Byoung Chul Chun

Comparison of thermal/mechanical properties and shape memory effect of polyurethane block-copolymers with planar or bent shape of hard segment

Abstract A series of shape memory polyurethane copolymers synthesized from 1,4-phenyldiisocyanate (PDI) and poly(tetramethyleneglycol) (PTMG) plus 1,4-butanediol as chain extender were compared with similar ones from 4,4′-methylene-bis-phenyldiisocyanate (MDI) and PTMG in various shape memory tests at temperature ranges of ±20 °C around glass transition temperature (Tg). Similarity and some differences between two series were found in other characterization methods such as IR spectra, phase transition, and mechanical properties. Especially, Tg increased with the content of hard segment (PDI or MDI) and both copolymers generally showed similar Tg at the same content of hard segment. In addition, vibration and shock absorption ability was investigated by measuring both loss tan δ and storage modulus with dynamic mechanical analyzer.

Characterization and low temperature test of the flexibly crosslinked polyurethane copolymer by poly(dimethylsiloxane)

Shape memory polyurethane (SMPU) flexibly crosslinked by a hydrophobic poly(dimethylsiloxane) (PDMS) spacer at its side through allophanate bonding was tested for shape recovery at −30 °C and compared to a linear SMPU. The SMPU was composed of 4,4′-methylenebis(phenylisocyanate) (MDI), poly(tetramethyleneglycol) (PTMG), 1,4-butanediol (BD), and PDMS that was connected to SMPU chains by a second MDI. The mechanical and shape memory properties of two types of SMPU, differing in PTMG and PDMS content, were compared. In the best case, a 283% increase in maximum stress compared to the linear polymer was attained without any sacrifice of strain. Shape recovery at 45 °C increased to 91% and remained at more than 80% after four cyclic tests. Shape recovery at 0 °C could be improved by increasing PDMS content, and the critical shape recovery temperature decreased with increasing PDMS content. The PDMS-crosslinked SMPU demonstrated instant shape recovery at −30 °C in comparison with the linear SMPU that must be warmed to room temperature for complete shape recovery. The extraordinary shape memory results were analyzed and are discussed together with differential scanning calorimetry and infrared data.

Structure-property relationship and shape memory effect of polyurethane copolymer cross-linked with pentaerythritol

Polyurethane block copolymers chemically cross-linked by pentaerythritol, a four-way cross-linker, are tested for the shape memory effect. One of the copolymers shows higher shape recovery than any other shape memory copolymer synthesized by us so far. The copolymer maintains a surprising 94 % shape recovery after the third cyclic test. The four-way cross-linking by pentaerythritol and interaction between hard segments are mainly responsible for the very high shape recovery. Tensile mechanical properties also significantly improve by cross-linking. Glass transition temperature (Tg) slightly increases with cross-linking content. Other characterization such molecular weight, IR, and X-ray diffraction is also carried out to understand the arrangement of copolymer chains.

Grafting of Shape Memory Polyurethane with Poly(ethyleneimine) and the Effect on Electrolytic Attraction in Aqueous Solution and Shape Recovery Properties

AbstractShape memory polyurethane (SMPU) grafted using polyethyleneimine (PEI) was tested for its electrolytic attraction in aqueous solution and shape memory effect. The PEI was connected through a second 4,4′-diphenylmethane diisocyanate anchored to the carbamate moiety of SMPU. Two series of SMPU that differed in soft segment polytetramethylene glycol and PEI content were prepared to compare their tensile and shape memory properties. Shape recovery was high as 99% and reduced only to 97% after four test cycles. The PEI group attached to the SMPU chain, if converted to the imminium salt form, was designed to work as an electrolyte during water electrolysis and was to be attracted toward the cathode to show SMPU movement in aqueous solution. The electrolytic attraction of SMPU in aqueous solution was demonstrated in an experiment in which a specimen moved to the cathode only when voltage above a minimum was applied. The mechanism and application of this finding are discussed.

Mechanical properties and fracture morphologies of poly(phenylene sulfide)/nylon66 blends—effect of nylon66 content and testing temperature

Poly(phenylene sulfide) (PPS) was melt blended with Nylon66 and the mechanical properties and corresponding fracture morphologies were investigated. The thermal distortion temperature (HDT) of PPS/Nylon 66 blend showed that the inherent thermal stability of pure PPS can be maintained up to 30 wt% Nylon66, but then it started to decrease linearly thereafter to that of pure Nylon66 based on the rule of mixtures relationship. Tensile tests of PPS/Nylon66 blends at testing temperatures of −30, 25, 75, and 150°C showed that the maximum stress decreased up to 30 wt% Nylon66, and started to increase thereafter. Strain at break showed little change at low nylon content regardless of testing temperature, however, a large strain at break increase could be observed at more than 30 wt% Nylon66 and at 150°C testing temperature. At the same testing temperatures, the impact strength of PPS/Nylon66 blends was investigated, and it was found that an impact strength increase at all testing temperatures could be observed at more than 30 wt% Nylon66.

Impact of cholesterol grafting on molecular interactions and low temperature flexibility of polyurethanes

A series of polyurethanes (PUs) containing grafted cholesterol (UA series) and a control series blended with free cholesterol (UB series) were prepared: the spectroscopic, thermal, tensile, shape memory, and low temperature flexibility properties of these series were compared with those of unmodified PU. For both the UA and UB series, the soft segment melting temperature (Tm) was not affected by the cholesterol content. Differential scanning calorimetry (DSC) results showed that the crystallization of the hard segment of the UA series was completely inhibited as the grafted cholesterol content increased, which were supported by dynamic mechanical analysis (DMA) results for the storage modulus. As the cholesterol content increased, the glass transition temperature (Tg) of the UA series increased and remained the same for the UB series. The tensile strength in the UA series sharply increased with the cholesterol content, unlike that of the UB series. The strain at break in the UA series remained the same as the cholesterol content increased, whereas that of the UB series decreased significantly. As the cholesterol content increased, the shape recovery of the UA series remained above 90% at 45 °C and sharply increased at 0 °C. Finally, the UA series containing grafted cholesterol demonstrated excellent low temperature flexibilities compared to the UB series and unmodified PU.

PEG-based surfactants that show high selectivity in disrupting vesicular membrane with or without cholesterol

Surfactants that are composed of different shapes of aromatic rigid unit and flexible PEG chain with variable chain length are synthesized, and their membrane-disrupting properties against phospholipid vesicles encapsulating fluorescence markers are compared with Trinton X-100, a well-known nonselective membrane disrupting surfactant, in the point of release behavior of fluorescence markers. Among the surfactants prepared, some surfactants show membrane disruption of DPPC vesicles, but vesicles with cholesterol was not affected by the surfactants at all. Structure and activity relationship of the selective surfactants and further modification for optimization are briefly discussed in this paper.

Highly pH-sensitive ion pair amphiphile vesicle

Several ion pair amphiphiles (IPA) that are composed of two positive parts (ammonium surfactants) and one negative part (dicarboxlyate surfactant) are prepared to test whether the IPA vesicles can sense the change in pH of surrounding solution and respond by reassembling their aggregation structures. Among IPA candidates, one with asymmetric chain length of positive parts showed high and reliable pH-sensitivity as compared with IPA with symmetric chain length, which can be utilized in switching-on of release of encapsulated fluorescence marker and design of pH-sensitive drug delivery vehicle. Reasons for such high pH-sensitivity are sought from combined physical properties such as turbidity, zeta potential, surface tension, and release rate of marker through the bilayer membrane.

Effect of high density polyethylene addition and testing temperature on the mechanical and morphological properties of polypropylene/ethylene-propylene diene terpolymer binary blends

High density polyethylene (HDPE) was added to the polypropylene (PP)/ethylene-propylene diene terpolymer (EPDM) binary blend, and the effect of testing temperatures on the modulus of elasticity, impact behavior and corresponding fracture morphology was analyzed. Modulus of elasticity generally decreased as the EPDM content increased regardless of the testing temperatures. However, it was found that the modulus of elasticity of PP/EPDM/HDPE ternary blend increased compared to PP/EPDM binary blend when tested at −30 and −60 °C. Notched Izod impact strength changed depending on the testing temperatures, however, there was not much difference between binary and ternary blends up to 20 wt% EPDM. However, at more than 30 wt% EPDM content, ternary blends showed higher impact strength compared to binary blends. Especially, at −30 °C, brittle-ductile transition was observed between 20 and 30 wt% EPDM. Subsurface morphology was also analyzed, and the relationship between the impact strength and the stress whitening zone was investigated. Scanning electron microscopy observation of impact fractured surfaces was conducted, and overall morphology was analyzed with respect to HDPE addition and testing temperature change.

Structural, thermal, and mechanical properties of polyurethane foams prepared with starch as the main component of polyols

In this study, rigid polyurethane foams were prepared using starch as the main component of polyols and their structural, thermal, and mechanical properties were investigated. The starch content in polyols was 30∼50 wt.%. The prepared polyurethane foams had a cell structure. When the starch content and -NCO/-OH molar ratio (TS4-05, TS3-07, and TS3-05) was low, polyurethane foams were not formed. To confirm the formation of a urethane linkage between -OH of the starch and -NCO of the 2,4-TDI, FT-IR spectroscopic analysis was performed. The thermal properties of polyurethane foams were analyzed by DSC and TGA. DSC thermograms showed two endothermic peaks: a sharp peak at a lower temperature and a broad peak at a higher temperature. Both peaks were shifted to higher temperature with starch content in polyols and -NCO/-OH molar ratio. Thermal degradation of polyurethane foams began at a lower temperature and ended at a higher temperature than that of starch. The impact resistance, compressive stress and modulus of polyurethane foams increased with -NCO/-OH molar ratio and starch content.