Polylactic acid (PLA) has always struggled with a low heat resistance. A popular example of its weakness is that a cup made from it would not be able to hold hot coffee. My fellow blogger "Th' Gaussling" has stated that this is due to the "low" Tg, previously noted in this blog as being about 60 oC  and that raising it would help. While there certainly is value in attempting to raise it , to correlate the weakness to Tg alone is incorrect. Polypropylene has a significantly lower Tg of about -17 oC and yet is able to handle plenty high temperatures. Why? Because of the crystallinity and its occurrence at such high levels that you can completely overlook that it is above its Tg in most circumstances.
If you are working with a purely amorphous material, yes, Tg is all important, but for semi-crystalline materials, it is only part of the description and it can be a very small part at that.
 Let's all play nicey-nice here and not get into any of the well-established arguments about measuring Tg, o.k.?
Here is latest effort but there are others going back at least 5 years.
PLA also suffers low thermal stability during processing (much worse than PET). As a polyester it also crystallizes slowly. Otherwise, it is indeed more favorable to extend the annealing time in order to obtain a high crystalline PLA product than to raise its Tg.
Issues with increasing the crystallinity would/could include a decrease in opacity and longer processing times.
My dear fellow. I certainly defer to your superior knowledge of rheology. I am an admitted and serial small-molecule chemist. The goal of raising the Tg was given to me upon my arrival to the PLA project. But you are certainly right about the weak correlation of Tg wih the suitability for holding hot liquids. Soon, we became more interested in other moving targets like comonomers that would lower the Tg and alter other properties in order to remove the brittle character of PLA films for packaging. Sharp corners were prone to fracture in PLA packaging. Rattle was viewed as undesirable by the marketing folks as well for film packaging applications. Actually, I spent most of my time working on identifying comonomers for lowering the crystallinity.
Our trained chemists work on custom projects designed specifically for the needs of each client. Polylactic acid
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