The effect of pressure pulses on crystallization of isotactic polypropylene: a dilatometric study

The effect of hydrostatic pressure changes on the crystallization kinetics of semi-crystalline polymers is of great importance in polymer physics and technology. The occurrence of step-like pressure changes during polymer solidification are often encountered in melt processing, like for instance in injection moulding. Similar to the effect of shear flow pulses, short-term pressurizations can be expected to accelerate the crystallization process due to the formation of additional nucleation precursors . Nonetheless, in contrast to the case of flow, the influence of pressure changes on nucleation and crystallization has received much less attentions so far. In this study, we investigate this hypothesis by means of high pressure dilatometry in a confining fluid apparatus to assess specific volume changes during isothermal crystallization upon pressure pulses. The dilatometric characterization of a commercial iPP homopolymer was carried out using a designed short-term pressure protocol . The samples were cooled from the melt to a temperature selected for the subsequent isothermal crystallization at constant rate and pressure (Pc = 10 MPa). After temperature stabilization, the pressure was increased to a set value and, after 60 s (tp), released back to Pc. The process was then monitored until complete crystallization at constant temperature and pressure. The pressurization time tp was chosen much shorter than crystallization time scale for the set undercooling condition, so that the nucleation event was clearly separated from growth of spherulites. The time evolution of the space filling and the crystallization half-time (t1/2) were evaluated from the decrease of the sample specific volume during the experiment . The experimental data show an enhancement of crystallization kinetics proportional to the magnitude of the pressure pulse. When increasing the pressure pulse from 20 to 70 MPa, t1/2 halves with respect to the isothermal experiment without pulse. On the other hand, the pulse magnitude does not have any effect on the final amount of crystallinity. To validate these observations, the results were matched with in-situ time resolved synchrotron X-ray measurements carried out with an analogous protocol. The X-ray analysis confirms the kinetics acceleration enlightened by dilatometry. Furthermore, it reveals the presence of a small and constant amount of the orthorhombic phase, together with the expected  monoclinic crystal. The observed acceleration is linked to an increase of the number of active nuclei after the short-term pressurization, as confirmed by ex-situ optical microscopy observations. Up to an order of magnitude increase in nucleation density is found, for pressure pulses around 60–70 MPa. The pressure-induced nucleating effect is interpreted in the light of classical nucleation theory. Lastly, to investigate a possible dependence of the observed trend of t1/2 on the crystallization conditions, experiments were performed at a higher Pc (30 MPa) but equal degree of undercooling. Also in this case crystallization kinetics are accelerated when increasing the magnitude of the pressure pulse, even though with a different proportionality relation between t1/2 and Pc. In light of this result, the dependence of the crystallization acceleration on Pc is currently being investigated.