NOVEL SOFT SURFACES WITH INTERESTING SURFACE AND BULK MORPHOLOGY

The goal of this research is to cover a broad set of scientific investigations of elastomeric materials based on polydimethylsiloxane (PDMS) and poly((3,3,3-trifluoroethoxymethyl)methyloxetane) diol. The scope of study covers five areas, well correlated with each other. The first study investigates the near surface morphology of condensation cured PDMS as a function of increasing the amount of siliceous phase. The appearance, disappearance and reappearance of untreated fumed silica nanoparticles at the PDMS near surface and their correlation with the volume fraction of siliceous phase have been studied. This research with PDMS nanocomposites has led to the development of an alternative route for improving mechanical strength of PDMS elastomers, conventionally known to have weak mechanical properties. The second study involves synthesis of a triblock copolymer comprising of four mutually immiscible phases, namely, soft segments comprising of fluorous and silicone domains, a diisocyanate hard segment and a glassy siliceous phase. Structure-property relationship has been established with an investigation of the interesting surface and bulk morphology. The highly improved mechanical strength of these soft materials is noteworthy. The dominance of silicone soft block at the triblock near surface has led to the third study which investigates their potential non-adhesive or abhesive characteristic in both a laboratory scale and in a marine environment. The peak removal stress and the removal energy associated with the detachment of a rigid object from the surface of these triblock copolymers have been measured. Results obtained from laboratory scale experiments have been verified by static immersion tests performed in the marine environment, involving the removal of adhered soft and hard fouling organisms. Gaining insights on the characteristics of an easy release surface, namely low surface energy and a low near surface modulus, a new way for controlling the near surface composition for elastomeric coatings have been developed. This technique involves an elastomer end-capped with a siliceous crosslinking agent and a tough, linear polyurethane. The basic concept behind the hybrid compositions is to develop a coating suitable for foul release applications, having a low energy surface, low surface modulus but good bulk mechanical strength. Henceforth, the fourth study deals with synthesis and characterization of the hybrid polymers over a wide range of composition and investigates their foul release characteristic in laborartory scale experiments. In our final study, attempts have been made in generating a silicone coating with antimicrobial property. A quaternary alkylammonium in different weight percents have been incorporated into a conventional, condensation cured polydimethylsiloxane (PDMS) elastomer. Antimicrobial assay has been performed on these modified silicone coatings to assess their biocidal activity against strains of Escherichia coli, Staphylococcus aureus and Pseudomonas aeruginosa. Surface accessibility of quaternary charges has been quantified by measuring the streaming potential of a modified coating. An effort has been made in improving the mechanical strength of the weak PDMS elastomers by adding treated fumed silica nanoparticles as reinforcements. The effect of adding fillers on the mechanical property (tensile), surface concentration of quaternary charge and on the biocidal activity of a representative sample has been investigated

PDMS–Fluorous Polyoxetane–PDMS Triblock Hybrid Elastomers: Tough and Transparent with Novel Bulk Morphologies

PDMS-3F-1.1-PDMS and PDMS-3F-4.5-PDMS triblock hybrid elastomers are investigated, wherein (1) 3F-1.1 and 3F-4.5 are poly­(3-methyl-3-trifluoroethoxymethyl)-1,3-propylene oxide with <i>M</i>_n = 1.1 or 4.5 kDa, (2) segments are linked by urethane/urea forming reactions, and (3) the intermediate PDMS-3F-PDMS aminopropyl end segments are end-capped with isocyanatopropyltriethoxysilane. After condensation cure, PDMS-3F-PDMS triblock hybrids (A-1.1 and A-4.5) form robust elastomers. In a second set, bis­(triethoxysilylethane), BTESE, was incorporated to probe effects of increased siliceous domain content (B-1.1, B-4.5). All compositions are optically transparent due to nearly identical refractive indexes for 3F and PDMS segments. TM-AFM images for A-4.5, A-1.1, and B-4.5 fracture surfaces reveal microscale bulk phase separation. The A-4.5 triblock hybrid shows a particularly interesting morphology comprised of 2–3 μm ovaloids (low modulus) surrounded by a higher modulus matrix. A model is proposed for this microscale morphology based on the relative rates of physical network formation (PN, H-bonding) and chemical network formation (CN, SiO_1.5) during coating deposition Despite low hard segment weight percents (2.6–3.5) the hybrid triblocks have moderate toughness with strain at break ranging from 260 to 492%. Triblock hybrid elastomer B-1.1 has the highest −SiO_1.5 wt % (mostly from BTESE) and lowest 3F wt % (3F-1.1). No sign of microscale phase separation is observed by TM-AFM imaging, and a separate <i>T</i>_g for the 3F segment is not detected by DMA; these findings are ascribed to network constrained phase separation of that results in 3F being incorporated in an “interphase”. The absence of a separate <i>T</i>_g for 3F leads to a gradual decrease in storage modulus (8 to 1.4 MPa) from −90 to 150 °C. In contrast to the complex bulk morphology, TM-AFM imaging shows the hybrid surfaces are devoid of microstructural features attributable to phase separation. Based on contact angle measurements and XPS analysis, the outermost surface for all PDMS-3F-PDMS hybrid triblocks elastomers is dominated by PDMS

Rigid Adherent-Resistant Elastomers (RARE): Nano‑, Meso‑, and Microscale Tuning of Hybrid Fluorous Polyoxetane–Polyurethane Blend Coatings

A series of hybrid elastomers were prepared by a combination of (1) condensation cured, −Si­(OC₂H₅)₃ end-capped, poly­(3-trifluoroethoxymethyl-3-methyloxetane), “3F”, and a siliceous domain “builder”, bis­(triethoxysilyl)­ethane, and (2) a linear 3F polyurethane “U-3F”. Hybrid compositions U-3F-<i>x</i> are designated by polyurethane weight percent “<i>x</i>”. Bulk characterization includes thermal transitions (DMA, DSC), mechanical properties (DMA, tensile), and susceptibility to swelling by a hydrocarbons (hexadecane). Increased mechanical properties, particularly toughness, was found with increasing U-3F wt %. Near surface U-3F depletion was established by ATR-IR spectroscopy with Ge and diamond crystals. To describe the length scale for near surface U-3F depletion, the term <i>mesosurface</i> is introduced, which in this context is a depth of ∼1000 nm. Peak removal force in shear (<i>P</i>_c‑s) for a rigid adherent, namely epoxied aluminum cylinders (ECs), was determined with a TA Instruments RSA III. A striking compositional dependence was found for EC adhesion. A U-3F-50 hybrid coating had the lowest adhesion (<i>P</i>_c‑s = 0.078 MPa) with good toughness (6.2 MPa). Bulk and surface characterization together with adhesion measurements established U-3F-<i>x</i> hybrid coatings, and U-3F-50 in particular, as new fluorous rigid adherent-resistant elastomers (RARE) that are tough, oil resistant, and optically transparent

Reversible HuR-microRNA binding controls extracellular export of miR-122 and augments stress response

microRNAs (miRNAs), the tiny but stable regulatory RNAs in metazoan cells, can undergo selective turnover in presence of specific internal and external cues to control cellular response against the changing environment. We have observed reduction in cellular miR-122 content, due to their accelerated extracellular export in human hepatic cells starved for small metabolites including amino acids. In this context, a new role of human ELAV protein HuR has been identified. HuR, a negative regulator of miRNA function, accelerates extracellular vesicle (EV)-mediated export of miRNAs in human cells. In stressed cells, HuR replaces miRNPs from target messages and is both necessary and sufficient for the extracellular export of corresponding miRNAs. HuR could reversibly bind miRNAs to replace them from Ago2 and subsequently itself gets freed from bound miRNAs upon ubiquitination. The ubiquitinated form of HuR is predominantly associated with multivesicular bodies (MVB) where HuR-unbound miRNAs also reside. These MVB-associated pool of miRNAs get exported out via EVs thereby delimiting cellular miR-122 level during starvation. Therefore, by modulating extracellular export of miR-122, HuR could control stress response in starved human hepatic cells

Studying the Mechanism of Hybrid Nanoparticle Photoresists: Effect of Particle Size on Photopatterning

Hf-based hybrid photoresist materials with three different organic ligands were prepared by a sol–gel-based method, and their patterning mechanism was investigated in detail. All hybrid nanoparticle resists are patternable using UV exposure. Their particle sizes show a dramatic increase from the initial 3–4 nm to submicron size after exposure, with no apparent inorganic content or thermal property change detected. XPS results showed that the mass percentage of the carboxylic group in the structure of nanoparticles decreased with increasing exposure duration. The particle coarsening sensitivities of those hybrid nanoparticles are consistent with their EUV performance. The current work provides an understanding for the development mechanism and future guidance for the design and processing of high performance resist materials for large-scale microelectronics device fabrication

High Antimicrobial Effectiveness with Low Hemolytic and Cytotoxic Activity for PEG/Quaternary Copolyoxetanes

The alkyl chain length of quaternary ammonium/PEG copolyoxetanes has been varied to discern effects on solution antimicrobial efficacy, hemolytic activity and cytotoxicity. Monomers 3-((4-bromobutoxy)­methyl)-3-methyloxetane (BBOx) and 3-((2-(2-methoxyethoxy)­ethoxy)­methyl)-3-methyloxetane (ME2Ox) were used to prepare precursor P­[(BBOx)­(ME2Ox)-50:50–4 kDa] copolyoxetane via cationic ring opening polymerization. The 1:1 copolymer composition and <i>M</i>_n (4 kDa) were confirmed by ¹H NMR spectroscopy. After C–Br substitution by a series of tertiary amines, ionic liquid C<i>x</i>-50 copolyoxetanes were obtained, where 50 is the mole percent of quaternary repeat units and “<i>x</i>” is quaternary alkyl chain length (2, 6, 8, 10, 12, 14, or 16 carbons). Modulated differential scanning calorimetry (MDSC) studies showed <i>T</i>_gs between −40 and −60 °C and melting endotherms for C14–50 and C16–50. Minimum inhibitory concentrations (MIC) were determined for Escherichia coli, Staphylococcus aureus, and Pseudomonas aeruginosa. A systematic dependence of MIC on alkyl chain length was found. The most effective antimicrobials were in the C6–50 to C12–50 range. C8–50 had better overall performance with MICs of 4 μg/mL, E. coli; 2 μg/mL, S. aureus; and 24 μg/mL, P. aeruginosa. At 5 × MIC, C8–50 effected >99% kill in 1 h against S. aureus, E. coli, and P. aeruginosa challenges of 10⁸ cfu/mL; log reductions (1 h) were 7, 3, and 5, respectively. To provide additional insight into polycation interactions with bacterial membranes, a geometric model based on the dimensions of E. coli is described that provides an estimate of the maximum number of polycations that can chemisorb. Chain dimensions were estimated for polycation C8–50 with a molecular weight of 5 kDa. Considering the approximations for polycation chemisorption (PCC), it is surprising that a calculation based on geometric considerations gives a C8–50 concentration within a factor of 2 of the MIC, 4.0 (±1.2) μg/mL for E. coli. C<i>x</i>-50 copolyoxetane cytotoxicity was low for human red blood cells, human dermal fibroblasts (HDF), and human foreskin fibroblasts (HFF). Selectivities for bacterial kill over cell lysis were among the highest ever reported for polycations indicating good prospects for biocompatibility