Crystal nucleation in molten polymers cooled below the equilibrium freezing point is technologically important — the "grain size" of the solid is set by the density of nuclei. Experiments suggest that PE and perhaps other polymers nucleate via a metastable rotator phase. How can we use theory to ask if that scenario is plausible?
Portion of simulated C23 alkane crystal, from atomistic MD simulation. The molecules arrange themselves in layers; within each layer, nearly parallel all-trans chain stems viewed from above appear packed in a herringbone fashion.
Crystal nucleation in supercooled molten polymers proceeds hundreds of times faster if the melt is exposed to a brief period of fast shear flow. Is the the aligning effect of the flow in lowering the entropy of the polymer chains enough to explain this effect? Or do minority components of especially long chains play a dominant role?
Optical micrographs of extrudate cross-sections, showing oriented "skin" (white layer) and spherulite "grain" size varying with radius (smaller in interior, where shear flow was weaker). (From Kumaraswamy et al. Shear-enhanced crystallization in isotactic polypropylene. 1. Correspondence between in situ rheo-optics and ex situ structure determination. Macromolecules (1999) 32, 7537.)
Long alkanes and polyethylene (PE) exhibit a partially ordered "rotator" phase, in which molecules adopt straight conformations and pack like pencils in a box, but with random rotations about their axes. The molecules do not turn "all at once"; instead, a localized "twist" zips along the molecule. These "twist solitons" also play a role in deformation of crystalline PE. What sets the size, energy, and mobility of these defects?
Schematic of known crystalline order X and proposed disorder in rotator R1 and R2 phases of normal alkanes. In R1, alkane all-trans chain stems are randomly rotated by 0 or 90 degrees relative to the crystal (bright or dark green). In R2, in-plane stem projections point randomly between nearest neighbors in any of three orientations (red, green blue).
How does flow affect structure and alignment in complex fluids, such as phase-separating polymer solutions and block copolymer mesophases? How can we perform efficient simulations of such mesoscale systems in flow?
Schematic of lamellar mesophase, represented with local phase-field variable defined only on the coarse grid of points (black dots).