This is the first time to report a facile strategy to fabricate galactoglucomannan-based latex with highly transparent, hydrophobic and flexible characteristics by combining etherification with subsequent emulsion polymerization.
Carbohydrate Polymers 291 (2022) 119565 Contents lists available at ScienceDirect Carbohydrate Polymers journal homepage: www.elsevier.com/locate/carbpol Synthesis of galactoglucomannan-based latex via emulsion polymerization Qiwen Yong a, b, *, Jiayun Xu b, c, Luyao Wang b, Teija Tirri b, Hejun Gao a, Yunwen Liao a, Martti Toivakka b, **, Chunlin Xu b, ** a Institute of Applied Chemistry, Chemical Synthesis and Pollution Control Key Laboratory of Sichuan Province, College of Chemistry and Chemical Engineering, China West Normal University, Nanchong 637009, China b Laboratory of Natural Materials Technology, Åbo Akademi University, Turku 20500, Finland c Tianjin Key Laboratory of Pulp and Paper, Tianjin University of Science and Technology, Tianjin 300457, China A R T I C L E I N F O A B S T R A C T Keywords: Hemicellulose Galactoglucomannan Bio-based latex Packaging application Etherification Semi-continuous emulsion polymerization This is the first time to report a facile strategy to fabricate galactoglucomannan-based latex with highly trans parent, hydrophobic and flexible characteristics by combining etherification with subsequent emulsion poly merization The allylated galactoglucomannans (A-GGM) and galactoglucomannan-based latexes (GGM-L) were prepared and their chemical structure, substitution degree, molecular weight, conversion rate, particle size and zeta potential were characterized by ATR-FTIR, 1HNMR, quantitative 13CNMR, HP-SEC, HPLC and zeta-sizer nanometer analyzer, respectively Furthermore, the effects of substitution degree on film surface roughness and homogeneity, water vapor permeability (WVP) and thermal stability were evaluated by AFM, SEM, WVP and TGA, respectively The optimal GGM-L film exhibited 91.3% transmittance and 0.43% haze, 117◦ water contact angle, 31.2% elongation at break and 30.9 MPa ultimate tensile stress The bio-based content of the GGM-L may reach about 99 wt%, which provides a promising avenue for polyolefin-based latex replacement for paper and paperboard applications Introduction Synthetic polymers or plastics are widely used for packaging, coating, paints, and other applications in our daily life, and are accu mulated in the landfills, oceans, waterways, and other natural envi ronment, being a severe burden to the nature The replacement of fossilbased resources by renewable and sustainable biomass is the basic idea behind the “Bio-economy”, which is gaining popularity in a wide range of industries at present (Lehtonen et al., 2016; Mankar, Pandey, Modak, & Pant, 2021) Agricultural- and forest-based biomass, such as cellulose, lignin and hemicelluloses, have attained widespread concern owing to their abundant availability, bioactivity, biocompatibility and biode gradability Moreover, they not compete with food supply in the production of biopolymers (Chio, Sain, & Qin, 2019; Schatz & Lecom mandoux, 2010) Hemicellulose (HC), the most abundant plant polysaccharide next to cellulose, accounts for 20–35% of the total wood mass, yet scarcely being exploited due to lack of economically feasible applications Use of hemicellulose-based films and coatings as green packaging materials to replace petroleum-based synthetic plastics can contribute to sustainable development In the soft wood, the main hemicellulose type is O-acetylgalactoglucomannan (GGM), which consists of a main backbone of randomly distributed (1 → 4)-linked β-D-mannopyranosyl and (1 → 4)linked β-D-glucopyranosyl units, while (1 → 6)-linked α-D-galactopyr anosyl units are attached to mannose units and mannose units are partially acetylated at C2 and C3 positions (Mikkonen, 2020; Zhao, Mikkonen, Kilpelainen, & Lehtonen, 2020) GGM has high water solu bility and is film-forming to some degree (Battista, Zuliani, Rizzioli, Fusco, & Bolzonella, 2021) The low oxygen permeability of GGM films make them promising candidates for substitution of traditional pack aging barrier materials, such as aluminum foil, PE, PP, PVDC or EVOH (Nechita & Roman, 2020) However, the pristine GGM films have some inherent drawbacks due to GGM's high number of hydroxyl groups, strong intermolecular hydrogen bonds and low molecular weights These result in, for instance, brittleness, low flexibility, high hydrophi licity, water/moisture sensitivity, and poor mechanical and thermal * Correspondence to: Q Yong, Institute of Applied Chemistry, Chemical Synthesis and Pollution Control Key Laboratory of Sichuan Province, College of Chemistry and Chemical Engineering, China West Normal University, Nanchong 637009, China ** Corresponding authors E-mail addresses: yongqiwen13@mails.ucas.ac.cn (Q Yong), Martti.Toivakka@abo.fi (M Toivakka), Chunlin.Xu@abo.fi (C Xu) https://doi.org/10.1016/j.carbpol.2022.119565 Received 17 January 2022; Received in revised form 27 April 2022; Accepted 29 April 2022 Available online May 2022 0144-8617/© 2022 The Authors Published by Elsevier Ltd This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) Q Yong et al Carbohydrate Polymers 291 (2022) 119565 properties (Khwaldia, Arab-Tehrany, & Desobry, 2010) Physical and chemical modifications of GGM by either surface or bulk modification are considered as effective ways to overcome the above-mentioned short-comings (Peng, Du, & Zhong, 2019) The phys ical blending of the hemicellulose with plasticizers including glycerol, sorbitol xylitol, or emulsifiers like sucrose ester, palmitic acid, or poly vinyl alcohol (PVOH) can greatly enhance the film-formation, reduce the brittleness and provide flexibility for GGM-based composite films However, the composite films are hydrophilic and sensitive to water and ănen, moisture, as reported previously (Mikkonen, Heikkilă a, Helen, Hyvo & Tenkanen, 2010) Various chemical modifications including oxida tion, reduction, etherification, esterification, ionization, amination, fluorination, acetylation, graft polymerization and crosslinking have been developed to improve the hydrophobicity, tensile property, ther mal stability and film-formation of GGM bio-based film materials (Kisonen et al., 2014; Liu et al., 2019; Markstedt, Xu, Liu, Xu, & Gate nholm, 2017; Yi, Xu, Wang, Huang, & Wang, 2020; Zoldners & Kiseleva, 2013) Nevertheless, the film flexibility/stretchability of modified GGMbased films are still hardly comparable to those of petroleum-derived synthetic polymer materials For example, Anette Larsson et al (Har delin, Bernin, Borjesson, Strom, & Larsson, 2020) recently reported etherified GGM films with high hydrophobicity but limited flexibility with tensile strain at break less than 9% Frederic Becquart et al (Farhat et al., 2018) introduced a long hydrophobic polycaprolactone (PCL) side chains to hemicellulose by ring-opening graft polymerization, providing a significant thermoplastic and hydrophobic alteration It not only increased the molecular weight of hemicellulose-based film, but it also broke hydrogen bonds between the molecular chains of hemicellulose, thereby improving film-forming ability and stretchability of hemicellulose-based product However, this graft polymerization was carried out in organic solvent, which is undesirable from the green chemistry point of view Therefore, developing sustainable hemicellulose-based packaging film materials with good transparency, high hydrophobicity and tensile property, excellent barrier property and thermal stability is still a considerable challenge Emulsion polymerization is an effective way to achieve free radical polymerization of water-insoluble organic monomers and water-soluble substances in an aqueous medium system with the help of emulsifiers In light of our previous research results (Kisonen et al., 2014; Yong & Liang, 2019), we anticipate that designing GGM-based biopolymer grafted by long-chain hydrophobic organic monomers may achieve both hydrophobicity and flexibility, due to the introduction of hydrophobic macromolecular side-chains and the increase of molecular weight of GGM-based latex In this paper, we first used allyl glycidyl ether (AGE) to etherify the native GGM between epoxy groups of AGE and hydroxyl groups of GGM, as ether bond is more stable than the ester and amide bonds, of which the latter are more easily hydrolyzed under acid/alkali conditions (Laine et al., 2013) Then a semi-continuous emulsion poly merization method was used to copolymerize between allylated GGM and hydrophobic acrylate monomers n-butyl acrylate (n-BA) under the assistance of emulsifier sodium dodecyl sulfate (SDS) and initiator ammonium persulfate (APS) It should be noted that n-butyl acrylate can be obtained from biomass resources based on the bio-based n-butanol fermented from glucose and the bio-based acrylic acid converted from ăkkilă lactic acid (Garcớa, Pa a, Ojamo, Muurinen, & Keiski, 2011; Niesbach, Fink, Lutze, & G´ orak, 2015; Yang et al., 2021) The bio-based content can reach up to 99% by weight, calculated from the solid matter of the latex, when the n-BA is obtained from biobased raw-materials In gen eral, a series of sustainable GGM-based latexes with different substitu tion degrees were prepared successfully The films exhibited good transparency, high hydrophobicity and flexibility, excellent barrier property and thermal stability, indicating that there is a promising substitution of conventional petroleum-based synthetic polymer mate rials in paper and paperboard coatings Experimental procedure 2.1 Materials Pressurized hot-water extracted galactoglucomannan from Norway spruce was obtained as a solution with concentration of 10 wt% from the Finnish Forest Research Institute Metla Allyl glycidyl ether (98%), so dium hydroxide (NaOH, AR), sodium dodecyl sulfate (AR), ammonium persulfate (95%), and n-Butyl acrylate (AR) were provided by SigmaAldrich Hydrochloric acid (HCl), ethanol (98%), acetone (99%) and methyl tert-butyl ether (MTBE, AR) were purchased from Altia Industrial 2.2 Methods 2.2.1 GGM extraction and characterization The native GGM dispersion was precipitated by industrial grade ethanol at a 2:8 (v/v) water-ethanol ratio Glass fiber filter was applied for vacuum filtration Then the filter cake was redissolved into water, and the ethanol precipitation and filtration were repeated After the third filtration, the filter cake was washed twice consecutively by flushing with pure ethanol, acetone and MTBE The purified GGM filtrate was dried overnight at ambient temperature, and further dried in a vacuum desiccator at 40 ◦ C for 48 h to remove the residual MTBE The purified GGM had a number average molecular weight (Mn) of 8.31 × 103 g/mol and a weight average molecular weight (Mw) of 19.80 × 103 g/mol (polydispersity ~2.38), as measured by a high-performance size exclusion chromatography (HPSEC) The sugar unit ratio of GGM was analyzed using a gas chromatography (GC), and it was at 0.67:1:3.26 (Galactose: Glucose: Mannose) The composition of GGM characterized by 2D HSQC-NMR was shown in Fig S6 and 13C–1H correlation signals were listed in Table S5 2.2.2 Synthesis of allylated GGM dispersions The GGM was modified by an etherification reaction with a bifunc tional monomer or crosslinker AGE at three molar ratios (see Table S1) between moles of anhyrohexose unit of GGM and moles of AGE used (i e., 1:0.25, 1:0.5, 1:1, 1:2 and 1:3, for the five reactions) (Hardelin et al., 2020; Laine et al., 2013) Specifically, 10 g of vacuum-oven dried GGM was mixed with 200 mL of deionized water and 2% of NaOH (based on the total weight of the reaction system) was added into a three-necked flask The reaction system was equipped with a condenser and pro tected by a flowing N2 atmosphere, and was also stirred by a magnetic stirrer at 600 rpm The temperature raised to 60 ◦ C and the AGE was added dropwise by a dropping funnel The 10 h reaction time commenced once all the AGE was added into the reaction flask After – C double bonds was cooled and wards, the modified GGM containing C– neutralized with HCl Finally, the allylated GGM (A-GGM) were further dialyzed with membranes with a molecular weight cut-off of 12– 14 × 103 g/mol for at least 72 h H NMR (500 MHz, DMSO‑d6): δ (ppm) = 2.0 (H-1), 4.0 (H-4), 4.5 (H-3, α), 5.1, 5.3 (H-6), 5.3 (H-3, β), 5.9 (H-5); 13 C NMR (125 MHz, DMSO‑d6): δ (ppm) = 21.5 (C-1′ ), 101.2 (C-3′ ), 116.0 (C-6′ ), 135.2 (C-5′ ), 170.1 (C-2′ ) 2.2.3 Synthesis of GGM-based latexes GGM-based latexes (GGM-L) were synthesized by semi-continuous emulsion polymerization technique The detailed recipes are in Table S2 In brief, g of above allylated GGM was concentrated into a volume of 100 mL by a rotary evaporator and then was transferred into a 250 mL three-necked round-bottom flask equipped with a condenser, a peristaltic pump with injection needle and a N2 circulation device 1% of emulsifier SDS (based on the weight of n-BA) was added to the flask and dissolved thoroughly The reaction system was heated to 80 ◦ C and 0.75% of APS (based on the weight of n-BA) was added to the flask Then, g of n-BA was added dropwise into the reaction system by the Q Yong et al Carbohydrate Polymers 291 (2022) 119565 Fig (a) Molecular structural formula of GGM and its etherification principle; (b) 1H NMR and (c) quantitative specimens; (d) ATR-FTIR and (e) partial enlarged ATR-FTIR spectra of native GGM and A-GGM specimens 13 C NMR spectra of native GGM and A-GGM Q Yong et al Carbohydrate Polymers 291 (2022) 119565 peristaltic pump during 30 The reaction temperature was kept at 80 ◦ C for h with the stirring speed of 600 rpm Finally, another 0.25% of APS initiator was added, and the reaction continued for h to ensure that the remaining monomers had completely reacted The prepared GGM-based latexes were stored in a cold room (4 ◦ C) for further use The solid content of the GGM-L latexes was about 12% and the apparent viscosity is reported in Fig S1 H NMR (500 MHz, DMSO‑d6): δ (ppm) = 0.9 (H-1), 1.3 (H-2), 1.5 (H-3), 2.4 (H-6), 5.1 (H-7), 5.3 (H-7), 5.9 (H-8) 13 C NMR (125 MHz, DMSO‑d6): δ (ppm) = 14.4 (C-1′ ), 18.9 (C-2′ ), 28.0 (C-3′ ), 35.7 (C-6′ ), 65.9 (C-4′ ), 116.0 (C-8′ ), 135.2 (C-7′ ), 174.8 (C5′ ) Table Allylation degrees and molecular weights of GGM and A-GGM Mw[×103 g/mol]b Mn[×103 g/mol]b PDIb GGM A1-GGM A2-GGM A3-GGM A4-GGM A5-GGM – 0.04 0.13 0.30 0.51 0.83 19.80 22.04 23.53 24.16 30.36 32.58 8.31 7.54 7.86 6.68 8.17 7.90 2.38 2.92 2.99 3.60 3.72 4.12 b Represents the DSal determined by quantitative 13CNMR Measured by HPSEC with DMSO/LiBr as solvent and eluent Theoretically, the full substitution degree of GGM is 3, which in dicates that all three hydroxyl groups of each anhydrosugar units have been grafted with AGE molecules Based on the quantitative 13C NMR spectra, we can calculate the DSal With the molar ratio of GGM:AGE increasing from 1:0.25 to 1:3, the DSal of GGM increased from 0.04 to 0.83 Compared to the DSal values of 1.1, 1.9, and 2.5 reported in (Hardelin et al., 2020), our substitution degree was lower This is due to the low molar ratio of AGE:GGM and the low amount of NaOH used in our work Moreover, our aim was to increase the bio-based content The detailed DSal data and molecular weight of GGM and A-GGM products can be found in Table The Mw and polymer dispersity index (PDI) of pristine GGM are 19.80 × 103 g/mol and 2.38, respectively (Hardelin et al., 2020) In general, the Mw and PDI of A-GGM increased with increasing the AGE contents This can be attributed to the introduction of more allyl chains into GGM backbones due to the continuously increasing of DSal The comparison of the ATR-FTIR spectra of GGM and allylated GGMs also identified the attachment of allyl groups onto the gal actoglucomannan backbone, as shown in Fig 1d and e In terms of un modified GGM, the wide band at 3300–3600 cm− represents the free and H-bonded -OH stretching vibrations of GGM (Liu, Renard, Bureau, & Le Bourvellec, 2021) The weak band attributed to the deformation vi brations of -OH is also displayed at 1631 cm− (Maleki, Edlund, & –O Albertsson, 2017) The band at 1725 cm− is attributed to the C– carbonyl stretching vibrations of the acetyl groups of GGM (Zasadowski, Yang, Edlund, & Norgren, 2014) With regard to the synthesized ally lated GGM, a strong wedge-shaped band at 1643 cm− is assigned to the – C stretching vibrations and a small band at 810 cm− is associated C– with an unsaturated C–H bending vibrations, confirming the successful etherified modification of AGE onto pristine GGM main chains More importantly, the increased band intensity at 1643 cm− also verifies the increasing content of double bonds attached onto the hemicellulose backbone, which is in good agreement with the degree of substitution determined by the use of carbon resonance signals in 13C NMR spectra 2.3 Characterizations Detailed information regarding instrumental setups and experi mental procedures employed for the characterization of pristine GGM, A-GGM and GGM-L latexes/ films is provided in Supporting Information Results and discussion 3.1 Characterization of A-GGM – C double bonds were grafted on GGM backbone to prepare an C– allylated GGM with suitable substitution degree to enable subsequent free radical copolymerization This was achieved through etherification reaction between hydroxyl groups of GGM and epoxy groups of AGE performed in aqueous NaOH solution The molecular structural formula of GGM and its etherification principle are shown in Fig 1a Meanwhile, the chemical structure and substitution degree of allylated GGM were confirmed by 1H NMR and quantitative 13C NMR spectra before and after modification, as shown in Fig 1b and c Prior to allylation, the single peak appearing at 2.0 ppm (1) in the 1HNMR spectra is ascribed to the methyl protons of pendant acetyl moieties of GGM, and the peaks at 3.1–5.3 ppm are corresponding to the protons of carbohydrate backbone (Chadni, Grimi, Bals, Ziegler-Devin, & Brosse, 2019) After allylation, the split peaks at 5.1 ppm and 5.3 ppm (6), as well as newly emerging signal at 5.9 ppm (5) originate from three unsaturated vinylene protons Moreover, the new resonance signal at 4.0 ppm (4) is attributed to the methylene protons connected to the vinyl group (Leppanen et al., 2014) All these results verify that the pendant allylated groups were success fully grafted onto the hemicellulose backbone With regard to the quantitative 13C NMR spectra, the resonances at 21.5 (1′ ) and 170.1 ppm (2′ ) can be separately assigned to methyl and carbonyl groups of acetyl moieties, which are diminished in the allylated GGM due to the hy drolysis reaction in alkali solution (Hardelin et al., 2020) In contrast, the new peaks at 116.0 (6′ ) and 135.2 ppm (5′ ) are ascribed to the vinyl groups of allylated GGM, which indicates that the success of ether ification between the hydroxyl groups of native GGM and epoxy groups of AGE The substitution degree of allylation (DSal) was calculated from the – C double bond peaks at 116.0 and 135.2 ppm relative integrals of the C– against the constant C1 peak at 101 ppm (3′ ) on quantitative 13C NMR spectra, according to Eq (1): (I116 + I135 )/2 I101 DSala a 2.2.4 Preparation of GGM-based films Films from the pristine GGM, A-GGM and GGM-based latexes were prepared in 10 cm diameter petri dishes with covers Known amounts of dispersion in petri dishes were dried at constant temperature of 23 ◦ C and humidity of 50% for days The thickness of the dried films was 80 ± μm based on ten measurements using a Lorentzen and Wettre Micrometer The prepared GGM-L films were kept in sealed bags and stored in a vacuum desiccator for further analysis DSal = Samples 3.2 Characterization of GGM-L – C double In the synthesis of GGM-based latexes, the unsaturated C– bonds of AGE grafted on the hemicellulose backbone were used as re action sites of the free-radical copolymerization of n-BA in semicontinuous emulsion polymerization process, which is sketched in Fig 2a The representative ATR-FTIR spectra of A-GGM and GGM-L products are compared in Fig 2b It can be seen that the newly emerging sharp band at 1735 cm− is attributed to the carbonyl stretching vibrations from the ester groups of n-BA after copolymeriza tion The emulsion polymerization also can be identified by the disap – C vibration at 1643 cm− 1, revealing that most of the pearance of C– – C double groups from either allylated GGM or n-BA are unsaturated C– consumed in the polymerization reaction H NMR and 13C NMR spectra were further to confirm the chemical structure of GGM-L specimens In the 1H NMR spectra of A-GGM and GGM-L (Fig 2c), The newly-emerged resonances at 0.9 (1), 1.3 (2), 1.5 (3) and 2.4 ppm (6) were originated from the n-BA monomer, especially the resonance at 2.4 ppm represented the hydrogen atoms connecting to (1) Q Yong et al Carbohydrate Polymers 291 (2022) 119565 Fig (a) Schematic outline of the synthesis of GGM-L latexes; (b) ATR-FTIR and (c) 1H NMR spectra of typical A-GGM and GGM-L specimens; (d) Hemicellulose biomass content of GGM-L specimens; (e) 13C NMR spectra of typical A-GGM and GGM-L specimens Q Yong et al Carbohydrate Polymers 291 (2022) 119565 ester group after copolymerization The extensive intensity decreased in resonances at 5.9 (7), 5.3 and 5.1 ppm (8), as well as the resonance at 4.0 ppm corresponding to vinyl groups and its neighbored methylene –C groups respectively, demonstrating that nearly all the unsaturated C– double bonds in the reactive system have participated in the copoly merization reaction In case of 13C NMR spectra of A-GGM and GGM-L (Fig 2e), the new resonances from 13C NMR spectrum of GGM-L at 174.8 (5′ ), 65.9 (4′ ), 35.7 (6′ ), 28.0 (3′ ), 18.9 (2′ ) and 14.4 ppm (1′ ) were from n-BA, which demonstrates that n-BA has introduced successfully into the system More convincingly, the disappearance of resonances at 135.2 (7′ ) and 116.0 ppm (8′ ) from 13C NMR spectrum of A-GGM in dicates that the double bonds of A-GGM had reacted completely with nBA monomer The resonance at 130.1 ppm in 13C NMR spectrum of GGM-L shows that there was still a small amount of unreacted n-BA Table Molecular weights of GGM-L and polymerization degrees of poly(n-butyl acrylate) Samples Mw[×104 g/mol]a Mn[×104 g/mol]a PDIa DPb GGM-L1 GGM-L2 GGM-L3 GGM-L4 GGM-L5 2.43 2.75 3.46 7.73 22.34 1.48 1.28 1.34 2.72 9.74 1.64 2.15 2.58 2.84 2.30 17.63 30.97 81.45 366.23 1488.80 a b Measured by HPSEC with DMSO/LiBr as solvent and eluent Estimated polymerization degree (DP) = (Mw (GGM-L) – Mw (A-GGM))/128.17 Fig (a) Photos of the prepared A-GGM and GGM-L dispersions; (b) and (c) are particle size distributions of the A-GGM and GGM-L specimens, respectively; (d) and (e) are Z-average nanoparticle sizes and zeta potentials of A-GGM dispersions and GGM-L latexes, separately Q Yong et al Carbohydrate Polymers 291 (2022) 119565 Fig (a) MilliQ water and ethylene glycol contact angles of GGM-L films; (b) Polarity force, dispersion force and surface free energy of GGM-L films; (c) AFM images and surface roughness of GGM-L1, GGM-L3 and GGM-L5 films, respectively The AFM images and surface roughness of GGM-L2 and GGM-L4 films are in Fig S4 Q Yong et al Carbohydrate Polymers 291 (2022) 119565 residual monomer in the system This result can be verified from Fig S7, which displays the reactive conversion rate of all GGM-L samples Compared to other GGM-L products, the reactive conversion rate of GGM-L1 was slightly lower Notably all of GGM-L specimens demon strate that the reactive conversion rates have exceeded 99% It is well accepted that biomass content is an important indicator in developing biomass-based products On the one hand, it will reduce the usage of fossil fuels in starting materials On the other hand, the more biomass materials we use, the easier the products based on them can biodegrade Fig 2d demonstrates the biomass content in the synthesized bio-based GGM-L products Obviously, with the increase of AGE content, the biomass content slightly decreased from 49.1% to 38.7% This is due to the fact that the DSal increased with an increase in AGE content, indicating that more AGE was consumed and grafted on GGM molecular structure In general, the usage of GGM which accounts for nearly 45 wt % of total mass, reveals a positive beginning in preparing highly hy drophobic hemicellulose-based coatings If we use commercially biobased n-butyl acrylate to substitute the fossil-based one, the biomass content of the GGM-L latexes can reach up to 99 wt%, which shows a tremendous perspective for green and sustainable future Table shows the average molecular weights of GGM-L latexes and the estimated degrees of polymerization (DP) of poly(n-butyl acrylate) When the allylated substitution degrees (DSal) of A-GGM samples increased from 0.04 to 0.83, the weight average molecular weight (Mw) of corresponding GGM-L samples increased from 2.43 × 104 g/mol to 22.34 × 104 g/mol and the number average molecular weight (Mn) increased from 1.48 × 104 g/mol to 9.74 × 104 g/mol Since the length of side chains on the GGM backbone is of critical importance for the properties of GGM-L, the average polymerization degrees of poly(nbutyl acrylate) attached on GGM backbone were estimated The DP values of GGM-L1 and GGM-L2 were 17.63 and 30.97, respectively, which indicates that the grafted poly(n-butyl acrylate) chains were not too long, and therefore it had less impact on properties of GGM-L This may be attributed to the low DSal of A1-GGM and A2-GGM samples With an increase of DSal from 0.30 to 0.51, the DP values of GGM-L increased from 81.45 to 366.23 With the long molecular chains of poly(n-butyl acrylate) attached on GGM backbone, GGM-L3 and GGM-L4 films showed enhanced hydrophobicity and stretchability compared with the native GGM In case of GGM-L5 sample, it had the highest Mw of 22.34 × 104 g/mol and Mn of 9.74 × 104 g/mol, as well as the highest DP of 1488.80 among all GGM-L samples It can be inferred that there were many branched poly(n-butyl acrylate) chains crosslinked each other due to the high DSal (0.83) of A5-GGM sample, leading to the sharp increase in molecular weight and degree of polymerization of GGM-L5 sample Therefore, we can conclude that the adjustment of DSal in the ether ification step is critical to prepare the GGM-L latexes with improved hydrophobicity and stretchability, and to avoid the fabrication of gels of GGM-L weighted average hydrodynamic diameter (Z-average) of GGM is mainly dependent on the GGM raw materials we used All of the A-GGM latexes display the particle size of around 100 nm The specific Z-average par ticle size data of A-GGM and GGM-L are in Fig 3d However, as shown in Fig 3c, the Z-average particle size of all GGM-L emulsions increases considerably when compared to the A-GGM dispersions, and the particle size increases with increasing the DSal of A-GGM Furthermore, the width of particle size distribution also increases also with the increase of substitution degree of the A-GGM These results are attributed to the higher the degree of substitution, the higher number of allylated groups grafted on the hemicellulose backbone Hence, the etherified GGM products were able to provide more reactive sites With polymerization taking place within in micelles where A-GGM and monomer are solu – C double bilized within clusters of SDS surfactant molecules, the C– bonds from both A-GGM and n-BA are initiated by a free radical, origi nating from the decomposition of the initiator APS, and can in turn add – C double bond to form large molecules of repeating on to another C– units The micelle particles obviously increase in size during the poly merization Particle stability is maintained by further adsorption of SDS molecules at the particle surface Zeta potential (ζ-potential) is caused by the net electrical charge contained within the region bounded by the hydrodynamic slipping plane The magnitude of the ζ-potential reveals the degree of electro static repulsion between adjacent charged particles in a dispersion Therefore, colloids with high ζ-potential (negative or positive) are electrostatically stabilized while colloids with low ζ-potential tend to coagulate or flocculate As we can see from Fig 3e, the absolute value of ζ-potential of all GGM-L latexes is much higher than that of all the AGGM dispersions, due to the introduction of SDS surfactant in the second stage of emulsion polymerization All the GGM-L specimens show a relatively high absolute ζ-potential value of over 25 mV, indicating that the synthesized GGM-L emulsions have a moderate electrostatic stability to avoid coagulation To further increase the ζ-potential and thereby also the emulsion stability, acrylic acid could be added in the emulsion polymerization process In general, the resultant GGM-L latexes having particle sizes of between 80 and 210 nm are suitable for technical use, for example, in paper and paperboard barrier coating applications 3.4 Film hydrophobicity and surface energy The hydrophobicity of a coating is a crucial parameter for packaging materials(Cunha & Gandini, 2010) The results in Fig 4a show that contact angles for both milliQ water and ethylene glycol increase with increasing the DSal of A-GGM from GGM-L1 to GGM-L4 films GGM-L5 film shows a small decrease in the contact angles compared to GGM-L4 film For instance, GGM-L1 (DSal 0.04) shows an average water contact angle of 62.9◦ which is still hydrophilic, whereas GGM-L4 (DSal 0.51) film demonstrate the water contact angle up to 117.2◦ , indicating an high hydrophobicity In the case of GGM-L5 specimen, the decrease of milliQ water and ethylene glycol contact angles may be due to the surface roughness of the GGM-L5 film (Farhat et al., 2018) This can be verified by AFM results shown in Fig 4c and Table S3 With an increase of DSal values, the GGM-L film surfaces became much rougher For example, GGM-L1 film had a film roughness of 5.25 nm of Ra and 6.92 nm of Rq, while GGM-L3 film possessed a surface roughness of 15.5 nm of Ra and 21.4 nm of Rq, respectively GGM-L5 film declared a DSal of 0.83, which demonstrated the highest roughness up to 54.9 nm of Ra and 67.9 nm of Rq respectively Therefore, the heteropolymer of A-GGM with a greater high DSal is copolymerized with n-BA monomer to form crosslinking and interpenetrating network between the molecular chains, leading to the gel formation of GGM-L5 product, which in turn shows a greater surface roughness on its film surface (Wenzel, 2002) The Owens-Wendt method (also known as the Kælble-Owens-Wendt method was used for calculating the surface energies of a series of GGML films and for resolving the surface energy into contributions from polarity and dispersion forces (Rudawska & Jacniacka, 2009; Wei, Zeng, 3.3 Particle size and zeta potential In the majority of coating applications, particle size and zeta po tential are highly important and readily measurable indicators that determine the properties of a polymer dispersion, such as flow behavior or stability In order to eliminate the impact of multiple scattering of the laser light and particle interactions, which influence diffusion, all sam ples were diluted to 0.01% w/w before the measurements As we can see from Fig 3a, all A-GGM and GGM-L specimens are brownish yellow in color, but the color of GGM-L latexes is slightly lighter than that of AGGM dispersions after emulsion polymerization Also, the GGM-L la texes are more viscous with increasing the molar ratio of GGM:AGE, which can be verified from data in Fig S1 This is due to the higher substitution degree that may result in a higher level of cross-linking between the macromolecular chains of GGM during the copolymeriza tion with n-BA In Fig 3b, the etherification modification shows little influence on the particle size and particle size distribution The intensity Q Yong et al Carbohydrate Polymers 291 (2022) 119565 Fig (a) Digital photos of GGM-L films covering on logos with copyright permission from Åbo Akademi University; (b) Optical transmittance and (c) transmission haze of GGM-L films & Yong, 2021) The polarity force is caused by the sum of polar, hydrogen, inductive and acid-base interactions The London dispersion force arises from instantaneous dipoles produced by the motion of electrons within the molecule The combination of Eqs (2) and (3) allow one to determine the surface free energy (SFE) and its components: accounts for a large proportion of total SFE, indicating that it contributes to a major part of the strength of GGM-L bio-based films γ S = γ dS + γ pS (2) )1/2 )1/2 ( ( γ L (1 + cosθ) = γ dS γdL + γ PS γ PL (3) Fig 5a shows digital photographs of the GGM-L films In general, the films show the color of light brown, but one can clearly see the logos beneath the films Fig 5b shows the measured light transmittance of the films as a function of wavelength All the GGM-L films were quite transparent in the visible light range, especially at high wavelength With increasing the DSal from 0.04 to 0.51, the light transmittance of GGM-L films increased slightly from 86.1% of GGM-L1 to the highest – C bond value of 91.3% for GGM-L4, which might be influenced by C– – C bonds either from A-GGM or n-butyl conversion rate Unreacted C– monomer reduces film transparency The slight decrease of GGM-L5 film transparency was not only related to conversion rate, but also influenced by its gel characteristic All the prepared GGM-L films show a steep drop of transmittance in the ultraviolet light range, i e., below 400 nm, indicating a superior block of UV light transmittance This result can be of benefit for protecting products in packaging coatings as the most pronounced detrimental effects of light are usually induced by ultravi olet light (Mikkonen et al., 2010; Song, Xu, Li, Chen, & Xu, 2021) Compared to the small change of light transmittance ranging from 86.1% to 91.3% at 550 nm, there are higher variations in the optical haze of the GGM-L films (in Fig 5c) Haze can be used to quantify the percentage of the forward light scattering (wide angle scattering), and is experimentally expressed as Eq (4): (Zhu et al., 2013) 3.5 Film transparency and haze where γdS denotes the component of SFE due to dispersion force, γpS de notes the component of SFE related to polarity force, γ S represents the total SFE of a solid surface The γL, γ dLand γ PL values of two measured liquids (milliQ water and ethylene glycol) can be obtained from Table S4 in supplementary materials As can be seen from Fig 4b, with the increase of DSal of A-GGM, the surface energy decreased from 40.6 mJ m− for GGM-L1 film to 10.3 mJ m− for GGM-L4 film We should always bear in mind that the SFE below 30 mJ m− for a coating means superior hydrophobicity of polymers such as polypropylene (Yong et al., 2016) The low SFE values of GGML3, GGM-L4 and GGM-L5 products confirmed the success of hydrophobic modification of the hydrophilic GGM raw material In the case of SFE components, the values of polarity force decreased from 19.4 mJ m− to 0.4 mJ m− in GGM-L1 ~ GGM-L4 samples, indicating that the film surface became less polarity due to the extension of polymer chains caused by the free radical polymerization with n-BA monomer Compared with the polarity component, London dispersion force Q Yong et al Carbohydrate Polymers 291 (2022) 119565 Fig (a) Diagram of spline preparation and clamping device; (b) Mechanical stress-strain curves of bio-based GGM-L films; (c) Elastic modulus and toughness of bio-based GGM-L films; (d) and (e) represent cross-section images of GGM-L4 and GGM-L5 films separately fractured by liquid Nitrogen The cross-sections of GGM-L1, GGM-L2 and GGM-L3 films can be seen in Fig S5 ( Haze (%) = ) T4 T3 − × 100% T2 T1 (4) Table The mechanical properties of the GGM-L films where T1, T2, T3 and T4 are defined in the Fig S3 With the increase of AGE dosage, the transmission haze of the GGM-L films at the wavelength of 550 nm demonstrates an obvious decreased trend, and the haze values are 3.94%, 2.05%, 0.96%, 0.43% and 1.26%, respectively The result is more likely due to the higher substitution degree of allylated groups leading to a high polymerization density that reduces the light scattering (Tong, Chen, Tian, & He, 2020) In the case of the GGM-L5 film, it was found that the haze slightly increased to 1.26%, which might be caused by the surface roughness Besides, the transmission haze values of these GGM-L films are well matched with the visual effect of Fig 5a, which can be directly judged by clear or cloudy appearance The results indi cate that the GGM-L films, especially for GGM-L4 film, possess high optical transparency and relatively low transmission haze, which are applicable for packaging barrier materials Sample Elongation at break (%)a, b Tensile stress at maximum load (MPa)a, b Elastic modulus (MPa) Toughness (MJ m− 3) GGML1 GGML2 GGML3 GGML4 GGML5 8.5 (2) 20.6 (2) 1265.1 157.4 14.4 (4) 28.0 (2) 1321.9 350.6 22.1 (3) 27.8 (1) 1108.1 523.2 31.2 (4) 30.9 (1) 882.3 701.5 3.6 (3) 18.5 (3) 922.0 56.2 a b All samples were tested in triplicate Standard deviations are presented in parentheses films are commonly very brittle (Huang, Zhong, Zhang, & Cai, 2017) The tensile strains at break of most reported natural polymer-based films are below 10% (Hardelin et al., 2020; Huang et al., 2017; Qi, Chang, & Zhang, 2009) Fig 6a displays the spline shape and clamping device that were used for tensile test Fig 6b and c shows the variation in the 3.6 Film flexibility and uniformity It is well established that films from natural biopolymers such as cellulose and hemicellulose, are difficult to form on their own and the 10 Q Yong et al Carbohydrate Polymers 291 (2022) 119565 Table The WVP properties of chemically modified hemicellulose-based films Material Chemical modification Physical blend Water vapor permeability [g • mm/m2 • d • kPa] Reference Galactoglucomannan – – (Mikkonen et al., 2010) Galactoglucomannan/PVOH – 25% PVOH Galactoglucomannan/CNW – Galactoglucomannan/G Crosslinking with 5% glyoxal 5% Cellulose nanowhiskers – Galactoglucomannan/G/S Crosslinking with 5% glyoxal 40% Sorbitol Galactoglucomannan/S – 40% Sorbitol Galactoglucomannan/AZC Crosslinking with 10% AZC – Arabinoxylan – – 1.7 ± 0.05 (RH 0/ 54%) 1.0 ± 0.1 (RH 0/ 54%) 2.0 ± 0.3 (RH 0/ 54%) 12.0 ± 0.9 (RH 0/ 54%) 1.1 ± 0.2 (RH 0/ 54%) 2.1 ± 0.09 (RH 0/ 54%) 21.8 ± 1.7 (RH 0/ 54%) 7.7 (RH 0/52%) Arabinoxylan Carboxymethyl xylan – Carboxymethylation, DS = 0.36 or 0.58 50% β-glucan – 9.9 (RH 0/52%) 19.0 ± 2, 38.0 ± Carboxymethyl xylan Carboxymethyl xylan/G Hydroxypropyl xylan Hydroxypropyl xylan/G Hemicellulose (67.8% Xylan) Carboxymethyl hemicellulose GGM (in this study) GGM-L1 Carboxymethylation, DS = 0.3 Carboxymethylation, DS = 0.3 Hydroxypropylation, DS = 1.1 Hydroxypropylation, DS = 1.1 – Carboxymethylation, DS = 0.51 or 0.85 – Etherification, DS = 0.04 and then graft emulsion polymerization Same as above, DS = 0.13 Same as above, DS = 0.30 Same as above, DS = 0.51 Same as above, DS = 0.83 – – 10% glycerol – 10% glycerol – – – 3.31 1.41 ± 0.32 2.23 ± 0.39 1.75 ± 0.23 5.2 5.9, 6.8 3.0 (RH 0/50%) 0.78 (RH 0/50%) – – GGM-L2 GGM-L3 GGM-L4 GGM-L5 Poly(vinyl alcohol) (PVOH) Low-density Poly(ethylene) (LDPE) – – – – – mechanical properties for a series of GGM-based films Table lists the data of the maximum elongation at break (ε), ultimate tensile stress (σ), elastic modulus and toughness of the prepared GGM-L films As the amount of AGE increased in first modification step, or the values of DSal increased, the extensional stress had a slight increase from 20.6 MPa of GGM-L1 to 30.9 MPa of GGM-L4, but the elongation at break revealed a pronounced enhanced trend, which was 31.2% for GGM-L4 film and only 8.5% for GGM-L1 film Generally, GGM-L4 specimen with allylated substitution degree of 0.51 yielded film with the optimal mechanical property, i.e., 31.2% of ε and 30.9 MPa of σ In addition, it possessed the lowest elastic modulus of 882.3 MPa and the highest toughness of 701.5 MJ m− among all prepared GGM-L samples The high toughness in dicates a high energy requirement for the sample to fracture, provided by the superior film flexibility of GGM-L4 sample Interestingly, as DSal further increased to 0.83, the GGM-L5 film only exhibited a ε value of 3.6% and a σ value of 18.5 MPa Even though the elastic modulus of GGM-L5 sample was not strong, the toughness value of GGM-L5 sample was only 56.2 MJ m− The result may be explained by that the branched chains of the hemicellulose were more likely to be crosslinked and tangled with each other during the second polymeri zation stage caused by the high substitution degree of allylated groups Therefore, a large amount of cross-linked network structure limited the movements between and inside macromolecular chains, leading to lower flexibility of the GGM-L5 film This also can be verified from Fig 6d and e, which show the freeze-fractured cross-sections of GGM-L4 and GGM-L5 films with 250×, 2500× and 5000× enlargements, respectively The surface morphology of GGM-L4 cross-sections is quite smooth and homogeneous, whereas in contrast the GGM-L5 film cross 0.45 (RH 0/50%) 0.41 (RH 0/50%) 0.30 (RH 0/50%) 0.22 (RH 0/50%) 0.5 ± 0.03 (RH 0/ 54%) 0.16 (Mikkonen et al., 2010) (Mikkonen et al., 2010) (Mikkonen, Heikkilă a, Willfă or, & Tenkanen, 2012) (Mikkonen et al., 2012) (Mikkonen et al., 2012) (Mikkonen, Schmidt, Vesterinen, & Tenkanen, 2013) (S arossy, Tenkanen, Pitkă anen, Bjerre, & Plackett, 2013) (S´ arossy et al., 2013) (Alekhina, Mikkonen, Alen, Tenkanen, & Sixta, 2014) (Sousa, 2016) (Sousa, 2016) (Sousa, 2016) (Sousa, 2016) (Geng et al., 2020) (Geng et al., 2020) (Mikkonen et al., 2010) (Hansen & Plackett, 2008) sections are relatively uneven and rough It should be noted that GGM-L5 latex has the unique and highest apparent viscosity among all GGM-L latexes (in Fig S1) The high apparent viscosity tends to affect the fluidity of latex, leading to an increase of surface roughness during the film formation This result suggests that excessive cross-linking between allylated GGM and monomer due to higher DSal might lead to instability within the emulsion system 3.7 Water vapor permeability Oxygen and water vapor barrier properties are considered crucial with respect to many packaging materials, notably for food packaging materials (Nechita & Roman, 2020) Hemicellulose has an intrinsic property of low oxygen permeability due to its specific molecular structure and intermolecular interactions, which makes it a prospective application in barrier coatings and films (Stark & Matuana, 2021) However, the disadvantages of moisture resistance and high water vapor permeability discourage its use in packaging films and coatings The setup for WVP test can be seen in Fig S2 As can be seen in Table 4, the reported water vapor permeability (WVP) of HC and HC-based modified films in literature data ranged from 1.0 to 38.0 g mm/m2 d kPa The WVP of the pure GGM film presented in this study is 3.0 g mm/m2 d kPa at 50% relative humidity and at 23 ± ◦ C This high permeability is mainly ascribed to the hydrophilic nature of GGM biopolymer with high hydration capacity After modification by etherification and emulsion polymerization, the WVP values of the GGM-L films produced in the present research greatly decreased, being about 10 times lower than for the neat GGM film The values were similar to the WVP value of Poly 11 Q Yong et al Carbohydrate Polymers 291 (2022) 119565 Fig (a) TGA and (b) DTG curves of the unmodified GGM and modified GGM-based films (vinyl alcohol) (PVOH) (0.5 g mm/m2 d kPa), and quite close to the lowdensity polyethylene (LDPE) (0.16 g mm/m2 d kPa) Increasing the substitution degree of the etherification reaction from DS = 0.04 to DS = 0.83 showed a positive influence on the moisture barrier properties of the GGM-L films All these results can be explained by the breakages of hydrated hydrogen bond through etherification modification and by the side chain extensions of GGM through emulsion polymerization The higher degree of substitution of the GGM contributed to a higher number – C double bond reaction sites for allylated GGM, which were pro of C– vided for subsequent emulsion polymerization with monomer, leading to more macromolecular branches and higher molecular weights polymerization process, the thermal stability of the bio-based modified GGM samples enhanced significantly compared to the unmodified GGM specimen Conclusions In summary, we developed a series of bio-based hemicellulose la texes, which can form films with high transparency, superior hydro phobicity and excellent stretchability In the first stage of modification, the A-GGM dispersions were prepared successfully by etherification with DSal values ranging from 0.04 to 0.83 In the second stage, the semicontinuous emulsion polymerization technique was used to achieve free radical polymerization between water soluble A-GGM and water insol uble n-butyl acrylate, thereby providing high molecular weight and long hydrophobic chains The resultant GGM-L coatings exhibited particle sizes between 100 nm and 200 nm and were moderately stable, and suitable for end-use applications such as packaging coatings on paper and paperboard Moreover, the optimal GGM-L film not only exhibited high transmittance of 91.3% and haze of 0.43%, but also demonstrated superior hydrophobicity arriving at 117.2◦ of water contact angle and while maintaining excellent stretchability, with elongation at break of 31.2% and ultimate tensile stress of 30.9 MPa Finally, the WVP of the prepared GGM-L films was very close to the commercial HDPE and showed a great increase in thermal stability compared to the pristine GGM specimen We anticipate that this newly-designed bio-based hemicellulose-modified latex with bio-based content up to 99 wt% can be industrially applied to replace the fossil-based polymer packaging materials, which simultaneously boosts the exploitation and utilization of hemicellulose wastes 3.8 Thermal stability Thermogravimetric analysis (TGA) was utilized to evaluate the thermal stability of the unmodified GGM and modified GGM-based latex films As shown in Fig 7a, an initial weight loss of less than 10% for all the samples due to water evaporation is observed when the heating temperature was below 220 ◦ C All the samples started to degrade from 220 ◦ C, but the degradation stage and rate were quite different between the unmodified GGM and modified GGM-L films, as indicated by the derivative weight analysis in Fig 7b The pristine GGM film only shows one large decomposition stage at around 300 ◦ C This might be associ ated with disintegration of the macromolecular chains of GGM such as the cleavage of the glycosidic bonds at 250 ◦ C, then followed by degradation of the hexoses ranging from 250 ◦ C to 350 ◦ C (Mendes et al., 2017) However, the GGM-L films show two distinct stages of decomposi tion, one at around 220– 350 ◦ C and the other at 350– 450 ◦ C The temperature corresponding to maximum decomposition rate (T1max) at around 300 ◦ C of GGM-based films increased with an increase of DS from 0.04 to 0.83 in the first stage, while in the second stage the temperature of maximum decomposition rate (T2max) at 400 ◦ C remained constant This is might be due to the activation energy of formulated n-butyl acrylate macromolecular main chains completely comprised of C–C bonds was much higher than that of GGM backbones containing C-O-C bonds (Hu, Chen, & Wang, 2004) Therefore, we can conclude that the first decomposition stage of GGM-L films ranging from 220– 350 ◦ C can be assigned to the degradation of GGM polymer backbone and the cleavage of substituents, and the second decomposition stage ranging from 350 ◦ C to 450 ◦ C is attributed to the degradation of n-butyl acrylate polymer chains Most of the degradation process was completed below 500 ◦ C In Fig 7a above 500 ◦ C, the unmodified GGM sample had a higher char yield compared to GGM-based films This was probably due to differences in the salts content of the initial samples (Wang, Xiao, & Lei, 2020) In general, through etherification and graft emulsion CRediT authorship contribution statement Conceptualization, experiment, data analysis and curation, writingoriginal draft, funding resources, Qiwen Yong; conceptualization, experimental facility, review & editing, guidance and supervision, funding resources, Martti Toivakka and Chunlin Xu；measurement, data analysis, review, Jiayun Xu, Luyao Wang and Teija Tirri; supervi sion and modification, Hejun Gao, Yunwen Liao; All authors have read and agreed to the published version of manuscript Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in 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Characterization of GGM-L – C double In the synthesis of GGM-based latexes, the unsaturated C– bonds of AGE grafted on the hemicellulose backbone were used as re action sites of the free-radical copolymerization... outline of the synthesis of GGM-L latexes; (b) ATR-FTIR and (c) 1H NMR spectra of typical A-GGM and GGM-L specimens; (d) Hemicellulose biomass content of GGM-L specimens; (e) 13C NMR spectra of typical