Rheology of polymer layered silicate nanocomposites
Introduction
Recently, nylon 6 based layered silicate nanocomposites with dramatically improved tensile strength and heat distortion temperature without significant loss of impact strength and with as little as 2 vol.% layered silicate have been demonstrated [1], [2], [3], [4]. Much interest has since focused on understanding and developing polymer layered silicate nanocomposites for a wide range of applications using commodity polymers [5], [6]••[7]. These systems have also rendered significant information regarding the static and dynamic properties of confined polymers [6], [8]••. Understanding the rheological properties of polymer melt layered silicate nanocomposites is crucial to gain a fundamental understanding of the processability and structure–property relations for these materials. In this review, recent developments in understanding the viscoelastic properties of such nanocomposites are considered.
Structurally, layered silicate based nanocomposites are characterized as immiscible, intercalated and exfoliated as depicted in Fig. 1. In an intercalated system, the polymer swells the galleries of the silicate layers but preserves the stacking of layers, while in an exfoliated system the silicate layers are dispersed in the polymeric matrix as individual layers. The response of such intercalated and exfoliated nanocomposites to external flow is vital in their processing, but would also provide a systematic study of the response of highly anisotropic layers suspended in a viscoelastic medium. Furthermore, viscoelastic measurements are highly sensitive to the nanoscale and mesoscale structure of the nanocomposites and appear to be a powerful method to probe the underlying structure in such materials.
Section snippets
Linear viscoelastic properties
The melt state linear dynamic oscillatory shear properties of intercalated and exfoliated nanocomposites have been examined for a wide range of polymeric matrices including nylon 6 [9]••[10]••, poly(ε-caprolactone) [6], [9]••, polystyrene (PS) [11]•, polystyrene–polyisoprene (PS–PI) block copolymers [12]•[13]•[14], and polypropylene (PP) [15]••[16]••. The most significant results of these works are summarized as:
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A transition from liquid-like to solid-like rheological behavior for nanocomposites
Non-linear dynamic response
As alluded to previously, prolonged application of large amplitude oscillatory shear results in the preferential orientation of the silicate layers and observation of liquid-like linear viscoelastic response. We note that in two dimensions the percolation threshold is significantly enhanced and a simple calculation based on the stack sizes estimated previously would require a silicate loading of over 40 wt.% before percolation occurred. The strain amplitude sensitivity of the viscoelastic
Steady shear behavior
As compared to the oscillatory shear data, relatively few studies have investigated the steady shear behavior of polymer nanocomposites [10], [15], [23]. Four salient points have been noted and are discussed below.
For nanocomposites with silicate loadings well above the percolation threshold, the viscosity at low shear rates diverge and the viscoelastic behavior is consistent with the presence of a finite apparent yield stress [12], [15], [23]. On the other hand, at high shear rates the
Flow orientation of nanocomposites
Layered silicate based nanocomposites, like other intrinsically anisotropic materials, exhibit the ability to orient the silicate layers in response to externally applied flow. This ability to orient, along with the quiescent mesoscale structure, appears to control the viscoelastic properties of such nanocomposites. The early studies on nylon 6 based end-tethered nanocomposites suggested that the silicate layers, upon injection molding of the nanocomposites, exhibit substantial parallel
Acknowledgements
We would like to thank Dr Vaia and Prof. Paul for providing us with manuscripts prior to publication. The help of Jiaxiang Ren and Cynthia Mitchell in the preparation of this manuscript is gratefully acknowledged. We would like to acknowledge financial support from the ExxonMobil Chemical Company, the donors of the Petroleum Research Fund administered by the American Chemical Society, and the Advanced Technology Program of the Texas Coordinating Board for Higher Education.
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