Polymer Crystallization from the Melt

The polymer we consider here is a semi-flexible chain which has some bending stiffness (Eq. 3). We first estimated the chain conformation in the melt. The calculated mean-square end-to-end distance R2n between atoms n-bond apart has shown that the chains have an ideal Gaussian conformation R2 is a linear function of n (see Fig. 35 given later). The value of R2 for n = 100 

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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. 

In polymers crystallized from the melt, in most cases spherulitic structures are observed spherical agglomerates of crystals and amorphous regions, grown from a primary nucleus via successive secondary nucleation (Figure 4.18). The dimensions of the spherulites are commonly between 5 pm and 1 mm. When spherulites grow during the crystallization process, they touch each other and are separated by planes. In a microtome slice they show a very attractive coloured appearance in polarized light. 

When a polymer crystallizes from the melt without disturbance, it normally forms spherical structures that are called spherulites [1,2]. The dimensions of spherulites range from micrometers to millimeters, depending on the structure of the polymer chain and the crystallization conditions, such as cooling rate, crystallization temperature, and the content of the nucleating agent. The structure of spherulites is similar regardless of their size they are aggregates of crystallites [1-6]. 

In making models, we must respect the principles of equilibrium statistical mechanics, but cannot wholly rely on them, since we have every reason to believe that in polymer crystallization we have only a frustrated approach towards thermodynamic equilibrium. Most of the models are in some way or another founded on their authors conceptions of the nature of the process of high-polymer crystallization from the melt. That is a process on which direct detailed information is hard to get, and some imaginative extrapolation from what one knows about related problems is almost unavoidable. 

Many polymers have the capability to crystallize. This capability basically depends on the structure and regularity of the chains and on the interactions between them. The term sernicrystalline state should be used rather than crystalline state, because regions in which the chains or part of them have an ordered and regular spatial arrangement coexist with disordered regions typical of the amorphous state. X-ray diffraction studies of samples of polymers crystallized from the melt reveal diffuse zones, char-... 

When polymers crystallize from the melt or solution, the crystalline regions may exhibit various types of polymorphic modification, depending on the cooling rate, evaporation rate of solvent, temperature and other conditions. These crystal modifications differ in their molecular and crystal structures as well as in their physical properties. Many types of crystalline modifications have been reported (Tashiro Tadokoro, 1987). (See also Chapter 4.)... 

Polymer crystallization from the melt takes place when system is cooled below the equilibrium melting point, T, i.e., to the crystallization temperamre, T < T. The difference D = T T, depends on the cooling rate and the nucleation. There are three mechanisms of the crystallization nucleation [Utracki, 1989] ... 

The thickness of the lamellae depends on the crystallization temperature and is usually in the order of 6-10 nm. The macromolecules are normal to the lamellae and are folded back and forth on themselves. A single-polymer molecule may belong to more than one lamella, especially in polymers crystallized from the melt. Such interlamellar bonding increases as the molecular weight increases. Chain-folding also exists in some of the fibrous or ribbon-like structures in which nylons may crystallize. These structures are presumed to be the degenerated forms of lamellae. The structure of the observed globular particles appears to be unknown. 

When polymers crystallize from the melt. X-ray diffraction studies show recognizable features in some of them. The Bragg reflections, however, appear broad and diffuse, as compared to those obtained from well developed single crystals. Such broadening could result from the crystals being... 

T. Yamamoto, Molecular dynamics modeling of polymer crystallization from the melt, 45 1357-1364 (2004). 

The growth rate of polymer spherulites crystallized isothermally from the melt, G, can be measured by a polarizing optical microscope with a hot-stage. The crystallization temperature, T., was controlled by the hot-stage. The following procedure is recommended for the measurement of the spherulitic growth rate of polymers crystallized from the melt ... 

The overall growth rate of the spherulites of polymers crystallized from the melt under isothermal conditions can be measured by means... 

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