str property course 2012.pdf

8/11/2019 Str Property Course 2012.PDF

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Model fibre silk, Molecular architecture, configuration, conformation , Molecular modelingamorphous and crystalline phases, glass transition, plasticization, crystallization, melting, factors affecting Tg and Tm.

Basic structure of a fibre structure of fibrils. Role of molecular entanglement on fibre formation.

Formation of structure in viscose and thermoplastic fibres (PET, Nylon, PP, Acrylic). Methods of investigating physical structure of fibres (WAXD, DSC, FTIR, birefringence, sonic modulus). Moisture absorption properties. Rate of moisture absorption, Heat of sorption, water retention and swelling. Theories of moisture absorption general view, absorption in crystalline and amorphous regions, quantitative theories.

Theories of mechanical properties of natural and man made fibres, viscoelastic behaviour, comparison of properties of various fibres. Fibre

friction. Optical properties, Introduction to electrical properties (dielectric properties and static charge generation). Thermal properties heatsetting.

1.Physical properties of Textile Fibres- Morton, Hearle

2. Hand book of Textile Fibres Gordon Cook, Vol-1,2

. - ,

1.Minor 1: 25%2.Minor 2 : 25%3.Quiz: 5-10%4.Major : 40%

Attendance policy:100% attendance is required1) Explanation

2) Downgrading

Textile fibres

Natural Manufactured

Vegetable BastFibre

Leaffibre

FlaxJuteHem

AbacaPineaple

Nutfibre

Coir

Animal Mineral

AsbestosCeramicCotton

Ramie

Hair WoolMohair cashmere

ProteinSilkCasein

Natural Fullysynthetic

Inorganicmaterials

Cellulose, Viscose Modified Cellulosic Acetate, triacetate Alginate, chitosan

Polyamide Polyester Polyacrylonitrile Polyolefin PVA

GlassCeramicMetallic

Apparel: Cotton, Wool, Polyester, Nylon, SilkDomestic: Carpets, Curtains & upholsteries, Bedding

Jute, Coir, hemp, sisal: stiff PP, PE, Nylon: hydrophobicPLA, PGA, casein, alginate: poor stability

(a) Medical Textiles :wound dressing, sutures, scaffolds fortissue engineering

(b) High tenacity: Tire cords, Belts, Ropes, Tents, Civilengineering, Sail cloth

(c) Protective Textiles: Bullet proof jackets, cut-resistance fabric(d) Smart Textiles: colour change, communication & monitoring

devices, electronics etc


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Polymers are long chains of organic molecules= Macromolecules

- Built up from smaller units called monomers

Poly = many mer = unit or part

- The nature/structure of polymers was a big question in early 1900s

Colloidal aggregatesMysterious forces vs

Macromolecules;small molecules held togetherby covalent bonds

Huge Debate:

Hermann Staudinger, 1920s (Nobel Prize 1953)for his discoveries in the field of macromolecular chemistry

Paul Flory (Nobel Prize 1974): tangled behavior of polymers.

Backboneor Main Chain

self-assembly

understanding of these structures at intermediate length scales, organizationof the polymers, properties

DNA and RNA

Proteins

A, T, G, C (4 bases) (Sugar phosphate backbone)

22 amino acids monomers

Poly(propylene)

CH 3

RepeatUnit Degree of

Polymerization

(DP)

Side Group(if long sidechain)

n

chain length, different side groups , chain branching,stereoregularity, chain flexibility, cross linking.


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Silk

Made at room temperature via self- assembly

Honeybee Wasp

Other producers of Silk(least known)


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Glycine Alanine Serine

What is a polypeptide

Polymer of a amino acids

R O R O

H2N CH C OH H2N CH C OH

R O O

H2N CH C HN CH C OH

R

Conc

(12-15%) C, D23-30% B

Dehydration

60%- 15%

Handbook of Fibre Chemistry

Shear stress

pH

ion conc


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FibroinFibroin

20~3020~30 mm

SericinSericin

Silk

alanine,glycine-alanine,

glycine-alanine-serine

primary sequences and secondary structures

Silk I:unordered structureSilk II: crystalline-sheets: contribute to the high tensile

strength of silk fibers-turn, helical structures: provide elasticity

Secondary structures , -helix and -sheet,have regular hydrogen-bonding patterns.

-helix -sheet


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Primary structure(Amino acid sequence)

Secondary structure

helix, sheet

Tert ary structure Three dimensional structure formed by assembly of

secondary structures

Quaternary structure

Structure formed by more than one polypeptide chains

Tertiary structure Quaternary structure

Bombyx mori silkworm

Silk fibroin is a high-molecular-weight block copolymer consisting of aheavy (370 kDa) and light (26 kDa) chain, linked together by adisulfide bond.

Nephila clavipes (spider silk)

,oligopeptides, separated by smaller charged and amorphous sequences.The hydrophobic domain is rich in alanine & glycine, while thehydrophilic spacers give the heavy chain a polyelectrolyte nature.The sequence of light chain is less repetitive, contains high conc ofglutamic and aspartic acid residues.

Assemblying folding packing


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Silk fibers Silk filmSilk gel Silk scaffold Supercontraction of spider silkContraction leads to changes in orientationof molecular chains due to rupture of Hbonds and conformations

deformational energy is stored in the formof conformational entropy

recoverable disorientation of the molecularchains in fibres oriented amorphous region.

During supercontraction, well-defined crystalline region retains

considerable order because solvent molecules cannot penetrateit, .. at the same time the degree of orientation in the orientedamorphous, as well as the poorly defined crystalline regionsdecreases appreciably


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During supercontraction, water plasticizes silk fibers bybreaking hydrogen bonds between polymer chains,allowing re-orientation of silk molecules to lower energylevels. This allows the random- coil macromolecularregions to move rapidly to more disordered, higherentropy configurations. This movement causes length-wise contraction of spider silk.

During drying, silk may also contract. Reformation ofhydrogen bonds results in organization of the silkproteins.

restores shape and tension after prey captureRecovers from slackness of the webs

Molecular architecture

Polymer architecture

- Microstructure : how individual monomers or atoms are arranged

- Alternating v. random copolymers- Stereochemical configurations

- Macrostructure : how the whole polymer molecule looks like

- Significant influence on properties ---> viscosity, etc.

CH 3 CH 3

nCH 3

nH

Conventional View

StereogenicCenter

Gross Fibrillar structure

CH 3n

H Hn

H3C

TWO distinct configurations

- These are different configurations

Configuration : fixed relative arrangements of atomsof a molecule in space

- Can not be interchanged by bond rotations- These may seem like a small difference

- Three important classes of configurations:

-Isotactic-Syndiotactic-Atactic

polymertacticity


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- Isotactic PP : - Regular stereochemistry-Chains can pack closely-Highly crystalline-Good mechanical properties-Tough, impact resistant-Opaque (scattering from crystalline

regions)

- Atactic PP :- Irregular stereochemistry

-Chains cannot pack closely- -,-Completely amorphous-Soft & waxy

- Syndiotactic PP : - Regular stereochemistryRegular alternation of the side

groups promotes close packing andcrystallization

-Has in between properties-Tough and clear

High DP: Longer chains make stronger polymers

greater resistanceto deformation, greater breakingload,

lower extension tobreak.

Molecular orientation /crystallinity

All polymers are not suitable as fibres

Optimum mol wt. to get mech. propts Adequate intermolecular forces & capacityo or en roug mo ecu ar a gnmen .

SpinnabilityOrientation , semi crystalline

Morphology- crystallite size- arrangement of crystalline & amorphous regions- fibrillar structure etc

fibril : an assembly of molecules

Structurefine structureinter molecular distancesinter planes distancesmolecular orientation etc


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Classification according to back bone Structure:- There are many ways to classify polymers by their structure :

- Linear Two end groups

Contour length

- ranc e

More than 2 end groups

Branch Points

branching has a substantial impact on properties: Crystallization, density,strength, Chemical activity etc.

Hyperbranched Polymers

Many, many end groups

-Very low density materials (e.g., ULDPE: ultra-low density polyethylene)

-Fillers in resin matrix, coating, additives

- Many branches, but branching in uncontrolled or random

-Many branches, butbranching isuncontrolled orrandom

H2N

Hyperbranched Polymers

N

NH

N

N

NH

HN

2

HN

N

2

NH

N

NHH2NN

NH2

NH

H2N

NH2

NH2

Poly(ethylene imine)

HN

n

Ring-openingpolymerization ofaziridine

Dendrimers or Dendritic Polymers

-Compared to hyperbranched polymer, Dendrimers havePERFECT branching emanating from a core

Tree-like Branching

- Every branch point has exactly three branches

-Specific number of endgroups

-Chemical detection (dye, radionucleotide, phamaceutical), drug delivery

- Globular, 3-D structure

~ 1 to 100 nm in diameter


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Ladder Polymers:

- Highthermal and oxidative stability- Must cut at least two bonds to break a chain

- Double-stranded

Crosslinked Polymers- Branched polymers

- If the branches link to the backbones of other polymer molecules

Crosslinked or Network Polymer

- Crosslinks can be short or long

- The material can be: - lightly crosslinked

- highly crosslinked

- Crosslinked polymers typically do not dissolve , melt , or flow

(But they may swell in certain solvents)

Structure vs. Conformation- Polymers are rarely stretched out in solution or in the melt

- Assume a random coilconformation

- The angle between the singly bonded carbon atoms is ~109 degreecarbon atoms form a zigzag pattern in a polymer molecule.

- while maintaining the 109o angle between bondspolymer chains can rotate around single C-Cbonds

- (double and triple bonds are very rigid).

- Random kinks and coils lead to entanglement

Isotactic PP 160 - 170 C

Syndiotactic PP 125 - 130 C

Atactic PP < 0 C

melting point = ratio of latent heat of melting to entropy

molar cohesion energy (of the whole molecule formonomers, or per chain unit for polymers),molecular flexibility (due to rotation around bonds),molecular shape effects

- The summation of subtle effects and weak forces play improles in properties


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Sir Norman Haworth

Hermann Emil Fischer Nobel prize 1902

Cellobiose

impart strength and rigidity to plant cell walls,can withstand high hydrostatic pressure gradients

Microfibrils:36parallel, interactingcellulose chains.

Chitosan

Alginate


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What is a polypeptide

Polymer of a amino acidsR O R O

H2N CH C OH H2N CH C OH

R O O

H2N CH C HN CH C OH

R

Molecular interactionsin wool fibre

Nylon 66 [-(CH2)6 -NH-CO-(CH)4 -CO-NH-]n

PAN


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In silico Structure Prediction ofPolymers-

Molecular Modeling

Polymer Structure

Linear

Branched

Crosslinked

Structure Prediction


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Mathematical Models

Ab Initio

Semi empirical

Molecular Dynamics

Quantum Mechanicsab initio Method

H = E

E is energy of the system

is the wave function

H is the Hamiltonian operator

pproximations

The Born-Oppenheimera roximation

Hartree-Fock approximation

Pros & Cons+++++

Predict structure of- Unsynthesized species cu or mposs e o so a e

-Ve Very large calculations Time consuming


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Semiempirical Methods

Several approximations

Some electron interactions areignored

Molecular mechanics

Force Field Theory

classical Newtonian physics

experimentally derived parameters

calculate geometry as a function of steric energy

Protein Structure

Primary Structure

Tertiary Structure Quaternary Structure

Primary Structure


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Secondary Structure Ramachandran Plot

Tertiary Structure Quaternary Structure


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Protein Structure Prediction XRD

Homology Modeling Homology


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Basic steps in Homology Modeling

Database searching

Alignment (Pairwise/ Multiple)

Model building

Model refinement

Identify Homologues in PDB

Database Search Alignment


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Specific Regions Structurally Conserved Regions

Structurally Variable Regions Generate Core Coordinates


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Model Building

Replace SVR (Loops)

Energy Minimization Geometry Optimization

Replace SVR (Loops)

Add Side Chains Energy Minimization


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Geometry Optimization Model Validation

Structure Validation Silk Fibroin (Bombyx Mori)

Two Peptide Chains

Heavy Chain


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Light Chain Sequence

ORIGIN1 mkpiflvllv atsayaapsv tinqysdnei prdiddgkas svisrawdyv

ddtdksiail61 nvqeilkdma sqgdyasqas avaqtagiia hlsagipgda caaanvinsy

tdgvrsgnfa 121 gfrqslgpff ghvgqnlnli nqlvinpgql rysvgpalgc agggriydfe

aawdailass181 dssflneeyc ivkrlynsrn sqsnniaayi tahllppvaq vfhqsagsit

dllrgvgngn241 datglvanaq ryiaqaasqv hv

//

Blast Report

Final 3-D Prediction Ramachandran Plot


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Model Quality Assessment Q- Mean Score Calculation

C_beta interaction energy: 1.00 (Zscore: 1.58)

Allatom pairwise energy: 7.36 (Zscore: 1.68)

Solvation energy: 618.74 (Zscore: 1.44)

Torsion angle energy: 3.01 (Zscore: 2.41)

. .

Solvent accessibility agreement: 54.9% (Zscore: 0.98)

Total QMEAN score: 0.306 (Zscore: 2.49)

(estimated model reliability between 01)

PDB Comparison HyperChem Prediction


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Heavy Chain SequenceORIGIN

1 agtgssgfgp yvanggysgy eyawssesdf gtgsgagags gagagsgaga gygagvgagy61 gagygagaga gygagagsgv asgagagags gagagsgaga gsgagagsga gagsgagags

121 gagagygaga gygagagyga gagvgygaga gvgygagagy gagagvgygagagsgaasga

181 gagsgagags gagagsgaga gsgagagsga gagsgagags gagagsgaga gygagagvgy241 gagagsgaas gagagsgaga gsgagagsga gagsgagags gagagsgaga

gsgagagsga301 s a a s a a a a a v a a a a a a v a a

gsgaasgaga361 gsgagagsga gagsgagags gagagsgags gagagsgaga gygagygagv

gagygagagv421 gygagygvga gagygagags gaasgagags gagagsgaga gsgagagsga

gagsgagsga481 gagygagags gaasgagaga gagtgssgfg pyvanggysr regyeyawss

ksdfetgsga541 asgagagags gagagsgaga gsgagagsga gaggsvsyga grgygqgags aassvssass601 rsydysrrnv rkncgiprrq lvvkfralpc vnc

//

Blast Report

Final 3-D Prediction Ramachandran Plot


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Model Quality Assessment Q- Mean Score Calculation

C_beta interaction energy: 1.00 (Zscore: 2.31)Allatom pairwise energy: 13.61 (Zscore: 3.10)Solvation energy: 156.14 (Zscore: 2.07)Torsion angle energy: 2.31 (Zscore: 2.22)Secondary structure agreement: 9.5% (Zscore: 5.82)

o vent access ty agreement: . score: . )

Total QMEANscore: 0.123 (Zscore: 5.81)(estimated model reliability between 01)

PDB Comparison


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Amorphous & crystalline phases,

Crystallization,

Effect of Crystallinity on properties of polymers

Why do polymers crystallize ?

Random coil : conformational entropy of arandom coiled chain is very large, due tosignificant number of accessible conformations.

crystallization

meltingLowest energy conformation

3D ordered latticeHigher entropy state

Only polymers of a regular configuration can crystallize (isotactic, syndiotactic)

1. Polymer crystals are formed by lateral alignment of extendedchains

2. Alignment is a 3D order

.crystal lattice

B. Chains will pack as close as possible. Distance of closestpacking is given by van der waals radii

C. Equivalent atoms of different monomer units along the chainaxis tend to assume equivalent positions wrt the atoms ofneighboring chains

Spherulite Growth

Nucleation-1

Nucleation-2

Kinetics of Polymer Crystals formation

A. Nucleation and Spherulitic growth

Branching Spherulite

B. Spherulite


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C. Spherulite structure havingradial growth & branching

D. Orientation of chainsin an enlarged portionof lamellae havingchain folding

Linear PE Nucleation at a reasonable rate of growth mustinvolve chain folding.

The fully extended chain crystal (most stable)requires unreasonably long time to form. Chainfolding is a compromise betweenthermodynamics and kinetics of crystallization.

So, polymer chains choose an alternative wayto crystallize, i.e. through chain folding.

E. Chain folding in the formation of single crystal lamella

100 Ao

Lamellae form because it is the fastest way for long molecules to crystallize

F. Ideal Stacking of lamellar crystal

G. Interwoven Structure of polymers through folded chain lamellae

H. Interlamellar amorphous str of semicrystalline polymer

I. Fringed micelle concept of partlycrystalline polymer

Shick-Kabab form

abcJ. Folded chain lamellar structure

Thinness of polymer lamellae is very imp tocrystallize polymers because of the manysurfaces it creates that directly affect theimp properties such as melting pt, chemicalreactivity, mechanical prop.Thickness is related to crystallization, ratherthan depending on chemistry of polymerchain. It is affected by H- bonding, itincreases with crystallization temp.

Drawn

FibrilsDrawn spherulite


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200 nmTEM: Silk fibrils

Microfibrils

ExtendedNoncrystallinemolecules

CrystallitesThe growth of these structuresimpeded by the presence ofentanglements and strainedregions, which constitute theamorphous phase

(A) Crystallization during polymerization

Fringed fibrillar modelFringed micelle modelFolded chain lamellar model

(B) Crystallization induced by orientationstretching of long chains to form fibrous crystalsdecrease in the conformational entropy

(C) Crystallization under quiescent condition(1) Crystallization from dilute solutions(2) Crystallization from the melt

melting temp to pre-determined crystallization temp

small-angle x-ray scattering

Density fluctuations (nucleation and growth processes)

Plain polarizing microscopy,Rheological and lightscattering studies

(very early stages of crystallinity development)

Onset of autocatalytic, observable crystallization(1) fringed micelle, (2) lamellar type of morphology

pseudo-equilibrium level of crystallinity

TEM, Birefringence


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Degree of crystallinity is determined by:

Molecular conformation & chain flexibility

Structural regularity & Chain configuration: linear polymerscrystallize relatively easily, branches inhibit crystallization,network polymers almost completely amorphous,crosslinked polymers can be both crystalline and amorphous

Complexity: crystallization less likely in complex structures.Simple polymers, such as PE, crystallize relatively easily

Rate of cooling during solidification: time is necessary forchains to move and align into a crystal structure

Isomerism: isotactic, syndiotactic polymers crystallize relatively

easily - geometrical regularity allows chains to fit together, atacticdifficult to crystallize

Copolymerism: easier to crystallize if monomer arrangementsare more regular - alternating, block can crystallize more easilyas compared to random and graft

Presence of Intermolecular forces: polarity, H-bonding etc

Branching / bulkyness of side groups:Short irregular branches tend to decrease crystallinity %Regular short branch, small side groups able to producecoiling, favours crystallization

Molecular weight: low mol wt favours mobility, highercrystallization

Impurities present

More crystallinity: higher density, more strength, higher resistance to dissolution andsoftening by heating

PMMA 100 oC 120 oC

GLASS TRANSITION, Tg

Rubber band vs PET bottle

Glassy stateStarts to soften Rubbery state

Tg = -70 oC, can we use it for making window pane ?Tg = 100 oC, can we use it for making car tire ?

Hardness is related to polymer mobilityBelow Tg, molecules are frozen in.

When polymer is heated above Tg, thermal

What happens on polymer molecules at Tg?

to overcome the energy barrier for translational& rotational motions.Onset of large scale motion

Entanglement restrictionsDegradation


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Definitions

Tg is the temp at which the polymermolecules start to perform large scale(translational & rotational motions) motionwhen we heat the polymer from glass state

Tg is the temp at which the polymermolecules are frozen into a state wherelarge scale motion is prohibited when wecool the polymer from the rubbery state

Does every material have a Tg?

Only amorphous materials

Crystals do not exhibit Tg, but shows melting phenomenonOnly vibrational motion is allowed

ow o we measure g

Glassy state

Rubbery stateSpecific

vol

TempTg

Glassy state

Rubbery stateEnthalpy

H

TempTg

Change of heat capacity (C p)Differential Scanning Calorimeter

Glassystate

Rubbery state

Cp

TempTg

Modulus

Stress/strain

Temp

Tg

ViscosityDielectric relaxationRefractive index


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1. Free volume theory does not involve microscopic description

2. Thermodynamic theory Tg is a thermodynamic 2nd order transition, which occurs as

Theories about Glass transition

the conformational entropy of polymer chains reaches zero.

3. Kinetics theory Tg is the point where the relaxation of molecules is unable to catch up with the experimental cooling rate, so the molecules are frozen into a non equilibrium state.

Free volume Vf = V V0

Vf = Free volumeV = total volumeV0 = occupied volume

Empty space = free volume

If the system has a larger free volume, the molecules have more space to undergo motion . The molecules have a larger mobility.

Doolittle equation

Free volume reaches a constant value at and below Tg. At Tg the fractional free volume is too small to allow large scale molecular motion to occur.

When the temp is further decreased, molecules can not move anymore , thus the fractional free volume is frozen in at the value of fractional free volume (f g).

Free volume theory is useful in interpreting effects of external factors (pressure, mol wt, etc) on Tg.

Concept underlying free vol may not be strictly true.

1. Effect of increasing pressure on Tg

V

P1 < P2

Tg1 Tg2

A Polymer has less free volume at higher pressure.So, a polymer has a higher Tg at higher pressure.


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2. Effect of Mol weight on Tg

Tg

Mol wt

Chain ends are connected only atone side, they have more freedomto move, compared to internalsegments

M1 < M2

Polymer with lower MW has more free volume.

Lower mol wt polymer has a lower Tg

Free volume

Tg1 Tg2Temp

Thermodynamic theory

Glassy state is thermodynamically equilibrium state. But properties like volume, enthalpy, mecha properties of glassy polymer change with time.

Consider phase transition from phase 1 to phase 2.

At phase transition temp T, Gibbs free energy G1 = G2But if their volumes & entropies are not equal, then the phase transition is called 1 st order transition.

Melting

CrystallizationVaporization

Condensation

S = - (dG/ dT) p

V = (dG/dP)T

1st order transition is the phase transition where propertiesrelated to 1 st partial derivatives of G exhibit discontinuities atthe transition temp.


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2nd order transition is the phase transition whereproperties related to 2nd partial derivatives of G exhibitdiscontinuities atthe transition temp.

Cp = Specific heat capacityat constant pressure

Tg is 2nd order transitionTg is the temp where the conformational entropy of apolymer approaches zero

No conformational arrangement will be possible.

Polymer molecules are randomly placed.

Equal probability

Can have any conformation

Trans: Low energyGauche: high energy conformation

Repulsive interaction between segments will makesome bond conformation less accessible effect ofinteraction energy .

At high temp a very great no of conformational statesare accessible to each chain at high T thermalenergy can easily overcome the hindrance to rotationand hence high energy conformation are accessible tothe chain..No of ways of arranging the polymermolecules and holes are large.

As T is lowered no of accessible conformation is ,drastically reduced (high energy conformations are nolonger accessible).When temp is reduced to Tg only ONE conformationis accessible to the chain. So during glass transitionfrom rubbery to glassy state occurs , Glassy stateshould be an eqlb state whose properties will notchange with time.

Criticism: 1. Glass state is a nonequilibrium state,rather than equilibrium state. Properties change withtime.

Vol

Cooling a polymer to Ta (below Tg)

Volume relaxation or physical ageing

2. Value of measured Tg is dependent on rate atwhich the experiment is done.Higher Tg with faster cooling rate.

Temp


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Kinetic theory

If Tg is measured in infinitely slow rate, it would be true 2nd order transition. But such extremely slow experiment is impossible to conduct.

Change of molecular mobility with temp

A system has an eqlbm state at every temp. If we cool a polymer from 100 degree C to 99 degree C polymer molecules will rearrange themselves to reach eqlbm state at 99 degree C.

Relaxation time: time reqd for polymer molecules to move to reach their eqlbm state at every temp.

At higher temp, molecules have higher mobility, so time reqd to reach eqlbm state is shorter.

Temp 100 95 91 90 89 88 85 79

Relax time

0.01 sec

1 sec

40sec

2 min

5min

18min

5hours

1 year

Structural parameters affecting Tg

1. Chain flexibility:stiffer chain has higher difficulty to performmotion (less free volume), hence has higher Tg.

O O PEEK

-(CH2-CH2)n- PE Tg = 145 oCo -

2. Side groups : Polymer containing bulky side groups willhave more difficulty to move chains (less free vol), hencehigher Tg

-(CH2-CH2)n--(CH2-CH)n-

Tg = - 100 oC Tg = 100 oC

3. Chain branching:

Branched chains polymers will have more chain ends,hence free volume of the molecule will be more, so Tgwill be less.

Chain branching will make the branch points in themolecule less mobile, as internal segments areconnected to each other. So free vol will be less, andhigher Tg.

4. Cross linking:


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1. One students was trying to measure % crystallinity byusing FTIR, DSC, XRD. He got different values usingdifferent technique. Explain Why?

2. Mention other methods for estimating % crystallinity

3. Tg of fibres can vary with RH (0% to 100%). Out of thefollowing fibres, which fibre will show this behaviour?

Polypropylene, cotton, nylon, polyester

Methods of Fibre preparation

1.Dry spinning: Solution of cellulose nitrate in aalcohol/ether solvent. Solidification by solventevaporation

wide range of properties depending on the processing conditions

2.Viscose : Wet spinning: solidification by chemicalcoagulation

3. PET, PP, Nylon: Melt spinning: solidification bycooling

Liquid crystal Conventional (PET) melt spg

Solution

r

Nematic structureLow entropy Random coil

High entropy

Structure formation during spinning

i

Solidstate

Extended chain structureHigh chain continuityHigh mechanical properties

Folded chain structureLow chain continuityLow mechanical properties

Dilute solution of (super) high molecular weight PEextruded into water by wet spinning, so that gel likefibers are formed.

Then hot drawing is applied (30 times in length)

Gel spinning

Classical PE drawn only up to 10 times in length.

Dyneema, Spectra, Tekmilon.


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Diameter

Birefriengence

Natural draw ratio

Maximum draw ratio

Stress

Drawing : as-spun fibres have poor elasticity, they undergo plasticdeformation on application of low stress. Polymer chains are in partiallyfolded conformation, can extend easily.

Heat setting: Polymer chains are unfolded, improved, more elasticnetwork of polymer is formed. Polymer chains assume extendedconformation, do not recover back to their original state when stress isreleased.

y zpoint Distance from spinneret

Elongation %

Nylon-6single phase rubber like amorphous network, having rubber likedeformation

Winding, conditioning:1. Orientation of crystalline phase: highly oriented amorphoussegments,

molecular chains get oriented in axial directionIm rovement in birefrin ence

2. substantial amount of crystallinity develops,even at low winding speed

Developed crystals are of alpha-type

-form : monoclinic unitcell(a = 9.56 ,b = 17.24 (fiber axis),c = 8.01 and = 67.5)with eightmonomeric units, andconsists of an extended-

sheet structure withhydrogen bonding betweenantiparallel chains

-form : monoclinic unit cell(a = 9.33 ,b = 16.88 (fiber axis),c = 4.78 and = 121 ).

Low speed spg: -crystalsHigh speed spg: -crystals

Drawing is easier as -aredeformable-form is more stable

Polyethylene fibersDyneema or SpectraOrientation > 95%Crystallinity up to 85%

Normal PEOrientation lowCrystallinity < 60%

The theoretical elastic modulus of the covalent C-C bond in the fullyextended PE molecule is 220 Gpa .

Experimental value in PE fibres - 170 Gpa.

Stretching

Entanglement network Fibrillar crystal


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Extended chain polyethylene

minimum chain folding

UHMPE fibre structure: (a) macrofibril consists of array of microfibrils;(b) microfibril; (c) orthorhombic unit cell; (d) view along chain axis

Intensity of deformationor

Strain= l / L

Elongation

Unstretched length=

Mechanical behaviour of polymers

1. Stress-Strain curve

Load intensity or Stress = Force P / cross sec area A

Rigid plastic Glassy

Flexible plasticleather like

Elastomers

Rubber like

Stress

Strain %

For small values of strain, stiffness or Youngs modulus (E) isequal to the tangent of the stress strain curve

E = tan

Adv:1.Stiffness, stress at which fracture occurs.2. relation between force and deformation at every point.

Limitation:1.Applicable to the particular mode of loading.

Diff in mechanical behaviour due to the mode of loading (tensionvs compression) are not so serious at small deformations but issignificant at large deformation.

1. Load: application of a load to a specimen in itsaxial direction causes a tension to be developed in thespecimen. The load may be expressed in Newtons (N)or in gm force.2. Breaking load: This is the load at which thespecimen breaks, usually expressed in gms, lbs orNewton.3. Stress: Load

Area of cross section dyn/ cm2, N/m2 or Pascal4. Specific stress: Cross section of many fibres areirregular in shape, and difficult to measure the area.Specific stress = Load / linear densityGm per tex or gm per denier


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5. Breaking length: length of the fibre which will justbreak under its own weight when hung vertically.

6. Strain: when a load is applied to a specimen acertain amount of stretching or elongation takes place.

Strain = Elongation / Initial length

7. Initial modulus: slope of the stress-strain curve atthe origin

2. Modulus temperature curves

Polymeric fibres are Viscoelastic, i.e. their mechanicalproperties depend on rate of loading or on time under load. Soit is necessary to specify the time interval under load at whichthe E-T data were obtained. 10 sec

1Tg

2 2

3

4 4

Tm

Crosslinked

Uncrosslinked

Amorphous

To

C

E

N/m2

Region 1: Glassy2. Transition or leatherlike3. Elastomeric orrubber like4. Liquid flow

E-T curves of amorphous (atactic),semicrystalline (isotactic) polystyrene

3. Regions of mechanical equivalence

Polymer at a given temp can be said to be in one of fourregions of mechanical equivalence.

(i)Glassy region: Modulus E takes values in the range 10 9 1010 N/m2 in this region which is found below Tg. A polymerbelow Tg is stiff, hard, brittle.

For many applications involving small deformations, polymersin region 1 can be designed by use of the theory of Linearelasticity.

This theory assumes that the stress-strain relation is linear and

that recovery of original shape, following unloading, isinstantaneous and complete, occuring along the identicalstress-strain path which was traversed during loading.

(ii) Transition region: The modulus of amorphous polymerdrops from about 10 9 106 N/m2 as the material is heatedabove Tg in the transition region between glassy (region 1)and rubber-like (region 3) behaviour.

The modulus of semicrystalline polymer drops only fromabout 109 107.5 N/m2 as the material . is heated from Tg toTm. Mechanical properties of semicrystalline polymer in

.

Polymers in region 2 are typically ductile. Their mechbehaviour is strongly time-dependent. So the branch ofmechanics which may be used to describe small-deformation behaviour in region 2 is the theory of linearviscoelasticity, a modification of theory of linearelasticity which accounts for time effects.


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(iii) Elastomeric (rubber like) region: Rubber like behavior ismonopolized by amorphous polymers. Crystallites stiffen thestructure and tie down macromolecules, thereby preventingtheir large-scale unfolding.

Characteristics of region 3: Extension 500-1000 %, moduli 10 5 106 N/m2

Crosslinked rubbers are capable of recovering their originalshape fully after being released from very large extensions ofindefinite duration. Uncrosslinked rubbers may recover fullyonly if stretched to a small extent over a short period of time.

Modulus of uncrosslinked suffers a dramatic drop with temp, &the polymer flows like a viscous liquid (Region 4). Butcrosslinking preserves the modulus almost intact until the temp

level above which 3D network undergoes chemicaldecomposition.

(iv) Liquid flow region: Crosslinked polymer decomposes ifheated to a high temp but it does not undergo flow.

But uncrosslinked amorphous polymer gradually loses itsability to recover from deformation while undergoingpermanent deformation (flow) as the temp is increased. At hightemp, amorphous polymer in region 4 behaves as a liquid withvery high viscosity, very limited elasticity.

When heated above Tm, semicrystalline polymers melt andbehave similarly to amorphous polymer in region 4.

In general, polymer fluids are non-Newtonian i.e., their flowproperties can be characterized by a nonlinear relationbetween shear stress and flow rate.

Monomer

T o C

E

N/m2 Mn = 140,000

Mn = 217,000

To C

E

N/m2

Crosslinkdensity

str property course 2012.pdf

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