Linear ABA Triblock Copolymers Research Articles

Theory of Microphase Segregation in ABA Triblock Comb-Shaped Copolymers: Lamellar Mesophase

A theory describing lamellar mesophases formed in the melt of ABA triblock copolymers with strongly incompatible comblike blocks is developed using a combination of a strong-stretching self-consistent field (SS-SCF) analytical approach and numerical self-consistent field calculations based on the Scheutjens-Fleer (SF-SCF) methods. The structural and thermodynamic properties of the lamellae are analyzed as a function of their architectural parameters, i.e., grafting density and polymerization degree of the main and side chains in the comb-shaped blocks. In particular, we distinguish between the loops and bridges formed by the middle comb-shaped block and demonstrate how a fraction of the bridges connecting the neighboring AB interfaces of the lamellar layers and shear modulus of the mesophase are mediated by the architectural parameters of the comblike blocks. In particular, we predict an increase in the shear modulus upon replacement of the linear ABA triblock copolymers by comblike ones with the same composition. The asymptotic analytical predictions of the theory are complemented by the results of numerical modeling.

Amphiphilic polymer co‐networks: 32 years old and growing stronger – a perspective

AbstractThis brief perspective overviews amphiphilic polymer co‐networks (APCNs), polymeric chemically crosslinked hydrogels also covalently hosting a hydrophobic segment, conferring upon these materials the important property of internal self‐organization in aqueous environments. APCN synthesis is most conveniently accomplished via the random crosslinking copolymerization of a hydrophilic monomer with a hydrophobic macro‐crosslinker, whereas better structures are attained via the end‐linking of amphiphilic linear ABA triblock copolymers or amphiphilic star diblock copolymers. Microphase‐separated morphologies, translucency‐to‐transparency, biocompatibility, low to moderate aqueous swelling, fair mechanical strength and possible stimulus‐responsiveness constitute the main physicochemical properties of APCNs, with their major commercial application being in silicone hydrogel soft contact lenses. Modeling the swelling, morphological and mechanical behavior of these intriguing materials is an area where more progress is necessary in order to improve our understanding of these networks and facilitate the design of next‐generation APCNs. © 2020 Society of Industrial Chemistry

Effect of Chain Architecture on Self-Assembled Aggregates from Cyclic AB Diblock and Linear ABA Triblock Copolymers in Solution.

The self-assembly behaviors of two block copolymers with the same chain length but different chain architectures (cyclic AB, linear ABA) in B-selective solvents are investigated using Monte Carlo simulations. A morphological transition sequence, from spherical micelles to cylindrical micelles, to vesicles and then to multicompartment vesicles, is observed for both copolymer systems when the interaction between the solvophobic A-block and the solvent is increased. In particular, toroidal micelles could be formed in triblock systems due to the presence of the bridging chains at the parameter region between cylindrical micelles and vesicles whereas disklike micelles are formed in cyclic systems. The simulation results demonstrated that the architecture of block copolymers could be used to regulate the structural characteristics and thermal stability of these self-assembled aggregates.

Phase Behaviors and Bridging Properties of Bolaform Tethered Nanoparticles

We investigate the bolaform tethered nanoparticles (P-H-P) in melt by using a computational method combining the self-consistent field theory for the flexible homopolymer (H) and the density functional theory for two hard spheres (P) chemically connected at each end of the homopolymer. The phase diagram of P-H-Ps is highly asymmetric even for the symmetric P-H-Ps with two identical nanoparticles. When the size difference between two particles is large enough, the P-H-Ps forming lamellae undergo a transition from the particle-aggregated state to the particle-separated state with increasing volume fraction fA of polymer or increasing Flory–Huggins parameter χ between the polymer and the particles. Compared with the linear ABA triblock copolymers with the same molecular size and component fractions, P-H-Ps have much higher fractions vB of the bridging conformation connecting two domains apart in distance. The bridging fraction vB, which is believed to be significant to the mechanical properties of the polyme...

Chain Architecture as an Orthogonal Parameter To Influence Block Copolymer Morphology. Synthesis and Characterization of Hyperbranched Block Copolymers: HyperBlocks

For block copolymers there is usually a strong correlation between the copolymer composition (volume fraction of each block) and the resulting solid state morphology. However, for a variety of potential applications, e.g., semipermeable membranes or templates, it might be desirable to vary the microphase morphology independently of copolymer composition. The use of chain branching is an additional and orthogonal parameter to influence morphology, independently of composition, and we explore for the first time the impact of a long-chain, hyperbranched architecture on the microphase-separated, solid state morphology of branched block copolymers. To this end, a series of functionalized linear ABA (polystyrene–polyisoprene–polystyrene) triblock copolymers (macromonomers), hyperbranched ABA triblock copolymers (HyperBlocks), and blends of HyperBlocks with a commercially available linear ABA triblock copolymeric thermoplastic elastomer were prepared. Moreover, the “macromonomer” approach is the only feasible route to prepare hyperbranched block copolymers. The solid state morphology of the resulting materials was investigated by a combination of transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS) which showed a dramatic impact of the chain architecture on the resulting morphology. While the linear ABA triblock copolymers showed the expected microphase-separated morphology with long-range order dependent upon composition, no long-range order was observed in the HyperBlocks. Instead, the HyperBlocks revealed a microphase-separated morphology without long-range lattice order, irrespective of macromonomer composition or molecular weight. Furthermore, when HyperBlocks were subsequently blended with a commercially available linear ABA triblock copolymer (Kraton D1160), the HyperBlock appeared to impose a microphase-separated morphology without long-range lattice order upon the linear copolymer even when the HyperBlock is present as the minor component in the blend at levels as low as 10 wt %.

Computer Simulation of Micellization for Ring-like Block Copolymers

The micellization behaviour of ring-like AB diblock copolymers in a selective solvent was compared with the corresponding linear ABA triblock copolymers via Monte Carlo simulation. The simulation results show that the difference in the critical micellization concentration (cmc) values between ring-like copolymers and linear copolymers with same compositions is closely related to the content of A blocks, f(A), and the strength of attractive interaction between B monomers, epsilon. When fA is relatively small and e is big, the cmc values of ring-like block copolymers are smaller than those of the corresponding linear ones; whereas when fA is big and e is small, the cmc values of triblock copolymers are smaller than those of the corresponding ring-like ones. To further understand the effect of fA and e on the micellization behavior, we calculated the entropy and the potential energy contributions to the micellization free energy, respectively. The results show that in the studied range of fA and 8, the entropy loss in micellization for ring-like block copolymers is always smaller than that for triblock copolymers, so it seems that the micellization of ring-like block copolymers is always easier than that of the linear ones if we only consider the entropy contributions to the free energy. But if we calculate the potential energy contributions to the free energy, it can be found when fA is relatively small and 8 is big, the potential energy decreases greatly during the micellization of ring-like block copolymers. Considering the less unfavorable contribution of entropy part to the free energy for ring-like block copolymers, the micellization of ring-like block copolymers should be much easier than that of the linear ones. When f(A), is big and e is small, the potential energy decreases greatly when the micelles form for linear block copolymers. Although the entropic contribution to the free energy is unfavorable for linear ones, the potential energy still shows the dominative contribution to the free energy and therefore the micellization of linear block copolymers is much easier than the ring-like ones. The above analysis of the free energy will help us understand the micellization behavior of ring-like block copolymers and linear block copolymers.

Design of Soft and Strong Thermoplastic Elastomers Based on Nonlinear Block Copolymer Architectures Using Self-Consistent-Field Theory

Thermoplastic elastomers (TPEs) that are soft at low extension yet strong at large extension are of great importance in a variety of technological applications. In ABA-triblock architectures, both the overall molecular weight and the composition of the glassy A-blocks correlate with TPE strength. The design space of current TPEs based on linear ABA triblock copolymers (e.g., polystyrene-b-polybutadiene-b-polystyrene) is restricted by the accessibility of the order−disorder transition temperature, limiting the molecular weight, and restricted by the maximum volume fraction for which glassy A-blocks will form discrete domains. Using self-consistent-field theory (SCFT), we designed in silico two new, nonlinear TPE architectures that significantly relax the composition restriction: radial (ABA′)n and A(BA′)n miktoarm star-block copolymers with chemically identical, but unequal, molecular weight A-blocks. Through a balance of end-block bidispersity, block extraction from the interface, and architectural asymmetry, these molecular architectures are able to stabilize phases with discrete A-rich domains at remarkably high overall A-monomer compositions (fA). In some cases the maximum fA achieved for phases with discrete A-rich domains surpasses twice that of conventional linear ABA TPEs.

Self‐Assembly of Cylinder‐Forming ABA Triblock Copolymers under Cylindrical Confinement

AbstractThe self‐assembly of two types of linear ABA triblock copolymers confined in cylindrical nanopores is studied using simulated annealing. The effects of pore size and block copolymer chain architecture on morphology, chain conformations and bridging fraction are investigated. For the bulk cylinder‐forming copolymers, novel structures such as helices and stacked toroids form, which depend sensitively on the pore size. Several significant differences between the two types of copolymers are predicted and explained based on the differences in their chain conformations and chain architectures. A simple model is proposed to explain the mean square radius of gyration for the bridge and loop chains.magnified image

Synthesis, characteristics and in vitro degradation of star-block copolymers consisting of l-lactide, glycolide and branched multi-arm poly(ethylene oxide)

Biodegradable star-block copolymers containing a hydrophilic 4- or 8-arm branched poly(ethylene oxide) (PEO) central unit and hydrophobic poly( l-lactide) (PLLA) or poly( l-lactide- co-glycolide) (PLLG) were synthesized by solution polymerization in toluene catalysed by aluminium triethylene. Using g.p.c. and laser light scattering analysis, a significant increase in the molecular weights could be demonstrated, corresponding to the ratio of the monomers relative to the multi-arm poly(ethylene oxide) in the feed. Polydispersities were of same order as those of the parent PEOs. N.m.r. spectra show that all hydroxy end groups of multi-arm PEO are esterified after copolymerization, which supports the star architecture of the products. The star-block copolymers have physico-chemical properties differing from their starting materials: branched PEO and PLLG or PLLA. The glass transition temperature, T g, and the crystallinity of star-block PLLA-PEO or PLLG-PEO were significantly reduced in comparison to the respective linear polymers, demonstrating the influence of the steric architecture of the star-block copolymers on the thermal properties. The in vitro degradation properties of star-block PLLG-PEO copolymers differ from those of linear ABA triblock copolymers of PLLG and PEO due to the steric architecture and shorter PLLG block. While mass loss and molecular weight decay are of comparable order in the initial phase, loss of branched PEO seems to be reduced. These novel biodegradable polyether-polyesters may have potential for parenteral protein delivery systems.