Date of Award
Doctor of Philosophy (PhD)
Materials Science and Engineering
Kornev , Konstantin
Mefford , Olin
Husson , Scott
Zdyrko , Bogdan
Future devices such as biomedical and microfluidic devices, to a large extent, will depend on the interactions between the device surfaces and the contacting liquid. Further, biological liquids containing proteins call for controllable interactions between devices and such proteins, however the bulk material must retain the inherent mechanical properties from which the device was fabricated from. It is well known that surface modification is a suitable technique to tune the surface properties without sacrificing the bulk properties of the substrate.
In the present study, surface properties were modified through temperature responsive polymer layers. After the modification, the surfaces gained switchability toward protein interaction as well as surface wettability properties. Poly(N-isopropylacrylamide) (PNIPAM), a well studied thermo-responsive polymer was utilized in the subsequent work.
Firstly, thermally responsive brushes made from well defined block copolymers incorporating NIPAM and the surface reactive monomer, glycidyl methacrylate (GMA) were fabricated in a single step process. Reaction of the PGMA block with surface hydroxyl groups anchors the polymers to the surface yet allows PNIPAM to assemble at the interface at high enough concentration to exhibit thermally responsive properties in aqueous solutions. Surface properties of the resulting brushes prepared the 1-step process are compared to characteristics of PNIPAM brushes synthesized by already established methods.
The thickness, swelling, and protein adsorption of the PNIPAM films were studied by ellipsometry. Chemical composition of the layer was studied by angle-resolved x-ray photoelectron spectroscopy. Film morphologies and forces of adhesion to fibrinogen were examined using atomic force microscopy (AFM) tapping mode and colloidal probe technique. Block copolymer (BCP) and conventional brush films were abraded and subsequently examined for changes in thermally responsive behavior. The results show that deposition of PNIPAM-b-PGMA provides an effective route to create thermally responsive brushes via a 1-step process, with properties equaling and surpassing that of traditional brushes obtained in multiple steps.
Further, the 1-step deposition of reactive BCPs can be extended to fabricate mixed block copolymer films. Well defined BCP containing ethylene glycol and GMA were deposited from a joint solution with PNIPAM-b-PGMA. Mixed brush films were also fabricated via a 2-step process for comparison of the resulting properties. PNIPAM BCP layers were utilized as the grafted primary layer with which end-functionalized PEG was grafted in a second step. Protein adhesion and adsorption of the resulting mixed brush films were studied by AFM colloidal probe technique and ellipsometry.
In the next part of the work reported, monolayers of PNIPAM containing nanogels were anchored to the surface of silicon wafers, glass slides, polyvinylidene fluoride (PVDF) fibers, and tungsten wires using a 'grafting to' approach. The particles of were synthesized with different diameters by free radical precipitation polymerization and reversible addition chain transfer polymerization (RAFT) techniques. The behavior of the synthesized grafted layers with the behavior of PNIPAM brushes (densely end-grafted) is compared. Indeed, the grafted monolayer swells and collapses reversibly at temperatures below and above the transition temperature of PNIPAM. AFM in aqueous environment was utilized to study the actuation behavior of the nanogel films. Wettability studies of the grafted layers were performed using various contact angle measurement methods to determine the contact angle changes on different substrates.
New methods for the development of thermally responsive polymer films are described. The methods enable the grafting of films with tunable film thickness, temperature response, and well defined biological interaction. The complete grafting of the responsive polymer films require no organic rinsing after grafting step.
Seeber, Michael, "SYNTHESIS AND CHARACTERIZATION OF THERMALLY RESPONSIVE POLYMER LAYERS" (2013). All Dissertations. 1138.