C

Co (d) 7

Zn 6 Mn 7

Mn Zn

Zn

Mn

 Co8

Zn 3 Mn 9

5

9

13

17

21

25

6 5 4 3 2 1 0

0

1

2

3

4

20

15

10

5

0

0

ρ nSLD solvent 2, n

2

 (10-6 ) -2 Å

nSLD solvent 1, 

ρ

n

1 (10

Information gain, 

Δ

SOFT MATTER

44 NIST CENTER FOR NEUTRON RESEARCH

Towards architecturally designed 

all-polymer based nanocomposites 

E. Senses,1 M. Tyagi,2,3 and A. Faraone2

O

ver the last few decades, advances in nanoparticle 

synthesis with precisely controlled size and shape have 

transformed the way we use polymers today. Dispersing 

these inorganic rigid nanoparticles in soft polymeric matrices 

resulted in new hybrid materials- polymer nanocompositeswith superior properties compared to those of the neat 

polymers [1]. Likewise, polymer chemists can now synthesize 

macromolecules with precise length and shape; stars, combs, 

bottlebrushes, rings, hyper-branched polymers have emerged 

with exciting new properties that cannot be provided by 

their linear chain analogs. In particular, the star-shaped 

polymers in which many linear chains share a common 

center are interesting as the monomer density changes along 

the radial direction. Therefore, these polymers can exhibit 

both soft/penetrable and hard/impenetrable sphere character 

depending on the length and number of the star arms (see 

schematic in Figure 1a). 

In our work [2], we dispersed high-Tg (≈ 373 K) star-shaped 

polystyrene (PS) at 20 % by mass in low-Tg (≈ 250 K) linear 

long poly (vinyl methyl ether) (PVME) matrices to obtain a new 

kind of nanocomposite in which the interpenetration between 

the nanofillers and the matrix, and therefore, the bulk 

rheological properties, can be systematically varied via filler 

architecture. As the filler structure transitions from flexible 

linear chains to hard spheres, the resulting nanocomposites 

reside between the traditional linear polymer blends and 

conventional polymer nanocomposites- a region that has 

not been explored before. Our aim was to investigate these 

nanocomposites using static (SANS) and quasielastic neutron 

scattering (backscattering and spin-echo) measurements over 

a wide range of time and length scales in the glassy and melt 

states of the composites, and relate their bulk flow behavior 

to the microscopic relaxation mechanisms. 

We used deuterated PS (d-PS) of various arm number and 

molecular weights to determine their conformation and 

dispersion in hydrogenated 20 kg/mol PVME (h-PVME) matrix 

by using small-angle neutron scattering (SANS). The coherent 

intensity results from the contrast between d- and h-chains, 

therefore, the stars are visible to neutrons. Figure 1b shows a 

peak appearing at intermediate wave-vectors, Q, in the SANS 

pattern of the 18-arms star shaped d-PS filler in h-PVME due 

to intra-chain correlation, confirming their compact particlelike nature. A linear d-PS chains of the same total molar 

mass retains its flexible Gaussian coil structure in the miscible 

h-PVME matrix.

Particularly important for polymers are the localized fast 

dynamics on sub-monomer level, the segmental dynamics 

at the monomer level and the entangled/collective dynamics 

at larger scales. To obtain an overview on the chain motions 

over a time scale of ≈ 1 ns using the High-Flux Backscattering 

Spectrometer (HFBS), we first measured the temperature 

dependence of the elastic scattering intensity (within the 

0.8 μeV resolution of the instrument). The incoherent intensity 

is dominated by the h-PVME matrix. The PS fillers are essentially 

frozen in the time-scale of the neutron experiments, the mobile 

component is the matrix. Figure 2a compares the normalized 

elastic intensities, I

el(Q,T)/Iel(Q,T = 15 K). Adding linear PS has 

a minority effect on sub-Tg dynamics of PVME, whereas the 

segmental dynamics is significantly slowed down above Tg due 

to increased monomeric friction due to the presence of frozen 

PS segments, as known from previous linear blend studies 

[3,4]. What we found different and interesting is that 

the 18-arms star PS significantly accelerated the localized 

dynamics of PVME below Tg, possibly due to additional 

free volume provided to the PVME chains by the free ends 

of the stars. This brings up exciting new opportunities 

for tuning glassy dynamics, which is important for highstrength applications. 

The segmental dynamics of PVME in presence of PS fillers with 

different architectures were measured by quasi-elastic neutron 

scattering (QENS) measurements on HFBS. The quasi-elastic 

broadening at low-Q is related to thermally driven Rouse 

motion of the chain segments, which is determined by the 

local monomeric friction. We found that PS chains slow down 

the Rouse relaxation of PVME; however, unlike the localized 

sub-Tg dynamics, the monomeric friction imposed by star-PS 

chains is the same as their linear counterparts, suggesting that 

the monomeric friction coefficient is mainly determined by 

chemistry rather than the shape of the macromolecules.

At larger scales and longer times, the chains feel the presence 

of other chains in the melt, and their motion is constrained by 

1 Department of Chemical and Biological Engineering, Koç University, Istanbul, 34450 Turkey

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

3 University of Maryland, College Park, MD 20742
A

12d

A

12d

A

12d

right-h

A

C

6

Å-2

)

solv

ρn

 

0.05

Momen

solv

ρn

 

SOFT MATTER

2019 ACCOMPLISHMENTS AND OPPORTUNITIES 45

entanglements (reptation motion). Such confined dynamics of 

chains is important for bulk rheological behavior of polymers 

as the rubbery plateau modulus is inversely proportional to the 

square of the distance between the entanglements, i.e. the 

reptation tube diameter. Using a mixture of d and h PVME 

chains, we highlighted the single h-PVME chains in the sea 

of deuterated background. NSE results suggest that star 

shaped particles do not much alter the apparent tube size of 

PVME whereas blending with linear chains reduces the tube 

diameter by ≈ 40 %. This reflects directly on the rheological 

behavior of the samples. Linear PS chains results in a 

monotonic and moderate increase of viscosity relative to the 

neat PVME, as expected from conventional polymer blends. 

The 13-nm diameter silica nanoparticles, representing the 

conventional nanocomposites, monotonically reinforce 

the system orders of magnitude compared to the neat 

matrix, again as expected. The compact star PS remarkably 

decreased viscosity at moderate concentrations as they are 

impenetrable and small, while at higher concentrations 

percolation of the glassy PS stars causes reinforcement as 

high as the level that silica can achieve. This dual effect, 

softening and stiffening, of star macromolecules is novel and 

brings along a new design parameter- the filler architecturefor all-polymer based nanocomposites.

In conclusion, high-resolution neutron spectroscopy on 

isotopically labeled components in the resulting ‘architecturally 

engineered’ all-polymer nanocomposites allowed us to directly 

observe the chain conformation and motions at the nanoscale. 

The unprecedented bulk rheological behavior was therefore 

related to very complex microscopic mechanisms at a broad 

range of time-scale (from sub-nanoseconds to hundred 

nanoseconds), and length-scales (from monomer size to 

entanglement mesh sizes).

References

[1] R. Krishnamoorti, R. A. Vaia, Polymer nanocomposites: 

synthesis, characterization, and modeling (American 

Chemical Society Washington, DC, 2002), Vol. 804.

[2] E. Senses, M. Tyagi, M. Pasco, A. Faraone, ACS Nano 12, 

10807 (2018).

[3] I. Cendoya, A. Alegria, J. Alberdi, J. Colmenero, H. 

Grimm, D. Richter, B. Frick, Macromolecules 32, 4065 

(1999).

[4] A. Arbe, J. A. Pomposo, I. Asenjo-Sanz, D. Bhowmik, O. 

Ivanova, J. Kohlbrecher, J. Colmenero, Macromolecules, 

49(6), 2354 (2016).

Disperse in a soft matrix

Architecturally

engineered

nanocomposites

Conventional

nanocomposites

Linear

blends

Star

polymers

Solid

spheres

Flexible

(a) coil

(b)

Q (Å-1)

Icoh

Q 2 (arb)

intra-star

linear d-PS-80 kDa

18-arms d-PS-80 kDa

0.0 0.1 0.2 0.0

0.2

0.4

FIGURE 1: (a) Schematic representation of different type of fillers. The glassy 

star polymers structurally reside in between two hard sphere and flexible coils; 

dispersing them in soft matrices results in new hybrids between conventional 

linear blends and nanocomposites. (b) Kratky plot for 80 kDa linear and 18-arms 

d-PS fillers dispersed in h-PVME matrix, showing the compact nanoparticle like 

structure of the high-functionality stars.

FIGURE 2: (a) Normalized scattered elastic intensities of PVME in neat forms 

and in composites with 20 % linear (80 kg/mol) and 18-arms (88 kg/mol) 

deuterated PS. (b) Normalized dynamic structure factors of the same samples 

at 363 K and at Q = 4.7 nm-1. The gray line is the resolution.

Glass

transition

I(Q,T )/I(Q,T=15 K)

T (K)

Methyl

dynamics

18 arms star

linear

neat matrix

0 100 200 300 400

0.4

0.6

0.8

1.0 (a)

(b)

Q = 0.47 Å-1 T = 363 K

neat PVME

Energy (hω) (μeV)

S(Q,ω)/

S(Q,ω)max

10-1

100

-10 -5 0 5 10