Week Five: Continued Polymer Synthesis

During week 5, the synthesis of starch/PLA composite films continued, as well as analysis of the results from the previous composites. The 29% starch composite from week 4 successfully formed a film, although it was more stiff to the touch compared to the 100% PLA films. As for the 6% starch composite, the resulting film did not result in a uniform thickness. One possible explanation for this is that the starch and PLA solutions may not have been properly mixed together before pouring them into the petri dish. To prevent such an issue from recurring, it was decided that the starch and PLA solutions would be combined and mixed together after both substances dissolved in separate chloroform solutions. With this procedural modification, three additional starch/PLA composite films were synthesized. The specific data for each film is shown in the table below. The films were composed of about 12%, 35%, and 6% starch. The reason for creating a second 6% starch composite was to replace the uneven 6% starch film that was created during week 4.

Finished Films from Previous Composites:

Starch/PLA Composite Films from Week 4

After creating the additional composite films, another starch film was synthesized using the same materials and methods as in week 4. One observation during week 4 starch film was that it was uneven and difficult to remove from its aluminum backing. This week, the mixture for the starch film was spread onto a variety of different surfaces to determine which surface would be most suitable for removal with the least amount of tearing. As shown in the figure below, the starch mixture was spread onto an aluminum sheet taped to a flat metal surface, wax paper, and the metal surface itself. Therefore, three separate films were made, each on a different surface.

Glycerin and Starch used to make Starch Films

Starch Film Mixture Prior to Heating

Starch Film Mixture Spread onto Surfaces

In addition to synthesizing polymer films, research was done to determine the chemical structure of PLA and the chemical processes occurring during the synthesis of 100% starch films. The repeating structural unit of PLA is shown below. As for the starch films, glycerin served as a plasticizer in order to increase the pliability of the film by acting like a molecular 'lubricator.' The vinegar helped to prevent the films from becoming too brittle by breaking down the branched amylopectin component of starch through a process called acid hydrolysis. By removing the amylopectin branch of starch, the flexible amylose component that is favorable for plastics would remain. See the post from week 1 for more information about the chemical structure of starch.

Chemical Structure of PLA
Source: http://bme240.eng.uci.edu/students/06s/shchung/