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Browsing by Author "Liu, Ran"

Now showing 1 - 3 of 3
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    Fearful Temperament in Middle Childhood and Anxiety Symptoms in Adolescence: The Roles of Attention Biases, Effortful Control, and Frontal EEG Asymmetry
    Liu, Ran (Virginia Tech, 2020-12-01)
    Fearful temperament represents one of the robust predictors of the development of child and adolescent anxiety. Not all children with fearful temperament unvaryingly develop anxiety, however. Diverse processes resulting from the interplay among attention, cognitive control, and motivational system drive the trajectories toward more adaptive or maladaptive directions. In this study, I examined various factors that underlie the association between fearful temperament at age 9 and adolescent anxiety symptoms including attention biases, different components of effortful control, and frontal EEG asymmetry. 78 children participated in this study. Results indicate that fearful temperament at age 9 significantly predicted adolescent anxiety symptoms. This association, however, was moderated by children's effortful control and frontal EEG asymmetry at age 9. Specifically, fear at 9 years predicted adolescent anxiety only when children had low attentional control, low inhibitory control, low activation control, and exhibited greater right activation from baseline to task. The associations between AB and fearful temperament as well as anxiety were not significant. The association between fear at 9 years and sustained AB during adolescence, however, was moderated by children's attentional control, inhibitory control, and frontal EEG asymmetry at age 9. Specifically, fear predicted attention biases away from threat when children had high attentional control, high inhibitory control, and showed greater left activation. The findings will be discussed in terms of the roles of attention biases in the development of anxiety and how different components of effortful control and frontal EEG asymmetry contribute to the resilience process.
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    Shyness and Internalizing Problems in Middle Childhood: The Moderating Role of Attentional Control, Inhibitory Control, and Frontal EEG Asymmetry
    Liu, Ran (Virginia Tech, 2017)
    Shyness is highly related to internalizing problems. However, not all shy children develop serious internalizing problems (IP). The aim of the current study was to identify the endogenous factors that might contribute to the resilience process from a self-regulation perspective. Participants included 73 children (33 boys; 40girls) who visited the lab at 6 and 9 years of age. Shyness, attentional control (AC), inhibitory control (IC), frontal electroencephalogram (EEG) asymmetry were measured at both 6 and 9 years using age appropriate questionnaires and tasks. Results indicated that age 6 shyness did not directly predict age 9 IP; instead it indirectly predicted IP through age 9 shyness. Neither AC, IC, frontal EEG asymmetry, nor the stability of frontal EEG asymmetry moderated the association between age 6 shyness and age 9 IP. However, there was a positive concurrent association between shyness and IP at 9 years. In addition, AC and IC moderated the shyness-IP association at age 9. Shyness was significantly associated with IP only when children had low AC or IC, but not when children had high AC or IC.
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    Synthesis and Characterization of Novel Polybenzimidazoles and Post-modifications for Membrane Separation Applications
    Liu, Ran (Virginia Tech, 2018-06-29)
    Polybenzimidazoles, a class of aromatic heterocyclic polymers, are well known due to their remarkable thermal stability, mechanical properties and chemical resistance which are often required in extreme operation conditions. Because of these properties, polybenzimidazoles are excellent candidates in various application areas including proton exchange membrane fuel cells, gas separation membranes, reverse osmosis and nanofiltration, and high performance coatings. The following studies are focused on the synthesis, characterization and related properties of polybenzimidazoles and polybenzimidazole based materials. A novel sulfonyl-containing tetraamino-substituted monomer (3,3',4,4'-tetraaminodiphenylsulfone) was synthesized and polymerized with three different diacid monomers to make polybenzimidazoles. The new monomer synthesis route with reduced steps relative to the existing literature method increased the overall yield by a factor of three. The sulfonyl-containing polybenzimidazoles have enhanced solubilities in common organic solvents including dimthylsulfoxide, dimethylacetamide and N-methyl-2-pyrrolidone in comparison with the commercial polybenzimidazole, Celazole®, poly(2,2'-(m-phenylene)-5,5'-bibenzimidazole). The improvements in solubility are attributed to the introduction of polar sulfonyl linking moiety in the monomer. Remarkable thermal stabilities (high Tg, > 428 °C) were demonstrated through Dynamic Mechanical Analysis (DMA) and Thermogravimetric Analysis (TGA). A well designed film casting process was investigated and established. Polybenzimidazoles were fabricated into transparent thin films (20-30 μm thick) for gas transport measurements. These novel polybenzimidazole films exhibited extraordinary gas separation properties, especially for H₂/CO₂ separation. There is a trade-off relationship between gas permeability and selectivity through dense, non-porous polymer membranes that was discovered by Robeson in 1991. The ultimate goal for developing gas separation membranes is to improve both permeability and selectivity simultaneously. Gas permeability is related to the free volume between polymer chains. In order to improve gas permeability, we hypothesized a concept that increasing free volume could be achieved by thermally degrading sacrificial components and volatilizing their byproducts from a glassy matrix. Volatile components were introduced into the films to preoccupy the spaces between polymer chains. Once they were degraded and removed through the thermal treatment, it was hypothesized that the preoccupied spaces would remain empty due to the glassy nature of the matrix at the heat treatment temperature, thus resulting in more free volume. Two post- modification strategies including grafting and blending were utilized to incorporate the volatile components, poly(propylene oxide) and poly(ethylene oxide). Post-modified polybenzimidazole films impressively showed significant enhancements in both gas permeability and selectivity for H₂/CO₂ separation. The H₂ permeability of the post-modified TADPS-OBA polybenzimidazole increased from 3.1-6.2 Barrers to 5.2-7.5 Barrers (up to 66% increase). The selectivity for H₂/CO₂ increased from 7.5-10.5 to 10.1-13.0 (up to 33% increase). The study on the potential effects of water vapor on the separation performance of PBI membranes was discussed in the appendix.