Role of Network Connectivity on the Mechanical Properties of Highly Cross-Linked Polymers

Abstract

The effects of mixed functionality and degree of curing on the stress-strain behavior of highly cross-linked polymer networks are studied using molecular dynamics simulations. The networks are made dynamically in a manner similar to epoxy network formation, and the average functionality of the cross-linker, f av, is systematically varied from 3 to 6 by mixing cross-linkers with functionalities f ) 3, 4, and 6. Stress-strain curves are determined for each system from tensile pull simulations. The range of strain of the plateau region (RP) in the stress-strain curve, failure strain ( f), and failure stress (σf) for fully cured networks are found to have a power law dependence on fav as ∼f av R . For RP and f, R is determined to be -1.22(3) and -1.26(4), respectively. The failure strain is equal to the strain needed to make taut the maximum of the minimal paths through the network connecting the two solid surfaces. The failure stress, however, shows two distinct regions. For f av R e 4, σf increases with increase in fav and R ) 1.22(5). In this fav regime, the work to failure is constant. For f av R g 4, the systems fail interfacially, σf becomes a constant, and work to failure decreases with fav. These mechanical properties are also found to depend on the degree of curing. With decrease in percentage of curing, failure stress decreases and failure strain increases. The mode of failure changes from interfacial to bulk.

Cavitation in crosslinked polymers: Molecular dynamics simulations of network formation

“…For our current choice of models and parameters, pores form above the glass transition temperature; while this may not be a universal feature of crosslinking systems, it is important because many crosslinked polymers are used in a temperature range around or below T g . Pore formation was seen in previous simulations of the 6-2 polymer system [10,11], but unexplored there because the focus was on the mechanical properties of the crosslinked network. Void formation and cavitation in LJ liquids has been extensively studied [6,7,[23][24][25][26][27] where it was found that void formation generally occurs at the spinodal and is held to be characteristic of liquids.…”

“…Simple, coarsegrained bead-spring models, which have been successfully used to treat bulk polymer melts and networks, have been used to study fracture and adhesion of highly crosslinked polymer networks [9][10][11]. Coarse-graining is useful because simulations are limited by computational resources; it allows simulations of networks on time scales on the order of microseconds and length scales on the order of tens of nanometers, which are necessary to study the mechanical response and network properties of these systems.…”

Molecular Modeling and Mechanics of Acrylic Adhesives on a Graphene Substrate with Roughness

“…Such models have successfully been used to investigate a broad range of polymer systems including basic dynamics and structure of amorphous polymers [15,16], adhesion of end-tethered polymers [17,18], and properties of highly crosslinked network polymer adhesives [19][20][21][22]. Here, we use a simple full atomic molecular dynamics model to directly investigate the extent to which the surface roughness of graphene can affect the adhesion force with a polyacrylic (PA) adhesive.…”

Synthesis and Characterization of a Single-Component Thermally Remendable Polymer Network: Staudinger and Stille Revisited

“…The resulting crude diacid was recrystallized from methanol to yield 92% (44.05 g, based on Na,) of Thiele's acid. 1 To this was added ≈500 mL of oxalyl chloride. This reaction mixture was allowed to stir at 55°C for 36 h. The reaction progress was monitored via CO 2 evolution and was considered complete when evolution ceased, and the reaction mixture had become a transparent light brown color.…”

“…1,2 In addition to these attributes, however, there also exist drawbacks: these materials experience a loss of recyclability and are usually brittle and rigid. Upon exposure to constant external stress, these systems experience thermal and mechanical fatigue that result in the formation of microcracks.…”