Polymeric Solid State: Amorphous and Crystalline

        The solid state of polymeric materials is, in some ways, more complex that the solid state of most ionic, metallic, and small covalent molecules. The complexity in polymers arises because solid polymeric materials can exist in two very distinct types of configurations. In one type, the polymer molecules are randomly coiled about each other with entanglement, much as cooked spaghetti would intertwine. (See Photo 3.1.) This structure type is called amorphous. In the second type of configuration, the polymer molecules can pack together into regular, repeating structural patterns. These regularly packed regions are called crystals or crystalline regions. Although no polymer is completely crystalline (having some amorphous regions), those with large concentrations of crystalline areas are said to be crystalline or, more accurately, semicrystalline.

Photo 3.1 Spaghetti showing entanglement.

        A semicrystalline polymer is pictured in Figure 3.1 in a two-dimensional representation. This representation, called the fringed micelle model, explains many, but not all, of the properties of crystalline polymers. One property not well explained by this model is the appearance of spherulite structures in the X-ray diffraction spectra. A model in which the molecules fold into platelike structures (see Figure 3.2), which grow from a central nucleation point, explains the growth pattern that forms the spherulites. This theory is called the laminar/spherulite model or more commonly, the folded chain model. Although the two models have not been fully reconciled, it may be that the folded chain model is simply the shape that emerges when the crystalline regions of the fringed micelle model are allowed to grow slowly into larger crystalline regions.

Figure 3.1 Crystalline and amorphous regions in a polymer structure.

Figure 3.2 Folded chain model or laminar/spherulite model of polymer crystallization.

        The most important (but not the only) feature of a polymer that determines whether it will be amorphous or crystalline is the shape of the polymer repeat unit. If the repeat unit is complex, especially with large pendant groups, the polymer cannot pack tightly together and will be amorphous. Some of the most common amorphous polymers are polystyrene, acrylic, polycarbonate, and most copolymers. Approximately half of the most common commercial plastics are amorphous.

        If the polymer repeat unit is simple and the pendant groups are small, the polymer may be able to pack tightly and crystalline regions could be formed. The regions of crystallinity are composed of folded chains held together by crystal bonds (secondary bonds). These bonding forces between the chains are localized to the tightly packed, crystalline areas and occur because the crystal structures, when they occur, represent structures with lower energy than a random, noncrystalline arrangement of the molecule. The lower energy is the result of the molecules forming bonds, which releases energy. The crystalline sections are scattered throughout the polymer with some nonstructured (amorphous) regions between them.

        The amount of crystallinity in the polymer depends upon several factors. As already mentioned, the most important is the size and regularity of the pendant groups (the groups attached to the main polymer backbone). If these pendant groups are relatively small and are regularly spaced along the polymer chain, they will not interfere with each other and the polymer chains can pack closer together. Forces of attraction and other similar interactions between polymers, such as hydrogen bonding, also increase crystallinity. Some important highly crystalline polymers are polyethylene (HDPE), acetal, and nylon.

        In addition to these structural factors, the crystallinity of polymers also depends upon molding or processing conditions. Crystallization in polymers takes time to occur. Therefore, factors such as cooling rate can have strong influences on the amount of the material that crystallizes, since below certain temperatures there is not sufficient molecular motion to allow the molecules to rearrange into a close packing configuration. In some polymers, mechanically stretching the polymer will draw the chains into close proximity and, therefore, induce crystallization. This phenomenon is the basis for the stretch blow molding process that is used to cause crystallization in soft-drink bottles, thus increasing their strength and resistance to gas diffusion over what they would be if amorphous.

        Some polymers will only form crystalline structures when polymerized under very special conditions. The most important of these, and the classical example of the type, is polypropylene. The relatively large pendant group on polypropylene prevented it from crystallizing and so the polymer lacked the strength and stiffness that would arise from the closely packed crystalline regions. Hence, early polypropylene had only limited applications. However, in the 1940s and 1950s, it was discovered that if polypropylene were polymerized using a special catalyst, crystallization would occur. (This is discussed further in Chapter 7.) Recent catalyst developments (especially with metallocenes) have indicated that many polymers once believed to only exist as amorphous polymers could be polymerized in such a way that crystalline structures would form.

        The amount of crystallinity, that is, the total number of atoms involved in a crystalline structure as opposed to the number in amorphous regions, can vary widely. In some polymers, no crystallinity takes place. In others, if all of the conditions are favorable, crystallinity can approach 100% but is more likely to be in the 60 to 70% range. As the material becomes more crystalline, it also becomes more dense. Especially in polyethylene and other plastics with a wide range of possible crystallinities, the density is the most common method of expressing crystallinity. For instance, polyethylene with a density of 0.97 grams per centimeter' would be high-density (HDPE) whereas a density of 0.92 g/cc would be low-density polyethylene (LDPE). The following methods are commonly used to measure specific gravity and density.

• Specific Gravity (ASTM D 792). This testdetermines the weight of a sample in airand then immersed in water. The ratio ofthe weights is the specific gravity, or thedensity of the material compared to thedensity of water. Most plastics have specific gravities in the 0.9 to 3.0 range. Smallsamples (about 1 inch3) are used in the test and are suspended in the water by a thinwire.
• Density-Gradient Technique (ASTM D 1505). The density of materials is determined by comparing the point at which a small sample (usually a pellet) will be suspended in a fluid of varying density with the suspension points of small glass floats of known densities. This method uses a density-gradient column prepared by adding two miscible liquids (usually water and ethanol) in varying concentrations so that more of the dense liquid is near the bottom of the column and more of the less dense liquid is near the top. The glass floats of known density are then carefully added to the column. The floats sink according to their densities and, by noting the depths, a plot of depth versus density can be established for the column. Then the plastic samples are added to the column and, by noting the depth at which they are suspended, their density can be read from the calibration chart. After some time (usually weeks), the liquids in the column mutually diffuse and the column must be refilled and recalibrated. A density-gradient column is illustrated in Figure 3.3.
• Bulk Density (ASTM D 1895). The bulk density is the apparent density of the material, that is, the density of the material without compaction or modification. This property is important when the plastic material is a powder or a flake. The test is conducted by carefully allowing the plastic to flow into a beaker or other container of known volume. The excess material is scraped off the top, and the container with the material is weighed. The weight of the container is subtracted from the total weight, and then density is calculated as the weight per volume. A related property is the bulk factor, which is the bulk density divided by the density of the plastic part after molding. Another related property is pourability, which is a measure of the time required for a standard quantity of material to flow through a funnel of specified dimension. Results are given as g/cc or lbs/foot³

Figure 3.3 Density-gradient column method for determining density, showing the column and a plot of the column calibration.

• Sieve-Analysis (Particle-Size) Test (ASTMD 1921), The size of the particles and thedistribution of the particle sizes in a particular batch can be important in some processes and in compounding. For instance,rotational molding fusion can be highly dependent on the sizes of the particles. Also,when large and small particles are mixedtogether, melting tends to be uneven andcan result in nonuniform mold filling andsurface defects such as orange peel. Thetest to determine the size distribution ofthe particles is simply to pour the powdered material through a series of sieveswith various opening sizes. The distribution is then determined by weighing the different sieves before and after the test. Ashaker usually is employed to facilitate passage of the material through the sieves.

        X-ray diffraction is useful in determining the degree of crystallinity because X-rays will develop a characteristic pattern when diffracted through a crystalstructure. (This same technique is used to investigate crystal properties in metals and ceramics.) Infrared spectroscopy can also be used to investigatecrystallinity because the vibrations and rotations of the atoms that are detected by infrared spectroscopy are affected by the crystal structure and, therefore, appear at slightly higher energy levels than do freely rotating and vibrating atoms. Hence, a shift in spectrographic pattern is detected when crystalline regions are present. Differential scanning calorimetry is another method that has been used, but not as frequently as the others.

        Because of the bonding forces within a crystal, crystallinity affects many physical properties in ways that are similar to other intermolecular attractions already discussed. Tensile strength and stiffness, for instance, are increased by crystallinity because of the high resistance to movement in the crystalline regions and the need to overcome the intermolecular (crystalline) forces. This resistance can be very high in some cases, resulting in a marked increase in these properties over amorphous polymer analogs. For instance, high-density polyethylene is strong and stiff enough to be self-supporting even when quite thin. It is used extensively for milk bottles. Low-density polyethylene is much more flexible and is used widely for trash bags. (Can you imagine the problems if milk bottles were made of the same plastic as trash bags?)

        The effect of crystallinity on impact toughness is somewhat more involved. The crystalline sections of a polymer are not as effective in absorbing and dissipating impact energy as are the amorphous regions because the atoms in the crystalline regions are not as free to rotate, vibrate, and translate. This restriction on atomic movement causes highly crystalline materials to be stiffer and more brittle. Therefore, even though the strength increases, the impact toughness often decreases for highly crystalline materials. Several other properties are also affected by crystallinity, as will be discussed in Chapter 5.

        For instance, solubility of the polymer is generally reduced in crystalline materials because of the compactness of the crystalline structure compared to the amorphous region. This compactness retards the access of solvent molecules to the bulk of the structure.

        Crystallinity is a basic property of plastics that should be considered in the selection of a polymer for any application. Many polymers can be obtained in a range of crystallinities, thus allowing the designer a wide choice of material properties. Another major effect of crystallinity is in thermal changes.