摘要:

Most structural studies of polymeric phase inversion membranes have focused on macrovoid formation, while the rich microstructure (size scale < 5 μm) has received little independent attention. We present a classification and qualitative description of possible microstructures on the basis of phase diagram, kinetic, and connectivity considerations. Special attention is given to the microstructural features resulting from a two-stage phase separation. Subsequently, our considerations are extended to the microstructure of the skin and that of the macrofinger boundaries. Finally, the features of actual phase inversion microstructures are described and interpreted.

ISSN:0022-2348    

DOI:10.1080/00222349908248101

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

Samples of polycarbonate (PC) before and after remelting and isothermal solidification treatment were characterized by wide-angle X-ray scattering (WAXS) and small-angle X-ray scattering (SAXS), microhardness, and stretching calorimetry techniques. Both PC samples could be regarded as structurally identical (X-ray amorphous); however, the isothermally solidified sample exhibited higher microhardness and Young's modulus, as well as a lower linear expansion coefficient and greatly reduced breaking strain. These results are attributed to the overlap of a network of ultramicrocrystals on the original entanglement network in the reference (untreated) PC sample.

ISSN:0022-2348    

DOI:10.1080/00222349908248102

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

A linear polyethylene (LPE) has been blended with each of three ethylene-co-4-methyl pentene-1 (E4MP1) copolymer fractions of differing branch content (2.0, 4.3, and 6.1 mol% 4-methyl pentene-1). These copolymers have Y-shaped branches, each containing four carbon atoms. The blends were studied using transmission electron microscopy (TEM) and differential scanning calorimetry (DSC) following experimental procedures that have been extensively used to investigate blends of LPEs with copolymers with alkane (linear) branches. When the copolymer is lightly branched, LPE/E-4MPl copolymer blend systems give, to a first approximation, morphology maps similar to those found for linear/branched polyethylene systems in which the branched polyethylene has linear alkane branches. Single morphologies are found for high LPE contents, but there are double morphologies (regions of thick crystals well separated in a matrix of thin crystals) at low LPE contents after quenching from within a restricted temperature range. As the branch density of the copolymer increases, the same changes in morphology map are seen for the LPE/E4MPl copolymer blend systems as for lineadalkane branched copolymer systems. The final morphology map, characteristic of a reasonably heavily branched copolymer, is reached at a lower branch density (near 6.1 mol%) for the LPE/E4MPl copolymer blend systems than for LPE/linear alkane branched copolymer systems. For LPE/E-4MP1 copolymer blend systems, as for linearlbranched polyethylene systems in which there are alkane branches, TEM is more sensitive than DSC detecting changes from single to double morphology.

ISSN:0022-2348    

DOI:10.1080/00222349908248103

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

The dynamic mechanical behavior of polyurethane networks based on liquid crystalline (LC) diol, 6,6′-[ethylenebis(1,4-phenyleneoxy)]dihexanol (D), 2(4)-methyl-1,3-phenylene diisocyanate (DI), and poly(oxypropylene)triol (T) at the stoichiometric molar ratio of isocyanate (NCO) and hydroxy (OH) groups was studied. Samples were prepared at various initial molar ratios of the reactive groups [OH]T/[NCO]DI/[OH]Dfrom 1/1/0 to 1/40/39. The gelation studies during the curing reaction in the LC and isotropic states showed that the critical gel structure at the sougel transition exhibits a power-law mechanical behavior; the relaxation exponent in the LC state is always higher than that in the isotropic state. From viscoelastic results, it follows that formation of the LC mesophase enhances the connectivity of the molecular structure at the gel point. Introduction of chemical cross-links in fully cured networks reduces the flexibility of the elastically active network chains (EANCs) and inhibits conformational arrangements required for LC ordering. Increasing the content of LC diol in networks, which leads to an increase of the length of EANCs, increases the concentration of permanent physical interactions (trapped entanglements) in the systems; the intensity of the slow relaxation process in the rubbery region also increases.

ISSN:0022-2348    

DOI:10.1080/00222349908248104

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

Cloud-point curves reported for the system polyethersulfone (PES)/phenoxy were calculated by means of the Sanchez-Lacombe (SL) lattice fluid theory. The one adjustable parameter ε12*/k (quantifying the interaction energy between mers of the different components) can be evaluated by comparison of the theoretical and experimental phase diagrams. The Flory-Huggins (FH) interaction parameters are computed based on the evaluated ε12*/k and are approximately a linear function of volume fraction and of inverse temperature. The calculated enthalpies of mixing of PES/phenoxy blends for different compositions are consistent with the experimental values obtained previously by Singh and Walsh [1].

ISSN:0022-2348    

DOI:10.1080/00222349908248105

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

A new interpretation ofMc, the critical molar mass for chain entanglement in polymers, is presented from the viewpoint of chain interpenetration in the melt. It is shown that, atM=Mc, the number of chains in the melt connected by chain interpenetration reaches approximately 10 being independent of chemical structure of the chain. This work also interprets the empirical rule of van Krevelen. Arguments are presented for the theory that the majority of the chain entanglements are not topological in nature, but rather a local interaction of cohesive nature.

ISSN:0022-2348    

DOI:10.1080/00222349908248106

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

The effect of simple shear flow on the miscibility and morphology of blends of poly(methyl methacrylate) (PMMA) and a styrene-acrylonitrile random copoly-mer with 29.5 wt% acrylonitrile (SAN-29.5) has been investigated using shear apparatus and transmission electron microscopy (TEM). The obtained data showed that only shear-induced mixing was observed for all of the composition ratios. The increase of the cloud point (or homogenization temperature) ΔT(γdot; =T(γdot;) -T(0) was investigated as a function of shear rate γdot;; in addition, the normalized shift in the cloud pointΔT(γdot;)/T(0) versus γdot; was also studied and compared with that of simple liquid mixtures and polymer solutions. The results showed that the polymer blends were more sensitive to the shear rate than both simple-liquid mixtures and polymer solutions. The morphology of the PMMN SAN(= 75/25) blend (the critical composition) indicated that shear-induced phase mixing occurred at a critical shear rate value, below which the two phases were highly oriented and elongated in the flow direction. Three regimes, depending on the applied shear rate values, were detected that were in good agreement with the literature data for polymer solutions. The effect of relaxation times after shear cessation showed a decrease in the orientation of the elongated particles, but it did not completely vanish even for 10 min after the shear cessation.

ISSN:0022-2348    

DOI:10.1080/00222349908248107

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

Poly(p-phenylene phenylthio-terephthalate) (PPTT) forms nematic melts and is highly crystalline in the solid state, despite the probable random 2- and 3-dispo-sition of the S-phenyl substituents. The X-ray pattern of melt-spun fibers of PPTT contains 24 Bragg reflections that are indexed by a monoclinic unit cell with dimensionsa= 28.6 Å,b= 4.81 Å,c= 12.57 Å (fiber axis), and γ= 101.6°. The cell contains monomer units of four chains that are arranged in pairs with the thiophenyl side chains interdigitated; successive pairs of chains are staggered by aboutc/2. We used molecular mechanics modeling to simulate arrays of chains with random 2- and 3-disposition of the side chains on the terephthalic acid units and compared the results with those for an idealized structure in which all the substituents were at the 2-position. The refined model for random substitution is more distorted, but the average separations of the monomer units are within the experimental errors of the observed unit cell dimensions, and their standard deviations are very similar to those derived from the line-broadening data. The potential energy of the model with random substitution is only about 1.9 kcal/mol of monomer higher than that for the model with all-2-substitution, indicating random substitution is not a major problem to the formation of an ordered structure. Compared to the structure formed by the analogous polyester, poly(p-phenylene phenylterephthalate) (PPT), the additional flexibility due to the thioether linkage between the backbone and phenyl side groups in PPTT allows better chain packing both within and between the layers of stacked chains.

ISSN:0022-2348    

DOI:10.1080/00222349908248108

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

Two organo-soluble aromatic polyimides have been synthesized by introducing pendant groups of trifluromethyl or methyl onto the 2- and 2′-positions of a biphenyl diamine and reacting each of them with biphenyltetracarboxylic dianhydride (BPDA): 2,2′-bis(trifluoromethyl)-4,4′-diaminobiphenyl (PFMB) or 2,2′-dimethyl-4,4′-diaminobiphenyl (DMB). These two polyimides, BPDA-PFMB and BPDA-DMB, are prepared via a one-step poly condensation method, and they can be fabricated into fibers, films, and other application forms. Our research shows that in fiber applications, these two fibers exhibit excellent mechanical properties and outstanding thermal and thermooxidative stability. Their long-term mechanical tensile performance at high temperatures is found to be critically associated with the type of side pendant groups at the 2- and 2′-positions of the diamines. High-resolution pyrolysis-gas chromatography/mass spectrometry (PyGC-MS) results show that the pyrorgams of these two polyimides possess more than 16 pyrolyzates. Based on the compositions and distributions of the pyrolyzates at different temperatures, the thermal degradation mechanisms for both of the polyimides are found to be the same.

ISSN:0022-2348    

DOI:10.1080/00222349908248109

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

The mechanical relaxation behaviors of oriented samples of a linear low-density polyethylene (LLDPE) and a high-density polyethylene (HDPE) were studied using tensile dynamic mechanical measurements. The anisotropy and activation energies of the relaxations were determined for several different samples to investigate why the α-relaxation in HDPE has characteristics similar in some respects to both the α- and β-relaxations in LLDPE. It is concluded that the anisotropy of the α-relaxation in LLDPE is determined byc-shear, whereas in HDPE it relates to interlamellar shear. The activation energy measurements, however, show that the thermally activated process for the α-relaxation in both the LLDPE and HDPE isc-shear. It is proposed that, in HDPE,c-shear has to occur before there is enough mobility at the fold surface for interlamellar shear. It is concluded that the β-relaxation is not observed in HDPE because the interlamellar regions are too constrained to allow interlamellar shear withoutc-shear.

ISSN:0022-2348    

DOI:10.1080/00222349908248110

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor