Photocrosslinkable Cellulose Derivatives for the Manufacturing of All-Cellulose-Based Architectures

Abstract

Replacing petroleum-based polymers with biopolymers such as polysaccharides is essential for protecting our environment by saving fossil resources. A research field that can benefit from the application of more sustainable and renewable materials is photochemistry. Therefore, cellulose-based photoresists that could be photocrosslinked via UV irradiation (λ = 254 nm and λ = 365 nm) were developed. These biogenic polymers enable the manufacturing of sustainable coatings, even with imprinted microstructures, and cellulose-based bulk materials. Thus, herein, cellulose was functionalized with organic compounds containing carbon double bonds to introduce photocrosslinkable side groups directly onto the cellulose backbone. Therefore, unsaturated anhydrides such as methacrylic acid anhydride and unsaturated and polyunsaturated carboxylic acids such as linoleic acid were utilized. Additionally, these cellulose derivatives were modified with acetate or tosylate groups to generate cellulose-based polymers, which are soluble in organic solvents, making them suitable for multiple processing methods, such as casting, printing and coating. The photocurable resist was basically composed of the UV-crosslinkable biopolymer, an appropriate solvent and, if necessary, a photoinitiator. Moreover, these bio-based photoresists were UV-crosslinkable in the liquid and solid states after the removal of the solvent. Further, the manufactured cellulose-based architectures, even the bulk structures, could be entirely regenerated into pure cellulose devices via a sodium methoxide treatment.

Keywords: UV curing; bio-based photoresist; biomaterial; cellulose derivative; cellulose regeneration; esterification; photocrosslinking.

Figure 1

Schematic representation of the synthesis pathways starting from cellulose as a raw material, leading to the reactants CDA and TSC. This employed unsaturated or polyunsaturated carboxylic acids or anhydrides, resulting in various photocrosslinkable cellulose derivatives: (A) 3-butenoic CDA, (B) linoleic CDA, (C) erucic CDA, (D) 10-undecenoic CDA, (E) oleic CDA, (F) sorbic CDA, (G) methacrylic CDA, (H) 4-pentenoic CDA, (I) methacrylic TSC, (J) 4-pentenoic TSC, (K) 3-butenoic TSC and (L) sorbic TSC.

Figure 2

FTIR spectra of MCC, TSC and MATSC. The spectra are shifted vertically for better readability.

Figure 3

FTIR spectra of CDA, erucic CDA and 4-pentenoic CDA. The spectra have been shifted vertically for better readability.

Figure 4

FTIR spectra of MACA, regenerated MACA and alpha cellulose. The spectra are shifted vertically for better readability.

Figure 5

Architectures manufactured with cellulose-based photoresists. (A) Casted film (generated with methacrylated CDA) with an imprinted diffraction pattern that produced brilliant coloration when irradiated with white light. (B) The SEM image of the pattern, which consists of regularly aligned pillars in the micrometer range. (C) The pattern structure still revealed brilliant coloration after the film was regenerated into cellulose with sodium methoxide treatment, indicating that the macrostructure of the cellulose-based device was not significantly affected by the reconversion process. (D) A BGP logo made from the cellulose derivative MACA, written by hand with a pipette. (E) Various shapes of trees manually printed using the derivative MACA (according to the synthesis described in [12], which caused the yellowish to brownish coloration of the biopolymer). Each tree consists of five individual layers that were UV-cured before the next layer was printed. (F) A butterfly structure printed with the biopolymer MACA. (G) Various bulky parts, a nut, a screw and a gear, fabricated using layer-by-layer casting of the derivative MACA [12] into a PDMS mold. Each layer was photocrosslinked for 15 min. (H) Same as (A), but produced with a methacrylated TSC derivative. (I) A butterfly structure generated from MACA and entirely converted into cellulose with sodium methoxide. (J–M) Several casted films with imprinted diffraction patterns, produced by the photocrosslinking of various cellulose derivatives such as (J) oleic CDA, (K) linoleic CDA, (L) 4-pentenoic CDA and (M) sorbic CDA.