Polymer, Vol.42, No.13, 5829-5839, 2001
Phase structures and morphologies determined by self-organization, vitrification, and crystallization: confined crystallization in an ordered lamellar phase of PEO-b-PS diblock copolymer
Phase morphology, crystal orientation, and overall crystallization kinetics as determined by self-organization of the diblock copolymer, vitrification of the amorphous block, and crystallization of the crystallizable blocks have been investigated for a lamellar-forming poly(ethylene oxide)-h-polystyrene (PEO-b-PS) diblock copolymer. The diblock copolymer has number-average molecular weights of 8700 g/mol for the PEO blocks and 9200 g/mol for the PS blocks. Based on wide angle X-ray diffraction (WAXD) observations, the PEO crystals possess the same crystal structure as pure PEG. When the crystallization temperature (T-c) is below 40 degreesC, the PEO crystals melt below 55 degreesC, which is much lower than the melting point of their homopolymer analogs. The glass transition temperature of the PS blocks is 62 degreesC, determined by the 50% heat capacity change during vitrification observed in differential scanning calorimetry (DSC). The order-disorder transition temperature is determined to be 160 degreesC using a one-dimensional (1D) small angle X-ray scattering (SAXS) experiment. The mean-field segmental interaction parameter is determined to be chi (EO/St) = 7.05 x 10(-3) + 21.3/T, based on the SAXS scattering intensity analysis for T > T-MF (crossover temperature from the concentration-fluctuation region to the mean-field region). Using real-time resolved simultaneous SAXS and WAXD techniques, the PEO block is observed to crystallize within the confined lamellae provided by the glassy PS layers. The crystal (the (c) over cap - axis) orientation within the confined lamellae is determined by combined two-dimensional (2D) SAXS and WAXD experiments. The PEO crystals are observed to tilt away from the lamellar surface normal ((n) over cap) at T-c < 35
C, while they are oriented parallel to a when crystallized above 35 degreesC. The thickness changes in the lamellar phase morphology after isothermal crystallization can also be observed when compared with that in the molten state using 1D SAXS. After the crystallization of PEO blocks, the phase lamellar thickness slightly decreases with decreasing T-c when T-c < 35 C. When T-c > 35 degreesC, the lamellar thickness slightly increases with increasing T-c. At T-c= 35 degreesC, the thickness remains identical to that in the molten state. The increase of phase lamellar thickness may be a result of both dimensional change of the crystal growth via thickening of PEO lamellar crystals, and weakening of the confinement of the PS glass layers. The isothermal crystallization kinetics and melting results observed in DSC also support these conclusions.