The Influence of the Comonomer Ratio and Reaction Temperature on the Mechanical, Thermal, and Morphological Properties of Lignin Oil–Sulfur Composites

Although lignin is a plentiful biomass resource, it continually exists as an underutilized component of biomass material. Elemental sulfur is another abundant yet underutilized commodity produced as a by-product resulting from the refining of fossil fuels. The current study presents a strategy for preparing five durable composites via a simple one-pot synthesis involving the reaction of lignin oil and elemental sulfur. These lignin oil–sulfur composites LOSx@T (where x = wt. % sulfur, ranging from 80 to 90, and T represents the reaction temperature in °C) were prepared via the reaction of elemental sulfur and lignin oil (LO) with elemental sulfur. The resulting composites could be remelted and reshaped several times without the loss of mechanical strength. Mechanical, thermal, and morphological studies showed that LOSx@T possesses properties competitive with some mechanical properties of commercial building materials, exhibiting favorable compressive strengths (22.1–35.9 MPa) and flexural strengths (5.7–6.5 MPa) exceeding the values required for many construction applications of ordinary Portland cement (OPC) and brick formulations. While varying the amount of organic material did not result in a notable difference in mechanical strength, increasing the reaction temperature from 230 to 300 °C resulted in a significant increase in compressive strength. The results reported herein reveal potential applications of both lignin and waste sulfur during the ongoing effort toward developing recyclable and sustainable building materials.

Both "upstream" and "downstream" processes contribute to efficient lignin valorization.Upstream processes involve separating and isolating lignin, lignin bioengineering, and catalytic conversion, whereas downstream methods involve the depolymerization and upgrading of lignin [31][32][33].Depolymerization yields bio-oils or lignin oils comprising mixtures of small molecular fragments of lignin and is among the most promising routes to lignin employment.Green methods of lignin depolymerization are therefore being hotly pursued.A few "green" solvents employed in extraction processes involve supercritical fluids, liquified gases, and bio-based solvents.Supercritical fluid extraction (SFE) requires an extraction solvent that can be removed easily, a shorter extraction time, and enhanced selectivity and efficiency [34].Bio-based solvents are derivative of a broad range of biomass sources, including the extraction of vegetable oils, the fermentation of carbohydrates, and the steam distillation of wood [35][36][37].Liquified gases are another green solvent.They require low temperature for easy evaporation, allowing for room-temperature liquid gas extractions to be performed at room temperature, consuming minimal energy and extracts containing an insignificant amount residual solvents [38,39].A recent report [40] discussing lignin's mild thermolytic solvolysis of yielding a solubilized form of lignin (termed lignin oil) garnered particular interest for the present study due to the process being performed at temperatures ranging from 100 to 350 • C by means of various alcohols as solvents [41].By heating in an ethanol solvent at a low temperature of 100 • C, this procedure accessibly accomplished the conversion of lignin (64 wt.%) to the desired product, lignin oil (LO).Ethanol is a preferred green solvent and is advantageous because of its low cost and availability [42][43][44][45].Lignin oil produced by this reported procedure was used in the studies described herein.
The search for a more sustainable S-C bond forming reaction to allow the direct utilization of lignin without the need for olefination has simplified the synthetic process toward lignin-sulfur composites and has improved the atom economy of the processes used for their preparation.Recently reported C-S bond-forming mechanisms during the reaction of sulfur with anisole derivatives (Scheme 1) such as O,O ′ -dimethylbisphenol A [106], guaiacol [64], and others [107] suggest that the direct reaction of lignin oil mixtures with sulfur was possible, as was validated in a recent proof-of-principle study detailing the crosslinking that takes place between sulfur and small lignin derivatives present in lignin oil [108].
Herein, five composites were prepared by heating lignin oil (LO) with elemental sulfur for 2 h.The composites, including LOS x @T (where x = wt.% sulfur, varying from 80 to 90, and T represents the reaction temperature in • C), were of interest to exploit the features of previously reported ELS 90 @180 and ELS 90 @230 due to S-C bond-forming at both olefinic and aryl sites after an increase in temperature.Previously studied model compounds with functional groups found within thermal degradation products or lignin subunits were reacted with elemental sulfur, implementing matching conditions [107] as reported to enhance the understanding of S-C bond-forming reactions within the lignin-sulfur composites prepared herein (Scheme 1a).The thermal, morphological, and mechanical properties were analyzed using powder X-ray diffraction (XRD), scanning electron microscopy accompanied by elemental mapping via energy-dispersive X-ray analysis (SEM-EDX), thermogravimetric analysis (TGA), flexural strength analysis, mechanical test stand analysis, and differential scanning calorimetry (DSC).Both "upstream" and "downstream" processes contribute to efficient lignin valorization.Upstream processes involve separating and isolating lignin, lignin bioengineering, and catalytic conversion, whereas downstream methods involve the depolymerization and upgrading of lignin [31][32][33].Depolymerization yields bio-oils or lignin oils comprising mixtures of small molecular fragments of lignin and is among the most promising routes to lignin employment.Green methods of lignin depolymerization are therefore being hotly pursued.A few "green" solvents employed in extraction processes involve supercritical fluids, liquified gases, and bio-based solvents.Supercritical fluid extraction (SFE) requires an extraction solvent that can be removed easily, a shorter extraction time, Herein, five composites were prepared by heating lignin oil (LO) with elemental sulfur for 2 h.The composites, including LOSx@T (where x = wt.% sulfur, varying from 80 to 90, and T represents the reaction temperature in °C), were of interest to exploit the features of previously reported ELS90@180 and ELS90@230 due to S-C bond-forming at both olefinic and aryl sites after an increase in temperature.Previously studied model compounds with functional groups found within thermal degradation products or lignin subunits were reacted with elemental sulfur, implementing matching conditions [107] as reported to enhance the understanding of S-C bond-forming reactions within the lignin-sulfur composites prepared herein (Scheme 1a).The thermal, morphological, and mechanical properties were analyzed using powder X-ray diffraction (XRD), scanning electron microscopy accompanied by elemental mapping via energy-dispersive X-ray analysis (SEM-EDX), thermogravimetric analysis (TGA), flexural strength analysis, mechanical test stand analysis, and differential scanning calorimetry (DSC).

Synthesis and Chemical Characterization of Composites
The lignin oil utilized herein was prepared and characterized as previously reported [108].
Composites LOSx@T (where x = wt.% sulfur, varying from 80 to 90, and T represents the reaction temperature in °C) were prepared by heating lignin oil (10,15, or 20 wt.%) and sulfur (90, 85, or 80 wt.%) to the requisite temperature in a sealed vessel for 2 h.The composites LOS80@230 and LOS85@230, and LOS90@230 [108] were prepared in thickwalled glass pressure vessels slowly heated to 230 °C and then held at 230 °C for 2 h.Composites LOS90@250 and LOS90@300 were prepared in a stainless steel autoclave reactor The composites were mechanically stirred and slowly heated to either 250 °C or 300 °C, respectively, and held at that temperature for 2 h.After cooling to room temperature, all the materials solidified into a dark brown, remeltable solid.After melting, the composite material was transferred into molds for shaping and cooled to room temperature (Figures 2 and 3).Scheme 1.(a) S-C bond-forming reactions for lignin-derived small molecules bearing some combination of −CH 3 , −OH, and −OCH 3 substituents [107] and (b) the major S-C bond forming reaction of composite GS 80 from sulfur and guaiacol.Polymeric sulfur is represented by -S x -with subscript letters w, x, y, and z signifying some amount of sulfur in the polymer chain.

Synthesis and Chemical Characterization of Composites
The lignin oil utilized herein was prepared and characterized as previously reported [108].Composites LOS x @T (where x = wt.% sulfur, varying from 80 to 90, and T represents the reaction temperature in • C) were prepared by heating lignin oil (10,15, or 20 wt.%) and sulfur (90, 85, or 80 wt.%) to the requisite temperature in a sealed vessel for 2 h.The composites LOS 80 @230 and LOS 85 @230, and LOS 90 @230 [108] were prepared in thick-walled glass pressure vessels slowly heated to 230 • C and then held at 230 • C for 2 h.Composites LOS 90 @250 and LOS 90 @300 were prepared in a stainless steel autoclave reactor The composites were mechanically stirred and slowly heated to either 250 • C or 300 • C, respectively, and held at that temperature for 2 h.After cooling to room temperature, all the materials solidified into a dark brown, remeltable solid.After melting, the composite material was transferred into molds for shaping and cooled to room temperature (Figures 2 and 3).
The sulfur present in high sulfur-content materials (HSMs) exists as both -S xchains that are linked covalently to the organic comonomers and entrapped sulfur species such as cyclo-S 8 , known as "dark sulfur", which is not linked covalently to the organic comonomers [109][110][111].Dark sulfur's relative quantity within an HSM can influence both its thermal and its mechanical properties.In this study, dark sulfur was extracted from each HSM using ethyl acetate.The wt. % of each sample that was soluble (dark sulfur) and the insoluble fraction (organic crosslinked network) are provided in Table 1.The majority of sulfur in the reported lignin-sulfur composites was stabilized as covalently attached crosslinking catenates, with all of the composites showing a similar contribution of dark sulfur over a narrow range of 22-31 wt.%.Elemental analysis confirmed that very little organic material was extractable, with soluble portions consisting of 94-99% sulfur.It is important to note that the extraction experiment was only conducted to assess the relative contributions of the dark sulfur and of the crosslinked network to the overall structure.All subsequent thermal, morphological, and mechanical testing was conducted on the complete composites, comprising the network with the entrapped dark sulfur.Representative photos of compressional cylinders of (from left to right) LOS80@230, LOS85@230, LOS90@230, LOS90@250, and LOS90@300.The sulfur present in high sulfur-content materials (HSMs) exists as both -Sx-chains that are linked covalently to the organic comonomers and entrapped sulfur species such as cyclo-S8, known as "dark sulfur", which is not linked covalently to the organic comonomers [109][110][111].Dark sulfur's relative quantity within an HSM can influence both its thermal and its mechanical properties.In this study, dark sulfur was extracted from each HSM using ethyl acetate.The wt. % of each sample that was soluble (dark sulfur) and the insoluble fraction (organic crosslinked network) are provided in Table 1.The majority of sulfur in the reported lignin-sulfur composites was stabilized as covalently attached crosslinking catenates, with all of the composites showing a similar contribution of dark sulfur over a narrow range of 22-31 wt.%.Elemental analysis confirmed that very little organic material was extractable, with soluble portions consisting of 94-99% sulfur.It is important to note that the extraction experiment was only conducted to assess the relative contributions of the dark sulfur and of the crosslinked network to the overall structure.All subsequent thermal, morphological, and mechanical testing was conducted on the complete composites, comprising the network with the entrapped dark sulfur.
Powder X-ray diffraction (PXRD) (Figures 4-6 and S1-S5, Supplementary Materials) was used for the initial qualitative assessment of crystallinity and species contributing to the crystalline domains of LOSx@T composites.As the amount of organic material increased, the PXRD data reveal the anticipated corresponding reduction in crystallinity: LOS90@230 is a crystalline polymer, LOS85@230 is a partially amorphous polymer, and Representative photos of compressional cylinders of (from left to right) LOS80@230, LOS85@230, LOS90@230, LOS90@250, and LOS90@300.The sulfur present in high sulfur-content materials (HSMs) exists as both -Sx-chains that are linked covalently to the organic comonomers and entrapped sulfur species such as cyclo-S8, known as "dark sulfur", which is not linked covalently to the organic comonomers [109][110][111].Dark sulfur's relative quantity within an HSM can influence both its thermal and its mechanical properties.In this study, dark sulfur was extracted from each HSM using ethyl acetate.The wt. % of each sample that was soluble (dark sulfur) and the insoluble fraction (organic crosslinked network) are provided in Table 1.The majority of sulfur in the reported lignin-sulfur composites was stabilized as covalently attached crosslinking catenates, with all of the composites showing a similar contribution of dark sulfur over a narrow range of 22-31 wt.%.Elemental analysis confirmed that very little organic material was extractable, with soluble portions consisting of 94-99% sulfur.It is important to note that the extraction experiment was only conducted to assess the relative contributions of the dark sulfur and of the crosslinked network to the overall structure.All subsequent thermal, morphological, and mechanical testing was conducted on the complete composites, comprising the network with the entrapped dark sulfur.
Powder X-ray diffraction (PXRD) (Figures 4-6 and S1-S5, Supplementary Materials) was used for the initial qualitative assessment of crystallinity and species contributing to the crystalline domains of LOSx@T composites.As the amount of organic material increased, the PXRD data reveal the anticipated corresponding reduction in crystallinity: LOS90@230 is a crystalline polymer, LOS85@230 is a partially amorphous polymer, and Table 1.Soluble (dark sulfur) and insoluble (crosslinked material) wt.% fractions of lignin-sulfur composites.

Materials
Dark Sulfur [a]  (wt.%) Crosslinked Material [a]  (wt.%) [a] Sum of soluble and insoluble fractions is greater than 100% due to rounding and errors associated with the measurements.
Powder X-ray diffraction (PXRD) (Figures 4-6 and S1-S5, Supplementary Materials) was used for the initial qualitative assessment of crystallinity and species contributing to the crystalline domains of LOS x @T composites.As the amount of organic material increased, the PXRD data reveal the anticipated corresponding reduction in crystallinity: LOS 90 @230 is a crystalline polymer, LOS 85 @230 is a partially amorphous polymer, and LOS 80 @230 is primarily amorphous.This trend reflects the diminished capacity for the crystalline packing of poly/oligosulfur crosslinking chains as they become progressively shorter in response to the availability of more organic crosslinkable sites.The sharp peaks are the crystalline sulfur, while broad baselines between 20 and 30 2θ, most noticeable for the amorphous LOS 80 @230 (Figure 6), are characteristic of the polymer.LOS 90 @230 is almost entirely crystalline sulfur within the polymer composite, as seen by the very tall, sharp peaks (Figure 4).While PXRD provides qualitative confirmation of a decreasing percentage of crystallinity with increasing organic comonomer, it only provides a semiquantitative estimate of the percentage of crystallinity.Due to the nature of the composite having numerous diffractions of crystalline sulfur, it is difficult to identify lignin's contribution within the pattern.Therefore, differential scanning calorimetry (DSC) data were used to calculate a more accurate percentage crystallinity for the composites (Table 2).We find that the percentage crystallinity values correlate quantitatively well with the PXRD patterns, which show increasing intensity of broad features about 20-30 2θ.
FT-IR spectra revealed further evidence of the formation of C-S bonds between the polymeric sulfur and lignin oil components (Figure 7).At 660 cm −1 , a visible peak indicates a C-S stretch [112,113].The IR spectra (Figures S6 and S7, Supplementary Materials) also revealed that LOS x @T composites preserved peaks characteristic of LO such as O-H stretches (3200-3500 cm −1 ), C-H stretches (2930 cm −1 ), and C-O stretches (1030-1200 cm −1 ) [114,115].Other important peaks to note are those characteristic of lignin bands such as the C=O aromatic skeletal vibration observed from 1600 to 1620 cm −1 and the unconjugated carbonyl groups observed from 1700 to 1720 cm −1 .As previously reported, the C-S stretch for LOS 90 @230 is predictably broadened because it has the lowest organic content and reaction temperature of the composites and therefore the longest average polysulfur chains between organic sites, leading to the greatest polydispersity of the possible bond stretch energies.The displacement of the bands observed can also be attributed to the reaction temperatures of the different composites.For example, slightly sharper peaks were observed in LOS 80 @230, LOS 85 @230, and LOS 90 @230 from 600 to 1200 cm −1 (Figure S8, Supplementary Materials) when compared to LOS 90 @250 and LOS 90 @300, which were reacted at higher temperatures.
Molecules 2024, 29, x FOR PEER REVIEW 6 of 22 LOS80@230 is primarily amorphous.This trend reflects the diminished capacity for the crystalline packing of poly/oligosulfur crosslinking chains as they become progressively shorter in response to the availability of more organic crosslinkable sites.The sharp peaks are the crystalline sulfur, while broad baselines between 20 and 30 2θ, most noticeable for the amorphous LOS80@230 (Figure 6), are characteristic of the polymer.LOS90@230 is almost entirely crystalline sulfur within the polymer composite, as seen by the very tall, sharp peaks (Figure 4).While PXRD provides qualitative confirmation of a decreasing percentage of crystallinity with increasing organic comonomer, it only provides a semiquantitative estimate of the percentage of crystallinity.Due to the nature of the composite having numerous diffractions of crystalline sulfur, it is difficult to identify lignin's contribution within the pattern.Therefore, differential scanning calorimetry (DSC) data were used to calculate a more accurate percentage crystallinity for the composites (Table 2).We find that the percentage crystallinity values correlate quantitatively well with the PXRD patterns, which show increasing intensity of broad features about 20-30 2θ.LOS80@230 is primarily amorphous.This trend reflects the diminished capacity for the crystalline packing of poly/oligosulfur crosslinking chains as they become progressively shorter in response to the availability of more organic crosslinkable sites.The sharp peaks are the crystalline sulfur, while broad baselines between 20 and 30 2θ, most noticeable for the amorphous LOS80@230 (Figure 6), are characteristic of the polymer.LOS90@230 is almost entirely crystalline sulfur within the polymer composite, as seen by the very tall, sharp peaks (Figure 4).While PXRD provides qualitative confirmation of a decreasing percentage of crystallinity with increasing organic comonomer, it only provides a semiquantitative estimate of the percentage of crystallinity.Due to the nature of the composite having numerous diffractions of crystalline sulfur, it is difficult to identify lignin's contribution within the pattern.Therefore, differential scanning calorimetry (DSC) data were used to calculate a more accurate percentage crystallinity for the composites (Table 2).We find that the percentage crystallinity values correlate quantitatively well with the PXRD patterns, which show increasing intensity of broad features about 20-30 2θ.
Molecules 2024, 29, x FOR PEER REVIEW 8 of 22 the C=O aromatic skeletal vibration observed from 1600 to 1620 cm −1 and the unconjugated carbonyl groups observed from 1700 to 1720 cm −1 .As previously reported, the C-S stretch for LOS90@230 is predictably broadened because it has the lowest organic content and reaction temperature of the composites and therefore the longest average polysulfur chains between organic sites, leading to the greatest polydispersity of the possible bond stretch energies.The displacement of the bands observed can also be attributed to the reaction temperatures of the different composites.For example, slightly sharper peaks were observed in LOS80@230, LOS85@230, and LOS90@230 from 600 to 1200 cm −1 (Figure S8, Supplementary Materials) when compared to LOS90@250 and LOS90@300, which were reacted at higher temperatures.Scanning electron microscopy (SEM, Figures S8-S12) imaging accompanied by element mapping by energy-dispersive X-ray analysis (EDX) was utilized to evaluate the dispersal of elements in the LOSx@T composites (Figure 8).SEM/EDX data revealed a homogeneous distribution of sulfur, carbon, and oxygen.Scanning electron microscopy (SEM, Figures S8-S12) imaging accompanied by element mapping by energy-dispersive X-ray analysis (EDX) was utilized to evaluate the dispersal of elements in the LOS x @T composites (Figure 8).SEM/EDX data revealed a homogeneous distribution of sulfur, carbon, and oxygen.
Scanning electron microscopy (SEM, Figures S8-S12) imaging accompanied by element mapping by energy-dispersive X-ray analysis (EDX) was utilized to evaluate the dispersal of elements in the LOSx@T composites (Figure 8).SEM/EDX data revealed a homogeneous distribution of sulfur, carbon, and oxygen.

Thermal and Morphological Properties of LOSx@T
The thermal stability of the reported LOSx@T composites was assessed using thermogravimetric analysis (Figure 9).The TGA data for lignin oil (LO) prior to its reaction with elemental sulfur are also provided in Figure 9, and the origin of the decomposition events in LO have been previously delineated [116].The TGA trace for each of the composites showed a single decomposition temperature (Td) ranging from 229 to 231 °C.These temperatures are characteristic of such HSMs due to the fact that elemental sulfur has a Td of 229 °C, which is attributable to the sublimation of sulfur in the LOSx@T composites.The smaller decomposition event at >300 °C in LOS90@230 and LOS90@250 is likely attributable to similar mechanisms leading to the corresponding decomposition events observed in lignin oil.The Td values for LOSx@T and other previously reported lignin-containing HSMs are summarized in Table 2.

Thermal and Morphological Properties of LOS x @T
The thermal stability of the reported LOS x @T composites was assessed using thermogravimetric analysis (Figure 9).The TGA data for lignin oil (LO) prior to its reaction with elemental sulfur are also provided in Figure 9, and the origin of the decomposition events in LO have been previously delineated [116].The TGA trace for each of the composites showed a single decomposition temperature (T d ) ranging from 229 to 231 • C.These temperatures are characteristic of such HSMs due to the fact that elemental sulfur has a T d of 229 • C, which is attributable to the sublimation of sulfur in the LOS x @T composites.The smaller decomposition event at >300 • C in LOS 90 @230 and LOS 90 @250 is likely attributable to similar mechanisms leading to the corresponding decomposition events observed in lignin oil.The T d values for LOS x @T and other previously reported lignin-containing HSMs are summarized in Table 2.The analysis of LOSx@T composites by differential scanning calorimetry (DSC) (Figures S13-S27, Supplementary Materials) showed thermal transitions that were concomitant with the presence of polymeric sulfur and cyclo-S8.LOS90@230, LOS90@250, and LOS90@300 exhibited the melting peaks (114 °C, 117 °C, and 119 °C) expected for cyclo-S8.However, LOS85@230 and LOS80@230 exhibited lower melting peaks at 106 °C and 105 °C, indicating an α-to-β transition of sulfur.A glass transition temperature (Tg) was also witnessed (Figure 10) at −35 °C or −36 °C for all composites other than LOS90@300, diagnostic for polymeric sulfur [117][118][119].Similar to the previously studied LS90-99 composites, LOS90@300 did not exhibit a glass transition temperature.This is likely due to the higher temperature allowing for more crosslinking, leading to shorter sulfur chains.
Melting and cold crystallization enthalpies determined from DSC data were employed for the calculation of the percentage crystallinity of the composites, further indi- The analysis of LOS x @T composites by differential scanning calorimetry (DSC) (Figures S13-S27, Supplementary Materials) showed thermal transitions that were concomitant with the presence of polymeric sulfur and cyclo-S 8 .LOS 90 @230, LOS 90 @250, and LOS 90 @300 exhibited the melting peaks (114 • C, 117 • C, and 119 • C) expected for cyclo-S 8 .However, LOS 85 @230 and LOS 80 @230 exhibited lower melting peaks at 106 • C and 105 • C, indicating an α-to-β transition of sulfur.A glass transition temperature (T g ) was also witnessed (Figure 10) at −35 • C or −36 • C for all composites other than LOS 90 @300, diagnostic for polymeric sulfur [117][118][119].Similar to the previously studied LS 90-99 composites, LOS 90 @300 did not exhibit a glass transition temperature.This is likely due to the higher temperature allowing for more crosslinking, leading to shorter sulfur chains.
Melting and cold crystallization enthalpies determined from DSC data were employed for the calculation of the percentage crystallinity of the composites, further indicating the presence of cyclo-S 8 , using Equation (1).
where ∆X c signifies the change in the percentage crystallinity with respect to sulfur, ∆H m signifies the melting enthalpy of the composite, ∆H cc represents the cold crystallization enthalpy of the composite, ∆H m(S) is sulfur's melting enthalpy, and ∆H cc(S) is sulfur's cold crystallization enthalpy.The percentage crystallinity of LOS 90 @230 is 57% relative to the crystallinity of pure elemental sulfur.Lower percentage crystallinity results in the reduced brittleness of HSMs.Compared to LS x composites (Table 1), LOS 90 @230 possesses a lower percentage crystallinity than any of the composites made from allylated lignin, for example, LS 90-99 (percentage crystallinity of 67-91%).However, LOS 90 @230 possesses a higher percentage crystallinity than that of composites LS 80 (8%) and LS 85 (40%), reflecting the trend more qualitatively revealed by PXRD data (vide supra).These trends are summarized in Figure 11.LOS 85 @230 also has a lower percentage crystallinity (19%) than that of LOS 90 @230 (57%) and LS 85-99 (40-91%).However, LOS 85 @230 possesses a higher percentage crystallinity than that of composite LS 80 (8%).The reaction temperature does not seem to have a significant effect on the percentage crystallinity of lignin-sulfur materials.
Melting and cold crystallization enthalpies determined from DSC data were employed for the calculation of the percentage crystallinity of the composites, further indicating the presence of cyclo-S8, using Equation (1).where ΔXc signifies the change in the percentage crystallinity with respect to sulfur, ΔHm signifies the melting enthalpy of the composite, ΔHcc represents the cold crystallization enthalpy of the composite, ΔHm(S) is sulfur's melting enthalpy, and ΔHcc(S) is sulfur's cold crystallization enthalpy.

Mechanical Properties and Environmental Impact Considerations
The LOSx@T composites studied herein must meet the specified mechanical strength requirements to be considered as potential replacements for less sustainable materials.For example, OPC requires a compressive strength greater than 17 MPa and a flexural strength greater than 3 MPa.The compressive strength of the LOSx@T composites reported exceeded that required of OPC with strengths ranging from 22.1 to 35.9 MPa.Other lignincontaining HSMs have been reported and compared to the LOSx@T composites.The composites ELSx@T (where x = wt.% sulfur in the reaction mixture and T represents the reaction temperature in °C), for example, were prepared by reacting sulfur with oleic-esterified lignin.ELS80@180 has a notably lower compressive strength (10.9 MPa) than LOSx@T composites.ELS80@180 comprises twice the amount of organic component as LOS90, Figure 11.A graphical representation of the general trend of the effect increasing the percentage of organic material has on the percentage crystallinity of LS x (blue triangles) and LOS x @T (orange circles).

Mechanical Properties and Environmental Impact Considerations
The LOS x @T composites studied herein must meet the specified mechanical strength requirements to be considered as potential replacements for less sustainable materials.For example, OPC requires a compressive strength greater than 17 MPa and a flexural strength greater than 3 MPa.The compressive strength of the LOS x @T composites reported exceeded that required of OPC with strengths ranging from 22.1 to 35.9 MPa.Other lignin-containing HSMs have been reported and compared to the LOS x @T composites.The composites ELS x @T (where x = wt.% sulfur in the reaction mixture and T represents the reaction temperature in • C), for example, were prepared by reacting sulfur with oleic-esterified lignin.ELS 80 @180 has a notably lower compressive strength (10.9 MPa) than LOS x @T composites.ELS 80 @180 comprises twice the amount of organic component as LOS 90 , LOS 90 @250, and LOS 90 @300.ELS 80 @180 was also reacted at a much lower temperature in comparison to LOS x @T composites, whose reaction temperatures ranged from 230 to 300 • C. A larger amount of organic material and lower reaction temperature possibly contributes to a lower sulfur rank, resulting in a lower compressive strength.The ELS 90 @180 and ELS 90 @230 composites exhibited similar compressive strengths as the LOS 90 @230, LOS 85 @230, LOS 80 @230, and LOS 90 @250 composites.The LOS 90 @300 exhibited a much higher compressive strength (35.9 MPa) than all other composites reported in Table 3 and compared graphically in Figure 12 (stress-strain plots are presented in Figures S28-S32, Supplementary Materials).This can possibly be attributed to the higher reaction temperature of 300 • C, which can lead to a higher sulfur rank and a higher compressive strength [65].While there was not a clear trend displaying how an increase in organic material affects the compressive strength, an increase in compressive strength is observed as the reaction temperature is increased (Figure 13).The preparation of the oleic-esterified lignin (the precursor for ELS x @T) is a much more time-and energy-intensive process compared to the process for preparing the lignin oil required for preparing LOS x @T composites.Therefore, preparing ligninoil-containing HSMs is a more realistic concept for acquiring comparable compressive strength features.Table 3. Mechanical properties of the lignin-sulfur composites LOS x @T with other previously reported lignin-containing HSMs composites for comparison.
affects the compressive strength, an increase in compressive strength is observed as the reaction temperature is increased (Figure 13).The preparation of the oleic-esterified lignin (the precursor for ELSx@T) is a much more time-and energy-intensive process compared to the process for preparing the lignin oil required for preparing LOSx@T composites.Therefore, preparing lignin-oil-containing HSMs is a more realistic concept for acquiring comparable compressive strength features.LOS90@230 [a]  22.1 ± 2.5 5.7/186 130 LOS85@230 [a]  26.0 ± 0.3 6.5/236 153 LOS80@230 [a]  22.6 ± 1.7 ND [k]  133 LOS90@250 [a]  22.1 ± 1.3 ND [k]  130 LOS90@300 [a]  35.9 ± 1.5 ND [k]  211 LS80 [b]  ND [k]  2.1/87 ND [k] LS85 [b]  ND [k]  1.5/76 ND Bars not displayed signify that the attendant value was not previously reported for the listed material or could not be obtained for the current research.
Figure 13.A graphical representation of the general trend of the effect increasing reaction temperature has on the compressive strength of ELSx@T (green squares) and LOSx@T (orange circles).

Materials
Sulfur powder (99.5%) was bought from Alfa Aesar (Haverhill, MA, USA).The chemicals employed did not undergo any further purification.The lignin oil used herein was produced via a reported method implementing the thermal solvolysis of kraft lignin (Sigma Aldrich, St. Louis, MO, USA) in the solvent ethanol following the reported procedure [40].A detailed description of the preparation and characterization of lignin oil was previously reported for the synthesis and analysis of LOS 90 @230 [108].The same lignin oil was used to prepare the remaining LOS x @T composites reported herein.

General Considerations and Instrumentation
Fourier transform infrared spectra were acquired using an IR instrument (Shimadzu IRAffinity-1S, Shimadzu Corporation, Columbia, MD, USA) equipped with an ATR attachment.Scans were collected over the range of 400-4000 cm −1 at ambient temperature with a resolution of 8 cm −1 .
SEM was attained using a Schottky Field Emission Scanning Electron Microscope SU5000 (Hitachi High-Tech, Tokyo, Japan) operating in variable pressure mode while using an accelerating voltage of 15 kV.
Compressional strength measurements were commenced with cylinders by means of a Mark-10 ES30 (Mark-10 Corporation, Copiague, NY, USA) Manual Test Stand equipped with a Mark 10 M3-200 Force Gauge (USA).The composites studied herein were melted and formed in molds (Smooth-On Oomoo ® 25 tin-cure, Oomoo Corp., Richmond, BC, Canada) and then allowed to cool to room temperature.The cylinders stood at room temperature for 4 d before compressive strength was tested.Tests for each composite were performed in triplicates, and the strength reported is the average of the three runs.The long-term stability of the reported materials is unknown.
Flexural strength analyses were performed using a Mettler Toledo DMA 1 STARe System (Mettler Toledo, Columbus, OH, USA) in single-cantilever mode.The samples were formed in silicon resin molds (Smooth-On Oomoo ® 25 tin-cure).The sample dimensions were 1.5 × 10.7 × 5.0 mm.The temperature was 25 • C with a clamping force of 1 cN m.Samples were assessed in triplicates, and the observed flexural strengths/moduli are an average of the three trials.DSC data were obtained by means of a Mettler Toledo DSC 3 STARe System (Mettler Toledo, Columbus, OH, USA) across a temperature range of −60 • C to 140 • C while implementing a heating rate of 10 • C•min −1 under a flow of N 2 (200 mL•min −1 ).DSC measurements were conducted over three cooling and heating cycles.Data from the third heating cycle are reported herein, with the first cycle removing solvent impurities.During the third cycle, the glass-transition temperature was observed.DSC data were utilized to calculate the percentage crystallinity using Equation (1).
UV-vis data were collected on Agilent Technologies Cary 60 UV-vis (Agilent Technologies, Inc., Santa Clara, CA, USA) using Simple Reads software (Cary WinUV Scan Application Version 5.1.0.1016) over the range of 400-600 nm.Dark sulfur was quantified by using the extinction coefficient of sulfur at 275 nm along with the absorbance of ethyl acetate soluble fraction of each composite.

Synthesis of LOS x @T Composites
LOS 80 @230 and LOS 85 @230 were prepared by adding elemental sulfur (8.005 g, 8.500 g) and lignin oil (2.001 g, 1.503 g) to a heavy-walled pressure flask sealed by a Viton O-ring and a PTFE stopper along with a PTFE stir bar and then placed in an oil bath at 180 • C under continuous magnetic stirring.Next, the temperature was increased to 230 • C. The mixture was heated under rapid continuous stirring for a duration of 2 h.During the reaction time, the reaction mixture appeared homogeneous and turned dark brown in color.Stirring was stopped, and the reaction was removed from the heat.The material was cooled to room temperature, forming a solid dark brown substance.Forming the compressive strength cylinders (Figure 2) and flexural strength rectangular prisms (Figure 3) required remelting the product at 180 • C and then pouring the product into molds, followed by cooling to room temperature.This method resulted in a 99.7% yield for LOS 80 @230 and a 99.8% yield for LOS 85 @230.For LOS 80 @230 elemental analysis, the theoretical values were C 12.86, H 1.22, and S 80; the actual values found were C 10.94, H 0.46, and S 84.85.For LOS 85 @230 elemental analysis, the theoretical were C 9.64, H 0.91, and S 85; the actual values found were C 3.76, H 0.05, and S 94.49.LOS 90 @230 was prepared as previously reported [108].
LOS90@250 and LOS90@300 were prepared by adding elemental sulfur (13.500 g, 13.505 g) and lignin oil (1.501 g, 1.500 g) to a stainless steel autoclave reactor and heated to 250 • C under rapid mechanical stirring.After 2 h, the heat and stirring was stopped, and the autoclave vessel cooled to room temperature.After cooling, the homogeneous, dark brown solid was removed from the reaction vessel.Forming the compressive strength cylinders (Figure 2) required remelting the product at 180 • C and then pouring the product into molds, followed by cooling to room temperature.For LOS 90 @250 elemental analysis, the theoretical values were C 6.43, H 0.61, and S 90; the actual values found were C 7.33, H 0.14, and S 91.24.For LOS 90 @300 elemental analysis, the theoretical values were C 6.43, H 0.61, and S 90; the actual values found were C 5.43, H 0.05, and S 93.06.

General Method for Dark Sulfur Quantification
The method used herein to determine the quantity of dark sulfur is based on a method previously reported by Hasell's group [109,110].This method uses absorbance to quantify dark sulfur in HSMs.A small fraction of the composite (6-7 mg) was weighed to +/−0.0001 g precision using a microbalance and placed in a 250 mL volumetric flask with ethyl acetate and stirred for 30 min.This duration of agitation was chosen due to the solubility of elemental sulfur.Ethyl acetate has a lower solubility for sulfur and not oligomeric fractions; therefore, it will extracts dark sulfur within the composite [110].The solution was then measured using a Carry-UV.A scan was taken over the range of 400-600 nm, and

Figure 11 .
Figure 11.A graphical representation of the general trend of the effect increasing the percentage of organic material has on the percentage crystallinity of LSx (blue triangles) and LOSx@T (orange circles).

Figure 12 .Table 3 .
Figure12.Graphical representation comparing the flexural strength (blue bars) and compressive strength (orange bars) of LOSx@T composites with lignin-sulfur composites (LSx), lignin/cellulosecontaining sulfur composites (ELSx@T, CLS80, mAPS95), and ordinary Portland cement (OPC).Bars not displayed signify that the attendant value was not previously reported for the listed material or could not be obtained for the current research.Table3.Mechanical properties of the lignin-sulfur composites LOSx@T with other previously reported lignin-containing HSMs composites for comparison.

Figure 12 .
Figure12.Graphical representation comparing the flexural strength (blue bars) and compressive strength (orange bars) of LOS x @T composites with lignin-sulfur composites (LS x ), lignin/cellulosecontaining sulfur composites (ELS x @T, CLS 80 , mAPS 95 ), and ordinary Portland cement (OPC).Bars not displayed signify that the attendant value was not previously reported for the listed material or could not be obtained for the current research.

Figure 13 .
Figure13.A graphical representation of the general trend of the effect increasing reaction temperature has on the compressive strength of ELS x @T (green squares) and LOS x @T (orange circles).
TGA data were recorded (Mettler Toledo TGA 2 STARe System, TA Instruments, New Castle, DE, USA) across the temperature range of 20-800 • C at a heating rate of 10 • C•min −1 under a flow of N 2 (100 mL•min −1 ).

Table 2 .
Thermal and morphological properties of lignin oil-sulfur composites, elemental sulfur, and other composites.