ZnO composites prepared by melt processing

Accepted Manuscript Original article Degradation of PLA/ZnO and PHBV/ZnO composites prepared by melt processing Alojz Anžlovar, Andrej Kržan, Ema Žagar PII: DOI: Reference:

S1878-5352(17)30131-4 http://dx.doi.org/10.1016/j.arabjc.2017.07.001 ARABJC 2113

To appear in:

Arabian Journal of Chemistry

Received Date: Accepted Date:

1 March 2017 1 July 2017

Please cite this article as: A. Anžlovar, A. Kržan, E. Žagar, Degradation of PLA/ZnO and PHBV/ZnO composites prepared by melt processing, Arabian Journal of Chemistry (2017), doi: http://dx.doi.org/10.1016/j.arabjc. 2017.07.001

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Degradation of PLA/ZnO and PHBV/ZnO composites prepared by melt processing Alojz Anžlovara*, Andrej Kržan a, Ema Žagara a

National Institute of Chemistry, Department for Polymers, D-07, Hajdrihova 19, SI-1000,

Ljubljana, Slovenia *

corresponding author; e-mail: [email protected], Tel: ++386 1 4760 204, Fax: ++386 1

4760 300

Abstract Composites of polylactide (PLA) or poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) and ZnO nanoparticles (nZnO) were prepared by melt processing. During extrusion and moulding nano ZnO formed aggregates with sizes between 0.5 and 5 μm in PLA and between 0.5 and 15 μm in PHBV. Nano ZnO acted as a disruptor of PLA crystallization process and shifted the polymer glass transition temperature to lower temperatures. This was explained by degradation of PLA polymer chains during melt processing. SEC, FTIR and 1H NMR confirmed that PLA degradation was correlated to nZnO concentration. The effect of nZnO on crystallization of PHBV matrix was much less intense which was shown by TGA. On the other hand, PHBV showed significantly lower thermal stability than PLA. ZnO participated as a reactant and an accelerator in the degradation reaction of PLA and at a smaller extent with PHBV. The results of this study revealed that addition of pure nZnO in concentrations higher than 0.1 wt.% is not recommended for the preparation of PLA/nZnO composites by melt processing while in the case of PHBV the nZnO concentration may be higher but it should not exceed 1.0 wt.%. The exposure time of these materials to high temperatures during melt processing should also be minimized.

Key words: Polylactide, Poly(3-hydroxybutyrate-co-3-hydroxyvalerate), Nano ZnO, Melt processing, Degradation stability

1. INTRODUCTION The environmental impact of plastic waste has become a major global concern due to the rapidly expanding application of plastics. Key points of concern are the use of non-renewable resources, plastic waste management, and plastic litter in the environment, particularly in marine and freshwater environments. Finding practical solutions to these issues is a high 1

R&D priority both in academia and industry. In particular, biobased and/or biodegradable polymers attract considerable attention (Drumright et al., 2000) as real alternatives to conventional synthetic polymers, if they have a favourable life-cycle balance and suitable properties (Thieras et al., 2012). Biodegradability is a specific functional property suitable for some applications (e.g. food packaging, mulching foils, drug delivery, bio-absorbable materials, etc.) (Lim et al., 2008). The market for biobased and biodegradable polymers is experiencing rapid growth which is expected to continue, especially in applications where biodegradability provides an advantage for customers and the environment (Murariou et al., 2011). Polylactide (PLA) is a biobased, compostable polymer with a relatively high production and is one of the most promising materials in this group. It is based on a natural monomer – lactic acid, which is produced via fermentation from renewable substrates such as sugar or corn starch (Hamad et al., 2015; Murariou et al., 2011). End-of-life options for PLA include composting, mechanical recycling or chemical recycling - e.g. LOOPLA process (Thieras et al. 2012). The disadvantages of PLA are some basic properties such as thermal and hydrolytic stability and a relatively narrow processing window. Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) is biobased polyester copolymer of microbial origin from the group of polyhydroxyalkanoates. It is a semi-crystalline thermoplastic material with interesting applications due to its anaerobic and aerobic degradability, biocompatibility as well as renewable nature (Ma et al., 2014). The introduction of 3-hydroxyvalerate units into poly(3-hydroxybutyrate) polymer chains improves mechanical properties and lowers the melting point thus reducing the degradation during processing. The degree of crystallinity is only slightly reduced due to isodimorphism of PHBV (De O. Patricio et al., 2013; Wang et al., 2010). Properties of PHBV are similar to those of polypropylene but there are some drawbacks that limit its application such as brittleness, poor processability and high production costs (Ma et al., 2014). Lack of specific modifications and additives together with a higher price compared to conventional polymers still limit greater penetration of both biopolymers in the market. Various approaches have thus been explored to broaden their applicability such as enhancement of properties by modification (Bugnicourt et al., 2014; Lim et al., 2008) and improvement of its processing methods and conditions (Bocz et al., 2016; Jaszkiewicz et al, 2014; Naphade and Jog, 2012). Zinc oxide (ZnO) and especially its nanostructures have attracted significant scientific attention as very promising materials for various applications (Yang et al., 2010; Anžlovar et al., 2010). Apart from its optical and electrical properties, ZnO is known to influence thermal 2

and mechanical properties of polymer composites (Djurišić and Leung, 2006). Nano ZnO (nZnO) can be synthesized through a number of chemical and physical routes including the solvothermal polyol (diol) method (Jezequel et al., 1995, Dong et al., 2015). Polyols (diols) are able to dissolve many inorganic compounds and reduce certain elements to the metallic state (Anžlovar et al., 2008; Anžlovar et al., 2014) and they are also suitable media for the synthesis of nZnO from precursors such as zinc acetate, zinc acetylacetonate (Famengo et al., 2009) or even zinc carbonate (Anžlovar et al., 2015). The addition of nZnO to PLA or PHBV can significantly change the polymer properties such as UV absorption, thermal stability and even mechanical properties. In addition, due to antibacterial properties of nZnO, its combination with PLA or PHBV results in biocompatible materials with antibacterial activity (Raquez et al., 2013; Diez-Pascual and Diez-Vincente, 2014). Such composites have high potential for use in packaging and biomedical applications (Castro-Mayorga et al., 2017; Fan et al., 2015; Murariou et al., 2011; Pantani et al., 2013; Thieras et al., 2012). Both, PLA and PHBV composites with nZnO have been widely studied but comparison of their properties has not been published, yet. Several recent publications focused on PLA or PHBV degradation problems during melt processing (Jaszkiewicz et al., 2014, Zheng et al., 2012). Various approaches towards reducing the degradation and enhancing properties of these materials are reported but this challenge is not yet fully resolved (Murariou et al., 2015; Hablot et al., 2008). The aim of our work was to prepare PLA/nZnO and PHBV/nZnO composites by extrusion and injection moulding using nZnO precipitated by the polyol method and deposited on PLA or PHBV pellets from dispersion of nZnO in methanol. We studied the effect of nZnO on physicochemical properties of the prepared composites and evaluated the influence of nZnO on PLA and PHBV degradation. Moreover stabilities to degradation of these two composite materials were compared.

2. EXPERIMENTAL 2.1 Materials: Polylactide used in the experiments was a commercial Ingeo 2003D grade (NatureWorks, USA, Mw = 150 kg/mol) intended for extrusion processing while poly(3-hydroxybutyrate-co3-hydroxyvalerate) used was ENMAT Y1000P (Ningbo Tianan Biologic Material Co LTD, Ningbo, China, Mw = 39 kg/mol) with low content of 3-hydroxyvalerate comonomer (~3 wt.%).

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2.2 Synthesis of nZnO ZnO nanoparticles (nZnO) with dimensions between 20 and 50 nm were synthesized by a previously published method involving hydrolysis of zinc acetate dihydrate in ethylene glycol (EG) using p-toluene sulphonic acid as an end-capping agent (Anžlovar et al., 2011, Anžlovar et al., 2012). The synthesis of ZnO in diols yielded nanoparticles with an organophilic character that were characterized by electron microscopy (Figure 1a Supp.Mat.), X-ray powder diffraction (Figure 1b Supp.Mat.) and infrared spectroscopy (FTIR) (Figure 1c Supp.Mat.). Characterized ZnO nanoparticles were further used for the preparation of PLA/nZnO composites by the extrusion and injection moulding.

2.3 Preparation of composites nZnO was deposited on the surface of PLA or PHBV granules by suspending nZnO in methanol followed by evaporation to constant mass at room temperature. PLA/nZnO and PHBV/nZnO granules were subsequently extruded on a Haake MiniLab II co-rotating twin screw extruder at 180 °C, retention time 15 min at 20 rpm. The resulting molten material was moulded on a Haake Mini Jet moulding device at 180 °C and 850 bars for 25 s to obtain specimens for further characterization.

2.4 Characterization techniques Scanning electron microscopy (SEM) micrographs were taken on a Zeiss Supra 35 VP electron microscope at an acceleration voltage of 20.0 kV and 4.5 – 5.0 mm working distance using a backscattered electrons detector. Samples were placed in a sample holder and coated with 10 nm C film. Differential scanning calorimetry (DSC) measurements were performed on a DSC-1 calorimeter (Mettler Toledo) in the temperature region from -30 to 250 °C, heating rate of 10 °C/min, and cooling rate of 200 °C/min. Two heating/cooling scans were performed. Thermogravimetric analysis (TGA) was performed on a TGA-1 analyzer (Mettler-Toledo) in the temperature range 50 - 600 °C with a heating rate of 60 °C/min. PLA and PHBV molar mass characteristics were determined by size exclusion chromatography (SEC) on a modular system composed of an isocratic pump – Hewlett Packard 1100 Series, a PLgel MIXED-D column with a precolumn, and an evaporative lightscattering detector – ELS, Polymer Laboratories. The solvent was CHCl3 with a flow rate of 1 mL/min. Polystyrene standards were used for column calibration. PLA and PHBV specimens

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were dissolved in CHCl3 at a concentration of 2.5 mg/mL. The injection volume of sample solutions was 100 µL. Transmission spectra in UV and visible spectrum (UV-VIS) of PLA/nZnO composites were measured on an Agilent 8453 UV-VIS spectrometer in a spectral range between 290 and 1000 nm. The impact of nZnO on PLA and PHBV degradation was studied on samples prepared by dissolving the respective polymer in CHCl3 (max. 10 wt.%) and adding 1 or 10 wt.% of nZnO vs. dry polymer weight. The suspension was homogenized by sonication and dried to constant mass prior to exposure to 185 °C for 24 h. Fourier transform infrared (FTIR) spectra were recorded on a FTIR spectrometer Spectrum One (Perkin Elmer) in transmittance mode in a spectral range 4000 - 400 cm-1 with a spectral resolution of 4 cm-1 using the KBr pellet technique. 1

H and

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C Nuclear magnetic resonance (NMR) spectra were recorded on a Varian Unity

Inova 300 MHz spectrometer under the following quantitative conditions

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C NMR: pulse –

90°, delay – 6.7 s, acquisition time – 3.7 s, and 1H NMR: pulse – 90°, delay – 5.0 s, acquisition time – 5.0 s. Signals were referenced to a CDCl3 resonance at 77.0 ppm (13C NMR) and 7.25 ppm (1H NMR), respectively. Samples were dissolved in CDCl3 and measured at room temperature.

3. RESULTS AND DISCUSSION 3.1 Nano ZnO distribution in PLA and PHBV matrix nZnO was synthesized by the previously published polyol method in ethylene glycol using p-toluene sulphonic acid as an end capping agent (Anžlovar et al., 2011; Anžlovar et al., 2012) and was then incorporated into PLA or PHBV by melt processing. The distribution of nZnO in the PLA matrix was studied by SEM microscopy (Lizundia et al., 2016a). SEM micrographs of the prepared PLA/nZnO composites (Figure 1a) showed a homogeneous distribution of ZnO particles with sizes between 0.5 and 5 μm while PHBV/nZnO composites showed larger ZnO particles with sizes between 0.5 and 15 μm (Figure 1b). Since the size of original nZnO particles was between 10 and 100 nm (Figure 1a Supp.Mat.) we concluded that the observed structures were aggregates of ZnO (Figure 1a and 1b) formed during either nZnO deposition on PLA or PHBV granules or during the melt processing. Additionally, EDS analysis of particle inclusions (at the exact location of SEM collection) was done for PLA/nZnO as well as for PHBV/nZnO samples containing 1 wt.% nZnO and results are shown in Figure 2 Supp.Mat. as well as in Figure 3 Supp.Mat., 5

respectively. EDS analysis confirmed that the majority of bright inclusions observed are ZnO while calculated wt.% of Zn confirmed that particle size in the PHBV matrix are on average much larger than in the PLA matrix (Table 1 Supp.Mat. and Table 2 Supp.Mat.).

Figure 1. SEM micrographs: (a) PLA/nZnO composite and (b) PHBV/nZnO composite both with 1 wt.% of nano ZnO. 3.2 Thermal properties of PLA/nZnO and PHBV/nZnO composites Thermal properties of all composites were studied by DSC and TGA. For PLA/nZnO the curves of the first heating showed superimposed endothermic peaks of the glass transition (Tg) (Figure 2a). Above the glass transition (between 85 and 125 °C) the curves showed the processes of cold crystallization, indicating disrupted PLA crystallization during cooling, caused by increasing concentration of nZnO (Figure 2a). The enthalpy change of PLA glass transition increased with higher content of nZnO (Figure 2a). Above the cold crystallization temperature, between 135 and 165 °C, we observed broad endothermic melting peaks, which were split into two poorly resolved peaks with increasing nZnO concentration. The two endothermic peaks indicate the presence of two crystal populations that differ in dimension and concentration of defects resulting from melting-recrystallization-melting phenomena (LeMarec et al., 2014; Sanchez et al., 2007). DSC curves of the first heating of PHBV showed an intense melting peak of PHBV at 175 °C which was shifted to 145 °C with addition of 0.1 wt.% nZnO and further to 140 °C with 1.0 wt.% nZnO. At the same time the melting peak splits into two and further into three peaks with the increase of nZnO from 0 to 1.0 wt.% (Figure 2b). Multiple melting peaks reveal that at least three different structures with various ratios of crystalline and amorphous domains were formed by the addition of nZnO. By increasing the nZnO concentration in PLA the crystallization degree is significantly reduced (Bussiere et al., 2012) as shown by intense cold crystallization while in PHBV it was only

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slightly reduced. In PLA, the melting process was split into two peaks indicating two crystal populations in the material while the temperature of the melting was not significantly shifted. On the other hand, in PHBV melting was split into three peaks indicating the same number of crystal populations and the melting process was shifted significantly towards lower temperatures. This indicated that nZnO particles functioned as crystallization nuclei forming larger number of PHBV crystallites with smaller sizes. The degree of crystallization was also reduced as indicated by the occurrence of cold crystallization at 35 °C (Figure 2b). The cold crystallization was much less intense than in PLA indicating that the crystallization of PLA was much slower (Gorrasi et al., 2013) than the crystallization of PHBV. The Tg values for both polymer matrices are given in Table 1. The glass transitions of PLA/nZnO composites with 1.0 wt.% nZnO showed a significant shift of the Tg towards lower temperatures, i.e. from 53 °C for pure PLA to 36 °C for PLA/nZnO composite (Table 1). This can be explained by degradation of PLA during the melt processing, more precisely by the plasticising effect of PLA degradation products. On the other hand, the Tgs of PHBV/nZnO composites barely showed any temperature shifts (Table 1). nZnO thus showed much higher effect on the thermal properties of composites with PLA than on those with PHBV. Therefore, Tg shifts also indicated that PLA degradation during melt processing was much more intense than PHBV degradation in the presence of nZnO.

Figure 2. Normalized DSC curves of the first heating as a function of nZnO concentration: (a) PLA/nZnO composites: A) PLA, B) 0.1 wt.%, C) 0.5 wt.%, D) 1.0 wt.% and (b) PHBV/ZnO composites: A) PHBV, B) 0.1 wt.%, C) 1.0 wt.%.

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Table 1. Glass transition temperatures of PLA/nZnO and PHBV/nZnO composites as a function of the nZnO concentration. nZnO concentration / wt.% Glass transition temperature / °C PLA PHBV 0 53 -5.2 0.1

50

-4.8

0.5

42

-

1.0

36

-8.7

Additionally, TGA curves were also measured in the temperature region between RT and 600 °C. Their first derivatives are shown in Figure 3 for PLA/nZnO composites (a) and for PHBV/nZnO composites (b) as a function of nZnO concentration. Results revealed that the onset and peak degradation temperature of PLA was shifted towards lower temperatures (Figure 3a) with the increase of nZnO concentration while degradation of PHBV showed no dependence on the nZnO concentration (Figure 3b). These results confirm the assumptions based on DSC results (Figure 2, Table 1) that nZnO affects PLA matrix much more than PHBV one. Comparing degradation temperatures of pure PLA and pure PHBV it is clear that neat PLA is thermally more stable (degradation peak = 338 °C) than neat PHBV (degradation peak = 279 °C).

Figure 3. TGA curves (first derivatives of weight loss) of PLA/ZnO composites (a) and of PHBV/ZnO composites (b) as a function of nZnO concentration: A) 0 wt.%, B) 0.1 wt.% and C) 1.0 wt.%.

3.3 Molar mass characteristics determined by SEC To study PLA or PHBV degradation during melt processing we measured their molar mass characteristics after extrusion and injection moulding as a function of nZnO concentration in PLA/nZnO and PHBV/nZnO composites. Results (Table 3 Supp.Mat.) showed that the

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degradation of PLA and PHBV occurred even in the absence of ZnO as indicated by significantly reduced Mw and Mn values as compared to the corresponding values of neat PLA and PHBV, respectively (Table 2). Comparing the Mw reduction of pure PLA and pure PHBV as a function of melt processing a significant difference in thermal and processing stability was observed since Mw was reduced by 20 % and 68 % for pure PLA and pure PHBV, respectively. This shows that thermal stability of PHBV is significantly lower compared to the stability of PLA confirming the results of TGA (Figure 3). The incorporation of nZnO into the PLA matrix accelerated PLA degradation (Table 3 Supp.Mat.), confirming assumptions based on the Tg shifts (Table 1) and TGA (Figure 3) (AlItry et al., 2012). It was reported that the temperature was a key factor in the hydrolytic degradation of PLA since degradation was significantly accelerated at temperatures above its Tg (Ndazi and Karlsson, 2011). The observed results are opposite to those of the hydrolytic PLA degradation study where nZnO was found to significantly retard hydrolytic degradation reaction (Benali et al., 2015). However, in this study silane coated nZnO was used which slowed water uptake and consequently reduced the PLA degradation rate in a phosphate buffer at 37 °C and these results can be only approximately compared with our results. Nevertheless, we think that surface coating of nZnO with inert materials such as siloxanes, silanes, silica or Zn silicate are beneficial for reducing the degradation of PLA (Kos et al., 2013; Murariou et al., 2011; Murariou et al., 2015; Podbršček et al., 2011; Thieras et al., 2012). SEC chromatograms of PHBV in PHBV/nZnO composites showed only negligible differences in Mw and Mn with increasing nZnO concentration indicating that nZnO did not contribute very much to degradation of PHBV which was in accordance with observed glass transition temperatures shifts (Table 1) and TGA results (Figure 3). This confirms the main difference between PLA and PHBV in relation to nZnO.

3.4 UV-VIS spectroscopy of PLA/nZnO composites Optical properties of PLA/nZnO composite samples were studied by UV-VIS spectroscopy. Plates (1 mm thickness) showed significant enhancement of UV absorption (almost 98% absorption up to 370 nm) as compared to pure PLA (Figure 4 Supp.Mat.). Visible light transparency of pure PLA test specimen (1 mm thickness) was rather poor, between 20 and 30 % (Figure 4A Supp.Mat.). It was additionally reduced by the addition of nZnO up to 1 wt.%, which we attributed to ZnO aggregation, in accordance with electron micrographs observations (Figure 4 B,C and D Supp.Mat.). Results reported in the literature (Lizudia et al., 9

2016b) showed significantly higher visible light transmittance and lower UV absorption due to significantly lower thickness of those test samples. Their samples were very thin (12 μm) while ours were relatively thick (1 mm) and most probably with reduced thickness our samples would also give spectra with relatively high visible transmittance at the cost of reduced UV absorption. PHBV is not a transparent material so UV-VIS spectra of these composites were not measured.

3.5 Degradation of PLA, PHBV and composites with nZnO studied by FTIR and NMR spectroscopy PLA/nZnO and PHBV/nZnO samples dried from CHCl3 suspensions and exposed to 185 °C for 24 h were analyzed by FTIR and NMR spectroscopies to study the role of nZnO on degradation of PLA and PHBV composites. When samples of PLA/ZnO composites were taken from the oven a clear difference was observed since the PLA films with nZnO were practically disintegrated while the pure PLA film showed only a slightly changed colour. A comparison of FTIR spectra for neat PLA and PLA exposed to 185 °C (Figure 4a) shows only small changes indicating that PLA is a relatively stable material at these conditions. On the other hand, by increasing the concentration of nZnO to 10 wt.%, the FTIR spectrum after thermal treatment showed intense absorption bands at 1606, 1424, 865, 770 and 564 cm-1 which are characteristic for Zn lactate (Figure 4a D) while the absorption bands of PLA were still visible. These results revealed that most of nZnO reacted with PLA forming Zn salts of PLA oligomers and Zn lactate during thermal treatment of the composite. This is supported by the significant decrease of the characteristic absorption band between 420 and 480 cm-1, indicating that nZnO is not just an accelerator of PLA degradation, but also a reactant in the degradation reaction (Figure 4b). FTIR spectra of PHBV/nZnO composites are given in Figure 5a. Comparing the spectra of PHBV (unexposed and exposed to high temperature - 185 °C for 24 h) (Figure 5a A and 5a B) it is clear that thermal exposure severely degraded the material even without nZnO as indicated by the appearance of new absorption bands of alkenes at 1392 and 800 cm-1 accompanied by the disappearance of PHBV ester absorption bands at 1726 and 1057 cm-1 (Žagar and Kržan, 2004). This indicates that thermal stability of PHBV is considerably lower than that of PLA (Hablot et al., 2008) most probably due to its auto-accelerated random degradation reaction (Ariffin et al., 2008). This is also in accordance with SEC results (Table 3 Supp.Mat.). The addition of nZnO further accelerated the decomposition of PHBV as

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indicated by the appearance of additional absorption bands of carboxylic or carboxylate, Zn carboxylate and alkene groups at 1660, 1550, 1424 and 969 cm-1 (Figure 5a C and D). Here again, most of the nZnO was consumed during PHBV degradation forming various Zn salts of carboxylic acids and indicating that ZnO was a reactant in the degradation process (Figure 5b). ZnO, as an amfotheric oxide, reacts with basic and also with acidic compounds such as PLA or PHBV degradation products. Namely, ZnO particles have hydroxyl groups on the surface as shown by FTIR spectroscopy (Figure 1 c Supp.Mat.) and these easily react with carboxylic groups.

Figure 4. FTIR spectra of PLA/nZnO composites degraded at 185 °C (a) as a function of nZnO concentration: A) neat PLA, B) 0 wt%, C) 1 wt%, D) 10 wt% and (b) PLA/nZnO composites: A) 10 wt.% - before degradation , B) 10 wt% - after degradation.

Figure 5. FTIR spectra of PHBV/nZnO composites degraded at 185 °C (a) as a function of nZnO concentration: A) neat PHBV, B) 0 wt%, C) 1 wt%, D) 10 wt% and (b) PHBV/nZnO composites: A) 10 wt.% - before degradation , B) 10 wt% - after degradation. Samples were also analyzed by 1H and

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C NMR spectroscopy. In addition to signals of

PLA, the NMR spectra of PLA with 10 wt.% nZnO showed additional signals (1H NMR: 1.44, 1.47, 4.37 and 5.02 – 5.08 ppm, and

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C NMR: 20.0, 20.4, 66.7, 174.9 and 177.5 ppm)

(Figures 6 and 7). New signals between 1.43 and 1.49 ppm originated from CH3 groups of 11

oligomer carboxyl ends while the signals at 4.34 and 4.36 ppm corresponded to methine protons of hydroxyl ends in PLA oligomers (Figure 6C). Signals between 4.98 and 5.05 ppm originated from methine protons of carboxylic ends in PLA oligomers. Correspondingly, we can explain new

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C signals (Figure 7B): signals at 20.0 and 20.4 ppm originating from CH3

groups of carboxylic ends in PLA oligomers, the signal at 66.7 ppm from CH groups of hydroxyl ends in PLA oligomers and the emerged signals at 175.0 and 177.5 ppm from carbonyl groups in PLA oligomers and in lactic acid, respectively. The additional signals in Figure 6C and in Figure 7B thus confirmed PLA degradation (Espartero et al., 1996). It is therefore, highly probable that nZnO with -OH groups and adsorbed H2O on the surface took part in the degradation process of PLA at elevated temperatures. Namely, two mechanisms of PLA degradation were proposed (Al-Itry et al., 2011): the first one based on hydrolytic degradation and the second on β-hydrogen transfer. In case of the second degradation mechanism the product should be vinyl terminated PLA, however in 1H or 13C NMR spectra (Figure 6 and 7) no signals of vinyl protons or carbons were observed, so this mechanism was ruled out.

Figure 6. 1H NMR spectra of PLA/nZnO composites degraded at 185 °C as a function nZnO concentration: A) 0 wt%, B) 1 wt%, C) 10 wt%.

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Figure 7. 13C NMR spectra of PLA/nZnO composites degraded at 185 °C as a function nZnO concentration: A) 0 wt%, B) 10 wt%. The 1H NMR spectrum of neat PHBV is given in Figure 8A. The spectrum showed signals of methyl protons at 1.26 ppm, and methylene protons between 2.4 and 2.6 ppm as well as signals of methine proton at 5.24 ppm in the polymer chain. Besides strong 3-hydroxybutyrate signals there were also weak signals of 3-hydroxyvalerate side chain at 0.88 and 1.6 ppm indicating a low mass share (~3 wt.%) of this comonomer in the PHBV copolymer (Wei et al., 2014). The spectrum of PHBV exposed to 185 °C showed significant changes of signals at 1.26, 2.5 and 5.25 ppm as well as the emerging signals at 1.84, 5.85 and 7.00 ppm indicating a severe thermal degradation of PHBV (Figure 8B) (Žagar and Kržan, 2004). The addition of 1.0 wt. % nZnO accelerated the degradation of PHBV since signals at 1.84, 5.85 and 7.0 ppm became the predominant signals (Figure 8C). By adding 10 wt.% of nZnO, PHBV was almost quantitatively degraded into 2-butenoic acid as indicated by the presence of well defined signals at 1.84, 5.85 and 7.0 ppm (Figure 8D) after 24 h at 185 °C. The results of NMR spectroscopy were not completely consistent with conclusions drawn from FTIR spectra most probably because NMR showed only the spectrum of the fraction soluble in CDCl3 (2butenoic acid) while FTIR spectrum shows all the degradation products in the sample. The most probable decomposition product is a mixture of 2-butenoic acid and its Zn salt as well as PHBV oligomers and their Zn salts (Li et al., 2003).

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Figure 8. 1H NMR spectra of neat PHBV and PHBV/nZnO composites degraded at 185 °C as a function nZnO concentration: A) neat PHBV, B) 0 wt%, C) 1 wt%, D) 10 wt%. Based on the results of FTIR and NMR results reaction mechanisms of nZnO participation in PLA or PHBV degradation were confirmed. In the case of PLA, two reaction mechanisms of nZnO participation in PLA degradation were proposed (Scheme 1 A and B Supp.Mat.). The first one was hydrolytic degradation of PLA producing oligomers with carboxylic end groups which reacted with hydroxyl groups on the surface of ZnO particles, forming the Zn salt (Scheme 1A Supp.Mat.). The second mechanism is a degradation reaction where hydroxyl groups on Zn directly attacks the carbonyl group of PLA, causing chain scission and formation of a Zn carboxylic salt (Scheme 1B Supp.Mat.) The formation of Zn salts shifts the reaction equilibrium in both cases strongly towards the PLA degradation and Zn salt formation and, thus, explained the accelerating effect of nZnO on the degradation of PLA (AlItry et al., 2012). In case of PHBV the first step was also hydrolytic degradation of the PHBV chain producing oligomers with carboxylic end groups on one side and unsaturated moieties on the other side, followed by the further reaction of carboxylic groups with Zn-OH on the ZnO surface forming Zn salts which shifted the equilibrium towards PHBV degradation thus accelerating the degradation process (Hablot et al., 2008). The final products of degradation were lactic acid and Zn dilactate in the case of PLA (Schwach et al., 1998) and 2-butenoic acid and its Zn salt in the case of PHBV (Li et al., 2003). From these results we conclude that nZnO behaves as an accelerator and a reactant in the degradation reaction of PLA and PHBV but its effect on PHBV degradation is 14

significantly lower than on PLA degradation. This is in agreement with the report that traces of metal ions including Zn2+ ion drastically decreased the thermal stability of PLA by catalyzing the degradation reaction in comparison to moisture which showed only minimal effect (Cam and Marucci, 1997), whereas Fan et al. (2004) found out that removal of metal ions significantly increased the thermal stability of PLA. Concerning the degradation stability in the presence of nZnO, experiments showed that ZnO has more pronounced effect on PLA than PHBV. This can be explained by higher polarity of PLA polymer chains in comparison to those of PHBV since the latter contain nonpolar -CH2-CH2- segments and longer side chains due to presence of valerate comonomer (Scheme 2 Supp.Mat.). On the basis of experiments we conclude that in PLA the addition of nZnO higher than 0.1 wt.% is not recommendable while in PHBV, nZnO in concentrations higher than 0.1 wt.% can be used, if the thermal and mechanical properties of pure PHBV after melt processing operations are acceptable for a potential application.

4. CONCLUSIONS

Composites of polylactide or poly(3-hydroxybutyrate-co-3-hydroxyvalerate) and ZnO nanoparticles were prepared by deposition of nZnO on PLA or PHBV granules and by subsequent melt processing. SEM micrographs showed that the ZnO nanofiller formed aggregates with sizes between 0.5 and 15 μm in PLA matrix and between 0.5 and 15 μm in PHBV matrix. Consequently, the transparency for visible light of PLA/ZnO composites significantly reduced even at the lowest nZnO concentration (0.1 wt.%). The absorption of UV light was higher than 95% at 1.0 wt.% of added nZnO. Nano ZnO acted as a disruptor in the crystallization process of PLA as shown by DSC measurements. The addition of nZnO shifted the Tg of PLA to a lower temperature which was explained by PLA degradation. In case of PHBV the effect of ZnO on its thermal properties was less pronounced with only slight changes of Tg and a slightly reduced degree of crystallinity. TGA curves clearly indicated that nZnO affected the degradation of PLA while there is little effect on the PHBV degradation during melt processing. TGA also revealed higher thermal stability of neat PLA than PHBV. Comparing the degradation of PLA and PHBV in the presence of nZnO, a more pronounced effect on thermal properties and crystallinity of PLA than on PHBV properties was observed. SEC chromatography showed that both PLA and PHBV were degraded during extrusion and injection moulding. The addition of nZnO significantly accelerated PLA degradation process 15

as indicated by the reduced Mw and Mn values while the effect of nZnO on degradation of PHBV was almost negligible. PLA and PHBV degradation as well as participation of nZnO as the reactant and the accelerator in the degradation process was also confirmed by FTIR and NMR spectroscopy of composites which were exposed to 185 °C for 24 h. At these conditions PHBV is much less thermally stable than PLA but the addition of ZnO caused a rather small additional degradation. We conclude that nZnO has a more pronounced accelerating effect on the degradation of PLA than on PHBV. Therefore, pure nZnO should not be used in concentrations higher than 0.1 wt.% for the preparation of PLA/nZnO composites, and the melt processing time should be minimized as much as possible. Alternatively, ZnO should be coated with an inert layer to prevent direct contact between the PLA and nZnO. When PHBV is used as a polymer matrix the concentration of nZnO can be higher (up to 1 wt.%), if the properties of pure PHBV after melt processing are acceptable for the intended application.

Acknowledgement The authors gratefully acknowledge the financial support of the Slovenian Research Agency and Ministry of Education, Science and Technology of the Republic of Slovenia (research core funding No. P2-0145).

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Legends of figures Figure 1. SEM micrographs: (a) PLA/nZnO composite and (b) PHBV/nZnO composite both with 1 wt.% of nano ZnO. Figure 2. Normalized DSC curves of the first heating as a function of nZnO concentration: (a) PLA/nZnO composites: A) PLA, B) 0.1 wt.%, C) 0.5 wt.%, D) 1.0 wt.% and (b) PHBV/ZnO composites: A) PHBV, B) 0.1 wt.%, C) 1.0 wt.%. Figure 3. TGA curves (first derivatives of weight loss) of PLA/ZnO composites (a) and of PHBV/ZnO composites (b) as a function of nZnO concentration: A) 0 wt.%, B) 0.1 wt.% and C) 1.0 wt.%. Figure 4. FTIR spectra of (a) PLA/nZnO composites degraded at 185 °C as a function of nZnO concentration: A) neat PLA, B) 0 wt%, C) 1 wt%, D) 10 wt% and (b) PLA/nZnO composites: A) 10 wt.% - before degradation , B) 10 wt% - after degradation. Figure 5. FTIR spectra of (a) PHBV/nZnO composites degraded at 185 °C as a function of nZnO concentration: A) neat PHBV, B) 0 wt%, C) 1 wt%, D) 10 wt% and (b) PHBV/nZnO composites: A) 10 wt.% - before degradation , B) 10 wt% - after degradation. Figure 6. 1H NMR spectra of PLA/nZnO composites degraded at 185 °C as a function nZnO concentration: A) 0 wt%, B) 1 wt%, C) 10 wt%. Figure 7. 13C NMR spectra of PLA/nZnO composites degraded at 185 °C as a function nZnO concentration: A) 0 wt%, B) 10 wt%. Figure 8. 1H NMR spectra of neat PHBV and PHBV/nZnO composites degraded at 185 °C as a function nZnO concentration: A) neat PHBV, B) 0 wt%, C) 1 wt%, D) 10 wt%.

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