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  • Author or Editor: J. Seferis x
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Abstract  

Until recently, the issue of the thermal gradients within TMDSC samples remained mostly a subject of theory and mathematical models — only the phase lag was subject to experimental verification, as this information is readily available from the analysis software of most instruments. There was no method to verify the transient behaviour and temperature gradients within a sample without making costly and intensive modifications to the equipment. Recently, however, a group of researchers were able to experimentally measure thermal profiles as a function of sample thickness with a high-speed, high-resolution infrared camera mounted on the TMDSC cell. Therefore, this paper is dedicated to comparing the predictions of our three-dimensional model with this newly available data.

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Abstract  

This investigation demonstrates that polyphenylene sulfide (PPS) crystallizes in a unique dual-mechanistic fashion in which 8% of the material by volume crystallizes instantaneously, while the remaining material crystallizes in a time dependent fashion. These rapid melt-crystallization kinetics are quantitatively modeled using a dual-mechanistic model approach which is based on the methodology first observed by Velisaris and Seferis in polyetheretherketone (PEEK) crystallization. The crystallization model is then used to accurately predict both isothermal and for the first time non-isothermal crystallization behavior over a wide spectrum of cooling rates utilizing the same model parameters. Specifically, this work identifies and models the initial fast crystallization kinetics of PPS. Additionally, the versatility of the Velisaris and Seferis dual-mechanistic model has been established with PPS, by simply showing that it is a special case of the generalized dual-crystallization kinetics methodology.

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Abstract  

Glass transitions of amorphous polystyrenes with low polydispersity were evaluated using the modulated Local Thermal Analysis mode of the TA Instruments 2990 TA and evaluating the thermomechanical signal. Transition temperature variance and fraction of transitions measured were compared for high molecular mass thermosetting materials and the melt of Nylon 6.6. The transition reproducibility was found to decrease as the molecular size of the polymer samples increased. Reproducibility also decreased for thermosetting materials when the experimental ramp rate was decreased. Heat transfer within the specimen was evaluated using finite element analysis, allowing scaling of microscale experimental results for comparison to bulk transitions.

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Abstract  

Three types of commercially available organophilic Montmorillonite (Cloisite 30B, 25A and 15A) were used to prepare VARTM epoxy resin nanocomposites in order to study the effect of the nanoclay organophilic modification on the epoxy matrix. The morphology of the dispersions was investigated through XRD and TEM analyses. The thermal stability of the nanocomposites was studied by means of HI-RES TG measurements and the influence of the nanoclay on the viscosity of the resin was investigated through rheological measurements. It was found that the nanoclay modification had no significant influence on the dispersion and on the thermal properties of the nanocomposites. Areas of exfoliated and intercalated morphology were observed. The viscosity of the resin furthermore did not exceed the critical value of the infusion process.

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Five epoxy resins of different chemistry and functionality were cured with DDS (4,4′-diaminodiphenyl sulfone) using 2, 8 and 14 h curecycles. Both Differential Scanning Calorimetry (DSC) and Thermomechanical Analysis (TMA) were used to characterize reaction behavior and cured properties of the resin systems. In addition, static mechanical tests and density measurements were integrated with the thermal characterization methods to correlate resin properties with process time. Flexural three-point bending experiments showed that the resins tended to have higher yield stress and toughness values at extended cure times. The improved mechanical properties could be attributed to the full development of the epoxy molecular structure, in the form of cross-linked networks and molecular rearrangement. These results suggest that extended cure times or high temperature post-curing may be required to obtain the resin's ultimate mechanical properties for high performance composites.

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The glass transition temperature,T g is a sensitive and practical parameter for following cure of reactive thermosetting systems. A new equation was developed for predicting theT g-conversion relationship based on the Dillman-Seferis viscoelastic compliance model. It assumes that the changes inT g are primarily due to changes in relaxation time as chain extension and crosslinking reduce the mobility of a polymer network. Such information is essential in combining kinetic and viscoelastic measurements, which monitor transformations of thermosets during cure. The equation derived from the viscoelastic model was shown to be applicable for a variety of experimental data. The success of the methodology was further demonstrated by comparing well-established relations, such as the Fox equation and the Di-Benedetto equation, to predictions made possible by adjusting two viscoelastic model parameters. Finally, the fitting power of the proposed equation was shown by fitting published epoxy data from the literature as well as experimental data on a relatively new resin system such as dicyanates used as a model in this study.

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