Electrochemical Aging of Poly (aniline) and Its Ring Substituted

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J. Phys. Chem. B 2008, 112, 10800–10805

ARTICLES Electrochemical Aging of Poly(aniline) and Its Ring Substituted Derivatives Waldemar A. Marmisolle´, Dionisio Posadas, and M. Ine´s Florit* Instituto de InVestigaciones Fisicoquı´micas Teo´ricas y Aplicadas (INIFTA). Facultad de Ciencias Exactas, UniVersidad Nacional de La Plata. Sucursal 4, Casilla de Correo 16, 1900 La Plata, Argentina ReceiVed: January 30, 2008; ReVised Manuscript ReceiVed: June 23, 2008

Aniline and methyl, ethyl, propyl, and methoxy ring substituted derivatives have been polymerized by electrochemical methods. The reduction of the film produces aging of the polymers. This is followed by the changes in the current/potential profile of the voltammetric response. The effect of the substituents produces changes in both the extent and the rate of aging. The rate of aging can be represented by a Roginsky-Zeldovich or Elovich type of kinetics characterized by a pseudo zero order rate constant and a self-inhibiting parameter. Both parameters and the extent of aging are related to the free volume available for the different types of polymers. 1. Introduction The physical aging of amorphous solids have attracted the attention of many workers in the field of polymers mainly due to the change of their mechanical properties during the aging process, and its obvious technological consequences, as well as the very wide range of behaviors linked to the same phenomenon. In short, the physical aging is a process that takes place when a material is submitted to a temperature below its glass transition temperature. It manifests itself as the change of some properties of the material, both macro structural or bulk properties, such as specific volume, enthalpy, mechanical, and dielectric response, as well as micro structural or molecular scale properties, which may be measured, for example, by spectroscopy and scattering techniques. Invariably, it is observed that the property studied changes linearly with the logarithm of time. This time dependence implies the aging process is selfinhibiting. A great variety of polymeric materials show this physical aging.1,2 The attention to the physical aging of polymers has been focused both in the theoretical aspects of the process as well as in the experimental consequences and its possible applications. From the theoretical point of view, the physical aging has been associated to the concept of free volume of the macromolecules in the amorphous state, the bigger free volume the higher the aging rate.1,2 Several reviews and articles on the subject are available in the literature.1–6 There is a class of polymers denominated “electroactive” because they can be reversibly oxidized and reduced. Usually, they are classified into redox and conjugated polymers. The first ones possess, either as a part of the monomer unit or as a coordination compound, chemical groups that participate in the redox reaction. Examples of this kind of polymers are poly(oaminophenol) (POAP) and osmium(II) bipyridile- poly(vinylpyridile), respectively. In the POAP the redox centers are the amino groups that oxidize to imino ones. In the other case, it is the coordinated Os(II), which is oxidized to Os(III). * To whom correspondence should be addressed. E-mail: mflorit@ inifta.unlp.edu.ar.

On the other hand, conjugated electroactive polymers are characterized by becoming electronic conductors when oxidized (positively doped) to a certain degree, but they are not conducting in the reduced state. Examples of this class of polymers are poly(aniline), poly(pyrrole) and poly(thiophene). These polymers, deposited as a film onto a base electrode, show a noticeable process of aging.7–16 When they are brought to the reduced state its properties change linearly with the logarithm of the time spent at this condition. The main difference between most of the physical aging experiments performed with amorphous solids in bulk and those carried out with conjugated polymers films covering electrodes is that, in the later case, the condition in which the polymer ages is achieved not by decreasing the temperature below the glass temperature but by poising the material to a reduced state. In the literature, the aging of electroactive conjugated polymers has received different names such as, “memory effect”,9 “slow relaxation”,10 and “first cycle effect”.12,13 The study of physical aging in polymer films supported on metallic electrodes offers the possibility of putting in evidence some aspects of the process. The first one is that the thickness of the sample is very small and it can be quite precisely controlled. The second one is that aging can be followed by simple electrochemical techniques. The third is that being the thickness of the samples so thin, the aging effects manifest themselves in a shorter time scale than for bulk samples. Furthermore, the effect of the solvent and the presence of different ions in solution on the aging process can be easily studied.14–18 On the other hand, there are the obvious disadvantages of working with very small amounts of sample. Nevertheless, electrochemical aging of conjugated polymers have been studied by numerous “in situ” techniques: ERS,9 electrochemical techniques,10–14,16–18 volume changes,19 UV-vis spectra,20,21 and XPS.15 Several models have been proposed to explain the electrochemical aging. Among them we may mention the following: (i) slow cis-trans isomerization kinetics of the polymer chains;22 (ii) slow adsorption/ desorption of ions23,24 (iii) changes in the polaron-bipolaron equilibrium constant;25 (iv) percolation of

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Electrochemical Aging of Poly(aniline) conducting clusters;26,27 (v) N-shaped free energy curves;28 (vi) Incomplete reduction of the polymer;13 (vii) changes in the local pH due to changes in the value of the potential at the outer Helmholtz plane.15 This effect is particularly notorious in conducting polymers derived from ring substituted aryl amines. Previously, it was found that the nature of the solvent has influence on the relaxation effect,16 the increase in the proton concentration of the external solution decreases the aging time,16,29 and also the effect of lowering the temperature has been studied.18 A previous work indicated that there are differences in the aging rate of poly(aniline) and poly(o-toluidine).18 For this reason, we consider interesting to study systematically the effect of the substituents on the electrochemical behavior during the aging process of several polymers derived from aniline. In this work, we study the effect of the different substituents in poly(aniline), at ortho position, on the electrochemical aging. We have chosen the ortho position because anilines with substituents at meta and para positions polymerize very little or do not polymerize at all.29–34 With this purpose, we have electrosynthesized and studied the following polymers: poly(aniline) (PANI), poly(o-methoxyaniline) (PMOA), poly(o-methylaniline) (PMA), poly(o-ethylaniline) (PEA), and poly(opropylaniline) (PPA). Finally, we propose a qualitative scheme, involving the three polymer states; Ox, the oxidized state; R*, the reduced state; R, the aged state. When a suitable negative potential is applied to the oxidized polymer, Ox, this is reduced to an R* form. This is not a stable form and tends to reach the equilibrium one, R. During this process its properties change in a linear way with the logarithm of the time elapsed after the reduction. Henceforth, we will name electrochemical aging to the transition between R* and R forms. 2. Experimental Section The polymers were obtained by electropolymerization on Au electrodes by cycling the potential at V ) 0.1 V s-1, between -0.2 V vs SCE, and the potential corresponding to that of the beginning of the oxidation of the monomer, around 0.8 V vs SCE, as described before.16 The films were grown until the desired thickness was reached. The synthesis solutions were 0.5 M in the monomer in aqueous solution of 3.7 M H2SO4. The area of the working electrode was 0.154 cm2. The voltammetric charge of the films employed in this work, Q, was determined from the integration of the i/E profiles of the voltammetric response. Although the film thickness could be derived from correlations between the anodic voltammetric charge and film thickness, previously established by ellipsometry for these types of polymers, for the reasons stated in previous works, we will quote the anodic charge corresponding to the first anodic peak, Q, as a measure of the film thickness.16 The electrolyte solutions were prepared from H2SO4 (97%) (Backer, p.a.); aniline; o-toluidine; o-anisidine (Fluka- Garantie, puriss. p.a.); o-ethylaniline; o-propylaniline (Aldrich. p.a.); MilliQ* water. The monomers were employed as received. The experimental setup for the voltammetric measurements was a three-electrode glass cell as described elsewhere.35,36 The auxiliary electrode was a cylindrical Pt foil. The reference electrode was a saturated calomel electrode (SCE). All potentials in the text, E, are referred to this electrode. Conventional voltammetry was performed using a PAR 273 potentiostat and a Philips PM 8134 X-Y1-Y2 recorder, at different sweep rates, V, ranging from 10-3 to 0.3 V s-1 and

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Figure 1. Steady state cyclic voltammograms of the different polymers in 3.7 M H2SO4, sweep rate, V ) 0.1 V s-1: PANI (b); PMA (O); PEA (1); PMOA (∆); PPA (9).

covering a potential region between -0.2 and 0.5 V. All experiments were carried out in 3.7 M H2SO4 solutions. The voltammetric experiments after holding the potential at the aging potential, Ea ) -0.2 V, were made at a sweep rate of V ) 0.1 V s-1. The thickness of all polymer films were in the range 5 mC cm-2 < Q < 30 mC cm-2. In the case of the electropolymerization of PPA, it was not possible to grow-up films thicker than 6.5 mC cm-2. 3. Results 3.1. The Voltammetric Response of the Different Polymers. In agreement with the information previously reported, the electrochemical response of the polymers studied in this work shows, in the potential range comprised between -0.2 V< E < +0.9 V vs SCE, two redox couples. The first corresponds to the transition leucoemeraldine/emeraldine and the second one to the pair emeraldine/pernigraniline. The potential region between them is the range where the polymers present the maximum conductivity value. The behavior of the different polymers in 3.7 M H2SO4 solutions is similar to that previously reported in other acid solutions.16–19,29–34,37–39 In this work, the polymers have been studied between the potential values of the reduced state, -0.2 V, and those corresponding at their own conducting states. The voltammetric parameter values, the peak potential, Ep, and peak current, Ip, are different for each one of them. They are presented in Figure 1. For the sake of comparison of polymers of different thickness, the voltammetric current was divided the by the current of the corresponding anodic peak, Ip,a. 3.2. The Aging of the Different Polymers. A typical electrochemical aging experiment consists of the following sequence: The potential applied to the polymer film coated electrode is cycled continuously between -0.2 V and the proper positive limit value in a free monomer acid solution. The typical steady state voltammetric response, for a particular polymer film, is shown in Figure 2. Then, the polymer is poised at a potential value corresponding to its reduced state, i.e., Ea ) -0.2 V, during an aging time, ta. After ta, the potential is swept again and it is observed that the voltammetric profile during the first positive going half-cycle differs from the steady state profile. The differences show up in the Ep and Ip values of the anodic peak, as shown in Figure 2. In order to normalize the peak current values, we will define the relative current change as:

Ir)(Ip,a(ta) – Ip,a(ta)0))/Ip,a(ta ) 0)

(1)

where Ip,a(ta) is the peak current after holding the potential at

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Figure 2. Aging of a PMA film, after holding the potential at Ea ) -0.2 V during ta ) 60 s in 3.7 M H2SO4, sweep rate, V ) 0.1 V s-1: first scan (O); second scan (∆); steady state (b).

Figure 3. Change of relative current as a function of the aging time, in solution 3.7 M H2SO4. Holding potential Ea ) -0.2 V, V ) 0.1 V s-1. Polymer film thickness, estimated as the voltammetric charge, Q ≈ 15 mC cm-2: PANI (b); PMA (O); PEA (1); PMOA (∆). Solid line, calculated values with eq 7, see below.

Figure 4. Relative current as a function of the logarithm of the aging time for the same polymer films presented in Figure 3: PANI (b); PMA (O); PEA (1); PMOA (∆).

time ta and Ip,a(ta ) 0) is the peak current in the steady state voltammetric response. It is interesting to remark that Ir is also a measure of the extent of the aging process. Figure 3 shows the Ir values as a function of the aging time for the different polymers. As it is referred in the literature, many material properties, both macro structural and micro structural, change as a linear function of the aging time, during the physical aging.1,2,4–6 In Figure 4, the Ir values are shown as a function of the logarithm of the aging time. There, it is seen that for intermediate times, Ir changes linearly with the logarithm of the aging time. The question arises about the time required to completely reduce the polymer and if it also ages during the reduction time. These questions have been answered, for PANI and PMA, in a

Marmisolle´ et al.

Figure 5. Effect of the polymer thickness on the aging of PMOA films: (1) 5.29 mC cm-2; (b) 13.72 mC cm-2; (O) 28.4 mC cm-2. 3.7 M H2SO4, V ) 0.1 V s-1, Ea ) -0.2 V.

previous work.16 For thin polymer films (Q ≈ 1-2 mC cm-2) the polymer completely reduces in one or two seconds. For thicker films (Q ≈ 150 mC cm-2) the time to reduce the polymer may take several minutes. These facts are known through the time required to obtain a constant value for the reduction charge of films. Besides, in the same previous work it was established that parts of the film, that have been reduced, age. In this work, polymer films of thickness between 5 and 28 mC cm-2 were studied. These films are completely reduced in around 30 s,16 a negligible value as compared to the time scale of the electrochemical aging. 3.3. Influence of the Film Thickness on Polymer Aging. As established previously,16 thicker films age more slowly than thinner ones. Figure 5 shows the change in the anodic peak current as a function of the aging time for three PMOA films of different thicknesses. 4. Discussion 4.1. The Voltammetric Response of the Different Polymers. In Figure 1, it is observed that the oxidation potential of the different polymer films, measured as the potential value at the oxidation peak of the first redox couple, increases in the sequence: PMOA < PANI < PMA < PEA < PPA. At this point, it is interesting to remark that the peak potential value of the first redox couple of PMOA is lower than that of PANI, but the peak potential value of the first redox couple of PMA is higher than that of PANI film. Methoxy and methyl groups have opposite effect in the peak potential of the corresponding polymer, referred to the PANI value. This fact points out that steric as well as electronic effects of the substituents must be taken into account in the evaluation of the electrochemical properties of these PANI derivatives polymers. The presence of some substituents, such as the methyl group, can induce some nonplanar conformation between the polymer units that increases the band- gap, giving higher redox potential values.29 Then, the shift of the peak potential of the first redox couple, to more positive values, could be related to higher torsion angles between the polymer units in the PMA as compared with the unsubstituted PANI.40,41 On the other hand, the lowest redox potential of PMOA can be related to the electronic effect of the methoxy group and also to the possibility for this polymer to adopt a more planar disposition than that of PMA. This behavior can be explained taking into account that the van der Waals radius of the oxygen atom, 0.14 nm, is smaller than that of the methyl group, 0.20 nm.42 These facts are also consistent with the electronic conductivity values for these polymers: 0.13 Ω-1 cm for PANI, 0.08 Ω-1 cm for PMOA, and 1.1 × 10-4 Ω-1 cm for PMA.29 The high conductivity of PANI is related to a

Electrochemical Aging of Poly(aniline) particular angle between polymer units that favors the extended π conjugation along the polymer.29 The presence of the alkyl groups in the macromolecule increases the torsion angle between adjacent rings yielding a decrease in the extension of the conjugation.31 The present results are consistent with theoretical calculations43,44 and also with the decrease of the electronic conductivity of the alkyl-substituted polymers previously reported.31 Thus, the methoxy group yields a more planar polymer, quite similar to PANI, with an electronic conductivity higher than that of the alkyl substituted polymers. 4.2. Qualitative Description of the Electrochemical Aging. Many years ago, it was showed that amorphous solids are not in thermodynamic equilibrium at temperatures below their glass transition temperature, Tg.1,2 Such materials have to be regarded as solidified supercooled liquids whose volume, enthalpy and entropy are greater than those corresponding to the equilibrium state. However, this non equilibrium state is unstable. Volume relaxation studies of glassy materials have revealed that they undergo slow processes to establish equilibrium, indicating that even below Tg molecular mobility is not quite zero.1,2,18,20 This gradual approach to equilibrium affects many properties of the material. Actually, these properties change with time and the material is said to undergo “physical aging”, distinguishing this aging process from chemical aging (thermal degradation, photooxidation, etc.). The term “physical aging” as applied to polymers cover a wide range of behaviors, all of which are linked by the same phenomenon. As it was stated above, the phenomenon is observed as a reversible change in a great number of properties of the material that include both macro as well as micro structural properties. Invariably, it is observed that the change of all these properties, happening along the aging process, is a linear function of the logarithm of the aging time.1,2,4–6 As we said in the Introduction, for polymer films coated electrodes the electrochemical aging can be followed by several methods. Perhaps the more relevant results are those coming from the measurement of the volume change during the redox switching, and after the aging in the case of PANI and PMA.19,45 While the polymer is being potential cycled, after reaching a steady state voltammetric profile, the volume differences between the reduced and the oxidized forms of the polymers are very small, but when the potential is held at a suitable reduction potential, the polymer volume dramatically decreases. This is a strong argument in favor that during the electrochemical aging, as it happens during the physical aging, the polymer undergoes conformational changes that decrease its volume. These experimental facts allow us to propose a qualitative scheme to describe the electrochemical aging of polymers; it is depicted in the Scheme 1. During the steady state redox switching, the polymer commutes between the reduced state, which we have called the R* form, and the oxidized one, Ox. The volume of R* is smaller than that of the Ox form.19,45 This reduced form, R*, is not at equilibrium since, if the potential is held at values belonging to the reduction range, its properties change with the aging time. Particularly, its volume continues decreasing.19,45 This experimental observation shows that conformational changes continue happening while the polymer asymptotically tends to achieve its equilibrium form, that we have named, R. As we said previously, electrochemical aging is called to be the transition from R* to R forms. It is important to note that the changes during this transition take place without any variation of the external parameters (temperature, potential, pressure, etc), but just holding the material

J. Phys. Chem. B, Vol. 112, No. 35, 2008 10803 SCHEME 1: Electrochemical Aginga

a Qualitative picture of the electrochemical aging showing the spatial disposition of the oxidized polymer (Ox), the just reduced polymer (R*), and the polymer after aging (R).

in its reduced state. The resulting process is quite similar to the physical aging described in the Introduction. Experimentally, it is observed that polymers with longer substituents achieve the R* state with a structure more open and then, the electrochemical aging becomes more relevant. There may be several reasons that justify this fact. First, it may happen that during the electrosynthesis process, the monomers possessing the bulkier substituent lead to polymers having more free volume. Moreover, it is well established that during the oxidation PANI, as well as the other polymers, expands, ejects protons, and incorporates water and anions; changes that are more important in substituted polymers. All these changes also affect the apparent standard redox potential of the polymers.46 4.3. The Extent of Aging. As it was mentioned in the Introduction, the features of the first anodic peak change during the aging time. The anodic peak potential, Ep,a, becomes more positive, the peak width at half-current, ∆E1/2, becomes narrower and the current peak, Ip,a, increases. These changes depend on the time spent at Ea. The electrochemical reasons for these changes have been discussed elsewhere.16,18,21 In the second positive going half-cycle, after the aging time, the current/ potential profile practically superimposes with that of the steady state cycling, Figure 2. In a certain aging time range, both voltammetric features, Ip,a and Ep,a, depend linearly on the logarithm of ta,10,11,16,18 according to the following eqs:

Ip,a(ta) ) I0 + δVI ln(ta)

(2)

Ep,a(ta) ) E0 + δVE ln(ta)

(3)

and

where δVI and δVE are the slopes of the linear portions of Ip,a(ta) vs ln(ta) and Ep,a(ta) vs ln(ta), respectively, and I0 and E0 are arbitrary constants.11 However, as the current peak is more sensitive to the phenomenon, we will employ this parameter to measure the aging effect. In Figures 3 and 4 are shown the relative current as a function of the aging time and as a function of the logarithm of the aging

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TABLE 1: Parameters Fitted to Eqs 4 and 7 for the Studied Polymer Films polymer

Q/mC cm-2

raI

Ra

β

k/s-1

Rb

PANI PANI PANI PMOA PMOA PMOA PMA PEA PPA

5.2 19.0 26.0 5.3 13.7 28.4 16.3 14.8 6.5

0.13 0.15 0.16 0.46 0.53 0.65 0.21 0.22 0.65

0.9995 0.9980 0.9992 0.9992 0.9997 0.9994 0.9998 0.9999 0.9994

16.95 14.51 13.51 4.93 3.91 3.26 10.31 9.35 1.54

0.013 0.0122 0.012 0.100 0.068 0.038 0.015 0.065 0.123

0.9995 0.9972 0.9992 0.9989 0.9997 0.9990 0.9994 0.9994 0.9984

a Correlation coefficient for the fit to eq 4. coefficient for the fit to eq 7.

b

only of the intermediate period of time but also of the initial moments of the aging process. So, the rate law may be written as:

V ) ∂Ir ⁄ ∂ta ) k exp(-βIr)

(5)

where, k and β are constants related with pseudo zero order initial rate and the activation energy, respectively. Obviously, this expression represents the kinetics far from equilibrium and does not apply for very long times. Equation 5 does truly represent a self-inhibitory process since while it progresses, the activation energy increases and can be expressed as follows:

∆G0,q ) ∆G0,q(Ir ) 0) + βIrRT

Correlation

(6)

The Elovich equation can be integrated to obtain: time, respectively, for different polymers of comparable thickness. In Figure 5 is shown the current and the relative current respectively, as a function of ta for different thicknesses of PMOA. The results for PPA films are not included in these figures because the thickness of this polymer is too small to be compared with the other ones. Then, for polymer films of similar thickness, it is observed that the extent of aging, at any ta, increases in the sequence PANI< PMA < PEA< PMOA, (Figure 3). Moreover, the final Ir value, corresponding to Ir(ta)∞), increases in the same order. It is interesting to note the highest extent of the aging process of the PMOA film. In the literature has been reported a significant difference between the 13C high resolution NMR characterizations of reduced PMOA and reduced PANI.42,47 In the case of reduced PMOA, the benzenic carbon signals are very broad and not as well defined as it has been reported for fully reduced PANI. This result indicates that some electronic conductivity is still present in the PMOA sample.47 Then, the great extent of aging of PMOA could be related to the fact that at the aging potential, the polymer film could be not fully reduced. In addition to these facts, it must also be considered that this polymer has a molecular disposition opener than that of PANI, due to the bulkier substituent. From all accounts, the highest extent of the aging process can be thought in terms of a bigger free volume and, as it was mentioned in the Introduction section, it should yield a faster aging process. For the other polymers, PEA and PMA, the extents of the aging effect are also greater than for PANI. These facts can be also explained taking into account that, as a consequence of the presence of rather bulky substituents, both polymers arrive to the R* state with an opener structure than that of PANI in the same state. 4.4. Quantitative Analysis of the Aging Kinetics. Following the nomenclature usually employed in the literature of physical aging of amorphous solids, we will define a current aging rate raI as the slope of the logarithmical plots (Figure 4). That is:

raI ) (1/Ip,a(ta ) 0))(∂Ip,a/∂[log(ta)]) ) 2.303(∂Ir/ ∂ [ln(ta)]) (4) The raI values were determined by linear regression in the interval where this type of plots is linear. They are assembled in Table 1. It is necessary to point out that raI is not a true rate, because it does not take account for the variation of Ir with time, but with its logarithm. In a previous work, we justified a logarithmic dependence of raI on ta, employing a Rogisnky-Zeldovich or Elovich type of pseudo zero order kinetics.18,48 This model takes account not

Ir ) 1/β [ln[(ta + t0)/t0]]

(7)

where t0 ) 1/kβ. For ta > t0, a plot of Ir vs. ln (ta) should give a straight line with slope (1/β). Equation 7 was fitted to the experimental results employing a least-squares non linear fit program. The fit was quite satisfactory in all cases and the resulting parameters are assembled in Table 1. 4.5. Analysis in Terms of the Substituent. The Values of β. The β values are slightly dependent on the charge but depend very much on the nature of the substituent. For comparable charges, a clear sequence of β values may be established as follows:

PANI > PMA > PEA > PMOA As may be expected, raI does correlate with 1/β, and the opposite sequence was obtained for this current aging rate. This sequence indicates that self-inhibition is bigger for PANI and approximately decreases with the size of the substituents. In other words, for bulkier substituent the bigger free volume is available, and the self-inhibition is smaller. The Values of k. The k values are also slightly dependent on the charge but depend very much on the nature of the substituent. For comparable charges, the following approximate order can be established:

PANI < PMA < PEA < PMOA The sequence indicates that at the beginning of the aging process PANI relaxes slower than PMOA, pointing out that less free volume would be available for PANI. We have not included the values of PPA in the sequence because for this polymer the thickness obtained is not comparable to the thickness of the other polymers. However, if the aging process of PANI and PMOA in the films of the smaller thicknesses (5mC cm-2) and PPA (6mC cm-2) is compared, the values of β and k obtained for this polymer will fit in the following sequences, respectively:

PANI > PMOA > PPA and

PANI < PMOA < PPA Moreover, if the data for PPA were extrapolated to the higher comparable thicknesses, their β and k values would fit in the observed sequence. With the β and k values assembled in Table 1, the Ir data as a function of ta were calculated with eq 7 (Figure 3). 5. Conclusions The potential at the oxidation peak of the first redox couple increases in the sequence: PMOA < PANI < PMA < PEA
PMA > PEA > PMOA. In turn, both of them should be related to the free volume available for each type of polymer. Acknowledgment. This work was financially supported by the Consejo Nacional de Investigaciones Cientı´ficas y Te´cnicas (CONICET), the Agencia Nacional de Promocio´n Cientı´fico Tecnolo´gica, and the Universidad Nacional de La Plata. M.I.F. and D.P. are members of the CIC of CONICET. W.A.M. is a fellowship holder of CONICET. References and Notes (1) Struik, L. C. E. Physical Aging in Amorphous Polymers and Other Materials, Elsevier: Amsterdam, 1978; p 1. (2) Hutchinson, J. M. Prog. Polym. Sci. 1995, 20, 703–760. (3) Hilhorst, H. J. In Fundamental Problems in Statistical Mechanics VII; van Beijeren, H., Ed.; Elsevier: Amsterdam, 1990; p 311. (4) Royal, J. S.; Torkelson, J. M. Macromolecules 1993, 26, 5331– 5335. (5) Priestley, R. D.; Broadbelt, L. J.; Torkelson, J. M. Macromolecules 2005, 38, 654–657. (6) Priestley, R. D.; Ellison, C. J.; Broadbelt, L. J.; Torkelson, J. M. Science 2005, 309, 456–458. (7) Genoud, F.; Guglielmi, M.; Nechtschein, M.; Genies, E.; Salmon, M. Phys. ReV. Lett. 1985, 55, 118–121. (8) Nechtschein, M.; Devreux, F.; Genoud, F.; Vieil, E.; Pernaut, J. M.; Genies, E. Synth. Met. 1986, 15, 59–78. (9) Villeret, B.; Nechtschein, M. Phys. ReV. Lett. 1989, 63, 1285–1287. (10) Odin, C.; Nechtschein, M. Phys. ReV. Lett. 1991, 67, 1114–1117.

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