*3.1. Additive–Polymer Compatibility*

Additive–polymer compatibility and the effects of photoactive additive incorporation on Tecoflex film properties were investigated with a variety of methods. ATR-IR analysis was performed to investigate the effect of C60 incorporation on infrared signature and probe the chemical composition of the polymer film. ATR-IR analysis confirmed that the low loading concentrations employed in this study allowed the chemical signature of polymer film to be maintained despite increasing concentrations of C60 and EO3–C60.

Effects of additive incorporation on Tecoflex films on thermal stability were investigated with TGA. The incorporation of C60 and EO3–C60 into Tecoflex films each resulted in an increased thermal stability (Table 1). The incorporation of EO3–C60 in Tecoflex resulted in increased thermal stability (temperature at 10% loss) that scaled linearly with loading concentration. The increase in initial thermal stability suggests that stabilizing intermolecular interactions are occurring between EO3–C60 and Tecoflex polymer. Generally, the incorporation of C60 into Tecoflex demonstrated a moderate increase in thermal stability. A weak inverse relationship was observed with increased loading of C60 and thermal stability, as opposed to the direct relationship of EO3–C60 loading. This suggests that with increased loading concentration, additional C60 aggregates to other C60 instead of interacting with polymer matrix. Additionally, the lessened effect on thermal stability upon C60 incorporation compared to EO3–C60 indicates that EO3–C60 interacts more strongly than C60 with Tecoflex polymer matrix. The greater favorable interactions between EO3–C60 and Tecoflex results from compatibility between the ethylene oxide tail of EO3–C60 and the ether regions of the butane diol constituents of the Tecoflex monomer.


**Table 1.** Thermal properties of Tecoflex films.

\* At 600 °C.

TGA analysis also indicated that incorporation of C60 into Tecoflex resulted in greater ultimate mass remaining than those loaded with EO3–C60. The alkoxy moiety of the EO3–C60 is susceptible to thermal degradation at a greater temperature than the fullerene cage of C60. Therefore, each comparable loading concentration of C60 (720 g/mol) and EO3–C60 (881 g/mol) contains a 1.22-fold molar excess of C60 to EO3–C60. Thus, the thermal degradation of the alkoxy tail of EO3–C60 below 600 °C results in only 82% remaining of the total loaded EO3–C60. Considering this, at 600 °C, there should be approximately a 1.5× excess remaining mass % of C60 relative to EO3–C60 at comparable loading concentrations. Indeed, such an excess was observed upon comparison of the 1 wt% loadings.

Interestingly, the ultimate wt% for each Tecoflex film containing additives was greater than the additive wt% loading concentration (Table 1). The amount of mass remaining from the thermal degradation of Tecoflex films containing C60 at 600 °C corresponded to approximately double the C60 loading concentration. This indicates one of two possibilities: a quantity of Tecoflex polymer is strongly adhered to the surface of C60 fullerene through intermolecular forces that require greater than 600 °C to dissociate or aggregation of C60 results in thermally stabilized Tecoflex polymer trapped within the aggregate. However, the lower thermal degradation onset temperature of Tecoflex films containing C60 rather than EO3–C60 revealed that intermolecular interactions were stronger for EO3–C60. The difference between the final mass remaining and loading concentration decreased with increased additive loading concentration of both C60 and EO3–C60, suggesting that with increased loading concentration, additional intermolecular C60 aggregation occurs instead of C60–polymer matrix interactions. The deviation was greater for C60 than it was for EO3–C60, indicating that aggregation is more prominent in C60 than EO3–C60.

DSC was performed on all Tecoflex films to investigate effects of additive incorporation on glass transition temperature (*T*g). The way in which additive incorporation affects *T*g can afford insight into intermolecular interactions between the additive and polymer. Increase in *T*g resulting from additive incorporation indicates increased intermolecular interactions that limit polymer mobility [36]. Unmodified Tecoflex exhibits a glass transition that spans a broad temperature range. Incorporation of additives, both C60 and EO3–C60, result in minor and insignificant effects on *T*g, thus indicating that the integrity of the coating was preserved.

For both neat C60 and Tecoflex films containing higher concentrations of C60, an endothermic transition was observed in the DSC thermograms at approximately í14 °C. The magnitude of the endothermic peak increases corresponding to C60 concentration (Figure 3). This endotherm corresponds to the phase transition of C60 crystals from simple cubic orientation ordering to face-centered cubic structure upon heating through the transition temperature [37,38]. The presence of this transition from Tecoflex films indicates a crystalline phase of C60 fullerene. C60 can only be in a crystalline phase when multiple fullerene molecules are in contact with one another, or aggregated, in a regular, repeated order. A polymeric film in which C60 fullerene was completely dispersed would not exhibit such crystalline phase transition. Furthermore, endotherm corresponding to the simple cubic to face-centered cubic phase transition was absent in the DSC analysis of neat EO3–C60 and the Tecoflex films containing EO3–C60. Therefore, aggregation, or at least the formation of crystallites, of C60 is inhibited by the covalent modification of C60 with ethylene oxide tails. Furthermore, the amphiphilic character of the EO3–C60 improves solubility of the additive in the Tecoflex solution and facilitates increased molecular dispersion of the additive throughout the polymer matrix.

X-ray diffraction analysis (Figure 4) confirmed crystallinity observed via DSC. Diffraction peaks corresponding to the (220), (331), (222), and (420) peaks of crystallized C60 were observed in the Tecoflex films containing unmodified C60 additive. The intensity of diffraction peaks increased with increased loading of C60 from 0.5 to 5.0 wt%. In contrast, control Tecoflex and Tecoflex loaded with EO3–C60 displayed only a broad peak resulting from the amorphous polymer. Therefore, C60 aggregates into crystals in Tecoflex matrix, while EO3–C60 is well dispersed.

**Figure 3.** Comparison of DSC thermograms for Tecoflex films containing C60 and neat C60.

**Figure 4.** XRD patterns of Tecoflex films containing additives.

Water contact angle measurements were performed on Tecoflex films containing C60 and EO3–C60, the results of which are shown in Figure 5. Addition of C60 in Tecoflex resulted in minor increases in water contact angle in loadings up to 2.5 wt% and ultimately a minor decrease at 5 wt% C60 loading. Correspondingly, the surface roughness of Tecoflex films increased with increased C60 loading. On the other hand, loading of EO3–C60 in Tecoflex resulted in a significant decrease in water contact angle accompanied with a linear increase surface roughness to a greater degree than C60. Therefore, comparison of contact angle and surface roughness indicates that surface roughness plays an insignificant role in the water contact angle, despite previous evidence to the contrary [39]. Thus the affect that additive loading has on water contact angle must result from the additive's effect on the surface energy of Tecoflex, instead of imparted surface roughness.

**Figure 5.** Water contact angle (**a**) and surface roughness (**b**), Sq, of Tecoflex films loaded with C60 and EO3–C60.

The covalent attachment of ethylene glycol moiety to C60 results in a molecule with amphiphilic character, *i.e.*, a surfactant. When incorporated into a solution of hydrophobic Tecoflex, the amphiphilic EO3–C60 has the potential to orient its hydrophilic moiety at the polymer–air interface in order to minimize solvophobic interactions between hydrophilic ethylene oxide and hydrophobic Tecoflex matrix. Indeed, a linear decrease in contact angle was observed between 0.25 and 1.0 wt% loadings of EO3–C60. Considering that the highest loading (5%) of C60 in Tecoflex resulted in decrease in water contact angle of only 4°, the significant decrease in water contact angle (increase in hydrophilicity) of 10° at only 1 wt% loading of EO3–C60 indicates that the additive is concentrated at the surface of the polyurethane film.
