ARTICLE Evaluation of microstructure, thermal, and mechanical properties of the green lignin-based polyurethane/ hydrophobic silica nanocomposite foam Raziye Mohammadpour | Gity Mir Mohamad Sadeghi Department of Polymer Engineering and Color Technology, Amirkabir University of Technology, Tehran, Iran Correspondence Gity Mir Mohamad Sadeghi, Department of Polymer Engineering and Color Technology, Amirkabir University of Technology, Tehran, Iran. Email: gsadeghi@aut.ac.ir Abstract In the present study, lignin-based polyurethane foam (LPUF) and hydrophobic silica LPUF (SLPUF) were synthesized using different concentrations of silica nanoparticles (SNP). The effect of SNP on the structure and properties of SLPUF samples was investigated and compared with LPUF through the scan- ning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT- IR), thermogravimetric analysis (TGA), dynamic mechanical thermal analysis (DMTA), and compressive tests. The FT-IR results showed changes in the H bonding interactions between the structures of SLPUF samples. Moreover, the SEM results indicated a decrease in the cell size of SLPUF samples. Incorpora- tion of SNP improved the thermal stability of SLPUF samples while the com- pressive strength of SLPUF samples decreased in comparison with LPUF. Furthermore, the DMTA results revealed a decrease in the glass transition temperature from 90  C (LPUF) to around 52  C (SLPUF samples). This means that applying the hydrophobic SNP changes the foam type from a rigid foam to soft one. Therefore, significant changes were observed in the physical– chemical properties of the SLPUF samples compared to the LPUF. KEYWORDS biopolymers, mechanical properties, microstructure, nanocomposite, thermal properties 1 | INTRODUCTION Polyurethane (PU), as a diverse group of polymer is often defined as “the bridging gap between the rubber and plastic.” 1 This polymer can be produced by the poly- addition reaction of diisocyanate groups with active hydrogen of compounds such as a polyol to create a ure- thane linkage (–NH–CO–O–). 2 The resulting polymer may be considered as a copolymer consisting of a soft seg- ment with a glass transition usually below ambient tem- perature (macrodiol) and a hard segment with a glass transition above ambient temperature (diisocyanate- chain extender sequences). 2,3 Physical, mechanical, and chemical properties of the PU can be easily manipulated by changing its molecular arrangements in the “soft segment” and “hard segment.” 4 Therefore, this polymer is able to satisfy the requirements of various applica- tions such as fibers, foams, 5 coating, 6 absorbents, 7 biomaterials, 8 and so on. Polyurethane foam (PUF) is one of the main trade products of PUs. PUF can be classified into rigid, semi- rigid, and flexible foams depending on the mechanical performance, physical properties, and cell morphology. The foams can also be defined as either closed or open cell foams. In fact, the PUFs consist of two phases of solid and gas. In the open cell foams, the cells are unconfined Received: 22 December 2019 Revised: 18 July 2020 Accepted: 19 July 2020 DOI: 10.1002/app.49864 J Appl Polym Sci. 2020;e49864. wileyonlinelibrary.com/journal/app © 2020 Wiley Periodicals LLC 1 of 14 https://doi.org/10.1002/app.49864