Tailoring Assemblies of Surface-Anchored Polymers by 'Grafting from' Free Radical Polymerization

TECHNICAL SUMMARY:

The principal goal of this project is to gain a detailed understanding of surface-initiated free radical polymerization, as well as its extension to controlled radical polymerization, and apply this knowledge towards preparing substrates with tailored and spatially-modulated distributions of polymeric grafts. There are four main objectives of this work:

First, free radical polymerization will be used to establish the relationship between the initiator density, polymer grafting density, molecular weight, and polydispersity index. Substrates with independently varied orthogonal gradients in polymer grafting density and molecular weight will be fabricated comprising polymers grown from substrate-bound initiators. The amount of activated vs. non-activated initiator sites and polymer grafting density and molecular weight of the grafts will be quantified by means of surface-sensitive analytical techniques. Second, azo-based initiator system will be employed to monitor the characteristics of polymer brushes grown via reverse atom transfer radical polymerization. While this transformation has been studied in bulk, it has never been performed in ?grafting from? polymerizations. Expected differences in polydispersity index and polymerization kinetics will be expedited through use of polymer grafting density/molecular weight orthogonal gradients at varying temperatures and transition metal salt concentrations. Third, the ability to UV-activate the azo-initiators will be exploited to form substrate-bound micrometer-sized macromolecular gradients. By utilizing microfluidic reactors microscale polymer patterning will be accomplished by locally intermixing two monomer streams along the microfluidic channel and by employing UV irradiation through photomasks. Fourth, as a seed project, microwave-induced free radical polymerization will be utilized to provide local substrate heating in order to de-couple the kinetics of propagation from the initiation step, which heretofore has not been possible, by concurrently cooling the monomer solution and heating the substrate locally by microwave.

NON-TECHNICAL SUMMARY:

The relationship between surface initiator density and grafted polymer density, which has never been investigated in a systematic way before, is of critical importance when considering the breadth and depth of applications of polymer brushes. While advanced materials have high value, economics limit their use, strictly confining them to those areas where they are absolutely necessary. To this end it is advantageous, from both cost and performance standpoint, to fabricate thin chemically and structurally tailored coatings that can be applied to various surfaces. By doing so, one can attain full control over the interface between the original substrate and its surroundings with just a few nm of engineered coating material. Applications of these polymeric structures range from preparing chemically and structurally tailored films preventing fouling of ships to controlling biomaterial interfaces in vivo, as well as countless other technological applications, including (but not limited to), lubrication, anti-flocculation, particle assembly, protein adsorption, cell signaling, and nanoscale molecular motion. Our outreach and education efforts, which complement the scientific aspect of the work, include: i) training graduate students, ii) outreach activities in an elementary school in Ararat, VA, and iii) attracting local K-12 teachers and students to take part in our interdisciplinary research endeavors through the Kenan Fellows and the Science House programs at NC State University, and iv) organizing national and international scientific meetings for undergraduate and graduate students at NC State University.