Christopher Bowman

Additive manufacture of lightly crosslinked semicrystalline thiol-enes for enhanced mechanical performance.

Anonymous — Wed, 26 Feb 2020 20:22:05 +0000

Additive manufacture of lightly crosslinked semicrystalline thiol-enes for enhanced mechanical performance. Anonymous (not verified) Wed, 02/26/2020 - 13:22

Photopolymerizable semicrystalline thermoplastics resulting from thiol-ene polymerizations were formed via fast polymerizations and achieved excellent mechanical properties. These materials have been shown to produce materials desirable for additive manufacturing (3D printing), especially for recyclable printing and investment casting. However, while well-resolved prints were previously achieved with the thiol-ene thermoplastics, the remarkable elongation at break (ϵ_max) and toughness (T) attained in bulk were not realized for 3D printed components (ϵ_max,bulk ~ 790%, T_bulk ~ 102 MJ m^-3 vs. ϵ_max,print < 5%, T_print < 0.5 MJ m^-3). In this work, small concentrations (5-10 mol%) of a crosslinker were added to the original thiol-ene resin composition without sacrificing crystallization potential to achieve semicrystalline, covalently crosslinked networks with enhanced mechanical properties. Improvements in ductility and overall toughness were observed for printed crosslinked structures, and substantial mechanical augmentation was further demonstrated with post-manufacture thermal conditioning of printed materials above the melting temperature (T_m). In some instances, this thermal conditioning to reset the crystalline component of the crosslinked prints yielded mechanical properties that were comparable or superior to its bulk counterpart (ϵ_max ~ 790%, T ~ 95 MJ m^-3). These unique photopolymerizations and their corresponding monomer compositions exhibited concurrent polymerization and crystallization along with mechanical properties that were tunable by changes to the monomer composition, photopolymerization conditions, and post-polymerization conditioning. This is the first example of a 3D printed semicrystalline, crosslinked material with thermally tunable mechanical properties that are superior to many commercially-available resins.

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Flocculation behavior and mechanisms of block copolymer architectures on silica microparticle and Chlorella vulgaris systems

Anonymous — Wed, 26 Feb 2020 20:18:39 +0000

Flocculation behavior and mechanisms of block copolymer architectures on silica microparticle and Chlorella vulgaris systems Anonymous (not verified) Wed, 02/26/2020 - 13:18

Hypothesis

Flocculation performance using polyelectrolytes is influenced by critical design parameters including molecular weight, amount and sign of the ionic charge, and polymer architecture. It is expected that systematic variation of these characteristics will impact not only flocculation efficiency (FE) achieved but that charge density and architecture, specifically, can alter the flocculation mechanism. Therefore, it should be possible to tune these design parameters for a desired flocculation application.

Experiments

Cationic-neutral and polyampholytic copolymers, exhibiting a range of molecular weights (10³–10⁶ g/mol), varying charge levels (0–100% cationic, neutral and anionic), and random or block copolymer architecture, were applied to dilute suspensions of silica microparticles (control) and Chlorella vulgaris. FE and zeta potential values were determined over a range of flocculant doses to evaluate effectiveness and mechanism achieved.

Findings

These different classes of copolymers provide specific benefits for flocculation, with many achieving >95% flocculation. Block copolymer flocculants exhibit a proposed, dominant bridging mechanism, therefore reducing flocculant dosage required for effective flocculation when compared to analogous random copolymer flocculants. Polyampholytic copolymers applied to C. vulgaris generally exhibited a bridging mechanism and increased FE compared to equivalent cationic-neutral copolymers, indicating a benefit of the anionic component on a more, complex, diversely charged suspension.

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Reaction Environment Effect on the Kinetics of Radical Thiol–Ene Polymerizations in the Presence of Amines and Thiolate Anions

Anonymous — Fri, 31 Jan 2020 20:46:18 +0000

Reaction Environment Effect on the Kinetics of Radical Thiol–Ene Polymerizations in the Presence of Amines and Thiolate Anions Anonymous (not verified) Fri, 01/31/2020 - 13:46

Because of facile implementation, quantitative conversions, and an insensitivity to oxygen, water, and most organic functional groups, radical-mediated thiol–ene coupling (TEC) reactions have emerged as a valuable tool in macromolecule synthesis. It was recently demonstrated that the kinetics and conversions of thiyl radical-mediated reactions are adversely affected in the presence of basic amines by the formation of retardive thiolate anions. Herein, the performance of TEC polymerizations is evaluated under a variety of reaction environments with the intention to aid in the optimal formulation design of TEC reactions in the presence of amines. Results from both bulk and aqueous-phase network photopolymerizations established that sensitivity to amine basicity and pH is dependent on the thiol acidity, although norbornene-type alkenes exhibit a unique ability to achieve high conversions, where allyl ethers, vinyl ether, and vinyl siloxanes are highly inhibited. Additionally, the protic solvents such as alcohols and acetic acid are established as ideal solvents or additives to suppress or eliminate amine-induced retardation.

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Combined Dynamic Network and Filler Interface Approach for Improved Adhesion and Toughness in Pressure-Sensitive Adhesives

Anonymous — Tue, 31 Dec 2019 17:58:47 +0000

Combined Dynamic Network and Filler Interface Approach for Improved Adhesion and Toughness in Pressure-Sensitive Adhesives Anonymous (not verified) Tue, 12/31/2019 - 10:58

Of importance for adhesive materials, particularly pressure-sensitive adhesive (PSA) systems, is the ability to increase bulk toughness without reduction of adhesion. Previous approaches for increasing PSA durability sacrifice permanent cross-linking or adhesive potential, limiting performance. In this work, covalent adaptable networks (CANs) derived from thiol-thioester exchange (TTE) are utilized as a basis for adhesive films. Tensile and single-lap shear tests were conducted for adhesive materials containing no filler, 15 wt % nanoparticles functionalized with thioester-containing acrylate, or 15 wt % nanoparticles functionalized with nonthioester-containing acrylate. Additionally, fatigue experiments were conducted on unfilled adhesives. Results indicate that TTE improves toughness, adhesion, and fatigue in unfilled materials. Filled adhesives with activated TTE showed a nearly fourfold increase in adhesion with slightly reduced toughness compared to uncatalyzed filled specimens. This work has implications in many industries, from biomedical to automotive, as toughness and fatigue resistance are important considerations for adhesive applications.

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Dynamic covalent chemistry (DCC) in dental restorative materials: Implementation of a DCC-based adaptive interface (AI) at the resin–filler interface for improved performance

Anonymous — Mon, 30 Dec 2019 20:41:03 +0000

Dynamic covalent chemistry (DCC) in dental restorative materials: Implementation of a DCC-based adaptive interface (AI) at the resin–filler interface for improved performance Anonymous (not verified) Mon, 12/30/2019 - 13:41

Objective

Dental restorative composites have been extensively studied with a goal to improve material performance. However, stress induced microcracks from polymerization shrinkage, thermal and other stresses along with the low fracture toughness of methacrylate-based composites remain significant problems. Herein, the study focuses on applying a dynamic covalent chemistry (DCC)-based adaptive interface to conventional BisGMA/TEGDMA (70:30) dental resins by coupling moieties capable of thiol–thioester (TTE) DCC to the resin–filler interface as a means to induce interfacial stress relaxation and promote interfacial healing.

Methods

Silica nanoparticles (SNP) are functionalized with TTE-functionalized silanes to covalently bond the interface to the network while simultaneously facilitating relaxation of the filler–matrix interface via DCC. The functionalized particles were incorporated into the otherwise static conventional BisGMA/TEGDMA (70:30) dental resins. The role of interfacial bond exchange to enhance dental composite performance in response to shrinkage and other stresses, flexural modulus and toughness was investigated. Shrinkage stress was monitored with a tensometer coupled with FTIR spectroscopy. Flexural modulus/strength and flexural toughness were characterized in three-point bending on a universal testing machine.

Results

A reduction of 30% in shrinkage stress was achieved when interfacial TTE bond exchange was activated while not only maintaining but also enhancing mechanical properties of the composite. These enhancements include a 60% increase in Young’s modulus, 33% increase in flexural strength and 35% increase in the toughness, relative to composites unable to undergo DCC but otherwise identical in composition. Furthermore, by combining interfacial DCC with resin-based DCC, an 80% reduction of shrinkage-induced stress is observed in a thiol–ene system “equipped” with both types of DCC mechanisms relative to the composite without DCC in either the resin or at the resin–filler interface.

Significance

This behavior highlights the advantages of utilizing the DCC at the resin–filler interface as a stress-relieving mechanism that is compatible with current and future developments in the field of dental restorative materials, nearly independent of the type of resin improvements and types that will be used, as it can dramatically enhance their mechanical performance by reducing both polymerization and mechanically applied stresses throughout the composite lifetime.

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Vinyl sulfonamide based thermosetting composites via thiol-Michael polymerization

Anonymous — Mon, 30 Dec 2019 20:27:30 +0000

Vinyl sulfonamide based thermosetting composites via thiol-Michael polymerization Anonymous (not verified) Mon, 12/30/2019 - 13:27

Objective

To assess the performance of thiol Michael photocurable composites based on ester-free thiols and vinyl sulfonamides of varying monomer structures and varied filler loadings and to contrast the properties of the prototype composites with conventional BisGMA-TEGDMA methacrylate composite.

Methods

Synthetic divinyl sulfonamides and ester-free tetrafunctional thiol monomers were utilized for thiol-Michael composite development with the incorporation of thiolated microfiller. Polymerization kinetics was investigated using FTIR spectroscopy. Resin viscosities were assessed with rheometry. Water uptake properties were assessed according to standardized methods. Thermomechanical properties were analyzed by dynamic mechanical analysis. Flexural modulus/strength and flexural toughness were measured on a universal testing machine in three-point bending testing mode.

Results

The vinyl sulfonamide-based thiol-Michael resin formulation demonstrated a wide range of viscosities with a significant increase in the functional group conversion when compared to the BisGMA-TEGDMA system. The two different types of vinyl sulfonamide under investigation demonstrated significant differences towards the water sorption. Tertiary vinyl sulfonamide did not undergo visible swelling whereas the secondary vinyl sulfonamide composite swelled extensively in water. With the introduction of rigid monomer into the polymer matrix the glass transition temperature increased and so increased the toughness. Glassy thiol-Michael composites were obtained by ambient curing.

Significance

Employing the newly developed step-growth thiol-Michael resins in dental composites will provide structural uniformity, improved stability and lower water sorption.

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Catalyst-free, aza-Michael polymerization of hydrazides: polymerizability, kinetics, and mechanistic origin of an α-effect

Anonymous — Tue, 03 Dec 2019 17:58:19 +0000

Catalyst-free, aza-Michael polymerization of hydrazides: polymerizability, kinetics, and mechanistic origin of an α-effect Anonymous (not verified) Tue, 12/03/2019 - 10:58

Despite the powerful nature of the aza-Michael reaction for generating C–N linkages and bioactive moieties, the bis-Michael addition of 1° amines remains ineffective for the synthesis of functional, step-growth polymers due to the drastic reduction in reactivity of the resulting 2° amine mono-addition adduct. In this study, a wide range of commercial hydrazides are shown to effectively undergo the bis-Michael reaction with divinyl sulfone (DVS) and 1,6-hexanediol diacrylate (HDA) under catalyst-free, thermal conditions to afford moderate to high molecular weight polymers with M_n = 3.8–34.5 kg mol⁻¹. The hydrazide-Michael reactions exhibit two distinctive, conversion-dependent kinetic regimes that are 2^nd-order overall, in contrast to the 3^rd-order nature of amines previously reported. The mono-addition rate constant was found to be 37-fold greater than that of the bis-addition at 80 °C for the reaction between benzhydrazide and DVS. A significant majority (12 of 15) of the hydrazide derivatives used here show excellent bis-Michael reactivity and achieve >97% conversions after 5 days. This behavior is consistent with calculations that show minimal variance of electron density on the N-nucleophile among the derivatives studied. Reactivity differences between hydrazides and hexylamine are also explored. Overall, the difference in reactivity between hydrazides and amines is attributed to the adjacent nitrogen atom in hydrazides that acts as an efficient hydrogen-bond donor that facilitates intramolecular proton-transfer following the formation of the zwitterion intermediate. This effect not only activates the Michael acceptor but also coordinates with additional Michael acceptors to form an intermolecular reactant complex.

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Hybrid Cerasomes Composed of Phosphatidylcholines and Silica Networks for the Construction of Vesicular Materials with Functionalized Shells

Anonymous — Tue, 03 Dec 2019 17:53:58 +0000

Hybrid Cerasomes Composed of Phosphatidylcholines and Silica Networks for the Construction of Vesicular Materials with Functionalized Shells Anonymous (not verified) Tue, 12/03/2019 - 10:53

A phosphatidylcholine-based cerasome (PC-cerasome) was prepared by a combination of a copper-catalyzed azide alkyne cyclo-addition (CuAAC) reaction and a sol–gel condensation process. In a 2 wt % urea aqueous solution, azide precursors containing a triethoxysilane group (TEOS) were coupled with an alkyne lysolipid (AL) through a CuAAC reaction to generate triethoxysilane triazole-phosphatidylcholine (TEOSTPC) molecules. TEOSTPC self-assembled into liposomes immediately after formation. Incubation of the liposomes at 50 °C led to a gradual hydrolysis and decomposition of urea, which caused a mildly basic condition. Simultaneously, a base-catalyzed sol–gel condensation of TEOS groups occurred and drove formation of a silica network in the bilayer membranes of the TEOSTPC-liposomes, resulting in cerasomes with an organic/inorganic hybrid shell composed of PC and an SO₂ network. Because of the presence of the SO₂ network in the bilayer membrane, the cerasomes exhibited good structural stability in the aqueous phase and maintained the vesicular structure even when dispersed in water-miscible organic solvents. Coexistence of the bilayer membrane and SiO₂ was verified, which is indicative of an intramembrane sol–gel condensation. The formation process of an SiO₂ network in the bilayer membrane was verified using spiropyran-containing triazole-phosphatidylcholine (SPTPC) as a molecular probe. The SPTPC was embedded in TEOSTPC-liposomes, and subsequent sol–gel condensation anchored the SPTPCs in the SiO₂ network. During sol–gel condensation, SP isomerized to merocyanine (MC), indicating that the polarity of the microenvironment in the bilayer membrane increased due to formation of the SiO₂ network. Moreover, the rigidity of the SiO₂ network prohibited MC-to-SP recovery, resulting in a cerasome with a MC-functionalized shell. Based on the structural stability of the PC-cerasome and the convenient preparation method combining CuAAC and sol–gel condensation, we anticipate that these PC-cerasomes will find broad utility in the construction of vesicular materials with functionalized shells.

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Shape-shifting material can morph, reverse itself using heat, light

Anonymous — Fri, 24 Aug 2018 06:00:00 +0000

Shape-shifting material can morph, reverse itself using heat, light Anonymous (not verified) Fri, 08/24/2018 - 00:00

Trent Knoss

A new material developed by CU Boulder engineers can transform into complex, pre-programmed shapes via light and temperature stimuli, allowing a literal square peg to morph and fit into a round hole before fully reverting to its original form.

The controllable shape-shifting material, described today in the journal Science Advances, could have broad applications for manufacturing, robotics, biomedical devices and artificial muscles.

“The ability to form materials that can repeatedly oscillate back and forth between two independent shapes by exposing them to light will open up a wide range of new applications and approaches to areas such as additive manufacturing, robotics and biomaterials”, said Christopher Bowman, senior author of the new study and a Distinguished Professor in CU Boulder’s Department of Chemical and Biological Engineering (CHBE).

Previous efforts have used a variety of physical mechanisms to alter an object’s size, shape or texture with programmable stimuli. However, such materials have historically been limited in size or extent and the object state changes have proven difficult to fully reverse.

The new CU Boulder material achieves readily programmable two-way transformations on a macroscopic level by using liquid crystal elastomers (LCEs), the same technology underlying modern television displays. The unique molecular arrangement of LCEs make them susceptible to dynamic change via heat and light.

To solve this, the researchers installed a light-activated trigger to LCE networks that can set a desired molecular alignment in advance by exposing the object to particular wavelengths of light. The trigger then remains inactive until exposed to the corresponding heat stimuli. For example, a hand-folded origami swan programmed in this fashion will remain folded at room temperature. When heated to 200 degrees Fahrenheit, however, the swan relaxes into a flat sheet. Later, as it cools back to room temperature, it will gradually regain its pre-programmed swan shape.

The ability to change and then change back gives this new material a wide range of possible applications, especially for future biomedical devices that could become more flexible and adaptable than ever before.

“We view this as an elegant foundational system for transforming an object’s properties,” said Matthew McBride, lead author of the new study and a post-doctoral researcher in CHBE. “We plan to continue optimizing and exploring the possibilities of this technology.”

Additional co-authors of the study include Alina Martinez, Marvin Alim, Kimberly Childress, Michael Beiswinger, Maciej Podgorski and Brady Worrell of CU Boulder and Lewis Cox and Jason Killgore of the National Institute of Standards and Technology (NIST). The National Science Foundation provided funding for the research.

[video:https://vimeo.com/286537992]

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