Diblock Copolymer Chains Research Articles

Micellar Formation and Organization in Thin Film Polymer Blends

We study the formation and organization of micelles, of polystyrene-b-poly(2-vinylpyridine) (PS-b-P2VP) diblock copolymer chains, in thin film hosts composed of (1) blends of long and short chain polystyrenes, PSL and PSS, of degrees of polymerization PL and PS, respectively (PS < PL), and (2) blends of PSL and a homopolymer with which it is compatible, tetramethyl bisphenol-A polycarbonate (TMPC), of degree of polymerization PTMPC (PTMPC < PL). The role of competing entropic and enthalpic interactions on the block copolymer micelle formation and organization is examined. We show that the average size of the micelle cores, Dcore, decreased with increasing weight fraction of the shorter chain PS component in PS(PS = 125)/PS(PL = 15400) mixtures, for PL≫NPS, NPS is the degree of polymerization of the PS block (the micelle corona). Dcore also decreased with increasing weight fraction of TMPC in TMPC(PTMPC = 122)/PS(PL = 15400) mixtures. The values of Dcore in the TMPC/PSL hosts were smaller than those in PSS/PSL mixtures, for the same TMPC, or PSS, weight fractions (PS ≈ PTMPC). This is due to more extensive mixing between the TMPC host chains, compared to the PSS chains, and the micelle corona. Furthermore, we show that the size and the organization of the micelles within the films may be controlled independently, through changing the relative fractions of PSS or TMPC and the value of PL. The fraction of TMPC, or PSS, mixed with the corona decreased as PL decreased from PL = 15400 to smaller values.

Dissipative particle dynamics simulation of the micellization–demicellization process and micellar shuttle of a diblock copolymer in a biphasic system (water/ionic-liquid)

We simulated the thermoreversible micellization–demicellization process and micellar shuttle of a poly(N-isopropylacrylamide-block-ethylene-oxide) (PNIPAM–PEO) diblock copolymer in a water/ionic-liquid (1-butyl-3-methylimidazolium hexafluorophosphate, [BMIM][PF6]) system by means of dissipative particle dynamics (DPD). The evolution of diblock copolymer chains (micellization–transfer–demicellization) in both water and the ionic liquid phase by the temperature effect reveals that it is a physical phenomenon, dependent on the solubility and interaction parameters of all chemical species involved in the multicomponent system. With the aid of a Monte Carlo simulation we calculated the Flory–Huggins interaction parameters χ of all the species. At room temperature the PNIPAM–PEO copolymer chains are miscible in the aqueous phase. At a higher temperature of T = 303 K the diblock copolymer shows the formation of micelles (micellization process). The micellar transfer to the ionic liquid phase was observed at T = 333 K. A further increase in temperature provokes the demicellization at T = 346 K. The process is reversible: reversing the temperature now to 333 K, shows the formation of the micelles. A further decrease in temperature makes the micelles go back to the water phase. All the simulation outcomes are qualitatively consistent with the experimental results, demonstrating that the DPD methodology may provide a tool for the investigation and analysis of the micellar transfer process in immiscible environments.

Thermodynamic Free Energy Behavior of Diblock Copolymer Chains Confined Between Planar Surfaces Having End-Tethered Flexible Polymer Molecules

A molecular model for the free energy of a confined system of diblock copolymer chains within a 2D slit with the interior surfaces having end-tethered chains is presented, based on a combined lattice and scaling theory approach. The thermodynamics of a model system, based on a constrained minimization of free energy, is explored as a function of the intermolecular energy parameters for interaction between the segments of block copolymer chains, end-tethered chains, and the surfaces. The effects of chain length and the block length ratio are investigated over a wide range of values. The results obtained are qualitative in nature; however, the model can be implemented to real systems provided appropriate parameterization of the model parameters to real systems can be performed. The phase diagrams obtained here provide ways for designing thermodynamically stable systems within the physical parametric variable space.

Conformational evolution under steady shear flow: a comparison between cyclic and linear block copolymers

Cyclic block copolymer is a special type of block copolymer having no free ends. Comparison of the dynamic behavior between cyclic and linear block copolymers enables an understanding of the role of chain ends in dynamics of the latter. In relation to this point, analysis was made on the conformational dynamics for a cyclic bead-spring type diblock copolymer chain, AoB, under the steady shear flow. Further comparison was made on the conformational behavior of the AoB chain and that of two symmetric linear triblock copolymer chains, A–B–A and B–A–B. For these chains, the mobility was set to be higher for the A segments than the B segments. Thus, for the AoB chain under the steady shear flow, the segments of the A block exhibit less orientational anisotropy than those of the B block. This orientational contrast is enhanced for the A–B–A chain partly because the constraint for the motion of the segments is less near the chain ends than near the chain center. Nevertheless, for the B–A–B chain, the segmental orientation over the A block becomes more anisotropic than that over the B block. Detailed analysis shows that this result is attributable to a high orientational correlation for the segments of two end B blocks, in particularly for those near the block junctions. The correlated B segments exert a tensile force on the A block thereby significantly enhancing the orientational anisotropy of the A segments.

Tethered-Polymer Structures in Thin Film Polymer Melts

We show that polystyrene-b-poly(2-vinylpyridine) (PS-b-P2VP) diblock copolymer chains aggregate to form micelles, composed of an inner P2VP core and an outer PS corona, as well as adsorb onto the substrate, forming brush layers, in supported thin film PS/PS-b-P2VP mixtures. The degrees of polymerization of the chains that comprised the corona and core were NPS and NP2VP, respectively. The diameter of the micelle cores, Dcore, increased with increasing degree of polymerization, P, of the PS host and became constant for large values of P. A decrease in the number density of micelles, nmicelle, accompanied the increase in Dcore; nmicelle reached a plateau in the same range of values of P where Dcore became constant. The organization of the micelles suggests the existence of attractive micelle–micelle interactions in the large P regime. Moreover, in this regime, the micelles preferentially migrated to the free surface. The morphology of this system is compared to thin film PS melts containing brush-coated nan...

EFFECT OF POLYDISPERSITY ON THE SELFASSEMBLY OF AMPHIPHILIC DIBLOCK COPOLYMER IN A SELECTIVE SOLVENT:A MONTE CARLO STUDY

The effect of polydispersity on the self-assembly of amphiphilic diblock copolymer in a selective solvent was studied by means of Monte Carlo simulation. The polydisperse diblock copolymer system was constructed via the mixing of three amphiphilic diblock copolymer chains different in length, i. e., A(2)B(2), A(2)B(6) and A(2)B(10). The simulation results show that the micelle morphologies formed by AB diblock copolymers in a selective solvent changed from vesicle to lamellae, then to spheres with an increase in polydispersity index (PDI) from 1. 0 to 1. 4. Through the analysis of the variations of the number and fractions of free single chains with the increase of PDI in each system, it is found that the number of short chains (i. e. A(2)B(2)) which aggregate into micelles increased with an increasing PDI. Meanwhile, the self-assembly processes of diblock copolymers with different PDI in a selective solvent reveal that the increasing number of short chains can increase the surface area of the micelles,leading to a disadvantage for the fusion between small micelles during the self-assembly. The increase of the surface area of micelles is the main reason of the morphological transition from large-sized vesicles to small-sized spheres when the PDI is increased. These simulation results indicate that the micelle morphologies and their mutual transitions in a selective solvent can be tuned by the control of the polydispersity of AB diblock copolymers.

The formation of core cross-linked star polymer and nanogel assemblies facilitated by the formation of dynamic covalent imine bonds

Reversible addition-fragmentation chain transfer (RAFT) polymerization has been utilised to prepare diblock copolymer chains possessing ‘inert’ blocks of polystyrene and ‘reactive’ blocks displaying aldehyde or amino functional groups. These polymer chains were shown to cross-link through the formation of imine bonds in organic solvents to form kinetically stable polymer assemblies possessing core-cross-linked star (CCS) polymers architectures which display a size-dependency upon the concentration at which the cross-linking reactions are performed. Evidence was found for the formation of a 2-armed species as a side-product in the formation of CCS polymers. RAFT polymerization was utilised to prepare a series of methacrylate copolymers possessing only ‘reactive’ blocks displaying a significantly lower density of aldehyde and amino functional groups than the diblock copolymers utilized in the formation of CCS polymers. These copolymers were shown to cross-link through the formation of imine bonds in organic solvents to form polymer assemblies possessing nanogel architectures. The lower density of functional groups was required to promote the formation of discrete nanogel assemblies. This cross-linking also displayed a size-dependency upon the concentration at which the cross-linking reactions are performed. Macroscopic gelation of these polymer chains was observed at higher concentrations of the copolymer chains. Both the CCS polymers and nanogels underwent structural reconfiguration to linear polymer chains through component exchange facilitated by trans-imination with a small molecule amine. We speculate that the formation of both the CCS polymers and nanogels is under kinetic control on account of the lack of reversibility of imine bonds in organic solvents.

Simulations of the gyroid phase in diblock copolymers with the Gaussian disphere model

Pure melts of asymmetric diblock copolymers are studied by means of the off-lattice Gaussian disphere model with Monte-Carlo kinetics. In this model, a diblock copolymer chain is mapped onto two soft repulsive spheres with fluctuating radii of gyration and distance between centers of mass of the spheres. Microscopic input quantities of the model such as the combined probability distribution for the radii of gyration and the distance between the spheres as well as conditional monomer number densities assigned to each block were derived in the previous work of F. Eurich and P. Maass [J. Chem. Phys. 114, 7655 (2001)] within an underlying Gaussian chain model. The polymerization degree of the whole chain as well as those of the individual blocks are freely tunable parameters thus enabling a precise determination of the regions of stability of various phases. The model neglects entanglement effects which are irrelevant for the formation of ordered structures in diblock copolymers and which would otherwise unnecessarily increase the equilibration time of the system. The gyroid phase was reproduced in between the cylindrical and lamellar phases in systems with box sizes being commensurate with the size of the unit cell of the gyroid morphology. The region of stability of the gyroid phase was studied in detail and found to be consistent with the prediction of the mean-field theory. Packing frustration was observed in the form of increased radii of gyration of both blocks of the chains located close to the gyroid nodes.

Synthesis and Characterization of Poly(methyl methacrylate)-b-Polystyrene/TiO2 Nanocomposites Via Reversible Addition-Fragmentation Chain Transfer Polymerization

A diblock copolymer, poly(methyl methacrylate)-b-polystyrene (PMMA-b-PS), was grafted onto the surface of nano-titania (nano-TiO2) successfully via reversible addition-fragmentation chain transfer (RAFT) polymerization. The surface of TiO2 nanoparticles was modified initially by attaching dithioester groups to the surface using silane coupling agent 3-(chloropropyl)triethoxy silane and sodium ethyl xanthate. The polymerization of methyl methacrylate and styrene were then initiated and propagated on the TiO2 surface by RAFT polymerization. The resulting composite nanoparticles were characterized by means of XPS, FT-IR, 1H NMR and TGA. The results confirmed the successful grafting of poly(methyl methacrylate) (PMMA) and diblock copolymer chains onto the surface of TiO2. The amount of PMMA grafted onto the TiO2 surface increased with the polymerization time. Moreover, the kinetic studies revealed that the ln([M]0/[M]), where [M]0 is the initial and [M] is the time dependent monomer concentrations, increased linearly with the polymerization time, indicating the living characteristics of the RAFT polymerization.

Cubic to Hexagonal Phase Transition Induced by Electric Field

The possibility of electric field induced phase transitions in soft matter systems was studied by means of small-angle X-ray (SAXS) and neutron (SANS) scattering measurements. By dissolving a diblock copolymer PS-b-PEP (polystyrene-block-poly(ethylene-co-propylene)) in a mixture of cyclohexane (CH) and dimethylformamide (DMF), it was possible to create a liquid 3D cubic structure in which spherical microdomains of DMF were embedded into a liquid CH (major component) matrix with the liquid−liquid interfaces covered by PS-b-PEP diblock copolymer chains. When sited under an external electric field, the experimental SAXS and SANS results revealed that the initial self-organized 3D cubic structure is converted into an hexagonal arrangement. The order-to-order transition was reached by the application of a relatively low dc electric field, ∼1.25 kV/mm. The electric field generates dipole moments in DMF-rich spherical microdomains that are deformed and further interconnected, leading to the formation of the hexagonal packed cylinders. The electric field strength Et needed to induce such transition depends on the magnitude of the generated dipole moment in the DMF-rich spherical microdomains and hence depends on their size and dielectric contrast irrespective of the surrounding liquid. The latter must have the lower dielectric constant for the transition to occur. Et also increases with increasing block copolymer concentration. The chain statistics does not change at the transition and always corresponds to that of a polymer in good solvent. The HEX-cylinders phase developed under external electric field is unstable, and as soon as the field is switched off, the cylinders undergo an order-to-order transition back to the cubic phase. Finally, another HEX-cylinders phase thermodynamically stable without electric field was created by dissolving a PS-b-PI diblock copolymer (polystyrene-block-polyisoprene) in a CH−DMF mixture. In this last case, the structure essentially does not feel the presence of an electric field of the same magnitude.

Non-Centrosymmetric Lamellar Structures in the Associating Blends of Tri- and Diblock Copolymers

We consider lamellar morphologies in the mixtures of AC diblock and ABC triblock copolymers and show that a proper functionalization of the end blocks of the triblock and diblock copolymer chains by donor and acceptor functional groups, respectively, could result in occurrence of the thermodynamically stable noncentrosymmetric lamellar structures in such mixtures. Depending on the chemical nature of the donor-acceptor bond (i.e., the values of the effective entropy Sbond and energy E per one bond), the typical phase diagrams could contain the upper and lower triple points where the pure diblock, pure triblock and noncentrosymmetric lamellae of a finite composition coexist. With increase of the association strength the triple points merge and the noncentrosymmetric lamellae become thermodynamically favorable at all temperatures. The boundary on the (Sbond ,E)-plane, which separates the regions corresponding to different types of the phase behavior, is found. The influence of the architecture of the system and the chemical nature of the donor-acceptor bond on the stability of the noncentrosymmetric lamellae is analyzed and typical phase diagrams are presented.

Surface-independent vertical orientation of cylindrical microdomains in block copolymer thin films directed by comb-coil architecture

Vertically oriented cylindrical microdomains of block copolymer thin films consisting of polystyrene and poly(methyl methacrylate) were fabricated regardless of the substrate by introduction of a grafted architecture into the diblock copolymer chains. A series of comb-coil block copolymers, poly(methyl methacrylate)-b-poly(2-(2-bromopropionyloxy)-ethyl acrylate)-g-polystyrene (PMMA-b-PBPEA-g-PS) with various lengths of PS were synthesized by combination of reversible addition-fragmentation chain transfer (RAFT) polymerization and atom transfer radical polymerization (ATRP). When the volume fraction of PS was 75%, microphase separation produced cylindrical microdomains of PMMA surrounded by PS matrix in a thin film state after thermal annealing. Analysis of the thin films after subsequent etching of PMMA domains by atomic force microscopy, cross-sectional scanning electron microscope images and grazing incidence X-ray scattering measurements showed that the microdomains were oriented perpendicular to various substrates including metals, silicon, polymers, and even patterned surfaces. Steric repulsion between the grafted chains during the phase separation was attributed to the driving force for perpendicular orientation of the cylindrical microdomains without the aid of external fields.

Polymer−Cisplatin Conjugate Nanoparticles for Acid-Responsive Drug Delivery

We report the synthesis of novel acid-responsive therapeutic nanoparticles (NPs) with sub-100 nm size consisting of polymer--cisplatin conjugates. The uniqueness of these drug delivery polymeric NPs lies in the covalent conjugation of each cisplatin drug to the hydrophobic segment of two biocompatible diblock copolymer chains through a hydrazone bond, resulting in highly differential drug release profile at different environmental acidity. We demonstrate that the synthesized polymer--cisplatin conjugates can readily precipitate to form sub-100 nm NPs in aqueous solution due to their very low critical micelle concentration (CMC). The resulting NPs show well-controlled cisplatin loading yield, excellent acid-responsive drug release kinetics, and enhanced in vitro cytotoxicity against ovarian cancer cells as compared to free cisplatin. As an environmentally sensitive drug delivery vehicle, these NPs can potentially minimize the drug loss during NP circulation in the blood, where the pH value is neutral, and trigger rapid intracellular drug release after the NPs are endocytosed by the target cells. This characteristic drug release profile holds the promise to suppress cancer cell chemoresistance by rapidly releasing a high dose of chemotherapy drugs inside the tumor cells, thereby improving the therapeutic efficacy of the drug payload.

"Smart" diblock copolymers as templates for magnetic-core gold-shell nanoparticle synthesis.

We report a new strategy for synthesizing temperature-responsive gamma-Fe(2)O(3)-core/Au-shell nanoparticles (Au-mNPs) from diblock copolymer micelles. The amphiphilic diblock copolymer chains were synthesized using reversible addition-fragmentation chain-transfer (RAFT) with a thermally responsive "smart" poly(N-isopropylacrylamide) (pNIPAAm) block and an amine-containing poly(N,N-dimethylaminoethylacrylamide) (DMAEAm) block that acted as a reducing agent during gold shell formation. The Au-mNPs reversibly aggregated upon heating the solution above the transition temperature of pNIPAAm, resulting in a red-shifted localized surface plasmon resonance.

Click Coupling Fullerene onto Thermoresponsive Water-Soluble Diblock Copolymer and Homopolymer Chains at Defined Positions

We report on the synthesis of well-defined thermoresponsive water-soluble diblock copolymer and homopolymers functionalized with controlled numbers of C60 moieties at predetermined positions via the combination of atom transfer radical polymerization (ATRP) and click chemistry. Azide-containing polymer precursors including monoazide-terminated and α,α-diazide-terminated poly(N-isopropylacrylamide), N3- PNIPAM and (N3)2-PNIPAM, as well as poly(ethylene glycol)-b-PNIPAM with one azide moiety at the diblock junction, PEG(-N3)-b-PNIPAM, were synthesized via ATRP using specific azide-functionalized small molecule and polymeric initiators. On the other hand, the reaction of 4-prop-2-ynyloxybenzaldehyde with pristine C60 in the presence of glycine afforded alkynyl-modified C60, alkynyl-C60. Subsequently, the click reaction of N3-PNIPAM, (N3)2-PNIPAM, and PEG(-N3)-b-PNIPAM led to the facile preparation of thermoresponsive diblock copolymer and homopolymers functionalized with controlled numbers of C60 at designed positions, including C60-PNIPAM, (C60)2-PNIPAM, and PEG(-C60)-b-PNIPAM. All the intermediate and final products were characterized by 1H NMR, Fourier transform infrared spectroscopy (FT-IR), UV−vis spectroscopy, thermogravimetric analysis (TGA), and gel permeation chromatograph (GPC) equipped with UV/RI dual detectors. C60-containing hybrid nanoparticles were then fabricated via supramolecular self-assembly of C60-PNIPAM, (C60)2-PNIPAM, and PEG(-C60)-b-PNIPAM in aqueous solution, which were characterized by dynamic and static laser light scattering (LLS) and transmission electron microscopy (TEM). These novel fullerenated polymers retain the thermoresponsiveness of PNIPAM-based precursors, and self-assembled hybrid nanoparticles exhibit thermo-induced collapse/aggregation behavior due to the lower critical solution temperature (LCST) phase transition of PNIPAM chains.

Conformation of Single Block Copolymer Chain in Two-Dimensional Microphase-Separated Structure Studied by Scanning Near-Field Optical Microscopy

The localization and orientation of the symmetric diblock copolymer chain in a quasi-two-dimensional microphase-separated structure were studied by scanning near-field optical microscopy (SNOM). In the monolayer of poly(isobutyl methacrylate)-block-poly(octadecyl methacrylate) (PiBMA-b-PODMA), the individual PiBMA subchains were directly observed by SNOM, and the center of mass (CM) and orientational angle relative to the phase interface were examined at the single chain level. It was found that the position of the CM and the orientation of the PiBMA subchain in the lamellar structure were dependent on the curvature of the PiBMA/PODMA interface. As the interface was bent toward the objective chain, the block chain preferred the CM position closer to the domain center, and the conformation was strongly oriented perpendicularly to the domain interface. With increase of the curvature, the steric hindrance among the block chain increases, resulting in the stretched conformation.

Conformational behavior of diblock comb copolymers

The conformational behavior of a single diblock copolymer chain with comb blocks is studied by computer simulation. The comb blocks have equal lengths of the backbones and side chains and differ only by the degree of grafting of the side chains. The collapse behavior is studied for two cases: (i) comb diblock copolymer with attracting backbones of the blocks and swollen side chains and (ii) comb diblock copolymer with attracting monomer units of the side chains of the blocks and swollen backbones. In both cases the conformation of a globule with a tail is observed when one of the block collapses while the other stays swollen. At high attraction micellar-type globules are formed. The core of the micelle consists of attracting monomeric units and the soluble segments (side chains or backbones) comprise the corona. It is shown that separation of monomeric units belonging to different blocks can take place both in the core and in the corona regions of the micelle.

Strain‐Induced Phase Morphology in Melt Drawn Ultrathin Highly Oriented Block Copolymer Films

AbstractA strain‐induced microphase morphology has been established by the melt drawing process in a high molecular weight asymmetric polystyrene‐block‐poly(vinyl‐2‐pyridine) (PS‐b‐P2VP) diblock copolymer. For the first time to the best knowledge of the authors, the melt drawing process has been applied to block copolymers to produce free‐standing, ultrathin block copolymer films with a thickness of ≈100 nm. Intriguingly, during the melt drawing of the polymer a global strain‐induced unidirectional order of the microphase separated needle‐like domains of the block copolymer was generated. This morphology consists of a PS matrix with embedded highly oriented P2VP needle‐like domains oriented parallel to the drawing direction. The needle‐like morphology is explained by a simplified extended chain model of the diblock copolymer chains. Annealing of the films leads to a transition from the strain‐induced needle‐like morphology toward the quasi‐equilibrium sphere‐like morphology.magnified image

Nanomechanical Fingerprints of Individual Blocks of a Diblock Copolymer Chain

Atomic force microscopy (AFM)-based single polymer chain pulling experiments have been used to study the structural transitions of individual homopolymer chains in water. Polystyrene (PS) showed a three-regime force-extension profile exhibiting a force plateau reminiscent of a first-order transition, as predicted theoretically, whereas poly(methyl methacrylate) (PMMA) showed a characteristic saw-tooth pattern reminiscent of multidomain disassembly behavior. The two distinct structural transtions provide fingerprints for the individual homopolymers, which can be used to identify individual blocks of symmetric and asymmetric PS- b-PMMA diblock copolymer chains.

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.