Jason J. Ge

Dianhydride architectural effects on the relaxation behaviors and thermal and optical properties of organo-soluble aromatic polyimide films

Abstract Dianhydrides of specific molecular architecture was designed and synthesized based on 2,2′-disubstituted 4,4′,5,5′-biphenyltetracarboxylic dianhydrides (2,2′-disubstituted BPDAs). Eight dianhydrides were polymerized with two 4,4′-diamino-2,2′-disubstituted biphenyl diamines (trifluoromethyl disubstituted groups, or PFMB, and methyl disubstituted groups, or DMB) to obtain two series of PFMB- and DMB-based aromatic polyimides. As the backbone structures of these two series of polyimides are unchanged throughout each series, the effects of the 2,2′-disubstituted groups of both the dianhydride and diamine constituents on the solubility and thermal and optical properties as well as the relaxation behavior of these polyimides can be identified. It was found that the PFMB-based polyimides with 2,2′-disubstituted BPDAs show excellent solubility while the DMB-based polyimides with the same dianhydrides are less soluble. The same trends can be found for both thermal and thermo-oxidative stability and optical transparency in the ultraviolet and visible light regions. These two series of polyimides exhibit glass transition temperatures ( T g ) which show a competition between chain rigidity and linearity with regards to molecular packing. When the size of the 2,2′-disubstituted groups is small and their shape is close to spherical, the T g initially increases with the size of these 2,2′-disubstituted groups. This is because of the fact that the steric hindrance of these groups prevents the appearance of a cis -conformation of BPDA. However, once these groups possess large size and exhibit anisotropic shapes, their effect on the molecular packing becomes dominant and the T g starts to decrease. Further, this is the first time that three relaxation processes (the β 1 , β 2 , and α processes) were observed above room temperature in these aromatic polyimides. We have identified that the β 1 process is attributed to the local motion of the diamine constituents while the β 2 process is caused by the local motion of the dianhydride constituents. The α process is associated with the glass transition. The cooperativity of the molecular motion associated with the β 1 and β 2 processes are also discussed.

Diamine architecture effects on glass transitions, relaxation processes and other material properties in organo-soluble aromatic polyimide films

Abstract A series of twelve aromatic diamines, 4,4′-diamino-2,2′-disubstitutedbiphenyls, has been designed and synthesized. These diamines were reacted with 2,2′-bis(3,4-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA) to form polyimides via a one-step polycondensation method. All of the resulting polyimides could be dissolved in common organic solvents and exhibited excellent film forming ability. At the same time, their inherent high thermal and thermo-oxidative stability of these polyimides was retained in the films. Because of the incorporation of disubstituted groups at the 2- and 2′-positions of these biphenyl diamines, their crystallinity was suppressed to the level that they were in complete amorphous state. Further, the conjugation of the phenylene and imide groups in these polyimide films was interrupted, leading to clear blue shifts during light transmission. As this series of polyimides possessed the same backbone, the chain rigidity and linearity changed very little throughout the series. However, the molecular packing was affected by the introduction of different disubstituted pendant groups. Each polyimide film exhibited an α relaxation process related to the glass transition. This relaxation changed significantly with the size and the shape of the disubstituted pendant groups. In addition to this process, each of these polyimide films displayed a sub-glass transition, the β relaxation process, which was initiated by motion of the 4,4′-diamino-2,2′-disubstituted biphenyls. This study provided an opportunity to investigate how disubstituted pendant groups affected the α and β relaxation behaviors of these polyimides. With an increase of the sizes and the shape anisotropy of the disubstituted pendant groups at the 2- and 2′-position, the nature of the motion regarding to the β relaxation was found to evolve from a non-cooperative process to a cooperative one, while the glass transition temperature (the α relaxation temperature) correspondingly decreased.

Surface studies of polyimide thin films via surface-enhanced Raman scattering and second harmonic generation

Two polyimides having the same backbone chemical structure and different pendant side groups at the 2- and 2′-positions of the diamine, the six methylene units capped with 4-cyanobiphenyl end groups and trifluoromethyl, were synthesized (6FDA-6CBO and 6FDA-PFMB). Surface-enhanced Raman scattering and surface optical second harmonic generation measurements show that after rubbing the major change in 6FDA-PFMB surface appears in the orientation of the dianhydride, which was originally planar, but becomes tilted with respect to the surface plane. In the case of 6FDA-6CBO, rubbing also causes the originally planar 4-cyanobiphenyls to tilt away from the surface and assume an azimuthally anisotropic distribution.

Phase transformations in a chiral main-chain liquid crystalline polyester involving double-twist helical crystals

Abstract A main-chain non-racemic chiral liquid crystalline polymer has been synthesized from (R)-(−)-4′-{ω-[2-(p-hydroxy-o-nitrophenyloxy)-1-propyloxy]-1-nonyloxy}-4-biphenyl carboxylic acid, abbreviated PET(R∗)-9. Based on differential scanning calorimetry, wide angle X-ray diffraction (WAXD) and polarized light microscopy experiments, this polymer undergoes at least three liquid crystalline (LC) transitions in addition to crystallization. The LC transition sequence is I ↔ 3.21 kJ/mol 185° C TGBA ∗ ↔ 1.27 kJ/mol 175° C S A ∗ ↔ 0.35 kJ/mol 130° C S C ∗ ↔ 37° C T g The transition sequence is reversible since they are LC phases, which are close to thermodynamic equilibrium. Among these phases, the twisted grain boundary smectic A (TGBA∗) phase is, for the first time, found in a main-chain LC polymer. The TGBA∗ is only stable within a temperature region of 10°C between the Smectic A∗ (S∗A) phase and the isotropic melt. Crystallization that takes place in the (S∗A) phase can form both flat-elongated and double-twist helical single lamellar crystals, as observed by transmission electron microscopy (TEM). Analysis of the WAXD fiber patterns of this polymer indicates that the crystal structure of PET(R∗)-9 in the bulk is orthorhombic, identical with that determined using selective area electron diffraction in TEM [Macromolecules 32 (1999) 524; Phys Rev B 60 (1999) 12 675].

Phase structures, transition behavior and surface alignment in polymers containing rigid-rod backbones with flexible side chains. Part VI Novel band structures in a combined main-chain/side-chain liquid crystalline polyester : From liquid crystal to crystalline states

Physical origins of banded structures appearing on different length scales have been investigated using polarized light and atomic force microscopies (PLM and AFM), polarized Fourier Transform infrared spectroscopy (FT-IR) and wide angle X-ray diffraction (WAXD) in a combined main-chain/side-chain liquid crystalline (LC) polyester, PEFBP(n). This series of PEFBP(n) polymers was synthesized from the polycondensation of 2,2′-bis(trifluoromethyl)-4,4′-biphenyldicarbonyl chloride with 2,2′-bis{ω-[4-(4-cyanophenyl)-phenyoxy]-n-alkoxycarbonyl]}-4,4′-biphenyl diol. In this paper, we focus on one polymer [PEFBP(n = 11)] of this series to illustrate the band structural formation on different length scales during the evolution from liquid crystal to crystalline states. Alternating bands of the films mechanically-sheared at 190 °C are formed with a spacing of 3 ± 0.5 μm in PLM, and recognized to be primary bands. PLM and AFM results show that these bands are seen due to the change of optical birefringence constructed mainly by alternating film thickness (and thus, retardation). Based on polarized FT-IR results, both the backbones and side chains of the polymers are orientated parallel to the shear direction. Secondary fibrillar bands develop within the primary bands after the sample is subsequently crystallized at 105 °C. These bands show a zigzag arrangement and possess a lateral size of 250 ± 50 nm determined by AFM. High resolution AFM observations illustrate that these bands consist of aggregated edge-on crystal lamellae having a thickness of approximately 20 nm. The lamellar crystals are assembled together and lie across the film thickness direction. The mechanism for the formation of these secondary zigzag bands originates from the expansion of the lattice dimension along the chain direction on a molecular scale during the nematic to crystalline phase transition and crystallization in the partially confined LC primary bands, which form macroscopic zigzag buckling.