S1

Supporting Information

Enhanced Mechanical Properties of Polymer

Nanocomposites Using Dopamine-Modified Polymers at

Nanoparticle Surfaces in Very Low Molecular Weight

Polymers

Na Kyung Kwon†, Hyunhong Kim†, Im Kyung Han†, Tae Joo Shin‡, Hyun-Wook Lee†,

Jongnam Park*,† and So Youn Kim*,†

†School of Energy and Chemical Engineering and ‡UNIST Central Research Facilities and

School of Natural Science, Ulsan National Institute of Science and Technology (UNIST), 50

UNIST-gil, Ulsan 44919, Republic of Korea

* E-mail: soyounkim@unist.ac.kr (S. Y. K.), jnpark@unist.ac.kr (J. P.)

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Experimental details

Materials. Poly(ethylene glycol) (PEG, average Mn = 400 g/mol), Poly(ethylene glycol)

methyl ether acrylate (PEGMEA, average Mn = 480 g/mol), azobisisobutyronitrile (AIBN),

dopamine hydrochloride (98%) and tetraethyl orthosilicate (TEOS ≥ 99.0%) were purchased

from Sigma-Aldrich. Ethyl alcohol anhydrous (99.9%) and ammonia solution (NH4OH, 28%)

was purchased from DAEJUNG CHEMICALS & METALS Co. and Wako Pure Chemical,

respectively. Deionized water (DIW) purified to 18.2 MΩ by Milli-Q purification system was

used. Reversible addition-fragmentation chain-transfer (RAFT) reagent and dibenzyl

trithiocarbonate was synthesized from the literature.1 All chemicals were used as received with

no further purification.

Particle and polymer synthesis. The silica nanoparticles (NPs) were synthesized by Stöber

method which involves the base-catalyzed hydrolysis and condensation of TEOS.2 The 1203

ml of ethanol, 32 ml of DIW, and 52 ml of ammonia solution was mixed and stabilized for 3 h

with 600 rpm stirring at 55 °C. Then, the reaction was run overnight after adding the 52 ml of

TEOS to the mixture. After synthesis, the mixture of particles and ethanol was concentrated

approximately 5-time by heating the mixture in a ventilation hood. Then, we transferred a

continuous phase of the silica suspension from ethanol to water by adding excess water to the

silica suspension. By heating the suspension, residual ethanol was removed, and aqueous silica

suspension was concentrated approximately 10 times. Then, the final silica suspension was

filtered through a hydrophilic MCE syringe filter with 0.45 um pores. The diameter of

synthesized silica NPs was 34.4 ± 4.4 nm for each synthesized batch and determined from the

fitting of the particle form-factor scattering from a dilute suspension.3

The dopamine-modified methylated PEG (DOPA-mPEG) polymer was synthesized by

RAFT polymerization according to the method previously reported with a modification.4

(Figure S1a) Dopamine methacrylamide (DMA) was synthesized from dopamine, following

the reported synthetic procedure.5 Dopmaine-HCl (20 mmol) was dissolved in 100 ml of Ar

bubbled 0.1 M sodium bicarbonate buffer (pH 8.2). 24 mmol of methacrylic anhydride in 20

ml of THF was added dropwise to the reaction solution. The reaction was kept at room

temperature for 12 h. The final solution was washed with 50 ml of ethyl acetate and then, the

pH of the aqueous layer decreased under 2. The washed reaction mixture was extracted three

S3

times with 50 ml of ethyl acetate. Afterward, the organic layer was evaporated to 20 ml and

recrystallized under 200 ml of hexane. To synthesis of DOPA-mPEG polymer, dibenzyl

trithiocarbonate (0.1 mmol), PEGMEA (1.6 mmol), DMA (0.4 mmol) and AIBN (0.05 mmol)

were prepared in 3 ml vial and 1 ml of dimethylformamide was added. The mixture was

dissolved completely and then, transferred to an ampule. The ampule was degassed by 4 freeze-

pump-thaw cycles by using Schlenk line to remove oxygen completely. The ampule was then

sealed under high vacuum using a torch. The reaction was performed at 70 °C in oil bath for

24 h. The final product was precipitated in diethylether twice and dried under vacuum.4

Sample preparation. First, silica NPs were physically grafted by DOPA-mPEG in aqueous

system. The silica NPs and DOPA-mPEG dispersed in the different aqueous solutions were

mixed and vigorously stirred for 5 hours at room temperature at a desired weight ratio (w/w)

of DOPA-mPEG to the silica; 0, 0.01, 0.02, 0.04, 0.1, 0.2, 0.4, 1.0 and 5.0 w/w. After grafting,

a known amount of PEG was added to the particle mixture to meet the final particle volume

fraction, ϕc, from 0.01 to 0.48. Then finally, the silica NPs were dispersed in the PEG 400 melts

after removing the solvent in a vacuum oven at 70 °C for 2 days. The entire sample preparation

procedure is fully described in Figure S1b.

Fourier transform infrared (FT-IR) spectroscopy. The FT-IR spectra were collected with a

Varian 670-IR spectrometer using mercury cadmium telluride (MCT) detector and single-

bounce attenuated total reflectance (ATR) accessory with 0.075 cm-1 of spectral resolution. All

samples were measured after solvent evaporation.

Thermogravimetric analysis (TGA). TGA experiment was conducted with Q500 of TA

instrument to examine the surface coverage density of DOPA-mPEG on silica surfaces. The

sample was heated from 30 °C to 900 °C with the heating rate of 10 °C/min in N2 atmosphere.

All samples were treated after solvent evaporation then loaded into a platinum pan. The sample

weight is measured after loading into the TGA pan in the chamber.

Transmission electron microscopy (TEM). To measure the thickness of grafted DOPA-

mPEG layer on silica surface, Tecnai G2 F20 X-twin from FEI was operated at 200 kV

acceleration voltages with 0.102 nm line resolution. Samples were prepared on lacey carbon

grids and fully dried in vacuum oven at 25 °C for overnight. All captured images were exposed

for 0.8 s.

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Scanning electron microscopy (SEM). The microstructure of silica NPs in PEG matrix is

characterized by the SEM images obtained by Hitachi S-4800 with a working distance of 5.2

mm at z-axis, 1 keV, and 10 A current.

Gel permeation chromatography (GPC). The molecular weight (MW) of DOPA-mPEG

polymer was measured by GPC using an Agilent 1200S system at 25 °C using tetrahydrofuran

(THF) as an eluting solvent. The MW was calibrated using polystyrene MW standards. Before

the measurement, the sample was dissolved in 700 ul of THF and filtered using PTFE syringe

filter with 0.2 um pores. The obtained MW of DOPA-mPEG polymer was Mw = 9306 g/mol

(PDI=1.16).

Nuclear magnetic resonance (NMR). DOPA-mPEG polymer was analyzed by 400 MHz FT-

NMR (ADVANCE III HD, Bruker). Sample was prepared by dissolution using chloroform-d.

Small-angle X-ray scattering (SAXS) measurement details. SAXS measurement was

performed at 6D UNIST-PAL beamline of the Pohang Accelerate Laboratory (PAL) to explore

the microstructure of silica NPs in PEG matrix. The employed radiation wavelength, λ, is 1.07

Å and a sample-to-detector distance (SDD) was 3.5 m. The scattered X-rays were recorded

with a charge-couple device (CCD) area detector (Rayonix, L.L.C., USA). The resulted two-

dimensional scattering patterns were azimuthally averaged to one-dimensional scattering

intensity, then the relative intensity was plotted as a function of scattering vector, q

(=4π·sin(θ/2)/λ), where θ is the scattering angle. As mentioned in the main text, the scattered

intensity were considered to arise from one-component system of silica NPs after subtracting

the corresponding background scattering intensity and it can be written as,

𝐼(𝑞, 𝜙𝑐) = 𝜙𝑐𝑉𝑐∆𝜌2𝑃(𝑞)𝑆(𝑞, 𝜙𝑐) + 𝐵

where ϕc is the NP volume fraction, Vc is the particle volume, Δρ is the excess electron

scattering-length density (SLD) of the particles relative polymer matrix, P(q) is the single-

particle form-factor, S(q,ϕc) is the collective particle-structure factor normalized to unity at

large q and B is the background scattering amplitude. S(q,ϕc) could be obtained by dividing

I(q,ϕc) from concentrated particle melts by that from dilute particle limit.

Rheology. Oscillatory shear rheology and viscometry experiments were carried out with

Malvern Kinexus Pro rheometer with a cone-and-plate geometry (20 mm diameter, 4° cone

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angle) at 25 °C and 75 °C. Before the measurement, all samples were stabilized for 30 min

after loading the sample at a fixed gap distance, 0.143 mm.

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Figure S1. (a) Reaction mechanism and final chemical structure of DOPA-mPEG and (b)

schematic illustration of the procedure for preparing polymer nanocomposites (PNCs) are

described.

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Figure S2. Normalized weight reductions from TGA analysis are plotted for varying the mixing

weight ratios of DOPA-mPEG to silica. The initial reduction (~ up to 80 °C) of weight came

from the residual solvents in the samples. The additional reduction of weight in grafted silica

compared to the bare silica is considered to be the result of DOPA-mPEG adsorption. Thus, the

amount of adsorption at each mixing ratio is obtained. Then, the coverage density, σ, is

calculated by dividing the total mass of adsorption with the total surface area of spherical silica

nanoparticles. The curve of Figure 1b in the main manuscript is fitted to y=Ae(-x/B)+y0 where A

is the grafted weight ratio, B is the maximum coverage ratio, and y0 is the intercept to calculated

the surface coverage rate, rσ.

Table S1. Calculated surface coverage density, σ, and surface coverage rate, rσ, for varying

mixing ratios between DOPA-mPEG and silica nanoparticles.

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1000 2000 3000 4000

0.00

0.03

0.06

0.09

1500 2000 2500

0.000

0.002

0.004

0.0061739 cm

-1

0.99

0.32

0.00

r

absorp

tion (

a.u

.)

wavenumber (cm-1)

Figure S3. The grafting of DOPA-mPEG on silica surfaces is confirmed by FT-IR results. The

absorption at 1739 cm-1 for ester-amide bond is increased with DOPA-mPEG grafting. The

black, red, and blue curves represent different surface coverage rate, rσ, of DOPA-mPEG on

silica surface of 0.00, 0.32, and 0.99, respectively.

Figure S4. TEM images of DOPA-mPEG grafted silica NPs with (a) rσ = 0.00 and (b) 0.51.

The wavy lines at the outer shell of silica NPs in (b) indicate the layer of DOPA-mPEG polymer

with the thickness of about 1.4 nm, whereas the surface of the silica NP in (a) contains the

similar amorphous lattice patterns as the inner part. The result shows that the grafted DOPA-

mPEG is well distributed at the surface of silica NPs with a homogeneous thickness of about a

few nanometers. The scale bar is 20 nm.

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Figure S5. Complex shear modulus (G*) of PNCs with ϕc = 0.07 is measured as a function of

(a) angular frequency, ω, at 0.1% strain, and (b) complex shear strain, γ, at 1 Hz with varying

rσ. All measurements are conducted at 75 °C.

Figure S6. Storage (G’) and loss (G”) shear moduli as a function of angular frequency, ω, are

plotted for (a) ϕc = 0.17 and (b) 0.38 at γ = 0.1%. All measurements are conducted at 75 °C.

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Figure S7. Complex shear modulus (G*) of PNCs with bare silica NPs is measured on various

volume fraction, ϕc = 0.005 to 0.48, as a function of angular frequency, ω.

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Figure S8. SEM images of DOPA-mPEG grafted silica NPs with rσ = 0.99 at ϕc = 0.17 in

PEG400 matrix. The scale bar is 500 nm. The SEM image of PNCs without DOPA-mPEG

grafting could not be obtained because of liquid-like nature of PNC with low MW polymers.

Figure S9. Storage (G’) and loss (G”) shear moduli as a function of complex shear strain, γ,

are measured for (a) ϕc = 0.17 at ω = 0.01 Hz and for (b) ϕc = 0.38 at ω = 1 Hz. The enlarged

image of G’’ in (b) at high strain is given in the inset. All measurements are conducted at 75 °C.

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Figure S10. The photographs of PNCs (left) without grafting, (middle) with grafting DOPA-

mPEG on silica surface, and (right) mixing with DOPA-mPEG in bulk. The composition of all

three PNCs is same for ϕc = 0.38. rσ of PNC in the middle was 0.51 and the same amount of

DOPA-mPEG was mixed in the bulk for PNC in the right. The photographs in the inset were

taken 1 year after the preparation and showed the stability of the PNCs. We note that the

original PNC’s yellowish color came from the vials not from the PNCs and the PNCs were

transferred to other vials later.

S13

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