Effect of strain on the thermodynamic melting temperature of polymers

X-ray diffraction, sonic velocity, and birefringence measurements were used to study the variation of the apparent melting point of strained natural rubber and polychloroprene vulcanizates with elongation ratio and crystallization temperature. The procedure of Hoffman and Weeks was employed to obtain the thermodynamic melting point, t_m, for each elongation ratio α. The parameter β relating to the distribution of fold lengths is unusually large for low elongation ratios and decreases into the usual range only at higher elongations. The observed variations of t_m with α for these two polymers are compared with the theoretical predictions of Flory and Roe and Krigbaum. Although the predictions of the Flory theory depend somewhat upon the value assigned for the number of repeating units per statistical link, and this parameter is not well known for polychloroprene, we nevertheless conclude that his treatment offers a better representation of the melting point elevation for high elongations. Due to the approximations introduced, the treatment of Flory is not valid for lower elongations. Any attempt to improve this treatment must begin by specifying the free energy of the semicrystalline system, which implies a knowledge of the distribution of crystallite orientations and how this distribution varies with strain and with the crystallization conditions.     

Preparation and chain extension of C100dicarboxylic acid

The selective degradation of polyethylene crystals with fuming nitric acid has been used to prepare a C₁₀₀ chain with terminal functional groups. After treating this product with concentrated sulphuric acid, it is shown by average molecular weight, titration and infra-red spectroscopy measurements that each molecule contains two carboxylic acid groups. This C₁₀₀ dicarboxylic acid is chain extended with several low molecular weight difunctional coupling agents. Block copolymers of this acid and poly(propylene glycol) have been prepared.     

Catalytic oxidative degradation of polyethylene crystals

Linear polyethylene crystals have been degraded with oxygen in the presence of a catalyst composed of cobaltous and bromide ions at temperatures between 60 and 130°. The number of carbonyl groups introduced into polyethylene increases as the oxidation temperature is raised. Infra-red spectroscopy and molecular weight evidence suggests that ketone groups are formed by a non-chain scission reaction, whereas aldehyde and carboxylic acid groups result from chain scission reactions. Molecular weight distributions of the degradation products show that reactions occur selectively in the amorphous regions at the crystal surfaces, when oxidations are performed below 110°. At higher temperatures, the chains are degraded randomly. The carbonyl groups introduced into polyethylene were oxidized to carboxylic acid groups with concentrated (～ 70 per cent) nitric acid. The results demonstrate that this two-stage process yields a polydisperse long-chain linear paraffin with carboxylic acid end-groups, when the catalytic oxidations are performed between 115 and 130°.     

Flocculation studies of non-aqueous poly(methyl methacrylate) dispersions stabilized by AB block copolymers of polystyrene and poly(dimethyl siloxane)

Summary Well defined non-aqueous dispersions of poly(methyl methacrylate) stabilized by AB block copolymers of polystyrene and poly(dimethyl siloxane) were prepared with a dispersion medium consisting of a binary mixture of n-heptane and ethanol. Flocculation was induced by cooling, and the lower critical flocculation temperature was studied as a function of the molecular weight and composition of the stabilizing copolymer, the particle size of the dispersion, and the surface coverage of the particles. The theta temperature for poly(dimethyl siloxane) homopolymer in the same binary liquid mixture was determined by two procedures. The lower critical flocculation temperature was found to be close to the theta temperature and was independent of the length of the stabilizing poly(dimethyl siloxane) chains for the molecular weight range 3200-48000.

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Zusammenfassung Gut definierte, durch AB-Block-Copolymere von Polystyrol und Polydimethylsiloxan stabilisierte nichtwässerige Dispersionen von Polymethylmethacrylat wurden mit binären Mischungen von Heptan und Äthylalkohol als Dispersionsmittel hergestellt. Die Flockung wurde durch Abkühlung herbeigeführt und die untere kritische Flockungstemperatur wurde als Funktion des Molekulargewichts und der Zusammensetzung des stabilisierenden Copolymeren, der Größe der dispergierten Teilchen und dem Bedeckungsgrad der Teilchenoberfläche studiert. Die Theta-Temperatur des Polydimethylsiloxan-Homopolymers in derselben binären flüssigen Mischung wurde durch zwei Verfahren bestimmt. Es wurde gefunden, daß die untere kritische Flockungstemperatur nahe an der ThetaTemperatur liegt und im Molekulargewichtsbereich 3200–48000 von der Länge der stabilisierenden Polydimethylsiloxan-Ketten unabhängig ist.

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With 2 figures and 2 tables     

An ultra-stable referenced interrogation system in the deep ultraviolet for a mercury optical lattice clock

We have developed an ultra-stable source in the deep ultraviolet, suitable to fulfil the interrogation requirements of a future fully-operational lattice clock based on neutral mercury. At the core of the system is a Fabry–Pérot cavity which is highly impervious to temperature and vibrational perturbations. The mirror substrate is made of fused silica in order to exploit the comparatively low thermal noise limits associated with this material. By stabilizing the frequency of a 1062.6 nm Yb-doped fiber laser to the cavity, and including an additional link to LNE-SYRTE’s fountain primary frequency standards via an optical frequency comb, we produce a signal which is both stable at the 10^?15 level in fractional terms and referenced to primary frequency standards. The signal is subsequently amplified and frequency-doubled twice to produce several milliwatts of interrogation signal at 265.6 nm in the deep ultraviolet.     

cyc-DEP: Cyclic immunofluorescence profiling of particles collected using dielectrophoresis

Cancer is a highly heterogenous disease that requires precise detection tools and active surveillance methods. Liquid biopsy assays provide an agnostic way to follow the complex trajectory of cancer, providing better patient stratification tools for optimized treatment. Here, we present the development of a low-volume liquid biopsy assay called cyc-DEP (cyclic immunofluorescent imaging on dielectrophoretic chip) to profile biomarkers collected on a dielectrophoretic microfluidic chip platform. To enable on-chip cyclic imaging, we optimized a fluorophore quenching method and sequential rounds of on-chip staining with fluorescently conjugated primary antibodies. cyc-DEP allows for the quantification of a multiplex array of proteins using 25 µl of a patient plasma sample. We utilized nanoparticles from a prostate adenocarcinoma (LNCaP) cell line and a panel of six target proteins to develop our proof-of-concept technique. We then used cyc-DEP to quantify blood plasma levels of target proteins from healthy individuals, low-grade and high-grade prostate cancer patients (n = 3 each) in order to demonstrate that our platform is suitable for liquid biopsy analysis in its present form. To ensure accurate quantification of signal intensities and comparisons between different samples, we incorporated a signal intensity normalization method (fluorescent beads) and a custom signal intensity quantification algorithm that account for the distribution of signal across hundreds of collection regions on each chip. Our technique enabled a threefold improvement in multiplicity for detecting proteins associated with fluid samples, opening doors for early detection, and active surveillance through quantification of a multiplex array of biomarkers from low-volume liquid biopsies.     

Expedient synthesis of N-methyl tubulysin analogues with high cytotoxicity

An optimized and highly efficient synthesis of potent, bioactive N-methyl tubulysin analogues 2 and 4 has been achieved with > 40% overall yields. This synthesis represents a significant improvement over previously reported syntheses of these and related tubulysin analogues. The stereoselective synthesis of the unnatural amino acid tubuvaline is accomplished using tert-butanesulfinamide chemistry. N-Alkylation to form N-methyl tubuvaline is performed without protection of the tubuvaline alcohol by implementing a unique N-methylation strategy via formation and reduction of a 1,3-tetrahydrooxazine heterocycle. Acylation of the hindered N-methyl tubuvaline amine utilizes a novel sequence of O-acylation followed by an O- to N-acyl transfer to form the hindered amide bond between N-methyl tubuvaline and isoleucine. This high-yielding synthesis should enable the production of large quantities of material for biological studies.     

Rate constants for the reactions of hydroxyl radical with several fluoroethers by the relative rate method

Relative rate experiments were used to measure ratios of chemical kinetics rate constants as a function of temperature for the reactions of OH with eight fluoroethers, including CF3OCF2CHF2, CF3OCF2CHFCF3, CHF2CF2OCHF2, CF3CHFCF2OCH2CF3, (CF3)2CHOCHF2, CF2HCF2OCH2CF3, CHF2CF2OCHFCF3, and CF3CH2OCH2CF3. The temperature ranges were about 270-400 K. Each compound was measured against at least two references. Results are compared with previous data where available. An approach using model compounds for the approximate estimation of rate constants for the fluoroethers is discussed. Observed temperature dependences for fluoroethers from the present work and some literature work are shown to be accurately predictable, based on a previously determined correlation of k298K with the pre-exponential factor, A, in the Arrhenius equation k = Ae(-E/RT).