BEHAVIOR OF POLYMERS - Shahid Chamran University of …engg.scu.ac.ir/_Engineering/Documents/4_20151226_091046.pdf · an amorphous polymer, ... This will be illustrated by a typical

BEHAVIOR OF POLYMERS

M. Khorasanian

RHEOLOGY OF POLYMERS

Rheology is a relatively young branch of natural science that deals with the relationships between forces(stresses) and deformations of material bodies.

Hence, it is also connected to the flow properties of polymers both in solution and in the melt, as well as the

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polymers both in solution and in the melt, as well as the reaction of materials in the solid state to mechanical stresses.

Most polymeric materials exhibit the combined reactions of both liquid and solid states, called viscoelasticity, a combination of the viscosity of a liquid and the elasticity of a solid.


Let's start with the problem of polymer flow.

This also happens to be an important practical issue, because, during shaping and processing, polymers must undergo fluid flow (the "plastic" statethe "plastic" state).

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must undergo fluid flow (the "plastic" statethe "plastic" state).

Long chains (which are not cross-linked) may slide and flow like any other liquid-when in solution or in a state that allows motion (above Tg, in the case of an amorphous polymer, or above Tm for a crystalline polymer).

TRANSITIONS

The performance of polymers may be best understood by considering the concept of "thermal "thermal transition"transition" that determines the transition between phases and states.

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The primary transition, between a crystalline solid phase and an amorphous liquid (polymers never appear as gases), is defined by the melting point, Tm.

TRANSITIONS

In this case, genuine changes in the primary thermodynamic properties (enthalpy, specific volume) occur, as with small molecules.

The major difference is that in polymers there is no

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The major difference is that in polymers there is no sharp transition point but a range of temperatures-depending on molecular weight distribution and on the degree of crystallinity.

TRANSITIONS

In essence, Tm defines the disappearance of the crystalline phase, being the temperature at which the last crystallites melt.

It increases with an increase of molecular weight,

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It increases with an increase of molecular weight, concentration and dimensions of the crystallites.

It is affected by the rigidity of chemical structure of the chain core, presence of strong intermolecular bonds, and bulky side groups.

TRANSITIONS

Needless to say, amorphous polymers lack the primary transition and, therefore, have no melting point.

They soften upon heating, however, and solidify

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They soften upon heating, however, and solidify glasslike at low temperatures.

The amorphous phase, however, exhibits a secondary transition temperature of extreme significance, the so-called glass transition temperature, Tg.

TRANSITIONS

Tg corresponds to a change in the slope of the thermodynamic function with temperature.

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Compared to simple liquids, polymers are very different and have extremely high viscosity and a special flow characteristic, which is termed "non-Newtonian."

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It is therefore appropriate to state Newton's lawNewton's law, in which the coefficient of viscosity appears.

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τ represents the shear stress (shear force per unit area) that acts on the fluid, and γ. represents the rate of shear (or velocity gradient) that expresses the rate of deformation (or strain rate).

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The physical interpretation of the unit of viscosity (η) is defined as equivalent to the force needed to affect the flow of a fluid that is bounded between two solid parallel plates (one stationary and the other moving with the fluid) while variables other moving with the fluid) while variables Idistance between the plates, velocity and cross-section area) are unity.


Only in the case of simple liquids, a constant viscosity coefficient (η) may occur, dependent only on temperature.

This dependence is mostly described by a typical This dependence is mostly described by a typical Arrhenius equation:


It is obvious that the viscosity of liquids drops exponentially with the temperature elevation.

E expresses an activation energy for flow, and its size dictates the sensitivity to temperature changes.size dictates the sensitivity to temperature changes.


In the case of polymers, it has been found that the viscosity is not a constant, but varies with the flow conditions.

Therefore, these liquids are termed nonnon--NewtonianNewtonian. Therefore, these liquids are termed nonnon--NewtonianNewtonian.


In Figure 4-2 we show the relationship between shear stress and shear rate (the so-called "flow curve").


The Newtonian liquid is represented by a straight line from the origin (1) while there may be several curves (2, 3, 4) representing non-Newtonian liquids.


In the straight line, 1, describing the Newtonian liquid, the viscosity is identical to the slope.

In curve 2, the viscosity gradually diminishes with increasing shear rate. This represents a shear-thinning (pseudoplastic) liquid, typical of most polymeric melts and solutions.

Curve 3 describes a shearthickening (dilatant) case, wherein Curve 3 describes a shearthickening (dilatant) case, wherein the viscosity increases with an increase of shear rate. This may appear in concentrated pastes.

Curve 4 is actually described by a straight line on a linear plot, but it does not start at the origin. This is a "Bingham liquid," wherein a threshold value of shear stress, τ0, appears, below which no flow occurs (this behavior is shown by toothpastes).

MECHANICAL PROPERTIES OF POLYMERS AND PLASTICS

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It is obvious that each material exhibits a different performance, while the crucial effects of time (rate) and temperature should not be disregarded.

This will be illustrated by a typical schematic plot of This will be illustrated by a typical schematic plot of a stress-strain relationship at various temperatures (Figure 4-24).

The polymer of choice is LDPE.


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On raising the temperature the elongation increases, while the tensile strength and modulus diminish.

Gradually the body converts from rigid and brittle Gradually the body converts from rigid and brittle (at low temperature) to soft and ductile.

Again the correspondence between temperature and time prevails, so that the type of response at low temperature is identical to that in a short time (high speed).

Characteristics of Typical Mechanical Properties

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Rigid and Brittle

A high modulus of rigidity, low elongation, breaks prior to yield.

A small area below the curve indicates low toughness. toughness.

Typical polymers that give such a response include polystyrene (unmodified) and most thermosets(unreinforced).

Brittleness occurs due to low elongation to break (mostly below 2% to 5%).


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Rigid and Strong

In this case, modulus and tensile strength are high,

while elongation is medium, exceeding yield.

A typical polymer is rigid PVC. A typical polymer is rigid PVC.


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Rigid and Tough

A high modulus, reasonable elongation and strength, and a high energy to break is the case for typical engineering polymers such as Nylon or typical engineering polymers such as Nylon or polycarbonate.


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Soft and Tough

Low modulus but exhibits high elongation and energy to break.

Typical polymers are polyethylene or flexible Typical polymers are polyethylene or flexible (plasticized) PVC.


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Elastomers

They have no yield point and therefore no necking, but their elongation and toughness are significantly high. high.

The deformation is basically elastic and therefore also recoverable.


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THERMOPLASTIC AND THERMOSETTING POLYMERS

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The response of a polymer to mechanical forces at elevated temperatures is related to its dominant molecular structure.

In fact, one classification scheme for these materials In fact, one classification scheme for these materials is according to behavior with rising temperature.

Thermoplastics (or thermoplastic polymers) and thermosets (or thermosetting polymers) are the two subdivisions.


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Thermoplastic PolymersThermoplastic Polymers

Thermoplastics soften when heated (and eventually liquefy) and harden when cooled - processes that are totally reversible and may be repeated.are totally reversible and may be repeated.

On a molecular level, as the temperature is raised, secondary bonding forces are diminished (by increased molecular motion) so that the relative movement of adjacent chains is facilitated when a stress is applied.


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Irreversible degradation results when a molten thermoplastic polymer is raised to a too high temperature.

In addition, thermoplastics are relatively soft. Most In addition, thermoplastics are relatively soft. Most linear polymers and those having some branched structures with flexible chains are thermoplastic.

These materials are normally fabricated by the simultaneous application of heat and pressure.


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Most linear polymers are thermoplastics. Examples of common thermoplastic polymers include polyethylene, polystyrene, poly(ethylene terephthalate), and poly(vinyl chloride).


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Thermosetting PolymersThermosetting Polymers

Thermosetting polymers are network polymers.

They become permanently hard during their formation, and do not soften upon heating. formation, and do not soften upon heating.

Network polymers have covalent crosslinks between adjacent molecular chains.


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During heat treatments, these bonds anchor the chains together to resist the vibrational and rotational chain motions at high temperatures.

Thus, the materials do not soften when heated. Thus, the materials do not soften when heated.

Crosslinking is usually extensive, in that 10 to 50% of the chain repeat units are crosslinked.


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Only heating to excessive temperatures will cause severance of these crosslink bonds and polymer degradation.

Thermoset polymers are generally harder and stronger than thermoplastics and have better stronger than thermoplastics and have better dimensional stability.

Most of the crosslinked and network polymers, which include vulcanized rubbers, epoxies, and phenolics and some polyester resins, are thermosetting.


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