Interfacial reactions involving polymers are far more complex than reactions involving small molecule reactants because of the broad range of variables that can affect their molecular structure and behavior at interfaces. While interfacial reactions are a part of many polymer applications, a fundamental understanding of the parameters affecting them is generally lacking. The scientific goal of the proposed research plan is therefore to provide a fundamental and quantitative understanding of the factors that influence the reactions of polymers at organic interfaces. The achievement of this goal is the deliverable of this project: a fundamental basis for both the molecular design of polymer interfacial reactions and for the subsequent control of the interfacial properties they bring about. Two model reaction systems have been selected for study to represent the wide variety of reactions that may be encountered in practice: . a "standard" reaction between a functional polymer and an organic surface containing a complementary functional group . a photografting reaction between a polymer in contact with an organic surface, one of which contains a photoactive group capable of grafting the polymer to the surface. The functional polymers required for the research will be prepared by atom transfer radical polymerization. The proposed research investigates how interfacial reactions are influenced by factors such as: solvent quality and concentration, including neat polymers; molecular weight; areal density of functional groups; location of functional groups along the polymer backbone; the nature of the surface interaction parameter, state of the polymer (i.e., glassy or rubbery), conditions of the photochemical reaction and reaction time. The organic interfaces employed will be prepared on either silicon wafers or gold-coated wafers in order to enable characterization of the reactions by a barrage of techniques including: x-ray photoelectron spectroscopy, grazing incidence reflection infrared, ellipsometry, contact angle, MALDI-TOF mass spectrometry, fluorescence spectroscopy and gel permeation chromatography. The research program also includes validation of a new concept, covalent layer-by-layer assembly, in which multiple polymer layers are successively built up on an organic substrate wherein each layer is covalently bound to the previous layer.
NON-TECHNICAL SUMMARY:
The nature of how long chain molecules react at interphases is an important underlying aspect of many applications involving polymers and biomacromolecules. Such reactions are used to control surface properties of materials and coatings such as adhesion and wettability, are employed to make magnetic nanoparticles soluble in water for potential use as magnetic resonance imaging contrast enhancement agents, and are used in the fabrication of biosensors wherein DNA, proteins, antibodies and antigens are chemically bound to substrates to form diagnostic microarrays. The proposed research program will provide a scientific basis for the design of such reactions and their optimization for use in these applications. This enabling technology will play an important role in the manufacturing of inexpensive DNA sequencing chips, clothing that can filter out harmful pathogens and rapid diagnostics that can identify dangerous pathogens and antigens. The proposed research will also have a broad impact on research and education through a number of programs: the training of undergraduate and graduate researchers, hosting of a summer high school chemistry teacher researching interfacial reactions and with that teacher, the development of modules on polymer chemistry suitable for use in his classes. The P.I. will also continue his outreach efforts to Columbia University undergraduate students at large through his activities as a Professor in Residence, living amongst the students in a dormitory and hosting informal dinners with undergraduates and a special guest talking on a topic related to the series theme "Society and Technology".
Long chainlike molecules, both synthetic and natural polymers, are often linked by their ends to certain substrates to enable important engineering applications. These materials are referred to as polymer brushes and are used in a myriad of important products ranging from adhesives to lubricants to biomaterials that resist biofouling. Natural polymers such as DNA, proteins and carbohydrates are often tethered to surfaces to create biological sensors. All of these applications require a facility to fabricate polymer brushes with precise control of their molecular length and the spacing between the polymer molecules on the surface. The design of manufacturing processes for these applications also requires a fundamental understanding of the factors that influence the interfacial reactions that are used to link the polymer brushes to the surfaces of interest. The research funded by this proposal therefore has two goals: 1) To develop methods to control the spacing between surface functional groups (i.e., their areal density) to which polymers will be linked to form a polymer brush and 2) To understand the fundamental factors that affect polymer interfacial reactions. Both of these tasks are challenging because they require methods that can interrogate the composition of a single molecular layer on a surface that is only nanometers in thickness. The methods developed to attain both of these goals were chosen to be extremely versatile and directly relevant to commercial applications. We have chosen to use a type of so-called "click" chemistry to tether polymers to surfaces, because it is quantitative, highly chemoselective, that is it works in virtually any chemical environment, and because the ease in adding "click"-reactive functional groups to almost any surface or polymer renders the method almost universally applicable to commercial practice. We have specifically chosen to use the "click" reaction between alkyne groups and azide groups for this purpose because the course of these reactions can be monitored with infrared spectroscopy by measuring the absorbance of the azide group. The first two outcomes of this research pertain to the first goal: two versatile methods for controlling the areal densities of surface alkyne and azide that are applicable to a wide variety of both flat and nanoparticle surfaces. In the case of alkyne groups, we found that mixed monolayers of alkane-silanes and alkyne-silanes were successful for controlling alkyne density on germanium, silica and iron oxide substrates and nanoparticles. We also developed two new techniques to prove that the alkyne groups were randomly located at the surface and that there was no preferential adsorption of one component in the mixed monolayer. In the case of surface azide groups, we found that the mixed monolayer method did not work, and developed a new method which controlled the areal density of azides kinetically. A monolayer of bromo-silane was first deposited and the areal density of azide was controlled kinetically through the conversion of bromine to azide upon the addition of a sodium azide reagent. To attain the second goal, an understanding of the factors that influence polymer interfacial reactions, we developed an infrared spectroscopy technique that is capable of monitoring the reaction of a single molecular layer of polymer by measuring changes in the absorbance of surface azide groups upon reaction with alkyne groups at the polymer chain end. The experiment employs a technique referred to as total internal reflection, in which a monolayer of azide-silane is formed on the surface of a germanium plate through which the infrared beam passes. A spectrum of the monolayer absorbance is obtained as the beam bounces back and forth inside of the Ge plate. Changes in the azide absorbance upon reaction with the alkyne terminus of the polymer allow the course of the polymer interfacial reaction to be measured. Using this method we have made the first direct measurements of polymer interfacial reactions, an example of which is shown in the figures for 11,000 Dalton poly(n-butyl acrylate), where the areal density, s, is plotted against the square root of time and the log of time. The first regime of reaction rate is found to be diffusion controlled (s ≈t0.5) as the polymer must move to the surface to react. Afterwards, the reaction rate slows, scaling with the log of the time as polymer must penetrate the initial brush to further react. These two regions are consistent with the predictions of theory. Additional unpredicted regions of behavior beyond these two are found and their origins are currently being studied in more detail. The transitions between regions of behavior are found to scale universally with the size of the polymer molecule. The availability of these new research results provides an important framework for the design and optimization of applications based upon the formation of polymer brushes at substrate surfaces.
