7 research outputs found
Unraveling the Dynamics of Nanoscopically Confined PVME in Thin Films of a Miscible PVME/PS Blend
Broadband
dielectric spectroscopy (BDS) was employed to investigate the glassy
dynamics of thin films (7ā200 nm) of a polyĀ(vinyl methyl ether)
(PVME)/polystyrene (PS) blend (50:50 wt %). For BDS measurements,
nanostructured capacitors (NSCs) were employed, where films are allowed
a free surface. This method was applied for film thicknesses up to
36 nm. For thicker films, samples were prepared between crossed electrode
capacitors (CECs). The relaxation spectra of the films showed multiple
processes. The first process was assigned to the Ī±-relaxation
of a bulklike layer. For films measured by NSCs, the rates of Ī±-relaxation
were higher compared to those of the bulk blend. This behavior was
related to the PVME-rich free surface layer at the polymer/air interface.
The second process was observed for all films measured by CECs (process
X) and the 36 nm film measured by NSCs (process X2). This process
was assigned to fluctuations of constraint PVME segments by PS. Its
activation energy was found to be thickness-dependent because of the
evidenced thickness dependency of the compositional heterogeneity.
Finally, a third process with an activated temperature dependence
was observed for all films measured by NSCs (process X1). It resembled
the molecular fluctuations in an adsorbed layer found for thin films
of pure PVME, and thus, it is assigned accordingly. This process undergoes
an extra confinement because of frozen adsorbed PS segments at the
polymer/substrate interface. To our knowledge, this is the first example
where confinement-induced changes were observed by BDS for blend thin
films
Unveiling the Dynamics of Self-Assembled Layers of Thin Films of Poly(vinyl methyl ether) (PVME) by Nanosized Relaxation Spectroscopy
A combination of
nanosized dielectric relaxation (BDS) and thermal spectroscopy (SHS)
was utilized to characterize the dynamics of thin films of polyĀ(vinyl
methyl ether) (PVME) (thicknesses: 7ā160 nm). For the BDS measurements,
a recently designed nanostructured electrode system is employed. A
thin film is spin-coated on an ultraflat highly conductive silicon
wafer serving as the bottom electrode. As top electrode, a highly
conductive wafer with nonconducting nanostructured SiO<sub>2</sub> nanospacers with heights of 35 or 70 nm is assembled on the bottom
electrode. This procedure results in thin supported films with a free
polymer/air interface. The BDS measurements show two relaxation processes,
which are analyzed unambiguously for thicknesses smaller than 50 nm.
The relaxation rates of both processes have different temperature
dependencies. One process coincides in its position and temperature
dependence with the glassy dynamics of bulk PVME and is ascribed to
the dynamic glass transition of a bulk-like layer in the middle of
the film. The relaxation rates were found to be thickness independent
as confirmed by SHS. Unexpectedly, the relaxation rates of the second
process obey an Arrhenius-like temperature dependence. This process
was not observed by SHS and was related to the constrained fluctuations
in a layer, which is irreversibly adsorbed at the substrate with a
heterogeneous structure. Its molecular fluctuations undergo a confinement
effect resulting in the localization of the segmental dynamics. To
our knowledge, this is the first report on the molecular dynamics
of an adsorbed layer in thin films
Molecular Mobility and Physical Aging of a Highly Permeable Glassy Polynorbornene as Revealed by Dielectric Spectroscopy
Polymeric
membranes represent a cost- and energy-efficient solution
for gas separation. Recently superglassy polymers with high free volume
outperform many conventional dense polymers in terms of gas permeability
and selectivity. However, such polymers are prone to pronounced physical
aging, resulting in a dramatic reduction in the gas permeability.
Molecular mobility of polymer segments plays an important role in
the physical aging and the gas transport performance of polymeric
membranes. Molecular mobility and physical aging of a representative
superglassy polynorbornene with very high gas permeability, PTCNSi2g,
was monitored by using dielectric spectroscopy with state-of-the-art
high-resolution analyzers. This work helps to shed some light on the
structureāproperty relationship of superglassy polymers on
a molecular level and to provide practical ādesign rulesā
for the development of high performance polymers for gas separation
In Situ Nanocalorimetric Investigations of Plasma Assisted Deposited Poly(ethylene oxide)-like Films by Specific Heat Spectroscopy
In
recent years, highly cross-linked plasma polymers have started
to unveil their potential in numerous biomedical applications in thin-film
form. However, conventional diagnostic methods often fail due to their
diverse molecular dynamics conformations. Here, glassy dynamics and
the melting transition of thin PEO-like plasma assisted deposited
(ppPEO) films (thickness 100 nm) were in situ studied by a combination
of specific heat spectroscopy, utilizing a pJ/K sensitive ac-calorimeter
chip, and composition analytical techniques. Different cross-linking
densities were obtained by different plasma powers during the deposition
of the films. Glassy dynamics were observed for all values of the
plasma power. It was found that the glassy dynamics slows down with
increasing the plasma power. Moreover, the underlying relaxation time
spectra broaden indicating that the molecular motions become more
heterogeneous with increasing plasma power. In a second set of the
experiment, the melting behavior of the ppPEO films was studied. The
melting temperature of ppPEO was found to decrease with increasing
plasma power. This was explained by a decrease of the order in the
crystals due to formation of chemical defects during the plasma process
Molecular Mobility and Gas Transport Properties of Mixed Matrix Membranes Based on PIMā1 and a Phosphinine Containing Covalent Organic Framework
Polymers with intrinsic
microporosity (PIMs) are gaining attention
as gas separation membranes. Nevertheless, they face limitations due
to their pronounced physical aging. In this study, a covalent organic
framework containing Ī»5-phosphinine moieties, CPSF-EtO,
was incorporated as a nanofiller (concentration range 0ā10
wt %) into a PIM-1 matrix forming dense films with a thickness of
ca. 100 Ī¼m. The aim of the investigation was to investigate
possible enhancements of gas transport properties and mitigating effects
on physical aging. The incorporation of the nanofiller occurred on
an nanoaggregate level with domains up to 100 nm, as observed by T-SEM
and confirmed by X-ray scattering. Moreover, the X-ray data show that
the structure of the microporous network of the PIM-1 matrix is changed
by the nanofiller. As molecular mobility is fundamental for gas transport
as well as for physical aging, the study includes dielectric investigations
of pure PIM-1 and PIM-1/CPSF-EtO mixed matrix membranes to establish
a correlation between the molecular mobility and the gas transport
properties. Using the time-lag method, the gas permeability and the
permselectivity were determined for N2, O2,
CH4, and CO2 for samples with variation in filler
content. A significant increase in the permeability of CH4 and CO2 (50% increase compared to pure PIM-1) was observed
for a concentration of 5 wt % of the nanofiller. Furthermore, the
most pronounced change in the permselectivity was found for the gas
pair CO2/N2 at a filler concentration of 7 wt
%
First Clear-Cut Experimental Evidence of a Glass Transition in a Polymer with Intrinsic Microporosity: PIMā1
Polymers
with intrinsic microporosity (PIMs) represent a novel,
innovative class of materials with great potential in various applications
from high-performance gas-separation membranes to electronic devices.
Here, for the first time, for PIM-1, as the archetypal PIM, fast scanning
calorimetry provides definitive evidence of a glass transition (<i>T</i><sub>g</sub> = 715 K, heating rate 3 Ć 10<sup>4</sup> K/s) by decoupling the time scales responsible for glass transition
and decomposition. Because the rigid molecular structure of PIM-1
prevents any conformational changes, small-scale bend and flex fluctuations
must be considered the origin of its glass transition. This result
has strong implications for the fundamental understanding of the glass
transition and for the physical aging of PIMs and other complex polymers,
both topical problems of materials science
First Clear-Cut Experimental Evidence of a Glass Transition in a Polymer with Intrinsic Microporosity: PIMā1
Polymers
with intrinsic microporosity (PIMs) represent a novel,
innovative class of materials with great potential in various applications
from high-performance gas-separation membranes to electronic devices.
Here, for the first time, for PIM-1, as the archetypal PIM, fast scanning
calorimetry provides definitive evidence of a glass transition (<i>T</i><sub>g</sub> = 715 K, heating rate 3 Ć 10<sup>4</sup> K/s) by decoupling the time scales responsible for glass transition
and decomposition. Because the rigid molecular structure of PIM-1
prevents any conformational changes, small-scale bend and flex fluctuations
must be considered the origin of its glass transition. This result
has strong implications for the fundamental understanding of the glass
transition and for the physical aging of PIMs and other complex polymers,
both topical problems of materials science