Non-isothermal Crystallization Experiments Research Articles

Crystallization of Polymers with a Reduced Density of Entanglements

Since methods for reducing macromolecule entanglements have been developed, it has become possible to better understand the impact of polymer chain entanglement on the crystallization process. The article presents basic information about the disentangling of macromolecules and the characterization of the degree of entanglement. The basic knowledge of polymer crystallization was also presented. Then, it was discussed how polymers crystallize during their disentangling. Non-isothermal and isothermal crystallization experiments using disentangled polymers, and for comparison using entangled polymers, are described in more detail. The influence of disentangling on both nucleation and crystal growth is highlighted. It is also shown how the crystallization of polymers changes when macromolecules re-entangle.

New Insights into Crystallization of Heterophasic Isotactic Polypropylene by Fast Scanning Chip Calorimetry.

The crystallization kinetics of metallocene-catalyzed heterophasic isotactic polypropylene composed of a matrix of isotactic polypropylene (iPP) and rubbery particles made of random ethylene–propylene copolymers (EPC), often denoted as heterophasic iPP copolymers, was analyzed as a function of the cooling rate and supercooling in nonisothermal and isothermal crystallization experiments, respectively. Fast scanning chip calorimetry (FSC) allowed assessing crystallization at processing-relevant conditions, and variation of the content (0–39 wt %) and composition (0–35 wt % propylene counits) of the EPC particles revealed qualitatively new insight about mechanisms of heterogeneous crystal nucleation. For neat iPP homopolymer, the characteristic bimodal temperature dependence of the crystallization rate due to predominance of heterogeneous and homogeneous crystal nucleation at high and low temperatures, respectively, is reconfirmed. At high temperatures, in heterophasic iPP, the here studied ethylene-(C2)-rich EPC particles accelerate crystallization of the iPP-matrix, with the acceleration or nucleation efficacy correlating with the EPC-particle content. The crystallization time reduces by more than half in presence of 39 wt % EPC particles. An additional nucleating effect of the EPC particles on iPP-matrix crystallization is detected after their crystallization, suggesting that liquid/rubbery particles are less effective than solid/semicrystalline particles in affecting crystallization of the surrounding iPP-matrix. At low temperature, homogeneous crystal nucleation in the iPP-matrix outpaces all heterogeneous nucleation effects, and the matrix-crystallization rate is independent of the sample composition. The obtained results lead to the conclusion that the crystallization kinetics of iPP can be affected significantly by the content and composition of EPC particles, even towards superfast crystallizing iPP grades.

Thermal and crystallization behaviour of epoxidized high cis-polybutadiene rubber

High cis-polybutadiene (HCBR) was epoxidized by using in situ formed formic acid and hydrogen peroxide in the absence and presence of Tween 20 at 50 °C at different reaction times. Epoxidized HCBR (EHCBR) with degree of epoxidation from 3.7 to 38.8% was obtained. Low degree epoxidation EHCBR are semi-crystalline rubbers presenting lower crystallization temperatures than the non-epoxidized rubber on cooling and a cold crystallization and melting temperature on heating, differently from high epoxidation content rubbers (over 10%) which are amorphous materials. Epoxidation increases the glass transition temperature and decreases the melting temperature and the degree of crystallinity of the polybutadiene. The increase of epoxidation also decreases its thermal stability, which drops about 28 °C when the content of epoxidation is increased about 30%. Non-isothermal crystallization experiments of HCBR and EHCBR were systematically carried out at cooling rates of 5, 10, 20, and 30 °C/min to evaluate the crystallinity developed upon cooling. By increasing the cooling rate, the crystallization enthalpy increases for HCBR while it decreases for EHCBR.

Effect of supercooling on crystal structure of nucleated isotactic polypropylene

The present study focuses on the crystallization behavior and structure formation of isotactic polypropylene (iPP) containing polymeric nucleating systems. Poly(vinyl cyclopentane) (PVCP) and poly(vinyl cyclohexanе) (PVCH) were pre-polymerized on the catalyst particle before propylene polymerization. Non-isothermal crystallization experiments were performed by standard differential scanning calorimetry (DSC) and by fast scanning chip calorimetry (FSC) at cooling rates in the range from 10−1 to 103 K/s. Wide angle X-ray scattering (WAXS) was employed to collect information regarding the crystal structure of samples with a specific cooling history prepared in FSC. Both nucleating agents, PVCP and PVCH, led to acceleration of crystallization and increase of the temperature range of α-phase crystallization of iPP. Most striking was the result that in presence of PVCH at concentrations lower than 30 ppm, it was possible to crystallize iPP in the stable monoclinic form in the complete range of processing relevant cooling rates.

Thermal stability of CaCO3/polyethylene (PE) nanocomposites

Calcium carbonate (CaCO3) nanoparticles in polymer matrix cause to improvement in polymer performance, including thermal stability and mechanical properties. The main goal of this article is to investigate the effect of different weight percentage of nanoparticles of CaCO3 on thermal stability and mechanical properties of polyethylene (PE) nanocomposites. The morphological structure of CaCO3 nanoparticles and nanocomposites was investigated by transmission electron microscopy and scanning electron microscopy. The thermal stability of PE and its nanocomposites was also determined by differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and thermomechanical analysis. Nonisothermal crystallization experiments by DSC test showed that the incorporation of nanoparticles increased the crystallinity, glass transition temperature, and the effective energy barrier for crystallization process. Besides, degradation behavior was evaluated by TGA. The onset mass loss temperature shifted to higher value in the presence of nanoparticles.

Crystallization Behavior of Polyvinyl Alcohol with 1,2-Diol Side-Chains

Abstract The crystallization behavior of a newly developed polyvinyl alcohol with 1,2-diol side-chains (DPVA) was investigated for different 1,2-diol side-chain, magnesium acetate, and sodium acetate contents. Crystallization by stepwise cooling, isothermal and non-isothermal crystallization (the actual cooling process in melt molding) experiments were conducted. Upon crystallization analysis by stepwise cooling, the end surface energy of DPVA increased as the 1,2-diol side-chain content was increased. The isothermal crystallization process exhibited longer half crystallization times at high 1,2-diol side-chain and magnesium acetate contents. Upon Kissinger analysis of the non-isothermal crystallization results, the effect of the acetate salts was found to be smaller than that for isothermal crystallization, while the smaller number of repeating units in the lamellar structure, reflecting a higher side-chain 1,2-diol content, resulted in a smaller activation energy for DPVA crystallization. In addition, analyses by the Ozawa method and the Kissinger Plot suggested possible changes in the mode and dimension of the crystal growth of DPVA.

Crystallization and melting characteristics of iPP nucleated by a sustainable eggshell powder-supported β-nucleating agent

The β-form crystals in isotactic polypropylene (iPP) are proved to be favorable and effective for developing general iPP plastics with high performance. Herein, sustainable eggshell power decorated with calcium pimelate (DES), an effective β-form nucleating agent for iPP, was fabricated via the interaction between pimelic acid and Ca2+ on the surface of eggshell powder at elevated temperature. DSC and FTIR analysis identified the reaction occurred and formation of calcium pimelate on surface of the eggshell powder. Then, the effects of the functionalized eggshell powder on the nonisothermal and isothermal crystallization behavior, melting characteristics and polymorph structures were investigated. From the nonisothermal crystallization experiments, the incorporation of supported β-form nucleator greatly enhanced the crystallization temperature of iPP. The content of β-form crystals (Kβ) could reach a value as high as 87% at 5 mass% DES load. Moreover, the crystallization and melting characteristics of iPP nucleated by DES were not changed under multiple scan conditions with melt temperature range from 190 to 250 °C. Interestingly, from the investigation of isothermal crystallization and subsequent melting behavior, it was found that DES is a temperature-dependent selective nucleator for inducing iPP crystallization. Main α-form crystals or β-form crystals could form when using high or low crystallization temperatures, respectively. Particularly, these results demonstrated a possibility to tailor the ratio of α-form crystals to β-form crystals by simply choosing isothermal crystallization conditions. Then the crystallization mechanism based on thermodynamics and kinetics factors was further discussed.

Cristalização não isotérmica de poliamida 66: Determinação dos parâmetros da Equação de Nakamura via Método da Curva Mestre

RESUMO Poliamida é uma família de termoplásticos utilizada na fabricação de peças com tolerância dimensional relativamente estreita, como engrenagens em produtos eletrônicos, pequenos filtros de combustíveis na indústria automobilística e etc. O controle dimensional está intimamente relacionado à contração que acompanha o processo de cristalização do polímero em peças moldadas por injeção. Assim, o propósito do presente trabalho foi determinar a constante de cristalização não-isotérmica de amostra de poliamida 66 empregando-se o Método da Curva Mestre visando seu emprego em conjunto com a equação de Nakamura para simulação do processo de solidificação na moldagem por injeção. Para tanto, experimentos de cristalização não isotérmica foram realizados em um Calorímetro Diferencial de Varredura, DSC, em várias taxas de resfriamento, na faixa de 5 a 40º C/min. As curvas originais de DSC foram corrigidas em termos da defasagem de temperatura entre a amostra e o forno do DSC antes da aplicação do método. A equação de Nakamura e a constante de cristalização não isotérmica obtida pelo método da Curva Mestre foram empregados para simular as curvas de cristalinidade relativa em função da temperatura para as diferentes taxas de resfriamento. Também foi avaliada a influência do uso de diferentes valores de cristalinidade relativa inicial nas simulações via modelo de Nakamura, análise esta normalmente não encontrada na literatura. As curvas geradas pela constante de cristalização calculada via Método da Curva Mestre apresentaram uma boa concordância com os dados experimentais para esta amostra de poliamida 66 usando-se 10-5 como cristalinidade relativa inicial para a forma diferencial da equação de Nakamura. Desse modo, o modelo de Nakamura, juntamente com sua constante determinada pelo Método da Curva Mestre, se mostrou uma boa opção para a simulação do processo de cristalização na moldagem por injeção de poliamida 66.

Crystallization of Polyethylene at Large Undercooling.

Extremely fast crystallization of high-density polyethylene and random copolymers of ethylene with up to 16 mol % 1-octene was observed for the first time by ultrafast scanning calorimetry. In order to account for the inherently high crystallization rate of polyethylenes, in nonisothermal and isothermal crystallization experiments cooling rates up to 1 000 000 K/s and crystallization times as short as 10 μs, respectively, were employed. It was possible to supercool the melt of high-density polyethylene down to 57 °C and the melt of a random ethylene/1-octene copolymer with 16 mol % 1-octene down to -33 °C, without prior crystallization. At these temperatures, the characteristic time of the primary crystallization process is of the order of magnitude of 100 μs. Complete vitrification of the liquid would require cooling even faster than 1 000 000 K/s. Compared to the homopolymer, the cooling-rate dependence of the crystallization temperatures and the temperature dependence of the characteristic time of primary crystallization of random ethylene/1-octene copolymers both are nearly parallel shifted to lower temperatures. Fast crystallization under conditions of reduced linear crystal growth rate is possibly caused by boosting homogeneous nuclei density up to 1027 m-3 and urgently requires further investigation.

Determination of non-isothermal crystallization rate constant of a rotational molding grade LLDPE

The purpose of the present paper was to test the validity of the nonlinear regression method for calculating the non-isothermal crystallization rate constant of the Nakamura's model of a rotational molding grade LLDPE directly from non-isothermal crystallization experiments carried out in a single cell DSC. Cooling rates of 50, 40, 30, 20, 10 and 5 oC/min were used with samples of 3.0 mg under nitrogen atmosphere. Here, good agreement was observed between the experimental relative crystallinity curves and the simulated ones using the calculated parameters by nonlinear regression. It shows that this method can be used to determine the Nakamura's non isothermal rate constant for using in simulation of the cooling phase of rotational molding. In this paper it was used 10-3, 10-4 and 10-14 as the initial crystallinity in the Nakamura's model. However the best average results for all cooling rates was obtained when 10-4 was used. Average spherulitic dimensions of LLDPE studied in this paper did not change significantly with different cooling conditions.

Effect of comonomer partitioning on the kinetics of mesophase formation in random copolymers of propene and higher α-olefins

The effect of counit type on the kinetics of mesophase formation has been investigated by means of chip-calorimetry in propene/α-olefin random copolymers, containing counits which show large differences in their co-crystallization behavior with propene, i.e. 1-butene and 1-hexene. Non-isothermal crystallization experiments indicated that the minimum cooling rate at which mesophase formation is observed is directly related to the kinetic of α-phase crystallization, which is lower for the copolymer with the bulkier 1-hexene counit. Isothermal structuring was probed in a wide temperature range, revealing that a double bell-shaped curve is required to describe the temperature dependence of crystallization times of the two polymorphs. The ordering kinetics of the mesophase is the fastest in i-PP homopolymer and decreases with increasing comonomer bulkiness, analogous to what happens for the monoclinic phase. The results are discussed by considering the effect of comonomer on the driving force for mesophase formation, also at the light of new WAXD and density evidences, which prove different extents of inclusion of 1-butene and 1-hexene in the ordered phases.

Crystallization of poly(lactic acid) accelerated by cyclodextrin complex as nucleating agent

Poly(lactic acid) (PLA) is a well-known biodegradable and biocompatible polyester with intrinsically slow crystallization rate. To extend its applications to the field where heat resistance is required, increasing the crystallization rate of the material becomes critical. In this note, the nucleation effect of supramolecular inclusion complex (IC), organized by non-covalent interactions through threading α-cyclodextrin molecules onto PLA chains, on the crystallization of PLA was investigated by differential scanning calorimetry (DSC) and polarized optical microscopy. The formation of IC was confirmed by wide-angle X-ray diffraction and DSC measurements. It was found that the presence of PLA-IC significantly promoted the crystallization of PLA from both the non-isothermal and isothermal crystallization experiments. The nucleation mechanism was also discussed to some extent.

Crystallization behavior of poly(lactic acid)/elastomer blends

Differential Scanning Calorimetry (DSC) was used to evaluate the crystallization behavior of poly(lactic acid) and its blends with elastomer. It has been observed that the cold crystallization temperature of the blends decreased as the weight fraction of elastomer increased as well as the onset temperature of cold crystallization also shifted to lower temperature. In non-isothermal crystallization experiments, the crystallinity of poly(lactic acid) increased with a decrease in the heating and cooling rate. The melt crystallization of poly(lactic acid) appeared in the low cooling rate (1, 5 and 7.5 °C/min). The presence of low elastomer tends also to increase the crystallinity of poly (lactic acid). The DSC thermogram at ramp of 10 °C/min showed the maximum crystallinity of poly(lactic acid) is 36.95% with 20 wt% elastomer contents in blends. In isothermal crystallization, the cold crystallization rate increased with increasing crystallization temperature in the blends. The Avrami analysis showed that the cold crystallization was in two stages process and it was clearly seen at low temperature. The Avrami exponent (n) at first stage was varying from 1.59 to 2 which described a one-dimensional crystallization growth with homogeneous nucleation, whereas at second stage was varying from 2.09 to 2.71 which described the transitional mechanism to three dimensional crystallization growth with heterogeneous nucleation mechanism. The equilibrium melting point of poly(lactic acid) was also evaluated at 176 °C.

Effect of PTW on crystallization kinetics of toughened PBT/PC blends

AbstractPoly(butylenes terephthalate) (PBT)/polycarbonate (PC)/poly(ethylenebutylacrylate- glycidyl methacrylate copolymer) (PTW) blends containing PTW as toughening modifier were obtained by extrusion and injection molding. The effect of PTW on the melt rheology of PBT/PC blend was examined by means of capillary calorimetry. The rheological properties of the blends suggested high reactivity between epoxy groups in PTW and terminal carboxylic acid group and/or hydroxyl group of PBT inhibited transesterification between PBT and PC. The effect of PTW on the crystallization of PBT in the PBT/PC/PTW blends was investigated by crystallization kinetics analysis under isothermal and nonisothermal conditions. Isothermal crystallization experiments were performed at crystallization temperature and analyzed based on the Avrami equation. A decrease of the half time of PBT crystallization t1/2, as well as an increase of the overall crystallization rate of PBT in the PBT/PC/PTW blends have been observed. Nonisothermal crystallization experiments were performed at given cooling rate. The Ozawa analysis is used to describe its nonisothermal crystallization behavior. It is observed that at a given cooling rate, the PTW increases the overall crystallization rate of PBT in the PBT/PC/PTW blends. This is consistent with the result obtained under isothermal crystallization condition.

Cellular Anorthite Glass-Ceramics: Synthesis, Microstructure and Properties

Highly permeable cellular anorthite glass ceramics with porosity of ∼95% were obtained by a simple replication technique using near-stoichiometric glass powders and a polymeric foam as sacrificial template material. Impacts of sintering conditions and additions of minor constituents, respectively, on microstructural evolution and resulting macroscopic properties of the derived foams were investigated by X-ray diffraction, scanning electron microscopy (SEM), differential thermal analysis and X-ray microcomputer tomography. Apparent activation energies of crystallization and the Avrami coefficient were estimated from nonisothermal crystallization experiments to evaluate the impact of titania and zirconia, respectively, as potential nucleation agents. Consistent with SEM analyses, it was found that crystal growth occurs in two dimensions. While TiO2 primarily acts on the viscosity of the precursor glass and, thus, on the process of sintering by viscous flow, ZrO2 is found to exhibit at least some nucleation efficiency. As compared with sintering of ceramic powders and solid-state reactions in general, the glass ceramic route enables significant reduction in sintering time and temperature.

Study of clay nanocomposites of the biodegradable polyhexamethylene succinate. Application of isoconversional analysis to nonisothermal crystallization

AbstractNanocomposites of organomodified montmorillonites and the biodegradable polyester derived from hexanediol and succinic acid were prepared by the solution‐casting method using chloroform as solvent. Samples were studied by means of X‐ray diffraction and transmission electron microscopy. Intercalated structures differentiated by the stacking mode between silicate layers were observed. The highest variability in interlayer spacing was found when C30B organoclay was added. In this case, hydroxyl groups of the modifier could interact with polar carbonyl groups of the polyester. Thermal stability and crystallization behavior under both isothermal and nonisothermal conditions were evaluated. The overall crystallization rate of the intercalated nanocomposites was higher than that of the neat polyester due to a significant increase in their nucleation density, which compensated for their lower crystal growth rate. Isoconversional analysis was used to determine effective activation energies and to estimate nucleation and transport energy parameters from nonisothermal hot crystallization experiments. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 2234–2248, 2008

Transparent Hafnia‐Containing β‐Quartz Glass Ceramics: Nucleation and Crystallization Behavior

Heterogeneous nucleation and crystallization of lithium alumosilicate glasses with hafnia‐containing nucleation agents was explored. Kinetic, thermodynamic, and structural data were considered to assess nucleation efficiency and to characterize the crystallization process. It is shown how lattice parameters and, particularly, anisotropy of the nuclei phase depend on the amount of ZrO2–HfO2 substitution in a specific base glass. For a given crystallization treatment, the size of the derived crystallites of β‐quartz structure is used as a measure of nucleation rates. A nonlinear dependence of nucleation efficiency on HfO2 content was established, with the supposedly most efficient nucleation corresponding to the lowest degree of anisotropy of the nuclei crystallites, i.e., when about 20%–30% of ZrO2 was substituted by HfO2. Apparent activation energies and estimates of the Avrami coefficient were determined from nonisothermal crystallization experiments for selected compositions to highlight the differences between hafnia and zirconia. Hafnia can be used alone or in combination with other agents to nucleate nanocrystalline glass ceramics with a low coefficient of thermal expansion.

The effect of nanoclays on the nucleation, crystallization, and melting mechanisms of isotactic polypropylene

AbstractThe influence of organomodified nanoclay (montmorillonite) on the crystallization and melting mechanisms of isotactic polypropylene (iPP) was studied. Films of both neat polymer and clay nanocomposites were prepared after molecular weight optimization through melt extrusion. Products exhibited the tactoidlike morphology since no compatibilizers were used. The effect of introduction of nanoclay within the polymer was studied through isothermal crystallization and linear heating. The results indicated that low nanoclay concentrations induce the formation of the β‐crystalline structure, its formation being inhibited with high nanoclay contents. Dynamic nonisothermal bulk crystallization experiments indicated that nanoclays act as nucleating agents of iPP. Isothermal, bulk crystallization studies indicated that the crystallization process (induction time and crystallization kinetics) is proportionally accelerated by the nanoclay presence. There was also an accelerated formation of secondary crystallization when nanoclays were present in high concentrations. POLYM. ENG. SCI., 47:1889–1897, 2007. © 2007 Society of Plastics Engineers

Determinação da constante cinética de cristalização não-isotérmica de polipropilenos modificados com ácido acrílico e anidrido maleico

O propósito do presente trabalho foi testar a validade do Método da Curva Mestre para a determinação da constante de cristalização não-isotérmica de polipropilenos modificados com anidrido maleico e ácido acrílico. Experimentos de cristalização não-isotérmica foram realizados a diversas taxas de resfriamento por meio de Calorimetria Exploratória Diferencial (DSC). Para serem usadas no Método da Curva Mestre, as curvas originais obtidas em DSC foram corrigidas em termos da defasagem de temperatura entre a amostra polimérica e o forno do equipamento. A equação de Nakamura e a constante de cristalização determinada foram utilizadas para simular as curvas de cristalinidade relativa em função da temperatura para as diversas taxas de resfriamento. As curvas simuladas apresentaram boa concordância com as curvas experimentais para ambas as amostras de polipropileno modificado, mostrando que o Método da Curva Mestre pode ser empregado com sucesso para este polímero.

Crystallization kinetics for simulation of processing of various polyesters

AbstractThe approach to determine crystallization kinetic parameters based on the DSC nonisothermal crystallization experiments is applied to poly(butylene terephthalate) (PBT) and poly(ethylene‐2,6‐naphthalate) (PEN). The differential form of the Nakamura equation and master curve approach are used. The isothermal induction times are obtained from nonisothermal induction times according to the concept of induction time index. The correction of temperature lag between the DSC furnace and the sample is incorporated. The corrected nonisothermal crystallization kinetic data is shifted with respect to an arbitrarily chosen reference temperature to obtain the master curve. By fitting the obtained master curve with the Hoffman‐Lauritzen equation, the model parameters for the crystallization rate constant are obtained. The relative crystallinity measured at different cooling and heating rates is described by these model parameters. © 2006 Wiley Periodicals, Inc. J Appl PolymSci 102: 2847–2855, 2006