What are the physical properties that will distinguish the cross-linked chain polymer, linear polymer and.....

What are the physical properties that will distinguish the cross-linked chain polymer, linear polymer and branched polymer?





And, if any, what are their similar physical properties?





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What are the physical properties that will distinguish the cross-linked chain polymer, linear polymer and.....
Polymer bulk properties are strongly dependent upon their structure and mesoscopic behavior. A number of qualitative relationships between structure and properties are known.





Chain Length





Increasing chain length tends to decrease chain mobility, increase strength and toughness, and increase the glass transition temperature (Tg). This is a result of the increase in chain interactions such as Van der Waals attractions and entanglements that come with increased chain length. These interactions tend to fix the individual chains more strongly in position and resist deformations and matrix breakup, both at higher stresses and higher temperatures.





Branching





Branching of polymer chains also affect the bulk properties of polymers. Long chain branches may increase polymer strength, toughness, and Tg due to an increase in the number of entanglements per chain. Random length and atactic short chains, on the other hand, may reduce polymer strength due to disruption of organization. Short side chains may likewise reduce crystallinity due to disruption of the crystal structure. Reduced crystallinity may also be associated with increased transparency due to light scattering by small crystalline regions. A good example of this effect is related to the range of physical attributes of polyethylene. High density polyethylene (HDPE) has a very low degree of branching, is quite stiff, and is used in applications such as milk jugs. Low density polyethylene (LDPE), on the other hand, has significant numbers of short branches, is quite flexible, and is used in applications such as plastic films. The branching index of the polymer is a parameter that characterizes the effect of long-chain branches on the size of a branched macromolecule in solution.





Chemical Cross-linking





Cross linking tends to increase Tg and increase strength and toughness. Cross linking consists of the formation of chemical bonds between chains. Among other applications, this process is used to strengthen rubbers in a process known as Vulcanization, which is based on crosslinking by sulfur. Car tires, for example, are highly crosslinked in order to reduce the leaking of air out of the tire and to toughen the tires durability. Eraser rubber, on the other hand, is not cross linked to allow flaking of the rubber and prevent damage to the paper.


2-Linear polymers. The are called linear polymers because of their linear structure. A linear molecule is not completely straight in shape. Generally, a polymer consists of more than one type of structure. Thus a linear polymer may contain some branched and cross-linked chains. As a result of branching and cross-linking, the polymer’s properties can change.





Branched polymers. The properties of a polymer depend not only on their arrangement in the molecular structure. In branched polymers, side-branched chains are attached to the main chain during the synthesis of the polymer . Branching interferes with the relative movement of the molecular chains. As a result, resistance to deformation and stress-crack are affected. Also, the density of branched polymers is lower than that of linear-chain polymers as branches interfere with the packing efficiency of polymer chains. The behavior of branched polymers can be compared to that of linear-chain polymers by making an analogy with a pile of tree branches (branched polymers) and a bundle of straight logs (linear). Notice that it is more difficult to move a branch within the pile of branches than to move a log in its bundle. The three-dimensional entanglements of branches make movements more difficult, a phenomenon akin to increased strength.





Cross-linked polymers. Generally, three-dimensional in structure, cross-linked polymers have adjacent chains linked by covalent bonds . Polymers with cross-linked chain structure are called thermosets, or thermosetting plastics, such as epoxies, phenolics, and silicones. Cross-linking has a major influence on the properties of polymers (generally imparting hardness, strength, stiffness, brittleness, and better dimensional stability).





Network polymers. Network polymers consist of spatial (three-dimensional) networks of three active covalent bonds . A highly cross-linked polymer is also considered a network polymer. Thermoplastic polymers that have already been formed or shaped can be cross-linked to obtain greater strength by subjecting them to high-energy radiation, such as ultraviolet, x-rays, or electron beams. However, excessive radiation can cause degradation of the polymer.