Enhancing the Mechanical Properties of Matrix‐Free Poly(Methyl Acrylate)‐Grafted Montmorillonite Nanosheets by Introducing Star Polymers and Hydrogen Bonding Moieties

Abstract

both polystyrene-and poly(methyl methacrylate)-grafted silica nanoparticles and produced a material consisting only of particles and surface-tethered polymer chains. They could observe a dependency of the materials Young's modulus, fracture toughness, and hardness on the degree of polymerization (DP) of the grafted polymer. [8] Crucial for the successful preparation of such materials is the presence of interactions between different particles through entanglement of polymer chains bound to different particles. This is given if the chains are long enough to entangle with each other or if the chains are cross-linked. [6] Additionally, research strives for achieving an order of the particles as it is hoped to produce materials with, for example, superior optical properties. [6,8] One naturally occurring "matrix-free" material with exceptional mechanical properties is nacre, which consists of 95 vol.-% of aragonite platelets and 5 vol.-% of organic biopoly mer. [9] They form a composite with a "brick-and-mortar"-like structure, which exhibits exceptional mechanical properties. The Young's modulus is known to be up to 135 GPa accompanied by a fracture toughness of up to 1.8 MJ m −3 . These properties can be assigned to the unique structure of this special material. [9,10] A biomimetic approach to produce a similar synthetic material is the use of layered nanoparticles, substituting the aragonite, and synthetic organic molecules to replace the natural biopolymer. Different nanoparticles, such as graphene oxide, [11,12] Al 2 O 3 platelets, [13] or nanoclays [4,10,14,15] have been used so far.In this publication, we report the successful preparation of a nacre-like material using montmorillonite (MMT) nanosheets and for the first time PMA to form self-standing films and explore new strategies to tailor their mechanical properties by modifying the polymer functionalization of the platelets. Using the soft and ductile PMA allows for the formation of nanocomposites that show an exceptionally high modulus and tensile strength without being brittle. The mechanical properties of the novel material are further enhanced by cross-linking of the surface-grafted polymer, which is either achieved via radical addition-fragmentation chain transfer (RAFT) star-polymerization upon the platelet surface or via introducing hydrogen-bonding moieties to the surface-grafted polymer. Matrix-free nanocomposite films of poly(methyl acrylate) (PMA) and montmorillonite (MMT) nanosheets that imitate the microscopic structure of nacre are developed and their mechanical properties are studied in detail via tensile testing. The exfoliated MMT nanosheets are grafted with PMA via a grafting-through radical addition-fragmentation chain transfer polymerization in presence of a surface-anchored ionic monomer. The mechanical properties are precisely tailored a) via the variation of the degree of polymerization of the surface-grafted polymer, illustrating the impact of chain entanglement and the MMT content on the performance of the material, and b) via cros...

Increasing the Gas Barrier Properties of Polyethylene Foils by Coating with Poly(methyl acrylate)-Grafted Montmorillonite Nanosheets

“…The CEA content after the statistical copolymerization of CEA and MA was determined using NMR. We demonstrated in an earlier publication that a low CEA content is already sufficient to achieve a significant impact on the mechanical performance of the material, while a content that is too high would result in the material turning brittle [ 14 ]. Via thermogravimetric analysis (m), it was guaranteed that all polymer-covered MMT nanosheets were grafted with exactly the same amount of polymer (65 wt%) in order to ensure that differences observed in the gas permeability measurements can be attributed only to structural differences of the three different kinds of particles, but not to a different ratio of polymer to nanosheets in the coating.…”

“…MMT nanosheets grafted with linear PMA, a mixture of linear and star-shaped PMA, and linear PMA with the hydrogen bonding comonomer CEA were prepared according to the protocol published recently by our group [ 14 , 15 ]. Therefore, MMT was grafted with an ionic monomer (VB16) via ion exchange as presented by Salem and Shipp [ 17 ].…”

“…However, when arranged tightly in a “brick-and-mortar” structure [ 11 , 12 ] as a separate layer or coating, the packing density and with that also the mean path length for diffusion of gas molecules can be significantly increased [ 2 , 13 ]. As “mortar”, synthetic polymers can be used and f they are synthesized via controlled polymerization techniques, e.g., by reversible addition-fragmentation chain transfer (RAFT) polymerization, their properties can be precisely tailored [ 14 , 15 ]. They do not only provide stability to the MMT layer but can also be used to ensure sufficient interaction between different layers.…”

“…We also presented in these studies how the topology of the grafted PMA can be altered from linear to a mixture of star-shaped and linear PMA and how this strategy can be used to further optimize the mechanical properties of the nanocomposites. Additionally, we showed that by addition of a hydrogen-bonding comonomer to the grafted linear PMA the properties of the composites can be further enhanced [ 14 ]. We demonstrated that these changes in the grafted polymer allow for an increase in mechanical stability of the arranged polymer/nanosheet structure.…”

“…They do not only provide stability to the MMT layer but can also be used to ensure sufficient interaction between different layers. We have recently presented a reliable strategy to graft linear poly(methyl acrylate) (PMA) to MMT nanosheets via a surface-confined grafting-through RAFT polymerization [14,15]. The resulting exfoliated and polymer-covered MMT nanosheets were employed as advanced filler material that is capable of tuning the mechanical properties of PMA over a wide range.…”