Soonja Choe

Size control of polystyrene beads by multistage seeded emulsion polymerization

Micron-sized monodisperse crosslinked polystyrene (PS) beads have been prepared by a multistage emulsion polymerization using styrene monomer, divinylbenzene crosslinking agent, and potassium persulfate initiator in the absence of emulsifier. In the first stage of the reaction, the lower the reaction temperature, the larger the bead size obtained. In the later stages of the reaction, the particle size is increased with the initiator concentration and monomer content. Particle nucleation of the preexisting polymer seed of 0.7–0.8 μm in diameter is prepared at 60°C, then the monodisperse crosslinked PS beads > 2 μm are synthesized up to the third stage of the reaction. As the particle size grows, the number of free radicals in the growing particles increases, and the conversion of the next stage is continuously increased. The reaction mechanism is suggested that the continuous polymerization be conducted due to the diffusion of monomer into the preexisting particles to induce spherical beads in the later stages of the reaction. Otherwise, phase separation or the formation of protrusion by the capture of free radicals will be taking place.

Tensile property and interfacial dewetting in the calcite filled HDPE, LDPE, and LLDPE composites

Abstract Mechanical properties and complex melt viscosity of unfilled and the calcite (calcium carbonate: CaCO 3 ) filled high density polyethylene (HDPE), low density polyethylene (LDPE), and linear low density polyethylene (LLDPE) composites using dumbbell bar and film specimens are studied. In addition, the formation of air holes between calcium carbonate and the resin matrix was investigated from the phase morphology and interfacial behavior between the above constituents upon stretching using scanning electron microscopy. The tensile stress and the complex melt viscosity of the calcite filled (50%) polyethylene composites were higher than that of unfilled ones, implying that the reinforcing effect of calcium carbonate. The crack was initiated up to first 50% elongation along the transverse direction and the formation of air holes was originated by dewetting occurring through machine direction in the interface between calcium carbonate surface and HDPE. The propagation mechanism of the air hole formation was proposed to firstly originate by dewetting up to 300% elongation, and enlarged not only by breaking of a superimposed fibril structure, but also by merging effect air holes between fibrous resin matrix. However, the crack propagation was not observed at the very beginning elongation for the calcite filled LDPE and LLDPE systems. Less fibril structure was observed in LLDPE, then LDPE composites. The observed shape and the average size of the air holes were different from system to system. This sort of different interfacial behavior and mechanical properties may arise from different configuration of polyethylene.

Solid-state relaxations in linear low-density (1-octene comonomer), low-density, and high-density polyethylene blends

Extensive thermal and relaxational behavior in the blends of linear low-density polyethylene (LLDPE) (1-octene comonomer) with low-density polyethylene (LDPE) and high-density polyethylene (HDPE) have been investigated to elucidate miscibility and molecular relaxations in the crystalline and amorphous phases by using a differential scanning calorimeter (DSC) and a dynamic mechanical thermal analyzer (DMTA). In the LLDPE/LDPE blends, two distinct endotherms during melting and crystallization by DSC were observed supporting the belief that LLDPE and LDPE exclude one another during crystallization. However, the dynamic mechanical β and γ relaxations of the blends indicate that the two constituents are miscible in the amorphous phase, while LLDPE dominates α relaxation. In the LLDPE/HDPE system, there was a single composition-dependent peak during melting and crystallization, and the heat of fusion varied linearly with composition supporting the incorporation of HDPE into the LLDPE crystals. The dynamic mechanical α, β, and γ relaxations of the blends display an intermediate behavior that indicates miscibility in both the crystalline and amorphous phases. In the LDPE/HDPE blend, the melting or crystallization peaks of LDPE were strongly influenced by HDPE. The behavior of the α relaxation was dominated by HDPE, while those of β and γ relaxations were intermediate of the constituents, which were similar to those of the LLDPE/HDPE blends.

Influence of film preparation procedures on the crystallinity, morphology and mechanical properties of LLDPE films

Abstract Influence of various film preparation procedures on the crystallinity, morphology and mechanical properties of pure linear low-density polyethylene and its calcite filled composite films has been studied using differential scanning calorimeter (DSC), wide-angle X-ray diffractometer (WAXRD), atomic force microscope (AFM) and ultimate tensile testing machine (UTM). The film preparation procedures include variation in cooling rates such as quenching, force (fan) and natural cooling and in techniques such as extrusion followed by melt squeezing and compression molding. The heat of fusion (from DSC), the degree of crystallinity (from WAXRD) and the crystallite size (from WAXRD and AFM) are found to be the highest for naturally cooled specimen, followed by fan cooled and quenched ones. The AFM images of surface topology exhibit stacked lamellar morphology for forcefully cooled (fan cooled and quenching) samples and spherulitic ‘lozenges’ for naturally cooled ones. The Young’s modulus and yield stress (from UTM) are the highest for naturally cooled samples, followed by fan cooled and quenched ones. Amongst the calcite filled composites, the ‘base film’, which is prepared by extrusion followed by melt squeezing and natural cooling, exhibits the lowest heat of fusion, degree of crystallinity and Young’s modulus, but the highest yield stress, elongation at break and tensile strength compared to the compression molded ones.

Miscibility of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) and poly(vinyl chloride) blends

Abstract The miscibility behaviour of biodegradable poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHB-HV) blended with poly(vinyl chloride) (PVC) was investigated by using differential scanning calorimetry, dynamic mechanical thermal analysis, Fourier-transform infra-red spectroscopy and a mechanical testing system. A blend of PHB-HV containing 8% HV (PHB-8HV) with PVC was immiscible, showing two separate Tg values in all compositions: whereas a blend of PHB-HV containing 18% HV (PHB-18HV) with PVC was miscible, showing a melting-point depression and a single Tg in the whole range of compositions. For the PHB-18HV/PVC system, the C-O-C stretching vibration at 1183 cm−1 of PHB-18HV and the CHCl deformation at 1254cm−1 of PVC were shifted, indicating that there exists a specific intermolecular interaction between the two components. In addition, as the PVC component was increased, tensile strength and Youngs modulus were increased, while the inverse behaviour was observed in elongation at break.

Living radical dispersion photopolymerization of styrene by a reversible addition - fragmentation chain transfer (RAFT) agent

In this study, an addition – fragmentation chain transfer agent bearing dithioester group is synthesized and applied to conventional dispersion photopolymerization of styrene in ethanol medium in the presence of poly(N-vinylpyrrolidone) stabilizer with varying amounts of the RAFT agent and optionally with conventional initiator, azobisisobutyronitril (AIBN) at various temperatures. Monomer conversion, molecular weight evolution, polydispersity index (PDI), and final particle sizes are measured. The PDI of the formed polymer is between 1.5 and 2.5 in the presence of RAFT agent. Higher concentration of RAFT agent or elevated temperature leads to the acceleration of the polymerization rate resulting in fast conversion, and reducing molecular weight and PDI. Stable polystyrene beads above 1 mm in diameter are successfully prepared by means of RAFT method applied in dispersion polymerization. The weight average particle sizes are between 1.08 and 2.04 mm, and the uniformity ðDw=DnÞ is ranged from 1.26 to 2.51. q 2003 Elsevier Ltd. All rights reserved.

Blends of ethylene 1-octene copolymer synthesized by Ziegler–Natta and metallocene catalysts. I. Thermal and mechanical properties

The thermal, viscoelastic, and mechanical behaviors of three binary blends of ethylene 1-octene copolymer (EOC) regarding the melt index and density, one component made by Ziegler–Natta (FA and RF in abbreviation of the commercial name) and the other by metallocene catalysts (FM, EN and PL in abbreviation of the commercial name), have been investigated to understand molecular mechanism of the blends. Thermal studies reveal that the two constituents in the three blends exclude one another during crystallization, implying a phase separation. Viscoelastic properties show a single β or γ transition in all the compositions, suggesting a miscibility in the amorphous region. The tensile modulus, yield stress, maximum strength at break, and elongation at break follow the rule of mixtures if the comonomer content does not differ too much. Otherwise, the modulus and yield stress are negatively deviated, whereas elongation at break is positively deviated from the weight-average value. The tensile properties of film at yield and break in the machine direction is increased with an addition of FM in the FA + FM blend. Although all three blends form separate crystals in the crystalline state, a correlation exists between the mechanical properties and the density of EOCs.

Hybrid blends of similar ethylene 1-octene copolymers

Abstract Two binary blends of FA+FM and SF+FM comprising ethylene 1-octene copolymers (EOC), one component prepared by Ziegler–Natta and another by metallocene catalysts were investigated in terms of the thermal, viscoelastic, rheological, mechanical, and morphological properties. The big difference between the Ziegler–Natta and metallocene catalyzed EOCs is the distribution and the length of the side chain branching. Each component in FA+FM has similar melt index (MI), density, and comonomer content, while that of the second pair (SF+FM) has similar MI and density, but differs in comonomer content. Both the melt and solution blended materials exhibit two distinct melting and crystallization peaks, implying that the constituents exclude one another during crystallization. A single β relaxation shifted to lower temperature with the content of metallocene EOC, indicates miscibility in the amorphous region, while the γ transition is observed in the same position within experimental error. Rheological observations suggest the FA+FM to be miscible, but not SF+FM, implying that the difference in the distribution and the length of the side chain branching influences the melt properties of the EOC blends regardless of the similarity in the density and MI. In addition, no dependency of comonomer contents and the difference in the side chain branching on the mechanical properties is observed. Morphological studies observed from the slow cooled specimens show large spherulitic diameter and ring space for the Ziegler–Natta EOC. In particular, grass like spherulitic sheaf structure is dominated in the blend by the addition of metallocene EOCs. Hence the properties of the hybrid blends consisting of similar MI and density are influenced by not only the distribution of the comonomer, but also the length of the side chain branching.

Blends of ethylene 1-octene copolymer synthesized by Ziegler–Natta and metallocene catalysts. II. Rheology and morphological behaviors

The rheological and morphological behaviors of commercially available three binary blends of ethylene 1-octene copolymer (EOC) regarding the melt index (MI), density and comonomer contents, one component made by the Ziegler–Natta and the other by the metallocene catalysts, were investigated to elucidate miscibility and phase behavior. Miscibility of the EOCs blend in a melt state was related to the value of the MI, density, and comonomer content. If the comonomer contents are similar, then the melt viscosity is weight average value, otherwise it is positively or negatively deviated. The microtomed surface prepared by two different cooling processes—one is fast cooling and the other is slow cooling—indicated that all the blends were not homogenous regardless the density, MI, and comonomer content. The Ziegler–Natta catalyzed EOCs exhibited bigger spherulitic diameter and larger ring space than those of the metallocene EOCs prepared by a cooling process. The blends consisting of similar MI showed banded spherulites with different diameter, whereas the blend consisting of different MI and density takes place of explicit phase separation and phase inversion at 1 : 1 blend composition. The melt rheology appeared to influence the mechanical and film properties in the solid state.

Miscibility and processibility in linear low density polyethylene and ethylene‐propylene‐butene‐1 terpolymer binary blends

Blends of linear low density polyethylene (ethylene-octene-1 copolymer) and ethylene-propylene-butene-1 terpolymer ( ter-PP ) mixed in a twin-screw extruder have been characterized by using differential scanning calorimetry (DSC), dynamic mechanical thermal analysis, scanning electron microscopy (SEM), rheometric mechanical spectrometry, a capillary rheometer, and a universal test machine. Melting and crystallization behaviors by DSC and the α, β, and γ dynamic mechanical relaxations proposed that the blend is immiscible in the amorphous and crystalline phases by observing the characteristic peaks arised solely from those of the constituents. The lack of interfacial interaction between the components was suggested by the SEM study. A strong negative deviation of melt viscosity from the additive rule and the Cole-Cole plot confirmed the immiscibility in melt state. Incorporation of ter-PP induced a reduction in melt viscosity, shear stress, and final load. Flexural modulus and yield stress were linearly increased with ter-PP content, while the tensile strength and elongation at break were more or less changed. Although this blend system is immiscible in the solid and melt states, addition of less than 20 wt % ter-PP in the blend is viable for engineering applications with the advantages of improved processibility and mechanical properties.