摘要:

Gas chromatography, thermal expansion, dynamic mechanical relaxation and thermally stimulated current measurements were used to study multiple transitions in isotactic polypropylene. The crystalline phase relaxation Tαc was decomposed into two components, Tαc1 and Tαc1, in order of decreasing temperature. The activation energy of the Tαc mode was found to be lower than that of the relaxation observed around the glass transition temperature. Thermally stimulated current (TSC) measurements revealed the existence of an apparent double liquid-liquid transition at 39°C and 68 °C in undrawn polypropylene and at 28°C and 55°C in drawn polypropylene. These TSC peaks correspond to a relaxation time following a Vogel equation whose critical temperatures let us predict an apparent double glass transition in polypropylene.

ISSN:0022-2348    

DOI:10.1080/00222348008212795

出版商:Taylor & Francis Group    

年代:1980

数据来源: Taylor

摘要:

The morphology and crystallization kinetics of oligoamidophosphates of molecular weight between 300 and 1800 prepared with various reaction times (1–12 hr) and different mole ratios of poly-caproamide and orthophosphoric acid (PA/AC = 0.5–1.6) has been studied. Very large, thin (several microns thick) or three-dimensional (diameter up to 8–10 cm) spherulites from aqueous solutions of oligoamidophosphates have been observed. Fully transparent lamellae with perfect angles, about 0. 2 cm thick, 1–12 cm wide, and 6–8 cm long can be formed too. The investigation of the kinetics of isothermal crystallization of oligoamidophosphates with various molecular weights in the temperature range of −5 to +90°C showed the well known bell-shaped curves. The dependence of the spherulite growth rate on the molecular weight generally observed was confirmed. It has been found that the morphologic and kinetic changes, established in the crystallization of the products prepared under various conditions (duration of treatment, mole ratio, acid concentration), are the exclusive result of differences in the molecular weight of the oligomers obtained.

ISSN:0022-2348    

DOI:10.1080/00222348008212796

出版商:Taylor & Francis Group    

年代:1980

数据来源: Taylor

摘要:

The permeability of various ionic salts through poly(γ-methyl L-glutamate) (PMLG) membrane, which consists of noncharged α-helical molecules, was studied. Based on the results, the mechanism of ion transport and the degree of hydration of ions in the membrane and their relation to salt rejection is discussed. The permeability coefficient of salt, PS, is almost independent of salt concentration over a wide range of concentration in aqueous solution, which contrasts with the case of charged membranes. The activation energy for PS is about 15 kcal/mole, which is almost half of that for cellulose acetate membranes. The ionic diffusion coefficient Di. decreases with increase of ionic radius, but it does not obey Stokes' law. The behavior is analyzed by the free volume theory, and it is found that the degree of ion hydration in PMLG membranes is depressed especially in the case of strongly hydrated ions in aqueous solution. This fact also explains the good permeability of salt through the rather hydrophobic polypeptide membranes. According to the analysis of the results of separation experiment, the low rejection of salt by PMLG membranes is also due to a coupling effect, i. e., to ionic flow accompanied by water flow. In this case, too, the hydration tendency of the ions in the membrane has a significant influence on the behavior.

ISSN:0022-2348    

DOI:10.1080/00222348008212797

出版商:Taylor & Francis Group    

年代:1980

数据来源: Taylor

摘要:

Two forms of the poly(β-benzyl L-aspartate) were prepared: the α form, with hexagonal packing of the α helix (3. 6 residue/turn) and the ω form, with tetragonal packing of the ω helix (4. 0 residue/turn) and intramolecular benzene ring stacking. The α and ω forms were analyzed by infrared absorption spectra, x-ray diffraction, and thermal analysis. The sorption isotherm of carbon dioxide gas for the α and ω forms was measured in the temperature range from 30 to 40°C. The solubility coefficient of the α form is larger than that of the ω form, reflecting the benzene ring stacking in the side chains of the ω form. The permeability coefficient of carbon dioxide gas for the α and ω forms was studied in the temperature range from 0 to 165°C. The permeability coefficient of the α form was larger than that of ω form. It decreased markedly at the α-ω transition. Such behavior is interpreted on the basis that the side chains of the α form are not arranged regularly, while that of the ω form are arranged to form the regular benzene ring stacking. The diffusion coefficient was evaluated in the temperature range from 0 to 50°C on the basis of the solubility coefficient and the permeability coefficient obtained in this study. The difference in diffusion coefficients for the α and ω forms was interpreted in terms of the distance between helices of the α and ω forms and the free volume concept.

ISSN:0022-2348    

DOI:10.1080/00222348008212798

出版商:Taylor & Francis Group    

年代:1980

数据来源: Taylor

摘要:

Results of polarization and thermally stimulated depolarization studies in cellophane thermoelectrets for different polarizing conditions are reported in this paper. A method of separating the spontaneous current, for isolating the actual depolarization current, with the help of two different electrodes having a small difference in work functions, is given. Up to 10 kV/cm, the peak current and total charge increase linearly with field, indicating uniform polarization. Depolarization kinetic parameters, viz. activation energy, relaxation time, etc., for different polarizing conditions have been calculated both by initial rise and whole curve methods. A Bucci plot for the present results gives a straight line, indicating that a uniform process was operative during polarization or depolarization. Main chain scission and degradation starts at ⋍320°C. Apart from dipole orientation, charge/ion migration with subsequent trapping has been identified as the major component of polarization.

ISSN:0022-2348    

DOI:10.1080/00222348008212799

出版商:Taylor & Francis Group    

年代:1980

数据来源: Taylor

摘要:

The weight fraction crystallinity (Wc) of a polymer sample can be determined from the relation Wc = △Hf/△Hf°. However to obtain a more accurate determination of Wc the term △Hf should be corrected for various excess enthalpy terms. These corrections should include terms related to the nature of the “fold surface,” i. e., the manner in which units in the fold structure are distributed between true folds and cilia. They should also take into account the nature of the crystal, e.g., the possibility of an enthalpy term associated with mosaic block side surfaces in dried-down crystals. The magnitude of the appropriate correction factors (and their affect on Wc) has been estimated for various fold/cilia models. From the derived equations it is apparent that the y-intercept of a plot of the form △Hf/Wc versus (crystal core thickness)-1 is equal to △Hf° only when dealing with crystals kept in suspension. For dried single crystals the y-intercept is shown to be some function of △Hf° . Since the vast majority of past △Hf work has been concerned with dried crystals, one must view the reported △Hf° values with some suspicion.

ISSN:0022-2348    

DOI:10.1080/00222348008212800

出版商:Taylor & Francis Group    

年代:1980

数据来源: Taylor

摘要:

A recent development of existing thermodynamic theory has been applied to the analysis of heat of fusion (△Hf) data from both “dry” and “suspension” polyethylene) - 1] were constructed for suspension samples assuming various fold models. Irrespective of the model chosen, the perfect crystal heat of fusion was found to be approximately 72 cal/g. From analysis of the slopes of these plots it was concluded that a 10-carbon-atom fold model best fits the suspension △AHf, data. Melting points for suspension crystals were also obtained. Analysis according to the Hoffman-Weeks equation reveals that the equilibrium melting point is somewhat lower than the usual range of accepted values. The discrepancy between the value determined here and that reported by other authors is discussed. The magnitude of the derived surface free energy from melting point data also indicates that PE crystals consist of folds containing approximately 10 methylene units on average. This conclusion is in excellent agreement with the results from recent small-angle x-ray studies.

ISSN:0022-2348    

DOI:10.1080/00222348008212801

出版商:Taylor & Francis Group    

年代:1980

数据来源: Taylor

摘要:

The adsorption isotherms of n-decane and n-dodecane on amorphous poly(cyclohexyl methacrylate) in its glassy state have been determined from the profiles of gas chromatographic peaks. The isotherms obtained are characteristic of a low surface energy solid. The possible influence of the experimental variables flow rate and sorbate sample size on the isotherms thus determined has been investigated. The BET model of multilayer adsorption, when applied to the isotherms, yields an estimated specific surface area of the adsorbent which is too large in comparison with literature values for other polymers. This discrepancy is attributed to the penetrable nature of our polymer. Its cyclohexyl side group should impart enough free volume to the solid below T to allow for the penetration of the sorbate molecule.

ISSN:0022-2348    

DOI:10.1080/00222348008212802

出版商:Taylor & Francis Group    

年代:1980

数据来源: Taylor

摘要:

Viscoelastic polymers exhibit stress relaxation, and it is therefore predicted that their stress-strain curves will be nonlinear at all strains. It is shown in this report that this behavior is observed only with straight and properly clamped specimens; when a specimen is crimped or badly clamped, the initial part of the curve exhibits “toe-in” (caused by straightening of the specimen), which obscures the viscoelastic curvature. In this case, a pseudo-linear region, strictly an inflection region, is observed at low strains, which arises from opposing viscoelastic and toe-in curvatures. There is thus no Hookean region in the stress-strain curve of (straight or crimped) viscoelastic polymers, and the concept of a Young's modulus is inapplicable; however, the inflection modulus, derived from the slope of the inflection region has limited applicability, particularly when the toe-in is not excessive. Instead of a Young's modulus, a unique stiffness parameter is proposed, termed the initial secant modulus, which is based on the secant near the origin of the “material stress-strain curve,” i. e., the curve in the absence of toe-in. It is proposed that in the case of keratin the term “Hookean region” be replaced by the term “pre-yield region.”

ISSN:0022-2348    

DOI:10.1080/00222348008212803

出版商:Taylor & Francis Group    

年代:1980

数据来源: Taylor

摘要:

By using the concept of a microheterogeneous instantaneous structure of amorphous flexible-chain polymers, empirical equations relating melt surface tension with effective free-volume fraction, feef = (va - vc)/va, as well as with activation energy for viscous flow and compressibility, are derived, and the obtained results are compared with existing correlations.

ISSN:0022-2348    

DOI:10.1080/00222348008212804

出版商:Taylor & Francis Group    

年代:1980

数据来源: Taylor