Crystallization Kinetics of semicrystalline Polyesters

To begin with, in this blog entry, we will elaborate on some background information about crystallization why it is important to understand the crystallization process, and how to use this knowledge to improve semicrystalline polymers and their properties. First of all, one needs to understand what a crystal is and how it distinguishes i from an amorphous material. In simplest words, one can see a crystal as a kind of a package of spaghetti. Within the package, the spaghetti (polymer chains, which are built up by repeating units (see previous post)) are aligned to get as many into that package as possible and to minimize the free volume between the chains. This is a very ordered structure. Opposite to that amorphous polymers are the same package of spaghetti, but cooked. The free volume between the cooked spaghetti is very high. This picture resembles well the structure-property relationship of crystalline vs. amorphous polymers. Amorphous polymers are usually tough (= it is hard to pull out a single noodle, as they are entangled) and hard but brittle crystalline polymers (= if you try to bend the non-cooked package of spaghetti it does not deform, it breaks). A semicrystalline polymer combines both characteristics and they are divided in “domains”, which are connected. The larger the content of one domain in the polymer is, the larger the influence. If many crystalline domains are present in a polymer material it becomes stiff, more temperature resistant but brittle. If amorphous domains prevail the material is soft, low melting but tough. One might now ask why crystals form at all because the universe expands and according to Boltzmann’s entropy theory amorphous materials should be more favored being in a higher entropy state than ordered structures. Good question. The answer to this is that crystallization is an energetically favored process that is kinetically hindered. That is the crystallization process releases energy which makes a spontaneous process possible. Polymer chain entanglement at too high chain movement (high crystallization temperature), however, are hindering that process. Physics are in order again.

Having understood this process, we can control the material properties of e.g. bio-based and biodegradable semicrystalline polyesters, such as PLA. While the rate of crystallization depends on the polymer itself, it can still be influenced. This process is called nucleation. The introduction of a nucleus provides the still-moving polymer chains guidance for alignment to form crystallites, which can further grow overcoming the kinetic hindrance.

Considering the two graphs in Figure 6 it becomes obvious how tremendous the right additives can accelerate the crystallization. While neat PLA crystallizes very slowly, the addition of PLLA/PDLA stereocrystals accelerates the crystallization and increases the degree of crystallization greatly.

However, PDLA is very high priced and limited in availability. Therefore, other nucleation agents were tested together with PLA. As seen from Figure 6 the introduction of CaCO3 improves the crystallization rate slightly, while the addition of talcum and LAK301 have a tremendous effect on the crystallization, observable by the narrower and lower minima of the curves. While being also dependent on the processing, the bare addition of nucleating agents can change the material property significantly. This behavior follows the same physics as e.g. nucleated PP in the conventional material regime. These principles can be translated to the final product in controlling the crystallization time in the process (e.g. injection molding using a hot mold) or by post crystallization as result of a thermal treatment.

A complementary method to induce crystallization is by the processing itself. We considered a crystal being assembled by aligned polymer chains. This alignment can be either induced by nucleation or by “physically forcing” the polymer chains to align. This can be realized by shear forces in the mold in injection molding, drawing during haul-off in film- or fiber extrusion or thermoforming. This process is commonly known as strain hardening. Obviously, the crystallization efficiency in greatly different between the processes, which is also due to the surface/volume ratio. Thin fibers are usually highly crystalline, while non-crystallized injection molding parts are rather amorphous.

Continuing our journey through the world of biopolymers we will cover Biopolymer Blends in our next blog post in this section.

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Dr. Rudi Eswein

Director of Sustainability