POLYISOPRENE – NANOSTRUCTURED CARBON COMPOSITE

(PNCC) MATERIAL FOR VOLATILE ORGANIC

COMPOUND DETECTION

Gita Sakale, Maris Knite, Valdis Teteris

Institute of Technical Physics, Riga Technical University, Azenes iela 14/24, Riga, Latvia

Velta Tupureina

Institute of Polymer Materials, Riga Technical University, Riga, Latvia

Keywords: Polymer-high structured carbon black composite, Volatile organic compound sensors.

Abstract: Our scientific group has chosen the elaboration of conductive composite material, which could be useful for

volatile organic compound detection, as one of research areas. It was found out that the most sensitive

composite material consists of polyisoprene and 10 mass parts of nanostructured carbon black. The electric

resistance changes of the composite in presence of 10 different saturated organic solvents vapour were

measured. Results obtained form our mass-sorption experiments indicated that electrical resistance of the

composite increases because of volatile organic compound (VOC) molecule absorption in the composite

matrix material. We also evaluated VOC compatibility with PNCC matrix material and estimated how the

PNCC resistance change velocity (

v ) versus organic solvent vapour molecule diameter varies.

1 INTRODUCTION

Available statistical data evidence about people

exposed to organic solvent daily at their workplaces,

but there are no monitoring devices used to control

VOC concentration in the room. There is also a

necessity to protect environment and equipment

from exposure to VOC. Above mentioned denotes

that there is an urgent need for VOC sensor

materials.

Devices (sorbent polymer films, metal oxide

semiconductors, quartz microbalance (quartz

resonator), laser gas sensors ect.) in the market can

not still be compared with mammal’s olfactory

system. Scientists are trying to design a prototype of

sensor which in sensing capability of different gases

could be close to mammals olfactory system and

even could be improved for practical applications.

We think that the desirable result of VOC

detection can be achieved by using polymer –

carbon black composites as gas sensor materials

because polymer matrix can be selected for direct

volatile compound detection and identifying.

In our opinion a candidate sensor material for gas

sensing should fulfil the following criteria: not

expensive constituent materials; simple production;

fast and reversible response; in-situ control of VOC;

small dimensions of sensing element and ability to

function for a long period of time.

The mechanism how polymer – carbon black

composite detects VOC is as fallows: i) the sample

of the composite material is exposed to VOC,

molecules of VOC adsorbe on the surface of

composite and diffuse into the matrix material; ii)

intermolecular chains in the polymer matrix weaken

and form intermolecular chains between VOC

molecules and macromolecules of matrix material;

iii) the matrix material swells; iv) electroconducting

pathways break down because distance between

carbon black aggregates increases; v) at the same

time tunnelling currents between carbon aggregates

in thin layers of matrix decreases and the electrical

resistance of the composite increases.

2 EXPERIMENTAL

Polyisoprene – nanostructured carbon composite

material was made by rolling highly structured nano-

size carbon black PRINTEX XE2 (specific surface

117

Sakale G., Knite M., Teteris V. and Tupureina V. (2009).

POLYISOPRENE – NANOSTRUCTURED CARBON COMPOSITE (PNCC) MATERIAL FOR VOLATILE ORGANIC COMPOUND DETECTION.

In Proceedings of the International Conference on Biomedical Electronics and Devices, pages 117-122

DOI: 10.5220/0001557301170122

 SciTePress

Figure 1: The change of relative resistance vs. time for the sample, held in saturated vapours of different solvents.

950 m

/g, mean diameter of primary particles 25 nm,

DBP absorption 380 ml/100g) and necessary

additional ingredients – sulphur and zinc oxide –

into a Thick Pale Crepe No 9 Extra polyisoprene

matrix using cold rolls. Then follows vulcanization

process (under 30BAR pressure at 150˚C for 15

minutes) when not only sulphur crossbonds form but

also possibly chemical bonds between carbon black

nanoparticles and matrix macromolecules form. As

better carbon black particle electrocodutive grid is

connected with polyisoprene macromolecules as

better sensing element to any kind of deformation is

achieved.

Preparation of samples and schematic illustration

of the experimental set-up is described in (Knite,

2007).

2.1 The Change of PNCC Electrical

Resistance Due to VOC Presence

Samples were exposed to vapour for 30 seconds and

then held in open air for relaxation processes to let

go on until the composite material reaches its initial

resistance R

. Then the measurement was repeated.

Composite material response to different saturated

organic solvents vapour can be seen in Fig. 1. In the

figure

denotes electrical resistance change

velocity (KΩ/min). It is obvious that the largest

resistance change velocity is obtained when PNCC

is exposed to tetrahydrofuran vapour followed by

benzene, ethylacetate and dichloroethane ect. In the

case of propanol vapour PNCC resistance did not

change at all. The composite can not sense the

presence of propanol vapour. In these experiments

PNCC samples with dimensions 50 x 5 x 1 mm

(length x width x thickness) were used.

Figure 2: Resistance change of PNCC vs. toluene vapour

concentration.

We carried out experiments with PNCC samples

with dimensions 50 x 5 x 0,25 mm also. The

composite capability to sense different toluene

vapour concentrations were tested (Fig. 2.).

Resistance of PNCC increases proportionally to

toluene vapour concentration. Here we would like to

mention that PNCC can sense concentrations which

are equal to or lower then TWA limits (TWA

indicates a time-weighted average concentration for

up to a 8-hour workday during a 40-hour workweek)

(http://www.cdc.gov). In the case of toluene vapour

TWA limit is 200 ppm.

2.2 The Relaxation Process of PNCC

Electrical Resistance

After samples were subjected to saturated organic

solvents vapour electrical resistance relaxation

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process in air were carried out. Typical PNCC

sample with thickness 1mm relaxation process is

shown in Fig.3.

Experimentally obtained data were fitted with

theoretical curve which can be characterised by

equation:

01 2

exp( ) exp( )

RR A A

=+ − + −

(1)

, where

– initial electrical resistance

R – transient electrical resistance

, A

– constants

, τ

– relaxation time (h).

From equation (1) we calculated relaxation time τ

and τ

. For all tested organic solvent vapour the

relaxation process can be divided into fast and slow

processes.

Figure 3: Electrical resistance relaxation of specimen in

open air. The sample was exibited to tetrahydrofuran for

30s.

In the case of 250μm thick samples electrical

resistance relaxation process is about one hour if the

sample has been exposed to 108,7 ppm toluene

vapour concentration.

2.3 Mass – sorption Experiments

Experiments of the change of the sample mass as a

function of time the sample is kept in organic

solvent vapour were used to find out the mechanism

of the change of resistance. For example, specimens

of pure polyisoprene, PNCC, and a pellet of

compressed high-structured carbon black powder

were held in toluene vapour for ~ 48 hours and a

mass as a function of time (sorption curve) was

recorded (Fig. 4.). Sorption of vapour in carbon filler

in the initial period (first 15 min) is approximately

three times (approximately 1,5 times after up to 104

min) as big as that of pure polyisoprene rubber. Yet

the sorption of vapour in PNCC material in the

initial period is around 1,3 times shorter if compared

with pure polyisoprene rubber, although it seems

that carbon filler should increase the vapour sorption

in the PNCC material. That can be explained, firstly,

by the fact that in the PNCC composition there are

only 10 mass parts of carbon and, secondly, in

processing (mixing and vulcanizing) the PNCC

compounds insulating polyisoprene layers are

formed between the carbon nano-particles. So, even

near the percolation threshold (10 mass parts of

carbon), when electro-conductive channels are

formed, very thin polyisoprene intermediate layers

between the nano-particles still exist and tunnelling

currents may emerge between the channels.

Figure 4: The change of mass for three materials

(polyisoprene rubber, HSCB and its composite) as

functions of the time of exposure to toluene vapour.

Only after approximately 28 h the sorption of vapour

by the PNCC material noticeably exceeds the

sorption by polyisoprene rubber, which indicates

that only after this period of time the sorption in the

filler begins to play substantial role. Consequently,

1) the obtained result indirectly proves the existence

of quantum effect – tunnelling currents in the PNCC

material, 2) sorption of vapour in the polyisoprene

matrix plays absolutely uppermost role in effect of

gas sensing (due to sorption of vapour molecules

and swelling of the matrix the distance between

carbon nano-particles increases and tunnelling

currents rapidly decrease) (Knite, Shakale 2007).

POLYISOPRENE – NANOSTRUCTURED CARBON COMPOSITE (PNCC) MATERIAL FOR VOLATILE ORGANIC

COMPOUND DETECTION

119

3 DISCUSSION

3.1 Electrical Resistance as a Function

of VOC Molecule Diameter

Previously we have made positron annihilation

lifetime spectroscopy measurements for PNCC in

collaboration with scientists from Monash

University in Australia (Knite, 2006). The purpose

of these experiments was to evaluate free volume

cavities dimensions in the composite material when

it is stretched and in normal (unloaded) state. As it

can be seen from Fig. 5 free volume mean radius for

PNCC containing 10 mass parts of carbon black in

normal state is 3,305Å. Then diameter of free

volume cavities is 6,61Å. When molecule diameters

of VOC are smaller than the diameter of free volume

cavities in the composite material then there is no

need for extra activation energy for molecule

diffusion into the matrix material.

Figure 5: Mean radius of free volume cavities in PNCC as

a function of carbon black content in normal and stretched

(Δl =15 mm) states measured by PALS.

Molecule volume of VOC was calculated using

equation (Askadcli, 1983):

⋅

=Δ

∑

(2)

, where

k – Atoms packing density

– Avogadro number, mol

-1

∑

V - Van der Valse volume of VOC molecule,

which consists of discrete atom volume sum, cm

M – Molar mass of VOC, g/mol

– Density of VOC, g/cm

After calculation of VOC molecule volume we

accepted approximation that all molecules take the

form of sphere. Finally we calculated diameters of

VOC molecules. We suppose with increasing

molecule diameter of organic solvent vapour the

maximum PNCC electrical resistance change

ΔRmax/R0 should be decreased.

In Fig. 6 we can see that above mentioned

realizes only partly. ΔRmax/R0 increases with

increasing molecule diameter starting from acetone

vapour until tetrahydrofuran vapour. In our opinion

this means that ΔR

max

is not dependent only from

organic solvent vapour molecule dimensions.

Figure 6: Electrical resistance change of PNCC vs. organic

solvents vapour molecule diameter.

ΔR

max

decreases with increasing molecule

diameter of organic solvents vapour in Fig. 6 from

tetrahydrofuran vapour until o- and p-xylene vapour.

In the case of toluene and chlorbenzene an exception

has to be made. Toluene vapour causes greater

ΔR

max

change than chlorbenzene vapour, while

molecule diameter of toluene molecule is lager than

that of chlorbenzene.

3.2 VOC Compatibility with Polymer

Matrix Material

The law “like dissolves the like” is already well

enough known. Polar solvents dissolve in polar

solvents and analogous non-polar dissolve in non-

polar solvents. This also can be attributed to organic

solvents and polymer materials.

To evaluate organic solvents compatibility with

polyisoprene matrix material we compared solvents

dielectric permeability (ε) values to polyisoprene ε

value. As ε value of organic solvents is closer to

polyisoprene ε as the matrix material better absorbs

solvent molecules and then greater

change is

observed.

From literature data (Brandrup, 1989) we know

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that polyisoprene dielectric permeability value is

2,68. Organic solvent vapour dielectric permeability

values are summarized in Table 1.

Table 1: VOC dielectric permeability values.

Substnace ε

Type of

solvent

P-xylene 2,2

Non-polar

Benzene 2,3

Toluene 2,3

O-xylene 2,5

Chlorobenzene 5,6

Weakly polar Ethyl acetate 6

Tetrahydrofurna 7,4

Dichloroethane 10,6

Polar Propanol 20,1

Acetone 20,7

From Fig. 7 we can see that PNCC resistance

response to organic solvent vapour is concentrated

into three groups. First let us begin with polar

organic solvents group. As we can see in this group

small electric resistance change is observed while

molecule dimensions of these solvents are the

smallest from all tested organic solvents. It can be

explained by acetone, propanol and dichlorethane

vapour non-compatibility with the composite matrix

material. Thus, we can conclude that resistance

change in this group is more dependent of organic

solvents vapour compatibility with the composite

matrix material.

Figure 7: PNCC resistance change vs. organic solvents

dielectric permeability values.

We will continue discussion with weakly polar

and non-polar solvents group. From Fig. 6 and Fig. 7

we can see that in these groups molecule dimensions

and dielectric permeability compensate each other

and both affect the composites response to organic

solvents vapour. For example, tetrahydrofuran

vapour molecule diameter is the smallest from these

two groups and the largest ΔR

max

value is

obtained, while ε value is 7,4.

Above we compared toluene and chlorbenzene

vapour caused electrical resistance change versus

vapour molecule dimensions. If we look at Fig. 7

then we can see that chlorbenzene belongs to weakly

polar solvents group. Chlorbenzene vapour is less

compatible with the composite matrix material. For

that reason PNCC resistance change in chlorbenzene

vapour is smaller then in toluene vapour. If we

compare mass of molecule for toluene and

chlorbenzene then for toluene it is 15,30·10

-23

g and

for chlorbenzene it is 18,69

-23

g. From kinetic-

molecular theory we know that for molecules with

smaller mass the motion velocity is higher. This also

explains why resistance change of PNCC is larger in

the case of toluene vapour.

Figure 8: Relaxation time τ

vs. resistance change in

organic solvents vapour.

Composites organic solvent vapour sensing

mechanism is based on matrix swelling, which

causes increased distance between carbon black

aggregates and resistance change of the composite.

As larger is value ΔR

max

as larger is amount of

organic solvent vapour molecules the composite has

absorbed and as longer should be relaxation process.

According to afore said relaxation time τ

and τ

should be proportional to ΔR

max

change.

Concerning Fig. 8 we can say that τ

are nearly

proportional to ΔR

max

, but there are also some

exceptions which are related to previously described

molecule dimensions and organic solvents

compatibility with polymer matrix material. But

from Fig. 9 we can not find any mathematical

relationship between ΔR

max

change and τ

values.

We conclude that fast relaxation processes (τ

)

are determined by vapour molecule diffusion from

swelled matrix interior layers to the surface and

crosslinked macromolecule relaxation. Slow

relaxation processes (τ

) are related to

electroconductive grid relaxation made of carbon

black particles.

POLYISOPRENE – NANOSTRUCTURED CARBON COMPOSITE (PNCC) MATERIAL FOR VOLATILE ORGANIC

COMPOUND DETECTION

121

Figure 9: Relaxation time τ

vs. resistance change in

vapour of organic solvents.

4 CONCLUSIONS

In conclusion we can say that for the production of

PNCC we used not expensive and available

materials. The production of the composite is rather

simple if the production procedure is strictly

performed. When PNCC is exposed to VOC the

electrical resistance of the composite increases

rapidly and the effect is reversible. Electric

resistance change velocity is dependent of both VOC

molecule diameters and VOC compatibility with

PNCC matrix material.

Better sensitivity PNCC exhibits when it is

exposed to non-polar or weakly polar solvent

vapour. So, we can declare that PNCC sensing of

VOC is selective.

Further, in our work, we are going to try to

decrease relaxation time of electrical resistance to

couple of minutes or even seconds. We suppose that

PNCC can be used for VOC detection with some

improvements, for example, reducing dimensions of

the composite to optimal size. Else, functioning of

PNCC needs to be evaluated in long time period.

REFERENCES

Knite, M., Klemenok, I., Shakale., G., Teteris, V., and

Zicans, J., 2007. J. Alloys Comp.

Knite, M., Shakale, G., Klemenoks, I., Ozols, K., Teteris,

V., 2007. Journal of Physics: Conference Series 93,

012031.

Askadcli, A. A., Matveev, J. I., 1983. Chemical structure

and physical characteristics of polymers, Kimija:

Moscow.

Knite, K., Hill, A.J., Pas, S.J., Teteris, V., Zavickis, J.,

2006. Materials Science & Engineering C, V 26.

Brandrup, J., Immergut, E.H., 1989. Polymer handbook,

John Wiley&Sons: New York, Chichester, Brisbane,

Toronto, Singapore, 3

edition.

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