. Also shown in this table are the results for isothermal stability studies conducted on both these polymers in aqueous solutions of urea and of sodium chloride. In general, solutions of urea at a concentration of 8 mol dm-3 are regarded as breakers of water structure and solutions of sodium chloride was water structure formers. Superficiallythe appearance of t h e p t curve illustrated in the figure resembles the profile anticipated for an autocatalytic rea~ti0n.I~When the data were analyzed according to method proposed by Hansen et al.,I9 appropriately linear plots result but the derived constants are uninformative. This is presumably due to the assumption inherent in the autocatalytic model of the existence of a single process. It is apparent from inspection of the Figure that there may well be at least three processes involved in the overall transition. There appears to be an endothermic process initiated early in the relaxation process as is evidenced in the figure. As noted above, however, the fact that the first hour or so of the experiment is 'hidden" because of the instrumental equilibration time required before data acquisition can commence means that the existence of this endotherm is suggested rather than proved. The figure also reveals that there are two distinct exothermic peaks in thep-t curves implying the existence of a further two discrete, but overlapping processes. Hence the transition may be autocatalytic but the analysis presented here cannot confirm this as the observation of multiple processes requires a more complex deconvolutionthan the present theory permits. The microcalorimetricoutput has been shown to be reproducible in terms of concentration effects since a plot of the area under the experimental curve (determined as described above) was reasonably linear (square of the regression coefficient = 0.90) for solution concentrations ranging from 4 to 20 mg cm-3. Furthermore the time scales and pt profiles observed were constant over this concentration range. The reproducibility of each individual enthalpy associated with each experiment was, however, necessarily more modest given the method of calculation of the enthalpy of the process. It should, however be noted that the enthalpies associated with these transitions (at least as measured) are very small; less than the average H-bond strength. It is, as yet, not possible to base these values on a more detailed descriptionof the particular components of the polymeric species involved in the observed transitions although the processes involved-breaking of waterlwater H bonds, separation of the ABA interactions, and formation of ABA/water interactions can be imagined. Similar results and consequences have been discovered from the study of F88 which was conducted only in aqueous solution. Moreover, only very limited study of this polymer has been made to date. The effect of urea was to remove all experimental indication of the existence of a phase transition-it remains possible that the transition does indeed occur but on a much more rapid time scale such that it has happened before microcalorimetricobservation is possible. Sodium chloride solution presented evidence for the existence of the phase transition but the transition itself
2036 The Journal of Physical Chemistry, Vol. 97, No. 9, 1993
was complete by 6 h, and thus no quantitative data could be assembled for this system. These results emphasisethe importance of water structure, the role of the relatively hydrophobic and hydrophilic portions of the polymer molecules and their hydrated states in determining the equilibrium structures present in aqueous solution. Naturally these microcalorimetric results for the enthalpies associated with the conformational changes undergone by these polymers in dilute aqueous solution do not give any information on the nature of the events at the molecular level that give rise to the observations reported here. Previous temperature-dependent studits'4~lS~20 using, in the main, noncalorimetrictechniques have suggested that transitions are associated with molecule to micelle processes necessarily associated with dehydration. The isothermal results reported here were conducted at the lowtemperature side of the correspondingtransition for the studied polymers.9 Indeed Devenand and Selser2Ihave indicated that in dilutesolution and at temperaturesof 298 and 303 K polyethylene oxidedoes not aggregatein water. Light-scatteringmeasurements of particle mass and high precision density measurements of partial molar volumes on the experimental solutions used in this work confirm that a temperature driven process does occur for pluronic F87. Importantly for the work reported here these same results show that, under isothermal conditions at 298 K,the system is largely monomeric (Mw= 2430; corresponding to the PO fraction) with only a very small proportion of aggregated material present. We will present elsewhere, in a more complete form light scattering, surface tension, densimetric,small-angle X-ray, FIIR studiesand molar mass determinationsby vapor phaseosmometry to indicatethat the isothermaltransitions reported here are indeed associated only with monomeric species of the polymers. The conclusion is, therefore, that the relaxation process under observation in these isothermal studies relates to conformational changes associated with the slow hydration of the polymer following the departure of a molecule from the solid phase to solution. Thus we suppose that the poly(oxypropy1ene) moiety (the hydrophobic portion of the molecule) slowly hydrates under isothermal conditions and this, in the main, is responsible for the observations reported here. The conformation of the polymer molecule entering solution is, therefore, not the equilibrium structure and hence relaxes rather slowly to its preferred orientations upon interaction with water. It is this relaxation which is properly called "aging" and whose time course and relative thermodynamic parameters are reported here for the first time.
Additions and Corrections The HSDSC results reported p r e v i ~ u s l y ~are ~ ~associated, J~ therefore, with the dehydration of this equilibriumconformation of the PO fraction of these ABA copolymers. It remains part of our concerns to demonstrate the general or specific nature of these observations. Further work from this laborabory will explore this issue. Aclrwwledgwat. The award of a Fulbright Scholarship to J.J.I. is gratefully acknowledged. References a d Notes (1) See for example: (a) Krezanoski, J. Z. U S . Patent No 4,188373, 1980. (b) Illum, L.; Hunneyball, I. M.; Davis, S.S . In?. J . Phurm. 1986,29, 53. (c) Park, T. G.; Cohen, S.; Langer, R. Pharm. Res. 1992. 9, 37. (d) Douglas, S. J.; Davis, S. S.;Illum, L. In?. J . Phurm. 1986, 34. 145. (e) Johnston, J. P.; Punjabi, M. A.; Froelich, C. J . Phurm. Res. 1992,9,425. (f) Guzman, M.; Garcia, F. F.; Molpeceres, J.; Aberturas, M. R.Inr. J . Phurm. 1992, 80, 119. (2) Carter, C.; Fisher, T. C.; Hamai, J.; Johnson, C. S.;Meiselman, H. J.; Nash, G. B.; Stuart, J. Clin. Hem. 1992, 12, 109. (3) Kumar, V.;Laouar, L.; Davey, M. R.; Mulligan. B. J.; Lowe, K. C. J . Exp. Bo?. 1992. 43, 487. (4) Schmolka, I. J . Am. Oil Chem. Soc. 1987,54, 110. (5) Gilbert, J. C.; Washington, C.; Davics, L. C.; Hadgraft, J. In?. J . Phurm. 1987, 40, 93. (6) Blou. D.; Hergeth, W. D.; Doring, E.; Witkowski, K.; Waterwig, S . Acta Polym. 1989, 40, 260. (7) Mitchard, N. A.; Beezer, A. E.; R e a . N.; Mitchell, J. C.; Leharne, S.;Chowdhry, B. Z.; Buckton, G., J . Chem. Soc.. Chem. Commun. 1990,900. (8) Beezer, A. E.; Mitchell, J. C.; Rees, N. H.; Armstrong, J. K.; Chowdhry, B. 2.;Leharne, S.;Buckton, G., J . Chem. Res. ( S ) 1991, 254. (9) Mitchard, N. A.; Beezer, A. E.; Mitchell, J. C.; Armstrong, J. K.; Chowdhry, B. Z.; Leharne, S.;Buckton, G., J . Phys. Chem., in press.
(IO) Beezer, A. E.; Mitchard, N. A.; Mitchell, J. C.; Armstrong, J. K.; Chowdhry, B. 2.;Leharne, S.;Buckton, G., J . Chem. Res. (S) 1992. 236. (11) Cole, S. C.; Chowdhry, B. 2. Tibrech 1989. (12) Williams, R. K.; Simard, M. A.; Jolicoeur, C. J. Phys. Chem. 1985, 89, 178. (13) Lianos, P.; Brown, W. J . Phys. Chem. 1992,96, 6439. (14) Yang. L.; &dells. A. D.; Attwood, D.;Booth, C. J. Chem. Soc., Furaduy Trans. 1992,88, 1447. (15) Brown, W.;Schillen, K.; Almgren, M.; Hvidt, S.;Bahadur, P. J . Phys. Chem. 1991, 95, 1850. (16) Privalov, P. L.; Gill, S . J. Adu. Protein Chem. 1989, 39, 191. (17) Privalov, P. L. Ann. Reu. Biophys. Chem. 1989, 18, 47. (1 8) Doi, M.; Edwards, S . F. The Theory of Polymer Dynumics; Oxford University Press: Oxford, UK, 1986. (19) Hansen, L. D.; Lewis, E. A.; Eatough, D. J.; Bergstrom, R.;DegraftJohnson, D. Pharm. Res. 1989, 6, 20. (20) Deng, Y.; Yu,G.; Price, C.; Booth, C. J . Chem. Soc.,Faraduy Truns. 1992. 88, 1441. (21) Devenand, K.; Selser, J. S . Nuture 1990, 343, 739.
ADDITIONS AND CORRECTIONS
1992, Volume 96 Makoto Aratono,' Takanori Takiue, Noribiro Ikeda, Akira Nakamura, nod Kinsi Motomura : ThermodynamicStudy on the Interface Formation of Water-Long-chain Alcohol Systems. Page9423. In Figure4,-0.01,-0).02,-0).03,-0.04, and-0.05 of the ordinate should be changed respectively to -0.02, -0.04, -0.06, -0.08, and -0.10.
