Article pubs.acs.org/Macromolecules
Sensitive Characterization of the Influence of Substrate Interfaces on Supported Thin Films Jie Xu, Lei Ding, Jiao Chen, Siyang Gao, Linling Li, Dongshan Zhou, Xiang Li, and Gi Xue* Department of Polymer Science and Engineering, School of Chemistry and Chemical Engineering, Key Laboratory of High Performance Polymer Materials and Technology, and The State Key Laboratory of Coordination Chemistry, Nanjing University, Nanjing, 210093, P. R. China S Supporting Information *
ABSTRACT: The perspective by Ediger and Forrest stated that, while we know that the dynamics of polymers in ultrathin films can be significantly altered by substrate interfaces, our understanding of how this depends on the polymer structure and the particular interfaces is rudimentary. Here, we show that fluorescence nonradiative energy transfer (NRET) is an extremely sensitive method for characterizing the interfacial adsorption of polystyrene onto silicon dioxide, even though their interaction is often suggested to be weak. We observed that tensile stress was generated in the supported film by substrate adsorption, which imposes constraints on molecular motion and prevents a reduction of the glass transition temperature (Tg). Furthermore, our investigation suggests that modifying the surface chemistry of the substrate can change the film conformation and dynamics when the film is thinner than 40 nm.
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INTRODUCTION
understanding of how these changes depend on polymer structure and the particular interfaces is rudimentary.19 Simulations have revealed that, close to a confining interface, the conformations of polymer chains are significantly perturbed.20−22 As expected by de Gennes, polymer chains are slightly swollen and strongly segregated in two dimensions.25 Small-angle neutron scattering measurements on thin polymer films have directly shown that the radius of gyration of a polymer chain (Rg) in the plane of the film is unaffected.23,24 Both of them imply that polymer chains in ultrathin films have reduced intermolecular interpenetrations.24−28 Because of the difficulty of separating surface and interior scattering, conflicting results have been obtained using scattering techniques.29−31 Several other techniques have been utilized to probe the configuration of polymer chains in thin films, such as scanning near-field optical microscopy techniques32 and vibrational sum frequency generation spectroscopy.33,34 Nevertheless, these techniques are only applicable for structural analysis in the vicinity of the surface or interface.35 New instruments and methods for making these measurements are continuously emerging in this research field.35,36 Nonetheless, it remains experimentally challenging to characterize polymer structure in ultrathin films, let alone to unambiguously determine changes in structure when introducing an interface or changing the surface chemistry of the substrate.
Polymers confined at the nanoscale have been shown to have anomalous behavior with respect to the bulk.1−6 Recently, interest in investigating confinement has been amplified by the growth of nanotechnology, which aims to alter or create new properties by modifying materials at the nanoscale. Polymer films are of particular interest because they can be used in a large range of applications in emerging areas, such as nanoprinting, electronics, and sensors.7−9 In the case of ultrathin polymer films, where the contribution of interfacial layers becomes relevant, the behavior of polymers can be greatly affected by the presence of the interface. As an important work by Forrest, Dalnoki-Veress, and co-workers demonstrated, the depression of the glass transition temperatures (Tg’s) of free-standing polystyrene (PS) films could be greatly diminished by transferring the same films onto SiOX substrates.3,10−12 Meanwhile, the chemistry of the substrate surface may dominate the differences in dynamics observed between confined and bulk polymers. Poly(methyl methacrylate) (PMMA) thin films exhibit an elevated Tg on native silicon surfaces because of hydrogen bonding, while their Tg is depressed on gold because of the weaker interfacial interactions.10 These results, along with previous studies by Tsui and Russell13 as well as by Napolitano,14 Priestley,15 and others,16−18 indicate that Tg changes with changes in the interfacial interactions or energy for ultrathin polymer films supported on a controlled substrate. The perspective by Ediger and Forrest stated that, while we know that polymer dynamics can be significantly altered near surfaces and interfaces, our © 2014 American Chemical Society
Received: April 25, 2014 Revised: July 25, 2014 Published: September 15, 2014 6365
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ultrathin films. Herein, we prove that NRET is a powerful and highly sensitive method for the direct characterization of polymer conformation in nanoscale confined systems.
PS is an important polymer in thin film research. Compared with PMMA or poly(2-vinylpyridine) (P2VP), nonpolar polymer PS is often suggested to have weaker interactions with substrate surfaces. However, researchers have still observed altered Tg variations in PS thin films on controlled substrate surfaces.13,14,16,17 Distinguishing subtle changes in the PS chain structure caused by the interfacial chemistry greatly challenges the sensitivity of the characterization method. The fluorescence nonradiative energy transfer method (NRET) is a powerful and highly sensitive technique that uses the response of probe molecules to provide information about the polymer chains in a microenvironment. This method has been applied extensively to characterize polymer conformation, interpenetration, entanglement, and diffusion.37−42 In the conventional procedure for NRET, small amounts of a fluorescence energy donor and an energy acceptor are attached to different polymer chains. When the polymer blends are excited at a wavelength that is only absorbed by the donor and not by the acceptor, the excitation energy of the donor molecules can transfer over a distance to the acceptor molecules via a resonance dipole−dipole interaction mechanism. According to the theory formulated by Förster,43 the efficiency E of this nonradiative energy transfer can be expressed by
E = [1 + (r /R 0)6 ]−1
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EXPERIMENTAL METHOD
Materials. Two types of PS molecules were employed as our probes: PS-Cz, which has a carbazolyl (Cz) moiety, and PS-An, which has an anthryl (An) moiety. Carbazolyl- and anthryl-labeled PS (the native PS was purchased from Polymer Source Inc.) were prepared according to the procedure of Liu et al.46 The structures of the two types of polymers are shown in Figure 1, and the characteristics of the PS samples are summarized in Table 1.
(1)
where r is the distance between the donor and acceptor and R0 is the distance at which the probability of transferring the excitation energy is 50%.38 For the pair of fluorophores used in our work, R0 is 2.875 nm.44 According to this theory, even if there is a subtle change in the polymer chain penetration, the energy transfer between the donor and acceptor will dramatically increase/decrease. Thus, the relative emission intensity (IA/IC) of the acceptor (IA) and donor (IC) can be used as a spectroscopic ruler to detect the separation of these two moieties. Morawetz et al. evaluated the level of polymer interpenetration in solutions by measuring the energy transfer represented by the ratio of IA/IC from pellets pressed from freeze-dried solutions.38 Torkelson et al. experimentally demonstrated the correlation hole effect by detecting interpolymer label−label energy transfer, quantified by the ratio of IA/IC.37 Recently, using the NRET method, we demonstrated that freeze-dried PS with a loose packing density shows increased segmental mobility under uniaxial compression.45 In this work, we will use the NRET method to amplify the sensitivity of interchain proximity measurements in ultrathin polymer films. Here, we demonstrated direct characterization of the structural perturbations of polymer chains by interfaces in ultrathin films using the NRET and correlated the structural changes with the dynamics of the supported films. We first investigated changes in chain conformation for an originally spin-coated thin film, a free-standing film (which was detached from a supporting substrate by water floatation), and a transferred film (which was prepared by the water-casting technique to transfer the free-standing film back to the original substrate). Second, the same thin films were transferred to substrate surfaces with different chemistries to explore the effects of interfacial adsorption strength on chain conformation and the depth to which the surface property effects could extend. Furthermore, the glass transition temperatures of all of the films were measured to correlate the changes in the polymer chain configuration to the polymer dynamics in the
Figure 1. Chemical structures of PS-An and PS-Cz. The terms x and y represent the fractions of the probe group.
Table 1. Characterization of Polystyrene Used in this Study polymer
Mn (g/mol)
Mw/Mn
mol % of labela
PS-An PS-Cz
1.05 × 10 1.05 × 106
1.05 1.05
0.33 0.96
6
a
The label concentrations on the PS were determined by UV absorption.47
Film Preparation. Supported PS films with thicknesses ranging from 12 to 150 nm were prepared by spin-casting PS (An:Cz = 1:1) from filtered toluene solutions onto cleaned natural quartz (as shown in Figure 2A) and annealing at Tg + 10 °C for 24 h in a vacuum. Freestanding films were prepared by floating the supported films onto a purified water surface, and they were captured on an aluminum plate with a circular opening that was 1.5 cm in diameter (as shown in Figure 2B). Several fine copper wires filled in the opening to support the ultrathin film, as shown in Figure S1 (Supporting Information). The free-standing films were dried overnight before Tg measurement to remove the residual water. Transferred films were prepared by transferring the free-standing film back onto the target substrates (as shown in Figure 2C). Film Characterization. Fluorescence spectra were measured on a Photon Technology International spectrofluorometer at 25 °C. The band-pass excitation and band-pass emission slits were both 1 μm. The excitation wavelength was 294 nm, and the fluorescence emission intensity was monitored at 310−500 nm. The films were oriented at 45° to the excitation beam, as shown in Figure S2 (Supporting Information). We applied this transmission optical pathway for the measurements to avoid interference between the fluorescence from the film and the light reflected from the surface of the substrate.40 The film thicknesses were verified by transferring the film onto a silicon slide and then by measuring the thickness on a spectroscopic ellipsometer (M-50, JASCO Co., Ltd.). 6366
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Figure 2. Schematic of the preparation of supported, free-standing, and transferred films. When the PS films prepared from the mixture of the two labeled PSs were excited at 294 nm, which was preferentially absorbed by Cz rather than An, the Cz fluorescence (IC) was found to be quenched dramatically, while the An fluorescence (IA) increased significantly, as shown in Figure 3. This result indicates that the excitation energy of
present the first characterization of chain conformation changes caused by adhesion between an ultrathin film and a substrate. Figure 4 shows the NRET results for a 25 nm single PS membrane that was either spin coated onto a quartz substrate, spin coated and then detached to produce a free-standing film, or spin coated, detached, and then transferred back to the original substrate. Here, it is important to note that that there was no additional high-temperature treatment during the detachment and transferring processes. First, we find that the NRET ratio IA/IC for a supported thin film is much lower than the bulk sample (shown in Figure S4, Supporting Information), indicating the separation of the donor and acceptor molecules due to the stretching effect during the spin-coating process. After we removed the supported substrate from the film by water floatation, the ratio IA/IC increased significantly, indicating a closer donor−acceptor proximity in the freestanding ultrathin film (schematic shown in Schematic S1, Supporting Information). The original supported film was prepared by the spin-coating method. During the spin-coating process, interfacial adsorption as well as the freezing-in of nonequilibrated polymer chain conformations caused by the fast evaporation of solvent could result in tensile stress within the film.51−53 Reiter and co-workers provided many valuable works on the residual stresses generated by fast solvent evaporation during spin coating.52−55 They found that these stresses tend to disappear as the chainlike molecules adopt conformations closer to their equilibrium during aging or annealing above Tg.52,53 However, in our case, we simply removed the supported substrate gently from the polymer film without any annealing process. Thus, the changes in the average interchain proximity in the same films observed here can be mainly attributed to the substrate/film interfacial adsorption. The free-standing thin film showed a closer average interchain proximity after the removal of the substrate, suggesting the existence of tensile stress in the supported film caused by the interfacial adsorption. This interfacial adsorption is attributed to pinning of the molecules at the contact with the wall, which prevents shrinkage from occurring freely. To confirm our hypothesis, we simply transferred this freestanding film back to the original substrate by the water-casting technique. Surprisingly, the NRET ratio decreased back to the same value of the original supported film, as shown in Figure 4, indicating that the average donor−acceptor distance was enlarged and returned to the original supported state during
Figure 3. Fluorescence spectra of 25 nm thick free-standing PS-Cz, PS-An, and PS-mix films. the donor molecules was transferred by a resonance dipole−dipole interaction mechanism to the acceptor molecules. Because the film thickness and chromophore concentrations were low enough, radioactive energy-transfer effects were negligible in our measurements.48
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RESULTS AND DISCUSSION The Influence of the Presence of a Substrate Interface on Chain Conformation. Baumchen12 performed an interesting experiment that showed that simply transferring a free-standing film onto a Si substrate can cause a significant increase in Tg, suggesting the importance of a free surface for Tg reduction in ultrathin films.12 On the basis of their work and the previous work of others, the Tg of a free-standing film with two free surfaces is reduced to less than half of that of a supported film.1,3,11,12,49,50 We hypothesized that, in the glassy state, the presence of a supported interface not only reduces the amount of free surface but also changes the polymer chain configuration and somehow constrains the mobility of the polymer chains in supported film. However, experimental work has rarely been performed to directly investigate the substrate effects on polymer structure in glassy ultrathin film. Here, we 6367
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Figure 4. (left) NRET ratio (IA/IC) of a single thin membrane that was originally spin coated and then floated up and transferred to a quartz substrate. The error bars show the standard error of the mean, which was calculated by dividing the standard deviation of the ratio by the square root of 5 (i.e., the number spectra measured). (right) Fluorescence spectra of spin-coated (black), free-standing (orange), and transferred (green) films.56
the adhesion process. It is generally accepted that, to prepare a high quality transferred film, we must first float the freestanding film on water.57 When we pulled films to the substrate smoothly and maintained their integrity, the shrinkage of the polymer is partly constrained by the substrate adsorption. Thus, a tensile stress is generated in the plane of the transferred film on the substrate,58,59 which alters the chain conformation and the donor−acceptor proximity. Because the same substrate was used, the strength of the interfacial adsorption was unchanged, meaning that the tensile stress trapped by the interfacial adsorption should have been the same both in the original supported film and the transferred film, consistent with our NRET characterizations. Here, we would like to make it clear that the obvious changes in the average interchain proximity in ultrathin film observed by the NRET method were due to immense amplifications at distances in the range 2−3.5 nm.38 Therefore, this observation may not be so apparent by other techniques. In this regard, we would like to make it clear that the quartz substrate itself did not alter the fluorescent intensity of IC or IA in our work. We performed fluorescence measurements on pure PS-Cz and PS-An thin films with/without quartz substrate separately. As shown in Figure 5, the fluorescence intensity at 365 nm (IC) of a pure PS-Cz film on a quartz substrate was almost the same as was observed for the free-standing film. Furthermore, when we excited the pure PS-An thin films at 365 nm, the fluorescence intensities at 414 nm (IA) in the spectra of films with/without a quartz substrate were nearly the same. This result confirms that the quartz substrate itself does not perturb the fluorescence intensities IC or IA in the films and, therefore, contributes little to the ratio changes in our work. The Correlation between Substrate Influences on Chain Configuration and Polymer Dynamics in Ultrathin Films. Determining the interfacial structure in ultrathin polymer coatings is important for predicting failure due to delamination and cracking. Thermal dynamic properties are also important for the coating and electronics industries, especially when segmental dynamics of thin films or coatings are critical for their final performance.7,60−62 Recent models
Figure 5. (a) Fluorescence spectra of pure PS-Cz films supported on quartz (black, left) and as free-standing films (red, right). The excitation wavelength was 294 nm. (b) Fluorescence spectra of pure PS-An films supported on quartz (black, left) and as free-standing films (red, right). The excitation wavelength was 365 nm. Every fluorescence spectrum was taken for an average of three spectra.
and experiments have suggested that changes in the interfacial free volume induced by adsorption onto a solid substrate can 6368
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affect dynamic aspects as well.63−65 The dynamic properties of molecules close to a hard wall can be strongly affected by adsorptive interactions with the substrate and entropic constraints. To understand the correlation between substrate interface influences on polymer chain configuration and polymer dynamics in ultrathin films, we measured the Tg of the original supported film, a free-standing film, and a transferred film using the fluorescence method developed by Torkelson.4,50 The PS films were doped with trace amounts of pyrene dye (
