Reorientation Of Side Groups Research Articles

Polymorphism of Paracetamol: A New Understanding of Molecular Flexibility through Local Methyl Dynamics

This study focuses on the interplay of molecular flexibility and hydrogen bonding manifested in the monoclinic (form I) and orthorhombic (form II) polymorphs of paracetamol. By means of incoherent inelastic neutron scattering and density functional theory calculations, the relaxation processes related to the methyl side-group reorientation were analyzed in detail. Our computational study demonstrates the importance of considering quantum effects to explain how methyl reorientations and subtle conformational changes of the molecule are intertwined. Indeed, by analyzing the quasi elastic signal of the neutron data, we were able to show a unique and complex motional flexibility in form II, reflected by a coupling between the methyl and the phenyl reorientation. This is associated with a higher energy barrier of the methyl rotation and a lower Gibbs free energy when compared to form I. We put forward the idea that correlating solubility and molecular flexibility, through the relation between pKa and methyl rotation activation energy, might bring new insights to understanding and predicting drug bioavailability.

Study on O-Terphenyl/Polystyrene Blends: How Plasticizer Affects the Glass Transition of Polystyrene?

The effect of plasticizer o-terphenyl on the glass transition of polystyrene was investigated by Fourier transform infrared spectroscopy from a new aspect. The peak areas of four conformation insensitive bands as a function of temperature were studied, these being assigned to the vibrational modes of main chain groups and side groups of polystyrene. It was shown that the reorientation relaxation temperature of the main chain around glass transition was lower than that of the side groups when polystyrene was plasticized by o-terphenyl. It was explained on the basis of cohesional entanglements of polystyrene chains. The reorientation relaxation region of the side groups was nearly the same as the macroscopically observable glass transition region of polystyrene, implying that the glass transition process of polystyrene was dominated by the reorientation of side groups.

Fabrication of super-hydrophobic film from PMMA with intrinsic water contact angle below 90°

Super-hydrophobic film is commonly prepared from hydrophobic materials. Poly(methyl methacrylate) (PMMA) is considered as a hydrophilic polymer (intrinsic contact angle below 90°) with the water contact angle (CA) of ∼68°. However, a super-hydrophobic PMMA film with CA of ∼154° was obtained by treating polystyrene (PS) and PMMA blended film in a warm selective solvent, cyclohexane. The surface was characterized by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and infrared spectroscopy (FT-IR). The results show that the conversion from the hydrophilic surface to a super-hydrophobic one is due to the cooperation of the micro-, nano-structure and the side group reorientation in the PMMA chains at the topmost.

Solid-state exchange NMR characterization of segmental dynamics in glassy poly(alkyl methacrylate)s

Slow side-group and main-chain β-relaxation dynamics in a series of glassy poly(alkyl methacrylate)s (PRMAs) with varying side-group sizes were studied by one-dimensional (1-D) centerband-only detection of exchange (CODEX) and static pure-exchange (PUREX) nuclear magnetic resonance (NMR) techniques. The13COO signal in 1-D PUREX experiments was analyzed to characterize the flips and small-angle motions of the side groups quantitatively. By comparing the13COO exchange intensities obtained from 1-D PUREX and CODEX experiments, it was possible to show that the small-angle reorientations vary between backbone segments. Analysis of the backbone13CH2 CODEX data supplied direct and quantitative information about the amplitudes of the main-chain motion, which is also reflected in the side-group reorientation due to the coupling between side group and backbone. The data obtained from both NMR exchange techniques were correlated, providing an accurate, consistent description of the backbone and side-group motion. The effects of side-group size and temperature on the fraction of flipping side groups were studied systematically by 1-D PUREX NMR. The flipping fraction at ambient temperature decreases with increasing the side-group size. It increases gradually with temperature for two PRMAs with conformationally flexible side groups, while it is constant over wide temperature ranges for poly(methyl methacrylate) and poly(cyclohexyl methacrylate).

Molecular interpretation of the main relaxations found in dielectric spectra of cellulose – experimental arguments

ID dielectric spectra of cellulose two relaxation processes dominate ID the low temperature range (−120 °C–0 °C). For application of the dielectric method as a diagnostic tool, these relaxations must be assigned to molecular motions of the polymeric system. This paper summarizes and discusses all experimental facts found by dielectric spectroscopy, which can help to solve this interpretation problem. ID ID necessary to include two other relaxation processes ID this discussion: the βwet- relaxation found ID all wet materials and the γ-relaxation evidently found ID all derivatives. The main result of our investigations ID that the dominant process ID the dielectric spectra at low temperatures (called β-relaxation) ID the segmental motion of the chain and not methylol side-group reorientation. Additionally, for the first time, the dielectric spectra of Valonia (and also of bacterial cellulose) clearly indicate that the methylol side-group relaxation ID represented ID the dielectric spectra of pure polysaccharides. This γ-process ID only masked by the β-process for the most pure celluloses. For low molecular weight saccharides and derivatives of cellulose both processes are simultaneously found ID the dielectric spectra. For the first time a correlation ID presented between the intensity of the local motion mode (β-relaxation) and the degree of crystallinity for various cellulosic materials.

The side-group reorientations of polypropylene and poly(methyl acrylate) during conformational state transition

Conformational contour maps of polypropylene and poly(methyl acrylate) have been examined by using full relaxation minimization under MM2 force field. Low energy paths along the potential surface were found for the meso and the racemic dyads. The side-group reorientation along the low energy pathways for both polymers was obtained. A 20° rotation was found for each methyl group of polypropylene. A large cooperative rotation, ∼130°, is needed for the two ester groups of poly(methyl acrylate) to climb the energy barrier.

Subglass relaxations. Intermolecular packing and the relaxation times for ester side-group reorientation: a molecular dynamics simulation

A model for the experimentally studied methyl acrylate(MA)-ethylene copolymer was considered. Free energy barriers for the rotation of a MA side group in a polyethylene matrix were generated for a number of molecular dynamics(MD)-generated glass structures. The distribution of rotational energy barriers determined from the MD study is very broad and resembles closely the experimental distribution. The variations of barrier height are found to correlate with close range packing effects in the glass.

Relaxation in polymers

Various molecular motions known to occur in high polymers are described and illustrated in connection with new magnetic resonance relaxation results. The motions include side-group reorientations, main-chain reorientation and translation and glass-transition phenomena. Classical hindered rotation and tunnelling are contrasted in connection with methyl group reorientation in polyvinyl acetate. A number of other methyl rotation situations are described. Polar side-group reorientation is discussed and the site-model parameters are evaluated for the well-characterized ester-group motion in polymethyl methacrylate. Main-chain reorientations are illustrated for several systems. In some cases all chains rotate and in others only a fraction of the chains are sufficiently free. This motion can become apparent abruptly at a first-order transition, as in linear polytetrafluoroethylene and polybutene-1, or gradually increase in frequency as the temperature is raised, as in hexafluoropropylene-tetrafluoroethylene copolymers and polychlorotrifluoroethylene. Translations parallel to the main chain axes are indicated for some fluorinated polymers. Fibre studies for tetrafluoroethylene polymers reveal these two types of main chain motions clearly. Glass-transition phenomena are described in connection with measurements on the extensively-studied orthoterphenyl. It is proposed that the rival configurational entropy and free-volume theories are about equally empirical.