In order to obtain crosslinked poly(butylene succinate) (PBS) foams with a closed-cell structure, a commercial-grade PBS was first modified in the melt using two different branching agents to increase the melt viscosity. The rheological properties of the branched and crosslinked PBS were examined by varying the amount of the branching agents. The complex viscosity of the crosslinked PBS increased with increasing amount of the branching agent. However, it decreased with increasing frequency. When 2 phr of the branching agent was added to PBS, the storage modulus (G') was higher than the loss modulus (G") throughout the entire frequency range, showing that the addition of a branching agent increases the melt viscosity and elasticity of PBS effectively. Closed-cell PBS foams were prepared by mixing the chemical blowing agent with the crosslinked PBS. The effect of the foaming conditions such as temperature and time, and the amount of the crosssslinking agent on the structure of the expanded PBS foams were also investigated.
Nanocomposites of organophilic montmorillonite clay (OMMT) and polyepichlorohydrin (PECH) were intercalated by a solvent-casting method using dichloromethane as a solvent. The intercalation of PECH segments in the interlayers of the clay was confirmed by X-ray diffraction, and the intercalation spacing was calculated. The increase in the onset temperature of the thermal degradation indicated the enhancement of thermal stability of PECH due to intercalation. Rheological properties of the PECH/OMMT nanocomposites were investigated using a rotational rheometer in a steady shear mode. The steady shear viscosity increased with the clay loading, and the shear thinning viscosity data were fitted well with the Carreau model. From the normalized shear viscosity analysis, a critical shear rate that is a crossover from a Newtonian plateau to a shear thinning region was found to approximately equal the inverse of the characteristic time of the nanocomposites.
Synthetic aliphatic biodegradable poly(butylene succinate) (PBS) nanocomposites with multiwalled carbon nanotube (MWNT) were investigated to study the effects of the preparation method and MWNTs on physical characteristics of biodegradable polymer nanocomposite foams. PBS nanocomposites were prepared by the solution blending and melt mixing methods. Nanocomposites of PBS/MWNT were also prepared by the SOAM method, where the solution-blended nanocomposites were further mixed in the melt state. The dispersion of MWNTs in the PBS matrix was characterized by FT-Raman spectroscopy, field-emission scanning electron microscopy (FE-SEM), and field-emission transmission electron microscopy (FE-TEM). The mechanical and thermal properties of the PBS nanocomposites were measured using a universal test machine (UTM), thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC). Nanocomposite foams with high-density closed cells were prepared by using a chemical blowing agent (CBA) that acted as a cell-nucleating agent. The effects of the foaming conditions and MWNTs on the blowing ratio, cell size distribution, and morphology of the expanded PBS foams were investigated. The effect of the CBA content on the blowing ratio, cell size distribution, and morphology was also investigated.
ABSTRACT:Low dielectric polyimides were synthesized from a new diamine, bis(4-aminophenyl)-1-adamantyl phosphine oxide (DAAPO) containing adamantane cage structured group in the pendant position. Thermomechanical properties were investigated using differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and thermomechanical analyzer (TMA). The thickness dependence of dielectric constant was measured using a LCR meter, and the results were compared with those of other polyimide systems reported. Variation of stress during film coating and annealing processes was analyzed using a bending beam curvature measurement system complemented with an in-situ film thickness measurement system, the interferometer. Stress was developed as solvent evaporated during film baking process, and the residual stress was obtained when the coated films were prepared. The stress behavior was significantly affected by ramping and annealing schedules.KEY WORDS Polyimide / Dielectric Constant / Adamantane / Cage Structure / Stress / Aromatic polyimides have been used as dielectric, coating, and adhesive layers in microelectronics fabrications, because they have excellent electrical, chemical, thermal, and mechanical properties compared to other organic polymers. [1][2][3][4][5] In order to fabricate a high performance chip, however, solutions to some problems should be addressed. One is about the dielectric performance. As dielectric constants of commercial polyimides (PIs) are reported to be higher than 3, it should be lowered to transport more signals through more integrated circuits. The residual stress is another problem, especially in the thin polymer film application for chip and wire packaging. Stress is usually developed by thermal expansion difference between the film and substrate. Organic/inorganic pairs like PI/silicon wafer imposed high stress due to relatively high thermal expansion difference. When it is considerable in multiplayer fabrication process, it sometimes results in cracking, delamination, or bending of films.Among many trials to reduce the dielectric constant and residual stress, one has recently proposed introduction of nano or molecular foams in PI matrix. This was based on the idea that the dielectric constant of air is close to 1 and that the stress is significantly affected by relative density of polymer foams, ρ/ρ 0 ; moduli of foams are known to be proportional to (ρ/ρ 0 ), 2, 6 under the circumstances that thermal expansion coefficients of bulk and foam polymers are almost the same. More than 10 year trials for insertion of nano or molecular foams in PIs are summarized by two approaches: i) the phase separation by heat and non-solvent during preparation of PI systems, 7 and ii) the synthesis of block copolymer composed of polyimide and thermally degradable polymer. 8 Thermal degradation of the second component leaves nano-scaled pores behind it. Not many of these, however, were successfully industrialized, because their preparation process was too complicated and the resulting material pr...
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