Extensional flow-induced crystallization of isotactic poly-1-butene using a filament stretching rheometer

Abstract

A filament stretching rheometer is used to investigate the extensional flow-induced crystallization of two commercial grade isotactic poly-1-butene samples. The degree of crystallinity of the stretched fibers is quantified using differential scanning calorimetry measurements as a function of extension rate and accumulated Hencky strains. All the measurements are performed using the Janeschitz-Kriegel protocol. The samples are first melted to erase their thermal and mechanical history. They are then quickly quenched to T = 98°C after which the stretch is imposed. The deformed filament is then allowed to crystallize fully at T = 98°C. The extensional rheology of both the samples shows only minimal strain hardening. For the case of the lower molecular weight sample, the percent crystallinity increases from 46% under quiescent conditions to a maximum of 63% at an extension rate of = 0.05 s −1. This corresponds to an increase of nearly 50% above the quiescent case. The high molecular weight sample shows similar trends achieving an increase in crystallinity of 25%. The experiments show an optimal extension rate for which the extensional flow has the maximum impact on the polymer crystallinity. The percent crystallinity of both the samples is observed to increase with increasing strain for a fixed extension rate. Small angle X-ray scattering shows that the observed increase in crystallinity is likely due to the increasing orientation and alignment of the polymer chains in extensional flows which enhances the thread-like precursors responsible for the formation of the crystals in the shish-kebab morphology.

Extensional-flow-induced crystallization of isotactic polypropylene

“…However, for polydisperse systems, shish-kebab morphologies can be generated if the specific work imposed by the flow is large enough to deform the higher molecular weight fractions of the melt (Mykhaylyk et al 2010). This was recently observed in extensional flows by Chellamuthu et al (2011) for a polydisperse poly-1-butene sample that developed a shish-kebab morphology even for extension rates less than one over the Rouse time,ε < 1/λ R .…”

“…As a result, the strain requirement for flow-induced crystallization is less in extension than it is in shear (Sentmanat et al 2010). Even though extensional flows are able to generate greater molecular alignment than shear flows (Hadinata et al 2007) and therefore more effectively enhance the resulting crystallization kinetics and morphological changes, very few studies have systematically investigated the effect of pure extensional flows on crystallization (Chellamuthu et al 2011;Hadinata et al 2007;Sentmanat et al 2010).…”

Flow-induced crystallization studied in the RheoDSC device: Quantifying the importance of edge effects

“…Hence, the values reported here are in the order of what is expected. The total heat release decreases with increasing shear rate, from 25 J/g at the lowest shear rate to 18 J/g at the highest shear rate, whereas Chellamuthu et al observed that total crystallinity increased with increasing flow strength (Chellamuthu et al 2011). Possibly, there is some heat leakage into the rotor of the rheometer, which becomes more noticeable at faster crystallization rates.…”

“…4a. The heat of fusion of the form II modification of poly(1-butene) was reported to be 58-65 J/g (Alfonso et al 2001), and crystallinity for melt-crystallized poly(1-butene) samples was reported to be in the order of 40 % (Chellamuthu et al 2011;Azzurri 2003). Hence, the values reported here are in the order of what is expected.…”

Polymeric Drift Control Adjuvants for Agricultural Spraying

“…Here ̇ is the strain rate applied to the polymer solution, with the coil-stretch transition believed to occur for flows at Wi = 0.5. [72][73][74] The longest relaxation time can be calculated from both Rouse (λ ~ Mw 2 ) and Zimm (λ ~ Mw 1.5 ) models. [75] Both of these models, like Equation (5), point to a decrease in the critical strain rate for coil-stretch transition with an increase in polymer molecular weight.…”