The classical division of polymeric materials into thermoplastics and thermosets based on covalent network structure often implies that these categories are distinct and irreconcilable. Yet, the past two decades have seen extensive development of materials that bridge this gap through incorporation of dynamic crosslinks, enabling them to behave as both robust networks and moldable plastics. Although their potential utility is significant, the growth of covalent adaptable networks (CANs) has obscured the line between “thermoplastic” and “thermoset” and erected a conceptual barrier to the growing number of new researchers entering this discipline. This Perspective aims to both outline the fundamental theory of CANs and provide a critical assessment of their current status. We emphasize throughout that the unique properties of CANs emerge from the network chemistry, and particularly highlight the role that the crosslink exchange mechanism (i.e., dissociative exchange or associative exchange) plays in the res...

Adaptable Crosslinks in Polymeric Materials: Resolving the Intersection of Thermoplastics and Thermosets.

https://pubs.rsc.org/en/content/articlepdf/2019/py/c9py01260e

Dynamic covalent chemistry in polymer networks: a mechanistic perspective

Vitrimers: Permanently crosslinked polymers with dynamic network topology

The development of more sustainable materials with a prolonged useful lifetime is a key requirement for a transition towards a more circular economy. However, polymer materials that are long-lasting and highly durable also tend to have a limited application potential for re-use. This is because such materials derive their durable properties from a high degree of chemical connectivity, resulting in rigid meshes or networks of polymer chains with a high intrinsic resistance to deformation. Once such polymers are fully synthesised, thermal (re)processing becomes hard (or impossible) to achieve without damaging the degree of chemical connectivity, and most recycling options quickly lead to a drop or even loss of material properties. In this context, both academic and industrial researchers have taken a keen interest in materials design that combines high degrees of chemical connectivity with an improved thermal (re)processability, mediated through a dynamic exchange reaction of covalent bonds. In particular vitrimer materials offer a promising concept because they completely maintain their degree of chemical connectivity at all times, yet can show a clear thermally driven plasticity and liquid behavior, enabled through rapid bond rearrangement reactions within the network. In the past decade, many suitable dynamic covalent chemistries were developed to create vitrimer materials, and are now applicable to a wide range of polymer matrices. The material properties of vitrimers, however, do not solely rely on the chemical structure of the polymer matrix, but also on the chemical reactivity of the dynamic bonds. Thus, chemical reactivity considerations become an integral part of material design, which has to take into account for example catalytic and cross-reactivity effects. This mini-review will aim to provide an overview of recent efforts aimed at understanding and controlling dynamic cross-linking reactions within vitrimers, and how directing this chemical reactivity can be used as a handle to steer material properties. Hence, it is shown how a focus on a fundamental chemical understanding can pave the way towards new sustainable materials and applications.

https://pubs.rsc.org/en/content/articlepdf/2020/SC/D0SC01069C

Vitrimers: directing chemical reactivity to control material properties

In this report, we merge block copolymers with vitrimers in an effort to realize the prospect of higher-order, nanoscale control over associative cross-link exchange and flow. We show the use of co...

Block Copolymer Vitrimers.

Vitrimers are a class of polymers that flow when heated but are insoluble. Vitrimers are made of covalent networks with dynamic links and/or cross-links that undergo an associative exchange reaction. These dynamic cross-links enable vitrimers to have interesting mechanical/rheological behavior, self-healing, adhesive, and shape memory properties. We demonstrate that vitrimers can self-assemble into complex meso- and nanostructures when cross-links and backbone monomers strongly interact. Vitrimers featuring polyethylene (PE) as the backbone and dioxaborolane maleimide as the cross-linkable moiety were studied in both the molten and semicrystalline states. We observed that PE vitrimers macroscopically phase separated into dioxaborolane maleimide-rich and -poor regions and characterized the extent of phase separation by optical transmission measurements. This phase separation can explain the relatively low insoluble fractions and overall crystallinities of PE vitrimers. Using synchrotron-sourced small-angle...

https://hal.archives-ouvertes.fr/hal-02350857/document

Phase Separation and Self-Assembly in Vitrimers: Hierarchical Morphology of Molten and Semicrystalline Polyethylene/Dioxaborolane Maleimide Systems

For vitrimer systems obtained by dynamic cross-linking of polymer chains, incompatibility effects between the cross-links and polymer backbone can lead to microphase separation, resulting in a network made of cross-linked aggregates. Additionally, when there is a wide distribution of the number of cross-links per chain, macrophase separation can occur, mostly due to entropic effects. Here, we investigate the linear viscoelasticity and flow of a polyethylene (PE) vitrimer that has cross-linkable dioxaborolane maleimide grafts, which self-assemble into a hierarchical nanostructure. To elucidate the role of self-assembly, we first studied dioxaborolane graft functionalized PE that was non-cross-linked. It had a terminal relaxation time that was orders of magnitude larger than both neat PE and partially peroxide cross-linked PE. When dioxaborolane cross-linker was added to form the vitrimer, the resulting material could not achieve terminal relaxation within 8 h. The soluble and insoluble portions of the PE v...

/pdf/linear-viscoelasticity-and-flow-of-self-assembled-vitrimers-3naooejxyp.pdf

Linear Viscoelasticity and Flow of Self-Assembled Vitrimers: The Case of a Polyethylene/Dioxaborolane System

Many pharmaceutical drugs in the marketplace and discovery pipeline suffer from poor aqueous solubility, thereby limiting their effectiveness for oral delivery. The use of an amorphous solid disper...

Recent Advances in Understanding the Micro- and Nanoscale Phenomena of Amorphous Solid Dispersions.

We explored the use of transmission electron microscopy (TEM) to evaluate the crystallinity of griseofulvin (GF)/hydroxypropyl methylcellulose acetate succinate (HPMCAS) solid dispersions. TEM, which provides both real-space images and electron diffraction patterns, was used to unambiguously identify GF crystals in spray dried GF. Using TEM, we were also able to detect GF crystals in a physical mixture of spray dried GF particles and spray dried HPMCAS particles with an overall crystallinity of ∼3 vol %, below the practical lower limit of detection for laboratory-scale wide-angle X-ray scattering (WAXS). Using TEM and WAXS, we did not find crystals in GF/HPMCAS solid dispersions with GF loadings of 5, 10, and 50 wt %. However, we detected GF crystals in annealed 5 wt % GF solid dispersion using TEM, whereas we did not detect crystals using in situ WAXS and modulated differential scanning calorimetry, thereby establishing the superior crystal detection sensitivity of TEM. We also performed TEM analysis of ...

Detection of pharmaceutical drug crystallites in solid dispersions by transmission electron microscopy.

Vitrimers are polymer networks that engage in dynamic associative exchange reactions. Their covalent cross-links preserve network connectivity but permit topology fluctuations, making them both ins...

/pdf/unentangled-vitrimer-melts-interplay-between-chain-942ayjpw2r.pdf

Ralm G. Ricarte

Papers

Phase Separation and Self-Assembly in Vitrimers: Hierarchical Morphology of Molten and Semicrystalline Polyethylene/Dioxaborolane Maleimide Systems

Linear Viscoelasticity and Flow of Self-Assembled Vitrimers: The Case of a Polyethylene/Dioxaborolane System

Recent Advances in Understanding the Micro- and Nanoscale Phenomena of Amorphous Solid Dispersions.

Detection of pharmaceutical drug crystallites in solid dispersions by transmission electron microscopy.

Unentangled Vitrimer Melts: Interplay between Chain Relaxation and Cross-link Exchange Controls Linear Rheology