摘要:

A summary of our work on a triblock copolymer under steady shear is presented. The experiments were conducted using small-angle neutron scattering (SANS), as a function of shear rate and temperature, and transmission electron microscopy (TEM) on quenched specimens. The triblock copolymer is composed of polystyrene-d8/polybutadiene/polystyrene-d8, and the ordered microstructure normally consists of hexagonally packed cylinders. Two temperature-dependent, characteristic shear rates, -γCIand γC2, are identified from the scattering results. The first characteristic shear rate identifies the shear rate required to go from a disordered state to an ordered state, while the second characteristic shear rate is interpreted as the shear rate necessary to produce a different ordered morphology. This latter transition was previously identified as being analogous to a martensitic transition in metal alloys. The supporting SANS and TEM evidence for the new ordered phase is presented. Dynamical aspects of the structural transition between different ordered triblock morphologies are discussed using the modei of a martensitic-like transformation.

ISSN:0022-2348    

DOI:10.1080/00222349608220391

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

Miscible polymer blends containing one crystallizable component and exhibiting liquid-liquid phase separation at elevated temperatures [lower critical solution temperature (LCST) behavior] offer an excellent possibility of controlling morphology and thus mechanical properties. For instance, if a homogeneous mixture of dissimilar polymers is allowed to undergo a rapid temperature jump from below LCST to above LCST, spinodal decomposition takes place and a highly interconnected two-phase morphology with uniform domain size (so-calledmodulated structure) develops. By quenching the phase-separated system below the glass transition temperature after an appropriate time of phase separation, one is able to fix this characteristic morphology [1]. By quenching the phase-separated blend below the melting point of the crystallizable component to different supercooling depths, it is possible to control the number of nuclei and thus the spherulite's size, creating more or less ordered structures.

ISSN:0022-2348    

DOI:10.1080/00222349608220392

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

The effect of transcrystallinity in carbon fiber reinforced poly(p-phenylene sulfide (PPS) composites on the apparent shear strength was investigated with the single fiber pull-out test. Transcrystalline zones around the reinforcing fibers do not seem to improve the adhesion level significantly. Neighbor fibers hinder the formation of the transcrystalline zone and a ductile fracture behavior can be observed. However, the apparent strength level is slightly higher for composites containing such reinforcing neighbor fibers compared with single fiber composite samples. During annealing a brittle interface can be formed in the multifiber composite yielding a higher level of the apparent shear strength.

ISSN:0022-2348    

DOI:10.1080/00222349608220393

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

An original method for measuring droplet deformation in polymer melts during uniaxial elongational flow has been developed. It is based on the observation of a limited number of drops, before and after elongation in the melt. The shape of the elongated drops was frozen by fast quenching. Two PS samples for the continuous phase, plus two HDPE, and one PMMA for the drops, allowed a wide range of viscosity ratios (0.0046 <p< 13). Experiments at high capillary number (Ca) values were in good agreement with Taylor's linear newtonian theory up to deformations of about λ = 4: viscous drops (p> 1) deform less than the surrounding matrix, whereas the opposite is observed for low viscosity drops (p< 1) with a limiting ratio of drop vs. matrix deformation of 5/3 at vanishing drop viscosity. Experiments carried out atCavalues of the order of unity showed that the drop deformation increases linearly with their initial radius in agreement with the linear theory. In some cases, the agreement with the data could be improved by using Palierne's theory for viscoelastic systems. Analytical expressions could be obtained for maxwellian fluids and high capillary numbers.

ISSN:0022-2348    

DOI:10.1080/00222349608220394

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

Dielectric relaxation spectroscopy measurements have been performed during isothermal curing of semi-interpenetrating polymer networks (semi-IPNs) and the related pure networks at frequencies between 3 kHz and 3 MHz and curing temperatures between 313 and 393 K. The pure networks consist of diglycidyl ether of bisphenol A (DGEBA) cross-linked with diaminodiphenylmethane (DDM) and the semi-IPNs contain in addition 10 or 20 wt% of polysulfone PSn) as the linear component. Temperature dependent dielectric measurements have been performed on the fully cured samples in the temperature range from 130 to 550 K for the same frequencies. For the pure networks, the imaginary part of the complex permittivity ε″ shows a decrease with curing timetcurefollowed by a peak. For the semi-IPNs this peak is much broader for all cases and can be resolved into two maxima for several curing conditions. The decrease in ε″ is connected to the decrease in the dc-conductivity due to gelation, whereas the peak is related to the relaxation of dipoles. The existence of two maxima in the time dependence of ε″ is an indication for two different relaxations in a phase separated structure. This is supported by temperature dependent measurements on the fully cured samples. The characteristic relaxation times ι, which are extracted from the maxima of ε″(tcure) for different frequenciesfusing the relation ωτ = 1(ω = 2πf), increase during curing by several orders of magnitude for both the pure networks and the semi-IPNs. In order to extract the characteristic times for gelationtgand the timest0where the dc-conductivity σdcapproaches a singularity, σdcwas fitted by a power law, σdc= σ0[(tg-t)/tg]p, and an exponential function, σdc=Aexp [B/(t-to)], respectively. In the error limits of the experiments both procedures lead to similar results. Thetgandtovalues are in good agreement with those measured mechanically and no significant differences between pure networks and semi-IPNs cured at the same temperature were found.

ISSN:0022-2348    

DOI:10.1080/00222349608220395

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

The morphology of polyethylene-silicone blends has been investigated by combining scanning electron microscopy with measurements of permeability to methanol vapor and selective extraction of the silicone phase by diethyl ether. The results show that scanning electron microscopy is the most suitable technique to characterize the microstructure of the blends (size and shape of the dispersed phase). On the other hand, permeability measurements provide the most useful information about the way morphology is changing with composition of the blends and about the composition corresponding to the maximum level of co-continuity of the two phases. Selective extraction of the dispersed phase is not a reliable technique in the system investigated because the extraction process induces physical damage in the matrix.

ISSN:0022-2348    

DOI:10.1080/00222349608220396

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

A comparative study of the structures of two thermotropic liquid crystal copolyesters (LCP) was carried out. The first copolymer, LCP-1, comprises three different monomer units of equal length, whereas the second copolymer, LCP-2, contains two monomer units which differ significantly in length. Both copolymers exhibit random structure. It was established that despite the irregular structure both copolymers have the ability to crystallize. Moreover, during crystallization of LCP-1 microphase separation into two phases occurs. Each phase has an orthorhombic crystal structure but with slightly different parameters of the unit cell. The reason for this difference is probably associated with configuration isomerism because of possible head-to-head or head-to-tail additions during synthesis of the copolyester. Upon annealing, the maximum degrees of crystallinity are 35% and 25% for LCP-1 and LCP-2, respectively. The noncrystalline fraction of the material, whose content exceeds appreciably the content of the crystalline phase, shows a LC smectic structure. However, the fundamental difference between the two copolymers is that, in the case of LCP-1, the periodic layer structure is observed, whereas for LCP-2 the layer structure is aperiodic. The above phenomena are universal, and this makes it possible to predict the structure and character of temperature behavior of any representative of the class of fully aromatic rigid-chain copolyesters only from the chemical formula of the copolymer.

ISSN:0022-2348    

DOI:10.1080/00222349608220397

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

The mechanism of cold drawing is described by a model of inhomogeneous deformation. The fundamental assumption of the model is that a random polymer coil subjected to an external stress forms a highly oriented strand and a residual rest coil that remains isotropic after a phase of elastic deformation. In this coil-strand transition model, the strand is growing during deformation by reduction of the volume part of the rest coil. In the volume-creating model, the constant orientation difference between rest coil and strand gives rise to a linear energy and entropy law and thus to a constant tensile stress. In the volume-conserving model, a decreasing power law for the stress-strain relationship is found, which is able to explain certain features of the cold drawing process.

ISSN:0022-2348    

DOI:10.1080/00222349608220398

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

Recent advances in the analysis of the fracture behavior of polyethylene have been reviewed specifically to establish the nature and extent of morphological changes. It has been shown that in the case of linear low-density polyethylene (LLDPE), reorganization and orientation of lamellae occur in the shear lips region, the pattern being influenced by differences in molecular structure. Crazes, on the other hand, are characterized by the presence of voiding, with no significant effect on the original morphology.

ISSN:0022-2348    

DOI:10.1080/00222349608220399

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

Transmission electron microscopy (TEM) of amorphous and semicrystalline isotactic polystyrene (iPS) thin films deformed well belowTg, suggests the same crazing mechanism to operate in both cases. Therefore, by analogy with the amorphous case, highly entangled semicrystalline polymers, such as poly(ether ether ketone) (PEEK) should craze less readily in the glassy-semicrystalline state than iPS, which has a low degree of entanglement. Since this is confirmed by observation, it is reasonable to extend this analogy, and invoke entanglement in order to account for the high fracture resistance of highly entangled semicrystalline polymers, such as polyoxymethylene, using models previously applied to glassy amorphous polymers. There are nevertheless often significant decreases in fracture toughness in polyoxymethylene as the crystallization temperature is raised and/or the molecular weight is reduced, which we attribute to entanglement loss during lamellar folding.

ISSN:0022-2348    

DOI:10.1080/00222349608220400

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor