Multi-scale modeling of soft nanostructured materials based on polymer adhesives
Soft nanostructured materials based on polymers overwhelm the modern adhesives industry due to a spectrum of unique properties such as excellent control on glass transition temperature, low plateau modulus, competitive cost, superior stability, and greater resistance to oxidation (because of their saturated backbone) compared to rubber-based analogues. Within these materials, interfaces pose specific challenges, since they are usually diffuse and can transfer stress through chain entanglements. For example, acrylic PSAs are based on complex formulations employing random copolymers of a long-chain acrylic (e.g., n-butyl acrylate, BA) characterized by a low glass transition temperature (Tg) with a short side-chain acrylic (such as methyl acrylate) to adjust Tg and acrylic acid (AA) to improve adhesion and optimise elongational properties. When they are prepared by emulsion polymerisation in water, they result from the drying and coalescence of separate latex particles so that the memory of the original interface is retained. In applications involving a contact between the soft polymer and a hard substrate, interfacial bonding also constitutes a fundamental problem, since it is at the root of the mechanisms governing failure upon deformation. In our lab, we try to get a fundamental understanding of the role of all these interfaces in the performance of acrylic adhesives by developing sophisticated hierarchical models addressing:
- the mechanism(s) of stress transfer at internal interfaces between soft latex particles containing polymers with different topologies; this needs to be addressed at different scales from the molecular to entanglement network level
- the mechanism of stress transfer at hard-soft interfaces between the substrate and the soft adhesive, as a function of chemical composition of the PSA formulation and the chemistry of the substrate
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- the effect of the presence of multiple internal interfaces on the macroscopic deformation behaviour of the adhesive
- how cavitation and the development of the fibrillar structure in the final adhesive product are affected by changes in the properties of the initial formulation
- the respective role played by the polymer rheology
- the correct type of boundary conditions (b.c.’s) at the polymer/substrate interface; this needs to be derived from non-equilibrium statistical mechanics
- a more representative constitutive law for the finite isotropic and anisotropic elastic behaviour of these materials