Materials Research and Science Engineering Center - Renewal 02

Project: Research project

Project Details

Description

Statement of Work related to the IRG led by Heinrich at University of Chicago
It is well known that composite materials, such as reinforced concrete or carbon fiber polymer blends can have greatly enhanced mechanical properties relative to the individual material components [1]. For standard solids (those well-described by linear elasticity), Eshelby theory describes this relative strengthening as a function of the elastic properties of the inclusions [2]. However, the situation is more complex in materials with soft interactions (comparable to kT), such as gels. Liquid inclusions in a stiff gel cause it to become more compliant (as expected), but these same inclusions in a soft gel lead to the opposite behavior: stain stiffening [3]. Thus far, most studies of soft composite materials have focused on materials in which the inclusions are locked into place either through physical or chemical means. Here we would like to explore a new kind of material in which the inclusions are free to rearrange; our goal is to use material training to guide these rearrangements to shape composite elastic properties.

To study the behavior of mobile inclusions we will fabricate gel samples embedded with colloidal particles, as shown in Figure 1. Co-PI Driscoll's lab has extensive experience working with a variety of hydrogels, as well as expertise in colloidal synthesisWe will use agarose hydrogels for this study, as they offer two key advantages: (1) this is a well-characterized system due to its extensive use in gel electrophoresis; and (2) gel mesh size can be varied across a wide range (20 nm - 1000 nm) by adjusting agarose weight fraction [4,5]. Before gelation, a small volume fraction (0.1-1%) of silica particles will introduced into the aqueous agarose solution; these particles will thus be uniformly dispersed in the polymerized gel (Fig. 1b). The Driscoll lab has the facilities and technical expertise to manufacture silica particles in a variety of sizes, from 300 nm - 1000 nm: monodisperse silica spheres (Fig. 1a) are created using the standard Stöber technique [6]; their size is increased via repeated additions of TEOS (tetra-ethyl orthosilicate), APS ((3-amino-propyl)triethosilane), and ammonia [7] (Fig. 1a).. Thus, the Driscoll lab can fabricate samples in which we can adjust the size of the embedded inclusions and the gel mesh size independently.

Once fabricated, the composite samples will be clamped and subjected to various training protocols, defined as cycles of stretching, compression, or shear. The cyclic stress will enable migration of the colloidal inclusions within the gel, subject to steric hindrance by the gel's polymer network. As we can easily tune both particle size and network mesh size, we can study how these parameters control migration, and how this migration can most effectively occur in response to stress. Polymer networks in a gel are highly heterogenous [8], and thus there will be preferentially locations in which particles will accumulate; they will migrate through larger pores until becoming sterically hindered at small pores, see schematic in Figure 1c. Concentrations of particles will function as effectively larger inclusions, and thus can highly modify bulk material properties [9]. Our overarching goal is to identify and understand how the stress protocol we apply is connected to the resulting changes in material properties.

This system presents additional opportunities to study how particle-gel attractions can modify training and response. The OH groups present on the silica particles are w
StatusActive
Effective start/end date9/1/208/31/26

Funding

  • University of Chicago (AWD101244 (SUB00000356)//2011854)
  • National Science Foundation (AWD101244 (SUB00000356)//2011854)

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