Crystaf: Crystallization analysis fractionation. A new approach to the composition analysis of semicrystalline polymers

Crystallization Analysis Fractionation is a new technique for the analysis of composition distribution in semicrystalline polymers; more specifically for the analysis of branching distribution in Polyethylene and tacticity in Polypropylene type resins. CRYSTAF as well as TREF (Temperature Rising Elution Fractionation) are separation techniques which fractionate species of differing crystallizability by slow cooling of a polymer solution. TREF however, demands in addition to the crystallization step a second temperature cycle, elution step, to obtain the polymer composition. CRYSTAF, on the other hand, extracts the information in the crystallization cycle by monitoring the solution concentration depression as temperature goes down; thus reducing significantly the analysis time and simplifying the hardware needs. CRYSTAF principles are discussed and various applications are described.     

Physical characterization of blends of isotactic poly(butene-1) with ethylene-propylene copolymer

Melt-mixed blends of isotactic poly(butene-1) (PB) with an ethylene-propylene copolymer (EPM) containing 60 wt% PP were studied over the complete composition range. Phase-contrast polarizing microscopy and dynamic mechanical spectroscopy revealed that the blend is heterogeneous. DSC studies of quenched and annealed blends for both PB modifications indicate that total blend crystallinity decreases linearly with the EPM content. Pure PB crystallinity is enhanced to a small degree in the presence of EPM. Tensile behaviour of the blends was good up to moderate EPM levels. It was also demonstrated that blends containing EPM with increased PP content showed synergism in tensile behaviour not exhibited by blends with EPM of lower PP content. Appropriate mechanical models tested over the complete composition and temperature range suggest that the rubbery phase adheres strongly to the PB matrix. Overall, the experimental results support the contention that the system is mechanically compatible, possibly a result of component miscibility at elevated temperatures as predicted on thermodynamic grounds.     

Crystallization kinetics of poly(ethylene oxide) confined in semicrystalline poly(vinylidene) fluoride

Polymer blends based on poly(vinylidene fluoride) (PVDF) and poly(ethylene oxide) (PEO) have been prepared to analyze the crystallization kinetics of poly(ethylene oxide) confined in semicrystalline PVDF with different ratios of both polymers. Both blend components were dissolved in a common solvent, dimethyl formamide. Blend films were obtained by casting from the solution at 70 °C. Thus, PVDF crystals are formed by crystallization from the solution while PEO (which is in the liquid state during the whole process) is confined between PVDF crystallites. The kinetics of crystallization of the confined PEO phase was studied by isothermal and nonisothermal experiments. Fitting of Avrami model to the experimental DSC traces allows a quantitative comparison of the influence of the PVDF/PEO ratio in the blend on the crystallization behavior. The effect of melting and further recrystallization of the PVDF matrix on PEO confinement is also studied. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018 , 56, 588–597     

Isothermal melt and cold crystallization kinetics of poly(aryl ether ketone ether ketone ketone)

The isothermal melt and cold crystallization kinetics of poly(aryl ether ketone ether ketone ketone) are investigated by differential scanning calorimetry over two temperature regions. The Avrami equation describes the primary stage of isothermal crystallization kinetics with the exponent n ≈ 2 for both melt and cold crystallization. With the Hoffman–Weeks method, the equilibrium melting point is estimated to be 406 °C. From the spherulitic growth equation proposed by Hoffman and Lauritzen, the nucleation parameter (K_g) of the isothermal melt and cold crystallization is estimated. In addition, the K_g value of the isothermal melt crystallization is compared to those of the other poly(aryl ether ketone)s. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 1992–1997, 2000     

AFM-tip-induced crystallization of poly(ethylene oxide) melt droplets

The AFM-tip-induced crystallization of poly(ethylene oxide) (PEO) melt droplets was studied. The melt droplets with a height of 50–100 nm and a lateral size of 2–3 μm were obtained by melting the PEO ultra-thin films on a mica surface. For the PEO samples with average molecular weights (M _n) ranging from 1.0 × 10³ g/mol to 1.0 × 10⁴ g/mol, the lateral perturbation from the AFM tip in the hard-tapping or nanoscratch modes could not induce the growth of the flat-on lamellae. In contrast, under AFM nanoindentation mode, the tip-induced crystallization occurred when a sufficiently high vertical tip force was applied to the melt droplets of PEO with M _n ⩾ 1.0 × 10⁴ g/mol. Moreover, the experimental results indicated that the AFM-tip-induced crystallization of PEO in the nanoindentation process had molecular weight dependence. Translated from Acta Polymerica Sinica, 2006, (4): 553–556 (in Chinese)     

Kinetics of polymer crystallization from the melt (calorimetric approach)

Crystallization kinetics for 12 polymers including polyolefins, polyesters, polyurethanes, polysiloxanes was measured by the evolution of heat in a modified Calvet-type calorimeter over wide temperature ranges. The results are analyzed in terms of the Avrami equation and a comparison between calorimetric and dilatometric results is carried out. It is concluded that, although in the majority of cases experimental results do not obey the Avrami equation, for some polymers the agreement is rather good. The Avrami parameter obtained, however, depends on the experimental technique. Possible reasons for this disagreement are discussed. Analysis of the calorimetric crystallization rate in the vicinity of the melting point by using the kinetic theory of crystallization shows that the growth is controlled by surface (two-dimentional) nucleation. Energy parameters for the crystallites were determined and it is shown that the surface energy of the crystallites depends on the molecular structure of the polymer. Temperature dependence of the calorimetric crystallization rate of the polymers for which crystallization rates could be determined above and below the maximum rate are analyzed using a kinetic equation with common approximations for the transport term. The influence of melting conditions on the crystallization rate was studied. The results indicate heterogeneous nucleation in the polymer melt. It is concluded that this may be due both to impurities and to high regularity of macromolecules in the polymer melt.     

Micromechanical modeling of the deformation kinetics of semicrystalline polymers

The mechanical behavior of semicrystalline polymers is strongly dependent on their crystallinity level, the initial underlying microstructure, and the evolution of this structure during deformation. A previously developed micromechanical constitutive model is used to capture the elasto‐viscoplastic deformation and texture evolution in semicrystalline polymers. The model represents the material as an aggregate of two‐phase layered composite inclusions, consisting of crystalline lamellae and amorphous layers. This work focuses on adding quantitative abilities to the multiscale constitutive model, in particular for the stress‐dependence of the rate of plastic deformation, referred to as the slip kinetics. To do that, the previously used viscoplastic power law relation is replaced with an Eyring flow rule. The slip kinetics are then re‐evaluated and characterized using a hybrid numerical/experimental procedure, and the results are validated for uniaxial compression data of HDPE, at various strain rates. A double yield phenomenon is observed in the model prediction. Texture analysis shows that the double yield point in the model is due to morphological changes during deformation, that induce a change of deformation mechanism. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 49: 1297–1310, 2011     

New insight into melting and crystallization behavior in semicrystalline poly(ethylene terephthalate)

After isothermal crystallization, poly(ethylene terephthalate) (PET) showed double endothermic behavior in the differential scanning calorimetry (DSC) heating scan. During the heating scans of semicrystalline PET, a metastable melt which comes from melting thinner lamellar crystal populations formed between the low and the upper endothermic temperatures. The metastable melt can recrystallize immediately just above the low melting temperature and form thicker lamellae than the original ones. The thickness and perfection depends on the crystallization time and crystallization temperature. The crystallization kinetics of this metastable melt can be determined by means of DSC. The kinetics analysis showed that the isothermal crystallization of the metastable PET melt proceeds with an Avrami exponent of n = 1.0 ∼ 1.2, probably reflecting one‐dimensional or irregular line growth of the crystal occurring between the existing main lamellae with heterogeneous nucleation. This is in agreement with the hypothesis that the melting peaks are associated with two distinct crystal populations with different thicknesses. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 53–60, 2000     

Micromechanics of semicrystalline polymers: Yield kinetics and long-term failure

The process of plastic deformation in semicrystalline polymers is complicated due to the operation of a variety of mechanisms at different levels and is strongly dependent on their underlying microstructure. The objective of this work is to establish a quantitative relation between the microstructure and the mechanical performance of semicrystalline polymers, as characterized by elasto-viscoplastic deformation. To do that, a micromechanically based constitutive model is used. The model describes the material as an aggregate of two-phase layered composite inclusions, consisting of crystalline lamellae and amorphous layers. The starting point for adding quantitative abilities to the model, in particular for the yield kinetics, is formed by experimental observations on both the yield kinetics and the time-to-failure of polyethylene at different temperatures, which reveal the contribution of two relaxation processes. To predict the thermo-rheologically complex short-term and long-term failure behavior, the crystallographic slip kinetics and the amorphous yield kinetics are re-evaluated, and the Eyring flow rule is modified by adding a temperature shift function. To enable the prediction of both tension and compression, a non-Schmid effect is added to the constitutive relation of each slip system. The creep behavior of polyethylene is then simulated directly using the multiscale, micromechanical model, predicting the time-to-failure, controlled by plastic deformation. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

Molecular orientations in semicrystalline poly(trans 1-octenylene)

Infrared dichroic ratios of drawn poly (trans 1-octenylene) for five absorption bands (722, 968, 1070, 1362 and 1468 cm⁻¹) were measured and compared with birefringence results, leading to a value of 44° for the angle between the dipole moment vector of the 1070 cm⁻¹ band and the chain axis. The increase of the orientation function 〈P₂(cos θ )〉 with the draw ratio rougly fits to the pseudo-affine scheme of deformation.     

Chromatography of functional polymers: A new approach to the characterization of reactive polymers obtained by chemical modification

High-performance liquid chromatography (HPLC) has been used to complement size-exclusion (gel permeation) chromatography (SEC) for the characterization of functional polymers. Whereas SEC is unable to detect compositional changes, HPLC in an appropriate interacting medium can provide detailed information on compositional changes occurring during chemical modification of a polymer. The method has been demonstrated using a normal-phase column consisting of porous monodisperse 10 μm poly(2,3-dihydroxypropyl methacrylate-co-ethylene dimethacrylate) beads that have a homogeneous coverage of aliphatic hydroxyl groups for the analysis of brominated poly(isobutylene-co-4-methylstyrene). Differences of well below 1 mol % of bromomethylstyrene units are easily detected and quantified. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35: 1173–1180, 1997     

Analysis of crystallization kinetics of poly(ether ether ketone)

An optical microscope equipped with a video photograph system was used to follow the growth of spherulites. Under nitrogen atmosphere, the growth rates at 290 and 300°C suggest that when the melt of PEEK has been equilibrated for 15 min at 400°C, the subsequent crystallization behavior was nearly independent of the prior thermal history. Linear growth rates of crystallization of PEEK have been measuredin the temperature range of 260–325°C for melt-pressed films and solvent cast films. Detailed kinetic analysis indicated that PEEK exhibited an unmistakable regime II → III transition at 296 ± 1°C. The II → III transition was clearly present irrespective of the rather drastic changes in U*. It is interesting that the branching and crosslinking retarded the growth rate of PEEK, but a transition from regime II to regime III still existed. For melt-pressed films after equilibration at 400°C for 15 min, values of σ and q suggest that U* should be taken nearer to 1500 cal/mol in the case of T_∞ = T_g − 30 K and 2000 cal/mol in the case of T_∞ = T_g − 51.6 K. The K_g(III)/K_g(II) ratio (1.32) was not as close to the predicted value of 2 as was Hoffman's ratio. For PEEK, the Thomas-Staveley constant (β) should be closer to 0.25 or 0.3 instead of 0.1. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1335–1348, 1998     

Solution crystallization analysis of poly(lactic acid) by Scalls: A facile approach for thermal analysis of polymers in solution

Non-isothermal solution crystallization and dissolution behaviour of both enantiomers (D and L) of poly(lactic acid) (PLA) and their blends were studied by the unique Solution Crystallization Analysis by Laser Light Scattering (Scalls) method. For the first time, we have investigated the crystallization of this biopolymer in solution, as well as the subsequent dissolution or “solution melting”. It was found that addition of the D-enantiomer (PDLA) to the L-enantiomer (PLLA) in solution resulted in the formation of stereocomplex crystals (SC), and the nucleation-effect of the crystals was intensified with increase in PDLA content, leading to an earlier onset of crystallization and increased crystallization peak area. Differential Scanning Calorimetry (DSC) analysis confirmed the formation of SC during solution crystallization. Large re-crystallization events were seen for the pristine polymers, indicative of their low crystallization rates. Overall, results obtained by Scalls provided promising information regarding PLA crystallization kinetics, which significantly influences practical applications of this biopolymer.     

Isothermal and nonisothermal crystallization kinetics of poly(propylene terephthalate)

The kinetics of crystallization of poly(propylene terephthalate) (PPT) samples of different molecular weights were studied under both isothermal and nonisothermal conditions. The Avrami and Lauritzen–Hoffmann treatments were applied to evaluate kinetic parameters of PPT isothermal crystallization. It was found that crystallization is faster for low‐molecular‐weight samples. The modified Avrami equation, and the combined Avrami–Ozawa method were found to successfully describe the nonisothermal crystallization process. Also, the analysis of Lauritzen–Hoffmmann was tested and it resulted in values close to those obtained with isothermal crystallization data. The nonisothermal kinetic data were corrected for the effect of the temperature lag and shifted alone with the isothermal kinetic data to obtain a single master curve, according to the method of Chan and Isayev, testifying to the consistency between the isothermal and corrected nonisothermal data. A new method for ranking of polymers, referring to the crystallization rates, was also introduced. This involved a new index that combines the maximum crystallization rate observed during cooling with the average crystallization rates over the temperature range of the crystallization peak. Furthermore, the effective energy barrier of the dynamic process was evaluated with the isoconversional methods of Flynn and Friedmann. It was found that the energy barrier is lower for the low‐molecular‐weight PPT. The effect of the catalyst remnants on the crystallization kinetics was also investigated and it was found that this is significant only for low‐molecular‐weight samples. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 3775–3796, 2004     

A new controllable approach to synthesize hyperbranched poly(siloxysilanes)

A new controllable approach to synthesize hyperbranched poly(siloxysilanes) via hydrosilylation of A₂‐ and B′B_x‐type monomers was developed in this work. A₂ monomers (dimethylbis(dimethylsiloxy)siloxane and tetramethyldisiloxane), B′B_x monomers (methylvinyldiallylsilane and vinyltriallylsilane), and the resultant hyperbranched poly(siloxysilanes) were well characterized using FTIR, ¹H NMR, ¹³C NMR, ²⁹Si NMR, and SEC/MALLS. The In situ FTIR results indicate that the controllable polymerization can be carried out quickly and the reaction process was obviously performed in two stages. At the first stage, silicon hydride selectively reacts with vinyl silane groups, which produces intermediate structures with one Si? H and two (or three) allyl groups. Consequently, at the second stage, these intermediates act as new AB₂ (or AB₃) type monomers and continue to be self‐polymerized to generate hyperbranched polymers. By this novel controllable approach, molecular weights and their polydispersity of the resulted hyperbranched poly(siloxysilanes) can be conveniently regulated via adjusting the process parameters, such as feeding ratio of two monomers. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 2708–2720, 2008