Elucidation Of Micro Pore Structure In Historic Silk And Development Of Targeted Conservation Methodology

Silk textile, a great invention and innovation initiated by ancient Chinese people, has contributed outstandingly to the enrichment of material civilization of mankind. Copious historic silk artifacts account for an important portion in the cultural heritage of our country, and also serve as meaningful entities for the research of Chinese culture in ancient times. However, a large number of historic silk textiles have become brittle in mechanical properties, because the microstructure of fibroin, the main constituent substance of silk, is destructed during a long period of preservation. They are in grave danger of perishment, so conservation techniques are urgently required. So far, elucidation of the microstructure of historic silk has been far from elaborate, and theoretical basis concerning this topic has been extremely flimsy. As a result, it is impossible to develop effective conservation technique against the defect of fibroin microstructure at a molecular level. In light of this situation, based on review of massive literature, this dissertation established a novel research direction for characterization of the microstructure of historic silk from the perspective of pore structure analysis, performed in-depth studies with original experimentations, and then developed corresponding consolidation material and technical craft aimed at the pore structure characteristics of historic silk.*H NMR cryoporometry was introduced to investigate the pore structure of silk for the first time. In this study, silk samples were treated as a porous system containing a mobile species, namely bound water, restricted in a rigid matrix of fibroin. Based on the principle of Gibbs-Thomson equation,1H NMR data under various temperatures, combined with theoretical calculation, was utilized to elucidate the micropore size distribution within historic and fresh silk, laying the scientific groundwork for adoption of consolidating materials with certain molecular dimensions. Also, in order to corroborate the obtained pore size distribution, solid-state13C cross-polarization (CP) magic-angle spinning (MAS) NMR spectroscopy and Fourier transform infrared spectroscopy (FT-IR) were used to analyze the feature of higher-order structures and functional groups within silk samples, and three-dimensional optical microscope and scanning electron microscope (SEM) were applied in the observation of surface morphology of silk samples. The results showed that the pore size distribution of fresh silk is strongly asymmetric with a well-defined maximum at1.1nm. Compared with fresh silk, the volume of pores around1.1nm decreased distinctly in the historic silk, and more pores with dimensions of30-60nm emerged accordingly, accompanied by scission of amino bond and loss of random coil conformation.Carr-Purcell-Meiboom-Gill (CPMG) pulse sequence was adopted to characterize spin-spin relaxation time (T2) of water molecules confined in the micropores of historic and fresh silk samples. Through characterization of the mobility of pore water, quantitative monitoring of the pore structure of silk was achieved. It is showed in the results that a singular relaxation time (T22=92.18μs) exists in fresh silk. On the other hand, a new relaxation component (T22=2-5ms) emerges in historic silk samples, whose relaxation time and relative intensity increases with the deepening of ageing degree. By analysis of the mobility and chemical environment of water proton, it was reflected that the molecular structure of silk fibroin becomes more disintegrated in the degradation process, which compares well with the results of micropore size analysis. Based on this, the newly-generated spin-spin relaxation time (T22) was demonstrated to be a sensitive indicator for the degradation degree of historic silk. The degradation process was imitated by alkaline hydrolysis, following this methodology a series of artificially aged samples with different preservation statuses were prepared. By means of1H NMR relaxation time analysis, combined with the mechanical properties tested by dynamic mechanical analysis (DMA), it is found that the1H NMR results of artificially aged samples and historic samples express a consistent trend, and the1H NMR results correlates well with mechanical properties. Therefore,1H NMR relaxation time can. be an effective tool for monitoring the preservation status of historic silk, and alkaline hydrolysis under certain conditions can obtain samples of similar micropore structure with historic silk.It is of great importance to silk artifacts conservation that the mechanical properties of waterlogged silk excavated in archaeological site will be restored after the fade away of water. Near-infrared spectroscopy was used to analyze the micro mechanism of this interesting phenomenon. In order to simulate it, historic and fresh silk samples were soaked completely in deionized water, then dried at room temperature. Through processing and comparison of NIR spectra of samples in both statuses, and utilizing second derivative spectra and peak fitting to determine the secondary structure in silk fibroin, we aimed to explain the mechanism of the dehydration process of water-saturated historic silk at molecular level. As shown in the results, for all the silk samples, after transition from water-saturated status to dry status, the peak of combination mode of amide A and amide II shifted to larger wavelength, indicating that the variation of moisture in silk fibers can lead to rearrangement of hydrogen bonds in silk fibroin polymeric system, i.e. the formation of stronger hydrogen bonds. Through deconvolution of the peak in the range of2130-2240nm, it is found that after dehydration, the conformations of fibroin molecular chain was altered, i.e. incompact random coil converted into relatively stable p-sheet. It is just because waterlogged silk in archaeological site lost the plasticization effect of water after dehydration, forming stronger hydrogen bond and more compact conformation, their mechanical properties were restored. Therefore, this research revealed that by utilizing the dehydration effects, the fragile water-saturated silk artifacts in the archaeological sites can successfully be extracted and transported to conservation laboratories without damage.Aiming at the feature of newly-generated micropores in historic silk, consolidation materials of polymerization type with the molecular dimension between the size of ever-existing and newly-generated micropores, namely sodium caseinate and transglutaminase were adopted, and the possibility of application of this enzyme mediated polymerization was demonstrated. Due to the rarity and irregular shape of genuine historic silk fabrics, alkaline hydrolyzed samples were used as substitutes in a large number of repeated tests. Orthogonal experiment and range analysis were utilized to determine the optimal technical conditions of this consolidation methodology. Dynamic mechanical analysis, color difference analysis, thermogravimetric analysis (TGA) and sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) were used to characterize the reaction and consolidation effects. The results showed that the optimal technical conditions of this consolidation methodology are water bath heating temperature50癈, sodium caseinate concentration2%, water bath heating time3h, transglutaminase activity units to sodium caseinate mass ratio30U/g. It is found that biopolymers with molecular weight higher than260kDa were formed in silk fibers due to the polymerization. The mechanical properties and thermal stability of the silk fabrics were improved significantly without an obvious change in the appearance of the samples, showing great potential of application.Aiming at the underlying problem that gas in the newly-generated micropores of aged silk may prevent the consolidation materials from entering, a vacuum experimental equipment was designed for the purpose of improving the effectiveness of transglutaminase mediated consolidation. Two kinds of vacuum consolidation mode (successive vacuum consolidation and synchronous vacuum consolidation) were studied, with non-vacuum consolidation mode as a reference. By using1H NMR cryoporometry and1H NMR relaxation analysis to characterize the changes of the micropore structure in silk samples, combined with the results of scanning electron microscope, dynamic mechanical analysis and color difference analysis, the merits and deficiencies of the two modes were compared, thus the suitable vacuum type for transglutaminase mediated consolidation was determined. As shown in the results, compared with successive vacuum consolidation and non-vacuum consolidation mode, synchronous vacuum consolidation can obviously enhance the mechanical properties of silk samples, and reduce the content of newly-generated micropores and relaxation component. Then dry thermal and hygrothermal ageing were used to verify the excellent ageing-resistance of this consolidation methodology on silk. On this basis, this mature consolidation technique was applied on genuine historic silk, and achieved favorable effects.The research findings in this dissertation about micropore structure of historic silk provided novel ideas and methods for studying the microstructure of silk, promoted a more comprehensive understanding of the information in historic silk, benefited the establishment of the fragility theory about historic silk, and afforded theoretical foundation for the development of conservation techniques. The application and popularization of the consolidation methodology developed in this dissertation is supposed to salvage a large number of vulnerable historic silk artifacts, thus preserve important entities for studying ancient civilization.