Gary W. Ceska Sincloir-Koppers Co. Monaco, Pennsylvania 15061
I
I
hnulsion Polvmerization and Film Formation o f Dispersed Polymeric Particles
The following experiment served to introduce some fundamentals of polymer science to a group of science-oriented Explorer Scouts (mostly high school seniors). The project aptly illustrates the principles of emulsion polymerization, copolymerization, glass transition temperature (T,) and the effect of T, on polymer properties. The operations can he conveniently carried out in one or two sessions in an undergraduate organic or polymer laboratory. Each member of the group prepared an emulsion polymer selected from the formulations listed in Table 1. Samples of the emulsion polymers (copolymers) were placed in a Teflon muffin pan and allowed to dry (1 wk). The results were discussed a t the next session. Results and Discussions
Descriptions of the dried polymeric material are listed in Table 2. The increased tendency toward fusion (film formation) of the dispersed particles with increasing hutyl acrylate content and the gradual change from brittle solid to elastomeric film are clearly evident and directly relatable to the glass transition temperature of the two polymeric components; polystyrene, T, = 100DC; polyhutyl acrylate, T p = -55%. Assuming random copolymerization, the glass transition of any copolymer composition may he calculated by the expression'
where WI and Wz are the respective weight fractions of the copolymers and T, is the glass transition in "K. Drying the prepared samples a t higher temperatures in an oven illustrates the relationship between temperature, T e and fusion of disnersed varticles. Vanderhoff has written an excellent review of the mechanism of particle fusion and film formation of synthetic emulsion polymers.2 Conclusion
This experiment illustrates a basic technique of polymer technology: the combination of "hard" (high glass transition) polymeric components with "soft" (low glass transition) polymers to achieve a balance of properties for a specific application. The so-called "hard" polymers (polystyrene, polymethylmethacrylate) have high tensile strengths and high modulus hut are brittle compared to "soft" polymers (polyhutadiene, polyhutylacrylate) which have low tensile strengths and modulus. The proper combination, either physical or chemical, of the two is essential in many applications in the plastics, fibers, and coatings industries. Experimental
The polymerizations were run in quart pop bottles in a rotary battle polymerizer, but could he carried out in a 3-neck flask,
Table 1. Emulsion Polymerization Formulations Containing Various Ratios of Stvrene and Butvl Acrvlate
Components Styrene Butvl Amlate ~mkoniuh Persulfate Sodium Lauryl Sulfate Water
-Grams 1 50 .. .
2 40 10
of Material in Sample3 4 5 30 20
25 25
6
10 40
...
50
0.4
0.4
0.4
0.4
0.4
0.4
1.0 200
1.0 200
1.0 200
1.0 200
1.0 200
1.0 200
Table 2. Relationship between Polymeric Film Formation and Glass Transition Temperature
GlassTransition Sample Temperature, 'C 1
100
2
54
3
18
4
2.0
5
- 35
6
-55
Observation of Air-Dried Polymer Samples No particle fusion; brittle, opaque solid No particle fusion; brittle, opaque solid Partial particle fusion; slight transparency Good particle fusion; toueh. trans~arentnlastie ~oodparticlefusion; ' ela.&meric, transparent Good particle fusion; elastamerie,transparent
making sure that the reaction mixture is provided vigorous agitation and a nitrogen atmosphere. Two hundred grams of distilled water were charged to the bottle fallowed hy 1.0 g of sodium lauryl sulfate and 0.40 g of ammonium persulfate. After the surfactant and initiator were dissolved, 50 g of the appropriate monomer or monomer mixture were charged to the bottle. The bottle was thoroughly swept with nitrogen (essential for the high aerylate samples), capped, and put in the polymerizer at 70°C. All runs were finished in 2 hr. The extent of the reaction may he followed gravimetrieally by driving off water and monomers from a weighed sample in an oven set at 175'C. Theoretical solids content is approximately 20% when the reaction is complete. The emulsion polymers were dried down in a Teflon muffin pan. The high acrylate samples will adhere tenaciously to most other metal or glass surface materials. Sinclair-Koppers polymer grade styrene and Rohm and Haas polymer grade hutyl acrylate were used. High purity sodium lauryl sulfate was obtained from Alcolac Chemical Co. The ammonium persulfate was Baker Chemical reagent grade.
'
Meares, P., "Palymers: Structure and Bulk Properties," Van Nostrand Co., London, 1965,p. 262-265. Vanderhoff, J . W., Paint Vorn. Prod., 60.25 (1970).
Volume 50, Number 11, November 1973
/
767
