Surfactant-Induced Mesomorphic Structures in Poly(1-vinylimidazole)-Alkanoic Acid Complexes Hua Jiao, S. H. Goh,* and S. Valiyaveettil Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543 Received June 29, 2001. In Final Form: November 5, 2001 Mesomorphic structures of supramolecular systems based on poly(1-vinylimidazole) (PVI) and alkanoic acids with chain lengths (n) of 10-18 carbon atoms have been studied. FTIR studies show the existence of hydrogen-bonding interaction and a low level of ionic interaction. POM measurements show that all the complexes are mesomorphic, and the isotropization temperature increases with increasing chain length of the acid. In addition, the isotropization temperature increases with decreasing acid content in the complex. DSC studies show that, besides isotropization transition, two melting transitions exist in complexes containing alkanoic acids with chain lengths n g 12. On the basis of XRD studies at room temperature and elevated temperatures, the complexes are grouped into two types: melted lamellar liquid crystal phase with interdigitating layer structure; crystallized lamellar phase with partial interdigitating layer structure. For PVI(PA)x and PVI(MA)1.0 (MA: myristic acid, n ) 14) complexes, these two types of structures are interconvertible upon heating/cooling with a change in layer thickness of about 10 Å. As shown by the present studies, the transition temperature and the thickness of layer can be tailored by varying the acid type and by changing the acid content in the complex. Introduction The use of specific interactions for the design and preparation of self-organized materials has attracted much attention. 1,2 Intensive investigations have been conducted on the use of ionic interaction, 3-5 metal co- ordination, 6 and hydrogen bonding 7-13 to generate su- pramolecular nanostructures based on polymer/amphi- phile systems. Systems based on poly(4-vinylpyridine) (P4VPy) hydrogen-bonded to amphiphiles exhibit very interesting phase structures. 7-11,13 In general, when there is a good balance between association interaction and polar-nonpolar repulsion, microphase-separated meso- morphic states exist in these systems. The structure of this state usually consists of lamellar layers with polar sublayers consisting of polymer and the polar head of surfactant and nonpolar sublayers consisting of the alkyl chains of surfactant. For the various nonmesogenic amphiphiles investigated so far, alkanoic acids with different chain lengths seem to be interesting because of their easy availability. Although the alkanoic acid-pyridine interaction is known to stabilize liquid crystallinity, 14 in some systems it results in only partial miscibility and at levels insufficient to support liquid crystallinity. 15,16 For benzoic acid deriva- tives, 17 molecular mixing occurs up to an acid mole fraction of ca. 0.3, while for alkanoic acid derivatives molecular mixing occurs below an acid mole fraction of ca. 0.2. 18 The partial miscibility of acids with polymers was ascribed to strong self-association of the acid to form dimers. 17 For P4VPy-alkanoic acid systems, the lack of mesomorphic structures was believed to be the results of both mac- rophase separation and weak repulsive polar-nonpolar interaction. 11 The phase behavior of polymer-amphiphile systems can be tailored by modifying the attractive and repulsive interactions in the systems. The latter can be realized by modifying the length of alkyl tail of the amphiphile or by adjusting the polarity of the polymer through the intro- duction of charges. 9 Imidazole (pK b ) 7.05) is a stronger base than pyridine (pK b ) 8.75). Therefore, polymers containing imidazole groups are likely to interact strongly or even to induce proton transfer with carboxylic acids. 19-21 We have recently reported that poly(1-vinylimidazole) (PVI) interacts more strongly with carboxyl-containing polysiloxanes than P4VPy does. 21 Thus, it is expected that the strong interactions between PVI and alkanoic acids with long alkyl tails will improve the miscibility and also * To whom correspondence should be addressed. E-mail: chmgohsh@nus.edu.sg. (1) Kato, T.; Frechet, J. M. J. Macromol. Symp. 1995, 98, 311. (2) Lehn, J.-M. Makromol. Chem., Macromol. Symp. 1993, 69, 1. (3) MacKnight, W. J.; Ponomarenko, E. A.; Tirrell, D. A. Acc. Chem. Res. 1998, 31, 781. 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