Deterioration of metal–organic framework crystal structure during fabrication of poly(l‐lactic acid) mixed‐matrix membranes

Poly(l ‐lactic acid) (PLLA) and metal–organic framework (MOF) mixed‐matrix membranes were prepared by melt extrusion of PLLA with 5% (w/w) of either activated or water‐saturated Cu₃(BTC)₂ (Cu₃(C₉H₃O₆)₂(H₂O)₃·xH₂O, HKUST‐1). The morphology and the stability of injection‐molded samples were evaluated using thermogravimetric analysis, differential scanning calorimetry, gel permeation chromatography, X‐ray diffraction (XRD) and scanning electron microscopy (SEM). The presence of activated and saturated MOF crystals increased the cold crystallization onset temperature as compared to neat PLLA. This can be attributed to the MOF crystals incorporated in the PLLA matrix, which decreased the mobility of PLLA and thus impeded the crystallization process. According to the XRD results, the activated MOF crystals were successfully incorporated into the PLLA matrix without altering the crystal structure of the MOF. Moreover, the findings from permeability and tensile tests as well as SEM imaging indicated good interfacial interactions between PLLA and activated MOF. However, during melt extrusion of PLLA with saturated MOF, water molecules from the saturated MOF altered the MOF crystal structure and contributed to the degradation of the PLLA polymer by reducing its molecular weight by around 21%. © 2013 Society of Chemical Industry