Piezoelectricity in Chitosan—How Does Crystallization Influence Piezoelectric Response?

In recent years, there has been a growing interest in sustainable technology, which has led to increased attention to biopolymers due to their diverse physical and chemical properties. Chitosan, the second most abundant natural semi-crystalline polymer after cellulose, stands out as a low-cost, bioderived, and biodegradable material with significant potential for biomedical engineering, pharmaceuticals, and environmental science applications. Recent studies have indicated that a particular crystal polymorph of chitosan may possess notable electromechanical properties [1], suggesting the possibility of using it as an active piezoelectric material in flexible device design [1,2]. However, achieving piezoelectric performances comparable to those of classical fully crystalline piezoceramics is challenging for biopolymers due to their semi-crystalline nature, making it difficult to create functional piezoelectric devices.<br/>Over the past decade, significant efforts have been directed towards enhancing the crystallinity and piezoelectricity of biopolymers. This study takes a step further by focusing on enhancing chitosan's crystal formation and investigating the piezoelectric behavior of these crystals. A unique approach was taken, using a water solution of chitosan and formic acid, and studying the crystallization process on the surface of chitosan spin-coated nanofilm, induced by thermal annealing and NaOH neutralization or by their combination. NaOH, known for its ability to induce chemical changes and the formation of crystalline phases by promoting stronger chitosan interchain hydrogen bonding, was a key component of this innovative method [3,4]. The study also aims to explore whether different treatments lead to distinct chitosan crystal polymorphisms, possibly correlated with different piezoelectric responses [5]. The surface topography of the spin-coated chitosan films was examined using Atomic Force Microscopy (AFM), and the piezoelectric responses were measured using the Piezo response mode of the same instrument (PFM). This mode enables the local characterization of piezoelectricity at the nanoscale, allowing for accurate discrimination between crystalline and amorphous responses. Additionally, the instrument is used to characterize local dielectric properties, which are expected to differ significantly between the crystal and amorphous phases. The nanofilms' structural properties and crystalline form are analyzed using additional techniques such as transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS), respectively.<br/>AFM topographical characterization revealed the presence of distinct ellipsoidal-shaped crystals even without any post-fabrication treatment. After neutralization treatment, these crystal structures numerically increased, while with the annealing treatment, no topographical effects indicating the formation of crystals are observed. A first, preliminary, assessment of the films piezoelectric response confirms the presence of a significant signal on the neutralized samples with respect to the not neutralized or thermal annealed ones.<br/>The study will serve as a foundation for optimizing chitosan crystallinity, a crucial step in enhancing its piezoelectric output in macroscopic devices. The potential implications of this research are far-reaching, with the possibility of revolutionizing the field of sustainable technology and opening new avenues for the use of biopolymers in piezoelectric applications.<br/><br/>[1] de Marzo, et al., Adv. Elect. Mat., 2023<br/>[2] Praveen, et al., RSC advances, 2017<br/>[3] Chang, Wei, et al., Food Hydrocolloids, 2019<br/>[4] Takara, Eduardo Andres, Jose Marchese, and Nelio Ariel Ochoa. "NaOH treatment of chitosan films: Impact on macromolecular structure and film properties." Carbohydrate polymers 132 (2015): 25-30.<br/>[5] Baklagina, et al., Crystallography Reports, 2018