Extensional flow of stiff‐chain polymer solutions in the semidilute regime

Abstract

Stiff-chain polymers are preponderant in biology as structural components of tissue and living cells. Hence, understanding their solution behavior under different flow histories is fundamental. Here, the extensional flow under capillary breakup of isotropic semidilute solutions of stiff-chain poly-n-butyl isocyanate (PBIC) and hydroxypropyl cellulose (HPC) has been studied. PBIC-chloroform and HPC-water were studied as the polymer-solvent interaction tunes the molecular conformation (Romo-Uribe, J. Appl. Polym. Sci. 2021, 138, e49712).PBIC in chloroform exhibits a rod-like conformation, and capillary breakup at constant concentration and temperature initially showed an exponential decay of filament diameter D(t) followed by a sudden decrease of D, and finally slow filament thinning. Increasing the molecular weight by threefold increased the breakup time τ b by twofold. HPC in water adopts a semiflexible conformation and the filament thinning also exhibited three stages, however the breakup time was shorter than PBIC. Hence, the extensional viscosity as a function of strain of PBIC and HPC solutions exhibited three regimes, where initial constant extensional viscosity η E was followed by strain hardening and finally a sudden decrease of extensional viscosity as strain increased. It was found that the filament breakup time τ b is a function of the radius of gyration R g .

Conformation changes in stiff‐chain poly‐n‐butyl isocyanate. The dilute regime

“…In nature, cross‐linked networks of stiff‐chain biopolymers are structural components in cytoskeletal filaments of tissue and living cells and in collagen networks, which are the core structure of bone 1–4 . There is, therefore, intensive theoretical and experimental research aiming to increase our understanding of the dynamics of stiff‐chain polymers 10–16 …”

“…In the concentrated isotropic regime, c >> c*, the macromolecular static correlations become important and the polymers tend to orient in the same direction as its neighbors. Further increase of the concentration would produce an ordered liquid crystalline phase 32 …”

“…[1][2][3][4] There is, therefore, intensive theoretical and experimental research aiming to increase our understanding of the dynamics of stiff-chain polymers. [10][11][12][13][14][15][16] Stiff chain polymers can exhibit conformational changes due to external stimuli from the natural rod-like shape to a random coil shape. This conformation transition has been observed in DNA and polypeptides, which exhibit helical conformation, and the transition is termed helix-coil.…”

“…Romo-Uribe 31,32 recently investigated the shear and extensional viscosity of PBIC solutions in the semi-dilute regime, where the shear viscosity exhibits scaling predicted for rod-like polymers. 33 These investigations also showed that the chain conformation can be tuned by the utilized solvent, and chlorinated solvents favored the most extended conformation whereas benzene promoted a semi flexible conformation.…”

“…33 These investigations also showed that the chain conformation can be tuned by the utilized solvent, and chlorinated solvents favored the most extended conformation whereas benzene promoted a semi flexible conformation. 31,32 The behavior of solutions of stiff-chain macromolecules is dependent on the concentration of the solution, and the concentration regime has been defined as dilute, semi-dilute, concentrated isotropic, and concentrated anisotropic (i.e., liquid crystalline). [31][32][33] The threshold concentration c* defines the transition from dilute (c < c*) to semi-dilute isotropic regime (c > c*), where c* = 1/[η].…”