Cercetarea in Catedra de Fizica a Universitatii Tehnice din Cluj-Napoca

Them:
CORRELATION OF THE MACROSCOPIC, MESOSCOPIC AND MICROSCOPIC PROPERTIES OF VARIOUS CLASSES OF NANOCOMPOSZITES POLYMERS AND POLYMER
MAMBRANES USED FOR FUEL CELLS

Project abstract:

Polymer membranes have been used for many years as proton conductors in fuel cells. Many structure-properties in the presence of nanoparticles are not well known and are of fundamental importance for improving they properties. The research project aims to establish the structure/properties relationship on the macroscopic, mesoscopic and microscopic scales for the nanocromosites polymers and proton conducting polymer membranes. Organo-clay, nanotubes, carbon black and silica nanofilers will be employed. New and standard multidimensional nmr method will be used to the structural and dynamical investigation that include: 1) NMR spectrscopy of multiple-quanta, 2) dipolar correlation fucntion under high-resolution conditions, 3) new dipolar fileters for spin-diffusion experiments, and 4) magnetization exchange under fast MAS. These nmr methods will be corrobaorated with mechanical measurements, dsc, wasx, tem, and afm techniques on: 1) nanocomposites elastomers, 2) semicristalline polymers with nanocomposites, 3) polymer memmbranes with proton conduction.

Research team:

Prof. Dr. Dan Demco - Project Manager
Ass. Prof. Dr. Radu Fechete
Asist. Drd. Ramona Chelcea
Res. Asist. Drd. Dumitrita Moldovan

Scientific objectives for the year 2007:

1.1 The preparation and characterization of different elastomers with various nano-composites filers.

1.2 The development of a theoretical model of dipolar correlation functions measured in high field NMR.

1.3 The correlation of NMR parameters with structural, mechanic and thermic properties of nano-composites elastomers.

All objectives and activities for the year 2007 were fulfilled completely.

Results: A new filter based on chemical shift was developed. This is associated with functional groups witch compose the monomer of polymeric networks. The optimized pulse sequences used under MAS is presented in figure a). The mixing time (t_m) is synchronization with rotor frequency and is introduced after implementation of chemical shift filter. During this time period a longitudinal magnetization transfer process between different functional groups take place. This transfer is investigated by the double-quantum (DQ) spectroscopy. One example of pulse sequence that using chemical shift filter is presented in figure b). DQ spectra are obtain using BABA (back to back) pulse sequences application, was presented in figure c).
The analytical expression of two-dimensional DQ spectra intensity can be obtained using the statistical density operator, r_DQ,spectrum, where r_CS,the initial density operator characterize the system state after application of chemical shift filter. The propagator is a product of several terms and describe an three-dimensional NMR experiment developed to measure the residual dipolar couplings between and among different functional groups.
Example of ¹H DQ two-dimensional spectra filtered by dipolar coherences registered under MAS at n = 5 kHz, for NR1. Some characteristic features can be highlighted: 1) in horizontal direction is represented the chemical shift, while on the vertical direction is represented the dipolar coupling, 2) the observed diagonal peaks corresponds to CH₂ and CH₃ functional groups. The peak corresponding to CH group is missing since it can not present DQ coherences, 3) the interactions between functional groups are represented by extra-diagonal peaks "cross-peaks" which as a feature of these types of experiments are found at the middle distance in the double quantum dimension.
Scheme of spin diffusion experiment with DQ filter. The first two pulses stimulate the double-quantum coherences which evolve for a short period of time, t_DQ. The following two pulses are designed to convert these coherences in longitudinal magnetizations, along the direction z. The spin diffusion takes place during the time interval, t_d. The NMR signal is acquired after the last pulse which brings the z magnetization in transverse plane. The ¹H NMR spectra recorded for different diffusion time t_d and edited by the spin diffusion are presented in the neighbouring figure for TPU (thermoplastic polyurethane).
The spin-diffusion equation in the cylindrical coordinate is given by: The probe magnetization is labeled by u and depends on the diffusion time and distance r from the centre of cylinder. D is the spin self-diffusion coefficient. The numerical solution of spin-diffusion equation implies a discretization procedure. Finally the Cranck and Nicholson algorithm was applied to obtain a stabile solution. The discrete spin-diffusion equation is given by:
The model shown in the neighbouring picture was used to determine the complex morphologies of Nylon 6 phases. The flow direction of magnetization is from crystalline domains towards amorphous less mobile domains. This process is considered to be three-dimensional. The distance between fibers was labeled with D. The amorphous less mobile domains separate the fibers from aggregates. It is assumed that the fibers axes are oriented roughly parallel to the main size of interface and amorphous regions. The fibers can be regarded as lamellar stack of amorphous less mobile domain and crystalline domains. They are very well organized along a distance considered the length of coherence, L_c.

Scientific objectives for the year 2009: 1.1. Determination of phase composition for a series of PE/C and iPP/PAOS samples with different concentrations. 1.2. Determination of changes in molecular dynamics in PE/C and iPP/PAOS samples. 1.3. Determination of dimensions of rigid, interface and amorphous regions in PE/C si iPP/PAOS samples. 2.1. Determination of proton diffusion coefficients and correlation with electrical conductibility. 2.2. Investigation of the hydration effect and the degree of sulphonation. All objectives and activities for the year 2009 were fulfilled completely. Results: Nanocomposites are a new class of composites, which are particle-filled polymers for which at least one dimension of the dispersed particles is in the nanometer range. The very large commercial importance of polyethylene (PE) and isotactic polypropylene (iPP) has also been driving an intense investigation of PE and iPP nanocomposites reinforced by particulates, fibers, and layered inorganic fillers (clay and hereafter PE/C and iPP/PAOS). Among all these nano-compsite precursors, those based on clay and layered silicates have been more widely investigated probably because the starting clay materials are easily available and because their intercalation chemistry has been studied for a long time. The combination of low levels of exfoliated natural and synthetic layered silicates with high aspect ratio in polyethylene matrix results in mechanical properties, flammability properties and barrier properties that are far superior to those of the base material.
Proton wide-line solid-state NMR is a valable tool for measuring the phase composition in semi-crystalline polymers like PE and iPP in the presence of organoclay or silica particles. An example of such investigation is shown in Fig. 1. It is evident that the presence of nanofilers in our case SiO2 particles will change the content of the rigid, interfacial, and amorphous fractions as compared to the non doped PE or iPP. The fraction of the rigid phase decreases in the presence of nanofilers due to the more restiction for making ordered chains. The interfacial and amorphous regions increases at the expence of rigid phase. Moreover, organo-clay as well as silica particles are expected to be localized more in the amophous and interfacial domains.
The presence of organocaly in PE nanocomposites will affect the mechanical and thermal properties. These changes are related to the degree of exfoliation of the organoclay in our case bentonite organo modified. Moreover, the degree of exfoliation could depend on the presence of the silica particles. The NMR measurements of the T1 parameter by saturation recovery was shown to be an indicative parameter for the degree of exfoliation. The T1 is strongly affected by the presence of Fe3+ paramagentic impurities in organoclay. Figure 2 shows that the degree of exfoliation exfoliation increase with the increase in the content of PAOS.
Proton spin-diffusion repesents a viable NMR method to measure the average domain sizes of the rigid, interface and mobile regions in a semicrystalline polymer. We use in this investigation a dipolar filter based on the double-qauntum coherences for the initial selection of the rigid domain. The evolution of the magnetization in the mobile domain can be measured from the decomposition of 1H wide-line in different components. Such an integral intensity of NMR signal as a function of square root of the diffusion time is shown in figure. The fit of the decay and buildup curves in Fig. 4 using the solution of the spin-diffusion equation leads us to the values of the rigid and amorphous domains. These values are shown in the table as a function of the content in PAOS, i.e., silica particle content. As expected the size of the amorphous domains increases due to more disorder induced in the semicrystalline polymer.
Poly(arylene ether ketone)s are high performance polymers characterized by excellent thermal, mechanical and chemical stability. These polymers are insoluble in most organic solvents. However, if their backbone is chemically modified as, for example, by ionic groups, their solubility is enhanced. For example, the functionalization of poly(ether ether ketone) (PEEK) by the substitution with sulfonic acid groups (sulfonation) paved the way for its use in advanced functional polymer membranes. Thanks to the hydrophilic character conferred by the sulfonic acid groups, sulfonated PEEK (SPEEK) has been used in blends with polysulfone to improve the anti-fouling properties of ultrafiltration/nanofiltration membranes.
The diffusion coefficients for the series SPEEK/SiO2 at T = 295 K and T = 353 K are shown in figure as a function of the PAOS content. For the samples SPEEK and SP-PE.50 where two diffusion coefficients were measured, the coefficient Dslow is reported. The presence of ultrasmall silica particles (10 nm or smaller) influences positively the water diffusion at room temperature. Second figure shows the conductivities of sample SPEEK, SPEEK.PAOS.10, SP-PE.20 and SP-PE.50 measured during the third heating cycle from 40 to 120 °C (90 % RH). For each sample the conductivity increases with temperature. However a drop in conductivity is observed from 80 to 100 °C for samples SP-PE.10 and SP-PE.50. This effect could be due either to some intrinsic phenomena occurring within these samples in the temperature range between 80 and 100 °C (for these samples similar sudden drops were observed also in the other two heating cycles)
The variations in the electrical conductivity of SPEEK after the first heating cycle (temperature range = 80-140 °C, RH = 75 %) and they proposed as possible explanation that, for a given relative humidity, the interaction forces between the polymeric chains weaken with increasing temperature, thus favoring growing hydration that is not lost during successive cycles, accounting for higher conductivity. In the higher temperature range (starting from 80 °C), the proton conductivity of the SPEEK-silica membranes at 90 % RH was basically independent from the silica content. The proton conductivities of the samples prepared from the lowest PAOS content (10 and 20 wt%) were more stable upon successive heating/cooling measuring cycles at 90 % RH, showing less dependency on membrane hydration than the pure SPEEK.

Results dissemination:

Participation to International Conferences
1. D. Moldovan, R. Fechete, D. E. Demco, E. Culea and B. Blümich, Monte - Carlo simulations of the two-dimensional NMR T₂-T₂ exchange of fluids in porous media, of the 9^th International Conference from Bologna (MRPM9 - Conference Magnetic Resonance in Porous Media), in the period of 13 - 17 july, 2008.
2. R. Fechete, D. Moldovan, D. E. Demco and B. Blümich, Laplace inversions applied to multi-component T₂ - T₂ exchange experiments, of the 9^th International Conference from Bologna (MRPM9 - Conference Magnetic Resonance in Porous Media), in the period of 13 - 17 july, 2008.
3. D. Moldovan, T2-T2 Exchange Processes in Ultra-Dynamical Systems, Two-Dimensional NMR Experiments and Monte Carlo Simulations, Oral presentation at 2^nd Transylvanian NMR Workshop in Cluj-Napoca September 18 -21, (2009)
4. Radu Fechete, Nano-structured Biomaterials: A NMR study of collagen and keratin, Oral presentation at 2^nd Transylvanian NMR Workshop in Cluj-Napoca September 18 -21, (2009)

Participation to the residential course

In the period of 1 - 24 September 2008, Prof. Dr. Dan Demco was invited professor to Ecole Normale Superior du Paris, Franta.
In the period 17.07.2009-31.07.2009 Conf. Dr. Radu Fechete and Research Asist. Moldovan Dumitrita have attended to a research stage at RWTH-Aachen University, Germany, at the invitation of Prof. Bernhard Blümich. The cooperation subject was related to the relaxation and spin-diffusion studies for the determination of phase composition, molecular dynamics and morphology for PE/C and iPP/PAOS nano-composite materials.

Publications
1. Maria Baias, Dan E. Demco, Irene Colicchio, Bernhard Blümich, and Martin Möller, Proton Exchange in Hybrid Sulfonated Poly(Ether Ether Ketone) - Silica Membranes by 1H Solid-State NMR, Chem. Phys. Lett. volume 456, Issues 4-6, 5 May 2008, Pages 227-230.
2. N. O. Goga, D. E. Demco, J. Kolz, R. Ferencz, A. Haber, F. Casanova, and B. Blümich, Surface UV aging of elastomers investigated with microscopic resolution by single-sided NMR, J. Magn. Reson. volume 192, Issue 1, May 2008, Pages 1-7.
3. R. I. Chelcea, R. Fechete, E. Culea, D. E. Demco and B. Blümich, Distributions of transverse relaxation times for soft-solids measured in strongly inhomogeneous magnetic fields, J. Magn. Reson., 196, 178-190 (2009).
4. Maria Baias, Dan E. Demco, Crisan Popescu, Radu Fechete, Claudiu Melian, Bernhard Blümich, and Martin Möller, Thermal Denaturation of Hydrated Wool Keratin by ¹H Solid-State NMR, J. Phys. Chem. B, 113, 2184-2192, (2009).
5. Irene Colicchio, Dan E. Demco, Maria Baias, Helmut Keul, Martin Möller, Influence of the silica content in SPEEK-silica membranes prepared from the sol-gel process of polyethoxysiloxane: Morphology and proton mobility, Journal of Membrane Science, 337 (1-2), 125-135 (2009).
6. Maria Baias, Dan E. Demco, Bernhard Blümich and Martin Möller, State of water in hybrid sulfonated poly(ether ether ketone) - silica membranes by ¹H solid-state NMR, Chem. Phys. Lett., 473 1-3, 142-145 (2009).
7. C. Melian, D.E. Demco, M. Istrate, A. Balaceanu, D. Moldovan, R. Fechete, C. Popescu, M. Möller, Morphology and side-chain dynamics in hydrated hard alpha-keratin fibers by 1H solid-state NMR, Chem. Phys. Letters, 480, 300 (2009)
8. D. Moldovan, R. Fechete, D. E. Demco, E. Culea and B. Blümich, Monte - Carlo simulations of the two-dimensional NMR T₂-T₂ exchange of fluids in porous media, Diffusion Fundamentals, 10 20.1 - 20.3 (2009).
9. R. Fechete, D. Moldovan, D. E. Demco and B. Blümich, Laplace inversions applied to multi-component T₂-T₂ exchange experiments, Diffusion Fundamentals, 10 14.1 - 14.3 (2009).
10. D. Moldovan, R. Fechete, D. E. Demco, E. Culea, B. Blümich, V. Herrmann, M. Heinz, Heterogeneity of nanofilled EPDM elastomers investigated by inverse Laplace transform ¹H NMR relaxometry and rheometry, Acceptat Macr. Chem. Phys. (2010).

Results: The elastomer samples with different types of fillers were prepared from natural rubber (NR), SMR 10 (Malaysia) and EPDM. The used filer types like: carbon black N121, Ecorax 1720, and N990 have different specific activities. The SiO₂ filers are represented by Ultrasil, Ultrasil+Si69 and Coupsil 8113. The last filer category is based on CaCO₃
The total spin Hamiltonianul under MAS which describe the ¹H-¹H interactions belonging to intra- and inter- functional groups must consider also the average over local motions of polymer chains Kuhn segments, the dipolar coupling modulated by the rotor coherent motion and by the incoherent motion due to the fluctuations of polymeric chains:
The variation of the c₁ quantity (which is also a measure of the ¹H residual dipolar interaction) as a function of the square of cross-link density, C².
The Laplace inversion of a measured Hahn echo decay from CPMG pulse sequence for EPDM elastomers with 70 phr concentration of CaCO₃ filers. The number of detected components can be easily correlated with the microscopic behaviour of polymeric chains.

PN II IDEI 1102/2007