## 4 Conclusions  
We have successfully used THz time-domain spectroscopy to characterize multi-block, co-polyester, graphene nanocomposite materials, synthesized using kilo-scale (suitable for Research and Development, and prototyping) in-situ polymerization and processed using industrially/economically viable compression molding. We present an experimental procedure and step-by-step data workup method used to characterize the FLG nanofiller within an insulating matrix material for the case where the filler is at relatively low density (1 wt%). We have shown that one can extract the refractive index, the dielectric constant, and the complex conductivity of the graphene-polymer nanocomposite material, as well as the effective dielectric constant of graphene nanoflakes within the matrix. In the THz-TDS method, the dielectric properties represent a sample average of all the materials in an almost cm\({}^{2}\) cross-section beam path; consequently, these data contain \"global\" information regarding the dispersion of the filler (in our case FLG) and its final retained \"quality.\" In our study, the Drude-Smith backscatter parameter, which is close to \(-1\), and the negative imaginary part of the complex conductivity both indicate that our nanofiller flakes are fully isolated in the polymer matrix. The quality of the dispersion is implied by the high value of the conductivity and moderate effective dielectric constant that is retained by the graphene nanoflakes. High THz conductivity typically corresponds to high carrier mobility within individual graphene flakes, which in turn suggests that flakes are structurally intact, without a significant amount of scattering defects introduced. The results of our THz study are consistent with the HRTEM image analysis, and indicate that THz-TDS characterization is uniquely suited to probe the dispersion and in-situ electronic quality of nanofilers such as graphene or, e.g., carbon-nanotubes in various, not only polymer, composite materials. In the latter context, however, it is important to stress that tested elastic PET-DLA 4060 copolymer nanocomposites are analogous to experimental hemispherical pneumatic membranes of a prototype extra-corporeal heart assist device (Figure 1). Consequently, this work, as well as other similar THz studies, serves as an important proof of concept of THz spectroscopy of nanocomposites in the product development chain. Non-destructive measurements could be carried out at different stages of the process: Aafter polymer nanocomposite synthesis, following processing into a given prototype component, and even after \"field\" testing (either mechanical or functional).  
Conceptualization, M.E.F. and R.S.; methodology, A.K., Z.S., P.S., and C.P.R.; investigation, A.K., G.C., K.M.G., Z.S., and A.L.; formal analysis, A.K., G.C. P.S., and C.P.R.; validation, A.N.V., M.E.F., A.C., and R.S.; visualization, A.K., G.C., K.M.G., and A.L.; resources, A.N.V., M.E.F., A.C., and R.S.; writing--original draft preparation, A.K., G.C., P.S., C.P.R., and R.S.; writing--review and editing, A.K., A.N.V., P.S., M.E.F., A.L., C.P.R., and R.S.; supervision, R.S.; project administration, A.N.V., M.E.F., A.C., and R.S.; funding acquisition, A.N.V., M.E.F., A.C., and R.S. Funding: This research was funded in part by the PumpPrimerII Program at the University of Rochester. A.N.V. acknowledges support from the Air Force Office of Scientific Research (FA9550-16-1-0020). M.E.F. acknowledges support from the Polish National Centre for Research and Development (PBS1/A5/2/2012).  
[PERSON] thanks [PERSON] from the West Pomeranian University of Technology for their helpful discussions.  
The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.