CRYSTALLINITY IN POLYMERS Solids CRYSTALLINE SOLIDS A crystal

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CRYSTALLINITY IN POLYMERS

CRYSTALLINITY IN POLYMERS

Solids • CRYSTALLINE SOLIDS : A crystal or crystalline solid is a solid material

Solids • CRYSTALLINE SOLIDS : A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. • AMORPHOUS SOLIDS : An amorphous solid is any non-crystalline solid in which the atoms and molecules are not organized in a definite lattice pattern. Such solids include glass, plastic, and gel.

Basic difference CRYSTALLINE SOLIDS • They have characteristic geometrical shape • T hey have

Basic difference CRYSTALLINE SOLIDS • They have characteristic geometrical shape • T hey have sharp melting point • Physical properties of crystalline solids are different in different directions. This phenomenon is known as Anisotropy. • When crystalline solids are rotated about the axis their appearance does not change. This shows that they are symmetrical. • Crystalline solids cleavage along particular direction at fixed cleavage planes. AMORPHOUS SOLIDS • Solids that don't have definite geometrical shape • They melt over a wide range of temperature • Physical properties of amorphous solids are same in different direction. amorphous solids are isotropic • Amorphous solids are unsymmetrical. • Amorphous solids don't break at fixed cleavage planes.

Introduction • Properties of textile fibers are determined by their chemical structure degree of

Introduction • Properties of textile fibers are determined by their chemical structure degree of polymerization, orientation of chain molecules, crystallinity, package density and cross linking between individual molecules. • Polymer crystallinity is one of the important properties of all polymers. Polymer exists both in crystalline and amorphous form Figure shows how the arrangement of polymer chain forming crystalline and amorphous regions. • It can be seen that part of molecules are arranged in regular order, these regions are called crystalline regions. In between these ordered regions molecules are arranged in random disorganized state and these are called amorphous regions. • Crystallinity is indication of amount of crystalline region in polymer with respect to amorphous content.

Amorphous Crystalline

Amorphous Crystalline

Why plastics are transparent, opaque and translucent? Semicrystallin e Amorphous Crystalline region

Why plastics are transparent, opaque and translucent? Semicrystallin e Amorphous Crystalline region

Why plastics are transparent, opaque & translucent? • Plastics are transparent due to amorphous

Why plastics are transparent, opaque & translucent? • Plastics are transparent due to amorphous region • Plastics are translucent due to crystalline regions • Opaque plastics are due to crystalline region • Translucency and opaqueness are due to the amount of crystallinity

Characteristics of amorphous polymers • Relatively low resistance to heat • Soften gradually as

Characteristics of amorphous polymers • Relatively low resistance to heat • Soften gradually as temperature rises • Translucent /transparent Little shrinkage as they cool • Tough at low temperatures • Low dimensional stability (easily deformed) • Tendency to creep Crystalline polymers show opposite characteristics

Properties of Polymers • Mechanical properties : Formation of spherulites affects many properties of

Properties of Polymers • Mechanical properties : Formation of spherulites affects many properties of the polymer material; in particular, crystallinity, density, tensile strength and. Young's modulus of polymers increase during spherulization. This increase is due to the lamellae fraction within the spherulites, where the molecules are more densely packed than in the amorphous phase. • Optical properties : Spherulites can scatter light rays and hence the transparency of a given material decreases as the size of the spherulites increases. Alignment of the polymer molecules within the lamellae results in birefringence producing a variety of colored patterns when spherulites are viewed between crossed polarizers in an optical microscope.

Properties affected by Crystallinity • HARDNESS : The more crystalline a polymer, the more

Properties affected by Crystallinity • HARDNESS : The more crystalline a polymer, the more regularly aligned its chains. Increasing the degree of crystallinity increases hardness and density. • YOUNG’S MODULUS : There is steep increase in young's modulus with increase in amount of crystalline component in the sample. • TENSILE STRENGTH : This property is directly proportional to the crystalline structure of a component. • PERMEABILITY : Crystalline polymers are far less permeable than the amorphous variety. It means as the polymer crystallinity increases with decrease in permeability.

List of Polymers • Crystalline Polymers – – Syndiotactic PP Nylon Kevlar Polyketones •

List of Polymers • Crystalline Polymers – – Syndiotactic PP Nylon Kevlar Polyketones • Amorphous Polymers – PMMA – PS (Atactic) – Poycarbonate – Polybutadiene – Polyisoprene

 • No polymer is completely crystalline. If you're making plastics, this is a

• No polymer is completely crystalline. If you're making plastics, this is a good thing. Crystallinity makes a material strong, but it also makes it brittle. A completely crystalline polymer would be too brittle to be used as plastic. The amorphous regions give a polymer toughness, that is, the ability to bend without breaking. • Amorphousness and Crystalline

Crystallinity refers to the degree of structural order of a solid. In a crystal,

Crystallinity refers to the degree of structural order of a solid. In a crystal, the arrangement of atoms or molecules is consistent and repetitive. Many materials such as glass ceramics and some polymers can be prepared in such a way as to produce a mixture of crystalline and amorphous regions.

Crystallisabilty • Crystallisabilty is the maximum crystallinity that a polymer can achieve at a

Crystallisabilty • Crystallisabilty is the maximum crystallinity that a polymer can achieve at a particular temperature, regardless of the other conditions of crystallization. • Crystallisablity at a particular temperature depends on the chemical nature of the macromolecular chain, its geometrical structure, molecular weight and molecular weight distribution

Degree Of Crystallinity • The degree of crystallinity is defined as the fractional amount

Degree Of Crystallinity • The degree of crystallinity is defined as the fractional amount of polymer that is crystalline and it is either expressed in terms of the mass fraction or the volume fraction. • For semi-crystalline polymers, the degree of crystallinity is one of its most important physical parameters since it reflects the sample’s morphology and determines various mechanical properties, such as the Young modulus, yield stress as well as the impact strength. • Differential scanning calorimetry is widely used to determine the amount of crystalline material. It can be used to determine the fractional amount of crystallinity in a polymer sample. Other commonly used methods are X-ray diffraction, density measurements, and infrared spectroscopy.

Crystallographic structures of polymers • Configuration: defined in terms of its chemical repeat unit

Crystallographic structures of polymers • Configuration: defined in terms of its chemical repeat unit and a statement of molecular architecture • Local conformation: refer to geometrical arrangements of neighboring groups in molecule, which can be altered only by rotation about primary valence bonds • Molecular packing: refer to the arrangement of the molecules in the crystal in terms of unit cell and its contents

CRYSTALLIZATION MECHANISMS • Crystallization from melt • Crystallization from solution evaporation • Crystallization during

CRYSTALLIZATION MECHANISMS • Crystallization from melt • Crystallization from solution evaporation • Crystallization during stretching

CRYSTALLIZATION FROM SOLUTION • Polymers can also be crystallized from a solution or upon

CRYSTALLIZATION FROM SOLUTION • Polymers can also be crystallized from a solution or upon evaporation of a solvent. This process depends on the degree of dilution. • In dilute solutions, the molecular chains have no connection with each other and exist as a separate polymer coils in the solution. • Increase in concentration which can occur via solvent evaporation, induces interaction between molecular chains and a possible crystallization as in the crystallization from the melt. • Crystallization from solution may result in the highest degree of polymer crystallinity.

Crystallization from melt • Ordered structure from disordered structure • When polymer melt is

Crystallization from melt • Ordered structure from disordered structure • When polymer melt is cooled to or below the melt tend to align and form small ordered regions (nuclei) • The small nuclei grow by addition of more chains • Nucleation can be homogeneous-small nuclei form randomly throughout the melt and heterogeneous. Nuclei formation on foreign bodies (dust particles etc. ). • Number of nuclei formed depends on the temperature of crystallization : At low undercooling sporadic nucleation , few large spherulites are formed • At high undercooling, large nulei are formed , many smaller spherulites are formed.

Crystallization during stretching Stress induced crystallization Amorphous polymers can be made partially crystalline by

Crystallization during stretching Stress induced crystallization Amorphous polymers can be made partially crystalline by stretching

POLYMER CRYSTALLISATION • Crystallization of polymers is a process associated with partial alignment of

POLYMER CRYSTALLISATION • Crystallization of polymers is a process associated with partial alignment of their molecular chains. • These chains fold together and form ordered regions called lamellae, which compose larger spheroidal structures named spherulites. • Polymers can crystallize upon cooling from the melt, mechanical stretching or solvent evaporation. Crystallization affects optical, mechanical, thermal and chemical properties of the polymer.

FOLDING OF CHAIN DURING CRYSTAL FORMATION A single polymer chain may be partly in

FOLDING OF CHAIN DURING CRYSTAL FORMATION A single polymer chain may be partly in a crystalline lamella, and partly in the amorphous state. Some chains even start in one lamella, cross the amorphous region, and then join another lamella. These chains are called tie molecules. • For a standard polymer, the lamellar thickness is around 100 Å and the molecular chain length is around 1000 to 10000 Å. • The accommodation of the long chain into the narrow lamella is by assuming that chain folding takes place during the process of crystallization. • Many experimental techniques such as electron diffraction prove beyond any reasonable doubt that the chains in a crystal are folded and oriented perpendicular to the plane of the polymer crystal lamella.

Crystallization of polymers • Crystal form by folding of polymer chains • Chains are

Crystallization of polymers • Crystal form by folding of polymer chains • Chains are much longer than the dimensions of the crystals they belong to. • Same chain can pass through many crystals • Re 0 entry of chain in the same crystal depends on chain flexibilty • Small & straight chain tends to form crystals more easily than long chain Coiled & entangled

Polymers form stacks of these folded chains. There is a picture of a stack,

Polymers form stacks of these folded chains. There is a picture of a stack, called a lamella, right below.

Of course, it isn't always as neat as this. Sometimes part of a chain

Of course, it isn't always as neat as this. Sometimes part of a chain is included in this crystal, and part of it isn't. When this happens we get the kind of mess you see below. our lamella is no longer neat and tidy, but sloppy, with chains hanging out of it everywhere!

Polymers crystallised from melts Polymers crystallized from the melt seem to maintain the two

Polymers crystallised from melts Polymers crystallized from the melt seem to maintain the two most prominent structural features of single polymer crystals: aggregates of 100 Å-thick lamellae of different degree of perfection are observed, and the chains are oriented perpendicular to the face of the lamellae so that chain folding must also be inherent in melt-crystallized materials

Formation of spherulites Crystallization from Melt In concentrated solution and melts , polymer chain

Formation of spherulites Crystallization from Melt In concentrated solution and melts , polymer chain are entangled Crystallization can result in polymer chain being incorported in more than one crystal In the melt chains are highly entangled Spherulites are formed by nucleation at different points and then grow

Models to explain crystallinity • Lamellar structures – Fringed micelle model – Flory Switch

Models to explain crystallinity • Lamellar structures – Fringed micelle model – Flory Switch board model • Spherulite structures • Helical structures • Single Crystal

Lamellar structure • Lamellar structures or microstructures are composed of fine, alternating layers of

Lamellar structure • Lamellar structures or microstructures are composed of fine, alternating layers of different materials in the form of lamellae. • Such conditions force phases of different composition to form but allow little time for diffusion to produce those phases equilibrium compositions. • Fine lamellae solve this problem by shortening the diffusion distance between phases, but their high surface energy makes them unstable and prone to break up when annealing allows diffusion to progress.

Fringed micelle model • X-ray diffraction showed their dimensions to be on the order

Fringed micelle model • X-ray diffraction showed their dimensions to be on the order of several hundred Angstroms • The crystallites were though to serve as mechanical crosslinks and to affect the physical properties in much the same way as chemical crosslinks in vulcanized rubber

Fringed Micelle Model The molecules passed successively through a number of these crystalline and

Fringed Micelle Model The molecules passed successively through a number of these crystalline and intervening amorphous region The crystallites were pictured as sheaves of chains aligned in a parallel fashion X-ray diffraction pattern showed their dimensions to be on the order of several hundred angstroms

Amorphous Crystalline

Amorphous Crystalline

Flory Switch Board Model Regular folding : Adjacent re-entry Irregular folding: Random re-entry

Flory Switch Board Model Regular folding : Adjacent re-entry Irregular folding: Random re-entry

HELICAL STRUCTURES • To facilitate closer packing of molecules in the crystalline phase ,

HELICAL STRUCTURES • To facilitate closer packing of molecules in the crystalline phase , many polymers tend to assume a helical structure. • Isotactic vinyl polymers has helical structures. • Helical structure has a special significance in polymers of biological origin. • DNA structure also have helical structures. • This DNA structures was determined by Watson and Crick. • Hydrogen bonding plays an important role in the formation of the double helix of the DNA molecules.

SPHERULITES • Spherulites are spherical semicrystalline regions inside nonbranched linear polymers. • Their formation

SPHERULITES • Spherulites are spherical semicrystalline regions inside nonbranched linear polymers. • Their formation is associated with crystallization of polymers from the melt and is controlled by several parameters such as the number of nucleation sites, structure of the polymer molecules, cooling rate, etc. • Spherulites are composed of highly ordered lamellae, which result in higher density, hardness, but also brittleness of the spherulites as compared to disordered polymer. • The lamellae are connected by amorphous regions which provide certain elasticity and impact resistance.

Spherulite • Alignment of the polymer molecules within the lamellae results in birefringence producing

Spherulite • Alignment of the polymer molecules within the lamellae results in birefringence producing a variety of colored patterns. • Birefringence is the optical property of a material having a refractive index that depends on the polarization and propagation direction of light. • If a molten polymer such as polypropylene is made into thin film between to hot glass plates and cooled, it is seen that, from different nucleation centres, spherulites are formed

POLYMER CRYSTALLINITY MEASUREMENTS • BY DIFFERENTIAL SCANNING CALORIMERTY (DSC) • BY X-RAY DIFFRACTION (XRD

POLYMER CRYSTALLINITY MEASUREMENTS • BY DIFFERENTIAL SCANNING CALORIMERTY (DSC) • BY X-RAY DIFFRACTION (XRD

X-RAY DIFFRACTION(XRD) • X-Ray diffraction is also used to measure the nature of polymer

X-RAY DIFFRACTION(XRD) • X-Ray diffraction is also used to measure the nature of polymer and extent of crystallinity present in the Polymer sample. • Crystalline regions in the polymer seated in welldefined manner acts as diffraction grating. • So the Emerging diffracted pattern shows alternate dark and light bands on the screen. • X-ray diffraction pattern of polymer contain both sharp as well as defused bands. • Sharp bands correspond to crystalline orderly regions and defused bands correspond to amorphous regions

XRD • Crystalline structure is regular arrangement of atoms. Polymer contains both crystalline and

XRD • Crystalline structure is regular arrangement of atoms. Polymer contains both crystalline and amorphous phase within arranged randomly. • When beam of X-ray passed through the polymer sample, some of the regularly arranged atoms reflect the x-ray beam constructively and produce enhanced intense pattern. • Amorphous samples gives sharp arcs since the intensity of emerging rays are more, where as for crystalline samples, the incident rays get scattered. • Arc length of diffraction pattern depends on orientation. If the sample is highly crystalline, smaller will be the arc length

Crystallinity Calculations • The crystallinity is calculated by separating intensities due to amorphous and

Crystallinity Calculations • The crystallinity is calculated by separating intensities due to amorphous and crystalline phase on diffraction phase. • Computer aided curve resolving technique is used to separate crystalline and amorphous phases of diffracted graph. • After separation, total area of diffracted pattern is divided into crystalline (Ac) and amorphous(Aa). • Small Angle X-ray Scattering (SAXS), Infrared Spectroscopy, can also be used to measure crystallinity. • Percentage of crystallinity Xc % is measured as ratio of crystalline area to total area. Xc = Ac /(Ac +Aa) Ac = Area of crystalline phase Aa = Area of amorphous phase

Single crystals or linear PE • All polymer single crystals seem to have the

Single crystals or linear PE • All polymer single crystals seem to have the same general appearance and structure. In their simplest form, they appear in the electron microscope as thin, flat platelets on the order of 100 to 200 Å thick and several microns in lateral dimensions

Arrangement of chains in unit cell of Polyethylene • Parallelepiped with axes a, b,

Arrangement of chains in unit cell of Polyethylene • Parallelepiped with axes a, b, c (T dependent) and angle α, β, γ • a = 7. 41 Å b = 4. 94 Å c = 2. 55 Å(chain axis) • α=β=γ= 90 º • Length of c-axis = crystallographic repeat unit

Factors affecting crystallinity • Length of chain (structure of polymers) Long chains with high

Factors affecting crystallinity • Length of chain (structure of polymers) Long chains with high degree of polymerization are less likely to crystallize More likely to get entangled and form amorphous regions: Linear and branched chains: Chain flexibilty; polar groups Side chains Intermolecular interactions Tacticity

Tacticity & Copolymerization Tacticity • Atactic : Crystallinity not possible • Syndiotactic: Crystalline polymers

Tacticity & Copolymerization Tacticity • Atactic : Crystallinity not possible • Syndiotactic: Crystalline polymers • Isotactic : Crystalline polymers Copolymers: • Random: not crystalline • Block: not crystalline • Graft: Not crystalline • Alternate: Crystalline

Effect of polymer structure on Melting temperatures (Tm) • Backbone stiffness : Increases melting

Effect of polymer structure on Melting temperatures (Tm) • Backbone stiffness : Increases melting temperatures Tm (ºC) -CH₂137 -146 -CH₂-CO-O- 122 -CH₂-O 67 -CH₂- Ph-CH₂397

Effect of polymer structure on Melting temperatures • Backbone symmetry and pendant group regularity

Effect of polymer structure on Melting temperatures • Backbone symmetry and pendant group regularity on Tm (ºC) - {CH₂-CH X}n. For X equal to - H 137 -146 - CH₃ 187 - CH₂-CH₃ 125 - CH₂-CH₃ 78 - CH₂-CH(CH₃ )₂ 235 - Ph 240

Effect of polymer structure on Melting tempertures (Tm) • Polarity : Increases melting temperatures

Effect of polymer structure on Melting tempertures (Tm) • Polarity : Increases melting temperatures Tm (ºC) -CH₂- C 0 -NH 330 -CH₂ -CO-NH 260 -CH₂-CO-NH 257 -Nylon 6, 6 267 Nylon 6, 10 222