ISSUES TO ADDRESS...

How do these features dictate room T tensile response?

Hardening, anisotropy, and annealing in polymers.

How does elevated temperature mechanical response compare to ceramics and metals?

CHAPTERS 15:

POLYMER APPLICATIONS, &

PROCESSING

Stress Strain Behavior The Mechanical characteristics of polymers are

highly sensitive to

Rate of deformation (strain rate)

Temperature

Chemical nature of environment (presence of water,

oxygen, organic solvents etc)

Stress-strain character for

a brittle polymer fractures while deforming elastically.

a plastic material is that initial deformation is elastic,

which is followed by yielding and a region of plastic

deformation.

elastomers is totally elastic, rubber like elasticity (large

recoverable strains produced at low stress levels)

Stress Strain Behavior For plastic polymers

the yield point is taken as the point which occurs just beyond the termination of linear elastic region.

The tensile strength corresponds to the stress at which fracture occurs. Tensile strength may be greater than or less than yield strength.

Stress strain behavior of polymethyl methacrylate at several temperatures between 4C and 60C (fig.15.3) shows that, increase in temperature produces Decrease in elastic modulus

Reduction in tensile strength

Enhancement of ductility

Decreasing the strain rate (rate of deformation) has the same influence on the stress-strain characteristics as increasing the temperature

8

Decreasing T... --increases E

--increases TS

--decreases %EL

Increasing strain rate... --same effects

as decreasing T.

Adapted from Fig. 15.3, Callister 6e. (Fig. 15.3 is from T.S. Carswell and J.K. Nason, 'Effect of Environmental Conditions on the

Mechanical Properties of Organic Plastics", Symposium on Plastics, American Society for Testing and Materials, Philadelphia, PA, 1944.)

T AND STRAIN RATE: THERMOPLASTICS

Macroscopic deformation Tensile stress strain curve for a semi crystalline

material shows upper and lower yield points followed by a near

horizontal region.

At upper yield point a small neck forms within the gauge section of specimen.

Within this neck, the chains become oriented, i.e., chain axes become aligned parallel to the elongation direction, which leads to localized strengthening.

Consequently there is a resistance to continued deformation at this point, and specimen elongation proceeds by the propagation of this neck region along gauge length; the chain orientation phenomenon, accompanies this neck extension.

This tensile behavior is in contrast to that found in ductile metals wherein once neck is formed, all subsequent deformation is confined to neck region

Macroscopic deformation An amorphous polymer may behave like

a glass at low temperatures, a rubbery solid at intermediate temperatures, and a viscous liquid as the temperature is further raised.

For relatively small deformations, the mechanical behavior at low temperatures may be elastic, at higher

temperatures viscous or liquid-like behavior prevails. For intermediate temperatures is found a rubbery solid (viscoelastic)

Elastic deformation is instantaneous ie. strain occurs the instant the stress is

applied or released (fig.15.5b)

For a totally viscous behavior deformation is delayed in response to applied stress. (fig.15.5d)

Viscoelastic behavior results in an instantaneous elastic strain, followed by viscous

time dependent strain. (fig.15.5c)

Viscoelastic relaxation modulus Viscoelastic behavior of polymeric materials is

dependent on both time and temperature.

In stress relaxation measurements specimen is initially strained rapidly in tension to a

predetermined and relatively low strain level.

The stress necessary to maintain this strain is measured as a function of time, while temperature is held constant.

Stress is found to decrease with time due to molecular relaxation processes that take place within the polymer.

Relaxation modulus is defined as the time dependent elastic modulus for viscoelastic

polymers as ratio of measured time dependent stress to the strain level, which is maintained constant

Relaxation modulus decreases with time

Lower values of relaxation modulus occurs with increasing temperature.

)0(

)()(

ttEr

9

Stress relaxation test:

Er (t )

(t )

o

--strain to o and hold. --observe decrease in

stress with time.

Relaxation modulus:

Data: Large drop in Er for T > Tg.

(amorphous

polystyrene)

Sample Tg(C) values:

PE (low Mw)

PE (high Mw)

PVC

PS

PC

-110

- 90

+ 87

+100

+150

Adapted from Fig.

15.7, Callister 6e. (Fig. 15.7 is from

A.V. Tobolsky,

Properties and Structures of Polymers, John Wiley and Sons,

Inc., 1960.)

Selected values

from Table 15.2,

Callister 6e.

TIME DEPENDENT DEFORMATION

Viscoelastic creep Viscoelastic creep

Many polymeric materials are susceptible to time dependent deformation when stress level is maintained constant.

Viscoelastic creep deformation may be significant even at room temperature and under modest stresses that lie below

the yield strength of material.

In creep tests on polymers stress is applied instantaneously and is maintained at constant level

while strain is measured as a function of time. Tests are performed under isothermal conditions.

Time dependent creep modulus may be defined as the ratio of constant applied stress to the time dependent strain.

Creep modulus diminishes with increasing temperature.

Creep modulus increases as the degree of crystallinity increases.

)(

0)(t

c tE

Deformation of Semicrystalline Polymers

(Spherulitic structure) Mechanism of elastic deformation in response to

the tensile stress is the elongation of chain molecules from their stable

configurations, in the direction of applied stress, by bending and stretching of strong chain covalent bonds.

In addition slight displacement of adjacent molecules resisted by weak secondary or van der Waals bonds.

Elastic modulus may be taken as some combination of moduli of crystalline and amorphous phases.

Deformation of Semicrystalline Polymers

(Spherulitic structure) Mechanism of plastic deformation is due to

The tie chains within the amorphous regions become extended.

In the second stage deformation occurs by tilting of the lamellae so that the chain folds become aligned with the tensile axis.

Next, crystalline block segments separate from the lamellae.

In the final stage, the blocks and tie chains become oriented in the direction of tensile axis.

4

Stress-strain curves adapted from Fig. 15.1, Callister 6e. Inset figures along plastic response curve (purple) adapted from Fig. 15.12, Callister 6e. (Fig. 15.12 is from J.M. Schultz, Polymer Materials Science, Prentice-Hall, Inc., 1974, pp. 500-501.)

TENSILE RESPONSE: BRITTLE & PLASTIC

Factors that influence the Mechanical

Properties of Semicrystalline Polymers Increasing the temperature or decreasing the strain

rate leads to decrease in the tensile modulus, reduction in tensile

strength and an enhancement of ductility.

Tensile modulus rises as both the secondary bonding strength and chain alignment increase. Extensive chain entanglements or a significant amount

of intermolecular bonding inhibit relative chain motions.

Significant intermolecular forces result from the formation of large number of van der Waals inter chain bonds.

Molecular weight Tensile strength increases with increasing molecular

weight.

This is due to increased chain entanglement with rising number average molecular weight.

Factors that influence the Mechanical

Properties of Semicrystalline Polymers Degree of crystallinity:

Tensile modulus increases significantly with degree of crystallinity.

Degree of crystallinity affects the extent of intermolecular secondary bonding.

Extensive secondary bonding ordinarily exists between adjacent chain segments for crystalline regions in which molecular chains are closely packed in an ordered and parallel arrangement.

Predeformation by drawing: Strength and tensile modulus are improved by

deforming the polymer in tension (drawing).

During drawing the molecular chains slip past one another and become highly oriented.

For materials drawn in uniaxial tension, tensile modulus and strength are significantly greater in the direction of deformation than in other directions.

5

Drawing... --stretches the polymer prior to use

--aligns chains to the stretching direction

Results of drawing: --increases the elastic modulus (E) in the

stretching dir.

--increases the tensile strength (TS) in the

stretching dir.

--decreases ductility (%EL)

Annealing after drawing... --decreases alignment

--reverses effects of drawing.

Compare to cold working in metals!

Adapted from Fig. 15.12,

Callister 6e. (Fig. 15.12 is from J.M. Schultz, Polymer Materials Science, Prentice-Hall, Inc., 1974, pp. 500-501.)

PREDEFORMATION BY DRAWING

Factors that influence the Mechanical

Properties of Semicrystalline Polymers Heat treating (Annealing) For undrawn materials

increasing annealing temperature leads to Increase in tensile modulus

Increase in yield strength

Reduction in ductility

Annealing effects in semicrystalline polymers are opposite to that observed in metallic materials

Deformation of Elastomers

In an unstressed state An elastomer will be amorphous and composed of

molecular chains that are highly twisted, kinked and coiled.

Elastic deformation upon application of tensile load Is partial uncoiling, untwisting, and straightening and

the resultant elongation of the chains in the stress direction.

Driving force for elastic deformation is entropy which is a measure of disorder within a system. Entropy increases with increasing disorder. As an elastomer is stretched and the chains become

more aligned, the system becomes ordered.

If the chains return to the original kinked and coiled contours, entropy increases.

6

Compare to responses of other polymers: --brittle response (aligned, cross linked & networked case)

--plastic response (semi-crystalline case)

Stress-strain curves

adapted from Fig.

15.1, Callister 6e. Inset figures along

elastomer curve

(green) adapted from

Fig. 15.14, Callister 6e. (Fig. 15.14 is from Z.D. Jastrzebski, The Nature and Properties of Engineering Materials, 3rd ed., John Wiley and Sons,

1987.)

TENSILE RESPONSE: ELASTOMER CASE

Deformation of Elastomers

Criteria that must be met for the polymer to be elastomeric are: It must not easily crystallize (elastomeric chains are

amorphous)

Chain bond rotations must be relatively free for the coiled chains to respond to an applied force

For relatively large elastic deformations, onset of plastic deformation should be delayed. Motion of chains past one another should be restricted by cross

linking. Cross links act as anchor points between chains and prevent chain slippage from occurring.

The elastomer must be above its glass transition temperature. Below its glass transition temperature, an elastomer becomes

brittle.

Vulcanization: Crosslinking process in Elastomers

Vulcanization is achieved by a nonreversible chemical reaction ordinarily carried out at

an elevated temperature.

In most vulcanizing reactions, sulfur compounds are added to the heated elastomer chains of sulfur atoms bond with adjacent polymer

backbone chains and crosslink them.

Unvulcanized rubber is soft and tacky and has poor resistance to abrasion.

Modulus of elasticity, tensile strength are enhanced by vulcanization. The magnitude of modulus of elasticity is proportional

to the density of cross links.

To produce rubber of large extensions without rupture of primary chain bonds there must be relatively few cross links and these must

be widely seperated.

Crystallization

Crystallization is a process upon cooling, an ordered solid phase is

produced from a liquid melt having highly random molecular structure.

Chain folded layers Upon cooling through the melting temperature nuclei

form wherein small regions of the tangled and random molecules become ordered and aligned in the manner of chain folded layers

Increase in chain folded layers or spherulite radius Subsequent to nucleation and during crystallization

growth stage, nuclei grow by continued ordering and alignment of additional chain segments i.e., chain folded layers increase in lateral dimensions, or for spherulitic structures there is an increase in spherulite radius.

Melting

Transformation of a polymer crystal having an ordered structure of aligned molecular chains, to a viscous liquid in which the structure is highly random.

Melting of polymers take place over a range of temperatures. Because every polymer is composed of molecules

having a variety of molecular weights and Tm depends on molecular weight.

Melting of specimen depends on the temperature at which it crystallized.

Thicker the chain folded lamellae, higher the melting temperature

Increase in the rate of heating, results in elevation of melting temperature

Melting and Glass Transition

Glass transition Temperature at which polymer experiences the

transition from rubbery to rigid states is termed as glass transition temperature Tg.

Melting and Glass transition temperatures In a crystalline material there is a discontinuous change

in specific volume at melting temperature

Totally amorphous material experiences a slight decrease in slope at glass transition temperature

In a semicrystalline polymer both melting and glass transition are observed

Factors that influence the Melting (Tm) Temperatures

Chain stiffness, controlled by the ease of rotation about the chemical bonds along chain has a pronounced effect. Presence of double chain bonds and aromatic groups

lower chain flexibility and cause increase in melting temperature

Size and type of side groups influence the chain rotational freedom and flexibility Bulky or large side groups tend to restrict molecular

rotation and raise melting temperature Polypropylene has a higher melting temperature than

polyethylene, the CH3 methyl side group for polypropylene is larger than H atom found in polyethylene.

the presence of polar side groups (Cl, OH, CN) leads to significant intermolecular bonding forces and relatively high Tm. Tm for polyvinyl chloride is higher than polypropylene.

Factors that influence the Melting (Tm)

Temperatures

Increasing average molecular weight (or chain length) raises Tm.

Introduction of side branches introduces defects into the crystalline material and lowers the melting temperature. High density polyethylene, a linear polymer has a

higher melting temperature than low density polyethylene which has branching.

Factors that influence the and Glass Transition

(Tg) Temperatures

Chain flexibility is diminished and Tg is increased by: Presence of bulky side groups

Polar side atoms or groups of atoms

Double chain bonds and aromatic chain groups, which tends to stiffen the molecular backbone.

Increasing the molecular weight raises Tg.

High density of branches raise Tg due to reduced chain mobility

Crosslinked amorphous polymers elevate Tg Crosslinks restrict molecular motion.

With high density of crosslinks molecular motion is disallowed to the degree that glass transition is not experienced by crosslinked amorphous polymers.

Tg lies between 0.5 to 0.8 Tm.

7

Thermoplastics: --little cross linking

--ductile

--soften w/heating

--polyethylene (#2)

polypropylene (#5)

polycarbonate

polystyrene (#6)

Thermosets: --large cross linking

(10 to 50% of mers)

--hard and brittle

--do NOT soften w/heating

--vulcanized rubber, epoxies,

polyester resin, phenolic resin

Callister, Fig. 16.9

T

Molecular weight

Tg

Tmmobile liquid

viscous liquid

rubber

tough plastic

partially crystalline solid

crystalline solid

Adapted from Fig. 15.18, Callister 6e. (Fig. 15.18 is from F.W. Billmeyer, Jr., Textbook of Polymer Science, 3rd ed., John Wiley and Sons, Inc., 1984.)

THERMOPLASTICS VS THERMOSETS

10

General drawbacks to polymers: -- E, y, Kc, Tapplication are generally small.

-- Deformation is often T and time dependent.

-- Result: polymers benefit from composite reinforcement.

Thermoplastics (PE, PS, PP, PC): -- Smaller E, y, Tapplication -- Larger Kc -- Easier to form and recycle

Elastomers (rubber): -- Large reversible strains!

Thermosets (epoxies, polyesters): -- Larger E, y, Tapplication

-- Smaller Kc

Table 15.3 Callister 6e:

Good overview

of applications

and trade names

of polymers.

SUMMARY

Reading:

Core Problems:

Self-help Problems:

0

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