A new opportunity to prepare mechanically strong isoporous membranes with sub 10 nm pores for molecular separation from symmetric triblock copolymers

Isoporous membranes produced from diblock copolymers commonly display a poor mechanical property that shows many negative impacts on their separation application. It is theoretically predicted that dense films produced from symmetric triblock copolymers show much stronger mechanical properties than those of homologous diblock copolymers. However, to the best of our knowledge, symmetric triblock copolymers have rarely been fabricated into isoporous membranes before, and a full understanding of separation as well as mechanical properties of membranes prepared from triblock copolymers and homologous diblock copolymers has not been conducted, either. In this work, a cleavable symmetric triblock copolymer with polystyrene as the side block and poly(4-vinylpyridine) (P4VP) as the middle block was synthesized and designed by the RAFT polymerization using the symmetric chain transfer agent, which located at the center of polymer chains and could be removed to produce homologous diblock copolymers with half-length while having the same composition as that found in triblock copolymers. The self-assembly of these two copolymers in thin films and casting solutions was first investigated, observing that they displayed similar self-organized structures under these two conditions. When fabricated into isoporous membranes, they showed similar pore sizes (5-7% difference) and comparable rejection performance (∼10% difference). However, isoporous membranes produced from triblock copolymers showed significantly improved mechanical strength and higher toughness (2-10 times larger) as evidenced by the compacting resistance, strain-stress determination, and nanoindentation testing, suggesting the unique and novel structure-performance relationship in the isoporous membranes produced from symmetric triblock copolymers. The above finding will guide the way to fabricate mechanically robust isoporous membranes without notably changing the separation performance from rarely used symmetric triblock copolymers, which can be synthesized by the controlled polymerization as facilely as that found for diblock copolymers.

Comparison of Solubilization of Hydrocarbons in (PEO–PPO) Diblock versus (PEO–PPO–PEO) Triblock Copolymer Micelles

Summary: We have performed Monte Carlo simulations to study the bridging of symmetrical or asymmetrical triblock copolymers confined between two similar or different solid surfaces based on a simple lattice model. The influence of the molecular structure, surface separation, adsorption energy, chain composition, and the chain concentration on the fractions of chains with bridge, loop and dangling configurations are reported in detail. The results show that the largest bridging fraction is given only when symmetrical triblock copolymers are confined between two parallel surfaces with the same adsorption energy. The bridge fraction is decreased so long as the asymmetry of the copolymers or the difference between the two surfaces is enhanced. It was found also that the bridging fraction increases as the adsorption energy increases. The bridging fraction Ωbridge under different separations, Lz, can be expressed as $\Omega _{{\rm bridge}} = A(1 - L_z /L_{z.\max } )^\alpha $ in various situations. On the other hand, by introducing a symmetry index ν, the influence of molecular structure of copolymers on the bridges can be illustrated approximately by a relation $(1 - \Omega _{{\rm b,asym}} /\Omega _{{\rm b,symm}} ) = B(1 - \nu )^\zeta $ when the two surfaces are similar and the adsorption energy is not too high. Combining the two expressions, data of the bridge fractions for copolymers of different symmetries confined between surfaces with different separations can be described with a single equation, which, in some occasion, can be used for prediction.Influence of molecular structure on the bridging fraction for ${\rm A}_{r_{{\rm A1}} } {\rm B}_{{\rm 40}} {\rm A}_{r_{{\rm A2}} }$.imageInfluence of molecular structure on the bridging fraction for ${\rm A}_{r_{{\rm A1}} } {\rm B}_{{\rm 40}} {\rm A}_{r_{{\rm A2}} }$.

Nitroxide-mediated polymerization to form symmetrical ABA triblock copolymers from a bidirectional alkoxyamine initiator

The structure of a thin film of symmetric ABA triblock copolymer melts in an external in‐plane DC or AC electric field is studied theoretically. The situation is considered when the triblock copolymer forms a hexagonal morphology of standing cylinders in bulk in the absence of an external field. Self‐consistent field theory calculations are carried out to determine the most thermodynamically favorable thin film structure among the cylindrical phases perpendicular and parallel to the substrate and the lamellar phase perpendicular to the substrate. The results are presented as phase diagrams with the film thickness and electric field energy on the axes and as distributions of the local composition, which serve as an order parameter in the system. It is confirmed that electric fields only weakly affect the spinodal curves of block copolymers but they can reorient or markedly modify microphase‐separated morphologies in those systems. Such restructuring is consistent with a considerable modulation of the free surface of a copolymer film and it can lead to the coexistence of different phases that appear in the film areas with different local thicknesses.

A comparison of beta-casein and symmetrical triblock copolymer (PEO-PPO-PEO and PPO-PEO-PPO) adsorption layer properties at the air-water interface has been carried out by bubble tensiometry and ellipsometry. It has been verified that the equation of state parameters (pi approximately gamma(y)) obtained from surface pressure (pi) and ellipticity in Brewster conditions (rhoB), which is proportional to the surface concentration (gamma) data, are the same as those obtained from dilational modulus epsilon and pi data. These two consistent approaches give further support to the theoretical model of block copolymers which has been previously developed for protein adsorption at fluid interfaces. It is shown that the interfacial behavior of the copolymer adsorption layer changes strongly as a function of the length of the hydrophilic and hydrophobic block sequences. The theoretical model may be used for the interpretation of the adsorption properties of the synthetic copolymers only when the size of the blocks is large enough. In the case of block copolymers, the coil is in a self-avoiding walk conformation (y = 3) whatever the temperature, while in the case of beta-casein, the polypeptide chain is partly collapsed at room temperature due to thermolabile noncovalent bonds. At the end of the first semidilute regime, there is clear evidence for a crossover toward a second semidilute regime for synthetic copolymers as well as for beta-casein but it is presently only partially characterized.

We study the lamellar phase formed by symmetric ABA triblock copolymers with degree of polymerization N and A monomer fraction f. Employing a mean-field, lattice formalism, we calculate the fraction of copolymers which bridge the B-rich lamellae. This fraction is found to be significant, although it is strictly bounded above by l/~ within the formalism. Once the lamellar phase is strongly segregated, the fraction of bridges decreases slowly with increasing N, increasing Flory parameter x, and decreasing f. In the strong-segregation limit, the lamellar spacing D is found to scale as D - N0xb with a = 0.68 and b = 0.19. The interfacial width 5 does not scale as a power of N but does scale with x as 5 - xc, where c = -0.51. Both D and 5 are nearly independent off. 187

Monte Carlo simulations based on a modified bond-fluctuation and vacancy-diffusion algorithm on a simple cubic lattice were employed to examine the morphology of thin films of the symmetric AmB2nAm triblock copolymer confined between two hard homogeneous parallel walls. The walls preferred either segment A or segment B. Parallel lamellae, parallel cylinders and perpendicular cylinders morphologies, dependent on the composition, film thickness and interaction energies, were identified in these simulations, in agreement with the experimental observations of several researchers.

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.

The phase behavior of symmetric ABA triblock copolymers containing a semiflexible midblock is studied by lattice Monte Carlo simulation. As the midblock evolves from a fully flexible state to a semiflexible state in terms of increase in its persistence length, different phase behaviors are observed while cooling the system from an infinite high temperature to a temperature below T(ODT) (order-disorder transition temperature). Within the midblock flexibility range we studied (l(p)N(c)<or=0.105), a lamellar structure is formed at equilibrium state as the situation for fully flexible chains. The fraction of bridge chain is evaluated for the lamellar structures. We find that the increase in midblock rigidity indeed results in the increase in bridge chain fraction within the range from 44.9% to 51.8%. In order to elucidate phase behavior evolution observed in our simulation, a detailed conformation distribution analysis is also given. Our results bridge a gap of different phase behaviors between rod-coil block copolymer and coil-coil block copolymer and show a necessity to investigate rigidity influence on phase diagram.

ABSTRACTHere, we report the morphology variation in a series of PS‐b‐PI‐b‐PS' asymmetric triblock copolymer and PS homopolymer (hPS) blends, where PS' and PS are polystyrene blocks with a molecular weight ratio of approximately 0.11 and PI is poly(isoprene). We find that adding a small amount of hPS results in significant order–order transition (OOT) boundary deflection toward higher PS volume fractions fPS, which is accompanied by morphology re‐entry. For example, the neat triblock copolymer with a PS + PS' volume fraction of fPS = 0.38 exhibits a lamellar microphase; adding a small amount of hPS reverts the morphology into a hexagonal phase with PS cylinders, while further increasing the hPS fraction leads to normal OOTs from PS cylinders to lamellae, to PI cylinders and finally to spheres. The morphology variation reported here is significantly different from that reported in binary blends of diblock or symmetric triblock copolymer with homopolymer. While the domain features of the LAM structure can be correctly reproduced by self‐consistent field theory (SCFT), the observed morphology re‐entry is absent in the theoretical SCFT phase diagram. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 169–179

This review focuses on the application of triblock copolymers as designed templates to synthesize nanoporous materials with various compositions. Asymmetric triblock copolymers have several advantages compared with symmetric triblock copolymers and diblock copolymers, because the presence of three distinct domains can provide more functional features to direct the resultant nanoporous materials. Here we clearly describe significant contributions of asymmetric triblock copolymers, especially polystyrene-block-poly(2-vinylpyridine)-block-poly(ethylene oxide) (abbreviated as PS-b-P2VP-b-PEO).

The self-consistent field theory (SCFT) complemented with the Poisson-Boltzmann equation is employed to explore self-assembly of polyelectrolyte copolymers composed of charged blocks A and neutral blocks B. We have extended SCFT to dissociating triblock copolymers and demonstrated our approach on three characteristic examples: (1) diblock copolymer (AB) melt, (2) symmetric triblock copolymer (ABA) melt, (3) triblock copolymer (ABA) solution with added electrolyte. For copolymer melts, we varied the composition (that is, the total fraction of A-segments in the system) and the charge density on A blocks and calculated the phase diagram that contains ordered mesophases of lamellar, gyroid, hexagonal, and bcc symmetries, as well as the uniform disordered phase. The phase diagram of charged block copolymer melts in the charge density--system composition coordinates is similar to the classical phase diagram of neutral block copolymer melts, where the composition and the Flory mismatch interaction parameter χ(AB) are used as variables. We found that the transitions between the polyelectrolyte mesophases with the increase of charge density occur in the same sequence, from lamellar to gyroid to hexagonal to bcc to disordered morphologies, as the mesophase transitions for neutral diblocks with the decrease of χ(AB). In a certain range of compositions, the phase diagram for charged triblock copolymers exhibits unexpected features, allowing for transitions from hexagonal to gyroid to lamellar mesophases as the charge density increases. Triblock polyelectrolyte solutions were studied by varying the charge density and solvent concentration at a fixed copolymer composition. Transitions from lamellar to gyroid and gyroid to hexagonal morphologies were observed at lower polymer concentrations than the respective transitions in the similar neutral copolymer, indicating a substantial influence of the charge density on phase behavior.

Influence of block lengths and symmetries of block copolymers on phase behavior of polymer A/polymer B/block copolymer ternary blends

Morphology evolution of multi-responsive ABA triblock copolymers containing photo-crosslinkable coumarin molecules