Effect of chain architecture on the compression behavior of nanoscale polyethylene particles
© Wu et al.; licensee Springer. 2013
Received: 14 May 2013
Accepted: 6 July 2013
Published: 15 July 2013
Polymeric particles with controlled internal molecular architectures play an important role as constituents in many composite materials for a number of emerging applications. In this study, classical molecular dynamics techniques are employed to predict the effect of chain architecture on the compression behavior of nanoscale polyethylene particles subjected to simulated flat-punch testing. Cross-linked, branched, and linear polyethylene chain architectures are each studied in the simulations. Results indicate that chain architecture has a significant influence on the mechanical properties of polyethylene nanoparticles, with the network configuration exhibiting higher compressive strengths than the branched and linear architectures. These findings are verified with simulations of bulk polyethylene. The compressive stress versus strain profiles of particles show four distinct regimes, differing with that of experimental micron-sized particles. The results of this study indicate that the mechanical response of polyethylene nanoparticles can be custom-tailored for specific applications by changing the molecular architecture.
Polymers play an indispensable and ubiquitous role in daily life. One approach to produce high-performance or multifunctional polymer materials is to blend chemically different monomers, add advanced fillers, and synthesize specific molecular architectures. It is well known that varying molecular architecture through branching and networking strongly influences the mechanical, dielectric, and thermal properties of polymers. For example, cross-linked molecular architectures enhance the strength and modulus of polymers but generally reduce their fracture toughness [1–3]. However, it has been recently shown that polymer hydrogels that form ionically and covalently cross-linked networks and have fracture energies of 9,000 J/m2 can withstand stretches of over 20 . Thus, tuning the molecular architecture can provide opportunities to custom-tailor polymer material properties for specific applications.
On the other hand, polymers at nanoscale dimension are a novel class of materials that offer diverse properties, which can be distinguished from their bulk counterparts. Nanoscale polymeric particles have attracted extensive attention from both the scientific community and industry. Polymeric nanoparticles are featured prominently in a wide variety of applications such as toners, coatings, adhesives, instrument calibration standards, column packing materials for chromatography, biomedicine, and biochemical analysis [5–7]. An emerging application focuses on metal-coated conductive polymeric particles for anisotropic conductive adhesives used in liquid crystal displays and microsystems. The use of these particles could reduce package sizes and manufacturing costs and entirely eliminate the use of lead in these systems [8–11]. The continued expansion of polymeric nanoparticles to new applications has revealed unexpected behaviors and potential shortcomings. Therefore, a complete understanding of their properties is of great importance for their successful use.
Most of the previous research on nanoscale polymers have been focused on properties of thin polymer films due to their relatively easy preparation, characterization, and established applications. It has been explicitly shown that the glass transition temperature (Tg) of polymer thin films is reduced from that of the bulk due to the presence of a free interface, and Tg is found to be strongly dependent on the film thickness and chain architecture [12–15]. Several studies have been conducted on the thermal properties of polymeric particles and reached similar conclusions as with thin films [16–18]. However, few studies have been performed on the mechanical characterization of freestanding polymeric nanoparticles because of their small size and spherical geometry. Recently, a nanoindentation-based flat-punch experimental technique was developed to characterize the mechanical properties of isolated micron-sized polymeric particles [19, 20]. The mechanical response was shown to be highly dependent on the particle size and cross-link density [21, 22].
A limited number of computational studies have been carried out to investigate structure and properties of polymeric nanoparticles at the molecular level. Fukui et al.  developed a method based on molecular dynamics (MD) to generate polymeric nanoparticle models with linear chain architectures in a layer-by-layer manner. Their results indicated that structural and thermal properties are dependent on particle size. Hathorn et al.  investigated the dynamic collision of polyethylene (PE) nanoparticles containing linear molecular architectures. Very recently, our group has studied the effect of size on the mechanical properties of PE nanoparticles via coarse-grained MD simulation (Zhao JH, Nagao S, Odegard GM, Zhang ZL, Kristiansen H, He JY: Size-dependent mechanical behavior of nanoscale polymer particles through coarse-grained molecular dynamics simulation. submitted). Although these pioneering experimental and simulation works have provided insight into the macroscopic properties of polymeric particles, fundamental knowledge of molecular-level behavior is still missing, particularly for different chain architectures.
In this paper, a novel method to construct MD simulation models of ultrafine and stable PE nanoparticles with different molecular architecture is introduced. The MD models are used to examine the compressive flat-punch behavior of PE nanoparticles with linear, branched, and cross-linked chains. It is shown that the chain architecture has a significant effect on the compression behavior of freestanding individual PE nanoparticles.
Potential functions and parameters of united atom force field
r c (Å)
k b (kcal/(mol·Å 2 ))
r 0 (Å)
k θ (kcal/mol)
θ 0 (deg)
A 0 (kcal/mol)
A 1 (kcal/mol)
A 2 (kcal/mol)
A 3 (kcal/mol)
CH x … CH y
(x = 1, 2, 3; y = 2, 3) 
CH x -CH y
CH x -CH2-CH y
CH x -CH2-CH2-CH y
(x, y = 1, 2, 3) 
(x, y = 1, 2, 3) 
(x, y = 1, 2, 3) 
CH… CH 
CH x -CH-CH y
CH x -CH-CH2-CH y
(x, y = 2) 
(x, y = 2) 
where R and R0 are the instantaneous and initial radius of the spherical wall, respectively, S is a positive constant, and n has progressive values of positive integers corresponding to elapsed time of the simulation (i.e., n = 1, 2, 3, …). For the simulations described herein, S was 0.99 and n increased by a value of 1 for every 5 ps of simulation time. Prior to this dynamic relaxation, the confined PE models were first quasi-statically relaxed to a local minimum potential energy configuration using the conjugate gradient method. For each increment of the subsequent dynamic compression, the systems were simulated in the NVT ensemble at 1,000 K, and the density of the polymeric particle was monitored. When the density reached 1.0 g/cm3, the compression was terminated. The confined nanoparticle were annealed at 1,000 K for 200 ps to reach a favorable energy configuration and then cooled down to 50 K at a rate of 2.375 K/ps in the absence of the spherical wall. The isolated nanoparticle was heated to 600 K at a rate of 1.1 K/ps, followed by cooling down to 200 K at a rate of 2 K/ps. Finally, 200 ps NVT runs were performed for relaxing the system, and the ultrafine PE nanoparticles were complete.
Results and discussion
Tensile and compressive modulus of bulk and particle PE with different chain architectures
MD models of ultrafine monodisperse polymeric nanoparticles with networked, branched, and linear chain architectures were developed using simulated spherical hydrostatic compression of groups of coarse-grained PE molecules. The mechanical response of these nanoparticles subjected to a simulated flat-punch compression test was predicted and compared to that predicted from a 3D bulk simulation of PE. It was determined that the network configuration yielded stronger nanoparticles than those with branched or linear chain configurations. These findings were consistent with the predictions of the bulk PE models. It was also shown that the nanoparticles have a relatively uniform mass density and that individual chains have unique morphologies for high values of compression for the three different architecture types. The results of this study are important for the understanding of chain architecture on the behavior of polymeric nanoparticles that are used in a wide range of engineering applications. The mechanical properties of these particles can be tailored to specific levels simply by adjusting the chain architecture between network, branched, and linear systems. While it is evident that the network architecture yields nanoparticles with a stiffer response, the linear system results in nanoparticles with lower compressive loads for a given compressive strain.
This work is supported by the Research Council of Norway (RCN) under NANOMAT KMB (MS2MP) project no. 187269 and the U.S.-Norway Fulbright Foundation. The computational resources are provided by the Norwegian Metacenter for Computational Science (NOTUR).
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