High Resolution Tem of Organic Aerogels and Inorganic Aerogels
- PDF / 6,273,972 Bytes
- 6 Pages / 420.48 x 639 pts Page_size
- 34 Downloads / 226 Views
HIGH RESOLUTION TEM OF ORGANIC AEROGELS AND INORGANIC AEROGELS GEORGE C. RUBEN AND *RICHARD W. PEKALA Department of Biological Sciences, Dartmouth College, Hanover, NH 03755 *Chemistry & Materials Science Department, Lawrence Livermore National Laboratory, Livermore, CA 94550 ABSTRACT A base catalyzed silica aerogel and three base catalyzed resorcinol-formaldehyde (RF) aerogels were imaged with a new vertical platinum-carbon (Pt-C) replication technique. In deep gels single molecular chains can be visualized with transmission electron microscopy (TEM) with 9-10 A thick Pt-C replicas backed with 110-130 A of carbon [2]. The three RF aerogels produced with [Resorcinol]/[Catalyst] ratios (R/C) of 50, 200, and 300 had densities of 0.103, 0.063, and 0.065 grm/cc and had surface areas of 905, 580, and 390 m2/gm, respectively. Thegel connectors and junctions in RF 300 and RF 200 were composed of 210±30 Aand 140±10 Abeads which were joined by polymeric chains covering an area less than half the bead's diameter and within a bead diameter's distance. The gel connectors and junctions in RF 50 were frequently composed of beads of about 120±10 A joined over an area larger than half the bead's diameter. Similar to the silica aerogel (0.065 gm/cc, 113±10 A beads), the beads were frequently connected over several bead diameter's distance from each other. INTRODUCTION The ultrastructure of resorcinol-formaldehyde (RF) and silica aerogels largely depends upon the catalyst conditions used in their respective sol-gel polymerizations. Silica gels are usually produced from the hydrolysis and condensation of tetramethylorthosilicate (TMOS) or tetraethylorthosilicate (TEOS) in the presence of an acid or base catalyst. Under acidic conditions, linear or slightly branched siloxane chains are formed which entangle and then crosslink to form a gel. Under alkaline conditions, highly branched "clusters" are formed. These "clusters" eventually crosslink through their surface silanol groups (Si-OH) to produce a gel. Each of the above reaction pathways has been mapped onto a simple physical growth model. Under acid catalyzed conditions, reaction limited cluster-cluster growth is favored and leads to aerogels that are mass fractals. In contrast, monomercluster growth is promoted under alkaline conditions, leading to aerogels of compact interconnected particles. Organic aerogels formed from the polycondensation of resorcinol with formaldehyde can only be produced under base catalyzed conditions. The [Resorcinol]/[Catalyst] ratio (R/C) is the major factor that controls the size of RF "clusters" generated in solution. These "clusters" contain surface hydroxymethyl groups (-CH 2 OH) that condense to form a gel. [Resorcinol]/[Catalyst] ratios of 50300 provide an acceptable range in which transparent gels can be produced. The particle size, surface areas, cell size, density, and mechanical properties of the resultant RF aerogels depend upon the RIC ratio [1]. Organic aerogels are generally composed of interconnected spherical particles (30-200 A diameter
Data Loading...
