Melt Memory Effect Research Articles

Analysis of the influence of the injection molding process on the crystallization kinetics of a HDPE

ABSTRACTThermal analyses of microparts in high density polyethylene (HDPE) have shown that the specific processing conditions used in microinjection molding have irreversible consequences on the polymer morphology. This result has been demonstrated with the analysis of the non‐isothermal crystallization behavior of a HDPE with different thermal histories. The evolution of the absolute crystallinity has been analyzed with a relevant model able to separate the primary and secondary mechanisms all over the crystallization duration. This model has emphasized that the evolution of the primary crystallinity with time is different for the microparts compared to the conventional objects. These differences were attributed to variations of the crystallization mechanisms, especially within the nucleation phase, where a persistent melt memory effect of the former chains orientation/extension was assumed. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016, 133, 44239.

Communication: Theory of melt-memory in polymer crystallization.

Details of crystallization processes of a polymer at the crystallization temperature Tc from its melt kept initially at the melt temperature Tm depend profoundly on the nature of the initial melt state and often are accompanied by memory effects. This phenomenon is in contrast to small molecular systems where the supercooling (Tm (0)-Tc), with Tm (0) being the equilibrium melting temperature, and not (Tm - Tc), determines the nature of crystallization. In addressing this five-decade old puzzle of melt-memory in polymer crystallization, we present a theory to describe melt-memory effects, by invoking an intermediate inhomogeneous melt state in the pathway between the melt and crystalline states. Using newly introduced dissolution temperature T1 (0) for the inhomogeneous melt state and the transition temperature Tt (0) for the transition between the inhomogeneous melt and crystalline states, analytical formulas are derived for the nucleation rate as a function of the melt temperature. The theory is general to address different kinds of melt-memory effects depending on whether Tm is higher or lower than Tm (0). The derived results are in qualitative agreement with known experimental data, while making predictions for further experiments on melt-memory.

Effect of Heterogeneous Short Chain Branching Distribution on Acceleration or Retardation of the Rate of Crystallization from Melts of Ethylene Copolymers Synthesized with Ziegler‐Natta Catalysts

SummaryEthylene 1‐alkene copolymers with the same molar mass, same molar mass distribution and very similar average comonomer content of 3.8 +/− 0.4 mol% but different broad interchain comonomer content distribution (CCD), are found to display different extents of melt memory effect on crystallization rate. With decreasing the initial melt temperature, all broadly distributed copolymers show enhanced crystallization followed by a decrease of crystallization rate. However, copolymers with a higher content of molecules in the medium composition range (6‐2 mol% comonomer) and a lower content of lightly branched molecules (<2 mol% comonomer), display the highest effect of memory of a prior crystallization in their high temperature melts. The increase in crystallization peak temperature is up to 4 °C. The acceleration and further retardation of the crystallization are associated with self‐seeds that remain in one‐phase or two liquid phases melts respectively. The magnitude of the effect on crystallization is proportional to the number of surviving seeds in the initial melt.

Melt memory of a spherulite nucleus formed through a seeding process in the crystal growth of isotactic polystyrene

The melt memory effect on the crystal growth of isotactic polystyrene is investigated using optical microscopy. After a spherulite is melted at a slightly higher temperature than the melting point and completely disappears, a polymer crystal nucleates and grows at the same position as the original spherulite when the temperature is decreased below the melting point. After the melting–recrystallization processes are repeated by raising and lowering the temperature, the melt memory effect becomes weak. This effect completely disappears at ~40 °C above the equilibrium melting point when the melt temperature is gradually increased during the melting–recrystallization processes. The formation of spherulite nuclei is promoted by the seeding process, which consists of the following consecutive procedures: quenching a sample to a temperature below the glass transition temperature and annealing the sample slightly above the glass transition temperature. At the initial stage, the number density of spherulites rapidly decreases with the cumulative melting time. After the initial stage, the number density of spherulites exponentially decays with the cumulative melting time and approaches a finite value. This asymptotic value decreases with the melt temperature.

Interplay between a Strong Memory Effect of Crystallization and Liquid–Liquid Phase Separation in Melts of Broadly Distributed Ethylene–1-Alkene Copolymers

Narrow metallocene-made random ethylene copolymers display a strong memory effect of crystallization above their equilibrium melting point akin to the melt memory effect observed in model ethylene–1-butene copolymers. The onset temperature for self-nucleation or surviving self-seeds displays a bell shape with increasing comonomer content with a maximum at ∼2 mol % branches. Self-seeds do not survive at temperatures above the equilibrium melting point for homopolymers and copolymers either with very low branching or with a branching content >4.5 mol %. The self-seeds are associated with clusters of ethylene sequences that remain in the melt in close proximity and accelerate a subsequent crystallization, as observed by higher crystallization peak temperatures and higher nucleation density. Contrasting this behavior, commercial ethylene–1-alkene copolymers with a broad, bimodal comonomer distribution display an inversion of the crystallization rate in a range of melt temperatures where narrow copolymers show...

A facile route to ordered γ phase in poly (vinylidene fluoride) via melt annealing and re-crystallization

The formation of γ phase in poly (vinylidene fluoride) is not rare from solution casting and high temperature melt crystallization. However, it is still a challenge to obtain γ phase with high melting point in a facile route. This study demonstrates that it can be readily realized via melt annealing of solution-cast films at 180°C, followed by subsequent re-crystallization upon rapid cooling in air. The re-crystallization into ordered γ phase arises from the formation of long TTTGTTTG' sequences in the melt due to transformation of TTTT conformation of initial β phase, rather than from the melt memory effects of original γ phase.

Melt memory effects on recrystallization of polyamide 6 revealed by depolarized light scattering and small‐angle X‐ray scattering

AbstractMelt memory effects on recrystallization of polyamide 6 were studied in a length scale from nm to μm using small‐angle X‐ray scattering (SAXS) and depolarized light scattering (DPLS). The memory effects on recrystallization rate were discussed in terms of the incubation period before nucleation and the half time of the crystallization, which were measures of the nucleation rate and the growth rate, respectively. Both rates are almost independent of the annealing temperature of the melt in the remelting process for the short term annealing below ∼ 3 min while they are slowed as the annealing temperature increases for the long term annealing, showing that the relaxation of melt memory takes very long even above the equilibrium melting temperature T. Extrapolating the incubation period to infinite annealing, time we found that it was very hard to attain the fully relaxed state in polyamide 6 even above the equilibrium melting temperature. This must be due to the strong hydrogen bonding in polyamide 6. We also discuss the memory effects on the final structure after recrystallization based on the SAXS and DPLS profiles. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011

Origin of the melt memory effect in polymer crystallization

Reproducibility of repeated crystallization experiments from melts is obtained after annealing the melt at a sufficiently high temperature for short time, or at lower temperatures for longer time. This annealing process erases the polymer melt memory. Its physical origin remains elusive, but it is linked to precursor structures for crystallization. What precursor structures are, and how they are affected by shear flow and melt temperature is also unclear. We identify in this work two well-defined melt states: the fully relaxed melt and the melt sheared up to a steady state. Crystallization from fully relaxed melts is slowest while from melts sheared up to the steady state is fastest. We demonstrate that polymers crystallized at the same temperature from the two different melt states have similar average spherulite size, but melts sheared up to steady state have lower viscosity and low number of entanglements, this being the reason for the acceleration of crystallization kinetics in these melts. Annealing of the melts sheared up to the steady state slows down the crystallization kinetics until it becomes comparable with that of fully relaxed melts.

Shear-induced nonisothermal crystallization of low-density polyethylene

The effect of melt memory on the shear-induced nonisothermal crystallization of a low-density polyethylene melt is analyzed with a shear DTA instrument. The melt state responsible for the saturation of shear-induced isothermal crystallization was identified previously as the steady state in steady shear flows and the strain applied to the melt was identified as the controlling factor for that saturation. Here it is shown that the same strain that saturates the isothermal crystallization also saturates the shear-induced nonisothermal crystallization. Regardless of the cooling rate, the nonisothermal crystallization, starting from the same sheared melt temperature, saturates at the same strain. The contribution of the melt memory effect to the overall nucleation density is estimated, and it is concluded that the majority of nuclei results from the thermal-induced crystallization.

Manipulation of Athermal Nuclei in Aqueous Poly(ethylene oxide) by Scanning Activity Gravimetric Analysis

We show that polymer solutions exhibit memory effects as they undergo repeated dissolution (deliquescence) and crystallization (efflorescence) transitions, similar to those previously observed in polymer melts. These memory effects have been probed using scanning activity gravimetric analysis (SAGA) on aqueous poly(ethylene oxide). SAGA is a new method that enables manipulation of the thermodynamic state of a polymer/solvent system under isothermal conditions. The measurements have been performed on individual micron-sized (∼10 ng) polymer particles that are levitated electrostatically and subjected to programmed changes in the activity of solvent vapor. The particle mass rapidly changes due to sorption or evaporation of solvent in response to changes in vapor activity, providing insights into the changing state of the polymer/solvent system. Repeated cycling of solvent activity within a levitated particle reveals systematic shifts up and down in the deliquescence and efflorescence activities depending upon the extreme activities reached. Melt memory effects have previously been attributed to remnants of lamellar crystals that persist beyond the nominal melting temperature. The present observations suggest that the thickness of these remnants evolves in solution, changing the supersaturation at which they may serve as athermal nuclei. An extension to the classical thermodynamic model of lamellae that includes a reduced interfacial tension at the perimeter of the high energy fold surface predicts the existence of a metastable radius that allows remnants to slowly increase or decrease in thickness under certain conditions, thereby rationalizing the ability to manipulate efflorescence behavior. We also present an analysis that suggests that SAGA can be extended to other polymer/solvent systems with the appropriate choice of temperature.

Effect of annealing time on the self‐nucleation behavior of semicrystalline polymers

AbstractIt is widely known that when a polymer is heated just above its melting point and is kept at a given temperature (denoted Ts) for a short time, when it is cooled down its nucleation density increases and its peak crystallization temperature shifts to higher temperatures, as detected for instance by differential scanning calorimetry (DSC). The Ts temperature range where the described process occurs has been named Domain II self‐nucleation (SN) because the selected Ts temperatures are high enough to melt the polymer without causing detectable annealing of any remnant crystals by DSC. Experimental results obtained by DSC, polarized light optical microscopy (PLOM), and rheology indicate that these techniques are unable to detect any remaining crystal fragments in Domain II. Our kinetic results demonstrate that Domain II SN is a transient phenomenon that can even disappear if enough time at Ts is allowed. Results of the study of the time dependence of the SN effect indicates two possibilities: (a) if crystal fragments are present (even if undetected by the employed techniques) their final melting is a very slow process (in the order of hours); (b) if all crystallites have melted in Domain II, then it may be more plausible to reinterpret self‐nuclei as arising from “precursors” whose detail nature has not been the subject of this investigation but that can be regarded as either a residual segmental orientation in the melt (i.e., a melt memory effect) or a mesophase in a preordered state. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 1738–1750, 2006

Relationship between magnetic alignment and the crystallization condition of isotactic polystyrene

The relationship between a magnetic alignment and crystallization conditions of isotactic polystyrene (iPS) was studied by means of DSC, POM, WAXD, FT-IR, and birefringence measurements. The onset temperature for crystallization during the cooling process was found to depend on the melting temperature. From observations using a polarized optical microscope, theproportion of spherulite decreased in cases where melting occurred at greater than 265 °C. These results suggest that melting at less than 265 °C causes self-seeding, which is known as the ‘melt memory effect’ in iPS, leading to a high nucleation density. The maximum intensity of transmitted light that could be observed by birefringencemeasurements was also dependent on the melting temperature. The high nucleation density that was obtained by melting at less than 265 °C was suitable for achieving higher orientations. In situ infrared measurements in a magnetic field were carried out in a home-built apparatus into which an autorotation polarizer unit had been installed. This allowed us to automatically acquire simultaneous polarized infrared spectra both parallel and perpendicular to the magnetic field. Orientation was confirmed for the crystalline band, while no orientation was observed for the amorphous band. The dichroic difference increased during isothermal crystallization with increasing crystallinity. No orientation effects were observed in the melted state in the present study.

Crystallization of poly(ethylene-co-octene): II Melt memory effects on first order kinetics

Dilatometric and X-ray scattering experiments of the crystallization kinetics of a sample of poly(ethylene-co-octene) show pronounced melt memory effects, i.e., the shapes of isotherms and characteristic times vary systematically with the temperature of the melt prior to cooling to the crystallization temperature. The temperature range of the effect is limited; crystallization kinetics remains constant below a melt temperature T(m)l and above a melt temperature T(m)h and varies only in-between. Analysis shows that the melt memory effect is caused by a variation of the characteristic time of a first order crystallization process. The process can be assigned to the in-filling of crystallites into objects of a previously generated precursor structure.

Melt memory effects in polymer crystallization

AbstractPolymer melts submitted to local shear flow fields, as well as isotropic solid samples, were treated in the melt at different temperatures and for various duration of time. The effects of thermal history on kinetics and morphological characteristics of subsequent isothermal crystallization was investigated. Results suggest that subcritical pseudocrystalline aggregates, behaving as predetermined athermal nuclei when temperature is brought below melting point, can survive in the melt for long time. Their concentration decays exponentially with a single characteristic relaxation time. For all investigated polymers, temperature dependence of relaxation times can be fitted by an Arrhenius law with high apparent activation energy. Experiments performed with narrow molecular weight fractions of poly(ethylene oxide) suggest that relaxation times relevant to destruction of oriented nucleation precursors are proportional to M1,5.