Browsing by Subject "Glass transition"
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Item A thermodynamical framework for the solidification of molten polymers and its application to fiber extrusion(Texas A&M University, 2006-04-12) Kannan, KrishnaA thermodynamical framework is presented that describes the solidification of molten polymers to an amorphous as well as to a semicrystalline solid-like state. This framework fits into a general structure developed for materials undergoing a large class of entropy producing processes. The molten polymers are usually isotropic in nature and certain polymers crystallize, with the exception of largely atactic polymers, which solidify to an amorphous solid, to an anisotropic solid. The symmetry of the crystalline structures in the semicrystalline polymers is dependent upon the thermomechanical process to which the polymer is subjected to. The framework presented takes into account that the natural configurations associated with the polymer melt (associated with the breaking and reforming of the polymer network) and the solid evolve in addition to the evolving material symmetry associated with these natural configurations. The functional form of the various primitives such as how the material stores, dissipates energy and produces entropy are prescribed. Entropy may be produced by a variety of mechanisms such as conduction, dissipation, solidification, rearragement of crystalline structures due to annealing and so forth. The manner in which the natural configurations evolve is dictated by the maximization of the rate of dissipation. Similarly, the crystallization and glass transition kinetics may be obtained by maximization of their corresponding entropy productions. The restrictions placed by the second law of thermodynamics, frame indiference, material symmetry and incompressibility allows for a class of constitutive equations and the maximization of the rate of entropy production is invoked to select a constitutive equation from an allowable class of constitutive equations. Using such an unified thermodynamic approach, the popular crystallization equations such as Avrami equation and its various modifications such as Nakamura and Hillier and Price equations are obtained. The predictions of the model obtained using this framework are compared with the spinline data for amorphous and semicrystalline polymers.Item Evaluation of viscoelastic materials: The study of nanosphere embedment into polymer surfaces and rheology of simple glass formers using a compliant rheometer(Texas Tech University, 2008-08) Hutcheson, Stephen Anthony; McKenna, Gregory B.; Rasty, Jahan; Simon, Sindee L.; Weeks, Brandon L.Viscoelasticity is a fundamental property of many materials such as polymers, inorganic glasses, biological materials, small molecule glass formers, and composites. This fundamental property is what links the research presented here. There are two focuses that will be presented: 1. A background of nanoparticle is presented and a viscoelastic model is applied to determined the actual rheological behavior of materials. An atomic force microscope (AFM) is used to measure the embedment depth as nanoparticles are pulled into the surface by the thermodynamic work of adhesion. 2. Instrument compliance effects caused by both the transducer and entire instrument itself can induce large errors on shear measurements of viscoelastic properties of materials. Examples of instrument compliance effects on the measurement of the material properties of small molecular glass formers and a commercially available polydimethysiloxane (PDMS) rubber using a commercial rheometer are presented. A technique is presented and applied to correct for compliance effects in stress relaxation experiments and dynamic frequency sweep experiments. Recommendations are made for both experimental and instrument design to avoid and/or minimize compliance effects.Item Glass transition kinetics of amorphous polymers(Texas Tech University, 2007-12) Badrinarayanan, Prashanth; Simon, Sindee L.; McKenna, Gregory B.; Dai, Lenore L.; Quitevis, Edward L.The glass transition temperature (Tg), an important characterizing parameter for amorphous materials, is correctly measured only on cooling, whereas the limiting fictive temperature (Tf') is measured on heating. Both parameters depend on the rate of cooling. In this work a comparison of the values of Tg measured on cooling and Tf' measured on heating is performed for a polystyrene sample using both capillary dilatometry and DSC. The results indicate that Tg is systematically lower than Tf', presumably due to the breadth of relaxation on cooling. The Tool-Narayanaswamy-Moynihan (TNM) model is used to fit the experimental data in order to ascertain the origins of higher value of Tg compared to Tf'. The values of Tg and Tf' are used to examine the relationship between the timescales of volume and enthalpy relaxation. The analysis in this study suggests that both properties exhibit similar timescales at temperatures above and below the nominal Tg. The divergence of times required to reach equilibrium noted in the literature at temperatures several degrees below than nominal Tg is attributed to the effect of nonlinearity. Experimental results are presented that corroborate this hypothesis. The data from dilatometry and DSC measurements is also used to apply an isoconversion analysis to determine the variation of activation energy throughout the glass transition. Although isoconversion methods have been used in the literature to determine the variation of activation energy with conversion through the glass transition using DSC heating curves, the results in this work demonstrate that the method should be applied on cooling rather than heating. In addition, it is shown that the conversion dependence is simply due to the non-Arrhenius temperature dependence known to be exhibited by glass-forming materials near Tg. Finally, the ability of the TNM model to describe the enthalpy relaxation data obtained at low heating rates for polycarbonate blends is examined in order to test the sensitivity of the model parameters to changes in chemical composition.Item The glass transition and reaction kinetics under nanoconfinement(2010-12) Koh, Yung P; Simon, Sindee L.; McKenna, Gregory B.; Khare, Rajesh; Quitevis, Edward L.The glass transition temperature (Tg) of the bulk state has been extensively studied, however there is no adequate theory of the glass transition. Furthermore, under nanoconfinement, Tg decreases, increases, or remains the same compared with that of the bulk, with results depending on starting material, confinement medium, sample preparation methods, and measurement technique. An adequate explanation of the diverse results has not been developed yet. Hence, well-designed experimental approaches under nanoconfinement may not only enrich current experimental facts but also may help establish an inclusive understanding of Tg under nanoconfinement. Once Tg under nanoconfinement is elucidated, this solution may be applicable to solve the bulk Tg problem also. Reactivity under nanoconfinement also changes from the bulk reaction. Unlike Tg, reaction kinetics under nanoconfinement has not been well studied. Recently, the enhance reactivity under nanoconfinement was found and possible reasons were suggested to be incomplete conversion, side reaction, changes in reaction mechanism, reduced activation energy, and/or higher collision efficiency, but prior to the work done here, the origin of the changes was still unclear. A primary goal of this work is to study the Tg behavior under the nanoconfinement geometry of thin films and in nanopores. In addition, a second goal is to study the reaction kinetics under nanopore confinement and to investigate the origin of the enhanced reactivity.Item The glass transition in confined and heterogeneous systems(Texas Tech University, 2008-08) Zheng, Wei; Simon, Sindee L.; Quitevis, Edward L.; Weeks, Brandon L.; McKenna, Gregory B.The glass transition (Tg) is an interesting but challenging problem. Although this phenomenon has been studied for over half a century, Tg is still not well understood especially at the molecular level. One important feature which seems to be missing in the current understanding is its inherent heterogeneous dynamics. The aim of this work is to study the Tg behavior under various conditions such as for material freeze-dried from dilute solutions, confined in nanopores, and blended with other components, and to examine the relation between the dynamic heterogeneity and the observed Tg behavior. Freeze-dried materials from dilute solutions show different Tg behavior from the bulk; however, the origin of the difference remains unclear. In this work, the residual solvent effect on the calorimetric Tg of freeze-dried polystyrene is investigated. A linear correlation is found between the Tg depression and the residual solvent concentration, in agreement with data in the literature, indicating that the Tg depression observed for polymers freeze dried from dilute solution is due to residual solvent. Confinement at the nanoscale is also found to affect the glass transition behavior. Two hydrogen-bonded liquids, glycerol and propylene glychol, confined in silanized and unsilanized nanopores are studied to elucidate the confinement effects on Tg. Upon confinement, these two materials show similar behavior except that an additional Tg is observed for propylene glycol. We find that the confinement effects strongly depend on the competition between size effects and surface effects. Recently, a self-concentration model was proposed to predict the segmental dynamics of misicible polymer blends. To test this model, in this work, the dynamic properties of athermal blends of poly(รก-methyl styrene) with its oligomer is examined. The effective Tgs of the components determined from the calorimetric transition can be described by the self-concentration model. However, the self-concentration value obtained is much lower than the theoretical prediction, indicating weak chain connectivity effects in the athermal mixture. Moreover, compared to the pure materials, the blends exhibit considerably broadened transitions and depressed enthalpy overshoots, presumably resulting from their broader relaxation time distribution. In addition to studies dealing with confined or heterogeneous systems, calorimetric measurements of Tg and the limiting fictive temperature, Tf', as a function of cooling and heating rates are performed for a polystyrene to examine the relationship between the Tg and Tf' and to examine the conversion dependence of the apparent activation energy.