The influence of pulsed CO 2 laser irradiation on the optical properties of PMMA

In this paper, effect of the pulsed CO2 laser irradiation on optical properties and surface morphology of PMMA was investigated. The samples were irradiated by a CO2 pulsed laser with the wavelength of 9.55 micron and the pulse duration of about 100 ns. UV-Visible spectroscopy and Scanning Electron Microscopy (SEM) were used for investigating the change in absorption, reflection, refractive index and surface morphology, respectively.


INTRODUCTION
Optical properties modification of polymers has been extensively investigated for achievement of desired properties for using in modern optical devices, such as polymeric waveguide technologies (Eldada & Shacklette, 2000).Optical properties of polymer are dependent on the specific polymer, the formulation of polymer and crystallinity.However, chemical or mechanical degradation or even temperature treatment can change the optical properties of polymers and influence their formulation.Depending on initial photon energy, photons will couple into the available vibrational or electronic states.For example, in insulators or semiconductors the absorption of light usually occurs through resonant excitation such as transition of valance bond to the conduction bond or within bonds (Baeuerle, 2000).Energy can be transferred from excited bonds to lattice phonons.If the energy of a photon is lower than the energy gap, it can't be absorbed.Nevertheless defects, multiphoton absorption and impurities may cause absorption (Baeuerle, 2000).Laser irradiation is used as a standard method for surface modification and optical properties changes in different materials especially polymers.Following irradiation different chemical, optical and morphological alterations occur dependent on the irradiation parameters like the laser wavelength, the fluence, the pulse number, pulse repetition rate and the pulse duration.The most important parameter affects laser-material interaction at a given wavelength is the laser fluence.At high fluences, large temperature gradient and small localized laser heating can lead to rapid self-quenching (Kim, Park, & Hong, 2007) of the material.Thermal stress can be induced by the rapid generation of large temperature gradients (Wang & Xu, 2001).These stresses can lead to cracking, surface morphology modification or optical properties changing.On the other hand, radical formation and nanostructuring of the surface are two important results of laser irradiation at fluences below the ablation threshold.Poly (methyl methacrylate) (PMMA) is one of the polymeric materials which is of interest because of its attractive properties.It is inexpensive, biocompatible, and thus ideal for disposable devices in clinical, biological, and chemical applications (Scully, Baum, Liu, & Perrie, 2012;Spasojevic et al., 2015;Zahedi & Dorranian, 2013).Again, its appropriate absorption at CO2 laser wavelength facilitates its use in CO2 laser micromachining for manufacturing microfluidic and optofluidic devices.In this research, optical properties modification and surface morphological changes of PMMA following pulsed CO2 laser irradiation are investigated.

EXPERIMENTAL WORK
Poly (methyl methacrylate) was cut in to pieces with a thickness of 1mm.Samples were irradiated by pulsed CO2 laser with 100 ns pulse duration and 9.55 µm wavelength at different fluences and with various number of pulses.Optical absorption and transmission spectra of pristine and irradiated PMMA were recorded in transmission mode with a step size of 2 nm, in the wavelength range of 400 to 900 nm using a UVvisible spectrometer (UV 2100, Border Company) at room temperature.Surface morphology was investigated by scanning electron microscopy (SEM).The reflection (R) and the refractive index (n) were calculated by using the following equations (Al-kadhemy, Saeed, Kadhum, Mazloum, & Aied, 2014;Zahedi & Dorranian, 2013).

RESULTS AND DISCUSSIONS
The fluence for the single pulse ablation threshold of PMMA samples was obtained experimentally to be about 1.5 J/cm 2 .Then samples were irradiated with different number of pulses at fluences above and below the ablation threshold.Fig. 1 a, b shows the change in transmission, reflectance and refractive index of the samples irradiated at the fluence of 0.5 J/cm 2 with different number of pulses and Fig. 2 and 3 show the results for samples irradiated at the fluence of 10, 30 and 50 J/cm 2 .As it can be seen from the figures, for all the samples and with increasing the number of pulses, the surface transmission decreases; while, the changes in reflection and the refractive index are dependent on the laser fluence and the number of pulses.The changes in the refractive index and the reflectance are the same; Both reflection and refractive index increase with increasing the number of pulses at the fluence of 0.5 J/cm 2 , while they increase first at the fluence of 10 and 30 J/cm 2 and then decrease.Both  and  increase continuously at the fluence of 50 mJ/cm 2 with number of irradiated pulses used in this experiment.Following laser irradiation, different mechanisms may lead to modification of optical properties like transmittance, reflectance and refractive index.These are including production of radicals, bond breaking due to laser irradiation, melting and morphology changing.The changes in surface morphology is very important parameter affect the surface properties.The changes in surface morphology is a result of laser irradiation with high number of pulses at fluences below the ablation threshold or lower number of pulses at fluences above the ablation threshold.Jelvani, & Abolhosseini, 2012;Pazokian, Jelvani, Mollabashi, Barzin, & Farahani, 2011).The radical or new bond formation is also caused UV-Vis spectrum intensity changes (Rai, Mukherjee, & Jain, 2016).The changes in spectrum will be saturated due to decreasing the number of radicals when laser irradiation continues.For example, elimination of C=O and O-H bonds may cause new bond formation between H2O in air and oxygen atom in PMMA.Higher number of pulses can induce higher sample temperatures and it may result in the elimination of C=O and O-H bonds.On the other hand, the ablation is saturated with lower number of pulses at higher fluences.Nevertheless, different reasons may affect the optical properties changing which needs further investigations.Cross-linking following irradiation, new bonds formation or oxidation, crystallization, molecular weight changing, self-quenching, thermal stresses, multi-photon absorption and nonlinear optical effects at high fluences, small produced cracks, defects, microstructures and surface morphology changing are some of the reasons for optical properties modification.On the other hand, for polymers irradiated at IR or NIR, a photothermal mechanism is found.In this case, four classes of mechanisms may appear: random chain scission, end-chain scission, chainstripping, and cross-linking (Kappes, 2012).Generally, if the temperature of polymer is more than glass transition (Tg), devitrification will occur (Pazokian et al., 2012), causing increasing density (Qi et al., 2013).A change in density leads to refractive index modification.

CONCLUSION
PMMA polymer was irradiated with a pulsed CO2 laser to investigate the effect of the laser irradiation on its optical properties.The results of the experiments show that the absorption increases for all the sample irradiated at fluences below and above the ablation threshold.However, the changes in reflection and refractive index depend on the laser fluence.The formation of different radicals on the surface following irradiation below the ablation threshold and a change in surface morphology upon irradiation above the ablation threshold are supposed to be the most important factors affect the optical properties changes.However, the other reasons were suggested for the explanation of the modification of the spectrum intensity, which need further investigations.
Fig.1 a) Transmission and b) Reflectance and refractive index at the fluence of 0.5 J/cm2 with different number of pulses

Fig. 3 Fig. 4
Fig.3 Refractive index and reflectance of the samples irradiated at the fluence of , a-10, b-30 and c-50 J/cm2.