摘要:

AbstractThe complex polymorphic behavior of syndiotactic polystyrene (s‐PS) in melt‐crystallized samples is altered by blending with poly (2,6‐dimethyl‐1,4‐diphenylene oxide) (PPO). In particular, to render the beta form in these blends, starting with samples containing the α or γ froms, requires much lower temperatures and shorter melting times than for pure s‐PS. On the basis of the results, it is suggested that this phenomenon is due to more rapid loss of the memory of the α form, for the same temperature and time, in the melt in the presence of

ISSN:0887-6266    

DOI:10.1002/polb.1991.090290301

出版商:John Wiley&Sons, Inc.    

年代:1991

数据来源: WILEY

摘要:

AbstractCold‐drawn Nylon‐11 films, which were prepared by stretching the melt‐quenched films to a draw ratio of 2.8: 1 at room temperature, were found to exhibit an electric displacement versus electric field hysteresis loop. The results confirmed that the Nylon‐11 film exhibited ferroelectric behavior at or below room temperature. The coercive fields were 65, 98, 125, 160, and 215 MV/m at 20, 0, −20, −40, and −60°C and the remanent polarization at −20°C (where there was considerably less dc conduction) was 56 mC/m2. Switching of the polarization was almost compl

ISSN:0887-6266    

DOI:10.1002/polb.1991.090290302

出版商:John Wiley&Sons, Inc.    

年代:1991

数据来源: WILEY

摘要:

AbstractWe have discovered recently that melt‐quenched and cold‐drawn Nylon ‐ 11 films exhibit very clear ferroelectric hysteresis behavior. In the present study, a remanent polarization as high as 86 mC/m2has been found in Nylon ‐ 7 samples; this is significantly higher than that usually observed in poly (vinylidene fluoride) films. The effect of annealing on the electric displacement versus electric field characteristics of both Nylon ‐ 11 and Nylon ‐ 7 films have been studied and show an increased coercive field (62 to 115 MV/m for Nylon ‐ 11 and 79 to 97 MV/m for Nylon ‐ 7) and a decreased remanent polarization (51 to 17.3 mC/m2for Nylon ‐ 11 and 86 to 70.5 mC/m2for Nylon ‐ 7) with increasing annealing temperatu

ISSN:0887-6266    

DOI:10.1002/polb.1991.090290303

出版商:John Wiley&Sons, Inc.    

年代:1991

数据来源: WILEY

摘要:

AbstractThe overall crystallization and crystal melting of one low‐molecular mass poly(ethylene oxide) (PEO) fraction (MW 3000) have been investigated by differential scanning calorimetry (DSC) and in situ small‐angle x‐ray scattering (SAXS). The salient new results indicate that initial transient crystals with nonintegral folding (NIF) chain lengths form over a wide range of crystallization temperatures. This NIF structure subsequently transforms into crystal forms with integral folding (IF). The PEO IF crystals consist of the extended chain (n = 0) crystal and the once‐folded chain (n = 1) crystal, while the NIF has an intermediate fold length. The NIF → IF transformation occurs either by lamellar thickening or thinning. The NIF crystal is less stable than the IF(n = 1) crystal, but its growth is more rapid. Crystallization of the PEO (MW 3000) fraction is thus recognized as a compromise between the direction of the thermodynamic driving force and the kinetic pathway. Some potential consequences of these observations are also

ISSN:0887-6266    

DOI:10.1002/polb.1991.090290304

出版商:John Wiley&Sons, Inc.    

年代:1991

数据来源: WILEY

摘要:

AbstractDifferential scanning calorimetry (DSC) and in situ small‐angle x‐ray scattering (SAXS) indicate that in an α ω‐methoxy‐poly(ethylene oxide) (MPEO) fraction (MW 3000) a transient nonintegral folding (NIF) crystal initially forms during crystallization throughout a wide range of crystallization temperatures. Subsequent transformations of the NIF to IF (integral folding) crystals at low temperatures occur mainly through isothermal thickening or thinning via perfection processes or, at higher temperatures, through primary crystal formation. The NIF crystal is thermodynamically the least stable state among the crystal forms, but its growth is the most rapid. The overall crystallization and crystal melting of this MPEO fraction reveal that the NIF crystal and the NIF → IF crystal transformations are common to low‐molecular mass PEO fractions without regard to the end group. Nevertheless, diffusion coefficient and viscosity measurements provide clear evidence of an end‐group effect in PEO and MPEO fractions. The difference in the overall crystallization and isothermal thickening and thinning kinetics of low‐molecular mass PEO and MPEO fractions can lead to further understanding of

ISSN:0887-6266    

DOI:10.1002/polb.1991.090290305

出版商:John Wiley&Sons, Inc.    

年代:1991

数据来源: WILEY

摘要:

AbstractCorrelations of the linear crystal growth rates with the change in crystal morphology for poly (ethylene oxide) (PEO) (MW 3000) and α,ω‐‐methoxy‐poly (ethylene oxide) (MPEO) (MW 3000) fractions have been established over a wide range of supercooling (ΔT= 25 K). Two linear crystal growth rates were measured, namely, the linear crystal growth rate of spherulites or hedrites and the lateral (linear) crystal growth rate of single lamellar crystals along different crystalline planes (below ΔT= 11–12 K). At a low supercooling of 5 K, the crystal growth rate of the MPEO fraction passes through a minimum. Subsequently, the rate increases abnormally and reaches a maximum at even lower supercooling. This crystal growth retardation has been attributed to the competition between differing chain conformations during crystal growth. In this case, particularly, these conformations are the extended chain and the once‐folded chain conformations. This retardation is not observed in the PEO fraction, since in this low supercooling region any once‐folded chain conformations formed during crystal growth are of the double‐layer lamella type. The change in morphology in this region supports this judgement. Above ΔT= 7.6 K, the crystal growth behavior in these two fractions can be described by the present nucleation theory on flat crystal growth faces. Below that ΔT, however, different crystal growth mechanisms can be clearly seen. This may be associated with the crystal growth on the highly serrated growth faces in

ISSN:0887-6266    

DOI:10.1002/polb.1991.090290306

出版商:John Wiley&Sons, Inc.    

年代:1991

数据来源: WILEY

摘要:

AbstractGas transport properties are reported for a series of films prepared from thermotropic poly (ethylene terephthalate‐co‐p‐oxybenzoate), or PET/PHB, having compositions of 60 and 80 mol % PHB. The mesomorphic and crystalline morphology of the copolyester films was examined by cross‐polarized light microscopy, differential scanning calorimetry (DSC), and x‐ray diffraction. Melt‐processed films of both compositions appeared to exhibit an entirely anisotropic morphology with low levels of conventional crystallinity. Solution‐cast films prepared from the 60 mol % material were found to contain a large fraction of isotropic regions, which become ordered upon annealing above the glass transition. Permeability measurements were made for He, H2, O2, N2, and CO2at 35°C and the diffusivities were computed from time‐lag data. The largely anisotropic films exhibit good barrier properties resulting from very low solubility coefficients. The partially isotropic 60 mol % films show much higher permeability coefficients driven primarily by increased solubility coefficients, while diffusivity is affected to a lesser extent. These results appear to contrast with what is observed in semicrystalline systems where increased crystalline order results in more dramatic reductions in pe

ISSN:0887-6266    

DOI:10.1002/polb.1991.090290307

出版商:John Wiley&Sons, Inc.    

年代:1991

数据来源: WILEY

摘要:

AbstractThe solubility of CO2and CH4in five polyimides was measured at 35.0°C and at pressures up to 10 atm (147 psia). The concentration of the penetrant gases dissolved in the polymers can be represented satisfactorily as a function of penetrant pressure by the “dual‐mode sorption” model. The solubility coefficients for CO2and CH4, S(CO2) and S(CH4), increase in the polyimide order:\documentclass{article}\pagestyle{empty}\begin{document}$ {\rm PMDA - }4,4'{\rm - }m{\rm - APPS} < 6{\rm FDA - }4,4'{\rm - }m{\rm - APPS} < 6{\rm FDA - }4,4'{\rm - }p{\rm - APPP} < 6{\rm FDA - CDA} < 6{\rm FDA - 4,4' - }p{\rm - APPS} $\end{document}The magnitude of the solubility coefficients appears to depend primarily on the intermolecular forces between the penetrant gases and the polymers. The values of these coefficients are greater for the polyimides with larger mean interchain spacings, but no one‐to‐one correspondence appears to exist in this respect. The lower solubility of CO2in PMDA‐4,4'‐m‐APPS compared with that in the 6FDA polyimides may be due to the lower “excess” free volume of the former polymer. The ratio S (CO2)/S (CH4) varies relatively little for a variety of PMDA

ISSN:0887-6266    

DOI:10.1002/polb.1991.090290308

出版商:John Wiley&Sons, Inc.    

年代:1991

数据来源: WILEY

摘要:

AbstractX‐ray diffraction studies have been undertaken on aqueous solutions of hydroxy propylcellulose (HPC) over a wide range of the scattering vector Q. The experiments revealed only modest differences in local structure on a distance scale ca. 5–300 Å despite the fact that they covered concentrations generally interpreted as ranging from the isotropic (35.1 wt %) to the anisotropic liquid crystalline (LC) phase (53.5 wt %). Several models were used to interpret the small‐angle scattering data, and each gave similar structural parameters and extrapolated intensities (Q→ 0) for both solutions. Peaks were observed withd‐spacings ca. 12–17 Å in both materials. Wide‐angle x‐ray scattering (WAXS) showed slightly increased local order over a size range ca. 5–20 Å for the anisotropic solution, and this is consistent with a greater intensity of the 13 Å peak in this material. It is difficult to reconcile these findings with an interpretation of the LC state involving major differences with the isotropic phase and a high degree of orientational order extending ov

ISSN:0887-6266    

DOI:10.1002/polb.1991.090290309

出版商:John Wiley&Sons, Inc.    

年代:1991

数据来源: WILEY

摘要:

AbstractBimodal elastomeric networks were prepared by tetrafunctionally endlinking mixtures of short and long hydroxyl‐terminated poly (dimethylsiloxane) chains having number‐average molecular weights of 500 and 18,000 g/mol−1, respectively, over a composition range of 0–98 mol % of the short chains. Biaxial extension (compression) measurements were carried out by inflation of circular sheets of these materials at 23°C. The unimodal network (0 mol % short chains) showed the same behavior reported by other workers for noncrystal‐lizable networks: as the compression increased, the reduced stress or modulus [f*] went through a rather slight maximum followed by a gradual leveling off to the rupture point. The bimodal networks, however, showed much more pronounced maxima with another, significant increase in [f*] at very high compressions. This final increase is presumably due to non‐Gaussian effects from the very limited extensibility of the short chains, and thus parallels the upturns in [f*] frequently reported for bimodal networks at very high

ISSN:0887-6266    

DOI:10.1002/polb.1991.090290310

出版商:John Wiley&Sons, Inc.    

年代:1991

数据来源: WILEY