Molecular Structure of Hydrophobic Alkyl Side Chains at Comb Polymer-Air Interface

Abstract

Introduction. Polymers with long side chains (comb polymers) provide unique opportunities to tailor surface properties of polymer films for applications in the areas of adhesion, friction, membrane transport, and biocompatibility. [1][2][3][4][5][6][7][8] In particular, polymers containing long alkyl or fluorinated alkyl side chains have been used to generate hydrophobic surfaces with surface energies that are comparable to those created by well-ordered self-assembled monolayers (SAM). 1-2,9-10 Comb copolymers with alkyl side chains typically show two thermal transitions: one associated with smectic liquid crystalline order where the d spacing is proportional to the length of the side groups 1,2 and the other associated with crystallization of the alkyl side chains. The surface energy of alkyl side chain comb polymers decreases with increase in the length of the alkyl side groups until a certain critical chain length, after which it asymptotes to a constant value. 1 These observations are reminiscent of results for SAM's where the length of the alkyl chains, packing, and mobility of the chains control the surface energies. 9,10 Although, considerable work has been done in the past in understanding the molecular ordering of SAM, very little is known about the molecular structure of alkyl chains at the comb polymer-air interface. 11 In this Communication, we report the first spectroscopic measurement of the molecular structure of octadecyl alkyl side chains at poly(vinyl octadecyl carbamatesco-vinyl acetate)-air interface (the polymer will be referred to as octadecyl polymer for simplicity) using the surface sensitive sum-frequency generation (SFG) technique. SFG involves mixing a visible high-intensity laser beam of frequency ω 1 , with a tunable infrared wavelength source of frequency ω 2 . According to the dipole approximation, generation of a SFG photon at (ω 1 + ω 2 ) is forbidden in the centrosymmetric bulk and is nonzero at interfaces where the inversion symmetry is broken. 12-16 SFG intensities are resonantly enhanced when ω 2 overlaps with the resonant frequency of a molecular vibrational mode that is both infrared and Raman active. The intensity, polarization, and resonance frequency of the SFG signal yield chemical and orientational information on molecules at interfaces. [12][13][14][15][16][17][18] Here, we compare the differences in the surface structure of 65 nm thick octadecyl polymer films prepared by spin-coating to a 7 nm polymer layer prepared by solution dipping on a mica substrate. In both cases, the SFG signals show strong methyl asymmetric and symmetric stretching vibrations indicating ordered methyl terminal groups. For the adsorbed octadecyl films, the SFG signal associated with methylene vibrations is higher indicating increased gauche defects in

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“…As we stated above, the SFG signal intensity is proportional to the square of the vibration’s second-order nonlinear susceptibility ${\chi }_{\mathit{eff}}^{(2)}·{\chi }_{\mathit{eff}}^{(2)}$ vanishes when a material has inversion symmetry under the dipole approximation (Bain, 1995; Buck et al, 2001; Chen and Chen, 2006; Chen and Clarke, 2005; Chen and Shen, 2002; Eisenthal, 1992; Hirose and Akamatsu, 1992a; Hirose and Akamatsu, 1992b; Hirose and Yamatoto, 1993; Gautam et al, 2001; Lambert et al, 2005; Miranda et al, 1999; Richmond, 2001; Shen, 1984, Shen, 1989; Tadjeddine et al, 1996; Wang and Chen, 2001; Wang and Gan, 2005; Williams et al, 2002; Zhuang et al, 1999). The majority of bulk materials exhibits inversion symmetry, thus they do not generate SFG signals.…”

“…Different components of ${\chi }_{\mathit{eff}}^{(2)}$ can be probed using different polarization combinations of the input and output laser beams. From such measurements, orientation information of surface molecules and functional groups can be deduced (Hirose and Akamatsu, 1992a; Hirose and Akamatsu, 1992b; Hirose and Yamatoto, 1993; Gautam et al, 2001; Kim and Somorjai, 2003; Shen, 1984; Wang and Chen, 2001). More details about SFG theory and data analysis can be found in the appendix.…”

“…The frequency dependence of ${\chi }_{\mathit{eff}}^{(2)}$ can be written as: $${\chi }_{\mathit{eff}}^{(2)}(\omega )={\chi }_{NR}^{(2)}+{\chi }_R^{(2)}={\chi }_{NR}^{(2)}+\sum _q\frac{A_q}{\omega -{\omega }_q+i{\mathrm{normal\Gamma }}_q}$$ where A q , ω q , and Γ q are the strength, resonant frequency, and damping coefficient of the vibrational mode q respectively, ${\chi }_{NR}^{(2)}$ is the nonresonant background, and ${\chi }_R^{(2)}$ is the resonant contribution (Hirose and Akamatsu, 1992a; Hirose and Akamatsu, 1992b; Hirose and Yamatoto, 1993; Gautam et al, 2001; Kim and Somorjai, 2003; Shen, 1984; Wang and Chen, 2001). …”

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“…The two main peaks in the PVNODC=Air interface 130 SSP spectrum are the methyl symmetric stretch ($2873 cm À1 ) and Fermi resonance ($2931 cm À1 ) [23]. This indicates that the surface is methyl terminated with well-ordered long alkyl side-chains in predominantly all-trans conformations [22,23]. The annealed PS=PVNODC PVNODC interface spectrum also includes a methylene symmetric 135 peak at 2844 cm À1 , a methylene asymmetric peak at 2920 cm À1 , a methyl asymmetric peak at $ 2960 cm À1 , and a n 20b asymmetric phenyl peak $ 3024 cm À1 [13].…”

“…SFG spectra for PS=air and PVNODC=air have been published before [13,[21][22][23]. Vibrational assignments and discussion of functional group orientation have also been discussed previously 125 for these interfaces [13,[21][22][23].…”