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Spectrophotometric studies of the interaction between 4-vinyl pyridine (4VP) block copolymers with methanofullerene (PCBM)
Author
Afshan Parveen Hamid
Supervisor
Gan, Leong Huat
Abstract
The aim of the research project is to study the interaction between a fullerene derivative and poly-4-vinyl pyridine di-block copolymers. In this thesis the first chapter introduces the P4VP di-block copolymers, their synthesis, chemistry and applications. The chemistry of [6,6]-phenyl C61-butyric acid methyl ester (PCBM) which is a fullerene derivative is also discussed in the introduction.
Chapter two describes the experimental and techniques acquired in the project. The UVVisible spectrophotometric studies of the reaction between the di-block copolymers were carried out. Chapter three contains the discussion on the results found.
P4VP di-block copolymers are good electron donors due to the presence of pyridine unit and PCBM is a good electron acceptor. It is well established that such C60 molecules in PCBM can accept up to six electrons and can form complex with more than one pyridine unit. The aim of the current work is to study the kinetics of interaction between PCBM and P4VP di-block copolymers by spectrophotometric analysis. The effect of chain length of the P4VP unit in the di-block copolymer on the kinetics was also investigated.
Two di-block copolymers of different P4VP chain length; MPEG45-b-P4VP25 and MPEG45-b-P4VP6 were used for the reactions with PCBM. The solutions of the polymers and PCBM were prepared in THF. The reactions were carried out at different temperatures.
All the kinetic runs were carried out under pseudo-first order conditions, with the pyridine units in large excess. For the reaction between MPEG45-b-P4VP25 and PCBM at 50 °C, the reaction was complete in 4 minutes. The pseudo first-order rate constant (kapp) calculated at two different wavelengths at 310 nm and 329 nm were found to be 0.350 s-1 and 0.336 s-1 respectively. On the other hand, the time taken for completion of interaction between MPEG45-b-P4VP6 and PCBM at the same temperature was 23 minutes. The pseudo-first order rate obtained at 310 nm and 329 nm were 0.164 s-1 and 0.184 s-1 respectively. The second order rate constant (k2) was calculated by dividing the value of kapp by the concentration of the pyridine units in the block copolymers.
The values of activation energy (Ea) of the reactions were obtained from the Arrhenius plot of lnk2 versus 1/T. The values of enthalpy of activation (∆H≠) and entropy of activation (∆S≠) were also evaluated.
Similar kinetic studies were also carried out for the reaction between 4-t-butyl-pryidine, a simple pyridine, with PCBM. The rates of the reaction were very much slower (by a factor of > 20) than those with the two di-block copolymers studied. This can be explained by the effect of micellar-catalysis. In aqueous solution, the di-block copolymer self-assembles into micelles of core-shell structure. The hydrophobic 4VP segments form the core and the hydrophilic PEG groups form the shell of the micelles. These self-assembled aggregates function as a micro-reactor. For the reaction to occur, the hydrophobic PCBM will solubilize into the 4VP micellar core, where it reacts with the pyridine units. As both the reactants are now confined to a much smaller volume, the effective concentrations of the reactants are greatly increased. This is the reason for the vast enhancement of rate, which is consistent with a micellar- catalysis model.
Chapter two describes the experimental and techniques acquired in the project. The UVVisible spectrophotometric studies of the reaction between the di-block copolymers were carried out. Chapter three contains the discussion on the results found.
P4VP di-block copolymers are good electron donors due to the presence of pyridine unit and PCBM is a good electron acceptor. It is well established that such C60 molecules in PCBM can accept up to six electrons and can form complex with more than one pyridine unit. The aim of the current work is to study the kinetics of interaction between PCBM and P4VP di-block copolymers by spectrophotometric analysis. The effect of chain length of the P4VP unit in the di-block copolymer on the kinetics was also investigated.
Two di-block copolymers of different P4VP chain length; MPEG45-b-P4VP25 and MPEG45-b-P4VP6 were used for the reactions with PCBM. The solutions of the polymers and PCBM were prepared in THF. The reactions were carried out at different temperatures.
All the kinetic runs were carried out under pseudo-first order conditions, with the pyridine units in large excess. For the reaction between MPEG45-b-P4VP25 and PCBM at 50 °C, the reaction was complete in 4 minutes. The pseudo first-order rate constant (kapp) calculated at two different wavelengths at 310 nm and 329 nm were found to be 0.350 s-1 and 0.336 s-1 respectively. On the other hand, the time taken for completion of interaction between MPEG45-b-P4VP6 and PCBM at the same temperature was 23 minutes. The pseudo-first order rate obtained at 310 nm and 329 nm were 0.164 s-1 and 0.184 s-1 respectively. The second order rate constant (k2) was calculated by dividing the value of kapp by the concentration of the pyridine units in the block copolymers.
The values of activation energy (Ea) of the reactions were obtained from the Arrhenius plot of lnk2 versus 1/T. The values of enthalpy of activation (∆H≠) and entropy of activation (∆S≠) were also evaluated.
Similar kinetic studies were also carried out for the reaction between 4-t-butyl-pryidine, a simple pyridine, with PCBM. The rates of the reaction were very much slower (by a factor of > 20) than those with the two di-block copolymers studied. This can be explained by the effect of micellar-catalysis. In aqueous solution, the di-block copolymer self-assembles into micelles of core-shell structure. The hydrophobic 4VP segments form the core and the hydrophilic PEG groups form the shell of the micelles. These self-assembled aggregates function as a micro-reactor. For the reaction to occur, the hydrophobic PCBM will solubilize into the 4VP micellar core, where it reacts with the pyridine units. As both the reactants are now confined to a much smaller volume, the effective concentrations of the reactants are greatly increased. This is the reason for the vast enhancement of rate, which is consistent with a micellar- catalysis model.
Date Issued
2010
Call Number
QD382.B5 Afs
Date Submitted
2010