Arm-First Synthesis of Star Polymers with Wedge Polymer Arms Using Ring-Opening Metathesis Polymerization and Bifunctional Crosslinkers

Recently, wedge polymers with high molecular weight, size, and rigidity have been designed and synthesized. To date, the complexity of wedge polymers has been limited to the diblock copolymer architecture. This work presents a two-step, one-pot process to make star polymers with wedge polymer arms. After the wedge polymer arms are synthesized, crosslinker species are added to the reaction, rapidly forming star polymers. Crosslinker species with different conformational freedom were designed and synthesized and their performance was evaluated. Mass conversion up to 92% and stars with up to 17 arms were synthesized with the most rigid crosslinker. The effects of arm molecular weight and molar ratio of crosslinker to arm on mass conversion and arms per star were explored further. Finally, the size scaling between the wedge homopolymer and star architectures were compared, verifying theoretical results regarding star polymers with arms much larger than their core

Control of hierarchical polymer mechanics with bioinspired metal-coordination dynamics.

In conventional polymer materials, mechanical performance is traditionally engineered via material structure, using motifs such as polymer molecular weight, polymer branching, or block copolymer design. Here, by means of a model system of 4-arm poly(ethylene glycol) hydrogels crosslinked with multiple, kinetically distinct dynamic metal-ligand coordinate complexes, we show that polymer materials with decoupled spatial structure and mechanical performance can be designed. By tuning the relative concentration of two types of metal-ligand crosslinks, we demonstrate control over the material's mechanical hierarchy of energy-dissipating modes under dynamic mechanical loading, and therefore the ability to engineer a priori the viscoelastic properties of these materials by controlling the types of crosslinks rather than by modifying the polymer itself. This strategy to decouple material mechanics from structure is general and may inform the design of soft materials for use in complex mechanical environments. Three examples that demonstrate this are provided

Droplet Control in Aqueous and Hydrocarbon Fluids: Long, End-Associative Polymers Dictate Fluid Behavior Under Elongational Flows

Modifying elongational flows seen in sprayed mists, turbulent flows, and droplet spreading and retraction following impact, is of interest in diverse industries, including agriculture and aviation. Long flexible polymers (with fully extended lengths 1 to 10 µm) modify the elongational flow behavior of a fluid to which they are added. At low concentrations (1 to 10% of their overlap concentrations), their effect is mild under shear flow (shear viscosity increases &lt; 50%), but dramatic under elongational flows (extensional viscosity increases ≥ 300). These long polymers are not widely used in practice because they degrade under strong flows, such as passing through pumps and filters, that typically precede spray. Pairwise end-associative polymers can overcome this limitation. Pulling apart non-covalent associative bonds under such strong flow conditions relieves the tension along the polymer backbone. The pairwise end-associative polymers that are effective in mist control and drag reduction are individually short enough to avoid chain scission in flows that would break long covalent polymers, yet long enough that 6 to 8 associative polymers connected end-to-end create supermolecules that are as effective as their long covalent counterparts. This thesis systematically compares the effect of long covalent and long end-associative polymers on the fluid’s extensional flow properties and the polymers' performance in controlling droplet impact and spray breakup. To measure the elongational flow properties, I implemented and enhanced the Dripping onto Substrate Extensional Rheometry (DoSER) technique (Chapter 2) and applied it to long covalent polymers (Chapter 3) and to end-to-end associative polymers (Chapter 4). Preparing solutions in which the polymers negligibly affect the interfacial tension (&lt; 10%) allows us to explore the relationship between extensional flow properties and droplet impact (Chapter 5) and spray (Chapter 6). By combining the quantitative measurements of extensional viscosity and extensional relaxation time with the corresponding behavior in impact and spray, I correlate the structure of polymers to the solution behavior in droplet rebound and spray breakup. This work has the potential to reduce pesticide contamination of soil, water, and air from agricultural sprays and fire hazard associated with hydrocarbon lubricants.</p

Engineering mechanical dissipation in solid poly(ethylene glycol) hydrogels with bio-inspired metal-coordinate crosslinks

Thesis: S.B., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2015.Cataloged from PDF version of thesis.Includes bibliographical references (page 32).Growing evidence supports that the unique mechanical behavior of mussel byssal threads, such as high toughness and self-healing, rely on an intricate balance of permanent covalent and reversible metal coordination bonds. Inspired by this material crosslink chemistry balance, we synthesized polyethylene glycol (PEG) hydrogels with two crosslinked networks; a primary permanent network composed of covalently crosslinked 4-arm PEG and a secondary network composed of 4-arm PEG functionalized with histidine on each arm. The histidine decorated PEG forms a mechanically reversible network via metal ion coordinated crosslinks. Using rheometry, we study the contribution of the metal-coordinate network to the bulk gels mechanics and find that we can control both the amplitude and the frequency of peak mechanical dissipation with the histidine: metal ion ratio and the choice of metal ion, respectively. Furthermore, we can control the mechanical contribution of metal coordinate bonds by changes in pH. These simple bio-inspired gels promise to serve as a new model system for further study of opto-mechanical coupling of metal-coordinate soft materials.by Robert Learsch.S.B

Durable, low-voltage electroactive polymers formed from polyionic complexes

Development for refreshable braille devices has recently shifted to electroactive polymers (EAP). This paradigm benefits from greater precision, smaller size, and lower cost assocd. with modern electronics, opening the door for higher resoln. and less expensive devices. Displays with resoln. finer than required to display braille characters will enable representation of non-text information such as figures, tables, or diagrams. However, adoption of EAPs for responsive displays has encountered problems such as high required field strength (kV/cm), insufficient pressure (2 kPa generated), and poor durability. This work presents a new type of electroactive polymers, based on poly ionic complexes. In contrast to previously reported electroactive polymers, these gels do not rely on a solvent bath, respond to a field that is on the order of V/cm, and expand in a direction parallel to the applied elec. field. Ultimately, the expansion is driven by electrostatic interactions: The polyionic gels are overall charge neutral, however, when an elec. field of sufficient strength is applied, the ionic bonds are broken and the polycation is drawn towards the plate. This reveals the charged backbone of both the polyanions and the polycations, which causes the repeat units to repel one another and the gel to expand. This mechanism is confirmed using cyclic voltammetry and impedance spectroscopy. With the understanding of the mechanism, the gels are made to expand rapidly and tolerate 100s of cycles. These properties are controlled through the identity of the ionic repeat units and further tuning the crosslinking and processing of the gels. The ionic crosslinks within the gel impart desirable qualities such as self-healing and high toughness a through a cooperative effect, yielding durable gels that are the much stronger than their component parts. The low power requirements and resilient nature of these EAPs make them attractive for use as refreshable braille displays. This methodol. could be adapted to other actuators such as soft robotic joints or grippers

Data set VII related to "Evolutionary assembly of cooperating cell types in a multi-compound chemical defense system"

We deploy single-cell RNA sequencing (10x) and tissue specific ultra-low input RNAseq methods to delineate the molecular evolution of a rove beetle gland. We deconstruct the molecular assembly of new cellular functions comprising a biosynthetic organ novelty and uncover how gland function was pieced together from ancestral molecular source material through the assembly of two novel secretory cell types, each of which synthesize distinct compound classes. --- Code used for data analyse

Data set I related to "Evolutionary assembly of cooperating cell types in a multi-compound chemical defense system"

We deploy single-cell RNA sequencing (10x) and tissue specific ultra-low input RNAseq methods to delineate the molecular evolution of a rove beetle gland. We deconstruct the molecular assembly of new cellular functions comprising a biosynthetic organ novelty and uncover how gland function was pieced together from ancestral molecular source material through the assembly of two novel secretory cell types, each of which synthesize distinct compound classes. Analyzed tissue-specific transcriptome (SMARTseq) data. Raw sequencing data has been deposited in the NCBI Sequence Read Archive (SRA). See bioproject: PRJNA70701

Data set VI related to "Evolutionary assembly of cooperating cell types in a multi-compound chemical defense system"

We deploy single-cell RNA sequencing (10x) and tissue specific ultra-low input RNAseq methods to delineate the molecular evolution of a rove beetle gland. We deconstruct the molecular assembly of new cellular functions comprising a biosynthetic organ novelty and uncover how gland function was pieced together from ancestral molecular source material through the assembly of two novel secretory cell types, each of which synthesize distinct compound classes. --- experimental data related to RNAi experiments, survival assays, in vitro enzyme dat

Data set V related to "Evolutionary assembly of cooperating cell types in a multi-compound chemical defense system"

We deploy single-cell RNA sequencing (10x) and tissue specific ultra-low input RNAseq methods to delineate the molecular evolution of a rove beetle gland. We deconstruct the molecular assembly of new cellular functions comprising a biosynthetic organ novelty and uncover how gland function was pieced together from ancestral molecular source material through the assembly of two novel secretory cell types, each of which synthesize distinct compound classes. --- alignment files, tree files, and phylogenetic tree