Programming of Shape Memory Natural Rubber for Near-Discrete

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Programming of Shape Memory Natural Rubber for Near-Discrete Shape Transitions Joerg C Tiller ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/am507184c • Publication Date (Web): 23 Dec 2014 Downloaded from http://pubs.acs.org on December 28, 2014

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Programming of Shape Memory Natural Rubber for NearDiscrete Shape Transitions

Journal: Manuscript ID: Manuscript Type: Date Submitted by the Author: Complete List of Authors:

ACS Applied Materials & Interfaces am-2014-07184c.R3 Article 21-Dec-2014 Quitmann, Dominik; University of Dortmund, Bio- and Chemical Engineering Reinders, Frauke; University of Dortmund, Bio- and Chemical Engineering Heuwers, Benjamin; University of Dortmund, Bio- and Chemical Engineering Katzenberg, Frank; University of Dortmund, Bio- and Chemical Engineering Tiller, Joerg; University of Dortmund, Bio- and Chemical Engineering

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Programming of Shape Memory Natural Rubber for Near-Discrete Shape Transitions Dominik Quitmann†, Frauke M. Reinders†, Benjamin Heuwers†, Frank Katzenberg† and Joerg C. Tiller*† †

Chair of Biomaterials & Polymer Science Department of Biochemical & Chemical Engineering, TU Dortmund, D-44221 Dortmund, Germany

Email corresponding author: [email protected] Keywords: narrow shape-transition, SMNR, shape memory, cold-programming, aging

ABSTRACT

Typical shape memory polymers are hot-programmed and show a shape transition over a broad temperature-range of 10 K and more. Cold-programmed shape memory natural rubber (SMNR) recovers more than 80% of its original shape within 1 K. The trigger-point can be increased upon aging the stretched SMNR over several weeks without losing the narrow trigger-range. This process can be accelerated by treatment of the stretched SMNR with non-affine solvent vapors. Affine solvent vapors of low concentrations afford even higher trigger-points than that achieved by aging. This way even higher cross-linked natural rubber can be cold-programmed. 1 ACS Paragon Plus Environment


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INTRODUCTION Shape memory materials are capable to memorize a temporary shape unless they are exposed to an external stimulus that triggers them to recover their permanent shape1. Although stimuli like chemical vapors2,3 or liquids4–7, mechanical stress8,9, IR-/UV-light10 or alternating magnetic fields11 have been described, the majority of all shape memory materials is triggered thermally by heating above a certain, material dependent temperature, which is referred to as triggertemperature Ttrig.12 Generally shape memory polymers (SMPs) do not recover their shape in full at this temperature but over a fairly broad range of several K. For SMPs based on glassy polymers the trigger-range is caused by the naturally broad glass transition. SMPs composed of semi-crystalline polymers are triggered at the melting point Tm of the crystals, which usually stretches over a broad temperature range.13,14 This is due to the dependence of Tm on crystal size and the naturally broad size distribution of those crystals. The same is true for liquid crystalline shape memory polymers.15,16 Even shape memory alloys are often described to have a fairly broad trigger-range17. Such a behavior is sufficient for most applications of shape memory materials. However there are several applications of smart materials that benefit from a discrete transition point such as safety systems, e.g., valves and switches as well as shape adaptive medical devices.18,19 In contrast to most other SMPs, the recently discovered shape memory natural rubber (SMNR) fully recovers within 1 K when being cold-programmed.20 The highest achievable triggertemperature upon cold-programming with the highest possible straining rate and the lowest possible degree of crosslinking (XC) was found to be 26 °C, which is too low for most applications.

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One common way to increase the trigger-temperature is to program at higher temperatures21–24. A more recently discovered way of obtaining higher trigger-temperatures of already programmed SMNRs is re-crystallization of the constrained sample upon solvent exposure25. Both methods allow increasing the trigger-temperature of SMNR to up to 42 °C but to the expense of loss of the exceptionally narrow trigger-range of cold-programmed SMNR. A possible explanation for the discrepancy between the trigger-range of the cold-programmed SMNR, and hot- as well as solvent-programmed SMNR, respectively, is that in the first case all crystals start growing simultaneously with the same rate. In the latter two scenarios only a fraction of the crystals grow at the programming temperature or in the swollen network. After cooling or drying more crystals of different size are formed which broadens the melting-range and thus the trigger-range. In the cause of investigations on SMNR, we observed that programmed samples show a higher TTrig after several days of storage. In order to explore this in detail, SMNR with a XC=0.2% was stretched to 800% and kept under ambient conditions for up to 49 d. In intervals samples were explored regarding their trigger-behavior25. As seen in Figure 1 aging of the samples results in a significant increase in trigger-temperature without losing the exceptionally narrow trigger-range for up to 12 d. After this the change of trigger-temperature is negligible but the trigger-range significantly broadens.



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Figure 1. a) Trigger-processes of a SMNR samples (XC=0.2%, ɛ=800%) directly after stretching and after 1, 4, 12 and 49 d of aging. b) Depiction of the dependency of the trigger-temperature on the days of aging after stretching. Additionally higher cross-linked not cold-programmable NR with a XC=0.42% was stretched to 560% and clamped at ambient conditions for 4 d. When releasing the stress of the sample after 1 h the NR completely recovered its original shape. In contrast unclamping the sample after 4 d resulted in a fixity of 70%, i.e. the non-programmable NR was cold-programmed upon aging the stretched network.

Figure 2. Trigger-processes of stretched NR samples (XC=0.42%, ɛ=560%), directly after stretching and after 4 d of aging, respectively.


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To figure out if the effect is truly a shifting of the trigger point the same experiment was repeated and the samples were both cooled with liquid nitrogen prior to unclamping. Then the samples were continuously heated in a TMA measuring the trigger-temperature by monitoring the thickness increase. As seen in Figure 2 the aging process indeed causes a trigger-temperature shift for 4 K (see Table 1). Table 1. Tabulation of the respective trigger-temperatures and fixity ratios Rf of SMNR (XC=0.2) after 0, 1, 4, 12 and 49 d of aging respectively and natural rubber (XC=0.42) after 0 and 4 d of aging respectively. TTrig(XC=0.2)/ TTrig(XC=0.42) [°C]

Rf(XC=0.2)/Rf(XC=0.42)

0

23.7/18.3

0.69/0.69

1

25.6/-

0.62/-

4

26.2/22.5

0.67/0.71

12

27.8/-

0.53/-

49

28.6/-

0.55/-

days of aging

A possible explanation for this behavior is the literature known lamellar thickening26,27 which increases the melting point of crystals with time. In order to explore if this is the case for our samples aswell we monitored the width of the (002)-reflection of stretched SMNR with time using wide angle x-ray scattering (WAXS) experiments (see Figure 3). According to these preliminary experiments no significant lamellar thickening but a slight increase in crystallinity was observed.



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Figure 3. Wide angle x-ray scattering (WAXS) measuring of the (002) reflection (carried out with Bruker Nanostar) of stretched SMNR (XC=0.2, ɛ=800%) using a bin-normalized integration (∆χ=1°, 0°

Programming of Shape Memory Natural Rubber for Near-Discrete

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