Order to Disorder Transition at The Polymer-Air Interface for Hydrophobic Octadecyl Side Chain Copolymers
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Order To Disorder Transition At The Polymer-Air Interface For Hydrophobic Octadecyl Side Chain Copolymers Keshav S. Gautam and Ali Dhinojwala Department of Polymer Science, University of Akron, Akron, OH 44325, USA
Abstract Molecular orientation of octadecyl alkyl side chains at the poly (vinyl octadecyl carbamates-co-vinyl acetate) polymer-air interface has been studied using surface sensitive sum frequency generation (SFG) spectroscopy. At 280C, below the side chain crystalline transition temperature, the SFG spectra show strong methyl vibrations indicating ordered alkyl side chains at the polymer-air interface. In the liquid state (980C), the SFG spectra show higher contributions from the methylene vibrations indicating higher gauche defects at the interface. This surface order to disorder transition is gradual and spans over a broad temperature range (50-900C), where the bulk is in the smectic liquid crystalline state. Introduction Surface and interfacial properties are determined by the chemical structure and orientation of the molecules at the interface [1]. Comb polymers with long hydrophobic side chains have been widely used as surface modifiers in applications such as adhesion, friction, membrane transport, coatings, and biomaterials [2-9]. These polymers typically show side chain crystallinity and smectic liquid crystalline states at low temperatures [2,3]. We have recently shown that below the side chain melting temperatures, the surface octadecyl chains are well ordered and have a predominantly trans structure [10]. This orientational ordering at the surface, measured by sum frequency generation (SFG) spectroscopy, correlated well with the high water contact angles (110±20) measured below the bulk melting transition temperatures. In this paper, we investigate the changes observed for the ordered surface chains when the polymer is heated above the side chain melting temperature into the smectic and finally the liquid state. The surface structure of alkyl side chains is studied using surface sensitive sum frequency spectroscopy (SFG). SFG is a second order non-linear optical technique that is forbidden in centrosymmetric bulk state and is non-zero at interfaces, which are inherently non-centrosymmetric [11-15]. SFG involves the overlapping of two high intensity laser beams, one in the visible with frequency ω1, and the other tunable in the infrared, with frequency ω2. The SFG signal at (ω1+ω2) is resonantly enhanced when the tunable infrared wavelength (ω2) overlaps with a molecular vibrational frequency, which is both infrared and Raman active. The position of the resonance peaks and the polarization of the SFG signals yield chemical and orientational information of molecules at interfaces [11-16].
KK6.3.1
SFG Background The SFG intensity is related to the second order non-linear macroscopic susceptibility as follows:
I (ω1 + ω IR ) ∝ [ χ tot (ω1 + ω IR )] ∝ χ eff , NR + ∑
Aq
2
q
ω IR − ω q − iΓ q
2
(1)
The first term in Equation (1), χeff, NR, corresponds to the non-resonance contribution tha
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