Browsing by Subject "Glass transition temperature"
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Item ARCHITECTURE EFFECTS ON THE BULK AND SHEAR RHEOLOGY AND PVT BEHAVIOR OF POLYMERS(2011-08) Guo, Jiaxi; Simon, Sindee L.; McKenna, Gregory B.; Hedden, Ronald C.; Quitevis, Edward L.; Wang, ShirenThe viscoelastic bulk modulus [K(t)] plays an important role in residual stress development during polymer and composite processing and application, and in developing relationships among the four fundamental material functions, bulk modulus, shear modulus, Young’s modulus, and Poisson’s ratio. In addition, the origins of viscoelastic bulk and shear moduli are still unresolved. However, while the viscoelastic shear modulus has been widely studied, only a handful of investigations can be found in the literature concerning the viscoelastic bulk response. In order to investigate the viscoelastic bulk modulus, pressure relaxation responses were measured in a custom-built pressurizable dilatometer capable of making K(t) and pressure-volume-temperature (PVT) behavior measurements. The architectural effects on the bulk and shear relaxation responses of two polycyanurate networks have been studied and suggest that the shift factors used to construct the reduced curves are identical in the liquid states. Furthermore, comparisons of retardation time spectra indicates that bulk and shear responses have similar underlying molecular mechanisms at short times since the slopes are similar for the spectra; however, long-time mechanisms that are available to the shear are not available to the bulk. In addition, the architectures are found to have negligible effects on the bulk response; on the other hand, the relaxation/retardation time distributions for the shear are observed to increase with decreasing the crosslink density. The architecture effects were also studied on the bulk and shear responses for linear and star shape polystyrenes. The shift factors are also found to be identical for the bulk and shear responses of the two polymers in the liquid state; moreover, by comparing the bulk and shear retardation time spectra, shear deformations are found to have long-time mechanisms that are not available for the bulk. The pressure-volume-temperature (PVT) behavior of the thermosetting networks is studied to investigate the pressure-dependent glass transition temperature (Tg) and the architecture effects on the PVT behavior. The results show that although the Tg values are different, the two networks have similar values of dTg/dP. By comparing the PVT data calculated from Tait equation with best fits to the experimental data for the two networks, the most important variable governing the PVT behavior of the thermosetting materials is found to be the glass transition temperature, which strongly depends on crosslink density. Finally, the temperature- and pressure-dependent shift factors which are related to the relaxation times are reduced using a thermodynamic scaling, where Tau= ƒ(T^-1V^-gamma), and compared the results to the T – Tg scaling, where Tau = ƒ(T – Tg). The thermodynamic scaling law successfully reduces the data for all of the samples; however, polymers with similar structures, but with different Tg and PVT behavior, i.e., the two polycyanurates, cannot be superposed unless the scaling law is normalized by TgVg^gamma. On the other hand, the T – Tg scaling successfully reduced the polymers having similar microstructures.Item The dynamic mechanical response of polymer-based nanocomposites and network glasses(2004) Putz, Karl William; Green, Peter F. (Peter Fitzroy)The structural entities, atomic or molecular, that constitute a material and their spatial organization within the material largely determine the properties of the material. Different classes of materials respond to external forces often in ways that are fundamentally connected to their structure. Polymer melts exhibit time-dependent viscoelastic behavior in response to external stresses. If the deformation is rapid then the response of the polymer is largely elastic, whereas for sufficiently slow deformations the response is time-dependent and dissipative. In the solid state, an amorphous polymer responds to sufficiently small forces through local relaxations of segments of the molecules. Inorganic network glass melts typically exhibit viscoelastic behavior in the vicinity of the glass transition temperature, Tg, whereas below Tg ionic entities that compose the structure undergo hopping processes in response the external perturbations. This dissertation is largely devoted to understanding the response, or relaxations, of two different classes of materials: polymer based nanocomposites and inorganic network glasses, to external periodic mechanical perturbations. Polymer nanocomposites, materials in which a polymer serves as the host for nanomaterials, may possess properties that differ substantially from those of the pure polymer. Interactions between polymer segments and the foreign particles and the geometry, size, and dispersion of the foreign particles profoundly influence the properties of the composite. Small concentrations (~ few percent) of particles of nano-scale dimensions have been shown to be particularly effective at modifying the properties of the polymer because of the large interfacial area. In this dissertation the influence of C60 fullerenes and of single walled carbon nanotubes on the dynamical mechanical and rheological properties of PS and PMMA is examined. The second problem examined was the mechanical response of inorganic network glasses based on tellurium oxide to mechanical perturbations. Often alkali oxides are included in the glass composition because they improve processability of the glass melt by decreasing Tg. Through a series of dynamic mechanical and rheological experiments, it is shown that the relaxations which characterize the response of the glass increase by up to an order of magnitude when two distinct types of alkali ions are present compared to one.Item Effects of confinement on the glass transition of polymer-based systems(2004) Pham, Joseph Quan Anh; Green, Peter F. (Peter Fitzroy)Item Interfacial instabilities and the glass transition in polymer thin films(2006) Besancon, Brian Matthew; Green, Peter F. (Peter Fitzroy); Ganesan, VenkatItem Polymer behavior under the influence of interfacial interactions(2008-05) Kropka, Jamie Michael, 1976-; Green, Peter F. (Peter Fitzroy); Ganesan, VenkatThe properties of polymers, thin films or bulk, are profoundly influenced by interactions at interfaces with dissimilar materials. Thin, supported, polymer films are subject to interfacial instabilities, due largely to competing intermolecular forces, that cause them to rupture and dewet the substrate. The addition of nanoparticles (such as clay sheets, metallic or semiconductor nanocrystals, carbon nanotubes, etc.) to polymers can substantially affect bulk properties, such as the glass transition and viscosity, and influence the processability of the material. In this dissertation, we contribute to a fundamental understanding of the role of interfacial interactions on both the instabilities exhibited by polymer thin films and the properties displayed by polymer-nanoparticle mixtures. While conditions under which the destabilization of compositionally homogeneous thin films occurs are relatively well understood, the mechanisms of film stabilization in many two-component thin film systems are still unresolved. We demonstrate that the addition of a miscible component to an unstable film can provide an effective means of stabilization. The details of the stabilization mechanism are understood in terms of the compositional dependence of both the macroscopic wetting parameters and the effective interface potential for the system. We find that the suppression of dewetting in the system is not an equilibrium stabilization process and propose a mechanism by which the increased resistance to dewetting may occur. There is also significant interest in understanding the extraordinary property enhancement of polymers that are enabled by the addition of only small concentrations of nanoparticles. If these effects could be distilled down to a few simple rules, they could be exploited in the design of materials for specific applications. In this work, the influence of C60 nanoparticles on the bulk dynamical properties of three polymers is examined. Based on the findings from a range of measurement techniques, including differential scanning calorimetry, dynamic mechanical analysis, dynamic rheology and neutron scattering, we propose that the changes in the glass transition temperature for the polymer-C₆₀ mixtures can be understood in terms of a percolation interpretation of the glass transition. The proposed mechanism is also characterized computationally.Item Single molecule studies of heterogeneous dynamics near the glass transition(2002-05) Deschenes, Laura Alice; Vanden Bout, David A.Item The Effect of Moisture Absorption on the Physical Properties of Polyurethane Shape Memory Polymer Foams(2012-07-16) Yu, Ya-JenThe effect of moisture absorption on the glass transition temperature (Tg) and stress/strain behavior of network polyurethane shape memory polymer (SMP) foams has been investigated. With our ultimate goal of engineering polyurethane SMP foams for use in blood contacting environments, we have investigated the effects of moisture exposure on the physical properties of polyurethane foams. To our best knowledge, this study is the first to investigate the effects of moisture absorption at varying humidity levels (non-immersion and immersion) on the physical properties of polyurethane SMP foams. The SMP foams were exposed to differing humidity levels for varying lengths of time, and they exhibited a maximum water uptake of 8.0 percent (by mass) after exposure to 100 percent relative humidity for 96 h. Differential scanning calorimetry results demonstrated that water absorption significantly decreased the Tg of the foam, with a maximum water uptake shifting the Tg from 67 ?C to 5 ?C. Samples that were immersed in water for 96 h and immediately subjected to tensile testing exhibited 100 percent increases in failure strains and 500 percent decreases in failure stresses; however, in all cases of time and humidity exposure, the plasticization effect was reversible upon placing moisture-saturated samples in 40 percent humidity environments for 24 h.Item The viscoelastic properties of thin films(2013-05) Wang, Jin Hua; McKenna, Gregory B.; Vaughn, Mark W.; Weeks, Brandon L.; Quitevis, Edward L.There is a considerable interest in studying material properties at micro-, nanometer size scale. Thin films exhibit significant different behaviors from that of bulk systems. Both liquid dewetting technique and particle embedment method have been used to investigate viscoelastic properties of thin films. The liquid dewetting technique which uses liquid as substrate is a direct mechanical method to study the viscoelastic properties of polymer (polystyrene (PS) and polycarbonate (PC)) thin films. Results show that the glass transition temperature (Tg) dependence for PS and PC thin films on liquid surface is more like the supported film rather than the freely standing film. Both Tg depression and film stiffening were observed when PS film thickness is less than 20 nm. No molecular weight effect was found for the dewetting behaviors of the PS films prior to the terminal flow regime, where the majority of the current work was carried out. For PC thin films, similar Tg was observed when film thickness is less than 35 nm. As the film thickness increases during the dewetting experiment, the present results show that the film dewetting process for the polymer/glycerol couple has to be described as a “non-isothermal” experiment. A numerical method is proposed to correct the experimental data to the “isothermal” and constant thickness condition. A novel temperature-step method has been described to direct obtain the Tg of thin films from the liquid dewetting measurement based on the liquid dewetting experiment is a “non-isothermal” experiment. Different from all of the previous methods in literature used to obtain the glass transition temperature of thin films, we can get polymer thin film glass transition temperature-thickness dependence only from one sample within a single experiment by change operating temperature steps. Two liquids (glycerol and 1-Butyl-3-methylimidazolium trifluoromethanesulfonate) have been chosen to study the liquid substrate effects on polymer thin film dewetting behaviors by this temperature-step mehod. It was found that PS thin film dewet faster on glycerol than that on ionic liquid and its Tg has a stronger film thickness dependence compared to PS thin film on ionic liquid. A sub-micron particle embedment technique has been used to determine the shear modulus of 1, 2-dipalmitoyl-Sn-glycero-3-phosphotidylcholine (DPPC) lipid multibilayers by two different particles (PS particle and silica particle) from atomic force microscope (AFM). Methods used to determine particle size has been discussed and results show that AFM might be the best way to obtain particle size in the present work. The standard JKR model was used to relate the shear modulus of the lipid multibilayer films with the particle embedment depths. The so-determined modulus of the DPPC is consistent with the reported literature values for DPPC, while silica particles give more reliable results compare to polystyrene particles which has a small amount of surfactant in the sample.Item Volume recovery of an epoxy material: resolution of the tau effective paradox(Texas Tech University, 2003-12) Kolla, SrinivasNot available