In-situ monitoring of the mechanical properties during the photopolymerization of acrylate resins using particle tracking microrheology

The fundamentals of the photopolymerization process are not well understood. As a result, issues affecting the cure speed and overall quality of the final product (shape, size, and surface finish) of photopolymerization impose significant limitations on applications that require fast processing and high spatial resolution. To address this issue, microrheology was employed to perform in-situ monitoring of the liquid-to-gel transition during free-radical photopolymerization. Photosensitive acrylate and hydrogel resins were exposed to ultraviolet light, while the Brownian motion of micrometer sized, inert fluorescent tracer particles was tracked via optical videomicroscopy. Statistical analysis of particle motion yielded the rheological properties of the embedding medium as a function of time and location, thereby relating UV exposure to the progress of polymerization and gelation. The microrheological setup enabled a detailed study of three-dimensional gelation profiles; other experimental parameters that were initially varied include photoinitiator concentration, monomer composition, and light intensity. Significant changes in gelation time were observed with varying UV intensity and UV penetration depth into the sample. In addition, oxygen inhibition was found to significantly impact the cure speed of monomeric resins. The preliminary results were used to test the accuracy of the energy threshold model, which is often used to empirically predict the outcome of photopolymerization reactions. By using lithographic masks to generate well-defined UV illumination patterns with characteristic dimensions of tens of micrometers, it could be shown unambiguously that the diffusion of oxygen, an inhibitor, plays a critical role in the polymerization reaction. The experiments are in excellent agreement with a simple two-step model of oxygen consumption followed by polymerization. The use of high-speed electronic shutters in the UV light path enabled us to control the illumination time of the samples with high precision. Microrheological analysis could be used to reconstruct three-dimensional profiles of partially polymerized samples. Traditional photorheometry is not capable of resolving the evolution of sample rheology with such spatial resolution. In addition, experiments with pulsed illumination were used to quantify the role of dark reactions due to residual free radicals after termination of UV illumination.Ph.D.Committee Chair: Dr. Victor Breedveld; Committee Member: Dr. Clifford Henderson; Committee Member: Dr. David Rosen; Committee Member: Dr. Peter Ludovice; Committee Member: Dr. Sai Kuma

An Assessment of Surface Properties and Moisture Uptake of Nonwoven Fabrics from Ginning By-products

Greige (raw) cotton by-products resulting from cotton ginning and mill processes have long been bleached for use in absorbent nonwoven products. The potential to use greige cotton by-products as an economical source for absorbent nonwoven blends is explored. The nonwoven hydroentanglement of greige cotton lint with cotton gin motes and comber noils blends was analyzed for fiber surface polarity, swelling, and absorbance to assess properties with potential usefulness in absorbent nonwovens. The electrokinetic analysis of the fabric surface gives a composite picture of the relative hydrophilic/hydrophobic polarity absorbency and swelling properties. Nonwoven fabrics made with cleaned greige cotton lint separately blended with comber noils and ginning motes at 40:60 and 60:40 blend ratios demonstrated charge, swell, and percent moisture uptake profiles that are characteristic of the fabrics’ crystalline/amorphous cellulosic content with some variance in swelling properties. However, cellulose crystallite size varied. X-ray diffraction patterns of the three different cotton constituents displayed similar crystalline cellulose compositions. An electrochemical double-layer analysis of charge based on a pH titration (ζplateau) was employed to measure the relative fiber and fabric surface polarity which varied slightly between -21 and -29 mV. A relationship of fiber swelling (∆ζ) and percent moisture content is apparent when greige cotton lint and other fibers are blended. The blended nonwoven materials possess absorbent properties characterized by similar moisture uptake (7.1-9.5 %) and fiber polarity, but some variation in swelling is based on the by-product additive and its percent content. The crystallinity, electrokinetic, and water binding properties of the nonwoven by-product materials are discussed in the context of the molecular features water, cellulose, and greige cotton components that enhance potential uses as absorbent nonwoven end-use products

In-situ Monitoring of Photopolymerization Using Microrheology

Photopolymerization is the basis of several multi-million dollar industries including films and coating, inks, adhesives, fiber optics, and biomaterials. The fundamentals of the photopolymerization process, however, are not well understood. As a result, spatial variations of photopolymerization impose significant limitations on applications in which a high spatial resolution is required. To address these issues, microrheology was implemented to study the spatial and temporal effects of free-radical photopolymerization. In this work a photosensitive, acrylate resin was exposed to ultraviolet light, while the Brownian motion of micron sized, inert fluorescent tracer particles was tracked using optical videomicroscopy. Statistical analysis of particle motion yielded data that could then be used to extract rheological information about the embedding medium as a function of time and space, thereby relating UV exposure to the polymerization and gelation of monomeric resins. The effects of varying depth, initiator concentration, inhibitor concentration, composition of the monomer, and light intensity on the gelation process were studied. The most striking result is the measured difference in gelation time observed as a function of UV penetration depth. The observed trend was found to be independent of UV light intensity and monomer composition. The intensity results were used to test the accuracy of energy threshold model, which is used to empirically predict photo-induced polymerization. The results of this research affirm the ability of microrheology to provide the high spatial and temporal resolution necessary to accurately monitor the photopolymerization process. The experimental data provide a better understanding of the photo-induced polymerization, which could lead to expanded use and improved industrial process optimization. The use of microrheology to monitor photopolymerization can also aid in the development of predictive models and offer the ability to perform in-situ quality control of the process.M.S.Committee Chair: Breedveld, Victor; Committee Member: Henderson, Cliff; Committee Member: Ludovice, Pet

Structural Effect of Phosphoramidate Derivatives on the Thermal and Flame Retardant Behaviors of Treated Cotton Cellulose

The present research is aimed at studying the structural effect of two phosphoramidate derivatives diethyl 3-hydroxypropylphosphoramidate <b>EHP</b> and dimethyl 3-hydroxypropylphosphoramidate <b>MHP</b> as flame retardants for cotton fabric. <b>EHP</b> and <b>MHP</b> were obtained in very high yield and purity by one step procedures. Cotton twill fabrics treated with the two compounds at different add-ons (5–20 wt %) were characterized. Vertical flammability, limiting oxygen index, thermogravimetric, and microscale combustion calorimeter analyses were performed, and all resulted in better flame retardancy and thermal behavior for <b>MHP</b> compared to <b>EHP</b>. A study of the functional groups which appeared on the treated fabrics by attenuated total reflection infrared spectroscopy revealed different binding mechanisms between each compound and cotton cellulose. Analysis of the released gas products by thermogravimetric analysis-Fourier transform infrared spectroscopy showed some distinctive details in the degradation of the treated fabrics during the burning process