Turning Rubber into a Glass: Mechanical Reinforcement by Microphase Separation

Abstract

Supramolecular associations provide a promising route to functional materials with properties such as self-healing, easy recyclability or extraordinary mechanical strength and toughness. The latter benefit especially from the transient character of the formed network, which enables dissipation of energy as well as regeneration of the internal structures. However, recent investigations revealed intrinsic limitations in the achievable mechanical enhancement. This manuscript presents studies of a set of telechelic polymers with hydrogen-bonding chain ends exhibiting an extraordinarily high, almost glass-like, rubbery plateau. This is ascribed to the segregation of the associative ends into clusters and formation of an interfacial layer surrounding these clusters. An approach adopted from the field of polymer nanocomposites provides a quantitative description of the data and reveals the strongly altered mechanical properties of the polymer in the interfacial layer. These results demonstrate how employing phase separating dynamic bonds can lead to the creation of high-performance materials.

Critical Role of the Interfacial Layer in Associating Polymers with Microphase Separation

“…To analyze the effect of phase separation on the viscoelastic properties of these samples, we constructed master curves from the shear modulus spectra using time–temperature superposition. The results for storage ( G ′) and loss ( G ″) moduli for the PDMS-UU sample are shown in Figure together with earlier results for PDMS-COOH samples. , …”

“…However, the difference increases strongly with decreasing DP, reaching ∼100 times for the sample with DP = 13. As it was shown in our earlier analysis, this difference cannot be explained by a hard-filler effect due to the presence of the glassy clusters. This difference clearly indicates that the interfacial layer has a much higher modulus than the bulk-like PDMS matrix.…”

“…However, it is unlikely that many connecting chains will have the minimum length. The reason is that there are many chain ends per cluster, , and most of them will be distributed across the entire surface of the cluster. Naively, we would expect that the average R ee for all chains connecting two neighbor clusters should be between ∼ d – R cluster and ∼ d .…”

“…In analogy with the well-known case of PNC, we can ascribe the observed behavior of the dielectric and rheological spectra to the existence of an interfacial polymer layer surrounding the “glassy” clusters of the associating chain ends. In our earlier studies, we used this idea to estimate a possible thickness of the interfacial layer using the plateau modulus by assuming the modulus in the interfacial layer to be ∼1 GPa. However, the BDS spectra provide an independent approach to estimate the thickness of the interfacial layer. ,, Knowing the thickness of the interfacial layer, we will be able to determine its modulus from the rheological data (Figure ).…”

“…In this studies, we employed small-angle X-ray scattering (SAXS), BDS, and rheology to study the structure, segmental relaxation, and viscoelastic properties of telechelic associating polymers with a poly­(dimethyl siloxane) (PDMS) backbone and two different types of associating end groups. Our previous studies revealed strong microphase separation in some of these polymers, which is connected to an unusually high rubbery plateau modulus, and demonstrated that their viscosity is controlled by structural relaxation in the clusters of the end groups. − There, we speculated that the high rubbery modulus might be explained by a higher modulus in the polymer interfacial layer . Here, we present a detailed analysis of the microstructure, the interfacial layer volume fraction, and thickness in these systems using an approach developed for the analysis of the dielectric relaxation spectra in PNCs.…”