摘要:

AbstractNormally ductile polymers may exhibit unexpected brittle fractures because of a phenomenon termed surface embrittlement. The problem may arise whenever a thin brittle layer is present on the surface, which may result from the application of brittle paint or, more commonly, from surface degradation caused by exposure to elevated temperatures, ultra‐violet radiation, and stress‐cracking agents. In rubber‐modified polymers such as acrylonitrile‐butadiene‐styrene and high‐impact polystyrene, exposure to the outdoor environment inevitably results in the formation of a thin surface layer containing cross‐linked rubber particles in a matrix of reduced molecular weight. Although the depth of material adversely affected is typically small compared to the bulk, a drastic reduction in ductility nonetheless has been observed. To better understand the mechanism of embrittlement, this paper examines the criterion for embrittlement by considering the mechanism for enhanced energy absorption of rubber‐toughened plastics and elementary concepts of fracture mechanics. It has been shown that a critical coating thickness exists when multiple crazing is essentially inoperative and the deformation is localized, to the tip of a fast moving sharp crack restricting strain energy dissipation to a relative

ISSN:0032-3888    

DOI:10.1002/pen.760261702

出版商:Society of Plastics Engineers    

年代:1986

数据来源: WILEY

摘要:

AbstractStudies with phenylalanine diffusing in well‐characterized poly(2‐hydroxyethyl methacrylate) (PHEMA) membranes swollen in water at 37°C were conducted. Films were prepared by reaction of HEMA monomer with the cross‐linking agent ethylene glycol dimethacrylate (EGDMA) at cross‐linking ratios, X, of 0.005, 0.01, 0.0128, 0.025, and 0.050 mole EGDMA/mole HEMA in the presence of 40 weight percent water at 60°C for 12 hours. These membranes were subsequently swollen in water at 37°C and their structure analyzed using a modified Gaussian distribution equation of swelling. The calculated values of M̄cvaried between 1,700 and 3,425 daltons, which corresponded to a correlation length of the mesh size, egr, of 24 to 35 A. The phenylalanine solute diffusion coefficient varied from 0.17 × 10−6to 0.97 × 10−6cm2/s, and depended on the aforementioned structural parameters

ISSN:0032-3888    

DOI:10.1002/pen.760261703

出版商:Society of Plastics Engineers    

年代:1986

数据来源: WILEY

摘要:

AbstractBisphenolic resins have been prepared by alkali catalyzed condensation of bisphenol‐F, bisphenol‐A, and bisphenol‐C, respectively, with furfural. The resin samples have been characterized by elemental analysis, dilute solution viscosity measurement, and measurement of number‐average molecular weight. Curing reactions of these bisphenolic resins with hexamine have been investigated by differential scanning calorimetry. The overall apparent activation energies for the curing have been found to be 24 ± 2, 34.2 ± 1.9, and 36.8 ± 2.3 kcal/mole for bisphenol‐F‐furfural, bisphenol‐A‐furfural, and bisphenol‐C‐furfural resin, respectively. The curing reactions have been found to follow an overal

ISSN:0032-3888    

DOI:10.1002/pen.760261704

出版商:Society of Plastics Engineers    

年代:1986

数据来源: WILEY

摘要:

AbstractThe development of silicon carbide‐silicon nitride (SiC‐Si3N4) fibers by the pyrolysis of polycarbosilazane precursors that was carried out in this laboratory is reviewed. Precursor resin, which was prepared by heating tris(N‐methylamino)methylsilane or tris(N‐methylamino)phenyl‐silane to about 520°C, was drawn into fibers from the melt and then made unmeltable by humidity conditioning at 100°C and 95 percent relative humidity. The humidity‐treated precursor fibers were pyrolyzed to ceramic fibers with good mechanical properties and electrical resistivity. For example, SiCSi3N4fibers derived from tris(N‐methyl‐amino)‐methylsilane had a tensile rupture modulus of 29 × 106psi and electrical resistivity of 6.9 × 108Ω‐cm, which is 1012times greater than a value ob

ISSN:0032-3888    

DOI:10.1002/pen.760261705

出版商:Society of Plastics Engineers    

年代:1986

数据来源: WILEY

摘要:

AbstractLow density polyethylene was chlorinated via a mixed solution‐suspension technique at three different temperatures. The products obtained at different conditions had properties ranging from hard brittle to tough elastomeric consistencies. Static mechanical, chemical, and thermal characterization of the products were made and the possibility of two different crystal structures is pointed out, one of which is consumed preferentially during chlorinatio

ISSN:0032-3888    

DOI:10.1002/pen.760261706

出版商:Society of Plastics Engineers    

年代:1986

数据来源: WILEY

摘要:

AbstractThe presence of a weldline generally reduces the mechanical strength of injection molded parts. A typical remedy to eliminate the problem of weak weldline structure has been to increase the melt temperature. This, however, is not an acceptable solution in some situations. A general solution to the weak weldline problem requires an in‐depth understanding of the thermomechanical history of the injection molding process. A theoretical model for the strength of weldlines is presented that provides a comprehensive physical insight of the bonding process at the weldline interface. The model is based on the self‐diffusion of molecular chains across the polymer‐polymer interface and the frozen‐in orientation that remains parallel to the interface. Both factors are analyzed separately and then superimposed to predict the strength of weldlines from known processing conditions and geometry. Experimental results show good correlation with pred

ISSN:0032-3888    

DOI:10.1002/pen.760261707

出版商:Society of Plastics Engineers    

年代:1986

数据来源: WILEY

摘要:

AbstractThis paper reports studies on polystyrene/poly‐(vinylmethylether) blends to elucidate phase decomposition phenomena. Small angle neutron scattering (SANS) and temperature jump light scattering (TJLS) techniques were used to study both static and kinetic properties. The SANS methods were used to characterize the blends with respect to phase diagram, the Flory‐Huggins interaction parameter, and the critical fluctuation length as a function of temperature. From the TJLS experiments, we have demonstrated the possibility of two kinds of phase decomposition mechanisms which should lead to distinct morphology and material propert

ISSN:0032-3888    

DOI:10.1002/pen.760261708

出版商:Society of Plastics Engineers    

年代:1986

数据来源: WILEY

摘要:

AbstractA simple andin‐situbending‐beam technique has been used to investigate the stress‐temperature relationships for three different polymer coatings (viz, poly(methyl methacrylate), FR‐4 epoxy resin, and an amide‐imide polymer) during thermal cycling. With this technique, we were able to detect stress relaxation near the polymer's glass transition or caused by cracking of the polymer. Knowing the Poisson's ratio and Young's modulus for the polymer, this technique also allows calculation of the polymer's thermal expansion coefficient from measured thermal stress data. The calculated thermal expansion coefficient of poly(methyl methacrylate) (7 × 10−5/°C) is in good agreement with literature values (5 to

ISSN:0032-3888    

DOI:10.1002/pen.760261709

出版商:Society of Plastics Engineers    

年代:1986

数据来源: WILEY

摘要:

AbstractAt 35°C miscible blends of poly(vinylidene fluoride) (PVF2), and poly(methyl methacrylate) (PMMA), may be semicrystalline or wholly amorphous and rubbery or glassy depending on composition and prior history. Ethanol is rather soluble in PMMA and plasticizes it, while sorption into PVF2is rather small. The equilibrium sorption of ethanol into these blends decreases in a well defined manner as the PVF2content increases and is only slightly affected by prior history of the sample. On the other hand, the kinetics of ethanol sorption varies in a complex manner with composition, ranging from Case II kinetics for glassy blends to Fickian diffusion for rubbery ones, and is a strong function of prior history. Quantitative descriptions are given and discussed in terms of the various mechanistic processes known to occur

ISSN:0032-3888    

DOI:10.1002/pen.760261710

出版商:Society of Plastics Engineers    

年代:1986

数据来源: WILEY

ISSN:0032-3888    

DOI:10.1002/pen.760261701

出版商:Society of Plastics Engineers    

年代:1986

数据来源: WILEY