Unravelling the multiple dynamics in supramolecular interpenetrating polymer networks using rheology and dielectric spectroscopy

Supramolecular chemistry has opened up an impressive toolbox for the development of novel polymeric systems. Whether exploited to create self-assembled architectures in dilute solutions, or to serve as non-permanent crosslinks in more concentrated polymer networks or hydrogels, supramolecular interactions induce superior, often stimuli-responsive properties. For biomedical applications such as drug delivery and tissue engineering, the thermoreversible hydrophobic association of poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO) block copolymers in aqueous solutions has emerged as a promising interaction type. The microphase separation of such amphiphilic molecules does not only govern the broadly tunable microstructure and associated rheological properties, but also enables efficient encapsulation and controlled release of therapeutic agents within the self-assembled hydrophobic cores. The current literature extensively reports on the fundamentals and applications of PEO-PPO-PEO triblock copolymers (commercially available as Pluronics), while other copolymer architectures such as multiblock copolymers are poorly discussed. Therefore, this dissertation focuses on gaining a fundamental understanding of the behavior of alternating PEO-PPO multiblock copolymers in increasingly complex aqueous media, thus demonstrating their potential as attractive alternatives to triblock copolymers.

In the first part of this dissertation, their behavior in simple aqueous solutions is elucidated. Different characterization techniques including rheology, scanning and titration calorimetry and light transmittance are used to determine the phase transitions and associated energetic contributions of two types of alternating PEO-PPO multiblock copolymers (commercially available as ExpertGel) with different hydrophobicity. In addition, the morphological evolution upon micellization is examined using light and X-ray scattering. The combination of these techniques allows the identification of the relevant temperature and concentration regimes in which polymer solutions, micellar solutions, micellar networks or macroscopic phase separation are obtained. Not only the importance of hydrophobicity in dictating the phase behavior, but also that of the copolymer architecture, i.e. the configuration and amount of the PEO and PPO blocks, is highlighted by comparing their behavior with that of triblock copolymers and longer multiblock copolymers with similar hydrophobicity. The subsequent chapters further explore the micellar networks formed at higher concentrations and temperatures to establish the relationships between their microstructure and rheological properties. Unlike Pluronic triblock copolymers, multiblock copolymers contain intermediate PEO blocks that can act as elastic bridges connecting two micellar PPO cores. The as such obtained polymer networks have the rheological characteristics of a transient network. Due to the presence of multifunctional micellar crosslink interactions between multisticker polymers, they are of particular interest to generate fundamental insights in transient networks of multisticker polymers. A mechano-statistical transient network model originally developed for telechelic triblock copolymers having hydrophobic end groups, is therefore extended and generalized to alternating multiblock copolymers bearing multiple hydrophobic blocks along their polymer backbone. Hereto, a combinatorics approach is used that takes into account the configurational complexity of multisticker polymers. The combination of the theoretical model with experimental input about the spatial distribution of micellar nodes as inferred from small-angle X-ray scattering, allows the microscopic network topology to be translated into macroscopic elasticity. The good agreement between model predictions and rheological elasticity reveals a concentration-dependent evolution from loop- and superstructure-dominated networks with limited elasticity to bridge-dominated networks with high node functionalities and increased elasticity. The rheological fingerprints of the micellar networks exhibit broad, sticky Rouse-like relaxation and reflect the need for multiple sticker dissociation events to allow complete chain relaxation. The scaling of the characteristic relaxation times with concentration questions the assumptions made in conventional transient network relaxation frameworks and mandates a novel microscopic view thereon. A hybrid model combining concepts from other transient network models with the insights from our mechano-statistical network model, thereby not neglecting the intrinsic chain dynamics, can explain the peculiar relaxation behavior.

The second part of this dissertation broadens the versatility and expands the complexity by combining PEO-PPO multiblock copolymer networks with a second supramolecular network in a thermoresponsive double network. The addition of a labile metal-ligand network of 4-arm PEO-star polymers crosslinked by telechelic zinc(II)-terpyridine complexes induces two distinct modes of relaxation, on which temperature has a contrasting effect and which can therefore be linked to the single network contributions. As a result of the topological constraints imposed by the presence of a second network and the possible formation of internetwork entanglements, the double network has a synergistic elasticity with significantly delayed relaxation dynamics as compared to the single networks. Unfortunately, the rheological reproducibility of the double networks is poor, mainly due to the microstructural complexity of the network formed by multiblock copolymers. In summary, this work demonstrates the broad tunability and potential of hydrophobically associating multiblock copolymers by highlighting the relationships between the building block architecture, the microstructure and the final rheological properties.