Photochemical behavior of PVA as an oxygen-barrier polymer for solar cellencapsulation

Julien Gaume,ab Pascal Wong-Wah-Chung,bc Agnes Rivaton,ab Sandrine Therias{*ab and Jean-Luc Gardetteab

Received 24th June 2011, Accepted 18th August 2011

DOI: 10.1039/c1ra00350j

Polyvinyl alcohol (PVA) is a water-soluble polymer that is anticipated to be a good candidate for

incorporation into multilayer coatings of organic solar cells due to its high transparency and ability to

form a barrier to oxygen. Because a long lifetime is a prerequisite for successful applications, it was

necessary to study the photochemical behavior of PVA under solar light. PVA films were exposed to

UV-visible light irradiation (l . 300 nm) in accelerated aging conditions representative of natural

ageing. Modifications in the chemical structure of aged samples irradiated at ambient air were

recorded. Due to the low oxygen permeability of PVA films, it was shown that the photooxidative

degradation of PVA films is restricted to the surface (,5 mm) and results in a large amount of chain

scissions, with a progressive erosion of the surface of the irradiated material. The oxidation products

formed along the macromolecular chains, and low molecular weight species trapped in the matrix or

emitted in the gas phase were also identified. An oxidation mechanism was then proposed to account

for these modifications. However, irradiation in the absence of oxygen demonstrated the high

photostability of PVA films, which permits the use of PVA as a sublayer in inorganic/organic

multilayer encapsulation systems.

1. Introduction

Polyvinyl alcohol (PVA) is extensively used in paper coating,

textile sizing, and in packaging as flexible water-soluble films1

for its oxygen barrier effect. PVA is also used in numerous

fields because of its biodegradability and environmentally

friendly processing.2,3 Indeed, PVA is a water-soluble polymer

that permits the formation of transparent films through the

evaporation of an aqueous solution.

It is well known that organic solar cells (OSCs) are sensitive to

the oxygen and water present in the atmosphere and therefore

require encapsulation by efficient barriers.4–6 To obtain flexible

organic solar cells with long lifetimes, it appears necessary to

develop encapsulation with a large range of properties: a high

barrier to water and oxygen, optical transparency in the visible

domain, flexibility and environmentally friendly processing.

Moreover, all these properties have to be preserved in use condi-

tions under sunlight exposure (l . 300 nm). Regarding the high

barrier properties to oxygen4,5 (PO2PVA = 3 6 10217 cm3 cm/cm2 s

Pa compared to PO2PET = 3 6 10215 cm3 cm/cm2 s Pa) and the

transparency of PVA, this polymer was anticipated to be a good

candidate for incorporation into a multilayer inorganic/organic

system for organic solar cell (OSC) encapsulation.6–9 The water

barrier property would be provided by the alternating inorganic

layers. However, the photochemical behavior of pure PVA must

be studied before its integration into an inorganic/organic multi-

layer structure.

To our knowledge, no study has focused on the photo-

degradation of PVA under UV-visible light irradiation in

conditions representative of outdoor exposure (l . 300 nm).

Some papers report on the phototransformation of PVA

irradiated at short wavelength (254 nm) in the presence of

collagen,10,11 montmorillonite12 or graphite oxide.13 Another

paper focused on the photochemical behavior of dichromated

PVA upon exposure at 365 nm.14 On the basis of published

results, it seems that PVA could be considered a photostable

polymer and could be a good candidate in multilayer encapsula-

tion systems for OSCs with long lifetimes.

The present study focuses on the photochemical behavior of

PVA and on the modifications of the chemical structure of PVA

resulting from photo-thermal ageing. PVA thin films (20–30 mm

thick) were submitted to UV-visible light irradiation (l . 300 nm,

60 uC) in ambient air or were thermally oxidized at 60 uC. Chemical

modifications were investigated by X-ray photoelectron spectro-

scopy XPS, IR and UV-visible spectroscopies. The photoproducts

were identified by various techniques combining physical and

chemical derivatization treatments of oxidized films coupled with

aClermont Universite, Universite Blaise Pascal, LPMM, BP 10448,F-63000 Clermont-Ferrand, France.E-mail: sandrine.therias@univ-bpclermont.fr; Fax: +33 (0)4 73 40 77 00;Tel: +33 (0)4 73 40 71 43bCNRS, UMR 6505, LPMM, BP 80026, F-63171 Aubiere, FrancecClermont Universite, ENSCCF, LPMM, BP 10448, F-63000Clermont-Ferrand, France{ Current address: Laboratoire de Photochimie Moleculaire etMacromoleculaire, UMR CNRS/Universite Blaise Pascal 6505, 24avenue des Landais, BP 80026, 63171 Aubiere Cedex, France.

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IR spectroscopy. Identification of the low molecular weight pro-

ducts was performed by head-space solid-phase micro-extraction

(HS-SPME)-coupled with GC-MS analysis and ionic chromato-

graphy experiments. The obtained data allowed us to propose a

detailed degradation mechanism that accounted for the formation

of the major oxidative species. Depth profiling was used to

determine the extent of the oxidative process throughout the

polymer films. Lastly, the degradative changes of PVA submitted

to irradiation in the absence of oxygen was also analyzed. The

results indicated the main parameters that must be managed for

PVA to serve as a good candidate for insertion in a multilayer

system for OSC encapsulation.

2. Experimental

2.1. Materials

It is important to note that PVA is obtained by the hydrolysis of

polyvinyl acetate. Usually, the reaction is not complete, and

acetate functions can remain in the polymeric structure.

Consequently, PVA with different hydrolysis percentages in the

range 70–99% are commercially produced.

In this study, PVA (98% hydrolyzed, weight-average mole-

cular weight # 16 000 g mol21) (Fig. 1) was purchased from Sp2

(Scientific Polymer Products). Before use, the PVA was purified

with methanol in a Soxhlet apparatus for two days.

The purified PVA was then dissolved in water (5 wt%) at 80 uCunder stirring for 3 h. Then the PVA aqueous solution was

deposited on a Teflon sheet with an Erichsen Coatmaster 809

MC (thickness of the liquid deposition y 300 mm). Free-

standing films with thicknesses in the range 20–30 mm were

obtained after drying in ambient air for one day plus a few hours

at 60 uC.

2.2. UV-visible irradiation

The UV-visible light irradiation (l . 300 nm) of the thin films

was performed in a SEPAP 12/24 unit, which was designed for

studying polymer photodegradation in artificial ageing with

medium-accelerated conditions.15 The chamber consisted of a

square reactor equipped with four medium-pressure mercury

lamps (Novalamp RVC 400W) situated vertically at each corner

of the chamber. Wavelengths below 300 nm are filtered by the

glass envelope of the lamps. In the center of the chamber,

the samples are fixed on a 13 cm-diameter rotating carousel

that could hold 24 samples. In this set of experiments, the

temperature at the surface of the samples was set at 60 uC.

Irradiation in the absence of oxygen (photolysis experiments)

was performed on samples introduced into borosilicate tubes and

sealed under a vacuum of 1024 Pa with a diffusion vacuum line.

The samples in tubes under vacuum were then placed in the

SEPAP 12/24 device.

Low-temperature thermooxidation experiments were carried

out in a ventilated oven at 60 uC.

2.3. Characterization

Changes in the UV-visible spectra were tracked with a Shimadzu

UV-2101PC spectrophotometer equipped with an integrating

sphere.

IR spectra were recorded in transmission mode with a Nicolet

6700 Fourier transform infrared (FTIR) spectrophotometer

operated with the OMNIC software. The spectra were obtained

with 32 scan summations at 4 cm21 resolution. IR-ATR

(Attenuated Total Reflectance) spectra were recorded in the

reflection mode with a Nicolet 380-FTIR spectrophotometer

equipped with a Thunderdome-ATR (4 cm21, 32 scans). The

Thunderdome is a single reflection ATR accessory with a

germanium crystal (depth analysis # 1 mm). Oxidation profiles

were determined by recording spectra in the IR-ATR mode after

successive abrasions of the film surface.

Most of the oxidation products were identified by performing

chemical derivatization treatments that selectively convert

oxidation products into chemical groups with different IR

characteristics.16,17 Ammonia (NH3) reacts with carboxylic acids

to generate carboxylate ions. NH3 also reacts with esters to give

amides. NH3 treatment was performed at room temperature in

simple flow reactors that could be sealed off to allow the reaction

to proceed. Irradiated films were also treated with a solution

of 2,4-dinitrophenylhydrazine (DNPH) in methanol for 18 h.

DNPH reacts with aldehydes and ketone groups to give

dinitrophenylhydrazones. Before DNPH treatment, the samples

have to be first immersed in methanol solution to eliminate low

molecular weight oxidation products.

X-Rray photoelectron spectroscopy (XPS) experiments were

conducted in a PHI Quantera SXM. The diameter of the

analyzed area was 200 mm, and the depth of analysis was less

than 10 nm.

Ionic chromatography analyses (Dionex AS11 column for

anions, eluant NaOH from 0.5 mmol L21 to 25 mmol L21,

injection 100 mL at 30 uC) were performed in a water immersion

solution of a photooxidized PVA film.

Films of PVA were also irradiated in sealed vials to collect the

volatile photodegradation products. Carboxen–PDMS fiber

(75 mm) purchased by Supelco (Bellefonte, PA, USA) was used

to extract the volatile products. The extraction time was 5 min at

80 uC. The volatile compounds were analyzed by gas chromato-

graphy/mass spectrometry (GC-MS) analysis with a 6890N

Agilent GC coupled to a 5973 Agilent mass detector. The GC

was equipped with a SupelcowaxTM 10 column (30 m 60.25 mm 6 0.25 mm) from Supelco. Splitless injections were

used (2 min). The temperature was programmed from 35 uC(10 min hold) to 60 uC at 5 uC min21 and then at 10 uC min21

to 200 uC (15 min hold). Helium was used as a carrier gas

with a constant pressure of 38 kPa. The temperature of the

splitless injector was 280 uC (2 min duration), and the flow was

50 mL min21. The transfer line temperature was 280 uC, and the

ion source temperature was kept at 230 uC. The ionization

occurred with a kinetic energy of the impacting electrons ofFig. 1 Chemical structure of PVA.

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70 eV. The detector voltage was 70 eV. Mass spectra and

reconstructed chromatograms (total ion current, TIC) were

acquired under the electron ionization mode (EI) at 70 eV and

recorded from 20 to 400 m/z. The compounds were identified

either by comparison of the retention times and mass spectra

with standards or with the spectral library.

3. Results and discussion

3.1. Thermooxidation of PVA at 60 uC

Thermal degradation of PVA has been reported in the literature

at a high temperature (T . 200 uC),18 which is significantly

higher than those expected for OSCs (,80 uC). It was shown that

the thermal degradation involves the elimination of hydroxyl

side groups together with the formation of isolated and

conjugated polyenes and carbonyl groups. To discriminate the

influence of temperature and light on the oxidation provoked by

exposure to UV light in the artificial ageing unit, thermooxida-

tion experiments were performed in a ventilated oven at 60 uC.

After 7000 h of thermooxidation, no significant change of the

IR spectra was observed. The only modifications concerned the

UV-visible spectra (Fig. 2), which showed an absorption band

at 280 nm and a shoulder at 330 nm, which were assigned to

the p–p* transitions of –(CHLCH)2–CO– and –(CHLCH)3–CO–,

respectively.19,20

The products that resulted from oxidation and dehydration of

PVA were formed at low concentrations and were only observed

by UV-visible analysis because the extinction coefficient of the

involved electronic transitions is important. As a consequence

of the weak concentrations, no modification of IR spectra

corresponding to these products was observed.

3.2. Photolysis of PVA at long wavelengths (l . 300 nm)

The irradiation of the PVA was performed in the absence of

oxygen (photolysis) at 60 uC. After 6 000 h in the SEPAP 12/24

unit, only a fairly weak dehydration (elimination of absorbed

water) of the samples was observed. Otherwise, no significant

changes of the UV-visible and IR spectra in the chemical

structure were observed.

3.3. Photooxidation of PVA at long wavelengths (l . 300 nm)

3.3.1. Changes in the UV-visible domain. The UV-visible spec-

trum of PVA before irradiation, reported in Fig. 3, shows that thin

films (e # 25 mm) have a high transparency in the UV-visible

region (no significant absorption at wavelengths above 250 nm).

Fig. 3 also shows that photooxidation of PVA films resulted in

a gradual increase of light absorption from 300 to 500 nm

without any maximum. The increase in absorbance at 400 nm

after 6000 h of exposure was not higher than 0.06, which

indicates that no significant discoloration was observed. In

comparison to thermooxidation experiments, no absorption

bands corresponding to the formation of polyenes were detected.

Although conjugated products could be formed, they would be

rapidly oxidized under irradiation and then disappear, as

observed in the case of poly(vinyl chloride).21,22

3.3.2. Analysis of the bulk oxidation.

3.3.2.1.Infrared analysis of the chemical changes in the polymer

matrix. Modifications in the bulk of PVA films were analyzed by

transmittance-IR spectroscopy after 6000 h of irradiation. The

analysis of changes in IR spectra requires that the absorption bands

composing the spectrum of native PVA are correctly identified. The

main spectral features are presented in Table 1.14,23–25

Fig. 2 UV-Visible spectra of a PVA thin film during thermooxidation

at 60 uC.

Fig. 3 UV-visible spectra of a PVA thin film during photooxidation in

the SEPAP 12/24 device at l . 300 nm and 60 uC

Table 1 Assignments of the main IR features of native PVA

Wavenumbers (cm21) Assignments

3340 OH stretching2942 CH2 stretching2914 C–H stretching2853 C–H stretching1730 CLO stretching of free ester groups1713 CLO stretching of ester groups linked by

H bonds to hydroxyl groups of PVA1420 CH2 bending1377 CH3 wagging1246 C–O–C stretching1096 O–H bending918 CH2 rocking853 C–C stretching

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Photooxidation resulted in noticeable modifications in the IR

spectrum of the PVA (Fig. 4), which can be observed in the

subtracted spectra.

The vibration bands corresponding to –OH groups (3340 cm21)

and the (CH) polymer backbone (2940 and 2910 cm21) strongly

decreased in intensity upon irradiation. In the carbonyl region, one

can observe the formation of a complex envelope containing a

maximum at 1740 cm21 and a shoulder at around 1785 cm21 in the

subtracted spectra. The formation of a band with a maximum at

approximately 1180 cm21 was also observed.

3.3.2.2. Identification of oxidation products by chemical derivatiza-

tion treatments. To identify the oxidation products detected by IR

analysis, which are formed on the macromolecular chains or

trapped in the bulk, chemical derivatization treatments were

performed on photooxidized PVA samples. The combination of

IR data with derivatization treatments is known to be particularly

efficient for the identification of carbonyl compounds.17

NH3 reacts with carboxylic acids to generate carboxylate ions

and with esters, anhydrides and lactones to generate amides. It

was first verified that no reaction with NH3 was observed

with PVA before photooxidation. Fig. 5 shows the subtracted

spectra of a PVA sample exposed for 5000 h compared to a non-

irradiated sample before and after NH3 treatment. The IR

spectrum monitored after treatment indicates the formation

of carboxylate (major) and amide (minor) groups, with the

concomitant disappearance of oxidation photoproducts centered

at 1785 cm21, 1740 cm21 and 1715 cm21.

On the basis of the numerous publications concerning

derivatizations by NH3,17 the oxidation photoproduct observed

at 1715 cm21, which reacted with NH3 to give the carboxylate

group observed at 1570 cm21, was assigned to a carboxylic acid

in the dimer form. The formation of an amide group (shoulder at

1670 cm21) can be assigned to the NH3 reaction with anhydrides

or lactones observed at 1785 cm21. In the case of the oxidation

photoproduct observed at 1740 cm21, this frequency is usually

associated with ester functions, which react with NH3 to give

amide groups. In the case of PVA, this absorption band could

also reflect the formation of a-hydroxylated carboxylic acid.

To check this hypothesis, a carboxylic acid (propanoic acid

Fig. 4 (left) Direct IR spectra of a PVA film photooxidized at l . 300 nm and 60 uC in the SEPAP 12/24 device for 6000 h. (right) Subtracted spectra

in the carbonyl region.

Fig. 5 Subtracted spectra of photooxidized PVA (5000 h) (compared to non-irradiated PVA) before and after NH3 treatment and the subtracted

spectra (after—before NH3 treatment).

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CH3CH2COOH) and an a-hydroxylated carboxylic acid (lactic

acid CH3CHOHCOOH) were introduced in two different

samples of native PVA film that were then treated with NH3.

Table 2 gives the absorption maxima of propanoic acid and

lactic acid in PVA and the absorption bands of the derivative

products after NH3 treatment.

One can see that the presence of a –OH group in the

a-hydroxylated carboxylic acid (lactic acid) resulted in a shift

(Du = 17 cm21) of the vibration band attributed with the pure

a-hydroxylated carboxylic acid. When these acids were intro-

duced in PVA, the shift was even more significant (Du = 28 cm21)

due to hydrogen bonds. Carboxylate derived products also gave

a shift (Du= 35 cm21) after NH3 treatment. Based on these

results, it can be assumed that the oxidation products observed

at 1740 cm21 in the photooxidized PVA film could be assigned to

ester groups and a-hydroxylated carboxylic acids.

After NH3 treatment, some unreacted oxidation products

remain in the PVA film. A 2,4-DNPH treatment was conducted

on irradiated PVA films, previously immersed in methanolic

solution (to extract the low molecular weight species) and then

soaked in a methanolic solution of 2,4-DNPH for 18 h. It is

important to note that 2,4-DNPH treatment gives qualitative

information about the presence of ketone and aldehyde.26,27 The

reaction with 2,4-DNPH (Fig. 6) led to a decrease in intensity

of the 1717 cm21 absorption band (before the treatment with

DNPH, it was verified that the decrease of this band did not result

from the immersion of the irradiated films in the solvent only). At

the same time, two bands at 1618 and 1592 cm21 appeared, which

can be assigned to the corresponding phenylhydrazone.

The formation of phenylhydrazone could also be followed

by UV-visible spectroscopy with the appearance of an intense

absorption at 365 nm (Fig. 6). To identify the chemical nature

(ketone or aldehyde) of the product reacting with 2,4-DNPH, a

physical treatment was performed on a photooxidized PVA film.

This film was placed above water vapor to eliminate low

molecular weight acid groups and then submitted to a photolysis

experiment (irradiation in the absence of oxygen). The sub-

tracted difference spectrum (after—before photolysis) revealed a

decrease in the vibration band at 1717 cm21 upon photolysis.

Due to the well-known photolytic instability of ketone according

to the Norrish reaction, the band at 1717 cm21 was attributed to

a ketone. Ketone functions are generally detected at approxi-

mately 1720 cm21. However, it is well-known that hydrogen

bonding between carbonyl and hydroxyl groups shifts the CLO

stretching vibrations to lower wavenumbers.14

3.3.2.3. Identification of oxidation products by ionic chromato-

graphy. A PVA film photooxidized for 4000 h was immersed in

2 mL of water during 2 min. The film was analyzed with IR

spectroscopy before and after immersion, and the subtracted

spectra to native PVA are reported in Fig. 7.

The immersion of photooxidized PVA in water leads to a 45%

decrease of the IR absorption of the carbonyl envelope, which

indicates the extraction of some low molecular weight products

trapped in the film. As observed by NH3 treatment, the

Table 2 IR bands of propanoic acid and lactic acid in PVA before andafter NH3 treatment

Wavenumber (cm21)

Acid Pure Acid in PVA Derived product

Propanoic 1713 1713 1565Lactic 1730 1745 1600

Fig. 6 (Left) IR difference spectra (after—before) the 2,4-DNPH treatment. (Right) UV-visible spectra (after—before) the 2,4-DNPH treatment.

Fig. 7 Subtracted spectra (after exposure—t0) (black line), (after water

immersion—t0) (dotted line) and (after—before) water immersion

(grey line)

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subtracted spectrum shows the decrease of three maxima

centered at 1785 cm21, 1740 cm21 and 1715 cm21. To identify

the extracted products, the water solution was analyzed with

ionic chromatography (results reported in Table 3). By com-

parison with standards, the primary released products were

identified as acetic, oxalic, malonic and succinic acids.

a-Hydroxylated carboxylic acids such as lactic and tartaric acids

were also present in minor proportions.

3.3.2.4. XPS analysis. The analysis of the bulk oxidation with IR

spectroscopy shows weak photoproduct concentrations after

2500 h. The modifications detected by IR analysis in transmis-

sion mode correspond to the mean concentrations of photo-

products within the PVA films. Due to the high oxygen barrier

of PVA, it can be anticipated that most of the oxidation

photoproducts are localized at the surfaces of the irradiated

films. The chemical modifications of the surface after 2500 h of

exposure were studied with XPS. XPS is commonly accepted as

one of the most powerful tools to monitor chemical changes in

polymer surfaces (to a depth of approximately 10 nm). The

investigation of the core electronic structures of the elements

gives precise information about the chemical environment of the

different atoms. An XPS spectrum of a PVA sample was

recorded. As expected, carbon (C1s at 285 eV) and oxygen (O1s at

532 eV) atoms were found as the two major constituents. Using

the C1s and the O1s peak areas, the oxygen/carbon atomic ratio

(O/C) was found to be 0.47, which is in good agreement with the

ideal stoichiometry of PVA (0.50). The C1s spectrum consisted of

four subpeaks at 285.0, 286.4, 287.6 and 289.1 eV (Fig. 8). The

lowest binding energy was assigned to C–C/C–H bonds, while

the subpeak at 286.4 eV was assigned to C–O bonds (alcohol

functions). The two weaker subpeaks at 287.6 and 289.1 eV were

assigned to CLO and O–CLO bonds, respectively, originating

from residual acetate groups present in PVA.

After 2500 h of exposure, modifications to the XPS spectrum

were observed. A significant increase of the O/C ratio (0.57) due

to oxygen fixation and the formation of oxidized products was

observed. The C1s spectrum of the photooxidized sample was

resolved using the same subpeaks as those for an un-irradiated

PVA and adding subpeaks corresponding to the formation of

oxidation products. Table 4 summarizes the C1s deconvolution

results before and after irradiation.

Photooxidation led to the decrease of C–C and C–H content

and the decrease of C–O bonds. This suggests that the oxidation

of the alkyl chains and the chain scission of C–O bonds with the

elimination of alcohol function occur simultaneously upon

exposure. The peaks situated at 287.6 and 289.1 eV were

assigned to the CLO and O–CLO of acetate groups present in

PVA and are not affected by photooxidation. As previously

suggested by Akhter et al.,28 a peak at 285.9 eV must be added to

Table 3 Concentrations of carboxylic acids extracted from irradiated PVA obtained with ionic chromatography analysis

Compound Formula Molecular weight (g mol21) C (mg L21) C (mM)

Formic acid H–COOH 46.03 220 4.78Acetic acid H3C–COOH 60.05 3560 59.28Propionic acid H3C–CH2–COOH 74.08 150 2.02Oxalic acid HOOC–COOH 90.03 2140 23.77Malonic acid HOOC–CH2–COOH 104.06 1890 18.16Succinic acid HOOC–CH2–CH2–COOH 118.09 1610 13.63Lactic acid H3C–CHOH–COOH 90.08 300 3.33Tartaric acid HOOC–CHOH–CHOH–COOH 150.09 670 4.46

Fig. 8 C1s fitted curves of pristine PVA (left) and after 2500 h of photooxidation (right).

Table 4 Summary of C1s peak deconvolutions (%)

BE (eV) Assignment 0 h 2500 h

285 C–C and C–H 49.9 45.5286.4 C–O 38.9 11.2287.6 CLO 4.1 4.1289.1 O–CLO 7.1 7.1285.9 C–O–C — 14.8287.2 CLO — 5.5288.7 O–CLO — 11.8

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correctly fit the data. This supplementary peak was tentatively

assigned to ether C–O–C functions induced by radical recombi-

nation. The formations of the subpeaks at 287.2 and 288.7 eV

correspond to the formation of carbonyl products. Although few

products were detected in the bulk, the surface was already

oxidized after 2500 h of exposure. This confirms, therefore, that

the oxidation process leads to the lower intensity of the C–H/C–C

subpeak at 285 eV. The first subpeak (287.2 eV) was assigned to

ketone formation (CLO), whereas the second subpeak (288.7 eV)

was assigned to carboxylic acids formation (O–CLO).

3.3.2.5. Oxidation photoproducts concentrations. The oxidation

products assigned to the increase in IR absorbance at 1715 cm21,

1717 cm21, 1740 cm21 and 1785 cm21 after photooxidation of

PVA are listed in Table 5.

Based on these results, it should be possible to compare the

concentration of carbonyl products present in the PVA film and

detected by IR analysis to the (C–H) consumption. Clearly, the

oxidation reaction of polymers involves a hydrogen abstraction

by a radical process leading to the loss of C–H bonds with the

formation of CLO bonds.29 The carbonyl envelope developed in

the IR spectra upon irradiation was deconvoluted with three

contributions. The first contribution was fixed at 1715 cm21 and

was attributed to carboxylic acids and ketones. The second

contribution was fixed at 1745 cm21 and was attributed to ester

and carboxylic acids, whereas the third contribution was fixed at

1785 cm21 and was attributed to lactones and anhydrides. Each

contribution was represented by a Gaussian curve with a fixed

width at half height (FWHM = 47 cm21). To obtain the (C–H)

concentration, the decrease of the vibration band at 2942 cm21

was measured. The average concentrations were determined

using the Beer–Lambert law (Abs = ecl). The molar extinction

coefficients depend on the nature of the products. Extinction

coefficients of some model carbonyl compounds, as measured by

Carlsson and Wiles,30 range from 350 L mol21 cm21 for ketones

to 650 L mol21 cm21 for acids. We have shown that the

absorption bands can result from two different products at the

same time (acids and ketones at 1715 cm21, acids and esters at

1745 cm21). For this reason, we used an average value for the

extinction coefficient of these bands. The concentrations of the

products at 1715 cm21 were then calculated with a coefficient of

500 L mol21 cm21, and the concentration of the products at

1745 cm21 were calculated with a coefficient of 550 L mol21 cm21.

In the case of the products at 1785 cm21, we used a coefficient

of 450 L mol21 cm21. On the basis of the infrared spectra of

several different polymer films with a known thickness, the

extinction coefficient of the (CH) groups at 2940 cm21 was

evaluated to be approximately 10 L mol21 cm21.

Fig. 9 shows an example of the fit for a PVA film after 5600 h

of exposure and the kinetics of carbonyl products formation and

(C–H) consumption in the PVA film upon irradiation.

One can observe that the loss of (CH) groups is significantly

higher than the accumulation of (CLO) groups in PVA irradiated

films. For instance, after 6000 h of irradiation, a C–H concentra-

tion decrease of about 25 mol L21 was obtained, whereas the

concentration of oxidation products formed in the PVA film was

less than 1.5 mol L21. These results indicate that some of the

oxidation products that were formed are not taken into account

by the IR analysis of the irradiated PVA film. This result

suggests the formation of low molecular weight species

susceptible to migration in the gas phase upon irradiation, as

reported for other polymers.31,32

3.3.2.6. Identification of volatile products: SPME. To identify the

volatile photoproducts, HS-SPME-GC-MS experiments were

performed. The GC-MS chromatogram (TIC), shown in Fig. 10,

shows that various low molecular weight products have migrated

in the gas phase after 4000 h of exposure.

Table 5 Oxidation products formed by the irradiation of PVA

Wavenumber (cm21) Oxidation products Identification

1715 Carboxylic acids NH3 treatment, ionicchromatography

1717 Ketones 2,4-DNPH treatmentand photolysis

1740 Esters and carboxylicacids

NH3 treatment, ionicchromatography

1785 Lactones and anhydrides NH3 treatment

Fig. 9 (Left) Fitted curves of subtracted carbonyl envelop after 5600 h of exposure. (Right) Kinetics of carbonyl products formation (&) and (C–H)

consumption (%) in PVA film during photooxidation.

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The retention times, tret, the main observed fragments, the

molecular weights of the compounds and the identification of the

products were given in Table 6.

According to the total ion abundance, the results given in

Table 6 indicate that the products with retention times of 2.9,

22.1 and 23.6 min were the main products detected during the

photooxidation process. These three volatile products were

identified as acetone, acetic acid and propanoic acid, respec-

tively. Other products, such as butanone, butanoic acid or

butyrolactone, were detected in lower quantities.

3.3.3. Distribution of photoproducts. ATR-IR analysis was used

to characterize surface oxidation within the first micrometres.

The ATR-IR spectra of unoxidized and oxidized PVA films

(after 3000 h and 5000 h of exposure) are shown in Fig. 11. The

modifications detected by IR-ATR analysis were similar to those

reported above in the case of the bulk analysis performed by IR

spectroscopy in transmission mode. The only difference con-

cerned the relative variations of the absorption bands. One can

observe that the increase in absorbance of the CLO band is

accompanied by a dramatic decrease of the absorbance of the

C–H bands at 2942 cm21 and 2914 cm21. This indicates that the

oxidation photoproducts were mainly formed at the surface of

the exposed samples.

To determine the distribution of the oxidation products within

the sample thickness, the IR-ATR spectra were recorded after

successive abrasions of the surface. The ratio oxidation products/

C–H are presented in Fig. 12.

The shape of the profile shows a marked concentration

gradient. The oxidation photoproducts were formed at the

surface of the film within a layer that is less than 5 mm from the

exposed surface. This profile reflects the high barrier of PVA

to oxygen,4 which limits the oxidation processes in the bulk

of PVA.

3.3.4. Photooxidation mechanism. The experimental results

reported above show that degradation products resulting from

PVA photooxidation are mainly composed of carboxylic acids in

the form of chain products (–CH2COOH and a-hydroxylated

carboxylic acids) and in the form of low molecular weight species

(e.g., acetic, oxalic, malonic and succinic acids). Ketones, esters,

anhydrides and lactones are formed at a lower rate. The

following mechanism accounting for the oxidation on the

tertiary carbon (C1) of PVA can be proposed (Scheme 1).

It is well known that the hydrogen atom on a tertiary carbon

atom is a preferential site for a radical attack. Such behavior has

been clearly demonstrated in the degradation mechanism of

numerous polymers, including common polymers such as

polyvinyl chloride21 or polypropylene.33 Hydrogen abstraction

at the tertiary carbon atom (C1 in Scheme 1) leads to the

formation of a macroalkyl radical, and after subsequent oxygen

Fig. 10 HS-SPME-GC-MS total ion chromatogram of a headspace of a

PVA film after 4000 h exposure.

Fig. 11 IR-ATR spectra of a pristine PVA and oxidized PVA (3000 h

and 5000 h).

Fig. 12 Photooxidation profile (CO/CH ratio) measured by IR-ATR

spectroscopy after successive abrasions of a PVA film irradiated for

3000 h at l . 300 nm and 60 uC.

Table 6 Volatile organic compounds released from irradiated film asidentified by headspace-SPME-GC-MS analysis

Numbertret

(min)Main fragment,m/z

Molecular weight(g mol21) Identification

1 2.9 58, 43, 27 58.1 Acetone2 3.4 72, 57, 43, 29 72.1 Butanone3 22.1 60, 43, 29 60.0 Acetic acid4 23.6 101, 75, 57, 41, 29 74.1 Propanoic acid5 24.8 88, 73, 60, 42 88.1 Butanoic acid6 25.1 86, 56, 42, 29 86.1 Butyrolactone

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Scheme 1 Oxidation mechanism of the PVA with a primary attack on the tertiary carbon (C1).

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fixation and hydrogen abstraction, hydroperoxides are formed as

primary oxidation products. The photochemical decomposition

of the hydroperoxides leads to alkoxy radicals (R1). A b-scission

of the C–O bond (A) on R1 may occur, forming a chain ketone

(K1) and a OHN radical. A b-scission of C–C bond (B) on R1 also

occurs and forms a carboxylic acid (A1) and an alkyl primary

macroradical (R3). K1, R3 and A1 are formed as the secondary

products. The formation of acetone can be attributed to the

oxidation of K1, which involves two successive oxidations on two

C1’s. These reactions also give A1. A Norrish I reaction on K1

can also occur, which leads to the formation of carbon dioxide

and R3. As shown in the case of polypropylene,33 isomerization

to a more stable tertiary radical occurs on R3. The oxidation of

this radical following a classical mechanism is the route to

produce acetic acid, which regenerates the primary alkyl radical

R3. The recombination of two R3 radicals, followed by two

successive oxidations on two carbon 1’s, is responsible for the

formation of propanoic acid and regenerates R3 and A1.

Hydrogen abstraction at carbon 1 on A1 leads to the formation

of a macroradical, which can evolve in two different ways. A first

b-scission (1) provokes the formation of malonic acid and R3.

The second b-scission (2) regenerates A1 and forms a radical,

which evolves by recombination to form succinic acid or by

hydrogen abstraction to form acetic acid. It should be noted that

b-scission on a C–O bond can also occur to form the chain

ketone K1. This path is not represented in Scheme 1. Hydrogen

abstraction at C2 on A1 can also occur. By the same b-scissions

previously described (3 and 4), oxalic acid and formic acid are

formed. Even if less favorable, radical attack on the secondary

carbon atom (carbon 2) can also occur. The following

mechanism accounting for the oxidation on the secondary

carbon (C2) of PVA can be proposed (Scheme 2).

Scheme 2 Oxidation mechanism of the PVA with a primary attack on the secondary carbon (C2).

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A hydrogen abstraction at C2 leads to the formation of a

macroalkyl radical, which forms hydroperoxides as primary

oxidation products. The photochemical decomposition of the

hydroperoxides leads to alkoxy radicals (R2). b-Scission on R2

occurs to form an aldehyde, which can be oxidized to an

a-hydroxylated carboxylic acid (A2). A cage reaction on R2 also

occurs to form a chain ketone (K2) and water. K2 and A2 are

formed as the secondary products in this way. Norrish I

reactions on K2 can form carbon dioxide and regenerate A2.

As previously observed for A1, A2 can evolve in two different

ways. The first implies hydrogen abstraction on carbon 1

followed by chain scission to regenerate A2 and form a radical,

which creates tartaric acid by recombination. Hydrogen abstrac-

tion on C1 can also occur and is responsible for the formation of

lactic acid. This last reaction also produces A1, which can be

oxidized following Scheme 1. These two schemes show that A1,

A2 and R3 are always regenerated during photooxidation (A2 can

also produce A1). The propagation of these reactions leads to the

formation of notable concentrations of low molecular weight

products, which can migrate from the solid polymeric matrix. R3

is mainly responsible for the formation of acetic acid, whereas A1

principally causes the formation of oxalic, malonic and succinic

acids. These products are the major products detected in ionic

chromatography.

4. Conclusion

In this paper, a comprehensive study of PVA photooxidation led

to a proposal for the mechanism to account for the modifications

of the chemical structure of PVA upon irradiation. The obtained

results indicate that oxidized products are formed in the first

5 microns at the surface of the exposed films and are mainly

carboxylic acids. Furthermore, numerous low molecular weight

products were identified that can be trapped in the film or

migrate in the gas phase. This could provoke a progressive

erosion of the surface. The strongest point is the high stability of

PVA upon irradiation in the absence of oxygen, even after long

exposure in conditions of accelerated ageing. This result is of

prime importance for the incorporation of PVA in a durable

multilayer encapsulation system for OSCs. Therefore, PVA

appears to be a good organic candidate for OSC encapsulation

with high durability, as long as the PVA layer is protected from

air by an inorganic layer as a first outside layer in the inorganic/

organic multistack.

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