Mechanical and dielectric properties of segmented polyurethane elastomers containing chemical crosslinks in the hard segment

Mechanical and dielectric properties of two series of segmented polyurethanes having soft segment concentration of 50 and 70% and a varying degree of crosslinking through the hard segment were studied. The degree of crosslinking in each series was varied by varying the butane diol/trimethylol propane ratio in the chain extender mixture. Tensile strength, elongation at break decrease, but elastic recovery increases monotonically with increasing crosslinking. The plateau modulus in the dynamic mechanical test decreases and then increases with increasing TMP content. Crosslinking causes broadening of the soft segment glass transition as seen by permittivity and loss factor measurements. It also affects high temperature behavior (above the glass transition of the hard segment); it lowers permittivity, loss factor, and ionic conductivity. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36 : 237–251, 1998     

Comparison of the properties of waterborne polyurethane/multiwalled carbon nanotube and acid‐treated multiwalled carbon nanotube composites prepared by in situ polymerization

A series of waterborne polyurethane (WBPU)/multiwalled carbon nanotube (CNT) and WBPU/nitric acid treated multiwalled carbon nanotube (A‐CNT) composites were prepared by in situ polymerization in an aqueous medium. The optimum nitric acid treatment time was about 0.5 h. The effects of the CNT and A‐CNT contents on the dynamic mechanical thermal properties, mechanical properties, hardness, electrical conductivity, and antistatic properties of the two kinds of composites were compared. The tensile strength and modulus, the glass‐transition temperatures of the soft and hard segments (T_gs and T_gh, respectively), and ΔT_g (T_gh − T_gs) of WBPU for both composites increased with increasing CNT and A‐CNT contents. However, these properties of the WBPU/A‐CNT composites were higher than those of the WBPU/CNT composites with the same CNT content. The electrical conductivities of the WBPU/CNT1.5 and WBPU/A‐CNT1.5 composites containing 1.5 wt % CNTs (8.0 × 10⁻⁴ and 1.1 × 10⁻³ S/cm) were nearly 8 and 9 orders of magnitude higher than that of WBPU (2.5 × 10⁻¹² S/cm), respectively. The half‐life of the electrostatic charge (τ_1/2) values of the WBPU/CNT0.1 and WBPU/A‐CNT0.1 composites containing 0.1 wt % CNTs were below 10 s, and the composites had good antistatic properties. From these results, A‐CNT was found to be a better reinforcer than CNT. These results suggest that WBPU/A‐CNT composites prepared by in situ polymerization have high potential as new materials for waterborne coatings with good physical, antistatic, and conductive properties. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 3973–3985, 2005     

Functionalized SWNT/polymer nanocomposites for dramatic property improvement

In this paper, we present results for polymer nanocomposites of poly‐ (methyl methacrylate) (PMMA) and amide‐functionalized SWNTs. The results demonstrate that even at very low loadings, 1 wt % (0.5 vol %), the mechanical and electrical properties are significantly improved. The improvement over PMMA properties exceeds the theoretical bounds for composites with the same volume fraction loading of randomly oriented, straight, individually dispersed nanotubes. The modeling and experimental results thus suggest that the nanotube bundles are well dispersed in the polymer matrix, that the functionalization significantly improves interaction with polymer, and that the interphase formed has improved mechanical properties over that of the matrix material. Loss modulus results indicate a significant difference between functionalized and nonfunctionalized tubes in the composite. Functionalized tubes result in a composite in which relaxation mechanisms are shifted by 30 °C from that of the matrix material, indicating extensive interphase regions and absence of PMMA with bulk properties. Unfunctionalized composites demonstrate a broadening of relaxation modes, but still retain the signature of bulk PMMA properties. These data suggest a morphological difference with a discrete interphase layer in unfunctionalized composites and a fully transformed matrix in the case of functionalization. This difference is consistent with electrical and mechanical property data. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 2269–2279, 2005     

Effects of graphene and carbon nanotube fillers on the shear properties of epoxy

The effects of quantity of graphene and carbon nanotube‐based fillers and their pendant functional groups on the shear properties of a thermoset epoxy were investigated. Two novel functionalized graphenes, one with epoxy functionality and the other with an amine, are synthesized for this purpose. Nanocomposites are prepared at concentrations of 0.5, 1, 2, 3, 5, and 10 wt % and the effects of functionalization on the homogeneity of dispersion and the shear mechanical properties are investigated. The properties of the epoxy nanocomposites containing epoxy‐ and amine‐functionalized graphene are compared with those containing graphene oxide, Claisen‐functionalized graphene, neat multiwalled carbon nanotubes (MWNTs), three types of epoxy‐functionalized MWNT (EpCNT), and the unfilled epoxy. One of the EpCNT ( EpCNT3 ) was found to increase the plateau shear storage modulus by 136% (1.67–3.94 MPa) and the corresponding loss modulus by almost 400% at a concentration of 10 wt %. Several other fillers were also found to increase shear properties at certain concentrations. A hybrid system of EpCNT3 and graphite was also studied, which improved the storage modulus by up to 51%. SEM images reveal a correlation between thorough dispersion of the additive and enhancement of shear modulus. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013 , 51, 997–1006     

Molecular mobility of free‐radical‐functionalized carbon‐nanotube/siloxane/poly(urea urethane) nanocomposites

Multiwalled carbon nanotubes (MWNTs) were functionalized by a free‐radical reaction of vinyltriethoxysilane and were blended with poly(urea urethane) (PUU) containing poly(dimethylsiloxane) as a soft segment. PUU was end‐capped with aminopropyltriethoxysilane (A‐silane) or phenyltriethoxysilane (P‐silane).A‐silane‐end‐capped PUU was covalently bonded to functionalized MWNTs, whereas P‐silane‐end‐capped PUU was noncovalently bonded to pristine MWNTs by a π–π interaction. Fourier transform infrared, Raman spectra, and thermogravimetric analysis confirmed the functionalization of MWNTs. The results showed that the optimal reaction time of the functionalization of MWNT was 8 h, and the organic content of the modified carbon nanotubes reached 35.22%. Solid‐state nuclear magnetic resonance and dynamic mechanical analysis were used to investigate the molecular structure and molecular mobility of the carbon‐nanotube/PUU nanocomposites. A‐silane PUU covalently bonded to MWNTs showed a considerable reduction in the molecular motion of the soft segment, which led to the glass‐transition temperature decreasing from ?117 to ?127 °C as MWNTs were incorporated. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 6084–6094, 2005     

Preparation and electromagnetic interference shielding characteristics of novel carbon‐nanotube/siloxane/poly‐(urea urethane) nanocomposites

Poly(urea urethane) (PUU) with a poly(dimethylsiloxane) soft segment was synthesized. Different types of conductive fillers—carbon nanotube (CNT), silver‐coated carbon nanotube (CNT–Ag), and nickel‐coated carbon nanotube (CNT–Ni)—were blended with PUU to form partially conductive polymer composites. The results showed that highly conductive metals could improve the conductivity of CNTs significantly. When the filler contents were 3, 4, and 5 parts per hundred parts of resin (phr), the PUU/CNT composites possessed electromagnetic interference shielding effectiveness (SE) at 8.5, 28.4, and 26.0 dB as the electromagnetic wave frequencies were 12.3, 16.2, and 15.9 GHz, respectively. SE of the composites that contained CNT–Ni and CNT–Ag increased with the filler loading. At the same modified‐CNT loading, the CNT–Ni‐filled composites had a higher SE than those filled with CNT–Ag. Tensile stresses ranged from 5.7 to 15.6 MPa (a 177.3% increase) when the CNT concentration reached 8 phr. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 345–358, 2005     

Thermal and mechanical characteristics of polyurethane/curaua fiber composites

Polyurethane composites reinforced with curaua fiber at 5, 10 and 20% mass/mass proportions were prepared by using the conventional melt-mixing method. The influence of curaua fibers on the thermal behavior and polymer cohesiveness in polyurethane matrix was evaluated by dynamic mechanical thermal analysis (DMTA) and by differential scanning calorimetry (DSC). This specific interaction between the fibers and the hard segment domain was influenced by the behavior of the storage modulus E′ and the loss modulus E″ curves. The polyurethane PU80 is much stiffer and resistant than the other composites at low temperatures up to 70°C. All samples were thermoplastic and presented a rubbery plateau over a wide temperature range above the glass transition temperature and a thermoplastic flow around 170°C.     

Segmented polyurethanes derived from novel siloxane–carbonate soft segments for biomedical applications

A novel macrodiol based on mixed silicone and carbonate chemistries was synthesized and used as a soft segment precursor in the synthesis of two series of segmented polyurethane (PU) copolymers varying in hard segment content and soft segment molecular weight. The hard segments in these copolymers were derived from 4,4‐methylene diphenyl diisocyanate and 1,4‐butane diol. The phase transitions, microphase separation behavior, and mechanical properties of the copolymers were investigated using a variety of experimental methods. When compared with segmented PU copolymers having predominately poly(dimethyl siloxane) soft segments, these siloxane–carbonate soft segment copolymers exhibit enhanced intersegment mixing, and consequently relatively low mechanical modulus. With relatively low modulus and siloxane units in the soft phase, the siloxane–carbonate PUs have potential for use in cardiac and orthopedic biomedical applications. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

Theoretical modeling of the relationship between Young's modulus and formulation variables for segmented polyurethanes

We describe a new modeling approach to prediction of Young's modulus of segmented polyurethanes. This approach combines micromechanical models with thermodynamic considerations based on the theory of block copolymers. The resulting model predicts both the equilibrium morphology and the “ideal” Young's modulus of a segmented polyurethane polymer as a function of its formulation (hard segment chemical structure, hard segment weight fraction, soft segment equivalent weight) and temperature. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 2123–2135, 2007     

Metallized nanotube polymer composites via supercritical fluid impregnation

Although many metal decorated nanotubes and nanowires appear in the literature, well‐dispersed metal decorated nanotube polymer composites have rarely been reported because of the excessive density mismatch between the decorated nanotubes and polymer matrix. Here, we report a novel method to prepare well‐dispersed, highly functional, metallized nanotube polymer composites (MNPCs) that possess remarkably improved electrical conductivity and mechanical toughness. The MNPCs are prepared by supercritical fluid impregnation of an organometal compound into a premade well‐dispersed single wall carbon nanotube‐polymer composite film. The infused precursor preferentially migrates towards the nanotubes to undergo spontaneous reduction and form nanometer‐scale metal particles leading to an increase in the conductivity of the MNPC films. The environmentally friendly supercritical fluid impregnation process significantly improved the toughness of the composite films, regardless of the presence of metal. Additional functionality can be imparted into the resulting MNPC by infusing other precursors such as magnetic and catalytic metal compounds. © 2011 Wiley Periodicals, Inc.* J Polym Sci Part B: Polym Phys, 2012     

Morphology and tensile properties of model thermoplastic polyurethanes with MDI/butanediol based monodisperse hard segments

Morphology and tensile properties of model thermoplastic polyurethanes (TPUs) containing polyisobutylene (PIB) or poly(tetramethylene oxide) (PTMO) based soft segment and 4,4‐methylene bis(phenyl isocyanate) (MDI) and 1,4‐butanediol (BDO) based monodisperse hard segments (HSs), consisting of exactly two to four MDI units extended by BDO, were investigated. Using FT‐IR spectroscopy, increased hydrogen bonded C?O fraction was observed in model TPUs as the HS size increased. The hydrogen bonded C?O fraction was higher in PIB based TPUs compared with PTMO based TPUs, indicating higher phase separation in PIB based TPUs. The morphology of TPUs was investigated using AFM phase imaging, which showed ribbon‐like or interconnected hard domains in PTMO based model TPUs and randomly dispersed hard domains in PIB based model TPUs. SAXS revealed that the degree of phase separation in the model TPUs was higher than in their polydisperse analogues. Domain spacing as well as interfacial thickness increased with the increasing HS size, and both values were higher in PTMO based TPUs. The tensile analysis indicated that model TPUs exhibited higher modulus and slightly higher elongation compared with their polydisperse analogues. Only in PTMO based model TPUs, strain induced crystallization was observed above 300% elongation. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 2485–2493     

Synthesis and characterization of a novel biodegradable,thermoplastic polyurethane elastomer

A series of biodegradable, thermoplastic polyurethane elastomers poly(?‐caprolactone‐co‐lactide(polyurethane [PCLA–PU] were synthesized from a random copolymer of L ‐lactide (LA) and ?‐caprolactone (CL), hexamethylene diisocyanate, and 1,4‐butanediol. The effects of the LA/CL monomer ratio and hard‐segment content on the thermal and mechanical properties of PCLA–PUs were investigated. Gel permeation chromatography, IR, ¹³C NMR, and X‐ray diffraction were used to confirm the formation and structure of PCLA–PUs. Through differential scanning calorimetry, tensile testing, and tensile‐recovery testing, their thermal and mechanical properties were characterized. Their glass‐transition temperatures were below ?8 °C, and their soft domains became amorphous as the LA content increased. They displayed excellent mechanical properties, such as a tensile strength as high as 38 MPa, a tensile modulus as low as 10 MPa, and an elongation at break of 1300%. Therefore, they could find applications in biomedical fields, such as soft‐tissue engineering and artificial skin. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 5505–5512, 2006     

Structure and deformation recovery of the thermoplastic polyurethane spherulite

We investigated the structure and deformation behavior of the thermoplastic polyurethane (TPU) spherulite by optical microscopy, tensile testing, Hv light scattering, and small angle X‐ray scattering. The TPU spherulite structure obtained by melt crystallization was coarse consisting of bundle‐like structure containing hard segment (HS) lamellar domain in which the HS domains were stacked and the HS chain direction was perpendicular to the longitudinal axis of the HS domain. By stretching, the spherulite was deformed to ellipsoidal one and the stacked HS lamellar domains were tilted in the stretching direction. The deformed spherulite and the tilted HS domain in the spherulite were recovered to the unstretched state by retraction. The recovery of the structure is ascribed to the characteristic spherulite structure consisting of rubbery soft segment matrix physically cross‐linked with the stacked HS domain. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 1585–1594     

Aspect ratio effects of multi‐walled carbon nanotubes on electrical,mechanical, and thermal properties of polycarbonate/MWCNT composites

Two multi‐walled carbon nanotubes (MWCNTs) having relatively high aspect ratios of 313 and 474 with approximately the same diameter were melt mixed with polycarbonate (PC) in a twin‐screw conical micro compounder. The effects of aspect ratio on the electrical, mechanical, and thermal properties of the PC/MWCNT composites were investigated. Electrical conductivities and storage moduli of the filled samples are found to be independent of the starting aspect ratio for these high aspect ratio tubes; although the conductivities and storage moduli are still significantly higher than values of composites made with nanotubes having more commercially common aspect ratios of ～100. Transmission electron microscopy results suggest that melt‐mixing reduces these longer nanotubes to the same length, but still approximately two times longer than the length of commercially common aspect ratio tubes after melt‐mixing. Molecular weight measurements show that during melt‐mixing the longer nanotubes significantly degrade the molecular weight of the polymer as compared to very similar nanotubes with aspect ratio ～100. Because of the molecular weight reduction glass transition temperatures predictably show a large decrease with increasing nanotube concentration. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014 , 52, 73–83     

Oxygen permeability in thermoplastic polyurethanes

The thermal and oxygen transport properties of a series of thermoplastic polyurethanes (TPUs) based on 4,4′‐methylene diisocyanate (MDI) and 1,4‐butanediol (BD) as hard segments, and poly(tetramethylene glycol) (PTMG) or poly(butylene adipate) (PA) as soft segments, are studied. Oxygen permeabilities (P) of both polyester‐based and polyether‐based TPUs increase with decreasing hard segment fractions. Oxygen solubility (S) and diffusivity (D) can be derived from permeation curves. S correlates with the amount of excess free volume as determined by the difference between glass‐transition and testing temperatures (i.e., the degree of super cooling) and decreases with the increased T_g in polyester‐based TPUs. The intensity of low temperature gamma transition reflects the activation energy for D; the higher the intensity is, the lower D is annealed TPU samples exhibited higher oxygen permeabilities as well as lower storage moduli at room temperature, despite modest increases in overall crystallinity. Dedensification of the soft segment phase during annealing/crystalline phase growth is the most likely explanation for loss of mechanical and barrier properties after annealing as partially confirmed by Fourier transform infrared spectroscopy. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

Preparation and characterization of N‐methyl‐substituted polyurethane

We prepared N‐methyl‐substituted polyurethanes with different substitution degrees from sodium hydride, methyl p‐toluene sulfonate, and polyether–polyurethane containing poly(oxytetramethylene) glycol, 4,4′‐diphenylmethane diisocyanate, and 1,4‐butanediol. The chemical structures were characterized with Fourier transform infrared and ¹H NMR. To investigate the effects of the N‐substitution degree on the morphology, thermal stability, and mechanical properties, we used differential scanning calorimetry, thermogravimetric analysis, and a universal testing machine. As the substitution degree increased, the new free (1708 cm^?1) and bonded (1650 cm^?1) carbonyl peaks increased. There was no bonded carbonyl peak in fully substituted polyurethane because the urethane groups had no hydrogen. At a small substitution degree, we observed a slight increase in the glass‐transition temperature and decrease in the endotherms of soft‐segment and hard‐segment domains due to the decrease in the hard‐segment domain and the increase in the urethane groups in the soft‐segment domain. The hard‐segment domain decreased and then disappeared as the N‐methyl substitution degree increased. These changes in the morphology resulted (1) in decreased modulus and tensile strength for the films because of the decrease in physical crosslinking points and (2) improved thermal stability as the substitution degree increased. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 4077–4083, 2002     

Functionalization of multiwalled carbon nanotube via surface reversible addition fragmentation chain transfer polymerization and as lubricant additives

Polymer‐grafted multiwalled carbon nanotube (MWCNT) hybrid composite which possess a hard backbone of MWCNT and a soft shell of brush‐like polystyrene (PSt) were synthesized. The reversible addition fragmentation chain transfer (RAFT) agents were successfully immobilized onto the surface of MWCNT first, and PSt chains were subsequently grafted from sidewall of MWCNT via RAFT polymerization. Chemical structure of resulting product and the quantities of grafted polymer were determined by Fourier transform infrared, thermal gravimetric analysis, nuclear magnetic resonance, and X‐ray photoelectron spectra. Transmission electron microscopy and field emission scanning electron microscopy images clearly indicate that the nanotubes were coated with a polymer layer. Furthermore, the functionalized MWCNT as additives was added to base lubricant and the tribological property of resultant MWCNT lubricant was investigated with four‐ball machines. The results indicate that the functionalization led to an improvement in the dispersion of MWCNT and as additives it amended the tribological property of base lubricant. The mechanism of the significant improvements on the tribological properties of the functionalized MWCNT as additives was discussed. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 3014–3023, 2008     

Amine-quinone polyurethanes. I. Preparation of polyurethane block copolymers containing 2,5–bis(N-2-hydroxyethyl-N-methylamino)-1,4–benzoquinone diol monomer

The amine-quinone monomer, 2,5–bis(N-2-hydroxyethyl-N-methylamino)-1,4-benzoqui-none (AQM-1), was prepared by the multiple-step condensation of 2-(N-methylam-ino)ethanol with benzoquinone in the presence of oxygen. This crystalline monomer was used to prepare a series of amine-quinone polyurethanes by condensation polymerization, either in the melt or in solution (THF or DMF), with a diisocyanate (MDI, TDI, or IPDI) and an oligomeric diol [poly(caprolactone) or poly(1,2-butylene glycol)]. The amine-quinone functional group was stable under the polymerization conditions, and was incorporated into the main chain, giving red-brown polyurethanes that had molecular weights in the range of 11,000–90,000 and were soluble in THF, MEK, DMF, and DMSO. The thermal properties were consistent with a two-phase morphology with an amorphous soft segment, containing the oligomeric diol, and a microcrystalline hard segment, containing AQM-1. The polymers having a low hard segment content (<50%) were rubbery (soft segment T_g <?25°C); polymers having a high hard segment content (>50%) were thermoplastic (hard segment T_g>150°C). © 1995 John Wiley & Sons, Inc.     

Morphological and mechanical properties of carbon nanotube/polymer composites via melt compounding

The mechanical properties and morphology of multiwall carbon nanotube (MWNT)/polypropylene (PP) nanocomposites were studied as a function of nanotube orientation and concentration. Through melt mixing followed by melt drawing, using a twin screw mini‐extruder with a specially designed winding apparatus, the dispersion and orientation of MWNTs was optimized in PP. Tensile tests showed a 32% increase in toughness for a 0.25 wt % MWNT in PP (over pure PP). Moreover, modulus increased by 138% with 0.25 wt % MWNTs. Transmission electron microscopy and scanning electron microscopy demonstrated qualitative nanotube dispersion and orientation. Wide angle X‐ray diffraction was used to study crystal morphology and orientation by calculating the Herman's orientation factor for the composites as function of nanotube loading and orientation. The addition of nanotubes to oriented samples causes the crystalline morphology to shift from α and mesophase to only α phase. Furthermore, the addition of nanotubes (without orientation) was found to cause isotropization of the PP crystal, and drawing was shown to improve crystal orientation through the orientation factor. In addition, differential scanning caloriometry qualitatively revealed little change in overall crystallinity. In conclusion, this work has shown that melt mixing coupled with melt drawing has yielded MWNT/PP composites with a unique combination of strength and toughness suitable for advanced fiber applications, such as smart fibers and high‐performance fabrics. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 864–878, 2006     

Study of slabstock flexible polyurethane foams based on varied toluene diisocyanate isomer ratios

The morphological features of three flexible slabstock polyurethane foams based on varied contents of 2,4 and 2,6 toluene diisocyanate (TDI) isomers are investigated. The three commercially available TDI mixtures, that is, 65:35 2,4/2,6 TDI, 80:20 2,4/2,6 TDI, and 100:0 2,4/2,6 TDI were used. The foams were characterized at different length scales with several techniques. Differences in the cellular structure of the foams were noted with scanning electron microscopy. Small‐angle X‐ray scattering was used to demonstrate that all three foams were microphase‐separated and possessed similar interdomain spacings. Transmission electron microscopy revealed that the aggregation of the urea phase into large urea‐rich regions decreased systematically on increasing the asymmetric TDI isomer content. Fourier transform infrared spectroscopy showed that the level of bidentate hydrogen bonding of the hard segments increased with the 2,6 TDI isomer content. Differential scanning calorimetry and dynamic mechanical analysis (DMA) were used to note changes in the soft‐segment glass‐transition temperature of the foams on varying the diisocyanate ratios and suggested that the perfection of microphase separation was enhanced on increasing the 2,6 TDI isomer content. The preceding observations were used to explain why the foam containing the highest content of the symmetric 2,6 TDI isomer exhibited the highest rubbery storage modulus, as measured by DMA. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 258–268, 2003