Abstract
The microstructure of physically assembled gels depends on mechanical loading and
environmental stimuli such as temperature. Here, we report the real-time change in the
structure of physically assembled triblock copolymer gels that consist of 10 wt% and
20 wt% of poly(styrene)-poly(isoprene)-poly(styrene) [PS-PI-PS] triblock copolymer in
mineral oil (i) during the gelation process with decreasing temperature, (ii) subjected
to large oscillatory deformation, and (iii) during the stress-relaxation process after the
application of a step-strain. The presence of loosely bounded PS-aggregates at temperatures higher than the rheometrically determined gelation temperature (Tgel) captures
the progressive gelation process spanning over a broad temperature range. However,
the microstructure fully develops at temperatures suciently lower than Tgel, and the
storage modulus (G0
) also reaches a plateau at those temperatures. The microstructure
orients in the stretching direction with the applied strain. In an oscillation strain cycle, such oriented structure has been observed at low-strain. But, at large-strain, the oriented structure splits, and only a fraction of midblock participates in load-bearing. This
has been attributed to the endblock pullout from the aggregates, likely caused by the
strain localization in the samples. Both microstructure recovery and time-dependent
moduli during the stress-relaxation process after the application of a step-strain can be
captured using a stretched-exponential model. However, the microstructure recovery
time has been found to be two orders of magnitude slower than the stress-relaxation
time at room temperature, indicating a complex nature of relaxation process involving
midblock relaxation, endblock pullout and reassociation process. Due to their viscoelastic nature, these gels' mechanical responses are sensitive to strain, temperature, and
rate of deformation. Therefore, insights into the microstructural information as a function of these parameters will assist these gels' real-life applications and design new gels
with improved properties
Supplementary materials
Title
10%Gel 27C Strain1
Description
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Title
20%Gel 27C Strain1
Description
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