The search for new, lightweight, and low-cost structural materials that have nevertheless good mechanical properties is an active area of materials research. A promising option is the use of low-density waste-particulate materials, such as fly ash (FA) and cenosphere (CS), as reinforcing agents in a polymer matrix.^1,2 These are byproducts of coal combustion in thermal power plants. Parallel efforts are under way to find new ways of characterizing the rather complex microstructures of such materials and aid optimization of their final properties. This is crucial for material design for specific applications such as packaging and construction. Our recent studies^3–5 show that the fractional free volume and average free-volume size of cavities in the amorphous domains of polymeric composites can serve as useful internal material parameters for characterizing the composites' mechanical properties.

Our study specifically focuses on finding a correlation between a composite's free-volume size and its mechanical properties. We prepared polymer composites with epoxy (a thermoset polymer) and polystyrene (PS, a thermoplastic polymer) as matrix phase, and FA, CS, and calcium aluminosilicate (CAS) as filler particles. We also prepared a second set of polymer composites that contained the same fillers, but with epoxy-resin matrix modified with amine-containing silicone. We characterized these composites based on three types of measurements. These included free-volume size and number density (determined using positron-annihilation lifetime spectroscopy: PALS,³ the most sensitive free-volume microprobe currently available), as well as mechanical properties, such as tensile strength and modulus. We also characterized the elongation at break—determined by universal testing machine—and the glass-transition temperature (T_g), using differential-scanning calorimetry (DSC).

DSC scans (see Figure 1) of the various composites showed that, in general, T_g increases with increasing filler load. The increase is significant for CS-filled composites. CS is a low-density, hollow filler and is, hence, dispersed evenly in the polymer matrix. This results in an appreciable increase in the composites' thermal stability. The T_g increase was also quite significant for a PS matrix containing CAS filler.

The chemical nature of the filler and its concentration in the matrix determines the composites' mechanical properties (see Figures 2, 3, and 4). Two general trends were evident. First, the low-cost fillers appeared to interact favorably with pure-epoxy, modified-epoxy, and PS matrices, in turn preventing deterioration of their mechanical properties. In fact, some properties were enhanced, particularly at higher loads. Second, the magnitude of improvement of mechanical properties depends on the silica content of the fillers: for higher silica concentrations the resulting properties were better.

A comparison between the free-volume size (V_f) and/or fractional free-volume content (F_v) of the composites at various loads and the corresponding mechanical properties suggests that, for better mechanical properties, V_f and F_v will be lower. Thus, the mechanical and free-volume properties are anticorrelated. In addition, this observation enables us to estimate the optimum level of filler loading required for maximum improvement of the composites' thermal and mechanical properties.

In summary, we have shown that evaluation of free-volume parameters using PALS represents a unique method for characterizing the microstructures of polymer composites, specifically of particulate fillers that influence the mechanical properties. This is useful for fabricating new polymer composites with certain desired end properties, which we will pursue next.