摘要:

AbstractA report of measurements of Ar emission during the loading of polystyrene and high impact polystyrene in vacuum is presented. Argon was introduced into the material prior to the experiment by storing the samples in an Ar atmosphere. The development of crazes during loading was monitored by videotaped visual observations and scattered light measurements. Increased Ar emission is observed at the onset of crazing, provided that the crazes intersect the surface. The strength of the Ar signal depends upon the extent of crazing; especially intense signals are observed from samples which display significant crazing prior to fracture. High‐impact polystyrene shows intense emissions at yield which soon decay due to the depletion of Ar from the near surface material. The emission intensity rises again prior to fracture, when surface crazes become connected to crazes in the bulk. Thus the emission of volatile species during deformation reflects the growth of crazes intersecting the surface, as well as changes in the “connectivity” of the craze network. © 1993 John Wiley&Son

ISSN:0887-6266    

DOI:10.1002/polb.1993.090311101

出版商:John Wiley&Sons, Inc.    

年代:1993

数据来源: WILEY

摘要:

AbstractThe effects of postannealing aging time on the brittle fracture behavior of polystyrene were studied. A combination of mechanical properties, including creep and creep rupture under constant load and the behavior under constant extension rate deformation were examined for polystyrene samples of different prior aging times (from 1h to 2 months). The specimens and fracture surfaces were examined by optical microscopy and SEM to observe any change in the fracture behavior. It was found that longer aging times caused not only a change in the time‐dependent modulus of the material but also a significant decrease in the creep rupture life and a decrease in strain to failure. It was found that the reasons for this are that although aging delays craze formation, craze breakdown and ultimate failure are accelerated by aging. The importance of these findings are discussed, particularly in relation to failure criteria involving the use of critical strains. © 1993 John Wiley&Sons, I

ISSN:0887-6266    

DOI:10.1002/polb.1993.090311102

出版商:John Wiley&Sons, Inc.    

年代:1993

数据来源: WILEY

摘要:

AbstractThe morphology of thermoreversible polyacrylonitrile–propylene carbonate (PAN‐PC) gels was examined using solid‐state carbon‐13 nuclear magnetic resonance (NMR) spectroscopy and x‐ray diffraction. Following complete dissolution of the polymer at elevated temperature and cooling of the concentrated PAN‐PC solutions, a gel was formed. The PAN‐PC gels consisted of regions of mobile polymer chains, rich in PC, “cross‐linked” by regions of rigid polymer. The mobile regions of the gels showed solution‐type NMR spectra with resolution of tacticity effects. The rigid component detected by NMR would correspond to the crysttallites detected previously by x‐ray diffraction. Wide‐angle x‐ray diffractograms of the gels showed different peaks when compared with the dry polymer powder. After solvent extraction and drying of the gel, the diffractogram reverted to that of the original dry powder. This new result is the strongest evidence to support the view advanced earlier that the new peaks found in the diffraction pattern of the wet gels arises from solvated polymer crystallites rather than from ordinary polymer crystallites.

ISSN:0887-6266    

DOI:10.1002/polb.1993.090311103

出版商:John Wiley&Sons, Inc.    

年代:1993

数据来源: WILEY

摘要:

AbstractThe dynamics of the changing microenvironment of the fluorescent probe pyrene in slightly cross‐linked networks of poly(diallyldimethylmmonium bromide) during diffusion of sodium dodecyl sulfate (SDS) in the gel phase has been investigated by fluorescence spectroscopy. Values of the spectral ratioI3/I1for pyrene monomer included in SDS micelles in the swollen networks fall between the corresponding values for pyrene in water and for pyrene dissolved in SDS micelles in aqueous solution. In the narrow interval of the surfactant concentrations in the gel phase, the formation of pyrene excimers is observed. The values of the critical micelle concentration in the gel phase (ca. 5 × 10−4to 8 × 10−4mol/L) are tenfold lower than in aqueous solutions of the surfactant. The effective micellar diffusion coeffecient D in the gel phase increases with increasing swelling of the network. © 1993 John Wiley&S

ISSN:0887-6266    

DOI:10.1002/polb.1993.090311104

出版商:John Wiley&Sons, Inc.    

年代:1993

数据来源: WILEY

摘要:

AbstractTo overcome the difficulties in measuring high‐frequency shear constants of polymeric materials by ultrasonic or Brillouin scattering technques, we extrapolate results from oriented materials to zero birefringence. Shear constantsC44in the high‐frequency limit (GHz) are determined for polyethylene, poly(ethylene terephthalate), poly(vinyl chloride), poly(methyl methacrylate), and polycarbonate using Brillouin scattering. Accurate values of Poisson's ratio are derived. The extrapolation to full orientation using an amended Moseley relation gives upper bounds for the “intrinsic birefringence.” Changes in the character of the orientation process are easily detected by monitoring the mode numbers, which are defined by analogy to Poisson's ratio. Extrapolation of these ratios to their upper bound 0.5 gives an independent check of the maximum intrinsic birefringence. © 1993 John Wiley&S

ISSN:0887-6266    

DOI:10.1002/polb.1993.090311105

出版商:John Wiley&Sons, Inc.    

年代:1993

数据来源: WILEY

摘要:

AbstractThe aim of this study was to introduce a new method for determining the surface composition and surface order of PP/EPDM (propylene/ethylene–diene–monomer rubber) blends, based on surface‐enhanced infrared spectroscopy. PP/EPDM blends were characterized by Fourier transfer IR–attentuated total reflectance (FTIR‐ATR) using two different internal reflection elements, Ge and KRS‐5. These elements provide information at different depths and show differences in relative band intensities. A linear relationship between the ratio of the bands at 2850 and 2920 cm−1EPDM content was obtained when well‐defined compositions of PP and EPDM mixtures were used. Changes in crystallinity were measured by the ratio of the bands at 2959 and 2951 cm−1. For comparison, bands in the lower region of the spectrum and DSC and WAXS measurments were used. It was shown by two calibration methods that FTIR‐ATR can provide information on surface composition and surface crystallinity at very low penetration depths. This new method indicates that injection‐molded sample show a lower surface order and lower EPDM content at the surface than is seen in bulk. © 199

ISSN:0887-6266    

DOI:10.1002/polb.1993.090311106

出版商:John Wiley&Sons, Inc.    

年代:1993

数据来源: WILEY

摘要:

AbstractExpressions for the entropy and free energy of mixing a solvent with a mixture of linear and cyclic polymer molecules are derived. The entropy of mixing is deduced from the number of ways of arranging on a honeycomb lattice a mixture of totally flexible molecules made up ofNRrings andNCchains. An equation is obtained through the combination of two independent expressions for the number of ways of arranging rings and chains. The free energy of mixing is deduced from the entropy and the enthalpy of mixing, using two distinct interaction parameters for ring and for chain molecules. The chemical potentials for solvent, ring polymer, and linear polymer are derived from the free energy of mixing. These quantities are found to be functions of the mole fraction of rings in the polymer mixture. © 1993 John Wiley&Sons, Inc

ISSN:0887-6266    

DOI:10.1002/polb.1993.090311107

出版商:John Wiley&Sons, Inc.    

年代:1993

数据来源: WILEY

摘要:

AbstractA mathematical model was developed to explain the anomalous penetrant diffusion behavior in glassy polymers. The model equations were derived by using the linear irreversible thermodynamics theory and the kinematic relations in continuum mechanics, showing the coupling between the polymer mechanical behavior and penetrant transport. The Maxwell model was used as the stress–strain constitutive equation, from which the polymer relaxation time was defined. An integral sorption Deborah number was proposed as the ratio of the characteristic relaxation time in the glassy region to the characteristic diffusion time in the swollen region. With this definition, an integral sorption process was characterized by a single Deborah number and the controlling mechanism was identified in terms of the value of the Deborah number. The model equations were two coupled nonlinear differential equations. A finite difference method was developed for solving the model equations. Numerical simulation of integral sorption of penetrants in glassy polymers was performed. The simulation results show that (1) the present model can predict Case II transport behavior as well as the transition from Case II to Fickian diffusion and (2) the integral sorption Deborah number is a major parameter affecting the transition. © 1993 John Wiley&Sons, I

ISSN:0887-6266    

DOI:10.1002/polb.1993.090311108

出版商:John Wiley&Sons, Inc.    

年代:1993

数据来源: WILEY

摘要:

AbstractA method has been proposed for evaluating the lattice distortion that yields broad X‐ray scattering peaks. The long‐period structure of thermally stabilized acrylic fibers has been analyzed by applying this method to the meridional small‐angle X‐ray scattering. The period length of the acrylic fibers increases during stabilization, while the distortion of the period remains unchanged, maintaining a coefficient of variation of about 0.3. It is concluded from a simulation using a model lattice that the cyclization of molecular segments proceeds faster in disordered regions than in crystalline regions, and that the cyclization in the crystalline regions proceeds preferentially from crystallities separated by smaller cyclized regions. © 1993 John Wiley&S

ISSN:0887-6266    

DOI:10.1002/polb.1993.090311109

出版商:John Wiley&Sons, Inc.    

年代:1993

数据来源: WILEY

摘要:

AbstractCompressed pellets of partly crystalline, chemically synthesized, doped (Cl−and FeCl −4) polybithiophene (PBTd), poly(3‐methylthiophene) (P3MTd), and their neutral (dedoped) forms (PBTn and P3MTn) were studied by wide‐angle x‐ray diffraction and positron annihilation lifetime spectroscopy. As synthesized, PBTd and P3MTd polymers have a helicalsynconformation they crystallize in the hexagonal system. On dedoping, PBT macromolecules change their helicalsynconformation in a rodlikeanticonformation and crystallize in the orthorhombic or monoclinic system, whereas P3MT macromolecules retain their helicalsynconformation. Chemical doping–dedoping cycles lead to amorphous PBT and P3MT in either doped or dedoped states. The P3MT helical macromolecule behaves like a spiral spring; by doping, it becomes axially compressed. The unit‐cell volume of P3MTd is smaller than that of P3MTn. The positron lifetime spectra for all polymers were resolved, without constraint, into three components. The τ1lifetime is attributed to free‐positron annihilation events, the τ2lifetime to positrons annihilating trapped in voids, and the τ3lifetime to positrons annihilating as o‐Ps trapped in cavities located inside the polymer grains for P3MTn and at the surface of the grains for PBTd, PBTn, and P3MTd. Most positrons annihilate when trapped in voids, both in doped and dedoped PBT and P3MT. The doping apparently increases the concentration of the voids and their mean diameter in P3MT, and probably also in PBT. Cavities anchored in the bulk are produced by dedoping. © 19

ISSN:0887-6266    

DOI:10.1002/polb.1993.090311110

出版商:John Wiley&Sons, Inc.    

年代:1993

数据来源: WILEY