Contribution of soft segment entanglement on the tensile properties of silicone–urea copolymers with low hard segment contents

Novel, segmented thermoplastic silicone–urea (TPSU) copolymers based on rather high molecular weight aminopropyl terminated polydimethylsiloxane (PDMS) soft segments (<M_n> 10,800 and 31,500 g/mol), a cycloaliphatic diisocyanate (HMDI) and various diamine chain extenders were synthesized. Copolymers with very low urea hard segment contents of 1.43–14.4% by weight were prepared. In spite of very low hard segment contents, solution cast films showed very good microphase separation and displayed reasonable mechanical properties. Tensile strengths of TPSU copolymers showed a linear dependence on their urea hard segment contents, regardless of the structure of the diamine chain extender used. The modulus of silicone–urea copolymers is dependent on the urea concentration, but not on the extender type or PDMS molecular weight. When silicone–urea copolymers with identical urea hard segment contents were compared, copolymers based on PDMS-31,500 showed higher elongation at break values and ultimate tensile strengths than those based on PDMS-10,800. Since the critical entanglement molecular weight (M_e) of PDMS is about 24,500 g/mol, these results suggest there is a significant contribution from soft segment chain entanglement effects in the PDMS-31,500 system regarding the tensile properties and failure mechanisms of the silicone–urea copolymers.     

Influence of soft segment molecular weight on the mechanical hysteresis and set behavior of silicone-urea copolymers with low hard segment contents

Effect of polydimethylsiloxane (PDMS) soft segment molecular weight (M_n = 3200, 10,800 and 31,500 g/mol) and urea hard segment content (2.0-11.4% by weight) on the hysteresis and permanent set behavior of segmented silicone-urea (TPSU) copolymers were investigated. In spite of very low hard segment contents, all copolymers formed self-supporting films and displayed good mechanical properties. When the mechanical hysteresis and set behavior of the silicone-urea copolymers with similar hard segment contents (around 7.5% by weight) but based on PDMS-3K, PDMS-11K and PDMS-32K were compared, it was very clear that as the PDMS molecular weight increased, hysteresis and instantaneous set values decreased significantly. Copolymers based on the same silicone soft segment (PDMS-11K or PDMS-32K) but with different hard segment contents showed a linear increase in hysteresis and a slight decrease in the instantaneous set as a function of hard segment content. Constant initial stress creep experiments also showed lower creep as the PDMS segment molecular weight increased for copolymers with similar urea contents. Since the critical entanglement molecular weight (M_e) of PDMS is stated to be 24,500 g/mol, our results tend to suggest important contribution of chain entanglements on the hysteresis and instantaneous set of these silicone-urea copolymers.     

Structure–property relationships of poly(urethane–urea)s with ultra-low monol content poly(propylene glycol) soft segments. Part II. Influence of low molecular weight polyol components

Segmented poly(urethane–urea)s have been synthesized with mixed soft segments of ultra-low monol content poly(propylene glycol) (PPG) and tri(propylene glycol) (TPG) which allows the fabrication of quality elastomers without crosslinking. The narrow molecular weight distribution of the ultra-low monol content PPG polyols allows for the probing of the influence of the low molecular components of the molecular weight distribution through the inclusion of low molecular homologs of PPG such as TPG. Structure–property relationships for these materials were investigated as average soft segment molecular weight was varied by blending 8000 g/mol PPG with TPG to achieve molecular weights of 2500, 2000, and 1500 g/mol. Morphological features such as microphase separation, interdomain spacing and interphase thickness were quantified and revealed with SAXS. AFM was utilized to verify the microphase separation characteristics inferred by SAXS. The thermal and mechanical behavior was assessed through applications of DMA, DSC, and conventional mechanical tests. It was found that as the average soft segment molecular weight was decreased through the addition of TPG, the interdomain spacing distinctly increased contrary to the trend seen for decreasing soft segment molecular weight in PPG based systems without TPG. Additionally, the inclusion of TPG in the poly(urethane–urea) formulations resulted in the formation of larger hard domains as evidenced by AFM. These results and supporting evidence from DMA, DSC, birefringence, and mechanical testing led to the conclusion that TPG apparently acts more as a chain extender as well as, or in contrast to, a soft segment.     

Thermal,mechanical, and fracture properties of copolyureas formed by reaction injection molding: Effects of hard segment structure

A series of copolyureas containing 50% by weight hard segment have been formed by RIM. The hard segment structure was systematically varied to investigate the effects of urea group density, hard segment crosslinking, relative reaction rates, and to compare the properties of aromatic and aliphatic hard segment materials. In each case the soft segment was based on a 2000 molecular weight polyether diamine. The RIM materials formed ranged from flexible elastomers to brittle plastics depending on composition and were characterized by SAXS, DSC, DMA, tensile stress–strain and fracture mechanics studies. SAXS, DSC, and DMA showed that microphase separation had occurred to give materials with a non-equilibrium morphology. DMA and tensile stress–strain studies showed the small strain properties to be very sensitive to the volume fraction of glassy material whereas the ultimate properties were dependent on chemical structure of the hard segment. Fracture properties were determined using the single-edge notch technique. In most cases ductile failure occurred with G_c > 2.5 kJ m^?2 and the fracture surfaces showed gross yielding and tearing. In the case of the copolyurea with the highest urea group content, brittle fracture occurred with G_c = 0.06 kJ m^?2.     

Tensile and elastic properties of triblock copolymer based on aramide end‐segments and polyether mid‐segments

The tensile and elastic behavior of triblock copolymers containing uniform aramide (TΦB) hard end‐segments (HS) and poly(tetramethylene oxide) (PTMO, M_n = 2900 g/mol) soft segments (SSs) was studied. The molecular weight of the copolymer was varied by changing the length of the soft mid‐segment; by extending the PTMO₂₉₀₀ with terephthalate units, the SS length was increased from 2900 g/mol to 21,000 g/mol and concurrently the aramide concentration decreased from 18 to 3 wt %. The mechanical properties were investigated by means of tensile testing, stress relaxation (SR) experiments, and cyclic tensile set (TS) tests. The E‐modulus was found to increase with increasing aramide content. The low molecular weight copolymers were brittle whereas the high molecular weight copolymers displayed large fracture strain values. The transition from brittle to ductile seemed to occur at a triblock copolymer molecular weight of 6600 g/mol. A strain‐induced crystallization was observed at strains above 250%, and both the fracture strain and stress were found to be highly dependant on the molecular weight of the copolymer. Cyclic tensile experiments showed that the materials had low TS values up to the strain hardening point. On the other hand, the SR data at 10% strain seemed to be little dependant on the molecular weight. The higher molecular weight copolymers did not display lower SR values than their low molecular weight counterparts. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Properties of polyisobutylene polyurethane block copolymers: 3. Hard segments based on 4,4′-dicyclohexylmethane diisocyanate (H12MDI) and butane diol

A series of polyisobutylene (PIB) polyurethanes based on 4,4′-dicyclohexylmethane diisocyanate (H₁₂MDI) have been synthesized and their structure-property relationships have been investigated. The PIB glycol was synthesized by the ‘inifer’ technique. Sample compositions were designed for independent investigation of the effects on physical properties of hard segment content and soft segment molecular weight and for comparison with corresponding 4,4′-diphenylmethane diisocyanate (MDI) based PIB polyurethanes. Increasing hard segment content resulted in improved dynamic and tensile modulus while elongation at break was unaffected. Increasing soft segment molecular weight led to decreased mechanical properties attributed to larger domain sizes as indicated by small angle X-ray scattering (SAXS). Both the soft segment T_g and the extent of interfacial mixing as measured by SAXS were unaffected by hard segment content and soft segment molecular weight suggesting that the materials were highly phase separated. In comparison with corresponding MDI based materials the H₁₂MDI based polyurethanes exhibited less hard segment ordering, slightly less interfacial mixing, smaller domain sizes, and slightly better ultimate tensile properties.     

Structure-property behavior of poly(dimethylsiloxane) based segmented polyurea copolymers modified with poly(propylene oxide)

Poly(propylene oxide) (PPO) was incorporated in a controlled manner between poly(dimethylsiloxane) (PDMS) and urea segments in segmented polyurea copolymers and their solid state structure-property behavior was investigated. The copolymers contained PDMS segments of MW 3200 or 7000 g/mol and an overall hard segment content of 10-35 wt%. PPO segments of MW 450 or 2000 g/mol were utilized. Equivalent polyurea copolymers based on only PDMS as the soft segment (SS) component were used as controls. The materials (with or without PPO) utilized in this study were able to develop microphase morphology as determined from dynamic mechanical analysis (DMA) and small angle X-ray scattering (SAXS). DMA and SAXS results suggested that the ability of the PPO segments to hydrogen bond with the urea segments results in a limited inter-segmental mixing which leads to the formation of a gradient interphase, especially in the PPO-2000 co-SS containing copolymers. DMA also demonstrated that the polyureas based on only PDMS as the SS possessed remarkably broad and nearly temperature insensitive rubbery plateaus that extended up to ca. 175 °C, the upper temperature limit depending upon the PDMS MW. However, the incorporation of PPO resulted in more temperature sensitive rubbery plateaus. A distinct improvement in the Young's modulus, tensile strength, and elongation at break in the PPO-2000 and PDMS-7000 containing copolymers was observed due to inter-segmental hydrogen bonding and the formation of a gradient interphase. However, when PPO was incorporated as the co-SS, the extent of stress relaxation and mechanical hysteresis of the copolymers increased relative to the segmented polyureas based on the utilization of only PDMS as the soft segment component.     

Influence of morphology on the properties of segmented block copolymers

A comparison was carried out regarding the structure and properties of segmented block copolymers with either non-crystallisable or crystallisable rigid segments. The flexible segment in the block copolymers was a linear poly(propylene oxide) end capped with poly(ethylene oxide), with a segment molecular weight of 2300 g/mol. The rigid segments were either non-crystallisable or monodisperse crystallisable polyamides of varying lengths. The morphologies were studied by TEM and AFM, the thermal mechanical properties by DMA and the elastic properties by compression set and tensile measurements. A direct comparison was made of segmented block copolymers with either liquid-liquid demixed or crystallised structures. The crystallised amide segments were more efficient in increasing the modulus and improving the elastic properties than the non-crystallisable ones. The copolymers with crystallised structures were transparent, had a low glass transition temperature of the polyether phase and a modulus that was independent of temperature between T_g and T_m. These copolymers also displayed a very low loss factor (tan δ), suggesting excellent dynamic properties. The hard phase in segmented block copolymers should thus preferably be crystalline.     

Polyurethane tri‐block copolymers—Synthesis,mechanical, elastic,and rheological properties

A series of polyurethane tri‐block copolymers were synthesized by reacting a 4,4′‐methylenebis(phenyl isocyanate) (MDI)‐endcapped poly(tetramethylene oxide) (PTMO, M_n = 2,000 g/mol) with a monoamine‐diamide (6T6m) hard segment (HS). The concentration of the HS in the copolymer was varied between 9 and 33 wt % by changing the length of the soft mid‐block segment. The structure of the copolymers was analyzed by nuclear magnetic resonance, the amide crystallinity was investigated by Fourier transform infra‐red and the thermal properties were studied by differential scanning calorimetry. The mechanical and elastic properties of the tri‐block copolymer were subsequently explored by dynamic mechanical analysis, compression set and tensile experiments, and the melt rheological behavior was studied by a parallel plate method. The amide end groups displayed a high crystallinity and the modulus of the tri‐block copolymers was relatively high. The fracture strain increased strongly with the molecular weight and the copolymers demonstrated a ductile fracture behavior for molecular weights above 6000 g/mol. Good compression set values were obtained for the tri‐block copolymers despite their low molecular weight. In the molten state, the tri‐block polymers displayed a gelling effect at low frequencies, which was believed to be a result of a clustering of the end‐segments. POLYM. ENG. SCI., 2010. © 2009 Society of Plastics Engineers     

Comparison of properties of thermoplastic polyurethane elastomers with two different soft segments

Two series of thermoplastic polyurethane elastomers [poly(propylene glycol) (PPG) based PP samples and poly(oxytetramethylene)glycol (PTMG) based PT samples] were synthesized from isophorone diisocyanate (IPDI)/1,4-butanediol (BD)/PPG and IPDI/BD/PTMG. The IPDI/BD based hard segments contents of polyurethane prepared in this study were 40–73 wt %. These polyurethane elastomers had a constant soft segment molecular weight (average M_n, 2000) but a variable hard segment block length (n, 3.5–17.5; average M_n, 1318–5544). Studies were made on the effects of the hard segment content on the dynamic mechanical thermal properties and elastic behaviors of polyurethane elastomers. These properties of PPG based PP and PTMG based PT samples were compared. As the hard segment contents of PP and PT samples increased, dynamic tensile modulus and α-type glass transition temperature (T_g) increased; however, the β-type T_g decreased. The permanent set (%) increased with increasing hard segment content and successive maximum elongation. The permanent set of the PT sample was lower than that of the PP sample at the same hard segment content. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 69: 1349–1355, 1998     

Properties and phase separation of reaction injection molded and solution polymerized polyureas as a function of hard block content

A series of amine terminated polypropylene oxide based thermoplastic polyureas with hard segment contents of 30%, 50%, and 70 percent were synthesized via solution polymerization and reaction injection molding (RIM). Amine terminated polypropylene oxide (PPO-NH₂) of M_n = 2000 was used as the soft segment and 4,4′-diphenylme-thanediisocyanate (MDI) extended with diethyltoluenediamine (DETDA) as the hard segment. These polyureas are linear, amorphous, and phase separated. Polymers were characterized by gel permeation chromatography (GPC), differential scanning calorimetry (DSC), dynamic mechanical spectroscopy (DMS), small angle X-ray scattering (SAXS), and tensile testing. RIM polyureas had significantly lower molecular weights than solution polymerized polyureas, but their mechanical properties did not suffer, RIM polyureas have poorer phase separation than solution polyureas as evidenced by DSC, DMS, and SAXS, especially at high hard segment levels. SAXS shows phase separation levels of up to 100 percent for low hard segment polyureas and down to 10 percent for high hard segment RIM polyurea. DSC found no evidence of a hard segment glass transition, and the evidence from DMS was inconclusive. In addition to polymer characterization, demolding behavior was studied. The 30 percent hard segment was always tough and elastomeric, while the 70 percent hard segment was always very brittle. The 50 percent hard segment showed the greatest variation in properties, ranging from very brittle to very though as mold temperature and in-mold time were increased. Demold brittleness is explained by the presence of low molecular weight DETDA/MDI oligomers on demolding, which continue to react on aging.     

A comparative study of the structure-property behavior of highly branched segmented poly(urethane urea) copolymers and their linear analogs

The solid-state structure-property behavior of highly branched segmented poly(urethane urea) (PUU) copolymers and their linear analog was investigated. A limited study of their solution rheological behavior was also undertaken. The linear PUUs were synthesized by the two-step prepolymer method, whereas the oligomeric A₂+B₃ methodology was utilized to synthesize the highly branched materials. The soft segments (SS) were either poly(tetramethylene oxide) (PTMO) or poly(propylene oxide) (PPO). All copolymers utilized in this study, with one exception, contained 28 wt% hard segment (HS) content. DMA, SAXS, and AFM studies indicated that the linear as well as the highly branched PUUs were microphase separated. The SS T_g of the highly branched PUUs was nearly identical to that of their respective linear analogs. However, the linear copolymers exhibited broader and less temperature sensitive rubbery plateaus, both attributed to one or both of two reasons. The first is better hydrogen bonding organization of the HS phase as well as greater HS lengths than in the highly branched analogs. The second parameter is that of a potentially higher chain entanglement for the linear systems relative to the branched analogs. Tapping-mode AFM phase images confirmed the microphase morphology indicated by SAXS and DMA. Ambient temperature strain-induced crystallization was observed in the PUU based on PTMO 2040 g/mol at a uniaxial strain of ca. 400%, irrespective of the chain architecture. Stress-strain, stress relaxation, and mechanical hysteresis of the highly branched copolymers were in general slightly poorer than that of their linear analogs. Ambient temperature solution viscosity of the highly branched materials in dimethyl formamide was substantially lower that that of the linear samples of nearly equal molecular weight.     

Influence of ionic charge placement on performance of poly(ethylene glycol)-based sulfonated polyurethanes

Poly(ethylene glycol) (PEG)-based sulfonated polyurethanes bearing either sulfonated soft segments (SSSPU) or sulfonated hard segments (SHSPU) were synthesized using sulfonated monomers. Differential scanning calorimetry (DSC) revealed that sulfonate anions either in the soft segments or hard segments both increased the glass transition temperatures (T_g’s) of the soft segments and suppressed their crystallization. Moreover, dynamic mechanical analysis (DMA) and tensile analysis demonstrated that SSSPU possessed a higher modulus and tensile strength relative to SHSPU. Fourier transform infrared (FTIR) spectroscopy revealed that hydrogen bonding interactions in SHSPU were suppressed compared to SSSPU and noncharged PU. This observation suggested a high level of phase-mixing for SHSPU. In addition, atomic force microscopy (AFM) phase images revealed that both SSSPU and noncharged PU formed well-defined microphase-separated morphologies, where the hard segments phase-separated into needle-like hard domains at the nanoscale. However, SHSPU showed a phase-mixed morphology, which was attributed to increased compatibility of polar PEG soft segments with sulfonated ionic hard segments and disruption of hydrogen bonds in the hard segment. The phase-mixed morphology of SHSPU was further demonstrated using small angle X-ray scattering (SAXS), which showed a featureless X-ray scattering profile. In contrast, SAXS profiles of SSSPU and noncharged PU demonstrated microphase-separated morphologies. Moreover, SSSPU also displayed a broad ionomer peak ranging in q = 1–2 nm^?1, which resulted from the sodium sulfonate ion pair association in the polar PEG soft phase. Morphologies of sulfonated polyurethanes correlated well with thermal and mechanical properties.     

Ionene segmented block copolymers containing imidazolium cations: Structure-property relationships as a function of hard segment content

Imidazolium ionene segmented block copolymers were synthesized from 1,1′-(1,4-butanediyl)bis(imidazole) and 1,12-dibromododecane hard segments and 2000 g/mol PTMO dibromide soft segments. The polymeric structures were confirmed using ¹H NMR spectroscopy, and resonances associated with methylene spacers from 1,12-dibromododecane became more apparent as the hard segment content increased. TGA revealed thermal stabilities ≥250 °C for all imidazolium ionene segmented block copolymers. These ionene segmented block copolymers containing imidazolium cations showed evidence of microphase separation when the hard segment was 6-38 wt%. The thermal transitions found by DSC and DMA analysis found that the T_g and T_m of the PTMO segments were comparable to PTMO polymers, namely approximately −80 °C and 22 °C, respectively. In the absence of PTMO soft segments the T_g increased to 27 °C The crystallinity of the PTMO segments was further evidence of microphase separation and was particularly evident at 6, 9 and 20 wt% hard segment, as indicated in X-ray scattering. The periodicity of the microphase separation was well-defined at 20 and 38 wt% hard segment and found to be approximately 10.5 and 13.0 nm, respectively, for these ionenes wherein the PTMO soft segment is 2000 g/mol. Finally, the 38 and 100 wt% hard segment ionenes exhibited scattering from correlations within the hard segment on a length scale of approximately 2-2.3 nm. These new materials present structure on a variety of length scales and thereby provide various routes to controlling mechanical and transport properties.     

Influence of microstructure on micro-/nano-mechanical measurements of select model transparent poly(urethane urea) elastomers

Morphology of 4,4′-dicyclohexylmethane diisocyanate–poly(tetramethylene oxide) (PTMO)–diethyltoluenediamine based poly(urethane urea) (PUU) elastomers is investigated by atomic force microscopy (AFM) and compared with elastic modulus data measured from AFM-enabled indentation, dynamic nanoindentation (nanoDMA), and dynamic mechanical analysis (DMA). These measurements highlight the effect of altering the molecular weight (M_w) of PTMO, which is used as a soft segment (SS), on the microstructure. In particular, at SS M_w 2000 g/mol, a strong microphase-separated morphology is observed, whereas a phase-mixed dominated microstructure is noted in PUU with SS M_w of 1000 and 650 g/mol. These observations are also consistent with DMA tan δ results. Furthermore, instrumented impact indentation is also utilized for elucidation of dynamic damping characteristics in these PUUs.     

Properties of polyisobutylene polyurethane block copolymers: 2. Macroglycols produced by the ‘inifer’ technique

The structure-property relationships of a series of 4,4′-diphenylmethane diiscoyanate (MDI) based polyisobutylene (PIB) polyurethanes were investigated. The PIB glycol was synthesized via the ‘inifer’ technique and had a narrow functionality distribution with a number average functionality of 2.0. The use of a PIB glycol with improved functionality and solution polymerization of the polyurethane led to improved mechanical properties compared with previously studied PIB polyurethanes. However, the mechanical properties were still low compared with conventional polyurethanes; the absence of soft segment strain-induced crystallization and compositional heterogeneity due to reactant incompatibility are cited as possible causes of low mechanical properties. Sample compositions were designed for independent investigation of the effects of hard segment content and soft segment molecular weight on the properties of the materials. Increasing hard segment content resulted in improved dynamic and tensile modulus, lower elongation at break, and larger hard segment domains. Increasing soft segment molecular weight led to larger domains and reduced mechanical properties. The degree of phase separation as measured by the soft segment T_g and the amount of interfacial mixing measured by small angle X-ray scattering (SAXS) were unaffected by hard segment content and soft segment molecular weight and were indicative of a high degree of phase separation compared with conventional polyurethanes.     

Phase separation of diamine chain-extended poly(urethane) copolymers: FTIR spectroscopy and phase transitions

As part of our continuing effort to understand microphase separation of poly(urethane urea) block copolymers, FTIR spectroscopy and thermal techniques (DSC and DMA) were used to investigate the phase behavior of two series of MDI-polytetramethylene oxide soft segment copolymers, chain-extended with ethylene diamine or a diamine mixture. Due to the complex nature and multiple absorbances in the carbonyl and N-H regions of the FTIR spectra, quantitative analysis was not possible. However, qualitative trends could be discerned, and the spectral changes were found to be in excellent agreement with our previous quantitative analysis of the same copolymers using small-angle X-ray scattering. DSC and DMA experiments both indicate that the soft phase T_g decreases with increasing hard segment content. This is contrary to increased hard segment mixing in the soft phase, but can be rationalized by taking into consideration soft segment crystallinity and the concentration of ‘lone’ MDI units in the soft phase.     

Phase Behavior and Mechanical Properties of Siloxane-Urethane Copolymer

Two series of siloxane-urethane copolymers were prepared from polydimethylsiloxane (PDMS) with a molecular weight of 1000 or 1800 which was used as a soft segment, 4,4′-diphenylmethane diisocyanate (MDI) and 1,4-butanediol (1,4-BD). Differential scanning calorimetry (DSC) demonstrated that the position (T_gs) and breadth (ΔB) of soft-segment glass transition of copolymers remained constant as the hard-segment content increased. Heat capacities at soft-segment glass transition of the copolymer (ΔCp) were 0.195∼0.411 J/g^○C and heat capacities of pure PDMS (ΔCp₀) were 0.571∼0.647 J/g^○C, leading to the various ΔCp/ΔCp₀ ratios. The ΔCp/ΔCp₀ ratios decreased as the increasing of hard-segment content, showing poor phase separation. The FTIR spectrum confirmed the occurrence of hydrogen bonding in ether end-group of pure PDMS. The ether group of the soft segment led to interfacial mixing between soft and hard segments. The tan δ of the soft segment determined by dynamic mechanical testing (DMA) also identified the mixing of soft and hard segments. The mechanical properties of the copolymer were directly related to either the soft and hard segment contents or the chain lengths of soft and hard segments. The hard segment that reinforced the soft segment and interfacial thickness between soft and hard segment dominated the mechanical properties.     

Preparation and properties of UV-curable polyurethane acrylates

UV-curable polyurethane (PU) acrylates have been synthesized from polypropylene glycol (PPG), isophoron diisocyanate (IPDI), and three types of reactive diluents, i.e., 2-hydroxyethylacrylate (HEA), tripropyleneglycol diacrylate (TPGDA), and trimethylolpropane triacrylate (TMPTA). The effects of soft segment length, type, and concentration of reactive diluent on the mechanical and dynamic mechanical properties have been determined. When the soft segment length was short (750) tensile strength (σ_b) decreased, and elongation at break (ϵ_b) generally increased with increasing HEA concentration, due respectively to the inferior strength of HEA homopolymer, and increased molecular weight between crosslinks (M_c). Initial modulus (E) and σ_b increase and elongation at break (ϵ_b) decreased with the increase of TPGDA concentration, and the effect was more pronounced as the soft segment length decreased. The hardness and σ_b increase with diluent concentration in PPG 2000-based materials was more pronounced with higher functionality diluent, due to the increased crosslinking density. The lower temperature glass transition peak of PU was not influenced by the TPGDA incorporation, whereas the higher temperature one moved toward still higher temperature. This was interpreted in terms of possible compatibility of hard segments and acrylates due to their similar polarity and hydrogen bonding. © 1996 John Wiley & Sons, Inc.     

Effect of the degree of soft and hard segment ordering on the morphology and mechanical behavior of semicrystalline segmented polyurethanes

The hierarchical microstructure responsible for the unique energy-absorbing properties of natural materials, like native spider silk and wood, motivated the development of segmented polyurethanes with soft segments containing multiple levels of order. As a first step in correlating the effects of crystallinity in the soft segment phase to the hard segment phase, we chose to examine the morphology and mechanical behavior of polyurethanes containing polyether soft blocks with varying tendencies to crystallize and phase segregate and the evolution of the microstructure with deformation. A series of high molecular weight polyurethanes containing poly(ethylene oxide) (PEO) (1000 and 4600 g/mol) and poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) (1900 g/mol) soft segments with varying hard segment content were synthesized using a two-step solution polymerization method. The presence of soft segment crystallinity (PEO 1000 g/mol) was shown to improve the storage modulus of the segmented polyurethanes below the T_m of the soft block and to enhance toughness compared to the PEO-PPO-PEO soft segment polyurethanes. We postulate that this enhancement in mechanical behavior is the result of crystalline soft regions that serve as an additional load-bearing component during deformation. Morphological characterization also revealed that the microstructure of the segmented polyurethanes shifts from soft segment continuous to interconnected and/or hard domain continuous with increasing hard segment size, resulting in diminished ultimate elongation, but enhanced initial moduli and tensile strengths. Tuning the soft segment phase crystallinity may ultimately lead to tougher polyurethane fibers.