Optimal radiation shielding capacity and thermal properties of poly(methyl methacrylate) films enhanced with different metal complexes

Abstract

Samples of pure poly(methyl methacrylate) (PMMA) and PMMA enhanced with metal complexes of Zn, Cd, and Hg were synthesized via catalytic chain transfer polymerization. Structural analysis, by means of XRD, showed the amorphous nature of the studied samples which meant high homogenous dissolution of the metals (Zn, Cd and Hg) into the polymer matrix. Density values increased linearly with respect to atomic radius of the incorporated metals. Addition of such elements causes an increase in mass attenuation coefficients and a remarkable decrease in the Half Value Layer (HVL) values, in comparison to existing standard shielding materials. Thermal studies by DSC indicated highly homogenous internal structure of the studied samples, with slight decrease in the T_g value when introducing metals due to heterogeneous nucleation agent role of the Cd, Zn and Hg. All observations and results suggest the studied materials to be used as promising and proper aprons for medical radiation shielding applications.

Introduction

Polymers and polymers reinforced with nanomaterials have shown improved workability in radiation protection, due to their enhanced mechanical properties and elevated thermal stability.^1–5 Basically, any material with tolerable thickness, immense dense structure, high atomic number, and consistent thermal stability can serve for radiation shielding. On the other hand, poly(methyl methacrylate) (PMMA) is a robust acrylic glass with enhanced mechanical properties and elevated thermoplastic and hardness aspects, in addition to its transparency to light. It is being employed in a broad range of applications, including its use in radiation shielding.^6–9 PMMA was employed as an enhancing agent in lead-free radiation shielding sheets and showed improved aspects. ¹⁰ In principle, the functionality of radiation shielding aprons is related to the density and molecular weight distribution of their constitute components. ¹¹ In this regard, the preparation route employed for the synthesis of shielding sheets plays main role in the properties of the product. Catalytic Chain Transfer (CCT) is a free radical polymerization route that was discovered in 1975. ¹² In CCT molecular weight of the produced polymer can be controlled by introducing catalysts that enhances effectively the process of chain transfer. ¹³ The present study aimed at developing lightweight radiation shielding aprons based on PMMA metal complexes. PMMA samples were examined as an alternative to traditional radiation shields, because of its lightweight, improved hardness, cost-effectiveness, and most importantly its diverse uses in everyday applications.^8,14 PMMA was chosen to study its shielding and thermal properties. Also, the effect of the addition of Hg, Zn and Cd carbazone complex as chain transfer agents through the polymerization process on the relevant properties was investigated.

Experimental details

Transparent thermoplastic complexes of poly (methyl methacrylate) were prepared through a catalytic chain transfer polymerization process using zinc (Zn), mercury (Hg), and cadmium (Cd) carbazone complexes as the chain transfer agent.^15,16 The complex concentration in each sample was determined spectrophotometrically, ¹⁷ using a Jenway 6405 UV-VIS spectrophotometer. Concentration values were 52, 56, and 65 µg/g monomer for Zn, Cd, and Hg carbazone complexes respectively. Preparation and concentration tests details are reported elsewhere. ¹⁶ Solution casting method was employed to form pure PMMA and PMMA complexes films onto glass dishes. Each cast film was dried under ambient conditions for 24 hours and then placed in a vacuum oven of temperature about 70 ± 2°C to remove any residual solvent.

Powder X-ray diffraction (XRD) charts for pure PMMA and PMMA complexes, containing different metal ion, were recorded at room temperature using a PW 1830 diffractometer with Cu Kα radiation (40 KV × 25 mA) and a graphite monochromator, with 2θ values from 10 to 90 degrees.

Dry density (g/cm³) was assessed based on experimental data of sample volume and mass, using selected parts of the sheets with homogenous dimensions of about 4.0 cm² surface area and about 0.5 cm thickness. Film mass (g) was measured and film volume (cm³) was calculated based on its dimensions.

Capacity of radiation shielding of the prepared films was assessed by means of gamma photon irradiation. Backscattered flux of gamma photons (with energies of 662, 1173 and 1333 keV) was obtained from the radioactive source ¹³⁷Cs (half-life of 30.17 years) and ⁶⁰Co (half-life of 5.6 years) having strength 10 µCi and detected using 2 × 2 thallium-doped sodium iodide NaI(Tl) scintillation detector, after their interaction with the specimens.

Glass transition temperature (T_g) for each sample was assessed using standard differential scanning calorimeter (DSC), from Shimadzu. Samples were ground into fine powder, then 10 mg was placed in an aluminum crucible and checked in temperatures up to 520°C, in an argon medium with a flow rate of 50 ml/min.

Results and discussion

Figure 1 shows XRD diffractograms of pure PMMA and PMMA metal complexes (Zn, Cd, and Hg). One high intensity broad band is observed for all samples between 2θ = 15 and 35 degrees, which is typical for PMMA amorphous structure. ¹⁸ Another low intensity hump is observed about 2θ = 48 degree, such hump characterizes the short range ordered structure of the materials. The center of both humps is not affected by the incorporation of the metallic elements. However, no sharp peaks corresponding to different content of metal were detected. This implies a good homogenous incorporation of these metals into the polymer matrix. The only observed difference due to the incorporation of the metals is the slight increase in the absorption intensity with increasing the sample density.

Figure 1.

XRD diffractograms for all samples.

Density is an important physical characteristic of polymeric materials. Since it is a function of the molecular mass of the material and is considered as a critical factor that affects production and processing costs. ¹⁹ As shown in Figure 2, PMMA density increased linearly with the addition of different metals. In other words, the presence of metals carbazone in PMMA matrix causes a sort of structural rearrangement of the polymer chain that results in an increased density. It is obvious that PMMA-Hg complex has the highest density, which implies the conversion of PMMA into a more compact structure when adding different metal carbazone to its matrix that resulted in changes in the cross-linking density. The increase in the density is attributed to the formation of new linkages in the structure of the polymer. Physicochemical properties of the polymer are interrelated, and hence any increase in density is associated with an increase in the molecular weight of the compounds. ²⁰ In other words, our PMMA-Hg complex has the highest density, and consequently it has the highest molecular weight.

Figure 2.

The dependence of density for PMMA with different metals carbazone.

Gamma ray shielding of studied complexes was carried out experimentally at fixed energies of different values, (0.662, 1.173 and 1.333 MeV). Figure 3 shows the relationship between logarithmic relative radiation intensity (LRRI) ratio, $lnI/I_0$ , and sample thickness, for the studied complexes, at fixed gamma energy of 1.173 MeV. Where I and I₀ are transmitted and initial photon intensities, respectively. Obviously, the value of LRRI decreased linearly with increasing sample thickness, in an agreement with Beer-Lambert law, $I/I_0=e^{-\mu .t}$ . Where t is the thickness, and μ is the linear attenuation coefficient, which is defined as the fraction of gamma photons removed per unit thickness of the material, and is a function of its density. ²¹ For the same thickness, values of the LRRI ratio showed a linear decrease with increasing sample density. Using values of both density and linear attenuation coefficient, the mass attenuation coefficient (cm²/g), which is the ratio between the linear attenuation coefficient and the density of shield material (μ/ρ), was calculated then plotted as seen in Figure 4. Where a linear relationship between the mass attenuation coefficient and the physical properties (linear attenuation coefficient, density, and atomic radius) was observed. Such behavior is most obvious in case of PMMA enhanced with metals than the pure PMMA films.

Figure 3.

Ln I/I_o ratio, at fixed gamma energy of 1.173 MeV and different film thickness (t), PMMA, Zn-PMMA, Cd-PMMA, and Hg-PMMA complexes.

Figure 4.

The mass attenuation coefficient for sheets of PMMA with different metal complexes, at different energies.

The Half Value Layer (HVL), in cm, at which the radiation intensity reduced by half, was calculated using linear attenuation coefficient (µ) values according to the following relation equation^9,22:

$HVL=ln0.5/\mu =0.693/\mu$

Figure 5, manifest two important features. One is the very low value of the HVL in case of PMMA complexes over the pure PMMA. Also, shows a relative decrease in the HVL value between samples with different metals. The other feature is the dependence of the HVL layer on the applied energy, which showed a slight increase in the HVL with increasing the energy, which agrees with the literature.^23,24 Our results for pure PMMA agrees with recently published work, in which they reported HVL value of about 10 cm for pure PMMA at about 0.400 MeV. ²² Interestingly, in the present work HVL values for all three enhanced PMMA samples are very low, at around 5.0 mm for the Co-60 source, which is less than quoted values for Lead at around 12.5 mm. ²⁵

Figure 5.

The HVL for pure and enhanced PMMA (Zn, Cd, and Hg). Inset is a zoomed in view on the data for the enhanced PMMA samples.

Thermal characteristics are attributable and good indicator to the internal structure of materials and their response to heating. Differential scanning calorimetry (DSC) studies were performed on PMMA samples to determine the glass transition temperatures, (T_g), and crystallization, (T_c), with a heating rate of 5°C/min up to 520°C as shown in Figure 6 and Table 1. DSC analysis showed only one crystallization exothermic peak for pure PMMA, while in the presence of metals carbazone two crystallization exothermic peaks have appeared. Both pure and PMMA containing metals exhibit a single endothermic peak attributed to the transition temperature range, which represents the strength or rigidity of the structure and full crystal growth. The appearance of a single peak due to the T_g transition temperature in DSC pattern indicates the high homogeneity of the studied samples. Crystallization temperatures given in the experimental procedure were selected from the DSC curves depending on the endothermic and exothermic reaction temperatures. The appearance of two crystallization peaks ( $T_{c1}$ and $T_{c2}$ ) on the DSC curve implied that at least two different crystal phases were formed during the heat treatment. This didn’t appear in the XRD data because it was performed at room temperature. At the same time, a slight decrease in T_g was obviously observed as shown in Table 1. The difference in T_g may be attributed to the heterogeneous nucleation agent role of the Cd, Zn and Hg particles that weakens the bonds between atoms in PMMA as shown in the T_g temperatures of DSC curves. ²⁶ It is found that the nature of the PMMA is very sensitive to its composition, where T_g decreases gradually due to the presence of metal carbazones. The quantity, $\Delta T=T_{c1} - T_g$ is often used as an estimation of the polymer stability.

Figure 6.

DSC curves of PMMA specimens without and with different metal complexes in temperatures range 27–527°C.

Table 1.

The glass transition temperatures (T_g), the crystallization (T_c) and the glass stability (ΔT).

Sample	Tg (°C)	$T_{c1}$ (°C)	$T_{c2}$ (°C)	ΔT (°C)
PMMA	119	197	—	78
Zn-PMMA	116	196	430	80
Cd-PMMA	112	194	432	82
Hg-PMMA	110	192	431	82

Conclusions

In this work, the effect of using Zn, Cd and Hg metal ion complexes as chain transfer agent through the polymerization of methyl methacrylate on the shielding and thermal properties of poly(methyl methacrylate) sheets have been studied. The change of density reveals that PMMA sheets in the presence of these metal complexes are denser than pure PMMA. The gamma ray shielding performance of the investigated polymer complexes was studied through mass attenuation coefficients and half value layer. Interestingly, HVL values for all three enhanced PMMA samples are very low at about 5.0 mm in comparison to quoted values for Lead at around 12.5 mm. The PMMA enhanced with Hg showed best gamma shielding capacity. The results also, manifested high thermal stability and low glass transition temperature of the investigated materials. The present study reveals that the PMMA loaded with Hg, Cd, and Zn have the capability to be used in shielding applications in several radiation shielding technologies.