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Background Postnatal piglet survival is important both in economic and animal welfare terms. It is influenced by the piglet’s own direct genetic effects and by maternal genetic effects of the dam, associated with milk production and mothering abilities. These genetic effects might be correlated, affected by other non-genetic factors and unfavourably associated with other reproduction traits such as litter size, which makes the development of optimal breeding strategies a challenge. To identify the optimum selection strategy for piglet survival, a selection experiment was carried out to compare responses in survival and reproduction traits to selection on only direct, only maternal, or both genetic effects of postnatal survival. The data of the experiment were recorded from outdoor reared pigs, with first- and second-generation sires selected based on their estimated breeding values for maternal and direct effects of postnatal survival of indoor reared offspring, respectively, with the opportunity to identify potential genotype-by-environment interaction. Results A Bayesian multivariate threshold-linear model that was fitted to data on 22,483 piglets resulted in significant (Pr(h2 > 0) = 1.00) estimates of maternal and direct heritabilities between 0.12 and 0.18 for survival traits and between 0.29 and 0.36 for birth weight, respectively. Selection for direct genetic effects resulted in direct and maternal responses in postnatal survival of 1.11% ± 0.17 and − 0.49% ± 0.10, respectively, while selection for maternal genetic effects led to greater direct and maternal responses, of 5.20% ± 0.34 and 1.29% ± 0.20, respectively, in part due to unintentional within-litter selection. Selection for both direct and maternal effects revealed a significant lower direct response (− 1.04% ± 0.12) in comparison to its expected response from single-effect selection, caused by interactions between direct and maternal effects. Conclusions Selection successfully improved post- and perinatal survival and birth weight, which indicates that they are genetically determined and that genotype-by-environment interactions between outdoor (experimental data) and indoor (selection data) housed pigs were not important for these traits. A substantially increased overall (direct plus maternal) response was obtained using selection for maternal versus direct or both direct and maternal effects, suggesting that the maternal genetic effects are the main limiting factor for improving piglet survival on which selection pressure should be emphasized.

Highlights • Minimally invasive cardiac surgery is limited in small children. • Femoral cannulation has high risks in children with body weight, Objective To evaluate the effectiveness and safety of right anterolateral mini-thoracotomy without inferior vena cava (IVC) cannulation for closing atrial septal defect (ASD) in small children. Methods From February 2016 to August 2017, 10 patients (the mean age was 18.5 ± 10.1 months and the mean weight was 8.3 ± 2.1 kg) underwent ASD closure via right anterolateral mini-thoracotomy. The superior vena cava cannula was placed through the right internal jugular vein. A 3–4 cm incision was made on the right chest. The pleural and pericardial cavities were filled with CO2 and the heart was beating during the surgery. Blood returned from IVC was drained by a right heart sucker. All ASDs were closed using artificial patch, continuous suture. Mean follow-up was 18 months (range, 15–22 months). Results No post-operative complications or deaths occurred. Mean operation time and mean cardiopulmonary bypass time were 140.5 ± 27.8 min and 50.3 ± 16.5 min, respectively. These patients were extubated within the first 6 h. The intensive care unit stay time and the post-operative hospital stay time were 19.6 ± 2.6 h and 7.1 ± 1.2 days, respectively. Follow-up transthoracic echocardiography showed no residual shunts or lung atelectasis. Conclusions The right anterolateral mini-thoracotomy without IVC cannulation is feasible for repairing ASD in small children. This technique is effective and safe and can be used as a therapeutic option for ASD.

Contains fulltext : 215333.pdf (Publisher’s version ) (Closed access) BACKGROUND: Patients with stable coronary artery disease and diabetes with previous percutaneous coronary intervention (PCI), particularly those with previous stenting, are at high risk of ischaemic events. These patients are generally treated with aspirin. In this trial, we aimed to investigate if these patients would benefit from treatment with aspirin plus ticagrelor. METHODS: The Effect of Ticagrelor on Health Outcomes in diabEtes Mellitus patients Intervention Study (THEMIS) was a phase 3 randomised, double-blinded, placebo-controlled trial, done in 1315 sites in 42 countries. Patients were eligible if 50 years or older, with type 2 diabetes, receiving anti-hyperglycaemic drugs for at least 6 months, with stable coronary artery disease, and one of three other mutually non-exclusive criteria: a history of previous PCI or of coronary artery bypass grafting, or documentation of angiographic stenosis of 50% or more in at least one coronary artery. Eligible patients were randomly assigned (1:1) to either ticagrelor or placebo, by use of an interactive voice-response or web-response system. The THEMIS-PCI trial comprised a prespecified subgroup of patients with previous PCI. The primary efficacy outcome was a composite of cardiovascular death, myocardial infarction, or stroke (measured in the intention-to-treat population). FINDINGS: Between Feb 17, 2014, and May 24, 2016, 11 154 patients (58% of the overall THEMIS trial) with a history of previous PCI were enrolled in the THEMIS-PCI trial. Median follow-up was 3.3 years (IQR 2.8-3.8). In the previous PCI group, fewer patients receiving ticagrelor had a primary efficacy outcome event than in the placebo group (404 [7.3%] of 5558 vs 480 [8.6%] of 5596; HR 0.85 [95% CI 0.74-0.97], p=0.013). The same effect was not observed in patients without PCI (p=0.76, pinteraction=0.16). The proportion of patients with cardiovascular death was similar in both treatment groups (174 [3.1%] with ticagrelor vs 183 (3.3%) with placebo; HR 0.96 [95% CI 0.78-1.18], p=0.68), as well as all-cause death (282 [5.1%] vs 323 [5.8%]; 0.88 [0.75-1.03], p=0.11). TIMI major bleeding occurred in 111 (2.0%) of 5536 patients receiving ticagrelor and 62 (1.1%) of 5564 patients receiving placebo (HR 2.03 [95% CI 1.48-2.76], p

Objectives The optimal selection of antibacterials during polymicrobial infections is poorly defined. The objective of the current investigation was to quantify the pharmacodynamics of relevant antimicrobials during co-culture of Pseudomonas aeruginosa with two separate Staphylococcus aureus phenotypes. Methods Time–kill experiments were conducted against co-cultures of the P. aeruginosa strain PA01 paired with either the normal phenotype (NP) MRSA isolate COL or the small colony variant phenotype (SCVP) MRSA isolate Ia48. The killing by levofloxacin, gentamicin, clindamycin, vancomycin and polymyxin B was evaluated to investigate drugs with activity against one or both pathogens. A Hill-type function and a mechanism-based model were used to describe bacterial killing. Results P. aeruginosa attenuated the activity of clindamycin against NP MRSA, with a reduction in the Emax (maximal killing) from 3.67 (95% CI 2.79–4.56) in monoculture to 1.86 (95% CI 1.35–2.37) during co-culture, whereas a significant protective effect was not observed for other antibacterials. The reduction in NP MRSA killing by clindamycin was described well by a mechanism-based model that generated a maximal killing rate constant of clindamycin against the susceptible NP MRSA subpopulation of 0.267 h−1 in monoculture and 0.0395 h−1 in the presence of P. aeruginosa. During exposure to gentamicin, P. aeruginosa was the dominant organism in co-culture experiments regardless of the drug concentration or S. aureus phenotype; however, the SCVP MRSA was able to dominate the joint population beginning at a levofloxacin concentration of 1.5 mg/L. Conclusions The anti-staphylococcal activity of clindamycin was attenuated by the presence of P. aeruginosa.

© 2017 IEEE. In order to increase spectrum efficiency, two-dimensional signal space in conventional modulation schemes is expanded by LED transmitter indices in the Optical Spatial Modulation (OSM) scheme, and a combination of transmitter and modulation indices in the Index-based Optical Spatial Modulation (IOSM) scheme. In this paper, we proposed a novel Spatial Modulation called Optical Color-Transmission modes Index-based Spatial Modulation scheme (OCTI-SM) which explore one more dimension - color dimension by using white Laser Diode (LD) light bulb which consists of four color laser sources. With the color space exploration, the proposed scheme achieves higher data rate than the SM schemes. Using computer simulations and mathematical analysis, the system performance of the proposed OCTI-SM is compared with the conventional OSM and the IOSM schemes, and results show that the OCTI-SM achieves a significant improvement in increasing spectral efficiency.

BACKGROUND No studies have examined the independent effects of current and longer-term dietary zinc intakes on zinc absorption. OBJECTIVE We determined the effects of current compared with longer-term zinc intake on fractional zinc absorption (FZA). DESIGN We studied 9 men whose usual zinc intakes were >11 mg/d. FZA was measured at baseline, depletion (0.6 mg Zn/d for 1 wk and 4 mg Zn/d for 5 wk), and repletion (11 mg Zn/d for 4 wk with 20 mg supplemental Zn/d for first 7 d). During 2 successive days after each dietary period, subjects consumed either adequate-zinc meals (11 mg Zn/d) with a zinc stable isotope tracer for 1 d, followed by low-zinc meals (4 mg Zn/d) with zinc tracer, or vice versa. Five days after oral dosing, a zinc tracer was infused intravenously. FZA was measured with the use of a modified double isotope tracer ratio method with urine samples collected on days 5-7 and 10-12 of absorption studies. RESULTS Plasma and urinary zinc did not vary by dietary period. Mean FZA was greater from low-zinc meals than from adequate-zinc meals (60.9% +/- 13.8% compared with 36.1% +/- 8.9%; P < 0.0001), whereas mean total absorbed zinc was greater from adequate-zinc meals than from low-zinc meals (3.60 +/- 0.91 compared with 2.48 +/- 0.56; P < 0.0001), regardless of the longer-term dietary period. CONCLUSIONS FZA was inversely related to current zinc intake, but there was no detectable effect of longer-term dietary zinc. If longer- term zinc intake does modify FZA, such changes are smaller than those caused by current zinc intake, or they occur only after more severe zinc depletion.

Because of the unequal reactivity of the two amine groups of L-lysine hydrochloride, thermal polymerization of this asymmetrical AB2 monomer results in hyperbranched polymers, which contain ?2.5 times more N?-linked linear compared to N?-linked linear structural units. This report discusses the feasibility of three approaches to control polymer architecture during the thermal hyperbranched polymerization of L-lysine hydrochloride. The reactivity of the more reactive ?-NH 2 group was modulated by introducing temporary protective groups that preferentially block the ?-NH2 position. This was achieved by (i) addition of o-vanillin to the polymerization, (ii) copolymerization of N?-benzylidene-L-lysine, and (iii) copolymerization of ?-amino-?-caprolactam. Analysis of the degree of branching (DB) and the average number of branches (ANB) of the obtained polymers did not provide evidence for any significant structural changes. Analysis of the distribution of structural units, in contrast, revealed major structural changes and indicated a rearrangement from N?-linked linear structural units to N ?-linked linear structural units upon addition of o-vanillin and N?-benzylidene-L-lysine. Unlike o-vanillin and N ?-benzylidene-L-lysine, ?-amino-?-caprolactam was found to be rather ineffective in modulating polymer architecture. © 2007 American Chemical Society.

This contribution reports on the synthesis of hyperbranched polylysines via thermal polymerization of L-lysine hydrochloride. Polymerization of L-lysine hydrochloride in the presence of one equivalent KOH at 150°C resulted in polymers with a number-average molecular weight of 4600 g/mol and a polydispersity of 2.6 after 48 h. The rate of polymerization could be significantly enhanced and the polymer molecular weight improved by carrying out the polymerization with 3 mol % of an amidation catalyst. Among the different catalysts that were investigated Zr(OnBu)4 was found to be the most effective. Unequivocal support for the branched architecture of the polymers was obtained from 1H NMR spectroscopy, which allowed the identification and quantification of the four different structural units that constitute the polymer, viz. Nα and Ne linked linear units, dendritic units and terminal units. The structure of the polymers was found to be relatively independent of the reaction conditions. The degree of branching and the average number of branches varied between 0.35-0.45 and 0.15-0.25, respectively. © 2007 Wiley Periodicals, Inc.

Summary For some polymer gels applied in reservoirs to control water flow, a favorable disproportionate permeability reduction (DPR) occurs in which permeability to water is reduced to a much greater extent than it is to oil. Permeability reduction in sandpacks by partially hydrolyzed polyacrylamide-chromium acetate gels was studied as a function of gel composition and the pressure gradients imposed on the gels. For the range of parameters studied, increased gel composition increased the factors by which the permeabilities to water and oil were reduced. Increased gel composition also increased selectivity, a measure of the water-permeability reduction with respect to oil-permeability reduction. Applied pressure gradients during steady-state flows had little effect on oil permeability and a moderate effect on water permeability. Material balances on phases and components in the sandpacks provided insights into mechanisms responsible for the development of flow channels through gelled sandpacks and mechanisms contributing to favorable DPR. Increased pressure gradient during channel development decreased the selectivity of the treatment.

A new class of water-soluble, amphiphilic star block copolymers with a large number of arms was prepared by sequential atom transfer radical polymerization (ATRP) of n-butyl methacrylate (BMA) and poly(ethylene glycol) methyl ether methacrylate (PEGMA). As the macroinitiator for the ATRP, a 2-bromoisobutyric acid functionalized fourth-generation hyperbranched polyester (Boltorn H40) was used, which allowed the preparation of star polymers that contained on average 20 diblock copolymer arms. The synthetic concept was validated by AFM experiments, which allowed direct visualization of single molecules of the multiarm star block copolymers. DSC and SAXS experiments on bulk samples suggested a microphase-separated structure, in agreement with the core−shell architecture of the polymers. SAXS experiments on aqueous solutions indicated that the star block copolymers can be regarded as unimolecular micelles composed of a PBMA core and a diffuse PPEGMA corona. The ability of the polymers to encapsulate and releas...

The structure and the associated dynamics of a series of poly(gamma-benzyl-L-glutamate)-b-poly(dimethyl siloxane)-b-poly(gamma-benzyl-L-glutamate) (PBLG-b-PDMS-b-PBLG) triblock copolymers were investigated using small- and wide-angle X-ray scattering, NMR, transmission electron microscopy, and dielectric spectroscopy, respectively. The structural analysis revealed phase separation in the case of the longer blocks with defected alpha-helical segments embedded within the block copolymer nanodomains. The alpha-helical persistence length was found to depend on the degree of segregation; thermodynamic confinement and chain stretching results in the partial annihilation of helical defects.

This paper reports the synthesis and characterization of a novel class of dendronized polymers which are based on styrene functionalized dendritic L-lysine macromonomers. Free radical polymerization of zeroth and first generation macromonomers afforded dendronized polymers with number-average degrees of polymerization (DP) between 50 and 240, corresponding to number-average molecular weights between 50 × 103 and 240 × 103 g/mol. In contrast, only short oligomers (DP = 5-15) were obtained on polymerization of the second generation dendritic L-lysine macromonomers. Molecularly resolved ATM images allowed first insight into the structure of these peptide dendronized polymers. Depending on the degree of polymerization, either rods, short rods, or globular structures were found. © 2005 American Chemical Society.

This report presents the results from IUPAC Working Party IV.2.2 of the global trial within the framework of IUPAC Commission IV.2, “Characterization of Commercial Polymers”. The results were compared on the basis of molecular weight obtained by size exclusion chromatography (SEC)using different techniques practiced in participating laboratories, the majority of which were materials suppliers. The practical methodologies used different solvents for the polymers, in particular, benzyl alcohol, 1,1,1,3,3,3 hexafluoropropan-2-ol and tetrahydrofuran; the latter solvent was used after chemical modification of the polyamides, in general with trifluoroacetic anhydride. Eight laboratories participated in the trial. The repeatability for molecular weight in each laboratory was good, whatever technique was used, the relative standard deviation averaged over all laboratories being around 3%. The deviations in distribution of molecular weights with different experimental methodologies were broader, but were reasonably good despite the diversity of methods. The differences in the distribution correspond to a confidence interval of about 30% in molecular weight.

Polymers obtained by Atom Transfer Radical Polymerization (ATRP) possess an ω-halogen end-group that may be converted to other functional groups by appropriate chemical transformations. Here we report a versatile new method for end-functionalizing polymers with a thiol. The bromine end-group of the polystyrene obtained from ATRP is converted into an isothiouronium salt by reaction with thiourea. Subsequent treatment of this salt with a base permits the cleavage to a thiol. This method allows the preparation of thiol functional polystyrene of narrow polydispersity with a high degree of functionalization and control of the molecular weight.

Gel permeation chromatography (GPC) viscometry with universal calibration (UC) has been assessed as a means of characterizing the absolute molecular weight distribution of hydroxyl terminated hyperbranched aliphatic polyesters (HBPs) of aliphatic compounds. To generate a valid UC curve, it is necessary to work under conditions of strict size‐exclusion separation. This requirement was met when both dimethyl formamide (DMF) and tetrahydrofuran (THF) were used as eluants. All the HBPs were highly soluble in DMF at room temperature, but its use may necessitate special columns. Dissolution of the HBPs in THF, on the other hand, which is compatible with a variety of stationary phases, was relatively difficult, requiring prolonged heating above 50°C. As an alternative to direct characterization, the HBP hydroxyl end‐groups were end‐capped with trimethylsilane prior to the GPC measurements. Near quantitative replacement of the hydroxyl groups was achieved and the modified HBPs remained stable for several...

A series of trimethylsilyl end-functionalized aliphatic hyperbranched polymers has been used to template porosity in polymethylsilsesquioxane films prepared by heat treatment of a spin cast methylsilsesquioxane precursor. By varying the extent of the end-functionalization, closed pore foams with controlled pore sizes and pore contents of up to 40 vol% were obtained by chemically-induced phase separation and thermal degradation of the hyperbranched polymers during the heat treatment.

Synthetic polyamides (PA) owe many outstanding properties to their semicristalline morphology and to the intermolecular hydrogen bonding of the amide groups. Although the strength of the Van der Waals network confers an exceptional solvent resistance to this class of polymers, it may lead to serious difficulties in their solution characterization, such as the determination of molecular weight distribution (MWD). Over the years, a large number of dissolution schemes have been developed for the GPC characterization of PA. A recent IUPAC round-robin test on the GPC of aliphatic PAs nevertheless showed large variations in the experimental data not only between laboratories employing different methods, but also between laboratories adopting similar protocols. The purpose of the present investigation is to understand the origins of these disparities by comparing GPC-Viscometry data obtained with some frequently used eluant systems: a. benzyl alcohol at high temperature; b. hexafluoro-isopropanol, pure or mixed ...

A variety of cobalt(II/III) and iron(II/III) complexes were prepared containing the ligands 4-fluoro-2,2′-bipyridine (fbpy), 2,2′-bipyridine (bpy), 1,10-phenanthroline (phen) and water, and anions hexafluorophosphate, chloride, perchlorate and sulfate. These complexes were characterized by a combination of 19 F , 1 H , 13 C , 15 N NMR and elemental analysis, and used for a mechanistic study of electron transfer in the CoII(bpy)32+–CoIII(fbpy)33+ system.

Molecular weight distribution obtained by gel permeation chromatography (GPC) conceals a wealth of mechanistic information which can be disclosed only after adequate data treatment. This paper discusses how GPC chromatograms may be used for the elucidation of polymer degradation kinetics. The case of ultrasonic degradation of dilute polystyrene solutions was investigated in detail. The extent of bond scission was determined as a function of sonication time and initial polymer molecular weight. Different bond scission mechanisms were evaluated by comparing calculated molecular weight distributions with experimental data. Best agreement was obtained when multiple fragmentation was included in the degradation scheme, a result experimentally supported by spectroscopic chain ends titration. The results are rationalized by establishing a parallelism between transient elongational flow and ultrasonic degradation.

Transient elongational flow, created by forcing a polymer solution across a narrow contraction, is characterized by a high strain rate of limited duration. Due to an inherent short residence time, this type of flow generally is considered as being less efficient in extending isolated flexible molecular coils than stagnation point elongational flow. Rheo-optical measurements revealed, nevertheless, a readily detectable birefringence zone above a critical strain rate in the immediate orifice entrance. Birefringence was studied for dilute PS solutions (100-400 ppm) in decalin as a function of fluid strain rate (e = 1000-38,000 s -1 ) and polymer molecular weight (M = 1.93-l0.2.10 6 ). Transient elongational flow is complicated by the presence of local orientation distribution along the different streamlines. To account for this effect, a numerical technique has been devised to compute local birefringence (Δn) from experimental retardation (δ). Results show a uniform birefringence distribution across the capillary entrance and a steep decrease with the axial distance. Molecular extension ratio calculated with the Kuhn-Grun theory suggests that polymers may uncoil up to one third of the chain contour length at the approach of capillary entrance. Although extension ratio determined at the inlet could be fitted with an affine deformation model, notable departure from this simple representation is observed when molecular strain is calculated along the streamline.

The kinetics of ultrasonic degradation has been investigated for dilute polystyrene solutions in decalin. The increase in the scission rate constant ( K ) with initial molecular chain length was systematically investigated over 2 decades of molecular weight by using narrow polymer fractions with M = 50 × 10 3 − 8.5 × 10 6 Da. An empirical relation of the form K ∼ ( M - M lim ) 1.9 , with M lim = 30 000, was found for 50 × 10 3 ≤ M ≤ 2.2 × 10 6 . The rate constant becomes independent from M at higher molecular weight. This complex behaviour is rationalized by establishing a parallelism between transient elongational flow and ultrasonic degradation.

Routine MW analysis of polymers is generally performed in solution by Gel Permeation Chromatography. The technique, however, presents increasing difficulties in the ultra-high and ultra-low MW range. Moreover, GPC is not applicable to most fluorinated and»high performance«thermoplastics which require dissolution temperatures close to the polymer melting point. In response to these challenges, polymer scientists have developed new methods of MWD determination. Although these methods have not yet reached the versatility of GPC, they present potential for further development and are interesting as complementary techniques to GPC. For sparingly soluble polymers, melt viscoelastic properties have been used to obtain information on polymer MWD. Flow birefringence, based on the orientation and extension of long flexible molecular chains in a shear field, has been applied to the characterization of ultra-high MW polymers. For low MW polymers and oligomers, the MALDI mass spectrometry technique, with its unique capabilities to resolve individual oligomers within a MWD, proves to be an interesting alternative to conventional GPC characterization.

Birefringence measurements are reported in transient and stagnation point elongational flows created by forcing a polymer solution through a narrow contraction. A strongly molecular weight dependent birefringence was observed and attributed to segmental orientation caused by individual chain extension. The birefringence was also found to depend on the position within the flow field, the solvent type, thermodynamic quality and viscosity, the polymer composition, and the type of flow field. Two distinct concentration regimes, corresponding to the dilute and semi-dilute polymer regions, were observed. The transition was marked by a dynamic critical overlap concentration which was lower than that observed under quiescent conditions. In the dilute regime, a laminar flow profile was observed and intermolecular interactions were negligible. The modest local orientation and overall chain deformation however indicate that intramolecular hindrances to extension exist. Transient elongational flows were characterized by a ‘tube-like’ birefringence which had a diameter equal to that of the orifice and extended several orifice diameters into the solution. In contrast, stagnation point elongation flows possessed both the tube-like birefringence and the classical highly localized birefringent ‘lines’. The transition between the tubular and linear birefringent zones is a function of the viscous coupling between the polymer coil and the deforming fluid element. This coupling is itself a complex function of temperature, molecular parameters and the flow conditions. Measurements in stagnation point elongational flows have also indicated that the maximum degree of local orientation does not occur at the stagnation point, and data indicate that chains continue to accelerate until a region very close to the orifice. In transient elongational flow the polymer dynamics follow a coil-to-deformed coil transition with birefringence saturation not observed.

Models for the dynamics of dilute polymer solutions under elongational flow have been examined in the light of new experimental data. In particular birefringence measurements on the polystyrene-decalin and poly(ethylene oxide)—water systems in both stagnation point and transient elongational flows are the most revealing as to the nature of polymer chain uncoiling and fracture. Both the deformation and fracture behaviour have been observed to depend on the type of elongational flow (transient, stagnation point) and the chemistry of the polymer chain. The data on the critical strain rate for chain fracture along with observations of precise midchain scission (as estimated from size exclusion chromatography), a limited overall chain deformation, and the role of accumulated energy (strain) in mechanochemical degradation, tend to imply that a hybrid model of polymer dynamics in elongational flows may be realistic. One such model, a ‘multi-stranded yo-yo with surface energy dissipation and solid like stress response’ has been developed as a general hypothesis to account for the deformation, stretching and fracture behaviour of polymer chains in both transient and stagnation point elongation flows. This modified yo-yo is found to be an energetically favourable alternative to an affinely deforming chain over certain degrees of elongation. It has been shown to qualitatively explain the apparent difference in behaviour of polymer chains in both stagnation point and transient elongational flows. The viscous coupling between the solvent and the polymer chain is also used as a qualitative parameter to account for the deformation and fracture behaviour of polymer chains under transient and stagnation point elongation flows. Based on these analyses, a set of recommendations as to future experiments are presented.

Chain scission was observed during the crystallization of p-xylene in dilute polystyrene solutions. Degradation yields were determined by gel permeation chromatography, as a function of the number of freeze-and-thaw cycles, polymer concentration, and initial polymer molecular weight (M). The rate constant for chain scission Kc increases with the polymer chain length, from 0.021%/cycle at M = 110·103 to 4.7%/cycle at M = 8.5·106. Over the two decades range of investigated molecular weights, Kc follows an empirical scaling law of the form Kc ∼ (M − Mlim)1.17578, where Mlim is a limiting molecular weight ≅ 29,000 g. mol−1 below which no degradation could be induced. Some propensity for midchain scission was detected, although this tendency was much weaker in comparison to flow-induced degradation. A chain scission model based on crack propagation failed to reproduce the experimental results. To explain the observed dependence of Kc with the square of the radius of gyration, an interfacial stress transmission mechanism between the crystallization fronts and the polymer coil has been proposed. © 1994 John Wiley & Sons, Inc.

Polymer chains undergo degradation through a variety of mechanisms, including shear action, chemical attack, thermal activation, electromagnetic (γ, UV) and ultrasonic irradiation. In many macromolecular systems, the breaking of bonds does not occur randomly but depends upon the position within the chain and the total chain length. Before the advent of GPC, the distribution of bond scission probability along the chain could only be inferred from the evolution of some molecular weight (MW) average as a function of extent of degradation. Over the past few years, gel permeation chromatography has evolved as a reliable analytical tool to measure not only the different MW averages, but also the molecular weight distribution (MWD) of the degraded samples. Obviously, the possibility of determining change, over the whole MWD, gives more insight into the mechanisms involved than do MW averages. The information content of the GPC traces could, however, be usefully exploited, but only after extensive data treatment. The general procedure starts with the elaboration of a degradation scheme and the integration of the corresponding set of differential kinetics equations. The rate constants were then used as fitting parameters until good agreement was obtained between the calculated MWD and the experimental curves. Case studies taken from the field of mechanochemical degradation are presented to illustrate the possibilities and limitations of the technique. In one instance, during the freezing-and-thawing degradation of polystyrene, it is found that good concordance between calculations and experiments could only be obtained if ternary bond scission is included in the degradation scheme. The formalism developed for single scission kinetics is extended to the case of a concerted multiple fracture process.

Cationic complexes PF4(N-N)+, where N-N = 2,2′-bipyridine (bpy), 4-fluoro-2,2′-bipyridine (fbpy), and 1,10-phenanthroline (phen), were prepared in modest yield and identified by their A2B2 and A2BC fluorine NMR spectra, which are similar to those of the isoelectronic SiF4(N-N) adducts. The ligands bpy, fbpy, and phen may be useful for monitoring reactions of HF and H2O, or other fluorinated compounds, with glass apparatus because any liberated SiF4 is then trapped as SiF4(N-N), which can be readily detected by NMR spectroscopy.

Rheology and birefringence measurements showed conclusively that quasi-full chain extension could be achieved in stagnant extensional flow above some critical strain rate of the order of the reciprocal of the chain characteristic time. Degradation in this highly expanded state could be rationalized with a model of chain scission through frictional loading in which the fracture event is controlled by the force balance between polymer solvent viscous friction and the breaking strength of the bond under consideration. In contrast to the process of chain extension, which could be observed only under rather restrictive conditions of flow, polymer degradation is ubiquitous and is readily detected in situations as diverse as convergent flow, ultrasonic irradiation, solution crystallization, turbulence and flow across porous media. Although some extensional flow component was always present in these types of degradation, the residence time in the high strain-rate region is so short that only a limited degree of coil expansion could be expected. Degradation in transient elongational flow indicated a behaviour definitely distinct from the degradation of fully extended chains under stagnant conditions. In particular, the following experimental facts cannot be explained by the model of fully extended frictionally loaded chains: dotef ≊ M−1 (where dote is the critical strain rate for chain fracture); dotef ≊ ηs−0.25 (where ηs, is the solvent viscosity); degradation yield is a fairly unique function of average fluid velocity or entrance pressure drop and is almost independent of differences in elongational strain rate introduced by nozzle geometry. These facts lead us to the conclusion that the fundamental parameter which governs the rate of degradation of partly deformed coils is not the magnitude of the local stress, but rather the deformation energy accumulated by the coil up to the moment of rupture.

Polymer molecules respond to the application of stress by disentanglement, chain orientation and bond rupture. Being the last step in this series of events, the efficiency of flow-induced mechanochemical degradation depends markedly on the details of the pervading flow field. From the results of fluid mechanics, it is usual to distinguish degradation in simple shear flow from degradation in elongational flow. Polymer dynamics require a further subdivision between transient elongational flow, characterized by a short residence time (less than the chain relaxation time τ 1) in the high strain rate region, from quasi-steady-state elongational flow which possesses a stagnation point and a long residence time Open image in new window . This chapter reviews the main factors which influence the degradation kinetics in transient elongational flow, exemplified by flow through a narrow contraction and ultrasonic irradiation. The experimental results reveal a fundamental difference in the kinetics of chain scission in transient and stagnant conditions. Differences in the characteristic residence time prevalent in each type of flow permits one to probe the different stages of the chain unravelling process up to the point of bond rupture. It is suggested that the intricate degradation behavior observed in transient elongational flow stems from the compact nature of the weakly deformed molecular coil. This favors intramolecular interactions during the dynamics of deformation.

Transient elongational flow was created by forcing dilute PS solutions across a narrow contraction. Rheo-optical measurements revealed a localized birefringence zone above a critical strain-rate in the immediate orifice entrance. Birefringence was studied with a polarization-modulation technique for dilute PS solutions (50–500 ppm) as a function of fluid flow rate (1000-38000 s-1), polymer molecular weight (M = 1.93–24 • 106) and solvent quality (decalin and 1-methyl-naphthalene). Transient elongational flow is complicated by the presence of local orientation distribution along the different streamlines. To account for this effect, a numerical technique has been devised to compute local birefringence (An) from experimental retardation (δ). Orientation correction turned out to be unimportant except at the dosest distances from orifice entrance. Results show a steep decrease in birefringence with axial distance. Birefringence profiles determined in a direction perpendicular to the flow show a maximum, not at the center, but in the vicinity of the orifice walls. This effect was explained by means of flow simulation which indicated the presence of extreme strain rates near re-entrant corners. Molecular extension ratio calculated with the Kuhn-Grun theory suggests that polymers may uncoil up to one third of the chain contour length at the approach of capillary entrance. Notable departure from the affine deformation model is, however, observed with multiple birefringence saturation levels changing with flow rate conditions.

The crack-heating behaviour of γ-irradiated poly (styrene-co-acrylonitrile) (SAN) was investigated as a function of radiation dose over the range 0–800 Mrad. The predominant response of SAN to γ-irradiation is crosslinking as evidenced by the radiation yields Gx(crosslink) = 0.077 and Gs (main chain scission) = 0.055. The presence of crosslinks considerably increased the time required for complete reheal of compact tension (CT) specimens. Fracture mechanics testing on non-irradiated samples showed that the energy release rate, G1, increased linearly with the square root of the rehealing time, tp, until the original value of the energy release rate, G10, of the virgin material was restored. In contrast, for crosslinked samples, the initial increase in energy release rate, Gl, was followed by a plateau region at a value of Glp which was less than Glo. A further recovery was observed over a long time scale, some minutes to one hour after the first plateau (Glp) was reached. The ratio of the plateau value and the original fracture toughness ( G lp G lo ) was found to vary with the square root of the average molecular weight between cross-links, M x . This phenomenon may be correctly explained in terms of the average length of free chains formed during the fracture process. A simple double-layer diffusion model describes well the existence of the two recovery regions, it is not yet sufficient, however, since the form of the transition from the first to the second recovery mechanism is not adequately represented by this model.

A recently developed theory permits the determination of peak broadening in size-exclusion chromatography by combining the simultaneous signals obtained from two different detectors, one being sensitive to the mass concentration and the other to the molecular weight of the solute eluted. Based on this methodology, a new experimental technique employing polymer-bound chromophores and a standard UV detector was developed to provide information normally obtained with an absolute detector. The advantages of the method are its simplicity and the utilization of a single cell for measurements, which relieves much of the detection geometry problems inherent to other techniques. Fractions of polystyrene labelled with azobenzene groups and synthesized by radical polymerization were used. The experimental broadening parameters are critically evaluated with respect to the possible sources of error and compared with results obtained from quasi-monodisperse polymer fractions.

Recent experimental investigations have shown that large macromolecules can be fully stretched and fractured in an extensional flow. In this situation, the critical strain-rate for bond scission was found to depend on molecular weight as (M W ) −2 , in agreement with the theoretical predictions of the beadrod model. One of the conditions prerequisite to full chain extension is that the residence time at the appropriate strain-rate must be much larger than the terminal relaxation time of the macromolecule. If this requirement is not fulfilled, chain fracture could still occur at sufficiently high strain-rate, but in a partially uncoiled state. In the present studies we have measured the critical strain-rate for chain scission in transient extensional flow of extremely sharp PS fractions dissolved in dekalin at the θ-temperature. The molecular weight range investigated varied from 2.86 × 10 6 to 426,000. The critical strain-rate for chain scission was found experimentally to scale as (M W ) −0.95 instead of (M W ) −2 as predicted for stagnant extensional flow. Our results are in good accord with a recent theory for rupture of partly extended coils. Even in the partially uncoiled state, the degraded macromolecules showed a remarkable propensity for chain halving, indicating that midchain scission in flow is a general property that is not uniquely reserved to the fully extended chain.

An expression of the kinetics describing the rate of bond scission under stress is proposed and applied to the case of degradation of polymers in transient elongational flow. The new equation permits to take into account the existence of critical values, either for molecular weight or strainrate, below which no chain scission occurs. Compared with the other types of mechano-chemical degradation, depolymerization in transient elongational flow is characterized by a short residence time and an inhomogeneous flow field in which the scission rate constants change drastically with the spatial coordinates. The evolution of molecular size distribution during the degradation process has been investigated using a general polymer fragmentation scheme. The set of differential equations was solved exactly with a matrix technique. Excellent concordance between calculations and experimental results from dilute polystyrene solutions was obtained, providing the spatial distribution of the strain-rate was properly incorporated into the degradation kinetics.

The design of a satellite communications system in an orbit lower than GEO is described. Two sun-synchronous orbits which lie in the equatorial plane have been selected: (1) the apogee at constant time-of-day equatorial orbit, a highly eccentric orbit with five revolutions per day, which allows 77-135 percent more satellite mass to be placed in orbit than for GEO; and (2) the sun-synchronous 12-hour equatorial orbit, a circular orbit with two revolutions per day, which allows 23-29 percent more mass. The results of a life cycle economic analysis illustrate that nongeostationary satellite systems could be competitive with geostationary satellite systems.

Narrow polydispersity polystyrene with a molecular weight of 1.03 x 10 6 has been functionalized with anthracene groups using the metallocene technique. Side reactions such as chain coupling and bond scission can be limited by working in THF at -75 °C. Gel permeation chromatography combined with on-line viscometric, refractive index and fluorescence detection allows us to verify that degradation did not exceed 1 bond scission/ 180000 monomer units. The degree of grafting increases with the reaction time but remains modest (< 5% after 7 h) at low temperature.