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. 2017 Jun 18;9(6):233.
doi: 10.3390/polym9060233.

Photo-Induced Vertical Alignment of Liquid Crystals via In Situ Polymerization Initiated by Polyimide Containing Benzophenone

Fei Wang  1 , Leishan Shao  2 , Qiyao Bai  3 , Xinyuan Che  4 , Bin Liu  5 , Yinghan Wang  6
Affiliations

Photo-Induced Vertical Alignment of Liquid Crystals via In Situ Polymerization Initiated by Polyimide Containing Benzophenone

Fei Wang et al. Polymers (Basel). .

Abstract

Vertical alignment of liquid crystal (LC) was achieved in an easy and effective way: in situ photopolymerization of dodecyl acrylate (DA) monomers initiated by polyimide based on 3,3',4,4'-benzophenonetetracarboxylic dianhydride and 3,3'-dimethyl-4,4'-diaminodiphenyl methane (BTDA-DMMDA PI). The alignment behavior and alignment stabilities were characterized by a polarizing optical microscope (POM), which showed a stable vertical alignment after 12 h of thermal treatment. The chemical structures, morphology, and water contact angles of alignment films peeled from LC cells with and without DA monomers were analyzed by means of a Fourier transform infrared spectrometer (FTIR), a scanning electron microscope (SEM), and a contact angle tester, separately. The results confirmed that the DA monomers underwent self-polymerization and grafting polymerization initiated by the BTDA-DMMDA PI under ultraviolet irradiation, which aggregated on the surfaces of PI films. The water contact angles of the alignment films were about 15° higher, indicating a relative lower surface energy. In conclusion, the vertical alignment of LC was introduced by the low surface free energy of PI films grafted with DA polymer and intermolecular interactions between LC and DA polymers.

Keywords: alignment stability; photopolymerization; surface free energy; vertical alignment.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure 1
Figure 1
(A) FTIR spectra of polyimide based on 3,3′,4,4′-benzophenonetetracarboxylic dianhydride and 3,3′-dimethyl-4,4′-diaminodiphenyl methane (BTDA-DMMDA PI) films peeled from liquid crystal (LC) cells with different weight ratios of dodecyl acrylate (DA) monomers: a: DA-0, b: DA-2, c: DA-1, and d: DA-0.5; (B) FTIR spectrum a (black line) and its peek fitted with Lorentzian functions curves (red dash line and bold black lines).
Figure 2
Figure 2
Polarizing optical microscope (POM) microphotographs of LC cells with 2 wt % DA monomers (A), 1 wt % DA monomers (B), and 0.5 wt % DA monomers (C) after UV irradiation.
Figure 3
Figure 3
POM microphotographs of LC cells with 2 wt % DA monomers (A), 1 wt % DA monomers (B), and 0.5 wt % DA monomers (C) after UV irradiation and thermal annealing at 120 °C for 30 min.
Figure 4
Figure 4
POM microphotographs of LC cells comprising BTDA-DMMDA PI film (A), polyimide based on 3,3′,4,4′-benzophenonetetracarboxylic dianhydride and 4,4-diaminodiphenyl ether (BTDA-ODA PI )film (B), and polyimide based on 4,4′-Oxydiphthalic anhydride and 4,4-diaminodiphenyl ether (ODPA-ODA PI) film (C) with 1 wt % DA monomers after UV irradiation.
Figure 5
Figure 5
Scheme of the photoreaction of BTDA-DMMDA PI and DA monomers.
Figure 6
Figure 6
POM microphotographs of LC cells with 2 wt % DA monomers (A), 1 wt % DA monomers (B), and 0.5 wt % DA monomers (C) after UV irradiation and thermal annealing at 120 °C for 12 h.
Figure 7
Figure 7
SEM photographs of PI films from LC cells with 0 wt % DA monomers (A), 2 wt % DA monomers (B), 1 wt % DA monomers (C), and 0.5 wt % DA monomers (D) after UV irradiation and thermal annealing at 120 °C for 30 min.
Figure 8
Figure 8
Water contact angles and total surface free energy as functions of DA monomer contents.
See this image and copyright information in PMC

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