Flow-induced Precursors Research Articles

Flow-induced crystalline precursors in entangled Poly(vinyl alcohol) aqueous solutions

Flow-induced formation of crystalline precursors was systematically investigated for entangled poly (vinyl alcohol) (PVA) aqueous solutions. The preshear flow was applied at 25 °C, a temperature well above the crystallization temperature. The crystalline precursors formed when the Weissenberg number (WiR=γ˙τR, defined with respect to the Rouse time) became larger than one such that polymer chains are stretched. The precursors lead to an additional relaxation process that was much slower than the chain relaxation process in the linear viscoelasticity. The precursors were very stable at 25 °C, which could be effectively smeared by application of a secondary shear flow that was much slower than the preshear, but faster than the inverse of a characteristic time scale of the slow process (arising from the crystalline precursors). During the secondary shear flow, the stress growth coefficient shows a strong stress overshoot, indicating that the flow-induced crystalline precursors had a structure akin to the associative network, probably owing to the disruption of the protective hydration shell around PVA molecules by the preshear flow, allowing the interchain hydrogen bonds to form.

Delay of re-entanglement kinetics by shear-induced nucleation precursors in isotactic polypropylene melt

Rheological measurements were carried out to in-situ monitor the re-entanglement process of large amplitude oscillatory shear flow (LAOS) modified partially disentangled isotactic polypropylene (i-PP) melt. The characteristic entanglement recovery time (τent) was essentially longer than the average reptation time (τD) of fully entangled melt. At temperatures above 200 °C, the Arrhenius temperature dependence of τent and τD are the same, displaying a normal chain diffusion-controlled mechanism with flow activation energy of 41.4 kJ/mol. However, when the temperature is lower than 200 °C, τent is unexpectedly longer than expectation extrapolated from high temperatures, and a much higher activation energy of 98.6 kJ/mol is observed for the re-entanglement process. Based on our understanding, the re-entanglement process of shear-induced disentanglements in i-PP melt was postponed by the existence of quasi-ordered clusters created by shear at relatively low temperatures and their dissipation turns to be the rate-determining step for the whole recovery. Moreover, morphological study under polarized optical microscope showing much higher nucleation density which originates from the flow-induced precursors corroborates our hypothesis. Meanwhile, the surviving time of LAOS-generated nucleation precursors is comparable to the recovery time of disentangled chains, suggesting that the delay effect of nucleation precursors on chain diffusion may last over the entire re-entanglement process. Notwithstanding the obstruction of “extra” physical entanglements for chain diffusion, the molecular weight (Mw) dependence of entanglement recovery still follows the reptation model-based power law: τent=(a+b)Mw3.4, where a and b is the contribution of chain diffusion and dissipation of precursors, respectively.

Shear-Induced Isotropic-Nematic Transition in Poly(ether ether ketone) Melts.

In a previous work on a poly(ether ether ketone) (PEEK) melt, above its nominal melting temperature (Tm ≅ 335 °C), a severe Cox-Merz rule failure was observed. The abrupt decrease in the apparent shear viscosity was ascribed to the formation of flow-induced crystallization precursors. Here shear rheology and reflection polariscope experiments are utilized to unravel the structural changes occurring under shear on a similar PEEK melt above Tm. Three regimes of the flow curve were identified from low (0.01 s-1) to high shear rates (1000 s-1): (I) an isotropic structure with weak birefringence due to polymer chain orientation and mild shear thinning for γ̇ < 1 s-1, (II) an isotropic-nematic transition accompanied by strong birefringence, two steady-state viscosities, and large nematic polydomain director fluctuations, and (III) shear-thinning behavior with an η ∼ γ̇-0.5 dependence for γ̇ > 20 s-1, typically found in nematic fluids. The findings reported in this experimental work suggest that the nematic phase may represent the early stage of the formation of shear-induced crystallization precursors.

Quiescent and flow-induced crystallization in polyamide 12/cellulose nanocrystal composites

Understanding the crystallization kinetics and microstructure that result after an imposed shear flow in an additive-containing polymeric system is imperative for the development of robust polymer composites suitable for advanced engineering applications. Both nucleating agents and flow accelerate the crystallization kinetics as well as alter the ultimate polymer microstructure. During melt processing, polymers are subject to shear flow prior to the solidification of the melt. As an unsheared baseline in this study, the addition of 5 wt % cellulose nanocrystals (CNC) into quiescent polyamide 12 (PA 12) revealed that CNCs act as a natural nucleating agent to the quiescent PA 12 during slow cooling and at high temperatures, as measured by standard differential scanning calorimetry. To evaluate the role of shear work in promoting flow-induced crystallization, both neat and PA 12/CNC composite were subjected to known amounts of shear work. Then fast scanning calorimetry was used to differentiate the nucleation activity from both flow-induced precursors and CNC particles during isothermal crystallization across a wide temperature range. It was found that the addition of the CNC accelerated crystallization in the heterogeneous nucleation regime (T > 100 °C) in the quiescent material. With the addition of shear, the neat system displayed a reduced crystallization peak time with increasing shear history. In the nanocomposite system, the CNCs are an extremely efficient nucleating agent, achieving a saturating limit for nucleation of crystallization, such that shear was only a factor at low supercooling, specifically at temperatures greater than 140 °C.

Flow-Induced Precursor Formation of Poly(l-lactic acid) under Pressure.

For the first time, the influences of two inevitable processing fields (pressure and flow fields) on the crystallization of a semirigid molecular chain polymer, that is, poly(l-lactic acid) (PLLA), were explored using a homemade pressuring and shearing device. The results reveal that the shear rate facilitated the generation of precursor because it induced oriented segment formation. It was found that the most sensitive shear temperature for the generation of PLLA precursor under 100 MPa was 180 °C. When the shear temperature was higher (e.g., 190 °C), the relaxation of shear-induced oriented segments was too quick to induce the generation of PLLA precursor. Oppositely, at a lower shear temperature (170 °C), the oriented segments were hard to relax within the whole shear rate range (3.1–31.4 s–1). Annealing treatment was infaust to the PLLA precursor formation because it promoted the relaxation of oriented segments. Different from the shear and annealing, pressure played a more complicated role in the formation of PLLA precursor. Pressure decreased the free volume between PLLA molecular chains and meantime increased the supercooling of PLLA melt. In addition, PLLA chains tended to form locally oriented segment bundles to adapt to the pressurized state, which facilitated the formation of PLLA precursor and the following crystallization process. These two factors lowered the movability of PLLA chains and suppressed the relaxation of chain, so shear-induced orientation facilitated PLLA precursor formation under pressure. In that case, pressure and shear flow showed a synergetic promoting effect on the generation of PLLA precursor and the following crystallization process. These meaningful results could be helpful for comprehending the relationship between crystallization conditions and the crystallization behavior of PLLA and thus would provide guidance to fabricating the final products through controlling the crystallization process of PLLA.

Two Distinct Morphologies for Semicrystalline Isotactic Polypropylene Crystallized after Shear Flow

Application of shear flow to molten, highly isotactic polypropylene (iPP) results in two different morphology transitions: (1) above a certain shear rate but below a critical shear stress σ*, flow-induced precursors nucleate many small crystallites; (2) for shear stress above σ*, shish precursors nucleate highly oriented shish-kebab morphology. Herein we study flow-induced crystallization (FIC) in iPP with different molecular weights, using rotational and capillary rheometry. Since precursors created by shear are quite stable, we can also use differential scanning calorimetry (DSC) and polarized optical microscopy (POM) to study crystallization, melting, and morphology of iPP samples with different shear histories. Above a critical shear rate (inverse of long-chain relaxation time 1/τ), the onset of crystallization on cooling shifts to higher temperatures compared to unsheared samples. POM micrographs see a clear border between the regions affected by FIC (with γ̇ > 1/τ) and regions crystallizing as though they had not been sheared. FIC results in much smaller crystallites, so-called rice grains of order 1 μm in size. Above a critical shear stress (σ* ∼ 0.11 MPa) in the rotational rheometer, the morphology transitions to a shish-kebab structure. Shish appear in micrographs as highly aligned birefringent regions; in DSC, flow-induced shish further accelerate the onset of crystallization. In the rheometer, sheared samples with σ < σ* at 170 °C (above Tm) behave as a viscoelastic liquid identical to unsheared samples, whereas strongly sheared samples with σ > σ* behave as weak gels, revealing the presence of a percolating network of shish. In capillary rheometry, samples sheared above this threshold stress likewise show an abrupt increase in apparent viscosity.

Stability of flow-induced precursors in poly-1-butene and copolymer of 1-butene and ethylene

Intervals of shear flow can induce the formation of crystallization precursors, thereby facilitating crystallization of semicrystalline polymers. This effect can be gradually removed through annealing the presheared samples at high temperature. This study compares the flow-induced crystallization and annealing behavior of a poly-1-butene (PB) sample and a 1-butene/ethylene copolymer (PBE) having a similar molecular weight and polydispersity, with the latter containing 9.88 mol. % ethylene comonomers. For the PB sample, intervals of steady shear flow accelerated the isothermal crystallization that followed, as detected by linear viscoelastic time sweep measurement, and annealing the presheared sample at sufficiently high T effectively removed this acceleration effect. In contrast, for the PBE sample, annealing the presheared sample showed three regimes in which crystallization time, as defined by linear viscoelasticity, first increased, then decreased, and finally increased again with the increase in the annealing temperature. A decrease of crystallization time with increased annealing temperature is quite counterintuitive. In situ X-ray scattering suggested that a cocrystallization of form I′ and form II crystals and a change in their ratio with annealing temperature should account for the three-regime annealing behavior.Intervals of shear flow can induce the formation of crystallization precursors, thereby facilitating crystallization of semicrystalline polymers. This effect can be gradually removed through annealing the presheared samples at high temperature. This study compares the flow-induced crystallization and annealing behavior of a poly-1-butene (PB) sample and a 1-butene/ethylene copolymer (PBE) having a similar molecular weight and polydispersity, with the latter containing 9.88 mol. % ethylene comonomers. For the PB sample, intervals of steady shear flow accelerated the isothermal crystallization that followed, as detected by linear viscoelastic time sweep measurement, and annealing the presheared sample at sufficiently high T effectively removed this acceleration effect. In contrast, for the PBE sample, annealing the presheared sample showed three regimes in which crystallization time, as defined by linear viscoelasticity, first increased, then decreased, and finally increased again with the increase in the a...

Effect of Random Ethylene Comonomer on Relaxation of Flow-Induced Precursors in Isotactic Polypropylene

The effect of comonomer on structure and relaxation of flow-induced precursors was investigated in a series of isotactic polypropylene and random propylene–ethylene copolymers. The polymers were subjected to flow by fiber pulling and allowed to relax above their nominal melting temperature for specific times. The type of morphology developed after cooling revealed whether flow-induced precursors were still present or the melt had fully re-equilibrated. Precursors were long-lived and, at fixed temperature, decayed significantly faster with higher ethylene content. The critical time for precursor relaxation followed an Arrhenius-type dependence with temperature. The apparent energy of activation for precursor dissolution decreased with increasing comonomer content, indicating that the rate-limiting step of the relaxation process becomes less difficult with higher ethylene fraction. This effect is attributed to ethylene counits acting as disruptors of precursor structure and is discussed in terms of quasi-cr...

Entropy-Driven Segregation and Its Competition with Crystal Nucleation in the Binary Blends of Stretched and Free Guest Polymers.

Enhancing the dynamic asymmetry between liquid-like and solid-like components leads to spontaneous segregation. As a typical example, the binary blends of stretched and free guest polymers were investigated by our dynamic Monte Carlo simulations. The results evidenced an entropic driving force for the strain-induced segregation between two components, similar to that for Onsager's lyotropic liquid crystals. In addition, with the decrease of strain rates, its competition with strain-induced crystal nucleation results in variable compositions in crystal precursors. The scenario helps to settle down the controversial arguments on the flow-induced precursors of shish-kebab crystallites in the melt.

Transition in Crystal Morphology for Flow-Induced Crystallization of Isotactic Polypropylene

Deformation applied to a semicrystalline polymer melt prior to quenching can dramatically increase the concentration of nuclei and even transform the final crystalline morphology, phenomena collectively known as flow-induced crystallization (FIC). Using polarized optical microscopy and atomic force microscopy, we image the changes in morphology of an isotactic polypropylene (iPP) sample previously sheared in a rotational rheometer. Sufficiently large deformations promote the formation of “rice grain” shaped crystalline domains. These anisotropic structures are randomly oriented and are about 1.5–3.0 μm long (depending on applied work), with an aspect ratio of about 2:1. Well below the critical specific work threshold Wc, the crystalline morphology of our iPP sample is predominately composed of spherulites, with a fraction of rice grains that increases as the applied work increases. Above Wc, rice grains predominate, with no visible spherulites. The size of spherulites appears to be independent of applied work. In contrast, the size of rice grains decreases with increasing applied work, up to a saturation specific work Wsat, remaining constant thereafter. Similarly, the isothermal crystallization time decreases with increasing applied work up to Wsat and is constant thereafter. The effects of flow-induced precursors persist after repeated melting and recrystallizations, observable by elevated freezing temperatures in DSC experiments. Here, we probe directly the robustness of these precursors in optical microscopy, in which we repeatedly melt, anneal, and recrystallize a thin slice of an FIC sample. We observe rice grain domains appearing over and over, with the first domains to crystallize tending to reappear near the same locations.

Flow-Induced Crystallization of PEEK: Isothermal Crystallization Kinetics and Lifetime of Flow-Induced Precursors during Isothermal Annealing.

The role of an interval of shear flow in promoting the flow-induced crystallization (FIC) for poly(ether ether ketone) PEEK was investigated by melt rheology and calorimetry. At 350 °C, just above the melting temperature of PEEK (Tm), a critical shear rate to initiate the formation of flow-induced precursors was found to coincide with the shear rate at which the Cox-Merz rule abruptly begins to fail. In cooling the sheared samples to 320 °C, FIC can be up to 25× faster than quiescent crystallization. Using rheology and differential scanning calorimetry, the stability of FIC-induced nuclei was investigated by annealing for various times at different temperatures above Tm. The persistence of shear-induced structures slightly above Tm, along with complete and rapid erasure of FIC-induced nuclei above the equilibrium melting temperature, suggests that FIC leads to thicker lamellae compared with the quiescently crystallized samples.

Lifetime of Flow-Induced Precursors in Isotactic Polypropylene

Brief intervals of strong flow stretch chains in a semicrystalline polymer melt to form flow-induced precursors, which accelerate crystallization kinetics and transform the morphology. Using commercial isotactic polypropylene, the persistence and lifetime of flow-induced precursors were investigated, focusing on the effects of specific work and annealing time and temperature. Precursors were formed by shearing polypropylene in a rotational rheometer and then quenching to a desired temperature. The crystallization time and crystallization temperature of the sheared samples were investigated using novel rheology and DSC experiments. For sufficiently large shear rates, the appearance of flow-induced precursors is controlled by the applied work. A qualitative change in many properties related to precursors takes place at a critical work value Wc, about 7 MPa for our iPP material. To erase the persistent “memory” of flow-induced nuclei, samples must be annealed for a very long time below 210 °C (>3000 min) or ...

Onset of Flow-Induced Crystallization Kinetics of Highly Isotactic Polypropylene

Brief intervals of shear prior to a temperature quench accelerate crystallization, resulting in much smaller spherulites. Crystallization kinetics of five commercial linear isotactic polypropylenes were investigated, using a rheometer to impose shear and monitor crystallization after quenching. Shear and quench temperatures, shear rate, and duration were all systematically varied. The crystallization rate increases with increasing applied work, up to a value independent of undercooling beyond which the rate remains constant. This saturation is consistent with a maximum number of nuclei, possibly set by the concentration of heterogeneous impurities. The crystallization rate likewise increases with increasing shear rate, saturating at about 1 s–1 for all grades studied. Only chains in the high molecular weight tail, above about 104 kg/mol, are stretched at this shear rate. Faster crystallization after shear was observed for grades with lower isotacticity. Flow-induced crystallization persists even when shear is applied well above the equilibrium melting temperature (187 °C), finally weakening above the Hoffman–Weeks temperature (210 °C), perhaps because flow-induced precursors are no longer metastable.

The thermodynamic properties of flow-induced precursor of polyethylene

Flow-induced preordering or precursor (FIP) has been studied in a series of lightly cross-linked high-density polyethylene with a combination of extensional rheology and in situ synchrotron radiation small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) measurements. Based on the incipient strains of SAXS and WAXD signals during extension in a large temperature range, strain-temperature diagrams for flow-induced preordering and nucleation were constructed and revealed that flow-induced crystallization (FIC) undergoes two stages: melt-precursor transition (MPT) and precursor-nuclei transition (PNT). At different temperatures, FIP with different inner structures and morphologies can be induced by strain; these embryos have shape and structure that are related to those of the corresponding critical nuclei. With the strain-temperature diagrams, the thermodynamic properties of FIP are deduced, which shows that compared with the relative nuclei the FIP always has a lower bulk free energy (ΔH) and a much lower surface free energy (σ e ). In extreme cases (high temperature), the σ e of FIP can be negligible. The quantitative estimation of the thermodynamic parameters suggests the existence of variant FIPs, which plays a vital role for the subsequent progress of PNT and the whole process of FIC.

Formation and stability of flow-induced nucleation precursors in PCL in the presence of a rigid miscible component

The influence of a miscible rigid component (styrene-co-acrylonitrile copolymer, SAN) on the formation and relaxation of flow-induced crystal nucleation precursors (FIPs) in poly(ɛ-caprolactone) (PCL) melt was investigated. In situ polarized optical microscopy (POM) experiments suggested that SAN promoted the formation of FIPs but lowered their regularity probably by restricting the mobility of PCL chains and incorporating some shorter molecular chains into FIPs. Sheared blends containing SAN formed fibril FIPs with lower thermal stability but exhibited stronger shear memory effect during subsequent crystallization process. Two dimensional wide-angle X-ray diffraction (2D-WAXD) results showed the orientation degree of crystals in sheared samples decreased with increasing SAN content, probably due to the depressed growth kinetics of kebabs and the relaxation of orientated PCL chains.

Flow-induced crystallization precursor structure in high molecular weight isotactic polypropylene (HMW-iPP)/low molecular weight linear low density polyethylene (LMW-LLDPE) binary blends

The formation of initial crystallization precursor induced by flow was investigated by synchrotron-based wide- and small-angle X-ray scattering techniques (WAXS/SAXS), using polymer blends consisting of high molecular weight isotactic polypropylene (HMW-iPP) and low molecular weight linear low density polyethylene (LMW-LLDPE) as model systems. LMW-LLDPE, the dominant component, served as the amorphous matrix. Crystallization behavior of the blends with three different HMW-iPP concentrations, i.e., 3, 6 and 9 wt%, were examined at temperatures higher than the blends' cloud points, in a time-resolved manner. It was found that the formation of a flow-induced precursor of crystallization was strongly influenced by the concentration of HMW-iPP in the blends. The blend with low HMW-iPP content (3 wt%) was not able to form a shear-induced crystalline structure. As the concentration of HMW-iPP increased, crystallization kinetics, crystallinity, and lamellar orientation were greatly enhanced.

In Situ Observations of Flow-Induced Precursors during Shear Flow

In this work, the formation of flow-induced precursors (FIPs) during and immediately after applying a shear flow in isotactic polypropylene (iPP) melt is studied at high spatial resolution and high time resolution with a high-speed polarized optical microscope (HSPOM) and simultaneous small/wide-angle X-ray scatterings (SAXS/WAXS). A number of studies have reported the formation of FIPs in the early stage of flow-induced crystallization. However, reports on direct observations of the formation of precursors during shearing are scarce. We find that FIPs are observed using HSPOM after a certain incubation time during shearing. After cessation of flow, some FIPs dissolve gradually and others transfer to crystals. This suggests a competition between chain relaxation and crystallization. In SAXS measurements, the formation of FIPs during shearing is indicated by weak equatorial streaks normal to the flow direction after a specific incubation time. The intensity integrated from 2D SAXS images shows a sudden dec...

Pressure Quench of Flow-Induced Crystallization Precursors

We developed a novel protocol to study the mutual influence of shear flow and pressure on crystallization of polymers. Here, we have applied this protocol, named “pressure quench”, to polyethylene with a bimodal molecular weight distribution. With pressure quench, the undercooling, required to initiate crystallization of flow-induced precursors generated at high temperature, is obtained by increasing pressure, i.e., leaving the specimen isothermal. We find that pressure enhances the effect of shear. In particular, results show that the pressure quench effectively “lightens up” shear-induced precursors which otherwise are not observable, even with high-resolution synchrotron X-ray scattering. A pressure quench in combination with SAXS and WAXD gives insight into the early stages of crystallization. In this paper we focus on the use of WAXD since it provides all the information required to demonstrate our main issues. We conclude that precursors with different stability can be formed during shear and, with annealing, the least stable ones relax back to the melt. Finally, it is demonstrated that when pressure is released after crystallization, an “inverse quench” takes place and crystalline structures partially melt, similar to an increase of the temperature.

Effects of particles on stability of flow-induced precursors

The effect of two colorant particles with different surface geometries on the stability of shear-induced precursors in isotactic polypropylene was studied after the cessation of shear flow at 140 °C. In the absence of particles, the shear-induced precursors survived for at least 100 s after the shear flow ended. The presence of particles was found to stabilize lower molecular weight chains assisting in the formation of additional shear-induced precursors. The precursors thus formed in the samples containing particles contained two oriented clusters with different molecular weights. Incorporation of lower molecular weight chains in the precursors led to increased dissolution rates of the shear-induced precursors. Particle surface geometry was found to influence precursor dissolution, with planar particles stabilizing the shear-induced precursors to a much greater extent than curved particles. The particles investigated thus act like structural probes to follow quantitatively the dissolution process of precursors after shear and importantly to infer the formation of precursors during shear.

A stretch-based model for flow-enhanced nucleation of polymer melts

A phenomenological model for flow-enhanced nucleation in crystallizing polymers is developed and validated by short-term shear experiments [Housmans et al., Macromolecules 42, 5728–5740 (2009); Hristova et al., Proceedings of the 228th ACS National Meeting, Philadelphia (2004)]. The model extends earlier work on flow-induced oriented crystallization [Custódio et al., Macromol. Theory Simul. 18, 469–494 (2009); Peters et al., Macromol. Symp. 185, 277–292 (2002); Peters, Polymer Crystallization: Observations, Concepts and Interpretations, edited by G. Reiter and J.-U. Sommer (Springer, Berlin, 2003), pp. 312–324; Zuidema, Ph.D. thesis, Technische Universiteit Eindhoven (2000); Zuidema et al., Macromol. Theory Simul. 10, 447–460 (2001)] to flow-enhanced pointlike nucleation, which can lead to number densities of spherulites multiplied by orders of magnitude. Excellent agreement between simulations and experimental data is obtained, for a range of rates and durations of shearing, with only two adjustable parameters: a prefactor to the creation rate of flow-induced nucleation precursors and a parameter that governs their influence on the relaxation dynamics of the high-molecular weight (HMW) fraction of the melt. The two main conclusions of this paper are, first, that the creation of flow-induced precursors is driven by the average stretch, not by the average orientation, of the primitive paths of chains in the HMW tail of the molecular weight distribution, and second, that nucleation of these precursors is impeded by flow.