Chapter 6 Molten State

Transcription

1 Chapter 6 Molten State

2 Rheology ( 流變學 ) study of flow and deformation of (liquid) fluids constitutive (stress-strain) relation of fluids shear flow shear rate ~ dγ/dt ~ velocity gradient dv 1 = dx 1 /dt γ = dx 1 /dx 2 shear flow

3 stress state in this flow case simple shear ~ σ 12 only, all others = 0 σ 1, σ 2, σ 3? ~ normal stress by shear flow relations in shear flow ~ Newton s law ~ 1 st normal stress coeff N 1 ~ 2 nd normal stress coeff N 2

4 Normal stress difference normal stress caused by shear flow 2 retractive force 3 1 σ 1 σ 2 = N 1 > 0 ~ 1st normal stress difference σ 2 σ 3 = N 2 0 ~ 2nd normal stress difference results of NSD Weisenberg effect rod-climbing die swell

5 Elongational flow v = dx/dt, a = dε/dt = dx/xdt 2 uniaxial elongational flow σ 1 only, all other stresses = for incompressible fluid (ΔV = 0) elongational viscosity, 0 η E = σ/(dε/dt) spinning, contraction flow

6 (balanced) biaxial elongation σ 1 = σ 2, all other stresses = biaxial elongational viscosity η E ballooning, film blowing

7 Dynamic viscosity from dynamic mechanical measurements η* ~ complex viscosity η ~ dynamic viscosity

8 Non-Newtonian behavior Newtonian ~ constant viscosity many solutions and melts non-newtonian dilatant ~ shear thickening suspensions pseudoplastic ~ shear-thinning polymer melts chains aligned to shear direction zero-shear-rate viscosity, η 0 Bingham body PP at dγ/dt = 0, N 1 N 1,0 Bingham plastic ~ yielding slurries, margarine

9 power-law expression σ 21 Newtonian η PP η 0 dγ/dt log σ n n=1 ~ Newtonian PP log (dγ/dt) n < 1 and n const in 1-2 decades only

10 time-dependence dγ/dt τ rheo thixotropic decrease in η with shearing time polymer melts, inks seldom in polymers, more in colloids thixotropic is pseudoplastic; PP is not necessarily thixo rheopectic (anti-thixotropic) gypsum and soils rheopectic is dilatant; dilatant is not necessarily rheo t thixo t

11 measurement of rheological properties shear flow Couette flow parallel-plate, cone-and-plate, two-cylinder η by velocity and torgue N by plate-separating force η* by oscillation Poiseuille flow capillary, slit η by pressure drop and flow rate N by non-zero exit pressure elongational flow difficult to perform expt possible only at small strain rate

12 melt index (MI) or melt flow index (MFI) melt indexer ~ a simple capillary viscometer M(F)I = g of resin/10 min at specified weight and temperature high MI ~ low η ~ low MW of a polymer

13 Viscoelastic fluid linear viscoelasticity only at small strain and shear rate little use in polymer processing condition useful for comparison of materials and molecular factors (MW, MWD) Boltzmann superposition principle for smooth strain history starting expt at time 0 t t = s, for steady flow (dγ/dt = const) η = σ/(dγ/dt), η 0 at low shear rate limit

14 (stress) relaxation modulus G(t) = σ(t)/γ 0 glassy ~ solid rubbery plateau region due to entanglement physical crosslink plateau modulus G N0 = ρrt/m e M e ~ entanglement mol wt avg mol wt betw entanglements terminal zone ~ flow ~ liquid G N 0 A ~ monodisperse, M w < M C B ~ monodisperse, M w > M C C ~ polydisperse, M w > M C (M C ~ 2 3 M e )

15 creep compliance J(t) = γ(t)/σ 0 steady-state compliance, J e 0 constant shear rate at long times J e 0 J(t) = J e0 +t/η 0 J N 0 J N 0

16 recovery compliance recovery test σ σ 0 γ γ r (t) recoil function or recovery compliance, R(t) = γ r (t)/ σ 0 lim (t= ) [R(t)] = J e0 ~ steady-state recovery compliance

17 dynamic η and VE as ω 0 (large t, small dγ/dt) η 0 η, η* G = ωη 0, G = ωη 0 η η = G /ω η 0 Newtonian η as ω (small t, large dγ/dt) η η G G ωη Hookean G ~ η log t, log ω, or log (dγ/dt)

18 rheometric and VE functions rheometric ftns η, N 1 ( η 0, N 1,0 ) linear (Newtonian) non-linear (non-newtonian) as dγ/dt viscoelastic ftns η, η* viscous elastic as ω stress ratio N 1 /τ (>1) a measure of elasticity

19 Behavior of polymeric liquids polymeric liquids dilute solution ~ as conc n 0, Newtonian concentrated sol n ~ behaves as melt melt ~ Newtonian as shear rate 0 η and N 1 shear thinning η = K (dγ/dt) n-1 (n < 1) N 1 > 0 N 1 > τ η E not much dep on dε/dt log η log η E log N 1 log τ at dε/dt 0, η E 3 η 0 log (dγ/dt)

20 effect of temp at T g < T < T g K ~ WLF eqn log η = log η Tg C 1 (T-T g )/(C 2 +T-T g ) at T > T g K ~ Arrhenius relation η = A exp[e/rt] effect of pressure η = A exp[bp] conversion factor, (ΔT/ΔP) h example p107

21 effect of mol wt η at M w < M c, η 0 M at M w > M c, η 0 M 3.4 M c ~ 2 3 M e J e 0 M e depends on chemical structure of chain chain stiffness and interactions PE ~1200, PS ~20000, PC ~2500 at M w < M c, J e0 = (0.4)M w /ρrt at M w > M c, J e0 = (0.4)M c /ρrt M c ~ 5 10 M e G N0 J e 0 = constant ~ 3

22 branching when M b < M c smaller <s 2 > 0 lower η 0, J e 0 when M b > M c smaller <s 2 > 0, but larger reptation time higher η 0, J 0 e η 0 = (M w ) k, k > 6

23 Macromolecular Dynamics Motions in polymers Defomation in bond angle and length ~ elastic Change in conformation ~ segmental motion ~ viscoelastic Translational motion ~ viscous

24 Models for macromolecular dynamics Rouse ( Bueche Zimm) model bead (friction) and spring (elastic) single chain with completely flexible repeat units moving in a medium three forces ~ friction, elastic, and Brownian not for M w > M c for M w < M c correctly describes η 0, J 0 e does not describe shear thinning

25 Reptation (de Gennes ( Doi Edwards)) model chain and obstacles (entanglements) chain reptates between obstacles friction M chain in a tube tube disappears and regenerated diffusion of tube M 2 successfully describes effect of entanglement effect of branching predicts higher η 0 with lower power (3 instead of 3.4) other mechanism should exist

26 Rheology of liquid crytals solution M melt