Improved experimental characterization of crystallization kinetics

Polymer solidification occurring in many processes, like for instance injection molding, compression molding and extrusion, is a complex phenomenon, strongly influenced by the thermo-mechanical history experienced by the material during processing. From this point of view, characterization of polymer crystallization in the range of processing conditions, i.e. including high cooling rate, is of great technological and academic interest. Quiescent, non-isothermal crystallization kinetics of two polypropylene resins were investigated using a new method, based on fast cooling of thin samples with air/water sprays and optical detection of the crystallization phenomenon. The range of cooling rates attained in this experimental study is considerably larger than that achieved by traditional methods. Quiescent crystallization kinetics of the resins is also investigated by the means of DSC, operated under isothermal conditions with a limited degree of under-cooling and for constant cooling rates up to about 1 K s⁻¹. The results demonstrate the importance of performing fast cooling experiments to gather reliable crystallization kinetics data.     

Crystallization kinetics of virgin and processed poly(lactic acid)

Poly(lactic acid) (PLA) is an emerging material mainly because it can be synthesized from renewable resources and is thus environmentally and ecologically safe. The mechanical properties, above all the thermal resistance of PLA are determined by the crystalline content: the heat deflection temperature of crystalline PLA can reach 100 °C, whereas amorphous PLA loses mechanical properties at temperatures slightly higher than 60 °C. However, PLA has a low crystallization rate, so that after processing it remains mostly amorphous. This characteristic heavily limits the use of PLA for commercial applications. Many studies have been recently published on the crystallization kinetics of PLA. The effect of processing on this feature is however often neglected. In this work, the significance of processing on the crystallization kinetics of a commercial PLA was investigated. Two processing methods were explored: extrusion and injection moulding. The obtained materials, and the starting pellets of virgin polymer, were analyzed by calorimetry in order to obtain the crystallization kinetics. Two protocols were adopted to determine the crystallization rates during cooling from the melt or heating from the solid. The parameters of a kinetic equation were determined for all the materials and protocols adopted and it was thus possible to describe the evolution of crystallinity during heating and during cooling.     

An experimental method for studying nonisothermal crystallization of polymers at very high cooling rates

A new technique based on light depolarizing microscopy was developed for studying non-isothermal crystallization of polymers at average cooling rates up to about 5000°C/min. The polymer is cooled down by a gaseous cooling medium supplied at a constant temperature. The temperature of polymer is measured by a thermocouple imbedded directly in the sample. A heat transfer analysis was used to establish appropriate sample geometry to assure that, under the applied cooling condition, the temperature distribution along the sample thickness can be neglected. A light-scattering effect, which occurs when crystallization is carried out under high cooling rates, was observed. This required the development of a method to correct the depolarized light intensity for the effect of light scattering. An appropriate correction method was developed based on both a theoretical and an experimental analysis of the light intensity measurement. This provided a means to measure the overall crystallization kinetics. Examples of such measurements for iPP, HDPE, and LDPE are presented. In addition to the overall crystallization kinetics, the developed technique includes a video camera and VCR system used for measurements of spherulite growth rates during crystallization under high cooling rates. Constant spherulite growth rates were observed for isotactic polypropylene crystallized under very non-isothermal conditions. © 1996 John Wiley & Sons, Inc.     

Phenomenological approach to compare the crystallization kinetics of isotactic polypropylene and polyamide‐6 under pressure

Reliable experimental data for semicrystalline polymers crystallized under pressure are supplied on the basis of a model experiment in which drastic solidification conditions are applied. The influence of the pressure and cooling rate on some properties, such as the density and microhardness, and on the product morphology, as investigated with wide‐angle X‐ray scattering (WAXS), is stressed. Results for isotactic polypropylene (iPP) samples display a lower density and a lower microhardness with increasing pressure over a wide range of cooling rates (from 0.01 to 20 °C/s). Polyamide‐6 (PA6) samples exhibit the opposite behavior, with the density and microhardness increasing at higher pressures over the entire range of cooling rates investigated (from 1 to 200 °C/s). A deconvolution technique applied to iPP and PA6 WAXS patterns has allowed us to evaluate the final phase content and to assess the crystallization kinetics. A negative influence of pressure on the α‐crystalline phase crystallization kinetics can be observed for iPP, whereas a slightly positive influence of pressure on the crystallization kinetics of PA6 can be noted. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 40: 153–175, 2002     

Nonisothermal crystallization behavior, and morphology of poly(trimethylene terephthalate)/polyethylene glycol copolymers

Poly(trimethylene terephthalate)/polyethylene glycol (PTT/PEG) copolymers, with PEG content ranging from 27.2 to 47.4 wt%, were synthesized by melt copolycondensation. Wide-Angle X-ray diffractometer revealed that all copolymers had the same crystal structure of homo-PTT at room temperature. All copolymers could form ring-banded spherulites, and band spacing increased with increasing PEG content at a given crystallization temperature. Nonisothermal crystallization morphology of copolymers was greatly influenced by cooling rate. When the cooling rate was 2.5 °C/min or lower, banded patterns were absent, whereas when the cooling rate was 20 °C/min or higher, a novel crystal morphology composed of non-banded spherulites (central part) and ring-banded spherulites with decreasing band spacing along the radial growth direction was observed. Moreover, the size of the non-banded spherulitic part decreased with increasing cooling rate. Finally, the nonisothermal crystallization kinetics of copolymers were analyzed and only the Mo method was satisfactory to accurately describe this system.     

Hybrid thermal management of lithium-ion batteries using nanofluid,metal foam,and phase change material: an integrated numerical–experimental approach

Safety issues of Li-ion batteries imposed by unfavorable thermal behavior accentuate the need for efficient thermal management systems to prevent the runaway conditions. To that end, a hybrid thermal management system is designed and further investigated numerically and experimentally in the present study. The passive cooling system is fabricated by saturating copper foam with paraffin as the phase change material (PCM) and integrated with an active cooling system with alumina nanofluid as the coolant fluid. Results for various Reynolds numbers and different heating powers indicate that the hybrid nanofluid cooling system can successfully fulfill safe operation of the battery during stressful operating conditions. The maximum time in which all PCM field is changed to the liquid phase is defined as the onset of the stressful conditions. Therefore, the start time of stressful conditions at 41 W and Re 420 is increased from 3700 s with nanofluid composed of 1% volume fraction nanoparticles (VF-1%) to 4600 s with nanofluid VF-2% during high current discharge rates. Nanofluid cooling extends the operating time of the battery in comparison with the water-based cooling system with 200-s (nanofluid with volume fraction of 1%) and 900-s (nanofluid with volume fraction of 2%) increases in operating time at Reynolds of 420. Using nanofluid, instead of water, postpones the onset of paraffin phase transition effectively and prolongs its melting time which consequently leads to a decrease in the rate of temperature rise.

     

Interpretation of the nonisothermal crystallization kinetics of polypropylene using a power law nucleation rate function

A nucleation rate function is proposed for use in analyzing the overall crystallization kinetics of polymers. This function allows for the possibility that the nucleation rate varies substantially during the crystallization. This feature is particularly useful in analyzing nonisothermal crystallization, but it can be used to analyze isothermal crystallization as well. The nucleation rate function was used in the derivation of a modified transformation kinetics equation of the Avrami type. The modified Avrami equation was found to be suitable for kinetics analysis for the data obtained from nonisothermal crystallization at rapid cooling rates. Kinetics parameters used to describe nonisothermal crystallization under rapid cooling rates are presented and discussed. These include crystallization induction time, plateau (crystallization) temperature, crystallization half-time, crystallization rate constant, Avrami index, and newly defined quantities called nucleation index, geometric index, and nucleation rate constant. The procedure used to obtain the nucleation rate constant and nucleation index for the nucleation rate function is described and illustrated by application to the analysis of the crystallization kinetics of polypropylene. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 1077–1093, 1997     

Nonisothermal bulk crystallization studies of high density polyethylene using light depolarizing microscopy

The quiescent nonisothermal bulk crystallization kinetics of two high-density polyethylene resins were investigated by a modified light-depolarizing microscopy (LDM) technique. The technique allows studies at average cooling rates up to 2500°C/min. The polymer was found to crystallize at a pseudo-isothermal temperature even at these very high cooling rates. The overall bulk crystallization rate increased rapidly as the cooling rate and supercooling increased. Crystallization kinetics was analyzed by Avrami analysis. Avrami exponents near 3 suggested spherical growth geometry and instantaneous nucleation at predetermined sites. Observation of spherulites by optical microscopy together with a number density of spherulites that changed little with increase in cooling rate or supercooling supported this model of crystallization behavior. Analysis of the half-time of crystallization based on the Lauritzen and Hoffman secondary nucleation theory indicated that the regime II-III transition was found to occur at a degree of supercooling of approximately 22°C. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 681–692, 1998     

Crystallization kinetics of isotactic polypropylene nucleated with octamethylenedicarboxylic dibenzoylhydrazide under isothermal and non-isothermal conditions

Octamethylenedicarboxylic dibenzoylhydrazide (TMC-300) was used as a nucleating agent for isotactic polypropylene (iPP) for the first time. The Avrami method and the Caze method were used to analyze the isothermal and non-isothermal crystallization kinetics of iPP incorporated with TMC-300, respectively. During isothermal crystallization, the half crystallization time at 130 °C reduces from 130 s of virgin iPP to 44 s after addition of TMC-300, which reflects that TMC-300 increased the crystallization rate of iPP obviously. The crystallization activation energy decreases from 382.5 kJ mol^?1 of virgin iPP to 275.3 kJ mol^?1 of iPP/TMC-300. During non-isothermal crystallization, the crystallization peak temperature of iPP nucleated with TMC-300 was increased by 5.1 °C when compared to that of virgin iPP at the cooling rate of 20 °C min^?1, and both the reduction of half crystallization time and the increase in peak crystallization temperature also justified that the addition of TMC-300 accelerated the crystallization of iPP.

     

Two‐phase crystallization kinetics of syndiotactic polystyrene

In this work, a two phase crystallization model based on the extension of the Kolmogoroff approach was proposed and verified by comparison with experimental isothermal and nonisothermal crystallization data of Syndiotactic Polystyrene (sPS) in a very wide range of cooling rates, up to 600 °C/s. To investigate the effects of high cooling rate on the sPS crystalline structure, a homemade apparatus was adopted. The morphology in solid samples was analyzed by densitometry, IR spectroscopy, and X‐rays diffraction. The coupling of these techniques allows the determination of the fractions of different crystalline phases. In agreement with melt‐crystallization studies of sPS proposed by different authors, either α and β forms could be produced depending on the thermal history of the sample. Results show that the stable β form is favored for specimens solidified at higher temperature or under low cooling rates, whereas α and mesomorphic forms are favoured at low temperature or high cooling rates. The proposed multiphase crystallization kinetics model successfully described all the range of experimental data. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1757–1766, 2010     

Study of the crystallization of leucite in feldspar glass matrix

Feldspathic glass–ceramics reinforced with leucite are usually used in dental prosthesis. This study focused on leucite crystallization kinetics due to its importance to the end product of a dental crown processing. Leucite grains were nucleated and grown from feldspathic glass frit powders with particle size smaller than 45 μm. The nucleation and crystallization kinetics of leucite crystals in the glass matrix was investigated under isothermal and non-isothermal conditions through differential thermal analysis. The samples were also characterized by X-ray diffraction and scanning electron microscopy. The temperature of maximum nucleation rate was determined from the DTA curves of samples heat treated at different temperatures. The activation energy (E) of leucite crystallization was determined by the Kissinger method and the Avrami parameter (n) indicated that surface crystallization is the dominant mechanism in the glass.     

Cooling rate effect on the phase transitions in a polymer liquid crystal: DSC and real-time MAXS and WAXD experiments

The importance of the cooling rate for the structural transformations in a main-chain poly(hexamethylene-4,4′-bibenzoate) has been presented. Detailed analysis of the phase transitions, main structural parameters and their temperature changes has been performed by differential scanning calorimetry, real-time middle-angle X-ray scattering and wide-angle X-ray diffraction methods. The thermodynamic nature of the initial transformation into a smectic A phase has been discussed. The material in the smectic state is supposed to be organized in smectic domains. The crystallization from the smectic phase depends strongly on the kinetics. The crystallization inside the smectic domains results into different final structures determined by the cooling rate applied. At the highest cooling rates, only one crystalline form has been observed. Different possible modifications have been discussed for the case: either a γ-polymorphic form or still some mesophase of high order, as a frozen metastable state. There is a possibility that the phase might be also identified as a condis crystal. At decreasing cooling rates, a new crystalline form, named α∗, appears together with the first one. Lowering the cooling rate, the volume fraction of the α∗-polymorph gradually increases, at the expenses of the first form. The interesting feature of the new observed α∗-polymorph is that it has some similarities with α- and δ-phases of the same material. Contrary to the previous observations, no γ?α^∗ transformation has been observed neither during the course of single crystallization nor during the subsequent heating. A model describing the gradual transformation of the material during its temperature treatment has been proposed.     

Bias in isothermal time-to-event studies due to approach to test temperature

Oxidative induction time (OIT), constant temperature stability (CTS) and isothermal crystallization are examples of isothermal time-to-event (TTE) measurements obtained using differential scanning calorimetry. In TTE experiments, a test specimen is heated/cooled at a constant rate from the setup temperature to an isothermal test temperature. Once the test temperature is achieved, a clock is started and the time to the thermal event (e.g., onset to oxidation, thermal decomposition or crystallization exotherm peak) is measured. Such TTE values may be used to rank stability of the material at the test temperature. Some portion of the reaction of interest, however, takes place during the pre-isothermal period as the test specimen approaches the test temperature. This amount of reaction is unmeasured and represents a bias in the resultant TTE value. An equation has been derived and numerically integrated to estimate this bias. This approach shows that the bias is dependent upon the activation energy of the test reaction, the heating/cooling rate used and the temperature range between the melting temperature and the test temperature. For commonly used heating rates, the bias for OIT and CTS tests is small. Further, the myth that isothermal crystallization kinetics determinations required high cooling rates is dispelled with the bias of less than 0.9 min resulting from heating rates as low as 10°C min^–1. Knowledge of magnitude of this bias permits the selection of experimental conditions without the expense of high heating/cooling rate apparatus or extra cost cooling accessories.     

Shape memory effect and reversible phase crystallization process in SMPU ionomer

It is the first attempt to reveal the effect of reversible phase crystallization process on shape memory effect in shape memory polyurethane (PU) ionomer. Thereof the cyclic tensile testing was conducted with various cooling time to fix the temporary deformation for assessing shape memory function. The crystallization process of the reversible phase, poly (ε‐caprolactone) (PCL) in shape memory PU ionomers composed of different ionic group contents, 1,4‐butanediol, 4,4′‐methylenebis(phenyl isocyanate) and PCL, was investigated by using isothermal crystallization kinetics under the thermal routine similar to that for the cyclic tensile testing. The results demonstrate that the ionic groups within hard segments significantly slow down the crystal growth of the reversible phase. When the physical crosslink is strong enough, the crystallization rate would be a predominant factor determining the shape fixity ratio after various cooling time. Instead, when physical crosslink is weakening, the influence of crystallization rate is much less on the cooling time dependence of fixity. Copyright © 2007 John Wiley & Sons, Ltd.     

Effect of cooling rate on the crystal/mesophase polymorphism of polyamide 6

The solidification of the quiescent polyamide 6 (PA 6) melt has been analyzed as a function of the cooling rate in a wide range between 1.67 × 10⁻² and close to 2 × 10² K s⁻¹, by means of differential scanning calorimetry at a low cooling rate of up to about 1 K s⁻¹, and by the recording of continuous cooling curves and time-resolved X-ray diffraction on cooling at a higher rate. The performed experiments allowed for the first time to establish the relationship between the cooling rate, the crystallization temperature, and the X-ray structure of PA 6. The exclusive formation of monoclinic α-crystals is only detected if the crystallization temperature is higher than about 430 K or if the cooling rate is slower than about 5 K s⁻¹, respectively. The formation of α-crystals is increasingly replaced by the development of mesophase with increasing cooling rate, accompanied with a decrease of the temperature of crystallization/ordering. Finally, completely amorphous samples were obtained on cooling faster than about 10² K s⁻¹. The continuous decrease of the temperature of crystallization with increasing cooling rate, regardless of the specific structure formed, precludes a primary effect of the nucleation mechanism on the α-crystal/mesophase polymorphism of PA 6. A preliminary discussion of the effect of molar mass of PA 6 on the cooling rate-dependent polymorphism is also included.     

Greatly accelerated crystallization of poly(lactic acid): cooperative effect of stereocomplex crystallites and polyethylene glycol

Stereocomplex crystallite (SC) between enantiomeric poly(l-lactic acid) (PLLA) and poly(d-lactic acid) (PDLA), with largely improved thermal resistance and mechanical properties compared with PLLA and PDLA, is a good nucleating agent for poly(lactic acid) (PLA). The effects of SC and/or polyethylene glycol (PEG) on the crystallization behaviors of PLA were investigated. The non-isothermal and isothermal crystallization kinetics revealed that SC and PEG can separately promote the crystallization rate of PLA by heterogeneous nucleation and increasing crystal growth rate, respectively. However, their promoting effect is limited when used alone, and the modified PLA cannot crystallize completely under a cooling rate of 20 °C/min. When SC and PEG are both present, the crystallization rate of PLA is greatly accelerated, and even under a cooling rate of 40 °C/min, PLA can crystallize completely and get a high crystallinity owing to the excellent balance between simultaneously improved nucleation and crystal growth rate.     

非等温结晶对PLLA的热行为和形貌的影响

将聚L-乳酸(PLLA)熔化非等温熔融结晶, 采用DSC、POM、SEM等技术研究了降温速率对PLLA的热行为和形貌的影响. PLLA在低降温速率(2 ℃·min-1)下的结晶在118 ℃伴随有结晶机制的转变. 玻璃化温度和结晶度随着降温速率的降低而增大. 随着降温速率的降低, 球晶尺寸增大, 当降温速率为10 ℃·min-1 时, PLLA 为无定型材料. 采用模压成型的方法并控制降温速率制备了具有球晶结构的条状PLLA 生物材料, 与高降温速率下制备的PLLA相比,低降温速率下获得的具有球晶结构的PLLA材料的断面更光滑和致密, 但脆性增强.     

Thermal properties of poly-ε-caprolactam and copolyamides based on ε-caprolactam

The copolyamides consisting of ε-caprolactam and 6.1–24.5 wt.% of nylon salt prepared from adipic acid and 1-(2-aminoethyl) piperazine were synthesized. Physical and thermal characteristics of polyamide 6 and the copolyamides were compared. Nylon salt does not influence the polyreaction equilibrium so it is possible to prepare the copolyamides with high molecular weight and with the content of low-molecular compounds comparable with that of pure PA 6. Melting temperatures of the copolyamides are lower in comparison with PA 6 and decrease proportionally to the amount of the nylon salt. The thermal stability of the copolyamides is good and equal to that of PA 6. The melting enthalpies indicate that the process of crystallization of the copolyamides is influenced by the time of crystallization and the amount of comonomer present. Longer time of the crystallization assures higher degree of crystallization. The kinetics and the level of crystallization are positively influenced by the mobility of copolyamide segments mainly up to 10 wt.% of comonomer.     

Effects of pressure on the compressibility and crystallization of poly(ethylene terephthalate)

The effects of pressure on the compressibility and crystallization of poly(ethylene terephthalate) (PET) have been investigated. The Instron capillary rheometer was adapted as a high-pressure dilatometer to perform experiments up to 40,000 psi. Compressibilities of solid and molten PET were measured. The increases in compressibility with increase in temperature for the solid state are discussed in terms of free-volume theory. Results obtained for the melt are explained by invoking the second law of thermodynamics and the effect of pressure on the Gibbs free energy. The effects of temperature and compression rate on the pressure of crystallization (P_c) were also studied. As the crystallization temperature was increased from 240 to 286°C, P_c increased by about 16,000 psi. As the compression rate was raised from 1%/min to 8%/min, P_c increased 10,000 psi. At some undetermined compression rate above 8%/min it seemed impossible to induce crystallization in the melt, even with pressures up to 40,000 psi. Analysis of data on the kinetics of crystallization of PET melt under high pressures revealed low Avrami exponents, for which no unequivocal explanation is offered. It is possible, however, that crystallization at high pressure promotes the formation of a morphology made up of a certain percentage of “extended chains.” The alteration in the attendant spatial geometry involved in the crystallization might explain the lower Avrami exponents found. In another set of experiments, crystallization temperatures (T_c) were measured by slowly cooling PET melt under high pressures. As the pressure was raised from 3000 to 15,000 psi, T_c increased from about 246 to 282.5°C. These results are consistent with thermodynamic theory.