Combining fast-scan chip-calorimeter with molecular simulations to investigate superheating behaviors of lamellar polymer crystals Huanhuan Gao a , Jing Wang a , Christoph Schick b , Akihiko Toda c , Dongshan Zhou a , Wenbing Hu a, * a Key Laboratory of High Performance Polymer Materials and Technology, State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Nanjing University, 210093 Nanjing, China b University of Rostock, Institute of Physics, 18051 Rostock, Germany c Graduate School of Integrated Arts and Sciences, Hiroshima University, Higashi-Hiroshima 739-8521, Japan article info Article history: Received 1 February 2014 Received in revised form 16 June 2014 Accepted 18 June 2014 Available online 24 June 2014 Keywords: Melting Chip-calorimeter Molecular simulations abstract We studied the power-law heating-rate dependence of superheating for the melting of alpha- and beta- crystals of isotactic polypropylene by means of chip-calorimeter, and expanded our parallel observation to higher heating rates by means of molecular simulations. We observed that, at low heating rates, the melting of lamellar crystals after thickened via melting-recrystallization exhibits no power-law- dependent superheating; at medium heating rates, the melting of crystals after thickened via chain- sliding diffusion exhibits the power-law-dependent superheating with the power indexes sensitive to chain mobility in the crystals; while at high heating rates, the zero-entropy-production melting of crystals without further thickening maintains the power-law-dependent superheating but with the power indexes uniform at an upper-limit 0.375. We attributed the index 0.375 to a result combining local intramolecular nucleation and global roughening growth at the lateral surface of lamellar crystals, which dominate the kinetics of crystal growth and melting of polymer crystals at high temperatures. © 2014 Elsevier Ltd. All rights reserved. 1. Introduction Crystallization of polymers from quiescent melt or solutions endows crystal morphologies a lamellar feature, with their melting points depressed by the limited lamellar thicknesses [1]. Super- heating of crystal melting refers to the phenomenon that crystals can be heated up to a temperature well above their size-dependent melting points without immediate melting. Under a certain super- heating, the equilibrium situation between melting and crystalli- zation will be replaced by the out-of-equilibrium process of melting, with its kinetics reflected by the degree of superheating. Because crystal melting is commonly initiated at the crystal surface limited in sizes [2], superheating is mainly attributed to the slow kinetics of crystal melting at that surface [3], unlike supercooling for crystal- lization which is mainly attributed to the slow kinetics of primary nucleation in the bulk phase [4]. Therefore, the study of super- heating will facilitate our better understanding on the melting and growth kinetics at the lateral surface of lamellar polymer crystals. Superheating of crystal melting is commonly monitored during the temperature scan with a constant heating rate in Differential Scanning Calorimetry (DSC) [5]. Melting of nascent crystals without any further structural reorganization at the size-limited melting point is often referred to zero-entropy-production (ZEP) melting [5]. However, monitoring the superheating for ZEP melting of polymer crystals is not an easy task. The reason is that upon heating to higher temperatures the unstable lamellar crystals commonly thicken into a more stable state, either via chain-sliding diffusion or via melting- recrystallization, before reaching the eventual melting of polymer crystals. This annealing behavior will shift up the melting point as well as the apparent superheating [5]. Fig. 1 demonstrates the shifting-up of melting points of unstable polymer crystals due to crystal thickening via chain-sliding diffusion in as-grown crystals, or via melting-recrystallization under slow-enough temperature scanning, from the melting point of zero-entropy-production upon heating towards melting [4,5]. Therefore, three melting paths exist upon heating scanning with various heating rates. One may observe the melting of crystals thickened via melting-recrystallization at small heating rates, the melting of crystals thickened via chain- sliding diffusion at medium heating rates, and the melting of crys- tals with zero-entropy-production at high heating rates. * Corresponding author. E-mail address: wbhu@nju.edu.cn (W. Hu). Contents lists available at ScienceDirect Polymer journal homepage: www.elsevier.com/locate/polymer http://dx.doi.org/10.1016/j.polymer.2014.06.048 0032-3861/© 2014 Elsevier Ltd. All rights reserved. Polymer 55 (2014) 4307e4312