Abstract
A new diamine monomer 4-((4-dodecyloxy)phenyl)-2,6-di((4-amino)phenyl)pyridine (DPDAPP) was successfully synthesised and polymerised with 4,4ʹ-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) and diamine 2,2-bis[4-(4-aminophenoxy)phenyl]propane (BAPP) via the conventional two-step chemical imidisation method to produce a series of various alkyl-chain content polyimides (PIs). X-ray diffraction (XRD), nuclear magnetic resonance spectrometry (1H-NMR) and Fourier-transform infrared spectroscopy (FT-IR) were applied to study the structure of PIs. Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) were employed to measure the thermal properties. Results show that the solubility of the PIs was significantly improved by the introduction of DPDAPP at low concentration (≤50%). However, with further increasing concentration of DPDAPP, the amount of alkyl-chain increased and then partial crystallisation appeared, resulting in decreased solubility. The glass transition temperature (Tg) and decomposition temperature (Td) of these PIs were above 215°C and 450°C, respectively, which can meet the demands as liquid crystal (LC) alignment layers. Furthermore, the surface of these PIs showed strong hydrophobicity and more stable vertical alignment abilities, and hence the pretilt angle of LC was nearly 90° even with a low content of alkyl-chain.
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