Multipod structures of lamellae‐forming diblock copolymers in three‐dimensional confinement spaces: Experimental observation and computer simulation

The three‐dimensional (3D) confinement effect on the microphase‐separated structure of a diblock copolymer was investigated both experimentally and computationally. Block copolymer nanoparticles were prepared by adding a poor solvent into a block copolymer solution and subsequently evaporating the good solvent. The 3D structures of the nanoparticles were quantitatively determined with transmission electron microtomography (TEMT). TEMT observations revealed that various complex structures, including tennis‐ball, mushroom‐like, and multipod structures, were formed in the 3D confinement. Detailed structural analysis, showed that one block of the diblock copolymer slightly prefers to segregate into the particle surface compared with the other block. The observed structures were further elaborated using cell dynamics computer simulation. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 1702–1709     

Polymer‐regulated pattern formation in pseudo‐2D arrays of a fullerene derivative at the solution—Air interface

Self‐assembly of nanoparticles (NPs) into nonclose‐packed (ncp), semi‐two‐dimensional (2D) arrays is of interest in a variety of applications. Of special interest are photochemically active surfactant‐like fullerene derivatives [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (PCBM). The study presented here characterizes the morphology and structure of patterns formed by a mixture of PCBM NP and an amphiphilic block‐copolymer tethered at the water–air interface (a surface brush) as a function of the concentration of poly(ethylene oxide) (PEO) dissolved in the liquid subphase. Theoretical modeling of the system shows that the concentration of PEO in the subphase mediates the dimensions of the surface brush and at high PEO concentrations induces a collapse of the brush at the solution–air interface. The state of the surface brush is suggested to tune the semi‐2D patterns observed in the experiments via lateral depletion interactions and, in particular, induce lateral phase separation of the PCBM‐block copolymer. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

Volume transition and internal structures of small poly(N‐isopropylacrylamide) microgels

Monodispersed poly(N‐isopropylacrylamide) (PNIPAM) nanoparticles, with hydrodynamic radius less than 50 nm at room temperature, have been synthesized in the presence of a large amount of emulsifiers. These microgel particles undergo a swollen–collapsed volume transition in an aqueous solution when the temperature is raised to around 34 °C. The volume transition and structure changes of the microgel particles as a function of temperature are probed using laser light scattering and small angle neutron scattering (SANS) with the objective of determining the small particle internal structure and particle–particle interactions. Apart from random fluctuations in the crosslinker density below the transition temperature, we find that, within the resolution of the experiments, these particles have a uniform radial crosslinker density on either side of the transition temperature. This result is in contrast to previous reports on the heterogeneous structures of larger PNIPAM microgel particles, but in good agreement with recent reports based on computer simulations of smaller microgels. The particle interactions change across the transition temperature. At temperatures below the transition, the interactions are described by a repulsive interaction far larger than that expected for a hard sphere contact potential. Above the volume transition temperature, the potential is best described by a small, attractive interaction. Comparison of the osmotic second virial coefficient from static laser light scattering at low concentrations with similar values determined from SANS at 250‐time greater concentration suggests a strong concentration dependence of the interaction potential. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 849–860, 2005     

Synthesis of TiO2–SiO2/polymer core–shell microspheres with a microphase‐inversion method

A novel microphase‐inversion method was proposed for the preparation of TiO₂–SiO₂/poly(methyl methacrylate) core–shell nanocomposite particles. The inorganic–polymer nanocomposites were first synthesized via a free‐radical copolymerization in a tetrahydrofuran solution, and the poor solvent was added slowly to induce the microphase separation of the nanocomposite and result in the formation of nanoparticles. The average particle sizes of the microspheres ranged from 70 to 1000 nm, depending on the reaction conditions. Transmission electron microscopy and scanning electron microscopy indicated a core–shell morphology for the obtained microspheres. Thermogravimetric analysis and X‐ray photoelectron spectroscopy measurements confirmed that the surface of the nanocomposite microspheres was polymer‐rich, and this was consistent with the core–shell morphology. The influence of the synthetic conditions, such as the inorganic composition and the content of the crosslinking monomer, on the particle properties was studied in detail. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 3911–3920, 2006     

Swelling of a polymer brush by a poor solvent

The addition of a small amount of a poor solvent impurity (methanol) to a theta solvent (cyclohexane) is found to cause appreciable swelling (≈30% increase of the average brush height) in a model end‐grafted polystyrene (PS) brush layer. This unusual type of swelling is not observed if octadecyltrichlorosilane (OTS) is first grafted to the portion of the silicon substrate uncovered by the grafting end‐groups of the PS chains. Brush swelling in the absence of OTS surface protection is interpreted as arising from a segregation of methanol to the solid substrate and the resulting modification of the polymer–surface interaction. We also observe that the addition of a small amount of methanol to an adsorbed PS layer exposed to cyclohexane causes rapid film delamination from the silicon substrate. Together these observations imply a strong influence of surface active impurities on the structure and adhesive stability of polymer layers. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 4126–4131, 2004     

Controlled dispersion of polystyrene‐capped Au nanoparticles in P3HT:PC61BM and consequences upon active layer nanostructure

Numerous recent publications detail higher absorption and photovoltaic performance within organic photovoltaic (OPV) devices which are loaded with Au or Ag nanoparticles to leverage the light management properties of the localized surface plasmon resonance (LSPR). This report details the impact upon film morphology and polymer/nanoparticle interactions caused by incorporation of polystyrene‐coated Au nanoparticles (Au/PS) into the P3HT:PC₆₁BM bulk heterojunction film. Nanostructural analysis by transmission electron microscopy and X‐ray scattering reveals tunable Au/PS particle assembly that depends upon the choice of casting solvent, polymer chain length, film drying time, and Au/PS particle loading density. This Au/PS particle assembly has implications on the spectral position of the Au nanoparticle LSPR, which shifts from 535 nm for individually dispersed particles in toluene to 650 nm for particles arranged in large clusters within the P3HT:PC₆₁BM matrix. These results suggest a critical impact from PS/P3HT phase separation, which causes controlled assembly of a separate Au/PS phase in the nanoparticle/OPV composite; controlled Au/PS phase formation provides a blueprint for designing AuNP/OPV hybrid films that impart tunable optical behavior and potentially improve photovoltaic performance. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 709–720     

The effect of nanoparticle shape on polymer‐nanocomposite rheology and tensile strength

Nanoparticles can influence the properties of polymer materials by a variety of mechanisms. With fullerene, carbon nanotube, and clay or graphene sheet nanocomposites in mind, we investigate how particle shape influences the melt shear viscosity η and the tensile strength τ, which we determine via molecular dynamics simulations. Our simulations of compact (icosahedral), tube or rod‐like, and sheet‐like model nanoparticles, all at a volume fraction ? ≈ 0.05, indicate an order of magnitude increase in the viscosity η relative to the pure melt. This finding evidently can not be explained by continuum hydrodynamics and we provide evidence that the η increase in our model nanocomposites has its origin in chain bridging between the nanoparticles. We find that this increase is the largest for the rod‐like nanoparticles and least for the sheet‐like nanoparticles. Curiously, the enhancements of η and τ exhibit opposite trends with increasing chain length N and with particle shape anisotropy. Evidently, the concept of bridging chains alone cannot account for the increase in τ and we suggest that the deformability or flexibility of the sheet nanoparticles contributes to nanocomposite strength and toughness by reducing the relative value of the Poisson ratio of the composite. The molecular dynamics simulations in the present work focus on the reference case where the modification of the melt structure associated with glass‐formation and entanglement interactions should not be an issue. Since many applications require good particle dispersion, we also focus on the case where the polymer‐particle interactions favor nanoparticle dispersion. Our simulations point to a substantial contribution of nanoparticle shape to both mechanical and processing properties of polymer nanocomposites. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1882–1897, 2007     

Interactions between polymer-grafted particles and bare particles for biocompatibility applications

We use self-consistent field theory (SCFT) to study the interactions between a polymer-grafted spherical particle and a bare spherical particle and explore how these interaction energies depend on the radii of the two particles and the grafting density. We find that the magnitude of the interaction energies increases with the radii of both the grafted and bare particles and with increasing grafting density. We also find a universal scaling law for the interaction potential which exhibits a power-law dependence on both particle sizes, a linear dependence on grafting density, and a logarithmic dependence on interparticle distance with a range of interaction that scales with brush height. We compare our numerical results to those obtained using the Derjaguin approximation. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 2566–2577, 2009     

Tuning the mechanical properties in model nanocomposites: Influence of the polymer‐filler interfacial interactions

This article presents a study of the polymer‐filler interfacial effects on filler dispersion and mechanical reinforcement in Polystyrene (PS)/silica nanocomposites by direct comparison of two model systems: ungrafted and PS‐grafted silica dispersed in PS matrix. The structure of nanoparticles has been investigated by combining small angle neutron scattering measurements and transmission electronic microscopic images. The mechanical properties were studied over a wide range of deformation by plate–plate rheology and uni‐axial stretching. At low silica volume fraction, the particles arrange, for both systems, in small finite size nonconnected aggregates and the materials exhibit a solid‐like behavior independent of the local polymer‐fillers interactions suggesting that reinforcement is dominated by additional long range effects. At high silica volume fraction, a continuous connected network is created leading to a fast increase of reinforcement whose amplitude is then directly dependent on the strength of the local particle–particle interactions and lower with grafting likely due to deformation of grafted polymer. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

Effect of temperature on the frictional forces between polystyrene brushes

Normal and shear forces between opposing polystyrene (PS) brushes made from preferentially assembled PS–polyvinylpyridine diblock copolymers were measured in toluene and in near‐theta cyclohexane at 32, 40, and 50 °C, using a modified surface forces apparatus. In cyclohexane, over the temperature window probed, the normal forces of interaction are repulsive and the range of those force profiles changes only slightly; however, for both of the PS brushes studied, the onset of shear forces in near‐theta cyclohexane is strongly influenced by changes in temperature: As the temperature is increased, the onset of the frictional interactions between the brushes in cyclohexane shifts to smaller distances, approaching the distances where frictional forces are observed for brushes in the good solvent toluene. The pattern of behavior seen in the frictional response between the limits of good and theta condition is attributed to composition fluctuations, which increase near the theta condition because of the decrease in excluded volume interactions. These fluctuations may give rise to increased drag during shearing motion because of interfacial roughness or interchain coupling across the brush–brush interface. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 649–655, 2006     

The effect of electrosteric interactions on the effective charge of thermoresponsive ionic microgels: Theory and experiments

In this work, the influence of counterion valence and salt concentration on the effective charge of two types of thermoresponsive ionic microgel particles has been studied. The effective charge of the microgel at different swelling states has been experimentally determined from electrophoretic mobility measurements by solving the electrokinetic equations of the solvent for a single polyelectrolyte brush in the presence of an electric field, taking into account the friction of the solvent inside the polymer network. The experimental results have been compared to those obtained by means of the Ornstein‐Zernike integral formalism within the HNC relation. Results show that microgel bare charge is screened by the combined effect of counterion condensation and permeation inside the microgel particle. In addition to the electrostatic interaction, the steric exclusion exerted by the polymer plays an important role on the local ionic concentrations, especially for shrunken configurations. This steric term is responsible for the strong increase of the microgel effective charge experimentally observed when particles shrink for temperatures above the lower critical solution temperature. We also observe that, in the internal region of the microgel, charge electroneutrality is fulfilled, so the effective charge mainly arises from the region close to the microgel surface. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 2038–2049.     

Mean‐field theoretical analysis of brush‐coated nanoparticle dispersion in polymer matrices

The equilibrium dispersion of nanoparticles with grafted polymer chains into polymer matrices, of the same chemical structure as the brush, is studied through the device of mean‐field theory. Our results show that the disperion of brush‐coated nanoparticles into a matrix polymer is improved with (i) decreasing particle radius and (ii) increasing brush chain length. Both of these aspects can be understood based on the fact that, unlike the case of planar surfaces, homopolymer chains end‐grafted to spherical nanoparticle surfaces tangentially spread away from the surface thus alleviating the packing frustration that is created by the relatively high grafting densities. This permits significant brush/matrix overlap, even at high grafting densities, a regime that has only recently become experimentally available due to advances in polymer synthesis (i.e., the “grafting‐to” methods). © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 351–358, 2008     

Multiple levels hydrophobic modification of polymeric substrates by UV‐grafting polymerization with TFEMA as monomer

The hydrophobic solid surface modification with fluorine‐containing monomers has received tremendous attention because of its unique structure and excellent property. However, these hydrophobic films normally suffer from two major problems: one is weak interface interaction between fluoropolymers and substrates, and the other is the high cost of fluorine‐containing monomers. Herein, with the aim of feasible industrial application, a facile in situ UV photo‐grafting method is reported, which could ensure the formation of chemical bonds between fluoropolymer‐grafted layer and substrate with a low cost commercial 2,2,2‐trifluoroethyl methacrylate (TFEMA) as monomer. With low‐density polyethylene (LDPE) film as a model substrate, four kinds of poly‐TFEMA‐grafted layer are fabricated on LDPE films with different surface morphologies: polymer brush, polymer network, crosslinked nanoparticles, and a micro‐ and nanoscale hierarchical structure. The experimental results showed that the water contact angles (CAs) of the LDPE films grafted with polymer brush, polymer network, and crosslinked nanoparticles were (103 ± 2)°, (95 ± 2)°, and (122 ± 2)°, respectively, which were much higher than that of LDPE film. The introduction of micro‐ and nanoscale hierarchical structures can dramatically improve the surface roughness, which will further enhance the film hydrophobicity, and the water CA can reach as high as (140 ± 1)°. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 1059–1067     

Effect of an anionic monomer on the pickering emulsion polymerization stabilized by titania hydrosol

Polystyrene (PS) nanocomposite particles with high titania content are prepared by Pickering emulsion polymerization. A self‐made titania hydrosol modified by an anionic monomer sodium styrene sulfonate (NaSS) is used as a stabilizer and photocatalyst. The stability of the emulsion system is greatly improved by the electrostatic interaction between negatively charged NaSS and positively charged titania nanoparticles. The nanocomposite spheres with the diameter of around 120 nm are highly charged, indicating titania‐rich surfaces of latex particles. It is also proven by the field‐emission transmission electron microscope and field‐emission scanning electron microscope images. The well‐defined core‐shell structure of the obtained PS/titania composite particles is confirmed by the formation of fragile hollow titania nanospheres after thermogravimetric analysis tests. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 5728–5736, 2009     

Design of free‐standing microstructured conducting polymer films for enhanced particle removal from non‐uniform surfaces

Efficient removal of particles from topologically‐complex surfaces is of significant import for a range of applications (e.g., explosive residue removal in security arenas). Here, we synthesize next‐generation polymeric particle removal swabs with tuned structural features to elucidate the influence of the polymer microstructure on the removal of trace particles from surfaces. Specifically, microstructured free‐standing films of the conducting polymer polypyrrole (PPy) were synthesized through template‐assisted electropolymerization techniques. The removal of polystyrene microspheres from representative aluminum surfaces of varying roughness was evaluated as a function of the PPy microstructure. PPy‐based microstructured swabs displayed increased particle trapping properties relative to non‐textured PPy‐based swabs and current commercial swabs. This increased effectiveness occurred from the more intimate particle‐swab contact, leading to increased van der Waals interactions for the microstructured swabs. Therefore, this effort provides critical design rules for the production of microstructured conducting polymer materials for their application toward advanced particle removal technologies. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 1968–1974     

Morphology and interactions of polymer brush‐coated spheres in a polymer matrix

Using a real space implementation of the self‐consistent field theory, we calculated the morphology and interactions of spherical nanoparticles with radius R_p that are grafted by polymer chains of N monomers immersed in a chemically identical polymer melt of polymerization index P. The calculation shows that, for big particles (R_p ? N^1/2a, with a the segment size), the interactions and density profiles of the grafted layers are that of brushes at flat interface; While for small particles (R_p ? N^1/2a), the interactions and density profiles are characteristic of star polymers. In the case of intermediate grafted chain lengths, that is, R_p ～ N^1/2a, we found that the grafting density of the polymers and the radius of the spherical nanoparticles are both important in determining the structure and interactions of the grafted layers. Our findings suggest possible ways to tailor the structure and interactions of the nanoparticles to benefit the fabrication of polymeric nanocomposites. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 2811–2820, 2006     

Silica nanoparticle dispersions in homopolymer versus block copolymer

Silica nanoparticles (17 ± 4 nm in diameter) were modified by grafting polystyrene chains to the surfaces using atom transfer radical polymerization (ATRP). The molecular weight of the grafted chains ranged from 8 to 48 kDa. These modified nanoparticles were mixed in solution with poly(styrene) homopolymer (18–120 kDa) and symmetric poly(styrene‐b‐butadiene) (PS‐PB) diblock copolymer (34–465 kDa) and the states of dispersion in the dried composites were characterized by transmission electron microscopy (TEM). In the so‐called wet brush limit, when the graft molecular weight equals or exceeds the matrix value, the silica particles form a uniform random dispersion in poly(styrene). Increasing the homopolymer matrix, molecular weight above the graft value results in particle clustering and macroscopic‐phase separation. Mixtures of the lamellar forming block copolymer and nanoparticles exhibit a very different trend, with particle clustering at the lower PS‐PB molecular weights and dispersion at the highest value. This latter finding is rationalized on the basis of packing constraints associated with lamellar order and the effective particle dimensions, and the degree of solvation at ordering, both of which favor higher molecular weight block copolymers. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 2284–2299, 2007     

Particle and breath figure formation of triblock copolymers having self‐complementary hydrogen‐bonding units

Novel triblock copolymers having self‐complementary hydrogen‐bonding units were synthesized by using reversible addition–fragmentation transfer polymerization. As characterized by dynamic light scattering and atomic force microscopy, these polymers formed noncovalently crosslinked polymer particles and showed an aggregation behavior by intermolecular and intramolecular interactions. At low concentration, polymers formed nanoparticles, and the particle diameter increased with increasing polymer concentration. Well‐ordered hexagonal microstructures were prepared by “Breath Figure” technique with the triblock copolymers. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Brushless and controlled microphase separation of lamellar polystyrene‐b‐polyethylene oxide thin films for block copolymer nanolithography

Creating perpendicular alignment in lamellar block copolymer (BCP) systems has considerable industrial and commercial significance, most importantly for generating nanowire structures in electronic devices. In general, these lamellar systems require careful interface engineering to obtain vertical orientation of the blocks. To avoid the strong preferential adsorption of one block to either the substrate or the polymer/air interface, the surface must be “neutralized” by chemical brushes or external forces, for example, solvent fields. Reported here is a stepwise thermo/solvent annealing process allowing the formation of perpendicular domains of polystyrene‐b‐polyethylene oxide lamellar structures while avoiding brush or other surface modifications. This BCP has a relatively small minimum feature size and can be used to generate substrate patterns for use in fabrication of nanowire electronic device structures. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

Nanosegregated polymeric domains on the surface of Fe3O4@SiO2 particles

Multifunctional, biocompatible, and brush‐grafted poly(ethylene glycol)/poly(ε‐caprolactone) (PEG/PCL) nanoparticles have been synthesized, characterized, and used as vehicles for transporting hydrophobic substances in water. For anchoring the polymer mixed brushes, we used magnetic‐silica particles of 40 nm diameter produced by the reverse microemulsion method. The surface of the silica particle was functionalized with biocompatible polymer brushes, which were synthesized by the combination of “grafting to” and “grafting from” techniques. PEG was immobilized on the particles surface, by “grafting to,” whereas PCL was growth by ROP using the “grafting from” approach. By varying the synthetic conditions, it was possible to control the amount of PCL anchored on the surface of the nanoparticles and consequently the PEG/PCL ratio, which is a vital parameter connected with the arrangement of the polymer brushes as well as the hydrophobic/hydrophilic balance of the particles. Thus, adjusting the PEG/PCL ratio, it was possible to obtain a system formed by PEG and PCL chains grafted on the particle's surface that collapsed in segregated domains depending on the solvent used. For instance, the nanoparticles are colloidally stable in water due to the PEG domains and at the same time are able to transport, entrapped within the PCL portion, highly water‐insoluble drugs. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 2966–2975