IR-visible sum-frequency generation (SFG) spectroscopy has been used in a total internal reflection geometry to study the molecular structure of polystyrene (PS) at PS/sapphire and PS/air interfaces, simultaneously. The symmetric vibrational modes of the phenyl rings dominate the SFG spectra at the PS/air interface as compared to the antisymmetric vibrational modes at the PS/sapphire interface. This indicates approximately parallel orientation of the phenyl rings at the PS/air interface while nearly perpendicular orientation at the PS/sapphire interface, with respect to the surface normal.

Infrared visible sum frequency generation (SFG) spectroscopy has been used to study structure and melting transition temperatures of alkyl-side chain-acrylate comb polymers at air and solid interfaces. At the air interface, the SFG spectra show methyl bands and two transitions are observed: the first, near the bulk melting temperature, T(m), and the second 10-20 degrees C higher than T(m). The shorter the alkyl side chain, the larger the difference between the two transition temperatures. In contrast, methylene bands are observed at sapphire interface with a single transition near T(m) (C18).

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

When two immiscible polymer surfaces are contacted, it is expected that their surface functionality will arrange or structure to minimize free energy. Understanding the structure of polymer/ polymer interfaces is important in areas of adhesion, polymer blends, and nanocomposites. The current understanding of polymer/ polymer interface structure is based on interfacial energies, either measured or calculated on the basis of the Flory-Huggins parameter, which accounts for unfavorable interaction of the different monomer repeat units. 1,2 However, directly probing the structure and orientation at polymer/polymer interfaces has not been possible due to insufficient contrast between the bulk and the interface. In this work we have taken advantage of recent developments in infrared-visible sum frequency spectroscopy (SFG) to study polymer/polymer interfaces. This technique has been recently used to study a variety of polymer interfaces including polymer/ air, polymer/liquid, and polymer/solid interfaces. [3][4][5][6] Here we report the first SFG measurement of molecular structure at poly(vinyl-N-octadecylcarbamate-co-vinyl acetate)/poly(styrene) (Comb/PS) polymer/polymer interface. The carbamate Comb polymers are widely used as release coatings for pressure sensitive adhesives, and their low adhesion at polymer/polymer interfaces with adhesives is attributed to their interfacial structure. 7 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 3 ) ω 1 + ω 2 ) is forbidden in the centrosymmetric bulk and is nonzero at interfaces where inversion symmetry is broken. The SFG is resonantly enhanced when ω 2 overlaps with the resonant frequency of a molecular vibrational mode that is both infrared and Raman active. The SFG output is also enhanced by 1 to 2 orders of magnitude when the incidence angle of the input beams is close to the critical angle for total internal reflection (explained by higher Fresnel coefficients of the reflected SFG signal near critical angles). The enhancement in SFG signal near critical angles has been used recently to study liquid/ liquid 8 and polymer/solid interfaces. 5,6 Further details of SFG theory and the details of SFG spectrometers have been discussed previously. 9,10 Samples for SFG were prepared by spin coating a film ∼300-nm thick from a 5 wt % solution of hPS (M w ) 108 kg/mol, M w / M n ≈ 1.06 or deuterated PS, M w ) 125 kg/mol M w /M n ≈ 1.13) in toluene onto the surface of a sapphire prism and annealing the film at 110°C under vacuum for 3 h. A second film ∼200 nm thick was then spin-cast on top of the first from a warm 6 wt % solution of Comb (M w ) 70 kg/mol M w /M n ≈ 3.0) in 50/50 mixture of octanol and butanol (both nonsolvents for PS), and the bilayer sample (Figure 1) was annealed again for 3 h at 110°C under vacuum. For comparison in separate experiments a film ∼200 nm thick was spin-cast from 6 wt % solution of Comb in t...

In this paper, we show that a noncrystalline but ordered smectic-like phase exists above the bulk-melting temperature (T(m)) at poly(n-alkyl acrylates)-air interface. The surface ordered phase is one monolayer thick and undergoes a sharp transition from order to disorder 10 degrees C above T(m) for n=22. The presence of a surface phase that does not exist in the bulk has important implications in the design of thermally responsive adhesives.