Towards a Molecular View of Glass Heterogeneity

Environmentally Sensitive Luminescence Reveals Spatial Confinement, Dynamics, and Their Molecular Weight Dependence in a Polymer Glass

Polymer glasses have an irregular structure. Among the causes for such complexity are the chemically distinct chain end groups that are the most abundant irregularities in any linear polymer. In this work, we demonstrate that chain end induced defects allow polymer glasses to create confined environments capable of hosting small emissive molecules. Using environmentally sensitive luminescent complexes, we show that the size of these confinements depends on molecular weight and can dramatically affect the photoluminescence of free or covalently bound emissive complexes. We confirm the impact of chain end confinement on the bulk glass transition in poly(methyl acrylate) (pMA) and show that commonly observed T g changes induced by the chain ends should have a structural origin. Finally, we demonstrate that the size and placement of luminescent molecular probes in pMA can dramatically affect the probe luminescence and its temperature dependence, suggesting that polymer glass is a highly irregular and complex environment, marking its difference with conventional small molecule solvents. Considering the ubiquity of luminescent glassy materials, our work lays down a blueprint for designing them with structural considerations in mind, ones where packing density and chain end size are key factors.

Color-Based Optical Detection of Glass Transitions on Microsecond Timescales Enabled by Exciplex Dynamics

Every measurement technique operates on a given timescale and measurements using emissive small molecule sensors are no exception. A family of luminescent sensors providing first optical characterization of dynamic phenomena in polymers at a timescale of several microseconds is described. This performance originates from the dynamics manifested in the excited state of the sensor molecules where diffusioncontrolled events select the emission color while radiative phenomena define the global operation timescale. Since the mechanism responsible for signal generation is confined to the short lived excited state of emissive probe, it is possible observe an unprecedented link between the timescale of sensory action and that of photoluminescence. An application of this new methodology is demonstrated by performing general, short timescale detection of glass transitions in a temperature ranges precluding the informative range of conventional techniques by tens of degrees.

Examining dynamics in a polymer matrix by single molecule fluorescence probes of different sizes

Rotational motion of single fluorescence probes with different sizes doped inside films of polyvinylacetate is investigated by defocused single molecule fluorescence microscopy. Discrete vibration modes in the rotational motion of individual probes are clearly exposed in the power spectra of the rotation trajectories, reflecting multiple relaxation dynamics and also the difference in dynamics among separate locations. The power spectra show a strong dependence on the probe size and temperature. By sampling the rotation of a large number of probes, the averaged power spectra show that the activated rotation of the probes falls into the frequency range of the α-relaxation and data analysis shows that activation of the vibration modes with all probes corresponds to the maximum population of rotating probes, and this made the comparison between single molecule data and the ensemble data meaningful (differential scanning calorimetry, as an example). Such an analysis shows a coincidence between the temperature of a significant occurrence of glass transition and the activation of rotational motion of all probes with one specific size, indicating that such a dimension correlates with the size of the cooperative rearrangement region - a volume of approximately 1.0 nm(3).

A multi-property fluorescent probe for the investigation of polymer dynamics near the glass transition

In addition to the commonly observed single molecule fluorescence intensity fluctuations due to molecular reorientation dynamics, a perylene bisimide-calixarene compound (1) shows additional on-off fluctuations due to its ability to undergo intramolecular excited state electron transfer (PET). This quenching process is turned on rather sharply when a film of poly(vinylacetate) containing 1 is heated above its glass transition temperature (T g), which indicates that the electron transfer process depends on the availability of sufficient free volume. Spatial heterogeneities cause different individual molecules to reach the electron transfer regime at different temperatures, but these heterogeneities also fluctuate in time: in the matrix above T g molecules that are mostly nonfluorescent due to PET can become fluorescent again on timescales of seconds to minutes.

The two different mechanisms for intensity fluctuation, rotation and PET, thus far only observed in compound 1, make it a unique probe for the dynamics of supercooled liquids.

Raman spectroscopy and density functional theory analyses of the melt structure in a Li2B4O7 crystal growth system

Raman spectroscopy and density functional theory methods were applied to study the Li₂B₄O₇ melt structure; a new boron–oxygen chain structure was found for the first time.