Polyethylene-like crystals

An understanding of both the microscopic structure and dynamics of matter is essentia] for a full description of its macroscopic properties. In this regard, recent studies of the internal dynamics of paraffin- and polyethylene-like crystals by simulation with supercomputers have established a link between microscopic motion and macroscopic effect [1-9]. It was shown that conformational, rotational, and diffusional motion can start considerably below the melting or disordering transition temperatures. These types of motion underlie much of the observed macroscopic properties of polymers, and thus it is essential to develop an understanding of how such motion occurs on the microscopic level (mechanisms) and the rates of changes it introduces (kinetics). 

Most of the early proposed models satisfied only one or several of the above requirements, but not all Since deformation and relaxation involves the dynamics of the polymer chains, it is by using the molecular dynamics method, quantum dynamics, or variants thereof that one can begin to derive a clear and complete description of what is happening inside a polymer crystal. In this regard, we have performed a number of detailed, systematic dynamics computations on parafiin- and polyethylene-like crystals. 

Fig. 8. Top view of the chains of a polyethylene-like crystaL The lamellar thidcness is 100 CHj-groups and the crystal consists of 19 inner, mobile chains, surrounded by a ring of 18 rigid chains...

Recently it has become possible to establish the link to the microscopic, thermal motion of macromolecules by simulation on supercomputers. By solving the equation of motion (Hamiltonian) for a system of thousands of atoms in a crystal, considering all intermolecular interactions and internal potentials, it was possible to produce a detailed movie of molecular motion in a polyethylene-like crystal. Figure 1.10 shows excerpts of this motion at low and high temperature. The lower 40 atoms of 7 dynamic chains are represented from a crystal that contained a total of 100 chain atoms. The (CH2-) groups are located at the corners of the indicated zig-zag lines at times zero. By comparison of the successive frames, various skeletal vibrations can be... 

Accelerated aging and crystal transformation rates have also been traced to high residual moisture content. Ando et al. studied the effect of moisture content on the crystallization of anhydrous theophylline in tablets [9]. Their results also indicate that anhydrous materials convert to hydrates at high levels of relative humidity. In addition, if hygroscopic materials (e.g., polyethylene glycol 6000) are also contained in the formulation, needle-like crystals form at the tablet surface and significantly reduce the release rate of the theophylline. 

At the initial stage of extrusion, the parallel lamellae rotated or tilted to form inclined lamellar stacks and the perpendicular lamellae tended to increase in number. The perpendicular lamellae increased up to around EDR 4.6 in the second stage where the inclined lamellae started to transform steadily into fibrils. This perpendicular lamellar orientation characteristic for extrusion drawing is likely caused by the compressive force applied for extrusion. Such an effect of the compressive force has bran more clearly observed in our previous study of crystalline-state extrusion of polyethylene single crystal powder (21). 

The macromolecular crystals of interest to the present review are largely polyethylene-like, so that electronic defects, such as electron holes, interstitial electrons, and exitons, need not be considered. 

The crystallization of isolated polyethylene chains was first reported by Kavassalis and Sundararajan (204) using MD with the Nos Hoover thermostat. More recently, single polyethylene chains driven by Langevin dynamics have been used by Muthukumar and co-workers (205,206) to model the early stages of nu-cleation and growth of crystals in solution. But even simpler models of chains will form ordered domains, and hence they have been explored as a means to understand some of the aspects of crystallization (122,138). A lattice model has been used in conjunction with an MC technique to model crystallization in short polyethylene-like chains (207). The crystallite-amorphous interface is foimd to be rough, and to have a relatively low concentration of regular folds for even this lattice model. 

A nnmber of techniques are appropriate to investigate the hierarchy of structnres formed by crystalline polymers. Crystallized polymer chains form crystal structures with lattices built up by translation of unit cells, just like crystals formed by low molar mass compounds. The space group symmetry depends on the polymer under consideration and also the conditions of the sample. For example, polyethylene usually forms a structure belonging to the orthorhombic crystal system, but at high pressures it is possible to obtain a hexagonal structure. Because it can adopt more than one crystal structure, polyethylene is said to be polymorphic. The best way to determine the crystal structure of a polymer is to perform wide-angle x-ray scattering (WAXS) experiments. WAXS on oriented polymers also provides information on the orientation of crystalline stems (chains). 

Fig. 14. Schematic view of the cross section of polyethylene single crystals. Note that there is an all trans chain segment of a specific length D connected to adjacent segments by chain folding at the surface of the crystal. The chain folds decouple the accordion-like vibrations of the all-trans segment so that the frequency of die vibration of a crystal segment is very close to that of a short chain hydrocarbon of...

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