Irradiation effects of Cu ions on the electrical and morphological properties of polypropylene

Abstract

The irradiation of metallic ions on polymeric surfaces may change the chemical, electrical and physical properties of polymers. In this article, we have investigated the effects on the electrical and morphological properties of polypropylene caused by irradiating with copper (Cu) ions generated from laser-produced plasmas. For this purpose Nd: YAG laser (1064 nm) was employed to produce Cu ions. The flux of ions was controlled by varying the number of laser shots in the range 100–400, with a step size of 100. The changes in the electrical conductivity of the irradiated sample were studied using a four-point probe technique, while the surface features were examined by using optical microscopy. The electrical conductivity changed from 4.758 × 10⁻⁸ Ω⁻¹ cm⁻¹ to 5.003 × 10⁻⁸ Ω⁻¹ cm⁻¹ for unexposed and exposed (with 400 shots) samples, respectively.

Introduction

The properties, structure and surface of polymers can be modified using various methods and techniques. These include low-temperature corona discharge, glow discharge plasma treatment, chemical etching and charged particle bombardments. ^{1 –4} Low-energy ion irradiation is particularly useful for polymer surface modification, since these ions induce different phenomena in the near-surface layer of the polymers. ⁵ The exposed polymers with low-energy ions are useful for various applications. For example, the surface modification of polymers can lead to the metal–polymer interface formation that increases metal-to-polymer adhesion, which is generally poor because the cohesive energy of polymers is about two orders of magnitude lower than that of metals. ^{6 –8}

Over the last few decades, polymers have become increasingly important and are considered to be promising candidates for the semiconductor industry, superconducting thin films, medicine and other technologies and research applications. ^{9 –11} However, their use is still limited in the micro- and optoelectronics industries, due to their inherent dominant insulating properties. ¹² Polypropylene, in particular, is one of the most common and widely-used polymers in areas such as construction, wiring and automotive due to its attractive properties which include lightweight, mechanical flexible, suitable and easy processability, less expensive, feasible mechanical properties and so on. ¹³ It is therefore important to study the properties of polypropylene to make it more reliable and useful for long-term future applications. ¹⁴

In the present investigation, we have used ions available from laser-produced plasma (LPP) for irradiation of polypropylene. This method is found to be very promising for surface modification and to alter the structure and properties of polymers. ¹⁵ When the energetic ions interact with polymers, they produce damages in the polymer structures and surface due to cross-linking, chain scission, irreversible bond cleavage, formation of free charge carriers, carbonisation and so on. This results in changes in electrical, optical, thermal, mechanical and magnetic properties. ^{12,16 –19} Previously, several authors have reported results for polymers irradiated with LPP-generated ions of various metals. More recently, carbon, iron, molybdenum, nickel, copper (Cu), titanium and zinc ions have been used to study the optical and surface morphological properties of different polymers. ^12,15,20 . To the best of our knowledge, the ion irradiation effects on polypropylene using LPP have not yet been reported, which motivated us to choose this polymer for the present investigation. In this work, we report ion irradiation effects on polypropylene and investigate the ion surface morphological and electrical properties of this polymer.

Experiment and data analysis

The present investigation was performed using similar experimental set-up as described in several previous studies. ^20,21 Briefly, we used Nd: YAG laser to produce plasma/ions from pure Cu target. The laser operated in its first harmonic with a wavelength of 1064 nm, pulse energy of 10 mJ and pulse duration of 10 ns at full width half maximum. The laser beam was focused on the target by using an infrared lens with a focal length of 220 mm. The intensity of the laser beam was 3 × 10¹⁵ W m⁻² with a spot size of about 12 µm.

We have chosen polypropylene samples with a thickness of 6 µm and dimensions of 12 × 12 mm. For producing Cu ions, we used pure Cu metal in the form of discs. Both polypropylene sample and target were placed in air at room temperature. The laser to target angle was kept at 45° and the substrate was kept exactly parallel to the target so that the entire ions ejected from the target could be collected from the surface of the substrate. The distance between sample and target was kept very small (i.e. 10 mm) to expose the samples with maximum number of ions generated from the target. The flux of the emitted ions was varied by varying the laser shots from 100 to 400, with a step size of 100. The variation of laser shots proved to be very useful for analysis and to see the changes in the properties of the sample.

Electrical properties of the exposed and unexposed samples were obtained from the resistance measure using the four-probe technique. ²² This technique involves bringing four equally spaced probes into contact with the material of unknown resistance. Typical arrangements involve a linear array of four conducting tips with two outer probes providing source current, while the two inner probes sensing the resulting voltage drop across the sample. The volume resistivity was obtained using the following equation

$\rho =2\pi a\left(\frac{V}{I}\right)$

1

where a, V and I represent the sample thickness, measured potential and source current, respectively. The conductivity was measured at several different randomly-selected regions for each sample, and their values were found to be within the uncertainties of about ±4%. The electrical conductivity which is defined as σ = 1/ρ is plotted in Figure 2 as a function of the number of laser shots.

The surface morphological properties of the polypropylene were investigated by optical microscopy using an STM 6 optical measuring microscope (Hamamatsu, Japan). In the current study, the optical micrographs of the samples were taken at 50× resolution.

Results and discussion

As discussed above, the interaction of energetic ions with a polymer surface may produce several complex mechanisms like cratering, structural changes along latent tracks, radiochemical changes, bond breaking and bond formation. The radiochemical changes lead to changing electrical behaviour of polymers while bond breaking and formation may lead to chain scission and cross-linking. ²³ The changes in the electrical properties of polypropylene samples are given in Figure 1 and Table 1.

Figure 1.

Measured electrical conductivity as a function of laser shots of polypropylene sample irradiated with Cu ions. The dotted line is drawn to guide the eyes.

Table 1.

Electrical conductivity values for polypropylene sample exposed with Cu ions.

No. of laser shots	Electrical conductivity (Ω−1cm−1)
0	4.758 × 10−8
100	4.798 × 10−8
200	4.866 × 10−8
300	4.940 × 10−8
400	5.003 × 10−8

Cu: copper.

Electrical conductivity

As can be seen in Figure 1, the electrical conductivity of the unexposed sample is lower than that of the exposed sample and it increases with the number of laser shots. This is due to the fact that with the increase of laser shots, the number of emitted ions increases from the sample; as a result, the sample is exposed to a high dose of ions, resulting in an increase of electrical conductivity.

The electrical conductivity of the polymers on which ions are bombarded depends on different factors and parameters, and the ion flux is one of them. ^12,24 It is also interesting to note that the electrical conductivity changes almost linearly with the increase of the number of shots. This suggests that the dosage of ions increases linearly as the number of laser shots is increased with fixed intervals of 100, 200, 300 and 400 shots. The exact values of electrical conductivity are given in Table 1 which shows an increasing shift in electrical conductivity of polypropylene from 4.758 × 10⁻⁸ Ω⁻¹ cm⁻¹ (unexposed sample) to 5.003 × 10⁻⁸ Ω⁻¹ cm⁻¹ (when exposed with 400 shots).

Optical microscopy

Optical microscopy was used to explore and investigate surface morphological changes due to irradiation of ions. Exposure of polymer surfaces to ions results in change in their surface morphology. Cross-linking, for example, changes the nature of the polymer material and chain scissoring leads to changes in the polymeric structure. Other possible processes involve ion–polymer interaction, radiation-induced viscous flow and local melting. These processes are present in our samples, as described below, but on a very small scale due to the low ion flux.

Figure 2 shows optical micrographs of polypropylene, both unexposed and after it was irradiated with 100, 200, 300 and 400 laser shots. One can see a clear difference between exposed and unexposed samples. Also the surface appearance is different when the sample was irradiated with different numbers of laser shots.

Figure 2.

Optical micrographs of unexposed and exposed samples of polypropylene with 100, 200, 300 and 400 laser shots. The micrographs were taken at ×50 resolution.

At 100 laser shots, there were observable micro-holes and scratches at the surface of the substrate, while irradiation of sample with ions at 200 shots induced micro-holes as well as local melting at the sample surface, as indicated by circles. Similarly, when 300 shots were used to create Cu ions, the irradiated sample seems to have several micro-holes, scratches and the local melted area on the surface. At 400 laser shots, there is even more damage on the surface of the polypropylene material due to the exposure of the sample to a large number of ions.

Conclusions

We have reported modification of electrical and surface morphological properties of polypropylene when irradiated with Cu ions generated from LPPs. It was found that the electrical conductivity increased with the number of laser shots on the Cu target due to the increase in ions fluence on the polypropylene sample. The results of the optical microscopy showed that the surface of the polypropylene was effectively modified as a result of the ion irradiation. These changes are present in the form of micro-holes, ion chains, scratches, localised melting and so on at the exposed polymer substrates.