Large strain requirements for shear-induced crystallization of isotactic polypropylene

We studied strain effects on the crystallization of a series of isotactic polypropylenes (iPP) of various molecular weights using rheology, rheo-optical measurements, differential scanning calorimetry (DSC), and wide-angle X-ray diffraction (WAXD). The samples were pre-sheared and then crystallized both at the same temperature. Transmitted light intensity measurements demonstrate that the effect of pre-shear on crystallization rate keeps increasing with strain up to surprisingly large strain levels, much beyond strains that are required to reach steady shear flow (at given We). Crystal orientation sets in at a total strain of about or higher. WAXD and DSC measurements corroborated the light transmission results. Total shear strains to 1000 were applied to the iPP samples at the beginning of a crystallization experiment, after the samples had reached the crystallization temperature of 145°C (under-cooled state). A constant Weissenberg number We=1 (We is defined as the product of shear rate and a relaxation time) was maintained for all pre-shearing of this study. We=1 corresponds to the onset of shear thinning in steady shear. Deborah number values were low, De<<1, indicating that steady shear flow had been reached in all pre-shearing runs (De is defined as the ratio of relaxation time to pre-shearing time). Further studies are needed at high We as there are indications that strain requirements are much reduced at high We. A fundamental understanding is still missing.     

Rheooptical study of the early stages of flow enhanced crystallization in isotactic polypropylene

The effect of a shear flow on the early stages and the kinetics of isothermal crystallization of an isotactic polypropylene has been studied experimentally. In the shear rate region where crystallization proceeds through point-like precursors, the magnitude of the shear rate, the shearing time as well as the instant in time at which the deformation starts have all been varied, in combination with rheooptical measurements. These include depolarized light intensity and birefringence. In agreement with previous work, above a critical shear rate and a critical shearing time, the crystallization kinetics are enhanced. Somewhat surprisingly, below a characteristic time, t_0,max, the kinetics are not affected by the instant in time at which flow is applied or stops. As long as flow takes place before this critical dwell time, only the shearing time and primarily the magnitude of the shear rate seem to matter. When flow is started only after t_0,max, its effect to accelerate crystallization kinetics becomes less efficient. The range over which the different parameters have an effect have been compared to the rheological relaxation times and to the measurements of global chain extension. To investigate the effects of flow on the early stages in more detail, time resolved Small-Angle Light Scattering experiments were used to detect changes in the density and orientation fluctuations. Measurements explicitly compare the effect of temperature and shear flow on the kinetics and the intensity of the density fluctuations.Electronic supplementary material to this paper can be obtained by using the Springer Link server located at     

Slippage and shear inhomogeneity in shear-induced crystallization of isotactic polypropylene on metal substrates

Substrate effect, slippage, and shear inhomogeneity in the shear-induced crystallization of an isotactic polypropylene are studied by rheological and optical experiments. Significant wall slip that reflects the polymer chain desorption from the wall was observed for the supercooled isotactic polypropylene (iPP) melt at 131 $^{\circ }$ C in both the pre-shear and the subsequent small-amplitude oscillatory shear that monitor the crystallization of iPP. Crystallization of the iPP melt on aluminum substrate is faster than that on stainless-steel substrate with the same pre-shearing condition. Because the surface energy of aluminum plate is higher than that of stainless-steel plate, when using the aluminum plates, the slip during the pre-shearing is smaller; thus, the real shear rate (or shear strain, shear work) exerted to the melt is higher and so is the shear-induced nucleation density. By using the observed nucleation density and the estimated nucleus growth rate, the Kolmogoroff equation can yield correct orders of magnitudes of crystallization rates of iPP. Assuming that higher shear rate induces higher nucleation density in the iPP melt, the shear inhomogeneity during the pre-shearing can be inferred based on the optical observation on the crystallized iPP samples.     

The impact of blue organic and inorganic pigments on the crystallization and rheological properties of isotactic polypropylene

Viscosity behavior is used to illustrate the relative sensitivity of the rheological properties to the morphological structure imposed by the presence of organic copper phthalocyanine and inorganic ultramarine blue pigments in isotactic polypropylene. While the inorganic pigment showed a small effect, the organic pigment was responsible for greatly increasing the nucleation rate and rapidly increasing the viscosity. This was also accompanied by a lowering of the degree of supercooling and the formation of a fibrous morphology. The results indicate that copper phthalocyanine is an excellent nucleating agent, drastically reducing the onset time of crystal formation and increasing the crystallization kinetics. Dynamic rheological properties also suggest that the organic pigment acts as a softening agent in the supercooled melt. Percent concentration and shear rate were not found to be major factors accelerating the rate of crystallization. This study details contributing factors including the particle size differences of the pigments under scanning electron microscopy, which provides further insight into the non-spherulitic morphology observed under a Linkam device. A possible correlation between the pigment morphology, the crystallization kinetics and rheology is discussed.     

Modeling flow induced crystallization in film casting of polypropylene

Data from iPP film casting experiments served as a basis to model the effect of flow on polymer crystallization kinetics. These data describe the temperature, width, velocity and crystallinity distributions along the drawing direction under conditions permitting crystallization along the draw length.In order to model the effect of flow on crystallization kinetics, a modification of a previously defined quiescent kinetic model was adopted. This modification consisted in using a higher melting temperature than in the original quiescent model. The reason for the modification was to account for an increase of crystallization temperature due to entropy decrease of the flowing melt. This entropy decrease was calculated from the molecular orientation on the basis of rubber elasticity theory applied to the entangled and elongated melt. The evolution of molecular orientation (elongation) during the film casting experiments was calculated using a non-linear dumbbell model which considers the relaxation time, obtained from normal stress difference and viscosity functions, to be a function of the deformation rate.The comparison between experimental distributions and model based crystallinity distributions was satisfactory.     

Rheological scaling and modeling of shear-enhanced crystallization rate of polypropylene

Shear-induced isothermal crystallization of a commercial isotactic polypropylene (iPP) has been investigated by using a rotational rheometer at the steady shear rates ranging from 0.00012 s⁻¹ to 1 s⁻¹, and the temperatures from 135 to 145 °C. Two time scales can be utilized to characterize the crystallization rates: one is the level-upturn onset time of the viscosity; another is that of the normal force. Plotting the onset times against the corresponding onset strain, a common critical value for all the undercooling temperatures can be identified, below which the shear flows have no significant effect on the crystallization rates. Furthermore, we propose a concept of dimensionless onset work; this parameter can make the normalized onset times approximately temperature-invariant in the range of our experiment. Our modeling of the quiescent crystallization is based on the nucleation theory of Ziabicki; the results indicate two-dimensional crystallite growth on pre-existing nuclei. The shear enhanced crystallization is modeled by estimating the excess free energy induced by the flow, and using the rheological model recently proposed by Marrucci, in which the required relaxation times are derived from our rheological measurements. The results imply that the crystallization under the present low shear rates is still two-dimensional crystallite growth on pre-existing nuclei, thus supporting the athermal nucleation theory proposed by Janeschitz-Kriegl. Compared with the experimental data, the modeling is only partially successful. Further improvements encompassing the effects of shear flows on the non-linear increase of the number density of athermal nuclei and on the acceleration of polymer chain disentanglement are needed.     

Aspects of crystallization fouling

Crystallization fouling or scale formation is temperature dependent, and a degree of supersaturation is required before precipitation occurs. In general, a nucleus is necessary around which crystals can form. The mechanism of crystallization on surfaces is complex, and it is difficult to explain in mathematical terms, although attempts have been made. The paper concludes with a brief review of scale formation under boiling conditions and the deposition of wax crystals from organic liquids.     

Rheology of carbon nanofiber-reinforced polypropylene

The rheological properties of two different nanocomposite systems consisting in the dispersion of carbon nanofibers (CNFs) in polypropylene are investigated. The nanoreinforced systems were identically prepared with two CNFs that differ only in the length of the fibers being otherwise identical to analyze the effect of fiber aspect ratio. Linear dynamic viscoelasticity and the steady-state rheology of the two different nanocomposites are presented. The system reinforced with CNFs with larger aspect ratio shows several rheological features that resemble peculiarities of rodlike polymers in the nematic liquid crystalline phase.     

Rheological characteristics of glass-fibre-filled polypropylene melts

The rheological properties of glass fibre-filled polypropylene melts have been investigated. A high pressure capillary rheometer has been used for the experimental study. The effect of shear rate, temperature, and fibre concentration on the melt viscosity and viscoelastic properties have been studied. An equation has been proposed to correlate the melt viscosity with shear rate, temperature and fibre content. A master curve relation on this basis has been brought out using the shift factora _T . a _T shift factor (=/ _r ) - A _i coefficients of the polynomical of eq. (1) (i = 0, 1, 2, ,n) - B constant in the AFE equation (eq. (2)) (Pa s) - B constant in eq. (3) - D extrudate diameter - d capillary diameter - activation energy at constant shear rate (kcal/mole) - E activation energy at constant shear stress (kcal/mole) - T melt temperature (K) - X fraction glass fibre by weight - shear rate (s^–1) - shear viscosity (Pa s) - normal stress coefficient (Pa s²) - ₁ – ₂ first normal-stress difference (Pa) - shear stress (Pa) - r at reference temperature     

Non-linear mechanical behaviour of oriented polypropylene

The paper is concerned with the study of isothermal extensional deformations of an oriented polypropylene monofilament under time-dependent loading. The monofilament has been subjected to various loading programmes, each containing a number of loading steps. It is found that the monofilament exhibits non-linear memory dependent behaviour. The question of mathematical representation of the observed non-linear behaviour is considered. It is assumed that the elongation at a given time is a non-linear functional of the stress-rate history to which the filament has been subjected prior to this instance. It is then shown that the experimental results can be represented, with reasonable accuracy, by approximating this functional by the sum of a linear and third order hereditary functional. It is seen that the loading programmes employed in the study provide substantial information for the construction of the kernels defining the two functionals.     

Structure development during crystallization of polycaprolactone

In this paper, the quiescent crystallization of polycaprolactone (PCL) melts is studied by rheological measurements coupled to calorimetry and optical microscopy. Based on a comparison between the different techniques, we find that the increase in viscoelastic properties during crystallization starts only when a relatively high degree of crystallinity is reached, which corresponds to a much developed crystalline microstructure. Like other semicrystalline thermoplastic polymers, the crystallization of PCL can be seen as a gelation process. In this case, however, we find a peculiar critical gel behavior, as the liquid-to-solid transition takes place at a very high (~20%) relative crystallinity, and this value is independent of temperature. These facts, and the comparison with optical microscopy observations, suggest that the microstructure at the gel point is controlled by the interactions between the growing crystallites. The gel time (from rheometry) and the half-crystallization time [from differential scanning calorimetry (DSC)] both show an Arrhenius-like behavior and have the same pseudoactivation energy. A practical implication of this parallel behavior of t _gel and t _0.5 is that the rheological measurements can be used to extend to higher temperatures the study of crystallization kinetics where DSC is not sufficiently sensitive.This paper was presented at the second Annual European Rheology Conference (AERC) held in Grenoble, France, 21–23 April 2005.     

Quiescent and shear-induced crystallization of polyprophylenes

In this paper, the effect of shear on the flow-induced crystallization (FIC) of several polypropylenes of various macrostructures was studied using rheometry combined with polarized microscopy. Generally, an increase in strain and strain rate or decrease of temperature is found to decrease the thermodynamic barrier for crystal formation and thus enhancing crystallization kinetics at temperatures between the melting and crystallization points. Secondly, popular models based on suspension theory which are used to relate the degree of crystallinity to normalized rheological functions (such as viscosity) are validated experimentally. For this purpose, the space filling of crystals in the polarized micrographs determined from image processing was plotted as a function of normalized viscosity under various shear rates. It is found that the constant(s) of various suspension models should be dependent on the flow parameters in order for the suspension models to describe the effect of shear on FIC, particularly at higher shear rates.     

Modeling crystallization of a binary alloy

Translated from Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi Fiziki, No. 4, pp. 39–45, July–August, 1989.     

Recovery of shear modification of polypropylene melts

Long-chain branched polypropylenes (LCB-PP) of different degrees of branching (up to 1 branch/10⁴ carbon atoms) and a linear polypropylene (L-PP) are deformed at different shear conditions (rate, time and deformation) leading to reversible modifications of the entanglement structure. These modifications recover with time. Because of it the intensity of the modification and the rate of recovery are studied. At shear rates between 1?s^?1 and 10?s^?1 lower rates modify stronger. The intensity increases with shear time to a maximum at times of about 1 h where the final deformation does not control the intensity. Obviously, the disentanglement created by shearing competes with the Brownian motion coupling entanglements. Also, the intensity increases with the degree of LCB where the increase is stronger at low degrees. The rate of recovery not influenced noticeably by the initial modification strongly depends on the degree of LCB. The pertinent recovery functions grow exponentially to the limiting value of the unmodified state. Three different recovery processes are found. The fastest one with a recovery time shorter than 10³?s is assigned to linear chains. The process with a time of about 5·10³?s independent of the degree of LCB is assumed to describe the recovery of the backbones. The times for the very slow recovery of the side chains increase with the degree of LCB (between 10⁴ and 10⁵?s for the investigated samples). The recovery strength reflects the initial modification and depends on the degree of LCB. By that, the recovery behaviour provides information on the molecular structure.     

Anisotropic thermal conductivity in sheared polypropylene

We discuss the anisotropy of the thermal conductivity tensor in polymer flow in this paper. Isotactic polypropylene (iPP) specimens were deformed by injection moulding at high shear rates and by steady shear at low shear rates, and were then quenched. The thermal conductivities parallel and perpendicular to the shear direction were measured using modulated differential scanning calorimetry (MDSC) in accordance with the ASTM E1952-01. The measured results showed that the thermal conductivity of the sheared polymer was anisotropic with an increase in the shear direction. The thermal conductivity can be regarded as varying either with the strain or the stress, as suggested by Van den Brule (1989). In addition to the Van den Brule mechanism, crystallization during flow also changes the thermal conductivity and this effect may often be dominant. Suggestions for procedures in processing computations, based on both effects, are given.     

A rheological study of shear induced crystallization

The isothermal crystallization of three isotactic polypropylene (iPP) types, with different molar mass (distributions), was studied after a well defined shear treatment of the melt at an elevated temperature and a subsequent quench to the crystallization temperature. For these experiments a standard rheometer of the cone and plate configuration was used. The development of the crystallization was monitored by dynamic oscillatory measurements. Shearing in the melt was shown to enhance subsequent crystallization at lower temperatures. Not only the total shear at constant rate is of importance, but also the chosen combination of rate and shearing time. Moreover, a pronounced influence of molar mass was detected. The exploration of the melting temperatures and times which are necessary for an erasion of the memory effects showed that the effect of shearing could not completely be erased, possibly as a consequence of mechanical degradation.