Preparation and Characterization of a Styrene−Isoprene Undecablock Copolymer and Its Hierarchical Microdomain Structure in Bulk

Dynamic Self-Consistent Field Approach for Studying Kinetic Processes in Multiblock Copolymer Melts

The self-consistent field theory is a popular and highly successful theoretical framework for studying equilibrium (co)polymer systems at the mesoscopic level. Dynamic density functionals allow one to use this framework for studying dynamical processes in the diffusive, non-inertial regime. The central quantity in these approaches is the mobility function, which describes the effect of chain connectivity on the nonlocal response of monomers to thermodynamic driving fields. In a recent study, one of us and coworkers have developed a method to systematically construct mobility functions from reference fine-grained simulations. Here we focus on melts of linear chains in the Rouse regime and show how the mobility functions can be calculated semi-analytically for multiblock copolymers with arbitrary sequences without resorting to simulations. In this context, an accurate approximate expression for the single-chain dynamic structure factor is derived. Several limiting regimes are discussed. Then we apply the resulting density functional theory to study ordering processes in a two-length scale block copolymer system after instantaneous quenches into the ordered phase. Different dynamical regimes in the ordering process are identified: at early times, the ordering on short scales dominates; at late times, the ordering on larger scales takes over. For large quench depths, the system does not necessarily relax into the true equilibrium state. Our density functional approach could be used for the computer-assisted design of quenching protocols in order to create novel nonequilibrium materials.

Mesoscopic Detection of the Influence of a Third Component on the Self-Assembly Structure of A2B Star Copolymer in Thin Films

The most common self-assembly structure for A2B copolymer is the micellar structure with B/A segments being the core/corona, which greatly limits its application range. Following the principle of structure deciding the properties, a reformation in the molecular structure of A2B copolymer is made by appending three segments of a third component C with the same length to the three arms, resulting (AC)2CB 3-miktoarm star terpolymer. A reverse micellar structure in self-assembly is expected by regulating the C length and the pairwise repulsive strength of C to A/B, aiming to enrich its application range. Keeping both A and B lengths unchanged, when the repulsion strength of C to A is much stronger than C to B, from the results of mesoscopic simulations we found, with a progressive increase in C length, (AC)2CB terpolymer undergoes a transition in self-assembled structures, from a cylindrical structure with B component as the core, then to a deformed lamellar structure, and finally to a cylindrical structure with A component as the core. This reverse micellar structure is formed with the assistance of appended C segments, whose length is longer than half of B length, enhancing the flexibility of three arms, and further facilitating the aggregation of A component into the core. These results prove that the addition of a third component is a rational molecular design, in conjunction with some relevant parameters, enables the manufacturing of the desired self-assembly structure while avoiding excessive changes in the involved factors.

Precise syntheses of structurally possible all tetrablock quaterpolymers by a methodology combining living anionic polymerization with linking chemistry using 1 : 1 addition reaction

A methodology combining living anionic polymerization with linking chemistry using 1 : 1 addition reaction has been developed for the syntheses of tetrablock quaterpolymers composed of A, B, C, and D blocks.

The limits of precision monomer placement in chain growth polymerization

Precise control over the location of monomers in a polymer chain has been described as the ‘Holy Grail' of polymer synthesis. Controlled chain growth polymerization techniques have brought this goal closer, allowing the preparation of multiblock copolymers with ordered sequences of functional monomers. Such structures have promising applications ranging from medicine to materials engineering. Here we show, however, that the statistical nature of chain growth polymerization places strong limits on the control that can be obtained. We demonstrate that monomer locations are distributed according to surprisingly simple laws related to the Poisson or beta distributions. The degree of control is quantified in terms of the yield of the desired structure and the standard deviation of the appropriate distribution, allowing comparison between different synthetic techniques. This analysis establishes experimental requirements for the design of polymeric chains with controlled sequence of functionalities, which balance precise control of structure with simplicity of synthesis.

Chemists increasingly seek to control monomer sequencing in aperiodic copolymers. Here, the authors show that the statistical nature of chain growth strongly limits the achievable control, and establish parameters for polymer design that balance precise control with simplicity of synthesis.

Formation of Hierarchical Lamellae-in-Lamella Nanostructures from Polymer Blends Via Controlled Nonequilibrium Freezing

The creation of hierarchical nanostructures in polymeric materials has been intensively studied due to the great potential to tailor their physicochemical properties. Although much success has been achieved over the past decades in block copolymers, hierarchical structure engineering in polymer blends remains a great challenge. Here, the formation of hierarchical lamellae-in-lamella nanostructures from polymer blends via controlled nonequilibrium freezing is reported. Polymer blends are first dissolved in molten hexamethylbenzene (HMB) to form a homogeneous melt. When cooled to below its melting temperature, the HMB is crystallized and depleted, and the polymers are directionally solidified. This process is rapid enough that phase separation of the polymer blends is kinetically trapped at the nanoscale level. Then, the polymer blend epitaxially crystallizes onto the HMB inside the nanophase, resulting in the hierarchical lamellae-in-lamella structure. This structure is stable under ambient conditions and tunable depending on the annealing temperature and blending ratio.

Aperiodic Copolymers

Current polymer terminology only describes very simple copolymer structures such as block, graft, alternating periodic, or statistical copolymers. This restricted vocabulary implies that copolymers exhibit either segregated (i.e., block and graft), regular (i.e., alternating and periodic), or uncontrolled (i.e., statistical or random) comonomer sequence distributions. This standard classification does not include many new types of sequence-controlled copolymers that have been reported in recent years. In this context, the present viewpoint describes a new category of copolymers: aperiodic copolymers. Such structures can be defined as copolymers in which monomer sequence distribution is not regular but follows the same arrangement in all chains. The term aperiodic can be used to describe encoded comonomer sequences in monodisperse sequence-defined copolymers but also the block sequence of some multiblock copolymers. These new types of copolymers open up very interesting perspectives for the design of complex materials. Some recent relevant literature on the topic is discussed herein.

Hierarchical self-assembly of two-length-scale multiblock copolymers

The self-assembly in diblock copolymer-based supramolecules, obtained by hydrogen bonding short side chains to one of the blocks, as well as in two-length-scale linear terpolymers results in hierarchical structure formation. The orientation of the different domains, e.g. layers in the case of a lamellar-in-lamellar structure, is determined by the molecular architecture, graft-like versus linear, and the relative magnitude of the interactions involved. In both cases parallel and perpendicular arrangements have been observed. The comb-shaped supramolecules approach is ideally suited for the preparation of nanoporous structures. A bicontinuous morphology with the supramolecular comb block forming the channels was finally achieved by extending the original approach to suitable triblock copolymer-based supramolecules.

Poly(Acrylic Acid-b-Styrene) Amphiphilic Multiblock Copolymers as Building Blocks for the Assembly of Discrete Nanoparticles

In order to expand the utility of current polymeric micellar systems, we have developed amphiphilic multiblock copolymers containing alternating blocks of poly(acrylic acid) and poly(styrene). Heterotelechelic poly(tert-butyl acrylate-b-styrene) diblock copolymers containing an α-alkyne and an ω-azide were synthesized by atom transfer radical polymerization (ATRP), allowing control over the molecular weight while maintaining narrow polydispersity indices. The multiblock copolymers were constructed by copper-catalyzed azide-alkyne cycloaddition of azide-alkyne end functional diblock copolymers which were then characterized by (1)H NMR, FT-IR and SEC. The tert-butyl moieties of the poly(tert-butyl acrylate-b-styrene) multiblock copolymers were easily removed to form the poly(acrylic acid-b-styrene) multiblock copolymer ((PAA-PS)(9)), which contained up to 9 diblock repeats. The amphiphilic multiblock (PAA-PS)(9) (M(n) = 73.3 kg/mol) was self-assembled by dissolution into tetrahydrofuran and extensive dialysis against deionized water for 4 days. The critical micelle concentration (CMC) for (PAA-PS)(9) was determined by fluorescence spectroscopy using pyrene as a fluorescent probe and was found to be very low at 2 × 10(-4) mg/mL. The (PAA-PS)(9) multiblock was also analyzed by dynamic light scattering (DLS) and transmission electron microscopy (TEM). The hydrodynamic diameter of the particles was found to be 11 nm. Discrete spherical particles were observed by TEM with an average particle diameter of 14 nm. The poly(acrylic acid) periphery of the spherical particles should allow for future conjugation of biomolecules.

Microphase separation of comb copolymers with two different lengths of side chains

The phase behavior of the monodisperse AB comb copolymer melt contained the macromolecules of special architecture is discussed. Each macromolecule is assumed to be composed of two comb blocks which differ in numbers of side chains and numbers of monomer units in these chains. It is shown (by analysis of the structure factor of the melt) that microphase separation at two different length scales in the melt is possible. The large and small length scales correspond to separation between comb blocks and separation between monomer units in repeating fragments of blocks, respectively. The classification diagrams indicated which length scale is favored for a given parameters of chemical structure of macromolecules are constructed.

Hierarchically ordered microstructures self-assembled from comb-coil block copolymers

Using the real-space implemented self-consistent field theory, we undertook an investigation on the hierarchical self-assembly behaviors of comb-coil block copolymers. The comb-coil block copolymers can self-assemble into hierarchical microstructures with two different length scales. Various structure-within-structure morphologies, such as parallel and perpendicular lamella-within-lamella, cylinder-within-lamella, lamella-within-cylinder, and cylinder-within-cylinder, were observed. In the hierarchical structures, the large-length-scale structures are produced by segregation between the coil blocks and comb blocks, and the small-length-scale structures are formed by microphase separation within the comb blocks. Effects of interaction strength and coil block length on the hierarchical phase behaviors were studied, and the phase diagram was mapped out accordingly. Furthermore, the large-length-scale lamellar period as a function of interaction strength was examined. It was found the lamellar periods are greatly dependent on interaction strengths.