Biaxial Stretch-Induced Crystallization of Polymers: A Molecular Dynamics Simulation Study

Abstract

Molecular dynamics (MD) simulations are employed to study biaxial stretch-induced crystallization of polymers, during which the individual roles of chain conformation and orientation on crystal nucleation and growth are clarified. Systems with different stiffness and orientations are constructed by changing the stretch ratios of the x-and y-axis, which allow us to figure out the individual contributions of chain conformation and orientation to flowenhanced nucleation. The results show that nucleation occurs in areas with high segment orientation, and the higher orientation corresponds to the shorter nucleation induction period. The relationship between the nucleation induction period and orientation is quantitatively expressed, which indicates that orientation plays a dominant role in flow-enhanced nucleation. On the other hand, the results show that chain stiffness exhibits a negative correlation with nucleation in biaxial stretch, supporting that conformational entropy reduction is not the main driving force in flow-induced crystallization of polymers. With the secondary nucleation model, the crystal growth rates in different directions correlate well with the orientation at the growth front of the clusters, further confirming the decisive role of orientation in crystal nucleation and growth. Finally, crystal cluster merging is proposed to be a way to form shish structures in highly oriented melts.

Molecular Dynamics Simulation of the Nascent Polyethylene Crystallization in Confined Space: Nucleation and Lamella Orientation

“…There was a compromise in competition between adjacent crystal nuclei; as the initial nuclei are unstable, adjacent nuclei will gradually “swallow” each other. Although the high undercooling bulk system can also reduce the nuclear energy barrier to generate a large number of nuclei, the sidewall-induced one was more similarly oriented, making it easier to achieve the merging of crystal clusters …”

“…Although the high undercooling bulk system can also reduce the nuclear energy barrier to generate a large number of nuclei, 54 the sidewall-induced one was more similarly oriented, making it easier to achieve the merging of crystal clusters. 28 During the nucleation period, SC-1D and EC showed similar trends in OZ c ; however, after 30 ns, they showed opposite trends to each other. SC-1D gradually increased from 0.6 to 0.8 as crystal growth progressed.…”

“…The molecular motion and crystallization in the branched chain, block copolymers, , and cyclic polymers show different states, while, in industrial processes, polymers are always under an unstable or heterogeneous system. Under the flow field, chain segments are arranged and straightened in the flow direction, forming regularly oriented crystal nuclei through flow-induced crystallization (FIC) , while changing the entangled state of the system. , Another way of modulating polymer properties is the addition of nanoparticles to form polymer nanocomposites with a specific structure by changing the crystalline behavior of the polymer. Nanoparticles with different dimensions produce can effectively control orientation and crystallization processes .…”

Structure–Mechanics Relation of Natural Rubber: Insights from Molecular Dynamics Simulations

“…Polymer crystals are considered as aggregates of stems, which are straight polymer segments. The local direction of the polymer chain is given by the chord vector , where r⃗ is the coordinate of the bead in the molecular chain. We can define an order parameter λ for a segment of length 2 k + 1 by …”

Specific deformation behavior of isotactic polypropylene films under a multiaxial stress field

“…The importance of multiaxial deformation, such as biaxial, [13][14][15][16][17][18][19][20] compression, 21 bulge, 22 lateral indentation, 23 and flow, 24 in terms of understanding the physical properties of polymers is well known. This is because (1) the polymers have a string-shaped structure, (2) they can become entangled, and (3) the strain density energy function of the materials can be obtained.…”