Enthalpies and entropies of fusion have been determined for two series of semicrystalline polyurethanes. The H series is based on 1,6-hexamethylene diisocyanate and various α, ω-diols containing from 2 to 10 methyle...
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Enthalpies and entropies of fusion have been determined for two series of semicrystalline polyurethanes. The H series is based on 1,6-hexamethylene diisocyanate and various α, ω-diols containing from 2 to 10 methylene units. The DP series is based on 4,4′-methylene diphenyl diisocyanate and various α, ω-diols containing from 2 to 10 methylene units. Differential Scanning Calorimetry (DSC) was used in conjunction with X-ray analysis (X-ray method) and low molecular-weight diluents (diluent method) to determine the enthalpies of fusion. Agreement between the X-ray method and the diluent method was *** between the enthalpies and entropies of fusion of the H and DP series could be largely attributed to differences in intermolecular hydrogen bonding in the melts of the two series. On the other hand, differences in the entropies of fusion between the H series and corresponding polyamides arise largely from differences in backbone-chain stiffness between the two series.
A dynamic mechanical study has been made of two series of partially hydrolyzed ethylene-vinyl acetate copolymers. Series A contains 75 mole % ethylene and series B contains 59 mole % ethylene. The frequencies employed...
A dynamic mechanical study has been made of two series of partially hydrolyzed ethylene-vinyl acetate copolymers. Series A contains 75 mole % ethylene and series B contains 59 mole % ethylene. The frequencies employed were 3.5, 11, and 110 Hz, and the temperature range was from −180° to +120°C. Three relaxation regions are discernable in this range and these are labeled α, β, and γ in order of decreasing temperature. The α relaxation is associated with the glass transition of the copolymers. The β relaxation is absent in the unhydrolyzed copolymers and is dependent on hydroxyl content. It is assigned to local motions of hydrogen bonded groups. The γ relaxation is insensitive to hydroxyl content and is assigned to local motions of methylene sequences. The above assignments are supported by the effect of water added to the polymers. Added water decreases the temperatures of the α and β relaxations but does not affect the γ relaxation significantly. The magnitude of the β relaxation is considerably enhanced by water.
The glycerol acetal of methyl azelaaldehydate is an ω-hydroxy ester that exists as a mixture of dioxolanyl and dioxanyl isomers, each having two geometric isomers. Each of the four isomers was isolated by chromatogra...
The glycerol acetal of methyl azelaaldehydate is an ω-hydroxy ester that exists as a mixture of dioxolanyl and dioxanyl isomers, each having two geometric isomers. Each of the four isomers was isolated by chromatographic (gas–liquid and column) and fractionating (crystallization and distillation) methods. Structural assignments were made on the basis of nuclear magnetic resonance and infrared spectral data. Linear poly(ester-acetals) were prepared from each of the cis and trans forms of the dioxanyl isomers and from a mixture of the dioxolanyl geometric isomers. Physical properties of these polymers were correlated with their structures. When prepared with basic condensation catalysts, the polymers retained the geometric configuration and structural identity of the monomer. When prepared with lead acetate, structural rearrangement as well as polycondensation occurred, resulting in an enrichment of the dioxolanyl isomer and simultaneous increase in polymer crystallinity. The enrichment took place at elevated temperatures and also, unexpectedly, at room temperature upon long standing of the polymer. Isomer redistribution at room temperature appears to be antithermodynamic in the reaction sense, yielding a polymer of unusually high dioxolanyl unit content. The driving force for this isomerization may be crystallization of these units by a phenomenon termed a “crystallization-induced reaction.”
Diester and hydroxy ester monomers containing internal five-, six-, and seven-member acetal rings were prepared and converted into linear polyesters, copolyesters, and copolyamides. The polymers so obtained were chara...
The dielectric relaxation of amorphous bulk poly(2, 6‐dimethyl phenylene oxide) (PPO) has been measured between −160° and 260°C over a frequency range extending from 0.1 to 10 kHz. Two broad loss peaks of c...
The dielectric relaxation of amorphous bulk poly(2, 6‐dimethyl phenylene oxide) (PPO) has been measured between −160° and 260°C over a frequency range extending from 0.1 to 10 kHz. Two broad loss peaks of comparable intensities are observed‐a high‐temperature α relaxation identified with the glass transition and a low‐temperature γ relaxation of uncertain origin. The intensity of the α relaxation can be well accounted for in terms of a loss of orientational dipolar correlation in a calculation based on a consideration of a rotational isomeric model and of the geometry of the PPO chain.
A study has been made of the relaxation phenomena in three series of glassy polyurethans in the temperature range from -190 to -100° and at various frequencies up to 110 Hz using dynamic mechanical and dielectric...
Blends of poly(2,6‐dimethyl‐1,4‐phenylene ether) (PPO) and atactic polystyrene (PS) have been prepared by mechanically mixing powders of the two polymers and subjecting the mixtures to three different thermal treat...
Blends of poly(2,6‐dimethyl‐1,4‐phenylene ether) (PPO) and atactic polystyrene (PS) have been prepared by mechanically mixing powders of the two polymers and subjecting the mixtures to three different thermal treatments. Three different compositions were studied by the dynamic mechanical and DSC techniques. The weight fractions of PPO in the mixtures were 0.25, 0.50 and 0.75. The dynamic mechanical measurements indicate that partial mixing took place but that two distinct phases, one rich in PS and the other in PPO, exist in all the mixtures studied. Each phase exhibits a characteristic relaxation peak associated with the glass transition of that phase. DSC measurements, on the other hand, reveal only a single glass transition apparently characteristic of the PS rich phase in each case. The results indicate that a given type of experiment will indicate compatibility or incompatibility depending upon the size of the molecular process it represents.
This investigation provides the first reported viscosity measurements on cholesteryl myristate. Viscosities were measured as a function of both temperature and shear over temperatures corresponding to the isotropic li...
This investigation provides the first reported viscosity measurements on cholesteryl myristate. Viscosities were measured as a function of both temperature and shear over temperatures corresponding to the isotropic liquid, the cholesteric mesophase, and the smectic mesophase. The myristate ester was chosen for study because its multiple phase transitions have been previously shown to be reproducible. The phases also persist over relatively long temperature ranges. The viscosity measurements were made at lower shear rates than previously reported for esters of cholesterol by using a Weissenberg Rheogoniometer. These are the first cone-and-plate viscosity measurements reported for any esters of cholesterol. The shear rates in this cone-and-plate viscometer are homogeneous and variable. The results show large breaks in viscosity behavior at the independently-measured thermodynamic transitions between the crystal, smectic, cholesteric, and isotropic states. Viscosities for the isotropic state are Newtonian throughout. The viscosities for both the cholesteric and smectic mesophases are prominently non-Newtonian with a sharp transition between the two. This is the first time a change in rheology has been reported at a smectic-cholesteric transition. A prominent viscosity maximum is observed near the cholesteric-isotropic transition which depends on shear rate and perhaps on time of shearing.
This volume contains papers presented at the Second Symposium on Ordered Fluids and Liquid Crystals held at the 158th National Meeting of the American Chemical Society, New York, Sep tember, 1969. The Symposium ...
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ISBN:
(数字)9781468482140
ISBN:
(纸本)9781468482164
This volume contains papers presented at the Second Symposium on Ordered Fluids and Liquid Crystals held at the 158th National Meeting of the American Chemical Society, New York, Sep tember, 1969. The Symposium was sponsored by the Division of Colloid and Surface Chemistry. The proceedings for the first symposium on this subject were published by the American Chemical Society in the Advances in Chemistry Series. In the preface to the volume for the first meet ing held four years ago, we noted that research on liquid crystals had gone through tremendous fluctuations, with peaks of activity around 1900 and again in the early 1930's. The present period of high activity which started about 1960 has continued to exhibit acceleration. The reason for the persistent growth in the field is due to the increasing recognition of the important role played by liquid crystals in both biological systems and in items of commerce as diverse as detergents and electronic components. Addi tionally, more powerful and sophisticated instrumentation is pro viding a basis for understanding the properties of the liquid crystalline state as weIl as yielding inc~s~ve tests for the theories of mesophase structure which are only now reaching astate of maturity. Julian F. Johnson Roger S. Porter v CONTENTS Thermal Phase Transitions in Biomembranes • . • • • • • . • • 1 Joseph M. Steim Conditions of Stability for Liquid-Crystalline Phospholipid Membranes .
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