Molecular weight dependence of the intrinsic size effect on T g in AAO template-supported polymer nanorods: A DSC study

Isaacson

et al. 2019

Adv Funct Materials

Polymer confinement is realized in hybrid nanocomposites where individual polymer molecules are confined by a nanoporous matrix to dimensions less than the molecular size of the polymer. Here it is shown that by functionalizing the interior pore surfaces of a nanoporous organosilicate matrix, the pores can be filled with polystyrene molecules to achieve extreme levels of molecular confinement not previously possible. This provides opportunities for unique thermal and mechanical properties. It is shown that pore surface functionalization markedly impacts the polymer mobility during polymer infiltration by affecting the polymer-pore surface interaction, addressing the challenge of filling high-molecular-weight polymer molecules into nanoscaleconfined spaces. This allows for achieving extreme levels of molecular confinement with the loss of interchain entanglement and extensive polymer elongation along the pore axis. The glass transition temperature of the polymer is suppressed compared to bulk polymer melt, and is significantly affected by the polymer-surface interaction, which changes the polymer segmental mobility. The polymer-surface interaction also affects the interfacial polymer-pore sliding shear stress during polymer pullout from the nanopores, markedly affecting the fracture resistance of the nanocomposite.of the polymer, which has been shown to reduce entanglements, [4] form ordered morphology, [5] influence T g , [6] improve polymer diffusivity, [7] and enhance fracture resistance of the nanocomposites. [8] A fundamental limit of these studies, however, has been to achieve extreme levels of molecular confinement and the ability to control the interfacial interaction of the confined polymer with the matrix pores.Here we demonstrate that pore surface chemical functionalization allows us to achieve extreme levels of molecular confinement and manipulate the nanocomposite thermal and mechanical properties by controlling the interaction of the polymer with the pore surface (Figure 1). We show that the polymer filling rate can be significantly improved through selection of appropriate surface chemistry, which enables filling polymers with unprecedentedly high molecular weight (M w ) into the porous matrix to create extreme levels of molecular confinement not previously possible. This induces polymer hyperconfinement [[8a]] in which polymer chains entirely lose interchain entanglements and elongate extensively along the pore axis. [9] We also show that T g of the polymer is decreased by hyperconfinement, and can be significantly affected by tuning the polymer-surface interaction to impact the polymer segmental mobility.In addition, we show that the polymer-surface interaction affects the fracture of the nanocomposites where molecular bridging that involves the pullout and stretching of individual confined polymer chains from the nanopores leads to a marked increase in fracture resistance. The surface chemical functionalization significantly changes the sliding shear stress at the polymer/pore surface interfac...

Section: Resultssupporting

confidence: 91%

Section: Resultssupporting

confidence: 90%

Section: Introductionmentioning

confidence: 99%

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Surface Chemical Functionalization to Achieve Extreme Levels of Molecular Confinement in Hybrid Nanocomposites

Isaacson

et al. 2019

Adv Funct Materials

J Polym Sci B Polym Phys

“…However, several recent studies have reported differences in confinement behavior associated with chain length, stiffness, entanglements, and chain architecture that are unexpected given the primarily enthalpic understanding developed over the years. This suggests that entropic effects may also play a modifying role in understanding confinement effects.…”

Section: Introductionmentioning

confidence: 99%

Unexpected Molecular Weight Dependence to the Physical Aging of Thin Polystyrene Films Present at Ultra‐High Molecular Weights

Thees

Roth

2019

The physical aging behavior, time-dependent densification, of thin polystyrene (PS) films supported on silicon are investigated using ellipsometry for a large range of molecular weights (MWs) from M w = 97 to 10,100 kg mol −1 . We report an unexpected MW dependence to the physical aging rate of h < 80-nm thick films not present in bulk films, where samples made from ultra-high MWs ≥ 6500 kg mol −1 exhibit on average a 45% faster aging response at an aging temperature of 40 C compared with equivalent films made from (merely) high MWs ≤ 3500 kg mol −1 . This MW-dependent difference in physical aging response indicates that the breadth of the gradient in dynamics originating from the free surface in these thin films is diminished for films of ultra-high MW PS. In contrast, measures of the film-average glass transition temperature T g (h) and effective average film density (molecular packing) show no corresponding change for the same range of film thicknesses, suggesting physical aging may be more sensitive to differences in dynamical gradients. These results contribute to growing literature reports signaling that chain connectivity and entropy play a subtle, but important role in how glassy dynamics are propagated from interfaces.

Macro Chemistry & Physics

“…Ellipsometry, capillary dilatometry, and X‐ray reflectivity require relatively flat, continuous films to accurately measure T g . While DSC has been used to study 2D‐confined (where two dimensions are restricted to the nanoscale) polymer systems through anodized aluminum oxide (AAO) template‐supported nanorods and nanotubes, sample geometry and interfacial interactions were somewhat limited by template constraints. Additionally, these techniques do not enable location‐specific measurements in the presence of a free surface without costly and time‐intensive scattering and reflectivity studies.…”

Section: Fluorescence To Measure Tg and Physical Aging In Single‐compmentioning

confidence: 99%

21st Century Advances in Fluorescence Techniques to Characterize Glass‐Forming Polymers at the Nanoscale

Burroughs

Christie

Gray

et al. 2017

properties. [9,10] While fluorescence has been used extensively to characterize bulk properties of polymers, including glass transition temperature (T g ) [11][12][13] and polymerization conversion, [13] it is particularly well-suited to study polymers at the nanoscale. This is largely due to the fact that fluorescence techniques enable sensitive and location-specific measurements with high resolution.In this review, we focus on the use of both steady-state and transient fluorescence techniques to characterize physical properties of polymers over nanoscale dimensions. This length scale can refer to film thickness in single-component polymer films, interparticle spacing in nanocomposites, domain spacing in phaseseparated block copolymers, and distance from the polymer-polymer interface in multicomponent immiscible blends. Several geometries characterized via fluorescence are shown in Figure 1. Prior to delving into specific contributions, the basic principles and techniques of fluorescence are presented as well as a brief introduction to the behavior of confined polymers. We then present recent advances in fluorescence techniques for polymer physical characterization under confinement with regards to the glass transition temperature, physical aging, mobility and diffusion, and mechanical response. Through this examination of the literature, we illustrate the unique ability of fluorescence to characterize polymers in confined and complex geometries, providing insights into the influences of interfaces and nanostructure on material properties through sensitive, location-specific measurements. We conclude with a view toward future areas of research in which fluorescence has the potential to be impactful. Introduction to FluorescenceWhen a molecule absorbs energy via light, an electron is excited to a higher energy state, returning to the ground state through either radiative or nonradiative deactivation. This electronic excitation and return to the ground state is a combination of three photophysical processes: absorption, luminescence, and nonradiative decay. [33] Fluorescence is a luminescent process in which the excited-state electron returns to the ground state by emitting a photon, i.e., light. As shown in a simplified Jablonski Polymer CharacterizationCharacterization of polymers at the nanometer-length scale has become increasingly important with the growth and expansion of nanotechnology. Due to limitations of sensitivity and specificity that persist with traditional materials characterization techniques, there is a growing need to develop new tools to measure the properties of confined polymer systems. Within the past 20 years, fluorescence characterization techniques have emerged to address this challenge. This review focuses on the employment of fluorescence techniques such as temperature-and time-dependent steady-state intensity, fluorescence recovery after photobleaching, and nonradiative energy transfer to study polymer behavior at the nanoscale. Properties discussed include glass transition temperatur...