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Chapter: Mechanical - Engineering materials and metallurgy - Non-Metallic Materials

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Polymer Architecture

An important microstructural feature determining polymer properties is the polymer architecture.The simplest polymer architecture is a linear chain: a single backbone with no branches. A related unbranching architecture is a ring polymer.

POLYMER ARCHITECTURE

 


 

Branch point in a polymer

An important microstructural feature determining polymer properties is the polymer architecture.The simplest polymer architecture is a linear chain: a single backbone with no branches. A related unbranching architecture is a ring polymer. A branched polymer molecule is composed of a main chain with one or more substituent side chains or branches. Special types of branched polymers include star polymers, comb polymers, brush polymers, dendronized polymers, ladders, and dendrimers.

 

Branching of polymer chains affects the ability of chains to slide past one another by altering intermolecular forces, in turn affecting bulk physical polymer properties. Long chain branches may increase polymer strength, toughness, and the glass transition temperature (Tg) due to an increase in the number of entanglements per chain. The effect

 

of such long-chain branches on the size of the polymer in solution is characterized by the branching index. Random length and atactic short chains, on the other hand, may reduce polymer strength due to disruption of organization and may likewise reduce the crystallinity of the polymer.

 

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 both long and short branc hes, is quite flexible, and is used in applications such as plastic films.


 

Dendrimers are a special case of polymer where every monomer unit is branched. This tends to reduce intermolecular chain entanglement and crystallization. Alternativel y, dendritic polymers are not perfectly branched but share similar properties to dendrimers due to their high degree of branching.

 

The architecture of the polymer is often physically determined by the functionality of the monomers from which it is formed. This property of a monomer is defined as the number of reaction sites at which may form chemical covalent bonds. The basic functionality required for forming even a linear chain is two bonding sites. Higher functionality yields branched or even crosslinked or networked polymer chains.

 

An effect related to branching is chemical crosslinking - the formation of covalent bonds between chains. Crosslinking tends to increase Tg and increase strength and

 

toughness. 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 ex ample, are highly crosslinked in order to reduce the leaking of air out of the tire and to toughen their durability. Eraser rubber, on the other hand, is not crosslinked to allow flaking of the rubber and prevent damage to the paper.

 

A cross-link suggests a branch point from which four or more distinct chains emanate. A polymer molecule with a high degree of crosslinking is referred to as a polymer network. Sufficiently high crosslink concentrations may lead to the formation of an infinite network, also known as a gel, in which networks of chains are of unlimited extent—essentially all chains have linked into one molecule.

 

Chain length

 

The physical properties of a polymer are strongly dependent on the size or length of the polymer chain. For example, as chain length is increased, melting and boiling temperatures increase quickly. Impact resistance also tends to increase with chain length, as does the viscosity, or resistance to flow, of the polymer in its melt state.

 

Chain length is related to melt viscosity roughly as 1:103.2, so that a tenfold increase in polymer chain length results in a viscosity increase of over 1000 times. Increasing chain length furthermore 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.

 

A common means of expressing the length of a chain is the degree of polymerization, which quantifies the number of monomers incorporated into the chain . As with other molecules, a polymer's size may also be expressed in terms of molecular weight. Since synthetic polymerization techniques typically yield a polymer product including a range of molecular weights, the weight is often expressed statistically to describe the distribution of chain lengths present in the same. Common examples are the number average molecular weight and weight average molecular weight. The ratio of these two values is the polydispersity index, commonly used to express the "width" of the molecular weight distribution. A final measurement is contour length, which can be understood as the length of the chain backbone in its fully extended state.

 

The flexibility of an unbranched chain polymer is characterized by its persistence length.

 

Monomer arrangement in copolymers


Monomers within a copolymer may be organized along the backbone in a variety of ways.

 

•Alternating copolymers possess regularly alternating monomer residues: [AB...]n (2). II Periodic copolymers have monomer residue types arranged in a repeating sequence: [AnBm...] m being different from n .

Statistical copolymers have monomer residues arranged according to a know statistical rule. A statistical copolymer in which the probability of finding a particular type of monomer residue at an particular point in the chain is independent of the types of surrounding monomer residue may be referred to as a truly random copolymer (3). •Block copolymers have two or more homopolymer subunits linked by covalent bonds (4). Polymers with two or three blocks of two distinct chemical species (e.g., A and B) are called diblock copolymers and triblock copolymers, respectively.  Polymers with three blocks, each of a different chemical species (e.g., A, B, and C) are termed triblock terpolymers. Graft or grafted copolymers contain side chains that have a different composition or configuration than the main chain.(5)

Tacticity

Tacticity describes the relative stereochemistry of chiral centers in neighboring structural units within a macromolecule. There are three types: isotactic (all substituents on the same side), atactic (random placement of substituents), and syndiotactic (alternating placement of substituents).

 

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