Smart Hydrogels based on Responsive Star-Block Copolymers
- The work presented in this thesis is focused on the synthesis of double-responsive star-shaped block copolymers and their formation of smart hydrogels in response to different external stimuli, specifically temperature and pH. Our concept was based on (AB)x diblock copolymer stars where both blocks are responsive to temperature and pH. This approach led to physically crosslinked hydrogels, which could form/disintegrate in response to a first trigger, i.e. the outer B blocks are alternating between hydrophilic and hydrophobic. The mechanical properties of the gels could still be manipulated by a second, independent trigger, i.e. upon applying the second trigger, the inner A blocks contract, leading to a change in mechanical properties.
The first part deals with the synthesis and characterization of linear and star-shaped poly((2 diethylamino)ethyl methacrylate) (PDEA) to investigate its double-responsive behavior and its potential for the design of double-responsive gelators. This polymer responds to variations in both pH and temperature, just like the analogous poly((2 dimethylamino)ethyl methacrylate) (PDMA). At a given temperature PDEA exhibits a critical pH value above which the chains collapse and aggregation takes place. The temperature-responsive behavior of PDEA does not depend on molecular weight or architecture, i.e. arm number. However, the cloud point does strongly depend on pH, as it affects the overall charge of the star.
For the second part we combined PDMA and PDEA to create double-responsive star-shaped block copolymers (DMA-DEA)x where both blocks are responsive to pH and temperature. The collapse of the PDEA outer blocks is first selectively triggered by heating. This has been proven by dynamic light scattering and is due to the significantly lower cloud point of PDEA with respect to that of PDMA at identical pH. The gelation behavior was investigated in dependence on block length and arm number. At high concentrations hydrogel formation was observed under conditions where only the PDEA outer blocks are insoluble. Rheology measurements showed that a minimum DEA fraction is necessary for gel formation and that the DEA fraction strongly influences the properties of the gels. Another factor controlling the gelation behavior of the diblock copolymer stars is the pH value, as the sol-gel transition temperature at a given concentration is shifted to lower values upon increasing the pH. The mechanical properties of some gel can be manipulated, as a decrease in the storage modulus was only observed for soft gels, if the temperature is increased above the transition temperature of the inner PDMA block, i.e. when the PDMA blocks contract. Thus, we successfully created double-responsive star-shaped gelators which formed reversible hydrogels that were still able to respond to a second trigger. However, the aggregation and hydrogel formation turned to out to be quite complex, due to the high number of parameters controlling them.
Finally, our concept was extended to other polymers and simplified by changing the outer block of the block copolymer stars to a polymer that is only responsive to temperature. This allows for an easier tuning of the sol-gel transition, as only one parameter is involved. The new diblock stars are comprised of PDMA inner blocks and outer blocks of poly(diethylene glycol methyl ether methacrylate) (PDEGMA), which can be triggered independently of each other as confirmed by turbidimetry and dynamic light scattering. They form hydrogels at relatively low concentrations upon heating above the transition temperature of PDEGMA independent of the pH value. The fraction of DEGMA is an important parameter for the gelation behavior of the (DMA-DEGMA)x stars, the same as the DEA fraction was for the (DMA-DEA)x stars. Unexpectedly, the mechanical properties of these gels can also not be changed by heating above the transition temperature of PDMA at pH values around 8. The gels formed in this pH region are strong and too rigid to be affected, similar to strong gels formed from (DMA-DEA)x stars. Only when the pH is increased close to 9 and the subsequently formed gels are softer, a decrease in the moduli is observed.
We also quaternized the inner PDMA blocks of the (DMA-DEGMA)x stars to transform them it into strong polycations. This leads to an increase in the effective volume fraction of the stars and consequently to a significant decrease of the critical gelation concentration. The quaternization opens our concept up to the introduction of light sensitivity through multivalent counterions and the incorporation of nanoparticles.