Depth-dependence of cross-linking on photo-oxidation ofpolyoctenamer

A. Kumar, S. Commereuc*, V. Verney

Laboratoire de Photochimie Moleculaire et Macromoleculaire (UMR CNRS 6505), Universite Blaise Pascal et ENS

Chimie de Clermont-Ferrand, F-63 177 Aubiere Cedex, France

Received 6 January 2003; received in revised form 13 February 2003; accepted 28 February 2003

Abstract

Photo-ageing of polymeric materials induces physical and chemical modifications. The non-uniformity of photo-oxidation acrosspolyoctenamer films has been investigated. While a previous study was devoted to the spatial distribution of photo-products, the

present paper deals with the heterogeneity of molecular structure changes. Melt viscoelastic experiments provide evidence thatmolecular weight increases rapidly as photo-oxidation proceeds, involving cross-linking which is characterized as being depthdependent. From our results, we assume that the depth dependence of the molecular structure change of the material throughphoto-ageing is correlated to the chemical profile in layers beyond 15 mm.

# 2003 Elsevier Science Ltd. All rights reserved.

Keywords: Gel time; Melt viscoelasticity; Elastomer; Photo-ageing; Degradation profile

1. Introduction

It is well-known that elastomeric materials are espe-cially sensitive to oxidative degradation. The mainmechanisms of the photoinitiated oxidation of dienicpolymers are fairly well understood [1–3]. However,heterogeneous oxidation is an important phenomenonthat commonly occurs when polymers are aged in air[4–9]. Inhomogeneous degradation can result frominitially inhomogeneous material (for instance morph-ology difference, interphase presence). Heterogeneousoxidation can also arise when the environmental stressinteracts nonuniformly with the material. Possibilitiesinclude the attenuation of UV light as it passes throughthe sample. Finally, the most widespread cause of hetero-geneity results from oxygen-diffusion-limited effects [7,8].In a previous study, we decided to focus our attention

in this area by investigating heterogeneous oxidationthrough photo-ageing of a previously studied elastomericmaterial [10].

We have observed the occurence of a heterogeneousoxidation in polyoctenamer films through photo-oxida-tion. This result is not surprising in elastomeric materialand it has been already ascribed to oxygen-diffusioneffects [4,7]. Nevertheless authors usually reported atendency toward initially homogeneous oxidation, witha delayed onset of heterogeneous degradation.From our experimental results, we claimed that the

chemical oxidation profile arises from the very begin-ning of irradiation of polyoctenamer at l >300 nm.Actually, no photo-product profile is initially observedby IR spectroscopy while ROOH titration reveals achemical oxidation gradient through the film cross-sec-tion from the earliest stage of photo-ageing. On theother hand, we assume that the distribution of by-products is uniform in the superficial layer in the range3–15 mm [10].Moreover, during photo-ageing, an oxygen perme-

ability coefficient decreasing with the extent of sampledegradation has been observed, which has been asso-ciated with cross-linking and presumably causes theoxidation to become limited in the later stages ofdegradation [7,11,12]. Thus, some investigations suggestthat crosslinking largely controls photo-oxidation ofelastomers [5,7,11,13]. Our finding is that the material is

0141-3910/03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved.

doi:10.1016/S0141-3910(03)00104-6

Polymer Degradation and Stability 81 (2003) 333–339

www.elsevier.com/locate/polydegstab

* Corresponding author. Tel. +33-4-7340-7174; fax +33-4-7340-

7095.

E-mail address: sophie.commereuc@univ-bpclermont.fr

(S. Commereuc).

considered fully cross-linked when photo-products aredetected by FTIR (at about 5 h of exposure) [14]. Now,it appears necessary to monitor the depth dependence ofthe molecular structure changes involving cross-linkingof the material through photo-ageing in order to corre-late the estimated chemical profile to a possible physicalprofile.It is well-known that melt viscoelasticity measure-

ments can provide information about molecular para-meters through structure relationships. Hence, theevolution of rheological properties directly reflects thechanges of such molecular parameters as molecularweight (Mw), and molecular weight distribution(MWD). Through photo-ageing, these changes can becorrelated to chain scissions and/or recombinationswhich involve cross-linking in case of elastomers.The aim of the current study was to investigate the

molecular structure changes of polyoctenamer throughphoto-oxidation versus sample thickness, using meltviscoelastic experiments. These attemps were expectedto give a detailed picture of the heterogeneity of thecross-linking extent in order to compare with the spatialdistribution of photo-products previously described[10].

2. Experimental

2.1. Material

The polymer used was a polyoctenamer rubber namedVestenamer1 8012, produced by Creanova. This is alow-molecular-weight polymer with a broad molecularweight distribution, made from cyclooctene by meta-thesis polymerization. It consists of linear as well ascyclic macromolecules which are unbranched and con-tains one double bond per eight carbon atoms with 80%of the double bonds arranged in a trans configuration(based on NMR determination).It was precipitated twice from chloroform solution

into methanol to remove possible additives. Films ofdifferent thicknesses (in the range 25–100 mm) were pre-pared by compression moulding between two polyestersheets at 60 �C under 100 bar for 1 min.

2.2. Photo-oxidation procedure

Films were fixed on aluminium holders and then irra-diated in a polychromatic set up. A ‘‘medium pressure’’mercury source filtered by a borosilicate envelope(Mazda type MA 400) supplies radiation of wavelengthslonger than 300 nm. This source is located along thefocal axis of a cylinder with an elliptical base. Samplefilms rotated around the other focal axis. The inside ofthe chamber is made of highly reflective aluminium.Sample temperature is controlled by a thermocouple

connected with a temperature regulator device whichcontrols a fan. All experiments were carried out at 35 �C.Films were analysed after various exposure times.

2.3. Rheological experiments

The changes in viscoelastic properties of photo-oxi-dized polyoctenamer were followed in oscillatory shearmode using a rotational controlled stress rheometer(StressTech/Rheologica) equipped with a parallel plategeometry. The plate diameter was 10 mm and the gapbetween the plates was about 1 mm. In all cases, thevalues of the stress amplitude were checked to ensurethat all measurements were conducted within the linearviscoelastic region. At different time during the UVexposure, a frequency sweep extending from 0.01 to 30Hz was performed. Different temperatures have beentested, but the low value of activation energy involves asmall extension of the frequency range. So, all experi-ments were carried out at 90 �C. The stability of oxi-dized samples at 90 �C with respect to the measurementduration has obviously been verified.

2.3.1. Background: basic melt viscoelasticity [15]An ideal elastic body obeys Hooke’s law: applied

stress (�) and induced strain (") are related by a uniquemodulus value E:

� ¼ E":

The elastic deformation is instantaneous and inde-pendent of time (wholly recoverable when the stress isremoved).A completely viscous response is that of Newtonian

fluid, whose deformation (") is linear with time whilestress (�) is applied and is completely irrecoverable:

� ¼ � d"=dtð Þ

where � is the viscosity of the fluid.Most polymeric materials have both viscous and

elastic character; that is, they exhibit a viscoelasticbehaviour. Viscoelastic properties are investigated usingrheological experiments such as dynamic mechanicaltesting, which offers a convenient way to assess timedependence of mechanical properties of polymers.Hence, the material is subjected to an oscillatory

stress, and the corresponding oscillatory response ismonitored and analysed. The effect of time dependencein linear viscoelastic material is exemplified by the phaselag observed between stress and strain, which variesaccording to test conditions (frequency, temperature). Ifthe time-dependent stress imposed on a polymeric sam-ple is given as

� tð Þ ¼ �0sin !tð Þ:

334 A. Kumar et al. / Polymer Degradation and Stability 81 (2003) 333–339

(This function depicts a stress that oscillates from

+�0 to ��0 with angular frequency !.)Then the resulting sinusoidal strain will lag the stress

by some amount of time:

" tð Þ ¼ "0sin !tþ �ð Þ

where "0 is strain amplitude.For an elastic material (at any frequency), the stress

and the strain maxima occur at the same time (�=0);stress and strain are in phase. For a viscous liquid, thestrain maximum lags the stress maximum by a phasedifference of /2. Thus, changes in the phase angle �reflect the time dependence of the viscoelastic propertiesof the polymer.No single parameter can be used to characterize the

stress–strain relationship in a viscoelastic material. Thecomplex dynamic modulus (G*) is resolved into twocomponents using complex notation:

G�

¼ �=" ¼ G0 þ iG00

The real part of the complex modulus (G0) describesstress-strain relationships that are in phase. G0 is calledthe storage modulus (or elastic modulus). The imaginarycomponent (G00) characterizes the out-of-phase compo-nent and is named the loss modulus (or viscous modulus).Dynamic viscosity (�*) is related to the complex

modulus by

�� ¼ �= d"=dtð Þ ¼ G�

= i!ð Þ ¼ �0 � i�00

with �0=G00/! and �00=G0/!. Then, the real componentof the complex viscosity (�0) describes the viscous dis-sipation in the sample, while the imaginary component(�00) represents the stored elastic energy.Furthermore, the tangent of the phase angle (tan �)

describes the balance between the viscous and elasticbehaviours in a polymer melt:

A. Kumar et al. / Polymer Degrad

tan� ¼ G00=G0 ¼ �0=�00:

It is well-known that the changes of the rheologicalmaterial properties directly reflects changes in molecularparameters. Thus, melt rheology provides a convenienttool to view the particular behaviour of a cross-linkingsystem. The linear viscoelastic properties in dynamicexperiments are sensitive to the formation of the three-dimensional network and can be used to preciselydetermine the gel point indicating the transition fromthe liquid to the solid state.

3. Results

In order to investigate the depth dependence of themolecular structure changes of the polyoctenamerthrough photo-ageing, samples of different thickness(ranging from 30 to 100 mm) have been irradiated atl>300 nm at 35 �C so as to expose just one side (theother side is covered with a black surface) to UV irra-diation. In case of the thinnest film, the two sides havebeen irradiated too. Let us notice that a thin film (30 mm)with two sides exposed to irradiation could be consideredas two thin films (15 mm) with only one side exposed.

3.1. Change of molecular weight in the pregel regime

The evolution of the storage component of thedynamic viscosity (�00) through photo-oxidation ofpolyoctenamer is illustrated in Fig. 1.While the frequency dependence curve shows a liquid-

like behaviour at the beginning of irradiation, the shapeof �0(!) changes over the course of photo-oxidation. Asphoto-ageing proceeds, �00(!) exhibits a power-law var-iation with respect to the frequency of oscillation, whichis a characteristic feature of cross-linked system.The changes of the dynamic viscosity give evidence

that cross-linking occurs through photo-oxidation ofpolyoctenamer. Gelation is the phenomenon by which acrosslinking polymeric material undergoes a phasetransition from liquid to the solid state at a criticalpoint, named the gel point.Moreover, the average molecular weight increases as

the crosslinking reaction proceeds, and diverges at thegel point [16]. This property could be used to measure theevolution of an incipient gel near the sol-gel transition. Itis well-known that the zero shear viscosity �0 depends onthe molecular weight and obeys a power law [17]:

�0 /MW

Fig. 1. Storage component of dynamic viscosity (�00) as a function of

frequency ! plotted at different UV exposure times (from 0.5 to 3 h)

for polyoctenamer films (30 mm), open symbols: two sides irradiated,

dark symbols: only one side irradiated.

ation and Stability 81 (2003) 333–339 335

The zero shear viscosity �0 can be obtained from thecomplex viscosity �*(!):

�� ¼ G�

!ð Þ=i! ¼ �0 � i�00

and ��j j!� > 0 ¼ �0j j!� > 0 ¼ �0An empirical rheological model used to fit

dynamic data is the Cole–Cole distribution expressedby [18–20]:

�� !ð Þ ¼ �0= 1þ i!l0ð Þ1�h

� �

where l0 is the average relaxation time and h the para-meter of the relaxation-time distribution.In the complex plane this model predicts the variation

of the viscosity components (�00 versus �0) to be an arc ofa circle. From this representation it is easy to determinethe parameters of the distribution: �0 is obtainedthrough the extrapolation of the arc of the circle on thereal axis and the distribution parameter h through themeasurement of the angle �=h/2 between the real axisand the radius going from the origin of the axis to thecentre of the arc of the circle.Fig. 2 diplays the change of the zero shear viscosity �0

through the photo-oxidation of polyoctenamer. In thepregel state, the zero shear viscosity increases from thebeginning of UV exposure indicating an increase of themolecular weight due to chain recombination reactions.It is worthwhile to note that no depth dependence isrevealed by melt viscoelasticity until about 120 min ofirradiation. It is important to point out that this specificirradiation time corresponds to the gel time of the thin-ner sample (15 mm). Hence, from the earliest stage ofphoto-oxidation, the molecular structure evolution isassumed to be homogeneous.

However, after 150 min of exposure, strong differ-ences in the molecular changes are exhibited versus boththe irradiation method (one or two sides) and thethickness of the sample. The zero shear viscosity of thinfilms rapidly increases and becomes infinite for less than200 min of irradiation, indicating the vicinity of the gelpoint, while, �0 of the most thick film is still measurableafter 400 min of UV exposure. Thus, the �0 evolutionshows that oxidation is not uniform throughout thetotal film cross-section.The molecular weight change clearly appears as a

depth dependent process through photo-ageing. So, thepolymer reaches the gel point at a critical yield of therecombination reactions involving cross-linking [16].Hence, upon photo-ageing, the gel point corresponds toa critical point of irradiation, termed gel time (tgel). It isnow necessary to characterize the incipient gel in orderto accurately determine the gel time (tgel).

3.2. Determination of the gel time

Winter and Chambon have shown that G0(!) andG00(!) exhibit a power-law behaviour with a commonexponent (n) at the gel point. This definition of the sol-gel transition implies that tan � (gelation variable) losesits dependency on frequency and converges at the gelpoint [21–23].A multifrequency plot of tan � versus irradiation time

of polyoctenamer sample exhibits a critical time corre-sponding to the convergence of the values of tan �. Analternative way to determine gel time is by plotting the‘‘apparent’’ viscoelastic exponents n0 and n00 (G0/!n

0

,G00!n

00

), obtained from the approximate scaling laws ofthe frequency dependence of G0(!) and G00(!) versusirradiation time. So, curves become congruent and thecrossover at n0=n00=n is an indicator of the gel point[14].However, in case of an elastomeric material, the

amplitude of the evolution of the storage (G0) and loss(G00) moduli through photo-oxidation is rather slight inthe terminal zone around the modulus plateau value.Hence, this behaviour induces incertainty in the eval-uation of the gel time (tgel).Hence, a new method to accurately determine the gel

point of elastomeric material has been proposed, basedon the use of the Cole–Cole representation [24], appliedto the change of an elastomer through ageing [25,26].Fig. 3 shows the changes of complex viscosity com-

ponents (�0, �00) through photo-oxidation of poly-octenamer using the complex plane model. In case of auncrosslinked material (t=0 h), this model predicts thevariation of viscosity components to be an arc of circle.As the gelation process occurs, the curves (�0 versus �00)become straight lines in the vicinity of the gel point.Thus, the slope of this line depends on the irradiation

time and obeys a sigmoidal law. Moreover, the inflexion

Fig. 2. Evolution of Newtonian viscosity (�0) through photo-oxida-

tion of polyoctenamer films for different thickness, ranging from 30 to

100 mm, so as to irradiate just one side irradiated (1side) or the two

sides (2sides). �0 was measured at 90 �C.

336 A. Kumar et al. / Polymer Degradation and Stability 81 (2003) 333–339

point of the resulting curve (slope versus irradiationtime) corresponds to the gel point which could be accu-rately determined from the derivative of the curve. Fig. 4illustrates the application of this method in the case of aphoto-oxidized polyoctenamer film (thickness 30 mm,two sides irradiated).Fig. 5 displays the sigmoidal curves, obtained from

the complex plane model, for different polyoctenamersamples of different thickness. The kinetic curves of themolecular change upon irradiation of only one side of athin film (30 mm) is quite different from those of thickfilm (about 100 mm). Thus, the influence of the irra-diated thickness is clearly shown.The derivatives of these sigmoidal curves allow

assessment of the exact gel time (tgel) in all cases (seeFig. 6). The gel time appears as a depth-dependentparameter since tgel varies significantly from 150 mn fora thin film (30 mm with two sides exposed, considered as2�15 mm) to more than 300 mn for a thick film (100 mmwith only one side exposed). It is important to point outthat oxidation is not uniform even in thin films. Hence,the Fig. 7 clearly reveals an actual cross-linking extentgradient through the depth from layers beyond 15 up to60 mm.

4. Discussion

Non-uniformity of the cross-linking extent in poly-octenamer through photo-oxidation has been investi-gated by melt viscoelasticity.The increase of the zero shear viscosity (�0) in the

pregel regime provides evidence a molecular structureevolution, implying molecular weight increase as photo-oxidation proceeds, involving cross-linking. Moreover,while �0 rapidly increases for thin films, �0 graduallyraises in case of thick films. Hence, the cross-linking isnot uniform through the depth of the sample andreveals a depth dependence of the molecular changesupon UV exposure.The characterization of the incipient gel confirms the

non-uniformity of the oxidation. The irradiation time(tgel) at which gelation phenomenon occurs stronglydepends on the depth of the irradiated sample. Thus,the critical point named gel point (tgel) appears as arelative parameter.A significantly different gel time is recorded even for

thin films (thickness 30 mm with just one side irradiatedcompared to those with two sides exposed, equivalent to15 mm irradiated). Based on our results, the superficial

Fig. 3. Changes of complex viscosity components (�0, �00) through

photo-oxidation of polyoctenamer using the Cole–Cole representa-

tion; (�0 versus �00) for different irradiation times. The two sides of the

film (30 mm thickness) were irradiated.

Fig. 4. Method of determination of gel time based on the use of the Cole–Cole representation applied in case of the photo-oxidation of a poly-

octenamer film (30 mm thickness), two sides irradiated for 30 mn: (A) Cole–Cole representation; (B) evolution of the slope obtained from the Cole–

Cole representation versus irradiation time.

Fig. 5. Sigmoidal curves obtained from the complex plane model for

different film thickness. Slope of the Cole–Cole representation versus

irradiation time of polyoctenamer films: (!) 30 mm two sides irra-

diated, (&) 100 mm only one side irradiated, (^) 30 mm only one side

irradiated.

A. Kumar et al. / Polymer Degradation and Stability 81 (2003) 333–339 337

layers will be considered as overcrosslinked while thedeeper one will remain uncrosslinked.Hence, from our experimental results, we assume that

an oxidation profile exists through the irradiation ofpolyoctenamer, involving cross-linking depth-profileacross layers up to 15–20 mm detected from irradiationduration longer than the specific time (120 mn).Elsewhere, it is worthwhile to note that these rheologi-

cal results are in quite good accord with the chemicaldepth-profile data previously published [10]. Non-uni-formity of photochemical oxidation of polyoctenamerhas been previously investigated by several IR spectro-scopic analyses including transmission, emission andreflection modes. Our finding is that a chemical peroxi-dation profile arises from the very beginning of theirradiation at l>300 nm at 35 �C, while the photo-pro-duct gradient is observed by FTIR with a delayed onset(from around 10 h of UV exposure).Concerning the distribution of photo-products

throughout the total film cross-section, a strong depthprofiling of by-products is detected in bulk layers (>20mm) and no photo-products are revealed by FTIRspectroscopy in the core of oxidized sample (>50 mm).

However, the photochemical evolution of poly-octenamer is considered as uniform in a superficial layerfrom 3 to about 15 mm. Unfortunately, any gradient ofcross-linking cannot be evaluated by melt viscoelasticityexperiments in the very superficial layers (<15 mm).

5. Conclusion

The occurrence of a heterogeneous oxidation in poly-octenamer films through photo-ageing was observed.From our results, we claim that both chemical oxidationand depth dependence of cross-linking extent occurthrough irradiation of polyoctenamer at l>300 nm.While the molecular structure change of the material

through photo-ageing in the pregel regime occurs fromthe very beginning of photo-ageing, a depth dependenceof the cross-linking is detected after 120 min of irradia-tion beyond 15 mm, which can be correlated to thechemical profile.Nevertheless, while the distribution of by-products is

shown to be uniform in the superficial layer (in therange 3–15 mm), the possible gradient of cross-linkingcannot been evaluated by melt viscoelasticity experi-ments in the very superficial layers (<15 mm). Therefore,it would be interesting to monitor the possible physicalprofile by local microthermal analysis as it has beenverified as a powerful technique for this purpose [27].

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