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Journal of Non-Crystalline Solids 351 (2005) 2593–2598

Physical aging of atactic polystyrene as seen by dielectricrelaxational and low-frequency vibrational Raman spectroscopies

A. Wypych a,b,c, E. Duval b, G. Boiteux c, J. Ulanski a,*, L. David c, G. Seytre c,A. Mermet b, I. Stevenson c, M. Kozanecki a, L. Okrasa a

a Department of Molecular Physics, Technical University of Lodz, Zeromskiego 116, 90-924 Lodz, Polandb Laboratoire de Physico-Chimie des Materiaux Luminescents, UMR CNRS 5620, Universite Claude Bernard Lyon 1,

43, Bd. du 11 Novembre, 69622 Villeurbanne cedex, Francec Laboratoire des Materiaux Polymeres et Biomateriaux, UMR CNRS 5627/IMP, Universite Claude Bernard Lyon 1,

43, Bd. du 11 Novembre, 69622 Villeurbanne cedex, France

Available online 1 August 2005

Abstract

The physical aging of atactic polystyrene at a temperature close to the glass transition temperature and at room temperature(storage conditions) was investigated by low-frequency Raman scattering, broadband dielectric spectroscopy and differential scan-ning calorimetry. A consistent relation between changes in molecular dynamics and nanostructure caused by physical aging has beenfound. The results suggest that the structure after aging becomes more cohesive and more homogeneous at the nanoscale, while thea-relaxation seems to be mainly affected by the mentioned structural changes.� 2005 Elsevier B.V. All rights reserved.

PACS: 61.43.Fs; 63.50.+x; 77.22.Gm; 65.50.+m

1. Introduction

Amorphous polymers in the glassy state are far fromtheir thermodynamic equilibrium and are subject to theso-called �structural relaxation�, a series of molecularrearrangements leading to lower energy states. This pro-cess is also called �physical aging�, to distinguish theseeffects from other aging processes such as chemicalreactions or degradations. The aging phenomenon inglasses is a long-standing issue, which was extensivelystudied due to the technological problems it causes[1–9]. This process occurs during long-term storage oruse of polymers and may limit their applications by

0022-3093/$ - see front matter � 2005 Elsevier B.V. All rights reserved.doi:10.1016/j.jnoncrysol.2005.04.077

* Corresponding author. Tel.: +48 42 6313 216; fax: +48 42 6313218.

E-mail addresses: alwypych@p.lodz.pl (A. Wypych), julanski@p.lodz.pl (J. Ulanski).

changing their properties. On a more fundamental pointof view, the study of the properties of polymers or glass-forming systems, connecting the dynamics to thestructural state at the nanoscale is one way to under-stand better the relaxation phenomena and the glasstransition.

In the present work, the aging processes were studiedfor atactic polystyrene (PS) annealed at a temperatureclose to the glass transition temperature (Tg) and storedat room temperature. Aging effects were investigated bylow-frequency Raman scattering (LFRS), broadbanddielectric spectroscopy (BDS) and differential scanningcalorimetry (DSC). The low-frequency vibrational den-sity of states (VDOS) in glasses is related to vibrationalmodes extended over nanometric lengths. The excess ofVDOS in comparison with the Debye regime is calledthe Boson peak and it is reflected in the excess in LFRS.The Boson peak is a characteristic feature of amorphous

2594 A. Wypych et al. / Journal of Non-Crystalline Solids 351 (2005) 2593–2598

matter and its frequency can be correlated to the size ofheterogeneities in a structure, as assumed by differentmodels. In the LFRS of polymers, Boson peak is inthe spectral range 2–30 cm�1. According to the modelof heterogeneous nanostructure of glasses proposed byDuval et al., the more strongly bonded regions (cohesivenanodomains) are separated by weakly bonded channels(soft zones) [10].

Complementary LFRS and BDS studies should allowto correlate the changes in the nanostructure in PS dur-ing the physical aging with changes in moleculardynamics.

40 60 80 100 120 140

-0.30

-0.25

-0.20

Endo

cHea

t Flo

w (W

/g)

40 60 80 100 120 140-0.35

-0.30

-0.25

-0.20

Hea

t Flo

w (W

/g)

40 60 80 100 120 140-0.4

-0.3

-0.2

-0.1

Hea

t Flo

w (W

/g)

b

Temperature (°C)

a

Fig. 1. The DSC thermograms for: (a) reference sample, (b) sampleafter aging close to Tg and (c) sample after 10 months aging at 20 �C.The solid line corresponds to the first run and the dotted line to thesecond run.

2. Experimental

2.1. Materials

The atactic PS studied in this work was manufacturedDwory (Poland) as an extrusion grade pellets. The aver-age molecular mass Mn is close to 120000 g/mol withIp = 2.1 as deduced by size exclusion chromatography(SEC) measurements. The glass transition temperatureTg was determined close to 95 �C (m.p., DSC measure-ments with heating rate 10 K/min). The LFRS andDSC measurements were performed on extrusion gradepellets, while the BDS measurements were performed onfilms. Before the aging, all the pellets were heated aboveTg in nitrogen atmosphere (30 min at 140 �C) in order toerase its earlier thermal and mechanical history and thenquenched to room temperature (reference sample). Thecooling rate was about 75 K/min.

The PS films with a thickness of ca. 0.1 mm werecompression-moulded at 150 �C during ca. 10 min underthe pressure of ca. 4 bars. To eliminate the internal stres-ses, the films were annealed additionally without pres-sure at 150 �C for about 5 min. The annealed sampleswere quenched to room temperature, similarly as men-tioned before for pellets. Some of them were investigatedimmediately after the quenching as the reference, notaged samples. Other samples were aged by: annealingat 80 �C (i.e. 15 �C below Tg) in nitrogen atmosphereduring three weeks, or storage at room with controlledtemperature (T = 20 �C) for about 10 months. The pel-lets and the films were aged simultaneously in the sameconditions.

2.2. Measurements

The DSC measurements were made using the DSC2920-TA instrument. For each sample two temperaturescans were performed in the temperature range 20–140 �C at a heating rate 10 K/min.

The LFRS was analyzed with a high-resolution five-grating monochromator equipped with a photon count-ing system. The sample was illuminated with an argon

laser beam of wavelength 514.5 nm. The polarizationof the detected scattered light was perpendicular to inci-dent light polarization (HV configuration). The spectrawere taken at room temperature and at about 20 K,using an helium flow cryostat.

The dielectric measurements were performed in thefrequency range: 0.01 Hz–1 MHz and in the temperaturerange: �150 �C to +140 �C (in steps of 5 K) using aNovocontrol Broadband Dielectric Spectrometer. Goldelectrodes with 34 mm diameter were evaporated ontothe polymer films (sandwich-type configuration).

3. Results

Fig. 1 shows DSC curves for the PS samples with dif-ferent thermal histories. From Fig. 1(a) the glass transi-tion temperature was determined and it is equal to 95 �C(mid point). The small peak visible on a second scan inthe region of glass transition is an effect of applied cool-ing rate after first scan (10 K/min). As an effect of theaging, an endothermic peak appears in the DSC thermo-grams, in Fig. 1(b) and (c), during the first run (pointedby arrows). This peak increases in magnitude and islocated at higher temperature for the sample annealedat 80 �C (Fig. 1(b)).

20 40 60 80 100 120 140 160 180 2000

20

40

60

80

100

I n (a

.u.)

Wavenumber (cm-1)

10 20 30 40 50

100

120

140

160

I n (a

.u.)

Wavenumbers (cm-1)

Fig. 3. Normalized LFRS spectra at low temperature (T < 20 K) forPS: reference sample (solid line), sample annealed three weeks at 80 �C(dotted line) and sample after 10 months aging at 20 �C (dashed line).Inset: spectra for the same samples collected at room temperature.

A. Wypych et al. / Journal of Non-Crystalline Solids 351 (2005) 2593–2598 2595

The physical aging affects the macroscopic thermo-physical properties (enthalpy and entropy level) ofamorphous polymers and simultaneously it may changethe polymer structure at the nanoscale range. To inves-tigate this effect in PS the LFRS was used since it en-ables an indirect investigation of the structure (and itsevolution) at nanometric level by observing the Bosonpeak (the band located between 10 and 30 cm�1). Gen-erally, the LFRS of glasses consists of two components:(a) Boson peak due to the harmonic vibrational mo-tions, and (b) quasi-elastic light scattering due to anhar-monic (or relaxational) motions.

Two LFRS spectra for the reference PS sample areshown in Fig. 2. One of them was performed at roomtemperature (solid line) and the second one (dotted line)at low temperature (T < 20 K). As one can see, the lowtemperature measurements are necessary, because ofstrong quasi-elastic contribution at room temperature.At low temperatures the quasi-elastic scattering at thefoot of the Rayleigh line is negligible and the observedLFRS spectra are only due to harmonic vibrations.

In Fig. 3 the normalized spectrum for the referencePS sample (solid line) is compared with the normalizedspectra: for the sample annealed three weeks at 80 �C(dotted line) and for the sample aged 10 months at20 �C (dashed line). It is seen that the Raman scatteringaround the Boson peak (at ca. 20 cm�1) is weaker for theaged samples, and that the effect seems to be similar forthe sample aged at temperature close to Tg and for thesample aged at room temperature during long time.Additionally some changes in the region of higher fre-quency (100 cm�1–200 cm�1) were observed due to weakfluorescence, which slightly decreases after aging. Forthis reason the low temperature Raman spectra werenormalized at 80 cm�1, where the intensity of harmonicvibrations is relatively high and constant for all samplesand fluorescence is negligible.

0

50

100

150

200

250

300

20 40 60 80 1001201401601802000

20

40

60

80

100

120

140

160

180

I n (a

.u.)

I n (a

.u.)

Wavenumber (cm-1)

Fig. 2. Normalized LFRS spectra for the reference PS samplemeasured at room temperature (solid line) and at T < 20 K (dottedline).

The inset in Fig. 3 shows the spectra obtained atroom temperature. The strong quasi-elastic scatteringoverlaps the Raman scattering by the harmonic vibra-tions, nevertheless the effect of aging is still clearly visi-ble and the same tendency can be found in these spectra.

The evolution of molecular mobility induced by agingin PS samples was investigated using the broadbanddielectric spectroscopy in a wide temperature range.The representation of tan d(=(e00/e 0) was chosen sincethis is not sensitive to changes of samples geometry.Fig. 4 shows the tan d spectra for the reference sample(solid line), sample annealed at 80 �C (dotted line) andsample aged 10 months at 20 �C (dashed line) as re-corded at a frequency of 100 Hz. Three relaxationprocesses can be found in this temperature range: thea-relaxation associated to the glass transition and

-150 -100 -50 0 50 100 150

1E-3 γβ

α

Temperature [oC]

tg δδ

Fig. 4. Dielectric spectra of tan d vs. temperature as measured atfrequency 100 Hz for the reference sample (solid line), sample annealedthree weeks at 80 �C (dotted line) and sample aged 10 months at 20 �C(dashed line).

2596 A. Wypych et al. / Journal of Non-Crystalline Solids 351 (2005) 2593–2598

located at ca. 110 �C and the secondary relaxations: band c, which are visible at �15 �C and �120 �C, respec-tively. The b-relaxation can be assigned to rotation ofthe phenyl groups, while the c-relaxation could be re-lated with motions of methylene sequences in the back-bone chain close to head-to-head or tail-to-tail defects[11]. The identification of the exact motions of second-ary relaxations is difficult and probably can not beascribed entirely to a precise moiety reorientation.Moreover, we observed a large difference between thedielectric b-relaxation presented here and the mechani-cal b-relaxation appearing at higher temperatures muchcloser to the a-process. This will be discussed in a futurepaper. The dielectric activation map for the a-, b- and c-relaxations for the reference and aged samples is shownby Fig. 5.

One of the general facts observed on dielectric spectraof aged samples is a decrease of the loss tangent as wellas a decrease in permittivity in the glassy state (not pre-sented in Fig. 4). As one can see, the effect of aging isdifferent for different molecular relaxations in PS. Thea-relaxation is strongly affected at temperatures belowTg (glassy state), so that after aging the width of thea-relaxation peak apparently decreases. Similar resultswere obtained from dynamic mechanical analysis forthe same samples (not shown here). The b-process isslightly shifted to lower temperature after the agingand its activation energy decreases from 73 ± 2 kJ/mol(reference sample) to 65 ± 2 kJ/mol for the sample aged10 months at 20 �C, and to 58 ± 2 kJ/mol for the sampleaged three weeks at 80 �C. The second sub-Tg relaxa-tion, the c-process is practically not affected by the agingin case of sample aged at 80 �C. Its activation energy isequal to 35 ± 2 kJ/mol for both the reference and theaged samples and no significant changes of amplitude

2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5

2.5

3.0

3.5

4.0

4.5

5.0

5.5

γβ

1000/T [K-1]

-log

τ

α

Fig. 5. The activation map for dielectric relaxation processes in PSreference sample (j), sample aged three weeks at 80 �C (�) and sampleaged 10 months at 20 �C (h). The points are taken from the e00(T)diagrams obtained for different frequencies.

were observed. In a case of the sample aged at 20 �Cduring 10 months the c-relaxation seems to be moreaffected by aging and its amplitude as well as activationenergy (Ea) increase as compared with the referencesample. The Ea value for this relaxation is 43 ± 2 kJ/mol, i.e. it is 8 kJ/mol higher than the Ea value obtainedfor the reference sample (35 ± 2 kJ/mol).

4. Discussion

Above Tg a polymer is free to explore all statesappropriate to the instantaneous temperature, so it isin conformational equilibrium with its surroundings.Quenching from the liquid to glassy state leads to highviscosity and lower mobility of the polymer systemand brings non-equilibrium state with an excess of en-thalpy and entropy. During physical aging polymerchains relax in a time scale to reach thermodynamicequilibrium, so the excess values mentioned before arereduced.

The relaxation of the enthalpy caused by physicalaging was monitored during the first DSC run and it isvisible as an endothermic peak which disappears in thesecond run. The position and magnitude of this effectdepend on aging temperature and aging time [12–14],and observed changes of enthalpy are the most pro-nounced when the aging was performed at temperatureclose to Tg. In Fig. 1(b) and (c) these peaks appear inthe a-relaxation thermal zone, which directly indicatesthat this relaxation is involved in physical aging process.

Changes of enthalpy detected by DSC, i.e. the differ-ences in the heat capacities of the aged and unaged sam-ples reflect the freezing of molecular motions and consistof different contributions like: conformational or config-urational changes of polymer elements as well as vibra-tional contribution. The vibrational heat capacity excessis associated with generation of low-frequency modeswith structural excitation. The existing in quenched state(reference sample) structural excitations, which exhibitthe entropy-rich low-frequency modes, decrease duringaging consistently with decreasing of VDOS, as onecan deduce from smaller intensity of the Boson peakin LFRS spectra measured at 20 K. Simultaneously,relaxational motions allow the system to minimalizethe configurational entropy by reorganization of smallerparts of polymer chain. It is difficult to estimate the ex-act contribution of the vibrational part in heat capacityin the present studies, but as it was discussed by Angellet al. [15], this component in hyperquenched glasses isresponsible for about 60% of the jump of heat capacityat Tg.

The Boson peak observed in LFRS spectra is relatedto the VDOS excess, which depends on nanostructure,hence we can gain some information on the supermolec-ular structure of glasses. Such a relation is expressed by

A. Wypych et al. / Journal of Non-Crystalline Solids 351 (2005) 2593–2598 2597

Eq. (1), which gives the LFRS intensity I(x) for Stokesscattering [16]:

IðxÞ ¼ CðxÞgðxÞ nðxÞ þ 1

x; ð1Þ

where C(x) is the light-vibration coupling coefficient,g(x) is the VDOS, x is the frequency and n(x) is theBose factor. The light-vibration coupling coefficientcan be calculated from a comparison between LFRSand VDOS obtained from inelastic neutron scattering.It is approximately proportional to x in the spectralrange of the Boson peak [17,18].

Raman spectra are usually presented in normalizedform, as one can observed in Fig. 3, which is given byEq. (2):

InðxÞ ¼IðxÞ

½nðxÞ þ 1�x . ð2Þ

The Boson peak is associated with the maximum inVDOS due to partially localized vibrations in disorderedstructure of glasses, what was evidenced by LFRS andinelastic neutron scattering experiments performed inthe same frequency range [18,19]. There are a few mod-els which explained the existence of Boson peak. Someof them allow for the spatial correlation between fre-quency of Boson peak and the size of heterogeneitiesin a glass structure. The obtained by us LFRS spectrawere analyzed in terms of the model of non-continuousstructure of amorphous polymers, whereby the Bosonpeak is interpreted as a signature of more cohesive nan-odomains surrounded by softer zones [10]. According tothis model the intensity of the Boson peak decreaseswith decreasing contrast between the cohesive nanodo-mains and the soft surrounding medium (softerchannels).

Thus the decrease of the Boson peak intensity withaging observed in Fig. 3 suggests that the glass nano-structure becomes more homogeneous with aging.According to the nanoheterogeneity cohesion model,the decrease of Boson peak after aging in spectra re-corded at 20 K (Fig. 3) is attributed to smoothing ofthe elastic constant contrast at the nanometric scale.The softer zones are believed to become more cohesiveafter aging, i.e. their elastic constant increases and ismore comparable with elasticity of cohesive nanodo-mains. The increase of interaction in softer channelscan be realized by a better packing, i.e. by the decreaseof free volume holes during aging, what was proposedby Struik long time ago [9] and evidenced recently bypositron annihilation lifetime spectroscopy measure-ments (PALS) [20,21].

The decrease of LFRS can be also observed in the in-set in Fig. 3. However in the case of spectra performedat room temperature the visible intensity consists oftwo components: harmonic (as seen by Boson peak)and anharmonic, i.e. quasi-elastic scattering (QELS).

This additional contribution to Raman scattering isdue to rapid, localized motions which are thermally re-leased. Such fast molecular motions are not themselvesthermally activated as it was proved during experimen-tal studies on another polymer glass [22]. However, theintensity of QELS is strongly dependent on temperature(as it is visible in Fig. 2) and decreases rapidly as freevolume is reduced during cooling up to very low temper-atures. In order to verify if physical aging, despite of itsinfluence on Boson peak, affects partially also QELS, itis necessary to discriminate these two contributions.The intensity of LFRS spectrum recorded at 20 K(I(x,T=20K)) can be regarded as purely vibrational, whilethe intensity of LFRS at 300 K (I(x,T=300K)) is a sum ofRaman scattering from harmonic vibrations and QELS.After substracting:

IQELSðx;T¼300KÞ ¼ I ðx;T¼300KÞ � I ðx;T¼20KÞ;

the QELS intensity at 300 K was determined for PSsamples with different thermal histories. From compari-son between the reference and the aged samples, wefound that physical aging does not affect intensity ofQELS. In such situation, we can conclude, that thephysical aging in PS causes a decrease of harmonicVDOS, as clearly observed by the decrease of LFRSintensity recorded at 20 K in Boson peak region.

The broadband dielectric spectra clearly show, thataging significantly affects the a-relaxation process attemperature below Tg and more specifically the interme-diate regime between the a- and b-processes. The effectsobserved in Fig. 4 can be associated with the molecularmobility due to residual �free volume� or �defective (mo-bile) sites� [13] below Tg that allows structural recovery.

From LFRS measurements we concluded that aginginduces a more homogeneous nanostructure at the scaleof the soft domains, but also at a smaller scale causes adecrease of local defective regions like free volume holes[20,21] or more generally defective sites in amorphousmatter. This results in a lower molecular mobility as re-vealed by the decrease of the loss in tand in dielectricspectra, while the decrease is more pronounced whenthe aging was performed at temperature 80 �C. AboveTg, all samples reach the same equilibrium structuralstate and the dielectric dynamic behavior is not sensitiveto thermal history anymore. It is why in the relaxationmap in Fig. 5, all data points situated above Tg (corre-sponding to the a-relaxation) for the reference and agedsamples, are located on the same curve. Because in PSthe a- and b-relaxations are overlapping, the reducedmobility of the a-process after aging induces a decreaseof the overlapping. This effect is more pronounced forthe sample aged at higher temperature. Such effect canbe responsible for smaller activation energies for b-pro-cess in the aged samples. Unexpectedly, the c-relaxationseems to be also affected by aging, as it was found in thesample aged 10 months at 20 �C, that is revealed as an

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increase of its amplitude and of the activation energy va-lue. Concerning the fact, that the exact motion of atomgroups corresponding to this relaxation is not well de-fined, it is rather difficult to draw any conclusion fromthe observed phenomena.

In the frame of the non-continuous nanostructuremodel the decrease of the mobility mentioned beforecan be associated with a decrease of the fluctuation ofthe cohesive energy density at the scale of a few nanome-ters. It is thus assumed that the structural relaxation attemperatures between the a- and b-relaxations corre-sponds to the decrease of free volume or defective sitesoccurring more specifically or more efficiently in thesofter zones. As a result, the structural changes inducedby aging can not be captured by a description focusedon only one length scale, but at least extends from theinteratomic distances (free volume, defective �quasi-punc-tual� sites [13]) to larger nanometric scales and involves thepicture of heterogeneous nanostructure of glasses [10].

5. Conclusion

Low-frequency Raman scattering and broadbanddielectric spectroscopy show that physical aging affectsboth nanostructure and molecular mobility of glassyatactic polystyrene. We suggest that the physical agingmakes cohesion more homogenous at the nanometricscale, also in relation with a structural state of lower en-thalpy and entropy. The overlap of the a- and b-relaxa-tion in the case of PS diminishes during aging, which ismainly associated with a decrease of the mobility of pri-mary relaxation amplitude. The precise effects of physi-cal aging on b-process are still under investigations.

Acknowledgments

This work was partially supported by French pro-gram: Mobilite Internationale Rhone-Alpes (M.I.R.A.),

KBNProject No. 4 T09 A 147 25 (Poland) and Poloniumproject No. 07551ZM (Poland–France). The authorsthank Dr J.M. Lucas for performing SECmeasurements.

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