摘要:

AbstractThe absorption and swelling of poly(aryl‐ether‐ether‐ketone) (PEEK) in toluene as a function of resin morphology and temperature in the range 35–95°C was investigated. In all cases the weight gain curves exhibit three characteristics: (1) an induction period, which is a strong function of both temperature and initial crystallinity, (2) a main absorption region, which is linear with square‐root time, and (3) a final equilibrium value, namely, solubility. The solubility of amorphous PEEK decreases with temperature and the heat of solution is −0.93 kcal/mol. The induction period varies with the fourth power of the crystallinity and decreases with temperature with an apparent activation energy of 50 kcal/mol. The strength of the interaction between the crystalline regions is markedly reduced at temperatures greater than 80°C. Swelling accompanying the absorption of the toluene is highly anisotropic with most of the dimensional changes occurring in the thickness direction. The deswelling process, however, is essentially isotropic. The concentration of toluene in solution has a strong effect on the transport process; the equilibrium solubility of toluene in amorphous PEEK immersed in a toluene/iso‐octane mixture is a linear function of toluene concentration; but the pseudo‐diffusion coefficient for the absorption of toluene varies approximately with the fourth power of

ISSN:0887-6266    

DOI:10.1002/polb.1992.090300201

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY

摘要:

AbstractGlassy polymers are considered as inhomogeneous with regions in which the gas sorption follows Henry's law and others where it follows Langmuir's law. It is assumed that the linear dimensions of these regions are small compared with the macroscopic length of interest but large compared with the mean free path of the penetrant gas molecules. Applying an homogenization method it is shown that the average flux is directly proportional to the concentration gradient in the polymer. This relationship can be expressed in terms of an effective diffusion coefficientDeff, which depends on the details of the microstructure.Deffis evaluated in the framework of the effective medium theory and compared with experimental data for diffusion of five vapors in ethylcellulose.

ISSN:0887-6266    

DOI:10.1002/polb.1992.090300202

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY

摘要:

AbstractBinary mixtures of a rodlike poly(p‐phenylene pyromellitimide) (PMDA‐PDA) and a flexible 6F‐BDAF polyimide synthesized from hexafluoroisopropylidene diphthalic anhydride and 2,2‐bis(4‐aminophenoxy‐p‐phenylene) hexafluoropropane were prepared by solution‐blending of the meta‐PMDA‐PDA poly(amic ethyl ester) and 6F‐BDAF poly(amic acid) precursors, followed by solvent evaporation and thermal imidization. Mixtures containing different molecular weights of 6F‐BDAF poly(amic acid) were studied. The size scale of the phase separation, as measured by light scattering, isca.1 μm or smaller in most cases. The domain size is primarily set by the demixing of the precursor polymers during solvent evaporation, with no significant coarsening observed during the thermal imidization. The observed variation of the domain size with molecular and process parameters such as composition, molecular weight, and film thickness is discussed in terms of the miscibility of the precursor polymers, rate of solvent evaporation, and solidification. Dynamic mechanical thermal analysis and dielectric relaxation measurements indicate that the glass transition temperature of 6F‐BDAF is unaffected in all of the mixtures studied, indicating complete demixing of rodlike and flexible polyimides in agreement with theory. X‐ray photoelectron spectroscopy results show a strong surface segregation of 6F‐BDAF in mixtures containing as low as 10% by weight of the 6F‐BDAF component in the bulk. The mixtures with PMDA‐PDA as the major matrix component therefore exhibit excellent mechanical toughness, dimensional stability up to 500°C, low coefficients of thermal expansion (

ISSN:0887-6266    

DOI:10.1002/polb.1992.090300203

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY

摘要:

AbstractWe adapt a recently proposed model for non‐Fickian diffusion of penetrants into polymers and use it to study a drug‐delivery problem. The model modified Fick's diffusion equation by the addition of stress‐induced flux. A stress evolution equation incorporating aspects of the Maxwell and Kelvin‐Voigt viscoelastic stress models completes the model. The relaxation time in the polymer is taken as a function of the penetrant concentration. The system is studied under the assumption that the diffusivity is large. Singular perturbation techniques are used to show that the concentration and stress evolve diffusively for small time, but exhibit steep fronts in a narrow region within the domain for larger time. These predictions are verified numerically for specified parameter values. Finally, the equations are studied in the steady state and are found to predict the evolution of

ISSN:0887-6266    

DOI:10.1002/polb.1992.090300204

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY

摘要:

AbstractIn this study, we report the melting behavior of poly(phenylene sulfide), PPS, which has been cold‐crystallized from the rubbery amorphous state. We find that the crystallization kinetics are faster for cold‐crystallized PPS than for melt‐crystallized material, due to formation during quenching of a short‐range ordered, but noncrystalline, structure. We observe that the endothermic response of cold‐crystallized PPS at a large undercooling consists of a low temperature endotherm, followed by an exothermic region, and by the main higher melting endotherm. The lower melting peak temperature of cold‐crystallized PPS increases as the crystallization temperature increases, but the main upper melting peak temperature remains almost the same. The size of the exothermic region is strongly related to the degree of undercooling, and must be taken into account in order properly to determine the degree of crystallinity of the material prior to the scan. When the crystallization time is varied, we see a systematic decrease in the size of the main endotherm, and an increase in size of the lower melting endotherm. This suggests that a portion of the main endothermic response is due to reorganization during the scan. Annealing will not only increase the degree of crystallinity but also improve the crystal perfection; therefore the ability of an annealed sample to reorganize decreases as the annealing time increases. However, an additional third melting peak is seen when a cold‐crystallized sample is annealed at high temperature for a sufficiently long residence time. The existence of the third melting peak suggests that more than one kind of distribution of crystal perfection may occur when PPS has been cold‐crystallized and subseq

ISSN:0887-6266    

DOI:10.1002/polb.1992.090300205

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY

摘要:

AbstractThe recent introduction of multiangle light‐scattering detectors for size‐exclusion chromatography has made possible the measurement of the root mean square radius of gyration (Rg) and molecular weight (M) of polymer fractions eluting from a size‐exclusion chromatography column. The characterization of the dimensions of a polymer may be accomplished with only a few milligrams. The dimensions of a polyimide precursor prepared by the condensation of themeta‐diethyl ester of pyromellitic dianhydride withpara‐phenylene diamine have been measured with this technique. The dependence ofRgonMacross the distribution is compared with that predicted for a freely rotating chain, and with other similar polymers measured with hydrodynamic t

ISSN:0887-6266    

DOI:10.1002/polb.1992.090300206

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY

摘要:

AbstractThe total photoabsorption cross section for poly(methyl methacrylate) (PMMA) is measured experimentally, using synchrotron radiation, over the photon energy range 40

ISSN:0887-6266    

DOI:10.1002/polb.1992.090300207

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY

摘要:

AbstractDipole moments of poly(p‐chlorostyrene) were determined from dielectric and refractive index measurements performed on their dilute benzene, isopropyl benzene andn‐propyl benzene solutions over a wide range of temperatures. Experimental results are expressed in terms of the dimensionless dipole moment ratios,Dx= 〈μ2〉/xμ02. The temperature coefficients of dipole moments,dIn 〈μ2〉/dTwere calculated. The presence and importance of the specific solvent‐segment interactions are demonstrated by considering the temperature dependence of the dipole moment ratio in the vicinity of the theta temperature. These results are compared with dipole moments and temperature coefficients which were calculated by Bahar et al. using the appropriate rotational isomeric state model to study the local solvent effect for the poly (p‐chlorostyre

ISSN:0887-6266    

DOI:10.1002/polb.1992.090300208

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY

摘要:

AbstractThermally Activated fragmentation of copolycarbonates PC(TxA1‐x) of bisphenol A (unit CA) and the Heat‐sensitive diol 1,1,2,2‐tetraethyl‐1,2‐di‐(p‐hydroxy) phenylethane (unit CT) was studied in the bulk, i.e., in the pure copolymers and in their blends with the polycarbonates of bisphenol A (PCA) or tetramethyl bisphenol A (TMPC). Fragmentation proceeds via dissociation followed by disproportionation at the central CC bond of the unit CT. The reaction has rates that are convenient to study near the glass transition temperature. The “chemical” time constants τ for the entire reaction and τ2for the disproportionation step compete with the “physical” time constants δαfor segmental motion and δqfor fragment diffusion. A cage effect is observed below τ2= δαand effects of delayed matrix response below τ = δαand τ = δq. Owing to the two latter effects, parameters such as the glass transition temperature and the structure factor of concentration fluctuations do not respond primarily to the fragmentation, but rather to subsequent relaxation and diffu

ISSN:0887-6266    

DOI:10.1002/polb.1992.090300209

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY

摘要:

AbstractUniaxially oriented polymer networks are produced by photoinitiated bulk polymerization of liquid‐crystalline diacrylates. The order parametersS2andS4of the mesogenic moieties in these networks are simultaneously determined by means of the polarized Raman scattering technique. It yields an estimate of the orientation distribution function for different polymerization temperatures and for different lengths of the alkyl chain, present as a spacer in the liquid‐crystalline diacryl

ISSN:0887-6266    

DOI:10.1002/polb.1992.090300210

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY