Lecture 15: Processing Approaches: Suspension (Bead) Polymerization Processes, Polyvinyl Chloride via Precipitation Polymerization, Polyethylene via Radical Polymerization

10.569 Synthesis of Polymers Prof. Paula Hammond Lecture 15: Processing Approaches: Suspension (bead) Polymerization Processes, Polyvinyl Chloride via Precipitation Polymerization, Polyethylene via Radical Polymerization surfactant R⋅ free radicals from redox (H2O soluble) Droplet reservoirs of monomer empty micelles micelle with monomer Rate of conversion, emulsion polymerization n pn = k p ⎡⎣M ⎤⎦ Δt = k p ⎡⎣M ⎤⎦ ρ handout In general, Rp = − n 2 dm 1 = k p ⎡⎣M ⎤⎦ dt 2 Rp = n k p ⎡⎣M ⎤⎦ 2 ρ (radicals) n pn = k p ⎡⎣M ⎤⎦ Δt = k p ⎡⎣M ⎤⎦ ρ Emulsion Polymerization: monomer → porous particles 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. introduce plasticizer more readily processible, aerodyn. in powder form Common emulsion polymers: styrene + copolymers vinyl chlorides ex. Pleather butadiene vinylidene chloride vinyl acetate vinyl acrylates (acrylics) methyl acrylates Advantages: - low η (viscosity) - great T control - final product → fine powder or water form ⇒ coatings Disadvantages: - a lot of soap as impurity ex. In medical applications, can be irritant Suspension Polymerization: (Pearl of Bead) “Ingredients” monomer – organic phase aqueous phase initiator (soluble in monomer phase) stabilizer, often a polymer, such as PVOH hydrophobic backbone CH2CH OH polar, hydrophilic sidegroup stirred PVOH is absorbed at interface of droplet organic droplets OH OH CH2CH OH much more soluble in water 10.569, Synthesis of Polymers, Fall 2006 Prof. Paula Hammond Lecture 15 Page 2 of 8 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. Organic droplets: ∼ 1 μm – 1 cm Drop size determined by impeller speed Within each droplet, have - initiator - monomer ⇒ Batch reactor within droplet Kinetics are identical to typical large scale free radical polymerization • initiation • propagation • termination • steady state assumption M M M • • M • • • M Concerned with avoiding drop coalescence → premix initiator + monomer → agitate H2O phase + add organic phase ∼ 20 – 30 % vol → adjust impellar speed to get desired drop size → add stabilizer (PVOH) → continue stirring at more gentle speed and increase T to 40oC → 80oC depending on which initiator you’re using → initiator activation → go to near complete conversion (may need to increase T for final % π) Products: - glassy rigid beads often called “latex beads” - very uniform - nice spherical shapes x-linked network + styrene divinyl benzene (tetrafunctional) 10.569, Synthesis of Polymers, Fall 2006 Prof. Paula Hammond Lecture 15 Page 3 of 8 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. vary surface chain: SnCl4 bead CH2Cl ClCH2OCH3 1. N(CH3)3 CH2 2. NaOH N OH anion exchange or H2SO4 SO3 Na cation exchange Form Pores: - add non-solvent + monomer w/organic phase Form connected channels of non-solvent → go to high π flash off non solvent If monomer (e.g. styrene) is well-dissolved in solvent, but solvent is poor for high MW polymer (e.g. cyclohexane) PS, Tg ∼ 100oC styrene cyclohexane 20 – 30 % pockets of cyclohexane PS spheres 10.569, Synthesis of Polymers, Fall 2006 Prof. Paula Hammond Increase T to 120oC ⇒ expansion – foaming effect → very large increases in size and decreases in density (mostly air) ⇒ STYROFOAM® Lecture 15 Page 4 of 8 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 following figures are adapted from page 250 of Polymer Synthesis: 1 1020 I II III n n, n* 1015 1010 n* 105 droplet phase end II [M] I I n k p [M ] 2 pn = k p [M ] n ρ (instantaneous) n↑ pn particles no longer can replenish monomer II Rp Rp = Equilibrium mon. conc [M]o n, [M] const I III [M]↓, const n II n↑ n, [M] const III III [M]↓, ν↓ time Fig 12.2. Diagrams showing the three stages of emulsion polymerization: (I) Micelles increasing; (II) micelles exhausted, droplet phase remains; (III) droplet phase exhausted. n number of particles per unit volume n* number of micelles per unit volume [M] monomer concentration in the droplets and in the particles Rp overall rate of polymerization pn instantaneous degree of polymerization 1 Rempp, P. and Edward W. Merrill. Polymer Synthesis. Second Edition. New York, NY: Hüthig and Wepf, 1991. page 250. 10.569, Synthesis of Polymers, Fall 2006 Prof. Paula Hammond Lecture 15 Page 5 of 8 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. Rate of Conversion, Emulsion Polymerization n pn = k p ⎡⎣M ⎤⎦ Δt = k p ⎡⎣M ⎤⎦ ρ rate of production of free radicals mol/l In general: Rp = n k p ⎡⎣M ⎤⎦ = 2 # particles per cm3 monomer molecules per sec per cm3 l/mol-sec * (recall that the rate, -dm/dt of disappearance of monomer molecules per particle 1 per second is k p ⎣⎡M ⎦⎤ ) 2 In general, for conversion: dπ n 1 −dm 1 = k p ⎣⎡M ⎦⎤ ⋅ = n⋅ dt 2 C dt mo =monomers/sec-cm3 C = initial # of monomer molecules charged ΘN A ρm = initial # of monomers in total volume Mu How much you’ve added where Θ = cm3 monomer charged/cm3 of total volume = volume fraction ρ = density of monomer (g/cm3) NA = Avogadro’s number = 6.023x1023 molecules/mole Mu = MW of monomer (repeat) unit For Stage I: ⎡⎣M ⎤⎦ ≅ ⎡⎣M ⎤⎦eq = ⎡⎣M ⎤⎦o (1 − Θ2 ) where [M]o = pure monomer Θ2 = vol. Fraction of polymer in particles at equilibrium n = n(t) b/c number of particles ↑ πI t kp 1 I n (t ) ⇒ ∫ dπ = ⎡M ⎤ (1 − Θ2 ) dt ∫ 2 ⎣ ⎦o C 0 0 10.569, Synthesis of Polymers, Fall 2006 Prof. Paula Hammond Lecture 15 Page 6 of 8 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. For Stage II: Surfactant consumed, droplets still exist Growing particles ⎡⎣M ⎤⎦ = constant = ⎡⎣M ⎤⎦o (1 − Θ2 ) n = constant π II t 1 II n d k p ⎣⎡M ⎦⎤o (1 − Θ2 ) dt π = ∫ C t∫I 2 πI constant ⇒ π II − π I = nk p ⎡⎣M ⎤⎦o (1 − Θ2 ) 2C (t II − tI ) Stage III: Droplets exhausted, only particles On avg, particles all same size, grow at same rate Here, monomer is consumed w/in particles, as monomer → polymer, [M]↓ w/in particle. ⎡⎣M ⎤⎦ = ⎡⎣M ⎤⎦o (1 − π ) ⇒ π 1 πII ⇒ t ∫ dπ = C ∫ k p t II −Δ ln (1 − π ) Δt = n ⎡⎣M ⎤⎦o (1 − π ) dt 2 k p ⎡⎣M ⎤⎦o n 2C 10.569, Synthesis of Polymers, Fall 2006 Prof. Paula Hammond Lecture 15 Page 7 of 8 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. III II Asymptotic value for monomer π I time For Stage I: dm ⎞ ⎛ n = f ⎜ C s ,as , ρ, ν n , dt ⎟⎠ ⎝ where Cs = surfactant concentration as = area of surfactant molecule (of polar head) ρ = rate of radical production νn = vol of a single repeat dm/dt = rate of monomer polym. Empirical expression: 2 3 dm ⎞ ⎛ n ≈ 0.53 ( C s as )5 ρ 5 ⎜ −ν n q dt ⎟⎠ ⎝ q= vol polymer + vol monomer vol polymer 10.569, Synthesis of Polymers, Fall 2006 Prof. Paula Hammond inside the particle (related to [M]) Lecture 15 Page 8 of 8 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.