Anomalous dewetting dynamics in active entangled polymer films: flexible chains.
basic_science · Level V
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- Also identified by DOI 10.1039/d6sm00359a.
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Abstract
Spin-coated polymer films store molecular recoiling stress in kinetically trapped chain conformations, providing an extra driving force for dewetting beyond capillarity. I develop an analytical framework connecting this preparation-induced nonequilibrium stress, continuum viscoelastic thin-film hydrodynamics, and the activity-enhanced elasticity of active entangled polymer melts containing flexible chains. Activity generates grip forces at entanglement junctions, enhancing the elastic plateau by (1 + <i>α</i>)-where <i>α</i> is the activity parameter, growing linearly with chain length and Péclet number-and opening a fast grip-force relaxation channel on the timescale <i>τ</i><sub>g</sub> ∼ <i>L</i>, far shorter than the reptation time <i>τ</i><sub>d</sub> ∼ <i>L</i><sup>3</sup>. The molecular recoiling stress decays biexponentially: the fast grip channel releases on <i>τ</i><sub>g</sub>, while the slow channel relaxes on the segmental timescale <i>τ</i><sub>eff</sub> ≪ <i>τ</i><sub>d</sub>, set by conformational rearrangements within the spin-cast film rather than by whole-chain reptation. Dewetting hole growth in all three rim-geometry regimes obeys <i>R</i><sup><i>p</i></sup> = <i>K</i><sub>p</sub><i>Φ</i><sup>act</sup>(<i>t</i>) with the same integrated active stress function <i>Φ</i><sup>act</sup>; the early-stage small-hole regime gives exponent <i>n</i> ≈ 1, consistent with polystyrene film experiments. Four unambiguous activity signatures emerge: a biexponential decay in sequential hole nucleation velocities with a kink at <i>t</i><sub>inc</sub> ∼ <i>τ</i><sub>g</sub> and slow slope measuring <i>τ</i><sub>eff</sub>; a reversed morphological fingerprint in which both rim height and rim width are suppressed-because activity raises both the hole growth driving stress and the restoring modulus by the same factor (1 + <i>α</i>); a double maximum in the rim-width time trace above a universal activity threshold <i>α</i> > <i>e</i><sup>2</sup> ≈ 7.4, with the first peak <i>W</i><sub>max,1</sub> = [(1 + <i>α</i>)<i>τ</i><sub>g</sub>/<i>τ</i><sub>d</sub>]<sup>2/3</sup> < 1 and the second peak recovering the passive value exactly for all <i>α via</i> the algebraic identity (1 - <i>f</i>)(1 + <i>α</i>) = 1; and a shift in slip length from <i>b</i><sub>pass</sub> ∼ <i>L</i><sup>3</sup> to <i>b</i><sub>act</sub> ∼ <i>L</i><sup>2</sup>, reducing the rim width by a factor <i>L</i><sup>2/3</sup>.