Endurance of linear and cross-linked shape memory polyurethane under rigorous hydrolysis conditions

Linear shape memory polyurethane (SMPU) copolymers synthesized from 4,4′-methylene bis(phenylisocyanate) (MDI), poly(tetramethyleneglycol) (PTMG), and 1,4-butanediol (BD), and cross-linked SMPU by glycerol were being tested under the severe hydrolysis conditions for the tensile mechanical properties and the shape memory effect for 6 months. Tensile mechanical properties and shape memory effect were not substantially decreased after 6 months of storage, and SMPU structure was not affected during the long-term hydrolysis test as evidenced by IR, DSC, contact angle, and viscosity. Hydrolysis test was important in proving the durability of SMPU before application of SMPU in aqueous surrounding.     

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.     

Flexible cross-linking of shape memory polyurethane by a long polyethyleneglycol spacer

Flexible linking of shape memory polyurethane (SMPU) was tried with a long polyethyleneglycol (PEG) spacer (PEG-600 or PEG-1000) longer than the one tested before (PEG-200). The SMPU was composed of MDI, polytetramethyleneglycol (PTMG), 1,4-butanediol (BD), glycerol, and PEG as a spacer. PEG connected the glycerol hydroxyl group of polyurethane chain via MDI as a connecting agent. Significant increase in both maximum stress and strain at break was attained simultaneously, because of the flexible structure of PEG. Especially, it was shown in the stress-strain curve that the PEG-linked SMPU showed similar behavior and superior tensile mechanical properties to natural rubber. Shape recovery of the PEG-linked SMPU went up to 96 %, and shape retention remained low compared to linear one. PEG-1000 was better than PEG-600 in enhancing the overall tensile strength and shape memory of SMPU.     

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.     

Synthesis of thermotropic polyurethanes containing imide units and their mesophase behavior

Thermotropic polyurethanes were synthesized from 1,6-hexane diisocyanate (HDI) as a diisocyanate, 1,6-hexane diol (HD), and rigid diols containing imide unit such as N,N′-bis(4-hydroxyphenyl)-3,4,3′,4′-biphenyl-dicarboxyimide (BPDI) or bis-N-(4-hydroxyphenyl)-4,4′-oxydiphthalimide (ODPI). The effects of structure difference between BPDI and ODPI and composition of HD/BPDI (ODPI) on the thermal and liquid crystalline behavior were studied. Thermotropic polyurethanes with an inherent viscosity of 0.59–0.70 were obtained. The melting temperature of BPDI-based polyurethanes were in the range of 150–290°C, however, those of ODPI-based polyurethanes were in the range of 150–190°C. All the polyurethanes based on ODPI (25–100 mole %) clearly exhibited a stable liquid crystalline phase, and BPDI-based polyurethane having 5–25% of BPDI showed a mesophase. The melting and isotropization temperatures (T _m , T _i ) andΔT(T _i −T _m ) increased with increasing BPDI and ODPI content. The polyurethanes based on BPDI has higher melting points and thermal stability compared to ODPI-based polyurethanes.     

Preparation and properties of waterborne polyurethanes based on triblock glycol (CL)4.5-PTMG-(CL)4.5 for water vapor permeable coatings: Effect of soft segment content

A series of waterborne polyurethanes (WBPU) were prepared from 4,4-dicyclohexylmethane diisocyanate (H₁₂MDI), 2,2-bis(hydroxylmethyl) propionic acid (DMPA), ethylenediamine (EDA), triethylamine (TEA), and triblock glycol [TBG, (ε-caprolactone)_4.5-poly(tetramethylene ether) glycol (MW=2000)-(ε-caprolactone)_4.5: (CL)_4.5-PTMG-(CL)_4.5, MW=3000] as a soft segment. Two melting peaks of TBG at about 14°C and 38°C were observed indicating the presence of two different crystalline domains composed of CL and PTMG dominant component. The effect of soft segment content (60–75 wt%) on the colloidal properties of dispersion, and thermal and mechanical properties of WBPU films, the water vapor permeability (WVP) and water resistance (WR) of WBPU-coated Nylon fabrics, and the adhesive strength of WBPU-coated layer and Nylon fabrics was investigated. As soft segment contents increased, the water vapor permeability of WBPU-coated Nylon fabrics increased from 3615 to 4502 g/m²day, however, the water resistances decreased from 1300 to 500 mmH₂O.     

Morphology and mechanical properties of thermo-molded bioplastics based on glycerol-plasticized wheat gliadins

Glycerol-plasticized wheat gliadin bioplastics were prepared through thermo-molding method. The effect of glycerol content on the morphology and the mechanical properties of wheat gliadin bioplastics was studied. Morphology, tensile properties (tensile strength and elongation at break), dynamic mechanical properties and rheological properties were evaluated in relation to glycerol content. Experimental results reveal that the morphology, the glass transition temperatures (T_g) of both the gliadin-rich and the glycerol-rich domains and the tensile properties are closely linked to the glycerol content. The time–temperature superposition (TTS) fails to be applied to the dynamic loss modulus G″ (all temperatures) and the dynamic storage modulus G′ (above 80 °C) of wheat gliadin bioplastics.     

Comparision of the properties of UV-cured polyurethane acrylates containing different diisocyanates and low molecular weight diols

UV-curable polyurethane acrylate prepolymers were prepared from diisocyanates [isophorone diisocyanate (IPDI), 2,4-toluene diisocyanate (TDI), or 4,4′-dicyclohexylmethane diisocyanate (H₁₂MDI)], diols [ethylene glycol (EG), 1,4-butane diol (BD), or 1,6-hexane diol (HD)], polypropylene glycol as a polyol. UV-curable mixtures were formulated from the prepolymer (90 wt%), reactive diluent monomer trimethylol propane triacrylate (10 wt%), and photoinitiator 1-hydroxycyclohexyl ketone (3 wt% based on prepolymer/diluent). The effects of different diisocyanates/low molecular weigh diol on the dynamic mechanical thermal properties and elastic recovery of UV-cured polyurethane acrylate films were examined. The tensile storage modulus increased a little in the order of EG>BD>HD at the same diisocyanate. Two loss modulus peaks for all samples are observed owing to the glass transition of soft segments (T _gs ) and the glass transition temperature of hard segments (T _gh ). For the same diisocyanate,T _gs decreased, however,T _gh increased, in the order of HD>BD>EG. The elastic recovery also increased in the order of HD>BD>EG at the same diisocyanate. In case of same diols,T _gh increased in the order of H₁₂MDI>TDI>IPDI significantly. The ultimate elongation and elastic recovery increased in the order of TDI>IPDI>H₁₂MDI at the same diol.     

Synthesis and properties of shape memory polyurethane nanocomposites reinforced with poly(ɛ-caprolactone)-grafted carbon nanotubes

Nanocomposites of polyurethane (PU) and multi-walled carbon nanotubes (MWNTs) were prepared via in-situ polymerization of poly(ɛ-caprolactone)diol (PCL)-grafted-MWNTs, 4,4′-methylene bis(phenyl isocyanate), and 1,4-butanediol. The grafting of PCL onto MWNTs was confirmed by Fourier transform infrared (FT-IR) spectroscopy and transmission electron microscopy (TEM). The nanocomposites showed more improved mechanical properties compared to conventional nanocomposites with the same MWNT loading. The thermo-responsive shape recovery as measured in a cyclic tensile test was observed to be approximately 80 % for in-situ nanocomposites, though it showed a reduced trend as the wt% of MWNTs increased. X-ray diffraction investigation also showed that the addition of MWNTs into the polyurethane increased the crystallinity. Scanning electron microscopy and TEM measurements showed better dispersion of MWNTs in the nanocomposites synthesized using in-situ method. Consequently, the presence of PCL-g-MWNTs made an important contribution to the enhancement of the mechanical and shape memory properties of polyurethane.     

Thermoplastic starch modified during melt processing with organic acids: The effect of molar mass on thermal and mechanical properties

Thermoplastic starch (TPS) was modified with ascorbic acid and citric acid by melt processing of native starch with glycerol as plasticizer in an intensive batch mixer at 160 °C. It was found that the molar mass decreases with acid content and processing time causing the reduction in melting temperature (T_m). As observed by the results of X-ray diffraction and DSC measurements, crystallinity was not changed by the reaction with organic acids. T_m depression with falling molar mass was interpreted on the basis of the effect of concentration of end-chain units, which act as diluents. FTIR did not show any appreciable change in starch chemical compositions, leading to the conclusion that the main changes observed were produced by the variation in molar mass of the material. We demonstrated that it is possible to decrease melt viscosity without the need for more plasticizer thus avoiding side-effects such as an increase in water affinity or relevant changes in the dynamic mechanical properties.     

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.     

Dynamic mechanical analysis of randomly oriented short bagasse/coir hybrid fibre-reinforced epoxy novolac composites

Hybrid composites of epoxy novolac reinforced with short bagasse fibres and short coir fibres were prepared. The mechanical and dynamic mechanical properties of these bagasse-coir hybrid fibres reinforced epoxy novolac composites were investigated with reference to different layering patterns of the composites. The tensile properties of the tri-layer composites are recorded higher than those of the bi-layer composites, whereas the flexural properties of the tri-layer composites are lower than bi-layer composites. The tensile strength of the intimate mix composite is comparable with trilayer composite having bagasse as skin material. The effect of layering pattern on storage modulus (E′), damping behavior (tan δ), and loss modulus (E″) was studied as a function of temperature and frequency. The E′ values of the bi-layer composites are recorded lower than those of tri-layer (bagasse/coir/bagasse) and intimately mixed hybrid composites. The minimum E′ value is obtained for the composites made with coir as skin layer. Bi-layer composite shows maximum damping property. The theoretical modeling showed good correlation with experimental results at above glass transition temperature (T _g ), while theoretical model deviates experimental data at lower T _g . The Arrhenius relationship has been used to calculate the activation energy of the glass transition of the composites.     

Development of biodegradable films based on blue corn flour with potential applications in food packaging. Effects of plasticizers on mechanical,thermal, and microstructural properties of flour films

In the present study, blue corn flour films were developed. The cereal grain’s total composition (excluding the pericarp) is used to obtain the films. The plasticizing effects of two different polyols such as glycerol and sorbitol on the mechanical, thermal, and microstructural properties of flour films were researched. The results showed that films plasticized with sorbitol had better mechanical properties and less affinity for water than those plasticized with glycerol. The sorbitol-plasticized films were more rigid and did not lose their integrity when immersed in water. The ATR-FTIR spectra of blue corn flour plasticizer with sorbitol showed the presence of the additional band at 1745 cm⁻¹ characteristic of the vibrational carbonyl peak, which confirms the chemical linkages between sorbitol and a polymeric matrix. The effect of the plasticizer on the glass transition temperature (T_g) was characterized using differential scanning calorimetry (DSC). T_g decreased as the plasticizer content increased. Plasticized glycerol films showed lower T_g values than those with sorbitol. SEM observations showed that it was necessary to add plasticizer to maintain film integrity. The sorbitol-plasticized flour films revealed better adhesion between phases, and these films showed a compact structure.     

The effect of an external electric field on solid-state phase transition of (P(VDF/TrFE)(72/28)

The copolymers of vinylidene fluoride and trifluoroethylene (P(VDF/TrFE)) with VDF content of 50–80 mole % can be applied to the field of nonvolatile ferroelectric polymeric random access memory (FePoRAM) devices, since they exhibit stable ferroelectricβ-phase at room temperature with spontaneous polarization of the C-F dipoles towards an external electric field greater than the coercive field. Many researchers have already reported the molecular structures and dynamics of the ferroelectric (F) crystalline phase and the unique change in chain conformation between polarF phase and non-polar paraelectric (P) phase near their Curie transition temperature (T _c) which is dependent on factors such as VDF content and annealing treatment conditions. The effect of external electric field strength on theF⇔P crystalline phase transition in P(VDF/TrFE)(72/28) random copolymer samples of nanometer thickness was investigated. Capacitance of 250 nm thick sample measured as a function of heating-cooling under varying external electric field strength exhibited increasingT _c’s during heating (T _c ^↑ ) and cooling (T _c ^↓ ) under an applied electric field of more than 0.03 MV/cm. Applying cyclic bias electric field (+1 to −1 MV/cm) for samples kept isothermally at just above theirT _c(T _c ^↓ ) during cooling, we were able to observe the field-inducedP→F phase transition. With increasing cycles of the applied electric field for sample maintained just above (T _c ^↓ ), the bistableC-E hysteresis was observed and the phase change fromP→F is irreversible even after the electric field is removed. However, for samples kept well above (T _c ^↓ ) and nearT _m (100 °C and 120°C respectively) during cooling, theF-phase initially formed through the field-induced phase transition is reversibly transformed to theP-phase when the applied electric field is removed. Drastic changes were observed in both coercive field (E _c) and remanent polarization (P _r) values during heating and cooling near theT _c range due to theF⇔P phase transition and the results are reported in detail here.     

Water solubility,thermal characteristics and biodegradability of extruded starch acetate foams

Starch based foams have been studied as replacements for non-degradable expanded polystyrene (EPS) as loose-fill packaging material because of starch’s total degradation and low cost. However, starch’s hydrophilicity, poor mechanical properties and dimensional stability limited their applications. Acetylated starch with a high degree of substitution (DS) is an alternative. Starch acetates with DS 1.11, 1.68, and 2.23 were extruded with either water or ethanol as solvents. The effects of DS and type of solvent on the starch acetate foam’s water absorption index (WAI), water solubility index (WSI), thermal behavior (glass transition temperature [T_g], melting temperature [T_m], and thermal decomposition temperature), and biodegradability were investigated. There was a significant interaction (P < 0.05) between solvent type and DS on WAI and WSI of the foams. As DS increased from 1.11 to 2.23, WAI and WSI increased when ethanol was used as solvent and decreased when water was used as solvent. The T_g values of starch decreased with acetylation and with increasing DS, but increased with extrusion. Acetylation and extrusion increased the thermal stability of the foams. The rate of biodegradation of the foams decreased with increasing DS. The foams, extruded with ethanol, had higher degradation rates than those with water.     

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.     

Characterization and mechanical properties of prepolymer and polyurethane block copolymer with a shape memory effect

The prepolymer and the final polyurethane (PU) block copolymer were synthesized by reacting 4,4-methylene bis(phenylisocyanate) with poly(tetramethylene glycol) and the prepolymer with 1,4-butanediol as a chain extender, respectively, to investigate the relation between phase separation and it’s resulting properties. According to FT-IR data, the phase separation of hard and soft segments in the prepolymer and the PU block copolymer grew bigger by increasing the hard segment content, and the PU showed more dominant phase separation than the prepolymer. The heat of fusion due to soft segments decreased in both the prepolymer and the PU by increasing the hard segment content, whereas the heat of fusion due to hard segments increased in the PU did not appear in the prepolymers. The breaking stress and modulus of the prepolymer increased by increasing the hard segment content, and the elongation at break decreased gradually, and the PU showed the highest breaking stress and modulus at 58 % hard segment content. However, the best shape recovery of the PU was obtained at 47 % hard segment content due to the existence of proper interaction among the hard segments for shape memory effect. Consequently, the mechanical properties and shape memory effect of the PU were influenced by the degree of phase separation, depending on the incorporation of chain extender as well as the hard segment content.     

The acid doping behavior of polybenzimidazole membranes in phosphoric acid for proton exchange membrane fuel cells

The acid doping behavior of poly(2,2′-(m-phenylene)-5,5′-bibenzimidazole) (PBI) membranes in aqueous phosphoric acid was studied at room temperature. It was found that doping phosphoric acid in the membrane obeyed a multimolecular layer absorption mechanism proposed in this work. Equation, i.e., 1/[L ^T ] _B =(1 − C ₀/17.5)/2.1, was presented to describe the relationship of the acid doping level of membranes and the concentration of the doping acid in a range of 2–14 mol L⁻¹. The acid doping kinetics as well as the influence of the doped acids on the conductivity and mechanical strength of the PBI membranes was investigated.     

Synthesis of hydrophilic copolyesters and the characterization of their moisture-related cooling properties

A series of copolyesters (Co-PETs) containing poly(ethylene glycol) (PEG), 5-sodiumsulfodimethyl isophthalate (DMS), and dimethyl isophthalate (DMI) were synthesized via the conventional two-step melt-polycondensation method. The synthesized Co-PETs were characterized by ¹H-NMR spectroscopy, FT-IR spectroscopy, differential scanning calorimeter (DSC), and thermogravimetric analyzer (TGA). The DSC results showed that the melting temperature (T _m) and the heats of fusion (ΔH _m) of Co-PETs decreased with increasing the DMS content in Co-PET, while the inclusion of PEG did not affect their thermal properties significantly. The water absorption and the water contact angle of the Co-PET films were found to be significantly affected by the DMS content rather than PEG content. The moisture-related cooling properties of the fabric samples made of Co-PET 5 as well as PET were evaluated by using liquid moisture management tester (MMT) and Q _max measurements. The MMT and Q _max results indicated that Co-PET 5 fabric containing DMS 1.0 mol% and PEG 10.0 wt% in Co-PET seemed to be a good candidate for the fabric having durable cooling effects.     

Miscibility with thermal and mechanical properties for poly(pentamethylene 2,6-naphthalate) and poly(heptamethylene 2,6-naphthalate) blends

We have prepared the blends of poly(pentamethylene 2,6-naphthalate) (PPN) with poly(heptamethylene 2,6-naphthalate) (PHepN) by solution blending method and investigated their glass transition behaviour, melting behaviour, and tensile properties. It was observed that the blends of PPN/PHepN(9/1) and PPN/PHepN(1/9) have a single glass transition, reflecting a homogeneous phase, whereas those of PPN/PHepN(7/3), PPN/PHepN(5/5), and PPN/PHepN(3/7) exhibit double glass transitions, representing the existence of two phases. The PPN homopolymer annealed below 90 °C shows triple melting peaks (T _m1, T _m2, and T _m3). It was proved that T _m1 is attributed to melting of thin lamellar formed during secondary crystallization process, T _m2 to melting of thick lamellar created during primary crystallization, and T _m3 to melting of crystals recrystallized after melting the primary crystals at T _m2. For the annealed PHepN homopolymer, double melting endotherms (T _m1 and T _m2) were observed, caused by dual lamellar population with different thickness, i.e. T _m1 corresponding to the melting of secondary crystal and T _m2 to primary one. The Hoffman-Weeks plots, applied to the melting of primary crystals (T _m2s), indicate that the equilibrium melting temperatures of PPN homopolymer, PPN/PHepN(9/1), and PPN/PHepN(7/3) blends are same to be 147 °C, and those of PHepN homopolymer, PPN/PHepN(1/9), and PPN/PHepN(3/7) blends to be 145 °C. Both the glass transition and melting behaviours demonstrate that the PPN/PHepN blend system is partially miscible. In addition, both the modulus and strength for the blends almost follow additive rule against blend composition, indicating that the PPN/PHepN blends are mechanically compatible over all blend compositions.