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Building Porosity in Polymers: How and Why?

This conference discusses the various methods and applications of creating porous polymers. Topics include physical aggregation, suspension/emulsion polymerization, membrane casting, and more. The use of porous polymers in biomedical, separation, and optoelectronic applications is also explored.

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Building Porosity in Polymers: How and Why?

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  1. BUILDING POROSITY IN POLYMERS : HOW AND WHY ? International Conferences on Emerging Trends in Chemical Sciences, VIT University, Vellore December 5, 2013 DR. S. SIVARAM A 201, Polymers & Advanced Materials Laboratory, National Chemical Laboratory,Pune-411 008, INDIATel : 0091 20 2589 2614Fax : 0091 20 2589 2615Email : s.sivaram@ncl.res.in

  2. POROUS POLYMERS : WHY ? An important class of organic materials with a diverse range of applications • Biomedical prosthesis and implants • Separation media (RO, MF, NF and PV, gas separation) • Super-absorbing materials • Selective ion transporters ( Proton, Lithium ion) • Ion exchange resins • Catalyst and enzyme supports • Sensors • Optoelectronic devices • Insulators

  3. POROUS MATERIALS ARE UBIQUITOUS ! Alveoli in the lungs Skin Porous Carbon electrode Dialysis Membrane Polyacrylonitrile Polyurethane Floor mop Zeolite 4A Polystyrene foam cup

  4. BUILDING POROSITY IN POLYMERS : HOW ? • Physical aggregation of pre-formed polymer particles into objects with specific shape and geometry • Use of suspension / emulsion / HIPE polymerization with or without porogens • Membrane casting a polymer solution containing a porogen followed by extraction of porogen • Membrane casting a polymer solution with an inorganic metal salts capable of forming weak complexes with polymers followed by extraction of salt • Use of hard ( e.g silica spheres) or soft template (porogen) followed by removal of the template • Intrinsically micro-porous polymers through chemical synthesis • Phase inversion using a solvent and a non-solvent • Nanoporous membranes derived from self assembled block copolymers containing a sacrificial segment

  5. Physical aggregation of pre-formed polymer particles

  6. POROUS POLYETHYLENES • CHARACTERISTICS: • Particle size ~ 400-1000 microns (application specific) • Particles of narrow size distribution fused together • Porosity ~ 40-55% • Pore size ~ 100-150 microns • Interconnected pores

  7. POROUS POLYETHYLENE IMPLANTS Porous polyethylene implants are used by surgeons for non-weight bearing, volume-filling applications in the maxillo-facial, cranial and ocular regions. They are used for reconstructive and cosmetic surgery – often in patients who have met with serious accidents or have required surgical intervention due to tumors. Key features: Biocompatible (no adverse reaction by the body), Bio-integration (cells/ tissues migrate into implant), Low weight and not fragile (unlike ceramics), Suturability, Limited shape-ability, Customized implants are possible to make.

  8. POROUS POLYETHYLENE IMPLANTS – IN THE MARKET! Sphere with suture tunnels Orbital floor plate Floor plate (part non-porous) Malar implant Orbital rim Nasal augmentation sheet Mandibular implant Chin implant Nasal dorsum Start-up company www.biopore.in Pterional implant Mastoid

  9. Use of metal complexing agents to create weak networks in polymers

  10. POROUS POLY(ACRYLONITRILE) MEMBRANES Membrane preparation By phase inversion of a soluble complex of metal halides (salts of bivalent alkali metals) with poly(acrylonitrile) followed by washing the cast membrane with water Total membrane thickness : 9 - 11 mil Membrane Cross Section(SEM)

  11. CHARACTERISTICS OF UF MEMBRANE • Average water flux: 50 lmh at 0.5 bar • 5 log reduction for viruses • 7-9 log reduction for bacteria • Molecular Weight Cut Off : ~ 60 k Dalton • BSA rejection > 90 % • Total membrane thickness : 9 - 11 mil

  12. UF MEMBRANE TECHNOLOGY :FROM CONCEPT TO MARKET • Discovery of a unique process to control membrane porosity - Reject smallest known pathogenic species (virus); - Still be able to operate at tap water pressure (0.4 bar) • Prototype preparation, demonstration & performance evaluation - Designed various easy to use prototypes - Demonstration & rigorous performance evaluation • Technology transfer - Technology licensed to Membrane Filters India Ltd., Pune, a start up enterprise incubated at NCL - Product in the market since 2007; Current sales turnover of the company ~ US$ 15 million

  13. School near Pune Tsunami Affected Camp Open Well School in Pune Army Camp-A’ Nagar Lake @ Pune Bore well- Lucknow Typical Installations

  14. High Internal Phase Emulsion Polymerization ( HIPE)

  15. HIGH INTERNAL PHASE EMULSION POLYMERIZATION ( HIPE)

  16. Water phase Oil phase W/O emulsion Poly (HIPE) 50 μm POROUS HIPE POLYMERS Beaded porous polymers polystyrene-DVB High internal phase emulsion (HIPE) monolith polyurethane HIPE stationary phase for HPLC and GC Optical microscopy SEM Schematic diagram of HIPE polymerization

  17. ALL ACRYLIC HIPE POLYMERIZATION LAYER 1 LAYER 2 • Mixing control (Constant cell size for kinetics ) • Temperature control (65ºC) • Controlled time for polymerization • Water: Oil ratio EHA EHMA EGDMA EMULSIFIERS EHA EGDMA EMULSIFIERS HC layer AY layer Characterization Mechanical Properties Yield stress, compression Residual Monomer analysis stability Cell size Distribution (SEM) P & G – CSIR confidential

  18. HEIRARCHIAL STRUCTURES WITH GRADED LAYER POROSITIES FUNCTIONAL ABSORBING MATERIALS Layer 1 and 2 combined Layer 1 Water : oil ratio  27: 1 Layer 2 Water : oil ratio  24: 1

  19. Hierarchical structures with graded layer porosities Always, the world’s leader in feminine protection, is dedicated to helping women embrace womanhood positively—from the very beginning of puberty through their adult lives. The Always brand is behind some of the biggest innovations in feminine hygiene history, including the introduction of winged pads in 1985 and Ultra thin pads in 1990. Continuing its goal of improving women’s lives across the world, Always recently introduced Always Infinity, a pad made with a new-to-the-world material that enables women to have the magical combination of absorbency, amazing softness, and flexibility all in one pad.

  20. Porous polymers : Selective ion transporters ( Proton, Lithium ion)

  21. Conversion of chemical energy to into electrical energy first demonstrated over 150 year ago ! • W.R.Grove, Phil.Mag.Ser. • 314,127-130 (1839) INTERNALS OF A FUEL CELL • However, in spite of the attractive system efficiencies and environmental benefits, it has proved difficult to develop the concept into commercially viable industrial products Proton Conducting Membrane Greatest unmet challenge : Lack of appropriate materials and scalable manufacturing technologies

  22. COMPONENTS OF A FUEL CELL • Produces electricity from the electrochemical oxidation of hydrogen • A fuel cell stack comprises of identical repeating unit of cells, called Membrane Electrode Assembly (MEA) • The MEA electrodes are attached to a solid polymer proton conducting membrane that conducts protons, not electrons • Hydrogen is oxidized at the anode and oxygen is reduced at the cathode • The entire assembly is compressed by bipolar plates that introduce gaseous reactants and coolants to the MEA M K. Debe Nature, 486, 43-51 (2012)

  23. ATTRIBUTES OF A PROTON CONDUCTING MEMBRANE FOR FUEL CELL APPLICATIONS • High proton conductivity • Low electronic conductivity • Low permeability to fuel • Low electro-osmotic drag coefficient • Good chemical stability • Good mechanical property Ideal membrane material is a compromise between performance, durability and cost

  24. POROUS POLYMERS FOR SELECTIVE TRANSPORT OF LITHIUM IONS • Current material of choice : Polyolefins (PO) • Polyolefins are hydrophobic and, hence, intrinsically less compatible with liquid electrolytes ; have low retention capacity to hold organic solvents with high dielectric constant • PO separators have poor wettability characteristics in polar electrolytes, such as, ethylene carbonate (EC), propylene carbonate (PC), and γ-butyrolactone (GBL) owing to their low polarity. • Polyolefins have a Tm ~ 150 to 160 C ; Pores tend to collapse near Tm, causing shrinkages and shorting • Polyolefins are also flammable

  25. POROUS POLYMERS FOR SELECTIVE TRANSPORT OF LITHIUM IONS • Need : A new porous polymer material for selective transport of lithium ions • Desirable Features • Retention of porosity at high temperature • Amorphous polymers to prevent shrinkage at high temperatures and provide high dimensional stability • High surface wettability for polar electrolytes; ability to form hydrogen bonds with electrolytes • Ability to bind Lithium ions for facilitated ionic conduction

  26. HIGH TEMPERATURE PEM-FC Improved carbon monoxide tolerance (~185oC, up to 3% CO in H2 can be tolerated) Simple cooling system 100% single water vapour phase Electro-osmotic drag of water = 0; conductivity not dependent on humidity levels No need for humidification Unique proton conduction mechanism by self ionization /self dehydration; proton hopping from N-H to phosphate anions No “cathode flooding” problem Reduced methanol crossover Improved kinetics of reaction at both electrodes Better heat utilization / recovery Greater ease of integration of a reformer with fuel cell Most preferred material : Poly(benzimidazole)s (PBI) doped with phosphoric acid Q.Li, J.O.Jensen, R.F.Savinell, N.J.Bjerrum, Prog. Polymer Sci., 34, 449 (2009) IOCL, Delhi 110308

  27. POLYMER • STRUCTURE • Ring substitution electronic/ steric • Co-monomersflelxible / rigid • Porosity • Crosslinking POROSITY • PERFORMANCE • Chemical stability • Thermal stability • Proton conductivity • Gas permeability • PROPERTY • Molecular weight • Tg / free volume • Crystalline/amorphous • Acid binding sites • Water retention • Tensile strength and elongation STRUCTURE – PROPERTY – PERFORMANCE MATRIX FOR FUEL CELL MEMBRANES

  28. ACID DOPED PBI BY SOL-GEL PROCESS(DENSE MEMBRANES) ~ 5 molecules of H3PO4 per repeat unit for PBI; 2 molecules of H3PO4 bonded to two nitrogen atoms of repeat units, the rest of the acid is unbonded “free acid” Presence of free unbound acid necessary for proton conductivity Inherent Viscosity : ~ 2 dl/ g Tg = 425 – 436oC No weight loss in air at 500oC pKa = 5.5 16-24 hours H.Vogl and C.S.Marvel, J.Polym.Sci.,50, 511 (1961) J.S.Wainright, J.T.Wang, D.Wang, R.F.Savinell and M.Litt J.Electrochem.Soc., 142, L121 (1995)

  29. Proton conductivity of PA doped PBI by sol-gel process Sol-gel Tensile strength(circles) and elongation at break(squares)as a function of polymer IV Nafion Conventional Phosphoric acid doped PBI : single phase transparent sol from which membranes can be cast by phase inversion process

  30. POROUS POLY(BENZIMIDAZOLE)S : PROOF OF CONCEPT 25% wt solution Doping with 11M Phosphoric Acid, Porogen: Dibutyl Phthalate MeOH 25° C Porogen : Dibutyl Phthalate 70 % DBP : 0.05 S/cm at 25° C Dense PBI : 0.0015 S/cm Dense PBI : 440 mol % PA per RU Porous PBI :1460 mol % PA per RU (70% porogen)

  31. SEM micrographs of fractured porous PBI membranes prepared from PBI / dibutyl phthalate films containing (a) 25 wt % DBP, (b) 50 wt % DBP, (c) 70 wt % DBP, and (d) 80 wt % DBP. Poorly controlled macropores

  32. Intrinsically micro-porous polymers through chemical synthesis

  33. THERMAL REARRANGEMENTS OF AROMATIC POLYIMIDES Glassy polymers with excellent thermal and mechanical properties, resistance to chemicals and easy processibility Workhorse materials for separation membranes, nmely, RO, MF, UF, PV and gas separation Precursor poly(amic) acids can be imidized using heat (3000 C), chemical methods using acetic anhydride and a base or an azeotropic imidization at 1600 C Thermal treatment of polyimides with an ortho functional group can lead to rigid aromatic polymers like poly benzoxazole, polybenzothiazole or polybenzimidazole Thermally rearranged (TR) polyimides possess some novel topologies which result is unusual increase in fractional free volumes

  34. THERMAL REARRANGEMENT OF OF ortho-HYDROXYPHENYL PHTHALIMIDES TO BENZOXAZOLES G.L.Tullos, J.M. Powers, S.J. Jeskey and L.J. Mathias, Macromolecules, 32, 3598 (1999)

  35. THERMAL IMIDIZATION OF BPDA−HAB POLYAMIC ACID AND THERMAL CONVERSION TO AROMATIC POLYBENZOXAZOLES 3,3I –Dihydroxy-4,4I-diamino biphenyl (HAB) + 3,3I ,4,4I –Biphenyl tetracarboxylic dianhydride (BPDA) Reasons for insolubility not explored; chemistry has remained unexploited !

  36. TR POLYIMIDES : AN EASY ROUTE TO POLYMERS WITH INTRINSIC MICROPOROSITY X : -O-, -NH-, -S- • Change of chain conformation: meta- and para-linked chains can be created • Spatial relocation due to chain rearrangement in confinement, leading to generation of free volume elements

  37. THERMAL REARRANGEMENT OF ortho-HYDROXYPHENYL PHTHALIMIDES TO BENZOXAZOLES : REEXAMINATION

  38. POSSIBLE ORIGINS OF BIS-BENZOXAZOLES Cause : Inadvertent moisture Reaction generates water ! Under carefully purified and dry Conditions of reactions, the formation of Bis benzoxazoles can be completely avoided

  39. THERMAL REARRANGEMET OF ortho--AMINOPHENYL PHTHALIMIDES

  40. POLYBENZIMIDAZOLES VIA THERMAL REARRANGEMENT Key step : hydrolysis of Polypyrollone 1 M tetrabutyl Ammonium hydroxide solution at 100 C

  41. PBI via TR of POLYIMIDES WITH ORTHO AMINO GROUPS PBI synthesized TR of polyimides derived from 3,3I,4,4I-tetramino biphenyl (TAB) and 3,3I ,4,4I–biphenyl tetracarboxylic dianhydride (BPDA) PBI produced exhibits the same solubility characteristics as obtained from TAB and isophthalic acids PBI doped with phosphoric acid exhibits a conductivity of 0.3 – 0.35 S/cm at 1400 C PBI obtained via TR of polyimides exhibits better acid uptake properties and is also able to sequester water Detailed characterization is in progress This approach provides an ability to access hetero-aromatic polymers of diverse structures and explore structure – property relationships

  42. CYCLOPHANE AS BUILDING BOLCKS FOR INTRINSICALLY POROUS POLYMERS Polymer properties under investigation

  43. APPROACHES TO REVERSE THE CROSSLINKING REACTION

  44. ANOTHER APPROACH TO CYCLOPHANE BEARING POLYMERS

  45. HT PEM FC : 20 CELL, TARGET 200 W WITH H2/AIR Bud pins – Vcell Anode side Inlet gas at RT Gas out sent to Arbin for exhaust and pressure measurement Cathode side Thermocouples Pad heaters on all four sides Insulation Cooling duct Blower 100-200 slpm Vstack

  46. BENCHMARKING WITH COMPETITION • Stoichiometry • Competitor: H2/air  1.35/2.5 • Our MEA: H2/air  3.5/2.5 @ 800 mA/cm2 • Temperature: ~165°C

  47. Developed knowhow for making all key material components Performance of MEAs benchmarked against competitive materials. Durability demonstrated at single cell and stack levels for 500 - 700 h. Built up to 1 kW prototype PEFC plants including BOP. POLYMER ELECTROLYTE FUEL CELLS A TEAM CSIR EFFORT

  48. SUMMARY • Porous polymers are a versatile material platform for creating new properties in existing polymers • They are relatively easy to prepare and tailored porosities with surface properties useful for a given application can be designed • Diverse applications in health care, ultra-filtration and separation processes and energy related applications e.g. fuel cell membranes and as electrode material for batteries

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