Plastics 1 Plastics 2 Observations about Plastics Plastics They can take almost any shape They can be clear, translucent, or opaque They can tear or shatter They can be hard, soft, elastic, fiberous They can form by mixing chemicals They can form by evaporating solvents Turn off all electronic devices Plastics 3 Plastics 4 5 Questions about Plastics 1. How do plastics differ from ordinary molecules? 2. How does temperature affect plastics? 3. Why are some plastics clear, others translucent? 4. Why are some plastics unable to melt? 5. How do plastics form from simpler chemicals? 6. Why are some plastics so strong? Plastics 5 Question 1 Q: How do plastics differ from ordinary molecules? A: Plastics consist of giant molecules Plastic molecules are enormous Many are long linear chains Others are branched or networked They can become entangled Plastics 6 Question 2 Q: How does temperature affect plastics? A: Thermal energy allows local and distant mobility. Local Mobility Acrylic plastics (Plexiglas, Lucite) Polystyrene P l (Styrofoam, (S f plastic l i cups)) PET and PETE (Mylar, soda bottles, plastic cups) Plastics can exhibit five distinct mobilityy regimes g With increasing temperature, plastics go through: Glassy solid: not even local mobility Leathery solid: some local mobility Elastic solid: local mobility, but not long long--range mobility Rubbery flow: some long long--range mobility Liquid flow: extensive longlong-range mobility Local mobility is governed by molecular adhesion Some plastic molecules cling together tightly Others cling weakly Polyethylenes (milk jugs, grocery store bags) Natural rubber Silicones 1 Plastics 7 Plastics 8 Long--Range Mobility Long Long-range mobility is governed by reptation LongThermal energy causes chain motion small molecules that are compatible with the plastics go into solution in the plastics (or vice versa) increase in r llocall and nd llonglong n -range r n mobilities m biliti Chicle (chewing gum) Silicones Plastics 9 Examples of plasticized plastics: Solvent-based glues and paints SolventWet hair, fabrics, paper, noodles, bread Vinyl upholstery fabrics Polyethylenes (jugs, bags) Other plastics disentangle Plastics can be softened by chemical plasticizers Chain motion is called reptation R Reptation i allows ll chains h i to di disentangle l themselves h l Some plastics stay tangled Plasticizers Plastics 10 Question 3 Question 4 Q: Why are some plastics clear, others translucent? A: Some are partly crystalline, others all amorphous Q: Why are some plastics unable to melt? A: Their molecules are crosslinked in one network Some p plastics are all amorphous p They are homogenous throughout Light is undisturbed; they’re clear Reptation cannot disconnect or disentangle them They remain in the elastic regime They can’t flow, so they don’t melt They are “thermosets “thermosets”” (set shapes at all temperatures) Meltable plastics are “thermoplastics” (variable shapes) Other plastics are partly crystalline They are inhomogenous Light scatters at boundaries; translucent Plastics 11 Crosslinks tack p polymer y chains to one another Plastics 12 Question 5 Question 6 Q: How do plastics form from simpler chemicals? A: Molecular chain reactions assemble them. Q: Why are some plastics so strong? A: If all the molecules work together, they’re strong Most p plastics begin g as monomer molecules Monomers are small building block molecules Monomers bind together in chains to form polymers Plastics can have one monomer or several Plastics can be linear or branched Plastics can be orderly or more complicated Aligning g g polymer p y chains into fiber gives g strength g Organizing those chains can yield extreme strength Liquid crystal fibers are naturally organized Aramids,, Kevlar Aramids Melt--drawn fibers are organized during formation Melt Spectra 2 Plastics 13 Summary about Plastics Plastics consist of giant molecules Temperature affects local and longlong-range mobility Entanglements limit longlong-range mobility Crosslinks can prevent longlong-range mobility 3
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