1 Introduction

Polymer composite dielectrics have attracted numerous attention for outstanding flexibility, easy-to-process, and lightweight compared with ceramic materials, and they are considered the ideal choice for many power electronics, power conditioning, and pulsed power applications [1,2,3,4,5,6,7]. Yet, most of petroleum-based polymer is difficult to degrade. The waste electrical equipment based on polymer composite dielectrics can lead to “white pollution.” Poly(lactide) (PLA) is a biodegradable polymer synthesized by ring-opening polymerization of lactide, which can be derived from renewable sources such as corn, and PLA has been regarded as one of the most promising polymer matrix for polymer composite dielectrics owing to its excellent biodegradability, renewability, high strength, and easy processibility [8,9,10].

However, the poor dielectric properties of PLA restrict its application in dielectric materials. Blending PLA with particles which have excellent electrical properties is an effective way to improve the dielectric properties. The specific functional fillers, such as some carbon materials and transition metal oxides, exhibit good electrical properties [11,12,13,14,15]. Among the functional particles, carbon nanotubes (CNTs) have excellent thermal, electrical, and mechanical properties; hence, CNTs as conductive fillers are increasingly used to improve the dielectric properties of PLA composites [16,17,18]. The mechanical and dielectric properties of PLA composites containing CNTs are strongly dependent on the CNTs-polymer interactions and the dispersion of CNTs in polymer matrix, but it has been reported that usually at the interface between the CNTs and the polymer, there is just a van der Waals interaction that is able to produce only a weak normal forces and almost null shear strength [19]. In order to improve the CNTs-polymer interactions and refine the dispersion of CNTs in polymer matrix, various attempts have been made. Surface functionalization of CNTs is an effective way to improve the interaction between CNTs and polymer matrix. Raja et al. introduced UV/ozone-modified carbon nanotubes into polyurethane (PU)/PLA blends, and it was found that the composites containing modified CNTs exhibit better properties compared to the pristine CNTs loaded system, which is attributed to the higher polymer-CNTs interactions and the fine dispersion of the modified CNTs in the matrix [20]. Seligra et al. prepared PLA-based nanocomposites reinforced by Fenton reaction CNTs, and the excellent dispersion of the fillers in the PLA matrix was observed [21]. Sun and He synthesized CNT-g-poly(d-lactide) via ring-opening polymerization initiated by modified CNTs, and the results indicated that stereocomplex crystallites can be formed between the PLA matrix and CNT-g-poly(d-lactide), which leads to the strong covalent/stereocomplex CNT-matrix interactions and well dispersion of CNTs [22].

On the other hand, the brittleness of PLA also restricts its application in dielectric materials. Blending PLA with flexible biodegradable polymers such as poly(butylene adipate-co-terephthalate) (PBAT), polycaprolactone (PCL), and polybutylene succinate (PBS) is efficient in improving the toughness of PLA [23,24,25,26]. PLA/PBAT blends are highly promising materials due to the considerable mechanical strength and the extreme toughness, but the PLA/PBAT blends are thermodynamically immiscible owing to their low interfacial adhesion, and the compatibility of PLA and PBAT is the key technical point to be addressed in order to obtain materials with excellent performance. Incorporating compatibilizers into the PLA/PBAT blends can improve the phase interface affinity of PLA and PBAT, and the compatibilizers can be mainly divided into two types that include physical compatibilizer and reactive compatibilizer.

The physical compatibilizers are commonly block copolymers or graft copolymers that can emulsify the phase interface to improve the interfacial adhesion of PLA/PBAT blends. Wang et al. investigated the effect of the chain length of PLA-PBAT-PLA tri-block copolymers on the compatibilization of PLA/PBAT blends, and the results showed that the tri-block copolymer with long-chain PLA blocks achieved more effective compatibilization of PLA and PBAT compared to the short-chain PLA blocks [27]. Reactive compatibilizers can form a chemical bonding between the PLA and PBAT, thus enhancing the interfacial adhesion between the polymer phases. Wang et al. employed a multifunctional epoxy oligomer (ADR) as a reactive compatibilizer for improving the compatibility of PLA and PBAT, and it was revealed that ADR can induce the in situ formation of PLA-g-PBAT-branched copolymers and enhance the compatibility between PLA and PBAT [28]. Han et al. used environmentally friendly epoxidized soybean oil (ESO) to improve the compatibility of PLA and PBAT, and the results indicated that the ESO could react with PLA and PBAT to form a chemical bonding interface [29].

Although the pretreatment of CNTs and the introduction of compatibilizers are demonstrated to be important in improving the interaction between PLA and PBAT in PLA/PBAT blends or the interactions between CNTs and PLA in PLA/CNTs composites, to evaluate a facile way to realize the simultaneously effective control of the polymer–polymer interaction and CNTs-polymer interactions microstructure of PLA/PBAT/CNTs composites is still a challenge.

When exposed to the irradiation of high energy rays, free radicals can be formed inside the polymer matrix, and they may couple with each other to form covalent bonds, which is helpful in enhancing the interfacial adhesion between the polymer components [30, 31]. Jeon et al. blended poly(ε-caprolactone) (PCL) into PLA and compatibilized the PLA/PCL blends by electron-beam irradiation in the presence of glycidyl methacrylate (GMA), and the results showed that the electron-beam irradiation can induce the occurrence of cross-copolymerization at the PLA/PCL interface in the presence of GMA, which will lead to the improvement of interfacial adhesion in PLA/PCL blends [32]. Meanwhile, it has been reported that high energy ray irradiation has the potential to enhance the interaction force between nanofiller and polymer matrix. Tarawneh et al. prepared PLA-based thermoplastic elastomer containing CNTs and montmorillonite nanoclay, and the results indicated that γ-ray irradiation promoted better interactions and improved the compatibility between the nanofiller and TPE [33].

However, according to our knowledge, there are few works utilizing γ-ray irradiation to control the microstructure and properties of PLA/PBAT/CNTs composites. In this work, we focus on whether the γ-ray irradiation can lead to the in situ enhancement of the component interaction in the composites. An epoxy chain extender (ADR) was introduced into the composites in order to improve the processing stability of PLA and PBAT during melting blending. Trimethylolpropane trimethacrylate (TMPTMA), an irradiation sensitizer, was incorporated into the system for the in situ formation of the crosslink network inside composites during γ-ray irradiation. The effects of γ-ray irradiation on component interaction, mechanical, and dielectric properties of PLA/PBAT/CNTs composites were investigated.

2 Experimental section

2.1 Materials

PLA (4032D) was procured from Nature Works LLC, the USA, which possesses a weight average molecular mass (Mw) of 195,000 g/mol. PBAT (FBX 7011) was provided from BASF Corporation, Germany, which possesses a Mw of 40,000 g/mol. ADR (4468) was obtained from BASF Corporation, Germany, with a Mw of 7250 g/mol and an epoxy equivalent of 310 g/mol. CNTs with a 10–20-nm outer diameter ranger and 10–30-µm length ranger were purchased by Nanjing XFNANO Material Technology Co., Ltd., with a purity of over 95%. TMPTMA was provided from KPX Green Chemical Co., Ltd.

2.2 Sample preparation

The PLA/PBAT/CNTs composites were prepared using a melt compounding process employing HAPRO rheometer (RM-200C) at 180 °C and 50 rpm. The CNTs, TMPTMA, and ADR contents are fixed at 4 wt%, 3 wt%, and 2 wt%, and the composite had a PLA:PBAT composition of 70:30. The melt-blended composites were put into a vacuum flatbed vulcanizing machine and hot-pressed at 180 °C and 5 MPa.

2.3 Irradiation conditions

The samples were irradiated in vacuum at ambient conditions with a 60Co γ source at the University of Science and Technology of China, Hefei, China, at an average dose rate of approximately 3 kGy/h. Target doses of 0, 6, 12, and 24 kGy were applied. The sample preparation process is shown in Fig. S1.

2.4 Characterization

The tensile properties of the PLA/PBAT/CNTs composites were investigated by using an electrical universal testing machine (CMT4304, MTS) according to the ISO 527–3 standards. The dielectric permittivity was measured using an Agilent E4980A LCR meter at a temperature of 30 °C. The rheological behavior of the composites was obtained by a rotating rheometer (DHR-1, TA, USA) with a parallel plate geometry of 25 mm in diameter and a gap of 0.9 mm at 180 °C. The morphologies of the cryofracture surfaces of PLA/PBAT/CNTs composites were observed by using a SU-8020 field emission scanning electron microscopy (FESEM; Hitachi, Japan) at 5 kV. The atomic force microscopy (AFM) measurements were carried out using tapping mode on a Dimension ICON AFM (Bruker, USA), and AFM pictures of the cryofracture surfaces were collected in the air temperature without any extra preparation. A Talos F200X G2 high-resolution transmission electron microscopy (HRTEM; Thermo Fisher Scientific, USA) was also employed to observe the structure of the composites. The nonisothermal crystallization behavior was performed by DSC (TA Q2000). The samples were firstly heated from 40 to 200 °C and maintained for 5 min to eliminate thermal history, then cooled down to − 60 °C at a rate of 10 °C/min, and heated to 200 °C again. Thermal gravimetric analysis (TGA) was conducted to determine the thermal stability of the composites under nitrogen atmosphere. A TA Q500 model TGA instrument was employed, and samples were heated from room temperature to 600 °C at a heating rate of 10 °C/min. The dynamic mechanical analysis (DMA) test was performed from 30 to 140 °C at a heating rate of 3 °C/min and frequency of 1 Hz by using the DMA Q800 (TA, USA).

3 Results and discussion

3.1 Mechanical properties, dielectric behavior, and rheological behavior of the composites

Figure 1a, b shows the mechanical properties of PLA/PBAT/CNTs composites with different γ-ray irradiation doses. The unirradiated PLA/PBAT/CNTs composites exhibit a tensile strength of about 31.9 MPa and an elongation at break of about 160%. When the irradiation is 6 kGy, the tensile strength of the composites reaches about 42.1 MPa, and the elongation at break of the composites reaches about 230%, which suggests that the mechanical properties of PLA/PBAT/CNTs composite can be significantly improved after receiving a low dose of γ-ray irradiation. As the irradiation dose further increases, tensile strength of the composites is not obviously improved, but the elongation at break of the composites is decreased. It may be because that when the irradiation dose is further increased, the formation of more crosslinking network inside composites owing to the radical reactions induced by γ-ray can hinder the movement of polymer chain. Meanwhile, chain scission reactions can also occur inside the crosslinking network; thus, the elongation at break of the composites is decreased [34,35,36].

Fig. 1
figure 1

Mechanical properties, dielectric behavior, and rheological behavior of PLA/PBAT/CNTs composites with different irradiation dose: a, b stress–strain curve and mechanical parameters; c, d dielectric behaviors; e, f, g rheological behavior

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The dielectric constant (ε′) and dielectric loss (Tan δ) of the PLA/PBAT/CNTs composites are presented in Fig. 1c, d, individually. Compared to the unirradiated samples, the ε′ of the composites increases slightly when the irradiation dose is 6 kGy, but the Tan δ decreases significantly. Above phenomenon can be related to the enhancement of the polymer-CNTs interaction. As discussed before, when under γ-ray irradiation, the radicals can be formed in the composites, and the CNTs can trap the generated radicals, leading to the increase in the interaction between polymer matrix and CNTs. The adjacent CNTs can act as electrodes, and the polymer can act as a dielectric to assemble the microcapacitor, which greatly increases the interfacial polarization and enhances the ε′ [37,38,39,40]. In addition, the enhanced interaction between the matrix and the filler can effectively reduce the Tan δ. When the irradiation dose further increases to 12 kGy, the ε′ and Tan δ decrease compared to the composite with an irradiation dose of 6 kGy. It is because when the irradiation dose increases, the chain segment motion of the polymer matrix is also inhibited owing to the more densely crosslinking networks in the polymer matrix; thus, the Tan δ of the composites is reduced. Moreover, the structural change of CNTs may lead to the change in the interfacial structure between polymer matrix and CNTs, resulting in the reduced interfacial polarization; hence, the ε′ of the composites is decreased. Interestingly, when the irradiation dose reaches 24 kGy, the ε′ of the composites increases significantly, while the Tan δ remains lower than that of the unirradiated samples. It is possible that high-dose irradiation can further change the interfacial structure between CNTs and polymer matrix [41]. Meanwhile, further aggravation of chain scission reactions can increase the molecular dipole orientation [42]. The smaller molecular size increases the number of interfaces per unit volume, which favors an increase in interfacial polarization, leading to an increase in the ε′ [43,44,45,46]. According to rheological results, a three-dimensional network of CNTs is formed when the irradiation dose reaches 24 kGy, indicating the formation of a conductive pathway, which causes an increase in leakage current, and ultimately a relatively high Tan δ compared to the irradiated sample [37, 47,48,49]. Overall, γ-ray irradiation has a strong effect on the interfacial structure between CNTs and polymer matrix, which leads to charge accumulation and induces interfacial polarization, resulting in lower Tan δ [43, 50].

Figure 1e−g displays the storage modulus (G′), loss modulus (G″), and complex viscosity (η*) of unirradiated and irradiated PLA/PBAT/CNTs composites, respectively. It should be pointed out that when the irradiation dose reached 24 kGy, a clear platform appeared in the low frequency region according to the curves of G′ of PLA/PBAT/CNTs composites with scanning angular frequency versus the logarithm of sweep frequencies, and G′ is not sensitive to the change in frequency, indicating that the motion of long-range polymer chains is significantly restricted [51]. The significant restriction of the motion of polymer chain is owing to the formation of the crosslinking network inside polymer matrix and the enhancement of the interaction between polymer matrix and CNTs. The G″ of the composites is also significantly decreased compared to other irradiated samples. It is because the solid network of CNTs in the polymer matrix is more stable when the irradiation dose reaches 24 kGy; thus, the internal energy dissipation of the filler/polymer interphase is decreased [52,53,54]. According to mechanical properties and dielectric behavior, it may be assumed that when the irradiation dose is 24 kGy, the optimized interfacial structure between the polymer matrix and CNTs is obtained, although the structural change of the crosslinking network inside polymer matrix can weaken the mechanical properties of the composites.

In conclusion, the γ-ray irradiation has a strong effect on the mechanical properties, dielectric behavior, and rheological behavior of the composites, which might be attributed to the enhancement of the CNTs-polymer interactions and polymer–polymer interactions induced by the irradiation.

3.2 Morphological analysis of the composites

The SEM, AFM, and TEM micrographs of cryogenically fractured surfaces of PLA/PBAT/CNTs composites are presented in Fig. 2. As can be observed from the SEM micrographs, compared to the unirradiated composites, the irradiated composites show a rough fracture surface over the entire micrographs. Moreover, more CNTs are pulled out from the polymer matrix with the increase of irradiation dose. The roughness observed for the fractured surface of the irradiated samples could evidence vigorous interfacial interactions between PLA and PBAT phases. Meanwhile, AFM micrographs suggest that the phase structure of the composites is significantly changed after irradiation. When the composites are under irradiation, macromolecular radicals can be formed in the polymer matrix; they can couple with each other to form a crosslinking network structure. The reactions between PLA macromolecular radicals and PBAT macromolecular radicals can enhance the interaction between PLA and PBAT, especially the TMPTMA located at the interface between two polymer phases can promote the crosslinking reactions between PLA and PBAT chains, which can further improve the compatibility of two polymers [55]. Thus, the phase structure of the composites and the interaction between the polymer matrixes are obviously changed after irradiation. On the other hand, since the CNTs can act as a radical scavenger and anchor radicals on CNTs, the interaction between CNTs and polymer matrix can also be improved after irradiation [56]. Take consideration that CNTs tend to migrate into the low-viscosity PBAT phase, the covalent interactions between CNTs and PBAT macromolecular radicals may play a dominant role in the composites [57]. Owing to the simultaneously enhancement of the polymer–polymer interaction and polymer-CNTs interaction, the microstructure of the PLA/PBAT/CNTs composites is significantly changed after irradiation. It should be mentioned that the structure of CNTs is not significantly changed after irradiation according to the TEM micrographs; hence, the enhancement of the mechanical and dielectric properties of the composites is mainly due to the change in the polymer–polymer interactions and polymer-CNTs interactions under irradiation.

Fig. 2
figure 2

SEM, AFM, and TEM micrographs of PLA/PBAT/CNTs composites with different irradiation doses: a, e SEM images (0 kGy); b, f SEM images (6 kGy); c, g SEM images (12 kGy); d, h SEM images (24 kGy); i, j AFM image (0 kGy); k, l AFM image (24 kGy); m, n TEM image (0 kGy); o, p TEM image (24 kGy)

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3.3 Nonisothermal crystallization behavior and thermal stability of the composites

The DSC and TG curves of the unirradiated and irradiated composites are exhibited in Fig. 3, and the relevant parameters are listed in Tables 1 and 2. The results show the γ-ray irradiation has a significant influence on the crystallization behavior of the PLA/PBAT/CNTs composites. As can be seen in Fig. 3a, PLA does not exhibit any crystallization peak during the cooling process. The cold crystallization temperature (Tcc) of irradiated samples is all increased during the secondary heating process in Fig. 3b, while the melting point (Tm) of PLA is also decreased. It is because the formation of crosslink network can restrict the PLA chain segment motion for crystallization, so that PLA segments are hard to stack regularly, and the Tcc of the PLA is increased. On the other hand, imperfect crystallites are formed inside the composites after irradiation because the crosslinking network interferes with the crystallization process; hence, the Tm of the PLA is decreased [58,59,60]. It should be mentioned that although the crystallization ability of PLA is weakened, the glass transition temperature of PLA is significantly enhanced, which is benefit for the improvement of the thermal stability of the composites [61].

Fig. 3
figure 3

Nonisothermal crystallization behavior and thermal stability of PLA/PBAT/CNTs composites with different irradiation dose: a, b DSC curves; c TG curves; d DTG curves

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Table 1 Thermal characteristics data from DSC analysis of PLA/PBAT/CNTs composites with different irradiation doses
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Table 2 Thermal stabilities characterization date of PLA/PBAT/CNTs composites with different irradiation doses
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The residual mass of the composites after thermal decomposition can be used to characterize the actual amounts of inorganic fillers added. As can be seen in the TG curves, the residual mass of all composites is essentially the same at 6%, indicating the presence of residual carbon formed by polymer cracking in addition to CNTs [62,63,64]. On the other hand, there is a slight drop in the mass of the composite around 200–300 °C for unirradiated composite, which may correspond to the decomposition of TMPTMA. After irradiation, the slight drop in mass disappears; it is because that the covalent bond is formed between TMPTMA and polymer chains inside composites owing to the irradiation-induced radical reactions, which improve the thermal stability of TMPTMA. As the irradiation dose increases, the thermal stability of the composites does not significantly change. Although the formation of crosslinked polymer network inside composites induced by irradiation is helpful for the improvement of thermal stability of the composites, the concurrent chain scission reactions can also weaken the thermal stability of the composites; hence, the thermal stability of the samples is not obviously improved [65,66,67].

3.4 Dynamic mechanical analysis of the composites

Temperature dependence of storage modulus and loss factor (Tan δ) for the composites with different irradiation dose are shown in Fig. 4. The storage modulus of the composites is obviously increased and then decreased during irradiation. On the other hand, all irradiated composites exhibit higher glass transition temperature determined for the maximum of the loss factor. Above phenomenon also indicates the enhancement of the CNTs-polymer interactions, and polymer–polymer interactions inside composites induced by irradiation can significantly restrict the movement of PLA and PBAT chains, which is consistence with the DSC and rheological results. When the irradiation dose reaches 24 kGy, the storage modulus of the composites is decreased; it may be because that the free chains formed at higher irradiation dose can improve the mobility of PLA and PBAT chains.

Fig. 4
figure 4

DMA curves of PLA/PBAT/CNTs composites with different irradiation doses: a temperature dependence of storage modulus, b temperature dependence of tan δ

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In summary, high performance PLA/PBAT/CNTs composites are obtained when exposed to the γ-ray. The structural evolution of the composites under irradiation is illustrated in Fig. 5. After irradiation, PLA and PBAT macromolecular radicals can be formed in the composites, and the reactions between PLA macromolecular radicals and PBAT macromolecular radicals can enhance the interaction between PLA and PBAT. The TMPTMA located at the interface can promote the crosslinking reactions between PLA and PBAT chains, thus further improve the interfacial adhesion between PLA and PBAT. Meanwhile, the TMPTMA can also facilitate the crosslinking process of PLA or PBAT, which is beneficial for the improvement of the mechanical properties of the composites. The interaction between CNTs and polymer matrix can be also improved after irradiation. Although CNTs tend to migrate into the low-viscosity PBAT phase and the irradiation may not significantly change the dispersion of CNTs in the polymer matrix, the simultaneously enhancement of the polymer–polymer interaction and polymer-CNTs interaction can obviously improve the properties of PLA/PBAT/CNTs composites, especially the change in interaction between polymer matrix and CNTs has a strong impact on the dielectric properties of the composites.

Fig. 5
figure 5

The structural evolution of the PLA/PBAT/CNTs composites under γ-ray irradiation

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4 Conclusion

PLA/PBAT/CNTs composites with excellent mechanical and dielectric properties were prepared without strict control of the pretreatment of CNTs, because the in situ enhancement of component interaction in the composites could be realized when the composites are exposed to γ-ray irradiation. The microstructure and properties of the composites can be effective controlled by simply adjusting the irradiation dose. When the irradiation dose is 6 kGy, the tensile strength of the composites increases from 31.9 to 42.1 MPa, and the elongation at break increases from 160 to 230% compared to unirradiated composites. As the irradiation dose reaches 24 kGy, the dielectric constant of the composites is significantly enhanced, and the dielectric loss is obviously decreased; meanwhile, the composites also possess good rigidity-toughness balance. The formation of crosslinking network and the enhancement of interaction force between CNTs and polymer matrix in the nanocomposites are proved by rheology tests, and SEM micrographs indicate the morphology of the composites is significantly changed with the γ-ray irradiation. In summary, this work provides a facile way to construct high-performance PLA blend composites without the pretreatment of CNTs, which can be applied in electronic fields.