POLYMER CHARACTERIZATION

Transcription

1 POLYMER ARATERIZATION Understand the use of common methods for determining chemical composition, microstructure, and molar mass distribution: Fractionation Light scattering Dilute solution viscometry olligative property measurement Size exclusion chromatography (GP) Mass spectrometry Absorption spectroscopy (UV-visible, IR, Raman,) Nuclear magnetic resonance spectroscopy ( 1 -, 13 -NMR) Know the optimal methods for specific characterizations Text Sources Topic Absorption spectroscopy NMR Mass spectrometry Fractionation End-group analysis Osmometry, ebulliometry, cryoscopy Light scattering Dilute solution viscometry Size exclusion chromatography (GP) hapter(s) , haracterization 6-1

2 What Influences Product Properties? Polymerization hemistry Monomers Mechansim onditions Polymer Structure omposition Branching Tacticity Mol. weight Polymer Properties Viscosity T g, T m hemical behavior Mechanical behavior Manufactured Article Tensile str. Modulus ardness Resilience Abrasion resistance Solvent resistance Traction Adapted from E.N. Kresge, W.W. Graessley, & G. VerStrate, 2000 Principles of Polymer Science ExxonMobil short course. hemical omposition & Microstructure Even if you know how a polymer was synthesized, you must confirm the chemical composition and structure of the material: omposition Elements present (,, O, N, P, Si, ) Mole ratios of elements (empirical formula: x y O z, ) Average chemical composition (copolymers: A x B y, ) Structure Repeat units Repeat unit sequence distributions (A-A-A-, A-B-A-B-, A-A-B-A-B, ) End groups Branch points haracterization 6-2

3 Fractionation Like dissolves like. Solubility parameters (δ) can be used to estimate the compatibility of polymers and solvents: Solvent δ, MPa ½ Polymer δ, MPa ½ n-exane Toluene 2 l 2 Acetone 3 N BR NR PETE PMMA Nylon-6, The smaller δ solvent δ polymer, the better the solvent. For a good solvent, (δ solvent δ polymer ) 2 < 4.0. Example: polyisoprene (NR) exane: (δ solvent δ polymer ) 2 = 3.2 Acetone: (δ solvent δ polymer ) 2 = 9.0 exane is a good solvent, acetone is not. Adapted from Thermodynamic onsiderations for Polymer Solubility ( Accessed 11 October, Fractionation (cont.) Polymer solubility is dependent on molecular weight (and temperature). Adding (or increasing the concentration of) a non-solvent causes parts of the polymer chains to become desolvated. The polymer chains begin to aggregate. igher-mw chains aggregate and precipitate from solution. Adding more non-solvent precipitates progressively lower- MW polymer: Fraction 6 13 O N P 6 13 O n M w 2,300, , ,000 26,000 Solvent: TF, non-solvent: ethanol. T = 15 Adapted from J. Bravo et al., 1991 Macromolecules, 24, haracterization 6-3

4 Static Light Scattering Molecules in solution scatter light: θ The intensity ratio of incident to scattered light ( R θ ) and the polymer concentration (c) are functions of the weightaverage MW: Kc 1 = R (θ 0,c 0) M w This method also allows determination of the radius of gyration (s or s 2 ½ ), which describes the average size and shape of a polymer molecule. Static Light Scattering (cont.) Useful range: M w = 20,000-5,000,000 Fraction M w s 2 ½, nm 6 13 O 1 2,300, N P 6 13 O n , , ,000 Adapted from J. Bravo et al., 1991 Macromolecules, 24, haracterization 6-4

5 Molecular Shape and Molecular Wgt Shape Example s 2 ½ = ƒ(m x ) Sphere Random coil ½ Rod 1 Rectangular Plate Square Plate ½-1 ½ Adapted from L.. Sperling, Introduction to Polymer Science, 2 nd Ed, 1992, p 247 Solution Viscosity The viscosity of a polymer solution (η) may be compared to that of the solvent (η 0 ) as the relative viscosity (η r ): η η r = η0 η r can be measured directly: capillary viscometer Since η r will be a number greater than one, it is convenient to use the specific viscosity (η sp, where 0.2 η sp 0.6): η sp = η r 1 haracterization 6-5

6 η sp = η r 1 The intrinsic viscosity ([η]) is related to both η r and η sp as a function of the polymer concentration (c): ln(η r ) [η] = c c 0 η sp [η] = c c 0 The Mark-ouwink (or Mark-ouwink-Sakurada or Kuhn- Mark-ouwink-Sakurada) equation relates the intrinsic viscosity to the viscosity average molecular weight: [η] = KM v a where the exponent a (0.5 a 2.0) is determined by the polymer, solvent, and temperature. Example: Poly(dihexoxyphosphazene) 6 13 O 2.2 N P 6 13 O n 2.0 in TF, 25 η sp /c η sp /c or ln(η r )/c, g/dl 1.8 [η] = 1.75, k = ln(η r )/c c, g/dl Adapted from J. Bravo et al., 1991 Macromolecules, 24, haracterization 6-6

7 Polymer Solvent T, Poor solvent (unperturbed coils) Polystyrene cyclohexane 34 Poly(Me methacrylate) acetonitrile 44 Good solvent (solvated coils) Polystyrene toluene 15 Poly(Me methacrylate) benzene 30 a To evaluate M v, K and a must be determined. This usually involves analyzing polymer samples with known M n or M w and narrow MW distributions Plot log[η] as a function of log(m). The slope of the line is a, the intercept is log(k). For typical polymers, M w > M v > M n. Adapted from.-g. Elias, An Introduction to Polymer Science, 2 nd Ed, 1997, p 207 Theory predicts the dependence of a on molecule shapes and segment distributions: Spheres: a = 0 Random coils: a = ighly-perturbed coils: a = 1 Rigid rods: a = 2 For most flexible polymers (example: polystyrene in cyclohexane), 0.5 < a < 0.8. For more rigid polymers (example: PV in TF), a > 0.8. For inherently stiff or highly extended chains (a 1), M v = M w. Q: Q: For For a given given M v, v, which shape gives gives the the least least viscous solution, the the most most viscous solution? haracterization 6-7

8 olligative Properties Functions of number-average MW (M n ). Vapor pressure lowering ( T/c) c 0 = K Rs /M n Boiling point elevation ( T/c) c 0 = K b /M n Freezing point depression ( T/c) c 0 = K f /M n Osmotic pressure (Π/c) c 0 = RT/M n orresponding techniques: Vapor phase osmometry M n 15,000 Ebulliometry M n 5,000 ryoscopy M n 15,000 Membrane osmometry 50, ,000 M n 500,000-1,000,000 Examples: M Π, torr* P vap *, torr + T b *, T f *, , , , ,000, *[solute] = 10 g/dm 3 in benzene. Using Using polymer colligative properties to to characterize large large molecules requires a high high level level of of detector sensitivity. haracterization 6-8

9 Size-Exclusion hromatography (GP) Polymer molecules in solution elute from a porous column packing in order of decreasing molecular size. Separation is based on the hydrodynamic volume (radius of gyration) of the molecules. Smaller molecule: Longer flow-path Longer residence time Larger molecule: Shorter flow-path Shorter residence time Elution time Size-Exclusion hromatography (cont.) Multi-column sets calibrated to provide molar mass as a function of elution volume. M w = 900,000 Effluent polymer detected by changes in refractive index, UV or IR absorption, light scattering. RI M w = 2,300,000 M w = 250,000 M w = 26, O N P 6 13 O n M n, M w, M z, polydispersity, modality determined from output data. Elution Volume logm Adapted from J. Bravo et al., 1991 Macromolecules, 24, haracterization 6-9

10 Mass Spectrometry Mass spectrometry measures the mass-to-charge ratio of charged particles. Mass spectrometry is used for determining Molecular mass, Elemental composition, and hemical structure. Mass spectrometry may be combined with separation techniques: Gas chromatography (G-MS) Liquid chromatography (L-MS) Mass spectra are plots of intensity (abundance) versus m/z (mass-to-charge ratio). Mass Spectrometry (cont.) Some mass spectrometry techniques (EI = electronionization) break analyte molecules into fragments: Rel. Abundance M m/z The fragmentation pattern is representative of the structural features of the molecule. Analyzing intact larger molecules is possible with matrixassisted laser desorption/ionization (MALDI). haracterization 6-10

11 Mass Spectrometry (cont.) Mass limit: kda General method: The polymer is mixed into a UV- or IR-absorbing matrix. The matrix + polymer are irradiated with laser light. The matrix fragments, producing a plume of ionized molecules that includes the polymer. The ionized molecules are analyzed using a time-of-flight (TOF) mass spectrometer. Result: a plot of intensity versus m/z (mass-to-charge ratio) for the unfragmented polymer molecules. Optimal for polymers with low MW and narrow mass distributions ( 1.2). Entanglements in synthetic polymer chains limits higher mass resolution. Example: Polyisoprene Supplier M n = MALDI M n = Signal Intensity M Adapted from T. Yalcin & D.. Schriemer, 1997 J. Am. Soc. Mass Spectrom., 8, haracterization 6-11

12 Example: Polystyrene M w = 11,980 (115n) M n = 11,980 (114n) M z = 12,060 (116n) MWD = 1.01 Signal Intensity n M Adapted from K. Rollins et al., 1990 Rapid ommun. Mass Spectrom., 4, Example: Poly(butylene adipate-co-butylene sebacate) A 6 B 5 A 5 B 6 A 7 B 5 A 6 B 6 A 5 B 7 A 3 B 7 A 7 B 4 A 4 B 7 A 8 B 4 A 3 B 8 A 9 B 4 A 4 B 8 A 8 B 5 A 8 B 3 A 2 B 8 A 9 B 3 A 2 B 9 A 10 B 3 Signal Intensity M Adapted from M.S. Montaudo et al, 1998 Rapid ommun. Mass Spectrom., 12, haracterization 6-12

13 Absorption Spectroscopy Light is electromagnetic energy Light has a wave structure: At a specific energy level, light s electric and magnetic fields vary at a fixed frequency (ν, s 1 or z) The distance between light wave maxima or minima corresponds to the wavelength (λ, Nm) Light waves are propagated at a fixed speed (c = m/s in a vacuum) λ = c/ν The peak-to-trough height of the electromagnetic wave is its amplitude Intensity or Amplitude 0 λ wavelength v = c frequency (ν) = c/λ Time, s or Distance, m haracterization 6-13

14 λ 1 l o n g wavelength ν 1 low frequency Intensity or Amplitude λ 2 short wavelength ν 2 high frequency Time, s or Distance, m Electromagnetic Spectrum λ, m X-ray UV IR Microwave FM TV AM electron transitions bond vibrations molecule rotations 400 nm 700 nm haracterization 6-14

15 σ* 2p π* 2p σ 2p π 2p UV absorbed IR absorbed σ* 2s σ 2s Beer-Lambert Law Sample I 0 I Light Source l Prism or Grating Detector Transmittance: Absorbance: I T = I0 I A = log = logt I 0 Absorbance (A) is proportional to the concentration of the absorbing chromophore and the optical path length: A = ε l haracterization 6-15

16 UV-Visible Spectroscopy A MW = 280,000 n λ, nm ast Films 3 MW = 19,000 n Electron transitions from highest-occupied molecular orbitals (OMO) to lowestunoccupied molecular orbitals (LUMO). π π* (stronger) n π* (weaker) onjugated systems absorb more strongly than non-conjugated systems. Useful for detecting unsaturated functional groups: =, =O, O-=O, =N, N, phenyl & heterocyclics. Adapted from L. Meal, 1990 J. Appl. Polym. Sci., 41, UV Electronic Transitions hromophore λ max, nm ε, L/mol cm Monomer ,400 Polymer ( 3 ) Olefin polymers lack conjugated functional groups, except as side groups weak ε. (Exceptions: certain polyesters, polyurethanes) UV spectroscopy is useful for qualitative analyses of structure, quantitative analyses of strongly-absorbing repeat groups or residual monomer. Adapted in part from NIST hemistry WebBook ( accessed 7 October, haracterization 6-16

17 IR Spectroscopy Identifies energy absorption by transitions between vibrational states. Dispersive method detects absorptions resulting from vibrations producing a change in dipole moment: X O==O O==O Most common absorptions occur in the region λ = μm (wavenumber ν = 1/λ = ν/c: cm 1 ). Useful for identifying a wide range of covalent bonds and functional groups. Stretching: In-Plane Bending: Out-of-Plane Bending: haracterization 6-17

18 IR Spectroscopy Functional group region ( cm -1 ): Absorptions assignable to specific structures and bond deformations. Fingerprint region ( cm -1 ): Absorptions assignable to some -O and - stretching vibrations, but generally unique to a particular material. Olefinic/aromatic region ( cm -1 ): Absorptions assignable to = or aromatic - out-ofplane bending vibrations, as well as carbon-halogen (-l, -Br) stretching. Distinctive absorptions Look for absorption bands in decreasing order of importance: 1. - absorption(s) between cm absorption between cm -1 : aliphatic. - absorption above 3000 cm -1 : =, either alkene or aromatic. onfirm aromatic ring by finding peaks at 1600 and 1500 cm -1 and - out-of-plane bending patterns below 900 cm -1. onfirm alkene with a peak at cm =O absorption (strong) between cm -1. Indicates aldehyde, ketone, carboxylic acid, ester, amide. Aldehyde confirmed by - absorption between cm O- or N- absorption (broad) between cm -1. Indicates alcohol, N- containing amine or amide, or carboxylic acid. A doublet if N O absorption (prominent, broadened) between cm -1. Indicates carboxylic acids, esters, ethers, or alcohols. Adapted in part from Quick Procedures for Infrared Analysis ( quickir.htm), accessed 8 October, haracterization 6-18

19 Distinctive absorptions (cont.) 5. N at cm -1. Moderate to weak, but well isolated. 6. Methyl - band at 1380 cm -1. A doublet for isopropyl (gemdimethyl). 7. Olefinic and aromatic - bands at cm -1. Prominent in styrene-based or otherwise highly unsaturated elastomers. 8. Aromatic overtone bands from cm -1. Adapted in part from Quick Procedures for Infrared Analysis ( quickir.htm), accessed 8 October, Related Techniques: FTIR, ATR Fourier Transform IR Spectroscopy Spectral data collected over a wide spectral range. Improved signal-to-noise ratios. Multi-second time scale permitting time-resolved analyses. Attenuated Total Reflectance onventional analysis: samples are analyzed in solution, as thin films, or as dispersions in KBr pellets. ATR: IR radiation passed through a crystal (ZnS, TlBr 0.4 I 0.6 ) sandwiched between polymer films, output beam analyzed. Sample TlBr 0.4 I 0.6 Direct analysis of polymer surfaces no interference fringes, peaks from solvent. haracterization 6-19

20 FTIR spectrum: Aromatic = str. Transmittance Aromatic - str Aliphatic - str bend Aromatic - bend, (monosubstituted ring) 770 n Wavenumber, cm 1 Adapted from A.. Kuptsov & G.N. Zhizhin, andbook of Fourier Transform Raman and Infrared Spectra of Polymers, Analysis of polypropylene tacticity: Atactic Transmittance Syndiotactic Isotactic Wavenumber, cm 1 haracterization 6-20

21 Raman Spectroscopy In conventional IR absorption, bonds are excited from ν 0 ν 1. hanges in bond dipole moment are detected. In Raman spectroscopy, laser-excited bonds decay to lowest excited vibrational states through inelastic light scattering. E ν1 ν 0 (Raman shift) is measured. hanges in bond polarizability are detected symmetrical vibrations identifiable, complementing conventional IR analysis. Practical Raman spectroscopy requires high-intensity monochromatic light laser. Distinctive Raman absorptions 1. = absorption between cm N=N absorption between cm N==O absorption between cm absorption between cm Aromatic ring breathing at about 1000 cm S absorption between cm S S absorption between cm -1. NOTE: Strengths of absorption bands common to both IR and Raman may change. haracterization 6-21

22 Raman spectrum: Aromatic ring breathing 1000 n Absorbance Aromatic - str Aliphatic - str Aromatic = str. Aromatic out-of-plane - bend Wavenumber, cm 1 Adapted from A.. Kuptsov & G.N. Zhizhin, andbook of Fourier Transform Raman and Infrared Spectra of Polymers, Spectrum comparison: FTIR Raman Wavenumber, cm 1 haracterization 6-22

23 Distinctive IR absorptions: unsat aliph olefinic = aromatic = below 1000 olefinic aromatic Transmittance O, N BROAD aromatic overtones ketone, ester, amide =O STRONG ~1380 methyl (doublet if isopropyl) O BROAD , N Wavenumber, cm 1 Distinctive Raman absorptions: = ~1000 aromatic ring breathing Transmittance N=N S S S N==O Wavenumber, cm 1 haracterization 6-23

24 Example: FTIR Spectrum Transmittance 1. Aliphatic 2. ~Olefinic 3. ~Olefinic = Wavenumber, cm 1 Adapted from A.. Kuptsov & G.N. Zhizhin, andbook of Fourier Transform Raman and Infrared Spectra of Polymers, Example (cont.): Raman Spectrum Aliphatic 2. Olefinic = 3. Absorbance Wavenumber, cm 1 Adapted from A.. Kuptsov & G.N. Zhizhin, andbook of Fourier Transform Raman and Infrared Spectra of Polymers, haracterization 6-24

25 Example (conclusion) Transmittance 1. Aliphatic 2. Olefinic 3. Olefinic = Wavenumber, cm 1 Exercise: Identify the Polymer Transmittance a) a) 1,4-cis-Polybutadiene b) b) Polystyrene Polystyrene c) c) Poly(methyl Poly(methylacrylate) d) d) Poly(ethylene Poly(ethyleneterephthalate) terephthalate) Wavenumber, cm 1 haracterization 6-25

26 Nuclear Magnetic Resonance In a magnetic field, certain nuclei ( 1, 2, 13, 14 N, 19 F) absorb electromagnetic radiation at a characteristic frequency for each isotope. Depending on the local chemical environment, a molecule s nuclei may resonate at slightly different frequencies. Frequency shifts are converted into a dimensionless value known as the chemical shift, expressed in parts per million (ppm) relative to an internal standard. Nuclei interact with their neighbors ( 3 bonds apart) through scalar spin-spin coupling. This causes certain NMR absorptions to be split into multiplets. NMR provides an absolute method for determining polymer chain composition, configuration, and conformation. Sample Sample in in magnetic magnetic field field is is irradiated irradiated with with a wide-band wide-band radio radio frequency frequency (RF) (RF) signal. signal. Sample Magnetic Field Magnet Magnet Radio Frequency Input Radio Frequency Output Spectrum Spectrum generated generated from from the the induced induced RF RF output. output. haracterization 6-26

27 NMR in Polymer Science Structural analysis Repeat unit structure ead-to-head, head-to-tail linkages haracterization of chain ends, branch sites Tacticity (configuration) Monomer sequence in copolymers hemical information Stereochemistry of initiation, propagation Monomer reactivity ratios hemical reactive sites Kinetics Physical information hain conformation and conformational transitions Phase transitions (T g, T m ) Representative polymer 1 chemical shifts: N Aromatic, Olefinic = Aliphatic O N =O O Aliphatic N =O Olefinic = N Aromatic = O F =O O =O Aromatic = l Aliphatic δ, ppm haracterization 6-27

28 Representative polymer 13 chemical shifts: O = R OR Olefinic = R 3 R O = R O R 2 R R=O N N R 2 R R 2 =O Aromatic = O R δ, ppm Example: Determination of MW Linear poly(ethylene oxide) d 1 -decoupled 13 -NMR spectrum (allows integration of peak areas) n = 30 b c a a b c d d d d c b a O O O-( 2 2 -O) n O O δ, ppm haracterization 6-28

29 Example: Determination of branching Poly(n-butyl acrylate) Normal repeat unit: a b 2 - O Branched repeat unit: O d c d b a O O O d Integration of 1 -decoupled peaks indicates 4.1 mol% branched structure. c δ, ppm Example: Determination of tacticity Linear polypropylene Isotactic 3 meso (m) Syndiotactic racemic (r) 3 100% isotactic polymer all meso configurations 100% syndiotactic polymer all racemic configurations 100% atactic (random) polymer [meso] = [racemic] haracterization 6-29

30 1 NMR: Isotactic Syndiotactic Atactic δ, ppm 13 NMR: Isotactic Syndiotactic Atactic δ, ppm haracterization 6-30

31 Sequence distributions: mmmm Example: mmmr pentad * rrrr m r 3 mmmr mmrr mmrm + rmrr rrrm * * * * * m m rmmr rmrm mrrm These structures affect polymer phase transitions such as T g and the abiltiy to crystallize. * Refers to the number of chiral centers. Overview of Optimal Methods Technique(s) Repeat unit structure, composition Repeat unit distribution Monomer sequence distributions Stereo-, regioregularity Tacticity Linear polymer MW, MWD Branched polymer MW, MWD IR, UV, 1 -NMR, 13 -NMR 1 -NMR, 13 -NMR 13 -NMR 13 -NMR 13 -NMR GP/SE Viscosity, light scattering haracterization 6-31

32 References and On-Line Resources General NIST hemistry WebBook ( accessed 7 October, ompendium of physical properties, including IR and UV-visible spectral data; accessed 12 October, Spectral Database for Organic ompounds, SDBS ( riodb01.ibase.aist.go.jp/ sdbs/cgi-bin/cre_index.cgi) ompendium of IR, Raman, NMR, and mass spectral data; accessed 12 October, UV-Visible Spectroscopy Visible and Ultraviolet Spectroscopy ( msu.edu/faculty/reusch/virttxtjml/spectrpy/uv-vis/spectrum. htm#uv4) accessed 8 October, IR Spectroscopy Infrared Spectroscopy ( reusch/virttxtjml/spectrpy/infrared/infrared.htm), accessed 8 October, Infrared Spectroscopy for Organic hemists Web Resources ( dq.fct.unl.pt/qoa/jas/ir.html), accessed 7 October, References and Resources Raman Spectroscopy D.J. Gardiner, Practical Raman Spectroscopy, Springer-Verlag, ISBN A.. Kuptsov & G.N. Zhizhin, andbook of Fourier Transform Raman and Infrared Spectra of Polymers, Elsevier, ISBN NMR Proton hemical Shifts ( handouts/nmr-h/hdata.htm), accessed 9 October, arbon hemical Shifts ( handouts/nmr-c13/cdata.htm), accessed 9 October, Mass Spectrometry (MALDI) S.D. anton, 2001 hem. Rev., 101, Mass Spectrometry of Polymers, G. Montaudo & R.P. Lattimer, eds., R Press, ISBN haracterization 6-32