Onset of Flow-Induced Crystallization Kinetics of Highly Isotactic Polypropylene

Composition Dependence of Flow-Induced Crystallization in High-Density Polyethylene/Isotactic Polypropylene Blends

Polyolefins, including high-density polyethylene (HDPE) and isotactic polypropylene (iPP), account for over half of the worldwide plastics market and have wide-ranging applications. Recycling of these materials is hindered due to separation difficulties as co-mingled blends of HDPE and iPP often exhibit brittle mechanical behavior because phase separated domains detach under stress due to low interfacial adhesion. Motivated to improve mechanical properties of mixed recyclates during processing, this work examines the effect of shear on the crystallization kinetics and rheological properties of HDPE-iPP blends utilizing a combination of differential scanning calorimetry (DSC), rheo-Raman spectroscopy, polarized optical microscopy, and scanning electron microscopy (SEM). In the quiescent experiments, the crystallization temperature as a function of blend composition exhibits a distinct decrease when the iPP forms the droplet phase, as expected, due to fractionated crystallization. In the presence of shear, we find elongated domains due to high capillary number. Unexpectedly, we find a compositional dependence to the flow-induced crystallization (FIC) of iPP: stronger FIC is observed in all blends compared to the pure iPP. Moreover, the flow completely counteracts the reduced crystallization arising from fractionated crystallization, indicating that the flow is able to induce nucleation in droplets to an extent such that it offsets the reduction in active nucleating agents in finite size droplets. We attribute these effects to differing microflow fields in the various morphologies as the iPP domains deform under shear.

Influence of the Ethanol Content of Adduct on the Comonomer Incorporation of Related Ziegler-Natta Catalysts in Propylene (Co)polymerizations

The aim of this work is to investigate the influence of the ethanol content of adducts on the catalytic behavior of related Ziegler-Natta (ZN) catalysts in propylene homo- and copolymerizations (with 1-hexene comonomer) in terms of activity, isotacticity, H2 response, and comonomer incorporation. For this purpose, three MgCl2.nEtOH adducts with n values of 0.7, 1.2, and 2.8 were synthesized and used in the synthesis of related ZN catalysts. The catalysts were thoroughly characterized using XRD, BET, SEM, EDX, N2 adsorption-desorption, and DFT techniques. Additionally, the microstructure of the synthesized (co)polymers was distinguished via DSC, SSA, and TREF techniques. Their activity was found to enhance with the adduct's ethanol content in both homo- and copolymerization experiments, and the increase was more pronounced in homopolymerization reactions in the absence of H2. Furthermore, the catalyst with the highest ethanol content provided a copolymer with a lower isotacticity index, a shorter meso sequence length, and a more uniform distribution of comonomer within the chains. These results were attributed to the higher total surface area and Ti content of the corresponding catalyst, as well as its lower average pore diameter, a larger proportion of large pores compared to the other two catalysts, and its spherical open bud morphology. It affirms the importance of catalyst/support ethanol-content control during the preparation process. Then, molecular simulation was employed to shed light on the iso-specificity of the polypropylene produced via synthesized catalysts.

Optimum processing conditions for the maximum crystallization rate of poly(3-hydroxybutyrate-co-3-hydroxyhexanoate)

The effect of thermal and shear histories on the crystallization rate of poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBHHx) was studied. As with other crystalline polymers, the shear history greatly affected the crystallization rate when the shear rate was beyond a critical value, i.e., the inverse of the Rouse relaxation time. Even after the formation of extended chain crystals, spherulite texture was clearly discernable. It grew from certain points on the extended chain crystals. Consequently, a row of spherulites appeared along the flow direction. The resin temperature in the molten state was also significant. When the sample was heated to 170 °C, which is beyond the main melting peak in the differential scanning calorimetry curve, unmolten crystals did not affect the linear viscoelastic properties. They acted as effective nucleating agents for the rest of the polymer during cooling. Therefore, the shear history hardly affected the crystallization rate and the number of spherulites.

Effect of Ultra-High-Molecular-Weight Molecular Chains on the Morphology, Crystallization, and Mechanical Properties of Polypropylene

The effects of the ultra-high-molecular-weight (UHMW) component of polypropylene (PP) on its rheological properties, crystallization behavior, and solid-state mechanical properties were investigated using various measurement techniques. The terminal relaxation time-determined by measuring the linear viscoelasticity-was increased by adding the UHMW component. The increase in the melt elasticity produced by adding the UHMW component was observed by measuring the steady-state shear flow, although the shear viscosity was not greatly affected. Owing to the long characteristic time of the Rouse relaxation of the UHMW component, PP with the UHMW component formed highly oriented structures through a shear-induced crystallization process. The addition of the UHMW component enhanced the orientation and regularity of crystalline structure for extruded films. Therefore, the Young's modulus, yield stress, and strength were higher in the PP film containing the UHMW component than in one without the UHMW component, irrespective of the direction of tensile deformation.

Isothermal Crystallization and Time-Temperature Transformation of Amorphous Nifedipine: A Case of Polymorphism Formation and Conversion

Crystallization of active pharmaceutical ingredients (APIs) from the supercooled liquid state is an important issue in determining the stability of amorphous pharmaceutical dispersions. In the present study, the isothermal crystallization from the supercooled liquid state of the pharmaceutical compound nifedipine was investigated by both rheological and differential scanning calorimetry (DSC) measurements, and the crystallization kinetics was fitted to the Johnson-Mehl-Avrami (JMA) equation. Both the crystallization induction time and completion time from the two methods were used to construct the time-temperature-transformation (TTT) diagram for nifedipine. A model based on a modification of classical homogeneous nucleation and crystal growth theory was employed to fit the induction and completion time curves. Both DSC and rheological methods give similar results for the crystallization kinetics of the nifedipine. From the crystallization kinetics modeling, the solid-liquid interfacial surface tension σ_SL of nifedipine was estimated and the value was found to be consistent with prior results obtained from melting point depression measurements as a function of crystal size. Evidence is shown that for temperatures below 110 °C, at the early stage of nucleation, NIF first nucleates into the metastable β'-form and later converts into the stable α-form during the isothermal crystallization. We are also able to report the heat of fusion of the γ'-NIF based on the calorimetric experiments.

Effect of Molar Mass on Critical Specific Work of Flow for Shear-Induced Crystal Nucleation in Poly (l-Lactic Acid)

The concept of specific work of flow has been applied for the analysis of critical shearing conditions for the formation of crystal nuclei in poly (l-lactic acid) (PLLA). Systematic variation in both time and rate of shearing the melt in a parallel-plate rheometer revealed that these parameters are interconvertible regarding the shear-induced formation of crystal nuclei; that is, low shear rate can be compensated for by increasing the shear time and vice versa. This result supports the view that critical shearing conditions can be expressed by a single quantity, providing additional options for tailoring polymer processing routes when enhanced nuclei formation is desired/unwanted. Analysis of PLLA of different mass-average molar masses of 70, 90, 120, and 576 kDa confirmed improved shear-induced crystal nucleation for materials of higher molar mass, with critical specific works of flow, above which shear-induced nuclei formation occurs, of 550, 60, 25, and 5 kPa, respectively.

Direct observation of long chain enrichment in flow-induced nuclei from molecular dynamics simulations of bimodal blends

Modelling of flow-induced nucleation in polymers suggest that long chains are enriched in nuclei, relative to their melt concentration. This enrichment has important consequences for the nucleation rate and mechanism, but cannot be directly observed with current experimental techniques. Instead, we ran united atom molecular dynamics simulations of bimodal polyethylene blends, comprising linear chains at a 50 : 50 mix of long (1000 carbon) and short (500-125 carbon) chains, under shear flow. We developed a method to extract the nucleus composition during a transient start-up flow. Our simulations show significant and systematic enrichment of long-chains for all nucleus sizes up to and beyond the critical nucleus. This enrichment is quantitatively predicted by the recent polySTRAND model [Read et al. Phys. Rev. Lett. 2020, 124, 147802]. The same model parameters also correctly capture the nucleus induction time in our simulations. All parameters of the model were fitted to a small subset of our data in which long chain enhancement was absent. We conclude that long-chain enrichment is central to the mechanism of flow-induced nucleation and that this enrichment must be captured to correctly predict the nucleation rate.

Inducing nano-confined crystallization in PLLA and PET by elastic melt stretching

Based on the widely studied poly(l-lactic acid) (PLLA) and polyethylene terephthalate (PET) that are brittle in their fully crystalline form, this Letter shows that they can be made to be super ductile, heat resistant and optically clear by creating nano-sized crystals while preserving the entanglement network. Atomic force microscopic images confirm the perceived nano-confined crystallization. Time-resolved X-ray scattering/diffraction measurements reveal the emergence of cold crystallization during either stress relaxation from large stepwise melt-stretching or annealing of pre-melt-stretched PLLA and PET above Tg. Mechanical tests show that these polymers in such a new state are rigid even well above Tg, e.g., at 100 °C.

PolySTRAND Model of Flow-Induced Nucleation in Polymers

We develop a thermodynamic continuum-level model, polySTRAND, for flow-induced nucleation in polymers suitable for use in computational process modeling. The model's molecular origins ensure that it accounts properly for flow and nucleation dynamics of polydisperse systems and can be extended to include effects of exhaustion of highly deformed chains and nucleus roughness. It captures variations with the key processing parameters, flow rate, temperature, and molecular weight distribution. Under strong flow, long chains are over-represented within the nucleus, leading to superexponential nucleation rate growth with shear rate as seen in experiments.

Rheology of crystallizing polymers: The role of spherulitic superstructures, gap height, and nucleation densities

A longstanding goal in polymer rheology is to develop a physical picture that relates the growth of mechanical moduli during polymer crystallization to that of a structure. Here, we utilize simultaneous mechanical rheology and optical microscopy, with augmentation by deterministic reconstruction and stochastic simulations, to study isothermal crystallization in isotactic polypropylene. We observe the nucleation and growth of the surface and bulk spherulites, which are initially isolated and then impinge to form clusters and superstructures that eventually span the gap. We find that spherulitic superstructures play a critical role in the rheology, especially in the characteristic sharp upturn in moduli. Both the rheology and the spherulitic superstructures show pronounced gap dependencies, which we explain via finite-size effects in percolation phenomena and via surface-induced nucleation. The modulus-crystallinity relationship can be described through a general effective medium theory. It indicates that for thicker gaps, the viscoelastic liquid to solid transition can be described via percolation, whereas for our thinnest gap, it is best described by the linear mixing rule. We describe our results in terms of dimensionless nucleation rates and spherulite size, which enable the estimation of when gap-dependent superstructure effects can be anticipated.

Effect of processing conditions on crystallization kinetics during materials extrusion additive manufacturing

Material extrusion additive manufacturing processes force molten polymer through a printer nozzle at high (> 100 s^-1) wall shear rates prior to cooling and crystallization. These high shear rates can lead to flow-induced crystallization in common polymer processing techniques, but the magnitude and importance of this effect is unknown for additive manufacturing. A significant barrier to understanding this process is the lack of in situ measurement techniques to quantify crystallinity after polymer filament extrusion. To address this issue, we use a combination of infrared thermography and Raman spectroscopy to measure the temperature and percent crystallinity of extruded polycaprolactone during additive manufacturing. We quantify crystallinity as a function of time for the nozzle temperatures and filament feed rates accessible to the apparatus. Crystallization is shown to occur faster at higher shear rates and lower nozzle temperatures, which shows that processing conditions can have a dramatic effect on crystallization kinetics in additive manufacturing.

Effect of Self-Assembly of Oxalamide Based Organic Compounds on Melt Behavior, Nucleation, and Crystallization of Isotactic Polypropylene

We report on the effect of an aliphatic oxalamide based nucleating agent (OXA3,6) on the melt and crystallization behavior of isotactic polypropylene (iPP) under defined shear conditions. Through polarized optical microscopy, we demonstrate that OXA3,6 self-assembles from the iPP melt into rhombic crystals whereas their size and distribution proved highly dependent on the employed cooling rates. The presence of 0.5 wt % of OXA3,6 in iPP results in a significant suppression in iPP melt viscosity, which could not be explained via molecular modeling. A possible cause for the drop in viscosity in the presence of OXA3,6 is attributed to the interaction (absorption) of high molecular weight iPP chains with the nucleating agent, thereby suppressing their contribution to the viscoelastic response of the melt. This proposed mechanism for the suppression in melt viscosity appears similar to that encountered by the homogeneous distribution of nanoparticles such as CNTs, graphene, and silica. Shear experiments, performed using a slit flow device combined with small-angle X-ray diffraction measurements, indicate that crystallization is significantly enhanced in the presence of OXA3,6 at relatively low shear rates despite its lowered sensitivity to shear. This enhancement in crystallization is attributed to the shear alignment of the rhombic OXA3,6 crystals that provide surface for iPP kebab growth upon cooling. Overall, the suppression in melt viscosity in combination with enhanced nucleation efficiency at low as well as high shear rates makes this self-assembling oxalamide based nucleating agent a promising candidate for fast processing.

Flow-Induced Crystallization of Polyamide-6

Flow-induced crystallization has been widely studied for a variety of polymers with the main focus on polyolefins. In this work, research has been conducted on the flow-induced crystallization of another important class of polymers, namely polyamides. Different polyamides-6 with varying molecular weight have been studied. At relatively modest values of shear rate, rheology has been used to study the kinetics of flow-induced crystallization. Typical scaling relations based on the longest relaxation time and the Rouse time – usually obtained for polyolefins – are tested for the polyamides under investigation in order to identify the different regimes of flow-induced crystallization. At high shear rates, a correct rheological signal was impossible to collect. However, the rheometer was used in this case to prepare the sample to be studied ex-situ by Wide Angle X-ray Scattering experiments to determine the onset shear rate for the formation of highly oriented shish-kebab structures.

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 (T_m), 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 T_m. The persistence of shear-induced structures slightly above T_m, 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.