ics

tAbstract

The thermal degradation mechanism of a novel polyvinyl alcohol/silica (PVA/SiO2) nanocomposite prepared with self-assembly and solu-tion-compounding techniques is presented. Due to the presence of SiO2 nanoparticles, the thermal degradation of the nanocomposite, comparedto that of pure PVA, occurs at higher temperatures, requires more reaction activation energy (E ), and possesses higher reaction order (n). ThePVA/SiO2 nanocomposite, similar to the pure PVA, thermally degrades as a two-step-degradation in the temperature ranges of 300e450

C and450e550 C, respectively. However, the introduction of SiO2 nanoparticles leads to a remarkable change in the degradation mechanism. Thedegradation products identified by Fourier transform infrared/thermogravimetric analysis (FTIR/TGA) and pyrolysis-gas chromatography/mass spectrometric analysis (Py-GC/MS) suggests that the first degradation step of the nanocomposite mainly involves the elimination reactionsof H2O and residual acetate groups as well as quite a few chain-scission reactions. The second degradation step is dominated by chain-scissionreactions and cyclization reactions, and continual elimination of residual acetate groups is also found in this step. 2007 Elsevier Ltd. All rights reserved.

Keywords: Polyvinyl alcohol; Silica; Nanocomposite; Thermal degradation

1. Introduction

The thermal resistance is one of the most dominative prop-erties for polymer materials, as it ultimately governs the me-chanical properties, durability, spectral stability, shelf lives,and life cycles of polymers [1e4]. Once the degradation be-gins, the above properties will gradually deteriorate.

The widely recognized polymers with inherent thermal re-sistance are linear single-strand polymers having a sequence ofcyclic aromatic or heterocyclic structures, ladder polymers,and inorganic or semiorganic polymers [5]. It is found that in-corporating highly stable, rigid aromatic, or heterocyclic rings

into the polymer chains can enhance the thermal stability ofpolymers [6]. By introducing inorganic nano-fillers into poly-mers, thermal resistance of polymer hosts has recently beendramatically improved [7,8]. Compared to virgin polymers,polymeric/inorganic nanocomposites (PINs) usually degradeat significantly higher temperatures, and demonstrate a sub-stantial decrease in the degradation rate [9]. Hence, the ther-mal resistance of polymer hosts can be markedly enhanced,even if only a small amount of nano-fillers is loaded [10].

The contribution of inorganic nano-fillers to thermal resis-tance of PINs has commonly been described as a barriermodel, which suggests that the thermal resistance is enhancedbecause of a strong polymeric-inorganic char [11e14]. Thischar is built up on the surface of the polymers as a massA thermal degradation mechannanocom

Zheng Peng a,b,*,a Chinese Agricultural Ministry Key Laboratory of Natural Rubber Pro

Academy of Tropical Agricultural Scienceb Center for Advanced Manufacturing Research, Universi

Received 9 November 2006; received in revised fo

Available online 2

Polymer Degradation and Stability* Corresponding author. Chinese Agricultural Ministry Key Laboratory of

Natural Rubber Processing, Agricultural Product Processing Research Institute

at Chinese Academy of Tropical Agricultural Sciences, Zhanjiang 524001,

Guangdong, China. Tel.: 86 759 2286933; fax: 86 759 2221586.E-mail address: zhengpeng8@yahoo.com (Z. Peng).

0141-3910/$ - see front matter 2007 Elsevier Ltd. All rights reserved.doi:10.1016/j.polymdegradstab.2007.02.012sm of polyvinyl alcohol/silicaposites

Ling Xue Kong b

essing, Agricultural Product Processing Research Institute at Chinese, Zhanjiang 524001, Guangdong, China

y of South Australia, Mawson Lakes SA 5095, Australia

rm 26 January 2007; accepted 15 February 2007

1 February 2007

92 (2007) 1061e1071www.elsevier.com/locate/polydegstaband heat transfer barrier, and limits the passage of degradationproducts from the matrixes. However, the detailed mechanismof such a remarkable effect is not well understood yet. For ex-ample, previous work seldom reported the kinetic aspects and

chemistry of the thermal degradation which are critical forunderstanding the processing energy barriers and degradationmechanisms [12,15].

Thermal analysis coupling systems provide powerful ap-proaches to reveal the thermal degradation mechanisms ofthe polymer hosts of PINs. Fourier transform infrared/ther-mogravimetric analysis (FTIR/TGA) allows simultaneousquantification and identification of evolved degradation prod-ucts, offering valuable information on thermal degradationof polymers [16]. Jang and Wilkie [17] studied the thermaldegradation of poly(acrylonitrile-co-styrene) and its claynanocomposites using TGA/FTIR. It was found that the nano-composite, compared to the pure poly(acrylonitrile-co-styrene),contains additional degradation steps: extensive random chainscission, evolving additional compounds that have an oddnumber of carbons in the chain backbones, and radical recom-bination, producing head-to-head structures. However, TGA/FTIR is not sufficient to investigate every aspect of thermaldegradation. Particularly when a large number of organic gas-eous species are generated, it is hard to use FTIR to identifythe structures for all degradation products due to intensiveoverlap of characterization peaks. Moreover, FTIR is power-less to identify the low concentration degradation productsdue to its inherent limitation of sensibility. Therefore, pyroly-sis-gas chromatography/mass spectrometric analysis (Py-GC/MS) has been used to collect more detailed information for un-derstanding the thermal degradation mechanisms [18]. Chenet al. [19] combined Py-GC/MS and FTIR/TGA methods toperform a comparative degradation study of polystyrene (PS)and polystyreneeclay composite. The results demonstratedthat the degradation mechanism of the PS/clay compositediffers from that of virgin PS and an unusually high yield ofa-methylstyrene indicates that intermolecular radical transferreactions are found in the nanocomposite.

The thermal degradation of polyvinyl alcohol (PVA) hasbeen initially investigated with thermal analysis [20e22]. Itwas found that PVA thermally degrades in two steps. The firstdegradation step mainly involves the elimination reactions,while the second one is dominated by chain-scission and cycli-zation reactions. Although it has been demonstrated in ourprevious work [3,13,23] that SiO2 reinforced PVA nanocom-posites have improved thermal resistance, the impact of SiO2nanoparticles on the thermal degradation of nanocompositeshas not been comprehensively studied. To understand howthe PVA/SiO2 nanocomposite exactly degrades, we will envis-age a thermal degradation mechanism of PVA/SiO2 nanocom-posite by combing three major experimental techniques TGA,FTIR/TGA and Py-GC/MS.

2. Experimental section

2.1. Materials

Polyvinyl alcohol (PVA) (averagemolecular weight: 67,000;polymerization degree: 1400; hydrolysis rate: 86.7e88.7 mol%),

1062 Z. Peng, L.X. Kong / Polymer Degradasilica nanoparticles (average diameter: 14 nm; surface area:200 25 m2/g) and polyallylamine hydrochloride (PAH)(average molecular weight: 70,000) were purchased fromSigma-Aldrich. All experimental materials were used asreceived.

2.2. Synthesis of PVA/SiO2 nanocomposite

In our previous work [3,13,23], we developed a novel pro-cess to prepare PVA/SiO2 nanocomposites by combining solu-tion compounding with self-assembly technique. Firstly, thenegatively charged SiO2 nanoparticles act as templates to ad-sorb positively charged PAH molecular chains through electro-static adsorption. PVA molecular chains are then assembled onthe surface of SiO2 nanoparticles through hydrogen-bondinginteraction between hydroxyl groups of PVA and aminogroups of PAH. Finally, the SiO2 nanoparticles covered withPAH and PVA molecules are uniformly distributed in thebulk PVA matrix, which is dried to form PVA/SiO2 nanocom-posite films. The key procedure of this process is the encapsu-lation of the SiO2 nanoparticles with PAH and PVA layers,aiming at suppressing the self-aggregation of SiO2 nanopar-ticle caused by strong particleeparticle interactions. Thenanocomposite contains 5 wt% SiO2.

2.3. Characterization

A Perkin Elmer TGA-7 thermogravimetric analyzer wasused for the thermogravimetric analysis (TGA). The measure-ment of the films (ca. 10 mg) was carried out from 100 to600 C with heating rates of 10, 20, 30, 40 and 50 C/min un-der nitrogen with a flow rate of 80 ml/min.

Fourier transform infrared/thermogravimetric analysis(FTIR/TGA) was performed on a combined Perkin ElmerSpectrum Gx-I FTIR and TGA-7 system. The decompositiongases from each degradation step were transferred fromTGA analyzer to FFIR/TGA interface, and measured byFTIR spectrometer with 4 cm1 resolution and 4000e600 cm1 scanning wave number range. To avoid the overlapof evolved gases from different degradation steps, the sampleswere heated at three temperature points, respectively. Firstly,the samples were heated at 110 C for 5 min to eliminate re-sidual water. The samples were then heated around the peakdegradation temperature (Tp) of the first degradation step(350 C) that is obtained from TGA analysis till the degrada-tion was completed. Finally, the samples were heated aroundTp of the second degradation step (470

C) till the degradationwas finished.

Pyrolysis-gas chromatography/mass spectrometric analysis(Py-GC/MS) was carried out with a Japan Analytical IndustryJHP-3S Curie Point Pyrolyzer coupled to a HP6890 gas chro-matograph linked to a 5973 Quadrupole mass spectrometer.The samples were pyrolyzed at 330 C for 10 s and 530 Cfor 15 s to simulate the two degradation steps that were con-ducted on TGA analysis. The carrier gas was high-purity nitro-gen at a flow rate of 50 ml/min. The GC column was a HP-5fused silica capillary column. The GC column temperature

tion and Stability 92 (2007) 1061e1071was initially held at 50 C for 2 min; then at a rate of 5 C/min, it was raised to 280 C and held there for 30 min. The

when the temperature increases too fast. Theoretically, theGCeMS interface was set at 230 C. Mass spectra wererecorded under the electron impact ionization energy of70 eV. The total flow was split at a ratio of 50:1. The massof each sample was 0.10e0.20 mg.

3. Results and discussion

3.1. Thermal degradation process

There are two distinct and well-separated turns (300e450 C and 450e550 C) in the thermogravimetric (TG)curves (Fig. 1a) and two corresponding weight-loss peaks inderivative thermogravimetric (DTG) curves for the pure PVA(Fig. 1b). Therefore, the thermal degradation of PVA can beroughly regarded as a two-step-degradation. Due to the pres-ence of SiO2, the TG/DTG curves of the nanocomposite shiftto higher temperatures (Fig. 2). Therefore, the PVA/SiO2nanocomposite is more thermally stable than the pure PVA.Correspondingly, at a given temperature, the degradation rateof the nanocomposite is significantly lower. Under a given

-48

-38

-28

-18

-8

2200 300 400 500 600

Temperature (C)

Derivative W

eig

ht %

(%

/m

in

)

0

20

40

60

80

100

200 300 400 500 600Temperature (C)

Weig

ht %

(%

)

TG

DTG

a

b

Step 1Step 2

Step 1: Tp = 1.68 + 355.6Step 2: Tp = 0.64 + 467.8

Fig. 1. TG (a) and DTG (b) curves of PVA in nitrogen with different heating

Z. Peng, L.X. Kong / Polymer Degradarates: B 10 C/min;O 20 C/min; , 30 C/min; > 40 C/min; C 50 C/min.heating rate, the TG curve of the nanocomposite is situatedat a higher temperature than that of the pure PVA. Here, wetake one of the most important degradation temperatures, thepeak degradation temperature with maximum weight lossrate (Tp), as an example to quantitatively investigate the differ-ence in degradation temperatures between pure PVA andnanocomposite.

As the thermal degradation in this study was conducted witha temperature-scanning model, the heating rate (b) has a strongeffect on the degradation. To truly identify the differencebetween the degradation of pure PVA and PVA/SiO2 nanocom-posite, the influence of heating rates was eliminated with amulti-heating rate method [13,24]. From Figs. 1b and 2b, it wasfound that degradation temperature Tp for both PVA and nano-composite linearly increases with the heating rate.

The linear increase of thermal degradation temperatureswith heating rate is caused by the heat hysteresis, i.e. the innerpart of the samples cannot follow the program temperature

-48

-38

-28

-18

-8

2200 300 400 500 600

Temperature (C)

Derivative W

eig

ht %

(%

/m

in

)

0

20

40

60

80

100

200 300 400 500 600Temperature (C)

Weig

ht %

(%

)

TG

DTG

a

bStep 1

Step 2

Step 1: Tp = 1.09 + 384.6Step 2: Tp = 0.75 + 479.3

Fig. 2. TG (a) and DTG (b) curves of PVA/SiO2 nanocomposite in nitrogen

with different heating rates: B 10 C/min;O 20 C/min; , 30 C/min; >40 C/min;C 50 C/min.

1063tion and Stability 92 (2007) 1061e1071slower the heating rate, the more accurate the degradation

presents. Therefore, by assuming the heating rate equals 0 C/min, the equilibrium peak thermal degradation temperature(Tp

) can be identified from equations in Figs. 1b and 2b.The Tp

of the PVA/SiO2 nanocomposite increase to 29.0 Cwhich is 11.5 C over the pure PVA at the first and second deg-radation steps, respectively.

In addition to the increase of thermal degradation tempera-tures, the difference in the degradation process between thepure PVA and nanocomposite is also obviously observed inthe DTG curves (Figs. 1b and 2b). Due to the improvementin thermal resistance, the whole degradation process of thenanocomposite is protracted, which is confirmed by theDTG curves where the degradation peak of nanocompositeis obviously wider than that of pure PVA. More importantly,there is a significant change in the shape of the degradationpeak of DTG. There are two obvious degradation peaks (aside peak accompanies the main peak) in the first degradationstep of the nanocomposite, while there is only one degradationpeak for the pure PVA (Figs. 1b and 2b). According to otherswork [20e22], at the first degradation step, the thermal degra-dation of PVA mainly involves the elimination reactions, andcorrespondingly there is only one peak in the DTG curve.The split of the first degradation peak in TG/DTG curves im-plies that the degradation mechanism of the nanocompositediffers from that of pure PVA. The side peak at lower temper-ature can be caused by the elimination reactions, while themain peak can be the overlap of the continual eliminationsand chain-scission reactions which need more energy andoccur at a higher temperature.

3.2. Thermal degradation kinetics

The kinetic analysis provides information on the energybarriers of the process and clues to the degradation mechanismfor polymers. The challenge for the study of thermaldegradation kinetics is to find a reliable approach. As single-heating-rate methods have identified shortcomings [25], multi-heating-rate methods have been extensively used to study thethermal degradation kinetics for polymers due to their reliabil-ity [14,26]. In this study, CoatseRedfern model [24] based onthe multi-heating rate method is employed to obtain reliablekinetic information on the thermal degradation of PVA/SiO2nanocomposite and pure PVA.

The reaction kinetic parameters are obtained by processingTG data in Figs. 1a and 2a. By integrating the reaction kineticequation

da=dt k1 an 1

and Arrhenius equation,

k AeE=RT 2

the following CoatseRedfern equations [24] can be obtained:

1 1 a1n 1 2RT=EAR E

1064 Z. Peng, L.X. Kong / Polymer DegradatlnT21 n ln bE RT ns1 3and

ln

ln 1 a

T2

ln

1 2RT=EARbE

ERT

n 1 4

where n is the reaction order, a the reaction degree, T the ab-solute temperature, b the heating rate, E the reaction activationenergy, R the gas constant and A the frequency factor. Whenns 1, a line can be obtained from the plot of ln11 a1n=T21 n versus 1/T, where the slope is E=R,and the intercept is ln1 2RT=EAR=bE. When n 1,a line can be obtained from the plot of lnln1 a=T2 ver-sus 1/T, of which the slope is E=R, and the intercept isln1 2RT=EAR=bE. Adopting the least square fittingmethod with different n, the n with maximum correlation co-efficient (r) is the apparent reaction order, and the correspond-ing E is the reaction activation energy.

Table 1 lists various kinetic parameters of the first thermaldegradation step for both PVA and nanocomposite. At the firstdegradation step, the average value of n of PVA is 3.6, whichis 0.8 lower than that of the nanocomposite. The reaction ac-tivation energy increases with the heating rate. Using the least-square linear regression method, E of the pure PVA and thenanocomposite can be described as E 0.66 b 129.9 andE 0.70 b 135.6, respectively. Eliminating the influenceof heating rates, i.e. b 0 C/min, the apparent reaction acti-vation energy (E0) for the pure PVA is 129.9 kJ/mol, and135.1 kJ/mol for the nanocomposite, respectively. At the sec-ond thermal degradation step, the average value of n forboth PVA and nanocomposite are 1.6 and 2.6, correspond-ingly. The E for PVA and nanocomposite are E 1.24b 145.6 and E 1.02 b 156.4, respectively (Table 2).

With the addition of SiO2, the thermal degradation mecha-nism tends to be more complex, the n for the nanocompositeis, therefore, higher than that of the pure PVA. Due to the re-tardant effect of PVA/SiO2 char, the nanocomposite, comparedto the pure PVA, is more difficult to be degraded, and more re-action activation energy is required during the reactionprocess.

The enhanced thermal resistance attributes to the introduc-tion of SiO2 nanoparticles. As illustrated in our previous work[3,13,23], when 5 wt% SiO2 is added to PVA, SiO2 nanopar-ticles are homogenously distributed throughout the PVA ma-trix as nano-clusters with an average size less than 30 nm.Because the size of SiO2 clusters is far below 100 nm, theseSiO2 nano-clusters with a huge relative surface area and greatpotential energy strongly interact with the PVA molecular

Table 1

Kinetic parameters of the first thermal degradation step

B (C/min) PVA Nanocomposite

n E (kJ/mol) n E (kJ/mol)

10 3.4 131.5 4.6 140.3

20 3.6 147.0 4.4 151.3

30 3.6 153.7 4.3 157.0

ion and Stability 92 (2007) 1061e107140 3.7 155.5 4.2 166.5

50 3.6 160.0 4.6 167.5

sition products from the bulk polymer to gas phase is sloweddown. Consequently, the nanocomposite has a pronounced im-provement in thermal resistance compared to the pure PVA.Another reason for this enhancement is the formation ofPVA/SiO2 char. While being heated, the SiO2 nanoparticles,due to their relatively low surface potential energy, migrateto the surface of the composites to form a SiO2/PVA char,which acts as a heating barrier to protect the PVA inside. Sim-ilarly, Gilman et al. [12] and Vyazovkin and Sbirrazzuoli [25]observed that a clay/polymer char greatly enhances the ther-mal resistance of host polymers.

However, the above theories seem somewhat weak to ex-plain why and how the PVA/SiO2 nanocomposite has muchbetter thermal resistance than the pure PVA. To give a moreillustrative insight into the thermal degradation, the degrada-tion chemistry was studied by identifying degradationproducts.

3.3. Thermal degradation chemistry of PVA/SiO2nanocomposite

To develop a degradation mechanism of the nanocomposite,the gaseous degradation products are characterized by FTIR/TGA and Py-GC/MS from two degradation steps as describedin Section 2.

3.3.1. The first degradation stepElimination reactions: The elimination reactions of linear

and aliphatic polymers including polyvinyl chloride (PVC)

+

OH

OH

OH O

OHScheme 1. Eliminatio4000 3500 3000 2500 2000 1500 10000.000

0.020

0.040

0.060

4000 3500 3000 2500 2000 1500 1000

0.000

0.008

0.016

0.024

Wavenumber (cm-1

)

Ab

so

rb

an

ce

c

b

Fig. 3. FTIR spectra of degradation products at different temperatures: (a)

110 C; (b) 350 C; (c) 470 C.

Elimination of nH2O

HOHO+

H

H2Ochains through various effects, such as branching effect, nucle-ation, size and surface effects. Thus, the diffusion of decompo-

Table 2

Kinetic parameters of the second thermal degradation step

B (C/min) PVA Nanocomposite

n E (kJ/mol) n E (kJ/mol)

10 1.6 163 2.3 169.8

20 1.4 167.0 2.8 174.6

30 1.6 176.7 2.8 184.7

40 1.6 198.7 2.9 200.2

50 1.7 209.3 2.2 209.1

4000 3500 3000 2500 2000 1500 1000

-0.5

0.5

1.5

2.5

3.5 a

1065Z. Peng, L.X. Kong / Polymer Degradation and Stability 92 (2007) 1061e1071n reaction of H2O.

nanocomposite should occur in the form of Scheme 2, wherethe elimination of H2O and residual acetate groups occurs si-multaneously as the acetate groups are randomly inserted be-tween hydroxyl groups. Therefore, in addition to water, a big

ene structures will act as intermediate products and be furtherdegraded into other products with lower molecular weights atthe next degradation step. Therefore, only a small amount oflow-molecular-weight polyenes is observed in the MS spectra(Table 3).

As the hydroxyl group is protonated under acidic condi-tions to form a substituent, namely eOH2

, that in turn givesrise to a more favourable leaving group, i.e. H2O, the firststage of PVA degradation can be catalyzed by acetic acid.From this point of view, the highly hydrolyzed PVA has betterthermal stability than PVA with a low degree of hydrolysis,and the addition of a compound with basic groups seems tobe suitable for improving the thermal stability.

Chain-scission reactions: Gilman et al. [21] and Alexy et al.[20] found that the first degradation step of PVA mainly con-tains the elimination reactions. In this study, it is howeverfound that the chain-scission reactions are also intensively in-volved at this stage when SiO2 nanoparticles are introducedinto PVA host. That is why the DTG curves of the first degra-dation of nanocomposite present two degradation peaks

Table 3

Identification of degradation products at 330 C for PVA/SiO2 nanocomposite

Retention

time (min)

Compound Molecular

formula

Molecular

weight

Content (%)

1.35 Acetaldehyde

H3C CH

O 44 18.0

1.52 AcetoneH3C C CH3

O58 3.1

1.74 Furan

O68 2.3

1.82e2.05 Acetic acidH3C C OH

O60 64.1and PVA in the first stage of thermal degradation mainly formpolyene structures via dehydrochlorination [27,28] or dehydra-tion [20e22]. Therefore, dehydration should be one of the ma-jor reactions involved in the first degradation step of the PVA/SiO2 nanocomposite, which well explains why a big amount ofwater is present in the FTIR/TGA analysis during the first deg-radation step of the nanocomposite (Fig. 3). Theoretically, ifa high-hydrolyzed PVA is employed, the dehydration ofPVA/SiO2 nanocomposite should generate two polyene struc-tures: conjugated polyenes and non-conjugated polyenes(Scheme 1).

However, as the present study employs a low hydrolyzed(86.7e88.7%) PVA and a big amount of acetate groups remainin PVA molecular chains, the eliminations of the PVA/SiO2

O

O

O

+

Elimination of acetate groups

HO

CH3

HO

O

H2OH3C C CH3

O++

Scheme 2. Dehydration and elimi

1066 Z. Peng, L.X. Kong / Polymer Degrada44.87 Polyenes e e 8.0amount of acetic acid is also observed during the first degrada-tion step. This is testified by the FTIR/TGA spectra (Fig. 3b)where narrow and acute peaks at 3550 cm1 and 1760 cm1

belong to the stretching vibration peaks of OeH and C]Oof gaseous acetic acids [13], respectively. Another evidenceis given by Py-GC/MS analysis, as Table 3 indicates that thecontent of acetic acid accounts for more than 64% of the gas-eous degradation products.

The high-molecular-weight polyenes presented in Scheme2 do not appear in the FTIR/TGA (Fig. 3b) and Py-GC/MSspectra (Fig. 4), due to their relatively high-molecular weights.The degradation temperature of the first degradation step is nothigh enough to break all the backbone chains of these poly-enes into low-molecular-weight polyenes. Most of these poly-

HOHO

Elimination of H2O

CH3

nations of residual acetate groups.

tion and Stability 92 (2007) 1061e1071(Fig. 2b). The first one occurs at lower temperatures and cor-responds to elimination reactions, while the other one at higher

1.20 1.60 2.00 2.40 2.80

150000

250000

350000

450000

550000

650000

1.15

1.35

1.521.60 1.74

1.821.95

2.05

20 40 60 80 100

20000

40000

60000

80000

100000

120000 44

284034 9478

30 40 50 60 70

200005000080000

110000140000170000200000 29

44

42312640 4633 6451 7855

20 40 60 80

4000

8000

12000

16000 43

58

3928 6637 46 7734 55 64

20 40 60 80 100

1000

4000

7000

10000

1300041

70

275032 36 57 9481

20 40 60 80 100 120

5000

20000

35000

50000

65000 45

60

29 105 122786852

a

b

c

d

e

f

1067Z. Peng, L.X. Kong / Polymer Degradation and Stability 92 (2007) 1061e1071Fig. 4. Pyrogramof the nanocomposite at (a) 330 Cand theMS spectra at different retention times: (b) 1.15 min; (c) 1.35 min; (d) 1.52 min; (e) 1.74 min; (f) 1.82 min.

temperature corresponding to the overlap of elimination andchain-scission reactions. As the introduction of SiO2 nanopar-ticles greatly improves the thermal resistance of the nanocom-posite, the first thermal degradation step of the nanocompositeoccurs at a higher temperature (Figs. 1 and 2), which enablesthe chain-scission reactions.

Tsuchiya and Sumi [29] reported that the dehydration ofPVA leads to the formation of the conjugated polyene struc-tures. However, we believe that the conjugated and non-conjugated polyenes are simultaneously formed during thedegradation of PVA/SiO2 nanocomposite (Scheme 2). At thetemperatures of first degradation step, the conjugated polyenesare unlikely to be degraded due to their relatively stable struc-tures, which are highly regular. However, the non-conjugatedpolyenes seem to be more likely degraded under this condition(Scheme 3). After the elimination reactions, lots of hydroxylgroups still anchor on the polyene molecular chains to form

+

HOHO

H3C

HO

H3C

H3C

CHOHO

H3C CHO

+

+

Chain-Scission

Chain-Scission

+ H

Scheme 3. Chain-scission reactions in the first degradation step.

O

O+

Intramolecular dehydration

Chain-Scission

OH

OH

1068 Z. Peng, L.X. Kong / Polymer DegradaScheme 4. Formation of furan.non-conjugated polyene structures. These hydroxyl groups de-stroy the structural regularity of polyenes and act as weaklinks, from where the non-conjugated polyenes can be snippedinto low molecular structure via chain-scission reaction ata relatively low degradation temperature. Therefore, a smallamount of polyenes are observed in the degradation products(Table 3).

Gilman et al. [21] proposed similar conjugated and non-conjugated polyene structures, and described the degradationmechanism of non-conjugated polyenes into cis and transmethyl-terminated polyenes via chain-scission reaction. How-ever, we envisage a different mechanism in the chain scissionof the nanocomposite at the first degradation step (Scheme 3).The non-conjugated polyenes are not directly degraded intolow-molecular-weight polyenes, but form some intermediates:methyl-terminated non-conjugated polyenes and carbonyl-ter-minated polyenes. The methyl-terminated polyenes are thensnipped into acetaldehyde and polyenes with carbocations.These polyenes are further degraded into cis and transmethyl-terminated polyenes via additional reactions. The char-acteristic peak of methyl terminal group around 1370 cm1 isclearly observed in the FTIR spectra of degradation products(Fig. 3b and c).

Other subsidiary reactions also contribute to the chain scis-sion. The appearance of a small amount of furan in the gaseousdegradation products (Fig. 4e) can be explained by the forma-tion of furan rings via intramolecular cyclizations. The dehy-dration reaction of two intramolecular hydroxyl groupsproceeds under the catalysis of acetate acid produced by theelimination of residual acetate groups (Scheme 4). As fore-mentioned reason, the first degradation step of the nanocom-posite occurs at relatively high temperatures, some of thechain scissions occur at the position of ortho-diol. This chainscission will generate some acetaldehyde and acetone struc-tures as indicated in Scheme 5.

3.3.2. The second degradation stepThe degradation products of the second degradation step are

acetaldehyde, acetic acid, polyenes, benzenoid derivatives anda small amount of furan (Table 4). As the second step of thermaldegradation occurs at higher temperatures compared to the firststep, it becomes more complex. At this step, the degradation isdominated by the chain-scission reactions, side-reactions andcyclization reactions. A continual degradation of residualacetate groups is also found.

The acetic acid is generated from the continued degradationof residual acetate groups as described in Scheme 2, while theacetaldehyde and acetone are attributed to the continual chain-scission reaction presented in Schemes 3 and 5. Furan, similarto the first degradation step, is formed by intramolecular cycli-zations (Scheme 4).

After the first step of degradation, the residuals from elim-ination reactions mainly include high-molecular-weight poly-enes, which are further degraded to low-molecular-weightpolyenes via chain-scission reactions (Scheme 6). That is

tion and Stability 92 (2007) 1061e1071why around 16% polyenes are found in the gaseous degrada-tion products (Table 4) and characterization peaks of polyenes

Another intramolecular cyclization reaction proposed byGilman et al. [21] is also involved (Scheme 8).

from that of pure PVA. In the first degradation step, the elim-ination of H2O and residual acetate groups generates water,non-conjugated polyenes and acetic acid; as the PVA/SiO2nanocomposite degrades at a higher temperature, differentfrom the pure PVA, quite a few chain-scission reactions arealso found in this step in which acetaldehyde, acetone and fu-ran are produced. The reactions in the second degradation stepare dominated by chain-scission reactions. Some side-reac-tions such as cyclization reactions and a continual degradation

Table 4

Identification of degradation products at 530 C for PVA/SiO2 nanocomposite

Retention

time (min)

Compound Molecular

formula

Molecular

weight

Content (%)

1.25 Acetaldehyde

H3C CH

O 44 24.0

1.34 Acetone

H3C C CH3

O 58 5.8

1.51 Furan

O

68 5.6

1.78 Acetic acid

H3C C OH

O 60 30.6

1.92e6.08 Low molecularpolyenes

e e 16.1

+ H Chain-scission

H3C

+ H Cat 16,400e1660 cm1 are observed in FTIR spectra during thesecond degradation step (Fig. 3c).

The Py-GC/MS results show that there are quite an amountof benzenoid derivatives (15.1%) in the degradation products(Table 4), which is also evidenced by the FTIR/TGA as peaksat 3050e3100 cm1 and 1610e1630 cm1 corrseponding tothe benzenoid derivatives are observed in the FTIR spectra(Fig. 3c). These benzenoid derivatives are generated fromthe cyclization reactions. DielseAlder reaction was frequentlyfound in the thermal degradation of polyvinyl chloride (PVC)[30,31]. As PVAs molecular structure is very similar to PVC,that is, they are all linear and aliphatic polymers, the cycliza-tion reaction of PVA should be the same as PVC (Scheme 7).

H3C CH

O

+

OH

OH

O

CHO

OH

OH

OH

OH

Scheme 5. Chain-

Z. Peng, L.X. Kong / Polymer Degrada4.40e9.98 Benzenoidderivatives

e e 15.14. Conclusion

The PVA/SiO2 nanocomposite exhibits a significantly im-proved thermal resistance. In comparison with the pure PVA,degradation temperatures of the nanocomposite are markedlyhigher, while its degradation rates at different degradationstages, correspondingly, are significantly lower; the thermaldegradation of the nanocomposite possesses higher reactionenergy and reaction order.

Similar to the pure PVA, the thermal degradation of thePVA/SiO2 nanocomposite can be roughly regarded as a two-step degradation. However, the introduction of SiO2 into thePVA matrix leads to a change of degradation mechanism

H3C CH3

O

+

+

H OH

OH

CH3

O OH

OH

scission reactions.

1069tion and Stability 92 (2007) 1061e10713

Scheme 6. Chain-scission reaction of polyenes.

of residual acetate groups are also involved. The main degra-dation products of this step are acetaldehyde, low-molecular-weight polyenes, benzenoid derivatives, furan, acetone andacetic acid.

Acknowledgment

We thank Prof. S.D. Li at Guangdong Ocean University,P.R. China, Prof. H.P. Yu at Agriculture Ministry Key Labora-tory of Natural Rubber Processing, P.R. China and Dr. P.Spiridonov in Center for advanced Manufacturing Researchat the University of South Australia for their helpful comments.

References

[1] Cho HJ, Jung BJ, Cho NS, Lee J, Shim HK. Synthesis and characteriza-

tion of thermally stable blue-light-emitting polyfluorenes containing

siloxane bridges. Macromolecules 2003;36:6704.

[2] Jin JY, Smith DW, Topping CM, Suresh S, Chen SR, Foulger SH, et al.

Synthesis and characterization of phenylphosphine oxide containing per-

fluorocyclobutyl aromatic ether polymers for potential space applica-

tions. Macromolecules 2003;36:9000.

[3] Peng Z, Kong LX, Li SD, Spiridonov P. Polyvinyl alcohol/silica nano-

composite: its morphology and thermal degradation kinetics. Journal of

Nanoscience and Nanotechnology 2006;12:3934.

[4] Xia R, Heliotis G, Campoy-Quiles M, Stavrinou PN, Bradley DDC,

Vak D, et al. Characterization of a high-thermal-stability spiroanthrace-

Intra-molecular

Cyclization

Aromatization-2H

Scheme 8. Cyclization reaction II.

Diels-Alder Reaction

-4H

Aromatization

Scheme 7. Cyclization reaction I.

1070 Z. Peng, L.X. Kong / Polymer Degradationefluorene-based blue-light-emitting polymer optical gain medium. Jour-

nal of Applied Physics 2005;98. Art. No. 083101.[5] Zhang HQ, Farris RJ, Westmoreland PR. Low flammability and thermal

decomposition behavior of poly(3,30-dihydroxybiphenylisophthalamide)and its derivatives. Macromolecules 2003;36:3944.

[6] Lin SH, Li FM, Cheng SZD, Harris FW. Organo-soluble polyimides:

Synthesis and polymerization of 2,20-bis(trifluoromethyl)-4,40,5,50-biphe-nyltetracarboxylic dianhydride. Macromolecules 1998;31:2080.

[7] Ji XL, Hampsey JE, Hu QY, He JB, Yang ZZ, Lu YF. Mesoporous silica-

reinforced polymer nanocomposites. Chemistry of Materials 2003;15:

3656.

[8] Tsai TY, Li CH, Chang CH, Cheng WH, Hwang CL, Wu RJ. Preparation

of exfoliated polyester/clay nanocomposites. Advanced Materials

2005;17:1769.

[9] Kong LX, Peng Z, Li SD, Bartold PM. Nanotechnology and its role in

the management of periodontal diseases. Periodontology 2000

2006;40:184.

[10] Vyazovkin S, Dranca I, Fan XW, Advincula R. Kinetics of the thermal

and thermo-oxidative degradation of a polystyrene-clay nanocomposite.

Macromolecular Rapid Communications 2004;25:498.

[11] Choi J, Kim SG, Laine RM. Organic/inorganic hybrid epoxy nanocom-

posites from aminophenylsilsesquioxanes. Macromolecules 2004;37:99.

[12] Gilman JW, Jackson CL, Morgan AB, Harris R, Manias E, Giannelis EP,

et al. Flammability properties of polymer-layered-silicate nanocompo-

sites, polypropylene and polystyrene nanocomposites. Chemistry of Ma-

terials 2000;12:1866.

[13] Peng Z, Kong LX, Li SD. Thermal properties and morphology of poly

(vinyl alcohol)/silica nanocomposite prepared with self-assembled mono-

layer technique. Journal of Applied Polymer Science 2005;96:1436.

[14] Vyazovkin S. Modification of the integral isoconversional method to ac-

count for variation in the activation energy. Journal of Computational

Chemistry 2001;22:178.

[15] Li SD, Peng Z, Kong LX, Zhong JP. Thermal degradation kinetics and

morphology of natural rubber/silica nanocomposites. Journal of Nano-

science and Nanotechnology 2006;6:541.

[16] Zhu J, Morgan AB, Lamelas FJ, Wilkie CA. Fire properties of polystyr-

eneeclay nanocomposites. Chemistry of Materials 2001;13:3774.

[17] Jang BN, Wilkie CA. The effects of clay on the thermal degradation

behavior of poly(styrene-co-acrylonitirile). Polymer 2005;46:9702.

[18] Sundarrajan S, Surianarayanan M, Srinivasan KSV, Kishore K. Thermal

degradation processes in polysulfide copolymers investigated by direct

pyrolysis mass spectrometry and flash pyrolysis-gas chromatography/

mass spectrometry. Macromolecules 2002;35:3331.

[19] Chen K, Susner MA, Vyazovkin S. Effect of the brush structure on the

degradation mechanism of polystyreneeclay nanocomposites. Macromo-

lecular Rapid Communications 2005;26:690.

[20] Alexy P, Bakos D, Crkonova G, Kolomaznik K, Krsiak M. Blends of

polyvinylalcohol with collagen hydrolysate thermal degradation and

processing properties. Macromolecular Symposia 2001;170:41.

[21] Gilman JW, VanderHart DL, Kashiwagi T. In: Nelson GL, editor. Fire

and Polymers II: Materials and tests for hazard prevention. Washington,

DC: American Chemical Society; 1994. p. 161.

[22] Zhao WW, Yamamoto Y, Tagawa S. Radiation effects on the thermal

degradation of poly(vinyl chloride) and poly(vinyl alcohol). Journal of

Polymer Science Part A Polymer Chemistry 1998;36:3089.

[23] Peng Z, Kong LX, Li SD. Non-isothermal crystallisation kinetics of self-

assembled polyvinylalcohol/silica nano-composite. Polymer 2005;

46:1949.

[24] Peng Z, Li SD, Huang MF, Xu K. Thermogravimetric analysis of methyl

methacrylate-graft-natural rubber. Journal of Applied Polymer Science

2002;85:2952.

[25] Vyazovkin S, Sbirrazzuoli N. Kinetic methods to study isothermal and

nonisothermal epoxyanhydride cure. Macromolecular Chemistry and

Physics 1999;200:2294.

[26] Brown ME, Maciejewski M, Vyazovkin S, Nomen R, Sempere J,

Burnham A, et al. Computational aspects of kinetic analysis. Part A:

The ICTAC kinetics project-data, methods and results. Thermochimica

Acta 2000;355:125.

n and Stability 92 (2007) 1061e1071[27] Millan JL, Martinez G, GomezElvira JM, Guarrotxena N, Tiemblo P. In-

fluence of tacticity on the thermal degradation of PVC. 8.

A comprehensive study of the local isotactic GTTG() conformationdependence of the mechanism of initiation. Polymer 1996;37:219e30.

[28] Starnes WH, Wallach JA, Yao HY. Six-center concerted mechanism for

poly(vinyl chloride) dehydrochlorination. Requiescat in pace? Macro-

molecules 1996;29:7631e3.[29] TsuchiyaY, SumiK.Thermaldecomposition products of poly(vinyl alcohol).

Journal of Polymer Science Part A-1: Polymer Chemistry 1969;7:3151.

[30] Garrigues C, Guyot A, Tran VH. Thermal dehydrochlorination

and stabilization of polyvinylchloride in solution. 9. Dialkyltin

maleates e DielseAlder reaction. Polymer Degradation and Stability

1994;43:307.

[31] German K, Kulesza K. The effect of poly(ethylene terephthalate) and

polyethylene as admixtures in, on dehydrochlorination of, poly(vinyl

chloride). Polimery 1999;44:548.

1071Z. Peng, L.X. Kong / Polymer Degradation and Stability 92 (2007) 1061e1071

A thermal degradation mechanism of polyvinyl alcohol/silica nanocompositesIntroductionExperimental sectionMaterialsSynthesis of PVA/SiO2 nanocompositeCharacterization

Results and discussionThermal degradation processThermal degradation kineticsThermal degradation chemistry of PVA/SiO2 nanocompositeThe first degradation stepThe second degradation step

ConclusionAcknowledgmentReferences