Thermodynamic Behavior of Poly(1,4-isoprene-b-DL-lactide) Diblock Copolymers

The phase behavior of a series of poly(1,4-isoprene-$b$-DL-lactide) (IL) diblock copolymers near the order-disorder transition temperature ($T_{ODT}$) was investigated using a combination of dynamic mechanical spectroscopy (DMS), small angle x-ray scattering (SAXS) and differential scanning calorimetry (DSC). IL copolymers of relatively low molecular weight ($\sim$ 2.5 -- 6 kg/mol) formed ordered phases with experimentally accessible $T_{ODT}$s due to the large segment-segment interaction parameter (\textit{$\chi _{IL}$}). The order-disorder transitions were accompanied by distinct signatures in the DSC traces, where the magnitude of the latent heat of the ODT ($\Delta H_{ODT}$) depends strongly on the ordered phase morphology. Symmetric compounds exhibited almost no hysteresis in the onset temperature for ordering and disordering, indicative of an absence of a nucleation barrier, while asymmetric IL diblocks displayed considerable hysteresis. These finding will be discussed in the context of composition fluctuation effects in the disordered melt near $T_{ODT}$.