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Employing a quartz crystal microbalance (QCM) as a MHz‐viscoelastic sensor requires extracting information from higher harmonics beyond the Sauerbrey limit, which can be problematic for rubbery polymer films that are highly dissipative because of the onset of anharmonic side bands and film resonance. Data analysis for QCM can frequently obscure the underlying physics or involve approximations that tend to break down at higher harmonics. In this study, modern computational tools are leveraged to solve a continuum physics model for the QCM's acoustic shear wave propagation through a polymer film with zero approximations, retaining the physical intuition of how the experimental signal connects to the shear modulus of the material. The resulting set of three coupled equations are solved numerically to fit experimental data for the resonance frequency Δf_nand dissipation ΔΓ_nshifts as a function of harmonic numbern, over an extended harmonic range approaching film resonance. This allows the frequency‐dependent modulus of polymer films at MHz frequencies, modeled as linear on a log–log scale, to be determined for rubbery polybutadiene (PB) and polydimethylsiloxane (PDMS) films, showing excellent agreement with time–temperature shifted rheometry data from the literature.

 

We demonstrate how the temperature dependence of perylene's fluorescence emission spectrum doped in bulk polymer matrices is sensitive to the local glass transition dynamics of the surrounding polymer segments. Focusing on the first fluorescence peak, we show that the intensity ratio I Ratio ( T ) = I Peak ( T )/ I SRR between the first peak and a self referencing region (SRR) has a temperature dependence resulting from the temperature-dependent nonradiative decay pathway of the excited perylene dye that is influenced by its intermolecular collisions with the surrounding polymers segments. For different polymer matrices, poly(methyl methacrylate) (PMMA), polystyrene (PS), poly(2-vinyl pyridine) (P2VP), and polycarbonate (PC), we demonstrate that I Ratio ( T ) exhibits a transition from a non-Arrhenius behavior above the glass transition temperature T g of the polymer to an Arrhenius temperature dependence with constant activation energy E below the T g of the polymer matrix, indicating perylene's sensitivity to cooperative α-relaxation dynamics of the polymer matrix. This transition in temperature dependence allows us to identify a perylene defined local T peryleneg of the surrounding polymer matrix that agrees well with the known T g values of the polymers. We define a fluorescence intensity shift factor in analogy with the Williams–Landel–Ferry (WLF) equation and use literature WLF parameters for the polymer matrix to quantify the calibration factor c f needed to convert the fluorescence intensity ratio to the effective time scale ratio described by the conventional WLF shift factor. This work opens up a new characterization method that could be used to map the local dynamical response of the glass transition in nanoscale polymer materials using appropriate covalent attachment of perylene to polymer chains. 

Polymers exist in the glass state for a wide range of applications. The slow and limited crystallizability of polymers means that solid polymer materials are either wholly or in part glassy, giving them great importance. The glass is a nonequilibrium amorphous state that occurs because the cooperative molecular dynamics become kinetically trapped on cooling as the available thermal energy for molecular motion decreases. This article aims to provide the reader with a molecular picture of what this packing frustration that causes glass formation means for polymers. Experimental considerations for accurately measuring the glass transition temperature 𝑇𝑔 given this nonequilibrium nature will be discussed. Basic concepts underpinning theoretical efforts to model the glass transition will be summarized to provide the reader with a lexicon and paradigm for understanding different approaches used in the field to capture the main characteristics of glasses. Current research areas of interest in polymer glasses will be briefly outlined. Hopefully, this article will provide the beginning investigator a starting point for their own studies. 

Polymers are increasingly being used in applications with nanostructured morphologies where almost all polymer molecules are within a few tens to hundreds of nanometers from some interface. From nearly three decades of study on polymers in simplified nanoconfined systems such as thin films, we have come to understand property changes in these systems as arising from interfacial effects where local dynamical perturbations are propagated deeper into the material. This review provides a summary of local glass transition temperature T g changes near interfaces, comparing across different types of interfaces: free surface, substrate, liquid, and polymer–polymer. Local versus film-average properties in thin films are discussed, making connections to other related property changes, while highlighting several historically important studies. By experimental necessity, most studies are on high enough molecule weight chains to be well entangled, although aspects that connect to lower molecule weight materials are described. Emphasis is made to identify observations and open questions that have yet to be fully understood such as the evidence of long-ranged interfacial effects, finite domain size, interfacial breadth, and chain connectivity. 

Recent studies suggest chain adsorption in the melt may be responsible for a number of property changes in thin films by making correlations between the residual adsorbed layer thickness h ads ( t ) measured after a given solvent washing procedure as a function of annealing time t of the film at an elevated temperature prior to this solvent rinse. This procedure, frequently called “Guiselin's experiment”, refers to the thought experiment proposed in a 1992 theoretical treatment by Guiselin that assumed chain segments in contact with the surface are irreversibly adsorbed whereby unadsorbed chains could be washed away by solvent without disturbing the adsorbed substrate contact points in the melt. In the present work, we review this recent literature, identifying and experimentally testing a common protocol for forming adsorbed layers h ads ( t ) from solvent washing melt films. We find h ads ( t ) curves to be far less reproducible and reliable than implied in the literature, strongly dependent on solvent washing and substrate cleaning conditions, and annealing at elevated temperatures is unnecessary as densification of films sitting at room temperature makes the glassy film harder to wash off, leaving behind h ads of comparable thickness. This review also summarizes literature understanding developed over several decades of study on polymer adsorption in solution, which experimentally demonstrated that polymer chains in solution are highly mobile, diffusing and exchanging on the surface even in the limit of strong adsorption, contradicting Guiselin's assumption. Preformed adsorbed layers of different thicknesses h ads are shown to not affect the average glass transition temperature or physical aging of 30 nm thick films. In summary, a number of open questions and implications are discussed related to thin films and polymer nanocomposites.