Surface -grafting of pH -responsive polymer layers

The goal of this work was to evaluate the pH-responsiveness of surface-tethered polymer chains. Solution grafting of end-functionalized polymers was the primary means of forming the surface-tethered layers, although other methods were also used, including surface-confined atom transfer radical polymerization (ATRP) (Appendix A) and melt grafting of end-functionalized polymers (Appendix B). The pH-responsive layers were formed by conversion of the tert-butyl ester groups of tethered poly(tert-butyl acrylate) (PtBA) to primary, secondary, and tertiary amines using a 'direct' reaction of diamines in the presence of ZnBr2. A series of diamine molecules was utilized to investigate the influence of pendant group functionality on the pH-responsiveness of the polymer layers.;The grafting and subsequent diamine conversion reactions were performed on silicon wafers and commodity copolymer film. Reactions were carried out on ethylene-acrylic acid (EAA) copolymer films where the acid groups were used as the grafting sites. To allow for better characterization of grafted polymer layer thicknesses, these reactions were also performed on silane-based self-assembled layers on silicon wafers. The chemical reactions were monitored using FTIR spectroscopy techniques, x-ray photoelectron spectroscopy (XPS), and static contact angle goniometry. The change in thickness of the tethered responsive layer as a function of pH was measured using in situ ellipsometry.;On EAA film, the acids were converted to acid chlorides with the use of PCl5 and the acid chlorides were then reacted with ethylenediamine (EDA) to provide amine functionality used to graft the end-functionalized PtBA. FTIR spectroscopy showed the progression of the reactions. Both FTIR and XPS data suggest that the EDA molecules are able to migrate in the EAA film and react in the subsurface region. Once the PtBA was grafted to the EAA film, the static water contact angle was 85°, which is in good agreement with 88° for neat PtBA. If the PtBA grafting reaction was allowed to proceed for 18 h (rather than 6 h), the tert-butyl ester groups were hydrolyzed and the grafted layer yielded a contact angle of 51°, which is close to the value for poly(acrylic acid) (PAA). After the EAA-g-PtBA specimens were reacted with dimethylethylenediamine (DMEDA), the films exhibited a contact angle of 74° as expected for a polymer with tertiary amine pendant groups.;Similar chemistry was then performed on Si wafers with a series of diamines. Survey and high-resolution C 1s XPS showed successful grafting of the PtBA to the amine-terminated silane layers. Subsequent to the diamine conversion reactions, XPS data revealed nitrogen content lower than expected and oxygen content greater than expected for a polyamine layer. Changes in layer thickness were then measured as a function of pH using in situ ellipsometry. The Si-g-PtBA/PAA (partially hydrolyzed) exhibited an increase in layer thickness with pH as expected due to the presence of acid pendant groups. Of the various diamines studied, the Si-g-PtBA→DMEDA showed the greatest thickness change (∼50%) over the pH range of 4 to 10.