Double replication for characterizing cracks
in surface-hardened PDMS
M. Leifels, A. Mayer, P. Görrn, H.-C. Scheer
Chair of Large Area Optoelectronics, University of Wuppertal,
D-42119 Wuppertal, Germany
mleifels@uni-wuppertal.deDue to its stretchability the well-known elastomer PDMS is highly interesting fora wide range of applications1. Typically, the surface of the PDMS has to bemodified to improve its adhesion to other materials. Common methods forsurface modification are plasma or (V)UV treatment; such tuning of the surface
energy is inherently coupled with surface hardening so that surface cracks may
occur, in particular under mechanical loading. In the majority of cases cracks
should be avoided2; however, controlled cracking may be of interest, too3,
provided that the cracks are well-characterized.
In our study the cracking of surface-hardened PDMS is induced in a motordriven clamping unit that ensures a uniaxial straining of the sample. Beforehand,
the PDMS is hardened by exposure to VUV light at 172 nm with an excimer
lamp. The impact of preparational issues is addressed, in particular the aging of
PDMS during storage and the effect of pre-evacuation before cross-linking. The
experimental procedure is as follows. First the critical strain for cracking is
detected, the strain at which the first cracks occur during stepwise stretching of
the PDMS (Fig. 1a). To freeze this state, a first replication into OrmoStamp isdone (Fig. 1b). To analyze the behavior of the cracks upon further stretching,additional replications at higher strain are performed. For quantifying the crack
characteristics (shape, width, depth) cross-sections are beneficial. Therefore, asecond replication into SU-8 on Si is conducted (Fig. 1c), which then is analyzed
in cross-section by SEM.
Fig. 2 illustrates the effect of the preparation conditions on the critical strain forcracking. Upon storage PDMS softens, where pre-evacuation stabilizes thecracking behavior. Fig. 3 shows examples of replicated cracks after furtherstretching. Fig. 4 gives typical evaluation results for the characteristic parameters
crack spacing, crack width and crack depth.
The study will show that reproducible cracking can be obtained when thepreparation conditions chosen are adequate. The evaluated parameters will be
discussed on the basis of crack theory4,5.
1

S. Wagner, S. Bauer, MRS Bull. 37, 207 (2012).
A. Polywka et al., Adv. Mater. 27, 3755 (2015).
3
R. Seghir, S. Arscott, Sci. Rep. 5, 14787 (2015); A. Polywka et al, Nanomaterials 6, 168 (2016).
4
Z. C. Xia, J. W. Hutchinson, J. Mech. Phys. Solids 48, 1107 (2000).
5J. Huang et al., J. Mater. Sci. 49, 255 (2014).
2

Fig. 1: Double replication forcharacterizing cracks.
a) Hardened PDMS with cracks,b) 1st replication in OrmoStamp,c) 2nd replication in SU-8 on Si
for preparing cross-sections.

Fig. 3: Cross-sectional view of cracks.
2nd replication (SU-8 on Si) of cracks at
critical strain (≈15%) and beyond (≈24%)(pre-evacuated before cross-linking, storage
time 1 day, 15 min/20 min VUV surface
treatment, similar scale bars).

Fig. 2: Critical strain for initialformation of cracks (ecr) after
different sample storage times.
(1 hour, 1 day, 3-5 days),
VUV treatment time 15/20 min,Full color: with pre-evacuation.
Shaded: without pre-evacuation.

Fig. 4: Parameters crack spacing, crack
width and crack depth at different strain.
(with pre-evacuation before cross-linking,storage time 1 day, VUV treatment time
blue: 15 min / red: 20 min),Left mark: at critical strain (ecr).
Right mark: at increased strain.