Reports: G7
47771-G7 Tracer Diffusion in Polymer Gels: Quenched Randomness and Thermal Fluctuations
The electromagnetic interference (EMI) remains a technical challenge for many electrical and electronic devices to function properly. EMI tends to degrade the interception of signals and adversely affect the equipment performance. Hence it is important to shield electrical and electronic devices over the EMI frequency spectrum to ensure the proper function. Metal sheeting was the conventional choice for this purpose; but seams commonly encountered in metal sheeting can cause radiation leakage and diminish the effectiveness of shielding. Polymer composites containing conductive fillers are attractive alternatives as EMI shielding material because seams are usually significantly reduced or completely eliminated. In addition, polymers possess general advantages of easy processing, low cost, lightweight, design flexibility, corrosion resistance and ease of mass production comparing to metals. We studied the elextromagnetic properties of single-walled carbon nanotube (SWNT)/poly(methyl methacrylate) (PMMA) composites prepared using coagulation method. We investigated the electrical conductivity as well as the electromagnetic interference (EMI) shielding of SWNT/PMMA composites over the X-band (8-12 GHz) and the microwave (200-2000 MHz) frequency range. The electrical conductivity of composites increases with SWNT loading by 11 orders of magnitude, from 10-15 to 10-4 mho/cm with a percolation threshold of ca. 3 wt% SWNTs. We also studied the effect of the sample thickness on the shielding effectiveness, which was correlated to the electrical conductivity of composites. The data suggest that SWNT/PMMA composites containing higher SWNT loading (above 10 wt%) be used for EMI shielding and those with lower SWNT loading be used for electrostatic charge dissipation. The composites studied here should find use in various electrical applications where EMI shielding and mechanical properties are desired criteria. The EMI SE of the composite with and above 15 wt% SWNTs is ~34-36 dB in GHz frequency range, showing promise for its commercial use as an EMI shielding material. We also analyzed the shielding mechanism of SWNT/PMMA composites. The interplay between percolation and the shielding is an interesting theoretical issue we will continue to study. This work was published in Polymer Engineering and Science 49, 1627-1634 (2009).
Crystallization of long-chain molecules has been extensively studied since 1930's. But still debate is going on the initial stage of polymer crystallization. The polymer crystallization from solution is very important, because the formation of small crystals in early stage play a decisive role in determining the properties of the solid in its final state and to predict and control the size, morphology and structures. We studied the crystallization of polyethylene (PE) in deuterated toluene solution using time-resolved ultra small angle neutron scattering (TR-USANS). We found that the average lateral dimension of crystallites grows linearly with time, while the degree of crystallinity increases as square of time. The results suggest that the number density of nuclei remains constant during the crystal growth. This is the first time that TR-USANS is used to investigate the early stage of the polymer crystallization. Interesting and important issues of data analysis arise. To obtain time-dependent structural information, we tried 1) de-smeared scattering spectra fitted using Guinier's law; and 2) using regression approach of the least square to find the best Guinier model fitting of the smeared data.. We found that method 2 to have better performance. This work is submitted to Polymer (2009).
Diffusion of a tagged particle in polymeric gels is a problem of both fundamental interest and practical significance. We are developing a molecular dynamics simulation that includes both quenched randomness and thermal fluctuations. Our model system is consisted of a fixed number of polymer chains, some of which are cross-linked, and a tracer particle. The solvent particles are not explicitly included in the simulations. Their effects are approximated at the level of Rouse model, namely producing friction and a random force to the tracer particle and the polymer chains. This work is on going.
