Phase separation in segmented polyurethanes derived from mixtures of polyether polyols with different functionality

A series of segmented polyurethanes containing 60 wt° of hard segments (HS) was prepared from MDI (4,4-diphenylmethane diisocyanate) ethylene glycol and mixtures of a polyoxyethylene end-capped polyoxypropylene triol and a polyoxyethylene end-capped polyoxypropylene diol. The effects of the content of polyether diol in polyether polyols on phase separation and properties was investigated by dynamic mechanical analysis (DMA), differential scanning calorimetry (DSC) and investigation of tensile properties. The DSC and DMA results indicate that the polyurethane derived from only polyether triol exhibits obvious phase separation and that the HS is immiscible with the SS, but that the HS is compatible with the HS for the polyurethane derived from polyether diol. As the content of polyether diol increases, the compatibility between HS and SS increases. As the content of polyether diol increases, the tensile strength. elongation. toughness and tear resistance of the polyurethanes increases. but their moduli decrease. The modulus-temperature dependence in the temperature region of –30 to 65 °C increases as the polyether diol content increases.     

Phase structure in segmented polyurethanes having fatty acid-based soft segments

Structural studies on two segmented polyurethanes prepared from modified oleic acid soft segments and butane diol/diphenylmethane diisocyanate hard segments, were performed. Polyurethanes were thermoplastic elastomers with 50% and 70% soft segment concentration (SSC) for TPU-50 and TPU-70, respectively. DMA revealed the co-continuous morphology in TPU-50 and dispersed hard domains in the soft matrix in TPU-70. AFM showed micron size globules in TPU-70 ascribed to hard segment-rich superstructures. Alternating small soft and hard domains were present in TPU-50, with ～15 nm lateral dimensions and local modulus deviations from 40 to 50 MPa, consistent with co-continuous morphology. The dispersed hard domains in the soft matrix of TPU-70, and co-continuous phases in TPU-50 morphology was confirmed. USAXS showed the domain spacing of TPU-50 was about 13.3 nm. TPU-70 demonstrated domain spacing of about 11.3 nm. USAXS revealed that the interface between phase-separated domains exhibited surface-fractal-like behavior.     

Composition-dependent properties of segmented polyurethanes

We studied segmented polyurethanes (SPU) from poly(diethylene glycol adipate) with molar masses M_SFT = 780, 1365, 1780 and 3200 g/mol (soft fragments, SFT), 2,4-toluene diisocyanate and 2,4-toluene diamine (stiff fragments, STF) with essentially monodisperse molar mass distribution of STF. They were characterized by WAXS, SAXS, heat capacity measurements, mainly from 150–450 K, and enthalpies of solution in dimethyl formamide at room temperature, ΔH_sol. The experimental results may be summarized as follows.

• The glass transition temperatures of SFT decrease with an increase of M_SFT, but remain essentially composition-invariant at fixed M_SFT, whatever the STF volume fraction φ.
• The surface-to-volume ratio of STF domains exhibits sudden, jump-like transitions at volume fractions φ* = 0.23 and φ** = 0.40, respectively.
• Exothermic (i.e., negative) values of ΔH_SOl exhibit a jump-like decrease at φ* = 0.23.

It has been shown that the main cause of the latter effect is the transition of SFT from a somewhat extended to an essentially unperturbed conformation.     

Thermal degradation of segmented polyurethanes

Thermal degradation of polyurethane samples was studied by a thermogravimetric method. The effect of soft-segment length and soft-segment concentration on activation energy of the degradation process was measured. Three methods of calculation gave activation energies at different stages of the very complex weight loss process. It was shown that at initial stages of the weight loss the process was dominated by hard-segment degradation. Activation energy of the whole process calculated by the Ozawa–Flynn method did not offer clear insight into the structure–stability relationship of polyurethanes. The second method showed that activation energy of the initial stage of degradation increased with decrease in hard-segment length. The Flynn method of calculating activation energy produced fairly constant activation energy of the first 40% degradation, at around 150 kJ/mol, for polymers in the C series. Generally, calculation of kinetic parameters of a complex degradation process as in polyurethanes gives results that are not easy to interpret. It has been shown qualitatively that polymers with higher soft-segment concentration display lower initial weight loss and higher onset temperatures of degradation. © 1994 John Wiley & Sons, Inc.     

Dielectric properties of segmented polytetramethyleneoxide-based polyurethanes

Three series of segmented polyurethanes, based on polytetramethylene oxide (PTMO) soft segments, having molecular weights of 650, 1000, and 2000 and MDI/butane diol hard segments, were synthesized and their dielectric properties examined. The effect of the soft segment length, soft segment concentration (ssc) as structural variables, and frequency and temperature as experimental variables, on relative permittivity and tan δ, were examined. The results were discussed in terms of the structural parameters such as the degree of phase separation and soft segment phase state. It was found that both soft segment length and ssc strongly affect dielectric behavior.     

Phase separation studies in RIM polyurethanes catalyst and hard segment crystallinity effects

Polyurethanes were prepared from pure 4,4′-diphenylmethane diisocyanate (MDI), 1,4-butane diol (BDO) or 1,2-ethane diol (EDO) and α,ω-hydroxyl poly(propylene oxide) (PPO) by reaction injection moulding (RIM). Hard segment (MDI + BDO or EDO) level was 45–50 wt%. The PPO had about 20% ethylene oxide copolymerized in at the chain ends to provide 80% primary OH end groups. M_n was varied from 2000 to 4000. Dibutyl tin dilaurate catalyst and mould temperature were varied. Dynamic mechanical, wide-angle X-ray, differential scanning calorimeter, molecular weight and tensile elongation measurements were made on the RIM polyurethanes. At low reaction rates (low catalyst or temperature) highly crystalline, well phase separated but low molecular weight polymers were produced. At high catalyst or temperature levels more poorly phase separated but high molecular weight, tough polymers resulted. Higher M_n PPO gave better phase separation and EDO gave higher melting temperatures. Preventing hard segment crystallinity by substituting asymmetric MDI or glycols resulted in phase compatibility.     

Interdependence of structure and properties in segmented polyurethanes

On the basis of Clough and Schneider's studies, who investigated segmented polyurethanes with the use of DSC, we assumed that warming may lead to morphological changes in these compounds. We warmed polymers for 10 h in argon atmosphere at 130°. Morphological changes occurring in polyurethanes at a modified temperature were confirmed with an electron microscope and were found to affect to a high degree the mechanical properties of the polymer investigated.     

Specific refractive index increments of segmented polyurethanes

Specific refractive index increments ν of polyester-based segmented polyurethanes in N,N-dimethyl formamide have been determined, and the quotient dν/df_d has been evaluated (where f_d is the weight fraction of hard-segment units). The results are in good agreement with the values calculated from group contributions to the molar refraction, using the Vogel or the Gladstone–Dale equations. The values calculated with the Lorenz–Lorentz equation are too low. A potential explanation of this fact is proposed. The same methods have been applied to reported ν values for polyether-based polyurethanes. An explanation is proposed for differences in dν/df_d for polyester- and polyether-based polyurethanes. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 68: 1917–1923, 1998     

Synthesis of surface-modifying macromolecules for use in segmented polyurethanes

Polyurethanes are one of the most important classes of thermoplastic elastomers and have been widely used in medical-device manufacturing as well as in other applications. However, their function can be limited, particularly under environmental conditions that render them susceptible to hydrolysis. Using polymeric additives that are hydrolytically stable may be one approach to modifying the surface of polyurethanes for the purpose of improving their hydrolytic resistance without compromising their structural features. In this paper, the development of a series of novel fluorine-containing polyurethane surface modifying macromolecules (SMMs) is described and their synthesis conditions are investigated. The material structure and mixing properties of the synthesized SMMs with base polyurethanes was dependent on the reactant stoichiometry and concentration for the SMM components, as well as the reaction temperature and the amount of catalyst used in the SMM synthesis. This study describes the use of low surface energy components (fluorinated tails) which showed selective migration towards the surface when added to a polyester-urea-urethane. These novel macromolecules generated a nonwettable surface while not significantly altering the apparent bulk structure of the base polymer. The advancing and receding contact angle results indicated that the surface of these modified polyurethanes showed wettability characteristics similar to that of Teflon. ^TM The differential scanning calorimetry thermograms for the mixtures of the SMM with the polyurethane showed that, at 5% w/w SMM in the base polyurethane, the thermal transitions were similar to that of the native base polyure-thane, indicating that the additives had no detectable effect on the polyurethane structure. © 1996 John Wiley & Sons, Inc.     

Determination of lamellae in segmented polyurethanes by electron microscopy

The morphology of segmented polyurethane was studied with an electron microscope. Polymers were obtained by the prepolymer method from poly(ethylene adipate), 4,4-diphenylmethane diisocyanate and 1,4-butanediol. The unstained sample showed spherulites with fibrillar structure. Attempts were made to enhance the contrast in order to study the fibrillar structure more thoroughly. The staining method used to study the structure of lamellae is described.     

Extraction fractionation of segmented polyurethanes according to composition

Extraction of solid polyester-based segmented polyurethanes by chloroform and its mixtures with an increasing amount of N,N-dimethylformamide is described. Unlike the precipitation in a solvent-nonsolvent system, where fractionation according to the molecular weight dominates, the extraction can resolve the copolymer into fractions differing in the content of diisocyanate units. © 1995 John Wiley & Sons, Inc.     

Properties of segmented polyurethanes derived from different diisocyanates

Various segmented polyurethane materials with a polyurethane hard segment (HS) content of 40 wt % were prepared by bulk polymerization of a poly(tetramethylene ether) glycol with M_n of 2000, 1,4‐butanediol, and various diisocyanates. The diisocyanates used were pure 4,4′‐diphenylmethane diisocyanate (MDI), 2,4‐toluene diisocyanate (T100), toluene diisocyanate containing 80% 2,4‐isomer and 20% 2,6‐isomer (T80), isophorone diisocyanate (IPDI), hydrogenated 4,4′‐diphenylmethane diisocyanate (HMDI), and 1,6‐hexane diisocyanate (HDI). The segmented polyurethane materials were characterized by differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), tensile properties, tear strength, and Shore A hardness. The DSC and DMA data show that the thermal transitions are influenced significantly by the diisocyanate structure. In the segmented polyurethane materials with aliphatic HS, the polyether soft segment (SS) is immiscible with the HS. However, in the segmented polyurethane materials with aromatic HS, the SS is partially miscible with the HS. The diisocyanate structure also influences the mechanical properties significantly and is described as the effect of symmetry and chemical structure of the HS. Various solution polymerized polyurethane resins with solid content of 30 wt % were also prepared and their thickness retention, water resistance, and yellowing resistance were determined for the evaluation of their usage as wet process polyurethane leather. The polyurethane resin with aliphatic HS show poorer thickness retention but better yellowing resistance. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 75: 167–174, 2000     

Electron-microscopical investigation of the morphology of segmented polyurethanes

Segmented polyurethanes obtained by two-step synthesis were studied by electron microscope on specimens prepared as replicas from the brittle surface of the polymer and on thin sections cut with a cryo-ultramicrotome. Each of the specimens showed two different kinds of dispersed phase. Information obtained on morphology complemented each other. The continueous phase observed on the thin section directly by microscope showed no structure of its own and was similar in all polymers investigated, whereas the surface of the brittle fracture showed two kinds of continuous phase.     

Biconstituent fibers from segmented polyurethanes and nylon 6

The characteristics of Monvelle, a new biconstituent fiber from nylon 6 and a segmented polyurethane, are reviewed briefly, and some of the technical problems inherent in producing such a fiber are discussed. The characterization of two series of polyurethanes which can be melt spun is given in detail. The chemical composition of the hard segment was maintained constant, being derived from 4,4′-diphenylmethane diisocyanate (MDI) and 1,4-butanediol, in all polymers. In one series using poly(butylene adipate) of MW 2000 as the soft segment, the average hard segment content was varied from 33% to 54%. In the other series, the hard segment content was held at 43%, and three additional soft segments, each at MW 2000, were used: poly(ethylene adipate), polycaprolactone and poly-1,4-oxybutylene glycol (PBG). Characterizations include molecular weight distributions, thermal analysis, rheological studies, and selected small-angle and wide-angle x-ray diffraction and polarized light microscopy. Crystallinity, melt viscosity, and activation energy of flow increased with increasing hardsegment content. Changes in the polyester soft segments had little effect on the properties studied, but with PBG the crystalline melting point of the polymer, without annealing, was higher and the melt viscosity was slightly higher than corresponding polyester-based samples, in agreement with previous reports of sharper phase separation in polyether urethanes, compared to polyester urethanes.     

Preparation and release profiles of cyclophosphamide from segmented polyurethanes

Supermolecular structure of drug delivery system on the basis of segmented polyurethane (SPU) has been determined to control the release of anticancer drug, cyclophosphamide (CPh). It has been established that the phase separation in SPU is essentially intensified by means of both the increase of molecular weight for SPU's soft segments and CPh incorporation in the monolithic systems of polyethylene glycol‐based polyurethane. Infrared and proton NMR data indicate that CPh is hydrogenicly associated with a urethane group of hard segments. It has been determined that the drug‐concentrated domains of hard segments are microheterogeneously dispersed in the amorphous soft segments. These results indicate that a supermolecular structure design of SPU allows for control of the CPh release from the polymer matrix. Medical‐biological tests of the prepared polyurethane device have shown reduced toxic action of the cytostatic drug compared with injections. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 75: 35–43, 2000     

Effect of silica nanoparticles on morphology of segmented polyurethanes

Two series of segmented polyurethanes having soft segment concentration of 50 and 70 wt%, and different concentrations of nanometer-diameter silica were prepared and tested. Atomic force microscopy revealed a strong effect of nanoparticles on the large-scale spherulitic morphology of the hard domains. Addition of silica suppresses fibril formation in spherulites. Filler particles were evenly distributed in the hard and soft phase. Nano-silica affected the melting point of the hard phase only at loadings >30 wt% silica. A single melting peak was observed at higher filler loadings. There is no clear effect of the filler on the glass transition of soft segments. Wide-angle X-ray diffraction showed decreasing crystallinity of the hard domains with increasing filler concentration in samples with 70 wt% soft segment. Ultra small-angle X-ray scattering confirms the existence of nanometer phase-separated domains in the unfilled sample. These domains are disrupted in the presence of nano-silica. The picture that emerges is that nano-silica suppresses short-scale phase separation of the hard and soft segments. Undoubtedly, the formation of fibrils on larger scales is related to short-scale segment segregation, so when the latter is suppressed by the presence of silica, fibril growth is also impeded.     

Physical and mechanical behavior of sterilized biomedical segmented polyurethanes

Changes in the physical and mechanical behavior induced by two sterilization methods, gamma irradiation and ethylene oxide, were determined on two commercial medical-grade segmented polyurethanes. The two materials have different chemical composition: one is an aromatic poly(ether urethane urea), Biospan^TM, and the other an aliphatic ether-free polyurethane, Chronoflex^TM. Properties before and after sterilization procedures were compared resulting in specific structural changes for each formulation. The thermal and mechanical properties were examined using differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), stress–strain measurements, and its hysteresis cycle. Molecular weight measurements were performed by gel permeation chromatography (GPC). Sterilized Biospan samples showed a decrease in the soft-segment glass transition temperature (T_g,s) and an increase in the soft segment crystallization heat along the quenching process. Sterilized Chronoflex materials showed the opposite behavior. The hysteresis percent and residual strain percent increased after sterilization. The same effect was observed when irradiation dose and strain level increased. Surface analysis performed by scanning electron microscopy showed magnification of original surface defects after sterilization. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 65:1193–1203, 1997     

Study of the structure and properties of phosphorinated segmented polyurethanes

The effect of an expanded set of physical bonds in rigid polyurethane segments has been studied by X-ray measurements, thermography, thermomechanics, etc. It has been shown that, by varying the nature and structure of quaternizing agent, the structure and properties of the segmented polyurethanes can be purposefully varied. Heat treatment (annealing) of polyurethanes, and polyurethane-ionomer samples has proved that the growth of hard domains is first of all due to the rigid segments associating with hard domains already existing.     

Crystallinity and morphology of segmented polyurethanes with different soft-segment length

A series of polycaprolactone/4,4′-diphenylmethane diisocyanate/1,4-butanediol (PCL/MDI/BDO) segmented polyurethanes of different compositions was synthesized by solution polymerization. The molecular weight of PCL diols used was in the range of 1600–7000. The crystallinity and morphology of these polymers were studied by using DSC, dynamic mechanical analysis, WAXD, and polarizing microscopy methods. It was found that the crystallinity of PCL prepolymers was depressed in segmented polyurethanes. A lower limit of PCL molecular weight was found, below which the PCL segments were not able to crystallize at the usual processing conditions. This limit of molecular weight is in the range of 2000–3000 and exhibits a slight increase with increasing hard-segment content of polyurethanes. The glass transition temperature related to the PCL segment regions in polyurethane specimens deviated from that of pure amorphous PCL prepolymer to a higher temperature. The deviation resulted from the crystallization of PCL segments and also the influence of hard segments. The formation of hard-segment domains becomes very difficult for polyurethanes having low hard-segment content and short hard-segment length. There is a lower limit of hard-segment content and segment length. Only above that limit do the polyurethanes have enough hard-segment domains acting as physical crosslinks at temperatures above the melting point of the PCL crystals. The structural characteristics of segmented polyurethanes which may exhibit a shape memory effect are also discussed. © 1996 John Wiley & Sons, Inc.