Nonselective Solvent Research Articles

Mixed micelles based on cationic and anionic amphiphilic diblock copolymers containing identical hydrophobic blocks

Through the use of the methods of turbidimetry, UV spectrophotometry, fluorescence spectroscopy, dynamic light scattering, and ultracentrifugation, micelle formation is studied for cationic (polysty-rene-poly-N-ethyl-4-vinylpyridium bromide) and anionic (polystyrene-sodium polyacrylate) diblock copolymers containing identical polystyrene blocks in dilute aqueous saline solutions. Mixing of aqueous dispersions of individual micelles is accompanied by the formation of only insoluble products, which likely are intermicellar interpolyelectrolyte complexes. At the same time, mixing of diblock copolymers in a nonselective solvent and its subsequent gradient replacement with water during suppressed interpolyelectrolyte interactions yields mixed diblock copolymer micelles, which are found to be dispersionally stable in an excess of charged units of any polymer component. The micelles are composed of an insoluble polystyrene core and a mixed interpolyelectrolyte corona, and their hydrodynamic characteristics are controlled by the ratio of charged units in the mixed diblock copolymers. The mixed micelles are found to be able to interact with the macromolecules of a homopolyelectrolyte, sodium poly(styrene sulfonate), in aqueous solutions and form ternary complexes. In this case, depending on the composition of the mixed micelles, ternary complexes can be dispersionally stable or can aggregate and precipitate.

Kinetics of Core‐Shell Nanoparticle Formation by Two‐Dimensional Nuclear Magnetic Resonance

We have studied the kinetics of polymeric nanoparticle formation for poly(styrene-block-4-vinylpyridine) [P(S-b-4-VPy)], chains in a non-selective solvent using 1,4-dibromobutane (DBB) as a cross-linker by means of different nuclear magnetic resonance (NMR) spectroscopy techniques. The kinetic process was followed using (1) H, (13) C, and 2-D Heteronuclear Single Quantum Correlation (HSQC) NMR experiments. The kinetic data obtained from 2-D HSQC and (1) H NMR experiments were in good agreement between them, proving the reliability of the 2-D HSQC NMR technique for the in situ study of the kinetics of core-shell nanoparticle formation. A value of 1.5 × 10(-5) s(-1) was determined for the apparent kinetic constant of the P(S-b-4-VPy)-DBB core-shell nanoparticle formation process.

Solution Self-Assembly and Photophysics of Platinum Complexes Containing Amphiphilic Triblock Random Copolymers Prepared by ROMP

Ring-opening metathesis polymerization was used to synthesize amphiphilic, phosphorescent block copolymers that self-assemble into nanometer-scaled aggregates when dissolved in a selective solvent. The triblock random copolymer architecture is built up by using an oligoglycol-substituted norbornene derivative for the hydrophilic part, 2,3-norbornenedicarboxylic acid dimethyl ester, as the first segment of the hydrophobic part and a statistical segment made using norbornene-functionalized dyes as the second segment of the hydrophobic part. The dyes comprise a blue fluorescent carbazole derivative serving as the host and a red phosphorescent platinum complex serving as the guest material. Emission properties of the amphiphilic triblock random copolymer dissolved in a nonselective solvent are exclusively determined by the host material. Only blue emission of the carbazole-derivative can be observed. When the polymer is dissolved in a selective solvent, polymer aggregates are formed and energy transfer occurs...

Multicompartment Core Micelles of Triblock Terpolymers in Organic Media

The formation of multicompartment micelles featuring a “spheres on sphere” core morphology in acetone as a selective solvent is presented. The polymers investigated are ABC triblock terpolymers, polybutadiene-b-poly(2-vinyl pyridine)-b-poly(tert-butyl methacrylate) (BVT), which were synthesized via living sequential anionic polymerization in THF. Two polymers with different block lengths of the methacrylate moiety were studied with respect to the formation of multicompartmental aggregates. The micelles were analyzed by static and dynamic light scattering as well as by transmission electron microscopy. Cross-linking of the polybutadiene compartment could be accomplished via two different methods, “cold vulcanization” and with photopolymerization after the addition of a multifunctional acrylate. In both cases, the multicompartmental character of the micellar core is fully preserved, and the micelles could be transformed into core-stabilized nanoparticles. The successful cross-linking of the polybutadiene core is indicated by 1H NMR and by the transfer of the aggregates into nonselective solvents such as THF or dioxane.

Hydrogen bond mediated supramolecular micellization of diblock copolymer mixture in common solvents

We report a new approach toward preparing self-assembled hydrogen-bonded complexes having vesicle and patched spherical structures from two species of block copolymers in nonselective solvents. Two diblock copolymers, poly(styrene- b-vinyl phenol) (PS- b-PVPh) and poly(methyl methacrylate- b-4-vinylpyridine) (PMMA- b-P4VP), were synthesized through anionic polymerization. The assembly of vesicles from the intermolecular complex formed after mixing PS- b-PVPH with PMMA- b-P4VP in THF was driven by strong hydrogen bonding between the complementary binding sites on the PVPH and P4VP blocks. In contrast, well-defined patched spherical micelles formed after blending PS- b-PVPh with PMMA- b-P4VP in DMF: the weaker hydrogen bonds formed between the PVPh and P4VP blocks in DMF, relative to those in THF, resulted in the formation of spherical micelles having compartmentalized coronas consisting of PS and PMMA blocks.

Combination of “living” nitroxide-mediated and photoiniferter-induced “grafting from” free-radical polymerizations: From branched copolymers to unimolecular micelles and microgels

We report on solution properties of lightly grafted copolymers composed of polystyrene (PS) backbone (degree of polymerization of PS backbone, N b = 95 ) and variable length of poly( tert-butyl methacrylate) P( tBuMA) side chains (degree of polymerization of side chains, N sc = 14 – 222 ) at fixed number of grafting sites n = 11 and polydispersity index ( M w / M n ) ranging from 1.05 to 2.63. Synthesis of these graft copolymers is based on a novel synthetic route [Gromadzki D, Makuška R, Netopilík M, Holler P, Lokaj J, Janata M, et al. Eur Polym J 2008;44:59–71] involving two independent controlled/“living” polymerization mechanisms, namely nitroxide-mediated radical polymerization (NMP) for the synthesis of the backbone and photoinduced “grafting from” iniferter process for building of P( tBuMA) branches. The viscosity-related contraction factors g ′ < 1 confirmed high degree of branching of the studied graft copolymers. Dilute solutions of graft copolymers in non-selective solvent (THF), examined by dynamic light scattering (DLS), small-angle X-ray scattering (SAXS) and viscometry, revealed a transition from comb-like conformation through wormlike-star to a microgel architecture under increasing number of monomeric units in side chains. These data were further supported by the structure factors R η / R h and R g / R h obtained by independent measurements and extrapolated to infinite dilution. Persistence lengths of the samples exhibiting comb-like topology were larger compared to linear polystyrene backbone and P( tBuMA) side chains in THF suggesting stiffening of the main chain with increasing size of the attached side chains. Unimolecular micelles were detected by DLS and SAXS in solvent selective for grafts, tert-amyl alcohol.

Ordering and reverse ordering mechanisms of triblock copolymers in the presence of solvent.

Self-consistent field theory is used to study the self-assembly of a triblock copolymer melt. Two different external factors (temperature and solvent) are shown to affect the self-assembly. Either one or two-step self-assembly can be found as a function of temperature in the case of a neat triblock melt, or as a function of increasing solvent content (for non-selective solvents) in the case of a triblock-solvent mixture. For selective solvents, it is shown that increasing the solvent content leads to more complicated self-assembly mechanisms, including a reversed transition where order is found to increase instead of decreasing as expected, and re-entrant behavior where order is found to increase at first, and then decrease to a previous state of disorder.

Mixed poly(methyl methacrylate)/poly(ethylene glycol) brushes: Study of switching behavior in selective solvent

AbstractThe mixed polymer brushes composed of two incompatible polymers, poly(methyl methacrylate) and poly(ethylene glycol), were successfully synthesized via a sequential grafting to method, which was confirmed by water‐contact angle and ellipsometric measurements. The resulted mixed polymer brushes could undergo conformational rearrangements upon exposure to different selective solvents, and then the lateral segregation in nonselective solvent and perpendicular segregation in selective solvent happened. As a result, the mixed polymer brushes exhibited the ripple morphology and dimple morphology in corresponding solvent, respectively, and which led to changes in water‐contact angle and surface composition as a function of bulk composition of the mixed polymer brushes. Moreover, the switching properties as the surface composition, wettability and topographical images could occur in a controlled and reversible fashion. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009

Self-consistent field theory for obligatory coassembly

We present a first-order model for obligatory coassembly of block copolymers via an associative driving force in a nonselective solvent, making use of the classical self-consistent field (SCF) theory. The key idea is to use a generic associative driving force to bring two polymer blocks together into the core of the micelle and to employ one block of the copolymer(s) to provide a classical stopping mechanism for micelle formation. The driving force is generated by assuming a negative value for the relevant short-range Flory-Huggins interaction parameter. Hence, the model may be adopted to study micellization via H bonding, acceptor-donor interactions, and electrostatic interactions. Here, we limit ourselves to systems that resemble experimental ones where the mechanism of coassembly is electrostatic attraction leading to charge compensation. The resulting micelles are termed complex coacervate core micelles (CCCMs). We show that the predictions are qualitatively consistent with a wide variety of experimentally observed phenomena, even though the model does not yet account for the charges explicitly. For example, it successfully mimics the effect of salt on CCCMs. In the absence of salt CCCMs are far more stable than in excess salt, where the driving force for self-assembly is screened. The main limitations of the SCF model are related to the occurrence of soluble complexes, i.e., soluble, charged particles that coexist with the CCCMs.

Characterization of amphiphilic hydrocarbon modified Poly(ethylene glycol) synthesized through ring-opening reaction of 2-(1-octadecenyl)succinic anhydride

Poly(ethylene glycol) (PEG) triblock and diblock amphiphilic block copolymers were synthesized from poly(ethylene glycol) and poly(ethylene glycol) monomethyl ether, respectively. The hydroxyl groups of PEG readily react with 2-(1-octadecenyl) succinic anhydride (OSA) at 140 °C through ring-opening reaction of the succinic anhydride. Both the PEG–OSA diblock and triblock copolymers are produced without use of any solvent or catalyst. The molecular structure of the copolymers was characterized by 1H NMR and FTIR spectroscopy, and the thermal properties by DSC. The behavior of the copolymers in selective and nonselective solvents was studied by 1H NMR spectroscopy in deuterium oxide and d-chloroform. The aggregation of the polymers in water was studied with a particle size analyzer and a transmission electron microscope (TEM) in bright field mode. The results show that the hydrophobic C 18 chain with intramolecular succinic anhydride linker can be attached to the hydrophilic PEG chain, an ester bond forming between the blocks. The copolymers exhibit flexible, liquid-like hydrophobic blocks even in water, which is a nonsolvent for OSA. PEG–OSA block copolymers self-organize in water, forming micellar polymer aggregates in nanoscale.

Transmission Electron Microscopy Study of Solvent-Induced Phase Morphologies of Environmentally Responsive Mixed Homopolymer Brushes on Silica Particles

We report in this work a transmission electron microscopy study of phase morphologies of environmentally responsive mixed poly( t-butyl acrylate) (P tBA)/polystyrene (PS) brushes and mixed poly(acrylic acid) (PAA)/PS brushes on 180 nm silica particles after treatments with nonselective good solvents and selective solvents, respectively. Mixed P tBA/PS brushes were grown from Y-initiator-functionalized silica particles by sequential atom transfer radical polymerization and nitroxide-mediated radical polymerization. Mixed PAA/PS brushes were prepared from mixed P tBA/PS brushes by removal of the t-butyl groups. For mixed P tBA/PS brushes with P tBA M n of 24.2 kDa and PS M n of 23.0 kDa and the corresponding mixed PAA/PS brushes, random worm-like, nearly bicontinuous nanostructures were formed from lateral microphase separation when the particles were cast from nonselective good solvents (chloroform for mixed P tBA/PS brushes and N, N-dimethylformamide for mixed PAA/PS brushes). The feature sizes were on the order of polymer chain root-mean-square end-to-end distances ( approximately 10 nm). In contrast, mixed P tBA/PS brushes with lower molecular weights (P tBA M n = 10.4 kDa and PS M n = 11.9 kDa) did not strongly phase separate after being cast from chloroform. After the solvents in the particle dispersions were gradually changed to selective solvents ( n-octane for mixed P tBA/PS brushes and H2O for mixed PAA/PS brushes), isolated microdomains with an average size of 14-19 nm were formed as one grafted polymer collapsed and associated to form isolated microdomains, which were shielded by another grafted polymer yielding surface-tethered micellar structures. These results confirmed the theoretical predictions of the formation of "rippled" nanostructures and surface micellar structures of mixed homopolymer brushes induced by nonselective and selective solvents, respectively.

Micellar Structures of Poly(styrene-b-4-vinylpyridine)s in THF/Toluene Mixtures and Their Functionalization with Gold

The effects of the block copolymer composition and the solvent selectivity on the micellar morphologies of poly(styrene- b-4-vinylpyridine)s (PS- b-P4VPs) and their functionalizations with gold were studied in 10 mg/mL solutions using small-angle X-ray scattering and transmission electron microscopy (TEM). The solvent selectivity for the PS block was controlled by toluene/tetrahydrofuran (THF) mixtures in which toluene and THF are selective for PS and nonselective, respectively. The micellar structure was strongly dependent on phi (wt % toluene in toluene/THF mixture) and the composition of the block copolymers. PS(12K)- b-P4VP(11.8K) (symmetric) showed spherical micelles in the entire range of phi except phi = 0 (THF, nonselective solvent). PS(3.3K)- b-P4VP(18.7K) (asymmetric, longer P4VP) showed multiple morphologies with transitions from spheres to cylinders and finally to vesicles with an increase in phi. PS(19.6K)- b-P4VP(5.1K) (asymmetric, longer PS) showed spherical micelles only at the narrow ranges of 90 wt % <or= phi <or= 100 wt % and an isotropic state at the other ranges. The micelles formed a complex with HAuCl 4 and were reduced with hydrazine in the solvent mixtures. The morphologies of micelles, hybrid micelles, and gold nanoparticles were investigated by using TEM and UV/vis spectroscopy. The possible mechanisms leading to these morphological changes and the formation of the nanosized gold particles were also discussed.

Control of Self-Assembly of Lithographically Patternable Block Copolymer Films

Poly(alpha-methylstyrene)-block-poly(4-hydroxystyrene) acts as both a lithographic deep UV photoresist and a self-assembling material, making it ideal for patterning simultaneously by both top-down and bottom-up fabrication methods. Solvent vapor annealing improves the quality of the self-assembled patterns in this material without compromising its ability to function as a photoresist. The choice of solvent used for annealing allows for control of the self-assembled pattern morphology. Annealing in a nonselective solvent (tetrahydrofuran) results in parallel orientation of cylindrical domains, while a selective solvent (acetone) leads to formation of a trapped spherical morphology. Finally, we have self-assembled both cylindrical and spherical phases within lithographically patterned features, demonstrating the ability to precisely control ordering. Observing the time evolution of switching from cylindrical to spherical morphology within these features provides clues to the mechanism of ordering by selective solvent.

Scaling Aspects of Block Co-Polymer Adsorption on Curved Surfaces from Nonselective Solvents

In this paper, we have developed a geometric-based scaling model that describes the adsorption of diblock copolymer chains from good solvents and theta-solvents onto reactive surfaces of varying curvatures. To evaluate the impact of particle size on the adsorption process, we probed the adsorption of poly(styrene-b-methymethacrylate) (PS-PMMA) diblock copolymers from solvents with different degrees of selectivity on aluminum oxide (Al(2)O(3)) surfaces belonging to particles of different sizes. When the adsorbed PMMA layer is dense enough (in the case of a theta-solvent for the PMMA block), our results show good correlation between the theory and experimental results, pointing to the formation of a PMMA adsorption layer and a brushlike PS layer. Conversely, when adsorption occurs from a nonpreferential solvent, particularly on particles with high curvature, the PMMA adsorption layer at the surface becomes less dense and the grafted PS moiety exhibits a transitional morphology consisting of several layers of increasingly sparsely spaced blobs.

Fabrication of fibril like aggregates by self-assembly of block copolymer mixtures via interpolymer hydrogen bonding

This paper describes the formation of fibril like aggregates from the self-assembly of block copolymer mixture (polystyrene-b-poly(4-vinylpyridine) (PS-b-P4VP) and polystyrene-b-poly(acrylic acid) (PS-b-PAA)) via interpolymer hydrogen bonding in nonselective solvent. The hydrogen bonding between P4VP and PAA in chloroform leads to the formation of complex. When all the pyridine units in P4VP were all hydrogen bonded to acrylic acid in PAA, the formed complex is insoluble, resulting in the formation of spherical micellar aggregates and nanorods. However, two kinds of supramolecules with insoluble or soluble complex are formed in the solution when PS-b-P4VP and PS-b-PAA are mixed with equal mole ratio. The fibril like aggregates can be formed from the self-assembly of supramolecule with soluble complex during spin-coating process. The effects of evaporation rate of solvent and solution concentrations on the formation of fibril like aggregates were investigated. The results prove that the kinetic factors play an important role in the formation of the fibril like aggregates.

Core−Shell−Corona Micelles by PS-b-P2VP-b-PEO Copolymers: Focus on the Water-Induced Micellization Process

It is now well established that amphiphilic PS-b-P2VP-b-PEO linear triblock copolymers can form multilayered assemblies, thus core-shell-corona (CSC) micelles, in water. Micellization is triggered by addition of a small amount of water into a dilute solution of the PS-b-P2VP-b-PEO copolymer in a non-selective organic solvent. However, the phenomena that take place at the very beginning of this process are poorly documented. How these copolymer chains are perturbed by addition of water was investigated in this work by light and neutron scattering techniques and transmission electron microscopy. It was accordingly possible to determine the critical water concentration (CWC), the compactness of the nano-objects in solution, their number of aggregation, and their hydrodynamic diameter at each step of the micellization process.

UV-induced crosslinking of ring opening metathesis block copolymer micelles

Statistical and amphiphilic block copolymers bearing cinnamoyl groups were prepared by ring opening metathesis polymerization (ROMP). The UV-induced [2 + 2] cycloaddition reaction of polymer bound cinnamic acid groups was studied in polymer thin films as well as in block copolymer micelles. In both cases, exposure to UV-light for 10 min led to a crosslinking conversion of about 60%, as determined by FT-IR spectroscopy and UV-vis absorption measurements. Time based IR-spectroscopy revealed a maximum conversion of 78% reached after an irradiation time of about 16 min. For micelles obtained from polymers bearing 5 mol % or more cinnamoyl groups, the crosslinking reaction proceeded smoothly, yielding in crosslinked particles which were stable in a non-selective solvent (CHCl 3 ). Diameters determined by dynamic light scattering in the selective solvent (MeOH) were similar for both, non-crosslinked and crosslinked micelles, whereas diameters of crosslinked micelles in the non-selective solvent (CHCl 3 ) were significantly larger compared to MeOH samples. This strategy of direct self assembly of block-copolymers in a selective solvent followed by clean crosslinking, without the need for additional crosslinking reagents or crosslinking initiators, provides a straight forward approach toward ROMP-based polymeric nano-particles.

Three-Dimensional Analysis of Switching Mechanism of Mixed Polymer Brushes

The inner structure of a polystyrene−poly(methyl methacrylate) (PS/PMMA) brush synthesized via two-step surface-initiated radical polymerization was studied by a combination of step-by-step oxygen plasma etching and atomic force microscopy (AFM, “nanotomography” approach). The brush adopts ripple and dimple morphologies upon exposure to toluene and acetone, respectively. The same locations on the brush samples, adopting these two morphologies, were scanned with AFM before and after each etching step. We found that the ripple morphology consists of depressed PS-rich and elevated PMMA-rich elongated domains. In the dimple morphology, the core of the cluster is rich in PS, while the very top layer and the valleys between clusters are rich in PMMA. These experimental observations agree with self-consistent field theoretical predictions of the phase-segregated morphologies in nonselective and selective solvents. The 3D structures of the mixed brushes in dry and swollen states were reconstructed using nanotomography data.

Influence of polymer structure on adsorption behavior at solid–liquid interface by Monte Carlo simulation

Monte Carlo simulations for the adsorption of polymers including random copolymer, homopolymer, diblock copolymer and two kinds of triblock copolymers, respectively, in nonselective solvent at solid–liquid interface have been performed on a simple lattice model. The effect of polymer structure on adsorption properties was examined. In simulations, all polymeric molecules are modeled as self-avoiding linear chains composed of two segments A and B while A is attractive to the surface and B is non-attractive. It was found that for all polymers, the size distribution of various configurations is determined by the linked sequence of segments and the interaction energy between segment and surface. The results of simulation show that the adsorbed amount always increases with increasing bulk concentration but the adsorption layer thickness is mostly dependent on the adsorption energy at a fixed fraction of segments A. On the other hand, diblock copolymer has always the highest surface coverage and adsorbed amount, while random copolymers and homopolymers give generally the smallest surface coverage and adsorbed amount. It is shown that the sequence of polymer chains, i.e. molecular structure, is the most important factor in affecting adsorption properties at the same composition and interaction between segment and surface. The results also show that the adsorption behavior of random copolymers is remarkably different from that of block copolymers, but acting like homopolymer.

Microphase Separation Study of Amphiphilic ROMP Block Copolymers by SAXS and TEM

The self-assembly of block copolymers to form well-ordered structures is of great interest for the design of hierarchical materials. In this contribution, the morphologies and properties of well-defined block copolymers with narrow molecular weight distributions prepared by ring-opening metathesis polymerization are studied. Two series of amphiphilic AB-block copolymers poly[(endo,exo[2.2.1]bicyclohept-5-ene-2,3-dicarboxylic acid dimethyl ester)-b-(endo,exo[2.2.1]bicyclohept-5-ene-2,3-dicarboxylic acid)] of different block ratios and lengths were investigated by thermal analysis, small-angle X-ray scattering, and transmission electron microscopy. As observed by thermal gravimetric analysis and FTIR spectroscopy, the free carboxylic acid functionalities lead to anhydride formation at higher temperatures so that direct observation of the melt morphologies is not possible. However, by drop-casting polymer solutions in THF, which is a nonselective solvent for this type of block copolymer, well-ordered films w...