Showing papers by "Alan J. Lesser published in 2004"

Non-Isothermal Crystallization of Poly(trimethylene terephthalate) (PTT)/Clay Nanocomposites

Abstract: The non-isothermal crystallization of intercalated poly(trimethylene terephthalate) (PTT)/clay nanocomposites was investigated quantitatively by different methods. Non-isothermal crystallization of pure PTT can be described by the Ozawa equation, but the Ozawa theory is not valid for PTT/clay nanocomposites. The addition of clay into PTT decreases the crystallization half-time while increasing the crystallinity and crystallization rate. The PTT/modified clay nanocomposites, in turn, have a higher crystallization rate parameter, k l/n , and a lower crystallization half-time compared to unmodified clay/PTT composites. The crystallization activation energy, calculated from Kissinger equation, of PTT, PTT/unmodified clay, and PTT/modilied clay composites are 100, 180, and 198-230 kJ.mol -1 respectively. Similarly, the nucleating activities of the unmodified clay and modified clays are 0.73 and 0.51-0.58, respectively. Although the addition of clay increases the crystallization activation energy, the clay still acts as an effective nucleating agent and increases the crystallization rate are crystallinity of PTT. Further, the modified nanoscale clays rate more effective nucleating agents than the unmodified counterpats, and the most effective nucleating concentration of nano-clay is between 1.0-3.0 wt.-%.

CO2-assisted polymer processing: A new alternative for intractable polymers

Abstract: CO2-assisted polymer processing is proposed as an alternative route for intractable and high molecular weight polymers based on the plasticization effects of CO2 and its direct effect on the melting behavior of semicrystalline polymers. A modified processing system was used to process a variety of polymers in the presence of high-pressure CO2. The system includes an extruder that was modified to allow for high pressures created by the injection of CO2. The new design includes a modified feed section that allows a given mass of polymer to interact with CO2 before and during the extrusion process. The inherent shear mixing and the presence of CO2 allow for a specific control over the extrudate morphology. Results suggest that this alternative design provides a new and easy route to melt process high melt viscosity polymers of commercial importance, such as polytetrafluoroethylene (PTFE), fluorinated ethylene propylene copolymer (FEP), and syndiotactic polystyrene (s-PS). The increased processability of these systems in CO2 is related to the plasticization effect of CO2 that was quantified through a depression in the glass-transition temperature according to the Chow model. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 1501–1511, 2004

Molecular parameters governing the yield response of epoxy‐based glassy networks

Abstract: This contribution considers to what extent two molecular parameters, the glass transition temperature and the cohesive energy density, relate to the yield behavior of epoxy-based glassy thermosets. Eight different formulations consisting of four aliphatic and four aromatic resins with varying molecular weights between crosslinks were investigated over a broad range of test temperatures. Additionally, one aromatic formulation is studied over a range of stress states and test temperatures. The results indicate that both the glass transition and cohesive energy density are governing parameters that relate to the yield response of these systems. A model is proposed to incorporate these parameters and to predict the yield response as a function of strain rate, temperature, and stress state. The functional form of the results also indicate that the activation energy density (i.e., the activation energy divided by the activation volume) may be the material characteristic that relates to yield of these systems rather than each term individually.

Characteristics of antiplasticized thermosets: Effects of network architecture and additive chemistry on mechanical fortification

Abstract: A new class of molecular additives is investigated for epoxy-based crosslinked polymers. These additives are shown to increase modulus and yield stress in the cured networks. In order to elucidate the mechanisms for reinforcement of epoxy thermosets, the effects of additive chemistry and network architecture are considered. Both model and commercial epoxy networks are studied, thereby probing the effect of molecular weight between crosslinks, Mc, on reinforcement. Additionally, the family of phosphates being studied ranges in molecular weight, solubility parameter and density. These parameters are demonstrated to strongly influence the degree of reinforcement and correlate with the ability of the additive to reduce mobility of the polymer network. Mechanisms of reinforcement are discussed. Polym. Eng. Sci. 44:2125–2133, 2004. © 2004 Society of Plastics Engineers.

Polymer-clay nanocomposites prepared in supercritical carbon dioxide

Abstract: An alternative route to prepare polymer-clay nanocomposites using supercritical carbon dioxide (scCO 2 ) is described. The presence of clay nanoparticles significantly influences the morphology, foaming process and crystallization of a polymer when processed in scCO 2 . Intercalated structures are successfully produced in the presence of scCO 2 even when favorable interactions between the polymer and the clay are not present. The effect of scCO 2 on the intercalation process is analyzed for a variety of polymer systems both with modified and unmodified clays. By controlling the hydrophilicity of the polymer and clay systems, specific understanding of the effect of scCO 2 on the structure and morphology of the nanocomposites is obtained. Experimental results show significant increases in the clays d-spacings for scCO 2 -treated samples. This behavior is consistent regardless of the nature of the polymer, showing significant amounts of intercalation even in purely hydrophobic polymers.

Influence of interchain forces and supermolecular structure on the drawing behavior of nylon 66 fibers in the presence of supercritical carbon dioxide

Abstract: The drawing behavior and mechanical properties of as-spun and highly oriented nylon 66 fibers drawn in supercritical carbon dioxide (SCCO2) were investigated. Conditions including different temperatures, CO2 pressures, and plasticizers with different polarity were systematically studied. Results indicate that CO2 is an efficient plasticizer for as-spun nylon 66 fibers as shown by decreases in the draw stress. In contrast, CO2 shows only a slight influence on the drawability of highly oriented nylon 66 fiber. The effect of other plasticizers such as water, methanol, and ethanol on the drawability of nylon 66 fibers is very similar to that of CO2. Tenacity and modulus of one-stage drawn fibers were less than 0.8 and 5.0 GPa, respectively. Fibers with the highest tenacity and modulus, 0.96/5.04 and 1.06/5.04 GPa, were obtained by two-stage drawing in SCCO2 from as-spun and drawn nylon 66 fibers, respectively. The main reason for the extremely low draw ratios (<6.0) of nylon 66 fibers was the presence of hydrogen bonds in the crystalline phase. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 2282–2288, 2004

Evaluation of a molecular based yield model for glassy networks

Abstract: Results are presented from an experimental investigation to evaluate the validity of a new, molecular based yield model. The proposed model incorporates effects of test conditions such as stress state, strain rate and temperature, as well as the effects of molecular architecture. The effects of molecular architecture are quantified by parameters considering stiffness and cohesive strength of the network. Previously published data along with new results of yield in compression and plane strain are used to assess the validity of the proposed model.