Micelle formation induced by photolysis of a poly(<Emphasis Type="Italic">tert</Emphasis>-butoxystyrene)-<Emphasis Type="Italic">block</Emphasis>-polystyrene diblock copolymer

We found the novel photolysis-induced micellization of the poly(tert-butoxystyrene)-block-polystyrene diblock copolymer (PBSt-b-PSt). PBSt-b-PSt with a molecular weight of Mn(PBSt-b-PSt) = 15,000-b-97,000 showed no self-assembly in dichloromethane and existed as isolated copolymers with a hydrodynamic diameter of 16.6 nm. Dynamic light scattering demonstrated that the copolymer produced micelles with a 63.0 nm hydrodynamic diameter when the copolymer solution was irradiated with a high-pressure mercury lamp at room temperature in the presence of bis(alkylphenyl) iodonium hexafluorophosphate, a photoacid generator. The ¹H NMR analysis revealed that the micellization resulted from the photolysis of the PBSt blocks into insoluble poly(vinyl phenol) blocks based on the fact that the signal intensity of the tert-butyl protons decreased over time during the irradiation. It was found that the micellization rapidly proceeded as the degree of the photolysis reached over 50% and was completed at 90%.     

Oxidation-induced micellization of a diblock copolymer containing stable nitroxyl radicals

Oxidation-induced micellization was attained for a diblock copolymer containing 2,2,6,6-tetramethylpyperidine-1-oxyl (TEMPO). Poly(4-vinylbenzyloxy-TEMPO)-block-polystyrene (PVTEMPO-b-PSt) showed no self-assembly in carbon tetrachloride, a nonselective solvent. Dynamic light scattering demonstrated that the copolymer self-assembled into micelles of 49.5-nm hydrodynamic diameter when chlorine gas was added to the copolymer solution. The UV and electron spin resonance (ESR) analyses verified that as TEMPO was oxidized into the one-electron oxidant, that is, oxoaminium chloride (OAC) by the chlorine, the nonamphiphilic block copolymer became amphiphilic in nature, and thus, the polymers underwent micellization. An investigation of the relation between the micellization and the oxidation degree of the TEMPO into the OAC revealed that the micellization was induced by only 16% of the OAC. It was confirmed that the POAC-b-PSt micelles were spherical in shape by transmission electron microscopy observation. The micelles served as a two-electron oxidizing agent for benzyl alcohol to quantitatively give benzaldehyde. The micellar structure was maintained after the oxidation of benzyl alcohol without any dissociation into unimers because the OAC was converted into an insoluble hydroxylamine–hydrochloride salt. On the other hand, the micelles reacted with N,N,N′,N′-tetramethyl-1,4-phenylenediamine (TMPD) to produce Wurster’s blue chloride by a one-electron transfer from TMPD to the OAC, converting themselves into PVTEMPO-b-PSt unimers.     

Control of micellization induced by disproportionation of 2,2,6,6-tetramethylpiperidine-1-oxyl supported on side chains of a block copolymer

The block copolymer micellization induced by the disproportionation of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) was performed using acids with different acid strengths. A poly(4-vinylbenzyloxy-TEMPO)-block-polystyrene diblock copolymer (PVTEMPO-b-PSt) produced micelles in 1,4-dioxane by the disproportionation of the TEMPO by HNO₃, HCl, HClO₄, and HSbF₆. The acid strength affected the efficiency of the micellization. The acid/VTEMPO molar ratio required for the micellization decreased with an increase in the acid strength: $ {\text{HNO}}_{\text{3}} < {\text{HCl}} < {\text{HClO}}_{\text{4}} < {\text{HSbF}}_{\text{6}} $ . The acid strength also made a difference in the hydrodyamic diameter of the micelles. The stronger acid provided larger micelles. This difference in the micellization was based on the difference in the solubility of the oxoaminium salt formed by the disproportionation of the TEMPO and on the steric hindrance of its counter anion.     

Micelle formation induced by photo-Claisen rearrangement of poly(4-allyloxystyrene)-block-polystyrene

A novel micelle formation induced by the photo-Claisen rearrangement was attained using a poly(4-allyloxystyrene)-block-polystyrene (PASt-b-PSt) diblock copolymer. The photoreaction was performed in cyclohexane at room temperature without a catalyst. The conversion of the 4-allyloxystyrene units reached 90% by irradiation for 24 h. The photo-Claisen rearrangement of PASt-b-PSt into poly(3-allyl-4-hydroxystyrene)-block-PSt quantitatively proceeded up to a 20% conversion; however, the elimination of the allyl groups competitively occurred over the 20% conversion. The degrees of the photorearrangement and elimination showed good agreement in their material balance throughout the course of the reaction. Both of the photorearrangement and elimination finally reached ca. 50% degrees over 60% conversion. The light-scattering studies demonstrated that the PASt-b-PSt copolymer with a 36-nm hydrodynamic diameter as unimers formed micelles with a 98-nm diameter by irradiation.     

Direct preparation of core cross-linked micelles in a nonselective solvent

This is the first light scattering study demonstrating that the size of micelles, the aggregation number, and the mobility of the core blocks of the micelles could be controlled by the length of the cross-linker in the micellar cores. The core cross-linked micelles were prepared using a poly[(4-pyridinemethoxy-methyl)styrene]-block-polystyrene (PPySt-b-PSt) diblock copolymer and perfluoroalkyl dicarboxylic acid. The PPySt-b-PSt copolymer formed the micelles in THF, a nonselective solvent, in the presence of the perfluoroalkyl dicarboxylic acid. The light scattering studies demonstrated that the micellar size and aggregation number were dependent on the chain length of the perfluoroalkyl dicarboxylic acid. Perfluoroazelaic acid produced micelles with a larger hydrodynamic radius and higher aggregation number than tetrafluorosuccinic acid. The micellization proceeded through the formation of the pyridinium carboxylate and the cross-linkage between the PPySt blocks via the dicarboxylic acid. The core cross-linked micelles were thermally stable and maintained its structure with changes in the temperature. A ¹H NMR analysis revealed that the micelles prepared by perfluoroazelaic acid had more mobility of the core blocks than those by tetrafluorosuccinic acid.     

Self-assembly control of a pyridine-containing diblock copolymer by perfluorinated counter anions during salt-induced micellization

The micelle formation of poly[(4-pyridinemethoxymethyl)styrene]-block-polystyrene (PPySt-b-PSt) was studied in the nonselective solvent using perfluoroalkyl carboxylic acids. PPySt-b-PSt showed no self-assembly into micelles in THF, because this solvent was nonselective for the copolymer. Dynamic light scattering demonstrated that the diblock copolymer formed the micelles in the solvent in the presence of perfluoroalkyl carboxylic acids in which the number of carbons in the perfluoroalkyl chains was over eight. ¹H NMR revealed that the micellization proceeded through the salt formation of the pyridinium perfluoroalkyl carboxylate and through the aggregation of the perfluoroalkyl chains in the counter anions. The hydrodynamic radius and the aggregation number of the micelles increased with an increase in the length of the perfluoroalkyl chain. The copolymer needed less carboxylic acid with longer perfluoroalkyl chain to form the micelles. The copolymer produced the micelles with lower aggregation number and higher critical micelle concentration at higher temperature, although the micellar size was almost independent of the temperature. The micelles were unstable with respect to the variation in the temperature, and were dissociated into the unimers with the increase in the temperature. The micelles, however, were reconstructed by decreasing the temperature. This dissociation–reconstruction of the micelles was controlled reversibly not only by the temperature but also by the concentration of the perfluoroalkyl carboxylic acid. An increase in the acid concentration suppressed the dissociation into the unimers, while promoting the reconstruction of the micelles.     

Photo-induced micellization of poly(4-pyridinemethoxymethylstyrene)-block-polystyrene using a photo-acid generator

The photo-induced micellization was attained for a poly(4-pyridinemethoxymethylstyrene)-block-polystyrene diblock copolymer using diphenyliodonium hexafluorophosphate, a photo-acid generator. Dynamic light scattering demonstrated that the copolymers with a 27.2-nm hydrodynamic diameter self-assembled into micelles with a 68.9-nm diameter by irradiation of a 1,4-dioxane solution of the copolymer using a high-pressure mercury lamp. The micellization was completed within 5 h based on the variation in the scattering intensity and the hydrodynamic diameter of the copolymer. It was found that the copolymer formed monodispersed spherical micelles because G₁(τ), the normalized time correlation function of the scattered field, showed a linear decay. Furthermore, the proton nuclear magnetic resonance analysis confirmed that the micelles had cores formed by the poly(4-pyridinemethoxymethylstyrene) blocks. It was suggested that the micellization occurred by electron transfer from the pyridine to the photo-acid generator in their excited states.     

Micelle formation of a diblock copolymer having pyridine as pendant groups by carboxylic acids in nonselective solvents

The micelle formation of a poly(4-pyridinemethoxymethylstyrene)-block-polystyrene diblock copolymer (PPySt-b-PSt) was investigated in nonselective solvents using bifunctional and trifunctional carboxylic acids. The copolymer showed no self-assembly in 1,4-dioxane and tetrahydrofuran (THF) because the PPySt and PSt blocks were solvophilic to the solvents. Dynamic light scattering studies demonstrated that the copolymer formed micelles in the nonselective solvents in the presence of bifunctional carboxylic acids. Oxalic acid, maleic acid, citric acid, and phospholic acid promoted the micellization, while malonic acid, succinic acid, and glutalic acid had no effect on the micellization. The micellar size, aggregation number, and critical micelle concentration were dependent not only on the acid strength but also on the type of acid and the functionality. The micellization was also affected by the solvent quality. The micellization proceeded more effectively in 1,4-dioxane than in THF. It was found that the micellization occurred by hydrogen bonding between the pyridine moiety and the carboxylic acid and by the interaction among the carboxylic acids. This is because the copolymer needed over an equivalent of the acid to the PySt unit to complete the micellization. Furthermore, monofunctional carboxylic acid such as trichloroacetic acid and trifluoroacetic acid promoted the micellization, although dichloroacetic acid had no effect on the micellization.     

TEM observation of nonamphiphilic copolymer micelles

Light scattering and transmission electron microscope (TEM) measurements were preformed for micelles of a nonamphiphilic poly(vinylphenol)-block-polystyrene diblock copolymer (PVPh-b-PSt) to determine the shape of the micelles. The micelles were prepared by the self-assembly of the copolymer in 1,4-dioxane, a nonselective solvent, in the presence of 1,4-butanediamine. The logarithm of the normalized time correlation function of the scattered field, lnG₁(τ), linearly decayed versus the delay time, τ. The diffusion coefficient measured in the range of the scattering angles from 30° to 150° was almost independent of the square of the magnitude of the scattering vector. The linear decay of lnG₁(τ) vs τ and the angular-independence of the diffusion coefficient suggested that the monodisperse spherical micelles were formed by the micellization. The TEM observations confirmed the formation of uniform spheres.     

Thermodynamics and kinetics on micelle formation of a nonamphiphilic poly(vinylphenol)-<Emphasis Type="Italic">block</Emphasis>-polystyrene by α,ω-diamine

A poly(vinylphenol)-block-polystyrene diblock copolymer (PVPh-b-PSt) forms micelles in the presence of 1,4-butanediamine (BDA) in 1,4-dioxane, a nonselective solvent. The micellization proceeds through the formation of hydrogen bond cross-linking between the PVPh blocks via BDA, and the dissociation and reconstruction of the micelles is reversibly controlled by temperature. We explored the thermodynamics and kinetics on the micellization of the nonamphiphilic PVPh-b-PSt copolymer by BDA. Light scattering studies demonstrated that an equilibrium existed between the micelles and the unimers. The equilibrium constants were determined for the dissociation and the reconstruction of the micelles on the basis of variation in the aggregation number of the micelles. The equilibrium constant of the dissociation showed a good agreement with the reciprocal of the equilibrium constant of the reconstruction. Based on the equilibrium constants, the standard Gibbs energy, enthalpy, and entropy of the dissociation and reconstruction were estimated. The standard enthalpy was Δ H° = 30–40 kJ mol⁻¹ for the dissociation. The enthalpy of the reconstruction was obtained as a negative value, however, there was a negligible difference in the absolute values of Δ H° between the dissociation and the reconstruction. The rate constant of the micellization was ca. 10² times larger than the back reaction, and increased with a decrease in the temperature.     

Preparation of micelles with azobenzene at their coronas or cores from ‘nonamphiphilic’ diblock copolymers

Micelles with azobenzene at the coronas or the cores were prepared by the micellization of nonamphiphilic diblock copolymers through hydrogen bond cross-linking. We used 4-(phenylazophenoxymethyl)styrene (AS) as the azobenzene. A poly(vinylphenol)-block-poly(AS-co-styrene) diblock copolymer (PVPh-b-P(AS-co-St)) was prepared by combination of the nitroxide-mediated living radical polymerization and the hydrolysis. The copolymer contained ca. 1 mol% of the azobenzene units in the P(AS-co-St) blocks on the basis of ¹H NMR analysis. The PVPh-b-P(AS-co-St) copolymer showed no micellization in 1,4-dioxane, the nonselective solvent. Dynamic light scattering demonstrated that the copolymer formed micelles in the presence of 1,4-butanediamine (BDA) in this solvent. ¹H NMR analysis revealed that the azobenzene moieties were located at the coronas of the micelles, because the signals of the aromatic protons originating from the azobenzene had no changes in the shape and the intensity by the micellization. UV analysis supported the presence of the azobenzene at the micellar coronas. The size of the PVPh-b-P(AS-co-St) micelles was independent of the copolymer concentration. On the other hand, the aggregation number of the micelles was dependent not only on the copolymer concentration but also on the kind of the diamine. A poly(AS-co-vinylphenol)-block-polystyrene diblock copolymer (P(AS-co-VPh)-b-PSt) formed the micelles with the azobenzene at the cores of the micelles by BDA. UV analysis demonstrated that the azobenzene at the micellar cores still had the potential to function as photorefractive switching.     

Reduction-induced micellization of a diblock copolymer containing stable nitroxyl radicals

A novel micelle formation induced by reduction was attained using a diblock copolymer supporting 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO). Poly(4-vinylbenzyloxy-TEMPO)-block-polystyrene (PVTEMPO-b-PSt) showed ultraviolet (UV) absorption at 467 nm as λ _max based on the TEMPO radicals. As the phenylhydrazine was added to the copolymer solution in benzene, the UV absorbance decreased. The decrease in the absorbance suggested that the TEMPO radicals were reduced to the colorless hydroxylamine by phenylhydrazine. The PVTEMPO-b-PSt copolymer showed no self-assembly in benzene due to the nonselective solvent. A light scattering study demonstrated that the scattering intensity of the copolymer increased with a decrease in the UV absorbance. The hydrodynamic diameter of the copolymer rapidly increased with the addition of phenylhydrazine and became almost steady over the molar ratio of phenylhydrazine to the VTEMPO unit of 0.2. It was found that the hydroxylamine in the micelles reverted to the TEMPO radicals by oxidation with oxygen.     

Reversible control of self-assembly of a diblock copolymer supporting Wittig reagent

The reversible control of self-assembly of a diblock copolymer supporting Wittig reagent was attained by changing the volume ratio of the mixed solvent. Poly(4-vinylbenzyltriphenylphosphonium chloride)-block-polystyrene (PPCl-b-PSt) self-assembled into micelles with the PPCl block cores in C₆H₆. The micelles were completely dissociated into unimers by the addition of CH₃CN at a 5/5 volume ratio of C₆H₆/CH₃CN. As a result of further increasing the CH₃CN, the reversed micelles with the PPCl block shells were produced. The copolymer served as the Wittig reagent for 9-anthracenecarboxaldehyde to produce a block copolymer with the pendent anthracene. The resulting copolymer also provided micelles in C₆H₆.     

Preparation of micelles with azo dye and UV absorbent at their cores or coronas using non-amphiphilic block copolymers

Micelles with azo dye and UV absorbent at their cores or coronas were prepared from non-amphiphilic random diblock copolymers by α,ω-diamine. Poly[4-(phenylazophenoxymethyl)styrene-ran-4-(2-hydroxybenzophenoxymethyl)styrene-ran-vinylphenol]-block-polystyrene (P(AS-r-HBS-r-VPh)-b-PSt) and poly(vinylphenol)-block-poly[4-(phenylazophenoxymethyl)styrene-ran-4-(2-hydroxybenzophenoxymethyl)styrene-ran-styrene] (PVPh-b-P(AS-r-HBS-r-St)) diblock copolymers were prepared by living radical polymerization mediated by 4-methoxy-2,2,6,6-tetramethylpiperidine-1-oxyl. The former copolymer had a molecular weight of Mn[P(AS-r-HBS-r-VPh)-b-PSt] = 10,000-b-250,000 by ¹H NMR and a molar ratio of AS:HBS:VPh = 0.01:0.01:0.98, while the latter had a molecular weight of Mn[PVPh-b-P(AS-r-HBS-r-St)] = 10,000-b-111,000 and a molar ratio of AS:HBS:St = 0.02:0.03:0.95. The copolymers showed no self-assembly in 1,4-dioxane because this solvent was non-selective to the copolymers. Dynamic light scattering demonstrated that the copolymers formed micelles in the solvent in the presence of α,ω-diamine. The hydrodynamic radii of the micelles slightly increased with the copolymer concentration decrease, while the aggregation numbers were almost independent of the copolymer concentration. It was found that P(AS-r-HBS-r-VPh)-b-PSt formed smaller micelles with a lower aggregation number than PVPh-b-P(AS-r-HBS-r-St) because of the steric hindrance of the AS and HBS units present at the micellar coronas.     

Synthesis, self-assembly and responsive properties of PEG-b-PDMAEMA-b-PMMAzo triblock copolymers

A novel amphiphilic copolymer poly(ethylene glycol)-block-poly(N,N-dimethylamino-2-ethylmethacrylate)-blockpoly[6-(4-methoxy-azobenzene-4’-oxy) hexyl methacrylate](PEG-b-PDMAEMA-b-PMMAzo) was prepared by ATRP polymerization.The self-assembly and responsive behaviors were investigated by SEM,TEM,LLS and UV-Vis spectra.The results indicated that the copolymers can self-assemble into spherical structures in aqueous media.The aggregate size can be tuned by pH and temperature.The trans-cis isomerization behavior of the formed aggregates was also examined.Upon irradiation with a linear polarized light,the elongation degree of the aggregates was increased with the irradiation time.     

Synthesis of Amphiphilic ABCBA-Type Pentablock Copolymer from Consecutive ATRPs and Self-Assembly in Aqueous Solution

Summary: A novel amphiphilic ABCBA-type pentablock copolymer with properties that are sensitive to temperature and pH, poly(2-dimethylaminoethyl methacrylate)-block-poly(2,2,2-trifluoroethyl methacrylate)-block-poly(ε-caprolactone)-block-poly(2,2,2- trifluoroethyl methacrylate)-block-poly(2-dimethylaminoethyl methacrylate) (PDMAEMA- b-PTFEMA-b-PCL-b-PTFEMA-b-PDMAEMA), was synthesized via consecutive atom transfer radical polymerizations (ATRPs). The copolymers obtained were characterized by gel permeation chromatography (GPC) and ¹H nuclear magnetic resonance (NMR) spectroscopy, respectively. The aggregation behaviors of the pentablock copolymers in aqueous solution with different pH (pH = 4.0, 7.0 and 8.5) were studied. Transmission electron microscopic images revealed that spherical micelles from self-assembly of the pentablock copolymer were prevalent in all cases. The mean diameters of these micelles increased from 34, 46, to 119 nm when the pH of the aqueous solution decreased from 8.5, 7.0, to 4.0, respectively.     

Model block–graft copolymer via anionic living polymerization: Preparation and characterization of polystyrene-block-[poly(p-hydroxystyrene)-graft-poly(ethylene oxide)]-block-polystyrene

A model graft copolymer in which position of graft points was set to the center of a backbone molecule was prepared via anionic living polymerization. Polystyrene-block-poly(p-tert-butoxystyrene)-block-polystyrene (PSt-b-PBSt-b-PSt) was prepared by three-stage sequential addition. The tert-butyl group was removed from PBSt by hydrogen bromide to yield PSt-b-PHSt-b-PSt, having a poly(p-hydroxystyrene) (PHSt) block. The hydroxyl group of PHSt was reacted with dimeric potassium dianions of 1, 1-diphenylethylene (DPE-K) or cumyl potassium (cumyl K) to yield the corresponding macromolecular initiators of PSt-b-PHSt⁻K⁺-b-PSt containing the potassium alkoxide ion of PHSt. The newly formed alkoxide groups and remaining initiators of DPE-K or cumyl K are capable of initiating the additionally introduced ethylene oxide (EO). Thus, two block–graft copolymers of polystyrene-block-[poly(p-hydroxystyrene)-graft-poly(ethylene oxide)]-block-polystyrene (PSt-b-(PHSt-g-PEO)-b-PSt) were prepared by a “grafting from” process (backbone initiation). A PSt-b-PHSt-b-PSt backbone (M_n = 1.7₅ × 10⁵ by osmometry and M_w/M_n = 1.0₈ by GPC), and two PSt-b-(PHSt-g-PEO)-b-PSt block–graft copolymers (M_n = 2.4₅ × 10⁵ by osmometry and M_w/M_n < 1.1₀ by GPC) had narrow molecular weight distributions. A relationship between nonquantitative metallation and spacing of the graft points on a backbone molecule was discussed in detail. Two benzene-cast films formed clear microphase-separated structures of lamellar structure. The dependence of composition on the morphology of the block–graft copolymers was found to differ from that of common block copolymers. A degree of crystallinity of PEO segment and lamellar thickness of PEO phase serving as graft molecule were also found to differ from those of homo PEO and/or PEO segment in common block copolymer. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 3021–3034, 1998     

Stimuli-responsive behavior of complex micelles based on double hydrophilic block copolymer and fluorescent indicator

The double hydrophilic block copolymer poly(ethylene glycol mono-methyl ether)-block-poly(4-vinylpyridine) (mPEG₄₃-b-P4VP₁₁₅) was synthesized by atom transfer radical polymerization. The structure, molecular weight and molecular weight distribution of mPEG₄₃-b-P4VP₁₁₅ were characterized by ¹H-NMR and gel permeation chromatography combined with laser light scattering technique. The complex micelles based on mPEG₄₃-b-P4VP₁₁₅ and the disodium 2-naphthol-3,6-disulfonate were obtained in acid aqueous solution. The morphologies of the complex micelles were observed by transmission electron microscopy. The revertible temperature and pH-responsive behaviors of complex micelles were studied by dynamic light scattering and fluorescence techniques.     

pH-responsive Micelles from a Blend of PEG-<Emphasis Type="Italic">b</Emphasis>-PLA and PLA-<Emphasis Type="Italic">b</Emphasis>-PDPA Block Copolymers: Core Protection Against Enzymatic Degradation

pH-responsive micelles with a biodegradable PLA core and a mixed PEG/PDPA shell were prepared by self-assembly of poly(ethylene glycol)-b-poly(lactic acid) (PEG-b-PLA) and poly(2-(diisopropylamino)ethyl methacrylate)-b-poly(lactic acid) (PDPA-b-PLA). The micellization status with different pH and the enzyme degradation behavior were characterized by ¹H-NMR spectroscopy, dynamic light scattering measurement and zeta potential test. The pH turning point of PDPA block was determined to be in the range of 5.5?7.0. While the pH was above 7.0, the PDPA block collapsed onto the PLA core and could protect the PLA core from invasion of enzyme, as a result, the micelle exhibited a resistance to the enzymatic degradation.     

Preparation and characterization of poly(styrene-alt-maleic acid)-b-polystyrene block copolymer self-assembled nanoparticles

Block copolymers poly(styrene-alt-maleic anhydride)-b-polystyrene (P(St-alt-MAn)-b-PSt) were synthesized via radical addition fragmentation chain transfer copolymerization. The maleic anhydride-containing segments of the block copolymer were hydrolyzed to form amphiphilic poly(styrene-alt-maleic acid)-b-polystyrene (P(St-alt-MA)-b-PSt). In aqueous solution, P(St-alt-MA)₇₃-b-PSt₈₁ and P(St-alt-MA)₅₈-b-PSt₁₃₀ formed stable dispersed spherical aggregates of approximately 25 and 40 nm, respectively. Particle size was stable under alkaline conditions and was little affected by the polymer concentration in the range of 0.025–1.0 mg mL^?1. The critical aggregation concentrations of the block copolymer self-aggregates were 1?×?10^?3 and 3?×?10^?3 mg mL^?1 for hydrophobic PSt block lengths of 130 and 81 monomer units, respectively. The nanoparticles had a negative surface charge at pH?>?2. Scanning electron microscopy images revealed that particle–particle coalescence did not occur upon drying of the film and the nanoparticles remained discrete. Controlled aspirin release from the nanoparticles was dependent on the structure of the block polymers and release medium.