ARTICLE pubs.acs.org/Biomac
Amorphous Characteristics of an Ultrathin Cellulose Film Eero Kontturi,*,† Miro Suchy,† Paavo Penttil€a,‡ Bruno Jean,§ Kari Pirkkalainen,‡ Mika Torkkeli,‡ and Ritva Serimaa‡ †
Department of Forest Products Technology, School of Science and Technology, Aalto University, P.O. Box 16300, 00076 Aalto, Finland Department of Physics, University of Helsinki, P.O. Box 64, 00014 Helsinki University, Finland § Centre de Recherche sur les Macromolecules Vegetales (CERMAV-CNRS), BP 53, 38041 Grenoble Cedex 9, France ‡
bS Supporting Information ABSTRACT: Swelling behavior and rearrangements of an amorphous ultrathin cellulose film (20 nm thickness) exposed to water and subsequently dried were investigated with grazing incidence X-ray diffraction, neutron reflectivity, atomic force microscopy, and surface energy calculations obtained from contact angle measurements. The film swelled excessively in water, doubling its thickness, but shrunk back to the original thickness upon water removal. Crystallinity (or amorphousness) and morphology remained relatively unchanged after the wetting/drying cycle, but surface free energy increased considerably (ca. 15%) due to an increase in its polar component, that is, the hydrophilicity of the film, indicating that rearrangements occurred during the film’s exposure to water. Furthermore, stability of the films in aqueous NaOH solution was investigated with quartz crystal microbalance with dissipation monitoring. The films were stable at 0.0001 M NaOH but already 0.001 M NaOH partially dissolved the film. The surprising susceptibility to dissolve in dilute NaOH was hypothetically attributed to the lack of hierarchical morphology in the amorphous film.
’ INTRODUCTION Many polymers, synthetic and native alike, exhibit a semicrystalline character.1 Semicrystallinity implies that highly ordered crystalline domains coexist with disordered or “amorphous” domains within their elementary supramolecular units. Cellulose, the main structural ingredient of plant cell walls, is somewhat similar. According to the so-called fringed-fibrillar model of native cellulose, the polysaccharide chains are arranged into longitudinal threads called microfibrils where crystalline and amorphous domains appear after each other at irregular intervals.2-6 The average length of the crystallites, that is, the average frequency of the amorphous regions, and the width of the microfibril depend on the botanical source of cellulose.6,7 The precise arrangement of crystalline and disordered domains with respect to each other is still under debate, and several reports have presented substantial evidence that most of the disordered cellulose, termed paracrystalline cellulose, resides on the microfibril surface.8-11 However, the concept of longitudinal disorder explains many macroscopic phenomena with cellulosic fibers, for example, their mechanical properties12,13 and behavior upon acid hydrolysis.5,7 Moreover, all disordered regions in cellulose reportedly show common low-energy dynamics, that is, homogeneous molecular mobility with inelastic neutron scattering14 and at least the very existence of disordered (amorphous) cellulose in both native and regenerated cellulose grades is widely accepted.6,15 Amorphous cellulose differs significantly from its crystalline counterpart. For example, its reactivity in heterogeneous reactions, r 2011 American Chemical Society
including susceptibility for degradation,5,16-18 and its mechanical properties19 are very different. Unlike crystalline cellulose, amorphous cellulose is accessible to water.14,20 As amorphous cellulose cannot be isolated from the native microfibrils, a need persists to prepare amorphous cellulose substrates for fundamental studies. As a result, several methods to prepare amorphous cellulose in bulk have been reported and its properties have been subsequently examined.21-24 One of the ways to prepare amorphous cellulose is by regenerating ultrathin films (
