Origin of high thermal conductivity in disentangled ultra-high molecular weight polyethylene films: ballistic phonons within enlarged crystals.

The thermal transport properties of oriented polymers are of fundamental and practical interest. High thermal conductivities ( ≳ 50 Wm-1K-1) have recently been reported in disentangled ultra-high molecular weight polyethylene (UHMWPE) films, considerably exceeding prior reported values for oriented films. However, conflicting explanations have been proposed for the microscopic origin of the high thermal conductivity. Here, we report a characterization of the thermal conductivity and mean free path accumulation function of disentangled UHMWPE films (draw ratio ~200) using cryogenic steady-state thermal conductivity measurements and transient grating spectroscopy. We observe a marked dependence of the thermal conductivity on grating period over temperatures from 30-300 K. Considering this observation, cryogenic bulk thermal conductivity measurements, and analysis using an anisotropic Debye model, we conclude that longitudinal atomic vibrations with mean free paths around 400 nanometers are the primary heat carriers, and that the high thermal conductivity for draw ratio ≳ 150 arises from the enlargement of extended crystals with drawing. The mean free paths appear to remain limited by the extended crystal dimensions, suggesting that the upper limit of thermal conductivity of disentangled UHMWPE films has not yet been realized.

Chain Dynamics of Ultrahigh Molecular Weight Polyethylene Composites with Graphene Oxide Nanosheets.

We investigate the melt chain dynamics of ultrahigh molecular weight polyethylene (UHMWPE) and its composites with graphene oxide (GO) nanosheets by means of neutron spin echo spectroscopy. For the GO concentrations explored, we observe hindered chain dynamics with respect to the pure UHMWPE. We propose a model where a fraction of the polymer is immobilized on top and at the bottom of GO sheets. This model enables us to provide a microscopic measurement of the adsorbed and free polymer fractions, as well as the thickness of the adsorbed layer. Our experiments provide experimental nanoscopic evidence of GO hindering entanglement formation in a polymer melt, a phenomenon that had been observed at the macroscale before via rheological measurements.

Synthesis of an UV-Curable Divinyl-Fumarate Poly-ε-Caprolactone for Stereolithography Applications.

The limited number of commercially available photocrosslinkable resins for stereolithography has often been considered the main limitation of this technique. In this manuscript, a photocrosslinkable poly-ε-caprolactone (PCL) has been synthesized by a two-step method starting from ring opening polymerization (ROP) of ε-caprolactone. Hydroxyethyl vinyl ether (HEVE) has been used both as the initiator of ROP and as photo-curable functional group to obtain a vinyl poly-ε-caprolactone (VPCL). The following reaction of VPCL with fumaryl chloride (FuCl) results in a divinyl-fumarate polycaprolactone (VPCLF). Moreover, a catalyst based on Al, instead of the most popular Tin(II) 2-ethylhexanoate, has been employed to reduce the cytotoxicity of the material. VPCLF has been successfully used, in combination with N-vinyl-pyrrolidone (NVP), to fabricate 3D porous scaffolds by micro-stereolithography (µ-SL) with mathematically defined architectures.

Petrographic and mineral-glass chemical dataset of igneous rock clasts from Early Oligocene Aveto-Petrignacola Formation (Northern Italy).

This dataset article contains petrographic and mineral-glass chemical data of igneous rock clasts from Early Oligocene Aveto-Petrignacola Formation (APF; Northern Italy). Methods for obtaining the dataset include optical microscopy, scanning electron microscopy and electron probe microanalysis. The APF volcanic rocks are basalts, basaltic andesites, andesites, dacites and rhyolites. Rare gabbroic cumulate nodules complete the dataset. Basalts are porphyritic, with calcic plagioclase (An72-92Ab7-27Or0-1), ferroan enstatite (En59-68Fs29-37Wo3-4) and augite (En38-39Fs18-20Wo41-44) phenocrysts, in a hypocrystalline groundmass made up of bytownite (An71-85Ab14-28Or1), augite (En37-38Fs19Wo43-44), ferroan enstatite (En62-68Fs30-35Wo1-4) and rare pigeonite (En46-50Fs37-42Wo7-17). The basaltic andesites are porphyritic to glomeroporphyritic with phenocrysts of zoned plagioclase (An44-67Ab32-55Or1), orthopyroxene, Mg-rich augite (En38-42Fs15-17Wo43-45), rare pargasite to edenite amphibole (Mg# 69-59) and very rare biotite in a hypocrystalline to holohyaline groundmass. Andesites are highly porphyritic with phenocrysts of plagioclase (An47-79Ab20-52Or0-1), pargasite to magnesio-hornblende (Mg# 72-67), Mg-rich augite (En43-46Fs12-17Wo41-43), subordinate ferroan enstatite (En68-74Fs23-29Wo3-4), biotite (Mg# 53) and Ti-magnetite (Usp29-41). Dacites (massive lavas and ignimbrites) are porphyritic, with phenocrysts and phenoclasts of plagioclase (An33-79Ab20-62Or0-4), calcic amphibole (Ti-pargasite, Mg-hornblende and edenite; Mg# 81-46), biotite (Mg# 67-56), very rare Mg-rich augite (En41-42Fs16-18Wo40-43) and resorbed quartz in hypohyaline to holohyaline groundmass with a dense mat of anhedral quartz, labradorite-andesine (An36-66Ab33-61Or1-4) and rare anorthoclase (An22Ab66Or12). Rhyolitic compositions have been found both as volcanic clasts (massive lava and ignimbrites) with andesine to oligoclase phenoclasts (An25-38Ab61-71Or1-4), quartz, biotite (Mg# 55-53) and Ti-magnetite (Usp18-77), and as interstitial glasses (residual melt drops) in other APF volcanic rocks. The cumulate nodules are olivine-gabbro and amphibole-gabbro/gabbronorite with a mineral paragenesis dominated by plagioclase (An41-73Ab26-57Or1-3), olivine (Fo68-72), Mg-rich augite to ferroan diopside (En41-45Fs12-15Wo42-45; Mg# 79-74), ferroan enstatite (En65-74Fs24-33Wo2-3; Mg# 76-68), magnetite (Usp15-28) and titanian pargasite (Mg# 67-65). The main cumulus phases are plagioclase, olivine and pyroxene, while intercumulus/postcumulus phases are titanian pargasite and magnetite. The dataset can be used to compare petrographic features and chemical compositions of calc-alkaline rocks emplaced in other subduction-related settings. Above all, it can represent a useful contribution in solving the problem linked to the identification of a hidden Early-Oligocene source of the thick volcaniclastic APF succession in the Alpine-Apennine belt geodynamic evolution.

Nanocomposites of Au/Disentangled UHMWPE: A Combined Optical and Structural Study.

The term disentangled refers to polymers with fewer entanglements in the amorphous regions, a metastable condition that can significantly affect the material's properties and processing behavior. The lower entanglement density in ultra-high molecular weight polyethylene (dis-UHMWPE) facilitates the solid-state processability into uniaxially-oriented specimens reaching very high draw ratios and crystallinities. In this study, Au/dis-UHMWPE nanocomposites were formulated and processed at variable draw ratios. Polarized light microscopy suggests gold nanoparticles are oriented in arrays following the drawing of polymer chains. The structural features, upon orientation, are studied by means of Raman spectroscopy, wide- and small-angle X-ray scattering, and near-infrared spectrophotometry. Crystallinity is found to increase by 15%, as calculated by wide-angle X-ray scattering. The change in optical absorbance in the visible spectrum indicates that, with orientation, the average size of gold aggregates increases, supported quantitatively by small-angle X-ray scattering. Since the gold nanoparticles are expected to be found within amorphous chain segments, the aforementioned findings are attributed to the increase of crystallinity and thus the decrease of available (amorphous) space.

Ballistic thermal phonons traversing nanocrystalline domains in oriented polyethylene.

Thermally conductive polymer crystals are of both fundamental and practical interest for their high thermal conductivity that exceeds that of many metals. In particular, polyethylene fibers and oriented films with uniaxial thermal conductivity exceeding 50 [Formula: see text] have been reported recently, stimulating interest into the underlying microscopic thermal transport processes. While ab initio calculations have provided insight into microscopic phonon properties for perfect crystals, such properties of actual samples have remained experimentally inaccessible. Here, we report the direct observation of thermal phonons with mean free paths up to 200 nm in semicrystalline polyethylene films using transient grating spectroscopy. Many of the mean free paths substantially exceed the crystalline domain sizes measured using small-angle X-ray scattering, indicating that thermal phonons propagate ballistically within and across the nanocrystalline domains; those transmitting across domain boundaries contribute nearly one-third of the thermal conductivity. Our work provides a direct determination of thermal phonon propagation lengths in molecular solids, yielding insights into the microscopic origins of their high thermal conductivity.

Synthesis and Electrospinning of Polycaprolactone from an Aluminium-Based Catalyst: Influence of the Ancillary Ligand and Initiators on Catalytic Efficiency and Fibre Structure.

In the present study, we investigated the catalytic performance of a 2,2'-methylenebis(6-tert-butyl-4-methylphenol) (MDBP)⁻aluminium complex for the ring-opening polymerisation (ROP) of ε-caprolactone in combination with various alcohols as initiators. Three different alcohols were investigated: 1-adamantanemethanol (A), 1H,1H,2H,2H-perfluoro-1-octanol (F) and isopropanol (I). Samplings of polycaprolactone (PCL) at various reaction times showed a linear increase in the polymer molecular weight with time, with very narrow polydispersity, confirming the living nature of the catalytic system. Scanning electron microscope (SEM) images of electrospun PCL fibre mats produced from 30 wt % dichloromethane/dimethyl sulfoxide solutions showed a high level of surface porosity with a reasonable homogeneity of fibre diameters. The values of the liquid absorption and water contact angle were measured for the electrospun mats, with the F-capped PCL consistently showing absorption values up to three times higher than those of PCL samples capped with the other two alcohols, as well as increased hydrophobicity. The nature of the alcohol can influence the surface hydrophobicity and absorption ability of electrospun fibres, demonstrating the possibility of tailoring material properties through controlled polymerisation.

Synthesis and characterization of divinyl-fumarate poly-ε-caprolactone for scaffolds with controlled architectures.

A vinyl-terminated polycaprolactone has been developed for tissue engineering applications using a one-step synthesis and functionalization method based on ring opening polymerization (ROP) of ε-Caprolactone, with hydroxyl ethyl vinyl ether (HEVE) acting both as the initiator of ROP and as photo-curable functional group. The proposed method employs a catalyst based on aluminium, instead of the most popular Tin(II) 2-ethylhexanoate, to reduce the cytotoxicity. Following the synthesis of the vinyl-terminated polycaprolactone, its reaction with fumaryl chloride (FuCl) results in a divinyl-fumarate polycaprolactone (VPCLF). The polymers obtained were thoroughly characterized using Fourier transform infrared spectroscopy (FTIR) and gel permeation chromatography (GPC) techniques. The polymer has been successfully employed, in combination with N-vinyl pyrrolidone (NVP), to fabricate films and computer-designed porous scaffolds by micro-stereolithography (µ-SL) with gyroid and diamond architectures. Characterization of the networks indicated the influence of NVP content on the network properties. Human mesenchymal stem cells adhered and spread onto VPCLF/NVP networks showing good biological properties and no cytotoxic effect. Copyright © 2016 John Wiley & Sons, Ltd.

Electrospun Nanofibres Containing Antimicrobial Plant Extracts.

Over the last 10 years great research interest has been directed toward nanofibrous architectures produced by electrospinning bioactive plant extracts. The resulting structures possess antimicrobial, anti-inflammatory, and anti-oxidant activity, which are attractive for biomedical applications and food industry. This review describes the diverse approaches that have been developed to produce electrospun nanofibres that are able to deliver naturally-derived chemical compounds in a controlled way and to prevent their degradation. The efficacy of those composite nanofibres as wound dressings, scaffolds for tissue engineering, and active food packaging systems will be discussed.

Effect of Ethyl Ester L-Lysine Triisocyanate addition to produce reactive PLA/PCL bio-polyester blends for biomedical applications.

We report in this paper the effects of Ethyl Ester L-Lysine Triisocyanate (LTI) on the physical-mechanical properties of Poly(lactide)/Poly(ε-caprolactone) (PLA/PCL) polyesters blends. The PLA/PCL ratios considered were 20/80, 50/50 and 80/20 (wt/wt %) and LTI was added in amounts of 0.0-0.5-1.0 phr. PLA and PCL reacted with LTI during processing in a Brabender twin screw internal mixer to produce block copolymers in-situ. The resulting blends have been characterized by torque measurements, uniaxial tensile tests, Differential Scanning Calorimeter, contact angle measurements with a Phosphate Buffered Saline (PBS) solution, ATR analysis and morphological SEM observations. Experimental results highlighted how LTI enhanced interaction and dispersion of the two components, resulting into a synergic effect in mechanical properties. Mechanical and physical properties can be tailored by changing the blend composition. The most noticeable trend was an increase in ductility of the mixed polymers. Besides, LTI decreased blend's wet ability in PBS and lowered the starting of crystalline phase formation for both polymers, confirming an interaction among them. These reactive blends could find use as biomedical materials, e.g. absorbable suture threads or scaffolds for cellular growth.

A hemi-metallocene chromium catalyst with trimethylaluminum-free methylaluminoxane for the synthesis of disentangled ultra-high molecular weight polyethylene.

Recently, it has been shown that by using a single-site catalytic system having titanium as a metallic center, it is possible to tailor the entanglement density in the amorphous region of a semi-crystalline ultra-high molecular weight polyethylene (UHMWPE). This route provides the possibility to make high-modulus, high-strength uniaxially and biaxially drawn tapes and films, without using any solvent during processing. In this publication, it is shown that a single-site catalyst having chromium as metallic center, proposed by Enders and co-workers, can also be tuned to provide control on the entanglement density during synthesis of the UHMWPE. However, to achieve the goal some modifications during the synthesis are required. The synthesized polymers can be processed in the solid state below the equilibrium melting temperature, resulting in uniaxially drawn tapes having tensile strength and modulus greater than 3.5 N/tex and 200 N/tex, respectively. Rheological studies have been performed to follow the increase in entanglement density in melt state with time.

Design of stereoselective Ziegler-Natta propene polymerization catalysts.

After five decades of largely serendipitous (albeit formidable) progress, catalyst design in Ziegler-Natta olefin polymerization, i.e., the rational implementation of new active species to target predetermined polyolefin architectures, has ultimately become a realistic ambition, thanks to a much deeper fundamental understanding and major advances in the tools of computational chemistry. In this article, we discuss, as a case history, a unique class of stereorigid C2-symmetric bis(phenoxy-amine)Zr(IV) catalysts with controlled kinetic behavior. A large variety of polypropylene microstructures have been obtained with these catalysts by modulating the steric demand of one key substituent, without altering the nature and symmetry of the ancillary ligand framework, under the guidance of computer modeling. This unusual achievement is relevant per se and for the perspective implications in catalyst discovery.

Reactivity of secondary metal-alkyls in catalytic propene polymerization: how dormant are "dormant chains"?

In this communication, we report on the direct measurement of dormant chain concentration and of the relative reactivity of authentic primary and secondary Zr-polymeryls toward propene, ethene, and H2 under practical conditions for a new highly regio- and stereoselective postmetallocene polymerization catalyst with controlled kinetic behavior. The results, in particular, confirm the poor reactivity toward propene of secondary M-polymeryls and the possible accumulation of dormant chains in propene homopolymerization.

Improving the performance of methylalumoxane: a facile and efficient method to trap "free" trimethylaluminum.

The presence of "free" trimethylaluminum (TMA) in methylalumoxane (MAO) solutions can be highly detrimental to the performance of metallocene and "post-metallocene" olefin polymerization catalysts. The most used strategy to remove "free" TMA is to evaporate MAO solutions to dryness, until a free-flowing white powder ("solid MAO") is left. This procedure is tedious and potentially hazardous, because in some cases the distillate is a concentrated hydrocarbon solution of TMA. Moreover, "solid MAO" is poorly soluble in common polymerization media, and once in solution it can regenerate TMA to some extent. This communication reports on a facile alternative, which consists in the controlled addition of a sterically hindered phenol, such as 2,6-di-tert-butylphenol, effectively trapping "free" TMA. We show here that 2,6-di-tert-butylphenol/MAO solutions activate equally well the dichloro-precursors of well-known zirconocene and bis(phenoxyimine)Ti catalysts, and that their use in propene polymerization results in a substantially higher productivity, polymer stereoregularity, and/or average molecular mass compared with activation by MAO alone.