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Biomacromolecules 2000, 1, 310-312
Articles Thermal and Structural Properties of Biologically Derived Monodisperse Hairy-Rod Polymers Seungju M. Yu† and David A. Tirrell* Department of Polymer Science and Engineering, University of Massachusetts, Amherst Massachusetts 01003; and Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125 Received May 10, 2000
Monodisperse derivatives of poly(γ-4-(hexadecyloxy)benzyl R,L-glutamate) (PHBG-X, X ) 3 or 4) with backbone sequence GluAsp(Glu17Asp)xGluGlu were prepared by reaction of 4-(hexadecyloxy)phenyldiazomethane with the corresponding monodisperse poly(R,L-glutamate) (PLGA) derivatives (PLGA-X, X ) 3 or 4). PHBG-3 and -4 exhibited strong endotherms near 45 °C and weak endotherms near 86 °C when analyzed by differential scanning calorimetry. X-ray diffraction suggested that these polymers aggregate to form layerlike solid structures at room temperature, with extended alkyl side chains forming paraffinlike crystallites. Most of the side chain order disappears at the first melting transition; however, the layerlike structure remains. Both polymers are isotropic above the second melting transition; no ordered melts were observed at higher temperatures, possibly due to the small aspect ratios of PHBG-3 and -4. In contrast, polydisperse poly(γ-4-(hexadecyloxy)benzyl R,L-glutamate) (PDI ) 1.2, DP ) 98) (PHBG-P1), prepared from commercial PLGA, formed liquid crystalline (LC) phases between 97 and 105 °C. Long-chain alkyl esters of poly(R,L-glutamate) (PLGA), e.g. poly(γ-alkyl R,L-glutamate) and poly(γ-alkoxybenzyl R,L-glutamate), are typical hairy-rod polymers.1-4 Such polymers are useful models for membrane proteins5 and have been proposed for applications in photonics, electronics, and chemical sensors.6,7 Self-assembly of hairy-rod polymers at the air-water interface produces ordered mono- and multilayer structures that can be transferred to solid substrates for engineering of controlled surface arrays.8,9 Hairy-rod polymers exhibit thermotropic liquid crystal (LC) behavior when the alkyl side chains are long enough to act as “solvents” for the main chain at temperatures above the sidechain melting transition.10,11 Unfortunately, interpretation of the LC properties of hairy-rod polymers and construction of ordered architectures for potential applications8,9 are complicated by the chain-length heterogeneity of samples prepared by chemical synthesis.12 Previously, we have reported the synthesis of monodisperse PLGA and poly(γbenzyl R, L-glutamate) (PBLG) derivatives via expression of appropriately designed artificial genes in bacterial cells.13,14 Monodisperse PBLG derivatives prepared in this way exhibit unusual lyotropic smectic liquid crystalline order.14,15 Here we report the synthesis of biologically derived hairy-rod polymers, specifically poly(γ-4-(hexadecyloxy)benzyl R,Lglutamate)s (PHBG-X, X ) 3 and 4), based on the * To whom correspondence should be addressed at the California Institute of Technology. † Present address: Department of Chemistry, University of Wisconsin, Madison WI 53706.
Scheme 1
corresponding monodisperse poly(R,L-glutamate) derivatives (PLGA-X, X ) 3 and 4). We also describe the thermal and structural properties of these polymers as determined by differential scanning calorimetry, polarized-light optical microscopy, and X-ray diffraction measurements. GluAsp(Glu17Asp)xGluGlu (PLGA-X, X ) 3 and 4) The precursor polymers PLGA-3 and 4 were prepared as described earlier.13,14 Two polydisperse (PDI ) 1.2) PLGA samples with average degrees of polymerization (DP) of 55 and 98 were purchased from Sigma. PHBG samples were prepared by reaction of PLGA with 4-(hexadecyloxy)phenyldiazomethane (Scheme 1), which was obtained by treatment of 4-(hexadecyloxy)benzaldehyde hydrazone with HgO in the presence of KOH (Scheme 2).16,17 The diazo compound produced in this fashion afforded more than 95% side chain substitution of PLGA-X. Although previous studies suggest that PLGA-derived hairy-rod polymers show several temperature-dependent LC phases, identification of each LC phase has been complicated
10.1021/bm000039w CCC: $19.00 © 2000 American Chemical Society Published on Web 08/18/2000
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Monodisperse Hairy-Rod Polymers Scheme 2
Table 1. DSC Data for PHBG transition temp, °C (enthalpy change, cal/g) polymer (PDI)a
DPb
first transition
second transition
PHBG-P1 (1.2) PHBG-P2 (1.2) PHBG-3 PHBG-4
55 98 57 78
38.8 (6.5) 38.4 (5.3) 44.9 (6.5) 46.9 (7.7)
94.6 (0.67) 85.8 (0.20) 87.9 (0.29)
a Polydispersity index of PLGA. b Degree of polymerization determined from PLGA.
by the polydispersity of the samples. For example, it has been suggested that both PHBG (DP ) 91)2 and poly(γoctadecyl R,L-glutamate) (POG, DP ) 60)11 may form smectic thermotropic LC phases. In Table 1, we list the thermal transitions of the PHBG samples examined in this work, as obtained by differential scanning calorimetry (heating rate, 10 °C/min). All four polymers show strong endotherms near 38-46 °C, and an additional weak endotherm is present at about 85-95 °C for all of the polymers except PHBG-P1. All of these transitions were reversible and appeared in the cooling curves as well. After the first melting transition the polymers became elastic, and the second transition was marked by an abrupt change from elastic to liquidlike behavior. In the case of PHBG-P1, the transition to liquidlike behavior seemed to take place continuously over the temperature range between 70 and 90 °C. For examination by polarized-light optical microscopy, PHBG samples were first heated above the isotropization temperature (110 °C), then cooled to 100, 70, or 25 °C, and finally annealed for 3-5 h before acquiring the photomicrographs. All polymers except PHBG-P2 showed similar temperature-dependent textures. When the samples were cooled from the isotropic state at 110 °C, small birefringent islands started to appear at 70 °C as shown in Figure 1a. In the case of PHBG-P2, a unique LC phase with a sheathlike texture was observed in the temperature region between 97 and 105 °C (Figure 1b). When this sample was cooled below 70 °C, birefringent islands appeared as in the other three samples. During heating, the sheathlike texture appeared immediately after the transition at 95 °C and was gradually transformed into an isotropic phase between 105 and 110 °C. These results are in apparent contradiction to the data reported by Iizuka and co-workers, who suggested that polydisperse PHBG of DP 91 shows an LC state up to 200 °C.2 Because the polydispersity of the samples examined by Iizuka and co-workers was not specified, it is difficult to comment on the reasons for the discrepancy in these results. In our hands, PHBG-P2 was the only polymer that showed an LC state at temperatures above that of the second transition, and it is likely that the higher molecular weight portion of the polydisperse sample contributed to formation of this phase. No ordered melts were observed above the
Figure 1. Polarized light optical micrographs (magnification ×200) of (a) PHBG-4 after annealing at 70 °C for 5 h and (b) PHBG-P2 after annealing at 101 °C for 3 h. Table 2. X-ray Diffraction Spacings (Å) of PHBG-4 at Different Temperatures 25 °C
70 °C
3.75 (s) 4.17 (vs)
3.74 (vw) 4.17 (vw) 4.5-5.5 (m, br) 14.1 (w) 17.9 (s) 35.9 (vs) 49.7 (w)
12.9 (w) 18.9 (s) 37.2 (vs) 47.620 (w)
110 °C
4.5-5.5 (m, br)
second melting temperature in any of the other polymers we examined, possibly owing to the insufficient aspect ratios of the chains. Temperature-dependent X-ray diffraction experiments on PHBG-4 yielded additional structural information. The results are summarized in Table 2. At room temperature, strong reflections with Bragg spacings of 3.75 and 4.17 Å were observed in the wide-angle diffraction pattern. These spacings are similar to those of the (10) and (01) reflections of the two-dimensional crystal lattices formed by alkyl chains of 10 or more methylene groups in poly(n-alkyl R,Lglutamate)s.3 In the small-angle region, a very strong reflection at 37.2 Å and the corresponding second-order signal at 18.9 Å are observed. The small-angle signals arise from layers formed by interdigitation of the alkyl side chains; the length of the side chains in extended form is ap-
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polydispersity may play an important role in the reported thermotropic LC behavior of low molecular weight hairyrod polymers. Acknowledgment. This work was supported by the National Science Foundation Materials Research Science and Engineering Center at the University of Massachusetts.
Figure 2. Schematic diagram of layered structure of PHBG-4 at 25 °C showing the origin of the 37.2 and 12.9 Å spacings.
proximately 32 Å.2 The well-defined second-order signal suggests that the layered structure is characterized by regular stacking. There is also a weak reflection at a spacing of 12.9 Å, very close to the reported interchain distance, 12.5 Å, of PBLG rods in the solid state.14 On the basis of the diffraction results, it is reasonable to suggest that the helical polypeptide rods are arranged in a layerlike structure with extended alkyl side chains forming paraffinlike crystallites within regularly stacked layers as shown in Figure 2. This structure is similar to those proposed for poly(γ-alkyl R,L-glutamate)s and for stoichiometric complexes of poly(L-lysine) and n-alkyl sulfates.3,18,19 When the diffraction pattern was recorded at 70 °C, the wide-angle reflections observed at room temperature were largely replaced by a broad halo; however, weak 3.75 and 4.17 Å reflections could still be detected. The interlayer spacing was reduced from 37.2 to 35.9 Å, and the 12.9 Å spacing increased to 14.1 Å. At 70 °C, therefore, the alkyl side chains have melted, allowing contraction of the layer thickness. When the sample was heated above 100 °C, only diffuse scattering was observed at around 4.5-5.5 Å, indicating formation of an isotropic liquid and confirming the results of polarized-light microscopy. In conclusion, monodisperse PHBG derivatives of DP e 78 show no evidence of LC phases; however they are able to maintain layerlike structures containing locally ordered aggregates at temperatures above the side-chain melting transition. A higher-temperature LC phase was observed only in the polydisperse PHBG sample of DP 98, suggesting that
Supporting Information Available. Text giving experimental details of the synthesis of PHBG and figures showing X-ray powder diffraction patterns for PHBG-4. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Sohn, D.; Yu, H.; Nakamatsu, J.; Russo, P. S.; Daly, W. H. J. Polym. Sci.: Polym. Phys. 1996, 34, 3025. (2) Iizuka, E.; Abe, K.; Hanabusa, K.; Shirai, H. Curr. Top. Polym. Sci. 1987, 1, 235. (3) Watanabe, J.; Ono, H.; Uematsu, I.; Abe, A. Macromolecules 1985, 18, 2141. (4) Hanabusa, K.; Sato, M.; Shirai, H.; Takemoto, K. J. Polym. Sci.: Polym. Lett. 1984, 22, 559. (5) Smith, J. C.; Woody, R. W. Biopolymers 1974, 12, 2657. (6) Vogel, A.; Hoffmann, B.; Schwiegk, S.; Wegner, G. Sensors Actuators 1991, B4, 65. (7) Mathy, A.; Mathauer, K.; Wegner, G.; Bubeck, C. Thin Solid Films 1992, 215, 98. (8) Mathauer, K.; Schmidt, A.; Knoll, W.; Wegner, G. Macromolecules 1995, 28, 1214. (9) Fukuto, M.; Heilmann, R. K.; Pershan, P. S.; Yu, S. M.; Griffiths, J. A.; Tirrell, D. A. J. Chem. Phys. 1999, 111, 9761. (10) Ober, C. K.; Jin, J.-I.; Lenz, R. W. AdV. Polym. Sci. 1984, 59, 103. (11) Watanabe, J. Ordering in Macromolecular Systems; Teramoto, A., Kobayashi, M., Norisuye, T., Eds.; Springer-Verlag: New York, 1993; p 99. (12) Kricheldorf, H. R. R-Amino Acid-N-Carboxy-Anhydrides and Related Heterocycles; Springer: New York, 1987. (13) Zhang, G.; Fournier, M. J.; Mason, T. L.; Tirrell, D. A. Macromolecules 1992, 25, 3601. (14) Yu, S. M.; Conticello, V. P.; Zhang, G.; Kayser, C.; Fournier, M. J.; Mason, T. L.; Tirrell, D. A. Nature 1997, 389, 167. (15) He, S.-J.; Lee, C.; Gido, S. P.; Yu, S. M.; Tirrell, D. A. Macromolecules 1998, 31, 9387. (16) Zollinger, H. Diazo Chemistry II; VCH: New York, 1995. (17) Murray, R. W.; Trozzolo, A. M. J. Org. Chem. 1964, 29, 1268. (18) Ponomarenko, E. A.; Waddon, A. J.; Tirrell, D. A.; MacKnight, W. J. Macromolecules 1996, 29, 8751. (19) Ponomarenko, E. A.; Waddon, A. J.; Tirrell, D. A.; MacKnight, W. J. Macromolecules 1998, 131, 1584. (20) A weak diffraction signal at 47.9 Å spacing is believed to originate from a similar supramolecular structure with less side chain overlap.
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