Accepted Manuscript

PLA-ZnO nanocomposite films: water vapor barrier properties and specific end-

use characteristics

Roberto Pantani, Giuliana Gorrasi, Giovanni Vigliotta, Marius Murariu,

Philippe Dubois

PII: S0014-3057(13)00414-X

DOI: http://dx.doi.org/10.1016/j.eurpolymj.2013.08.005

Reference: EPJ 6196

To appear in: European Polymer Journal

Received Date: 30 December 2012

Revised Date: 30 July 2013

Accepted Date: 4 August 2013

Please cite this article as: Pantani, R., Gorrasi, G., Vigliotta, G., Murariu, M., Dubois, P., PLA-ZnO nanocomposite

films: water vapor barrier properties and specific end-use characteristics, European Polymer Journal (2013), doi:

http://dx.doi.org/10.1016/j.eurpolymj.2013.08.005

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1

PLA-ZnO nanocomposite films: water vapor barrier properties and specific

end-use characteristics

Roberto Pantani[a]*

, Giuliana Gorrasi[a]

, Giovanni Vigliotta[b]

, Marius Murariu[c]

and Philippe

Dubois[c]

[a] Department of Industrial Engineering, University of Salerno, via Giovanni Paolo II, 84084 Fisciano (SA)-Italy

[b] Department of Chemistry and Biology University of Salerno, via Giovanni Paolo II, 84084 Fisciano (SA)-Italy

[c] Center of Innovation and Research in Materials & Polymers (CIRMAP), Laboratory of Polymeric and Composite Materials (LPCM), University of Mons & Materia Nova Research Centre, Place du Parc 20, 7000 Mons, Belgium

* e-mail: rpantani@unisa.it

PLA nanocomposite films with multifunctional characteristics such as mechanical, anti-UV,

antibacterial, electrical, gas barrier properties are potentially of high interest as packaging

biomaterials. Occasionally, desired and beneficial effects obtained by addition nanofillers come

along with some drawbacks, leading to the sharp drop in the molecular weights of the polyester

chains, and consequently an important loss of mechanical and thermal properties. Novel PLA-

ZnO nanocomposite films were produced by melt-compounding PLA with 0.5-3% ZnO rod-like

nanoparticles. The surface treatment of nanofiller by silanization (with triethoxy caprylylsilane)

was necessary to obtain a better dispersion and to limit the decrease of molecular mass of PLA.

The morphology, molecular, thermo-mechanical and transport properties to water vapor of PLA-

ZnO films were analyzed with respect to the neat PLA. According to DSC and to XRD, the

2

produced films were essentially amorphous. The changes in PLA permeation properties were

strongly dependent on temperature and nanofiller loading. The well dispersed ZnO nanoparticles

within the polyester matrix were effective in increasing the tortuosity of the diffusive path of the

penetrant molecules. The activation energy remained similar for PLA and PLA-1% ZnO, but was

found greater at higher loading of ZnO (3%), confirming the increased difficulty of travelling

molecules to diffuse through PLA. In comparison to the neat PLA (presenting no antimicrobial

efficacy), the nanocomposites were active against both Gram positive and Gram negative

bacteria, stronger antibacterial activity being evidenced after 7 days elapsed time. By considering

the multifunctional properties of PLA-ZnO nanocomposites, the films produced by extrusion can

be considered a promising alternative as environmental-friendly packaging materials.

Keywords: Poly(lactic acid); nanocomposite films; zinc oxide; packaging; films; water vapor

permeability.

3

1. Introduction

The need to extend the shelf life of packaged food has brought the research to innovative

solutions along with the changing needs of consumers, making the "packaging" a constantly

evolving field [1-3]. The current trend is directing the research towards the development of

innovative solutions both for functional packaging (active packaging and nanocomposite

materials) and low environmental impact (biodegradable materials, recyclable packaging with

reduced size). Most films, used to preserve food stuff, have been produced from synthetic

polymers. Nevertheless, for environmental reasons, attention has lately been focused on

biodegradable polymers for the preparation of food packaging films [4–13]. These films are

usually loaded with antimicrobial agents that come into contact with food stuff, act on food-born

microorganisms and inhibit their growth [14-21]. Therefore, the current research has been

focused on the search for new bactericides that can effectively reduce the harmful effects of

microorganisms. With the emergence of nanotechnology, the search for effective biocidal agents

has focused on the development of nanostructure of coinage metals like silver, copper, zinc and

gold [22]. However, the high cost of silver and gold metals has limited their use as antibacterial

agents on industrial basis. Therefore, currently metal oxide nanoparticles, such as ZnO, have

emerged out as a new class of important materials that are increasingly being developed for use

in research and health-related applications, because of their low cost, easy availability, and

unique chemical and physical properties. Recently, several reports have described the

antimicrobial activity of ZnO nanoparticles [23-26].

Polylactide (PLA) is an environmental friendly, economical and commercially available

polymer that offers great potential as disposable packaging material [27-31].

4

ZnO nanoparticles are well-known environmentally friendly and multifunctional inorganic

additives that could be considered as nanofillers for various polymers providing properties like

antibacterial effect or intensive ultraviolet absorption [32, 33]. In this context, it is certainly of

interest to endow PLA with antibacterial properties (and intensive ultraviolet absorption) and

addition of ZnO nanoparticles to PLA could represent a relevant approach [34-36].

Unfortunately, the addition of untreated ZnO nanoparticles into PLA at melt-processing

temperature leads to severe degradation of the polyester matrix, ascribed to the transesterification

reactions and ‘unzipping’ depolymerization of PLA. By contrary, as reported elsewhere [34],

ZnO adequately surface-treated by selected silanes can lead to PLA-based nanocomposites

characterized by quite good preservation of PLA intrinsic molecular parameters and related

physicochemical characteristics.

In this paper we report the preparation of PLA-ZnO nanocomposites in a composition range from

0.5 to 3% nanofiller, followed by the extrusion of films. This is one of the first studies

concerning the main characteristics of PLA-ZnO nanocomposite films, with a special focus on

the water vapor barrier properties resulted from nanofiller incorporation. The morphology of the

so-produced films and their thermal, mechanical and transport properties (sorption (S), diffusion

(D), permeability (P)) to water vapor were analyzed. The investigation of transport properties

was conducted at three different temperatures to evaluate also the activation energy of the

diffusion phenomenon. In order to test the used ZnO as antimicrobial agent in the prepared film

nanocomposites, two different types of bacteria (Gram-positive and Gram-negative) were used to

evaluate the degree of inhibition of bacterial growth depending on the incubation time and filler

amount.

5

2. Experimental

2.1 Materials

Poly(L,L-lactide) – hereafter called PLA, supplier NatureWorks LLC, was a grade designed

for realization of films (4032D) with Mn= 133 000, dispersity, Mw/Mn = 1.9, whereas according

to producer information the other characteristics are as follows: D isomer = 1.4%; relative

viscosity = 3.94; residual monomer = 0.14%. Commercially available ZnO nanofillers (rod-like

particles) were kindly supplied by Umicore Zinc Chemicals (Belgium) as Zano 20 Plus (surface

coated with a silane especially suitable for the treatment of metal oxides, i.e., triethoxy

caprylylsilane; ZnO content: 96.2 ± 0.5%, bulk density: 360 g/L). A thermal stabilizer, Ultranox

626A, (bis (2,4-di-t-butylphenyl) pentaerythritol diphosphite) was used at 0.3% in PLA.

Throughout this contribution, all percentages are given as wt %.

2.2 Preparation of nanocomposites and films

PLA-ZnO nanocomposites were produced by melt compounding PLA with up to 3% ZnO

nanofiller in a Leistritz twin-screw extruder (type ZSE 18 HP-40D, diameter of screws (D) = 18

mm, L/D = 40). The previously dried granules of PLA and additives were first mixed in a

Rondol turbo-mixer (2000 rpm, 2 min) with 0.5, 1, 2 and 3% ZnO, followed by dosing and melt

compounding in twin-screw extruder (throughput of 1.5 kg/h, speed of the screws = 100 rpm,

temperature of the molten polymer ~185 °C). For the sake of comparison, unfilled PLA

containing only thermal stabilizer was processed in similar conditions of melt-compounding. The

granules of unfilled PLA and PLA-ZnO nanocomposites were dried (at 80 °C overnight, under

vacuum) and used for the production of films by extrusion. Films with a thickness of about 150

µm were obtained using a DSM twin-screw microcompounder (batch-volume: 15 mL, speed of

6

screws: 70 rpm, temperature of molten polymer: 185-190 °C) equipped with a flat die (width: 35

mm, die opening: 0.4 mm) and a DSM Xplore microfilm device.

2.3 Methods of characterization

Molecular weight parameters (number average molar mass, Mn, and dispersity index, Mw/Mn)

of unfilled PLA and PLA-ZnO films were determined by size exclusion chromatography (SEC).

Recovery of PLA from selected compositions for molecular weight parameters determination

was carried out by firstly dissolving the samples in chloroform. The metallic residues were

removed by liquid–liquid extraction with a 0.1N HCl aqueous solution, step followed by

intensively washing with demineralized water. Finally, PLA was recovered by precipitation in an

excess of heptane. After filtration and drying, PLA solutions were prepared in chloroform (10

mg polymer / 5 ml solvent). Molecular weight parameters of pristine PLA and those of PLA

extracted from the studied nanocomposites were determined by SEC after a previously filtration

of PLA solutions using filters of 0.45 µm.

X-ray diffraction measurements (XRD) were performed with a Bruker diffractometer

(equipped with a continuous scan attachment and a proportional counter) with Ni-filtered Cu Kα

radiation (λ = 1.54050 Å).

Differential scanning calorimetry (DSC) analysis was carried out on samples with a mass

ranging between 8 and 12 mg. The tests were carried out by means of a DTA Mettler Toledo

(DSC 30) under nitrogen atmosphere. The samples were heated from -60 °C to 250 °C at 10

°C/min. To ensure reliability of the data, heat flow and temperature were calibrated with standard

materials, i.e., indium and zinc. The events of interest, i.e., the glass transition temperature (Tg)

and the associated enthalpy of relaxation (∆Hrel), cold crystallization temperature (Tc), enthalpy

of cold crystallization (∆Hc), melting temperature (Tm) and melting enthalpy (∆Hm), were

7

evaluated. The degree of crystallinity (χc) was determined by subtracting ∆Hc from ∆Hm, and by

considering a melting enthalpy of 93 J/g for 100% crystalline PLA [34].

Barrier properties (sorption, diffusion and permeability) were evaluated using a microbalance

SMS DVS Advantage-2 system. This system has a sensitivity of ±1.0µg, and allows the

measurements of mass changes due to sorption or desorption of vapor molecules. In this work

the chosen method consisted in submitting the sample to pressure steps at constant temperature.

The tests were conducted using water vapor in a nitrogen atmosphere at 30 °C. The starting

samples were dry, square films having a thickness of 150µm and a side of 15mm. The

experimental protocol considered steps of relative humidity from 0 to 80%. The chosen

temperatures were: 15; 30 and 45 °C.

Tensile testing measurements were performed using a Lloyd LR 10K tensile bench on strips

cut from films (63.5 x 10 x ~0.15 mm3) at a speed rate of 0.5 mm/min, with the distance of 40

mm between grips. All tests were carried out on specimens previously conditioned for at least 48

hours at 20 (±2) °C under a relative humidity of 50 (±3) % and the values were averaged out

over minimum five measurements from each sample.

Opacity measurements were performed using a Konica Minolta CM-2500d X-Rite SP60 Series

spectrophotometer. Following the ASTM E284 (“Terminology of Appearance”), the opacity was

defined as ability of a thin film to hide a surface behind and in contact with it, expressed as the

ratio of the reflectance factor (Rb) when the material is backed by a black surface to the

reflectance factor (Rw) when it is backed by a white surface (usually having a reflectance factor

of 0.89). The opacity (O) was calculated using the relationship:

O (%) = (Rb/Rw) x 100 1

8

In order to evaluate the antibacterial effect at different ZnO loading, Escherichia coli and

Staphylococcus aureus were pre-inoculated in Luria-Bertani (LB) medium (10 gL-1 trypton, 5

gL-1 yeast extract, 10 gL-1 NaCl) at 37 °C and grown for 12 h in aerobiosis at 250 rpm.

Subsequently, bacteria were collected by centrifugation for 10 min at 3500 g, re-suspended at

concentration of 0.005 OD600 in presence of 1 cm2 of polymeric films, each cut into four equal

parts, and incubated in aerobiosis to 37 °C at 250 rpm. E. coli more resistant to oligotrophic

conditions was re-suspended in distilled, sterile water, while S. aureus more sensitive to lack of

nutrients was re-suspended in diluted (50% vol/vol) peptone water (1 gL-1 peptone, 8.5 gL-1

NaCl). For cell survival determination at indicated times 50 µl of different dilutions of each

suspension were spread on LB agar dishes (15 gL-1 agar), incubated for 24 hours and colony

forming units (CFU) calculated. The obtained values were used to calculate the antibacterial

activity (A) as previously reported [34]. Briefly, was applied the formula:

A = F – G 2

F represents the growth values in presence of unfilled PLA samples (control/without ZnO),

while G corresponds to the growth values of filled samples (PLA-ZnO films). They are

calculated according to the formula F = Log (C24h/7d - Log C0) and G = Log (T24h/7d - Log T0). C

and T are CFU detected respectively for control and filled nanocomposite material at different

times, i.e., 0, 24 hours and 7 days.

Antibacterial efficacy was tested on films with 1, 2 and 3% ZnO. The PLA-ZnO films were

considered “antimicrobial” when achieving an A superior to 2 (reduction in bacteria number > 99

%).

Transmission electron microscopy (TEM): TEM images of nanofiller and selected PLA-ZnO

nanocomposites were obtained with a Philips CM200 apparatus using an accelerator voltage of

9

up to 120 kV. The nanocomposite samples (70–80 nm thick) were prepared with a Leica Ultracut

UCT ultracryomicrotome by cutting at -100 °C.

3. Results and discussion

Preliminary considerations

In order to make PLA matrix less susceptible to the catalytic action of ZnO nanoparticles

during the melt blending process and subsequent film processing, the nanofiller surface treatment

with silane agents was considered as a prerequisite, leading to significant enhancements of both

thermo-mechanical and molecular properties with respect to the use of untreated ZnO [34]. In

relation to the transport properties of PLA films, it is noteworthy to mention that according to H.

Tsuji et al. [37], the changes in Mn of PLLA films in the range of 9 104 - 5 105 g mol-1and D-

lactide unit content up to 50%, lead to insignificant effects on the water vapor transmission rate

(WVTR). In contrast, the WVTR of PLA films were found to decrease monotonically with

increasing the degree of crystallinity from 0 to 20%, while levelling off for values exceeding

30%.

Table 1 reports the modification of PLA molecular characteristics upon films production. PLA

is very sensitive to hydrolysis, shearing and to the contact with Zn-based products. The

molecular characterization of unfilled PLA and PLA-ZnO nanocomposite films confirms the

expected decreasing of Mn and changes of dispersity index after the melt-compounding and

following the second processing to produce films by extrusion.

The Mn of PLA was about 101 000Da, whereas the films containing 1 and 3% ZnO were

characterized by a Mn of 78 500Da and 68 200Da, respectively. Accordingly, a noticeable

10

decrease of Mn (about 32 000Da) and significant change of dispersity index are seen even for the

neat PLA film. However, as previously reported [34], once more it is confirmed that the addition

of silane treated ZnO does not lead to a dramatic drop of PLA molecular weights, which was not

the case of untreated nanofiller. Based on previous data reported in the literature [37], for an

easier interpretation of the results we will assume that the barrier proprieties of the films are

marginally influenced by the molecular parameters, whereas the information concerning the level

of crystallinity is presented in the next section.

Table 1. Determination of molecular weights and dispersity index of neat PLA and PLA-ZnO

films

Entry Sample (%, by weight) Dispersity index Mn [Da]

1 PLA (granules) 1.9 133 000

2 PLA (0% ZnO) 2.6 101 200

3 PLA- 1% ZnO 2.6 78 500

4 PLA- 3% ZnO 2.9 68 200

Morphology and crystallinity

In the case of the studied nanocomposite films, the system contains a continuous phase

represented by the PLA matrix and a dispersed phase, the ZnO nanofiller. Following the TEM

investigations (Fig. 1a) the nanoparticles are characterized by a rod-like morphology, typically

with diameters of ~15-30 nm and a length up to around 100 nm. Mineral surfaces covered with

hydroxyl groups such as ZnO are generally very receptive to the bonding with alkoxysilanes and

more than one (mono) layer of silane is usually applied onto the surface of the filler, which could

11

play the role of compatibilizer and dispersion agent. In the nanocomposites with 1% naanofiller

the ZnO nanoparticles are quite well distributed and dispersed through the PLA matrix (Fig. 1b

and c), since the presence of aggregates is difficult to be observed in the TEM pictures.

Furthermore, by increasing the loading of ZnO at 3% the distribution of the nanofiller is

remaining reasonably good (Fig. 1d), whereas the TEM images realized at higher magnification

(Fig. 1e) are proving that are only few zones where some ZnO nanorods are associated as small

clusters.

On the other hand, even though the loading (up to 3%) and consequently the volume fraction

of ZnO nanofiller is not so high in nanocomposite films, it is believed that the nanoparticles can

provoke a tortuosity to water vapor, pure or mixed gas, such in the case of other

(nano)composites, finally leading to changes of PLA inherent permeation properties. [38-40]

12

Figure 1 (a – e). TEM images at different magnifications to illustrate the morphology of ZnO

nanoparticles (a) and those of PLA nanocomposites filled with 1% ZnO (b and c) and 3% ZnO (d

and e)

13

Furthermore, in semi-crystalline polymers, the crystalline regions are considered to be

impermeable to the vapor molecules, and in this context it is of interest to have information

about the morphology and structures of initial samples as they are evaluated by the different

techniques. However, by considering the specific experimental procedure, it is assumed that the

PLA crystallinity of prepared films could be mainly influenced by the molecular characteristics,

the content of D-isomer, the parameters of processing by extrusion (residence time, shear,

drawing and cooling to produce the films), etc. The quantification of PLA crystallinity on films

was carried out mainly by DSC whereas XRD was used as additional tool of investigation.

Table 2. Calorimetric data from DSC thermograms of PLA and PLA-ZnO films

Sample Tg

(°C)

∆Hrel

(J/g)

Tc

(°C)

∆Hc

(J/g)

Tm

(°C)

∆Hm

(J/g)

χc

(%)

PLA 64 5.8 108 26.4 169 32.5 6.6

PLA- 0.5% ZnO 65 5.6 110 32.2 170 35.0 3.0

PLA- 1% ZnO 64 6.1 111 34.2 171 36.8 2.8

PLA- 2% ZnO 64 4.7 110 32.3 171 35.1 3.0

PLA- 3% ZnO 63 5.5 111 34.1 172 36.7 2.8

Figure 2 shows the DSC thermograms of all nanocomposites and of neat PLA, whereas the

calorimetric parameters are summarized in Table 2. All samples are characterized by a marked

exothermic crystallization as confirmation of the amorphous or low crystalline starting

structures. The glass transition temperature (Tg) and the endothermic enthalpy of relaxation

(∆Hrel) do not show significant modifications with nanofiller addition during the first DSC scan.

This phenomenon of enthalpic relaxation is typical for a polymeric material in the glassy state

that undergoes physical ageing [28, 41]. The samples display a similar cold crystallization

14

temperature (Tc), whereas the nanocomposites show higher enthalpy of cold crystallization (∆Hc)

with respect to neat PLA. However, it is of interest to note that all nanocomposite films were

characterized by a very low degree of crystallinity (χc), i.e., about 3%, while neat PLA showed a

slightly higher value, i.e., 6.5%. Accordingly, the analyzed films are largely characterized by an

amorphous structure, results that are not surprising by considering the poor crystallization ability

of PLA [42, 43]. Furthermore, it is assumed that the eventual orientations and rearrangements of

macromolecular chains into a crystalline structure under the specific extrusion conditions are

rather negligible. Following the DSC analysis, it is evident that the addition of up to 3% ZnO

(thus surface-treated with silane agents), does not lead in this typical experiment to an increase of

PLA crystallinity, a conclusion that is also supported by the results of XRD investigations.

Figure 2. DSC thermograms of neat PLA and all PLA-ZnO nanocomposites. The curves are

arbitrarily shifted along the vertical axis

Figure 3 shows the XRD patterns of neat PLA and all studied nanocomposite films. On one

hand, all samples show a broad intensity with a maximum appearing at approximately 2θ = 17°

15

indicating mainly an amorphous structure [44]. Furthermore, according to the XRD

measurements and consistent with the DSC results, the samples filled with 1 - 3% ZnO are

completely amorphous or only slightly crystalline. On the other hand, due to some traces of

crystallinity, only the neat PLA and the nanocomposites film containing the lower amount of

ZnO (0.5%) exhibit an evident crystallization peak (close to 2θ of ~ 16.5°) ascribed to the

diffraction of (200) and/or (110) planes of the typical orthorhombic crystal of PLA. [45].

Furthermore, the specific peaks evidenced at 2θ= 31.6° and 2θ= 36.2° can be ascribed

respectively to the diffraction planes of (100) and (101) of the crystalline form of ZnO, while

their intensity is clearly increasing with nanofiller loading [46]. Finally, because the investigated

PLA samples were mainly amorphous, in the discussion of results it was considered that the

crystallinity is not an additional parameter that can influence the transport properties.

Figure 3. X-ray diffraction patterns for PLA and all PLA-ZnO nanocomposites

Barrier properties

It is well known that biodegradable polyesters, like PLA, are moisture sensitive [47-48], so the

analysis of transport properties (sorption, diffusion and permeability) to water vapor is of

16

fundamental importance for developing new packaging materials. Water molecules, at different

activity and temperatures, could have effect on the microbial growth and influence the shelf life

of the packed products. On the other hand, the presence of filler could influence the transport

phenomena of small molecules through a polymeric matrix either from a thermodynamic or a

kinetic viewpoint. For this purpose, we performed the analysis of the thermodynamic parameter,

sorption (S), the kinetic parameter, diffusion (D), and their product, the permeability (P = S x D)

at three different temperatures (i.e., 15, 30 and 45 °C) on unfilled PLA and PLA filled with 1%

and 3% ZnO.

Table 3. Sorption parameters, S (wt%/atm), evaluated from isotherms of Fig. 4 using Eq. 3

Sample →

Temperature , °C↓

PLA PLA-

1% ZnO

PLA-

3% ZnO

15 44.90 46.15 46.05

30 20.58 20.50 20.22

45 9.94 10.00 9.98

Sorption

Some modifications of polymers by swelling, hydrolysis or plasticization can result from the

sorption of water, which can be accelerated by the presence of polar and hydrophilic groups. [47]

Sorption isotherms were obtained for PLA and nanocomposites loaded with 1% and 3% ZnO.

Figure 4 reports the equilibrium concentration of water vapor, Ceq (wt%), as function of water

partial pressure, P (atm) at the investigated temperatures for unfilled PLA and PLA-ZnO

nanocomposite films. All samples show an ideal behaviour at low pressure, that allowed

evaluating the sorption coefficient, S, from Henry’s law:

17

Ceq= S x P 3

For isotherms at 15 and 30 °C we considered the first four points, very well interpolated from

Henry’s model. In the case of isotherms evaluated at 45 °C, for the evaluation of sorption, we

have taken into account the first three points, which ensure an ideal interaction matrix-penetrant.

It is evident that, in all cases, at high pressure the isotherms deviate from linearity following a

Flory-Huggins mode of sorption [48] (see the arrows in the plots). According to this behaviour

there is preference for the formation of penetrant-penetrant pairs, so that the solubility coefficient

continuously increases with penetrant pressure. The first molecules sorbed tend to locally loose

the polymer structure and make it easier for the following molecules to enter. These isotherms

are observed when the penetrant effectively plasticizes the polymer, being a strong solvent or

swelling agent for the polymer, like is water vapour for PLA matrix. Table 3 reports the S

(wt%/atm) parameters, evaluated according to Eq. 3, for the analyzed samples at the three

studied temperatures. At constant temperature, there is not evident variation of S with increasing

ZnO loading. Comparing the data evaluated at different temperatures, it is possible to observe a

decreasing of sorption with increasing the temperature, in agreement with the thermodynamics of

the system.

18

Figure 4. Ceq of water vapor (wt%) vs. the partial pressure of water vapour at different

temperatures for PLA, PLA- 1% ZnO and PLA- 3% ZnO.

19

Table 4: Thermodynamic diffusion coefficients, Do (cm2/s), at different temperatures (15; 30, 45 °C) and diffusion activation energy, ED (kJ/mol) for PLA and nanocomposite films loaded with 1% and 3% ZnO. Do and ED values should be considered as having an accuracy of about ±10%. Sample

Temperature,

°C

Diffusion Coefficient, Do (cm2/s)

Diffusion Activation Energy (ED), kJ/mol

PLA

15

30

40

3.27 10-8

7.75 10-8

10.4 10-8

29.60

PLA- 1% ZnO

15

30

40

3.75 10-8

5.71 10-8

11.7 10-8

28.73

PLA- 3% ZnO

15

30

40

2.51 10-8

4.85 10-8

11.1 10-8

37.63

Diffusion

Following the increasing of sample weight as function of time, it was possible to evaluate the

diffusion coefficient, D, at different water pressures [49]. The equation used to evaluate the

diffusion parameter at different water vapor partial pressures is as follows:

4

in which M(t) is the mass of the sample at each time, M0 is the value at the beginning of the

test, M∞ is the value at equilibrium, h is the thickness of the sample. Eq. 4 is valid for times for

which the ratio [(M-M0)/(M∞-M0)] is larger than 0.4. Plotting ln(d Mt/dt) vs. time, the value of D

20

(cm2/s) is calculated at each partial pressure from the slope of the curve. Figure 5 shows an

example of the procedure adopted to determine the diffusivity. On the left axis it is possible to

read measurements of the normalized mass of PLA at 30 °C (step of humidity from 0 to 20%).

The squares show the calculations performed according to Eq. 4. The range of points to fit was

always determined by a mass change between 40% and 90%. Accordingly, for a time long

enough the considered model fits very well the experimental data. By the knowledge of sample

thickness, the diffusion coefficient is evaluated from the slope of the line.

Figure 5. Example to illustrate the procedure adopted to determine the diffusion coefficients.

The diffusion parameters obtained using Eq. 4 were plotted as function of water percentage

sorbed at the different pressures. For polymer-solvent systems, the diffusion parameter is usually

not constant, but depends on the vapor concentration, according to the empirical Eq. 5:

D = Do exp (γ Ceq) 5

where Do (cm2/s) is the zero concentration diffusion coefficient (related to the fractional free

volume and to the microstructure of the polymer); γ is a coefficient which depends on the

fractional free volume and on the effectiveness of the penetrant to plasticize the matrix.

21

Figure 6 reports the diffusion coefficient, D (cm2/s), as function of the equilibrium

concentration of water vapor, Ceq (wt%), for unfilled PLA and nanocomposite films (1 and 3%

ZnO) at 15, 30 and 45 °C. It is evident that the diffusion is independent of water vapor

concentration at any investigated temperature, so that the Do values extrapolated at Ceq= 0, with

very good degree of approximation can be considered equal to D at any vapor pressure. Do

values are reported in Table 4. As expected, on increasing the temperature, Do increases.

Therefore, the diffusion process is faster by increasing polymer free volume and at higher

mobility of polymeric chains. At constant temperature a slight decrease of D with ZnO loading

can be observed, more evident at T= 30 °C. A great number of data in literature suggest that the

diffusion coefficient of vapors in polymers depends on temperature, via an Arrhenius’ law on a

narrow range of temperatures following the Eq. 6:

6

where Do represents the limit value of the transport coefficient for T (temperature) →∞ and ED

(kJ/mol) is the diffusion activation energy, i.e., the energy level that a molecule must reach to

diffuse inside the polymeric matrix. In our case, D being constant with Ceq, it was possible to

plot Do vs. 1/T (T in K) to evaluate the activation energies (Figure 7) that are reported in Table 4.

It is evident that such values remain quite constant for PLA and PLA- 1% ZnO, but are higher

for the sample containing 3% ZnO, confirming the increased difficulty of the travelling

molecules to diffuse into the polyester matrix.

22

0.00 0.20 0.40 0.60 0.80 1.00

Ceq (wt%)

1.00E-8

1.00E-7

1.00E-6

D (

cm

/s)

2

PLA+3% ZnO

15°C

30°C

45°C

0.00 0.20 0.40 0.60 0.80 1.00

Ceq (wt%)

1.00E-8

1.00E-7

1.00E-6

D (

cm /s

)2

PLA+1%ZnO

15°C

30°C

45°C

0.00 0.20 0.40 0.60 0.80 1.00

Ceq (wt%)

1.00E-8

1.00E-7

1.00E-6

D (

cm

/s)

2

PLA

15°C

30°C

45°C

Figure 6. Diffusion coefficient, D (cm2/s), as function of Ceq of water vapour (wt%) at different

temperatures for neat PLA and nanocomposites loaded with 1% and 3% ZnO.

23

Permeability values to water vapor, reported in Table 5, were evaluated as product of sorption

and diffusion following the Eq. 7:

P = S x D 7

Noteworthy, as it comes out from the results shown in the Table 5, the increasing of

temperature triggers an increasing of permeability, while at constant temperature this parameter

does not show significant variation.

Table 5. Permeability parameters to water vapour, P (wt%/atm x cm2/s), for PLA and nanocomposite films loaded with 1% and 3% ZnO

3.00 3.10 3.20 3.30 3.40 3.50

1/T (K) x 10

1.00E-8

1.00E-7

1.00E-6

Do

(cm

/s

)2

PLA

PLA+1% ZnO

PLA+3% ZnO

3

Figure 7. Do (cm2/s) as function of 1/T (K).

Sample →

Temperature , °C↓

PLA PLA-

1% ZnO

PLA-

3% ZnO

15 8.26 10-7 9.81 10-7 6.43 10-7

30 14.7 10-7 10.4 10-7 9.30 10-7

45 14.6 10-7 18.7 10-7 17.5 10-7

24

Antibacterial properties

To assess the antimicrobial effects of ZnO, the nanocomposite films were evaluated against E.

coli and S. aureus, respectively Gram-negative and Gram-positive bacteria. Samples of unfilled

PLA and PLA-ZnO nanocomposites with a surface area of about 1 cm2 and thickness of ~ 150

µm were incubated with each bacterial suspension at 37 °C and constant agitation, harvested at

different times and plated on a rich medium and incubated for 24 h. Subsequently, the colony

forming units (CFU) were determined and the antibacterial activity (A) was measured as

differences between logarithm values of growth of untreated (neat PLA) and treated (PLA-ZnO)

samples.

First of all, it is noteworthy mentioning that the mechanisms responsible for the antibacterial

activity of ZnO nanoparticles are still not fully clear. However, the antibacterial effect is mainly

ascribed to the photocatalytic generation of a large number of reactive oxygen species

(superoxide anions and hydroxyl radicals, generation of hydrogen peroxide), to the formation of

Zn2+ ions or the capture of ZnO onto the cell membrane, etc. [26, 50].

The values of antibacterial activity are summarized in Table 6. On one hand, for E. coli at 24 h

it was found that in presence of ZnO the number of CFU decreased with respect to the control,

but the antimicrobial effect was low (> 70 % of killed bacteria). On the other hand, after 7 days

(see also Fig. 8a) the antibacterial effect was high for all concentrations and even very good for

nanocomposite films containing low amount of nanofiller, i.e., 1% ZnO. In fact following the

values of A that are higher than 2 after 7 days (Table 6) the antimicrobial performance is

considered good or very good, which means a reduction of bacterial population of 99% or more.

For sake of comparison, it is of interest to note that in the absence of ZnO, for the neat PLA the

number of CFU after a similar incubation time is mostly unchanged (see Fig. 8a).

25

In S. aureus antibacterial properties occurred only with films containing 3% ZnO. Similarly to

E. coli, significant reduction of population (>99%) was obtained after 7 days of treatment (A=

2.85) (Table 6 and Fig. 8b).

Table 6. Antibacterial activity (A) of selected PLA-ZnO films against E. coli and S. aureus after different incubation times

Following these results, it is assumed that under the specific experimental conditions the

antibacterial activities to both bacteria were time-dependent. Moreover, similarly to the behavior

of other metal-polymer formulations [51], it is believed that the release of antimicrobial

products, such as reactive oxygen species and/or of Zn2+ ions, can require longer time than 24h

to obtain the minimum inhibitory concentration and optimal effectiveness. The different

sensitivity of E. coli and S. aureus to the concentration of ZnO may be due to structural

differences between the cell wall of Gram positive and Gram-negative bacteria. However this

could also be due to the characteristic of S. aureus to form aggregates which would protect more

internal cells from lower doses of antimicrobial products. Finally as remark, growth (G) values

Sample Average (log CFU) Growth24h Growth 7 d (day) Antibacterial

Activity (24 h) (A24h)

Antibacterial

Activity (7 d) (A7d) 0 h 24h 7d

Escherichia coli

PLA 6.76 6.59 6.62 F= -0.17 F = -0.14 (A24h=F24h–G%_24h) (A7d = F7d – G%_7d)

PLA- 1% ZnO 6.66 5.89 3.20 G1.0% = -0.77 G1.0% = -3.46 0.60 3.32

PLA- 2% ZnO 6.76 5.84 4.38 G2.0% = -0.92 G2.0% = -2.38 0.75 2.24

PLA- 3% ZnO 6.70 6.00 4.30 G3.0% = -0.70 G3.0% = -2.40 0.53 2.26

Stapylococcus aureus

PLA 7.16 7.08 4.98 F = -0.08 F=-2.18 (A24h = F24h – G%_24h) (A7d = F7d – G%_7d)

PLA- 3% ZnO 7.11 6.81 2.08 G3.0% = -0.30 G3.0 = -5.03 0.22 2.85

26

resulted significantly negative for all nanocomposites, as attended by a toxic effect of ZnO,

however, slightly negative values were also found for control populations (F), indicating an

absence of growth and a reduction of the microbial populations in experimental conditions. This

occurred because to reduce interference of nanocomposite materials with the substances

dissolved in growth media, microorganisms were suspended in distilled water (E. coli) or in

diluted peptone water (S. aureus), both non suitable to support the bacterial growth. Nonetheless,

under these conditions the bacteria tested may remain viable several weeks. Thus, based on

antimicrobial activity, ZnO containing nanocomposites showed a high antibacterial effect toward

analyzed Gram positive and negative microorganisms, but the bacterial sensitivity to them was

different, with E. coli killed efficiently at concentrations apparently innocuous for Gram positive.

Mechanical properties and additional end-use characteristics

Mechanical parameters were evaluated from stress-strain curves following the tensile tests

performed at ambient temperature. As shown in Table 7, the addition of surface-treated ZnO into

PLA leads to the slight increase of the rigidity (Young’s modulus, E) whereas both tensile stress

at yield (σy) and at break (σb) reveal interesting values for packaging applications (e.g., σy is in

the range 39 - 44 MPa) as compared to the neat PLA matrix (σy of 45 MPa). However, especially

at high ZnO loading (3%), a slight reduction of the tensile stress was observed, that is reasonably

associated to some diminution of molecular weights and to formation of low molecular products

as aforementioned. Furthermore, like the neat PLA, the nanocomposite films show low

elongation at yield (εy) and break (εb), without an apparent correlation with the nanofiller

amount. To improve this parameter of interest in the perspective of further applications, the

addition of a plasticizer into PLA-ZnO nanocomposite films can be supplementary considered.

27

a

b

Figure 8 (a, b). Evolution of log10 CFU (colony forming units) at various time intervals of E.

coli (a) and S. aureus (b) on the surface of PLA and PLA-ZnO nanocomposite films

28

Table 7: Comparative mechanical properties of PLA and PLA–ZnO nanocomposite films (standard deviations are given in brackets)

*Distance between grips of 40 mm.

The opacity is a parameter that must be considered for packaging applications, since it affects

the appearance of the products. Figure 9 reports the experimental data obtained on films of about

150 micron thickness. The opacity is only slightly increased with a higher ZnO amount. The

opacities were ~10% higher for the films containing 3% ZnO. Such effect, due to the interference

of nanoparticles with the light, must be taken into account since this is relevant for packaging

applications.

Sample

(%- by weight)

E

(MPa)

σy

(MPa)

σb

(MPa)

εy *

(%)

εb*

(%)

PLA 2700 (±200) 45 (±3) 42 (±3) 2.5 (±0.3) 9.3 (±2.4)

PLA- 1% ZnO 2900 (±300) 44 (±5) 41 (±5) 3.3 (±0.7) 13.0 (±4.1)

PLA- 2% ZnO 3000 (±150) 42 (±4) 39 (±3) 1.9 (±0.1) 7.1 (±1.5)

PLA- 3% ZnO 2800 (±100) 39 (±2) 35 (±3) 2.0 (±0.3) 12.9 (±3.0)

29

Figure 9. Opacity of 150 micron-thick films vs. the ZnO content

Finally it is of interest to note that the additional investigations (thermogravimetric analyses

(TGA) and UV–Visible absorption spectra, results not shown here) confirm the previously

reported results [34], respectively, addition of ZnO is leading in some decrease of PLA thermal

stability, whereas once more, at above 1% ZnO the films were characterized by a total anti-UV

protection and good transmittance of the visible light (400–800 nm). By considering the

multifunctional properties that are specific for PLA-ZnO films (mechanical performances, anti-

UV, antibacterial, barrier characteristics) with respect to PLA or other PLA nanocomposites and

additional potential features, such as the possibility of utilization as self-cleaning materials, these

new nanocomposites can present a high interest in traditional and special applications as

biosourced packaging materials.

30

4. Conclusions

The work was focused on the production and characterization of PLA-ZnO films to highlight

their key-characteristics. PLA-ZnO nanocomposites were produced by melt-compounding PLA

and 0.5 - 3% surface-treated ZnO rod-like nanoparticles, step followed by the production of

films. It was demonstrated that it is possible to obtain competitive thin films characterized by

adequate ZnO dispersion and quite good preservation of molecular and mechanical properties

particularly when the polyester/nanofiller interface is adequately tuned via a triethoxy

caprylylsilane surface-treatment. According to DSC and consistent with XRD the produced films

were mostly amorphous. They were analyzed with special attention paid to the water vapor

barrier properties (sorption, diffusion, permeability) which resulted from the nanofiller

incorporation. It was assumed that the well dispersed nanoparticles through the polyester matrix

can provoke certain tortuosity to water vapor molecules. The changes in PLA inherent

permeation properties were mainly dependent on the temperature and nanofiller loading. The

values of energy activation were higher for the films containing 3% ZnO with respect to the neat

PLA, confirming the increased difficulty for molecules to diffuse through the PLA matrix. The

nanocomposite films were characterized by a good antibacterial activity against Gram-positive

and Gram-negative bacteria and the antibacterial performance was found to be time-dependent.

By considering the multifunctional properties of PLA-ZnO films (anti-UV, antibacterial,

mechanical, etc.), such nanocomposites produced by extrusion could be considered as a very

promising approach in the production of environmental-friendly packaging materials.

31

Acknowledgements

The authors from University of Salerno thank Ms Laura Bassi for carrying out part of the

experiments of transport properties during her work of thesis for bachelor in Chemical

Engineering.

The authors from UMONS & Materia Nova thank the Wallonia Region, Nord-Pas de Calais

Region and European Community for the financial support in the frame of the INTERREG –

NANOLAC project. They thank to Yoann Paint and Anne-Laure Dechief for assistance in the

preparation and characterization of samples, NANOLAC partners and all mentioned companies

for supplying raw materials.

CIRMAP acknowledges supports by the Région Wallonne in the frame of OPTI²MAT

program of excellence, by the Interuniversity Attraction Pole program of the Belgian Federal

Science Policy Office (PAI 6/27) and by FNRS-FRFC.

32

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Nanocomposites of PLA and surface-treated ZnO rod-like nanoparticles.

Changes in PLA inherent permeation properties dependent on temperature and loading

Good antibacterial activity against Gram-positive and Gram-negative bacteria

The antibacterial performance was found to be time-dependent.