| 1. |
- Andersson, Anna, et al.
(författare)
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Free volume and crystallinity of poly(ethylene naphthalate) treated in pressurized carbon dioxide
- 2010
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Ingår i: Polymer. - : Elsevier BV. - 0032-3861. ; 51:1, s. 146-152
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Tidskriftsartikel (refereegranskat)abstract
- The free volume and crystallization behaviour of poly(ethylene naphthalate) (PEN) treated in pressurized carbon dioxide was studied, using Positron Annihilation Lifetime Spectroscopy (PALS) and Differential Scanning Calorimetry (DSC). PALS probes the values of free volume cavity sizes in materials, thus making it possible to investigate the effect of pressurized carbon dioxide treatment on free volume hole sizes in PEN. The crystallinity and melting behaviour of PEN was analyzed with temperature modulated DSC. We found that the carbon dioxide pressure during treatment of PEN was the prime parameter affecting the value of free volume after the treatment. The free volume sizes were observed to be insensitive to the other two parameters, temperature and time. Increasing the time of the treatment however, increased the crystallinity of PEN. Interestingly, this was not coupled to a decrease in ortho-Positronium intensity as was expected, indicating that positronium may form in the crystalline fraction of the polymer as well as in the amorphous fraction. (C) 2009 Elsevier Ltd. All rights reserved.
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| 2. |
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| 3. |
- Barreto, C., et al.
(författare)
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Graphite nanoplatelet/pyromellitic dianhydride melt modified PPC composites: Preparation and characterization
- 2013
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Ingår i: Polymer. - : Elsevier BV. - 0032-3861. ; 54:14, s. 3574-3585
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Tidskriftsartikel (refereegranskat)abstract
- The properties of composites prepared by melt compounding from graphite nanoplatelets (GNP) and pyromellitic anhydride modified poly(propylene carbonate) (PPC) were investigated. GNPs of different morphologies and acid residue levels were selected. Acid residues in GNP grades were detrimental for PPC in terms of thermal stability, stiffness and conservation of the molecular weight. The exfoliation of graphite nanoparticles was challenging, and morphologies of combined tactoids with thicknesses both on the micro and nano scales were observed. The filler contributed to the thermal stability, and the mechanical reinforcement was observed from approximately 6 wt% GNP and accompanied by an increase in the glass transition temperature. For a specific GNP grade, at a 15 wt% filler content, the glass transition temperature increased by 10 degrees C and the storage modulus measured at 20 and 30 degrees C increased correspondingly by a factor of ca. 3 and 30, respectively. Solid state NMR spin-spin relaxation time T-2 measurements (Carr-Purcell-Meiboom-Gill pulse sequence) revealed three significantly different components. Using PLS, empirical models were established which enable the prediction of some viscoelastic parameters as a function of filler parameters and the average T-2.
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| 4. |
- Bayley, G. M., et al.
(författare)
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Large strain and toughness enhancement of poly(dimethyl siloxane) composite films filled with electrospun polyacrylonitrile-graft-poly(dimethyl siloxane) fibres and multi-walled carbon nanotubes
- 2011
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Ingår i: Polymer. - : Elsevier BV. - 0032-3861 .- 1873-2291. ; 52:18, s. 4061-4072
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Tidskriftsartikel (refereegranskat)abstract
- Unfilled cross-linked poly(dimethyl siloxane) (PDMS) is a weak material and is generally filled with high levels of particulate fillers such as silica, calcium carbonate and carbon black to improve its mechanical properties. The use of fibrous fillers such as electrospun nanofibres and multi-walled carbon nanotubes as fillers for PDMS has not been widely studied. In this study anew copolymer, polyacrylonitrile-graft-poly(dimethyl siloxane) (PAN-g-PDMS), is used as fibrous filler for PDMS. The graft copolymer is electrospun to produce the fibre filler material. It is shown how the PDMS content of the graft copolymer provides increased compatibility with silicone matrices and excellent dispersion of the fibre fillers throughout a silicone matrix. It is also shown that it is possible to include multi-walled carbon nanotubes in the electrospun fibres which are subsequently dispersed in the PDMS matrix. Fibre mats were used in the non-woven and the aligned forms. The differently prepared fibre composites have significantly different mechanical properties. Conventional composites using fibrous fillers usually show increased strength and stiffness but usually with a resultant loss of strain. In the case of the composites produced in this study there is a dramatic improvement in the extensibility of the non-woven PAN-g-PDMS fibre mat filled silicone films of up to 470%.
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| 5. |
- Bingöl, Bahar, et al.
(författare)
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Well-defined phosphonated homo- and copolymers via direct ring opening metathesis polymerization
- 2013
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Ingår i: Polymer. - : Elsevier BV. - 0032-3861. ; 54:25, s. 6676-6688
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Tidskriftsartikel (refereegranskat)abstract
- Phosphonated polymers with a well-defined molecular weight, composition, and architecture have been prepared via ring opening metathesis polymerization (ROMP) of diethyl phosphonate norbornene imides at room temperature for the first time. ROMP gave a broad range of phosphonated homopolymers, statistical, diblock and triblock copolymers based on poly(norbornene imide)s with low polydispersity (1.09-1.32) Complete hydrolysis of diethyl phosphonate poly(norbornene imide)s under mild conditions yielded the phosphonic acid derivatives. Thermogravimetric analysis indicated high thermal and thermo-oxidative stability of the polymers. Free standing and transparent films with good mechanical stability were obtained from the phosphonic acid functional homopolymers, diblock and triblock copolymers. Combining these basic properties with the advantages mentioned above make ROMP a promising pathway for accessing a wide diversity of phosphonated macromolecular structures. These new phosphonated polymers will open new perspectives in advanced application areas such as medicine and membrane science, which require a high level of control over polymer structure.
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| 6. |
- Börjesson, Anders, et al.
(författare)
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Molecular modelling of oxygen and water permeation in polyethylene
- 2013
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Ingår i: Polymer. - : Elsevier. - 0032-3861 .- 1873-2291. ; 54:12, s. 2988-
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Tidskriftsartikel (refereegranskat)abstract
- Monte Carlo and molecular dynamics simulations were performed to calculate solubility, S, and diffusion, D, coefficients of oxygen and water in polyethylene, and to obtain a molecular-level understanding of the diffusion mechanism. The permeation coefficient, P, was calculated from the product of S and D. The AMBER force field, which yields the correct polymer densities under the conditions studied, was used for the simulations, and it was observed that the results were not sensitive to the inclusion of atomic charges in the force field. The simulated S for oxygen and water are higher and lower than experimental data, respectively. The calculated diffusion coefficients are in good agreement with experimental data. Possible reasons for the discrepancy in the simulated and experimental solubilities, which results in discrepancies in the permeation coefficients, are discussed. The diffusion of both penetrants occurs mainly by large amplitude, infrequent jumps of the molecules through the polymer matrix.
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| 7. |
- Chelakara Satyanarayana, Kavitha, et al.
(författare)
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Molecular dynamics simulations of α- to β-poly (vinylidene fluoride) phase change by stretching and poling
- 2012
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Ingår i: Polymer. - : Elsevier Ltd. - 0032-3861 .- 1873-2291. ; 53:14, s. 2927-2934
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Tidskriftsartikel (refereegranskat)abstract
- The mechanism of inducing a phase change from α-poly(vinylidene fluoride) (α-PVDF) to β-PVDF is addressed using molecular dynamics simulations based on a molecular mechanics force field. The effect of applying a strain to the α-PVDF crystal along the axis of the molecules is investigated, as well as poling the crystal before or after stretching. Rather large (at least 1010 V/m) electric fields that are perpendicular to the axis of the PVDF molecules are required to induce α- to β-PVDF phase change when no strain is applied to the α-PVDF crystal. However, at a strain of 1.0475 (i.e., when the crystal is stretched by 4.75%) α-PVDF changes to a β-PVDF like structure, where the β-PVDF molecules orientate anti-parallel relative to each other. Transformation of the anti-parallel β-PVDF to β-PVDF can be induced by poling (even at the lowest electric field of 105 V/m studied here) or by thermal annealing.
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| 8. |
- Erdtman, Edvin, 1981-, et al.
(författare)
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Simulation of α- and β-PVDF melting mechanisms
- 2012
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Ingår i: Polymer. - : Elsevier. - 0032-3861 .- 1873-2291. ; 53:14, s. 2919-2926
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Tidskriftsartikel (refereegranskat)abstract
- Molecular dynamics (MD) simulations have been used to study the melting of α- and β-poly (vinylidene fluoride) (α- and β-PVDF). It is seen that melting at the ends of the polymer chains precedes melting of the bulk crystal structure. Melting of α-PVDF initially occurs via transitions between the two gauche dihedral angles (G ↔ G′) followed by transitions between trans and gauche dihedral angles (T ↔ G/G′). Melting of β-PVDF initially occurs via T → G/G′ transitions and via transitions of complete β- (TTTT) to α- (TGTG') quartets. The melting point of β-PVDF is higher than that of α-PVDF, and the simulated melting points of both phases depend on the length of the polymer chains used in the simulations. Since melting starts at the chain ends, it is important to include these in the simulations, and simulations of infinitely long chains yield melting points far larger than the experimental values (at least for periodic cells of the size used in this work), especially for β-PVDF. The simulated heats of fusion are in agreement with available experimental data.
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| 9. |
- Kroon, Renee, 1982, et al.
(författare)
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Effect of electron-withdrawing side chain modifications on the optical properties of thiophene-quinoxaline acceptor based polymers
- 2013
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Ingår i: Polymer. - : Elsevier BV. - 0032-3861. ; 54:4, s. 1285-1288
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Tidskriftsartikel (refereegranskat)abstract
- Four new thiophene-quinoxaline acceptor based polymers have been synthesized. The parent thiophene-quinoxaline acceptor based copolymer was modified by introducing different electron-withdrawing groups adjacent to the thiophene side group. The effect on the physical, electrochemical and optical properties of the polymer series is discussed with respect to the parent thiophene-quinoxaline acceptor based polymer. The side chain carbonyl from one modified monomer could conveniently be transformed to either a difluoromethylene or an -ylidene malononitrile group via carbonyl transformations. For all polymers incorporating an electron-withdrawing side group both the HOMO and especially the LUMO were significantly shifted away from vacuum while all exhibit enhanced light harvesting ability. Especially the incorporation of an -ylidine malononitrile side group increases both the spectral coverage and absorption coefficient. Incorporation of a difluoromethylene side group resulted in a twofold increase of the molecular weight compared to the parent polymer structure.
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| 10. |
- Li, Weiwei, et al.
(författare)
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Tailoring side chains of low band gap polymers for high efficiency polymer solar cells
- 2010
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Ingår i: Polymer. - : Elsevier Science B.V., Amsterdam.. - 0032-3861 .- 1873-2291. ; 51:14, s. 3031-3038
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Tidskriftsartikel (refereegranskat)abstract
- High efficiency organic solar cells (OSCs) require conjugated polymers with a low band gap, broad absorption in visible and IR region, high carrier mobility, and relatively high molecular weight as p-type donor materials. Flexible side chains on the rigid polymer backbone are crucial for the solubility of conjugated polymers. In this work, four polymers with the main chain structure of fluorene-thiophene-benzothiadiazole-thiophene and flexible side chains located on fluorene, thiophene, and benzothiadiazole moiety, respectively, have been synthesized by Suzuki-Miyaura-Schluter polycondensation. Photovoltaic device measurements with a device configuration of ITO/polymer:PC71BM blends/LiF/Al show that P1 carrying octyloxy chains on benzothiadiazole rings gives the best performance, with a power conversion efficiency of 3.1%.
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