JongChoo Lim

Effect of molecular structure of self-assembled monolayers on their tribological behaviors in nano- and microscales

The tribological behavior of self-assembled monolayers (SAMs) of ω-functional n-alkanethiol compounds were experimentally investigated at the nanoscale using friction force microscopy (FFM) and at the microscale using a microtribometer. Molecularly thin and well-ordered films of the surfactants with different terminal groups and chain length were prepared on gold substrates, gold-coated atomic force microscope (AFM) tips and steel balls by the spontaneous adsorption of the surfactant molecules. Nanoscale adhesion and friction forces between the films and gold-coated AFM tip were measured by FFM whereas microscale friction and wear properties between the films and gold-coated steel balls and uncoated steel balls were measured by the microtribometer. The nano- and microscale tribological behaviors between the films are compared and discussed in terms of the chemical structure such as terminal group and chain length of the film molecules. This investigation revealed that the molecular structure of the films significantly affects their tribological behaviors both in nano- and microscales.

Aging Behavior of Metal Oxide Aerogels

Aerogels of titania, zirconia, niobia, and silica were prepared by CO2 supercritical drying of the corresponding alcogels. Their porous texture could be modified by further aging of the alcogels. This kind of textural modification was found to be different according to the nature of the metal precursor and sol-gel conditions.

Fully biobased robust biocomposites of PLA with assisted nucleation by monodispersed stereocomplexed polylactide particles

Fully biodegradable biocomposites are a desirable choice among the synthetic plastics due to their increasing adverse ecological issues. Stereocomplexed polylactide submicronparticles (s-PLA SP), which provide dense nucleating sites to PLLA in the crystallization process, were used to prepare homogenous PLLA biocomposites to enhance the thermal and mechanical properties of PLLA. Herein, we report the synthesis of monodispersed s-PLA SP and preparation of biocomposites of PLLA incorporating s-PLA SP. The synthetic system for s-PLA SP is comprised of two steps. The first step involves solvating PLLA and PDLA in respective reactors. The second step is the stereocomplexation of the dissolved polymer in the mixing reactor. The polymer solvation conditions greatly affect the s-PLA particle size and distribution. Therefore, we studied the variation of s-PLA particle size and polydispersity index (PDI) due to the effect of the polymer solvation parameters, such as homopolymer concentration, solvation time, temperature, pressure and molecular weight, as the first step. The optimum conditions established were 50 °C, 300 b, with 5% homopolymer solvation for 12 h. In the second step, s-PLA SP formation was carried out at the previously established reaction conditions of 70 °C, 250 b for 2 h. The PLA homopolymers (Mn ∼ 57 kg mol−1) produced s-PLA SP smaller than 963 nm with a PDI of 0.12, as analyzed by DLS and SEM at the optimum reaction conditions. We cast completely biobased homocomposites (BC) for high performance by homogeneously dispersing s-PLA SP in the high molecular weight PLLA matrix (Mn ∼ 200 kg mol−1) in chloroform, followed by annealing. The solution casting DSC results show the enhanced crystallization of PLLA for various compositions of BC ranging from 0.5% to 5%. An increase in the concentration of s-PLA SP increases the thermal and mechanical properties of the PLA BC. Effective nucleation with decreased spherulite size was noticed for the 5% BC with 963 nm s-PLA SP incorporated into the casting solution, as inferred by SEM analysis. The tensile strength and Youngs modulus increased from 29 MPa to 73 MPa and 1.3 GPa to 3.2 GPa, respectively, whereas the Xcc increased from 20% to 35% and Tcc increased from 98 °C to 113 °C with the 5% composition involving 963 nm s-PLA SP.

Synthesis and surface activities of multi-ammonium cationics derived from n-octadecyl glycidyl

In this study, cationic surfactants having two, three or four hydroxyl groups were synthesized by the condensation reaction of n-octadecyl glycidyl ether and amine (methyl amine and dimethyl amine) followed by the quaternization with dimethyl sulfate. The structure of a resulting product was determined by 1H NMR and FT-IR spectroscopies. Interfacial tensions measured as a function of time for n-decane drops brought into contact with 1 wt. % surfactant solutions at 25°C indicated that the interfacial tension decreased over a period of about 10–15 min to an equilibrium value. The results of contact angle measurements indicate that C18-BADM surfactant having four hydroxyl groups is the best wetting agent among others studied. Moreover, the excellent adsorption capacity of C18-BADM system suggests that it can be used as a softening agent during a laundry process. The results of foam stability measurement were consistent with those of CMC and contact angle. The percentage of foam volume decrease was found to increase with the hydrophilicity of a surfactant.