Interface-induced crystallization via prefreezing: A first order prewetting transition

Ann-Kristin Flieger, M. Schulz, and T. Thurn-Albrecht

Experimental Polymer Physics, Institute of Physics, Martin Luther University Halle-Wittenberg, 06120 Halle, Germany

Interface-induced crystallization of a liquid on a solid substrate can either occur via heterogeneous nucleation or via prefreezing. Whereas heterogeneous nucleation takes place at finite supercooling below the melting temperature Tm, in prefreezing a crystalline layer is formed at the surface of a solid substrate already above Tm. Wetting theory predicts a jump in thickness at the formation and a divergence upon approaching coexistence. However, the thickness of the prefreezing layer has not been experimentally measured so far.

We studied ultrathin films of polycaprolactone (PCL) during the crystallization on graphite. With in-situ AFM-measurements we observe prefreezing instead of heterogeneous nucleation. The corresponding crystalline layer is formed at a temperature above the bulk melting temperature. Similar observations were already made for polyethylene on graphite [1]. In that case however, a direct measurement of the thickness of the prefreezing layer was not possible. Here, we show directly the finite thickness of the prefreezing layer for PCL. It forms with a thickness of a few nanometers which further increases during cooling. This observation demonstrates the transition is of first order, as expected for a prewetting transition.

The results prove that prefreezing can be described by common wetting theory. The studied system PCL-graphite is of importance for applications since graphitic materials are widely used as fillers for PCL.

[1] A.-K. Löhmann, T. Henze, and T. Thurn-Albrecht, PNAS 49, 17368-17372 (2014). (link)

Deformation and nano-void formation of β-phase isotactic polypropylene during uniaxial stretching

T. Kawai and S. Kuroda

Graduate School of Science and Engineering, Gunma University, Ota, Gunma 373-0057, Japan

Pseudo-hexagonal β-form is known to transform into thermodynamically stable monoclinic α -form during elongation. It is also reported that the nano-sized void is formed during deformation. Since the crystal deformation/void formation mechanism of β-iPP is not fully understood, we aim in this study to clarify the deformation behavior of β-iPP in both terms of crystal transformation (angstrom scale) and the void formation (nanometer scale). The film of β-iPP was prepared by melt crystallization of PP with 0.2% DCNDCA as a nucleating agent (kβ = 0.94). The samples were drawn uniaxially at 100ºC with fixed strain rate of 0.66 min-1. Synchrotron radiation WAXD/SAXS measurements were performed at BL40B2 in SPring-8, Japan. Deformation of β -iPP proceeded as follows; (i) at the yielding point of ε = 0.1 β-form started to decrease followed by increase in amorphous fraction. (ii) at ε = 0.4, α-form crystal with the chain orientation parallel to the stretching direction was formed. Importantly, as soon as α -form crystallized, formation of nano-sized void was initiated. Above findings strongly suggest that the β -form transforms to amorphous and/or mesomorphic state before recrystallization into α-form crystal. A detailed analysis on void structure by means of SAXS streak scattering is also to be discussed based on lamellar deformation during elongation.

Multiplicity of Morphologies in Poly (L-lactide) Bioresorbable Vascular Scaffolds

Julia A. Kornfield

California Institute of Technology, Chemistry & Chemical Engineering,  Pasadena CA 91125

Poly(L-lactide), PLLA, is the structural material of the first clinically approved bioresorbable vascular scaffold (BVS), a promising alternative to permanent metal stents for treatment of coronary heart disease. BVSs are transient implants that support the occluded artery for 6 months, and are completely resorbed in 2 years. Clinical trials of BVSs report restoration of arterial vasomotion and elimination of serious complications such as Late Stent Thrombosis. It is remarkable that a scaffold made from PLLA, known as a brittle polymer, does not fracture when crimped onto a balloon catheter or during deployment in the artery. X-ray microdiffraction revealed how PLLA acquired ductile character and that the crimping process creates localized regions of extreme anisotropy; PLLA chains in the scaffold change orientation from the hoop direction to the radial direction over micron-scale distances. The multiplicity of morphologies in the crimped scaffold enable a low-stress response during deployment, which avoids fracture of the PLLA hoops and leaves them with the strength needed to support the artery. Thus, the transformations of the semicrystalline PLLA microstructure during crimping explain the unexpected strength and ductility of the current BVS and point the way to thinner resorbable scaffolds in the future.

[1] Ailianou, A.; Ramachandran, K.; Kossuth, M.; Oberhauser, J.P.; Kornfield, J.A.*; “Multiplicity of Morphologies in Poly (L-lactide) Bioresorbable Vascular Scaffolds,” PNAS, 113, 11670-11675 (2016). (link)