JOURNAL OF
POLYMER SCIENCE
WWW.POLYMERCHEMISTRY.ORG
ARTICLE
considerably shorter cutoff wavelength (253 nm) than the
other PI-HD-based polyimides.
As the chemical imidization temperature of PI-HD is
decreased, its transmittance at 400 nm tends to gradually
increase, its cutoff wavelength decreases, and it becomes paler.
A film of PI-HD-40-10 showed up to 97% transmittance in the
visible region, a considerably low cutoff wavelength (253 nm),
and colorlessness. This was probably due to reduced or absent
conjugation and CTC formation, in conjunction with a lack of
yellowing at the low imidization temperature used. In addi-
tion, PI-HD-40-10 had good solubility and thermal stability,
and yielded thin films that were highly flexible.
It was also observed that the imidization temperature influ-
ences the color of polyimide films. As the imidization tem-
perature of PI-HD was decreased, the color of the resulting
PI-HD films gradually became pale; the PI-HD-40-10 film
was almost colorless [Fig. 1(c)]. Films of PI-HD-140-10 and
PI-HD-160-10 were yellow.
In this study, the transparency and color of polyimide film
were considered to be affected by the chemical structure of
the polyimide and by the imidization temperature. It is well
known that the opacity and color of polyimide films is attrib-
utable to intramolecular conjugation and intra- and intermo-
lecular CTC formation.3,35–37
REFERENCES AND NOTES
1 D. Wilson, H. D. Stenzenberger, P. M. Hergenrother, Polyi-
mides; Blackie: Glasgow and London, 1990.
2 C. E. Sroog, Prog. Polym. Sci. 1991, 16, 561–694.
Because PI-HD polymers are partially aliphatic, the PI-HDs
have reduced or no conjugation and CTC effects, and thus
show higher transparency and less color than fully aromatic
PI-PDs.
3 M. K. Ghosh, K. L. Mittal, Polyimides: Fundamentals and
Applications; Marcel Dekker, Inc.: New York, 1996.
4 M. Hasegawa, M. Fujii, J. Ishii, S. Yamaguchi, E. Takezawa,
T. Kagayama, A. Ishikawa, Polymer 2014, 55, 4693–4708.
5 M. Hasegawa, M. Horiuchi, Y. Wada, High Perform. Polym.
2007, 19, 175–193.
As described above, as imidization temperature is increased,
the transparency of the resulting polyimide films decreases
and they become yellow. One reason for the low transpar-
ency and yellow color of PI-HD-140 and PI-HD-160 films
could be a certain chemical change to the polyimides at
higher temperatures (140 and 160 8C) that cannot be
detected by IR or NMR spectroscopy. Another reason could
be that the decreased transparency and yellowing of the pol-
yimide films is caused by remaining solvent in the films. It
has been shown that polyimide films contain small amounts
of residual solvent.38–40 In this study, the imidization solvent,
NMP, became yellow under the reaction conditions [Fig.
1(d)]. It was assumed that the yellowed residual solvent was
partially responsible for making the films yellow.
6 H. Lim, W. J. Cho, C. S. Ha, S. Ando, Y. K. Kim, C. H. Park, K.
Lee, Adv. Mater. 2002, 14, 1275–1279.
7 L. Zhai, S. Yang, L. Fan, Polymer 2012, 53, 3529–3539.
8 A. S. Mathews, I. Kim, C. S. Ha, Macromol. Res. 2007, 15, 114–128.
9 T. Matsumoto, Macromolecules 1999, 32, 4933–4939.
10 J. Kato, A. Seo, S. Shiraishi, Macromolecules 1999, 32, 6400–6406.
11 A. S. Mathews, I. Kim, C. S. Ha, J. Appl. Polym. Sci. 2006,
102, 3316–3326.
12 Y. Watanabe, Y. Sakai, Y. Shibasaki, S. Ando, M. Ueda, Y.
Oishi, K. Mori, Macromolecules 2002, 35, 2277–2281.
13 W. Volksen, H. J. Cha, M. I. Sanchez, D. Y. Yoon, React.
Funct. Polym. 1996, 30, 61–69.
14 T. Yamashita, T. Miura, J. Photopolym. Sci. Technol. 2007,
20, 743–746.
Highly flexible films were obtained from most of the polyi-
mides prepared in this work. A representative example was
the highly flexible PI-HD-40-10 film (Fig. 7). There was no
significant change to the films’ appearance when they were
bent, twisted and rolled up many times.
15 E. S. Balcerzak, D. Sek, A. Volozhin, T. Chamenko, B.
Jarzabek, Eur. Polym. J. 2002, 38, 423–430.
16 S. Chisca, V. E. Musteata, I. Stoica, I. Sava, M. Bruma, J.
Polym. Res. 2013, 20, 111–121.
17 A. E. Eichstadt, T. C. Ward, M. D. Bagwell, I. V. Farr, D. L.
Dunson, J. E. McGrath, J. Polym. Sci. B: Polym. Phys. 2002, 40,
1503–1512.
CONCLUSIONS
18 D. J. Liaw, F. C. Chang, J. Polym. Sci. A: Polym. Chem.
2004, 42, 5766–5774.
HPMDA or PMDA was polymerized with an aromatic dia-
mine, DDPM, to give poly(amic acid)s. Chemical imidization
of the poly(amic acid)s was attempted at the low tempera-
tures of 40–160 8C. More than 90% imidization was
achieved, even at very low imidization temperature of 40 8C.
The degree of imidization increased with increasing imidiza-
tion temperature.
19 C. P. Yang, Y. Y. Su, S. J. Wen, S. H. Hsiao, Polymer 2006,
47, 7021–7033.
20 H. S. Jin, J. H. Chang, J. C. Kim, Macromol. Res. 2008, 16,
503–509.
21 T. H. Lee, J. H. Kim, B. S. Bae, J. Mater. Chem. 2006, 16,
1657–1664.
22 M. I. Bessonov, V. A. Zubkov, Polyamic Acids and Polyi-
mides: Synthesis, Transformations, and Structure, CRC Press,
Inc.: Boca Raton, 1993, p 2.
The imidization rate increases with temperature and the
imidization occurs in two steps: an initial rapid cyclization
and a subsequent slower cyclization process. Assuming that
the imidization was a first-order reaction, the activation
energy for the initial rapid process was determined to be 4.3
kJ/mol, whereas the activation energy for the slower process
was determined to be 4.8 kJ/mol.
23 K. L. Mittal, Polyimides and Other High-Temperature Poly-
mers: Synthesis, Characterization and Applications; BRILL: Lei-
den, 2009; Vol. 5, p 25.
24 H. F. Mark, Encyclopedia of Polymer Science and Technol-
ogy, Concise; John Wiley & Sons, 2013, p 914.
WWW.MATERIALSVIEWS.COM
JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2016, 54, 1593–1602
1601