Detection of Surface Enrichment Driven by Molecular
Weight Disparity in Virtually Monodisperse Polymers
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Abstract
The preference for a shorter chain
component at a polymer blend
surface impacts surface properties key to application-specific performance.
While such segregation is known for blends containing low molecular
weight additives or systems with large polydispersity, it has not
been reported for anionically polymerized polymers that are viewed,
in practice, as monodisperse. Observations with surface layer matrix-assisted
laser desorption ionization time-of-flight mass spectrometry (SL-MALDI-ToF-MS),
which distinguishes surface species without labeling and provides
the entire molecular weight distribution, demonstrate that entropically
driven surface enrichment of shorter chains occurs even in low polydispersity
materials. For 6 kDa polystyrene the number-average molecular weight
(<i>M</i><sub><i>n</i></sub>) at the surface is
ca. 300 Da (5%) lower than that in the bulk, and for 7 kDa poly(methyl
methacryalate) the shift is ca. 500 Da. These observations are in
qualitative agreement with results from a mean-field theory that considers
a homopolymer melt with a molecular-weight distribution matched to
the experiments