PERVAPORATION STUDIES ON POLY(DIMETHYL

SILOXANE) BLENDS AND COMPOSITES

By

PRABHAT GARG

Centre for Polymer Science and Engineering

Submitted

in fulfilment of the requirements of the degree of

Doctor of Philosophy

to the

Indian Institute of Technology Delhi

July, 2011

CERTIFICATE

This is to certify that the thesis entitled "PERVAPORATION STUDIES ON

POLY(DIMETHYL SILOXANE) BLENDS AND COMPOSITES" being submitted by Mr.

Prab11at Garg to the Indian Institute of Technology, Delhi, for the award of degree of

Doctor of Philosophy is a record of bonaFide research work carried out by him. Mr.

Prabhat Garg has worked under our guidance and supervision and has fultilied the

requirements for the submission of this thesis, which to our knowledge has reached the

requisite standard.

This work has not been submitted, in part or full, to any other University or Institute for

the award of any other degree or diploma.

4 Dr. R. P. Si gh, Sc'F' Co-supervisor Centre For fire, Explosive & Environment Safety

Defence R & U Organisation

'rint.tt'pt[r, Delhi-11g054

India

Prof. ( s.) Veena Choudllary Supervisor

Centre For Polymer Science & Engineering

Indian Institute vFTechuology, Delhi

Hauz Khas, New DeIhi•1100 G

India

ACKNOWLEDGEMENTS

This thesis could have not been completed without the generosity and support of many.

First I wish to express my thank to my supervisors, Prof. (Mrs.) Veena Choudhary, and

Dr. R. P. Singh for their invaluable guidance, constant encouragement and generous co-

operation at different stages of the work.

I express deep sense of gratitude to Dr A. B. Samui, Associate Director, Naval Materials

Research Laboratory, Ambernath for his constant support and encouragement during

my work.

It is of great pleasure to include thanks to Dr R. Vijayraghawan, Dr. A. K. Kapoor. Dr. D.

K. Dubey from Defence R&D Organization to allow me to carry out this work.

My special thanks to Dr P. K. Roy for his encouragement and help in getting this

opportunity.

I sincerely acknowledge the help received from Mr. Vijay Shankar Mishra, NMRL,

Ambernath.

I also pay my gratitude to Dr P. K. Gutch, DRDE, Gwalior for his support in arranging

testing of samples.

I would like to thank Mr Pravin Srivastava, Ms. Deeksha Gupta, Mrs. Anju Gupta, Mr.

Sandeep Tripathi, Mr. Satpal Singh at CPSE for their help and support whenever I

needed. I express my sincere thanks to my seniors at CPSE Dr. Rashmi Chauhan and Dr.

Pooja Chhabra for their guidance.

I thank my SRC members, Prof. A. K. Singh, Prof. Harpal Singh and Prof. S. N. Maiti for

their criticism, advice and support.

It is of great pleasure to include thanks to Mr. Vishal Dalvi and Mr. Jayesh Chavan, NMRL

who helped me at various stages.

iv

I would also like to thank all of my friends and loved ones for everything they've done

for me throughout the years; I could not have done this without them.

I dedicate this thesis to my late parents who always wished to see me in a high moral.

I would like to acknowledge the support received from my family members specially Mr.

Neerav Gupta.

Last but not least I would like to thank my wife and my loving daughter for their love,

support, help and encouragement at difficult times.

Finally, I thank the `ALMIGHTY GOD' for his blessings and further seek him to provide

me blessings, patience and strength to accomplish newer goals.

Prabhat Garg

July 2011, IIT Delhi

v

ABSTRACT

Pervaporation is the most promising technology on a molecular-scale in liquid/liquid

separations existing in biorefinery, petrochemical, pharmaceutical industries etc. for

being highly selective, economical, safe and eco-friendly. Theoretically, pervaporation

can be used for the separation of any type of liquid mixture but practically, it is meant

for the separation of azeotropes, close boiling mixtures and mixtures having very low

quantity of impurity. Generally, pervaporation is combined in series with other

conventional techniques of separation like distillation.

Performance of a pervaporation process is measured in terms of two factors; i)

pervaporation flux, amount of permeant per unit time and ii) pervaporation selectivity,

ratio of one component to the second component in permeate with respect to the ratio

in feed at a given time. Parameters such as pressure at permeate side, temperature,

module design etc affect pervaporation performance for a given system but membrane

is the most important hardware of the pervaporation process. The inadequacy of the

existing polymeric membranes hinders the full exploitation of the application

opportunities on the industrial-scale. This situation has motivated a substantial amount

of work to explore diverse polymers and their efficiency in current and potential

pervaporation fields. Pervaporation membranes have been prepared ranging from

pristine polymers like polystyrene, poly(dimethyl siloxane), polyimide, polyamide etc to

the inorganic materials like zeolites. Another approach for the membrane fabrication is

mixed matrix membrane which is made of a blend or composite material.

Polysiloxanes, especially poly(dimethyl siloxane) are a unique class of polymers that

can be crosslinked via a number of different routes to form elastomers with many

attractive properties, including high optical transparency, low surface energy, low

toxicity and high bio-compatibility and good chemical and thermal stability. As these are vi

extremely weak materials, therefore must be heavily reinforced with inorganic/organic

fillers.

Extensive studies have been reported on poly(dimethyl siloxane) (PDMS) membrane

used for pervaporation separation of hydrophobic/hydrophilic or nonpolar/polar liquid

mixture. PDMS is considered one of the most recommended material for this purpose as

it contains combination of properties of hydrophilicity due to Si-O-Si backbone and

hydrophobicity due to its methyl groups on the surface. Apart from poor mechanical

properties, another drawback with PDMS is its poor selectivity due to large pore size

specially when it swells in the presence of compatible organic solvent. Crosslinking is

one way to control the swelling of polymer chains, still it lacks the dimensional stability

in the presence of hydrophobic solvents like toluene and benzene.

The aim of this thesis was to modify PDMS by blending with polyimide or by

incorporation of nanofillers (to prepare composite membranes) and evaluating effect of

such additives on the performance properties i.e. in the pervaporation separation of

organic/water and organic/organic azeotropes. Thesis consists of six chapters.

Chapter I contains the introduction of pervaporation, its development, transport

mechanisms involved and some other fundamentals of pervaporation. The superiority

of poly(dimethyl siloxane) over other polymers in separation property was highlighted.

Thereafter, pros and cons of the various facets of poly(dimethyl siloxane) for

pervaporation application, crosslinking mechanism and modification to membrane

formation were analyzed. This chapter also illustrates the review of literature on the

basis of applications of pervaporation.

Chapter II explains the experimental details i.e. preparation of poly(amic acid) [PAA],

polyimide [PI] by reacting diaminodiphenylether with pyromelletic dianhydride using

dimethylacetamide as solvent, and its characteristics by FTIR. Preparation of PDMS vii

membranes in absence/presence of varying amounts of polyimide/ nanoclay [Cloisite

30B, Nanomer 1.30P]/ POSS[Trisilanolphenyl/Dodecaphenyl] is also described. This

chapter also illustrated the method adopted for characterization and evaluation of

membranes in detail.

In the first approach for the modification, PDMS was reinforced by incorporation of

polyimide (chapter III). Polyimides are also known for their excellent permeation

selectivity but the pervaporation flux is low in polyimides due to their compact

structure. Another problem with polyimides is that they cannot be molded easily as

these are insoluble and infusible. The combination of PDMS and polyimide was thought

to be an ideal combination for the preparation of pervaporation membrane. The blends

of these two polymers were prepared using two different approaches; in the first

approach, polyimide powder [prepared by direct polycondensation of dianhydride and

diamine followed by thermal cyclization of poly(amic acid)] was incorporated in PDMS

matrix ranging from 5 %(w/w) to 25 %(w/w) followed by crosslinking of PDMS.

Structural and morphological characterization of these membranes by FTIR and SEM

showed the uniform dispersion of polyimide powder in the PDMS matrix. Being

incompatible blends, wetting of polyimide particles by PDMS was also confirmed.

Effects of polyimide (PI) filler on pervaporation properties of poly(dimethyl siloxane)

(PDMS) were studied. PDMS membrane filled with 25 %(w/w) PI (SPI-25) was used for

the separation of benzene (Bz) and toluene (Tol) from the aqueous solution and the

results were compared with the neat PDMS membrane of similar thickness. The SPI-25

membrane showed normalized flux upto 1.2 kg µm/mzh for Bz and 1.48 kg µm/mzh for

Tol and selectivity of organics varied from 7.3 to 3.2 for Bz and 8.9 to 2.8 for Tol with

increasing concentration of organics. PI filler increases thermal as well as mechanical

stability of filled PDMS membranes. viii

In the second approach, poly(amic acid)(10 %w/v in DMAc) was directly mixed with

the liquid PDMS resin followed by simultaneous cyclization of poly(amic acid) and

crosslinking of PDMS which led to possible interpenetrating (IPN) blends. The ratio of

polyimide in the membranes was varied from 5 %(w/w) to 15 %(w/w). Thermal

stability of such membranes was compared by comparing the decomposition

temperatures at 10% mass loss [Tio]. Tio increased from 445 - 490°C in air and 410-

520°C in inert atmosphere upon incorporation of polyimide. Activation energies for the

decomposition, calculated using Coats and Redfern equation also showed an increase

upon incorporation of polyimide. Permeation properties of PDMS and IPN blends were

evaluated by water diffusion, measured by fourier transform-attenuated total

reflectance (FT-ATR) method and water vapour transmission rate (WVTR) as per ASTM

E 96. In case of samples having 15 %w/w in-situ generated polyimide content in PDMS

membrane (SPA-15) water diffusion and WVTR decreased significantly as compared to

PDMS. The tensile strength of PDMS also increased upon incorporation of polyimide.

IPN membranes prepared in this work were employed in pervaporation separation of

azeotrope forming toluene/methanol mixtures. The pervaporation properties could be

tuned by adjusting the blend composition. All the blend membranes tested showed a

decrease in flux with increasing polyimide content for methanol/toluene liquid

mixtures. Toluene permeated preferentially through all tested blend membranes, and

the selectivity increased with increasing polyimide content. The flux increased

exponentially with increasing toluene concentration in the feed mixtures, whereas the

selectivities decreased.

Chapter IV describes the properties of PDMS composite membranes using nanoclays as

filler. PDMS clay nanocomposite membranes were prepared by in-situ crosslinking of

PDMS resin in the presence of clay content varying from 1-10 %(w/w) in order to ix

evaluate the influence of layered silicate on pervaporation characteristics of PDMS. Two

commercial clays, Cloisite 30B and Nanomer 1.30P functionalised with polar and

nonpolar surfactants were chosen for this purpose. Structural, mechanical and thermal

characterization was done using FTIR, tensile testing system and thermogravimetric

analyser. Morphological characterization using X-ray diffraction and transmission

electron microscopy showed intercalation or partial exfoliation of silicate layers.

Surface characterization using scanning electron microscope showed a uniform

dispersion of nanoclays in PDMS matrix. Two nanocomposite membranes having

PDMS/nanoclay [Cloisite 30B/Nanomer 1.30P (10 %w/w)] were selected based on

their mechanical properties and evaluated for their performance in separating

azeotropic toluene/methanol mixture. Composite membranes showed higher selectivity

as compared to neat PDMS and toluene was a preferred permeant. The total flux for

composite membranes was lower as compared to PDMS membrane.

Chapter V describes the preparations of PDMS composite membranes using polyhedral

oligomeric silsesquioxanes (POSS) nanoparticles (1- 5 %w/w) as filler. Trisilanolphenyl

[TSP] POSS and dodecaphenyl [DCP] POSS were selected as filler in the PDMS matrix.

The presence of POSS particles was confirmed by structural characterization using

FTIR. XRD and SEM studies showed a uniform dispersion of POSS particles in PDMS

matrix but TEM studies suggested that though there was a homogeneous distribution of

trisilanolphenyl POSS in PDMS matrix whereas thick aggregates were seen in case of

composite membranes prepared using dodecaphenyl POSS as filler. This was also

supported by the improved mechanical strength of PDMS-TSP POSS composites while

there was no enhancement in tensile properties in case of PDMS-DCP POSS composites.

PDMS-TSP membrane having 5 %(w/w) TSP POSS[PT-5] was used in the pervaporation

x

separation of ethanol from aqueous solutions. These membranes were found to be more

selective towards ethanol.

The final summary and conclusion are given in Chapter VI of thesis. Suggestions for

future work are also made.

xi

TABLE OF CONTENTS

CERTIFICATE . iii

ACKNOWLEDGEMENT.................................................................................................................. iv

ABSTRACT....................................................................................................................................... vi

TABLE OF CONTENTS ................................................................................................................... xii

LISTOF FIGURES ......................................................................................................................... xvii

LISTOF TABLES ......................................................................................................................... xxi

LISTOF SCHEMES ....................................................................................................................... xxii

CHAPTERI

INTRODUCTION AND LITERATURE REVIEW.....................................................................1

1.1 Introduction........................................................................................................................1

1.2 Pervaporation.....................................................................................................................2

1.3 Fundamantals and mechanism of pervaporation............................................................4

1.3.1 Solution diffusion model..........................................................................................6

1.3.2 Pore flow model........................................................................................................8

1.4 Review of literature..........................................................................................................10

1.4.1 Dehydration of organic solvent.............................................................................10

1.4.2 Removal of dilute organics from aqueous streams.............................................12

1.4.3 Organic-organic mixtures separation...................................................................13

(i) Separation of polar/nonpolar solvent mixtures.............................................14

(ii) Separation of aromatic/alicyclic mixtures....................................................14

(iii) Separation of aromatic/aliphatic hydrocarbons..........................................16

(iv) Separation of isomers.....................................................................................16

1.5 Factors determining pervaporation performance.........................................................17

1.5.1 Feed composition and concentration....................................................................17

1.5.2 Feed and permeate pressure..................................................................................17

xii

1.5.3 Temperature ............................................................................................................17

1.5.4 Concentration polarization ....................................................................................18

1.5.5 Membrane ...............................................................................................................18

1.5.6 Plasticization effect of polymeric membrane .......................................................19

1.6 Material selection for development of pervaporation membrane .............................20

1.7 Membrane morphology ..................................................................................................24

1.8 Modification of membrane materials ...........................................................................26

1.8.1 Crosslinking ............................................................................................................2 7

1.8.2 Copolymerization ....................................................................................................27

1.8.3 Blending ..................................................................................................................28

1.8.4 Interpenetrating polymer network ......................................................................29

1.8.5 Polymer composites ...............................................................................................29

(i) Polymer layered silicate nanocomposites .......................................................30

(ii) Methods for preparation of polymer layered silicate nanocomposites......34

(iii) Polymer/POSS nanocomposites .....................................................................3 5

1.9 Commercial pervaporation membranes .......................................................................38

1.10 Silicones .........................................................................................................................39

1.11 Silicone cross-linking ...................................................................................................39

1.12 Polyimides .....................................................................................................................43

1.13 Research objectives and strategies .............................................................................45

1.14 Plan of work ..................................................................................................................46

1.15 Format of thesis ...........................................................................................................46

CHAPTER II

EXPERIMENTAL DETAILS ....................................................................................................49

2.1 Introduction ......................................................................................................................49

2.2 Experimental .....................................................................................................................49

2.2.1 Materials ..................................................................................................................49 xiii

2.2.2 Synthesis of polyimide ...........................................................................................51

2.2.3 Preparation of membranes ....................................................................................52

2.2.4 Characterization of membranes ............................................................................54

(i) Structural characterization ...............................................................................54

(ii) Morphological characterization .......................................................................55

(iii) Thermal characterization ................................................................................5 6

(iv) Mechanical properties ....................................................................................56

(v) Extent of crosslinking .......................................................................................56

(vi) Water diffusion measurements .......................................................................57

(vii) Water vapour transmission rate (WVTR) measurements ...........................57

(viii) Liquid Sorption studies .................................................................................59

(ix) Solvent uptake from binary solutions ............................................................60

(x) Pervaporation studies .......................................................................................60

CHAPTER III

THERMAL AND PERVAPORATION STUDIES USING MEMBRANES BASED ON

POLY(DIMETHYL SILOXANE)/POLYIMIDE BLENDS .......................................................65

3.1 Introduction .......................................................................................................................65

3.2 Results and discussion ......................................................................................................67

3.2.1 Characterization of polyimide and membranes ...................................................67

(A) PDMS-polyimide blends ..........................................................................................67

(i) FTIR studies .......................................................................................................67

(ii) Thermal & mechanical characterization ........................................................69

(iii) Morphological characterization .....................................................................71

(iv) Sorption studies ...............................................................................................72

(v) Pervaporation studies ......................................................................................74

(B) PDMS-PAA blends ....................................................................................................78

xiv

(i) FTIR studies .......................................................................................................79

(ii) Morphological studies ......................................................................................82

(iii) Crosslink density ...........................................................................................85

(iv) Thermogravimetric analysis ...........................................................................87

(v) Water diffusion measurements ........................................................................91

(vi) Sorption studies ...............................................................................................92

(v) Pervaporation studies .......................................................................................94

(C) Performance comparison of SPI-25 and SPA-15 membranes ............................98

3.3 Conclusions ........................................................................................................................99

CHAPTER IV

POLY (DIMETHYL SILOXANE) - CLAY NANOCOMPOSITE MEMBRANES:

CHARACTERIZATION AND EVALUATION .......................................................................101

4.1 Introduction ....................................................................................................................101

4.2 Results and discussion ....................................................................................................103

4.2.1 Curing of PDMS in presence of nano-clays .........................................................103

4.2.2 Characterization ...................................................................................................104

(i) Structural characterization ............................................................................104

(ii) Morphological characterization ...................................................................105

(iii) Mechanical properties ..................................................................................112

(iv) Thermogravimetric analysis ........................................................................113

4.2.3 Evaluation of membranes ....................................................................................116

(i) Sorption studies .............................................................................................116

(ii) Pervaporation studies ...................................................................................118

4.3 Conclusions .....................................................................................................................123

CHAPTER V

xv

POLY(DIMETHYL SILOXANE) - POSS NANOCOMPOSITE MEMBRANES:

CHARACTERIZATION AND EVALUATION .......................................................................12 5

5.1 Introduction ....................................................................................................................12 5

5.2 Results and Discussion ..................................................................................................128

5.2.1 Charcaterization ....................................................................................................128

(i) Structural characterization ............................................................................128

(ii) Morphological characterization ....................................................................130

(iii) Mechanical properties ..................................................................................13 6

(iv) Thermogravimetric (TG) analysis ...............................................................136

5.2.2 Evaluation of membranes in pervaporation studies .........................................139

(i) Effect of feed composition on pervaporation flux and selectivity ..............139

(ii) Effect of feed temperature on flux and selectivity .......................................141

(iii) Activation energy of pervaporation ............................................................143

(iv) Evaluation of PT-5 membrane in separation of toluene/methanol

Mixture...........................................................................................................143

5.3 Conclusions .....................................................................................................................144

CHAPTER VI

SUMMARY, CONCLUSIONS AND FUTURE SCOPE OF THE WORK ................................145

6.1 Introduction .....................................................................................................................145

6.2 Synthesis of poly(amic acid)/polyimide/membranes .................................................146

6.3 Characterization and evaluation of membranes based on PDMS/ polyimide

blends..............................................................................................................................147

6.4 Characterization and evaluation of PDMS-nanoclay composite membranes .............152

6.5 Characterization and evaluation of PDMS-POSS composite membranes ...................154

6.6. Performance comparison data of PDMS, blends and composite membranes .......................156

6.7 Conclusions ......................................................................................................................157

6.8 Suggestions for future .....................................................................................................158

REFERENCES........................................................................................................................161

xi