Shear thinning the polymer melt is so different! Molecular dynamics simulations reveal the molecular mechanism of shear-thinning

The polymer melt shear deformation and flow occur during processing, resulting in significant changes in its relaxed dynamics occur. Many studies have shown that, with deformation of the material, structural relaxation time and effectively reduce the viscosity, sometimes several orders of magnitude decrease. After a long period of \”aging\”, fluid dynamics relaxation deformation return to equilibrium state. Although there are a large number of experimental studies have reported, but from the perspective of the theory of the polymer melt relaxation dynamics of change is still very difficult.

The results presented in

Based on the above analysis, National Institute of Standards and Technology Professor Jack F. Douglas Task Force Based molecular dynamics simulations, α relaxation proposed kinetic model had a steady shear at coarse polymer melt. found that shear forces gradually suppressed α relaxation process of the polymer melt, resulting in a dominant relaxation at high shear rates inertia pure beta], this trend is similar to elevated temperatures. As the shear rate increases, the melt viscosity decreased 2 to 4 orders of magnitude, independent of temperature in the melt viscosity at high shear rates. Due to shear-thinning under shear, moment association between the macromolecules of the \”depolymerization\” clusters or particles are immobilized damage caused. 聚合物熔体剪切变稀原来是这么回事!分子动力学模拟揭示剪切变稀的分子机理

The polymer melt simulation method

In order to study the kinetics of the polymer melt under a steady shear relaxation process, researchers use molecular dynamics simulation, LAMMPS code calculation. Completely by a flexible chain of beads harmonic spring connection represents a polymer composition, each polymer chain has a 20-mer, the number of polymer chain analog of 500, a total of 10,000 monomer intermolecular force by truncating the LJ interaction potential of representation. Nose-Hoover using constant pressure and temperature allows the initial analog systems NPT ensemble equilibrium (a constant number of particles N, the pressure P and temperature T), the balance, in the xy plane at a fixed shear rate of shear deformation in the simulation unit. Simulation using periodic boundary conditions, the temperature range from T = 0.43 to T = 0.55, the shear rate range from 100 to 10-5. Of twelve independent copies of each state is calculated to obtain a statistical average.

The shear thinning effect of the polymer melt

聚合物熔体剪切变稀原来是这么回事!分子动力学模拟揭示剪切变稀的分子机理
Fig 1. Relationship between shear rate and viscosity of the structural relaxation time. All system effective viscosity which is a steady state value FIG. 5 illustration stress. With the increase, decrease shear thinning viscosity signal. At low shear rates, the viscosity observed strong temperature dependence, when the shear rate is higher than 10-1, the temperature dependency becomes inconspicuous. Illustration: The system derived from the relaxation time Fself (q0, t).

The researchers studied the structural relaxation of polymer melt viscosity and time relationship with shear rate. When the lowest shear rate, the viscosity of the polymer melt to reach equilibrium, with increasing shear rate, melt viscosity decreased 2 to 4 orders of magnitude, and at a high shear rate dependence of melt viscosity temperature almost disappeared . Shear thinning is a common phenomenon in complex fluids. They believe that shear-thinning structure is destroyed due to the fluid under shear, moment association between the macromolecules of the \”depolymerization\” or called clusters immobilized particles caused by destruction.

the relaxation time of the polymer melt

聚合物熔体剪切变稀原来是这么回事!分子动力学模拟揭示剪切变稀的分子机理
Figure 2 shows the intermediate scattering function α relaxation process is suppressed, the temperature of T = 0.43. = Brown curve labeled 0 is the reference balance system before shearing, circles analog data, a curve fitting the data to Equation 2, the fitting constants τf = 0.3, βf = 1.3, A = 0.73. Black dotted line indicates the start of the platform after the initial fast decay, which corresponds to the limit value relaxation time infinity in the intermediate scattering function. 0.73 height of the platform, a value consistent with the estimated parameter A at this temperature.

Researchers studied intermediate scattering function at the lowest temperature with time at different shear rates. It found that with increasing shear rate, is consistent with inhibiting [alpha] relaxation, relaxation time decreased [alpha], [alpha] together final relaxation and γ rapid relaxation, which increases the effect of the temperature reached. To quantify these results, they used to fit the data to the following equation: 聚合物熔体剪切变稀原来是这么回事!分子动力学模拟揭示剪切变稀的分子机理 The intermediate of formula scattering function consists of two parts: beta] relaxation (the relaxation time is independent of temperature) and α relaxation (the relaxation time associated with the temperature, and the amplitude independent of temperature).

聚合物熔体剪切变稀原来是这么回事!分子动力学模拟揭示剪切变稀的分子机理
Figure 3. The relationship between relaxation time and the tensile index β is dimensionless shear rate.

renormalization group supercritical fluid dynamic viscoelasticity theory shows that relaxation time describes the dynamic life of the clusters associated with the initial phase separation, gradually break the clusters in a stable under shear. When not reached steady shear, near critical fluid shear viscosity and normal stress diverging in powers of the correlation length of the correlation length describes the dynamic average size of the clusters. They found that in the simulation α relaxation time increases as the shear rate decreases, the gradual integration with β relaxation at a critical shear rate. This means that α is approximately equal to the relaxation time must be at a high shear rate, and temperature insensitive belonging to the relaxation time, in order to solve this limitation, researchers use a simplified form as follows: 聚合物熔体剪切变稀原来是这么回事!分子动力学模拟揭示剪切变稀的分子机理 When the shear strength tends to infinity relaxation time of 0.5 can be well fit the experimental data, due to the dimensionless time units is about 1 ps, and close sake.

The polymer melt fracture cluster

聚合物熔体剪切变稀原来是这么回事!分子动力学模拟揭示剪切变稀的分子机理
FIG. 4. T = 0.43 when restricted by the cage near the particle fraction C (t) over time It changes in the relationship. Clathrate particles decay time equivalent to the shear shown in FIG. 2 associated relaxation time. t = 1 transformation point is lowered due to the heterogeneity of the definition of the dynamic clathrate particles and high shear caused. Two snapshots were used to highlight the colored balls larger particles fall high shear and zero shear motility. The large cluster balancing system (upper right in the snapshot blue particles) is destroyed under high shear. Quantitative terms, the average cluster size of the two snapshots were 4.0 and 2.4.

Starr et al defines the autocorrelation function that defines the fixed particles remain \”cage\” persistent state. Researchers use this function, the study of how to change the duration of the shear is fixed particle clusters, the relationship between the scores obtained with a shear rate of clathrate particles. Balance analysis Transformation Temperature Fixed life clusters coincide, attenuation and structural relaxation time is quite time, and with the increase of shear rate becomes shorter. The researchers also gives a snapshot of the system at high shear and zero shear conditions. Although the balance fixing system shows a larger particle clusters, but in fact with increasing shear rate, they are broken down into progressively smaller and spaceA more uniform cluster, increasing the temperature and a considerable time the effect of the balance.

limit steady state flow of the polymer melt

聚合物熔体剪切变稀原来是这么回事!分子动力学模拟揭示剪切变稀的分子机理
Figure 5. The total shear stress and stress-strain behavior of the flow should be close to the limit of stability. At T = 0.55, a shear rate of 10-1, the system change with shear stress total strain curve.

researchers studied the limit behavior of the polymer melt under steady state flow. Found T = 0.55, a shear rate of 10-1, the system increases the shear stress total strain is increased, then leveled off. After an initial linear increase in shear stress occurs overshoot stress, total strain occurs in the vicinity of the maximum value of 2.6. After the stress decreased, a stable flow can be observed, the system quasi-equilibrium is established. Steady-related stress and temperature at low shear rate, the shear rate is high when the correlation disappears at this time 10-1.

Summary

In order to clarify the relaxation behavior of the polymer melt under steady state conditions, the US NIST professor Jack F. Douglas TF molecular dynamics simulation based on the relaxation effects of melt relationship between time and shear rate, molecular motion from the perspective of shear thinning behavior of the polymer melt. It found that as the shear rate increases, the melt viscosity decreased 2 to 4 orders of magnitude, independent of temperature in the melt viscosity at high shear rates. Due to shear-thinning under shear, moment association between the macromolecules of the \”depolymerization\” or called clusters are immobilized particles damage caused. As the shear rate increases, the relaxation is suppressed [alpha], together with the final flash γ relaxation. System shear stress increases linearly with the total strain in the initial, overshoot occurs stress total strain in the vicinity of 2.6, then the system reached a quasi-equilibrium state. Original link: https: //advances.sciencemag.org/content/6/17/eaaz0777

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