Thermochimica Acta 461 (2007) 137–144 Effect of water on the thermal properties of silk fibroin Xiao Hu a , David Kaplan b , Peggy Cebe a,∗ a Department of Physics and Astronomy, Tufts University STC-208, 4 Colby Street, Medford, MA 02155, United States b Departments of Biomedical Engineering and Chemical and Biological Engineering, Tufts University, Medford, MA 02155, United States Available online 14 December 2006 Abstract Silk fibroin films cast from water solution, and containing bound water, are quantitatively studied in this work. First, to obtain the solid and liquid heat capacities of the pure dry silk fibroin, cyclic heat treatment was used to monitor the process of removing the bound water. After water removal, the glass transition of pure non-crystalline silk was observed at 451 K (178 ◦ C). The solid and liquid heat capacities of the pure silk fibroin were then measured using differential scanning calorimetry (DSC), temperature-modulated DSC (TMDSC), and quasi-isothermal TMDSC, and found to be: C p (T) solid = 0.134 + 3.696 × 10 -3 T J/g K and C p (T) liquid = 0.710 + 3.47 × 10 -3 T J/g K over the temperature region from 200 to 450 K. These heat capacities were used to construct the underlying baseline heat capacity for the combined silk–water system. When the combined silk–water system is studied, bound water is lost from the film during heating, and the loss of mass is quantified using thermogravimetric analysis (TGA). Bound water in the silk film acts as a plasticizer, and a lower glass transition of the silk–water system is observed. Comparison of the measured heat capacity of the silk–water system to the calculated total baselines was made in the vicinity of the water-induced glass transition. Results show that the total solid specific heat capacity is in good agreement with the calculated solid baseline in the low-temperature region below about 240K. As temperature increases above the lower glass transition, all bound water eventually leaves the silk, and the free volume and the silk mobility are reduced. This allows the upper glass transition of the dried silk to be observed. © 2007 Elsevier B.V. All rights reserved. Keywords: Bombyx mori silk fibroin; Heat capacity; DSC; Bound water; Glass transition 1. Introduction Domesticated (Bombyx mori) silk worm fiber has been stud- ied for many years [1,2]. The filament of the silk fiber, silk fibroin, is coated with a water-soluble protein glue, sericin, to form the natural silk fiber. As the interior structural protein of silk fiber, silk fibroin has been used in many fields including bio- materials applications [3,4]. In the gland of B. mori silkworms, the water-soluble silk fibroin solution and the sericin are spun together into a fiber, leading to a new insoluble conformation due to a rapid change in structure of the fibroin. The sericin is added to the surface of silk fibroin during spinning, and the result is the formation of the silk fiber. The phase transitions of silk fibroin have been well studied and were a result of the formation of anti-parallel beta-sheet structure. Exposure of the aqueous solution of fibroin to organic solvents, mechanical stress and thermal treatment can induce formation of the insoluble beta-sheet structure in degummed silk ∗ Corresponding author. Tel.: +1 617 627 3365; fax: +1 617 627 3744. E-mail address: peggy.cebe@tufts.edu (P. Cebe). fibroin films [2,5–11]. However, the mechanism of beta-sheet- crystallization is still unclear. One reason for this uncertainty is that silk fibroin crystallizes rapidly, making the initial stages of crystallization difficult to study. We have used thermal analy- sis and infrared spectroscopy to determine the content of beta sheet after thermal crystallization [10,11]. Thermal crystalliza- tion provides a wide temperature region to study the structural transitions of the silk fibroin because of the high glass transition temperature, T g = 451 K (178 ◦ C) of this protein. In addition, its isothermal crystallization above T g is much slower than in the natural spinning process [10,11]. In the present work, we investigate the impact of water on the thermal properties of silk fibroin. Water in polymeric systems can be classified into three types [12–14]: freezing free water, freezing bound water, and non-freezing bound water. According to McGrath and co-workers [13], freezing free water in polymers is unbound water that has the same transition temperature at 273 K as bulk water. In our silk–water system, the freezing free water can be removed by keeping the samples in a vacuum oven for several days. Freezing bound water is attributed to the weak interaction between water and polymer and will also undergo the freezing transition. Non-freezing bound water results from 0040-6031/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.tca.2006.12.011