Thermal Transitions

7/30/2019 Thermal Transitions

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Dr. S. Ray_Chem. Engg._NIT Agtl 2/7/2013

Polymer Processing Engg. 1

A polymer is composed of a mixture of molecules having differentchain lengths (molecular weights) therefore, the transition betweenthe solid and liquid forms of a polymer is rather diffuse and occursover a temperature range whose magnitude (of the order of 2 to10C) depends on the poly-dispersity of the polymer

On melting, polymers become very viscous (viscoelastic) fluids, notfreely flowing (as in the case of low-molecular-weight materials).

On heating the molecular motion in a polymer sample is promotedby thermal energy and is opposed by the cohesive forces between

structural segments (groups of atoms) along the chain and betweenneighboring chains.

The cohesive forces and, consequently, thermal transitions inpolymers depend on the structure of the polymer.

Thermal transitions in polymers

Small molecules

Thermal transitions


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Polymers


In an amorphous polymer the solid-to-liquid transition occurs very gradually, goingthrough an intermediate rubbery state without a phase transformation and occursover a narrow temperature range referred to as the glass transition temperature. Inthe case of a partially crystalline polymer, the above transformation occurs only inthe amorphous regions, the crystalline zones remain unchanged and act asreinforcing elements.If heating is continued, a temperature is reached at which the crystalline

zones begin to melt. The equilibrium crystalline melting point, Tm, for polymerscorresponds to the temperature at which the last crystallite starts melting.

The value of Tm depends on the degree of crystallinity and size distribution ofcrystallites.

The thermal behavior of polymers is of considerable technological importance, asknowledge of thermal transitions is important in the selection of proper processingand fabrication conditions, the characterization of the physical and mechanicalproperties of a material, and hence the determination of appropriate end uses.



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(A)Liquid region;

(B)Viscous liquid with

some elastic response;

(C)Rubbery region;

(D)Glassy region;

(E) Crystallites in a

rubbery matrix;

(F) Crystallites in a

glassy matrix.

Specific volumetemperature curves

for a semi-crystalline polymer

As the amorphous polymer (line ABCD) is heated from the low-temperature region (region D), the volume expands at a constant rate.At a characteristic temperature, Tg, the rate of volume expansionincreases suddenly to a higher constant level, i.e., there is a change inthe slope of the volumetemperature curve from a lower to a highervolume coefficient of expansion. At the same time, there is an abruptchange in physical behavior from a hard, brittle, glassy solid below Tg(region D) to a soft, rubbery material above Tg (region C). On further

heating, the polymer changes gradually from the rubbery state to aviscous liquid (region B) whose viscosity decreases with increasingtemperature until decomposition sets in.

For a crystalline polymer, the changes at Tg are less drastic. This isbecause these changes are restricted mainly to the amorphousdomains while the crystalline zones remain relatively unaffected.Between the Tg and Tm (region E) the semi-crystalline polymer iscomposed of rigid crystallites immersed (dispersed) in a rubberyamorphous matrix. In terms of mechanical behavior, the polymerremains rigid, pliable, and tough. At the melting temperature, thecrystallites melt, leading to a viscous state (region B). Above Tm thecrystalline polymer, like the amorphous polymer, exists as a viscousliquid.

Thermal transitions of a semi-crystalline polymer


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Thermal transitions in amorphous polymer

Thermal transitions in semi-crystalline polymer


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The stability of the molecular system depends on the vibrationenergy of the chemical bonds.

In polymers, intramolecular bonds are due to primary valencebonds (covalent) while the intermolecular attractions usually aredue to secondary bonding forces. The intermolecular forces areopposed by thermal agitation, which induces vibration, rotation,and translation of a molecular system.

The thermal degradation occurs when the energy of vibration

exceeds the primary bonding between atoms, while thetransitional phenomena associated with crystalline melting point,the glass transition temperature, and the polymer deformationsare related to rotation and vibration of molecular chains.


At very low temperatures i.e., in the glassy state chain segments arefrozen in fixed positions; atoms undergo only low-amplitude vibratorymotion about these positions. As the temperature is increased, theamplitude of these vibrations becomes greater, thereby reducing theeffectiveness of the secondary intermolecular bonding forces.Consequently, the cooperative nature of the vibrations between

neighboring atoms is enhanced.

Below the Tg, in the glassy state, only atoms or small groups of atomssuch as short sections of the main chain or pendant/side groups moveagainst the local restraints of intermolecular interactions and result in othertransitions, designated as , , , etc., in order of decreasing temperature.

The fully extended chain, which is the conformation of minimum energy, isthe preferred conformation at low temperatures. Therefore, as themolecules straighten out, the free volume decreases. Consequently, flowbecomes difficult and the polymer assumes the characteristic hard andbrittle behavior of glasses.



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At the glass transition temperature, chain ends and a substantial number ofchain segments have acquired sufficient energy to overcome intermolecularrestraints and undergo rotational and translational motion. Therefore, theglass transition temperature is referred to as the onset of large-scalecooperative motion of chain segments (of the order of 20 to 50 consecutivecarbon atoms). Rotational and translational modes of motion provideimportant mechanisms for energy absorption. This accounts for glassy-to-rubbery transition and the tough nature of an amorphous polymer above itsglass transition temperature.

The molecular motion of the Tg is restricted only to segmental motion;entire molecular motion is barred by chain entanglements. However, abovethe Tg, in the rubbery state, there is a sharp increase in the number ofpossible conformations. The molecular motion in the rubbery state requires

more free volume, and this rise in the relative free volume leads to theobserved higher volume expansion coefficient above the Tg.

As heating is continued into the liquid region, molecules acquire increasedthermal energy, and the amplitudes of associated molecular motions alsoincrease. Translation, or slip of entire molecules, becomes possible; largechanges in conformation occur and elasticity virtually disappears.



Factors affecting Tg:

At the Tg there is a large-scale cooperative movement of chain segments,therefore it is expected that any structural features or externally imposedconditions that influence chain mobility will also affect the value of Tg

Internal variable:(a) Structural factors: chain flexibility;

stiffness,steric hindrance,polarity,inter-chain attractive forces;

(b) geometric factors;(c) copolymerization;(d) molecular weight,(e) branching; cross-linking;(f) crystallinity.

External variables: plasticization,pressure,

rate of testing.


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Factors affecting thermal transitions

1. Chain Flexibility

Chain flexibility is determined by the ease with which rotation occurs aboutprimary valence bonds. Polymers with low hindrance to internal rotation havelow Tg values.

Long-chain aliphatic groups ether and ester linkages enhance chain

flexibility, while rigid groups like cyclic structures stiffen the backbone.

Bulky side groups that are stiff and close to the backbone cause steric hindrance,decrease chain mobility, and hence raise Tg

The influence of the side group in enhancing chain stiffness depends on theflexibility of the group and not its size. In fact, side groups that are fairlyflexible have little effect within each series; instead polymer chains are forcedfurther apart. This increases the free volume, and consequently Tg drops.



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2. Geometric FactorsThe symmetry of the backbone and the presence of double bonds on the mainchain, affect Tg.

Polymers that have symmetrical structure have lower Tg than those withasymmetric structures.

Additional groups near the backbone for the symmetrical polymer areaccommodated in a conformation with a loose structure which result inincreased free volume and lower Tg

Double bonds in the cisform reduce the energy barrier for rotation of adjacentbonds, soften the chain, and hence reduce Tg

.


3. Inter-chain Attractive Forces

The intermolecular bonding in polymers is due to secondary attractiveforces. Consequently, it is to be expected that the presence of strongintermolecular bonds in a polymer chain, results in a high value ofcohesive energy density and significantly increase Tg


The secondary bonding forcesare effective only over shortmolecular distances. Therefore,any structural feature thattends to increase the distancebetween polymer chainsdecreases the cohesive energydensity and hence reduces Tg


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4. Molecular weight In a polymer, chain end segments restricted only at one end have relatively

higher mobility than the internal segments, which are constrained at bothends.

Thus, at a given temperature, chain ends provide a higher free volume formolecular motion.

Accordingly, as the number of chain ends increases the available freevolume increases, and consequently there is a depression of Tg.

The higher number of chain ends means a decrease in molecular weight(Mn)

The effect is more pronounced at low molecular weight, but as Mnincreases, Tg approaches an asymptotic value.

Empirical relation:


5. Cross-Linking and BranchingThe cross-linking involves the formation intermolecular connections throughchemical bonds. This process necessarily results in reduction in chainmobility, consequently, Tg increases.

For lightly cross-linked systems like vulcanized rubber, Tg shows a moderateincrease over the uncross-linked polymer, and Tg have a linear dependenceon the degree of cross-linking,


where Tg = the glass transition temperature of the uncross-linked polymerhaving the same chemical composition as the cross-linked polymer

Mc = the number-average molecular weight between cross-linked points

For highly cross-linked systems like resins, the glass transition is virtually infinite.This is because the molecular chain length between cross-links becomes smallerthan that required for cooperative segmental motion.

For long and flexible side chains, branching increases the separation betweenchains, enhances the free volume, and therefore decreases Tg.


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6. CrystallinityIn semi-crystalline polymers, the crystallites may be regarded as physical cross-links that tend to reinforce or stiffen the structure.

Accordingly, Tg increase with increasing degree of crystallinity, since thecohesive energy factors operative in the amorphous and crystalline regions arethe same and exercise similar influence on transitions.

Empirical relationship:

where Tg and Tm are in degrees Kelvin.


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Thermal Transitions