pH dependent thermoresponsive behavior of acrylamide–acrylonitrile UCST-type copolymers in aqueous media

Abstract

There are only a few non-ionic polymers known for showing thermoresponsivity of UCST-type. Copolymers of acrylamide (AAm) and acrylonitrile (AN) represent one of such thermoresponsive polymers.The present work shows pH-dependent UCST-transitions of this copolymer system. Herein, systematic studies were carried out to show hydrolytic stability and retention of UCST of the copolymer under various conditions. Regeneration of lost UCST-type transitions under extreme pH conditions could be achieved by changing pH, and by addition of electrolytes. Reversible addition fragmentation chain transfer (RAFT) was employed as a tool to synthesize copolymers of AAm and AN. Hydrolysis reactions were carried out intentionally under acidic and alkaline conditions, in order to analyze the chemical stability of the synthesized copolymers as well as to introduce carboxylic groups into the polymer structure. The obtained results showed high tolerance of poly(AAm-co-AN) samples under acidic conditions even after long periods of storage (25 days at pH 3) or after use of pH 0 and increased temperatures (40°C). In the case of base catalyzed hydrolysis, the thermoresponsive behavior was significantly influenced during hydrolysis in buffer solution of pH 9. Loss and regeneration of the phase transition temperature of these copolymers could be achieved by changing the pH from basic to acidic. Meanwhile, hydrolysis at pH 14 at 40°C influenced the thermoresponsive behavior and the chemical stability of the polymer, increasing the phase transition temperature over 30°C. Further, we observed that additives, e.g. formamide can act as a sacrificial agent for providing stable UCST-type transitions even under alkaline conditions as well as at high temperatures (85°C).

Tunable, concentration‐independent, sharp, hysteresis‐free UCST phase transition from poly(N‐acryloyl glycinamide‐acrylonitrile) system

“…In the last few years more and more attention has been given to the nonionic polymers with phase transitions dominated by hydrogen (H)‐bonding . Acrylamide and acrylonitrile random copolymers (poly(AAm‐co‐AN)) have emerged out as one of the promising systems with sharp and reversible phase transitions of UCST type and tunable cloud points (temperature at which phase separation occurs depending on polymer concentration) Acrylamide, small hysteresis, and tolerance toward electrolytes . Moreover, cross‐linked poly(AAm‐co‐AN) could provide thermophilic hydrogels .…”

“…19,20 Acrylamide and acrylonitrile random copolymers (poly(AAm-co-AN)) have emerged out as one of the promising systems with sharp and reversible phase transitions of UCST type and tunable cloud points (temperature at which phase separation occurs depending on polymer concentration) Acrylamide, small hysteresis, and tolerance toward electrolytes. 4,[19][20][21][22][23] Moreover, cross-linked poly(AAm-co-AN) could provide thermophilic hydrogels. 24 All these properties provide the poly(AAm-co-AN) system at present a place in the category of UCST-type polymers which is occupied by poly(N-isopropylacrylamide) (poly(NIPAM)) for polymers with LCST-type transitions.…”

A Synthetic, Transiently Thermoresponsive Homopolymer with UCST Behaviour within a Physiologically Relevant Window

“…[21][22][23][24][25] However,b iomedical applications require polymeric materials to be degraded into water-soluble products that can be cleared from the body.W hereas gelatine is ag ood example of ad egradable natural macromolecular UCST system, [24,26] degradable synthetic UCST systems that operate within ap hysiological window are extremely rare and only af ew examples have been reported. [27] Very recent work by Wurm and co-workers introduced an interesting class of phosphonate-based copolymers that show UCST behaviour at acidic pH in combination with degradability at alkaline pH. [28] However,because of the presence of ionisable groups, these polymers cannot exhibit UCST behaviour at physiological pH.…”

Synthetisch hergestellte, transient thermoresponsive Homopolymere mit einer oberen kritischen Lösungstemperatur für physiologisch relevante Anwendungen

“…Für biomedizinische Anwendungen sollten polymerbasierte Materialien jedoch in wasserlösliche Einheiten abgebaut werden, damit sie vom Körper ausscheidbar sind. Während Gelatine ein hervorragendes Beispiel für ein natürlich abbaubares Makromolekül mit UCST‐Verhalten ist, gibt es nur sehr wenige synthetische, abbaubare UCST‐Polymere, die auch unter physiologischen Bedingungen einsetzbar sind . Vor kurzem haben Wurm et al.…”

“…Während Gelatine ein hervorragendes Beispiel füre in natürlich abbaubares Makromolekülm it UCST-Verhalten ist, [24,26] gibt es nur sehr wenige synthetische,a bbaubare UCST-Polymere,d ie auch unter physiologischen Bedingungen einsetzbar sind. [27] Vork urzem haben Wurm et al eine interessante Klasse von phosphonatbasierten Copolymeren vorgestellt, die bei saurem pH ein UCST-Verhalten aufweisen und sich unter basischen Bedingungen zersetzen. [28] Doch wegen der Anwesenheit von ionisierbaren Gruppen kçnnen diese UCST-Polymere niemals unter physiologischen Bedingungen ein UCST-Verhalten ausbilden.…”