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Research Papers

Nonlinear Bending Mechanics of Hygroscopic Liquid Crystal Polymer Networks

[+] Author and Article Information
M. R. Hays

Department of Mechanical Engineering, Florida Center for Advanced Aero-Propulsion–FCAAP,  Florida A&M/Florida State University, Tallahassee, FL 32310-6046mrh04@fsu.edu

H. Wang

Department of Mechanical Engineering, Florida Center for Advanced Aero-Propulsion–FCAAP,  Florida A&M/Florida State University, Tallahassee, FL 32310-6046hongbo@eng.fsu.edu

W. S. Oates1

Department of Mechanical Engineering, Florida Center for Advanced Aero-Propulsion–FCAAP,  Florida A&M/Florida State University, Tallahassee, FL 32310-6046

1

Corresponding author.

J. Appl. Mech 79(2), 021009 (Feb 24, 2012) (10 pages) doi:10.1115/1.4005547 History: Received April 26, 2010; Revised August 25, 2011; Posted January 30, 2012; Published February 13, 2012; Online February 24, 2012

A chemically responsive liquid crystal polymer network is experimentally characterized and compared to a nonlinear constitutive model and integrated into a finite element shell model. The constitutive model and large deformation shell model are used to understand water vapor induced bending. This class of materials is hygroscopic and can exhibit large bending as water vapor is absorbed into one side of the liquid crystal network (LCN) film. This gives rise to deflection away from the water vapor source which provides unique sensing and actuation characteristics for chemical and biomedical applications. The constitutive behavior is modeled by coupling chemical absorption with nonlinear continuum mechanics to predict how water vapor absorption affects bending deformation. In order to correlate the model with experiments, a micro-Newton measuring device was designed and tested to quantify bending forces generated by the LCN. Forces that range between 1 and 8 μN were measured as a function of the distance between the water vapor source and the LCN. The experiments and model comparisons provide important insight into linear and nonlinear chemically induced bending for a number of applications such as microfluidic chemical and biological sensors.

FIGURES IN THIS ARTICLE
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Copyright © 2012 by American Society of Mechanical Engineers
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Figures

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Figure 1

Illustration of the bending mechanism caused by asymmetric swelling of the hygroscopic liquid crystal

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Figure 2

The experimental set-up that illustrates the technique used to indirectly measure forces using the stainless steel wire using an optical microscopy set-up

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Figure 3

Experimental results of bending forces generated by the LCN film that is measured with the stainless steel wire

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Figure 4

Variations in the elastic modulus based on classic beam theory of a cantilever under a point load. Multiple measurements were taken at each normalized distance x/L.

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Figure 5

Time dependent solutions of diffusion of water vapor into the liquid crystal network. Each line corresponds to a time step starting from zero initial conditions at t = 0. Results are plotted for a linear chemical potential where g ≠ 0 and h = 0. The linear fit is given at steady-state.

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Figure 6

Time dependent solutions of diffusion of water vapor into the liquid crystal network. Results are for a nonlinear chemical potential where g = 0 and h ≠ 0. Again, the polynomial fit is given at steady-state.

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Figure 7

Model predictions based on the moment calculated using the bending force data and the FEAP nonlinear shell model. These calculations are compared with the general stress couple model using 14 by varying the chemical potential parameters g and h. The dashed curve uses g = 9.0 × 1013 Nm3 /mol2 and h = 0 and the solid curve uses h = 1.35 × 1026 Nm9 /mol4 and g = 0. The linear and nonlinear regimes refer to the form of the chemical potential driving force.

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Figure 8

Bending deformation for the maximum bending force case where X = 0.25 mm. The results are given for the free displacement case (no external edge load) for the case where concentration is independent or dependent on the bending deformation of the film.

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Figure 9

The cross-section view of the shell model illustrating increases in bending with no external load. The displacements correspond to two experimental water vapor locations (X = 1 mm and X = 0.25 mm) previously illustrated in Fig. 7. The “true” solutions correspond to position dependent concentration while the “ideal” solution neglects position dependent concentration.

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Figure 10

The changes in free displacement bending was modeled by varying the chemical potential parameter h by factors of 2, 4, and 8. Bending monotonically increases as this parameter increases. The result is compared to the original case (smallest bending) previously plotted in Fig. 8.

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Figure 11

Optical microscopy comparison of the film bending in comparison with classic beam theory. The film is clamped on the left and the wire point load is shown on the right for the case of the smallest normalized point load location of 0.35 from Fig. 4.

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Figure 12

Similar comparison of beam deformation microscopy measurement with classic beam theory as illustrated in Fig. 1. The location of the point load was along the cantilever tip (i.e., normalized location of 1 in Fig. 4).

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