Due to light weight and low cost, many synthetic polymers based items like polypacks, polyfilms, polycontainers etc. are used for food packaging and disposed after use. These waste products cause garbage disposal problem, especially in developing countries, where there is no proper waste management system. Therefore, there is an urgent need to employ biodegradable natural polymers such as starch, cellulose, chitosan, poly(lactic acid) (PLA), poly(caprolactone) etc. extracted from the renewable natural resources (Kabir et al., 2020; Qasim et al., 2021; Reichert et al., 2020).
These bio-polymers have certain limitations such as poor mechanical, thermal, barrier properties etc (Asgher et al., 2020). The properties of bio-polymers can be enhanced using nanoscale reinforcing fillers (dispersed on bio-polymer matrix) to form nanocomposites. Reinforcing fillers not only improves the existing thermal and mechanical properties of the bio-polymers, but depending on their nature also impart certain new properties such as higher conductivity, EMI shielding, antimicrobial activity, UV resistivity etc. (Trifol et al., 2021).
Among the various bio-polymers, the PLA derived from lactic acid is widely used in the development of bionanocomposites, mainly in combination with organically modified silicates, cellulose nanocrystals, carbon based nanofillers and metal oxides etc.(Ghozali et al., 2020).
Among various popular nanofillers used in polymer nanocomposites, ZnO in the form of nanowires/nanorods/ nanoparticles have been reported to improve antimicrobial, optical and thermo-mechanical properties of the polymer. The interfacial interactions of ZnO nanofiller with polymer matrix are influenced by the morphology of the nanofiller, which in turn affects the physiochemical properties of the nanocomposites (S. Wang et al., 2020). Many synthesis techniques are available to produce hierarchical (or layered) ZnO, which is suitable for employment as nanofiller in synthesis of nanocomposites. These techniques include electro- deposition (C. Wang et al., 2019), chemical bath deposition (Abdulrahman et al., 2020), microwave methods (Wojnarowicz et al., 2020), chemical vapor deposition (CVD) (Choi et al., 2020), sonochemical method (Bao et al., 2020), hydrothermal methods (Agarwal et al., 2019) etc.
There are previous studies in literature that report synthesis of different hierarchical structures of ZnO such as nanosheets, nanoplates, nanoflowers, disc shaped and star like morphology, etc. (Chang et al., 2020; Zhu et al., 2018). These special morphologies of ZnO affect the physical properties and provide beneficial effect in various applications such as gas sensors, high performing photocatalysts, supercapacitors, EMI shielding, solar cells etc. Guo et al.(Guo et al., 2020) have investigated gas sensing ability of different morphologies of ZnO including nanoparticles, nanosheets and nanoflowers. As compared to other two nanostructures (particles and nanosheets), 3-D hierarchical flower like morphology of ZnO showed the most superior gas sensing properties. This property was attributed to the highest specific surface area of ZnO nanoflowers (BET surface area: 5.7 m2g1) against the other nanostructures, viz. nanoparticles (BET surface area: 2.8 m2g1) and nanosheets (BET surface area: 4.3 m2g1). Fan et al. (2019) synthesized two hierarchical structure of ZnO, viz. fire cracker like ZnO and flower shaped ZnO.
They have reported that flower shaped ZnO exhibited higher H2S sensitivity response (50 ppm H2S) at 180℃ and shorter response time (14 s) over fire cracker like ZnO (H2S sensitivity response: 50 ppm H2S, response time: 19 s) (Fan et al., 2019). Chang et al. (2020) have synthesized three different morphologies of ZnO including nanorods, nanoplatelets and multibranched flower like structures by varying pH value of precursor and growth time in hydrothermal reaction (Chang et al., 2020). Results indicated that multibranched flower shape ZnO showed better antifungal activity and significant photodecomposition of organic chemicals present in the soil, as compared to nanorods and nanoplatelets. Cai et al. (2016) have also fabricated different shape of ZnO nanoflowers viz., rod flowers, fusiform flowers, and petal flowers using hydrothermal process (Cai et al., 2016). Among all petal flowers exhibited highest antibacterial activity (petal flowers >
fusiform flowers > rod flowers). This is due to differences in microscopic parameters such as specific surface area (petal flowers: 7.21 m2g1, fusiform flowers: 2.72 m2g1 and rod flower: 3.28 m2g1), pore size (petal flowers: 38.31 nm, fusiform flowers: 46.43 nm and rod flower: 42.63 nm)
and rod shape ZnO nanoparticles into different polymer matrix. However, relatively fewer studies have been published on hierarchical ZnO based polymer nanocomposites (Sharma et al., 2020).
Pariona et al. (2020) have studied the effect of size and morphology on the antimicrobial activities of ZnO. They have found that ZnO nano platelets showed better antifungal activity than ZnO nanoparticles and nanorods (Pariona et al., 2020).
Polymer nanocomposites can be prepared by using various techniques viz. solution casting, melt mixing, in situ polymerization etc. Among these, solution casting is the simplest technique with least energy requirement and faster kinetics. Uniform dispersion of nanofiller in the polymer matrix is essential for attaining desired enhanced properties. Use of sonication (or ultrasound irradiation) has been attempted by many previous authors for achieving uniform dispersion of nanofiller in the polymer matrix (Hussein et al., 2019; Soltani et al., 2018).
Mallakpour et al. (2018) have fabricated nanocomposites of poly(vinyl alcohol) and ZrO2
nanoparticles by using ultrasound assisted solvent casting method (Mallakpour & Shafiee, 2018).
Dhatarwal et al. (2021) also reported ultrasound-assisted synthesis of poly(methyl methacrylate)/MMT nanocomposites using solution casting method (Dhatarwal & Sengwa, 2021).
Similarly, nanocomposites with different nanofillers have also been synthesized using ultrasound assisted emulsion polymerization process (Poddar et al., 2016). These nanocomposites possessed superior physical properties (as compared to conventionally synthesized nanocomposites) by virtue of uniform dispersion of nanofillers in the polymer matrix.
In the present study, we report synthesis of polylactic acid/ZnO nanocomposites with a special feature. This feature is in terms of the ZnO with morphology of “nanoflowers”. These ZnO nanoflowers have also been synthesized by us using facile sonochemical method with CTAB as a morphology directing agent. Effect of synthesis conditions (such as different molar concentration of the precursor zinc nitrate hexahydrate) on the morphology (or architecture) of flower-like ZnO was also studied. The ZnO nanoflowers were incorporated into PLA matrix by using ultrasound assisted solution casting. The resulting nanocomposite films were characterized for thermal,
mechanical and antimicrobial properties. As a consequence of enhanced interfacial interactions- attributed to special nanoflower morphology of ZnO, these nanocomposites possessed superior physical and antimicrobial properties, as explained in greater details in subsequent sections. To the best of our knowledge, no previous study has reported synthesis of bionanocomposites with flower shaped morphology of ZnO with poly(lactic acid) matrix.
2.2 Materials and Methods