Branching and crosslinking are two structural features that significantly affect the physical properties of polymers. Branching refers to the presence of side chains or irregularities in the polymer chain, while crosslinking refers to the formation of covalent bonds between different polymer chains. Both of these features can influence the glass transition temperature Tg , crystallinity, and mechanical properties of polymers.1. Glass transition temperature Tg : The glass transition temperature is the temperature at which a polymer transitions from a glassy, brittle state to a rubbery, more flexible state. - Branching: The presence of branches in a polymer chain can disrupt the regular packing of the polymer chains, leading to a decrease in the Tg. This is because the branches create more free volume and reduce the intermolecular forces between the polymer chains, making it easier for the chains to move and become more flexible at lower temperatures. For example, low-density polyethylene LDPE has a lower Tg than high-density polyethylene HDPE due to its higher degree of branching.- Crosslinking: Crosslinked polymers have covalent bonds between the chains, which restricts the movement of the chains and increases the Tg. For example, vulcanized rubber, which is crosslinked, has a higher Tg than non-crosslinked rubber, making it more resistant to deformation at higher temperatures.2. Crystallinity:Crystallinity refers to the degree of order and regularity in the arrangement of polymer chains.- Branching: Branching can disrupt the regular packing of polymer chains, leading to a decrease in crystallinity. The presence of branches makes it more difficult for the chains to align and form ordered, crystalline regions. For example, LDPE has a lower degree of crystallinity than HDPE due to its higher degree of branching.- Crosslinking: Crosslinking can also reduce the crystallinity of a polymer, as the covalent bonds between the chains can prevent them from aligning and forming ordered structures. However, the effect of crosslinking on crystallinity depends on the degree of crosslinking and the specific polymer system.3. Mechanical properties:Mechanical properties of polymers, such as tensile strength, modulus, and toughness, are influenced by branching and crosslinking.- Branching: The presence of branches can lead to a decrease in tensile strength and modulus, as the irregular packing of the chains reduces the intermolecular forces between them. However, branching can also increase the toughness of a polymer, as the increased free volume allows for more energy absorption during deformation. For example, LDPE is generally tougher but has lower tensile strength and modulus compared to HDPE.- Crosslinking: Crosslinked polymers generally exhibit improved mechanical properties, such as increased tensile strength, modulus, and toughness. This is because the covalent bonds between the chains restrict their movement and increase the intermolecular forces, leading to a more robust and resistant material. For example, vulcanized rubber has improved mechanical properties compared to non-crosslinked rubber, making it suitable for applications such as tires and seals.In summary, branching and crosslinking can significantly affect the physical properties of polymers, including their glass transition temperature, crystallinity, and mechanical properties. Branching generally leads to a decrease in Tg, crystallinity, tensile strength, and modulus, but an increase in toughness. Crosslinking typically results in an increase in Tg, tensile strength, modulus, and toughness, but a decrease in crystallinity. The specific effects of branching and crosslinking depend on the polymer system and the degree of branching or crosslinking.