title: Hybrid Bragg stacks - polymer-clay nanocomposites defined by ultraconfinement
Creators: name: Theresa, Dörres
Publisher: name: University of Bayreuth
Year: 2022
Subjects: subject: Nacre
subject: hectorite
subject: nanocomposites
subject: ultraconfinement
subject: mechanical properties
subject: anisotropy
subject: oxygen permeability
subject: 540

description: Natural composites like wood, bones or nacre always served as inspiration for the design of advanced and functional materials. All natural composites have one thing in common; the constituents are hierarchically arranged over multiple length scales. In case of nacre, the hierarchical arrangement of the strong and weak but tough constituents in a so-called “brick-and-mortar” structure is considered as a precondition for the combination of the technically mutually exclusive properties of strength and toughness. However, the realization of such highly ordered arrangements is not so easy to achieve for synthetic polymer-clay nanocomposites. Apart from the varying quality of clay and inadequate processing methods, phase segregation is a widespread problem. Phase segregation not only creates a problem with reproducibility but also renders a systematic variation of parameters and their influence on the material’s properties challenging. This work is based on the unique ability to produce highly homogeneously ordered one-dimensional (1D) crystalline nanocomposite films by spray coating, which serve as model systems for investigating the thermoelastic, permeability and mechanical properties. In literature, such 1D crystals are referred to as hybrid Bragg stacks. Within this work, two polymer/clay systems are presented, that enable the realization of a 1D crystalline arrangement of clay nanoplatelets for two different polymer contents: polyvinylpyrrolidone (PVP) and polyethylene glycol (PEG). One factor, enabling the synthesis is the used synthetic sodium fluorohectorite (Hec) and its unique and rare phenomenon of repulsive osmotic swelling. The resulting nanoplatelets of uniform thickness form nematic liquid crystalline phases that yield a perfectly homogeneous nematic hybrid phase when mixed with a polymer. By spray coating transparent, self-standing and monocrystalline films were obtained. Carefully adjusting the Hec to PVP content, the basal spacing can be tuned in the range from 1.9 to 3.8 nm. The unique long-range 1D ordered hybrid films in combination with advanced characterization methods allow to determine the direction-dependent thermoelastic properties. The anisotropy between the in-plane and cross-plane thermal conductivities was found to be exceptionally high for an electrically insulation hybrid material. Moreover, the structural anisotropy of the hybrid films is also displayed in the anisotropy of the direction-dependent Young’s moduli. Although the films experience strong polymer confinement, density, specific heat, in-plane thermal conductivity, and all mechanical moduli display an effective medium behavior. The cross-plane thermal conductivities however, do not obey a simple mixing rule, as the thermal transport in the cross-plane direction is highly governed by the clay/polymer interface. The hybrid Bragg stacks with a PVP content of 40 and 31 vol% only differ in the filler loading and are comparable with respect to filler type, aspect ratio, quality of texture and one-dimensional (1D) crystallinity. The basal spacing of 2.3 and 3.0 nm corresponds to a monolayer and bilayer of PVP, respectively. Consequently, PVP features strong confinement, as the interlayer height falls beneath the polymer’s radius of gyration. Therefore, the hybrid Bragg stacks represent a perfect model system for studying the impact of the polymer confinement on permeability. Cussler’s theory predicts that the permeability only non-linearly depends on aspect ratio and volume fraction of clay. However, within this investigation an additional nonlinear effect of confinement on permeability is established. Finally, the combination of mono and double layers of Hec and PEG is applied to fabricate hybrid Bragg stacks that differ in the thickness of Hec layers, polymer layer thickness and nanoplatelet diameter. The filler contents can be adjusted in the range from 73 to 91 wt%, of which the latter represent the record filler content in a clay nacre-mimetics. These highly filled hybrid Bragg stacks allow to study the bulk mechanics in a systematic way. However, only the hybrid Bragg stacks consisting of a monolayer of Hec and two layers of PEG behaves ductile with an elongation of 8% and a yield strength of 48 MPa. Summarizing, the ductility of the hybrid Bragg stack is lost when the thickness of the PEG decoupling layer is reduced, double layers of Hec are utilized, and the nanoplatelet diameter is increased.