Molecular Motions In Poly Research Articles

Molecular motion in poly(vinyl acetate) and poly(methacrylic acid)

NMR relaxation and IR measurements have been performed on poly(vinyl acetate) and poly(methacryl acid). The measurements are used to deduce the motions in polymers.

Molecular motions in poly(dimethyl siloxane) oligomers and polymers

The glass transition temperatures of fifteen samples of poly(dimethyl siloxane) polymers and oligomers have been measured by differential scanning calorimetry. Polymers with M n >2400 exhibit an asymptotic value of T g (∞)=148K, but for shorter chains the T g is lower. The thermomechanical behaviour has also been examined using a torsional braid analyser and this has served to confirm the chain length dependence of T g . A sub-glass transition, located in the temperature range 80–120K for short chain samples, has been attributed to methyl group rotation.

Effects of drawing on molecular motions in poly(ethylene terephthalate)

Effects of drawing on molecular motions in amorphous poly(ethylene terephthalate) (PET) were studied by the dielectric relaxation measurements. The rate of the segmental motion increases by small drawing and turns to decrease with draw ratio in large draw ratios. The local mode relaxation time in glassy PET was found to vary with draw ratio in a more complex manner than the segmental mobility does. These variations of molecular motions are explained in terms of the free volume in PET. A structural model for the drawn PET is postulated from the evidences on the variations of the density, the birefringence and the dielectric properties with the draw ratio. That is, the small drawing promotes the molecular orientation more or less but produces unstable chain conformations accompanied with a local volume increase, while the high drawing enhances the molecular aligment.

Dielectric relaxations and molecular motions in poly(vinylidene fluoride) with crystal form II

AbstractMolecular motions in poly(vinylidene fluoride) were studied by dielectric measurements. Of the three relaxation processes, the αa and the αc were studied. The αa relaxation was attributed to the molecular motions in the amorphous regions and the αc relaxation to molecular motions in the crystalline regions and their surfaces. The relaxation time and the magnitude of the αa absorption were analyzed on the basis of the Adam‐Gibbs theory of the temperature dependence of the size of the cooperatively rearranging region. As a result it was concluded that the molecular structure of crystal form II holds locally, even in the amorphous regions. The relaxation time and the magnitude for the αc absorption were analysed on the basis of the two‐site model. It was concluded that the αc relaxation is attributable not only to molecular motions in the folds of the lamellar crystals, but also to those in the interior of the crystal; the folded chain is relatively mobile, while the molecular chain in the interior of the crystal executes restricted rotation around the chain axis.

Molecular motions in poly(2,6‐diphenyl‐p‐phenylene oxide)

Molecular motions in poly(2,6‐diphenyl‐<i>p</i>‐phenylene oxide)

Dielectric relaxation and molecular motion in poly(vinylidene fluoride)

AbstractThe dielectric properties of poly(vinylidene fluoride) have been studied in the frequency range 10 Hz to 100 kHz at temperatures between −196 and 150°C. Three dielectric relaxations were observed: the α relaxation occurred near 130°C, the β near 0°C, and the γ near −30°C at 100 kHz. In the α relaxation the magnitude of loss peak and the relaxation times increased not only with increasing lamellar thickness, but also with decrease of crystal defects in the crystalline regions. In the light of the above results, the α relaxation was attributed to the molecular motion in the crystalline regions which was related to the lamellar thickness and crystal defects in the crystalline phase. In the β relaxation, the magnitude of the loss peak increased with the amount of amorphous material. The relaxation times were independent of the crystal structure and the degree of crystallinity, but increased slightly with orientation of the molecular chains by drawing. The β relaxation was ascribed to the micro‐Brownian motions of main chains in the amorphous regions. The Arrhenius plots were of the so‐called WLF type, and the “freezing point” of the molecular motion was about −80°C. The Cole‐Cole distribution parameter of the relaxation time α increased almost linearly with decreasing temperature in the temperature range of the experiment. The γ relaxation was attributed to local molecular motions in the amorphous regions.

Molecular motions in poly‐α‐methylstyrene

AbstractDynamic mechanical properties (sound velocity and damping factor) have been determined in poly‐α‐methylstyrene in the temperature range from 60 to 470°K., at frequencies in the range 103 ÷ 106 Hz. Two high dissipation regions are found in the damping‐temperature curves: one, at temperatures below Tg, characterized by a loss peak at about 140°K., and another, at temperatures above Tg (405°K.), characterized by a very rapid increase of losses with increasing temperatures. The loss peak at low temperature (δ) is probably due to rotational motions of the phenyl rings in the side chain; the corresponding activation energy is about 4000 cal.‐mole−1, much higher then that of polystyrene, because of the sterical hindrance of CH3 in the α position. A δ transition is also found in other vinyl polymers containing aromatic groups as side chains: by plotting the logarithm of the activation energy as a function of the reciprocal maximum damping temperature, determined in different polymers, points nearly lying on a straight line are obtained. The main transition is associated with cooperative motions of chain segments. Practically no difference is found between the dynamic mechanical properties of atactic and syndiotactic samples of poly‐α‐methylstyrene.

Poly(γ‐alkyl glutamates). I

AbstractGood yields of some crystalline γ‐alkyl esters of L‐glutamic acid were obtained by carrying out the esterfication with a small (20–50 mole‐%) excess of alcohol in aqueous hydrochloric acid or 60–80% sulfuric acid followed by neutralization with an alkaline solution. This new method made it possible to synthesize various γ‐alkyl L‐glutamates, including those higher than ethyl, and consequently, various poly(γ‐alkyl L‐glutamates) such as methyl, ethyl, n‐propyl, n‐butyl, isobutyl, and isoamyl. The conformation of these poly‐L‐glutamates in the solid state was determined by the infrared absorption method. The molecular motions of the polymers of γ‐methyl, ‐ethyl, ‐n‐propyl, ‐n‐butyl, and‐isoamyl L‐glutamates and poly(γ‐methyl‐D‐glutamate) in the solid state were studied by NMR, and dielectric and mechanical measurements. At temperatures up to 400°K., the NMR spectra of poly(γ‐methyl D‐glutamate) can be explained only by rotational motion of the side chain. Also, from NMR results, rotational motion of CO groups in the side chain of poly(γ‐methyl D‐glutamate) is expected near room temperature, and such a motion was examined by dielectric measurements. Rotation of CO groups in the side chains of polymers of γ‐methyl, γ‐ethyl, γ‐n‐propyl, γ‐n‐butyl, and γ‐isoamyl L‐glutamate was also observed near room temperature by dielectric measurements in the frequency range from 102 to 106 cps. Activation energies obtained by dielectric and mechanical measurements were similar to those for the side chain motions of the corresponding esters of poly(methacrylic acid). Although it has been noted that the molecular motion of poly(γ‐benzyl L‐glutamate) in the solid state at room temperature may be related to the motion of its back bone, the molecular motion in these poly‐L‐glutamates at these temperatures can be explained only in terms of side‐chain rotation.