The Green Group

Department of Chemical Engineering

Research

Here are descriptions of ongoing research projects in our lab: 

Sterically Stabilized Nanoparticles in Polymer Melts

In colloid science, a common technique to disperse particles in polymer melts and solutions is to chemically attach polymeric chains to the particle surfaces. At moderate graft densities, the grafted chains stretch and form a “brush” which is interpenetrated by the matrix polymer.  When this occurs, dispersion of nanoparticles is achieved as attractive forces between particles are screened due to the thermodynamic interactions of the grafted polymer and the matrix polymer.  The strength of attraction between nanoparticles may be fine-tuned by altering key parameters including graft density, grafted polymer chain length, and polymer matrix chain length.  In order create uniform dispersions that have optimally enhanced material properties, there is a need for an improved understanding of how surface-modified particles in a polymer melt interact and ultimately affect bulk properties.  Specifically, the following questions must be addressed:

· How does particle size affect a polymer brushes’ ability to interpenetrate (i.e. wet) the melt?

· How do grafting density and melt chain length affect dispersion?

· How does the strength of interparticle attraction relate to the bulk rheological properties of the system?

In order to create well-defined systems, monodisperse spherical silica particles are synthesized and the surfaces are modified by grafting polymers to the surface.  The particles are then suspended in polymer melts.  Rheological experiments yield information about the dynamic behavior of the bulk material providing insight into interparticle interactions.  

Compatibilized Immiscible Polymer Blends

The greatest potential for making new materials lies in combining existing components in new ways. This is the ultimate goal of the research on immiscible polymer blends. Immiscible polymers are made of chemically different monomers and display mixing properties similar to that of oil and water. To get these materials to form continuous systems, a third component called a compatibilizer must be added. This compatibilizer is a di-block copolymer which means that it is made of two distinct regions. Each region is chemically identical to one of the polymers present in the blend and has been bonded to the other through chemical reaction.  The copolymer orients itself at the interface of the two polymers, stabilizing the droplets that form as a result of shear flow. The interactions between the copolymer and both polymers at the surface of the droplets control the overall stability of the system. Without copolymer, the droplets would simply regroup over time. Thermodynamically stable dispersions of immiscible polymers will lead to materials with enhanced mechanical, thermal, and electrical properties depending on the components used in the blend.

 

 

The research done on this topic in the Green Lab is focused on the rheological and morphological response of these systems to steady shear flow conditions. Blends are studied for a model system over a range of molecular weights and copolymer composition to gain a fundamental understanding of how these systems function. Areas of interest include studying the effects of copolymer symmetry, matrix polymer molecular weight, and copolymer surface coverage.

Nanoparticle Stability in Polymer Solutions

Colloidal systems are routinely encountered in paints, inks, and milk products. Uniform distribution of colloidal particles in polymer solutions and melts is of great importance in biological, environmental, and industrial applications. One way to achieve homogeneity in these systems is by grafting polymers on the particles. The central theme of this research is to study the stability of colloidal particles in dilute, semidilute and concentrated polymer solutions and melts.

To conduct this study monodisperse silica particles are synthesized and polymer chains are grafted to their surface. Molecular weight of the graft polymer in varied while the graft density on the surface is kept constant, and these particles are then immersed in polymer solutions and melts. These solutions are then studied to check the stability/flocculation using static and dynamic light scattering and rheology. These techniques help to observe the ‘floc’ formation in unstable systems as a function of time. The cross over from stable to unstable suspensions is related to the graft and free polymer chain lengths and the concentration of the free polymer in the solution. This is directly related to the state of minimum free energy as a result of gain in conformational entropy of the polymer. The regions of particle stability and instability are corroborated with theoretical self-consistent field calculations.

Another part of this research is to study the effects of solvency on the stability of particles. This is done by varying the temperature of the solution from better-than-θ to worse-than-θ conditions.

Techniques such as scanning electron microscopy, light scattering, ultraviolet spectrophotometry, rheology, gel permeation chromatography, elemental analysis are used to conduct these studies.