Article pubs.acs.org/Macromolecules
Glass Transition Behavior in Thin Polymer Films Covered with a Surface Crystalline Layer Biao Zuo,† Yue Liu,† Yongfeng Liang,† Daisuke Kawaguchi,‡ Keiji Tanaka,*,§ and Xinping Wang*,† †
Department of Chemistry, Key Laboratory of Advanced Textile Materials and Manufacturing Technology of Education Ministry, Zhejiang Sci-Tech University, Hangzhou 310018, P. R. China ‡ Education Center for Global Leaders in Molecular Systems for Devices and §Department of Applied Chemistry, Kyushu University, Fukuoka 819-0395, Japan S Supporting Information *
ABSTRACT: Thin amorphous poly(ethylene terephthalate) (PET) films covered with/without a crystallized surface layer were prepared onto silicon wafers. In the former and latter cases, the surface mobility in the film was depressed and enhanced, respectively. The glass transition temperature (Tg) of the amorphous PET film decreased with the reduction of the film thickness, exhibiting a remarkable nanoconfinement effect. However, once the surface region of the thin film was crystallized, or frozen in terms of the segmental motion, Tg of the films recovered to that of the bulk. Concurrently, the apparent activation energy of the segmental motion in the surface-crystallized film was in good accordance with the bulk value as well. These results make it clear that the mobility in the surface region plays an essential role in the glass transition of the thin films.
■
INTRODUCTION Nanometer thick polymer films play an important role in the nanomanufacturing sector. The glass transition temperature (Tg) is a key factor determining the performance of polymeric nanodevices. Since 1994, with the first report by Keddie and Jones of the striking deviation of Tg, specifically the thermal Tga measure of how a supercooled liquid vitrifies on coolingfor thin polystyrene (PS) films using ellipsometry,1,2 a large number of experimental and simulation studies have revealed that the Tg of thin polymer films decreases as the thickness is reduced to below ∼100 nm, if the film is supported on a weakly attractive substrate or freely suspended. Recently, Forrest,3 Torkelson,4 and others5,6 have found that not only the Tg but also the activation energy and fragility index also decrease significantly for the thin polymer films. Such an intriguingly notable deviation in glass transition dynamics has inspired a great number of scientists working in this area to clarify the associated mechanisms. So far, several scenarios (e.g., finite size effect,7 distortion in macromolecular conformation,8 perturbation in entanglement,9,10 presence of a mobile surface,1,2,11−13 modification of packing density,14−16 nonequilibrium structures17,18 and easily attaining the thermodynamic equilibrium by a free volume holes diffusion (FVHD) mechanism19−22) have been put forward to account for the phenomenon of Tg reduction of thin polymer films, among which the presence of a mobile free surface might be the most reasonable model. It has been proposed that segments in direct contact with the free surface gain access to a larger free volume © XXXX American Chemical Society
due to the lack of neighboring interactions of segments and thus have extraordinarily high mobility and lowered Tg.11−13,23−25 The enhanced surface mobility propagates deeply into the interior, influencing the global dynamics of films of tens or maybe even hundreds of nanometers thick.11,26,27 Although supported by a range of experimental results, the free surface mechanism hypothesis has still met with a degree of skepticism because a large number of reports have also shown that polymers in nanoconfined environments with weak boundary interactions can exhibit unchanged28−38 or even increased Tg.39−42 In addition, several studies have recently revealed that thin polymer film can show remarkably reduced Tg whereas a bulklike segmental dynamic, which enormously complicates the interpretation of the glass transition behavior of thin polymer films.43−47 The decoupling between Tg and segmental dynamics in confinement was described by the FVHD model as developed by Cangialosi et al.,19−21,46,47 in which the surface was merely considered as the outlet of free volume trapped in the film interior. These inconsistences persisting in the literature motivate further studies to identify a direct relationship between the mobility of the surface layer and the unusual cooperative dynamics of polymers in nanostructured systems. Received: December 20, 2016 Revised: February 9, 2017
A
DOI: 10.1021/acs.macromol.6b02740 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
Figure 1. (a) Temperature dependence of ellipsometric angle (Δ) and its temperature derivative for a thin PET film (h = 20 nm, heating rate: 2 K/ min). (b) AFM topographic images of thin PET films that were heated to various temperatures (scale: 20 μm × 20 μm). (c) XRR curves of PET films that were heated to 350 and 376 K and the model density profile along the direction normal to the surface (inset). Both the XRR curves and the depth profile have been vertically shifted for clarity.
crystallization temperatures (Tc) during heating of the films. The Tc of the surface layer was about 20 K lower than that of the underlying materials. On the basis of the stratification crystallization of thin PET film, an amorphous PET film covered with a surface crystalline layer can be conveniently prepared by interrupting the heating at an intermediate temperature where the crystallization of the surface layer has been completed, while that of the underlying materials has not yet started. Moreover, the surface-crystallized films can recover to the original completely amorphous state by annealing them to above melting temperature (Tm) and then quenching. Since crystallization suppresses the molecular mobility, if the Tg of thin PET films reversibly changes along with the variation of the surface structure from an amorphous to a crystallized state, then the surface mobility must be decisive in determining the glass transition of the entire film.
The straightforward way to ascertain the effects that free surfaces have on the Tg is to modify the surface and study the resultant changes in the Tg, as suggested by Forrest et al.48,49 The approach commonly taken for this purpose is to manipulate the number of free surfaces by (1) evaporating a thin metal coating on the top surface of thin films,48 (2) stacking two supported thin films together in a face-to-face fashion,48 and (3) transferring a free-standing film onto a substrate surface.49 These methods reduce the number of air/ polymer free surfaces yet at the same time introduce a foreign interface (e.g., metal/polymer interface), another source of perturbation on the dynamics of the thin films. Napolitano et al.22,50 revealed that there is also an excess of free volume at the polymer/substrate interface, which would accelerate the interface dynamics. That is to say, the interface can play a similar role to the f ree surface in determining the dynamics of thin films, with the result that capped supported thin films (e.g., aluminum-capped PS films) where free surfaces are not present may also have a lowered Tg.48,50−52 The introduction of foreign interfaces with complex interfacial dynamics complicates the explanation of the results, making it less than ideal for studying the impact of the free surface on the Tg of thin films. Experiments that are able to modify the free surface properties without introducing foreign interfaces are thus desperately needed, and the results should become more conclusive, if such modifications can be tuned reversibly. In order to achieve this purpose, a crystallizable polymer, poly(ethylene terephthalate) (PET), was chosen as a rational candidate, since PET thin films exhibit stratification in crystallization kinetics, as revealed in our previous work.53 It was demonstrated that an 11 nm thick surface layer of PET has a faster crystallization rate than the remaining underlying materials.53 Crystallization of the surface layer and underlying materials was well-decoupled, exhibiting two individual cold-
■
EXPERIMENTAL SECTION
Materials and Film Preparation. Powder sample of PET (Mw = 30 kg/mol; Mw/Mn = 1.87) was purchased from Polymer Source Inc. (Montreal, QC, Canada) and used as received. Bulk Tg of the PET sample was 349 K, measured by differential scanning calorimetry (DSC) with a heating rate of 2 K/min. Amorphous ultrathin films of PET were prepared by spin-coating solutions of PET in a mixture (5:2) of trifluoroacetic acid and chloroform onto silicon wafers with a native oxide layer. The prepared films were then placed in a vacuum oven (pressure
