Enhancement of Mechanical Properties of PCL/PLA/DMSO2 Composites for Bone Tissue Engineering

Abstract

Bone tissue engineering shows potential for regenerating or replacing damaged bone tissues by utilizing biomaterials renowned for their biocompatibility and structural support capabilities. Among these biomaterials, polycaprolactone (PCL) and polylactic acid (PLA) have gained attention due to their biodegradability and versatile applications. However, challenges such as low degradation rates and poor mechanical properties limit their effectiveness. Dimethyl sulfone (DMSO₂) has emerged as a potential additive to address these limitations, offering benefits such as reduced viscosity, increased degradation time, and enhanced surface tension. In this study, we investigate tailored composites comprising PLA, PCL, and DMSO₂ to enhance mechanical properties and hydrophilicity. Through material characterization and mechanical testing, we found that the addition of DMSO₂ led to improvements in the yield strength, modulus, and hydrophilicity of the composites. PCL and DMSO₂ 10, 20, and 30 wt% were premixed, and 20 wt% PCL + 10, 20, and 30 wt% DMSO₂ were mixed with PLA. Specifically, PLA/PCL/DMSO₂ composites exhibited higher yield strengths and moduli compared to pure PLA, pure PCL, and PLA/PCL composites. Moreover, the hydrophilicity of the composites increased with DMSO₂ concentration, facilitating cell attachment. Fourier-transform infrared spectroscopy (FTIR) confirmed the presence of –COOH and –COH bands in PLA/PCL/DMSO₂ composites, indicating chemical interactions between DMSO₂ and the polymer matrix. Fractography analysis revealed enhanced interface adhesion in PLA/PCL/DMSO₂ composites due to the hydrogen bonding. Overall, this study demonstrates the potential of PLA/PCL/DMSO₂ composites in bone tissue engineering applications, offering improved mechanical properties and enhanced cell compatibility. The findings contribute to the advancement of biomaterials for additive manufacturing in tissue engineering and regenerative medicine.

1. Introduction

Bone tissue engineering has the potential to aid in the regeneration or substitution of injured bone tissues through the utilization of biomaterials [1,2,3]. Biomaterials are employed across diverse medical and biomedical contexts owing to their distinctive characteristics and harmonization with biological systems [4,5]. Biomaterials must fulfill the requirement of biocompatibility, biodegradability, strength, and durability to be used as structural support in medical implants, which did not cause side effects or rejection by the body [6,7,8,9,10,11,12]. Biomaterials can be gradually resorbed by the body, thus eliminating the need for surgical removal after the tissue regeneration process is complete [13,14]. Polycaprolactone (PCL) and polylactic acid (PLA) are frequently reported as the most promising biocompatible and biodegradable polymers synthesized and were verified as biomaterials by the US Food and Drug Administration [15]. Dimethyl sulfone (DMSO₂), well known as an organosulfur compound [16], is a non-hazardous and stable material [15]. In addition, DMSO₂ has high surface tension and precise geometric accuracy with low viscosity in additive manufacturing [17].

PCL and PLA have been widely utilized as biomaterials that have gained significant attention in recent years due to their broad range of applications, including biomedical uses [18,19], packaging [20,21,22,23], and 3D printing [24,25]. PCL is a polyester synthesized by the polymerization of the ε-caprolactone monomer, while PLA is a polyester derived from natural sources. PCL and PLA have good biocompatibility, various applications, and easy processing. On the other hand, both PCL and PLA have a low rate of degradation and low melt viscosity, leading to poor flow, low adhesion, and poor dimensional stability. In addition, PLA is relatively brittle [17], and PCL has relatively low strength and stiffness compared to other polymers [17,26]. Several studies have been conducted to investigate the properties of PLA (worked as a matrix) and PCL (worked as an additive) composites. Some studies showed that as PCL content was increased, the modulus and yield strength of PCL/PLA composites were decreased, and toughness was decreased as well [27,28].

Matta et al. [29] studied the rheological, thermal, mechanical, and viscoelastic properties of biodegradable PLA/PCL composites. The elongation, impact toughness, and loss factor decreased compared to pure PLA, while the strength and modulus decreased. SEM analysis showed that PCL was dispersed homogeneously in the PLA matrix. Luyt and Gasmi [30] reported on the influence of PCL on the crystallization behavior of PLA and PCL in various PLA/PCL composites. PCL in the composites significantly impacted the crystallization of PLA. The analysis of successive self-nucleation and annealing regarding PCL crystallization in the composites revealed a notable influence of the presence and quantity of PLA in the composites on the size distribution of crystals in PCL’s primary crystalline fraction. Aliotta et al. [31] researched a micromechanical analysis and the fracture mechanics of various PLA/PCL (from 10 up to 40 wt%) composites. Up to 20 wt% of PCL, debonding occurred, and void growth along the tensile direction occurred. When 40 wt% of PCL was applied, a significant enhancement in the elongation at break occurred due to the predominant shear strength. Hou and Qu [32] studied thermal behavior and mechanical properties. PLA/10 wt% PCL composites exhibited an optimum interfacial adhesion with an elongation at a break of 500.94% and a notched impact strength of 64.31 kJ/m². However, PCL/PLA composites have a limitation for the low rate of degradation. In order to solve this challenge, dimethyl sulfone (DMSO₂) has emerged as an additive to control degradation time.

Dimethyl sulfone (DMSO₂) is a naturally occurring organosulfur compound known for its non-hazardous and stable properties [15,33]. DMSO₂ has been reported to possess several advantageous properties that can mitigate the drawbacks of PCL/PLA composites, including a reduction in viscosity, an increase in degradation time, and improvements in geometric accuracy and surface tension [8,34,35]. Particularly, high surface tension enhances hydrophilicity, thereby increasing the composite material’s cell-attracting properties within the biomaterial matrix. However, an insufficient reporting of studies on DMSO₂, which can potentially alleviate the drawbacks of PCL/PLA composites, has been observed in the existing literature. Recently, Jang et al. [36] studied the material properties of PCL and DMSO₂ composites, with PCL acting as the matrix and DMSO₂ serving as a filler to customize the composite properties. The study identified a 15.5% decrease in the water contact angle of the PCL/PLA composites compared to the pure PCL and a decrease in yield strength along with an increase in modulus as DMSO₂ content was increased. The result was attributed to interface or adhesion challenges between PCL and DMSO₂. As a solution, they proposed the addition of a binder that functions as an adhesion promoter to increase the yield strength of PCL/DMSO₂ composites.

While various materials based on polymers have been studied, there are still limitations to their applications and manufacturing processes, especially in the context of utilizing biomaterials in additive manufacturing. In addition, the material properties of the composite system comprising a PCL/PLA matrix and DMSO₂ filler system have been subject to limited investigation. In this study, the tailored composites of a PLA + 20 wt% PCL + 10, 20, and 30 wt% DMSO₂ system were characterized, investigated to enhance mechanical properties, such as yield strength, modulus, and hydrophilicity, and compared to pure PLA, pure PCL, and PLA/PCL composites. The yield strengths, moduli, and hydrophilicities of PLA/PCL/DMSO₂ composites were increased with the concentration of DMSO₂ in the PLA matrix.

2. Materials and Methods

2.1. Materials

In this study, powder forms of PCL and PLA with a particle diameter of approximately 600 µm and the powder form of DMSO₂ with a particle diameter of 300 µm were utilized. The properties of the materials are listed in Table 1. A base composite PLA/20 wt % PCL and 10, 20, and 30 wt% DMSO₂ were used. PCL and DMSO₂ 10, 20, and 30 wt% were premixed, and 20 wt% PCL + 10, 20, and 30 wt% DMSO₂ were mixed with PLA. The compositions of PLA, PCL, and DMSO₂ are listed in Table 2. The materials were dried at 45 °C under vacuum for one day and physically mixed using an electric grinder at a rotation speed of 500 rpm for 10 min.

2.2. Specimens and Analysis

The specimens for the mechanical experiment and hydrophilicity experiment were fabricated using mold casting. The mold was designed according to the ASTM D790 [37] standard for a three-point bending test. The dimensions of the mold with three cavities were 127 mm × 25.4 mm × 3.2 mm, as shown in Figure 1. The target specimen size was 12.7 mm × 25.4 mm × 3.2 mm for the hydrophilicity experiment and 127 mm × 12.7 mm × 3.2 mm for a three-point bending test.

The powders of PLA, PCL, PLA/PCL composites, and PLA/PCL/DMSO₂ composites were placed in a mold and heated at 120 °C on a hot plate. Once fully melted, the materials were cooled to 20–25 °C for 2 h.

2.3. Hydrophilicity, Bending Test, Fractography, and Spectroscopy Analysis

Hydrophilicity was assessed through contact angle measurements using the sessile drop method, following the ASTM D7334 standard [38]. Distilled (DI) water was a common test liquid for this evaluation. In the test, a 10 μL droplet of DI water was placed on the specimen, and the angle between the droplet and the specimen was measured at the endpoints on the left and right sides. The test was repeated five times, and the contact angle was calculated as the average of these measurements.

Yield strength (0.2% offset) was determined using a three-point bending test. The tests were conducted using a bench-mounted universal testing machine, model 5ST (Tinius Olsen, Redhill, UK), which has a maximum force capacity of 5000 N and a load measuring accuracy of 0.2% within the range of 0.2% to 100% of the load cell capacity. Yield strength measurements were taken at a strain limit of 5%, with the deflection at the mid-span of the materials reaching 6.8 mm. Nine specimens per composition were tested to obtain the yield strengths.

A SEM analysis of specimen surfaces was conducted to examine the bonding mechanism. Additionally, SEM was employed to inspect the surfaces of the composites and analyze interface adhesion, both in their as-molded state and in cross-sections/surfaces after the bending test. The blade-cut surfaces of specimens were examined at the maximum deflection point following the bending tests. The surfaces of the composites were coated with Au for 4 min using an ion-sputter coater. The SEM analysis was performed using an SNE-4500M Plus system (SEC Co., Ltd., Suwon-si, Republic of Korea), while the surface coating was conducted with an MCM-100P device (SEC Co., Ltd., Suwon-si, Republic of Korea).

A spectroscopy analysis of specimen was performed to investigate chemical structure and composition after SEM analysis. The tests were conducted using FTIR equipment, specifically an iS10 Spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) equipped with a deuterated triglycine sulfate (DTGS) detector and a standard IR light source. Spectra were recorded over the range of 4000–400 cm⁻¹ with a spectral resolution of 4 cm⁻¹. An attenuated total reflectance (ATR) accessory with a diamond crystal was used for all measurements.

3. Results

3.1. Hydrophilicity of Composites

The water contact angles (WCAs) of pure PLA, pure PCL, PLA/PCL composites, and PLA/PCL/DMSO₂ composites were measured to investigate the hydrophilicity of the composites. The contact angle shapes and results of the composites are shown in Figure 2 and Figure 3.

The hydrophilicity of PLA/PCL/DMSO₂ composites was better than that of other composites (Figure 2). As the DMSO₂ content increases, the water contact angle is also decreased. As shown in Figure 3, the WCAs of pure PLA, pure, PCL, PLA/PCL composites, and PLA/PCL/10, 20, and 30 wt% DMSO₂ composites were 86.93° ± 0.60°, 88.52° ± 0.70°, 85.25° ± 0.49°, 81.88° ± 0.37°, 80.49° ± 0.65°, and 76.97° ± 0.29°, respectively. The WCAs of pure PLA, PLA/DMSO₂ composites, and PLA/PCL/DMSO₂ composites decreased by 1.8, 3.7, 7.5, 9.0, and 13% compared to that of pure PCL.

The WCA is one of the critical properties to investigate surface hydrophilicity that significantly influences biological activities, including cell attachment [39]. The WCA can vary depending on the molecular weight of the material, surface treatment, specimen structure, and other factors. In previous studies, the contact angles for PLA, PCL, and PLA/PCL composites were measured to be from 59.46° ± 0.43° to 100.0 ° ± 1.0°, from 75.40° ± 0.35° to 107.0 ° ± 2.0°, and from 66.60° ± 0.19° to 100.0° ± 3.0°, respectively, under various conditions [40,41,42,43,44]. The water contact angles measured in this study for PLA, PCL, and PLA/PCL composites fell within the contact angle ranges reported in previous studies. The contact angles for PLA, PCL, and PLA/PCL composites were observed to be in the following order: PCL > PLA > PLA/PCL composites. This study’s results, showing a decrease in the contact angle for composite materials, are consistent with previous findings [45,46].

A high contact angle, as observed in pure PCL and pure PLA, indicated low surface energy, which hampered the initial adhesion of cells [36,39,47]. DMSO₂ showed a decrease in contact angle due to its high surface energy. As DSMO₂ was added, the hydrophilicity was improved compared to ones of other composites.

Hydrophilicity can anticipate cell attachment. However, in recent studies, the results indicate that cell attachment cannot be predicted based on only hydrophilicity [48,49,50]. Some studies showed that the water contact angles were not correlated with cell attachment [45,51]. However, many studies have been researching the correlation between water contact angle and cell attachment [40,41,42,43,44,46]. Therefore, various surface modification studies aim to enhance the wettability and cell attachment properties of PLA- and PCL-based surfaces in the literature [52,53].

3.2. Mechanical Properties

The three-point bending tests for the specimens of the composites were conducted, and the stress–strain curves were plotted to determine 0.2% offset yield strengths and moduli for specimens. As shown in Figure 4 and Table 3, the highest yield strength was observed under the PLA/PCL/30 wt% DMSO₂ composite specimen. The yield strength for the PLA/PCL/30 wt% DMSO₂ composites was 2.81–239.49% higher than that of the other composites. The highest modulus was observed in the pure PLA specimen, and the modulus was 32.90–661.08% higher than that of the other composites. As DMSO₂ concentration was increased, yield strengths and moduli were increased.

In a previous study, the natural brittleness, poor elongation at break, and high modulus material for PLA and ductileness, high elongation at break, and low modulus material for PCL were verified [32,54,55]. The yield strengths and moduli for PLA, PCL, and PLA/PCL composites followed the following order: PLA > PLA/PCL composites > PCL. These findings were corroborated by previous research [32].

Yield strengths for pure PLA, pure PCL, and PLA/PCL composites vary from 21.5 MPa to 39.9 MPa, from 10.3 to 19.2 MPa, and from 16.75 MPa to 47.5 MPa, respectively, according to the literature [31,56,57,58,59,60]. The measured yield strengths for PLA, PCL, and PLA/PCL composites in this study were consistent with the yield strength ranges reported in previous studies. The moduli for pure PLA, pure PCL, and PLA/PCL composites were investigated to be about 3000 MPa, 417 MPa, and 2900 MPa, respectively [31,59,61]. The moduli measured in this study exhibited findings comparable to those reported in the literature review.

The elongation of PCL was 227.74% higher than that of PLA. However, the yield strength and modulus of PCL were 137.70% and 661.08% lower than those of PLA, respectively. The yield strength and modulus of PLA/PCL composites increased by 101.24% and 411.32%, respectively, compared to those of pure PCL. However, the elongation decreased by 220.50% compared to that of pure PCL. As the DMSO₂ content increased, the elongation increased by 12.50–14.96%, accompanied by increases in yield strength and modulus. Moreover, the elongation and yield strength of PLA/PCL/30 wt% DMSO₂ composites increased by 86.75% and 42.84%, respectively, compared to those of PLA.

3.3. Fractography Analysis

The surface and cross-sections of the specimens were analyzed before and after the three-point bending test to investigate the interface adhesion challenges using SEM. The SEM images are shown in Figure 5 and Figure 6.

Figure 5 shows the surface of the composites before the bending test. The surface of the PLA/PCL composites (Figure 5c) transitioned to a rough surface as PCL fillers were added into the PLA matrix. Pure PLA (Figure 5a) and pure PCL (Figure 5b) surfaces exhibited clear surfaces without any defects and agglomeration challenges [19]. However, the surface of the PLA/PCL composites (Figure 5c) became rough due to interface adhesion issues [19,31,62].

The surface of the PLA/PCL/DMSO₂ composites (Figure 5d–f) transitioned to a smooth surface as the DMSO₂ content increased compared to the surface of PLA/PCL composites (Figure 5c). This is the reason why PLA, PCL, and DMSO₂ composites were formed through a bonding reaction, whereas PLA and PCL composites were connected by a mechanical anchoring effect [36,63,64].

Figure 6 shows the cross-sectioned surfaces of the composites after the bending test. The cross-sectioned surface of the composites transitioned to a brittle–ductile combination state, characterized by a debonded PCL filler, as PCL was added into the PLA matrix. As shown in Figure 6a, the pure PLA matrix exhibited brittle characteristics, consistent with the references [32,65], and pure PLA demonstrated the highest yield strength and modulus [32,56]. On the other hand, as shown in Figure 6b and the stress–strain curve (Figure 4), the cross-sectioned surface exhibited ductile characteristics, with the matrix being elongated. Figure 6c shows the surface with a brittle–ductile combination characteristic. The PLA/PCL composites showed that the yield strength and modulus exhibited an increase compared to pure PCL, and the composites demonstrated greater ductility than pure PLA, as depicted in the stress–strain curve (S-S curve) and scanning electron microscopy (SEM) image. In addition, a debonded PCL particle was observed on the PLA matrix, consistent with previous studies by various researchers [19,29,31,32]. In this study, the debonding of the PCL filler on the PLA matrix was confirmed.

The cross-sectioned surface transitioned to a ductile state with an increase in DMSO₂ content. Figure 6d depicts a surface with both brittle and ductile characteristics. The brittle characteristics are attributed to PLA/PCL composites, while the addition of DMSO₂ induces ductile characteristics. As shown in Figure 6e,f, it is evident that the PLA matrix exhibits significant elongation compared to other composites, excluding pure PCL. The composite surface reveals numerous dimples and regions displaying plastic deformation, indicative of ductile fracture. Moreover, with an increase in DMSO₂ concentration, the ductility of the surface also increased. This outcome aligns directly with the enhancement in ductility (elongation) observed during bending tests. The presence of some porosities is attributed to the extraction of PCL particles [57].

Figure 7 shows the FTIR spectra of pure PLA, pure PCL, PLA/PCL composites, and PLA/PCL/DMSO₂ composites. As shown in Figure 7a–c, the presence of CH₂, C=O, CH₃, and C–O peaks are indicated. CH₂ stretching bands were observed at 2995–2915 cm⁻¹, a C=O stretching band appeared at 1760–1670 cm⁻¹, a CH₃ bending band was found at 1480–1346 cm⁻¹, and a C–O stretching band was noticed at 1210–1000 cm⁻¹. As shown in Figure 7d–f, the FTIR spectra were based on the spectrum of PLA/PCL composites, and –COOH bands and –COH bands, based on hydrogen at 1320–1210 cm⁻¹ and 1127–1042 cm⁻¹, respectively, were observed for PLA/PCL/DMSO₂ composites.

The FTIR spectra of PLA/PCL composites indicated the absence of any chemical bonding reactions. Maleki et al. [66] demonstrated that the FTIR spectra for PLA/PCL composites do not show the presence of any chemical bonding between PLA and PCL. In the results, no new peaks were observed for the PLA/PCL composites in the wavenumber range of 4000–400 cm⁻¹, indicating that PLA and PCL had no chemical interactions or chemical decomposition [66,67,68,69,70].

The bands associated with hydrogen bonding were observed in FTIR spectra analysis for PLA/PCL/DMSO₂ composites. According to previous studies [71,72,73,74], DMSO or SO₂ composites exhibited a –COOH or –OH hydrogen band in a range of 1320–1210 cm⁻¹ or 1366–1294 cm⁻¹, respectively, in the FTIR analysis. In this study, –COOH bands and –COH bands, corresponding to hydrogen stretching vibrations at 1320–1210 cm⁻¹ and 1127–1042 cm⁻¹, respectively, were observed for PLA/PCL/DMSO₂ composites [64,71,72,75]. The FTIR results confirmed that the hydrogen bonding mechanisms were formed in PLA/PCL/DMSO₂ composites. The spectra for the PLA/PCL/DMSO₂ composites revealed the presence of hydrogen bands, and as the DMSO₂ content increased, the intensity of these hydrogen bands also increased. Consequently, ductility and yield strength increased, attributed to the enhanced intensity hydrogen bands with an increase in DMSO₂ content [76]. Additionally, SO₂ can be copolymerized with other polymers to obtain polysulfides, which exhibit excellent properties due to the enriched diversity of polymers [64]. Therefore, the polymerized SO₂ could enhance the interface adhesion between PLA and PCL, resulting in improved mechanical properties. However, to identify the detailed bonding mechanisms, further studies are needed through the blend melt mixing process of the composites.

4. Conclusions

In this study, the tailored composites comprising PLA with 20 wt% PCL and 10, 20, and 30 wt % DMSO₂ were characterized and investigated to enhance mechanical properties, including yield strength, modulus, and hydrophilicity. The following conclusions were derived:

• When DMSO₂ was added, the hydrophilicity was improved due to its high surface energy. The highest hydrophilicity was observed in the PLA/PCL/30 wt% DMSO₂ composite. The contact angle of PLA/PCL/30 wt% DMSO₂ composites decreased by 4.57–14.99% compared to other composites. Therefore, in order to enhance the initial cell attachment, the addition of DMSO₂ was necessary.

• As DSMO₂ content increased, yield strengths and moduli were increased. The highest yield strength was observed in the PLA/PCL/30 wt% DMSO₂ composite specimen. The yield strength was 2.81–239.49% higher than that of other composites. The pure PLA specimen showed brittle behavior, and in the order of PLA/PCL composites, PLA/PCL/10,20, and 30 wt% DMSO₂ composites, and pure PCL, ductile behaviors were observed in stress–strain curves.

• In PLA/PCL/DMSO₂ composites, hydrogen bonding mechanisms were formed. The pure PLA specimen showed brittle characteristics, and the pure PCL specimen displayed ductile characteristics with the elongated matrix. In PLA/PCL composites, a PCL filler debonding challenge occurred on the PLA matrix. The PLA matrix mixed with PCL and DMSO₂ exhibited ductile characteristics with dimple features. Ductility and yield strength increased, attributed to the enhanced intensity hydrogen bands with an increase in DMSO₂ content. Therefore, the interface adhesion between PLA and PCL improved, resulting in improved mechanical properties.

For the PLA/PCL/DMSO₂ system, as DMSO₂ fillers were added and increased, yield strength and modulus were increased compared to other composites except for the pure PLA. Hydrophilicity and ductility were also improved. This study effectively customized the yield strength, hydrophilicity, and ductility for the PLA/PCL/DMSO₂ system. Therefore, the newly developed PLA/PCL/DMSO₂ composites have the potential to be extended for application as biomaterials in the field of bone tissue engineering.

Author Contributions

Conceptualization, K.-E.M., S.Y. and C.K.; methodology, K.-E.M., J.-W.J., S.Y. and C.K.; investigation, K.-E.M. and J.-W.J.; writing—original draft preparation, K.-E.M.; writing—review and editing, C.K. and S.Y.; project administration, C.K. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

Footnotes

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Figures and Tables

Figure 1. (a) Mold and (b) specimen size [36].

Figure 2. Contact angle shapes of pure PLA, pure PCL, PLA/PCL, and PLA/PCL/10, 20, and 30 wt% DMSO2 composites with DI water.

Figure 3. Contact angle of PLA, PCL, PLA/PCL, and PLA/PCL/10, 20, and 30 wt% DMSO2 composites with DI water.

Figure 4. Stress–strain curves of specimens of pure PLA, pure PCL, PLA/PCL composites, and PLA/PCL/10, 20, and 30 wt% DMSO2 composites.

Figure 5. As-molded SEM images: (a) pure PLA, (b) pure PCL, (c) PLA/PCL composites, (d) PLA/PCL/10 wt% DMSO2 composites, (e) PLA/PCL/20 wt% DMSO2 composites, and (f) PLA/PCL/30 wt% DMSO2 composites.

Figure 6. Fractography SEM images: (a) pure PLA, (b) pure PCL, (c) PLA/PCL composites, (d) PLA/PCL/10 wt% DMSO2 composites, (e) PLA/PCL/20 wt% DMSO2 composites, and (f) PLA/PCL/30 wt% DMSO2 composites (red circle: PCL particle debonded and blue circle: PCL particle pulled out).

Figure 6. Fractography SEM images: (a) pure PLA, (b) pure PCL, (c) PLA/PCL composites, (d) PLA/PCL/10 wt% DMSO2 composites, (e) PLA/PCL/20 wt% DMSO2 composites, and (f) PLA/PCL/30 wt% DMSO2 composites (red circle: PCL particle debonded and blue circle: PCL particle pulled out).

Figure 7. FTIR spectra: (a) pure PLA, (b) pure PCL, (c) PLA/PCL composites, (d) PLA/PCL/10 wt% DMSO2 composites, (e) PLA/PCL/20 wt% DMSO2 composites, and (f) PLA/PCL/30 wt% DMSO2 composites.

Table 1

Material properties of PLA, PCL, DMSO₂.

Materials	Phase at Room Temperature	Molecular Weight (g/mol)	Melting Point (°C)	Viscosity at Liquid (mPa·s)	Supplier
PLA	Solid	60,000	178 ± 2	400 @ 215 °C	NatureWorks, Waringtion, PA, USA
PCL	Solid	50,000	58 ± 2	3200 @ 100 °C	Polysciences, Waringtion, PA, USA
DMSO2	Solid	94.13	107 ± 2	1.14 @ 125 °C	Bergstrom Nutrition, Vancouver, WA, USA

Table 2

The compositions of PLA, PCL, DMSO₂.

Composite Name	Composite Ratio in Composites (wt%)
	PLA	PCL	DMSO2
PLA/PCL	80	20	0
PLA/PCL/10DMSO2	80	18	2
PLA/PCL/20DMSO2	80	16	4
PLA/PCL/30DMSO2	80	14	6

Table 3

The 0.2% offset yield strength of pure PLA, pure PCL, PLA/PCL composites, and PLA/PCL/10, 20, and 30 wt% DMSO₂ composites.

Process	0.2% Offset Yield Strength (MPa)(Mean ± Standard Deviation)	Modulus (MPa)(Mean ± Standard Deviation)
Pure PLA	32.56 ± 2.78	3227.97 ± 147.47
Pure PCL	13.7 ± 0.44	424.00 ± 2.94
PLA/PCL	27.57 ± 4.22	2168.97 ± 201.30
PLA/PCL/10 wt% DMSO2	37.81 ± 2.83	2278.63 ±109.39
PLA/PCL/20 wt% DMSO2	45.24 ± 3.13	2330.97 ± 81.72
PLA/PCL/30 wt% DMSO2	46.51 ± 1.53	2428.81 ± 26.15

References

1. Black, C.R.; Goriainov, V.; Gibbs, D.; Kanczler, J.; Tare, R.S.; Oreffo, R.O. Bone tissue engineering. Curr. Mol. Biol. Rep.; 2015; 1, pp. 132-140. [DOI: https://dx.doi.org/10.1007/s40610-015-0022-2]

2. Iranmanesh, P.; Gowdini, M.; Khademi, A.; Dehghani, M.; Latifi, M.; Alsaadi, N.; Hemati, M.; Mohammadi, R.; Saber-Samandari, S.; Toghraie, D. Bioprinting of three-dimensional scaffold based on alginate-gelatin as soft and hard tissue regeneration. J. Mater. Res. Technol.; 2021; 14, pp. 2853-2864. [DOI: https://dx.doi.org/10.1016/j.jmrt.2021.08.069]

3. Jang, J.-W.; Min, K.-E.; Kim, C.; Shin, J.; Lee, J.; Yi, S. Scaffold characteristics, fabrication methods, and biomaterials for the bone tissue engineering. Int. J. Precis. Eng. Manuf.; 2023; 24, pp. 511-529. [DOI: https://dx.doi.org/10.1007/s12541-022-00755-7]

4. Salgado, A.J.; Coutinho, O.P.; Reis, R.L. Bone tissue engineering: State of the art and future trends. Macromol. Biosci.; 2004; 4, pp. 743-765. [DOI: https://dx.doi.org/10.1002/mabi.200400026]

5. Amini, A.R.; Laurencin, C.T.; Nukavarapu, S.P. Bone tissue engineering: Recent advances and challenges. Crit. Rev. Biomed. Eng.; 2012; 40, pp. 363-408. [DOI: https://dx.doi.org/10.1615/CritRevBiomedEng.v40.i5.10]

6. Hou, Y.; Wang, X.; Yang, J.; Zhu, R.; Zhang, Z.; Li, Y. Development and biocompatibility evaluation of biodegradable bacterial cellulose as a novel peripheral nerve scaffold. J. Biomed. Mater. Res. Part A; 2018; 106, pp. 1288-1298. [DOI: https://dx.doi.org/10.1002/jbm.a.36330]

7. Gabbai-Armelin, P.R.; Souza, M.; Kido, H.W.; Tim, C.R.; Bossini, P.S.; Fernandes, K.R.; Magri, A.M.P.; Parizotto, N.A.; Fernandes, K.P.S.; Mesquit-Ferrari, R.A. Characterization and biocompatibility of a fibrous glassy scaffold. J. Tissue Eng. Regen. Med.; 2017; 11, pp. 1141-1151. [DOI: https://dx.doi.org/10.1002/term.2017]

8. Máca, J.; Vondrák, J.; Sedlaříková, M. Use of Dimethyl Sulfone as Additive in Aprotic Electrolytes. ECS Trans.; 2014; 48, 135. [DOI: https://dx.doi.org/10.1149/04801.0135ecst]

9. Nair, L.S.; Laurencin, C.T. Biodegradable polymers as biomaterials. Prog. Polym. Sci.; 2007; 32, pp. 762-798. [DOI: https://dx.doi.org/10.1016/j.progpolymsci.2007.05.017]

10. Mitragotri, S.; Lahann, J. Physical approaches to biomaterial design. Nat. Mater.; 2009; 8, pp. 15-23. [DOI: https://dx.doi.org/10.1038/nmat2344]

11. Marin, E.; Boschetto, F.; Pezzotti, G. Biomaterials and biocompatibility: An historical overview. J. Biomed. Mater. Res. Part A; 2020; 108, pp. 1617-1633. [DOI: https://dx.doi.org/10.1002/jbm.a.36930]

12. You, B.C.; Meng, C.E.; Nasir, N.F.M.; Tarmizi, E.Z.M.; Fhan, K.S.; Kheng, E.S.; Majid, M.S.A.; Jamir, M.R.M. Dielectric and biodegradation properties of biodegradable nano-hydroxyapatite/starch bone scaffold. J. Mater. Res. Technol.; 2022; 18, pp. 3215-3226. [DOI: https://dx.doi.org/10.1016/j.jmrt.2022.04.014]

13. Asghari, F.; Samiei, M.; Adibkia, K.; Akbarzadeh, A.; Davaran, S. Biodegradable and biocompatible polymers for tissue engineering application: A review. Artif. Cells Nanomed. Biotechnol.; 2017; 45, pp. 185-192. [DOI: https://dx.doi.org/10.3109/21691401.2016.1146731]

14. Wheelton, A.; Mace, J.; Khan, W.S.; Anand, S. Biomaterials and fabrication to optimise scaffold properties for musculoskeletal tissue engineering. Curr. Stem Cell Res. Ther.; 2016; 11, pp. 578-584. [DOI: https://dx.doi.org/10.2174/1574888X11666160614101037]

15. Hewlings, S.; Kalman, D. Sulfur in human health. EC Nutr.; 2019; 14, pp. 785-791.

16. Todo, M.; Park, S.-D.; Takayama, T.; Arakawa, K. Fracture micromechanisms of bioabsorbable PLLA/PCL polymer blends. Eng. Fract. Mech.; 2007; 74, pp. 1872-1883. [DOI: https://dx.doi.org/10.1016/j.engfracmech.2006.05.021]

17. Krishnan, S.; Pandey, P.; Mohanty, S.; Nayak, S.K. Toughening of polylactic acid: An overview of research progress. Polym.-Plast. Technol. Eng.; 2016; 55, pp. 1623-1652. [DOI: https://dx.doi.org/10.1080/03602559.2015.1098698]

18. Patrício, T.; Glória, A. Mechanical and biological behaviour of PCL and PCL/PLA scaffolds for tissue engineering applications. Chem. Eng Trans.; 2013; 32, pp. 1645-1650.

19. Todo, M.; Takayama, T. Fracture mechanisms of biodegradable PLA and PLA/PCL blends. Biomater.-Phys. Chem.; 2011; 19, pp. 375-394.

20. Lunt, J. Large-scale production, properties and commercial applications of polylactic acid polymers. Polym. Degrad. Stab.; 1998; 59, pp. 145-152. [DOI: https://dx.doi.org/10.1016/S0141-3910(97)00148-1]

21. Garlotta, D. A literature review of poly (lactic acid). J. Polym. Environ.; 2001; 9, pp. 63-84. [DOI: https://dx.doi.org/10.1023/A:1020200822435]

22. Auras, R.; Harte, B.; Selke, S. An overview of polylactides as packaging materials. Macromol. Biosci.; 2004; 4, pp. 835-864. [DOI: https://dx.doi.org/10.1002/mabi.200400043]

23. Averous, L.; Pollet, E. Environmental Silicate Nano-Biocomposites; Springer: Berlin/Heidelberg, Germany, 2012; Volume 1.

24. Yang, S.; Leong, K.-F.; Du, Z.; Chua, C.-K. The design of scaffolds for use in tissue engineering. Part II. Rapid prototyping techniques. Tissue Eng.; 2002; 8, pp. 1-11. [DOI: https://dx.doi.org/10.1089/107632702753503009]

25. Jariwala, S.H.; Lewis, G.S.; Bushman, Z.J.; Adair, J.H.; Donahue, H.J. 3D printing of personalized artificial bone scaffolds. 3D Print. Addit. Manuf.; 2015; 2, pp. 56-64. [DOI: https://dx.doi.org/10.1089/3dp.2015.0001]

26. Pitt, C.G. Poly-ε-caprolactone and its copolymers. Drugs Pharm. Sci.; 1990; 45, pp. 71-120.

27. Horak, Z.; Fortelny, I.; Kolarik, J.; Hlavata, D.; Sikora, A. Polymer blends. Kirk-Othmer Encycl. Chem. Technol.; 2005; 19, pp. 837-881.

28. López-Rodríguez, N.; López-Arraiza, A.; Meaurio, E.; Sarasua, J. Crystallization, morphology, and mechanical behavior of polylactide/poly (ε-caprolactone) blends. Polym. Eng. Sci.; 2006; 46, pp. 1299-1308. [DOI: https://dx.doi.org/10.1002/pen.20609]

29. Matta, A.; Rao, R.U.; Suman, K.; Rambabu, V. Preparation and characterization of biodegradable PLA/PCL polymeric blends. Proced. Mater. Sci.; 2014; 6, pp. 1266-1270. [DOI: https://dx.doi.org/10.1016/j.mspro.2014.07.201]

30. Luyt, A.; Gasmi, S. Influence of blending and blend morphology on the thermal properties and crystallization behaviour of PLA and PCL in PLA/PCL blends. J. Mater. Sci.; 2016; 51, pp. 4670-4681. [DOI: https://dx.doi.org/10.1007/s10853-016-9784-z]

31. Aliotta, L.; Gigante, V.; Geerinck, R.; Coltelli, M.-B.; Lazzeri, A. Micromechanical analysis and fracture mechanics of Poly (lactic acid)(PLA)/Polycaprolactone (PCL) binary blends. Polym. Test.; 2023; 121, 107984. [DOI: https://dx.doi.org/10.1016/j.polymertesting.2023.107984]

32. Hou, A.-L.; Qu, J.-P. Super-toughened poly (lactic acid) with poly (ε-caprolactone) and ethylene-methyl acrylate-glycidyl methacrylate by reactive melt blending. Polymers; 2019; 11, 771. [DOI: https://dx.doi.org/10.3390/polym11050771]

33. Wong, T.; Bloomer, R.J.; Benjamin, R.L.; Buddington, R.K. Small intestinal absorption of methylsulfonylmethane (MSM) and accumulation of the sulfur moiety in selected tissues of mice. Nutrients; 2017; 10, 19. [DOI: https://dx.doi.org/10.3390/nu10010019]

34. Markaryan, S.A.; Aznauryan, M.; Kazoyan, E. Physicochemical properties of aqueous solutions of dimethyl-and diethylsulfones. Russ. J. Phys. Chem. A; 2011; 85, pp. 2138-2141. [DOI: https://dx.doi.org/10.1134/S0036024411120211]

35. Loeser, R.F.; Goldring, S.R.; Scanzello, C.R.; Goldring, M.B. Osteoarthritis: A disease of the joint as an organ. Arthritis Rheum.; 2012; 64, 1697. [DOI: https://dx.doi.org/10.1002/art.34453]

36. Jang, J.-W.; Min, K.-E.; Kim, C.; Wern, C.; Yi, S. PCL and DMSO₂ composites for bio-scaffold materials. Materials; 2023; 16, 2481. [DOI: https://dx.doi.org/10.3390/ma16062481]

37. ASTM D790 Standard Test Methods for Flexural Properties of Unreinforced and Reinforced Plastics and Electrical Insulating Materials; ASTM: West Conshohocken, PA, USA, 1997.

38. D7334-08 Standard Practice for Surface Wettability of Coatings, Substrates and Pigments by Advancing Contact Angle Measurement; ASTM: West Conshohocken, PA, USA, 2013.

39. Menzies, K.L.; Jones, L. The impact of contact angle on the biocompatibility of biomaterials. Optom. Vis. Sci.; 2010; 87, pp. 387-399. [DOI: https://dx.doi.org/10.1097/OPX.0b013e3181da863e]

40. Koo, G.-H.; Jang, J. Surface modification of poly (lactic acid) by UV/Ozone irradiation. Fibers Polym.; 2008; 9, pp. 674-678. [DOI: https://dx.doi.org/10.1007/s12221-008-0106-1]

41. Sharifi, M.; Bahrami, S.H.; Nejad, N.H.; Milan, P.B. Electrospun PCL and PLA hybrid nanofibrous scaffolds containing Nigella sativa herbal extract for effective wound healing. J. Appl. Polym. Sci.; 2020; 137, 49528. [DOI: https://dx.doi.org/10.1002/app.49528]

42. Herrero-Herrero, M.; Alberdi-Torres, S.; González-Fernández, M.L.; Vilarino-Feltrer, G.; Rodríguez-Hernández, J.C.; Vallés-Lluch, A.; Villar-Suárez, V. Influence of chemistry and fiber diameter of electrospun PLA, PCL and their blend membranes, intended as cell supports, on their biological behavior. Polym. Test.; 2021; 103, 107364. [DOI: https://dx.doi.org/10.1016/j.polymertesting.2021.107364]

43. Liu, Q.; Jia, H.; Ouyang, W.; Mu, Y.; Wu, Z. Fabrication of antimicrobial multilayered nanofibrous scaffolds-loaded drug via electrospinning for biomedical application. Front. Bioeng. Biotechnol.; 2021; 9, 755777. [DOI: https://dx.doi.org/10.3389/fbioe.2021.755777]

44. Shahverdi, M.; Seifi, S.; Akbari, A.; Mohammadi, K.; Shamloo, A.; Movahhedy, M.R. Melt electrowriting of PLA, PCL, and composite PLA/PCL scaffolds for tissue engineering application. Sci. Rep.; 2022; 12, 19935. [DOI: https://dx.doi.org/10.1038/s41598-022-24275-6]

45. Oztemur, J.; Yalcin-Enis, I. Development of biodegradable webs of PLA/PCL blends prepared via electrospinning: Morphological, chemical, and thermal characterization. J. Biomed. Mater. Res. Part B Appl. Biomater.; 2021; 109, pp. 1844-1856. [DOI: https://dx.doi.org/10.1002/jbm.b.34846]

46. Lu, Y.; Chen, Y.-C.; Zhang, P.-H. Preparation and characterisation of polylactic acid (PLA)/polycaprolactone (PCL) composite microfibre membranes. Fibres Text. East. Eur.; 2016; 24, pp. 17-25. [DOI: https://dx.doi.org/10.5604/12303666.1196607]

47. Agrawal, G.; Negi, Y.S.; Pradhan, S.; Dash, M.; Samal, S. Wettability and contact angle of polymeric biomaterials. Characterization of Polymeric Biomaterials; Elsevier: Amsterdam, The Netherlands, 2017; pp. 57-81.

48. Zelzer, M.; Majani, R.; Bradley, J.W.; Rose, F.R.; Davies, M.C.; Alexander, M.R. Investigation of cell–surface interactions using chemical gradients formed from plasma polymers. Biomaterials; 2008; 29, pp. 172-184.

49. Herrwerth, S.; Eck, W.; Reinhardt, S.; Grunze, M. Factors that determine the protein resistance of oligoether self-assembled monolayers−internal hydrophilicity, terminal hydrophilicity, and lateral packing density. J. Am. Chem. Soc.; 2003; 125, pp. 9359-9366.

50. Ostuni, E.; Chapman, R.G.; Liang, M.N.; Meluleni, G.; Pier, G.; Ingber, D.E.; Whitesides, G.M. Self-assembled monolayers that resist the adsorption of proteins and the adhesion of bacterial and mammalian cells. Langmuir; 2001; 17, pp. 6336-6343. [DOI: https://dx.doi.org/10.1021/la010552a]

51. Celiz, A.; Smith, J.; Patel, A.; Langer, R.; Anderson, D.; Barrett, D.; Young, L.; Davies, M.; Denning, C.; Alexander, M. Chemically diverse polymer microarrays and high throughput surface characterisation: A method for discovery of materials for stem cell culture. Biomater. Sci.; 2014; 2, pp. 1604-1611.

52. Haddad, T.; Noel, S.; Liberelle, B.; El Ayoubi, R.; Ajji, A.; De Crescenzo, G. Fabrication and surface modification of poly lactic acid (PLA) scaffolds with epidermal growth factor for neural tissue engineering. Biomatter; 2016; 6, e1231276.

53. Oyane, A.; Uchida, M.; Yokoyama, Y.; Choong, C.; Triffitt, J.; Ito, A. Simple surface modification of poly (ε-caprolactone) to induce its apatite-forming ability. J. Biomed. Mater. Res. Part A Off. J. Soc. Biomater. Jpn. Soc. Biomater. Aust. Soc. Biomater. Korean Soc. Biomater.; 2005; 75, pp. 138-145.

54. Fortelny, I.; Ujcic, A.; Fambri, L.; Slouf, M. Phase structure, compatibility, and toughness of PLA/PCL blends: A review. Front. Mater.; 2019; 6, 206.

55. Wang, Y.; Wang, X.; Zhou, D.; Xia, X.; Zhou, H.; Wang, Y.; Ke, H. Preparation and Characterization of Polycaprolactone (PCL) Antimicrobial Wound Dressing Loaded with Pomegranate Peel Extract. ACS Omega; 2023; 8, pp. 20323-20331. [DOI: https://dx.doi.org/10.1021/acsomega.2c08180]

56. Shih, Y.-F.; Huang, C.-C. Polylactic acid (PLA)/banana fiber (BF) biodegradable green composites. J. Polym. Res.; 2011; 18, pp. 2335-2340. [DOI: https://dx.doi.org/10.1007/s10965-011-9646-y]

57. Zarei, M.; Shabani Dargah, M.; Hasanzadeh Azar, M.; Alizadeh, R.; Mahdavi, F.S.; Sayedain, S.S.; Kaviani, A.; Asadollahi, M.; Azami, M.; Beheshtizadeh, N. Enhanced bone tissue regeneration using a 3D-printed poly (lactic acid)/Ti6Al4V composite scaffold with plasma treatment modification. Sci. Rep.; 2023; 13, 3139. [DOI: https://dx.doi.org/10.1038/s41598-023-30300-z]

58. Eshraghi, S.; Das, S. Mechanical and microstructural properties of polycaprolactone scaffolds with 1-D, 2-D, and 3-D orthogonally oriented porous architectures produced by selective laser sintering. Acta Biomater.; 2010; 6, 2467. [DOI: https://dx.doi.org/10.1016/j.actbio.2010.02.002]

59. McKeen, L.W. The Effect of Long Term Thermal Exposure on Plastics and Elastomers; William Andrew: Norwich, NY, USA, 2021.

60. Solechan, S.; Suprihanto, A.; Widyanto, S.A.; Triyono, J.; Fitriyana, D.F.; Siregar, J.P.; Cionita, T. Characterization of PLA/PCL/Nano-Hydroxyapatite (nHA) biocomposites prepared via cold isostatic pressing. Polymers; 2023; 15, 559. [DOI: https://dx.doi.org/10.3390/polym15030559]

61. Farah, S.; Anderson, D.G.; Langer, R. Physical and mechanical properties of PLA, and their functions in widespread applications—A comprehensive review. Adv. Drug Deliv. Rev.; 2016; 107, pp. 367-392. [DOI: https://dx.doi.org/10.1016/j.addr.2016.06.012]

62. Ostafinska, A.; Fortelny, I.; Nevoralova, M.; Hodan, J.; Kredatusova, J.; Slouf, M. Synergistic effects in mechanical properties of PLA/PCL blends with optimized composition, processing, and morphology. RSC Adv.; 2015; 5, pp. 98971-98982. [DOI: https://dx.doi.org/10.1039/C5RA21178F]

63. Min, K.-E.; Jang, J.-W.; Yi, S.; Kim, C. Role of binder on yield strength of polycaprolactone/dimethylsulfone composites for bio-applications. J. Mater. Res. Technol.; 2023; 27, pp. 462-471. [DOI: https://dx.doi.org/10.1016/j.jmrt.2023.09.238]

64. Yue, T.-J.; Ren, W.-M.; Lu, X.-B. Copolymerization Involving Sulfur-Containing Monomers. Chem. Rev.; 2023; 123, pp. 14038-14083. [DOI: https://dx.doi.org/10.1021/acs.chemrev.3c00437]

65. Hendrick, E.; Frey, M. Increasing surface hydrophilicity in poly (lactic acid) electrospun fibers by addition of PLA-b-PEG co-polymers. J. Eng. Fibers Fabr.; 2014; 9, 155892501400900219. [DOI: https://dx.doi.org/10.1177/155892501400900219]

66. Maleki, F.; Jafari, H.; Ghaffari-bohlouli, P.; Shahrousvand, M.; Sadeghi, G.M.M.; Alimoradi, H.; Shavandi, A. Proliferation and osteogenic differentiation of mesenchymal stem cells on three-dimensional scaffolds made by thermal sintering method. Chem. Pap.; 2021; 75, pp. 5971-5981. [DOI: https://dx.doi.org/10.1007/s11696-021-01774-w]

67. Åkerlund, E.; Diez-Escudero, A.; Grzeszczak, A.; Persson, C. The effect of PCL addition on 3D-printable PLA/HA composite filaments for the treatment of bone defects. Polymers; 2022; 14, 3305. [DOI: https://dx.doi.org/10.3390/polym14163305]

68. Solechan, S.; Suprihanto, A.; Widyanto, S.A.; Triyono, J.; Fitriyana, D.F.; Siregar, J.P.; Cionita, T. Investigating the Effect of PCL Concentrations on the Characterization of PLA Polymeric Blends for Biomaterial Applications. Materials; 2022; 15, 7396. [DOI: https://dx.doi.org/10.3390/ma15207396]

69. Kemala, T.; Budianto, E.; Soegiyono, B. Preparation and characterization of microspheres based on blend of poly (lactic acid) and poly (ε-caprolactone) with poly (vinyl alcohol) as emulsifier. Arab. J. Chem.; 2012; 5, pp. 103-108. [DOI: https://dx.doi.org/10.1016/j.arabjc.2010.08.003]

70. Chee, W.K.; Ibrahim, N.A.; Zainuddin, N.; Abd Rahman, M.F.; Chieng, B.W. Impact toughness and ductility enhancement of biodegradable poly (lactic acid)/poly (ε-caprolactone) blends via addition of glycidyl methacrylate. Adv. Mater. Sci. Eng.; 2013; 2013, 976373. [DOI: https://dx.doi.org/10.1155/2013/976373]

71. Hesse, M.; Meier, H.; Zeeh, B. Spektroskopische Methoden in der Organischen Chemie; Georg Thieme: New York, NY, USA, 2005.

72. Devadoss, V.; Basha, C.A.; Jayaraman, K. Direct Oxidation of Dimethylsulphoxide and Reduction of Maleic Acid in Methanesulphonic Acid Medium. Int. J. Chem. React. Eng.; 2009; 7, pp. 1-20. [DOI: https://dx.doi.org/10.2202/1542-6580.2079]

73. Wen, Z.; Kim, W.; Yoo, S.J.; Chae, C.-G.; Seo, H.-B.; Bak, I.-G.; Changez, M.; Lee, J.-S. Highly ordered supramolecular structure built from poly (4-(4-vinylphenylpyridine)) and 1, 1′-ferrocenedicarboxylic acid via hydrogen bonding. Polym. Chem.; 2020; 11, pp. 2666-2673. [DOI: https://dx.doi.org/10.1039/D0PY00066C]

74. Kulichikhin, V.; Ilyin, S.; Mironova, M.; Berkovich, A.; Nifant’ev, I.; Malkin, A.Y. From polyacrylonitrile, its solutions, and filaments to carbon fibers: I. Phase state and rheology of basic polymers and their solutions. Adv. Polym. Technol.; 2018; 37, pp. 1076-1084. [DOI: https://dx.doi.org/10.1002/adv.21758]

75. Zhao, T.; Sha, F.; Xiao, J.; Xu, Q.; Xie, X.; Zhang, J.; Wei, X. Absorption, desorption and spectroscopic investigation of sulfur dioxide in the binary system ethylene glycol+ dimethyl sulfoxide. Fluid Phase Equilibria; 2015; 405, pp. 7-16. [DOI: https://dx.doi.org/10.1016/j.fluid.2015.06.047]

76. Baek, J.-H. Change of Properties of Epoxy Resin Blended with the Modified Epoxy; Pukyong National University: Busan, Republic of Korea, 2007.