Probe diffusion in homogeneous diblock copolymers

Forced Rayleigh scattering was used to investigate the diffusion of a photoreactive dye molecule in two homogeneous poly(styrene-b-isoprene) (SI) diblock copolymers with overall molecular weights of approximately 2000. Although diffusion rates were intermediate to TTI transport in homopolymer polystyrene (PS) and polyisoprene (PI), system dynamics appear to be largely dictated in each case by the PI block. The size of the polymer jumping unit, on the other hand, is evaluated from a free-volume analysis of the data, and is found to be governed predominantly by the PS component of the copolymer. The mechanism for tracer diffusion in low-molecular-weight block copolymers appears analogous to transport in a high molecular weight SI diblock copolymer (M_n = 13,600) that has been solvated sufficiently in toluene to be microstructurally disordered. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1739–1746, 1998     

Styrene and isoprene friction factors in styrene–isoprene matrices

Monomeric friction factors, Ξ, for polystyrene (PS), polyisoprene (PI), and a polystyrene–polyisoprene (SI) diblock copolymer have been determined as a function of temperature in four poly(styrene-b-isoprene-b-styrene-b-isoprene) tetrablock copolymer matrices. The Rouse model has been used to calculate the friction factors from tracer diffusion coefficients measured by forced Rayleigh scattering. Within the experimental temperature range the tetrablock copolymers are disordered, allowing for measurement of the diffusion coefficient in matrices with average compositions determined by the tetrablock copolymers (23, 42, 60, and 80% styrene by volume). Remarkably, for a given matrix composition the styrene and isoprene friction factors are essentially equivalent. Furthermore, at a constant interval from the system glass transition temperature, T_g, all of the friction factors (obtained from homopolymer, diblock copolymer, and tetrablock copolymer dynamics) agree to within an order of magnitude. This is in marked contrast to results for miscible polymer blends, where the individual components generally have distinct composition dependences and magnitudes at constant T − T_g. The homopolymer friction factors in the tetrablock matrices were systematically slightly higher than those of the diblock, which in turn were slightly higher than those of the homopolymers in their respective melts, when all compared at constant T − T_g. This is attributed to the local spatial distribution of styrene and isoprene segments in the tetrablocks, which presents a nonuniform free energy surface to the tracer molecules. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 3079–3086, 1998     

A theoretical and simulation study of the self-assembly of a binary blend of diblock copolymers

Pure diblock copolymer melts exhibit a narrow range of conditions at which bicontinuous and cocontinuous phases are stable; such conditions and the morphology of such phases can be tuned by the use of additives. In this work, we have studied a bidisperse system of diblock copolymers using theory and simulation. In particular, we elucidated how a short, lamellar-forming diblock copolymer modifies the phase behavior of a longer, cylinder-forming diblock copolymer. In a narrow range of intermediate compositions, self-consistent field theory predicts the formation of a gyroid phase although particle-based simulations show that three phases compete: the gyroid phase, a disordered cocontinuous phase, and the cylinder phase, all having free energies within error bars of each other. Former experimental studies of a similar system have yielded an unidentified, partially irregular bicontinuous phase, and our simulations suggest that at such conditions the formation of a partially transformed network phase is indeed plausible. Close examination of the spatial distribution of chains reveals that packing frustration (manifested by chain stretching and low density spots) occurs in the majority-block domains of the three competing phases simulated. In all cases, a double interface around the minority-block domains is also detected with the outer one formed by the short chains, and the inner one formed by the longer chains.     

Kinetically constrained block copolymer self-assembly a simple method to control domain size

In this work asymmetric polystyrene-block-polyethylene oxide (PS-PEO) diblock copolymers were blended with high and low molecular polystyrene (PS) homopolymer and spin cast, resulting in the rapid self-assembly of vertically oriented PEO cylinders in a matrix of PS. Due to the kinetically constrained phase separation of the system, increasing addition of homopolymer is shown to reduce the diameter of the PEO domains, even when the homopolymer was of significantly higher molecular weight than the PS block in the PS-PEO diblock copolymer and would be predicted to macro-phase separate from the copolymer. The outcomes of this study provide a novel method that requires the adjustment of a single variable to tune the size of vertically oriented PEO domains between 10 and 100 nm, with potential applications in a number of areas including membrane technologies.     

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     

Ozonolysis efficiency of PS‐b‐PI block copolymers for forming nanoporous polystyrene

A PS‐b‐PI diblock copolymer has been synthesized and blended with a homopolystyrene of a variable molecular weight at a variable weight ratio, resulting in different volume fractions of PS to PI and various morphologies. After being cast as a film, the double bonds in the PI microdomains were cleaved via ozonolysis resulting in the formation of a nanoporous PS. The ease of ozonolysis depended strongly upon the morphology of the PI microdomains. The degree of ozonization after 24‐h reaction was about 90% for lamellar microdomains, about 80% for bicontinuous microdomains, about 70% for cylindrical microdomains, and about 50% for spherical microdomains. These variations were attributed to the decrease in the contact area of PI microdomains with ozone and the total volume of PI microdomains accessible to ozone. All PS‐b‐PI/homopolystyrene samples have turned into nanoporous materials after the removal of PI nanodomains. SEM, AFM, and TEM images indicated that the resulting PS had a pore size of 20–30 nm, and thus was potentially useful for photonic crystals or fuel cell applications. © 2008 Wiley Periodicals, Inc. JPolym Sci Part A: Polym Chem 46: 1964–1973, 2008     

Periodic organic–organometallic microdomain structures in poly(styrene‐block‐ferrocenyldimethylsilane) copolymers and blends with corresponding homopolymers

A series of poly(styrene‐block‐ferrocenyldimethylsilane) copolymers (SF) with different relative molar masses of the blocks were prepared by sequential anionic polymerization. The bulk morphology of these polymers, studied by TEM and SAXS, showed well‐ordered lamellar and cylindrical domains as well as disordered micellar structures. Temperature‐dependent rheological measurements exhibited an order–disorder transition for SF 17/8 (the numbers refer to the relative molar masses in 10³ g/mol) between 170 and 180°C, and an order–order transition for SF 9/19 between 190 and 200°C. The morphologies of binary blends of the diblocks with homopolymer were also investigated. In the blends the molar mass of the homopolymer was always less than the molar mass of the matching block. Ordered spheres on a bcc lattice and double‐gyroid morphology were observed for the blends. The double‐gyroid morphology was found only in F‐rich diblock/homopolymer systems. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1009–1021, 1999     

3D Nanometer‐Scale Study of Coexisting Bicontinuous Morphologies in a Block Copolymer/Homopolymer Blend

Summary: Coexisting bicontinuous morphologies, one ordered and one disordered, are investigated in a macrophase‐separated poly(styrene‐block‐isoprene) diblock copolymer/homopolystyrene (SI/hS) blend. Two‐phase behavior is attributed to the relatively high hS/S mass ratio (0.92). According to its crystallographic signature and channel coordination as discerned from three‐dimensional (3D) images generated by transmission electron microtomography (TEMT), the ordered morphology is classified as gyroid. The 3D local and global topological characteristics of both bicontinuous morphologies as measured directly from TEMT images are reported. The disordered morphology is further compared with molecular‐field simulations to ascertain the spatial distribution of the constituent species within the blend, thereby demonstrating the utility of high‐resolution 3D imaging coupled with molecular‐level simulations.

     

Controlled supramolecular assembly of micelle-like gold nanoparticles in PS-b-P2VP diblock copolymers via hydrogen bonding

We report a facile strategy to synthesize amphiphilic gold (Au) nanoparticles functionalized with a multilayer, micelle-like structure consisting of a Au core, an inner hydroxylated polyisoprene (PIOH) layer, and an outer polystyrene shell (PS). Careful control of enthalpic interactions via a systematic variation of structural parameters, such as number of hydroxyl groups per ligand (N(OH)) and styrene repeating units (N(PS)) as well as areal chain density of ligands on the Au-core surface (Σ), enables precise control of the spatial distribution of these nanoparticles. This control was demonstrated in a lamellae-forming poly(styrene-b-2-vinylpyridine) (PS-b-P2VP) diblock copolymer matrix, where the favorable hydrogen-bonding interaction between hydroxyl groups in the PIOH inner shell and P2VP chains in the PS-b-P2VP diblock copolymer matrix, driving the nanoparticles to be segregated in P2VP domains, could be counter balanced by the enthalphic penalty of mixing of the PS outer brush with the P2VP domains. By varying N(OH), N(PS), and Σ, the nanoparticles could be positioned in the PS or P2VP domains or at the PS/P2VP interface. In addition, the effect of additives interfering with the hydrogen-bond formation between hydroxyl groups on Au nanoparticles and P2VP chains in a diblock copolymer matrix was investigated, and an interesting pea-pod-like segregation of Au nanoparticles in PS domains was observed.     

Formation and Reversible Morphological Transition of Bicontinuous Nanospheres and Toroidal Micelles by the Self‐Assembly of a Crystalline‐b‐Coil Diblock Copolymer

We herein report the formation of two complex nanostructures, toroidal micelles and bicontinuous nanospheres, by the self‐assembly of the single structurally simple crystalline‐b‐coil diblock copolymer poly[bis(trifluoroethoxy)phosphazene]‐b‐poly(styrene), PTFEP‐b‐PS, in one solvent (THF) and without additives. The nature of these nanostructures in solution was confirmed by DLS and cryo‐TEM experiments. The two morphologies are related by means of a new type of reversible morphological evolution, bicontinuous‐to‐toroidal, triggered by changes in the polymer concentration. WAXS experiments showed that the degree of crystallinity of the PTFEP chains located at the core of the toroids was higher than that in the bicontinuous nanospheres, thus indicating that the final morphology of the aggregates is mostly determined by the ordering of the PTFEP core‐forming blocks.     

Polymerization of n‐hexyl isocyanate with CpTiCl2(OR) (R = functional group or macromolecular chain): A route to ω‐functionalized and block copolymers and terpolymers of n‐hexyl isocyanate

Well‐defined ω‐cholesteryl poly(n‐hexyl isocyanate) (PHIC–Chol), as well as diblock copolymers of n‐hexyl isocyanate (HIC) with styrene, PS‐b‐PHIC [PS = polystyrene; PHIC = poly(n‐hexyl isocyanate)], and triblock terpolymers with styrene and isoprene, PS‐b‐PI‐b‐PHIC and PI‐b‐PS‐b‐PHIC (PI = polyisoprene), were synthesized with CpTiCl₂(OR) (R = cholesteryl group, PS, or PS‐b‐PI) complexes. The synthetic strategy involved the reaction of the precursor complex CpTiCl₃ with cholesterol or the suitable ω‐hydroxy homopolymer or block copolymer, followed by the polymerization of HIC. The ω‐hydroxy polymers were prepared by the anionic polymerization of the corresponding monomers and the reaction of the living chains with ethylene oxide. The reaction sequence was monitored by size exclusion chromatography, and the final products were characterized by size exclusion chromatography (light scattering and refractive‐index detectors), nuclear magnetic resonance spectroscopy, and, in the case of PHIC–Chol, differential scanning calorimetry. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 6503–6514, 2005     

Using neutron reflectivity to determine the dynamic properties of a copolymer in a homopolymer matrix

A protocol for using neutron reflectivity to monitor the dynamic properties of a copolymer in a homopolymer matrix is described. This technique may be used to monitor a broad range of systems, as long as the copolymer and homopolymer form a miscible blend at low copolymer concentrations. Moreover, with knowledge of the Flory–Huggins interaction parameter between the copolymer and homopolymer, the molecular dynamic parameters of the copolymer, such as the tracer diffusion coefficient, segmental friction factor, and longest relaxation time, can be quantitatively determined. This technique is demonstrated by the determination of these parameters for a series of styrene/methyl methacrylate alternating copolymers dispersed in a matrix of deuterated poly(methyl methacrylate). Interestingly, the segmental friction factor of these alternating copolymers is significantly different from that of similar diblock copolymers. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 3235–3247, 2004     

Effect of homopolymer matrix on diblock copolymer grafted nanoparticle conformation and potential of mean force: A molecular simulation study

We study the effect of homopolymer A or B matrix on the conformations and effective interactions of AB diblock copolymer grafted particles using coarse‐grained molecular dynamics simulations. In an A homopolymer matrix we observe patchy conformations within the AB diblock copolymer grafted layer, where the number of B patches is controlled by the A‐A attractive interaction strength. In a B homopolymer matrix the grafted particle takes on a core‐corona conformation, where the inner A block aggregates near the particle surface and the outer B block forms a corona that interacts with the B matrix. The potential of mean force (PMF) between two particles in an A homopolymer matrix has a long‐ranged attractive well with a minima at intermediate distances corresponding to the location of the outer B block patches. The PMF between two particles in a B homopolymer matrix has an attractive well at short interparticle distances corresponding to the size of the inner A block. We isolate the contribution of the homopolymer matrix on the PMF between the two diblock copolymer grafted particles, by deducting the PMF in the absence of a matrix, assuming the contributions of the grafted particle and matrix to the PMF to be additive. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 76–88     

Control of self‐organized low‐dimensional morphology in poly(styrene‐b‐4vinylpyridine)/polystyrene blend thin films

The surface morphologies of poly(styrene‐b‐4vinylpyridine) (PS‐b‐P4VP) diblock copolymer and homopolystyrene (hPS) binary blend thin films were investigated by atomic force microscopy as a function of total volume fraction of PS (?_PS) in the mixture. It was found that when hPS was added into symmetric PS‐b‐P4VP diblock copolymers, the surface morphology of this diblock copolymer was changed to a certain degree. With ?_PS increasing at first, hPS was solubilized into the corresponding domains of block copolymer and formed cylinders. Moreover, the more solubilized the hPS, the more cylinders exist. However, when the limit was reached, excessive hPS tended to separate from the domains independently instead of solubilizing into the corresponding domains any longer, that is, a macrophase separation occurred. A model describing transitions of these morphologies with an increase in ?_PS is proposed. The effect of composition on the phase morphology of blend films when graphite is used as a substrate is also investigated. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 3496–3504, 2004     

Elucidation of protein adsorption behavior on polymeric surfaces: toward high-density, high-payload protein templates

The elucidation of protein adsorption behavior on polymeric surfaces is very important, since their use as arrays and carriers of biomolecules is ever growing for a wide variety of bioapplications. We evaluate protein adsorption characteristics on chemically homogeneous and heterogeneous polymeric surfaces by employing polystyrene-block-polymethylmethacrylate (PS-b-PMMA) diblock copolymer, PS homopolymer, PMMA homopolymer, and PS/PMMA blend as protein templates. We also investigate distance-dependent protein adsorption behavior on the interfacial region between PS and PMMA. We observe selective protein adsorption exclusively onto PS areas for the chemically heterogeneous PS-b-PMMA and PS/PMMA blend templates. On blend films, protein adsorption is highly favored on the PS regions located near the PS:PMMA interface over that on the PS areas situated away from the interface. Protein density on PS domains is inversely proportional to the separation distance between two neighboring PS:PMMA interfaces. We also observe a higher protein density on the PS-b-PMMA than on the PS or PMMA homopolymer templates. This effect is due to the fact that chemically heterogeneous PS-b-PMMA presents periodically spaced PS:PMMA interfaces on the nanometer scale, whereas no such interfaces are present on homopolymer films. The density of protein molecules on the heterogeneous PS-b-PMMA surface is approximately 3-4-fold higher than on the homogeneous PS surface for the identical experimental conditions. These results demonstrate that self-assembling, chemically heterogeneous, nanoscale domains in PS-b-PMMA diblock copolymers can be used as excellent, high-payload, high-density protein templates. The unique advantages of the diblock copolymer may prove the spontaneously constructed protein nanotemplates to be highly suitable as functional substrates in many proteomics applications.     

Surface segregation of polypeptide-based block copolymer micelles: An approach to engineer nanostructured and stimuli responsive surfaces

We describe the surface segregation of polypeptide-based block copolymer micelles to produce stimuli-responsive nanostructures at the polymer blend/air interface. Such structures were obtained by simultaneous surface migration and self assembly at the surface of diblock copolymer/homopolymer blends. We employed blends composed of homopolymer (PS) and an amphiphilic block copolymer polystyrene-b-poly(l-glutamic acid) (PS-b-PGA). The surface was functionalized based on the preferential segregation to the polymer blend/air interface of the hydrophilic PGA block of the diblock copolymer upon annealing to water vapor. The surface migration of the diblock copolymer to the interface was demonstrated both by XPS and contact angle measurements. As a consequence, the PGA interfacial attraction leads to a large surface excess on diblock copolymer which in turn, through macrophase and microphase separation, produced separated domains at the surface with regions composed either of homo or block copolymer. Herein we demonstrate that the use of asymmetric diblock copolymers with a higher content in PS lead to spherical micellar assemblies randomly distributed at the surface. As observed by AFM imaging the blend composition, i.e. the amount of block copolymer within the blend influences the density of micelles at the surface. Finally, when exposed to water, the pH affects the surface morphology. The PGA segments are collapsed at low pH values and extended at pH values above 4.8, thus inducing variations on the topography of the films at the nanometer scale.     

Influences of organometallic polymer‐derived catalyst dispersion on SWNT growth

Catalyst formation kinetics of a ferrocene‐containing homopolymer, polyferrocenylethylmethylsilane (PFEMS), is investigated as it relates to the catalysis of single walled carbon nanotubes (SWNTs) through a chemical vapor deposition (CVD) process. The formation and efficiency of the PFEMS‐based iron catalyst is compared with that of the corresponding polystyrene (PS)‐b‐PFEMS diblock copolymer. The PFEMS homopolymer contains 23 wt % iron, while PS‐b‐PFEMS, with a 25 vol % PFEMS content, is only 6% iron. Despite its lower iron content, spin‐cast PS‐b‐PFEMS films on SiO₂/Si substrates produce more active iron sites than spin‐cast PFEMS films during CVD growth of SWNTs. This is related to the self‐assembly of the block copolymer, where PFEMS domains are well dispersed in the PS matrix, which degrades at a CVD temperature of 920 °C to leave catalytically active elemental iron behind. On the contrary, the pure PFEMS films contain a high percentage of iron and silicon, which tend to transform into ceramic‐coated iron at this high temperature, thus rendering the iron inactive towards SWNT growth. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 758–765, 2007     

Use of block copolymer-stabilized cadmium sulfide quantum dots as novel tracers for laser scanning confocal fluorescence imaging of blend morphology in polystyrene/poly(methyl methacrylate) films

This paper describes the first use of polymer-coated quantum dots (QDs) as fluorescent tracers for LSCFM imaging of phase morphology in polymer blends. Cadmium sulfide (CdS) QDs stabilized at the surface with a PS-b-PAA block copolymer are shown to be well dispersed via their polystyrene (PS) brush layer in the PS phase of solvent-cast 40/60 (w/w) PS/PMMA blends. The QDs are excluded from the PMMA phase, providing excellent fluorescence contrast for LSCFM imaging of the phase-separated blends. The presence of PS-b-PAA-stabilized QDs does not appear to affect the blend morphology, since the observed morphologies are the same when the percentage of QDs within the PS phase is varied from 10 to 50 wt %. These QD fluorescent tracers are used to characterize several aspects of blend morphology in solvent-cast 40/60 PS/PMMA blends containing PS homopolymer with either 100 (low molecular weight) or 1250 (high molecular weight) repeat units. In the PS(1250)/PMMA blends, a percolating distribution of PMMA droplets (2-25 mum) in a PS matrix is observed in the bulk, and a distinct inversion in the continuous phase is found near the glass substrate. In the PS(100)/PMMA blends, a "phase-in-phase" morphology is found, consisting of large PS domains (20-100 mum) dispersed in a PMMA continuous phase and small PMMA domains (1-2 mum) scattered throughout the larger PS droplets. The observed change in blend structure is attributed to a lower interfacial tension for the lower molecular weight PS.     

Structure and Dynamics of Blends of Diblock Copolymers

The melt state rheological behavior of a series of blends of a lamellae‐forming nearly symmetrical polystyrene‐polyisoprene (PS‐PI) diblock copolymer with a sphere‐forming highly asymmetrical PS‐PI diblock copolymer is correlated to their structure obtained from small‐angle neutron scattering (SANS). The exponential composition dependence of the relaxation of fluctuations associated with the disordered asymmetrical diblock is similar to that observed in solutions of star polymers. Further, the zero‐shear viscosity of the blends in the disordered state exhibits unusually strong composition dependence. These dynamic properties are closely linked with the SANS‐based structural data that suggest strong compositional fluctuations, even in the disordered state.     

3D analysis of the lattice defects in the gyroid network structure of a block copolymer/homopolymer blend – The line-like defects in the gyroid network structure

The line-shaped defects formed in the gyroid bicontinuous microdomain structures in the solution-casting process of a blend of polystyrene-block-polyisoprene (SI) diblock copolymer and homopolystyrene (HS) were investigated using 3D electron tomography. The inclusion-type lattice defects composed of segregated HS formed during the morphological transition from the preexisted sponge phase to the stable gyroid phase induced by the phase separation between SI and HS. The gyroid grains grew by consuming the sponge phase and expelling excess HS as well as by impinging each other. Consequently, some expelled HS was trapped as the line-like inclusions in the gyroid grains. Structural rearrangement occurred so that low energy planes of the gyroid phase are facing the HS inclusions while the gyroid structure became continuous across/around the inclusions.