Frank W. Harris

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.

Synthesis and Characterization of Polyimides Based on 3,6-Diphenylpyromellitic Dianhydride:

3,6-Diphenylpyromellitic dianhydride (DPPMDA) has been synthesized from 2,5-diphenyl-3,4-bis(4-methoxyphenyl)cyclopentadiene-1-one and dimethyl acetylenedicarboxylate. The monomer was polymerized with 11 aromatic diamines in refluxing m-cresol containing isoquinoline to afford two new series of phenylated polyimides. The polymers containing flexible ether or methylene linkages, meta-catenation, or cardo linkages were soluble in m-cresol, N-methylpyrrolidinone, and symtetrachloroethane. Their intrinsic viscosities ranged from 0.81 to 2.40 dl/g. The polymers prepared from 4,4′-diaminodiphenyl ether, 4,4′-diaminodiphenylmethane, and 1,3-diaminobenzene underwent extensive crystallization in concentrated m-cresol solutions. These polymers displayed weak glass-transition temperatures between 310°C and 320°C and melting points near 430°C. Three para-catenated, rigid-rod polymers were also prepared from DPPMDA and substituted 4,4′-diaminobiphenyls that displayed limited solubility in m-cresol. Solutions containing 8.5 to 15% (w/v) of the polymer prepared from 2,2′-bis(trifluoromethyl)-4,4′-diaminobiphenyl underwent reversible gellation near 85°C. Liquid-crystalline spherulites formed in the gels upon standing at ambient temperatures.

High-performance aromatic polyimide fibres: 2. Thermal mechanical and dynamic properties

Abstract A family of high-temperature, high-modulus aromatic polyimide fibres has been dry-jet wet spun from either its gel state or isotropic solution, followed by high-temperature drawing. In this report, thermal and dynamic mechanical properties of one of the family members, a segmented rigid-rod polyimide synthesized from 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA) and 2,2′-bis(trifluoromethyl)-4,4′-diaminobiphenyl (PFMB), are presented in detail. Mechanical properties of these BPDA-PFMB fibres can be improved remarkably by drawing due to drastic increases in overall orientation, crystal orientation and crystallinity. These three structural parameters, however, do not show parallel changes with increasing draw ratio. It has been observed that the linear coefficient of thermal expansion (CTE) of BPDA-PFMB fibres after drawing generally show negative values in the solid state when low stresses are applied during measurements. For as-spun fibres, the CTEs are constant over a certain applied stress region, which is on the same order of magnitude as CTEs of in-plane oriented BPDA-PFMB films along the film surface. This may be an indication that within this region the stress applied is at the same level as the internal stress frozen into the fibres during spinning and drawing. Glass transition temperatures (Tg) of as-spun fibres show a linear decrease at low applied stress region, then level off when the applied stress becomes high. Dynamic mechanical data indicate two relaxation processes in as-spun fibres above room temperature: an α relaxation corresponding to the glass transition and a β relaxation which is a subglass transition. In the fibres with a draw ratio of above three times, the α relaxation is totally suppressed. This reveals a rigid fraction (above Tg) dependence of this relaxation in the fibres. The β relaxation is, on the other hand, crystallinity dependent. The Arrhenius activation energy (about 160 kJ mol−1) of the β relaxation in as-spun fibres is about 50 kJ mol−1 lower than that of drawn fibres, indicating that the cooperativity of molecular motion in the fibre changes with orientation and crystallinity.

Synthesis and characterization of imide oligomers end-capped with 4-(phenylethynyl)phthalic anhydrides

Abstract A new series of phenylethynyl end-capped imide oligomers was synthesized and analysed for thermal stability, thermo-oxidative stability, glass transition temperature (Tg), cure temperatures and adhesive strength. Thus, 4-(phenylethynyl)phthalic anhydride (PEPA) and four substituted PEPAs containing electron-withdrawing groups were synthesized and used as end-capping agents for oligomers of 1,4-diaminobenzene and 2,2′-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride. The oligomers underwent an exothermic reaction above 350°C to afford an insoluble network. The cure reaction was apparently promoted by electron-withdrawing groups as the oligomers end-capped with the substituted PEPAs started to cure at lower temperatures. Their rate of cure was also faster, as evidenced by the faster rise in their Tgs. The cured resins had Tgs as high as 405°C and displayed good thermo-oxidative stability at 371°C. A cured sample of a PEPA end-capped oligomer with a calculated molecular weight of 4200 g mol−1 displayed moderate high-temperature adhesive strength.

Synthesis and Characterization of Reactive End-Capped Poiymide Oligomers

Abstract A series of reactive, end-capped, polyimide oligomers has been prepared for possible use as planarizing coatings in the electronics industry. Thus, 1,3-bis(3-aminophenoxy)benzene was treated with various excess amounts of 2,2-bis(3,4-dicarboxyphenyl)hexafluoro-propane dianhydride in m-cresol containing toluene and isoquinoline. The resulting anhydride-terminated amic-acid oligomers were thermally imidized and then allowed to react with 3-aminophenylethyne, 1-phenyl-4-(3-aminophenyl)buta-1-ene-3-yne, 1-phenyl-4-(3-amino-phenyl)-1,3-butadiyne, 2-aminobiphenylene, or 1-phenyl-2-(3-amino-phenyl)ethyne. Thermal imidization of these intermediates produced the corresponding end-capped polyimide oligomers. The white oligomers were soluble in organic solvents, such as diglyme, and had glass transition temperatures (Tgs) between 95 and 145°C. The Tg of an ethynyl-terminated oligomer was reduced from 105 to 62°C upon the incorporation of 20 wt % of the reactive plasticizer bis[2-(3-ethynylphenoxy)ethyl]eth...

Organo-soluble, segmented rigid-rod polyimide films: 2. Properties for microelectronic applications

Abstract The essential properties of polyimide films of importance in microelectronic applications are thermal and thermo-oxidative stability, dimensional stability, glass transition behaviour and the relative permittivity (dielectric constant e′). A segmented rigid-rod polyimide was synthesized from 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA) and 2,2′-bis(trifluoromethyl)-4,4′-diaminobiphenyl (PFMB) in order to develop new materials for microelectronic applications. The thermal and thermo-oxidative stability were investigated by determining the thermal degradation activation energy in air (210 kJ mol −1 ) and in nitrogen (303 kJ mol −1 ). The thermal stability was further studied through thermogravimetry-mass spectroscopy. The coefficient of thermal expansion, which indicates the dimensional stability, was measured via a tension mode of a thermomechanical analyser and doubly extrapolated to zero stresses, and was 6.98 × 10 −6 °C −1 for the BPDA-PFMB films. The glass transition temperature, measured thermomechanically, was found to be 287°C. The dielectric constant for the films, measured after ageing at 50% relative humidity for 48 h at 23°C, was between 2.8 and 2.9 in a frequency range from 0.1 kHz to 1 MHz. The temperature and frequency dependence of the dielectric behaviour is also discussed.

From crystals to columnar liquid crystal phases: molecular design, synthesis and phase structure characterization of a series of novel phenazines potentially useful in photovoltaic applications

It is known that in photovoltaic applications, columnar discotic liquid crystal (LC) phases of conjugated compounds are useful to align the molecules for improving their charge mobilities. However, conjugated compounds are usually either crystalline or amorphous. For compounds to form columnar discotic LC phases, specific molecular design is required for their ordered structural packing. In our recent report, a series of conjugated compounds, 6,7,15,16-tetrakis(alkylthio)quinoxalino-[2′,3′:9,10]-phenanthro[4,5-abc]phenazine (TQPP-[SCn]4) (n = 6, 8, 10 and 12), which display p-channel characteristics, were synthesized and characterized. This series of compounds was crystalline and did not exhibit LC behavior (S. Leng, B. Wex, L. H. Chan, M. J. Graham, S. Jin, A. J. Jing, K.-U. Jeong, R. M. Van Horn, B. Sun, M. Zhu, B. R. Kaafarani and S. Z. D. Cheng, J. Phys. Chem. B, 2009, 113, 5403–5411). In order to create a columnar LC phase with the lowest free energy within a broad applicable temperature region, we specifically designed and synthesized several series of electron-deficient phenazine derivatives to disrupt the molecular crystal packing and force the compounds to enter the columnar LC phase. These phenazine derivatives were designed to control the fused rigid ring size and shape as well as the location, lengths, and chemical structures of their flexible tails. These series include a series of 2,11-bis(1-methylethyl)-6,7,15,16-tetrakis(alkoxy)quinoxalino[2′,3′:9,10]phenanthro-[4,5-abc]-phenazines (TQPP-[t-Bu]2-[OR(B)]4), a series of 2,13-bis(1-methylethyl)-7,8,18,19-tetrakis(alkoxy)pyrazino[2,3-i]pyrazino[2″,3″:6′,7′]quinoxalino[2′,3′:9,10]phenanthro[4,5-abc]-phenazines (TPPQPP-[t-Bu]2-[OR(B)]4), and a series of 3,4,11,12,19,20-hexaalkoxy-2,5,7,8,10,13,15,16,18,21,23,24-dodecaazatri-anthracenes (HDATAN-[OR]6), where R is the alkyl chain in the substituents and B represents that they are branched structures. The different phase structures and transition behaviors of these series of compounds were studied, and based on the experimental results, we can conclude that tailoring the alkyl tail size, the core size, and the core shape leads to a promising way to design molecules that exhibit the columnar LC phase. In particular, changes in alkyl tail architecture affect the phase behaviors more significantly than changes in its length.

Organo-soluble polyimides: synthesis and polymerization of 2,2′-disubstituted-4,4′,5,5′-biphenyltetracarboxylic dianhydrides

Abstract The objective of this research was to investigate the effects of introducing pendant groups in the 2- and 2′-positions of 4,4′,5,5′-biphenyltetracarboxylic dianhydrides on the properties of polyimides produced from the dianhydrides. Thus, the work was begun with the syntheses of 2,2′-dibromo-4,4′,5,5′-biphenyltetracarboxylic tetracarboxylic dianhydride (DBBPDA) and 2,2′-diphenyl-4,4′,5,5′-biphenyltetracarboxylic dianhydride (DPBPDA). The two new dianhydrides were polymerized with several substituted 4,4′-diaminobiphenyls including 2,2′-bis(trifluoromethyl)-4,4′-diaminobiphenyl (PFMB) in refluxing m -cresol containing isoquinoline to afford two new series of soluble polyimides. The polyimides had intrinsic viscosities that ranged from 1.80 to 5.58 dl g −1 ( N -methyl-2-pyrrolidinone or m -rmcresol at 30°C). Their glass transition temperatures ( T g s) ranged from 322 to 351°C (thermal mechanical analysis). Several of the polymers obtained from the new dianhydrides were soluble in acetone and tetrahydrofuran. The polymers formed water-white, tough films that were transparent above 350 nm. The films displayed negative birefringence, i.e. their in-plane refractive indices ( n ∥ ) were higher than their out-of-plane refractive indices ( n ⊥ ). For example, the DBBPDA/PFMB polymer, which was soluble in acetone and had a T g of 330°C, formed films with n ∥ = 1.637 and n ⊥ = 1.564. The films had coefficients of thermal expansion that ranged from 1.05 × 10 −5 to 2.12 × 10 −5 °C −1 . The films may be useful as retardation layers in liquid crystal displays.

Phase behaviors and supra-molecular structures of a series of symmetrically tapered bisamides

A series of symmetrically tapered 1,4-bis[3,4,5-tris(alkan-1-yloxy)benzamido] benzene bisamides (CPhBA, where is the number of carbon atoms in the alkyl chains, = 10, 12 and 16), was synthesized in order to investigate the effect of alkyl chain length on supra-molecular ordered structures induced by hydrogen (H)-bonding and micro-phase separation. These bisamides consist of a rigid aromatic bisamide core with three flexible alkyl chains at each end of the core. Major phase transitions and their origins in CPhBA bisamides were studied with differential scanning calorimetry, one-dimensional (1D) wide angle X-ray diffraction (WAXD), infrared spectroscopy, and solid-state carbon-13 nuclear magnetic resonance experiments. The structures of these compounds in different phases were identified using 2D WAXD from oriented samples and were also confirmed by selected area electron diffractions in transmission electron microscopy from stacked single crystals and by computer simulations. All of the CPhBA bisamides in this series formed a highly ordered oblique columnar () phase and a low-ordered oblique columnar () phase, similar to a recent report on C14PhBA. The two main driving forces in the formation of these two supra-molecular columnar structures were identified: One was the H-bond formation between N-H and C[double bond, length as m-dash]O groups, and the other was the micro-phase separation between the bisamide cores and the alkyl chains. With increasing the length of alkyl tails, the isotropization temperature decreased, while the disordering temperature of the alkyl tails increased. The 2D lattice structures perpendicular to the columnar axis also increasingly deviated from the pseudo-hexagonal packing with increasing the alkyl tail length. However, the alkyl tail length did not have a significant influence on the packing along the columnar axis direction. Utilizing polarized optical microscopy, the phase identifications were also supported by the observation of texture changes and molecular arrangements inside of the micro-sized domains.