A network polymer, incorporating dynamic and reversible crosslinks, was synthesized using the Diels-Alder reaction. Fourier transform infrared (FTIR) spectroscopy was used to characterize the ...reaction rate and thermodynamic equilibrium over a broad temperature range. Equilibrium conversion of the furan and maleimide varied from 74% at 85°C to 24% at 155°C, demonstrating significant depolymerization via the retro-Diels-Alder reaction. The gel point temperature, as determined by rheometry using the Winter-Chambon criterion, was 92°C, corresponding to a gel-point conversion of 71%, consistent with the Flory-Stockmayer equation. The scaling exponents for the complex moduli, viscosity, and plateau modulus, in the vicinity of the gel-point, were determined and compared with experimental and theoretical literature values. Further, the material exhibited a low frequency relaxation owing to dynamic rearrangement of crosslinks by the Diels-Alder and retro-Diels-Alder reactions.
Since polymerization-induced shrinkage stress is detrimental in many applications, addition-fragmentation chain transfer (AFCT) was employed to induce network relaxation and adaptation that mitigate ...the shrinkage stress. Here, to form high glass transition temperature, high modulus polymers while still minimizing stress, multifunctional methacrylate monomers were incorporated into allyl sulfide-containing thiol-yne resins to provide simultaneously high glass transition temperatures and a facile mechanism for AFCT throughout the network. As a negative control, in an attempt to isolate just the effects of AFCT in the polymerization, a propyl sulfide-based diyne, which has a nearly identical chemical structure though absent any AFCT-capable functional group, was synthesized and implemented in place of the allyl sulfide-based diyne. The glass transition temperature of the ternary systems increased from 39°C to 79°C as the methacrylate content increased while the shrinkage stress of the optimal ternary resin was lower than either the binary thiol-yne resin or the pure methacrylate resin. The stress relaxation benefit associated with AFCT increased with increasing allyl sulfide concentration as shown by a decrease in the relative stress from 0.98 to 0.53. The allyl sulfide-based thiol-yne-methacrylate system exhibits stress relaxation up to 55% and increased T(g) up to 40°C compared with the control, AFCT-incapable thiol-yne. This ternary system has less than 1/3 of the stress of conventional dimethacrylate monomer resins while possessing similarly outstanding mechanical behavior.
To reduce shrinkage stress which arises during the polymerization of crosslinked polymers, allyl sulfide functional groups were incorporated into methacrylate polymerizations to determine their ...effect on stress relaxation via addition-fragmentation chain transfer (AFCT). Additionally, stoichiometrically balanced thiol and allyl sulfide-containing norbornene monomers were incorporated into the methacrylate resin to maximize the overall functional group conversion and promote AFCT while also enhancing the polymer's mechanical properties. Shrinkage stress and reaction kinetics for each of the various functional groups were measured by tensometry and Fourier-transform infrared (FTIR) spectroscopy, respectively. The glass transition temperature (T(g)) and elastic moduli (E') were measured using dynamic mechanical analysis. When the allyl sulfide functional group was incorporated into dimethacrylates, the polymerization-induced shrinkage stress was not relieved as compared with analogous propyl sulfide-containing resins. These analogous propyl sulfide containing monomers are incapable of undergoing AFCT while having similar chemical structure and crosslink density to the allyl sulfide containing methacrylates. Here, a monomethacrylate monomer that also contains a cyclic allyl sulfide (PAS) was found to increase the crosslinking density nearly 20 times as compared to an analogous monomethacrylate in which the allyl sulfide was replaced with an ethyl sulfide. Despite the much higher crosslink density, the PAS formulation exhibited no concomitant increase in stress. Thiol-norbornene resins were copolymerized in PAS to promote AFCT as well as to synergistically combine the ring opening benefits associated with the thiol-ene reaction. AFCT resulted in a 63% reduction of polymerization stress and a 45°C enhancement of the glass transition temperature in the allyl sulfide-containing thiol-norbornene-methacrylate system compared with rubbery dimethacrylates. When compared with conventional glassy dimethacrylates, this combined system has less than 10% of the typical shrinkage stress level while having similarly excellent mechanical properties.
Optical direct write lithography (ODWL) has the capacity for generating three dimensional arbitrary patterns. Here we examine principles for voxel refinement and relate several techniques for ...achieving nanoscale resolution. The influence of optics, gelation, and polymerization scaling behavior are expounded, demonstrating the necessity for adopting a multidisciplinary mindset to control both voxel dimensions and minimize out-of-focus reactions. Aspects of two-photon ODWL are reviewed and recent multi-beam ODWL approaches that draw inspiration from STED microscopy are examined.
Mechanophotopatterning on a photoreversible covalent adaptable network allows complex topo‐graphical features to be fabricated by performing sequential stretching and light exposure steps, report ...Christopher Kloxin and co‐workers on p. 1977. In this image, the material was stretched and exposed to a square‐array photomask, followed by a second exposure to a square‐array photomask rotated 45°. The lens quality of this material is demonstrated when backlit by an LCD display.
Elegant und flexibel: Mithilfe reversibler Umesterungsreaktionen im Rückgrat eines Polymernetzwerks wurde ein kovalent vernetztes Material erzeugt, das verarbeitet, ausgebessert und wiederaufbereitet ...werden kann. Die Studie erweitert den Anwendungsbereich kovalenter adaptiver Netzwerke und veranschaulicht den Spaltungs‐ und Neubildungsmechanismus von Bindungen, der der Anpassungsfähigkeit dieser Materialien zugrundeliegt (siehe Bild).
An approach for the design of functionalized cyclic peptides is established for use in 3D cell culture and in cell targeting. Sequential orthogonal click reactions, specifically a photoinitiated ...thiol-ene and strain promoted azide-alkyne cycloaddition, were utilized for peptide cyclization and conjugation relevant for biomaterial and biomedical applications, respectively.
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