Thermal insulation behavior of functionally graded aerogel: The role of novolac molecular-weight

• Published in: Polymer (Guilford) 2019-09, Vol.178, p.121575, Article 121575
• Main Authors: Noroozi, Mina, Panahi-Sarmad, Mahyar, Bahramian, Ahmad Reza
• Format: Article
• Language: English
• Subjects: Aerogel; Aerogels; Crosslinking; Functionally graded structure; Functionally gradient materials; Interfaces; Microstructure; Molecular weight; Molecular weight & structure evaluation; Phenols; Polymerization; Pore size; Porosity; Sol-gel processes; Solvents; Thermal conductivity; Thermal insulation; Thermal insulator
• ISSN: 0032-3861; 1873-2291
• DOI: 10.1016/j.polymer.2019.121575

In this research, a novel aerogel with a functionally-graded (FG) structure as a thermal insulator has been fabricated without interfaces through sol-gel polymerization. The effects of molecular-weight (Mw) and initial-solvent concentration as decisive parameters in the microstructure of phenol-formaldehyde aerogels are systematically studied. The results show that a broad spectrum of Mw created via the precise control of the cross-linking (CL) process of the resin. Moreover, the sol-size have been remarkably augmented when increasing the Mw. The porousness structure assessment illustrated that the porosity decreases with enhancing the amount of solvent, but increasing Mw has no significant effect on the porosity of specimens. In FG-aerogel microstructure, more homogeneous distribution in the sol-size and significant decrement in pore size is observed while the Mw and solution concentration are simultaneously increased throughout the structure. Therefore, FG-aerogel structure contains a wide range of porosity, thermal conductivity (0.029–0.042 (W⁄(m.K))) and pore size (200–2000 nm) that leads to unique structural features of aerogels. [Display omitted] •Developing a novel aerogel with functionally graded (FG) structure and also without any interface at the homogenous structure.•Excellent agreement between the cross-link density (as an index parameter for predicting the Mw) and average aggregate sizes.•Providing a correlation between the Novolac Mw and porous structure throughout the FG aerogel.•Decoding the effect of Mw of initial solvable sol and solvent concentration of the sol-gel solution on thermal insulation performance of aerogel structure.•Unique thermal insulation behavior through molecular-weight-concentration functionally graded (MC FG) aerogel was observed.