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This document provides an overview of techniques for characterizing the structure of semi-crystalline polymers, including WAXS (wide-angle X-ray scattering) and SAXS (small-angle X-ray scattering). WAXS is used to determine the crystalline unit cell structure, while SAXS can measure the lamellar spacing between crystalline and amorphous regions. A correlation function analysis of SAXS data can provide additional structural information beyond just the lamellar spacing, such as the average crystal thickness and interfacial surface area. The degree of crystallinity can be estimated from WAXS, SAXS, or DSC measurements, but accuracy depends on the material's crystallinity level.
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- 2. Semi-crystalline Polymers Most commercial thermoplastic polymers are part amorphous, part crystalline. Semicrystalline. Crystalline lamellae, separated by amorphous regions. Gives the ability to maintain shape but resist impact. On heating the lamellae melt and the polymer flows, usually with a high viscosity due to chain entanglement. 2
- 3. Polymer morphology A semi-crystalline polymer has a hierarchical structure. Some parts of the polymer chain crystallise. These regions give rise to a WAXS (Wide angle X-ray, XRD) diffraction pattern. 3
- 4. WAXS WAXS measures the interatomicspacingswithin the unit cell. The PE unit cell is orthorhombic (all angles 90°all sides different lengths). The plane separation for an orthorhombic cell is given by a,b,care the lengths of the sides. h,k,lare the Miller indices of the planes. Different unit cells have different formulae. 22222221111lckbhadhkl++= 4
- 5. WAXS The degree of crystallinity can be calculated from WAXS. The degree of crystallinity is related to the area under the crystalline and amorphous peaks. 5
- 6. WAXS Therefore WAXS will give you structural information about the unit cell. This is normally prior knowledge as most polymer structures have been determined. Its rare you get a completely new polymer to work on. WAXS gives a check on the structure and points to preferred orientation of the crystal lamella and the degree of crystallinity. To determine this you would run a Rietveldanalysis and possibly pole figures. The performs a fit to the pattern and determines a,b,cand the predicted intensities. 6
- 7. WAXS To perform any meaningful analysis via WAXS the polymer should be very crystalline. 70% is a very crystalline polymer 40% you will struggle to find more than one or two peaks Less than this and you are pushing the boundaries. There are some people who use WAXS to study amorphous systems, but I’m not one of them. 7
- 8. SAXS SAXS is ideal for studying polymers as the length scales of SAXS overlap those of semi-crystalline polymers. The following correlation function analysis is commonly applied to semi-crystalline polymers (see Strobl’sbook) However, it is equally applicable to any two phase system and is often used in polyurethane (PU)systems. Ophir, Wilkes, J Poly SciPoly Phys Ed1980 18p1469 It assumes a lamella morphology, but this is not as stringent as you may think, as PU is locally lamella. 8
- 9. Lamella SAXS measures the d spacing (d in the picture) The crystal does not have perfect edges, they are made up of folded chains There is a small transition zone from pure crystal to amorphous (greatly exaggerated on this slide) In addition not all the lamella will be the same size. As a result the SAXS peak is broadened. To determine the crystal morphology the correlation function is used. See Strobl, G. R. and Schneider, M. J., Polym. Sci.(1980) 18, 1343-1359 Also Strobl: The Physics of Polymers: Concepts for Understanding Their Structures and Behavior 9 Tie molecule Amorphous region Crystalline lamella Transition zone Transition zone Crystalline lamella Amorphous region Transition zone d
- 10. SAXS (Small Angle X-ray Scattering) SAXS can measure the lamella spacing in a semi- crystalline polymer. SAXS can do much more but due to time limitations this will be the main subject. SANS can also measure it. The position of the diffraction maximum is related to the lamella d spacing. The larger d the smaller the angle the scattering appears at. 10
- 11. SAXS In SAXS you would normally measure the scattered intensity (I) as a function of q rather than θ (angle). Usually q has units of Å-1 or nm-1(reciprocal Angstrom or reciprocal nanometres) 1Å =10-10m (Angstrom) 1Å-1 =1010m-1 1nm =10-9m 11
- 12. SAXS 12 Scattering close to beam stop not a real peak Scattering peak due to lamella structure Tail scattering tells you about the interface
- 13. Lorentz Correction There is a problem with scattering from 1 D stuff. Scattering is a 2D representation of a 3D system. Therefore the I(q) data from SAXS will give the wrong structural information. To resolve this issue the data must be multiplied by q2first. This is termed the Lorentz correction. 13
- 14. Lorentz Correction 14 D spacing from this peak D≈2π/q* = 2π/0.045 = 155Å Weak second order can now be seen
- 15. SAXS But just finding the d spacing tells you very little and you are throwing away a lot of information if this is all you do. To extract more information you should run a correlation function analysis. 15
- 16. Correlation function The one dimensional correlation function is useful for determining the structure of a semi-crystalline polymer. It assumes that the scattering is due to a linear arrangement of lamella stacks. While this may not always be the case it can still be applied to most semi- crystalline polymers. 16 density Mean density
- 17. Correlation function As the system only shows contrast in the z direction that’s all we need to be concerned with. We need to perform a Fourier transform on the scattering data to obtain the correlation function. ()∫ ∞ = 02.)(41dqqIqekzKziqπ 17 LIMITS extrapolation of data is problematic.
- 18. Correlation function limits LOW Q Usually Guinier Ignore the first 5 points from the beamstop Check the output Element of skill/experience HIGH Q Usually Porod Ignore the very high q Use tail end of a broad peakproblems with sharp peaks. Check the output Again experience 18
- 19. Symmetry As the structure is periodic in the z direction we can fold it in half and simplify the maths. This cosine transformation relies only on the fact that the structure is periodic. () ()∫ ∫ ∞ ∞ ∞− = = 022. )(4.cos2)(41dqqIqzqkdqqIqekzKziq ππ 19
- 20. Correlation function 1 ()eaeccBρρφ−=−2()()21eaecccQρρφφ−−= 20 ρ K dac dc dc dac z -B ρecis the crystalline electron density ρeais the amorphous electron density φcis the volume degree of crystallinity Q has a maximum at 50% crystallinity, this means that you can’t tell the difference between 40 and 60%. Similarly you cannot distinguish between the amorphous and crystalline d. However DSC or WAXS should give you a clue. Q
- 21. Correlation function 2 21 ρ K dc Average dac z -B ρ Q K Average dc Average dac z -B Q
- 22. Correlation Function 3 ()22eaecacOdzdKρρ−−= 22 ρ K Average dc Average dac z -B Q Slope gives Oac the “specific internal surface”. Area per unit volume of the interface separating crystalline and amorphous regions.
- 23. Good versus bad. A good correlation function will have 2-3 oscillations and asymptote to axis. If it does not asymptote then the Guinierprobably is wrongmove closer or further from the beamstop. Weak oscillations check the Porodfit and change the high q limit. 23
- 25. Degree of Crystallinity If the Bragg peaks are from the crystal then you should be able to calculate he degree of crystallinity from the WAXS pattern. In principle this is possible however the accuracy is not that good and its best limited to comparable measures. SAXS, density and DSC give more reliable answers. 25
- 26. Degree of Crystallinity The idea is that you can break the scattering up into regions. One from the crystals (C) One from the amorphous (A) And one from incoherent scatter. (IS) Then the ratio of C/(C+A) gives the degree of crystallinity. 26 q2I(q) q /Å-1
- 27. Degree of Crystallinity The problem with this method is that its difficult to determine the exact curve of the incoherent scatter. Ruland does give a functional form W. Ruland Acta Cryst 14 1180 (1961) But this is not exact. Therefore the method is best limited to comparing the effects of processing on a given material. 27
- 28. Degree of Crystallinity The next option is to measure a completely amorphous sample (quenched from high temperature) Use this as a background to establish the correct form of the amorphous halo and the incoherent scatter. Then perform the calculation of crystallinity as before. 28 x 0.5
- 29. WAXS and Crystallinity Most people seem to think that WAXS is the best general technique for determining the degree of crystallinity ITS NOT, it depends on the degree of crystallinity. Because of the uncertainty in the incoherent scattering. There have been many attempts to overcome this with varying degrees of success. If I were asked to determine the degree of crystallinityI would start with DSCthen WAXS/SAXS correlation function, and then density. But of course this only works for a thermoplastic material, but most thermosetsare amorphous anyway. Use SAXS and correlation function. 29
- 30. WAXS and Structure What WAXS is very good at is structure Not the amount of structure But what structures you have How the atoms pack in the crystal How big the crystals are What orientation the crystals are in What preferred growth directions occur in the crystals. For this information POLE figures are required. Pole figures have a reputation as a specialist technique and require a 3 circle goniometerfor the diffractometer. 30
- 31. Reference Books Otto Glatterand Kratky: Small Angle X-ray Scattering Thebook on SAXS. Leroy E. Alexander: X-ray Diffraction Methods in Polymer Science H.P. Krug and L.E. Alexander X-ray Diffraction Procedures All of these are old and out of print. But any good library will stock at least two of the three. I have yet to find a good new book (still in print) on the subject. Methods of X-ray and Neutron Scattering in Polymer Science (Topics in Polymer Science S.) by Ryong-JoonRoe is OK The Physics of Polymers: Concepts for Understanding Their Structures and Behaviorby GertR. Strobl This is good general polymer book and appendix A gives a good introduction to SAXS and the correlation function. 31
- 33. Block Copolymer/Lyotropicliquid Crystals. E=PEO water soluble B=polybutyleneoxide water insoluble Forms ordered phases in water 33
- 34. Hexagonal Phases 1°C /min Analyse via lfit LfitUNIX based program runs on SUN Working on new version. 34
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- 36. Caution P(q) has minima If S(q) has a max at P(q) min then you loose the peak. Frequently occurs close to 50/50 volume fraction for lamella systems. 36 2)()()(ηqPqSqI=