Assessing Fast Structure Formation Processes in Isotactic Polypropylene with a Combination of Nanofocus X-ray Diffraction and In Situ Nanocalorimetry.

Yaroslav Odarchenko,Emanuela Di Cola,Jaime J Hernandez,Mikhail Soloviev,Martin Rosenthal,Dimitri A Ivanov,David Doblas

doi:10.3390/nano11102652

Abstract

A combination of in situ nanocalorimetry with simultaneous nanofocus 2D Wide-Angle X-ray Scattering (WAXS) was used to study polymorphic behaviour and structure formation in a single micro-drop of isotactic polypropylene (iPP) with defined thermal history. We were able to generate, detect, and characterize a number of different iPP morphologies using our custom-built ultrafast chip-based nanocalorimetry instrument designed for use with the European Synchrotron Radiation Facility (ESRF) high intensity nanofocus X-ray beamline facility. The detected iPP morphologies included monoclinic alpha-phase crystals, mesophase, and mixed morphologies with different mesophase/crystalline compositional ratios. Monoclinic crystals formed from the mesophase became unstable at heating rates above 40 K s−1 and showed melting temperatures as low as ~30 K below those measured for iPP crystals formed by slow cooling. We also studied the real-time melt crystallization of nanogram-sized iPP samples. Our analysis revealed a mesophase nucleation time of around 1 s and the co-existence of mesophase and growing disordered crystals at high supercooling ≤328 K. The further increase of the iPP crystallization temperature to 338 K changed nucleation from homogeneous to heterogeneous. No mesophase was detected above 348 K. Low supercooling (≥378 K) led to the continuous growth of the alpha-phase crystals. In conclusion, we have, for the first time, measured the mesophase nucleation time of supercooled iPP melted under isothermal crystallization conditions using a dedicated experimental setup designed to allow simultaneous ultrafast chip-based nanocalorimetry and nanofocus X-ray diffraction analyses. We also provided experimental evidence that upon heating, the mesophase converts directly into thermodynamically stable monoclinic alpha-phase crystals via perfection and reorganization and not via partial melting. The complex phase behaviour of iPP and its dependence on both crystallization temperature and time is presented here using a time–temperature–transformation (TTT) diagram.

Highlights

Isotactic polypropylene combines simple chemical architecture with distinctive polymorphic behaviour and is one of the most intensely studied semicrystalline polymers.When isotactic polypropylene (iPP) is cooled down slowly from the melt it can often yield several crystalline polymorphs such as α-monoclinic, β-hexagonal, and γ-triclinic
Quenching iPP melt may produce a mesophase consisting of conformationally disordered crystals [1,2]
The formation of alpha-phase crystals following the slow cooling regime of 0.83 K s−1 (Figure 2), similar to the regime used in conventional differential scanning calorimetry (DSC), is in agreement with the results that we reported previously for polymer crystallized in bulk at a rate of 0.05 K s−1 [40]

Summary

Introduction

When iPP is cooled down slowly from the melt it can often yield several crystalline polymorphs such as α-monoclinic, β-hexagonal, and γ-triclinic. Quenching iPP melt (cooling rates higher than approximately 100 K s−1 ) may produce a mesophase consisting of conformationally disordered crystals [1,2]. The results of temperature-resolved X-ray scattering experiments that have been reported previously for slowly heated iPP (rates below 0.1 K s−1 ) or iPP annealed at elevated temperatures yielded mesophase transition temperatures of between 350 K and 360 K [6,7]. Some important aspects of the temperature dependence of the reorganization processes in mesophase that happen upon heating are not fully understood. Whether the mesophase converts into crystals directly or via partial melting and/or reorganization remains unknown

Methods

Results

Discussion

Conclusion