Linear Viscoelastic and Dielectric Relaxation Response of Unentangled UPy-Based Supramolecular Networks

Abstract

Supramolecular polymers possess versatile mechanical properties and a unique ability to respond to external stimuli. Understanding the rich dynamics of such associative polymers is essential for tailoring user defined properties in many products. Linear copolymers of 2-methoxyethyl acrylate (MEA) and varying amounts of 2-ureido-4[1H]-pyrimidone (UPy) quadruple hydrogen-bonding side units were synthesized via free radical polymerization. Their linear viscoelastic response was studied via small amplitude oscillatory shear (SAOS). The measured linear viscoelastic envelope (LVE) resembles that of a well entangled polymer melt with a distinct * To whom correspondence should be addressed † Technical University of Denmark ‡ University of the Basque Country ¶ Drexel University 1 plateau modulus. Dielectric relaxation spectroscopy (DRS) was employed to independently examine the lifetime of hydrogen bond units. DRS reveals a high frequency α-relaxation associated with the dynamic glass transition, followed by a slower α * -relaxation attributed to the reversible UPy hydrogen bonds. This timescale is referred to as the bare lifetime of hydrogen bonding units. Using the sticky Rouse model and a renormalized lifetime, we predict satisfactorily the LVE response for varying amounts of UPy side groups. The deviation from the sticky Rouse prediction is attributed to polydispersity in the distribution of UPy groups along the chain backbone. We conclude that the response of associating polymers in linear viscoelasticity is general and does not depend on the chemistry of association, but rather on the polymer molecular weight (MW) and MW distribution, the number of stickers per chain, n s , and the distribution of stickers along the backbone.

Linear shear and nonlinear extensional rheology of unentangled supramolecular side-chain polymers

“…These results are consistent with results for several other reported supramolecular polymers [63][64][65][66]. As discussed in the introduction, we predict that all our polymer samples are unentangled.…”

“…The model has been used to describe data on supramolecular polymers and it typically fits the experimental data quite well in the terminal regime [41,65,88]. However, a relatively large mismatch between data and theory can often be observed in the rubber plateau region [41,65], and we find the same to be true for the polymers in the present investigation.…”

“…Generally, it is thus important to understand the rheological response for a proper control of the material behaviour. The effect of UPy addition on the linear viscoelasticity of supramolecular polymers has previously been investigated [20,[63][64][65][66]. However, few studies exist where the rheological response is characterised for a systematic variation of supramolecular side-group density [41,66,67].…”

“…Here, the standard Rouse model for single chain dynamics is modified to take into account the effects of the sticker interactions on the viscoelastic properties of the supramolecular material; the associations and dissociations of the sticky groups are assumed to act as an extra friction between polymer chains and thus to delay the terminal relaxation. The sticky-Rouse model can be generalized to account both for polydispersity in the overall molecular weight and for variation in the total number of stickers per chain [65]. The model has been used to describe data on supramolecular polymers and it typically fits the experimental data quite well in the terminal regime [41,65,88].…”

“…The sticky-Rouse model can be generalized to account both for polydispersity in the overall molecular weight and for variation in the total number of stickers per chain [65]. The model has been used to describe data on supramolecular polymers and it typically fits the experimental data quite well in the terminal regime [41,65,88]. However, a relatively large mismatch between data and theory can often be observed in the rubber plateau region [41,65], and we find the same to be true for the polymers in the present investigation.…”