Amphiphilic Molecular Brushes with Regular Polydimethylsiloxane Backbone and Poly-2-isopropyl-2-oxazoline Side Chains. 2. Self-Organization in Aqueous Solutions on Heating

Abstract

The behavior of amphiphilic molecular brushes in aqueous solutions on heating was studied by light scattering and turbidimetry. The main chain of the graft copolymers was polydimethylsiloxane, and the side chains were thermosensitive poly-2-isopropyl-2-oxazoline. The studied samples differed in the length of the grafted chains (polymerization degrees were 14 and 30) and, accordingly, in the molar fraction of the hydrophobic backbone. The grafting density of both samples was 0.6. At low temperatures, macromolecules and aggregates, which formed due to the interaction of main chains, were observed in solutions. At moderate temperatures, heating solutions of the sample with short side chains led to aggregation due to dehydration of poly-2-isopropyl-2-oxazoline and the formation of intermolecular hydrogen bonds. In the case of the brush with long grafted chains, dehydration caused the formation of intramolecular hydrogen bonds and the compaction of molecules and aggregates. The lower critical solution temperature for solutions of the sample with long side chains was higher than LCST for the sample with short side chains. It was shown that the molar fraction of the hydrophobic component and the intramolecular density are the important factors determining the LCST behavior of amphiphilic molecular brushes in aqueous solutions.

Self-Organization in Dilute Aqueous Solutions of Thermoresponsive Star-Shaped Six-Arm Poly-2-Alkyl-2-Oxazines and Poly-2-Alkyl-2-Oxazolines

“…In order to estimate the aggregation degree m agg for large aggregates, one can use the previously proposed approach [ 52 ] based on comparing the translational friction coefficients for macromolecules and aggregates. By modeling unimolecular micelles by spheres, and large aggregates by ellipsoids of revolution, it is possible to show that the aggregation degree associated with the parameters of the ellipsoid by the equation [ 52 ]: m agg = V ell / V sph = p 2 R f 3 /8 a 3 where V ell is the volume of the modeling ellipsoid of revolution, V sph is the volume of the modeling sphere, p = a / b , a and b are the major and minor axes of ellipsoid. Herein, the dependences of a and b on p for a model ellipsoid of revolution with a translational friction coefficient f are described by the formulas a = f /(6πη 0 F ( p )) b = f /(6πη 0 pF ( p )) where aF ( p ) = ( p 2 −1) 1/2 /( p ln(( p + ( p 2 −1) 1/2 )/( p −( p 2 −1) 1/2 )) …”

“…In order to estimate the aggregation degree m agg for large aggregates, one can use the previously proposed approach [52] based on comparing the translational friction coefficients for macromolecules and aggregates. By modeling unimolecular micelles by spheres, and large aggregates by ellipsoids of revolution, it is possible to show that the aggregation degree associated with the parameters of the ellipsoid by the equation [52]:…”

Amphiphilic Molecular Brushes with Regular Polydimethylsiloxane Backbone and Poly-2-isopropyl-2-oxazoline Side Chains. 3. Influence of Grafting Density on Behavior in Organic and Aqueous Solutions

“…Our previous works described the synthesis, characterization, and conformational behavior of amphiphilic PDMSk- graft -PiPrOx molecular brushes with polydimethylsiloxane (PDMS) backbone and poly-2-isopropyl-2-oxazoline (PiPrOx) side chains, and the effect of PiPrOx side chain length on self-organization and aggregation in aqueous solutions was also investigated [ 42 , 43 ]. It was shown that the LCST for PDMSk- graft -PiPrOx with long side chains is higher than the LCST for samples with short PiPrOx chains, despite the fact that the M of the latter is almost two times lower.…”

Conformation, Self-Organization and Thermoresponsibility of Polymethacrylate Molecular Brushes with Oligo(ethylene glycol)-block-oligo(propylene glycol) Side Chains

“…Moreover, the architecture parameters of grafted copolymers strongly affect the solubility and the assembly behavior of macromolecules compared to linear diphilic block-copolymers [ 7 , 8 , 9 , 10 ]. For example, the shape of the molecules of dense amphiphilic brushes in a thermodynamically poor solvent for the backbone resembles a star-shaped molecule, the core of which is a collapsed main chain, and the grafted chains are arms [ 11 , 12 , 13 , 14 ] A decrease in the grafting density of the side chains leads to a decline in solubility and to aggregate formation due to the interaction of insoluble components [ 15 , 16 ]. In particular, diphilic molecular brushes are able to form ordered micellar structures by the self-assembly processes of macromolecules in selective solvents.…”

“…A large number of works are devoted to the analysis of the stimulus sensitivity of molecular brushes [ 12 , 13 , 17 ]. However, the number of systematic studies of the concrete classes of polymer brushes is not very large.…”