  Necking or cold drawing is a smoothed jump in cross-sectional area of long
and thin bars (filaments orfilms) propagating with a constant speed. The necks
in polymers, first observed about seventy years ago, arenow commonly used in
modern processing of polymer films and fibers. Yet till recently there was a
lack infundamental understanding of necking mechanism(s). For semi-crystalline
polymers with co-existingamorphous and crystalline phases, recent experiments
revealed that such a mechanism is related tounfolding crystalline blocks. Using
this idea, this paper develops a theoretical model and includes it in ageneral
continuum framework. Additionally, the paper explains the forced (reversible)
elasticity observedin slowly propagating polymeric necks, and also briefly
analyses the viscoelastic effects and dissipative heatgeneration when polymer
necks propagate fast enough.
