Utilization of bamboo biochar as a multi-functional filler of flexible poly(L-lactide)-b-poly(ethylene glycol)-b-poly(L-lactide) bioplastic

Abstract

Biodegradable poly(L-lactide)-b-poly(ethylene glycol)-b-poly(L-lactide) (PLLA-PEG-PLLA) triblock copolymer could potentially be used in bioplastic applications because it is more flexible than PLLA. However, investigations into modifying PLLA-PEG-PLLA with effective fillers are still required. In this work, bamboo biochar (BC) was used as an eco-friendly and cost-effective filler for the flexible PLLA-PEG-PLLA. The influences of BC addition on crystallization properties, thermal stability, hydrophilicity, and mechanical properties of the PLLA-PEG-PLLA were explored and compared to those of the PLLA. The PLLA-PEG-PLLA matrix and BC filler were found to have strong interfacial adhesion and good phase compatibility, while the PLLA/BC composites displayed weak interfacial adhesion and poor phase compatibility. For the PLLA-PEG-PLLA, the addition of BC induced a nucleation effect that was characterized by a decrease in the cold crystallization temperature from 76 to 71–75 °C and an increase in the crystallinity from 18.6 to 21.8–24.0%; however, this effect was not observed for the PLLA. When compared to pure PLLA-PEG-PLLA, the PLLA-PEG-PLLA/BC composites displayed greater thermal stability, tensile stress, and Young’s modulus. Temperature at maximum decomposition rate (T_d,max) of PLLA end-blocks increased from 315 to 319–342 °C. Ultimate tensile stress of PLLA-PEG-PLLA matrix improved from 14.5 to 16.2–22.6 MPa and Young’s modulus increased from 220 to 280–340 MPa. Based on the findings, the crystallizability, thermal stability, and mechanical properties of the flexible PLLA-PEG-PLLA bioplastic were all enhanced by the use of BC as a multi-functional filler.

Introduction

Bio-based plastics are currently attracting a lot of attention for the production of eco-friendly products because they have a lower carbon footprint than petrochemical-based plastics and reduced pollution from plastic wastes¹. One of the bio-based plastics that has attracted the most attention for use in medical, pharmaceutical, agricultural, packaging, sports and automotive applications is poly(L-lactic acid) or poly(L-lactide) (PLLA)^2–5 due to its good biocompatibility and biodegradability as well as potential for production scale-up^2,3,5. Low flexibility is the main disadvantage on the applications of PLLA⁵. Triblock copolymers of PLLA-b-poly(ethylene glycol)-b-PLLA (PLLA-PEG-PLLA) were reported to be more flexible and have a faster biodegradation rate than PLLA due to the flexibility and hydrophilicity of the PEG middle-blocks^6–9. Therefore, PLLA-PEG-PLLA has potential for use as flexible bioplastics.

The PLLA has been blended with a variety of bio-based fillers to improve and modify the properties of composites and/or reduce production costs. Bio-based fillers that have been employed include natural fibers, starch, alginate, and biochar (BC)^10–14. The use of BC as an eco-friendly filler of polymers has attracted a lot of interest as an alternative to other carbon fillers that are not sustainable environmentally, such as carbon black¹⁵ or have high-cost production, such as carbon nanostructures (carbon nanotubes and graphene)^10,16. Unfortunately, weak interactions between the BC particles and the PLLA matrix decreased the tensile properties of the PLLA matrix due to their different hydrophilicities^17–21. Space gaps between the surfaces of BC particles and PLLA matrix occurred suggesting that there was only weak interfacial adhesion^20,21. PEG has been used as a compatibilizer in PLLA/BC composites to improve the interfacial adhesion and to enhance phase compatibility between PLLA-BC²⁰.

In our pervious studies, the hydrophilic PEG middle-blocks of PLLA-PEG-PLLA were shown to have the potential to enhance the interfacial adhesion with hydrophilic fillers such as thermoplastic starch (TPS)^9,22–24 and CaCO₃²⁵. The thermal stability of PLLA end-blocks of PLLA-PEG-PLLA was improved by the addition of TPS and CaCO₃, suggesting strong interfacial adhesion between the PLLA-PEG-PLLA matrices and these fillers. Addition of CaCO₃ also increased the tensile stress and Young’s modulus of the PLLA-PEG-PLLA matrices²⁵. However, no information has been reported on PLLA-PEG-PLLA composited with BC. Therefore, the objective of this study was to investigate the effects of BC addition on the crystallization, thermal stability, hydrophilicity, and mechanical properties of PLLA-PEG-PLLA. For comparison, the PLLA/BC composites were also prepared in the same conditions.

Materials and methods

Materials

A chain-extended PLLA-PEG-PLLA triblock copolymer was synthesized through ring-opening polymerization using PEG (molecular weight of 20,000 Da) and stannous octoate as the initiating system and in situ chain-extension reaction as described in our previous work²⁵. Joncryl™ ADR4368 (BASF, Thailand) was used as a chain extender. Its number-averaged molecular weight (M_n) and dispersity (Ð) obtained from gel permeation chromatography (GPC) were 108,500 Da and 2.2, respectively. The EO/LLA ratio obtained from the ¹H-NMR spectrum, as shown in Fig. S1, was 40/60 mol% (EO is the ethylene oxide repeating unit of PEG middle-blocks and LLA is the L-lactide repeating unit of PLLA end-blocks). Peaks 1 (methine protons of LLA units) and 14 (methylene protons of EO units) were used to calculate the EO/LLA ratio. Additionally, the peaks 6–9 and 11–13 of a chain extender were also assigned⁷. The melt flow index (MFI) value of PLLA-PEG-PLLA measured at 190 °C under 2.16 kg load was 31 g/10 min. PLLA (3251D) with an MFI value of 29 g/10 min was purchased from the NatureWorks LLC (MA, USA). Bamboo biochar (BC) with a BET surface area of 64.2 ± 5.7 m²/g was prepared through pyrolysis at 800 °C and sieved with 400 mesh that was supplied by Community Enterprise, Organic Agriculture Learning Center and Herbal Product Development, Tha Khae Subdistrict (Lop Buri, Thailand). An SEM image of BC is shown in Fig. 1.

Figure 1.

SEM image of bamboo biochar powder (bar scale = 10 µm).

Preparation of PLLA/BC and PLLA-PEG-PLLA/BC composites

PLLA and BC were melt blended with an internal mixer (HAAKETM Polylab OS, Rheomix Thermo Scientific, Waltham, MA, USA) at 170 °C for 6 min using a rotor speed of 100 rpm. The PLLA and BC were dried at 50 °C in a vacuum oven overnight before use. BC content was 0.5 wt%, 1 wt%, 2.5 wt% and 5 wt%. PLLA-PEG-PLLA/BC composites were prepared by the same method. The ground particles of composites were hot pressed with a compression machine (Carver Auto CH, Wabash, IN, USA) at 170 °C for 3 min without compression force before compressing at 170 °C for 3 min under 5 MPa load. The obtained film was immediately cooled for 3 min under 5 Mpa load with water-cooled plates. The composites were dried at 50 °C in a vacuum oven overnight before compression molding. The film thicknesses were in range 0.2–0.3 mm.

Characterization of PLLA/BC and PLLA-PEG-PLLA/BC composites

Scanning electron microscopy

Phase morphology of the cryo-fractured films was observed by scanning electron microscopy (SEM). The images from a JSM-6460LV scanning electron microscope (JEOL, Tokyo, Japan) were taken at 15 kV under high vacuum mode. The samples were sputter coated with gold before SEM analysis.

Differential scanning calorimetry

A differential scanning calorimeter (DSC, Pyris Diamond, PerkinElmer, Waltham, MA, USA) was used to examine the thermal transition properties of composites. For DSC scans, composites were heated for three minutes at 200 °C to eliminate thermal history before quickly quenching the composites to 0 °C. Composites were then heated under a nitrogen gas flow from 0 to 200 °C with a rate of 10 °C/min. The degree of crystallinity of the composites as determined by DSC (DSC − X_c) of PLLA crystallites was calculated using the following equation.

$$DSC-X_c\left(\%\right)=[(\mathrm{\Delta }D{H}_m-\mathrm{\Delta }D{H}_{\mathit{cc}})/\left(93.6\times W_{\mathit{PLLA}}\right)]\times 100$$ 1

where ΔH_m and ΔH_cc are the enthalpies of melting and cold-crystallization, respectively. For 100%DSC-X_c of PLLA, the ΔH_m value is 93.6 J/g²⁰. W_PLLA is the weight fraction of PLLA.

X-ray diffractometry

The crystalline structures of the composites were collected through wide-angle X-ray diffractometry (XRD) using a D8 Advanced X-ray diffractometer (Bruker Corporation, Karlsruhe, Germany) from 5 to 30° with a scan speed of 3°/min and a CuKα radiation at 40 kV and 40 mA. The degree of crystallinity as determined by XRD (XRD − X_c) of PLLA crystallites was calculated using the following equation.

$$XRD-X_c\left(\%\right)=\left[\left(A_c\right)/\left(A_c+A_a\right)\right]\times 100\%$$ 2

where A_c is the peak area of PLLA crystallites and A_a is the halo area of the amorphous phase.

Thermogravimetric analysis

Thermogravimetric analysis (TGA) was used to determine the thermal decomposition behaviors of the composites using a SDT Q600 thermogravimetric analyzer (TA Instruments, New Castle, DE, USA) with a heating rate of 20 °C/min and a nitrogen gas flow at 100 mL/min.

Water contact angle measurement

The hydrophilicity of film surfaces was investigated through average values of water contact angles using an OCA 11 contact angle goniometer (DataPhysics Instruments GmbH, Filderstadt, Germany). For this purpose, a 2.5 µL distilled water droplet was placed on film surface for 15 s before a photograph was taken. The water contact angles between the sample surface and water were then determined. The average value was obtained from five different locations on the surface of each film sample.

Tensile testing

The tensile properties of the composite films were evaluated according to the ASTM D882 with a universal test machine (LY-1066B, Dongguan Liyi Environmental Technology Co., Ltd., Guangdong, China). A film width of 10 mm, a gauge length of 50 mm, a load cell of 100 kg, and a crosshead speed of 50 mm/min were chosen. At least 5 films of each sample were tested to obtain the averaged tensile values.

Results and discussion

Phase morphology

Phase compatibility between the PLLA matrix and BC particles and between the PLLA-PEG-PLLA matrix and BC particles was investigated from SEM images of the cryo-fractured films as shown in Figs. 2 and 3, respectively. The gaps between the surfaces of BC particles and the PLLA matrices are clearly observed in Fig. 2b–e indicating poor interfacial adhesion between the hydrophobic PLLA matrix and hydrophilic BC^20,26. The aggregation of BC particles with 5 wt% load is also clearly observed in Fig. 2e.

Figure 2.

SEM images of (a) cryo-fractured PLLA film and cryo-fractured PLLA/BC composite films with BC contents of (b) 0.5 wt%, (c) 1 wt%, (d) 2.5 wt%, and € 5 wt% (some BC particles were pointed out by white arrows, all bar scales = 10 µm).

Figure 3.

SEM images of (a) cryo-fractured PLLA-PEG-PLLA film and cryo-fractured PLLA-PEG-PLLA/BC composite films with BC contents of (b) 0.5 wt%, (c) 1 wt%, (d) 2.5 wt%, and € 5 wt% (some BC particles were pointed out by white arrows, all bar scales = 10 µm).

The cryo-fractured surface of the pure PLLA-PEG-PLLA in Fig. 3a exhibited rougher surface than that of the pure PLLA in the data in Fig. 2a suggesting that the PLLA-PEG-PLLA was more flexible than the PLLA. It should be noted that stretching of PLLA-PEG-PLLA matrix before breaking was also detected as small white needles on the fractured surfaces. Interestingly, the surfaces of BC particles of the PLLA-PEG-PLLA/BC composites were closely attached to the PLLA-PEG-PLLA matrices (Fig. 3b–e) indicating that there was strong interfacial adhesion between the PLLA-PEG-PLLA matrices and BC. This can be explained by the hydrophilicity of PLLA-PEG-PLLA being higher than the PLLA because of the hydrophilic PEG middle-blocks²². This conjecture will be confirmed later by determination of water contact angle.

Some interlocking of the PLLA and PLLA-PEG-PLLA phases with the porous structures of large BC particles is shown in Fig. 4. Strong interfacial adhesion between the surface of BC particles and PLLA-PEG-PLLA matrix is clearly observed in Fig. 4b but not for the PLLA matrix in Fig. 4a. Both the PLLA and PLLA-PEG-PLLA can be incorporated into the porous structures of BC during melt blending. In the cryo-fraction step, the some PLLA phases were pulled out from the porous structures of BC but the PLLA-PEG-PLLA did not pull out. This supports the conjecture of strong interfacial adhesion between the PLLA-PEG-PLLA matrices and the BC.

Figure 4.

Physical interlocking of (a) PLLA and (b) PLLA-PEG-PLLA phases with the porous structures of large BC particles of the composites contained 5 wt% BC (All bar scales = 10 µm).

Thermal transition properties

The thermal transition properties of the composites were investigated from DSC curves as shown in Fig. 5. The DSC curves displayed glass transition temperature (T_g), cold-crystallization temperature (T_cc), and melting temperature (T_m). All the T_cc peaks were higher than 70 °C and were associated with the cold crystallization of PLLA end-blocks because the T_m value of PEG was 66 °C (Fig. S2). These DSC results and degrees of crystallinity from DSC (DSC-X_c) of the composites are summarized in Table 1. It was found that the addition of BC did not affect the T_g (58–59 °C), T_cc (100–101 °C), T_m (166–167 °C), and DSC-X_c (8.4–10.1%) of the PLLA matrices. This may be have been due to the poor interfacial adhesion between the BC filler and PLLA matrix as described above in the SEM results.

Figure 5.

DSC curves of (above) PLLA/BC and (below) PLLA-PEG-PLLA/BC composites with various BC contents.

Table 1.

Thermal transition properties of PLLA/BC and PLLA-PEG-PLLA/BC composites.

BC content (wt%)	Tg (°C)	Tcc (°C)	Tm (°C)	DSC-Xca (%)
PLLA/BC composites
–	58	100	167	8.4
0.5	58	100	166	9.9
1	59	100	166	9.5
2.5	58	101	167	8.4
5	58	100	166	10.1
PLLA-PEG-PLLA/BC composites
–	33	76	160	18.6
0.5	32	75	159	20.8
1	31	73	158	22.0
2.5	32	73, 94	159	24.0
5	32	71, 94	158	21.8

^aCalculated from Eq. (1).

For the PLLA-PEG-PLLA/BC composites, the T_g (31–32 °C) and T_m (158–160 °C) values of PLLA-PEG-PLLA matrices did not significantly shift by the addition of BC. Their expanded DSC curves of T_g regions are presented in Fig. S3. However, the T_cc peaks shifted to lower temperature as the BC content increased suggesting that the BC enhanced crystallization of the PLLA-PEG-PLLA matrix by acting as a nucleating agent^15,17. Increasing the DSC-X_c values when the BC was incorporated supported the nucleation effect of the BC. It should be noted that the second T_cc peaks were found at 94 °C after addition of 2.5 wt% and 5 wt% BC. This may be explained by the high content of BC inhibiting the chain mobility of some PLLA-PEG-PLLA chains for crystallization during the DSC heating scan because of strong interfacial adhesion between PLLA-PEG-PLLA and BC. Two opposite phenomena, such as a nucleating effect and an interference on the cold crystallization process of PLLA-PEG-PLLA chains, were then found¹⁷. The DSC-X_c values decreased from 24.0 to 21.8% when the BC content increased from 2.5 to 5 wt%.

Crystalline structures

The crystalline structures of the composites were investigated from XRD patterns as shown in Fig. 6. The pure PLLA and PLLA/BC composites in Fig. 6(above) showed completely amorphous characteristics without XRD peaks suggesting that they had poor crystallizability. The addition of BC in the range of 0.5–5.0 wt% did not improve crystallization of the PLLA matrix. Nizamuddin et al.²⁷ reported that BC did not enhance the crystallization of PLLA. The pure PLLA-PEG-PLLA in Fig. 6(below) had a broad XRD peak at 2θ = 16.5° corresponding to the PLLA crystallites²⁵. The PEG middle-blocks of PLLA-PEG-PLLA induced plasticizing effects to enhance the crystallization of PLLA end-blocks⁷. This XRD peak at 2θ = 16.5° became the sharpest as the BC content was 2.5 wt%.

Figure 6.

XRD patterns of (above) PLLA/BC and (below) PLLA-PEG-PLLA/BC composite films with various BC contents.

The degree of crystallinity assessed from XRD (XRD-X_c) values of the pure PLLA-PEG-PLLA calculated from Eq. (2) was 12.9%. The XRD-X_c values of PLLA-PEG-PLLA/BC composites were 18.7%, 20.4%, 24.4%, and 23.9% for the BC contents of 0.5 wt%, 1 wt%, 2.5 wt%, and 5 wt%, respectively. The XRD-X_c values slightly decreased from 24.4 to 23.9% when the BC content increased from 2.5 to 5 wt% according to the above DSC results. This may have been due to the aggregation of BC particles at higher ratio causing a decreased nucleation effect because the surface area between BC particles and PLLA-PEG-PLLA matrices was reduced, and which was confirmed by SEM analysis as described above. It can be concluded from the XRD results that the addition of BC enhanced crystallizability of the PLLA end-blocks. The difference between the DSC-X_c and XRD-X_c values of both the pure PLLA and pure PLLA-PEG-PLLA may result from variations in the crystallization conditions during the samples’ cooling process^28,29. The film sample was cooled under a compression force for the XRD test to limit chain mobility for crystallization, whereas no force was applied during sample cooling for the DSC test. The different X_c values from XRD and DSC tests of PLLA/BC composites may be due to the possibility of chain rearrangement causing the PLLA crystals to increase in the temperature range between T_cc and T_m during the DSC heating scan. While the X_c values of PLLA-PEG-PLLA/BC composites from DSC and XRD tests were not much different, this may be due to the good interfacial adhesion between PLLA-PEG-PLLA and BC, which may cause BC to prevent chain rearrangement of PLLA-PEG-PLLA for crystallizing of PLLA blocks during the DSC heating scan.

Thermal decomposition

The thermal decomposition behaviors of the composites were investigated from thermogravimetric (TG) and derivative TG (DTG) thermograms as shown in Figs. 7 and 8, respectively. The TG thermograms of pure PLLA and PLLA/BC composites in Fig. 7(above) indicated that their thermal decomposition behaviors were similar with a single step in temperature range 300–450 °C due to the thermal decomposition of PLLA. The pure PLLA-PEG-PLLA and PLLA-PEG-PLLA/BC composites exhibited two thermal decomposition steps in the temperature ranges 250–350 °C and 350–450 °C [Fig. 7(below)] due to thermal decompositions of PLLA end-blocks and PEG middle-blocks, respectively^22,25. Decomposition temperature at 5% weight loss (5%-T_d) of the samples are summarized in Table 2. The 5%-T_d values of PLLA/BC composites (333–335 °C) were lower than the pure PLLA (345 °C) and slightly decreased as the BC content increased. This may have been due to the loss of the moisture in BC^30,31. However, the 5%-T_d values of pure PLLA-PEG-PLLA and PLLA-PEG-PLLA/BC composites were in the range of 281–282 °C. The 5%-T_d values of PLLA-PEG-PLLA/BC composites did not significantly decrease with the addition of BC. It was observed that the thermal decompositions of PLLA end-blocks of PLLA-PEG-PLLA/BC composites shifted to higher temperature as the BC content increased. As would be expected, the residue weight at 800 °C of the composites increased as the BC content increased as reported in Table 2.

Figure 7.

TG thermograms of (above) PLLA/BC and (below) PLLA-PEG-PLLA/BC composites with various BC contents.

Figure 8.

DTG thermograms of (above) PLLA/BC and (below) PLLA-PEG-PLLA/BC composites with various BC contents.

Table 2.

Thermal decomposition of PLLA/BC and PLLA-PEG-PLLA/BC composites.

BC content (wt%)	5%-Td a (°C)	Residue weight at 800 °Ca (%)	PLLA-Td,max b (°C)	PEG-Td,maxb (°C)
PLLA/BC composites
–	345	–	379	–
0.5	335	0.34	380	–
1	335	1.42	377	–
2.5	333	2.30	375	–
5	334	4.26	376	–
PLLA–PEG–PLLA/BC composites
–	282	–	315	417
0.5	282	0.44	319	418
1	281	1.18	323	419
2.5	282	2.04	332	416
5	281	4.53	342	418

^aObtained from TG thermograms in Fig. 7.

^bObtained from DTG thermograms in Fig. 8.

The DTG thermograms in Fig. 8 show peaks of temperatures at maximum decomposition rate (T_d,max) for PLLA (PLLA-T_d,max) and for PEG (PEG-T_d,max) as also summarized in Table 2. The PLLA-T_d,max value of pure PLLA-PEG-PLLA (315 °C) was lower than that of the pure PLLA (379 °C). This was because the PEG middle-blocks acted as plasticizing sites to reduce the intermolecular forces of PLLA end-blocks⁷. The PLLA-T_d,max peaks of pure PLLA and PLLA/BC composites were nearly all values in range 375–380 °C indicating that the addition of BC had no significant effect on the thermal decomposition of PLLA matrix.

It is interesting that the PLLA-T_d,max peaks of PLLA-PEG-PLLA/BC composites significantly shifted to higher temperatures when BC was incorporated and with increasing BC content. This suggests that the addition of BC improved thermal stability of the PLLA end-blocks. This may be explained by the PEG middle-blocks enhancing interfacial adhesion between the PLLA end-blocks and BC particles which was confirmed by the SEM results which revealed denser composite structures²⁰ and strong interactions between the polymer matrix and the BC³² to improve the thermal stability of the PLLA end-blocks. However, the PEG-T_d,max peaks did not significantly shift. This may be explained by the almost composite matrices becoming deteriorated because the phases of PLLA end-blocks were thermal decomposed. Then, interactions between the PLLA-PEG-PLLA matrix and BC may have disappeared. The PLLA-T_d,max values of PLLA-PEG-PLLA/BC composites were lower than those of the PLLA/BC composites because of the lower PLLA-T_d,max value of PLLA-PEG-PLLA matrix, as described above. Improvement in thermal stability of the PLLA end-blocks of PLLA-PEG-PLLA by the addition of BC expands its processing window (temperature range between melting and decomposition temperatures) in conventional melt processing.

Water contact angles

The effect of BC on the hydrophilicity of the composites was determined from contact angles of water droplets on the film surfaces, as shown in Fig. 9. The results of water contact angles are compared in Fig. 10. The pure PLLA and pure PLLA-PEG-PLLA had water contact angles of 81.5° and 69.5°, respectively suggesting that the PLLA-PEG-PLLA had higher hydrophilicity than the PLLA, due to the hydrophilic PEG middle-blocks²². Increasing the BC contents decreased the water contact angles (increased the hydrophilicity) of both the PLLA-based and PLLA-PEG-PLLA-based composites because of hydrophilicity of BC^19,33. Aggregation of BC particles at high content of BC did not affect the reduction of water contact angle for both the composite series. This may be due to the water molecules of water droplets still being able to diffuse into the hydrophilic BC aggregates. It should be noted that the water contact angle of PLLA-PEG-PLLA/BC composites was greatly reduced compared to the PLLA/BC composites for the same BC content. This may be explained by strong interfacial adhesion between the PLLA-PEG-PLLA and BC (as above described in SEM analysis), which also induced the reduction of the water contact angle.

Figure 9.

Images of water droplets on PLLA/BC (left column) and PLLA-PEG-PLLA/BC (right column) composite films with various BC contents.

Figure 10.

Water contact angles of PLLA/BC and PLLA-PEG-PLLA/BC composite films with various BC contents.

Tensile properties

The tensile properties of the composite films were investigated from tensile curves as shown in Fig. 11 and the tensile results of PLLA-based and PLLA-PEG-PLLA-based composite films are compared in Fig. 12. The ultimate tensile stress, strain at break, and Young’s modulus of the PLLA matrices decreased as the BC content increased. Although, it has been reported that the mechanical interlocking between the polymer and the filler through the porous structures of filler improved the mechanical properties of polymer matrix^20,34; in this work, the biochar was crushed prior to sieving and consequently, the biochar’s nearly porous structures were destroyed. The interfacial adhesion between the polymer matrix and the BC was the most important factor concerning the mechanical properties of the composites. Poor interfacial adhesion between hydrophobic polymer matrix and the hydrophilic fillers can lead to repulsion between them and to decreased mechanical properties of the polymer matrices^20,35,36. The gaps at interfaces between the BC particles and the PLLA matrices from above SEM analysis indicated that there was poor interfacial bonding between the PLLA matrix and the BC²⁷.

Figure 11.

Tensile curves of (above) PLLA/BC and (below) PLLA-PEG-PLLA/BC composite films with various BC contents.

Figure 12.

Tensile properties of PLLA/BC and PLLA-PEG-PLLA/BC composite films with various BC contents.

The ultimate tensile stress and Young’s modulus of the PLLA-PEG-PLLA-based films increased as the BC content increased until the BC content was 1 wt%. The 1% BC shows the best reinforcing effect for PLLA-PEG-PLLA matrix. For PLLA-PEG-PLLA/1%BC composite, ultimate tensile stress increased from 14.5 to 22.6 MPa (increased 55.86%), Young’s modulus increased from 220 to 340 MPa (increased 54.54%). This indicates the BC acted as a reinforcement filler of the PLLA-PEG-PLLA. The strong interfacial adhesion between the composite components caused stress transfer from the PLLA-PEG-PLLA matrix to BC. The mechanical properties of the composites were improved by the effective stress transfer between the polymer matrix and filler due to strong interfacial adhesion and good phase compatibility^25,26,37. However, the stress ultimate tensile and Young’s modulus decrease when the BC content was higher than 1 wt%. This may have been due to the BC particles aggregating at high ratios. BC agglomerates caused microcrack formation and restricted the effect of stress transfer between the polymer matrix and BC particles in order to reduce the mechanical properties of the composites^19,38. The PLLA-PEG-PLLA/5 wt% BC composite still had a greater stress ultimate tensile and Young’s modulus compared to the pure PLLA-PEG-PLLA. The strain at break of the PLLA-PEG-PLLA/BC composites steadily decreased as the BC content increased because the BC particles were rigid^20,26.

Conclusions

The effects of adding BC to the flexible PLLA-PEG-PLLA were examined and compared to PLLA in terms of phase compatibility, crystallization properties, thermal stability, hydrophilicity, and tensile properties. Good phase compatibility was induced between the PLLA-PEG-PLLA matrix and the BC due to strong interfacial adhesion, but there was weak interfacial adhesion between the PLLA matrix and the BC. The T_cc peaks of the PLLA-PEG-PLLA-based composites shifted to lower temperatures and both the DSC-X_c and XRD-X_c values increased as the BC content increased, suggesting the BC enhanced nucleating effects but these crystallization properties of the PLLA-based composites did not change. The addition of BC improved the thermal stability, ultimate tensile stress, and Young’s modulus of the PLLA-PEG-PLLA but not of the PLLA. As a result, the BC could be utilized in PLLA-PEG-PLLA as a heat stabilizer and a reinforcement filler. The results suggest that the strong interfacial adhesion between the PLLA-PEG-PLLA matrix and the BC lead to enhanced crystallization properties, thermal stability, and tensile properties of the PLLA-PEG-PLLA. Consequently, the eco-friendly and cost-effective BC showed promise as a multi-functional filler for the flexible PLLA-PEG-PLLA. The PLLA-PEG-PLLA/BC composites have the potential to be environmentally friendly, fully biodegradable, and flexible packaging materials. In addition, PLLA-PEG-PLLA may be considered for use as a compatibilizer between PLLA and BC.

Author contributions

PS and YB: conceptualization, designed, and methodology as well as wrote the first draft; PS, KP, YB, YS, TP, and AM: material preparation and analysis; PS, YB, PP and MY: review and editing. All authors read and approved the final manuscript.

Funding

This research project was financially supported by Mahasarakham University.

Data availability

Data is provided within the manuscript.

Competing interests

The authors declare no competing interests.

Supplementary Information

The online version contains supplementary material available at 10.1038/s41598-024-68638-7.