A perspective on the time‐dependent structure and properties of amorphous polymer

Abstract

A molecular mechanism of viscoelastic deformation is presented. Amorphous polymers are characterized as a heterogeneous network of nanoscale cells. A “cell” is a homogeneous region within the network. Each cell phase fluctuates between the glassy and elastomer states with a period τ. The values of τ vary from cell to cell over several orders of magnitude and are strong functions of temperature. Whether a given cell responds as glassy or elastomeric depends on the ratio between its period of phase fluctuation (τj) and the period of observation (1/f), where f is the test frequency. We express this ratio, the Deborah number, as (τj × f), and present equations for modulus and energy loss as functions of (τj × f). The condition (τj × f) = 1 defines the glass transition of a cell, which arises cell‐by‐cell over a range of temperature, or over a range of time under stress. Viscoelastic deformation occurs if a cell changes phase while under stress. Energy stored during glassy state deformation is lost if the cell fluctuates to the elastomeric state. Stress is out of phase with strain, because strain rate controls the level of stress in glassy cells between phase fluctuations.Highlights A molecular mechanism of viscoelasticity is presented that does not involve viscous flow. Amorphous polymer is a heterogeneous network of nanoscale cells. Cells fluctuate in phase between glassy and elastomeric states due to their size. Time‐dependent properties are caused by time‐dependent structure. Energy is lost when glass phase cells under stress fluctuate to the elastomer phase.