Surface Crystal Nucleation and Growth in Poly (ε-caprolactone): Atomic Force Microscopy Combined with Fast Scanning Chip Calorimetry.

Rui Zhang,René Androsch,Evgeny Zhuravlev,Mengxue Du,Christoph Schick

doi:10.3390/polym13122008

Abstract

By using an atomic force microscope (AFM) coupled to a fast scanning chip calorimeter (FSC), AFM-tip induced crystal nucleation/crystallization in poly (ε-caprolactone) (PCL) has been studied at low melt-supercooling, that is, at a temperature typically not assessable for melt-crystallization studies. Nanogram-sized PCL was placed on the active/heatable area of the FSC chip, melted, and then rapidly cooled to 330 K, which is 13 K below the equilibrium melting temperature. Subsequent isothermal crystallization at this temperature was initiated by a soft-tapping AFM-tip nucleation event. Crystallization starting at such surface nucleus led to formation of a single spherulite within the FSC sample, as concluded from the radial symmetry of the observed morphology. The observed growth rate in the sub-micron thin FSC sample, nucleated at its surface, was found being much higher than in the case of bulk crystallization, emphasizing a different growth mechanism. Moreover, distinct banding/ring-like structures are observed, with the band period being less than 1 µm. After crystallization, the sample was melted for gaining information about the achieved crystallinity and the temperature range of melting, both being similar compared to much slower bulk crystallization at the same temperature but for a much longer time.

Highlights

The in-situ combination of an atomic force microscope (AFM) with a fast scanning chip calorimeter (FSC) enables the visualization of the nanometer-scale morphology of polymer samples subjected to specific thermal histories
In a recently developed device, the FSC sensor is directly placed into an AFM and used as a hot-stage [1], yielding several advantages compared to earlier attempts of assessing the structure of polymer samples prepared in an FSC
Worth noting that such experiments can be performed at any temperature and time scale as the short time constant and high cooling capacity of the FSC sensor/hot-stage [7] allows freezing all intermediate steps of phase transformations by cooling the system to below the glass transition temperature (Tg ) for AFM scanning

Summary

Introduction

The in-situ combination of an atomic force microscope (AFM) with a fast scanning chip calorimeter (FSC) enables the visualization of the nanometer-scale morphology of polymer samples subjected to specific thermal histories. While in the past it was needed to destroy the FSC sensor holding the sample subjected to a well-defined thermal profile [2,3,4,5], with the new setup such sensor destruction is not required This allows calorimetric analysis of the sample after imaging for further correlation of the structure with thermal properties assessable in the FSC heating scans, including the temperature and enthalpy of melting or the glass transition temperature. A further advantage of the new device is the possibility of repeated imaging of a selected sample/sample-spot exposed to specific thermal treatments, permitting e.g., following structure evolution as a function of time during a phase transformation Worth noting that such experiments can be performed at any temperature and time scale as the short time constant and high cooling capacity of the FSC sensor/hot-stage [7] allows freezing all intermediate steps of phase transformations by cooling the system to below the glass transition temperature (Tg ) for AFM scanning. Such an approach has been applied to obtain the kinetics of homogeneous crystal nucleation of PA 66 in a wide range of temperatures [6]

Methods

Results

Conclusion