What happens upon annealing of pre-drawn semicrystalline polymers?*

As part of efforts to understand mechanical behavior semicrystalline polymers under large deformation, we investigate what happens to predrawn semicrystalline polymers upon annealing at elevated temperatures. For instance, LLDPE drawn at room temperature will neck to its characteristic draw ratio around 5.5 and will shrink by 10% when unloaded but will remain in this highly stretched state for many years. Such predrawn PE samples will either exhibit various levels of retractive stress when held at a fixed length or shrink by different amounts, depending on the annealing temperature. The physical meaning of this characteristic will be explored experimentally, first mechanically and eventually by in situ WAXS and SAXS measurements. Specifically we will search for evidence to support our concept that there is partial melting upon annealing due to the chain tension produced by the pre-cold-drawing, at temperatures well below the melting temperature. The universality of the phenomenology and its molecular interpretation is explored by making a similar investigation of a second semicrystalline polymer, poly(ethylene terephthalate), drawn above its glass transition temperature T = 70 Celsius.