Exchange Lifetimes of the Bound Polymer Layer on Silica Nanoparticles

Letter Cite This: ACS Macro Lett. 2019, 8, 166−171

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Exchange Lifetimes of the Bound Polymer Layer on Silica Nanoparticles Andrew M. Jimenez,†,§ Dan Zhao,†,§ Kyle Misquitta,† Jacques Jestin,*,‡ and Sanat K. Kumar*,† †

Department of Chemical Engineering, Columbia University, New York, New York 10027, United States Laboratoire Léon Brillouin, CEA Saclay, 91191 Gif-sur-Yvette Cedex, France

‡

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S Supporting Information *

ABSTRACT: Understanding the structure and dynamics of the bound polymer layer (BL) that forms on favorably interacting nanoparticles (NPs) is critical to revealing the mechanisms responsible for material property enhancements in polymer nanocomposites (PNCs). Here we use small angle neutron scattering to probe the temporal persistence of this BL in the canonical case of poly(2-vinylpyridine) (P2VP) mixed with silica NPs at two representative temperatures. We have observed almost no long-term reorganization at 150 °C (∼Tg,P2VP + 50 °C), but a notable reduction in the BL thickness at 175 °C. We believe that this apparently strong temperature dependence arises from the polyvalency of the binding of a single P2VP chain to a NP. Thus, while the adsorption−desorption process of a single segment is an activated process that occurs over a broad temperature range, the cooperative nature of requiring multiple segments to desorb converts this into a process that occurs over a seemingly narrow temperature range.

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techniques such as TGA, which assume that the BL has a meltlike density, report BL thicknesses in the 1−5 nm range, while methods such as dynamic light scattering (DLS, which make no such assumption) report dimensions comparable to the chain radius of gyration. Small angle neutron scattering (SANS) has been used to characterize the BL,29,35,36 but not its temperature-dependent exchange behavior in a free polymer matrix with the same chemistry. Here we specifically highlight the BL by using contrast matching methods, so that the NPs (∼53 nm diameter silica, number averaged), the BL (partially deuterated P2VP, d3-P2VP, the three hydrogens on the polymer backbone are deuterated), and the matrix (protonated P2VP, or h-P2VP, with a glass transition temperature of ∼100 °C) are distinguishable (Figure 1). The polymer molecular weights and material scattering length densities (SLDs) are shown in Table 1. We anneal these samples for different times at two representative temperatures. We find practically no exchange between the BL and the matrix at long times for 150 °C, but substantial chain exchange at 175 °C, even over tens of hours. We therefore suggest that this exchange process has a relatively sharp temperature dependence. (We do not perform experiments at much higher annealing temperatures due to concerns with polymer degradation.) We conjecture that the appearance of this relatively strong temperature dependence is probably

or decades it has been postulated that a layer of immobilized polymer forms between a polymer matrix and an attractive surface (both flat substrates and dispersed nanoparticles, NPs).1−19 This “bound layer” (BL) is thought to bridge the favorable mechanical properties of inorganic fillers with the surrounding polymer in polymer nanocomposites (PNCs)15,20−23 and yield reinforcement well beyond that expected by the Guth-Gold model.24 Thus, NPs can increase the PNC modulus by almost an order of magnitude, even at 10 vol % loading.25 It is common practice in the nanocomposite community to treat this BL as effectively immobile and temporally persistent.26,27 In a slightly different context, Granick et al. have however demonstrated a temperature-dependent exchange kinetics for adsorbed BLs at a flat surface.28 The question then is whether commonly termed “frozen” BLs on NPs can show such exchange behavior and, if so, under what conditions this occurs. The current understanding of BLs is that the adsorbed polymers assume the form of “trains”, “loops”, and “tails”, making up two regions: the tightly adsorbed, flat region in the immediate vicinity of the NP and the outer swollen tails that mix with the surrounding medium.29,30 Attempts to measure the effective thickness of the BL include dynamic scattering,29,31 thermogravimetric analysis (TGA),31 calorimetry,12,20,21 dielectric spectroscopy,32−34 small angle scattering,15 positron annihilation lifetime spectroscopy,12 and electron microscopy.31 Many of these studies employ different indirect assumptions to infer the effective BL length scale. Thus, © XXXX American Chemical Society

Received: November 12, 2018 Accepted: January 14, 2019

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DOI: 10.1021/acsmacrolett.8b00877 ACS Macro Lett. 2019, 8, 166−171

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ACS Macro Letters due to polyvalency (i.e., the adsorption of multiple monomers from a given chain onto the NP surface), which makes this process very cooperative (and, hence, behave akin to an on−off switch).

Figure 1. Schematic of the system studied. The silica core (black) has a well-defined BL (blue, d3-P2VP) and is well-dispersed in a polymer matrix (orange, h-P2VP). In TEM or SAXS, the BL (dark orange) is nearly indistinguishable from the matrix and, thus, the silica core is highlighted, whereas in SANS, the silica core with the partially deuterated BL (gray) effectively becomes an enlarged NP due to the similar SLDs between silica and d3-P2VP.

Table 1. Physical Characteristics of Polymer and NP silica h-P2VP d3-P2VP

Mw (kDa), Đ

neutron SLD (1 × 10−6 Å−2)

DH (nm)

100, 1.55 107, 1.23

3.47 1.87 3.73

53 10 10

To achieve the SANS contrast matching, composites of silica NPs with d3-P2VP BLs and h-P2VP matrices were prepared following a known protocol to provide good NP dispersion (Supporting Information).12 Briefly, we first mixed d3-P2VP and silica NPs in a cosolvent. The particle/polymer dispersions were then cast into films, dried and annealed at 150 °C for 5 days to realize the full formation of bound polymer layers. (The polymers adsorb to the NP during the casting process, but this procedure is known to depend on the casting solvent used. We chose this melt annealing protocol, used previously, to provide a reproducible, well-defined starting point.31,37,38) Afterward, the composites were washed multiple times to remove unbound polymers. TGA reveals a significant decrease of polymer for the first two washes, but little change after subsequent washes (Figure 2A). We determined the BL thicknesses by assuming a polymer with a melt-like density31 and found the effective BL size to be ∼30 Å, similar to that seen in previous work under similar assumptions. The silica NPs with the d3-P2VP BLs were collected via centrifugation and solution mixed with h-P2VP, then cast into nanocomposite films for SANS experiments. DLS in solution (Figure 2B) shows that the bare NP has a number average hydrodynamic diameter, DH,NP ∼ 53 nm. Upon casting with polymer, annealing, and redissolving, one observes a hydrodynamic diameter of ∼75 nm, roughly corresponding to DH,NP + 2RH,p (RH,p is the polymer’s hydrodynamic radius), independent of the number of washes. This is consistent with previous work, which showed that DLS reports the “real” extent of the bound layer in solution (proportional to the chain dimension). Based on the TGA and DLS results, we conclude that a BL of d3-P2VP was formed on the NP, with complete removal of unbound, free polymer chains on washing.

Figure 2. (A) Weight percent of silica remaining, from TGA, of as cast system (black) and after subsequent washings. Inset: Representative TGA curves for each corresponding washing, renormalized at 250 °C to account for initial solvent removal. (B) DLS number average size distributions of h-P2VP, NPs, and those with bound polymer chains throughout the washing process, in methyl ethyl ketone (MEK) with 3% pyridine.

Transmission electron microscopy (TEM) and small angle X-ray scattering (SAXS) show that the NPs are well-dispersed under all conditions (e.g., Figures 3A and 3B). The scattering data were fit to a polydisperse sphere form factor with a lognormal distribution, yielding the radius (R = 25 nm) and polydispersity (σ = 0.27) of the NPs in both the dilute NP solution and the low loading PNC sample (4 wt % silica); these numbers are consistent with manufacturer specifications. At loadings larger than 8 wt %, a Percus−Yevick structure factor was needed to account for the interparticle scattering. This indicates that the silica NPs are well-dispersed in the casting process, during which the bound layer forms. Despite attempts to maintain this good dispersion, the process of washing away the free polymer followed by recasting the NPs with d3-P2VP BLs in an h-P2VP matrix (at ∼1−3 wt % NP) caused minor agglomeration. Although no significant change is obvious in TEM, SAXS fits on the recast samples require minor adjustments in the R and σ values (Supporting Information, Figure S1). These structural parameters are used for subsequent SANS-based analysis. 167

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Figure 4. (A) SANS data for select samples prior to annealing (closed symbols) and after annealing (open symbols), with their corresponding fits in solid and dashed lines, respectively. Note that this is a zoomed in scale with regard to typical scattering q ranges in order to highlight the pertinent fitting region, and that the error bars in intensity are smaller than the size of the symbols. (B) SANS curve for an annealed sample showing the maximum q range probed (on a separate beamline) to demonstrate the consistent core−shell fit (dashed black line). (C) The same two representative samples in (A) are plotted on a linear-log scale, arbitrarily shifted, to demonstrate the significant drops in the low q intensity.

Figure 3. (A) SAXS of silica NPs in dilute MEK solution (gray, open symbols) and in h-P2VP matrices at loadings of 4, 8, 12, and 20 wt % (orange). Solid black lines are best fits. (B) TEM image of 8 wt % silica in h-P2VP (scale bar is 2 μm).

After recasting the d3-P2VP bound silica NPs into the hP2VP matrix, subsequent high temperature (150 and 175 °C) ex situ annealing in a vacuum oven allowed for thermal rearrangement well above the glass transition temperature of P2VP (Tg ∼ 100 °C). The samples were then measured with SANS after various time intervals. Since the SLD of the d3P2VP BL is close to that of the silica, SANS initially reports an apparently larger NP (Figure 1 and Table 1). By monitoring the change in size of this “enlarged NP”, we infer the change in the BL thickness. Figure 4 shows representative SANS data of the d3-P2VP coated silica in an h-P2VP matrix before and after annealing. Upon thermal annealing, the scattering intensity drops up to 40% in the low q (scattering vector) regime, presumably due to the exchange of the bound d3-P2VP with the matrix h-P2VP. Note that the decrease in the scattering intensity is more significant at 175 °C compared to 150 °C. To model the data, a polydisperse core−shell form factor with a rigid shell of thickness δ (i.e., only consisting of the bound d3-P2VP chains at a melt-like density) was used. With the radius and polydispersity of the silica core fixed from the SAXS fits, the only parameter that is left for the SANS fittings is δ. Note that due to the potential changes in the sample thicknesses during annealing, it was difficult to place the SANS data on an

absolute scale. Instead, the scattering curves were shifted so that they superimpose at high q, where there is little effect from the change in the shell thickness (Supporting Information, Figures S2−S4). We start from the preannealed sample, for which we assume all d3-P2VP chains were bound onto the silica particle surfaces. This yields a BL thickness of ∼30 Å, which is consistent with that obtained from TGA under the same assumption of a dense BL. Upon annealing, some bound d3-P2VP chains will desorb and exchange with the matrix hP2VP, resulting in changes in the compositions and thereby the contrast of the BL. (The low volume fraction of initially bound polymer produces essentially no changes in the SLD of the matrix even if it were to fully mix with the matrix.) More details about the fitting process are provided in Figure S5 (Supporting Information). Figure 5 shows the BL thickness, δ, as a function of annealing time at two representative temperatures. Each data point represents an average over several samples (Supporting Information, Figure S6). At 150 °C there is a relatively rapid (∼5 h) change in δ from its initial value of 30 Å to roughly 21 Å, beyond which it remains unchanged even after weeks of annealing. In contrast, at 175 °C this decrease continues until 168

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combination of these results establish that the BL is temporally persistent for low annealing temperatures, but that for higher temperatures it is dynamically active and can substantially exchange with the bulk polymer. We believe that this relatively strong temperature dependence arises from the polyvalency of the binding of a P2VP chain to a NP, i.e., due to the fact that each P2VP chain is adsorbed through multiple monomers. Thus, while the adsorption−desorption process of a single segment is an activated process that occurs over a broad temperature range, the cooperative nature of requiring multiple segments to desorb (to exchange BL chains with the bulk) converts this into a sharp process. This result is analogous to the binding of NPs grafted with multiple DNA strands to NPs with complementary single-stranded DNA chains, while the base-pairing transition of a single chain occurs over a 30−40 °C temperature range, the cooperativity associated with multiple chains binding sharpens this “transition” to occur over 5 °C.42 In conclusion, we have found that the BL itself is not a frozen object, but rather that it is prone to desorption; based on previous results, we expect that the BL would further desorb with small increases in the annealing temperature. However, these temperatures remain currently inaccessible due to concerns about thermal degradation upon long time annealing.

Figure 5. Average shell thickness, δ, vs annealing time at 150 °C (green) and 175 °C (red), starting from an average preannealed thickness of ∼30 Å (black). Green and red dashed lines are drawn as guides to the eye.

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the BL thickness drops to 6 Å, beyond which it appears to plateau as well. While the fits for the BL thickness could easily vary within a range of ±5 Å, depending on the definition of the “best fit” (Supporting Information, Figure S5), there is a clear trend of decreasing shell thickness with increasing time. Based on these data, there appears to be two processes for chain exchange. A relatively rapid, apparently temperature-independent process, occurring within a few hours. However, after that, the behavior of the two temperatures is very different. This is the major result of this work, and we shall discuss its consequences below. We postulate that the reduction in BL thickness is due to a combination of two mechanisms that occur simultaneously. Since the BL should have both tightly bound “trains” and the loosely bound “loops and tails,” we suspect that the polymer chains with few surface contacts are able to desorb from the surface relatively quickly at both temperatures. At the lower temperature the more tightly bound chains, with significantly reduced mobility relative to the bulk, have no practical chance of fully desorbing due to the high energy of adsorption from multiple interaction points between it and the NP surface.39 Meanwhile, at 175 °C the data imply that a large fraction of these strongly bound chains desorb. This suggests that there are surface bound chains with differently adsorbed populations of monomers, whereby 150 °C annealing reduces the volume of initially bound chains by 30%, while it is reduced by 80% at 175 °C. The desorbed fraction at the lower temperature is comparable to that seen by Koga et al. on flat surfaces where “loosely bound chains” desorb at fractions of 10−25%, depending on the affinity of the polymer to the surface, for solvent leaching processes.40 Our major conclusion is that exchange kinetics of commonly referenced “irreversibly” bound polymers appear at high enough annealing temperatures. There is essentially no longterm reorganization of the BL at 150 °C, but an obvious reduction of the BL thickness at 175 °C. This idea is further supported by Koga et al. with their nanocalorimetric demonstration of a significantly higher secondary Tg (∼205 °C) for P2VP adsorbed on a flat silica surface.41 The

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.8b00877. Detailed sample preparation protocols, SAXS of h-P2VP washing/redispersion, relative SANS intensity data, SANS modeling, SANS fitting details, and BL thicknesses for all samples prior to averaging (PDF).

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Andrew M. Jimenez: 0000-0001-7696-9705 Jacques Jestin: 0000-0001-7338-7021 Sanat K. Kumar: 0000-0002-6690-2221 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Author Contributions §

These authors contributed equally to the work as cofirst authors. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Science Foundation DMR-1629502. We thank NYULMC DART Microscopy Lab for assistance in TEM. We acknowledge the support of the National Institute of Standards and Technology, U.S. Department of Commerce, as well as Ralf Schweins and David Bowyer on D11 (ILL), and Arnaud Hélary for PACE (LLB) in providing the neutron research facilities used in this work. This 169

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research used resources of the Center for Functional Nanomaterials, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Brookhaven National Laboratory under Contract No. 35884.

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