Vibrational spectroscopy6.3.9 Qualitative analysisPurificationGet information as more as possibleKindsKinds of spectroscopyofspectroscopyFor a molecule CmHnOqNrXs,degree of unsaturation can be calculated according to the following equation: U=2m+2−(n−r+s)2Vibrational spectroscopy6.3.10 Quantitative analysisBeer-Lambert LawTransmittance:T=I/I0Percentagetransmittance:100×I/I0Absorbance:A=log(1/T).Vibrational spectroscopy6.3.10 Quantitative analysisForabsolutemeasurements,therearesomeproblems:¾Uniformfilmthickness¾Homogeneousdistributionofabsorbingspecies(particularlyadditives)¾Opticaleffectssuchasextraneouslightscattering※So great care must be taken when attempting absolute measurements.Vibrational spectroscopy6.3.10 Quantitative analysisBeer-Lambert LawA=0,noabsorption(I=I=∞,completeabsorption0)AA=logI(I=0).I=εcl0A:Absorbance;I0:Incidentlightintensity;I:Intensityatadepthlintheabsorbingmedium;c:theconcentrationoftheabsorbingspecies;ε:aconstantforthematerialtermedtheabsorbanceorextinctioncoefficient.※Mostspectrometersdisplaythespectrumaspercentagetransmittancevswavenumber.Vibrational spectroscopy6.3.10 Quantitative analysis¾In principle, if extinction coefficient is known, absorbance can be made by direct application of the Beer-Lambert Law ¾Quantitative analysis is best carried out in solution ywith appropriate concentration.¾Andmoreusuallythereisneedtoexaminepolymersinthesolidstate.(thinfilm,intheformofKBrdiscsorNujolmulls)Vibrational spectroscopy6.3.10 Quantitative analysisMore reliable results are usually obtained:¾Compare the sample under investigation with a standard reference material located in the reference beam of dual beam spectrometers;¾Use internal reference standards;Uitlftdd※Ex:thevinylacetatecontentofEVAcopolymercanbedeterminedbymeasurementoftherelativeintensitiesoftheacetateabsorptionat1020cm-1andthemethyleneabsorptionat724cm-1.1Vibrational spectroscopy6.3.11 Instrumentation(a) Dispersive infrared spectrometers¾Double beam mode¾The source emits radiation over the whole IR region (a black body source (globar))¾Use more than one monochromator,¾Whole infrared region¾FTIR instruments have largely replaced the traditional double beam instruments.¾Higher signal-to-noise ratios compared to dispersive instruments¾Capability of complex data handling¾Also permit investigation of transient structural changes during deformation of polymersVibrational spectroscopy6.3.11 Instrumentation(b)FouriertransformspectrometersThe splitter that is set at an angle of 45\" to the path of the collimated beam. The beam is partially transmitted Thebeamispartiallytransmittedand partially reflected at the thin dielectric beam splitter. The transmitted and reflected beams produce interference effects. The beam is then passed through the sample and on to the detector.Fig. 6-20 The structure of IR spectrometer. Vibrational spectroscopy6.3.11 Instrumentation(b)Fouriertransformspectrometers¾mirrors are located equidistant,the optical path difference is an IntegralIntegral number of numberofwavelengths¾the reflected beams are in phase¾produce constructive interferenceVibrational spectroscopy6.3.11 Instrumentation(b)FouriertransformspectrometersHigh intensity mercury lamp or heated wire Michelson interferometer(continuous spectrum)Two mirrors, one fixed and one movableFig. 6-19 Layout ofFourier transform spectrometersVibrational spectroscopy6.3.11 Instrumentation(b)Fouriertransformspectrometers¾constructive interference.¾destructive IR interference source¾oscillatory Samplepattern or interferogramVibrational spectroscopy6.3.11 Instrumentation(b)Fouriertransformspectrometers¾the optical path difference is an odd number of half wavelengths¾destructive interference2Vibrational spectroscopy(b)FouriertransformspectrometersInterferogram¾The interferogram is an intensity function of the path difference x;¾TheThe interferogram for a polychromatic source interferogramforapolychromaticsourceis a summation of many individual cosine interference patterns:I(x)=(1∫∞2π)0G(v)(cos2πvx)dvVibrational spectroscopy(c)RamanspectrometersHigh intensity monochromatic laser light sources: argon ion, krypton ion, helium-neon gas lasersRecorderRaman scattering: 1 in 109The scattered light is usually observed at 900to the incident beam.Vibrational spectroscopyInfraredDichroismFig. 6-21 Infrared dichroism Vibrational spectroscopy(b)FouriertransformspectrometersInterferogramThetrueabsorptionspectrumisderivedfromtheinterferogrambyaFouriertransformoftheformG(v)=(11/2∞2π)∫0I(x)cos((2πvx)dxG(v)isthespectraldistributionatagivenfrequency(v).ThetransformationiscarriedoutonarangeofintensityvaluesI(x)atdifferentpathlengthsxtogivethespectralintensityatthegivenfrequency.Vibrational spectroscopy(d)AttenuatedTotalReflectance(ATR)¾IRspectraofsurfaces¾ProvideinformationonsurfaceoxidationVibrational spectroscopyInfraredDichroismandRamanPolarization¾Use polarized light source;¾Determine the extent of IR dichroism and/orand/or Raman depolarization Ramandepolarization¾Provide structural information of molecular chain orientation.3Vibrational spectroscopyInfraredDichroism¾The maximum absorption of IR radiation occurs when the electric vibrating of the incident EM radiation is parallel to the dipole of the vibrating group; ¾Absorption of IR radiation is zero in the Fig. 6-21 Infrared dichroism perpendicular direction.Vibrational spectroscopyInfraredDichroismDichroic ratio:D = A///A⊥D0=2cot2αf=[3
-1]/2=(D-1)(D0+2)/(D0-1)(D+2)where A//and A⊥are the absorbances defined with respect to a reference axis in the sample, typically the orientation direction. fis the chain orientation factor. αis the transition moment angle, which is the angle between the direction of the nitrile group’s dipole moment and the chain axis. For PAN, it is 70°. And θis the angle of the molecular segment relative to the fiber axisVibrational spectroscopyRamanPolarizationFor randomly orientated molecules,the intensities of Raman lines are dependent on:¾The polarization of the incident beam;¾The symmetry of the vibrational modes.Vibrational spectroscopyInfraredDichroismTwo situations:¾Randomorientatedmolecules(The measured absorbance is independent of the polarization of the incident light.)g)¾Orientedspecimens(Theobservedspectrawilldifferdependingonthedirectionoftheplaneofpolarizationrelativetothemolecularorientation.)Vibrational spectroscopyRamanPolarizationThe directional properties of Raman spectra are determined by:¾The symmetry of the vibrationsyy¾The orientation of the molecules relative to the plane of polarization.Vibrational spectroscopyRamanPolarizationFororientedspecimens,theintensityofthescatteredlightisdependentalsoon:¾Theemolecularoecuaoorientationetatorelativeeatvetottheeplaneofpolarizationoftheincidentradiation;¾Thedirectionandplaneofpolarizationofthescatteredradiation.4Vibrational spectroscopyRamanPolarizationPolarization effects are expressed quantitatively by depolarization ratio(ρ)ρ=I//I⊥whereI//istheintensityofthelightscatteredwithitsplaneofpolarizationinthesamedirectionasthatoftheincidentlight;I⊥istheintensityoflighthavingitsplaneofpolarizationatrightanglestothatoftheincidentlight.Vibrational spectroscopyRamanPolarization¾Depolarization:ρ=0.75¾Polarization:ρ<0.755