Assessment the Exposure Effects of Polycarbonate with X-ray Radiation using Spectroscopic Techniques and Molecular Modeling Calculations

doi:10.21203/rs.3.rs-3642873/v1

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Assessment the Exposure Effects of Polycarbonate with X-ray Radiation using Spectroscopic Techniques and Molecular Modeling Calculations

https://doi.org/10.21203/rs.3.rs-3642873/v1

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In this article, the effects of 6 MeV energy of X-ray radiation on polycarbonate polymeric films were investigated. The induced alterations are assessed using several methodologies: FTIR spectroscopy, contact angle measurements, surface roughness assessment, UV/Vis spectroscopy, and luminescence emission spectroscopy. As well, using the basic sets in the ground state of the polycarbonate structure, the vibrational analysis has been carried out using the density functional theory (DFT). The FTIR spectra display that the X-ray irradiation produces surface chemical alterations in the irradiated films due to successive degradation mechanisms due to the decrease in the detected band peaks. The basis sets that were calculated using the DFT method are in good agreement with the experimentally observed spectra. The frontier molecular orbital energies are used to assess the molecule's energy gap (HOMO-LUMO). The value of the frontier energy gap reflects the chemical reactivity and intermolecular charge transfer that take place within the molecule. The surface wettability behaviors were amended due to the decrease in the contact angle values of irradiated films. This leads to an increase in the surface roughness and surface free energy. X-ray irradiation can enhance the surface goodness of polycarbonate films and control their surface properties to be used in biocompatibility applications. The optical properties of irradiated films show modifications in the studied optical parameters. The absorbance spectra exhibited a shift in the absorption edge of the irradiated samples compared with the pristine one. This shift indicates the decreases in the band gap energy of irradiated samples. For direct transitions, the band gap decreased from 4.03 to 3.125 eV, and for indirect transitions, it decreased from 3.5 0 to 2.65 eV. This result was attributable to the formation of defects and the creation of complex charge transfer due to X-ray irradiation. The photoluminescence emission spectra show that the peak intensities are obviously influenced by increasing the irradiation doses. This is attributable to the band-band transition, donor/acceptor pairs, and bound to free transition, which correlated to the received absorbed dose.

Nuclear Physics

X-ray radiation

Polycarbonate films

FTIR spectroscopy

DFT

Surface wettability

Optical properties

The physical and chemical properties of modified polymeric materials, like optical, electrical, mechanical, surface, and thermal have been used in several industrial and medical applications [1]. However, these modified properties are limited to undesired behaviors to be used in other important applications. Modifying the characteristics of polymeric materials has been done using different techniques, like chemical treatments, annealing temperatures, doping with suitable nanoparticles, and irradiation facilities [2–4]. Lately, the irradiation technique is an advanced tool employed to prepare and modify promising polymers that have unique physicochemical characteristics [5]. Different types of radiation can be used in this manner, like gamma rays, electron beams, ion implantation, UV radiation, and X-rays. Most of the time, ionizing radiation (such as gamma rays, X-rays, accelerated electrons, or ion beams) causes changes in the physicochemical properties of solid polymeric materials, leading to the formation of extremely reactive intermediates [6–10]. Numerous investigations into the effects of ionizing radiation on polymeric materials reveal multiple structural alterations in the macromolecular structure of these materials [9]. These alterations comprise carbonization, atom elimination, molecular emission, formation of oxidation products, polar components, creation of ionic chemical species, gaseous and water molecules production, loss of volatile fragments and the generation of free radicals that can cause scission and/or cross-linking mechanisms [11–13]. The two mechanisms are reliant on the polymer’s nature and the environmental conditions, and consequently, scissioning and crosslinking are competing phenomena. Moreover, the natural behaviors of these changes in the irradiated polymeric films depend on the interfering factors, like the composition of the polymer and its molecular weight, and also on the types of radiation and its energy [14–16]. Polymers are gaining popularity due to their numerous advantages, including ease of processing, and several degrees of elasticity or stiffness. They are also more affordable overall due to their durability, lubrication/adhesion, thermal and electrical insulation, transparency, opacity, and color, as well as their resistance to corrosive agents [17]. So, polymers are widely used at this time and have taken the place of conventional structural materials including metals, ceramics, wood, leather, textiles, and paper in many aspects of our daily life.

Polycarbonate films are one of these polymeric materials used in signs, windows, greenhouses, and other purposes. Polycarbonate films have lately gained more attention due to their enormous applications, including lightweight traditional eyewear, bulletproof jackets, bus shelters, aviation windows, machine guards, sports helmets, windshields, and aircraft canopies. Due to the limitation of the usage of polymeric materials in medical and industrial applications, this study concerns the investigation of the enhancements of polycarbonate films after exposure to different doses of X-ray radiation to be used in medical applications, especially in X-ray diagnostics and radio-graphical applications, through improving its surface and optical properties. Moreover, it can be employed in specific materials that are applicable, such as lenses, optical fibers, optical data storage media, protective shields, automotive parts, and others. To investigate this viewpoint, the chemical bond changes, surface behaviors, and optical and photoemission properties of irradiated films were investigated and discussed. Additionally, the molecular modeling calculations were investigated using DFT theory to compare with the experimental findings.

2.1. Polycarbonate films

Samples of small pieces with a 2 cm x 2 cm area were cut from a sheet of Polycarbonate film manufactured by Covestro Solution Center (Germany). The samples have a thickness 275 µm, and 1.2 g/cm³ of density. The optimized chemical construction of polycarbonate film is shown in Fig. (1).

2.2. Irradiation facility

The polycarbonate film samples were exposed to 6 MeV of energy using ELEKTA precise source by different irradiation doses from 1000 Gy and 5000 Gy. The irradiation method was performed in air at Zagazig University Hospital at the linear accelerator of the radiology department, Egypt.

2.3. Analytical methods

2.3.1. FTIR spectroscopy

The infrared spectra were performed in the wavenumber range 400–4000 cm^− 1 with SMART OMNI-TRANSMISSION Nicolet iS10 FTIR spectrometer (Thermo Scientific). The detection spectra resolution was ± 1 cm^− 1 and the accuracy was better than ± 4 cm^− 1 for the full range. The spectra scan were performed for each sample three times and the average of obtained results are evaluated.

2.3.2. Contact angle and surface roughness measurements

The hydrophobicity and hydrophilicity were carried out using Ossila Contact Angle Goniometer, (Ossila Digital Goniometer, Model L2004A1, Sheffield, UK) with a measurement precision of ± 1 to assess wettability of the pristine and irradiated polycarbonate films at room temperature. The evaluation platform was leveled and set at 0 degrees. Three drops of 5 µL deionized water and glycerol was dropped on the pristine and irradiated area of each sample on the evaluation platform. The average values of captured images for each measurement were analyzed using Ossila Contact Angle v.3.1.2.2, the goniometer’s software.

The influence of X-ray irradiation on the surface roughness and surface morphology were performed using Surface Roughness Tester (Model SRT-6600) and an Optical Microscope Trinocular analysis system (Model N-800M) attached with a programmable high-resolution digital camera, respectively. The roughness coefficient R_a was measured at different positions on the surface films for five times, where the contact method by a sensor was used. The roughness tester was connected to a computer and a software program, DataView V1.2, was used to evaluate the measured data.

2.3.3. UV/Vis spectroscopy

The information about the electronic transitions of the materials is analyzed by UV–Visible spectroscopy. The absorption spectra of irradiated polycarbonate films were detected. JASCO Model V-630 UV-Visible Double-Beam Spectrophotometer with a 200–1100 nm wavelength at room temperature was used.

2.3.4. Luminescence emission spectroscopy

The induced defects formed of the irradiated films can be studied by the luminescence emission. This luminescence spectrum were registered of pristine and irradiated polycarbonate films using double beam monochromatic spectrometer, Model RF-1501 Shimadzu, Japan. First, the absorption edge was used to determine the selected activation energy value or excitation wavelength from UV/Vis spectrum. The luminescence spectra were scanned at 350 nm excitation wavelength and in the emission mode, between 220 and 900 nm.

2.4.1. FTIR analysis

The vibrational modes of chemical bonds in the pristine and irradiated polycarbonate films were detected by the transmittance bands mode of FTIR spectroscopy. The FTIR spectra of transmittance bands of polycarbonate films are shown in Fig. (2). It is essential to understand how the transmittance of the observed peak connected to the current functional groups of polycarbonate bonds changed with respect to the estimated bands. These bands were determined based on the relative changes in the predicted bands' widths. The detected functional groups of the pristine polycarbonate film were as follows: 886 cm^− 1 for C–O–C bond, 1656 cm^− 1 for C = O bond, 2331 cm^− 1 for CO₂, 2879 cm^− 1 for CH₃ bond, 3049 cm^− 1 for C–H stretching bond, and 3528 cm^− 1 for OH bond [3, 6, 18]. Figures (3) displays the induced changes in these distinctive wavenumbers' band intensities. It can be noticed from Fig. (3) that the transmittance intensity of detected band peaks are decreased with increasing the irradiation doses. However, the hydroxyl group may be formed due to the chain scission of C = O stretch corresponding to the peak intensity 1656 cm^− 1 at the carbonate sites with the possibility of the elimination of carbon monoxide and carbon dioxide. Also, the slight decreases in the band intensity corresponding to the C–O–C that may be due to the scission process as a result of the breaking of C–O–C bonds. This indicates an increase in the scissioning process at carbonate sites and the release of gas during X-ray irradiation. The same result was obtained in the band intensity corresponding to the hydroxyl group due to the end group of macromolecules. Furthermore, the dissipation of carbonate linkages and the smashing of (C-H) bonds are responsible for the decrease in band intensity with increasing radiation dose and lead to the removal of –H from the polymer's backbone. It should be noted that the scission mechanism induced by X-ray is the dominant one, resulting in an increase in chain mobility and allowing some molecules to reorganize. The formation of carbon-rich clusters and carbonization of the irradiation area is responsible for all of these. Therefore, X-ray irradiation causes changes in peak intensity and breaks polymer chains, leading to micro-strain in polycarbonate polymers. In general, X-ray radiation interacts with polycarbonate films and causes chain scission [19], and the C-O bonds in the polycarbonate chain near the carbonyl group are weak bonds that lack the resonance stability of the phenyl group [20, 21]. This carbonate group selectively absorbs energy and undergoes chain scission, resulting in the formation of phenoxy and phenyl free radicals. Further, carbonyl, methyl, and methylene group transmittance also decreased, indicating the breakdown of the carbonate bond [22–24].

2.4.2. Molecular Modeling Calculations

This work was established to interpret the vibrational analysis of polycarbonate polymeric film. The vibrational and spectral implementations of polycarbonate structure were discussed by using density function theory (DFT) studies [25, 26]. To calculate the electronic structure of atoms, molecules, and solids, DFT is a quantum techniques (QM) method used in chemistry and physics. The understanding of chemical structure and reactivity is facilitated by electronic structure calculations. In addition, the DFT method and basis set 6-311g* are utilized in determining the conformation behavior of material molecules in both neutral and ionizing states [25]. The calculations that were acquired provide valuable information about the structure of molecules, which can be utilized to support experimental findings. Furthermore, the density functional theory (DFT) can be employed to study the molecular structure optimization and harmonic vibrational frequencies of the materials through the B3LYP/6-311g basis set. With DFT, it is possible to analyze and understand the electronic absorption as well as evaluate thermodynamic and non-linear optical properties, total dipole moment, HOMO–LUMO energies, natural population analysis, and global chemical reactivity descriptors. Over the last few years, the use of DFT computations to understand vibrational and structural features has increased dramatically. To define the entire vibrational influence on the structure of polycarbonate polymer, DFT computations will be used in conjunction with vibrational spectroscopy techniques. The energy gap of the molecule was calculated from the obtained HOMO-LUMO calculation. The polycarbonate structure has been studied theoretically using the molecular modeling software, Spartan’20 parallel suite [27]. The structure was optimized using density function theory (DFT) with basis set 6-311g* [25, 26] for obtain the thermochemical data of the polycarbonate molecules. The experimentally and theoretical FTIR spectra registered of polycarbonate structure are depicted in Fig. (4).

Figure (5) shows a correlation graphic between theoretical wavenumbers and experimental findings that is linear and is determined by the equation inset in the figure. By comparing the theoretical wavenumbers to recorded values, the basis sets computed using the B3LYP/6-311g* method is shown to be in good agreement with the experimental spectra. The HOMO-LUMO analysis was investigated in an attempt to predict the stability of the molecule and computed using lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO). The pictorial representation of HOMO and LUMO of polycarbonate using B3LYP/6–311g* is shown in Fig. (6). The molecular orbital energies are effective instruments that play a crucial role in the electric, optical, and quantum chemical properties. The HOMO and LUMO orbitals, two significant orbitals, are used to define conjugated molecules, which named as Frontier Molecular Orbitals (FMO). These FMOs exist at the outermost boundary between a molecule's electrons [24]. The FMO explains a variety of reactions, but one in particular involves an electron transferred from the (HOMO) to the LUMO. The energy of HOMO evaluates electronic donor behavior, whereas energy of LUMO evaluates the electron-acceptor behavior. Larger HOMO energy values are indicative of higher electron-donor capacities, and lower LUMO energy values are indicative of reduced electron-acceptance barriers. The HOMO and LUMO frontier energy separation is a significant factor in establishing the electrical transfer within molecules capabilities, which also reveals information about the molecules' kinetic stability, chemical reactivity, and optical polarizability. In Fig. (6), the HOMO-LUMO graphic representation is identified. The HOMO-LUMO plot describes data on molecular physical characteristics and sheds insight on the reactivity nature [29]. In Fig. (6) red to green color codes denote the positive and negative phases of a molecule, respectively. The molecular orbital energy gap at the frontier (ELUMO-EHOMO) plays an important role in implying the molecule stability with respect to subsequent chemical alterations after irradiation. The energy difference between HOMO and LUMO in this instance is estimated to be 6 eV. When the frontier orbital gap has a high value, the molecule has a poor chemical reactivity and eventually undergoes charge transfer within the material. Molecule with a high value of HOMO-LUMO gap indicates to the compound that is less polarizable and chemically stable [30, 31]. Moreover, FT-IR calculates the vibrational energy for each level, so the difference between the LUMO and HOMO gives the energy gap value for this molecule. After irradiation, a reactive molecule is capable of reacting with several molecules, and it has a tendency to decrease its energy significantly when perturbed. The irradiation can cause either external or internal perturbation, depending on whether it is caused by other molecules during irradiation or by the structural change of the molecule itself.

2.4.3. Surface wettability and surface roughness

Many industrial applications rely on this property, including lubrication, liquid coating, printing, sprays quenching, and oil recovery, as well, it's also employed in medical settings [32–35]. The contact angle, which indicates how wet the liquid is when it contacts the solid surface, can be quantified as primary important data. High wettability is indicated by a contact angle smaller than 90°, while low wettability is indicated by a large contact angle (≫90°). The measured contact angle of deionized water and glycerol on the pristine and irradiated polycarbonate films is illustrated in Table (1). It is evident that the contact angle values decrease as the X-ray doses increases for the two liquids. This is attributable to the creation of new hydrophilic groups and oxidized layers due to the correlation of the rate of oxidation changes on the irradiated polycarbonate surface during the irradiation causing the contact angle to decrease as a function of X-ray doses [36]. Following the irradiation process, the presence of oxygen causes the creation of hydroperoxides, which are eventually transformed into carbonyl groups. Furthermore, broken bonds cause radical species to form on the irradiated polycarbonate surface. These radical species interact with molecules in the air, particularly oxygen, resulting in a rise in the number of polar groups (like –OH, C = O, COO–, and COOH) on the irradiated surface films [37]. These groups lead to increased surface polarity and cause the liquid drop to spread across the surface polycarbonate films. This behavior of contact angles is referring the improvement in the surface wettability of the irradiated polycarbonate films.

Table 1

Contact angle (θ_CA) using water (W) and glycerol (G), and Surface Roughness (SR_a) of the pristine and irradiated polycarbonate films.
Doses (Gy)		0	1000	2000	3000	4000	5000
θ_CA (degree)	W	102.61	98.83	90.14	89.45	87.75	64.55
θ_CA (degree)	G	92.34	88.12	86.67	67.02	64.38	61.31
SR_a (µm)		0.05	0.138	0.141	0.208	0.713	1.778

The wettability, the contact angle of a liquid with a solid, and adhesion are correlated and impacted by roughness. An overarching principle states that roughness promotes wettability at low contact angles and decreases it at high contact angles. The deflection in one direction or the other is determined by the limiting contact angle of wettability, which is 90°. Wetting of the surface is energetically beneficial at contact angles lower than 90°. Due to the minute depressions, a rough surface has a bigger total surface area than a smooth one, providing a larger wetting area, and increasing the wettability. Table (1) shows the variations in surface roughness measurements of irradiation polycarbonate films. It is apparent that as the irradiation dose is increased, the surface roughness coefficient SR_a increases. This may be due to the fact that irradiation adds another layer, increasing surface energy and changing wettability and bonding strength [38]. Furthermore, after interacting with the polycarbonate surface, the X-ray radiation loses energy in localized areas on the near-surface layer. This may result in the creation of chemically active defects and clusters on the irradiation surfaces that are atomic in size. Therefore, the surface becomes more hydrophilic and has a greater surface area due to the increase in surface roughness. This indicates that there is a surface that traps air, increasing the interaction of polar components with liquid droplets and decreasing liquid contact angles.

The optical microscope images clarify the surface changes in the X-ray irradiated samples compared with the pristine one. Under identical circumstances, all images have been formatted. Figure (7) displays gradual transformations in the changes of the surface roughness with increasing X-ray doses. This alteration might be brought on by the production of free radicals after X-ray exposure on the irradiated surface, which also causes degradation and crosslinking.

2.4.4. UV/Vis Studies

The most effective method to detect some optical properties of materials is to measure the absorption of UV/vis spectrum. UV/Visible absorption spectroscopy may quantify the wavelength-dependent attenuation of light beam after it has passed through the substance. The Beer-Lambert Law states that the relationship between the intensities of the input I_o and output I can be used to calculate the absorbance (A) according to the relation [36]:

$$\frac{I}{{I}_{0}}= {e}^{-A} \left(1\right)$$

When a molecule absorbs a photon, its electrons are energized to move from one energy level to a higher energy level. These molecular electronic transitions include types: 𝜎 → 𝜎∗, n → 𝜎∗, n → 𝜋∗, and 𝜋 → 𝜋∗. These transitions were referred to as "band-to-band" or "exciting transitions." The basic absorption measured from the UV/vis spectrum's highest peak location and manifested by electron excitation from the valence band into the conduction band is named by the absorption edge [14, 39]. The absorbance (A) from UV/Vis spectra can be used to obtain the absorption coefficient ($a$), which is determined by Beer-equation Lambert's and depends on the frequency of absorbed photons (ʋ) [36, 40]:

$$\alpha = \frac{2.303 A}{x} \left(2\right)$$

where x is the film's thickness measured in centimeters (cm). Figures (8) displays the optical absorption measurements with a UV-visible spectrophotometer performed at room temperature on the pristine and irradiated polycarbonate films in the wavelength range of 195 to 1100 nm. It is observed that optical absorption of the pristine sample, which occurs at a wavelength of about 250 nm is attributed to π → π∗, and the carboxyl group transition of C = O. Thus, the structural deformation of the carboxyl group C = O is mostly responsible for the peak's apparent red shift ranged from 250–300 nm following X-ray irradiation. On the other hand, as X-ray radiation dose are raised, the optical absorption edge shows an increasing tendency [41]. An illustration shows that the absorption edge shifts toward the longer wavelength as the X-ray dose is increased. The development of free radicals and defects brought on by X-ray irradiation, which raises the conductivity of samples, can be used to explain this shift. Additionally, it may be deduced that the greater rate of electronic energy loss caused by the incident X-ray radiation affects the samples' optical absorption. The UV-visible spectrum is well known to be highly sensitive to the length of conjugated chains [42].

The dependence of the polycarbonate transmittance spectrum on X-ray doses is shown in figure (9). It is evident that, within a given spectral range, the transmittance increases as the wavelength increases, and then a plateau occurs. This is explained by the fact that as the doses were progressively raised, the color centers of the irradiated films changed, resulting in the formation of new electronic levels in the forbidden gap [13]. Also, as the X-ray dose increases, the irradiated films’ transmittance decreases. This results in the creation of extended systems of conjugate bonds (–C = C–), which in turn causes an increase in defects and carbon clusters. This is caused by the deposition energy that radiation transfers to the polymer chain [14].

The absorption edge shifting to higher wavelengths causes a change in the optical band gap. However, according to Tauc's relation, the optical band gap (E_g) and photon energy (h$\upsilon$) is connected to the absorption coefficient (α) in the following relation [14, 40]:

$${\left(\alpha h\upsilon \right)}^{\frac{1}{n}} =C\left(h\upsilon -{E}_{g}\right) \left(3\right)$$

where C is a constant, n is the kind of band gap transition that relies on the structure of the material, and the energy of the incident photon is hʋ. The n value displays (1/2) for direct transitions that are allowed, (2) for indirect transitions that are allowed, (3/2) for direct transitions that are forbidden, and (3) for indirect transitions that are forbidden. The band gap energy of the pristine and irradiated polycarbonate films was determined from the UV-visible absorption spectra by extrapolating the plot of (αhυ)^1/n versus (hʋ) on the hυ axes. (αhυ)² and (αhυ )^1/2 were plotted as functions of photon energy (hυ), respectively, to determine the direct and indirect energy band gaps. Considering the linear component of the UV-visible spectra's fundamental absorption edge is shown in Fig. (7). For pristine and irradiated polycarbonate films with varying X-ray doses, the direct and indirect band gaps have been calculated from the intercept of the fit lines on the hυ axis as shown in Figs. (10) and (11), respectively.

It is clear from Figs. (10) and (11) that X-ray irradiation causes the band gap for the polymer samples to drop, going from 4.05 to 3.25 eV for direct transitions at the highest irradiation dose and from 3.50 to 2.65 eV for indirect transitions at the highest irradiation dose, respectively. The results demonstrated that when the X-ray dose rises, the optical band-gap values drop. This might result from a change in the structure of the polymer matrix as evidenced by the rise in the number of induced defects that occur in the irradiated polymer films. This is associated with the creation of localized states in the band gap within the irradiated films and the carbon enriched clusters are generated due to the release of hydrogen molecules. Additionally, this alteration is likely attributable to the generation of free radicals and active products due to the interaction of energetic photons with the polymer chain causing scission and crosslinking. This is harmonious with the hypothesis of HOMO–LUMO [43] which is extremely sensible to the molecular states and impurities in polymeric chain networks. Overall, X-ray authorized extra charge carriers to be produced and consequently, the eventuality of ionization and excitation of the molecules increased as well as the increase of localized states existing in the band gap.

Thus, it can be concluded that the results of the current investigation demonstrate a strong relationship between the optical band gap energies of irradiated polymer samples and the X-ray dose. It is possible that the severe chain scissions and subsequent cross-linking process, which cause a large amount of disorder, are the cause of how the optical band gap varies with X-ray exposure. This is due to the fact that X-rays harm objects as they pass through due to three distinct processes: physical, physico-chemical, and chemical [39, 44]. One of the main factors investigated to improve the polymeric materials' optical capabilities material is the carbon cluster. The surface conductivity of the polymers is enhanced because of the large number of charge carriers in this cluster. The number of carbon clusters (M) may be calculated using the equation [14, 39]:

$$M={\left(\raisebox{1ex}{$34.3$}\!\left/ \!\raisebox{-1ex}{${E}_{g}$}\right.\right)}^{2} \left(4\right)$$

The numeric value (34.3 eV) is associated with π–π* optical transitions in band structure energy (-2.9 eV) of a pair of adjacent π bond sites in –C = C– ligation for hexagon rings in buckminsterfullerene form. The number of carbon atoms can be calculated using the Tauc's equation [45]:

$$N=\frac{2\pi \beta }{{E}_{g}} \left(5\right)$$

where 2β represents the band structure energy of a pair of adjacent π sites and β takes the value 2.9 eV for carbon ring. Figures (12) shows the relationship between the X-ray doses and the optical energy gap (E_g), the number of carbon clusters (M), and the number of carbon atoms (N) of the polymer detector (a) direct transition, and (b) indirect transition. It is evident that both M and N greatly rise in the direct and indirect transitions. This increase in M and N values is caused by hydrogen escaping from samples as a hydrogen molecule as a result of the breakdown of C-H bonds after irradiation process [39].

2.4.5. Photoluminescence spectroscopy

The most popular method for investigating induced defects in luminescence materials is photoluminescence spectroscopy. Three distinct processes are involved in luminescence: (a) excitons, or electron–hole (e–h) pairs, are excited by an external energy source; (b) the e–h pairs are thermalized toward thermal or epithermal equilibrium, and (c) the thermalized pairs undergo radiative recombination to generate electromagnetic emission. The recombination of excitons or band-to-defect level or band-to-band transitions is commonly associated with peaks in the photoluminescence spectrum [4, 15]. The radiative-recombination of the thermalized pairs of electron-hole (e–h) occurs in this technique as a result of the excitation and transfer of energy into the chromophoric sites [14]. Figures (13) displays the photoluminescence intensity spectra of pristine and irradiated polycarbonate films.

The obtained PL spectra have a primary broad emission luminous bunch in the 2.25–3.31 eV range, as seen in Fig. (13). It is evident that the main emission PL peak positions and knobs in the spectra of the pristine sample coincide with the maxima of other samples at 2000 Gy and 3000 Gy X-ray doses and shifted with slight values towards the shorter wavelength (higher photon energies) at 1000 Gy, 4000 Gy and 5000 Gy X-ray doses. These changes correspond to the blue shift, also known as the hypsochromic shift, which is defined as a change in the spectral band locations in the absorption, reflectance, transmittance, or emission spectrum of a molecule exposed to a shorter wavelength [14]. Furthermore, a change in the environment, such as a change in solvent polarity or exposure to radiation, might cause this shift. As a result, a hypsochromic shift can occur in a succession of structurally related compounds that occur in a substitution series. The emission spectrum of each sample is resolved and the resolved emission peak bands and knob positions of the pristine sample and irradiated polycarbonate films are illustrated in Table (2). It is evident that the PL intensity corresponding to the emission photoluminescence bands varies with the received dose. This could be due to the formation of free positive and negative radicals, which serve as defect, acceptor, and donor sites. As a result, as the radiation dose increases, the number of defects, acceptors, and donors increases as well. On the other hand, the radiative recombination of luminescence emission may be due to the band-band transition, donor/acceptor pairs, and bound to free transition, which correlated to the received absorbed dose. As the radiation dose is increased, new levels are introduced by breaking bonds and forming positive and negative radicals, which act as donor and acceptors. Furthermore, the energy gap is reduced at the donor and acceptor levels, allowing direct band-to-band transitions to occur. The emission energies (energy gaps) for irradiated films are closely related to those obtained from UV-Vis data calculations, with only a slight difference. Therefore, it can be concluded that the emission lines/peaks are equivalent to the observed absorption peaks. In the materials with a direct band gap transition, in which the electric dipole transition is permitted; these materials have a sturdy emission. Also, recombination of free electrons with holes trapped on the acceptor level or recombination of free holes with trapped electrons on the donor level are also included in the free-to-bound transitions. The defect, donor, and acceptor levels become increasingly close as the irradiation dose increases, eventually forming an intermediate defect band. As the dose is increased, the defect band may overlap with the conduction or valence bands, releasing the bound carrier from the impurity levels.

Table 2

Photoluminescence emissions detected of the pristine and irradiated polycarbonate films.
Doses (Gy)	Emission wavelength (nm)	Energy of emission photon (eV)	PL peak intensity (a.u)	Type of PL shift relative to pristine sample
Pristine	403	3.08	81.87	…….
Pristine	426	2.91	93.41	…….
1000	402	3.08	77.20	Hypsochromic - Hypochromic
	426	2.91	90.20	Hypochromic
	453	2.74	56.54	…….
2000	403	3.08	44.26	Hypochromic
	426	2.91	50.45	Hypochromic
	539	2.30	7.06	…….
3000	403	3.08	79.93	Hypochromic
3000	426	2.91	93.03	Hypochromic
4000	401	3.09	27.42	Hypsochromic
	425	2.92	30.74	Hypsochromic - Hypochromic
	530	2.34	5.77	…….
	538	2.30	5.47	…….
	549	2.26	5.01	…….
5000	399		10.68	Hypsochromic - Hypochromic
	427	2.90	11.50	Bathochromic - Hypochromic
	531	2.34	3.33	…….
	535	2.32	3.10	…….
	541	2.29	3.11	…….
	548	2.26	2.85	…….

The modification of polycarbonate films' properties has been explored through the study of the impact of 6 MeV X-rays at different doses. The results demonstrate that X-ray irradiation of polycarbonate films caused considerable modifications to the chemical transformations, contact angles, surface roughness, optical characteristics, and luminescence emission. Due to subsequent degradation and/or crosslinking mechanisms, the FTIR spectra show that X-ray irradiation causes surface chemical changes in the irradiated films. The induced structural degradation response is caused by chain bond breaking, as seen by the FTIR spectra of the irradiated films, highlighting the impact of the X-rays on the polymer chain. Due to the oxidized layer and the development of hydrophilic groups on the irradiation surface, the contact angle dropped from 102.61° and 92.34° for the pristine sample to 64.55° and 61.31° for water and glycerol, respectively, for the greatest irradiated dosage. Irradiation produces free radicals on the film surface, resulting in the production of hydrophilic groups, which subsequently interact with oxygen to produce polar groups. This behavior of the contact angles refers to an increase in the wettability of the surfaces of irradiated polycarbonate films. It is found that the surface roughness coefficient rises as the radiation dose increases; indicating an increase in the irregularity of the irradiated films' surfaces attributable to the formation of species products on the surface. Surface roughness parameter can be improved by using X-ray irradiation, which is considered an effective method. The promotion of desired adhesion may be achieved through an increase in roughness. As a result, the wettability property of the modified films is improved, which makes them suitable for usage in biocompatibility applications as well as other beneficial applications like paint and coating adherence.

As X-ray doses increase, the absorbance intensities in the UV/Vis spectra increase. The formation of intermediate levels as a result of irradiation causes the energy band gap to decrease. Thus, there is an increase in the number of carbon atoms and clusters as a result of the release of hydrogen molecules. Thus, the modified films can be used in optoelectronic devices and anti-reflection coating applications. The luminescence spectra exhibits the resultant emission is correlated with the X-ray doses. The occurrence of different emission behaviors, like hypsochromic, bathochromic, and hypochromic effects, confirms this modification. These findings may provide acquaintance on the band structure, inclusive of the band gap width and the efficiency of relative light output in several materials. Thus, these outcomes render the polycarbonate films a promising option for various technological uses that rely on the photoemission process, such as the investigation of solid surfaces concerning their characteristics in adsorption, catalysis, and photocatalysis, as well as radiation dosimetric uses such as solar ultraviolet radiation dosimetry.

Acknowledgment

The authors express their sincere thanks to Dr. Emad Mostafa, Medical Physicist at International Medical Center, IMC, Egypt for providing us with the irradiation facility for the polymer samples.

Ethical Approval

(not applicable)

Competing interests

No, I declare that the authors have no competing interests as defined by Springer, or other interests that might be perceived to influence the results and/or discussion reported in this paper.

Author Contributions

M.F. Zaki: Conceptualization, Formal analysis, Investigation, Methodology, Data curation, Writing – review & editing. A. Rasad: Computational, Formal analysis, Investigation, Writing – review & editing. Sh. I. Elkalashy: Investigation, Writing – review & editing. Tayseer I. Al-Naggar: Conceptualization, Methodology, Formal analysis, Data curation, Writing – review & editing.

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(No funding)

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Assessment the Exposure Effects of Polycarbonate with X-ray Radiation using Spectroscopic Techniques and Molecular Modeling Calculations

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Abstract

Figures

1. Introduction

2. Experimental Details

2.1. Polycarbonate films

2.2. Irradiation facility

2.3. Analytical methods

2.3.1. FTIR spectroscopy

2.3.2. Contact angle and surface roughness measurements

2.3.3. UV/Vis spectroscopy

2.3.4. Luminescence emission spectroscopy

2.4. Results and Discussion

2.4.1. FTIR analysis

2.4.2. Molecular Modeling Calculations

2.4.3. Surface wettability and surface roughness

2.4.4. UV/Vis Studies

2.4.5. Photoluminescence spectroscopy

2.5. Conclusion

Declarations

References

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