Abstract
Development of bio-based materials, especially from agricultural and forestry industrial byproduct streams, is urgent and necessary under the circumstances of sustainability and the environment. Industrial lignin is an underutilized biopolymer byproduct from both the pulping and cellulosic ethanol biorefinery industries with tremendous availability, showing potential as a substrate for producing biobased polyester materials because of its structure and abundance of hydroxyl functional groups.
In this study, lignin based copolymeric elastomers were synthesized by simple one pot two step polycondensation of methanol soluble (MS) fractions of industrial lignins and a series hyperbranched prepolymers (HBP) with different branching and core structures using various multifunctional monomers. Specifically, three industrial lignins (Indulin AT Kraft softwood (IN), Protobind 1000 (PB), and corn stover (CS)) were first subjected to methanol fractionation and then to a detailed chemical and thermal characterization. Fractionation and characterization of three industrial lignins were carried out to provide a hard (or netpoint) segment substrate for the copolymer synthesis. Correlations between chemical and thermal properties were observed in terms of condensation index, hydroxyl group, and molar mass versus the glass transition temperature (Tg).
A series of lignin-copolymers were prepared using the three different lignins and a HBP composed of triethanolamine (TEA, trifunctional) and adipic acid (AA, difunctional), A2B3) to evaluate the effects of lignin types and lignin contents. Tensile properties were dominated by HBP <45% lignin content while lignin dominated at 45% content. The copolymers Tg increased with lignin content, while lignin type did not play a significant role. Thermally-stimulated dual shape memory effects (Ts-SME) of the copolymers were obtained and quantified by cyclic thermomechanical tests. All copolymers had shape fixity rate >95% and shape recovery >90% for all copolymers. The copolymer shape memory transition temperature (Ttrans) increased with lignin content and Ttrans was 20 oC higher than Tg.
To obtain different HBP structure, a prepolymer was synthesized composing of long alkyl (C12) chain diacid (DDDA), TEA, and tris(hydroxymethyl)aminomethane (THAM, tetrafunctional), B3-A2-CB31). The C12 diacid contributed to a partially crystalline structure, while THAM contributed to more branching as well as forming stiff amide linkages in the prepolymer. Lignin-copolymers were synthesized with these prepolymers and PB lignin. Tensile properties were dominated by HBP <25% lignin content while lignin dominated >25% content. The Tgs of copolymers increased with lignin content. The lignin-copolymer with 30% lignin content demonstrated optimal mechanical properties (tensile strength 5.3 MPa, Young's modulus 8.9 MPa, strain at break 301%, and toughness 1.03 GPa). Good Ts-SME was obtained and could be tailored by lignin content and activation temperatures ranged between ambient and body temperature.
Finally, in order to pursue higher biobased content in the lignin-copolymers, HBP were prepared from AA, Glycerol (Gly, trifunctional), and enhanced by additions of diisopropanolamine (DIPA, trifunctional), or THAM, B2B32-A2, B2B32-DB42- A2, and B2B32-CB13- A2) to form branched to hyper branched structures. The higher branching crosslinkers, DIPA and THAM, were shown to influence the chemical and thermal properties of the prepolymers. The highly biobased lignin-copolymers demonstrated good shape memory and elastic properties. Ttrans could be tuned by variations of Gly, DIPA and THAM proportions for applications under different temperature circumstances.
Different branching and core structures of prepolymers (soft segment) were shown to influence the properties of prepolymers and corresponding lignin-copolymers. All the lignin-copolymers were shown to be elastomeric and possess great Ts-SME. The results demonstrated that properties of lignin-copolymers could be tuned through lignin type, lignin content, prepolymer structure, and monomer variations. This study demonstrated that lignin, a renewable byproduct, can be promisingly valorized to apply as a netpoint segment in biobased polymer systems with SME behavior.