Crystallinity in polymers

well explanation of crystallinity in polymers

1. Crystallinity in polymers Manjinder S ingh SC16M072

2. content : SOLIDS INTRODUCTION DEGREE OF CRYSTALLINITY CRYSTALLISABLITY POLYMER CRYSTALLISATION HELICAL STRUCTURES SPHERULITES LAMELLAR STUCTURES FOLDING OF CHAINS DURING CRYSTAL FORMSTION CRYSTALLIZATION MECHANISMS POLYMER CRYSTALLINITY MEASUREMENTS PROPERTIES AFFECTED BY CRYSTALLINITY

3. 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.

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

5. 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.

6. 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.

7. 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.

8. CRYSTALLISABLITY 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.

9. 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. 

10. Crystallization mechanisms Crystallization by stretching Crystallization from solution

11. Crystallization by stretching Crystallization occurs upon extrusion used in making fibers and films. In this process, the polymer is forced through, e.g., a nozzle that creates tensile stress which partially aligns its molecules. Such alignment can be considered as crystallization and it affects the material properties. Anisotropy is more enhanced in presence of rod-like fillers such as carbon nanotubes, compared to spherical fillers.  Polymer strength is increased not only by extrusion, but also by blow molding, which is used in the production of plastic tanks and PET bottles.

12. Some polymers which do not crystallize from the melt, can be partially aligned by stretching. Some elastomers which are amorphous in the unstrained state undergo rapid crystallization upon stretching.

13. 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.

14. The crystal shape can be more complex for other polymers, including hollow pyramids, spirals and multilayer dendritic structures.  The rate of crystallization can be monitored by a technique which selectively probes the dissolved fraction.

15. 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.

16. D ouble helix of the DNA molecule by Watson-crick

17. SPHERULITES Spherulites  are spherical semicrystalline regions inside non-branched 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.

18. 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 start developing.

19. SPHERULITES formation in self-nucleated trigonal isotactic poly ( 1-butene )

20. 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.

21. 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.

22. Left to right: spherulites; block copolymer microdomains; lamellar crystals; crystalline block unit cell. 

23. Folding of chain during crystal formation 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.

24. schematic representation of chain folding taking place during formation of crystal lamella

25. Polymer crystallinity measurements BY DIFFERENTIAL SCANNING CALORIMERTY (DSC) By X-RAY DIFFRACTION (xrd)

26.  DIFFERENTIAL SCANNING CALORIMERTY (DSC) DSC can be used to determine amount of crystallinity in a polymer. Instrument is designed to measure amount of heat absorbed or evolved from sample under isothermal conditions. DSC contains two pans, one reference pan that is empty and the other pan has polymer sample. In this method polymer sample is heated with reference to a reference pan. Both polymer and the reference pan are heated at same rate. The amount of extra heat absorbed by polymer sample is with reference to reference material

27. DSC curve of a PET bottle sample

28. 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 well-defined 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

29. Schematic diagram of X-ray diffraction pattern

30. 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.

31. X-ray diffraction pattern of (a) amorphous sample and (b) Semi crystalline polymer sample

32. 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 (A c ) and amorphous(A a ). Small Angle X-ray Scattering (SAXS), Infrared Spectroscopy, can also be used to measure crystallinity. Percentage of crystallinity X c % is measured as ratio of crystalline area to total area. X C = A C /(A C +A A ) A C = Area of crystalline phase A A = Area of amorphous phase

33. 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.