Isothermal Crystallization Research Articles

A new class of nucleating agents for poly(L-lactic acid): Environmentally-friendly metal salts with biomass-derived ligands and advanced nucleation ability

Adding nucleating agents has been a successful strategy to boost the heat resistance of poly(L-lactic acid) (PLLA) by increasing the crystallinity. In this study, a new series of bio-based complexes as nucleating agents for PLLA, including twelve combinations of three eco-friendly metal ions (Zn, Mg, Ca) and four biomass-derived α-hydroxy acids, were successfully synthesized to respectively investigate the effects of metal ions as well as ligands on nucleation capacity of complexes. By investigating the non-isothermal and isothermal crystallization at 135 °C of PLLA with 0.3 wt% loading of complexes, both zinc and magnesium salts of L-mandelic acid showed excellent nucleation capacities. And magnesium L-mandelate performed better, raising the crystallinity of PLLA to 44.4 % as well as minimizing its crystallization half-time from 73 min to 2.7 min. The growth and denser distribution of PLLA spherulites on the salt surface were also observed by POM, reflecting epitaxial nucleation as the possible mechanism. A novel inspiration, utilizing VESTA software to simulate the crystal structure of zinc L-mandelate (Zn(L-MA)2), was proposed to determine the nucleation mechanism. Also, using polyethylene terephthalate (PET) as a test protocol, the rationality of the model could be approved by checking the fitness of nucleating prediction and experiment results.

Synergistic effect of star‐shaped architecture and poly(ethylene glycol) moieties on poly(lactic acid) stereocomplex crystallization behaviors

AbstractStereocomplex (sc) crystal formation represents an important strategy to improve the physical properties of poly(lactic acid) (PLA), including enhanced thermal resistance, anti‐hydrolysis and mechanical performance. Therefore, understanding and controlling the sc crystallization behaviors is an important and widely studied research topic. Herein, we investigate the cooperative effect of star‐shaped PLA chain topology and poly(ethylene glycol) incorporation on sc formation. We synthesize a series of poly(lactides), including poly(l‐lactic acid) (PLLA), poly(d‐lactic acid), star‐shaped PLLA and poly(d‐lactic acid)‐poly(ethylene glycol)‐poly(d‐lactic acid) (DED) triblock copolymer, via ring‐opening polymerization of lactide, followed by solution blending to form PLA stereocomplexes (scPLAs). The chemical structure of the prepared scPLAs is characterized by Fourier transform infrared and nuclear magnetic resonance, whereas the crystallization behavior and spherulite morphology are investigated by wide‐angle X‐ray diffraction, DSC and polarized optical microscopy. Linear PLLA/star‐shaped poly(d‐lactic acid) presents rapid crystallization behaviors and high crystallinity due to the excellent nucleation ability of star‐shaped chain topology. For isothermal crystallization, star‐shaped PLLA/DED exhibits the fastest crystallization kinetics, the highest crystallinity and the lowest crystallization half‐time of all samples. It is found that addition of poly(ethylene glycol) segments can increase chain mobility and backbone flexibility, further facilitating the crystallization of scPLA containing star‐shaped chain architecture. These results demonstrate an effective approach to cooperatively promote scPLA formation, with potential use for practical applications. © 2022 Society of Industrial Chemistry.

Effect of compatibilizer addition on the surface nucleation of dispersed polyethylene droplets in a self-nucleated polypropylene matrix

A significant portion of the global plastics market encompasses the production of polyolefin materials and especially polypropylene (PP) and polyethylene (PE), as commodity polymers with a wide range of applications. However, the increase in the generation of unsustainable plastic waste requires a close technological look-up to address this challenge adequately. In this context, mechanical recycling is part of the strategies expected to contribute to the solution. Nevertheless, the melt blending process presents a challenge due to the immiscibility between PP and PE. Therefore, compatibilization strategies are meant to solve the problem effectively. In this paper, we employ a commercial ethylene-ran-methyl acrylate random copolymer as a compatibilizer for PP/PE blends. With the addition of the compatibilizer, it was possible to obtain a 44% reduction in PE domain size, while ductility increased by around ∼40% with respect to uncompatibilized blends. Interesting results were obtained concerning the crystallization behavior of the blends. The overall isothermal crystallization kinetics of the different blend components was studied, and a synergistic nucleation effect of the PP and the compatibilizer toward the PE phase was found. For the first time, the effect of the compatibilizer on the surface nucleation of PE in a self-nucleated PP matrix phase is reported. An enhancement in the crystallization rate of PE was found when the self-nucleation protocol was applied to the polypropylene matrix phase for neat and compatibilized blends. The nucleation efficiency was in the range of 120–124%, indicating a supernucleation behavior. The induced crystallization at the interface by the self-nucleated polypropylene is the reason for such high nucleation efficiencies. Surprisingly, a higher amount of compatibilizer decreases the overall crystallization rate of PE droplets. The compatibilizer segregates at the interface between both polymers, reducing the surface nucleation of the PE droplets on the PP matrix phase. The results presented in this paper lead the way toward improving the use of post-consumer recycled materials.

Investigation of polyamide 12 isothermal crystallization through the application of the phase‐field model

AbstractCrystallization of polyamide 12 (PA12) is an essential process requiring thorough investigation for evaluating the mechanical properties after the polymer parts are manufactured. The change in crystallization temperature results in different crystallization behaviors for PA12. Hence, the crystal morphology of PA12 achieved provides important information about crystallization behavior, especially for those produced through additive manufacturing due to its heterogenous cooling rate in a single print bed. Considering the need of investigating PA12 crystallization using phase‐field modeling, this paper aims to simulate the spherulite morphology of PA12 undergoing isothermal crystallization. This model is compared with the spherulite morphologies obtained from the optical microscopy test. The model shows that PA12 spherulites have thicker dendrites when the isothermal temperature is higher. The present phase‐field model can determine the spherulite morphologies of bulk printed PA12 based on the crystallization condition and be used to evaluate the properties of the printed part.

Confocal scanning laser microscopy investigation of crystallization behavior of hot blast furnace slag

The isothermal and non-isothermal crystallization behavior of blast furnace (BF) slag with varying CaO/SiO2 ratios was in-situ observed through a confocal scanning laser microscopy (CLSM). The crystallization kinetics was analyzed and precipitation of crystalline phases was discussed by theoretical analysis from FactSage 8.0 and X-ray Diffraction (XRD). The results showed that, with the increasing CaO/SiO2 ratio from 1.0 to 1.3, the crystals morphology changed first from rectangles and squares to small-size and irregular, and further to long strip shape at the cooling rate of 100 °C/min. The onset crystallization temperature in CTT diagram was higher at a larger CaO/SiO2 ratio due to the decrease of polymerization degree of the slag. For the isothermal crystallization process at 1250°C, the crystals morphology changed gradually from lamellar to long-trip or columnar, and the incubation period in TTT diagram became shorter with the increase of CaO/SiO2 ratio. Avrami index n decreased from appropriately 2.3 to 1.5, and the growth mode of crystals converted from two-dimensional to one- dimensional. XRD analysis showed that only merwinite existed in four slags at 1380°C. Akermanite and gehlenite were generated at 1350°C, and their amount first increased greatly and then decreased slightly with a continuous increase of CaO/SiO2 ratio. At the low temperature, the number of akermanite and gehlenite reduced, while that of merwinite increased obviously in the slags when CaO/SiO2 ratio was 1.2 and 1.3, indicating that increasing CaO/SiO2 ratio promoted the generation of merwinite.

Phase Sequence, Kinetics of Crystallization and Molecular Dynamics of the Chiral Liquid Crystalline Compound Forming a Hexatic Smectic Glass

The vitrification of the antiferroelectric hexatic smectic XA* phase and cold crystallization are reported for (S)-4′-(1-methylheptylcarbonyl)biphenyl-4-yl 4-[5-(2,2,3,3,4,4,4-heptafluorobutoxy) heptyl-1-oxy]benzoate. The kinetics of isothermal cold crystallization and melt crystallization are investigated, revealing that both are controlled mainly by diffusion, as indicated by decrease in the characteristic crystallization time with increasing temperature of crystallization, with an activation energy of 114 kJ/mol. A weak relaxation process is detected in a crystal phase, with an activation energy of 38 kJ/mol, implying the conformationally disordered crystal phase. The estimated fragility parameter of the investigated glass former is equal to 94.5, which indicates rather high fragility.

Influence of sample measuring cell on isothermal crystallization of liquid crystalline compound forming the smectic CA* phase

ABSTRACT Crystallization of liquid crystalline (S)-4′-(1-methylheptyloxycarbonyl) biphenyl-4-yl 4-[7-(2,2,3,3,4,4,4-heptafluorobutoxy) heptyl-1-oxy]-2,3-difluorobenzoate in isothermal conditions is investigated by polarizing optical microscopy and differential scanning calorimetry. It is shown that the sample container influences the crystallization kinetics, as a faster development of the crystal phase is observed on microscopic textures, for a sample in an electro-optic cell with a planarly aligning polymer layer, than for a sample in an aluminum pan used in a differential scanning calorimetry experiment. However, in both cases the total time of melt crystallization in the 291–303 K range is shorter than 10 min. Only at the highest investigated temperature, 305 K, the crystallization process is completed after 1.4 h because of a long initialization time. The difference with the glassforming (S)-4′-(1-methylheptyloxycarbonyl) biphenyl-4-yl 4-[7-(2,2,3,3,4,4,4-heptafluorobutoxy) heptyl-1-oxy]-2-fluorobenzoate is explained based on the thermodynamic driving force of crystallization.

Crystallization mechanism and mechanical properties of CF/PPS thermoplastic composites manufactured by laser-assisted automated fiber placement

In situ consolidation of the thermoplastic composites can be realized through laser-assisted automated fiber placement technology, and the properties of the composites are significantly affected by the crystallinity. Understanding the complicated crystallization process is very important for controlling the performance of the composites manufactured by automated fiber placement. In this work, the crystallization mechanism of carbon fiber/polyphenylene sulfide composites during the automated fiber placement process and its effect on the mechanical properties were investigated. Crystallization kinetics analysis indicated that the crystallization window of the carbon fiber/polyphenylene sulfide composites was 87–270°C. Furthermore, for carbon fiber/polyphenylene sulfide composites manufactured by automated fiber placement, the crystallinity was influenced by the parameters including laser temperature, placement speed and tool temperature, in which the tool temperature was the main factor. Increasing the tool temperature was an effective method to achieve high crystallinity. Meanwhile, when the tool temperature was in the range of the crystallization window, the composites could experience isothermal crystallization, which could further improve the crystallinity. With the increase of the tool temperature, the flexural strength and interlaminar shear strength were improved due to the enhanced self-adhesion of the matrix, while the Mode І fracture toughness was decreased because of the reduction of the matrix ductility. The combination of the kinetic method and experimental study was conducive to a better understanding of the crystallization mechanism and optimization of the processing conditions.

Material extrusion thermal model mapped across polyetheretherketone isothermal and continuous cooling transformation charts

This study provides an insight into the polyetheretherketone (PEEK) crystallinity progression throughout the material extrusion (MEX) additive manufacturing process as a function of time and temperature, comparing it with the isothermal and continuous cooling transformation charts created over a wide range of isothermal crystallisation temperatures and constant cooling rates. The isothermal and non-isothermal crystallisation kinetics were explored using Differential Scanning Calorimetry (DSC) and Fast Scanning Calorimetry (FSC). The half-time, onset and ending of crystallisation were obtained for isothermal crystallisation temperatures between 150 °C and 330 °C, while the crystallisation under constant cooling was obtained using rates between − 0.5 K s −1 and − 45 K s −1 . The results were used to draw the Continuous Cooling Transformation (CCT) and the Time-Temperature Transformation (TTT) diagrams and calculate the Avrami numbers using the parallel Avrami model. These results were then compared to the degree of crystallinity as a function of time and temperature for the MEX process. To evaluate the crystallisation within the MEX process a 1D transient transfer heat model was used to obtain the printing thermal profile, which was replicated using the FSC technique. The results showed that for the MEX printing process, the crystallisation usually is a product of a combination of rapid cooling and heating processes followed by periods of greater thermal stability which, depending on the nature of the process, can approach a quasi-isothermal crystallisation process. By superimposing the process thermal profile on the TTT and CCT diagrams and comparing the crystallinity values measured from each point in the thermal profile, it was possible to understand the crystallinity evolution and the remelting of the build surface promoted by the subsequent printed layers. • Temperature fluctuations in the MEX process are a combination of ultra-fast dynamic cooling steps with heating and remelting. • The FSC and standard DSC allowed the replication of the modelled thermal profile. • The crystallinity varies as a function of local temperature and the fluctuations caused by the multi-layer printing process. • Higher chamber temperature increased the crystalline phase remelting, resulting in greater mobility of polymeric chains. • Shorter return times improved overall crystallinity levels due to more frequently heat input in the building surface.

Slowing crystallization to enhance interlayer strength of 3D printed poly (ether ether ketone) parts by molecular design

The major challenge restricting the application of 3D printed PEEK parts is the weak interlayer strength. Herein, the fluorene-based poly (ether ether ketone)s (FD-PEEK) were designed and synthesized to solve the problem. A remarkable improvement in the interlayer strength of materials was achieved. By introducing 10 mol% fluorene groups into the polymer chain (10 %-FD-PEEK), the interlayer strength of 10 %-FD-PEEK was improved to 41 MPa, which is close to that of the commercial PEEK resin for 3D printing (Victrex, AM 200). The introduction of 15 mol% fluorene groups (15 %-FD-PEEK) enabled the interlayer strength and strain at break of 400 % and 500 % higher than that of PEEK, reaching 67 MPa and 11.23 %, respectively. The improvement of the interlayer strength was attributed to the slower crystallizing rate of FD-PEEK, which was verified by the isothermal and non-isothermal crystallization kinetics. Meanwhile, the chain rigidity caused by fluorene groups and the larger spherulite generated by the slower crystallization allowed FD-PEEK to reach a similar tensile strength to PEEK in the X-axis and Y-axis directions. Achieved by slowing crystallization and introducing rigid structure into the molecular chain, the results significantly promote the engineering application of 3D printing parts and provide new insights into the performance enhancement of semi-crystalline additive manufacturing materials.

Effect of self-nucleation on the crystallization kinetics and the microstructure of polypropylene parts manufactured by Laser Powder Bed Fusion

Laser Powder Bed Fusion (LPBF) of thermoplastic polymers is an additive manufacturing (AM) technique experiencing a remarkable growth in the recent years. Nowadays the most widely used materials are polyamides (PA12, PA11…) but massively available LPBF-compatible polyolefin powders recently arrived on the market. The need of understanding the crystallization process for these new materials is essential and this was addressed in the present work. Isothermal and non-isothermal crystallization kinetics was investigated by differential scanning calorimetry and parts microstructure was analyzed using polarized light optical microscopy, scanning electron microscopy, transmission electron microscopy and wide-angle X-rays diffraction experiments. It was shown that it is important to consider the significant nucleating effect of the sub-surface layers solidification during the building stage. Indeed, half-crystallization times of the PP material are consistent with experimental observations and time range in LPBF only if crystallization occurs from a self-nucleated molten state. Evidence of the presence of nuclei were given thanks to microscopic and wide-angle X-rays diffraction (WAXD) techniques. Alternate patterns of very finely microstructured, oriented transcrystalline regions and isotropic spherulitic regions every 100 µm ( i.e. the layer thickness during printing) were highlighted.

The Effect of Chain Topology on the Crystallization and Polymorphism of PVDF: Linear versus Star Molecules

AbstractWell‐defined linear, 3‐, 4‐, and 6‐arms polyvinylidene fluoride (PVDF) stars are synthesized by reversible addition−fragmentation chain transfer (RAFT) polymerization using mono‐, tri‐, tetra‐, and hexa‐functionalized chain transfer agents, respectively, and 1,1‐bis‐(tert‐butylperoxy)‐cyclohexane as initiator. The crystallization kinetics and the polymorphic character of PVDF stars are investigated by differential scanning calorimetry (DSC) and Fourier transform infrared (FTIR) . Using a cooling rate of 10 °C min−1, all samples form α‐ and β‐phases. However, the amount of β‐phase increases with respect to the α‐phase as the number of arms in the PVDF stars increases. This results from the increased topological complexity in the stars , which leads to the preferential formation of the less thermodynamically stable ferroelectricβ‐phase. When the cooling rate is decreased (1 °C min−1) and/or an isothermal crystallization procedure is applied, polymorphism is inhibited in the PVDF stars, and only the paraelectric α‐phase is formed. On the other hand, the linear PVDF sample is still capable of producing both paraelectric and ferroelectric phases after slow cooling or isothermal crystallization. The isothermal crystallization kinetics of the PVDF stars i faster than the linear as a result of their speedier nucleation, possibly promoted by their particular topology where the arms radiate from a common junction point.

Crystallization kinetics and nanomechanical behavior of biobased poly(ethylene 2,5‐furandicarboxylate) reinforced with carbon nanotubes

AbstractPoly(ethylene 2,5‐furandicarboxylate) (PEF) is one of the most widely known biobased polyesters due to its outstanding gas barrier and mechanical properties, making it the number one candidate to substitute poly(ethylene terephthalate) (PET). However, its crystallization from the melt is much slower compared to PET, affecting the overall processing of the material. For this reason, PEF nanocomposites containing various carbon nanotubes (CNTs) loadings (0–2.5 wt%) have been in situ synthesized in this work. Differential scanning calorimetry (DSC) measurements were conducted to evaluate all the prepared materials' isothermal and nonisothermal crystallization kinetics. Isothermal kinetics using the Avrami and Hoffman–Lauritzen (H–L) theories suggested that the nucleating effect of the CNTs is more profound during the crystallization from the melt, where the higher crystallization rate is observed in the case of PEF/2.5 CNTs nanocomposite. This is also confirmed by the nonisothermal crystallization analysis. The nucleation activity of the filler was estimated by Dobreva's method, which revealed that higher CNTs (1 and 2.5 wt%) content presents increased nucleation efficiency during the process. The nanoindentation results showed that the addition of CNTs in the PEF matrix enhanced the nanomechanical properties of PEF. Semicrystalline samples presented significantly higher hardness (H) and elastic modulus (E) values compared to their amorphous counterparts, where semicrystalline PEF/2.5 CNTs nanocomposite presented 135% increase of E compared to the amorphous neat PEF.

Isothermal crystallization kinetics in bulk of olefin-based multiblock copolymers

Isothermal crystallization kinetics of ethylene/1-octene (C2/C8) multiblock copolymers synthesized by chain shuttling technology is investigated. The samples are a reactor blend of segmented chains characterized by alternating crystalline and amorphous blocks with C8 content of 0.5 and 20 mol%, respectively, and statistical distribution of block number/chain and block length. The analysis is carried out after complete removal of a fraction (5–12 wt%), namely consisting of C8-rich blocks, through Kumagawa extraction with boiling diethyl ether. The resultant diethyl ether-insoluble fractions have similar average content of C8 units (≈13–14 mol%) and of crystalline blocks (≈23–27 wt%) but different molecular mass (the number average molecular mass Mn is ≈ 60–70 kDa for the samples 1,2 and ≈38 and ≈21 kDa for the samples 3 and 4, respectively). An additional sample with Mn ≈ 93 kDa, but a greater average content of C8 units (≈15 mol%) and a smaller content of crystalline blocks (15 wt%) is also analyzed. The crystallization half time of the samples increases with increase of Mn and, for each sample, its logarithm increases linearly with a decrease of the undercooling by a factor of -0.155/°C, for the samples 1–4 and −0.031/°C, for the sample 5. Using the classic kinetic crystallization model by Lauritzen and Hoffman, values of energy barrier constant due to contributions from primary nucleation KN and crystal growth KG are extracted. The KN contribution is esteemed to amount to ≈34% of the total barrier assuming regime II for the sample 5 and regime III (or I) for the samples 1–4, to ≈34% for the sample 5 and 67% for the samples 1–4, assuming regime II for all the samples. In all the cases, regardless of the assumed regimes, the KN values of the sample 5 are lower than those of the samples 1–4. As a final remark, the implications of crystallization kinetics on the solid-state morphology are also discussed, considering that transmission electron microscopy (TEM) images present a partially mesophase separated morphology for the samples 1,2, and 5 and a classic lamellar morphology for the samples 3 and 4.

Effect of multiphase content on temperature‐dependent electrical conductivity in silicone rubber composites

AbstractTo investigate how the multiphase structures affect the electrical conductivity in semi‐crystalline polymer composites, herein, an accurate multiphase content calculation method is proposed and verified, which combines amorphous phase information in broadband dielectric spectroscopy and crystalline phase information in differential scanning calorimetry. Taken aluminium hydroxide (ATH) filled silicone rubber as an example, it is found that the rigid amorphous fraction (RAF) corresponding to the chains constrained by crystals (RAFcry) is not linearly increased with crystalline fraction (CF). Compared to non‐isothermal crystallisation, RAF caused by ATH/silicone rubber interface (RAFint) after isothermal crystallisation at 213 K changes little, while mobile amorphous fraction and RAFcry is attenuated and CF is strengthened. Based on the calculated structures of ATH filled silicone rubber, activation energy of conductivities during cooling is dominated by the thermal transition for conductive ions and shortened distance among the conductive ions through shrunk volumes of the amorphous phase. Our findings deepen the understanding of multiphase content in semi‐crystalline polymer composites and its relationship with electrical conductivity, which can be applied in manipulating electrical performance of semi‐crystalline polymers by fillers.

Kinetics of cold crystallization in two liquid crystalline fluorinated homologues exhibiting the vitrified smectic CA* phase

Dielectric relaxation processes in the supercooled antiferroelectric smectic CA* phase and crystallization kinetics of two chiral fluorinated 5HF6 and 6HF6 compounds from the same homologous series are investigated. Fragility parameters are determined from the relaxation time of the α-process, including τHN from the Havriliak-Negami formula and τpeak denoting the position of the absorption peak. The coupling coefficient ξ between the characteristic time of isothermal cold crystallization and relaxation time of the α-process is obtained. Despite similar values of the fragility index, the even 6HF6 homologue undergoes cold crystallization much faster than the odd 5HF6 homologue, with significantly different ξ coefficients. Influence of the relaxation time of the PH process (anti-phase phason) in the smectic CA* phase on the crystallization kinetics is presumed.

An exact measurement of nucleation incubation times in isothermal crystallizations of liquid metal Al via configuration heredity

Nucleation induction time τ∗ is a crucial parameter in the classical nucleation theory, which is intimately associated with the steady-state nucleation rate I0 in the homogenous nucleation. To examine the impact of temperatures T on τ∗(T) in the isothermal crystallizations of liquid metal Al, a series of molecular dynamics simulations are performed at different undercoolings ΔTm. With the help of cluster type index method based on Honeycutt–Anderson bond-type index, a cluster analysis method based on the continuous heredity of crystal clusters is developed to further distinguish nuclei from various embryos. By means of the transient heredity of crystal clusters, a nucleation incubation time τc different from nucleation induction time, i.e., a sum of incubation time of embryo τe and its effective growth time τgeff prior to arriving the critical size, is introduced and exactly measured. For the critical nuclei emerged in the transient nucleation stage, the τc are found to be long compared with the steady-state nucleation stage and mainly depend on an effective growth of incubated embryos, while those in steady-state nucleation stage are mostly decided by a successful incubation of initial embryos. Also, relative to the τ∗(T) derived from I0(T) at different ΔTm, these measured τc(T) are very small, but their average times τ¯c(T) in the whole region are mirror symmetric to I0(T).

Comments on isothermal crystallization kinetics of polymers: Polypropylene at high supercooling

A robust procedure for isothermal kinetic evaluation of Differential Scanning Calorimetry (DSC) measurements of polymers using the Kolmogorov-Johnson-Mehl-Avrami (KJMA) equation is proposed. This procedure takes into account the experimental uncertainties for short (initial deflection) and long (secondary crystallization) measurement times. This procedure is applied to the isothermal crystallization of various commercially available isotactic polypropylenes (iPP) in the temperature range between 10°C and 120°C. Further measurements are performed on unfilled iPP and iPP highly filled with calcium carbonate. The measurements were performed using Fast Differential Scanning Calorimetry (FDSC).Evaluation of the crystallization kinetics indicates homogeneous nucleation in the temperature range of mesophase formation below 60°C. With decreasing temperature, the crystallization is more affected by diffusion control and/or termination of growth at an early crystallization stageDuring α-phase formation (above 60°C), crystallization is heterogeneously nucleated. Depending on the sample, growth occurs at a constant rate or is diffusion controlled.Polypropylene highly filled with calcium carbonate shows an additional α-phase crystallization process. The KJMA evaluation indicates that this process is homogeneously nucleated.

Crystallization behavior and secondary nucleation model of PP/GN nanocomposites under supercritical N2

Supercritical N2, as a physical blowing agent, is commonly used in microcellular foam injection molding to improve the crystallization kinetics and foaming performance of polymer-based composites. Herein, the crystal morphology and growth behavior of polypropylene/graphite nanosheet (PP/GN) nanocomposites under supercritical N2 were investigated using a self made in-situ high pressure microscope system. Due to the heterogeneous nucleation effect of GN, more irregular lamellar crystals inside the spherulites were generated and the stacking of lamellar crystals was more disordered. The intrinsic mechanisms of GN content on the growth rate of spherulites were investigated on the basis of the interfacial free energy induced by GN. The calculated results of the secondary nucleation model are in line with the results obtained from the isothermal crystallization experiments, indicating that the model can effectively predict the grain growth rate of PP/GN nanocomposites under supercritical N2. In addition, based on the crystallization kinetics of PP/GN composites, the action mechanism of GN on the foaming behavior of PP was explored. As expected, more cell nuclei were induced due to the refinement of grains and the increased of GN content.

Crystallization Kinetics in Atomic Layer Deposited (ALD) Thin Films: Opportunities of Time and Chemical Environment

Atomic layer deposition (ALD) frequently produces amorphous films, however crystalline phases are often desired for various electrical, chemical, and optical applications. Thus, understanding the fundamental crystallization kinetics of films deposited by ALD is important in designing process pathways to achieve low-temperature crystallization, ultra-thin film (<50 nm) crystallization, controlled microstructures, and epitaxial growth. This presentation will discuss our work to study the crystallization kinetics in ALD TiO2 thin films and challenge some pre-conceived notions about thickness and temperature requirements for ALD film crystallization. We will focus on a prototypical TiO2 ALD process using tetrakis(dimethylamino)titanium(IV) (TDMAT) and water at between 140 °C and 220 °C. Initially, we will discuss data collected on post-deposition, isothermal crystallization kinetics at temperatures ranging from 140 to 200 °C. These time-dependent transformation kinetics are fit to the Johnson-Mehl-Avrami-Kolmogorov (JMAK) model, revealing a continuous nucleation process and a two-dimensional growth mechanism, consistent with the observed grain sizes. We also use images collected over short annealing times to independently determine the crystal nucleation rate, which allows us to determine that the activation energies for nucleation (1.32–1.35 eV K-1 atom-1) are about one order of magnitude greater than the activation energies for grain growth (0.12–0.24 eV K-1 atom-1), sugggesting that nucleation is the more significant barrier to crystallization in this system. Finally, we will show how a modification to the process’s chemical environment can lower this barrier to nucleation leading to dramatic in situ crystallization (with no post-deposition annealing) at unusually low process temperatures and film thicknesses (< 15 nm).