摘要:

The evolution of LAM contours in the Raman spectrum of polyethylene fibers as draw ratio increases is studied. Profiles of the length distribution of straight-chain segments (SCS) have been determined from the spectra. In fibers up to 20 μm thick, bimodal SCS length distribution occurs. A double maximum has also been observed in the spectra of samples prepared from the blend of polyethylenes with molecular weights of 8 × 104and 2 × 106. In one-component polymer the peak of the SCS length distribution function relating to shorter regular sequences is attributed to the contribution of the surface layers enriched by the low-molecular fraction of the molecular mass. This conclusion is confirmed by the results of chain orientation studies carried out by infrared ATR spectroscopy: At a distance of 2–4 μm from the fiber surface, there has been detected a layer with an orientation which is considerably lower than that in the bulk of a sample.

ISSN:0022-2348    

DOI:10.1080/00222349308215468

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The structure of quenched isotactic polypropylene (iPP) films, including samples etched with fuming nitric acid (FNA), has been studied by infrared (IR) spectra, wide-angle x-ray diffraction (WAXD), small-angle x-ray scattering (SAXS), and differential scanning calorimetry (DSC) measurements. The changes of IR, SAXS, DSC, and WAXD results induced by annealing for etched samples have been compared with those for unetched ones. The IR absorbance spectrum of the quenched iPP etched by FNA did not change. In addition, the SAXS intensity did not increase when these samples were annealed, indicating that the total (IR) crystallinity (i.e., the content of chain segments in the helical conformations) of the etched samples does not increase. However, WAXD patterns of the samples changed in a manner similar to those of the unetched one, from the original two blurred diffraction peaks to the sharp crystal patterns of α-form iPP, suggesting that the mesomorphic (or liquid crystal-like) phase has reorganized to α-form crystals. It is concluded that the change of WAXD of quenched iPP films during annealing results mainly from transformation of order in the mesomorphic phase, rather than only from an increase of crystal size. In other words, mesomorphic-form iPP is not constituted by any known crystals (such as α or β crystals) in small sizes; its WAXD pattern reflects truly the degree of order in the mesomorphic phase.

ISSN:0022-2348    

DOI:10.1080/00222349308215469

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

An empirical equation for Young's modulus is proposed. It originates from thermal fluctuations and relates Young's modulus for highly oriented polymers,E, to thermal expansion strain of the segments of macromolecular helices, εT, and to measurement time γ:

ISSN:0022-2348    

DOI:10.1080/00222349308215470

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Miscibility characteristics of poly-p-phenylene benzobisoxazole (PPBO) and nylon 66 molecular composites have been investigated by means of differential scanning calorimetry (DSC), dielectric relaxation, and optical microscopy. The DSC studies show the lowering of melting temperature (Tm) of nylon 66 with increasing PPBO content. Beyond the 70 wt% PPBO content, the melting transition of nylon 66 is no longer discernible. The movement of the glass transition temperature (Tg) of the mixtures is observable in DSC scans but is not reliable for quantitative evaluation due to the influence of absorbed water by nylon. However, the dielectric loss tangent peak corresponding toTgshows systematic movement with composition, suggesting partial miscibility of the pair. The results of the melting point depression were analyzed in accordance with the Flory polymer-diluent theory. The interaction parameter χ estimated to be about −0.45 in the melt state. These mixtures are not stable and undergo thermally induced phase segregation in the vicinity of the onset ofTmof nylon 66.

ISSN:0022-2348    

DOI:10.1080/00222349308215471

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Miscibility characteristics of poly-p-phenylene benzobisoxazole (PPBO)/amorphous nylon (AN) molecular composites were studied by means of differential scanning calorimetry (DSC), dielectric relaxation, wide-angle x-ray diffraction (WAXD), and light scattering. The solution-mixed PPBO/AN exhibited a systematic movement of a single glass transition temperature (Tg) with composition in DSC and dielectric measurements, implying an entrapped single phase of the molecular composites. In the WAXD studies of the 50/50 mixture, two very weak reflection peaks of the PPBO phase and a small amorphous halo of AN were discerned. Light scattering studies showed no detectable level of orientation fluctuations. Upon heating, thermally induced phase separation occurred, with a cloud point phase diagram reminiscent of a lower critical solution temperature (LCST). As typical for entrapped single-phase blends, the phase diagram of LCP blends is irreversible. Nevertheless, several temperature jump experiments were undertaken from ambient temperature to a two-phase temperature region in order to gain further insight into the nonequilibrium behavior. The time-evolution of scattering profiles was analyzed in the context of nonlinear theories and dynamical scaling laws. The kinetic exponent of the phase growth was found to be consistent with the prediction of cluster dynamics proposed by Binder and Stauffer.

ISSN:0022-2348    

DOI:10.1080/00222349308215472

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

An epoxy-amine system (DGEBA-DDS) is cured isothermally at 150 °C under various pressures up to about 124.1 MPa (18,000 psi) and isobarically under 69.0 MPa (10,000 psi) at 135°, 150°. and 165 °C. The curing process is monitored by a high-pressure torsion pendulum (HPTP) which is newly designed, constructed, and tested for this research. Two macroscopic phenomena, namely the gelation and the vitrification, are monitored in the form of damping peaks at all cure pressures and temperatures. The phenomena are influenced by hydrostatic pressure in the form of changes of the time position, shape, and size of the two peaks. The time positions of the gelation peak and the vitrification peak are shifted to a shorter time scale with increasing pressure, but the amount of the shift of the vitrification peak is three times greater than that of the gelation peak. The size of the vitrification peak becomes relatively smaller compared to that of the gelation peak under pressure. FTIR is employed to obtain the percent extent of reaction (α) at gel point and after complete vitrification. The α at gel point increases with increasing pressure, but α after vitrification decreases up to 89.6 MPa (13.000 psi) and increases between 89.6 MPa (13,000 psi) and 124.1 MPa (18,000 psi).

ISSN:0022-2348    

DOI:10.1080/00222349308215473

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Cast poly(ethylene terephthalate) films were biaxially drawn and subsequently heat-set. All films were drawn 3.5×, at fixed width and at 90°C, in the machine direction (MD). and subsequently drawn to various degrees in the transverse direction (TD). The structure of the films, before and after heat treatment, was investigated using refractive index and density measurement, wide- and small-angle x-ray scattering, and transmission electron microscopy (TEM). The TEM study was performed using intrinsic phase-contrast imaging of thin foils microtomed at an angle to the MD. The MD-drawn material exhibits crystallization in the form of thin fibrillar crytals with chain axes along the MD. TD drawing disrupts this structure and ultimately produces a second crystal population with chain axes along the TD. Subsequent heat treatment produces a fibrillar-to-lamellar transition and a subsequent increase in the degree of crystallinity. Throughout these processes (100) is parallel to the film plane, while (010) lies normal to the film plane.

ISSN:0022-2348    

DOI:10.1080/00222349308215474

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Polyurethanes form an important class of materials largely due to their interesting properties arising from the phase-separated structures. An added advantage of polyurethanes is associated with the possibility of tailoring the properties by a judicious choice of components and composition. Substantial efforts have been expended to elucidate the structural features of polyurethanes (e.g., Refs. 1–8). Many diffusion studies have also been carried out in an attempt to obtain information on and insight into the microstructure of polyurethanes (9–13). Recently we have studied the diffusion of lipids through polyurethanes considering the blood-contacting applications of this class of materials (14–16). Our studies indicate that the lipids' diffusion is confined to the soft segment (SS). This communication addresses our further effort in this area, particularly the specific role of hard segment (HS) content on the activation energy for diffusion of a few lipids.

ISSN:0022-2348    

DOI:10.1080/00222349308215475

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor