crystal materials

8/10/2019 Crystal Materials

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Chapter 6

Noncrystalline and Semicrystalline Materials


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Introduction

The emphasis thus far has been on crystalline materials.

There are numerous engineering materials that lack the long rangetranslational periodicity of a crystalline material.

These non-crystalline materials are referred to as either- amorphous,glassy, or super-cooled liquids.

Theoretically, any material can form an amorphous structure if thecooling rate from the melt is sufficiently rapidto suppress crystalformation.

This chapter will emphasize the structural considerations thatfacilitate the development of an amorphous structure.


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N-th Order Phase Transition

The order of a phase transition can be understood by investigating the

behavior of derivatives of the Gibbs free energy (G).

An n-th order transition is the one in which discontinuities appear only in the

n-th and higher derivatives of G with respect to T and p.

The ordinary first-order transition has discontinuities in

Tp

GV

pT

GS

The ordinary second-order transition has discontinuities in

pTp p

GTVT

VV

11

TT

TpG

VpV

V

2

211

VpV

VT

G

TT

T

STC

pp

pT

GT

T

STC

2

2


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Glass Transition Temperature

3. Glassy State

A state of material in the absence of long-range order below the glass

transition temperaturelarge scale mobility is frozenatomic movement

requires time.

(Liquid-like structure and molecular mobility is frozen)

Window glass vs. Rubber bandWhat if you hammer them?

Glass transition temperature: the temperature below which the physical

properties of amorphous materials vary in a manner similar to those of a

solid phase (glassy state), and above which amorphous materials behave

like liquids (rubbery state).

2. Rubbery State

A state of material in the absence of long-range order above the glass

transition temperatureatomic movement takes shorter time.

(Liquid-like structure and liquid-like molecular mobility)

1. Liquid State


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Specific Volume for a Variety of Materials

Liquid to glass solid transformationin a

pure substance. The glass transition

temperature, Tg, is not an equilibrium

transformation temperature.

Liquid to crystalline solid transformation

for a pure substance. Tmis an

equilibrium transformation temperature

Glass Transition

The slope normalized by the volume V is the volumetric thermal expansion

coefficient (v)dTdV

Vv 1

Decrease of specific volume of liquid with decreasing temperature


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Glass Transition

Below Tm, material tends to crystallize.

The crystal formation (crystallization)

occurs over a period of time becausethe establishment of long-range order

(LRO) requires atomic rearrangement

by diffusion.

It is possible to avoid crystallization

by cooling at a sufficiently high rate

so as to suppress the diffusion

necessary to establish LRO in thecrystal.

The volume of the collection of

atoms continues to decrease with

the slope characteristic of the liquid

below the melting temperature,

forming a super-cooled liquid.


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The Effect of Cooling Rate

on the Glass Transition Temperature, Tg

SpecificVo

lume

Tg1Tg2 Tm

Temperature

T1

.

T2

.

Liquid

Solid

GlassSuper-cooled liquid

T1

.T2

.>

Glass Transition

Kinetic aspect of Tg

T1

.

T2

.

Super fast fast


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Liquidto Semi-crystallineSolidTransformation

Tm


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Structure and Properties of Amorphous and

Semicrystalline Polymers

Ethylene

Basic building

block

Poly (ethylene)

monomer

Poly (ethylene)

chain

Poly (ethylene) chains pack well

because the side groups are only

hydrogen

Formation (polymerization) of Poly (ethylene)from a Basic

Chemical Unit of C2H4

C=C

HH

H H

C=C

HH

H H

C C

H H

H H

.. C C

H H

H H

C C

H H

H H

.. C C

H H

H H

C C

H H

H H

C C

H H

H H

.. C C

H H

H H

C C

H H

H H

C C

H H

H H

C C

H H

H H

C C

H H

H H

C C

H H

H H

C C

H H

H H

C C

H H

H H

C C

H H

H H

..

C C

H H

H H

C C

H H

H H

C C

H H

H H

C C

H H

H H

C C

H H

H H

C C

H H

H H

C C

H H

H H

C C

H H

H H

C C

H H

H H

C C

H H

H H

C C

H H

H H

C C

H H

H H

C C

H H

H H

C C

H H

H H

C C

H H

H H

C C

H H

H H

C C

H H

H H

C C

H H

H H

C C

H H

H H

C C

H H

H H

C C

H H

H H

C C

H H

H H

C C

H H

H H

C C

H H

H H

C C

H H

H H

C C

H H

H H

C C

H H

H H

C C

H H

H H

C C

H H

H H

C C

H H

H H

C C

H H

H H

C C

H H

H H

Glass transition temperature: -78 C.

Melting temperature: 100 C.

Amorphous density at 25 C: 0.855 g/cm3.

Crystalline density at 25 C: 1.00 g/cm3.

Molecular weight of repeat unit: 28.05 g/mol

used in making translucent, lightweight, and

tough plastics, films, containers, insulation,

etc


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C

H

HC

H

HC C C C

H

H

H

H

H

H

H

H... ... C

H

HC

H

Hn

Degree of Polymerization (DP)

n: the number of monomeric unit

~10,000 to ~1,000,000

The degree of polymerization

C C

H

H

H

Cl

C C

H

H

H

Cl

C

C

C

C

C

C

C

C

C

C

Cl Cl Cl Cl Cl

H H H H HH H H H H

H H H H H

n

Another Example: Poly (vinyl chloride)


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Poly (ethylene terephthalate), PET or PETE, one of the polyesters

Film, fibers, clothing, drink bottle

Density 1370 kg/m3

Young modulus (E) 28003100 MPa

Tensile strength (t) 5575 MPa

Glass temperature 75 C

melting point 260 C

Poly (styrene), PS

Density 1050 kg/m

Specific Gravity 1.05

Young's modulus (E) 3000-3600 MPa

Tensile strength (st) 4660 MPa

Glass temperature 95 C

Melting point 240 C

Containers and toys
http://upload.wikimedia.org/wikipedia/commons/1/10/Polystyrene_formation.PNGhttp://upload.wikimedia.org/wikipedia/commons/5/5f/PET.png


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CH2

C

H2

CH2

C

H2

CH2

C

H2

NH

NH

C

O

CH2

C

H2

CH2

C

H2

C

O

n

Nylon 66, one of the polyamides

Carpet fiber, apparel, airbags, tires,

ropes, conveyor belts, and hoses

Poly (p-phenyleneterephthalamide), PPTA or Kevlar

Fibers and bulletproof vests

One of the most strong polymers

C

F

F

C

F

F

n

Poly (tetrafluoroethylene), PTFEbearings, bushings, gears, slide plates

One of the most hydrophobic polymers

With the lowest frictional coefficient


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semi-crystalline structure of polymer amorphous structure of polymer


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Thermoplastic Polymer and Thermoset Polymer

Thermoplastic polymer: capable of softening or fusing (melting) whenheated and of hardening again when cooled

e.g.) various linear polymers (no chemical crosslinking)poly (ethylene), poly (propylene), and Poly (ethylene terephthalate)

Thermoset polymer: not capable of softening or fusing (melting) whenheated and of hardening again when cooledThe curing (crosslinking)

process makes three-dimensional network structure in polymeric material

e.g.) various polymers with chemical crosslinkingVulcanized rubber

Bakelite, a Phenol Formaldehyde Resin (used in electrical insulators and plastic wear)Urea-formaldehyde foam (used in plywood, particleboard and medium-density fibreboard)

Melamine resin (used on worktop surfaces)

Polyester Resin (used in glass-reinforced plastics/fibreglass (GRP))

Epoxy Resin


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Thermoplastic Polymer and Thermoset Polymer

Polyester with saturated

bonds along the chain

Polyester with unsaturated

bonds along the chain

Crosslinking with a

polystyrene monomer

Thermoplastic

Thermoset polymerPET

PET

PET-based thermosetpolymer


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Structure of Cross-linked Rubber

Thermoplastic Polymer andThermoset Polymer

C C

R H

H

C C

H H

H

C C

R H

H

C C

H H

H

C C

R

H

H

C C

H

HH

C C

R

H

H

C C

H

HH

Doublebonds

C C

R H

H

C C

H H

H

C C

R H

H

C C

H H

H

C C

R H

H

C C

H H

H

C C

R H

H

C C

H H

H

C C

R

H

H

C C

H

HH

C C

R

H

H

C C

H

HH

Doublebonds

C C

R H

H

C C

H H

H

C C

R H

H

C C

H H

H

C C

R

H

H

C C

H

HH

C C

S H

H

C C

H S

H

SSCross-linkedPrimary Bonds

H H

C C

R H

H

C C

H H

H

C C

R H

H

C C

H H

H

C C

R

H

H

C C

H

HH

C C

S H

H

C C

H S

H

SSCross-linkedPrimary Bonds

H H

Unsaturated bonds are used to form cross-links with cross-linker.

Vulcanized rubber

Thermoset polymers do not crystallize well. Why?


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Factors Affecting Crystallinity in Polymers

The size of the side groups

The extent of chain branching

Tacticity

The complexity of the repeat unit

The degree of secondary bonding

Factors influencing the efficiency of packing polymer chains


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21


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Tacticity

crystallizable

The complexity of the repeat unit

C

H

H

C

H

H

n

V.S.

The degree of secondary bonding

CH2

CHn

OH

C

H

H

C

H

H

n

CH3

V.S.

C

O

O CH2

CH2

C

O

On

Not crystallizable

22


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Rubbers

CH

2

C

CH3

CCH

2

n

H

Cis-1,4-poly (isoprene)

Natural Rubber

Synthetic Rubber

CH2 CH CH CH2 n

CH2

CH CH CH2 n

CH2

CH CH CH2

CH2

CH CH CH2CH2 CH CH CH2

S S

S S

CH2

CH CH CH2Poly (butadiene)

Sulfur bridge

Sulfur cross-link

Poly(isoprene) : Tg=~-70C

Poly(butadiene) : Tg=~-70C
http://en.wikipedia.org/wiki/Image:20070104rubberlatex.jpghttp://en.wikipedia.org/wiki/Image:Ceylon_rubber.jpg

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