Article pubs.acs.org/Macromolecules
Plasticization in Ultrathin Polymer Films: The Role of Supporting Substrate and Annealing Hung K. Nguyen,*,†,§ Massimiliano Labardi,‡ Mauro Lucchesi,† Pierangelo Rolla,† and Daniele Prevosto*,‡ †
Dipartimento di Fisica “Enrico Fermi”, Università di Pisa, Largo Pontecorvo 3, 56127 Pisa, Italy CNR-IPCF, Consiglio Nazionale delle Ricerche, Istituto per i Processi Chimico-Fisici, c/o Dip. Fisica, Largo Pontecorvo 3, 56127 Pisa, Italy
‡
S Supporting Information *
ABSTRACT: The effects of supporting substrate and thermal annealing on plasticization of poly(vinyl acetate) ultrathin films by absorption of ambient moisture have been studied by local dielectric spectroscopy. Upon exposure to ambient moisture, the relaxation rate of the α-process increases to a different extent at variance of film thicknesses and supporting substrates. Namely, on hydrophobic gold substrates the speeding up is slightly reduced by decrease of film thickness down to about 20 nm. Moreover, the increase of the relaxation rate measured on a 21 nm thick film supported on gold is smaller compared to that measured on a film with similar thickness supported on the relatively hydrophilic aluminum. We interpret such results in terms of the presence of an interfacial polymer layer at the substrate surface with different contents of water in dependence of the substrate. Furthermore, plasticization effect in ultrathin films significantly decreases after annealing at high temperature (∼ Tg + 60 K) and for times longer than usual time scales of relaxation processes. Such results are consistent with the formation of a highdensity irreversibly adsorbed polymer layer at the interface, able to hinder the effect of the substrate. However, in ultrathin films the observed influence of moisture is in apparent contradiction. Kim et al.11 found an increase in glass transition temperature (Tg) respect to the dry condition of a wet 23 nm thick PVAc film supported on a silicon oxide. Such increase of Tg with humidity is in contrast with the results obtained on bulk PVAc samples14,15 and with the findings of O’Connell et al.13 on freely standing PVAc ultrathin films. In the latter, the Tg of a freely standing PVAc film with thickness of 28 nm was found to decrease with increasing the relative humidity (RH) of the ambient where the sample was equilibrated. Also, Serghei et al.8 found that relaxation dynamics of ultrathin PMMA films capped between two aluminum electrodes was strongly enhanced by humidity. However, up to date most of the studies related to the effect of ambient RH on the dynamics of ultrathin polymer films have not considered the role of the supporting substrate as well as of annealing. The former one has been evidenced to strongly affect the moisture absorption capacity into ultrathin films. A significant number of studies have focused on estimating the thickness and substrate dependence of water absorption in supported ultrathin polymer films using various techniques.5,16−19 Most of them evidenced the existence of a water concentration gradient near the supporting substrate, depend-
1. INTRODUCTION When confined to dimensions comparable to the length scales characterizing a polymer chain, chemical and physical properties of ultrathin polymer films can be significantly altered with respect to the bulk.1−5 Although it is often reported that interfacial interactions play a key role in determining the observed dynamics deviations between confined and bulk polymers,6,7 some other factors have been pointed out to produce a strong influence, including annealing at temperature much higher than the glass transition for times much longer than time scales of any relaxation process,8−10 and absorption of small molecules acting as plasticizers.11 Most of the phenomenology related to dynamic properties of ultrathin films can be regarded in terms of confinement-induced reduction of intermolecular cooperativity.12 Some phenomena specific of polymer/substrate interactions rather than related to the mechanisms of glass formation remain instead unexplained.8−11 The impact of water molecules on the change of glass transition dynamics in ultrathin polymer films compared to the bulk behavior has been recently investigated, especially when measurements are performed on polymer films containing hydrophilic groups, such as poly(methyl methacrylate) (PMMA)8 or poly(vinyl acetate) (PVAc).11,13 When exposed to the ambient atmosphere, such polymers can absorb a certain amount of moisture, usually causing plasticization of polymer chains leading to an enhancement of dynamics in the bulk.14 © XXXX American Chemical Society
Received: September 20, 2012 Revised: December 8, 2012
A
dx.doi.org/10.1021/ma301980w | Macromolecules XXXX, XXX, XXX−XXX
Macromolecules
Article
details on sample preparation can be found in previous publications.20,23 In order to obtain an equilibrium value of absorbed moisture, PVAc films were kept at fixed conditions of humidity and temperature for at least 4 h before each measurement. This time was considered sufficient to reach the equilibrium condition. In fact, we have verified that the time needed to reach the equilibrium state in bulk PVAc upon humidity exposure is about 4 h for grains with average diameter of about 1 mm. It is expected that this time should be much shorter for ultrathin PVAc films due to a larger fraction of surface area to volume in an ultrathin film compared to the bulk sample and to its smaller thickness. 2.2. Measurement Procedure. Dielectric relaxation of supported polymer films was measured in dry as well as humid conditions by using the LDS method. Further details on the working principle of the technique and the adopted experimental protocol can be found in previous reports.22,23,25 LDS was operated in controlled humidity within a homemade enclosure that allows to obtain a relative humidity as low as 3% by using desiccants and flowing nitrogen gas through the enclosure and as high as 80% by placing a dish full of distilled water inside the enclosure and discontinuing the nitrogen flow. The temperature of the ambient around the sample was stable for all our measurements within a range ±3 deg around 300 K. This introduces an uncertainty that is much less than the variation due to the imposed change of RH and can be consequently be neglected. In this RH interval PVAc sample is expected, on the basis of ambient temperature bulk data, to absorb up to a maximum of 3% w/w of water.14,15 The sample temperature was controlled using a thermal application controller (TAC, Veeco Instruments, Inc.). The actual temperature on the sample surface was calibrated using a PT100 sensor.
ing on the relative hydrophilicity of the polymer and the substrate material.5,19 In particular, if the substrate is more hydrophobic than the polymer, a lower amount of water molecules can accumulate at the substrate interface, whereas if the substrate is more hydrophilic than the polymer, a different trend occurs.5,16,19 Also annealing conditions have been found to influence polymers relaxation dynamics in confinement. Recent reports have evidenced a slowing down of relaxation dynamics of ultrathin polymer films after annealing at a temperature much higher than Tg, often in the range from Tg + 50 K to Tg + 100 K.8−10,20 This effect has been related to the formation of a highdensity polymer layer irreversibly adsorbed on the substrate surface during the annealing process.10,21 Both thickness and Tg of such layer increase with annealing time in dependence of the polymer molecular weight (Mw)10 and supporting substrate.20 All of these findings suggest that annealing can change the moisture absorption capacity of supported ultrathin films because the supposed high-density irreversibly adsorbed layer should be less permeable to water molecules. If verified, such interplay should lead to the possibility to control the moisture absorption into supported ultrathin films by using appropriate annealing conditions as well as to modulate relaxation dynamics of supported ultrathin films by use of appropriate plasticizers (in the present case, water) and annealing conditions. In the present work we use a state-of-the-art technique, namely local dielectric spectroscopy (LDS), which has been recently improved22,23 in order to study the effects of moisture absorption on relaxation dynamics of ultrathin PVAc films. The influence of supporting substrate and annealing is also investigated by using gold and aluminum substrates, which have different degrees of hydrophobicity, and by annealing polymer films for increasing periods of time at high temperature.
3. RESULTS AND DISCUSSION 3.1. Influence of Moisture Absorption on Relaxation Dynamics. The influence of moisture absorption on relaxation dynamics of supported PVAc films was measured using LDS. In this technique, loss-tangent spectra, tan(δv), are measured as a function of the applied frequency to characterize dielectric relaxation of polymer films.22 The advantage of using the LDS technique with respect to more conventional dielectric techniques is that the integrity of the polymer film can be easily checked through the imaging capability of the AFM just before the dielectric measurement, evaporation of a second electrode on the upper film surface is not needed, and absorption of ambient vapors is made easier, through the free upper surface, in comparison with capped geometries. The relation between these spectra and the dielectric permittivity of a polymer film is given by the following equations:
2. EXPERIMENTAL METHOD 2.1. Sample Preparation. Poly(vinyl acetate) (PVAc) with molecular weight Mw = 350 kg/mol, polydispersity index PDI = 2.80, and estimated gyration radius24 Rg ∼ 17 nm was purchased from Scientific Polymer Products, Inc. Ultrathin PVAc films were prepared by spin-coating solutions of PVAc in toluene onto gold and aluminum substrates. Gold layers of 30 nm thickness were obtained by thermal evaporation on glass disks previously evaporated with a ∼5 nm adhesion layer of chromium, whereas 50 nm thick aluminum layers were directly evaporated on glass disks. The roughness of substrate surfaces was measured by atomic force microscopy (AFM) in tapping mode, resulting 0.3 and 1.1 nm RMS on gold and aluminum, respectively. The thickness of polymer films was varied in the range from 21 to 128 nm by changing the concentration of polymer solutions as well as spinning speed. Prior to spin-coating, contact angles of water on each substrate were measured at room temperature using a KSV CAM 200 optical contact angle meter. The values of the contact angles, which were obtained from averaging five measurements for each sample, were 89.2 ± 0.9° and 27.9 ± 1.3° on gold and aluminum, respectively.20 Such values confirmed the well-known hydrophilicity difference between the two types of substrates. After spin-coating, all polymer films were annealed with a similar procedure, at 323 K (∼ Tg + 15 K) under vacuum (
