CHAPTER FOUR edited.pdf
- 1. CHAPTER FOUR crystalline and amorphous morphology of polymer
- 2. Amorphous polymer Morphology The bulk state, sometimes called the condensed or solid state, includes both amorphous and crystalline polymers Morphology : is a term used to describe the form or structure of the polymer chains of thermoplastic materials when they are in their frozen or solid state. For thermoplastic resins, there are two basic morphologies: AMORPHOUS and SEMI-CRYSTALLINE Amorphous polymers appear random and jumbled when allowed to cool in a relaxed state. They appear very similarly to their molten state, only the molecules are closer together. They can be described as being similar to a large pot of spaghetti noodles.
- 3. Amorphous polymer Morphology Amorphous materials are like cooked ramen noodles in that there is a random arrangement of the molecules and there are no crystals present to prevent the chains from flowing
- 4. Amorphous polymer Morphology It is important to remember that both materials have the random, unordered arrangement when molten. amorphous polymers exhibit different physical and mechanical behavior. Depending on temperature and structure, At low temperatures, glassy, hard, and brittle. As the temperature is raised, they go through the glass–rubber transition. Above Tg, cross-linked amorphous polymers exhibit rubber elasticity. An example is styrene–butadiene rubber (SBR) An amorphous polymer does not exhibit a crystalline X- ray diffraction pattern, and it does not have a first-order melting transition.
- 5. Amorphous polymer Morphology Example : Polyvinyl Chloride (PVC) General Purpose Polystyrene (GPPS) Polycarbonate (PC) Polymethylmethacrylate (PMMA or Acrylic) Acrylonitrile Butadiene Styrene (ABS a terpolymer)
- 6. Amorphous polymer Morphology Older literature referred to the amorphous state as a liquid state. However, polystyrene or poly(methyl methacrylate) at room temperature are glassy. Today, amorphous polymers in the glassy state are better called amorphous solids. >Tg, if the polymer is amorphous and linear, it will flow, albeit the viscosity may be very high. For most materials, we are concerned with the melting point and boiling point.
- 7. Glass Transition Temperature (Tg) The glass transition temperature (Tg) is defined as the temperature at which the polymer softens because of the onset of long-range coordinated molecular motion. For thermoplastic materials, we are concerned with: o Glass Transition Temperature o Melting Temperature In amorphous materials, it is the temperature at which material behaves more rubber-like than glass-like. Above Tg: The material stretches further when pulled (more ductile) The material absorbs more impact energy without fracturing when struck When the material does fail, it fails in a ductile manner as opposed to a brittle manner
- 8. The sample experienced a brittle failure The material broke like glass The sample broke in a ductile manner. The material yielded (stretched) before failure. The material behaved more like a rubber
- 9. Glass Transition Temperature (Tg) example: Polyethylene and Polypropylene both have low Tg’s. They are way below room temperature. That is why milk jugs and yogurt containers are flexible when you take them out of the refrigerator. Amorphous materials don’t truly have a Tm. They just continue to soften more until they behave more like a liquid. The molecules absorb enough energy and move far enough apart (increase the free volume) that the material can flow.
- 11. Melt Temperature (Tm) When we refer to the melt temperature for amorphous materials, it is usually the temperature at which we can process it. For S/C materials, the Tm is the temperature at which the crystals melt. If the polymer is crystalline Tm>Tg ideal temperature for growing crystals is approximately 2/3 of the way between the Tg and the Tm. Not in all cases, but in many, the degradation temperature for S/C materials is not much higher than the melt temperature.
- 12. Melting vs. Glass Transition Temp. What factors affect Tm and Tg? Both Tm and Tg increase with increasing chain stiffness Regularity (tacticity) affects Tm only
- 13. • The mechanical properties of polymers are sensitive to temperature changes Figure 1 Modulus versus temperature for an amorphous thermoplastic
- 14. Figure 2. Modulus versus temperature for crystalline thermoplastics
- 15. Figure 3. Modulus versus temperature for thermosetting resins
- 16. Figure 4 Modulus versus temperature for a reinforced crystalline thermoplastic
- 17. crystalline state • Both amorphous and crystalline areas in can exist in the same polymer. • Areas in polymer where chains packed in regular way. • X-ray scattering and electron microscopy have shown that the crystallites are made up of lamellae which,in turn, are built-up of folded polymer chains Schematic representation of (a) fold plane showing regular chain folding, (b) ideal stacking of lamellar crystals, (c) interlamellar amorphous model (d) fringed micelle model of randomly distributed crystallites (Plastic Technology Handbook)
- 18. crystalline state • The crystalline state is defined as one that diffracts X- rays and exhibits the first-order transition known as melting. A first-order transition normally has a discontinuity in the volume– temperature dependence crystalline region amorphous region
- 19. crystalline state Polymers crystallized in the bulk, however, are never totally crystalline, a consequence of their long-chain nature and subsequent entanglements. Crystallinity occurs when linear polymer chains are structurally oriented in a uniform three dimensional matrix. Three factors that influence the degree of crystallinity are: i) Chain length ii) Chain branching iii) Interchain bonding iv) the rate of cooling during solidification
- 20. Melting temperature observing Non regularity of structure first decreases the melting temperature and finally prevents crystallinity. crystalline materials have sharp X-ray pattern characteristic at Tm Ideally, the melting temperature constitutes a first order phase change, should give a discontinuity in the volume, with a connected sharp melting point. Due to polymer imperfections or very small size of the crystallites in bulk most polymers melt over a range of several degrees
- 21. Specific volume–temperature relations for linear polyethylene
- 22. The melting temperature is usually taken as the temperature at which the last trace of crystallinity disappears. melting temperature can be determined thermally. using the differential scanning calorimeter (DSC) gives the heat of fusion as well as the melting temperature Specific volume–temperature relations for linear polyethylene. Open circles, specimen cooled relatively rapidly from the melt to room temperature; solid circles, specimen crystallized at 130°C for 40 days, then cooled to room temperature
- 23. methods for determining the percent crystallinity most crystallizing polymers are semicrystalline; that is, a certain fraction of the material is amorphous, while the remainder is crys-talline The reason why polymers fail to attain 100% crystallinity is kinetic, resulting from the inability of the polymer chains to completely disentangle and line up properly in a finite period of cooling or annealing. 1. Determination of the heat of fusion of the whole sample by calorimetric methods such as DSC
- 24. DSC of a commercial isotactic polypropylene sample scanning calorimetry (DSC) which measures the heat flow into or from a sample as it is either heated, cooled heat of fusion ∆Hf is the area under the peak
- 25. 2. Determination of the density of the crystalline portion via X-ray analysis of the crystal structure, and determining the theoretical density of a 100% crystalline material.
- 26. ρ exptl experimental density ρ amorph density ρ100% cryst crystalline portions density
- 30. METHODS OF DETERMINING CRYSTAL STRUCTURE There are four basic methods in wide use for the study of polymer crystallinity: X-ray diffraction, electron diffraction, infrared absorption, and Raman spectra X-Ray Methods crystalline substances must to act as a three-dimensional diffraction grating for X-rays Bragg equation : By considering crystals as reflection gratings. λ the X-ray wavelength θ the angle between the X-ray beam and these atomic planes n the order of diffraction
- 31. X-Ray Methods d and λ are of the order of 1 Å. Such an analysis from a single crystal produces a series of spots. However, not every crystalline substance can be obtained in the form of macroscopic crystals. This led to the Debye–Scherrer for powdered crystalline solids or polycrystalline specimens. The crystals are oriented at random so the spots become cones of diffracted beams. can be recorded either as circles on a flat photographic plate or as arcs on a strip of film encircling the specimen
- 32. X-Ray Methods
- 33. The angle RSX is 2θ, where θ is the angle of incidence on a set of crystal plane X-Ray Methods
- 34. X-Ray Methods Diffraction spot or line depends on the scattering power of the individual atoms( the number of electrons in the atom) the arrangement of the atoms with regard to the crystal planes the angle of reflection the number of crystallographically equivalent sets of planes contributing the amplitude of the thermal vibrations of the atoms. intensities of the spots or arcs and their positions are required to calculate the crystal lattice
- 35. Electron Diffraction of Single Crystals Electron diffraction studies utilize single crystals. Since the polymer chains in single crystals are most often oriented perpendicular to their large flat surface, diffraction patterns perpendicular to the 001 plane are common. Tilting of the sample yields diffraction from other planes. The interpretation of the spots obtained utilizes Bragg’s law in a manner identical to that of X-rays.
- 36. Electron Diffraction of Single Crystals Required: Evacuated diffraction tube that contains an electron gun accelerating anode to provide a known energy to the electrons in the beam crystalline targets and screen
- 37. Infrared Absorption The information that infrared absorption spectra yield about crystallinity: 1. “crystallization sensitive bands.” The intensities of these bands vary with the degree of crystallinity and have been used as a measure of the crystallinity. 2. By measuring the polarized infrared spectra of oriented semicrystalline polymers, information about both the molecular and crystal structure can be obtained. Both uniaxially and biaxially oriented samples can be studied. 3. The regular arrangement of polymer molecules in a crystalline region can be treated theoretically, utilizing the symmetry properties of the chain or crystal
- 38. Raman Spectra 1. Since the selection rules for Raman and infrared spectra are different, Raman spectra yield information complementary to the infrared spectra. example, the S—S linkages in vulcanized rubber and the C=C bonds yield strong Raman spectra but are very weak or unobservable in infrared spectra. 2. Since the Raman spectrum is a scattering phenomenon, whereas the infrared methods depend on transmission, small bulk, powdered, or turbid samples can be employed. 3. the Raman spectra provide information equivalent to very low- frequency measurements, even lower than 10 cm-1. Such low frequency studies provide information on lattice vibrations.
- 39. Polymer Single Crystals crystallization is an allayment of molecular chain and folding of chain to get order region polymer single crystal formation 1) From precipitate (from dilute solutions, they form lamellar-shaped single crystals) 2) From melt
- 40. Polymer Single Crystals 1) From precipitate Ideas about polymer crystallinity start by preparing single crystals of polyethylene. These were made by precipitation from extremely dilute solutions of hot xylene. These crystals tended to be diamond-shaped and of the order of 100 to 200 Å thick
- 41. 2. CRYSTALLIZATION FROM THE MELT usually super cool to greater or lesser extents crystallization temperature may be 10 to 20°C lower than the melting temperature Supercooling arises from the extra free energy required to align chain segments spherulites are really spherical in shape only during the initial stages of crystallization
- 42. Spherulites of low-density polyethylene
- 43. Spherulites of low-density polyethylene
- 44. The evolution of the spherulite
- 45. KINETICS OF CRYSTALLIZATION Lamellae formation from bulk, are organized into spherulites or their predecessor structures, hedrites volume changes on melting; usually increasing the isothermal crystallization of poly(ethylene oxide) as determined dilatometrically:
- 46. The rate of crystallization increases as the temperature is decreased. This follows from the fact that the driving force increases as the sample is supercooled
- 47. KINETICS OF CRYSTALLIZATION Crystallization rates may also be observed microscopically : by measuring the growth of the spherulites as a function of time The isothermal radial(at 125°C) growth of the spherulites is usually observed to be linear
- 48. KINETICS OF CRYSTALLIZATION The increase in rate of crystallization as the temperature is lowered is controlled by the increase in the driving force temperature is lowered still further, molecular motion becomes sluggish as the glass transition is approached, and the crystallization rate decreases again. linear growth rate versus crystallization temperature for poly(ethylene terephthalate) .Tf = 265°C, and Tg = 67°C, at which points the rates of crystallization are theoretically zero.
- 49. KINETICS OF CRYSTALLIZATION Below Tg, the rate of crystallization effectively becomes zero. rule-of-thumb for determining a good temperature to crystallize a polymer if the melting temperature(Tf) is known. where Tf is in absolute temperature. At (8/9) Tf the polymer is supposed to crystallize readily.
- 50. Free energy of polymer crystallization The classic melting temperature is usually taken where the last trace of crystallinity disappears, point A in Figure b.
- 51. ∆Gf The free energy of fusion ∆Hf the molar enthalpy ∆Sf entropy of fusion At the melting temperature, ∆Gf equals zero, and smaller entropy or a larger enthalpy term raises Tf. the relative changes in ∆Hf and ∆Sf in going from the amorphous state to the crystalline state determine the melting temperature of the polymer Free energy of polymer crystallization
- 52. Properties Amorphous vs. S/C Chemical Resistance Optical Properties Impact Resistance Viscosity Weather Resistance Shrinkage
- 53. Chemical Resistance Plastic materials are used in virtually and contact with a wide variety of chemical substances that they need to resist As a general rule S/C materials are more resistant to chemical attack than amorphous materials. It is more difficult for the chemical media to penetrate the dense crystalline structure to damage the polymer chains. Polyethylene is used to store everything from detergent to mineral spirits to gasoline.
- 54. But Polypropylene is only slightly less chemically resistant than Polyethylene. Of the amorphous materials PVC is probably the best in chemical resistance, mainly due to the large chlorine atom that helps to protect the main polymer chain. Polycarbonate, Acrylic, Polystyrene and the other styrenics are all very susceptible to chemical attack, especially to mineral spirits and solvents like lacquer and paint thinners, alcohol, and gasoline.
- 55. Optical Properties Amorphous materials have a much higher clarity than S/C materials. and can be translucent/optical quality. If the crystallinity is disrupted by adding a copolymer or other additive or by quenching the material so quickly the crystals don’t have enough time to form, the material may appear somewhat clear. Amorphous Acrylic more commonly known as Plexiglas and Polycarbonate used in safety glasses and optical lenses are far superior in terms of optical properties
- 56. Impact Resistance The material structure determines the impact resistance, but as a general rule, S/C materials are more brittle than Amorphous. The chain portions that are folded up in the crystal restrict the polymer chains as they try to move past one another when a force is applied making the S/C materials more brittle. Polycarbonate is used in safety glasses, but General Purpose Polystyrene (GPPS) is very brittle – both are amorphous, but have different polymer structures. On the S/C side, Polyethylene is very ductile at room temperature because it is above its Tg, but Nylon and Polyester are brittle at room temperature.
- 57. Viscosity S/C materials by their very nature flow more easily than Amorphous materials. The same mechanism that allows the material to fold up into dense crystals allows the polymer chains to slide past one another easily in the melted state.
- 58. Weather Resistance The most damaging aspect of weathering is generally considered to be Ultraviolet light. The UV light breaks down the chains of the polymers making them more brittle, causing colors to fade or yellow, and causing additives in the polymers to migrate to the surface (chalking).
- 59. Amorphous polymers have better chemical resistance to weathering effects than S/C polymers. The crystals in the S/C polymers diffract the light so the UV rays spend more time within the polymer structure and do more damage. The clear amorphous polymers allow the damaging radiation to pass through doing less damage.
- 60. Shrinkage Because they fold up into crystal structures, S/C materials have higher shrinkage rates when compared to Amorphous materials. In injection molding most amorphous materials will shrink between 0.003-0.007 in/in (0.3-0.7%) S/C materials shrink differently depending upon the level of crystallinity that they achieve. Some will shrink over 0.025 in/in depending on processing variables, part thickness, and additives.
- 61. Broad soflening range thermal agitation of the molecules breaks down the weak secondary bonds. The rate at which this occurs throughout the formless structure varies producing broad temperature range for softening Sharp melting point the regular close-packed structure results in most of the secondary bonds being broken down at the same time.
- 62. Crystalline vs Amorphous Thermoplastics Crystalline (actually usually semi-crystalline): Atomic bonds regular and repeated Have a defined melting point Tm Can contain some degree of amorphous polymer Usually translucent to opaque
- 63. Amorphous Extensive chain branching All thermosets are amorphous Exhibit glass tranistion temperatures Tg Below Tg, polymer acts stiff and rigid Above Tg, polymer acts soft and rubbery Melt or liquefy over extended temperature range near Tg. Don’t have distinct Tm like crystalline polymers. Thermosetting polymers do not melt but degrade above Tg
- 65. Examples o amorphous and crystalline thermoplastics Amorphous • Polyvinyl Chloride (PVC) • Polystyrene (PS) • Polycarbonate (PC) • Acrylic (PMMA) • Acrylonitrile-butadiene- styrene (ABS) • Polyphenylene (PPO) Crystalline • Polyethylene (PE) • Polypropylene (PP) • Polyamide (PA) • Acetal (POM) • Polyester (PEW, PBTF’) • Fluorocarbons (PTFE, • PFA, FEP and ETFE)