Branched crystal morphology of linear polyethylene crystallized in a two-dimensional diffusion-controlled growth field

Abstract

The branched crystal morphology of linear polyethylene formed at various temperatures from thin films has been studied by atomic-force microscopy (AFM), transmission electron microscopy (TEM), electron diffraction (ED) pattern and polymer decoration technique. Two types of branched patterns, i.e. dendrite and seaweed patterns, have been visualized. The fractal dimension d(f) = 1.65 of both dendrite and some of seaweed patterns was obtained by using the box-counting method, although most of the seaweed patterns are compact. Selected-area ED patterns indicate that the fold stems tilt about 34.5( degrees ) around the b-axis and polymer decoration patterns show that the chain folding direction and regularity in two (200) regions are quite different from each other. Because of chain tilting, branched crystals show three striking features: 1) the lamella-like branches show two (200) regions with different thickness; 2) the crystals usually bend towards the thin region; 3) the thick region grows faster by developing branches, thus branches usually occur outside the thick region. The branched patterns show a characteristic width w, which gives a linear relationship with the crystallization temperature on a semilogarithmic plot.

Crystallization in Polymer Thin Films: Morphology and Growth

“…Similar techniques have been used to identify the dendrite morphology along with their growth. 26,27,34,35,38,39 Dendrites are single-crystal like and grow flat-on as compared to edge-on as confirmed by the ED technique. The branching occurs along the growth faces and is caused by a difference in the diffusion rate of the chain and the growth rate.…”

“…In the work of Zhang and coworkers, 39 the curvature occurs in the flat-on, dendritic branches. These crystals also exhibit two thicknesses along the (200) direction.…”

“…Above 10 nm, w is constant and then is linearly proportional with 1/d. Zhang and coworkers 39 show that the width of CS fingers has an exponential relationship with crystallization temperature at a film thickness of 50 nm.…”

“…39 In both of these studies, the branch width, w, was compared to the given parameter. For the film thickness (d ), Taguchi et al 35 showed that the branch width increases with one over the thickness (1/d ).…”

Studies on confined crystallization behavior of polycaprolactone thin films

“…Two types of branched patterns, i.e. dendrite and seaweed patterns were visualized by He et al [6] with AFM and TEM (electron microscopy) for linear polyethylene crystallized in thin films. Kressler and Wang [7] studied the structure formation of poly (e-caprolactone) thin films, during spin-casting and subsequent thermal annealing by scanning electron microscopy and atomic force microscopy.…”