Stimuli sensitive copolymer poly( N- tert-butylacrylamide- ran-acrylamide): processing into thin films and their transitional behaviour

Abstract

Temperature sensitive polymer hydrogels are being extensively studied because of their potential applications in biomedical, robotics, and chemical industry. However, major hurdles in their development have been their slow response, low efficiency, and poor mechanical properties. One of the main reasons for these shortcomings is the difficulty of processing them into mechanically fine structures in polymer gel form. In this work, a novel approach has been developed to process temperature sensitive copolymers based on acrylamide into mechanically stable thin films. A series of temperature sensitive random linear copolymers of N- tert-butylacrylamide (NTBA) and acrylamide (Am) were synthesized by solution polymerization method, using regulated dosing of comonomer Am having a higher reactivity ratio ( r Am=1.5) than NTBA ( r NTBA=0.5). Copolymers with varying feed ratios of NTBA and Am (80:20 to 20:80 mol%) were synthesized and characterized. The actual incorporation of less reactive comonomer NTBA was found to be lower than the feed and was found to vary between 75 mol% with feed of 80 and 11 mol% for a feed of 20%. Linear copolymer with 40:60 feed ratio of NTBA and Am monomers, with actual incorporation of NTBA to the extent of 27 mol%, was selected for processing. The copolymer films of thickness in the range of 10–200 microns could be obtained from aqueous solution in the presence of citric acid or 1,2,3,4-butane tetracarboxylic acid as crosslinkers and sodium hypophosphite as catalyst. Subsequently, the films were crosslinked at 150–160 °C to obtain mechanically strong insoluble films. The crosslinks were formed between reactive amide side groups of the acrylamide moiety of the polymer and the carboxylic acid group of the crosslinker. The transition temperatures of the crosslinked films were found to shift towards the lower temperature from 37 °C (in linear copolymer) to 22–25 °C. High surface to volume ratio of the prepared films lead to significant increase in swelling percentage from 490 to 2980% and faster response time from 1280 min (in the first cycle) to 5 min compared to polymerized-gel samples (2 mm disc) of the same composition.