Page 250 - Carrahers_Polymer_Chemistry,_Eighth_Edition

Free Radical Chain Polymerization 213

Copolymers of vinyl chloride with vinyl acetate and maleic anhydride are made more adhesive
to metals through hydrolysis of the ester and anhydride units.
Copolymers of vinyl chloride and vinylidene chloride are widely used (6.61). Two extremes are
employed. The lesser known has a high vinyl chloride content compared with the vinylidene chlo-
ride comonomer. These copolymers are more easily dissolved and have greater flexibility. The most

widely known copolymer has about 90% vinylidene chloride and 10% vinyl chloride and is known
as Saran. This copolymer has low permeability to gases and vapors and is transparent. Films are
sold as Saran Wrap. Because of potential health-related issues with Saran Wrap, it has been largely
replaced.
R
R
Cl
Cl Cl (6.61)

6.8 POLY(METHYL METHACRYLATE)
Poly(methyl methacrylate) (Equation 6.62) is the most widely employed of the alkyl methacrylates.
For the most part, commercial PMMA is sold as an atactic amorphous polymer that has good

light transparency (92%) and gives transparent moldings and films. General properties are given
in Table 6.10. The presence of two substituents on every alternate carbon atom restricts chain

mobility so PMMA is less flexible than the corresponding poly(alkyl acrylates). The presence of
the alpha methyl group increases the stability of PMMA toward light-associated and chemical
degradation.

CH 3
CH 3
R
H C
2
R
O (6.62)
H C O H C
3
3
O O

Unlike the poly(alkyl methacrylates) that degrade by random chain scission, PMMA undergoes
degradation through unzipping when heated.

TABLE 6.10
General Physical Properties of PMMA
o
Heat deflection temperature (1,820 kPa; C) 95

o
Maximum resistance to continuous heat ( C) 75
Coefficient of linear expansion (cm/cm- C, 10 ) 7.0
–5

o
Compressive strength (kPa) 1 × 10 5
Flexural strength (kPa) 9.6 × 10 4
Impact strength (Izod: cm-N/cm of notch) 21
Tensile strength (kPa) 6.5 × 10 4
Ultimate elongation (%) 4
Density (g/mL) 1.2

9/14/2010 3:39:41 PM
K10478.indb 213 9/14/2010 3:39:41 PM
K10478.indb 213