Synthesis and antimicrobial activity of quaternary ammonium-functionalized POSS (Q-POSS) and polysiloxane coatings containing Q-POSS Partha Majumdar a , Elizabeth Lee a , Nathan Gubbins a , Shane J. Stafslien a , Justin Daniels a , Clayton J. Thorson a , Bret J. Chisholm a, b, * a The Center for Nanoscale Science and Engineering, North Dakota State University,1805 Research Park Drive, Fargo, ND 58102, USA b Department of Coatings and Polymeric Materials, North Dakota State University,1735 Research Park Drive, Fargo, ND 58102, USA article info Article history: Received 16 September 2008 Received in revised form 5 January 2009 Accepted 7 January 2009 Available online 14 January 2009 Keywords: Polyhedral oligomeric silsesquioxane (POSS) Polydimethylsiloxane Antimicrobial abstract An array of quaternary ammonium-functionalized POSS (Q-POSS) compounds were synthesized and their antimicrobial properties toward the Gram-negative bacterium, Escherichia coli, and the Gram- positive bacterium, Staphylococcus aureus, determined in aqueous solution. Using Q-POSS compositions that exhibited broad spectrum antimicrobial activity in solution, the utility of the Q-POSS compounds as an antimicrobial additive for polysiloxane coatings was determined. The results of the investigation showed that Q-POSSs possessing a relatively low extent of quaternization and longer alkyl chain lengths provided the highest antimicrobial activity in solution. For polysiloxane coatings containing Q-POSS molecules as an antimicrobial additive, coating surface energy, surface morphology, and antimicrobial properties were found to be strongly dependent on Q-POSS composition. Coatings based on Q-POSSs possessing the lowest extent of quaternization displayed antimicrobial activity while analogous coatings produced using Q-POSSs possessing the highest extent of quaternization showed no antimicrobial activity. The lack of antimicrobial activity exhibited by coatings possessing Q-POSSs with a relatively high extent of quaternization was attributed to agglomeration of Q-POSS molecules through the formation of intermolecular interactions involving the quaternary ammonium moieties. Agglomeration would be expected to reduce diffusivity and inhibit interaction of the Q-POSS molecules with microbial cells. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction Silsesquioxanes, which have the general molecular formula [RSiO 1.5 ] n (n  4), were first reported in 1946 by Scott [1]. Silses- quioxanes exist in a variety of structures among which polyhedral oligomeric silsesquioxane (POSS) compounds are of particular interest since they possess a unique cage-like structure and nano- scale dimensions (1–3 nm in diameter) [2]. Effective incorporation of POSS into a polymer matrix to produce a nanocomposite can result in increased glass transition temperature [3], enhanced mechanical properties [4,5], increased use temperature [6], lower flammability, and enhanced rheological properties [7]. Function- alized POSS with multiple reactive functionalities are ideal for the production of unique organic–inorganic hybrid nanomaterials. Functional groups such as epoxy [8,9], amine [10], vinyl [11], alcohol, carboxylic acid, fluoroalkyl, imide, and halide have been successfully incorporated into POSS structures and used for a variety of applications. Most of these functionalized POSS compounds are commercially available. Quaternary ammonium compounds (QACs) are widely used as antimicrobial agents to inhibit microbial growth [12–15]. The antimicrobial activity provided by QACs results from both ionic and hydrophobic interactions between the QAC and components of the microbial cell wall that leads to cell death or malfunction in cellular processes [16–18]. The ability of a QAC to bind to the microbial cell wall and disrupt its function is dependent on various compositional factors such as charge density, amphiphilicity, molecular size, and molecular mobility. Since cell wall composition and structure varies from one microorganism to another, the effectiveness of a given QAC tends to vary from one microorganism to another. The complex relationship between QAC composition, microorganism species, and antimicrobial activity has been previously demon- strated by several investigators [19,20]. Traditionally, QACs containing one or two salt groups have been used as antimicrobial agents in commercial products; however, in nature, compounds containing multiple quaternary ammonium * Corresponding author. The Center for Nanoscale Science and Engineering, North Dakota State University,1805 Research Park Drive, Fargo, ND 58102, USA. Tel.: þ1 701 231 5328; fax: þ1 701 231 5325. E-mail address: bret.chisholm@ndsu.edu (B.J. Chisholm). Contents lists available at ScienceDirect Polymer journal homepage: www.elsevier.com/locate/polymer 0032-3861/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2009.01.009 Polymer 50 (2009) 1124–1133