Silica-silk fibroin hybrid (bio)aerogels: two-step versus one-step hybridization

Hajar Maleki,Nicola Huesing

doi:10.1007/s10971-019-04933-4

Hajar Maleki, Nicola Huesing

Open Access

https://doi.org/10.1007/s10971-019-04933-4

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Abstract

In this study, silk fibroin as a highly promising naturally occurring biopolymer extracted from silkworm cocoon is applied to mechanically reinforce silica aerogels. To this aim, two different approaches for the incorporation of silk fibroin into the silica network are compared: (1) a one-step acid catalyzed and (2) a two-step acid-base catalyzed sol–gel reaction. The total organosilane concentration, as well as the SF to silane mass fractions, regulated the hybridization process to proceed either through a one-step or two-step sol–gel reaction. In both processes, for an efficient chemical mixing the silk fibroin components with the silane phase, a silane coupling agent, 5-(trimethoxysilyl) pentanoic acid (TMSPA), comprising carboxylic acid groups and a pentyl hydrocarbon chain has been used. For a low organosilane content (3.4 mmol) along with a high SF to silane mass ratio (15–30%), the gelation of the silane and silk fibroin phases took place in a one-pot/one-step process in the presence of an acid catalyst in an entirely aqueous system. In the two-step synthesis approach, which was applied for high initial silane contents (17 mmol), and low SF to silane mass ratios (1–4%), first, the gelation of the silk fibroin phase was triggered by addition of an acid catalyst followed by a more pronounced condensation of the silane catalyzed by the addition of the base. Both synthesis approaches led to materials with promising mechanical properties—being 1) the one-step process resulting in gels with much better compressibility (up to 70% of strain), low density (0.17–0.22 g cm−3) and three orders of magnitude improvement in the Young’s modulus (13.5 MPa) compared to that of the pristine silica aerogel but with rather high shrinkage (30–40%). The two-step process in principle could result in the hybrid aerogel with interesting bulk density (0.17–0.28 g cm−3) with lower shrinkage (10%), but the resultant aerogel was stiff and fragile. Also, both approaches led to a significant reduction in the time required to prepare strong hybrid aerogels compared to conventional hybrid polymer-silica aerogels with the utilization of an entirely aqueous synthesis approach for a wide range of applications.

Highlights

Porous and lightweight silica-based aerogels are of interest for a multitude of high-performance applications, such as thermal insulation in the construction, building and aerospace sector, environmental cleaning, biomedical, and pharmaceutical applications to name only a few [1]
We present two synthesis approaches: a one-step procedure, in which hybridization of silk fibroin and the silica network is performed in a one-pot sol–gel reaction of silk fibroin with a small amount of the silanes, tetramethoxysilane and TMSPA, (3.4 mmol), and a high mass fraction of Silk fibroin (SF) to silane (SF: Si, 15:100 and 30:100) in the presence of an acid catalyst
This has been done by control of the gelation kinetics of SF by the acid catalyst concentration followed by the polycondensation and gelation of the hydrolyzed organosilane species in the presence of a base catalyst

Summary

Introduction

Porous and lightweight silica-based aerogels are of interest for a multitude of high-performance applications, such as thermal insulation in the construction, building and aerospace sector, environmental cleaning, biomedical, and pharmaceutical applications to name only a few [1]. Different approaches have been proposed to improve the mechanical strength of silica and silsesquioxane based aerogels, e.g., (a) by using ORganically MOdified SILanes, ORMOSILs, having nonhydrolyzable moieties in the network in order to replace the short siloxane (Si–O–Si) bonds with more flexible -Sicarbon chains, (b) by polymerization or crosslinking the silica network with flexible organic monomers/polymers, and (c) by using various fiber networks (i.e., carbon, ceramic and polymeric fibers) [2]. Amongst these approaches, the polymerization/cross-linking of the silica network skeleton with an appropriate organic monomer or cross-linker is counted as an elegant mechanical reinforcement method [3, 4]. In this approach tedious multi-step processes are required in which first hydrolysis and polycondensation of the silane phase is initiated, followed by polymerization/crosslinking of the organic monomer inside the silica and/or silsesquioxane network or the other way round [2]

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