Yong-Chan Chung

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.

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.

Polymerized ion pair amphiphile that shows remarkable enhancement in encapsulation efficiency and very slow release of fluorescent markers

N,N-dimethyl-N,N-dihexadecylammonium 12-(lipoyloxy)dodecanoate and three other analogues were synthesized as polymerizable ion pair amphiphiles (PIPA) to increase the stability of ion pair amphiphile (IPA) liposome and control the release of encapsulated materials. Among the four PIPA candidates made, only N,N-dimethyl-N,N-dihexadecylammonium 12-(lipoyloxy)dodecanoate showed reliable and stable liposome formation with remarkable increase in encapsulation efficiency, and the polymerized IPA liposome thus formed was stable enough to withstand the lysis from SDS or Triton X-100. In addition, the polymerization method was as easy and convenient as just shaking the solution at room temperature without any need for the addition of initiator and input of energy such as light. Physical properties were also checked in the point of phase transition, release rate of marker, and ion permeation through the bilayer. Reasons for such high encapsulation and future applications are briefly discussed.

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.

Thermomechanical properties and shape memory effect of PET-PEG copolymers cross-linked with pentaerythritol

Poly(ethylene terephthalate) (PET) and poly(ethylene glycol) (PEG) copolymers cross-linked with pentaerythritol, a four-way cross-linker, are prepared to compare their mechanical and shape memory properties with the one cross-linked by glycerol. Composition of PEG and pentaerythritol is varied to search for the one with the best mechanical and shape memory properties. The highest shape recovery rate is observed for the copolymer composed of 30 mol% PEG-200 and 2.5 mol% pentaerythritol. Four-way cross-linking by pentaerythritol significantly improves shape recovery rate and retention of high shape recovery rate after repeated use compared to the one cross-linked by glycerol, a three-way cross-linker, and difference and advantage of additional cross-linking point are discussed.

Characterization of a polyurethane copolymer covalently linked to graphite and the influence of graphite on electric conductivity

Polyurethane was linked to graphite modified with a phenol or 4-phenylethanol group on its surface, and the composite polymer was characterized regarding its thermal, spectroscopic, mechanical, and shape-memory properties as well as its electric conductivity. Graphite as a filler was covalently linked to a polyurethane backbone composed of 4,4′-methylenebis(phenylisocyanate) and 1,4-butanediol as hard segments and poly(tetramethyleneglycol) as a soft segment. The two different polyurethane series, differing in their modifying spacer, show similar structural characteristics and exhibit a small degree of cross-linking due to the agent used for the covalent linking of graphite to the polyurethane frame. The tensile mechanical strength and shape-recovery characteristics remain high as the graphite content is increased when compared to the linear polyurethane polymer without the linked graphite. Regarding the tensile mechanical properties, the maximum stress increases up to 436% compared to the linear polyurethane, and the maximum strain goes up to 1744%. Shape recovery reaches values as high as 93%, and shape retention gradually decreases as the graphite content increases. The electric conductivity of graphite-linked polyurethane exhibits a sharp increase as the graphite content increases, but the blended graphite polyurethane has very poor electric conductivity at higher graphite contents. The implication of this finding is discussed, and the synthesized polymers are compared with other conductive polymer composite.