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Structure-property-application relationship of new stimuli-responsive polymers based on N-acryloyl-N'-propylpiperazine
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Type
Thesis
Author
Mah, Chin Hao
Supervisor
Roshan Deen Gulam Rasool
Abstract
This thesis describes the structure-property-application relationship of a piperazinebased multiple stimuli-responsive polymer, poly(N-acryloyl-N’- propylpiperazine) (PAcrNPP). In order to achieve this goal, the polymer was synthesized in various architecture from the corresponding functional monomer N-acryloyl-N’-propylpiperazine (AcrNPP) through different polymerization methods. This polymer is responsive to more than one change in external stimuli such as pH, temperature, types of salt and concentration. This important property of multiple stimuli response is achieved in a homopolymer molecule without the need of co-monomers in the macromolecular architecture. The polymer in the following architectures were developed:
(i) Linear homopolymers and chemically crosslinked polymers in the form of hydrogels were synthesized by thermally initiated free-radical polymerization method. Partially quaternized linear homopolymers was prepared by reacting the homopolymers either with allyl bromide or octyl bromide.
(ii) Chemically crosslinked microgels were synthesized by surfactant-free emulsion polymerization method.
(iii) Crosslinked thin polymer films on glass substrates were developed by radiofrequency plasma polymerization method.
The linear and quaternized polymers were soluble in water. The linear polymer and crosslinked microgels exhibited lower critical solution temperature (LCST) phenomenon with phase transition at 37 ± 1 °C. The phase transition temperature was highly dependent on solution pH and ionic strength. The linear polymers, hydrogels, and microgels were highly responsive to pH changes due to the presence of free tertiary amine group in the piperazine ring. At low pH, the free amine groups are protonated leading to increase in solubility and increased electrostatic interactions between the charge groups thereby causing the phase separation to occur at higher temperature. The enthalpy of phase separation of the polymer in water and in acidic and basic solution was determined by differential scanning calorimetry (DSC). The effects of simple salts such as sodium chloride, sodium bromide and sodium iodide on the LCST of the linear homopolymer were studied. Sodium chloride and sodium bromide caused a salting-out effect while sodium iodide caused a salting-in effect, in accordance with Hoffmeister series of anions.
The linear quaternized polymers were effective against bacterial strains (Pseudomonas aeruginosa). The quaternized polymer with allyl bromide showed a minimal inhibitory concentration (MIC) of 0.4 µg ml−1 which was more effective than the reference antibiotic polymyxin (MIC = 1 µg ml−1). The crosslinked thin polymer films developed by plasma polymerization was also effective against the growth bacterial strain (Pseudomonas aeruginosa).The linear quaternized polymer and the thin polymer films show promise in the development of stimuli-responsive anti-bacterial surfaces.
The hydrogels were prepared using two different chemical crosslinkers such as N,Nmethylenebisacrylate (mba) and 1,4- butanediol diacrylate (bda). The gels were responsive to both pH and temperature. The effect of external stimuli and type of crosslinker on the equilibrium swelling behavior and dynamic swelling was investigated in detail in buffer solution of various pH and temperatures. The equilibrium swelling capacity of the gels was large in swelling medium at pH 3.0 than at pH 10.0 due to ionization of polymer network under acidic conditions. With increase in temperature from 25 °C to 45 °C, the gels exhibited negative temperature-responsive (thermoshrinking/thermophobic) behavior with negative activation energy for the diffusion of water. The thermodynamic parameters such as Gibbs’ free energy (ΔG), enthalpy (ΔH), and entropy (ΔS) for the swelling of gels as function of temperature were negative, indicating an exothermic swelling process. Water (media) transport mechanism and diffusion process in thin rectangular gels were studied. At pH 3.0, the diffusion process was non-Fickian (anomalous) while at pH 10.0 it was quasi-Fickian. The transport mechanism was partly influenced by the type of crosslinker in the gel.
The dynamic swelling data was analyzed using early-time, late-time and Etters diffusion models. From the equilibrium swelling studies, the average molecular weight between crosslinks (Mc), the crosslink density (ρc), and the mesh size (ξ) were determined. The Mc was large at pH 3 due to ionization of polymer and chain expansion. The experimental Mc was much larger than the theoretical Mc which implied that the gels were loosely crosslinked real networks. The mesh size of gels were between 447 and 786 Å in the swollen (ionized) state (pH 3.0), and between 100 and 231 Å in the collapsed (non-ionized) state (pH 10.0). The mesh size increased between three to four times during the pH-dependent swelling transition. The state of water in fully swollen hydrogels which influences many important biomaterial properties was determined by differential scanning calorimetry. The bound water content of gels increased linearly with increase in pH of the swelling medium while the unbound water decreased.
The solubility parameter of the hydrogels was evaluated by equilibrium swelling experiment in a series of twelve solvents with well-distributed range of solubility parameters. The hydrogels exhibited maximum equilibrium swelling ratio in 1-butanol. The experimental swelling data was analyzed by two methods, (i) non-linear curve fitting and (ii) least-square regression to determine the solubility parameter of the hydrogels. The solubility parameter determined by the two methods was in the range 11.45 – 11.85 (cal cm-3)1/2. The solubility parameter of the hydrogel containing the hydrophilic crosslinker (Mba) was higher than the hydrogel with a hydrophobic crosslinker (Bda), in all cases. This showed that the nature of chemical crosslinker influences the swelling and thus the solubility parameter of the hydrogels. The partial solubility parameter of the hydrogels related to the cohesive energy was evaluated by plotting the equilibrium swelling data against dispersion forces, polar forces and hydrogen bonding. It was observed that both polar forces and hydrogen bonding play an important role in the swelling of these cationic hydrogels. The solubility parameter and the related forces were also estimated theoretically using group contribution methods and compared with the experimental results.
The stimuli-responsive property of the microgels was studied using light scattering method in aqueous solution. At the phase transition temperature of 37.1 °C the microgels collapsed to a hydrodynamic radius of 34 nm. The microgel was effective in the removal of a toxic dye Congo red from aqueous solution. The removal efficiency was 62.6 %.
Given the multiple stimuli-responsive properties, different types of applications were investigated based on the piperazine-based hydrogels. These include (i) the removal of anionic dyes and heavy metals from solution, (ii) in the preparation of silver nanoparticles through self- reduction, and (iii) encapsulation and release of a model drug salicylic acid. The gels effectively adsorbed 97.4 % of an anionic dye, Alizarin Yellow, and 92.7 % of Cu2+ ions from aqueous solution. The efficiency of desorption was more than 85 % for both species, making the gels sustainable in terms of materials requirement. Silver and copper nanoparticles were successfully fabricated without the need for external chemical reducing agent in the hydrogel matrix through the self-reducing property of the polymer due to the presence of tertiary amine functional group. The size and morphology of the nanoparticles were studied using Scanning Electron Microscopy (SEM) and X-ray Photoelectron Spectroscopy (XPS). The gels were effective in encapsulating and releasing a model drug salicylic acid (burst release of 40 % at 1st hr and sustained-release for next 30 % ) making the polymer suitable for drug delivery applications.
This study has thus established the structure- property-application relationship of a piperazine-based multiple stimuli- responsive polymer, poly(N-acryloyl-N’-propylpiperazine) (PAcrNPP) in its various structural architecture.
(i) Linear homopolymers and chemically crosslinked polymers in the form of hydrogels were synthesized by thermally initiated free-radical polymerization method. Partially quaternized linear homopolymers was prepared by reacting the homopolymers either with allyl bromide or octyl bromide.
(ii) Chemically crosslinked microgels were synthesized by surfactant-free emulsion polymerization method.
(iii) Crosslinked thin polymer films on glass substrates were developed by radiofrequency plasma polymerization method.
The linear and quaternized polymers were soluble in water. The linear polymer and crosslinked microgels exhibited lower critical solution temperature (LCST) phenomenon with phase transition at 37 ± 1 °C. The phase transition temperature was highly dependent on solution pH and ionic strength. The linear polymers, hydrogels, and microgels were highly responsive to pH changes due to the presence of free tertiary amine group in the piperazine ring. At low pH, the free amine groups are protonated leading to increase in solubility and increased electrostatic interactions between the charge groups thereby causing the phase separation to occur at higher temperature. The enthalpy of phase separation of the polymer in water and in acidic and basic solution was determined by differential scanning calorimetry (DSC). The effects of simple salts such as sodium chloride, sodium bromide and sodium iodide on the LCST of the linear homopolymer were studied. Sodium chloride and sodium bromide caused a salting-out effect while sodium iodide caused a salting-in effect, in accordance with Hoffmeister series of anions.
The linear quaternized polymers were effective against bacterial strains (Pseudomonas aeruginosa). The quaternized polymer with allyl bromide showed a minimal inhibitory concentration (MIC) of 0.4 µg ml−1 which was more effective than the reference antibiotic polymyxin (MIC = 1 µg ml−1). The crosslinked thin polymer films developed by plasma polymerization was also effective against the growth bacterial strain (Pseudomonas aeruginosa).The linear quaternized polymer and the thin polymer films show promise in the development of stimuli-responsive anti-bacterial surfaces.
The hydrogels were prepared using two different chemical crosslinkers such as N,Nmethylenebisacrylate (mba) and 1,4- butanediol diacrylate (bda). The gels were responsive to both pH and temperature. The effect of external stimuli and type of crosslinker on the equilibrium swelling behavior and dynamic swelling was investigated in detail in buffer solution of various pH and temperatures. The equilibrium swelling capacity of the gels was large in swelling medium at pH 3.0 than at pH 10.0 due to ionization of polymer network under acidic conditions. With increase in temperature from 25 °C to 45 °C, the gels exhibited negative temperature-responsive (thermoshrinking/thermophobic) behavior with negative activation energy for the diffusion of water. The thermodynamic parameters such as Gibbs’ free energy (ΔG), enthalpy (ΔH), and entropy (ΔS) for the swelling of gels as function of temperature were negative, indicating an exothermic swelling process. Water (media) transport mechanism and diffusion process in thin rectangular gels were studied. At pH 3.0, the diffusion process was non-Fickian (anomalous) while at pH 10.0 it was quasi-Fickian. The transport mechanism was partly influenced by the type of crosslinker in the gel.
The dynamic swelling data was analyzed using early-time, late-time and Etters diffusion models. From the equilibrium swelling studies, the average molecular weight between crosslinks (Mc), the crosslink density (ρc), and the mesh size (ξ) were determined. The Mc was large at pH 3 due to ionization of polymer and chain expansion. The experimental Mc was much larger than the theoretical Mc which implied that the gels were loosely crosslinked real networks. The mesh size of gels were between 447 and 786 Å in the swollen (ionized) state (pH 3.0), and between 100 and 231 Å in the collapsed (non-ionized) state (pH 10.0). The mesh size increased between three to four times during the pH-dependent swelling transition. The state of water in fully swollen hydrogels which influences many important biomaterial properties was determined by differential scanning calorimetry. The bound water content of gels increased linearly with increase in pH of the swelling medium while the unbound water decreased.
The solubility parameter of the hydrogels was evaluated by equilibrium swelling experiment in a series of twelve solvents with well-distributed range of solubility parameters. The hydrogels exhibited maximum equilibrium swelling ratio in 1-butanol. The experimental swelling data was analyzed by two methods, (i) non-linear curve fitting and (ii) least-square regression to determine the solubility parameter of the hydrogels. The solubility parameter determined by the two methods was in the range 11.45 – 11.85 (cal cm-3)1/2. The solubility parameter of the hydrogel containing the hydrophilic crosslinker (Mba) was higher than the hydrogel with a hydrophobic crosslinker (Bda), in all cases. This showed that the nature of chemical crosslinker influences the swelling and thus the solubility parameter of the hydrogels. The partial solubility parameter of the hydrogels related to the cohesive energy was evaluated by plotting the equilibrium swelling data against dispersion forces, polar forces and hydrogen bonding. It was observed that both polar forces and hydrogen bonding play an important role in the swelling of these cationic hydrogels. The solubility parameter and the related forces were also estimated theoretically using group contribution methods and compared with the experimental results.
The stimuli-responsive property of the microgels was studied using light scattering method in aqueous solution. At the phase transition temperature of 37.1 °C the microgels collapsed to a hydrodynamic radius of 34 nm. The microgel was effective in the removal of a toxic dye Congo red from aqueous solution. The removal efficiency was 62.6 %.
Given the multiple stimuli-responsive properties, different types of applications were investigated based on the piperazine-based hydrogels. These include (i) the removal of anionic dyes and heavy metals from solution, (ii) in the preparation of silver nanoparticles through self- reduction, and (iii) encapsulation and release of a model drug salicylic acid. The gels effectively adsorbed 97.4 % of an anionic dye, Alizarin Yellow, and 92.7 % of Cu2+ ions from aqueous solution. The efficiency of desorption was more than 85 % for both species, making the gels sustainable in terms of materials requirement. Silver and copper nanoparticles were successfully fabricated without the need for external chemical reducing agent in the hydrogel matrix through the self-reducing property of the polymer due to the presence of tertiary amine functional group. The size and morphology of the nanoparticles were studied using Scanning Electron Microscopy (SEM) and X-ray Photoelectron Spectroscopy (XPS). The gels were effective in encapsulating and releasing a model drug salicylic acid (burst release of 40 % at 1st hr and sustained-release for next 30 % ) making the polymer suitable for drug delivery applications.
This study has thus established the structure- property-application relationship of a piperazine-based multiple stimuli- responsive polymer, poly(N-acryloyl-N’-propylpiperazine) (PAcrNPP) in its various structural architecture.
Date Issued
2019
Call Number
QD139.P6 Mah