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Blown polyethylene film was exposed to ultraviolet (UV) irradiation while under stress applied (i) parallel and (ii) perpendicular to the machine direction. Oxidation was followed using the FTIR carbonyl index and was found to accelerate under tensile stress. Some samples broke during UV-tension exposure and did so in a shorter time when stress was applied in the machine direction than when it was applied in the transverse direction. Samples with different TiO2 pigments gave different rates of carbonyl development; some showed greater acceleration when stress was applied parallel to the machine direction, others when it was applied transversely. The carbonyl index was approximately the same at break for all longitudinally stressed samples and a higher carbonyl index was found at the onset of cracking for transversely strained samples.
In this study, initially thick low density polyethylene / polyethylene terephthalate (LDPE/PET) multilayer blown films were biaxially stretched at different temperatures. The effect of draw temperature and draw speed as well as adhesion between the LDPE and PET layers, through a middle tie layer, on orientation and some properties is studied. The properties of interest are tensile properties, tear, impact strength and haze. The results indicate that the tie layer enhances the properties of stretched films and that toughness is strongly affected by draw ratio and little by draw speed and temperature in the ranges studied.
Adding small amounts (less than 10%) of High Pressure Low Density Polyethylene (HP-LDPE) or High Density Polyethylene (HDPE) to a Metallocene Linear Low Density Polyethylene (mLLDPE) led to a reduction in haze of the resulting blown films. It was found that for both systems the haze reduction was mainly due to the reduction of surface roughness. mLLDPE films shows a rough surface with large lamellar aggregates or spherulitic superstructure. By adding HP-LDPE or HDPE to mLLDPE, the lamellar aggregates became smaller or the structure became simply a row structure in HP-LDPE blends. Both changes led to smoother surfaces. Although majority of the haze came from the light scattering of the rough film surface, the similarity between surface morphology and bulk morphology suggests that haze could be correlated to the bulk structure. In fact, it was found that the haze showed a good correlation with lamellar orientation. Blends with a higher lamellar orientation showed lower haze. Attempt to correlate haze with overall melt elasticity was not successful. The results suggested that the overall melt elasticity was not the dominate factor for forming oriented lamellae. Instead, the formation of oriented lamellae is promoted by the small amounts of molecules with extremely long relaxation times, which behave as oriented nuclei.
By incorporating both polymer properties and processing variables, a robust model has been developed through multivariate statistical analysis for the MD tear strengths of some solution octene LLDPE resins produced using NOVA Chemicals' Advanced SCLAIRTECH technology. It is found that the model is applicable to different resins and processing conditions. The model suggests that MD tear strength is a nonlinear function of polymer properties, machine direction strain rate and process time. It has demonstrated its utility in product development at extrusion film lines of different dimensions, and to resins produced at both pilot and commercial scales.
Biaxial stretching of polymer films such as PET is a common process in the industry. Being able to monitor the stress-strain mechanical behavior of the films during stretching and linking this with the birefringence behavior provides a good tool to investigate stress and strain optical behavior of these films undergoing complex structural organization The results indicate that PET exhibits four stage stress optical behavior in simultaneous biaxial mode. The first stage is traditional linear stress optical behavior with stress optical constant (SOC) of 5.8 GPa-1 (5800 Brewsters). In the second stage birefringence rapidly increases and this is typically associate with the formation of poorly ordered crystals that are assumed to be at the nodes of the physical network. In the third stage the birefringence begins to level off with considerable negative slope variation with stress where the crystallinity further develops. The fourth stage was observed only at high stretching rates and in this distinct stage, birefringence become near constant while stress continues to increase as the chains have reached their full extensibilities.
Semi-crystalline PET/montmorillonite nanocomposite films were processed by melt-extrusion along with 0.5% maleic anhydride as a coupling agent. Mechanical, thermal, barrier and morphological properties of the nanocomposite films with and without the coupling agent were examined. The nanocomposites have demonstrated an intercalated/exfoliated morphology with the montmorillonite acting as a crystal-nucleating agent in all of the nanocomposite film samples. DSC experiments show this effect through faster nucleation rates and an increased overall degree of crystallization. A depressed glass transition temperature is observed in all nanocomposite samples along with an increased Young's modulus and decrease in film toughness.
The influence of molecular weight (Mw) on the structural evolution of as cast amorphous poly (ethylene naphthalene), PEN, during uniaxial stretching and further relaxation is investigated by real time measurement of the true mechanical and optical behavior of the material in the rubbery state. Uniaxial deformation behavior reveals that stress-optical behavior is composed of three regions: (I) traditional stress optical region where the polymer remains amorphous, (II) fast birefringence increase region that accompany rapid rise in crystallinity and (III) birefringence saturation region. Both materials follow the stress optical rule (SOR) at low deformation levels with a stress optical constant of 27.5 GPa-1. Stress-optical behavior deviates from linearity in both MW PEN's when crystallinity exceeds 7% and the material exhibit nematic order. The deviation from linearity occurs at lower stress levels for the low MW material, nevertheless. The mechanism of neck formation has a considerable influence on the crystallization of the material. After neck starts the crystallinity increases rapidly in both materials, reaching a saturation value of approx. 35% in crystallinity when strain hardening occurs. The preliminary studies revealed that the relaxation behavior of PEN involves a complete orientation recovery following the stress optical rule when the material is first stretched within the Region I and if stretched beyond the linear range the relaxation stage is found to accompany partial orientation relaxation followed by crystallization.
Blends of a Metallocene Linear Low Density Polyethylene (mLLDPE) with a variety of high pressure Low Density Polyethylene (HP-LDPE) or High Density Polyethylene (HDPE) at different blend ratios were investigated. Effects of these additions on blown film haze and toughness are discussed.Dramatic improvement in haze is achievable either by HP-LDPE addition or by HDPE addition. The improvement in haze, however, is usually accompanied with losses in mechanical property. Careful choice of blending partner and level is crucial for the best balance of haze and mechanical properties.
The influence of draw conditions on the morphology and physical performance of polyethylene-octene elastomer(POE) film were studied in this paper. The influence of draw conditions on the morphology of POE film was discussed via wide-angle X-ray diffraction (WAXD). The results showed that draw conditions caused POE film to produce a (110) plane crystalline peak in PE. When the draw ratio (DR) = 3, the interference intensity at draw temperature (DT) = 90? was greater than that at DT = 75?; and when DR = 6, the interference intensity at DT = 75? was greater than that at DT = 90 ? . Elastic recovery rate and stress-strain value were tested using an Instron multi-testing machine. The results were used to establish a dynamic deformation mechanism. The results showed that, after draw processing of POE, the stress value fell after approaching the saturation point with the increase in draw ratio. Furthermore, it was also shown that the elastic recovery rate and stress retention rate were better at DT = 75? than at DT = 90?.
Evolution of the morphology of dispersed PTFE dispersion particles on glass and mica as a function of melt time and temperature indicates that individual molecules wander" on the substrate crystallizing (from a chain- folded "mesomorphic" state it is suggested) in single and multi-molecule chain folded single crystals when cooled. Shearing of the particles results in nano-fibril production; annealing results in shish-kebab formation through single molecule addition. Nascent dispersion particle structure will also be considered."
Three sets of two-component blends from various narrow-MWD (molecular weight distribution), linear (no rheologically significant long branches) polyethylenes were prepared with multiple compositions in each set of blends. These blends were deliberately prepared such that the branching (from 1-hexene co-monomer) was present exclusively on either the high or the low molecular weight blend component. The average branching content in each blend component was verified to be uniform across its MWD. In this study, the influence exerted by such selective placing of the branching on the crystallization characteristics of the resulting blends will be discussed. Further, some new observations relating to the tensile stress-strain behavior of these blends will also be described.
The influence of blend composition on deformation as well as stress-optical behavior of cast amorphous PEN/PEI blends is investigated above their respected glass transition temperatures. To elucidate the structural changes occurring during stretching a real time birefringence technique coupled with true stress/strain measurement technique is used. These techniques are able to track fast changes in birefringence and stress levels as the film is being deformed. The results obtained by these two techniques permit determination of the stress optical coefficient and the limits of validity of Stress-Optical Rule (SOR). Further investigations by X-ray and DSC measurements help to understand and clarify the structure developed during the deformation process of the material. These studies indicate that the deformation of 95/5 and 90/10 PEN/PEI blends in rubbery state involves three main stages. In the first stage the material remains amorphous following the stress optical rule. In the second stage the birefringence rises rapidly and it is associated with the highly disordered nematic like structure formation. The final stage is attributed to finite chain extensibility resulting in little change in birefringence with significant stress increase. 80/20 PEN/PEI blend does not show up the final stage III in the deformation process and it seems it is delayed to higher stretch levels as the crystallization is highly suppressed due to dilution effect .
Ethylene copolymers are extensively used in the automotive industry as preferred plastomers for impact modification of polypropylene. Performance of these plastomers is strongly effected by the molecular architecture determined by the type and amount of comonomer, and the distribution of the co-units along the polymer backbone. In this paper we describe the analysis of a series of commercial and developmental metallocene random ethylene-octene copolymers with low and ultra-low density in the range 0.860 g/cm3 to 0.906 g/cm3, and melt index between 0.5 to 27. Thermodynamic properties and crystallization behavior are evaluated using differential scanning calorimetry (DSC) to determine the effect of crystalline and amorphous phase on the density and mechanical performance. Peak melting temperature and crystallinity are almost linearly related to the density while MI does not appear to have any effect on the crystalline phase. A thermal segregation technique is used to determine the influence of octene branches on the sequence length of crystallizable methylene units. With an increase in octene content, glass transition broadens and decreases with density as crystallization mechanism changes. Longer sequences form lamellar structures while presence of shorter sequence lead to a mixed crystalline morphology consisting of predominantly fringed-micellar crystals. Results of modelling of properties and performance will be discussed and compared with the previous analysis of a series of ethylene-butene plastomers.
While epoxy thermosets are commonly used and are best known for their high glass transition temperature (Tg), creep resistance, environmental resistance and high stiffness, they are extremely complicated and intractable to thorough investigation. This is in part due to the fact that these are curing systems and gelation marks a turning point in the system’s performance as well as ability to be probed for effective structure-property relationships. In addition, practical formulations often contain multiple components that have subtle but important interactions to the final performance.In this presentation we will cover work that was performed recently to quantitatively probe the effect of one such practical yet important effect, namely chain termination. The effect of the size as well as flexibility of the chain termination group will be examined via a controlled host matrix chemistry that comprises of DER™ 332 as the epoxy, bisphenol A (BA) as extender and tris(4- hydroxyphenyl)ethane (THPE) as a crosslinker. Data and trends pertaining to Tg, stiffness, yield strength, fracture toughness and thermal expansion coefficient will be discussed.
UV curable polymer systems are commonly characterized for degree and time of cure by thermal methods such as scanning photocalorimetry. In this work, we modified existing thermal and rheological instruments in order to apply a controllable dosage of UV radiation and compare the curing kinetics of two commercial polyester and epoxy powder coating systems differing in rate of cure. In general, there is an optimal temperature range whereby the material is completely cured. The optimal conditions are generally set for material heated above its flow temperature to cure within 5 seconds of UV radiation. Our thermal and rheological results are in good agreement with each other and with manufacturer's published data employing photocalorimetry. The data were analyzed for temperature dependence and efforts were made to correlate thermal and rheological results with time dependent structural changes monitored by FTIR spectroscopy.
A diglycidyl ether of bisphenol A (DGEBA) epoxy resin was chemically modified using a silanolterminated polymethylphenylsiloxane (PMPS) and two methoxy-terminated PMPS intermediates with high (3.3/1) and low (0.5/1) phenyl/methyl ratios. At room temperature, the methoxy-terminated PMPS with the high phenyl/methyl ratio was miscible with the epoxy resin and formed a one-phase system; however, both the silanolterminated PMPS and the methoxy-terminated PMPS with low phenyl/methyl ratio were incompatible with the epoxy resin and phase separation occurred. While, at 120?, resins modified with all three PMPS intermediates formed homogeneous solutions. A range of the tensile and the fracture properties of unmodified and modified epoxy systems was obtained by changing the modification method, the concentration and the type of PMPS modifiers, the type of curing agent, and the cure cycle. The modified epoxy systems have slightly decreased tensile moduli. The chemical modification method is more efficient in the improvement of the fracture toughness than the physical blending method. The fracture toughness increases with an increase in PMPS concentration. The silanol-terminated PMPS has a better toughening effect than the two methoxyl-terminated PMPS modifiers. The polyoxypropylene diamine (POPDA) cured epoxy systems have much higher fracture toughness values than the 1,3- phenylenediamine (MPDA) cured epoxy systems.
The effects of moisture absorption on products molded from polyamides are well documented. However, other high-performance materials such as polyetherimide and polyethersulfone also absorb significant amounts of moisture from the atmosphere. Early tests suggest that this moisture absorption influences the glass transition temperature of these polymers. This has implications for short-term elevated temperature service as well as creep resistance and fatigue resistance at lower temperatures. This paper quantifies the effects of absorbed moisture on the elevated temperature properties of polyetherimide and characterizes rates of moisture gain and loss at different environmental conditions.
In this paper we describe a comparison of the fibre orientation structures developed in a transverse ribbed plate during injection moulding, with those predicted using the Moldflow commercial software, for a model transverse ribbed component. Fibre orientation measurements were carried out using a large area image analysis system developed at the University of Leeds. Moldflow analyses of the ribbed plate were conducted and compared to measured fibre orientation distributions. Adjustments of the fibre interaction coefficient within Moldflow analyses have yielded significant improvements in model accuracy.
The calorimetric glass temperature was measured for three cyclic polystyrenes with apparent molecular weights ranging from 4.0 x 103 to 195.5 x 103 g/mol for both bulk material and for samples freeze-dried from dilute solution. Freeze-drying from dilute solution was found to reduce the glass temperature by 7 to 14 K depending on the sample. These Tg depressions are 5 to 12 K greater than those found previously for freeze-dried linear polystyrene. Annealing at 403.2 K and 443.2 K (130 °C and 170 °C) resulted in recovery of the Tg back to the bulk value with the time scales depending on both temperature and the magnitude of the Tg reduction; the low apparent activation energy dependence of the recovery of Tg precludes its being due to viscous flow.
Results are presented from an experimental investigation to evaluate the validity of a new, molecular based yield model. The proposed model incorporates effects of test conditions such as stress state, strain rate and temperature, as well as the effects of molecular architecture. The effects of molecular architecture are quantified by parameters considering stiffness and cohesive strength of the network. Previously published data along with new results of yield in compression and plane strain are used to assess the validity of the proposed model.
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