Extensional Strain Hardening Induced by $\pi \text{−}\pi$ Interactions in Barely Entangled Polymer Chains: The Curious Case of Poly(4-vinylbiphenyl)

Aromatic $\pi \text{−}\pi$ interactions between phenyl groups of adjacent chains in poly(4-vinylbiphenyl) (PVBP) have profound effects on the dynamics of this polymer. We report two unexpected nonlinear viscoelastic responses of PVBP when subjected to uniaxial flow. One is the unprecedented observation of extensional strain hardening (SH) in a barely entangled polymer melt. An even more intriguing finding is that SH of lightly (or even barely) entangled melts occurs at strain rates one order of magnitude below the coil-stretch transition predicted by Rouse theory (${\dot{ϵ}}_H=0.5/{\tau }_R$).We postulate that this behavior is due to a molecular rearrangement mechanism (supported by x-ray diffraction measurements) that involves flow-induced $\pi \text{−}\pi$ stacking of the phenyl groups, which results in an enhancement of the friction coefficient between polymer chains.

Figure 1

(a) Transient extensional viscosity during startup of extensional flow for the low PDI samples measured at $190°\mathrm{C}$ and at the indicated strain rates. (b) SHR versus ${\mathrm{Wi}}_R$ for all the samples for all the PVBP samples measured at $170°\mathrm{C}$ (open symbols) and 190° (closed symbols), and for a PS sample (with $Z=8.3$) measured at $120°\mathrm{C}$ (crosses). Inset: ${\tau }_R$ values measured at $190°\mathrm{C}$ versus $Z$.

Figure 2

Transient extensional viscosity for (a) PVBP71 measured at $190°\mathrm{C}$ and for (b) PVBP27 measured at $170°\mathrm{C}$. Solid lines are LVE predictions. The MWD of these two samples are shown in (c).

Figure 3

(a) 2D WAXS patterns of the annealed and stretched PVBP120 sample. (b) Annular averaged intensity versus azimuthal angle for the annealed and stretched sample. The averaging is performed using the data corresponding to the ring sectors spanning the $q$ ranges from 0.32 to $0.64{\AA }^{-1}$ (for the interchain peak) and from 1.25 to $1.7{\AA }^{-1}$ (for the phenyl-phenyl peaks).

Figure 4

(a) Cartoon of PVBP chains annealed and stretched showing schematically a detail of the characteristic distances measured by WAXS. (b) 1D WAXS profile of the annealed and stretched PVBP120 sample. The inset show the angular sectors used for the SD and TD averaged profiles.