Crystallization Kinetics Of Polymers Research Articles

Identification of three groups of polymers regarding their non-isothermal crystallization kinetics

The non-isothermal crystallization of 11 different homopolymers and one copolymer is measured by fast differential scanning calorimetry (FDSC) with various cooling rates. A new method to evaluate the kinetics of polymer crystallization based on the Gibbs free energy difference between liquid and crystal is proposed. As a result a surface parameter, κ, is introduced characterizing the surface free energy of the crystals in respect to the molecular packing density. Regarding this surface parameter the homopolymers can be grouped in three groups. Polyamides and polyesters behave differently compared to the other polymers investigated here. Inside a group of polymers the ratio between surface tension and crystalline package density is widely invariant regarding crystal perfectness and chemical structure.

Polymer crystallinity and crystallization kinetics via benchtop 1H NMR relaxometry: Revisited method, data analysis, and experiments on common polymers

Semi-crystalline polymers play an enormously important role in materials science, engineering, and nature. Two thirds of all synthetic polymers have the ability to crystallize which allows for the extensive use of these materials in a variety of applications as molded parts, films, or fibers. Here, we present a study on the applicability of benchtop 1H NMR relaxometry to obtain information on the bulk crystallinity and crystallization kinetics of the most relevant synthetic semi-crystalline polymers. In the first part, we investigated the temperature-dependent relaxation behavior and identified T = Tg + 100 K as the minimum relative temperature difference with respect to Tg for which the mobility contrast between crystalline and amorphous protons is sufficient for an unambiguous determination of polymer crystallinity. The obtained bulk crystallinities from 1H NMR were compared to results from DSC and XRD, and all three methods showed relatively good agreement for all polymers. In the second part, we focused on the determination of the crystallization kinetics, i.e., monitoring of isothermal crystallization, which required a robust design of the pulse sequence, precise temperature calibration, and careful data analysis. We found the combination of a magic sandwich echo (MSE) with a short acquisition time followed by a Carr-Purcell-Meiboom-Gill (CPMG) echo train with short pulse timings to be the most suitable for monitoring crystallization. This study demonstrates the application of benchtop 1H NMR relaxometry to investigate the bulk crystallinity and crystallization kinetics of polymers, which can lead to its optimal use as an in-situ technique in research, quality control, and processing labs.

A Generalized Avrami Equation for Crystallization Kinetics of Polymers with Concomitant Double Crystallization Processes

Concomitant double crystallization processes widely occur in semicrystalline polymers and their blends because of structural and morphological heterogeneity. In this work, a generalized kinetics model was established for the isothermal crystallization in the presence of concomitant double crystallization processes, based on the Avrami theory for single crystallization process. The different situations of the double crystallization processes, including independent, competitive, and composite types, are considered in the model by introducing the concept of “domains”. This model was applied to crystallization of triblock terpolymer with morphological defects, polymorphic poly(vinylidene fluoride)/nanoclay nanocomposite, isotactic polypropylene with a broad isotacticity distribution, and poly(ethylene terephthalate) ultrathin film. It is found that, besides the information on crystallization kinetics, other information such as morphological defect, nucleation density, cocrystallization, and structure of ultra...

Controlling Polymer Crystallization Kinetics by Sample History

AbstractIn this paper systematic changes in the isothermal crystallization kinetics (at Tc = 200 °C) of nonequilibrated isotactic polystyrene films, generated through spin coating are reported. Kinetics is tuned by annealing films at Ta = 250 °C, i.e., above their nominal melting temperature (TmDSC = 220 °C). An exponential decrease in growth rate and a significant decrease of crystal nucleation density by approximately three orders of magnitude are observed by increasing the time (ta) provided for equilibration at Ta. The characteristic exponential decay time is approximately four orders of magnitude longer than the equilibrium reptation time at Ta. Interestingly, with increasing ta, a change in the morphology of lamellar crystals varying from spherulites to hexagons of rather uniform height to finally hexagons exhibiting periodic modulations in height is observed. These observations reveal that understanding and tuning the crystallization kinetics of polymer films under quiescent conditions requires knowledge on how strongly chain conformations deviate from equilibrium.

The effect of a secondary process on polymer crystallization kinetics – 3. Co-poly (lactic acid)

The crystallization kinetics of a copolymer of l-lactic acid with 4% d-lactic acid has been studied using FTIR spectroscopy by measuring the absorbance of the crystalline carbonyl stretching band at 1759cm−1. Copolymerization greatly reduced the rate of crystallization and the technique directly measured the relative crystallinity over extended periods with sufficient accuracy to test the validity of the Avrami equation. This was modified to account for the simultaneous presence of a secondary process from the onset of crystallization, such that the overall fractional crystallinity, Xt, is related to the lapsed time, t, by.Xt=Xp,∞(1-exp-Zptn)1+kst12where Xp,∞ is the final fractional crystallinity achieved by the primary process, Zp is the primary composite rate constant incorporating nucleation and growth, and ks is the secondary rate constant.The additional crystallinity produced by the secondary processes is sufficient to account for the observed fractional constant n values on analysis of the total development of crystallinity with time. It was concluded that the analysis using the Avrami equation should be restricted to the time dependence of the crystallinity produced by the primary process alone.

Influence of Graphene Oxide on Crystallization Behavior and Chain Folding Surface Free Energy of Poly(vinylidenefluoride‐co‐hexafluoropropylene)

Graphene oxide (GO) filled poly(vinylidene fluoride‐co‐hexafluoropropylene) (PVDF‐HPF) copolymer nanocomposites as promising piezoelectric materials are developed, and their crystallization behavior and chain folding free energy are examined. Appropriate distribution of GO nanoparticles in the copolymer is confirmed by transmission electron microscopic analysis. The crystalline structure of nanocomposites is analyzed by wide‐angle X‐ray scattering and Fourier transform infrared spectroscopy. The results show an increase in the β‐phase content of PVDF‐HPF copolymer in the presence of GO. Non‐isothermal crystallization kinetics of the neat polymer and corresponding nanocomposites are studied by a multiple heating rate differential scanning calorimetry using modified Avrami–Jeziorny and Liu models. Moreover, barrier energy of crystallization is calculated by Friedman and Kissinger models. It is found that addition of GO to the copolymer increases nucleation activity of the nanocomposites. Investigation on linear crystal growth via Hoffman's theorem indicates an enhanced nucleation activity upon increasing GO content. The surface folding free energy of the neat polymer is changed from 5.79 × 10−2 to 7.27 × 10−2 J m−2 upon addition of 5 wt% of GO. A bundle‐like mechanism is proposed, which can explain the crystallite growth. Analysis based on Vyazovkin theorem reveals that the crystallization activation energy is independent of GO at elevated temperatures.

Tunable Multiscale Nanoparticle Ordering by Polymer Crystallization

While ∼75% of commercially utilized polymers are semicrystalline, the generally low mechanical modulus of these materials, especially for those possessing a glass transition temperature below room temperature, restricts their use for structural applications. Our focus in this paper is to address this deficiency through the controlled, multiscale assembly of nanoparticles (NPs), in particular by leveraging the kinetics of polymer crystallization. This process yields a multiscale NP structure that is templated by the lamellar semicrystalline polymer morphology and spans NPs engulfed by the growing crystals, NPs ordered into layers in the interlamellar zone [spacing of (10–100 nm)], and NPs assembled into fractal objects at the interfibrillar scale, (1–10 μm). The relative fraction of NPs in this hierarchy is readily manipulated by the crystallization speed. Adding NPs usually increases the Young’s modulus of the polymer, but the effects of multiscale ordering are nearly an order of magnitude larger than those for a state where the NPs are not ordered, i.e., randomly dispersed in the matrix. Since the material’s fracture toughness remains practically unaffected in this process, this assembly strategy allows us to create high modulus materials that retain the attractive high toughness and low density of polymers.

Sintering of Two Viscoelastic Particles: A Computational Approach

Selective laser sintering (SLS) is a high-resolution additive manufacturing fabrication technique. To fully understand the process, we developed a computational model, using the finite element method, to solve the flow problem of sintering two viscoelastic particles. The flow is assumed to be isothermal and the particles to be in a liquid state, where their rheology is described using the Giesekus and XPP constitutive models. In this work, we assess the parameters that define this problem, such as the initial geometry, the Deborah number and other dimensionless parameters present in the rheological models. In particular, the conformation tensor is considered, which is a measure for the polymeric strain and plays an important role in the crystallization kinetics of semicrystalline polymers like polyamide 12, usually used in SLS.

Crystallization kinetics of polymer fibrous nanocomposites

Through applying both a probabilistic approach and a combination of probabilistic and the Avrami ‘extended volume’ approaches we have derived a theory of overall crystallization kinetics of polymers reinforced with nanofibers. The theory describes the crystallization kinetics in the presence of straight or curved nanofibers, with different nucleation ability and orientation, and allows to account for their variable length. The analytic results are supported by computer simulations of spherulitic structures. The derived mathematical formulas are in exponential forms suggesting the use of the Avrami logarithmic coordinates for detailed analysis of experimental data. Experimental data on crystallization of several nanocomposites, including polypropylene reinforced with poly(tetrafluoroethylene) nanofibers and polyamide 12 with carbon nanotubes, are in a good agreement with the theoretical predictions.

Effect of shear flow on spherulitic growth and nucleation rates of polypropylene

The effect of flow on the crystallization kinetics of polymers is well known, at least from a qualitative point of view. Quantitative determinations are difficult to obtain, due to the experimental difficulties of obtaining direct measurements of nucleation and growth rates during flow. In this work, step shear tests were conducted in the Linkam device adopting a new protocol in which the flow is applied when the crystalline structures have already reached dimensions easily detectable by an optical microscope. This delayed protocol allowed to achieve a more accurate experimental analysis of the effect of flow on spherulite growth rate with respect to what already reported in the literature. The new results obtained for the growth rate were interpreted, with the equilibrium melting temperature function of the shear rate. Also the nucleation rate could be assessed and the existence of a correlation between nucleation rate and growth rate was confirmed.

Nucleating agents for high-density polyethylene-A review

A review of nucleating agent (NA) types and their effect on crystallization in high-density polyethylene (HDPE) is presented. The focus is on methods to improve the physical properties of HDPE due to its widespread use in commercial applications and high volume use in typical industrial processes including extrusion, injection molding, and blow molding. The crystallization process in HDPE significantly affects its final optical, mechanical, and thermal properties. The addition of NAs affects the physical properties of HDPE by controlling the crystallization from the melt state. Specific NAs improve properties such as clarity, cycle time, and modulus. NAs are more widely developed for polypropylene (PP) than HDPE as its slower crystallization rate allows greater control in achieving property improvements. While certain NAs are effective in improving property characteristics in HDPE, greater control over the crystallization process would achieve further improvements in specific properties. Research has progressed in identifying effective NAs for HDPE, though the magnitude of the effects remains lower than those generally observed in PP. Inorganic and organic NAs are reviewed with an emphasis on the mechanisms by which they function. Fundamentals of polymer crystallization and modeling kinetics during both isothermal and nonisothermal studies provide the necessary framework for characterizing the effects of a NA in HDPE. Finally, the interactions between HDPE, NA, and industrial processing conditions as related to practical applications are discussed. POLYM. ENG. SCI., 56:541–554, 2016. © 2016 Society of Plastics Engineers

Nanocomposite of Poly(Lactic Acid)/Cellulose Nanocrystals: Effect of CNC Content on the Polymer Crystallization Kinetics

The barrier and mechanical properties of polymer matrices are affected by the polymer crystallization kinetics and the incorporation of nanoparticles can modify the nucleation density as well as the rate and mechanism of crystallization. In this study, a nanocomposite of poly(lactic acid) (PLA) containing cellulose nanocrystals (CNC) was prepared in order to evaluate the effect of the CNC content on the crystallization kinetics of the polymer matrix. Dimethylformamide, a solvent of PLA, was used to replace the aqueous medium for the dispersion of the CNC, to facilitate the preparation of PLA films with dispersed CNC. Higher levels of CNC in the films induced an increase in the crystallization rate and reduced the degree of crystallinity without affecting the dimensions of the crystal lattice. In conclusion, the incorporation of CNC into PLA influences the crystallization kinetics, which significantly affects the PLA processing conditions.

Isothermal crystallization kinetics of AB2 hyper-branched polymer (HBP)-filled polypropylene (PP)

In this paper, the isothermal crystallization kinetics of pure polypropylene (PP) and AB2 hyper-branched polymer (HBP)/PP blends have been investigated by differential scanning calorimetry (DSC). During isothermal crystallization, the crystallization rates of the blends are higher than those of PP. Furthermore, in the blends with different HBP contents, the value of t 1/2 became smaller with increasing HBP content; however, the crystallization rate of the blend decreased slightly when the content of HBP is 5 %. An increase in the Avrami exponent means the addition of HBP influences the mechanism of nucleation and the growth of PP crystallites. In addition, the crystallization activation energy of pure PP and HBP/PP blends were also discussed, and the result showed that the crystallization activation energy has decreased remarkably in HBP/PP blends.

Confined crystallization of polymers within anodic aluminum oxide templates

In this article, a review of recent literature on confined crystallization within nanoporous anodic aluminum oxide (AAO) templates is presented. For almost all infiltrated polymeric materials, crystal orientation within the nanopores is a function of pore diameter. Tc and Tm usually decrease and are a function of pore size. When no pore interconnection remains, the crystallization occur at large supercoolings in heterogeneity free environments. Hence, the nucleation mechanism changes from heterogeneous to surface or homogeneous nucleation. The crystallization kinetics of infiltrated polymers should be close to first order, since in confined environments nucleation is the determining step of the overall crystallization and Avrami indexes (n) of ∼1 (or lower in some cases) should be obtained. Examples are provided where these conditions have been met and first order kinetics (n = 1) were measured as opposed to higher orders (n = 3−4) for the same polymer in the bulk. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014, 52, 1179–1194

Effects of supported (nBuCp)2ZrCl2 catalyst active center multiplicity on crystallization kinetics of ethylene homo- and copolymers

Two different supported zirconocene, that is, bis(n-butylcyclopentadienyl) zirconium dichloride (nBuCp)2ZrCl2, catalysts were synthesized. Each catalyst was used to prepare one ethylene homopolymer and one ethylene–1-hexene copolymer. Catalyst active center multiplicity and polymer crystallization kinetics were modeled. Five separate active center types were predicted, which matched the successive self-nucleation and annealing (SSA) peak temperatures. The predicted crystallinity well matched the differential scanning calorimetric (DSC) values for a single Avrami–Erofeev index, which ranged between 2 and 3 for the polymers experimented. The estimated apparent crystallization activation energy Ea did not vary with cooling rates, relative crystallinity α, and crystallization time or temperature. Therefore, the concept of variable/instantaneous activation energy was not found to hold. Ea linearly increased with the weight average lamellar thickness Lwav DSC-GT; and for each homopolymer, it exceeded that of the corresponding copolymer. Higher Ea, hence slower crystallization, was identified as a pre-requisite to attain higher crystallinity. Crystallization parameters were correlated to polymer backbone parameters, which are influenced by catalyst active center multiplicity.

Morphology and crystallization kinetics of polyethylene/long alkyl-chain substituted Polyhedral Oligomeric Silsesquioxanes (POSS) nanocomposite blends: A SAXS/WAXS study

The dispersal, quiescent crystallization kinetics and morphology of a series of unique polyethylene–polyhedral oligomeric silsesquioxanes (PE-POSS) nanocomposite blends is presented. POSS molecules with long linear alkyl-chain substituents were blended at one composition into a commercial low density polyethylene. Time-resolved Small- and Wide-Angle X-ray scattering (SAXS/WAXS) and thermal techniques were used to elucidate the affect that POSS and its substituent groups have on the dispersal and crystallization kinetics of the host polymer. The miscibility and dispersal of the POSS molecules was seen to increase with the increasing alkyl-chain length substituents suggesting increased compatibility and interaction with the host polymer chains. The POSS molecules act as nucleating agents increasing the crystallinity, crystallization kinetics and influencing the final lamellar morphology. Thus, these unique POSS compounds show great potential as nancomposite filler particles in polyolefins where the alkyl-chain substituent plays a vital role in its compatibility and subsequent improvement of physical properties in the host polymer.

Nonisothermal crystallization kinetics of AB2 hyperbranched polymer‐filled polypropylene

In this article, nonisothermal crystallization kinetics of polypropylene (PP) and AB2 hyperbranched polymer (HBP)‐filled PP have been investigated by differential scanning calorimetry. The Avrami analysis modified by Mandelkern and a method combined with Avrami and Ozawa equations were employed to describe successfully the nonisothermal crystallization kinetics of samples. The conclusion showed that HBP can prompt crystallization effectively. Furthermore, in blends of different HBP contents, the value of t1/2 became smaller with increasing HBP content; however, the crystallization rate of the blend decreased slightly when content of HBP is 5%. An increase in the Avrami exponent showed that addition of HBP influenced the mechanism of nucleation and the growth of PP crystallites. The possible explanation could be attributed to the fractal structure of HBP. The polarized micrographs showed that HBP acts as a heterogeneous nucleation agent, and the nucleation efficiency has increased remarkably in HBP/PP blends. POLYM. ENG. SCI., 53:2535–2540, 2013. © 2013 Society of Plastics Engineers

Recent research on conductive phase-change materials for energy storage

(PCM) for energy storage are materials that can store and release thermal energy through melting and solidification processes. Over the years a fair amount of work has been done to determine the suitability and properties of such materials. The first challenge was to find ways to prevent the leaking or flow of the molten paraffins. Some solutions e.g. encapsulation of the PCM by a thin polymeric shell and the formation of immiscible polymer/ PCM blends were investigated. The most recently investigated challenge is to improve the thermal conductivity of the systems, because both polymer and paraffin have relatively high thermal resistivities. A few papers were published during the past four years where the PCM was directly mixed with the conductive filler. These investigations included expanded graphite, carbon nanofibres and carbon nanotubes as fillers mixed into several paraffinbased PCMs. Generally the thermal conductivity increased, but several factors such as particle shape and orientation and particle dispersion were found to influence the extent of improvement in thermal conductivity. Another approach was the microencapsulation of a PCM by an inorganic conductive shell. The most investigated approach was the impregnation of thermally conductive, porous fillers with the liquid PCM. It was observed that the PCM was trapped inside the pores of the fillers, and generally no leakage was observed after melting of the PCM. The last approach, which involves polymers, is the formation of immiscible polymer/PCM blends containing conductive fillers. The effectiveness of such systems is determined by the extent of immiscibility of the polymer and PCM, and the strength of the interaction between the polymer or PCM and the conductive filler. To me the last approach is the most interesting from a polymer scientist’s viewpoint, because there are a number of factors that may determine the properties and effectiveness of such a phase-change system, e.g. crystallization kinetics of polymer and PCM, extent of co-crystallization, location and dispersion of conductive filler particles, extent of interaction of the filler particles with respectively the polymer and the PCM, to name a few. This may not be groundbreaking research, but the results of such research will certainly contribute to the finding of solutions for the provision of renewable energy.

Influence of the Calendering Step on the Adhesion Properties of Coextruded Structures

Abstract The coupled coextrusion calendering process of multilayer thermoplastic sheets has been studied. The structure of the film has been observed, the density of chemical links at the interface and the adhesion between the coextruded layers have been measured. There are some controversial features about the influence of calendering parameters. Several models of increasing complexity are proposed to master the process and its influence on adhesion. A multilayer thermal model accounting for the crystallization kinetics of both polymers allows defining qualitatively the temperature field especially at the interface between the two layers. A multilayer thermo-mechanical model provides quantitative figures on the stress, shear, elongation and temperature fields in the coextruded film and especially at the interface. Relationships between these parameters, density of chemical links at the interface, crystalline structure of the sample and peeling forces are discussed.

PBAT based nanocomposites for medical and industrial applications

Poly(butylene adipate-co-terephthalate) (PBAT) based nanocomposites were prepared by melt blending PBAT with 5 and 10wt.% of clay nanoparticles (unmodified and modified montmorillonites, unmodified and modified fluoro-hectorites, and unmodified sepiolites). All nanocomposites showed a good level of clay distribution and dispersion into PBAT, especially nanocomposites with high clay chemical affinity with the polymer matrix. DSC results showed that addition of layered silicates slightly hindered kinetics and extent of crystallization of PBAT; however, sepiolite particles were able to promote polymer crystallization kinetics and the transformation of the PBAT crystal structure to a more ordered form.Similar increases in the thermal stability of PBAT in nitrogen and air were obtained upon addition of all clays, due to a barrier effect of the clays toward polymer decomposition product ablation.Preliminary biocompatibility tests indicated that PBAT based materials with 10% clay content have good biological safety and display almost no cytotoxicity.The addition of all nanofillers increased the hardness of PBAT matrix. The DMA analysis showed that all nanocomposites presented higher E′ values than neat PBAT, indicating that addition of clays improved the mechanical properties of PBAT. For layered silicate nanocomposites, the main influencing factors on the thermo-mechanical properties appeared to be the aspect ratio and dispersion of clay nanoplatelets, rather than polymer/clay chemical affinity. The highest E′ values of sepiolite based nanocomposites make this nanoparticle the most attractive material for tissue engineering and environmental industrial applications.