10.569 Synthesis of Polymers Prof. Paula Hammond Lecture 14: Processing Approaches: Emulsion Polymerization Processes Tromsdorff Effect (or Auto-acceleration) Auto-acceleration usually occurs in radical chain polymerizations and normally in bulk polymerization. As conversion (π) increases, the viscosity (η) increases so much that the ends cannot find each other to terminate polymerization. This in turn lowers the effective termination rate (kt,eff) and termination rate (Rt) and thus, the average number weight of polymerization Pn increases. In other words: π ↑ Æ η ↑ kt,eff ↓ Æ Rt ↓ Æ Pn ↑ In o o o auto-acceleration, There is no change in kd because there are small molecules There is no change in kp because of small molecules as well But for kt, the rate of termination decreases because the two growing chain ends cannot find each other. We know from previous lecture that the rate of polymerization is: 1/ 2 ⎛ 2k f [I ] ⎞ ⎟⎟ R p = k p ⎜⎜ d ⎝ kt ⎠ [M ] And the number average degree of polymerization is: Pn = 2aν where ½ ≤ a ≤ 1 a = 1 for coupling a = ½ for disproportionation Where ν is the number of monomers added per effective free radical and given by: ν = k p [M ] (2k d fd t [I ])1 / 2 See Figure 3-15 in Odian for a plot that demonstrates autoacceleration. Emulsion Polymerization (See handout: Ch. 12) • In emulsion polymerization there are some key “ingredients”: o The monomer must be insoluble in water and polymerizable by free radicals o Water-soluble initiator o Water o Surfactant Citation: Professor Paula Hammond, 10.569 Synthesis of Polymers Fall 2006 materials, MIT OpenCourseWare (http://ocw.mit.edu/index.html), Massachusetts Institute of Technology, Date. Formation of micelles In water, soap (e.g. sodium palmitate) is added to the aqueous phase. For example: CH3(CH2)n – COO- Na+ Hydrophobic tail Hydrophilic head group The surfactant forms micelles at high concentrations: Add monomer and disperse as droplets large monomer drops (~mm) I I I micelle with monomer (1Å to 0.1μm) and initiator (I) I empty micelle • • • The initiator (I) can diffuse into the micelle particle and polymerize the monomer More monomer from the droplet diffuses over and polymerizes Thus the particle size increases 10.569, Synthesis of Polymers, Fall 2006 Prof. Paula Hammond Lecture 14 Page 2 of 5 Citation: Professor Paula Hammond, 10.569 Synthesis of Polymers Fall 2006 materials, MIT OpenCourseWare (http://ocw.mit.edu/index.html), Massachusetts Institute of Technology, Date. The three stages of emulsion polymerization: Stage I: o o o o The monomer diffuses to the empty micelle from droplet Polymerization initiated in micelles to become polymer particles New particles are generated as micelles are consumed This stage lasts for conversion π ~ 0-15% R• Monomer diffuses from droplet Stage II: o o o No more surfactant available to generate new particles Monomer diffuses into now a constant number of particles to maintain some equilibrium [M] with the particle The monomer reservoir drops get slowly consumed monomer droplet Polymer grows from 1-3nm to tens of nm o (Droplet much bigger than particles) An equilibrium between increasing interfacial tension within micelle and monomer/polymer dilution leads to a constant volume fraction where Ф2 is the volume fraction of the polymer and Ф1 is the volume fraction of the monomer [M ] = [M ]0 (1 − Φ 2 ) Stage III: o o This stage occurs when conversion π ~ 40-60% All the monomers exist in particles 10.569, Synthesis of Polymers, Fall 2006 Prof. Paula Hammond Lecture 14 Page 3 of 5 Citation: Professor Paula Hammond, 10.569 Synthesis of Polymers Fall 2006 materials, MIT OpenCourseWare (http://ocw.mit.edu/index.html), Massachusetts Institute of Technology, Date. o o o o The monomer droplets are exhausted [M] in particles continually decreasing as π increases Particle size is constant Conversion rate can reach up to 80-100% Smith-Ewart Model o The Smith-Ewart model states that one radical diffuses into a particle at a time and termination occurs when another radical R• diffuses in: R• o MM M R R There is 1-100 seconds between radical entry events Vs droplet o R particles Why does polymerization happen in the particle rather than the droplet? Answer: Large polymer particles have a much lower surface area to volume ratio than the small droplets. The probability of a free radical entering a droplet versus a micelle particle is very small. Kinetics: o o Assumptions: All radicals enter the micelle When radical enters a particle, there is instant initiation if there are no other radicals present. If there is a radical already present, termination occurs The number of monomer molecules converted to polymer per second per particle is given by: − dm 1 monomers = k p [M ] = dt 2 sec∗ radical ½ because the chain grows half the time (the other ½ is to terminate the polymer) Initiators (Ch. 12.1.3, p. 251 in handout) o Rate of production of initiator radicals is given by: 10.569, Synthesis of Polymers, Fall 2006 Prof. Paula Hammond Lecture 14 Page 4 of 5 Citation: Professor Paula Hammond, 10.569 Synthesis of Polymers Fall 2006 materials, MIT OpenCourseWare (http://ocw.mit.edu/index.html), Massachusetts Institute of Technology, Date. ρ= NA 10 3 (1 − Φ )k d ,r [ X ][Y ] = radicals cm 3 ∗ sec Where Ф is the organic (monomer) volume fraction kd,r is the kd for the radical [X] and [Y] are the concentration of the redox initiators o The number of particles in solution is given by: n = (# of particles)/cm3 ⎛ dν ⎞ n = (0.53)( c s a s ) 3 / 5 ρ 2 / 5 ⎜ ⎟ ⎝ dt ⎠ −2 / 5 Where cs is the concentration of the surfactant as is the area of the surfactant head group o Define Δt as the time between R• entry: Δt = n ρ is usually 1-100 sec p n = k p [ M ]Δt = k p [ M ] 10.569, Synthesis of Polymers, Fall 2006 Prof. Paula Hammond n ρ Lecture 14 Page 5 of 5 Citation: Professor Paula Hammond, 10.569 Synthesis of Polymers Fall 2006 materials, MIT OpenCourseWare (http://ocw.mit.edu/index.html), Massachusetts Institute of Technology, Date.