Cyclic block copolypeptoids : synthesis, self-assembly and macroscopic properties

This dissertation presents the first experimental efforts for synthesizing a new class of peptidomimetic polymers, cyclic block copolypeptoids, studying their self-assembly in dilute solution, and examining their macroscopic properties. Cyclic poly(N-methyl-glycine)-b-poly(N-decyl-glycine) (PNMG-b-PNDG) diblock copolymers were synthesized via sequential, N-heterocyclic carbene (NHC)-mediated ring-opening polymerization of NCA monomers with controlled chain length and compositions. Cryo-TEM shows that PNMG105-b-PNDG10 copolymers in methanol (1 mg/mL) form spherical micelles that organize to form cylindrical micelles over time. The formation of cylindrical micelles is attributed to the crystallization of PNDG chains, as revealed by SAED and microDSC analyses. Cyclic PNMG100-b-PNDG10 copolymers form thermo-reversible, free-standing gels at a concentration of 5 wt %. Rheological measurements of the gels reveal that cyclic copolymers produce harder gels than their linear counterparts. Variable-temperature 1H NMR suggests that thermo-reversible gelation is related to any transitions of morphologies formed from the copolymers upon heating and cooling. Analyses by WAXS and SAXS suggest that the cyclic copolymers favor to form cylindrical micelles more than the linear copolymers, leading to formation of harder gels due to more transient networking among the cylindrical micelles. The second part focuses on the synthesis of cyclic triblock terpolypeptoids and their self-assembly in solution. Cyclic poly(N-methyl-glycine)-b-poly(N-allyl-glycine)-b-poly(N-benzyl-glycine) triblock terpolypeptoids were synthesized via sequential addition of monomers. Studies by SEC and NMR suggest block copolymer compositions can be controlled by varying molar ratios of each monomer to NHC. Similarly, cyclic [poly(N-ethyl-glycine)-ran-poly(N-butyl-glycine)]-b-poly(N-methyl-glycine)-b-poly(N-decyl-glycine) triblock terpolypeptoids were synthesized. Analysis by DLS suggests that this terpolymer forms micelles with broad size distributions below the cloud point temperature (Tcp) of PNEG-ran-PNBG blocks, and it may form well-defined, larger micellar aggregates above the Tcp. The third part presents systematic studies of the melting and crystallization of cyclic or linear polypeptoids with various n-alkyl side groups. Studies by DSC show that polypeptoids with butyl or longer side groups have two endothermic melting transitions (Tm,1 \u3c Tm,2). Tm,1 increases and Tm,2 decreases, as the length of n-alkyl side groups increases. Studies by WAXS show that polypeptoids with butyl or longer side groups are well ordered. Tm,1 is attributed to side chain packing and Tm,2 to main polypeptoid packing

Controlling the Dispersion of Nanoparticles in Polymer Matrices

The dispersion of nanoparticles in a polymer matrix is a critical parameter to design and realize targeted morphologies and properties of a final polymer-based nanocomposite. This thesis presents experimental studies to investigate the dispersion of nanoparticles is polymer matrices using specific intermolecular hydrogen bonding between the nanoparticles and polymer matrices under two main topics. The first topic is the impact of sample preparation processes on the properties of polymer carbon nanotube nanocomposites. Polymer nanocomposites composed of poly(styrene-co-vinyl phenol) (PSVPh) copolymers and 5 wt % multi-walled carbon nanotubes (MWNTs) were prepared from three different methods, including melt-mixing and solution casting. The MWNTs were either oxidized to incorporate oxygenated defects or utilized as received. The mechanical properties of the nanocomposites were measured by DMA, and the extent of intermolecular hydrogen bonding between MWNTs and PSVPh was quantified by IR. Our DMA results suggest that melt-mixing leads to more stable morphologies of the final nanocomposites than solution casting does. Additionally, the IR analysis of the nanocomposites indicates melt-mixing can result in the formation of more intermolecular hydrogen bonding between the MWNTs and PSVPh than solution casting, and thus suggests that melt-mixing leads to more reproducible mechanical properties than solution casting. Our DMA and IR results may provide guidelines to realize the desired morphologies and to improve the properties of polymer carbon nanotube nanocomposites by optimizing intermolecular interactions between MWNTs and polymers. The other topic examined seeks to synthesize the starting materials to sequester surface-modified gold nanoparticles in a diblock copolymer matrix. The long term goal of this project is to investigate the impact of enthalpic attractions between the gold nanoparticles and one phase of the diblock copolymer on the nanocomposite morphologies. That is, this system is designed such that the hydroxyl groups on the gold nanoparticles can hydrogen bond with poly(2-vinyl pyridine) blocks of poly(styrene-b-2vinyl pyridine) diblock copolymers. Polystyrene-coated gold nanoparticles were synthesized using thiol-terminated polystyrene (PS-SH) as a stabilizing ligand. The low molecular weight PS-Sh was obtained by the thiolation of bromine-terminated polystyrene (PS-Br), which was synthesized by ATRP. TEM images of the gold nanoparticles shows that the size of the gold nanoparticles can be controlled using this PS-SH as a stabilizing ligand. Additionally, the synthesis of thiol-terminated poly(styrene-co-vinyl phenol) (PSVPh-SH) copolymers was attempted starting with bromine-terminated poly(styrene-co-4-tert-butoxystyrene) copolymers, followed by thiolation and hydrolysis. Our NMR results indicate that the conversion of the bromine-end groups of the copolymers to thiol-end groups by thiolation was complete. However, the conversion of the tert-butoxy groups of the thiolated copolymers to hydroxyl groups by hydrolysis using HCI was not successful. Furthermore, NMR data suggests that this hydrolysis may impact the thiol-end groups of the thiol-terminated copolymers. Our NMR results may provide guidelines such that carefully controlled reaction time and/or HCl amount would improve the success of the PSVPh-Sh synthesis

Tau functions as Widom constants

We define a tau function for a generic Riemann-Hilbert problem posed on a union of non-intersecting smooth closed curves with jump matrices analytic in their neighborhood. The tau function depends on parameters of the jumps and is expressed as the Fredholm determinant of an integral operator with block integrable kernel constructed in terms of elementary parametrices. Its logarithmic derivatives with respect to parameters are given by contour integrals involving these parametrices and the solution of the Riemann-Hilbert problem. In the case of one circle, the tau function coincides with Widom's determinant arising in the asymptotics of block Toeplitz matrices. Our construction gives the Jimbo-Miwa-Ueno tau function for Riemann-Hilbert problems of isomonodromic origin (Painlev\'e VI, V, III, Garnier system, etc) and the Sato-Segal-Wilson tau function for integrable hierarchies such as Gelfand-Dickey and Drinfeld-Sokolov.Comment: 26 pages, 6 figure

Effects of Distilled Cervi Pantotrichum Cornu and Rehmannia glutinosa Pharmacopuncture at GB21 (Jianjing) on Heart Rate Variability: A Randomized and Double-blind Clinical Trial

AbstractBackground/PurposeThe purpose of this study was to use heart rate variability (HRV) to investigate the effects of distilled Cervi Pantotrichum Cornu pharmacopuncture and Rehmannia glutinosa pharmacopuncture on the autonomic nervous system.Materials and methodsForty healthy male participants were divided into two groups: the participants of the C-group received distilled Cervi Pantotrichum Cornu pharmacopuncture and those of the R-group received Rehmannia glutinosa pharmacopuncture. The study design was a randomized, double-blind clinical trial. Each participant received one of the two solutions injected at GB21 (Jianjing). The changes in HRV were measured seven times using the QECG-3: LXC3203 system (LAXTHA Inc. Korea). Time-dependent changes in HRV for each group were analyzed using the paired t test (significance level: p < 0.05), and the difference in the HRV fluctuations between the two experimental groups was evaluated using the independent sample test (significance level: p < 0.05).Results and conclusionThe results showed that Cervi Pantotrichum Cornu pharmacopuncture and Rehmannia glutinosa pharmacopuncture tended to activate the autonomic nervous system within the normal range. Cervi Pantotrichum Cornu pharmacopuncture tended to activate the sympathetic nervous system, whereas Rehmannia glutinosa pharmacopuncture tended to activate both the sympathetic and parasympathetic nervous systems

Excess cost of non-remission among outpatients with major depressive disorder

Background: The purpose of this study was to assess the economic benefit of achieving remission among outpatients with major depressive disorder (MDD) who are currently employed in Korea. Methods: Cross-sectional observational study. A total of 337 outpatients with MDD with paid jobs were recruited from 14 psychiatric clinics in Korea and were then divided into three groups as follows: new visit group (n = 128), remitted group (n = 100) and non-remitted group (n = 109). The 17-item Hamilton Depression Rating Scale (HAM-D) was used to decide whether a patient should be assigned to the remitted or non-remitted group. Direct medical and non-medical costs were measured via interview with the subjects. The World Health Organization Health and Work Performance Questionnaire (HPQ) were applied in order to measure the lost productive time (LPT) and related productivity costs. Results: The three groups did not show a significant difference in direct medical cost. However, the difference between the remitted group and non-remitted group was statistically significant (25.49 ± 52.99 vs. 44.79 ± 126.55, χ2 = 12.99, p = 0.0015). The remitted group demonstrated a significant improvement in productivity (particularly presenteeism) when compared with the new visit group (Z = −3.29, p = 0.001). Although the non-remitted group received treatment at psychiatric clinics similar to the remitted group, it lost 33 more working hours per month, which is compatible to $332 per month. Conclusion: These results suggest the economic importance of achieving remission in treating depression

Two-gap and paramagnetic pair-breaking effects on upper critical field of SmFeAsO0.85_{0.85} and SmFeAsO0.8_{0.8}F0.2_{0.2} single crystals

We investigated the temperature dependence of the upper critical field [$H_{c2}(T)$] of fluorine-free SmFeAsO$_{0.85}$ and fluorine-doped SmFeAsO$_{0.8}$F$_{0.2}$ single crystals by measuring the resistive transition in low static magnetic fields and in pulsed fields up to 60 T. Both crystals show that $H_{c2}(T)$'s along the c axis [$H_{c2}^c(T)$] and in an $ab$-planar direction [$H_{c2}^{ab}(T)$] exhibit a linear and a sublinear increase, respectively, with decreasing temperature below the superconducting transition. $H_{c2}(T)$'s in both directions deviate from the conventional one-gap Werthamer-Helfand-Hohenberg theoretical prediction at low temperatures. A two-gap nature and the paramagnetic pair-breaking effect are shown to be responsible for the temperature-dependent behavior of $H_{c2}^c$ and $H_{c2}^{ab}$, respectively.Comment: 21 pages, 8 figure

Limits of Binaries That Can Be Characterized by Gravitational Microlensing

Due to the high efficiency of planet detections, current microlensing planet searches focus on high-magnification events. High-magnification events are sensitive to remote binary companions as well and thus a sample of wide-separation binaries are expected to be collected as a byproduct. In this paper, we show that characterizing binaries for a portion of this sample will be difficult due to the degeneracy of the binary-lensing parameters. This degeneracy arises because the perturbation induced by the binary companion is well approximated by the Chang-Refsdal lensing for binaries with separations greater than a certain limit. For binaries composed of equal mass lenses, we find that the lens binarity can be noticed up to the separations of $\sim 60$ times of the Einstein radius corresponding to the mass of each lens. Among these binaries, however, we find that the lensing parameters can be determined only for a portion of binaries with separations less than $\sim 20$ times of the Einstein radius.Comment: 5 pages, 3 figures, 1 tabl