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.