Showing papers by "Guy C. Berry published in 1997"

On the use of stretched-exponential functions for both linear viscoelastic creep and stress relaxation

Abstract: The use of the stretched-exponential function to represent both the relaxation function g(t)=(G(t)-G ∞)/(G 0-G ∞) and the retardation function r(t) = (J ∞+t/η-J(t))/(J ∞-J 0) of linear viscoelasticity for a given material is investigated. That is, if g(t) is given by exp (−(t/τ)β), can r(t) be represented as exp (−(t/λ)µ) for a linear viscoelastic fluid or solid? Here J(t) is the creep compliance, G(t) is the shear modulus, η is the viscosity (η−1 is finite for a fluid and zero for a solid), G ∞ is the equilibrium modulus G e for a solid or zero for a fluid, J ∞ is 1/G e for a solid or the steady-state recoverable compliance for a fluid, G 0= 1/J 0 is the instantaneous modulus, and t is the time. It is concluded that g(t) and r(t) cannot both exactly by stretched-exponential functions for a given material. Nevertheless, it is found that both g(t) and r(t) can be approximately represented by stretched-exponential functions for the special case of a fluid with exponents β=µ in the range 0.5 to 0.6, with the correspondence being very close with β=µ=0.5 and λ=2τ. Otherwise, the functions g(t) and r(t) differ, with the deviation being marked for solids. The possible application of a stretched-exponential to represent r(t) for a critical gel is discussed.

Static and Dynamic Light Scattering

Abstract: Static and dynamic light scattering methods for use in the characterization of dilute solutions of polymers or suspensions of dispersed particles are presented. The theoretical foundations are summarized to give the expressions most often utilized, central issues in the calibration and use of light scattering photometers are considered, and several examples are discussed, including the use of light scattering as a detector in connection with size exclusion chromatograpy.