Well-defined metal-polymer nanocomposites: The interplay of structure, thermoplasmonics, and elastic mechanical properties

Metal-polymer nanocomposites are hybrid materials combining the superior plasmonic, electrical, and thermal properties of metals with the good elasticity and manufacturability of polymers. This renders metal-polymer nanocomposites promising candidates for conductive filler and coating applications, where mechanical properties are optothermally coupled. Here, we study the interplay of nanostructure, thermoplasmonics, and elastic mechanical properties of silver-polystyrene nanocomposites (AgPS) by transmission electron microscopy, small-angle x-ray scattering, Brillouin light scattering (BLS), and other supplemental techniques. We utilize the well-known particle-brush architecture to ensure a homogeneous and isotropic nanoparticle distribution throughout the hybrid material. The effective longitudinal modulus of the as-prepared samples is found to decrease from 5.7 to 4.8 GPa with increasing Ag content from 0 to 4.4 vol.%. Temperature-dependent BLS measurements reveal the unique contribution of local thermoplasmonic heating that depends on the Ag nanoparticle composition. This thermoplasmonic effect results in a lower apparent glass transition temperature $\left(T_{\mathrm{g}}\right)$ and a stronger laser power dependence of the speed of sound. Exceeding moderate thermal annealing temperatures $\left(>150{}^{\circ }\mathrm{C}\right)$ leads to a strong structural rearrangement within the homogeneous nanocomposite material with a peculiar clustering-redispersion effect, which also translates into altered mechanical properties. The annealing-induced Ag nanoparticle aggregation results in an even stronger thermoplasmonic effect. We validate our experimental findings with complementary thermographic measurements and finite-element modeling. Overall, this work demonstrates the combined effects of composition and (reversible) aggregation on the mechanical and thermoplasmonic properties of metal-polymer nanocomposites. It not only deepens our understanding of the interaction between light, temperature, and mechanical properties in metal-polymer nanocomposites but also provides a guide for customizing AgPS nanocomposites for potential applications.

Figure 1

(a), (b) TEM images of the AgPS8k and AgPS18k samples with silver volume fractions, $\phi =0.05$ and 0.014, respectively. (c) SEM image (side view) of an AgPS18k film, where the bright spots indicate the well-dispersed Ag nanoparticles over the whole thickness of the film. (d) The polymer height determined from the TEM images is plotted as a function of the degree of polymerization, which shows a linear relation in the double-logarithmic plot. (e) SAXS patterns of the AgPS8k and AgPS18k films (symbols) and theoretical descriptions (lines; for details, see section S3 in Supplemental Material). (f) UV-Vis spectra of the AgPS8k and AgPS18k samples measured in solid films (solid lines) and toluene (dashed lines).

Figure 2

(a) Linear acoustic dispersion of the longitudinal phonon in the as-prepared AgPS18k and AgPS12k nanocomposite films (red circles and blue triangles), and bulk PS18k (black squares) recorded by BLS. Inset: BLS spectra of a 1.4-μm-thick AgPS18k film, recorded at three different wave numbers using a 532-nm laser at 45 mW, and represented by Lorentzian line shapes (solid red lines). (b) Effective longitudinal sound velocity $c_{\mathrm{L}\text{,}\mathrm{eff}}$ vs Ag volume fraction. Wood's law representation (red line) was conducted using Eq. (2) in section S4 in Ref. [29] without any adjustable parameter. The densities and sound velocities assume the values of bulk Ag and PS. (c) Computed effective longitudinal modulus of the AgPS nanocomposites in (b) as a function of the Ag volume fraction.

Figure 3

Longitudinal sound velocities in the as-prepared AgPS18k and AgPS8k nanocomposite films and the bulk PS18k film as a function of temperature. The shaded areas indicate the glass transition temperatures, and the four vertical lines denote the glass transition temperatures obtained from the DSC traces of bulk AgPS18k (solid red), AgPS8k (solid blue), PS18k (dashed red), and PS8k (dashed blue) films. Inset: Anti-Stokes BLS spectra of AgPS18k at three temperatures as indicated in the plot. The BLS spectrum in the lower panel was recorded at the end of the temperature scan after cooling the sample from 363 K back to 294 K. For the AgPS8k and AgPS18k films, the BLS spectra were recorded with laser powers of 45 and 60 mW, respectively.

Figure 4

Effective longitudinal sound velocity in the as-prepared AgPS18k (red circles) and AgPS8k (blue triangles) nanocomposite and PS18k (black squares) films as a function of the laser power at 532 nm. The red and blue solid lines denote a parabolic representation of the experimental sound velocities in AgPS18k and AgPS8k, respectively, whereas the solid black line indicates the power-independent sound velocity in bulk PS18k. Inset: Anti-Stokes BLS spectra at $q=0.017\mathrm{n}{\mathrm{m}}^{-1}$ at laser powers of 45 and 100 mW represented by Lorentzian line shapes.

Figure 5

The local temperature in the laser spot as a function of the laser power in the AgPS18k (red circles) and AgPS8k (blue triangles) films estimated from (a) the data of Fig. 3 and Fig. 4 as described in the text, and (b) using an IR camera. The solid lines indicate the simulation results (Supplemental Material, section S6, Fig. S8 [29]), whereas the horizontal dashed lines indicated the local $T_{\mathrm{g}}$ induced by the laser heating at ambient temperature.

Figure 6

(a)–(c) TEM images of the AgPS18k films (a) as prepared from spin coating, (b) after 1 min annealing at 200 °C, and (c) after 30 min annealing at 200 °C. (d) UV-Vis and (e) SAXS patterns (symbols) and fits (lines; details in Supplemental Material, section S3 [29]) of the as-prepared and annealed AgPS18k films.

Figure 7

Effective medium longitudinal sound velocity in annealed AgPS18k nanocomposite films as a function of the laser power, in comparison with the results for the as-prepared AgPS18k film (black squares). The data for the short and long annealing times are indicated by red circles (1 min) and blue triangles (30 min). The half-filled symbols represent the presence of a secondary softer phonon in the BLS spectra. Inset: Anti-Stokes BLS spectra at $q=0.0167\mathrm{n}{\mathrm{m}}^{-1}$ for the AgPS18k films annealed for 1 and 30 min. The spectra at 45 mW (black) and 100 mW (red) are represented by one (black) and two (green) Lorentzian peaks, respectively.