(85c) Creating Functional Polymer Surfaces with Block Copolymers

Many current applications for soft materials require an ability to quantitatively control the number and type of functional groups present at a surface or interface. When the substrate to be modified is glass or metal, this need has usually been fulfilled by heterobifunctional self-assembled monolayers (SAM). There are some drawbacks, however, to the use of SAM's. In order to control functional group surface density, mixed monolayer must be made. In some cases this approach can lead to an inhomogeneous spatial distribution of surface functional groups. In addition, it is often desirable to control the nature of the immediate layer underlying the functional groups of interest, a characteristic not provided by conventional SAM's. Finally, SAM's are not particularly useful for the modification of soft or polymeric surfaces. In this presentation, we focus on an alternative technology that can be applied to quantitatively functionalize metal, ceramic and polymeric substrates: functional block copolymer monolayers (FBCM). Like SAM's, block copolymers can be designed to spontaneously self-assemble at an air-substrate interface. The driving forces are different however. In the case of SAM's, a silane or thiol groups tethers the molecule to a glass or gold substrate, respectively, and attractive, in-plane van der Waals interactions drive the self assembly process. In the case of FBCM's, a low energy copolymer surface directs the copolymer to adsorb at the surface of the substrate, and repulsive out-of-plane interactions between the two copolymer sequences drive self-assembly into a bilayer structure. There are a number of methods to apply FBCM's onto substrates. For flat substrates, FBCM's are easily fabricated by standard spin coating techniques. In difference to SAM's, the thickness of FBCM's can be controlled by adjusting the concentration of the spin coating solution or the RPM. Since each copolymer chain has a controlled number of functional groups (i.e., usually one), the areal density of surface functional groups can thus be readily controlled through the spin coating process. Alternatively, the block copolymer can be employed as an additive to a polymeric matrix, wherein it will spontaneously adsorb and self-assemble at the air-polymer interface. We also demonstrate that FBCM's can be designed so as to be fabricated on surfaces of arbitrary shape through supercritical fluid processes. In this case, the areal density is readily controlled by changing the pressure. Finally, we show that the thermodynamic nature of the layer underlying surface functional groups can be readily controlled in FBCM's by syntheses of an appropriate block copolymer. One of the more exciting aspects of FBCM's with respect to this issue is that the block adjacent to the functional group can be prepared from a photoactive polymer. We take advantage of this property to create high energy functional surfaces by first self-assembling a low surface tension hydrophobic block copolymer and then photochemical transforming it into a functional reactive surface. The first example is the delivery of carboxylic acid groups to a polystyrene (PS) substrate. The block copolymer design for this application, prepared anionically, is poly(styrene-b-dimethyl siloxane-e-COOH) where the nomenclature ?-e-? denotes end group. The copolymer can be applied to the PS substrate either by spin coating or by deposition from supercritical CO2. The siloxane block is low surface tension and causes the copolymer to be surface active, while the PS block serves to anchor the FBCM at the surface by means of entanglement with the PS substrate. Once the block copolymer assembles at the surface, the carboxylic acid groups provide sites for bioconjugation to peptides and proteins. Herein we demonstrate that use of this technology to covalent bind a growth factor to the surface. The second example of FBMC technology is the delivery of multiple functional groups to a PS surface by use of a diblock copolymer containing a photoactive sequence. In this case the copolymer, poly(styrene-b-tert butyl acrylate) is synthesized by atom transfer radical polymerization (ATRP). The block copolymer self-assembles as a bilayer at the PS substrate because the t-butyl acrylate block (PtBA) has a low surface tension. Once self assembled at the surface, the layer is stabilized by entanglement with the PS substrate. The PtBA layer can subsequently be converted to poly(acrylic acid) (PAA) by borrowing from chemical amplification photoresist technology. The surface PtBA layer is coated with a photoacid generator (PAG) and then exposed to UV radiation. UV exposure creates a strong acid which hydrolyzes the t-butyl ester to yield a surface covered with carboxylic acid functional PAA brushes. Because the UV radiation can be patterned, the carboxylic acid decorated polymer brushes can also be patterned. We demonstrate that this method can be used to pattern the physisorption of a number of molecules as well as covalent bonding and biorecognition of streptavidin protein. The PtBA copolymers can also be delivered to surfaces using supercritical solvents.

This research has been funded by grants from the National Science Foundation (Division of Materials Research, IGERT program and MRSEC program) and by the Army Research Office.