L4 Polymer Structure

8/23/2019 L4 Polymer Structure

1/10

r. S.Ray_Chem Engg_NIT Agratala 1/23/2013

olymer Processing Engineering 1

A polymeric solid material is an aggregate of a large number ofpolymer molecules. Depending on the molecular structure, theprocess of molecular aggregation occurs essentially by either oftwo possible arrangements of molecules, leading to either acrystalline or amorphous material.

Irrespective of the type of molecular arrangement, the forcesresponsible for molecular aggregation are the intermolecularsecondary bonding forces.

Bonding energies due to secondary bonding forces range from 0.5to 10 whereas primary bonding forces are of the order 50 to 100

kcal/mol.

When molecules are large enough, the attractive forces resultingfrom the secondary inter-molecular bonding forces may build up tosuch a level that, in some cases, they become greater than theprimary valence forces responsible for intra-molecular bonds

Molecular structure and polymer properties

The secondary bonds consist of dipole, induction, van derWaals, and hydrogen bonds.

Dipole forces result from the attraction between permanentdipoles associated with polar groups. Induction forces arise

from the attraction between permanent and induced dipoles,and Van der Waals (dispersion) forces originate from the time-varying perturbations of the electronic clouds of neighboringatoms.

In general, the magnitude of the bond energies decreases fromhydrogen bond to dipole bond to Van der Waals (dispersion)forces.



2/10



A quantitative measure of the magnitude of secondary bondingforces is the cohesive energy density (CED), which is the totalenergy per unit volume needed to separate all intermolecularcontacts

For liquids of low molecular weight the energy necessary toseparate molecules from one another is evaluated from the heatof evaporation or from the dependence of vapor pressure ontemperature.

For polymers (which cannot be evaporated), the cohesiveenergy density is estimated indirectly by dissolution in liquids ofknown cohesive energy density.

Cohesive Energy Density

where = solubility parameter.

In the absence of strong interactions such as hydrogen bonding,a polymer 2 will dissolve in a solvent 1 if 1 2 1.7 2.0.

Ecoh is also dependent on the molar volume. For polymers it is the volumeoccupied by each repeat unit in the solid state. Thus Ecoh represents thecohesive energy per repeat unit volume, VR.

Ecoh allows predictions about what solvents will dissolve a given polymer.



3/10




When a polymer is cooled from the melt or concentrated from adilute solution, molecules are attracted to each other forminga solid mass. In doing so, two arrangements are essentiallypossible:

1. Molecules vitrify, with the polymer chains randomly coiledand entangled. The resulting solid is amorphous and is hardand glassy.

2. Individual chains are folded and packed in a regular mannercharacterized by three-dimensional long-range order. Thepolymer thus formed is said to be crystalline.



4/10



Polymers are made up of long molecules; therefore, theconcept of crystallinity in polymers must be viewed slightlydifferently from that in low-molecular-weight substances, socomplete parallel alignment is never achieved in polymericsystems, only certain clusters of chain segments are aligned toform crystalline domains. These domains, do not have theregular shapes of normal crystals.

The crystalline domains in polymers are much smaller in size,contain many more imperfections, and are connected with the

disordered amorphous regions by polymer chains that runthrough both the ordered and the disordered segments.

Consequently, no polymer is 100% crystalline.

Crystallinity in polymers

Intermolecular bonding in polymers, are effective only at veryshort molecular distances. Therefore, to maximize the effect ofthese forces in the process of aggregation of molecules to form acrystalline solid mass, the molecules must come as closetogether as possible. The tendency for a polymer to crystallize,therefore, depends on the magnitude of the inherentintermolecular bonding forces as well as its structural features.



5/10



In the process of association of polymer molecules to form a solid

mass, molecules must come as close together as possible. It follows that

any structural features of polymer molecules that can impede this

process will necessarily detract from crystallinity.

High structural regularity that permits close packing of the chains

coupled with the limited chain flexibility leads to high melting point,

relatively high rigidity, and low room-temperature solubility.

The value of these properties shows a significant reduction as

irregularities are introduced into the structure,

Regularity of polymer chains is not sufficient to ensure crystallizability

in polymers. The spatial regularity and packing are equally important.


Alignment of polymer molecules is a vital prerequisite forthe effective utilization of the intermolecular bondingforces, however, during crystallization, this alignmentand uniform packing of chains are opposed by thermalagitation, which tends to induce segmental, rotationaland vibrational motions. The potential energy barriershindering this rotation range from 1 to 5 kcal/mol, the

same order of magnitude as molecular cohesion forces.

It is to be expected, therefore, that those polymers whosechains are flexible will be more susceptible to thisthermal agitation than those with rigid or stiff chainstructure.

The flexibility of chain molecules arises from rotationaround saturated chain bonds.

Crystallinity in polymers Chain flexibility


6/10



Rapid conformational change due to ease of rotationaround single bonds occurs if such groups as below areintroduced into the main chain.

( COO) (OCOO) (CN)

If they are regular and/or if there exist considerableintermolecular forces, the materials are crystallizable,relatively high melting, rigid, and soluble with difficulty.

If they occur irregularly along the polymer chain, they areamorphous, soft, and rubbery materials.


Ether and imine bonds and double bonds in the cisformreduce the energy barrier for rotation of the adjacent bondsand soften the chain by making polymers less rigid, morerubbery, and more readily soluble than the corresponding

chain of consecutive carboncarbon atoms.

If such plasticizing bonds are irregularly distributed alongthe polymer chain length, crystallization is inhibited.

Cyclic structures in the backbone and polar group such as SO2, and CONH drastically reduce flexibility and enhancecrystallizability.



7/10




When molecules come together and aggregate into a

crystalline solid, a significant cohesion betweenneighboring chains is possible.

Consequently, polymer molecules with specific groupsthat are capable of forming strong intermolecular bonding,particularly if these groups occur regularly without

imposing valence strains on the chains, are crystallizable.

Crystallinity in polymers Polarity


8/10



In polyamide polymers (nylon), the regular occurrence ofamide linkages leads to a highly crystalline, high melting

polymer. These functional groups provide sites for hydrogenbonding whose energy ranges from 5 to 10 kal/mol.

Molecules whose backbone contains O units or with polarside groups(CN, Cl, F, or NO2) exhibit polar bonding. The bondingenergies of such dipoles or polarizable units are in the range

between hydrogen bonding and Van der Waals bonding.

If these groups occur regularly along the chain (isotactic andsyndiotactic), the resulting polymers are usually crystallineand have higher melting points




9/10



The vibrational and rotational mobility of intrinsically .exiblechains can be inhibited by bulky substituents; the degree ofstiffening depends on the size, shape, and mutual interaction ofthe substituents.

Small substituents (such as polypropylene [CH3] andpolystyrene [C6H5]) can crystallize if these pendant groups arespaced regularly on the polymer chain as in their isotactic andsyndiotactic forms. In their atactic forms, the randomly disposedpendant groups prevent the close packing of the chains intocrystalline lattice. The atactic forms of these polymers are

therefore amorphous.

Large or bulky substituents, on the other hand, increase theaverage distance between chains and, as such, prevent theeffective and favorable utilization of the intermolecular bondingforces.

Crystallinity in polymers Substituents

The presence of an aromatic side group in (polystyrene)considerably stiffens the chain, which has a stable helixform in the solid state.

The helices pack efficiently to allow greater interchaininteraction.

The stereo regularity, chain flexibility, polarity, and othersteric factors have profound influence on crystallizabilityand melting points and play an important role in the thermaland mechanical behavior of polymers.



10/10



Polymers are partially crystalline.The interpretation of these patterns was that polymers are

semicrystalline, consisting of small, relatively orderedregions the crystallites embedded in an otherwiseamorphous matrix.

This interpretation led to the fringed micelle model ofcrystalline polymers. The fringed micelle concept, whichenjoyed popularity for many years, held that, sincepolymer chains are very long, they passed successivelythrough the crystallites and amorphous regions. Thechains were thought to run parallel to the longer directionof the crystallites.

Although the fringed micelle model of polymer morphologyseemed to explain many of the properties ofsemicrystalline polymers,

Crystal morphology in polymers

Documents

L4 Polymer Structure