Nonlinear Mechanical Behavior of Scarcely Crosslinked Poly(dimethyl siloxane) Gel: Effect of Strand Length Polydispersity

Abstract

Nonlinear mechanical behavior was examined for a scarcely crosslinked poly(dimethyl siloxane) gel (referred to as Gel-1/1) under constant-rate elongation and large step shear strains. The average molecular weight of the gel strands evaluated from the equilibrium modulus in the linear viscoelastic regime was M c ¼ 190 Â 10 3 , and the strands had a significantly broad molecular weight distribution, M w =M n ¼ $ 600 as estimated by fitting the linear viscoelastic moduli with a Rouse network model. In the elongational test at constant elongational rates _ " " (¼ _ = ; = elongational ratio), the Gel-1/1 sample exhibited _ " "-insensitive strain hardening followed by rupture at max ¼ 4:5. This max was significantly smaller than the max nominally expected for a gel composed of monodisperse strands having M c ¼ 190 Â 10 3 ; max ¼ 53 and max = max ¼ $ 0:08 for those strands. In contrast, a reference experiment made for a Gel-U sample composed of monodisperse strands (M c ¼ 15 Â 10 3 ; including densely trapped entanglements) indicated that max of this gel was close to max ; max ¼ $ 14, max ¼ 16, and max = max ¼ $ 0:9 for Gel-U. These results suggested that the low-M fractions of the strands in the Gel-1/1 sample were highly stretched and broken at much smaller than the max defined for the average M c , thereby governing the nonlinear elongational behavior/rupture of Gel-1/1. Under large step shear strains (> 2), Gel-1/1 exhibited nonlinear decay of the shear stress with time. Analysis of the linear viscoelastic moduli of Gel-1/1 after imposition of large strains indicated that the stress decay under large strains reflected scission of the low-M fractions of the gel strands as well as the motion of scissionformed long strands occurring with time. This behavior was qualitatively similar to the nonlinear elongational behavior, although a delicate difference related to time-dependent cessation/motion of the scission-formed long strands remained between the nonlinearities under the large shear and elongation.KEY WORDS: Scarcely Crosslinked Siloxane Gel / Polydisperse Gel Strands / Large Deformation / Strand Scission / The rubber elasticity has been one of the most important subjects in the field of polymer physics, and the relationship(s) between the mechanical properties and the network structure of rubbers/gels has been studied over several decades. In the early studies, Flory 1 and James and Guth 2 showed that the fluctuation of the crosslinking points reduces the equilibrium modulus compared to the modulus expected for the affine displacements of these points. Langley 3 and Dossin and Graessley 4 demonstrated that the modulus is enhanced by the trapped entanglement (permanent knot) between the network strands, and Murakami et al. 5 suggested a chemo-rheological method for determining the molecular weight between crosslinks. After these pioneering studies, several new aspects have been revealed for natural rubbers (crosslinked cis-polyisoprenes). For example, Toki et al. 6 examined X-ray diffraction from natural rubbers e...

Brownian simulations for tetra-gel-type phantom networks composed of prepolymers with bidisperse arm length

“…They have summarized that the examined arm length contrast does not significantly affect the toughness of resultant gels. We note that if further contrast is introduced, the inhomogeneous stretching of strands probably depresses the toughness, as suggested by earlier studies for end-linked 17,18 and randomly cross-linked 19 poly (dimethylsiloxane) (PDMS) elastomers.…”

Nonlinear Elasticity of Ultrasoft Near-Critical Gels with Extremely Sparse Network Structures Revealed by Biaxial Stretching

“…The rheological properties of sparse polydimethylsiloxane (PDMS) networks were intensively investigated by Takahashi et al . ,− These PDMS gels were composed of an infinite sparse network with considerable amounts of soluble components (finite clusters) owing to the low degree of reaction extent. They showed that the sparse network structures enhanced the ultimate extensibility and decreased the gel modulus because of the long average chain length of network strands and low network backbone concentration. ,− However, the nonlinear elasticity characteristics of sparse networks still need to be characterized. As an extremely sparse network, one can consider a near-critical gel slightly beyond the gelation threshold.…”

“…The present study investigates the nonlinear elasticity of extremely sparse networks in near-critical gels employing biaxial stretching. Nonlinear stress–strain behaviors of various types of sparse networks have been reported. , However, they are limited to uniaxial stretching or compression. While uniaxial deformation is a special type among all accessible deformations of gels, unequal biaxial stretching with varying independently the strains in two orthogonal directions covers a wide range of deformations. ,, The biaxial stress–strain data provide a definite basis for a comprehensive understanding of the nonlinear elasticity of elastomers and gels, e.g., the unambiguous determination of strain energy density function. , Because near-critical gels are composed of an extremely sparse infinite network and a large number of finite clusters, the modulus is so low (typically of the order of 10 3 Pa) that they can undergo considerable bending deformation by their own weight, resulting in an obstacle for mechanical testing.…”

“…Nonlinear stress−strain behaviors of various types of sparse networks have been reported. 14,21 However, they are limited to uniaxial stretching or compression. While uniaxial deformation is a special type among all accessible deformations of gels, unequal biaxial stretching with varying independently the strains in two orthogonal directions covers a wide range of deformations.…”

Research on the crosslinking mechanism of polyacrylamide/resol using molecular simulation and X-ray photoelectron spectroscopy

“…On the contrary, organic crosslinkers can gel slowly and bear relatively high temperatures, prompting them to be candidates for chemical flooding in enhanced oil recovery. The different types of crosslinkers lead to discrepancies in rheological properties, salt resistance and temperature resistance; moreover, crosslinking density and junction size also have an effect on these properties, [13][14][15] hence comprehension of the structure-function relation has become increasingly important. However, it is difficult to characterize the structure and composition of the crosslinked products with poor solubility and meltability, hence the crosslinked structure and mechanism are not clearly understood.…”