II. Submolecular Structure Chapter Chemical Composition

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1 II. Submolecular Structure Chapter Chemical Composition

2 Chemical Composition atoms (elements) functional groups impurities additives (Table 2.1) 2-1. Chemical composition (atoms & functional groups) (Table 2-2. Periodic table) Electronegativity or positivity strength and types of bonds polymer reactivity thermal stability UV stability weather resistance aging polarity of the bonds electric properties refractive index solubility permeability surface energy surface tension, adhesion, friction

3 1. Carbon C-C bond by itself : neutral, stable (ex, diamond, graphite) attachment of other atoms to C polarity lowers stability (increase reactivity) C=C : less stable, reactive to oxygen, ozone, halogens, HX,.. can be used for vulcanization (X-linking) of rubber *terminal C=C : more reactive, polymerizable (sterically less hindered) *internal C=C : less reactive, mostly unpolymerizable, but, if activated, still usable (ex, unsaturated polyesters) ----CO-CH=CH-CO---- *conjugated C=C systems (alternating double bonds) above 6 conjugated C=C bonds absorb visible light (blue) : colored (yellow) ex) PVC discoloration (see Fig 2-10) *aromatic double bonds : much more stable due to resonance ring structure high thermal stability high temperature resins *extended conjugation delocalization of π bonds may cause electrical conduction (when doped) ex) polyacetylene, polythiophene, polypyrrole, polyaniline..

4 2. Hydrogen C-H : fairly neutral, (electronegativity, C 2.5; H 2.1), stable at normal condition more reactive than C-C point of attack chemical reactivity, instability heat-or light activated atmospheric oxidation halogenation possible w/o O 2 : sufficient thermal and UV stability reactivity (stability) depends on status of C primary < secondary < tertiary H sensitive to free radical attack, oxidation, UV scission, substitution, X-linking < α-carbon, allyl structure : carbon next to C=C bond vulcanization of rubber, oxidative curing.. ex) aging of polyester

5 aliphatic C-H : low surface energy, low surface tension low adhesion, low friction Table 2-4 to 6 aromatic C-H : quite stable (more stable than aliphatic C-H) moderate surface tension, higher adhesion OH, or NH 2 activate ortho, and para positions cure ofphenolicresins (novolac) oxidative polymerization of PPO O-H : highly reactive cellulose esters, polyesters, polyvinyl acetals polymerization and cure of urea, melamine, phenolic resins high polarity H-bonding increase dielectric properties water absorption

6 3. Oxygen O : e.n. 3.5 ; more electronegative than C, H C-O, H-O : more polar, reactive strong H-bonding : due to two pairs of unshared electrons on O high surface energy (surface tension) due to unshared electrons and polar bonds C-O-C (ether) aromatic ether : extremely stable (by resonance contribution) aliphatic ether : activates adjacent C-H bond easy peroxidation of α-carbon ex) acetals : sensitive to hydrolysis or oxidation epoxy : reactive C-O + highly strained 3-membered ring highly reactive easy cure of epoxy resins (good adhesives) C=O (carbonyl, ketone) : polar, resonating : increase reactivity of α-groups absorb near UV nm instability to UV aging can be used as additives for UV stabilizer CO-O (ester) : sensitive to hydrolysis : some polyesters use : PVAc to PVA

7 COOH : adhesion, latex, solubility in alkai, thermoplastic X-linking (ionomer) O-O (peroxide) : very unstable initiator of free radical polymerization during polymerization under O 2 post-polymerization atmo peroxidation thermal degradation 4. Nitrogen N : e.n. 3.0 (C 2.5, O 3.5) C-N : covalent, fairly strong, fairly polar strong adhesion to polar substrates one pair of unshared electrons of N H-bonding to adjacent H (Fig.2-3) quarternary ammonium compounds : generally stable ion exchange resin, cationic detergent

8 C=N :strong bond but weak to hydrolysis aromatic C=N : linear linkage useful for mesogenic liquid crystal & liquid crystalline polymer CN (nitrile group) : strongly ABS, SAN, arylic fiber, nitrile rubber highly exposed, linear v. strong polarity therefore, v. strong H-bonding hardness, stiffness heat resistance, chemical resistance PAN : heat may form ladder polymer : conjugation conductivity N-H : fairly reactive, polar, H-bonding formation of polyamide cure of epoxy, urethane, urea, melamine resins modification of nylons

9 CO-NH (amide) : polar, strong bond v. strong H-bonding : places for H-bonding with water absorption of water dimensional instability (fig. 2-6, 7) hydrolysis degradation (hydrophilic) high surface energy, surface tension adhesives aromatic polyamides : high performance polymers when para linked, liquid crystalline (lyotropic) phase be formed ex) kevlar : extended linear chain, strong intermolecular forces excellent mechanical, and thermal properties bullet proof Fibres of KEVLARR consist of long molecular chains produced from poly-paraphenylene terephthalamide. The chains are highly oriented with strong interchain bonding which result in a unique combination of properties.

10 N=C=O (isocyanate) : highly reactive (polymerizable) casting of polyurethane elastomers adhesives foaming and cure of polyurethane flexible and rigid foam isocyanate reactions (fig. 2-8) -NH-CO-O-(urethane group) : fairly stable one more oxygen than amide strong H-bonding hydrolysis and maydecoposeunder the heat more flexible linkage than amide kinked linkaged (not linear) good for elastomers and adhesives Heterocycles containing N: ex) melamine strong resonance stabilization mostly stronger than C aromatic systems new ultra high strength polymers unusual electric and optical properties -ONO2 (nitrate) : unstable oxidizing agent, high flammability some thin film applications only

5. Halogens electronegative small atomic radii F : smallest, most electronegative C-F : small bond length, strong bond extremely low surface tension (hydrophobic) nonwetting high lubricity stain

11 5. Halogens electronegative small atomic radii F : smallest, most electronegative C-F : small bond length, strong bond extremely low surface tension (hydrophobic) nonwetting high lubricity stain proof fabric nonstick cooking utensils mold lubricants self-lubricating bearing surfaces why? These unusual surface electrons are held closely and tightly about the fluorine nucleus and cannot be shared and or easily polarized thus shielding the nucleus and preventing the formation of weak bonds

12 chemically stable high thermal stability UV stability but may release HF under severe conditions for an old poly(tetrafluoroethylene), Teflon story : for commercial Teflons flame resistance due to F atom (and all halogen atoms) : why? Electric properties : low dielectric constant, high dielectric break down good insulator

13 Optical properties -no optical absorption from near UV to near infrared no absorption in near UV -> UV stability why? tightly bound electrons of F requires higher photon energy (shorter w.l.) for excitation no absorption in near IR useful for optical transmission medium in telecommunication wavelengths (1.3, 1.55 μm) plastic optical fibers, waveguides why? no overtone or combination band at those wavelengths C-H vibrational overtone bands absorb near IR region which means loss of the signal C-F : overtone bands shift to longer wavelength because fundamental vibration occurs at longer wavelength. (due to high mass of F) ν = transparent at near IR wavelengths

14 polymers w/ F polytetrafluoroethylene(ptfe) polyvinylidenefluoride(pvf2) polyvinylfluoride (PVF) polyhexafluoropropylene (PHFP) polyvinylethers many copolymers new amorphous polymers : Teflon AF, Cytop. Cl : larger and slightly less negative than F strong covalent C-Cl bond good flame resistance major reason for use of PVC in building, electrical, and transport applications why? Combustion : radical reaction flame propagation ( H, HO ) halogens prevent flame propagation because they form relatively stable (relatively less reactive radicals) intermediate RX + H R-H + X RX + HO R-OH + X stability of radicals : F < Cl< Br < I : increasing degree of effective flame resistance but, may form toxic dioxin (polychlorobenzodioxin) when burned

15 strong polarity H-bonding (Fig. 2-9) strength and plasticizer compatibility high surface tension and adhesion : due to high polarity : dipole-dipole interaction and relatively loose (polarizable) outer electrons of Cl: induced dipole-dipole interaction when Cl is on tertiary or allylic C : thermal, UV, chemical instability Fig Br larger and less negative than Cl fairly stable C-Br bond weak thermal and UV stability discoloration high flame resistance I (iodine) : largest and least electronegative of the halogens unstable C-I bond

16 6. Sulfur similar to O (in the same VI column) less electronegative and larger C-S-C (sulfide) bond aliphatic : sensitive to atmospheric or chemical reaction aromatic : stable strong bond high temperature resin (PPS) due to resonance and steric hindrance of benzene ring protecting sulfide group -Sx-: polysulfide rubber : fairly weak bond useful in vulcanization, but stress relaxation by bond rearrangment Fig SO2-(sulfone) : stable Oxygens strong H-bonding aromatic sulfones : increase conjugation (resonance) high termal stability aliphatic : v. low thermal stability depolymerize no practical use

17 7. Silicon larger and more electropositive than C Si-Si: unstable to UV (positive photoresist applicable), though to be unstable polysilane : recent some polysilanes show thermally stable structure unusual sigma electron delocalization along the backbone (vs pi e. delocalization of C) Si-H bond : unstable, very reactive useful in substitution of silicone derivatives Si-C (organosilane) : very stable, high temperature stability silicone polymers : for vulcanization use C-H bond break -CH3 groups on silicone low surface energy, high lubricity Si-OH (silanol) : very reactive intermediate polymerize rapidly silicone Si-OR (organosiloxane) : quite reactive, polymerize sol-gel reaction forming Si-O-Si bond : very stable to heat (ex, glass), silicone rubber

18 8. Phosphorus slighlty more electropositive than N well known flame retardant effects : most use of P due to formation of phosphorus-oxygen glass at the surface (coating theory) + PO radical traps H, HO radicals (radical scavenging) P-C bond : may be accompanied by toxicity, not commonlly used 9. Metals traditional use : Na+, Zn++ ionic X-linking w/ acid groups (thermoplastic X-linking, thermoplastic elastomer = ionomer) processability, toughness, adhesion new applications : utilize different properties of metals from polymers how to incorporate metals into the polymer structure? chelating polymers : binds metal ions by coordination bonds forming macromolecular complex passivation of particles (nanoparticles) electrical properties : electric conductivity, ionic conductivity

19 magnetic properties : organics : paired electrons - opposite spins diamagnetic repelled by magnetic fields * one exception :? metals : unpaired electron(s) - parallel spins paramagnetic attracted into M. field * ferromagnetic : strong cooperative interactions between spins optical properties : utilization of optical properties of metal ions fluorescence, phosphorescence, biological applications : most biological processes involve metal ions 10. Price C, H : cheapest Cl, O, N, S, Si, F : cost of raw materials and more synthetic work

20 2-2. Monomeric Ingredients i) impurity : come out shrinkage, crack monomer, solvent, water, ions, initiator, dispersant, mechanical, thermal, weatherability, electrical insulating properties ii) additives Plasticizer: monomoric liquid - improve processability (PVC, cellulose esters) should be mixable :similar structure in polarity, H-bonding should have low volatility, diffusability common choice : esters (polarity, H-bonding) w/ short chains of alkyl groups (good lubricity) major problem : not permanent comes out to surface (Table 2-8) migration, extractable by solvent softening chemical resistant, thermal,

21 Stabilizer antioxidant : oxidation of hydrocarbon polymers takes the form of? phenolic, aromatic amines (Fig. 2-12) - how they work? R-R 2R (by heat, light, shear force,,,) (1) R + O2 RO2 (fast) (2) RO2 + RH ROOH + R (propagation) (3) ROOH RO + OH (4) 2ROOH RO + RO2 + H2O (5) So, i) either (1) or (3) should be prevented or ii) those radical reactions should be terminated by interrupting the propagation cycles -we need something to stabilize the radicals

22 UV stabilizer : conjugated aromatics - absorb UV -prevent penetration of UV into the volume -conversion of energy into heat -scheme (b) below -fate of excited molecules A + energy (hν) A* (absorption) (1) Photophysical process (a) emission of energy (ex, fluorescence and phosphorescence) A* Ao + Energy emitted (light) (b) generation of heat (radiationless conversion) A* Ao + Heat (c) energy transfer A* + B Ao + B* (2) Photochemical process (d) consequent on energy transfer A* + B Ao + B* (e) reaction of excited molecules A* + B C

23 Filler : solid inorganic power : rigidity and (sometimes) cost fiber : strength, dimensional stability, trade off processability.. Clay, carbon black, graphite, metal fillers opaque, colored Cross-linking agent curing for thermosetting, x-linking of thermoplastics, vulcanization of elastomers required balance between pot life versus rate and extent of cure may introduce opacity types of X-linking (a) bridging agents (b) crosslinking initiator (c) catalytic crosslinking agents (d) active site generator Forming agent (chemical blowing agent) decompose to liberate gas - production of foams

24 Flame retarder organic phosphate (P), halogen compounds, antimony oxide thermal stability, UV stability, opacity, toxicity Lubricant improve processability reduce friction in the final product liquids, low melting waxes comes out to surface painting problem, contamination solids : graphites, MoS2 Antistatics : hold static charges ionic materials : quarternary ammonium compounds or hydrophilic materials (polyethoxylated) RO(CH2CH2O)H coated on surface, or added to polymer processing : should be semicompatible continuously migrate to the surface to work Biocides: should be semicompatible migrate to surface during use Colors : inorganic : more stable, toxic -incompatible -opaque colors organic: less stable, expensive, safer

25 * Reference on Additives, Plastics Materials, Chapter 7, J. A.Brydson,

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