Stephen Picken | Delft University of Technology (original) (raw)
Papers by Stephen Picken
Philosophical transactions - Royal Society. Mathematical, physical and engineering sciences, 2006
The unusual exhibition of a biaxial nematic phase in nonlinear thermotropic mesogens derived from... more The unusual exhibition of a biaxial nematic phase in nonlinear thermotropic mesogens derived from the 2,5-oxadiazole biphenol (ODBP) core is placed in a general context; the uniaxial nematic phase of the prototypical rod-like mesogen para-quinquephenyl does not follow the classical mean-field behaviour of nematics, thus questioning the utility of such theories for quantitative predictions about biaxial nematics. The nuclear magnetic resonance spectra of labelled probe molecules dissolved in ODBP biaxial nematic phases suggest that a second critical rotation frequency, related to the differences in the transverse diamagnetic susceptibilities of the biaxial nematic, must be exceeded in order to create an aligned two-dimensional powder sample. Efforts to find higher viscosity and lower temperature biaxial nematics (with lower critical rotation rates) to confirm the above conjecture are described. Several chemical modifications of the ODBP mesogenic core are presented.
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Molecular Crystals and Liquid Crystals, 2004
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ter verkrijging van de graad van doctor
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ACS applied polymer materials, Sep 2, 2021
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Tectonophysics, Apr 1, 2019
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Macromolecules, Jun 27, 2006
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Springer eBooks, Sep 27, 2005
A theory of the tensile strength of oriented polymer fibres is presented. From an analysis of the... more A theory of the tensile strength of oriented polymer fibres is presented. From an analysis of the observed fracture envelope it is shown that the criterion for fracture of the fibre is either a critical shear stress or a critical shear strain. Owing to the chain orientation distribution in the fibre, the initiation of fracture is likely to occur in domains whose symmetry axes have orientation angles in the tail of this distribution. By considering the fibre as a molecular composite, the tensile strength is calculated as a function of the modulus. The results are compared to the observed values of PET, POK, cellulose II, PpPTA, PBO and PIPD fibres. In addition, the relation between the ultimate strength and the chain length distribution is investigated. By using the critical shear strain as a fracture criterion in the Eyring reduced time model, relations are derived for the fibre strength as a function of the load rate, as well as for the lifetime under constant load. Moreover, this model predicts the dependence of the strength on the temperature. The theoretical relations are compared to the experimental results on PpPTA fibres.
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Macromolecules, Mar 1, 1991
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Macromolecules, Aug 1, 1990
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Langmuir, Jul 26, 2023
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Proceedings of SPIE, Dec 16, 1993
ABSTRACT
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Journal of Testing and Evaluation, Jul 1, 2014
In this paper, refractometry is used for characterization of the adhesion between bitumen and agg... more In this paper, refractometry is used for characterization of the adhesion between bitumen and aggregates. The main purpose of this paper is to find whether preferential adsorption occurs and can be demonstrated in pure and undiluted road-construction bitumen. For this, bitumens from different crudes with different penetration grades and six bitumens with different penetration grades from the same crude are examined in a refractometer. The measurements give indices of refraction of the different bitumens at the bitumen-aggregate interface. In this paper, it is concluded that the indices of refraction are strongly related to the penetration of the bitumens. Clear preferential adsorption was not found from the refractometric measurements at this stage. It is suggested that a change in the composition of the bulk of the bitumen results in a similar change in composition at the aggregate surface.
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High Performance Polymers, Apr 2, 2008
Liquid crystal polymers (LCPs) are known to exhibit slow and fast phase transitions upon heating ... more Liquid crystal polymers (LCPs) are known to exhibit slow and fast phase transitions upon heating and cooling. The kinetics of both types were investigated for an liquid crystal (LC) polyimide derived from pyromellitic dianhydride (PMDA) and 1,3-bis[4'-(4''-aminophenoxy) cumyl] benzene (BACB) (PMDA/BACB). The kinetics were compared with our previous results on poly(bistrifluorethoxyphosphazene) (PBFP), which also shows LC and non-LC transitions, and with nylon PA6, a non-LC reference sample. Transition kinetics were studied upon cooling using non-isothermal differential scanning calorimetry (DSC), wide angle X-ray diffraction (WAXD) and polarization microscopy. The DSC experiments were performed by cooling our samples from 400 to 100 °C using rates between 2.5 and 100 K min-1. We observed that the LC transitions show a linear dependency on the cooling rate. These transitions were solely governed by super-cooling and these observations are congruent with our earlier results on PBFP. Apparently, we may conclude that all LC transitions show this linear dependence. For PMDA/BACB, the fast LC transitions were related to the formation of a smectic phase. WAXD did not reveal a crystalline phase in PMDA/BACB. The non-LC transition was related to a thermally activated process, having an apparent activation energy, EA, of ≈ 360 kJ mol -1, indicating a re-arrangement of chains previously randomly oriented. Crystallization could proceed after cooling from the melt to room temperature followed by annealing based on a thermally activated process, with E A ≈ 160 kJ mol-1.
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Polymer, Dec 1, 2005
ABSTRACT
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Liquid Crystals, Apr 1, 2012
Liquid crystalline polymers (LCPs) are defined by their instantaneous liquid crystal phase transi... more Liquid crystalline polymers (LCPs) are defined by their instantaneous liquid crystal phase transitions, as compared with set heating/cooling rates in differential scanning calorimetry (DSC). This is well accepted for the case of main-chain (MC) LCPs. It is confirmed here for the case of two side-group (SG) LCPs as well. The DSC temperature calibration is usually based on melting of metals
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Journal of Applied Polymer Science, Feb 14, 2011
Structure and rheological properties of hybrids with polymer matrix and layered silicates as fill... more Structure and rheological properties of hybrids with polymer matrix and layered silicates as filler were studied. Solution containing 60% wt of hydroxypropylcellulose (HPC) in oligomeric polyethyleneglycol (PEG) was used as a matrix. The peculiarity of this study is that the matrix depending on temperature can form different phase states including liquid-crystalline (LC). So, a possibility of coexistence and superposition of different ordered structures can be realized at different temperatures. Three different fillers were used: natural Na-montmorillonite (MMT) and organoclays obtained by treating MMT with surfactants varying in polarity of their molecules. Depending on the type of clay, materials with different morphology can be obtained. X-ray data showed that PEG intercalates all types of clay used whereas penetration of HPC macromolecules into clay galleries during mixing does not occur. Clay modified with more polar surfactant should be treated as the most convenient material to be intercalated by PEG. Rheological studies (included steady-state and dynamic shear properties in a wide temperature range) demonstrate that composites are viscoplastic materials and the yield stress is observed already at 5% fillers loading. The level of the yield stress depends on the phase state of the matrix being induced by the superposition of structures formed by clay particles as well as by the LC phase (if it exists). The same conclusion can be made on the base of linear oscillatory measurements because the existence of the LC phase and/or the presence of filler lead to a pseudo solid-like behavior of a system as whole. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011
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Liquid Crystals, Jul 5, 2010
ABSTRACT
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Liquid Crystals, Jul 5, 2010
ABSTRACT
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Philosophical transactions - Royal Society. Mathematical, physical and engineering sciences, 2006
The unusual exhibition of a biaxial nematic phase in nonlinear thermotropic mesogens derived from... more The unusual exhibition of a biaxial nematic phase in nonlinear thermotropic mesogens derived from the 2,5-oxadiazole biphenol (ODBP) core is placed in a general context; the uniaxial nematic phase of the prototypical rod-like mesogen para-quinquephenyl does not follow the classical mean-field behaviour of nematics, thus questioning the utility of such theories for quantitative predictions about biaxial nematics. The nuclear magnetic resonance spectra of labelled probe molecules dissolved in ODBP biaxial nematic phases suggest that a second critical rotation frequency, related to the differences in the transverse diamagnetic susceptibilities of the biaxial nematic, must be exceeded in order to create an aligned two-dimensional powder sample. Efforts to find higher viscosity and lower temperature biaxial nematics (with lower critical rotation rates) to confirm the above conjecture are described. Several chemical modifications of the ODBP mesogenic core are presented.
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Molecular Crystals and Liquid Crystals, 2004
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ter verkrijging van de graad van doctor
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ACS applied polymer materials, Sep 2, 2021
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Tectonophysics, Apr 1, 2019
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Macromolecules, Jun 27, 2006
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Springer eBooks, Sep 27, 2005
A theory of the tensile strength of oriented polymer fibres is presented. From an analysis of the... more A theory of the tensile strength of oriented polymer fibres is presented. From an analysis of the observed fracture envelope it is shown that the criterion for fracture of the fibre is either a critical shear stress or a critical shear strain. Owing to the chain orientation distribution in the fibre, the initiation of fracture is likely to occur in domains whose symmetry axes have orientation angles in the tail of this distribution. By considering the fibre as a molecular composite, the tensile strength is calculated as a function of the modulus. The results are compared to the observed values of PET, POK, cellulose II, PpPTA, PBO and PIPD fibres. In addition, the relation between the ultimate strength and the chain length distribution is investigated. By using the critical shear strain as a fracture criterion in the Eyring reduced time model, relations are derived for the fibre strength as a function of the load rate, as well as for the lifetime under constant load. Moreover, this model predicts the dependence of the strength on the temperature. The theoretical relations are compared to the experimental results on PpPTA fibres.
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Macromolecules, Mar 1, 1991
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Macromolecules, Aug 1, 1990
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Langmuir, Jul 26, 2023
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Proceedings of SPIE, Dec 16, 1993
ABSTRACT
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Journal of Testing and Evaluation, Jul 1, 2014
In this paper, refractometry is used for characterization of the adhesion between bitumen and agg... more In this paper, refractometry is used for characterization of the adhesion between bitumen and aggregates. The main purpose of this paper is to find whether preferential adsorption occurs and can be demonstrated in pure and undiluted road-construction bitumen. For this, bitumens from different crudes with different penetration grades and six bitumens with different penetration grades from the same crude are examined in a refractometer. The measurements give indices of refraction of the different bitumens at the bitumen-aggregate interface. In this paper, it is concluded that the indices of refraction are strongly related to the penetration of the bitumens. Clear preferential adsorption was not found from the refractometric measurements at this stage. It is suggested that a change in the composition of the bulk of the bitumen results in a similar change in composition at the aggregate surface.
Bookmarks Related papers MentionsView impact
High Performance Polymers, Apr 2, 2008
Liquid crystal polymers (LCPs) are known to exhibit slow and fast phase transitions upon heating ... more Liquid crystal polymers (LCPs) are known to exhibit slow and fast phase transitions upon heating and cooling. The kinetics of both types were investigated for an liquid crystal (LC) polyimide derived from pyromellitic dianhydride (PMDA) and 1,3-bis[4'-(4''-aminophenoxy) cumyl] benzene (BACB) (PMDA/BACB). The kinetics were compared with our previous results on poly(bistrifluorethoxyphosphazene) (PBFP), which also shows LC and non-LC transitions, and with nylon PA6, a non-LC reference sample. Transition kinetics were studied upon cooling using non-isothermal differential scanning calorimetry (DSC), wide angle X-ray diffraction (WAXD) and polarization microscopy. The DSC experiments were performed by cooling our samples from 400 to 100 °C using rates between 2.5 and 100 K min-1. We observed that the LC transitions show a linear dependency on the cooling rate. These transitions were solely governed by super-cooling and these observations are congruent with our earlier results on PBFP. Apparently, we may conclude that all LC transitions show this linear dependence. For PMDA/BACB, the fast LC transitions were related to the formation of a smectic phase. WAXD did not reveal a crystalline phase in PMDA/BACB. The non-LC transition was related to a thermally activated process, having an apparent activation energy, EA, of ≈ 360 kJ mol -1, indicating a re-arrangement of chains previously randomly oriented. Crystallization could proceed after cooling from the melt to room temperature followed by annealing based on a thermally activated process, with E A ≈ 160 kJ mol-1.
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Polymer, Dec 1, 2005
ABSTRACT
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Liquid Crystals, Apr 1, 2012
Liquid crystalline polymers (LCPs) are defined by their instantaneous liquid crystal phase transi... more Liquid crystalline polymers (LCPs) are defined by their instantaneous liquid crystal phase transitions, as compared with set heating/cooling rates in differential scanning calorimetry (DSC). This is well accepted for the case of main-chain (MC) LCPs. It is confirmed here for the case of two side-group (SG) LCPs as well. The DSC temperature calibration is usually based on melting of metals
Bookmarks Related papers MentionsView impact
Journal of Applied Polymer Science, Feb 14, 2011
Structure and rheological properties of hybrids with polymer matrix and layered silicates as fill... more Structure and rheological properties of hybrids with polymer matrix and layered silicates as filler were studied. Solution containing 60% wt of hydroxypropylcellulose (HPC) in oligomeric polyethyleneglycol (PEG) was used as a matrix. The peculiarity of this study is that the matrix depending on temperature can form different phase states including liquid-crystalline (LC). So, a possibility of coexistence and superposition of different ordered structures can be realized at different temperatures. Three different fillers were used: natural Na-montmorillonite (MMT) and organoclays obtained by treating MMT with surfactants varying in polarity of their molecules. Depending on the type of clay, materials with different morphology can be obtained. X-ray data showed that PEG intercalates all types of clay used whereas penetration of HPC macromolecules into clay galleries during mixing does not occur. Clay modified with more polar surfactant should be treated as the most convenient material to be intercalated by PEG. Rheological studies (included steady-state and dynamic shear properties in a wide temperature range) demonstrate that composites are viscoplastic materials and the yield stress is observed already at 5% fillers loading. The level of the yield stress depends on the phase state of the matrix being induced by the superposition of structures formed by clay particles as well as by the LC phase (if it exists). The same conclusion can be made on the base of linear oscillatory measurements because the existence of the LC phase and/or the presence of filler lead to a pseudo solid-like behavior of a system as whole. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011
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Liquid Crystals, Jul 5, 2010
ABSTRACT
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Liquid Crystals, Jul 5, 2010
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