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The microscopic diffusion and the low frequency density of states (VDOS) of PIM-EATB(CH3) are investigated by inelastic and quasi-elastic neutron scattering where also the demethylated counterpart of PIM-EA-TB(H2) is considered. These intrinsic microporous polymers are characterized by large BET surface area values of several hundred m2/g and pore sizes between 0.5 and 2 nm. Detailed comparison is made to the archetype of polymers of intrinsic microporosity, PIM-1, and polynorbornenes also bearing a microporosity. Due to the wavelength of neutrons, the diffusion and vibrations can be addressed on microscopic length and time scales. From the inelastic neutron scattering experiments the low frequency density of states (VDOS) is estimated which shows excess contributions to the Debye-type VDOS known as Boson peak. It was found that the maximum frequency of the Boson peak decreases with increasing microporosity characterized by the BET surface area. However, besides the BET surface area, additional factors such as the backbone stiffness govern the maximum frequency of the Boson peak. Further the mean squared displacement related to microscopic motions was estimated from elastic fixed window scans. At temperatures above 175 K, the mean squared displacement PIM-EA-TB(CH3) is higher than that for the demethylated counterpart PIM-EA-TB(H2). The additional contribution found for PIM-EATB(CH3) is ascribed to the rotation of the methyl group in this polymer because the only difference between the two structures is that PIM-EA-TB(CH3) has methyl groups where PIM-EA-TB(H2) has none. A detailed comparison of the molecular dynamics is also made to that of PIM-1 and the microporous polynorbornene PTCNSi1. The manuscript focuses on the importance of vibrations and the localized molecular mobility characterized by the microscopic diffusion on the gas Transport in polymeric separation membranes. In the frame of the random gate model localized fluctuations can open or close bottlenecks between pores to enable the diffusion of gas molecules.
We report the results of a Versailles Project on Advanced Materials and Standards interlaboratory study on the intensity scale calibration of x-ray photoelectron spectrometers using low-density polyethylene (LDPE) as an alternative material to gold, silver, and copper. An improved set of LDPE reference spectra, corrected for different instrument geometries using a quartz-monochromated Al Kα x-ray source, was developed using data provided by participants in this study. Using
these new reference spectra, a transmission function was calculated for each dataset that participants provided. When compared to a similar calibration procedure using the NPL reference spectra for gold, the LDPE intensity calibration method achieves an absolute offset of ∼3.0% and a systematic deviation of ±6.5% on average across all participants. For spectra recorded at high pass energies (≥90 eV), values of absolute offset and systematic deviation are ∼5.8% and ±5.7%, respectively, whereas for spectra collected at lower pass energies (<90 eV), values of absolute offset and systematic deviation are ∼4.9% and ±8.8%, respectively; low pass energy spectra perform worse than the global average, in terms of systematic deviations, due to diminished count rates and signal-to-noise ratio. Differences in absolute offset are attributed to the surface roughness of the LDPE induced by sample preparation. We further assess the usability of LDPE as a secondary reference material and comment on its performance in the presence of issues such as variable dark noise, x-ray warm up times, inaccuracy at low count rates, and underlying spectrometer problems. In response to participant feedback and the results of the study, we provide an updated LDPE intensity calibration protocol to address the issues highlighted in the interlaboratory study. We also comment on the lack of implementation of a consistent and traceable intensity calibration method across the community of x-ray photoelectron spectroscopy (XPS) users and, therefore, propose a route to achieving this with the assistance of instrument manufacturers, metrology laboratories, and experts leading to an international standard for XPS intensity scale calibration.
The lead authors failed to name two collaborators as co-authors. The authors listed should include:
Miss Claudia L. Compean-Gonzalez (ORCID:
0000-0002-2367-8450) and Dr. Giacomo Ceccone (ORCID:
0000-0003-4637-0771).
These co-authors participated in VAMAS project A27, provided data that were analyzed and presented in this publication (and supporting information), and reviewed the manuscript before submission.
The glassy dynamics of poly(propylene glycol) (PPG) and poly(dimethyl siloxane) (PDMS) confined to a nanoporous host system revealed by dielectric spectroscopy, temperature-modulated DSC and neutron scattering is compared. For both systems the relaxation rates estimated from dielectric spectroscopy and temperature-modulated DSC agree quantitatively indicating that both experiments sense the glass transition. For PPG the segmental dynamics is determined by a counterbalance of adsorption and confinement effect. The former results form an interaction of the confined macromolecules with the internal surfaces. A confinement effect originates from an inherent length scale on which the underlying molecular motions take place. The increment of the specific-heat capacity at the glass transition vanishes at a finite length scale of 1.8 nm. Both results support the conception that a characteristic length scale is relevant for glassy dynamics. For PDMS only a confinement effect is observed which is much stronger than that for PPG. Down to a pore size of 7.5 nm, the temperature dependence of the relaxation times follows the Vogel-Fulcher-Tammann dependence. At a pore size of 5 nm this changes to an Arrhenius-like behaviour with a low activation energy. At the same pore size vanishes for PDMS. Quasielastic neutron scattering experiments reveal that also the diffusive character of the relevant molecular motions --found to be characteristic above the glass transition-- seems to disappear at this length scale. These results gives further strong support that the glass transition has to be characterised by an inherent length scale of the relevant molecular motions.
The effect of a nanometer confinement on the molecular dynamics of poly(methyl phenyl siloxane) (PMPS) was studied by dielectric spectroscopy (DK), temperature modulated DSC (TMDSC) and neutron scattering (NS). DK and TMDSC experiments show that for PMPS in 7.5 nm pores the molecular dynamics is faster than in the bulk which originates from an inherent length scale of the underlying molecular motions. At a pore size of 5 nm the temperature dependence of the relaxations times changes from a Vogel-Fulcher-Tammann like behavior to an Arrhenius one. At the same pore size Dcp vanishes. These results give strong support that the glass transition has to be characterized by an inherent length scale of the relevant molecular motions. Quasielastic neutron scattering experiments reveal a strong change even in the microscopic dynamic.
The glassy dynamics of poly(propylene glycol) (PPG) and poly(methyl phenyl siloxane) (PMPS) confined to nanoporous glasses (pore sizes 2.520 nm) investigated by dielectric spectroscopy, temperature modulated DSC and neutron scattering is compared. For both systems the relaxation rates estimated from dielectric spectroscopy and temperature modulated DSC agree quantitatively indicating that both experiments sense the glass transition.
For PPG the glassy dynamics in nanopores is determined by a counterbalance of an adsorption and a confinement effect where the temperature dependence of the relaxation times obeys the Vogel/Fulcher/Tammann (VFT-) equation. The former effect results from an interaction of the confined macromolecules with the internal surfaces which in general slows down the molecular dynamics. A confinement effect leads to an acceleration of the segmental dynamics compared to the bulk state and points to an inherent length scale on which the glassy dynamics takes place. The step of the specific heat capacity Deltacp at the glass transition vanishes at a finite length scale of 1.8 nm. This result supports further the conception that a characteristic length scale is relevant for glassy dynamics.
For PMPS down to a pore size of 7.5 nm the temperature dependence of the relaxation times follows the VFT-dependence and a confinement effect is observed like for PPG. At a pore size of 5 nm this changes to an Arrhenius-like behavior with a low activation energy. At the same pore size Deltacp vanishes for PMPS. This points to a dramatic change in the character of molecular motions responsible for glassy dynamics and supports further the relevance of a characteristic length scale on which it takes place.
Quasielastic neutron scattering experiments on PMPS reveal that the microscopic dynamics characterized by the mean square displacement depends on confinement above the glass transition. The diffusive character of the relevant molecular motions seems to disappear at a length scale of about 1.6 nm.
Inelastic neutron scattering for investigating the dynamics of confined glass-forming liquids
(2005)
Inelastic neutron scattering was employed over recent years to investigate the influence of spatial confinement on the dynamics of glass-forming systems. We review the common phenomena observed by neutron scattering in such different confining hosts like porous glasses, molecular sieves, clays or free standing polymer films, which impose a spatial limitation to the motion of small organic molecules, oligomers or polymers. Near the glass transition temperature the mean squared displacements of the confined molecules show clear deviations from the bulk behavior. The observed increase or decrease of the mean squared displacements confirms the high relevance of the interface interaction near walls of confining media without excluding additional real confinement effects. We show a new comparison of the mean squared displacement for PDMS and PMPS in bulk and in different type of restricting geometries, which evidence a weak influence of the restricting geometry on the local methyl group motion, but a strong influence on the glass transition dynamics, if wall interactions are taken into account. Strong wall interaction is also supported by the intermediate scattering function, measured either by combining neutron backscattering and time-of-flight experiments to cover 3 decades in time from ns to ps or by neutron spin echo, which reveal above Tg an increasing elastic fraction with decreasing pore size and a slowing down of the dynamics. Furthermore we show that a reduction of modes below the Boson peak frequency is a more general feature of confined glass-forming systems.
Dielectric spectroscopy in combination with temperature modulated differential scanning calorimetry and quasielastic/inelastic neutron scattering are employed to investigate the molecular (glassy) dynamics of poly(dimethyl siloxane) (PDMS) and poly(methyl phenyl siloxane) (PMPS) confined to random nanoporous glasses with nominal pore sizes between 2.5 nm and 20 nm. Inside the pores PDMS and PMPS have faster molecular dynamics than in the bulk state. Down to a pore size of 7.5 nm the temperature dependence of the relaxation times (or rates) obeys the Vogel/Fulcher/Tammann (VFT) equation where the data obtained from dielectric and thermal spectroscopy agree quantitatively. At a pore size of 5 nm this VFT-like temperature dependence changes to an Arrhenius behavior. At the same confining length scale the increment of the specific heat capacity at Tg normalized to the weight of the confined polymer vanishes. The results indicate that a minimal length scale seems to be relevant for glassy dynamics in both polymers although the estimated length scale of about 5 nm seems to a bit too large in comparison to other experimental results and theoretical approaches. Neutron scattering is employed to investigate methyl group reorientation and the fast segmental dynamics of both polymers in confinement. Although the methyl group rotation is a localized process these experiments show that a part of the methyl groups is immobilized by the confinement whereas the effects for PDMS are much more pronounced than for PMPS. With regard to the segmental dynamics, neutron scattering reveals a big difference in the behavior of both polymers. Whereas the data obtained for PMPS are in accord with a boundary layer formed at the surfaces of the nanopores, for PDMS a considerable amount of elastic scattering is observed. To explain this result it is assumed that some structure formation of PDMS takes place in the nanopores, although the thermal data show no crystallization or melting effects.
In this article, results on the microscopic dynamics of a low-molecular glass former and a polymer confined in nanoporous silica obtained by inelastic neutron scattering are presented. By combining time-of-flight spectroscopy and backscattering spectroscopy we are able to cover a large dynamical range, from the low-frequency vibrations to the relaxation. The most prominent effect was observed on the boson peak in the vibrational spectrum. A strong reduction of the modes at lowest frequencies could be observed in both cases. The main effect on the relaxation is a broadening for the salol system. For the polymeric system a difference of the confinement influence on the methyl-group rotation and the relaxation can be detected from a first qualitative interpretation of the data.