MW SP Relation

7/30/2019 MW SP Relation

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Dr. S.Ray_Chem Engg_NIT Agratala 1/17/2013

olymer Processing Engineering 1

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Structure-processing-property relationships


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Structure and properties of the polymer depends on:

The nature of bonds in monomers (chemical bonding)

The type of monomers that are capable of formingpolymers (functionality of monomers)

The mode of linking of monomers (polymerizationmechanisms)

The chemical composition of monomers and theproperties conferred on monomers as a result of theirchemical composition

Structure-property relationshipsin polymers


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Primary structure refers to the atomic composition andchemical structure of the monomer the building block ofthe polymer chain.

An appreciation of the nature of the monomer is fundamentalto understanding the structureproperty relationship ofpolymers.

The chemical and electrical properties of a polymer aredirectly related to the chemistry of the constituent monomers.

The physical and mechanical properties of polymers, on theother hand, are largely a consequence of the macromolecularsize of the polymer, which in itself is related to the nature ofthe monomer.


Chemical bonds may be classified as primaryor secondarydependingon the extent of electron involvement.

Valence electrons are involved in the formation of primary bonds, hencethese are strong bonds.

Valence electrons are not involved in the formation of secondary bondsleading to weak bonds.

Chemical bonds

1. Primary bondsa. Ionic electron transfer to attain the inert gas electron configurationb. Covalent - involve the sharing of valence electrons.

2. Secondary bondsa. Dipole arises if the electrons shared by two atoms spend more

time on one of the atoms due to differences in electro-negativity.b. Hydrogen - a particularly important kind of dipole interactionc. Induction forces due to induced relative displacements of the

electrons by permanent dipole.d. Van der Waals (dispersion) instantaneous fluctuating dipoles exist

even in non-polar materials, the orientations of fluctuating dipolesin proper alignment results intermolecular attractions


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1-Primary bonds : the covalent bonds that connect the atoms of the main chainhain

22-- Secondary bonds : nonSecondary bonds : non covalent bonds that hold one polymer chain to anothercovalent bonds that hold one polymer chain to anotherincluding hydrogen bond and other dipoleincluding hydrogen bond and other dipole dipole attractiondipole attraction

33--Crystalline polymer : solid polymers with a high degree of structural order andCrystalline polymer : solid polymers with a high degree of structural order andrigidityrigidity

44-- Amorphous polymers : polymers with a low degree of structural orderAmorphous polymers : polymers with a low degree of structural order

55--SemiSemi crystalline polymer : most polymers actually consist of both crystallinecrystalline polymer : most polymers actually consist of both crystallinedomains and amorphous domains with properties between that expected for adomains and amorphous domains with properties between that expected for apurely crystalline or purely amorphous polymerpurely crystalline or purely amorphous polymer

66--Glass : the solid form of an amorphous polymer characterized by rigidity andGlass : the solid form of an amorphous polymer characterized by rigidity and

brittlenessbrittleness

Amorphous Crystalline

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Van de Waals Forces ~ 2-20 kJ/mol

1. Dipole-Dipole Interaction KTRED 6

2

2

2

1

3

2 =

~ 13-21 kJ/mol

2. Induced Dipole-Dipole InteractionR

EI 6

2

222

11 += ~ 6-13 kJ/mol

3. Dispersion Interaction

+=

RII

IIEL 6

21

21

21

2

3 ~ 0.8-8 kJ/mol

dipole moment, polarizability, I ionization energy, R intermolecular distance


Intermolecular Interactions defining polymer structure and properties


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Hydrogen Bonding

Proton Donors:

Proton Acceptors:

Oxygen Nitrogen

Why are nylons strong and tough?

Why nylons have high meltingtemperature (~250C)?

-X-H Y- ~ 20-40 kJ/mol~ 4-5 (


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Chemical bond energies

Effect of Bonding on polymer properties

The primary bonds between the chains are strong mostly covalentbonds that you would expect to produce a

Strong Stiff

high melting temperature material.

The secondary bonds mean that chains can move with respect toeach other easily which makes polymers relatively

Weak low stiffness low melting temperature

Structure-property relationships in polymers


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Functionality of monomers

The functionality of a molecule issimply its interlinking capacity, orthe number of sites it hasavailable for bonding with othermolecules under the specificpolymerization conditions.

A molecule may be classified asmonofunctional, bifunctional, or

polyfunctional depending onwhether it has one, two, orgreater than two sites availablefor linking with other molecules.

The structural units resulting from the reaction of monomers may inprinciple be linked together in any conceivable pattern.

Bifunctional structural units can enter into two and only two linkageswith other structural units. This means that the sequence of linkagesbetween bifunctional units is necessarily linear.The resulting polymer is linear

The reaction between polyfunctional molecules results in structuralunits that may be linked so as to form nonlinear structures. In somecases the side growth of each polymer chain may be terminated beforethe chain has a chance to link up with another chain.The resulting polymer molecules may be branched.

In other cases, growing polymer chains become chemically linked toeach other, resulting in a cross-linkedsystem



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The chemical composition and atomic arrangement of an organic moleculeconfer certain properties on the molecule, one such property is the polarity.

In a diatomic molecule formed from two like atoms (e.g., H2 ), the electron pairlinking the two atoms is equally shared and the molecule is said to be non-polar.

But when molecules are formed from two unlike atoms (e.g., CO), the distributionof the electron cloud is concentrated on the more electronegative atoms andthere is a separation of positive and negative charge and the molecule is polar.


Polarity affects the intermolecular attraction between chain molecules, andthus the regularity and symmetry of polymer structure.

The properties such as the solubility and electrical nature of polymers, whichdepend on polymer structure, are intimately related to polarity

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Polar side groups like Chlorine or Flourine can increasethe strength of the secondary bonding.

Bulky side groups like those on polypropylene increasethe entanglement like barbed wire increases theentanglement of wire.


Polar Side Groups

Polymer Densityg/cc

UTSk-lb/si

EL%

Ek-lb/si

PVC 1.35 7.5 45 385

PP 0.950 5.0 150 300

PS 1.05 6.5 1.5 413

MDPE 0.936 2.5 750 90



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Effect of Backbone The all carbon backbone of PE is very flexible. The addition of N or O on the backbone can make the polymer stiffer

(harder to uncoil and slide past other chains (Nylon) The existence of ring structures on the chain makes the polymer stiff.


Polymer Densityg/cc

UTSk-lb/si

EL%

Ek-lb/si

PVC 1.35 7.5 45 385

PP 0.950 5.0 150 300

PS 1.05 6.5 1.5 413

MDPE 0.936 2.5 750 90

Nylon 1.14 12 15-300 230-550

PC 1.2 10 120 345

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The shape of the polymer molecule (molecular architecture) will be influencednaturally by the nature of the repeating unit and the manner in which these unitsare linked together


Configuration Arrangement fixed by primary valence bonds; can be altered onlythrough the breaking or reforming of chemical bonds

Conformation Arrangement established by rotation about primary valencebonds

Diene Polymerization - 1,2, and 1,4 additions:

In 1,2 addition the resulting polymer unsaturation is part of the pendant group,while in 1,4 addition the unsaturation is part of the backbone, thus in 1,4 addition,the backbone has a rigid structure and rotation is not free around it.

Predominant specie

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In addition to the molecular shape fixed by chemical bonding, variations in theoverall shape and size of the polymer chain may occur due to rotation aboutprimary valence bonds (conformation).

A polymer molecule may assume a large or limited number of conformationsdepending on:

Steric factorsWhether the polymer is amorphous or crystalline

Whether the polymer is in a solution state, molten state, or solid state


A single isolated polymer chain in dilute solution havingfour successive chain atoms; the first three of these definea plane, and the fourth can lie anywhere on the indicatedcircle which is perpendicular to and dissected by the plane.However, all bonds have to overcome certain rotationalenergy barriers whose magnitude depends on factors suchas steric hindrance, dipole forces, etc., accordingly, somepositions will certainly be more probable than others

The thermal energy of the molecular environment provides the energyrequired to overcome the rotational energy barrier, consequently, theshape (flexibility) of a polymer molecule is temperature dependent.


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stablegauche(+) gauche(-)

stable

~+15 ~-15

CH2CH2n

PolyethyleneCF2CF2

n

Polytetrafluoroethylene

CH3-CH2-CH2-CH3


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The different possible spatial arrangements are called the tacticityof the polymer.

For monosubstituted ethylene, such as a vinyl polymers, everyother carbon atom is a chiral center and are referred to aspseudochiral centers

If the R groups on successive pseudochiral carbons all have thesame configuration, the polymer is called isotactic

When the pseudochiral centers alternate in configuration from onerepeating unit to the next, the polymer is called syndiotactic.

If the pseudochiral centers do not have any particular order, but infact are statistical arrangements, the polymer is said to be atactic.

Polymer Tacticity


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In Fisher projections the R groups are placed either up or down.

a) all up (or all down) indicates the isotactic structure:

b) alternating up and down indicates syndiotactic:

c) random up and down indicates atactic:

Polymer Tacticity

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Polymer Tacticity

X=CH3Y=H

R R R R R R

isotactic

isotactic PP

R S R S R S

syndiotactic

syndiotactic PP

C

H3C CH2CH3

H Cl

S

C

H3C CH2CH3

Cl H

R

Atactic (non-atactic)

S R R R S R S S R S R

atactic PP


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The isotactic and syndiotactic (also known as stereo-regular)polymers are both crystallizable because of their regularityalong the chain. However, their melting temperatures are notthe same.

Atactic polymers, on the other hand, are usually completelyamorphous unless the side group is so small or so polar as topermit some crystallinity.

Polymer Tacticity structure-property relationship

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The same bonding forces (intra- and intermolecular) operate in both low- andhigh-molecular weight materials, however, polymers have unique properties.

The unique properties exhibited by polymers and the difference in behaviorbetween polymers and their low-molecular weight analogs are attributable totheir large size and flexible nature.

The properties that depend on molecular weight are:

mechanical properties:tensile strength,compressive strength,elongation at break,modulus,impact strength

other properties:softening point,solution viscositiesmelt viscosities,solubility



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Molecular Weight of Polymers

1. Molecular Weight (MW) = Degree of Polymerization (DP) M0

CH2CH21000

MW = 1000 M0 (28) g/mol = 28,000 g/mol

Number-average DP

MMDP nn 0/=

Weight-average DP

MMDP ww 0/=

Number-average Molecular Weight

===

iMiwi

iiiin N

MNMxM 1

Weight-average Molecular Weight

=

==

MN

MNM

MN

MNMwM

ii

iii

ii

iiiiw

2

2. Average Molecular Weights of Polymers

A set of values {M1, M2, . . ., Mn}. The probability of each value is {P1, P2, . . ., Pn}.

The average is . So, the average depends on the probability.

=1iiiMP

=

=

1

1

ii

iii

P

MP


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1. A number-average molecular weight Mn : divide chains into series of sizeranges and then determine the number fraction Ni of each size range

where Mi represents the mean molecular weight of the size range i, and N i is thefraction of total number of chains within the corresponding size range

To create a solid with useful mechanical properties the chain must be long !!One may describe chain length in terms of polymer average molecular weight,which can be defined in several ways:

Molecular weight averages

2. A weight average molecular weight Mw is based on the weightfraction wi within the size ranges:

Mn = Mi Ni/ Ni

Mw = Mi W i/ W i

(1) The number-average molecular weight for a

discrete distribution of molecular weights is given

as

where N is the total number of molecular-weight

species in the distribution.

(2) The weight-average molecular weight is given as


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Z-average Molecular Weight (melt elasticity)

==

==

MN

MN

Mw

MwM

Mw

MwMzM

ii

ii

ii

iii

ii

iiiiz 2

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Viscosity-average Molecular Weight (viscosity)

=

+

MN

MNM

ii

aii

a

v

1/1

ais viscosityparameterbetween 0.5 and 1.0

Generally:

MMMM zwvn 1 polydispersePolymer consisting of molecules with the variety of chain length


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0 200,000 400,000 600,000 800,000 1,000,000

4.0

3.0

2.0

1.0

0

Mi (g/mol)

104wi

MM pn = g/mol000,100

g/mol900,199=Mw

g/mol850,299=M z

g/mol000,165=Mv

1=wi


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Molecular Weight Distribution (MWD)

M

MMWD

n

w=

Perfectly MonodisperseMWD = 1.0

Monodisperse (general term)1.0 < MWD 1.1Polydisperse

MWD > 1.1

Living polymerization:1.0 < MWD 1.1

Radical polymerization:1.5 < MWD 2.0

Condensation polymerization:MWD ~ 2

Ziegler-Natta polymerization:5 < MWD < 30



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CH2CH2 n

polyethylene (PE)

MW = 105 g/molN = 7143 CH2a = 0.154 nm

random coil

extended chain

Rg = N0.5a= 13.0 nm

Rg (radius of gyration)

Lex = Na = 1100.0 nm= 1.1 m

An Example Polymer - Polyethylene

CH2=CH2ethylene

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Start Destination Miles

1: A 2: B 210

2: B 3: C 90

3: C 4: D 620

4: D 5: E 2790

Suppose that you want to travel from place A to place E, visiting yourfriends along the way. Unfortunately your friends live only on the easternand western coasts, leading to a somewhat unconventional journey.

An Exercise


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The number-average of the miles traveled in the four legs of thejourney is 928 miles. This is the average distance traveled in eachleg of the journey. It places equal emphasis on each leg.

An Exercise - continued

miles5.927

1111

2790162019012101

=

+++

+++=

==

N

MNMxM

i

iiiin

Number-average

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miles8.22152790162019012101

2790162019012101 2222

2

=+++

+++=

=

==

MN

MNM

MN

MNMwM

ii

iii

ii

iiiiw

The mass-average of the miles traveled in the four legs of thejourney is 2216 miles. This average places a greater emphasison the leg of the journey with the largest "mass" - that is thefourth leg in which you travel 2790 miles. It is morerepresentative of the major part of your journey.

Mass-average

An Exercise - continued


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Property - Molecular Weight Dependence

1. Glass Transition MW Dependence

10-4

Mn

Tg

(C)

PMMA

M

KTT

n

gg =

K is a polymer-related constant.

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2. Viscosity MW Dependence

M < Mc, no entanglement M > Mc, entanglement

Mc critical MW

3.4

1.0

topologicalentanglement



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Entanglements affects solubility of polymersPolymers chains tends to entangle beyond criticallength and when entangled, polymers cant movelaterally but move along their lengths (think chowmein)

Polymers that are long enough to entangleneed to disentangle before dissolution

Solvent diffuses into the solid and swells thematerial, allowing space for movement and thenslow longitudinal diffusion allows single chainsto dissolve

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States of Thermoplastics Polymers

Thermoplastics Polymers

Crystallinee.g. polyethylene,polypropylene, etc.

Amorphouse.g. polystyrene,polyacrylates, etc.

Liquid Crystallinethermotropic or lyotropic,main-chain or side-chain

LC polymers (LCPs)

Amorphous: No long range order, only short range order. Like gas and liquid.Important parameter: Glass Transition Temperature (Tg)

Glassy state (hard) Tg Rubbery state (soft)

Crystalline: Long range order, including positional, orientational, and conformational orders.Like crystals or solids.Important parameter: Melting Point (Tm)

crystallization

melting


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77 Crystalline melting temperature (TCrystalline melting temperature (Tmm ) : temperature at which crystalline) : temperature at which crystallinePolymer converts to a liquid or crystalline domains of a semi crystallinePolymer converts to a liquid or crystalline domains of a semi crystallinePolymer melt (increased molecular motion )Polymer melt (increased molecular motion )

88-- Glass transition temperature (Glass transition temperature (TTgg ) : temperature at which an amorphous) : temperature at which an amorphouspolymer converts to a liquid or amorphous domains of a semi crystallinepolymer converts to a liquid or amorphous domains of a semi crystallinepolymer meltpolymer melt

99 Thermoplastics (plasticsThermoplastics (plastics )) :polymers that undergo thermally reversible:polymers that undergo thermally reversibleInterconversionInterconversion between the solid state and the liquid statebetween the solid state and the liquid state

1010-- ThermosetsThermosets : polymers that continue reacted at elevated temperatures: polymers that continue reacted at elevated temperaturesgenerating increasing number ofgenerating increasing number of crosslinkscrosslinks such polymers do not exhibitsuch polymers do not exhibitmelting or glass transitionmelting or glass transition

1111-- LiquidLiquid crystalline polymers : polymers with a fluid phase that retainscrystalline polymers : polymers with a fluid phase that retains

some ordersome order1212-- ElastomersElastomers : rubbery , stretchy polymers the effect is caused by light: rubbery , stretchy polymers the effect is caused by lightcrosslinkingcrosslinking that pulls the chains back to their original statethat pulls the chains back to their original state

Temperature

3

9

67

8

4

5

Glass phase (hard plastic)

Rubber phase (elastomer)

Liquid

Leathery phase

Log-stiffn

ess


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Glass TransitionMelting

Property of the amorphous region Below Tg: Disordered amorphous

solid with immobile molecules Above Tg: Disordered melt A second-order-like transition

Property of the crystalline region Below Tm: Ordered crystalline solid Above Tm: Disordered melt A first-order transition

-like

Melting versusGlass Transition

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Higher Order Polymer Structures

Second Order Structures - Conformation

extended chain random coil folded chain chain helix

Third Order Structures Micromorphology

non-entangled entangled fringe micelle crystal chain-folded crystal

Fourth Order Structures Macromorphology

crystallinespherulites

double gyroid ofABC terpolymers

MW SP Relation

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