Stimuli-responsive polymers based on N-isopropylacrylamide and N-methacryloyl-L-leucine

Linear and cross-linked polymers capable of responding to both temperature and pH changes were synthesized by radical copolymerization of N-isopropylacrylamide (NIPAAm) and N-methacryloyl-l-leucine (MALEU) in different NIPAAm/MALEU molar ratios. The solution behavior of linear polymers was investigated in 0.1 m NaCl and in citrate buffers pH 4.0, 4.5, 5.0 and 6.0. A sharp increase in the cloud point temperature (TCP) was observed to occur around pH 4–5. Hydrogels were synthesized employing N,N ′-methylenebisacrylamide as cross-linking agent. The influence of composition and degree of cross-linking on water absorption was investigated in 0.1 m NaCl and in citrate buffers pH 4.0 and 5.0. A reversible phase transition within a wide range of temperatures, depending on the composition and the pH, in accordance with the behavior of linear polymers, was observed.