Final answer:
Increasing molecular weight is likely to increase the melting point of the polymer material as it leads to longer polymer chains, strengthened intermolecular forces, and potential enhancement in crystallinity, all contributing to a higher melting point.
Step-by-step explanation:
The question asks which of the following changes is likely to increase the melting point of the polymer material. The correct answer is: a. Increasing molecular weight. We can understand this by looking at the physical properties of polymers which indicate that longer chain lengths generally make the polymer stronger. Increasing the molecular weight means longer polymer chains, which enhances intermolecular forces such as Van der Waals forces and potentially increases the degree of crystallinity. Stronger intermolecular forces and higher crystallinity result in a higher melting point.
Adding plasticizers (b) would actually reduce the melting point by making the polymer more flexible and easier to melt. Introducing branching in the polymer chain (c) would make the chains pack less densely, reducing intermolecular forces and thereby lowering the melting point. Decreasing the degree of crystallinity (d) would also lower the melting point because crystalline regions are associated with more ordered structures that require more energy to break apart.