A Computer Model for Laser Photopolymerization

A computer model for a laser induced photopolymerization process has been established which simulates stereolithography. The model couples irradiation, chemical reaction, and heat transfer equations to provide insights into rate processes occurring in the volume element contacted by the laser beam. Quantities predicted include the spatial variation in conversion of monomer to polymer, depletion of photoinitiator, and local variations in temperature in and around the spot contacted by the laser. This allows predictions to be made about the laser dwell time, depth penetration and uniformity of the photopolymer formed in the process.Mechanical Engineerin

Material and Process Parameters that Affect Accuracy in Stereolithography

Experimental real time linear shrinkage rate measurements simulating stereolithography are used in an analysis of shrinkage during line drawing in stereolithography. While the amount of shrinkage depends on the polymerization kinetics, shrinkage kinetics and overall degree of cure, it also depends on the length of time to draw a line of plastic. A line drawn slowly will exhibit less apparent shrinkage than one drawn very quickly because much of the shrinkage is compensated for as the line is drawn. The data also indicates that a typical stereolithography resin in the green state may shrink to only 65% of its maximum, thus retaining considerable potential for shrinkage during post-cure. This infonnation can be used to predict the amount of shrinkage to be expected under certain exposure conditions and to fonnulate overall strategies to reduce shrinkage and subsequent warpage that causes shape distortion.Mechanical Engineerin

Polymer Matrix Nanocomposites by Inkjet Printing

This paper describes work on a continuing project to form functional composites that contain ceramic nanoparticles using a Solid Freeform Fabrication (SFF) inkjet printing method. The process involves inkjet deposition of monomer/particle suspensions in layers followed by curing each layer in sequence using UV radiation. The reactive monomer is hexanediol-diacrylate (HDODA); the polymer forming reaction proceeds by a free radical mechanism. The liquid monomer containing nanoparticles is essentially a printing ink formulation. Successfully suspending the particles in the monomer is critical. We have developed a surface treatment method for forming stable suspensions of the nanoparticles so that they remain discrete throughout the processing sequence. The SFF process involves careful control of the polymer cure so that the interface between layers is seamless and residual stresses in the composites are eliminated. An immediate use for such composites is in optical applications as gradient refractive index lenses (GRIN). GRIN lenses have planar surfaces, eliminating the need for costly grinding and polishing. The planar surfaces also eliminate optical aberrations that result at the edges of spherical lenses and diminish the accuracy of focus. If the appropriate nanoparticles are fully dispersed they will modify the polymer's refractive index without interfering with light transmission. The effect is additive with volume concentration. Using 'inks' of different compositions in a multiple nozzle inkjet printer allows the formation of composites with precise composition gradients. Since an object is built one planar layer at a time, changes can be made readily both within each layer and from layer to layer. Inkjet printing with picoliter resolution is ideal for this task. Working with SiC nanoparticles in HDODA as a model system for demonstrating the inkjet deposition process, nanocomposite films with a linear concentration gradient varying from 0 to 4.5% (wt) were fabricated on Silicon wafers. These composites are 30 layer films, which total 140µm in thickness. Each layer in the composite is about 5 µm in thickness. Analytical methods for characterizing the dispersion of the nanoparticles in the composite and some of the salient optical properties of the composites also were established. The status of the program is reviewed in this paper.Mechanical Engineerin

Thermal-Expansion and Fracture Toughness Properties ofParts made from Liquid Crystal Stereolithography Resins

Liquid crystal (LC) resins are a new kind ofstereolithography material that can produce parts with structured or ordered morphologies instead ofthe amorphous morphologies that result from standard resins. The LC molecules can be aligned before cure resulting in an anisotropic crosslinked network when the laser induced polymerization "locks-in" the alignment. Previous papers have explored liquid crystal orientation dynamics [1], the effects of orientation on viscoelastic and mechanical properties [2,3], and the processing ofLC resins by stereolithography [4]. This paper considers the effects ofmorphology on fracture toughness and thermal-expansion properties. Both toughness and thermal-stability continue to be important issues for stereolithography parts. The use ofLC resins may provide a way to significantly improve performance in both ofthese areas, and in addition result in parts with high upper use . temperatures.Mechanical Engineerin

Simulation of Laminated Object Manufacturing (LOM) with Variation of Process Parameters

A previously developed and verified thermal model for Laminated Object Manufacturing (LOM) was used to investigate the effects of various processing parameters on the temperature profile in a LOM part during the build cycle. The mathematical model, based on 3-dimensional transient heat conduction in a rectangular geometry LOM part, allows calculation ofthe transient temperature distribution within the part during the application of a new layer as well as during other periods ofthe LOM build cycle. The parameters roller temperature, roller speed, chamber air temperature, base plate temperature, and laser cutting time were independently varied, and the LOM process response simulated. The results were analyzed in order to gain insight into potential strategies for intelligent process control.Mechanical Engineerin

Development of polyisocyanurate pour foam formulation for space shuttle external tank thermal protection system

Four commercially available polyisocyanurate polyurethane spray-foam insulation formulations are used to coat the external tank of the space shuttle. There are several problems associated with these formulations. For example, some do not perform well as pourable closeout/repair systems. Some do not perform well at cryogenic temperatures (poor adhesion to aluminum at liquid nitrogen temperatures). Their thermal stability at elevated temperatures is not adequate. A major defect in all the systems is the lack of detailed chemical information. The formulations are simply supplied to NASA and Martin Marietta, the primary contractor, as components; Part A (isocyanate) and Part B (poly(s) and additives). Because of the lack of chemical information the performance behavior data for the current system, NASA sought the development of a non-proprietary room temperature curable foam insulation. Requirements for the developed system were that it should exhibit equal or better thermal stability both at elevated and cryogenic temperatures with better adhesion to aluminum as compared to the current system. Several formulations were developed that met these requirements, i.e., thermal stability, good pourability, and good bonding to aluminum

Does Social Support Diminish Depression in Students? Evidence from Athletes and Greek Life

Depression and stress are extremely prevalent in college students. The goal of the current study was to examine if social support from athletic teams and fraternities or sororities are related to lower levels of depression and stress than that found in students normally. Participants (N=134) were asked to fill out a questionnaire with scales of depression, stress and social support. It was hypothesized that members of Greek life and student-athletes will show higher levels of social support and lower levels of depression compared to a control group of students. The results supported these hypotheses and suggest that joining social groups might be an answer to creating a healthier lifestyle in college

It’s MORe exciting than mu: crosstalk between mu opioid receptors and glutamatergic transmission in the mesolimbic dopamine system

Opioids selective for the G protein-coupled mu opioid receptor (MOR) produce potent analgesia and euphoria. Heroin, a synthetic opioid, is considered one of the most addictive substances, and the recent exponential rise in opioid addiction and overdose deaths has made treatment development a national public health priority. Existing medications (methadone, buprenorphine, and naltrexone), when combined with psychosocial therapies, have proven efficacy in reducing aspects of opioid addiction. Unfortunately, these medications have critical limitations including those associated with opioid agonist therapies (e.g., sustained physiological dependence and opioid withdrawal leading to high relapse rates upon discontinuation), non-adherence to daily dosing, and non-renewal of monthly injection with extended-release naltrexone. Furthermore, current medications fail to ameliorate key aspects of addiction such as powerful conditioned associations that trigger relapse (e.g., cues, stress, the drug itself). Thus, there is a need for developing novel treatments that target neural processes corrupted with chronic opioid use. This requires a basic understanding of molecular and cellular mechanisms underlying effects of opioids on synaptic transmission and plasticity within reward-related neural circuits. The focus of this review is to discuss how crosstalk between MOR-associated G protein signaling and glutamatergic neurotransmission leads to immediate and long-term effects on emotional states (e.g., euphoria, depression) and motivated behavior (e.g., drug-seeking, relapse). Our goal is to integrate findings on how opioids modulate synaptic release of glutamate and postsynaptic transmission via α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid and N-methyl-D-aspartate receptors in the nucleus accumbens and ventral tegmental area with the clinical (neurobehavioral) progression of opioid dependence, as well as to identify gaps in knowledge that can be addressed in future studies

Functionally Graded Polymer Matrix Nano-Composites by Solid Freeform Fabrication: A Preliminary Report

A research program has been initiated to develop a Solid Freeform Fabrication (SFF) technology for combining nanosized particulate or fiber reinforcements with a photocurable thermoset matrix resin in order to produce functional graded composites. The composites that are being studied initially are optical components filled with nano-phase ceramic particles that form gradient refractive index lenses (GRIN). The Solid Freeform Fabrication (SFF) method employs an ink-jet deposition (IJD) process to form the composites. The IJD process has the advantage of incorporating nano-reinforcements into a low viscosity matrix resin that is relatively easy to process and rapidly photocures to produce functional polymeric parts. It also has the advantage that major modifications to the basic SFF processing methodology are not necessary. The emphasis in the program is on demonstrating the feasibility of this approach for fabrication of gradient refractive index lenses (GRIN), which are flat instead of the traditional spherical lens geometry. As a result these lenses will be less costly to produce than conventional curved lenses. SFF is an ideal technique for meeting the needs of GRIN lens fabrication because changes in composition can be made from layer to layer and even within each layer, allowing for the introduction of compositional and structural gradients. Thus it has the potential for creating the spatial material distributions required for designing computer optimized, custom made GRIN lenses. Integral to the SFF process are computer design procedures that specify the exact material deposition patterns that need to be employed in order to optimize the performance of the GRIN lens. The optical nano-composites will serve as a model system that we will use to work out the many challenges for implementing a viable SFF polymer composites technology. We then will make use of the information obtained and lessons learned from the work on optical composites and extend the development to structural composites that incorporate nano-particulate clays and carbon nanofibers.Mechanical Engineerin