Bowls, vases and goblets—the microcrockery of polymer and nanocomposite morphology revealed by two-photon optical tomography

Abstract

On the >1 µm scale the morphology of semicrystalline plastics like polyethylene or Nylon features spherulites, “shish-kebabs”, cylinddrites and other crystalline aggregates which strongly affect mechanical and other material properties. Current imaging techniques give only a 2D picture of these objects. Here we show how they can be visualized in 3D using fluorescent labels and confocal microscopy. As a result, we see spherulites in 3D, both in neat polymers and their nanocomposites, and observe how unevenly nanoparticles and other additives are distributed in the material. Images of i-polypropylene and biodegradable poly(lactic acid) reveal previously unsuspected morphologies such as “vases” and “goblets”, nonspherical “spherulites” and, unexpectedly, “shish-kebabs” grown from quiescent melt. Also surprisingly, in nanocomposite sheets spherulite nucleation is seen to be copied from one surface to another, mediated by crystallization-induced pressure drop and local melt-flow. These first results reveal unfamiliar modes of self-assembly in familiar plastics and open fresh perspectives on polymer microstructure.

Morphology of Shear-Induced Polymer Cylindrites Revealed by 3D Optical Imaging

“…The question is whether (i) those spherulites are induced directly by the shear flow or (ii) indirectly by the negative pressure at the growth front of the PLA cylindrite. In the former case (i), one would expect the spherulite nucleation density to increase close to the fiber due to the sharp increase in shear rate. , On the contrary, the situation actually reminds us of the behavior of spherulite growth in a quiescent melt film of nanoparticle-loaded PLA . There we have seen how spherulite nucleation is replicated at the opposite surface of the film and postulated that spherulite growth creates negative pressure particularly pronounced at the opposite polymer–glass interface, which initiates copycat nucleation at that interface.…”

“…In the 2PCM study, spherulites have been seen for the first time in 3D, both in neat polymers and in their composites with inorganic nanoparticles. The study revealed previously unsuspected morphologies such as “vases” and “goblets” as well as nonspherical “spherulites” . These first results have uncovered unfamiliar modes of self-assembly in familiar SCPs and opened new perspectives on a polymer microstructure.…”

“…An even bigger challenge is the visualization of the outlines of crystalline aggregates, such as spherulites, in fully crystallized polymers. SCP morphology on the super-μm scale has not been reported until our recent work in which we use two-photon confocal laser scanning microscopy (2PCM) on suitably labeled isotactic polypropylene (iPP) and poly­(lactic acid) (PLA). , The key step was the addition of a compatible fluorescent dye that segregated at spherulite boundaries either during crystallization or through postcrystallization penetration of the dye solution. In the 2PCM study, spherulites have been seen for the first time in 3D, both in neat polymers and in their composites with inorganic nanoparticles.…”

Real-Time Imaging of Stress in Single Spherulites and Its Relaxation at the Single-Particle Level in Semicrystalline Polymers

“…As shown in Figure a–c, they showed the limited 3D spatial movements in the first 5 min of isothermal crystallization, which was consistent with the trend of fluorescence intensity of the wrapped CD/SiO 2 particle probe (Figure b). Spherulite can preferentially nucleate at the surface often causing copycat nucleation at the opposite surface, and strong stress and melt flow normal to the solid substrate have been demonstrated in the crystallization . Subsequently, the moving CD/SiO 2 particle probe began to move along the Z -axis, indicating that the stress generation and relaxation have started from polypropylene nucleation.…”

“…Semicrystalline polymers account for 70% of polymeric materials worldwide, and their mechanical, thermal, and optical properties are closely dependent on the inevitable crystallization process. − Measuring and manipulating crystallization-induced stress and its relaxation during the crystallization process are one of the cornerstones for achieving the advanced properties to various applications. − In comparison to the easily measured macroscopic properties (e.g., crystallinity, transmittance, and stiffness), − the quantitative measurements of microscopic stress inside single spherulites and its relaxation outside the spherulites are a long-term challenge. Moreover, beyond individually characterizing microscopic stress and its relaxation, it is critical to understand how microscopic stress and its relaxation regulate each other in real time.…”