Morphology Of Diblock Copolymers Research Articles

Temperature Transferable and Thermodynamically Consistent Coarse-Grained Model for Binary Polymer Systems

For the simulations of polymer systems, coarse-grained (CG) models are often developed to tackle the length and time scale limitations that are not feasible by all-atom molecular dynamic simulations. However, due to the necessary simplification or reduction in atomistic degrees of freedom, CG models usually have poor transferability over various temperatures, compositions, or thermodynamic states. In this work, the structure-based iterative Boltzmann inversion (IBI) method is further developed to obtain temperature transferable and thermodynamically consistent CG potentials for binary polymer systems. In particular, the conventional IBI procedure is first utilized to optimize single-component CG potentials from homopolymer systems while cross-interaction potentials from a random block copolymer system. Afterward, a thermodynamic integration method is applied to refine the cross-interaction potential between two constituent components in binary systems until the Flory–Huggins parameter (χ) between two components is matched with experimental value. We apply the above optimization procedure for the polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) as an example, which is widely studied experimentally, and the χ value can be easily obtained in the literature. To validate the transferability of the obtained CG potential, systems of both PS and PMMA homopolymers, random block copolymers, diblock copolymers, and PS/PMMA blends with various compositions are simulated. Many properties, including the mass density, radius distribution function, and bonded distribution functions, between CG beads generated from CG simulations match very well with those from atomistic simulations in the simulated temperature range, i.e., from 453 to 500 K. More importantly, phase morphologies of diblock copolymers and phase diagram of PS/PMMA binary blends over wide temperature and compositional ranges generated from CG simulations using our model are in good agreement with experimental results and predictions from self-consistent field theory.

Aggregation behaviors of gradient and diblock copoly(2-oxazoline) monolayers at the air-water interface

This study reveals the different behavior of amphiphilic gradient and diblock copolymers based on aliphatic and aromatic 2-oxazolines. The copolymers were synthesized by the polymerization of hydrophilic 2-ethyl-2-oxazoline with hydrophobic 2-(4-octyloxyphenyl)-2-oxazoline. The aggregation behavior of gradient and diblock copolymers with different ratios between hydrophilic and hydrophobic units was explored. Surface pressure (Π)–surface area ( A ) isotherms were measured at the air-water interface. The gradient copolymers produced higher surface pressures and smaller limiting areas, while the diblock copolymers exhibit relatively flexible behaviors. Langmuir–Blodgett monolayers were transferred on the hydrophilic silicon wafer substrates at various surface pressure levels, and the nanostructure of the films was investigated by using atomic force microscopy and X-ray reflectivity measurements. It was observed that the morphologies of gradient and diblock copolymers were similar, indicating that the films form a uniform monolayer. The diblock copolymers produced relatively small changes in thickness upon the compression, compared to the changes in gradient copolymer monolayer films. The thickness of the gradient copolymers which have a high hydrophilic ratio at low surface pressure increases by increasing surface pressure and hydrophobic ratio. But the diblock copolymers do not change the thickness by increasing the surface pressure and hydrophobic ratio. The results indicate that the gradient copolymer monolayers have relatively rigid domains where hydrophilic and hydrophobic parts are smeared. On the other hand, the diblock copolymer has a flexible hydrophilic cushioning effect. This study provides insights into the development of novel materials based on hydrophilic-hydrophobic interactions. • The different behavior of amphiphilic gradient and diblock copolymers based on aliphatic and aromatic 2-oxazolines. • The aggregation behavior of gradient and diblock copolymers with different ratios between hydrophilic and hydrophobic units was explored. • The gradient copolymer monolayers have relatively rigid domains where hydrophilic and hydrophobic parts are smeared. • The diblock copolymer has a flexible hydrophilic cushioning effect.

Structure and Morphology of Crystalline Syndiotactic Polypropylene-Polyethylene Block Copolymers.

A study of the structure and morphology of diblock copolymers composed of crystallizable blocks of polyethylene (PE) and syndiotactic polypropylene (sPP) having different lengths is reported. In both analyzed samples, the PE block crystallizes first by cooling from the melt (at 130 °C) and the sPP block crystallizes after at a lower temperature. Small angle X-ray scattering (SAXS) recorded during cooling showed three correlation peaks at values of the scattering vector, q1 = 0.12 nm−1, q2 = 0.24 nm−1 and q3 = 0.4 nm−1, indicating development of a lamellar morphology, where lamellar domains of PE and sPP alternate, each domain containing stacks of crystalline lamellae of PE or sPP sandwiched by their own amorphous phase of PE or sPP. At temperatures higher than 120 °C, when only PE crystals are formed, the morphology is defined by the formation of stacks of PE lamellae (17 nm thick) alternating with amorphous layers and with a long period of nearly 52 nm. At lower temperatures, when crystals of sPP are also well-formed, the morphology is more complex. A model of the morphology at room temperature is proposed based on the correlation distances determined from the self-correlation functions extracted from the SAXS data. Lamellar domains of PE (41.5 nm thick) and sPP (8.2 nm thick) alternate, each domain containing stacks of crystalline lamellae sandwiched by their own amorphous phase, forming a global morphology having a total lamellar periodicity of 49.7 nm, characterized by alternating amorphous and crystalline layers, where the crystalline layers are alternatively made of stacks of PE lamellae (22 nm thick) and thinner sPP lamellae (only 3.5 nm thick).

Side-Chain Density Driven Morphology Transition in Brush–Linear Diblock Copolymers

We report the synthesis and self-assembly of brush-linear diblock copolymers with variable side-chain length and density. Poly(pentafluorophenyl acrylate-g-ethylene glycol)-b-polystyrene ((PPFPA-g-PEG)-b-PS) brush-linear diblock copolymers are prepared by sequential reversible addition-fragmentation chain transfer (RAFT) polymerization of PPFPA and PS, followed by postpolymerization reaction between the precursor PPFPA-b-PS diblock copolymer and amine-functionalized PEG. By controlling the PEG chain length and the degree of substitution, we obtained brush-linear diblock copolymers with different side-chain lengths and densities. The solid-state morphologies of the diblocks are then examined by small-angle X-ray scattering (SAXS). At low PEG side-chain density, the segregation of PEG and PS away from PPFPA leads to the formation of PEG and PS lamellar domains with PPFPA in the interface. At high PEG side-chain density, the segregation is between the PPFPA-g-PEG brush block and the PS linear block, and the domain morphology is determined by the composition of the brush block. A partial experimental phase diagram is presented, and it illustrates the importance of both side-chain length and density on the microdomain morphology of brush-linear diblock copolymers.

Morphology of Isotactic Polypropylene–Polyethylene Block Copolymers Driven by Controlled Crystallization

A study of the morphology of diblock copolymers composed of two crystalline blocks of isotactic polypropylene (iPP) and polyethylene (PE) is shown. The samples form phase-separated structures in th...

Confined Copolymer Structures

Bulk morphologies of diblock copolymers have been extensively studied in literature both experimentally and theoretically. Depending on the composition of the blocks it is comprised of, the bulk structures correspond to lamellae, gyroids, cylinder hexagonal arrangements and BCC structures of spheres. Recent research has shown that confinement is a powerful tool to break the symmetry of a structure, and obtain new shapes, different from those of the bulk.The process of forming new morphologies by confinement in nano-droplets, created from a dewetting process, was simulated. The structures obtained showed a great similarity with the experimental results present in the literature.The developed model captures the fundamental interactions that determine the dynamics of the phase separation process of the confined copolymer system between a rigid substrate and a free surface.

Ionic and Nonspherical Polymer Nanoparticles in Nonpolar Solvents

A series of ionic diblock copolymer nanoparticles was prepared in a typical nonpolar solvent (n-dodecane) via polymerization-induced self-assembly (PISA). A single cationic repeat unit was incorporated into the poly(stearyl methacrylate) (PSMA) stabilizer of otherwise uncharged poly(stearyl methacrylate)–poly(benzyl methacrylate) (PSMA–PBzMA) diblock copolymer nanoparticles. By using short PSMA stabilizer blocks, it was possible to obtain nanoparticles with the range of morphologies expected (spheres, worms, and vesicles). For nanoparticles where all stabilizer chains possessed an ionic group, higher-order morphologies were obtained at lower PBzMA degrees of polymerization than corresponding uncharged particles, and the particles were electrophoretic. For nanoparticles where only a fraction of the stabilizer chains contained an ionic group, higher-order morphologies were obtained at precisely the same PBzMA degrees of polymerization, and the electrophoretic response was greater than when the shell was fully ionic. These particles with a partially ionic shell are a fascinating system, providing morphologies that can be predicted from the existing knowledge of the diblock copolymer morphology yet with the highest possible electrophoretic mobility.

The size and affinity effect of counterions on self-assembly of charged block copolymers.

The effect of counterions' size and affinity on the microphase separated morphologies of neutral-charged diblock copolymers is investigated systematically using a random phase approximation (RPA) and self-consistent field theory (SCFT). The phase diagrams as a function of χAB and fA at different counterion sizes and different affinities to neutral blocks are constructed, respectively. Stability limits calculated using the RPA are in good agreement with the disorder-body-centered cubic phase boundaries from SCFT calculations. It was found that increasing the size of counterions causes the phase diagram to shift upward and leftward, which is attributed to electrostatic interactions and the intrinsic volume of counterions. The domain size of the ordered phase shows an unexpected tendency that it decreases with increasing counterions' size. The counterions' distributions in H and G phases demonstrate that it is electrostatic interaction, instead of packing frustration, that plays a leading role in such systems. For finite size counterions, with the increase in affinity between counterions and neutral blocks, the phase diagram shifts upward, indicating the improved compatibility between different blocks. Furthermore, the affinity effect between counterions and neutral blocks can be mapped into an effective Flory parameter χAB ' = χAB + 0.27χBC.

In Situ Small-Angle X-ray Scattering Studies During Reversible Addition-Fragmentation Chain Transfer Aqueous Emulsion Polymerization.

Polymerization-induced self-assembly (PISA) is a powerful platform technology for the rational and efficient synthesis of a wide range of block copolymer nano-objects (e.g., spheres, worms or vesicles) in various media. In situ small-angle X-ray scattering (SAXS) studies of reversible addition–fragmentation chain transfer (RAFT) dispersion polymerization have previously provided detailed structural information during self-assembly (see M. J. Derry et al., Chem. Sci.2016, 7, 5078–509030155157). However, conducting the analogous in situ SAXS studies during RAFT aqueous emulsion polymerizations poses a formidable technical challenge because the inherently heterogeneous nature of such PISA formulations requires efficient stirring to generate sufficiently small monomer droplets. In the present study, the RAFT aqueous emulsion polymerization of 2-methoxyethyl methacrylate (MOEMA) has been explored for the first time. Chain extension of a relatively short non-ionic poly(glycerol monomethacrylate) (PGMA) precursor block leads to the formation of sterically-stabilized PGMA-PMOEMA spheres, worms or vesicles, depending on the precise reaction conditions. Construction of a suitable phase diagram enables each of these three morphologies to be reproducibly targeted at copolymer concentrations ranging from 10 to 30% w/w solids. High MOEMA conversions are achieved within 2 h at 70 °C, which makes this new PISA formulation well-suited for in situ SAXS studies using a new reaction cell. This bespoke cell enables efficient stirring and hence allows in situ monitoring during RAFT emulsion polymerization for the first time. For example, the onset of micellization and subsequent evolution in particle size can be studied when preparing PGMA29-PMOEMA30 spheres at 10% w/w solids. When targeting PGMA29-PMOEMA70 vesicles under the same conditions, both the micellar nucleation event and the subsequent evolution in the diblock copolymer morphology from spheres to worms to vesicles are observed. These new insights significantly enhance our understanding of the PISA mechanism during RAFT aqueous emulsion polymerization.

Engineering Scale Simulation of Nonequilibrium Network Phases for Battery Electrolytes

Diblock copolymers play an important role in the fabrication of battery materials and fuel cells. For these applications, one block provides mechanical stability, whereas the other is conducting. The application characteristics of the material critically depend on the morphology of the multicomponent material, and three-dimensional, percolating domains of the conducting domains are preferred. In this work, we investigate the nonequilibrium morphology of diblock copolymers after a quench from the disordered phase. After the spinodal self-assembly, we observe three-dimensionally percolating network structures for volume fractions, f ≥ 8/32, of the conducting component even if the equilibrium phases exhibit different percolation properties. We quantify the conductivity and tortuosity of these structures via a simple random-walk model and observe that the conductivity of the nonequilibrium structures is significantly smaller than that of the equilibrium phases. We also find large but finite-sized, fractal-lik...

Correlation between morphology and anisotropic transport properties of diblock copolymers melts.

Molecular simulations of coarse-grained diblock copolymers (DBP) were conducted to study the effect of segregation strength and morphology on transport properties. It was found that in the strong segregation limit (i.e., high χN, where χ is the Flory-Huggins parameter and N is the degree of polymerization), the presence of the DBP interfaces imposes topological constraints similar to those of entanglements as manifested in the rheological signature of the polymer (i.e., a plateau modulus). Furthermore, compared to the behavior of isotropic melts, the crossover from Rouse to reptation scaling of the self-diffusion coefficient (D) parallel to the DBP interface takes place at a smaller N, an effect that depends on temperature and is more pronounced in the Lamellae morphology than in the hexagonal cylinder morphology. Additionally, it is shown that for an entangled melt (i.e., N ≫ Ne where Ne is the entanglement length) block retraction is instrumental for chains to diffuse parallel to the interface of lamellar layers. Lastly, it is found that the anisotropic viscosity of different morphologies is mostly affected by the orientation of the chains relative to the shear flow direction, exhibiting reduced values when chains align in the neutral or flow directions.

Transitions between Lamellar Orientations in Shear Flow

Shear flow is a versatile strategy to align microphase-separated morphologies of diblock copolymers over macroscopic scales. Details of the local mechanism of reorientation toward the steady, nonequilibrium state, however, are only incompletely understood. Using large scale molecular simulation as well as experiments, we study the shear-alignment mechanism using lamella-forming, symmetric, unentangled diblock copolymers in steady and oscillatory shear flow. First we study homogeneously oriented systems and investigate the stability of different orientations with respect to the shear flow by the Rayleighian. Second, we investigate the process of reorienting an unstable grain with parallel orientation embedded in a matrix of stable, perpendicularly oriented lamellae. We observe two different reorientation mechanisms as a function of the shear rate: a fast transition, which is comparable to experimental conditions in large-amplitude oscillatory shear (LAOS) tests, and a slower transition occurring at lower shear rates. We show that for high shear rates the long-range orientational order of the lamellae inside the unstable grain disintegrates, while the grain remains spatially structured with the same characteristic length scale (similar to a microemulsion). At lower shear rates, however, we observe a shrinking of the unstable grain, i.e., a directed movement of the grain boundaries. Additionally, we compare the results of steady shear with oscillatory shear.

Two New Triply Periodic Bicontinuous Network Structures for Molten Block Copolymers

The evolution of new triply periodic bicontinuous morphologies of molten diblock and triblock copolymers is theoretically studied in a mean-field level. The evolved mesophases are probed through characteristic peaks in the scattering functions and the real-space visualization. The ABC or AB copolymers at selected compositions and segregation levels exhibit holey network structures with $$Ia\bar 3d$$ and $$I\bar 43d$$ symmetries, where the channels are wholly connected with tetrapod units. The curvatures, surface areas, and genera g’s of their dividing surfaces are estimated by matching them to proper model surfaces. The obtained surface properties of the newly developed networks are discussed in comparison with those of the known bicontinuous networks such as double gyroids and double diamonds.

Ion Correlation Effects in Salt-Doped Block Copolymers.

We apply classical density functional theory to study how salt changes the microphase morphology of diblock copolymers. Polymers are freely jointed and one monomer type favorably interacts with ions, to account for the selective solvation that arises from different dielectric constants of the microphases. By including correlations from liquid state theory of an unbound reference fluid, the theory can treat chain behavior, microphase separation, ion correlations, and preferential solvation, at the same coarse-grained level. We show good agreement with molecular dynamics simulations.

Time-Resolved SAXS Studies of the Kinetics of Thermally Triggered Release of Encapsulated Silica Nanoparticles from Block Copolymer Vesicles.

Silica-loaded poly(glycerol monomethacrylate)-poly(2-hydroxypropyl methacrylate) diblock copolymer vesicles are prepared in the form of concentrated aqueous dispersions via polymerization-induced self-assembly (PISA). As the concentration of silica nanoparticles present during the PISA synthesis is increased up to 35% w/w, higher degrees of encapsulation of this component within the vesicles can be achieved. After centrifugal purification to remove excess non-encapsulated silica nanoparticles, SAXS, DCP, and TGA analysis indicates encapsulation of up to hundreds of silica nanoparticles per vesicle. In the present study, the thermally triggered release of these encapsulated silica nanoparticles is examined by cooling to 0 °C for 30 min, which causes in situ vesicle dissociation. Transmission electron microscopy studies confirm the change in diblock copolymer morphology and also enable direct visualization of the released silica nanoparticles. Time-resolved small-angle X-ray scattering is used to quantify the extent of silica release over time. For an initial silica concentration of 5% w/w, cooling induces a vesicle-to-sphere transition with subsequent nanoparticle release. For higher silica concentrations (20 or 30% w/w) cooling only leads to perforation of the vesicle membranes, but silica nanoparticles are nevertheless released through the pores. For vesicles prepared in the presence of 30% w/w silica, the purified silica-loaded vesicles were cooled to 0 °C for 30 min, and SAXS patterns were collected every 15 s. A new SAXS model has been developed to determine both the mean volume fraction of encapsulated silica within the vesicles and the scattering length density. Satisfactory data fits to the experimental SAXS patterns were obtained using this model.

Copolymer-Nanoparticles Mixture in a Cylindrical Pore

Computer simulation is carried out for investigating the effect of nanoparticles on diblock copolymer morphology under cylindrical confinement. The phase diagrams of polymer nanocomposites with nanoparticle-block wetting strength and concentration of nanoparticles are obtained in different nanopores. In small diameter nanopore, there is almost no influence of nanoparticles on the diblock copolymer morphology because of the stronger confinement effect; in middle diameter nanopore, the system can self-assemble into various novel structures due to the interaction between confinement effect and nanoparticles effect; in large diameter nanopore, due to the stronger effect of nanoparticles, a disorder-order-disorder phase transition occurs with the wetting strength and concentration of nanoparticles increasing. This result can be useful in designing new nanocomposites with advanced electrical conductivity and/or mechanical strength.

INTERRELATIONS BETWEEN MORPHOLOGY AND SOFTENING BEHAVIOR IN SELF-ASSEMBLED POLY (BUTADIENE-BLOCK-(STYRENE-STAT-BUTADIENE)) COPOLYMERS

ABSTRACTTwo series of poly(butadiene-block-(styrene-stat-butadiene) (PB-SBR) diblock copolymers were synthesized to study interrelations between microphase-separated state and relaxation dynamics. Series I consists of six symmetric PB-SBR samples (ΦSBR ∼ 0.50) with different styrene contents in the random SBR block (21 mol% ≤ xS,SBR ≤ 52 mol%). Series II contains six asymmetric PB-SBR diblock copolymers with almost constant styrene content in the SBR block (xS,SBR = 32 ± 4 mol%) and SBR volume fractions in the range of 0.20 ≤ ΦSBR ≤ 0.69. All PB-SBR diblock copolymers have a total molecular weight of about 200 kg/mol with 1,2-vinyl content of about 8.5 and 15.8 mol% in the soft PB and in the hard SBR block, respectively. A novel synthesis route has been developed so as to maintain low 1,2-vinyl contents in both blocks and a random distribution of the styrene units in the SBR block. The softening behavior of non–cross-linked PB-SBR diblocks was investigated by differential scanning calorimetry, providing first information about miscibility. The morphology of cross-linked PB-SBR diblock copolymers, which have been vulcanized with sulfur at 150 °C using standard procedures, was studied by atomic force microscopy. Dynamic shear measurements aimed at understanding the influence of morphological changes on the segmental dynamics of both phases, αPB and αSBR, were performed on vulcanized samples. Atomic force microscopy results and shear data indicate that symmetric diblock copolymers with styrene content higher than 27 mol% in the SBR block are well microphase separated. Strongly segregated states are also observed for asymmetric diblock copolymers with volume fractions 0.28 ≤ ΦSBR ≤ 0.60 showing cylindrical and lamellar morphologies. The origin of changes in the αPB and αSBR dynamics depending on morphology and segregation strengths is considered. Implications for the use of rubbery PB-SBR block copolymers in composites for tire treads are discussed.

Poly(N-2-(methacryloyloxy)ethyl pyrrolidone)-poly(benzyl methacrylate) diblock copolymer nano-objects via RAFT alcoholic dispersion polymerisation in ethanol

N-2-(methacryloyloxy)ethyl pyrrolidone (NMEP) is a highly polar monomer that is much more readily copolymerised with methacrylic monomers than its structural analogue, N-vinyl pyrrolidone. The RAFT solution polymerisation of NMEP has been conducted in ethanol at 70 °C while varying the degree of polymerisation (DP) from 70 to 400. Reducing the CTA/initiator molar ratio from 10.0 to 3.0 when targeting PNMEP100 led to a three-fold rate enhancement and increased the final monomer conversion from 72% to 98%. Targeting DPs of 200 or greater led to lower monomer conversions. DMF GPC analysis of a series of PNMEPx homopolymers confirmed a linear increase in molecular weight with conversion and relatively low dispersities (Mw/Mn < 1.26). A PNMEP50 macro-CTA was chain-extended with benzyl methacrylate (BzMA) via RAFT alcoholic dispersion polymerisation. High BzMA conversions (>90%) were obtained and electron microscopy studies indicated that a range of diblock copolymer morphologies, e.g. spheres, worms and vesicles, were produced by polymerisation-induced self-assembly (PISA). Finally, a PNMEP47-PBzMA243 diblock copolymer was synthesised via a convenient ‘one-pot’ protocol whereby a PNMEP47 macro-CTA was first prepared at 97% conversion, followed by in situ RAFT dispersion polymerisation of BzMA to produce diblock copolymer nano-objects. TEM analysis of aliquots taken during the diblock copolymer synthesis indicated a gradual evolution in copolymer morphology from spherical micelles after 1 h to a pure vesicle phase after 8 h via intermediate mixed phases (including worms).

Synthesis, liquid crystalline mesophases and morphologies of diblock copolymers composed of a poly(dimethylsiloxane) block and a nematic liquid crystalline block

ABSTRACTIn this study, a series of liquid crystalline diblock copolymers, composed of a soft poly(dimethylsiloxane) (PDMS) block with a defined length and a side-on liquid crystalline poly(3ʹʹ-acryloyloxypropyl 2,5-di(4ʹ-butyloxybenzoyloxy) benzoate) (P3ADBB) block with different lengths, are synthesised by the atom transfer radical polymerisation. The macromolecular structures, liquid crystalline properties and the microphase-separated morphologies of the diblock copolymer are investigated by 1H NMR, FT-IR, GPC, POM, DSC and TEM. The results show that the well-defined diblock copolymers (PDMSn-b-P3ADBBm) possess four different soft/rigid ratios (n = 58, m = 10, 25, 42, 66) and relatively narrow molecular distributions (PDI ≤ 1.30). P3ADBB blocks of the copolymers show nematic sub-phases, which are identical to the mesomorphic behaviour of the homopolymer P3ADBB. After being annealed at 90°C in a vacuum oven for 48 h, the copolymers form a lamellar morphology when m = 10 and morphologies of PDMS spheres embedded in P3ADBB matrix when m = 25, 42 and 66.

Impact of intrinsic backbone chain stiffness on the morphologies of bottle-brush diblock copolymers

Self-assembly methods are used for the production of photonic and nanoporous materials derived from block copolymers. In this context, bottle-brush copolymers have demonstrated a number of advantages over the respective linear copolymers, such as faster self-assembly kinetics and a richer morphology behavior. However, the effect of intrinsic molecular stiffness on the morphology of bottle-brush copolymers has been previously overlooked. Here, we investigate the role of the intrinsic backbone chain stiffness on the morphology behavior of bottle-brush diblock copolymers by using molecular dynamics simulations of a bead-spring coarse-grained model. We focus on bottle-brush macromolecules having blocks of the same volume fraction and asymmetric-in-length side chains. We find that an increase of the backbone stiffness triggers an order–order transition from hexagonal packed cylinders to lamellar morphologies with asymmetric domain spacings, which is of particular interest for the manufacturing of nanopatterning and semiconductor applications. The change in the morphology is due to the effective many-body attractions between the blocks resulting in a parallel stacking that disrupts the symmetry of the cylindrical morphology. We anticipate that our work will underline the significance of intrinsic molecular stiffness in the self-assembly of polymer systems, which has been previously neglected.