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ACS Omega
ACS Omega
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acsodf
ACS Omega
2470-1343
American Chemical Society

10.1021/acsomega.4c04407
Article
Research on the Stability and Response of Food Packaging Polystyrene Resin Materials to γ-ray Irradiation
Yan Jing †‡
https://orcid.org/0000-0001-8912-3004
Cheng Wencai †‡§
Ding Congcong †‡§
Qiu Ze †‡
Li Xiaoan ∥
https://orcid.org/0000-0003-4751-8408
Lu Xirui *†‡§
† State Key Laboratory of Environment-Friendly Energy Materials, Southwest University of Science and Technology, Mianyang 621010, P. R. China
‡ National Co-innovation Center for Nuclear Waste Disposal and Environmental Safety, Southwest University of Science and Technology, Mianyang 621010, P. R. China
§ Tianfu Institute of Research and Innovation, Southwest University of Science and Technology, Chengdu 610299, P. R. China
∥ Nuclear Medicine Laboratory of Mianyang Central Hospital, Mianyang 621010, P. R. China
* Email: luxiruimvp116@163.com.
08 09 2024
17 09 2024
9 37 3866838677
09 05 2024
28 08 2024
25 08 2024
© 2024 The Authors. Published by American Chemical Society
2024
The Authors
https://creativecommons.org/licenses/by-nc-nd/4.0/ Permits non-commercial access and re-use, provided that author attribution and integrity are maintained; but does not permit creation of adaptations or other derivative works (https://creativecommons.org/licenses/by-nc-nd/4.0/).

Radiation stability of food packaging materials is the key to ensuring food quality. In this study, 60Co γ-ray was selected to investigate the radiation resistance of food packaging polystyrene (PS) resin material, although the FTIR analysis showed that the intensity of several peaks decreased slightly. The gel permeation chromatography (GPC) results displayed that the value of peak molecular weight (Mp) of PS went from 2.68 × 105 g/mol down to 2.22 × 105 g/mol. Moreover, the residual mass (Res) of PS increased from 7.208 to 30.23%, indicating that the tendency of coking of PS was stronger after irradiation. In addition, the peak intensities of the three main pyrolysis products −CH2–, CH4, and CH2=CH2 increased by more than 30% compared to unirradiated PS, and a large number of them were detected in the whole pyrolysis process. Moreover, mechanical property analysis finds that both breaking strength and elongation data increased before irradiation dose of 50 kGy, then, decreased sharply with further increase of irradiation dose. The theoretical bond order analysis confirmed that the tertiary carbon bond attaching the benzene ring had the lowest bond energy. This study can give helpful guidance when using PS for food packing materials.

Department of Science and Technology of Sichuan Province 10.13039/501100004829 2022NSFSC1197 Tianfu Institute of Research and Innovation NA NA Tianfu Institute of Research and Innovation NA 2022ZY013 NHC Key Laboratory of Nuclear Technology Medical Transformation, Mianyang Central Hospital NA 2021HYX028 State Key Laboratory of Environmental-friendly Energy Materials 10.13039/501100019060 22fksy12 document-id-old-9ao4c04407
document-id-new-14ao4c04407
ccc-price
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pmc1 Introduction

Since December 2019, coronavirus disease 2019 (COVID-19), caused by a novel coronavirus (2019-nCoV), has spread worldwide, with enormous social and economic impacts in more than 200 countries worldwide.1−4 In general, the main mode of transmission of COVID-19 is through respiratory droplets.5,6 However, the possibility of transmission of the virus through other infected substances (including food and food packaging surfaces) cannot be completely ruled out.7

Since early July 2020, at least nine food contamination incidents have been reported across China where 2019-nCoV was found in imported food (mostly packaging).7 Although different quality checks are carried out at all stages of the food processing and supply chain, taking into account the consumer safety requirements for food packaging as a potential source of further inspection is still particularly necessary. Generally speaking, there are many different processing methods for food processing. Gamma irradiation as an environmentally friendly cold physical process is being used in more than 60 countries worldwide for a variety of socio-economic applications, such as food safety, quarantine, and disinfection of medical supplies and equipment, inactivation of bacteria and viruses in animal sera and biotherapeutics.8

Over the past 50 years, polymers have played a crucial role in the development of human society,9 driving the development of packaging, electronics, automobiles, medical, construction, and many other industries.10 Polystyrene (PS) is one of the most stable organic polymers, and the presence of a phenyl (C6H5) group is the key to its properties. These large rings prevent the polymer chains from stacking into a tight crystalline arrangement, so the solid polystyrene is transparent. In addition, the benzene ring limits the rotation of the chain around the carbon–carbon bond, thus making the polymer remarkably rigid.11 PS is widely used in many aspects of human life and industry because of its low cost, lightweight, ease of manufacture, multifunction, thermal efficiency, durability, and moisture resistance.12,13

In general, there are three types of common PS: general polystyrene (GPPS), reinforced polystyrene (HIPS), and expanded polystyrene (EPS). Among them, GPPS is often used to make foamed plastics and thermoplastic-forming materials for food and pharmaceutical packaging due to its high heat resistance temperature, high fluidity (easy-to-blow molding), and almost no additives.14,15 Moreover, HIPS plastics are produced by blending styrene-butadiene copolymer as an impact modifier and crystal PS (or general-purpose polystyrene, GPPS). It is a common material for packaging refrigerated dairy products.16 Finally, EPS is a kind of foam crystal polystyrene bead made by adding a foaming agent, which is mainly used in various electrical buffer packaging,17 food packaging, and live packaging of fresh fish.18,19 It can be seen that all three kinds of PS materials can be applied in the field of food packaging. However, when it is used as a food packaging material,20 it is inevitable that PS is sterilized during food processing or during a pandemic to eliminate the source of infection. When irradiation disinfection was selected for application in this field, PS is required to have good irradiation resistance stability.

Therefore, the radiation resistance of PS has also become the first part of our understanding. In 2000, Zhu et al21 used rapid Ar ion irradiation to chemically modify PS and found that the main chain and benzene ring of PS were destroyed at the core of orbit, at the same time, the material carbonization occurred. Then, in 2008, Singh et al22 investigated the effect of proton (3 MeV) irradiation on the optical structure of PS and showed that the band gap of UV–vis light of PS was reduced by 27.5%, the absorption intensity of the main characteristic bands in FTIR decreases. In the same year, Liu et al23 studied the anisotropic deformation of PS particles by using Au ion irradiation. The results showed that the shape of PS particles changed from spherical to elliptical, and the volume of PS particles decreased at the beginning of irradiation; the aspect ratio then increased with the energy density. In 2009, Mathad et al24 studied the effect of an 8 MeV electron beam on the structure of PS film, and the results showed that the film was resistant to electron beam irradiation in the range of radiation dose studied. In 2022, Cui et al25 have been applied to evaluate the differences in microplastics (Plastic particles less than 5 mm in diameter) generation behavior of α and β crystalline-structured polypropylene (α-PP, β-PP) under conditions of ultraviolet (UV) irradiation and natural forces. The results indicate that after UV aging for 1470–1500 and 1670–1700 h, respectively, α-PP generates 71% of microplastics and β-PP generates 25% of microplastics. In 2023, Huang et al26 reported a simulation method through associating plastic aging with mechanical failure on a time scale to predict MP generation and give an experimental verification. The results indicate that the proposed evaluation method has high accuracy for predicting MP generation from aged polystyrene foams. Under conditions of ultraviolet (UV) irradiation and heat for 1000 h, the aged polystyrene foam generates significant microplastics (6.78 × 106 particles/cm3) by water scouring force after the expected aging time (400 h). In the same year, Ferry et al27 investigated the effect of the atmosphere on the irradiation of random PS by fast heavy ions. The results showed that PS had a strong resistance to radiation under an inert atmosphere. In addition, the degradation degree of random PS was increased in an oxidizing atmosphere, but the oxidation degree was not high. It could be seen that many researchers had done PS radiation response-related studies.

However, there are few studies of the response mechanism of PS to 60Co-γ rays. In this study, the PS resin is selected as the subject. The radiation resistance and response mechanism are studied by means of SEM, FTIR, XRD, GPC, TG-DSC, TG-MS, and mechanical property analysis. The purpose is to provide a more comprehensive theoretical basis for the application of irradiation technology.

2 Material and Methods

2.1 Materials

PS (C8H8)n is a colorless, odorless, odorless, tasteless, and glossy transparent granular general-purpose thermoplastic PS resin produced by Shanghai Maclin Biochemical Technology Co., Ltd. The digital images are shown in Figure 1. To facilitate test characterization and increase radiation exposure rates, the PS resin, as shown in Figure 1a was crushed by a high-speed mill. Then, using the GB/T6003.1–2002 standard sieve (100 mesh) to handle them, the small particles, as shown in Figure 1b were obtained. Moreover, a more detailed sample handling procedure is displayed in Figure S1 (Supporting Information). In addition, Figure 1c shows the particle size distribution of PS powder after sieving. Similarly, the particle size distribution test raw data are shown in Table S1 (Supporting Information). The 60Co source was a cylindrical γ-ray source, and it was supplied by China Gold Irradiation Chengdu Co., Ltd.

Figure 1 Schematic diagram of a granular PS solid resin sample (a), schematic diagram of a powdered PS sample (100 mesh) (b), and particle size distribution of PS powder after sieving (c).

2.2 Irradiation Experimental Method

The samples of PS with sufficient doses of 0, 50, 100, 150, 200, and 250 kGy were prepared and sent to China Gold Irradiation Chengdu, LTD for irradiation treatment. The irradiation dose is the product of the irradiation time and irradiation intensity (the irradiation dose rate). In this study, the cumulative radiation dose was investigated, so the dose rate was set at 10 kGy/6 h, and different doses were processed at different times. Therefore, the irradiation time was set at 0, 30, 60, 90, 120, and 150 h for the samples at six dose points of 0, 50, 100, 150, 200, and 250 kGy, respectively. The final test was carried out after irradiation treatment.

2.3 Sample Characterization

The particle size distribution of the original PS powder was measured by a laser particle size analyzer (MS3000, Malvern, Britain). The microstructure of PS was observed by scanning electron microscopy (SEM, TM4000, Japan). The detailed structural characteristics of PS were tested by X-ray diffraction (XRD, Empyrean, Netherlands) and Fourier transform infrared spectrometer (FTIR, Spectrum One, USA). In addition, the molecular weight changes of PS were observed using gel permeation chromatography (GPC, Agilent PL-GPC220, USA. Test conditions: trichlorobenzene (high temperature); detector: refractive index detector; chromatographic column: PLgelMIXED-BLS300 × 7.5mmx2; flow rate: 1 mL/min; standard product: PS (polystyrene); experimental temperature: 150 °C; mobile phase: TCB (1,2, 4-trichlorobenzene)). Also, the tests of GPC were conducted by Shiyanjia Lab (www.shiyanjia.com). Thermal characteristic was analyzed by a synchronous thermal analyzer (TG-DSC, Netzsch STA 449 F3, Germany, air atmosphere) and thermogravimetric mass spectrometry (TG-MS, Netzsch STA 449 F3, Germany, air atmosphere. Heating rate: 10 °C/min; MS detection range: 1–300).

In addition, the mechanical properties of PS samples were tested according to “GBT 10400.1–2018 Determination of tensile properties of plastics Part 1: General Principles” and “GBT 10400.3–2006 Determination of tensile properties of plastics Part 3: Test conditions for films and sheets”. The shape of the sample in the test is dumbbell type, and its sample size is shown in Figure 2. Specifically, the mechanical properties test sample was obtained by melting and shaping the irradiated PS powder sample into a dumbbell compression groove mold in the tensile test laboratory. The specific production process is shown in Figure S2 (Supporting Information).

Figure 2 Schematic diagram of sample shape and size for mechanical properties analysis and testing of PS.

Finally, the Multiwfn procedure28 and Gaussian software were used to assist the electron localization function (ELF)29,30 and Laplacian bond order (LBO) analysis31 to clarify the irradiation degradation mechanisms.

3 Results and Discussion

3.1 Structure and Phase Analysis

We carried out the structure and the phase change analysis by FTIR and XRD. Before discussing the FTIR results of PS, it is pertinent to consider the structure. The structure of the polymer-repeating unit can be represented as [CH2C(C6H5)H]n.32 The presence of the phenyl (C6H5) groups is key to the properties of polystyrene. These large rings prevent the polymer chains from packing into close crystalline arrangements so that solid polystyrene is transparent. The chain is probably stiffened somewhat as a result of the interference between neighboring phenyl groups, but there is an indication33 that no regularity exists along the carbon backbone. In addition, the phenyl rings restrict the rotation of the chains around the carbon–carbon bonds, thus lending the polymer its noted rigidity.

The FTIR and XRD results are shown in Figure 3 and Table 1. FTIR spectrum analysis shows that the intensity of some characteristic peaks in the FTIR spectrum has a slight weakening change. The change of characteristic peaks formed by the out-of-plane unsaturation C–H symmetric bending vibration at 754.00 and 695.00 cm–1 are quite significant, especially at high irradiation doses. This indicates that although PS resin has a certain radiation resistance in the range of radiation dose studied, with the increase of radiation dose, the damage of γ-rays to PS is still slightly increased.

Table 1 FTIR Test Results of PS

wavenumber (cm–1)	functional group	references	
3059.73, 3025.48	aromatic C–H stretching	(36)	
2920.18	–CH2– asymmetrical stretching	(37)	
2850.19	–CH2– symmetrical stretching	(38,39)	
1600.10, 1542.18, 1491.53, 1448.93	benzene ring skeleton vibration	(36)	
1370.52	CH3 groups	(36,40)	
1181.48, 1154.15, 1067.27, 1026.47	–CH2– out-of-plane bending, aromatic C–H in-plane bending	(41)	
905.47, 754.00, 695.00	aromatic C–H out-of-plane bending	(36,41,42)	

Figure 3 FT-IR spectra of PS at different irradiation doses (a, b). XRD spectra of PS at different irradiation doses (c).

Finally, the XRD test results are listed in Figure 3c. The XRD results of the original sample and the samples at different irradiation doses demonstrate that the polymers are completely amorphous. This is mainly due to the random orientation of the phenyl group.34,35

3.2 Molecular Weight Change Analysis

Irradiation mainly caused cross-linking and degradation effects for polymers.43,44 The most immediate result shows a change in the molecular weight. The molecular weight change of PS is shown in Figure 4. The values of Mp, Mn, Mw, and PDI are given in Table 2. Seeing from Table 2 and Figure 4a, the Mp value of PS decreases from 2.68 × 105 g/mol down to 2.22 × 105 g/mol with the increase in irradiation dose. In addition, the Mn value decreases after a 100 kGy irradiation dose and then increases with the dose. Furthermore, Mw shows a slight increase from 3.37 × 105 to 3.56 × 105 g/mol. From the molecular weight distribution statistics in Figure 4c, it can be seen that the main reason for this change is that the molecular weight greater than 1.00 × 106 g/mol increases with the increase of irradiation dose.

Figure 4 Molecular retention time curves (a). Molecular weight distribution curves (b). Statistical result of the molecular weight distribution (c).

Table 2 GPC Results for PS (0, 100, and 200 kGy)a

irradiation dose (kGy)	Mp (g/mol)	Mn (g/mol)	Mw (g/mol)	PDI	
0	2.68 × 105	8.93 × 104	3.37 × 105	3.78	
100	2.48 × 105	7.79 × 104	3.38 × 105	4.35	
200	2.22 × 105	7.87 × 104	3.56 × 105	4.52	
a Mp is the peak molecular weight, the highest molecular weight of the molecular retention time curve; Mn is the number of average molecular weight, weighted by the number of statistical molecular weight results:;Mw is the weight average molecular weight; mass as the weight statistical molecular weight results:; PDI is the polydispersity index; ratio of weight average molecular weight to number average molecular weight: PDI = Mw/Mn.

Moreover, after exposure to a dose of 100 kGy, the increase is from 24.5 to 29.4% in the molecular weight range of 0.10–1.00 × 105 g/mol and the decrease is from 24.5 to 21.7% in the molecular weight range of 2.00–4.00 × 105 g/mol. These show that the cross-linking and degradation effects occur at the same time. This is also the reason why Mn decreases first and then increases with the increase of the irradiation dose. Finally, PDI is a parameter used to measure the molecular weight distribution breadth of polymers. The larger the PDI value, the wider the molecular weight distribution.45 It can be seen from Table 2 that the PDI value of PS increases after irradiation, indicating a wider molecular weight distribution. The molecular weight distribution in Figure 4b also illustrates this. In general, the cross-linking and degradation effects of PS by γ-ray irradiation are simultaneous. Also, the final effect of irradiation is the widening of the molecular weight distribution of PS. Also, the larger the irradiation dose, the more dispersed the molecular weight distribution.

3.3 Thermal Stability

To evaluate the effect of irradiation on its thermal stability, thermogravimetric analysis was carried out at 10 °C/min. Figure 5a,b displays the TG-DTG curves of PS. The degradation of the contrast PS commences at 229.93 °C attains its maximum at 489.41 °C and ends at 562.67 °C. At low temperatures, products mainly consist of liquid compounds (monoaromatic). At higher temperatures, gas and coke yields are higher and the liquid fraction has significant aromatics (dimer, trimer).46−48 Thus, the thermal decomposition of PS mainly proceeds through two processes. The two processes are labeled first and second.

Figure 5 TG-DTG curves of PS: (a) 0 kGy; (b) 200 kGy. DSC curves of PS: (c) 0 and (d) 200 kGy.

The ending temperature of the first (from 489.41 down to 461.99 °C) and second (from 562.67 down to 525.43 °C) decrease after irradiation, and the hump deformation formed by the second widens greatly. Meanwhile, the maximum weight loss rate temperature also decreases from 453.74 to 433.59 °C. Therefore, the pyrolysis of PS becomes easier after irradiation. Furthermore, Figure 5 displays that the residual mass (Res) from the pyrolysis of unirradiated PS is 7.208%, while the irradiated sample is up to 30.23%. Therefore, no matter whether the PS samples are irradiated or not, the PS samples have a tendency of coking,49 while the tendency of coking was stronger after irradiation. This should be related to the molecular content of molecular weight greater than 1.00 × 106 g/mol increases found in the GPC analysis.

Figure 5c,d illustrates the DSC curves for PS during the pyrolysis process. The DSC curve of PS can distinguish approximately 3 temperature regions. In region I, the thermal effect is endothermic. This enthalpy of absorption is related to the heat absorbed by the phase transition (melting) and the beginning of decomposition (i.e., the competition between intramolecular and intermolecular bond breaking) of PS; in region II, the thermal effect is exothermic, and the polymer weight loss mainly occurs in this region, so the exothermic effect in this region is mainly caused by the forging chain decomposition during PS decomposition. The thermal effect of the last stage III is endothermic, in which the weight loss is very low and gasification and coking are the main reactions. After irradiation, the absorption and release of heat in all three regions of PS increased by 46.25, 8.56, and 113.25%espectively. It is concluded that the molecular content of molecular weight greater than 1.00 × 106 g/mol increases and the increase of coking tendency after irradiation have a great influence on the endothermic and exothermic effects of PS pyrolysis.

The MS detection results are shown in Figure 6, which intuitively shows the distribution of pyrolysis gas-phase products of PS. It is clear from Figure 6a,b that the products are mainly distributed in the low molecular weight stage, and the characteristic peak intensity of each pyrolysis product molecule increases significantly after irradiation. Furthermore, molecules in the charge/mass ratio (m/z) range from 0 to 50 are plotted separately, as shown in Figure 6c,d. The main volatile products with m/z values of 14, 16, and 28 are −CH2–, CH4, and CH2=CH2, respectively. In addition, small amounts of H2O and CO2 with m/z of 18 and 44 were also detected. After irradiation, the peak intensities of −CH2–, CH4, and CH2=CH2 increased by 30.47, 38.67, and 36.24% of unirradiated PS, respectively. In addition, the intensity of the characteristic peak of O2 left behind after the reaction in the reaction system increased. (These indicate that the oxygen consumption of PS pyrolysis is reduced after irradiation.) Finally, the distributions of −CH2–, CH4, and CH2=CH2 molecules with temperature are shown in Figure 6e,f. Before irradiation, the three main products of PS are mainly detected in the range of 300–400 °C, while after irradiation, a large number of them are detected in the whole range of pyrolysis temperature. It is shown that the thermal stability of PS material after irradiation will decrease greatly in practical applications, and CH4 and CH2=CH2 are easily produced by slight heating. Therefore, the practical application potential of PS material after irradiation will be greatly reduced.

Figure 6 3D MS diagrams of gas product distribution of PS: (a) 0 and (b) 200 kGy. The maximum intensity of characteristic peak of each m/z product was detected in the range of 0–50 m/z: (c) 0 and (d) 200 kGy. Distribution of molecules with m/z ratios of 14, 16, and 28 with the temperature changing: (e) 0 and (f) 200 kGy.

3.4 Mechanical Performance Analysis

The microscopic images at different irradiation doses of PS are shown in Figure 7. It is obvious that the micromorphology of PS has little change after irradiation. However, the irradiation effect causes the samples to transform color from white to orange-yellow obviously (Figure 8a). The mechanical performance test results for PS are shown in Figure 8, and the raw data plot is shown in Figures S3–S8 (Supporting Information). Figure 8a–d shows schematic diagrams of the samples, and Figure 8e,f shows the curves of PS breaking strength and breaking elongation as the irradiation dose increases, respectively. It is obvious from Figure 8e,f that the two parameters have curves of PS breaking strength and breaking elongation as the irradiation dose increases, with similar variation laws. When the irradiation dose is below 50 kGy, it increases first with the increase of irradiation dose and then decreases when the irradiation dose reaches 50 kGy with the further increase of irradiation dose. Both breaking strength and breaking elongation reach the maximum value at 50 kGy: 144.50 N and 12.36%, respectively.

Figure 7 SEM images of PS at different irradiation doses (a–c); locally magnified image of some PS particles (a1–c1).

Figure 8 Schematic diagram of the tensile test sample: (a) prepared at different irradiation doses, (b) three parallel samples prepared for the original sample (0 kGy); (c) clamped on the testing machine before the test; (d) broken on the testing machine after the test; change curve of PS breaking strength with the increase of irradiation dose (e); change curve of PS breaking elongation with the increase of irradiation dose (f).

According to the above analysis, irradiation has two effects on PS: cross-linking and degradation. Therefore, it is believed that when the irradiation dose is lower than 50 kGy, the cross-linking effect of irradiation on PS is stronger, which enhances its mechanical properties. With the further increase of the irradiation dose, the degradation effect is dominant, which leads to the decrease of mechanical properties of the sample. In general, the decrease of breaking strength and elongation at break after the irradiation dose reaches 100 kGy is consistent with the above conclusion that the performance of PS decreases after the irradiation dose of 100 and 200 kGy.

3.6 Analysis of Irradiation Effect Mechanisms Based on ELF and LBO

The electron cloud density distributions of PS are shown in Figure 9a and Figure S9 (Supporting Information). In the projection shown in the image, the darker the red, the denser the electron cloud. The stick model is shown in Figure 9b, and the corresponding atoms are labeled. Moreover, LBO can be used to measure the strength of the chemical bonds. Also, the LBO of each chemical bond in PS is listed in Table 3. In the example PS molecule, the C–C bonds with the lowest LBO values are C7–C10 and C10–C12, both 0.897, indicating that these are the most easily broken. Then, the LBO value of C5–C7 and C12–C15 is 0.900. All C–C bonds that are mentioned before are formed by the involvement of the tertiary carbon attaching the benzene ring. It is because these C–C bonds break. The cross-linking and degradation of PS are realized, resulting in changes in molecular weight. In addition, the C–H bonds with the lowest LBO value are C5–H6, C7–C8, C7–H9, C10–H11, C12–H13, C12–H14, and C15–H16. These C–H bonds are formed by the secondary and tertiary carbon involvement, which are located in the main chain.

Table 3 LBO Values of the Chemical Bonds in the PS Molecule, for Example

chemical bond	LBO value	chemical bond	LBO value	chemical bond	LBO value	chemical bond	LBO value	
C1–C5	0.921	C25–C27	1.494	C1–H2	0.829	C22–H26	0.854	
C5–C7	0.900	C31–C32	1.445	C1–H3	0.827	C23–H28	0.861	
C5–C20	0.974	C31–C33	1.457	C1–H4	0.836	C25–H29	0.860	
C7–C10	0.897	C32–C34	1.496	C5–H6	0.818	C27–H30	0.861	
C10–C12	0.897	C33–C36	1.492	C7–H8	0.814	C32–H35	0.855	
C10–C31	0.969	C34–C38	1.493	C7–H9	0.814	C33–H37	0.862	
C12–C15	0.900	C36–C38	1.502	C10–H11	0.814	C34–H39	0.862	
C15–C17	0.921	C42–C43	1.446	C12–H13	0.814	C36–H40	0.862	
C15–C42	0.974	C42–C44	1.459	C12–H14	0.814	C38–H41	0.859	
C20–C21	1.459	C43–C45	1.496	C15–H16	0.818	C43–H46	0.854	
C20–C22	1.445	C44–C47	1.490	C17–H18	0.827	C44–H48	0.855	
C21–C23	1.490	C45–C49	1.494	C17–H19	0.829	C45–H50	0.860	
C22–C25	1.496	C47–C49	1.503	C17–H53	0.836	C47–H51	0.861	
C23–C27	1.503	 	 	C21–H24	0.855	C49–H52	0.861	

Figure 9 PS molecule example: CH3–CH(C6H5)–CH2–CH(C6H5)–CH2–CH(C6H5)–CH3: (a) electron cloud density distribution, (b) ball-and-stick model.

Generally speaking, β-cleavage is the most frequent reaction during the degradation of PS.50 There are two forms: the free radical break at the end of the chain and the free radical break in the chain. The former is the process of PS depolymerization, which is a reversible process (also can be cross-linked). In the process, the self-scission of PS fragment radical accelerates the midchain and end chain β-scission by attacking the polymer chain by a phenyl radical. This reaction ends in the formation of styrene and end-chain free radicals. The attack of free radicals on the secondary (C5, C10, C15) and tertiary carbons (C7, C12) by β-scission immediately forms the oligomers of styrene.50 This is the main way for PS degradation and cross-linking.

4 Conclusions

In this work, the irradiation resistance of PS irradiated by γ-rays in the range 0–250 kGy was investigated. The GPC results displayed that Mp value of PS went from 2.68 × 105 g/mol down to 2.22 × 105 g/mol. In this process, the molecular content of molecular weight greater than 1.00 × 106 g/mol increased, resulting in the increase of Mw. The results of thermal stability analysis showed that the pyrolysis process of PS has two main stages. The mechanical properties analysis showed that the PS has a maximum inflection point at 50 kGy. Eventually, the theoretical bond order analysis confirmed that the tertiary carbon bond attaching the benzene ring had the lowest bond energy. In conclusion, the results of this study give helpful guidance when using PS for food packing materials.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.4c04407.More detailed powder sample handling procedure; particle size; more detailed sample handling procedure of the mechanical properties test; results of theoretical calculations; GPC data (PDF)

Supplementary Material

ao4c04407_si_001.pdf

The authors declare no competing financial interest.

Acknowledgments

The authors appreciate the financial support from the Project of State Key Laboratory of Environment-friendly Energy Materials, Southwest University of Science and Technology (no. 22fksy12), the Open Foundation of NHC Key Laboratory of Nuclear Technology Medical Transformation (Mianyang Central Hospital) (2021HYX028), the Foundation from Tianfu Institute of Research and Innovation (2022ZY013), and the Sichuan Science and Technology Program (2022NSFSC1197).
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