Synthesis of Polymers Prof. Paula Hammond Lecture 14: Processing Approaches: Emulsion Polymerization Processes

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1 Synthesis f Plymers Prf. Paula Hammnd Lecture 14: Prcessing Appraches: Emulsin Plymerizatin Prcesses Trmsdrff Effect (r Aut-acceleratin) Aut-acceleratin usually ccurs in radical chain plymerizatins and nrmally in bulk plymerizatin. As cnversin (π) increases, the viscsity (η) increases s much that the ends cannt find each ther t terminate plymerizatin. This in turn lwers the effective terminatin rate (k t,eff ) and terminatin rate ( t ) and thus, the average number weight f plymerizatin P n increases. n ther wrds: π Æ η k t,eff Æ t Æ P n n aut-acceleratin, There is n change in k d because there are small mlecules There is n change in k p because f small mlecules as well But fr k t, the rate f terminatin decreases because the tw grwing chain ends cannt find each ther. We knw frm previus lecture that the rate f plymerizatin is: p = k p 2k d f [ ] k t 1/ 2 [M ] And the number average degree f plymerizatin is: P n = 2aν where ½ a 1 a = 1 fr cupling a = ½ fr disprprtinatin Where ν is the number f mnmers added per effective free radical and given by: k p [M ] ν = (2kd fd t [ ]) 1/ 2 See Figure 3-15 in Odian fr a plt that demnstrates autacceleratin. Emulsin Plymerizatin (See handut: Ch. 12) n emulsin plymerizatin there are sme key ingredients : The mnmer must be insluble in water and plymerizable by free radicals Water-sluble initiatr Water Surfactant Citatin: Prfessr Paula Hammnd, Synthesis f Plymers Fall 2006 materials, MT OpenCurseWare ( Massachusetts nstitute f Technlgy, Date.

2 Frmatin f micelles n water, sap (e.g. sdium palmitate) is added t the aqueus phase. Fr example: CH 3 (CH 2 ) n COO - Na + Hydrphbic tail Hydrphilic head grup The surfactant frms micelles at high cncentratins: Add mnmer and disperse as drplets large mnmer drps (~mm) micelle with mnmer (1Å t 0.1μm) and initiatr () empty micelle The initiatr () can diffuse int the micelle particle and plymerize the mnmer Mre mnmer frm the drplet diffuses ver and plymerizes Thus the particle size increases , Synthesis f Plymers, Fall 2006 Lecture 14 Prf. Paula Hammnd Page 2 f 5 Citatin: Prfessr Paula Hammnd, Synthesis f Plymers Fall 2006 materials, MT OpenCurseWare ( Massachusetts nstitute f Technlgy, Date.

3 The three stages f emulsin plymerizatin: Stage : The mnmer diffuses t the empty micelle frm drplet Plymerizatin initiated in micelles t becme plymer particles New particles are generated as micelles are cnsumed This stage lasts fr cnversin π ~ 0-15% Mnmer diffuses frm drplet Stage : N mre surfactant available t generate new particles Mnmer diffuses int nw a cnstant number f particles t maintain sme equilibrium [M] with the particle The mnmer reservir drps get slwly cnsumed mnmer drplet Plymer grws frm 1-3nm t tens f nm (Drplet much bigger than particles) An equilibrium between increasing interfacial tensin within micelle and mnmer/plymer dilutin leads t a cnstant vlume fractin where Ф 2 is the vlume fractin f the plymer and Ф 1 is the vlume fractin f the mnmer Stage : [M ] = [M ] 0 (1 Φ 2 ) This stage ccurs when cnversin π ~ 40-60% All the mnmers exist in particles , Synthesis f Plymers, Fall 2006 Lecture 14 Prf. Paula Hammnd Page 3 f 5 Citatin: Prfessr Paula Hammnd, Synthesis f Plymers Fall 2006 materials, MT OpenCurseWare ( Massachusetts nstitute f Technlgy, Date.

4 The mnmer drplets are exhausted [M] in particles cntinually decreasing as π increases Particle size is cnstant Cnversin rate can reach up t % Smith-Ewart Mdel The Smith-Ewart mdel states that ne radical diffuses int a particle at a time and terminatin ccurs when anther radical diffuses in: M M M There is secnds between radical entry events drplet Vs particles Kinetics: Why des plymerizatin happen in the particle rather than the drplet? Answer: Large plymer particles have a much lwer surface area t vlume rati than the small drplets. The prbability f a free radical entering a drplet versus a micelle particle is very small. Assumptins: All radicals enter the micelle When radical enters a particle, there is instant initiatin if there are n ther radicals present. f there is a radical already present, terminatin ccurs The number f mnmer mlecules cnverted t plymer per secnd per particle is given by: nitiatrs dm 1 mnmers = k p [M ] = dt 2 sec radical ½ because the chain grws half the time (the ther ½ is t terminate the plymer) (Ch , p. 251 in handut) ate f prductin f initiatr radicals is given by: , Synthesis f Plymers, Fall 2006 Lecture 14 Prf. Paula Hammnd Page 4 f 5 Citatin: Prfessr Paula Hammnd, Synthesis f Plymers Fall 2006 materials, MT OpenCurseWare ( Massachusetts nstitute f Technlgy, Date.

5 N = 10 A 3 (1 Φ) k d, r radicals [ X ][ Y ] = 3 cm sec initiatrs Where Ф is the rganic (mnmer) vlume fractin k d,r is the k d fr the radical [X] and [Y] are the cncentratin f the redx The number f particles in slutin is given by: n = (# f particles)/cm 3 n = (0.53)( c a ) 3 / 5 s s 2 / 5 d ν dt 2 / 5 Where c s is the cncentratin f the surfactant a s is the area f the surfactant head grup Define Δt as the time between entry: Δ t = n is usually sec p n n = k p [ M ] Δt = k p[ M ] , Synthesis f Plymers, Fall 2006 Lecture 14 Prf. Paula Hammnd Page 5 f 5 Citatin: Prfessr Paula Hammnd, Synthesis f Plymers Fall 2006 materials, MT OpenCurseWare ( Massachusetts nstitute f Technlgy, Date.