Time‐resolved small‐angle neutron scattering study of polyethylene crystallization from solution

Abstract

With time-resolved small-angle neutron scattering (TR-SANS), the crystallization kinetics of polyethylene from deuterated o-xylene solutions upon a temperature jump have been investigated. On the basis of a morphological model of coexisting lamellar stacks and coil chains in solution, experimental data have been quantitatively analyzed to provide structural information, such as the lamellar long period, the lamellar crystal thickness, the thickness of the amorphous layers between lamellae, the degree of crystallinity, and the crystal growth rate at various degrees of undercooling. The viability of TR-SANS for studying polymer crystallization is demonstrated through the consistency of these measurements and well-established knowledge of polyethylene crystallization from xylene solutions. One unique feature of this experimentation is that both the growth of lamellar crystals and the condensation of coil chains from solution are monitored simultaneously. The ratio of the crystal growth to the chain consumption rate decreases rapidly with a decreasing degree of undercooling. The Avrami analysis suggests that the growth mechanism approaches two-dimensional behavior at higher temperatures, and this is consistent with the observation of an increasing ratio of the sharp-surface area to the bulk crystal growth rate with temperature. The limitations, possible remedies, and potentials of TR-SANS for studying polymer crystallization are discussed.

A comparison on self-seeding and isothermal crystallization of polyethylene in solution using small angle neutron scattering

“…The crystallinity cannot estimated directly because the contrasts between the crystalline and amorphous layers in lamellar stacks are not known. Alternatively, the degree of crystallinity can be estimated from the scattering invariant (Q inv ) as follows [12]. In this calculation, scattering intensity from the incoherent background, and free coils in solution are subtracted from the measured intensity.…”

“…Narrow lamellae peaks at 0.025, 0.050 Å À1 are due to lamellae stacking which depict the 'structure factor' of lamellae stacking. The morphological parameters such as lamellae thickness (L c ), long period of lamellae stacking (L l ) and crystallinity are estimated from model fitting as developed [12]. L l represents the centre to center distance between two lamellae and lamellae stacking.…”

“…S 11 and S 33 are the partial structure factors consisting form, structure factors and coil chains. The detail equations involved in isothermal crystallization of polymer in solution are shown elsewhere [12,13]. We are able to modify model eqn.…”

“…Time sequence spectra at different T c were collected with time intervals varying from 1 min to 3 min for each spectrum. Total coherent scattering cross section per unit volume for polymer crystallization in solution as developed is expressed as [12,13,15] …”

“…0.00001 of the volume fraction degree of crystallinity (comparing to 0.001 typically found in small angle x-ray scattering), allowing for the measurement on the very early stage structural development [11]. Also, TR-SANS allows for simultaneous monitoring of both coils condensation and crystals growth in solutions [12], revealing new insights in polymer crystallization from solution. The details quantitative morphology in the early stage crystallization of PE from solution at different crystallization temperature with time was also estimated [11e13].…”