10.569 Synthesis of Polymers Prof. Paula Hammond Lecture 7: Crosslinking and Branching, Network Formation and Gelation, Carothers Equation, Pn Approach Polyimides O O O O CC C C H2NR NH2 + O R' O R N R' N CC C C O O O O Staged formation of Polyimides Stage 1: Formation of polyamic acid H2N O NH2 + HO C C C C O O O O O ring strain O O O more reactive H2N O HN C O O HN C O HOC C COH O HN HOC C C O O O O n Polyamic Acid • T ∼ 30oC – 50oC b/c anhydride is reactive • No side product I OO • Can CAST polyamic acid or SPIN into final form • Relatively soluble in many solvents Citation: Professor Paula Hammond, 10.569 Synthesis of Polymers Fall 2006 materials, MIT OpenCourseWare (http://ocw.mit.edu/index.html), Massachusetts Institute of Technology, Date.III Stage 2: Cyclization • reaction takes place in solid state or near solid state • H2O removal OO I -H2O Δ N C C C N O n C OO rigid rigid T > 150oC Kapton® Low P (vacuum) Pyralin® Final product is intractable Vespel® Aromatic Polyamides “Wholly” aromatic O O H H N Ar N C Ar C O Ar = H NAr C p-phenylene O O O H H H NAr N C Ar' C NAr" C m-phenylene • very high stiffness, modulus • high mechanical strength biphenyl • solvent resistance • “performance” polymers or• hydrogen bonding + regular structure ⇒ very stable crystallites “aramids” 10.569, Synthesis of Polymers, Fall 2006 Lecture 7 Prof. Paula Hammond Page 2 of 5 Citation: Professor Paula Hammond, 10.569 Synthesis of Polymers Fall 2006 materials, MIT OpenCourseWare (http://ocw.mit.edu/index.html), Massachusetts Institute of Technology, Date.C OO H H H-bondingC C N N β sheets ⇒ semicrystalline O O very high melting points H H N N C C rigid backbones ⇒ liquid crystal phases in soln Example: fibrils O O H H C Kevlar® (Dupont) N N C Compare to: high tensile steel Ultimate TS ε at break Energy at break Weakness: Low compressive strength. (analogy: broom straws) Kevlar 49 3.6 GPa 2.7% 25 MJ/m3 High tensile steel 1.5 GPa 0.8% 6 MJ/m3 How to react? (making aramids) • Bulk melt: Tm way too high! • Interfacial polymerization: possible Get product as precipitate at interface Works for partially aromatic polyamides NotSolvent: solvate low + mod MW’s possibleRemain phase separated from H2O • Solution polymerization: O + H2N reactive groups Cl highly reactive – allows dilution Reaction conditions: T ∼ 25oC – 50oC or lower Add Li2CO3, Na2CO3, CaOH Solvents: must be very polar, H-binding groups Advantageous if also sol basic ⇒ neutralize HCl 10.569, Synthesis of Polymers, Fall 2006 Lecture 7 Prof. Paula Hammond Page 3 of 5 Citation: Professor Paula Hammond, 10.569 Synthesis of Polymers Fall 2006 materials, MIT OpenCourseWare (http://ocw.mit.edu/index.html), Massachusetts Institute of Technology, Date.Often add LiCl or other Li salts to solvent ⇒ aid in H-bond break-up Common Solvents: CHCl3 Less polar CH2Cl2 CH3CN Cl-CH2-Br DMAc H3CC O N NMP N CH3 O DMSO More polar Kevlar®: Tm = 570oC Tg = ? Tdeg = 550oC in N2 Eo = 6000 – 8000 kg/mm2 Slight Change: go from p (para) to m (meta) linkages NH HN C C O O Tm = 435oC Tg = 272oC Eo =2000 kg/mm2 Stretches out more Branching and Network Formation So far: difunctional monomers: f = 2 When monomer functionality f ≥ 3 ⇒ + can form branches networks • crosslinks are +can form branchesnetworks• crosslinks areindividual junctionsindividual junctions• networks are• networks areinfinitely largeinfinitely largeExamples: 1. a—b + a—a → branches a 2. a—b + af + b—b → branches, then crosslinks 10.569, Synthesis of Polymers, Fall 2006 Lecture 7 Prof. Paula Hammond Page 4 of 5 Citation: Professor Paula Hammond, 10.569 Synthesis of Polymers Fall 2006 materials, MIT OpenCourseWare (http://ocw.mit.edu/index.html), Massachusetts Institute of Technology, Date.3. a—a + b—b + bf → branches, then crosslinks 4. af + bf → branches, crosslinked networks 10.569, Synthesis of Polymers, Fall 2006 Lecture 7 Prof. Paula Hammond Page 5 of 5 Citation: Professor Paula Hammond, 10.569 Synthesis of Polymers Fall 2006 materials, MIT OpenCourseWare (http://ocw.mit.edu/index.html), Massachusetts Institute of Technology,