Abstract
The discovery that the lower critical solution temperature (LCST) of poly(N-Isopropylacrylamide) (PNIPAM) in water is affected by the tacticity opens the perspective to tune the volume phase transition temperature of PNIPAM microgels by changing the content of meso dyads in the polymer network. The increased hydrophobicity of isotactic-rich PNIPAM originates from self-assembly processes in aqueous solutions also below the LCST. The present work aims to detect the characteristics of the pair interaction between polymer chains, occurring in a concentration regime close to the chain overlap concentration, by comparing atactic and isotactic-rich PNIPAM solutions. Using atomistic molecular dynamics simulations, we successfully modelled the increased association ability of the meso-dyad-rich polymer in water below the LCST, and gain information on the features of the interchain junctions as a function of tacticity. Simulations carried out above the LCST display the PNIPAM transition to the insoluble state and do not detect a relevant influence of stereochemistry on the structure of the polymer ensemble. The results obtained at 323 K provide an estimate of the swelling ratio of non-stereocontrolled PNIPAM microgels which is in agreement with experimental findings for microgels prepared with low cross-linker/monomer feed ratios. This study represents the first step toward the atomistic modelling of PNIPAM microgels with a controlled tacticity.
Highlights
Polymer microgels based on poly(N-isopropylacrylamide) (PNIPAM) are the most famous paradigm of stimuli-responsive soft microdevices [1]
The thermal sensitivity of PNIPAM microgels comes from the lower critical solution temperature (LCST) behaviour of PNIPAM aqueous solutions, that, in hydrated polymer networks, causes a volume phase transition (VPT) between a swollen state and a shrunk state at a VPT temperature similar to the LCST of PNIPAM in water
The temperature trigger was the first element exploited for the responsivity of PNIPAM microgels and it opens the perspective toward applications in biomedicine, since the VPT temperature, about 32 ◦ C, is close to the physiological temperature
Summary
Polymer microgels based on poly(N-isopropylacrylamide) (PNIPAM) are the most famous paradigm of stimuli-responsive soft microdevices [1]. Several variants of PNIPAM based microgel were proposed by introducing different polymers in the network to add other responsivity factors, such as pH and ionic strength [2,3]. Other options to change the VPTT are to modify the composition of the solution that swells the microgel, for example by using mixed solvents [4] or high ionic strength [5]. All these methods alter the chemical features of the original polymer matrix and the microgel biocompatibility with possible consequences on application
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