The thermal transport properties of oriented polymers are of fundamental and practical interest. High thermal conductivities ( ≳ 50 Wm −1 K −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.

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.

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.

Entanglement dynamics of uniaxially-oriented, disentangled ultra-high molecular weight polyethylene nanocomposites modified with gold nanoparticles are investigated, using dielectric spectroscopy, during the transition to melt state.The dc conduction is approximately calculated via the logarithmic derivative of dielectric permittivity and is observed to decrease with the formation of entanglements.As the draw ratio increases, the progressive formation of entanglements resulted in a stronger dc conductivity decrease due to the loss of orientation in the pre-melt crystalline and partially oriented amorphous chain segments.Additionally, a sharp peak is obtained in the absence of dc conductivity, attributed to the dipolar contribution of the Maxwell-Wagner-Sillars (MWS) interfacial polarization between the gold nanoparticles and the polymer chains.The relaxation time of the MWS interfacial polarization increases with the progressive formation of entanglements, as observed in a previous study.The results presented shed light on the process of entanglement formation in oriented ultrahigh molecular weight polyethylene nanocomposites.

The growing demand for renewable energy sources has prompted the development of dielectric materials with the ability to store and efficiently recover electrical energy. Here, we correlate the structure and thermal conductivity of uniaxially oriented disentangled ultra-high molecular weight polyethylene (dis-UHMWPE) composites reinforced with gold nanoparticles with their electrical properties and potential application as electrical energy storage devices. Stretching increases the orientation of the polymer chains and thus the crystallinity and reduces the aggregation of gold nanoparticles while the thermal conductivity enhances significantly along the orientation axis. The structural changes driven by stretching result in two competing effects; on the one hand, the crystallinity increase reduces the permittivity of the composites and increases the resistivity, while on the other hand the recovery efficiency of oriented materials excels that of unstretched samples by up to 6 times at 5 s. Therefore, our work shows the structure-property relationship in electrical energy storage materials.

The relaxation dynamics of disentangled ultrahigh molecular weight polyethylene (UHMWPE) were analyzed by means of torsional rheology in a broad frequency and temperature range. The disentangled specimens were compressionmolded at two different temperatures, solid state (125 degrees C) and melt state (160 degrees C), and the latter was compared with a melt-state-processed commercial UHMWPE specimen. Three different relaxation processes were observed, namely, alpha(-)(c), beta-, and gamma-relaxations, as expected for polyethylene. The relaxation strengths of the alpha(c)- and gamma-relaxations were found to be dependent on the crystallinity content, verified by means of differential scanning calorimetry. The relaxation molecular dynamics of the gamma-relaxation in the solid-state-compressed disentangled sample follows a Vogel- Fulcher-Tammann-Hesse trend, suggesting a dynamic glass-to-rubber transition. The same trend is not found for the gamma-relaxation of both melt-state-processed samples, thus suggesting a role of crystalline polydispersity and entanglement density in the free volume of the amorphous segments.

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.

PolyolefinsPolyolefins arePolyolefins thermoplastic polymers used in a wide range of applications, including medical implants, insulating materials, fabrics, and packaging. The two most popular representatives, polyethylene (PE) and polypropylene (PP), present linear chemical structures that yield these materials semicrystalline (except for atactic PPPolypropylene (PP)). The versatility of their synthesis enables the fabrication of different grades, covering a wide range of crystallinities which can reach up to 90%. However, because of their symmetric aliphatic structure, they do not present a permanent dipoledipole moment. Their nonpolar nature makes dielectric spectroscopy measurements challenging, as this technique relies on the relaxation of dipoles after the application of an external electric fieldelectric field. Here, we review different approaches that have been followed in order to introduce permanent dipoles and render polyolefins dielectrically active, including: (i) addition of probes with a permanent dipole moment, (ii) oxidation/chlorination to produce dielectrically active chains, or (iii) induced oxidation in the presence of metal oxide fillers. The introduction of dipoles, either intentionally or due to the presence of impurities, has enabled the characterization of the full relaxation spectra of polyethylene and polypropylene as well as the assignment of dielectric relaxations to their respective molecular mechanisms. We then turn our attention into PEPolyethylene (PE) and PP composites for electrical energy storage and insulation applications. We show how, in these materials, the effect of the polymer matrix and filler orientationorientation has been proven key to enhance their dielectric breakdown strengthdielectric breakdown strength.

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.

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.

With the help of Broadband Dielectric Spectroscopy, it has been possible to study the molecular dynamics of disentangled Ultra High Molecular Weight Polyethylene in a wide temperature and frequency range. Catalytic ashes of aluminum oxide act as dielectric probes, allowing the identification of five different processes: an alpha(c)-process due to movements in the crystalline phase, two gamma-processes attributed to amorphous chain portions close to the crystalline lamellae, and two beta-processes that we have attributed to the disentangled and entangled amorphous phases. The entanglement formation has been followed by isothermal runs and a model that predicts the energy spent to form entanglements as a function of time and temperature is thereby proposed. This model allowed us to calculate the associated activation energy of the entanglement process. Our work advances further the understanding of entanglement dynamics of ultra-high molecular weight polymers, and the proposed model could prove useful to describe other similar processes such as cross-linking. (C) 2018 The Authors. Published by Elsevier Ltd.

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.

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.

The polymerization of ethylene to Ultra-High Molecular Weight Polyethylene (UHMWPE) in certain reaction conditions allows synthesis of nascent powders with a considerably lower amount of entanglements: the material obtained is of great interest from both academic and industrial viewpoints. From an academic point of view, it is interesting to follow the evolution of the metastable melt state with the progressive entanglements formation. Industrially, it is valuable to have a solvent-free processing route for the production of high modulus, high strength tapes. Since the polymer synthesis is performed in the presence of a solvent, it is interesting to investigate the influence that the reaction medium can have on the catalyst activity, resultant molecular characteristics, and polymer morphology at the macroscopic as wells as microscopic level. In this paper, we present the effect that two typical polymerization solvents, toluene and heptane, and mixtures of them, have on the catalytic performance and on the polymer properties. The observations are that an unexpected increase of catalyst activity, accompanied by a significant improvement in mechanical properties, is found when using a carefully chosen mixture of solvents. A tentative explanation is given on the basis of the presented results.

Ultra High Molecular Weight Polyethylene with a reduced number of entanglements can be stretched in the solid state both uni-or biaxially to produce highly oriented tapes and films. The chain orientation, in combination with the reduced number of chain ends, is responsible for the high tensile modulus and tensile strength of the drawn materials, and, as we report here, also for the high thermal conductivity achieved through lattice movements. A property such as thermal conductivity in an electrical insulator makes UHMWPE tapes and films of great applicative interest. In-plane laser-flash thermal analysis has been applied to measure the thermal diffusivity of samples of different molecular weights stretched both uni-and biaxially, and a strong correlation has been found between the drawing ratio and the resulting in-plane thermal conductivity. Values of at least 40 W/m K have been achieved for UHMWPE having M-w comprised between 2 and 10 million, while higher values of 65 W/m K are observed for the higher M-w samples having relatively lesser number of chain ends. Surprisingly the biaxially stretched samples also show in-plane conductivity, with the highest value reaching 18 W/m K, comparable to stainless steel. (C) 2017 The Authors. Published by Elsevier Ltd.

Most plastics materials may be considered as electrical insulators, as they are able to withstand a potential difference between different points of a given piece of material with the passage of only a small electric current and a low dissipation energy. A notable and more recent exception is constituted by intrinsically conductive polymers, which are able to reach conductivities similar to metals. When assessing a polymeric material for its use in an electrical application, information on some properties will be required, such as dielectric constant/power factor, dielectric strength, volume resistivity, surface resistivity, tracking, and arc resistance. The chapter gives an overview of the most important aspects of electrical properties of polymers, including some new applications in the field. The optical properties of plastics are also considered in this chapter. How the structure of a plastic affects transparency and the significance of refractive index and birefringence are discussed. The influences on surface appearances and color are explored. How each of these aspects is quantified is also included.

The addition of ethyl ester l-lysine tri-isocyanate (LTI) to mixtures of poly(lactide)/poly(ε-caprolactone) has been reported to improve the compatibility of the resulting blends. In the present work, we have investigated the influence of adding increasing amounts of LTI to the mechanical and thermal properties of the blends. Torque trends, plate-plate rheology, differential scanning calorimetry, scanning electron microscopy, and uniaxial tensile characterization were conducted on samples with amounts of LTI comprised between 0.5 and 5 phr. Results suggest that by increasing the content of LTI over 0.5 phr the mechanical and thermal behaviour of the blends tend to change from that of a thermoplastic to that of a cross-linked, rubber-like material with outstanding mechanical properties. Morphological investigations show a very fine, well-dispersed morphology in all cases. Numerical models have been applied to rheological experiments to identify processes and phases in the studied blends, further supporting the hypothesis of a cross-linked phase formed for blends containing more than 0.5 phr of LTI.