Ageing of isotactic polypropylene due to morphology evolution, experimental limitations of realtime density measurements with a gradient column

Ageing in crystalline polymers is responsible for the deterioration of physical properties leading, for example, to a decrease in toughness and to dimensional changes that are to some extent responsible for warpage and scrap production in injection molding. Since, it depends on the mutual transformation of stable and metastable phases, being always related to changes in morphological organization, it is here preferred to call it 'Morphological ageing'.Although, one would expect the ageing regime to be determined by the complex morphology with amorphous phases of different mobility and eventually multiple crystalline phases, transformed into each other at an associated transition, existing literature always shows a more trivial linear with the logarithm of time dependence of every probe used to describe ageing.Existing literature always overlooks the initial morphology, often complex, or, adopts well equilibrated samples, not representative of processing conditions. In this work, ageing of iPP was addressed producing samples characterized by an homogeneous morphology even at the largest cooling rates adopted using a CCT approach. This paper describes few of the many attempts undertaken to quantitatively relate ageing to initial morphology in iPP melt solidified under conditions emulating polymer processing.Ageing was monitored by measuring the density time dependence offline, i.e. separately applying the ageing protocol, and online in a gradient column conditioned at the ageing temperature following samples' apparent density evolution. The offline method can give hints about the role of temperature and initial morphology but the results so obtained suffer from significant data scattering. The online method on the other hand, can provide a more accurate interpretation of ageing if due account is made of all the features of the fluid dynamic transient superimposed on the densification due to morphological ageing.Densification during ageing of the iPP samples used, solidified between 1 and 100 °C/s, takes place as a superposition of two phenomena: one is the linear increase of density with the logarithm of time, which is quantitatively related to the initial morphology, i.e. the cooling rate, and the other is a density jump that takes place at times much smaller than the available resolution and thus not quantitatively accessible.