Frank Laboratory of Neutron Physics

Joint Laboratory Seminar

Leader – V.N. Shvetsov

Date and Time: Tuesday, 7 November 2017, at 11:00 AM

Venue: Conference hall (3rd floor), Frank Laboratory of Neutron Physics

1. Seminar topic: «Study of molecular dynamics in heteronuclear systems by solid state NMR off-resonance»

   Speaker: Aneta Woźniak-Braszak (Adam Mickiewicz University, Poznan, Poland)

   Abstract:
   
   This work presents the study of molecular dynamics in the heteronuclear systems in the rotating frame. The solid-state NMR off-resonance experiments were carried out in order to study cross-relaxation effects in a system containing proton and fluorine nuclei. Both 1H and 19F nuclei are suitable for the investigation of cross-relaxation effects due to the similar high gyromagnetic ratio and a natural abundance of 100%. However, the relevant resonance frequencies of proton and fluorine are close, therefore a specially designed probe is required. The solid state NMR experiments were carried out on a homemade pulse spectrometer operating at the frequency of 30.2 MHz and 28.411 MHz for protons and fluorines, respectively. This spectrometer includes a specially designed probe which contains two independently tuned and electrically isolated coils installed in the coaxial position on the dewar. A unique probe design allows working at three slightly differing frequencies off and on resonance for protons and at the frequency of 28.411 MHz for fluorine nuclei with complete absence of their electrical interference. The probe allows simultaneously creating rf magnetic fields at off – resonance frequencies within the range of 30.2 ÷ 30.6 MHz and at the frequency of 28.411 MHz. Contrary to a large number of previous cross-relaxation studies [5, 6], in our experiments proton spins can be polarized in the magnetic field B0 or excited by rf pulses, while fluorine spins are continuously saturated for a long time. The saturation of fluorines is maintained throughout the whole duration of the experiment. It leads to a simplification of the mathematical analysis of the experimental results. Presented heteronuclear cross-relaxation off – resonance experiments in the rotating frame provide information about molecular dynamics. By studying the proton longitudinal polarizations without the saturation of the fluorine spins and with the saturation the nuclear Overhauser effect (NOE) and the correlation time of the molecular motion were obtained.
   
   

2. Seminar topic: «Multistage networking in Monte Carlo simulated block copolymers»

   Speaker: Sebastian Woloszczuk (Adam Mickiewicz University, Poznan, Poland)

   Abstract:
   
   While linear A-B-C triblock copolymers are known to form a plethora of dual-mode (i.e., order-on-order) nanostructures, bicomponent A-B-A triblock copolymers normally self-assemble into single morphologies at thermodynamic incompatibility levels up to the strong-segregation regime.
   
   In this study, we use a lattice Monte Carlo method known as the cooperative motion algorithm (CMA) to simulate molecularly asymmetric A1-B-A2 copolymers possessing chemically identical endblocks differing significantly in length (A1 long, A2 sort). These copolymers are simulated in a molten state, dissolved in a selective solvent at the different concentrations, and blended with a selective homopolymer of a different lengths and copolymer concentrations.
   
   Among classical order-disorder transition we observe (within the ordered phase) twostage networking process. First stage involves building bridges between neighboring domains, while the second one uses the phenomenon of localization of short A2 blocks (we call them dangles) within B-domain. This phenomenon has an entropic origin. Initially disordered dangles inside B-domain start to aggregate into the interstitial micelles (IM’s) as the temperature is decreased. In the limit of superstrong segregation in the lamellar nanostructures, interstitial micelles composed of the minority A2 endblock arrange into two-dimensional hexagonal arrays along the midplane of B-rich lamelae. When dealing with other structures (e.g cylinders, bicontinous morphologies etc.) interstitial micelles are observed in a characteristic for a given morphology order or without specific arrangement.
   
   Simulations performed here establish the coupled molecular-asymmetry and incompatibility conditions under which such micelles form, as well as the temperature dependence of their aggregation number. Beyond an optimal length of the A2 endblock, the propensity for interstitial micelles to develop decreases, and the likelihood for colocation of both endblocks in the A1-rich lamellae increases.
   
   These results can have many both practical and theoretical applications potentially.