Block Copolymer Composition Research Articles

Formation of phase structure and crystallization behavior in blends containing polystyrene–polyethylene block copolymers

The microphase separation structure in the molten state and the structure formation in crystallization from such ordered melt were investigated for the blends of polystyrene–polyethylene block copolymers (SE) with polystyrene homopolymer (PS) and polyethylene homopolymer (PE) and for the blends consisting of two kinds of SE with different copolymer compositions from each other, using synchrotron small-angle X-ray scattering techniques (SAXS). The copolymer compositions of SE block copolymers employed were 0.34, 0.58 and 0.73 wt. fraction of PE, and their melt morphologies were cylindrical, lamellar and lamellar, respectively. Macrophase separation or the morphology change in the melt occurred depending on the molecular weight and the blend composition, as reported so far. In crystallization from such macrophase-separated and microphase-separated melts, the melt morphology was completely kept for all the blends. Crystallization behavior was also investigated for the blends. The crystallization within the spherical and cylindrical domains surrounded by glassy PS was not observed for SE/PS blends. In the crystallization from the macrophase-separated melt, two exothermal peaks were observed in the DSC measurements, while a single peak was observed for other blends. For the blends with PS, the degree of crystallinity was depressed and the apparent activation energy of crystallization was high, compared to those for the corresponding neat SE. For SE/PE and SE/SE blends, those were changed depending on the blend composition.

Creating Surfactant Nanoparticles for Block Copolymer Composites through Surface Chemistry

A simple strategy to tailor the surface of nanoparticles for their specific adsorption to and localization at block copolymer interfaces was explored. Gold nanoparticles coated by a mixture of low molecular weight thiol end-functional polystyrene (PS-SH) (Mn = 1.5 and 3.4 kg/mol) and poly(2-vinylpyridine) homopolymers (P2VP-SH) (Mn = 1.5 and 3.0 kg/mol) were incorporated into a lamellar poly(styrene-b-2-vinylpyridine) diblock copolymer (PS-b-P2VP) (Mn = 196 kg/mol). A library of nanoparticles with varying PS and P2VP surface compositions (FPS) and high polymer ligand areal chain densities was synthesized. The location of the nanoparticles in the PS-b-P2VP block copolymer was determined by transmission electron microscopy. Sharp transitions in particle location from the PS domain to the PS/P2VP interface, and subsequently to the P2VP domain, were observed at FPS = 0.9 and 0.1, respectively. This extremely wide window of FPS values where the polymer-coated gold nanoparticles adsorb to the interface suggests a redistribution of PS and P2VP polymers on the Au surface, inducing the formation of amphiphilic nanoparticles at the PS/P2VP interface. In a second and synthetically more challenging approach, gold nanoparticles were covered with a thiol terminated random copolymer of styrene and 2-vinylpyridine synthesized by RAFT polymerization. Two different random copolymers were considered, where the molecular weight was fixed at 3.5 kg/mol and the relative incorporation of styrene and 2-vinylpyridine repeat units varied (FPS = 0.52 and 0.40). The areal chain density of these random copolymers on Au is unfortunately not high enough to preclude any contact between the P2VP block of the block copolymer and the Au surface. Interestingly, gold nanoparticles coated by the random copolymer with FPS = 0.4 were dispersed in the P2VP domain, while those with FPS = 0.52 were located at the interface. A simple calculation for the adsorption energy to the interface of the nanoparticles with different surface arrangements of PS and P2VP ligands supports evidence for the rearrangement of thiol terminated homopolymers. An upper limit estimate of the adsorption energy of nanoparticles uniformly coated with a random arrangement of PS and P2VP ligands where a 10% surface area was occupied by P2VP -mers or chains was approximately 1 kBT, which indicates that such nanoparticles are unlikely to be segregated along the interface, in contrast to the experimental results for nanoparticles with mixed ligand-coated surfaces.

Effect of Molecular Properties of Block Copolymers and Nanoparticles on the Morphology of Self-Assembled Bulk Nanocomposites

Self-assembly of thiol-terminated polystyrene-tethered Au nanoparticles in microphase-separated diblock copolymers composed of poly(styrene-b-2vinylpyridine) (PS−PVP) as a function of particle concentration and composition of block copolymers was investigated using anomalous small-angle X-ray scattering (ASAXS) and transmission electron microscopy (TEM). Results reveal that the self-assembly of nanoparticles in the PS domain causes swelling and increases the interfacial curvature that, in turn, induces order−order transitions. At intermediate loading, the presence of nanoparticles amplifies the local compositional fluctuations, hence the roughness at the PS and PVP interface, which creates conditions to induce disorder in the polymer morphology. The system thus undergoes an order−disorder transition. At high particle loading, packing constraints prevent all particles from assembling in the PS domain, and the excess nanoparticles undergo macrophase separation. The present systematic study augments experimental data to the scarce literature on the phase behavior of bulk nanocomposites. We present a generalized phase map for the bulk composites as a function of effective volume fraction of PS (FPS) for a given nanoparticle size. We believe that the results from this study will enable comparison of the phase maps from various studies and will serve to validate the simulation studies of inorganic particle/block copolymer composites.

Development and Optical Properties of Cadmium Sulfide and Cadmium Selenide Nanoparticles in Amphiphilic Block Copolymer Micellar-like Aggregates

CdS and CdSe nanoparticles were prepared in organic solvents using as templates inverse amphiphilic block copolymer micellar aggregates, which were induced by cadmium complexation. Amphiphilic block copolymers utilized in this work include poly(sulfonated styrene-block-tert-butylstyrene) (SPS−PtBS) and poly(styrene-block-sulfonated isoprene) (PS−SPI). Copolymers with different compositions and molecular weights were synthesized by anionic polymerization high-vacuum techniques and postpolymerization selective sulfonation reactions. Cadmium acetate was used as the cadmium precursor and thioacetamide or H2Se as the sulfur or selenium source, respectively. The nanostructured organic−inorganic hybrid materials were characterized in solution and in thin films by light scattering, UV−vis and fluorescence spectroscopy, infrared and Raman vibrational spectroscopy, and transmission electron microscopy (TEM). The influence of composition and molecular weight of block copolymers on the properties of nanoparticles was investigated at all stages of nanoparticle formation. It was shown that well-defined CdS and CdSe semiconductor nanoparticles with controllable sizes and optical properties can be formed by judicious choice of the experimental conditions following the suggested preparation protocol.

Toughness Properties of Lightly Crosslinked Epoxies Using Block Copolymers

AbstractSummary: In this work is discussed an alternative approach to the toughening of epoxy networks by the addition of acrylic block copolymers, composed of rigid and rubbery blocks. Once the reaction is completed, the initial self‐assembly of block copolymers in epoxy thermoset precursors produces rubbery domains: depending on the block copolymer structure and composition, these domains are of the micrometer or the nanometer size. Nanostructures are obtained when the rigid block is a random copolymer of methylmethacrylate and N,N‐dimethylacrylamide. The rubbery domains prevent rapid crack propagation and the highest toughness is obtained with filament‐like microparticles or wormlike micelles.

Self-Assembly in Mixed Aqueous Solutions of Amphiphilic Block Copolymers and Vesicle-Forming Surfactant

The self-assembly behavior of mixed solutions consisting of poly(isoprene-b-ethylene oxide) (IEO) copolymer micelles and vesicle-forming didodecyldimethylammonium bromide (DDAB) was investigated. Dynamic light scattering indicated the presence of two populations of nanoassemblies in the solutions. By aid of atomic force microscopy, the larger ones were identified as block copolymer modified surfactant vesicles (BCMSVs) and the smaller ones as surfactant-modified block copolymer micelles (SMBCMs). This identification is based on the amphiphilic character of the low and high molecular weight molecules and the notion that exchange of unimers of both types can take place between the initial nanoassemblies in aqueous solution. Electrophoretic light scattering experiments showed that the nanostructures carry positive charges originating from the surfactant. The sizes of the nanoassemblies depend on the relative concentrations of both components. The behavior of the mixed systems was also found to depend on block copolymer composition and temperature. Nanoassemblies of smaller sizes were formed at higher temperatures. BCMSVs and SMBCMs are thermosensitive, in contrast to the temperature stability of pure block copolymer micelles. On the other hand, BCMSVs showed lesser sensitivity to temperature increase compared to the pure DDAB vesicles. This indicates that incorporation of macromolecules into the DDAB bilayer increases the stability of the vesicles.

Block copolymers of dodecafluoroheptyl methacrylate and butyl methacrylate by RAFT miniemulsion polymerization

AbstractReversible addition‐fragmentation chain transfer (RAFT) miniemulsion polymerization of butyl methacrylate (BMA) and dodecafluoroheptyl methacrylate (DFMA) was carried out with 2‐cyanoprop‐2‐yl dithiobenzoate (CPDB) as chain transfer agent (CTA). Concentration effects of RAFT agent and initiator on kinetics and molecular weight were investigated. No obvious red oil layer (phase's separation) and coagulation was observed in the first stage of homopolymerization of BMA. The polymer molecular weights increased linearly with the monomer conversion with polydispersities lower than 1.2. At 75 °C, the monomer conversion could achieve above 96% in 3 h with [momomer]:[RAFT]:[KPS] = 620:4:1 (mole ratio). The results showed excellent controlled/living polymerization characteristics and a very fast polymerization rate. Furthermore, the synthesis of poly(BMA‐b‐DFMA) diblock copolymers with a regular structure (PDI < 1.30, PMMA calibration) was performed by adding the monomer of DFMA at the end of the RAFT miniemulsion polymerization of BMA. The success of diblock copolymerization was showed by the molecular weight curves shifting toward higher molar mass, recorded by gel permeation chromatography before and after block copolymerization. Compositions of block copolymers were further confirmed by 1H NMR, FTIR, and DSC analysis. The copolymers exhibited a phase‐separated morphology and possessed distinct glass transition temperatures associated with fluoropolymer PDFMA and PBMA domains. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 1585–1594, 2007

Controlling the Composition of Block Copolymers

Controlling the Composition of Block Copolymers

Surface nanopatterning of phosphorylcholine unit by well-defined block copolymer containing 2-methacryloyloxyethyl phosphorylcholine

Ordered surface patterning of biocompatible microdomain is potentially needed in many applications such as high sensitive biochips, biosensors, and nanobio devices for cell culturing. One of the efficient ways in the fabrication of patterned surface is using the microphase separation of well-defined block copolymers by controlling molecular weight and film formation process. Thus we synthesize several compositions of block copolymers containing 2-methacryloyloxyethyl phosphorylcholine (MPC) which is hydrophilic and shows excellent biocompatibility, and polydimethylsiloxane (PDMS) which has hydrophobic nature and usually used as soft lithography materials. All of the copolymers were synthesized by the atom transfer radical polymerization (ATRP) method at room temperature. The kinetics plot and molecular weight plot shows that the polymerizations of block copolymers were well controlled, thus the compositions of block copolymers were controllable. Two kinds of synthesized copolymers: the longest MPC segment and the shortest one, were cast on Si(100) substrate by spin casting or solvent casting. By using the IR-multichannel viewer and atom force microscope, we could conclude that the film formed by the longest MPC segment could not generate the well defined surface structure due to its long chain aggregation. By using the copolymer containing the shortest MPC segment, we could generate a nanoscale ordered surface patterning of phorsphorylcholine unit.

Nematic and Domain Order Parameters for Partially Oriented Isotropic/Liquid Crystalline Diblock Copolymers: A Dielectric Spectroscopy Study

Dielectric relaxation in a liquid crystalline (LC) side chain polymer and in a series of diblock copolymers with polystyrene is investigated using dielectric spectroscopy. The block copolymer morphology is cylindrical with the LC block forming the continuous matrix. Two processes are found in the nematic state with their relative dielectric relaxation strength strongly dependent on block copolymer composition. The spectra are described using the concept of a local order parameter S and a domain order parameter SD. Application of electric or mechanical fields induce a homeotropic orientation of the nematic domains in the homopolymer without significantly influencing the local order parameter. The block copolymers display a spontaneous orientation even in thick films (d < 300 nm).

Effect of Hydrophobicity inside PEO−PPO−PEO Block Copolymer Micelles on the Stabilization of Gold Nanoparticles: Experiments

In this paper we present the effect of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) block copolymer micelles and their hydrophobicity on the stabilization of gold nanoparticles. Gold nanoparticles were prepared by a method developed by Sakai et al. (Sakai, T.; Alexandridis, P. Langmuir 2004, 20, 8426). An absorption centered at 300-400 nm in time-dependent UV spectra provided evidence that the very first step of the synthesis was to form primary gold clusters. Then the gold clusters grew in size and were stabilized by block copolymer micelles. The stabilization capacities of the micelles were modulated by tuning the block copolymer concentration and composition and by adding salts. With good stabilization, gold particles were spherical and uniform in size with a diameter of 5-10 nm. Otherwise they were aggregates with irregular shapes such as triangular, hexagonal, and rodlike. The presence of a small amount of NaF significantly increased the stabilization capacity of the micelles and consequently modified the quality of the gold particles. Using FTIR and 1H NMR spectroscopy, micellization of the block copolymers and hydrophobicity of the micelles were proven very important for the stabilization. A higher hydrophobicity of the micelle cores was expected to favor the entrapment of primary gold clusters and the stabilization of gold nanoparticles.

Synthesis and properties of novel macromolecular coupling agents prepared by ATRP

AbstractNovel block and graft macromolecular coupling agents with well‐defined structures have been synthesized successfully by atom transfer radical polymerization (ATRP). The molecular weights and molecular weight distributions of those functional copolymers can be controlled because of the living/controlled ATRP. The structures and composition of block and graft copolymers with the monomers of butyl acrylate (BA), styrene (St), and 3‐methacryloxyl‐propyltrimethoxyl silicon (KH‐570) have been characterized by mean of 1H NMR, IR, GPC, and UV. Because the KH‐570 of macromolecular coupling agents owns strong affinity to surface hydroxyl of fillers, inorganic fillers that were treated by the macromolecular coupling agents possess some new properties, for example increasing the effect between matrix and fillers of composites. The mechanical properties and morphology of composites with macromolecular coupling agents have been investigated by the dynamic mechanical spectra and SEM. The results showed that the damping value and compatibility of composites with macromolecular coupling agents were improved greatly when compared with that of composite with small molecular coupling agent. Furthermore with different contents of BA, St, and KH‐570 in the macromolecular coupling agents, the damping value of the composites are different. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 3919–3926, 2006

Precise Tuning of Micelle, Core, and Shell Size by the Composition of Amphiphilic Block Copolymers Derived from ROMP Investigated by DLS and SAXS

Self-assembly of well-defined block copolymers in solution has gained attention not only for its scientific value but also for its potential application in nanotechnology. In this study, we present a comprehensive series of synthesized block copolymers that allows conclusions to be drawn how to design micelles by ROMP with defined core−shell geometry and controlled size. We used a general route for the preparation of well-defined block copolymers by ring opening metathesis polymerization (ROMP) with “Grubbs first generation catalyst” RuCl2(PCy3)2(CHPh) (Cy = cyclohexyl). In a first step, we sequentially polymerized endo,exo[2.2.1]bicyclo-2-ene-5,6-dicarboxylic acid dimethylester with endo,exo[2.2.1]bicyclo-2-ene-5,6-dicarboxylic acid di-tert-butyl ester, to obtain a high control of polymerization and complete characterization of the block copolymers. The polymers were characterized by 1H- and 13C NMR spectroscopy, FT-IR spectroscopy, by GPC, and by DSC. By cleavage of the tert-butyl group, the polymer was...

Preparation of poly(3-hydroxybutyrate- co-3-hydroxyvalerate) (PHBV) particles in O/W emulsion

In the present paper, we investigate the preparation of polymeric particles based on the biodegradable polyester poly(3-hydroxybutyrate- co-3-hydroxyvalerate) (PHBV). A new technique for PHBV particle preparation has been developed. This method utilizes the thermoreversible gelation of PHBV in toluene. Particles have been obtained by the secondary dispersion technique in a three-step procedure: (a) preparation of PHBV solution in toluene; (b) preparation of O/W emulsion by ultrasound followed by the gel formation in toluene/PHBV droplets; and (c) toluene extraction. In the present study we investigated the influence of the stabilizer type and its concentration in the aqueous phase, ultrasound power, and PHBV concentration in toluene on the size and stability of the formed droplets as well as the final PHBV particles. It has been found that PEO/PS block copolymers are the best stabilizers for the present system as compared to conventional tensides such as SDS or CTAB. It has been found that PEO/PS block copolymers allow obtaining PHBV particles with a regular shape and controlled dimensions after toluene extraction. The minimal size of the PHBV particles obtained by this technique was ca. 100 nm. The obtained particles exhibit a relatively broad particle size distribution and the particle shape is strongly affected by the block copolymer composition, ultrasound power and the way of toluene extraction.

Noncovalently Functionalized Block Copolymers Possessing Both Hydrogen Bonding and Metal Coordination Centers

Block copolymers containing both hydrogen bonding and metal coordination sites have been synthesized by ring-opening metathesis polymerization and subsequently functionalized using noncovalent interactions. The resulting block copolymers can be viewed as “universal polymer backbones”, as a wide variety of polymers with varying functionalities can be prepared by altering the noncovalent functionalization strategy of the same polymer backbone. The effect of degree of polymerization, block copolymerization, block copolymer composition, and metal coordination on the hydrogen bonding interaction has been investigated. In general, none of these variables have a profound effect on the strength of the hydrogen bonding interactions along the polymer backbones, suggesting that the metal coordination and hydrogen bonding are orthogonal to each other in block copolymers. Finally, the effect of the noncovalent functionalization on the thermal properties of the polymers was investigated. We found that the noncovalent functionalization of all copolymers via hydrogen bonding and/or metal coordination reduced the glass-transition temperature and the thermal stability of all copolymers.

Magnetic investigations of titanium‐doped gamma iron oxides dispersed in polymers

AbstractThe preparation and characterization of titanium‐doped iron oxides–styrene isoprene styrene block copolymer composites is reported. The precursor, as‐synthesized nanoparticles, contains akaganeite, maghemite, and titanium‐doped maghemite. Hysteresis loops obtained by SQUID measurements revealed an exchange biased field assigned to the presence of akaganeite nanoparticles. It was found that the coercive field and the exchange bias field decreases as the temperature of the sample is increased. The temperature dependence of the magnetization revealed a blocking temperature of about 220 K. The ESR spectra in the temperature range 150 K to about 300 K are single broad resonance lines. It was observed that the resonance line position is strongly temperature dependent due to the combined action of the external and molecular magnetic fields. The decrease of the resonance line width as the temperature is increased is governed by the reorientation of the magnetization. Thermally activated movements of magnetic nanoparticles within the soft phase (polyisoprene) of the matrix may also contribute to resonance line narrowing. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 3432–3437, 2005

Surface-grafted block copolymer gradients: Effect of block length on solvent response

We outline a method to fabricate gradient combinatorial libraries that explore architectural parameters of surface-grafted block copolymers (BCs). In addition, we demonstrate the utility of such libraries for the rapid, thorough assessment of the response of grafted BCs to solvent exposure. Our fabrication route uses surface-initiated controlled radical polymerization to produce a tethered polymer block with uniform length (in this case, poly( n-butyl methacrylate), PBMA), followed by a graded synthesis that adds a second block that varies in its length over the library (here, poly(2-( N,N′-dimethylamino)ethyl methacrylate), PDMAEMA). Our demonstration study maps the response of PBMA and PDMAEMA blocks to hexane and water, and defines regimes of behavior to this respect. Moreover, our study illuminates a narrow BC composition window that exhibits the strongest possible response to water and hexane treatment.

In vitro degradation and controlled release behavior of d, l-PLGA50 and PCL-b- d, l-PLGA50 copolymer microspheres

Blank and bovine serum albumin (BSA)-loaded microspheres based on poly(lactic-acid- alt-glycolic acid) ( d, l-PLGA50) and poly(ε-caprolactone)-b-poly(lactic-acid- alt-glycolic acid) (PCL-b- d, l-PLGA50) were successfully fabricated using water-in-oil-in-water (w/o/w) double-emulsion extraction/evaporation technique. In vitro degradation of the blank microspheres was characterized by techniques including nuclear magnetic resonance ( 1H NMR), gel permeation chromatography (GPC), differential scanning calorimetry (DSC) and scanning electron microscopy (SEM). The PCL-b- d, l-PLGA50 copolymer ( M n: number-average molecular weight, M w: weight-average molecular weight, M n = 44800, M w / M n = MWD = 1.24, ε-caprolactone (CL) % = 20.4% in molar ratio) had similar rate of molecular weight reduction compared with the d, l-PLGA50 copolymer before 5 weeks of in vitro degradation. The BSA % loading efficiency of microspheres was mainly controlled by both block copolymer composition and macromolecular architecture, while the sequence structure and the molecular weight of copolymer had no apparent effect on it. Significantly, The PCL-b- d, l-PLGA50 copolymer microspheres showed good release profiles with a nearly constant release during 20−110 days.

Unique Toroidal Morphology from Composition and Sequence Control of Triblock Copolymers

The mechanism by which the unique toroidal supramolecular assemblies were formed for triblock copolymers of acrylic acid (AA), methyl acrylate (MA), and styrene (S), PAA99-b-PMA73-b-PS66, was probed in this study by investigating the influences of the block copolymer compositions and sequences. Two triblock copolymers, PAA99-b-PMA73-b-PS66 and PAA99-b-PS76-b-PMA62, and two diblock copolymers, PAA99-b-PMA155 and PAA99-b-PS133, were studied under experimental solution-state conditions that involved a range of solvent/nonsolvent (tetrahydrofuran/water) compositions, each in the presence of 2,2'-(ethylenedioxy)bis(ethylamine). The resulting morphologies were determined by transmission electron microscopy. The failures to afford toroidal supramolecular assemblies from both diblock copolymers having comparable lengths of the total hydrophobic chain segment, either entirely PMA or entirely PS, and from the triblock copolymer having a reversed connection sequence for the hydrophobic (PMA and PS) segments demonstrate the unique self-assembly behaviors of triblock copolymers and the importance of the block copolymer sequence.

Investigation of the micropolarity in reverse micelles of nonionic poly(ethylene oxide) surfactants using 4-nitropyridine- N-oxide as absorption probe

The micropolarities of the reverse micelle (RM) interior of nonionic poly(ethylene oxide) surfactants of the alkyl ether type (poly(ethylene oxide)[4] lauryl ether (C 12E 4, Brij 30)), alkyl–aryl ethers (poly(ethylene oxide)[4] nonylphenyl ether (C 9ΦE 4), poly(ethylene oxide)[5] nonylphenyl ether (C 9ΦE 5), and poly(ethylene oxide)[5] octylphenyl ether (C 8ΦE 5)), and poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) (PEO–PPO–PEO) triblock copolymers (Pluronics P123, F127) were investigated as a function of the water content by applying the absorption probe technique, using 4-nitropyridine- N-oxide (NP) as a probe. The change in the micellar aggregate micropolarity in different solvents (cyclohexane, decane, n-butanol, and n-butyl acetate) at various water contents has been investigated. The research was focused on the determination of the effects of surfactant structure and solvent type on the hydration degrees of the PEO chains in the region at the core limit, where the NP probe was located. All results regarding the polarities in RM and PEO/water calibration mixtures have been expressed in terms of Kosower's Z values, using the linear dependence of E NP on Kosower's Z. The PPO/butanol mixtures have also been used for RM in butanol as a reference system. The data revealed that local polarity in RM is dependent on the surfactant type, block copolymer composition, solvent nature, and water content. At the same water content, the results clearly indicate a lower hydration degree of triblock copolymers, as compared to the surfactants of the alkyl ether and alkyl–aryl ether type, but for P123 and F127 Pluronics in n-butanol the hydration is higher owing to the behavior of butanol as cosurfactant and to its hydration.