DR. S. SIVARAMA 201, Polymers & Advanced MaterialsLaboratory, National Chemical Laboratory,Pune-411 008, INDIATel : 0091 20 2589 2614Fax : 0091 20 2589 2615Email : s.sivaram@ncl.res.in

BUILDING POROSITY IN POLYMERS : HOW AND WHY ?

International Conferences on Emerging Trends in Chemical Sciences, VIT University, Vellore

December 5, 2013

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

Alveoli in the lungs

Skin

Dialysis MembranePolyacrylonitrile

Polystyrene foam cup

PolyurethaneFloor mop

Porous Carbon electrode

Zeolite 4A

POROUS MATERIALS ARE UBIQUITOUS !

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

Physical aggregation of pre-formed polymer particles

POROUS POLYETHYLENES

0%

20%

40%

60%

80%

100%

0 200 400 600 800 1000 1200

Particle size, D (microns)

No

. o

f p

art

icle

s l

arg

er

tha

n D

(%

)

450 microns

600 microns

700 microns

800 microns

S value= 0.12

S value= 0.13

S value= 0.08

S value= 0.14

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

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.

POROUS POLYETHYLENE IMPLANTS – IN THE MARKET!

Sphere with suture tunnels Orbital floor plate Floor plate (part non-porous)

Malar implant

Mandibular implant

Chin implant

Orbital rim

Nasal augmentation sheet

Nasal dorsum

Start-up company

www.biopore.in

Pterional implant Mastoid

Use of metal complexing agents to create weak networks in polymers

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)

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

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

Typical Installations

School near Pune

Army Camp-A’’’’ Nagar

Lake

@

Pun

e

Bore well- Lucknow

School in Pune

Tsunami Affected Camp

Open Well

High Internal Phase Emulsion Polymerization ( HIPE)

HIGH INTERNAL PHASE EMULSION POLYMERIZATION ( HIPE)

POROUS HIPE POLYMERS

Beaded porous polymers polystyrene-DVB

polyurethane HIPE stationary phase for HPLC and GC

High internal phase emulsion (HIPE) monolith

Schematic diagram of HIPE polymerizationSchematic diagram of HIPE polymerization

Optical microscopy

Water phase

Oil phase

W/O emulsion Poly (HIPE)50 µm

SEM

P & G – CSIR confidential

ALL ACRYLIC HIPE POLYMERIZATION

Characterization

HC layer AY layer

EHAEHMAEGDMAEMULSIFIERS

EHAEGDMAEMULSIFIERS

LAYER 1 LAYER 2� Mixing control (Constant cell size

for kinetics )

� Temperature control (65ºC)

� Controlled time for polymerization

� Water: Oil ratio

Residual Monomer analysis

stabilityCell size Distribution(SEM)

MechanicalProperties

Yield stress, compression

Layer 1 and 2 combined

Layer 1

Water : oil ratio ⇒⇒⇒⇒ 27: 1

Layer 2

Water : oil ratio ⇒⇒⇒⇒ 24: 1

HEIRARCHIAL STRUCTURES WITH GRADED LAYER POROSITIES

FUNCTIONAL ABSORBING MATERIALS

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Hierarchical structures withgraded layer porosities

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

INTERNALS OF A FUEL CELL

� Conversion of chemical energy to into electrical energy first demonstrated over 150 year ago !W.R.Grove, Phil.Mag.Ser.314,127-130 (1839)

� However, in spite of the attractive system efficiencies and environmental benefits, it has proved difficult to develop the concept into commercially viable industrial products

Greatest unmet challenge : Lack of appropriate materials and scalable manufacturing technologies

Proton Conducting

Membrane

M K. Debe Nature, 486, 43-51 (2012)

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

Polymer Materials

(Membrane Electrode Assembly)

Stack Engineering

BoP

Control Electronics

Carbon and Carbon

composites

(GDL and Bipolar Plates)

Electro-catalysts

(Anode and Cathode)

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

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

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

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

IOCL, Delhi 110308

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)

POLYMERSTRUCTURE

• Ring substitution

electronic/ steric

• Co-monomers

flelxible / rigid

• Porosity

• Crosslinking

PROPERTY

• Molecular weight

• Tg / free volume

•Crystalline/amorphous

• Acid binding sites

• Water retention

•Tensile strength and

elongation

PERFORMANCE

• Chemical stability

• Thermal stability

• Proton conductivity

• Gas permeability

POROSITY

STRUCTURE – PROPERTY – PERFORMANCE MATRIX FOR FUEL CELL MEMBRANES

����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 = 5.5H.Vogl and C.S.Marvel, J.Polym.Sci.,50, 511 (1961)J.S.Wainright, J.T.Wang, D.Wang, R.F.Savinell and M.LittJ.Electrochem.Soc., 142, L121 (1995)

16-24 hours

Tensile strength(circles) and elongation at break(squares)as a

function of polymer IV

Proton conductivity of PA doped PBI by sol-gel process

Sol-gel

NafionConventionalPhosphoric acid doped PBI : single

phase transparent sol from which membranes can be cast by phase

inversion process

POROUS POLY(BENZIMIDAZOLE)S : PROOF OF CONCEPT

MeOH

Porogen : Dibutyl Phthalate

70 % DBP : 0.05 S/cm at 25°°°° CDense PBI : 0.0015 S/cm

Porogen, wt% Uptake, wt%

0 132

25 173

50 246

Doping with 11M Phosphoric Acid, Porogen: Dibutyl Phthalate

Dense PBI : 440 mol % PA per RUPorous PBI :1460 mol % PA per RU

(70% porogen)

25% wt solution

25°°°° C

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

Intrinsically micro-porous polymers through chemical synthesis

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

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)

THERMAL IMIDIZATION OF BPDA−HAB POLYAMIC ACID AND THERMAL CONVERSION

TO AROMATIC POLYBENZOXAZOLES

Reasons for insolubility not

explored; chemistry has remained unexploited !

3,3I –Dihydroxy-4,4I-diamino biphenyl

(HAB) +

3,3I ,4,4I –Biphenyl tetracarboxylic

dianhydride (BPDA)

TR POLYIMIDES : AN EASY ROUTE TO POLYMERS WITH INTRINSIC MICROPOROSITY

� 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

X : -O-, -NH-, -S-

THERMAL REARRANGEMENT OF ortho-HYDROXYPHENYL PHTHALIMIDES TO BENZOXAZOLES :

REEXAMINATION

POSSIBLE ORIGINS OF BIS-BENZOXAZOLES

Cause : Inadvertent moistureReaction generates water !

Under carefully purified and dryConditions of reactions, the formation of

Bis benzoxazoles can be completely avoided

THERMAL REARRANGEMET OF ortho--AMINOPHENYL PHTHALIMIDES

POLYBENZIMIDAZOLES VIA THERMAL

REARRANGEMENT

Key step : hydrolysis of Polypyrollone

1 M tetrabutylAmmonium hydroxide

solution at 100 C

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

NH2

NH2

H2N

H2N

CO2H

HO2C

PPA, 180 oC

2 hour

N

N

N

N

N

N

N

N

350 oC

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

CYCLOPHANE AS BUILDING BOLCKS FOR INTRINSICALLY POROUS POLYMERS

Polymer properties under investigation

N

N

N

N

N

N

N

N

CH3

CH3

N

N

N

N

N

N

N

N

H3C

H3C

N

N

N

N

N

N

N

NH3C H3C

H3C

H3C

250 oC

liquid phase hydrogen donor

APPROACHES TO REVERSE THE CROSSLINKING REACTION

ANOTHER APPROACH TO CYCLOPHANE BEARING

POLYMERS

HT PEM FC : 20 CELL, TARGET 200 W WITH H2/AIR

Cathode sideAnode side

Thermocouples

Bud pins – Vcell

Vstack

Blower100-200 slpm

Cooling duct

Insulation

Pad heaters on all four sides

Inlet gas at RT

Gas out sent to Arbin for exhaust and pressure measurement

BENCHMARKING WITH COMPETITION

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

0 100 200 300 400 500 600 700 800 900

Vo

lta

ge

(V

)

Current density (mA/cm2)

Sartorius

CelTek P

Advent (TPS)

Daposy (DPS)

NCL (20 cells average data)

NCL (best from 20 cells data)

Stoichiometry� Competitor: H2/air ���� 1.35/2.5� Our MEA: H2/air ���� 3.5/2.5 @ 800

mA/cm2

Temperature: ~165°°°°C

POLYMER ELECTROLYTE FUEL CELLSA TEAM CSIR EFFORT

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.

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

NCL 210706

THANK YOU