Introduction

Polystyrene (PS) is a polymer with applications such as electronic devices and mechanical components. PS undergoes degradation due to factors such as exposure to ultraviolet light and repeated recycling. Quantitatively evaluating the associated changes in physical properties is important for developing products made from PS and controlling their quality. One method for performing such evaluations of polymer raw materials involves the use of gel permeation chromatography (GPC) to determine the molecular weight distribution. When analyzing PS by GPC, tetrahydrofuran (THF) is typically used as an eluent.

In this study, we investigated the degradation of PS test samples caused by irradiation in a xenon accelerated weathering tester. Changes in the molecular weight distribution were evaluated using a GPC system equipped with a refractive index detector (RI-4030) and a high-performance analytical GPC column. For data analysis, we used the molecular weight distribution calculation program in ChromNAV. We created a molecular weight calibration curve using PS standard samples and then calculated the molecular weight distribution for degraded PS samples.

LC-4000 GPC system

Experimental

LC system
Pump:  PU-4180*
Autosampler:  AS-4150
Column oven:  CO-4060
Detector:  RI-4030
* with option units

LC conditions
Column:   GPC KF-806L  (8.0 mmI.D. x 300 mmL, 10 µm)
Eluent:   THF (stabilizer-free)
Flow rate:   1.0 mL/min
Column temp.:   40 ºC
Injection volume:   50 µL

Sample
◦Standard samples for creating molecular weight calibration curve
PS mixed sample
(two samples prepared for different molecular weight peaks (Mp))
Standard sample 1: Mp 1470000, 257000, 18100, 3090
Standard sample 2: Mp 778000, 64500, 6320
Each sample was dissolved and diluted in THF to 0.025 % (w/v)

◦Test samples for evaluation
PS test samples (approx. 3 mm, pellet-shaped, Standard Test Piece Co., Ltd.)

Structure

Keywords

PS, polystyrene, GPC, molecular weight distribution, molecular weight calibration curve, THF, RI detector

Results

Figure 1 shows the procedure used for the photodegradation test. For light irradiation, a xenon accelerated weathering tester (SOLARBOX 1500e, manufactured by Cofomegra, provided by JASCO INTERNATIONAL Co., Ltd.) was used, with irradiation performed at an irradiance of 60 W/m² and a temperature of 65 ºC. Three test samples were sequentially inserted into the weathering tester at different times to give total irradiation times of 10 days, 5 days, and 1 day for Samples 1, 2 and 3, respectively. After irradiation was completed, to measure the degraded layer of the test samples using GPC, a plane slicer (Slice Master KS-10, provided by JASCO Engineering Co., Ltd.) was used to remove the degraded layer from the irradiated surface of the test samples. The cut fragments were dissolved in THF to a concentration of 0.1% (w/v) for GPC measurement samples.

Fig. 1   Photodegradation test procedure

Figure 2 shows chromatograms for two PS samples (standard sample 1 and 2), and Figure 3 shows the resulting calibration curve.

Fig. 2   Chromatograms for PS standard samples
(A) Full curves (B) Enlarged view of peaks
(The values above the peaks represent the Mp for the PS standard samples.)

Fig. 3   Molecular weight calibration curve created using PS standard samples

Figure 4 shows chromatograms for the PS samples subjected to photodegradation testing, together with that for an unirradiated sample. Figure 5 shows the corresponding differential molecular weight distribution curves, where the horizontal axis represents the logarithm of the molecular weight to make it easier to observe changes in the distribution. It can be seen that the distribution shifts towards lower molecular weight as the irradiation time increases.

Fig. 4   Chromatograms for PS test samples

Fig. 5   Differential molecular weight distribution curves for PS test samples

Table 1 shows the equivalent average molecular weight calculation results, and Figure 6 shows the changes in the number-average molecular weight (Mn) and weight-average molecular weight (Mw) with irradiation time. As the irradiation time was increased, both Mp and Mw decreased while Mn increased slightly and then decreased. This suggests that the decomposition and volatilization of pre-existing unpolymerized monomers and short-chain oligomers occur preferentially upon light irradiation, after which the degradation of the main chain becomes dominant^1-3.

Table 1   Average molecular weight calculation results for PS test samples

Fig. 6   Change in average molecular weight with irradiation time
(A)  Number-average molecular weight (Mn)   (B)  Weight-average molecular weight (Mw)

Conclusion

In this study, we investigated the degree of photodegradation of PS test samples following irradiation in a xenon accelerated weathering tester. The molecular weight distribution was determined using a GPC system. As the irradiation time was increased, Mp and Mw decreased while Mn increased slightly and then decreased. This suggests that the decomposition and volatilization of pre-existing unpolymerized monomers and short-chain oligomers occur preferentially upon light irradiation, after which the degradation of the main chain becomes dominant. These measurements can be utilized for the quantitative evaluation of resin degradation.

References

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2.H. Kimukai, K. Koizumi, H. Taguchi, A. Okabe, K. Takatama, S. Y. Chung, B. G. Kwon, M. Nishimura, S. Mentese, K. Saido: ACS ES&T Water, 2, 1976 (2022). DOI: 10.1021/acsestwater.2c00023
3.X. Colin, J. Verdu: C. R. Chim., 9, 1380 (2006). DOI: 10.1016/j.crci.2006.06.004