Interparticle distance in monolayers controlled by soft spacers

Abstract

This paper addresses the use of graft polymer layers as spacers to control interparticle distance in 2-dimensional monolayers. Gold nanoparticles grafted with thermosensitive PNIPAM-based polymers with a large range of molecular mass and different degrees of hydrophobicity have been studied. The hydrophobicity of the polymer is adjusted by incorporation of a comonomer n-propylamine. The resulting copolymer, PNIPAM-co-NPAM, exhibits lower collapse transition temperature and increased cooperativity in the collapse process with n ≈ 150 compared to n ≈ 100 for PNIPAM, n being the number of monomers per collapse domain. Langmuir isotherms of these polymers under moderate compression follow closely a π ≈ c3 behavior with corresponding critical exponent ν = 3/4 as predicted for 2-dimensional polymer conformation in good solvent. Nanoparticles grafted with these polymers form stable Langmuir monolayers where the graft polymer chains adopt a 2-D stretched conformation that tethers the nanoparticles to the interface. The nanoparticle cores are thus isolated by the polymer shells resulting in nanoparticle areas that increase with polymer chain length. Correspondingly, the interparticle distance is found to vary with chain length as Dp ≈ N0.8. For the Au-PNIPAM-NPAM, a moderate increase in temperature to near-θ conditions decreases the nanoparticle area by about 30% through lateral collapse of the polymer layer. This thermally induced molecular collapse in a 2-D monolayer is an unusual and novel observation that may be attributed to cooperative effects of the collapse transition of the new copolymer PNIPAM-co-NPAM.