Mechanical behaviour of poly(methyl methacrylate)

A series of tensile and three-point bending studies was conducted at various temperatures and loading rates using a commercial poly(methyl methacrylate) (PMMA). Tensile properties and fracture toughness data were obtained for the various conditions. In general, both tensile strength and fracture toughness increase with increasing loading rate and decreasing temperatur E. However, when the temperature reaches the glass transition region, the relationships between fracture toughness, loading rate, and temperature become very complex. This behaviour is due to the simultaneous interaction of viscoelasticity and localized plastic deformation. In the glass transition region, the fracture mechanism changes from a brittle to a ductile mode of failure. A failure envelope constructed from tensile tests suggests that the maximum elongation that the glassy PMMA can withstand without failure is about 130%. The calculated apparent activation energies suggest that the failure process of thermoplastic polymers (at least PMMA) follows a viscoelastic process, either glass or transition. The former is the case if crack initiation is required.Deceased.     

The anomalous lowering of the glass transition of an epoxy resin by plasticization with water

The validity of the assertion that 6 to 7 wt% absorbed water lowers the glass transition,T _g, (~250° C) of a highly cross-linked, high temperature epoxy resin (NARMCO 5208) by 100 to 150° C was investigated by a number of thermoanalytical techniques. This plasticization is of significance to the use of organic matrix composition skins on supersonic aircraft which experience a sudden thermal spike, and supposedly a sudden loss in modulus. The study was complicated by the loss of water above 100° C during the tests and the decomposition of the polymer at about the temperature of its apparent glass transition. No glass transition could be observed by differential scanning calorimetry and the thermal mechanical analyser gave inconclusive results. The dynamic mechanical analyser gave a clear indication of a glass-like transition at about 250° C which was reversible, reappearing upon cooling and reheating. This transition could not be related to the decomposition of the sample. Absorption of 7 wt% water broadened and lowered the transition by about 50° C. However, after studying the rate of change of the modulus of the resin from the plasticized state to the dry state it was concluded that the absorbed water lowersT _g by only 50° C and not the 100 to 150° C claimed by others. Moreover, the mechanism of this apparent glass transition differs from that of a normalT _g, and may involve the breaking of hydrogen bonds.     

Modification of vinyl ester and vinyl ester–urethane resin-based bulk molding compounds (BMC) with acrylated epoxidized soybean and linseed oils

Acrylated epoxidized soybean and linseed oils of different characteristics were incorporated in the absence and presence of polymeric methylene diphenyl isocyanate (PMDI) in a vinyl ester (VE) resin-based bulk molding compound (BMC) up to 15 wt% (with respect to VE resin). The thermal, thermo-mechanical, static fracture mechanical, dynamic impact (Charpy), and thermal degradation properties of the BMC compounds were determined. With increasing amount of functionalized plant oils the glass transition temperature (T _g) of the matrix, the stiffness (E modulus) and Charpy impact strength of the BMCs decreased. The static fracture toughness was slightly increased and the fracture energy remained unaffected by the modification with increasing amount of oil. Additional crosslinking of VE, induced by PMDI, markedly enhanced the T _g but yielded a large drop in the glassy modulus. This finding was traced to resin dilution and to unfavored PMDI/kaolin interactions triggered by the water content of the latter. The thermal degradation of the BMCs was less affected, however, their degradation started earlier for the modifications either with functionalized plant oil or PMDI. Dilution of VE-based BMCs with acrylated epoxidized plant oils requires reworking of the corresponding recipes to keep the property degradation limited.     

Temperature dependence of fracture toughness of epoxy resins cured with diamines

The fracture toughness (critical stress intensity factor, K _Ic) of epoxy resins cured with four diamines has been measured as a function of temperature over the range from –35° C to above T _g. It was found that K _Ic for each epoxy-diamine system did not vary below room temperature, and in the higher temperature range K _Ic increased with increasing temperature to a maximum and then decreased. The temperature which maximized K _Ic, agreed with the temperature at which the flexural modulus of the epoxy resins abruptly dropped. This temperature was therefore considered as T _g. This temperature was found to be about 20° C lower than the heat deflection temperature under load (1.82 M Pa) of the resins.     

Effect of functional monomer GMA on the physical–mechanical properties of coatings from poly(BA–MMA) latexes

Functional acrylic latexes based on the terpolymer of butyl acrylate (BA)–methyl methacrylate (MMA)–glycidyl methacrylate (GMA) were prepared with different amounts of GMA via semi-batch emulsion polymerization at 75 °C, using potassium persulfate as an initiator, sodium dodecylbenzene sulfonate as an emulsifier and sodium bicarbonate as a buffer. The latexes were characterized by laser light scattering, apparent viscosity, and proton magnetic resonance (¹H NMR) and Fourier transform infrared (FTIR) spectroscopies. Physical and mechanical properties of cured and uncured latex films were investigated. For uncured films, gel content, solvent resistance, flexibility, and adhesion of uncured films were improved using higher GMA in the feed, while the glass transition temperature (T _g), surface hydrophobicity, water resistance, tensile modulus and strength, and hardness were decreased. For cured films, higher T _g values, more gel content, improved water and solvent resistances, and enhanced hardness and adhesion, were obtained with increasing the GMA contents. Curing the latex films resulted in hydrophilic surfaces.     

Influence of post-curing and temperature effects on bulk density,glass transition and stress-strain behaviour of imidazole-cured epoxy network

The effects of post-curing and temperature on the glass transition, bulk density and stress-strain behaviour in the glassy and rubbery state of 2-ethyl-4-methyl imidazole-cured epoxy network have been evaluated by differential scanning calorimetry (DSC), water displacement and tensile testing. The glass transition temperature,T _g, was found to increase with increasing post-cure temperature and the size of the base line shift in the glass transition region on the DSC thermogram can serve as an indicator of the extent of cure. At room temperature, the decrease in bulk density with increasing extent of cure may be attributed to the additional cross-linking, adding molecular constraints to the thermal constraints. Thus, a higher free volume atT _g can be expected to remain in the glassy state as the sample is slowly cooled through the glass transition temperature. In the investigation on the temperature dependence of the tensile mechanical properties, a fracture envelope was obtained. The tensile strength, Young's modulus and ultimate elongation in the glassy and rubbery state are discussed in detail.     

Improvement of tensile properties and toughness of an epoxy resin by nanozirconium-dioxide reinforcement

Zirconium dioxide (ZrO₂) nanoparticles were systematically added as reinforcement to a diglycidyl ether of bisphenol A (DGEBA)-based epoxy resin. A series of composites with varying amounts of nanoparticles was prepared and their morphology and mechanical properties were studied. The obtained nanocomposites were characterized by tensile tests, dynamic mechanical thermal analysis, and fracture toughness (K_IC) investigations; by standardized methods, to define the influence of the nanoparticle content on their mechanical and thermal properties. The morphological analysis of the composites shows that nanoparticles form small clusters, which are uniformly distributed into the matrix bulk. The tensile modulus (E) and the K_IC of the epoxy matrix increase at rising zirconia content. Improvements of more than 37% on modulus and 100% on K_IC were reached by the nanocomposite containing 10 vol.-% ZrO₂ with respect to the neat epoxy (E_o = 3.1 GPa, K_ICo = 0.74 MPam^0.5). The presence of nanoparticles produces also an increment on glass transition temperature (T _g). The epoxy resin added with 8 vol.-% ZrO₂ records a T _g approximately 8% higher than the unmodified matrix (T _go = 100.3 °C).     

The effect of nitrogen on viscosity of La-Si-Mg-O-N glasses by compressive creep and dilatometry

Viscosity as a function of temperature and nitrogen content and the glass transition temperature (T_g) were investigated in 20La-60Si-20Mg-O-N glasses with nitrogen contents varying from 0 to 28 eq.% (e/o) using compressive creep and dilatometric experiments. Although T_g obtained from dilatometry were 6–12°C lower than the lower limit of the transition temperature range from creep, both methods revealed identical and linear dependencies of T_g on N content. The average activation energy was 1184 ± 36 kJ/mol and viscous flow remains the deformation mechanism in all glasses regardless of nitrogen content. Addition of 28 e/o N in oxide glass resulted in an increase of T_g by 94–105°C and an increase in viscosity of around 5 orders of magnitude. These changes are greater than those reported in similar Al-containing glasses. Linear increase of T_g and compactness of the glass with nitrogen content result from enhanced cross-linking of the glass network.     

Structural transition in poly(methyl methacrylate) due to large deformation at temperatures below the equilibrium second-order transition temperature

To analyse the structural change in glassy polymers under large deformation at low temperatures, poly(methyl methacrylate) specimens were uniaxially compressed at temperatures below the equilibrium second-order transition temperature T ₂ with varying strain rates. The state of steady plastic flow which appeared in the lower yield range of the stress-strain curve was analysed using the Eyring equation in a novel way. This analysis provided the following results: at low temperatures, the volume of a flow unit decreased with temperature, probably approaching the least critical value; and a functional relation between the activation enthalpy H and the activation entropy S for the glass deviated gradually from that for the melt derived from the WLF equation with decreasing temperature. This deviation could be attributed to the structural transition of the glass into liquid-like structures of the melt at elevated temperatures above T _g + 100 K where the WLF equation is no longer available.     

Effects of BaO on the glass formation of the PbO-B2O3-TiO2-BaO system in relation to transition temperatures

The variation of the BaO content on the quaternary PbO-B₂O₃-TiO₂-BaO system's glass formation tendency was investigated in relation to transition temperatures, such as melt temperature (T _f), liquidus temperature (T _l), crystallization temperature (T _c) and glass transition temperature (T _g). Compositions were melted between 800°C and 1300°C. In order to obtain bulk glass samples, glass formation was carried out using a preheated cylindrical brass mould without forcing the cooling rate to increase. Glass formation tendency increased with increasing the temperature ratios of T _g/T _l and T _c/T _l and with decreasing T _g/T _c. Eutectic compositions preferentially formed glasses from melts due to their low melting temperatures.     

Tensile behaviour of blends of poly(vinylidene fluoride) with poly(methyl methacrylate)

Blends of poly(vinylidene fluoride) (PVF₂) and poly(methyl methacrylate) (PMMA) were prepared over a wide concentration range and tested in tension at the same relative temperature below the glass transition. Testing was performed at strain rates ranging from 10 to 0.01 min^–1 at test temperatures fromT _g-40 toT _g-10. By normalizing the test temperature to fixed increments belowT _g, blends and homopolymers can be compared on the basis of PVF₂ and PMMA composition and crystallinity. In nearly all blends, under conditions favouring disentanglement, (decrease in strain rate, or increase in test temperature), the yield stress and drawing stress decreased while the breaking strain increased. For materials with about the same degree of crystallinity, those with a higher proportion of amorphous PVF₂ exhibited brittle-like behaviour as a result of interlamellar tie molecules. In the semicrystalline blends, yield stress remains high as the test temperature approachesT _g, whereas in the amorphous blends the yield stress falls to zero nearT _g. Results of physical ageing support the role of interlamellar ties which cause semicrystalline blends to exhibit ageing at temperatures aboveT _g.     

Effect of surfactant treatment on thermal stability and mechanical properties of CNT/polybenzoxazine nanocomposites

The mechanical and thermo-mechanical properties of polybenzoxazine nanocomposites containing multi-walled carbon nanotubes (MWCNTs) functionalized with surfactant are studied. The results are specifically compared with the corresponding properties of epoxy-based nanocomposites. The CNTs bring about significant improvements in flexural strength, flexural modulus, storage modulus and glass transition temperature, T_g, of CNT/polybenzoxazine nanocomposites at the expense of impact fracture toughness. The surfactant treatment has a beneficial effect on the improvement of these properties, except the impact toughness, through enhanced CNT dispersion and interfacial interaction. The former four properties are in general higher for the CNT/polybenzoxazine nanocomposites than the epoxy counterparts, and vice versa for the impact toughness. The addition of CNTs has an ameliorating effect of lowering the coefficient of thermal expansion (CTE) of polybenzoxazine nanocomposites in both the regions below and above T_g, whereas the reverse is true for the epoxy nanocomposites. This observation has a particular implication of exploiting the CNT/polybenzoxazine nanocomposites in applications requiring low shrinkage and accurate dimensional control.     

Electrospun nanofibre toughened carbon/epoxy composites: Effects of polyetherketone cardo (PEK-C) nanofibre diameter and interlayer thickness

Polyetherketone cardo (PEK-C) nanofibres were produced by an electrospinning technique and directly deposited on carbon fabric to improve the interlaminar fracture toughness of carbon/epoxy composites. The influences of nanofibre diameter and interlayer thickness on the Mode I delamination fracture toughness, flexure property and thermal mechanical properties of the resultant composites were examined. Considerably enhanced interlaminar fracture toughness has been achieved by interleaving PEK-C nanofibres with the weight loading as low as 0.4% (based on weight of the composite). Finer nanofibres result in more stable crack propagation and better mechanical performance under flexure loading. Composites modified by finer nanofibres maintained the glass transition temperature (T_g) of the cured resin. Increasing nanofibre interlayer thickness improved the fracture toughness but compromised the flexure performance. The T_g of the cured resin deteriorated after the thickness increased to a certain extent.     

Synthesis and characterization of core–shell nanoparticles and their influence on the mechanical behavior of acrylic bone cements

Core–shell nanoparticles consisting of polybutyl acrylate (PBA) rubbery core and a polymethyl methacrylate (PMMA) shell, with different core–shell ratios, were synthesized in order to enhance the fracture toughness of the acrylic bone cements prepared with them. It was observed by TEM and SEM that the core–shell nanoparticles exhibited a spherical morphology with ca. 120 nm in diameter and that both modulus and tensile strength decreased by increasing the PBA content; the desired structuring pattern in the synthesized particles was confirmed by DMA. Also, experimental bone cements were prepared with variable amounts (0, 5, 10 and 20 wt.%) of nanoparticles with a core–shell ratio of 30/70 in order to study the influence of these nanostructured particles on the physicochemical, mechanical and fracture properties of bone cements. It was found that the addition of nanostructured particles to bone cements caused a significant reduction in the peak temperature and setting time while the glass transition temperature (T_g) of cements increased with increasing particles content. On the other hand, modulus and strength of bone cements decreased when particles were incorporated but fracture toughness was increased.     

Effect of norbornene content on deformation properties and hot embossing of cyclic olefin copolymers

The dynamic mechanical behaviour of a series of cyclic olefin copolymers (COCs) with varying norbornene content has been examined in the vicinity of the glass transition temperature, T _g. Using dynamic mechanical thermal analysis (DMTA), the temperature of the glass transition in COC increased linearly with increase in % norbornene. Above T _g, the magnitude of the elastic storage modulus, E′, decreased exponentially with rise in temperature for all of the copolymers. The loss modulus, E″, has also sharply decreased at temperatures above the transition with a levelling-off in E″ at ≥20 °C above T _g for all grades. The results of DMTA have been used in the identification of the optimum conditions for hot embossing experiments. Hot embossing of COC at ≥20 °C above the transition temperature in a region of viscous liquid flow has resulted in a full replication of channel depth without cracking or distortion.     

The β-relaxation in epoxy resins; the temperature and time-dependence of cure

Internal friction and creep measurements have been used to reveal the mechanism of cure in epoxy resins cross-linked with diethylene triamine (DETA). The -relaxation is associated with the main glass transition of the undercured resin network. The glass transition temperature (T _g) is about 40° C above the maximum temperature of cure and the curing reaction slows down about 2 h after each increase of the temperature. At 25° C the cure is only about half complete and since in this resin, when fully cross-linked, T _g is at about 140° C, temperatures of 100° C or over are needed to complete cure.     

Strain rate- and temperature-dependent tensile properties of an epoxy-based, thermosetting, shape memory polymer (Veriflex-E)

In this study, the inelastic deformation behavior of an epoxy-based, thermally triggered shape memory polymer resin, known as Veriflex-E, was investigated. The experimental program was designed to explore the influence of strain rate on monotonic loading at various temperatures which is needed to establish the design space of SMPs in load bearing applications. Thermally actuated shape memory polymers can be thought of as having two phases separated by the glass transition temperature (T _g ). At temperatures below the T _g , Veriflex-E exhibits a high elastic modulus and positive, non-linear strain rate sensitivity in monotonic loading. The Poisson’s ratio at room temperature is independent of the strain rate, but dependent upon the strain magnitude. As the temperature is increased, the strain rate sensitivity in monotonic loading decreases. Well above the T _g , the elastic modulus drops by several orders of magnitude. In this high temperature region, the material achieves strain levels well above 100% and Poisson’s ratio is constant at 0.5 regardless of strain rate or strain magnitude.     

The Influence of Different Plasticizers and Polymers on the Mechanical and Thermal Properties,Porosity and Drug Permeability of Free Shellac Films

ABSTRACT

The effect of triethyl citrate (TEC) and different molecular weights and concentrations of polyethylene glycol (PEG), in addition to the effect of different water-soluble polymers and dispersions at different levels, hydroxypropyl methylcellulose (HPMC), methylcellulose (MC), carbomer 940, polyvinyl alcohol (PVA), ethyl cellulose (EC), on the mechanical and thermal properties, drug permeability, and porosity of free shellac films were investigated. Shellac films were cast from aqueous solutions, and their mechanical properties were studied by tensile test. Thermal analyses were performed using differential scanning calorimetry (DSC).

The results showed that the addition of plasticizer caused a decrease in both elastic modulus and glass transition temperature (T_g) and an increase in elongation at break of free shellac films. This effect was related to the concentrations of plasticizers. Different molecular weights of PEGs have different plasticization mechanisms.

Moreover, the incorporation of different amounts of HPMC, MC, or carbomer in free shellac films caused an increase in the flexibility, decrease in T_g, and a marked increase in drug permeability of free shellac films, whereas the addition of PVA caused a decrease in flexibility and drug permeability and an increase in T_g. Addition of EC resulted in a slight decrease of the elasticity and a small decrease in drug permeability. However it does not show a considerable effect on the T_g. In addition, it was found that the drug permeability is directly related to the mechanical properties and T_g of shellac films.     

Physical ageing and glass transition in amorphous polymers as revealed by microhardness

Microhardness (MH) data as a function of temperature for two amorphous polymers [poly(methylmethacrylate) and poly(vinylacetate)] and two semicrystalline polymers [poly(ethyleneterephthalate) and poly(arylether ether ketone)] quenched into the amorphous state are presented. It is shown that MH can conveniently detect the glass transition temperature (T _g) for the above mentioned polymers. Molecular rearrangements taking place above and belowT _g, such as physical ageing leading to a more compact molecular packing, and thermal expansion can also be followed by means of MH measurements. Finally, the presence of a crystalline phase in these materials has been shown to shift theT _g value towards higher temperatures.     

Phenomenological study of a single craze propagating in several glassy polymers over a wide range of temperatures

The single craze which appears at the crack tip in some amorphous polymers has been observed between — 100 and + 100° C in five different polymers. The results show that the single craze exists only between two temperatures: a certain critical temperatureT _c and the glass transition temperature. In the case of oriented block co-polymers the structure also affects the formation of the craze. Below the critical temperatureT _c, a multiple craze appears ahead of the crack and the fracture toughnessK _c increases.