Structure and relaxation in cellulose hydrogels Zhiyong Xia, Marcia Patchan, Jeffrey Maranchi, Morgana Trexler The Johns Hopkins University, Applied Physics Laboratory, Laurel, Maryland 20723 Correspondence to: Z. Xia (E - mail: Zhiyong.Xia@jhuapl.edu) and M. Trexler (E - mail: Morgana.Trexler@jhuapl.edu) ABSTRACT: Cellulose-based hydrogels show great potential for a wide range of applications. However, the structure of these hydrated gels is not fully understood. The impact of moisture on the structure and stability of cellulose based hydrogels is reported in this article. Analytical data based on GPC, NMR, and rheology are discussed. It was found that moisture-induced gelation greatly reduces the crystallinity of the hydrogels, and the release of water from the hydrogels leads to permanent structural changes in the network structure due to the reformation of hydrogen bonding. V C 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 42071. KEYWORDS: cellulose; hydrogels; relaxation Received 9 December 2014; accepted 30 January 2015 DOI: 10.1002/app.42071 INTRODUCTION Cellulose-based hydrogels are three dimensional (3D) network structures with a large amount of trapped water. These hydrogels have found applications in tissue engineering, controlled drug release, contact lenses, and disposable diapers applications. 1–4 Compared to chemically crosslinked hydrogels which require covalent bonding for the formation of 3D networks, cellulose- based hydrogels are formed mostly by hydrogen bonding via a self-assembly process. Formation of cellulose-based hydrogels starts by the dissolution of crystalline cellulose powder. Due to the high intrinsic crystallinity and high level of hydrogen bonding between the anhydrous glucose main chains and within the main chains (Figure 1), strong organic solvents such as N,N-dimethyla- cetamide (DMAc) in the presence of LiCl salt are generally used for dissolution. 5–8 The gradual dissolution of cellulose starts by the formation of hydrogen bonding between the hydroxyl protons of the anhydroglucose unit in cellulose with the Cl 2 in LiCl. The Cl 2 is then attracted to the (LiDMAc) 1 macrocation. The latter leads to the formation of repulsive forces between different seg- ments of the host cellulose. The repulsive forces will then open more micropores, allowing more solvent to penetrate into the cel- lulose matrix. Due to the anhydrous nature of the above solution, the presence of moisture leads to the regeneration of hydrogen bonding between the dissolved cellulose segments, that is, gela- tion. Throughout this process, cellulose main chains assemble into a 3D network structure by reforming hydrogen bonds between various OH 2 groups on the anhydrousglucose units. Due to the fast reaction kinetics and high hydrophilicity of the anhydrousglucose units, large amounts of moisture are trapped in the gel. Recently, we reported the synthesis and crosslinking behavior of hydrogels based on Avicel PH101 cellulose powder (with a nom- inal particle size of 50 mm from FMC Biopolymer) for ocular bandage applications. 10,11 In this article, the microstructure and relaxation of these hydrogels is studied. The main focus of this work is to understand the effect of moisture on the structure of these hydrogels, which is of interest to guide incorporation of drugs into these ocular bandage gels. EXPERIMENTAL Hydrogel Preparation A detailed synthesis procedure has been previously reported. 10 Briefly, Avicel 101 cellulose powder (2, 3, and 5 g) was soaked in 100 mL N,N-DMAc for 24 h followed by the addition of 8 g of LiCl. The subsequent hydrogels are referred to as 2%, 3%, and 5% samples. The solution was then heated to 95  C under agitation to form a uniform clear solution. The anhydrous solu- tion was then poured into a silicone mold (4 mm 3 3 mm 3 40 mm). The mold containing the cellulose solutions was then exposed to 73% relative humidity at 35  C and left for 12 h for complete gelation. The gelled sample was then washed with deionized (DI) water at least five times to remove any excess LiCl and DMAc. Samples were then soaked in DI water before testing. Complete removal of Li 1 and DMAc was confirmed by inductively coupled plasma-optical emission spectroscopy (Var- ian VistaPro) and Fourier transform infrared spectroscopy (Per- kin Elmer Spectrum 100), respectively. 10 Nuclear Magnetic Resonance Analysis To understand the 3D structure of the hydrogels, solid state 13 C CPMAS NMR (nuclear magnetic resonance) was performed on three representative samples: (a) as-received Avicel 101 powder, V C 2015 Wiley Periodicals, Inc. WWW.MATERIALSVIEWS.COM J. APPL. POLYM. SCI. 2015, DOI: 10.1002/APP.42071 42071 (1 of 5)