Concept of hard clusters in the interpretation of thermal and mechanical properties of polyurethane and polyurethane acrylate networks

Polyurethane (PU) and polyurethane acrylate (PUA) networks based on hydroxyl-terminated polycaprolactone (PCL), 1,3-bis-2,2′(2-isocyanatopropyl)benzene (m-TMXDI), trimethylolpropane (TMP) for PU or hydroxyethyl methacrylate (HEMA) for PUA were synthesized. Glass transition temperature, T_g, dynamic mechanical relaxation, α, and equilibrium tensile modulus, E′, were measured to compare the two kinds of networks. To explain thermal and mechanical properties of networks, the concept of hard clusters has been introduced. PU networks exhibit a single-phase structure with modulus and T_g dependent on the concentration of elastically active network chains (EANC) per unit volume calculated by considering hard crosslink clusters. The rigidity of the clusters comes from small diisocyanate and trimethylolpropane units connected by urethane bonds. They are embedded in a continuous soft phase of macrodiol urethane. Physical equivalence between several kinds of network models has been demonstrated for full conversion of isocyanate-alcohol reaction. PUA networks exhibit thermodynamically one-phase structures that become a two-phase structure for high molar mass of macrodiol when the molar fraction of isocyanate groups increases. For those networks, the calculated modulus considering clusters based on polyacrylate chains seems to be a good way to approach the experimental value of the equilibrium modulus. For the same molar ratio of OH to NCO groups the range of dynamic moduli is larger for PUA than for PU. This difference can be explained by a different concentration of crosslinks in the networks. © 1996 John Wiley & Sons, Inc.     

Synthesis and characterization of poly(urethane‐benzoxazine) films as novel type of polyurethane/phenolic resin composites

Poly(urethane‐benzoxazine) films as novel polyurethane ( PU )/phenolic resin composites were prepared by blending a benzoxazine monomer ( Ba ) and PU prepolymer that was synthesized from 2,4‐tolylene diisocyanate (TDI) and polyethylene adipate polyol (MW ca. 1000) in 2 : 1 molar ratio. DSC of PU/Ba blend showed an exotherm with maximum at ca. 246 °C due to the ring‐opening polymerization of Ba, giving phenolic OH functionalities that react with isocyanate groups in the PU prepolymer. The poly(urethane‐benzoxazine) films obtained by thermal cure were transparent, with color ranging from yellow to pale wine with increase of Ba content. All the films have only one glass transition temperature (T_g ) from viscoelastic measurements, indicating no phase separation in poly(urethane‐benzoxazine) due to in situ polymerization. The T_g increased with the increase of Ba content. The films containing 10 and 15% of Ba have characteristics of an elastomer, with elongation at break at 244 and 182%, respectively. These elastic films exhibit good resilience with excellent reinstating behavior. The films containing more than 20% of Ba have characteristics of plastics. The poly(urethane‐benzoxazine) films showed excellent resistance to the solvents such as tetrahydrofuran, N,N‐dimethyl formamide, and N‐methyl‐2‐pyrrolidinone that easily dissolve PU s. Thermal stability of PU was greatly enhanced even with the incorporation of a small amount of Ba . © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 4165–4176, 2000     

Interpenetrating polymer networks of polyurethane and graft vinyl ester resin–polyurethane formed with diphenylmethane diisocyanate

For enhancing the compatibility and/or the interpenetration of the simultaneous interpenetrating networks (SINs) composed of polyurethane (PU) formed with uretonimine modified 4,4′‐diphenylmethane diisocyanate and vinyl ester resin (VER), a series of graft VERs consisting of different lengths of side chains were synthesized and characterized. It was found that there exists some limited short‐range order due to the strong hydrogen bonding in the graft VER network composed of butanol side chains (BO‐g‐VER). The graft VER network composed of poly(oxypropylene) (PPO) side chains (M_n: 200, 200‐g‐VER) showed compatible system, while the VER network consisting of longer PPO grafts (M_n: 390, 390‐g‐VER) exhibited microphase separated morphology. Based upon the DSC and FTIR measurements as well as the SEM and TEM observation, the lengths of side chains existing in graft VER network have great effect on the morphologies of PU/graft VER SINs. For PU/BO‐g‐VER SINs, there has been some interpenetration between the two networks because of the miscibility between the BO‐g‐VER network and the hard segments existing in the PU network. For PU/200‐g‐VER SINs, the good compatibility and/or the interpenetration between the two phases was observed, since the long‐range ordered structure of hard segments in PU phase was greatly suppressed, resulting from the excellent miscibility between the urethane groups as well as the PPO side chains existing in the 200‐g‐VER network and those in the PU network, respectively. Thus, the strong reinforcement effect of these two graft networks on the PU network and the excellent mechanical properties of the SIN systems were observed. However, the PU/390‐g‐VER SINs showed the complicated morphologies because of existing microphase‐ separated morphology of 390‐g‐VER network in itself. In this case, the enhancement effect of such a graft VER network on the PU network is limited. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 136–144, 2000     

Curing kinetics and glass‐transition temperature of hexamethylene diisocyanate‐based polyurethane

The curing process of hexamethylene diisocyanate‐based polyurethane has been monitored by applying FTIR and DSC methods. A general relationship between glass‐transition temperature (T_g) and conversion of curing process has been obtained. This suggests that the reaction path and the relative reaction rates are independent of the curing temperature. The reaction kinetics of the system is analyzed using the T_g data converted to the conversion of the curing process. A set of experimental data and one theoretical model of T_g versus chemical conversion are presented to prove the assumption where a direct one‐to‐one relationship between the T_g (as measured) and the chemical conversion is obtained. Apparent activation energies (E_a) obtained by applying three different methods suggest good agreement. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 2213–2220, 2000     

Novel method for the preparation of poly(urethane–imide)s and their properties

A polymer blend consisting of polyimide (PI) and polyurethane (PU) was prepared by means of a novel approach. PU prepolymer was prepared by the reaction of polyester polyol and 2,4-tolylenediisocyanate (2,4-TDI) and then end-capped with phenol. Poly(amide acid) was prepared from pyromellitic dianhydride (PMDA) and oxydianiline (ODA). A series of oligo(amide acid)s were also prepared by controlling the molar ratio of PMDA and ODA. The PU prepolymer and poly(amide acid) or oligo(amide acid) solution were blended at room temperature in various weight ratios. The cast films were obtained from the blend solution and treated at various temperatures. With the increase of polyurethane component, the films changed from plastic to brittle and then to elastic. The poly(urethane–imide) elastomers showed excellent mechanical properties and moderate thermal stability. The elongation of films with elasticity was more than 300%. The elongation set after the breaking of films was small. From the dynamic mechanical analysis, all the samples showed a glass transition temperature (T_g) at ca. −15°C, corresponding to T_g of the urethane component, suggesting that phase separation occurred between the two polymer components, irrespective of polyimide content. TGA and DSC studies indicated that the thermal degradation of poly(urethane–imide) was in the temperature range 250–270°C. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35: 3745–3753, 1997     

Chlorine degradation of polyether-based polyurethane

The tensile properties of polyether-based polyurethane (PU) filaments decrease with increasing chlorine concentrations as well as with treatment times. Fourier transform infrared (FTIR) results show the formation of quinoid, azo, and aldehyde groups in the chlorine-treated PU, and increased hydrogen bonding between the C O C in the soft segment and the N H in the hard segments. A breakdown mechanism involving chain cleavages along the ether linkages in the soft segments as well as at the urethane linkages of the hard–soft segment interfaces is proposed. Chlorine-treated PU showed increased solubility in tetrahydrofuran (THF). The molecular weight data of the THF-soluble portion of treated PU also support the proposed locations of chain scissions. The increased soft segment T_g and T_m with increasing chlorine concentrations are results of increased phase-mixing and hydrogen bonding. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35: 3263–3273, 1997     

Hydroxy-terminated poly(ε-caprolactone-co-δ-valerolactone) oligomers: Synthesis,characterization, and polyurethane network formation

Difunctional hydroxy-terminated poly(ε-caprolactone-co-ε-valerolactone) (PCV) oligomers were synthesized by the diol-initiated bulk copolymerization of ε-caprolactone (C) and δ-valerolactone (V). The two homopolymers were semicrystalline, with almost identical melting temperatures; copolymerization significantly lowered the melting point (T_m) compared to either homopolymer. Copolymer melting points were found to decrease with decreasing molecular weight and to be dependent on composition, i.e., the incorporation of a comonomer into either homopolymer resulted in a decrease in T_m, with the maximum decrease occurring at a copolymer composition of about 60 mol % ε-caprolactone. The molar compositions of the copolyesters were determined from ¹³C-NMR spectra. The reactivity ratios of the two monomers (M₁ = C, M₂ = V) were determined to the r₁ = 0.25 and r₂ = 0.49. Number average molecular weight (M?_n) of the PCV diols was inversely proportional to the initial diol concentration within the studied molecular weight range of 900 to 11,100 g/mol. Crosslinked polyurethane networks were prepared by reacting PCV diols with triphenylmethane triisocyanate. Network characterization included determination of sol content by solvent extraction, glass transition (T_g) and T_m by DSC, and tensile properties by stress-strain measurements. Completely amorphous networks resulted from PCV diols of M?_n ≤ 2,400; semicrystalline networks resulted from PCV diols of M?_n ≥ 3,600.     

Pressure‐temperature study of dielectric relaxation of a polyurethane elastomer

The effect of hydrostatic pressure up to 1,361 atms on the dielectric properties of a segmented polyurethane elastomer (Dow 2103‐80AE) is studied at temperatures from 0°C to 80°C. The experimental results show that the relaxation time for both the I–process, associated with the molecular motions in the hard segments, and the α–process, associated with the glass transition, increases with pressure, and this shift is more pronounced for the I–process. Besides the glass transition, it is found that the I–process can be described by the Vogel–Fulcher (V–F) and Williams–Landel–Ferry (WLF) relations. At atmospheric pressure, T_g and T₀ for the I–process are 235.9 K and 4.2 × 10³ K, respectively. Based on the V–F and WLF relations and experimental results, it is found that a parameter, C₁, in the WLF relation is independent of the pressure. Thus, a method is introduced to determine the values of both the characteristic transition temperature (T_g) and activation energy (T₀) for the processes at different pressures. As the pressure increases from atmospheric to 1,361 atms, the increase of T_g for the I–process is about 30°C. The results also show that, for both the I– and the α–processes, T₀ decreases with increasing pressure. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 983–990, 1999     

Preparation of high‐temperature polyurethane by alloying with reactive polyamide

A series of novel poly(urethane amide) films were prepared by the reaction of a polyurethane (PU) prepolymer and a soluble polyamide (PA) containing aliphatic hydroxyl groups in the backbone. The PU prepolymer was prepared by the reaction of polyester polyol and 2,4‐tolylenediisocyanate and then was end‐capped with phenol. Soluble PA was prepared by the reaction of 1‐(m‐aminophenyl)‐2‐(p‐aminophenyl)ethanol and terephthaloyl chloride. The PU prepolymer and PA were blended, and the clear, transparent solutions were cast on glass substrates; this was followed by thermal treatments at various temperatures to produce reactions between the isocyanate group of the PU prepolymer and the hydroxyl group of PA. The opaque poly(urethane amide) films showed various properties, from those of plastics to those of elastomers, depending on the ratio of the PU and PA components. Dynamic mechanical analysis showed two glass‐transition temperatures (T_g's), a lower T_g due to the PU component and a higher T_g due to the PA component, suggesting that the two polymer components were phase‐separated. The rubbery plateau region of the storage modulus for the elastic films was maintained up to about 250 °C, which is considerably higher than for conventional PUs. Tensile measurements of the elastic films of 90/10 PU/PA showed that the elongation was as high as 347%. This indicated that the alloying of PU with PA containing aliphatic hydroxyl groups in the backbone improved the high‐temperature properties of PU and, therefore, enhanced the use temperature of PU. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 3497–3503, 2002     

Use of blowing catalysts for integral skin polyurethane applications in a controlled molecular architectural environment: Synthesis and impact on ultimate physical properties

Polyurethane elastomers of a controlled molecular architecture were synthesized using a two‐step polymerization technique. The building blocks of the elastomeric materials included urea–urethane prepolymers end‐capped with diisocyanate groups and had an exact number of urea groups at both ends. Two‐dimensional bifurcated hydrogen‐bonding networks incorporating the urea groups were, with differential scanning calorimetric and dynamic mechanical thermal analyzer techniques, responsible for the increase in the glass‐transition temperature (T_g) of the hard block and sharp interface morphology between the pure “hard” domains and pure “soft” domains. The higher extent of the phase separation between the two phases contributed to higher elastic moduli for the hard blocks and higher tensile strength for the elastomeric samples. Higher elongation values were attributed to the liberation of the elastomeric chain ends that otherwise would have been constrained in the interface region. The higher T_g values of the hard blocks corresponded to an increase in the hardness values and a decrease in the tear‐strength values. The increase in the amount of urea groups within the hard segments, as a result of the increased amount of water and blowing catalyst, resulted in elastomeric foams with higher open‐cell content. This resulted in lower resilience values as measured using the pendulum rebound test and was attributed to the ability of the open cells to absorb and dissipate energy. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 2526–2536, 2002     

Effects of the hydroxyl value of polyol in rigid polyurethane foams

Rigid polyurethane foams (RPUFs) have been fabricated from crude MDI (CMDI) and polypropylene glycols (PPGs) of various hydroxyl values (OHVs) of 300–600 with an environmentally friendly blowing agent (HFC 365mfc). The closed cell content, compression strength, and dimensional stability of the foam increased with increase in the OHV due to the increased crosslink density, which was evidenced from the increased glass transition temperature (T_g). The cream time, gel time, tack‐free time, and density of the foam showed a minimum at 500OHV. The decrease and increase around the minimum were, respectively, interpreted in terms of increased mixture mobility and extensive allophanate crosslinks, which retard gelling as well as foaming reactions. The thermal conductivity also showed a small minimum at 500OHV. Copyright © 2008 John Wiley & Sons, Ltd.     

Temperature‐dependent changes in the hydrogen bonded hard segment network and microphase morphology in a model polyurethane: Experimental and simulation studies

Hydrogen bonding between hard segments has a critical effect on the morphology and properties of polyurethanes. Influence of temperature on hydrogen bonded urethane network and melting behavior of a model semicrystalline segmented polyurethane was investigated by experiments and simulations. Polyurethane was synthesized by the stoichiometric reaction between p‐phenylene diisocyanate and poly(tetramethylene oxide) (PTMO) with a molecular weight of 1000 g/mol. Simulations were carried out using dissipative particle dynamics (DPD) and molecular dynamics (MD) approaches. Experimental melting behavior obtained by various techniques was compared with simulations. DPD simulations showed a room temperature microphase morphology consisting of a three‐dimensional hydrogen‐bonded urethane hard segment network in a continuous and amorphous PTMO matrix. The first‐order melting transitions of crystalline urethane hard segments observed during the continuous isobaric heating in DPD and MD simulations (340–360 K) were in reasonably good agreement with those observed experimentally, such as AFM (320–340 K), WAXS (330–360 K), and FTIR (320–350 K) measurements. Quantitative verification of the melting of urethane hard segments was demonstrated by sharp discontinuities in energy versus temperature plots obtained by MD simulations due to substantial decrease in the number of hydrogen bonds above 340 K. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018 , 56, 182–192     

Prediction of the glass transition temperature of cycloaliphatic amine–epoxy networks

Stoichiometric, completely cured binary or ternary mixtures of the diglycidylether of bisphenol A (DGEBA), isophorone diamine (IPD), and trimethylcyclohexylamine (TMCA) monomers were studied by differential scanning calorimetry. The relations between the glass transition temperature T_g and the structure of the copolymer networks were investigated. Good predictions of T_g can be obtained on the basis of the following hypothesis: (1) the molar contributions M_iT^?1_gi to the “copolymer effect” of difunctional groups are additive, (2) the crosslinking effect can be expressed by the DiMarzio relation in which the molar contribution of crosslink mers is an increasing function of the stiffness of linear segments. For the seven systems under study, the deviation between calculated and experimental T_g values is within experimental scatter.     

Effects of methyl sidegroup on the extent of hydrogen bonding and modulus of polyurethane elastomers

The effects of methyl sidegroup on the properties of polyurethane elastomers, mainly hydrogen bonding and elastic properties, were studied. Polyurethanes of almost equal crosslinking density and urethane group concentration, but with different side methyl groups, were prepared by a prepolymer method. The extent of hydrogen bonding was determined by an infrared technique. The density and Young's modulus are decreased by the introduction of methyl sidegroup, swelling ratio and T_g are increased, but the extent of hydrogen bonding is not affected.     

One‐shot synthesis of high molar mass polyurethane in supercritical carbon dioxide

Well‐defined polyurethane–polydimethylsiloxane particles of tunable diameter in the range of 0.5–20 μm were synthesized in “one‐shot” by step‐growth polymerization using supercritical carbon dioxide (scCO₂) as a dispersant medium. Polymerizations were carried out at 60 °C and above 25 MPa, after the solubility of each reactant in scCO₂ has been determined in its typical reaction concentration. The synthesis of such copolymers was achieved by polyaddition between short aliphatic diols, that is, ethylene glycol, 1,4‐butanediol (BD) or polyethylene oxide (M_n = 200 g mol^?1), and tolylene‐1,4‐di‐isocyanate (TDI) in the presence of mono or di‐isocyanate‐terminated polydimethylsiloxane (PDMS) as reactive stabilizers and dibutyltin dilaurate as a catalyst. The nature of the diol used as well as the functionality of the reactive stabilizer incorporated was found to have a dramatic effect on the molar mass and the morphology of the resulting product. Thus, copolymers obtained from the polyaddition of BD and TDI in the presence of di‐isocyanate‐terminated PDMS exhibit molar mass up to 90,000 g mol^?1. Thermal behaviors of copolymers were also examined by differential scanning calorimetry. All samples exhibited only one glass transition temperature (T_g) and were found to be totally amorphous. A logical decrease of the T_g was observed as the length of the diol incorporated increased, that is, as the density of urethane linkages within the polymer decreased. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 5649–5661, 2007     

Segmental and crosslink point motion in networks

¹³C- and ³¹P-NMR spin lattice relaxation in the rotating frame have been measured on a series of networks prepared from monodisperse and deliberately bimodal poly(propylene glycols) (PPG) crosslinked with tris(4-isocyanatophenyl) thiophosphate. The T_1pC minima correspond to loss maxima in the DMTA (Dynamic Mechanical Testing) measured at 10Hz. The T_1p^P minima fall at higher temperatures than those of T_1p^C for the same network indicating that these crosslinks lag the segments in frequency of motion at a given temperature. The carbon relaxation is biphasic below T_g of the segments indicating two relaxation domains which we assign to bulklike PPG segments and PPG segments proximal to he crosslink. Lineshape analysis by a diffusional model indicates crosslink reorientation is not isotropic until well above T_g. Relaxation and lineshapes for the bimodal networks indicate that junctions are not uniformly plasticized by the segments.     

Microstructure to property relations in a family of millable polyurethane fluoroelastomers

A series of segmented polyurethane fluoroelastomers based on perfluoropolyether macromers and functionalized with allyl groups is presented. Their peroxidic vulcanization behaviour was studied on both unfilled and carbon black filled compounds by dynamic rheometry. The optimal amount of allyl groups and peroxy content was determined. Formation of polyphasic structure with phase segregation of a low T_g fluorinated moiety was shown by calorimetric analysis. The low T_g value is not affected by vulcanization which, on the other hand, is effective in the elimination of any residual crystalline order of the urethane phase. The vulcanized compounds were characterized by tensile tests, dynamic-mechanical analysis, solvent swelling, compression tests and non-isothermal stress relaxation measurements. Mastercurves were successfully built based on time-temperature superposition principle. It resulted that polyurethanes based on symmetric diisocyanates show better chemical resistance and low temperature viscoelastic behaviour, and are promising candidates for the development of high performance low temperature sealing materials.     

Morphology development during the polymerization of styrene within a polyurethane network: Investigations by small-angle X-ray and light scattering

Phase separation that takes place during the formation of semi-interpenetrating polymer networks based on crosslinked polyurethane and linear polystyrene was studied by small-angle X-ray scattering and light scattering. The kinetics of the chemical reactions was followed by Fourier transform infrared spectroscopy. The occurrence of broad peaks in the X-ray scattering curves was interpreted in terms of distances between the urethane crosslinks. Small modulations on these curves were assigned to sphere-like structures with a diameter of around 5 nm which might be related to the urethane crosslink regions. Small modulations on the light-scattering curves at the beginning of styrene polymerization were assigned to spheres with diameters of around 4.5 μm, which can be related to the polystyrene-rich phase. These modulations disappear with time, which might indicate an increasing polydispersity of the domain sizes. The final morphology was found to depend on the time at which polymerization of styrene is initiated with respect to the time of gelation of polyurethane.     

Effect of structure on properties of polyols and polyurethanes based on different vegetable oils

We synthesized six polyurethane networks from 4,4′‐diphenylmethane diisocyanate and polyols based on midoleic sunflower, canola, soybean, sunflower, corn, and linseed oils. The differences in network structures reflected differences in the composition of fatty acids and number of functional groups in vegetable oils and resulting polyols. The number average molecular weights of polyols were between 1120 and 1300 and the functionality varied from 3.0 for the midoleic sunflower polyol to 5.2 for the linseed polyol. The functionality of the other four polyols was around 3.5. Canola, corn, soybean, and sunflower oils gave polyurethane resins of similar crosslinking density and similar glass transitions and mechanical properties despite somewhat different distribution of fatty acids. Linseed oil–based polyurethane had higher crosslinking density and higher mechanical properties, whereas midoleic sunflower oil gave softer polyurethanes characterized by lower T_g and lower strength but higher elongation at break. It appears that the differences in properties of polyurethane networks resulted primarily from different crosslinking densities and less from the position of reactive sites in the fatty acids. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 809–819, 2004     

Effects of the type of crosslink on viscoelastic properties of natural rubber

The viscoelastic properties of various crosslinked natural rubbers, NR, were investigated by mechanical spectroscopy. The glass transition temperature, T_g, was found to be dependent on both the crosslink density and the crosslink type. Higher values of T_g were obtained for sulfur-crosslinked NR than for peroxide-crosslinked NR at the same crosslink density. The greater influence of the sulfur content on T_g may be attributed to polysulfidic crosslinks and cyclic sulfide structures favored at high sulfur contents. Sulfur-vulcanized NRs with monosulfidic crosslinks, favored at relatively high accelerator/sulfur ratios, have properties more similar to the peroxide-cured NR with simple carbon(SINGLE BOND)carbon crosslinks covalent bonds, resulting in only small shifts in T_g. A qualitative analysis of monosulfidic crosslinks and polysulfidic structures was performed with ¹³C solid-state NMR spectroscopy. The storage modulus, E′, in the rubbery plateau region increased with increasing crosslink density. However, the crosslink type did not influence the moduli values as much as it influenced the T_g values. Different methods of detecting the crosslink density were also discussed. © 1996 John Wiley & Sons, Inc.