As the application of polymer films in the field of new energy continues to increase, there is a growing emphasis on developing polymer films with high strength, substance exchange, and applicability. At a microscopic level, understanding the structural basis of material properties will play an essential guiding role in the design and preparation of high-performance materials. In my Ph.D. research, I have utilized molecular dynamics simulations to conduct a comprehensive study on models with entangled polymer chains. My research includes three-dimensional isotropic models and two-dimensional free-standing films. The research findings reveal the relationship between polymer chains entanglements and polymer concentrations, polymer film thickness, and mechanical properties, providing a det...[ Read more ]

As the application of polymer films in the field of new energy continues to increase, there is a growing emphasis on developing polymer films with high strength, substance exchange, and applicability. At a microscopic level, understanding the structural basis of material properties will play an essential guiding role in the design and preparation of high-performance materials. In my Ph.D. research, I have utilized molecular dynamics simulations to conduct a comprehensive study on models with entangled polymer chains. My research includes three-dimensional isotropic models and two-dimensional free-standing films. The research findings reveal the relationship between polymer chains entanglements and polymer concentrations, polymer film thickness, and mechanical properties, providing a detailed analysis of the ultra-thin film preparation process at the microscopic level.

In this doctoral thesis, three progressive works are presented:

3-D entangled model with flexible chains in athermal solvents: Through large-scale molecular dynamics simulations in a 3-D entangled model, we have confirmed the theories proposed by de Gennes and Kavassalis et al. that for flexible polymer chains dissolved in athermal solvents, the entanglement length N_e satisfies the relation N_e~ Ф^−1.25 with respect to the polymer concentration Ф , and the chain-mean-square end-to-end distance < R²_ee> satisfies the relationship < R²_ee>~ N∗ Ф^−1/4 with respect to the chain length N and concentration Ф. Our findings reveal a precise concentration-scaling relationship for geometric entanglement length N_e, demonstrating N_e= AФ^−1.25+B where A and B are chain length independent constants. The introduction of constants A and B allows the scaling relationship to be adaptable to the entanglement length in systems ranging from low to high concentrations. We have also identified a critical concentration Ф_c above which the solution exhibits long-lived entanglement effects. Our simulation results indicate that in entangled models, solvent-induced swelling of polymer chains is localized, which we refer to as the local swelling effect. By combining the modified entanglement length N_e with the local swelling effect, we have proposed a new expression for accurately quantifying the plateau modulus of flexible chains dissolved in athermal solvents.

2-D free standing film with relaxed chains: In this section, we classify the entanglements into two categories: single-point entanglement (SPE) and multi-coupled entanglement (MCE). SPE refers to an entanglement consisting of geometrical constraints between only two polymer chains, whereas MCE implies that at least three chains are entangled two by two with each other, resulting in a coupled structure. To explore multi-coupled entanglements, we construct three-dimensional isotropic blocks with periodic boundaries, as well as two-dimensional free-standing film models. Unlike single-point entanglements, we find that in 3-D models the multicoupled entanglements depend on chain length, whereas in free-standing films, the percentage of multi-coupled entanglements increases as the film thickness decreases. Additionally, we have observed that MCE plays a crucial role in maintaining the mechanical stability of the film, enabling us to successfully correlate geometrical properties in the entangled regime with mechanical features.

Ultra-thin topological film: Biaxial stretching includes asynchronous and synchronous process of semi-crystalline bulk films is simulated in this section, with the X-and Y-axes defined as the stretching axes. Our simulations have shown that ultra-high molecular weight polyethylene film exhibits excellent mechanical properties even under asynchronous stretching. During stretching along the X-axis, the PE lamellas initially rotate towards the stretching direction, resulting in a rapid increase in the order parameter along the stretching direction. During the rotation process, the lamellas maintain the stability of their structure. As the strain increases further, the lamellas become disrupted, but the polyethylene chains begin to align parallel to the stretching direction, resulting in the formation of fibrous crystals. By fixing the X-axis boundary and stretching along the Y-axis, the stresses and order parameters in the Y-direction eventually surpass those in the X-direction, demonstrating anisotropy in performance. During synchronous stretching, the film showed consistent mechanical and geometric properties in all directions, we discovered that during the stretching process, as the voids inside the film changed from circular shape to triangular shape, there was a transition in the relationship between the conformational energy and the strain, similar to a material phase transition.

The nanofilm consists of stretched PE fibers that form a pattern of polygons and triangles, and the entanglement points are primarily located at the vertices of the fibers. By conducting global and local topological analysis, we have successfully established a correlation between this unique structure and the stability of ultra-thin films.