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Conformational Changes of Methacrylate-Based Monomers at the Air–Liquid Interface Due to Bulky Substituents

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journal contribution
posted on 10.07.2017 by Uvinduni I. Premadasa, Narendra M. Adhikari, Susil Baral, Ahmed M. Aboelenen, Katherine Leslee Asetre Cimatu
Functionalization of monomers is important for various applications of polymers from electronics to surface coatings. Polymer chemists utilize methacrylate-based monomers to design polymers with wider application features. In this work, ethyl methacrylate-based monomers are functionalized with varying bulky substituents, methoxy (−OCH3), ethoxy (−OEt), isopropoxy (−OiPr), tert-butoxy (−OtBu), and phenoxy (−OPh), at the ethyl end position. These substituents are selected to understand the steric consequences of modification with a bulkier group. This study allows the determination of how the substitution affects the overall conformation of the monomer at the air–liquid interface using the sum frequency generation spectroscopic (SFGS) technique. The SFG spectral results were different for all monomers. To emphasize, the SFG intensity profile at ∼2910 cm–1 (assigned as the methyl symmetric stretch, −CH3 SS) is more noticeable in the SSP spectra compared to other vibrational modes in the SSP spectra. On the basis of the spectral and global fitting, the change in the intensity of the ∼2910 cm–1 peak was affected by an increase in the number of methyl groups in the chemical structure of the monomers. This observation has become more visible for −OiPr- and −OtBu-substituted monomers. Orientation distribution analysis was performed for the −CH3 SS of substituted −OCH3 and −OPh monomers using the calculated amplitude ratios of SSP and PPP polarizations. This analysis shows a narrow distribution angle of <30° for average tilt angles near the surface plane for the −OCH3-substituted monomer. Meanwhile, narrow distribution angles were obtained for the average tilt angles closer to the surface normal for the −OPh-substituted monomer. On the basis of the results, the change in the intensity is affected by the number density of the vibrational modes present at the interface and/or the orientation distribution of the interfacial molecules. The SFG spectral results were compared to our measured surface tension (ST) results. The ST values followed a trend with increasing number of −CH3 groups of the substituted monomers according to −OCH3 > −OEt > −OiPr > −OtBu. The −OPh-substituted monomer, 2-phenoxyethyl methacrylate (PhEMA), had a high ST value, which is the result of the intermolecular forces, including π-stacking interactions. The ST value of PhEMA favored the SFG spectral data because the aromatic CH stretch was observed and can only be detected if the −OPh group is oriented perpendicular to the surface plane. These results facilitate understanding the effects of the substituents on the conformations of the monomers. Moreover, these results will help correlate these effects with the physicochemical properties of the respective polymers.