J. Pablo Tomba

Spectroscopic Slicing to Reveal Internal Redox Gradients in Electricity-Producing Biofilms†

Fil: Robuschi, Luciana. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Centro Cientifico Tecnologico Mar del Plata. Instituto de Investigacion en Ciencia y Tecnologia de Materiales (i); Argentina. Universidad Nacional de Mar del Plata. Facultad de Ingenieria; Argentina

Calculation of polymer blend compositions from Raman spectra: A new method based on parameter estimation techniques

A simple method to determine polymer blend compositions from their Raman spectra is presented. The method is based on expanding linearly the experimentally measured Raman spectrum of the blend, in terms of Raman spectra of pure components. A smooth function has also been included in the linear expansion to take into account the fluorescence interference, inherent to Raman spectroscopy. The coefficients of the linear expansion that give the best fit to the experimentally measured Raman spectrum of the blend are found by using a standard method of parameter estimation (Marquardt–Levenberg). These coefficients are directly related to the blend composition via a simple calibration procedure. Unlike standard methods of processing Raman spectra as deconvolution and curve-fitting—which use Gaussian and/or Lorentzian functions to approximate the spectrum bands—the proposed method does not require either baseline correction or previous knowledge of peak parameters. Also, this method requires less CPU time than deconvolution and curve-fitting procedures, and it is easy to automate. The proposed method has been applied to blends made out of two polymers: Polystyrene (PS) and poly(phenylene oxide) (PPO), to test its precision and consistency. Excellent agreement was found between calculated and expected blend compositions. Also, the reconstructed spectra agree very well with the experimentally measured blend spectra.

Limited-supply diffusion in the liquid polystyrene–glassy poly(phenylene oxide) pair. Further results in extended times scale

Diffusion between a liquid polystyrene and a glassy poly(phenylene oxide) matrix is experimentally studied over a wide range of temperatures and diffusion times, using confocal Raman microspectroscopy. A specially designed experimental setup allows precise direct following of time evolution of the chemical composition profiles along the diffusion path. A direct and precise quantification is made for the experimental errors involved in two methods used for Raman measurements. An already proposed diffusion model is used to predict the time evolution of the advancing composition profiles along the diffusion path, and gives precise results. Experimental thermodynamic and kinetic data taken from literature are used for the model calculations, and excellent agreement with experimental results is obtained. Diffusion slow down is confirmed at the lowest diffusion temperature used, and probable causes are discussed.

Calculation and experimental verification of concentration profiles for selected species at the interphase generated by diffusion in polymer pairs

A method for calculating diffusion rates for individual species in concentrated regime is outlined. The effects of monomeric friction coefficient, Flory-Huggins thermodynamic interaction parameter, individual species molecular weights, local molecular weights distribution, and local T g are precisely calculated. The method is used to calculate individual concentration profiles generated by diffusion of multicomponent polymer blends, and experimentally tested. Polystyrene with a bimodal molecular weight distribution is allowed to diffuse in a blend of polyphenylene oxide and polystyrene. Local physical properties change markedly along the interdiffusion path and, therefore, this is a demanding test for the proposed calculation method. The simulated concentration profiles are compared with results obtained by using two independent experimental techniques: Raman spectroscopy and dynamic mechanical analyzer (DMA). The total polystyrene (PS) concentration profiles, calculated using the proposed method, agree well with Raman spectroscopy results. Simulated DMA results-which are sensitive to the PS species molecular weight distribution-obtained using the concentration profiles, calculated for each PS molecular weight species agree well with the experimental DMA results. Calculations based on average molecular weights give incorrect results.

A generalized method to calculate diffusion rates in polydisperse systems. Further results on Rouse dynamics in the concentrated regime

Experiments designed to thoroughly test a recently proposed generalized method to calculate diffusion rates in polydisperse systems have been carried out. Polydisperse polystyrene (PS) samples were allowed to diffuse in a poly(phenylene oxide) (PPO) matrix. Designed blends were made from anionically polymerized PS with molecular weights which cover most of the ranges where Rouse dynamics control the diffusion processes. The diffusion temperatures range from (T g - 1 K) to (T g + 105 K), causing the monomeric friction factor values for PS to change by up to seven orders of magnitude along the diffusion coordinate. Calculations performed with the above mentioned method agree with Raman and DMA experimental data.

Finite element model of interdiffusion and reaction in polydisperse polymer blends

Numerical modelling of polymer blends production and processing, while technologically important, poses a considerable computational challenge when one or more of the polymer species is polydisperse. The mathematical formulation of such problems, where molecular weight distribution may range over several hundreds—which are very frequent in practice—leads to a huge set of coupled differential equations. In this work a numerical technique is presented that reduces the problem to a manageable number of degrees of freedom while keeping the essential physics of the original statement. For this purpose the distributions of molecular degrees of polymerization are approximated by continuous variables, and the set of concentration profiles replaced by a continuous field. The resulting equations are then reexpressed in weak form as a Galerkin integral identity and discretized using finite elements. Mass conservation and interchange chemical reactions as transesterification are treated using Lagrange multipliers. The accuracy of the technique—which can also be applied to the monodisperse case—is assessed through a number of simulations, and comparisons with experimental results published earlier. Copyright