Novel Tough and Transparent Ultra-Extensible Nanocomposite Elastomers Based on Poly(2-methoxyethylacrylate) and Their Switching between Plasto-Elasticity and Viscoelasticity

Abstract

Novel stiff, tough, highly transparent and ultra-extensible self-assembled nanocomposite elastomers based on poly(2-methoxyethylacrylate) (polyMEA) were synthesized. The materials are physically crosslinked by small in-situ-formed silica nanospheres, sized 3–5 nm, which proved to be a very efficient macro-crosslinker in the self-assembled network architecture. Very high values of yield stress (2.3 MPa), tensile strength (3.0 MPa), and modulus (typically 10 MPa), were achieved in combination with ultra-extensibility: the stiffest sample was breaking at 1610% of elongation. Related nanocomposites doubly filled with nano-silica and clay nano-platelets were also prepared, which displayed interesting synergy effects of the fillers at some compositions. All the nanocomposites exhibit ‘plasto-elastic’ tensile behaviour in the ‘as prepared’ state: they display considerable energy absorption (and also ‘necking’ like plastics), but at the same time a large but not complete (50%) retraction of deformation. However, after the first large tensile deformation, the materials irreversibly switch to ‘real elastomeric’ tensile behaviour (with some creep). The initial ‘plasto-elastic’ stretching thus causes an internal rearrangement. The studied materials, which additionally are valuable due to their high transparency, could be of application interest as advanced structural materials in soft robotics, in implant technology, or in regenerative medicine. The presented study focuses on structure-property relationships, and on their effects on physical properties, especially on the complex tensile, elastic and viscoelastic behaviour of the polyMEA nanocomposites.

Self-Healing and Super-Elastomeric PolyMEA-co-SMA Nanocomposites Crosslinked by Clay Platelets

“…Phase separation during synthesis: As described and discussed in previous works [ 52 , 53 ], the formation of polyMEA from the aqueous monomer solution always is accompanied by phase separation, as the monomer is moderately hydrophilic, but the polymer is predominantly hydrophobic. The micro-precipitation followed by coagulation also occurs in case of polyMEA/clay nanocomposites [ 52 , 53 ]. This effect leads to the appearance of turbidity and to a subsequent formation of a partly hydrated solid raw product, which has a cottage cheese consistence.…”

“…1 nm). The nanofiller served as polyfunctional physical crosslinker (see Scheme 2 and discussion in previous works [ 48 , 49 , 52 , 53 ]). The ionic co-monomer SMA was added in order to modify and enhance the internal interactions in the nanocomposite elastomers, and was expected to raise their toughness.…”

“…Nanocomposites with more than 10 wt.% of the RDS clay were difficult to prepare, because the synthesis mixture becomes relatively viscous, so that increasing challenges arise, especially with homogeneous admixing of co-initiators. The employed concentrations of the latter (see Experimental Part) were based on previous experience with polyMEA systems [ 52 , 53 ] and were chosen so that very long polymer chains were favored, which in turn was useful for obtaining super-elastomeric properties.…”

“…In Scheme 2 , the basic structural features of the prepared nanocomposite elastomeric networks are shown (they were studied in detail in previous work [ 52 , 53 ]): Polymer chains anchored by adsorption on clay platelets ( Scheme 2 a) are the most basic feature, which also is the basis of the distantly related ultra-extensible and tough poly(N-isopropylacrylamide) (PNIPAm)/clay hydrogels, mentioned in the Introduction. The very efficient adsorption of a long chain segment is the result of the previous adsorption of monomer, prior to its polymerization, and of starting the polymerization from equally adsorbed TEMED co-initiator molecules, as mentioned further above (Synthesis).…”

“…The latter elastomer, similarly to some PNIPAm/clay hydrogel varieties [ 11 , 12 ], indicated a potential of self-healing of disrupted samples at selected compositions [ 52 ] (which was not deeper studied), as well as considerable self-recovery of internal mechanical damage. In a subsequent work [ 53 ], the authors prepared highly transparent, tough, and ultra-extensible polyMEA/SiO 2 nanocomposites, which exhibited very attractive elasto-plastic, or plasto-elastic behavior, as well as related polyMEA/clay/SiO 2 systems. Another important recent work [ 54 ] was dedicated to Haraguchi-type gels, which were doped by methacrylate anion repeat units neutralized by different trivalent cations.…”