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Research Papers

# Time and Temperature Dependent Recovery of Epoxy-Based Shape Memory Polymers

[+] Author and Article Information
Francisco Castro, Kristofer K. Westbrook, Dae Up Ahn, Yifu Ding

Department of Mechanical Engineering, University of Colorado, Boulder, CO 80309

Jason Hermiller

Cornerstone Research Group, Dayton, OH 45440

H. Jerry Qi1

1

Corresponding author.

J. Eng. Mater. Technol 133(2), 021025 (Mar 24, 2011) (9 pages) doi:10.1115/1.4003103 History: Received December 06, 2009; Revised November 01, 2010; Published March 24, 2011; Online March 24, 2011

## Abstract

Shape memory polymers (SMPs) are a group of adaptive polymers that can recover the permanent shape from a temporary shape by external stimuli on demand. Among a variety of external stimuli for polymer actuation, temperature is the most extensively used. In SMP applications, one of the major design considerations is the time necessary to recover the shape without external deformation constraints, or free recovery, and the amount of the recoverable strain. This paper investigates the amount of the recoverable strain and the recovery rate of an epoxy-based SMP (Veriflex® E, VFE1-62 (CRG, Dayton, OH)) under different thermal conditions. In particular, the free recovery behaviors of the SMPs under two experimental protocols, isothermal and shape memory (SM) cycle, are studied. It is found that free recovery in isothermal experiments is much faster than that in a SM cycle at the same recovering temperature and the material is fully recoverable at the temperature above differential scanning calorimetry $Tg$. Furthermore, for the recovery in SM cycle experiments, reshaping the sample at a low temperature and recovering from the deformation at a high temperature yield the fastest recovery rate, while reshaping at a high temperature and recovering at a low temperature cannot recover the original shape within this work’s experimental time frame. The possible mechanism for these observations is discussed.

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## Figures

Figure 1

Experimental setups. (a) Overall setup, including the testing frame, the thermal chamber, and the laser extensometer. (b) Sample and compression platens.

Figure 2

Schematic plots of thermal and strain profiles for (a) isothermal experiments and (b) SM cycle experiments

Figure 3

DMA, CTE and DSC measurements to determine Tg of the SMP. (a) Storage modulus and tan δ as a function of temperature from the DMA experiment scanned at a thermal rate of 1°C/min. (b) CTE measurement during cooling at 1°C/min. (c) DSC measurement at 10°C/min.

Figure 4

Stress-strain behavior at isothermal conditions. (a) 40°C and 60°C and (b) 80°C and 100°C. The arrows indicate direction of loading.

Figure 5

Free recovery in isothermal experiments: strain recovery ratio

Figure 6

(a) Free recovery at different temperatures as a function of prescribed temperatures. (b) Recovery ratio as a function of time. Recovery onset points are indicated by an arrow. The point where the temperature reaches TH2 is indicated by a vertical dashed line.

Figure 7

(a) Comparison of recovery ratio as a function of times from isothermal experiments and SM cycle. (b) Recovery ratios at 20 min after the end of heating.

Figure 8

Temperature rate effects during free recovery, TH1=100°C. (a) Recovery ratio versus time since beginning of heating and (b) recovery ratio versus temperature. In (b), the recovery ratios at 60 min are indicated by the symbol “o” for the cases with 10°C/min heating rate and by the symbol “×” for the cases with 2.5°C/min heating rate.

Figure 9

Recoveries in SM cycle experiments with the same TH2 but different TH1

Figure 10

Recoveries in the experiments that used TH1=TH2 in one SM cycle

Figure 11

Comparison of recovery ratios of SM cycle experiments at different TH1 and TH2. (a) With TH1 or TH2 being 60°C or 100°C and (b) with TH1 or TH2 being 80°C or 100°C.

Figure 12

(a) Recovery ratio at 60 min after heating as a function of TH1 and TH2. (b) Time for 95% recovery ratio as a function of TH1 and TH2.

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