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SOFT MATTER

40 NIST CENTER FOR NEUTRON RESEARCH

T

he addition of nanoparticles (NPs) to a polymer matrix, 

forming a polymer nanocomposite (PNC), can significantly 

enhance the thermal, mechanical, and functional properties 

of the host matrix. As a result, PNCs are appealing materials for 

a variety of fields, industries, and applications [1]. The dynamics 

of polymer segments are particularly important because they 

significantly influence or dictate the processability, applications, 

glass transition temperature (Tg), and various macroscopic 

properties (such as creep, toughness, and transport) of the 

PNC. Despite decades of research on various PNC systems, 

additional fundamental studies are needed to explore the 

complex parameter space and understand the underlying physics 

of interfacial and confined polymer dynamics. In this work, we 

use quasi-elastic neutron scattering (QENS) and temperature 

modulated differential scanning calorimetry to present a 

systematic and detailed study of segmental dynamics in highly 

attractive polymer nanocomposites comprised of poly(2-vinyl 

pyridine) (P2VP) and colloidal silica (SiO2) NPs as a function of 

temperature, NP concentration, and P2VP molecular weight.

There are several methods that can be used to analyze 

segmental dynamics including nuclear magnetic resonance 

(NMR), dynamic mechanical analysis (DMA), broadband 

dielectric spectroscopy (BDS), temperature-modulated 

differential scanning calorimetry (TMDSC), neutron spin 

echo (NSE), and quasi-elastic neutron scattering (QENS) [2]. 

The approximate timescales associated with many of these 

measurement techniques, as they pertain to segmental 

dynamics, are schematically represented in Figure 1 with 

measurements of neat P2VP and a global fit to Vogel-FulcherTammann (VFT) temperature dependence. QENS, the focus of 

this paper, has the advantage of spanning short time scales 

(≈ 1 ns), providing simultaneous temporal and spatial information 

to capture the timescales and geometries of measured segmental 

motions, and measuring the fraction of relaxing segments 

through the elastic incoherent structure factor.

First, by monitoring the elastic scattering intensity in a fixed 

window scan (FWS) as a function of temperature, the meansquared displacement (‹x2

›) of P2VP segments was found to 

decrease for all T > Tg as a function of NP concentration (Figure 

2a). Similar results were observed by analysis of the QENS 

spectra for T >> Tg. Specifically, we observe the full-width at 

half max (FWHM) of quasi-elastic broadening increases linearly 

as a function of q2 for all NP concentrations, which is indicative 

of translational diffusive motion over these length and time 

scales (≈ 1 nm and ≈ 1 ns). The extracted diffusion coefficient 

of the alpha process (Da) obtained from this simple model 

(FWHM ~ Daq2) is plotted as a function of inverse temperature 

in Figure 2b, and the relaxation time for neat P2VP aligns 

with extrapolations from BDS and TMDSC measurements 

(Figure 1). However, as attractive SiO2 NPs are added to the 

polymer matrix, Da is slower than bulk by as much 80% at NP 

concentrations of ≈ 50 vol%, and despite the slowed timescale, 

the temperature dependence remains largely unchanged (Figure 

2b). A similar q-dependence of FWHM with a simultaneous 

slowing of mobility indicates that segmental dynamics in 

PNCs are perturbed much more temporally than spatially, i.e. 

the segmental relaxation process is slowed but segments relax 

in a similar manner. Interestingly, all polymer segments in the 

50 vol% PNC are in close proximity to a NP surface because 

the interparticle distance is ≈ 2.5 nm. Thus, our measurement 

of segmental relaxations shows that interfacial segments are 

dynamically active, albeit slowed, at these high temperatures. 

Segmental diffusion in attractive 

polymer nanocomposites: A quasi-elastic 

neutron scattering study

E. J. Bailey,1 P. J. Griffin,1 M. Tyagi,2,3 and K. I. Winey1

FIGURE 1: Primary segmental relaxation times (α-process) of bulk P2VP

measured via TMDSC (red circle), BDS (blue circle), and QENS (green circle). 

Approximate time scales for five techniques, and their corresponding 

temperatures, are depicted by shaded regions along the relaxation curve.

T

1 University of Pennsylvania, Philadelphia, PA 19104

2 NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, MD 20899

3 University of Maryland, College Park, MD 20742
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SOFT MATTER

2019 ACCOMPLISHMENTS AND OPPORTUNITIES 41

We next probe the effect of P2VP molecular weight on 

segmental dynamics in PNCs by studying P2VP/SiO2 (25 vol% 

SiO2) with unentangled to well entangled P2VP (10 kg/mol, 

40 kg.mol, 190 kg/mol). All bulk and PNC materials exhibit 

classic characteristics of translational segmental diffusion 

(FWHM ~ Daq2) that differ only in the associated timescale. 

In Figure 3, the extracted diffusion coefficient of the PNC 

is normalized to that of bulk to highlight the impact of 

molecular weight on segmental diffusion in PNCs. For all 

P2VP molecular weight, Da in the PNC is reduced relative to 

neat P2VP by a factor of ≈ 2, although a slight trend can be 

observed where NPs are more impactful in lower molecular 

weight P2VP. In contrast, a strong molecular weight dependence 

on segmental dynamics has been observed in P2VP/SiO2

PNCs using BDS and TMDSC [3]. However, not only do these 

techniques probe and sample segmental dynamics differently, 

QENS measurements are at much higher temperatures which 

may indicate that the interfacial structure and dynamics have a 

strong temperature dependence or may indicate an existence of 

an equilibrium structure that other techniques do not access [2].

In summary, we systematically study P2VP/SiO2 PNCs using 

QENS as a function of NP concentration, temperature, and 

P2VP molecular weight and compare to measurements from 

other techniques (e.g. TMDSC and BDS). We show that 

motions of P2VP segments on ≈ 1 nm length scales and ≈ 1 ns 

time scales are well described by classic translational diffusion, 

even at NP concentrations of ≈ 50 vol% where the average 

interparticle spacing is ≈ 2.5 nm. The average segmental 

diffusion coefficient decreases with increasing NP concentration 

by up to a factor of ≈ 5 and is nearly independent of temperature 

over the studied temperature range. In contrast to the welldocumented molecular weight dependence of segmental 

diffusion in the deeply supercooled regime, our measurements of 

the same dynamic process at higher temperatures show slowed 

segmental dynamics that are largely independent of matrix 

molecular weight [3]. Finally, by comparing TMDSC, BDS, and 

QENS, our results suggest that temperature has a significant 

impact on the NP-induced perturbation to segmental dynamics in 

PNCs and highlight the unique and complementary insights that 

can be provided by QENS.

References

[1] S. K. Kumar et al., Macromolecules 50, 714 (2017).

[2] C. G. Robertson, C. M. Roland, Rubber Chem. Technol. 

81(3), 506 (2008).

[3] S. Cheng et al., Phys. Rev. Lett. 116(3) 1 (2016).

<

x2> (Å2

) Dα (cm2/s)

T - T

g

bulk (K)

1000/T (K-1)

T

g

bulk

FIGURE 2: (a) Average segmental mean-squared displacement (<x2>) obtained 

from FWS of bulk 40 kg/mol P2VP and P2VP/SiO2 PNCs with concentrations of 

25 vol% and 50 vol%. (b) Extracted segmental diffusion coefficient (Da) as a 

function of inverse temperature for neat polymer and PNCs. 

Dα (PNC) / 

Dα (neat)

Molecular weight (kg/mol)

T ≈ Tg + 160 K

FIGURE 3: Diffusion coefficient of P2VP segments (Da) in PNC normalized to Da

in neat P2VP as a function of polymer molecular weight for 25 vol% SiO2 (blue) 

and 50 vol% SiO2 (red).