Converting Magazine

Eldridge M. Mount

Crystal orientation 2: How do the crystals rotate?
July 8, 2008

The question that should arise from my last posting should be, "How do the crystals rotate?"

Understanding this requires knowledge of the way crystals and amorphous phase polymers are combined together in the bulk polymer. In an amorphous polymer all of the chains can be readily imagined as a plate of intertwined spaghetti, but what does a plate of semicrystalline polymer look like? I guess a good analogy would be a plate full of string pom-poms. How to I get to that point?

Well, when a polymer crystallizes it has three levels of scale which are need to describe the crystal. First is the unit cell, the smallest repeat unit of the crystal which describes the dimensions and the habit of the crystal. Next, as the crystal grows it forms into a lamella crystal, a flat, thin crystal which is longer and wider than it is thick. In fact, it is so thin that a single polymer molecule is many times longer than the lamella is thick. We know this because single crystals have been isolated by recrystallization of polymers. Because of this we know that each polymer molecule must be folded onto itself to fit into the lamella crystal. This still does not look like a pom-pom. Well when lamella crystals grow from the melt they can branch and when they branch it causes a branch which goes off at a different direction than the rest of the lamella. As the polymer continues to crystallize, the lamella branch out more and more and the polymer crystal grows and fills space, trapping amorphous material between the branches of the lamella. This is what is called a spherulite and it can resemble a sheaf of wheat which is tightly bound at the center with the ends splayed apart, you can get the same effect by tying a bunch of short yarn strings together to form a pom-pom. 

Many crystals form at once and therefore there are many spherulites growing and they continue until they impinge against each other and stop growing, giving a spherulitic structure to the bulk polymer and my plate of pom-poms.  However, if we look inside the spherulite at the amorphous polymer trapped between the branches of the lamella, we find that there are portions of polymer chain which are part of one lamella, then go into the amorphous phase, and then become part of an adjacent lamella crystal, tying them together. These are called tie molecules.   You can see a picture of this in the Dec 15 post, “source of Internal haze”.

As the polymer is stretched, the spherulites will deform under the force of stretching, but looking closer we find that the amorphous phase is deforming the most and the amorphous molecules are elongating. As the deformation increases sooner or later some of the tie molecules are stretched tight, because they are held at two points (or ends) by different lamella crystals. As stretching continues they pull on the crystal and the crystal begins to rotate in the direction of the forces. The resistance to the rotation of the crystal is the mechanism of strengthening of a polymer by the crystal phase and gives rise to the polymers stiffness. I will make or find a good drawing of this in a day or so.

Posted by Eldridge M. Mount on July 8, 2008 | Comments (0)