Indiana Section of the Society for Applied SpectroscopyIndiana Section of the Society for Applied Spectroscopy
1999-2000 Seminar Abstracts
MASS
SPECTROMETRY OF BLOCK COPOLYMERS
David
M. Hercules
Department of Chemistry, Vanderbilt University, Nashville, Tennessee 37235
TOF-SIMS has been used to examine the surfaces of symmetric poly(styrene-co-isoprene) (PS-PI) diblock copolymer films on silver. The low mass and high-mass spectral regions were compared. High-mass spectra contain only large silver cationized polyisoprene (PI) fragment ion peaks; no polystyrene (PS) fragments ions are observed. Absolute high-mass ion intensity measurements performed on PS and PI homopolymers show that PS has an 8.8-fold higher ion yield than PI. Since PS has higher ion yield than PI, the presence of only PI fragment ions in the PS-PI diblock spectra is evidence that the PI block forms a surface layer. In the low-mass region, the relative intensities of the characteristic PS and PI peaks indicate that the surface is enriched with PI, but that a detectable amount of PS is also present in the surface region. This contradiction may be interpreted as evidence of greater surface sensitivity of the high-mass region. High-mass ions probably originate from the less energetic and shallower regions within the collision cascade. It is possible to estimate the high-mass escape depth at 5 ≤ Å. Another explanation for the absence of PS fragment ions in the high-mass region is that PI constitutes thinner domains and therefore is preferentially cationized by silver during the collision cascade event. The high-mass region of the diblock spectra contain repeat peak patterns characteristic of PI. The relative intensity of the two most prominent clusters is sensitive to the molecular weight of the PI block. Atomic microscopy (AFM) was used to obtain images of the diblock surfaces. AFM revealed that the topography of the PS-PI diblocks changes with molecular weight.
Selective chemical degradation combined with MALDI analysis was used for the characterization of polyether and polyester polyurethanes (PUR’s). Two selective chemical degradation reagents were used. Ethanolamine was applied for the recovery of polyether (pTHP) soft-blocks. MALDI analysis of the degraded polymers indicates recovery of a representative oligomer distribution. Allowing only partial reaction enables identification of the diisocyanate; ions containing the urethane linkage are observed in the MALDI spectrum. The polydispersity indices determined by MALDI are in reasonable agreement with the expected values based in the pTHF synthesis reaction.
Phenylisocyanate, which does not cleave ester bonds, was applied for the analysis of polyester PUR’s. MALDI showed that the pBA oligomer distribution is recovered, along with minor degradation products containing the urethane linkage, providing identification of both the polyester and the diisocyanate. Comparison of the degraded pBA-PUR oligomer distributions with the unreacted pBA material indicates that smaller oligomers are less abundant in the degraded samples. SEC-MALDI was used to obtain more accurate MWD determinations than MALDI alone. The polydispersity indices determined using SEC-MALDI are higher than MALDI determined PD indices. The results presented indicate that the combination of selective degradation, combined with SEC-MALDI analysis is a viable means for polyether and polyester polyurethane soft-block characterization.
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