Modulus Of Polymer Research Articles

Parylene cantilevers integrated with polycrystalline silicon piezoresistors for surface stress sensing

Polymer based microcantilevers are known to have higher sensitivity than their silicon counterparts for surface stress sensing, owing to the lower Young’s modulus of polymers. This letter reports the development of parylene cantilevers integrated with polycrystalline silicon piezoresistors for the detection of chemical or biological molecules based on the surface stress sensing principle. The justification of using parylene is presented with theoretical calculations along with finite element simulation. The fabrication process employed for making these devices is also reported. The functionality of the sensor was preliminarily proven by the detection of octanethiol vapors. The stability of the sensor in water was demonstrated as well.

New Approaches to Improved Polymer Implant Toughness and Modulus

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.

Characterisation of alginate gels using quasi-static and dynamic methods

The rheological behaviour of the calcium salts of several commercial mannuronic and guluronic alginates in the time and frequency domain was assessed by quasi-static (uniaxial compression and compressive stress relaxation) and oscillatory dynamic tests. Both tests gave almost the same range (1–10%) of apparent linear viscoelasticity. Use of the Friedrich and Heymann model [Friedrich, Chr., & Heymann, L. (1988). Extension of a model for crosslinking polymer at the gel point. Journal of Rheology, 32, 235–241] allowed the transient responses of the network of alginate gels to be approximately characterised by a single relaxation function, its empirical parameters being independent of the experimental test provided that the applied strains were kept within the elastic and proportional limit. The linear interrelation between the material compressive ( S α ) and shear ( S β ) parameters was roughly checked using an independent estimate of the Poisson’s ratio via the NOL test [Warfield, R. W., Cuevas, J. E., & Barnet, F. R. (1970). Single specimen determination of Young’s and bulk moduli of polymers. Rheologica Acta, 9, 439–446] Since the information collected from simple compression or shear tests in the linear viscoelasticity regime was inter-convertible, the small amplitude oscillatory dynamic tests, which are simpler, quicker, less material consuming and thus less expensive than the quasi-static ones, might be recommended to discriminate between alginates of different origin and composition.

Effects of Modulus and Surface Chemistry of Thiol-Ene Photopolymers in Nanoimprinting

Thiol-ene photopolymers were studied as patternable resins for nanocontact molding imprint lithography. Photopolymerizable thiol and ene monomer mixtures were used, and after molding, patterned thiol-ene polymer features the size and shape of the original molds were replicated. Adhesion and release were examined and controlled by manipulating the surface chemistry of the substrate and mold. A direct correlation between cured thiol-ene polymer modulus and pattern fidelity was observed.

Mechanical Characterization of Polymers on a Nanometer Scale through Nanoindentation. A Study on Pile-up and Viscoelasticity

The analysis of nanoindentation force curves collected on polymers through the common Oliver and Pharr procedure does not lead to a correct evaluation of Young's modulus. In particular, the estimated elastic modulus is several times larger than the correct one, thus compromising the possibility of a nanomechanical characterization of polymers. Pile-up or viscoelasticity is usually blamed for this failure, and a deep analysis of their influences is attempted in this work. Piling-up can be minimized by indenting on a true nanometer scale, i.e., at penetration depth smaller than 200 nm. On the other side, it is common knowledge that fast indentations minimize the effect of viscoelasticity. However, changing the indentation time in a broad range of contact time (fractions of second up to hundreds of seconds) did not allow the correct estimation of Young's modulus for the polymers used in this work. The final result is that the Oliver and Pharr procedure as well as any other procedure analyzing the unloading curve with elastic contact mechanics models cannot be employed to measure Young's modulus of polymers because its application is incorrect from a theoretical point of view, unless the analysis is limited to the very first nanometers of penetration depth when the contact is perfectly elastic. Viscoelastic contact mechanics models should instead be employed to characterize these materials.

Degradation and calcification in vitro of new bioresorbable terpolymers of lactides with an improved degradation pattern

Experimental implants produced from poly( l-lactide/ɛ-caprolactone/glycolide) 80/10/10% (polymer 1), poly( l-lactide/ dl-lactide/glycolide) 80/10/10% (polymer 2) and poly( l-lactide/ dl-lactide/glycolide) 80/5/15% (polymer 3) were subjected to in vitro degradation in a buffer solution at 37 °C and pH = 7.4 and calcification in vitro was performed at 37 °C in a simulated body fluid for 4, 8, 12, 16, 24, 28, 32 and 36 weeks. In vitro degradation was performed in static and pseudodynamic modes. Samples from poly( l-lactide) were used as a control. The changes in the materials during the course of degradation were assessed from the measurements of molecular weight, mechanical properties and crystallinity. The changes in the appearance of the materials upon degradation and calcification were observed using a scanning electron microscope with an EDAX attachment. The decrease of molecular weight at 4 weeks was 66% for polymer 1, 56% for polymer 2 and 20% for polymer 3. Samples retained 55% of their bending strength at 4 weeks (polymer 1), 50% at 12 weeks (polymer 2) and 99% at 12 weeks (polymer 3). The bending modulus of polymer 3 remained practically unchanged during the first 12 weeks of degradation. Subsequently it increased by 44% at week 16 and remained unchanged up to 24 weeks and next decreased to 33% of the initial value at the end of the experiment at week 32. The bending modulus of polymer 2 decreased 35% at week 8 and subsequently increased to 44% of the initial value at week 16 and remained at this level until week 20. Next the modulus decreased to 84% of the initial value at week 24. The bending modulus of polymer 1 progressively decreased over the first 12 weeks of degradation to 40% of the initial value. The maximum crystallinity attained by the samples at the end of the experiments was 60% for polymer 1 and 38% for polymers 2 and 3. In the static mode the pH remained constant up to week 8 for polymer 1, week 20 for polymer 2 and week 28 for polymer 3. It decreased to 3.8 at weeks 12, 20 and 36 for polymers 1, 2 and 3, respectively. All the samples underwent calcification from week 16 of the experiments with the Ca/P ratio ranging from 0.92 to 1.20.

Modeling changes in the modulus of poly(ɛ-caprolactone) due to hydrolysis and plasticization

The evolution of the flexural modulus of poly(e-caprolactone), prepared with a tin(II) 2-ethylhexanoate catalyst, was measured in terms of the polymer molecular weight and moisture content. Changes in the polymer molecular weight were observed, but these did not result in a loss of modulus. Fickian diffusion was used to characterize the absorption of moisture into the polymer. The maximum moisture content and moisture diffusivity constant of the polymer were determined to be 0.62% and 2.039 × 10−5 mm2/s, respectively. Reductions in the polymer modulus from 0.5 to 0.4 GPa were attributed to water plasticization. An empirical expression for the polymer modulus as a function of immersion time was developed. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008

Loss of Polymer-Modified Binder Durability with Oxidative Aging

In recent years, there has been an increased use of polymers to modify asphalt binders, mainly to decrease pavement rutting but also to improve binder failure strain in direct tension. With binder oxidation, however, failure strain improvements (relative to the unmodified binder) disappear, and two mechanisms are suspected: polymer degradation and base binder embrittlement. The extent to which each of these mechanisms is responsible for loss of modified-binder performance is important and bears on maintenance treatments and recycling methodologies. Changes in styrene-butadiene-styrene (SBS) polymer-modified binder properties due to oxidation were analyzed by means of dynamic shear rheometry, ductility, force ductility, and gel permeation chromatograph. Previous literature reports using size exclusion chromatography showed that degradation of the molecular weight profile of SBS accompanied the loss of ductility in polymer-modified asphalt (PMA). Yet base binder embrittlement also occurred, as evidenced by ductility and force ductility. Testing aged PMA binders at higher temperatures to soften the base binder restored the polymer modulus to the force ductility measurements, as did blending with a softer deasphalted oil. The conclusion from these measurements is that the primary cause of PMA degradation with aging is base binder embrittlement rather than polymer degradation.

Preparation of poly (styrene–methyl methacrylate)/SiO 2 composite nanoparticles via emulsion polymerization. An investigation into the compatiblization

Inorganic/organic nanocomposite systems, in which inorganic particles are encapsulated into the polymer matrix, are new classes of polymeric materials. These materials combine the properties of both components. It means that polymer component with excellent optical property, flexibility and toughness could improve the brittleness of inorganic particles and besides, inorganic particles could increase the strength and modulus of polymers. There are various methods to make these inorganic/organic nanocomposites. One of them is the chemical process, in which polymerization is performed directly in the presence of the inorganic particles. Examples of miniemulsion, suspension or dispersion polymerization can be found in the literature but emulsion polymerization is by far the technique most frequently used. In this work, latex containing nanostructure hybrid of copolymer (styrene, methyl methacrylate, acrylic acid) and inorganic nanoparticles (silica) with core/shell structure was prepared via semi-batch emulsion polymerization. At first, silica nanoparticles were dispersed in water phase in an ultrasound bath to prevent the aggregation of nanoparticles, and then emulsion polymerization was performed in the presence of silica nanoparticles. Related tests and analysis confirmed the success in synthesis of nanostructure hybrids. Induced coupled plasma (ICP) analysis and thermal gravimetric analysis (TGA) showed the presence and amount of silica nanoparticles in the final latex. Dynamic light scattering (DLS) analysis confirmed the presence of 25–35 nm particles in the system and transmission electron microscopy (TEM) showed the core/shell morphology of nanoparticles. It has been shown that with an appropriate surfactant, adjusting the pH of media, using suitable monomers and under controlled conditions, it would be possible to produce stable organic/inorganic composite nanoparticles with core/shell structure. In another attempt and in order to investigate the effect of compatiblizing system, styrene–methyl methacrylate was copolymerized in the presence of modified silica particles with oleic acid as the inorganic dispersed phase at the same condition. Similar characterizations were performed in order to have a worthwhile comparison. The results for the late procedure show the effect of oleic acid in formation of aggregates as the core for polymeric nanocomposite particles.

Accurately evaluating Young’s modulus of polymers through nanoindentations: A phenomenological correction factor to the Oliver and Pharr procedure

The Oliver and Pharr [J. Mater. Res. 7, 1564 (1992)] procedure is a widely used tool to analyze nanoindentation force curves obtained on metals or ceramics. Its application to polymers is, however, difficult, as Young’s moduli are commonly overestimated mainly because of viscoelastic effects and pileup. However, polymers spanning a large range of morphologies have been used in this work to introduce a phenomenological correction factor. It depends on indenter geometry: sets of calibration indentations have to be performed on some polymers with known elastic moduli to characterize each indenter.

Physico-chemical properties of plasma-polymerized tetravinylsilane

Plasma polymer films of tetravinylsilane were deposited on silicon wafers using an RF glow discharge operated in pulsed mode. Microscopic and spectroscopic techniques revealed that physico-chemical properties of plasma polymer depend on the effective power used if the flow rate was constant. The deposition rate (8–165 nm min − 1 ) and surface roughness (2.0–5.8 nm) of films varied with the RF power. An organic/inorganic character (C/Si ratio) of films and a content of vinyl groups could be controlled by the effective power. The refractive index of films increased with power from 1.63 (0.05 W) to 1.75 (10 W) at a wavelength of 633 nm and the extinction coefficient rose sharply with the power from 0.09 (0.05 W) to 0.37 (10 W) at a wavelength of 240 nm. The elastic modulus and hardness of plasma polymer could be varied from 11 GPa (0.05 W) to 30 GPa (10 W) and from 1.4 to 5.9 GPa, respectively.

The Effect of Interphase on the Elastic Modulus of Polymer Based Nanocomposites

The elastic modulus of interphase in polymer based nanocomposites is investigated. A new three-dimensional unit cell model has been developed for modeling three constituent phases including particle, interphase and matrix. The elastic modulus of the interphase as a function of radius is then evaluated with the help of mathematical models. The average value of interphase elastic modulus is defined and the effect of interphase thickness and particle and matrix elastic modulus on interphase is investigated.

On the Use of the Nanoindentation Unloading Curve to Measure the Young's Modulus of Polymers on a Nanometer Scale

AbstractSummary: The nanoindentation test is a fundamental tool to assess the link between morphology and mechanical properties. The preliminary results of a more exhaustive study about the applicability to polymers of the most used procedure to determine elastic modulus by indentation are reported in this short communication. A departure of the experimental conditions from the theoretical assumptions and results that give rise to the Oliver and Pharr analysis is shown to occur under a wide range of experimental conditions, with applied loads and penetration depths covering several orders of magnitude and using different indenter geometries. Unloading curves with exponents significantly larger than 2 are observed in disagreement with the contact mechanics approach used by Oliver and Pharr.An AFM image obtained in non contact mode of an indentation induced by a sharp AFM tip with a maximum applied load of ca 1.2 µN on amorphous PET.magnified imageAn AFM image obtained in non contact mode of an indentation induced by a sharp AFM tip with a maximum applied load of ca 1.2 µN on amorphous PET.

Wrinkling of a two-layer polymeric coating

Wrinkling of a two-layer coating, comprised of a glassy polyvinyl alcohol top layer and a rubbery polyethyl acrylate bottom layer, was studied. The coatings were prepared on polyethylene terephthalate substrates by depositing and drying the layers in sequence. Fine, short-wavelength (180–200μm) wrinkles formed when the two-layer coating was exposed to humid atmosphere after the second drying treatment and coarse, long-wavelength (450–500μm) wrinkles formed when the two-layer coating was heated to a temperature above the glass transition temperature of the top layer. Both types of wrinkles must have arisen from compressive stress in the top layer. The compressive stress developed by either moisture absorption (fine wrinkles) or differential thermal expansion on heating (coarse wrinkles). When compressive stress is high enough, the top layer deforms out of plane without detaching from the bottom layer to produce wrinkles. The balance of forces in systems of three adhering layers of different thicknesses and moduli in static mechanical equilibrium was analyzed, and it was found that the layer with the highest in-plane axial stiffness, the product of elastic modulus by thickness, typically dictates the dimension of the entire composite. When the mismatch of thermal expansivities of the layers generates the compressive stress that produces wrinkling, the difference between the thermal expansivity of the top layer and the axial stiffness-weighted average thermal expansivity of the three layers governs the compressive stress in the top layer. Because polymer modulus falls abruptly at the glass transition temperature Tg, the critical compressive stress required for wrinkling falls abruptly at Tg. Therefore, the two-layer coating atop a stiff substrate is more susceptible to wrinkling when the top layer is above its Tg. When absorption of moisture in the top layer generates the compressive stress that generates wrinkling, that stress goes through a minimum because modulus falls and compressive strain rises with rising moisture content. However, the critical compressive stress for wrinkling falls because modulus falls with rising moisture content. Therefore, the two-layer coating atop a stiff substrate is more susceptible to wrinkling the greater the moisture content in the top layer. The relative humidity (RH) of the sample atmosphere was raised to raise the moisture content in the top layer. The wrinkle wavelength fell with rising RH because the modulus, and therefore the bending stiffness, of the top layer fell with rising RH. The measured wavelengths were qualitatively predicted at low humidities and quantitatively predicted at high humidities.

Gas foaming of segmented poly(ester amide) films

Biodegradable segmented poly(ester amide)s, based on dimethyl adipate, 1,4-butanediol and N, N′-1,2-ethanediyl-bis[6-hydroxy-hexanamide], with two distinct melting transitions were gas foamed using carbon dioxide (CO 2). Polymer films were saturated with CO 2 at 50 bar for 6 h after which the pressure was released. The samples were immersed in octane at the desired temperature after which foaming started immediately. Just above the lower melt transition the polymers retain adequate mechanical properties and dimensional stability, while the chain mobility increased sufficiently to nucleate and expand gas cells during the foaming process. In this way semi-crystalline poly(ester amide)s can be gas foamed below the flow temperature. Two poly(ester amide)s with 25 mol% (PEA2,5-25) and 50 mol% (PEA2,5-50) of bisamide segment content were foamed at 70 and 105 °C, respectively. The storage modulus ( G′) of both pure polymers at the onset foaming temperature is 50–60 MPa. Closed-cell foams were obtained with a maximum porosity of ∼90%. The average pore size of PEA2,5-25 ranges from 77 to 99 μm. In contrast, the average pore size of PEA2,5-50 is in between 2 and 4 μm and can be increased to 100 μm by lowering the CO 2 saturation pressure to 20 bar. The porosity of PEA2,5-50 foams using this saturation pressure decreased to 70%.

DEMOLDING OF HIGH ASPECT RATIO POLYMERIC MICRO-PATTERNING

UV embossing for polymeric micro-patterning thin film is an emerging replication technique. This paper investigates UV curable multifunctional acrylates pre-polymer resin patterned by a micro-structured mold and subsequently cured by UV irradiation. To further enhance this duplication method for high aspect ratio production, demolding must be reliable and repeatable without damage to the embossing or mold. Previously, it has been reported that UV embossed patterns for aspect ratios as high as 14 have been achieved experimentally. Finite element analyses for patterns with aspect ratios of 5 using parallel demolding between two parallel plates have also been reported. However, the parallel demolding method may not be suitable for large area patterns as forces generated were high. As such, an alternative demolding method, namely peel demolding, for micro-patterns with an aspect ratio of 14 was investigated and key parameters identified. The parameters governing the demolding process were the peel angle, the pre-crack condition, shrinkage, interface fracture toughness, tensile strength and modulus of polymer. A pre-crack between the polymer and mold was introduced before peel demolding. Numerical analyses in terms of Cohesive Zone Modeling (CZM) were used to simulate the demolding process. Shrinkage caused by UV exposure was represented by thermal strain effects and the fully cured polymer was peeled off using displacement control. The ultimate tensile strength (U.T.S) of the cured polymer was used as a failure criterion. The stresses involved were crucial for determining clean demolding. As peeling progressed, stresses experienced in the polymer matrix increased rapidly in the region ahead of the crack with little or no stress at the cracked region. When stresses experienced by the polymer were below the U.T.S, demolding was deemed to be successful.

A unified model for stiffness modulus of amorphous polymers across transition temperatures and strain rates

In many applications, polymer materials undergo a large variety of mechanical loading conditions, wherein the influences of temperature and strain rate are of prime importance. Mahieux and Reifsnider [Mahieux CA, Reifsnider KL. Polymer 2001;42:3281. [19]] have proposed a statistical model to describe the stiffness variation of polymers over a wide range of temperatures. However, this model does not consider any frequency/strain rate dependence of the stiffness modulus. Starting from this consideration, we propose here to transform this latter model into a robust physically based model for the prediction of the stiffness modulus for a wide range of temperatures and frequencies/strain rates. This new formulation has been successfully validated for two amorphous polymers, polymethylmethacrylate (PMMA) and polycarbonate (PC), using dynamic mechanical analysis and uniaxial compression testing. Good agreement has been found between theory and experiment for the non-linear behavior of the initial Young's modulus, at the very high strain rates.

Micromechanical interpretation of the modulus of ethylene–(meth)acrylic acid copolymers

Statistical incorporation of a comonomer into polyethylene is well known to reduce its crystallinity, and consequently the small-strain modulus. However, we have found that when the comonomer is methacrylic or acrylic acid, the modulus of homogeneous copolymers initially decreases and then increases with increasing comonomer content. The modulus increase is traced to an elevation of the amorphous phase glass transition temperature (Tg) with comonomer content; Tg passes through room temperature at comonomer contents sufficiently low that substantial crystallinity remains. When the modulus is measured at temperatures above Tg, such that the amorphous phase is fully relaxed, a monotonic increase in modulus with crystallinity is both anticipated and observed. Several two-phase composite models are investigated to describe the modulus vs. crystallinity data above Tg; the Davies model provides a good quantitative description for the copolymers examined. The Davies model may also be applied at temperatures where the amorphous phase is incompletely relaxed, to extract its contribution to the semicrystalline polymer's modulus.

Reliability of Anisotropic Conductive Film Joints Using Bumpless Chip—Influence of Reflow Soldering and Environmental Testing

This paper presents the reliability of anisotropic conductive film (ACF) joint tested under reflow soldering and environmental test effect. The ACF joint behaved differently under different reflow soldering profiles. The lower reflow temperature resulted in more reliable ACF joints by maintaining low contact resistance. By contrast, high contact resistance was found in assemblies treated with higher reflow. Under humidity aging (85°C/85%RH), bumpless chips proved to be unreliable due to corrosion mechanism. Moreover, ACF had shown degradation in chemical and physical properties, including modulus reduction, Tg depression, polymer hydrolysis, and surface swelling after exposing to humidity aging. The deterioration in reliability of aged ACF joint during reflow process was mainly caused by hydroscopic swelling-induced stress of ACF.

Modelling of Elastic and Strength Properties of Structural Plastics Based on Composites of Polymers with Fiber and Other Fillers

Modelling is considered for the elastic and strength properties of filled polymer composites taking account of the increase in the elasticity modulus of polymer absorbed at the surface of the filler as a result of polarization.