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Finite shear strain behavior of a crosslinking polydimethylsiloxane near its gel point (1990)

Venkataraman, S. K. ; Winter, H. H.

Springer

Rheologica acta 29 (1990), S. 423-432

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ISSN: 1435-1528

Keywords: Gelpoint ; gelequation ; polydimethylsiloxane ; shear modification

Source: Springer Online Journal Archives 1860-2000

Topics: Chemistry and Pharmacology , Physics

Notes: Abstract A power law distribution of relaxation times, large normal stress differences, and physical rupture of molecular network strands dominate the shear behavior of polymers at the gel point (critical gels). This is shown in a series of well-defined experiments with increasing magnitude of shear on a model-network polymer system consisting of a linear, telechelic, vinyl-terminated poly-dimethylsiloxane (PDMS) and a four-functional siloxane crosslinker. Stable samples were prepared by stopping the crosslinking reaction at different extents of reaction in the vicinity of the gel point (GP). The Gel Equation has been shown to be valid up to strains of about 2 when using a finite strain tensor. Larger strains have been found to disrupt the network structure of the crosslinking polymer, and introduce a mechanical delay to the gel point. A sample that was crosslinked beyond the gel point (p〉p c ) can be reduced from the solid state to a critical gel, or even to a viscoelastic liquid, depending on the magnitude of shear strain. As a consequence, the relaxation exponent of a critical gel created under the influence of shear is less than that of a quiescently crosslinked critical gel.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/BF01376793

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Exploiting the Gibbs-Duhem equation in separation calculations (1986)

Venkataraman, S. ; Lucia, Angelo

Hoboken, NJ : Wiley-Blackwell

AIChE Journal 32 (1986), S. 1057-1066

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ISSN: 0001-1541

Keywords: Chemistry ; Chemical Engineering

Source: Wiley InterScience Backfile Collection 1832-2000

Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology

Notes: Various ways of building quasi-Newton matrix approximations that satisfy the special form of the Gibbs-Duhem equation are studied. Partition symmetry, the separability of the functions in γ and in φ, and the method of iterated projections are used in order to develop thermodynamically consistent matrix approximations with good secant information. Many examples are presented which show that exploiting the special form of the Gibbs-Duhem equation results in improved numerical performance. Ways of exploiting the Gibbs-Helmholtz equation in addition to the special form of Gibbs-Duhem equation, and thus the isobaric form of the Gibbs-Duhem equation, are also discussed.

Additional Material: 3 Ill.