Polymorphous structures and their phase relationships in a main-chain/side-chain liquid crystalline polyester

Jr-Jeng Ruan, Jason J. Ge, Anqiu Zhang, Jin Shi, Shy Yeu Wang, Frank W. Harris, Stephen Z.D. Cheng

Research output: Contribution to journalArticlepeer-review

27 Citations (Scopus)

Abstract

A series of specially designed polyesters consisting of aromatic main-chain/side-chain liquid crystalline (LC) polymers for flat-panel display applications have been synthesized via polycondensation of 2,2′-bis(trifluoromethyl)-4,4′-biphenyldicarbonyl chloride with 2,2′-bis{ω-[4-(4-cyanophenyl) phenyoxyl]- n-alkoxycarbonyl}-4,4′-biphenyldiol (PEFBP(n), where n is the number of methylene units in the side chains). For a PEFBP polyester of which a 4-cyanobiphenyl mesogen in each side chain is coupled with the aromatic backbone via seven methylene units, PEFBP(n=7), complicated polymorphism has been identified during its structural evolutions. On the basis of differential scanning calorimetry, wide-angle X-ray diffraction, electron diffraction (ED), polarized light, and transmission electron microscopy (TEM) experiments, it is found that this polymer possesses three triclinic crystalline (Kt1, Kt2, and Kt3) phases having the same symmetry but different unit cell dimensions, one highly ordered LC smectic phase having an orthorhombic (SKO) structure, and one nematic (N) phase. Both of the triclinic Kt1 and Kt3 structures are also confirmed by the ED results. Investigations of thermodynamic phase stability relationships indicate that when PEFBP(n=7)is crystallized from the N phase, the Kt1 phase forms. During heating, this Kt1 phase transfers to the Kt3 phase via reorganization and melting/recrystallization before it enters the N phase. However, when the sample is first cooled to below 130°C to form the SKO phase, the Kt2 phase develops during heating. Further heating the Kt2 phase leads to a transformation to the Kt3 phase at high temperatures via, again, reorganization and melting/recrystallization. Nevertheless, no apparent transformation between the Kt1 and Kt2 phases is experimentally observed. Polarized infrared spectroscopy experimental results indicate that the aromatic polyester backbones and 4-cyanobiphenyl mesogens are parallel to each other and packed into one crystal unit cell. TEM observations show different morphological characteristics of the Kt1 and Kt3 phases. It is concluded that the Kt3 phase is the thermodynamically most stable phase, and the Kt1 and Kt2 phases are metastable. These two phases can be experimentally accessed only because the crystallization rate of the Kt3 phase is much slower than those of the Kt1 and Kt2 phases.

Original languageEnglish
Pages (from-to)736-745
Number of pages10
JournalMacromolecules
Volume35
Issue number3
DOIs
Publication statusPublished - 2002 Jan 29

All Science Journal Classification (ASJC) codes

  • Organic Chemistry
  • Polymers and Plastics
  • Inorganic Chemistry
  • Materials Chemistry

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