PS-PVP-PEO micelles

Some time ago, we investigated the behavior PEO-containing PE terpolymer PS-PVP-PEO in aqueous solutions [88, 89], The micellization of this copolymer is strongly pH-dependent because PVP is protonized and therefore soluble in acidic solutions at pH lower than 4.8, but is deprotonized and therefore water-insoluble at higher pH. The PS-PVP-PEO micelle is a three-layer nanoparticle in which the PVP blocks form a middle layer between the rigid PS core and the PEO shell. The PVP layer is either collapsed at high pH, so that PS-PVP-PEO micelles resemble onion micelles formed in mixtures of PS-PVP and PVP-PEO diblock copolymers, or it is partially protonized, swollen, and flexible in acid aqueous media, so that the PVP layer becomes a soluble inner shell between the core and the outer PEO shell. 

The most surprising feature of the behavior of PS-PVP-PEO micelles with water-soluble PVP (protonized) and PEO blocks in acidic media is their aggregation in the region of low pH. Because it is a rather unexpected phenomenon, we studied it in more detail. The distributions of relaxation times obtained by DLS are bimodal (Eig. 8). Angular dependences (not shown) prove that both fast and slow relaxation modes correspond to diffusive processes. The intensity of the slow mode decreases with increasing pH and decreasing copolymer concentration. At very low copolymer and HCl concentrations, the slow mode disappears completely. The DLS measurements thus show that PS-PVP-PEO solutions contain two types... 

Fig. 9 Hydrodynamic radius, Rh, of PS-PVP-PEO micelles in acidic aqueous solutions, as a function of pH...

Fig. 13 Time-resolved Stokes shift, C(t), of patman in PS-PVP-PEO micelles at pH values of 2, 3, and 4, as indicated on the corresponding curves. Inset Time-dependent halfwidth, 5(t), of the time-resolved emission spectra of patman in PS-PVP-PEO micelles at different pH values, as indicated...

Fig. 18 Particle number, (iV), as a function of the fluorescent probe concentration, cqrb, for PS-PVP-PEO micelles in 0.01 M HCl. Inset Typical correlation curve for an FCS measurement of ORB-labeled PS-PVP-PEO micelles O.OIM HCl...

The second example concerns the multidisciplinary study of the micelliz-ing block copolymer polystyrene-( -poly(2-vinylpyridine)-ft-poly(ethylene oxide) (PS-PVP-PEO), which shows a high tendency to aggregation and the formation of micellar clusters [88,89]. It shows the application of SRM for studying the mobility and structural details of different domains in micelle-like polymeric nanoparticles. The fluorescence technique reveals interesting features of studied systems that are hardly accessible by other techniques. Section 3.3 is devoted to the development of the methodology of the solvent relaxation technique for studying nanostructured self-assembling systems. 

The studied triblock copolymer PS-PVP-PEO was purchased from Polymer Source (Dorval, Canada). The number-average molar masses of PS, PVP, and PEO blocks were 2.1 x 10 , 1.2 x 10 , and 3.5 x 10 g mol , respectively, and the poly-dispersity index of the sample was 1.10. The copolymer is insoluble in aqueous media, but the micelles can be prepared indirectly both in acidic and alkaline aqueous solutions by dialysis from 1,4-dioxane-methanol mixtures [88]. The micelles can be transferred from acidic to alkaline alkaline solutions and vice versa, but the addition of a base together with intense stirring promotes aggregation. Two factors contribute to the destabilization of micelles after the pH increase (a) In alkaline media, the PVP blocks become insoluble, collapse and form an upper layer of the core. Since the cores of micelles are kinetically frozen, the association number does not change. The mass of insoluble cores increases, while the length of soluble shellforming chains decreases, which results in a deteriorated thermodynamic stability of micellar solutions, (b) The PVP middle layer shrinks and PEO chains come close to each other, which worsens the solubility due to insufficient solvation of PEO blocks. 

As we intended to study the pH-dependent hydration of PEO in triblock copolymer micelles, we measured the solvent relaxation for patman embedded in PS-PEO micelles both in acidic (0.01 M HCl) and alkaUne (0.01 M NaOH) solutions for comparison. Because we found only marginal differences in the relaxation behavior, we can conclude that the dye itself does not exhibit any pH-dependent changes after binding to micelles and that the solvation of short PEO does not change much with pH (it is very important to emphasis that the PEO blocks are significantly shorter than those in the studied PS-PVP-PEO copolymer). 

Prochazka et al. [31] studied solvent relaxatitm in the shells of polystyrene-WocA -poly(ethylene oxide) (PS-PEO), polystyrene-Woc)t-poly(2-vinylpyridine) (PS-PVP), and polystyrene-Woc)t-poly(2-vinylpyridine)-Woc)t-poly(ethylene oxide) (PS-PVP-PEO) in aqueous solutions, using Patman as a fluorescent probe. While in the case of the PEO shell of the PS-PEO micelle, a slow relaxation was observed because the mobility of water molecules in the shell was hindered by the strong hydrogen bonding to PEO chains, the solvent cage of the probe in the shell of PS-PVP micelles appeared to relax much faster. Interestingly, water in PS-PVP-PEO terpolymer micelles exhibited a slow relaxation as in the case of the PS-PEO diblock, but the relaxation time became pH responsive due to the presence of the weak polyelectrolyte PVP block. 

The potential of hollow mesoporous silica nanoparticles as a drug carrier has been actively researched in vitro. For example, hollow mesoporous silica nanospheres with uniform sizes of 31-33 nm, prepared using PS-PVP-PEO triblock copolymer micelles, exhibited a higher storage capacity of a model drug (ibupro-fen) than the conventional mesoporous silica particles [111]. A more sustained release was achieved by propylamine functionalization, which is effective for controlled release of drug molecules. 

Consequently, the solvent relaxation was studied in PS-PVP micelles in 0.01 M HCl solution. The micelles are stable in acidic solution, where they are positively charged. Nevertheless, our earlier studies suggest that the PVP layer partially collapses around the core because PVP is only slightly protonized close to the nonpolar PS core [132], A fairly high value of the residual anisotropy (around 0.2) measured at the maximum emission intensity (467 nm) suggests that patman is embedded in considerably rigid and little protonized domains close to the PS-PVP interface, which means that its location in PS-PVP micelles is similar to that in PS-PEO micelles. 

At low pH, the swollen shell-forming PVP chains are partially protonized and we assume that the aliphatic chain of patman, which prefers the PS core to the partially protonized PVP, tries to pull the fluorophore closer to the core. However, the headgroup, which bears the same positive charge as the protonized PVP units, prefers the location in the PEO outer shell to the PVP middle shell due to electrostatic repulsion. Therefore, both effects roughly compensate each other and the location of patman hardly varies with pH. This assumption is supported by the experimental observation that in low pH solutions (below pH 4), both C(t) and 5(t) show similar relaxation behavior as in PS-PEO micelles, which indicates the same microenvironment and supports the location of the headgroup in the PEO layer. 

Studies have been carried out on multi-component systems such as water— toluene-propan-2-ol, containing copolymers of (a) polyethylene oxide (PEO) and poly(2-vinylpyridine) (PVP) and (b) PEO and polystyrene (PS). In both cases the molecules were found to aggregate into micelles with the PEO blocks on the outside. 

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