Containing Azobenzene Side Chain Thermotropic Liquid Crystals Of Poly Peptide Synthesis And Phase Transition Behavior

Polypeptides usually exsit in living systems in liquid crystalline states, so the structures and functions of liquid crystalline polypeptides are concerned by researchers. Theoretical and applied studies have been conducted extensively on thermotropic liquid crystalline polypeptides. By modifying the side chains, the phase transition behaviors of polypeptides can be altered. Cholesteric phase is the most common phase in liquid crystalline polypeptides. As the cholesteric pitch can reflect the circularly polarized light selectively and is sensitive to temperature, cholesteric polypeptides are promising materials used in electrooptical displays and temperature indicators. Light is a kind of clean energy which can be controlled rapidly, accurately and remotely, therefore photoresponsive materials are becoming the new research topics. If the chromophores are introduced into the thermotropic cholesteirc liquid crystalline polypeptides, their photoisomerization will lead to the response to the external stimuli. This kind of materials is promising to be developed as novel optical materials which can be controlled by temperature or light.First, lyotropic cholesteric liquid crystalline polypeptide, poly(y-benzyl D-glutamate) (PBDG), was synthesized by N-carbonyl Anhydride (NCA) method. Except the general structural characterizations of the product, the effect of reaction conditions such as initiator and reaction temperature on molecular weight and molecular weight distribution was also investigated. The results give a good selection for the reaction condition of thermotropic cholesteric liquid crystalline polypeptide.Based on the above research, thermotropic cholesteric liquid crystalline polypeptide poly(γ-dodecyl L-glutamate) (PDLG) was synthesized by NCA method under the suitable reaction condition. A series of novel thermotropic liquid crystalline polypeptides AzoPDLGs which incorporate mesogens both in main chains and side chains were synthesized by attaching mesogenic azobenzene groups to the mesogenicα-helical main chain of PDLG.The thermotropic phase transitions of AzoPDLGs were investigated. It was found that thermotropic liquid crystalline polypeptides with different azobenzene contents showed different phase transition behaviors. When the azobenzene content was lower than 20%, the thermotropic behavior of product was similar with that of PDLG, but the rigid azobenzene mesogens increased its phase transition temperature. When the azobenzene content was higher than 40%, the stacking of azobenzene groups induced the formation of layered crystalline phase at the lower temperatures. On heating, azobenzene groups randomly dispersed around the main chain, which resulted in cholesteric phase based on the cc-helical mesogenic groups. When the azobenzen content was 100%, layered crystalline phase and smectic phase formed in the lower temperature range. However, isotropic phase instead of cholesteric phase formed in the higher temperature range.The photoresponsive behaviors of AzoPDLGs were also investigated. It was found that when the azobenzene content was in the range from 40% to 70%, the liquid crystalline phase of AzoPDLGs could be changed reversibly between crystalline phase and cholesteric phase by irradiation of UV or visible light. The reason was that the azobenzene groups transformed from rod-like trans form to bent cis form upon UV irradiation, which destabilizes the initial phase structure and lowers the phase transition temperature obviously. It should be pointed out that when the azobenzene content was 100%, UV irradiation could induce the transition from smectic phase to isotropic phase but could not induce an apparent transition of the crystalline phase. These results infer that the content of long alkyl side chains has a great impact on the photoresponsive behavior of liquid crystalline polypeptides. When the azobenzene groups are separated from each other by the dodecyl side chains, they are not able to stack too tightly. Therefore, the azobenzene groups take sufficient free volume to undergo photoisomerization and photochemical phase transition. This study is the first example of the photochemical phase transition of liquid crystalline polypeptides induced by the photochemical reaction of chromophoric groups which enables the phase transition of the material to take place at lower temperatures.