It's my life! / Science

Most of us just ‘polymorphed’ away from Question 4…

The skeletal formula of polylactic acid

Image via Wikipedia

It’s 11.03am and there goes my second last paper, BN3301 – Introduction to Biomaterials.

As I was leaving the exam hall, I eavesdropped several of my batchmates anxiously talking about Question 4. I heard a few snickers here and there, and the underlying message was, “Who, in their right frame of mind, would do Question 4?” “… Hmm, maybe just the Dean-Listers. Or perhaps, the material engineering students.”

I shared their sentiments. Question 4 was the shortest but, toughest question in the paper. As we had to return the question booklet, I spent a few minutes during the exam burning that question in my memory. My curiosity had to be appeased.

The question was something like this:

You are a researcher and you want to blend two biodegradable polyesters together using the melt blending process. While you are doing so, you discover that crystallization was rampant initially, and towards the end of the process, you discover that the product can no longer crystallize. Explain the observed behaviour in your own words. (20 marks)

Wow. All I knew was it had something to do with the stereoregularity of polyesters. Let me illustrate further with an example: poly(lactic acid) (PLA). PLA is a biodegradable polyester and it is hot stuff in science research. Examples of ongoing research on PLA are biodegradable bioplastics for eco-friendly products and biodegradable sutures in biomedical applications.

Depending on where its side-branches are located, PLA can exist in two major forms: D-lactide or L-lactide. But, let’s leave chiral molecules and enantiomers aside. The only relevant information we need to know for this question is that racemic mixtures of both D- and L-lactides are less crystalline than pure D- and L-lactide polymers. And, this question is all about biodegradable polyester’s ability to polymorph (according to Wiki, polymorphism is the ability for a structure to exist in more than one form, or in this case, to crystallize).

I found a research article that answers this question. Now, I hardly have the time to read it as I have another exam to prepare for, but I’ll write another post about this as soon as I can.

For now, this would have to do:

PAN, P. & INOUE, Y. 2009. Polymorphism and isomorphism in biodegradable polyesters. Progress in Polymer Science, 34, 605-640. Retrieved from:


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