Rheological study of structural transitions in triblock copolymers in a liquid crystal solvent

Rheological properties of triblock copolymers dissolved in a nematic liquid crystal (LC) solvent demonstrate that their microphase separated structure is heavily influenced by changes in LC order. Nematic gels were created by swelling a well-defined, high molecular weight ABA block copolymer with the small-molecule nematic LC solvent 4-pentyl-4-cyanobiphenyl (5CB). The B midblock is a side-group liquid crystal polymer (SGLCP) designed to be soluble in 5CB and the A endblocks are polystyrene, which is LC-phobic and microphase separates to produce a physically cross-linked, thermoreversible, macroscopic polymer network. At sufficiently low polymer concentration a plateau modulus in the nematic phase, characteristic of a gel, abruptly transitions to terminal behavior when the gel is heated into its isotropic phase. In more concentrated gels, endblock aggregates persist into the isotopic phase. Dramatic changes in network structure are observed over small temperature windows (as little as 1 °C) due to tccche rapidly changing LC order near the isotropization point. The discontinuous change in solvent quality produces an abrupt change in viscoelastic properties for three polymers having different pendant mesogenic groups and matched block lengths

Director dynamics in liquid-crystal physical gels

Nematic liquid-crystal (LC) elastomers and gels have a rubbery polymer network coupled to the nematic director. While LC elastomers show a single, non-hydrodynamic relaxation mode, dynamic light-scattering studies of self-assembled liquid-crystal gels reveal orientational fluctuations that relax over a broad time scale. At short times, the relaxation dynamics exhibit hydrodynamic behavior. In contrast, the relaxation dynamics at long times are non-hydrodynamic, highly anisotropic, and increase in amplitude at small scattering angles. We argue that the slower dynamics arise from coupling between the director and the physically associated network, which prevents director orientational fluctuations from decaying completely at short times. At long enough times the network restructures, allowing the orientational fluctuations to fully decay. Director dynamics in the self-assembled gels are thus quite distinct from those observed in LC elastomers in two respects: they display soft orientational fluctuations at short times, and they exhibit at least two qualitatively distinct relaxation processes

Self-Assembly of Coil/Liquid-Crystalline Diblock Copolymers in a Liquid Crystal Solvent

Diblock copolymers having a random-coil polymer block (polystyrene, PS) connected to a side-group liquid crystal polymer (SGLCP) self-assemble in a nematic liquid crystal (LC), 4-pentyl-4′-cyanobiphenyl, into micelles with PS-rich cores and SGLCP-rich coronas. The morphologies of block copolymers with varying PS content are characterized as a function of temperature and concentration using small-angle neutron scattering, rheometry, and transmission electron microscopy. Unlike conventional solvents, the nematic LC can undergo a first-order transition between distinct fluid phases, accessing the regimes of both strong and slight selectivity in a single polymer/solvent pair. Micelles dissolve away above a microphase separation temperature (MST) that is often equal to the solution’s isotropization point, TNI. However, increasing or decreasing the polymer’s PS content can shift the MST to be above or below TNI, respectively, and in the former case, micelles abruptly swell with solvent at TNI. Comparable effects can be achieved by modulating the overall polymer concentration

Well-Defined Liquid Crystal Gels from Telechelic Polymers

Well-defined liquid crystal networks with controlled molecular weight between cross-links and cross-link functionality were prepared by “click” cross-linking of telechelic polymers produced by ring-opening metathesis polymerization (ROMP). The networks readily swell in a small molecule liquid crystal, 5CB, to form LC gels with high swelling ratios. These gels exhibit fast, reversible, and low-threshold optic switching under applied electric fields when they are unconstrained between electrodes. For a given electric field, the LC gels prepared from shorter telechelic polymers showed a reduced degree of switching than their counterparts made from longer polymer strands. The reported approach provides control over important parameters for LC networks, such as the length of the network strands between cross-links, cross-linker functionality, and mesogen density. Therefore, it allows a detailed study of relationships between molecular structure and macroscopic properties of these scientifically and technologically interesting networks

De retornos y ruedas: Configuración de la identidad cultural en mexicanos retornados de Estados Unidos pertenecientes a clubes de lowriders en el Área Metropolitana de Guadalajara.

La migración de retorno es una faceta que podríamos considerar como emergente dentro del marco de los estudios migratorios globales. La dinámica del retorno en el corredor fronterizo México/Estados Unidos en el que se insertan las personas que regresan a su país de origen, en este caso México, trae consigo una serie de cambios estructurales y simbólicos en los espacios a los cuales retornan, lo cual resulta en la creación de ciertos territorios específicos, los cuales son significados a través de la producción simbólica de determinados grupos como es el caso de los lowriders. En el presente trabajo de investigación, se busca observar y dar cuenta de cómo la experiencia/trayectoria de migración de retorno, ya sea por deportación o voluntaria, configura la identidad cultural de los mexicanos retornados de Estados Unidos que se insertan en los clubes de lowriders del Área Metropolitana de Guadalajara.ITESO, A.C

Study of the fatigue behaviour of some Fe-based metallic glass wires.

Fe78_C.,r xSiioBj2 and Fe77S. xCr-,S i7S. Bjs (with x values ranging from 0 to 8) glassy metallic alloys have been produced in the form of both wires and ribbons using the rotating water bath melt spinning process and chill block melt spinning process, respectively. It was found that the Fe77.5C.,r,, ,S ii. 5B15s eries had better amorphousw ire forming ability than the Fe7g_XCrXSiloBs1e2ri es. However, in ribbon form, both series showed good castability as amorphous alloys. Selected mechanical properties of the glassy samples have been compared with those of high tensile steel wire. Both thermal and mechanical properties were found to be composition-dependent, especially on the Cr content, with the Fe77.X5.C rrSi7.5B1s5e ries wires being thermally more stable and strongert han their Fe7gX. CrrSiloB12c ounterparts. A new bend type fatigue testing machine that uses two pulleys (DP) has been developed to perform tests on the basis of tension-compression loading cycles. The results have been compared with those obtained by using an existing single pulley (SP) machine that performs only tensile-tensile loading cycles. Although the DP machine can impose compressive to tensile bend stresses at different constant mean stress, it was observed that, for low stress ranges and large constant mean stresses the machine performed equivalent bend tests to those using the SP machine. The fatigue performance of amorphous wires was improved on substituting Fe by Cr and on decreasing the constant mean stress. The high tensile (HT) steel wire apparently had better fatigue performance than amorphous wires in terms of stress range; however, when the results were expressed in terms of strain range, the fatigue performance of amorphous wires was slightly better than HT steel wires, as had been demonstrated in previous studies. It was observed that, when the DP machine performed compressive to tensile bending stress cycles, the fracture surfaces differed from those observed in the SP machine. Under tensile to tensile conditions, the fracture surfaces observed for samples tested in the DP machine were similar to those tested in the SP machine. Fracture mechanics analysis of the results suggests that embrittlement is occurring ahead of growing fatigue cracks in these amorphous wires

Enhanced External Counterpulsation as a Novel Treatment for Post-acute COVID-19 Sequelae

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is the virus responsible for the coronavirus disease 2019 (COVID-19) pandemic. As patients recover from COVID-19, some continue to report persisting symptoms weeks to months after acute infection. These effects have been referred to as post-acute sequelae of SARS-CoV-2 infection (PASC). We report the case of a 38-year-old woman suffering from PASC symptoms following acute COVID-19 in October 2020. During her acute infection phase, she had a home recovery and reported her predominant symptoms as fatigue, headaches, body pain, and shortness of breath. After most of her symptoms were resolved, she continued to have periodic episodes of fatigue and headaches, along with random shortness of breath while at rest and during activities for months beyond the acute phase of the illness. She also noted the presence of “brain fog,” as if lacking the same clarity that she had prior to her illness. These symptoms persisted for three months before the patient underwent enhanced external counterpulsation (EECP) therapy in one-hour sessions, three times per week. This therapy was chosen based on the mechanism of action of EECP benefiting patients with ischemic cardiovascular diseases. After one week, her “brain fog” had improved, with shortness of breath improving after 1.5 weeks. The patient reported returning to pre-COVID health and fitness after approximately five weeks of EECP treatment. To our knowledge, this is the first case of using EECP for post-COVID shortness of breath, fatigue, and “brain fog.

Kinetics of crystallization of FeB-based amorphous alloys studied by neutron thermo-diffractometry

Kinetics of crystallization of two amorphous alloys, Fe70Cr10B20 and Fe80Zr10B10, have been followed up by neutron thermodiffractometry experiments performed in the two axis diffractometer D20 (ILL, Grenoble). The structural changes are directly correlated with the temperature dependence of the magnetization. Fe70Cr10B20 crystallizes following a two-step process: an eutectic crystallization of alfa-Fe (bcc) and the metastable tetragonal phase (Fe0.8Cr0.2)3B followed by another eutectic transformation to the stable phase (Fe0.75Cr0.25)2B and more segregation of alfa-Fe. These tetragonal phases are magnetically anisotropic, giving rise to a large increase of the coercivity. This behaviour is similar to that of Fe80B20 alloys, with Cr atoms replacing the Fe positions in both crystalline phases. Fe80Zr10B10 shows also a two-step process in which two polymorphic transformations take place.Comment: 3 pages. Proceedings International Workshop Non-Crystalline Solids 2006, Gijon (Spain