The solubility and dialysis of deoxypeganine hydrochloride and natural polyampholite were studied by IR spectra and x-ray diffraction analysis. Inclusion complexes form during mechanical treatment of a 1:1 mixture of deoxypeganine hydrochloride and natural polyampholite相似文献
The effects of high-temperature curing and overcuring on the cure characteristics, crosslink structure, physical properties and dynamic mechanical properties (DMPs) of gum and carbon black (N330) filled natural rubber (NR) vulcanizates cured with conventional (CV), semi-efficient (SEV) and efficient (EV) cure systems, which have about the same total crosslink densities under a moderate curing temperature of 150°C, were investigated. The gum NR vulcanizates cured with CV, SEV and EV curing systems have about the same glass transition temperature (Tg) and tan δ values below the temperature of about 0°C, but showed some apparent differences in the tan δ values increasing in the order CVG′ and tan δ values above Tg higher than those of the gum NR vulcanizates.
High-temperature curing and overcuring cause decreases to various extents in the cure plateau torque, Shore A hardness, 300% modulus and tensile strength, and lead to apparent changes in the DMPs. Typically, there is an increase in Tg of all three kinds of gum and N330-filled NR vulcanizates because of changes in the total crosslink densities and crosslink types. The CV vulcanizates show the most significant change in cure characteristics, physical properties and DMPs since the highest content of polysulfidic crosslinks appears in the CV vulcanizate, causing the highest level of reversion and having a dominant effect on the properties. 相似文献
The activation energy associated with the glass transition relaxation of an epoxy system has been determined by using the
three-point bending clamp provided in the recently introduced TA Instruments DMA 2980 dynamic mechanical analyzer. A mathematical
expression showing the dependency of modulus measurements on the sample properties and test conditions has also been derived.
The experimental results showed that the evaluation of activation energy is affected by the heating rate and test frequency,
as well as the criterion by which the glass transition temperature (Tg) is established. It has been found that the activation energy based on the loss tangent (tanδ) peak is more reliable than
on the loss modulus (E2) peak, as long as the dynamic test conditions do not cause excessive thermal lags.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
The relationship between copolymer composition and transition temperatures was studied by means of differential scanning calorimetric
analysis and dynamic mechanical spectroscopy. Six samples of ethylene vinyl acetate (EVA) copolymers containing from 5 to
40 mass per cent of vinyl acetate (VA) were studied. The differential scanning calorimetric analysis revealed that each EVA
copolymer displays two endothermic peaks (Tm1 and Tm2 ) in the melting zone. Dynamic mechanical spectroscopy was used to determine the primary relaxation temperature (Tα ) for EVA copolymers. This latter characteristic is relatively insensitive to the level of vinyl acetate contained in the
copolymer and is influenced by the pulsation frequency ω, also named the angular frequency.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
The physical properties of chemicals are strongly influenced by their protonation state, affecting, for example, solubility
or hydrogen-bonding characteristics. The ability to accurately calculate protonation states in the form of pKas is, therefore, desirable. Calculations of pKa changes in a series of substituted pyridines are presented. Computations were performed using both ab initio and semiempirical
approaches, including free energies of solvation via reaction-field models. The selected methods are readily accessible with
respect to both software and computational feasibility. Comparison of calculated and experimental pKas shows the experimental trends to be reasonably reproduced by the computations with root-mean-square differences ranging
from 1.22 to 4.14 pKa units. Of the theoretical methods applied the best agreement occurred using the second-order M?ller–Plesset/6-31G(d)/isodensity
surface polarized continuum solvation model, while the more computationally accessible Austin model 1/Solvent model 2 (SM2)
approach yielded results similar to the ab initio methods. Analysis of component contributions to the calculated pKas indicates the largest source of error to be associated with the free energies of solvation of the protonated species followed
by the gas-phase protonation energies; while the latter may be improved via the use of higher levels of theory, enhancements
in the former require improvements in the solvation models. The inclusion of alternate minimum in the computation of pKas is also indicated to contribute to differences between experimental and calculated pKa values.
Received: 27 April 1999 / Accepted: 27 July 1999 / Published online: 2 November 1999 相似文献
We have integrated scientific research of polymer blends on the base of poly-3-hydroxybutyrate (PHB and its copolymers) with bench testing in blend preparation by both solvent casting and melt extrusion. As a second component, we have used traditional synthetic macromolecules with various hydrophilicity degree and hence with different morphologies and physical behavior. Besides, variation of polymer hydrophilicity permits to control both the service characteristic and the rate of (bio)degradation operating in the presence of water. Therefore, a substantial part of our work is devoted to water transport in the parent PHB and its blends. Combining the morphology knowledge (SEM, WAXS, FTIR tecynique), transport characteristics (permeability cells and McBain spring microbalance), and mechanical testing, we propose that blending of the parent biodegradable polymer with synthetic macromolecules is a perspective tool to design novel materials with improved characteristics. Both the water transport coefficients and the mechanical characteristics are essentially sensitive to structure and morphology of the blends. Hydrophilicity variation in the order LDPE < SPEU < PVA at blending with PHB shows that the morphology transformation in immicsible or partly miscible blends shifted along the PHB concentration scale as LDPE (at ∼16 wt% PHB) < PVA (∼30 wt% PHB) < SPEU (∼50 wt% PHB) Our instrumental monitoring the structural hierarchy of parent polymers and their blends as well as , simultaneously, the study of transport processes, their modeling, and computer simulating open up the way to understanding the precepts of polymer operation in corrosive and bioactive media. 相似文献
A mechanical model of liquid crystals (LCs) is applied to
study the polymorphism of homologous series of terphenyl compounds. With
a semi-experimental molecular orbit method, we calculate the moment of inertia
which represents the rotation state to describe the phase transition
temperature obtained from experimental data. We propose a novel
explanation of the phase sequence or polymorphism of LC materials using
the two key
parameters, the moment of inertia and critical rotational velocity. The effect
of molecular polarity on the appearance of liquid crystalline is also discussed. 相似文献