junhan cho - Profile on Academia.edu (original) (raw)

Papers by junhan cho

Bulletin of the American Physical Society, 2011

Ordered structures in thin films of ABC triblock copolymers are studied experimentally and theore... more Ordered structures in thin films of ABC triblock copolymers are studied experimentally and theoretically in order to be applied to fabrication of nanoscale electronic devices. A field-theoretic simulation method based on the self-consistent field theory is used to generate useful nanopatterns ...

Temperature-Pressure Superposition in SANS chi for an AbB Diblock Copolymer

Macromolecules, 2011

PS-b-PMMA copolymer is one of the most useful nanoscopic materials that can be used as passive el... more PS-b-PMMA copolymer is one of the most useful nanoscopic materials that can be used as passive electronic materials, and also as templates and scaffolds. It is then clear that the better knowledge on the strength of intermonomer interactions for the PS-b-PMMA is of great importance in fabricating nanomaterials from it. Using a compressible random-phase approximation (RPA) theory, we discuss mainly

Anomaly in SANS χ for polydisperse polystyrene-b-poly(isooctyl acrylate)

ABSTRACT Small-angle neutron scattering (SANS) measurements have been performed on a disordered b... more ABSTRACT Small-angle neutron scattering (SANS) measurements have been performed on a disordered block copolymer from deuterated polystyrene (dPS) and self-adhesive poly(isooctyl acrylate) (POA) in order to elicit the effective Flory–Huggins χ, which carries the essence of the copolymer phase behavior. The copolymer sample for the measurement was prepared by blending two polydisperse dPS-b-POAs of different molecular weights, where the overall average size of the blend was low enough to ensure to be in the mean-field region but high enough to have discernible scattering intensities. The SANS profiles for the copolymer were fitted to Leibler's scattering function for a polydisperse copolymer system described by Schulz-Zimm distribution. The resultant χ as a function of inverse temperature was shown to have a strong entropic contribution and a weak enthalpic contribution. By adopting Sanchez-Balasz or ten Brinke-Karasz-type simple analysis for specific interactions, it was found that the entropically dominated χ for dPS-b-POA arises from the steric hindrance of long alkyl side groups of POA.

Physical review. E, Statistical, nonlinear, and soft matter physics, 2014

The thermodynamic and conformational properties of catenated double-ring A-B copolymer melts are ... more The thermodynamic and conformational properties of catenated double-ring A-B copolymer melts are investigated through lattice Monte Carlo simulations. The topological constraint on the catenated copolymers is shown to suppress demixing of A and B monomers. This action results in their order-to-disorder transition (ODT) at an increased segregation level and the lamellae below ODT with reduced order, when compared to diblock copolymers of linear or single-ring topology. The A and B rings are pulled closer by catenation in the copolymer, which leads to its smaller gyration radius, lamellar domain spacing, and distance between mass centers of the two rings than for the diblock copolymers. With increasing segregation tendencies, the gyration radii of the A rings of the catenated copolymers stretch along the direction normal to lamellae, while the A-block conformations of the single-ring copolymers change their shapes from ellipsoid to sphere.

simulations. The topological constraint on the catenated copolymers is shown to suppress demixing... more simulations. The topological constraint on the catenated copolymers is shown to suppress demixing of A and B monomers. This action results in their order-to-disorder transition (ODT) at an increased segregation level and the lamellae below ODT with reduced order, when compared to diblock copolymers of linear or single-ring topology. The A and B rings are pulled closer by catenation in the copolymer, which leads to its smaller gyration radius, lamellar domain spacing, and distance between mass centers of the two rings than for the diblock copolymers. With increasing segregation tendencies, the gyration radii of the A rings of the catenated copolymers stretch along the direction normal to lamellae, while the A-block conformations of the single-ring copolymers change their shapes from ellipsoid to sphere.

Macromolecules, 2008

The phase behavior of symmetric polystyrene-block-poly(n-butyl-ran-n-hexyl) methacrylate copolyme... more The phase behavior of symmetric polystyrene-block-poly(n-butyl-ran-n-hexyl) methacrylate copolymers (PS-b-Pn(B-r-H)MA) with various molecular weights was investigated by small-angle X-ray scattering (SAXS), rheometry, polarized optical microscopy (POM), and static birefringence. PS-b-PnBMA exhibited a lower disorder-to-order transition (LDOT), whereas PS-b-PnHMA exhibited an order-to-disorder transition (UODT). However, when a random copolymer of Pn(B-r-H)MA was used as one of the blocks, PS-b-Pn(B-r-H)MA showed closed-loop phase behavior having both LDOT and UODT, which was seen in PS-b-poly(n-pentyl methacrylate) copolymer, when the total molecular weight was judiciously controlled. The phase behavior change by including a random copolymer was explained by an argument based on a compressible random phase approximation. We also found that the pressure coefficient of both transition temperatures of PS-b-Pn(B-r-H)MA was much larger than that of neat PS-b-PnBMA and PS-b-PnHMA.

Macromolecules, 2006

The phase diagrams for a binary mixture of symmetric polystyrene-block-poly(n-butyl methacrylate)... more The phase diagrams for a binary mixture of symmetric polystyrene-block-poly(n-butyl methacrylate) copolymer (PS-b-PnBMA) exhibiting lower disorder-to-order transition (LDOT) and PS homopolymer and a binary mixture of PS-b-PnBMA and PnBMA homopolymer were investigated by using polarizing optical microscopy, turbidity measurement, small-angle X-ray scattering, rheology, and transmission electron microscopy. Emphasis was placed on the effect of the molecular weight of homopolymers on the phase behavior of the mixtures. We found that when the molecular weight of a homopolymer was smaller than the half of molecular weight of the corresponding block, LDOT of the mixture increased (thus miscibility was enhanced). However, the changes of microdomains for the mixtures containing PS homopolymer are different from those for the mixtures containing PnBMA homopolymer, especially when the molecular weight of homopolymer becomes large. For instance, for the mixture of PS-b-PnBMA and PnBMA with a molecular weight of 65000, lamellar microdomains structure was destroyed even at the addition of 20 wt % of PnBMA homopolymer. On the other hand, for the mixture of PS-b-PnBMA and PS with a molecular weight 68000, the lamellar microdomain structure was maintained up to 40% of PS homopolymer. The phase diagrams of the mixtures were compared with predictions developed based on the compressible random phase approximation.

Effective Interaction Parameter for Homologous Series of Deuterated Polystyrene-block-Poly (n-alkyl methacrylate) Copolymers

…, 2007

Bulletin of the American Physical …, 2007

We investigate the scattering behavior of some LDOT-type styrenic block copolymers measured by sm... more We investigate the scattering behavior of some LDOT-type styrenic block copolymers measured by small angle neutron scattering (SANS). The absolute scattering intensities and the resultant correlation lengths for the disordered samples are obtained as a function of temperature ...

Polymers

Using field-theoretic simulations based on a self-consistent field theory (SCFT) with or without ... more Using field-theoretic simulations based on a self-consistent field theory (SCFT) with or without finite compressibility, nanoscale mesophase formation in molten linear AB and ABC block copolymers is investigated in search of candidates for new and useful nanomaterials. At selected compositions and segregation strengths, the copolymers are shown to evolve into some new nanostructures with either unusual crystal symmetry or a peculiar morphology. There exists a holey layered morphology with Im3 symmetry, which lacks one mirror reflection compared with Im3m symmetry. Also, a peculiar cubic bicontinuous morphology, whose channels are connected with tetrapod units, is found to have Pn3m symmetry. It is shown that there is another network morphology with tripod connections, which reveals P432 symmetry. The optimized free energies of these new mesophases and their relative stability are discussed in comparison with those of double gyroids and double diamonds.

Determining basic thermodynamic properties for some poly(n-alkyl methacrylates)

Cover: A field-theoretic simulation is performed on the response of a loop-forming diblock copoly... more Cover: A field-theoretic simulation is performed on the response of a loop-forming diblock copolymer film to pressure. Based on a compressible random-phase approximation in the spirit of the Ginzburg-Landau formalism, this method is capable of visualizing patterns inside the copolymer film in between selective surfaces at ambient conditions and also upon pressurization. Further details can be found in the article by J. Cho on page 8.

Pressure Effects on Baroplastic Diblock Copolymer Films

ABSTRACT The effect of pressure on the phase behavior of diblock copolymer films is studied by us... more ABSTRACT The effect of pressure on the phase behavior of diblock copolymer films is studied by using a field-theoretic simulation based on a RPA approach for compressible polymer mixtures in the spirit of the TDGL formalism. Some selected baroplastic copolymers exhibiting pressure-induced suppression of ordering are taken as a model system. The simulation provides a precise description of patterns inside the pressurized baroplastic copolymer films and also of residual layering triggered by selective surfaces. In addition, the TDGL character of the simulation gives the dynamic pathway toward lower-energy states for the given block copolymer films upon pressurization. The suggested field-theoretic simulation may be of great use for designing pressure-related baroprocessing of thin copolymer films.