摘要:

AbstractWe will review experimental results obtained recently on the determination of the laws governing the growth process of polymer clusters as the gel point is approached. The exponent γ which characterizes the increase of the mean weight‐average molecular weightMwas the gel point is approached and the exponent τ which characterizes the mass distribution were measured on different chemical systems. They were found to be independent of the chemical system (within experimental error) and very close to exponent values calculated by computer simulations following the percolation model. Therefore, the sol‐gel transition is a critical phenomenon of connectivity belonging to the same class of universality as percol

ISSN:0258-0322    

DOI:10.1002/masy.19910450104

出版商:Hüthig&Wepf Verlag    

年代:1991

数据来源: WILEY

摘要:

AbstractThe linear viscoelasticity of polymers near the gel point can be described by two scaling laws. The material at the gel point has a power‐law linear viscoelastic relaxation modulus, and the relaxation exponent has been found to vary with the composition of the precursor materials, i.e., it is not universal for gelation. A second scaling law describes the evolution of the linear viscoelastic properties through the gel point. The rate of change of the dynamic mechanical modulus/viscosity is observed to scale as a power‐law function of frequency. This power‐law function defines a dynamic critical exponent, and this has been found to be independent of precursor composition for end‐linking poly(dimethylsiloxane) polymers and equal to κ = 0.21 ± 0.02. This exponent may be a universal measure of gelation. The technique of Time Resolved Mechanical Spectroscopy is used to observe the evolution of linear viscoelastic properties of crosslinking polymersin situin the rheometer. A stretched exponential relaxation modulus describes the evolution of mechanical properties in the vicinity of the gel point very well. The exponents which characterize the divergence of the zero‐shear viscosity and the equilibrium modulus are not universal, since they are related to the relaxation exponent and the dynamic critic

ISSN:0258-0322    

DOI:10.1002/masy.19910450105

出版商:Hüthig&Wepf Verlag    

年代:1991

数据来源: WILEY

摘要:

AbstractRheological measurements have been performed on pectin biopolymers close to the sol‐gel transition. From these measurements scaling exponents were determined independently for the viscosity, s = 0.82(5), for the elastic modulus, t = 1.93(8), for the frequency‐dependent modulus, Δ = 0.71(2), and for the relaxation times below and above the transition, vz = 2.67(12) and vz' = 2.65(9). The exponents satisfy the scaling relations predicted by the theory and their numerical values agree with those from scalar elasticity percola

ISSN:0258-0322    

DOI:10.1002/masy.19910450106

出版商:Hüthig&Wepf Verlag    

年代:1991

数据来源: WILEY

摘要:

AbstractUltrasonics measurements were performed during the cure of epoxy resins. The results show some different comportments if there is only the gelation process which passes through the frequency window of the study or if there is also a vitrification phenomenon. The last case occurs with a high glass transition temperature epoxy resin like DGEBA‐DDS. with which the increasing glass transition temperature of the resin is rapidly higher than the study temperature. The former case happens for low Tg epoxy resin like BDGE‐HMDA. But in any case the method is very sensitive to the mechanical properties evolution of the mater

ISSN:0258-0322    

DOI:10.1002/masy.19910450107

出版商:Hüthig&Wepf Verlag    

年代:1991

数据来源: WILEY

摘要:

AbstractWe report a first set of results of Molecular Dynamics simulations of phase separation in a two‐dimensional system of identical particles bearing attachment sites at their surface. Morphology of regions of aggregation so obtained appear similar to images of biologically self‐assembled structures. Also, they evidence the relevance of the region of thermodynamic instability and of the presence and range of interaction of specific attachment sites. Particles of our system are apt to represent indifferently, though grossly, globular proteins or rigid cells, bearing specific and fixed adhesion sites at their surface. Results illustrate some basic aspects of the origin of the morphology of extended biomolecular/cellular structures, self‐organized from initially homogeneous solu

ISSN:0258-0322    

DOI:10.1002/masy.19910450108

出版商:Hüthig&Wepf Verlag    

年代:1991

数据来源: WILEY

摘要:

AbstractThe peculiarities of the structurisation of polydimethylsiloxane with carboxyl side groups are investigated by studying its viscosity, elasticity and thermomechanical behaviour over a large temperature interval. The structurisation exhibited in the transition from initial liquid to elastomeric state is shown to be thermoreversible. It occurs more intensively as temperature increases. The cooling of the polymer to room temperature leads to the liquefaction of the elastomer proceeding over a long time; this time depending on the temperature and the length of time of heating. The reasons for these peculiarities lie in the relation between intra‐ and intermolecular interactions between COOH‐groups and the specific changes of polydimethylsiloxane conformations with temperat

ISSN:0258-0322    

DOI:10.1002/masy.19910450109

出版商:Hüthig&Wepf Verlag    

年代:1991

数据来源: WILEY

摘要:

AbstractIn contrast to common curing reactions, the anhydride curing of epoxies follows a living anionic chain growth. The resulting consequences of this mechanism, i.e. (1) DPn= a[Mo]/[Io], (2) first‐order kinetics and (3) Poisson chain‐length distribution were tested with the phenyl glycidyl ether/phthalic acid anhydride system, using l‐methyl imidazole. Overall agreement was found and the observed deviations could be explained with a modified Poisson process.Conformational properties of the resins were measured by static and dynamic light scattering and by viscometry. These were compared with the quantities of a corresponding branched system prepared with a mixture of phenyl glycidyl ether and bisphenol‐A diglycidyl ether. Typical deviations to smaller dimensions were observed at high molar masses as a result of increasing br

ISSN:0258-0322    

DOI:10.1002/masy.19910450110

出版商:Hüthig&Wepf Verlag    

年代:1991

数据来源: WILEY

摘要:

AbstractThe copolymerization of an epoxy resin [1, 4‐butanediol diglycidyl ether (DGEB) (Note a)] with an anhydride [cis‐1, 2‐cyclohexanedicarboxylic anhydride (CH)]in the presence of N, N‐benzyldimethylamine (CA) as a catalyst produces a branched epoxy polymer. We show that the branching kinetics of the copolymerization reaction and the molecular weight distribution of the branched polymers can be approximated by using Smoluchowski's coagulation equation. In the simplest relevant application of this equation to our problem, the overall rate kernel w(u, v) that describes the branching probability in the equation turns out to be proportional to the sum of active sites on the two polymers with a time dependent coefficient. The molecular weight distribution (MWD) and the weight average molecular weight of the branched copolymers at different reaction stages before the gelation threshold are calculated theoretically. The calculated values are then compared with the experimental results obtained by using small angle X‐ray scattering (SAXS), laser light scattering (LLS), and chemical analysis. Satisfactory agreement between experimental results and the use of the coagulation equation is attained when it is assumed that the distribution of epoxy polymer molecules is exponential in the number of branching points or, equivalently, active sites, at an early stage of the polymerization

ISSN:0258-0322    

DOI:10.1002/masy.19910450111

出版商:Hüthig&Wepf Verlag    

年代:1991

数据来源: WILEY

摘要:

AbstractGel fraction and equilibrium elastic modulus of networks formed from α, ω‐dihydroxypoly(oxypropylene) and bis(4‐isocyanatophenyl)methane (MDI) as a result of side reactions were compared with theoretical predictions based on the theory of branching processes (Macromolecules, 23, 1774 (1990)). If isocyanate is in excess, trifunctional allophanate and biuret groups are formed. The experimental sol fraction can be correlated with theoretical predictions. The resulting networks are rather weak and the measured equilibrium modulus can be correlated with the theoretical values calculated from the concentration of elastically active network chains if the value of the front factor is close to that of a phantom network without a trapped entanglements contrib

ISSN:0258-0322    

DOI:10.1002/masy.19910450112

出版商:Hüthig&Wepf Verlag    

年代:1991

数据来源: WILEY

摘要:

AbstractNetworks were prepared by copolycyclotrimerization of aromatic dicyanates with diphenols in bulk. The main reactions and structural elements were found and the network building process was modelled by cascade theory combined with a model of the reaction kinetics. Good agreement between theory and experiment is shown.

ISSN:0258-0322    

DOI:10.1002/masy.19910450113

出版商:Hüthig&Wepf Verlag    

年代:1991

数据来源: WILEY