Functionalization of PLA Aerogels with TiO2 Nanoparticles

This study was aimed to prepare material with high porosity and photocatalytic activity by immobilization of TiO2 nanoparticles (NPs) onto poly(lactic acid) (PLA) aerogels. PLA aerogels were prepared in three steps: (1) dissolution of polymer in chloroform at 22 °C, (2) chloroform replacement with ethanol, and (3) supercritical CO2-drying at pressure 19 MPa and temperature 39 oC. Immobilization of TiO2 NPs was performed by in situ and ex situ methods. Obtained samples were characterized using SEM, EDX, and FTIR analysis. Photocatalytic activity of developed material was tested by following decolorization of dye C.I. Acid Orange 7 in water solution. It was shown that the morphology of PLA aerogels was slightly affected by TiO2 NPs immobilization. PLA aerogels with TiO2 NPs immobilized by ex situ method sustained floatability during test period and provided a complete decolorization of dye solution after 330 minutes of illumination. High photocatalytic activity of the sample was preserved within three repeated cycles of dye decolorization.


INTRODUCTION
In the last years, poly(lactic acid) (PLA) gained significant attention as a biodegradable and biocompatible material, produced from renewable resources, which could be used in medicine and food packaging industry [1,2]. In order to increase number of PLA applications, several methods have been explored such as chemical or thermal processing as well as addition of different organic and inorganic substances [3][4][5][6]. Functionalization of PLA films, foams or fabrics, can be performed by addition of natural bioactive components [7,8], metal oxides [9], carbon nanotubes [3], drugs [10], proteins [11], clay [12], etc. However, this study tested functionalization of PLA aerogels by immobilization of TiO2 nanoparticles (NPs) for application in treatment of colored wastewaters. For that purpose, PLA aerogels were prepared in three steps: (1) dissolution of PLA in appropriate solvent, (2) so-lvent replacement with ethanol, and (3) supercritical CO2 (scCO2) drying.
These steps allow formation of highly porous polymer materials [13,14]. ScCO2 is the most appropriate medium for polymer gel drying and aerogel preparation since it has mild critical conditions, liquid like density, gas like diffusivity, GRAS status (Generally Recognized as Safe), and near zero surface tension [13].
Additionally, variation in scCO2-assisted process pressure and temperature allows control of material morphology. TiO2 NPs were selected for PLA aerogel functionalization due to its high photoactivity, high stability, and non-toxicity. Owing to these properties, TiO2 NPs are appropriate for development of material that can be used for treatment of wastewater [15][16][17].
Through this study, we showed that the scCO2drying method enabled preparation of porous material and that the method of TiO2 NPs immobilization determines material photocatalytic activity. Properties of materials were assessed by SEM, EDX, and FTIR analysis. Activity of the obtained materials was tested by following decolorization of dye C.I. Acid Orange solution under sun light illumination. TEHNIKA -NOVI MATERIJALI 30 (2021) 4

MATERIALS AND METHODS
PLA beads with density of 1250 kg/m 3 , melting temperature 155°C, and molecular weight of 116 kg/mol were supplied by NatureWorks (USA). Beads were dissolved in chloroform (Lachema, Czech Republic) at a ratio of 1:10 (at temperature 22°C, during 4 h mixing using a magnetic stirrer). PLA solution was poured in a petri dish where absolute ethanol (p.a, Zorka Pharma, Serbia) was added for solvent replacement. After 24 h, obtained alcogels were cut into a disc shape (V~95 mm 3 ).
PLA discs were dried in a high-pressure view cell (Eurotechnica GmbH, Germany) using scCO2 at pressure 19 MPa and temperature 39 ºC [2] during 2 h and 40 min by a combination of static and dynamic mode. In this manner aerogels were obtained. The drying process was finished by release of CO2 from the system with a rate of 2 MPa/min. TiO2 NPs (Degussa P25, Aeroxide, Germany) were immobilized onto prepared PLA aerogels by ex situ method. Namely, 0.5 g of PLA aerogels was immersed in 55 mL of a dispersion containing TiO2 NPs (0.1 M) for 2 h. This method of TiO2 NPs im-mobilization was also applied for PLA beads (control sample). Afterwards, samples were dried at room temperature, rinsed with deionized water, air-dried, and tested as floating photocatalysts. For comparison, TiO2 NPs were also immobilized onto/into PLA matrix by in situ method. Namely, 20 mg of TiO2 NPs was added into 10 mL of previously prepared PLA solution. After intensive mixing for 1 h, the obtained suspension was poured into a petri dish, and absolute ethanol was added for solvent replacement. Obtained alcogels were cut into discs shape and dried using scCO2 (at the same conditions as previously descri-bed).
The morphology of dry PLA aerogels was analyzed by field emission scanning electron microscopy (FESEM, Tescan Mira3 FEG, Czech Republic). The samples were fractured using liquid nitrogen to pre-serve its morphology and coated with a thin layer of Au/Pd (85/15) prior to the analysis.
The presence of Ti on the surface of prepared material was confirmed by energy-dispersive X-Ray spectroscopy (EDX) using a JEOL JSM 5800 SEM with a SiLi X-Ray detector (Oxford Link Isis series 300, UK).
The photocatalytic activity of prepared samples with TiO2 NPs was examined by following disco-loration of acid dye C.I. Acid Orange 7 (AO7, Cassella, Germany). PLA samples with TiO2 NPs (0.25 g) were placed in AO7 aqueous solutions (25 mL of 10 mg/L) and shaken in the water bath under sunlike illumination (ULTRA-VITALUX lamp, 300 W, Osram). Optical power was 30 mW·cm 2 (determined by R-752 Universal Radiometer Readout with sensor model PH-30, DIGIRAD). The concentration of AO7 was determined using UV-vis spectrophotometer (Cary 100 Scan, Varian) by measuring absorption intensity at 484 nm. Test was repeated two times.

RESULTS
PLA beads were transformed into disc shape alcogels (Figure 1a), which were dried using scCO2 at 19 MPa and 39°C enabling preparation of disc-shape aerogels (Figure 1b). Compared to the volume of alcogels, the volume of obtained aerogels decreased only slightly during scCO2-assisted drying implying that the morphology of wet gel was preserved. a) b) Figure 1 -Images of PLA samples: a) alcogel and b) aerogel (bar size 1 cm) TiO2 NPs were further immobilized onto obtained aerogels (ex situ method). Immobilization of NPs was also tested by addition of TiO2 NPs during PLA solution preparation (in situ method). Addition of nanoparticles did not affect appearance of PLA aerogels. The same behaviour was previously reported for immobilization of TiO2 NPs onto/into PLA aerogels prepared by using scCO2 at 15 MPa and 35 °C [18].
Morphology of prepared PLA samples is presented in Figure 2. SEM images confirm that scCO2-assisted drying enabled preparation of porous PLA material (Figure 2a). It can be seen that the surface of aerogels is non-uniform with pores smaller than 1 μm. On the contrary, internal pores are uniformly distributed. Aerogel interior has two size pores (around 10 μm and around 0.5 μm). Immobilization of TiO2 NPs by ex situ method led to formation of NPs agglomerate. Addi-tionally, these agglomerates formed continued layer of NPs in some sections of the aerogel surface (Fig. 2b). Presence of small number of NPs in this material cross section is noticed only close to the surface. On the other hand, smaller agglomerates of TiO2 NPs were presented through the surface and cross-section of the PLA material obtained by in situ method (Fig. 2c). In addition, it can be seen that the surface of aerogels with TiO2 NPs immobilized by ex situ method contains larger amount of nanoparticles on its surface. Also, the size of aerogels' interior pores was not significantly affected upon nanoparticles immobilization. The EDX spectra of PLA+TiO2 aerogel samples, presented in Figure 3, revealed the presence of Ti. Picks corresponding to Ti originate from TiO2 NPs. Additionally, it can be seen that the intensity of Ti picks is significantly larger on the surface of the sample obtained by ex situ method (Figure 3a). These results confirm findings of SEM analysis.
The photocatalytic activity of materials with NPs prepared by ex situ method (PLA aerogels and PLA beads with immobilized TiO2 NPs), PLA aerogels with NPs prepared by in situ method, as well as neat PLA aerogels was tested in AO7 aqueous dye solution under sun-like illumination. The possible photodegradation of dye was investigated by following the decolorization of dye solution. First, it was determined that the dye AO7 was not prone to photolysis. Test revealed that neat PLA aerogels did not induce any dye sorption or dye photodegradation. The results also revealed that PLA beads with TiO2 NPs immobilized by ex situ method as well as PLA aerogels with TiO2 NPs immobilized by in situ method did not show any photocatalytic activity. Only PLA aerogels with TiO2 NPs immobilized by ex situ method enable disco-lo-ration of dye solution. The same behaviour was pre-viously reported for PLA aerogels with TiO2 NPs pre-pared using scCO2 at 15 MPa and 35°C [18]. a) b) Figure 3 -EDX spectra of aerogels: a) PLA+TiO2 ex situ and b) PLA+TiO2 in situ Kinetic of dye solution decolorization is presented in Figure 4. It can be seen that complete discoloration of AO7 dye solution, during first cycle, is obtained after 330 min of illumination. Hsieh et al. [19] reported that for complete discoloration of AO7 dye solution with graphene+TiO2 composites it took more than 6 h. However, our previous publication [18] reported PLA aerogels obtained by scCO2-drying at 15 MPa and 35 °C with immobilized TiO2 NPs that completely discolor AO7 dye solution after 240 min of illumination. conse-quently affected TiO2 immobilization and photocatalytic activity of obtained material.
After dye solution discoloration, aerogels were separated from the liquid, dried and tested again in fresh dye solution. Reusability of aerogels was con-fir-med in two additional cycles. The second cycle of pho-todegradation resulted in dye solution decolorization after 240 min, while the third cycle resulted in dye solution discoloration after 360 min.
FTIR spectra of neat PLA aerogel, PLA with TiO2 NPs immobilized by ex situ method (PLA+TiO2 ex situ), and PLA with TiO2 NPs immobilized by ex situ method after three cycles of dye solution decolo-rization (PLA+TiO2 ex situ after three cycles of illu-mination) are presented in Figure 5. Bands observed at 2996 cm −1 , 1751 cm -1 , and 1455 cm -1 are assigned to C-H from aliphatic CH3 groups, to carbonyl stretching C=O in the ester group, and to asymmetric -CH3 deformation vibrations, respectively [20][21][22]. In addition bands observed at 1182 cm −1 and 754 cm −1 are attributed to the symmetric C-O-C stretching of the PLA ester groups and to O-CH-CH3 ester groups, respectively [20][21][22]. Namely, these bands that are characteristic for neat PLA can be seen in spectra of all samples.
Hence, it can be concluded that immobilization of TiO2 NPs did not affect chemical stability of PLA aerogels. Figure 5c also revealed that illumination and use of samples for dye solution decolorization did not induce any change in chemical stability of PLA during tested period.
Proposed method of PLA processing enabled preparation of porous material with desired properties.

CONCLUSION
The results of this study indicated that scCO2-drying process at 19 MPa and 39 ˚C and for 2 h 40 min led to preparation of porous PLA material. Obtained porous PLA aerogels could be successfully used as carriers for TiO2 NPs. It was shown that ex situ method of TiO2 NPs immobilization was superior compared to in situ method for preparing of a floating photocatalyst. FTIR analysis indicated that TiO2 NPs immobilization and illumination did not affect chemical stability of PLA. PLA aerogels with TiO2 NPs immobilized by ex situ method decolorized dye AO7 solution after only 330 min of illumination. Excellent photocatalytic activity of prepared materials was retained after three illumination cycles. These results indicated that environmentally friendly material was prepared for potential use in treatment in color wastewater.

REMARK
The paper was presented at the International Conference on Aerogels for Biomedical and Envi-ron-mental Applications, 18-20 February 2020. Santiago de Compostela (Spain).