摘要:

AbstractIsothermal melt crystallization kinetics were investigated by differential scanning calorimetry (DSC) for virgin and α‐chloronaphthalene solvent‐treated poly(phenylene sulfide) (PPS) systems. The overall crystallization rates were found to be much faster for the solvent‐treated PPS than for the virgin neat PPS. Additionally, the Avrami crystallization plot for the solvent‐treated PPS samples appeared as two straight portions with an apparent discontinuity, but as a continuous straight line for the virgin PPS system. After the treatment of solvent dissolution and subsequent drying, the residual trace α‐chloronaphthalene, upon being quenched to the crystallization temperatures, initiated some localized solvent‐induced nuclei‐like crystals in PPS. It was the nuclei that enhanced secondary crystallization in treated PPS during the second stage, and the higher extents of secondary crystallization in the solvent‐treated PPS caused the apparent discontinuous break in the Avrami plots. The causes for the difference were explained and the mechanism of the sequential primary/secondary crystallization kinetics for the solvent‐treated PPS was satisfactorily described with a proposed series‐parallel crystallization model. ©199

ISSN:0887-6266    

DOI:10.1002/polb.1995.090331401

出版商:John Wiley&Sons, Inc.    

年代:1995

数据来源: WILEY

摘要:

AbstractStructural changes of tussah (Antheraea pernyi) silk fibroin films treated with different water‐methanol solutions at 20°C were studied as a function of methanol concentration and immersion time. X‐ray diffraction measurements showed that the α‐helix structure, typical of untreated tussah films, did not change for short immersion times (2 min), regardless of methanol concentration. However, crystallization to β‐sheet structure was observed following immersion of tussah films for 30 min in methanol solutions ranging from 20 to 60% (v/v). IR spectra of tussah films untreated and methanol treated for 2 min exhibited strong absorption bands at 1265, 892, and 622 cm−1, typical of the α‐helix conformation. The intensity of the bands assigned to the β‐sheet conformation (1245, 965, and 698 cm−1) increased for the sample treated with 40% methanol for 30 min. Raman spectra of tussah films with α‐helix molecular conformation exhibited strong bands at 1657 (amide I), 1263 (amide III), 1106, 908, 530, and 376 cm−1. Following α → β conformational transition, amide I and III bands shifted to 1668, and to 1241, 1230 cm−1, respectively. The band at 1106 cm−1disappeared and new bands appeared at 1095 and 1073 cm−1, whereas the intensity of the bands at 530 and 376 cm−1decreased sign

ISSN:0887-6266    

DOI:10.1002/polb.1995.090331402

出版商:John Wiley&Sons, Inc.    

年代:1995

数据来源: WILEY

摘要:

AbstractFormation of dilatation bands has been found in a series of moderately crosslinked thermosets with molecular weights between crosslinks, Mc, ranging from 560 to 1000 g/mole. These dilatation bands, although they do not show distinct craze fibrils inside the band, do behave mechanically like the well‐known crazing phenomenon observed in polystyrene. Although the cause for the formation of dilatation bands is still largely unknown, the occurrence of the craze‐like fracture phenomenon (dilatation bands) is found to be strongly influenced by the stress state the matrix experiences, by the type of loading applied to the specimen, and by the type/size of toughener particles incorporated in the matrix. The tendency for dilatation band formation in engineering thermosets can be greatly enhanced by the use of craze‐prone toughener particles, such as nylon particles. The toughening effect via these newly observed dilatation bands is found to be quite impressive, and in some cases, can approach that via the shear‐banding process. The potential significance of the present finding for a better design of toughened thermosets for structural applications is discussed. ©1995 John Wiley&S

ISSN:0887-6266    

DOI:10.1002/polb.1995.090331403

出版商:John Wiley&Sons, Inc.    

年代:1995

数据来源: WILEY

摘要:

AbstractIsothermal crystallization kinetics for random copolymers of vinylidene chloride (VDC) with methyl acrylate (MA) is reported. Syntheses of many semicrystalline polymers follow heterogeneous reaction paths in which the macromolecule chains phase separate from the reaction mixtures. The internal particle morphology (the internal structure of the resin bead) from this type of reaction is granular and porous, as a result of the demixing processes accompanying polymer formation. Demixing in these polymers involves either liquid‐liquid (L‐L) phase separation followed by liquid‐solid (L‐S) transformation (crystallization) or L‐S transformation alone. Crystallization (L‐S transformation) must be an indispensable part of the process if a porous granular structure is to be expected. This is because L‐S transformation is the most probable means by which the demixed structure can be stabilized against complete coalescence or agglomeration, which would lead to totally fused bead internal structure. This is particularly true if the glass transition temperature (Tg) is lower than the polymerization temperatures, as is the case with the VDC‐MA copolymers. Copolymers that crystallize the fastest will have the finest (most porous) resin bead morphology. The result of this work is consistent with expectation. The homopolymer (PVDC) that crystallizes the fastest has the finest resin bead internal morphology. The copolymers show slower crystallization rates with increasing noncrystallizable MA content. Correspondingly, resin morphology measured by specific surface area decreased with increasing amounts of the noncrystallizable (MA) comonomer unit in the copolymer. This is clearly seen in SEM photographs of the internal bead structures of these copolymers. ©1995 John

ISSN:0887-6266    

DOI:10.1002/polb.1995.090331404

出版商:John Wiley&Sons, Inc.    

年代:1995

数据来源: WILEY

摘要:

AbstractWater sorption by amorphous (meth)acrylic poly(zwitterions)‐bearing quaternary ammonioalkoxydicyanoethenolate side groups , dipole moment μ(D) = 25.9 and 30.8 forp= 2 and 3, respectively was studied at 23°C over a broad range of water activitya(0.14–0.98). Water diffusion is nearly Fickian (Ds= 5.9 10−7cm2.s−1fora= 0.63) and the sorption isotherms may be quantitatively analyzed according to the Guggenheim‐Anderson‐De Boer equation for a multilayer process characterized by a number of site‐bound water molecules per monomeric unit,nm⋑ 0.7. The Flory χ interaction parameter is a strongly increasing function of the water content in the glassy hydrated systems and it always remains higher than 0.75. Clustering of water molecules (Zimm‐Lundberg theory) is never observed. Differential scanning calorimetry allows to quantify nonfreezable bound water (type I) of strong plasticization efficiency,n(I) = 2.8 mol. of water per monomeric unit, and it points out the quasisimultaneous emergence of low amounts of freezable bound water (type II) crystallizing at −40°C and melting at −1°C and of bulkfree water (type III,n(II)/n(III) ⋑ 0.1). All these typical features distinguish these rather hydrophobic poly(zwitterions) from their hydrophilic homologues of the quaternary ammoniopropanesulfonate type .

ISSN:0887-6266    

DOI:10.1002/polb.1995.090331405

出版商:John Wiley&Sons, Inc.    

年代:1995

数据来源: WILEY

摘要:

AbstractCellulose derivatives with low degrees of substitution (i.e., DSC6), and they haveTgs within 5–20°C of theirTms. This proximity is unusual for a semicrystalline material, and it interferes with the crystallization process that occurs betweenTmandTg. Triesters of cellulose with mixed acyl substituents (one smaller and one larger) allow not only unambiguous observation ofTgs andTms but also an adjustable Δ(Tm−Tg) window that depends upon the size and the DS of the larger substituent. The materials studied including cellulose acetate butyrate triesters (DSbu0.8–2.6), cellulose acetate hexanoate triesters (DShex0–3.0), and cellulose acetate (DSac2.44), revealed that only the mixed esters, in which the bulkier acyl group is in the range of DS 0.3–1.0, had a Δ(Tm−Tg) value in excess of 40°C. Although theTmof cellulose acetate hexanoate declined by ca. 150°C per unit of DShexas DShexrose from 0 to 1, this was only ca. 25°C between DShexof 1 and 3. Frequently observed dual‐melt endotherms were attributed to two separate crystal morphologies. ©1995

ISSN:0887-6266    

DOI:10.1002/polb.1995.090331406

出版商:John Wiley&Sons, Inc.    

年代:1995

数据来源: WILEY

摘要:

AbstractThe thermal conductivity and thermal expansivity of a thermotropic liquid crystalline copolyesteramide with draw ratio λ from 1.3 to 15 have been measured parallel and perpendicular to the draw direction from 120 to 430 K. The sharp rise in the axial thermal conductivityKpar;and the drastic drop in the axial expansivity α∥at low λ, and the saturation of these two quantities at λ>4 arise from the corresponding increase in the degree of chain orientation revealed by wide‐angle x‐ray diffraction. In the transverse direction, the thermal conductivity and expansivity exhibit the opposite trends but the changes are relatively small. The draw ratio dependences of the thermal conductivity and expansivity agree reasonably with the predictions of the aggregate model. At high orientation,Kpar;of the copolyesteramide is slightly higher than that of polypropylene but one order of magnitude lower than that of polyethylene. In common with other highly oriented polymers such as the lyotropic liquid crystalline polymer, Kevlar 49, and flexible chain polymer, polyethylene, αpar;of the copolyesteramide is negative, with a room temperature value differing from those of Kevlar 49 and polyethylene by less than 50%. Both the axial and transverse expansivity show transitions at about 390 and 270 K, which are associated with large‐scale segmental motions of the chains and local motions of the naphthalene units, respectively. ©1995 John Wi

ISSN:0887-6266    

DOI:10.1002/polb.1995.090331407

出版商:John Wiley&Sons, Inc.    

年代:1995

数据来源: WILEY

摘要:

AbstractThe structure and morphology of Nylon‐12, 10 lamellar crystals has been investigated using transmission electron microscopy, selected area electron diffraction, and x‐ray diffraction. Additional data have been obtained from uniaxially oriented fibers. The unit cell parameters of two crystalline structures have been determined. They are similar to those usually found in other polyamides (α and β form). Calorimetric (DSC) studies on nylon 12, 10 were also carried out. Melting curves indicate that changes in the internal structure occur when scanning speeds less than 80°C min−1are used. ©1995 John Wiley&

ISSN:0887-6266    

DOI:10.1002/polb.1995.090331408

出版商:John Wiley&Sons, Inc.    

年代:1995

数据来源: WILEY

摘要:

AbstractHomogeneous precursor/precursor solutions with various compositions were obtained with appreciably high solid contents inN‐methyl‐2‐pyrrolidone from soluble poly(amic diethyl ester) precursors of rodlike poly(p‐phenylene biphenyltetracarboximide) (BPDA‐PDA) and flexible poly(4,4′‐oxydiphenylene biphenyltetracarboximide) (BPDA‐ODA), which are hydrolytically more stable as well as more soluble than the corresponding poly(amic acid)s being equilibrated with the constituent monomers. Both optical microscopic and light scattering measurements showed that the dried precursor blend films and resultant polyimide composite films were optically transparent, regardless of compositions and process conditions. The composite films showed a singleTgbehavior. However, for the composite of 30 wt % BPDA‐PDA dispersed in the matrix of 70 wt % BPDA‐ODA, a smectic crystalline‐like aggregation of the BPDA‐PDA component was detected on wide‐angle x‐ray diffraction patterns, indicative of microscopic phase separation between the two components. This phase separation was not detected on the optical microscopy, light scattering, and dynamic mechanical thermal analysis because of their resolution limits: Optical microscopy has a resolution of submicrometers, whereas dynamic mechanical thermal analysis and light scattering have a resolution of ca. 50 Å. Therefore, it is speculated that in the composite films BPDA‐PDA and BPDA‐ODA polyimide molecules have demixed on the scale of a few nanometers. The mean long periodicity, which was estimated from the small‐angle x‐ray scattering pattern, varied from 134 to 170 Å as the content of BPDA‐ODA component increased. In addition, mechanical properties of the composite films were ch

ISSN:0887-6266    

DOI:10.1002/polb.1995.090331409

出版商:John Wiley&Sons, Inc.    

年代:1995

数据来源: WILEY

摘要:

AbstractThe rheological behavior of solutions containing blends of poly(γ‐benzyl‐L‐glutamate) (PBLG) and either the free acid or the zinc salt of lightly sulfonated polystyrene (SPS) was studied as a function of blend composition, polymer concentration, degree of sulfonation of the SPS, and the polypeptide molecular weight. The zinc salt of SPS formed a transition metal complex with the amine‐end groups of the PBLG, and this resulted in an enhancement of the solution viscosity relative to a weighted average of the viscosities of the individual polymer solutions. The ZnSPS/PBLG solutions showed no anomalous time or shear dependencies. In contrast, solutions containing PBLG and the sulfonic acid derivative of SPS also had enhanced viscosities, but in addition, they exhibited time‐dependent viscosities (thixotropic behavior) and shear thickening (dilatant behavior). This was attributed to a nonequilibrium structure of the interpolymer complex due to a competition between acid‐amine and acid‐ester interactions. Although the acid‐amine interaction is enthalpically favored, when sufficient sulfonic acid groups were available, interactions between the sulfonic acid and the glutamate ester side groups of PBLG developed and this interaction promotes a helix‐to‐coil transition of the PBLG. ©1995

ISSN:0887-6266    

DOI:10.1002/polb.1995.090331410

出版商:John Wiley&Sons, Inc.    

年代:1995

数据来源: WILEY