摘要:

AbstractThe effect of crack tip sharpness on crack propagation in vulcanized rubbers has been studied. For very sharp cracks, tearing is found to occur on a small scale at very low energies not far above the threshold required for the onset of mechanical crack growth. The “small‐scale” tearing energies show relatively little variation for rubbers that differ widely in tear strength as normally measured. Thus the latter property appears to be strongly influenced by variations in the ability of rubbers to promote tip blunting. The small‐scale tear behavior is of relevance to other fracture phenomena, including cutting by sharp objects and tensile failure. Natural variations in tip sharpness occur during cyclic or time‐dependent mechanical crack growth and influence the form of the crack growth charac

ISSN:0887-6266    

DOI:10.1002/polb.1987.090250601

出版商:John Wiley&Sons, Inc.    

年代:1987

数据来源: WILEY

摘要:

AbstractTheoretical expressions for describing the weight‐average molecular weights of the soluble fractions from polymerizations obtained beyond the gel point were tested experimentally. The theory of branching processes and a recursive approach essentially based on the method of Macosko and Miller were found to be virtually equivalent. The soluble fractions produced from the stoichiometric polymerization of 1,3,5‐benzenetriacetic acid (BTA) with decamethylene glycol (DMG) gave molecular weights and distributions in excellent qualitative agreement with the theory. The results are interpreted in terms of intramolecular cyclization, diffusion, and the presence of microgel partic

ISSN:0887-6266    

DOI:10.1002/polb.1987.090250602

出版商:John Wiley&Sons, Inc.    

年代:1987

数据来源: WILEY

摘要:

AbstractThe effect of molecular weight distribution (MWD) (the low‐molecular weight “tail” in particular) on the deformation behavior of low‐density polyethylene (LDPE) during uniaxial stretching was studied over the temperature range from 293 K to the melting point. Four types of film samples were used: untreated polyethylene (PE), the same PE subjected either to solvent extraction or to thermal degradation, and PE with extracted fractions replaced by a low‐molecular weight paraffin analog. Graphs of elongation at break εbversus temperature always exhibited a minimum that occurred in the neighborhood of 370 K. The shape of the curve, the amount that εbdecreases with increasing temperature, as well as the absolute values of εbstrongly depend on the type of sample. The εbfor a degraded PE is markedly lower than that for an untreated one. On the other hand, the extraction of low‐molecular weight fractions leads to a higher εb, especially in the premelting region. The low‐molecular weight “tail” has a different effect from that of added paraffin. This appears to be due to the plasticizing effect of the latter. The effect of temperature is much more pronounced on the work of deformation (Ad) than on εb. The results obtained were interpreted on the assumption that structural (phase) inhomogeneity of LDPE is the key factor determining the deformation behavior of polymer over the st

ISSN:0887-6266    

DOI:10.1002/polb.1987.090250603

出版商:John Wiley&Sons, Inc.    

年代:1987

数据来源: WILEY

摘要:

AbstractAbout 40 epoxide‐amine network polymers with glass transition temperatures ranging from 0 to 232°C were investigated, about one‐third being reported for the first time. The glass transition temperatureTglof the corresponding linear copolymers was first calculated by use of an additivity law whose physical validity was carefully checked. Then the contribution of crosslink mers was determined, and various physical and empirical approaches of the effect of crosslinking onTgwere compared. The results gave evidence in favor of the DiMarzio approach. A predictive relationship based on these considerations is prop

ISSN:0887-6266    

DOI:10.1002/polb.1987.090250604

出版商:John Wiley&Sons, Inc.    

年代:1987

数据来源: WILEY

摘要:

AbstractThe dynamic mechanical behavior of monodisperse atactic polystyrene (mol. wt. 98,000) has been measured in the frequency range, 10−5to 10 Hz and temperature range 359–374K. The time‐temperature superposition of the entire data in the frequency range of overlap seems less satisfactory in both the real and imaginary components of the complex shear modulus,G′ andG″, respectively. The lack of adequate superposition becomes pronounced in the tan ϕ (G″/G′) plots. The tan ϕ plots provide a more discerning criteria for the superposition than theG′ orG″ spectra.An analysis based on an earlier model for anelastic deformation shows that of the several changes that may occur in the dynamic mechanical behavior on heating of polystyrene, the predominant ones are both an increase in the size of the microshear domains and the correlations of movement of segments near entanglements. These decrease the contribution to the modulus on heating nearTgso that the time‐temperature supe

ISSN:0887-6266    

DOI:10.1002/polb.1987.090250605

出版商:John Wiley&Sons, Inc.    

年代:1987

数据来源: WILEY

摘要:

AbstractMeasurements of both real and imaginary parts of the dielectric constant at a fixed microwave frequency have been performed onn‐butyl acrylate/poly(n‐butyl acrylate) mixtures. The dielectric constant of the mixtures has been compared with that of neatn‐butyl acrylate during UV photoinitiated polymerization. The values of the imaginary part of the dielectric constant thus obtained have allowed determination of the instantaneous monomer concentration and verified the kinetic equation for the photoinitiated polymerization process. The limits of validity of the method as well as its capability of providing detailed kinetic information are also disc

ISSN:0887-6266    

DOI:10.1002/polb.1987.090250606

出版商:John Wiley&Sons, Inc.    

年代:1987

数据来源: WILEY

摘要:

AbstractPermeability coefficientsPfor He, O2, N2, CO2CH4, C2H4, C2H6, and C3H8in 12 different silicone polymer membranes were determined at 35.0°C and pressures up to 9 atm. Values ofPfor CO2, CH4, and C3H8were also determined at 10.0 and 55.0°C. In addition, mean diffusion coefficientsDand solubility coefficientsSwere obtained for CO2, CH4, and C3H8in 6 silicone polymers at 10.0, 35.0, and 55.0°C. Substitution of increasingly bulkier functional groups in the side and backbone chains of silicone polymers results in a significant decrease inPfor a given penetrant gas. This is due mainly to a decrease inD, whereasSdecreases to a much lesser extent. Backbone substitutions appear to have a somewhat lesser effect in depressingPthan equivalent side‐chain substitutions. The selectivity of a silicone membrane for a gas A relative to a gas B, i.e., the permeability ratioP(A)/P(B), may increase or decrease as a result of such substitutions, but only if the substituted groups are sufficiently bulky. The selectivity of the more highly permeable silicone membranes is controlled by the ratioS(A)/S(B), whereas the selectivity of the less permeable membranes depends on both the ratiosD(A)/D(B) andS(A)/S(B). The permeability as well as the selectivity of one silicone membrane toward CO2were significantly enhanced by the substitution of a fluorine‐containing side group that increased the solubility of CO2in that p

ISSN:0887-6266    

DOI:10.1002/polb.1987.090250607

出版商:John Wiley&Sons, Inc.    

年代:1987

数据来源: WILEY

摘要:

AbstractThe rotational mobility of anionic spin probes in water‐containing nylon 6 film was investigated by means of electron spin resonance (ESR) measurements for comparison with the results for nonionic spin probes reported previously. The extrema separation of the ESR spectra, 2Az′ increased with time owing to the evaporation of water. In the higher temperature region, 2Az′ increased steeply with time at first, and then more slowly, whereas for the nonionic spin probes, 2Az′ increased gradually and monotonically with time. This fact suggests that the anionic probe molecules are more strongly affected by water than the nonionic ones, i.e., the former probes are located in hydrophilic regions and the latter in hydrophobic regions.T50G, which can be empirically correlated with the glass transition temperature of the polymerTgdecreased with increasing water content. The decreasing tendency for the anionic spin probes was stronger than that for the nonionic ones. This fact also indicates that the local environment around the probe molecules varies from probe to probe. The rotational correlation time τRdecreased markedly with an increase in water content. The Arrhenius plots of τRshowed two crossover points. The crossover points in the higher temperature regionTndecreased greatly with increasing water content. The difference inTnbetween dried and water‐containing films was larger than that forT50G. The activation energy for rotation,E aR, also decreased with increasing water content. It is suggested that water concentrates around the anionic spin probes and makes their rotatio

ISSN:0887-6266    

DOI:10.1002/polb.1987.090250608

出版商:John Wiley&Sons, Inc.    

年代:1987

数据来源: WILEY

摘要:

AbstractThe two‐dimensional cell model is defined by the following conditions of its formation: athermal nuclei may be randomly (Poisson) distributed in a thin amorphous film. From these nuclei spherulites may start to grow instantaneously, circularly, and at the same rate. Where two spherulites touch growth stops, and all points of contact form a straight intersection between neighboring spherulites. Finally, if the film contains spherulites only, each spherulite (cell) is completely limited by straight intersections, forming an irregular polygon. All irregular polygons of the film together form a planar irregular net of straight intersections. In quantitative microscopy this net is characterized by a “lineal analysis.” A straight line (Rosiwal's line) is arbitrarily put into the plane of the net. The net divides Rosiwal's line into chord intercepts of different lengths. The distribution of these lengths is analytically derived in the present paper by use of the theory of probab

ISSN:0887-6266    

DOI:10.1002/polb.1987.090250609

出版商:John Wiley&Sons, Inc.    

年代:1987

数据来源: WILEY

摘要:

AbstractStructural changes in poly(L‐alanine)(PLA), a model compound related to tussah silk fibroin, induced by heat treatment have been studied by differential thermal analysis, x‐ray diffraction, and infrared spectroscopy. PLA heated below 300°C shows x‐ray patterns very similar to those of the α‐helix crystalline phase, in addition to the diffraction patterns due to the β structure. Samples heated at 368°C exhibit predominantly the diffraction patterns due to the β crystalline phase. From infrared spectra, PLA samples heated below 280°C are found to be composed of all three molecular conformations: β sheet, random coil, and α helix. The intensity of the infrared absorption at 650 cm−1(amide V) assigned to the random coil conformation is decreased by heat treatment above 325°C. The content of the β‐sheet structure remains almost constant when the specimen is heated below 325°C, and increases abruptly on heating to around 335°C, while the random coil content is decreased abruptly by heat treatment above 335°C. The α‐helix content does not change, regardless of heat treatment. It is suggested therefore that the random coil conformation of PLA transforms directly into the β sheet o

ISSN:0887-6266    

DOI:10.1002/polb.1987.090250610

出版商:John Wiley&Sons, Inc.    

年代:1987

数据来源: WILEY