Stretched Exponential Stress Relaxation in a Thermally Reversible, Physically Associating Block Copolymer Solution

Abstract

The shear stress relaxation of a thermally reversible, physically associating solution formed from a triblock copolymer in solvent selective for the mid-block was found to be well described over a broad temperature range by a stretched exponential function with a temperature independent 'stretching exponent', ≈ 1/3. This same exponent value has been suggested to have particular significance in describing structural relaxation in a wide range of disordered viscoelastic materials ranging from associating polymer materials ('gels') to glass-forming liquids. We quantify the temperature dependence of the high frequency, or short time, shear modulus as function of temperature and find that this property also follows a variation often observed in gels and glass-forming materials.

Mechanical Properties and Failure Behavior of Physically Assembled Triblock Copolymer Gels with Varying Midblock Length

“…Consequentially, with a decrease in temperature, solvent quality becomes poor for the endblock, while the midblock solubility does not change significantly . As a result of poor solubility of PS, a few of PS blocks collapse together to form aggregates, while the PI blocks bridge those aggregates, forming a 3D network of physically crosslinked gel swollen in the solvent . The gelation behavior observed here is similar to that observed for the ABA‐type block copolymers in a midblock selective solvent.…”

“…As the chosen organic liquids often have low‐vapor pressure, these gels are more stable during applications in comparison to the conventional hydrogels. These physically assembled gels behave like soft, stretchable solid at room temperature and based on the structural variation can display significantly different modulus, toughness, and failure behavior . Although low‐strain mechanical‐properties of physical gels are widely studied, the failure behavior of these gels is not well‐understood, particularly, if the bridging blocks or the load‐bearing chains are entangled.…”

“…The gelation behavior and structure of physical gels that consist of ABA triblock copolymer dissolved in a B‐selective solvent have been widely studied in the literature . Here, in the selected solvents, the A‐ and B‐blocks have different relative solubility as a function of temperature.…”

“…Here, both endblocks of a polymer chain remain in the same aggregate . However, at a relatively high polymer concentration, the interaggregate distance becomes less and the midblocks can bridge those aggregates (acting as crosslinkers), leading to the formation of a three‐dimensional network or gel . These gels are often thermoreversible in nature, as with increasing temperature the A‐block solubility increases, resulting in a viscous solution.…”

“…The mechanical properties of these gels have been widely explored and explained on the basis of their structure . Particularly, the change of mechanical properties linking to the size of aggregates (crosslinks), aggregate shape, block length or molecular weight, solvent quality, temperature, polymer volume fraction, and inclusion of nanoparticles has been studied.…”