Chang Dae Han

Transient rheological behavior of a thermotropic liquid‐crystalline polymer. I. The startup of shear flow

The transient rheological responses upon startup of shear flow were investigated for a well‐characterized thermotropic liquid‐crystalline polymer in the nematic region using a cone‐and‐plate rheometer. For the study, an aromatic polyester, poly[(phenyl sulfonyl)‐p‐phenylene 1,10‐decamethylene‐bis(4‐oxybenzoate)] (PSHQ10), was synthesized via solution polymerization in our laboratory. The PSHQ10 was found via gel permeation chromatography to have the weight‐average molecular weight of 45 000 relative to polystyrene standards and a polydisperity index of 2, and via differential scanning calorimetry to have (a) a glass transition temperature of 88 °C; (b) a melting point of 115 °C; and (c) a nematic–isotropic transition temperature (TNI) of 175 °C. The initial morphology (i.e., initial conditions) for the startup of shear flow of PSHQ10 in the nematic region was controlled by first heating a solvent‐cast specimen to 190 °C (i.e., to the isotropic region), shearing at a rate of 0.085 s−1 for 5 min, and then c...

Phase equilibria in mixtures of block copolymer and homopolymer

Abstract Phase diagrams were constructed experimentally for mixtures of a polystyrene-block-polyisoprene-block-polystyrene (SIS) copolymer and a homopolymer polystyrene. For the study, two SIS triblock copolymers having spherical and cylindrical microdomain structures, respectively, and a series of homopolymer polystyrenes were synthesized via anionic polymerization. In constructing phase diagrams, we combined the results of dynamic viscoelastic measurement (i.e. logarithmic plots of dynamic storage modulus versus dynamic loss modulus), which allowed us to determine the boundary between the mesophase and homogeneous phase, and turbidity measurement, which allowed us to determine cloud point curves for liquid-liquid (macrophase) separation. Experimental results are compared with predictions made by the theory of Hong and Noolandi. We have found that predicted results are at variance with experimental results.

Transient rheological behavior of a thermotropic liquid‐crystalline polymer. II. Intermittent shear flow and evolution of dynamic moduli after cessation of shear flow

The intermittent shear flow behavior in the nematic state of a well‐characterized thermotropic liquid‐crystalline polymer, poly[(phenylsulfonyl)‐p‐phenylene 1,10‐decamethylene‐bis (4‐oxy‐ benzoate)] (PSHQ10), was investigated using a cone‐and‐plate rheometer. For the study, PSHQ10 was synthesized in our laboratory. In order to erase the thermal history associated with polymerization and sample preparation and control initial morphology, an as‐cast specimen was first heated to the isotropic region (190 °C), sheared there at a rate of 0.085 s−1 for 10 min, and then cooled slowly down to a predetermined nematic state (130, 140, 150, or 160 °C). A fresh specimen was used for each intermittent shear flow experiment, thus circumventing the effect of previous shear history on subsequent transient rheological responses. Upon startup of shear flow, the growths of shear stress σ+(t,γ) and first normal stress difference N+1(t,γ) were recorded as functions of applied shear rate, temperature, and rest period. We fou...

Rheology and phase behaviour of blends of poly(ethylene oxide) with poly(vinyl acetate) and with poly(vinyl acetate-ran-ethylene)

Abstract The dynamic viscoelastic properties of blends of poly(ethylene oxide) (PEO) with poly(vinyl acetate) (PVAc) and with poly(vinyl acetate-ran-ethylene) (PVAE) were measured, using a cone-and-plate rheometer in the oscillatory shear mode. For the study, two grades of PEO, one with weight-average molecular weight MW = 20 000 (PEO20) and the other with MW = 100 000 (PEO100), were used. The PVAE used was found to be amorphous, as determined by differential scanning calorimetry, and it had MW = 12 600, as determined by gel permeation chromatography, and 73.4 wt% vinyl acetate, as determined by elemental analysis. It was found that the amorphous phase of the PEO20/PVAc blend system has one glass transition temperature Tg over the entire range of blend compositions, whereas the amorphous phase of the PEO100/PVAc blend system has one Tg for blend compositions up to 50 wt% PVAc, but two Tg values for blends with greater amounts of PVAc. Melting-point depression measurements were conducted to determine the Flory interaction parameter χ to be: (a) −0.211 for PEO20/PVAc pair; (b) −0.069 for PEO100/PVAc pair; (c) −0.027 for PEO20/PVAE pair; and (d) −0.024 for PEO100/PVAE pair. Our experimental results show that plots of the logarithm of zero-shear viscosity versus blend composition at constant temperature exhibit negative deviations from linearity for the PEO20/PVAc blend system, but positive deviations from linearity for both PEO20/PVAE and PEO100/PVAE blend systems. These experimental observations are interpreted using a molecular viscoelasticity theory recently developed by Han and Kim.

Small-angle X-ray scattering and rheological studies on the ordering process of cylindrical microdomains in a polystyrene-block-polyisoprene copolymer

Abstract Ordering of cylindrical microdomains in a low molecular weight polystyrene- block -polyisoprene (SI diblock) copolymer was investigated using small-angle X-ray scattering (SAXS) and rheology. The SI diblock copolymer was found to have (i) hexagonally packed cylindrical microdomains of polystyrene phase in a polyisoprene matrix in the ordered state, and (ii) an order—disorder transition temperature T ODT of ca. 90°C. To investigate the ordering process from the disordered state, a specimen was quenched rapidly from the disordered state to a temperature below its T ODT , and then the time evolution of the SAXS intensity and dynamic moduli ( G ′ and G ″) (i.e. the ordering process) was monitored. We have found that the ordering process in the block copolymer, when quenched to an ordered state close to its T ODT , takes place very slowly. Specifically, we found that when a specimen was quenched rapidly from 100°C (10°C above T ODT ) to 80°C (10°C below T ODT ), the ordering into the hexagonally packed cylinders occurs via a nucleation-growth process and the attainment of the equilibrium microdomain morphology took about 6 h when monitoring G ′ and G ″ and about 8 h when monitoring the SAXS intensity.

Rheological Behavior of Thermotropic Liquid-Crystalline Polymers: Effects of Thermal and Deformation Histories

Abstract The effects of thermal and deformation histories on the rheological behavior of thermotropic liquid-crystalline polymers (TLCPs) were investigated using a cone-and-plate rheometer. For the study, two TLCPs were used: (1) the copolyester consisting of 73 mol % hydroxybenzoic acid (HBA) and 27 mol % 6-hydroxy-2-naphthoic acid (HNA) (Vectra A900) having the clearing temperature (Tcl) close to or above thermal degradation temperature (ca. 350°C) and (2) an aromatic polyester, poly [(phenylsulfonyl)-p-phenylene 1,10-decamethylene-bis(4- oxybenzoate)] (PSHQ10) having a Tcl of 175°C, which was synthesized in our laboratory. In the use of PSHQ10, we were able to control the initial conditions of the specimens before being subjected to shear flow in the nematic region by first heating a solvent-cast specimen to the isotropic state and then cooling slowly to the nematic region. This was not possible, however, for Vectra A900. We found that the first normal stress difference (N1) of PSHQ10 in the nematic st...

Viscoelastic Behavior, Thermodynamic Compatibility, and Phase Equilibria in Block Copolymer-Based Pressure-Sensitive Adhesives

Abstract The viscoelastic behavior, thermodynamic compatibility, and phase equilibria in block copolymer-based pressure-sensitive adhesives were investigated. The block copolymers investigated were: (1) polystyrene-block-polybutadiene-block-polystyrene (SBS) copolymer (KRATON® D-1102, Shell Development Company) and (2) polystyrene-block-polyisoprene-block-polystyrene (SIS) copolymer (KRATON® D-1107, Shell Development Company). The tackifying resins investigated were: (1) WINGTACK® 86 (Goodyear Tire & Rubber Company) and (2) PICCOTAC® 95BHT (Hercules Inc.). Samples of various compositions were prepared by a solution-casting method with toluene as solvent. Measurements of dynamic storage modulus (G″), dynamic loss modulus (G″), and loss tangent (tan δ) were taken, using a Rheometrics Mechanical Spectrometer. It was found that: (1) both WINGTACK 86 and PICCOTAC 95BHT were equally effective in decreasing the plateau modulus (G O N ), and increasing the glass transition temperature (Tg ) of the polyisoprene mi...

Transient rheological behavior of a thermotropic liquid‐crystalline polymer. III. Step strain experiment and shear stress relaxation modulus

Shear stress relaxation modulus Gs(t,γ) was determined for a well‐characterized thermotropic liquid‐crystalline polymer, poly[(phenylsulfonyl)‐p‐phenylene 1,10‐decamethylene‐ bis(4‐oxybenzoate)] (PSHQ10) by conducting step strain experiments for a range of shear strains (γ). A fresh specimen was employed for each step strain experiment. We found that while time‐strain factorability [i.e., Gs(t,γ)=G(t)h(γ), where G(t) is the memory function and h(γ) is the damping function] was applicable to the isotropic region, it was not to the nematic region of PSHQ10. We further found that preshearing of a specimen decreased the magnitude of Gs(t,γ) considerably, suggesting that variations in the morphological state (i.e., domain texture) took place during shearing. The failure of time‐strain factorability for the PSHQ10 in the nematic region is believed to be attributable to variations in its morphology as affected by the level of applied shear strains.

Effect of annealing in the nematic phase on the transient shear response of thermotropic liquid-crystalline polymers

The effect of isothermal annealing, prior to start-up of shearing flow, in the nematic region on transient shear responses was investigated using a glassy thermotropic liquid-crystalline polymers (TLCP), PSHQ9, and a crystalline TLCP, PSHQ10. It was observed that the peak value of first normal stress difference (N1,peak+) of PSHQ9 remains constant, independent of the duration of annealing prior to start-up of shearing flow, whereas the N1,peak+ of PSHQ10 increases steadily with increasing period of annealing in the nematic phase prior to start-up of shearing flow. On the other hand, it was observed that the peak value of shear stress (σpeak+) remains constant in both PSHQ9 and PSHQ10, independent of the duration of annealing in the nematic phase prior to start-up of shearing flow. The observed dependence of N1,peak+ for PSHQ10 on the duration of isothermal annealing in the nematic phase is found, via differential scanning calorimetry, to be attributable to the perfection (or growth) of the crystals in PSH...