Elastomeric properties of networks prepared from high-cis and high-trans 1,4-polybutadienes

Networks were prepared from a sample of 1,4-polybutacliene having an extremely high cis content (98.5 mol percent) and from other samples having relatively high trans contents (72 to 80 mol percent). The cross-linking techniques employed were gamma irradiation, ultraviolet irradiation, and peroxide thermolysis. The resulting elastomers were studied in elongation in the unswollen state at several temperatures, to their rupture points, and in swelling equilibrium in a good solvent. Values of the elasticity constant 2C₁ obtained from the stress-strain isotherms at low and moderate elongations gave values of the molecular weight M, between cross-links in at least approximate agreement with those obtained from the swelling equilibrium data. The other elasticity constant 2C₂ generally had relatively large values, suggesting inhomogeneous cross-linking, but approached zero for small M_c as expected from reduced configurational interpenetration. At high elongations, the reduced stress for the high-cis polymer showed marked upturns due to strain-induced crystallization.     

Elastomeric latex domain-interpenetrating polymer networks: Physical and rheological properties

The combination of rubbery and rigid polymers in a multiphase structure using staged emulsion polymerization has yielded materials with properties ranging from reinforced elastomers to high impact plastics. The many different particle morphologies that result from a two-stage latex (TSL) polymerization include core/shell, domain, interpenetrating polymer networks (IPN), and various combinations thereof. The sequence of polymerization, crosslinking, grafting, and composition are among the significant parameters that determine the particle morphology. Elastomeric TSL with soft polyacrylates (PA) as the seed particles and polystyrene (PS) as the second stage, with each stage lightly crosslinked, may yield IPN-microdomain particles. The particle morphology has been elucidated through a combination of microscopy and mechanical property analyses. The significant modulus of elastomeric latex interpenetrating polymer networks (LIPN) results from reinforcement by PS intra-particle microdomains and their significant tensile strength from a strength forming mechanism of PS inter-particle microdomains. The increase in the PA seed crosslinking increases the crosslinked PS (xPS) level of molecular mixing with, and grafting via residual unsaturation to, the crosslinked PA (xPA) network and decreases particle deformnability. At higher xPS concentrations the formation of an xPS-rich shell enhances xPS continuity in the molded material through the partial coalescence of the shells, diminishing the PA continuity, and yielding more PS-like properties. The submicron lightly crosslinked latex particles with these different morphologies flow as a pseudoplastie material through a particle slippage flow mechanism exhibiting neither a Newtonian plateau nor a yield stress at low shear rates. The deformable lightly crosslinked particles with interchangeable PS ties which disintegrate at elevated temperatures retain their identity and regain their shape at the cessation of shear. The LIPN can be processed using standard thermoplastic methods and machinery, with power law constants and shear insensitive flow activation energies that are similar to those of thermoplastics at high levels of shear. Uncrosslinked PS shells around crosslinked PA seed particles, on the other hand, completely coalesce upon molding to form a continuous thermoplastic PS matrix that may essentially flow through molecular deformation.     

Elastomeric properties of bimodal networks prepared by simultaneous curing-filling technique

If the tetraethylorthosilicate (TEOS) used to end link hydroxyl-terminated poly(dimethylsiloxane) chains is present in excess, there are two effects on the resulting network structure. First, some of the excess TEOS hydrolyzes to give in situ precipitation of reinforcing silica particles. In addition, some can cause extension of the polymer chains, particularly of the shorter chains in the case of a bimodal network. In the present investigation, the ultimate strength and toughness of such bimodal networks was found to go through a maximum with increase in the amount of excess TEOS used in the curingfilling procedure.     

Particle Adhesion to Elastomeric Substrates and Elastomeric Substrates with Semi-Rigid Coatings

The force needed to remove micrometer-size polystyrene particles from elastomeric substrates having Young's moduli of 3.8 and 320 MPa was measured using atomic force techniques. It was found that the removal force was approximately an order of magnitude less for the more rigid substrate than for the more compliant substrate. In both cases the removal force was independent of applied load. However, when the more compliant material was overcoated with the stiffer material, the particle removal force was found to increase with increasing pressure, with the limit at low pressure commensurate with the removal force observed for the stiffer substrate and commensurate with the more compliant material at higher pressures. The results are interpreted in terms of the penetration depth of particle asperities into the substrates.     

Elastomeric alloys of PVC

The performance standards for flexible PVC compounds continue to become more demanding as manufacturers pursue more value-added applications traditionally dominated by rubbers and thermoplastic elastomers. Blending of PVC with other polymers has proven to be a viable approach in developing higher performance flexible PVC-based materials. This paper will review some of the typical approaches to producing PVC-based elastomers, and show how these standard flexible PVC blends compare to a newly developed family of PVC alloys. The advantages of these materials will be discussed, and some examples of commercial materials and their application will also be reviewed.     

Elastomeric properties of ethylene/propylene copolymers prepared by zirconocene/methylaluminoxane catalysts

Ethylene/propylene copolymers (EPM) have been prepared by a liquid propylene suspension process, using homogeneous catalysts based on nonbridged zirconocenes and methylaluminoxane (MAO). When bis(η⁵-cyclopentadienyl)zirconium dichloride (Cp₂ZrCl₂), bis(η⁵-cyclopentadienyl) dimethyl zirconium (Cp₂ZrMe₂) and bis(η⁵-cyclopentadienyl), dibenzyl zirconium (Cp₂ZrBz₂) were used as catalyst components, EPM having low average molecular weight (MW) were in general obtained in low yield. On the contrary, a very large improvement of productivity and MW was observed when bis(η⁵-indenyl)zirconium dichloride (Ind₂ZrCl₂) and bis(η⁵-indenyl) dimethyl zirconium (Ind₂ZrMe₂) as well as bis(η⁵-tetrahydroindenyl)zirconium dichloride [(IndH₄)₂ZrCI₂] were employed in combination with MAO. In particular, this last catalytic system allowed one to obtain EPM with a narrower distribution of MW in addition to the above characteristics. Better performances, in terms of rheological and elastomeric properties, were also found for the crude and vulcanized samples prepared by (IndH₄)₂ZrCl₂/MAO catalyst.     

Thermal history and mechanical properties

An account is given of three separate experiments in which the creep modulus and the recovery after creep are shown to be strongly affected by the thermal history. The tensile creep modulus of several amorphous plastics in their glassy state increases with storage time; the recovery after creep can be very much worse than normal under some quite ordinary operating conditions; the modulus in flexure of polypropylene at 20°C is remarkably sensitive to minor thermal treatment (e.g. quenching to 0°C after a short period at 40°C). The results, including those on the polypropylene, are all consistent with the concept of slow molecular re-ordering leading to a state of lower free volume. The implications for materials evaluation programmes are discussed.     

Elastomeric matrix-short fibers adhesion

This paper analyzes the effects of the addition of a three-component dry type adhesive system on the properties of composite SBR (styrene-butadiene rubber) material containing 20 percent short polyamide (5 mm length and 60 μm diameter). The curing times are extended and green strength is doubled. The replacement of the silica component in the system by a natural silicate is not only feasible, but proves to be favorable for certain properties, such as the storage modulus E′, which becomes less sensitive towards temperature changes. Dynamic property measurements provide evidence of an additional dispersion corresponding to the matrix-fiber interface located around 40 to 50°C, which does not occur in the composites without adhesive. The activation energies of the main relaxation process of these materials are determined, amounting to 175 KJ/mol for the matrix and 250 KJ/mol for adhesive-containing composites, as a consequence of the formation of strong bonds between both phases.     

磺化乙丙橡胶离聚体的熔融流动性及力学性能

研究了不同因素包括相转移催化剂、磺化度、中和度、中和用阳离子及增塑剂等对磺化乙丙橡胶离聚体的熔融流动性及力学性能的影响。结果表明,在合适条件下可得到Ｂｒａｂｅｎｄｅｒ扭矩为３５～４０Ｎｍ、拉伸强度为２０～２８ＭＰａ、断裂伸长率为５００％～６００％、永久变形小于１０％的热塑性弹性体     

聚丙烯基纳米复合材料的制备方法与力学性能

综述了聚丙烯(PP)基纳米复合材料的制备方法和力学性能的研究进展，介绍了目前国内外研究的以PP为基体与粘土层状物、无机、金属纳米粒子复合制备的复合材料的表面处理、制备方法与材料力学性能的关系。用传统的表面处理方法可改善纳米粒子的分散性与力学性能，少量纳米粒子可使PP同时获得增强增韧。     

Swelling and mechanical properties of polyvinylalcohol hydrogels

Summary The swelling and mechanical properties of Poly(vinylalcohol) hydrogels were examined. It was found that the degree of swelling of PVA hydrogels depends on annealing temperature, but is almost independent of the initial polymer concentration. Mechanical properties of the hydrogels were also influenced by the degree of swelling. A shoulder was observed in double-logarithmic plots of stress vs. strain for the hydrogels, and became clearer as annealing temperature increased. This shoulder was closely related to the breakdown of the microcrystalline domains acting as crosslinks. Also, the shape of stress-strain curves plotted double-logarithmically for the hydrogels changed with the extension rate.     

Microstructure and mechanical properties of acicular mullite

Porous acicular mullite (ACM) ceramics are known to be mechanically robust even at high porosities. This study was undertaken to better understand what aspects of acicular mullite's needle-like microstructure affect the overall mechanical properties and how the microstructure might be modified to improve mechanical performance. ACMs with a variety of porosities, pore sizes, and needle diameters were produced, and their elastic moduli, flexure strengths, and fracture toughnesses were measured. Three-dimensional image analysis was an invaluable tool in determining the needle diameters of these complex 3D network structures. It was found that porosity was the most dominant factor in determining the mechanical properties of ACM and that its behavior could be described using the Gibson–Ashby foam model.     

Porosity and mechanical properties of cement mortar

The effect of volume fraction porosity on the mechanical properties of cement mortar is studied. It is shown that both the Young's modulus and fracture toughness decrease with porosity. Although the flexural strength also decreases with porosity the linear relationship is largely fortuitous. Maximum size pores do not act as critical flaws in controlling flexural strength. The critical crack size is several times larger than the maximum pore size due to stable crack growth according to the crack growth resistance curve concept applied to cement mortar.     

Physical and mechanical properties of henequen fibers

A study of morphology, physical, and mechanical properties of henequen (Agave fourcroydes) fibers have been performed in this article. It has been concluded that properties of the fibers are more uniform in their middle section. As other natural hard fibers, henequen has a relative high tenacity, low elongation at break and a low modulus. These properties suggest that the fiber could be used as reinforcing agent in composite materials.     

Thermal and dynamic mechanical properties of polyurethaneureas

Measurements were made of the thermal and dynamic mechanical properties of 22 polyurethaneureas of varying diol molecular weight, type of aromatic chain extender, diol molecular weight distribution, and chain extender stoichiometry. The dynamic mechanical data, obtained as a function of temperature and frequency (in the kHz region), were used to construct master curves of shear modulus and loss factor over a wide range of reduced frequencies. Based on these master curves, interpreted in conjunction with the thermal analysis results, it was found that: Soft segment crystallization occurs at the higher diol molecular weights, dynamic mechanical properties are well correlated with the soft segment glass transition, diol molecular weight influences dynamic mechanical properties by affecting the degree of phase separation and hence glass transition temperature, and neither diol molecular weight distribution nor chain extender stoichiometry have a significant effect, in the ranges studied, on transition temperatures or dynamic mechanical properties.     

Rheological and mechanical properties of recycled polypropylene

In order to verify the possibility of manufacturing good quality articles by recycled polypropylene (PP), a study on the effect of many recycling operations on the rheological and mechanical properties of PP was conducted. The amount of degradation occurring during the reprocessing was estimated in the melt state by means of rheological measurements and the results obtained were correlated with the weight average molecular weight M_w and the molecular weight distribution MWD. Since the properties in the solid state are strictly dependent upon the molecular structure parameters and the morphology, mechanical properties were checked on films produced from virgin and recycled PP with a cast technology. Moreover, in order to estimate the effect of film composition and of the number of reprocessings on the final properties of the manufactured articles, blend films having different percentages of PP recycled several times and virgin PP were prepared.     

Dynamic mechanical properties of polymers

A technique, employing samples in the form of tuning forks, to measure the mechanical properties of polymers is presented. Results for low density polyethylene, polypropylene, and polycarbonates are shown. A large transition is observed for polypropylene at approximately +10°C and a small transition at ?100°C for the polycarbonates. Polycarbonate data has also been obtained from 20 to 150°C, at approximately 70 cps. Effects of time, temperature, and history are presented.     

Processing and mechanical properties of magnesium foams

An open-cellular magnesium foam has been manufactured by using powder metallurgy technology basing on space holder fillers in the present study. Depending on the volume fraction and the diameter of the carbamide particles, the porosity and pore size can be controlled in the range of 40–80% and 0.5–2.0 mm, respectively. Quasi-static compressive tests indicate that the mechanical behavior of the present magnesium foam is in good agreement with the Gibson–Ashby model when the porosity is over 45%. The most outstanding mechanical feature, however, may be its long and flat plateau region that is favorable for energy absorbing applications.