Sensing of Acetone Vapours using Polymer Composite

Introduction

Gas detection instruments are increasingly needed for industrial health and safety, environmental monitoring and process control. To meet this demand, considerable research into new type of sensors is underway to enhance the performance of traditional devices. Polymer ceramic composites have superior characteristics and play vital role in gas sensing applications. Composites are materials obtained by bonding two or more materials together. The characteristics of the resulting materials are not that of the components in isolation. Polymer composites which fit into particular engineering applications can be designed by selecting the correct composition and choosing the appropriate manufacturing process1. Piezoelectric ceramics have good detection and output characteristics and thus used as sensors. Pure piezoelectric ceramics are often too stiff and brittle. If piezoelectric ceramic particles are incorporated within a polymer matrix, it will result in composite sensors containing both piezoelectric and visco elastic properties. Electrically conducting organic polymers are important in sensing devices^2-4. Polymers used in sensor devices either participate in sensing mechanisms or immobilize the component responsible for sensing the analyte. The emission of gaseous pollutants such as sulphur oxide, nitrogen oxide and toxic gases from related industries has become a serious environmental concern. Thus sensing devices need to be installed in such places. These devices should be cheap, reagentless and able to quantify the levels of gases in a rapid manner, at room temperature with low power consumption⁵. The room temperature gas sensing property is very attractive for many applications^6,7. Acetone is a commonly used chemical reagent in industry, For instance it is used to dissolve plastics, purify paraffin, dehydrate tissues and for pharmaceutical applications. Several techniques have been  developed  to  measure  the  concentration  of acetone,  such  as  gas/liquid chromatographic  analysis, spectroscopy  and  sensors^8-10. PMMA/CNT polymer composites ^11,12 and conductive polymer composites¹³ have been developed as sensors.The present work reports the results of the gas sensing behavior of a sensor developed using thick film which shows a  maximum response to the test gas at room temperature.

Materials used

Polymethyl methacrylate (PMMA)  (Alfa Aesar), Titanium dioxide (TiO₂) (Fischer Scientific, India) and Chloroform   (CHCl₃) (Fischer Scientific, India).All chemicals used were of Analar grade and were used as received from the supplier without further purification.

Preparation of PMTi Composites

A definite quantity of PMMA was dissolved in chloroform followed by the addition of a known quantity of TiO₂ and then it was made into a paste in an agate mortar and was subjected to heat at 110 °C for 30 minutes in a hot air oven and made into a powder. PMTi composites were prepared in the following proportions of PMMA and TiO₂: PMTi 1 – 9:1, PMTi 2 – 8:2, PMTi 3 – 7:3, PMTi 4 -6:4, PMTi 5 – 5:5 and PMTi 6 – 4:6¹⁴.

Characterisation Techniques

The X-ray diffraction pattern (XRD) and Scanning Electron Microscopy (SEM) are the basic techniques required to find out the phase and purity of the composite materials. The XRD patterns of polymers and polymer composites were recorded using Philips X’Pert pro diffractometer. The FTIR  spectrums of polymers and polymer composites were recorded using Shimadzu FTIR spectrophotometer. The SEM images of polymers and polymer composites were recorded using Hitachi Scanning Electron Microscope SU1510.

Results and Discussion

The XRD pattern for PMMA (Figure 1) show peaks at 2θ 14.50, 22.49, 29.45 and 41.41° and relative intensitites obtained for the polymer match with the JCPDS Card no. 13-0835 file, identifying it as PMMA. The average crystallite size of PMMA is determined using Xpert’ High Score plus software and it is found to be 0.1344 µm.

 

Figure 1: XRD pattern of PMMA Click here to View Figure

 

The peak positions (2θ = 25.30 (1 0 1), 38.57 (1 1 2), 48.04 (2 0 0), 53.88      (1 0 5), 62.68 (2 0 4), 70.29 (2 2 0), 75.05 (2 1 5) and 83.16^o  (3 1 2) and relative intensitites obtained for  TiO₂ match with the  JCPDS Card no. 78-2486 file, identifying it as TiO₂ with anatase phase (Figure 2). The average crystallite size is found to be 0.1858µm.

Figure 2: XRD pattern of TiO2 Click here to View Figure

 

The XRD patterns of PMTi composites is shown in the (Figures 3(i) – (iii)). From the figures, we can say that the crystallinity of the polymer has been considerably decreased upon the addition of TiO₂ and the intensity of the crystalline peaks of PMMA decreases and broadens. This reduction in crystallinity upon the addition of TiO₂ is attributed to small particles of TiO₂ changes the chain re-organization and facilitates for higher ionic conduction^15-19. The average crystallite size is found to be 0.1465 µm.

Figure 3: (i)  XRD patterns of PMTi 1 and 2 Click here to View Figure

 

Figure 3: (ii) XRD patterns of PMTi 3 and 4 Click here to View Figure

 

Figure 3: (iii) XRD patterns of PMTi 5 and 6 Click here to View Figure

 

An absorption band in the range of 1500-700 cm^-1 comes from the -C-O stretching vibration(1270-990 cm^-1), -C-H bending vibration (1450-1350 cm^-1), and CH₂ rocking (810, 750 cm^-1) ¹⁵. The bands at 1387 and 1456 cm^-1correspond to symmetrical bending vibration and asymmetrical bending vibration of methyl group respectively (Figure 4). Broader and stronger bands in the region 1300-1000 cm^-1 correspond to C-O stretching vibrations, which usually consists of two asymmetric coupled vibrations, i.e. C-C(=O)-O and O-C-C. Peak at 749 cm^-1 corresponds to out of plane C-H bending²⁰. Strong peak appearing in the region 1731 cm^-1corresponds to C=O stretching vibrations (Figure 4).

Figure 4: FTIR spectrum of PMMA Click here to View Figure

 

In the FTIR spectrum of TiO₂ , Ti–O bands in the range of 800-450 cm⁻¹ corresponds to the stretching mode vibration of Ti–O and the peak at 511  cm⁻¹, the characteristic of Ti-O anatase crystalline phase¹⁸. The peaks at 550 and 1479 cm^-1 corresponds to the stretching vibration of Ti-O and Ti-O-Ti respectively (Figure 5).

 

Figure 5: FTIR spectrum of TiO2 Click here to View Figure

 

The comparison between the two spectra namely PMMA and the composite PMTireveals that the the shape and position of ester carbonyl groups (C=O stretching mode) expanded in the band (1800-1500 cm^-1). The peak position for the C=O stretching (1730 cm^-1) in pure PMMA shows slight shifting (1722 cm^-1). The shifts in the pure PMMA and pure TiO₂ indicate that some interactions have occurred. (Figure 6(i) – (iii)).

Figure 6: (i) FTIR spectra of PMTi 1 and 2 Click here to View Figure

 

Figure 6: ii) FTIR spectra of PMTi 3 and 4 Click here to View Figure

 

Figure 6: (iii)  FTIR spectra of PMTi 5 and 6 Click here to View Figure

 

The SEM images of PMMA and  PMTi 6 are shown in the figure 7. SEM images reveal the porous nature of polymer composites. Incorporation of TiO₂ into the PMMA matrix, reduces the crystallinity and alters the original polymer structure²¹. When the concentration of ceramic oxides are increased, dark spots are observed in the images which indicate the amorphization of the composites. The dark spot in the SEM image is due to the metal oxide particles. And the area of dark spots increases as the concentration of oxide is increased. It shows the crystallinity of the composites, which decreases as the concentration of oxide increases and it resembles the XRD pattern.

Figure 7:(a)  SEM image of  PMMA  Click here to View Figure

 

Response of composites to acetone vapours

The responsiveness, S, of the composites to acetone vapours can be determined from the equation,

S = R_t / R_o                                                        

Where R_o is  the initial resistance of the sensor value, and R_t is the maximum steady state response value of the sensor when it was exposed to the analyte vapour. In the present study, the vapours of acetone were detected by the sensor. The response time is defined as the time required for the sample resistance variation to reach 90% of equilibrium value following a step increase in the concentration of the testing gas. The resistance, responsiveness and sensitivity of the composite-based sensor obtained from the response and recovery curves are summarized in table 1. The sensing mechanism can be explained on the basis of volume expansion and polar interaction of these vapours on the polymer composite’s surface²². The sample resistance increases in acetone ambience and reaches a saturation at a particular value and the sample resistance decreases in the air ambience. The sensing cycle is reversible upto to two cycles. The sensitivity increases with increasing concentration of TiO₂ in the composite samples. The response time of the sensor is found to be 10 seconds while some recent studies report a higher response time²³ and the recovery time was 900 seconds. It can be seen from the figure 8 that the response of the sensor is fast and the samples  PMTi 6 shows highest sensitivity when compared to other polymer composite samples. This is due to higher proportion of in TiO₂ in polymer composites. In order to validate the selectivity of the gas sensor, polymer composite plates were exposed to ammonia and ethanol vapours which do not show any response. The adsorption and desorption mechanism very well works when the sensor is exposed to acetone while the high conductivity of the sensor opts out ammonia and ethanol from sensing them. Hence, the PMTi sensor is selectively sensitive to acetone vapours^24-25.

Figure 8: Response-recovery plots of PMTi composites to acetone vapour  Click here to View Figure

 

Table 1 : Sensor response of PMTi composites to acetone vapour

PMMA/TiO2 composites	Ro MΩ (Initial resistance)	Rt MΩ (Maximum steady state resistance)	Rt/Ro (Sensitivity)
PMTi 1	40	3056	76
PMTi 2	27	2094	77
PMTi 3	25	1984	79
PMTi 4	23	1876	81
PMTi 5	21	1800	85
PMTi 6	18	1760	97

 

Conclusions

A gas sensor based on PMTi has been developed and the experimental results are evaluated. The resistance of the polymer composite got extremely increased in acetone vapours and reached equilibrium over time. The addition of  TiO₂  to  PMMA  has increased  the  conductivity  of  the  PMTi  composites. The  results  have  shown  that  PMTi  composites  have potential  application  for  detecting  acetone  selectively  at room temperature.

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