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Communication

Thermal and Mechanical Properties of Guaiacol–Fatty Acid–Sulfur Composites

by
Charini P. Maladeniya
,
Nawoda L. Kapuge Dona
,
Ashlyn D. Smith
and
Rhett C. Smith
*
Department of Chemistry and Center for Optical Materials Science and Engineering Technology, Clemson University, Clemson, SC 29634, USA
*
Author to whom correspondence should be addressed.
Macromol 2023, 3(4), 681-692; https://doi.org/10.3390/macromol3040038
Submission received: 10 July 2023 / Revised: 17 August 2023 / Accepted: 19 September 2023 / Published: 25 September 2023
(This article belongs to the Special Issue Advances in the Mechanical Behavior of Biopolymers, Biodegradable Polymers, Their Blends and Composite Materials)
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Abstract

:
A series of six composites was prepared from the reaction of lignin-derived guaiacol, fatty acids, and sulfur. In this preparation, the organic comonomers undergo C–S bond-forming reactions to establish a highly crosslinked network material in which some non-covalently incorporated sulfur species are also entrapped. Both monounsaturated oleic acid and diunsaturated linoleic acid were used as fatty acid components to assess the influence of their unsaturation levels on composite properties. The ratio of organics and the proportion of sulfur (70 or 80 wt%) was also varied to assess the effect on thermal, morphological, and mechanical properties. Thermogravimetric analysis showed that composites exhibited good thermal stability up to ~220 °C. Differential scanning calorimetry revealed that the materials generally exhibit melting features for entrapped cyclo-S8, cold crystallization features for some materials, and a composition-dependent glass transition temperature. The flexural and compressive strengths of the composites revealed that some of the composites exhibit strengths significantly higher than those required of Portland cements used in residential housing fabrication and may be more sustainable structural materials. The thermal and mechanical properties could be tailored by changing the degree of unsaturation of the fatty acid comonomer or by altering the percentage of fatty acid in the monomer feed. The highest mechanical strength was achieved with greater amounts of monounsaturated oleic acid comonomer.

1. Introduction

Leveraging lower-value, waste, or bio-based materials to replace ecologically harmful mineral and plastic structural goods is vital for the success of a green economy [1,2]. Lignin, for example, is the most underutilized lignocellulosic biomass constituent, so lignin oils, comprising small molecular lignin breakdown products like guaiacol, are hotly pursued as chemical feedstocks for more sustainably sourced commodities [3,4,5,6,7,8,9,10,11,12]. Another abundant industry byproduct is elemental sulfur. Sulfur is a product of fossil fuel refining whose production outpaces all current economically viable uses by over 7 Mt per year [13,14,15,16,17,18,19]. Based on the abundance and affordability of sulfur, various high sulfur-content materials (HSMs) have been pursued for applications such as sulfur cements [20,21,22,23,24,25]. Owing to the thermal reversibility of S–S bond breakage and formation such HSMs are often thermally recyclable over many cycles without any loss in mechanical strength [26,27,28,29,30,31]. More sustainable sulfur composites can be prepared when petroleum-derived olefins are replaced with biologically produced monomers, such as fatty acids [26,32,33] or lignin derivatives [4,5,6,34]. Reaction temperatures for preparing or shaping HSMs can be lowered by using catalysts [35], mechanochemical methods [36], ternary mixtures [37], pre-formation of more reactive sulfur species [37,38,39], nucleophiles to fuse materials through S–S metathesis [40], or by compression-molding of materials [41]. Promising photochemical routes to C–S bond formation towards preparing or healing HSMs also hold potential for lowering the energy requirements of these processes [42].
HSMs have most often been prepared by inverse vulcanization (Scheme 1) [43,44,45,46,47], a convenient process whereby thermally generated polysulfur radicals add across olefins, typically at 159–180 °C, to create highly crosslinked networks comprising oligo- and poly-sulfur chains [13,14,15,16,17,18]. Other routes to HSMs have been developed for C–S bond formation between elemental sulfur and organic thermal decomposition products, requiring higher temperatures of 220–320 °C [5,34].
We have previously reported a guaiacol–sulfur composite (GS80, Scheme 2) formed through the reaction of guaiacol and sulfur at 230 °C to facilitate the formation of a crosslinked network via S–Caryl bond-formation [34]. GS80 is a relatively soft and flexible composite as compared to most HSMs, with relatively poor compressive strength (<10 MPa). HSMs with fatty acid crosslinkers like oleic or linoleic acid, on the other hand, are quite brittle but can exhibit compressive strengths (up to 19.4 MPa) higher than the 17 MPa required for Portland cements used in residential housing foundations [19,26,32,33].
Given the contrasting properties endowed to HSMs by guaiacol versus fatty acid monomers, we hypothesized that reacting elemental sulfur with a monomer blend comprising both guaiacol and a fatty acid may lead to composites exhibiting intermediary or tunable properties suitable for a broader range of applications than accessible to either guaiacol–sulfur or fatty acid–sulfur composites. Herein we report composites of sulfur and guaiacol with either oleic acid or linoleic acid to give composites GOxSy or GLxSy, respectively (Scheme 3, x = wt% fatty acid, y = wt% sulfur, with the mass balance of guaiacol). The influences of constituent ratios on thermomorphological properties, flexural strength, and compressive strength are discussed.

2. Materials and Methods

2.1. Instrumentation

Thermogravimetric analysis (TGA) was recorded on a Mettler Toledo TGA 2 STARe System from 25–800 °C with a heating rate of 5 °C min−1 under a flow of N2 (20 mL·min−1).
Differential scanning calorimetry (DSC) data were acquired using a Mettler Toledo DSC 3 STARe System from −60 to 140 °C, with a heating rate of 5 °C min−1 under a flow of N2 (200 mL·min−1). Each DSC measurement was carried out over three heat–cool cycles to screen out thermal history. The data reported were taken from the third cycle of the experiment to minimize thermal history effects and allow for their comparison to the reported data for other HSMs discussed in this report.
Compressive measurements were acquired on cylinders (Figure 1) using a Mark-10 ES30 Manual Test Stand equipped with a Mark 10 M3-200 Force Gauge by a modified ASTM C39 standard. Terpenoid–sulfur composite materials were aged for 4 d prior to compressive strength testing. The 4 d aging period was selected after assessing material properties over shorter and longer times for one set of samples and the properties were leveled off after 4 d (Figure S2, Supplementary Materials).
Flexural strength was determined via dynamic mechanical analysis (DMA) using a Mettler Toledo DMA 1 STARe System in single cantilever mode. DMA samples were cast from silicone resin molds using a commercial Smooth-On Oomoo® 30 tin-cure kit. Samples were manually sanded to ensure uniform dimensions of approximately 15 × 8 × 1.5 mm. Sample dimensions were measured using a digital caliper with 0.01 mm resolution. The force was varied from 0 to 10 N with a ramp rate of 0.2 N·min−1 measured isothermally at 25 °C.
SEM was acquired on a Schottky Field Emission Scanning Electron Microscope SU5000 operating in variable pressure mode with an accelerating voltage of 15 keV.

2.2. Materials and Precautions

Guaiacol (98%, TCI America, Portland, OR, USA), elemental sulfur (99.5%, Alfa Aesar, Ward Hill, MA, USA), linoleic acid (98%, Acros Organics, Verona, Italy), and oleic acid (99%, Alfa Aesar, Ward Hill, MA, USA) were used without further purification.
CAUTION: Heating elemental sulfur with organics can result in the formation of H2S gas. H2S is toxic, foul-smelling, and corrosive. Heating sealed tubes presents significant danger of the vessel bursting and such reactions must be carried out behind ballistic glass or other shielding that can withstand vessel failure [48,49,50].

2.3. Synthesis

To a heavy-walled glass pressure tube (Chemglass CG-1880-01, maximum pressure rating 150 PSI) sealed with a Viton O-ring-equipped Teflon stopper were added a Teflon-coated magnetic stir bar, elemental sulfur, and the requisite organics according to the ratios provided in Table 1. The flask was sealed under an atmosphere of N2 in a glove box and then transferred to an oil bath at 230 ± 5 °C. The reaction mixture was stirred for 24 h, after which the reaction mixture was a black, visually homogenous solution. Cooling to room temperature gave the requisite materials in quantitative yield. Amounts used to prepare each composite are indicated below.

2.3.1. GO1S70

This material was made according to the general procedure described above using 7 g of sulfur, 0.1 g of oleic acid, and 2.9 g of guaiacol, and was isolated in quantitative yield as a black solid. IR spectra and TGA traces are provided in the Supporting Information. DSC data are provided in Table 2 and Figure 2.

2.3.2. GO5S70

This material was made according to the general procedure described above using 7 g of sulfur, 0.5 g of oleic acid, and 2.5 g of guaiacol, and was isolated in quantitative yield as a black solid. IR spectra and TGA traces are provided in the Supporting Information. DSC data are provided in Table 2 and Figure 2.

2.3.3. GO10S80

This material was made according to the general procedure described above using 8 g of sulfur, 1 g of oleic acid, and 1 g of guaiacol, and was isolated in quantitative yield as a black solid. IR spectra and TGA traces are provided in the Supporting Information. DSC data are provided in Table 2 and Figure 2.

2.3.4. GL1S70

This material was made according to the general procedure described above using 7 g of sulfur, 0.1 g of linoleic acid, and 2.9 g of guaiacol, and was isolated in quantitative yield as a black solid. IR spectra and TGA traces are provided in the Supporting Information. DSC data are provided in Table 2 and Figure 2.

2.3.5. GL5S70

This material was made according to the general procedure described above using 7 g of sulfur, 0.5 g of linoleic acid, and 2.5 g of guaiacol, and was isolated in quantitative yield as a black solid. IR spectra and TGA traces are provided in the Supporting Information. DSC data are provided in Table 2 and Figure 2.

2.3.6. GL10S80

This material was made according to the general procedure described above using 8 g of sulfur, 1 g of linoleic acid, and 1 g of guaiacol, and was isolated in quantitative yield as a black solid. IR spectra and TGA traces are provided in the Supporting Information. DSC data are provided in Table 2 and Figure 2.

3. Results and Discussion

3.1. Synthesis and Surface Properties

Preparation of guaiacol–fatty acid–sulfur composites was accomplished by the reaction of guaiacol, the requisite fatty acid, and elemental sulfur in the ratios shown in Table 1 under conditions we previously reported for the preparation of composite GS80 (Scheme 2). These reactions all produced quantitative yields of dark solids that could be readily remelted when heated above 150 °C, facilitating their fabrication by simply pouring the molten materials into silicone molds and allowing them to cool to room temperature (25 °C). In this way, variously shaped samples needed for mechanical tests, such as cylinders for compressional strength testing (Figure 1a), and rectangular prisms for flexural strength testing (Figure 1b) were prepared.
Because the oleic acid-containing composites proved most promising from the standpoint of mechanical strength (vide infra), the surface properties of those composites were further investigated. Scanning electron microscopy with elemental mapping by energy dispersive X-ray analysis (SEM-EDX) was used to analyze composites GO1S70, GO5S70, and GO10S80 (Figure 3). The elemental mapping from EDX shows the expected abundance of elements C, O and S, and good distribution of carbon and oxygen throughout the majority of the sulfur composites without significant observable bulk macroscale phase separation other than the aforementioned sulfur crystallites in GO1S70.
The composites were also characterized by Fourier-transform infrared spectroscopy (FTIR, Figure S1, Supplementary Materials). Most notably, IR spectra revealed consumption of olefin units as evidenced by the disappearance of out-of-plane alkene C–H bends, which appear at ~933 cm−1 in both oleic and linoleic acid, but which are absent or very weak in the composites.

3.2. Thermal Properties

The thermally induced mass-loss properties of the composites were assessed by thermogravimetric analysis (TGA, Table 2, original traces for all materials are provided in Figure S2, Supplementary Materials). HSMs comprising fatty acids and majority constituent sulfur generally exhibit a Td (5% mass loss under N2) at around the same temperature (229 °C), for example compounds ZOS90, ZPLS90, ZPLS95 and ZPLS99 in Table 2 (Td values of 220–231 °C) [24,30]. This trend holds for materials comprising ≥80 wt% S in the current series as well. Lower Td values were observed, however, for several of the materials having the highest percent composition of the organic components, with two examples having Td below 200 °C, specifically for GO1S70 (Td = 159 °C) and GO5S70 (Td = 184 °C), reflecting the lower thermal stability of the organic component. The fatty acid has a notable impact on the Td, with higher values (by 16–60 °C) uniformly observed in each GLxSy composite when compared to its GOxSy analogue. The improved thermal stability may be attributable to the greater crosslinking afforded by linoleic versus oleic acid. Such a trend in Td values of HSMs has been noted in some previous cases [30,51,52], but is not universally observed [41].
The thermomorphological properties of the composites were assessed by differential scanning calorimetry (DSC, Figure 2 and Table 2). Polymeric sulfur domains within the crosslinked network produced a diagnostic glass transition temperature (Tg) near −37 °C, while the melting temperature (Tm) around 118 °C reflects the presence of entrapped cyclo-S8 [53,54,55]. In the oleic acid-containing composites, the cyclo-S8 melting feature was only observed in GO1S70, consistent with the observation of cyclo-S8 crystals in the SEM-EDX image for this composite (Figure 3). Cold crystallization features were also generally observed in HSMs due to morphological changes in partially organized oligomeric/polymeric sulfur catenate domains [41,56,57,58,59,60]. The cold crystallization temperatures (Tcc) for the current guaiacol–fatty acid–sulfur composites ranged from 21 to 44 °C, somewhat lower than observed in the composites of 80–90 wt% sulfur with guaiacol (GS80, Tcc = 57 °C) or linoleic acid (ZPLS90, Tcc = 87 °C). Several of these notable morphological changes underwent progressive changes as the monomer composition was varied (Figure 2). For composites containing linoleic acid, when the organic component was 30 wt%, composites showed significant cold crystallization features. In contrast, for composites comprising 20 wt% organics, the cold crystallization and melting features were absent as longer sulfur chains and lower crosslink density dictated material properties. Results from oleic acid-containing composites exhibited this same general pattern. Oleic acid provides fewer crosslinking sites (one unsaturation) than linoleic acid (two degrees of unsaturation), so oleic acid-containing composites have longer average sulfur crosslink lengths [2]. As a result, cold crystallization features were not observed in the GO5S70 but were prominent in its homolog GL5S70.

3.3. Mechanical Properties

A wide range of organic monomer mixtures has been employed in preparing HSMs for which mechanical strength properties have been reported, but knowledge of how comonomer composition and additives may be used to tune mechanical properties is in its early stages, bolstered by insightful recent studies by the Chalker and Hasell groups [16,61,62,63]. The compressive and flexural strengths of the composites were thus assessed for comparison to those of other HSMs and to delineate any predictable trends in these metrics with composition (Table 3: representative stress-strain plots are provided in Figure S3 for compressional and Figure S4 for flexural analyses). All measurements were performed in triplicate and the errors reported were the standard deviations of the set of measurements for each material.
Portland cement used for building foundations is required by the American Concrete Institute (code 332.1R-06) to have a minimum compressive strength of 17 MPa and flexural strength ranges from 2 to 7 MPa [43]. The current composites generally have low flexural strength and are brittle materials. However, several of the materials exhibit promising compressive strengths for load-bearing applications, notably GO10S80, which has a compressive strength of 30.4 ± 0.5 MPa.
An interesting trend in compressive strength with respect to the amount and identity of the added fatty acid was observed (Figure 4). First, oleic acid-containing composites outperform linoleic acid-containing composites for a given percent of fatty acid composition. Second, a higher amount of fatty acid relative to guaiacol significantly improves compressive strength performance. When compared to the DSC data, these trends suggest that fewer sites for crosslinking, which generally correlates with longer sulfur crosslinking chains [2,32], leads to materials with greater compressive strength. Flexural strength does not follow this same trend. In fact, composites having the highest fatty acid content have by far the lowest flexural strengths of all the composites. Prior studies show that aryl-crosslinked HSMs generally have the highest flexural and tensile properties among HSMs [42,54,57]. Although it is clear that replacing aryl guaiacol with fatty acid crosslinkers diminishes the flexural strength in the current case as well, a molecular basis for this phenomenon is unclear.

4. Conclusions

The current study employs lignocellulosic biomass-derived guaiacol, low nutritional value fatty acids, and fossil fuel processing by-product sulfur to prepare durable composites simply by heating the components together in a single reaction stage. During this process, the fatty acid-derived olefins undergo inverse vulcanization with sulfur to form new C–S bonds and create a crosslinked network that appears primarily responsible for dictating the mechanical strength of the composites. Guaiacol undergoes thermal decomposition during heating to form additional C–S bonds within the network. The resultant materials are best described as composites in which some non-covalently incorporated sulfur species is entrapped within the network. The materials show uniform distribution of elements on the micron level, ruling out any macroscopic phase separation. The extent to which cold crystallization features are observed tracks with the degree of unsaturation of the organic crosslinking components, but the observed Tg values reflect the presence of oligomeric and polymeric sulfur chains as major contributors to the thermomorphological transitions in all cases. Both the degree of unsaturation and amount of the fatty acid monomer added to the composite formulation influence on the overall composite compressive strengths, with the highest observed compressive strengths achieved when greater amounts of monounsaturated oleic acid comonomer were employed. The flexural and compressive strengths of the composites revealed that some of the composites exhibit strengths significantly higher than that of Portland cements used in residential housing fabrication. The utility of more sustainable feedstocks and waste materials/by-products in the current formulation may thus provide insight into ways to achieve more sustainable structural materials.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/macromol3040038/s1. Figure S1: (A) Region from 650–1850 cm−1 of the FTIR spectra of composites; Figure S2: TGA traces for GOxSy (A) and GLxSy (B); Figure S3: Representative stress-strain plots for compressive strength analysis of GLxSy and GOxSy; Figure S4: Representative stress-strain plots for flexural strength analysis of GLxSy and GOxSy.

Author Contributions

Conceptualization, R.C.S.; methodology, R.C.S.; formal analysis, C.P.M.; investigation, C.P.M. (lead) and N.L.K.D. (supporting); resources, R.C.S.; data curation, C.P.M.; writing—original draft preparation, R.C.S.; writing—review and editing, all authors; supervision, R.C.S.; funding acquisition, R.C.S. and A.D.S. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by The National Science Foundation grant number CHE-2203669 and a seed grant from the Animal Coproducts Research and Education Center.

Data Availability Statement

Data supporting the manuscript are provided in the Supplementary Materials.

Conflicts of Interest

The authors declare no conflict of interest.

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Macromol 03 00038 sch001
Scheme 1. Inverse vulcanization occurs when elemental sulfur reacts with olefins, resulting in high sulfur-content materials (HSMs) comprising crosslinked networks.
Scheme 1. Inverse vulcanization occurs when elemental sulfur reacts with olefins, resulting in high sulfur-content materials (HSMs) comprising crosslinked networks.
Macromol 03 00038 sch001
Macromol 03 00038 sch002
Scheme 2. The reaction of guaiacol with elemental sulfur leads to formation of composite GS80.
Scheme 2. The reaction of guaiacol with elemental sulfur leads to formation of composite GS80.
Macromol 03 00038 sch002
Macromol 03 00038 sch003
Scheme 3. The reaction of guaiacol and fatty acids with elemental sulfur leads to formation of (a) composites GOxSy and (b) GLxSy. This is a random polymerization, so organic units attached to each end of a sulfur chain are expected to be variable.
Scheme 3. The reaction of guaiacol and fatty acids with elemental sulfur leads to formation of (a) composites GOxSy and (b) GLxSy. This is a random polymerization, so organic units attached to each end of a sulfur chain are expected to be variable.
Macromol 03 00038 sch003
Macromol 03 00038 g001
Figure 1. (a) Representative photos of compressive cylinders and (b) rectangular prisms for flexural strength testing of the guaiacol–fatty acid–sulfur composites. All of the composites have the same visual appearance. Samples of GO10S80 are shown as an example.
Figure 1. (a) Representative photos of compressive cylinders and (b) rectangular prisms for flexural strength testing of the guaiacol–fatty acid–sulfur composites. All of the composites have the same visual appearance. Samples of GO10S80 are shown as an example.
Macromol 03 00038 g001
Macromol 03 00038 g002
Figure 2. DSC traces for composites.
Figure 2. DSC traces for composites.
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Figure 3. Surface analysis of the composites by SEM (top row) with elemental mapping by EDX (rows 2–4). The scale bar in each image is 100 microns.
Figure 3. Surface analysis of the composites by SEM (top row) with elemental mapping by EDX (rows 2–4). The scale bar in each image is 100 microns.
Macromol 03 00038 g003
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Figure 4. A plot of compressive strength versus fatty acid content for composites comprising 70 wt% sulfur.
Figure 4. A plot of compressive strength versus fatty acid content for composites comprising 70 wt% sulfur.
Macromol 03 00038 g004
Table 1. Monomer feed mass ratios used to prepare guaiacol–fatty acid–sulfur composites.
Table 1. Monomer feed mass ratios used to prepare guaiacol–fatty acid–sulfur composites.
Materials [a]Guaiacol
(wt%)		Oleic Acid
(wt%)		Linoleic Acid
(wt%)		Sulfur
(wt%)		Reference
GO1S70291070This work
GO5S70255070This work
GO10S801010080This work
GL1S70290170This work
GL5S70250570This work
GL10S801001080This work
GS80200080[42]
ZOS90 [b]010090[24,30]
ZPLS90 [b],[c]001090[30]
ZPLS95 [b],[c]00595[30]
ZPLS99 [b],[c]00199[30]
[a] Material abbreviations are as reported in the referenced publications to allow interested readers to readily refer to those references. [b] 1 wt% of ZnO was added. The remaining wt% values still add up to 100% due to rounding. [c] In the referenced work the authors used pure and technical grade linoleic acid. The ‘P’ in these abbreviations indicates the authors used pure (>95%) linoleic acid as was used in the current work.
Table 2. Thermal properties of sulfur and composites.
Table 2. Thermal properties of sulfur and composites.
MaterialsTd/°CTg/°C Tm/°CTcc/°CReference
S8229NA119NA [a][2]
ZOS90220–39109NA [a][24,30]
ZPLS90231–3910787[30]
ZPLS95231–3911734[30]
ZPLS99252–39118NA [a][30]
GS80264–3010757[42]
GO1S70159–2310744This work
GO5S70184–13114NA [a]This work
GO10S80201–3399NA [a]This work
GL1S70209–3111229This work
GL5S70204–3511521This work
GL10S80217–37112NA [a]This work
[a] Not applicable (the feature is not observed in DSC analysis for the material).
Table 3. Mechanical properties of composites and sulfur.
Table 3. Mechanical properties of composites and sulfur.
MaterialsCompressive Strength
(MPa)		Flexural Strength
(MPa)		Reference
S8ND [a]<0.3[2]
GL10S8015.4 ± 0.20.3This work
GO10S8030.4 ± 0.50.3This work
GL5S706.8 ± 0.12.3This work
GO5S7012.5 ± 0.21.9This work
GL1S705.2 ± 0.22.4This work
GO1S706.2 ± 0.11.9This work
GS803.2 ± 0.2ND[42]
ZOS9019.4 ± 1.8<0.3[24,30]
ZPLS90ND [a]2.0[30]
ZPLS95ND [a]1.6[30]
ZPLS99ND [a]1.3[30]
[a] Not determined. These data were not reported in the publication where the material was reported.
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Maladeniya, C.P.; Kapuge Dona, N.L.; Smith, A.D.; Smith, R.C. Thermal and Mechanical Properties of Guaiacol–Fatty Acid–Sulfur Composites. Macromol 2023, 3, 681-692. https://doi.org/10.3390/macromol3040038

AMA Style

Maladeniya CP, Kapuge Dona NL, Smith AD, Smith RC. Thermal and Mechanical Properties of Guaiacol–Fatty Acid–Sulfur Composites. Macromol. 2023; 3(4):681-692. https://doi.org/10.3390/macromol3040038

Chicago/Turabian Style

Maladeniya, Charini P., Nawoda L. Kapuge Dona, Ashlyn D. Smith, and Rhett C. Smith. 2023. "Thermal and Mechanical Properties of Guaiacol–Fatty Acid–Sulfur Composites" Macromol 3, no. 4: 681-692. https://doi.org/10.3390/macromol3040038

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