Deuterium NMR Studies of Segmental Dynamics of Anopore-Adsorbed Poly(Methyl Acrylate)

Studies of segmental dynamics of polymers at interfaces provide a basis for understanding the properties of composite materials. Interfacial phenomena in multi-phase systems need to be understood as devices made from multi-phase polymeric systems become smaller. Various techniques have been used to investigate interfacial polymers at the air-polymer-solid interface; for example, ellipsometry,1 X-ray,2 and neutron reflectometry.3 These techniques typically characterize the structure (often the thickness) of the polymer layer and are normally used to determine the glass-transition temperature (Tg) through a break in the thicknesstemperature curve. There have been a few reports where the dynamics of polymers at the surface-air interface has been probed directly or inferred by NMR4 or ESR5. Using deuterium quadrupolar echo techniques on poly(vinyl acetate)-d3, Blum et al.6 showed that a deuterated methyl group could be used to probe the dynamics of adsorbed polymers. Lin and Blum7,8 observed that molecular mass affected segmental dynamics in both bulk and silica-adsorbed poly(methyl acrylate) (PMA-d3). As temperature increased, both high and low molecular mass samples showed increased motional rates, with significantly faster motional rates in the high molecular mass samples. They also reported that lower adsorbed amounts exhibited relatively slower motional rates than higher adsorbed amounts did. Anopore is an inorganic membrane that is produced by anodic oxidation of aluminum.9 Since the process is electrochemical, the conditions can be controlled and reproducible pore structures with narrow pore-size distributions can be obtained. They are available in 4.7 cm discs, 60 um thick, with 0.2 or 0.02 um diameter pore sizes. The anopore membranes are usually used for filtration purposes. Anopore has been used as a confining substrate in studies of polymer adsorption.10-12 However, not much is known about the adsorption mechanism or the state of the polymer in them. In this paper, we examine the adsorption of PMA-d3 on anopore by 2H NMR. We look at the process of adsorption, the effect of adsorbed amounts, and the effect of pore size. The results show little difference in the effect of pore size, or adsorbed amounts on the mobilities of PMA-d3 segments

Dynamics of Adsorbed PMA-d₃ - Effect of Substrate

In the last few years, our group has focused much of our attention on studying the dynamics of polymers adsorbed at interfaces. Much of our work, to date has been on labeled poly(vinyl acetate)-d3 (PVAc-d3)1 and poly(methyl acrylate)-d3 (PMA-d3)2 on silica. We have been able to probe the effects of adsorbed amount,3 molecular mass,4,5 and the effect of overlayer.6 These studies have provided a view of the adsorbed polymer consistent with a motional gradient in the layer with the more mobile segments being those at the air-polymer interface and the less-mobile segments at the substratepolymer interface. However, we have not probed the effect of the interaction with the substrate. In the present work, we describe the dynamics of PMA-d3 adsorbed on different substrates with a focus on how the substrate affects the dynamics of the polymer. In particular, we examine silica- and alumina-based substrates. For silica we explored the behavior of PMA-d3 on Cab-O-Sil silica, both in its native and hydrophobic form. For alumina we have probed the behavior on both alumina powder and also anopore membranes. We find that the dynamics of the adsorbed polymer depends on the nature of the substrate

Deuterium NMR Studies of Bulk and Silica-Absorbed Polystyrene

Thin polymer films are used in applications such as electronics, biomedical devices and coatings. As the trend to make the devices smaller continues, a film\u27s influence on a material\u27s overall properties becomes more prominent. Previous studies have shown that thin polymer films have characteristics slightly different from those of bulk polymers. Film behavior has been characterized by examining thermal expansion, diffusion, conformation, density and glass transitions. The dynamics of glassy polystyrene has been investigated using a wide variety of theoretical and experimental methods. Schaefer et al., using 13C NMR, reported that ~7% of phenyl rings in polystyrene undergo -flips. Using deuterium NMR on ring-deuterated polystyrene, E. Rossler, et al. determined that a powder pattern of a rigid solid is obtained at room temperature. At 383 K, two low intensity peaks are present in the middle of the powder pattern and, at 433 K, a Lorentzian line was found. While the broad powder pattern was characteristic of deuterated polystyrene with no chain motion, the two low intensity middle peaks were attributed to the phenyl rings undergoing -flips. Kulik and Prins also showed that the deuterium powder pattern for polystyrene contained two weak middle peaks.4 Upon altering the relaxation delay, Kulik et al. determined that, for a partially relaxed sample (short relaxation delay), the two middle peaks were dominant over the \u27outer horns\u27 but, when given enough time for complete relaxation, the converse is true. They concluded that the line shapes are superpositions of roughly two contributions from phenyl rings performing relatively slow and fast flipping motions. Zhao et al., using deuterium NMR on ring-deuterated polystyrene, found a strong dependence of the line shape on temperature and echo delay, indicating motion at the microsecond time scale at the temperatures studied. In this study, we investigate the dynamics of polystyrene in bulk and silica-adsorbed samples as temperatures approach the glass transition temperature (Tg)

Molecular Dynamics of Polystyrene Solutions in Microwave Fields

Equilibrium and nonequilibrium molecular dynamics simulation techniques were used to assess the influence of an applied microwave field on the dynamics of methylamine-methanol and methylamine-dimethylformamide (DMF) solutions bound within atactic polystyrene over a range of polymer densities from 35 to 96 wt % polymer. Atomistically detailed systems were studied, ranging from 3000 to 10 644 particles, using previously established potential models. Structural and dynamical properties were determined in the canonical (NVT) ensemble at 298 K. The simulated DMF self-diffusion coefficients in polystyrene solutions were compared with the zero-field experimental results established with pulsed-gradient spin-echo NMR spectrometry. A simulated external microwave field, with a rms electric field intensity of 0.1 V/Å, was applied to these systems and the simulated dynamical results over field frequencies up to 104 GHz were compared with the zero-field values. Simulated evidence of athermal effects on the diffusive characteristics of these mixtures is reported

A Comparative Study of the Properties of Polar and Nonpolar Solvent/Solute/Polystyrene Solutions in Microwave Fields Via Molecular Dynamics

The influence of an applied microwave field on the dynamics of methylamine-dichloromethane (DCM) mixtures bound within atactic polystyrene (a-PS) over a range of polymer densities from 30 to 94 wt % polymer was examined using atomistic molecular dynamics simulations. This study is an extension of previous studies on methylamine transport in relatively polar polystyrene solutions of methanol and dimethylformamide [M. J. Purdue et al., J. Chem. Phys. 124, 204904 (2006)]. A direct comparison is made across the three types of polystyrene solutions. Consideration is given to both solvent and reagent transport within the polymer solutions under zero-field conditions and in an external electromagnetic field in the canonical ensemble (NVT) at 298.0 K. Various frequencies up to 104 GHz and a rms electric field intensity of 0.1 V/Å were applied. The simulation studies were validated by comparison of the simulated zero-field self-diffusion coefficients of DCM in a-PS with those obtained using pulsed-gradient spin-echo NMR spectrometry. Athermal effects of microwave fields on solute transport behavior within polymer solutions are discussed

Polymer dynamics in bulk, adsorbed and solution by NMR methods

This work consists of two topics. The first part deals with deuterium nuclear magnetic resonance (²H NMR) study of bulk and adsorbed polymers. Segmental dynamics of poly(methyl acrylate) is examined in bulk and when adsorbed to anopore, alumina and silica as a function of temperature. The second part investigates diffusion of small molecules in polymer solutions by pulsed field-gradient proton nuclear magnetic resonance (PGSE ¹H NMR). The diffusion of solvents in polymer solutions is investigated. The diffusion of N-N, dimethylformamide in polystyrene is studied as a function of concentration and temperature. The diffusion coefficients of solvents in binary solvent mixtures of methylene chloride/toluene and chloroform/acetonitrile in poly(methyl methacrylate) solutions as functions of concentration are also reported --Introduction, page 1

Developing a Color-Based Molecular Sensing Device: DETECHIP®

DETECHIP® is a detection system made of various sensors that has been shown to detect and discriminate between small molecules of interest, including various illicit drugs for and over- the-counter medications, explosives, pesticides and food spoilage metabolites. DETECHIP® employs an array of sensors from which an analyte is tested and based on changes in color; a code specific to the analyte is developed. DETECHIP® offers possibilities for a simple, sensitive, selective, and affordable alternative to costly immunoassays. Results from NMR, UV-VIS and fluorimetry studies suggest that the color and fluorescence changes are a result of intermolecular interaction between the analytes and sensors ranging from non-covalent covalent bonding of supramolecular structure to proton exchange between the analyte and sensor molecules. Current efforts are focused on miniaturization of DETECHIP® to the micro and nanoscale

Segmental Dynamics of Poly(methyl Acrylate)-d₃ Adsorbed on Anopore: A Deuterium NMR Study

The segmental dynamics of poly(methyl acrylate-d3) (PMA-d3) adsorbed in the pores of anopore membranes has been investigated using deuterium NMR over the temperature range 25-80 C. The onset of the NMR glass-transition temperature (Tg) for the adsorbed samples was approximately 15 C higher than that for the bulk sample. The adsorbed polymer contained segments with restricted mobility (glassy), even at the highest temperatures studied, at which the bulk polymer showed only mobile segments. The spectra from samples with different adsorbed amounts of PMA-d3, between 1.1 and 4.2 mg/m2, were similar in their temperature-dependent mobilities. Neither was there much difference in the spectra of PMA-d3 on anopore samples with pore sizes of 0.2 and 0.02 m. However, for a solvent-washed sample with an adsorbed amount of 0.7 mg/m2, additional restriction in PMA-d3 mobility was observed

Applied Circular Dichroism: A Facile Spectroscopic Tool for Configurational Assignment and Determination of Enantiopurity

In order to determine if electronic circular dichroism (ECD) is a good tool for the qualitative evaluation of absolute configuration and enantiopurity in the absence of chiral high performance liquid chromatography (HPLC), ECD studies were performed on several prescriptions and over-the-counter drugs. Cotton effects (CE) were observed for both S and R isomers between 200 and 300 nm. For the drugs examined in this study, the S isomers showed a negative CE, while the R isomers displayed a positive CE. The ECD spectra of both enantiomers were nearly mirror images, with the amplitude proportional to the enantiopurity. Plotting the differential extinction coefficient (Δε) versus enantiopurity at the wavelength of maximum amplitude yielded linear standard curves with coefficients of determination (R2) greater than 97% for both isomers in all cases. As expected, Equate, Advil, and Motrin, each containing a racemic mixture of ibuprofen, yielded no chiroptical signal. ECD spectra of Suphedrine and Sudafed revealed that each of them is rich in 1S,2S-pseudoephedrine, while the analysis of Equate vapor inhaler is rich in R-methamphetamine