Nanostructure and Rheology of Hydrogen-Bonding Telechelic Polymers in the Melt: From Micellar Liquids and Solids to Supramolecular Gels

Polymers with hydrogen-bonding groups in the melt state often combine the ability to form specific supramolecular bonds with a tendency for unspecific aggregation and microphase separation. Using a combination of small-angle X-ray scattering and shear spectroscopy, we present a study of structure formation and rheological properties of such a case, an exemplary series of telechelic polyisobutylenes, functionalized with hydrogen-bonding end groups. Unspecific interaction between hydrogen-bonding groups leads to the formation of micelles. For monofunctional samples, we observe ordering at lower temperatures, induced by a temperature dependent concentration of the micelles. The rheological properties of these systems can be mapped to the behavior of a concentrated colloidal fluid or solid. For bifunctional polymers with complementary hydrogen-bonding groups, interaction between micellar aggregates leads to network formation and solidlike properties at lower temperatures induced by gelation without ordering. Only in this case the supramolecular bonds directly determine the rheological properties

What Controls the Structure and the Linear and Nonlinear Rheological Properties of Dense, Dynamic Supramolecular Polymer Networks?

We investigated a series of telechelic polyisobutylenes, previously shown to exhibit self-healing, by means of small-angle X-ray scattering and rheology. All samples form a dense, dynamic network of interconnected micelles resulting from aggregation of the functional groups and leading to viscoelastic behavior. The dynamic character of this network manifests itself in the appearance of terminal flow at long time scales. While the elastic properties are distinctly molecular weight dependent, the terminal relaxation time is controlled by the functional end groups. The yielding properties under large deformation during startup shear experiments can be understood by a model of stress activation of the dynamic bonds. Stress relaxation experiments help to separate the nonlinear response into two contributions: a fast collapse of the network and a slow relaxation, happening on the time scale of the terminal relaxation. The latter is also known to control self-healing of the collapsed structure

Influence of Chain Topology on Polymer Dynamics and Crystallization. Investigation of Linear and Cyclic Poly(ε-caprolactone)s by ¹H Solid-State NMR Methods

We report on the investigation of cyclic and comparable linear poly(ε-caprolactone)s (PεCL) with molecular weight between 50 and 80 kg/mol with regard to chain mobility in the melt and crystallinity using low-field solid-state ¹H NMR. Our results from NMR Hahn echo and more advanced multiquantum measurements demonstrate a higher segmental mobility of cyclics in the melt as compared to their linear counterparts. Rheological experiments indicate that the cyclics are less viscous than the linear analogues by about a factor of 2, confirming the NMR results. FID component analysis shows higher crystallinities of the cyclic samples by some percent under the condition of isothermal crystallization at 48 °C, suggesting that due to their enhanced overall mobility in the melt, the cyclics reach a more perfect morphology leading to higher crystallinity. We compare this finding with results from DSC measurements obtained under identical conditions and critically evaluate the applicability of polymer crystallinity determination from nonisothermal crystallization investigations by DSC. We further highlight the use of nucleating agents to investigate the particular effect of crystal growth on (nonisothermal) crystallization, separated from the influence of nucleation. These experiments indicate a faster crystal growth for cyclic samples