Polymer Diffusion from Attractive and Athermal Substrates

Mar 22, 2017 - Interestingly, the dPS tracer with an intermediate molecular weight (M = 168 kDa; heq = 8.6 nm) exhibits a bimodal profile (Figure 2b) ...
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Polymer Diffusion from Attractive and Athermal Substrates Jihoon Choi,† Nigel Clarke,‡ Karen I. Winey,§ and Russell J. Composto*,§ †

Department of Materials Science and Engineering, Chungnam National University Daejeon, South Korea Department of Physics and Astronomy, University of Sheffield, Sheffield, United Kingdom § Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States ‡

ABSTRACT: Given the exceedingly high interfacial area-to-volume ratios in polymer nanocomposites and the ability to manipulate the polymer/nanoparticle interfacial interactions, manipulating the chain dynamics at these interfaces has immense potential for impacting macroscopic properties. There, the polymer center-of-mass tracer diffusion coefficient (D) from attractive (hydroxylterminated) and athermal (phenyl-terminated or polymer-grafted) substrates was measured over a range of temperatures and tracer molecular weights using elastic recoil detection. The tracer polymer diffusion slows significantly relative to the bulk when polymers are in direct contact with an attractive substrate and exhibits a weaker molecular weight dependence, D ∼ M−1.4. For polymers without direct contacts on the attractive substrates and for athermal substrates, the diffusion coefficients are similar to the bulk case. The temperature dependence of these diffusion coefficients indicates that the slower diffusion at the interfaces is coupled to differences in polymer conformation and smaller fractional free volumes. These deviations from bulk are more pronounced for higher molecular weights and more attractive interfaces.

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INTRODUCTION The properties of polymer nanocomposites (PNCs) such as thermomechanical strength or NP dispersion depend on the local interactions (i.e., chemical and physical) between polymer chains and the surface of the nanoparticle (NP).1−3 Enhanced physical properties can be achieved by attractive polymer/ nanoparticle interactions resulting in the formation of a bound polymer layer.4−7 Recent experimental and theoretical efforts focused on the presence of bound polymer chains as well as their molecular architecture.8−10 In particular, Gin et al. showed that bound layers are composed of inner flattened chains and outer immobile chains that are only loosely adsorbed at the surface.11 Moreover, the desorption kinetics showed that the equilibrium thickness (heq) of the inner flattened layer is smaller than the radius of gyration (Rg) of unperturbed polymer chains.11 The interactions between nanoparticles and polymer can also affect polymer dynamics because the local relaxation of chains can be hindered by the adsorption of polymer segments onto NP surfaces.10,12 When the NP−segmental attraction is several times stronger than thermal energy (kBT), the formation of an irreversibly bound layer on the NP surface leads to perturbations from Gaussian chain conformations due to the formation of trains, loops, and tails.13,14 Hence, polymer dynamics depend on the topology of adsorbed chains when the polymer desorption energy is proportional to the number of segment−surface contacts (i.e., trains on particle). For example, Zheng et al. reported that polystyrene (PS) in contact with silicon oxide diffuses ∼100 times slower than in the bulk, consistent with adsorption on an attractive substrate.15 In © XXXX American Chemical Society

addition, when poly(2-vinylpyridine) strongly adsorbs to silica nanoparticles, the nanoparticles diffuse slower in a manner that can be quantified by accounting for their larger effective size.16 Understanding the impact of polymer/nanoparticle interfacial interactions is critically important for controlling the properties of polymer nanocomposite (viscosity, toughness, etc.). In this article we systematically study polystyrene (PS) diffusion, including temperature dependence, from attractive and athermal substrates. Silicon substrates are terminated with hydroxyl groups, functionalized with phenyl groups, or grafted with long PS brushes. Conformational differences between the chains adjacent to the substrate and chains slightly further from the substrate (∼2Rg) may impact polymer diffusion. These experimental studies provide new insights that relate the molecular conformations of substrate-bound polymer chains to the diffusive motion of polymer chains near rigid interfaces and have important implications for polymer dynamics in polymer nanocomposites.

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EXPERIMENTAL SECTION

Bilayer samples (Figure 1a) were prepared with a thick top layer of PS (hPS; M = 650 kDa; 700 nm) and a thin bottom layer of dPS (M = 23, 49, 168, 532, and 1866 kDa; ∼13 nm) adjacent to the hydroxyl (−OH), phenyl, and PS-brush substrates. Gin et al. showed that the equilibrium thickness (heq) of an adsorbed polymer layer is proportional to Rg (∼0.78Rg), indicating that the bound layer Received: January 15, 2017 Revised: March 3, 2017

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DOI: 10.1021/acs.macromol.7b00086 Macromolecules XXXX, XXX, XXX−XXX

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dPS/hPS/substrate). Figure 1e compares the dPS depth profiles from an OH-terminated substrate and free surface. Under the same annealing condition, dPS (1866 kDa) diffused much further in the bulk (black) than from the OH-terminated substrate. This pronounced slowing down of dPS diffusion from the OH-terminated substrate is consistent with the observation by Zheng et al. in 1995,15 although it is important to note that their PS (M = ∼700 kDa) tracer film is only ∼5 nm, which is ∼4 times thinner than the unperturbed Rg (22.4 nm) and thereby consists solely of adsorbed chains. Next we explore a range of dPS molecular weights at a fixed dPS thickness, which systematically varies the fraction of adsorbed dPS. Attractive monomer−substrate contacts introduce additional friction and slow dPS segments adjacent to the substrate relative to the bulk. Figure 2a shows the tracer diffusion coefficients for M values from 23 to 1866 kDa. For the two highest molecular weights (heq532K = 15.6 nm and heq1866K = 28.9 nm), the dPS chains in the tracer film (∼13 nm) are

Figure 1. Schematic drawings of the (a) bilayer (thick hPS layer of M = 650 kDa, over a thin dPS layer with M = 23, 49, 168, 532, and 1866 kDa) deposited on silicon substrates with hydroxyl groups, phenyl groups, or PS brushes, as shown in (b−d), respectively. (e) Depth profile of dPS (M = 1866 kDa) from an OH-terminated substrate (red) and bulk (black) after annealing at 170 °C for 48 h. Inset shows the ERD geometry from (a) with incident 3 MeV He+ impinging on the bilayer and the subsequent recoiling deuterium coming from different depths below the surface. Bulk tracer diffusion profiles employed the conventional sample geometry with a thin layer of dPS on top of hPS. Solid lines are fit to the experimental profiles using Fick’s second law to determine the tracer diffusion coefficients of dPS. thickness increases as M1/2.11 By investigating tracer films of various molecular weights and a fixed initial dPS film thicknesses, which varies from thinner than to thicker than heq, we systematically control the ratio of adsorbed to free chains within the tracer dPS film. To prepare OH-terminated substrates, silicon (Si) wafers were exposed to UVozone for 20 min to create silanol groups (Figure 1b). Phenyl-capped Si substrates (Figure 1c) were prepared by spin-coating chloro(dimethyl)phenylsilane (Sigma-Aldrich, 98%) on OH-terminated substrates. PS brushes with a grafting density (ρ) of 0.152 chains/ nm2 and molecular weight (M) of 37 kDa were grafted from silicon using surface-initiated atom transfer radical polymerization (SI-ATRP) (Figure 1d). These surfaces were characterized by contact angle measurements. The hPS films were spin-coated on silicon substrates, floated off on water, and then picked up by a dPS film on a silicon substrate. The thickness of each polymer layer was measured by ellipsometry (Auto EL II, Rudolph Technologies).

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RESULTS AND DISCUSSION Bilayers were annealed to allow the tracer (dPS) to diffuse up into the matrix (hPS) layer in a vacuum oven for times ranging from a few minutes to hours. Tracer diffusion coefficients (D) of deuterated polystyrene (dPS) near the substrates were determined by fitting the volume fraction profiles measured by elastic recoil detection (ERD) following standard procedures (Figure 1e).17,18 To investigate the temperature dependence of polymer diffusion, five annealing temperatures (150, 160, 170, 180, and 190 °C) were studied. For comparison, dPS tracer diffusion in the bulk was also measured using the same molecular weights, and the conventional geometry where a thin film of dPS was deposited on top of a thick matrix of hPS (i.e.,

Figure 2. (a) Tracer diffusion coefficients of dPS with an initial thickness (ddPS) of 13 nm for M = 23, 49, 168, 532, and 1866 kDa at 170 °C into a hPS matrix (700 nm). Open and closed circles denote bulk diffusion and diffusion from an attractive OH-terminated substrate, respectively. Inset shows how the relative amounts of bound (bold) and free chains depend on the dPS molecular weight at a fixed tracer film thickness, ddPS; matrix PS is not shown in schematics. (b) Depth profile of dPS (M = 168 kDa) in hPS fit with two diffusion coefficients: D = 1.1 × 10−14 cm s−2 corresponds to bound chains (bold line); D = 1.3 × 10−13 cm s−2, which is comparable to the bulk diffusion coefficient, corresponds to dPS chains further from the attractive substrate (thinner black line). The error bars are smaller than the symbol size in all cases. B

DOI: 10.1021/acs.macromol.7b00086 Macromolecules XXXX, XXX, XXX−XXX

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terminated (triangles) and PS-brush substrates (inverted triangles) are very similar for all molecular weights and scales as M−2.1. Also, the dPS concentration profiles for these athermal substrates were readily fit with a single diffusion coefficient. Note that dPS diffusion for the highest molecular weights (532 and 1866 kDa) is somewhat slower than in the bulk case. However, this reduction in diffusion is weaker than for the attractive OH−substrate case (Figure 2a), indicating that the phenyl groups and grafted PS chains significantly limit the adsorption of dPS chains on the silicon substrate. Interestingly, and in contrast to an earlier study by Zheng et al.,19 long-range effects between the diffusing polymer chains and the substrate were not observed for the two athermal cases studied here. Our results clearly demonstrate that direct monomer− substrate contacts slow polymer diffusion for high molecular weight polymers near both attractive and athermal substrates. This has important implications for polymer diffusion in polymer nanocomposites. As previously reported, dPS tracer diffusion in athermal polymer nanocomposites (i.e., a mixture of PS and phenyl-capped silica nanoparticles) is slow compared to the bulk diffusion.20 When the interparticle distance (ID) is less than 2Rg (i.e., confined polymer), this reduction is particularly significant with up to a 10-fold decrease in D. A more recent study by Tung et al. on temperature-dependent polymer diffusion in these nanocomposites suggests that the configurational entropy barriers imposed by nanoparticles are key to understanding polymer diffusion.21 Thus, our tracer diffusion experiments confirm that the enthalpic contribution to the excess free energy barrier is negligible for the polymer nanocomposites containing the phenyl-capped silica NPs (i.e., athermal interactions) (see Figure 3). Nonetheless, as the NP concentration and the tracer molecular weight increase, the polymers become highly confined polymers, the number of monomer−nanoparticle interactions per polymer increases, and chain diffusion requires simultaneous desorption of polymer chains from several neighboring NPs, which hinders diffusion. This may contribute to the strongly reduced polymer diffusion for ID/2Rg < 1 observed in polymer composites.12,17,20,22 By studying the temperature dependence of diffusion, the role of free volume and chain organization in the bound layer can be investigated. Using neutron reflectivity, Gin et al. reported that polymer chains adsorbed on an attractive substrate form an inner layer of flattened chain conformations that have a higher density than bulk polymer.11 To understand the relation between chain conformations adjacent to a substrate and in the bulk, the temperature dependence of the dPS diffusion coefficient, D, was measured in the bulk and from phenyl- and OH-terminated substrates (Figures 4a−c). This temperature dependence of D is related to the free volume and thermal expansion coefficient, f = α(T − T∞), where f is the fractional free volume, α is the thermal expansion coefficient of free volume, and T∞ is the Vogel temperature. Using the Williams−Landel−Ferry (WLF) relationship, D/T is given by

mostly in contact with the substrate and the measured tracer diffusion coefficient is lower than the bulk case (open symbols) (Figure 1e). In contrast, for the two lowest tracer molecular weights the tracer film is much thicker than the adsorbed layers (heq23K ∼ 3.1 nm and heq49K ∼ 4.7 nm), and thus most of dPS chains are not expected to be in direct contact with the substrate and can freely diffuse. Thus, the diffusion coefficients are nearly identical for the bulk and near OH-terminated substrates for M = 23 and 49 kDa. Interestingly, the dPS tracer with an intermediate molecular weight (M = 168 kDa; heq = 8.6 nm) exhibits a bimodal profile (Figure 2b) of slower (bold black line) and faster (thinner black line) diffusing chains. We attribute this behavior to the 13 nm dPS film having both flattened chain conformations in direct contact with the substrate and loosely adsorbed chains that are freer to diffuse into the PS matrix. For M = 168 kDa, the higher D for the faster or freely diffusing species (free) is in good agreement with the diffusion coefficient for the bulk. Figure 2a demonstrates that the bound equilibrium thickness (heq) relative to the tracer film thickness determines the partitioning of the slower (bound) and faster (free) diffusing species. The molecular weight dependence of the tracer diffusion coefficient is D ∼ M−2.0 for the bulk and D ∼ M−1.9 for the 13 nm dPS film at the substrate when M ≤ 168 kDa (Figure 2a). These data are consistent with simple reptation (D ∼ M−2.0). The molecular weight dependence of D when diffusion is slowed by the attractive substrate (M ≥ 168 kDa) is D ∼ M−1.4. This weaker dependence of M is consistent with a modified reptation model15 that predicts −1.5 scaling because friction per chain results from both bulk monomer−monomer friction (∼f 0M) and monomer−substrate friction (∼f M0.5). Thus, these studies show that direct segment−substrate contact plays a crucial role in determining polymer diffusion at an attractive substrate. To evaluate the importance of polymer−substrate attractions, dPS tracer diffusion was measured from two nonattractive substrates, namely silicon modified with phenyl groups (Figure 1c) and PS brushes (Figure 1d). Figure 3 shows that the dPS tracer diffusion coefficients from the phenyl-

log

D 1 = C1′ − C2 T T − T∞

(1) 21

where C1 and C2 are empirical constants. In eq 1, C2 is proportional to fractional free volume, ∼f/α, and can be obtained by fitting D/T data in Figure 4a−c. In the bulk C2 decreases slightly with increasing M, and these values are similar to that determined from viscosity (C2 = 710 K).23 For the athermal and attractive substrates, C2 decreases significantly as M increases. The decrease is more pronounced for the OH-

Figure 3. Tracer diffusion coefficients of dPS (M = 23, 49, 168, 532, and 1866 kDa) from phenyl-terminated silica (triangles) and PSgrafted silicon (inverse triangles) at 170 °C. Circles denote the bulk diffusion coefficients. Insets: ddPS is the original dPS tracer film thickness (13 nm), and bold curved lines are bound molecules when M = 532 or 1866 kDa and heq > ddPS. C

DOI: 10.1021/acs.macromol.7b00086 Macromolecules XXXX, XXX, XXX−XXX

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important implications for polymer nanocomposites with attractive polymer/nanoparticle interactions.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (R.J.C.). ORCID

Jihoon Choi: 0000-0003-2162-8895 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was primarily supported by the National Science Foundation NSF/EPSRC Materials World Network DMR1210379 (R.J.C., K.I.W.), the EPSRC EP/5065373/1 (N.C.), and NSF Polymer Programs DMR15-07713 (R.J.C.). Support was also provided by the NSF/MRSEC-DMR 11-20901 (K.I.W., R.J.C.), and Dupont Central Research and Development (R.J.C.) as well as by the National Research Foundation of Korea (NRF) funded by the Korean government (No. NRF2015R1C1A1A01052865).

Figure 4. Temperature dependence (D/T) of tracer diffusion for dPS with M = 168 (squares), 532 (circles), and 1866 kDa (triangles) diffusing (a) in bulk, (b) from phenyl-terminated substrates, and (c) from OH-terminated substrates as a function of 1/(T − T∞) using T∞ = 322 K.23 (d) Molecular weight dependence of C2 is determined by a linear fitting of the data in (a−c) using the WLF equation.24

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REFERENCES

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terminated substrates, which is consistent with the adsorption of polymer chains on an attractive substrate that increases density and decreases the fractional free volume (and C2 ∼ f). This argument assumes that the thermal expansion coefficient is nearly constant. For 532 and 1866 kDa, the values of C2 for the phenyl-terminated substrate are between the values for the bulk and OH-terminated substrates. This suggests that the phenyl groups reduce the number of segment−substrate contacts, and as a result the interfacial layer is more weakly bound to the substrate than in the presence of OH-terminated substrates.

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SUMMARY In summary, we have measured the diffusion coefficients of dPS tracer chains (M = 23−1866 kDa) diffusing from attractive (OH-terminated) and athermal (phenyl-terminated or PSbrush grafted) substrates. Polymer diffusion from the attractive substrate is partitioned into an inner bound layer and an outer layer with weakly adsorbed chains, such that when these populations are present in nominally equal amounts, two diffusion coefficients are evident. Strongly adsorbed polymer chains attributed to the direct contact with the attractive substrate exhibit significant slowing down of polymer diffusion at the interface compared to the bulk case, whereas the diffusion of loosely adsorbed chains is indistinguishable from the bulk. We have also shown that athermal substrates limit the adsorption of polymer chains forming inner bound layer, such that polymer diffusion at the interface is closer to that of the bulk. The temperature dependence of the tracer diffusion coefficient was used to attribute the decrease in diffusion coefficient at the interface to a reduction in fractional free volume within the bound layers. The molecular weight and tempearture dependencies of dPS diffusion from attractive and athermal substrates provide new insights into how adsorbed polymer chains affect the longest relaxation times within D

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DOI: 10.1021/acs.macromol.7b00086 Macromolecules XXXX, XXX, XXX−XXX