 Award ID(s):
 2116298
 NSFPAR ID:
 10400542
 Date Published:
 Journal Name:
 ACS Macro Letters
 Volume:
 12
 ISSN:
 21611653
 Page Range / eLocation ID:
 362 to 368
 Format(s):
 Medium: X
 Sponsoring Org:
 National Science Foundation
More Like this

We study the effect of interfiber adhesion on the mechanical behavior of crosslinked ran dom fiber networks in two dimensions. To this end, we consider networks with connectiv ity number, z , below, at, and above the isostaticity limit of the structure without adhesion, z c . Fibers store energy in the axial and bending deformation mode and the crosslinks are of freely rotating type. Adhesive forces lead to fiber bundling and to a reduction of the total volume of the network. The degree of shrinkage is determined as a function of the strength of adhesion and network parameters. The mechanical response of these struc tures is further studied in uniaxial tension and compression. The stressstrain curves of networks without interfiber adhesion exhibit an initial linear regime, followed by strain stiffening in tension and strain softening and strain localization in compression. In pres ence of adhesion, the response becomes more complex. The initial linear regime persists, with the effective modulus decreasing and increasing with increasing adhesion in cases with z > z c and z < z c , respectively. The strain range of the linear regime increases signif icantly with increasing adhesion. Networks with z > z c subjected to tension strainstiffen at rates that depend on the adhesion strength, but eventually enter a large strain/stress regime in which the response is independent of this parameter. Networks with z < z c are stabilized by adhesion in the unloaded state. Beyond the initial linear regime their tangent modulus gradually decreases, only to increase again at large strains. Adhesive interactions lead to similar effects in compression. Specifically, in the z > z c case, increasing the adhe sion strength reduces the linear elastic modulus and significantly increases the range of the linear regime, delaying strain localization. This first investigation of the mechanics of crosslinked random networks with interfiber adhesion opens the door to the design of soft materials with novel properties.more » « less

Fiberbased materials are prevalent around us. While microscopically these systems resemble a discrete assembly of randomly interconnected fibers, the network architecture varies from one system to another. To identify the role of the network architecture, we study here cellular and fibrous random networks in tension and compression, and in the context of large strain elasticity. We observe that, compared to cellular networks of same global parameter set, fibrous networks exhibit in tension reduced strain stiffening, reduced fiber alignment, and reduced Poisson’s contraction in uniaxial tension. These effects are due to the larger number of kinematic constraints in the form of crosslinks per fiber in the fibrous case. The dependence of the small strain modulus on network density is cubic in the fibrous case and quadratic in the cellular case. This difference persists when the number of crosslinks per fiber in the fibrous case is rendered equal to that of the cellular case, which indicates that the different scaling is due to the higher structural disorder of the fibrous networks. The behavior of the two network types in compression is similar, although softening induced by fiber buckling and strain localization is less pronounced in the fibrous case. The contribution of transient interfiber contacts is weak in tension and important in compressionmore » « less

The mechanical properties of a polymer network reflect the collective behavior of all of the constituent strands within the network. These strands comprise a distribution of states, and a central question is how the deformation and tension experienced by a strand is influenced by strand length. Here, we address this question through the use of mechanophore force probes with discrete molecular weights. Probe strands, each bearing a mechanochromic spiropyran (SP), were prepared through an iterative synthetic strategy, providing uniform PDMSfunctionalized SP force probes with molecular weights of 578, 1170, and 2356 g/mol. The probes were each doped (9 mM) into the same silicone elastomer matrix. Upon stretching, the materials change color, consistent with the expected conversion of SP to merocyanine (MC). The critical strain at which measurable mechanochromism is observed is correlated with the strain hardening of the matrix, but it is independent of the molecular length of the probe strand. When a network with activated strands is relaxed, the color dissipates, and the rate of decoloration varies as a function of the relaxing strain ((ε_r ) ̅); faster decoloration occurs at lower (ε_r ) ̅. The dependence of decoloration rate on (ε_r ) ̅ is taken to reflect the effect of residual tension in the onceactivated strands on the reversion reaction of MC to SP, and the effect of that residual tension is indistinguishable across the three molecular lengths examined. The combination of discrete strand synthesis and mechanochromism provides a foundation to further test and develop molecularbased theories of elasticity and fracture in polymer networks.more » « less

We combine a slipspring model with an ‘entangled kink dynamics’ (EKD) model for strong uniaxial extensional flows (with Rouse Weissenberg number W i R ≫ 1 ) of long ( M w > 1 Mkg / mol for polystyrene) entangled polymers in solutions and melts. The slipspring model captures the dynamics up to the formation of a ‘kinked’ or folded state, while the kink dynamics simulation tracks the dynamics from that point forward to complete extension. We show that a singlechain slipspring model using affine motion of the slipspring anchor points produces unrealistically high tension near the center of the chain once the Hencky strain exceeds around unity or so, exceeding the maximum tension that a chain entangled with a second chain is able to support. This unrealistic tension is alleviated by pairing the slip links on one chain with those on a second chain, and allowing some of the large tension on one of the two to be transferred to the second chain, producing nonaffine motion of each. This explicit pairing of entanglements mimics the entanglement pairing also used in the EKD model, and allows the slip spring simulations to be carried out to strains high enough for the EKD model to become valid. We show that results nearly equivalent to those from paired chains are obtained in a singlechain slipspring simulation by simply specifying that the tension in a slip spring cannot exceed the theoretical maximum value of ζ ′ ϵ ˙ L 2 / 8 where ζ ′ , ϵ ˙ and L are the friction per unit length, strain rate and contour length of the chain, respectively. The effects of constraint release (CR) and regeneration of entanglements is also studied and found to have little effect on the chain statistics up to the formation of the kinked state. The resulting hybrid model provides a fast, simple, simulation method to study the response of high molecular weight ( M w > 1 Mkg / mol ) polymers in fast flows ( W i R ≫ 1 ), where conventional simulation techniques are less applicable due to computational cost.more » « less

The classic picture of soft material mechanics is that of rubber elasticity, in which material modulus is related to the entropic elasticity of flexible polymeric linkers. The rubber model, however, largely ignores the role of valence (i.e., the number of network chains emanating from a junction). Recent work predicts that valence, and particularly the Maxwell isostatic point, plays a key role in determining the mechanics of semiflexible polymer networks. Here, we report a series of experiments confirming the prominent role of valence in determining the mechanics of a model system. The system is based on DNA nanostars (DNAns): multiarmed, selfassembled nanostructures that form thermoreversible equilibrium gels through base paircontrolled crosslinking. We measure the linear and nonlinear elastic properties of these gels as a function of DNAns arm number, f, and concentration [DNAns]. We find that, as f increases from three to six, the gel’s highfrequency plateau modulus strongly increases, and its dependence on [DNAns] transitions from nonlinear to linear. Additionally, highervalence gels exhibit less strain hardening, indicating that they have less configurational freedom. Minimal strain hardening and linear dependence of shear modulus on concentration at high f are consistent with predictions for isostatic systems. Evident strain hardening and nonlinear concentration dependence of shear modulus suggest that the lowf networks are subisostatic and have a transient, potentially fractal percolated structure. Overall, our observations indicate that network elasticity is sensitive both to entropic elasticity of network chains and to junction valence, with an apparent isostatic point
$5<{f}_{c}\le 6$ in agreement with the Maxwell prediction.