Highly biobased thermally-stimulated shape memory copolymeric elastomers derived from lignin and glycerol-adipic acid based hyperbranched prepolymer
Graphical abstract
Introduction
Lignin, an underutilized biopolymeric byproduct from both the pulping (about 5 × 1010 kg produced in 2004) and cellulosic ethanol industries, shows potential as a substrate for producing bioplastic materials because being abundant in supply and hydroxyl functional groups present for substitution (Zakzeski et al., 2010). Lignin’s utilization is impaired by its high glass transition temperature (Tg) and brittleness due to its rigid aromatic backbone, high intra/inter hydrogen bonding, and heterogeneity (Yoshida et al., 1987). Applications for lignin has been to incorporate it as a filler (Mousavioun et al., 2013), extender or as a reinforcing pigment in rubber (Wang et al., 1992) and in thermoset resins (Feldman, 2002). Another alternative to endow thermoplasticity properties to lignin has been conducted by various chemical modifications, such as alkylation (Li and Sarkanen, 2002), benzylation (McDonald and Ma, 2012), and oxypropylation (Cui et al., 2013, Li and Ragauskas, 2012, Sadeghifar et al., 2012, Wu and Glasser, 1984), which inhibits hydrogen bonding and thus lower’s its Tg and more processable. More recent work has been using lignin as a hard segment in the preparation of highly and hyper branched smart elastomeric copolymers with tunable shape memory properties (Li et al., 2015, Sivasankarapillai and McDonald, 2011, Sivasankarapillai et al., 2012). The properties were easily tailored by changing the prepolymer branching structure and lignin content.
Highly branched, or hyperbranched, polymers (HyBP) are attractive types of dendritic polymers due to their high branching structures, less tedious preparations, as well as affordable up-scale pathways (Gao and Yan, 2004, Hult et al., 1999, Muscat and van Benthem, 2001, Sivasankarapillai et al., 2012, Voit and Lederer, 2009). The term “hyperbranched polymers” was first introduced in 1988 to describe the structure of an imperfect dendrimer by preparation of polyphenylenes with ABx type monomers (Kim and Webster, 1988). HyBP based on polyesters were then become increasingly attractive due to incorporating commercial availability of monomers with more diverse functionalities (e.g., symmetric diacid (A2) and trihydroxy (B3)) and ease of preparation by standard melt polycondensation methodology (Muscat and van Benthem, 2001). Combinations of various branched structures were involved such as A2, B3 and trihydroxy-monoamino (CB31) monomer units to increase the structural diversity from branched to HyBP (Sivasankarapillai et al., 2012). The key of this approach is the employment of multifunctional monomers with suitable unequal reactivity, where both B and C can react with an A group, as a result, hyperbranched poly(ester-amide)s with amine cores and amide linkages can be formed (Li et al., 2004, Li et al., 2005, Li et al., 2006, Sivasankarapillai et al., 2012). Furthermore, a series of lignin HyBP-copolymers have been produced by melt condensation with lignin using the monomers: adipic acid (AA, A2), triethanolamine (TEA, B3) and tris(hydroxymethyl) aminomethane (THAM, CB31). The advantage of this system was that TEA acted as the reaction solvent and catalyst, and THAM introduced the amide linkage and increased the complexity of the structure. In addition, these lignin HyBP copolymers showed interesting thermal properties and elasticity (Sivasankarapillai and McDonald, 2011, Sivasankarapillai et al., 2012).
The global biodiesel production by 2016 is estimated as 1.4 × 1011 L according to the report, which leads to approximately 1.5 × 1010 L of crude glycerol (Gly) available as a byproduct for utilization into biobased products (Anand and Saxena, 2012, Yang et al., 2012). Gly with trihydroxy functionality, consequently, makes it a sustainable alternative crosslinker for synthesis of HyBPs. Other benefits of Gly are that it can act as a solvent and has a high boiling point. Gly based HyBPs have been synthesized as early as 1929 (Kienle and Hovey, 1929). Stumbe and Bruchamann first synthesized HBP with Gly and AA using a tin catalyst at 150 °C (Stumbe and Bruchmann, 2004). An alternative to tin based catalysts, Kulshrestha et al. (2005) produced glycerol copolyesters using a lipase catalyst to control branching and molar mass. Wyatt et al. 2006 synthesized a series of Gly based highly branched oligomers for use in coatings (Wyatt et al., 2006a, Wyatt et al., 2011. More complex polymers based on poly(oleic diacid-co-glycerol) have been synthesized by Yang et al. (2011) In polyurethanes, Gly is also utilized as a polyol cross-linker (Chun et al., 2007).
AA is a high volume (2.6 × 109 kg) commodity chemical traditionally produced from petrochemicals, primarily used in nylon 66, is now being produced via several bio-based routes (De Guzman, 2010). One commercial approach by Verdezyne has been to convert vegetable or coconut oil to AA (Beardslee and Picataggio, 2012). An alternate process by Rennova has been to oxidize glucose to gluconic acid, which is then hydrogenated to AA. These new technologies clearly show a need for biobased building blocks.
Therefore, the aim of this study was to develop a new series of hyper branched prepolymers based on bio-derived Gly and AA by a simple approach with addition of different crosslinkers such as diisopropanolamine (DIPA, DB42) or THAM (CB31) to evaluate their effect on properties. Corresponding high biobased-content lignin copolymers were synthesized based on these prepolymers. The effect of prepolymer composition on thermal, mechanical, and shape memory effect (SME) properties of the prepolymers and lignin copolymers were investigated.
Section snippets
Materials
Protobind 1000 soda lignin was supplied by ALM India Pvt., Ltd. The methanol soluble fraction of the soda lignin was used for copolymer synthesis because of its low Tg and good dispersability in the prepolymer (lignin content = 93.3%, Mw = 958 g/mol (ESI-MS), aromatic/aliphatic hydroxyl group ratio = 0.93, Tg = 132 °C (DSC)) (Li and McDonald, 2014, Li et al., 2015). Glycerol (Gly), tris(hydroxymethyl) aminomethane (THAM), diisopropanolamine (DIPA), adipic acid (AA), methanol and tetrahydrofuran (THF) were
Monomer selections
With the purpose of pursuing highly biobased polymers and value-added products from biobased starting materials (lignin, Gly, and AA), lignin copolymeric elastomers were synthesized containing as high as 100% bio-derived materials (Table 2). In detail, HyBP prepolyesters (G1 and G2) were first prepared by incorporating Gly to react with AA, and then lignin copolymers (LG1 and LG2) were synthesized. The advantage or starting point herein was that Gly possessing trihydroxy functionalities, is a
Conclusions
A series highly to hyper branched Gly based prepoly(ester-amide) s with varying branching structures (i.e., B2B32–A2, B2B32–DB42–A2, and B2B32–CB13–A2) were successfully synthesized via melt polycondensation reactions. Lignin-copolymers were subsequently synthesized via a bulk polycondensation reaction to afford materials with Ts-SME dual shape memory behavior with as high as 100% biobased materials. The lignin-copolymer properties (tensile, thermal, and thermomechanical) were able to be tuned
Acknowledgements
We would like to acknowledge (i) the financial support from a USDA-NIFA Wood Utilization Research grant number 2010-34158-20938, (ii) Dr. Alexander Blumenfeld for his technical help with NMR, (iii) USDA-CSREES grant 2007-34158-17640 for supporting the DSC/DMA, (iv) Thermoscientific for the LCQ-Deca mass spectrometer, and (v) Liqing Wei for her technical help with rheological measurements.
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