Abstract:Geometric nanoconfinement,
in one and two dimensions, has a fundamental
influence on the segmental dynamics of polymer glass-formers and can
be markedly different from that observed in the bulk state. In this
work, with the use of dielectric spectroscopy, we have investigated
the glass transition behavior of poly(2-vinylpyridine) (P2VP) confined
within alumina nanopores and prepared as a thin film supported on
a silicon substrate. P2VP is known to exhibit strong, attractive interactions
with confining surfaces… Show more
“…Separately, there are tendencies for the chain ends, which possess more free volume than chain segments do, to segregate to the free surface and for the density of chain entanglement near the surface to be smaller than that in the bulk. ,− All of these factors can lead to enhancements in the near-surface mobility ,, and reductions in the T g of thin polymer films ( T g film ). − ,,, On the other hand, the surface chemistry ,,− and stiffness of the substrate supporting the films, and the near-substrate chain conformations , can affect the dynamics of the nearby polymer. Combinations of the free surface and substrate effects can produce a variety of thickness dependences of the thin film dynamics as found in experiments. ,,,, …”
Section: Introductionmentioning
confidence: 89%
“…Many studies involving polymer films with thicknesses ( h ) below ca. 100–1000 nm have revealed the so-called confinement effect, namely, variations in the physical properties of the films with h , including the glass-transition temperature ( T g ), − viscosity, − and elastic modulus. − The thickness dependences of these properties have generally been attributed to perturbations by the free (or air) surface and substrate interface to the mobility of the near-surface segments. ,,− At the free surface, the density of the polymer decreases rapidly to zero within a small distance of ∼1 nm . Furthermore, the average number of nearest neighbors per segment in this region is reduced by a large fraction from that in the bulk.…”
Section: Introductionmentioning
confidence: 99%
“…Combinations of the free surface and substrate effects can produce a variety of thickness dependences of the thin film dynamics as found in experiments. 7,12,24,38,39 The notable influence of the free surface on the physical properties of thin polymer films has motivated numerous efforts to measure the surface dynamics of polymers. Among these efforts, some involve the development of innovative methods to measure the surface glass-transition temperature (T g surf ).…”
Surface glass-transition temperature (T
g
surf) and
transition
width (W) within 1 nm of the surface were measured
by monitoring a qualitative change in the contact angle or density
of end-groups (by time-of-flight secondary ion mass spectrometry)
with temperature. Polystyrene (PS) films with various thicknesses
(h) and molecular weights were studied. For unannealed
PS supported on oxide-coated silicon or poly(dimethyl siloxane) with h > ∼60 nm, T
g
surf approached a plateau value
of ∼25 K below the bulk T
g; below
60 nm, T
g
surf decreased with decreasing h. Separately, W exhibited a stepwise increase when h was decreased below the radius of gyration of the polymer.
Upon thermal preannealing or deposition on a PS brush or adsorbed
layer, the films ceased to exhibit T
g
surf reductions
or stepwise change in W. We discuss how an out-of-equilibrium
density profile with a deficit near the substrate and de Gennes’
sliding mode may explain these observations.
“…Separately, there are tendencies for the chain ends, which possess more free volume than chain segments do, to segregate to the free surface and for the density of chain entanglement near the surface to be smaller than that in the bulk. ,− All of these factors can lead to enhancements in the near-surface mobility ,, and reductions in the T g of thin polymer films ( T g film ). − ,,, On the other hand, the surface chemistry ,,− and stiffness of the substrate supporting the films, and the near-substrate chain conformations , can affect the dynamics of the nearby polymer. Combinations of the free surface and substrate effects can produce a variety of thickness dependences of the thin film dynamics as found in experiments. ,,,, …”
Section: Introductionmentioning
confidence: 89%
“…Many studies involving polymer films with thicknesses ( h ) below ca. 100–1000 nm have revealed the so-called confinement effect, namely, variations in the physical properties of the films with h , including the glass-transition temperature ( T g ), − viscosity, − and elastic modulus. − The thickness dependences of these properties have generally been attributed to perturbations by the free (or air) surface and substrate interface to the mobility of the near-surface segments. ,,− At the free surface, the density of the polymer decreases rapidly to zero within a small distance of ∼1 nm . Furthermore, the average number of nearest neighbors per segment in this region is reduced by a large fraction from that in the bulk.…”
Section: Introductionmentioning
confidence: 99%
“…Combinations of the free surface and substrate effects can produce a variety of thickness dependences of the thin film dynamics as found in experiments. 7,12,24,38,39 The notable influence of the free surface on the physical properties of thin polymer films has motivated numerous efforts to measure the surface dynamics of polymers. Among these efforts, some involve the development of innovative methods to measure the surface glass-transition temperature (T g surf ).…”
Surface glass-transition temperature (T
g
surf) and
transition
width (W) within 1 nm of the surface were measured
by monitoring a qualitative change in the contact angle or density
of end-groups (by time-of-flight secondary ion mass spectrometry)
with temperature. Polystyrene (PS) films with various thicknesses
(h) and molecular weights were studied. For unannealed
PS supported on oxide-coated silicon or poly(dimethyl siloxane) with h > ∼60 nm, T
g
surf approached a plateau value
of ∼25 K below the bulk T
g; below
60 nm, T
g
surf decreased with decreasing h. Separately, W exhibited a stepwise increase when h was decreased below the radius of gyration of the polymer.
Upon thermal preannealing or deposition on a PS brush or adsorbed
layer, the films ceased to exhibit T
g
surf reductions
or stepwise change in W. We discuss how an out-of-equilibrium
density profile with a deficit near the substrate and de Gennes’
sliding mode may explain these observations.
“…The glass transition temperature, T g , is a key characteristic of polymer materials because many physical and mechanical properties change markedly when a polymer undergoes a glass transition from a rubbery state to a glassy state or vice versa. , For polymer films confined at the nanoscale, perturbations originating at the polymer/free surface and the polymer/solid interfaces often result in deviations of the measured T g from the bulk T g . − Such nanoconfinement behavior is important because it can affect the commercial application of ultrathin polymer films and may aid in understanding the underlying mechanisms of glass transition behavior, which has been a long-standing, unresolved challenge. − Indeed, the T g -confinement behavior of polymers has been the subject of intense research since 1994 when Keddie et al , first reported that, relative to bulk T g , the T g s of polystyrene (PS) and poly(methyl methacrylate) ultrathin films supported on silicon decrease and increase, respectively, with decreasing nanoscale thickness.…”
Nanoconfined polystyrene (PS) films exhibit substantial
reductions
in glass transition temperature (T
g) from
bulk T
g. By incorporating 2–6 mol
% 2-ethylhexyl acrylate (EHA) into styrene (S)-based random copolymers
and characterization via ellipsometry, we show that the T
g-confinement effect for films supported on silicon wafers
is eliminated within experimental uncertainty down to a 15 nm thickness.
Previous studies have neutralized this confinement effect by the copolymerization
of minority levels of styrene with majority levels of a comonomer
that can undergo hydrogen bonding with hydroxyl groups on a substrate
surface, thus counteracting the free-surface-based T
g reduction with a T
g increase
near the substrate interface. In contrast, the T
g-confinement effect is eliminated in our 2–6 mol %
EHA copolymers independent of the presence of substrate surface hydroxyl
groups. Thus, polymer–substrate interfacial hydrogen bonds
play no significant role in neutralizing the T
g-confinement effect in the S-based copolymers with 2–6
mol % EHA. Instead, the neutralization must come from suppressing
free-surface effects via very low levels of an EHA monomer in the
copolymer. Importantly, this simple approach to eliminate the T
g-confinement effects with as little as 2 mol
% EHA is accompanied by only a minor change in bulk T
g and no change in thermal expansivity within the experimental
error relative to PS. Furthermore, this approach cannot be generalized
to other acrylate comonomers, such as n-butyl acrylate.
It neither requires complex syntheses to achieve dense brush, bottlebrush,
or cyclic or ring polymer topologies nor the addition of a plasticizer
or a surfactant to the polymer, earlier approaches that suppressed
the T
g-confinement effect in PS. For a
99:1 mol % S/EHA copolymer, the confinement effect is nearly identical
to that of PS, indicating that a critical yet low EHA level is needed
to eliminate the effect.
“…However, when the materials are confined by their thickness, they show a lot of anomalous behavior compared to their bulk counterparts. This can include various properties such as its density, − dynamics, − crystallization, − stability, , and so forth. This is where the challenges in the practical implementation of a confined system lie.…”
The substrate roughness is a very important parameter that can influence the properties of supported thin films. In this work, we investigate the effect of surface roughness on the properties of a vapor-deposited glass (celecoxib, CXB) both in its bulk and in confined states. Using dielectric spectroscopy, we provide experimental evidence depicting a profound influence of surface roughness on the αrelaxation dynamics and the isothermal crystallization of this vapor-deposited glass. Besides, we have verified the influence of film confinement on varying values of surface roughnesses as well. At a fixed surface roughness value, the confinement could alter both the dynamics and crystallization of vapor-deposited CXB.
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