Frank S. Bates

Block Copolymers—Designer Soft Materials

Block copolymers are all around us, found in such products as upholstery foam, adhesive tape and asphalt additives. This class of macromolecules is produced by joining two or more chemically distinct polymer blocks, each a linear series of identical monomers, that may be thermodynamically incompatible (like oil and vinegar). Segregation of these blocks on the molecular scale (5–100 nm) can produce astonishingly complex nanostructures, such as the “knitting pattern” shown on the cover of this issue of PHYSICS TODAY. This striking pattern, discovered by Reimund Stadler and his coworkers, reflects a delicate free‐energy minimization that is common to all block copolymer materials.

Polymer-Polymer Phase Behavior

Different polymers can be combined into a single material in many ways, which can lead to a wide range of phase behaviors that directly influence the associated physical properties and ultimate applications. Four factors control polymer-polymer phase behavior: choice of monomers, molecular architecture, composition, and molecular size. Current theories and experiments that deal with the equilibrium thermodynamics and non-equilibrium dynamics of polymer mixtures are described in terms of these experimentally accessible parameters. Two representative molecular architectures, binary linear homopolymer mixtures and diblock copolymers, exhibiting macrophase separation and microphase segregation, respectively, are examined in some detail. Although these model systems are fairly well understood, a myriad of mixing scenarios, with both existing and unrealized materials applications, remain unexplored at a fundamental level.

Multiblock polymers: panacea or Pandora's box?

Getting Around the Block Diblock copolymers provide a rich variety of morphologies that depend on the length of the polymer blocks, the overall fraction of each block, and their chemical dissimilarity. New synthetic methods have made it possible to make copolymers with three or more components and in a range of chemical architectures. However, this growth in design choices can offer too many variables to work with, and rational design is important, especially when trying to transform small-scale products in engineered commodities. Bates et al. (p. 434) review the opportunities and complexities that exist when working in this expanded playground of block copolymers. Advances in synthetic polymer chemistry have unleashed seemingly unlimited strategies for producing block polymers with arbitrary numbers (n) and types (k) of unique sequences of repeating units. Increasing (k,n) leads to a geometric expansion of possible molecular architectures, beyond conventional ABA-type triblock copolymers (k = 2, n = 3), offering alluring opportunities to generate exquisitely tailored materials with unparalleled control over nanoscale-domain geometry, packing symmetry, and chemical composition. Transforming this potential into targeted structures endowed with useful properties hinges on imaginative molecular designs guided by predictive theory and computer simulation. Here, we review recent developments in the field of block polymers.

Self-Assembly of Janus Dendrimers into Uniform Dendrimersomes and Other Complex Architectures

Janus Drug Delivery Vehicle Efficient drug delivery vehicles need to be produced in a limited size range and with uniform size distribution. The self-assembly of traditional small-molecule and polymeric amphiphiles has led to the production of micelles, liposomes, polymeric micelles, and polymersomes for use in drug delivery applications. Now, Percec et al. (p. 1009) describe the self-assembly of Janus-type (i.e., two-headed) dendrimers to produce monodisperse supramolecular constructs, termed “dendrimersomes,” and other complex architectures. The structures, which showed long-term stability as well as very narrow size distributions, were easily produced by the injection of an ethanolic solution of the dendrimer into water. The dendrimersomes could be loaded with the anticancer drug doxorubicin and exhibited controlled drug release with changing pH. Amphiphilic, spherically shaped polymers self-assemble into larger hollow complexes that could be used for drug delivery. Self-assembled nanostructures obtained from natural and synthetic amphiphiles serve as mimics of biological membranes and enable the delivery of drugs, proteins, genes, and imaging agents. Yet the precise molecular arrangements demanded by these functions are difficult to achieve. Libraries of amphiphilic Janus dendrimers, prepared by facile coupling of tailored hydrophilic and hydrophobic branched segments, have been screened by cryogenic transmission electron microscopy, revealing a rich palette of morphologies in water, including vesicles, denoted dendrimersomes, cubosomes, disks, tubular vesicles, and helical ribbons. Dendrimersomes marry the stability and mechanical strength obtainable from polymersomes with the biological function of stabilized phospholipid liposomes, plus superior uniformity of size, ease of formation, and chemical functionalization. This modular synthesis strategy provides access to systematic tuning of molecular structure and of self-assembled architecture.

Polymer vesicles in vivo: correlations with PEG molecular weight

PEG-modified lipid vesicles have already shown considerable utility in delaying vesicle clearance from the circulation. They are, however, limited in their ability to stably integrate high molar ratios of PEG-lipid due to the high curvature and micellar preference of the very large hydrophilic PEG chain. Polymersomes, by contrast, are vesicles composed entirely of PEG-based block copolymer amphiphiles that are not only more proportionately designed, but also have already been shown to considerably broaden the range of vesicle properties (e.g. stability). Here, polymersomes composed of varying length copolymer chains were injected into rats and found to have in vivo circulation times, tau(1/2), up to about two-fold longer than PEGylated, or Stealth, liposomes. The dependence of tau(1/2) on PEG molecular weight is nonetheless limited by uptake into the liver and spleen-as with liposomes. In vitro incubations of polymersomes in plasma indicate gradual opsonization through plasma protein adsorption, such that, when vesicles are held in an optical trap and presented to a phagocyte, rapid engulfment occurs only after incubation times of similar magnitude to tau(1/2). The stealthiness introduced to liposomes through PEGylation is thus extended here with completely synthetic polymersomes.

Block copolymer microstructures in the intermediate-segregation regime

A detailed examination of the intermediate-segregation regime of diblock copolymer melts is presented using the incompressible Gaussian chain model and self-consistent field theory (SCFT). We find that the competition between interfacial tension and chain stretching used to describe behavior in the strong-segregation regime also explains behavior in this regime. Phase transitions from lamellae (L) to cylinders (C) to spheres (S) occur due to the spontaneous curvature produced as the asymmetry in the diblock composition increases. Complex phases, gyroid (G), perforated lamellar (PL), and double diamond (D), have curvatures between those of L and C, and therefore they compete for stability along the L/C boundary. Nevertheless, only G exhibits a region of stability. To explain why, we recognize that interfacial tension prefers the formation of constant mean curvature (CMC) surfaces to reduce interfacial area, and chain stretching favors domains of uniform thickness so as to avoid packing frustration. While t...

Fluctuation effects in a symmetric diblock copolymer near the order–disorder transition

The thermodynamic and dynamic properties of a partially deuterated poly(ethylene‐propylene)–poly(ethylethylene)(PEP–PEE) diblock copolymer containing 55% by volume PEP were characterized above and below the order–disorder transition (ODT) by small‐angle neutron scattering (SANS) and rheological measurements, respectively. Both experimental techniques produced unambiguous evidence of composition fluctuations well above TODT(T−TODT≲50 °C) in the disordered state, which increase in magnitude as the weak first‐order transition is approached. Based on the SANS results, which are nearly predicted by a recent fluctuation theory, we conclude that the (equilibrium) instantaneous morphology in the disordered state closely resembles a spinodally decomposed binary mixture. Below TODT, long‐range order can be obtained by the application of a shear field as evidenced by the resulting highly anisotropic (one‐dimensional) SANS pattern. As the ODT is approached in the shear‐oriented ordered state, an isotropic scattering ...

Spinodal decomposition of a symmetric critical mixture of deuterated and protonated polymer

A nearly symmetric critical mixture (φc=0.486) of perdeuterated and protonated 1,4‐polybutadiene exhibiting an upper critical solution temperature Tc=61.5±1.5 °C has been quenched from the homogeneous state (≂75 °C) to various temperatures between 25 and 57.5 °C. Light scattering measurements document the subsequent spinodal decomposition process which we describe based on a four‐stage model: early, intermediate, transition, and final. The early stage is accounted for by the Cahn theory, yielding initial correlation lengths and effective diffusion coefficients in quantitative agreement with mean‐field predictions. Nonlinear effects mark the beginning of the intermediate stage, which exhibits a simple power‐law growth of heterogeneity length Lm(t)∼tneff, but with a temperature dependent exponent neff. As the composition fluctuation amplitude approaches the equilibrium values, the spinodal decomposition process enters the transition stage, characterized by a decreasing interfacial thickness and an increasin...

Layer Structure Preservation during Swelling, Pillaring, and Exfoliation of a Zeolite Precursor

MCM-22(P), the precursor to zeolite MCM-22, consists of stacks of layers that can be swollen and exfoliated to produce catalytically active materials. However, the current swelling procedures result in significant degradation of crystal morphology along with partial loss of crystallinity and dissolution of the crystalline phase. Fabrication of polymer nanocomposites and coatings with MCM-22 for separation, barrier, and other applications requires a swelling method that does not alter drastically the crystal morphology and layer structure and preserves the high aspect ratio of the layers. Here, we demonstrate such a method by swelling MCM-22(P) at room temperature. The low-temperature process does not disrupt the framework connectivity present in the parent MCM-22(P) material. By extensive washing with water, the swollen material, MCM-22(PS-RT), evolves to a new ordered layered structure. Interestingly, the swelling procedure is reversible and the swollen material can be restored back to MCM-22(P) by acidification of the sample. The swollen material can also be pillared to produce an MCM-36 analogue. It can also be exfoliated, and layers can be incorporated in a polymer matrix to make nanocomposites.

Fluctuations, conformational asymmetry and block copolymer phase behaviour

Phase behaviour near the order–disorder transition (ODT) of 58 model hydrocarbon diblock copolymers, representing four different systems, is summarized. Six distinct ordered-state microstructures are reported, including hexagonally modulated lamellae (HML), hexagonally perforated layers (HPL) and a bicontinuous cubic morphology with Iatext-decoration:overline3d space group symmetry. Two non-classical parameters, Iµ and text-decoration:overlineN, control the occurrence and distribution of these phases, in addition to the classical variables ƒ and χN, where ƒ, χ and N are the composition, segment–segment interaction parameter and degree of polymerization, respectively. Iµ accounts for differences in the conformational and volume-filling characteristics of each block. Conformational asymmetry, Iµ≠ 1, produces an asymmetric phase diagram around ƒ= 1/2. The importance of fluctuation effects are inversely related to the magnitude of text-decoration:overlineN, a type of Ginzburg parameter that is proportional to N. As text-decoration:overlineN decreases, the bicontinuous Iatext-decoration:overline3d phase appears adjacent to the ODT. Development of this cubic phase can be rationalized based on chain-packing frustration near the lamellar hexagonal state. Apparently the Iatext-decoration:overline3d cubic state is stabilized by fluctuations since it disappears when text-decoration:overlineN becomes large. These findings provide new insights into the origins of phase complexity in condensed soft matter.