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.     

Microphase separation and crystallization in mixtures of polystyrene-poly(methyl methacrylate) diblock copolymer and poly(vinylidene fluoride)

This study examined the microdomain structures and the crystallization behavior in binary blends consisting of an asymmetric block copolymer and a homopolymer using small-angle X-ray scattering, optical microscopy and differential scanning calorimetry. A polystyrene-block-poly(methyl methacrylate) copolymer (PS-b-PMMA) was mixed with a low molecular weight poly(vinylidene fluoride) (PVDF), where the PS-b-PMMA had a 0.30 wt fraction of the PMMA block. At a PVDF concentration of <13.0 wt%, the PVDF was completely miscible with the PMMA microdomains, and the blends had a cylindrical structure. The addition of PVDF altered the morphology from a PMMA-cylindrical structure to a lamellar structure and finally to a PS-cylindrical structure. When the PVDF concentration was <23.0 wt%, the PVDF was distributed uniformly within the PMMA microdomains. After adding more PVDF, some of the PVDF was locally dissolved in the middle of the PMMA microdomains. The addition of PVDF also affected the ordered microstructure in the blends, leading to a well-defined microdomain structure. However, PVDF crystallization significantly disturbed the pre-existing microdomain structure, resulting in a poorly ordered morphology. In the blends, PVDF had unique crystallization behavior as a result of the space constraints imposed by the microdomains.     

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.     

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.     

Perpendicular lamellae induced at the interface of neutral self-assembled monolayers in thin diblock copolymer films

We obtained perpendicular lamellar orientations in thin films of symmetric polystyrene-block-poly(methyl methacrylate), PS-b-PMMA, on self-assembled monolayers (SAMs) of 3-(p-methoxyphenyl)propyltrichlorosilane (MPTS) prepared on silicon wafers. In contrast to completely parallel lamellae on silicon wafers having a native oxide layer, perpendicular lamellae at the MPTS interface with parallel lamellae at the air interface were directly observed by transmission electron microscopy (TEM) in cross-sectional view. The perpendicular lamellae at the MPTS interface were attributed to the non-preferential (neutral) MPTS-covered substrate to both PS and PMMA blocks. The neutrality of the SAMs of MPTS was confirmed by the similar interfacial tension values of the SAMs of MPTS with PS and PMMA, estimated by contact angle measurements.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

Complicated phase behavior and ionic conductivities of PVP-co-PMMA-based polymer electrolytes

We have used DSC, FTIR spectroscopy, and ac impedance techniques to investigate the interactions that occur within complexes of poly(vinylpyrrolidone-co-methyl methacrylate) (PVP-co-PMMA) and lithium perchlorate (LiClO₄) as well as these systems' phase behavior and ionic conductivities. The presence of MMA moieties in the PVP-co-PMMA random copolymer has an inert diluent effect that reduces the degree of self-association of the PVP molecules and causes a negative deviation in the glass transition temperature (T_g). In the binary LiClO₄/PVP blends, the presence of a small amount of LiClO₄ reduces the strong dipole-dipole interactions within PVP and leads to a lower T_g. Further addition of LiClO₄ increases T_g as a result of ion-dipole interactions between LiClO₄ and PVP. In LiClO₄/PVP-co-PMMA blend systems, for which the three individual systems—the PVP-co-PMMA copolymer and the LiClO₄/PVP and LiClO₄/PMMA blends—are miscible at all compositional ratios, a phase-separated loop exists at certain compositions due to a complicated series of interactions among the LiClO₄, PVP and PMMA units. The PMMA-rich component in the PVP-co-PMMA copolymer tends to be excluded, and this phenomenon results in phase separation. At a LiClO₄ content of 20 wt% salt, the maximum ionic conductivity occurred for a LiClO₄/VP57 blend (i.e., 57 mol% VP units in the PVP-co-PMMA copolymer).     

Friction spot welding of PMMA with PMMA/silica and PMMA/silica-g-PMMA nanocomposites functionalized via ATRP

In the present study, the feasibility of Friction Spot Welding (FSpW) of a commercial-grade poly(methyl methacrylate) (PMMA) (PMMA GS) and PMMA 6N/functionalized silica (SiO₂) nanocomposites was investigated. The silica nanoparticles were functionalized via atom transfer radical polymerization (ATRP) with PMMA chains to achieve a uniform dispersion in the polymer matrix. The successful functionalization of silica nanoparticles with PMMA chains via ATRP was evaluated by ATR-FT-IR and TGA measurements. Rheological investigations of the silica nanocomposites showed a plateau of the storage modulus G′ at low frequencies (0.01–0.03 rad/s) as a result of elastic particle–particle interactions. Overlap friction spot welds consisting of PMMA GS and a 2 wt% SiO₂-g-PMMA nanocomposite were successfully prepared and compared to spot joints of PMMA GS welded with PMMA 6N and PMMA 6N/silica nanocomposite with 2 wt% unfunctionalized silica nanoparticles. Raman mappings of selected areas of cross-sectional plastographic specimens revealed an increased mixing behavior between the two polymer plates in the case of PMMA GS/2 wt% SiO₂-g-PMMA joints. Although the joints welded with PMMA 6N/silica nanocomposites showed a reduction of 22% in lap shear strength and 21% displacement at peak load compared with the neat PMMA spot welds, they can compete with other state-of-the-art PMMA welding techniques such as thermal bonding and ultrasonic welding, which indicates the potential of friction spot welding as an alternative fabrication technology for joining future nanocomposite engineering parts.     

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.     

Effects of additives on the morphologies of thin titania films from self-assembly of a block copolymer

The effects of additives of poly(methyl methacrylate) (PMMA) and HAuCl₄ on the morphologies of hybrid titania films formed via co-assembly of polystyrene-block-poly(ethylene oxide) (PS-b-PEO) copolymers, titania sol-gel precursor in a selective solvent were investigated. The results show that addition of PMMA or HAuCl₄ has an important influence on the morphologies of hybrid titania films. Addition of PMMA or HAuCl₄ can induce the morphology transition of the PS-b-PEO/titania sol-gel mixture from spherical micelles to vesicles. Therefore, the morphologies of the hybrid films formed on silicon substrate surfaces by spin-coating can be controlled by the addition of homopolymer (PMMA) or inorganic precursor (HAuCl₄) into the PS-b-PEO/titania sol-gel mixtures, allowing access to nanoparticles or nanoporous films. After removing the polymer matrix, nanoparticle aggregates or nanobowl-like structures are left behind on the substrate surfaces.     

Formation and properties of multivariant assemblies of surface-tethered diblock and triblock copolymers

We present methodologies for fabricating block copolymer assemblies grafted onto flat solid substrates, where each block of the copolymer possesses a systematic and gradual variation of molecular weight as a function of the position on the substrate. We demonstrate the utility of this technique on two case studies. In the first project, we generate surface-tethered poly[(2-hydroxyethyl methacrylate)-b-(methyl methacrylate)] (PHEMA-b-PMMA) diblock copolymer brushes and study systematically morphological transitions associated with collapsing either the top PMMA or the bottom PHEMA block while keeping the other block solvated. Scanning force microscopy studies of systems having the top block collapsed reveal the presence of either flat (F), or micellar (M) or bicontinuous (BC) morphologies, whose locus in the phase diagram agrees with theoretical predictions and results of computer simulations. The second case study demonstrates the extension of the deposition method to the case of surface-anchored triblock copolymer brushes. Specifically, we present results pertaining to the formation of poly[(2-hydroxyethyl methacrylate)-b-(methyl methacrylate)-b-(dimethylaminoethyl methacrylate)] brushes with independent variation of all three block lengths.     

High-pressure solution blending of poly(?-caprolactone) with poly(methyl methacrylate) in acetone + carbon dioxide

The miscibility of blends of poly(?-caprolactone) (PCL, M_w = 14,300) with poly(methyl methacrylate) (PMMA, M_w = 15K or 540K) in acetone + CO₂ mixed solvent has been explored. The liquid-liquid phase boundaries at different temperatures have been determined for mixtures containing 10 wt% total polymer blend, 50 wt% acetone and 40 wt% CO₂. The PCL and PMMA contents of the blends were varied while holding the total polymer concentration at 10 wt%. The polymer blend solutions all displayed LCST-type behavior and required higher pressures than individual polymer components for complete miscibility. Complete miscibilities were achieved at pressures within 40 MPa. The DSC scans show that the blends are microphase-separated. The blends display the melting transition of PCL and the glass transition temperature of the PMMA phases. The presence of PMMA is found to influence the crystallization and melting behavior of PCL in the blends. The DSC results on heat of melting and the FTIR spectra, specifically the changes at 1295 cm⁻¹ band show the changes (decrease) in overall crystallinity of the blend upon addition of PMMA.     

Synthesis and properties of polyolefin graft copolymers by a grafting “onto” reactive process

The synthesis of graft copolymers by the grafting “onto” process in the molten state was described. Functional oligomers obtained by telomerization or by ATRP were reacted onto maleic anhydride grafted polypropylene (PP-g-MAH) and poly(ethylene-ter-maleic anhydride-ter-methyl acrylate) (P(E-ter-MAH-ter-MeA)) to obtain PP-g-PMMA and P(E-ter-MAH-ter-MeA)-g-PMMA graft copolymers, respectively. The grafting of different mono-functional oligomers bearing hydroxyl, aliphatic amine or aromatic amine functions was investigated at 180 °C and at 200 °C. The grafting efficiency was very low in the case of hydroxyl-terminated PMMA, while the amine-terminated PMMA led to high yields. In the last part, PP-g-PMMA and P(E-ter-MAH-ter-MeA)-g-PMMA graft copolymers were synthesized by the reaction of aliphatic amine functional PMMA oligomers onto PP-g-MAH and P(E-ter-MAH-ter-MeA), respectively. The influence of the molecular weight of PMMA oligomers was investigated and showed that he grafting efficiency slightly decreases with the increasing molecular weight. However, this process allows the synthesis of PP-g-PMMA graft copolymers containing 6-45 wt% of PMMA side chains. The microstructure of the nanostructured PP-g-PMMA and P(E-ter-MAH-ter-MeA)-g-PMMA graft copolymers was investigated by TEM and SEM. This was established that the addition of PP-g-PMMA in PP/PMMA binary blends allows to control their morphologies and stabilities.