Novel photodynamic polymers: Azobenzene-modified cellulose (azocellulose) and azobenzene-modified poly(L-glutamic acid) (AZOPLGA)

• Main Author: Yang, Suizhou
• Format: Dissertation
• Language: English
• Subjects: chemistry, polymer; plastics technology
• ISBN: 9780493433745; 0493433740

This thesis research presents the synthesis and characterization of two new kinds of photodynamic polymers: azobenzene-modified cellulose (azocellulose) and azobenzene-modified poly(L-glutamic acid) (AZOPLGA). The photoinduced deformation process on the thin films of these novel polymers was also investigated.
Azocellulose polymers with ultrahigh and low molecular weight were first successively synthesized by covalently linking azobenzene chromophores to natural cellulose via Mitsunobu reaction. By varying the molar ratios of the reactants, the azocellulose polymers with varying degree of substitution (DS) were obtained. These polymers were characterized using FT-IR, LTV-vis, and solid-state 13C CP/MAS NMR spectroscopies. Experimental results provided evidence that the coupling reaction occurs preferentially at the C6 carbon site and the primary alcohol is the predominant reactive group among the hydroxyl groups (one primary and two secondary alcohol groups) in the D glucopyranse unit of cellulose. The thermal cis-trans isomerization of azocellulose polymers is composed of two processes: (a) a fast process due to the relaxation of azobenzene chromophores trapped in a strain conformation at temperatures below its glass transition temperature, and (b) a slow process which follows a single exponential decay. A linear increase of photoinduced birefringence with the azobenzene chromophore concentration in these photodynamic polymers was observed. The maximum birefringence of 0.11 was achieved for an azocellulose polymer with a DS of 0.99.
It has been reported that SRGs cannot be efficiently inscribed with polymers of high molecular weight because of large number of entanglements of polymer chains. However, SRGs on the films of our azocellulose polymers of ultrahigh molecular weight with different functionality were successfully inscribed in a single step at different grating spacing. Surface modulation depth of the gratings increased with the degree of substitution of the azobenzene chromophores in the cellulose as well as with the grating spacing. This is the first report of SRGs on the polymers with ultrahigh molecular weight. Under the same condition, inscribing SRGs on the film of azocellulose with low molecular weight is more easily performed than that for high molecular weight. Also, the SRGs of low molecular weight azocellulose are more smoothly inscribed than that of high molecular weight azocellulose.
Another class of synthesized photodynamic polymers are azobenzene-modified poly(L-glutamic acid) (AZOPLGA) by the reacting of 4-methoxy-4′ -aminoazobenzene with poly(L-glutamic acid) (PLGA). Experiment data indicate that azobenzene chromophores were covalently linked to the side chain of PLGA. The thermal cis-trans isomerization of AZOPLGA polymers also shows two distinct relaxation mechanisms: (a) a fast process attributed to the relaxation of azobenzene chromophores trapped in a strained conformation at temperatures below its glass transition temperature, and (b) a slow process which follows a single exponential decay. The main chains of 35%AZOPLGA polymer are in random coils in the film cast from TFA solution but in α-helix conformation in the film cast from pyridine solution. And the modulation depth of surface relief gratings on this film in which the polymer chain in random coil is much higher than on the film in which the polymer chains are in a-helix conformation. Inscribing surface relief grating on the films of 22%AZOPLGA is not efficiently due to the low azobenzene chromophore content and tightly hydrogen-bonded conformation (α-helix or β-sheet) of the polymer. This is the first report of the effect of conformation on the surface relief gratings.