Shear-induced Crystallization Behavior Research Articles

Shear-induced crystallization of polypropylene/low-density polyethylene blend

Shear-induced crystallization behavior was studied using a phase-separated blend comprising a polypropylene continuous phase and a low-density polyethylene (LDPE) dispersion, which is known to show strain hardening in transient elongation viscosity. In this study, crystallization behavior was mainly evaluated by the light intensity transmitted through a transparent parallel-plate geometry. It was found that shear-induced crystallization was greatly accelerated by the addition of LDPE, resulting in a high crystallization temperature and a highly oriented structure. When the sample was cooled slowly, however, shear-induced crystallization was not accelerated by LDPE. Furthermore, extrusion was performed using a capillary rheometer. The molecular orientation in the strands collected after cooling in the air was found to be enhanced by the LDPE addition, suggesting that flow-induced crystallization was accelerated even at capillary extrusion.

Preparation of modified hydrotalcite based on click chemistry and its effect on the shear induced crystallization, rheological behavior and thermal degradability of poly (lactic acid) film

ABSTRACT In this work, Zn-Mg-Al-LDHs was prepared by co-precipitation. LDHs-B-OM was prepared with Zn-Mg-Al-LDHs by coupling method and thiol-ene click chemical reaction. The optimum conditions of thiol-ene click reaction were determined by orthogonal experiment as follows: reaction time of 5 h, mass ratio of OM to B-LDHs of 1: 1, mass ratio of LDHs-B to initiator of 100: 9. LDHs-B-OM was characterized by Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy, scanning electronic microscope, and static contact angle, which showed that its surface modification was confirmed successfully. Then, PLA/LDHs-B-OM films were prepared by blow molding process using LDHs-B-OM as fillers and PLA as matrix. The mechanical properties, rheological properties, and crystallization behavior of PLA/LDHs-B-OM films were studied, which showed that the elongation at break, the tensile strength of PLA0 and PLA4 (0.7 wt% LDHs-B-OM) were 14.20%, 36.47 MPa, 74.09%, and 46.99 MPa, respectively, which demonstrated that LDHs-B-OM had a significant strengthening and toughening effect on PLA. Rheological behavior results showed that LDHs-B-OM effectively promoted the movement of PLA molecular chain. The shearinduced crystallization behavior and thermal degradation was analyzed by simultaneous rheology and FTIR technology, which showed that the carbonyl peak shifted from amorphous region to crystalline phase region. The reason was that LDHs-B-OM acted as a nucleating agent to accelerate the crystallization process of PLA. The carbonyl index reduction of PLA0 and PLA4 before and after thermal degradation was 14.93% and 7.97%, respectively, which provided sufficient evidence that the addition of LDHs-B-OM improved thermal stability of PLA.

Concentration Effect of a Bis-amide Nucleating Agent on the Shear-Induced Crystallization Behavior of Isotactic Polypropylene

The concentration of nucleating agent (NA) and flow fields are both important factors affecting the crystalline behavior and final properties of semicrystalline polymers. In this work, the concentration effect of N,N′-dicyclohexylterephthalamide (DCHT), an effective β-NA for isotactic polypropylene (iPP), on the shear-induced crystallization of iPP was investigated by comparing the crystallization temperatures (Tcs) of iPP/DCHT under unsheared, shear-unrelaxed, and relaxed conditions via rheological measurement. For iPP with an extremely low concentration of 0.01 wt %, Tc greatly improved after shearing and distinctly reduced after relaxation of 30 min, which was similar to that of pure iPP. For iPP with moderate concentrations of 0.05–0.3 wt %, the Tc could still increase after shearing but remained nearly constant after relaxation. In this concentration range, the increment in Tc induced by shear gradually decreased with increasing concentration. Interestingly, for iPP with high concentrations of 0.5 and 1 wt %, the Tc hardly changed regardless of applying shear or relaxation. The results of Fourier transform infrared suggested that DCHT existed in different forms including “free” molecules only and the coexistence of “free” DCHT and the associated assembly in various iPP/DCHT samples, and polarized optical microscopy observation showed that the DCHT assembly exhibited diverse morphologies at different concentrations. Combining the roles of “free” DCHT molecules and the associated assembly with diverse morphology in the shear-induced crystallization of iPP, a possible mechanism explaining the effect of DCHT concentration was proposed and experimentally verified.

Distributive mixing of carbon nanotubes in poly(caprolactone) via solution and melt processing: Viscoelasticity and crystallization behavior versus mixing indices

Different mixing processes give rise to significant differences in the agglomeration and deagglomeration of nanoparticles and how they are mixed and distributed within a polymeric matrix. Here poly(caprolactone), PCL, was compounded with carbon nanotubes, CNTs, at a weight fraction of 0.5% (volume fraction, Φ = 0.0027), using both solution and melt processing methods. Microscopy, image analysis, and thermogravimetric analysis, TGA, were used to define and characterize the distributions of the CNT-rich domain sizes and to obtain quantitative mixing indices at different scales of examination. The presence of the CNTs led to the shear-induced crystallization of PCL under conditions at which pure PCL would not crystallize and the microstructures obtained upon shear-induced crystallization were documented via X-ray diffraction, differential scanning calorimetry, and rheological characterization. Shear-induced crystallization occurred at a faster rate (smaller induction times) when the nanocomposites exhibited greater mixing index values. The shear-induced crystallization behavior was found to be a more sensitive indicator of the distributive mixing states of the CNTs in comparison with the widely used linear viscoelastic rheological material functions. The demonstrated methodologies can be used as complementary tools for processing and development of nanocomposites to achieve consistent and reproducible microstructures at various scales of examination and functional properties. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 2254–2268.

Shear-induced β -form polypropylene in long chain branching isotactic polypropylene

Long chain branching polypropylene (LCBPP) with different long chain branching (LCB) contents were prepared by reactive extrusion in the presence of styrene and benzoyl peroxide, and their shear-induced crystallization behaviors were investigated. The results indicated that the LCB structure extended the relaxation time of LCBPP in the molten state, which led to the formation of β-form polypropylene under shear and high cooling rate. The content of β-form (Kβ) increased with the increase of LCB content, shearing rate and cooling rate. The Kβ value of LCBPP3 whose weight average molecular weight was 920,000 g mol−1 could be up to 52.0% with a shear rate of 60 s−1 associated with a cooling rate of 280°C min−1. This study is expected not only to have a deeper understanding of the shear-induced crystallization behavior of LCBPP, but also provide a new strategy to obtain high level β-form polypropylene. POLYM. ENG. SCI., 56:240–247, 2016. © 2015 Society of Plastics Engineers

Quiescent and shear-induced crystallization of linear and branched polylactides

The quiescent and shear-induced isothermal crystallization behavior of linear and long-chain branched (LCB) polylactides (PLAs) was investigated at a temperature of 130 °C. LCB-PLAs were produced by the reaction with a multifunctional chain extender, Joncryl©. In quiescent crystallization, the presence of the LCB structure accelerated the nucleation process and reduced the induction time, depending on the level of branching. The impact of shear strain and shear rate on crystallization was also examined. The shear-induced crystallization of the linear and LCB-PLAs was affected by both the total shear strain and shear rate. The crystallization kinetics of the LCB-PLAs was more affected by shear than that of the linear PLA. The crystalline morphology of the linear and LCB-PLAs under quiescent and step shear rate conditions was examined using a Linkam optical shearing system. An increase in the spherulite density was observed in the strained melt of both linear (33 %) and LCB-PLAs (15 %), in comparison with those of unstrained counterparts. Optical micrographs confirmed that the crystal nucleation was affected by the shear flow. Long-chain branching significantly promoted the nucleation density (6.7 times), although it diminished the crystal growth rate from 4.4 to 2.0 μm/min.

Wall Slip Effect on Shear-Induced Crystallization Behavior of Isotactic Polypropylene Containing β-Nucleating Agent

Shearing is unavoidable during the polymer process, and isotactic polypropylene (iPP) is one of the most used commercial polymers. iPP mixed with β-nucleating agent TMB-5 was isothermally crystallized at 135 °C from melts under various shear conditions and investigated via synchrotron radiation small-angle and wide-angle X-ray scattering (SAXS and WAXS) techniques with CCD detector. The shear-induced crystalline structure and the orientation of the samples were studied according to the obtained WAXS and SAXS patterns. The results showed that both the crystallinity of α-iPP and β-iPP are in direct proportion to the orientation degree rather than shear rate especially at high shear rate, which proves that wall slip should not be neglected when taking shear effect or rheological behavior into consideration.

Shear-induced crystallization and rheological behavior of syndiotactic polystyrene

Abstract

SHEAR-INDUCED CRYSTALLIZATION OF ISOTACTIC POLYPROPYLENE

Shear-induced crystallization behavior of isotactic polypropylene(iPP)was investigated using a rotational rheometry covering a shear rate range of 1.2×10-4～1 s-1 and temperatures between 135℃ and 145℃.As the specimen was sheared at a constant rate,the viscosity and normal force were constant at first,then started to increase sharply after a certain time.At higher shear rates the transition of the normal force occurred considerably earlier than that of the viscosity,and at medium shear rates the two transition times were coincident.However at extremely low shear rates the data of normal force became noisy and only the data of viscosity was reliable.It was suggested that the earlier time in transitions for the normal force and viscosity was used to define an onset time(t_ on)which characterized the crystallization process.Furthermore,it is proposed that the plot of t_ on versus shear rate can be converted into a temperature-invariant curve by dividing the y axis by quiescent onset time t_ on,q and multiplying the x axis by t_ on,q.

Deformation-Induced Crystallization and Associated Morphology Development of Carbon Nanotube-PVDF Nanocomposites

Poly(vinylidene fluoride) (PVDF) is a semicrystalline thermoplastic polymer that is of interest for sensor, actuator and biomedical applications because of its piezoelectric and pyroelectric properties, as well as outstanding mechanical and chemical properties. Although it is known that the shear-induced crystallization behavior of nanocomposites can be significantly affected by the presence of nanoparticles, the effects of the incorporation of carbon nanotubes on the deformation-induced crystallization and associated morphology development of PVDF have not been previously investigated. Here the dynamics of the shear-induced crystallization of carbon nanotubes incorporated in PVDF were investigated using simple shear flow. The shear-induced crystallization behavior was affected by the deformation rate, temperature, and the concentration of the carbon nanotubes. Time-dependence of linear viscoelastic properties indicated that the presence of multi-walled carbon nanotubes (MWNTs) in PVDF greatly altered the shear-induced crystallization kinetics of PVDF, while no significant changes in crystallization behavior were observed for pure PVDF samples sheared under similar conditions. Upon increase of the concentration of the MWNTs the crystal size of PVDF decreased while its rate of crystallization increased in conjunction with an increase of the beta phase crystallization. Overall, these findings suggest that the shear-induced crystallization of PVDF nanocomposites (and in general flow-induced crystallization effects associated with the thermo-mechanical history experienced by the nanocomposite during processing) should be integral parts of attempts to generate a comprehensive understanding of the development of the microstructural distributions and the coupled ultimate properties of polymer nanocomposites.

Effects of Multiwalled Carbon Nanotubes on the Shear-Induced Crystallization Behavior of Poly(butylene terephthalate)

The effects of the incorporation of multiwalled carbon nanotubes (MWNT) with a diameter range of 10-30 nm on the shear-induced crystallization behavior of poly(butylene terephthalate) (PBT) were investigated under myriad shearing and loading conditions employing principally the small-amplitude oscillatory shear flow. Upon shearing, the presence of MWNTs leads to the crystallization of the PBT nanocomposites at temperatures that are higher than the crystallization temperature of unfilled PBT. The Avrami analysis of the shear-induced crystallization data of PBT nanocomposite samples indicated that the kinetics of the crystallization depend on both the temperature and the concentration of the MWNTs. When the MWNTs were replaced with ∼70 µm graphite particles at similar volume loading levels the crystallization behavior of the PBT/graphite suspension samples did not differ from that of pure PBT. These findings emphasize that one primary mechanism associated with the significant changes in the mechanical properties observed upon the compounding of nanoparticles into various semicrystalline polymers is the change in the crystallization behavior of the polymer as affected by the presence and the concentration of the nanoparticles and the associated changes in the microstructural distributions of the nanocomposite.

Combined effect of shear and fibrous fillers on orientation-induced crystallization in discontinuous aramid fiber/isotactic polypropylene composites

The shear-induced crystallization behavior in isotactic polypropylene (iPP) composite melt containing short aramid fibers was investigated by means of WAXD (wide-angle X-ray diffraction) and SAXS (small-angle X-ray scattering) techniques using synchrotron radiation. The study was carried out in a post-shear isothermal crystallization mode at temperatures of 140–160°C. Parameters pertaining to the crystallization morphology and kinetics were analyzed, including total crystallinity, orientated crystalline and amorphous fractions, dimensions of the formed shish–kebab structure, as well as induction time and rate of crystallization. The individual contributions of shear and fibers were evaluated and the combined effect was compared. The results clearly indicated that the effect is synergistic rather than additive.

Formation of cylindrite structures in shear-induced crystallization of isotactic polypropylene at low shear rate

The shear-induced crystallization behavior of isotactic polypropylene (iPP) at low shear rate and low temperature was investigated by in situ optical microscopy, time-resolved small-angle light scattering (SALS) and ex situ atomic force microscopy (AFM). Some new details and insight of the cylindrites were observed, which are important to delineate the formation mechanism of the cylindrite structures. Optical microscopic measurements showed that the core of cylindrite was formed rapidly after shear cessation, at late stage the quantity of the newly formed crystals increased and these crystals impinged into each other rapidly due to the space limitation. Measurement of the length of cylindrite core from optical microscopic observation showed that the length of cylindrite core did not change or changed very slightly with crystallization time. SALS results indicated that immediately after shear flow, the highly oriented structure emerged and the scattering intensity increased with crystallization time, and eventually the scattering patterns became circularly symmetric and the anisotropic scattering decreased, indicating the impingement of different cylindrite structures. AFM observation showed that the crystalline lamellae grew both perpendicular to and along the cylindrite core. In addition, a much wider cylindrite core was observed. Compared with the radius of gyration of a molecule, the process of shear-induced crystallization may have involved a large number of entangled molecules under a low shear rate. These evidences clearly indicate that the cylindrite core comes from the stretched bundles of the entangled network strands but not from the extended crystals of stretched single chains. A model based on the above observations has been proposed to explain the mechanism of the cylindrite developing process at low shear rate and low temperature.

In situ optical microscope study of the shear-induced crystallization of isotactic polypropylene

The shear-induced crystallization behavior of isotactic polypropylene (i-PP) has been investigated by in situ optical microscope under various thermal and shear histories. A shish-kebab structure during growth was observed under a well controlled, however, weak shear field. According to our results under these weak shear and thermal history, a modified model was proposed for the growing process of melt shear-induced crystallization of i-PP. Furthermore, many physical insights were provided on several still unsettled issues such as extended chain crystals, row nuclei, and smectic ordering questions.

Comparison of shear-induced crystallization behavior of PB-1 samples with different molecular weight distribution

Shear-induced crystallization behavior of three grades of PB-1 having different molecular weights and breadths of molecular weight distribution was investigated by means of rotational rheometry covering a shear rate range of 0.0001 up to 0.3 s−1 and temperatures between 99 °C and 107 °C. The up-shoot in viscosity after a certain time is used to define an onset time ton for both quasiquiescent and shear-induced crystallization. The plot of ton versus the shear rate can be converted into a temperature-invariant curve by dividing the y axis by quiescent onset time ton,q and multiplying the x axis by the square root of ton,q. The shape and location of the T-invariant curves for the three PB-1 samples are compared with respect to their molecular parameters. An additional normalization step by multiplying the abscissa by the square root of the characteristic retardation time of the melt yields a dimensionless representation of the T-invariant curve. Furthermore, by this procedure the three curves are shifted into one region, where they have the same threshold value, indicating the onset of shear-induced crystallization. A connection of this T-invariant curve to the morphology of the crystallites formed are addressed.

Shear-Induced Crystallization of Poly(butylene terephthalate): A Real-Time Small-Angle X-ray Scattering Study

The shear-induced crystallization behavior of poly(butylene terephthalate) (PBT) has been investigated by in-situ small-angle X-ray scattering at temperatures of 198, 203, and 208 °C as well as by ex-situ atomic force microscopy. Increasing the shear rate and/or the shear strain enhances the crystallization kinetics. The acceleration of the crystallization saturates with increasing shear rate. A pronounced effect of the shear field on the Avrami exponent is observed at 198 °C but not at 203 and 208 °C, even though at the latter temperatures the relative reduction of the crystallization time induced by the shear is twice the value at 198 °C. Compared to some other polymers like isotactic polypropylene, the shear field leads only to a weak orientation of the PBT crystals. The Avrami exponent and the orientation induced by the shear field are interpreted on the basis of a microshish model. Microshish structures are due to the breaking up of shear-induced row nuclei at high temperatures. Finally, the shear field is found to lead to a decrease of the long period.