Although aromatic polyimides (PIs) belong to the most important thermally stable polymers, their application was limited due to their infusibility and insolubility making them difficult to process. To overcome these difficulties, various modified PIs such as poly(amide-imide)s (PAIs) and poly(ether-imide)s. etc., have been developed. PAIs which have amide and imide groups in the polymer backbone, are amorphous, injection-moldable thermoplastics that exhibit outstanding mechanical properties at high temperature with good dimensional stability, excellent creep, impact and chemical resistance.
Generally, PAI is prepared by low-temperature-solution polycondensation of trimellitic anhydride chloride (TMAc) with aromatic diamines. TMAc is an asymmetric monomer containing two functional groups in which the reactivity to the symmetric monomer is different. The polycondensation of a symmetric monomer with an asymmetric monomer gives rise to PAIs with different microstructures which can be expressed in terms of imide-amide sequence (hereafter referred to head-to-tail, HT), and imide-imide (head-to-head) or amide-amide (tail-to-tail) sequence which is noted as HH.
In spite of the great importance of PAIs, there is a lack of information about the microstructure and the corresponding properties. Aromatic PAIs with different microstructures, based on trimellitimide and 4,4-diphenylether in 4,4''-oxydianiline (ODA) or 1,3,5-trimethylbenzene in 2,4-diaminomesitylene (DAM), were prepared with varying the reaction conditions and methods.
PAIs containing different microstructures were prepared by two different methods, ⅰ) direct polycondensation reaction using triphenylphosphite (TPP) as a condensing agent in the presence of pyridine and lithium chloride and ⅱ) low-temperature solution polycondensation. The microstructures of PAIs were examined by 500MHz $^1H$-NMR spectroscopy. The microstructures were dependent on the reaction temperature, polymerization methods and the addition mode of the monomer.
The head-to-tail PAI based on ODA, ODA-HT, revealed a recognizable peak at around 2θ = 20 degree, while ODA-HH and ODA-copolymer (ODA-Co) showed broad halo, meaning that ODA-HT had more ordered structure due to better packing of polymer backbones than ODA-HH and random copolymers. In FT-IR spectra of the PAI polymers the hydrogen bonding between -C=O and -NH- group also ensure the above results. The -C=O stretching band of ODA-HT shifted from $1674 cm^{-1}$ to $1663 cm^{-1}$ due to stronger hydrogen bonding than that of ODA-HH and ODA-Co with random sequence. Most of the PAIs were readily soluble in amide type solvents such as DMF, DMAc except for ODA-HT. The copolymers revealed better solubility than ODA-HT or ODA-HH. ODA-HT has the highest density value, $T_g$ of 308℃, 20℃ higher than ODA-HH or ODA-Co, tensile strength and modulus compared to the ODA-HH and ODA-copolymers. The storage moduli of ODA-based PAIs are in the order of ODA-HT, ODA-HH, and ODA-Co in the temperature range between 25℃ to 250℃, i.e., the ODA-HT has the highest rigidity, resulting from the restricted chain mobility due to the most ordered structure as seen in WAXD.
DAM-based PAIs with bulky $CH_3$ substituents, on the contrary, showed no significant differences among DAM-HH and DAM-HT in WAXD, FT-IR, density, solubility, and tensile properties due to the difficulties in the ordering tendency of the polymer chains. The $T_gs$ of the DAMs were not detected by DMTA or DSC below 400℃, where the polymers begin to decompose in air or nitrogen atmosphere.
New apparatus based on continuous flow techniques was successfully applied for measuring the gas permeation properties of various PI membranes. The results were found similar to the previously reported values. The transport properties of the ODA based PAIs for $O_2, N_2, CO_2, CH_4$, and He, however, could not be measured due to their low permeabilities of the membranes except for He gas. In the case of DAM based PAIs with bulky side groups, the permeabilities of DAM-HT were similar to those of DAM-HH as expected from the physical properties explained before.
To investigate the effect of bulkiness between the imide-imide in the main chains of PAIs, various HH-PAIs with 1,2,4,5-tetramethylbenzene unit in 2,3,5,6-tetramethyl-1,4-phenylenediamine (TMPD) were prepared and their permeation properties were measured. The permeability of the HH-PAIs was in the following order :
TDA(2,6-tolunendiamine) -TMPD < FDA[4,4''-(9-fluorenylidene)dianiline]-TMPD ＜ (DAM-HH) ＜ TMC[1,1-bis(4-amino-3,5-dimethylphenyl) cyclohexane]-TMPD ＜ DAM-TMPD ＜ APF[4,4''-(hexafluoroisopropylidene)dianiline]-TMPD.
The increasing order in the permeability showed the same trend as that of the fractional free volume calculated from the densities and van der Waals volume of the PAIs.'
