Short Block Copolymers Research Articles

Lithium Salt Distribution and Thermodynamics in Electrolytes Based on Short Perfluoropolyether-block-Poly(ethylene oxide) Copolymers

Wide-angle X-ray scattering (WAXS) was used to study the distribution of salt in short-chain disordered block copolymer electrolytes with concentration fluctuations on the length scale of 1 nm. The electrolytes were binary mixtures of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt and a short block copolymer comprising perfluoroether (PFE) segments covalently bonded to ethylene oxide (EO) segments. We develop a method to analyze scattering data from mixtures of block copolymers and salt where salt distribution is not known beforehand, using a minimal number of adjustable parameters. The WAXS peak due to scattering from disordered concentration fluctuations is fit to a random phase approximation (RPA) model that includes a parameter to describe the partitioning of salt between the perfluoropolyether (PFPE)-rich and poly(ethylene oxide) (PEO)-rich concentration fluctuations. This method enables quantification of the salt distribution and the effective Flory–Huggins interaction parameter between polymer segments. We posit that attractive interactions between the TFSI– anion and PFPE (fluorous effect) drive some of the salt into the PFPE-rich fluctuations. On the other hand, the attractive interactions between Li+ and EO segments drive the remaining salt molecules into the PEO-rich fluctuations. We use WAXS to quantify LiTFSI partitioning between the PFE and EO segments in the block copolymer. The segregation between blocks, quantified by an effective Flory–Huggins interaction parameter between polymer segments, decreases with increasing salt concentration, behavior that is atypical for block copolymer electrolytes.

Block Copolyesters Containing 2,5-Furan and trans-1,4-Cyclohexane Subunits with Outstanding Gas Barrier Properties.

Biopolymers are gaining increasing importance as substitutes for plastics derived from fossil fuels, especially for packaging applications. In particular, furanoate-based polyesters appear as the most credible alternative due to their intriguing physic/mechanical and gas barrier properties. In this study, block copolyesters containing 2,5-furan and trans-1,4-cyclohexane moieties were synthesized by reactive blending, starting from the two parent homopolymers: poly(propylene furanoate) (PPF) and poly(propylene cyclohexanedicarboxylate) (PPCE). The whole range of molecular architectures, from long block to random copolymer with a fixed molar composition (1:1 of the two repeating units) was considered. Molecular, thermal, tensile, and gas barrier properties of the prepared materials were investigated and correlated to the copolymer structure. A strict dependence of the functional properties on the copolymers’ block length was found. In particular, short block copolymers, thanks to the introduction of more flexible cyclohexane-containing co-units, displayed high elongation at break and low elastic modulus, thus overcoming PPF’s intrinsic rigidity. Furthermore, the exceptionally low gas permeabilities of PPF were further improved due to the concomitant action of the two rings, both capable of acting as mesogenic groups in the presence of flexible aliphatic units, and thus responsible for the formation of 1D/2D ordered domains, which in turn impart outstanding barrier properties.

Topology and Shape Control for Assemblies of Block Copolymer Blends in Solution

We study binary blends of asymmetric diblock copolymers (AB/AC) in selective solvents with a mesoscale model. We investigate the morphological transitions induced by the concentration of the AC block copolymer and the difference in molecular weight between the AB and AC copolymers, when segments B and C exhibit hydrogen-bonding interactions. To the best of our knowledge, this is the first work modeling mixtures of block copolymers with large differences in molecular weight. The coassembly mechanism localizes the AC molecules at the interface of A and B domains and induces the swelling of the B-rich domains. The coil size of the large molecular weight block copolymer depends only on the concentration of the short block copolymer (AC or AB), regardless of the B–C interactions. However, the B–C interactions control the morphological transitions that occur in these blends.

The influence of one block polydispersity on phase separation of diblock copolymers: The molecular mechanism for domain spacing expansion

Comprehensive dissipative particle dynamics simulations are performed to investigate the effect of A-block polydispersity on the phase behavior of AB-diblock copolymers. The experimental results of Lynd and Hillmyer on polydispersity induced domain spacing expansion at different segregations (Macromolecules 2005, 38, 8803) are well reproduced and explained in terms of interfacial free energy and molecular stretching in domain center. Short block copolymers in polydisperse system are found to preferably accumulate at A/B interface as compatibilizers causing a reduction in interfacial free energy. At the same time, domain centers are found mainly occupied by long blocks which in turn determining the spacing of the corresponding domain. More importantly, increase in polydispersity will introduce more long blocks into system and therefore will enhance the molecular packing and stretching in the domain center. Such chain stretching in domain center and reduction in interfacial free energy are found to be more severe at stronger segregation, which will lead to more severe domain spacing expansion with increasing polydispersity.

Brownian Dynamics Simulation of Comicellization of Amphiphilic Block Copolymers with Different Tail Lengths

Brownian dynamics simulations were performed to study the self-assembly of mixed linear amphiphilic block copolymers. The molecules consist of similar hydrophilic heads and hydrophobic tails of different lengths. The model amphiphilic diblock copolymers have been selected to gain insight into the comicellization process in concentrated regimes, and the micelles were not kinetically frozen on the time scale of simulation. The critical micelle concentration (cmc), micelle size distribution, radius of gyration distribution, density profile of comicelles, shape anisotropy, and dynamics of comicellization have been studied as a function of the varying molar fraction of components. The cmc's of systems rich in the molar fraction of each type were found to be close to the cmc of that component. It has also been found that at a certain concentration comicellization affects the cmc in mixed systems. The weight-average aggregate size distribution of mixed copolymers was found to be between the aggregate distributions of short and long copolymers and becomes broader because of mixing. Moreover, values of the most probable aggregate size and radius of gyration of comicelles follow the mixing rule. Results show that small aggregates are mainly made from shorter block copolymers, whereas longer block copolymers form the major portion of large clusters. Furthermore, the cores of the micelles are mainly composed of longer block copolymers, and in all cases, the concentrations of shorter block copolymers are more dominant in the outer part than in the interior regions. In addition, the dynamics of polymeric micelles was studied using tracer and extraction autocorrelation functions and their relaxation times. The tracer correlation time increases with increasing longer copolymer concentration and deviates positively from the mixing rule. We also find that the total extraction correlation time increases exponentially from short to long copolymers, but the presence of long block copolymers linearly increases the short copolymer extraction correlation times. Short block copolymers, however, linearly decrease the long copolymer correlation times.

Well‐defined N‐Isopropylacrylamide Dual‐Sensitive Copolymers with LCST ≈38 °C in Different Architectures: Linear, Block and Star Polymers

AbstractReversible addition‐fragmentation chain transfer (RAFT) polymerization is used to prepare temperature‐ and pH‐sensitive statistical copolymers with lower critical solution temperature (LCST) close to 38 °C at pH 7.4 based on N‐isopropylacrylamide and methacrylic acid derivative comonomers with a pKa close to 6. Statistical copolymers are re‐activated to prepare amphiphilic block copolymers and star polymers with cross‐linked core. The LCST is maintained by varying the architecture; however, the LCST originated behaviour changes due to self‐aggregation. Statistical copolymers and short block copolymers show complex aggregation, whereas mid‐size block copolymers and star polymers show shrinkage of aggregate dimensions. The pH of the medium has a profound impact on the self‐assembling behaviour of the different polymer architectures.

Microstructure of Block Copolymers near Selective Surfaces: Theoretical Predictions and Configurational-Bias Monte Carlo Simulation

We report a new density functional theory (DFT) for block copolymers and its performance for representing the microscopic structure has been tested with the configurational-bias Monte Carlo simulation. The segmental density profiles are calculated for systems containing short block copolymers, represented by tangent-sphere square-well chains of sequences AABB, AAAABBBB, ABBBBA, ABBBBBBA, BBAABB, and BBBBAABBBB confined in slit pores where the planar surfaces attract A segments but repel B segments. The detailed segmental density profiles are calculated for block copolymers confined in slit pores where the planar surfaces attract A segments but repel B segments. For all conditions considered, the microscopic structures predicted by the DFT are in excellent agreement with the simulation results. Interesting loop and tail conformations of the block copolymers are observed near the selective surfaces. The effects of surface energy, chain length, and copolymer backbone structures on the microscopic segregation of AB segments are also investigated.

Self‐assembly in mixtures of block copolymers: “co‐surfactant effects” on morphology control

AbstractWe found that competing long‐range interactions of the two block chains in their binary mixtures give intriguing effects on ordered microdomain structures. One effect is a co‐surfactant effect observed when the two blocks are mixed with their junctions on common interfaces, giving rise to such effects as (i) the two lamella‐forming block copolymers give hexagonal cylinders and consequently to the increase of the interface curvatures, or (ii) a short symmetric block copolymer mixed with a large asymmetric block copolymer effectively decreases the interface curvatures, depending upon composition and molecular weight of the short block copolymer. The other effect is the one which causes two blocks macroscopically phase‐separate into different ordered domains.

Competition between Micro- and Macrophase Separations in a Binary Mixture of Block Copolymers. A Dynamic Density Functional Study

Using the dynamic density functional (DDF) simulation technique, we studied the domain morphology and dynamics of phase separation in a mixture of long and short block copolymers. The simulation results well reproduce the recent results of the X-ray scattering experiment and the TEM observation on a mixture of long and short polystyrene−polyisoprene block copolymers. The DDF simulation clarifies that the three-dimensional complex domain structures and the mechanism of the dynamics of the phase separation are arising from the competition between the microphase separation and the macrophase separation.

Off-lattice Monte Carlo methods for coarse-grained models of polymeric materials and selected applications

This brief review presents a general introduction to the Monte Carlo simulation of coarse-grained models for flexible macromolecules under various conditions: from the solution to the melt, in the bulk as well as in geometries confined by walls etc. In the first part, the general concepts for the construction of such coarse-grained models for the simulation of cooperative phenomena in multi-chain systems and `technical' aspects of Monte Carlo simulations are discussed, as well as the limitations that this methodology still has to face. In the second part, typical applications are briefly reviewed, including the dynamics of chains adsorbed on surfaces or confined between plates, the scattering function from semidilute polymer solutions, and the formation of micelles from solutions of short block copolymers in the bulk and at the surface. Finally, a brief comparison to related but somewhat different approaches (Monte Carlo simulation of lattice models, Molecular Dynamics of bead-spring models) is given.

Formation of Block Copolymer Micelles in Solution: A Monte Carlo Study of Chain Length Dependence

Short block copolymers in a selective solvent (bad for A-block, good for B-block) are modeled by flexible bead−spring chains, where beads interact with short-range Morse potentials of variable strength. In particular, treating the strength EAA of attraction between monomers of the A-block as a variable, we study the mass distribution of the micelles that are formed under conditions that correspond to the vicinity of the critical micelle concentration (cmc). Choosing a composition f = NA/N = 1/4 for the block copolymers, we vary their chain length N from N = 4 to N = 32. Only such relatively short chains can be used in thermal equilibrium, since the relaxation times of the system increase dramatically with increasing length. We show that in the regime of parameters accessible to our study, the number of chains per micelle is rather small and almost independent of chain length, implying that the core radius scales as NA1/3 in this regime. We compare our results with existing theoretical predictions and with...

Formation of Surface Micelles from Adsorbed Asymmetric Block Copolymers: A Monte Carlo Study

The properties of surface micelles formed from flexible short block copolymers of composition AfB1-f with f = 1/4 and chain lengths of N = 8 and N = 16 effective monomers are obtained from Monte Carlo simulation of a coarse-grained bead spring model. We consider the case where the surface has a short-range attraction to the A monomers but is repulsive for the B monomers. Depending on the surface coverage and the strength EAA of the attractive energy between the A monomers, the adsorbed polymers may form “mushrooms”, or a polymer brush, or surface micelles. We show that a rather well-defined critical micelle concentration occurs, similar to that for micelle formation in bulk solutions, and we study the size distribution of the micelles as well as their internal structure. Large micelles exhibit a very dense solid core and are essentially immobile. Equilibration then is still possible via the exchange of single mobile chains between micelles.

Monte Carlo simulation of micelle formation in block copolymer solutions

Short block copolymers in selective solvents (bad for A-block, good for B-block) are modeled by flexible bead-spring chains, where beads interact with short range Morse potentials of variable strength. It is shown that already very short chains (N A = N B = 2) exhibit a rather well-defined critical micelle concentration (cmc). The mass distribution of the micelles and their gyration tensor components as well as their internal structure are studied. It is shown that the relaxation time increases exponentially with the strength E AA of the attractive energy between the A-monomers, and thus frozen-in micelles of medium size are obtained when E AA is chosen too large. Our results are compared to studies of related but somewhat different models.

Simulation of Lamellar Phase Transitions in Block Copolymers and Surfactants

Abstract Simulations of the lamellar phase transitions of symmetric amphiphilic chains are carried out on a cubic lattice, with the amphiphilic chain length N varied from 6 to 48 lattice sites, corresponding to lengths ranging from surfactants to short block copolymers. We find that the effective interaction energy parameter χN (which incorporates the effect of added solvant) at which the transition from the lamellar ordered state to the disordered state occurs is roughly equal to 18-21. While this result is consistent with an extrapolation of the Fredrickson-Helfand weak-segragation theory to N values in the range of the simulations, the amplitude of the sinusoidal compositional wave in the ordered state near the transition is large for all N studied, in disagreement with the weak segregation theories. Thus, for values of N up to 48, the transition occurs in a “moderate,” rather than weak-segregation regime. Near the disordering transition, fluctuating “bridge” or “hole” defects in the lamellae spontaneo...