摘要:

AbstractThe free‐volume theory for the temperature dependence of transport properties of glass‐forming polymers is extended to obtain their relationship to the extent of cure. This treatment centers on the unifying role of molecular mobility and yields a model which connects extent of reaction, viscosity, diffusivity, ionic conductivity and dipole relaxation time. The temporal dependence of these properties is expressed by coupling the extended free‐volume model with a relationship for the rate of cure, which included diffusional limitations. Analyses based on this model are applied to the observed behavior of a model epoxy‐amine resin system. The intrinsic kinetics of this model system are shown to be first order. It is shown that diffusional limitations strongly affected the progress of the reaction in the final stages of cure. The diffusion‐modified rate expression predictions agree with extent of reaction versus time data over the range of experimental temperatures. The temporal dependence of viscous behavior of the curing resin is measured. The extended free‐volume model accurately describes the evolution of resin viscosity during cure. The dielectric behavior is similarly characterized and is in close agreement with the predictions of the general free‐volume expression. The results of this study indicate that the free‐volume theory modified to account for molecular weight effects allows prediction of resin properties with a two‐parameter model. The results show that a power‐law relationship exists between viscosity and ionic conductivity. This result suggests that electrical properties may be used for on‐line measurement of resin

ISSN:0887-6266    

DOI:10.1002/polb.1990.090280701

出版商:John Wiley&Sons, Inc.    

年代:1990

数据来源: WILEY

摘要:

AbstractA general expression for the magnetization decay of a multipulse group is derived. This formula is applied to a three‐component model of molecular motions in polymer melts. The influence of the several components on the magnetization decay is discussed. The relation of the effective nuclear magnetic relaxation timeT2eto the Anderson‐Weiss formula is also shown, and an analytical expression for the transverse relaxation in melts is derived. FinallyT2eis compared with the relaxation time in the rotating frameT1ρin the melt. The theoretical results forT2eare tested with measurements of frequency dependence in polyethylene m

ISSN:0887-6266    

DOI:10.1002/polb.1990.090280702

出版商:John Wiley&Sons, Inc.    

年代:1990

数据来源: WILEY

摘要:

AbstractStatic and dynamic mechanical properties, morphology, and thermal behavior of polycarbonate (PC)/plasticized cellulose acetobutyrate (CAB) blends were investigated to determine whether the plasticizer of the CAB modifies the miscibility of the blend and the mechanical properties of this essentially incompatible blend. In spite of the lack of transparency of the blends, both dynamic mechanical and thermal analysis results show the presence, at all blend compositions studied, of a single glass transition temperature which varies with the composition of the blend. Considering the ternary nature of the blends, we propose that plasticizer migration and the difficulty of discerning the presence of one or two peaks in a narrow temperature range may account for the observed behavior. Scanning electron microscopy confirms the immiscibility of the blends. The blends show large positive deviations of the tensile moduli from linearity and very low ductility. The reported tensile strength data are discussed in terms of several different equations for composites. This mechanical behavior is explained as a consequence of the migration of the plasticizer and of its subsequent antiplasticizing effect on the properties of the blend.

ISSN:0887-6266    

DOI:10.1002/polb.1990.090280703

出版商:John Wiley&Sons, Inc.    

年代:1990

数据来源: WILEY

摘要:

AbstractAn attempt was made to measure the effects of molecular entanglements on the rheological properties of polymer melts. Two classes of polymers were studied; glassy atactic polystyrene polymers covering a 60‐fold range in molecular weight, and semicrystalline high‐density polyethylene from two sources covering about a twofold range in molecular weight. The entanglements initially present were removed or greatly reduced in number by freeze drying the polystyrene polymers from dilute solutions below and aboveC*, the critical overlap concentration, and by slowly crystallizing the polyethylene from very dilute solutions. Since only minor rheological changes were observed with polystyrene, it would appear that the initially isolated coils interpenetrate more rapidly than is indicated by the results of Liu and Morawetz, or that the rheological behavior is rather insensitive to whether the flow obstacles are intermolecular or intramolecular. The enhancement of the viscosity and elasticity observed with polyethylene polymers indicate the importance of the crystallization step on the local melt topology of the polymer cha

ISSN:0887-6266    

DOI:10.1002/polb.1990.090280704

出版商:John Wiley&Sons, Inc.    

年代:1990

数据来源: WILEY

摘要:

AbstractMesophase transitions in liquid‐crystalline (LC) polyesters were studied by dynamic x‐ray diffraction using a synchrotron radiation source. Powder and fiber samples were examined by continuous heating from 50°C to 270°C in a hot stage. The polymer systems consisted of two types of thermotropic polyesters with mesogenic cores composed of combinations of substituted terephthalate, oxybenzoate and hydroquinone units combined with aliphatic spacers placed in the main chain. One of these samples was a chemically homogeneous LC polyester (HTH12) while the other LC polyester possessed chemical heterogeneity (BP6). BP6 could also be processed to form fibers which showed thermal transition behavior by x‐ray diffraction and no detectable melting or clearing transition by thermal measurements. LC textures were observed using polarized light microscopy. Results of the dynamic x‐ray diffraction studies of these two LC polymers are

ISSN:0887-6266    

DOI:10.1002/polb.1990.090280705

出版商:John Wiley&Sons, Inc.    

年代:1990

数据来源: WILEY

摘要:

AbstractAn unexpected feature of melt flow behavior has been identified in high‐molecular weight (≥ 4.105) polyethylenes such as are being considered unprocessable at conventional temperatures (>160°C) and at practicable extrusion rates (>1 cm/min). In addition to a lower temperature window of smooth extrudability, lying in the range of 138–155°C already observed in previous works, we now discovered within this window a narrow temperature interval (150–152°C) of minimum flow resistance (viscosity). The new effect has the attributes of being associated with a new phase of increased fluidity. This, in turn, we attribute to a transient mesophase arising through the chain‐orienting effect of the elongational flow within the extrusion orifice; from the experiments presented here, this mesophase depends critically on both molecular weight and strain rate. The hexagonal form of polyethylene, known to exist under other circumstances, is suggested as this mesophase. The relevance of the new findings for applications (extrusion, melt rheology) and for fundamentals (orientation‐induced phase transformations, liquid crystals from flexible chains in particular) should be obvious and are according

ISSN:0887-6266    

DOI:10.1002/polb.1990.090280706

出版商:John Wiley&Sons, Inc.    

年代:1990

数据来源: WILEY

摘要:

AbstractThe influence of nematiclike interactions, that is, preference for the alignment of neighboring chain segments, is considered for a globally deformed polymer network. The local order is characterized by an orientation factor which is the average of second Legendre polynomial, being a function of segment orientations, calculated with the actual distribution of chain segment orientations. For the description of interactions between segments, a molecular mean‐field approach is used. The orientation‐dependent potential is assumed proportional to cos2θ, where θ is the angle between the direction of the orienting field and the chain segment vector. The local orientation factor of deformed polymer network, is calculated for various values of interaction strength and network structural parameters (contraction coefficent, length of network chains). For some values of these parameters, the local order can be conside

ISSN:0887-6266    

DOI:10.1002/polb.1990.090280707

出版商:John Wiley&Sons, Inc.    

年代:1990

数据来源: WILEY

摘要:

AbstractA series of measurements of integraln‐hexane vapor sorption at 25°C and moderate activity, in polystyrene microspheres of varying radii, exhibits typical non‐Fickian behavior: nonoverlap of curves of fractional uptake versus (square root of time)/radius. The data are examined in light of a sorption isotherm indicating hexane solubility in excess of that predicted by the Flory–Huggins equation, up to the hexane activity at which the glass transition apparently occurs. A transport analysis is developed based on the assumption that below the transition temperatureTgthe rate of sorption is limited by the rate of polymer chain relaxations induced by the penetrant, which facilitate hexane entry into, and immobilization in, glassy micr

ISSN:0887-6266    

DOI:10.1002/polb.1990.090280708

出版商:John Wiley&Sons, Inc.    

年代:1990

数据来源: WILEY

摘要:

AbstractThe crystalline structures of “microlayer” and “nanolayer” polyethylene have been examined in coextruded films comprised of alternating layers of high‐density polyethylene and polystyrene. Transmission electron microscopy (TEM), small‐angle x‐ray scattering (SAXS), and wide‐angle x‐ray scattering (WAXS) reveal that microlayer polyethylene, where the layer thickness is on the order of several microns, crystallizes with the normal unoriented lamellar morphology. In nanolayer films, where the film thickness of tens of nanometers is on the size scale of molecular dimensions, lamellae are oriented with the long axes perpendicular to the extrusion direction in a row‐nucleated morphology similar to structures described in the literature. The lamellae are partially twisted about the long axes. The preferred twist angles of ±40° orient the lamellar surfaces normal to the layer surface. The row‐nucleated morphology imparts highly anisotropic mechanical properties to the

ISSN:0887-6266    

DOI:10.1002/polb.1990.090280709

出版商:John Wiley&Sons, Inc.    

年代:1990

数据来源: WILEY

摘要:

AbstractThe surface of graphitized carbon fibers is oxidatively treated by nitric acid. Fourier transform infrared attenuated total reflection spectroscopy has been utilized to monitor the influence of the surface species on the curing of an epoxy system. A curve‐fitting program is used to deconvolve the overlapped infrared bands. The integrated intensities of the ester, anhydride, epoxy, and carboxylic acid, plotted against the curing time, clearly show the dependence of the crosslinking process on the concentration of the surface species. The surface groups themselves act as a catalyst: there exists an optimal concentration below which an activation of the curing process occurs. Above that concentration, a retardation of the curing process has been detecte

ISSN:0887-6266    

DOI:10.1002/polb.1990.090280710

出版商:John Wiley&Sons, Inc.    

年代:1990

数据来源: WILEY