#################################################### # Benjamin Anderson # requesting 4 days, at minimum=3 days # beamline 33ID, Microstructure of silica nanoparticles in low MW PEG # instrument 33ID-D USAXS #################################################### # top:/home/www/beamtime-requests/req00879.txt # UNICAT Member Beam Time Request #879 # created Wed Nov 23 11:53:52 CST 2005 #################################################### apsrun: 2006-01 beamline: 33ID collaboration: Yes contact: banders3@uiuc.edu days: 4 description: Filled polymers are an important class of materials where particulate matter is dispersed in a polymer matrix to improve strength and durability over the neat polymer. These improvements are understood to be linked to a change in polymer dynamics in the interphase region between the particle surface and the bulk polymer. Such materials find application in carbon filled rubber, nanocomposites, and coatings. The development of systematic studies aimed at understanding the microscopic physio-chemical effects that manifest themselves in macroscopic material properties has been difficult. Relaxation times are long for high molecular weight polymers making filled polymers intractable to an equilibrium suspension. It is, therefore, difficult to characterize these materials. The ability to obtain good particle dispersion is another hindrance. Particles tend to agglomerate which reduces surface area for particle/polymer interaction and lowers the ability of the filler to improve mixture mechanical properties. It is believed that there must be a preference for the polymer to wet the particle surface. In order to circumvent these issues, we propose investigating silica nanoparticle dispersions in low molecular weight solvents of increasing molecular weight where the solvent is known to adsorb to the particle surface. The phase behavior of particle/polymer mixtures will be governed by enthalpic and entropic contributions and could result in a variety of mixture phase behavior: homogeneous fluid, phase separation, or nonequilibrium gel. The competition between enthalpic and entropic contributions will drive the organization of the particles in the polymer matrix. Several types of particle organization have been proposed: entropic depletion flocculation, steric stabilization, local bridging flocculation, and tele-bridging flocculation. [1] Mixture phase behavior is believed to depend on physical properties such as particle size, particle loading, and the polymer molecular weight (MW) and on chemical properties such as strength and range of attraction between polymer segments and the particle surface. [1] We employ dispersions of monodisperse Stober silica nanoparticles in polyethylene glycol (PEG) 400-12000 MW as a model system to investigate particle/polymer mixtures. Low MW PEG has a melting pt range of -4 to 66 C for 400-12000 MW allowing investigation under melt conditions at moderate temperatures. PEG is also known to adsorb to bare silica which offers a specific polymer/particle attraction considered important for particles to form stable suspensions in a polymer melt. We have found 50 nm silica to form stable liquids in low MW PEG opening the possibility of probing equilibrium suspension properties. We have also found that particles in low MW PEG gel at specific elevated particle concentrations which are a function of PEG MW. We believe that this behavior is caused by polymer bridging flocculation. We are interested in understanding the microstructural changes in particle dispersion that lead up to this liquid/gel transition. Our experiments can be summarized as follows: 1) We want to measure interparticle structure factors, S(q), while varying particle size (50nm and 100nm), polymer MW and particle concentrations leading up to the gel transition. Experiments will require a q range of 0.005 to .5 nm-1, possible with USAXS. These data can be compared to mixture macroscopic properties such as viscosity and phase behavior. 2) We are interested in calculating second virial coefficients, B2, to quantify how the polymer matrix interacts with the particle surface and mediates particle/particle interactions. Experiments will gather scattering intensity of mixtures with particle volume fractions ranging from 0.02 to 0.10. B2 values are obtained from extrapolating scattering data to zero angle and require low q (0.005 nm-1) scattering measurements, possible with USAXS. Our data can be analyzed by comparing experimental structure factors and second virial coefficients with recent theory of Hooper and Schweizer at the University of Illinois who analytically predict the structure and phase behavior of filled polymer systems. Our comparison depends on scattering data obtained at low q. USAXS at APS offers an incident intensity and a q range of 0.005 to .5 nm-1 able to probe interparticle interaction and structure of our particles. [1] J.B. Hooper and K.S. Schweizer, Contact Aggregation, Bridging and Steric Stabilization in Dense Polymer-Particle Mixtures, Macromolecules 38, 8858-8869 (2005). equipment_required: NONE experiment: Microstructure of silica nanoparticles in low MW PEG foreign_nationals: NONE hazards: Samples will consist of low MW PEG and silica nanopartilces. Neither are considered hazardeous. instrument: 33ID-D USAXS instrument_other: minimumdays: 3 name: Benjamin Anderson new_request: ON nonmembers: NONE submit: Submit unacceptable_dates: NONE z34ID_details: #REMOTE_HOST: banders.scs.uiuc.edu #REMOTE_ADDR: 130.126.231.151 #CONTENT_LENGTH: 5241 #HTTP_REFERER: http://www.uni.aps.anl.gov/admin/unireq.html #HTTP_USER_AGENT: Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1; .NET CLR 1.1.4322)