Miscibility and Double Glass Transition Temperature Depression of Poly(l‑lactic acid) (PLLA)/Poly(oxymethylene) (POM) Blends

The poly­(l-lactic acid)/poly­(oxymethylene) (PLLA/POM) blends have been prepared by simply melt blending. The phase diagram, miscibility, glass transition temperatures, and physical properties have been investigated systematically. The PLLA/POM blends exhibit typical lower critical solution temperature (LCST) behaviors. PLLA and POM are miscible in the melt state at low temperature and become phase-separated at elevated temperatures. It was found that the weak interactions between the carboxyl groups of PLLA and methylene groups of POM (weak C–H^...O hydrogen bonding) account for the miscibility of the two components. Although the PLLA/POM blends are homogeneous at the melt state in the miscible temperature region, two distinct glass transition temperatures are observed for the all blends when quenched from the homogeneous state. More surprisingly, both POM and PLLA exhibit the apparent glass transition temperature (<i>T</i>_g) depression in the blends, compared with <i>T</i>_gs of the neat polymers. The behaviors are totally different from other reported miscible or partially miscible polymer blends, in which <i>T</i>_gs shift to each other or merge into one glass transition temperature. The investigation indicates that the crystallization of POM in the blend induces the phase separation of PLLA/POM blends and forms novel morphologies with the interpenetrated (cocontinuous) PLLA and POM phases. The double glass transition temperature depression of both PLLA and POM in the blends originates from the mismatch thermal shrinkage during cooling down from the high temperature. Moreover, we observed the improved ductility of the PLLA/POM blends as compared with the neat PLLA and POM, which has been attributed to higher molecular mobility due to the glass transition temperature depression for both PLLA and POM in the blends