Free volume nanocomposites

Electron behavior, optical properties, catalytic properties, conductivity, and magnetic properties of nanocomposites were discussed in an extensive review pa-per. Complementary use of electron paramagnetic resonance and nuclear magnetic resonance helped to understand chain mobility in nanocomposites obtained from poly(ethylene oxide) encapped with triethoxy silicon. This nanocomposite is composed of PEO chains attached to silica clusters. It was found that chain fragments close to the silica clusters have hindered mobility due to the reduction of local free volume. The length of this hindered segment is estimated as three ethylene oxide units. 

Merkel et al. [2002, 2003] carried out studies of gas and vapor permeability and PALS free volume in a poly(4-methyl-2-pentyne) (PMP)/fumed silica (FS) nanocomposite. It was observed that gas and vapor uptake remained essentially unaltered in nanocomposites containing up to 40 wt% FS, whereas penetrant diffusivity increased systematically with the spherical nanofiller content. The increased diffusivity dictates a corresponding increase in permeability, and it was further established that the permeability of large penetrants was enhanced more than that of small penetrants. PALS analysis indicated two o-Ps annihilation components, interpreted as indicative of a bimodal distribution of free-volume nanoholes. The shorter o-Ps lifetime remained unchanged at a value T3 2.3 to 2.6 ns, with an increase in filler content. In contrast, the longer lifetime, T4, attributed to large, possibly interconnected nanoholes, increased substantially from 7.6 ns to 9.5 ns as FS content increased up to 40 wt%. 

FIGURE 12.8 Mean free-volume cavity diameter, determined from o-Ps lifetime, as a function of temperature for organically modified clay (T ), polyamide 6 ( ), and 65 wt%/35 wt% polyamide 6/organically modified clay nanocomposite (o). The dashed line represents the mean of a weight average of cavity sizes in a physical mixture of polymer and clay with no changes in the free-volume sizes in either polymer or clay. (Adapted from Winberg et al. [2005 a].)... 

A common finding in many of the PALS studies of nanocomposites described above is an increase in the nanohole size and/or the o-Ps intensity, indicative that the free volume is increased as a result of interaction between the filler and the polymer. However, several PALS studies have reached the opposite conclusion that the free volume of a polymer matrix decreases when filler particles are added. Thus, Stephen et al. [2006]... 

Becker, O., Cheng, Y.-B., Varley, R. J., and Simon, G. R, Layered silicate nanocomposites based on various high functionality epoxy resins the influence of cure temperature on morphology, mechanical properties and free volume, Macmmolecules, 36,1616-1625 (2003). 

Merkel, T. C., Freeman, B. D., Spontak, R. J., He, Z., Pinnau, I., Meakin, R, and Hill, A. J., Sorption, transport, and structural evidence for enhanced free volume in poly (4-methyl-1-pentyne/fumed silica nanocomposite membranes, Chem. Mater., 15, 109-123 (2003). 

Utracki, L. A., Free volume of molten and glassy polystyrene and its nanocomposite, J. Polym. Sci. Polym. Phys., 46, 2504-2518 (2008). 

Wmberg, R, DeSitter, K., Dotremont, C., Mullens, S., Vankelecom, 1. F. J., and Maurer, R H. J., Free volume and interstitial mesopores in siUca-filled poly(l-trimethylsilyl-l-propyne) nanocomposites. Macromolecules, 38, 3776-3782 (2005b). 

FREE VOLUME IN MOLTEN AND GLASSY POLYMERS AND NANOCOMPOSITES... 

---