Probing the mobility of polymers grafted on cosmetic pigments using NMR and EPR spectroscopies
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Presented by: Olivier Lafon
Introduction
In 2017, the European Commission requested the European Chemical Agency (ECHA) to assess the scientific evidence for taking regulatory action at the EU level on microplastics that are intentionally added to products. Accordingly, the current definition of microplastics is described in the opinion of the Committee for Risk Assessment (RAC) and the Committee for Socio-economic Analysis (SEAC) on 11th June 2020.
Some cosmetic make-up pigments currently on the market use polymers as processing materials in order to improve the ‘feel’ and water (sweat) resistance.
According to the definition mentioned above, such liquid polymers, are not microplastics. However, it is unknown whether this polymer remains in a ‘liquid-like’ state after the surface treatment process has been performed.
Considering the origin of the regulation is related to environmental impact mitigation, attempts were made herein to clarify whether the aforementioned surface treatment of pigments with polymers results in microplastic formation. Therefore, the mobility of the polymer grafted onto the pigment was measured using electron paramagnetic resonance (EPR) and solid-state nuclear magnetic resonance (NMR) spectroscopies.
Methods
Solid-state 1H and 13C NMR experiments were recorded at 9.4 T (400 MHz) using a 4 mm magic angle spinning (MAS) probe spinning at 12.5 kHz. One-dimensional (1D) 1H NMR spectra were acquired using the DEPTH pulse sequence. 1H relaxation times, T1 and T2’, were measured using saturation recovery and Hahn-echo sequences, respectively. 1D 13C NMR spectra were recorded using direct excitation as well as transfers of 1H polarization to 13C nuclei via dipolar interaction or scalar coupling using CPMAS and J-RINEPT sequences, respectively.
Continuous wave X-band EPR spectra were recorded at room temperature using a Bruker ELEXYS E580. Amplitude modulation and microwave power were respectively set to 1 G and 1 mW. EPR spectra were simulated with Easyspin toolbox to extract the rotation correlation time.
Results
The 1H NMR spectrum of polymer treated pigment is dominated by an intense and sharp peak at ~0 ppm ascribed to the liquid polymer. This spectrum also exhibits a low-intensity broad resonance assigned to hydroxyl protons from the pigment surface. The narrow linewidth of the 1H signal for grafted polymer indicates that these protons are highly mobile. This result is confirmed by the measured 1H T1 and T2’ values, which are affected by the atomic-level dynamics of the polymer chains.
We also recorded 1D 13C direct excitation, 1H-13C J-RINEPT and 1H-13C CPMAS NMR spectra of pigment treated with polymer. 1D 13C direct excitation allows the observation of all 13C nuclei in the sample, whereas 1H-13C J-RINEPT and 1H-13C CPMAS NMR allows the selective observation of 13C NMR signals of the sample’s mobile and rigid regions, respectively. Comparison of the 13C direct excitation and 1H-13C J-RINEPT spectra reveals two similar lineshapes centred at ~0.5 ppm, suggesting that most of 13C nuclei of the grafted polymer are highly mobile. Conversely, 1H-13C CP MAS spectra show much broader lineshapes ascribed to rigid part of the polymer, probably close to the particle surface. Nevertheless, this broad lineshapes is hardly visible in the 13C direct acquisition. This observation confirms that most of the polymer chains are highly mobile.
EPR spectra of treated and untreated pigment show the presence of paramagnetic defects in the material. In order to discriminate between solid or liquid phases, we added a small amount of nitroxide probe (TEMPO) to the treated pigment. We also prepared another sample by mixing 2 µL of TEMPO with the ungrafted polymer and pigment. The two experiments show that the nitroxide probe is very mobile with correlation times of 3.1 and 1.8 ns for the treated pigment and the mixture, respectively. These measurements confirms that the polymer forms a liquid phase.
Conclusion
Both EPR and 1H and 13C solid-state NMR data demonstrate that polymer chains grafted at the surface of pigments are highly mobile with dynamics similar to that of ungrafted liquid polymers. Therefore, the surface treatment of pigments with polymers does not result in microplastic formation.
In 2017, the European Commission requested the European Chemical Agency (ECHA) to assess the scientific evidence for taking regulatory action at the EU level on microplastics that are intentionally added to products. Accordingly, the current definition of microplastics is described in the opinion of the Committee for Risk Assessment (RAC) and the Committee for Socio-economic Analysis (SEAC) on 11th June 2020.
Some cosmetic make-up pigments currently on the market use polymers as processing materials in order to improve the ‘feel’ and water (sweat) resistance.
According to the definition mentioned above, such liquid polymers, are not microplastics. However, it is unknown whether this polymer remains in a ‘liquid-like’ state after the surface treatment process has been performed.
Considering the origin of the regulation is related to environmental impact mitigation, attempts were made herein to clarify whether the aforementioned surface treatment of pigments with polymers results in microplastic formation. Therefore, the mobility of the polymer grafted onto the pigment was measured using electron paramagnetic resonance (EPR) and solid-state nuclear magnetic resonance (NMR) spectroscopies.
Methods
Solid-state 1H and 13C NMR experiments were recorded at 9.4 T (400 MHz) using a 4 mm magic angle spinning (MAS) probe spinning at 12.5 kHz. One-dimensional (1D) 1H NMR spectra were acquired using the DEPTH pulse sequence. 1H relaxation times, T1 and T2’, were measured using saturation recovery and Hahn-echo sequences, respectively. 1D 13C NMR spectra were recorded using direct excitation as well as transfers of 1H polarization to 13C nuclei via dipolar interaction or scalar coupling using CPMAS and J-RINEPT sequences, respectively.
Continuous wave X-band EPR spectra were recorded at room temperature using a Bruker ELEXYS E580. Amplitude modulation and microwave power were respectively set to 1 G and 1 mW. EPR spectra were simulated with Easyspin toolbox to extract the rotation correlation time.
Results
The 1H NMR spectrum of polymer treated pigment is dominated by an intense and sharp peak at ~0 ppm ascribed to the liquid polymer. This spectrum also exhibits a low-intensity broad resonance assigned to hydroxyl protons from the pigment surface. The narrow linewidth of the 1H signal for grafted polymer indicates that these protons are highly mobile. This result is confirmed by the measured 1H T1 and T2’ values, which are affected by the atomic-level dynamics of the polymer chains.
We also recorded 1D 13C direct excitation, 1H-13C J-RINEPT and 1H-13C CPMAS NMR spectra of pigment treated with polymer. 1D 13C direct excitation allows the observation of all 13C nuclei in the sample, whereas 1H-13C J-RINEPT and 1H-13C CPMAS NMR allows the selective observation of 13C NMR signals of the sample’s mobile and rigid regions, respectively. Comparison of the 13C direct excitation and 1H-13C J-RINEPT spectra reveals two similar lineshapes centred at ~0.5 ppm, suggesting that most of 13C nuclei of the grafted polymer are highly mobile. Conversely, 1H-13C CP MAS spectra show much broader lineshapes ascribed to rigid part of the polymer, probably close to the particle surface. Nevertheless, this broad lineshapes is hardly visible in the 13C direct acquisition. This observation confirms that most of the polymer chains are highly mobile.
EPR spectra of treated and untreated pigment show the presence of paramagnetic defects in the material. In order to discriminate between solid or liquid phases, we added a small amount of nitroxide probe (TEMPO) to the treated pigment. We also prepared another sample by mixing 2 µL of TEMPO with the ungrafted polymer and pigment. The two experiments show that the nitroxide probe is very mobile with correlation times of 3.1 and 1.8 ns for the treated pigment and the mixture, respectively. These measurements confirms that the polymer forms a liquid phase.
Conclusion
Both EPR and 1H and 13C solid-state NMR data demonstrate that polymer chains grafted at the surface of pigments are highly mobile with dynamics similar to that of ungrafted liquid polymers. Therefore, the surface treatment of pigments with polymers does not result in microplastic formation.