Phase behavior and structure formation in linear multiblock copolymer solutions by Monte Carlo simulation

Abstract

The solution phase behavior of short, strictly alternating multiblock copolymers of type (A(n)B(n))(m) was studied using lattice Monte Carlo simulations. The polymer molecules were modeled as flexible chains in a monomeric solvent selective for block type A. The degree of block polymerization n and the number of diblock units per chain m were treated as variables. We show that within the regime of parameters accessible to our study, the thermodynamic phase transition type is dependent on the ratio of m / n. The simulations show microscopic phase separation into roughly spherical aggregates for m / n ratios less than a critical value and first-order macroscopic precipitation otherwise. In general, increasing m at fixed n, or n at fixed m, promotes the tendency toward macroscopic phase precipitation. The enthalpic driving force of phase change is found to universally scale with chain length for all multiblock systems considered and is independent of the existence of a true phase transition. For aggregate forming systems at low amphiphile concentrations, multiblock chains are shown to self-assemble into intramolecular, multichain clusters. Predictions for microstructural dimensions, including critical micelle concentration, equilibrium size, shape, aggregation parameters, and density distributions, are provided. At increasing amphiphile density, interaggregate bridging is shown to result in the formation of networked structures, leading to an eventual solution-gel transition. The gel is swollen and consists of highly interconnected aggregates of approximately spherical morphology. Qualitative agreement is found between experimentally observed physical property changes and phase transitions predicted by simulations. Thus, a potential application of the simulations is the design of multiblock copolymer systems which can be optimized with regard to solution phase behavior and ultimately physical and mechanical properties.

Micelle formation, gelation and phase separation of amphiphilic multiblock copolymers†

“…Consistently with the different cases for P sub = 0.2 and 0.5, simulations of pentablock copolymers in solution by Anderson and Travesset 32 have provided evidence for what they call a swollen gel (a network of loosely connected micelle cores) and for a giant cylindrical micelle (a single tubular core made of several copolymers) when increasing the ratio of hydrophobic to hydrophilic monomers in the chain. Also, Gindy et al 34 observed in simulations, at P sub = 0.5, the formation of a different kind of gel network for what they call microscopically or macroscopically phase separating copolymer, depending on the lengths of the H blocks, the P blocks and the copolymers.…”

“…Glotzer et al 33 showed in a twodimensional model calculation that multiblock copolymers, unlike diblock copolymers, micro-phase separate and form a gel network. Gindy et al 34 focussed on the role of the number m and length n of the blocks in (H n P n ) m multiblock copolymers containing the same number of H and P monomers. They showed that these copolymers may either evolve by microphase separation or by macroscopic phase separation, depending on the m/n ratio.…”

Collapse transitions in thermosensitive multi-block copolymers: A Monte Carlo study

“…Which scenario will be favored depends on copolymer architecture, composition and concentration, as well as on solvent quality and selectivity for the two blocks. 4,5 The intermolecular aggregation is similar to the manner that surfactant molecules behave in water, i.e., by clustering in multi-chain aggregates -micelles-formed with cores of collapsed hydrophobic blocks surrounded by a corona of hydrated hydrophilic blocks. An example of intrachain aggregation is that of proteins dissolved in water: in the simplest description, they undergo a very specific intrachain aggregation -protein folding-with hydrophobic segments clustering in the inner part of the collapsed coil leaving most of the hydrophilic parts on their exterior and exposed to water.…”

“…Towards this end, computer simulations have recently made substantial strides 4,19,20,21,22,23,24,25,26,27,28,29,30,31 focusing on solutions of linear multiblock copolymers of the (A n B n ) m type, where n is the degree of polymerization of each block and m is the number of blocks. Systematic explorations within the large parameter space established quantitative relations on how copolymer architecture and intermolecular interactions determine the macroscopic phase behavior and the related chain conformations changes.…”

Coarse-grained models for macromolecular systems

“…Using bead-spring models, the formation of micelles has been investigated either in solution [15] or at surfaces [16]. Lattice models can also prove helpful in the study of the micellization and phase separation of surfactants and multiblock copolymers [17][18][19]. Protein folding and aggregation is another field of application of these models, where they are used in view of elucidating the generic mechanisms involved [20].…”