This part is the link between our biological work and the 3D printer. After production and purification of the monomer molecules in E. coli, they need to be connected to short polymers, the prepolymers. Those fluid prepolymers are used in our 3D printer as a resin, the basis from which the final and hardened 3D structures emerge.
We used different monomers in varying proportions to produce the prepolymers. We were able to create a lot of alternative prepolymers with different physical properties. Those can be utilized to adjust the physical properties of the final polymers.
Itaconic acid (IA)
This is the crucial monomer, because it is photoactive. It contains a carbon-carbon double bond that undergoes the light-initiated crosslinking reaction during the final printing process. The other important chemical functions are the two acid groups of the molecule. In the chemical generation of the prepolymer they react with the alcohol groups of the other monomers in an esterification reaction while water is released.
Ethylene glycol (EG) / Polyethylene glycol (PEG)
Ethylene glycol is the basic provider of alcohol groups for the esterification reaction. It also serves as a spacer to increase the distance between the itaconic acid molecules in the prepolymer. By changing the distance between the photoactive compounds it is possible to adapt the density of light-initiated crosslinking and thus the physical properties of the final polymer. For this reason we also used polyethylene glycol of different length.
EG/PEG have two terminal alcohol groups and itaconic acid has to terminal acid groups. They can be connected to polymers which do not possess side chains. This makes the adjustment of the reaction and the determination of the prepolymer rather easy, but it also means that only the length of the chains can be adapted.
To obtain a broader scope of different possible prepolymers we decided to use xylitol as an additional compound in our preploymer. Besides its two primary alcohol groups it also contains three secondary ones. It can react with more than two itaconic acid molecules and thus induce further crosslinking in addition to the light-initiated crosslinking in the printing process afterwards. By using different proportions of (P)EG and xylitol the density of this crosslinking step can be adapted.
During the experiments we designed our own test procedure. To our surprise we were able to produce polymers under much easier reaction conditions than in the literature [Referenz]. In contrast to literature data we did not use vacuum and an argon atmosphere to get comparable polymers. This procedure was used for all of our following experiments (please see the material and methods part for further details).
1. Pre-dried (water free) monomers were mixed:
Since we use two different monomers a molar ratio of exactly 1:1 and a high chemical conversion close to 100% are necessary to reach high degrees of polymerization (compare Carothers-Equation). Therefore, careful weighing of the monomers is extremely important
2. Melting at 145°C:
PEG/EG is a liquid, while IA and xylitol are solids which need to be melted.
Heating to 145°C while stirring at atmospheric conditions for 24 hours or more. The reaction time depends on the batch size and it is very important to have an open system because the resulting water needs to escape.
Mixing the prepolymer with an UV-dependent photoinitiator and validating the crosslinking in an UV-chamber.
To our knowledge only the use of polyethylene glycol (PEG) and not EG is described in literature for the production of photoactive polymers. We performed polymerization experiments with EG and received a prepolymer. The resulting resin was too viscous and therefore not usable for printing. When using EG the double bonds of itaconic acid seemed to be too close to each other and therefore the polymer got brittle.
In the following experiments we used polyethylene glycol (PEG200 or PEG400) instead of EG. We performed crosslinking experiments with all promising prepolymers by using an UV-dependent photoinitiator and an UV-chamber. Prepolymers containing PEG200 showed the fastest crosslinking and resulted in the hardest final polymer. Thus they were used in our 3D printer for the production of objects.
The prepolymer with the best properties: IA + PEG 200
Addition of xylitol
We also performed experiments with xylitol, PEG and IA among the same reaction conditions. If the amount of xylitol was too high compared to PEG the resin already got hard during the polyesterification. Using small amounts of xylitol (5 to 10%) resulted in liquid resins which were unfortunately also not usable for the printing process because they did not lead to satisfying polymers during UV-crosslinking experiments.
For the characterization of our produced prepolymers we used 1H-NMR-Spectroscopy to check if the double bonds of the IA are retained. Differential Scanning Calorimetry (DSC) was used to analyze the glass transition temperature of the prepolymers which were compared to literature data as well as among themselves. Furthermore we ascertained the average molecular weight through Gel Permeation Chromatography (GPC).
The resins we produced out of PEG and IA show a honey-like viscosity, which is much higher than the viscosity of pure PEG.