Supporting information

Study on foamability and electromagnetic interference shielding effectiveness of

supercritical CO2 foaming epoxy/rubber/MWCNTs composite

Xun Fan, Guangcheng Zhang*, Jiantong Li, Zhengyang Shang, Hongming Zhang,

Qiang Gao, Jianbin Qin, Xuetao Shi**

Department of Applied Chemistry, MOE Key Lab of Applied Physics and Chemistry

in Space, College of Science, Northwestern Polytechnical University, Xi’an, 710072,

China

*Corresponding author.

**Corresponding author. Tel.: +86-29-88431672; Fax: +86-29-88431672.

E-mails: zhangguc@nwpu.edu.cn (G. Zhang), shixuetao@nwpu.edu.cn (X. Shi)

1.1 CO2 absorption

CO2 absorption was measured by weighting the sample before and after

saturating. The amount of gas soaped in polymer was calculated by equation S1:

M=( M 2−M 1 )

M 1×100 Equation S1

Where M, M1 and M2 represent the percentage of CO2 concentration, the weight of

pristine composite and the mixed weight of composite and CO2 after saturating in

autoclave.

1.2 Densities and volume expansion ratio of epoxy composite and composite foam

The densities of samples were determined by calculating the results involving

weighting solids and foams in water and in air, according to formula S2, which was

deduced by Archimedes law.

ρ f=M a

M a−M w× ρw

Equation S2

Where Ma and Mw are obtained by weighting solids and foams in water and in air,

respectively; ρf and ρw are the densities regarding to foamed sample and water.

Moreover, the volume expansion ratio (VER) is defined as the ratio of bulk

density (ρs) of composite to corresponding foam (ρf), and is calculated as follows:

VER=ρs

ρf

Equation S3

1.3 Conductivity analysis

The specimens with diameter of 70mm and thickness of 2mm were prepared for

conductivity analysis using Fischer-Elektronik Tera-Ohmmeter TO-3o, Germany,

according to ASTM D257 [1]. The equation S4 was as follow to calculate the

conductivity of foamed and unfoamed epoxy composites.

S= 4 hRπ (d+g )2

Equation S4

Where the diameter (d=50.4mm) of inner circle and the distance (g=9.4mm) between

inner and outer ring electrodes are available parameters of equipment; π is a constant;

R and h measured are representatives of volume resistance and sample thickness.

1.4 Bubble nucleation

For classical nucleation theory, the critical free energy of heterogeneous

nucleation (ΔGhet*) was clearly dropped by nanofillers depending on the factor of

contact angle (θ) and relative curvature (w)[2]:

∆ Ghet¿ =16 π σ3

3∆ P2f (m,w )

2 Equation S5

m=cosθ Equation S6

f ( m, w )=1+[ 1−m w3

g ]+3 m w2[ w−mg −1]+w3 {2−3[ w−m

g ]+[ w−mg ]

3} Equation S7

g= (1+w2−2m w2)12 Equation S8

Where σ and ΔP present the surface energy of matrix and the pressure difference of

between gas pressure inside and outside the nucleated bubble. Thus, we directly

observe from formulas S5～S8 that smaller f(m, w) can cause lower ΔGhet*, indicating

the addition of nanotube dramatically reducing the critical free energy of

heterogeneous nucleation and then inducing lots of heterogeneous nuclei in composite

matrix.

1.5 Simon formalism

The electromagnetic interference shielding effectiveness (EMI SE) was

represented by Simon formalism as following equation:

SE=5+10 log( σf )+1,7 t √σf Equation S9

where σ(S/m) is the conductivity of sample, f refers to the measured frequency and t

(mm) is the thickness of material.

Figure S1 SEM micrographs for EP/fMWCNTs and EP/CTBN/fMWCNTs

composites at (a, e) 0.3 wt% fMWCNTs, (b, f) 1.0 wt% fMWCNTs, (c, g) 2.0 wt%

fMWCNTs, and (d, h) 5.0 wt% fMWCNTs, respectively

Figure S2 DSC curves regarding on (a) cure reaction and (b) Tg of epoxy and epoxy

composites. In the graphs, neat epoxy resin, epoxy with 10wt% CTBN, epoxy

modified with 1.0wt.% fMWCNTs and epoxy along with 10wt.% CTBN and 1.0wt.%

fWMCNTs were marked as EP, EP-10-0, EP-0-1.0 and EP-10-1.0, respectively.

Figure S3 DSC curves regarding on (a) cure reaction and (b) Tg of epoxy and epoxy

composites. In the graphs, epoxy with 10wt% CTBN and epoxies modified10wt.%

CTBN along with 0.3wt.% fWMCNTs, 1.0wt.% fWMCNTs, 2.0wt.% fWMCNTs,

5.0wt.% fWMCNTs were marked as EP, EP-10-0, EP-10-0.3, EP-10-1.0, EP-10-2.0,

EP-10-5.0, respectively.

In this section, the fundamental investigations, such as cure reaction and gas

absorption, are discussed here due to both of them having crucial impact on 3D cross-

linking networks and foamability of epoxy and epoxy composites [3,4]. The heat flow

as a function of temperature was measured in Fig. S3. Fig. S3a shows that each single

exothermic peak is occurred in DSC curves and their peak-temperatures were same

and irrespective of the reaction systems, indicating no influence of variation of

fMWCNTs content on curing reaction. Otherwise, in Fig. S3b, when CTBN content is

stable (10 wt.%) the reduction of Tg exhibited the increment of fMWCNTs had a

negative effect on thermal property of epoxy composites with content, but all of these

systems show higher Tg compared to that of EP/CTBN system. There are the

following reasons: on the one hand, the nanotube bundles form with the content of

nanotube increased, witch reduce the Tg of composites; on the other hand, adding

nanotube into epoxy resin toughened with CTBN is capable of fixing or constraining

the movement of chain segments, leading to an increased Tg.

Figure S4 Schematic of foaming process including cell nucleation, cell growth and

cell stabilization and solidification.

Fig. S4 illustrates the foaming process for EP/CTBN/fMWNCTs samples composed

of gas sorption in autoclave, depressurization during 30s, heating foaming in oil bath

and stabilization at ice water. First of all, the samples are placed in autoclave to obtain

homogenous polymer/CO2 solution after saturating samples for 3 days at 60oC and

15MPa. Subsequently, release pressure quickly and then takes samples out of

autoclave to cool at room temperature. Simultaneously, depressurization resultes in

cell nucleation in CTBN domains because of the lower Tg of CTBN domains in

comparison with saturated temperature [5]. Thirdly, gas swelling causes cell growth

when putting samples into oil bath at 110 oC to foam. At this step, owing to foaming

at 110 oC, which higher than Tg of CTBN and epoxy resin, both of them are able to

form cellular morphologies. Additionally, when foaming temperature is closed to Tg

of epoxy, the cell nucleation induced by fMWCNTs would happen, resulting in

smaller cell size than that of CTBN domains [6-8]. Finally, the cellular morphology is

controlled by immersing foamed samples foamed at various foaming time into ice

water to solidify molecular chain mobility.

Figure. S5 SEM micrographs on fracture surfaces of EP/CTBN/fMWCNTs composite foams loaded with 1.0wt fMWCNTs. The foaming time

was set as 5s, 10s, 20s and 40s.

Figure S6 Stress-strain curves of pure epoxy, EP/CTBN and EP/CTBN/fMWCNTs (a)

before and (b) after foaming process

Table S1 Compatative characteristics of foams produced from neat epoxy, epoxy

toughened with CTBN, epoxy modified by fMWCNTs and epoxy together with

CTBN/fMWCNTs hybrids, respectively

SampleCTBN (wt. %)

fMWCNTs (wt. %)

Foam density (g/cm3)

Volume expansion ratio

Average cell size

(μm)

Cell density (cells/cm3)

EP 0 0 0.86 1.37 32.54 1.41×107

EP-0-0.3 0 0.3 0.55 2.18 3.58 1.39×1010

EP-0-1.0 0 1.0 0.59 2.08 2.24 2.91×1010

EP-0-2.0 0 2.0 0.62 2.03 2.01 6.20×1010

EP-0-5.0 0 5.0 0.65 2.02 1.73 5.23×1010

EP-10-0 10 0 0.48 2.42 1.41 5.63×1010

EP-10-0.3 10 0.3 0.51 2.31 1.79/0.390.87×1010//1.41×1012

EP-10-1.0 10 1.0 0.54 2.24 1.54/0.451.67×1010//1.21×1012

EP-10-2.0 10 2.0 0.57 2.16 1.72/0.461.54×1010//2.27×1012

EP-10-5.0 10 5.0 0.61 2.10 2.16/0.371.19×1010//2.43×1012

Table S2 Comparison of tensile strength and Young’s modulus of epoxy and its composites before and after foaming process

MaterialsTensile strength/MPa Young’s modulus/GPa Deformation/%

Solid Foam Solid Foam Solid FoamEP 43.20 42.70 2.90 2.27 1.95 3.07

EP-10-0 34.55 29.85 2.57 1.49 2.13 7.23EP-10-1.0 45.55 46.28 3.05 1.94 3.56 5.46

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