For the samples of the PLGA 75:25 microspheres, we observe a similar trend to the PLGA 50:50 microspheres. The value of T g,β increases from 179 K for the blank microspheres, increases further to 192 K for the low exendin-4 loaded microspheres, and rises to T g,β = 215 K, for the high exendin-4 loaded microspheres (Figure ). Notably, we have previously shown that the methyl side group of PLGA 75:25 inhibits polymer mobility and introduce steric hinderance. Thus, for PLGA 75:25, the steric hinderance caused by the lactide monomer can restrict the intermolecular interactions between exendin-4 and PLGA, limiting the sites that are able to participate in hydrogen bonding, most likely making the carbonyl group of PLGA more favourable for hydrogen bonding. With limited sites for hydrogen bonding, less hydrogen bonds can form between exendin-4 and PLGA, resulting in PLGA chain entanglement, lower mobility, and reduced free volume. Our measurement that the PLGA 75:25 microspheres exhibit higher T g,β values compared to the 50:50 microspheres could therefore be explained by steric effects.