Microstructure elucidation of historic silk (Bombyx mori) by nuclear magnetic resonance.

(1)H NMR cryoporometry and solid-state (13)C cross-polarization (CP) magic-angle spinning (MAS) NMR spectroscopy were used to characterize the microstructure of historic and fresh silk samples. Silk is a polymeric bicomponent material composed of fibroin and water located in micropores. According to the (1)H NMR cryoporometry method, the intensity of the water resonance as a function of the temperature was used to obtain the pore size distribution, which was strongly asymmetric with a well-defined maximum at 1.1 nm. Compared with the fresh silk samples, the volume of pores around 1.1 nm decreased distinctly in the historic silk, and more pores larger than 2 nm emerged accordingly. In addition, these results correlated well with solid-state (13)C CP/MAS NMR spectroscopy as the percentage of random coil in the historic silk sample was much less than that in the fresh silk samples. Therefore, it is suggested that the water-filled microvoids grow larger as the random coil conformation fades away in the degradation process.