Unexpected Molecular Weight Effect in Polymer Nanocomposites

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Unexpected Molecular Weight Effect in Polymer Nanocomposites

Shiwang Cheng, Adam P. Holt, Huiqun Wang, Fei Fan, Vera Bocharova, Halie Martin, Thusitha Etampawala, B. Tyler White, Tomonori Saito, Nam-Goo Kang, Mark D. Dadmun, Jimmy W. Mays, and Alexei P. Sokolov

Phys. Rev. Lett. 116, 038302 – Published 22 January 2016

Abstract

The properties of the interfacial layer between the polymer matrix and nanoparticles largely determine the macroscopic properties of polymer nanocomposites (PNCs). Although the static thickness of the interfacial layer was found to increase with the molecular weight (MW), the influence of MW on segmental relaxation and the glass transition in this layer remains to be explored. In this Letter, we show an unexpected MW dependence of the interfacial properties in PNC with attractive polymer-nanoparticle interactions: the thickness of the interfacial layer with hindered segmental relaxation decreases as MW increases, in sharp contrast to theoretical predictions. Further analyses reveal a reduction in mass density of the interfacial layer with increasing MW, which can elucidate these unexpected dynamic effects. Our observations call for a significant revision of the current understandings of PNCs and suggest interesting ways to tailor their properties.

Received 4 November 2015

DOI:https://doi.org/10.1103/PhysRevLett.116.038302

Physics Subject Headings (PhySH)

Interfaces Polymer-nanoparticle composites

Polymers & Soft Matter

Authors & Affiliations

Shiwang Cheng¹, Adam P. Holt², Huiqun Wang³, Fei Fan³, Vera Bocharova¹, Halie Martin³, Thusitha Etampawala³, B. Tyler White¹, Tomonori Saito¹, Nam-Goo Kang³, Mark D. Dadmun³, Jimmy W. Mays³, and Alexei P. Sokolov^1,2,3,*

¹Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA
²Department of Physics and Astronomy, University of Tennessee, Knoxville, Tennessee 37996, USA
³Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996, USA

^*Corresponding author. sokolov@utk.edu

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Vol. 116, Iss. 3 — 22 January 2016

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Images

Figure 1
(a) Comparison of dielectric loss spectra of neat PVAc40K (red circles) and PVAc40K-20% (blue squares) at $T = 373 K$ . The dashed lines are the fits to Havriliak-Negami functions for the $α$ process (dashed-red), $α^{'}$ process (dashed-blue), and the Maxwell-Wagner-Sillar polarization process (dashed-green). Solid lines present the total fit. (b) Relaxation time distribution for neat PVAc40K (red circles) and PVAc40K-20% (blue squares), where the $α^{'}$ process can be clearly identified.
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Figure 2
Normalized dielectric loss spectra $ϵ^{''} (ω)$ (a) and derivative spectra ${ϵ^{'}}_{der} (ω)$ (b) of PNCs with 3 selected MWs in comparison to the neat polymer PVAc85K spectrum at $T = 353 K$ . The spectra of the $α$ process in the studied neat polymers are identical, regardless of MW. (a) and (b) The dashed-red line, neat PVAc85K; blue line or squares, PVAc12K-20%; green line or diamonds, PVAc85K-20%; black line or triangles, PVAc253K-20%. (c) TMDSC of different PNCs and neat polymers. The $y$ axis is arbitrarily shifted for clarity.
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Figure 3
(a) The estimated $l_{int}$ by HMA (red circles) and HN fittings (blue squares). The $h_{loop}$ is calculated by SFT with different MWs. (b) The average interfacial slowing down, $τ_{α^{'}} / τ_{α}$ , for different MWs estimated from both the HMA and the HN fitting at $T = 353 K$ . The inset shows the relaxation time distribution spectra of $α$ and $α^{'}$ for different PNCs, where the dashed arrow indicates an increase in MW. (c) The relative average density of the matrix, ${\bar{ρ}}_{mat} / ρ_{neat}$ , for different PNCs at $T = 293 K$ . The filled diamond is reanalyzed data from Ref. [15] on PVAc167K-28.2% at 293 K and 10 MPa. The inset presents the mass density (error bars are within the symbols) of $PVAc 40 K / {SiO}_{2}$ PNCs with different loadings up to $φ_{NP} = 31.9 %$ . The black line is a prediction of the two phase mixing rule: $ρ_{PNC} = ρ_{NP} φ_{NP} + ρ_{PVAc}^{neat} (1 - φ_{NP})$ . A deviation occurs at $\sim 17 vol %$ , where $d_{IPS} \sim 12 nm$ is about $2 R_{g}$ of PVAc40K ( $R_{g} \sim 5.7 nm$ ).
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Figure 4
The interfacial chain packing in low and high MW PNCs. The reddish brown background in (i) and (iii) denotes the solvent molecules. (i) and (iii) represent the packing of the adsorbed chain (red) and free chains (green) in solution. The black dashed lines indicate the height of loops. (ii) and (iv) represent the chain packing of low and high MW PNCs after solvent evaporation. The black lines and orange lines indicate the height of the loops and the thickness of the DIL.
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