Graduate Thesis Or Dissertation
Formation of Microstructures in Crosslinked Polymer Films by Photocrosslinking of Monomers in Evaporating Solvents Public Deposited
Crosslinked microstructured polymer films have received much attention due to their potential applications. Currently, these polymer films can be fabricated through a range of processes and lithographic techniques. However, they often require multiple fabrication steps and expensive instruments. In this thesis, we report the structure formation of films obtained via a simple, low cost one-step synthesis/fabrication process, by in situ photocrosslinking of precursors during the evaporation of solvents.
Under controlled dry N2 flow, the forced convective evaporation resulted in highly hierarchical film morphology, featuring a skin layer atop of a layer of microspheres. This unique morphology is determined by the interplay between fast solvent evaporation and reaction induced phase separation between the reactive precursor and the remaining solvent. For the first time, the skin layer formed during the evaporation was directly observed after the complete evaporation of the solvent. The thickness of the skin layer is dependent on the processing parameters including the N2 flow rate, UV intensity and precursor concentrations. Under the free convection or by means of adding non-solvent, the skin layer formation could be suppressed. A model is presented that can qualitatively describe the skin layer formation and its dependence on the processing parameters, providing a mechanistic understanding of the photocrosslinking-induced phase separation under evaporative environments. We further used N-isopropylacrylamide/carbon disulfide (CS2) as a model system to study the evolution of the composition and morphology within the solution drop. Within seconds of the process, fast evaporation of solvent dominated, which resulted in the formation of a concentration gradient that led to a skin layer later. Subsequently the evaporation rate slowed down and excessive solvent activity due to the reaction resulted in the phase separation that not only interrupted the further growth of the capping skin layer but also generated a particulate layer underneath.
Using the same fabrication process but in the presence of moisturized N2 flow, we successfully produced honeycomb like porous structures in the crosslinked polymer films. This is based on the "breath figure" formation during the in situ crosslinking of reactive monomer solutions. A range of precursor solutions, including tert-butyl acrylate/tetraethylene glycol dimethacrylate (TEGDMA) in chloroform, methyl methacrylate/TEGDMA in CS2 and Norland optical adhesive 65 (NOA65) in CS2 was crosslinked upon UV radiation under a moist and nitrogen saturated chamber and micro-size pores with reasonably uniformity were obtained in these crosslinked polymer films, illustrating the generality of this approach. The size, shape, uniformity and ordering of the pores show significant dependences on the casting conditions. In contrast, the pore morphology remains similar despite varying precursor compositions, suggesting that the pore-fixing process is still dominated by the solvent evaporation similar to that of conventional breath figure process, and is less dependent on the crosslinking process. We further observed the pores at the peripheral regions appeared more uniform in size and showed higher degree of ordering, due to the interplay between the solvent evaporation and crosslinking reaction. Detailed characterizations of the chemical and physical properties, including the glass transition temperature, thermal stability, and surface wettability of the porous films were carried out.
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