Free volume and free volume distribution have long been used to explain differences in the gas transport properties of polymeric materials. However, only a few experimental techniques allow a comprehensive evaluation of polymeric void space. Through the use of computer simulations, the free volume was characterized of two polyester systems used for beverage packaging and polynorbornene, a unique polymer with possible applications in both microelectronic fabrication and membrane separations. Delaunay Tessellation was used to calculate the fractional free volume (FFV) of both polyethylene terephthalate (PET) and polyethylene naphthalate (PEN) molecular models. It was hypothesized that differences in the FFV distributions could be used to explain the higher experimental O2 solubility in PEN relative to PET. The analysis showed that there was no statistical difference between the FFV distributions for O2 sized penetrants. Clustering analysis was performed based upon the tetrahedra formed by Delaunay Tessellation to examine the connectivity of free volume pockets. These results show that there is a statistically larger number of small (containing less than 10 tetrahedra/cluster and between 20-30 and #506;3 in volume) clusters in PEN. It is this difference in small clusters which provides for the 30% higher O2 solubility in PEN. The free volume of a representative high molecular weight amorphous model of Hexafluroalcohol Subsituted Polynorbornene (HFA-PNB) was also characterized in to examine the shape of the free volume cavities and to draw correlations with the mean lifetime of ortho-positronium (o-Ps) from Positron Annihilation Lifetime Spectroscopy (PALS). Delaunay Tessellation and clustering analysis indicated that the free volume clusters in high molecular weight HFA-PNB are slightly non-spherical. Correcting lifetimes for the somewhat non-spherical shape of these free volume clusters was insufficient to reproduce experimentally measured positron annihilation lifetimes because the clusters contained many tortuous connections within the clusters. Inclusion of this connectivity information does produce a more accurate estimate of the measured life times. This indicates that the o-Ps does sample many tetrahedra in these static clusters, but does not freely sample every section of these clusters.

No description available.

No description available.

Diffusion behavior of spin cast polymer thin films was studied in detail as a function of film thickness. Diffusion coefficients of water molecules in poly(methyl methacrylate) (PMMA) were found to decrease from 10-8 cm2/s in thick films to 10-13 cm2/s in ultra-thin films. In order to probe if there is a characteristic length scale set by the polymer chain size, the effect of PMMA molecular weights on this behavior was tested and deviation of diffusion coefficient from bulk was observed in all molecular weights of PMMA investigated. Diffusion coefficients in these films was also studied as a function of aging time at 25°C and was not found to change significantly over a time period of approximately four months. The impact of residual casting solvent in thick and thin films was studied and found to have no influence in the diffusion behavior. Positron Annihilation Lifetime Spectroscopy (PALs) was used to probe the free volume (FV) pocket size and its distribution within the film as a function of film thickness in PMMA. Decrease in FV pocket size was found to be one of the general underlying causes for such thickness dependent diffusion behavior observed in thin polymer films. In addition, Protracted Colored Noise Dynamics (PCND) that enables efficient sampling of phase space and faster relaxation of the systems compared to Molecular Dynamics (MD) was investigated for its extensibility to three dimensional systems and was found to be sensitive to initial conformation.

Within a polymer thin film, free-volume elements have a wide range of size and topology. This broad range of free-volume element sizes determines the ability for a polymer to perform molecular separations. Herein, the free volume and transport properties (diffusion, permeability, and selectivity) in both rubbery and glassy polymers were simulated using fully atomistic models. Extension of the computational tool to study the void structure in proteins is also included in this thesis. Six permeable thermally rearranged (TR) polymers and their precursors were studied. Using atomistic models, cavity size (free volume) distributions determined by a combination of molecular dynamics and Monte Carlo methods were consistent with experimental observation that TR polymers are more permeable than their precursors. The cavity size distributions determined by simulation were also consistent with free volume distributions determined by positron annihilation lifetime spectroscopy. The diffusion, solubility and permeation of gases in TR polymers and their precursors were also simulated at 308 K, with results that agree qualitatively with experimental data. A new hybrid Monte Carlo/Molecular Dynamics method is developed for estimating the slow diffusion processes of light gases transporting in glassy polymers. Diffusion coefficients, as small as 10⁻⁵ to 10⁻⁹ cm²/s are estimated for penetrants in four different polymers at 298 K. In all cases, agreement between literature experimental data and values obtained from the fast hybrid molecular dynamics method ranges from good to excellent. A new technique is developed using Monte Carlo methods to characterize the cavity size distribution and surface atoms in globular proteins. New statistical metrics have been defined for the structural characterization of globular proteins. Some of these metrics include volume, surface area, asymmetry ratio, interior cavity size distribution, and the identification of percolation channels. Wild-type (WT) myoglobin (Mb) and 5 Mb mutants have been studied in this research as examples. An analysis of cavity statistics provides an efficient method to quantify local properties such as packing density and transport pathways. The average cavity sizes of WT Mb and its mutants are around 4.0-5.0 Å. / text

The increasing use of polymer-based materials (PBMs) across all types of industry has not been matched by sufficient improvements in understanding of polymer tribology: friction, wear, and lubrication. Further, viscoelasticity of PBMs complicates characterization of their behavior. Using data from micro-scratch testing, it was determined that viscoelastic recovery (healing) in sliding wear is independent of the indenter force within a defined range of load values. Strain hardening in sliding wear was observed for all materials-including polymers and composites with a wide variety of chemical structures-with the exception of polystyrene (PS). The healing in sliding wear was connected to free volume in polymers by using pressure-volume-temperature (P-V-T) results and the Hartmann equation of state. A linear relationship was found for all polymers studied with again the exception of PS. The exceptional behavior of PS has been attributed qualitatively to brittleness. In pursuit of a precise description of such, a quantitative definition of brittleness has been defined in terms of the elongation at break and storage modulus-a combination of parameters derived from both static and dynamic mechanical testing. Furthermore, a relationship between sliding wear recovery and brittleness for all PBMs including PS is demonstrated. The definition of brittleness may be used as a design criterion in selecting PBMs for specific applications, while the connection to free volume improves also predictability of wear behavior.

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No description available.

The physical and biophysical properties of a biological membrane model, phosphatidylcholine bilayers, were investigated using novel spiropyran/merocyanine molecular probes. The femtosecond to second dynamics of this system's photochemistry enabled bilayer viscosity and free volume to be studied over a broad time scale. Spiropyrans/merocyanines with different polarity were synthesized by changing the substitution of the indole moiety enabling determination of the trans-membrane properties of the bilayer. In addition, transient grating spectroscopy was used to study thermal energy transfer in phospholipid bilayers on a picosecond time scale.Femtosecond transient absorption spectroscopy was used to study the photo-induced spiropyran ring-opening and isomerization reactions that produce the highly polar merocyanine species. The hindered rotation of the merocyanine bridge results in several metastable merocyanine isomers. The merocyanine ground state was determined to be populated predominantly by two isomers (TTC and TTT). Selective photoexcitation of these isomers results in excited state isomerization producing a third isomer (τ = 60 ps). Merocyanine thermal ring-closing was observed on a seconds time scale. Reaction kinetics, and solvatochromic and photochromic properties of merocyanines and spiropyrans were used to determine the bilayer physical properties. Bilayer viscosity was determined from merocyanine isomerization kinetics. Phospholipid bilayer free volume (the unoccupied volume enclosed in the bilayer) was determined from a modified Kramers' analysis. The greatest free volume was found in the extreme interior of the bilayer, while the head-group region exhibited the least free volume in qualitative agreement with molecular dynamics simulations of these bilayer systems. Free volumes determined via ps experiments were lower than those determined on a seconds time scale due to reduced acyl chain dynamics on the ps time scale.Femtosecond transient grating spectroscopy was used to study the rate of thermal energy transfer from photo-excited porphyrin molecules to the surrounding solvent. Thermal energy transfer was observed as photo-acoustic waves propelled through the system upon relaxation of photo-excited porphyrin molecules in aqueous solution and embedded in bilayers. For liposome solutions, a bimodal energy transfer model was developed. The determined rate constants suggest that energy transfer occurs predominantly via thermal diffusion and vibrational energy transfer, while lipid dynamics (isomerizations) are not involved.

Polímeros com grupos azobenzênicos exibindo fotoisomerização reversível trans-cis-trans tem sido usado para armazenamento óptico, chaves ópticas e produção de grades de relevo. A fotoisomerização produzida por luz linearmente polarizada cria uma orientação molecular perpendicular a polarização da luz (hole-burning angular) que induz a birrefringência na amostra. Neste trabalho, medimos a birrefringência foto induzida em filmes de poliestireno dopado com corante vermelho disperso 1 (DR1) em função da temperatura. A amplitude da birrefringência aumenta com a temperatura entre 20 e 180 K, atinge um máximo e diminui a zero próximo da temperatura de transição vítrea do polímero, a qual é da ordem de 373 K. A amplitude da birrefringência é proporcional a concentração de DR1 e também depende da história térmica da amostra. Propõe-se um modelo teórico que leva em conta a mudança de volume livre do polímero com a temperatura, representando um passo além dos modelos conhecidos. Assumimos que as moléculas de DR1 ocupam cavidades do polímero e a função distribuição Gama é usada para descrever a distribuição dos volumes das cavidades, enquanto a função Gaussiana descreve as flutuações térmicas de volume em torno de seu valor médio. A isomerização das moléculas de DR1 só podem ocorrer em cavidades com volume maior que um valor crítico. A comparação entre o modelo e os dados experimentais mostra uma razoável concordância. O modelo prevê corretamente a dependência da birrefringência com o tempo, com a temperatura e com a história térmica da amostra, já que o volume livre do polímero depende dessa história. / Polymers with azobenzene groups exhibiting reversible trans-cis-trans photoisomerization characteristics have been used for optical storage, optical switching and production of surface relief gratings. The photoisomerization produced by a linearly polarized light leads to a molecular orientation perpendicular to the light polarization (angular hole burning), which induces a birefringence on the sample. In this work we report on the photoisomerization of films of polystyrene (PS) doped with disperse red 1 (DR1), performed at various temperatures. The birefringence amplitude rises with temperature from 10 to 270 K, goes through a maximum and decays to zero near the polystyrene transition temperature, which is 370 K. The birefringence amplitude, at a given temperature, is proportional to the DR1 content and also depends on the sample thermal history. We proposed a model that accounts for the change in free volume of the polymer with temperature though representing an improvement to well-known models. We assume that the azobenzene group is inside a local free volume and the Gamma distribution function is used to describe a local free volume distribution in the sample while the Gaussian distribution function gives the thermal free-volume fluctuation. Isomerization of the azobenzene group only occurs if the local free volume is larger than a critical value. Comparison with the experimental data shows that the model explains the temporal evolution, the temperature dependence of the birefringence and how the birefringence is affected by the sample thermal history, as the sample free volume of the polymer depends on such history.