Introduction

Polyacetal (POM) is a polymer with applications such as mechanical components. POM undergoes degradation due to factors such as exposure to ultraviolet light and repeated recycling. Quantitatively evaluating such associated changes in physical properties is important for developing products made from POM 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 POM by GPC, hexafluoroisopropanol (HFIP) is typically used. However, since HFIP is an extremely expensive solvent, excessive solvent consumption becomes problematic. One means of reducing solvent consumption is to perform the analysis on a semi-micro scale. This also offers the advantage of shorter analysis times.

In this study, we investigated the degradation of POM test samples caused by irradiation in a xenon accelerated weathering tester. Changes in the molecular weight distribution were evaluated using a GPC system. This was a semi-micro scale GPC system compatible with HFIP, equipped with a refractive index detector (RI-4035) 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 polymethyl methacrylate (PMMA) as the standard sample and calculated the molecular weight distribution for POM samples.

LC-4000 GPC system

Experimental

LC system
Pump:  PU-4185
Autosampler:  AS-4150*
Column oven:  CO-4060
Detector:  RI-4035
* with option units

LC conditions
Column:   GPC LF-404  (4.6 mmI.D. x 250 mmL, 6 µm)
Eluent:   5 mmol/L sodium trifluoroacetate  in HFIP
Flow rate:   0.15 mL/min
Column temp.:   40 ºC
Injection volume:   20 µL

Sample
<Standard samples for creating molecular weight calibration curve>
Polymethyl methacrylate (PMMA) mixed sample (two samples prepared for different molecular weight peaks (Mp))
– Standard sample 1: Mp 772000, 51900, 6900, 645
– Standard sample 2: Mp 211000, 21700, 2200
Each sample was dissolved and diluted in the mobile phase to 0.025 % (w/v)

<Test samples for evaluation>
POM test samples (approx. 3 mm, pellet-shaped, Standard Test Piece Co., Ltd.)

Structure

POM

Keywords

POM, GPC, molecular weight distribution, PMMA, molecular weight calibration curve, HFIP, semi-micro scale, 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 the mobile phase to a concentration of 0.1% (w/v) for use as the GPC measurement samples.

Fig. 1   Photodegradation test procedure

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

Fig. 2   Chromatograms for PMMA standard samples
(The number above each peak represents the corresponding peak molecular weight Mp)

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

Figure 4 shows chromatograms for the POM 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 shift towards lower molecular weight as the irradiation time increases.

Fig. 4   Chromatograms for POM test samples

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

     Table 1 shows the PMMA-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 is increased, Mp decreases while Mn and Mw increase slightly and then decrease. This suggests that oxidation reactions and cross-linking reactions occur preferentially upon light irradiation, after which the degradation of the main chain becomes dominant^{1, 2}.

Table 1   PMMA-equivalent average molecular weight calculation results

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 POM test samples irradiated in a xenon accelerated weathering tester. The molecular weight distribution was determined using a semi-micro scale GPC system. As the irradiation time was increased, Mp decreased while Mn and Mw increased slightly and then decreased. This suggests that oxidation reactions and cross-linking reactions 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

1. V. M. Archodoulaki, S. Lüftl, T. Koch, S. Seidler: Polym. Degrad. Stab., 92, 2181 (2007). DOI: 10.1016/j.polymdegradstab.2007.02.024
2. X. Colin, J. Verdu: C. R. Chim., 9, 1380 (2006). DOI: 10.1016/j.crci.2006.06.004