Molecular Probe Diffusion in Thin Polymer Films - ACS Publications

Mar 12, 2018 - Department of Physics, University of Ioannina, 45110 Ioannina, Greece. •S Supporting Information. ABSTRACT: We studied experimentally...
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Letter Cite This: ACS Macro Lett. 2018, 7, 425−430

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Molecular Probe Diffusion in Thin Polymer Films: Evidence for a Layer with Enhanced Mobility Far above the Glass Temperature Mahdis Hesami,† Werner Steffen,† Hans-Juergen Butt,† George Floudas,*,‡ and Kaloian Koynov*,† †

Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany Department of Physics, University of Ioannina, 45110 Ioannina, Greece

‡

S Supporting Information *

ABSTRACT: We studied experimentally the influence of interfaces on the dynamics in thin polymer films at temperatures far above the glass temperature (Tg + 80 °C). Polyisoprene (PI) was employed as a model system. We examined glass substrate supported films with thicknesses (d) spanning the range from 10 μm to 10 nm that correspond to d/Rg from 400 to 1, where Rg is the polymer radius of gyration. We employed fluorescence correlation spectroscopy (FCS) to monitor the translational diffusion of small fluorescent tracer molecules, dispersed at nanomolar concentrations in the PI matrix. In thick films, a single diffusion process correlated to the bulk segmental dynamics of the matrix polymer was present. However, when the film thickness was smaller than the normal dimension of the FCS observation volume, a second, faster diffusion process appeared, reflecting enhanced segmental dynamics near the free surface. Our results provide direct experimental evidence for the existence of a layer with enhanced mobility near the free surface of supported PI films at temperatures as high as 80 °C above the bulk Tg. n polymer films, the presence of interfaces (solid substrate or the free surface) may affect the dynamics of those chain segments that are in the vicinity of the interface. As a consequence, material characteristics related to segmental dynamics, such as the glass temperature, thermomechanical properties, or ion conductivity, may be different in thin polymer films as compared to bulk polymer. A large number of experimental1−25 and theoretical26−30 work dedicated to this phenomenon were undertaken in the last two decades. With decreasing film thickness, the contribution of the interfacial regions to the overall properties increases. Therefore, many previous studies investigated the dependence of polymer film properties, such as the average glass temperature Tg, on film thickness. Several works3−5,8,11 found a reduction of Tg with decreasing film thickness, especially for free-standing films5,8 and thus indicated faster mobility near the free surface. However, the magnitude of the Tg change, as well as the effect of annealing toward the equilibrium state, are still debated.15,25 In addition to the thickness-dependent Tg measurements, enhanced mobility near the free surface of polymer films was observed using methods based on atomic force microscopy,1,16 nanoparticles embedding,12−14,17 monitoring the translational7 or rotational18,20 diffusion of tracer molecules by fluorescence recovery after photobleaching, resonance enhanced DLS,23 and ellipsometry.19 In a recent article, M. Ediger and J. Forrest presented an overview of the current state of understanding on the subject and discussed future research directions and remaining open questions.22 The open question that we address in this paper is “How surface mobility evolves as temperature is raised above Tg?” As

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© XXXX American Chemical Society

discussed above, a number of experimental studies1,12−14,16−18,20 indicate the existence of a “liquid-like” or “enhanced mobility” layer near the free surface of polymer glasses. The mobility of this surface layer was reported to be many orders of magnitude faster than the bulk at temperatures below the bulk Tg. However, with the increase of temperature, the difference between surface and bulk mobility decreases17,18 and above the bulk Tg it could not be resolved with most experimental techniques. To the best of our knowledge, faster dynamics at the surface layer at temperatures above Tg was experimentally observed only in few studies of thin polystyrene7,16,21 and polybutadiene23 films. On the other hand, computer simulations suggested that the existence of enhanced mobility layer on the free surface of a polymer film is a more general phenomenon that should be present also at temperatures well above the bulk glass temperature.26,27 In order to get further insight, and better correlate theory and experiment, new experimental approaches are needed that can monitor locally the mobility in polymer films with high sensitivity and at temperatures far above the glass temperature. In this respect, fluorescence correlation spectroscopy (FCS) is a promising method. FCS is a sensitive and selective technique for studying the mobility of fluorescent species, such as, small molecules, macromolecules or nanoparticles, in various environments.31 While initially developed,32 and still predominantly used as a Received: February 7, 2018 Accepted: March 12, 2018

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ACS Macro Letters tool in molecular and cell biology,33,34 during the past decade FCS has also become an established technique in polymer, colloid, and interface science.35−39 With respect to monitoring the mobility in polymers, FCS has been employed earlier in studying the diffusion of small fluorescent tracers in bulk homopolymers with different chain topologies such as linear and star polyisoprenes,40 in poly(dimethylsiloxane),41,42 poly(nbutyl acrylate),43 in miscible42 and phase-separated44 polymer blends, as well as to follow the polymerization process.45 It was shown that the translational diffusion coefficient of tracers depends strongly on the glass temperature of the studied polymers. Hence, tracers can be employed as reporters for changes in the local segmental dynamics. In this Letter, we describe the first FCS study of the translational diffusion of fluorescent tracers in thin polymer films as a function of film thickness. By taking advantage of the high sensitivity and small observation volume of FCS, we were able to resolve faster segmental mobility near the free surface of films of high molecular weight polymers at temperatures more than 80 °C above the bulk Tg. As a model system we employ, 1,4-cis-polyisoprene (in-house synthesis) with molecular weight of 700 kDa and polydispersity index of 1.05. The polymer was selected for its low glass temperature, Tg ≈ −60 °C, which allowed the FCS studies to be performed conveniently at room temperature. A terrylene dye, N,N′-bis(2,6-diisopropylphenyl)-1,6,9,1 tetraphenoxyterrylene-3,4:11,12-tetracarboxidiimide (TDI, in-house synthesis) was used as fluorescent molecular tracer in the polymer host, because it is well dispersible in organic materials, provides high quantum yield, and shows good photostability.46,47 Moreover, earlier studies have shown40,42 that the diffusion coefficient of TDI tracers in bulk PI is strongly coupled to the matrix polymer segmental dynamic and glass temperature. The chemical structures of TDI and PI are shown in Scheme S1 of the Supporting Information (SI). Chloroform solutions of PI and TDI were prepared by shaking for 24 h. PI concentration was tuned in the range from 10 to 100 mg/mL. TDI concentration was adjusted to be in the 50 nM range in the dry films. Thick polymer films (μm range) were prepared by drop-casting of a concentrated solution while thin films (nm range) were prepared by spin coating of diluted solutions. All films were prepared on cleaned microscope glass slide, with diameter of 25 mm and thickness of 0.15 mm. In order to remove the solvent, samples were kept in vacuum at 45 °C overnight. The final film thickness was measured by profilometry as described in the SI. For the FCS studies the samples were mounted in an Attofluor Cell Chamber (Thermo Fisher Scientific) with the polymer film facing upward. First, we studied the tracer diffusion in the middle of several μm thick PI films. FCS measurements were performed on a commercial setup (Carl Zeiss) consisting of the module ConfoCor2 and an inverted microscope model Axiovert 200. The laser beam of a HeNe laser (λ = 633 nm) was tightly focused into the polymer film via a high numerical aperture microscope objective (alpha Plan-Fluar 100×/1.46 Oil, Carl Zeiss). The emitted fluorescence was collected with the same objective and after passing through a dichroic mirror, a LP650 long pass emission filter and a confocal pinhole, directed to an avalanche photodiode detector operating in single-photon counting mode. This arrangement resulted in the formation of a subfemtoliter observation volume, Vobs, with a Gaussian ellipsoid shape. Only fluorescence emitted from species inside Vobs is detected. The Brownian diffusion of the fluorescent TDI

molecules in and out of the observation volume creates fluctuations of the detected fluorescence intensity compared to its temporal average value, δF (t) = F (t) − ⟨F (t)⟩, that were recorded and evaluated in terms of an autocorrelation function: G (τ ) = 1 +

⟨δF(t )δF(t + τ )⟩ ⟨F(t )⟩2

(1)

For all studied samples, a series of FCS measurements with duration of 3 min each were performed at least at five different lateral positions and the results were averaged as described in the SI. A typical autocorrelation curve measured in a 10 μm thick PI film at room temperature is shown (symbols) in Figure 1. The

Figure 1. Experimental FCS autocorrelation curve (symbols) measured in the middle of a 10 μm thick PI film. The position of the observation volume is schematically shown in the inset. The line in the upper panel represents the corresponding single component fit (eq 2, m = 1). The lower panel shows the corresponding residuals.

FCS observation volume, was positioned in the middle of the film as schematically shown in the inset of Figure 1. Thus, the diffusion of the observed tracer molecules was not affected by the presence of interfaces. It corresponds to the bulk polymer behavior. As has been shown theoretically, for an ensemble of m different types of freely diffusing fluorescence species, the autocorrelation function has the following analytical form:31 ⎡ fT −t / τ ⎤ 1 G (t ) = 1 + ⎢1 + e T⎥ ⎢⎣ ⎥⎦ N 1 − fT m

∑ i=1

⎡ ⎢⎣1 +

t ⎤ ⎦ τDi ⎥

fi 1+

t S 2τDi

(2)

Here, N is the average number of diffusing fluorescence species in the observation volume, f T and τT are the fraction and the decay time of the triplet state, τDi is the lateral diffusion time of the ith species, f i is the fraction of component i, and S is the socalled structure parameter, S = z0/r0, where z0 and r0 represent the axial and radial dimensions of the observation volume. Furthermore, the lateral diffusion time, τDi, is related to the respective diffusion coefficient, Di, through31 τDi = 426

r02 4Di

(3) DOI: 10.1021/acsmacrolett.8b00103 ACS Macro Lett. 2018, 7, 425−430

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ACS Macro Letters

We studied a large number of thin films with thicknesses in the range from 10 to 300 nm (see Figures S1 and S2 for further examples of autocorrelation curves) and found similar behavior. The results of the fitting in terms of diffusion times and fractions of the slow and the fast processes for all studied films are plotted versus d/Rg on Figure 3. Here d denotes the film thickness and Rg = 26.3 nm is the polymer radius of gyration.

As the dimensions of the observation volume are not known a priori they were determined from reference FCS measurements of TDI diffusion in toluene as described previously.42 The calibration yielded values of r0 = 0.2 μm and z0 = 1.4 μm. The experimental autocorrelation curves measured in thick PI films could be fitted well (solid line on Figure 1) with the model function for one type of freely diffusing fluorescent species (eq 2, m = 1). The fit yielded values of the lateral diffusion time τD = 39 ± 2 ms and the diffusion coefficient D ≈ 2.6 × 10−9 cm2/s for the TDI molecules in the PI matrix. These results are in good agreement with earlier studies42 that have demonstrated a single diffusion process with a similar diffusion coefficient for the same tracers in PI melts. Thus, we conclude that the polymer dynamics in the middle of few μm thick films has the usual bulk behavior. Next, we studied the tracer diffusion in thin PI films. A typical autocorrelation curve recorded in a 30 nm thick film is shown in Figure 2. The film thickness is significantly smaller

Figure 3. Diffusion times (a) and fractions (b) resulting from two components fitting of the FCS autocorrelation curves measured in PI films as a function of the film thickness (d). Rg is the radius of gyration of PI chains. Figure 2. Experimental FCS autocorrelation curve (symbols) measured in 30 nm thin PI film. The position of the observation volume is schematically shown in the inset. The lines in the upper panel represent the corresponding single component (red solid line) and two component (blue dashed line) fits with a 2D diffusion model function (eq 2, S → ∞). The lower panel shows the corresponding residuals.

The values of the slow diffusion time measured in the thin films agree with the value of the lateral diffusion time measured in the middle of the 10 μm thick film (also shown in Figure 3a for comparison). This indicates that in thin PI films a large fraction of the TDI tracers are diffusing through the FCS observation volume with the same diffusion coefficient as in bulk, that is, tracers experience bulk polymer mobility. However, a smaller fraction of tracers is able to diffuse through the FCS observation volume 5−10× faster, that is, they experience significantly enhanced polymer mobility. As such faster process is observed only in thin films, i.e. when the interfaces of the film are also probed by the FCS observation volume, we conclude that the enhanced polymer mobility is originating from the interface regions. Two interfacial regions are present: one near the free surface and another near the glass substrate. In previous studies, performed below the polymer glass temperature, the presence of a free surface was commonly correlated to enhancing the polymer segmental dynamics. On the other hand, the proximity of a solid substrate was considered to either speed-up or slowdown the dynamics, depending on the polymer−substrate interactions. In order to decouple the influence of the two interfaces on the polymer dynamics, we performed additional experiments with samples without free surface. To this end thin PI films “sandwiched” between two glass substrates were prepared by either (i) capping a thin film spin-coated on a glass substrate

than the extension of the observation volume in the normal direction as schematically illustrated in the inset of Figure 2. Thus, TDI diffusion in normal direction does not cause fluctuations of the measured fluorescence signal, because the tracers cannot leave the observation volume in this direction. Such fluctuations are caused only by the lateral diffusion of the TDI molecules through the observation volume. This situation is reminiscent to the one used to study 2D diffusion in cell and model lipid membranes48 or at solid/fluid35 or fluid/fluid49,50 interfaces. In such cases, a 2D diffusion analytical model function, that is, eq 2 with S → ∞, is used to represent experimentally measured autocorrelation curves.48 A single component 2D diffusion model function fit (eq 2 with m = 1, S → ∞) to the experimental FCS autocorrelation curve fails to represent well the experimental data, as evidenced from the structured residuals (Figure 2). In contrast, a two component fit (eq 2 with m = 2, S → ∞) represent the data well with reduced and uniform residuals. This fit yields two diffusion times: τD,slow ≈ 40 ± 3 ms and τD,fast ≈ 6 ± 0.5 ms. The corresponding fractions were fslow ≈ 75% and f fast ≈ 25%. 427

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ACS Macro Letters with another glass substrate or (ii) by spin-coating thin films on two individual substrates and pressing them against each other, as described in the SI. As these geometries are lacking a free surface one would expect a single bulk-like diffusion time under the premise that the fast diffusing process is originating from the free surface. A typical autocorrelation curve measured in single “sandwiched” film (i) with a thickness of ≈65 nm is shown in Figure 4 together with the corresponding single and

an assumption is not realistic, because the tracer molecules may also diffuse in the normal direction and thus experience a mobility gradient. There is no analytical model that can account for this effect and proper quantitative evaluation will require numerical approaches for generating tracer trajectories across the FCS observation volume and fitting of the experimental autocorrelation curves as in earlier studies.51 Such rigorous fitting is beyond the scope of this paper. Instead, we choose a simple two component model to qualitatively represent the existence of two types of average tracer trajectories: such that are mostly close to the free surface and thus probe enhanced mobility, and such that are mostly away from the free surface and thus probe the combined effects of bulk and possibly even slower, near substrate, segmental dynamics. Despite its simplicity, this two-component model represents well the underlying physics and even allows for a rough estimation of the mobility enhancement at the surface layer. Indeed, the diffusion time τD,fast of the fast component in the thin films (Figure 3) provides an estimate of the surfacelayer glass temperature. The diffusion time (about 5 ms) corresponds to a diffusion coefficient D ≈ 2 × 10−8 cm2/s. This value is slightly higher than the reported one42 for the same TDI tracer in bulk PI with molecular weight of 1.5 kDa. Thus, tracers diffusing in the surface layer of PI with Mw = 700 kDa experience a mobility at least as fast as the one they experience in a bulk PI with a molecular weight of Mw = 1.5 kDa. Since the glass temperature of a 1.5 kDa PI is Tg ≈ −80 °C42 our results indicate a decrease of the glass temperature in the surface layer of the PI films (with Mw = 700 kDa) by at least 20 °C, compared to the rest of the film, which maintains its bulk Tg (−60 °C). On the other hand, it is important to emphasize that our data do not allow estimation of the thickness of the enhanced mobility layer. The fraction of the fast diffusion process (Figure 3) should not be considered as the fraction of the film thickness near the free surface in which fast mobility is present. As discussed above, this simplified model is used only to represent the underlying physics but cannot provide a full quantitative description of the layer thickness. Finally, it is worth considering the fact that for very thin films (d/Rg < 1.2 in Figure 3) the diffusion time of the slow process increases to values larger than those measured in bulk PI. This may suggest that in such very thin films tracer diffusion is affected by the substrate as well. Thus, within the simple twocomponent model considered here, the slow component contains contributions from the bulk film and from a layer in the proximity of the solid substrate. In conclusion, we showed that due to its high sensitivity and small observation volume the fluorescence correlation spectroscopy technique can monitor the translational diffusion of small fluorescent tracers in thin polymer films for a broad range of film thicknesses and at temperatures far above the glass temperature. Because the diffusion of tracers is directly related to the local segmental dynamics of the polymer matrix, such studies can be used to address the issue of an enhanced mobility layer at the free surface of polymer films at temperatures well above the glass temperature. Using this approach, we provide the first experimental evidence for a mobile surface layer in a series of supported films of entangled polyisoprenes at temperatures as high as 80 °C above the glass temperature. At such temperatures, annealing effects play a smaller, if any, role and the polymer is closer to equilibrium.

Figure 4. Experimental FCS autocorrelation curve (symbols) measured in thin PI film sandwiched between two glass substrates as schematically shown in the inset. The lines in the upper panel represent the corresponding single component (red solid line) and two component (blue dashed line) fits with a 2D diffusion model function (eq 2, S → ∞). The lower panel shows the corresponding residuals.

two component fits with a 2D diffusion model. As can be seen, a single component fit (red line) represents the data very well. This fit yields a lateral diffusion time of τD = 43 ± 3 ms that is slightly longer than in bulk PI. Two component fitting (blue dashed line in Figure 4) could not represent the data better, even when the fast diffusion time was fixed to τD,fast ≈ 4 ms as obtained for a film with similar thickness on a single glass substrate. Furthermore, the fraction of the fast component, f fast, resulting from such fitting was less than 2% and τD,slow was 44 ± 3 ms. Similar behavior was also observed for the “sandwiched construct” formed by pressing against each other two thin films spin coated on individual substrates (Figure S3). These results unambiguously show that the fast diffusion process observed in the uncapped supported PI film films is related to the presence of a free surface. Thus, the presence of a fast diffusion process in thin supported films (Figure 3) is evidence for the existence of a layer of enhanced mobility near the free surface at temperatures 80 °C above the bulk glass temperature. It is tempting to use the data summarized in Figure 3 for a quantitative estimation of the degree of mobility enhancement and for estimating the thickness of the mobile surface layer. However, one should keep in mind that the two-component 2D diffusion model used for the fitting is a simplified approximation only and does not represent, quantitatively, the diffusion of the TDI molecules. For a rigorous application of this model one would have to assume that certain fraction of tracers diffuse laterally through the FCS observation volume (∼400 nm) without never leaving the surface layer in normal direction (inset in Figure 2). Such 428

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.8b00103. Experimental details and supporting scheme and figures (PDF).

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (K.K.). *E-mail: gfl[email protected] (G.F.). ORCID

Hans-Juergen Butt: 0000-0001-5391-2618 George Floudas: 0000-0003-4629-3817 Kaloian Koynov: 0000-0002-4062-8834 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank A. Best for technical assistance and K. Peneva and K. Müllen for the synthesis of the TDI dye. The financial support of the HFSP (RGP0013/2015) and DAAD (M.H.) is gratefully acknowledged.

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