Nature of self-seeding polyethylene crystal nuclei

Abstract

In a limited temperature range above the clearing point of crystal suspensions, submicroscopic entities representing unmeasurably small portions of the original crystal population may survive whose existence is revealed by crystals nucleated on subsequent cooling [l]. Factors influencing the number of these nuclei and the nature of these nuclei themselves were explored in the case of polyethylene.It was found that the number of nuclei a s assessed from the dimensions of the uniform-sized seeded crystals depends on the temperature to which the suspension was heated (Ts), the rate of heating to Ts, the morphology of suspension before dissolution, and the molecular weight distribution. It was not affected by the time the suspension was kept at Ts o r by the dilution of the starting suspension, and only slightly by the subsequent crystallization conditions. trusions within self -seeded crystals. They were also detected and analyzed by light scattering above the clearing point of the suspensions. The picture emerges that they consist of high molecular weight material which has been appropriately stabilized by refolding on heating up. The observed correspondence of mass and number of nuclei with the high end tail of the molecular weight distribution of the starting material is consistent with each nucleus being a monomolecular crystal. Furthermore, the observed temperature depenThe nuclei were identified electromicroscopically a s central pro-* P r e s e n t address: Petrochemical and Polymer Laboratory, Imperial Chemical Industries LM., Runcorn, Cheshire, England.

Fast Scanning Calorimetry

“…The melting of crystals at low temperatures near T g retains nuclei that can enhance cold-crystallization. For high melting temperatures such nucleus retention has in the past been called selfnucleation and the nuclei were found to survive heating above the equilibrium melting temperature [226][227][228][229][230][231]. This research has, thus, opened a new direction of inquiry to gather quantitative information about nucleation and growth of polymer crystals as it affects the amorphous and crystalline phases and their interrelationship.…”

Growth kinetics of polyethylene single crystals: (II) Growth of {110} faces of crystals from dilute solutions in decalin and in octane

“…1. It is known that the T8 varies not onty with the molecular weight of the polymer, but is also dependent on the previous temperature (Tez) at which the crystals were formed (8). When crystallizing from decalin solution Tez = 80 °C was used, whilst with octane Tet = 95 °C.…”

“…Growth rates were measured at various temperatures and concentrations using experimental techniques identical to those described earlier (1). As before, the selfseeding method (7,8) was used to initiate crystal growth at the desired temperature and concentration. The dissolution temperatures, T,, to which crystal suspensions were heated in order to leave a suitable density of nuclei are shown in tab.…”

Thermal properties of polycarbonate grade bisphenol A

“…Such a procedure allowed some crystals to be left intact so that during the cooling cycle samples crystallized with the remaining crystals acting as self-seeded nuclei. (This seeding phenomenon is the subject of the paper by Blundel and Keller [11].) When the samples were analyzed on heating, smooth DSC traces were recorded in all cases showing the melting process at 157~ and crystallinities close to 100%.…”