Microphase-separated State Research Articles

Integration of Recognition Elements with Macromolecular Scaffolds: Effects on Polymer Self-Assembly in the Solid State

Polystyrene scaffolds were grafted with model functionalities featuring strongly interacting hydrogen bonding and aromatic stacking elements. Both glass transition temperatures and degree of microphase separation in functionalized block copolymers depend on the nature of the functionality and in particular on the strength of intermolecular interactions. The polymers under study were amorphous; it was found, however, that domain periodicities of functionalized diblock copolymers in the microphase-separated state are extremely sensitive to local interactions between functionalities and can express even subtle differences in interaction strength. The results emphasize the ability to fine-tune polymer microstructure and thermomechanical behavior using supramolecular chemistry.

Phase behavior and structure formation of hairy-rod supramolecules.

Phase behavior and microstructure formation of rod and coil molecules, which can associate to form hairy-rod polymeric supramolecules, are addressed theoretically. Association induces considerable compatibility enhancement between the rod and coil molecules and various microscopically ordered structures can appear in the compatibility region. The equilibria between microphase-separated states, the coil-rich isotropic liquid and the rod-rich nematic are discussed in detail. In the regime where hairy-rod supramolecules with a high grafting density appear as a result of the association, three phase diagram types are possible depending on the value of the association energy. In the low grafting density regime only the lamellar microstructure is proven to be stable.

Phase Separation in a Polymer Solution Induced by Steady and Large Amplitude Oscillatory Shear Flow

Neutron- and light-scattering measurements reveal that large-amplitude oscillatory shear flow can drive entangled polymer solutions to a completely microphase-separated state with sharp interfacial boundaries while steady shear flow does not. For steady shear flow, interfacial boundaries remain less well-defined for all shear rates measured, indicating less complete phase separation. Results are discussed within the context of recent theoretical models.

Self-assembly of globular particles in a nematic dispersion of colloidal rods

Small spherical guest particles added to a nematic liquid-crystalline dispersion of colloidal rods self-assemble linearly into chain-like aggregates. We argue that the formation of these chains is induced by the excluded-volume coupling of the globules to the nematic matrix, and that pretransitional fluctuations in the mixture give rise to the structural reorganization of the linear aggregates observed in experiment. Ultimately, the repulsive interactions between the globules and the rods either promote large-scale demixing or a transition to a microphase-separated lamellar state, depending on the concentration of rods in the host dispersion.

Morphologies and Mechanical Properties of a Series of Block-Double-Graft Copolymers and Terpolymers

Morphological characteristics and mechanical properties of a series of block-double-graft (BDG) copolymers and terpolymers polystyrene−[1,2-polybutadiene-g-X2] (X = 1,4-polybutadiene, polyisoprene, polystyrene, and polystyrene-b-polyisoprene diblocks) were investigated by transmission electron microscopy (TEM), small-angle X-ray scattering (SAXS), and tensile testing. All BDG materials have linear polystyrene−1,2-polybutadiene (PS-b-1,2-PBD) diblock copolymer backbones. Two identical branches are grafted at every randomly distributed tetrafunctional junction point on the 1,2-PBD part of the backbone. Standard microstructures, such as body-center-cubic spheres, hexagonally packed cylinders, and lamellae, are obtained at different total PS volume fractions. It is found that when the branches are polydienes, the BDG molecules form the same morphologies as their linear diblock counterparts. In such cases, phase separation occurs between the polystyrene domain and a combined diene microdomain formed by the backbone 1,2-PBD part and the polydiene branches. In BDG materials in which the branches are polystyrene−polyisoprene diblock copolymers, lamellae are obtained at a total PS volume fraction close to 0.50. It is found that the domain spacings of these materials are predominately determined by the molecular weights of the diblock branches instead of the backbones. A lamellae-forming BDG terpolymer with an average of three tetrafunctional junction points per molecule exhibited characteristic thermoplastic elastomer properties with a stress at break of 32 MPa and strain at break of 1000%. It is proposed that the high strength of this BDG terpolymer is attributed to the chain conformation in the microphase-separated state. The elastic PBD blocks of the backbone bridge adjacent PS domains through multiple junction points, resulting in the enhanced elastomeric properties. Several parameters are found to influence the mechanical properties of these BDG materials: (1) the existence of backbone PS, (2) the molecular weight of the branches, and (3) the number and functionality of branch points on the 1,2-PBD part of the backbone.

Comb Copolymer Brush with Chemically Different Side Chains

An investigation of side chain microphase separation within a single comb copolymer molecule containing chemically different A and B side chains has been carried out. Expressions for the transition point in a good ( ∼ N-3/8), marginal ( ∼ N-1/2), θ ( ∼ N-2/3), and poor ( ∼ N-1) solvent are derived both by a mean field calculation and by scaling arguments. Properties of the system below and above the transition point are described. Some unusual “bowlike” conformations are predicted for a single molecule in the microphase separated state in a good solvent.

Phase Behavior and Dynamics of the ABA Triblock Copolymer Poly(ethylene glycol) Distearate Doped with Alkali Metal Salts

The ABA triblock copolymer poly(ethylene glycol) distearate (PEGD), average Mn ca. 930, complexed with lithium and sodium perchlorates has been studied by 1H, 7Li, 13C, and 23Na solid-state nuclear magnetic resonance (NMR), SAXS, DSC, and polarized-light optical microscopy. Unlike other solid polymer electrolytes, highly Li+-doped PEGD samples exhibit sharp 7Li NMR quadrupolar powder patterns even at temperatures well above the melting point, indicating that this triblock copolymer is microphase separated and the dynamics in the PEG phase are anisotropic. Measurements of the 7Li central transition line width in highly doped samples show three distinct line narrowings, due to the poly(ethylene glycol) glass transition (∼−20 °C), the stearate melting point of the polymer (∼35 °C), and an order−disorder transition (∼72 °C). 23Na NMR measurements yield similar results. SAXS, DSC, and optical microscopy with polarized light confirm the presence of a microphase-separated state up to ∼72 °C. 13C and 1H NMR show that the segmental mobility in the ordered state is reduced compared to the isotropic melt. The results confirm the previously proposed order−disorder model to explain the dependence of the ionic conductivity on the lithium concentration for Li+-doped PEGD samples.

Steady-shear rheology of block copolymer melts and concentrated solutions: Defect-mediated flow at low stresses in body-centered-cubic systems

We investigate the low-deformation-rate behavior of sphere-forming block copolymers in the microphase-separated state by both creep and dynamic viscoelastic measurements. Polystyrene-polyisoprene and polystyrene-poly(ethylene-alt-propylene) diblock, triblock, and starblock systems of varying molecular weights and compositions, all of which form a polycrystalline body-centered-cubic (bcc) mesophase, are studied as melts and as concentrated solutions in a matrix-selective solvent. At low stresses, the bcc lattice is preserved, and steady-shear measurements reveal a Newtonian viscosity behavior. Following models for high-temperature creep in metals and ceramics, and incorporating the thermodynamic barrier for endblock pullout from spherical microdomains, we have been able to collapse the zero-shear viscosity versus temperature data for diblock melts of a given composition onto a master curve. By analogy with the behavior of polycrystalline metals and ceramics, we anticipated a substantial effect of thermal history on viscosity, through its impact on the grain size. However, only a modest thermal history effect was observed.

Compositionally modulated phase in crystalline n-alkane mixtures.

We report an x-ray scattering study of the compositionally modulated microphase separated state (muPSS) of binary n-alkane (C23H48:C28H58) mixtures. By employing a quenching technique, we obtained many orders of sharp lamellar and superlattice reflections. The muPSS is a regular superstructure consisting of pure C23 layers, and layers rich in C28 but containing up to approximately 14% C23 plus voids. A monolayer of the minority component regularly alternates with one or more layers of the majority. The mechanism for thermodynamic stability of the muPSS may be related to the entropy associated with longitudinal displacements of the molecules.

A Molecular Dynamics Simulation on the Self-Assembly of ABC Triblock Copolymers, 1. Effects of Block Composition in Symmetric Triblock Copolymers

The self-assembly of ABC triblock copolymers in the microphase-separated state is investigated using an isothermal-isobaric molecular dynamics simulation. For the validation of our simulation scheme, ABA triblock copolymers are also simulated. We examine the effect of the composition (f B ) of symmetric triblock copolymers on the morphology realized in these copolymers, keeping other parameters fixed. For ABA triblock copolymers, the transition from lamellar to cylindrical morphologies is observed with increasing the composition from f B = 0.5 to f B = 0.75, and such behavior is supported by calculation results of scattering putterns. These simulated results agree well with experimental and theorotical ones, validating our simulation method. More complex structures are predicted for ABC triblock copolymers. If midblock B is the minor component, its structures are changed from lamellar, cylindrical, to spherical morphology at the interface between A/C lamellae as f B decreases. For ABC triblock copolymers with the mldblock B as the major component, the morphology of end blocks in the matrix composed of the midblock is changed from tricontinuous to spherical structures as f B increases.

Phase Coherence and Microphase Separation Transitions in Diblock Copolymer Thin Films

In thin films of diblock copolymers it was found that microphase separation and the establishment of phase coherence can occur as two distinct events in diblock copolymer thin films, at widely separated temperatures. At temperatures far removed from the bulk microphase separation transition temperature increasing overlap of the decaying concentration profiles from the substrate and air interfaces leads to a sharp onset of phase coherence throughout a thin film and the discretization of the film thickness, despite only weak microphase separation in the center of the film. A further transition to a fully microphase separated state occurs at temperatures close to the microphase separation transition temperature, but this transition becomes broader and less distinct with decreasing film thickness. In thicker films, on the other hand, phase coherence is not established until the ordering transition temperature is reached, due to the first-order nature of the transition and the finite coherence length in the disordered state. In thick films, phase coherence throughout the film is established either simultaneously or after complete microphase separation has occurred.

Melting Behavior of Poly(ε-caprolactone)-block-Polybutadiene Copolymers

The melting behavior of poly(e-caprolactone)-block-polybutadiene (PCL-b-PB) copolymers, crystallized at various temperatures Tc from the melt (homogeneous state or microphase-separated state), has ...

Effect of chain end group hydrophobicity onsurface aggregation structure of poly(styrene- block-4-vinylpyridine) symmetric diblock copolymer films

Proton-terminated polystyrene- block-poly(4-vinylpyridine) (P(St- b-4VP)-H) and 3,3,3,-trifluoropropyl-dimethylsilane-terminatedpolystyrene- block-poly(4-vinylpyridine) (P(St- b-4VP)-C 2C F) symmetric diblock copolymers were synthesized by a living anionic polymerization. Differential scanning calorimetry (DSC) measurements revealed that both diblock copolymers were in a microphase-separated state in the bulk. The results of X-ray photoelectron spectroscopy (XPS) measurements revealed that in the case of the P(St- b-4VP)-H film, the surface weight fraction of poly(4-vinylpyridine) (P4VP) decreased drastically on annealing at 432 K for 90 h in order to minimize the magnitude of the free energy at the air-polymer interface, whereas in the case of the P(St- b-4VP)-C 2C F film, the surface P4VP weight fraction was almost the same as the bulk value even after annealing at 423 K for 90 h. Also, scanning force microscopy observations (SFM) were in good agreement with the XPS results. These results clearly indicate that the surface molecular aggregation state of the hydrophobic-hydrophilic diblock copolymer film can be controlled by the hydrophobicity of the chain end groups.

Phase behaviour in binary mixtures of diblock copolymers as analysed by the random phase approximation calculations

Binary mixtures of diblock copolymers are interesting materials for controlling the size and the morphology of the microdomains in novel ways. We report here simulation results for the phase behaviour in binary mixtures of the diblock copolymers. The random phase approximation calculations were conducted for the binary mixtures of diblock copolymers, α: (A-B) 1 and β: (A-B) 2 to discuss stability of the homogeneous (α + β) mixture. The parameter values used for the simulations are that total degree of polymerization, N(= N α = N β ), is 1000, the radius of gyration, R g(= R g,α = R g,β), is l0nm, and the segmental volume, v A = v B = 100 cm 3 mol −1 for monodisperse and polydisperse samples ( 1.0 ≤ MW Mn ≤ 1.5 ). The fraction of A in α(f A α) and that in β(f A β) are varied in such a way to satisfy f A α + f A β = 1. As a result, it is predicted that the homogeneous mixture undergoes microphase separation as the segregation increases, for f A α larger than a particular value (f A α, crit) which is dependent on the values of M w M n . On the other hand, for f A α > f A α, crit the macroscopic phase separation between α and β is expected to occur prior to the microphase separation. It is also found that the wavelength of the dominant concentration fluctuation in the homogeneous mixture increases gradually as f A α approaches f A α, crit from the upper side of f A α. At f A α = f A α, crit, the wavelength diverges, indicating the macrophase separation. These results give some implications to the unit size of the microdomains formed upon the disorder-to-order transitions (ODT) for f A α > f A α, crit. Namely, the unit size might be larger than those of the component pure diblocks. The phase diagram of the ODT for the binary mixtures are found to depend on the compositions of α and β, i.e. the values of f A α and f A β. The phase boundary between the homogeneous (disordered) state and the microphase separated state for the binary mixture is found to be different from the one for the pure block copolymer. The result shows that the miscibility (disordered state) is suppressed in the binary mixture.

Morphology study of poly(γ-benzyl L-glutamate) and polyisoprene copolymer by pulsed proton NMR

Morphology study of poly(γ-benzyl L-glutamate) (PBLG) and polyisoprene (PI) block copolymer using pulsed NMR and electron microscope revealed that the solid block copolymer is in microphase separated state. NMR measurement detected the interface of the block copolymer. Spin-lattice relaxation time T 1 of the block copolymer gave the domain size which is in good agreement with the result obtained from the electron microscopy.

A Simplified Confined-Chain Model for AB Diblock Copolymers

A simplified confined-chain model for AB diblock copolymers in the lamellar microphase-separated state is presented. Block end-to-end distances and segment distribution functions are calculated by random flight statistics in one dimension without solving the diffusion equations as in the traditional approach. A simple expression to evaluate lamellar domain periods is obtained in terms of polymer pure density data and molecular weights. Numerical results are in close agreement with experimental data on domain sizes. A simple criterion for determining the equilibrium microdomain morphology is presented and compared with other theories and data from the literature.

Microphase Separation in Star Block Copolymers of Styrene and Isoprene. Theory, Experiment, and Simulation

The order−disorder transition (ODT) and the ordering kinetics have been studied in four-arm star diblock copolymer melts of styrene (corona) and isoprene (core) by rheology and SAXS. The results are compared with the corresponding linear diblock and show that the transition shifts in the direction predicted by theory. The mean-field structure factor is used to describe the SAXS profiles at T ≫ TODT and to extract the temperature dependence of the interaction parameter χ(T). Because of the weak T dependence of χ, we can explore only a small region of the phase diagram, namely, 16.5 < χN0 < 14.6, for fPS = 0.25. The microphase-separated state consists of spherical microdomains of the minor component (PS) which resembles a bcc phase, however, not with the full symmetry. To explore the morphology over a broader T range, we use the results of computer simulation on equivalent chains, which for fPS = 0.25 provide the following succession of phases: hexagonal − a bicontinuous structure which resembles the gyroi...

A lower critical ordering transition in a diblock copolymer melt

DIBLOCK copolymers, comprised of two distinct homopolymers covalently bonded together at one end, undergo a transition on cooling from a state in which the segments of the blocks are homogeneously mixed to one in which they are segregated locally1,2. This microphase separation is driven by the enthalpy of unfavourable interactions between segments. Here we report the microphase separation of a diblock copolymer melt on heating. Similar in nature to the lower critical solution temperature seen in polymer mixtures, this lower critical ordering transition is driven by entropic factors—specifically, by a negative volume change on mixing of the blocks. The transition to the microphase-separated state alters the rheological and mechanical properties of the copolymer markedly, the material gaining a non-zero equilibrium modulus above the ordering transition. This suggests potential technological applications of these copolymer systems as 'smart' materials.

Effect of hydrophilic soft segment side chains on the surface properties and blood compatibility of segmented poly (urethaneureas)

Segmented poly(urethaneureas) with hydrophilic side chains were prepared from poly(tetramethylene oxide) (PTMO), 4,4'-diphenylmethane diisocyanate (MDI), ethylene diamine (ED) and a diol with a long hydrophilic side chain comprised of an ethylene oxide-proplene oxide copolymer. The end groups of the hydrophilic chains were either sodium sulfonate or methoxy groups. The state of microphase separation showed a small dependence on the fraction of long-chain hydrophilic diol. Surface analysis by means of static underwater contact angle and dynamic contact angle measurements revealed that the graft chains were at the aqueous interface in the hydrated state. An ex vivo A-V shunt experiment revealed that a more thrombogenic blood-material response was correlated with an increase in the concentration of polymeric hydrophilic side chain incorporation. The polyurethane containing a long chain diol with methoxy end groups exhibited a higher level of thrombogenicity than the similar polymers possessing a sulfonate terminated side chain.

Dynamics and rheology of diblock copolymers quenched into microphase-separated states.

We propose a theoretical interpretation of observed anomalous rheological behavior of diblock copolymers quenched into microphase-separated states having locally lamellar morphology with disorder. The proposed mechanism of the anomaly is associated with the second-sound modes occurring in a locally lamellar structure that are overdamped in a wide wave-vector region due to large shear viscosity. The resulting complex shear modulus is proportional to (i\ensuremath{\omega}${)}^{1/2}$, which contains no adjustable parameter and is consistent with the experimental result by Bates et al. [J. Chem. Phys. 92, 6255 (1990)].