Characterization of silica–polymer aerogel composites by small-angle neutron scattering and transmission electron microscopy

Journal of Non-Crystalline Solids 288 (2001) 184±190

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Characterization of silica±polymer aerogel composites by small-angle neutron scattering and transmission electron microscopy Xiangjun Hu a, Kenneth Littrel b, Shuang Ji a, D.G. Pickles c, W.M. Risen Jr a,* b

a Department of Chemistry, Brown University, Providence, RI 02912, USA Intense Pulsed Neutron Source Division, Argonne National Laboratory, Argonne, Illinois 60439, USA c Sullivan Park Research Center, Corning Incorporated, Corning, NY 14831, USA

Received 10 January 2001

Abstract In this work a new type of silica±biopolymer aerogel composite containing chitosan has been studied with smallangle neutron scattering (SANS) and transmission electron microscopy (TEM). A new small-particle mass-fractal model scattering function, derived from the Teixeira mass-fractal scattering function, was used to ®t the SANS data. As the content of chitosan was increased, the fractal dimension was decreased from 2.9 to 2.5, and the cut-o€-length increased from 3.2 to 6.0 nm. This indicates that chitosan helps form a more open aerogel structure. It supports a structural model in which there are primary particles that connect with each other closely to form clusters, and these clusters serve as a secondary structural unit to form the chitosan-reinforced aerogel network. It also indicates that chitosan reinforces the inter-particle connections. This picture is consistent with the TEM measurements, which reveal that there are 1.5 nm wide entities with long dimensions in the 3±10 nm range. Ó 2001 Elsevier Science B.V. All rights reserved.

1. Introduction Aerogels are highly porous solid materials with unusually low densities and high speci®c surface areas [1,2]. They usually are prepared by the supercritical drying of highly cross-linked inorganic or organic gels. The most commonly studied aerogels contain only silica, but studies also have been carried out on silica-based gels that contain organic compounds [3±5]. To incorporate organic

*

Corresponding author. E-mail address: [email protected] (W.M. Risen Jr).

modi®ers e€ectively, the silica component of the gel must be prepared under carefully controlled conditions so that it leads to microstructures that can interact with these modi®ers. The microstructure of the silica component of the aerogel and the sol±gel from which it is made is sensitive to preparation conditions, such as the pH, and the ratio of the silica precursor to the solvent in the sol±gel processing, and other factors. The interaction between the silica network and its modi®ers also may a€ect the resulting aerogel structure [3]. In this work, we employed small-angle neutron scattering (SANS) and transmission electronic microscopy (TEM) techniques to study the microstructure of a new type of biopolymer-modi®ed

0022-3093/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 0 1 ) 0 0 6 2 5 - 1

X. Hu et al. / Journal of Non-Crystalline Solids 288 (2001) 184±190

silica aerogels. These transparent, monolithic aerogels contain the biopolymer chitosan, which is introduced through the formation and subsequent supercritical drying of a homogeneous silica± chitosan gel. Chitosan is the deacylated form of chitin. It is a polysaccharide with amine groups on its polymer chains. The chitosan was incorporated into silica aerogels to introduce amine groups that can act as ligands for metal ions. There are also hydroxyl groups and C±O±C groups on the chitosan chain, and they too can interact with the metal ions and the silica network. The structures of silica aerogels often are characterized as being three-dimensional networks composed of silica particles [2,3]. In this view, the particles are viewed as being connected to each other to form `strings of pearls'-like structures. These `strings', in turn, are connected with each other to form porous three-dimensional structures. The particle size and the pore size depend on the preparation conditions [6]. When the initial sol±gel transformation is carried out under basic conditions, relatively large particles (10±100 nm) form. However, under acidic conditions smaller particles tend to form (<10 nm). In this structural model, the silica particles are the primary and, indeed, the only structural unit. Whether there are secondary or higher order structural units in the model is not clear. In SANS studies, the structure of the network often is represented by the mass-fractal model [7± 9], in which the mass of the particle clusters is related to their linear dimension by Eq. (1) M…r† / rDF :

…1†

Here, DF is referred to as the fractal dimension. For three-dimensional clusters with no density variation, DF is 3. For a mass-fractal cluster, DF is less than three. The magnitude of DF is related to the openness of the structure. The more open the structure becomes, the smaller DF becomes. The mass-fractal structure can only exist over a certain length scale. Above this scale the density variation can no longer be observed. This length is usually called the cut-o€-length, f. For aerogels, f is in the nanometer to micron range.

185

SANS has been applied widely to the study of microstructure of fractal systems. As pointed out by Teixeira [7], SANS is one of the most appropriate ways to determine the fractal dimension and the cut-o€-length. The scattering in aerogel systems is described by the overall equation I…q† ˆ UV 2 …qS

2

q0 † P …q†S…q†:

…2†

In Eq. (2), I…q† is the neutron scattering intensity as a function of q, the neutron scattering vector. This intensity can be measured through SANS experiments. The parameters U and V are the number density and volume of the scatters, and qS and q0 are the scattering length density of the particle and the embedding medium, respectively. The form factor of the particle is P …q†. It describes the structure of the particles and satis®es the requirement P …q ˆ 0† ˆ 1. The structure factor is S…q†. It describes the interaction between the particles and therefore contains information about the organization of the particles. For silica aerogels, it is assumed that the particles are spherical and that the form factor of spherical particles can expressed as 3 2

P …q† ˆ ‰3‰sin…qr†

qr cos…qr†Š=…qr† Š :

…3†

The structure factor for silica aerogels can take a mass-fractal form expressed as [7,8] S…Q† ˆ 1 ‡

DF C…DF

1† sin‰…DF 1† arctan…Qn†Š :  …DF 1†=2 D …QR† F Q21n2 ‡ 1 …4†

In this study, the SANS analysis was based generally on the Teixeira mass-fractal model described above. However, in this study preliminary investigation [10] showed that it is necessary to introduce a modi®ed scattering function in order to make the ®tting more physically reasonable. The structural information obtained from the SANS study was analyzed in light of the images obtained from TEM measurements. The combined SANS and TEM results have provided a useful understanding of the microstructure of this material. This appears to be the ®rst study on polymer-

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containing silica aerogels combining both techniques. 2. Experimental 2.1. Preparation of chitosan-containing silica aerogel The preparation of the chitosan-containing silica aerogels has been described in detail elsewhere [11]. Brie¯y, a silica aerogel containing 10 wt% chitosan was prepared through the following procedure. First, 12.5 ml of tetraethoxy silane (TEOS) and 40.0 ml of aqueous chitosan solution, which contained 1.0 wt% chitosan and 1.0 wt% acetic acid, were combined with 0.9 ml glacial acetic acid and mixed at 20°C. The mixture was stirred for 7 h and over the period a clear sol was formed. The sol gelled at about 12 h from the beginning of the reaction. The wet gel was formed into ®lms of ca. 0.5±1 mm thick by pouring it into polystyrene molds and then aging it for 10 h. Then each gel ®lm sample was soaked in absolute ethanol containing 0.6 wt% concentrated NH4 OH solution to neutralize the acetic acid in the gel. Additional absolute ethanol was used to accomplish complete exchange of the water in the gel by ethanol. Then the ethanol was extracted with supercritical CO2 . The typical resulting aerogel is a monolithic, transparent material about 40  40  1 mm3 in size. It has a density of 3 0:25 g=cm and a BET surface area of 6  102 m2 =g. Silica±chitosan aerogels also were prepared at 2.5%, 5%, 15%, and 20% chitosan content. Silica aerogels were made under exactly the same conditions but without chitosan in the aqueous acetic acid solution. These correspond to `0%' chitosan, or simply, silica aerogels.

64  64 spatial channels. This instrument provides data in a q range 0.0005±0.035 nm 1 in a single measurement. The aerogel ®lms were cut into pieces and placed in between two quartz windows for the measurements. The scattering from the quartz windows was used for background correction for the samples. 2.3. TEM measurements TEM measurements were performed at Corning. TEM sample preparation of the aerogel samples consisted of grinding the sample into a powder. The powder was then placed into a test tube with ethanol, the test tube was then further dispersed ultrasonically. Following the sonication the solution was atomized onto a coated carbon grid. TEM examination was completed using a JEOL 2000FX, operating at 200 kV, with 0.28 nm resolution. All images were captured as bright ®eld images. 3. Results and discussion The SANS experiments were performed on a series of silica±chitosan aerogels that contain 0%, 2.5%, 5%, 10%, 15%, and 20% (w/w) chitosan, respectively. The log±log plots of the measured neutron scattering intensity I…q† versus q of these measurements are shown in Fig. 1. These scatter-

2.2. SANS experiments SANS experiments were performed at the Intense Pulsed Neutron Source at Argonne National Laboratory. The instrument uses neutrons from a solid CH4 moderator at 24 K with a wavelength from 0.05 to 1.4 nm. The scattered neutrons are detected by a 20  20 cm2 3 He area detector with

Fig. 1. SANS scattered neutron intensity, I…Q† …cm 1 † versus  1 †, for silica aerogels containing 0, neutron wave vector, Q …A 2.5, 5, 10, 15, 20% (m/m) chitosan.

X. Hu et al. / Journal of Non-Crystalline Solids 288 (2001) 184±190

Fig. 2. SANS data for chitosan±silica aerogels with 0% and 10% chitosan and their ®ts with Eq. (5).

ing curves all show a power-law correlation in the higher q range, which is typical for fractal systems. On the low q end of the curves, the scattering curves show a consistent variation over the chitosan concentration. This is indicative of systematic e€ects of chitosan on the structure of the aerogel. The SANS data were ®tted ®rst with the Teixeira mass-fractal model [7] using the scattering function of Eq. (5). The ®tting of two scattering curves measured on 0% and 10% chitosan aerogels is shown in Fig. 2. I…Q; n; DF ; R† 0

1

 PSphere …Q; R† ‡ IBackground :

is the radius of the fundamental spherical particles. The parameters calculated from the best ®t of the data to Eq. (5) are listed in Table 1. Although the quality of this ®tting is satisfactory, and the parameters obtained are reasonable, there is a fundamental problem with this ®tting. Namely, the I0S obtained from this equation is fairly small and it is indistinguishable from the background intensity IBackground . That is consistent with the assumption, based on TEM data (vide infra), that the particles are very small and have a very low number density because of the low density of the aerogel. Those should give rise to a low I0S . However, having an I0S that is indistinguishable from the background intensity IBackground makes its meaning problematic. To overcome this problem, a new scattering function has been derived from the above equation. This model, which will be referred as the small-particle mass-fractal model, is a valid approximation for conditions in which the scattering from the form factor for the fundamental sphere is not visible or, alternately, whenever n  R and Qmax R  1. Here Qmax is the value of Q at which the power-law scattering fades into the background scattering. This small-particle mass-fractal scattering function is given by Eq. (6) I…Q; n; DF † ˆ I0A

B sin‰…DF 1† arctan…Qn†Š C C ˆ I0S B @1 ‡  DF2 1 A DF 1 …QR† ‡1 Q2 n2

187

sin‰…DF …DF

1†Qn…Q2 n2 ‡ 1†…DF

…5†

31:7  0:3 50:1  0:7

…6†

where I0A is related to I0S by the expression I0A ˆ I0S …DF

1†C…DF

 DF n 1† : R

…7†

Fig. 3 shows the ®tting of the scattering curves of the aerogel samples with di€erent chitosan contents using Eq. (6). The parameters obtained by this ®tting are listed in Table 2. All the ®ttings are

Table 1 Parameters for silica±10% chitosan and silica aerogels obtained by ®tting SANS data with Eq. (5)   Sample DF f (A) R (A) I0S 2:88  0:01 2:49  0:01

1†=2

‡ IBackground ;

In this function, the factor UV 2 …qS q0 †2 in Eq. (2) is substituted with I0S . It represents the overall intensity scattered by the fundamental sphere extrapolated to q ˆ 0. The term IBackground was added to o€set the background intensity of the instrument. Recall that DF and f are fractal dimension and cut-o€-length, respectively. In the equation, R

0% Chitosan±silica 10% Chitosan±silica

1† arctan…Qn†Š

7:1  0:7 3:3  5:1

0:04  0:01 0:04  0:14

IBackground 0.010.01 )0.100.01

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X. Hu et al. / Journal of Non-Crystalline Solids 288 (2001) 184±190

Fig. 3. SANS data for chitosan±silica aerogels and their ®ts with Eq. (6). (a) 0% Chitosan±100% silica; (b) 2.5% chitosan±97.5% silica; (c) 5% chitosan±95% silica; (d) 10% chitosan±90% silica; (e) 15% chitosan±85% silica, with % by mass.

of good quality, as shown by the reduced chisquared. The fractal dimensions fall in the range between 2.95 and 2.40. The cut-o€-length is between 3.2 and 6.0 nm. The clear general trends are that the fractal dimension decreases and the cut-o€-length increases as the chitosan content increases. This indicates that the introduction of chitosan helps to

Table 2 Parameters for silica±x% chitosan and silica aerogels obtained by ®tting SANS data with Eq. (6)  f (A) Sample DF 0% Chitosan±100% silica 2.5% Chitosan±97.5% silica 5% Chitosan±95% silica 10% Chitosan±90% silica 15% Chitosan±85% silica

2:86  0:01 2:93  0:01 2:69  0:01 2:49  0:01 2:47  0:01

31:5  0:3 32:2  0:3 37:6  0:3 50:1  0:6 58:6  0:6

X. Hu et al. / Journal of Non-Crystalline Solids 288 (2001) 184±190

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Fig. 4. TEM images of chitosan±silica aerogels with (a) 0% chitosan and (b) 10% (m/m) chitosan.

form a more open aerogel structure and increase the inter-particle correlation. There are experimental variations in the sol±gel processing, supercritical extraction processes, and perhaps in the long-time relaxation phenomena that can lead to minor variations in structure among samples. For example, the SANS data on several 10% chitosan silica samples have been ®tted with DF of 2.49 to 2.52 and f of 5:38  0:36 nm. As mentioned earlier the structure of aerogels often is represented as a three-dimensional network of strings of silica particles. In this model it is clear that silica particles are the primary structure unit. While it is not clear whether there are any secondary or higher structure units, the cut-o€length is better explained with the existence of a secondary structure unit. This unit is a cluster of strings of particles. It has a mass-fractal structure and its size corresponds to the cut-o€-length. These clusters connect with each other to form the ultimate aerogel network. With this picture in mind it is easy to understand the e€ect of chitosan on the aerogel structure. At the beginning of the

sol±gel processing, silica particles of small sizes are formed because the reaction is carried out under acidic conditions. Then these particles aggregate to form the clusters. When chitosan is present in the solution, it can interact with the particles through a variety of interactions, including Si±O±C bond formation and several types of hydrogen bonding. The e€ect is to hold the particles relatively close to each other so that the particle clusters form readily. When the concentration of chitosan increases, the clusters grow bigger and faster, resulting in a larger cut-o€-length and lower fractal dimension. This structural role of chitosan helps to explain why the gelation reaction is much faster when chitosan is present. Another question is how silica clusters connect directly to each other. One possibility is that they are in direct contact and are connected through a number of ±Si±O±Si± bonds. However, Ruben et al. [12] reported another type of connection in silica aerogel. With high resolution TEM they observed linear- or ladder-like polymeric silica chains between silica particles. Therefore, the

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connection between the clusters also can be of this type. For example, it takes about 120 h for the sol to gel when there is no chitosan in the solution, but it only takes about 15 h under the same conditions when there is 10% chitosan in the solution. Presumably both chitosan chains and silica chains can serve as the polymeric connections between the particles and clusters. Fig. 4 shows the TEM pictures of silica aerogel and 10% chitosan silica aerogel. It can be seen that the network is built up with `chains' with breadth of about 1±2 nm, averaging about 1.5 nm. On close examination there also seem to be cluster-like entities that are of somewhat larger scale order and are joined together with relatively loose connections. Although it is dicult to determine their actual size, it can be estimated that they are on the order of 10±20 nm. All these observations are consistent with the particle±cluster picture of the aerogel microstructure. 4. Conclusion The small-particle mass-fractal model derived from the general mass-fractal model of Teixeira has been applied to analyze the SANS data of silica aerogels containing the biopolymer chitosan. The cut-o€-length and fractal dimensions determined from this analysis indicates that chitosan helps to form a more open structure of the aerogel. Chitosan chain can interact with the primary silica particles and help them aggregate into clusters. This conclusion is in agreement with the TEM

measurements. It helps to explain the formation of monolithic transparent silica±chitosan aerogels.

Acknowledgements We are grateful for the cooperation and assistance of Dr Arthur Yang of the Industrial Science and Technology Network. We thank Dr Pappanan Thiyagarajan and Mr Denis Wozniak of IPNS, Argonne National Laboratory for their help in the SANS experiments and data analysis.

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