Nonlinear Mechanics of Triblock Copolymer Elastomers: From Molecular Simulations to Network Models

Stress Chain Analysis for an ABA Triblock Copolymer Using Principal Component Scores

Researchers characterize the mechanical properties of ABA triblock copolymers by structures such as chain conformation. During elongation, bridge chains are stretched and act as a stress chain. Some loop chains also act as a stress chain because of the transmission of stress through an entanglement of loop chains. The stress chain, including the entangled loop chains, in an ABA triblock copolymer that exhibits a body-centered cubic structure was analyzed by principal component analysis (PCA), using the physical data for the B block obtained by coarse-grained molecular dynamics simulations. Local deformation of the A domains caused by the stress chains was also analyzed by PCA of the A block. The dynamics of the stress chain strongly corresponded to the recombination of the A domains; shrinkage because of domain breakage, replacement of stress chains, and biased stress distribution as well as its time dependence were observed.

An extensible density-biasing approach for molecular simulations of multicomponent block copolymers

A node-density biased Monte Carlo methodology is proposed for the molecular structure generation of complex block copolymers. Within this methodology, the block copolymer is represented as bead-spring model. Using self-consistent field theory, a density field for all monomer species within the system is calculated. Block copolymers are generated by random walk configuration biased by the density fields. The proposed algorithm then modifies the generation process by taking the global structure of the polymer into account. It is then demonstrated that these global considerations can be built into the sampling procedure, specifically through functions that assign a permissible difference in density field value between relevant monomer species to each step of the random walk. In this way, the random walk may be naturally controlled to provide the most appropriate conformations. The overall viability of this approach has been demonstrated by using the resulting configurations in molecular dynamics simulations. This new methodology is demonstrated to be powerful enough to generate molecular configurations for a much wider variety of materials than the original approach. Two key examples of the new capabilities of the method are viable configurations for ABABA pentablock copolymers and ABC triblock terpolymers.

Computational Study of Mechanochemical Activation in Nanostructured Triblock Copolymers

Force-driven chemical reactions have emerged as an attractive platform for diverse applications in polymeric materials. However, the microscopic chain conformations and topologies necessary for efficiently transducing macroscopic forces to the molecular scale are not well-understood. In this work, we use a coarse-grained model to investigate the impact of network-like topologies on mechanochemical activation in self-assembled triblock copolymers. We find that mechanochemical activation during tensile deformation depends strongly on both the polymer composition and chain conformation in these materials. Activation primarily occurs in the tie chains connecting different glassy domains and in loop chains that are hooked onto each other by physical entanglements. Activation also requires a higher stress in materials having a higher glassy block content. Overall, the lamellar samples show the highest percent activation at high stress. In contrast, at low stress, the spherical morphology, which has the lowest glassy fraction, shows the highest activation. Additionally, we observe a spatial pattern of activation, which appears to be tied to distortion of the self-assembled morphology. Higher activation is observed in the tips of the chevrons formed during deformation of lamellar samples as well as in the centers between the cylinders in the cylindrical morphology. Our work shows that changes in the network-like topology in different morphologies significantly impact mechanochemical activation efficiencies in these materials, suggesting that this area will be a fruitful avenue for further experimental research.

Compression-induced anti-nematic order in glassy and semicrystalline polymers

We provide new insights into the molecular origin of the asymmetry between uniaxial tensile and compressive deformation of glassy and semicrystalline polymers using molecular dynamics simulations. The difference between the two responses strongly depends on the chain length and is the largest at intermediate chain lengths. Irrespective of chain length, the intra- and interchain organization of polymers under extension and compression are remarkably distinct. The chains align along the tensile axis leading to a global nematic order of the bonds and end-to-end vectors, whereas compression reorganizes polymers to lie in planes perpendicular to the compressive axis resulting in the emergence of an anti-nematic order and destruction of crystallinity. Regardless of the initial glassy or semicrystalline structure, the deformed state of polymers at large strains converge towards the same kind of structure that only depends on the deformation mode.

Modeling the Contact Mechanics of Hydrogels

A computationally lean model for the coarse-grained description of contact mechanics of hydrogels is proposed and characterized. It consists of a simple bead-spring model for the interaction within a chain, potentials describing the interaction between monomers and mold or confining walls, and a coarse-grained potential reflecting the solvent-mediated effective repulsion between non-bonded monomers. Moreover, crosslinking only takes place after the polymers have equilibrated in their mold. As such, the model is able to reflect the density, solvent quality, and the mold hydrophobicity that existed during the crosslinking of the polymers. Finally, such produced hydrogels are exposed to sinusoidal indenters. The simulations reveal a wavevector-dependent effective modulus E * ( q ) with the following properties: (i) stiffening under mechanical pressure, and a sensitivity of E * ( q ) on (ii) the degree of crosslinking at large wavelengths, (iii) the solvent quality, and (iv) the hydrophobicity of the mold in which the polymers were crosslinked. Finally, the simulations provide evidence that the elastic heterogeneity inherent to hydrogels can suffice to pin a compressed hydrogel to a microscopically frictionless wall that is undulated at a mesoscopic length scale. Although the model and simulations of this feasibility study are only two-dimensional, its generalization to three dimensions can be achieved in a straightforward fashion.

Scattering patterns and stress-strain relations on phase-separated ABA block copolymers under uniaxial elongating simulations

To develop molecularly based interpretations of the two-dimensional scattering patterns (2DSPs) of phase-separated block copolymers (BCPs), we performed coarse-grained molecular dynamics simulations of ABA tri-BCPs under uniaxial stretching for block-fractions where the A-segment (glassy domain) is smaller than the B-segment (rubbery domain), and estimated the behaviour of their 2DSPs. In BCP stretching experiments, mechanical properties are generally evaluated using a stress-strain curve. We obtained 2DSPs with different contrasts for the A- and B-segments, which are indicative of the differences between X-ray and neutron scattering experiments. The small- and wide-angle behaviours of the 2DSPs originate from the morphologies of the phase-separated domains and local bond orientations, respectively. When the block-fractions are changed for a constant stress value on the stress-strain (SS) curve, the brightness of the spots in the wide-angle region of the A- and B-segment-dominant 2DSPs decreases and increases with increasing strain, respectively. We can regard the systematic changes in the small-angle 2DSPs of the glassy domain and the wide-angle 2DSPs of the rubbery domain with changes in the SS-curve as a structure-property relationship.