Shear Flow Induced Formation And Crystalline Transformation Of Polybutene-1 Fibrillar Crystal

The flow field plays a key role in processing polymeric materials.Investigations of this problem are of both fundamental and practical importance.In particular,a deep understanding of flow-induced crystallization(FIC)is important in guiding the processing of semicrystalline polymer materials,like extrusion,injection,and blow molding processes.When the flow field is sufficiently strong,the chains of semicrystalline polymers are highly oriented and stretched,and immediately quenching these chains would lead to the formation of highly oriented fibrillar crystals that usually exhibit the shish-kebab structure.Isotactic polybutene-1(PB-1)is a semi-crystallized polyolefin possessing high mechanical performance and resistance to creep over a wide temperature range.These features make PB-1 extremely useful in highly demanding applications including feed pipes,hot water heaters and tanks,compression packaging,and so on.These characteristics are related to the polymorphic nature:form Ⅱ is kinetically favored while form Ⅰ is thermostatically more stable,and form Ⅱ crystals usually form first,and transform into form Ⅰ crystals spontaneously.The transformation usually takes weeks and is accompanied with volume shrinkage owing to the different unit cell parameters of the two crystal forms,leading to the buildup of internal stress and accordingly the undesired shape change.Therefore,it is crucial to control the transformation kinetics.This thesis places a focus on the shear-induced formation of precursors of the fibrillar crystals of form Ⅱ,and the following form Ⅱ to form Ⅰ transformation.The contents are summarized in three parts as below.The first part characterized the linear and nonlinear rheological behavior of PB-1.The linear viscoelastic(LVE)moduli were measured and the LVE master curve was constructed using the time-temperature superposition principle.The master curve was fitted to the Double reptation model and Rouse model and two fitting parameters,the entanglement molecular weight Me and chain segment relaxation time τ0,were determined.From these parameters,the reptation time τd and Rouse relaxation time τR at 125℃ were calculated,and the critical shear rates for chain orientation(～1/τd)and stretching(～1/τR),were estimated.The damping function of PB-1 upon application of step strain was examined.The stress growth coefficient θ+(t,γ)during the step shear rate was reasonably predicted based on the linear viscoelasticity and the damping function using the K-BKZ model that assumed the strain-time separability.The CoxMerz rule was further demonstrated to be valid for PB-1.The second part examined the flow-induced precursors of fibrillar crystals upon applying the strong shear flow(shear rate>1/τR).The formation of the precursors was accompanied by the growth of the transient shear stress and normal stress after the steady stress had been achieved.To illuminate this phenomenon,the stress growth was stopped at different stages,and stress relaxation measurements were followed.The sample was quenched after the stress relaxation process,and the small-angle X-ray scattering(SAXS)and wide-angle X-ray diffraction(WAXD)measurements were conducted to characterize the crystalline structure formed after the preshear and the stress relaxation processes.A combination of the stress relaxation and scattering results revealed that for the precursors formed at the early stage,the stress fully relaxed and the orientation of the subsequent crystals(formed upon quenching the sample at different stress levels)decays gradually with the relaxation.In comparison,for those precursors formed at the late stage,the stress hardly relaxes durirg～104s,and the orientation of the subsequent crystals hardly decays,suggesting that the precursors have been somehow stabilized during the shear.These features have been discussed with respect to the percolation of the precursors to form a reversible network.The third part examined the shear-induced formation of fibrillar form Ⅱ crystals and subsequent form Ⅱ to Ⅰ transformation of an isotactic polybutene-1 sample through a combination of rheology,polarized optical microscopy(POM),and small-and wideangle X-ray scattering(SAXS and WAXS)measurements.Highly oriented crystals were created by the shear flow,leading to the birefringent region shown in the POM images.The form Ⅱ to Ⅰ transformation,traced by WAXS,was greatly accelerated with an increasing shear rate.This trend was attributed to the formation of fibrillar crystals and accordingly a large amount of highly oriented chains tethered between the crystal lamellae.The stress sustained by these tethered chains facilitated the nucleation of formⅠ thereby accelerating the form Ⅱ to Ⅰ transformation.