Glucose cross-linking of polyurethane copolymer and its impact on the elevation of mechanical properties and shape memory effect

Effect of glucose cross-linking on the shape memory and mechanical properties of polyurethane (PU) block copolymer was investigated. Glucose was selected due to its large number of free hydroxyl groups, easy availability, miscibility with other reactants, and cyclic structure. The glycerol cross-linking did not affect the molecular interaction and phase separation of hard and soft segments in polyurethane structure as judged from IR and DSC analysis. Viscosity of glucose cross-linked PU increased after cross-linking due to the cross-linked structure. Maximum stress drastically improved with the adoption of glucose as a cross-linker together with a slight increase in strain at break. Shape recovery also increased with the adoption of glucose as a cross-linker, and shape recovery was not diminished after four cyclic shape recovery tests. In contrast, shape retention significantly decreased if glucose was included for two different hard segment contents. Finally, glucose cross-linking was compared with other cross-linkers used in shape memory polymer and the advantage of glucose cross-linking was discussed.     

Low temperature shape recovery effect of polyurethane copolymer grafted with pendant <Emphasis Type="Italic">n</Emphasis>-butyl group

Effect of the pendant n-butyl group on shape recovery and tensile properties of polyurethane (PU) block copolymer was investigated. The grafted n-butyl group was intended to keep PU chains away and to deter molecular interaction between PU chains by its flexible chains, and thus improve shape recovery at subzero temperature while maintaining high and reproducible tensile properties and shape recovery at ambient temperature. The attachment of n-butyl group did not make any change in the molecular interaction and phase separation of hard and soft segments in PU structure as judged from IR and DSC analysis. Cross-link density and intrinsic viscosity increased with the increase of n-butyl content due to the partial cross-linking by the linking reagent. Shape recovery and shape retention were not diminished after cyclic shape memory tests. Finally, the effect of n-butyl group on low temperature shape recovery was compared with linear ones and the potential application of this finding was discussed.     

Synthesis of polypropylene carbonate polyol-based waterborne polyurethane modified with polysiloxane and its film properties

Waterborne polyurethane (WPU) prepolymer was synthesized using polypropylene carbonate polyol as the soft segment, dimethylolpropionic acid as a hydrophilic chain extender and isophorone diisocyanate. The prepolymer was modified with aminoethyl aminopropyl dimethicon (AEAPS) to prepare a series of WPU emulsions and films. The structures and the films properties of the WPUs were characterized by Fourier transform infrared spectrometry, gel permeation chromatography, atomic force microscopy, X-ray diffraction, thermogravimetric analysis, dynamic thermomechanical analysis, X-ray photoelectron spectroscopy, water contact angles and water absorption. It was found that pure polypropylene carbonate WPU had a wide molecular weight distribution and its microphase separation was not apparent between its hard and soft segments. The WPU also had a high glass transition temperature (24.5 °C) and its film had a high damping property (tan δ>0.40) from 12 °C to 42 °C. Modification with polysiloxane had enlarged the molecular weight, narrowed the molecular weight distribution and resulted in the microphase separation between the hard and soft segments of WPUs, and this amplified the damping temperature of the WPU films. Along with the increasing utilization of polysiloxane the thermolysis, water resistance and water contact angles of WPU films were improved while the orientation of their structure regularity declined.     

Grafting of Triphenylmethyl Group onto Polyurethane and the Impact on the Shape Recovery and Flexibility at Extremely Low Temperature

The bulky and rigid triphenylmethyl group was grafted onto polyurethane (PU) to reduce the molecular attractions between hard segments and to improve the mobility of the PU chain under freezing conditions. The triphenylmethyl-grafted PU exhibited improvement in the cross-link density, solution viscosity, maximum tensile stress, shape recovery at 10 °C, and low temperature flexibility compared with the plain PU. The soft segment melting was not affected by the grafted triphenylmethyl group, whereas the soft segment crystallization disappeared with the increase of the triphenylmethyl group content. The glass transition temperature (T_g) increased with the increase of the triphenylmethyl group content. The rapid increase of the tensile strength and shape recovery at 10 °C resulted from the cross-linking effect, whereas the strain at break and shape retention at -25 °C slightly decreased with the increase of the triphenylmethyl group content. The triphenylmethylgrafted PU displayed an excellent low temperature flexibility even at -50 °C due to the improved mobility of the PU chain compared to ordinary PU.     

Preparation and characterization of thermoplastic polyurethanes using partially acetylated kraft lignin

Thermoplastic polyurethanes were prepared using 90 % acetylated softwood kraft lignin, polyethylene glycol and 4,4-methylene diphenyl diisocyanate. Due to the glass transitions of the compatible soft and hard segment mixtures, the polyurethanes exhibited the first glass transitions at ?40 to 10 °C, and the transition temperatures increased with increasing hard segment content. Due to the glass transitions of the microphase separated hard domains, the second glass transitions occurred at 150 °C. The viscous responses during the second transitions decreased as the separated hard domains-induced chemical and physical crosslinks increased. The Young’s modulus and tensile strength increased with increasing hard segment content, whereas the breaking strain decreased. The phase morphology changed from an isolated hard domain structure to an interconnected one as the physical crosslinks increased, which caused drastic changes in the increasing or decreasing tendency of the tensile strength or breaking strain. Because of the phase morphology, the polyurethanes exhibited viscoplasticity or viscoelasticity.     

Compressive viscoelastic properties of softwood kraft lignin-based flexible polyurethane foams

Softwood kraft lignin (SKL)-based water-blown flexible polyurethane foams were prepared using SKL as a crosslinking agent and a hard segment polyol. Polyethylene glycol (PEG) as a soft segment diol and 2,4-toluene diisocyanate (TDI) were used. While increasing hard segment content caused the increase in crosslink density in foams, the foams became more and more viscous with increasing hard segment content due to the distinctive phase heterogeneity in foams. In this case, the contributiveness of the filler-like behaviors of separated hard segments always overtook the crosslinking effects derived from SKL in terms of overall viscoelasticity, thus the resultant viscometric properties such as tanδ _max and hysteresis loss increased as hard segment content increased. Furthermore, increasing M _n,PEG caused the severer microphase separation and intensified the filler effects in foams, thus the foams became more viscous with increasing M _n,PEG. The 25 % and 65 % CFD values and Young’s moduli of foams increased with increasing hard segment content due to the increase in crosslink density for foams, and the properties also increased with increasing foam density. Most of foams showed the support factors in the range of 2–3, which are suitable values for cushioning use. Even though the microscopic deformation behaviors in foams are irrelevant to foam density, the cyclic compressive tests showed that the higher foam density possess the better shape recovery performances.     

Laterally crosslinked polyurethane copolymers with polycarbonate as a soft segment

The polyurethane (PU) copolymer was laterally crosslinked with an extra MDI, in which a more rigid polycarbonatediol replaced the conventional poly(tetramethyleneglycol) as a soft segment. What is the impact of the possible molecular interaction between polycarbonate soft segments and lateral crosslinking. The structural change after crosslinking and the impact of new soft segment were followed by infrared spectra, crosslink density, UV-VIS spectra, and relative viscosity. The tensile stress could improve as much as 554 % by selecting the polycarbonate soft segment and the lateral crosslinking. Shape recovery was over 90 % for the entire series and reproducible for four test cycles. The adoption of polycarbonate soft segment and lateral crosslinking significantly could improve the tensile strength and shape recovery compared to PU with polyetherdiol or polyesterdiol soft segment.     

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 (T _g ) 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.     

Shape memory effects of polyurethane block copolymers cross-linked by celite

The shape memory polyurethane (PU) copolymers cross-linked by celite, a porous inorganic material with enormous surface area and hydroxyl groups on the surface, were prepared to see if the shape memory effect and the mechanical properties were improved. The PU copolymers with different celite contents were compared and characterized by IR, DSC, DMA, and UTM. The melting temperatures of PU soft segment were around 20 oC independent of celite content. The shape memory effect and mechanical properties were dependent on the celite content, and the celite addition into the reaction mixture should be made in the middle of polymerization to get the best shape memory and mechanical properties. The best mechanical properties were found at 0.2 wt% celite content and its shape retention rate went up to 98 %. The inclusion of celite as a cross-linker increased both shape memory effect and mechanical properties. The reasons underlining the improvements by adopting celite as a cross-linker are discussed in this paper.     

Effect of glycerol cross-linking and PDI on the shape memory effect and mechanical properties of polyurethane

A series of shape memory polyurethane (PU) copolymers synthesized from 1,4-phenyldiisocyanate (PDI), poly(tetramethyleneglycol) (PTMG), 1,4-butanediol (BD) as a chain extender, and glycerol as a cross-linking agent were tested for the mechanical properties and the shape memory effect at the temperature 20 °C above melting temperature (T _m), and were compared with other PUs synthesized from 4,4′-methylene-bis-phenyldiisocyanate (MDI), PTMG, and BD. Mechanical properties and shape memory effect were improved substantially by adopting both PDI and glycerol. Interestingly, enthalpy of melting and T _m were not affected by the glycerol content. Vibration and shock absorption ability was investigated by measuring both loss tan δ and storage modulus with dynamic mechanical analyzer (DMA).     

Core-sheath polyurethane-carbon nanotube nanofibers prepared by electrospinning

The core-sheath nanofibers consisting of polyurethane (PU) core and PU composites sheath with multi-walled carbon nanotubes (MWNTs) were prepared by electrospinning. At low MWNT concentration, MWNTs appeared highly aligned along the fiber axis with some curving in nanotubes, whereas in case of high concentration, some aggregation of MWNTs appeared due to difficulty in full dispersion of nanotubes. In comparison of the single component nanofiber webs, the core-sheath nanofiber webs showed much better mechanical properties of modulus and breaking stress, including an exceptional elongation-at-break. It indicates that the CNT-incorporated core-sheath structure is very effective for enhancing the mechanical properties of nanofiber webs. In addition, the core-sheath nanofibers demonstrated the fast shape recovery, compared with one component fibers of pure shape memory PU and PU/MWNTs, which provides the possibility of fabricating more sensitive intelligent materials.     

Effect of cross-linking agent structure on the shape memory property of polyurethane block copolymer

Effect of cross-linking agent on the shape memory and mechanical property of polyurethane (PU) block copolymer is comprehensively investigated. The selected chemical cross-linking agents are glycerol, 1,2,6-trihydroxyhexane, and 2,4,6-trihydroxybenzaldehyde that are differentiated from each other in having remote hydroxyl group and aromatic ring. Significant increase in maximum stress was observed for all of the cross-linked PUs, although the cross-linker structure was different. Structural change of PU after cross-linking as evidenced by differential scanning calorimetry and infrared spectra was not detected, suggesting that interaction between PU chains remained intact. Shape recovery went to as high as 95 % after cross-linking for all of the cross-linking agents, and shape retention did not improve even if cross-linker was used. The remarkable increase in shape recovery and maximum stress definitely originated from the employment of a cross-linking agent, and the effect of different cross-linker structure on shape memory and mechanical property is discussed.     

Acid-sensitivity and physical properties of polymethylmethacrylate and polyurethane films containing polymeric styryl dye

Solution-cast films of polymethylmethacrylate (PMMA) and polyurethane (PU) containing polymeric styryl dye up to 5% by weight were prepared to investigate their acid-sensitivity and mechanical and thermal properties. Original red samples due to styryl dye turned out yellow very rapidly as they were exposed to acid vapor of hydrochloric acid or p-toluenesulfonic acid. According to UV/VIS spectroscopic measurements, characteristic peak intensities near 430 nm and 520 nm increased or decreased relatively with exposed amounts of acid, respectively. Both PMMA and PU samples showed uniform color distribution due to a good miscibility between polymer and dye which can be evidenced by measurements of glass transition temperature. No significant difference in acid-sensitivity was found between PU and PMMA except relative absorbance. However, dependence of their mechanical properties on dye content was somewhat different with PU or PMMA. In case of PMMA, modulus and breaking stress increased up to about 50% with increasing dye content, whereas those of PU samples showed only slight increase. It was ascribed to whether the matrix polymer was in the glassy or rubbery state.     

The grafting of recycled polyol from waste polyurethane foam onto new polyurethane and its impact on shape recovery and water vapor permeation

Recycled polyols from waste polyurethane (PU) foams were grafted onto PU to improve the properties such as tensile strength, shape recovery, low-temperature flexibility, and water compatibility. The recycled polyol was either purified by column chromatography before grafting or was used directly for grafting. The soft segment melting temperature of PU did not notably increase with the addition of polyol, whereas the glass transition temperature increased with increased polyol content. The tensile strength sharply increased at low polyol content and decreased at high polyol content, while the strain at break did not significantly change with an increase in polyol content. The shape recovery at 10 oC notably improved compared with unmodified PU and remained high after four cyclic tests. Polyol-grafted PU demonstrated better lowtemperature flexibility and reduced the water vapor permeability of PU membranes. Overall, grafting recycled polyol onto PU significantly improved the tensile stress, shape recovery, and low-temperature flexibility of PU.     

Properties of polyurethane-poly(2,2,3,3-tetrafluoropropyl acrylate) triblock copolymer aqueous dispersion and its film cast from the dispersion

Polyurethane-poly(2,2,3,3-tetrafluoropropyl acrylate) (PU-PTFPA) triblock copolymer aqueous dispersions were synthesized by three-step polymerization. Infra-red (IR) data verify the copolymerization between PU and TFPA. The properties of copolymer aqueous dispersion and its film cast from the dispersion have been investigated by transmission electron microscopy (TEM), dynamic light scattering (DLS), and some other physical testing methods. TEM observations indicate that the morphologies of copolymer particles formed in water are almost irregular spherical shape with core-shell structure. DLS results verify that the introduction of TFPA monomer changes the average particle size of copolymer particles. The experimental data demonstrate that the factors influencing the properties of PU-PTFPA triblock copolymer aqueous dispersion and its film cast from the dispersion mainly involve PU content, DMPA content and PTFPA content.     

The exceptional low temperature flexibility of polyurethane copolymer grafted with dimethylphenyl group

The grafted 3,5-dimethylphenyl group remarkably improved the low temperature flexibility of a polyurethane (PU) copolymer. The rigid and blunt shape of 3,5-dimethylphenyl was designed to interrupt molecular interactions and to disturb the close contact between PU chains and, thus, to improve the flexibility at extremely low temperature, while maintaining high and reproducible tensile and shape memory properties at ambient temperature. The effect of the 3,5-dimethylphenyl group on the flexibility of PU was tested at −30 °C together with a linear PU, and the reason for the exceptional flexibility at low temperature is discussed.     

The effects of blend composition and blending time on the ester interchange reaction and tensile properties of PLA/LPCL/HPCL blends

PLA/LPCL/HPCL blends composed of poly(lactic acid) (PLA), low molecular weight poly(ε-caprolactone) (LPCL), and high molecular weight poly(ε-caprolactone) (HPCL) were prepared by melt blending for bioabsorbable filament sutures. The effects of blend composition and blending time on the ester interchange reaction by alcoholysis in the PLA/LPCL/HPCL blends were studied. Their thermal properties and the miscibility due to the ester interchange reaction were investigated by¹H-NMR, DSC, X-ray, and UTM analyses. The hydroxyl group contents of LPCL in the blends decreased by the ester interchange reaction due to alcoholysis. Thus, the copolymer was formed by the ester interchange reaction at 220 °C for 30–60 minutes. The thermal properties of PLA/LPCL/HPCL blends such as melting temperature and heat of fusion decreased with increasing ester interchange reaction levels. However, the miscibility among the three polymers was improved greatly by ester interchange reaction. Tensile strength and modulus of PLA/LPCL/HPCL blend fibers increased with increasing HPCL content, while the elongation at break of the blend fibers increased with increasing LPCL content.     

Effect of chemical structure on the properties of UV-cured polyurethane acrylates films

The effect of compositions of isophorone diisocyanate (IPDI)/4,4′-diphenylmethane diisocyanate (MDI) and polypropylene oxide diol (PPG,M _w : 3000)/1,4-butane diol (BD) on the properties of UV-cured polyurethane acrylate films based on 2-hydroxyethyl acrylate (HEA) was examined. UV-curable polyurethane acrylates were formulated from the prepolymer, trimethylol propane triacrylate (TMPTA) as a reactive diluent, and 1-hydroxycyclohexyl ketone (Irgacure 184) as a photoinitiator. Dynamic mechanical thermal properties and elastic properties of UV-cured polyurethane acrylates was found to depend on the chemical composition of IPDI/MDI and PPG/BD. As the BD content increased, the tensile storage modulus of all series samples increased significantly. The storage modulus increased in the order of samples A (IPDI based samples)>samples B (IPDI/MDI (7/3 molar ratio) based samples)>samples C (IPDI/MDI (5/5 molar ratio) based samples at the same composition. Two distinct loss modulus peaks for all samples are observed owing to the soft segment glass transition temperature (T _gs ) and hard segment glass transition temperature (T _gh ). The difference betweenT _gs andT _gh (ΔT _g ) increases in the order of A>B>C at the same composition. In cycle test, the initial onset strain (%) was found to decrease with increasing BD content in PPG/BD and with increasing MDI content in IPDI/MDI.     

The characterization and effects of the alizarin series dyes grafted to polyurethane copolymers on the photoluminescence and low temperature flexibility

Alizarin-series dyes (alizarin, alizarin red S, alizarin yellow GG, or mordant orange) are grafted onto shape memory polyurethane (PU) through an allophanate bonding, and the photoluminescence and the low-temperature flexibility of the resulting PUs are tested. The PU is mainly composed of 4,4′-methylenebis(phenylisocyanate) (MDI), poly (tetramethyleneglycol) (PTMG), and 1,4-butanediol (BD), and the dye is connected, through another MDI, to the carbamate moiety of the PU chain. The PUs with different dye contents are characterized, and their shape recovery and photoluminescence properties are compared. With respect to the tensile mechanical properties, the maximum stress increases up to 50 MPa, and the strain remains above 1000 % even after the dye is grafted onto the PU. The shape recovery is as high as 99 %, and the shape retention improves as the dye content increases. Finally, the photoluminescence of the PUs is demonstrated by the luminescent light emission test, and the dye-grafted PU shows excellent low-temperature flexibility compared with that of linear PU.     

Small diameter Polyurethane vascular graft reinforced by elastic weft-knitted tubular fabric of polyester/spandex

Small diameter vascular grafts were fabricated from pure Polyurethane (PU) as well as PU reinforced with a tubular weft-knitted fabric. The tensile properties of the reinforced composite vascular grafts were compared with that of the tubular fabric itself and the pure PU vascular grafts. The elasticity and strength of the reinforced vascular grafts were improved compared with the tubular fabric. Strength of the reinforced vascular grafts was 5–10 times of the strength of the pure PU vascular grafts. Expanding the tubular fabric to increase the inner diameter of the reinforced vascular graft reduced the graft’s strength and initial modulus, but the difference was reduced as the PU content was increased. For grafts of the same inner diameter, increasing the PU content increased the thickness and strength of the graft wall, which led to a general increase in the strength and initial modulus of the composite vascular grafts.