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Molecular weight and polydispersity estimation of adsorbing polymer brushes by atomic force microscopy

TitleMolecular weight and polydispersity estimation of adsorbing polymer brushes by atomic force microscopy
Publication TypeJournal Article
Year of Publication2004
AuthorsGoodman, D, Kizhakkedathu, JN, Brooks, DE
JournalLangmuir
Volume20
Pagination3297-3303
Date PublishedApr
Type of ArticleArticle
ISBN Number0743-7463
KeywordsADSORPTION, AFM, CHAIN ELONGATION, ELASTIC PROPERTIES, PHASE, POLY(ACRYLAMIDE), POLY(N-ISOPROPYLACRYLAMIDE), SPECTROSCOPY, SURFACES
Abstract

{We have estimated the molecular weight, M-n, and polydispersity, PDI, of densely grafted poly(N-isopropylacrylamide) (PNIPAM) brushes using a novel atomic force microscopy (AFM) approach. When compression of a polymer brush induced adsorption of multiple chains to an AFM tip, the resulting decompression force profile exhibited a maximum attractive force at a separation, L-m, that decayed to zero with increasing tip-sample separation. We have found that the separation L approximates the average contour length, L,, determined by gel permeation chromatography (GPC). The detection of a decaying attractive force at separations larger than L, suggests that chains of above average length sequentially break free from the tip as they are stretched away from the grafting surface. The shape of the decompression profile in this region approximately paralleled the cumulative weight fraction of the grafted chains determined by GPC. The fraction of chains of a given molecular weight determined from a single force curve fit a log-normal distribution, having a standard deviation that provided an estimate of the PDL We have characterized two PNIPAM brushes by this AFM technique as well as by GPC coupled to a multiangle laser light-scattering detector (MALLS). The values obtained by AFM-(1) Mn,(AFM) = (3.8 +/- 0.5) X 104

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