Comparison of crystallization kinetics in various nanoconfined geometries

Phase morphological effect on crystallization kinetics in various nanoconfined spaces in a polystyrene-block-poly(ethylene oxide) (PS-b-PEO) diblock copolymer with a PEO volume fraction of 37 vol% was investigated. The phase morphology was characterized by small-angle X-ray scattering and transmission electron microscopy techniques. When the sample was cast from chloroform solution and annealed at 150 °C, a double gyroid (DG) phase was obtained. After it was subjected to a large-amplitude reciprocating shear, the sample transformed to an oriented hexagonal cylinder (Hex) phase. To obtain a lamellar confined geometry, lamellar single crystals were grown from dilute solutions. The crystallization in the lamellar (Lam) phase was one-dimensionally (1D) confined, while it was two-dimensionally (2D) confined in the DG and Hex phases, although they had different structures. Differential scanning calorimetry (DSC) was employed to study the crystallization kinetics using the Avrami analysis for these three nanoconfined geometries. Heterogeneous nucleation was found in all three samples in the crystallization temperature (T_c) regions studied. DSC results indicated that the crystallization kinetics in the Lam phase was the fastest, and the PEO crystals possessed higher thermodynamic stability than in the DG and Hex phases. For the crystallization kinetics in two 2D-confined phases, at low T_c (<35 °C) the PEO crystallization rates in the DG and Hex phases were similar, while at high T_c (>35 °C) the PEO crystallization was slower in the DG phase than in the Hex phase. The Avrami exponent n-values for the DG and the Hex samples were similar (∼1.8), yet the values of lnK in the DG phase were smaller than those in the Hex phase. This suggested that the linear growth rate was slower in the DG phase than in the Hex phase due to continuous curved channels in the DG phase.     

Opening the black box of academic entrepreneurship: a bibliometric analysis

Scientometrics - Interdisciplinary research is vital in addressing complex real-world problems. To understand how the scientific workforce is being engaged in the interdisciplinary research, it is...     

Comparison of poly(ethylene oxide) crystal orientations and crystallization behaviors in nano-confined cylinders constructed by a poly(ethylene oxide)-b-polystyrene diblock copolymer and a blend of poly(ethylene oxide)-b-polystyrene and polystyrene

A poly(ethylene oxide)-b-polystyrene (PEO-b-PS) diblock copolymer with a number average molecular weight of PEO blocks, =8.8 kg/mol, and a number average molecular weight of PS blocks, =24.5 kg/mol, (volume fraction of the PEO blocks, f_PEO, was 0.26) exhibited a hexagonal cylinder (HC) phase structure. Small angle X-ray scattering results showed that the PEO cylinder diameter was 13.3 nm, and the hexagonal lattice was a=25.1 nm. The cylinder diameter of this HC phase structure was virtually the same as that in the blend system constructed by a PEO-b-PS diblock copolymer (=8.7 kg/mol and =9.2 kg/mol) and a PS homo-polymer (=4.6 kg/mol) in which the f_PEO was 0.32. The cylinder diameter in this blend sample was 13.7 nm and the hexagonal lattice was a=23.1 nm. Comparing crystal orientation and crystallization behaviors of this PEO-b-PS copolymer with the blend, it was found that the crystal orientation change with respect to crystallization temperature was almost identical. This is attributed to the fact that in both cases the PEO block tethering densities and confinement sizes are very similar. This indicates that when the of PS homo-polymer is lower than the PS blocks, the PS homo-polymer is located inside of the PS matrix rather than at the interface between the PEO and PS in the HC phase structure. On the other hand, a substantial difference of crystallization behaviors was observed between these two samples. The PEO-b-PS copolymer exhibited much more retarded crystallization kinetics than that of the blend. Based on the small angle X-ray scattering results, it was found that in the blend sample, the HC phase structure was not as regularly ordered as that in the PEO-b-PS copolymer, and thus, the HC phase structure contained more defects in the blend. This led to a suggestion that the primary nucleation process in the confined crystallization is a defect-controlled process. The mean crystallite sizes were estimated by the Scherer equation, and the PEO crystal sizes are on the scale of the confined size.