摘要:

AbstractStudies of the thermal and photodegradation of two commercial polymers, stabilized and unstabilized low‐density polyethylene, show the effects of weathering on differential scanning calorimetry (DSC) curves. The shape and the size of the melting peak vary significantly in thermal aging and in accelerated photoirradiation, but no change was observed in natural weathering, implying that the crystallinity remains steady in outdoor exposures. The melting temperatures remain steady for all exposure tests. The fall of mechanical properties and the evolution of chemical structure, followed by IR spectroscopy results, especially near the carbonyl regions, were also observed. The resistance to UV light irradiation as probed by deformation at break was superior in stabilized LDPE compared to unstabilized LDPE. Simple correlations observed between the fall of mechanical properties, the rate of oxidation, and morphology. © 1993 John Wiley&Sons, I

ISSN:0021-8995    

DOI:10.1002/app.1993.070471101

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1993

数据来源: WILEY

摘要:

AbstractThe aim of this paper was to study the plasticizing effect of subjecting native/commercial gutta‐percha samples to controlled levels of different humidities. It was expected that this study would provide the practicing endodontist with invaluable insight into the mechanical properties of commercial gutta‐percha cones (i.e., retrievability, malleability, flexibility, and torsional and bending ability). The “conditioning” of the gutta‐percha samples depended greatly on the percentage of compatible/incompatible substances. The mixtures of native gutta‐percha with compatible (i.e., wax) showed greater elongation, tensile strength, and number of rotations, but lower deformation angle than the mixtures of native gutta‐percha with incompatible substances (i.e., salts, metal oxides). The observed lower values for tensile strength and torsional strain, relative energy to break, and the Young's modulus and the higher values for elongation and number of rotations for wet samples compared to the dry ones were attributed to the plasticizing effect of gutta‐percha due to the insertion of water molecules in the polymer chains. © 1993 John

ISSN:0021-8995    

DOI:10.1002/app.1993.070471102

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1993

数据来源: WILEY

摘要:

AbstractA novel class of cyano‐substituted maleimides were prepared and polymerized. Particularly, 4‐aminobenzonitrile reacted with maleic or nadic anhydride to yield the amic acid, which was then cyclodehydrated to the corresponding maleimide or nadimide. In addition, the monomaleamic acids prepared from an aromatic diamine reacted with 4‐cyanobenzaldehyde to afford maleamic acids that were cyclodehydrated to maleimides. All maleimides and nadimide as well as their intermediate amic acids were characterized by IR and1H‐NMR spectroscopy. The curing behavior of polymer precursors was investigated by DTA. Upon curing at 240°C for 15 h, network polymers were obtained both by cross‐linking through the maleimido olefinic bonds and the trimerization reaction of the cyano groups. They were stable up to 330–310°C in N2or air and afforded anaerobic char yield of 60–40% at 800°C. © 1993 Joh

ISSN:0021-8995    

DOI:10.1002/app.1993.070471103

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1993

数据来源: WILEY

摘要:

AbstractComposite polymers of polystyrene and gelatin have been prepared using a gel pathway. The gels have been obtained by mixing at 50°C an aqueous solution containing gelatin and a surfactant (SDS) with styrene containing an initiator (AIBN). The obtained emulsions have the appearance of gels and are very stable both at room temperature and at 50°C. At 50°C, these gels are liquidlike with high viscosity; at room temperature, solidlike gels are formed when the gelatin content is sufficiently high. Polymerization of the gels can be achieved in several days at room temperature and in 24 h at 50°C. Composite polymers with different water‐absorbing capacities have been produced by controlling the polymerization temperature and duration. These composite polymers are insoluble in water, ethanol, octane, and cyclohexane, but exhibit some swelling in these liquids. Additional thermal treatment at relatively high temperatures increases the mechanical strength of the composites. © 1993 John Wiley&Son

ISSN:0021-8995    

DOI:10.1002/app.1993.070471104

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1993

数据来源: WILEY

摘要:

AbstractThe permeability coefficient of a wide range (0/100–100/0 octanelactam/laurolactam [OL/LL] mol/mol) of novel copolyamides (nylon 8/nylon 12) was determined and related to the diffusion rate over a range of temperatures:Tg–30 toTg+ 40°C. An inflection in the plot of permeability vs. 1/T(K) provided an indirect indication of glass transition value and agreed satisfactory with DSC and DMTA measurements. Both uniaxially drawn and undrawn membranes were evaluated and the permeability values were correlated with the % crystallinity. The reduction in solubility, diffusivity, and permeability with increasing draw ratio was attributed to a substantial increase in % crystallinity. Finally, the effect of absorbed water on gas permeability of polyamide films was examined. In most homopolyamides and copolyamides, the higher the water absorption of films the higher the permeability values. © 1993 John Wiley&Sons

ISSN:0021-8995    

DOI:10.1002/app.1993.070471105

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1993

数据来源: WILEY

摘要:

AbstractTwo hexafluoro‐substituted aromatic polyimides, 6FDA‐p‐PDA and 6FDA‐4,4′‐ODA, were synthesized. The influence of some synthetic conditions on physical properties and gas permeabilities of the 6FDA polyimide membranes were studied. Gas permeabilities and permselectivities of six different gases for the homogeneous films have been determined at 15–35°C and at pressures up to 10 atm. 6FDA polyimide composite membranes with asymmetric structure were prepared by the solution‐casting method. The membranes showed exceptionally high permeation fluxes and permselectivities. No capability deterioration of asymmetric membrane was observed during 19 days operation with pure gases. © 1993 Joh

ISSN:0021-8995    

DOI:10.1002/app.1993.070471106

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1993

数据来源: WILEY

摘要:

AbstractVarious α‐keratin fibers that had been treated with an 11MLiBr solution containingN‐ethylmaleimide showed typical rubberlike elasticity in a solution composed of equal volumes of 8MLiBr and diethylene glycol mono‐n‐butyl ether. Stress—strain relations of the swollen fibers were treated with a two‐phase model: a mechanically stable phase of higher cross‐linked domains and a rubber phase with lower cross‐link density. Stress—strain curves for a variety of keratins (three different human hairs, six different wools, mohair, cashmere, llama, alpaca, angora, and opossum) were analyzed by applying non‐Gaussian chain statistics to the swollen keratin network, including microdomains, which act as reinforcing filler particles in rubber. The phase structures of unswollen domains and swollen rubber were considered to originate from different structural components characteristic of α‐keratin, namely, the high‐sulfur matrix and the low‐sulfur microfibrils being randomized by swelling. It has been suggested that (1) the modulus of swollen fibers increases with increase of the content of disulfide (SS) in keratins, (2) the volume fraction of high‐sulfur domains increases with increase of SS content, and (3) the number of intermolecular cross‐links in the rubber region of low‐sulfur proteins is virtually the same among keratins and reaches about 65–75% of the SS linkages in the corresponding proteins. Some discussion has been made on the SS bondingin situ, namely, SS linkages between the low‐sulfur proteins, between the low‐sulfur and the high‐sulfur proteins, and between the high‐sulfur protei

ISSN:0021-8995    

DOI:10.1002/app.1993.070471107

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1993

数据来源: WILEY

摘要:

AbstractA novel artificial extracellular matrix derivatized with a cell adhesive peptide, Arg—Gly—Asp—Ser (RGDS) tetrapeptide, which is the central peptidyl sequence of the adhesive site of fibronectin, was designed. RGDS coupling was achieved via isocyanation of surface hydroxyl groups of poly(vinyl alcohol) (PVA) film and subsequent conversion to activated ester. The surface‐modified PVA film was quantitatively analyzed by ESCA. The surface density of RGDS was partly controlled by the degree of isocyanation of the PVA film. Bovine endothelial cells (ECs) adhered and grew well on the RGDS‐derivatized PVA film, irrespective of the presence or absence of the serum. The adhesion and growth of ECs were enhanced with an increase in the surface density of RGDS. When a sufficient amount of RGDS was added to the medium, the adhered ECs were delaminated. This indicated that the adhesion of ECs on an RGDS‐derivatized PVA surface is mediated by the RGD—ligand/receptor interaction. Thus, a peptidyl artificial matrix via surface derivatization was developed. © 1993 John W

ISSN:0021-8995    

DOI:10.1002/app.1993.070471108

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1993

数据来源: WILEY

摘要:

AbstractWe have studied development of the in‐plane distribution of “amorphous orientation” during sequential and simultaneous biaxial drawing of poly(ethylene terephthalate) film, using polarized intrinsic fluorescence. The machine direction (MD) draw ratio was always fixed at 3.5, and the transverse direction (TD) draw ratio was varied. The rate of increase in the proportion of TD‐oriented chains with increasing TD draw ratio is almost identical in the sequential and simultaneous processes up to a draw ratio of 2.7. At this point, sequential drawing starts to involve transverse realignment of MD‐oriented chains, which accelerates redistribution of orientation from the MD to the TD. Consequently, in sequential drawing, a “balanced” biaxial orientation distribution is achieved at a TD draw ratio significantly below the MD draw ratio, whereas at the same TD draw ratio in the simultaneous process, MD orientation remains dominant. At equal MD and TD draw ratios, the non‐crystalline chains in sequentially drawn film are predominantly oriented along the TD, but their orientation distribution is isotropic in simultaneously drawn film. High‐temperature annealing at fixed dimensions diminishes the proportion of TD‐oriented chains in films with transverse draw ratios<2.5. We attribute this to a more highly developed crystallite network in the MD, which constrains orientational relaxation along the MD. A balanced distribution of amorphous orientation is directly responsible for achieving balanced tensile strength and balanced extensibility. © 1993

ISSN:0021-8995    

DOI:10.1002/app.1993.070471109

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1993

数据来源: WILEY

摘要:

AbstractThe sedimentation behavior of alumina powder has been studied in the presence of poly‐vinylpyrrolidone (PVP) and poly(vinylpyrrolidone‐co‐vinyl acetate) (PVP/VA) in both thermodynamically “good” and “poor” solvents for the PVP homopolymer. PVP/VA provides higher sediment densities than does its PVP homopolymer counterpart. Solutionstate13C‐T1spin‐lattice relaxation measurements were made on analogous mixtures both with and without alumina powder. The NMR results suggest that the PVP/VA copolymer is anchored to the alumina powder surface by means of VA moieties, whereas the PVP moieties extend into the continuous phase of the slurry medium. Thus, the higher settling densities that are observed in the presence of PVP/VA can be attributed to a steric stabilization mechanism. © 1993 Jo

ISSN:0021-8995    

DOI:10.1002/app.1993.070471110

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1993

数据来源: WILEY