Engineering the interfacial chemistry and mechanical properties of cellulose-reinforced epoxy composites using atomic layer deposition (ALD)

Automotive and aerospace industries require new lightweight materials that enhance payload and improve efficiency via vehicle weight reduction. Employing composites, such as fiber-reinforced polymers, is a common approach to reducing vehicle component weight. In this work, we examine the use of atomic layer deposition (ALD) to alter the interfacial chemistry in cellulose-reinforced epoxy composites. As produced, most cellulosics are hydrophilic and immiscible in industrially relevant hydrophobic polymers. In this study, a variety of ALD-derived surface modification schemes are explored to improve resin permeation within a fibrous, cellulose-based paper preform and to increase interfacial adhesion between the epoxy and the cellulose preform. Specifically, we consider surface modification of the cellulose paper with the ALD precursors trimethylaluminum (TMA) and titanium tetrachloride (TiCl 4 ) with a water oxidant to form aluminum oxide and titanium oxide-based surface chemistries. Few cycle ALD treatments (2-cycles) combined with an additional post-deposition heating step are found to make the cellulose preforms more hydrophobic. X-ray photoelectron spectroscopy verifies the presence of the metal oxide surface treatments and points towards high concentration of adsorbed adventitious carbon as the source for surface hydrophobicity. Tensile testing of laminated epoxy composites made from these cellulosic preforms indicates two mechanical property regimes depending on surface treatment: (1) high toughness and high strain for preforms that underwent only an ALD coating and (2) high modulus and high strength for preforms ALD coated and then heated at 120 °C in air. ALD treatments resulted in an 80% increase in toughness and a 47% increase in strain at break. ALD treatments with post-deposition heating resulted in a 16% increase in the elastic modulus and a 27% increase in the ultimate tensile strength. Here we propose a combination of cellulose/epoxy mechanical interlocking and interfacial adhesion as mechanisms to explain the difference in mechanical properties of the explored composites.