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Major Roles of Blend Partner Fragility and Dye Placement on Component Glass Transition Temperatures: Fluorescence Study of Near-Infinitely Dilute Species in Binary Blends

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journal contribution
posted on 23.10.2012, 00:00 by Christopher M. Evans, John M. Torkelson
We measure the glass transition temperatures (Tgs) of near-infinitely dilute (0.1 wt %), well-dispersed polystyrene (PS) components in seven blend partners using intrinsic and extrinsic fluorescence. The PS Tgs span a 150 °C range when PS is dispersed in partners with Tgs of −6 to 150 °C. We calculate self-concentrations for PS (ϕselfPS) of ∼0 to 0.72 via the Lodge–McLeish model, the largest range ever reported for a blend component. Our study reveals that perturbations to PS Tg, which may be quantified by ϕselfPS calculations, correlate with partner fragility rather than partner Tg, with higher fragility partners resulting in higher ϕselfPS values. In particular, for 0.1 wt % PS in poly­(vinyl chloride) (PVC), the partner with the highest reported fragility but an intermediate Tg, the PS Tg is only weakly perturbed with ϕselfPS = 0.72. These results indicate that partner fragility plays a key role in determining the strength of Tg perturbations to a blend component. Also noteworthy is 0.1 wt % PS in poly­(4-vinylpyridine) (P4VP) where the PS Tg is slaved to that of P4VP (ϕselfPS ∼ 0). Fluorescent label location on the backbone does not significantly influence the measured Tg. However, the distance separating a label from the backbone is crucial in determining Tg and thereby ϕselfPS. In PVC, ϕselfPS decreases from 0.72 to ∼0 when a dye is attached to the PS backbone by a methyl ester or butyl ester group, respectively, indicating a highly heterogeneous environment within ∼0.45 nm of the backbone.