Langmuir films of polystyrene-b-poly(alkyl acrylate) diblock copolymers

Thin Solid Films 354 (1999) 136±141 www.elsevier.com/locate/tsf

Langmuir ®lms of polystyrene-b-poly(alkyl acrylate) diblock copolymers S. Li, C.J. Clarke*, A. Eisenberg, R.B. Lennox Department of Chemistry, McGill University, 801 Sherbrooke Street West, Montreal, Quebec, Canada H3A 2K6 Received 17 April 1999; received in revised form 2 July 1999; accepted 2 July 1999

Abstract We report studies of the self-assembly of amphiphilic diblock copolymers, polystyrene-b-poly(alkyl acrylate) at the air water interface. Using the Langmuir ®lm balance, unusual multiple plateau/in¯ection transitions in surface pressure / molecular area isotherms have been observed. Transmission electron microscopy of the corresponding Langmuir±Blodgett ®lms has shown the presence of surface micelles. We have studied the effect of a range of different alkyl blocks on the isotherms. Possible mechanisms for the multiple in¯ections observed in the isotherms are discussed. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Nanostructures; Phase transitions; Polymers; Surface pressure

1. Introduction The manner in which polymer molecules spread at the air-water interface to form monolayer or multilayer thin ®lms is of interest both for its fundamental physics and because of potential applications [1]. One system which has been extensively studied in our laboratories and elsewhere consists of diblock copolymers which have a hydrophobic block and an ionic or polar one. These have been shown to spread spontaneously at the air±water interface [1±13]. Unlike most homopolymers these diblocks spontaneously self-assemble to form 'surface micelles', typically 100 nm in size, with the hydrophobic chains aggregating to form the core, surrounded by a corona of hydrophilic chains. The surface micelles can be of three morphologies: circular, ribbons or large `pancake-like' aggregates, usually referred to as lamellar, by analogy with the morphologies found in 3D block copolymer micelles [2]. They can be imaged via transmission electron microscopy (TEM) and atomic force microscopy (AFM) of Langmuir±Blodgett (LB) ®lms. The correspondence between the ®lm formed at the water surface and the LB ®lm has been con®rmed by X-ray scattering studies [10]. The aggregation numbers, morphologies and transitions of these micelles have been investigated [2±9]. Recently, Kumaki and Hashimoto have reported studies of a related system, in which both blocks of a diblock

* Present address: Unilever Research, Colworth House, Sharnbrook, Beds MK44 1LQ, UK. E-mail address: [email protected] (C.J. Clarke)

copolymer are adsorbed at the surface. However in this case the morphology is much less regular [14]. Homopolymers such as polyesters and polyalkylesters spread easily on the surface of water and have been studied over the past 50 years using surface tension measurements and the Langmuir ®lm balance technique [15,16]. In this paper, we explore the surface pressure-molecular area (p± A) isotherms of diblock copolymers composed of a poly (alkyl acrylate) (PRA), a modestly hydrophilic block and polystyrene (PS), a strongly hydrophobic block. The PS-bPRA diblocks which have been studied have R ˆ C3 , C4, C5, C6, C8 and C10. Particular attention has been focused on the butyl system because its p±A isotherms have an unusual multiplicity of in¯ections and plateaus. In these regions, the slope of the p±A curve markedly decreases over a certain area. This is often interpreted in terms of phase transitions within the ®lm, although the precise nature of these phases is rarely known.

2. Experimental section Poly(tert-butyl acrylate) (PtBA) homopolymers and PSb-PtBA diblocks were synthesized by sequential anionic polymerization. Copolymerization was carried out in a fashion analogous to that employed by Zhong et al. [17] following the work of TeyssieÂ's group [18]. The other PS-b-PRA diblocks used in this study were obtained by trans-esterifying PS-b-PtBA. Transesteri®cation was typically performed by dissolving 0.6 g of PS-b-PtBA in 60 ml of toluene, followed by adding 40 ml of the appropriate alcohol and

0040-6090/99/$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S00 40-6090(99)0057 0-2

S. Li et al. / Thin Solid Films 354 (1999) 136±141

0.9 g p-toluenesulfonic acid. The reaction solution was stirred and maintained at re¯ux for 48 h. After completion of the reaction, the solution was concentrated by evaporation, and precipitated in methanol. The products obtained were washed with excess methanol and dried under vacuum at 708C for 24 h. A LAUDA Model D ®lm balance with a maximum surface area of 927 cm 2 was used. During compression of a ®lm, the temperature of the water subphase was kept constant (at 258C unless otherwise stated) with a circulating water bath. The Te¯on trough was ®lled with freshly puri®ed water (from a Millipore Milli-Q water treatment system, 18 MV resistivity) and the water surface was repeatedly cleaned by aspiration before the isotherms were measured during continuous compression of the polymer ®lm. In a typical measurement, 0.1 ml of a polymer stock solution (5 mg of polymer dissolved in 1 ml of chloroform) was spread dropwise on the water surface with a syringe. Fifteen minutes elapsed to allow for solvent evaporation before compression was initiated. The rate of change of area was 40 mm 2/min. LB ®lms were prepared by the method described previously for similar systems [8,9]. Transmission electron microscopy (TEM) was performed on a JEOL-CX100-TEMSCAN. All samples were shadowed with a platinum / palladium mixture at an angle of 158 to 208 in order to provide contrast and height information in the TEM experiment. 3. Results PS does not spread to form ®lms at the air-water interface [19] whereas PtBA does. Fig. 1 shows isotherms of two homopolymers, PtBA(150) and PtBA(1150) (the numbers in parentheses indicate the number of units in the polymer). Both homopolymers form compressible ®lms on water and show a plateau at p ˆ 23 mN/m before collapse occurs at small areas. Unlike most spreadable polymers, these isotherms are molecular weight dependent, with the higher

Fig. 1. Isotherms of two different PtBA homopolymers, compared with the isotherm of a PS-b-PtBA diblock copolymer in terms of tBA residue area.

137

Fig. 2. The isotherms of a range of PS(305)-b-PtBA(0, 12, 47, 85, 138, 222) diblocks. The numbers denote the PtBA corona chain length.

molecular weight material forming the more expanded ®lm. This suggests that although the individual segments are located on the water surface, the area expressed by the longer chain includes a signi®cantly higher trapped or 'dead' area than the lower molecular weight material. At the start of the plateau, the tert-butyl side groups begin to reorient from prone to vertical until eventually all of the side groups have adopted a vertical conformation [15]. Further compression leads to ®lm collapse. Fig. 2 shows isotherms for diblocks with a ®xed PS block length (305 units) and a range of PtBA block lengths (0± 222). PS-b-PtBMA diblocks form very different monolayer ®lms from the related homopolymers. The diblock material is signi®cantly expanded compared to the homopolymers: the area at which the surface pressure ®rst begins to rise for the diblock ®lm is 0.65 nm 2/monomer, c.f. the area of 0.39 nm 2 for a monomer residue. The isotherms exhibit multiple in¯ections and plateaus, except for the most highly asymmetric diblocks. Since up to 4 in¯ections are seen, 4 ®lm states may in fact be accessible in this system. This number of transitions is very unusual in a Langmuir ®lm. Fig. 3 shows TEM images of LB ®lms of PS(305)-bPtBA(222, 138, 85, 47, 12, 0) dipped at a pressure of 2 mN/m. The ®rst three images, corresponding to the longest PtBA units, reveal a highly correlated pattern of circular surface micelles with core diameters and corona lengths of approximately 50 and 35 nm, respectively. The other previously observed surface micelle morphologies, i.e. ribbon (Fig. 3d) and lamellar (Fig. 3e) are found in the more asymmetric diblocks. In situ X-ray re¯ectivity studies of a related system, quaternized polystyrene-b-poly(4-vinyl pyridinium), have established that the structures as observed in the transferred ®lms also exist on the water surface [10]. It has also been established in previous work on similar systems, that these micelles persist over the whole range of surface pressures of the isotherm up to ®lm collapse [2±10]. Once a micelle has formed during the solvent casting process, the hydrophobic core is well below its glass

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S. Li et al. / Thin Solid Films 354 (1999) 136±141

Fig. 3. TEM photographs of LB ®lms of PS(305)-b-PtBA(x) surface micelles: (a) x ˆ 222, circular morphology; (b) x ˆ 138, circular; (c) x ˆ 85, circular; (d) x ˆ 47, ribbons; (e) x ˆ 12, lamellae; (f) x ˆ 0, lamellae.

transition. Chains are then stuck within a particular micelle, and the core size and aggregation number remain unchanged during compression, while the micelles move closer together. Finally PS homopolymer agglomerates are shown (Fig. 3f). The in¯ections and plateaus in the isotherms are only seen when the micelles have the circular or ribbon morphology. The p±A isotherms of large lamellar surface micelles for example, are featureless. This has been previously observed for quaternized polystyrene-b-poly(4-vinyl pyridinium) diblocks [2±7]. The difference between these complex isotherms and the featureless isotherms must arise because the corona chains in the circular micelles are anisotropically oriented to a greater extent than in either

the homopolymer or the very asymmetric diblocks. The surface pressure of the transitions are similar for all PtBA block lengths, and are summarized in Table 1. Experiments were performed to study the temperature dependence and reversibility of the p±A isotherms of the PS(305)-b-PtBA(222) diblock. The effect of increasing the Table 1 The transition pressures as a function of x in PS(305)-b-PtBA(x) PtBA block length (x)

222

138

85

47

p1 (mN/m) p2 (mN/m) p3 (mN/m)

12.2 17.8 22.3

13.2 18.8

13.0 17.6

13

S. Li et al. / Thin Solid Films 354 (1999) 136±141

Fig. 4. The isotherms of PS(305)-b-P(n-BA, s-BA, i-BA, t-BA)(222) diblocks.

temperature is to decrease transition pressure values (at constant area). This is the opposite of what would be expected for an entropy associated surface pressure [20] and is rare, although this has been observed in isotherms of both quaternized polystyrene-b-poly(4-vinyl pyridinium) surface micelles [3] and isotactic PMMA [21]. It can be explained by temperature-dependent lateral cohesive interactions in the ®lm [21]. Melting of the alkyl side chains as the temperature is raised could produce a more liquid like ®lm with weaker cohesive interactions, and hence a lower transition pressure. Compression-expansion hysteresis experiments were performed by compressing newly formed ®lms to a range of surface pressures (7, 14, 19, 28, and 50 mN/m), expanding them to their initial area, and then recompressing (not shown). Every subsequent re-compression of a ®lm retains the original onset area and is superimposable on the previous isotherms, even after compression beyond the third transition. This clearly establishes that the transitions are reversible. A range of PS-b-PRA diblocks (n-C3, i-C3, n-C4, i-C4, sec-C4, tert-C4, n-C5, n-C6, n-C8, n-C10) with identical block lengths were also studied. For the butyl esters, four different side group isomers are available. This makes it possible to study the effects of both packing (different structural isomers) and side group volume (different esters). We ®rst focus on the butyl esters, whose p±A isotherms are shown in Fig. 4. They all show multiple in¯ections as described above for the tert-butyl ester. The nature of the butyl group clearly affects the surface pressures at which the in¯ections are observed; these are given in Table 2.

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Fig. 5. The effect of the ester group length of PS(305)-b-PRA(222) (R ˆ C3 , C4, C5, C6, C8, C10) diblocks on the isotherms.

The pressure at which the ®rst transition occurs (p1) follows (in order of decreasing pressure): tert-BA . iBA . sec-BA . n-BA. This is also the order of the packing ef®ciency of the butyl groups. For both the second and third transitions, the transition pressures for the s-butyl ester are the highest, whilst those of the other isomers are lower and similar. These observations suggest that the side group has a greater effect on the ®rst transition than the second or third; thus the latter transitions may be associated with conformational changes of the backbone while the former involves reorientation of the side groups. Fig. 5 shows the isotherms for six samples of n-alkyl acrylate (n ˆ 3, 4, 5, 6, 8, 10). The transition parameters are listed in Table 3. The C3 isomer shows only one low pressure transition (the trends in the transition pressures suggest that this may correspond to the second transition though the transition pressure in this case lies between p2 and p3 of the C4). The C5 and C6 samples show three transitions (like the C4 discussed above). The C8 and C10 samples do not exhibit the ®rst transition. For the rest, p1 increases slightly with side group length. The second and third transitions occur at similar pressures for all the side group lengths, i.e. 16.1±17.8 and 19.2 to 20.8 mN/m, respectively, and all decrease slightly with increasing group length, i.e. showing the opposite trend from p1. C3 and C4 also show a high pressure transition at ,50 mN/m.

Table 3 The transition parameters for PS(305)-b-PnRA(222)

Table 2 The transition pressures of the butyl ester isomers in PS-b-PBA

Alkyl group

Isomer

t-BA

i-BA

s-BA

n-BA

p1 (mN/m) p2 (mN/m) p3 (mN/m)

12.2 17.8 22.3

12.1 17.7 21.5

9.4 22.4 24.7

8.9 17.8 21.0

p1 p2 p3 p4

(mN/m) (mN/m) (mN/m) (mN/m)

C3

C4

C5

C6

C8

C10

19.4 53.9

8.9 17.8 21.0 47.3

10.5 16.9 20.5

11.6 16.2 20.8

16.1 19.6

16.1 19.2

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S. Li et al. / Thin Solid Films 354 (1999) 136±141

4. Discussion There are several different interactions in the system which combine to produce the complex observed behaviour, e.g. side group - side group interactions, backbone - backbone and backbone - water. Side group reorientation is known to cause the plateau in the PtBA homopolymer isotherms, and we expect this process to occur also in the corona chains in the surface micelles. However the radial orientation and con®nement of one chain end at the core of the micelle make the situation in the diblock materials more complicated. One possibility is that the multiple transitions correspond to the single transition seen in the homopolymer system (i.e. the prone-to-vertical side group reorientation) divided into several transitions, each corresponding to different density regions in the micelle. Simple geometric arguments (using the micelle dimensions from TEM images of LB ®lms) show that the surface density for PS(305)-bPtBA(222) approximately doubles between the perimeter of the corona and the outer edge of the core. The ®rst transition would thus correspond to reorientation of the alkyl groups near the micelle core where the surface density is highest. The macroscopic surface pressure required to produce the transition is thus lower than in the homopolymer system. Subsequent transitions would then re¯ect the reorientation taking place in regions of the corona further from the core. However this explanation would require the surface density to be divided into a series of discrete regions as a function of distance from the core, which seems unlikely. Neither does it explain some of the experimental observations (Tables 2 and 3) such as why p1 increases with alkyl group length while p2 decreases and p3 shows no obvious trends. This difference suggests that the origin of the ®rst transition is different from the others. An alternative explanation is that the ®rst transition is due to to side group reorientation, and the others to further changes in the conformation of the corona chains. The trend in p1 in the butyl esters follows the order of their packing ef®ciency, indicating that more dense groups need more energy to produce the transition (as expected for side chain reorientation). In the n-alkyl series C3, C8 and C10 do not show the ®rst transition (i.e. the one at ca. 10 mN/m). For C3, this may be because the side group is short, so that there is little difference energetically between the prone and vertical orientations, i.e. the small propyl ester groups are easily reoriented into the air from the water surface. C8 and C10 do not exhibit the ®rst transition, possibly because the side groups are so large that the enforced proximity to other chains in the micelle causes the them to be vertical before compression. For the others p1 increases slightly with group length. It remains to explain the `extra' in¯ections seen in most of the samples. The relevant experimental observations regarding these transitions are: ² Multiple transitions are only occur for circular and

² ² ²

²

²

ribbon micelles and not for the homopolymer. Therefore the spatial con®nement and orientation of the chains imposed by the micelle structure is important. The transitions are completely reversible, so that multilayering seems unlikely. The transition pressures show no obvious trend with corona chain length. In the butyl esters, the secondary isomer has the highest transition pressure for both the second and third transitions whilst those of the other three isomers are both lower and similar. The nature of the side group has a smaller effect on pt in the second and third transitions than in the ®rst. The pressures of the second and third transitions are almost the same for all the n-alkyl samples, i.e. 16.1± 17.8 and 19.2 to 20.8 mN/m, respectively. There is however a slight decrease in both p2 and p3 as the side group length increases. Again, the side group has a smaller effect on pt in the second and third transitions than in the ®rst. We have previously studied circular surface micelles of a related diblock, PS(180)-b-PtBMA(70) which has stiffer methacrylate corona chains, and for which the isotherm exhibits only one plateau [8]. Thus it appears that a ¯exible chain is required for multiple transitions.

These observations suggest that some form of backbone condensation in the monolayer is probably responsible for the second and third transitions. This has previously been observed in PMMA monolayers, where Brinkhuis and Schouten attributed a transition in the p±A isotherm to the formation of double helices [22]. Inter-micelle corona chain condensation seems unlikely since this would require the interpenetration of coronal chains on adjacent micelles, and there are no de®nitive examples within the polymer literature where this occurs. Intra-micelle condensation could occur when compression forces the chains into a series of states, each marked by an in¯ection/plateau in the isotherm. We also note that multiple transitions have been observed (but not discussed) in the p±A isotherms of atactic low molecular weight poly(octadecyl methacrylate) [23]. It is dif®cult to infer with any con®dence the structural and conformational origin of transitions from the isotherms alone, without any supporting evidence. Further experiments, such as grazing incidence IR spectrometry, X-ray and neutron re¯ectivity will be required to determine the structure of the ®lm and the nature of the transitions. At our current level of understanding, the pressure-induced organization of polymer backbones is the favoured mechanism. 5. Summary The surface self-assembly of polystyrene-b-poly(alkyl acrylate) diblock copolymers at the air-water interface has been studied by measuring p±A isotherms and performing

S. Li et al. / Thin Solid Films 354 (1999) 136±141

TEM imaging on LB ®lms. A number of interesting in¯ections/plateaus are observed in the isotherms. Nearly symmetric diblocks show three or four `phases' while the corresponding homopolymer has only one transition, at relatively high surface pressure. These multiple transitions were found to be reversible and are very unusual. Transesteri®cation of the PS(305)-b-PtBA(222) diblock copolymers was performed, to investigate the effects of the packing ef®ciency of different butyl ester isomers and the ester group length. From a series of experiments on different esters, we can observe some trends which give clues to the origin of the transitions. We tentatively suggest that the ®rst transition corresponds to side group reorientation, as in the homopolymers, and that the other transitions may be due to some form of backbone condensation or organization. References [1] J.K. Cox, A. Eisenberg, R.B. Lennox, Curr. Opin. Coll. Interface Sci. 4 (1999) 52. [2] J. Zhu, A. Eisenberg, R.B. Lennox, J. Am. Chem. Soc. 113 (1991) 5583. [3] J. Zhu, R.B. Lennox, A. Eisenberg, Langmuir 7 (1991) 1579. [4] J. Zhu, A. Eisenberg, R.B. Lennox, Macromolecules 25 (1992) 6547.

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