Pressure and shear rate dependence of the viscosity and stress relaxation of polymer melts

Reynolds, C. D.; Thompson, Richard L.; McLeish, Tom

doi:10.1122/1.5012969

Cited by 22 publications

(14 citation statements)

References 41 publications

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“…Interdigitation due to enhanced polymer chain mobility at the polymer–vapor interface has previously been proposed to occur for PS at temperatures as low as 50 °C below the bulk T g and could therefore be expected to occur for the low-MW PS in this work ( T g – T = 47 °C), especially since the low-MW PS has a relatively large concentration of chain ends at the interface, which are known to enhance interdigitation . Although interdigitation would seemingly occur throughout the entire contact area during an adhesion cycle, compressive stress can increase the viscosity and relaxation time(s) of polystyrene. − The JKR theory predicts the stress distribution across the contact where r is the radial distance from the center of contact, plotted in Figure S8. Equation shows that the maximum compressive stress occurs at the center of the contact area and that a transition from compressive to tensile stress occurs at the edge of the contact area.…”

Section: Resultsmentioning

confidence: 77%

Surface Damage Influences the JKR Contact Mechanics of Glassy Low-Molecular-Weight Polystyrene Films

et al. 2019

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Using a surface forces apparatus (SFA), we quantitatively study the influence of surface damage on the contact mechanics of self-mated glassy polystyrene (PS) films. We use the SFA to measure the contact radius, surface profile, and normal force between the films, including the adhesion force. The molecular weight (MW) of the polymer influences the repeatability of the adhesion measurements and the effective surface energy calculated using the Johnson−Kendall−Roberts (JKR) theory. For low-MW PS (MW = 2.33 kDa), the effective surface energy increases over repeated adhesion cycles as the films become progressively damaged. For high-MW PS (MW = 280 kDa), the effective surface energy is constant over repeated adhesion cycles, but hysteresis is still present, manifested in a smaller contact radius during compression of the surfaces than during separation. Our results demonstrate that while the JKR theory is appropriate for describing the contact mechanics of glassy polymer thin films on layered elastic substrates, the contact mechanics of low-MW polymer films can be complicated by surface damage to the films.

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Section: Resultsmentioning

confidence: 77%

Surface Damage Influences the JKR Contact Mechanics of Glassy Low-Molecular-Weight Polystyrene Films

et al. 2019

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“…In addition, investigations using dielectric measurements to determine the dynamics of the chain segments show that the influence of pressure on the relaxation of orientations cannot be ignored [ 36 ]. Recent investigations by Reynolds et al show a first approach to determine the pressure dependence of the relaxation of the molecular orientations, in the pressure range of 1–100 bar, as a function of the shear rate of a stationary flowing melt with a two-piston rheometer [ 37 ]. For this purpose, the capillary between the pistons was equipped with a quartz glass window, whereby the stress birefringence caused by the orientations could be visualized and its temporal development could be recorded and evaluated by a camera.…”

Section: Introductionmentioning

confidence: 99%

“…The relaxation times measured from the decay of the fringes have been compared with relaxation times, determined from the decay of the pressure signals after the stop of both pistons. Within these investigations a pressure dependence of the relaxation time was found, but no shear rate dependence [ 37 ].…”

Section: Introductionmentioning

confidence: 99%

Pressure Equilibrium Time of a Cyclic-Olefin Copolymer

Roth

Drummer

2021

Polymers

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Integrative simulation techniques for predicting component properties, based on the conditions during processing, are becoming increasingly important. The calculation of orientations in injection molding, which, in addition to mechanical and optical properties, also affect the thermal shrinkage behavior, are modeled on the basis of measurements that cannot take into account the pressure driven flow processes, which cause the orientations during the holding pressure phase. Previous investigations with a high-pressure capillary rheometer (HPC) and closed counter pressure chamber (CPC) showed the significant effect of a dynamically applied pressure on the flow behavior, depending on the temperature and the underlying compression rate. At a constant compression rate, an effective pressure difference between the measuring chamber and the CPC was observed, which resulted in a stop of flow through the capillary referred to as dynamic compression induced solidification. In order to extend the material understanding to the moment after dynamic solidification, an equilibrium time, which is needed until the pressure signals equalize, was evaluated and investigated in terms of a pressure, temperature and a possible compression rate dependency in this study. The findings show an exponential increase of the determined equilibrium time as a function of the holding pressure level and a decrease of the equilibrium time with increasing temperature. In case of supercritical compression in the area of a dynamic solidification, a compression rate dependency of the determined equilibrium times is also found. The measurement results show a temperature-invariant behavior, which allows the derivation of a master curve, according to the superposition principle, to calculate the pressure equilibrium time as a function of the holding pressure and the temperature.

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“…The speed of the intramolecular space change is determined by the free volume between the polymer chains. [ 18 ] Accordingly, higher melt temperatures [ 19 ] or lower pressures [ 20 ] lead to shorter relaxation times due to the higher free volume. Thus, in Meister, [ 14 ] when investigating the stress birefringence of tensile bars made of PC with different cavity materials, a reduced degree of orientation was found for poorly thermally conductive mold materials.…”

Section: Introductionmentioning

confidence: 99%

“…In Pantani et al, [ 21 ] a significant increase in molecular orientations in injection‐molded plates made of PP could be attributed to an inhibition of relaxation due to the restriction of molecular mobility as a result of a higher holding pressure level. Furthermore, in Reynolds et al, [ 20 ] by means of an adapted two‐piston rheometer with sapphire glass window, pressure‐dependent relaxation times could be determined by evaluating the path difference caused by residual stress and molecular orientation induced birefringence. A clear pressure dependence of the relaxation times was found in the measured pressure ranges from 1 to 100 bar.…”

Section: Introductionmentioning

confidence: 99%

Isotropy of mechanical properties and environmental stress crack sensitivity in injection‐ and injection‐compression molding of polystyrene with different mold temperature

Roth

Drummer

2021

Polymer Engineering & Sci

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In addition to conventional injection‐compression molding and injection molding with dynamic process temperature control, a synergistic combination of both processes has recently been applied in dynamic temperature‐controlled injection‐compression molding. The two‐dimensional holding pressure effect in combination with the long maintenance of the flowability of the melt due to high mold temperature enables particularly large flow path to wall thickness ratios. In the most cases, only the optimized molding of microstructures and aspect ratios is considered without including the changed internal structure of such manufactured components. In the course of this investigation, the influence of different process strategies under variation of the mold temperature on production‐related anisotropies in the mechanical properties and stress crack sensitivity of thin‐walled polystyrene components was examined. The determined mechanical properties are significantly below the values given in the data sheet of the material in the adapted process variant of injection‐compression molding with high mold temperature. However, the results also show a clear homogenization of the direction‐ and flow path‐dependent mechanical properties. In contrast, components produced in this way tend to show increased environmental stress crack sensitivity. This could be attributed to significantly reduced orientations as a result of the favorably proceeding orientation relaxation.

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Pressure and shear rate dependence of the viscosity and stress relaxation of polymer melts

Cited by 22 publications

References 41 publications

Surface Damage Influences the JKR Contact Mechanics of Glassy Low-Molecular-Weight Polystyrene Films

Surface Damage Influences the JKR Contact Mechanics of Glassy Low-Molecular-Weight Polystyrene Films

Pressure Equilibrium Time of a Cyclic-Olefin Copolymer

Isotropy of mechanical properties and environmental stress crack sensitivity in injection‐ and injection‐compression molding of polystyrene with different mold temperature

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