Article pubs.acs.org/Macromolecules
Enthalpy Recovery in Nanometer to Micrometer Thick Polystyrene Films Virginie M. Boucher,*,† Daniele Cangialosi,† Angel Alegría,†,‡ and Juan Colmenero†,‡,§ †
Centro de Física de Materiales (CSIC-UPV/EHU), Paseo Manuel de Lardizabal 5, 20018 San Sebastián, Spain Departamento de Física de Materiales, Universidad del País Vasco (UPV/EHU), Apartado 1072, 20080 San Sebastián, Spain § Donostia International Physics Center, Paseo Manuel de Lardizabal 4, 20018 San Sebastián, Spain ‡
ABSTRACT: The physical aging of polystyrene (PS) free-standing films has been investigated as a function of the films thickness, ranging from several micrometers to tens of nanometers. In this range of thicknesses, unchanged segmental dynamics in comparison to the bulk was previously reported. This study has been carried out through differential scanning calorimetry (DSC), by following the enthalpy recovery to monitor the physical aging process at different temperatures. The temperature marking the onset of nonequilibrium effects, that is the onset of the glass transtion temperature, Ton g , was also assessed, at different cooling rates. An acceleration of the physical aging process, and consequently a depression of Ton g , is found with decreasing the films thickness, already for thicknesses in the micrometer range. Moreover, the onset of nonequilibrium effects is shown to be cooling rate dependent, this being more pronounced when the PS films get thinner. The thickness effects on the typical signatures of the out-of-equilibrium dynamics of the films, namely their physical aging and Ton g , can be well accounted for by assuming an equilibration mechanism based on volume holes diffusion toward the interfaces of the films. The temperature dependence of the diffusion coefficient obtained within this framework is found to crossover from Vogel−Fulcher− Tammann (VFT) to Arrhenius when decreasing the temperature. The implications of these results are discussed.
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acceleration of physical aging under confinement,8−11 others conclude to a slowing down, or even a suppression of the same process.12,13 Concerning the Tg, marking the transition from the equilibrium to out-of-equilibrium states, it has been shown to be depressed in comparison to the bulk, unless strong interactions with a substrate are present.14−22 Furthermore, whereas the physical aging behavior of PS ultrathin films (i.e., less than 100 nm thick) has been widely studied,9,12,9,23−26 that of PS films with thicknesses above 100 nm remains marginally explored.25,27 The literature studies about the physical aging behavior of the latter mainly deal with polycarbonate28 and other polymers generally used for gas separation membranes applications.2,10,29−31 A recent study on PS films also investigated the thickness dependence of physical aging in a wide range of thicknesses with particular attention to the effect of stress induced by cooling.27 Interestingly, these studies indicate a thickness effect on the physical aging behavior in the submicrometer and micrometer range. This qualitatively agrees with the results reported in previous works on the effect of fillers concentration on the physical aging of PS in nanocomposites,32−34 in which the characteristic length scales, defined as the ratio of the volume of the polymer to the area of the interface polymer/particles, are above 100 nm. This observation supports the previously proposed equivalence between polymer nanocomposites and polymer thin films.35
INTRODUCTION Amorphous polymer glasses always experience a slow evolution of their thermodynamic state toward equilibrium by a loss of the excess in the thermodynamic properties (volume, enthalpy, entropy).1 This phenomenon, referred to as physical aging, induces an alteration of the materials engineering properties depending on the thermodynamic properties (e.g., mechanical, dielectric, diffusive properties),2−4 and thus may result in technological troubles. These aging phenomena occur in laboratory times at temperatures some tens of degrees below the glass transition range, separating the metastable equilibrium state of the supercooled liquid from the out-of-equilibrium glassy state. This temperature range usually extends over few degrees but is conveniently characterized by a temperature referred to as the glass transition temperature, Tg. Physical aging is a general feature of glassy systems, and is of special relevance in polymers.1 Of particular interest nowadays is the physical aging behavior of polymers with thicknesses ranging from the micrometer to the nanometer range because of the great variety of technological applications based on glassy polymer films (nanoimprinting, optical coatings, photovoltaics, memory storage devices, etc.).5−7 Consequently, a better fundamental understanding of this phenomenon in polymer materials confined to the micro- or nanoscale is required. Despite the vast interest of the scientific community for nanostructured systems in the last few decades, their physical aging and glass transition behaviors are still a matter of debate. In both cases, experimental and theroretical investigations have given rise to opposing results. While several works report an © 2012 American Chemical Society
Received: March 27, 2012 Revised: May 12, 2012 Published: June 7, 2012 5296
dx.doi.org/10.1021/ma300622k | Macromolecules 2012, 45, 5296−5306
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Article
mg. The so-prepared DSC pans were subsequently dried at 343 K under vacuum during one day, in order to eliminate the water used to remove the films from the glass substrates. The DSC measurements were carried out on a DSC-Q2000 calorimeter from TA-Instruments, calibrated with melting indium. The measurements were performed under nitrogen atmosphere on samples placed in aluminum pans. The Ton g , as defined in the Introduction, has been determined for two cooling rates: 0.2 and 20 K·min−1. In all cases, the samples were first equilibrated at 428 K. Then the following procedure was employed: (i) samples were cooled down at 0.2 K·min−1/20 K·min−1 to a selected temperature, then cooled down until 293 K at a rate of 20 K·min−1 before being heated up at the selected temperature, where they were annealed for 2 h. Samples were subsequently cooled down at 20 K·min−1 to 293 K and a heating scan at 20 K·min−1 was recorded until 428 K; (ii) Afterward, samples were cooled down at 0.2 K·min−1/20 K·min−1 to the temperature at which they were previous annealed and, once this temperature was reached, immediately cooled down at 20 K·min−1 to 293 K, and a heating scan at 20 K·min−1 was recorded until 428 K. The procedure followed for the two cooling rates is schematized in Figure 1. In both cases, stages i and ii delivered two specific
In the same way, much effort has been devoted to the understanding of the influence of confinement on the glass transition behavior of ultrathin polymer films.15,19,36,37 It has been put forward a significant depression of Tg with decreasing thickness of free-standing thin film,15,37,38 often explained by an enhanced mobility near the film surfaces, and an increase in Tg with decreasing the thickness of supported thin film,39,40 supposed to result from attractive interactions at the interface. In this study, we have investigated the effect of film thickness on the enthalpy recovery, occurring during the physical aging of PS free-standing films in a very wide range of thicknesses, ranging from 30 nm to several micrometers. This has been carried out by using conventional differential scanning calorimetry (DSC), on the same samples as those investigated in a previous work where no changes in the average equilibrium dynamics, as probed by broadband dielectric spectroscopy (BDS) and AC-calorimetry, were found with decreasing the film thickness, for different films geometries.41 The enthalpy recovery study of PS thin films was completed by accurately determining the onset temperature for nonequilibrium effects, Ton g , by applying a protocol based on the determination of the temperature at which physical aging effects, after cooling down at a given rate, start to appear. This temperature, unequivocally marking the loss of equilibrium, constitutes a well-defined measure of Ton g , namely the temperature of the onset of the glass transition, on cooling. The enthalpy recovery results have been described by a mechanism based on the diffusion of free volume holes, previously employed to explain similar outcomes in polymer nanocomposites.32,42 Furthermore, the simple assumption of the equilibration driven by a diffusion mechanism also reavealed to be able to account for the thickness and cooling rate dependences of Ton g .
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EXPERIMENTAL PART
Polymer thin films were prepared from a high molecular weight linear polystyrene (Mw = 1408 kg/mol, I = 1.17), from Polymer Source. Toluene (99.8%; Sigma-Aldrich) was used to dissolve PS. The films with thicknesses h ≥ 1 μm were prepared by solvent casting PS solutions into freshly cleaned Petri dishes of varying sizes (Duran Group), placed on a horizontal bench under the fume hood. After casting of the PS solution, the samples were left at room temperature under the fume hood during at least 12 h before being transferred to a vacuum oven at 433 K, where they were dried during 4 days. After annealing, the solvent casted films were removed from the Petri dishes by the floating on water method. The thickness of the soprepared samples was verified by scanning electron microscopy (SEM). The films with thicknesses h < 1 μm were prepared by spin coating PS solutions onto ultraflat glass slips (18 × 24 mm; Menzel-Gläser), previously rinsed in acetone. Spin-coating was realized with a speed of 50 rpm, during 30 s, under the fume hood. The thickness of the films was controlled by varying polymer concentration, according to the work of Schubert and Dunkel.43 As-deposited PS films were annealed in vacuum at 433 K for 4 days. The thickness of the so-prepared samples was checked by atomic force microscopy (AFM), which also revealed no inhomogeneities at the surface of the films. After annealing, the dried thin films were removed from the glass substrates by floating on water, and by using a razor blade.
Figure 1. Schematization of the thermal procedure applied to PS thin films for the determination of their Ton g by DSC at two different cooling rates.
heat versus temperature plots, for the aged and the unaged samples, respectively. If the two plots overlap, the selected temperature is above the Tgon. Conversely if the plot corresponding to the aged sample displays an excess specific heat overshoot in comparison to the one corresponding to the unaged sample, the selected aging temperature lays below the Ton g . Thus, this method is able to provide a conceptually sound evaluation of the equilibrium to out-of-equilibrium transition. Note that the annealing time at the selected temperature was arbitrarily chosen to be 2 h. However, this annealing time, ta, has no influence on the results, provided that this is larger than the time scale of temperature stabilization imposed by the applied cooling rate. As estimated from the literature,44 the time scale for interdiffusion for a very high molecular weight PS, is extremely large, which ensures to avoid interdiffusion of the layers in the time scale of the DSC experiments and at the highest achieved temperature. In particular, in all cases, the stacked films reached a maximum temperature of 428 K for times shorter than 1 min. This corresponds to a negligible length scale of interdiffusion (of the order of 1 Å).44 The absence of interdiffusion of the stacked PS layers was also verified by the reproducibility of the
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DSC MEASUREMENTS For the preparation of DSC pans, the thin films were repeatedly stacked until the weight of the sample was high enough (>2 mg) to get a reasonable signal by differential scanning calorimetry (DSC). Thus, for the thinnest films, about 100 ultrathin films had to be stacked to obtain one DSC sample of 2 5297
dx.doi.org/10.1021/ma300622k | Macromolecules 2012, 45, 5296−5306
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Ton g measurements on the same sample after several temperature cycles.
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PHYSICAL AGING MEASUREMENTS For the study of physical aging, the samples underwent a heating ramp to a temperature of 428 K, with a stabilization period of 1 min, to erase the materials previous thermal history. They were subsequently cooled down at a programmed rate of 20 K·min−1 to reach 293 K before stabilizing at the temperature used for structural recovery, Ta. Then, they were aged for times from several minutes to several days before being cooled down to 293 K at a cooling rate of 20 K·min−1, prior to reheating at 10 K·min−1 for data collection. For the measurements of the enthalpy relaxation at short aging times (ta < 48 h), the aging of the samples was carried out in the calorimeter; for longer aging times (ta ≥ 48 h), the annealing of the samples was performed in an external oil-free vacuum oven. Second scans were run immediately after a new quench at 20 K·min−1. The complete thermal procedure applied to the samples for the structural recovery study has been schematized in previous works.33,45 For all films thicknesses, the investigated annealing temperatures were 358 and 369 K. In the case of the 100 nm thick film, physical aging was performed also at 343, 353, and 363 K. This provided a detailed picture of the temperature dependence of enthalpy recovery. The amount of enthalpy relaxed during aging of a glass for a period of time ta at a given temperature Ta was evaluated by integration of the difference between the thermograms of the aged and unaged samples subsequently recorded, according to the relation:3 ΔH(Ta , ta) =
∫T
Ty
x
C pa(T ) − Cpu(T ) dT
(1)
In this equation, Cap(T) and Cup(T) are the heat capacity measured after the annealing and that of the unannealed sample, respectively, whereas Tx and Ty are reference temperatures (Tx < Tg < Ty), at which the unaged and the aged sample scan superimpose below and above T g , respectively. Typically, these temperatures were 368 and 393 K.
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RESULTS Physical Aging of Thin Films. The physical aging of PS films was studied by following the enthalpy recovery of the materials after their annealing at a temperature Ta (
