Scholars' Mine Doctoral Dissertations Student Theses and Dissertations Fall 2013 Polybenzoxazine aerogels: synthesis, characterization, conversion to porous carbons, and energetic composites Shruti Mahadik-Khanolkar Follow this and additional works at: https://scholarsmine.mst.edu/doctoral_dissertations Part of the Chemistry Commons Department: Chemistry Recommended Citation Mahadik-Khanolkar, Shruti, "Polybenzoxazine aerogels: synthesis, characterization, conversion to porous carbons, and energetic composites" (2013). Doctoral Dissertations. 1821. https://scholarsmine.mst.edu/doctoral_dissertations/1821 This thesis is brought to you by Scholars' Mine, a service of the Missouri S&T Library and Learning Resources. This work is protected by U. S. Copyright Law. Unauthorized use including reproduction for redistribution requires the permission of the copyright holder. For more information, please contact [email protected]. POLYBENZOXAZINE AEROGELS: SYNTHESIS, CHARACTERIZATION, CONVERSION TO POROUS CARBONS, AND ENERGETIC COMPOSITES by SHRUTI MAHADIK-KHANOLKAR A DISSERTATION Presented to the Faculty of the Graduate School of the MISSOURI UNIVERSITY OF SCIENCE AND TECHNOLOGY In Partial Fulfillment of the Requirements for the Degree DOCTOR OF PHILOSOPHY in CHEMISTRY 2013 Approved by: Dr. Nicholas Leventis, Advisor Dr. Chariklia Sotiriou-Leventis, Co-Advisor Dr. Manashi Nath Dr. Jeffrey G. Winiarz Dr. Lokeswarappa R. Dharani ii 2013 Shruti Mahadik-Khanolkar All Rights Reserved iii DEDICATED TO MY BELOVED FATHER Late Mr. SURYAKANT G. MAHADIK iv PUBLICATION DISSERTATION OPTION This dissertation consists of two articles that have been prepared for publications as follows: Paper I found on pages 35-100 is inteneded to submit in Chemistry of Materials Paper II found on pages 101-156 is intended to submit in Chemistry of Materials v ABSTRACT Aerogels are nanoporous, low-density bulk objects, consisting of three- dimensional assemblies of nanoparticle. Structured similarly, polymeric aerogels are emerging as a mechanically strong alternative to traditional silica aerogels, which are fragile. Amongst polymeric aerogels, those based on polybenzoxazine (PBO - a type of phenolic resin), are extremely robust and comprise an economic alternative to resorcinol- formaldehyde aerogels, also a class of phenolic resins, as the main source of carbon aerogels. The drawback of the PBO chemistry has been the long (days) processing time at high-temperatures (>130 oC). Herewith, we have developed an energy- and time-efficient process to PBO aerogels by inducing acid-catalyzed gelation at room-temperature completed in a few hours. The new aerogels are compared directly with their conventional counterparts and are found equivalent or better in terms of mechanical strength, thermal insulation value, surface area and carbonization yield. Hexahydrated iron chloride (FeCl3.6H2O) is a fairly strong Brønsted acid, which, based on the above, catalyzes formation interpenetrating networks of PBO and iron oxide nanoparticles (PBO-FeOx). Pyrolysis of that intimate mixture of a carbon source (PBO) and iron oxide undergoes smelting to highly porous (>90% v/v) monolithic metallic iron aerogels. The porous network was loaded with oxidizers (e.g., LiClO4) into a new class of energetic materials (thermites, explosives, pyrotechnics). The PBO aerogels developed here comprise a wide-base platform for use as thermal insulators in civil and transportation applications (PBO aerogels themselves), electrodes for fuel cells, lithium ion batteries (nanoporous carbons), catalysts and energetic materials (PBO-FeOx). vi ACKNOWLEDGEMENTS I would like to express my sincere gratitude to my advisor, Prof. Nicholas Leventis and Co-advisor Prof. Chariklia Sotiriou-Leventis for their continuous guidance, encouragement and support throughout my graduate studies at Missouri S & T. They have taught me to be systematic and persistent to excel in the field of research. Especially, the passion and enthusiasm of Prof. Leventis towards research, his eagerness to learn new things, and his critical thinking has always inspired me to go an extra mile in my work. I am fortunate to have them as my advisors. I would like to thank my committee members, Dr. M. Nath, Dr. J. Winiarz, and Dr. L. R. Dharani for providing useful suggestions for my research. I thank the Department of Chemistry, Missouri S&T for providing financial assistance and resources. I am grateful to Mr. J. Counsil for his timely help with all the instruments and key insights in my projects. I would also like to thank Dr. E. Bohannan, Dr. A. Choudhury, and C. Wisner for crucial characterizations of samples for my thesis work. I highly appreciate the help, guidance, and emotional support of my lab colleagues (Abhishek, Chakri, Dhiru, and Anand). Our brainstorming discussions helped me to nurture my research aptitude. I also want to extend my gratitude to all my friends for their moral support during tough as well as good times. Most importantly, I would like to thank my father (baba) Late Mr. S. G. Mahadik. I would not have pursued my goals without his belief and support. I am forever grateful to my mother (aai) Mrs. S. Mahadik and my sister (tai) Mrs. A. Sawant for their unconditional love and support. I am lucky to have found my soul mate, Mr. A. Khanolkar in Rolla. His love, understanding and constant motivation has given me strength to complete my Ph.D. vii TABLE OF CONTENTS Page PUBLICATION DISSERTATION OPTION ................................................................... iv ABSTRACT……………………………………………………………………………….v ACKNOWLEDGEMENTS ............................................................................................... vi LIST OF ILLUSTRATIONS ............................................................................................. xi LIST OF SCHEMES..........................................................................................................xv LIST OF TABLES ........................................................................................................... xvi SECTION 1. INTRODUCTION ...................................................................................................1 1.1. AEROGELS ......................................................................................................1 1.2. THE SOL-GEL SYNTHESIS OF SILICA AEROGELS .................................3 1.3. SURFACE MODIFICATION OF SILICA AEROGELS WITH POLYMERS (X-LINKING) .............................................................................6 1.4. ORGANIC AEROGELS DERIVED FROM PHENOLIC CHEMISTRY .......8 1.4.1. Resorcinol Formaldehyde Aerogels ........................................................8 1.4.2. Base Catalyzed and Acid Catalyzed Gelation of Resorcinol Formaldehyde .......................................................................8 1.4.3. The Effect of Synthetic Parameters on the Morphology of RF Aerogels ......................................................................................12 1.4.4. Polybenzoxazine Chemistry..................................................................14 1.4.5. Porous Polybenzoxazines......................................................................18 1.5. CARBON AEROGELS ..................................................................................21 1.6. POROUS METALS FROM AEROGELS .....................................................27 viii PAPER I. POLYBENZOXAZINE AEROGELS I: HIGH-YIELD ROOM-TEMPERATURE ACID-CATALYZED SYNTHESIS OF ROBUST MONOLITHS, OXIDATIVE AROMATIZATION AND CONVERSION TO MICROPOROUS CARBONS ...............................................................................35 Abstract ........................................................................................................................35 1. INTRODUCTION .................................................................................................36 2. RESULTS AND DISCUSSION ............................................................................39 2.1. Materials Synthesis .........................................................................................39 2.2. Chemical Transformation along Processing ...................................................42 2.3. PBO Aerogel Characterization .......................................................................51 2.3.a. General material properties .................................................................51 2.3.b. The porous structure ...........................................................................53 2.3.c. The skeletal framework and interparticle connectivity .......................55 2.4. Carbonization ..................................................................................................58 3. CONCLUSION .....................................................................................................63 4. EXPERIMENTAL. ................................................................................................64 4.1. Materials .........................................................................................................64 4.2. Synthesis of the Benzoxazine Monomer (BO Monomer)...............................64 4.3. Preparation of Polybenzoxazine (PBO) Aerogels...........................................65 4.3.a Via heat-induced polymerization at 130 oC ......................................65 4.3.b.Via acid-catalyzed polymerization at room temperature ..................66 4.4. Carbonization of PBO Aerogels .....................................................................67