Molecular dynamics in core-shell filled epoxy nanocomposites

Abstract

In this study, core-shell and hollow nanoparticle architecture types were synthesised and utilised as epoxy fillers with the main objective of understanding the molecular dynamics of these nanocomposites. Secondly to understand the effect of different characteristics of these nanoparticles such as the structure, crystallinity, core and shell particle nature on the dielectric properties of the bulk system by comparing them with commercially obtained conventional core-type nanoparticles and amongst themselves. The first step of the study was to select suitable combinations of core and shell and establish a reproducible method for the synthesis of the core-shell and hollow nanoparticle. For this, initially, to understand the effect of the shell crystallinity, commercially available silica (SiO₂), silica-silica (SiO₂-SiO₂) and hollow-silica (h-SiO₂) were selected. Commercially obtained SiO₂ was calcined at 800 °C and crystalline. Alternatively, both SiO₂-SiO₂ and h-SiO₂ were vacuum dried at 60 °C and the resulting shell was amorphous. For the synthesis of SiO₂-SiO₂, commercially obtained SiO₂ was used as the core whereas for the synthesis of h-SiO₂, poly (acrylic acid-sodium salt) (PAA-Na) was used as a template and the shell was deposited via the sol-gel method; subsequently, the PAA-Na template was removed to obtain hollow architecture via heat treatment. These nano-powders were characterised by transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC) and broadband dielectric spectroscopy (BDS). TEM graphs showed the successful synthesis of the nanoparticle where a shell formation was observed in
the case of SiO₂-SiO₂ nanoparticles and hollow centre in the case of h-SiO₂. FTIR measurement revealed a characteristic Si - O - Si absorbance band along with an overlapping absorbance band at higher wavelengths which were attributed to ν_as-transverse optic (TO) of crystalline and ν_as-longitudinally optic (LO) of amorphous structures in the nanoparticles. ν_as-LO absorbance intensity was the least in SiO₂ followed by SiO₂-SiO₂ and h-SiO₂ implying the increase in structural disorder following the same trend. Afterwards, epoxy nanocomposites were prepared whilst maintaining the stoichiometric ratio between the resin and hardener. To obtain optimal dispersion the nanocomposites were prepared by there different methods, namely, bath sonication, probe sonication and solvent mixing. TEM graphs were obtained to evaluate the dispersion and solvent mixing which demonstrated better dispersion was selected to prepare the nanocomposites. Since we are interested in the surface characteristics it is important to maintain comparability across all samples. Therefore, the fillers were loaded based on their total surface area; three different surface areas were selected. No significant changes in the unreacted epoxide groups were observed. Secondly, with increased disorder of the shell, an increasing concentration of hydroxyl groups originating from the surface silanols was observed. DSC measurements showed no significant changes in the glass transition temperature (T_g ). Correspondingly, from BDS measurements no significant changes in the alpha (α) and gamma (γ) relaxation were observed. Alongside beta (β) relaxation an additional relaxation was observed in SiO₂-SiO₂ and h-SiO₂ nanocomposite termed omega (ω) relaxation. The behaviour of β and ω relaxation were similar which implied the involvement of similar chemical
groups i.e. hydroxyl (-OH). As the concentration of surface silanols increased the ω relaxation became more pro-eminent. SiO2-SiO2 nanocomposite has an overlapping β and ω relaxation where it is segregated towards higher frequencies in the h-SiO₂ nanocomposite. The activation energy of the β and ω was similar. Thus, the ω relaxation was attributed to the hydrogen bond interaction between the hydroxy ether group of the polymer main chain and the surface silanol groups. Additionally, towards lower frequencies, interfacial polarization phenomena were observed. SiO₂ and h-SiO₂ had a single interfacial peak whereas SiO₂-SiO₂ had two interfacial peaks. The second peak was relatively weak and was hypothesised to be originating from the core-shell interface within the nanoparticle.
Furthermore, To confirm the hydrogen bond interactions, dynamic mechanical analysis (DMA) measurements were performed. For this, additionally, four more epoxy nanocomposites filled with Al₂O₃ - SiO₂, TiO₂ - SiO₂, Al₂O₃ and TiO₂ were prepared. Alumina-silica (Al₂O₃ - SiO₂) and titania-silica (TiO₂ - SiO₂) were selected due to the significant contrast between their relative permittivity and to study of the effects of the core particle. Both were synthesised following the sol-gel method. Analysis of tan δ for α and β relaxation revealed the presence of hydrogen bonds in core-shell nanoparticles. The absorbance band of the symmetric and asymmetric vibrations of the CH₂ and CH₃ groups were also affected by the core-shell structures via OH - π interactions. OH - π primarily depended upon the structure of the surface of the nanoparticles. Finally, the hypothesis related to the core-shell interface was tested. BDS analysis of metal oxide nanocomposites at interfacial polarization frequencies via Havriliak - Negami (HN) and Cole - Cole formulation revealed two pro-eminent interfacial relaxations. Low-frequency relaxation was attributed to the core-shell interface and higher-frequency relaxation was attributed to the conventional particle-polymer interface. Additionally, to maintain consistency between α and β
parameters at varying temperatures whilst performing HN analysis a MATLAB plugin software was developed.

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Identifiers

ORCID for Sunny Chaudhary: orcid.org/0000-0003-2664-7083

ORCID for Thomas Andritsch: orcid.org/0000-0002-3462-022X

ORCID for Alun Vaughan: orcid.org/0000-0002-0535-513X

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