摘要:

Cellulose and its chemical analogs chitin and chitosan are abundant and technologically important fibrous polysaccharides. Cellulose and chitin are, respectively, the first [1] and second [2] most abundant natural polysaccharides. Chitosan, though less prevalent in nature, is a useful and easily accessible derivative of chitin. All three polymers are biodegradable, renewable resources with versatile chemical and physical properties. As such, they are the subject of active scientific and commercial scrutiny.

ISSN:1532-1797    

DOI:10.1080/15321799708018366

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

About 8 years ago, Brinker and Robinson [l] invented the first aliphatic polybenzimidazoles. Two years later, aromatic polybenzimidazoles with impressive thermal properties were synthesized by Vogel and Marvel [2, 3] at the University of Illinois and later at Du Pont. Since then, high performance polybenzimidazoles received a great deal of attention by academia, U. S. government, and industry. Various new polybenzimidazoles have been invented, as reviewed in Refs. 4–12. Hoechst Celanese commercialized polybenzimidazole fiber for thermal protective clothing and fireblocking applications in 1983. In the mid 1980s, both Hoechst Celanese and Alpha Performance Company developed the technology to mold polyben- zimidazole parts (under the trade name of Celazole®). Alpha took over Celazole® technology completely in 1995 and continued marketing polybenzimidazole molded parts as sealing elements in high-temperature corrosive environments. Poly (2,2′-(m-phenylene)-5,5′ bibenzimidazole) is their product composition and is referred to as PBI.

ISSN:1532-1797    

DOI:10.1080/15321799708018367

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

There are basically three types of particle-producing polymerization methods: emulsion, dispersion, and suspension. All three methods involve a particle of polymer stabilized by a dispersing agent of some type. In emulsion polymerization the monomer is insoluble in the polymerization medium (usually water) and the polymerization occurs within a surfactant micelle. Suspension polymerization involves the use of the monomer droplets in water and is carried out with an initiator that is soluble in the monomer. The resulting polymer particles are stabilized by a polymeric suspending agent. Dispersion polymerization differs from the other forms of heterogeneous polymerization described above in that the monomer is soluble in the polymerization medium and the polymer is not. As the polymerization proceeds, the growing chains become insoluble and precipitate from the solution. Flocculation is prevented by the presence of an amphipathic graft or block copolymer. The average particle size achieved in this type of process is smaller than the products of suspension polymerization, but larger than the products of emulsion polymerization.

ISSN:1532-1797    

DOI:10.1080/15321799708018368

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

In semicrystalline (SC) polymers, the mechanical and dielectric loss spectra generally show (at least) one absorption peak between the melting temperature Tm, of the crystalline phase [1–14] and the glass transition temperature Tg, of the amorphous phase. The first peak just below Tm, is called αc its amplitude increases with crystallinity and therefore is attributed to reorientation of the chains in the crystalline phase. This reorientation is commonly identified with the flipflop mechanism and/or the screw motion of methylene group in the crystal lattice. According to the soliton like model of Manfield and Boyd [12, 13] collective distortion of the C-C skeleton can propagate through the crystal lattice and in doing so causes trans- port of matter and hence diffusion. Infrared and Raman spectroscopy studies support this model [14]. This transition involving cooperative movements (of the whole crystalline stem) is quite different from the various phase transitions (first order) observed in paraffin crystals (for example, the rotator phases). It is well known that well below the melting temperature annealing effects occurs (change in density, crystallite thickness, long period). Several authors have described these effects as a gradual and re- versible passage toward melting and premelting [15–22]. In the Tαc region the mobility of the crystalline chain has been observed by NMR [18–22], and only a few authors [5, 6, 18] realized that the annealing effect so widely studied appears at that temperature. Obviously, the αc transition is unique to SC polymer; its characteristics will explain the unusual properties of these materials.

ISSN:1532-1797    

DOI:10.1080/15321799708018369

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

This is a scanned image of the original Editorial Board page(s) for this issue.

ISSN:1532-1797    

DOI:10.1080/15321799708018365

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor