What will happen when thermoresponsive poly(N-isopropylacrylamide) is tethered on poly(ionic liquid)s?

Luo, GM (Luo, Guangmei)^[ 1 ] ; Guo, YK (Guo, Yakun)^[ 1 ] ; Liu, CG (Liu, Chonggao)^[ 1 ] ; Han, G (Han, Guang)^[ 2 ] ; Ma, XD (Ma, Xiaodong)^[ 3 ] ; Zhang, WQ(Zhang, Wangqing)^[ 1,4 ]

RSC ADVANCES, 2019, 9(23): 12936-12943

DOI: 10.1039/c9ra01849b

Abstract

The thermoresponsive ionic liquid diblock copolymer of poly[1-(4-vinylbenzyl)-3-methylimidazolium tetrafluoroborate]-block-poly(N-isopropylacrylamide) (P[VBMI][BF4]-b-PNIPAM) containing a hydrophilic poly(ionic liquid) block of P[VBMI][BF4] is prepared by sequential reversible addition-fragmentation chain transfer (RAFT) polymerization. This P[VBMI][BF4]-b-PNIPAM exhibits an abnormal thermoresponsive phase transition at a temperature above the phase transition temperature (PTT) of the PNIPAM block. For P[VBMI][BF4]-b-PNIPAM including a short P[VBMI][BF4] block, its aqueous solution becomes turbid at a temperature above the PTT of the thermoresponsive PNIPAM block, whereas for P[VBMI][BF4]-b-PNIPAM containing a relatively long P[VBMI][BF4] block even in the case of a relatively long PNIPAM block, the aqueous solution remains transparent at a temperature far above the PTT of the PNIPAM block, although a soluble-to-insoluble phase transition of the PINIPAM block is confirmed by dynamic light scattering (DLS) analysis and variable temperature H-1 NMR analysis. The reason that P[VBMI][BF4]-b-PNIPAM exhibits an abnormal thermoresponse is discussed and ascribed to the highly hydrophilic and charged poly(ionic liquid) block of P[VBMI][BF4] leading to the formation of small-sized micelles at a temperature above the PTT.