Final answer:
The factors that increase a polymer's melting point are increasing its molecular weight and its crystallinity. So, the correct options are (a) Increasing molecular weight and (c) Increasing crystallinity.
Step-by-step explanation:
The student is asking which factors are likely to increase the melting point of a polymer material. The options that would lead to an increase in a polymer's melting point are:
Increasing molecular weight: As the molecular weight of the polymer increases, the intermolecular forces between the chains become stronger, which typically results in a higher melting point.
Increasing crystallinity: A higher degree of crystallinity indicates more orderly packing of polymer chains, which strengthens intermolecular interactions and raises the melting point.
Plasticizers increase the distance between polymer chains, weakening the intermolecular forces and reducing the melting point. So, adding plasticizers actually decreases the melting point, as they reduce the intermolecular forces between polymer chains, making them more flexible. Decreasing cross-linking would also lower the melting point because cross-linked polymers are generally harder and more difficult to melt, due to the covalent bonds linking chains together. Therefore, decreasing cross-linking tends to lower the melting point.