Module 6 Network formation

When gelation happens what causes the dramatic change of the system? What are the sol and gel fractions?

2 Module objectives

  • Understand the role of the largest molecule in a gelation process.
  • Understand the significant changes of the system when gelation happens.
  • Understand sol-gel fractions when network is formed.

3. Classroom implementation ideas

Experiments:

Set the number of monomers as \(10^5\) and the ratio between A and B as 1. Change the ratio between A3 and AA and perform simulations. The values of A3 to AA are set as 0.1, 0.5, 1, 1.5, and 2.

Set the number of monomers as \(10^5\), ratio between A and B as 0.95, and the ratio between A3 and AA as 0.1, perform the simulation.

Questions:

  1. Observe the change of the mean DP and the mean DP without the largest molecule in each simulation. Conclude what happens to the system at the gelation point.

  2. How do the DPs of the second, third, fifth, tenth largest molecules change with the extent of reaction? What do you observe at the gelation point?

  3. Compare the sol fractions under the conditions: (1) the ratio between A3 and AA is 0.1 and the ratio between A and B is 0.95; and (2) the ratio between A3 and AA is 0.1 and the ratio between A and B is 1. Explain why these two conditions lead to such different results.

4. Example practice questions

  1. Calculate the critical extent of reaction when gelation would occur based on Carothers’ theory and the statistical theory, given the ratio between A and B is 1 and the ratio between A3 and AA is 0.5.

  2. Based on the simulations, describe what happens to the sytem when gelation occur.

  3. Which of the following statement is true?

  1. If the quantities of two types of functional groups are imbalanced, gelation would never occur.
  2. If the quantities of two types of functional groups are imbalanced, the sol fraction would be relatively large at the same extent of reaction after gelation occurs.
  3. When gelation occurs all molecules in the system merge into one huge molecule.
  4. When gelation occurs the largest molecules, i.e., the largest, the second largest, the third largest, …, all grow bigger exponentially.