Electroresponsive characteristics of highly substituted phosphate starch

Abstract

Electrorheological (ER) fluids, suspensions typically formed by dispersing polarizable particles in an insulating oil, respond rapidly and reversibly to external electrical fields, leading to a sharp increase in shear viscosity [1] due to polarized and chain-like structures, which span the electrodes as a result of attractive forces generated between the dipoles. Because of their physicochemical stabilities, drybase anhydrous ER systems have attracted much attention as compared to the wet-base hydrous ER systems. Anhydrous ER materials include various semiconducting polymers [2–4], biopolymers [5, 6], and polymer/clay nanocomposites [7, 8], as well as modified cellulose and chitosan [9]. These ER materials possess either branched polar groups, such as amine, hydroxyl, and amino-cyano, or semiconductive repeating groups. The polar groups may affect the ER behavior by playing the role of the electronic donor under the external electric field. Starch has also been used as a wet-base ER material at an early stage of an ER research, in which the water coats the starch surface to induce polarization [10]. We adopted an esterification process using a mixture of ortho-phosphoric acid and urea to substitute the phosphate groups at ambient temperature [11]. The phosphate group in amylose of the potato starch is substituted in order to produce more polar groups in the potato starch under an applied electric field, and is adopted as a potential candidate for anhydrous ER material. Starch as one of the polysaccharides is comprised of both amylose and amylopectin. Although the phosphate in potato starch is located in the amylopectin portion, the substituted phosphate groups from the esterification are added to the amylose group. Potato tuber starches have a high content of phosphate relative to cereal starches. The phosphate groups are located as monoesters at the C-6 (∼70%) and at the C-3 (∼30%) positions of the glucose residues. In general, native potato starch has 0.3–0.4% of the glucose residues in the amylopectin [12]. To substitute pristine potato starch with a high degree of potato starch phosphate (HPSP), we used a mixture of disodium hydrogen phosphate and sodium hydrogen phosphate with water. Initially, 2.5 mole fraction of phosphate was dissolved in distilled water at 35 ◦C and controlled to make pH 6 using hydrochloric acid and sodium hydroxide. The potato starch powder was added in the salt solution and stirred for 20 min. This starch slurry was then filtered with a glass funnel and dried for 12 hr. To enhance phosphorylation, this mixture was heated at 150 ◦C for 3 hr and cooled in 50% methanol aqueous solution at 25 ◦C. This product was filtered again and washed with ethanol three times. The final powdered product, potato starch phosphate, was sieved using a 100-μm molecular sieve after drying in a vacuum oven for 3 days. ER fluids were prepared by dispersing the HPSP particles in an insulating silicone oil. ER properties were examined by a rotational rheometer (Physica MC120, Germany) equipped with a high-voltage generator using a Couette geometry. Fig. 1 shows the scanning electron microscope (SEM) image of HPSP, which is oval-shaped, and the sizes are in the range of 50–70 μm. The HPSP particles were dried to remove any trace of water for an anhydrous ER application. SEM micrographs also indicate that the phosphorylation of high substitution does not change the general granular morphology of potato starch. The phosphorus content of HPSP was measured with an inductively coupled plasma mass spectrometer (ICPMS, Perkin Elmer Elan 6100, USA). The degree of substitution (DS) of potato starch phosphate was calculated by the following Paschall’s equation [13]: