Radiation Chemistry Rules: Crystalline vs Amorphous Polymer Structures

amorphous polymer structures
A question we are often asked is “How are the polymers in my product going to respond to irradiation.” The answer is . . . not short, so we will have to answer it in multiple entries.

Some of the factors that affect the response of polymers during irradiation are the structure or morphology of the polymer, additives and the irradiation conditions.

Polymer Structure

Let’s start by looking at the polymer’s structure, and what makes them more likely to crosslink, degrade or be radiation stable. Several factors of the polymer structure are worth considering, and it is really tempting to delve into the effect of this pendant group or that backbone structure, but I want to start with a 10,000 foot view, or at least zoomed out a bit…

Crosslinking is often the desired outcome when electron beaming polymers. Crosslinking occurs when there is a generation of secondary radicals in the amorphous polymer region at the rubbery state with mobile polymer chains that have secondary radicals. (Secondary radical is when the carbon atom bearing the unpaired electron is bonded to two other carbon atoms)

Said differently, when a polymer has a combination of enough secondary radicals and chain mobility, a new bond will be created between the backbones, resulting in crosslinking.

What this means is crosslinking will not occur in the crystalline region of the polymer, as it is too rigid. And it will not occur in the amorphous polymer region at a glassy state, as again it is too rigid.

Crosslinking and Glass Transition Temperature

How do we know if a given polymer will have enough chain mobility to possibly crosslink? Knowing the glass transition temperature of the polymer can be helpful. In general terms, the glass transition temperature is the measure of where the polymer moves from a glassy, brittle state to a softer, more rubbery state. Below the glass transition temperature (Tg) the polymer chains are not able to move and rotate, which is why this is not ideal for radiation crosslinking. Above the glass transition temperature, the chains are able to move, creating a better environment to allow crosslinking.

glass transition temperatureSo how much of a relation is there between polymer crosslinking and the glass transition temperature? Let’s look at the following figure, simplified from Radiation Processing of Polymer Materials and it Industrial Applications. The G value is a relation of the chemical yield resulting from radiation. G(X) is crosslinking and G(S) is chain scissioning. In the figure, we are looking at the results of irradiation conducted at room temperature and can see the lower the glass transition temperature, the higher the G(X) or amount of crosslinking. Of course, there are other factors involved, and we will talk about more of these factors in a future post.

In the meantime, you can learn even more about polymer crosslinking through our series of videos related to different aspects of E-BEAM’s crosslinking process.

And, as always, reach out to us with questions! Call 1-877-41E-BEAM or complete our online contact questionnaire.

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