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 $\left(M_n\right)$ of 11 000 g/mol in the PEO blocks and an $M_n$ of 5200 g/mol in the PS blocks has been synthesized (with a volume fraction of the PEO blocks of 0.66 in the molten state). Differential scanning calorimetry results show that this copolymer possesses a single endotherm, which is attributed to the melting of the PEO-block crystals. Based on real-time resolved synchrotron small-angle x-ray scattering (SAXS) observations, the diblock copolymer is in a disordered state above the glass transition temperature of the PS-rich phase ${(T}_g^{\mathrm{PS}}),$ which has been determined to be 44.0 °C during cooling using dilatometer mode in thermomechanical measurements. The order-disorder transition temperature ${(T}_{\mathrm{ODT}})$ for this diblock copolymer is thus experimentally inaccessible. Depending upon different isothermal crystallization temperatures quenched from the disordered state ${(T}_{q}s),$ four cases can be investigated in order to understand the phase relationships among self-organization, crystallization of the PEO blocks, and vitrification of the PS-rich phase: the region where the $T_q$ is above the $T_g^{\mathrm{PS}},$ the regions where the $T_q$ is near but slightly higher or lower than the $T_g^{\mathrm{PS}};$ and the region where the $T_q$ is below the $T_g^{\mathrm{PS}}.$ Utilizing simultaneous SAXS and wide angle x-ray-diffraction experiments, it can be seen that lamellar crystals of the PEO blocks in the first case grow with little morphological constraint due to initial disordered phase morphology. As the $T_q$ approaches but is still slightly higher than the $T_g^{\mathrm{PS}},$ as in the second case, the PEO-block crystals with 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 $T_g^{\mathrm{PS}},$ the crystallization takes place largely within the existing phase morphology. Only a gradual shift of the L towards smaller q values can be found with increasing time, which implies that the initial phase morphology is disturbed by the crystallization of the PEO blocks. In the last case, the PEO blocks crystallize under a total constraint provided by the disordered phase morphology due to rapid vitrification of the PS-rich phase. Substantial decrease of crystallinity can be observed in this case. This study also provides experimental evidence that the PS-rich phase size, which is down to 7–8 nm, can still retain bulky glassy properties.