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Química de Polímeros · Structure, properties and degradation

Propriedades, degradação e reciclagem de polímeros

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O que você vai aprender

  • Relate the glass transition and melting temperatures of a polymer to chain stiffness, side groups, plasticizers and molar mass.
  • Explain how tacticity, branching and comonomers control crystallinity, and calculate crystallinity from DSC or density data.
  • Classify copolymers and crosslinked networks and explain how vulcanization and physical crosslinks make elastomers.
  • Draw acid- and base-promoted hydrolysis of polyester linkages and the radical autoxidation and chain scission of polypropylene.
  • Choose and draw chemical recycling routes such as PET glycolysis and PLA depolymerization, and predict the monomers they return.

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Glass transition and melting

An amorphous polymer below its glass transition temperature Tg is a rigid glass: the chains are frozen in place. Above Tg, segments of 10–50 backbone atoms can rotate cooperatively, and the material becomes rubbery or, if not crosslinked, a viscous melt. Tg is not a melting point. There are no crystals to melt and no latent heat; DSC shows only a step in heat capacity. Crystalline regions melt at a higher temperature Tm, with an endothermic peak. Anything that makes backbone rotation harder raises Tg: rigid rings in the chain (polycarbonate, about 150 °C), bulky or polar side groups (polystyrene, about 100 °C; PVC, about 80 °C). Flexible Si–O or C=C–C backbones give low values (PDMS about −125 °C, natural rubber about −70 °C). Long flexible side chains space the backbones apart and lower Tg, so poly(n-butyl methacrylate) (about 20 °C) is much softer than PMMA (about 105 °C). A plasticizer does the same from outside the chain. Chain ends add free volume, so Tg falls at low molar mass: Tg = Tg∞ − K/Mn (Flory–Fox). For a random copolymer the Fox equation, 1/Tg = w₁/Tg,₁ + w₂/Tg,₂ with temperatures in kelvin, gives a Tg between those of the homopolymers.

Crystallinity, tacticity and branching

Only regular chains pack into crystals. Linear high-density polyethylene is 60–80 % crystalline (density 0.94–0.97 g/cm³). Short branches in low-density polyethylene and the comonomer branches of linear low-density polyethylene disrupt packing, lowering crystallinity, density and Tm. Tacticity matters for chains with stereocentres. In isotactic polypropylene every methyl group has the same configuration, so the chain forms a regular helix and crystallizes (Tm about 165 °C). Atactic polypropylene, with random configurations, cannot crystallize and is a soft, tacky solid. Polymers are never fully crystalline, so crystallinity is measured. By DSC, the mass fraction crystalline is χ = (ΔHm − ΔHcc)/ΔHm°, where ΔHcc is any cold-crystallization exotherm formed during the scan and ΔHm° is the melting enthalpy of a perfect crystal (about 93 J/g for PLA, 140 J/g for PET, 293 J/g for polyethylene). By density, χ = ρc(ρ − ρa)/[ρ(ρc − ρa)]. A quenched, glassy PET sample heated in a DSC shows Tg near 75 °C, then a cold-crystallization exotherm near 130 °C as the now-mobile chains order, then melting near 255 °C.

Copolymers, crosslinks and elastomers

Copolymers are named by sequence: random (…ABBABAAB…), alternating (…ABAB…), block (…AAAA–BBBB…) and graft (B chains attached as branches to an A backbone). Incompatible blocks separate into nanometre domains. In SBS, a polystyrene–polybutadiene–polystyrene triblock, glassy polystyrene domains (Tg about 100 °C) tie together rubbery polybutadiene chains (Tg about −90 °C), so SBS behaves as a rubber at room temperature yet flows when heated: a thermoplastic elastomer. Permanent covalent crosslinks give a thermoset or a vulcanized rubber, which swells in solvent but does not dissolve or melt. Sulfur vulcanization links chains through short polysulfide bridges at allylic positions next to the C=C bonds that diene rubbers keep. Butyl rubber is mostly polyisobutylene, which has no C=C bonds, so about 2 % isoprene is copolymerized in to give cure sites. A rubber is elastic only above its Tg, and for an ideal network the shear modulus is G = ρRT/Mc, where Mc is the molar mass between crosslinks: more crosslinks, smaller Mc, stiffer rubber.

Hydrolytic degradation of polyesters

Polymers with C–C backbones (polyethylene, polystyrene, PVC) have no backbone bond that water can cleave. Polyesters such as PLA, poly(glycolic acid) and PET hydrolyse by ordinary acyl substitution. In base, hydroxide adds to the ester carbonyl and the tetrahedral intermediate expels the alkoxide; deprotonation of the carboxylic acid makes the cleavage irreversible. In acid, the protonated carbonyl is attacked by water and the alcohol leaves after proton transfers. Each scission creates a new carboxylic acid end, which catalyses further hydrolysis, so polyester hydrolysis is autocatalytic. In a thick PLA part, the acidic oligomers formed inside cannot diffuse out, and the centre degrades faster than the surface. Poly(glycolic acid) lacks PLA's methyl group, takes up water more readily and loses strength in weeks, while PLA takes many months or years. Every random chain scission adds one chain, so the average number of scissions per original chain is Mn,0/Mn − 1. PLA is industrially compostable: near 58 °C, around its Tg, abiotic hydrolysis cuts the chains to oligomers and lactic acid that microbes consume. In cold soil or seawater it persists for years.

Oxidative degradation of polyolefins

Heat, UV light and air degrade polyolefins by a radical chain. A peroxyl radical ROO• abstracts a hydrogen; in polypropylene it takes the tertiary hydrogen, the weakest C–H bond in the chain (about 400 kJ/mol against about 410 kJ/mol for secondary C–H). The tertiary chain radical adds O₂ to give a new peroxyl radical, which continues the chain, and the hydroperoxides that accumulate break at the weak O–O bond to start new chains. The tertiary alkoxyl radical formed this way undergoes β-scission: the backbone C–C bond breaks, a methyl ketone forms at one chain end and a primary alkyl radical at the other. Polypropylene therefore loses molar mass and becomes brittle, while polyethylene, with no tertiary hydrogens, oxidizes more slowly. IR spectra of aged samples show a growing carbonyl band near 1715 cm⁻¹ and a broad O–H band near 3400 cm⁻¹. Hindered phenol antioxidants donate their O–H hydrogen to ROO• and stop the chain.

Recycling chemistry

Mechanical recycling melts and reshapes the polymer, so the chains must survive the melt. Damp PET hydrolyses at its melt temperature (about 280 °C) and loses molar mass, so flakes are dried first; PVC contamination releases HCl that degrades PET. Chemical recycling breaks the polymer back to monomers. For PET, excess ethylene glycol near 190 °C with a catalyst cleaves every ester by transesterification (glycolysis) and gives bis(2-hydroxyethyl) terephthalate (BHET, drawn); methanol gives dimethyl terephthalate and ethylene glycol; water gives terephthalic acid. Each is an equilibrium driven by a large excess of the cleaving reagent. Polymers made by ring-opening can be unzipped to the ring: PLA heated with a catalyst under vacuum releases lactide as the chain-end alkoxide attacks the ester two units along. Lactide is distilled out, and above the ceiling temperature, Tc = ΔHp/ΔSp (where ΔGp = 0), the entropy gained by releasing a small molecule wins. Nylon 6 gives back ε-caprolactam the same way, and PMMA heated above 300 °C, well above its ceiling temperature, unzips to methyl methacrylate in high yield. Polyethylene breaks by random C–C scission into a mixture of waxes, oils and gases, so it cannot return its monomer cleanly.

What to carry forward

Chain stiffness, regularity and crosslinks set a polymer's Tg, crystallinity and elasticity. Degradation follows ordinary mechanisms: acyl substitution cuts ester backbones, and radical autoxidation with β-scission cuts polyolefins at tertiary carbons. Chemical recycling runs the same acyl substitutions with glycol, methanol or water in excess, or unzips a ring-opened polymer back to its ring.

Problemas de prática (40)

Praticar é grátis com uma conta.

Desenhe o mecanismo 6

Hydrolytic and oxidative degradation

  • Alkaline hydrolysis of a PET ester linkageDraw the hydroxide-promoted hydrolysis of a PET-type ester and explain why the final proton transfer makes base hydrolysis irreversible.
  • Acid-catalysed hydrolysis of a PLA ester linkageDraw the acid-catalysed hydrolysis of a lactate ester linkage with hydronium and water as the only acid and base, and explain why polyester hydrolysis is autocatalytic.
  • Autoxidation of polypropylene at a tertiary carbonDraw the propagation cycle of polypropylene autoxidation with fishhook arrows: hydrogen abstraction from the tertiary carbon by a peroxyl radical, then O₂ capture that regenerates a peroxyl radical.
  • β-Scission cuts the polypropylene backboneDraw homolysis of a polypropylene hydroperoxide and the β-scission of the tertiary alkoxyl radical that breaks the polymer backbone into a methyl ketone and a primary alkyl radical.

Recycling chemistry

  • Glycolysis of PET to bis(2-hydroxyethyl) terephthalateDraw the base-catalysed transesterification in which 2-hydroxyethoxide cuts a PET ester linkage, and explain why excess ethylene glycol converts the polymer into bis(2-hydroxyethyl) terephthalate.
  • Backbiting depolymerization of PLA to lactideDraw the intramolecular transesterification in which a PLA chain-end alkoxide releases a six-membered lactide ring, and explain why heat and vacuum drive chemical recycling to the monomer.

Preveja o produto 4

Copolymers, crosslinks and elastomers

  • More comonomer in an ethylene copolymerPredict how comonomer branch content changes the density and melting point of an ethylene copolymer.

Recycling chemistry

  • Chemical recycling of nylon 6Predict the monomer recovered when nylon 6 is depolymerized with steam and acid.
  • Methanolysis of PETPredict the products of PET methanolysis by applying transesterification at each ester linkage.
  • What alkaline hydrolysis of polycarbonate returnsPredict the aromatic monomer recovered by alkaline hydrolysis of bisphenol A polycarbonate.

Classificar 4

Copolymers, crosslinks and elastomers

  • What kind of copolymer is HIPS?Classify a copolymer as random, alternating, block or graft from its chain architecture.
  • Which material is a thermoset?Classify a material as a thermoset from its covalent network structure.

Hydrolytic and oxidative degradation

  • Which backbone can water cut?Identify the polymer whose backbone contains hydrolysable linkages.

Thermal transitions and crystallinity

  • A step in the DSC trace of polystyreneIdentify the glass transition as the onset of segmental motion in an amorphous polymer and distinguish it from melting.

Comparar 7

Hydrolytic and oxidative degradation

  • Why do polyester sutures dissolve but nylon sutures last?Compare the hydrolysis rates of ester and amide linkages in polymers using carbonyl electrophilicity.
  • Poly(glycolic acid) versus poly(lactic acid)Explain why poly(glycolic acid) hydrolyses faster than poly(lactic acid).

Copolymers, crosslinks and elastomers

  • cis- versus trans-polyisopreneExplain how double-bond geometry decides whether polyisoprene is a rubber or a hard crystalline solid.

Thermal transitions and crystallinity

  • Branched versus linear polyethyleneCompare the crystallinity, density and melting point of branched and linear polyethylene.
  • Methacrylate side chains and TgRank poly(n-alkyl methacrylate)s by Tg using internal plasticization by the ester side chain.
  • Rank polymers by glass transition temperatureRank polymers by Tg from backbone flexibility and the size and placement of side groups.

Recycling chemistry

  • Why PMMA unzips and polyethylene does notCompare depropagation and random chain scission in the pyrolysis of PMMA and polyethylene.

Calcular 8

Recycling chemistry

  • Mass of BHET from 1 kg of PETCalculate the theoretical mass of BHET obtained by complete glycolysis of a given mass of PET.
  • Ceiling temperature of methyl methacrylateCalculate the ceiling temperature of methyl methacrylate and use it to explain why PMMA can be recycled by thermal unzipping to monomer.

Thermal transitions and crystallinity

  • Tg of a low-molar-mass polystyreneUse the Flory–Fox equation to calculate how a low molar mass lowers Tg.
  • Crystallinity of polyethylene from densityCalculate the mass-fraction crystallinity of polyethylene from its density and the amorphous and crystal densities.
  • Crystallinity of a PLA part from DSCCalculate the initial crystallinity of a polymer from DSC melting and cold-crystallization enthalpies.

Copolymers, crosslinks and elastomers

  • Tg of a styrene–butyl acrylate copolymerUse the Fox equation to calculate the Tg of a random copolymer from weight fractions.
  • Stiffness of a crosslinked rubberCalculate the shear modulus of an ideal rubber network from the molar mass between crosslinks.

Hydrolytic and oxidative degradation

  • How many cuts per chain?Calculate the average number of chain scissions per original chain from the change in Mn.

Encontre o erro 1

  • Dark, brittle recycled PETDiagnose the chemistry by which PVC contamination degrades PET during melt recycling.

Interprete as evidências 2

Hydrolytic and oxidative degradation

  • New IR bands in weathered polypropyleneInterpret the IR bands that grow during photo-oxidation of polypropylene.

Thermal transitions and crystallinity

  • Events in the DSC scan of quenched PETOrder the glass transition, cold crystallization and melting of a quenched semicrystallizable polymer on heating.

Trabalhe de trás para frente 1

  • Which monomers make PET?Disconnect the ester linkages of PET to identify its diacid and diol monomers.

Planeje uma síntese 2

Copolymers, crosslinks and elastomers

  • Giving butyl rubber cure sitesChoose the comonomer that leaves C=C bonds in a saturated rubber so it can be vulcanized with sulfur.

Recycling chemistry

  • Preparing PET flakes for re-extrusionPlan the pretreatment that prevents hydrolytic chain scission when recycled PET is remelted.

Explique por quê 5

Hydrolytic and oxidative degradation

  • Why PLA needs an industrial composterExplain why PLA biodegrades in industrial compost but persists in cold environments.

Thermal transitions and crystallinity

  • What a plasticizer does to PVCExplain how a small-molecule plasticizer lowers the glass transition temperature of PVC.
  • Isotactic versus atactic polypropyleneExplain how tacticity determines whether polypropylene can crystallize.

Copolymers, crosslinks and elastomers

  • Why SBS needs no vulcanizationExplain how glassy end-block domains act as physical crosslinks in a thermoplastic elastomer.
  • What vulcanization does to rubberDescribe the chemical and physical changes when natural rubber is vulcanized with sulfur.

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