Robert E. Lowry

Novel fluorescence method for cure monitoring of epoxy resins

Abstract The fluorescence spectra of organic dyes dissolved in epoxy resins are sensitive to local viscosity. The excimer forming dyes are particularly useful as probes since the monomer emission can be used as an internal standard in the measurement. In this case, the probability of excimer formation is related to molecular mobility and hence to the microviscosity. This approach has been demonstrated on epoxy resins. In another approach, trace amounts of 1-(4-dimethylaminophenyl)-6-phenyl-1,3,5-hexatriene (DMA-DPH) and 9,10-diphenylanthracene (DPA) are added to an epoxy resin. The fluorescence intensity of DMA-DPH increases with the increase in local viscosity while the fluorescence intensity of DPA is insensitive to local viscosity and can be used as an internal standard. The ratio of the fluorescence intensities of DMA-DPH and of DPA has been measured to monitor the cure of epoxy resins.

Novel excimer fluorescence method for monitoring polymerization: 1. Polymerization of methyl methacrylate

Abstract An excimer is formed by the association of an excited molecule with another molecule in its ground state. Such an excimer is characterized by a broad structureless fluorescence which is shifted to longer wavelengths compared to the fluorescence spectrum of the isolated molecule. Intramolecular excimer fluorescence has been observed in solutions of pyrene-labelled alkanes such as 1,3-bis-(1-pyrene)propane and 1,10-bis-(1-pyrene)decane. We have measured the solvent-viscosity dependence of the intensity ratio F M F D for solutions of these pyrene-labelled alkanes in mixed solvents made of ethyl acetate and glycerol tripropionate. Here FM and FD are, respectively, the fluorescence intensity of the unassociated pyrene groups and that of the intramolecularly formed pyrene excimers. We have found that for each of the two pyrene-labelled alkanes, the ratio F M F D increases with the increase in solvent viscosity. Further, we have shown that by adding a trace amount of 1,3-bis-(1-pyrene)propane or 1,10-bis-(1-pyrene)decanee to a polymerizing system, we can measure the ratio F M F D to monitor in situ the polymerization reaction. polymerization;

Self-diffusion in concentrated polystyrene solutions measured by fluorescence recovery after photobleaching

Abstract A polystyrene polymer of narrow molecular weight distribution was carboxylated, then reduced, and finally esterified with NBD-aminohexanoic acid [6-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)-aminohexanoic acid]. The self-diffusion of the NBD-labelled polystyrene polymer in concentrated solutions of the unlabelled polystyrene polymer was measured by the method of fluorescence recovery after photobleaching over a concentration range from 0.017 g/ml to 0.41 g/ml at room temperature. In the semi-dilute region, the concentration dependence of diffusion coefficient was found to be in agreement with the predictions of scaling concepts.

Process Monitoring of Polymer Matrix Composites Using Fluorescence Probes

On-line process monitoring and control are prerequisites for more efficient and reliable manufacture of polymer matrix composites. The cure process involves complex chemical and physical changes that must be adequately controlled to consistently produce high quality products. Of various changes that occur, the viscosity is particularly important as it influences fiber wetting, uniformity of resin distribution and consolidation of the reinforcement plies. In the usual cure cycle, the viscosity of the resin initially decreases as the temperature of the resin rises owing to the higher temperature of the autoclave and heat released by the exothermic chemical reactions of the cross-linking process. Ultimately, the growing molecular weight of the polymer increases the viscosity beyond the range where flow will occur. If the resin viscosity becomes too low during this period, excess resin will seep out of the part producing an inferior product owing to resin deficiency. On the other hand, if the viscosity increases too fast the flow may be insufficient to achieve good consolidation of the plies.

Picosecond excimer fluorescence spectroscopy: applications to local motions of polymers and polymerization monitoring

Abstract The local motions of α,ω-bis-(1-pyrene)alkanes and pyrene-labelled poly(methyl methacrylate) polymers in solution were characterized by picosecond excimer fluorescence spectroscopy. The experimental results showed that 1,3-bis-(1-pyrene)propane and 1,10-bis-(1-pyrene)decane have similar local motions that bring two pyrene groups together to form excimers. Further, poly(1-pyrenylmethyl methacrylate) and a copolymer of methyl methacrylate and 1-pyrenylmethyl methacrylate in solution were found to have similar local motions that lead to excimer formation. In addition, the viscosity change during the polymerization of methyl methacrylate was monitored by measuring with picosecond fluorimetry the fluorescence lifetime of a trace amount of 1,3-bis-(1-pyrene)propane dissolved in methyl methacrylate.

Non-Destructive Evaluation Measurement Technology for Polymer Processing Based on Fluorescence Spectroscopy

We are employing fluorescence spectroscopy as a tool to monitor polymer processing parameters which are important for understanding process behavior. The measurements involve the detection of fluorescence spectra from fluorescent dyes which have been doped into the processed polymer material. The character of the fluorescence, i.e. its intensity, polarization, and wavelength distribution, yields information about the state of the polymer matrix. We have concentrated on developing concepts and methods to measure molecular orientation, shear stress, shear rate, non-Newtonian viscosity, velocity, residence time distribution, flow instabilities, quality-of-mix of ingredients, and intersegmental mixing. Work on each of these measurement problems is ongoing and in various stages of development.1-2 In this paper, we describe some recent work on quality-of-mix and intersegmental mixing.