Role of Interfacial Interaction in Orientation of Poly(N-isopropylacrylamide) Chains on Silicon Substrate.

Studies of self-assembled PNIPAM films on hydrophilic and hydrophobic substrates that are grown below and above lower critical solution temperature (LCST) have been performed to understand the role of intramolecular and polymer–substrate interactions in self-assembly using near edge X-ray absorption fine structure (NEXAFS) spectroscopy, atomic-force microscopic (AFM) and density functional theory (DFT). The NEXAFS spectra suggest that the polymer chains are oriented similarly on hydrophilic as well as hydrophobic substrate although the chains were having different morphologies in the solution, whereas, AFM studies shows that the morphology of the polymer chains on the two substrates are widely different. NEXAFS absorption spectra for free molecule and that with effects of substrate were calculated using DFT. The results show that on hydrophobic substrate polymer–substrate interaction is stronger as compared to that with hydrophilic one, however, for both cases this interaction plays dominant role over the intramolecular interaction for the orientation of the chains on the substrate. Matching of the calculated and experimental NEXAFS spectra shows on both types of substrates only a fraction of the attached molecules are oriented. Effect of external force on polymer substrate attachment was studied with the spin-coated films on hydrophilic substrate. Our results show that in this case substrate attachment plays an important role unlike the case of self-assembly on hydrophilic substrate, where the polymer–substrate interaction is weak.