A SYNTHETIC ROUTE TO ORGANIC AEROGELS - MECHANISM, STRUCTURE, AND PROPERTIES

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A SYNTHETIC ROUTE TO ORGANIC AEROGELS - MECHANISM, STRUCTURE, AND PROPERTIES

R. Pekala, F. Kong

To cite this version:

R. Pekala, F. Kong. A SYNTHETIC ROUTE TO ORGANIC AEROGELS - MECHANISM, STRUCTURE, AND PROPERTIES. Journal de Physique Colloques, 1989, 50 (C4), pp.C4-33-C4- 40. �10.1051/jphyscol:1989406�. �jpa-00229481�

REVUE DE PHYSIQUE APPLIQUÉE

Colloque C4, Supplément au n°4. Tome 24, Avril 1989 C4-33

A SYNTHETIC ROUTE TO ORGANIC AEROGELS - MECHANISM, STRUCTURE, AND PROPERTIES

R.W. PEKALA and F.M. KONG

Lawrence Livermore National Laboratory Livermore, CR 94550, U.S.A.

Résumé - En catalyse basique, la réaction en voie aqueuse du résorcinol avec le formaidehyde est assimilable à une voie sol-gel, dans laquelle des "clusters" de polymère à surface fonctionnelle sont formés. La nature covalente de la reticulation ntre ces "clusters" produit des gels qui sont séchés sous des conditions hypercri- tiques pour obtenir des aerogels organiques de faible densité (< 200 mg/cc). Les aerogels sont transparents et sont constitués de cellules dont la taille est infé- rieure à 1000 Â. Leur microstructure est constituée par des particules quasi colloï- dales interconnectées, dont le diamètre varie entre 30 et 100A. La taille de la par- ticule, la taille de la cellule, la surface spécifique et la densité des aerogels de résorcinol-formaidehyde (R.F.) sont grandement déterminées par la concentration en catalyseur du mélange. Les aerogels R.F. sont sous plusieurs angles similaires à ceux de silice, encore qu'une comparaison de leurs propriétés mécaniques respectives sug- gère l'existence de subtiles différences

Abstract - The base catalyzed, aqueous reaction of resorcinol with formaldehyde follows a sol-gel pathway in which surface functionalized polymer "clusters" are formed. The covalent crosslinking of these "clusters" produces gels which are dried under supercritical conditions to obtain low density, organic aerogels (< 200 mg/cc).

The aerogels are transparent and have cell sizes less than 100A. Their microstructure consists of interconnected colloidal-like particles with diameters of 30-100 A. The particle size, cell size, surface area, and density of resorcinol-formaldehyde (RF) aerogels are largely determined by the catalyst concentration in the mixture. RF aerogels are similar to silica aerogels in many ways, yet a comparison of their mechanical properties suggests subtle morphological differences.

1 - INTRODUCTION

Sol-gel processing of metal alkoxides in the common route for the production of two different materials : (i) inorganic xerogels which are heat treated to form coatings, fibers, or composites /1,4/ and / 2 / inorganic aerogels which have lqy/ density, high poro- sity, and ultrafine cell sizes /5,6/. In the first process, the xerogel is formed from the slow evaporation of solvent from an alcogel. Large capillary forces are exerted as the liquid-vapor interface moves through the gel, and these forces cause macroscopic shrinkage.

The resulting xerogel has high surface area and continuous porosity. These parameters allow the material to be sintered into full density coatings at relatively low temperatures.

In the formation of inorganic aerogels, shrinkage of the alcogel is minimized by super- critical drying. Above the critical point of the solvent which occupies the pores of the gel, capillary forces are reduced to zero / 7 / . Inorganic aerogels retain the morphology of the original alcogels. Their high surface area, continuous porosity, and low density lead to potential applications as phonic and thermal insulators, filters, and catalyst supports /8,10/.

After reviewing the sol-gel chemistry of alkoxysilanes (i.e. TM0S and TEOS), our research group concluded that organic chemistry could be used to produce new low density materials in a similar fashion /ll,13/. In our view, four important requirements must be satisfied to produce gels via a sol-gel route*. These requirements are outlined in Table I.

The synthesis of organic aerogels via a sol-gel route involves the polycondensation reaction of resorcinol (1,3 dihydroxybenzene) with formaldehyde under alkaline conditions.

In this reaction, resorcinol serves as a trifunctional monomer capable of electrophilic aromatic substitution in the 2,4 and 6 ring positions. Formaldehyde is difunctional and forms covalent bridges between the resorcinol rings leading to high crosslinking densities.

The resultant sol is composed of polymer "clusters" with diameters of 30-100 A. Favorable solvent-polymer and ionic interactions are responsible for the stabilization of these

"clusters". The "clusters" crosslink to form gels through the condensation of surface functional groups such as the hydroxymethyl (-CH20H) species.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1989406

The resorcinol-formaldehyde (RF) gels are red in color and transparent indicative of both a small cell (pore) size and "clusters" size. The supercritical extraction of solvent from the pores of the gel results in organic aerogels. This paper describes the chemistry, structure, and properties of RF aerogels in detail.

TABLE I

Sol -uel requirements

Mu1 tifonct ional monomers High degree of crosslinking Surface functional groups Particle stabilization 2

-

2.1 - GEL PREPARATION AND SUPERCRITICAL DRYING

Gel preparation has been described previously /14/. Briefly, resorcinol and formaldehyde were mixed in deionized and distilled water at a molar ratio of 1:2. Sodium carbonate was then added as the base catalyst. The RF solution was poured into 23 x 85 mm glass vials or ampules which were tightly sealed and placed into an oven to cure at 85-95°C. Depending upon the % solids and catalyst level, gel times varied from several hours to days. Typically, the RF gels were cured for 7 days and then removed from their glass containers and placed in a dilute acid solution at 45°C. The acid treatment increased the modulus of the gels through additional cross1 inking provided by the condensation of hydroxymethyl groups to form ether bridges.

In preparation for supercritical drying, the gels were exchanged into an organic solvent which was compatible with liquified carbon dioxide (e.g. acetone, methanol, or isopropanol).

The solvent-filled gels were placed into a jacketed pressure vessel (Polaron Equipment Ltd., Watford, England) which was then filled with carbon dioxide. After the carbon dioxide was completely exchanged for the solvent in the pores of the gels, the vessel was heated 15°C above the critical temperature (Tc = 31"). The samples were maintained at 46'C for a minimum of 4 hours, and then the pressure was slowly released through an exit valve. At atmospheric pressure, the samples were removed and held in front of a light source to ensure that the aerogels were transparent.

2.2 - AEROGEL CHARACTERIZATION

All mass and dimensional measurements were made after the aerogels were dessicated for 24 hours at room temperature. The high surface area of the aerogels and the presence of many hydrophilic groups caused a 2-8 % mass increase if the materials were not dried properly.

Selected samples were submitted for surface area analysis and microscopic examination. BET surface areas were obtained from nitrogen adsorption analysis (Digisorb 2600). Prior to analysis, samples were evacuated (lo-= Torr) for 24 hours at 25°C to remove adventitious moisture and contaminants from the surface of the aerogels. Some samples were dried under vacuum at 200°C for 4 hours ; however, no significant differences in surface areas were noted between these two procedures. The standard deviation for surface area mesurements was generally

+

The morphology of the RF aerogels was examined with scanning electron microscopy (Hitachi S-800) and transmission electron microscopy (Jeol 200 CX). SEM was performed on uncoated samples because the sputter deposition of a thin, conductive gold coating was found to distort the aerogel structure. Samples were prepared for TEM by atomizing an RF aerogel/

methanol mixture on to a Formvar grid. Thin sections of the aerogel were analyzed at an accelerating voltage of 80 kV. Re-wetting of the RF aerogels with methanol did not damage the microstructure observed with TEM.

2.3 - MECHANICAL PROPERTIES

The strength and modulus of RF aerogels were measured in uniaxial compression at a strain rate of O.l%/sec using an Instron (Model # 1125). The aerogels were machined into

10 x 10 x 10 mm3 rectangular prisms for testing. All measurements were conducted under ambient conditions at 22-25°C. The relative humidity was generally 50-70 % during the experiments, and no special precautions were taken to prevent moisture adsorption by the aerogels.

The compressive strength of RF aerogels was calculated at the point on the stress-strain curve where the curve deviated from linearity by 0.2 % strain. Although no fundamental reason exists for choosing the 0.2 % strain offset, we have found it to be a convenient way to characterize the strenght of many different low density materials which exhibit a variety of responses at higher compressive strains. The compressive modulus of the aerogels was calculated from the slope of the stress-strain curve in the linear elastic region.

3 - RESULTS AND DISCUSSION

3.1 - REACTION CHEMISTRY AND KINETICS

Resorcinol reacts with formaldehyde under alkaline conditions to form mixtures o f addition and condensation products. These intermediate products can react further to form a tightly cross1 inked polymer network. The principal reactions include /1/ the formation of hydroxy- methyl derivatives at the 2,4, and/or 6 ring positions of resorcinol, and /2/ the condensation of the hydroxymethyl derivatives to form methylene and methylene ether bridged compounds /15,16/. Figure 1 shows a schematic diagram of the reaction of resorcinol with formaldehyde under conditions used in this study.

OH OH

Crosslinked Polymer

Figure 1. A crosslinked polymer network from the reaction of resorcinol with formaldehyde.

. - . . . . - . - ^{-. .} , --

A Resorclnol

Gel polnt

I

0.01

I

0 250 500 750 1M)O 1250 1500 1750 Polymerlzatlon time (mlns)

Figure 2. Free monomer in the RF reaction as a function of the polymerization time at 85°C.

Sample was prepared at a target density of 50 mm/cc with [[Resorcinol] /[Catalyst] = 200.

The catalyst concentration affects both the reaction kinetics and resultant aerogel properties. Figure 2 shows the amounl; of unreacted formaldehyde and resorcinol in a 5 %

solids solution as a function of the polymerization time at 85°C /14,17/. Approximately 60 percent of the original formaldehyde is consumed after 200 minutes, and an abrupt change in the rate of formaldehyde consumption follows. At this point in the polymerization,

approximately 90 percent o f t h e i n i t i a l r e s o r c i n o l i s reacted. These data i n d i c a t e t h a t the n u c l e a t i o n and growth o f polymer c l u s t e r s " i s n e a r l y complete. Henceforth, t h e r a t e - l i m i t i n g step becomes t h e i n t e r c o n n e c t i o n o f t h e " c l u s t e r s " . Eventually, a gel i s formed a t approximately 950 minutes, b u t formaldehyde i s s t i l l a b l e t o p a r t i c i p a t e i n a d d i t i o n and cross1 i n k i n g r e a c t i o n s .

Substituted RF "CluSers" RF Gel

resorclnol 30-100A diameter lndlvldual beads

30100A dlameter

F i g u r e 3. A shematic diagram o f t h e RF g e l a t i o n mechanism.

The use o f a d i f f e r e n t c a t a l y s t c o n c e n t r a t i o n i n t h e RF f o r m u l a t i o n changes t h e absolute amount o f formaldehyde remaining i n s o l u t i o n a t v a r i o u s p o l y m e r i z a t i o n times ; however, t h e same t r e n d s f o r n u c l e a t i o n , growth, and g e l a t i o n a r e observed. F i g u r e 3 d e p i c t s t h e s o l - g e l chemistry o f an RF m i x t u r e .

3.2 - STRUCTURE AND PROPERTIES

F i g u r e 4 d i s p l a y s an RF aerogel a f t e r s u p e r c r i t i c a l d r y i n g . The aerogel i s dark r e d i n c o l o r and t r a n s p a r e n t . The l a t t e r p r o p e r t y i n d i c a t e s t h a t both t h e c e l l s i z e and c h a r a c t e r i s t i c p a r t i c l e s i z e ( r e f e r r e d t o as " c l u s t e r " s i z e i n t h e gel phase) o f t h e aerogel are l e s s than 1/20th t h e wavelength o f v i s i b l e 1 ig h t .

RF aerogels were examined by SEM t o r e v e a l t h e i r morphology. F i g u r e 5 shows t h a t t h e aerogels have an open c e l l s t r u c t u r e w i t h continuous p o r o s i t y . The observed c e l l s i z e i s l e s s than 1000 A

---

Figure 4. A transparent RF aerogel on a perforated metal screen.

TEM r e v e a l s t h a t t h e aerogel m i c r o s t r u c t u r e i s composed o f i n t e r c o n n e c t e d c o l l o i d a l - l i k e p a r t i c l e s . The s i z e o f these p a r t i c l e s i s c o n t r o l l e d t o a l a r g e e x t e n t by t h e c a t a l y s t c o n c e n t r a t i o n used i n t h e RF f o r m u l a t i o n . F i g u r e 6 shows t h e m i c r o s t r u c t u r e o f aerogels synthesized from a 5% s o l i d s m i x t u r e a t two d i f f e r e n t c a t a l y s t concentrations. As t h e c a t a l y s t c o n c e n t r a t i o n i s increased by a f a c t o r o f 4 , t h e p a r t i c l e s a r e observed t o decrease i n diameter f r o m approximately 100 A t o 30 A.

Figure 5. Scanning electron micrographs of an RF aerogel.

Figure 6. Transmission electron micrographs of RF aerogels synthesized at a target densities of 50 mg/cc wifh 2 different catalyst concentrations. [Resorcinol]/[Catalyst] ratio equals (a) 50 and (b) 200.

The effect of the catalyst concentration upon the interconnected particle size and presumably the size of "clusters" formed prior to gel ation suggests an autocatalytic growth mechanism. After the base catalyst activates the resorcinol ring through hydrogen abstraction, formaldehyde can add to the ring in either the 2,4, or 6 positions. The reaction rate of this substituted resorcinol molecule toward bridge formation or further substitution is faster than the original addition of formaldehyde to resorcinol. After a bridge is formed between two resorcinol rings, this dimer is tetra-functional and has a higher probabil ity of further reaction than an individual resorcinol molecule in solution.

The above mechanism leads to the preferential growth of polymer "clusters", the total number of which are determined by the initial catalyst concentration.

The catalyst concentration also affects the final density of aerogels synthesized from gels with the same solids content. Figure 7 shows that aerogels dried from gels synthesized at high catalyst concentrations have higher densities than their low catalyst counterparts. The high catalyst gels experience more shrinkage during supercritical drying even though their modulus is higher than the low catalyst 9els. Presumably, the higher modulus results from better interconnection between "clusters in the high catalyst gels. Nevertheless, the increased shrinkage during supercri tical drying suggests that the interparticle cross1 inking density is low. This discrepancy is under further investigation.

The above data indicate that RF aerogels will most closely approximate their theoretical densities at low catalyst concentrations. Figure 8 compares actual RF aerogel densities to

C4-38 REVUE DE PHYSIQUE A P P L I Q U ~ E

Figure 7. The e f f e c t of t h e c a t a l y s t concentration upon f i n a l RF aerogel d e n s i t i e s . All samples were synthesized a t a t a r g e t d e n s i t y of 50 mg/cc.

Figure 8 . A comparison of f i n a l RF aerogel d e n s i t i e s t o t h e i r t a r g e t d e n s i t i e s . All samples were synthesized with a [Resorcinol]/[Catalyst] r a t i o = 200.

t h e i r t h e , o r e t i c a l values a t a [Resorcinol]/[Catalyst] r a t i o of 200. The s o l i d l i n e of t h e p l o t r e p r e s e n t s t h e ideal cake where t h e f i n a l d e n s i t i e s match t h e t h e o r e t i c a l d e n s i t i e s . Above 100 mg/cc, t a r g e t d e n s i t i e s a r e r e a d i l y achieved. Below 50 mg/cc, t h e aerogels a r e approximately 33 % higher i n d e n s i t y than t h e i r expected values. Lower concentrations of c a t a l y s t can be used, but e v e n t u a l l y a point i s reached where only opaque g e l s o r p r e c i p i t a t e s a r e formed.

In o r d e r t o f u r t h e r c h a r a c t e r i z e t h e above a e r o g e l s , BET s u r f a c e a r e a s were measured. Figure 9 shows a 60% i n c r e a s e i n t h e s u r f a c e area o f an aerogel a f t e r t h e c a t a l y s t concentration was r a i s e d f o u r - f o l d . The TEM d a t a show t h a t t h e average p a r t i c l e s i z e decreases from approximately 100 h t o 30 h over t h i s same range. This decrease i n p a r t i c l e diameter p a r t i a l l y e x p l a i n s t h e increased s u r f a c e area per gram of aerogel ; however, o t h e r f a c t o r s such as t h e p o r o s i t y of individual p a r t i c l e s and t h e s i z e of necks between them must be q u a n t i f i e d before a d e t a i l e d model of t h e microstructure can be proposed.

I f t h e [Resorcinol]/[Catalyst] r a t i o i s held c o n s t a n t f o r gel formulations varying i n s o l i d s c o n t e n t , t h e s u r f a c e a r e a s of t h e r e s u l t a n t a e r o g e l s remain r e l a t i v e l y c o n s t a n t . Figure 10 d i s p l a y s t h e s u r f a c e a r e a s of a e r o g e l s ranging i n d e n s i t y from 30-200 mg/cc. These d a t a i n d i c a t e t h a t t h e average p a r t i c l e s i z e i s approximately t h e same f o r each aerogel;

however, t h e average c e l l s i z e must be increasing a s t h e d e n s i t y i s lowered.

The mechanical p r o p e r t i e s of RF a e r o g e l s were measured in uniaxial compression as a function o f d e n s i t y . As expected, both t h e modulus and s t r e n g t h increased with t h e aerogel bulk d e n s i t y a s shown i n Figures 11 and 12. The l i n e a r log-log p l o t i n each c a s e demonstrates a power-law d e n s i t y dependence t h a t has been observed in many o t h e r low d e n s i t y m a t e r i a l s . This r e l a t i o n s h i p i s expressed a s

E o r S ( d ) n (1

where d i s t h e bulk d e n s i t y and n i s a non-integer exponent t h a t u s u a l l y ranges from 2-4 120-231. For highly r e g u l a r , geometric foam s t r u c t u r e s t h e exponent u s u a l l y f a l l s very c l o s e t o 2.0, while f o r i r r e g u l a r , f r a c t a l t y p e s t r u c t u r e s t h e value g e n e r a l l y exceeds 3.0. In t h e c a s e of t h e RF a e r o g e l s , t h e exponent i s 2 . 5

+

In comparison, t h e modulus of s i l i c a aerogels has been reported t o show a power-law dependence with an exponent equal t o 3.7 t 0.3 [24]. Thus, our mechanical property d a t a suggest t h a t d i f f e r e n c e s e x i s t between t h e m i c r o s t r u c t u r e of t h e RF aerogels and s i l i c a a e r o g e l s . Although t h e s e d i f f e r e n c e s a r e not g e n e r a l l y observable with TEM o r SEM, we p r e d i c t t h a t f u t u r e s c a t t e r i n g experiments w i l l provide i n s i g h t concerning t h i s matter.

[Resorcinol] 1 [Catalyst] RF Aerogel Density (mglcc)

Figure 9. The effect of the catalyst concentration upon the BET surface areas of RF aerogels. Measurements were performed on the samples from Figure 7.

Figure 10. BET surface areas for RF aerogels ranging in density from 30-200 mg/cc. All samples were synthesized with a [Resorcinol]/[Catalyst] = 200.

Figure 11. A log-log plot of compressive modulus vs. density.

Figure 12. A log-log plot of compressive strength vs. density.

10.0

!i

-

5

H s

g

4 - SUMMARY

Organic aerogels can be successfully synthesized from the base catalyzed reaction of resorcinol with formaldehyde. The catalyst concentration of this mixture dramatically affects the growth rate and size of the polymer "clusters" which crosslink to form gels. The catalyst effect manifests itself in the measurements of BET sur~face area, particle size, and density of the final aerogel. Although the RF aerogels appear similar to silica aerogels, a comparison of the mechanical properties of these materials suggests subtle morphological differences.

5 - ACKNOWLEDGMENTS

The authors would like to Yhank the following individuals for their contributions to this project : Walt Bell (TEM), Suzy Husley (BET), Jim Le May (Mechanical Properties), Ben Mendoza (Sample Preparation), Fred Priebe (Precision Machining), and Jim Yoshiyama (SEM).

This work was pqrformed under the auspices of the U.S. Department of Energy by the Lawrence Livermore National Laboratory under contract #W-7405-ENG-48.

50 100 250 50 106 250

Density (mglcc) Density (mglcc)

I I

- -

- 11 -

I I

200

g

5 i? 50

f g

S

- - REFERENCES

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