Hello! My name is Kathleen Dewan and I am a rising junior at Northwestern studying materials science and engineering. This summer, I am working with Yuepeng Zhang, whose group specializes in the synthesis of nanofibers using electrospinning. In my project, I am aiming to use electrospinning for the conservation of paper artifacts.
The preservation of cultural heritage is vital to the perpetuation of unique identities and communities within society, as it serves to maintain the histories and records of these cultures for many generations into the future. Paper is one of the oldest substrates used by man for record-keeping, as it was first invented in China around 100 B.C., and a great number of historically pertinent documents are paper-based. There is currently a great emphasis placed on the preservation of these artifacts from chemical degradation, as cellulose-based paper is subject to acidification and oxidation over extended periods of time. However, there are far fewer techniques that exist to protect paper from mechanical wear, which can include tearing, water damage, degradation from UV radiation, and exposure to contaminants.
In order to protect paper artifacts from this kind of damage, the ideal conservation mechanism would be clear, tensily strong, hydrophobic, UV resistant, and able to provide a barrier against common airborne particulates. Due to these requirements, we looked towards electrospinning a blend of polyvinylidene fluoride with hexafluoropropylene (PVDF/HFP) to deposit a near-invisible membrane of nanofibers directly onto a paper substrate. The electrospinning process produces nanofibers by using a voltage source to draw a solution of opposite charge onto it, and this results in the formation of a nanofiber mesh. This mesh structure is ideal for our applications, as it provides a barrier against potential contaminants while still remaining porous enough to maintain the ambient atmospheric conditions that are required for the paper to remain chemically stable. We chose PVDF/HFP for our solution because it is colorless, meaning that the legibility of the paper can be maintained, as well as highly hydrophobic and UV resistant, meaning that it can serve to protect the paper from water and UV damage.
Below is an image of the experimental setup used for electrospinning.
Figure 1: Electrospinning experimental setup
The first step in creating an appropriate membrane is to optimize the morphology of the fibers produced by electrospinning. Because electrospinning is a process that involves many parameters, including spinning time, voltage, injection rate of solution, and concentration of solution, this required the controlling and variation of many variables in order to get the desired results. Overall, the trends that I observed are that higher spinning times resulted in thicker membranes, higher voltages resulted in better overall coverage, and higher injection rates resulted in a greater amount of fibers. For our applications, we needed our fibers to be numerous and provide adequate coverage, while having the membrane still thin enough as to not alter the legibility of the document. It was sometimes challenging to alter the parameters appropriately to achieve these desired results, which I will discuss further below.
We first deposited the fibers onto aluminum foil, in order to provide a initial estimate of the required parameters. We then deposited the fibers onto a paper substrate, in which we found that certain adjustments needed to be made in order to account for the reduced conductivity of the paper and to maintain the legibility of the text. From these experiments we found that the ideal spinning time would be around 3 minutes, as this would allow the fibers to be just visible enough to identify an appropriate area to take an SEM sample, while still thin enough to preserve the legibility of the text.
Figures 2, 3, and 4: Visual determinations of membrane thickness
We then performed analyses of the fibers using SEM imaging in order to examine their microstructures. SEM imaging is necessary to observe the actual morphology of the fibers, as even if the membrane seems thin enough from the naked eye, we need to ensure that they provide even coverage of the substrate. Below are some examples of how altering certain parameters can change the morphology of the fibers.
The sample below was spun on an aluminum substrate at a voltage of 16 kV and an injection rate of 0.2 mL/hr, with varying spinning times. These images showed us that the injection rate of 0.2 mL/hr was too low, as their coverage is not sufficient, even at 3 minutes.
Figure 5: SEM analysis of samples with varying spinning times
The sample below was spun on a paper substrate for 3 minutes at 0.5 mL/hr, with varying voltages. After increasing the injection rate to 0.5 mL/hr, we began to see more effective coverage at 16 kV and 20 kV, but there are still beads of solution that are present on the fibers. This resulted from instability during the electrospinning process, which was likely caused by the solution concentration, and thus viscosity, being too low, as well as the decrease in the conductivity of the voltage source introduced by the paper substrate [1].
Figure 6: SEM analysis of samples with varying voltages
The most recent experiment that I performed used the same spinning time, voltage, and injection rate conditions, but this time with double the concentration of PVDF/HFP in the solution. Because we increased the solution concentration, we hypothesize that the bead formation will decrease due to the increase of the solution viscosity. However, further SEM imaging is still necessary to determine if these conditions will be optimal.
After the fibers have been sufficiently optimized, I can perform an ImageJ analysis of the SEM images in order to determine the sizes of the spaces in between the fibers in the mesh. I can then compare this information to the average diameters of typical airborne contaminants, which can be found in the table below [2]. This will allow me to make a qualitative estimate of whether or not the membranes will be effective in protecting the paper from these contaminants.
Table 1: Size of common pollutants or contaminants and aerodynamic diameter of common gases.
In the coming weeks, I would also like to performed certain tests on both paper with and without the PVDF/HFP membranes. I would first like to perform tensile tests in order to compare the tensile strength and elongation of each sample, as well as contact angle tests in order to compare the hydrophobicity of each sample. If I still have enough time, I can also perform porosity testing on the paper sample with the membrane in order to determine what kinds of gases the membrane will allow to pass through; this information, in conjunction with Table 1, will help me to determine if the appropriate ambient atmospheric conditions can be maintained with the membrane.
References
1: H Fong, I Chun, D.H Reneker, Beaded nanofibers formed during electrospinning, Polymer, Volume 40, Issue 16, 1999, Pages 4585-4592, ISSN 0032-3861, https://doi.org/10.1016/S0032-3861(99)00068-3.
2: Qinglian Li, Sancai Xi, Xiwen Zhang, Conservation of paper relics by electrospun PVDF fiber membranes, Journal of Cultural Heritage, Volume 15, Issue 4, 2014, Pages 359-364, ISSN 1296-2074, https://doi.org/10.1016/j.culher.2013.09.003.