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Novel azobenzene-containing polymeric materials
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Type
Thesis
Author
Sin, Sau Leng
Supervisor
Gan, Leong Huat
Abstract
The focus of this research is the development of new azobenzene- containing polymeric materials. In this connection, two vastly different azobenzene-containing polymeric systems were investigated. The thesis is divided into two sections. Section I presents the works on azobenzene- containing conducting polymers via electrochemical and chemical polymerizations. The synthesis of azobenzene-containing 3-carbonylmethylenethiophenes is described. The electrochemical oxidation of four different azobenzenecontaining 3-carbonylmethylene-thiophenes, and their corresponding derivatives without the thiophene moiety, in acetonitrile solutions containing LiClO4 or methylene chloride solutions containing tetrabutylammonium tetrafluoroborate (TBABF4) has been studied using cyclic voltammetry at platinum and gold anodes. The resulting polymer films were characterized by X-ray Photoelectron Spectroscopy (XPS), UV-visible Spectroscopy, Electrochemical Quartz Crystal Microbalance (EQCM) and Scanning Electron Microscopy (SEM). Good polymer films were found to be difficult to obtain owing to instability of the azobenzene moiety under the anodic oxidation experimental conditions employed. However, for the azobenzene monomers substituted with electron withdrawing –NO2 and -CN groups, thin polymers films were obtained.
Chemical polymerizations of these 3- carbonylmethylene-thiophenes did not produce high molecular weight polymers. The presence of the redox-active azobenzene group in the side chain and the steric hindrance factor were found to be the reasons for the difficulties that were encountered during the polymerization process.
Section II describes the syntheses of azobenzene-containing polymers via atom transfer radical polymerization (ATRP). In this connection, homopolymers of azobenzene (azo) methacrylates with different substituents and their diblock copolymers with poly[2-(dimethylamino)ethyl methacrylate] p(DMAEMA) have been synthesized. Controlled/“living” ATRP of azo methacrylates were achieved with up to ~30% conversion. The copolymerization rate of 6-[4-phenylazo]phenoxy]hexylmethacrylate (PPHM) from p(DMAEMA) macroinitiator was found to be almost identical to that for the homopolymerization of PPHM monomer, with kapp ~ 0.0078 min-1. For copolymerizations, several novel well-defined diblock copolymers p(DMAEMA-b-Azo) were successfully synthesized. Almost complete incorporation of the azo methacrylate monomers could be obtained with a low molecular weight macroinitiator (PDMAEMA)-Cl, whereas a macroinitiator of long chain length did not give full conversion owing to possible chain entanglement and partial loss of the terminal halogen. Diblock azo copolymers with methyl methacrylate (MMA) were also synthesized. Unfortunately, several attempts to fabricate the polymers for holographic study were not successful because high quality transparent polymer films could not be obtained.
The azo homopolymers and azo diblock copolymers exhibited some interesting thermal and structural properties. Only azobenzene substituted with cyano and butoxy groups exhibited liquid crystalline textures. The glass transition behavior of the new azo diblock copolymers with DMAEMA was investigated. It was found that the Fox equation could not be used to satisfactorily predict the Tg for the block copolymers, even for those with very short second block. It appears that the short azo segment could act as a plasticizer causing the Tg to be lower.
Because azo monomers are highly hydrophobic, only the diblock copolymers with short azo segments are soluble in water. In aqueous solution, these copolymers self-assemble into micellar particles. The trans- cis photoisomerization and the thermally-induced reverse cis-trans isomerization in solid films and in solutions were studied in detail. Both the isomerization processes could be fitted by the first order exponential decay function. It was found that the rate of photoisomerization in aqueous solution was comparable to that in solid film. This result could be explained by the formation of core-shell micelles in aqueous solutions with the hydrophobic azo block forming the core and the hydrophilic DMAEMA segments forming the shell. The azo block in the core has restricted reorientation movements, similar to those in the solid; hence comparable photoisomerization rates were observed.
The effect of photo- induced trans-cis isomerization on the lower critical solution temperature (LCST) and surface tension were studied. The LCST of the diblock copolymers increased upon irradiation by UV light owing to the cis conformers being more hydrophilic. However, the trans-cis isomerization had only a small effect on the CMC and γcmc of the azo methacrylate block copolymers, owing to the formation of a compact core of the micelles. The formation of core-shell micelles was established from laser light scattering (LLS) and transmission electron microscopy (TEM). All of the three azo methacrylate amphiphilic block copolymers formed hard core-shell micelles with relatively small Rh values of 31 nm for p(DMAEMA172-b-BPHM7), 26 nm for p(DMAEMA172-b-CPHM7) and 32 nm for p(DMAEMA172-b- PPHM9). However for the azo acrylate copolymer, p(DMAEMA172-b- BPHA6), large micelles with Rh ~78nm with a loose core were formed.
Chemical polymerizations of these 3- carbonylmethylene-thiophenes did not produce high molecular weight polymers. The presence of the redox-active azobenzene group in the side chain and the steric hindrance factor were found to be the reasons for the difficulties that were encountered during the polymerization process.
Section II describes the syntheses of azobenzene-containing polymers via atom transfer radical polymerization (ATRP). In this connection, homopolymers of azobenzene (azo) methacrylates with different substituents and their diblock copolymers with poly[2-(dimethylamino)ethyl methacrylate] p(DMAEMA) have been synthesized. Controlled/“living” ATRP of azo methacrylates were achieved with up to ~30% conversion. The copolymerization rate of 6-[4-phenylazo]phenoxy]hexylmethacrylate (PPHM) from p(DMAEMA) macroinitiator was found to be almost identical to that for the homopolymerization of PPHM monomer, with kapp ~ 0.0078 min-1. For copolymerizations, several novel well-defined diblock copolymers p(DMAEMA-b-Azo) were successfully synthesized. Almost complete incorporation of the azo methacrylate monomers could be obtained with a low molecular weight macroinitiator (PDMAEMA)-Cl, whereas a macroinitiator of long chain length did not give full conversion owing to possible chain entanglement and partial loss of the terminal halogen. Diblock azo copolymers with methyl methacrylate (MMA) were also synthesized. Unfortunately, several attempts to fabricate the polymers for holographic study were not successful because high quality transparent polymer films could not be obtained.
The azo homopolymers and azo diblock copolymers exhibited some interesting thermal and structural properties. Only azobenzene substituted with cyano and butoxy groups exhibited liquid crystalline textures. The glass transition behavior of the new azo diblock copolymers with DMAEMA was investigated. It was found that the Fox equation could not be used to satisfactorily predict the Tg for the block copolymers, even for those with very short second block. It appears that the short azo segment could act as a plasticizer causing the Tg to be lower.
Because azo monomers are highly hydrophobic, only the diblock copolymers with short azo segments are soluble in water. In aqueous solution, these copolymers self-assemble into micellar particles. The trans- cis photoisomerization and the thermally-induced reverse cis-trans isomerization in solid films and in solutions were studied in detail. Both the isomerization processes could be fitted by the first order exponential decay function. It was found that the rate of photoisomerization in aqueous solution was comparable to that in solid film. This result could be explained by the formation of core-shell micelles in aqueous solutions with the hydrophobic azo block forming the core and the hydrophilic DMAEMA segments forming the shell. The azo block in the core has restricted reorientation movements, similar to those in the solid; hence comparable photoisomerization rates were observed.
The effect of photo- induced trans-cis isomerization on the lower critical solution temperature (LCST) and surface tension were studied. The LCST of the diblock copolymers increased upon irradiation by UV light owing to the cis conformers being more hydrophilic. However, the trans-cis isomerization had only a small effect on the CMC and γcmc of the azo methacrylate block copolymers, owing to the formation of a compact core of the micelles. The formation of core-shell micelles was established from laser light scattering (LLS) and transmission electron microscopy (TEM). All of the three azo methacrylate amphiphilic block copolymers formed hard core-shell micelles with relatively small Rh values of 31 nm for p(DMAEMA172-b-BPHM7), 26 nm for p(DMAEMA172-b-CPHM7) and 32 nm for p(DMAEMA172-b- PPHM9). However for the azo acrylate copolymer, p(DMAEMA172-b- BPHA6), large micelles with Rh ~78nm with a loose core were formed.
Date Issued
2005
Call Number
QD383.A95 Sin
Date Submitted
2005