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.