Abstract
The possibility to respond to external stimuli with change in physicochemical properties of a polymeric system is appealing to a broad range of scientists as it may result in the preparation of exciting new ‘smart’ materials. Whilst numerous reports exist on polymers which respond to single stimulus such as temperature, light, pH, redox, etc., the examples of multiple stimuli responsive polymers are limited.
This study deals with the preparation and characterisation of polymers comprising of calix[4]arene or dibenzo-18-crown-6-ether units, which have been joined by azo-bridges. The objectives and synthetic pathways towards monomer and polymer formation are evaluated and the best methods are highlighted as being versatile and promising for the synthesis of a wide range of main chain azopolymers.
As a result of their intricate design, the polymers are capable of response to irradiation by means of reversible trans-to-cis photo-isomerization of the azo-linkages. The extent of the photo-switchability in various solvents is evaluated by UV-Vis spectroscopy and the structural changes upon isomerization are resolved with 1H NMR analysis.
The choice of the calix[4]arene and crown ether units as major chemical components enables interaction of the polymers with low molar mass guests (evidenced by titration experiments monitored by means of 1H NMR, UV-Vis spectroscopy). This supramolecular activity is shown to be influenced by the photo-stationary state of the polymers in the case of poly(azocalix[4]arene)s and thus a serial photo-tuning of the extent of the host-guest interaction is possible. For the poly(azodibenzo-18-crown-6-ether)s addition of guests leads to phase separation of the polymers from certain solutions.
By incorporating tetraethyleneglycol monomethyl ether chains into the lower rim of the calixarene units, thermo-responsive behaviour could be induced onto the poly(azocalix[4]arene) framework. Lower critical solution (LCST) type transitions in water and upper critical solution (UCST) type transitions in alcohols were observed, which are strongly dependent on the degree of polymerization, concentration and choice of solvent. The latter was also demonstrated for the poly(azodibenzo-18-crown-6-ether)s. In both cases, the UCST type behaviour may be influenced by irradiation. The reversible trans-to-cis photo-isomerization decreases the demixing temperature of the solutions proportionally to the trans content of the samples. Thermal relaxation reverts the isomerization and the cloud point values from prior to irradiation are regained. For the tegylated poly(azocalix[4]arene)s the isomerization may also be induced in the phase separated state, leading to dissolution of the polymers, as the UCST value is shifted. This may be done locally in a thin 1 mm quartz cuvette, yielding areas of translucency in the polymer dispersion upon irradiation and a possible photo-writing application.
The thermo-responsive properties of the crown ether polymers may also be influenced by the addition of barium ions, which act as dynamic intermolecular crosslinkers increasing the cloud point values. These polymers are also shown to be solvatochromic and pH-sensitive.
This study deals with the preparation and characterisation of polymers comprising of calix[4]arene or dibenzo-18-crown-6-ether units, which have been joined by azo-bridges. The objectives and synthetic pathways towards monomer and polymer formation are evaluated and the best methods are highlighted as being versatile and promising for the synthesis of a wide range of main chain azopolymers.
As a result of their intricate design, the polymers are capable of response to irradiation by means of reversible trans-to-cis photo-isomerization of the azo-linkages. The extent of the photo-switchability in various solvents is evaluated by UV-Vis spectroscopy and the structural changes upon isomerization are resolved with 1H NMR analysis.
The choice of the calix[4]arene and crown ether units as major chemical components enables interaction of the polymers with low molar mass guests (evidenced by titration experiments monitored by means of 1H NMR, UV-Vis spectroscopy). This supramolecular activity is shown to be influenced by the photo-stationary state of the polymers in the case of poly(azocalix[4]arene)s and thus a serial photo-tuning of the extent of the host-guest interaction is possible. For the poly(azodibenzo-18-crown-6-ether)s addition of guests leads to phase separation of the polymers from certain solutions.
By incorporating tetraethyleneglycol monomethyl ether chains into the lower rim of the calixarene units, thermo-responsive behaviour could be induced onto the poly(azocalix[4]arene) framework. Lower critical solution (LCST) type transitions in water and upper critical solution (UCST) type transitions in alcohols were observed, which are strongly dependent on the degree of polymerization, concentration and choice of solvent. The latter was also demonstrated for the poly(azodibenzo-18-crown-6-ether)s. In both cases, the UCST type behaviour may be influenced by irradiation. The reversible trans-to-cis photo-isomerization decreases the demixing temperature of the solutions proportionally to the trans content of the samples. Thermal relaxation reverts the isomerization and the cloud point values from prior to irradiation are regained. For the tegylated poly(azocalix[4]arene)s the isomerization may also be induced in the phase separated state, leading to dissolution of the polymers, as the UCST value is shifted. This may be done locally in a thin 1 mm quartz cuvette, yielding areas of translucency in the polymer dispersion upon irradiation and a possible photo-writing application.
The thermo-responsive properties of the crown ether polymers may also be influenced by the addition of barium ions, which act as dynamic intermolecular crosslinkers increasing the cloud point values. These polymers are also shown to be solvatochromic and pH-sensitive.
| Original language | English |
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| Qualification | Doctor Degree |
| Awarding Institution |
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| Supervisors/Advisors |
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| Award date | 22 Aug 2014 |
| Publisher | |
| Print ISBNs | 978-951-51-0041-2 |
| Electronic ISBNs | 978-951-51-0042-9 |
| Publication status | Published - 2014 |
| MoE publication type | G5 Doctoral dissertation (article) |
Keywords
- polymer Chemistry