Chemiluminescence of Polypropylene

Introduction

Chemiluminescence (CL) is the term used to describe the emission of (usually) visible light as a result of a chemical reaction. In the 1960s, different polymers were also studied with respect to this phenomenon. Under natural conditions, most polymers degrade or decompose as a result of oxidation due to atmospheric oxygen. In a first step, unstable alkyl radicals are formed through the action of heat, mechanical stress or the effect of light.

The radicals then react with oxygen to form peroxide radicals. Peroxide radicals can also be produced during the production of plastics and be present in the plastic as an undesirable secondary product. In the presence of oxygen, the peroxide radicals accelerate the decomposition of the polymer through a chain reaction mechanism.

The step of the reaction in which chemiluminescence occurs has not been fully clarified. A mechanism often described in the scientific literature assumes that it takes place when two peroxide radicals recombine, whereby, besides oxygen, an excited carbonyl radical is formed (Russell mechanism [2]). Chemiluminescence measurements allow specific studies to be made on the oxidation of polymers and hence on the influence of stabilizers. This article shows how this can be done using an HP DSC827^e.

Experimental Details

Chemiluminescence measurements can be made with (non-imaging) photomultipliers or with highly sensitive CCD cameras. The latter technique produces images and has the advantage that you can “see” the individual centers where chemiluminescence occurs. This is of particular interest, for example, if you want to investigate the influence of impurities or inhomogeneity in the sample on its decomposition behavior. In this study, we used a CCD camera (SensiCam from PCO.imaging) together with an objective of high light-gathering power (Navitar, f-number 0.95, focal length 50 mm) to investigate the chemiluminescence of a sample in the HP DSC827e. The instrumental set-up used for these measurements is shown schematically in Figure 2.

The experiments were performed with small samples cut from a polypropylene (PP) film. Each sample weighed about 0.4 mg and had an area of 4 mm2. Two such samples were arranged side-by-side in a crucible and a small piece of copper placed on the upper surface of one of the samples. The measurements were performed in the HP DSC at different temperatures in an oxygen atmosphere (50 ml/min). The chemiluminescence was measured by integrating the light intensity for different periods of time depending on the measurement temperature used, that is 5 minutes at 150 °C, 10 minutes at 140 °C and 130 °C, and 15 minutes at 120 °C. In each experiment, the first image measured was used as a “background image” and subtracted from the images that followed (blank correction). The chemiluminescence curves were constructed by plotting the mean gray values of the two samples individually as a function of time.

Results

Figure 3 shows images taken at different times of samples measured at 140 °C. The first signs of chemiluminescence become apparent after 60 to 70 minutes. The dark triangle on the images is due to the small piece of copper that was placed on one of the two samples. The images show that the oxidation clearly develops from this piece of copper - as if the copper acts as a kind of nucleus for decomposition. The second piece of film (without copper) only begins to exhibit chemiluminescence after about 110 minutes. It is noticeable that the chemiluminescence intensity over the surface of the film is not uniform. “Centers of decomposition” apparently form from which oxidation of the sample develops.

Figure 4 shows the chemiluminescence (CL) intensity curves for the samples at 140 °C together with the measured DSC signal. Induction times (OIT) can then be determined from the CL intensity curves of the two samples. For the sample with copper, the OIT is about 60 minutes, and for the sample without copper about 100 minutes. The determination of the OIT from the DSC curve is however not so straightforward because the baseline is not horizontal at any point. 

The reason for this is that polypropylene undergoes significant recrystallization at higher temperatures, which affects the shape of the DSC curve. In contrast, this has no influence on the chemiluminescence so that the OIT can be determined with good reproducibility. An OIT of about 80 minutes can be estimated from the DSC curve.

Conclusions

The decomposition of polymers can be observed by means of chemiluminescence measurements. This method allows the influence of stabilizers or other additives on the stability of polymers to be studied. 

Compared with pure DSC measurements, chemiluminescence experiments with a CCD camera offer a number of interesting advantages.

1. Chemiluminescence occurs specifically only during the decomposition of the polymer. It is therefore appreciably more selective and specific than DSC, which measures the sum of different processes that proceed at the same time (e.g. in the above example, recrystallization and decomposition).
   
2. Chemiluminescence can be measured very efficiently with the detectors available today. This allows decomposition processes to be studied at lower temperatures, which is of course more relevant in practice.
   
3. The distribution of the observed chemiluminescence allows propagation and nucleation phenomena relating to the decomposition to be studied.
   
4. Chemiluminescence can be measured even with very small samples.
   

Measurements performed on a polypropylene film as an example showed that the decomposition spreads out from “decomposition nuclei”. Using polymer samples on which a small piece of copper had been placed, it was clearly demonstrated that at higher temperatures the copper functioned as a “decomposition nucleus”. At lower temperatures, the nucleating effect of the copper was insignificant.

_{Chemiluminescence of Polypropylene |  Thermal Analysis Application No. UC 204 | Application published in METTLER TOLEDO Thermal Analysis UserCom 20}