Structure-property relationships in thermoplastic elastomers: I. Segmented polyether-polyurethanes

Segmented poly(ether-b-urethanes) have been synthesized with 2000 MW polypropylene oxide coupled with diisocyanates and diol type chain extenders. The diisocyanates used were symmetric rigid 4, 4′-diphenylmethane diisocyanate (MDI), linear aliphatic hexamethylene diisocyanate (HDI), and unsymmetric rigid toluene-2, 4-diisocyanate (TDI). The chain extenders were symmetric N, N′-bis(2-hydroxyethyl) terephthalamide (BT) and N, N′-bis(2-hydroxyethyl)-hydroquinone (BH) unsymmetric N, N′-bis(2-hydroxyethyl)isophthalamide, and linear aliphatic 1, 4-butanediol (B). Hard segment contents ranged from 20 to 40 wt percent. The thermal behavior of these materials is consistent with phase separation into separate hard and soft domains, In order of increasing temperature above the soft segment T_g, there are transitions which occur in the regions ?56 to ?36°C (T_a), 70 to 90°C (T_b), and 138 to 168°C (T_m). The former is probably associated with soft segment change from a viscoelastic to an elastomeric state. Values of T_a are ～ ?51 C and ?56°C for the MDI-BT and HDI-BT polymers, respectively, and are independent of hard segment content. Microscopy showed that the former polymers have spherulitic morphology, so these materials have good microphase separation and exhibit crosslinked elastomeric properties. The TDI-BT or BI and MDI-B polyurethane have composition-independent T_a values of ?41 and ?36°C, respectively. These materials probably have considerable “domain-bound-ary-mixing”. At low hard segment content the MDI-B polymers behave as non-crosslinked elastomers. Only the MDI-BI polymers have _Ta values, which are strongly affected by composition, increasing in magnitude with increasing of hard segment content. This is interpreted as significant “mixing-in-domains” and is supported by morphology observed by microscopy. The next higher transition, T_b, probably involves dissociation of interdomain hydrogen bonding. In the case of the MDI-BT polyurethanes, the spherulites associated with the hard domains had disappeared at 141°C and the few small spherulites in the MDI-BI polymers disappeared at 130°C. The T_b values are 70, 83 to 90, and 100°C for the MDI-B, HDI-BT, and HDI-BI polymers, respectively. The melting transitions occurred between 138 to 168°C for the various polyurethanes except for the MDI-BT systems which decompose before melting. Thermal decomposition is a two-stage process. Hard segments decompose between 200 and 300°C. The initial decomposition temperatures are lowered in the presence of strong acid. Soft segments decompose at higher temperatures. The mechanical properties of the MDI-BI polyurethanes are charateristic of crosslinked elastomer, the results of which will be presented in a subsequent paper.     

Polyurethane elastomers containing polybutadiene and aliphatic diols: Structure-property relationships

The effect of aliphatic diols on the structure and some mechanical properties of polyurethane elastomers containing hydroxyl-terminated polybutadiene and three different diisocyanates was studied. Differential scanning calorimetric studies revealed the existence of several thermal transitions, characteristic of structures of multiphased elastomers. Three transition temperatures, a subzero transition and two high temperature transitions, were found in some of the elastomers. The higher-high temperature transition reflects ordered domains, also supported by X-ray diffraction, Higher degree of order was achieved with longer diols. The mechanical behavior is affected by the multiphase nature of the elastomers, especially by the morphology of the hard segment domains. The structureproperty relationships for the three component polyurethane elastomers in question thus have been established.     

Segmented polyether ester copolymers—A new generation of high performance thermoplastic elastomers

This paper reviews a series of high-melting thermoplastic polyether esters prepared by transesterification from dimethyl terephthalate, polytetramethylene ether glycol (MW 600–2000), and 1,4-butanediol. The resulting copolyesters exhibit a two-phase domain structure consisting of amorphous polyether ester soft segments and crystalline tetramethylene terephthalate hard segments. By proper selection of the relative amounts of soft and hard segments, polymers ranging from relatively soft elastomers to impact resistant elastoplastics may be obtained. The preparation, polymer structure as well as the physical and environmental properties of polyether esters are discussed. The combination of good melt flow properties, excellent melt stability, and rapid hardening rates permits the processing of these polymers by a wide variety of methods. The excellent processing characteristics in conjunction with the unusual physical and mechanical properties of segmented polyether esters has led to their wide acceptance.     

PVC thermoplastic elastomers

The paper describes the development and properties of a new family of thermoplastic elastomers based on polyvinyl chloride. Comparisons of the new products with traditional flexible PVC's will be presented, pointing up differences in specific gravity, low temperature properties, heat resistance, melt viscosity, and general physical properties such as tensile modulus and elongation. The elastomeric properties of the new materials are explored and compared to some typical cured elastomers—chloroprene, chlorosulphonated polyethylene. The common thermoplastic elastomers based on block copolymers and polyurethanes are also considered in relation to the new compositions. Such properties as compression-set, flex and fatigue resistance, oil and chemical resistance, weatherability, and performance at elevated temperatures are detailed. The unique feature of this technology that allows one to tailor the surface characteristics from typically high PVC gloss to the fine dull satin finish of cured elastomers is discussed.     

Structure-property relationships in nanocomposites

A numerical finite-difference model is presented for the study of the factors controlling the properties of composites reinforced with platelets and fiber-like nano-inclusions. The approach provides a comprehensive treatment of the dependence of composite modulus and strength on the shape of the inclusions and the interrelated effects of their orientation, volume fraction, aspect ratio, modulus and interfacial properties with the matrix. At the same volume fraction, we find that platelets are generally more efficient than fibers in improving composite modulus. This is rationalized through our model finding that fibers have a typically low critical aspect ratio value, which puts an upper limit to their reinforcement potential. Platelets also turn out to be superior to fibers in all nanocomposites characterized by a poor orientation of the inclusions. We also find that low interfacial adhesion and poor dispersion of the inclusions lead to a decrease in reinforcement efficiency. Turning to comparison with experiment, a good agreement is found between our model predictions and modulus data on nanocomposites reinforced with montmorillonite platelets and carbon nanotubes.     

Structure-property relationships for thermoplastic block terpolymers containing poly(styrene-p-tert-butylstyrene) copolymer end segments

Terpolymers containing polybutadiene as a central segment and diblock or random copolymer terminal units of equimolar amounts of styrene and p-tert-butylstyrene have been synthesized and evaluated. The structure property relationships were examined by a combination of transmission electron microscopy (morphology), dynamic viscoelasticity measurements, and melt rheological evaluations. The presence of p-tert-butylstyrene was found to lead, under certain circumstances, to thermoplastic elastomers exhibiting good phase separation and mechanical properties at ambient temperature along with Newtonian melt viscosities at low shear rates at elevated temperatures. This latter behavior is in contrast with that exhibited by conventional poly(styrene-diene-styrene) triblock copolymers.     

Interfacial studies of crosslinked urethanes: Part III. Structure-property relationships in polyester waterborne polyurethanes

These studies examine crosslinking reactions of polyurethanes (PURs) using attenuated total reflectance Fourier-transform (ATR FTIR) spectroscopy and show that higher relative humidity (RH) accelerates the crosslinking reactions leading to the formation of polyurethane and polyurea. Concentration levels of unreacted isocyanate (NCO) are greater at the film-air (F-A) interface than the film-substrate (F-S) interface. In contrast to the previous studies on polyacrylate emulsion urethanes, no stratification was detected between 0.65 to 1.14 μm near the F-A and F-S interfaces. This behavior is attributed to equivalent weight differences, 3100 g/eq for polyacrylate and 1140 g/eq for polyester. Solvent evaporation experiments show that approximately 10% of the initial water concentration remains in the film for extended periods of time, resulting in reactions leading to the formation of urea near the F-S interface. PUR film formation occurs in two stages, a solvent vapor pressure controlled stage, followed by a diffusion controlled stage. The duration of each stage depends on several factors, including the amount of shear induced on the shear thinning waterborne urethanes, which subsequently affects the exposure of isocyanate aggregates to water. Increased RH significantly affects structure-property relationships of waterborne PURs due to urea formation, which alters the glass transition temperature, storage modulus, crosslink density, and film hardness. Dept. of Polymers and Coatings, Fargo, ND 58105.     

Structure-property relationships in amorphous polyamides

New methods are put forward to explain the numerical values of some useful bulk physical properties of amorphous copolyamides, in terms of parameters related to monomer structure. Polyamides based on adipic, tetramethylsuberic, iso- and tere-phthalic acids, and diamines including isophorone, xylylene, cyclohexane, hexamethylene and its trimethyl derivatives, methylnonane and dodecamethylene diamines were studied. ε-Caprolactam and 12-aminododecanoic acid were also used as comonomers. In Part I, an empirical rule is proposed, based on experimental observations which predicts whether a copolyamide has an amorphous or crystalline character. The rule is based on the individual stereochemical contributions of the constituent monomers to the overall polymer chain structure. A relationship between Vicat softening point and monomer composition is derived from experimental data, which seems to be generally applicable to all amorphous polyamides of the diacid/diamine type. Each monomer makes a molar contribution which has been determined experimentally for all the materials studied. An arbitrary set of simple structural rules has been devised which enables the molar contributions of monomers to be related to their chemical structure. This procedure provided a method of predicting the contributions of other monomers for which molar constants had not been measured, and was successfully tested for a limited number of materials. A modified relationship was obtained experimentally to explain the effect of amino-acids on Vicat softening point.In Part II, the relationships outlined in Part I are combined on the basis of experimental evidence to provide an empirical relationship between composition and impact strength. This relationship predicted the impact strengths of the majority of eighty copolyamides, of widely different chemical structure, with a reasonably good accuracy. Substantial inaccuracy occurred only when a large proportion of a long chain aliphatic monomer was present in the polyamide. Tentative correlations between tensile strength and carbon chain length were observed from a limited number of measurements which suggests that tensile strength may be a constitutive property. The main conclusions of this work are: (a) polyamides are naturally crystalline with high melting points if more than 80% of the monomer units are symmetrical; (b) Vicat softening point and tensile strength decrease linearly with increasing monomer chain length; the softening point is particularly affected by the presence of substituent groups; (c) amino-acids reduce symmetry, impact strength and Vicat softening points of copolyamides; (d) Charpy impact strength increases with the proportion of symmetric monomer units, the rigidity of the acid and flexibility of the diamine structures; (e) tensile strength and flexural modulus correlate with each other in copolyamides of diacids and diamines and both increase as the amount and chain length of aliphatic monomer is reduced; (f) by using the empirical relationships developed in this work, it has proved possible to formulate amorphous polyamides with outstanding combinations of physical properties, when compared with commercially available polymers.     

New fluorinated thermoplastic elastomers

New fluorinated thermoplastic elastomers (FTE) with perfluoropolyether (PFPE) blocks have been synthesized by reacting a fluorinated macrodiol with aromatic diisocyanates in the presence of a solvent, followed by subsequent chain extension with low molecular weight aliphatic or aromatic diols. Tensile properties measurements and dynamical-mechanical analysis (DMA) have been carried out and the relationship between chemical structure and final properties has been determined. These new thermoplastic fluorinated polyurethanes show an elastomeric behavior over a wide temperature range (between −75 and 100°C), thanks to their multiphase morphology consisting of a continuous fluorinated phase with a very low T_g (−120°C) and a dispersed high melting hydrogenated hard phase, as verified by a calorimetric and dynamic-mechanical analysis. At the same time, some of the outstanding properties of fluorinated oligomers, such as chemical inertness and low surface tension, are retained in the final polymers. Thanks to these characteristics this new class of polymeric materials provides new opportunities for the application of thermoprocessable elastomers in advanced technological fields. © 1996 John Wiley & Sons, Inc.     

世界热塑性弹性体发展现状

综述了世界上各种热塑性弹性体的发展现状及趋势,论述了它们在橡胶工业的应用情况及前景。     

Structure-property relationships in dendritic encapsulation

Several molecular structure-property relationships are presented and compared to illustrate our current understanding of macromolecular encapsulation using dendrimers. Specifically, the effect that dendrimer architectures have on encapsulating photoactive and redox-active units fixed at the molecular core is considered.     

Structure-property relationships in rubber-toughened epoxies

Epoxies toughened with two reactive liquid rubbers, an epoxy-terminated butadiene acrylonitrile rubber (ETBN) and an amino-terminated butadiene acrylonitrile rubber (ATBN), were prepared and studied in terms of their structure property relationships. A two-phase structure was formed, consisting of spherical rubber particles dispersed in an epoxy matrix. A broad distribution of rubber particles was observed in all the materials with most of the particles ranging in size from 1 to 4 μm, but some particles exceeding 20 μm were also found. Impact strength, plane strain fracture toughness (K_IC), and fracture energy (G_IC) were increased, while Young's modulus and yield strength decreased slightly with increasing rubber content and volume fraction of the dispersed phase. Both G_IC and K_IC were found to increase with increasing apparent molecular weight between crosslinks and decreasing yield strength. The increased size of the plastic zone at the crack tip associated with decreasing yield strength could be the cause of the increased toughness. An ATBN-toughened system containing the greatest amount of epoxy sub-inclusion in the rubbery phase demonstrated the best fracture toughness in this series. In the present systems, rubber-enhanced shear deformation of the matrix is considered to be the major toughening mechanism. Curing conditions and the miscibility between the liquid rubber and the epoxy resin determine the phase morphology of the resulting two-phase systems. Kerner's equation successfully describes the modulus dependence on volume fraction for the two-phase epoxy materials.     

Segmented polyurethane elastomers by nonisocyanate route

Novel segmented polyurethane elastomers were successfully synthesized by a nonisocyanate route using dicyclic carbonates as precursors for both soft and hard segments. The hard segment was prepared from the dicyclic carbonate of bisphenol A and m‐xylylenediamine as chain extender. The soft segment was poly(tetramethylene ether) glycol of molecular weight 1000. Three polyurethanes with different morphologies were made with soft segment concentration of 70, 50, and 30%. Method of synthesis consisted in preparation of dicyclic carbonate of bisphenol A from a commercial epoxy resin and carbon dioxide, while dicyclic carbonate of poly(tetramethylene ether) glycol (PTMEG) was made from previously prepared diglycidyl ether of PTMEG and carbon dioxide. Polymers were prepared by reacting dicyclic carbonates with m‐xylylenediamine by one‐pot process. The monomers and polymers were characterized by Fourier transform infrared spectroscopy, differential scanning chromatography, thermogravimetric analysis, ¹H NMR, ¹³C NMR, scanning electron microscopy, and mechanical measurements. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42492.     

Photomechanical degradation of thermoplastic elastomers

The photodegradation of thermoplastic elastomers designed for outdoor applications was studied with laboratory ultraviolet (UV) exposure in the unstrained state and under tensile strain (25 and 50%). Strained exposure caused a reduction of the strain to failure in subsequent tensile tests. For some combinations of material and exposure conditions, some recovery of extensibility occurred between 2 and 4 weeks. Microscopic examination revealed that this was probably due to embrittlement of the surface region that was sufficiently severe that surface cracks did not propagate into the interior and that the observed recovery did not correspond to repair or improvement of the material. Shielding the sample surface from UV irradiation reduced the formation of surface cracking very significantly, and it was deduced that the principal cause of degradation was photooxidation rather than ozone attack. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 99: 150–161, 2006     

Finite elasticity of thermoplastic elastomers

Strain energy density is developed for a network of flexible chains with weak excluded-volume interactions between segments. Constitutive equations (involving three to four material constants) are derived for an incompressible network of self-repellent chains at finite strains. These relations are applied to study the elastic response of thermoplastic elastomers at uniaxial tension. Good agreement is demonstrated between experimental data and results of numerical simulation at uniaxial deformations with elongation ratios up to 1100%.     

Structure-property relationships of chlorinated polyethylenes

This experimental study of suspension-chlorinated linear polyethylenes (CPE), (degree of chlorination 25 to 48% Cl), covers dynamic mechancial behavior at 3.5 and 110 Hz in the temperature range ? 100 to 120°C. The semicrystalline samples, with a maximum degree of crystallinity of 25%, showed the main relaxations α and β. The effect of thermal treatment was examined. In the amorphous specimens, in addition to the Tg relaxation, other low temperature damping peaks were observed. The mechancial spectra indicate structure heterogeneity and increasing stiffness with increasing chlorine substitution. For the most prominent β relaxation, the apparent activation energy was determined from the frequency shift of tan δ and E″ maxima. The effect of structure, crystallinity and frequency on Young's moduli are also discussed. For the amorphous γ ray-Crosslinked elastomeric samples the photoelastic properties were examined at near equilibrium conditions between 30° and 80°C at various degrees of crosslinking. The polarizability anisotropy of the optical link and the Mooney-Rivlin elasticity constants were determined and an attempt was made to relate the result to the specimens' structure. A compatibility study was also made for a 42% Cl, CPE/high cis polybutadiene polyblend. The damping mechanical spectra indicate a noninteracting system whose Young's moduli can be correlated with those of the pure components using a phenomenological model proposed by Takayanagi.     

Structure-property relationships in elastomer-modified terpolymer systems

A number of terpolymers, incorporating as the elastomer phase polybutadiene, polyisoprene, poly-2,3-dimethylbutadiene, poly(butadiene-co-styrene), and poly(butadiene-co-2-methyl-5-vinypyridine), were studied. Matrices were composed of poly(styrene-co-acrylonitrile) (SAN), poly(α-methylstyrene-co-acrylonitrile), and poly(styrene-co-acenaphthylene). At constant elastomer content and elastomer molecular weight in systems employing a SAN matrix, Izod impact resistance was found to vary inversely with rising elastomer-glass transition temperature. In systems of various matrix composition, using a polybutadiene elastomer, heat deflection temperatures were found to very directly and impact resistance inversely with rising matrix-glass transition temperature. In acrylonitrile–butadiene–styrene (ABS), systems of constant matrix composition and elastomer content, varying the elastomer molecular weight from 0.6 to 2.6 × 10⁵ resulted in increasing the Izod impact resistance from 0.67 to 12.8 ft-lb/in. of notch.     

Structure-property relationships in cross-linked polyester-clay nanocomposites

Crosslinked polyester-clay nanocomposites were prepared by dispersing organically modified montmorillonite in prepromoted polyester resin and subsequently crosslinked using methyl ethyl ketone peroxide catalyst at several different clay concentrations (1.0, 2.5, 5.0, and 10.0 wt%). X-ray diffraction studies revealed the formation of a nanocomposite in all cases with the disappearance of the peak corresponding to the basal spacing of the pure clay. Transmission electron microscopy was used to study the morphology at different length scales and showed the nanocomposite to be comprised of a random dispersion of intercalated/exfoliated aggregates throughout the matrix. Thermal degradation of the nanocomposites was found to be slightly but progressively hastened compared to the pure crosslinked polymer, loss and storage modulus were monotonically shifted toward higher frequency values, and the tensile modulus was found to decrease with increasing clay content. These unexpected results were rationalized based on the decrease in the degree of crosslinking of the polyester resin in the nanocomposite, in the presence of clay. In particular, the nanocomposite containing 2.5 wt% clay consistently demonstrated a drop in properties far greater than that observed at other clay concentrations, and was attributed to the greater degree of exfoliation seen in this case which presumably leads to a greater decrease in the degree of crosslinking. Oxygen permeability rates in the polyester nanocomposites decreased with increasing clay content, as expected, and was satisfactorily reproduced using a tortuosity based model.     

热塑性聚酯弹性体研究进展

介绍了热塑性聚酯弹性体的结构、性能及国内外研发状况     

耐热耐油的新型热塑性弹性体橡胶

聚偏氟乙烯(PVDF)/丙烯酸酯橡胶(ACM)并用体经动态硫化后能得到一种耐热和耐油的热塑性弹性体(TPE)。用小角度X射线散射(SAXS)、动态机理分析(DMA)和透射电子显微镜研究了TPE的形态。研究结果表明,相分解发生在动态硫化过程中,TPE由PVDF富含基料和ACM富含亚微米粒径的交联粒子组成,其中PVDF晶片是衍变而来的。该TPE具有优异的力学性能,即具有较高的拉伸强度、较大的扯断伸长率和优异的高形变状态下的应变回复率,同时还具有优异的耐油、耐热性。在ACM富含的粒子中PVDF晶片的存在可能是赋与优异耐油性的原因。