Melting of polymer crystals

Why does the melting of polymer crystals exhibit a wide temperature range ... 

S. Rastogi, D.R. Lippits, G.W.M. Peters, R. Graf, Y.F. Yao, H.W. Spiess, Heterogeneity in polymer melts from melting of polymer crystals. Nature Materials 4, 635-641 (2005). 

Parallel processes should be inhibited. Melting of polymer crystals is accompanied by crystal thickening (parallel process). Further details about this particular case are given in section 10.3.1. 

The melting of polymer crystals exhibits many instructive features of non-equilibrium behaviour. It has been known since the 1950s that the crystals of flexible-chain polymers, e.g. polyethylene (PE), are lamella-shaped with the chain axis almost parallel to the normal of the lamella. The lamellar thickness (LJ is of the order of 10 nm, corresponding to approximately 100 main chain atoms, which is considerably less than the total length of the typical polymer chain. This fact led to the postulate that the macroconformation of the chains must be folded. The... 

The melting of polymer crystals is essentially the reverse of crystallization, but it is more complicated than the melting of low molar mass crystals. There are several characteristics of the melting behaviour of polymers which distinguishes them from other materials. They can be summarized as follows. 

Chapter 1 covers experimental techniques widely used in studies of polymer crystalhzation. Chapter 2, Chapter 3, Chapter 4, and Chapter 5 are devoted to the structure of crystalline polymers and also to the kinetics of nucleation and growth of the crystaUine phase. Chapter 6 is focused on molecular modeling of polymer crystallization, whereas Chapter 7 describes overah crystalhzation kinetics, with special reference to the theories widely used in practice. Chapter 8 covers the subject of epitaxy. Chapter 9 is dedicated to melting of polymer crystals. Chapter 10, Chapter 11, and Chapter 13 describe the crystahization in copolymers, miscible and immiscible polymer blends, and also polymer composites. Chapter 12 is focused on phenomena related to the confinement of polymer chains. Chapter 14 describes the effect of flow on crystahization, and finally Chapter 15 covers the crystalhzation in processing conditions. 

ACS, Polymeric Materials Science Engineering Fall Meeting 1999. Volume 81. Conference proceedings. New Orleans, La., 22nd-26th Aug. 1999, p.236-7 CRYSTALLISATION AND MELTING OF POLYMER CRYSTALS REVEALED BY T-M DSC Toda A Oda T Tomita C Hikosaka M Arita T Samyama Y... 

Temperature-modulated differential scanning calorimetry (T-MDSC) applies a thermal modulation in temperature to a conventional DSC mn and determines a dynamic heat capacity from the relationship between the modulation components of temperature and of heat flow. Primary application of this technique has been the measurement of specific heat capacity and the examination of the anomaly in a relaxation process such as alpha process related to the glass transition. An application to the first-order phase transitions of crystallisation and melting of polymer crystals has recently been suggested. The method and typical results are described. 13 refs. 

Fig. 17 B/E-p dependence of the critical temperatures of liquid-liquid demixing (dashed line) and the equilibrium melting temperatures of polymer crystals (solid line) for 512-mers at the critical concentrations, predicted by the mean-field lattice theory of polymer solutions. The triangles denote Tcol and the circles denote T cry both are obtained from the onset of phase transitions in the simulations of the dynamic cooling processes of a single 512-mer. The segments are drawn as a guide for the eye (Hu and Frenkel, unpublished results)...

Finally, we were led to the last stage of research where we treated the crystallization from the melt in multiple chain systems [22-24]. In most cases, we considered relatively short chains made of 100 beads they were designed to be mobile and slightly stiff to accelerate crystallization. We could then observe the steady-state growth of chain-folded lamellae, and we discussed the growth rate vs. crystallization temperature. We also examined the molecular trajectories at the growth front. In addition, we also studied the spontaneous formation of fiber structures from an oriented amorphous state [25]. In this chapter of the book, we review our researches, which have been performed over the last seven years. We want to emphasize the potential power of the molecular simulation in the studies of polymer crystallization. 

Real polymer processes involved in polymer crystallization are those at the crystal-melt or crystal-solution interfaces and inevitably 3D in nature. Before attacking our final target, the simulation of polymer crystallization from the melt, we studied crystallization of a single chain in a vacuum adsorption and folding at the growth front. The polymer molecule we considered was the same as described above a completely flexible chain composed of 500 or 1000 CH2 beads. We consider crystallization in a vacuum or in an extremely poor solvent condition. Here we took the detailed interaction between the chain molecule and the substrate atoms through Eqs. 8-10. 

With decreasing temperature, the density oscillation becomes very pronounced and grows into a deeper melt region. At 300 K, for example, we can see at least 5 layers after 1.28 ns. Within the layers, as will be shown later, definite order in chain orientation and chain packing is observed suggesting the growth of polymer crystals. 

The microscopic structure of the undercooled melt has been a subject of great interest in studies of polymer crystallization. There have been long arguments in favor of the presence of mesoscopic local order in the melt or at the crystal-... 

The bundle model of polymer crystallization will be discussed first. This mean-field approach describes the metastable configurational equilibrium of the undercooled polymer chain in solution or in the melt, and we will summarize and update previous work of ours [7-9]. The concept of the bundle, i.e. an aggregate of a few parallel polymer segments or stems connected by folds (see Fig. 1 see also [10]) and stabilized by attractive crystal-like interac-... 

Kaji K, Nishida K, Kanaya T, Matsuba G, Konishi T, Imai M (2005) Spinodal crystallization of polymers Crystallization from the unstable melt. Adv Polym Sci (in this volume)... 

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