Conformation and orientation effects in the X-ray photoelectron spectra of organic polymers

Abstract

Conformation and orientation effects in the XPS spectra of organic polymers are small but can be observed with modern high performance equipment. This paper discusses some of the experimental factors that should be considered when attempting to detect such effects, and describes several recent studies which illustrate the subtle phenomena that can now be observed. Conformation effects, revealed by melting semi-crystalline polymer samples in the analysis chamber of an XPS spectrometer, are reported for the valence band and C 1s spectra of nylon 12 and poly(ethylene adipate), and for the valence band spectrum of poly(ethylene succinate). For nylon 12 the changes in the C 2s region of the valence band spectrum are interpreted in terms of disordering of the planar zig-zag conformation of the (CH 2) 11 segments of the polymer chain. Pendant group surface orientation effects, detected by angle resolved XPS (with emission angles as low as 5° relative to the sample surface), are reported for poly(2-chloroethyl methacrylate) (PCEMA), poly(lauryl methacrylate) (PLMA) and poly(2-hydroxyethyl methacrylate) (PHEMA). For PCEMA and PLMA the uppermost surfaces are enriched with –CH 2CH 2Cl and –(CH 2) 11CH 3 pendant groups, respectively, whereas for PHEMA the data suggest relatively few –CH 2CH 2OH pendant groups at the surface. The C 1s and Cl 2p spectra of PCEMA reveal surface core level binding energy shifts of about +0.2 eV, and the Cl L 23M 23M 23 spectrum a surface Auger kinetic energy shift of about −0.6. The C 1s spectra of PLMA and PHEMA also reveal surface core level shifts and for PLMA this is interpreted in terms of a disordered and open arrangement of the –(CH 2) 11CH 3 pendant groups at the polymer surface.