As I have understood, PLA is not entirely hydrophobic... About why it has some hydrophobicity, I think is becuase the methyl groups which PLA has, and this is also related to its high crystallinity.... sorry about my poor answer, maybe you can check these documents
http://www.journalamme.org/papers_vol43_1/43118.pdf
http://cdn.intechopen.com/pdfs/26368/InTech-Poly_lactic_acid_based_biomaterials_synthesis_modification_and_applications.pdf
As all aliphatic polyhydroxyacids it is hydrophobic because of its chemical structure. There is no sites for effective interaction with water molecules, but more sites for hydrophobic interactions. By the way, erosion of the polymer via hydrolysis leads to increasing of hydrophilic carboxylic groups but also lowering the MW.
Due to the chiral nature of lactic acid (Contains R and S form), PLLA has a crystallinity of around 37%. PLLA contains ester linkages in its backbone and end groups contains COOH groups. So the Hydrophobilicity can also be directly correlated with polydispersity of this polymer or number average molecular weight.
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