Kathleen M. Flynn

Continuous flow enzyme-catalyzed polymerization in a microreactor.

Enzymes immobilized on solid supports are increasingly used for greener, more sustainable chemical transformation processes. Here, we used microreactors to study enzyme-catalyzed ring-opening polymerization of ε-caprolactone to polycaprolactone. A novel microreactor design enabled us to perform these heterogeneous reactions in continuous mode, in organic media, and at elevated temperatures. Using microreactors, we achieved faster polymerization and higher molecular mass compared to using batch reactors. While this study focused on polymerization reactions, it is evident that similar microreactor based platforms can readily be extended to other enzyme-based systems, for example, high-throughput screening of new enzymes and to precision measurements of new processes where continuous flow mode is preferred. This is the first reported demonstration of a solid supported enzyme-catalyzed polymerization reaction in continuous mode.

Optical coherence tomography of glass reinforced polymer composites

Abstract Optical coherence tomography (OCT) is a nondestructive and noncontact technique to image microstructure within scattering media. The application of OCT to highly scattering materials such as polymer composites is especially challenging. In this work, OCT is evaluated as a technique to image fiber tows and voids in two materials: an epoxy E-glass-reinforced composite and a vinyl-ester E-glass-reinforced composite. Features detected using OCT are compared with optical microscopy. Fiber architecture and voids of glass-reinforced polymer composites can be successfully imaged using OCT. The quality of the OCT image is strongly affected by the refractive index mismatch between the fibers and reinforcement. The largest sources of noise in the images arise from fiber lens effects, interference from within the sample, and a very large reflection at the surface.

Swelling in crosslinked natural rubber: experimental evidence of the crosslink density dependence of χ

Mechanical and swelling measurements were carried out on samples of dicumyl-peroxide-crosslinked natural rubber. The balance of the elastic free energy and the mixing free energy at swelling equilibrium was used to calculate the value of the Flory-Huggins χ parameter for a series of crosslinked rubbers swollen in an excess of different solvents. The results show that χ for the crosslinked rubber is greater than that of the uncrosslinked counterpart. Other evidence from the literature, based on swelling activity parameter measurements, which supports this conclusion, is discussed. We also find that the value of χ in the crosslinked rubber is a linear function of the crosslink density. We examine these results in the context of the Freed and Pesci lattice model.

The use of evanescent wave fluorescence spectroscopy for control of the liquid molding process

Fluorescence has been demonstrated to be an accurate measurement of resin cure and is measured using an evanescent wave fiber-optic sensor. An economical optical fiber sensor has been developed with a refractive index in excess of 1.6, permitting evanescent wave monitoring of epoxy resins. In this work, the fluorescence wavelength-shift, which has been correlated with monomer conversion, is monitored during the liquid molding process. A hierarchical control strategy is being developed for liquid molding processes that will use the cure measurement provided by the evanescent wave fluorescence sensor. Single input/single output control loops are implemented at the base level to regulate the mold temperature, the vacuum pressure in the mold, either the inlet flow or pressure during injection, and the final mold pressure. A model-based feedback controller is implemented to control the chemical cure and manipulates the setpoint of the temperature controller to achieve cure control. A general high-level controller is under development to optimize the process under a wide range of operating conditions.

Fourier Transform Near-Infrared Monitoring of Reacting Resins Using an Evanescent Wave High-Index Fiber-Optic Sensor

In this work, a high-index silica-based fiber-optic mini-bundle sensor was constructed and implemented with a Fourier transform near-infrared spectrometer in the spectral region from 10 000 to 4500 cm−1. The refractive index of the multimode step index fiber was 1.618. This arrangement allows the propagation of waveguiding modes when the sensor is immersed in most resin systems. The reactions of a polyisocyanurate resin system and an epoxy resin system were monitored and peak assignments were made and discussed with respect to their potential for use in real-time analysis to be applied to cure control. Last, cure monitoring with this sensor was successfully demonstrated in a glass-reinforced resin transfer molded epoxy part.

A General Method for Quantitative Measurement of Molecular Mass Distribution by Mass Spectrometry

A method is presented to test whether the conversion of the mass spectrum of a polydisperse analyte to its molecular mass distribution is quantitative. Mixtures of samples with different average molecular masses, coupled with a Taylor’s expansion mathematical formalism, were used to ascertain the reliability of molecular mass distributions derived from mass spectra. Additionally, the method describes how the molecular mass distributions may be corrected if the degree of mass bias is within certain defined limits. This method was demonstrated on polydisperse samples of C60 fullerenes functionalized with ethylpyrrolidine groups measured by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry; however, it is applicable to any polydisperse analyte and mass spectrometric method as long as spectrum resolution allows individual oligomers to be identified. Mass spectra of the derivatized fullerenes taken in positive ion mode were shown to give an accurate measurement of the molecular mass distribution while those taken in negative ion mode were not. Differences in the mechanisms for ion formation are used to explain the discrepancy.Official contribution of the National Institute of Standards and Technology; not subject to copyright in the United States of America.

Synthesis of Polymerizable Cyclodextrin Derivatives for Use in Adhesion-Promoting Monomer Formulations

The synthesis of the cyclodextrin derivatives reported herein was assisted by extensive literature research together with structure-property relationships derived from three-dimensional molecular modeling. These studies led to the hypothesis that many of the 21 hydroxyl groups on beta-cyclodextrin molecules could be derivatized to form a closely related family of analogous chemical compounds containing both polymerizable groups and hydrophilic ionizable ligand (substrate-binding) groups, each attached via hydrolytically-stable ether-linkages. The vinylbenzylether polymerizable groups should readily homopolymerize and also copolymerize with methacrylates. This could be highly useful for dental applications because substantially all contemporary dental resins and composites are based on methacrylate monomers. Due to hydrophilic ligands and residual hydroxyl groups, these cyclodextrin derivatives should penetrate hydrated layers of dentin and enamel to interact with collagen and tooth mineral. Analyses indicated that the diverse reaction products resulting from the method of synthesis reported herein should comprise a family of copolymerizable molecules that collectively contain about 30 different combinations of vinylbenzyl and hexanoate groups on the various molecules, with up to approximately seven of such groups combined on some of the molecules. Although the hypothesis was supported, and adhesive bonding to dentin is expected to be significantly improved by the use of these polymerizable cyclodextrin derivatives, other efforts are planned for improved synthetic methods to ensure that each of the reaction-product molecules will contain at least one copolymerizable moiety. The long-term objective is to enable stronger and more durable attachments of densely cross-linked polymers to hydrated hydrophilic substrates. Capabilities for bonding of hydrolytically stable polymers to dental and perhaps other hydrous biological tissues could provide widespread benefits.

Model-assisted feedback control for liquid composite molding

A model-assisted feedback control algorithm, a type of generic model control, is implemented to control cure in resin transfer molding. This control algorithm calculates an apparent temperature of reaction based on the cure data input form a sensor, and this temperature is used to compare the actual rate of reaction to the desired rate and to calculate the mold set-point temperature. The model input into the control algorithm is an empirical cure model of a pre-ceramic polymer with an Arrhenius temperature dependence from 55 to 95 °C. In this work, the effect of varying control parameters is evaluated through cure simulations and experiments. Also, the effect of noise on the controller robustness is evaluated through simulation and experiment. Control parameters are evaluated for 55 and 95 °C.

Quantitative measurement of the polydispersity in the extent of functionalization of glass‐forming calix[4]resorcinarenes

The polydispersity in the degree of functionalization for two calix[4]resorcinarenes was determined by measuring quantitatively their molecular mass distribution with matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. A mathematical method for polydisperse materials is described that creates a calibration curve to correct the ion signal intensities in the mass spectrum to give a more reliable molecular mass distribution. Correction is required due to various sample preparation and instrumental effects that may produce a systematic mass bias in the number of oligomers measured. This method employs gravimetric mixtures of analytes with different degrees of functionalization. One calix[4]resorcinarene was found to give accurate molecular mass distributions with little correction, while another, having a very similar molecular structure, was found to exhibit strong over-counting of the oligomers having a high degree of functionalization.

A Test Method to Determine the Fiber and Void Contents of Carbon/Glass Hybrid Composites

Hybrid composites are increasingly being used in infrastructure, oil recovery, and aerospace applications. These hybrids generally combine two different types of fibers to reinforce the resin, thereby gaining some of the advantageous properties of both fibers. Typically, carbon and glass are used since this combines the high performance of the carbon with the low cost of the glass. The performance of such materials depends on a number of factors, including the mix ratio of the fibers as well as the fiber and void contents. At present, there is no simple way to determine these features. Although a number of techniques exist for measuring such parameters when only a single reinforcement is present, extension of these methods to hybrids can be difficult. The work here, however, shows that one technique, the burn off test in ASTM D 3171-99, can be extended to characterize carbon/glass hybrids. To verify this procedure, data were obtained for a series of samples with known compositions, and the agreement was excellent. The proposed method has minimal equipment requirements and provides a simple way to obtain important compositional information.