Crystallization of polyethylene

Semicrystalline polymers are those that consist of two or more sohd phases, in at least one of which molecular chain segments are organized into a regular three-dimensional array, and in one or more other phases chains are disordered. The nonciystalline phases form a continuous matrix in which the crystalline regions are embedded. Most polyolefins are semicrystalline their specific morphology is governed by molecular characteristics and preparation conditions. Polyethylene is no exception to this mle it is all but impossible to prepare a solid specimen of polyethylene that is not semicrystalline. All commercial polyethylene products are semicrystalline. The physical properties exhibited by polyethylene products are governed by the relative proportions of the crystalline and noncrystalline phases and their size, shape, orientation, connectivity, etc. with respect to one another. 

The ordered phase of semicrystalline polyethylene consists of crystallites in which molecular chain segments are packed in regular arrays. The thickness of crystallites in molded high density polyethylene samples is commonly in the range of 80-200 A with lateral dimensions of up to several micrometers. Low density and linear low density polyethylene samples typically have somewhat thinner crystallites with smaller lateral dimensions. The nonciystalline regions separating crystallites can vary from approximately 50 A to 300 A. Linearly 

The concept of semicrystallinity is important because polyethylene can be considered to be a composite of ciystalline and nonciystalline regions. Polyethylene that consisted solely of ciystalline matrices would be a fiiable material, and a totally amorphous sample would be a highly viscous fluid. In practice, of course, polyethylene is a tough, resilient material. The arrangement of the three phases with respect to each other, their relative proportions, and their degree of connectivity determine the properties of a polyethylene sample. Neither pure ciystalline nor pure amorphous polyethylene samples are available, so the properties of each phase must be extrapolated from those of partially crystalline samples. 

Given the estimated properties of each phase and assuming a model of connectivity via the interface, it is possible to explain the mechanical behavior of polyethylene samples. To carry out this type of analysis it is desirable to have an accurate knowledge of the relative proportions of each of the three phases. In practice the single term degree of crystallinity is frequently used to characterize the semicrystalline nature of polyethylene samples. Quantification of the three phases of polyethylene can be made experimentally by several methods [1-3], the degree of crystallinity by many more [4-6], The experimental determination of the relative amounts of each of the phases is addressed in Chapter 6. Some of the most commonly used and most important descriptors of a polyethylene sample, such as density and stifiness, are closely related to its ciystallin- 

The unit cells of most nonpolymeric compounds conatain an integral number of complete molecules. In contrast, polymeric unit cells contain short segments from one or more molecular chains. By convention, the c axis of a polymeric unit cell is designated as being parallel with the chain axis of its molecular segments. 

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Magonov, S.N., Yerina, N.A., Godovsky, Y.K., and Reneker, D.H., Annealing and recrystallization of single crystals of polyethylene on graphite An atomic force microscopy study, J. Macromol. Sci. Part B Phys., 45, 169, 2006. 

Figure 8.28. Demonstration of a CDF. Data recorded during non-isothermal oriented crystallization of polyethylene at 117°C. Surface plots show the same CDF (a) Linear scale viewed from the top. (b) Linear scale viewed from the bottom, (c) Viewed from the top, logarithmic scale. Indicated are the determination of the most probable layer thickness, lt, and of the maximum layer extension, le. (d) Viewed from the bottom, logarithmic scale. The IDF in fiber direction is indicated by a light line in (a) and (b) (Source [56])...

Takeda, H., Ehara, M Sakai, Y. and Choi., Thermal crystallization of polyethylene terephthalate) and its copolyesters effect of degree of polymerization and copolymerized components, Textile Res. J., 61, 429-432 (1991). 

When polyethylene is crystallized from its solutions in p-xylene at suitable temperatures (around 70°C), single crystals of polyethylene are formed as... 

If crystallization is carried out from concentrated solutions, multilamellar aggregates are formed. In particular, melt crystallization of polyethylene gives bunched-up lamellae with an overall spherical symmetry. The space between the lamellae contains uncrystallized amorphous polymer. These objects are called spherulites, and their radii grow linearly with time, in spite of their intricate morphological features [9]. Another remarkable feature of spheruhtes formed by linear polyethylene is that they are gigantically chiral, although the molecules are achiral. 

As an example, we consider crystallization of polyethylene from a melt. As mentioned above, crystallization proceeds with the initial formation of isolated spherulites, which then grow until their mutual impingement with further slow crystallization. Time (t)-dependent measurements [26] of the density of the crystallizing melt at different temperatures are given in Figure 1.3 as a plot of degree of crystallinity versus logarithm of time. 

With large single crystals of polyethylene, the gelation dose is some 10 times greater than for the same polymer grown under conventional conditions (14). This is not caused by any inherent difference in the effect of radiation since both the radical concentration (deduced from ESR measurements) and hydrogen production are similar. We must therefore assume that most of the links produced in the single crystal are internal links which do not influence solubility. This is understandable in a crystal where each molecule folds backward on itself. 

Wunderlich, B. and Melillo, L. Morphology and growth of extended chain crystals of polyethylene. Makromol. Chemie 118, 250 (1968)... 

Siegmann, A. and Harget, P. J. Melting and crystallization of polyethylene terephthalate) under pressure. J. Polymer Sci., Polymer Phys. Ed. 18, 2181 (1980)... 

Calculation of the heats of fusion of 100% crystalline polymers from calorimetric data is not clear cut. Because of the partial crystallinity of all polymers (a possible exception are single crystals of polyethylene and other polymers, but these single crystals have not yet been investigated calorimetrically), calorimetric measurements do not yield the true heat of fusion AHf, in calg-1, but only AH where these two quantities are related by the expression... 

Fig. 4 Transmission electron micrograph of a replicate of a single crystal of polyethylene decorated with polyethylene vapour. With permission from Wiley, New York [30]...

The single crystals of polyethylene grown from solution showed a number of other important features ... 

Fig. 14.9 Snapshots of a system of twenty 100 carbon atom long polyethylene chains deformed at 300 K. The initial slab at the top rapidly deforms with the applied stress in the x dimension of the slab, roughly doubling in the first 500 ps to / — 2.64 (second image from the top) then the rate of deformation is slower and doubles again in 1500ps to X — 5.15 (third image from the top). Beyond this point the cell deforms even more slowly to reach a final deformation of X = 6.28 (bottom image). In absolute values, the initial cell of dimensions 1.88 x 5.32 x 5.32 nm deforms to 11.8 x 2.23 x 1.96nm. [Reprinted by permission from M. C. Levine, N. Waheed, and G. C. Rutledge, Molecular Dynamics Simulation of Orientation and Crystallization of Polyethylene during Uniaxial Extension, Polymer, 44, 1771-1779, (2003).]...

M. C. Levine, N. Waheed, and G. C. Rutledge, Molecular Dynamics Simulation of Orientation and Crystallization of Polyethylene during Uniaxial Extension, Polymer, 44, 1771-1779 (2003). 

Fig. 4 Schematic free energy diagram for the crystallization of polyethylene from the melt showing the specific Gibbs function (chemical potential) for melt (m), hexagonal (h), and orthorhombic (o), phases as function of temperature...

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