Differential scanning calorimetry analysis of morphological changes in segmented elastomers

Investigations of morphological changes which are induced in segmented elastomers by annealing and quenching are reported. Four different polymers were studied each based on the same soft segment—1000 or 2000 molecular weight poly(tetramethylene oxide). The hard segments were 4,4′-diphenylmethane diisocyanate (MDI) chain extended with 1,4-butane diol (ET series), piperazine coupled with 1,4-butane diol bischloroformate (BN-1,4), or dimethyl terephthalate condensed with 1,4-butane diol (H-50). Following annealing at various temperatures (120, 150, 170, or 190°C), the polymers were quenched to ambient conditions, and their properties measured by differential scanning calorimetry (DSC) as a function of time following the quench. DSC measurements taken immediately after the quench show that the soft segment T_g is higher than that of the control, suggesting that the applied thermal history promoted increased mixing of hard and soft segments. As time passes after quenching, the T_g values decrease and approach an equilibrium value. This effect is much smaller for those samples having crystalline hard segments. Endotherms attributed to the disruption of long range ordering in the hard segment domains resulted from the annealing process. These endotherms appeared at higher temperatures for higher annealing temperatures. The positions of crystalline melting endotherms were independent of the annealing/quenching conditions investigated.     

The thermal response of the polyether soft segment chain conformation in a polyurethane block copolymer measured by small-angle neutron scattering

The conformation of the soft segment chains in a pair of polyether polyurethanes was studied as a function of temperature using small angle neutron scattering. The samples were synthesized from 3 moles of methylene bis(p-phenyl isocyanate), 2 moles of butanediol, and one mole of a poly(tetramethylene oxide) (PTMO) blend. The PTMO blend was composed of 0.325 moles of deuterated PTMO (d-PTMO) and 0.675 moles of hydrogenous PTMO. This degree of deuterolabelling was chosen so that there would be no interphase scattering in the final sample; only intrachain scattering from the labelled soft segments comprised the coherent part of the total scattering. At room temperature, the average soft segment was found to be in an extended conformation. As the temperature was raised from room temperature, the radius of gyration of the soft segments decreased. This was attributed to the stress exerted by the extended soft segments on the adjoining hard segments increasing as the temperature was increased. The increased stress causes some of the hard segments to pull out of the hard domain into the soft phase, thereby allowing the soft segments adjacent to the extracted hard segment to relax to a more compact conformation. As the temperature was increased above 160°C, the soft segment radius of gyration increased slightly. This behaviour is ascribed to an increased degree of mixing between the phases. The presence of substantial amounts of hard segment material in the soft phase causes the increase in the soft segment R_g due to the greater compatibility between the soft and hard segments in the soft phase at these elevated temperatures. This effect is similar to a homopolymer being swollen by a small amount of a good solvent, where the chain conformation is a random coil, but the radius of gyration is larger than that measured for the pure material.     

Structure and properties of segmented polyether-esters. II. Crystallization behavior of polyether-esters with random distribution of hard segment length

Six different segmented copolyether-esters based on polybutyleneterephthalate as the hard and oligotetramethylene oxide glycols as the soft segments were studied. The weight fraction of the hard segments varied between 0.26 and 0.72 and the soft segment had an average molecular weight of either 1000 or 2000. The melting, recrystallization, and annealing behavior was studied as well as the relaxation behavior via measurement by a torsion pendulum. The DSC-data indicate that only a small fraction of all hard segment sequences crystallize. Sequences shorter or longer than the average sequence length do not crystallize but together with the soft segments form a homogeneous amorphous matrix in which the crystalline domains are embedded. These domains are completely destroyed by cold flow under stress but are formed again on annealing the stretched sample. An exponential increase of the long spacing is observed with increasing annealing temperature without change in melting temperature. The sequence distribution does not seem to influence the annealing behavior.     

Effects of aggregation structure on rheological properties of thermoplastic polyurethanes

The effects of the molecular aggregation structure on the rheological properties of thermoplastic polyurethane (TPU) were investigated. The TPU was composed of poly{(tetramethylene adipate)-co-(hexamethylene adipate)} glycol as the soft segments, 4,4′-diphenylmethane diisocyanate and 1,4-butanediol as the hard segments. The TPU sheets prepared by injection molding were annealed at various temperatures from 23 to 120 °C to vary the molecular aggregation structure. Glass transition temperature of the soft segment and melting points of the hard segment domains of the TPUs decreased and increased, respectively, with increasing annealing temperature. The results of DSC, solid-state NMR spectroscopy and dynamic viscoelastic measurements revealed that the degree of micro-phase separation of the TPUs becomes stronger with increasing annealing temperature due to the progress of formation of well-organized hard segment domains. The dynamic temperature sweep experiments for molten TPUs revealed that the temperature at critical gel point, which is defined as the temperature at which the dynamic storage modulus coincides with the loss storage modulus, in the cooling process increased with the progress of aggregation of the hard segments in the TPUs observed in the solid state. The uniaxial elongational viscosity measurements showed that TPUs exhibited an obvious strain hardening behavior with strain rate owing to residual hard segment domains at an operating temperature. It was revealed that the formation of well-organized hard segment domains had a profound effect on the rheological properties of TPUs, in particular on their elongational viscosity.     

软段结构对聚氨酯弹性体性能的影响

分别以聚四氢呋喃二醇(PTMG)、聚己内酯二醇(PCL)、高顺式端羟基聚丁二烯(HTPB)和自由基聚合制得的端羟基聚丁二烯(FHTPB)为软段,采用溶液聚合两步法制得了四种聚氨酯弹性体(PUE)。通过拉伸试验、动态力学性能分析(DMA)、差示扫描量热(DSC)和热重分析等手段,考察了软段结构对它们室温及低温下力学性能、热性能等的影响。结果表明,四种PUE低温(-30℃)下的拉伸强度和断裂伸长率均大于室温下的对应值。这不仅与低温下软段诱导结晶所产生的自增强效应有关,也与软、硬两段的微相分离程度增大有关。相较于其他三种PUE,HTPB-PUE软段不仅玻璃化温度(T _g)最低,而且极性也最弱,因而微相分离程度高,具有优异的柔性,-30℃下其断裂伸长率仍达660%以上。PCL-PUE和PTMG-PUE因软段易结晶,且软段与硬段的微相分离程度低,则刚性强。低温循环拉伸试验表明,-30℃下HTPB-PUE和FHTPB-PUE有较强的弹性恢复能力,而PCL-PUE和PTMG-PUE则相对较差。DSC和DMA结果显示HTPB-PUE的T _g远低于其他三种PUE,其T _g(DSC)低至-103℃。此外,四种PUE的初始分解温度十分相近,均在270℃左右。     

Effect of annealing on phase separation and mechanical properties of epoxy/ATBN adhesive

Amine-terminated butadiene acrylonitrile (ATBN) was applied as curing agents for diglycidyl ether of bisphenol A epoxy resin without any accelerating agent. ATBN weight percentage of 59–82 wt% was used, so that the soft ATBN domains in the cured samples formed a continuous phase, while the hard epoxy domains formed a discontinuous phase. Mechanical properties were tested in the means of strain-stress and adhesive strength. The results showed that the samples had excellent toughness at temperatures above the flexible segment glass transition temperature (T_g1), and it was well maintained after annealing at 150 °C. However, adhesive strength of the annealed sample decreased dramatically when the testing temperature was close to the rigid segment glass transition temperature (T_g2). It was observed that (T_g2) decreased and phase separation became weaker after the annealing. Real-time Fourier transform infrared (FTIR) measurement indicated that this phenomenon was related to the disassociation of hydrogen bonding within the hard domain caused by the increased mixing of the hard segments into the soft domains by the high temperature annealing. It was confirmed by transmission electronic microscope (TEM) test.     

The effect of annealing on the thermal and dynamic mechanical properties of para-tetramethyl xylene diisocyanate-based polyurethanes

A series of polyurethane elastomers based on the para-Tetramethyl Xylene diisocyanate group were annealed at temperatures from 100 to 180°C. These high-temperature exposures were found to raise significantly the peak melting point of the urethanes that contained crystalline hard segments. The corresponding enthalpy of fusion was found to decrease after annealing at the higher temperatures. In most cases, the annealings caused a slight increase in the soft-segment glass transition temperature, indicating that some increase in phase mixing had occured. Dynamic mechanical measurements indicated that the annealings caused a slight reduction in the Young's modulus and a broadening of the loss–tangent peak. The broadening of the loss–tangent peak can be attributed to the increased phase mixing induced by the annealings. The large increases in melting point induced by annealing do not appear to significantly affect the subsequent dynamic mechanical properties.     

Temperature dependent microphase mixing of model polyurethanes with different intersegment compatibilities

In this paper we explore the temperature dependence of segregation of hard and soft segments of selected segmented polyurethane copolymers using synchrotron small-angle X-ray scattering (SAXS). The copolymers are composed of the same hard segments but three different soft segment chemistries, of particular interest in biomedical device applications. Hard segments are formed from 4,4′-methylenediphenyl diisocyanate and 1,4-butanediol, and soft segments from an aliphatic polycarbonate [poly(1,6-hexyl 1,2-ethyl carbonate)], poly(tetramethylenoxide), or a mixed soft segment synthesized from hydroxyl-terminated poly(dimethylsiloxane) [PDMS] and poly(hexamethylenoxide) macrodiols. The changes in SAXS relative invariants and interdomain spacings are indicative of gradual dissolution of phase separated hard and soft segments with increasing temperature. All copolymers investigated herein, even those containing PDMS soft segments, transform to the single-phase state at a temperature determined by the soft segment chemistry (and hard segment content). The SAXS findings, along with those from parallel temperature-controlled Fourier Transform infrared spectroscopy measurements, also facilitate assignment of the origin of the thermal events observed in the DSC thermograms of these materials.     

Phase transition and domain morphology of siloxane‐containing hard‐segmented polyurethane copolymers

The phase transitions and the morphology of hard‐segment domains of those siloxane‐containing hard‐segmented polyurethane copolymers are studied by differential scanning calorimetry (DSC). The NH‐SiPU2 copolymer, which comprises a siloxane–urea hard segment and a polytetramethylene ether glycol soft segment (PTMG2000), exhibits a high degree of phase‐separation and a highly amorphous structure. Therefore, NH‐SiPU2 copolymer proceeds with a melt‐quenching process and with various annealing conditions, to examine the morphologies and the endothermic behaviors of the siloxane‐containing hard‐segment domains. DSC thermograms of further annealed NH‐SiPU2 indicate that the first endotherm (T₁) at around 75°C is related to the short‐range ordering of amorphous siloxane hard‐segment domains (Region I), and the second endotherm (T₂) at around 160°C is related to the long‐range ordering of amorphous siloxane hard‐segment domains (Region II). The DSC thermograms at annealing temperatures below and above T₁ demonstrate that both the temperature and the enthalpy of T₁ linearly increase with the logarithmic annealing time (log t_a). This result shows that the endothermic behavior of T₁ is typical of enthalpy relaxation, which is caused by the physical aging of the amorphous siloxane hard segment. Additionally, the siloxane hard segments in Region I are movable, and can merge with the more stable Region II under suitable annealing conditions. Transmission electron microscopy shows that Regions I and II are around 200 and 800 nm wide, and that the Region I can be combined with the stable Region II, under suitable annealing conditions. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 4242–4252, 2006     

Polyurethane elastomers through multi-hydrogen-bonded association of dendritic structures

A series of hydrogen bonding-rich polyurea/malonamide dendrons have been utilized as building blocks for the synthesis of novel dendritic polyurethane elastomers. Based on the resulting microstructure of soft segments reinforced by the rigid dendritic domains, the hydrogen bonding enforced phase separation of segmented polyurethanes was explored. DSC and FT-IR results indicate that a certain degree of phase separation between dendritic and poly(tetramethylene oxide) (PTMO) domains. The domain size of phase separation are less than 100 nm based on the results obtained from the atomic force microscopy (AFM) and small-angle X-ray scattering (SAXS). The analysis of tensile measurements indicates that the incorporation of various contents of different dendrons as the hard segments allows these polymers to exhibit drastically different mechanical properties. Furthermore, low complex viscosity is observed at medium temperatures (above 130 °C) via the rheological analysis. With good mechanical properties at room temperature and low melt viscosity at medium temperatures, these thermoplastic elastomeric polyurethanes are suitable for applying in hot-melt process.     

Polyätherester-Block-Copolymere – Eine Gruppe neuartiger thermoplastischer Elastomerer

High melting polyether esters can be prepared by ester interchange from readily available starting materials such as dimethyl terephthalate, polyalkylene ether glycols and linear short chain diols. The resulting block copolymers exhibit a continuous two-phase domain structure consisting of amorphous polyether ester soft segments and crystalline short chain polyester hard segments. By proper selection of the relative amounts of polyether and polyester segments, polymers ranging from fairly soft elastomers to impact resistant elastoplastics may be obtained. The preparation, morphology and physical characterization of polyether esters as well as the effect of the structures of the soft and hard segments on polymer properties are described. Polytetramethylene terephthalate based on polyether esters are particularly suited as thermoprocessable high performance elastomers offering an unusual combination of physical and chemical properties characterized by high abrasion, tear and solvent resistance as well as excellent low and high temperature properties. Because of their good melt stability, low melt viscosity and high crystallization rates, these polymers can be processed within a wide temperature range by all methods commonly used in the plastics industry.     

Relationships between synthesis and mechanical properties of new polyurea materials

New segmented polyureas were prepared from 4,4′-diisocyanato dicyclohexylmethane and amino terminated polyoxypropylene. Different cycloaliphatic and aromatic diamines were used as chain extenders as well as to synthesize the corresponding model hard segments (HSs). The competition between polycondensation and HS crystallization requires a sufficiently fast reaction (e.g., cycloaliphatic diamines associated with a catalyst, if necessary in solution), although low reactivity monomers would be necessary to avoid demanding processing techniques such as reaction injection molding; otherwise, materials with isolated (i.e., not chemically linked) HS are obtained and they display poor mechanical properties. In contrast, when an appropriate synthesis procedure is used, elastomers with high molar masses and narrow molar mass distributions can be obtained. These characteristics, associated with the high melting temperature of the hard domains, result in very good mechanical behavior, up to high enough temperatures (∼ 180–190°C). The presence of low molar masses can be responsible for a rather low but continuous energy dissipation between the relaxations of the soft and hard domains (and particularly at room temperature), but can be well limited by a short thermal treatment at high temperature. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 65: 2265–2280, 1997     

聚醚型聚氨酯弹性体的合成及其动态力学行为

采用两步合成法,以4,4′-二苯基甲烷二异氰酸酯(MDI)和1,4-丁二醇(BDO)为硬段,相对分子质量分别为1000、2000、4000的聚氧化丙烯二元醇(PPG)为软段,制备了一系列聚醚型聚氨酯(PUR)弹性体,研究了预聚体异氰酸酯指数R及软段相对分子质量对PUR动态力学性能的影响。结果表明,预聚体R值增大,即PUR的硬段含量增加,储能模量G′提高,软段相的玻璃化转变温度(Tg)升高,软硬相区的相容性增大;软段相对分子质量增加,PUR的G′下降,软段相的Tg降低,并出现硬段相的玻璃化转变,软硬相区的相分离程度增大。     

Effect of annealing on poly(urethane‐siloxane) copolymers

Poly(urethane‐siloxane) copolymers were prepared by copolymerization of OH‐terminated polydimethylsiloxane (PDMS), which was utilized as the soft segment, as well as 4,4′‐diphenylmethane diisocyanate (MDI) and 1,4‐butanediol (1,4‐BD), which were both hard segments. These copolymers exhibited almost complete phase separation between soft and hard segments, giving rise to a very simple material structure in this investigation. The thermal behavior of the amorphous hard segment of the copolymer with 62.3% hard‐segment content was examined by differential scanning calorimetry (DSC). Both the T₁ temperature and the magnitude of the T₁ endotherm increased linearly with the logarithmic annealing time at an annealing temperature of 100°C. The typical enthalpy of relaxation was attributed to the physical aging of the amorphous hard segment. The T₁ endotherm shifted to high temperature until it merged with the T₂ endotherm as the annealing temperature increased. Following annealing at 170°C for various periods, the DSC curves presented two endothermic regions. The first endotherm assigned as T₂ was the result of the enthalpy relaxation of the hard segment. The second endothermic peak (T₃) was caused by the hard‐segment crystal. The exothermic curves at an annealing temperature of above 150°C exhibited an exotherm caused by the T₃ microcrystalline growth. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102:5174–5183, 2006     

Microstructural organization of polydimethylsiloxane soft segment polyurethanes derived from a single macrodiol

Segmented polyurethane (PU) block copolymers were synthesized using 4,4′-methylenediphenyl diisocyanate and 1,4-butanediol as hard segments and oligomeric ethoxypropyl polydimethylsiloxane (PDMS) as the soft segments, with hard segment contents ranging from 26 to 52 wt%. The microphase separated morphology, phase transitions, and degrees of phase separation of these novel copolymers were investigated using a variety of experimental methods. Like similar copolymers with mixed ethoxypropyl PDMS/poly(hexamethylene oxide) soft segments, PU copolymers containing only ethoxypropyl PDMS soft segments were found to consist of three microphases: a PDMS matrix phase, hard domains, and a mixed phase containing ethoxypropyl end group segments and dissolved short hard segments. Analysis of unlike segment demixing using small-angle X-ray scattering demonstrates that degrees of phase separation increase significantly as copolymer hard segment content increases, in keeping with findings from Fourier transform infrared spectroscopy measurements.     

Morphology of polyurethane–isocyanurate elastomers

The dynamic viscoelastic properties and thermal transition behavior of reaction injection molding (RIM) and cast polyurethane—isocyanurate elastomers have been studied as a function of various segments (soft and hard urethane, and hard isocyanurate) content. RIM and cast elastomers were prepared at different concentrations of soft and hard urethane, and hard isocyanurate segments. RIM elastomers with the higher isocyanate index (lower hard urethane and greater isocyanurate segment content) displayed an unchanged T_g (glass transition temperature of soft segment) and increasing T_gh (glass transition temperature of hard segment) related to the hard urethane and isocyanurate segments. This is due to the phase separation between the soft and the hard segments. Cast elastomers synthesized from the higher amount of 1,4-butanediol (greater hard urethane and less hard isocyanurate segment content) showed an increasing T_gs, decreasing T_gh of hard urethane segments, and an unchanged T_gh of isocyanurate segments. This is related to the phase mixing between the soft and the hard urethane segments and the phase separation of hard isocyanurate and hard urethane segments.     

Segmented copolymers with monodisperse crystallizable hard segments: Novel semi-crystalline materials

Segmented block copolymers with short monodisperse crystallizable hard segments have interesting structures and properties. In the melt, such short monodisperse segments are miscible with the matrix segments. Moreover, upon cooling, they crystallize fast, demonstrating a very high crystallinity, and only a small crystallization window is needed. The melting temperature of the short segments is high, provided that they can H-bond and/or contain aromatic groups. The melting temperature was found to decrease with increasing matrix segment concentration, due to the solvent effect of the matrix segments. At concentrations of crystallizable segment of 4-35 wt%, good dimensional and solvent stabilities were obtained.The monodisperse segments crystallized into nano-ribbons with uniform thickness and high aspect ratio, and these dispersed nano-ribbon crystallites constituted physical crosslinks, while acting also as reinforcing fillers. At concentrations of the monodisperse segments below 20 wt% no spherulitic ordering took place, and the semi-crystalline polymers were transparent. The monodisperse crystallizable segments can be used in combination with matrix segments of either low or high glass transition temperature, and may even contain (bio)functional units.     

Aliphatic polycarbonate‐based polyurethane elastomers and nanocomposites. II. Mechanical,thermal, and gas transport properties

Thermal, thermomechanical, tensile and gas transport properties of aliphatic polycarbonate‐based polyurethanes (PC‐PUs) and their nanocomposites with bentonite for organic systems were studied. Hard segments are formed from hexamethylene diisocyanate and butane‐1,4‐diol. All PC‐PUs and their nanocomposites feature high degree of the phase separation. Three phase transitions were detected by temperature‐modulated differential scanning calorimetry (TMDSC) and dynamic mechanical thermal analysis. TMDSC revealed the filler affinity both to soft and hard segments, even though the affinity to hard segments is much stronger. Elongation‐at‐break at ambient temperatures is mostly over 700%, which leads together with high tensile strength (in some cases) to very high toughness values (over 200 mJ/mm³). The addition of 1 wt % of bentonite does not practically affect mechanical properties implying its very good incorporation into the PU matrix. Permeabilities and other gas transport properties depend on regularity of PC‐diol and on hard segment content, but the variations are insignificant. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Morphology control of segmented polyurethanes by crystallization of hard and soft segments

Segmented polyurethanes (SPUs) have been designed with controlled hard to soft segment ratios. The confinement effect of the SPU blocks is induced by phase separation of the SPU segments and has been harnessed to selectively control crystallization. Hard segment (HS) concentrations greater than 50 wt.% allowed for the study of morphological changes and mechanical properties associated with confinement of the soft segment (SS). It was observed that crystallization temperature and normalized percent crystallinity were reduced with increasing HS content, creating a largely amorphous PEG SS at ambient temperature. High temperature annealing further confined the SS because the HS had more time to crystallize, which increased confinement. Considerable insight has been gained through the manipulation and characterization of the SS and HS, in an SPU, towards the design of impact absorbing and structural materials.     

Segmental orientation behavior of flexible water-blown polyurethane foams

The ambient temperature structure–property orientation behavior in two different polyureaurethane polymers (one cross-linked and one linear) was measured by using infrared dichroism along with mechanical response. Thin films (plaques) thermally compression-molded from TDI-polypropylene (PO) flexible water-blown polyurea-urethane foams and solution-cast TDI–PO polyurea–urethane elastomers were studied. Segmental orientation was measured as a function of elongation and relaxation, as well as of hysteresis behavior. The level of strain was 50–70% for the plaques and up to 240% for the elastomer. The soft segments for both materials exhibited a low state of orientation with elongation. Small changes in orientation with time and upon cyclic straining were also observed for the soft segments. Significant transverse orientation upon stretching was observed in the hard segments of the plaques and up to elongations of 100% for the elastomer. The transverse behavior of the hard segments in the plaques pressed from the foams was attributed to both the smaller hard domains as well as to the polyurea aggregates that have been reported to be present in flexible foams. This transverse behavior also suggested that the smaller hard domains and the polyurea aggregates possess a lamellarlike structure. At low strain levels (up to 50%), only small amounts of orientation hysteresis as well as mechanical hysteresis were observed for the hard segments of the plaques as well as for the elastomer. No significant relaxation in orientation was detected for the hard segments of both materials at a 30% strain level.