Surface chemical analysis of poly (ε-caprolactone -perfluoropolyether- poly (ε-caprolactone) triblock copolymers by X-ray photoelectron spectroscopy

The air-side surface composition of a series of poly(ε-caprolactone)-perfluoropolyether-poly(ε-caprolactone) triblock copolymers with different compositions and block lengths have been studied by angle-dependent X-ray photoelectron spectroscopy (XPS). The weight percentage of the perfluoropolyether (PFPE) and polycaprolactone (PCL) blocks, and ethylene oxide linker (R_H) has been calculated in different ways: from C1s, O1s and F1s photoemission peaks and by line fitting of the C1s and O1s envelopes. The atomic sensitivity factors and the parameters used to fit the peak envelopes have been experimentally determined using some reference materials. A critical discussion of the different methods used in the surface characterization and the degradation of PFPE segments, induced by irradiation beam, have been also reported. A large excess of PFPE with respect to the bulk composition was observed in all samples, and the angular dependence of the XPS signal demonstrated that the content of the fluorinated block segment increased by decreasing the sampling depth. The PFPE surface concentration was also decreased by increasing the PCL/PFPE ratio, but the surfaces of the samples were still dominated by PFPE segments for copolymers with a bulk PFPE composition lower than 10%. Moreover, copolymers with similar PCL/PFPE bulk ratios but with different PFPE block lenghts, showed similar PFPE surface composition when the number-average molecular weight (M_n) was 2000 and 3200 g mol^-1, while that observed for copolymers containing PFPE block with M_n 900 g mol^-1 was lower.