S. H. Woo

Pohang University of Science and Technology, Andong, North Gyeongsang, South Korea

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Publications (4)8.2 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: Soluble poly(p-phenylene biphenyltetracarboxamic acid) precursor was converted to the polyimide by thermal imidization at various conditions. The structure evolution being occurred during its thermal imidization over 25–400°C was investigated as a function of imidization temperature and time by X-ray scattering with synchrotron radiation sources of Pohang Accelerator Laboratory in Korea and with conventional radiation sources. In addition, properties in the polyimide films were investigated by dynamic mechanical thermal analysis, stress-strain analysis, prism coupling, and residual stress analysis.
    Macromolecular Symposia 03/2011; 118(1):213 - 220.
  • [Show abstract] [Hide abstract]
    ABSTRACT: A soluble poly(amic acid) precursor solution of fully rod-like poly(p-phenylene pyromellitimide) (PMDA-PDA) was spin cast on silicon substrates, followed by soft bake at 80–185°C and subsequent thermal imidization at various conditions over 185–400°C in nitrogen atmosphere to be converted to the polyimide in films. Residual stress generated at the interface was measured in situ during imidization. In addition, the imidized films were characterized in the aspect of polymer chain orientation and ordering by prism coupling and X-ray diffraction. The soft-baked precursor film revealed a residual stress of 16–28 MPa at room temperature, depending on the soft bake condition: higher temperature and longer time in the soft bake gave higher residual stress. The stress variation in the soft-baked precursor film was not significantly reflected in the final stress in the resultant polyimide film. However, the residual stress in the polyimide film varied sensitively with variations in imidization process parameters, such as imidization temperature, imidization steps, heating rate, and film thickness. The polyimide film exhibited a wide range of residual stress, −7 MPa to 8 MPa at room temperature, depending on the imidization condition. Both rapid imidization and low-temperature imidization generated high stress in the tension mode in the polyimide film, whereas slow imidization as well as high temperature imidization gave high stress in the compression mode. Thus, a moderate imidization condition, a single- or two-step imidization at 300°C for 2 h with a heating rate of < 10 K/min was proposed to give a relatively low stress in the polyimide film of < 10 μm thickness. However, once a precursor film was thermally imidized at a chosen process condition, the residual stress–temperature profile was insensitive to variations in the cooling process. All the films imidized were optically anisotropic, regardless of the imidization history, indicating that rod-like PMDA-PDA polyimide chains were preferentially aligned in the film plane. However, its degree of in-plane chain orientation varied on the imidization history. It is directly correlated to the residual stress in the film, which is an in-plane characteristic. For films with residual stress in the tension mode, higher stress films exhibited lower out-of-plane birefringence, that is, lower in-plane chain orienta-tion. In contrast, in the compression mode, higher stress films showed higher in-plane chain orientation. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1261–1273, 1998
    Journal of Polymer Science Part B Polymer Physics 12/1998; 36(8):1261 - 1273. · 2.22 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Rigid/crosslinked flexible polyimide composites were prepared from soluble polymeric and oligomeric precursors: poly(4,4′-oxydiphenylene pyromellitamic acid) [PMDA-ODA(PAA)], poly(4,4′-oxydiphenylene pyromellitamic diethyl ester) [PMDA-ODA(ES)], acetylene-terminated isoimide oligomer of 3,3′,4,4′-benzophenonetetracarboxylic dianhydride/1,3-bis(3-aminophenoxy)benzene (BTDA-APB) and acetylene-terminated imide oligomer of 2,2′-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride/1,3-bis(3-aminophenoxy)benzene (6FDA-APB). PMDA-ODA(PAA) precursor was found to react with BTDA-APB isoimide oligomer in N-methyl-2-pyrollidone (NMP), resulting in gels. Furthermore, PMDA-ODA(PAA) exhibited a limited miscibility in making its homogeneous solution with 6FDA-APB imide oligomer in NMP, consequently leading to large domains in the softbaked blend films as well as in the resultant composite films. In contrast, for PMDA-ODA(ES) blends with 6FDA-APB and BTDA-APB, homogeneous ternary solutions with a relatively high solids content of ≤ 30 wt% were easily achieved in NMP over the entire composition range. Dried blend films were optically clear except a few compositions. For some compositions, the optical transparency in the dried films was retained through the thermal curing process, providing optically transparent semi-interpenatrating network (semi-IPN) composites. However, for the other compositions, phase separation took place through thermal curing, forming domains of submicrometre scales in the resultant composites. Properties of the polyimide composites were investigated by dynamic mechanical thermal analysis (d.m.t.a.), stress-strain analysis and self-adhesion measurement. In particular, the self-adhesion property of PMDA-ODA was significantly improved by the composite formation with 6FDA-APB, but not improved by the composite formation with BTDA-APB. The good self-adhesion in the PMDA-ODA/6FDA-APB composite was characterized by X-ray photoelectron spectroscopy and attributed mainly to the favourable molecular interdiffusion through the 6FDA-APB component segragated on the surface. In addition, the residual stress and relaxation behaviour were measured.
    Polymer 01/1998; 39(12):2521-2529. · 3.77 Impact Factor
  • M. Ree, K. Kim, S. H. Woo, H. Chang
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    ABSTRACT: Several high temperature polyimides, as model polymers with various chain rigidities and chain orders, were synthesized through the polycondensation of p-phenylene diamine (PDA) with five different aromatic dianhydrides [i.e., pyromellitic dianhydride (PMDA), biphenyltetracarboxylic dianhydride (BPDA), 4,4′-oxydiphthalic anhydride (ODPA), benzophenonetetracarboxylic dianhydride (BTDA), and 4,4′-hexafluoroisopropylidenediphthalic anhydride (6F)], and investigated in terms of structure and properties. Both fully rodlike PMDA-PDA and ether-linked ODPA-PDA polyimides showed excellent orientation and poor ordering of chain in x-ray diffraction patterns, whereas both biphenyl-linked (BPDA-PDA) and carbonyl-linked (BTDA-PDA) polyimides exhibited excellent orientation and high ordering of chain. However, hexafluoroisopropylidene-linked 6F-PDA polyimide is structureless. The coherence length along the chain axis, which is a measure of chain rigidity and ordering, was estimated from (00l) peaks of x-ray diffraction patters: 6F-PDA (could not be measured) <ODPA-PDA (95 Å)<BTDA-PDA (98 Å)<BPDA-PDA (104 Å)<PMDA-PDA (130 Å). In films prepared on substrates, all polyimides were preferentially oriented in the film plane. However, the degree of molecular in-plane orientation strongly depended on the coherence length. Higher coherence length, i.e., higher chain rigidity and ordering, caused higher in-plane orientation of chains. However, the chain rigidity is more critically responsible for molecular in-plane orientation, in comparison to the chain ordering. Higher in-plane orientation of chains led to larger anisotropies of refractive indices and dielectric constants, higher in-plane modulus, lower in-plane thermal expansion coefficient, and lower residual stress. © 1997 American Institute of Physics.
    Journal of Applied Physics 01/1997; 81(2):698-708. · 2.21 Impact Factor