45688-AC7
How Does Substrate Geometry Affect the Surface-Initiated 'Living' Polymerization?
Our goal is to investigate how substrate geometry and solution conditions affect controlled radical polymerization (“grafting from”) of selected monomers from substrates.
In order to probe the effect of confinement imposed by the geometry during “grafting from” polymerization, poly(methyl methacrylate) (PMMA) anchored chains were grown on porous silicon (p-Si) substrates (pore diameter ~5 nm) and anodically-etched aluminum oxide (AAO) substrates (pore diameter ~200 nm) via surface-initiated atom transfer radical polymerization (ATRP). Using hydrogen fluoride, the chains could be cleaved from the substrates, as evidenced by infrared spectroscopy. The molecular weights and molecular weight distributions of PMMA could be analyzed directly on these substrates (after cleaving the chains from the support) using direct ionization mass spectrometry (DIOS-MS) and matrix assisted laser desorption ionization mass spectrometry (MALDI-MS). Two principal conclusions were drawn from the study. First, matrix-free DIOS-MS was effective at direct analysis of the polymers up to a molecular weight of ~6 kDa; the signal-to-noise ratio for heavier polymer chains diminished rapidly. The utilization of DIOS-MS as an analytical tool for establishing the molecular weight distribution of low molecular weight polymers is unprecedented and potentially very useful as it removes the limitations of the “matrix effect” frequently encountered in MALDI-MS experiments. Second, under the same polymerization conditions, PMMA grown on both p-Si and AAO substrates had a much lower molecular weight and a broader molecular weight distribution than that grown in solution. By comparing the experimental results of polymerization in 3 different geometries: solution, flat substrate, and concave substrate it is concluded that these systematically increased the confinement of the growing chains which ultimately lead to a decrease in the molecular weight of the grown polymer.
We have also studied the polymerization kinetics of surface initiated poly(N-isopropylacrylamide) (PNIPAAm) grown by ATRP at different copper salt ratios ([CuBr2]/[CuBr]) and temperatures. Here CuBr and CuBr2 act as an activator and deactivator, respectively, in the ATRP process. We have accomplished this by measuring the dry thickness of the grown polymers by means of spectroscopic ellipsometry. Previous reports on the solvent ATRP polymerization of (meth)acrylamides showed that these reactions are not controlled due to 1) deactivation of copper catalyst due to complexation with nitrogen containing polymers and monomers, 2) possible displacement of terminal halide atoms, and 3) low deactivation rate relative to the rate of propagation. For polymerizations of PNIPAAm performed in a water/methanol solvent mixture it is known that PNIPAAm exhibits changes in its transition temperature that depends on the composition of the solvent. Specifically, going from pure water to 50% (v/v) water the transition temperature lowers to -5°C, an effect known as co-nonsolvency. Our results indicate that co-nonsolvency can play a role in the ATRP polymerization of NIPAAm, possibly contributing to the “non-living” character o this reaction. Our results indicate that as the [CuBr2]/[CuBr] ratio is increased the rate of polymerization and the final dry thickness is decreased, as expected. This is because the presence of CuBr2 increases the rate of deactivation, resulting in a small amount of chains propagating at any given time. In addition, as the temperature is increased the polymerization rate decreases markedly; suggesting either increased and less controlled reaction rates at higher temperatures leading to fast termination, or higher temperatures being closer to the collapse transition temperature of PNIPAAm chains leading to chain collapse. To discriminate between these two scenarios we plan to map the transition temperature of PNIPAAm as a function of methanol, water and NIPAAm monomer composition by performing cloud point measurements (in bulk) and liquid cell spectroscopic ellipsometry (on the surface).
