Page 251 - Carrahers_Polymer_Chemistry,_Eighth_Edition
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214                                                    Carraher’s Polymer Chemistry


                    The most widely used acrylic plastics are PMMA (such as Lucite) and copolymers of methyl
                 methacrylate that contain small amounts (2%–18%) of methyl or ethyl acrylate (such as Plexiglas).
                 These materials are available as sheets and molding powders. PMMA is more resistant to impact
                 than PS or glass but its scratch resistance is inferior to glass. PMMA is widely used in the creation
                 of automotive (light) lenses for tail lights and some front light casings.
                    Poly(methyl methacrylate) is resistant to nonoxidizing acids, bases, and salts at ordinary temper-
                 atures but is attacked by oxidizing acids at room temperature. It is resistant to highly polar solvents
                 such as ethanol but is soluble in less-polar solvents such as toluene.
                    Polymers such as the methacrylates play an essential role as photoresists. To make a photoresist,
                 the methacrylate polymer is deposited onto silicon dioxide. A mask, which shields specifi c regions
                 from subsequent exposure to light, is placed over the methacrylate polymer resist. The combination
                 is exposed to light of such a strength as to induce methacrylate polymer bond breakage producing a
                 somewhat degraded methacrylate product that is more soluble in organic liquids, allowing the pref-
                 erential removal of the exposed photoresist. These methacrylate polymers are especially designed
                 to allow both easy degradation and subsequent easy removal. One of these is a copolymer with
                 adamantane and lactone-containing units (6.63).


                                                                                 O
                                         CH 3               O
                                                   O
                                                                          O
                                                             R

                                   R
                                                      CH 3
                                                                                            (6.63)
                                       O         O
                                H C
                                 3







                                     Modified methacrylate copolymer
                    This type of photoresist is called a “positive photoresist” since it is the exposed area that is removed.
                    Negative photoresists are formed from polymers that undergo reactions that decrease their solu-
                 bility when exposed to radiation. Thus, polymers such as cis-1,4-polyisoprene (6.64) are used that
                 crosslink when exposed to the appropriate radiation giving insoluble products.

                                            R                         R


                                                                                            (6.64)

                                                H 3 C
                    Since the 1950s, synthetic polymers have been used as art binders. The most common groups are
                 referred to as the vinyls and acrylics by artists. Both groupings represent a wide variety of polymers
                 and copolymers and are inaccurately, or at best not accurately, named. For polymer scientists, the
                 term vinyl generally refers to PVC, but to artists it may refer to many other materials. Even so, most
                 of the synthetic paint market today is based on acrylics. Acrylic paints are typically water emulsions







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         K10478.indb   214                                                                    9/14/2010   3:39:44 PM
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