The Materials Analyst, Part 44: I can't believe it's not crystalline (Part 1)The Materials Analyst, Part 44: I can't believe it's not crystalline (Part 1)

Many of us who have spent time in the business of manufacturing anything plastic have become familiar with the fact that some of the materials we work with have an amorphous structure while others are crystalline, or more properly semicrystalline. If the sentence you have just read is Greek to you, run—don't walk—to a good short course on material properties or find and read an older version of an engineering properties and design guide produced by any of the major material suppliers. 

If, however, you are nodding your head in knowing agreement, read on. The distinction between amorphous and semicrystalline polymers is so fundamental that it influences decisions we make about material selection, processing, part design, and mold design—in other words, everything that matters in developing and producing a molded plastic part. 

The details that determine whether a material will be semicrystalline or amorphous are fascinating but beyond the interest of most of us because they involve an understanding of chemical structure. Experience has shown that discussions involving chemical structure give rise to a unique brain state where the listener's eyes remain open but it can be clinically proven that he or she is thinking about golf. This is understandable in light of the fact that the instruction most of us received in chemistry was so poorly thought out and dispensed with such disinterest in our comprehension. So, let's revisit this territory and see if we can revive some sense of the wonder that should attend this subject. 

Crystallinity in Polyethylene 
To begin, most substances form crystals when they are in the solid state. Salt, sugar, water, aluminum, and almost everything else we come in contact with on a daily basis form crystals when cooled to the point of solidification. A crystal is the result of a regular arrangement of atoms or molecules and it gives rise to a distinctive geometry within the material. This geometry repeats itself over a relatively large area compared to the size of the molecules. Material scientists call this repeating pattern long-range translational order because that's the way material scientists talk. When a material is made of small molecules, these molecules organize quickly and virtually completely as the material changes from a liquid to a solid. 

Not so with polymers. Because of the large size and extended chain shape of the molecules and the resulting high viscosity of the molten material, the mobility required to achieve an organized structure in polymers is limited. In cases where this organization is possible at all, it is optimized by a streamlined molecular shape and a prolonged opportunity for molecular motion. 

This is where the chemistry comes in. First, let's look at the function of the shape of the molecule. Some materials naturally have a very regular and streamlined structure because of the arrangement of atoms in the individual chains. The best example of this is polyethylene. Polyethylene is nothing more than a chain of carbon atoms with hydrogen atoms attached along the side. Hydrogen atoms are the smallest known atoms, so their presence does not create much of an obstacle to the close approach and alignment needed for crystal formation. 

About the only thing that hinders the crystallization process in polyethylene is branching off the main chain. Branches are just what they sound like, sections of the chain that head off in a new direction. These create an irregular geometry. Figure 1 shows a schematic of a short section of three different types of polymer chains. We understand today that crystal formation is promoted primarily by the folding of the chains back upon themselves. It is easy to see that this folding is most easily accomplished in the linear system and occurs with the greatest difficulty in the long-branched structure. 

When polyethylene branches become long and plentiful, we end up with a soft, flexible material that we call low-density or Type I polyethylene. The structure of polyethylene is so streamlined that even with all of the branches the material still forms crystals, but not as many as it would if there were no branches. When polyethylene was first created during World War II, it was all produced in the form of low-density material (LDPE). Of course it was not called low-density polyethylene because there was no polyethylene of higher density to which to compare it. 

Through the miracle of catalyst technology, polymer chemists discovered ways to reduce the number and length of the branches to create a polyethylene with a very different set of properties. This polyethylene was stiffer and stronger and had a higher melting point. At the time, the method of manufacturing was more of a focus than the resulting structure and the low-density material was called high-pressure polyethylene because of the extreme conditions used during polymerization. The new material, which was developed in the 1950s, was labeled low-pressure polyethylene because the new catalysts permitted a much more manageable set of manufacturing conditions. Today we know it as high-density polyethylene (HDPE). 

The linear and short-branched models in Figure 1 are a reasonable representation for HDPE. The long-branched model is associated with the low-density materials. Table 1 compares some short-term properties for representative materials from the ranks of low-density and high-density polyethylene. Notice that we have used materials of the same melt index so as not to skew the comparison with differences in molecular weight. 

It is important to stress that while the structure and properties of these two materials are quite different, the actual composition of the two polymers is chemically the same. The difference is in the arrangement of the atoms in the chains. A large number of relatively straight chains gives rise to a high degree of regularity and greater opportunity for crystal formation. Branches disrupt this regularity. The study of these arrangements is called stereochemistry, and it is a critical consideration in understanding polymers. 

Moving Atoms 
In the early 1980s, chemists devised a way to control more precisely the placement and length of the branches along the polyethylene chain and gave birth to a material that we know today as linear low-density polyethylene (LLDPE). But that was just a warning shot compared to the revolution in catalyst technology that occurred in the 1990s with the advent of something called metallocenes. While these new catalysts are starting to affect a broader array of materials (which we will discuss in later articles), the primary benefit has been in the world of polyethylene. These new catalysts have given polymer chemists unprecedented control over the arrangement of the atoms in the polymer molecule. 

With this newfound control, they have exploded the simple picture of low-, medium-, and high-density polyethylene and have created compounds with combinations of properties that were unthinkable a decade ago. All of these new compounds leverage the marvels of stereochemistry. And crystallinity is one of the key properties that is manipulated. 

Because polymers are complicated structures, we never reach a point where all of the polymer chains are completely included in the crystal structure. Instead, even in a material like high-density polyethylene, we end up with regions of crystallized material surrounded by areas where crystals do not form—amorphous regions. In a simplified view of structure, the crystals provide the load-bearing properties like strength and stiffness while the amorphous areas give us the mechanical damping that is so important to impact properties. 

This trade-off can be seen in Table 1. The HDPE has substantially higher strength and modulus, but it lacks the impact resistance of the low-density material. The notched Izod test does a poor job of highlighting the difference in toughness; a better gauge is the elongation at break. 

This partially crystallized state leads to a term that has no meaning in the world of small molecules. We speak of a degree of crystallinity. While the degree of crystallinity can be varied substantially in the family of materials we call polyethylene, the maximum possible degree is very high because of the streamlined shape of the molecules. In general, the degree of crystallinity increases with density, although the creativity of the new chemistry makes this rule more difficult to apply today than a decade ago. The density of polyethylene can vary from a low of about .860 g/cu cm to a maximum of .980 g/cu cm. Over that density range the degree of crystallinity can vary from 15 to 90 percent. 

At the beginning of this article we said there were two key parameters that determine the potential for crystallization. The first is the shape of the molecule, which we have discussed. The second parameter is the opportunity for molecular motion. This gets a little deeper into the chemistry and will have to wait until next month. In next month's article, we will also explain how to measure this degree of crystallinity and we will dig a little deeper into the relationship between structure and properties by looking at materials other than polyethylene. We will also begin to approach the role of the processor in creating the crystalline structure in the molded part.