The collapse transition of poly(styrene-b-(N-isopropyl acrylamide)) diblock copolymers in aqueous solution and in thin films

Abstract

The thermal behavior of a poly(styrene-b-N-isopropyl acrylamide) diblock copolymer was studied in aqueous solution as well as in thick and in thin films. The polymer was synthesized using reversible addition-fragmentation chain transfer. The critical micelle concentration in aqueous solution was determined using fluorescence correlation spectroscopy. The lower critical solution temperature (LCST) of micellar solutions was detected using microcalorimetry and turbidimetry at 31 °C. Using dynamic light scattering, the collapse of the micelles at the LCST as well as their clustering above was observed. These findings were corroborated with small-angle X-ray scattering. In thick films immersed in water, similar findings were made. In a thin film, however, the LCST is depressed and is found at 26-27 °C

Temperature‐Responsive Aqueous Micelles From Terpyridine End‐Capped Poly(N‐Isopropylacrylamide)‐Block‐Polystyrene Diblock Copolymers

“…In our case, one could reasonably assume that the terpyridine groups located at the end of the PNIPAM chains are the reason for the lowering in cloud point for PNIPAM. The insoluble PS micellar cores are not thought to have a major influence on the cloud point of the PNIPAM coronal chains, as already reported by other authors 8, 11, 14, 36…”

“…A widely used LCST polymer is poly( N ‐isopropylacrylamide) (PNIPAM) that exhibits an LCST at 32 °C 4. Examples from the literature focused on the formation of temperature‐responsive micelles and micellar gels from diblock copolymers comprising a PNIPAM block and a hydrophobic block 5–11. ABA triblock copolymers containing a central PNIPAM B block and two outer hydrophobic blocks were also synthesized, leading to flower‐like micelles in the dilute regime, with the PNIPAM central block forming loops in the corona,12, 13 and to thermoresponsive gels in more concentrated solutions 14…”

Poly(N-isopropyl acrylamide)-block-poly(n-butyl acrylate) thermoresponsive amphiphilic copolymers: Synthesis, characterization and self-assembly behavior in aqueous solutions

“…It was shown in many studies that the temperature-induced coil-to-globule transition of PNIPAm can be influenced by various parameters such as the tacticity of the polymer [12], pressure [13], the presence of salts [14] or the presence of surfactants [15]. Amphiphilic diblock copolymers in which PNIPAm is the hydrophilic block, such as poly(N-isopropyl acrylamide)-block-polystyrene [16,17] form (PNIPAm-b-PS) micelles with thermoresponsive shells in aqueous solutions. Although the micelles with PNIPAm shells can form multimicellar aggregates above the LCST of PNIPAm [18], it was also reported for some systems that no aggregation occurs in contrast to the PNIPAm homopolymer [17].…”

“…Amphiphilic diblock copolymers in which PNIPAm is the hydrophilic block, such as poly(N-isopropyl acrylamide)-block-polystyrene [16,17] form (PNIPAm-b-PS) micelles with thermoresponsive shells in aqueous solutions. Although the micelles with PNIPAm shells can form multimicellar aggregates above the LCST of PNIPAm [18], it was also reported for some systems that no aggregation occurs in contrast to the PNIPAm homopolymer [17]. It has been suggested that the aggregation is suppressed due to the available contact area of the tethered PNIPAm chains which is too small to allow the shells of two colliding micelles to form entanglements which would lead to the formation of aggregates [18,19].…”

The micellization and dissociation transitions of thermo-, pH- and sugar-sensitive block copolymer investigated by laser light scattering

“…As the temperature increases to 37°C, the f(R h ) becomes narrow with <R h > 28 nm, suggesting the collapse of micelles. We note that besides the micelles in solution, there must be a small amount of free PAAPBA-b-PNIPAM chains, which are usually not detectable in the presence of micelles by DLS [42]. The actual morphology of the micelles has been obtained by TEM observation (Figure 3), which clearly reveals the presence of irregular micelles with average diameters of 50~65 nm at 20°C and spherical micelles with average diameters of 20~30 nm at 37°C, respectively.…”