High oxygen barrier, clay and chitosan-based multilayer thin films: an environmentally friendly foil...

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ice | science

Green MaterialsVolume 1 Issue GMAT1

High oxygen barrier, clay and chitosan-based multilayer thin fi lms: an environmentally friendly foil replacementLaufer, Priolo, Kirkland and Grunlan

Pages 4–10 http://dx.doi.org/10.1680/gmat.12.00002Short CommunicationReceived 22/03/2012 Accepted 25/07/2012Published online 30/07/2012Keywords: coatings/nanoscale materials/packaging/thin fi lms

ICE Publishing: All rights reserved

High oxygen barrier, clay and chitosan-based multilayer thin fi lms: an environmentally friendly foil replacement

Galina Laufer BS Department of Mechanical Engineering, Texas A&M University, College Station, TX, USA

Morgan A. Priolo PhDDepartment of Mechanical Engineering, Texas A&M University, College Station, TX, USA

Christopher KirklandDepartment of Mechanical Engineering, Texas A&M University, College Station, TX, USA

Jaime C. Grunlan PhD*Department of Mechanical Engineering, Texas A&M University, College Station, TX, USA

Multilayered thin fi lms, with high oxygen barrier, were deposited from water using chitosan (CH), polyacrylic acid

(PAA), and montmorillonite (MMT) clay. Layer-by-layer assembly of ten “quadlayers” of CH/PAA/CH/MMT (<100 nm

thick) on polylactic acid (PLA) and polyethylene terephthalate (PET) fi lms, commonly used for food packaging, reduced

the oxygen transmission rate (OTR) of PLA and PET fi lms by two orders of magnitude under dry conditions. At 38°C

and 90% RH, the OTR of 500 μm PLA was reduced from 50 to 4·6 cc/(m2 day atm), which is lower than 179 μm PET fi lm

under the same conditions. This high gas barrier is believed to be due a to a nanobrick wall structure present in this

thin fi lm, where clay platelets act as bricks held together by polymeric mortar. These assembled thin fi lms are also very

transparent, which combined with ambient processing and the use of renewable and food contact approved ingredi-

ents, makes this a promising foil replacement technology.

1

2

3

4

*Corresponding author e-mail address: [email protected]

1 2

1. IntroductionThe permeation of oxygen through food packaging often leads

to spoilage, making oxygen barrier crucial for achieving longer

shelf life.1,2 Additionally, there is strong interest in develop-

ing environmentally-friendly and transparent barrier materials.3

Layer-by-layer (LbL) assembly of multilayered thin fi lms, through

electrostatic attraction of oppositely charged polyelectrolytes or

particles on a substrate,4,5 can impart these characteristics to pack-

aging fi lms. This technique has been used to grow multilayered

fi lms with various properties including hydrophobicity,6,7 control-

led drug release,8,9 chemical sensing,10,11 antimicrobial,12,13 and fi re

retardancy.14–16 LbL assembly has also been used to construct thin

transparent fi lms that signifi cantly reduce the oxygen permeability

of polyethylene terephthalate (PET).17,18 The most impressive bar-

rier properties result from the incorporation of clay in these thin

fi lms,18–20 but only one study prepared these layers exclusively with

food contact approved ingredients.20

In the present work, multilayer nanocoatings made with three food

contact improved components (chitosan (CH), poly(acrylic acid)

(PAA), and montmorillonite (MMT) clay) were deposited onto

PET and polylactic acid (PLA) substrates. CH is a biodegradable

and benign polysaccharide that is derived from crustacean exoskel-

etons.21 It was approved by the United States Food and Drug

Administration (FDA) as a feed additive in 1983 and has recently

been designated as GRAS (Generally Recognised As Safe) compo-

nent.22 At low pH (≤ 6), CH is a cationic and water-soluble polymer.

Anionic poly(acrylic acid) and clay have been used before in food

contact materials that have been granted approval by the FDA.23,24

Depositing ten “ quadlayers” (QL) of CH/PAA/CH/MMT (< 100

3 4



5

nm thick) on PLA or PET fi lm, using LbL assembly, reduces the

oxygen transmission rate (OTR) of these substrates by two orders

of magnitude under dry conditions. The OTR of coated PLA and

PET remains more than an order of magnitude below that of the

neat substrates at 38°C and 90% RH. These thin fi lms offer a

unique, environmentally-friendly foil replacement alternative for

food packaging.

2. Material and methods

2.1 MaterialsCH (Aldrich, Milwaukee, WI, USA) (MW 50–190 kDa, 75–85%

deacetylated) was dissolved in deionized water (18·2 MΩ) to cre-

ate a 0·1 wt% solution. The pH of this solution was increased to

6 with 1 M sodium hydroxide (NaOH). Poly(acrylic acid) (PAA)

(Aldrich) (MW 100 kDa) was used as a 0·2 wt% solution in deion-

ized water. The pH of this solution was increased to 4 with NaOH.

Sodium MMT clay (tradename Cloisite® Na+, Southern Clay

Products, Inc., Gonzales, TX, USA) suspensions were prepared at

2·0 wt% in deionized water. Single-side-polished (1 0 0) silicon

wafers (University Wafer, South Boston, MA, USA) were used as

the substrate for fi lm thickness characterization and 125 μm poly-

styrene (PS) fi lm (Goodfellow, Oakdale, PA, USA) was used for

TEM imaging. PLA with 500 μm thickness (trade name BioWare

PLA, produced by Huhtamaki Forchheim), provided by Faerch

Plast (Holstebro, Denmarkand), and 179 μm thick poly(ethylene

terephthalate) (trade name ST505, produced by Dupont-Teijin),

purchased from Tekra (New Berlin, WI), were used for oxygen

barrier testing.

2.2 Layer-by-layer depositionPrior to deposition, plastic substrates were corona treated with a

BD-20C Corona Treater (Electro-Technic Products, Inc., Chicago,

Figure 1. Schematic of layer-by-layer deposition of food contact

approved ingredients on substrate (e.g. polylactic acid or polyethylene

terephthalate fi lm) (a). Film thickness of as a function of cycles depos-

ited of (CH/PAA)2n and (CH/PAA/CH/MMT)n (b). Mass as a function of

quadlayers deposited, as measured by quartz crystal microbalance,

where (CH/PAA/CH) mass deposition is denoted as unfi lled points and

MMT as fi lled points (c).

−−

− −−

PAA

2

41 3

− −

−−

−MMT

++

+ ++CH

++

+ ++CH

1 quadlayer

(a)

(b) (c)

Substrate

MMT

CH PAAO OH

n

00

2

4

6

8

Mas

s (µ

g/c

m2 )

10

12

14

16

18

2 4Quadlayers

6 8 10

CH2OH

OH

OHOH

O SlAl

OH

OO O

NH3

CH2OH

OHO

NH3

CH2OH

OHO

NH3n

0 2 4Number of cycles (n)

6 8 10

200180160140120100

Thic

knes

s (n

m)

806040200

(CH/PAA/CH/MMT)n(CH-PAA)2n



6

USA) to create a negative surface charge. All fi lms were depos-

ited on a given substrate using the procedure shown schemati-

cally in Figure 1(a). Substrates were alternately dipped into CH,

polyacrylic acid (PAA), CH, and MMT. One cycle of CH/PAA/

CH/MMT is referred to as a QL. Initial dips were 5 min each and

subsequent dips were 1 min. Each dip was followed by rinsing with

deionized water and drying with air.

2.3 Characterization of fi lm growth, structure, and properties

Film thickness was measured with an alpha-SE Ellipsometer (J.

A. Woollam Co., Inc., Lincoln, NE, USA). The weight per depos-

ited layer was measured with a Maxtek Research Quartz Crystal

Microbalance (RQCM) (Infi nicon, East Syracuse, NY, USA), with

a frequency range of 3·8−6 MHz, in conjunction with 5 MHz quartz

crystals. Cross sections of CH-based assemblies were imaged with

a JEOL 1200 EX TEM (JEOL Ltd., Tokyo, Japan), operated at

110 kV. Samples were prepared for imaging by embedding a piece

of PS supporting the LbL fi lm in epoxy and sectioning it with a

microtome equipped with a diamond knife. OTRs were measured

by MOCON (Minneapolis, MN, USA) in accordance with ASTM

D-3985, using an Oxtran 2/21 ML instrument.

3. Results and discussionThe CH/PAA/CH/MMT assembly grows linearly as a function of

QLs deposited, as shown in Figure 1(b). A similar system, grown

with polyethylenimine (PEI) as the polycation instead of CH,

showed exponential growth.25,26 For exponential growth to occur,

a given polymer must diffuse in and out of the previously depos-

ited layer.27 CH’s structure consists of a double helix of polysac-

charide rings, which makes it much more rigid than PEI.28 This

relatively high stiffness, as evidenced by a large persistence

length (> 10 nm), prevents CH from interdiffusing into underly-

ing layers and the associated exponential growth.29 Growth of

CH-PAA in the absence of MMT is also shown in Figure 1(b) to

highlight the linear growth of these two polymers. This polymer-

only growth further demonstrates that clay layers are not limiting

polymer interdiffusion and therefore preventing the exponen-

tial growth of the system. Nevertheless, in a QL system, PAA

is replaced by MMT in every other bilayer, which contributes

only 1–2 nm of thickness because of complete clay exfoliation.

Additionally, clay platelets provide a new, relatively fl at depo-

sition surface each time they are deposited. This new surface

diminishes growth because less polymer deposits onto fl at plate-

lets than it does on top of another polymer layer with more nano-

topology. As a result, assemblies with clay are thinner than those

grown with polymers only.Linear growth of these CH-based

QLs is also observed using a quartz crystal microbalance, where

every QL contains approximately the same mass (Figure 1(c)). A

10 QL fi lm contains approximately 37 wt% clay, which is very

high compared to conventional bulk composites.30,31 In fact, when

the clay concentration exceeds ~10 wt% in bulk materials, their

mechanical and optical properties start to degrade due to nano-

particle aggregation.32,33 UV−vis spectroscopy reveals that this

same 10 QL fi lm has an average light transmission of 98% across

the visible light spectrum (390−750 nm), as shown in Figure 2(a).

This transparency is more apparent when examining the inset

images of quartz slides with and without the nanocoating. High

transparency is attributed to the high level of clay orientation and

exfoliation within the deposited fi lm. A TEM image of the cross

section of a 10 QL fi lm shows this high level of clay orientation

(Figure 2(b)). Individual clay platelets can be seen as dark lines

in this micrograph, which reveals a “nanobrick wall” structure.25

This nearly perfect alignment of the platelets is expected to pro-

vide excellent gas barrier to the underlying substrate. It has been

previously shown that at high clay concentration (i.e. deposition

solution concentration > 0·2 wt%) there is a higher degree of

lateral packing of clay in each layer, including more overlapping

of neighboring platelets.34

These CH-based nanocoatings were deposited on PET, which is

frequently used for food packaging, and PLA fi lm. PLA has gained

notoriety for its biodegradability, but it tends to have a higher OTR

as compared to more traditional fi lms like PET, which signifi cantly

limits its use in packaging.1,35 Improving the oxygen barrier of PLA,

relative to that of PET, will make it more useful for food packaging

and reduce waste due to its biodegradability.36 Figure 3(a) shows

that OTR signifi cantly decreases, for both PET and PLA, when a

10 QL assembly is deposited. Depositing less than 100 nm of CH/

PAA/CH/MMT reduced the OTR of both substrates by two orders

of magnitude under dry conditions. These values are well below

the required OTR values for packaging processed meat and cheese

(3·1–15·5 cc/(m2 day atm)) or snack foods (30 cc/(m2 day atm)).37

It is also important to note that PLA coated with 10 QL can achieve

a lower OTR than uncoated PET, making it more competitive for

similar applications.

It is true that adding clay directly into a polymer matrix improves

oxygen barrier of the bulk composite, but these OTR reductions

are relatively modest.38,39 Permeability of the composite material

is predicted to be a function of the aspect ratio of the fi ller and its

orientation.40 The thickness of a single MMT platelet is roughly

1 nm, while the length is in the range of 200–500 nm.41 It is very

diffi cult to achieve complete exfoliation of these high aspect ratio

nanoplatelets in bulk nanocomposites, which reduces their effec-

tiveness in reducing permeability values.42 This is emphasized

in the literature values presented in Table 1, where the addition

of clay to PLA and PET provides only a moderate reduction in

oxygen permeability. LbL assembly provides near perfect control

of orientation and exfoliation of clay platelets and this creates

an extremely tortuous path for permeating molecules. Highly-

oriented layers of tightly-packed, impermeable platelets cause

a rerouting of oxygen molecules along the fi lm thickness direc-

tion, resulting in a lower transmission rate through the thin fi lm

composite.18



7

Another useful tool to evaluate the barrier properties of thin fi lms

is the barrier improvement factor (BIF), where BIF equals the per-

meability of pure polymer divided by the permeability of the com-

posite (or coated substrate). As shown in Table 1, a 10 QL coating

has a BIF of 263 on PET and 60 on PLA under dry conditions,

which is signifi cantly greater than values reported in the literature

for bulk composites. Only another LbL system, made with bilayers

Film composition Oxygen permeability

(×10−16 cm3 cm/cm2 s Pa)

BIF Ref.

PET 17·6a This paper

25·4b This paper

PLA 177·2a This paper

288·7b This paper

10 QL on PET 0·067a 263 This paper

0·68b 37 This paper

10 QL on PLA 2·96a 60 This paper

26·87b 11 This paper

Chitosan 1770000a 46

PLA/10 wt% clay 52·8a 2 47

PET/3 wt% clay 43·4a 2 48

PS/38·8 wt% clay 114a 20 49

70 bilayers CH/MMT on PLA

2·95d 69 20

93·8e 1·9 20

Wood hydrolysate/40 wt% CH on PET

3·8c 4 50

a23 C/0% RHb38 C/90% RHc23C/50% RHd23C/20% RHe23C/70% RHBIF, barrier improvement factor; CH, chitosan; MMT, montmorillonite; PAA, polyacrylic acid; PET, polyethylene terephthalate; PLA, polylactic acid.

Table 1. Oxygen permeability of CH/PAA/CH/MMT assemblies

deposited on PET and PLA and of other barrier materials.

88390 490

Wavelength (nm)590 690

90

92

94W/out coating 10 QL coating

96

% T

ran

smis

sio

n

98

100(a) (b)

50 nm

Figure 2. Visible light transmission as a function of wavelength for

a 10 QL CH/PAA/CH/MMT fi lm deposited on a fused quartz slide (a).

The inset images in (a) show quartz slides with (right) and without

(left) the 10 QL nanocoating. Transmission electron microscope cross

section of 10 QL fi lm deposited on a polystyrene substrate (b).

00 10

Quadlayers

5

10 8·60

30·54

0·02 0·51

15

OTR

(cc

/(m

2 ·d

ay·a

tm)

20

25

30(a)

(b)

PETPLA

00 10

Quadlayers

10

12·40

49.76

0·334·63

OTR

(cc

/(m

2 ·d

ay·a

tm)

20

40

30

50 PETPLA

Figure 3. Oxygen transmission rate of 175 μm polyethylene

terephthalate and 500 μm polylactic acid with and without a 10 QL

CH/PAA/CH/MMT nanocoating. Measurements were made at 23°C

and 0% RH (a) or 38°C and 90% RH (b).



8

of CH and MMT, provided comparable BIF values.20 It should be

noted that this high barrier requires 70 CH/MMT bilayers, which

is more than triple the number of layers needed for the present

QL system. This difference is likely due to greater clay spacing in

the QL system presented here, where clay layers are separated by

three polymer layers (CH/PAA/CH) versus a single CH layer in the

bilayer system. Increased clay spacing has been shown to improve

oxygen barrier in LbL thin fi lms.18 Another possible reason for

the superior performance of the QL system is the interactions of

oppositely charged polyelectrolytes (CH and PAA) which results in

more dense polymer chain packing than CH alone. This effect was

observed in assemblies of polyethylenimine and PAA,17 which also

showed good barrier at high humidity. Furthermore, the BIF of CH/

MMT system drops sharply, from 69 to 1·9, as humidity increases

from 20 to 70%. Even so, this is an interesting demonstration of a

fully renewable gas barrier recipe.

The infl uence of temperature and relative humidity on OTR is

especially important for practical applications. Oxygen barrier is

expected to diminish with increasing humidity and temperature

due to swelling of the fi lm and an associated increase in free vol-

ume.43 Despite an increase in OTR, Figure 3(b) shows that PLA

and PET maintain an oxygen barrier that is an order of magnitude

better than uncoated fi lms at 90% RH and 38°C. It is expected

that these nanocoatings will be further improved with crosslink-

ing, which has already been shown to reduce moisture sensitivity

in LbL nanocoatings.17,25,44 None of the other systems highlighted

in table have a BIF greater than 10 when OTR is measured under

high humidity (≥ 70% RH).

4. ConclusionsFully transparent nanocoatings with remarkable oxygen barrier

properties were deposited on PLA and PET fi lm using biodegrad-

able, food contact approved materials. LbL assembled thin fi lms,

made with CH, PAA and natural MMT clay, have near perfect clay

platelet orientation and exfoliation that result in exceptional oxy-

gen barrier. Ten CH/PAA/CH/MMT QLs reduces OTR of PLA and

PET by two orders of magnitude, under dry conditions, and more

than one order of magnitude at 38°C and 90% RH. The ability to

produce such low permeability thin fi lms from water (and under

ambient conditions), with a relatively small number of layers and

GRAS materials, should make this an interesting foil replacement

technology for various food packaging applications. As an added

benefi t, these fi lms may possess antimicrobial properties due to CH

being a natural biocide.45

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10

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nanocomposites with various organoclays. I. Thermomechanical

properties, morphology, and gas permeability. Journal of Polymer Science, Part B: Polymer Physics 2003, 41, 94–103.

48. Ke, Z.; Yongping, B. Improve the gas barrier property of PET

fi lm with montmorillonite by in situ interlayer polymerization.

Materials Letters 2005, 59, 3348–3351.

49. Dunkerley, E.; Schmidt, D. Effect of composition,

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10536–10544.

50. Edlund, U.; Ryberg, Y. Z.; Albertsson, A-C. Barrier fi lms

from renewable foresty waste. Biomacromolecules 2010, 11,

2532–2538.

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