SynopsisA high molecular weight poly(ethy1ene oxide) is gelled by radiation from a @' Co source. At concentrations of 0.25 to I we%, doses of 4.5 to 7.5 X lo4 rad cause gelation. The storage modulus of the gel formed increases linearly with additional dose to a plateau value and then remains constant during further irradiation. The dose needed to reach the plateau, D, in rads, and the plateau modulus G, in dynes per square centimeter, are related to the concentration c, in weight per cent: D, = 1.05 X 10% and G, = 1.75 X 1 0 4~2 .~. The energy absorbed by the polymer per crosslink formed is about 80 f 10 kcal/mol. The crosslink density produced by radiation is calculated from the measured modulus by using the theory of rubber elasticity. Both modulus and loss are measured in a special air-bearing, recording torsion pendulum with a coneplate geometry.

SynopsisReticulation of a dilute aqueous solution of poly(ethy1erie oxide) in a WCo gammaradiation field yields a weak gel which is characterized by its damped sinusoidal oscillation in a free-swinging torsion pendulum. A storage and a loss modulus are calculated from the log decrement and frequency of oscillation, and the crosslink density is obtained from the storage modulus using the kinetic equation of rubber elasticity. The number of crosslink added to the polymer per 100 eV of energy absorbed in the polymer above and below the gel point and the fraction of gamma energy absorbed directly by the polymer were determined from the dose-to-gel and modulus data. The effect of oxygen on the crosslinking reaction and the temperature on the gel's mechanical properties were experimentally observed.

SynopsisA recording torsion pendulum has been designed which allows the measurement of storage modulus and loss modulus for weak gels. The gel is held in a cone-plate geometry. The 5" cone, along with the inertial elements of the pendulum, is supported by an air-lubricated bearing. The decay of oscillations is sensed by a Proximeter which does not touch the moving apparatus and which converts the amplitude of oscillation into a dc signal of several volts. Storage moduli as low as 40 dyn/cm2 and log decrements as low as 0.0054 have been measured.The dynamic mechanical properties of weak, fragile gels are not easily measured in conventional instruments. With self-healing gels such as those formed by soaps in lubricating oil, many kinds of penetrometers and consistometers are available which measure gel strength, rigidity, and recovery. However, with gels of covalently crosslinked polymers in water, only small strains and stresses can be imposed without irreversibly disrupting the network structure. The torsion pendulum, properly designed and operated, can yield useful results within the limits of a shear strain less than 0.5 and with shear moduli down to 40 dyn/cm2.When a sample of a material is used as the restoring element in a torsion pendulum, the dynamic modulus and loss for the material can be estimated from the geometry of the sample together with the frequency and the decay rate of the oscillations. If the experiment can be confined to the region of linear viscoelasticity and if the damping of the oscillations is not excessive, the frequency of oscillation f (in radians per second) will be a function only of the moment of inertia of the system 1 (in g-cm2) and the stiffness of the sample as torque k per unit of twist (in dyne-centimeters per radian) :A useful measure of the damping properties of the material is t,he log decrement A, which is the decay rate of maximum amplitude em with cycle number n :