BackgroundIntroduction to PolymersDynamic Light ScatteringMaterialsLab ProcedureBackground Materials Lab ProcedureDynamic Light Scattering Study ofPolymer Chain DimensionsProfessor Geoffrey BeachJoy PerkinsonMIT 3.014 Module AFall 2009Background Materials Lab Procedure3.014 Lab Module APurpose: to measure polymer dimensions using dynamic lightscattering (DLS) and to relate the particle size to the number ofmonomers.Additional goals:Learn about light scatteringIntroduction to polymer behaviorGain laboratory experienceBackground Materials Lab ProcedureWhy do we care?danallen.comBackground Materials Lab ProcedureOutlineBackgroundIntroduction to PolymersDynamic Light ScatteringMaterialsLab ProcedureBackground Materials Lab ProcedureIntroduction to PolymersPolymers are long molecules composed of repeating unitsknown as monomers.Molecular weights are very high – typically between 10,000 and10,000,000 g/mol!Background Materials Lab ProcedureIntroduction to PolymersPolymers are long molecules composed of repeating unitsknown as monomers.Molecular weights are very high – typically between 10,000 and10,000,000 g/mol!Background Materials Lab ProcedurePolymers in SolutionIn solution, polymers generally form coils:They swell in good solvents and shrink in poor solvents.Background Materials Lab ProcedureRandom Walk ModelMathematician’s random walk:- each step is independent from the previous step- path can cross over itself100 step simulationBackground Materials Lab ProcedureRandom Walk ModelChemist’s random walk:- some restrictions on bond angle- path cannot cross over itselfBackground Materials Lab ProcedureRandom Walk Model“Self-avoiding random walk" leads to effective swelling:100 step simulationBackground Materials Lab ProcedureScaling lawsMathematician’s model:R21/2= N1/2lSelf-avoiding model:R21/2= N1/2C1/2∞lR21/2is the RMS end-to-end distance, N is the number ofsteps, l is the step length, and C1/2∞depends on the polymer.Background Materials Lab ProcedureTypes of SolventIn general,R21/2∼ NνGood solvent:ν > 1/2 (generally around 3/5)Bad solvent: ν < 1/2“Theta" solvent: ν = 1/2Background Materials Lab ProcedureTypes of SolventIn general,R21/2∼ NνGood solvent: ν > 1/2 (generally around 3/5)Bad solvent:ν < 1/2“Theta" solvent: ν = 1/2Background Materials Lab ProcedureTypes of SolventIn general,R21/2∼ NνGood solvent: ν > 1/2 (generally around 3/5)Bad solvent: ν < 1/2“Theta" solvent: ν = 1/2Background Materials Lab ProcedureChoice of SolventHildebrand solubility parameter:∆Hm∼ (δ1− δ2)2In this lab:δPS= 18.5 × 103J1/2m−3/2δTHF= 18.5 × 103J1/2m−3/2δacetone= 19.7 × 103J1/2m−3/2Low values of ∆Hmpredict favorable mixing.Background Materials Lab ProcedureScatteringIntuitive viewpoint: light deflected by particles in its trajectoryTechnical viewpoint:Background Materials Lab ProcedureRayleigh ScatteringParticle size is << light wavelength.I ∼ 1/λ4Blue scattered more than red – this is why the sky looksblue!Looking at the sun, you see yellow and red, the colors thatare scattered lessAt sunset, light moves through more atmosphere, creatinga greater yellow/red color gradientBackground Materials Lab ProcedureRayleigh ScatteringParticle size is << light wavelength.I ∼ 1/λ4Blue scattered more than red – this is why the sky looksblue!Looking at the sun, you see yellow and red, the colors thatare scattered lessAt sunset, light moves through more atmosphere, creatinga greater yellow/red color gradientBackground Materials Lab ProcedureDynamic Light Scattering (DLS)Light is scattered from particles solutionParticles move due to “Brownian motion," which dependson particle size, solution viscosity, temperature, andparticle densityBackground Materials Lab ProcedureStokes-Einstein EquationRh=kT6πηsDTerminology:k = Boltzmann constantT = temperatureηs= solvent viscosityD = Diffusion coefficient ofparticlesRh= “Hydrodynamic radius"The larger the particle, the slower the diffusion.Background Materials Lab ProcedureHydrodynamic RadiusRadius of particle in solution, including solvent molecules thatmay be clustered around particleBackground Materials Lab ProcedureOverview of DLSDirect laser at dilutesolution of particlesMeasure intensityfluctuationsInfer diffusion coefficientfrom speed of intensityfluctuationsCalculate hydrodynamicradius from diffusioncoefficientBackground Materials Lab ProcedureIntensity CurvesBackground Materials Lab ProcedureCorrelationBasic idea: fit an exponential correlation curve to the intensitygraph, and determine how long it takes for the correlation curveto decay.G(τ ) =R∞0I(t)I(t + τ) dxIf the particles are large, the signal will be changing slowly andthe correlation will persist for a long time.If the particles are small and moving rapidly then the correlationwill disappear more rapidly.Background Materials Lab ProcedureCorrelationBackground Materials Lab ProcedureDLS SummaryParticles in a dilute solution move due to Brownian motionLight scattered from particles is detected, creating anintensity curveRate of correlation curve decay is related to diffusioncoefficient of particlesDiffusion coefficient determines particle size(hydrodynamic radius)Background Materials Lab ProcedureLab GoalsGoals of 3.014 module A:Make dilute polymer solutionsMeasure hydrodynamic radius of particles using DLSDerive scaling law of polymer chains in solutionBackground Materials Lab ProcedureMaterialsTetrahydrofuran (THF)AcetonePolystyreneBackground Materials Lab ProcedureLab SafetyAcetone and tetrahydrofuran (THF) are both extremelyflammable. Additionally, THF is moderately reactive, causesirritation to skin, eyes, and respiratory systems, and affects thecentral nervous system. Solvents should only be handled withina fume hood. Additionally, the following safety equipmentshould be worn when handling chemicals:- Long pants, closed-toed shoes- Lab coat- Goggles- Heavy (blue) nitrile glovesBackground Materials Lab ProcedureSolutionsPolystyrene MwTarget Concentration Solvent(g/mol) (g/mL)90,000 0.01 THF & THF/Acetone152,000 0.009 THF200,000 0.007 THF & THF/Acetone390,000 0.006 THF575,000 0.004 THF & THF/Acetone900,000 0.003 THF2,000,000 0.001 THFAll solutions will be 5mL, so you will need to calculate theappropriate amount of polymer to use.Background Materials Lab ProcedureProcedureMake dilute polymer solutionsGo to 8-206 in small groups for DLSClean upDerive scaling law of polymer chains in THF