Langmuir 2005, 21, 9199-9205
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Langmuir-Blodgett Films of Preformed Polymers Containing Biphenyl Groups Frank Davis,*,† Philip Hodge,*,‡ Richard H. Tredgold,‡ and Ziad Ali-Adib‡ Institute of Bioscience and Technology, Cranfield University at Silsoe, Silsoe, MK45 4DT, United Kingdom, and Chemistry Department, University of Manchester, Oxford Road, Manchester M13 9PL, United Kingdom Received April 15, 2005. In Final Form: July 18, 2005 Polymers containing liquid crystal groups have been studied previously as waveguides, and they have been deposited as spacer materials with various chromophores to form alternating films However, only a few members of this group of materials have been studied in any detail, and very little structural information has been obtained so far. Therefore, a more detailed examination of these materials as monoand multilayers was undertaken. A new group of materials including the same mesogenic group, polymeric sulfones, was also studied. The polymers gave steep isotherms with high collapse pressures, indicating good packing of the monolayers, and could be deposited to form thick multilayers. X-ray diffraction showed that an ordered multilayer was formed, and the effects of the polymers chemical nature on the structure of the LB films are discussed. It appears that the dominant factor in monolayer structure is the nature of the polymer backbone rather than that of the liquid crystal side chains, which play a secondary role.
1. Introduction Langmuir-Blodgett (LB) films have been known since 1935,1 but most LB films of classical materials, such as stearic acid, are not sufficiently stable or robust for use in commercial devices. This has prompted an interest in polymeric LB films, though such films are, of course, also of considerable fundamental interest in themselves because they are one of the simplest types of ordered polymeric materials. It is recognized that the films obtained from preformed polymers may not be as highly ordered as the films prepared from low molecular weight amphiphiles, but for many applications the order is quite acceptable. If regular LB multilayers are to be obtained from preformed amphiphilic polymers, it would be expected that the hydrophilic and lipophilic parts should be frequently and regularly placed along the polymer chain. Polymers that have structures of this type and have been studied as monolayers on water2 include polyacrylates,3 polymethacrylates,3-6 poly(viny1 ester)s,7 poly(viny1 ether)s,8 and acetals prepared from poly(vinyl alcohol).9,10 However, in many of these polymers, the hydrophilic * Corresponding authors. F.D.: phone, +44(0)1525 863455; fax, +44 (0)1525 863533; e-mail,
[email protected]. P.H.: phone, +44(0)161 2754707; fax, +44 (0)1524 793252; e-mail, philip.hodge@ man.ac.uk. † Cranfield University at Silsoe. ‡ University of Manchester. (1) Blodgett, K. B. J. Am. Chem. Soc. 1935, 57, 1007. (2) Breton, M. J. J. Macromol. Sci.; Rev. Macromol. Chem. 1981, C21, 61. (3) Mumby, S. J.; Swalen, J. D.; Rabolt, J. F. Macromolecules 1986, 19, 1054. (4) Puggelli, M.; Gabrielli, G. Colloid Polym. Sci. 1985, 263, 879. (5) Mumby, S. J.; Rabolt, J. F.; Swalen, J. D. Thin Solid Films 1986, 133, 161. (6) Duda, G.; Schouten, A. J.; Arndt, T.; Lieser, G.; Schmidt, G. F.; Bubek, C.; Wegner, G. Thin Solid Films 1988, 159, 221. (7) Puggelli, M.; Gabrielli, G. J. Colloid. Interface Sci. 1977, 61, 420. (8) Lovelock, B. J.; Grieser, F.; Sanders, J. V. J. Colloid. Interface. Sci. 1985, 108, 297. (9) Watanabe, M.; Kosaka, Y.; Oguchi, K.; Sanui, K.; Ogata, N. Macromolecules 1988, 21, 2997. (10) Oeuchi, K.: Yoden, T.: Kosaka, Y.; Watanabe, M.; Sanui, K.; Ogata, N. Thin Solid Films 1988, 161, 305.
groups are relatively weakly hydrophilic and the preparation of ordered LB multilayers is not always easy, although, for example, multilayers can be constructed from materials such as polyoctadecyl acrylate or methacylate.3 Also a variety of substituted rodlike polymers such as polyglutamates11 and polyphthalocyanines12 have been successfully deposited as LB multilayers. Our early work in this area was concerned mainly with copolymers of styrene and maleic anhydride, which are easily available and can be reacted with water, alcohols, amines, etc. to give a wide variety of derivatives.13 Although these polymers form monolayers on water that display steep isotherms with high collapse pressures, the LB films prepared from them are not in general highly ordered, unless the anhydride residues have been reacted with a long-chain alcohol.14 Even then, when studied by lowangle X-ray scattering, they showed just one Bragg peak. Derivatives of long chain alkene/maleic anhydride copolymers were, however, much more satisfactory and gave much better ordered films (up to three Bragg peaks).14-16 Other workers later studied the formation of LB films of a variety of maleic anhydride copolymers, for example, monolayers of maleic anhydride copolymer and their interactions with metal ions in the subphase and incorporation into LB films.17 Other workers reacted octadecene-maleic anhydride copolymers with species such as 4-aminopyridine18 or histamine19 and studied complex formation with a variety of metal ions. Similar polymers (11) Mabuchi, M.; Ito, S.; Yanamoto, M.; Miyamoto, T.; Schmidt, A.; Knoll, W. Macromolecules 1998, 31, 8802. (12) Silerova, R.; Kalvoda, L.; Neher, D.; Ferencz, A.; Wu, J.; Wegner, G. Chem. Mater. 1998, 10, 2284. (13) Tredgold, R. H.; Winter, C. S. J. Phys. D Appl. Phys. 1982, 15, L55. (14) Tredgold, R. H.; Vickers, A. J.; Hoorfar, A.; Hodge, P.; Khoshdel, E. J. Phys. D Appl. Phys. 1985, 18, 1139. (15) Hodge, P.; Davis, F.; Tredgold, R. H. Philos. Trans. R. Soc. 1990, A330, 153. (16) Davis, F.; Hodge, P.; Towns, C. R.; Z. Ali-Adib, Z. Macromolecules 1991, 24, 5695-5703. (17) Lee, B.-J.; Choi, G.; Kwon, Y.-S. Thin Solid Films 1996, 284285, 564. (18) Oertel, U.; Nagel, J. Thin Solid Films 1996, 284-285, 313. (19) Jeong, H.; Lee, B.-J.; Cho, W. J.; Ha, C.-S. Polymer 2000, 41, 5525.
10.1021/la051011e CCC: $30.25 © 2005 American Chemical Society Published on Web 08/23/2005
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could also be substituted with pyrenyl groups and their exciton energy transfer studied20 or reacted with a perfluorinated alcohol to give LB films of extremely low dielectric constant.21 LB films of these types of polymers were also suitable matrixes for the incorporation of nanoparticles of materials such as PbS22 and iron oxide.23 Maleic anhydride can copolymerize with a wide variety of olefins; therefore, a range of olefins derived from commercial liquid crystals were synthesized24 and shown to form monolayers and deposit to form LB films suitable for use as waveguides.25 This class of polymer has also been incorporated as spacer layers combined with various chromophores in nonlinear optical devices.26,27 Besides our work, a series of liquid crystalline maleic acid copolymers containing biphenyl and cyanobiphenyl units has also been studied as monolayers and one of them deposited as a multilayer.28 Sulfur dioxide has also been shown to form alternating copolymers with a variety of olefins. Previous work has included the synthesis of copolymers with mesogenic olefins29 and deposition of LB films of a 2-methylpentene copolymer.30 Other polymers containing sulfone groups in the side chains have been deposited to form LB films.31,32 In this paper we present more detailed studies of Langmuir films and LB films formed from derivatives of copolymers of biphenyl-containing olefin with either maleic anhydride or sulfur dioxide. The studies include the effect of varying the comonomer, the length of the spacer chain between the biphenyl and the olefinic group, the linker between the chain and biphenyl unit, and the size of the headgroups above the biphenyl. The purpose of this work is to attempt to determine which are the major contributors to polymeric LB film structure: is either the backbone structure or the nature of the side chains and their packing interactions the dominant factor, or is it a combination of the two? Many of the polymers previously studied for LB work have not been deposited as thick films and their postulated regular structure been proved by diffraction methods. These measurements are crucial since they show whether the ordered structures found in, for example, fatty acid films are repeated in polymeric species, since many desirable properties of LB films such as second-harmonic generation absolutely require a regular structure.26,27,31 2. Materials and Methods Solvents were of HPLC grade and supplied by Aldrich. The structures of the various materials used are given in Scheme 1. (20) Oertel, U.; Appelhans, D.; Friedel, P.; Jehnichen, D.; Komber, H.; Pilch, B.; Hanel, B.; Voit, B. Langmuir 2002, 18, 105. (21) Onah, E. J. Chem. Mater. 2003, 15, 4104. (22) Li, L. S.; Qu, L.; Wang, L.; Lu, R.; Peng, Z. Langmuir 1997, 13, 6183. (23) Lee, D. K.; Kang, Y. S.; Lee, C. S.; Stroeve, P. J. Phys. Chem. B 2002, 106, 7267. (24) Vickers, A. J.; Tredgold, R. H.; Khoshdel, E.; Hodge, P.; Girling, I. R. Thin Solid Films 1985, 134, 43. (25) Tredgold, R. H.; Young, M. C. J.; Hodge, P.; Khoshdel, E. Thin Solid Films 1987, 151, 441. (26) Young, M. C. J.; Jones, R.; Tredgold, R. H.; Lu, W. X.; Ali-Adib, Z.; Hodge, P.; Abbasi, F. Thin Solid Films 1989, 182, 319. (27) Young, M. C. J.; Lu, W. X.; Tredgold, R. H.; Hodge, P.; Abbasi, F. Elect. Lett. 1990, 26, 993. (28) Nieuwhof, R. P.; Kimkes, P.; Marcelis, A. T. M.; Sudholter, E. J. R.; Opitz, R.; de Jeu, W. H. Langmuir 2001, 17, 78. (29) Braun, D., Arnold, N.; Liebmann, A.; Schmidtke, I. Macromol. Chem. 1993, 194, 2687. (30) Dovek, M. M.; Albrecht, T. R.; Kuan, S. W. J.; Lang, C. A.; Emch, R.; Grutter, P.; Frank, C. W.; Pease, R. F. W.; Quate, C. F. J. Micros. 1988, 152, 229. (31) West, D.; Dunne, D.; Hodge, P.; McKeown, N. B.; Ali-Adib, Z. Thin Solid Films 1998, 323, 227. (32) Razna, J.; Hodge, P.; West, D.; Kucharski, S. J. Mater. Chem. 1999, 9, 1693.
Davis et al. Scheme 1. Structures of the Polymers Based on (A) Maleic Anhydride and (B) Sulfur Dioxide
Table 1. Structures and Molecular Weights of the Various Polymers polymer
R
n
X
Mna
Mwa
1 2 3 4 5 6 7 8 9 10
H H C5H11 C5H11 C5H11 H C5H11 H OC5H11 C5H11
0 2 2 8 10 3 2 8 2 9
CdO CdO CdO CO2 CO2 CO2 CdO CO2
14000 22000 5000 5000 4000 3500 2000 14000 3900 2500
23000 33000 9000 10000 6000 5500 2500 25000 4500 3900
a GPC of parent anhydride copolymer based on polystyrene standards. The synthesis of the monomers, their copolymerization with maleic anhydride, and reaction with methanol have been previously described.16,24 Copolymers with sulfur dioxide were synthesized as previously published.29 Since the degree of polymerization varies from polymer to polymer, their molecular weights were measured using gel permeation chromatography (GPC), carried out on a Waters Associates Model 440 instrument using high molecular weight polystyrene columns, THF as the eluent, and polystyrene standards (Table 1). The LB trough used in this work was designed and built inhouse and has been extensively described in previous work.13,14,16 Specific details of the experimental conditions used in the present work are as follows. The subphase was deionized, double-distilled, 0.2-µm-filtered water at room temperature, pH 5.3-5.6. For X-ray experiments CdCl2 was added to the subphase (to a concentration of 2.5 × 10-4 M). Surface films were prepared by dissolving the polymer in ethyl acetate (toluene for polymers 11, 12) to a concentration of 0.1-0.3 mg/mL. An appropriate amount of this solution was carefully placed on the water surface and the solvent allowed to evaporate. The film was compressed to 30 mN m-1 and then expanded, and this procedure was repeated several times to check that reproducible isotherms were obtained. Unless indicated otherwise, LB films were deposited on silicon wafer slices (treated to make them hydrophobic as previously described)16 operating at a surface pressure of 30 mN/m and a dipping speed of 5 mm/min. For the magnetically aligned films, solutions of the polymers in ethyl acetate were cast onto glass slides and then heated to 140 °C inside a 5 T magnetic field as previously described.33,34 For low-angle X-ray studies, thick layers were deposited on silicon wafers using an automated trough.16 Bragg peaks were detected and measured with a Raymax RX3D X-ray diffractometer using nickel-filtered Cu KR radiation, calibrated using the (111) reflection from a silicon wafer. The scan rate was 0.25° min-1 with a spot size of 1 mm.
3. Results 3.1. Isotherms of the Polymers. Isotherms of the polymers 1-10 are given in Figures1 and 2. They were (33) Tredgold, R. H.; Ali-Adib, Z. J. Phys. D. Appl. Phys. 1988, 21, 1467. (34) Ali-Adib, Z.; Tredgold, R. H.; X.; Hodge, P.; Davis, F. Langmuir 1991, 7, 363.
LB Films of Polymers Containing Biphenyl Groups
Langmuir, Vol. 21, No. 20, 2005 9201 Table 2. Monolayer and Multilayer Behavior of the Polymers
Figure 1. Isotherms of the maleic anhydride copolymers 1-5.
Figure 2. Isotherms of the maleic anhydride copolymers 6-10.
measured on pure water subphases, at pH 5.4-5.6 and room temperature. As can be seen, the polymers have steep isotherms with collapse pressures over 40 mN m-1 and slightly larger areas per repeat unit than the straight chain analogues.16 The polymers gave reproducible isotherms upon expansion and recompression, providing the collapse pressure was not attained, indicating that there is good redispersion of the materials upon expansion and that excessively rigid monolayers are not formed, similar to previous results.16 We found the isotherms of the polymers to be very insensitive to the presence of cadmium in the subphase, the isotherms measured on 2.5 × 10-4 M CdCl2 being essentially identical to those on water, indicating a minimal effect of Cd ions on the packing of the polymer at the air-water interface. This is in contrast to the behavior of fatty acid monolayers,1 indicating again that it is the nature of the polymer backbone at the airwater interface that is the dominant factor in monolayer behavior. The areas per repeat units and isotherm shapes are similar to those obtained for maleic acid copolymers previously reported.28 For almost all the polymers, the isotherms become very steep at pressures of more than 5 mN m-1, probably because a tightly packed structure is being formed. The isotherms are fairly featureless otherwise, except for polymer 8, which has a kink at 25 mN m-1, which could indicate that at lower pressures there is some interaction between the hydrophilic ester group adjacent to the biphenyl unit and the water surface, a similar interaction being noted by other workers for polymers containing cyanobiphenyl groups.28 This did not
polymer
area/repeat unit at 30 mN m-1
bilayer spacing (nm)
number of Bragg peaks
1 2 3 4 5 6 7 8 9 10 11 12
0.41 0.38 0.40 0.34 0.38 0.27 0.31 0.33 0.38 0.32 0.26 0.19
2.49 2.95 3.88 4.25 4.34 2.77 3.89 3.46 4.05 4.48 3.87 2.34
1 1 1 2 2 1 1 2 2 2 1 2
occur for similar polymers 6 and 7, probably because these have much shorter and less flexible spacer chains. This kink remained upon repeated compression of the monolayer and was insensitive to the presence of cadmium ions in the subphase and also to the pH, seeing that dilute HCl was added to the subphase to give a pH of 1.8 but caused minimal change in the isotherm. All the materials formed stable monolayers at 30 mN m-1. The areas per repeat unit at this pressure are given in Table 2. The side chain of the polymer is made up of several parts. All the polymers contain the biphenyl unit. The biphenyl unit is linked to the main chain by short or long spacers and is linked to the spacer by a variety of groups (ester, ketone, or methylene). In some cases, it is also substituted with a second short alkyl or alkoxy chain. Variation of the linking group, spacer chain, and substituent all had effects on the area per repeat unit. In general, however, the polymers with an ester linking group tended to give lower areas per repeat unit than with the ketone group, both of which were less than their hydrocarbon analogues. This is possibly indicative of the amount of interaction there is between neighboring side chains. Addition of an alkyl substituent to the biphenyl only increased the area per repeat unit by a small amount, and the addition of an alkoxy substituent slightly lowered the surface area. The length of the spacer chain is also found to have some effect; by comparing 1 with 2, 3 with 4, or 7 with 10 it is apparent that the longer spacer polymers give lower areas per repeat unit suggesting tighter packing. This could be due to the bulky biphenyl unit being at a larger distance from the backbone, not effecting its packing, or the longer, more flexible spacer allowing better ordering within the monolayer, similar to findings by other workers.28 The differences in surface area per repeat unit, although dependent on the side chain structure, are in many cases quite small, indicating that the packing of the polymer backbone and headgroups on the surface is a major factor in determining the nature of the monolayers. 3.2. Deposition of the Polymers and Multilayer Structures. LB films could be prepared from all this class of polymer. Some of the polymers in general did not dip as well as the straight chain analogues. The problem appeared to be greater rigidity in the film than for the earlier polymers. Increasing the temperature (up to 35 °C) or varying the dipping pressure (10-40 mN m-1) did not appear to have any effect. However multilayers of the polymers could be constructed and X-ray diffraction measured. The polymers were deposited similarly to the derivatives of the R-olefin-maleic anhydride copolymers16 at a surface pressure of 30 mN m-1. Thick films (300 layers)
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Davis et al. Table 3. Modeling of Polymer Spacing
polymer 1 2 3 4 5 6 7 8 9 10
Figure 3. Low-angle X-ray diffraction trace for 300 layers of polymer 8.
Figure 4. Proposed structure of the bilayer of polymer 1.
were deposited on hydrophobic silicon wafers using subphases containing 2.5 × l0-4 M CdCI2. Polymers 3, 4, 5, and 10 all gave reproducible Y-type deposition. The remainder gave Y-type deposition but with slightly inferior transfer ratios (0.8-1.0) on upstrokes. The films were usually clear and even in texture to the naked eye, although films of 1, 6, and 7 were slightly cloudy in appearance. Low-angle X-ray studies were made on the films, and this technique shows Bragg peaks which can be used to determine the bilayer spacing. The number and height of peaks also gives a rough indication of the order in the films, well-ordered films of materials such as cadmium stearate giving at least nine peaks.16 A typical diffraction trace is shown for polymer 8 (Figure 3). As was found with the derivatives of the R-olefin-maleic anhydride copolymers,14,16 longer side chains led to greater order in the film. Polymers containing these longer chains gave more intense peaks in the X-ray studies and also always showed a second-order peak. The number of peaks was never more than two, however, compared with three for some of the R-olefin-maleic anhydride copolymers.16 From previous work16 with the derivatives of the R-olefin-maleic anhydride copolymers,16 we deduced a thickness of 1.17 nm for the maleic anhydride residues within the bilayer (Figure 4). If it is assumed that the structure of the maleic anhydride-derived residue in the LB film is the same as previously determined,16 the thickness of a biphenyl bilayer can be derived from the thickness of a bilayer of polymer 1 (2.49 nm) to be 1.32 nm. Measurement of a CPK molecular model gave a length for a single biphenyl unit of 1.10 nm. Therefore, it can be
thickness of side chain bilayer(nm) X-ray CPK 0.66 0.89 1.35 1.54 1.58 0.80 1.36 1.14 1.44 1.65
1.10 1.40 1.86 2.51 2.60 1.62 2.08 2.10 2.02 2.72
angle of side chain to substrate (deg) 37 39 47 38 37 30 41 33 45 37
calculated that the biphenyl unit is tilted at about 37° to the substrate, similar to the linear olefin-maleic anhydride copolymers.16 Similar measurements give the calculated angles between the side chain and the substrate for the other material; see Table 3. Tilting of biphenylcontaining side chains has also been observed for maleic acid copolymers.28 Several points can be seen from the angles calculated. First, all the tilt angles are relatively similar, indicating that variation of the side chains does not cause major changes in film structure. The tilt angles quoted are of course calculated average tilt angles consistent with the X-ray spacing and do not actually prove existence of a close packed crystalline structure. However the chains must be consistently tilted to within a few degrees of each, because if they were to be widely different, then the structure of the multilayer would be so disrupted that no regular layer structure would be present to give rise to X-ray diffraction. Since the area per repeat unit is greater than the cross-section of a biphenyl unit or alkyl chain, there is probably free volume within these films. Earlier FTIR studies tended to indicate that these chains exist in a liquidlike environment.16 Although the X-ray data does not indicate formation of a close-packed crystalline structure, we can still deduce some information about the alignment of groups within the LB film. The smallest tilt from the vertical appears to occur with polymers 3 and 9, both of which are very similar in structure. Combined with the isotherms, this indicates that the ether oxygen in polymer 9 has a negligible effect on the film structure. Replacing the chain with a hydrogen atom leads to a small increase in tilt (compare 1 with 3 or 5 with 8). The length of the spacer also has an effect, the shorter spacer polymers being less tilted than the polymers with longer spacers, as can be seen for polymers 3 and 4 or 7 and 10. Alteration of the functional group via which the spacer is attached to the biphenyl group appears to have a negligible effect on the tilt, possibly because in the multilayer packing considerations dominate over the effects of this group; for example, compare the similar polymers 4, 5, and 10. Studies using X-ray diffraction and FTIR on the derivatives of the R-olefin-maleic anhydride copolymers16 showed a tilted structure with no interdigitation. The biphenyl-containing polymers appear to have a very similar structure to the derivatives of the R-olefin-maleic anhydride copolymers.16 Only polymers 6 and 8 are tilted at an angle of less than 37° to the substrate. Some of the materials also have areas per repeat unit large enough to allow interdigitation of the alkyl side chains, such as polymers 1 and 3. Whether this occurs or not is not known; however, the X-ray spacings are too large for this interdigitation to be complete, except for polymer 6 and possibly 8. Partial interdigitation, for example, just the alkyl chain substituted on the biphenyl ring, could be
LB Films of Polymers Containing Biphenyl Groups
Figure 5. Isotherms of two versions of maleic anhydride copolymer 3.
occurring. This would mean that the angle between the substrate and the side chain would be larger than calculated. Although we do not have absolute proof whether interdigitation occurs, we think a tilted noninterdigitated structure is the most likely, as observed for the R-olefin-maleic anhydride copolymers16 and also a similar polymer deposited by other workers.28 From the area per repeat unit and bilayer spacings it is possible to estimate the volume of a polymer repeat unit and also its density. The polymers tend to have densities in the range 1.0-1.2; however, two materials, the similar polymers 3 and 9, have lower densities (both about 0.9). Again this indicates a somewhat disorganized material with a large amount of free volume within the system. From our studies, therefore, it appears that the major factor in determination of film structure is the structure of the polymer backbone and headgroups, the side chains only having a smaller effect since the isotherms and the side chain tilt angles are so similar to each other and those of the linear R-olefin-maleic anhydride copolymer derivatives.16 A higher molecular weight version of polymer 3 (Mn ) 18 000, Mw ) 32 000) was tested as above. Although the area per repeat unit was slightly lower for the high molecular weight version (isotherms given in Figure 5), the X-ray diffraction trace was almost identical. This confirms that the molecular weight of the polymer, at least between certain limits, has only a negligible effect on the film structure, as found for the corresponding linear R-olefin derivatives.16 It was somewhat disappointing that the liquid crystalline derivatives did not form better films than the linear olefin copolymers. This is thought to be due to the difficulty in dipping the films and the dominance in the backbone packing for the polymers with biphenyl groups in the chains. 3.3. Magnetic Alignment of the LB Films. From the work described, it appears that during formation of LB layers, the backbone structure and its interaction with the air-water interface is the major deciding factor in the structure of the final LB film. Langmuir-Blodgett films are not the only way of aligning polymer molecules; many liquid crystals which can be electrically or magnetically aligned contain biphenyl units. Therefore, it was decided to investigate whether cast films of our materials, when heated, would form an ordered liquid crystal phase, indicating interactions between the mesogenic groups and whether they could then be aligned by use of a high
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magnetic field. Magnetic fields have been used to align polymers cast from solution.33,34 It is thought that to align the polymers in this way, a nematic liquid-crystalline phase must be formed.34 Previous studies on this range of polymers have shown that only 4 and 5 form nematic phases, the others remaining crystalline or forming smectic phases.34 Polymers 4 and 5 have been studied by casting them as films from solution and heating inside a 5 T superconducting solenoid. Polymer 4 was found (by X-ray and birefringence studies)34 to align in a magnetic field similar to an azobenzene-containing polymer previously studied.34 Polymer 5 did not show this behavior. Differential scanning calorimetry (DSC) studies of both polymers indicated formation of a mesophase at about 140 °C. However, polymer 5 did not align. This could be due to the fact that a larger dipole across the biphenyl link due to the ketone group exists for polymer 4, helping it to align in a magnetic field. LB films (100 layers) of both polymers were dipped on glass and heated in a magnetic field in an attempt to align them also. No alignment could be observed by birefringence studies. It is thought that the order in an LB film prevents reorientation of the mesogenic units to form a mesophase.34 3.4. LB Films of Sulfur Dioxide Copolymers. Sulfur dioxide forms alternating 50:50 copolymers with many olefins in the manner of maleic anhydride.29,30 The formation of LB films from these polymers was attempted. 3.4.1. Isotherms of the Polymers. Copolymers of sulfur dioxide with various R-olefins were synthesized, but upon studying them for LB film qualities, they were found to be unsuitable. The polymers when spread gave very irreproducible isotherms. The olefins used ranged in length from C10 to C18. Stable monolayers could not be formed even at very low pressures (5 mN m-1). Therefore, no detailed study could be made of these materials. Monolayers of a copolymer of 2-methyl-1-pentene with SO2 have been deposited, however, indicating that polymers with this type of structure can be deposited.30-32 Two copolymers containing liquid crystal groups, polymers 11 (n ) 8, Mn ) 8000, Mw ) 13 000) and 12 (n ) 2, Mn ) 6000, Mw ) 10 000), were found to be more suitable for study than the copolymers with linear olefins. The structure of these polymers can be found in Scheme 1. Table 2 gives the areas and bilayer spacings of these materials. The polymers had their isotherms measured as previously described; they are shown in Figure 6. Both materials had rather low surface areas per repeat unit compared with the maleic anhydride derivatives. This is probably due to the smaller size of one sulfone headgroup compared to the two headgroups of each anhydride residue, indicating again that the structure of the polymer backbone is a major factor in determining the packing of the compounds. The isotherms were very steep, indicating a high degree of order in the monolayer. The surface area per repeat unit for the polymers is very close to the crosssectional area of the biphenyl unit as measured with CPK molecular models. This indicates that the side chains must be packed very tightly together. 3.4.2. Deposition of the Polymers. The polymers were dipped at 30 mN m-1 on a subphase of pure water. The substrate was hydrophobic silicon. Polymer 11 gave somewhat irregular Y-deposition, with a deposition ratio of 0.95-1.05 on upstrokes and 0.4-1.1 on downstrokes. The film was, however, of good quality to the naked eye. Polymer 12 gave reproducible Z-deposition, as found by
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manner that helps stabilize the film whereas in the other systems the packing is either prevented by the larger distance between the units, or occurs and disrupts the monolayer. Fewer backbone atoms separating the mesogenic units should decrease tilt and better align the biphenyl units, increasing π-π interactions. It would be interesting to see what effect varying the average distance between the mesogenic units would have on the film-forming properties. Possible ways of achieving this include variation of the nonmesogenic monomer, substituting the anhydride unit with a long alkyl or mesogenic chain, or introducing a third nonmesogenic comonomer, for instance hexadec-1-ene or oct-1-ene. 4. Conclusions
other workers for their biphenyl polymer,28 and appeared by the naked eye to give a poorer quality multilayer. Thick LB films were constructed on a pure water subphase and examined by low angle X-ray diffraction. Cadmium was not included in the subphase because the absence of acid groups in the polymer suggests that there would be no uptake of cadmium ions. The peaks were of low intensity, probably because the heaviest atoms present in the film were sulfur atoms. However, polymer 11 gave peaks indicating a Y-type structure and polymer 12 a peak indicating a Z-type structure, when compared with similar maleic anhydride copolymer derivatives. Work on many nonpolymeric species has showed that usually Z-deposition is followed by rearrangement to give a Y-type structure.1,28,35 This does not appear to occur with polymer 12, possibly because the sheer size and bulk of the polymer inhibits rearrangement. These polymers also gave much less tilted structures, the measured thickness for a bilayer of 11 being 5.02 nm (indicating a tilt angle of 50°) and a Z-layer of 12 having a calculated thickness of 2.28 nm, indicating that the side chains are standing vertically, as expected for such a tightly packed structure. Overall then, the sulfur dioxide copolymers did not form as good, well-ordered LB films as the maleic anhydride derivatives, possibly due to having less hydrophilic headgroups and also only one headgroup per hydrophobic side chain. However they appeared to be quite different than the maleic anhydride derivatives, displaying much smaller surface areas per repeat unit and much less tilted structures in the multilayer. This leads us to the conclusion that the structure of the polymer backbone and headgroups is a major factor in determining the monolayer and multilayer structure. It is interesting to note that the presence of liquidcrystalline groups appears to improve the quality of the mono/multilayers over their linear analogues rather than, in the case of the maleic anhydride copolymers, reduce it. It is not clear why this occurs, but a possibility is that for the maleic anhydride copolymers (backbone repeat unit four carbons long) the mesogenic units are held further apart than in the SO2 copolymers (two carbons + one sulfur). This difference could be crucial in that for the SO2 copolymers the mesogenic units may pack together in a
A variety of maleic anhydride copolymers with side chains containing biphenyl groups were studied. The isotherms of these materials generally indicated a compact, well-ordered monolayer. The films were somewhat rigid and difficult to transfer to substrates, but multilayers could be deposited. The multilayers were in general not as well-ordered as those made from other polymers, at best giving only two Bragg peaks compared with up to five orders of Bragg peaks for other polymeric systems.36 Longer side chains generally appeared to confer more order on the system. The area per repeat unit of the multilayer appeared to be too small to allow too great an amount of interdigitation. The best model for the polymer structure is one similar to that of the R-olefin copolymers i.e., one with the side chains tilted at an angle to the substrate. For most of the polymers, this angle was calculated from the bilayer spacings to be in the range 37-45°, indicating formation of very similar structures, with one or two materials showing smaller calculated angles, perhaps evidence of partial interdigitation. One material from this family, polymer 4, formed a liquid-crystal phase and could be aligned by heating a solution cast film in a 5 T magnetic field. This alignment did not occur when an LB film was heated, perhaps because the tight packing in the LB film prevents such rearrangement of the structure. Besides maleic anhydride copolymers, the film-forming properties of some polysulfones were investigated. Sulfur dioxide forms alternating copolymers with olefinic species in the same way as maleic anhydride. Therefore, a series of R-olefin-SO2 copolymers was investigated. Unfortunately they were found to be totally unsuitable for LB work. They did not spread well to form a monolayer, leading to irreproducible isotherms. The monolayers were very unstable. Two copolymers of SO2 with biphenyl-containing olefins similar to those copolymerized with maleic anhydride were found to be somewhat better. Both polymers formed monolayers that had steep isotherms and high collapse pressure. The monolayers were very compact, the areas per repeat unit indicating that the side groups must be packed very closely together. These polymers did not dip quite as well as their maleic anhydride counterparts, but multilayers could be constructed. X-ray diffraction showed that one of the polymers (11) appeared to form quite wellordered Y-type layers (two Bragg peaks) and the other (12) a less well-ordered Z-type layer (one peak). The final conclusion that we draw from this work is that the behavior of the polymers in monolayer and multilayers is mainly determined by the headgroup
(35) Tredgold, R. H. Order in Thin Molecular Films; Cambridge University Press: New York, 1994.
(36) Ali-Adib, Z.; Hodge, P.; Owen, G. J.; Tian, H. J. J. Mater. Chem. 1998, 8, 383.
Figure 6. Isotherms of the sulfur dioxide copolymers 11 and 12.
LB Films of Polymers Containing Biphenyl Groups
structure, the larger maleic anhydride residues headgroup leading to formation of structures with larger surface areas per repeat unit and preventing close packing of the polymer side chains, leading to LB films with highly tilted structures and probably large amounts of free volume in the film. Smaller sulfone headgroups allow much tighter packing, leading to films with more efficient side chain packing and much less tilted structures. Although other factors such as side chain length and structure do effect
Langmuir, Vol. 21, No. 20, 2005 9205
the film structure, this is much less than caused by headgroup variation. Acknowledgment. The authors wish to thank I. R. Girling, Dr. E. Khoshdel, and Dr. J. Obuchowicz for synthesis of the polymers and their precursors. LA051011E
