A theory of yielding of glassy polymers by thermally-activated production of local molecular kinks is described. It is possible to obtain the activation free enthalpy of this process by modeling the intermolecular energy barrier as resulting from the stress fields of two equal and opposite closely spaced wedge disclination loops extending over the molecular cross section at the points of rotation of the molecular kinks. The theory predicts the yield stress at absolute zero to be dependent only on the shear modulus and the Poisson's ratio, and is capable of describing the temperature, pressure, and strain rate dependences of the flow stress from absolute zero to near the glass transition temperature. Comparison of the theory with the available experimental, results on polystyrene, polyethylene-terephthalate, polycarbonate of bisphenol A, and poly-methyl-methacrylate shows excellent agreement in nearly all respects.

A theory for the low-temperature plastic deformation of glassy polymers

Publisher Summary This chapter focuses on measurements and theories pertaining to homogeneous nucleation under cooled liquids and glasses that transform to phases of the same composition. The dynamical model describing the cluster evolution appears to be quantitatively correct. Although there are some experimental and theoretical difficulties with the classical theory, there are also many approaches to rectify these problems. At present, there are no experiments that can demonstrate or can refute conclusively the validity of the classical theory in the regime where it is expected to be valid. This is partially because of a lack of knowledge of the correct values for the parameters that are fit by the theory. Better measurements of the steady-state and time-dependent nucleation rates and the free-energy difference as a function of temperature, and the design of new experiments such as the multistep annealing treatments discussed in the chapter are used to test more rigorously nucleation theories. Time-dependent nucleation and the interface kinetics are incorporated into the field-theoretic approaches to give theories that are as widely applicable and, hence, testable as the classical theory.

Crystal Nucleation in Liquids and Glasses

The nucleation of crystals in liquids is one of nature’s most ubiquitous phenomena, playing an important role in areas such as climate change and the production of drugs. As the early stages of nucleation involve exceedingly small time and length scales, atomistic computer simulations can provide unique insights into the microscopic aspects of crystallization. In this review, we take stock of the numerous molecular dynamics simulations that, in the past few decades, have unraveled crucial aspects of crystal nucleation in liquids. We put into context the theoretical framework of classical nucleation theory and the state-of-the-art computational methods by reviewing simulations of such processes as ice nucleation and the crystallization of molecules in solutions. We shall see that molecular dynamics simulations have provided key insights into diverse nucleation scenarios, ranging from colloidal particles to natural gas hydrates, and that, as a result, the general applicability of classical nucleation theory...

Crystal Nucleation in Liquids: Open Questions and Future Challenges in Molecular Dynamics Simulations.

Nucleation in solids: The induction and steady state effects

Recent theoretical work on the energetics and formation kinetics of helium bubbles in metals is reviewed. The energetics are discussed in particular for bubbles with radii between 10 and 1000 A containing helium under very high pressures (nonideal gas bubbles). For this size range, the bubble formation free energy is split into three parts: a helium bulk free energy which is considered for solid and fluid helium (high-density equation of state), a bubble-matrix interface free energy for which curvature corrections are introduced, and a relaxation energy which is treated within the elastic continuum approach. The formation kinetics are considered for two extreme cases. For high helium production rates and low temperatures, helium clustering is associated with athermal processes such as metal interstitial emission and dislocation loop punching. For dislocation loop punching, an interpolation between computer simulation and continuum-theory results is discussed. For low helium production rates and h...

K.C Russell

Papers

Nucleation of voids in irradiated metals

The theory of void nucleation in metals

Linked flux analysis of nucleation in condensed phases

Grain boundary nucleation kinetics

Particle stability with recoil resolution