## 5 Thermomechanical Composite Properties
### Elastic modulus, tensile strength, flexural strength and impact performance  
Hot compression molding of surface modified SiO\({}_{2}\) (primary particle size 12 nm) or Al\({}_{2}\)O\({}_{3}\) (primary particle sizes 15 and 90 nm), mixed with micron sized PEEK powder, was used for the fabrication of test specimens for mechanical testing [73]. The authors investigated the influence of different mixing methods (direct mechanical mixing, liquid-solid mechanical dispersing, sonification, ball milling) and particle sizes on the composite properties. In agreement to results listed earlier the addition of the nanofillers to PEEK caused an increase of TG but no remarkable change of the melting behavior. The amount of crystalline domains in the polymer decreased. The authors found, that tensile and compressive strength increased with Al\({}_{2}\)O\({}_{3}\) particle diameter while the flexural strength was not affected. Comparable trends could be detected for the Al\({}_{2}\)O\({}_{3}\) and SiO\({}_{2}\) fillers load increase. Both materials showed an optimized concentration of 5 wt % for a raise of the tensile, compressive and impact strength; but a further filler concentration increase yielded a strength reduction. A nanofillersurface modification using different coupling agents did not result in a unique trend of mechanical property reinforcement or weakening. A clear influence of the dispersing method on the investigated mechanical properties was not found [73]. Surface modified nanosized Al\({}_{2}\)O\({}_{3}\) (primary particle size 39 nm), dispersed in PMMA, caused an increase in strain-to-failure over 28% at a solid load of 5 wt % enabling a ductile flow in the glassy state [117]. Thin films, consisting of up to 22.8 wt % SiO\({}_{2}\) and polyamide 6, were investigated with respect to their mechanical properties [118]. A significant change of the viscoelastic properties (increase of storage and loss modulus) with increasing load as well as of the Young's modulus as function of solid load and temperature was reported. The mechanical properties of an _in situ_ synthesized and surface modified nanosized TiO\({}_{2}\) using acrylates as reactive resin matrix were investigated after polymerization [82]. The sol-gel based nanoparticle synthesis and the _in situ_ composite formation avoided particle agglomeration guaranteeing a homogenous particle distribution in the matrix. Increasing TiO\({}_{2}\) content caused an increase of hardness (from 0.030 GPa for the pure polymer and 0.198 GPa for a TiO\({}_{2}\) content of 10 wt %) and Young's modulus (from 2.83 GPa for the pure polymer and 4.98 GPa for a TiO\({}_{2}\) content of 10 wt %). The thermal stability was improved also, an increase of the refractive index in the visible range from 1.5 up to 1.8 (TiO\({}_{2}\) load 50 wt %) due to the high intrinsic refractive index value of TiO\({}_{2}\) accompanied with a good optical transmittance was measured [82]. The effect of the nanoparticle size and amount on resulting mechanical composite properties were measured by [PERSON] _et al._[72]. Fracture images showed a strong bond between the surface-modified SiO\({}_{2}\) and the polymer matrix. Tensile tests showed an improvement of the Young's modulus with solid load, in case of untreated, hydrophilic SiO\({}_{2}\) no reinforcement was observed. Hence, for a numerical description of the mechanical behavior using FEM-methods an interfacial layer between the filler and the matrix had to be assumed. [PERSON] and coworkers found, that a pronounced dependence of the mechanical properties like Young's modulus and tensile strength is given [119]. Surface coated nanosized Fe\({}_{2}\)O\({}_{3}\) with a nominal particle size of 9 nm, dispersed in an epoxy matrix, induced an improved thermal stability of the resulting composite, a significant TG increase of 15 \({}^{\circ}\)C at a 10 wt % solid load as well as an improved pencil hardness [120]. The properties of nanosized SiO\({}_{2}\)-latex-composites are described by [PERSON] [121].