Joy P. Dunkers

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.

Interfacial shear strengths of dental resin-glass fibers by the microbond test

OBJECTIVES The aim of this study was to investigate the feasibility of using the microbond test (MBT) to probe the durability of the bond between a polymerized dental resin with differently silanized E-glass fibers. METHODS The E-glass fibers were silanized with equivalent amounts of two types of acrylic-silane coupling agents: 3-methacryloxypropyltrimethoxysilane (MPTMS) and 10-methacryloxydecyltrimethoxysilane (MDTMS), a more hydrophobic silane coupling agent than MPTMS. Unsilanized E-glass fibers were used as the control. Microdroplets of a photo-activated dental resin were applied on the fiber and photocured with visible light irradiation (470 nm). Subsequently, the specimens were tested in shear after 24h storage in air at 23 degrees C or water at 60 degrees C. RESULTS The mean interfacial shear strength (tau) and the standard deviation in MPa for the three systems in 23 degrees C in air (n>7) were: 33.8(10.1), 33.7(8.9) and 15.3(4.2) for the MPTMS silanized, MDTMS silanized, and unsilanized fibers, respectively. When the three types of fibers were first exposed to 60 degrees C water for 24h prior to having the microdroplets of the resin bonded to them, the strength values of the MDTMS silanized fibers and the control fibers remained essentially unchanged at (n> or =7) 31.8(7.7) and 17.5(4.9)MPa respectively; the MPTMS specimens showed a significant decrease to 15.8(4.8)MPa. Similar trends were observed when the fibers had microdroplets of the resin bonded to them prior to aqueous exposure. SIGNIFICANCE These results indicate that the microbond test has the sensitivity to measure changes at the interface between polymerized dental resins and variously silanized E-glass fibers. It appears that surface modification of the fibers with the more hydrophobic silane coupling agent MDTMS promotes enhanced resistance to degradation from exposure to water. The microbond test has the potential for studying dental adhesion involving small bonded areas under a variety of conditions with different adhesive systems and substrates.

The Application of Optical Coherence Tomography to Problems in Polymer Matrix Composites

Abstract The Composites Group at the National Institute of Standards and Technology has found optical coherence tomography (OCT) to be a powerful tool for non-destructive characterization of polymer matrix composites. Composites often exhibit superior properties to traditional materials such as wood and metal. However, the barrier to their widespread infiltration into consumer markets is cost. Composites can be made more cost competitive by improved composite design, process optimization, and quality control. OCT provides a means of evaluating the three aforementioned areas. OCT is a very versatile technique that can be applied to a variety of problems in polymer composites such as: microstructure determination for permeability and mechanical property prediction, void, dry spot, and defect detection, and damage evaluation. Briefly, OCT uses a low coherence source such as a superluminescent diode laser with a fiber optic based Michelson interferometer. In this configuration, the composite is the fixed arm of the interferometer. Reflections from heterogeneities within the sample are mapped as a function of thickness for any one position. Volume information is generated by translating the sample on a motorized stage. Information about the location and size of a feature within the composite is obtained. In this work, the power of OCT for imaging composite microstructure and damage is presented. An example of permeability prediction using the composite microstructure imaged from OCT is demonstrated. The effect of image processing on the value of permeability is discussed. Using the same sample, OCT imaging of composite impact damage is compared to more traditional techniques, X-ray computed tomography and confocal microscopy.

Fiber Optic Flow and Cure Sensing for Liquid Composite Molding

The Polymer Composites group at the National Institute of Standards and Technology has efforts in both on-line flow and cure sensing for liquid composite molding. For our flow program, a novel fiber optic real-time sensor system has been developed that can sense resin at various locations on a single fiber using long-period gratings and a polychromatic source. The sensor operation and characterization will be discussed along with sensor performance during mold filling with various types of reinforcement. The cure sensing program focuses on the interface-sensitive fluorescence response of a dye molecule grafted to a high-index glass fiber. The fluorescence emission of the fluorophore undergoes a blue shift as the resin cures. The fluorescence sensor is made by grafting a silane functional fluorophore onto the surface of the glass with close attention to layer thickness. Fluorescence emission of the grafted fluorophore film is shown to be sensitive to epoxy resin cure, co-silane, and layer thickness. The response of the grafted fluorophore to cure on a high-index fiber is demonstrated.

Collinear optical coherence and confocal fluorescence microscopies for tissue engineering.

Tissue engineered medical products (TEMPs) are often three-dimensional (3D) hybrid materials consisting of a porous scaffold upon which the tissue is grown. However, monitoring of the developing tissue deep within the scaffold is hampered by its turbidity. We have sought new ways to probe the interior of the scaffold with the same resolution as conventional laser scanning confocal microscopy but with greater sensitivity. We present a novel application of optical coherence microscopy (OCM) by combining it with confocal fluorescence microscopy (CFM) togather simultaneous structural and functional information on TEMPs in a registered fashion. In this work, we describe the collinear OCM and CFM instrument. We demonstrate the utility of this dual-mode technique for TEMPs by imaging fluorescently stained osteoblasts cultured in a polymeric TEMP.

Comparison of optical coherence tomography, x-ray computed tomography, and confocal microscopy results from an impact damaged epoxy/e-glass composite

Optical coherence tomography (OCT) is an emerging technique for imaging of synthetic materials. OCT is attractive because it combines high sensitivity (>90 dB), high resolution (5 µm to 20 µm), and low cost, approximately US

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

75 k. The value of any new technology is evaluated by how well it compares with existing methods. In this work, impact damage of an epoxy/E-glass composite is imaged using OCT, and the results were compared with micro-focus X-ray computed tomography. This technique is a good benchmark to compare with OCT because both techniques have the ability to locate features precisely and have comparable resolutions. OCT is considered to be a confocal technique so it was also compared to laser scanning confocal microscopy (LSCM). Contrast mechanisms, sensitivity, resolution, depth of penetration, and artifacts among the techniques are compared and contrasted. Also, impact damage features revealed using OCT are briefly discussed.

A mid-infrared attenuated total internal reflection cure sensor for control of resin transfer moulding of a pre-ceramic polymer

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

Abstract An attenuated total internal reflection (ATR) mid-infra red (m.i.r.) sensor has been constructed and interfaced with a resin transfer moulding (RTM) system to monitor the progression of chemical reactions during processing. Software, resident on the microcomputer used to control the Fourier transform infrared (FT-i.r.) spectrometer, automatically collects the spectra and converts the m.i.r. spectra to a single number indicative of the ‘degree of cure’. This index is then sent to the process control computer. Programmable cure is achieved by using the ‘degree of cure’ signal from the FT-i.r. in a model-based feedback cure controller cascaded to the base-level temperature controller. Thus, the cure cycle is expressed as a cure history, not as a temperature history as is typical in composites processing. Cure control was simulated to test the effectiveness of the control algorithm.

Single cell viability measurements in 3D scaffolds using in situ label free imaging by optical coherence microscopy

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.