On the equivalence between the thermodynamic and dynamic measurements of the glass transition in confined polymers
Introduction
The study of confined amorphous polymers, those with a reduced length scale in one or more dimensions, has become increasingly important because these systems enable numerous technologies in which miniaturization is paramount. As an example, thin polymer films (with thickness (h) < 100 nm) are being exploited for use as templates in microelectronics [1], active layers in photovoltaic cells [2], non-biofouling protective coatings [3], and membranes in separation technologies [4]. Polymer nanoparticles (with diameter (d) < 500 nm) are being explored for use as vehicles in drug delivery [5], components in fluorescent imaging [6], performance reinforcing additives [7], and components in photonic structures [8]. Polymer nanocomposites (with interparticle distance (ID) < 100 nm) are being engineered to enhance structural, barrier, flame resistance, electro-optical, and bactericidal properties, among many others [9], [10]. If the physical properties of polymers change due to physical confinement or interfacial effects, our understanding of such effects will be essential in assessing their potential use in nanotechnology, including the above mentioned.
The discovery, in the early 1990s, that the glass transition temperature (Tg) can deviate significantly from the bulk for nanoconfined molecular [11] and polymer [12] glass formers exposed a new aspect of glassy behavior that continues to provide challenges for fundamental understanding [13]. For the case of polymer glasses, the material of attention for this perspective, it is now recognized that systematic deviations in Tg may be observed for thin films [14], [15], [16], [17], [18], nanoparticles [19], [20] and nanocomposites [21], [22]. Deviations in Tg with confinement are generally explained to be a result of (or lack of) interfacial interactions between the polymer and the interface [12], [16], [19], [22], [23], [24], [25]. When attractive interactions between the polymer and the interface (substrate) persist, enhancements in Tg may be observed [22], [23], [24]. On the other hand, repulsive or free/soft interfaces can lead to a reduction in Tg with confinement [12], [16], [19], [25]. By tuning the interfacial interactions between the substrate and confined polymer, it is possible to systematically change Tg [24], [26]. This effect is highlighted in Fig. 1 [26]. For 10 nm thick films supported on a silica substrate, ΔTg is ~ 35 K and ~− 20 K for poly(vinyl pyridine) (P2VP) and polystyrene (PS), respectively. Nevertheless, a 70:30 P2VP–PS copolymer exhibits a Tg that is invariant with film thickness. We note, that recent work has opened to the possibility of tuning Tg by varying the adsorption degree, while keeping interfacial interactions and thickness constant [27].
How interfaces modify Tg remains an intriguing and open question [13], [28], [29]. One possible scenario is that the presence of the free surface acts to locally reduce the requirement for cooperativity of segmental dynamics, thereby reducing Tg while, in contrast, the presence of attractive polymer–substrate or polymer–nanoparticle interactions, e.g., hydrogen bonds, increases the requirement for cooperativity in the dynamics associated with the glass transition, leading to a Tg increase [23], [30], [31]. Because deviations in Tg have been reported for thin films and nanoparticles with thicknesses and diameters greater 100 nm and 400 nm, respectively, perturbations to dynamics originating from interfaces must propagate over a long length-scale (h > 10 nm), a fact that would require a completely new picture of glassy dynamics (the cooperative length scale is generally predicted to be of the order of nanometers [32]). Understanding how interfaces may influence glassy dynamics over tens-of-nanometers remains a challenge and brings into question our understanding of structural relaxation, which is believed to be localized to a few nanometers.
As it sets the practical use temperature of amorphous polymers, characterizing and understanding Tg of confined polymers are immensely important. The descriptive understanding of the glass transition, in bulk, suggests that a 10-degree change in Tg would be accompanied by an ~ 1000-fold change in molecular mobility [32]. Thus, it seems intuitive to expect that a change in Tg due to confinement necessitates a corresponding change in molecular dynamics, and that such change in dynamics may be predicted based on our understanding of the glass transition. The aim of this perspectives article is to examine whether or not segmental dynamics change in accordance with the value of Tg for confined polymers based on bulk rules. Here, we define the dynamics Tg as one assigned via the measurement of molecular dynamics, e.g., dielectric relaxation spectroscopy. The thermodynamic value of Tg is assigned based on monitoring a thermodynamic property (or proxy) as a function of temperature, e.g., calorimetry and dilatometry. We first briefly discuss the phenomenology of glass formation. Next, we highlight prior and recent findings that examined the relationship between Tg and molecular dynamics of confined polymer and an anomalous decoupling between translational (diffusion) and rotational (segmental) motion taking place in the proximity of attractive interfaces. We discuss the results within the context of our understanding of the bulk glass transition. Finally, we end with concluding remarks.
Section snippets
Dynamics description
The glass transition is a dynamic event. The viscosity of liquids above their melting temperature, Tm, generally exhibits Arrhenius temperature dependence. This behavior changes considerably once the liquid is cooled down below Tm, provided that crystallization is avoided, that is when a supercooled liquid is formed. In this case the viscosity and the relaxation time (τ) of the associated thermal fluctuations enter a regime with considerably more pronounced temperature variations. τ may vary
Highlights of past and recent observations
When polystyrene is confined to the nanometer length scale its glass transition temperature can deviate dramatically from the bulk. In fact, suppressions in the Tg of confined PS have been observed in freestanding films, supported films, nanoparticles, and nanocomposites by a range of dissimilar techniques. In essence, the influence of size on Tg of PS is near universally observed [29], [52], [53]. Are accompanying changes in molecular dynamics universally observed [13]? This question has
Discussion of observed decoupling
The previous section demonstrated that the out-of-equilibrium dynamics in terms of Tg variations in confinement couldn't be exclusively described based on a modification of the molecular mobility. Recently, Ediger and Forrest elaborated on one interpretation for the decoupling between Tg and segmental dynamics in thin films in the following manner [13]. There is a region near the free surface of thin films with enhanced segmental mobility. The thickness of the mobile layer, which is thought to
Conclusions
This perspectives article has focused on emphasizing the correlation or lack thereof between the thermodynamic and dynamic measurements of the glass transition temperature of confined polymers. Specifically, we highlighted past and recent literature contributions that undertook investigations in which simultaneous measurements of the thermodynamic and dynamic Tg were obtained. The results illustrate either a less-than-expected or lack of positive correlation between the size-dependence of Tg as
Acknowledgments
RDP acknowledge support from the National Science Foundation (NSF) Materials Research Science and Engineering Center program through the Princeton Center for Complex Materials (DMR-0819860), the NSF through a CAREER Award (DMR-1053144) and the AFOSR through a YIP Award (FA9550-12-1-0223). SN acknowledges financial support from the Defay Foundation and the funds FER of the Université Libre de Bruxelles. DC acknowledges the University of the Basque Country and Basque Country Government (Ref. No.
References (97)
- et al.
Biodegradable polymeric nanoparticles as drug delivery devices
J. Control. Release
(2001) - et al.
Polymer nanotechnology: nanocomposites
Polymer
(2008) - et al.
The glass transition of organic liquids confined to small pores
J. Non-Cryst. Solids
(1991) - et al.
Thickness and composition dependence on the glass transition temperature in thin random copolymer films
Polymer
(2004) Heterogeneity at the glass transition: a review
J. Non-Cryst. Solids
(1999)Enthalpy relaxation and recovery in amorphous materials
J. Non-Cryst. Solids
(1994)- et al.
The glass transition in thin polymer films
Adv. Colloid Interface Sci.
(2001) - et al.
Cooperative and non-cooperative dynamics in ultrathin films of polystyrene studied by dielectric spectroscopy and capacitive dilatometry
J. Non-Cryst. Solids
(2006) - et al.
Accounting for the thickness dependence of the Tg in supported ps films via the volume holes diffusion model
Thermochim. Acta
(2014) - et al.
Mobility and glass transition temperature of polymer nanospheres
Polymer
(2013)
Structural relaxation and dynamic fragility of freely standing polymer films
Polymer
Observing the twinkling fractal nature of the glass transition
J. Non-Cryst. Solids
Polymer
Enhanced physical aging of polymer nanocomposites: the key role of the area to volume ratio
Polymer
Downscaling of self-aligned, all-printed polymer thin-film transistors
Nat. Nanotechnol.
Nanoscale morphology of high-performance polymer solar cells
Nano Lett.
Highly efficient non-biofouling coating of zwitterionic polymers: poly((3-(methacryloylamino)propyl)-dimethyl(3-sulfopropyl)ammonium hydroxide)
Langmuir
Nanometric thin film membranes manufactured on square meter scale: ultra-thin films for CO2 capture
Nanotechnology
Multifunctional nanoparticles for imaging, delivery and targeting in cancer therapy
Expert Opin. Drug Deliv.
Synthesis, characterization, and application of novel polymeric nanoparticles
Macromolecules
Colloidal systems: a promising material class for tailoring sound propagation at high frequencies
J. Phys. Condens. Matter
Polymer nanocomposites containing carbon nanotubes
Macromolecules
Size-dependent depression of the glass transition temperature in polymer films
Europhysics
Dynamics near free surfaces and the glass transition in thin polymer films: a view to the future
Macromolecules
Direct measurement of molecular motion in freestanding polystyrene thin films
J. Am. Chem. Soc.
Two simultaneous mechanisms causing glass transition temperature reductions in high molecular weight free-standing films as measured by transmission ellipsometry
Phys. Rev. Lett.
Effect of free surfaces on the glass transition temperature of thin polymer films
Phys. Rev. Lett.
The distribution of glass transition temperatures in nanoscopically confined glass formers
Nat. Mater.
Glass transitions and dynamics in thin polymer films: dielectric relaxation of thin films of polystyrene
Phys. Rev. E.
Glass transition temperature of polymer nanoparticles under soft and hard confinement
Macromolecules
Fragility of an isochorically confined polymer glass
J. Phys. Chem. Lett.
Quantitative equivalence between polymer nanocomposites and thin polymer films
Nat. Mater.
Model polymer nanocomposites provide understanding of confinement effects in real nanocomposites
Nat. Mater.
Interface and surface effects on the glass transition temperature in thin polymer films
Faraday Discuss.
Effects of nanoscale confinement and interfaces on the glass transition of a series of poly(n-methacrylate) films
Aust. J. Chem.
Glass transition temperature of freely-standing films of atactic poly(methyl methacrylate)
Eur. Phys. J. E
The lifetime of the deviations from bulk behaviour in polymers confined to the nanoscale
Nat. Commun.
Confined polymer properties of nanoparticles
J. Polym. Sci. B Polym. Phys.
Effects of confinement on material behaviour at the nanometre size scale
J. Phys. Condens. Matter
Influence of a reduced mobility layer on the structural relaxation dynamics of aluminum capped ultrathin films of poly(ethylene terephthalate)
Langmuir
Interplay of surface and confinement effects on the molecular relaxation dynamics of nanoconfined poly(methyl methacrylate) chains
Eur. Phys. J. E
The Glass Transition
Das temperatur-abhangigkeitsgesetz der viskositat von flussigkeiten
Phys. Unserer Zeit
Analysis of recent measurements of the viscosity of glasses
J. Am. Ceram. Soc.
Die abhangigkeit der viskositat von der temperatur bei unterkuhlten flussigkeiten
Z. Anorg. Allg. Chem.
Physical Aging in Amorphous Polymers and Other Materials
A multiparameter approach for structural recovery of glasses and its implication for their physical properties
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