Surface Layer Matrix-Assisted Laser Desorption Ionization Mass Spectrometry Imaging: A Surface Imaging Technique for the Molecular-Level Analysis of Synthetic Material Surfaces.

Surface layer matrix-assisted laser desorption ionization mass spectrometry imaging (SL-MALDI-MSI) is a powerful new surface sensitive imaging technique to establish surface component localization of multicomponent polymer materials. This study demonstrates the ability of SL-MALDI-MSI to image defects from foreign materials, material absence, mechanical scribing, and solvent perturbation at the surface of low-molecular-weight poly(methyl methacrylate) and polystyrene thin films. The surface specificity of the SL-MALDI-MSI technique is validated by imaging polystyrene on poly(methyl methacrylate) bilayer films; only polystyrene ions are detected from the surface of the unperturbed polystyrene layer. A key process enabling SL-MALDI-MSI is the solvent-free sublimation of matrix and salt uniformly on the sample's surface.

Detection of Surface Enrichment Driven by Molecular Weight Disparity in Virtually Monodisperse Polymers.

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 (Mn) 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.

Subtle End Group Functionalization of Polymer Chains Drives Surface Depletion of Entire Polymer Chains.

The surface of a blend of 6 kDa polystyrene and 6 kDa polystyrene functionalized with hydroxymethyl ends not only is depleted of the higher energy end groups but also is depleted of any segments belonging to the functionalized chains. This is demonstrated using the emerging technique of surface layer matrix-assisted laser desorption ionization time-of-flight mass spectrometry (SL-MALDI-ToF-MS), which detects entire chains that have any repeat unit at the outer surface, and requires no labeling. Detecting entire chains provides information about the relationship of chain functionalization to surface segregation behavior of entire chains. That the surface is depleted of interior segments of functionalized chains as well as of the ends is remarkable, since the functionality at the single chain end involves less than 0.5 wt % of the functionalized polymer chain.