Surface Plasmon Resonance Studies of Pullulan and Pullulan Cinnamate Adsorption onto Cellulose Abdulaziz Kaya, † Xiaosong Du, † Zelin Liu, † Jessica W. Lu, † John R. Morris, † Wolfgang G. Glasser, ‡ Thomas Heinze, § and Alan R. Esker* ,† Department of Chemistry and Department of Wood Science and Forest Products, Virginia Tech, Blacksburg, Virginia 24061, and Center of Excellence for Polysaccharide Research, Friedrich Schiller University of Jena, Humboldtstrae 10, Jena, 07743 Germany Received April 12, 2009; Revised Manuscript Received June 17, 2009 Surface plasmon resonance studies showed pullulan cinnamates (PCs) with varying degrees of substitution (DS) adsorbed onto regenerated cellulose surfaces from aqueous solutions below their critical aggregation concentrations. Results on cellulose were compared to PC adsorption onto hydrophilic and hydrophobic self-assembled thiol monolayers (SAMs) on gold to probe how different interactions affected PC adsorption. PC adsorbed onto methyl- terminated SAMs (SAM-CH 3 ) > cellulose > hydroxyl-terminated SAMs (SAM-OH) for high DS and increased with DS for each surface. Data for PC adsorption onto cellulose and SAM-OH surfaces were effectively fit by Langmuir isotherms; however, Freundlich isotherms were required to fit PC adsorption isotherms for SAM-CH 3 surfaces. Atomic force microscopy images from the solid/liquid interfaces revealed PC coatings were uniform with surface roughnesses <2 nm for all surfaces. This study revealed hydrogen bonding alone could not explain PC adsorption onto cellulose and hydrophobic modification of water-soluble polysaccharides was a facile strategy for their conversion into surface modifying agents. Introduction Bone and wood as biological structural materials have been classical examples of complex composite materials. 1 In con- sideration of the relatively poor properties of the basic building blocks, the resulting composites demonstrated remarkable mechanical properties required for their function. 1,2 These natural composites consisted of a polymer matrix reinforced with either crystallites or fibers, were hierarchically organized on different scales from nano to micrometer levels, and were sources of inspiration for the design of new materials. 3-5 These factors along with increased environmental consciousness and legislative mandates have driven interest in materials with the focus on renewable raw materials which mimicked natural composites. 4,6 Cellulose fibers have received greater attention for use in biocomposites, with substitution of one or more man-made materials in composite materials with a biologically derived component. However, the natural fiber-polymer interfaces have presented a formidable challenge for cellulose-based biocom- posites. Because of the presence of hydroxyl groups and other polar substances in various natural fibers, moisture absorption in biocomposites has led to poor interfacial bonding between polyhydroxyl fiber surfaces and the hydrophobic matrix component. 6,7 Hence, modification of wood fiber surfaces has been a significant challenge for the production of novel biocomposites. Bonding between components could often be increased through derivatization of cellulosic fibers with hy- drophobic moieties via an ester linkage. This process yielded improved compatibility between the cellulosic fibers and thermoplastics; however, derivatization of cellulose cleaved the glucan chain and disrupted the extensive hydrogen bond network. As a consequence, derivatization undermined two important contributors to the strength of native cellulose materials. Because the successful utilization of cellulose materi- als in many biocomposite applications required the retention of the crystalline character of the cellulose, more gentle modification of cellulose was required. 8 In this context, surface modification of cellulose fibers with adsorbed molecules has been an attractive option for the creation of better interfaces between cellulose and thermoplastics. The self-aggregation of hydrophobically modified pullulan (HMP) has been previously studied in aqueous solutions, 9-17 as has HMP adsorption at the air/liquid 15,16,18,19 and solid/liquid interfaces. 20,21 Akiyoshi et al. observed that chloresterol-bearing pullulan (CHP) self-aggregated in aqueous solutions and formed stable nanoparticle hydrogels. The aggregation numbers calcu- lated from the molecular weights of the aggregates corresponded to 10-12 for all CHPs regardless of their degrees of substitution (DS) by cholesterol groups. 13 In another study, HMP modified with dodecanoic acid showed a more compact conformation than the starting pullulan, and reduced the contact of alkyl groups with water. 16 When amphiphilic HMPs were monitored at the air/water interface by surface tension measurements, longer equilibration times were observed for the formation of the adsorbed layer. 16,19,22 Deme et al. 22 attributed the slow kinetics of the surface tension change to the reorganization of the adsorbed polymer chains at the air/water interface. Studies concerning HMP adsorption onto polystyrene at the solid/liquid interface involved hydrophobically modified 6-carboxypullulan 20 and carboxymethylpullulan. 21 Both of these studies revealed strong adsorption originated from hydrophobic interactions between the polymer and the polystyrene. 20,21 Nonetheless, systematic studies of HMP adsorption at solid/liquid interfaces, where the solid surfaces were both hydrophilic and hydrophobic, have largely been absent. * To whom correspondence should be addressed. Fax: (540) 231-3255. E-mail: aesker@vt.edu. † Department of Chemistry, Virginia Tech. ‡ Department of Wood Science and Forest Products, Virginia Tech. § Friedrich Schiller University of Jena. Biomacromolecules 2009, 10, 2451–2459 2451 10.1021/bm900412g CCC: $40.75  2009 American Chemical Society Published on Web 07/27/2009