Aggregation in Aqueous Poly(N-isopropylacrylamide)-block-poly(ethylene oxide) Solutions Studied by Fluorescence Spectroscopy and Light Scattering

Abstract

Block copolymers poly(N-isopropylacrylamide)-block-poly(ethylene oxide), PNIPA-b-PEO, were synthesized by free radical polymerization using macroazoinitiators bearing PEO chains with two different chain lengths (M w = 550 or 1900 g/mol). The molar mass of the PNIPA block varied from 0.4 × 105 to 7.3 × 105 g/mol. The cloud points of the aqueous copolymer solutions shifted to slightly higher temperatures only with the samples having the longer PEO block. Above the lower critical solution temperature (LCST) of PNIPA the block copolymers formed aggregates with a spherical core−shell structure sterically stabilized by a PEO shell. The formation and especially the shape of the aggregates were influenced by the length of the PNIPA block, the molar ratio of the repeating units of PNIPA and PEO, and the polymer concentration. A fluorescent probe 4-(dicyanomethylene)-2-methyl-6-(p-(dimethylamino)styryl)-4H-pyran, 4HP, was localized inside the polymer differently depending on the method of sample preparation. The microviscosity in the surroundings of the probe increased with the length of the PNIPA block. However, depending on the sample preparation method, the length of the PEO block was also observed to have an effect on the polarity of the environment around the probe. In general, increasing the block length of PEO affected critically the size and shape of the aggregates as well as the mass distribution within them above the LCST, owing to the enhanced solubilizing effect of PEO on the collapsing PNIPA and the improved steric stabilization of the aggregates induced by a PEO shell.

Thermo-responsive polymers and hydrogels in tissue engineering

“…Several other groups have also studied PNiPAAm/PEG copolymers with different polymer architecture. Maeda et al [38], Virtanen et al [39] and Topp et al [40] synthesised block-copolymers, whereas Virtanen et al also described the preparation of graft copolymers by the 'graftingonto' method [41,42]. The block-copolymers have a volume phase transition around 32°C, but the graft copolymers exhibit a shift in the transition temperature T tr towards higher values similar to our observations.…”

A polymeric micelle system with a hydrolysable segment for drug delivery

“…In the first step, 4,4′-azobis(4-cyanopentanoic acid) (ABCPA) was treated with SOCl 2 at 100 °C for 20 minutes and converted to the corresponding acid chloride: 4,4′azobis(4-cyanopentanoyl chloride) (ABCPC). In the second step, the PEO macroinitiator was prepared by a condensation reaction of ABCPC and PEO in dry dichloromethane in the presence of an excess amount of triethylamine for 24 hr as described by Virtanen et al [10].…”

Thermosensitive Block Copolymers Consisting of Poly(N-isopropylacrylamide) and Star Shape Oligo(ethylene oxide)

“…The thermally induced micellization of PEGb-PNIPAM has been studied by several groups. [7][8][9][10][11][12][13][14] Below the LCST of PNIPAM, both blocks are hydrophilic and soluble in water. Above the LCST of PNIPAM, the copolymer becomes amphiphilic to form a micelle.…”