## 5 Thermomechanical Composite Properties
### Glass transition temperature and coefficient of thermal expansion  
The [PERSON] group of [PERSON] investigated the influence of coated and uncoated Al\({}_{2}\)O\({}_{3}\) (average particle size around 39 nm) on the glass transition (TG) behavior of PMMA [110,111]. They found, that uncoated Al\({}_{2}\)O\({}_{3}\) caused, at concentrations greater than 0.5 wt %, a significant TG-drop of around 25 \({}^{\circ}\)C. The silanization of the Al\({}_{2}\)O\({}_{3}\) enabled a constant TG even at higher Al\({}_{2}\)O\({}_{3}\) load of 10 wt %. In contrast, the Hu group found that the addition of hydrophobic nanosized SiO\({}_{2}\) (Aerosil(r)) to PMMA yielded a pronounced increase of the glass transition temperature up to 15 \({}^{\circ}\)C at a solid load of 4 wt % due to a strong attractive interaction of the non-polar nanoparticle with a huge specific surface area up to 200 m\({}^{2}\)/g and the non-polar polymer [112]. The decomposition temperatures of the composites were elevated remarkably even at low SiO\({}_{2}\) contents. Both groups used sonification in MMA as dispersing method prior to the polymerization to the final PMMA-based composite [110,112]. The addition of two different nanosized Al\({}_{2}\)O\({}_{3}\) (primary particle sizes 13 and 38 nm, solid load up to 10 wt %) to a methylmethacrylate based reactive resin caused after polymerization only a slight drop of TG [113]. \begin{table}
\begin{tabular}{l|l|l} \hline
**Item** & **Polymer-filler interaction** & **Impact** \\ \hline Elastic modulus & Attractive/repulsive & Increase with volume fraction \\ \cline{2-3}  & Attractive/repulsive & Increase with size decrease \\ \hline Density/volume & Attractive & Increased volume as size decreases \\ \cline{2-3}  & Repulsive & n.a. \\ \hline Glass transition temperature & Attractive & Decrease with addition of particles \\ \cline{2-3}  & Repulsive & n.a. \\ \hline Crystallinity & Attractive/repulsive & No major effect \\ \hline \end{tabular}
\end{table} 
Table 4: Impact of nanoparticles on composite properties with semicrystalline polymer matrix. \begin{table}
\begin{tabular}{l|l|l} \hline
**Item** & **Polymer-filler interaction** & **Impact** \\ \hline Elastic modulus & Attractive/repulsive & Increase with volume fraction \\ \cline{2-3}  & Attractive/repulsive & Increase with size decrease \\ \hline Density/volume & Attractive & Increased volume as size decreases \\ \cline{2-3}  & Repulsive & n.a. \\ \hline Glass transition temperature & Attractive & Increase with size decrease \\ \cline{2-3}  & Repulsive & Level until 0.5\%, drops off level from 1–10\% \\ \hline \end{tabular}
\end{table} 
Table 3: Impact of nanoparticles on composite properties with amorphous polymer matrix. The coefficient of thermal expansion (CTE) was more affected by the 13 nm Al\({}_{2}\)O\({}_{3}\), which can be explained by the larger specific surface area of 107 m\({}^{2}\)/g than the 38 nm Al\({}_{2}\)O\({}_{3}\) with only 34 m\({}^{2}\)/g enabling pronounced polymer chain immobility. Although a TG-increase could not be detected in PMMA/SiO\({}_{2}\) nanocomposites by thermal analysis (DSC), dielectric and IR-spectroscopy gave evidence for a strong attractive interaction of the nanoparticles with the polymer chains by means of reduced chain movement and longer relaxation times [88]. The direct chemical bonding of monodisperse nanosized SiO\({}_{2}\) to a polymer backbone, here polystyrene (PS), via grafting, caused a TG increase up to 6 \({}^{\circ}\)C at 2 wt %, while the simple physical mixing via sonification generated a slight TG decay [114]. Composites, consisting of nanosized Al\({}_{2}\)O\({}_{3}\) and the semicrystalline high performance polymer polyetheretherketone (PEEK), fabricated by wet chemical methods followed by hot pressing, showed an elevated decomposition temperature in comparison to the neat polymer [115]. In contrast, the melting temperature was not affected by the addition of the nanosized Al\({}_{2}\)O\({}_{3}\) even at very high load (30 wt %). The crystallization temperature increased slightly with load.