Download - 2013-Mechanical Properties Polymer(1)
-
8/11/2019 2013-Mechanical Properties Polymer(1)
1/24
12/21/2013
Mechanical Properties
Outline:
Mechanical properties
Stress-Strain Behavior
Crystallization , Melting, Glass Transition
Thermoplastic and Thermosetting Polymers
Viscoelasticity
2
e orma on an as omers
-
8/11/2019 2013-Mechanical Properties Polymer(1)
2/24
-
8/11/2019 2013-Mechanical Properties Polymer(1)
3/24
-
8/11/2019 2013-Mechanical Properties Polymer(1)
4/24
12/21/2013
Mechanical Properties
Mechanical Properties
-
8/11/2019 2013-Mechanical Properties Polymer(1)
5/24
12/21/2013
Mechanical Properties
Mechanical Properties
-
8/11/2019 2013-Mechanical Properties Polymer(1)
6/24
12/21/2013
.
A polyethylene sample necking undertension.
Mechanical Properties
-
8/11/2019 2013-Mechanical Properties Polymer(1)
7/24
12/21/2013
Mechanical Properties
Toughness
-
8/11/2019 2013-Mechanical Properties Polymer(1)
8/24
12/21/2013
Mechanical Properties
Stress Strain Behavior (III)
Moduli of elasticity Polymers: ~ 10 MPa - 4 GPa Metals: ~ 50 - 400 GPa
Tensile strengths Polymers: ~ 10 - 100 MPa Metals: 100s - 1000s MPa
16
onga on Polymers: up to 1000 % in some cases Metals: < 100%
-
8/11/2019 2013-Mechanical Properties Polymer(1)
9/24
-
8/11/2019 2013-Mechanical Properties Polymer(1)
10/24
12/21/2013
Tensile Response: Brittle & Plastic
brittle failure
lastic failure
(MPa)x
fibrillar
structure
nearfailureonset of
necking
Near Failure
xInitial
aligned, networked
unload/reload
regionsslide
crystallineregions align
semi-crystalline
case
cross-linkedcase
case
amorphousregions
elongateStress-strain curves adapted from Fig. 15.1, Callister 7e. Inset figures along plastic response curve adapted fromFigs. 15.12 & 15.13, Callister 7e . (Figs. 15.12 & 15.13 are from J.M. Schultz, Polymer Materials Science , Prentice-Hall, Inc., 1974, pp. 500-501.)
Stress-strain curvesadapted from Fig. 15.1,Callister 7e. Insetfigures along elastomer
Tensile Response: Elastomer Case
(MPa)brittle failurex
curve (green) adaptedfrom Fig. 15.15, Callister7e . (Fig. 15.15 is fromZ.D. Jastrzebski, TheNature and Properties ofEngineering Materials ,3rd ed., John Wiley andSons, 1987.)
x
final: chainsare straight,
stillcross-linked
elastomer
plastic failurex
Compare to responses of other polymers:-- brittle response (aligned, crosslinked & networked polymer)-- plastic response (semi-crystalline polymers)
initial: amorphous chains arekinked, cross-linked.
Deformationis reversible!
-
8/11/2019 2013-Mechanical Properties Polymer(1)
11/24
-
8/11/2019 2013-Mechanical Properties Polymer(1)
12/24
12/21/2013
Stress-Strain Test
23
machine
Tensile Test
24
-
8/11/2019 2013-Mechanical Properties Polymer(1)
13/24
12/21/2013
Important Mechanical Propertiesfrom a Tensile Test
Young's Modulus: This is the slope of the linearportion of the stress-strain curve, it is usuallyspec c to eac mater a ; a constant, nown va ue.
Yield Strength: This is the value of stress at theyield point, calculated by plotting young's modulusat a specified percent of offset (usually offset =0.2%).
Ultimate Tensile Strength: This is the highest
25
value of stress on the stress-strain curve.
Percent Elongation: This is the change in gaugelength divided by the original gauge length.
Melting vs. Glass Transition Temp.What factors affect Tm
and Tg? Both T and T increase with
increasing chain stiffness
Chain stiffness increased by1. Bulky sidegroups2. Polar groups or
sidegroups3. Double bonds or aromatic
c a n groups
Regularity (tacticity) affectsTm only
Adapted from Fig. 15.18,Callister 7e .
-
8/11/2019 2013-Mechanical Properties Polymer(1)
14/24
12/21/2013
The Glass Transition
The Glass Transition
The glass transition is a property of onl y the amorphous portion of asemi-crystalline sol id 34 . The crystalline portion remains crystalline duringthe glass transition.
a ow empera ure e amorp ous reg ons o a po ymer are n e g assystate . In this state the molecules are frozen on place. They may be able tovibrate slightly, but do not have any segmental motion in which portions ofthe molecule wiggle around. In the glassy state, the motion of the redmolecule in the schematic diagram at the right would NOT occur. When theamorphous regions of a polymer are in the glassy state, it generally will behard, rigid, and brittle.
-
8/11/2019 2013-Mechanical Properties Polymer(1)
15/24
12/21/2013
The Glass Transition
If the polymer is heated it eventually will reach its glass transitiontemperature . At this temperature portions of the molecules can start towiggle around as is illustrated by the red molecule in the diagram above.
.and flexibility to a polymer.You may have experienced the glass transition of chewing gum. At bodytemperature the gum is soft and pliable, which is characteristic of anamorphous solid in the rubbery state. If you put a cold drink in your mouthor hold an ice cube on the gum, it becomes hard and rigid. The glasstransition temperature of the gum is somewhere between 0 oC and 37 oC.
The Glass Transition
-
8/11/2019 2013-Mechanical Properties Polymer(1)
16/24
12/21/2013
The Glass Transition
The Glass Transition
-
8/11/2019 2013-Mechanical Properties Polymer(1)
17/24
12/21/2013
Factors Influencing T g
Factors Influencing T g
-
8/11/2019 2013-Mechanical Properties Polymer(1)
18/24
12/21/2013
Factors Influencing T g
Stronger intermolecular forces lead to a higher T g. PVC has
Intermolecular Forces
of the dipole-dipole forces from the C-Cl bond.
Factors Influencing T gPendant Groups
1. Bulky pendant groups , such as a benzene ring, cancatch on neighboring chains like a "fish hook" and restrict
. g.
-
8/11/2019 2013-Mechanical Properties Polymer(1)
19/24
12/21/2013
Factors Influencing T g2. Flexible pendant groups , such as aliphaticchains, tend to limit how close chains can pack.
This increases rotational motion and lowers T .
Factors Influencing T g
The presence of cross-links between chains restricts rotational motion andraises Tg.
Cross-Linking
Plasticizers
Plasticizers are low molecular weight compounds added to plastics to increasetheir flexibility and workability. They weaken the intermolecular forces betweenthe polymer chains and decrease T g. Plasticizers often are added to semi-crystalline polymers to lower the value of T g below room temperature. In thiscase the amorphous phase of the polymer will be rubbery at normaltemperatures, reducing the brittleness of the material.Plasticizers are added to the lastic used for automobile u holster . In older automobiles, the plasticizer may be distilled from the upholstery during hotweather so that it becomes brittle over time.Some plasticizers have been identified as major health and environmentalproblems. Before 1977, Polychlorinated biphenyls (PCBs) were used asplasticizers in paints and plastics. Because they are toxic and possibleendocrine disruptors, PCBs no longer are used.
-
8/11/2019 2013-Mechanical Properties Polymer(1)
20/24
12/21/2013
2
Crystallization, Melting, Glass Transition
39
Dependence of melting and glass transitiontemperatures and polymer properties on molecular weight
Thermoplastics:-- little crosslinking-- ductile
Thermoplastics vs. Thermosets
Callister,Fig. 16.9
T
Tmmobileviscous
liquidrubber
-- soften w/heating-- polyethylene
polypropylenepolycarbonatepolystyrene
Thermosets:i i i ill
Molecular weight
Tgliquid oug
plastic
partiallycrystallinesolidcrystallinesolid
-- large crosslinking(10 to 50% of mers)
-- hard and brittle-- do NOT soften w/heating-- vulcanized rubber, epoxies,
polyester resin, phenolic resin
i . . , . i . . i . . ill ,Jr., Textbook of Polymer Science , 3rd ed., John Wiley and Sons, Inc.,1984.)
-
8/11/2019 2013-Mechanical Properties Polymer(1)
21/24
12/21/2013
2
Polymer Additives
Improve mechanical properties, processability,durability, etc.
Added to improve tensile strength & abrasion
resistance, toughness & decrease cost ex: carbon black, silica gel, wood flour, glass,
limestone, talc, etc.
Plasticizers Added to reduce the glass transition
temperature Tg commonly added to PVC - otherwise it is brittle
Polymer Additives Stabilizers
Antioxidants UV protectants
Lubricants Added to allow easier processing slides through dies easier ex: Na stearate
Colorants
Dyes or pigments
Flame Retardants Cl/F & B
-
8/11/2019 2013-Mechanical Properties Polymer(1)
22/24
12/21/2013
2
Processing of Plastics
Thermoplastic can be reversibly cooled & reheated, i.e. recycled heat till soft, shape as desired, then cool ex: polyethylene, polypropylene, polystyrene, etc .
Thermoset when heated forms a network degrades (not melts) when heated mo e prepo ymer en a ow ur er reac on ex: urethane, epoxy
Processing Plastics - Molding Compression and transfer mold ing
thermoplastic or thermoset
Adapted from Fig. 15.23,Callister 7e . (Fig. 15.23 is fromF.W. Billmeyer, Jr., Textbook ofPolymer Science, 3rd ed.,John Wiley & Sons, 1984. )
-
8/11/2019 2013-Mechanical Properties Polymer(1)
23/24
12/21/2013
2
Processing Plastics - Molding
Injection molding
Adapted from Fig. 15.24,Callister 7e . (Fig. 15.24 is fromF.W. Billmeyer, Jr., Textbook ofPolymer Science, 2nd edition,John Wiley & Sons, 1971. )
-
8/11/2019 2013-Mechanical Properties Polymer(1)
24/24
12/21/2013
Processing Plastics Extrusion
Adapted from Fig. 15.25,Callister 7e . (Fig. 15.25 is fromEncyclopdia Britannica, 1997.)
Blown-Film Extrusion
Adapted from Fig. 15.26, Callister 7e .(Fig. 15.26 is from EncyclopdiaBritannica, 1997.)