摘要:

Electron‐diffraction investigation of single crystals of &ggr;‐poly (vinylidene fluoride) grown from the melt has revealed streaking of 0k0 reflections. The three‐dimensional structure of the spikes was studied by specimen tilting during electron diffraction; streaking is maximized in theb*c* view and disappears in thea*b* view, indicating that the reciprocal‐lattice spikes are directed parallel toc. After exclusion of effects associated with the shape transform of our &ggr; crystals, the streaks are attributed to stacking faults parallel to theabplane; such faults are consistent with incorporation of &agr; sequences as defects within &ggr; chains during crystallization. Conformational changes between &agr; and &ggr; arrangements are examined using a number of models. The simplest motions (flip‐flops or rigid crankshafts) require simultaneous rotation about two noncolinear axes and are, therefore, less favored than mechanisms involving flexible chain motions as described in this report.

ISSN:0021-8979    

DOI:10.1063/1.328522

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

X‐ray studies of unpoled and poled &ggr;‐form Nylon 11 films indicate that the polarization mechanism is not due to a field‐induced crystal transformation from the &ggr; to the &agr; form, suggesting the existence of a polar &ggr; form. The temperature dependence ofd31shows no decrease in polarization at the &agr;‐&ggr; transition temperature (∼95 °C), indicating that the hydrogen bonds in the poled film are not able to randomize. These results suggest the possibility that our understanding of the &ggr;‐form structure of Nylon 11 is incomplete.

ISSN:0021-8979    

DOI:10.1063/1.328523

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

We use scaling arguments to construct the ’’phase diagram’’ of an athermal bimodal solution; i.e., a ternary system composed of long chains, short chains, and solvent. Qualitatively, the presence of short chains reduces the effective solvent quality as experienced by the long chains. We obtain asymptotic forms for the radii of gyration of both chains throughout the phase diagram.

ISSN:0021-8979    

DOI:10.1063/1.328524

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

The intra‐ and inter‐chain structural analyses of proposed poly (acetylene) (PA) valence geometries were performed. The semi‐empirical molecular methods CNDO/2, MINDO/3, and NDDO were used in the intramolecular conformational analysis. A molecular‐mechanics based lattice packing formalism was used to compute the interchain energies. The structures proposed by Baughman and coworkers are predicted to be most stable in observed orthorhombic lattices. The all‐trans chain packs with the lowest energy and the trans‐cisoid structure is more stable than the cis‐transoid. However, a monoclinic set of lattices, not yet observed, are predicted to be about 50% lower in energy than the corresponding orthorhombic lattices for all three chain conformations.

ISSN:0021-8979    

DOI:10.1063/1.328525

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

A theory of crack healing in polymers is presented in terms of the stages of crack healing, namely, (a) surface rearrangement, (b) surface approach, (c) wetting, (d) diffusion, and (e) randomization. The recovery ratioRof mechanical properties with time was determined as a convolution product,R=Rh(t)*&fgr;(t), whereRh(t) is an intrinsic healing function, and &fgr;(t) is a wetting distribution function for the crack interface or plane in the material. The reptation model of a chain in a tube was used to describe self‐diffusion of interpenetrating random coil chains which formed a basis forRh(t). Applications of the theory are described, including crack healing in amorphous polymers and melt processing of polymer resins by injection or compression molding. Relations are developed for fracture stress &sgr;, strain &egr;, and energyEas a function of timet, temperatureT, pressureP, and molecular weightM. Results include (i) during healing or processing att<t∞, &sgr;,&egr;∼t1/4,E∼t1/2; (ii) at constantt<t∞, &sgr;,&egr;∼M−1/4,E∼M−1/2; (iii) in the fully interpenetrated healed state att=t∞, &sgr;,&egr;∼M1/2,E∼M; (iv) the time to achieve complete healing,t∞∼M3, ∼exp P, ∼exp 1/T. Chain fracture, creep, and stress relaxation are also discussed. New concepts for strength predictions are introduced.

ISSN:0021-8979    

DOI:10.1063/1.328526

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

The effect of temperature on vibrational frequency shifts in mechanically stressed polymer chains is analyzed. A single chain is modeled as a weakly coupled, one‐dimensional series of anharmonic diatomic oscillators. The anharmonic potential is described as a perturbation of the harmonic potential with cubic and quartic terms. Expressions are derived for the dominant, quasiharmonic contribution to the temperature‐dependent frequency shifting coefficient &agr;c, and distinctions are drawn between the vibrational mode ’’mixing contribution’’ and the ’’anharmonic contribution.’’ &agr;cis derived in terms of the Gru¨neisen parameter; and the resulting expression is −&agr;c= &agr;0+&bgr;0T, where &agr;0is the extrapolatedT= 0 °K value of the frequency shifting coefficient, and &bgr;0is constant. The predicted linear dependence of &agr;con temperature was found to be in excellent qualitative agreement with experimental studies on stress‐induced infrared frequency shifts versusTin polypropylene.

ISSN:0021-8979    

DOI:10.1063/1.328527

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

The fatigue endurance of unnotched samples of a rubber modified, high impact polystyrene (HIPS) was determined under tension‐compression cycling. The role of the rubber phase inclusions in the fatigue fracture process was further studied by detailed examination of the fracture surfaces by optical methods and by scanning electron microscopy. The observed morphology is compared with that obtained on HIPS samples subject to simple tension bonding and with that obtained on the unmodified polystyrene homopolymer. The dispersed rubber phase particles lead to an increased fatigue crack propagation resistance and, if comparison is made on the basis of the same relative stress, the rubber modified polymer leads also to increased fatigue lifetimes. The influence of various environmental variables such as elevated temperature annealing, surface condition, and surface treatments is discussed. Silicon oil coatings of low viscosity tend to significantly reduce fatigue lifetimes, but lifetimes increase with increased viscosity of oil coating. Significant enhancement in fatigue endurance can be realized by use of elastomeric coatings, such as a thin layer of nitrile rubber.

ISSN:0021-8979    

DOI:10.1063/1.328528

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

The glass transition pressurePgfor a polyurethane elastomer (Solithane 113, 50/50 resin‐catalist ratio, manufactured by Thiokol Chemical Co.) is located at 2.5 kbar at room temperature and the glass transition temperatureTgis at −20 °C. Mechanical behavior of the elastomer, namely the tensile and the compressive stress‐strain behavior, in the glassy state as well as in the rubbery state has been determined. The Young’s modulus increases from ∼107dyn/cm2in the rubbery state to ∼1010dyn/cm2in the glassy state. The tensile fracture strain increases rapidly from 60% at atmospheric pressure to greater than ∼200% at 1 kbar and higher. In the glassy state, the samples exhibit yielding, yield drop, and cold drawing. The yield drop is not accompanied by necking. Rather the samples undergo uniform drawing throughout the entire gage length. A series of sequential loading, unloading, and reloading tests in the plastic range was also conducted in the glassy state. It was observed that the plastic strain recovers as a function of time, that the yield maximum reappeared and grew after a delay time, and that the Young’s modulus in subsequent loadings was higher than the initial values and increased steadily with time. Various loading histories can be completely erased upon returning to a rubbery state by removal of applied pressure. The recovery of the plastic deformation, or the viscoplastic behavior, occurs at essentially the same rate at all pressures tested, and thus the data were superimposable to form a master curve nearPg. A molecular explanation for the various new phenomena observed is given.

ISSN:0021-8979    

DOI:10.1063/1.328529

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

Recent investigations have shown that unoriented phase I poly(vinylidene fluoride) films can be produced by pressure quenching. The poling field dependence of the pyroelectric coefficientPy, the piezoelectric coefficientsdp,d31ande31, and the dielectric response is investigated for poling fields up toEp= 2.75×106V/cm. These quantities exhibit a plateau region where the initial rapid increase with increasingEpdecreases. This behavior is similar to that observed for unoriented and biaxially oriented phase II poly(vinylidene fluoride) films. The effect of draw ratio ondpandPyfor draw ratios up to 6:1 is also examined.dpincreases from 6.5 to 19 pC/N as the draw ratio goes from 1:1 to 6:1 (at constantEp).Pyincreases from 1×10−9to 5×10−9C/cm2 K.

ISSN:0021-8979    

DOI:10.1063/1.329817

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

FM‐9 (the commercial name of an ICI product) in organic solvents or poly(methacrylic acid) (PMMA) in aqueous solution exhibit strong time‐dependent shear‐thickening behavior. The induction time needed for the onset of the thickening depends markedly on shear rate, solvent used, concentration, and molecular weight. Slightly altering the nature of the solvent profoundly affects the behavior. The effects are similar to those obtained by changing the concentration or molecular weight. The nature of thickening behavior is not well understood. The effect will be illustrated (using FM‐9 solutions) as a function of shear rate, concentration, and temperature. Defining a critical shear rate &ggr;˙cas that at which the induction time for thickening becomes so short as to be immeasurable within the resolution of the viscometer, &ggr;˙cfor this polymer is found to be inversely proportional to the zero shear‐rate viscosity of the solution for the temperature and concentrations studied.

ISSN:0021-8979    

DOI:10.1063/1.329818

出版商:AIP    

年代:1981

数据来源: AIP