Sharp Crystal-Melt Interface

The interface between the lamellar crystals and the non-crystalline, interlamellar region was studied using the technique of the Gibbs dividing surface. In so doing, one is able isolate the effects of the interface alone, irrespective of thickness of the lamellae and, to some degree, of the interlamellar domain. Therefore, the properties attached to the sharp interface can be used in a three-component model with arbitrary composition, which accounts for the interface contribution explicitly, in addition to the crystal and melt bulk contributions. 

Throughout the manuscript, we carefully specified the variables under control. In particular when taking temperature derivatives, we observed differences between keeping pressure constant or the geometry of the system. We point out that such book keeping is essential, because it is not only responsible for quantitative differences, e.g., Cy vs. Cp, but it can even result in a alteration of the overall sign, as shown in Study 2 of the interface energy and stresses. 

---

Much of the interesting physics in semicrystaUine materials is hidden in the transition region between the crystalline domain and the melt-like domain. In particular in polymeric systems with a certain degree of stiffness of the backbone, the chain connectivity between both phases results in a rather wide transition region. In the following, we focus on the characterization of the crystal-melt interface by invoking the Gibbs construction of a sharp... 

The nucleation theory just described is referred to as classical nucleation theory. It relies on the capillarity approximation, in which crystallites of microscopic size are treated as if they are macroscopic, and in which the kinetics is described as the stepwise attachment of single molecules across the crystal-melt interface. In fact, this approximation may not be valid under realistic conditions. A small crystallite may not achieve bulk properties at its center, and its interface may be so strongly curved that the planar value Ysl no longer applies. The interface may be diffuse rather than sharp, so that the description of the kinetics as resulting from addition of solid particles one after another may not be valid instead, a collective fluctuation may result in the simultaneous incorporation of a larger number of molecules in a loosely structured crystallite. 

Above Tm, the viscosity of the melt has Arrhenius-type dependence, decreasing (exponentially) with increasing temperature. Therefore a sharp transition is observed in both mechanical and viscous properties of semicrystalline polymers at Tm, resulting in a physical situation that is closer to the classic melting interface of monomeric crystals where, on one side, there is a viscous liquid, and on the other side, an elastic solid. 

These apparent restrictions in size and length of simulation time of the fully quantum-mechanical methods or molecular-dynamics methods with continuous degrees of freedom in real space are the basic reason why the direct simulation of lattice models of the Ising type or of solid-on-solid type is still the most popular technique to simulate crystal growth processes. Consequently, a substantial part of this article will deal with scientific problems on those time and length scales which are simultaneously accessible by the experimental STM methods on one hand and by Monte Carlo lattice simulations on the other hand. Even these methods, however, are too microscopic to incorporate the boundary conditions from the laboratory set-up into the models in a reahstic way. Therefore one uses phenomenological models of the phase-field or sharp-interface type, and finally even finite-element methods, to treat the diffusion transport and hydrodynamic convections which control a reahstic crystal growth process from the melt on an industrial scale. 

Temperature-resolved measurements were revealed to be very instructive for the characterisation of the interface interaction. For LMW material we detected a complete change of crystal unit cell with increasing temperature. Moreover, the phase transition between solid and melt changes as a function of temperature. A continuous drop of intensity was found for thick films but there was a sharp drop of intensity for the thinnest films. The film thickness gives a rough... 

Table 14.3 summarizes the most important results of our MC simulations in terms of thermal and mechanical properties. It is striking that no mechanical response data, e.g., stiffness or compliance data, are available for the sharp interface. In that respect we mention that measuring the change in the interface stresses with respect to small deformations (tensile, shear) in directions of the interface plane with reasonable accuracy is difficult, as discussed in more detail in [30]. However, we have the feeUng that such data would be useful for calculating the average mechanical response of semicrystaUine polyethylene, in conjunction with similar data for the melt and crystal phases [60] and appropriate multiphase models. 

Jump-like creep was also studied for UHMWPE films with intentionally varied structural organization of interfaces between fibrils at the expense of their different preparation (gel-casting, crystallization from the melt), various draw ratios X (from 7 to 119), and the special cross-linking of fibrils [315,317]. The fibrils were weakly connected and loosely packed, weakly tied but closely packed, or cross-linked by long molecular segments or connected by short crosslinks, in these samples. As a result, absolutely different jump-like creep rate vs deformation curves and jump sharpness parameter h vs strain dependencies were obtained for these model samples with different interfacial structures. Short interfibrillar crosslinks provided the largest effect on creep behavior. Creep occurred basically through shear of fibrillar structural units relative to one another in an acceleration-deceleration way deceleration was due to slip resistance by some stoppers. ... 

---