摘要:

N-Methylmorpholine-N-oxide (NMMO) technology provides new possibilities for shaping fibers and films from cellulose. We discuss a blow-extrusion technique can be applied to a cellulose-amine oxide dope, yielding blown cellulosie films. Process parameters, such as the draw down ratio and the blow ratio, can be used to control the mechanical film properties in both the machine and transverse directions. In this way, a wide range of properties is covered, and a completely balanced film can be manufactured. The film thickness can be made as small as 5 μm, and the membrane properties can be varied by the conditions of precipitation. The blown cellulosic films exhibit a partially crystalline supermolecular structure and, depending on the coagulation conditions, a symmetrical or asymmetrical homogeneous morphology and pore structure. Generally, a uniplanar orientation type was found, the chains being parallel to the film surface. Around the surface normal, the chain orientation can be varied from nearly random to strongly uniaxial, in this way determining the mechanical properties in the machine and transverse directions. As compared with the conventional viscose processing route of cellophane, the blown film's NMMO processing route is less complicated and friendlier to the environment. Blown films can be made much thinner, and the mechanical properties are superior to viscose films. Possible application fields of blown cellulose films are food casings, particularly small sausage casings, packaging, and membranes.

ISSN:0022-2348    

DOI:10.1080/00222349908248124

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

The structure of the polymers polypropylene (PP), polyethylene (PE), and poly-(oxymethylene) (POM) and the blends PE-PP and PE-POM containing carbon black (CB) were studied. It was found that spatial distribution of CB depends on the interface interactions between the components of composites. It is possible to obtain three cases of filler spatial distribution: Filler can be distributed randomly within the polymer matrix, can be contained in one of the polymer components, or can be localized on the polymer-polymer boundary. The conditions of various filler distribution in the heterogeneous polymer matrix are given. The correlation between morphology of the composites and their percolation conductivity was found.

ISSN:0022-2348    

DOI:10.1080/00222349908248125

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

In applications like space satellites, high-energy physics experiments, and nuclear power stations, epoxy structural adhesives are normally used in an ionizing radiation environment. To check the effects of γ-irradiation on room temperature epoxy adhesives, mechanical measurements were undertaken for three different resins up to the dose of 3 MGy. Both dumbbell and single-lap shear tests were performed. To correlate the measured radiation effects on these mechanical properties with the molecular modifications of the resins, outgassing and calorimetric tests were performed on one of the tested adhesives. As a result of these tests, the mechanical modifications were associated with the combination of reticulation, network scission, and production of low-weight molecules due to radiation. Differences in shear and tensile strength behaviors were associated with the presence of the adhesive-adherend interface.

ISSN:0022-2348    

DOI:10.1080/00222349908248126

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

Fracture mechanical terms related to the crack initiation and propagation, respectively, were assessed by the essential work of fracture (EWF) method for compression-molded polypropylene (PP) blends and elastomeric polypropylenes (ELPPs). The resistance to crack initiation (RCI) was characterized by the essential work of fractureweTheweof the PP blends with 20 vol% ethyiene-co-butylene (EB) copolymers of various butylene (B) contents (58 and 90 wt%) and morphology (EB58 is amorphous, whereas EB90 is semicrystalline) increased with increasing butylene content and thus crystallinity. At the same time, in the slope of the EWF resistance curves, which inform about the resistance to crack propagation (RCP) of the PP/EB blends, an adverse change was observed. For the ELPPs of various crystallinity and morphology, the opposite tendency was found, that is, the RCI decreased, but the RCP increased with increasing crystallinity. The above findings were interpreted by considering the morphology and its rearrangement due to loading. It was suggested that the RCI is improved by the order of the crystalline structure and by the density of tie molecules that transfer the stress from the amorphous toward the crystalline phase. On the other hand, the RCP is controlled by a stress redistribution process that is influenced by possible changes in the local morphology.

ISSN:0022-2348    

DOI:10.1080/00222349908248127

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

This study was devoted to the instrumented falling weight impact (IFWI) behavior of injection-molded a-and β-phase isotactic polypropylene (iPP) homopolymers. The perforation impact response of iPP with various melt flow indices (MFIs), and thus molecular weight (MW) characteristics, was studied at two different temperatures (T= 23°C andT=−40°C) and incident impact speeds (vinc= 5 and 10 m/s). The impact resistance of β-iPP was superior to the α-modification. The absolute resistance to perforation increased with increasing MW or decreasing MFI, whereas the relative toughness improvement between the β-and α-iPPs followed an opposite tendency. The molding-induced skin-core morphology did not affect practically the out-of-plane response of the impacted plaques. Changes in the fractograms (viz. force-time curves) under various experimental conditions were traced to variations in the failure mode, showing a competition between radial and circumferential cracking with respect to the clamping ring. In the case of the more ductile β-iPP. circumferential cracking was favored.

ISSN:0022-2348    

DOI:10.1080/00222349908248128

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

The influence of tensile deformation on the crystalline properties of ethylene copolymers (ethylene-vinyl acetate [EVA] copolymers) was investigated by differential scanning calorimetry (DSC). The consequence of drawing on the mobility of the amorphous phase also was investigated through the study of the glass transition temperature. The results indicate that more disorganized crystals, melting at a lower temperature, are present after the tensile deformation, reducing the mobility of the amorphous chains, as shown by an increase of the glass transition temperature. For the lowest molecular weight copolymer. less crystalline changes are observed after the tensile test, probably due to the fact that no stiffening appears during the drawing.

ISSN:0022-2348    

DOI:10.1080/00222349908248129

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

The present work demonstrates the possibility of determining and differentiating the elastic and plastic material properties (like the Young's modulus, the ball hardness under load, and the plastic hardness) by applying the dynamic ball hardness indentation test. In Ref. 1, the elastic properties are neglected. Nevertheless, the obtained hardness number includes both elastic and plastic parts. Now, the continuous data acquisition allows the determination of the elastic modulus of the polymer and also its dynamic and thermal dependence. Furthermore, a way of specifying a plastic hardness number is shown. Using the approach of Oliver and Pharr [2] enables the separation of the real material property of plastic hardness. Topographic measurements allowed taking the wall formation during a hardness test into account while analyzing the impression. It turned out that the elastic modulus determined in the manner described is independent of the penetration rate, but decreases with increasing temperature or caoutchouc mass content. Also, the dynamic and thermal dependence of the hardness are discussed.

ISSN:0022-2348    

DOI:10.1080/00222349908248130

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

Strain rate effects on Hardness and Young's modulus of two glassy polymers, poly(diethylene glycol bis allyl carbonate) (CR39) and bisphenol-A polycarbonate (PC), were studied in the nanoscale range. Before analyzing material behaviors, we focused on a particular phenomenon prevailing at the first stage of the contact between the surface of these polymers and the Berkovitch diamond tip used in the experiments, leading to an apparent increase of the tip defect (i.e., the missing tip of the diamond from having a shape equivalent to a perfect cone). The common methods based on calibration functions of the tip appear to be inaccurate to calculate correctly the contact area at the nanoscale range for these polymers. A new method based on Loubet et al.'s model to calculate the contact area by taking account of the apparent tip defect is proposed. The hardness values obtained this way were compared to the compressive yield stress using Tabor's relationship. A hardness-yield stress ratio close to 2.0, as expected on such polymers, was found. A strain-rate effect on the load-depth curve for these two polymers is interpreted as an increase of the hardness with the strain rate. The results from quasi-static and dynamic (the continuous stiffness method) measurements are compared. The strain-rate effect on Young's modulus in dynamic conditions should be taken into account in the hardness calculation.

ISSN:0022-2348    

DOI:10.1080/00222349908248131

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

Quasi-isothermal temperature-modulated differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) measurements (TMDSC and TMDMA, respectively) permit determination of heat capacity and shear modulus as a function of time during crystallization. Nonreversible and reversible phenomena in the crystallization region of polymers can be observed. The combination of TMDSC and TMDMA yields new information about local processes at the surface of polymer crystals, like reversible melting. Reversible melting can be observed in the complex heat capacity and in the amplitude of sheer modulus in response to temperature perturbation. The fraction of material involved in reversible melting, which is established during primary crystallization, remains constant during secondary crystallization for polycaprolactone (PCL) and polyether ether ketone (PEEK). This shows that, also after long crystallization times, the surfaces of the individual crystallites are in equilibrium with the surrounding melt. Simply speaking, polymer crystals are “living crystals.”

ISSN:0022-2348    

DOI:10.1080/00222349908248132

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

Time-resolved Fourier transform infrared (FTIR) spectroscopy with polarized light was employed to study the structure and mobility of a homologous series of ferroelectric liquid crystalline polymers (FLCPs) and ferroelectric liquid crystalline elastomers (FLCEs) in response to an external electric field. The chemical composition of the samples, besides the cross-linking units, is similar. For the elastomers, two different cross-linking architectures are realized: “intralayer” cross-linking leads to the formation of two-dimensional networks, whereas “interlayer” cross-linking forms three-dimensional networks. Due to its specificity, FTIR spectroscopy enables analysis of the reorientational dynamics for the different molecular moieties in detail, thus revealing information about reorientation times, angular excursion, and the phase relationship in the rearrangement of the various molecular groups. In comparison to the un-cross-linked FLCP, both elastomeric samples exhibited smaller reorientation angles and an increase of the reorientation times. In the case of the interlayer cross-linked FLCE, an elastic memory effect was observed: For the reversal from negative to positive field polarity, the reorientation times were longer than for those in the opposite direction. For the intralayer cross-linked sample, it was shown that the backbone molecules reorient slower than the other molecular units (“locomotive effect”). For the un-cross-linked FLCP and the two FLCE samples, different coupling mechanisms between the network and the mesogenic parts are derived from the measurements.

ISSN:0022-2348    

DOI:10.1080/00222349908248133

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor