Weathering Effects on Degradation of Low-Density Polyethylene-Nanosilica Composite with Added Pro-oxidant.

Nanomaterials are increasingly used in polymer composites to enhance their properties, such as mechanical performance and durability, increased electrical conductivity, and improved optical clarity. Here results are presented of a study simulating effects of weathering on degradation of a nanosilica-low-density polyethylene (LDPE) composite. Release of nanosilica from LDPE composites is a potential source of toxic SiO2. Nanosilica based LDPE composites were weathered under carefully controlled conditions by exposure to simulated sunlight. The effects of an added pro-oxidant on weathering was examined. Weathering of the composites with pro-oxidant was determined by quantifying changes in infrared spectroscopic properties (Fourier transform infrared spectroscopy / FTIR); mechanical properties, atomic force microscopy (AFM), scanning electron microscopy and other procedures. Wavelength effects on weathering rates were determined in a series of irradiations using simulated solar radiation passed through light filters that blocked different parts of the ultraviolet spectral region. Rates and spectral irradiance were then analyzed to develop spectral weighting functions (SWFs) that quantify wavelength effects on the sunlight-induced weathering of the pro-oxidant amended composites.

Degradable Polycaprolactone and Polylactide Homopolymer and Block Copolymer Brushes Prepared by Surface-Initiated Polymerization with Triazabicyclodecene and Zirconium Catalysts.

Surface-initiated ring-opening polymerization (SI-ROP) of polycaprolactone (PCL) and polylactide (PLA) polymer brushes with controlled degradation rates were prepared on oxide substrates. PCL brushes were polymerized from hydroxyl-terminated monolayers utilizing triazabicyclodecene (TBD) as the polymerization catalyst. A consistent brush thickness of 40 nm could be achieved with a reproducible unique crystalline morphology. The organocatalyzed PCL brushes were chain extended using lactide in the presence of zirconium n-butoxide to successfully grow PCL/PLA block copolymer (PCL-b-PLA) brushes with a final thickness of 55 nm. The degradation properties of "grafted from" PCL brush and the PCL-b-PLA brush were compared to "grafted to" PCL brushes, and we observed that the brush density plays a major role in degradation kinetics. Solutions of methanol/water at pH 14 were used to better solvate the brushes and increase the kinetics of degradation. This framework enables a control of degradation that allows for the precise removal of these coatings.

A dynamic duo: pairing click chemistry and postpolymerization modification to design complex surfaces.

Advances in key 21st century technologies such as biosensors, biomedical implants, and organic light-emitting diodes rely heavily on our ability to imagine, design, and understand spatially complex interfaces. Polymer-based thin films provide many advantages in this regard, but the direct synthesis of polymers with incompatible functional groups is extremely difficult. Using postpolymerization modification in conjunction with click chemistry can circumvent this limitation and result in multicomponent surfaces that are otherwise unattainable. The two methods used to form polymer thin films include physisorption and chemisorption. Physisorbed polymers suffer from instability because of the weak intermolecular forces between the film and the substrate, which can lead to dewetting, delamination, desorption, or displacement. Covalent immobilization of polymers to surfaces through either a "grafting to" or "grafting from" approach provides thin films that are more robust and less prone to degradation. The grafting to technique consists of adsorbing a polymer containing at least one reactive group along the backbone to form a covalent bond with a complementary surface functionality. Grafting from involves polymerization directly from the surface, in which the polymer chains deviate from their native conformation in solution and stretch away from the surface because of the high density of chains. Postpolymerization modification (PPM) is a strategy used by our groups over the past several years to immobilize two or more different chemical functionalities onto substrates that contain covalently grafted polymer films. PPM exploits monomers with reactive pendant groups that are stable under the polymerization conditions but are readily modified via covalent attachment of the desired functionality. "Click-like" reactions are the most common type of reactions used for PPM because they are orthogonal, high-yielding, and rapid. Some of these reactions include thiol-based additions, activated ester coupling, azide-alkyne cycloadditions, some Diels-Alder reactions, and non-aldol carbonyl chemistry such as oxime, hydrazone, and amide formation. In this Account, we highlight our research combining PPM and click chemistry to generate complexity in polymer thin films. For the purpose of this Account, we define a complex coating as a polymer film grafted to a planar surface that acts as a template for the patterning of two or more discrete chemical functionalities using PPM. After a brief introduction to grafting, the rest of the review is arranged in terms of the sequence in which PPM is performed. First, we describe sequential functionalization using iterations of the same click-type reaction. Next, we discuss the use of two or more different click-like reactions performed consecutively, and we conclude with examples of self-sorting reactions involving orthogonal chemistries used for one-pot surface patterning.

Direct grafting of poly(pentafluorophenyl acrylate) onto oxides: versatile substrates for reactive microcapillary printing and self-sorting modification.

Poly(pentafluorophenyl acrylate) was covalently attached to silicon oxide through the direct coupling of the reactive ester to surface silanol groups. Subsequently, reactive microcapillary printing (R-µCaP) and a one-pot, self-sorting postpolymerization modification reaction were used to generate patterns of spatially resolved chemical functionality.

Self-sorting click reactions that generate spatially controlled chemical functionality on surfaces.

This Article describes the generation of a patterned surface that can be postpolymerization modified to incorporate fragile macromolecules or delicate biomolecules without the need for special equipment. Two monomers that undergo different click reactions, pentafluorophenyl acrylate (PFPA) and 4-(trimethylsilyl) ethynylstyrene (TMSES), were sequentially polymerized from a silicon surface in the presence of a shadowmask with UV light, generating 12.5 and 62 µm pitch patterns. Two different dyes, 1-aminomethylpyrene (AMP) and 5-azidofluorescein (AF), were covalently attached to the polymer brushes through aminolysis and dual desilylation/copper(I)-catalyzed alkyne/azide cycloaddition (CuAAC) in one pot. Unlike most CuAAC reactions, the terminal alkyne of TMSES was not deprotected prior to functionalization. Although a 2 nm thickness increase was observed for poly(PFPA) brushes after polymerization of TMSES, cross-contamination was not visible through fluorescence microscopy after functionalization.

Rate determination of azide click reactions onto alkyne polymer brush scaffolds: a comparison of conventional and catalyst-free cycloadditions for tunable surface modification.

The postpolymerization functionalization of poly(N-hydroxysuccinimide 4-vinylbenzoate) brushes with reactive alkynes that differ in relative rates of activity of alkyne-azide cycloaddition reactions is described. The alkyne-derived polymer brushes undergo "click"-type cycloadditions with azido-containing compounds by two mechanisms: a strain-promoted alkyne-azide cycloaddition (SPAAC) with dibenzocyclooctyne (DIBO) and azadibenzocyclooctyne (ADIBO) or a copper-catalyzed alkyne-azide cycloaddition (CuAAC) to a propargyl group (PPG). Using a pseudo-first-order limited rate equation, rate constants for DIBO, ADIBO, and PPG-derivatized polymer brushes functionalized with an azide-functionalized dye were calculated as 7.7 × 10(-4), 4.4 × 10(-3), and 2.0 × 10(-2) s(-1), respectively. The SPAAC click reactions of the surface bound layers were determined to be slower than the equivalent reactions in solution, but the relative ratio of the reaction rates for the DIBO and ADIBO SPAAC reactions was consistent between solution and the polymer layer. The rate of functionalization was not influenced by the diffusion of azide into the polymer scaffold as long as the concentration of azide in solution was sufficiently high. The PPG functionalization by CuAAC had an extremely fast rate, which was comparable to other surface click reaction rates. Preliminary studies of dilute solution azide functionalization indicate that the diffusion-limited regime of brush functionalization impacts a 50 nm polymer brush layer and decreases the pseudo-first-order rate by a constant diffusion-limited factor of 0.233.

Failing finals is often a surprise for the student but not the teacher: identifying difficulties and supporting students with academic difficulties.

Weak students often continue with little guidance and feedback, and often have ongoing difficulties. Early support may stop students experiencing a cycle of failure. Key to supporting struggling students is to identify reasons for poor performance. We explored the reasons for poor performance in a cohort of fifth year students who failed their final clinical examinations. Qualitative methods (interviews and a focus group) identified several themes. Many of the students had experienced personal problems or issues. They regarded themselves as competent students although they had significantly greater problems with earlier exams than the year mean, and significantly lower scores in formative assessments during the year leading up to these exams than their peers. Factors relating to the exam itself were seen as relevant to failure. More specific support and feedback throughout the MBcHB was seen as desirable. They tended to take little personal responsibility for their performance and were reluctant to seek help. We conclude that while the onus is on Faculty to identify and support failing students, the results indicate that students would benefit from support in developing self-reflection skills in such a way to support life-long learning.