Cooperative End-to-End and Lateral Hydrogen-Bonding Motifs in Supramolecular Thermoplastic Elastomers

Abstract

To a large extent, the mechanical properties of polymers are determined by the strength of the physical interactions between chains. Thermoplastic elastomers (TPE’s) make sophisticated use of reversible physical interactions.1 They consist of polymer chains in which strongly interacting segments alternate with weakly interacting segments to give rise to microphase-separated materials with a soft block/hard block morphology. When the hard blocks contain urethane and/or urea groups, strong and specific hydrogen-bonding interactions2 lead to useful properties, such as a high modulus for a given hard block content. In recent years, highly directional physical interactions have been applied in a fundamentally different way to form supramolecular polymers.3 In this novel class of materials, endfunctionalization of unimers with functional groups that associate via noncovalent interactions such as multiple hydrogen bonds4,5 or coordinative bonds6 results in a strong increase of the virtual molecular weight and in concurrent changes of mechanical and rheological properties. The use of directional hydrogen bonding within the hard blocks of TPE’s has been studied by Stadler and co-workers,7 who used telechelic hydrogen-bonded polyisobutylenes with relatively weak end-to-end interactions between urazole groups and between benzoic acid groups. Recently, Bouteiller et al.8 have studied poly(dimethylsiloxane)s with bisurea end groups which aggregate laterally. The authors concluded that aggregation of the end groups into 3-D crystalline domains results in elastomeric behavior, while the formation of hydrogen bonds without crystallization was not sufficient to obtain tensile properties. Similar conclusions were drawn by Rowan et al.,9 who noticed that even very weak end-to end interactions can be used to obtain polymers with film-forming properties when the end groups phase segregate. Here, we study the effect of combining very strong end-toend association via ureidopyrimidinone (UPy) quadruple hydrogen bonding10 and directional lateral aggregation via the urea (U) and urethane (T) hydrogen bonding motifs11 to give supramolecular thermoplastic elastomers with 1-D aggregation of dimerized end groups. Hydroxy-telechelic poly(ethylene butylene) (PEB, Mn ) 3500 g/mol, Mw/Mn ) 1.06, degree of functionalization 1.92) was functionalized with lateral hydrogen-bonding functionalities (UU-PEB-U-U and U-T-PEB-T-U), with end-to-end hydrogenbonding functionalities (UPy-PEB-UPy), and functionalized with both lateral and end-to-end functionalities (UPy-U-PEB-U-UPy and the previously reported5 UPy-T-PEB-T-UPy) (Scheme 1). The resulting materials were characterized with DSC, AFM, dynamic mechanical measurements, and tensile testing. The parent hydroxy-telechelic PEB is a liquid at room temperature which shows purely viscous behavior in oscillatory shear experiments. Incorporation at the chain ends of functional groups capable of lateral aggregation via 3 or 4 hydrogen bonds leads to the formation of elastic solids, with melting points that increases from 45 °C for U-T-PEB-T-U to 129 °C for the U-UPEB-U-U material. The former is a single segment of a segmented, PEB-based poly(urethane-urea) recently reported by Wilkes et al.,2b while the latter material can be considered a PEB analogue of the bisurea-functionalized PDMS reported by Bouteiller.8 In line with the highly directional nature of the hydrogen bonding of UPy moieties, direct functionalization of PEB with the UPy quadruple hydrogen bonding unit in UPyPEB-UPy gives rise to a noticeable increase in viscosity at 40 °C from 10 to 7 × 103 Pa‚s, but it does not lead to the formation of a material with a discernible melting point. The viscosity changes are in line with end-to-end linking of PEB by directional quadruple hydrogen bonds between UPy functional groups, considering that the degree of functionalization of 1.92 of the starting material limits the average number of end-linked unimers to ∼50. A master curve of oscillatory shear experiments on UPy-PEBUPy (Figure 1) confirms the directional end-to-end nature of UPy-UPy hydrogen bonding as the storage and loss moduli show terminal relaxation behavior with slopes of 0.96 and 2.05, demonstrating the absence of long-lived lateral interactions. Nevertheless, the formation of supramolecular polymer chains by linear association of unimers results in an entanglement network with characteristic lifetime of 1 s at 40 °C, evident from a viscoelastic transition at higher frequencies. A dramatic enhancement of mechanical properties was observed when end-to-end and lateral interactions were combined in UPy-U-PEB-U-UPy. In contrast to UPy-PEB-UPy, this material is an elastic solid, with a melting point of 129 °C. The * Corresponding authors. E-mail: r.p.sijbesma@tue.nl; e.w.meijer@tue.nl. Scheme 1. Hydrogen-Bonded Telechelic PEB’s