Browsing by Subject "Physical aging"
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Item Carbon dioxide plasticization and conditioning of thin glassy polymer films monitored by gas permeability and optical methods(2012-05) Horn, Norman Randall; Paul, Donald R.; Freeman, Benny; Biewlawski, Chris; Ellison, Christopher J.; Sanchez, IsaacThis research project investigated physical aging and carbon dioxide plasticization behavior of glassy polymer films. Recent studies have shown that thin glassy polymer films undergo physical aging more rapidly than thick films. This suggests that thickness may also play a role in the plasticization and conditioning responses of thin glassy films in the presence of highly-sorbing penetrants such as CO₂. The effect of film thickness on CO₂ permeation and sorption was studied extensively through carefully defined and controlled methods that provide a basis for future study of plasticization behavior. Thin films are found to be more sensitive than thick films to CO₂ exposure, undergoing more extensive and rapid plasticization at any pressure. The response of glassy polymers films to CO₂ is not only dependent on thickness, but also on aging time, CO₂ pressure, exposure time, and prior history. Thin films experiencing constant CO₂ exposure for longer periods of time exhibit an initial large increase in CO₂ permeability, which eventually reaches a maximum, followed by a significant decrease in permeability for the duration of the experiment. Thick films, in contrast, do not seem to exhibit this trend for the range of conditions explored. For a series of different polymers, the extent of plasticization response tracks with CO₂ solubility. There is little data available for gas sorption in thin glassy polymer films. In this work, a novel method involving spectroscopic ellipsometry is used to obtain simultaneously the film thickness and CO₂ sorption capacity for thin glassy polymer films. This allows a more comprehensive look at CO₂ permeability, sorption, and diffusivity as a function of both CO₂ pressure and exposure time. Like the gas permeation data, these experiments suggest that thin film sorption behavior is substantially different than that of thick film counterparts. Dynamic ellipsometry experiments show that refractive index minima, fractional free volume maxima, and CO₂ diffusivity maxima correlate well with observed CO₂ permeability maxima observed for thin Matrimid® films. These experiments demonstrate that plasticization and physical aging are competing processes. Aging, however, dominates over long time scales. Over time, CO₂ diffusivity is most affected by these competing effects, and the evolution of CO₂ diffusivity is shown to be the main contributing factor to changes in CO₂ permeability at constant pressure.Item Evaluation of transport and transport stability in glassy polymer membranes(2014-05) Czenkusch, Katrina Marie; Paul, Donald R.; Freeman, B. D. (Benny D.); Sanchez, Isaac C; Ellison, Christopher J; Li, WeiBoth novel membrane materials with better separation characteristics and a better fundamental understanding of membrane transport stability are needed to improve the competitiveness of commercial membrane separations. In this work, the effect of a novel moiety, hexafluoroalcohol, on the gas transport properties of an aromatic polyimide membrane are evaluated. The hexafluoroalcohol group increases the membrane’s fractional free volume, which increases the membrane’s permeability to all gases. Additionally, the HFA-containing polyimide shows resistance to plasticization by carbon dioxide. However, ideal selectivity for several gas pairs is unchanged by the inclusion of hexafluoroalcohol and the increase in the polymer’s fractional free volume. This lack of selectivity loss with increasing free volume is attributed to hydrogen bonding between the hexafluoroalcohol and imide groups, which reduces chain mobility. The ethanol dehydration characteristics of a so-called “TR” polymer are also evaluated in this work. TR polymers are heterocyclic, aromatic polymers synthesized by a solid-state, high temperature condensation from ortho-functional polyimides. Pervaporation studies on a representative TR polymer film demonstrate that the material has separation properties that exceed those of a commercial ethanol dehydration membrane. The transport properties of the TR film, combined with high thermal and chemical stability characteristic of these materials, make TR polymers promising materials for high-temperature, high-water content ethanol dehydration. Finally, the physical aging and plasticization of cellulose triacetate, the dominant natural gas purification membrane, is presented. Although this material has been used industrially for over 30 years, the physical aging and plasticization of the material, particularly in sub-micron films, has never been studied. Although cellulose triacetate does show physical aging behavior, as observed by permeability decreases over time, cellulose triacetate thin films do not show accelerated aging. Furthermore, the plasticization of thin cellulose triacetate films is reduced, rather than increased as seen in other polymers. The unusual transport stability of thin cellulose triacetate films may be due to their complex, semi-crystalline morphology, which, due to the thermal instability of the material, may not be thermally controlled.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 Physical aging of glassy polymers in confined environments(2012-12) Murphy, Thomas Matthew; Paul, Donald R.; Freeman, B. D. (Benny D.); Ellison, Christopher J; Vanden Bout, David A; Sanchez, Isaac CThis research project investigated the physical aging of glassy polymers in confined environments. Many recent studies of aging in glassy polymers have observed that aging behavior is often strongly affected by confinement. Understanding aging in confined environments (e.g., thin polymer films and nanocomposites) is vital for predicting long-term performance in applications that use confined glassy polymers, such as gas separation membranes and advanced nanocomposite materials. Aging in bulk and layered films produced via layer-multiplying co-extrusion was studied using gas permeability measurement and differential scanning calorimetry (DSC). The layered films consisted of polysulfone (PSF) and a rubbery co-layering material, with PSF layers ranging in thickness from ~185 nm to ~400 nm. Gas permeation aging studies at 35 °C revealed that the PSF layers in layered films aged in a manner that was similar to bulk PSF and independent of layer thickness. This finding differs from what was observed previously in freestanding PSF films, in which aging depended strongly on thickness and was accelerated relative to bulk. Isothermal aging studies at 170 °C and cooling rate studies were performed on both bulk and layered samples using DSC. The aging of the PSF layers was similar to aging in bulk PSF for films having PSF layer thicknesses of ~640 nm and ~260 nm, while the film with 185 nm PSF layers showed a slightly higher aging rate than that of bulk PSF. The results of the DSC studies generally support the conclusions of our gas permeation aging studies. The absence of strong thickness dependence in aging studies of layered films tends to support the idea that the effect of film thickness on physical aging stems from interfacial characteristics and not merely thickness per se. The physical aging of thin polystyrene (PS) films at 35 °C was also investigated using gas permeation techniques. PS films of 400 nm and 800 nm did not exhibit aging behavior that was highly accelerated relative to bulk or strongly dependent on film thickness. At the thicknesses and aging temperature considered, the aging of PS shows much weaker thickness dependence than that seen in polymers like PSF and Matrimid.Item Physical aging of thin and ultrathin glassy polymer films(2010-05) Rowe, Brandon William; Paul, Donald R.; Freeman, B. D. (Benny D.); Ganesan, Venkat; Eldridge, R B.; Kulkarni, SudhirThis research effort investigated the influence of confinement on the physical aging behavior of thin and ultrathin glassy polymer membranes. Membrane permeability changes with time due to physical aging, and for reasons not completely understood, the rate of permeability change can become orders of magnitude faster in films thinner than one micron. Special experimental techniques were developed to enable the study of free standing, ultrathin glassy polymer films using gas permeability measurements. The gas transport properties and physical aging behavior of free-standing glassy polysulfone (PSF) and Matrimid® films from 18-550 nm thick are presented. Physical aging persists in glassy films approaching the length scale of individual polymer coils. The membranes exhibited significant reductions in gas permeability and increases in selectivity with aging time. Additionally, the influence of physical aging on the free volume profile in thin PSF films was investigated using variable energy positron annihilation lifetimespectroscopy (PALS). The films exhibited decreasing o-Ps lifetime during physical aging, while o-Ps intensity remained constant. The o-Ps lifetime was reduced at lower implantation energies, indicating smaller free volume elements near the film surface. Thin films aged dramatically faster than bulk PSF and the PALS results agree favorably to behavior tracked by gas permeability measurements. The physical aging behavior of ultrathin films with different previous histories was also studied. The state of these materials was modulated by various conditioning treatments. Regardless of the previous history, the nature of the aging response was consistent with the aging behavior of an untreated film that was freshly quenched from above Tg, i.e., permeability decreased and pure gas selectivity increased with aging time. However, the extent of aging-induced changes in transport properties of these materials depended strongly on previous history. The properties of these ultrathin films deviate dramatically from bulk behavior, and the nature of these deviations is consistent with enhanced mobility and reduced Tg in ultrathin films, which allows them to reach a lower free volume state more quickly than bulk material. The Struik physical aging model was extended to account for the influence of film thickness on aging, and was shown to accurately describe the experimental data.Item Structural recovery and physical aging in polymer glasses in plasticizing environments(Texas Tech University, 2006-12) Banda, Lameck; McKenna, Gregory B.; Quitevis, Edward L.; Dai, Lenore L.; Simon, Sindee L.Small molecule plasticizers impart significant effects on the viscoelastic and mechanical responses of polymer glasses. The effects of plasticization on the structural recovery and mechanical responses of polymer glasses have been investigated using carbon dioxide as a small molecule plasticizer and an epoxy resin and polystyrene as model polymer glasses. This work reports the first physical aging results on a polymer glass subsequent to carbon dioxide pressure jumps. Also reported are the first structural recovery results of a polymer glass subsequent to small molecule plasticizer jumps. Consistent with the hypothesis that for a polymer glass, a change in plasticizer concentration is similar to a change in temperature, the three signatures of structural recovery; intrinsic isopiestics, asymmetry of approach, and the memory effect were constructed and shown to be qualitatively similar to those observed after temperature jumps. However, quantitative and anomalous differences were also observed. This work was modeled and illustrated the limitations of current phenomenological models. Subsequent work showed that, in fact, concentration glasses are fundamentally different from temperature-hyperquenched glasses. An attempt to investigate the real-time monitoring of mass change during the structural evolution of a polymer glass was made. The results report serious fundamental errors in the use of piezoelectric devices for the investigation of mass changes in compliant materials. Since these devices have been used as sensitive mass sensing devices, these results exposed their limitations. A semi-quantitative model of errors induced is also presented. The anomalous differences reported above led to the motivation to complete the thermodynamic surface of polymer glasses by studying the calorimetric response after carbon dioxide pressure jumps. To date there are no results reported in the literature on a direct measurement of the enthalpy recovery for glassy polymers subsequent to changes in temperature or small molecule concentration. This work reports the first measurements of the enthalpy recovery responses of polymer glasses subsequent to plasticizer concentration changes by measuring the heat flow under isothermal conditions as the structure evolves. The first intrinsic isopiestics, asymmetry of approach, and memory effects in enthalpy recovery after carbon dioxide jumps for polystyrene and the epoxy resin are reported.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.