摘要:

AbstractThe crystallization of highly oriented homopolymer melts (or glasses) is modeled. It is shown that in such cases heat flow controls the kinetics and microstructure of the transforming material. The situation is modeled similarly to the growth of a thermal dendrite, with the inclusion of large and variable concentrations of defects in the fibrillar crystals. Expressions relating the undercooling, growth velocity, filament tip radius, and defect concentration to a normalized tensile force are derived. Example predictions for the case of isotactic polystyrene are given.

ISSN:0098-1273    

DOI:10.1002/pol.1984.180220201

出版商:John Wiley&Sons, Inc.    

年代:1984

数据来源: WILEY

摘要:

AbstractSingle‐crystal x‐ray diffraction analysis is performed on a model compound (bisurethane of diphenylmethane‐4–monoisocyanate and butanediol‐1,4) for the thermoplastic polyurethane (TPU) hard segment formed from diphenylmethane‐4,4′‐diisocyanate and butanediol‐1,4. The resulting structure is compared to structure models of corresponding TPU hard segments, especially to the structure proposed by Blackwell and Ross. Our results confirm this structure model showing a planar zigzag of the (CH2)4group and planar hydrogen bonding between the urethane groups of adjacent molecules. X‐ray diffraction analyses of polymeric TPU hard segments and of TPU elastomers with noncrystallizing soft segments lead to a revision of the dimensions of the proposed lattice cell, resulting in a more plausible value of 1.3226g/cm3for

ISSN:0098-1273    

DOI:10.1002/pol.1984.180220202

出版商:John Wiley&Sons, Inc.    

年代:1984

数据来源: WILEY

摘要:

AbstractPhase diagrams of low‐molecular‐weight polyethylene (M= 1000, 2000, 6500, 16,000) and polyethylene (M ≈ 1,000) after fuming nitric acid treatment have been determined from 20 to 300°C up to pressures of about 10 kbar. The fractions with molecular weights 1000 and 2000 do not exhibit the hexagonal phase, but the others do. Effects of molecular weight and fuming nitric acid treatment on the phase diagrams are discussed in terms of the entropy of th

ISSN:0098-1273    

DOI:10.1002/pol.1984.180220203

出版商:John Wiley&Sons, Inc.    

年代:1984

数据来源: WILEY

摘要:

AbstractAttention is called to an early observation of Case II diffusion kinetics during penetration of a liquid into a uniaxially prestretched polymer film along the axis of stretching. Application of a previously developed transverse differential swelling stress model to these experimental conditions is described. It is shown that the model can predict Case II diffusion kinetics in a physically meaningful manner and will also explain the transition of the kinetics of the above system into normal or hindered diffusion regimes as either the degree of stretching or its direction relative to that of penetration is changed.

ISSN:0098-1273    

DOI:10.1002/pol.1984.180220204

出版商:John Wiley&Sons, Inc.    

年代:1984

数据来源: WILEY

摘要:

AbstractThe temperature, strain‐rate, and pressure dependences of the yield stress have been determined for two polyethylene homopolymers of differing molecular weights and for a polyethylene copolymer. Samples were prepared by slowly cooling from the melt, and also by quenching in order to assess the effects of morphology on the yield behavior. The data have been analyzed on the basis of two Eyring processes acting in parallel. Comparison of all data sets reveals the existence in general of three distinct activated processes each with its own temperature, strain‐rate, and pressure dependences. The relative contribution of each process is dependent on the molecular weight, morphology, etc. Also discussed is the relation of these three yield processes to the well‐known loss processes of linear viscoelast

ISSN:0098-1273    

DOI:10.1002/pol.1984.180220205

出版商:John Wiley&Sons, Inc.    

年代:1984

数据来源: WILEY

摘要:

AbstractThe effect of diluents and temperature on segmental motion in poly(vinyl acetate) was investigated by both NMR and ESR spectroscopy. Three classes of diluents were studied: chloroform, a thermodynamically good solvent; water, a poor solvent which slightly swells the polymer and lowers its glass transition temperature; and decane, a nonsolvent so poor it does not appear to swell the polymer nor lower the calorimetric glass transition temperature. At all temperatures investigated each type of diluent increased the segmental motion of the polymer over that of the bulk sample. Under the conditions studied,13C and2H NMR and nitroxide spin‐label ESR data gave similar views of segmental motion of the polymer, indicating that in this amorphous polymer the segmental motion of the polymer may be safely inferred from spin‐label d

ISSN:0098-1273    

DOI:10.1002/pol.1984.180220206

出版商:John Wiley&Sons, Inc.    

年代:1984

数据来源: WILEY

摘要:

AbstractPoly(tetramethyl‐p‐silphenylene siloxane) crystal mats initially prepared from benzene/methanol (2:1 v/v), when annealed in small amounts of solvent undergo considerable thickening in the chain direction. When the crystals are annealed above their formation temperature, their physical properties change rapidly at first before reaching an asymptotic limit commensurate with annealing time and type of solvent. Changes in melting temperature, heat of fusion, small‐angle x‐ray spacing, and wide‐angle x‐ray scattering patterns have been monitored for three solvents of varying solvent power, ranging from very good to extremely poor. Upon solution annealing, the original crystals mats equilibrate to more stable dimensions compatible with their environment. The activation energy of crystal thickening in contact with a liquid is estimated to be about an order of magnitude lower than that deduced from dry annealing data. It appears that the crystal surface and the crystalline core of the crystals comprising the mats must participate in the measured severalfold increase in long period noted after annealing. The lower surface (or interfacial) energy of the liquid annealed mats compared to isothermally melt‐crystallized polymer of similar molecular weight has a direct bearing on the polymer morphology and

ISSN:0098-1273    

DOI:10.1002/pol.1984.180220207

出版商:John Wiley&Sons, Inc.    

年代:1984

数据来源: WILEY

摘要:

AbstractPrevious interpretations of gas transport data in crosslinked networks have been hindered by an inability to accurately control and evaluate the network parameters. We have recently prepared a series of model networks by reacting poly(propylene glycol) with a triisocyanate crosslinking agent. The poly(propylene glycol)s had narrow molecular weight distributions and average molecular weights between 425 and 3000, so the resulting networks had uniform average molecular weights between crosslinks. Hydrogen and carbon monoxide permeabilities in membranes formed from these networks increase with decreasing crosslink density. These results indicate increased cooperative molecular motions in the networks with longer average chain lengths between crosslinks. Increasing the average molecular weight between crosslinks also reduces the discrimination between these two gases so that the separation factors decrease. For networks prepared from mixtures of poly(propylene glycol)s with different molecular weights the gas permeabilities (but not the separation factors) depend on the molecular weight distribution.

ISSN:0098-1273    

DOI:10.1002/pol.1984.180220208

出版商:John Wiley&Sons, Inc.    

年代:1984

数据来源: WILEY

摘要:

AbstractThe effects of poling temperature on piezoelectricity and its thermal stability were investigated on the basis of the thermal molecular motion associated with the crystalline region. This was done by using a film of highly oriented poly(vinylidene fluoride) containing form‐I crystals. The film was prepared by a zone‐drawing apparatus of the forced‐quenching type. The piezoelectric stress constante31is a monotonically increasing function of the poling temperature which becomes steeper above ca. 320 K and again at ca. 400 K. The degree of orientation of the crystalbaxis generated by poling also increases more steeply with poling temperature above ca. 320 K and again at 400 K. These temperatures correspond, respectively, to the crystalline dispersion temperature at 11 Hz, designated as αc, and the initiation temperatureTpmof large‐scale molecular motion corresponding to premelting of form‐I crystals. Thus the effect of poling temperature on piezoelectricity closely reflects the moleculer motion in form‐I crystals. The annealing temperatureT'aat whiche31decreases to 70% of that of unannealed sample by annealing a poled sample increases with the poling temperature and again this increase is steeper above poling temperatures of ca. 320 K and ca. 400 K. Thus the decay of piezoelectricity depends on both the αctempe

ISSN:0098-1273    

DOI:10.1002/pol.1984.180220209

出版商:John Wiley&Sons, Inc.    

年代:1984

数据来源: WILEY

摘要:

AbstractA new chlorination mechanism has been suggested for polyethylene crystals. Based on electron microsocopy and infrared spectroscopy of chlorinated high‐density polyethylene single crystals, it is proposed that beyond the initial chlorination of the lamellae surfaces, which accounts for only 2‐3% chlorine uptake, the reaction proceeds through the lamellar side faces from the periphery inward. Thus at intermediate degrees of chlorination two major phases coexist: the undamaged interior regions of the lamellae and the surrounding chlorinated material. This mechanism differs from that of oxidative degradation, and also from the generally Accepted mechanism of chlorination. Rather than proceeding through the reacted fold surface into the crystalline core, it is proposed that the direction of chemical attack is prependicular to that suggested previously. This mechanism probably describes any chemically nondestructive attack on the crystalline regions of polymers in gene

ISSN:0098-1273    

DOI:10.1002/pol.1984.180220210

出版商:John Wiley&Sons, Inc.    

年代:1984

数据来源: WILEY