11,549 research outputs found

    Segmented copolymers with polyesteramide units of uniform length: structure analysis

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    Segmented poly(ether esteramide) copolymers with short (M = 382) partially aromatic esteramide units of uniform length and segments of poly(tetramethylene oxide) (PTMO) have beem synthesized in the melt. The polymers show phase separation into two or three phases. The influence of the PTMO segment length on the following properties was studied: the Tg of the amorphous phase, the Tm of the crystalline PTMO and the melting and crystallization behaviour of the uniform polyesteramide units. Volume fraction and density of each phase were determined. The polyesteramide units crystallize in lamellar structures; their sizes were studied using WAXD and SAXS. The polymers were melt processed and their mechanical properties were investigated using dynamic mechanical thermal analysis (d.m.t.a.) and tensile tests. Polyesteramide crystallinity, crystalline structure and crystallite size were found to be almost independent of PTMO segment length. The decrease in hard-segment melting temperature with increasing PTMO segment length is explained as being due to a `solvent¿ effect of the soft phase. The copolymers crystallize very fast, and the modulus in the rubber region is essentially independent of temperature. The copolymers with long PTMO segments (M = 2000 and 2900) have a low glass transition temperature (¿65°C); the materials are very soft and have an elongation at break of over 1000%. The copolymer with the shortest PTMO segments (M = 250) has a glass transition temperature of 43°C and the material is hard at room temperature

    Oxidative degradation of polylactide (PLA) and its effects on physical and mechanical properties

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    The thermo-oxidative degradation of polylactide (PLA) films was studied between 70 and 150 C. It was shown that the oxidative degradation of PLA leads to a random chain cission responsible for a reduction of the molar mass. These molar mass changes affect Tg and the degree of crystallinity, and it was found that Tg decreases according to the Fox–Flory theory whereas the degree of crystallinity increases due to a chemicrystallization process. A correlation between molar mass and strain at break during oxidation has been established: PLA displays a brittle behavior when Mn falls below 40 kg mol 1 in agreement with relationships linking the critical value for embrittlement with the molar mass between entanglements

    Structural Dependence of the Molecular Mobility in the Amorphous Fractions of Polylactide

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    Fragility index and cooperativity length characterizing the molecular mobility in the amorphous phase are for the first time calculated in drawn polylactide (PLA). The microstructure of the samples is investigated from wide-angle X-ray scattering (WAXS) whereas the amorphous phase dynamics are revealed from broadband dielectric spectroscopy (BDS) and temperature-modulated differential scanning calorimetry (TMDSC). The drawing processes induce the decrease of both cooperativity and fragility with the orientation of the macromolecules. Post-drawing annealing reveals an unusual absence of correlation between the evolutions of cooperativity length and fragility. The cooperativity length remains the same compared to the drawn sample while a huge increase of the fragility index is recorded. By splitting the fragility index in a volume contribution and an energetic contribution, it is revealed that the amorphous phase in annealed samples exhibits a high energetic parameter, even exceeding the amorphous matrix value. It is assumed that the relaxation process is driven in such a way that the volume hindrance caused by the thermomechanical constraint is compensated by the acceleration of segmental motions linked to the increase of degrees of freedom. This result should also contribute to the understanding of the constraint slackening in the amorphous phase during annealing of drawn PLA, which causes among others the decrease of its barrier properties

    Investigation of Thermal Properties of Radiation Grafted Polystyrene / Polytetrafluoroethylene (PTFE) Copolymer Films

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    Grafting was carried out using 7-radiation from a 60Co source at dose rates of 1.32- 15.0 kGyh"! at room temperature. The chemical structure and the crystallinity ofthe grafted PTFE films were studied by means of Fourier transform infrared (FTIR). Structural investigation of polystyrene grafted polytetrafluoroethylene) (PTFE) membranes prepared by radiation-induced grafting of styrene onto commercial PTFE films carried out by Fourier transform infrared spectrometer (FTIR) and the thermal stability of thermal gravimetric analyzer (TGA). The effect of the structural changes taking place in the film by grafting and the variation of the degree of grafting on melting temperature (Tm), glass transition temperature (Tg), heat of melting (AHm), anddegree of crystallinity was studied by using differential scanning calorimetry (DSC). Polystyrene grafted films prepared by radiation-induced grafting of styrene onto PTFE films were found to undergo considerable structural changes. The degree of crystallinity decreased upon grafted as well as deviation of the degree of grafting. The changes in melting temperature (Tm) was found to be insignificant regardless of the degree of grafting unlike glass transition temperature (Tg). The decrease in crystallinity was suggested mainly due to the effect of dilution on the original crystalline structure by incorporation of the amorphous polystyrene grafts. Furthermore based on the results and discussion and those on the original and grafted films is that the PTFE film surface undergoes structural changes in terms of chemical composition as a result of styrene grafting. The effect of grafting of polystyrene onto PTFE membrane on the thermal stability of the PTFE matrix is studied. It is found that the fluorinated-structure of the PTFE matrix has a thermal stabilityup to ~550°C followed by one-step degradation. Grafting of styrene, which led to the formation of polystyrene grafts in PTFE matrix introduces a two-step degradation pattern. The polystyrene grafts start to degrade at ~400°C and continue until ~480°C. This is followed by the degradation ofPTFE backbone, which starts at ~550°C and continues to ~680°C. Styrene, which radiochemically grafted onto PTFE films leading to the formation of polystyrene grafted PTFEfilms. The grafted films were found to be very stiff compared to the original PTFE fimi.From DSC thermograms, the high value of Tg compared to that reported in literature (>20°C) [10,11], thus indicates that commercial PTFE film contains certain additive (filler such as glass fibres) that decreases the polymer chain mobility and gives rise to Tg. The incorporation of polystyrene side chain grafts into PTFE film caused a shift in the Tg of the grafted film to higher temperature while significant changes took place in Tm. Grafting of styrene were found to reduce the heat of melting and the degree of crystallinity of PTFE films. In addition, the degree of crystallinity decreased in the original PTFE film to the grafted films

    On the Molecular Origin of the Cooperative Coil-to-globule Transition of Poly(N-isopropylacrylamide) in Water

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    By means of atomistic molecular dynamics simulations we investigate the behaviour of poly(N-isopropylacrylamide), PNIPAM, in water at temperatures below and above the lower critical solution temperature (LCST), including the undercooled regime. The transition between water soluble and insoluble states at the LCST is described as a cooperative process involving an intramolecular coil-to-globule transition preceding the aggregation of chains and the polymer precipitation. In this work we investigate the molecular origin of such cooperativity and the evolution of the hydration pattern in the undercooled polymer solution. The solution behaviour of an atactic 30-mer at high dilution is studied in the temperature interval from 243 to 323 K with a favourable comparison to available experimental data. In the PNIPAM water soluble states we detect a correlation between polymer segmental dynamics and diffusion motion of bound water, occurring with the same activation energy. Simulation results show that below the coil-to-globule transition temperature PNIPAM is surrounded by a network of hydrogen bonded water molecules and that the cooperativity arises from the structuring of water clusters in proximity to hydrophobic groups. Differently, the perturbation of the hydrogen bond pattern involving water and amide groups occurs above the transition temperature. Altogether these findings reveal that even above the LCST PNIPAM remains largely hydrated and that the coil-to-globule transition is related with a significant rearrangement of the solvent in proximity of the surface of the polymer. The comparison between the hydrogen bonding of water in the surrounding of PNIPAM isopropyl groups and in bulk displays a decreased structuring of solvent at the hydrophobic polymer-water interface across the transition temperature, as expected because of the topological extension along the chain of such interface

    Influence of chain topology (cyclic versus linear) on the nucleation and isothermal crystallization of poly(L-lactide) and poly(D-lactide)

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    In this paper, ring closure click chemistry methods have been used to produce cyclic c-PLLA and c-PDLA of a number average molecular weight close to 10 kg/mol. The effects of stereochemistry of the polymer chains and their topology on their structure, nucleation and crystallization were studied in detail employing Wide Angle X-ray Scattering (WAXS), Small Angle X-ray Scattering (SAXS), Polarized Light Optical Microscopy (PLOM) and standard and advanced Differential Scanning Calorimetry (DSC). The crystal structures of linear and cyclic PLAs are identical to each other and no differences in superstructural morphology could be detected. Cyclic PLA chains are able to nucleate much faster and to produce a higher number of nuclei in comparison to linear analogues, either upon cooling from the melt or upon heating from the glassy state. In the samples prepared in this work, a small fraction of linear or higher molecular weight cycles was detected (according to SEC analyses). The presence of such “impurities” retards spherulitic growth rates of c-PLAs making them nearly the same as those of l-PLAs. On the other hand, the overall crystallization rate determined by DSC was much larger for c-PLAs, as a consequence of the enhanced nucleation that occurs in cyclic chains. The equilibrium melting temperatures of cyclic chains were determined and found to be 5 ºC higher in comparison with values for l-PLAs. This result is a consequence of the lower entropy of cyclic chains in the melt. Self-nucleation studies demonstrated that c-PLAs have a shorter crystalline memory than linear analogues, as a result of their lower entanglement density. Successive self-nucleation and annealing (SSA) experiments reveal the remarkable ability of cyclic molecules to thicken, even to the point of crystallization with extended collapsed ring conformations. In general terms, stereochemistry had less influence on the results obtained in comparison with the dominating effect of chain topology.“UPV/EHU Infrastructure: INF 14/38”; “Mineco/FEDER: SINF 130I001726XV1/Ref: UNPV13–4E–1726” and “Mineco MAT2014-53437-C2-P”, 'Ministerio de Economia y Competitividad (MINECO), code: MAT2015-63704-P (MINECO/FEDER, UE) and by the Eusko Jaurlaritza (Basque Government), code: IT-654-13. O.C acknowledges financial support from the European Commission and Région Wallonne FEDER program (Materia Nova) and OPTI²MAT program of excellence, by the Interuniversity Attraction Pole Program (P7/05) initiated by the Belgian Science Policy office and by the FNRS-FRFC. OC is Research Associate of the F.R.S.-FNRS. Organic Synthesis and Mass Spectrometry Laboratory thanks F.R.S.-FNRS for the financial support for the acquisition of the Waters QToF Premier and Synapt-G2Si mass spectrometers and for continuing support. Finally, all authors would like to acknowledge Research and Innovation Staff Exchange (RISE) H2020-MSCA-RISE-2017-778092, project BIODEST for promoting cooperation between the Mons team and the UPV/EHU team

    Synthesis, structure, crystallization and confinement of isodimorphic PBS-ran-PCL copolyesters

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    363 p.In this thesis, the preparation of PBS-ran-PCL biodegradable and biocompatible copolyesters by ring opening-melt polycondensation polymerization with three different molecular weight is presented. The copolymers have been characterized by proton and carbon nuclear magnetic resonance (NMR), gel permeation chromatography (GPC), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), polarized light optical microscopy (PLOM), scanning electron microscopy (SEM), Raman, FT-IR, GIWAXS and wide angle X-ray scattering (WAXS). The PBS-ran-PCL copolyesters were able to crystallize in the entire composition range and displayed a pseudo-eutectic region. Most copolymers away from the pseudo-eutectic region exhibited a single crystalline phase (PBS-rich or PCL-rich crystalline phase), while within the pseudo-eutectic region the copolymers were double crystalline. Obtained results demonstrated that the copolymers are isodimorphic. Isothermal crystallization kinetics showed that both nucleation density and spherulitic growth rate of the copolyesters are determined by the component that constitutes the majority phase and was a strong function of composition and supercooling. For the infiltrated random copolyesters, results revealed that the nucleation process changed from heterogeneous in bulk to surface or homogeneous nucleation in the nanopores.Polyma

    ION TRANSPORT IN POLYMER ELECTROLYTES

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    Batteries with superior performance will advance many technologies, such as the field of energy storage and electrochemical devices. Traditional lithium ion batteries based on liquid electrolytes have intrinsic problems such as leaking, dendrite growth, and those problems are associated with fire or even explosion hazard. Extensive efforts have been devoted into the development of solid polymer electrolytes (SPEs), which would not only reduce the size and weight of the batteries, but also solve safety related issues. However, none of current dry SPEs have reached the desired conductivity of 10-3 [0.001] S/cm at ambient temperature. The ion conductivity is controlled by two parameters, the free ion concentration and ion diffusivity. Despite the generally accepted theory that ion diffusion is facilitated by the segmental relaxation of the polymer, the mechanism of ion transport in SPEs is not completely understood. In this dissertation, the ion transport in different SPEs systems were studied with a combination of experimental techniques: dielectric spectroscopy, differential scanning calorimetry and rheology. The ion transport mechanism was investigated in poly(propylene glycol) (PPG) doped with LiClO4 [lithium perchlorate]. A comprehensive analysis was performed by systematically varying the temperature, pressure, polymer molecular weight and salt concentration. It was found that the ion transport was controlled by the segmental relaxation of the “ion-rich” phase in the system, which obeyed the traditional theory. On the contrary, decoupling was observed in several carbonate and styrene based polymer electrolytes. Analysis indicated that the decoupling feature might be related to the packing frustration in those systems. Polymerized ionic liquids (PolyILs) offer an opportunity of combining the high conductivity of ionic liquids and the superior mechanical strength of polymer. Unlike their small molecule analogue-aprotic ionic liquids, decoupling feature was observed in studied PolyILs. The variation of the pendant group structures altered the fragility index of the samples and thus the degree of decoupling. Unraveling the mechanisms of the ion transport and structure-property relationship in SPEs is of obvious fundamental and industrial importance. Findings in this work suggested new routes for future polymer electrolytes design of desired properties
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