2013 Theses Doctoral

Kinetics of Polymer Interfacial Reactions

Zhang, Shuo

The reaction of a functional polymer at an interface has very broad applications in industry. Surface-bound polymers have been employed to control surface properties ranging from wettability and adhesion to friction and biocompatibility. More recently, the applications of functional polymer films in microelectronics, optical, nanocomposites, DNA microarrays, and enzyme immobilizations have drawn a lot of attention. There are a wide range of influencing factors associated with the kinetics of polymer interfacial reaction, and these challenges are especially prominent in the field of surface modification. In this thesis, different challenges were addressed to explore click reaction kinetics of end functional reactants (small molecules and polymers) reacting to self-assembled monolayers (SAMs) on solid substrates: first of all, functional surfaces modified with azide SAMs are presented in Chapter 3. Secondly, a precise control of the areal density of surface `click' functionality is proposed in Chapter 4. Thirdly, a kinetic investigation of reaction between functional polymers and functionalized interface is described in Chapter 5. In Chapter 3, the reactive surface of a Germanium substrate is prepared by a high-quality azide-functional SAM. These azide-functional substrates enable interfacial `click' reactions with complementary alkyne-functional molecules to be studied in situ by attenuated total reflectance infrared spectroscopy (ATR-IR). To demonstrate their potential utility for kinetic studies, we show that, in the presence of copper (I) catalyst, the azide-modified surfaces react rapidly and quantitatively with 5-chloro-pentyne to form triazoles via a 1,3-dipolar cycloaddition reaction. Time-resolved ATR-IR measurements indicate that the interfacial click reaction is initially first order in azide concentration, and then transitions to apparent second order dependence, when the surface azide and triazole concentrations become similar. The reaction achieves an ultimate conversion of 50% consistent with the limit expected due to steric hindrance of the 5-chloro-pentyne reactant at the surface. In Chapter 4, two approaches are developed to control the `click' functionality on the surface: a common approach by mixed monolayers constituting a fraction of the functional alkyne silanes and a fraction of chemically similar nonfunctional alkane silanes, and a new kinetic approach by tailoring surface azides through quenching SN2 azide substitution reaction at a specific time. In Chapter 5, the ATR-IR technique is further manipulated to directly measure the kinetic trends of the reaction between azide functional monolayers modified on Germanium crystal surface and alkyne end-functional poly (n-butyl acrylate) (PnBA) and polystyrene (PS), via a copper-catalyzed 1,3-dipolar cycloaddition reaction. Time-resolved ATR-IR measurements distinguish four regimes rather than the two predicted by theory in the absence of segmental physisorption. The first two regimes correspond well to the theory. In the first regime, the rate is rapid and controlled by Brownian diffusion of polymer through the solvent, scaling with the square root of time. In the second regime, the rate slows considerately because of the energy barrier when the polymer chains have to penetrate a covalently bound polymer brush to reach the surface, and the rate is proportional to logarithm of time. There appear another two transition regimes before the ultimate saturation, where the reaction rate reduces and then accelerates briefly. A possible explanation for this behavior is that the tethered polymer layer contracts lateral during the transition from "mushroom" to "brush" and thereby provides additional space for a few incoming polymer chains. A number of factors that influence the kinetics of the polymer interfacial reactions are examined including polymer nature, molecular weight and surface tension. Some general observations are summarized to show consistent kinetic behavior for different polymers with a wide range of molecular weights.