摘要:

AbstractThermal degradation of polyacrylonitrile (PAN) has been studied by combined differential thermal analysis, thermogravimetry, and XRD analysis. On the basis of the experimental observations, it has been suggested that the intramolecular polymerization of nitrile groups to a conjugated polyimine structure is initially limited to only the amorphous zone of PAN, which involves little evolution of off‐gases. Polymerization of nitrile group in the crystalline zone starts with the melting of crystallites with the evolution of off‐gases from the amorphous‐crystalline zone interface. ©1995 John Wiley&Son

ISSN:0887-6266    

DOI:10.1002/polb.1995.090331201

出版商:John Wiley&Sons, Inc.    

年代:1995

数据来源: WILEY

摘要:

AbstractBlock copolymers, when dissolved in a selective solvent, form spherical micelles. These micelles can selectively solubilize organic molecules otherwise insoluble in the pure solvent. In this study, we report solubilization of organic molecules by styrene‐methacrylic acid block copolymer micelles in aqueous buffers. A light scattering technique was developed to determine the extent of micellar solubilization. Our results indicate that the extent of micellar solubilization depends on the chemical nature of organic molecules, specifically, on the interactions between the organic compound and polystyrene. A thermodynamic model has been developed to describe micellar solubilization. The theoretical calculation agrees reasonably well with the experimental results for two micellar samples examined. ©1995 John Wiley&Sons, I

ISSN:0887-6266    

DOI:10.1002/polb.1995.090331202

出版商:John Wiley&Sons, Inc.    

年代:1995

数据来源: WILEY

摘要:

AbstractThe influence of hydrogenated olio(cyclopentadiene) (HOCP) on tensile mechanical properties of its blends with high density polyethylene (HDPE) has been studied at 20, 80, and 100°C. The nominal stress‐strain curves performed at 20°C indicate an increase of the modulus and a conspicuous decrease of the ultimate properties increasing the HOCP content in the blend. Blends with HOCP content>20%‐wt are found to be very fragile at 20°C. These behaviors have been related to the presence of the HOCP‐rich phase that has the glass transition at about 55°C. At 20°C the HOCP oligomers act as a hardener for the HDPE. The nominal stress‐strain curves performed at 80 and 100°C show a strong decrease of the modulus (compared to the value at 20°C), plastic deformation for all the blends tested, and rupture of HDPE and blend samples at the beginning of the cold‐drawing. Moreover, the modulus and the ultimate parameters are found to decrease with the composition. The behaviors at 80 and 100°C have been related to several factors: the higher mobility of HDPE molecules at these temperatures compared to that at 20°C; the HOCP‐rich phase that is at temperatures higher than its glass transition, and so it acts as a plasticizer for the HDPE; and the decreases of crystallinity and number of entanglements when the HOCP is added to the HDPE component. ©1995

ISSN:0887-6266    

DOI:10.1002/polb.1995.090331203

出版商:John Wiley&Sons, Inc.    

年代:1995

数据来源: WILEY

摘要:

AbstractThe concentration dependence of cryogenic gelation for aqueous solution of poly(vinyl alcohol) was studied by measuring the apparent gel fractionGand the swelling ratioQof the gel formed by freezing and thawing. It was found that for the gelation process there were three distinct regions of solution concentration bounded by two concentrationsCgelandC gel*. The gel started to form atC=Cgel, while no visible gel could be detected even upon repeated freezing and thawing of the extremely dilute solutions ofCC gel*. In the intermediate concentration region,Cgel

ISSN:0887-6266    

DOI:10.1002/polb.1995.090331204

出版商:John Wiley&Sons, Inc.    

年代:1995

数据来源: WILEY

摘要:

AbstractThe electrical and dielectric properties of poly(hydroxyethyl acrylate), PHEA, hydrogels were studied by means of dielectric relaxation spectroscopy in wide ranges of frequencies (5–2 × 109Hz), temperatures (173–363 K) and water contents (0.065–0.46, g of water per gram of dry material). The secondary dipolar mechanisms (γ and βsw) and the dc conductivity mechanism were studied in detail by analyzing the dielectric susceptibility data within the complex permittivity formalism, the modulus formalism, and the complex impedance formalism. For both mechanisms molecular mobility was found to increase with increasing temperature or increasing water content (T‐f‐h superposition principle). The energy parameters and the shape parameters of the response were determined for both mechanisms at several water contents and temperatures. The temperature dependence of dc conductivity was found to change from Vogel‐Tamman‐Fulcher (VTF) type to Arrhenius type at water contents of ca. 0.30. At water contents lower than about 0.30 the hydrogels are homogeneous whereas at higher water contents a separate water phase appears. In terms of the strong/fragile classification scheme our results suggest that the PHEA hydrogels are fragile systems. ©1995 Joh

ISSN:0887-6266    

DOI:10.1002/polb.1995.090331205

出版商:John Wiley&Sons, Inc.    

年代:1995

数据来源: WILEY

摘要:

AbstractTransmission electron microscopy (TEM) was used to examine the morphology of blends of nylon 6 and polypropylene (PP) containing various maleated polypropylenes (PP‐g‐MA). The size of the dispersed polypropylene particles decreases as the content of maleic anhydride in the PP‐g‐MA increases for binary blends of nylon 6 and the maleated polypropylenes. Ternary blends of nylon 6, PP, and PP‐g‐MA show morphologies that depend on the content of maleic anhydride of the PP‐g‐MA and on the miscibility of PP and PP‐g‐MA. Blends where PP and PP‐g‐MA are immiscible show a bimodal distribution of particle sizes. Miscibility of the PP and PP‐g‐MA was determined by TEM using a special staining technique. Experimental observations of miscibility were further corroborated by thermodynamic calculations. The morphology of the ternary blends was also found to be dependent on the ratio of PP/PP‐g‐MA. By changing this ratio it was possible to induce drastic changes of morphology, going from a continuous nylon 6 phase to a continuous PP phase at a fixed composition. The mechanical properties of these blends were found to be dependent on their morphol

ISSN:0887-6266    

DOI:10.1002/polb.1995.090331206

出版商:John Wiley&Sons, Inc.    

年代:1995

数据来源: WILEY

摘要:

AbstractDuring the polymerization of tetrafunctional monomers, a heterogeneous distribution of species exists throughout the reaction mixture and dramatically complicates the network structural evolution. This work attempts to quantify this heterogeneity using kinetic gelation simulation predictions of a defining parameter called the heterogeneity index. The heterogeneity index directly measures how heterogeneous the polymerization is with respect to interactions of like species or pairs of species. Examples of the species are polymer segments, monomeric double bonds, pendant double bonds, initiator molecules, radicals, and free volume. By implementing this index, it is clear that the heterogeneity in the polymer network dramatically changes as a function of conversion and polymerization conditions. In particular, microgel formation and monomer pooling were characterized and quantified with the heterogeneity index. In addition to characterizing the structural heterogeneity, the influence of the heterogeneity on the initiator efficiency and trapping of free radicals was also studied and qualitatively compared to experimentally observed behavior. ©1995 John Wiley&Sons, Inc

ISSN:0887-6266    

DOI:10.1002/polb.1995.090331207

出版商:John Wiley&Sons, Inc.    

年代:1995

数据来源: WILEY

摘要:

AbstractThis is a series of articles that deals with fundamental aspects of the effects of the structure of latex particles of acrylic copolymers on their adhesion behavior. Specifically, relationship or analogy between rheological properties and adhesion performance of the acrylic copolymers was demonstrated. The first part of this series concerns the synthesis and characterization of latex particles with desired structures and compositions, and the experimental results of peel adhesion. The second part develops an analogy between the peel adhesion performance of the adhesives and rheological properties of the corresponding copolymers. The third part addresses the generalities and particularities of three major tests for adhesion: peeling, blistering, and spontaneous peeling. Three types of structured latex particles were synthesized by three different emulsion polymerization processes: the first type had a uniform composition over the entire particles with a glass transition temperature (Tg) varying between −40°C and 0°C, depending upon the compositions of monomers involved in the copolymer; the second type was of core‐shell structure. As for the third type, the composition of monomers varied gradually across the particle radii. The glass transition behavior and the dynamic mechanical properties in the solid state of the copolymers confirmed the structures of the corresponding latex particles. On the other hand, the peel adhesion performance of the films of these latex particles varied with the dynamic mechanical properties of the corresponding copolymers. This implies that a correlation could be found between the structure of the latex particles, dynamic mechanical properties in the solid state of the corresponding copolymers, and the peel adhesion performance of the adhesive films. ©1995 John Wiley&Son

ISSN:0887-6266    

DOI:10.1002/polb.1995.090331208

出版商:John Wiley&Sons, Inc.    

年代:1995

数据来源: WILEY

摘要:

AbstractThis article, the second part of this series, concerns the development of an analogy between the peel behavior of pressure‐sensitive adhesives and the dynamic mechanical properties of the corresponding copolymers. The adhesive copolymers used were synthesized by emulsion polymerization processes. Their physical and dynamic mechanical properties were characterized and presented in Part I of this series. In this study, an analogy was built up between the force in a peel test as a function of peel velocity,Fp(vp), and the loss modulus of the adhesive as a function of the angular frequency in a dynamic mechanical experiment,G″(ω). This was done by superimposing the curves ofFpversusvpand those ofG″ versus ωβ0/β, where β0/β is a shift factor with β being a parameter in the Kaelble theory and β0being some reference value of the Kaelble parameter. When the curves ofFp∼vpand those ofG″ ∼ ωβ0were plotted together, they followed the same trend of variation. This analogy betweenG″(ωβ0/β) andFp(vp) was further confirmed by the fact that the apparent activation energies of the primary glass transition forG″(ω) andFp(vp) are virtually the same, suggesting that the analogy betweenG″(ω) andFp(vp) is dictated by the glass transition. The existence of the above‐mentioned analogy betweenG″(ω) andFp(vp) shows that the performance of an adhesive can be evaluated or predicted from the dynamical loss modulus of the corresponding (c

ISSN:0887-6266    

DOI:10.1002/polb.1995.090331209

出版商:John Wiley&Sons, Inc.    

年代:1995

数据来源: WILEY

摘要:

AbstractThe stress in films of semirigid and rigid polyimides (PIs) on silicon (Si) substrate has been measured in situ during curing using a newly modified bending beam apparatus. By using the apparatus, the onset of residual stress in the initially solvent‐rich films has also been investigated. The stress characteristics of the PI films are strongly ramp rate dependent. Different ramp rates result in markedly different stress patterns, thermo‐mechanical properties, and film morphologies. The residual stress at room temperature after curing is scattered around 23–31 MPa for the films of pyromellitic dianhydride‐4,4′‐oxydianiline (PMDA‐ODA). For pyromellitic dianhydride‐p‐phenylenediamine (PMDA‐PDA), it systematically increases from −6 to 28 MPa for ramp rate increasing from 0.5 to 10°C/min. The residual stress is very low in the slowly cured PMDA‐PDA films. However, these films have gone through a very high‐stress transient state during curing. The maximum transient stress is comparatively higher in the films of PMDA‐PDA than in PMDA‐ODA. Slowly cured PMDA‐PDA films exhibit high structural ordering, high in‐plane anisotropy, and low coefficient of thermal exp

ISSN:0887-6266    

DOI:10.1002/polb.1995.090331210

出版商:John Wiley&Sons, Inc.    

年代:1995

数据来源: WILEY