• Macromolecular Research
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• v.16 no.4
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• pp.329-336
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• 2008

Core/shell ($\pm$)PS/(-)silica nanocomposite particles were synthesized by dispersion polymerization using an amphoteric initiator, 2,2'-azobis [N-(2-carboxyethyl)-2,2-methylpropionamidine] ($HOOC(CH_2)_2HN$(HN=) $C(CH_3)_2CN$=NC $(CH_3)_2C$(=NH)NH $(CH_2)_2COOH$), VA-057. Negatively charged (-6.9 mV) silica was used as the stabilizer. The effects of silica addition time and silica and initiator concentrations were investigated in terms of polymerization kinetics, ultimate particle morphology, and size/size distribution. Uniform hybrid microspheres with a well-defined, core-shell structure were obtained at the following conditions: silica content = 10-15 wt% to styrene, VA-057 content=above 2 wt% to styrene and silica addition time=0 min after initiation. The delay in silica addition time retarded the polymerization kinetics and broadened the particle size distribution. The rate of polymerization was strongly affected by the silica content: it increased up to 15 wt% silica but then decreased with further increase in silica content. However, the particle size was only marginally influenced by the silica content. The zeta potential of the composite particles slightly decreased with increasing silica content. With increasing VA-057 concentration, the PS microspheres were entirely coated with silica sol above 1.0 wt% initiator.

• Carbon letters
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• v.14 no.1
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• pp.45-50
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• 2013

Polyacrylonitrile (PAN) copolymers of different molecular weights were synthesized by a suspension polymerization and precipitation polymerization method. The rheology behaviors of the synthesized PAN copolymers were investigated in relation to their molecular weight, solid content and melting temperature. The influence of "historical effects" on the spinning solution of PAN was studied by analyzing the laws of viscosity considering the diversification time and temperature. The viscosity disciplines of each spinning solution conformed well to the rheological universal laws in a comparison of the suspension polymerization product with that of precipitation polymerization. Viscosity changes in the swelling process of dissolution were gentler in the suspension polymerization product; a small amount of water will quickly debase the solution viscosity, and high-speed mixing can greatly shorten the time required by the spinning solution to reach the final viscosity.

• Restorative Dentistry and Endodontics
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• v.35 no.1
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• pp.51-58
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• 2010

Dental composites have improved significantly in physical properties over the past few decades. However, polymerization shrinkage and stress is still the major drawback of composites, limiting its use to selected cases. Much effort has been made to make low shrinking composites to overcome this issue and silorane-based composites have recently been introduced into the market. The aim of this study was to measure the volumetric polymerization shrinkage kinetics of a silorane-based composite and compare it with conventional methacrylate-based composites in order to evaluate its effectiveness in reducing polymerization shrinkage. Five commercial methacrylate-based (Beautifil, Z100, Z250, Z350 and Gradia X) and a silorane-based (P90) composites were investigated. The volumetric change of the composites during light polymerization was detected continuously as buoyancy change in distilled water by means of Archemedes' principle, using a newly made volume shrinkage measurement instrument. The null hypothesis was that there were no differences in polymerization shrinkage, peak polymerization shrinkage rate and peak shrinkage time between the silorane-based composite and methacrylate-based composites. The results were as follows: 1. The shrinkage of silorane-based (P90) composites was the lowest (1.48%), and that of Beautifil composite was the highest (2.80%). There were also significant differences between brands among the methacrylate-based composites. 2. Peak polymerization shrinkage rate was the lowest in P90 (0.13%/s) and the highest in Z100 (0.34%/s). 3. The time to reach peak shrinkage rate of the silorane-based composite (P90) was longer (6.7 s) than those of the methacrylate-based composites (2.4-3.1 s). 4. Peak shrinkage rate showed a strong positive correlation with the product of polymerization shrinkage and the inverse of peak shrinkage time (R = 0.95).

• Proceedings of the Polymer Society of Korea Conference
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• 2006.10a
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• pp.71-72
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• 2006

We investigated a simultaneous living anionic polymerization process of isoprene (I) and 4styrene-d_8$ (S) in $benzene-d_6$ as a solvent with sec-buthyllithium as an initiator into polyisoprene(PI)-block-poly($styrene-d_8$)(PS) and the polymerization-induced molecular self-assembling process. This process was observed in-situ by time-resolved small-angle neutron scattering (SANS) experiment. The SANS profiles measured exhibited three time regions, where (i) the selective growth of PI chains occurs; (ii) the living chain ends switch from isoprenyllithium to styryllithium, and (iii) the SANS exhibited the polymerization induced disorder-to-order transition and order-to-order transition.

• Macromolecular Research
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• v.13 no.5
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• pp.391-396
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• 2005

The polymerization of methyl methacrylate (MMA) was carried out using CuBr/bidentate phosphorus ligand catalyst systems. MMA polymerization with CuBr/phosphine-phosphinidene (PP) exhibited high conversion ($\~80\%$) in 5 h at $90^{\circ}C$ along with a linear increase of ln($[M]_0/[M]$) versus time, indicating constant concentration of the propagating radicals during the polymerization. The molecular weight of the prepared PMMA tended to increase with conversion, suggesting the living polymerization characteristic of the system. On the other hand, a large difference between the measured and theoretical molecular weight and a broad molecular weight distribution were observed, implicating possible incomplete control over the polymerization. This may have been caused by the low deactivation rate constant ($\kappa_{deact}$) of the system. The low $\kappa_{deact}$, would result in irreversible generation of radicals instead of reversible activation/deactivation process of ATRP. Polymerizations performed at different ligand to CuBr ratios and different monomer to initiator ratios did not afford better control over the polymerization, suggesting that the controllability of CuBr/phosphorus ligand system for ATRP is inherently limited.

• Journal of the Korean Chemical Society
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• v.19 no.1
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• pp.53-64
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• 1975

The polymerization of methyl methacrylate initiated by cobalt(II) chloride in tetrahydrofuran has been studied. The conversion was increased liearly with polymerization time at initial stage of 6${\sim}$7% conversion and then the rate of polymerization was slightly reduced, and after that the autoacceleration was observed. The rate of polymerization was increased with the concentration of cobalt(Ⅱ) chloride, but at the concentration of cobalt(II) chloride higher than $3.4{\times}10^{-4}$ mole/l the rate of polymerization was decreased. The rate of polymerization was dependent on the 1.38 power of the concentration of methyl methacrylate. The effect of the radical inhibitor DPPH was observed and the inhibition time was increased with the concentration of DPPH. The apparent overall activation energy was calculated to be 13.2 kcal/mole. In the copolymerization of methyl methacrylate ($M_1$) and styrene ($M_2$), the monomer reactivity ratio was found to be $r_1$ = 2.35, $r_2$ = 0.78. From the above results. the initiation mechanism of polymerization was discussed and it was considered that the polymerization proceeds mainly via diradical mechaism.

• Journal of Korean society of Dental Hygiene
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• v.9 no.3
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• pp.369-379
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• 2009

This study investigated the polymerization inhibition effect of latex gloves on addition silicone impression material. Three different kinds of addition silicone impression materials and a natural latex gloves were used in this study. The results were as follows. 1. Compared to the control group, all of those three kinds of impression materials took longer curing time in order of unwashed, alcohol and washed group, on the other hand, degloving group had shorter curing time than control group(p<0.05). 2. By the type of impression materials, there was no significant difference observed between Exafine and Twinz in ungloved group, Exafine and Imprint II in unwashed group, and Exafine and Twinz in degloving group(p>0.05). 3. The degree of polymerization at 6 minutes after mixing impression materials was evaluated by dividing its range into score 1 to 5. All of the impression materials got score 5 in control group and degloving group, which implies perfect polymerization. In unwashed group, most of them appeared to be score 2 while score 3 were most frequently observed in alcohol group and score 4 in washed group. Thus each group showed differences in the degree of polymerization(p<0.05).

• Journal of the korean academy of Pediatric Dentistry
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• v.26 no.2
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• pp.296-309
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• 1999

Autopolymerized resin facilitates a more rapid and easier means for the construction of removable orthodontic appliances than heat cured resin. But many reports reveal that more unreacted monomer is found in autopolymerized resin. It is very important to achieve maximum degree of polymerization because if polymerization is inadequate, high level of unreacted monomer has been shown to adversely affect mechanical and physical properties, and also the question of allergy or toxicity to methylmethacrylate must be considered. The purpose of this study was to compare the degree of polymerization according to curing method and curing time. Five groups were desinged ; Group 1 was polymerizied at room temperature($28^{\circ}C$) ; Group 2 in $28^{\circ}C$ water ; Group 3 in $28^{\circ}C$ water under 30psi pressure ; Group 4 in $43^{\circ}C$ water ; Group 5 in $43^{\circ}C$ water under 30psi pressure for 10 minutes, 1 hour 12 hours, 1 day and 3 days. The degree of polymerization was measured by means of Fourier Transform Infrared spectroscopy. The results were as follows: 1. The degree of polymerization increased constantly in accordance with curing time in all groups and after curing for 10 minutes, Group 1 showed significantly higher degree of polymerization after 12 hours and Group 2, Group 3, Group 4, Group 5 after 1 hour(p<0.05). 2. The degree of polymerization decreased in the order of Group 5, Group 4, Group 3, Group 2, Group 1 except when the curing time was 1 hour and 12 hours(p<0.05). 3. The degree of polymerization of Group 4, Group 5 cured at $43^{\circ}C$ showed significantly higher degree of polymerization than Group 2, Group 3 at $28^{\circ}C$ except when the curing time was 1 day(p<0.05). 4. Among Group 2, Group 3 and Group 4, Group 5, the pressure had no effect on polymerization except when the curing time was 12 hours(p<0.05). 5. Between Group 1 and Group 2, the method of storage had no effect on polymerization except when the curing time was 1 hour(p<0.05).

• Elastomers and Composites
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• v.31 no.3
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• pp.175-182
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• 1996

Polymer latices, prepared from the emulsion polymerization of vinyl monomer compounds, are widely used for many industrial applications. Included among these are uses in paints, adhesives, flocculants, and heavy-duty plastics as well as their original use in synthetic rubber compounds. The emulsion polymerization process with chemical initiator has chemical disadvantage such as removal of initiator which was left after polymerization. In this study, polystyrene latex was prepared by using ultrasonic irradiation which generate the free radical, and then it was analyzed by GPC. Reaction temperature hardly have an effect on average molecular weight. Average molecular weight is increased by increasing amount of surfactant, i.e. SDS, but polydispersity is decreased. After 90 minutes of reaction time, increase and decrease of average molecular weight Is repeated. It is proposed that monodisperse polymer is obtained by controlling ultrasonic irradiation time and surfactant concentration.

• Electrical & Electronic Materials
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• v.10 no.6
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• pp.576-580
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• 1997

In this paper, the viewing angle characteristics of amorphous (a) - twisted nematic (TN)-liquid crystal display (LCD) on poly(vinyl)cinnamate (PVC) surface with UV (ultraviolet) light irradiation were investigated. It was found that the threshold voltage increases with increasing the polymerization of PVC surfaces. We observed that the domain size of a-TN-LCD decreases with increasing the photo polymerization by increasing UV light irradiation time on PVC films. Also, we observed that the viewing angle of a-TN-LCD increased with increasing the photo polymerization on PVC surfaces. Finally, we consider that the viewing angle of a-TN-LCD on PVC surfaces is large compared to a-TN-LCD on polyimide (PI) surface.