(225f) Kinetics Studies of Polymer Interfacial Reaction in Solutions

The grafting of polymers onto various substrates has become a common and versatile method for the control of surface properties ranging from wettability and adhesion to friction and biocompatibility. Polymers grafted at their chain end to form monolayers, referred to as polymer brushes, have been the subject of broad interest both academically and industrially as a utile method for surface modification. Because of the importance of covalently tethered brushes, it has become necessary to understand the polymer interfacial reactions by which they are formed. Theoretical work and simulations predict two regimes of kinetics while experimental results observe three-regime kinetics. The first two regimes agree with theoretical predictions followed by a third regime, which shows an enhancement in polymer grafting rate. This third regime is not fully understood and is ascribed to the creation of free space due to lateral contraction of chains. Here, we propose a correction for the analysis of the kinetics and an explanation for this unexpected enhancement in polymer grafting behavior using a blob model.

Polymer interfacial reactions are far more complex than the analogous reactions involving small molecules because of the wide range of factors that can influence the molecular structure and behavior of polymers at interfaces. While there are a large number of experimental studies on the structure and physical properties of grafted polymer layers, especially polymer brushes, there are relatively few studies of the kinetics of the grafting reactions by which they are formed and most of this data was collected by indirect methods. We introduce two reaction models wherein the kinetics of polymer grafting reactions can be directly determined. The first model comprises of azide-modified silica nanoparticles and alkyne-terminated polystyrene. The grafting rate of polystyrene onto silica nanoparticles is determined by measuring the amount of grafted polystyrene over time using thermo-gravimetric analysis. The second model is time-resolved ATR-IR measurements of alkyne-terminated polystyrene grafted onto azide-modified Germanium substrates. By following the reduction of azide peaks in ATR-IR spectra, the rate of reaction can be monitored quantitatively. These two research models provide versatile platforms for the investigation of a wide range of factors that effect polymer interfacial reactions. Among these factors are molecular weight effect, surface curvature effect, solvent effect, and surface loop formation effect. Understanding the influence of these factors on polymer interfacial reactions provides important insights in design, fabrication, and modification of surfaces with desired properties.