Phase structures and morphologies determined by competitions among self-organization, crystallization, and vitrification in a disordered poly(ethylene oxide)-b-polystyrene diblock copolymer

A poly(ethylene oxide)-b-polystyrene (PEO-b-PS) diblock copolymer having a number-average molecular weight ((M) over bar(n)) of 11000 g/mol in the PEO blocks and an (M) over bar(n) of 5200 g/mol in the PS blocks has been synt hesized (with a volume fraction of the PEO blocks of 0.66 in the molten sta te). Differential scanning calorimetry results show that this copolymer pos sesses a single endotherm, which is attributed to the melting of the PEG-bl ock crystals. Based on real-time resolved synchrotron small-angle x-ray sca ttering (SAXS) observations, the diblock copolymer is in a disordered state above the glass transition temperature of the PS-rich phase (T-g(PS)) whic h has been determined to be 44.0 degrees C during cooling using dilatometer mode in thermomechanical measurements. The order-disorder transition tempe rature (T-ODT) for this diblock copolymer is thus experimentally inaccessib le. Depending upon different isothermal crystallization temperatures quench ed from the disordered state (T(q)s), four cases can be investigated in ord er to understand the phase relationships among self-organization, crystalli zation of the PEO blocks, and vitrification of the PS-rich phase: the regio n where the T-q is above the T-g(PS), the regions where the T-q is near but slightly higher or lower than the T-g(PS) ; and the region where the T-q i s below the T-g(PS) . Utilizing simultaneous SPXS and wide angle x-ray-diff raction experiments, it can be seen that lamellar crystals of the PEO block s in the first case grow with little morphological constraint due to initia l disordered phase morphology. As the T-q approaches but is still slightly higher than the T-g(PS) , as in the second case, the PEG-block crystals wit h a greater long period (L) than that of the disordered state start to grow . The initial disordered phase morphology is gradually destroyed, at least to a major extent. When the T-q is near but slightly lower than the T-g(PS) , the crystallization takes place largely within the existing phase morphol ogy. Only a gradual shift of the L towards smaller q values can be found wi th increasing time, which implies that the initial phase morphology is dist urbed by the crystallization of the PEO blocks. In the last case, the PEO b locks crystallize under a total constraint provided by the disordered phase morphology due to rapid vitrification of the PS-rich phase. Substantial de crease of crystallinity can be observed in this case. This study also provi des experimental evidence that the PS-rich phase size, which is down to 7-8 nm, can still retain bulky glassy properties. [S0163-1829(99)01138-8].