Effect of annealing time on the self-nucleation behavior of semicrystalline polymers

It is widely known that when a polymer is heated just above its melting point and is kept at a given temperature (denoted T-s) for a short time, when it is cooled down its nucleation density increases and its peak crystallization temperature shifts to higher temperatures, as detected for instance by differential scanning calorimetry (DSC). The T-s temperature range where the described process occurs has been named Domain II self-nucleation (SN) because the selected T-s temperatures are high enough to melt the polymer without causing detectable annealing of any remnant crystals by DSC. Experimental results obtained by DSC, polarized light optical microscopy (PLOM), and rheology indicate that these techniques are unable to detect any remaining crystal fragments in Domain II. Our kinetic results demonstrate that Domain II SN is a transient phenomenon that can even disappear if enough time at T-s is allowed. Results of the study of the time dependence of the SN effect indicates two possibilities: (a) if crystal fragments are present (even if undetected by the employed techniques) their final melting is a very slow process (in the order of hours); (b) if all crystallites have melted in Domain II, then it may be more plausible to reinterpret self-nuclei as arising from "precursors" whose detail nature has not been the subject of this investigation but that can be regarded as either a residual segmental orientation in the melt (i.e., a melt memory effect) or a mesophase in a preordered state. (c) 2006 Wiley Periodicals, Inc.