Dispersion of polymer-grafted magnetic nanoparticles in homopolymers and block copolymers

The dispersion of magnetic nanoparticles (NPs) in homopolymer poly(methyl methacrylate) (PMMA) and block copolymer poly(styrene-b-methyl methacrylate) (PS-b-PMMA) films is investigated by TEM and AFM. The magnetite (Fe₃O₄) NPs are grafted with PMMA brushes with molecular weights from M = 2.7 to 35.7 kg/mol. Whereas a uniform dispersion of NPs with the longest brush is obtained in a PMMA matrix (P = 37 and 77 kg/mol), NPs with shorter brushes are found to aggregate. This behavior is attributed to wet and dry brush theory, respectively. Upon mixing NPs with the shortest brush in PS-b-PMMA, as-cast and annealed films show a uniform dispersion at 1 wt%. However, at 10 wt%, PS-b-PMMA remains disordered upon annealing and the NPs aggregate into 22 nm domains, which is greater than the domain size of the PMMA lamellae, 18 nm. For the longest brush length, the NPs aggregate into domains that are much larger than the lamellae and are encapsulated by PS-b-PMMA which form an onion-ring morphology. Using a multi-component Flory-Huggins theory, the concentrations at which the NPs are expected to phase separate in solution are calculated and found to be in good agreement with experimental observations of aggregation.     

Comparison of morphologies and mechanical properties of crosslinked epoxies modified by polystyrene and poly(methyl methacrylate)) or by the corresponding block copolymer polystyrene-b-poly(methyl methacrylate)

Polystyrene (PS, M_n=28,400, PI=1.07), poly(methyl methacrylate) (PMMA, M_n=88,600, PI=1.03), and PS (50,000)-b-PMMA (54,000) (PI=1.04), were used as modifiers of an epoxy formulation based on diglycidyl ether of bisphenol A (DGEBA) and m-xylylene diamine (MXDA). Both PS and PMMA were initially miscible in the stoichiometric mixture of DGEBA and MXDA at 80 °C, but were phase separated in the course of polymerization. Solutions containing 5 wt% of each one of both linear polymers exhibited a double phase separation. A PS-rich phase was segregated at a conversion close to 0.02 and a PMMA rich phase was phase separated at a conversion close to 0.2. Final morphologies, observed by scanning electron microscopy (SEM), consisted on a separate dispersion of PS and PMMA domains. A completely different morphology was observed when employing 10 wt% of PS-b-PMMA as modifier. PS blocks with M_n=50,000 were not soluble in the initial formulation. However, they were dispersed as micelles stabilized by the miscible PMMA blocks, leading to a transparent solution up to the conversion where PMMA blocks began to phase separate. A coalescence of the micellar structure into a continuous thermoplastic phase percolating the epoxy matrix was observed. The elastic modulus and yield stress of the cured blend modified by both PS and PMMA were 2.64 GPa and 97.2 MPa, respectively. For the blend modified by an equivalent amount of block copolymer these values were reduced to 2.14 GPa and 90.0 MPa. Therefore, using a block copolymer instead of the mixture of individual homopolymers and selecting an appropriate epoxy-amine formulation to provoke phase separation of the miscible block well before gelation, enables to transform a micellar structure into a bicontinuous thermoplastic/thermoset structure that exhibits the desired decrease in yield stress necessary for toughening purposes.     

Morphology of polystyrene/polystyrene-block-poly(methyl methacrylate)/poly(methyl methacrylate) composite particles

Polystyrene/polystyrene-block-poly(methyl methacrylate)/poly(methyl methacrylate) (PS/PS-b-PMMA/PMMA) composite particles were prepared by releasing toluene from PS/PS-b-PMMA/PMMA/toluene droplets dispersed in a sodium dodecyl sulfate aqueous solution. The morphology of the composite particles was affected by release rate of toluene, the molecular weight of PS-b-PMMA, droplet size, and polymer composition. ‘Onion-like’ multilayered composite particles were prepared from toluene droplets of PS-b-PMMA and of PS/PS-b-PMMA/PMMA, in which the weights of PS and PMMA were the same. The layer thicknesses of the latter multilayered composite particles increased with an increase in the amount of the homopolymers. PS-b-PMMA/PS composite particles had a sea-islands structure, in which PMMA domains were dispersed in a PS matrix. On the other hand, PS-b-PMMA/PMMA composite particles had a cylinder-like structure consisting of a PMMA matrix and PS domains.     

Effects of substrate roughness on the orientation of cylindrical domains in thin films of a polystyrene-poly(methylmethacrylate) diblock copolymer studied using atomic force microscopy and cyclic voltammetry

This paper describes the orientation of cylindrical domains in thin films of a polystyrene-poly(methylmethacrylate) diblock copolymer (PS-b-PMMA; 0.3 as the PMMA volume fraction) on gold and oxide-coated Si substrates having different surface roughness. Atomic force microscopy images of PS-b-PMMA films having thickness similar to the domain periodicity permitted us to study the effects of substrate roughness and block affinity on domain orientation. PS-b-PMMA films on gold substrates showed metastable vertical domain orientation that was attained more slowly on rougher substrates. In contrast, the domains were horizontally oriented on oxide-coated Si regardless of surface roughness and the annealing conditions examined. In addition, cyclic voltammetry data for PS-b-PMMA films on gold substrates whose PMMA domains were etched suggested that the metastable vertically oriented domains reached the underlying substrates. These results indicate that PS-b-PMMA films containing vertically oriented cylindrical domains can be obtained by using rough gold substrates upon annealing under controlled conditions.     

Nanophase morphology and crystallization in poly(vinylidene fluoride)/polydimethylsiloxane‐block‐poly(methyl methacrylate) blends

A polydimethylsiloxane‐block‐poly(methyl methacrylate) (PDMS‐b‐PMMA) diblock copolymer was synthesized by the atom transfer radical polymerization method and blended with a high‐molecular‐weight poly(vinylidene fluoride) (PVDF). In this A‐b‐B/C type of diblock copolymer/homopolymer system, semi‐crystallizable PVDF (C) and PMMA (B) block are miscible due to favorable intermolecular interactions. However, the A block (PDMS) is immiscible with PVDF and therefore generates nanostructured morphology via self‐assembly. Crystallization study reveals that both α and γ crystalline phases of PVDF are present in the blends with up to 30 wt% of PDMS‐b‐PMMA block copolymer. Adding 10 wt% of PVDF to PDMS‐b‐PMMA diblock copolymer leads to worm‐like micelle morphology of PDMS of 10 nm in diameter and tens of nanometers in length. Moreover, morphological results show that PDMS nanostructures are localized in the inter‐fibrillar region of PVDF with the addition of up to 20 wt% of the block copolymer. Increase of PVDF long period by 45% and decrease of degree of crystallization by 34% confirm the localization of PDMS in the PVDF inter‐fibrillar region. © 2018 Society of Chemical Industry     

Nanophase morphology and crystallization in poly(vinylidene fluoride)/polydimethylsiloxane‐block‐poly(methyl methacrylate)‐block‐polystyrene blends

An approach to achieve confined crystallization of ferroelectric semicrystalline poly(vinylidene fluoride) (PVDF) was investigated. A novel polydimethylsiloxane‐block‐poly(methyl methacrylate)‐block‐polystyrene (PDMS‐b‐PMMA‐b‐PS) triblock copolymer was synthesized by the atom‐transfer radical polymerization method and blended with PVDF. Miscibility, crystallization and morphology of the PVDF/PDMS‐b‐PMMA‐b‐PS blends were studied within the whole range of concentration. In this A‐b‐B‐b‐C/D type of triblock copolymer/homopolymer system, crystallizable PVDF (D) and PMMA (B) middle block are miscible because of specific intermolecular interactions while A block (PDMS) and C block (PS) are immiscible with PVDF. Nanostructured morphology is formed via self‐assembly, displaying a variety of phase structures and semicrystalline morphologies. Crystallization at 145 °C reveals that both α and β crystalline phases of PVDF are present in PVDF/PDMS‐b‐PMMA‐b‐PS blends. Incorporation of the triblock copolymer decreases the degree of crystallization and enhances the proportion of β to α phase of semicrystalline PVDF. Introduction of PDMS‐b‐PMMA‐b‐PS triblock copolymer to PVDF makes the crystalline structures compact and confines the crystal size. Moreover, small‐angle X‐ray scattering results indicate that the immiscible PDMS as a soft block and PS as a hard block are localized in PVDF crystalline structures. © 2019 Society of Chemical Industry     

Reaction-induced microphase separation in thermosetting blends of epoxy resin with poly(methyl methacrylate)-block-polystyrene block copolymers: Effect of topologies of block copolymers on morphological structures

Polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) block copolymers with linear and tetra-armed star-shaped topological structures were synthesized via sequential atomic transfer radical polymerization (ATRP). With pentaerythritol tetrakis(2-bromoisobutyrate) as the initiator, the star-shaped block copolymers with two sequential structures (i.e., s-PMMA-b-PS and s-PS-b-PMMA) were prepared and the arm lengths and composition of the star-shaped block copolymers were controlled to be comparable with those of the linear PS-b-PMMA (denoted as l-PS-b-PMMA). The block copolymers were incorporated into epoxy resin to access the nanostructures in epoxy thermosets, by knowing that PMMA is miscible with epoxy after and before curing reaction whereas the reaction-induced phase separation occurred in the thermosetting blends of epoxy resin with PS. Considering the difference in miscibility of epoxy with PMMA and/or PS, it is judged that the reaction-induced microphase separation occurred in the systems. The design of these block copolymers allows one to investigate the effect of topological structures of block copolymers on the morphological structures of the thermosets. By means of atomic force microscopy (AFM) and small-angle X-ray scattering (SAXS), the morphology of the thermosets was examined. It is found that the nanostructures were formed in the thermosets containing l-PMMA-b-PS and s-PS-b-PMMA block copolymers. It is noted that the long-range order of the nanostructures in the epoxy thermosets containing l-PMMA-b-PS is obviously higher than that in the system containing s-PS-b-PMMA. However, the macroscopic phase separation occurred in the thermosetting blends of epoxy resin with s-PMMA-b-PS block copolymer.     

A new approach in the study of tethered diblock copolymer surface morphology and its tethering density dependence

A poly(l-lactic acid)-block-polystyrene-block-poly(methyl methacrylate) (PLLA-b-PS-b-PMMA) triblock copolymer was synthesized with a crystalline PLLA end block. Single crystals of this triblock copolymer grown in dilute solution could generate uniformly tethered diblock copolymer brushes, PS-b-PMMA, on the PLLA single crystal substrate. The diblock copolymer brushes exhibited responsive, characteristic surface structures after solvent treatment depending upon the quality of the solvent in relation to each block. The chemical compositions of these surface structures were detected via the surface enhanced Raman scattering technique. Using atomic force microscopy, the physical morphologies of these surface structures were identified as micelles in cyclohexane and “onion”-like morphologies in 2-methoxyethanol, especially when the PS-b-PMMA tethered chains were at low tethering density.     

The effect of polystyrene-block-poly(4-vinylpyridine) prepared by a RAFT method in the dispersion polymerization of MMA

The dispersion polymerization of methyl methacrylate (MMA) has been carried out using polystyrene-block-poly(4-vinylpyridine) copolymer [P(S-b-4VP)], which was prepared by a reversible addition-fragmentation chain transfer (RAFT) method, as a steric stabilizer in an alcohol media. The stable polymer particles were obtained when the block copolymer concentrations increased from 1 to 10 wt% relative to the monomer and the average particle sizes decreased from 5.3 to 3.4 μm with the increasing concentration of the block copolymer. In particular, the incorporation of 2 wt% polystyrene-block-poly(4-vinylpyridine) produced 4.3 μm of monodisperse PMMA particles with 2.14% of C_v. Thus, the P(S-b-4VP) block copolymer prepared by the RAFT method is working not only as a steric stabilizer, but also in providing monodisperse micron-sized PMMA particles.     

Synthesis and analysis of phase segregation of polystyrene-block-poly(methyl methacrylate) copolymer obtained by Steglich esterification from semitelechelic blocks of polystyrene and poly(methyl methacrylate)

Here, an alternative route to successfully synthesize polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) is reported. Steglich esterification was used as an effective, metal free approach for coupling carboxylic terminated PS and the hydroxyl end-functionalized PMMA chains obtained by nitroxide-mediated polymerization and atom transfer radical polymerization, respectively. α-Functionalization was obtained using 4,4′-azobis(4-cyanovaleric acid) and 2,2,2-tribromoethanol as initiators. The synthesis of PS-b-PMMA was confirmed by gel permeation chromatography and nuclear magnetic resonance (NMR), while the dependence of the diffusion coefficients of the polymers (PS, PMMA, PS/PMMA blend, and PS-b-PMMA) with their corresponding molecular weights was discussed based on the results of atomic force microscopy-based infrared spectroscopy, differential scanning calorimetry, and spectra of diffusion-ordered NMR spectroscopy. Differently from PS-b-PMMA, a partial segregation was observed for the PS/PMMA blend, affecting its thermal behavior and diffusion coefficient. The study here presented provides an easier and efficient strategy for the synthesis of PS-b-PMMA and new insights into the diffusion of polymers.     

Transition behavior of PS-b-PMMA films on the balanced interfacial interactions

The thickness dependence of the order-to-disorder transition (ODT), measured by in situ grazing-incidence small-angle X-ray scattering (GISAXS), has been investigated in thin films of a symmetric polystyrene-b-poly(methyl methacrylate) (PS-b-PMMA) on a random copolymer (P(S-r-MMA)) grafted to the substrate where the interfacial interactions are balanced. With decreasing film thickness less than 25L₀, the ODT significantly decreases to 193 °C for film of 10L₀ in thickness, because the interfacial interactions by a random copolymer grafted to the substrate provide a surface-induced compatibilization toward two block components. However, a plateau of the ODT at ∼213 °C for films thicker than 25L₀ was observed above the bulk value of 200 °C. The elevation of this ODT indicates a suppression of compositional fluctuations normal to the film surface, more than likely because the dominant orientation of the lamellar microdomains was found to be parallel to the film surface.     

Tuned phase behavior of weakly interacting polystyrene-block-poly(n-pentyl methacrylate) by selective solvent

We investigated, via small angle X-ray scattering, depolarized light scattering, rheometry, and transmission electron microscopy, the phase behavior of the mixture of a symmetric polystyrene-block-poly(n-pentyl methacrylate) copolymer (PS-b-PnPMA) showing the closed-loop phase behavior and excellent baroplasticity, and dodecanol, a PnPMA-selective solvent. We found that the addition of a selective solvent is simple, but very effective to obtain various microdomains including hexagonally packed cylinders and gyroids. Also, with increasing temperature, the mixtures showed multiple ordered-to-ordered transitions (OOTs) in addition to upper ordered-to-disordered transition (UODT). The first observation of gyroid microdomains in PS-b-PnPMA is very important, although they have been widely reported in many block copolymers, for instance, PS-block-polyisoprene copolymer (PS-b-PI) and PS-block-poly(d,l-lactide) copolymer (PS-b-PLA). Since the gyroid microdomains of PS-b-PnPMA show excellent baroplasticity, external pressure instead of temperature could easily change the microdomains.     

Crystallization, melting and morphology of PEO in PEO/MWNT-g-PMMA blends

The dispersion of multi-walled carbon nanotubes (MWNTs) in crystalline poly(ethylene oxide) (PEO) is significantly improved by grafting with poly(methyl methacrylate) (PMMA) on surface of MWNTs via emulsion reactions. The synthesized MWNTs-g-PMMA is soluble in solvents that can dissolve PMMA and is well dispersed in PEO. The effects of the MWNTs-g-PMMA on PEO crystallization and its use as a reinforcement for PEO are investigated using DMA, DSC, POM, and SAXS. DMA data show that the PEO/MWNTs-g-PMMA blends containing up to 30 wt% MWNTs-g-PMMA are compatible. DSC data show the crystallization of PEO is enhanced by the MWNTs-g-PMMA, accompanying with a decreased thickness of crystal layers and an increased thickness of amorphous layers of the PEO lamellar stacks, in combination with SAXS data.     

Corona structure on demand: Tailor-made surface compartmentalization in worm-like micelles via random cocrystallization

We present a straightforward approach to well-defined 1D patchy particles utilizing crystallization-induced self-assembly. A polystyrene-block-polyethylene-block-poly(methyl methacrylate) (PS-b-PE-b-PMMA) triblock terpolymer is cocrystallized in a random fashion with a corresponding polystyrene-block-polyethylene-block-polystyrene (PS-b-PE-b-PS) triblock copolymer to yield worm-like crystalline-core micelles (wCCMs). Here, the corona composition (PMMA/PS fraction) can be easily adjusted via the amount of PS-b-PE-b-PMMA triblock terpolymer in the mixture and opens an easy access to wCCMs with tailor-made corona structures. Depending on the PMMA fraction, wCCMs with a mixed corona, spherical PMMA patches embedded in a continuous PS corona, as well as alternating PS and PMMA patches of almost equal size can be realized. Micelles prepared by cocrystallization show the same corona structure as those prepared from neat triblock terpolymers at identical corona composition. Thus, within a certain regime of desired corona compositions the laborious synthesis of new triblock terpolymers for every composition can be circumvented.     

Sequence distribution affect the phase behavior and hydrogen bonding strength in blends of poly(vinylphenol-co-methyl methacrylate) with poly(ethylene oxide)

Experimental results indicate that the PEO was miscible with PVPh-r-PMMA copolymers as shown by the existence of single composition-dependent glass transition temperature over the entire compositions. However, the PVPh-b-PMMA copolymer with PEO shows a like closed loop phase-separated region in this copolymer/homopolymer blend system. Furthermore, FTIR reveals that at least three competing equilibrium are present in these blends; self-association (hydroxyl-hydroxyl), interassociation (hydroxyl-carbonyl) of PVPh-co-PMMA, and hydroxyl-ether interassociation between PVPh and PEO. Based on the Painter-Coleman Association Model (PCAM), a value for inter-association, K_C=300 is obtained in PVPh-b-PMMA/PEO blend system at room temperature. Although the relative ratio of interassociation equilibrium constant of PEO to PMMA is larger in PVPh-b-PMMA/PEO blend system, the PVPh-r-PMMA/PEO blend system has greater Δν and greater homogeneity at the molecular scale than the PVPh-b-PMMA/PEO blend system because of the ΔK effect.     

Solvent vapor induced dewetting in diblock copolymer thin films

The dewetting pattern development of thin film of poly(styrene)-block-poly(methyl methacrylate) (PS-b-PMMA) diblock copolymer has been studied after ‘annealing’ in the PMMA block selective solvent vapor. Initially, typical circular dewetted holes are observed. Further annealing, however, results in the formation of fractal-like holes. The heterogeneous stress induced by the residual solvent remaining in the film after spin-coating induces the anisotropy of the polymer mobility during the annealing process, which triggers the formation of the intriguing surface patterns.     

Ordering and microdomain orientation in block copolymer films by thermal deprotection

Ordering and microdomain orientation for the films of symmetric polystyrene-b-poly(tert-butyl methacrylate)s (PS-b-PtBMAs) was investigated by in-situ grazing incidence small-angle X-ray scattering (GISAXS) and the electron microscopy. During thermal deprotection at higher temperature (200 °C), functional tert-butyl ester units in the PtBMA block component are integrated into inter- or intra-molecular anhydride linkages. It was observed that this process causes an increase in the Flory-Huggins interaction parameter (χ) between the two block components for disordered PS-b-PtBMA film, leading to a modulated nonequilibrium structure. Interestingly, for lamella-forming PS-b-PtBMA film, a significant chain stretching in lateral direction during thermal deprotection resulted in a characteristic strain-induced perpendicular orientation in the middle of the film confined between two parallel orientations of lamellar microdomains.     

Compatibilized polymer blends with nanoscale or sub-micron dispersed phases achieved by hydrogen-bonding effects: Block copolymer vs blocky gradient copolymer addition

Addition of styrene (S)/4-hydroxystyrene (HS) block, blocky gradient, or blocky random copolymer to 80/20 wt% polystyrene (PS)/polycaprolactone (PCL) blends is examined as a compatibilization strategy. Four copolymers are synthesized by controlled radical polymerization, each with an S block and the other block being a HS block or S/HS random or gradient copolymer. Compatibilization depends on copolymer level and HS sequence distribution and content. Using a two-step solution-mixing/melt-mixing process, addition of 2 wt% and 5 wt% nearly symmetric S/HS diblock copolymer leads to compatibilization with average PCL domain diameters of 390-490 nm and 90-110 nm, respectively. In contrast, adding 0.25-0.75 wt% copolymer leads to microscale dispersed-phase domains and only reduced melt-state coarsening. Results with 2-5 wt% added copolymer indicate that a major reduction in interfacial tension is facilitated by hydrogen bonding of HS units and PCL carbonyl groups. Nanoscale confinement of normally semi-crystalline PCL within blends with 100 nm dispersed phases impedes PCL crystallizability, yielding liquid-state PCL domains at room temperature and demonstrating that properties of nanostructured blends and microstructured blends can differ greatly. Polystyrene/PCL blends are also made by one-step melt mixing with low mol% HS copolymers. Adding 5 wt% blocky gradient S/HS copolymer (86/14 mol% S/HS) leads to compatibilization with an average dispersed-phase diameter of 360-420 nm. In contrast, adding 5 wt% blocky random (87/13 mol% S/HS) or 5 wt% diblock (81/19 mol% S/HS) copolymer yields microscale dispersed-phase domains and only reduced coarsening. Crystallization in these blends is less hindered than in blends containing 2-5 wt% nearly symmetric S/HS diblock copolymer, indicating that both hydrogen bonding and confinement suppress PCL crystallization.     

Nanostructured polystyrene-block-poly(4-vinyl pyridine)(pentadecylphenol) thin films as templates for polypyrrole synthesis

Polypyrrole has been chemically synthesized on thin film nanostructures obtained from comb-shaped supramolecules of polystyrene-block-poly(4-vinyl pyridine) (PS-b-P4VP) hydrogen bonded with pentadecylphenol (PDP). PDP was washed from thin films of cylindrical and lamellar self-assembled comb-copolymer systems, which resulted in removal of the upper layers of microdomains, leaving single cylindrical and lamellar layers covering a substrate, with P4VP segregated at the bottom as well as at the free air interface. This P4VP was complexed with Cu²⁺ ions, after which chemical oxidation polymerization of pyrrole resulted in a thin polypyrrole layer covering the nanostructured block copolymer. The use of a catalytic amount of bipyrrole greatly improved the quality of the obtained product. The conductivity was measured to be ∼0.7 S cm⁻¹.