Langmuir 1996,11,414-421
414
The Langmuir Lectures ~
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Structure and Dynamics of Block Copolymer Colloids Benjamin Chu Department of Chemistry, State University of New York at Stony Brook, Stony Brook, New York 21794-3400 Received September 23, 1994@ Polymer colloids offer great potential in creating tailor-madesupramolecules because ofwider variations in the molecular architecture when compared with small detergent molecules. The delicate balance between inter- and intramolecularinteractionstogether with interfacelsurface and geometricalcontraints challenges both theory and experiment. Many experiments and applicationshave been made. A short-chain triblock copolymer is used as an example to discuss the methodology developed to investigate the structure and dynamics of such polymer colloids and to see the type of information which can be made available by using a combination of physical techniques
Introduction Micelles of block and graft copolymers in solution have been reviewed extensively by Tuzar and Kratochvil with 206 references.l The topic is extremely rich and offers great potential because the colloidalbehavior of block and graft copolymers is in many respects an extension of that of soaps and surfactants of small molecules. Block and graf? copolymers, however, offer much wider variations in the molecular architecture which are potentially our crude attempts to mimic nature, such as in protein folding2 and self-assembled biological systems. At the present time, our synthesis ability is still limited to a small number of diblock and triblock copolymers whereby we can vary the chain length (total copolymer molecular weight), the chain ratio, and the block rearrangement, such as ABA, BAB, and ABC triblocks or ABBAB-type multiblocks with A, B, and C being the respective polymer blocks. Nevertheless, by adding variations in solvent quality, i.e., whether the solvent is a good, marginal, poor, or nonsolvent for A, B, or C block(s), together with external variables such as temperature and pressure, the resultant interactions are already sufficiently complex so that quantitative theories have been developed only to a limited extent. More importantly, instead of seeking universal behavior which is a trademark of basic research, the supramolecular formation of block copolymers in solution can be controlled by delicate variations in the intra- and intermolecular interactions, as well as by geometrical considerations of the building blocks, i.e., of the block copolymers, such that specific results can be achieved. Thus, the ability to perform intricate maneuvers has indeed provided the necessary means to create new products for industrial applications. Several avenues of further development appear to be promising. For examples, one can functionalize one or both blocks in a diblock o r a triblock copolymer in order to promote or influence polymer-polymer or polymersolvent interactions. The polymer block can be made of stiffer chains. By variation of the persistence length of the block copolymer segments, the energy minimization requirements are changed so that geometrical factors could
become more important, leading toward possible supramolecules with sizes in the mesoscopic scale. Such colloids could very well become ingredients of new molecular composites. The speculative approach to the design and creation of new supramolecules which are actually controlled by the nanostructures of the functionalized block copolymers clearly suggeststhe promising nature of block polymer colloids, a t least in the near future, and offers new challenges to colloid science. In this article, the aim is to demonstrate how the structure and dynamics of block copolymer colloids can be characterized. In fact, it will be difficult to discuss all possible approaches, especially those which are not familiar to this lecturer. Thus, the discussion will be limited to a number of selected methods which are coherently self-consistent and provide the means to study the structure and dynamics ofblock copolymer colloidsin solution. The model example deals mainly with a relatively short-segment length triblock copolymer of the EPE type (P and E represent poly(oxypropy1ene) and poly(oxyethylene), respectively). Water-soluble triblock copolymers of the EPE type, often abbreviated as PEO-PPO-PEO or (EO),(PO)dEO),, are commercially available nonionic macromolecular surfactants, with the commercial names for these surfactants being denoted as Poloxamers (IC11 or PluroniC polyols (BASF) and a , b, and c representing the number of repeat units in the copolymer. In a recent review article on “Poloxamers in the Pharmaceutical Industry”,3Schmolka stated that there were over 1000 published articles on many of the different applications of the poloxamers in medical and pharmaceutical industries alone. The Pluronic polyols find widespread industrial applications in their uses as emulsifying, wetting, thickening, coating, solubilizing,stabilizing, dispersing, lubricating, and foaming agents. In many applications, the material may have two or more functions. The Pluronic polyols are available in large quantities. However, it is not an ideal model system because one has to expect the presence of polydispersity in size and in chemical comp~sition.~ Nevertheless, in most cases, the self-assembly behavior can be studied in some detail
Abstract published in Advance ACS Abstracts, December 15, 1994. (1)Tuzar, Z.; Kratochvil, P. InSurfaceand ColloidScience; Matijevic, E., Ed.; Plenum Press: New York, 1993; Vol. 15, Chapter 1,pp 1-83. (2) Chan, H.S.; Dill, K. A. Phys. Today 1993,46, 24.
(3) Schmolka, I.R.In Polymers for Controlled Drug Delivery; Tarcha, P. J., Ed.; CRC Press: Boca Raton, Ann Arbor, Boston, 1991; Chapter 10, pp 189-214 with 187 references. (4)For example, see Zhou, Z.; Chu, B. Macromolecules 1987,20, 3089; 21, 2548.
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0743-746319512411-0414$09.00/0 0 1995 American Chemical Society
Langmuir, Vol. 11, No. 2, 1995 415
The Langmuir Lectures without worrying about the anomalous effects due to chemical composition inhomogeneity. In the past few years, basic studies of the colloidal behavior of PEOPPO triblock copolymers have been quite active. A brief, but by no means exhaustive, survey suggests that there are a t least a dozen research groups working on many aspects of the self-assembly properties of the triblock copolymers using water, oil (e.g. o-xylene), or a mixture of the two as the solvent. The physical techniques used are listed in Table 1where representative (5) Reddy, N. K.; Fordham, P. J.; Attwood, D.; Booth, C. J . Chem. Soc., Faraday Trans. 1990, 86, 1569. (6) Wang, Q.-G.; Price, C.; Booth, C. J . Chem. Soc., Faraday Trans. 1992,88,1437. (7) Yu, G.-E.; Deng, Y.; Dalton, S.; Wang, &.-G.;Price, C.; Booth, C. J . Chem. Soc., Faraday Trans. 1992,88,2537. ( 8 ) Samii, A. A,; Karlstrom, G.; Lindman, B.Langmuir 1991,7,1067. (9) Tiberg, F.; Malmsten, M.; Linse, P.; Lindman, B. Langmuir 1991, 7, 2723. See other experimental techniques for the adsorption phenomena mentioned in the Introduction. (10)Malmsten, M.; Lindman, B. Macromolecules 1992,25, 5446. (11)Malmsten, M.; Lindman, B. Macromolecules 1993, 26, 1282. (12) Penders, M. H. G. M.; Nilsson, S.; Piculell, L.; Lindman, B. J . Phys. Chem. 1994,98, 5508. (13) Mortensen, K. Europhys. Lett. 1992, 19, 599. Note beautiful diagrams of shear-oriented EPE-type triblocks in water. (14) Mortensen, K.; Pedersen, J. K. Macromolecules 1993,26, 805. (15) Mortensen, K.; Brown, W. Macromolecules 1993,26, 4128. (16) Mortensen, K. Prog. Colloid Polym. Sci. 1993, 91, 69. (17) SchillBn, K.; Brown, W.; Kofiak, C. Macromolecules 1993, 26, 3611. (18) Brown, W.; SchillBn, K.; Almgren, M.; Hvidt, S.; Bahadur, P. J . Phys. Chem. 1991,95, 1850. (19)Almgren, M.; Bahadur, P.; Jansson, M.; Li, P.; Brown, W.; Bahadur, A. J . Colloid Interface Sci. 1992, 151, 157. (20) Brown, W.; Schillen, K.; Hvidt, S. J . Phys. Chem. 1992,96,6038. (21) Schillen, K.; Glatter, 0.;Brown, W. Prog. Colloid Polym. Sci. 1993, 93, 66. (22) Almgren, M.; Stam, J.; Lindblad, C.; Li, P.; Stilbs, P.; Bahadur, P. J . Phys. Chem. 1991,95, 5677. (23) Almgren, M.; Alsins, J.; Bahadur, P. Langmuir 1991, 7, 446. (24) Bahadur, P.; Pandya, K. Langmuir 1992, 8, 2666. (25) Bahadur, P.; Pandya, K.; Almgren, M.; Li, P.; Stilbs, P. Colloid Polym. Sci. 1993, 271, 657. (26) Pandya, K.; Bahadur, P.; Nagar, T. N.; Bahadur, A. Colloids Surf., A 1993, 70, 219. (27) Malmsten, M.; Linse, P.; Cosgrove, T. Macromolecules 1992,25, 2474. (28) Fleischer, G. J . Phys. Chem. 1993, 97, 517. (29) Alexandridis, P.; Holzwarth, J. F.; Hatton, T. A. Macromolecules 1994.27. 2414. ~. . (30jBiosz:P.; Hergeth, W.-D.;Wohlfarth, C.;Wartewig, S.Makromol. Chem. 1992,193, 957. (31) Fleischer, G.: Blosz, P.;Hergeth, W.-D. Colloid Polym. Sci. 1993, 271, 217. (32) Hergeth, W.-D.; Alig, I.; Lange, J.; Lochmann, J. R.; Scherzer, T.; Wartewig, S.Makromol. Chem., Macromol. Symp. 1991, 52, 289. See also, Wartewig, S.; Alig, I.; Hergeth, W.-D.; Lange, J.; Lochmann, R.; Scherzer, T. J . Mol. Struct. 1990, 219, 365. (33) Alig, I.; Ebert, R.-V.; Hergeth, W.-D.; Wartewig, S. Polym. Commun. 1990,31, 314. (34) Wanka, G.; Hoffmann, H.; Ulbricht, W. ColloidPolym. Sci. 1990, 268, 101; Macromolecules 1994, 27, 4145. (35) Mitchard, N.; Beezer, A,; Rees, N.; Mitchell, J.; Leharne, S.; Chowdhry, B.; Buckton, G. J . Chem. SOC.,Chem. Commun. 1990,900. (36) Beezer, A. E.; Mitchell, J. C.; Rees, N. H.; Armstrong, J. K.; Chowdhry, B. Z.; Leharne, S.; Buckton, G. J . Chem.Res. (S) 1991,254. (37) Beezer, A. E.; Mitchard, N.; Mitchell, J. C.; Armstrong, J. K.; Chowdhry, B. Z.; Leharne, S.; Buckton, G. J . Chem. Res. ( S )1992,236. (38) Turro, N. J.;Kuo, P.-L.Langmuir 1987,3,773,andearlier papers; ibid. 1986,2,438;J . Phys. Chem. 1986,90,4205. Turro, N. J.;Chung, C. Macromolecules 1984, 17, 2123. (39) Rassing, J.;Attwood, D. Int. J. Pharm. 1983, 13, 47. (40) Rassing, J.;McKenna, W. P.; Bandyopadhyay, S.;Eying, E. M. J.Mol. Liq.1984, 27, 165. (41) Zhou, Z.; Chu, B. J . Colloid Interface Sci. 1988, 126, 171. (42) Tontisakis, A,; Hilfiker, R.; Chu, B. J . Colloidlnterface Sci. 1990, 135, 427. (43) Wu, G.-W.; Zhou, Z.-K.; Chu, B. Macromolecules 1993,26,2117. (44) Wu, G.-W.; Zhou, Z.-K.; Chu, B. J . Polym. Sci., Part B: Polym. Phys. 1993, 31, 2035. (45) Wu, G.-W.; Chu, B. Macromolecules 1994,27, 1766. (46) Zhou, Z.-K.; Chu, B. Macromolecules 1994,27, 2025. (47) Chu, B.; Wu, G.-W.; Schneider, D. K. J . Polym. Sci., Part B: Polym. Phys. 1994, 32, 2605. (48) Wu, G.-W.; Chu, B.; Schneider, D. K. J . Phys. Chem., in press.
references from different research groups on the use of different methods can be detected by denoting the simultaneous appearance of the references listed in Table 1. The methods, denoted by the italics, will be discussed in more detail in later sections. The remarks are made only on those methods which are in italics and act as a brief note on the capabilities of the physical methods to be discussed. Only selected r e f e r e n ~ e s lafter ~ - ~ ~1990have been included. Earlier references, such as those by Turro and his c o - w ~ r k e r on s ~ ~excimer formation, by Rassing and Attwood on ultrasonic velocity and light scattering studies,39 and by Rassing et al. on ultrasonic and I3CNMR studies,40have been excluded. Our own interest in the triblock (PEO-PPO-PEO) copolymers started in the late 19809, first in aqueous solution^^,^^ and then in solvent mixtures of xylene and water.42 Results on recent s t ~ d i e sare ~ ~also - ~referenced ~ in Table 1and have been reviewed partially.49s50Results from diblock (PS-PE0,51 PS-PtBS,52 and SPS-PtBS53,54 with tBS and SPS denoting the tert-butylstyrene and the sulfonated styrene polymer blocks, respectively) and triblock (PtBS-PS-PtBS)55 copolymers with longer chain lengths, as well as from a triblock (PEO-PBO-PEO with BO denoting the more hydrophobic middle oxybutylene block when compared with the oxypropylene block in PEO-PPO-PEO)56 copolymer, show similar findings as the relatively shorter-chain PEO-PPO-PEO triblocks and will not be discussed. Finally, it should be noted that recent theoretical t r e a t m e n t P j 9 on the (PEO-PPOPEO) triblocks have shown promising predictions, in reasonable agreement with experimental findings and, therefore, should not be overlooked.
Sketches of Micellar Structures Polymer aggregation processes leading to supramolecular structures depend not only on solvent quality but also on molecular architecture. Here, the solvent quality is presented in a broad sense by including all solvent polymer interactions. Thus, one can change the solvent quality of one or more blocks in a block copolymer by changing the temperature orland the pressure as well as by introducing interactive groups, such as ionic pendant groups, to the polymer backbone of one of the blocks. The latter variation is also related to changes in molecular architecture which impose geometrical contraints that can influence the supramolecular formation and the resultant morphologies. The number of variations is very great, making polymer colloids an extremely rich but also complex topic because from the brief discussion above we note that the variables for solvent quality control are often coupled and cannot be treated as independent ones. Most studies have been concerned with the supramolecular formation of an AB diblock or a BAB triblock (49) Chu, B.; Zhou, Z.-K.; Wu, G.-W. J . Non-Cryst. Solids 1994,172174, 1094. (50) Wu, G.-W.; Chu, B. Macromol. Symp., in press. (51)Xu. R.-L.: Winnik. M. A.: Riess. G.: Chu, B.: Croucher, M. D. Macromolecules 1992, 25, 644. (52) Zhou, Z.-K.; Chu, B.; Peiffer, D. G. Macromolecules 1993, 26, 1876. (53) Zhou, Z.-K.; Chu, B.; Wu, G.-W.; Peiffer, D. G. Macromolecules 1993,26, 2968. (54) Zhou, Z.-K.; Peiffer, D. G.; Chu, B. Macromolecules 1994, 27, 1428. (55) Zhou, Z.-K.; Chu, B.; Peiffer, D. G. J . Polym. Sci., Part B : Polym. Phys. 1994, 32, 2135. (56) Luo, Y.-Z.; Nicholas, C. V.; Attwood, D.; Collett, J. H.; Price, C.; Booth. C.: Chu., B.:, Zhou. Z.-K. J . Chem. Soc., Faraday Trans. 1993,89 (31, 539. (57) Wang, Y.; Mattice, W. L.; Napper, D. H. Macromolecules 1992, 25.> 4073. (58) Rodrigues, K.; Mattice, W. L. Langmuir 1992, 8, 456. (59) Linse, P. J . Phys. Chem. 1993, 97, 13896. I
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416 Langmuir, Vol. 11, No. 2, 1995
Table 1. Physical Methods Used in PEOPPO Triblock Copolymer Solution Studies methodsa
referencesC
remarksb
A. Scattering Techniques 1. laser light scattering (LLS) static light scattering (SLS) dynamic light scattering (DLS) Rayleigh Brillouin scattering 2. Small angle X-ray scattering (SAXS) 3. Small angle neutron scattering (SANS)
molar mass, Rg, Az translational diffusion similar as SLS; internal structure similar as SAXS H-D substitution
43,44 5 , 7 , 18,23,25, 32-34,43-46 7, 15, 17-20, 24, 26, 34 17 21,24 13-16,34,47,48
B. Other Light-Related Techniques
4. Light transmittance 5. Transient electric birefringence (TEB) 6. Fluorescence quenching and excimer formation 7. Dye solubilization
12 34,44
shape
19,22-24 29
C. Spectroscopic Techniques 8. IR and Raman Spectroscopy 9. NMR
local environment
D. Thermodynamic and Macroscopic Properties 10. Differential scanning calorimetry (DSC) hydrodynamic volume 11. Viscosity and rheological measurements 12. Sedimentation 13. Ultra-sound velocimetry 14. Gel permeation chromatography (GPC) 15. Adsorption by ellipsometry unimer molecular weight 16. Vapor pressure osmometry (VPO) 17. Surface tension 18. Vapor sorption E. Observations (Microscopy) macroscopic phase behavior 19. Determination o f cloud pointl phase diagram
32 7, 10, 12, 19,22, 28, 31, 34,35, 36,43 7, 33, 35, 37 11,20,23-26, 30, 32, 34,43 26 21,26, 32, 33 6 9,27 30,43 5, 24,34 30 8, 11,25
a Methods expressed in italics are discussed in the lecture. Remarks are made only on those methods which are in italics. Selected references have been grouped arbitrarily in perceived research teams. -100 basic research publications on the PEOPPO triblock copolymers in solution have appeared since 1990.
copolymer in a selective solvent which is a good solvent for the B block but a poor solvent (or nonsolvent) for the A block. Triblock copolymers with poorly solvated end (B) blocks can also form aggregates.6o In order to bring the two end (B) blocks into the core of the micelle, the coronal (A) block has to form a loop. Micelle formation of BAB triblock copolymers in solvents that preferentially dissolve the A block has been investigated by Balsara et a1.61and by Raspaud et a1.62 Figure 1shows sketches of possible structures of diblock and triblock copolymers in selective solvents. The sketches61,62 have been expanded to include those of unimolecular micelles. Furthermore, a distinction has been made concerning the miscibility of the A,B blocks and the solvent quality varying from nonsolvent to good solvent. From Figure 1.1,the following observations can be summarized. Unimolecular Micelles. 1.l.a.i. In an AB diblock copolymer, if the solvent is a good solvent for the A block and a nonsolvent for the B block, and if the A,B blocks are nonmiscible, the A block is solvated t o an expanded coil while the B-block is collapsed to a globule. It is important to realize that if the solvent is truly a nonsolvent for the B block, the B block coil forms an amorphous liquid-like (or even quasi-crystalline) structure with no solvent molecules inside the B globule. Then, the B block coil, because it has to be forced into solution by the A coil, is likely to be much smaller in size than the A block coil. If supramolecular formation is to take place, the critical micelle concentration (cmc) is likely to be low since the B (60)Krause, S. J.Phys. Chem. 1964, 68, 1948. (61) Balsara, N. P.; Tirrell, M.; Lodge, T. P. Macromolecules 1991, 24. 1975. See references therein. (62)Raspaud, E.;Lairez, D.; Adam, M.; Carton, J.-P. Macromolecules 1994,27,2956.
chains like each other much more than the solvent molecules. Furthermore, at concentrations slightly higher than the cmc, the equilibration time between the micelles and the unimolecular micelles is likely to be long. From our experience with diblock copolymers of sulfonated polystyrene (an ionomer) and tert-butylpolystyrene (a neutral polymer; with molecular weights of the order of lo5 g/mol) in a polar ~ o l v e n t ,the ~ ~equilibration ,~~ time can be of the order of many months. Thus, in many experiments it is worthwhile to take note of the equilibration time and to make sure that the experimental results deal with systems at thermodynamic equilibrium but not controlled by kinetic processes. Otherwise, different interpretations are needed. 1.l.a.ii. If the solvent is a good solvent for the A block and a poor solvent for the B block, and if the A,B blocks are nonmiscible, the A block is solvated to an expanded coil, while the B block is contracted (or collapsed) to a globule. However, in a poor solvent, the B block coil (or globule) usually contains a large amount of solvent63and does not exist in a liquid-like structure. For the sketch in Figure l.l.a.ii, the B-block size is smaller than the A block size with A,B blocks of equal initial segment length. 1.l.a.iii. Ifthe solvent is a good solvent for both Aand B blocks and if the A,B blocks are not miscible, one can visualize the two blobs of comparable size being linked together by a covalent bond which forms the AB diblock copolymer. However, if the A,B blocks are miscible, one can think of an interpenetrating coil. In the case of BAB triblock copolymers, a more complex variation exists even for the unimolecular micelles (or (63)Chu, B.; Ying, Q.; Grosberg, A. Yu. Macromolecules, submitted for publication.
The Langmuir Lectures
Langmuir, Vol. 11,No. 2, 1995 417
1. Unimolecular micelles Apolymer n, B polymer solvent w
a. diblocks
B-block contracted A-block solvated poor solvent for B good solvent for A A.B not miscible
B-block collapsed A-block solvated nonsolvent for B good solvent for A A,B not miscible
b. triblocks
A, B block solvated
solvent good for both
A-B not miscible
miscible
polymer solution
2. Diblock andTriblock copolymers in selective solvents A-B
B-A-B both diblock & triblock
a. Solvent good for B:
diblock
b. Solvent good for A:
triblock
(i)
The coronal block must form a loop Entropy penalty
(ii)
@
poorly solvated into solution (less stable; but possible) Ex.polydispersity
(iii)
poorly solvated block well solvated block loose-branched structure
Figure 1. Sketches of micellar structures.
just denoted as unimers). Here, we use the definition of a unimer or unimolecular micelle to be distinct from that of a monomer which denotes a single segment (or repeat unit) in the copolymer chain. 1.l.b.i. For the BAB triblockcopolymer under the same condition as l.l.a.i, two possibilities exist: if the B end blocks come together, a ring is formed; if the B end blocks are sufficientlysmall so that the A block can force the two ends separately into solution, a dumbbell-type unimer can exist. The two forms are, however, not mutually exclusive. Again, if the B block is in a nonsolvent, the tendency for ring formation is enhanced.
Figure 1.l.b.ii shows similar conformationsas sketched in Figure l.l.b.i, except for the fact that the blocks are more expanded and that the sizes of the B blocks can be larger with the copolymer still being able to be solubilized before aggregation occurs. The B blocks are contracted but contain solvent as in Figure 1.l.a.ii. Finally, if the A,B blocks are both solvated in a good solvent, the sketches, as shown in Figure l.l.b.iii, schematically represent the two cases for A,B blocks: not miscible and miscible. Micelle Formation. Based on Figure 1.1,the micelles that can be formed by diblock and triblock copolymer in a closed association process can be summarized as follows. 1.2.a. If the solvent is a good solvent for B, the micellar core consists predominantly of A blocks, while the solvated B blocks form the micellar corona. For diblock and triblock copolymers, the coronal B chains are “tethered” at one end to the core-corona interface, while the other end is free to meander within the limits of the corona.61 There is a difference between nonsolvent and poor solvent, as shown in Figure 1.1; i.e., the coil size for A and the amount of solvent in the micellar core depend on solvent quality. If the solvent is a good solvent for the A block, a distinction between diblock and triblock copolymers exists. For the diblock copolymer, only the role of A and B is reversed, as shown in Figure 1.2.b for the diblock copolymer. However, for the triblock copolymer, three possible structures, as shown in Figure 1.2.b.i-iii, could exist. Indeed, the experimental existence for dominance in Figure 1.2.b.P and Figure 1.2.b.iiP2 has been reported. The sketches above implicitly emphasize on long-chain blocks with hundreds of repeat units for each block. The behavior of short-chain block copolymers can bridge between that of long-chain block copolymers and of the small detergent molecules. The example provided below, Pluronic L64, has a nominal value of 13 segments each for the two PEO end chains and of 30 segments for the middle PPO block. It is clear that less entanglement of the polymer segments should occur and the looping constraint exerted on the middle solvated block (e.g., see Figure 1.2.b.i) could become more important for shortchain block copolymers.
Characterization of Micellar Structures On the basis of the methods listed in italics in Table 1, we subdivide the characterization procedure by starting with unimers (SLS, DLS, SANS, viscosity). The phase diagram is determined partially by visual observation. At concentrations higher than the cmc, the micellar size and shape are characterized by SLS, DLS, SAXS, SANS, and TEB including the local environment by NMR and the internal structures by SAXS and SANS. Finally, at high copolymer concentrations, the supramolecular formation of large aggregates and gel-like structures can be characterized by scattering techniques. Pluronic L64 is used as the model copolymer to demonstrate how polymer colloids in solution can be characterized. Pluronic L64 (Synthesis,Nomenclature, Properties). The synthesis of PEO-PPO-PEO triblock copolymers is shown in Figure 2 where one notes that these triblocks are not very monodisperse and may contain chemical composition inhomogeneities. Further purification of copolymers by precipitation or GPC for more quantitative studies is recommended. Nevertheless, the anomalous behavior due to chemical composition inhomogeneities depends on many factors, varying from very delicate balances to inconsequential effects. For the following discussion on methodology, no further purification is required. The Pluronic L64 poly01 is based on the notation used by the BASF Wyandotte Corporation with L, 6, and 4
418 Langmuir, Vol. 11, No. 2, 1995
P
HOCHCHnOH
+
FHS
FHg
r-=
FHg
HO(CHCH,O)&
‘d
propT1one glycol
HO(CHCH,O),H
(OH-)
(b-1)qH-FHh
The Langmuir Lectures
propylene oxide
t (a+c) ethylene ollde
Reaction conditions: Catalyst NaOH or KOH; under N, atomsphere. Controlled a t 12OoC by adjusting monomer feeding rate.
Product treatment: Using H,PO, to neutralize alkaline catalyst t o pH-7.
Figure 2. Synthesis of PEO-PPO-PEO copolymers.
denoting the physical appearance of liquid for the copolymer, a PPO molar mass of 1750, and 40 w t % PEO in the copolymer,respectively. The copolymer has a nominal average molar mass of 2900 and a HLB value of 15. Unimolecular Micelles (or Unimers). From static scattering technique^,^^ one can obtain the followingtype of information. 1. Absolute scattered intensity I (q = 0; C = 0, with q [=(4z/A) sin(8/2)1being the magnitude of the scattering wave vector) yields a weight-average molar mass M, for homopolymer and an apparent molar mass M* for the block copolymer. For a diblock copolymer M*(dddCI2 = M,(dn/dC),(dddC),
+ [(dddc),’
-
(dn/dC),(dn/dC),]W,M,,, -t [(dddC)B2(dn/dC),( dddC),I W B M , ~ ( 1 where M,,A and Mwm are the weight-average molecular weights of blocks A and B and WAand WBare the weightaverage weight fractions of A and B, respectively. The refractive indices of PEO and PPO blocks are similar and so are their densities. Thus, M* and M , are estimated to be about the same. 2. Interference effect I (q; C = 0) yields a radius of gyration R,. For block copolymer, a n apparent R, is obtained. However, for micelles with core-shell or other well-defined geometrical structures, the size can again be determined uniquely. 3. Concentration dependence I (q = 0; C ) yields information on intermolecular interactions, such as the second virial coefficient A2. From dynamic light scattering,@the intensity-intensity has the form time correlation function G(2)(t)
G‘2’(t)= A ( l
+Plg‘”(~)1~)
obtained with f‘ = Dq2 and JTG(T)dT. pz = J(T r)zG(T)dT with variance = p a 2 . At infinite dilution, the hydrodynamic radius Rh can be computed from D by using the Stokes-Einstein relation. As the size of the unimers is quite small, SLS with visible incident radiation cannot produce an interference effect for particle size determination. Thus, SAXS and SANS are more appropriate. For PEO-PPO-PEO in water, the electron density contrast is fairly small. Thus, if we want to know the unimer size, only DLS and SANS (by taking advantage of the scattering length difference of DzO and L64 or xyleneD and L64) are feasible methods. Figure 3 shows SANS of L64 in DzOat 8.4 “C. The results of L64 unimers in o-xylene and in water by VPO,viscosity, LLS, and SANS are listed in Table 2. From M , and M,, the L64 copolymer appeared to be very homogeneous in size. This suggestion should not be taken too seriously based on the synthesis method as shown in Figure 2. Rather, we can safely conclude that the sample is reasonably “monodisperse” and M , is higher than the nominal value specified by the manufacturer. From a plot of Rayleigh ratio versus concentration (Figure 4 of ref 43) for L64 in xylene, the hard sphere model [S(q=O,C) = (1- #I4( 1 24)-2 and 4 = ( ~ Z R H S ~ / ~with ~ N 4, ANA, C and RHSbeing, respectively, the volume fraction of hard spheres, Avogadro’s number, and the hard sphere radius] fits the experimental SLS datavery well. The same should hold true for L64 in water since Rh is comparable in either solvent. From Figure 3 and Table 2, L64 unimers are shown to exist as coils, but not as Gaussian coils, and the scattering behavior can be represented as that from uniform spheres.
+
Micelle Formation Micelle formation can be induced by temperature. For example, L64 in water forms unimers at low temperatures (-6 “C)because water is a good solvent for both PEO and PPO. With an increase in the temperature, the solvent quality ofwater with PPO becomes poorer. Then, micelles are formed. Micelles can also be formed by changing the solvent quality of the PEO blocks in L64. L64 is soluble in o-xylene. By addition of water, which is not miscible with xylene, to the L64 in xylene solution, the water molecules interact with PEO and thus change the solvent quality of the hydrated PEO. Water-induced mic e l l i ~ a t i o n is ~ ~the - ~model ~ system presented here as an illustration to show how micelles can be characterized. The solubilization of water in L64/0-xylene~~ is shown in Figure4. The solid line in Figure 4 can be approximately expressed by
(2)
where t is the delay time, A is the background, and ,8 is the coherence factor. The measured electric field correlation function Ig(l)(t)lis related to the normalized characteristic line width distribution function G(T)by the Laplace equation
(3) Laplace inversion of eq 3yields information on G(T)which can be related to translational diffusive or/and internal motions. If G(T)measures purely translational motions, the z-average translational diffusion coefficient D is (64) Chu,B . Laser Light Scattering: Basic Principles and Practice, 2nd ed.; Academic Press: Boston, MA, 1991; 343 pp, (65) Wu, G.-W.;Ying, &.-C.; Chu, B. Macromolecules 1994,27,5758.
where Z,, and 20(=0.15) are the maximum molar ratio of solubilized water to EO in the micelles and in the unimers, respectively. The solid curve is the fit with cmc = 0.086 g/mL and Z,, = 2.7. It should be noted that although the information on micelle formation and phase behavior is tedious to obtain, it invariably becomes the fundamental basis from which we can probe deeper on supramolecular structures. Its importance should not be overlooked. In this respect, the determination of cmc represents only a necessary step to study polymer colloids in a closed association process. In order to understand the interaction between solubilized water and L64 molecules, lH and 13C NMR spectroscopy is used to study the local environment of the copolymer and water molecules. On the basis of chemical
Langmuir, Vol. 11, No. 2, 1995 419
The Langmuir Lectures
Table 2. Characterization of L64 Unimer in Water and in 0 - X y l e n e ~ 1 ~ ~
Mw
solvent
M,
R
RHS
2.3
1.4 1.2
RE
Rh
R,
A2
temp, "C
1.8 1.3b(29 "C)
1.7 (21 "C) 1.4
1.6
0.0045
8.4 26
~
DzO 3.7
xylene
3.4
103
103
M expressed in g mol-l, R in nm, and Az in mol cm3 g+.
* S A N S of L64 in o-xylene. Mw: SLS X=400 n m ; 26.2 "C. R,: DLS 0=90".
100
-Ll12 8
'=
1,
0.0
u.9 a
'
,
b 1
0.5
3
3 0
1
,
a
1
1 .o
'
6
1.5
q /nm-' Figure 3. Structure of L64 unimer in water as determined by SANS of L64 in DzO. Solid line denotes uniform sphere of radius R = 2.3 nm. Dashed line denotes an initial slope with R, = 1.8 nm. C = 0.080 g/mL. The SANS experiment was performed at 8.4 "C. The intermolecular interactionhas been corrected by using the hard sphere approximation for the structure factor S(q,C). The plot is for the form factor P ( q ) as a function of q. (Reprinted with permission from ref 48.)
4 2
.A-
25
!"'"']
l l l l l l l l r , l l l l , l l
.-.--=
. . e . -
-*-*-*-* -*'
15'
*+./
3 d
10
0.0 0.5 1.0 1.5 2.0 2.5 H,O/EO in Micelles(2)
Figure 5. Effect of HzO/EO on M w and Rh. (Reprinted with permission from ref 44. Copyright 1993 Wiley.) 100
0 HzO/EO=2.4
-- -Sphere -Prolate
10-1
2.5 -Oblate 3.0 cdLC=0.128g/mL 29.0 "C
2 10-8 a 10-~ 104
100
10'
100
temperatures (23-24 "C). The solid line denotes the fitting results according to eq 4. Open symbols were measured by adding water to the solution until the solution became cloudy. The three open symbols represent three independent measurements. The filled symbol was measured by diluting the water-solubilized solution. (Reprinted with permission from ref 43.)
shifts, solubilized water can be separated into two fractions: bound water and free water. As the water molecules can be bound to the EO units, local environment of EO units is 2 dependent, while that of PO units is only very slightly affected by water. From 13C NMR, the conformation of C in EO and PO units remains essentially the same before and after micellization and is independent of 2. We can determine the molar mass and the hydrodynamic radius of the micelles as a function 2 as shown in Figure 5 by means of laser light scattering. M, increases with increasing 2 while Rh remains constant for 2 5 1.3 and then increases with increasing 2. SAXS and SANS can be used to show that for small 2 (S1.3) spherical micelles are formed, while for large 2 there is a shape change. Micellar Shape Determination If the micelles undergo a shape change, the form factor P ( q )can be used to fit many geometrical shapes, such as sphere, ellipsoids of revolution (prolate or oblate), cylinder, etc. This determination of micellar shape is feasible by using the scattering techniques only if the polydispersity effect is tolerable. For the water-induced micellization of L64 in o-xylene, DLS measurements show a variance of
- Sphere
-Prolate 2.5 - Oblate 3.5
4 Figure 4. Apparent maximum ratio of solubilized water to EO against the concentration of L64 in o-xylene at room
HO/E0=2.6
0 -.
'
-Oblate C*=0.043 23 O C
3 l
a
1 100
4.0 g/mL
10'
qR,
Figure 6. Micellar shape by SANS (upper figure) and SAXS
(lower figure). (Reprinted with permission from ref 48.)
s0.2 (e.g., see Figure 9 of ref 431, implying a reasonably narrow size distribution of micelles. This narrow size distribution will in turn permit us to do shape analysis of the scattering functionP(q1. Figure 6 shows SANS and SAXS experiments for the micellar shape of L64 in xyleneD (SANS) and xylene (SAXS), in the presence of water. The intermicellar interactions have been corrected by using the hard sphere (HS)approximation. The combined scattering results show that the micellar shape a t high 2 values (m2.5) is equivalent to that of an oblate ellipsoid with a small axial ratio of -3-4. If the micelles are not spheres at high 2 values, an applied electric field should orient the asymmetric micelles. Indeed, at E = 3.0 kV/cm, no optical anisotropy (An)could be observed for L64 in xylene with 2 1.0. However, for 2 = 2.6, TEB signals clearly exist. Figure 7 shows a typical TEB trace with 2 = 2.6. The symmetrical rise and decay curves imply that no permanent dipoles existed in water-induced L64 reverse micelles. To be more precise, the measured rotational diffusion coefficient should be extrapolated to the cmc in order to eliminate the effect of intermicellar interactions. By combination of the extrapolated rotational diffusion coefficient with the extrapolated translational diffusion coefficient, the
-
420 Langmuir, Vol. 11, No. 2, 1995 10 8 (0
'
4:
1
E=3.0 kV/cm
4
The Langmuir Lectures D2OIL64lxylene
some D fl
6
' 4
D20
d a 2
Derived Contrast xylene
0
some xylene 0
200
400
600
800
time /ps Figure 7. TEB signals for L64 i n xylene with 2 = 2.6 a t 26.2 "C. Cmic = 0.24g/mL. The upper curve is a n electric field profile with a field strength of 3.0 kV/cm. Solid curves in t h e lower p a r t represent t h e theoretical fitting based on AnR = An0 (1exp( -68t)) for t h e rise curve a n d AnD = An0 exp( -68t) for t h e decay curve, with An a n d 8 being t h e birefringence and t h e
hollow center
D20/L64/xylene( D)
r, PEO
rotational diffusion coefficient. Contribution from ,u2 term is small. (Reprinted with permission from ref 44. Copyright 1993 Wiley.)
no solvent
some D .$I
Hydrogeddeuterium substitution: D20 no polymer
Xylene c)Xylene(D) H20 ,&O Micelle profile
.07 H2 O/L64/xylene(D)
Main scattering contrast Figure 9. Sketches of SANS contrast based on Figure 8.
Scattering length density difference
DLS: A=488 nm; 90';
r+ I+ O/L64/xylene
CONTIN.
O/L64/xylene(D)
Figure 8. Contrast profiles of micelles for SANS based on hydrogen deuterium substitution of xylene/xyleneD a n d H20/ D20.
size and shape of the micelle can again be determined. The slight polydispersity effect can be taken into account by using the size distribution from the DLS experiment. One should also note that the intensity weighting factors for DLS and TEB measurements are different.44The TEB/ DLS result confirms the shape change as determined by SANS and SAXS. SANS Study for WaterKylene Profiles A unique feature of SANS is the large scattering length difference between hydrogen and deuterium. Figure 8 shows a schematic diagram of the contrast profiles of micelles by exchanging xylene with xyleneD or/and H20 with D20. The resultant SANS profiles can yield information on density distributions of xylene and of water, as shown in Figure 9. In Figure 9, the main scattering contrast is depicted on the left three figures. SANS results of the upper two left figures yield the derived contrast of the upper right figure, while SANS results of the lower two left figures yield the derived contrast of the lower right figure. The lightly dotted lines linking the upper left figure to the lower right figure signifythat conclusions of the lower right figure can be checked independentlyby the result of the upper left figure. Similarly, conclusions
io1i02
io3
io1i02
io3 10: r /s-
10' i o 6 i 0 6
io6i06
1 0 9 0 ~io3 10' io6i08
1 0 ~ 1 0io3 ~ 10:
r /s-
io6i06
Figure 10. Formation of large aggregates as observed by DLS with CONTIN analysis. (Reprinted with permission from ref 45.)
of the upper right figure can be checked independently by the results of the lower left figure. On the basis of results from SANS contrast variation, we can obtain density profiles of L64, xylene, and water, and infer that (1)water exists not only in the micellar core but also in the corona, with the volume fraction of water in core to that in corona to be -40, (2) volume fraction of polymer segments in the corona is rather low, i.e., -0.2, with 0.8 being that ofxylene,
Langmuir, Vol. 11, No. 2, 1995 421
The Langmuir Lectures
Table 3. Chain Architecture Effect on Micelle Formati~n~?~~ L64 17R4
400
-
300
composition temperaturePC
200
HLB cmdg mL-' (40 "C) aggregation no. Adcm3 mol g+ Rhhm m/kJmol-l ASAJ mol-' K-l(40.0 "C)
100 0
42.5 9.0 15 10-4
88 -6.0 10.2 210 0.74
10-4
POi4EOz4POi4 40.0 7-12 9.1 x 10-2 10 -6.0 x 10-5 4.0 115 0.40
a Water is a relatively good solvent for PEO, the end blocks of L64 or the middle block of 17R4. Micellization of L64 is much easier than that of 17R4 as reflected from cmc values, implying that micellization in good solvent for the middle block is more difficult, probably due to the entropy penalty for looping.
= 10' 0.0
0.5
1.0
1.5
q /nm-'
Figure 11. SAXS profile of L64 in xylene/HzOmixtures. Peak ratio of 2:1as indicated by arrows suggests a lamellar structure.
(Reprinted with permission from ref 65.) im
E013P030EOi3
n
0
" " 1.o " " " 1.5
q
/ nm-'
Figure 12. SANS profile of L64 in xylene/DzO mixtures. qdql z 2 again suggests a lamellar structure as shown in Figure 11.
(Reprinted with permission from ref 65.)
and (3)the interface between core and corona is not very sharp. The scheme is applicable especially when intermicellar interactions are negligible,i.e., when cmc is small. For systems with large cmc values, the contribution due to the structure factor could be difficultto take into account properly.
Secondary Large Aggregate versus Gel Formation45 At higher micellar concentrations, secondary large aggregates could form. Figure 10 depicts the formation of large aggregates by increasing the water to EO molar ratio 2 from 0.7 to 2.0 (upper left to lower left), by increasing the L64 concentration Cofrom 0.331 to 0.472 g/mL (upper right to lower left). The amount and the size of the large aggregates decrease with increasing temperature (lower left to lower right). The secondary large
aggregates have a relatively broad size distribution and can coexist with micelles and unimers. At even higher concentrations, the solution viscosity becomes very high, probably because of micelle/aggregate overlap to yield a gel-like system even though no specific rheological measurements have been performed to show the presence of a yield point. SAXS and SANS experiments suggest the internal structure ofthe secondary large aggregates and of the gel-like system to be lamellar in nature, as shown in Figures 11and 12. Further study is needed.
Chain Architecture Effect on Micelle Formation& Table 3 lists a comparison of LLS results between L64 and 17R4 in aqueous solution. Water is a relatively good solvent for PEO, the end blocks of L64, or the middle block of 17R4. From the cmc, we see that micellization of L64 is much easier than that of 17R4,implying that micellization in a good solvent for the middle block is more difficult, probably due to the entropy penalty for looping. Summary By a combination of techniques, we can characterize fairly complex polymer colloids. It is important to experimentally determine the phase behavior so that we have a thermodynamic basis for further fundamental studies. The scattering techniques (SLS, DLS, SANS, and SAXS)permit us to investigate the molar mass, size, shape, and internal structure of supramolecules and lH and 13CNMR spectroscopy to study the local environment of solvents and polymer chains. Unimers have scattering behavior of hard spheres but exist as coils, while micelles can be represented by a corestar model with fuzzy interface. Micellar shape can change in order to accommodate more water in the L64Ixylene solution. The shape can be determined by a combination of DLSPTEB, SLSISAXS, or SANS experiments. Acknowledgment. The author gratefully acknowledges support of this work by the Department of Energy (DEFG0286ER45237.010), the Polymers Program, National Science Foundation (DMR9301294), and the U.S. Army Research Office (DAAH0494G0053). LA940761D
