Difference between revisions of "Team:TU Darmstadt/Project/Chem"
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<p>The reaction time is dependent on the preparation size and it’s very important to have an open system because the resulting water has to escape</p> | <p>The reaction time is dependent on the preparation size and it’s very important to have an open system because the resulting water has to escape</p> | ||
| − | <h4>4 | + | <h4>4: Photocuring</h4> |
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| + | <p>Mixing the prepolymer with an UV-dependent photoinitiator (Igracure 1173) and validatind the crosslinking in an UV-Chamber. <p> | ||
<h2>Results</h2> | <h2>Results</h2> | ||
Revision as of 23:17, 18 September 2015
Introduction
This part is the link between our Biological work and the 3D printer. After we have produced our monomer molecules in E. coli and purified them subsequently, 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 for produce the prepolymers. We’d be able to create a lot of alternative prepolymers with different physical properties. Those could be utilized to adjust the physical properties of the final polymers.
Schematic overview
Itaconic acid (IA)
This is the crucial monomer, because it is photoactive. It contains a carbon double bond that undergoes the light-initiated crosslinking during the final printing process. The other important chemical functions are the two acid groups at the ends of the molecule. In the chemical generation of the prepolymer they react with the alcohol groups of the other monomers in an esterification reaction and 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 to increase the coverage of our spacer.
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.
Xylitol
In order to get a broader scope of different possible prepolymers we decided to use xylitol as an additional compound in our preploymer. Besides its two terminal it contains three additional alcohol groups. It can react with more than two itaconic acid molecules and thus induce further crosslinking additionally to the light-initiated crosslinking in the printing process afterwards. By using different proportions between (P)EG and xylitol the density of this crosslinking step can be adapted.
Procedure
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 didn’t 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:
Because of the Carothers-Equation, a molar monomer ratio of exactly 1:1 is necessary to reach high degrees of polymerization high chemical conversion. 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 and need to be molten.
3. Heating to 145°C with stirring at atmospheric conditions for at least 24 hours
The reaction time is dependent on the preparation size and it’s very important to have an open system because the resulting water has to escape
4: Photocuring
Mixing the prepolymer with an UV-dependent photoinitiator (Igracure 1173) and validatind the crosslinking in an UV-Chamber.
Results
Only the use of polyethylene glycol (PEG) and not EG used for the Production of resin was described in literature. We performed polymerization experiments with EG and received a prepolymer. But in the light-initiated crosslinking couldn’t produce a hard polymer. The resulting resin was very doughy 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 recalcitrant.
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 did the fastest crosslinking and resulted in the hardest final polymer. Thus they were used in our 3D printer for the production of objects.