Micro Structures in Polymers

8/13/2019 Micro Structures in Polymers

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Micro Structures in PolymersChapter 3

Professor Joe GreeneCSU, CHICO

September 20, 1999MFGT 041


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Chapter 3 Objectives

Objectives Polymer length, molecular weight, molecular weight

distribution (MWD) Physical and mechanical property implications of

molecular weight and MWD Melt Index Amorphous and crystalline structures in polymers Thermal transitions in plastics (thermoplastics and

thermosets Steric (shape) effects


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Molecular Weight Average Molecular Weight

Polymers are made up of many molecular weights or adistribution of chain lengths.

The polymer is comprised of a bag of worms of the same

repeating unit, ethylene (C 2H4) with different lengths; somelonger than others.

Example, Polyethylene -(C 2H4)-1000 has some chains (worms) with 1001 repeating

ethylene units, some with 1010 ethylene units, some with 999 repeatingunits, and some with 990 repeating units.

The average number of repeating units or chain length is 1000 repeatingethylene units for a molecular weight of 28*1000 or 28,000 g/mole .


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Distribution of values is useful statistical way tocharacterize polymers.

For example, Value could be the heights of students in a room.

Distribution is determined by counting the number of students in theclass of each height.

The distribution can be visualized by plotting the number of students onthe x-axis and the various heights on the y-axis.

Histogram of Heights of Students

0

510

15

20

25

60 70 80

Height, inches

F r e q u e n c y

Series1


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Molecular Weight Molecular Weight Distribution

Count the number of molecules of each molecular weight The molecular weights are counted in values or groups that have

similar lengths, e.g., between 100,000 and 110,000 For example,

Group the heights of students between 65 and 70 inches in one group,70 to 75 inches in another group, 75 and 80 inches in another group.

The groups are on the x-axis and the frequency on the y-axis. The counting cells are rectangles with the width the spread of

the cells and the height is the frequency or number of molecules Figure 3.1 A curve is drawn representing the overall shape of the plot by

connecting the tops of each of the cells at their midpoints.

The curve is called the Molecular Weight D istri bution (M WD)


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Determined by summing the weights of all of the chainsand then dividing by the total number of chains.

Average molecular weight is an important method ofcharacterizing polymers.

3 ways to represent Average molecular weight Number average molecular weight Weight average molecular weight

Z-average molecular weight


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Gel Permeation Chromatography GPC Used to measure Molecular Weights

form of size-exclusion chromatography smallest molecules pass through bead pores, resulting in

a relatively long flow path

largest molecules flow around beads, resulting in arelatively short flow path chromatogram obtained shows intensity vs. elution

volume

correct pore sizes and solvent critical


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Number Average Molecular Weight, M n

where Mi is the molecular weight of that species (on the x-axis) where Ni is the number of molecules of a particular molecular

species I (on the y-axis).

Number Average Molecular Weight gives the same weight to all polymer lengths, long and short. Example, What is the molecular weight of a polymer sample in which the

polymers molecules are divided into 5 categories. Group Frequency 50,000 1 100,000 4 200,000 5 500,000 3

700,000 1

......

321

332211

N N N

M N M N M N

N

M N M

i

ii

n

000,260

)13541()700(1)500(3)200(5)100(4)50(1

.. ... .

321

332211

n

n

i

iin

M

K K K K K M

N N N M N M N M N

N

M N M


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Molecular Weight Number Average Molecular Weight. Figure 3.2

The data yields a nonsymmetrical curve (common) The curve is skewed with a tail towards the high MW The Mn is determined experimentally by analyzing the

number of end groups (which permit the determination of

the number of chains) The number of repeating units, n, can be found by the

ratio of the Mn and the molecualr weight of the repeatingunit, M

0, for example for polyethylene, M

0 = 28 g/mole

The number of repeating units, n, is often called thedegree of polymerization, DP.

DP relates the amount ofmonomer that has been converted to polymer.

0 M

M n n


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Weight Average Molecular Weight, M w

Weight Average Molecular Weight, M w Favors large molecules versus small ones

Useful for understanding polymer properties that relate tothe weight of the polymer, e.g., penetration through amembrane or light scattering.

Example, Same data as before would give a higher value for the

Molecular Weight. Or, M w = 420,000 g/mole

......

332211

233

222

211

2

M N M N M N

M N M N M N

M N

M N M

ii

ii

w


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Z- Average Molecular Weight

Emphasizes large molecules even more than Mw Useful for some calculations involving mechanical

properties.

Method uses a centrifuge to separate the polymer

.. .

...233

222

211

333

322

311

2

3

M N M N M N

M N M N M N

M N

M N M

ii

ii

z


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Molecular Weight Distribution

Molecular Weight Distribution represents thefrequency of the polymer lengths

The frequency can be Narrow or Broad, Fig 3.3

Narrow distribution represents polymers of aboutthe same length. Broad distribution represents polymers with varying

lengths MW distribution is controlled by the conditions

during polymerization

MW distributions can be symmetrical or skewed.


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Physical and Mechanical PropertyImplications of MW and MWD

Higher MW increases Tensile Strength, impact toughness, creep resistance, and

melting temperature.

Due to entanglement, which is wrapping of polymerchains around each other.

Higher MW implies higher entanglement which yieldshigher mechanical properties.

Entanglement results in similar forces as secondary orhydrogen bonding, which require lower energy to breakthan crosslinks.


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Physical and Mechanical Property Implicationsof MW and MWD

Higher MW increases tensile strength Resistance to an applied load pulling in opposite directions Tension forces cause the polymers to align and reduce the

number of entanglements. If the polymer has manyentanglements, the force would be greater.

Broader MW Distribution decreases tensile strength Broad MW distribution represents polymer with many shorter

molecules which are not as entangled and slide easily.

Higher MW increases impact strength Impact toughness or impact strength are increased with longer

polymer chains because the energy is transmitted down chain.

Broader MW Distribution decreases impact strength Shorter chains do not transmit as much energy during impact


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Thermal Property Implications of MW & MWD Higher MW increases Melting Point

Melting point is a measure of the amount of energy necessaryto have molecules slide freely past one another.

If the polymer has many entanglements, the energy requiredwould be greater.

Low molecular weights reduce melting point and increase easeof processing.

Broader MW Distribution decreases Melting Point Broad MW distribution represents polymer with many shorter

molecules which are not as entangled and melt sooner. Broad MW distribution yields an easier processed polymer

MW MW

* Decomposition


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Example of High Molecular Weight Ultra High Molecular Weight Polyethylene (UHWMPE)

Modifying the MWD of Polyethylene yields a polymer with Extremely long polymer chains with narrow distribution Excellent strength Excellent toughness and high melting point.

Material works well in injection molding (though high melt T) Does not work well in extrusion or blow molding, which

require high melt strength. Melt temperature range is narrow and tough to process. Properties improved if lower MW polyethylene

Acts as a low-melting lubricant Provides bimodal distributions, Figure 3.5 Provides a hybrid material with hybrid properties


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Melt Index Melt index test measure the ease

of flow for material Procedure (Figure 3.6)

Heat cylinder to desired temperature (melt temp)

Add plastic pellets to cylinder and pack with rod Add test weight or mass to end of rod (5kg) Wait for plastic extrudate to flow at constant rate Start stop watch (10 minute duration) Record amount of resin flowing on pan during time

limit Repeat as necessary at different temperatures and

weights


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Melt Index and Viscosity Melt index is similar to viscosity

Viscosity is a measure of the materials resistance to flow. Viscosity is measured at several temperatures and shear rates Melt index is measured at one temperature and one weight.

High melt index = high flow = low viscosity Low melt index = slow flow = high viscosity Example, (flow in 10 minutes)

Polymer Temp Mass HDPE 190C 10kg Nylon 235C 1.0kg PS 200C 5.0Kg


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Melt Index and Molecular Weight Melt index is related closely with average molecular weight

High melt index = high flow = small chain lengths = low M n Low melt index = slow flow = long chain lengths = high M n

Table 3.1 Melt Index and Average Molecular WeightMn Melt Index* (g/10min)

100,000 10.00

150,000 0.30 250,000 0.05* Note: PS at T= 200C and mass= 5.0Kg


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Amorphous- Molecular structure is incapable of forming

regular order (crystallizing) with molecules or portions ofmolecules regularly stacked in crystal-like fashion. A - morphous (with-out shape)

Molecular arrangement is randomly twisted, kinked, andcoiled

States of Thermoplastic Polymers


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Amorphous Materials

PVC Amorphous PS Amorphous Acrylics Amorphous

ABS Amorphous Polycarbonate Amorphous Phenoxy Amorphous PPO Amorphous SAN Amorphous Polyacrylates Amorphous


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Crystalline- Molecular structure forms regular order

(crystals) with molecules or portions of molecules regularlystacked in crystal-like fashion. Very high crystallinity is rarely achieved in bulk polymers Most crystalline polymers are semi-crystalline because

regions are crystalline and regions are amorphous Molecular arrangement is arranged in a ordered state

States of Thermoplastic Polymers


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Crystalline Materials LDPE Crystalline HDPE Crystalline PP Crystalline PET Crystalline

PBT Crystalline Polyamides Crystalline PMO Crystalline

PEEK Crystalline PPS Crystalline PTFE Crystalline

LCP (Kevlar) Crystalline


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Form of Polymers

Thermoplastic Material: Amaterial that is solid, that possessessignificant elasticity at roomtemperature and turns into a viscous

liquid-like material at some highertemperature. The process is reversible

Polymer Form as a function oftemperature Glassy: Solid-like form, rigid, and

hard

Temp

Glassy

Rubbery

Melt

Polymer

Form

Tm

Tg


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Glass Transition Temperature, T g Glass Transition Temperature, Tg: The temperature

by which: Below the temperature the material is in an immobile

(rigid) configuration

Above the temperature the material is in a mobile (flexible) configuration

Transition is called Glass Transition because the properties below it are similar to ordinary glass.

Transition range is not one temperature but a rangeover a relatively narrow range (10 degrees). T g isnot precisely measured, but is a very importantcharacteristic.


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Crystalline Polymers: T m

Tm: Melting Temperature

T > T m, The order of the molecules is random (amorphous) T < T m >T g, Crystallization begins at various nuclei and the order

of the molecules is a mixture of crystals and random polymers

(amorphous). Crystallization continues as T drops until maximumcrystallinity is achieved. The amorphous regions are rubbery anddont contribute to the stiffness. The crystalline regions areunaffected by temperature and are glassy and rigid.

T < T g, The amorphous regions gain stiffness and become glassy

Polymer Form

Temp

Glassy

Rubbery

MeltTm

Tg


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Crystalline Polymers T g Tg: Affected by Crystallinity level

High Crystallinity Level = high Tg Low Crystallinity Level = low Tg

Modulus(Pa)or

(psi)

Temperature-50C 50C 100C 150C 200C 250C

Tg

High Crystallinity

Medium Crystallinity

Low Crystallinity


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Thermal Properties

Table 3.2 Thermal Properties of Selected Plastics

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Micro Structures in Polymers