摘要:

AbstractThis paper reviews the modern ideas on the process of gelation which have arisen from the analogy between gelation and percolation. The basic features which are common to all types of gels, colloidal or polymeric systems are first recalled; then after a simplified presentation of the percolation model, a few examples illustrating these concepts are chosen (for chemical gelation, a copolymerisation reaction and for physical gelation, the gelatin sol‐gel transition). In conclusion, electron micrographs of gel networks (gelatin gel and an inorganic gel of thorium phosphate) are shown, which reveal the great diversity and complexity of the structure

ISSN:0959-8103    

DOI:10.1002/pi.4990250302

出版商:John Wiley&Sons, Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractAn attempt was made to clarify the change in the viscosity‐average degree of polymerisation (Pv) of a bacterial cellulose within a cell cycle using the technique of synchronous static culture conditions of an acetic acid bacterium (Acetobacter xylinum), formerly established by us (British Polymer J.,22(1990) 167). Under these conditions the yield of bacterial cellulose and itsPvwere found to increase stepwise within a cell cycle. Furthermore,Pvof the cellulose increased rapidly in the cultivation stage where the production rate of cellulose is quite low. As soon as the cultivation went into the first stage wherePvof the bacterial cellulose increased very slowly, its infrared absorbance for C—O stretching with parallel infrared dichroism strongly increased. The maximumPvattainable for cellulose obtained under the synchronous culture conditions used here wasc. 1500. The time length of a cell cycle for the synchronous culture was controlled by adjusting the air sup

ISSN:0959-8103    

DOI:10.1002/pi.4990250303

出版商:John Wiley&Sons, Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractThe structural modifications and the phase state of the gelatin matrix during the sol‐gel transition and gelatin gel deswelling were studied by optical rotation and wide‐angle X‐ray scattering (WAXS) methods. It is shown that the quenching of a solution results in rapid renaturation of macromolecular fragments of gelatin and in collagen‐like structures forming junction zones in the polymer network. Microcrystallites in the polymer matrix of gelatin gels were not revealed which shows that the aggregation of renaturated helices and the formation of collagen‐like fibrillar crystals do not occur. However, the crystalline phase appears upon gel drying or its deswelling in a n

ISSN:0959-8103    

DOI:10.1002/pi.4990250304

出版商:John Wiley&Sons, Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractA styrene‐arsenic sulphide complex in DMSO at 85 ± 0.1°C initiates radical polymerization of methyl acrylate over 90 min under a N2blanket. Conversion reaches 83% without gelation and results in the formation of polymer having average degree of polymerization 710. Analysis of the kinetic data (up to 29% conversion) indicates that the system follows ideal kinetics. The reaction is retarded by hydroquinone. The overall activation energy and value ofk2p/ktare 96 kJ mol−1and 6.9 × 10−3litre mol−1s−1,

ISSN:0959-8103    

DOI:10.1002/pi.4990250305

出版商:John Wiley&Sons, Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractSensitive components of foods may undergo degradation reactions when subjected to light. The packaging of the food may slow down such reactions by absorbing and reflecting a fraction of the incident light. Riboflavin is a typical lightsensitive nutrient and undergoes first‐order degradation when subjected to light. Rate constants of this reaction were evaluated under 25W, 60W, 100W and 200W white light bulbs at pH = 8, at 20±1°C, with no light shielding or by shielding the light with a single fold of high density polyethylene or two folds of low density polyethylene films. Numerical values of the reaction rate constants were proportional to the light source power and varied with the number of polymer films. The proportionality constant was regarded as the ‘protection index’ of the specific film. The results implied that the smaller the protection index of the film the smaller was the degradation reaction rate constant. Although this study was done by employing two different kinds of polymer films and one light‐sensitive chemical only, this approach may be employed for comparing the packaging efficiency of polymers used for wrapping foods containing light‐sensitive

ISSN:0959-8103    

DOI:10.1002/pi.4990250306

出版商:John Wiley&Sons, Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractThe DC electrical conductivity of pure and Fe3+‐doped polystyrene films has been studied at various bias field strengths in the temperature range 304–392K. Doped films showed increased conductivity compared with undoped films up to 0.5% dopant concentration; beyond this concentration there was a decrease in the conductivity. The increase in conductivity for lower doping concentrations (0.25% and 0.5%) is attributed to the formation of charge transfer complexes. The decrease in the electrical conductivity and the increase in the activation energy at more than 0.5% dopant concentration is due to the formation of molecular aggregates. For undoped films the activation energies in the low temperature region were found to be higher than those at higher temperatures. Also, with increasing bias field strength, the activation energies showed a decrease at all temperatures studied. It is suggested, on the basis of the magnitudes of the activation energies, that electronic conduction is the dominant charge transport mechanism both at low and high temperatu

ISSN:0959-8103    

DOI:10.1002/pi.4990250307

出版商:John Wiley&Sons, Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractPolyurethane‐polystyrene sequential interpenetrating polymer networks have been synthesized from castor oil, diphenylmethane diisocyanate, styrene, ethylene glycol dimethacrylate as a crosslinker and benzoyl peroxide as an activator. The interpenetrating polymer networks were obtained as tough and transparent films by a transfer moulding technique. These IPNs were characterized by mechanical properties, viz. tensile strength, Young's modulus, elongation at break and Shore A hardness, and crosslink density. The morphological behaviour was studied by dynamic mechanical analysis and scanning electron microscop

ISSN:0959-8103    

DOI:10.1002/pi.4990250308

出版商:John Wiley&Sons, Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractIt was found that radical polymerization ofN‐cyclohexylmaleimide (CHMI) occurred readily under various polymerization conditions, and that the resulting poly(CHMI) had excellent thermal stability; i.e. the onset of decomposition was 362°C and the maximum rate of decomposition was observed at 430°C by thermogravimetric analysis in nitrogen. Subsequently, radical copolymerizations of CHMI with various vinyl monomers were performed, and the monomer reactivity ratios were determined from the copolymer composition curves obtained. It was found that CHMI copolymerized alternatingly with electron‐donating monomers such as styrene and isobutene. It was also discovered that facile copolymerizations proceeded with methyl acrylate and methyl methacrylate as electron‐accepting monomers, and with vinyl acetate and vinyl chloride as non‐conjugated monomers. It was verified that the introduction of the CHMI unit increased the thermal stability of common vinyl

ISSN:0959-8103    

DOI:10.1002/pi.4990250309

出版商:John Wiley&Sons, Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractThe enzyme alcohol dehydrogenase (ADH) from horse liver (EC 1.1.1.1), cross‐linked through the bifunctional reagent glutaraldehyde, was immobilized in nylon tubing (35 μg/cm2internal surface). The intrinsicv([S]) kinetic behaviour of the immobilized ADH, analysed graphically and by non‐linear statistical fitting, was found to fit a polynomial quotient rate equation in [S]of minimum degree

ISSN:0959-8103    

DOI:10.1002/pi.4990250310

出版商:John Wiley&Sons, Ltd    

年代:1991

数据来源: WILEY

ISSN:0959-8103    

DOI:10.1002/pi.4990250312

出版商:John Wiley&Sons, Ltd    

年代:1991

数据来源: WILEY