Synthesizing monodisperse polymer colloids is usually the first step toward the study of self-assembling processes and especially important for the fabrication of nanostructured materials. The current available polymer colloids include polystyrene, polymethyl methacrylate (PMMA) and poly-N-isopropylacrylamide (PNIPAM) spheres and their derivatives. The objective of this proposed project is to create new monodisperse, thermo-responsive polymer colloids based on poly(ethylene glycol) derivative polymers, joining in these well-known colloids of polystyrene, PMMA, and PNIPAM. The central idea is to synthesize these colloids with copolymerization of poly(ethylene glycol) ethyl ether methacrylate (PEGETH2MA), poly(ethylene glycol) methyl ether methacrylate (PEGMEA), poly(ethylene glycol) acrylate (PEGA) and their derivatives using precipitation polymerization method. The first two components give the low critical solution temperature (LCST) near the physiological temperature while the third component (PEGA) provides a functional group. Under proper chemical compositions and reaction conditions, PEG derivative microgels with a very narrow size distribution may be obtained. As a result, these PEG derivative microgels can be used as building blocks to fabricate hydrogels with colloidal crystal structures. This proposed project consists of five specific aims. The first is to synthesize and characterize PEG derivative microgels that should be monodisperse, thermoresponsive, and with functional groups. The second aim is to synthesize biodegradable PEG derivative microgels by first synthesizing biodegradable PEG-polylactic acid macromer and then using them as crosslinkers for microgels. Self-assembling processes of the PEG derivative microgels will be explored by establishing the relationship between crystallization kinetics with the softness (low mechanical modulus) of the microgels. (Aim 3) Hydrogels with colloidal crystalline structures will be synthesized using PEG derivative microgels as both crosslinkers and as a light diffraction lattice (Aim 4). Aim five will focus on preparation of PEG-microgels-based colloidosomes and build a theoretical model that will describe swelling kinetics of a gel shell such as colloidosomes.
NON-TECHNICAL SUMMARY:
This proposed project is innovative because if successful, it will lead to a new class of polymer colloids that have thermal responsive properties and monodisperse size distribution. In contrast to polystyrene spheres and PMMA spheres that are hydrophobic, the proposed PEG derivative particles are hydrophilic and have a thermally responsive volume phase transition near the physiological temperature. Different from PNIPAM microgels that are extremely soft in terms of elastic modulus, the PEG derivative microgels will be denser, harder and easier to form a crystalline structure as revealed by the feasibility study. Furthermore, in the past two decades the most research on thermally responsive polymer microgels has focused on PNIPAM and its derivatives. However, the extraordinary thermo-sensitive properties of PNIPAM have not been transferred into a biomedical breakthrough. One of the major hurdles is that PNIPAM monomer is carcinogenic or teratogenic. Thus, finding a biocompatible polymer microgel replacement of PNIPAM will be one of the major advancements in this field. The proposed project will provide direct support for two graduate students and broaden the participation of underrepresented groups. This program will integrate its undergraduate educational efforts with three existing programs that promote research experiences: the Texas Academy of Mathematics and Science, the Ronald F. McNair Post-baccalaureate Achievement Program at UNT, and NSF-UNT REU summer program. From this proposed inter-disciplinary project, both graduate and undergraduate students will gain valuable experimental and analytical skills in the rapidly growing fields of polymers, colloids and nanostructured materials. The basic sciences established in this research will have impacts not only in polymer sciences but also in biomedical technology.
The project outcomes or findings that address the intellectual merit: Synthesis of monodispersed polymer colloids has been one of the major themes in the field of polymer research in the past many decades. The current interests focus on using polymer colloids and silica spheres as building blocks to form photonic crystals. The representative systems include hydrophobic spheres of poly(methyl methacrylate) and polystyrene (PS) made by emulsion and hydrophilic spheres of poly-N-isopropylacrylamide (PNIPAM). These major polymer colloids plus inorganic colloids of silica have found a broad range of applications in the fields ranging from drug delivery to biodiagnostics to phase transitions. The one of the major outcomes of this project is that the microgel colloids based on poly-oligo(ethylene glycol) methacrylates (POEGMA) polymers have been successfully synthesized and used as building blocks for preparation of photonic hydrogels. These hydrophilic particles not only have thermal responsive behavior like PNIPAM particles but also can self-assemble into crystalline structures like PS, silica or PNIPAM particles. In contrast to PS, silica and PNIPAM particles, the new particles were made of POEGMA polymers that are nontoxic, anti-immunogenic and have a potential for applications in biomedical filed. History of the development of polymer colloids points out the most useful colloids should have multi-functionalities such as self-assembling to various structures and easily being modified to incorporating other molecules or polymers. The multi-functionalities of POEGMA have been produced in this project by synthesizing novel core-shell microgels. The core was responsive to the temperature change while the shell is responsive to both pH and temperature stimuli. Furthermore, stimuli-responsive phosphorescent microgels have been synthesized by incorporating a water-soluble phosphorescent gold complex into the polymer network. Results of pH and temperature-dependent luminescence titrations show that the sensitization is further magnified at physiological conditions, which is desirable for biomedical applications that will include bioimaging and drug delivery. The suspensions of microgels with designed structures have been used as a model to gain foundamenrtal understanding of various phase formations. For example, aqueous suspensions of soft microgel particles with a structure of two interpenetrating polymer networks were explored for the role of the particle elasticity in the dynamic arrest of these suspensions. The results demonstrate the remarkable similarities between the behaviors of colloidal and molecular glass-formers. It is found that the rate at which colloids solidify depends on the softness — or elasticity — of the colloids, which means elastic energy plays an important role in glass formation. Furthermore, stimuli-dependent microgels have also been used as a model system to study colloidal crystal/melt interface. These microgels are first assembled into large crystals using temperature fields. The structure of the colloidal crystal/melt interface is then imaged directly in three dimensions. Interface reconstructions allow direct connection of the structure and interface free energy. Another major outcome of this project is that a theoretical model has been proposed and experimentally tested for swelling kinetics of a microgel with a shell structure. The boundary condition at the inner surface is obtained from the minimization of shear elastic energy. The swelling of Tanaka's solid spherical gel is recovered as a special case of this general solution where the inner radius approaches zero. In another limiting case it is found that the swelling of a thin shell like a balloon is described simply by a single exponential term. The project outcomes or findings that address broader impacts of the work: Major education activities include active participation of the research from three graduate students and one postdoc and involvement of other one graduate student. They have synthesized and characterized new polymer materials using various instruments and gained valuable experimental and analytical skills in this rapidly growing field of polymers colloids research. They have also disseminated the research results to peer-reviewed journals. One graduate student successfully defended his dissertation in May 2011 and received a Ph.D. in Materials Science and Engineering. Another two students are expected to defend their Ph.D. dissertation next year. One graduate student won 2011 UNT Toulouse Graduate School Thesis/Dissertation Fellowship Award. UNT is investing $25 million over a five-year period to further faculty research on high profile issues affecting society by supporting six multidisciplinary research or clusters. Results from this project have contributed to the formation of the Bio/Nano photonics cluster, one of the fortunate six at UNT, composed of 14 faculty members from Chemistry, Physics, Biology, Materials Science and Engineering and UNT Health Center at Fort Worth.
