Organic compatible and surface reactive ester-functionalized cellulose nanofibrils (CNFs) have been facilely produced with reaction-dependent and easily tunable levels of surface functionalities. The optimal synthesis yielded 70.9% ester-functionalized CNFs in averaged 4.7 nm thickness, 29.3 nm width, and approximately 1 um length. The surface hydroxyl to ester conversion was determined by proton nuclear magnetic resonance (1H NMR) to be 0.62. With crystalline cores (CrI = 47.8%) and amphiphilic surfaces, these ester-functionalized CNFs have demonstrated to be excellent polyols or chain crosslinker in polyurethanes (PUs). The ester-functionalized CNFs served as polyols along with polytetramethylene ether glycol (PTMEG, Mn=2,900 Da) to be capped by methylene diphenyl diisocyanate (MDI) as pre-polymer synthesis, then extended with 1,4-butanediol (1,4-BD) to PU. With mere 0.3% DMF compatible ester-functionalized CNF in PU, significant increases in the elastic modulus from 2.61 to 8.25 MPa (3.2x), tensile strength from 5.4 to 21.1 MPa (3.9x), and strain-to-failure from 490 to 755% (1.5x). Most impressively, chain extending with 5.5% ester-functionalized CNF in 1,4- BD increased tensile modulus from 2.5 to 170.8 MPa (70x) and strength from 4.3-17.0 MPa (4.0x). These ester-functionalized CNFs exhibited complete reversibility from isotropic to oriented alignment upon uniaxial stretching to 300%, then returned to isotropic at 0 % strain, demonstrating strong effects of chemical bonding with MDI whereas unaffecting the soft domains. Furthermore, the outstanding strength enhancement in these PUs was complimented by reducing MDI from 31-15.5%.

One-pot synthesis of ester-functionalized cellulose nanofibrils for high modulus and strength polyurethanes

ACS Spring 2022

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