On the morphology of poly(vinylidene fluoride) crystals in blends

Change in Concentration of Amorphous Region Due to Crystallization in PTT/PET Miscible Blends

In a miscible crystalline/crystalline blend of poly(trimethylene terephthalate) (PTT) and poly(ethylene terephthalate) (PET), the PET spherulites grew at 240 °C when the PTT content was 30 wt% or less. The growth rate of PET spherulites decreased with time due to the exclusion of PTT from the growth front of PET spherulites into the amorphous region, resulting in a three-stage crystallization process. Due to the exclusion, the spherulite growth stopped before the volume filling of the PET spherulites, causing the formation of an excluded PTT amorphous region. When the temperature was lowered from 240 °C to 210 °C, the PTT spherulites grew in the excluded PTT amorphous region. The spherulite growth rate of PTT in the excluded PTT amorphous region was equivalent to that of a blend of 60-70 wt% PTT in 30/70 PTT/PET. These results suggest a significant change in the PTT concentration in the amorphous region, from the initial PTT content of 30 wt% to 60-70 wt%, due to the exclusion of PTT during the melt crystallization of PET at 240 °C.

Lamellae Assembly in Dendritic Spherulites of Poly(l-lactic Acid) Crystallized with Poly(p-Vinyl Phenol)

Lamellar assembly with fractal-patterned growth into dendritic and ringed spherulites of crystallized poly(l-lactic acid) (PLLA), of two molecular weight (MW) grades and crystallized at (temperature of crystallization) Tc = 120 and 130 °C, respectively, are evaluated using optical and atomic-force microscopies. The results of surface-relief patterns in correlation with interior microscopy analyses in this work strongly indicate that the observed birefringence changes in PLLA polymer dendritic or ringed spherulites (from blue to orange, or to optical extinction) need not be definitely associated with the continuous helix twisting of lamellae; they can be caused by sudden and discontinuous lamellae branching at intersected angles with respect to the original main lamellae, as proven in the case of dendritic and zig-zag rough-ringed spherulites. Intersection angles between the main stalks and branches tend to be governed by polymer crystal lattices; for PLLA, the orthorhombic lattice (α-form) usually gives a 60° angle of branching and hexagonal growth. The branching lamellae then further bend to convex or concave shapes and finally make a 60⁻90° angle with respect to the main stalks. Such mechanisms are proven to exist in the straight dendritic/striped high-molecular weight (HMW)-PLLA spherulites (Tc = 120 °C); similar mechanisms also work in circularly ringed (Tc = 130 °C) HMW-PLLA spherulites.

Growth pulsations in symmetric dendritic crystallization in thin polymer blend films

The crystallization of polymeric and metallic materials normally occurs under conditions far from equilibrium, leading to patterns that grow as propagating waves into the surrounding unstable fluid medium. The Mullins-Sekerka instability causes these wave fronts to break up into dendritic arms, and we anticipate that the normal modes of the dendrite tips have a significant influence on pattern growth. To check this possibility, we focus on the dendritic growth of polyethylene oxide in a thin-film geometry. This crystalline polymer is mixed with an amorphous polymer (polymethyl-methacrylate) to "tune" the morphology and clay was added to nucleate the crystallization. The tips of the main dendrite trunks pulsate during growth and the sidebranches, which grow orthogonally to the trunk, pulsate out of phase so that the tip dynamics is governed by a limit cycle. The pulsation period P increases sharply with decreasing film thickness L and then vanishes below a critical value L(c) approximately 80 nm. A change of dendrite morphology accompanies this transition.