摘要:

AbstractThe growth of (110) twinned crystals of a sharp fraction of linear polyethylene (Mw/Mn= 1.10) of moderate molecular weight (Mw= 17,000) is followed during crystallization by the isochronous decoration method. New morphological features are observed. The fast‐growing tip of our laths presents, in addition to the two (100) facets usually observed, a possibly stable small reentrant (110) corner. This is a situation intermediate between the facies described by Dawson and Keller. Moreover, the slow tip of our laths presents various degrees of asymmetry with respect to the junction plane. A new characteristic lengthLn=j/iis introduced to explain our morphological observations on (110) twinned crystals:jis the nucleation rate at a reentrant corner andithe nucleation rate on a smooth facet. Three linear growth ratesGhklare calculated as a function of the lengthLof the face (hkl):GhklandGĥklorG′hklare respectively the growth rates of a face bordered by two salient corners and by a reentrant corner. A distinction betweenGĥklandG′hklis introduced to take into account the relative sizes of the two faces of the reentrant dihedral angle. The major points of the discussion concern (i) the stability of the (110) reentrant corner of the fast tip of the lath, (ii) the nearly constant shape of the twinned crystals, (iii) the effects of dislocations incorporated in the fast edge of the laths, and (iv) the various asymmetries observed in the slow tip of our laths. Theories of surface nucleation‐controlled growth explain our various morphological observations on (110) twinned PE crystals, and growth usually proceeds in

ISSN:0887-6266    

DOI:10.1002/polb.1986.090240601

出版商:John Wiley&Sons, Inc.    

年代:1986

数据来源: WILEY

摘要:

AbstractThe application of29Si solution and solid‐state cross‐polarization/magic‐angle spinning (CP/MAS) nuclear magnetic resonance (NMR) techniques to the study of structural features in polydimethylsiloxane (PDMS) model endlinked elastomeric networks is explored. The relationship between the topological (network) functionality of a structural moiety, which determines network mechanical properties, and its chemical (spectral) functionality, which is reported by the NMR, is discussed. The second‐order spectral shifts corresponding to topographical functionality variation within a chemical functionality class are usually sufficiently well resolved in these networks to allow positive identification of a variety of structural features. The basic PDMS repeat unit, OSi(CH3)2, is found to possess an axially symmetric chemical shift tensor with σ∥= −56.8 ppm downfield from TMS, and σ⊥= −4.4 ppm. This axial symmetry does not result from rapid reorientation about the chain axis. The NMR spectrum reveals defects in model endlinked networks. In the case of vinyl‐endlinked systems, the defects are ascribed to the formation of elastically ineffective loops. Hydroxyl‐endlinked systems contain either loops or else trifunctional junctions (hydrolyzed before chain coupling could take place) and dangling chain ends. The CP/MAS technique provides an order‐of‐magnitude reduction over standard solution techniques in the time to acquire a spectrum from a network not containing paramagnetic doping.13C spectra of PDMS systems are not

ISSN:0887-6266    

DOI:10.1002/polb.1986.090240602

出版商:John Wiley&Sons, Inc.    

年代:1986

数据来源: WILEY

摘要:

AbstractThe crystalline transition induced by immersion in a methanol/water mixture of tussah silk fibroin (fromAntheraea pernyi) film obtained by casting from a 1% solution was studied by x‐ray diffraction, differential scanning calorimetry, and infrared spectroscopy. The molecular conformation of the fibroin, consisting mostly of the α‐helix and random‐coil forms, was transformed into a random‐coil and β‐rich conformation containing only a small amount of α‐helix after immersion for no more than 5 min. The intersheet packing of the β‐crystal of the original tussah silk fibroin film was imperfect in the early stage of immersion. However, crystallization proceeded further when the immersion time exceeded 10 min. As a result the sheets in the β‐form crystal became closely packed because of the decrease in the content of the random‐coil form. The exothermic peak, which appeared at 226°C in the original fibroin and was attributed to the random‐coil → β‐structure transition, disappeared comple

ISSN:0887-6266    

DOI:10.1002/polb.1986.090240603

出版商:John Wiley&Sons, Inc.    

年代:1986

数据来源: WILEY

摘要:

AbstractThe results of MNDO geometry optimizations on selected H(SiH2)nH polysilane model compounds are presented. Near energetic degeneracy is indicated for all‐trans(T), alternatinggauche–trans(GT), and all‐gauche(G+G+) models (n= 10). The most stable (T) and least stable (G+G+) conformations are separated by only ca. 0.11 eV. The existence of low‐energy barriers to moderate structural distortion is also suggested. Orbital localizations and charge density distributions along the “polymer” backbone are found to be sensitive functions of such distortion. The ground‐state electronic distribution of the saturated all‐transsilane chains are calculated to be considerably more polarizable than the fully conjugated H(CH)nH π‐electron framework of comparable length. The one‐electron HOMO → LUMO excitation can be viewed essentially as an in‐plane Si 3p → Si3s + H1s intramolecular charge transfer transition. The qualitatively different atomic orbital character of the HOMO and LUMO levels yields transition moment components for the separate repeat units which are relatively small. In the case of the rigidlytransconformation, the phase relationships of the transition moment terms are such as to constructively sum to a large net value reflecting strong optical absorpt

ISSN:0887-6266    

DOI:10.1002/polb.1986.090240604

出版商:John Wiley&Sons, Inc.    

年代:1986

数据来源: WILEY

摘要:

AbstractPositronium annihilation spectroscopy (PAS) has been used to study the microstructural properties of amine‐cured epoxy polymers. We have determined the free‐volume “hole” sizes in these polymers by comparing the observedortho‐positronium lifetimes with the known lifetime–free volume correlation for low‐molecular‐weight systems. The free volumes for four epoxies with different crosslink densities are found to vary significantly over the temperature range between −78° and 250°C. The free‐volume holes for these polymers are found to range from 0.025 to 0.220 nm3. Two important transition temperatures were found: one corresponds to the glass transition temperatureTgdetermined by differential scanning calorimetry (DSC), and the other occurs about 80–130°C belowTg. The sub‐Tgtransition temperature is interpreted tentatively as being where hole size reaches dimensions adequate for positronium trapping or else the onset temperature for local mode or side‐chain motions. These two transition temperatures plus two additional onset temperatures are found to be correlated with crosslink densities c

ISSN:0887-6266    

DOI:10.1002/polb.1986.090240605

出版商:John Wiley&Sons, Inc.    

年代:1986

数据来源: WILEY

摘要:

AbstractDielectric relaxation studies have been used as a means of investigating the behavior of aromatic molecules dissolved in a semicrystalline polymer. All molecules show a relaxation in the vicinity of the γ loss peak of polyethylene, while several show an additional peak at lower temperatures. The latter is interpreted as an indicator of molecular motions below the glass transition and the former is to be associated with the onset of molecular motions on a local scale as the glass transition is approached. The α loss process of polyethylene is seemingly unaffected by the presence of the molecules. Integration of loss curves shows that, at most, 50% of the molecules are able to relax but that the fraction relaxing increases with increasing temperature, suggesting the presence of a wide distribution of sites within the polyme

ISSN:0887-6266    

DOI:10.1002/polb.1986.090240606

出版商:John Wiley&Sons, Inc.    

年代:1986

数据来源: WILEY

摘要:

AbstractMechanical and dielectric β relaxations were investigated for bisphenol‐A‐type epoxide resin cured with succinic anhydrides with and without substituent linear or branched alkyl side chains. The dielectric β relaxation is due solely to the motion of the diester segment which is formed in the network from the reaction with the acid anhydride. The mechanical β relaxation is attributed to the motion of both the diester segment and another nonpolar segment, i.e., the linear alkyl side chain. Thus it is concluded that the long alkyl side chain in the diester segment not only restricts the motion of that segment, but also introduces additional motion of the side chain

ISSN:0887-6266    

DOI:10.1002/polb.1986.090240607

出版商:John Wiley&Sons, Inc.    

年代:1986

数据来源: WILEY

摘要:

AbstractQuantitative thin‐section electron microscope studies were made on compositional fractions of ethylene copolymers crystallized under several extreme conditions. The co‐units chosen were such that the chains contained either ethyl, hexyl, or acetate side groups. Certain general features are independent of the chemical nature of the co‐unit. It was found that up to about 2 mol % co‐unit content lamellae are observed in all copolymers which were slowly cooled from the melt. The lateral dimensions of lamellae are not as large as in corresponding homopolymers and the lamellae are segmented. As the branch content increases the lamellae become shorter and eventually degrade into small crystallites. Within this broad outline of behavior, differences between the copolymer types are discerned and discussed. By comparison with the Raman LAM it is shown that quantitative values for the crystallite size distribution are obtained by the electron microscopy method used. Furthermore, when the specimen consisted of well‐ordered lamellar stacks the degree of crystallinity obtained from the electron micrographs was in quantitative agreement with the enthalpy

ISSN:0887-6266    

DOI:10.1002/polb.1986.090240608

出版商:John Wiley&Sons, Inc.    

年代:1986

数据来源: WILEY

摘要:

AbstractA series of epoxy resins derived from diglycidyl ethers of bisphenol A with differing initial linear molecular chain lengths have been studied during and after curing with the diamines MDA (4,4′‐methylene dianiline) and DDS (4,4′‐diamino diphenyl sulfone). The properties that were measured during curing were the volume, the fictive temperatureTf, the gel fraction, the viscosity, and the equilibrium compliance. Graphs ofTfas a function the time of curingtcobtained at four curing temperatures between 40 and 100°C have been reduced to a common curve. After curing, creep compliance curvesJ(t) were determined which characterize the viscoelastic response from the glassy compliance level to a rubbery equilibrium compliance level. The change in properties that occurs during the time‐dependent spontaneous densification below the glass temperatureTgwas monitored with repeated measurements ofJ(t). Time‐scale shift factors as a function of volume contraction obtained during this physical aging belowTgwere reduced to a

ISSN:0887-6266    

DOI:10.1002/polb.1986.090240609

出版商:John Wiley&Sons, Inc.    

年代:1986

数据来源: WILEY

摘要:

AbstractThe kinetics of crystallization of quenched poly(ethylene terephthalate) (PET) films during the imbibition of methylene chloride (MeCl2) vapor is studied by density measurements. The effects of film thickness (0.0025–0.086 cm) and temperature (0–38°C) were examined. The data suggest that MeCl2transport controls the crystallization in thick films and at elevated temperatures, but that spherulite growth controls in thin films and at reduced temperatures. The application of a mathematical model developed previously supports this mechanistic interpretation of the

ISSN:0887-6266    

DOI:10.1002/polb.1986.090240610

出版商:John Wiley&Sons, Inc.    

年代:1986

数据来源: WILEY