Crosslinking Amine-Modified Silica Aerogels with Epoxies: Mechanically Strong Lightweight Porous Materials

2005·6.1 MB·English

by  Meador, Mary Ann B.

Crossinkiny Amine-Moditied Silien Aerogels with Kposies: Mechonically Strong

ightoreight Parus Mate

Mary Aan 8, Meador’, tive P: Katwieia’, Paes thas, Armales Daso®, Groat Zhsany” Pca

Viwsilarov!, J. Chere dona? crc ichebas Levene

NASA Glenn Rescureh Center, 21000 Brookpark Re, Clovelaml OH 441

Univorsity of Missow-Retl, Rolla, Missouri 6540: and Ohio Aerospace Institute, 22800 Cetur

Poin Ra, Clavelné OFT 44.42

Abstract
The mesupaus surfaces af TMOSleivell sition angels

bean modified with amines by

co polymeizeton of TMOS with APTES. The cmine siter have become anchors to:

teronslinking the nanoparticiew of the skeletal harkbone nf the neroy

by allachment of dics tris

ani “eta-teactional epoxies. The retulting contornal co

9 ineeease “he deactry of the native
ore thar
(+0 arders of magnitude. Processing variables such as semmnt of APTRS nied to take the gels,

neragels by & factor af 2-3 bul Ihe sircngth of the resulting masterinls may inevesse

The epeny lyre am

tion used for crosslinking, as well ss the ctossivking temperame

and lume weie vane uckonding ta u :nullivasiahle desigceotesperiineeus (BOR) mle. 1 wnan

mis

Gouend that white elastic smawlulue foliows a similar trend avin dkmsily. maximum siteog"

attaincé acitber at the maxitoum density roe a. te highest eonventration of Nip yrours,

Ling surface suluraion offeets. Serogels envwslinked wih the tri-fimetiomal epee always

show imavaved stcagth compared witl aotagels cxosslinked with he oth poxides unser

bow

idsctivu! vouitious. Solid °C NMIK studies show rosidual enrswsted epoxides, which condsnse

wath ons ano:hor by heating exosslankead gezogels al 150%

1

Ono Aorospaze Eines

Sis as uf Wess Hulls
4 NSYALERCE Sinan imera
* Covspanding Author
1. tatroduetion

Siliea aoroge’s are nniny the Zaswest density, Lowest dhermtal conduetivity moterialy
boon, with smany possible aerorsutic and space applications, ixeluding vehicles. aubitats,
plamsiary ravers and sqace suits, The use af aerogels as thermal inselacion, anlar semnotural

smatetials cou'e lead te significant weight reductions. enablirg fituzo sgace missions. Lo date the

tose ov Tica tents as heen Ty Uhre tree gy

Jost as dispersion of inorgacic azopareles in polymers hive hesn shown to improve
smcchanical, thermal andforclecuical proporiss af other matsials Zoheducing 8 polymer inva
the Tragle msc of a silica aeroyel may add stengd co ike evecall conspusite. Funbernores

chemmisad bonding (hah covalenk aml noa-cavalenl) between a ler anal a palyner improves

reatetial

compatibility, enhancing certain properties of a polymeric matrix abowe aud beyoud

what is uecoreplshed hy simple doping with he Giller?" Ry anehigy, eavelertly boning the

polymer 0 tae surfues uf tne aagel particles saonll havea sila eft

“We lave recently demenstsced chat templated polymetizetion of i- snd t-isocyanates

ont

surdtee of the nanoparticle build

locks oF silica aero ineren

strength of the

mateaa's by a lactor of 30 for a nominal taroe-foll inezcase sn density.” Crosslinking silica

Js with isocsanales effectively mimics pulyurethane chemistry, where « poly unetional

isoeynae reacts ath a polylimetional alkohol uy fires cursamales (uretoanes. Tn the euse of
sulica acrogels, ths role of the functional polyol is played by che hydroxy! eroups on te surface of

vilfica, Thus, it as swore thes ithe swace of shea is devoratee wath ache I

palty, for

sxample amines, them oper

y chewisiry could br ublived, where a polyfimerionail glycidyl ether

world react with the

fix, a polylutional cpoxl of silica playing the cole of a

polytimetional anne (he harlerer) ki farm a hand esa esi
Herein, we deseribe a system where an amine functionality is inttadvesd itso silica

eeiogels hy vopulymorization and gelation a2 tstrewethosysilene (TOS) and

reninopropsltetboxysulare (APTES in up to 1:1 voune sotios, ‘Lhe vesulling gels are nested

sits dr stza-unetional eponies in variows concentrations, The reaction condoms (ime
‘and temperetate) ate vatied slong vita the APES concenteation, and the epoxy spe and

eomentalios, uning a slaltieal experimental danign slraleny ly reduce the mauher af

exporicncars, The rssuling erees'inked zels ane died vith supererieal carbo cioxice into

satogsis. which ee cha

ccctizod macwwscopically, mictoscop.cally, and eccurielly. “Lhe most

isin. property of polyiner enwolinked asragels is the greatly improved mecharial properties
compare ta unive uerngels, Therefore, special emphasis is gives lo Uheir strength as Ramen

of the chemical composition, prov

ng conditions and resulting deasiy

2 Ryperimental

Moterials. ‘Te:ranrethyloiesilcate (TMOS). amnosrupsltecthxysilone (APTES),

Aigdycidybmiting —Yl-epony), NATAlighyeidyl-ulycidytonvanitine — (rieposy, 44

mothy.snsbis(."-diplvoidylmiline) itet-ezoxy) aud amenimm: hycroxide (ACS xagout

glade) weve uso as seosived Irom Alekich C:

eraieal Compan

Solvents used in the piodctan

(of erosilicked epoxy aomye's included metherol (ACS reagent grade), lete'yleuran (PHP:

ACS ronger: grade}

ankylerus wtrahydrofosm (ACS reagent yraie purified ancl dried using @

sols

et putificarioa system by Innovative Tecnology, ine), aad acctone (ACS respent acai

Supercritical yi

vas perforesell using Tiquid earhin ion’

tartan] T ype tanh with a

siphon tehe, AGA Ine,, Cleveland OH).
Statiteal Amalyas. Exyernental desig and andlyis was curral out ning. the
RSSories for Windous, including RSM. Version 61, and RS Discover and RS/Explove Release
4.1, availabe (hon Domain Manuacuring Corporation, Buctinyton, MA

Methods. Preparation of wes gels thydrogeish AIL gels were arepaced by mixing
loge twa solutions, A and H. Socio A contained x vatisale volume mio of IMOS te

APLES in meshanok, solution B contsined aa cqual amount e'metharol, he app-opiale wuneunt

bef water and the gelubon catalyst, W'oeceseary. Wel gels willoul APTIS ware grepured By uri

TMOS, deiorved wouter and methamol in the molar ratio of 1:38

gelation wns catalyzed by
adding, © jel of NHsOH (284) for every evel af TMOS. Thus, in a typical procedure solution
A comeing 45 mL oF methunal, ged 3.85 mil. of TMOS, Solution B youtaius 48 mb af

methane’, 15 mL of watesand 40 pL

f the NHOH salucon.
‘We: gels with APTES were prepara hy replecing 2595 or 50% wv nf TMOS in solurion

> wth a equal volume of ATES. Tnmportarly, the base catalyst, NH.OHL, can be alintiaated

trom the prepaation proceduce when APTES is used, because the wine gioup ef APTES is

servos wt the gelation eubalyat© Tr Gas, 10 shine coon the APTRS:

catuyva yelion prneess
tong eniigh to allow: pouring the sol into the appropsiate molds, solutioas A and B had to be
cooled ina dry ioe aceroue beth pice to maxing, Imumediatey ater anxing the cold solutions 3.6
sil oC each enieote was poared into eslindreal mols (Whostom Polypropylone Cna-Vieo, Pare

Do, 225402) appronimataly t om in diameter to foun the desined epliadsies! monoliths for

testing. ‘The molds were wavered will paraliln and the gels were allowed lo aye fir 48 By

Subsequently, wel yok wore sareflly removed ftom the molds acd wero wasaed approximately

covery 12 hwich mneehano! (20 mL.) tora of loau times, ‘This ise verve NII OUT if. present, bt

sya dhs fbr the sice al consistency ir all yes, Wet gels were then woised Fur more elmes

covery 12, ovics wi: THE (20 mL) and a

‘with snaytrous THF (20 mL).
calon i

hire ditlenent percent volun wae Zh epevid in unity THI (188

459% and 7874 wv) were

J for omeslinking Because of the high viscosity of the epoxies,

dissolution was f

rated wit Weating ate. SO-°C and vigorous shaking, hee the slums

ore hamogs

1s, 1ae wat gels were placed info the desires epoxy solution (20 mL), gently

stieed and allowed to equilibrate toe 48}. Subsequently, end while stil in the corresponding,

spowide solution, wel gels wore placed in an oven and sllewd to react at specified tomporsnmes

(50°C 7

6-95) ated Hengtt of tines (16 b, 44 k oF 72 W) as predeterotined by a statistical
experienental desige procedure. Allerwards, ezonslis‘ed gels were alluwel cca] 1 mom
lemperalure and each ye! sas fat sashod fore Fimas with nokytrous TH (20) mL sach ims) i

x

2 b intervals co remove che unreacted spenico, acd then four times with acctone (28 mL eae
lune), Aer the Unal acetone wis, emis were trunsfimed together with the fins] acetone
sash salu inl an autoclave (SPT-DRY Jexnbss Supercritical Beir Darien, SPI Swaplics, Le.

‘West Chester, 14) and ware driod fiom: supetcsiticel CO3(40°C and-~100 bar)?

Chearacteresition ce erassTnead wes acre

ie. Chemisal characterization 0°

spony scosslinke aerogel monoliths was comlusted Sy iniaged, CNM aud "SLNMR

specuoscopy. ‘Tae dry aecogel samples were groand by ball-uulkng foc 5 min using a SPEX
5300 Mixer Mil (SPEX Inc, Edison, XJ}. Solid samples were mined with KR, prewel nla 4
pol ul, und infrared spectou obluingd with u Nicole FTIR Spovremeter Model 759, Solid"
NM and ’SHNMR agcotns wero obiainod on a Bruker Avance 300 Speemometer, asing cress
polarization and magic-angle spinning at 7kEI2, The acquisivan sia emplayed spinning

sidcbsu¢ eupprossion usiag a CHSELTICS soquense, Iho solid "C spoctts wi

xtennlly

seierenced to the carbonyl of glycine 1176.1 me relative us tetramehylslane, TMS). The solid

“Si spectca Were externally raferenred to ‘he sili pos of the sodiom sodivm salt of

Uiecthylsilyipropion’c ecid ¢¢ ppm.
Physical Garauerivation af rative nnd erossEnked silica astogel santa Was once
by thormogrevintric analysis (GA). scannug eeewos micrescopy GTM}, and wizeagen
adsoption poeoometry. TCA was penfonned using 9 TA Instrauens Model 2950 {Res
insinnmen:, Samples were run at a temperanne ramp sate uf 11°C min | uneler wilrogen ov ain,

‘Sampies for SEM were coated with

nl and sniroseupy was conducted with & Mitac: $8700
Teld-emission mrigmosone. Hor witmgen adsozptice. poresimety. camples weve oeugaseed at
IPC for 72 h enter yas and stocies Ware conducted with ao ASAP 2O0U Surface Areao1e
Disesibotina analyzer (Micromeitcs Inslrument Corp

Three-point Nexural vending tests were perlotnsed adv

A dElexural Propertios of Unvciniinced ansl Reinturced Plaion and Ploctrical knsuleing

‘Mafia, using on Instron 4269 universal teeing machine fame-with a 2 KN load esl! Ustrin

09 in. span ged 28-e9m toler

park rumber 252:

IR) and g chroo-poli bord xtuee, wi

29

diameter (asuon part aon Typical samples were eylinerical, 1 em in diamew

and -4em is enyth. The crosshond speed seas scat (G4 bs mir!. Taco samples were tesel

foreach eorsposifem amd the average of the :hrse Values ave resenlel in Tabi 1

Diclootrc chiscactevization was venducted hy mytsuriny hz expacitanes of then 9 am

dlametot, +L mma thick) disks. The thin lises were cut tiem cylindrical monoliths will a
diawor wafer blake Mudson, Cove and, OH) and saw Usomet 11-1180, Low Spot Sa:
Huchlsr lat. evemslor, LL, ‘he lateral area oF each Gise sas masked with duet tape ane the tem

and boom surfaces were spur

could with yold using & Baltek MEE 20 Sputtering System.

Copper wires were Sealva!™ raped on the two surfaces, end were cormestall to an MEMS

SWIVRE analyzer (MEI Trverprises, Tre, Mississippi State, M18), rmaning ia eamctance

rreasnremart mule, Capacitance ucazurcaments were made at hice different reyuencins (72,

115 oud 174 MHz). Lhe average capuctlance values were corrected for frings-ficld cumers* ‘The
relive dFclostrie constant gf ou warmple was caleulalel using the corrected capacitance, area,

and tuekness ofthe samplos Ihe accuracy of the dielesriv constant measurements ws Sevkess

upuriag, the dislecris constant of a kneven maleial, Pkag’as. The racasured velno was

2.21 G2, while the reposted value is 2.55 0.13."

3 Results and Di

BI Preparation of epoxy ceusstinked weragels
‘The firs. class af raochanica’ly strom silica aorayels ws reported relics on sonfourally

seating the silica nacopariclss of the skelstal framework of the serogel wita a potynter dezrved.
frou diisoeyseates, which bind to sie twough retier: wilh iby suelice yell groups! The

remaining Foe isveyanale is Rysalyzed hy the water adored on the sur

Filion prod

an amie that reac further with mor> dlsocyanate fom tac solution, filling the mesopoves. sis
recess huilds tte (ex. pontamers} between skeletal nanepertcles
Analogous tw ensslinking, silien aeraely ovth issxymaios inepired “ay polyurethace

shemisty, we rumed © the epoxy chetaisuy that involves reaction of a diamine with a

slieponide.?” This approach would xaquire u surfeve of siiou rich with arrinen aa wl souk ost

a polyfimstional epuside intruluced in the messes, Wi knew thal amine 24

lass crn be obtained by froslment with APTES.' A similne approsch would also work well for
cour purposes, but past reamieet ofsiliea hydrogels with APTES uanecessarily introduces several

time-conaurning wash steps. Allemalively, itis knowa that owing to che elawer hydrolysis of

APTES relative to TMOS. including APTES in the so: together with IMOS leads to a regular

Siva huektaone, whieh is

sfuce-mul fil wilh the APTRS amines? This became the fevoted

peach, but the main challenge was the amtocer!

te role of AVIES (owing to the base
properties of the antine finctonality), which, ie carcbiation with the high corentrations used

jn this sy, causes iastantaeous coven tarperalure wclaion spon mixing of solucions A aud B
(Gee Scheme 1}. TL was found, however. that cooling the two solutions rerards gelation. ong

enough 10 allow time tin pouring tire sel int the apupriane ids. Once the amine

fiscelionalizeal gels have bon formed, the

¥y aac exposed © 3 polsfietional epoxide leading
eventually 10 a strong ‘ightieight snateriol anclagaus Ww Uheir isneyanetocrossinkeal

eounterparls, Sehure | summerizes the pruguaion proealine of Ihe epaxyeornsslinked sites

TFor comparison purposes, previously published pro:oenlk were fallawed for cong as

possibie in the process. It is xoted, however, that altnougk: in the case of ctosslinsed acrogels

several of the orocessing steps could become signiticant’y shoder us eliorinated altogether, ne

such eli was made al Cais sage, This study is eather iseuse om dhe reabionship etweon the
crasalinking chersisiry wil tho propertics of

resulting acmycls, Thotefoee, she Haetors

ccpiozed in orossLnkiug arnins-modificd silica wth potylaneioaal epoxides ane: Cie amine
cancentrtion on tite sities backbone, the chemical idenliyy andl eanueninuton of the opoxie in
ths mesoperes, sm finally the xeaction temperanice and sims, De to chs poor gensralsalubility
‘ofthe cpowises in msthauol, THE was uscd as Le salve (Tu thi content, i is ne agi tha
several was seps could be eliminate, shortening the process of Scheme 1 significantly, xf
metho napacl hy F111 the get solve)

‘ee arsine concentration om the silica backbone was contolled by the eletive stio of
APTES 10 TMOS in che sl, Thus the APTES conuuntrcn, a, wus evalu from 9% up Wo

0% ev Au ths total volume of TOS

-AMLES, Higher conesruzations of APIES €o not

produce stable gels. Vor example. using a 3:) We ratio af APTES to TMOS (75% APTS
produces upiyue Waits, yeiainony (os oppose 10 rigid) aud catromtly fragile wot gels. while

100% substitution of APLES for EMOS

ults w sols thar (al to gel altogetter. As amextionell

shove, hydrolyscs al! APTES is a much sinwer priest than hyetralysis of TMOS: chus one
Jalnwnus texture ul 3:1 viv cain of APTES to TMOS (75% APTES) could

cexplanstion dr the 3p

vomnlrol

sinount of TMOS is to¢ sacall to form

be the fact chat

stable gels, Mowe

ng Une relative ratio wl APTESITMOS to L:3 wy wile ceducing

expusimuns shows thas by Kes

joven when the total xmeunt of

the total smount of ANTHS1TMOS én tke sok prodecss ust

APTESITM(

is reduced by 20 times. Presumably. high concemeatous of AIIES in ce sol

may algo rosult in estic inotfecencss to gelation, Honse, while il was deemed retscmuble thal

Tngher concsntiations of APTES nugix siold the most sits for reaction with ar: epoxide, lnwer
eoncentrations of ADPTRS read produce a stronger silica Gamework wo Iuskl upon,
Thos differen: epoxy scosstrieens were camsidened: diglycidslaniline. a dilunstiomal

epoxide: &.A-diglveidyl-t-g-yeidvloxyaniline. a trifmesional oposides wel 44"-anethyleuc-

DisGW.Artigheeidylanitine). a etealiewttonal epoxide. Fer simplicity, these monomess will

benesfomh he relermal in ws disgpimy, I-eposy aml tetroepery, cespectively. ann for

lotr

i epoxy

Alaistcal movleting purpaces they are peprssarad hy the discrete vaviable, epoxy type, e. It is

sxpested that more epoxy groupe per molecule might lore to more extensive croselinking

hheiween neighboring amine sitcs on tho skelctal tamework. However, it 1s also deemed
reasounble tial as the number af epoxy grmups increases the mulecales migh: hecome ae

cconstesinod tor any inecemental beacfit fiom the additioue! reactive sites. Hythe sacae token, of
{be concentration of the epoxide i the mesoporess too bigs, then itis expeceet tt single point
itachmes woul oveur ever whun there are na slrartural eatin for rmultple attachment of
the epoxide on the skelevs emnework,

Thus should be

foc: of the chemical idetity ofthe eposide om the aeragel properties

studied sn patel to its concentration, c, m.she THIF bat, har voucentration was vacied from

15% to 75% wy oF the earrespunding epoxide to THIF. Finally, Ure ceacian stmt, aad the

sesotion sazpcrature, 8 whica, at first apoznxitation, both contra! the nile of incomonition af

the epoxide into the sien backbone stocld also be investiga

sd for ane ining, begat

lumporstuns eauld possibly reduce the epaxcde incorporetion time, making the process more

officiont, Lemperslure 3 consimined, however, lo a range bobween 50 °C, Velo shiv te

reactinn is found empincally to be exeremely slow; and 95 °C, above whica wo experience

sxcossive solvent heilafl Ths, it was theught chet the inability to

tempersrue indefinitely could bo compeusated with longer reaetina mes ‘The reaction time wns

varie frum 16 00 72 12,

3.2 Phssicad properties of epoxy crostinked aeragels

Lypica: apoxy crosslinked aevogels are shown in Figure 1, along with n sample prepatcd

Si: APTS to APIS! IMOS volume ratio and re epeny ot

ink (Sample furthest <0

fhe lel), Te general, epoxy envetinked weruyels ure trumsiacont, The yellowish eoloraticm

observed with uncansslitked samples is cartie¢ aves ta crosslinked sows as well, and comes

from a trace amount oT bese-catalyzed polymerization of acetone, dhe solvent that is used io Cie

final washes hefors siporvtioal Mut drying" Roe NHOH eluate, unis eploaton ie

vine

avoigeé by removing the base wita alcohol washes belore inasxiuring acetone.” or ADTES

u