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44161-GB5
Photopolymerization by Evanescent Waves and Spectroscopic Study of Mechanochemical Response of the Polymer Network on an Optical Fiber

Sergey V. Kazakov, Pace University

Goal Statement. The ultimate goal of the research is to fabricate a cylindrical polymer network (hydrogel) on the side-surface (not the distal end) of an optical fiber core and characterize spectroscopically its swelling/de-swelling ability in response to different stimuli.

Year 1 brief summary. (1) Composition of the hydrogel forming solution, proper photoinitiator, and transmittance of fiber optics at the wavelength of light source were optimized using procedure for bulky-hydrogel preparation as a model. (2) A unique reactor-container was designed to control the polymerization process both spectroscopically and microscopically. (3) To study photopolymerization within microscopic volumes, the giant unilamellar vesicles (GUV, ~5-300 µm) have been used as soft microreactors. (4) Kinetics of proton intake has been studied in suspensions of artificial hydrogel particles of micrometer size (~20-200 µm) and natural gel-like multilayered structures (bacterial spores). (6) Three oral talks have been presented by the undergraduate students involved into these studies at 55th ACS Undergraduate Research Symposium (NY, May 2007).

Year 2 progress. Considerable efforts have been undertaken to work out a robust procedure for hard polymer cladding removal in order to make a side-open fiber core. Eventually, a method for reproducible fabrication of a cylindrical polymer network on a fiber core using polymerization by evanescent waves was developed (Figure 1). For example, the average thickness of the film after 2 hour polymerization has been estimated to be ~175 µm in wet state, ~60 µm after 6 hours of drying, and ~30 µm after overnight drying on air.

Figure 1. Optical micrographs (x10) of the hydrogel on optical fiber core: (a) wet hydrogel film on the optical fiber core in air after polymerization and washing (UV light passes through the fiber core), (b) hydrogel film on the optical fiber core on air dried overnight ((light scattering in dried hydrogel film is much intensive than in wet one), (c) a portion of a dried hydrogel film on the fiber core.

Spectral analysis of light passing through the fiber provided the method for the cylindrical hydrogel film detection and characterization. Particularly, kinetics of hydrogel film swelling in the course of dry-wet transition was studied. Two unexpected phenomena have been discovered as a result of the study of kinetics of polymerization and swelling of the hydrogel film: (1) scattering of evanescent waves on the successively formed polymer layers can result in cylindrical film around fiber core with thickness significantly higher than the depth of evanescent field penetration out of fiber (0.1l), and (2) in the course of shrinking, temperature sensitive poly–N–(isopropylacrylamide) hydrogel supported by fiber core absorbs the light of different spectral composition (Figure 2): at the phase transition temperature (~32°C), ultraviolet begins to absorb, at higher temperatures, spectral range of absorbed light expands to the visible and near infrared domains. The last results have been submitted for presentation at the 2009 Pittsburg Conference.

Figure 2. Transmittance of the fiber core covered by the cylindrical temperature sensitive hydrogel (PNIPA) as a function of temperature (numbers assigned to each curve).

Our results are of great potential for hydrogen storage on ion-sensitive polymer network. For the first time, gigantic proton capacity (average number of ionizable groups) and particular type of the binding sites (apparent binding constant) have been obtained for synthetic ionic reservoirs (hydrogels) and for ionic polymer networks uniformly fabricated by Nature (bacterial spores). It was shown that the plurality of steps comprising the uptake of protons inside spores may be attributed to their internal multi-layered microstructures.

Conclusions and future perspectives. It was demonstrated that an optical fiber with open side surface of its core connected to a spectrometer is an apparatus for polymerization and analytical device in one. A cylindrical polymer network itself is a prerequisite for a supported 2D-single macromolecule with the world's highest and the record fastest level of expansion and contraction (energy conversion). A light emitting polymer, cross-linked around a fiber core, is a prerequisite for an optically pumped organic laser (energy-efficient lighting). A polyelectrolyte network on a fiber core is a prerequisite for a hydrogen storage container with a high ionic capacity and spectroscopic control of accumulation and release of hydrogen ions (alternative energy source).

Impact to the PI and students career. These results on ionic reservoirs were published in Journal of Physical Chemistry and NIST Nanotech 2008, presented at the following conferences: PITTCON 2008, MARM 2008, SIGMA-XI Students Research Society 2008, NANOTECH 2008, 236th ACS National Meeting. New results were submitted to PITTCON 2009. Four oral talks have been presented by undergraduate students involved in these studies at 56th Undergraduate Research Symposium (NY, April 2008) and 27th Annual Meeting of The Dyson Society of Fellows. The PI has been promoted to the rank of Associate Professor in 2007 and received his tenure in 2008. The results were used to complete the research grant application submitted to the NSF.

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