DETERMINATION OF THE DISTRIBUTION OF POLY(DIMETHYLSILOXANE) SEGMENT LENGTHS AT THE SURFACE OF POLY[(DIMETHYLSILOXANE)-URETHANE]-SEGMENTED COPOLYMERS BY TIME-OF-FLIGHT SECONDARY-ION MASS-SPECTROMETRY

Time-of-flight secondary ion mass spectra were recorded from submonola yer thin films of aminopropyl end-capped poly(dimethylsiloxane) (PDMS) and from thick films (ca. 50 mu m) of poly-[(dimethylsiloxane)-uretha ne] (PU-DMS)-segmented copolymers. Ions detected and assigned to fragm ents in the low-mass range (m/z less than or equal to 300) provided st ructural information about the repeat units and the end groups. The hi gh-mass spectrum of the PDMS homopolymer yielded a series of ions assi gned to Ag+-cationized oligomers; this enabled determination of the mo lecular weight distribution in comparison with GPC measurement. In the high-mass (m/z = 800-3500) spectra of thick PU-DMS films, the peak se ries was assigned to a simple fragmentation process. That process woul d yield ions where the intact PDMS segment is present; it therefore ca n be used to evaluate the PDMS segment length distribution at the surf ace of the copolymer. It was found that the distribution of PDMS segme nt lengths segregated at the surface of the thick film was almost iden tical with that in the bulk for PU-DMS with PDMS nominal molecular wei ght of ca. 1000 Da. These results allow the development of an analysis of ion structure and a stepwise procedure for evaluating the segment length distributions in the near-surface region of siloxanes.