Published: September 29, 2011 r2011 American Chemical Society 8042 dx.doi.org/10.1021/ma201797k | Macromolecules 2011, 44, 8042–8055 ARTICLE pubs.acs.org/Macromolecules Influence of Amorphous Component on Melting of Semicrystalline Polymers Anurag Pandey, †,‡ Akihiko Toda, || and Sanjay Rastogi* ,†,‡,§ † Department of Materials, Loughborough University, Loughborough, LE11 3TU, U.K. ‡ The Dutch Polymer Institute (DPI), P.O. Box 902, 5600 AX Eindhoven, The Netherlands § Department of Chemical Engineering, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands ) Graduate School of Integrated Arts and Sciences, Hiroshima University, Higashi-Hiroshima 739-8521, Japan b S Supporting Information ’ INTRODUCTION Melting in semicrystalline polymers is a complex process, where melting transition is not sharp and covers a broad temperature range and is correlated to the distribution of the crystal lamellae thickness. Thermodynamically, melting in solids is defined as a first-order transition (sharp) at the intersection of the Gibbs free energy of the solid and liquid state, which is only true if we consider equilibrium and infinite size of the phases involved. However, these conditions are not met in semicrystal- line polymers such as polyethylene, either crystallized from solution or melt. These polymers possess lamellae crystal thick- ness of finite dimensions (1030 nm) with lateral dimensions of at least an order of magnitude larger. 16 GibbsThompson equation in its simple form has been widely used to quantitatively describe the maximum melting temperature correlating the lamellae thickness T m ¼ T ∞ m 1  2σ lFΔH m   ::: ð1Þ where T m is the experimentally determined melting tem- perature, T m ∞ is the equilibrium melting temperature for infinite size crystal (141.5 °C for polyethylene 7,8 ), σ is the end surface free energy for the folded planes, l is the crystal lamellae thickness, F is the crystal density, and ΔH m is the heat of fusion per unit mass. Recently, considerable questions have been raised on the validity of GibbsThomson equation for semicrystalline polymers. For example, recent studies by Muthukumar 9,10 demonstrate that extended chain crystals may not be the requisite for the thermodynamically equilibrium state. Fol- lowing the entropic calculations the author shows that the thermodynamically favorable structure would be the chain folded crystals where the chains are adjacently re-entrant. These findings are in accordance with the recent studies performed by H€ ohne 11 on linear polyethylene that also suggest that melting temperature will be influenced by the number of CH 2 units involved in crystal to liquid transition not the crystal stem length only. Thus, the melting tempera- ture will strongly depend on the topological constraints that may involve more than one stem, thus greater the number of CH 2 units involved in conformational changes from trans to gauche, higher will be the melting temperature  independent of the crystal thickness. Considering these concepts H€ ohne Received: June 16, 2011 Revised: September 8, 2011 ABSTRACT: Unlike inorganic and organic molecules, in semi- crystalline polymers melting gets complicated because of the requirement of conformational transformation of the chain segments, where part of the same chain resides in the crystal and also in the amorphous phase. The chain segment residing in the amorphous part can be constrained, either due to adjacent or nonadjacent re-entry leading to a different nature of chain folding, and arising differences are observed in local chain mobility due to differences in topological constraints. Thus, different conformational possibilities in the amorphous region of the semicrystalline polymer has implications on melting temperature and the processes involved in the order to the disorder phase transformation. With a series of experiments on ultra high molecular weight polyethylene, where the topological constraints are tailored by adopting a different synthesis route, it is shown that melting behavior cannot be fully explained by GibbsThomson equation only. Nonlinearity in melting temperature on heating rate invokes kinetics in the melting process, where depending on the heating rate melting can occur either via successive detachment of chains and their diffusion in the melt or by cluster melting. The role of superheating on melting process is also addressed.