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ABSTRACT: The potential of polystyrene/polymethylphenylsiloxane (PS/PMPS) blends as a matrix for nanocomposites is investigated. It
was proven by dynamic rheometry and conductivity measurements that PMPS effectively disperses carbon nanotubes, as was already
known for polydimethylsiloxane (PDMS). The phase behaviour of PS/PMPS blends was investigated using differential scanning
calorimetry or modulated temperature differential scanning calorimetry. The blends were found to exhibit partial miscibility,
in contrast to the known immiscible behaviour of PS/PDMS blends. A miscibility window exists for PS/PMPS blends containing
less than approximately 10wt% PMPS.
KeywordsPartial miscibility–Nanocomposites–Percolation threshold–Polysiloxanes
Journal of Thermal Analysis and Calorimetry 04/2012; 105(3):775-781. · 1.60 Impact Factor
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ABSTRACT: A newly developed hyphenated technique is presented that combines an existing rheometer and differential scanning calorimeter
(DSC) into a single experimental setup. Through the development of a fixation accessory inside the cell of the calorimeter
and the introduction of an add-on unit for the rheometer, the simultaneous calorimetric and rheological measurement inside
the well-controlled thermal environment of a Tzero™ DSC cell opens new experimental possibilities. The evolution of thermal
and flow properties of a material can be simultaneously monitored during steady or oscillatory shear flow and regular or modulated
temperature DSC measurements. The technique offers interesting opportunities for the investigation of flow-induced transitions,
such as crystallization or phase separation, and provides a possibility for high-throughput screening of materials. The signal
quality of the novel technique in comparison to the stand-alone techniques is demonstrated by the evaluation of the calibration
factors and by measurements on standard materials. Finally, combined rheological and calorimetric melting and crystallization
experiments on polycaprolacton are performed.
Journal of Thermal Analysis and Calorimetry 04/2012; 98(3):675-681. · 1.60 Impact Factor
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Jun Zhao,
Sabine Bertho,
Joke Vandenbergh,
Guy Van Assche,
Jean Manca,
Dirk Vanderzande,
Xiaoqing Yin,
Jingdan Shi,
Thomas Cleij,
Laurence Lutsen, Bruno Van Mele
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ABSTRACT: In this work the phase behavior of [6,6]-phenyl C(61)-butyric acid methyl ester (PCBM) blends with different poly(phenylene vinylene) (PPV) samples is investigated by means of standard and modulated temperature differential scanning calorimetry (DSC and MTDSC) and rapid heat-cool calorimetry (RHC). The PPV conjugated polymers include poly(2-methoxy-5-(3',7'-dimethyloctyloxy)-1,4-phenylene vinylene) (MDMO-PPV), High T(g)-PPV which is a copolymer, and poly((2-methoxy-5-phenethoxy)-1,4-phenylene vinylene) (MPE-PPV). Comparisons of these PPV:PCBM blends with regioregular poly(3-hexyl thiophene) (P3HT):PCBM blends are made to see the different component miscibilities among different blends. The occurrence of liquid-liquid phase separation in the molten state of MDMO-PPV:PCBM and High T(g)-PPV:PCBM blends is indicated by the coexistence of double glass transitions for blends with a PCBM weight fraction of around 80 wt%. This is in contrast to the P3HT:PCBM blends where no phase separation is observed. Due to its high cooling rate (about 2000 K min(-1)), RHC proves to be a useful tool to investigate the phase separation in PPV:PCBM blends through the glass transition of these crystallizable blends. P3HT is found to have much higher thermal stability than the PPV samples.
Physical Chemistry Chemical Physics 07/2011; 13(26):12285-92. · 3.57 Impact Factor
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ABSTRACT: We provide the first experimental evidence for the existence of a low concentration narrow UCST miscibility gap, which is the second of two narrow miscibility gaps constituting together adjacent UCST phase behavior. The existence of adjacent UCST miscibility phase behavior in aqueous polymer solutions has been theoretically predicted, but so far escaped experimental verification. We have been able to determine the low concentration narrow UCST miscibility gaps from the changes in apparent heat capacity observed in stepwise isothermal cooling modulated temperature differential scanning calorimetry (MTDSC) experiments. In wide-angle X-ray diffraction (WAXD) experiments it was unambiguously demonstrated that the MTDSC heat capacity changes are not due to crystallization. Together with earlier experimental results on the high concentration narrow UCST miscibility gap, the bimodal LCST miscibility gap and the sigmoidal melting line of ice the experimental results reported in this work complete the full experimental verification of the predicted complex phase behavior of aqueous solutions of PVME. As far as the authors are aware, this is the first system where adjacent UCST phase behavior is found. On the basis of the theoretical predictions the observed phase behavior could be more general and other systems may exist where similar behavior can be observed.
01/2011;
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ABSTRACT: N-Isopropylpropionamide (NiPPA), which can self-associate via hydrogen bonds, was found to undergo a solid-solid transition as identified by DSC and X-ray diffraction. Below the melting temperature of 51 °C NIPPA adopts a plastic crystalline state with a tetragonal unit cell until it transforms into an ordered crystal with a monoclinic structure at temperatures ≤10 °C. Dielectric spectroscopy was used to characterize the dynamics of the system, determining the activation parameters for the plastic to crystalline phase transition. The activation enthalpy is relatively high, as expected for a system that involves hydrogen bonds. However, most of the activation energy as the plastic phase assumes a more crystalline state is due to the activation entropy, suggesting that the increased cooperativity observed in the relaxation processes is due to a steric locking of the molecules.
The Journal of Physical Chemistry B 10/2010; 114(44):13944-9. · 3.70 Impact Factor
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ABSTRACT: The demixing and remixing kinetics of aqueous solutions of poly(2-isopropyl-2-oxazoline) (PIPOZ) with four different chain lengths (number-average molar mass of 3300−13000 g mol−1) is studied by means of modulated temperature differential scanning calorimetry (MTDSC) in both nonisothermal and quasi-isothermal modes. The nonisothermal measurements show that the aqueous solutions of all four PIPOZ samples follow the lower critical solution temperature (LCST) phase behavior. Both the LCST and the corresponding polymer weight fraction (fw) decrease with increasing molar mass: 33.8 ± 0.1 °C at 29.8 wt % for 3300 g mol−1 shifts to 26.2 ± 0.1 °C at 19.8 wt % for 13000 g mol−1. This typical type I phase behavior of aqueous solutions of PIPOZ is in contrast to the typical type II phase behavior of poly(N-isopropylacrylamide) (PNIPAM) which has isomeric repeat units. The demixing and remixing kinetics throughout the phase transition is studied by quasi-isothermal heat capacity measurements. Overall, the response for PIPOZ solutions is markedly faster than for PNIPAM solutions.
07/2010;
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ABSTRACT: Besides chemical functionalisation, the use of surfactants can be applied to debundle and disperse carbon nanotubes before further application in polymer nanocomposites. In this work we present a theoretical analysis of the interaction between single-walled carbon nanotubes and sodium dodecyl sulfate as surfactant and/or polystyrene as polymer matrix using semi-empirical AM1 calculations. Results indicate that the use of short potassium sulfate-terminated polystyrene chains as an extra component can help to remove the surfactant from the nanotube surface within the matrix, resulting in improved electronic properties of the nanocomposite.
Physical Chemistry Chemical Physics 12/2009; 11(47):11121-6. · 3.57 Impact Factor
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ABSTRACT: The demixing and remixing kinetics in aqueous dispersions of poly(N-isopropylacrylamide) (PNIPAM) brushes covalently bound to the surface of the gold nanoparticles, which are denoted as Au−PNIPAM, is studied by means of modulated temperature differential scanning calorimetry (MTDSC) in both nonisothermal and quasi-isothermal modes. The nonisothermal measurements show double demixing peaks in both the heat flow and the heat capacity traces for dispersions with a weight fraction of Au−PNIPAM (fw) below 50 wt %. The lower phase transition corresponds to an inner layer of PNIPAM segments on the surface of the gold core, while the upper transition corresponds to an outer layer. The dispersions follow a lower critical solution temperature phase behavior with a threshold demixing temperature at about 11.4 °C for fw of 45.3 wt %. Comparison with aqueous solutions of PNIPAM with different molar masses shows that the gold core reduces the miscibility of PNIPAM with water. The process kinetics throughout the phase transition is studied by quasi-isothermal heat capacity measurements, through the effect of the modulation frequency, by changing the heating rate and repeated heating−cooling cycles. Overall, the response for Au−PNIPAM dispersions is markedly faster than for PNIPAM solutions, which might be the result of water remaining finely dispersed within the polymer matrix. Close to the gold core, the restricted collapse of the PNIPAM chains, as a result of steric hindrance by neighboring anchored chains, might result in the presence of water inside the collapsed nanoparticle. Notwithstanding the occurrence of partial vitrification in the polymer-rich phase during heating, the Au−PNIPAM dispersions retain their thermoresponsive behavior after repeated heating−cooling cycles.
07/2009;
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ABSTRACT: In this work, the phase diagram of poly(3-hexyl thiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) blends is measured by means of standard and modulated temperature differential scanning calorimetry. Blends were made by solvent-casting from chlorobenzene, as blends cast from toluene or 1,2-dichlorobenzene prove to retain effects of phase segregation during casting, hindering the determination of the phase diagram. The film morphology of P3HT/PCBM blends cast from chlorobenzene results from a dual crystallization behavior, in which the crystallization of each component is hindered by the other component. A single glass transition is observed for all compositions. The glass transition temperature (Tg) increases with increasing concentration of PCBM: from 12.1 degrees C for pure P3HT to 131.2 degrees C for pure PCBM. The observed Tg defines the operating window for the thermal annealing and explains the long-term instability of both the morphology and the photovoltaic performance of the P3HT/PCBM solar cells.
The Journal of Physical Chemistry B 02/2009; 113(6):1587-91. · 3.70 Impact Factor
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ABSTRACT: The state diagram of a blend consisting of a copolymer containing ethylene oxide and propylene oxide, P(EO-ran-PO), and poly(ether sulfone), PES, is constructed by using modulated-temperature differential scanning calorimetry (MTDSC), T(2) NMR relaxometry, and light scattering. The apparent heat capacity signal in MTDSC is used for the characterization of polymer miscibility and morphology development. T(2) NMR relaxometry is used to detect the onset of phase separation, which is in good agreement with the onset of phase separation in the apparent heat capacity from MTDSC and the cloud-point temperature as determined from light scattering. The coexistence curve can be constructed from T(2) values at various temperatures by using a few blends with well-chosen compositions. These T(2) values also allow the detection of the boundary between the demixing zones with and without interference of partial vitrification and are in good agreement with stepwise quasi-isothermal MTDSC heat capacity measurements. Important interphases are detected in the heterogeneous P(EO-ran-PO)/PES blends.
Chemistry 01/2009; 15(5):1177-85. · 5.93 Impact Factor
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ABSTRACT: During nonisothermal crystallization of highly dispersed polypropylene/carbon nanotube (CNT) composites, considerable heterogeneous nucleation is observed to an extent scaling with the CNT surface area. Saturation occurs at higher loadings, reaching a plateau value for the crystallization onset which is 15 °C higher than in the unfilled matrix. Polymorphic behavior does not occur, as revealed from wide-angle X-ray diffraction. Upon subsequent heating, an increase in the melting temperature is observed due to increased crystalline perfection in the presence of CNTs. The complex multiple melting behavior is interpreted in terms of recrystallization phenomena. A study at varying heating and cooling rates reveals that CNTs affect the chain segment mobility of the matrix and largely inhibit recrystallization upon heating. TEM observation of the nanocomposite morphology evidences the occurrence of a transcrystalline layer around the CNTs. A structure model is presented, in which individually dispersed CNTs are separated from a bulklike polymer phase by a highly ordered crystalline interface with reduced polymer mobility.
07/2008;
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ABSTRACT: Chlorinated polyethylene (CPE) nanocomposites were synthesized by melt blending clay-rich/poly(epsilon-caprolactone) (PCL) masterbatches to CPE matrices. The masterbatches were prepared following two synthetic routes: either PCL is melt-blended to the clay or it is grafted to the clay platelets by in situ polymerization. The microscopic morphology of the nanocomposites was characterized by X-ray diffraction, atomic force microscopy, transmission electron microscopy, and modulated temperature differential scanning calorimetry. When using free PCL, intercalated composites are formed, with clay aggregates that can have micrometric dimensions and a morphology similar to that of the talc particles used as fillers in commercial CPE. PCL crystallizes as long lamellae dispersed in the polymer matrix. When using grafted PCL, the nanocomposite is intercalated/exfoliated, and the clay stacks are small and homogeneously dispersed. PCL crystallizes as lamellae and smaller crystals, which are localized along the clay layers. Thanks to the grafting of PCL to the clay platelets, these crystalline domains are thought to form a network with the clay sheets, which is responsible for the large improvement of the mechanical properties of these materials.
Langmuir 04/2008; 24(5):2072-80. · 4.19 Impact Factor
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ABSTRACT: The cross-linked polystyrene grafted organotin [P-H](1−t)[P-(CH2)11-SnCl3]t, in which [P-H] represents the monomeric unit of the nonfunctionalized polymer and t is the functionalization degree, has been synthesized and characterized by 1D and 2D 1H, 13C, and 119Sn high-resolution magic angle spinning (HRMAS) NMR spectroscopy, elemental analysis, and IR spectrometry. Its catalytic activity was assessed in the transesterification model reaction between ethyl acetate and n-octanol. The catalyst displays an average conversion degree of 76% after 2 h and can be recycled at least nine times. The experimental concentration profiles were fitted to a mechanistic model, from which a turnover frequency in the range of 10−2 s−1 was calculated. Residual tin contents on the order of 5 ppm in the reaction products were assessed by inductively coupled plasma/atomic emission spectroscopy (ICP/AES). The combined use of 1H, 13C, and 119Sn HRMAS NMR and modulated DSC enables one to assess the chemical integrity of the catalyst upon recycling. Detailed analysis of 119Sn HRMAS NMR spectra under various chemical conditions mimicking the reaction mixtures allows gaining a better insight into the catalysis mechanism.
12/2007;
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ABSTRACT: Polystyrene−single-wall carbon nanotube (PS−SWNT) nanocomposites were prepared by directly mixing aqueous suspensions of exfoliated SWNTs and PS latex particles. After freeze-drying and compression molding, homogeneous polymer films were obtained with well-dispersed carbon nanotubes, as evidenced by scanning electron microscopy imaging. The nanocomposite films display a low percolation threshold and high levels of electrical conductivity. Simultaneously, a considerable increase in the glass-transition temperature of PS is achieved, provided that a sufficient amount of low-molar-mass PS is present in the matrix material. It is suggested that a certain extent of molar mass segregation occurs in the samples, with shorter PS chains preferentially adsorbed onto the nanotube surface. The latter wetting mechanism is indispensable for obtaining favorable electrical and thermal properties.
06/2007;
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ABSTRACT: The effect of poly(oxyethylene), PEO, on the thermal response rate of aqueous solutions of poly(N-isopropylacrylamide), PNIPAM, block and graft copolymers has been discussed. The PNIPAM-b-PEO/water system reveals thermoresponsive properties similar to the PNIPAM/water system. In PNIPAM-g-PEO/water solutions, however, PEO provides hydrophilic channels, facilitating the diffusion of water molecules through the collapsed polymer aggregate at temperatures above the demixing temperature of the aqueous copolymer solution. As a result, the thermal response of the aqueous PNIPAM-g-PEO system is significantly faster than in the case of either pure PNIPAM or PNIPAM-b-PEO, indicating the influence of the macromolecular architecture. An attempt has also been made to correlate the thermal response kinetics of the aqueous solutions of different copolymers with the miscibility of the polymer constituents, i.e., PEO and the thermoresponsive PNIPAM backbone, which can vitrify during phase separation.
04/2007;
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ABSTRACT: Proper filler-matrix compatibility is a key factor in view of obtaining nanocomposites with well-dispersed nanofillers displaying enhanced properties. In this respect, polymer-filler interaction can be improved by a proper combination of matrix and nanofiller polarities. This is explored for matrices ranging from nonpolar high density poly(ethylene) to ethylene-vinyl acetate (EVA) copolymers with varying vinyl acetate contents, in combination with several types of organoclay or carbon nanotubes. A novel in situ characterization methodology using modulated temperature differential scanning calorimetry is presented to evaluate the matrix-filler interaction. During quasi-isothermal crystallization of the matrix, an “excess” contribution is observed in the recorded heat capacity signal because of reversible melting and crystallization. Its magnitude considerably decreases upon addition of nanofiller in case of strong interfacial interaction, whereas the influence is moderate in case of a less interacting matrix-filler combination. It is suggested that the “excess heat capacity” can be used to quantify the segmental mobility of polymer chains in the vicinity of the nanofiller. Hence it provides valuable information on the strength of interaction, governed by the physical and chemical nature of matrix and filler. Heating experiments subsequent to quasi-isothermal crystallization point at a certain degree of molecular ordering, responsible for crystal nucleation in EVA copolymers. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1291–1302, 2007
Journal of Polymer Science Part B Polymer Physics 04/2007; 45(11):1291 - 1302. · 1.53 Impact Factor
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ABSTRACT: Supported by theoretical predictions based on the Wertheim Lattice Thermodynamic Perturbation Theory, modulated temperature differential scanning calorimetry (MTDSC) was used to further the knowledge of the phase behavior of aqueous poly(vinyl methyl ether) (PVME) solutions. Using a narrowly dispersed low molar mass PVME, we determined the following phase boundaries: (i) a bimodal lower critical solution temperature (LCST) miscibility gap at physiological temperature (around 37 degrees C), (ii) an upper critical solution temperature (UCST) two-phase area at sub-zero temperatures and high polymer concentration, and (iii) the melting line of the solvent across the entire concentration range, showing a peculiar stepwise decrease with composition. The location of the glass transition region and its influence on the crystallization/melting behavior of the solvent is discussed.
The Journal of Physical Chemistry B 03/2007; 111(6):1288-95. · 3.70 Impact Factor
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ABSTRACT: A range of hydrophilic poly(methyl vinyl ether) (PMVE) polymers was synthesized by living cationic polymerization of methyl vinyl ether (MVE), having different hydrophilic or hydrophobic chain-end functionalities. The dissimilar end-groups were either introduced by end-capping of the growing polymer chain with LiBH4, methanol, and water or by functional initiation with 2-bromo-(3,3-diethoxy-propyl)-2-methylpropanoate. The synthesized PMVEs were characterized by 1H NMR, size exclusion chromatography, and matrix-assisted laser desorption ionization time of flight, displaying a narrow polydispersity. Modulated temperature DSC was applied to study the influence of the nature of the end-groups on the solubility behavior of PMVE in water. Terminal-modification with a hydroxyl function improves the solubility, whereas a Br-containing end-group causes the polymer to be insoluble in water at room temperature; however, the special type III lower critical solution temperature demixing behavior being maintained. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 461–469, 2006
Journal of Polymer Science Part B Polymer Physics 12/2005; 44(2):461 - 469. · 1.53 Impact Factor
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ABSTRACT: The Wertheim lattice thermodynamic perturbation theory is used to calculate the liquid−liquid and solid−liquid coexistence data of a model polymer solution. The theory predicts bimodal LCST phase behavior as well as an unusual step with composition in the solid−liquid equilibrium of the solvent. These theoretical predictions are discussed in relation to the experimental results obtained for the poly(vinyl methyl ether) (PVME)/D2O system. The apparent heat capacity signal from modulated temperature DSC (MTDSC) is used to measure the onset of LCST phase separation along with the melting temperature of D2O in the presence of PVME. The experimentally observed trace of the melting endotherm allows calculating the complete melting line of the solvent, in agreement with theory. Moreover, an alternative approach, employing Fourier transform infrared spectroscopy, is established from which the equilibrium melting line of D2O could be determined, again confirming theoretical predictions. The peculiar concentration dependence of the melting curve of ice provides a new explanation for (i) the double melting endotherm observed in (MT)DSC and (ii) the inhibited crystallization in highly concentrated aqueous PVME mixtures. Finally, the existence of a low-temperature UCST miscibility gap is suggested via an insightful examination of the glass transition region.
10/2005;
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ABSTRACT: The heat capacity signal from modulated temperature DSC can be used to measure the onset of phase separation in aqueous poly(N-isopropylacrylamide) (PNIPAM) solutions, showing a type II LCST demixing behavior. Quasi-isothermal measurements through the phase transition show large excess contributions in the (apparent) heat capacity, caused by demixing and remixing heat effects on the time scale of the modulation. These excess contributions and their time-dependent evolution are useful to describe the kinetics of phase separation and to follow the related morphology development. Partial vitrification of the polymer-rich phase slows down the remixing kinetics.
11/2004;
