Non-isothermal Crystallization Process Research Articles

Effective attainment of a substantial proportion of the PbTiO3 crystalline phase through a non-isothermal crystallization process

The crystallization kinetics studies were performed on a lead–titanium borate glass with a composition 50PbO–25TiO2–25B2O3 (mol %) to well understand the crystallization process by a non-isothermal method in an attempt to obtain a substantial proportion of the lead titanate (PbTiO3) crystalline phase. Based on the DTA result, the glassy material was heat-treated to induce crystallization and then produced a glass-ceramic. The broad hump observed in the XRD patterns of a glassy sample affirmed its amorphous nature. However, XRD studies conducted on a glass-ceramic revealed the evolution of the crystalline phase of PbTiO3 and a minor phase of TiO2. The phase relationship of PbTiO3 resulting from the reaction between PbO and TiO2 during a non-isothermal crystallization process has been examined. The activation energy for the crystallization of PbTiO3 is evaluated to be 135 kJ/mol (at PbO/TiO2 ratio = 2/1 mol %). When the Pb/Ti ratio exceeds 1, the process progresses nearly to completion by the first crystallization peak, and the PbTiO3 phase formation is easier. The fraction of PbTiO3 phase in the present glass is 98.3 % at PbO/TiO2 ratio = 2/1 mol %. In addition, FTIR spectra provided the presence of Ti4+ ions in the glass-ceramic. According to the values of crystal growth and Avrami index, surface crystallization and one-dimensional growth mechanisms are suitable mechanisms for contributing to the crystallization of this glassy sample. The obtained findings provided details on the crystallization behaviour of this glassy sample and excellent insight into obtaining the PbTiO3 crystalline phase in a large proportion, which is of great importance in future technological applications.

Composition design and crystallization behavior of Zr–Cu–Ni–Al bulk metallic glasses

The Zr–Cu–Ni–Al system was explored in this study, and seven new compositions of Zr-based bulk metallic glasses (BMGs) were designed by combining different proportions of binary eutectic phases Zr38Cu62, Zr76Ni24 and Zr51Al49, and rod-like shape samples were prepared with different diameters using the copper mold casting method. Modifying the proportion coefficient of these eutectic units, the width of the supercooled liquid region was increased from 79 to 95 K, and the thermal stability of the prepared metallic glass was improved. Differential scanning calorimetry was used to investigate the non-isothermal and isothermal crystallization behavior of the prepared samples. Noticeably, there was a small low-temperature exothermic peak before the glass transition temperature Tg in the non-isothermal crystallization process, which was related to the structural relaxation. The crystallization transition activation energy fitted by the Kissinger equation had the same variation trend as that obtained by the Arrhenius equation. Among the four metallic glasses studied, the glassy phase of Zr55·5Cu23·25Ni9Al12.25 was more stable, showing higher glass transition activation energy and longer incubation time. In addition, the isothermal crystallization processes of four metallic glasses were studied according to the Johnson-Mehl-Avrami (JMA) model. The calculated average Avrami exponents were >2.5, indicating that the nucleation rate increased during the isothermal crystallization process.

The effect of tetra‐needle‐like zinc oxide whisker as additive on the crystallization kinetics of polybutylene adipate terephthalate/polylactic acid blend

AbstractHerein, we investigate the effect of the addition of tetra‐needle‐like zinc oxide whisker (T‐ZnOw), a novel reinforcing additive, on the melt crystallization behavior of biodegradable polybutylene adipate terephthalate (PBAT)/polylactic acid (PLA) polymer blends. PBAT and PLA were blended in a Haake mixer at a fixed weight ratio of 70/30, and the contents of T‐ZnOw varied from 0 to 7 wt%. The melt thermal properties and crystallization behavior of PBAT/PLA blends and PBAT/PLA/T‐ZnOw composites were studied using differential scanning calorimetry. The kinetics of melt crystallization have been quantitatively explained using a range of mathematical formalisms. The Avrami equation is adopted to analyze the isothermal crystallization process and kinetics of PBAT/PLA/T‐ZnOw composites. For its nonisothermal crystallization process and kinetics, the Ozawa equation cannot describe the entire process, while the Jeziomy‐modified Avrami equation can only describe the initial stage of its nonisothermal crystallization. Using Mo‐analysis, the impact of T‐ZnOw contents on the composite's nonisothermal crystallization kinetic has been thoroughly examined. Given that the PBAT/PLA blend system's crystal formation may be inhibited by T‐ZnOw, the observation demonstrates that T‐ZnOw only marginally alters this process. The work is of significant scientific value and adds a new concept to the development of sustainable thin film for packaging.Highlights T‐ZnOw is used as a novel additive for PBAT/PLA polymer blends. The crystallization of PBAT/PLA/T‐ZnOw composite systems is thoroughly examined. T‐ZnOw marginally reduces the kinetics of the crystallization process of PBAT/PLA blends. PBAT/PLA/T‐ZnOw composites follow volume nucleation with mixed 2D/1D growth. The nonisothermal crystallization of PBAT/PLA/T‐ZnOw composites is validated by Mo‐analysis.

Stereocomplex crystallization of equimolar Poly(L-lactic acid)/Poly(D-lactic acid) blends from melt with lowered chain entanglements

Understanding polymer crystallization at the molecular level is one of the key open questions of polymer physics. Here, high molecular weight equimolar poly(L-lactic acid)/poly(D-lactic acid) (PLLA/PDLA) blends with lowered chain entanglements are prepared by the freeze-drying method. The role of chain entanglements in stereocomplex (SC) crystallization is explored. A significant increase in the crystallization kinetics of SC is observed with decreasing the entanglement density, and thus highly crystallized SC is obtained in the PLLA/PDLA blends by a process of non-isothermal crystallization. With reducing the chain entanglements of PLLA/PDLA blends, an intriguing phenomenon that SC crystallization stops at a high temperature around 200 °C and starts again at ∼160 °C on continuous cooling is observed, verifying the new concept of “poisoning by purity”. Further studies reveal that the PLLA/PDLA blends with lowered chain entanglements are more likely to form regularly folded SC lamellae, which nucleate and grow at higher temperatures. Meanwhile, the reduced chain entanglements also reduce the activation energy for chain diffusion. These two effects may be the main reason for the significantly accelerated crystallization kinetics of SC. This study provides a new idea for producing highly crystallized SC in PLLA products to improve their thermal resistance and other properties.

Isothermal and nonisothermal crystallization kinetics of modified hydroxyapatite-enhanced poly(3-hydroxybutyrate-co-4-hydroxybutyrate)

Poly(3-hydroxybutyrate-co-4-hydroxybutyrate) (P34HB) is a biodegradable material, and efforts have been devoted to enhancing its mechanical properties and biocompatibility for biomedical applications. However, there is a lack of research confirming the influence of nucleating agents on its crystallization behavior. In this study, the isothermal crystallization process was described using the Avrami model. The non-isothermal crystallization process was investigated using the Avrami-Jeziorny and Mo models. The effects of different calcium hydroxyapatite (CA-HA) contents on the nucleation activity, crystallization activation energy, and equilibrium melting point of P34HB/CA-HA composites were compared. The impact of CA-HA on the crystallization rate, nucleation rate, and crystal growth rate of the polymer matrix was discussed. The results indicate that with an increase in CA-HA content, the crystallization rate initially decreases and then increases. The minimum crystallization rate is observed at a CA-HA content of 9 wt%. Moreover, as the CA-HA content increases, nucleation activity is enhanced, suggesting that CA-HA acts as a nucleating agent, facilitating the nucleation process. When the CA-HA content exceeds 9 wt%, the increase in crystallization rate may be attributed to the further enlargement of molecular clusters formed by CA-HA, leading to a reduced dispersion of CA-HA in the polymer matrix. Furthermore, when the CA-HA content surpasses 9%, a significant decrease in crystallization activation energy is observed, indicating a reduction in the mobility of polymer chain segments.

Non-isothermal crystallization kinetics behavior of ethylene-methyl acrylate copolymer and ethylene-methyl acrylate copolymer/chloroprene rubber thermoplastic vulcanizate

The non-isothermal crystallization kinetics of the pure ethylene-methyl acrylate copolymer (EMA) and EMA/chloroprene rubber (CR) thermoplastic vulcanizate (TPV) prepared by dynamic vulcanization were extensively investigated using differential scanning calorimetry (DSC). The Avrami method modified by the Jeziorny, the Ozawa and the Mo methods were employed to characterize the non-isothermal crystallization process. The results indicated that the Avrami method modified by the Jeziorny and Mo methods effectively characterized the non-isothermal crystallization behavior of both pure EMA and EMA/CR TPV, while the behavior could not be described by the Ozawa modification method.

Polypropylene’s crystallization morphology during the non-isothermal crystallization process and its electrical properties

Polypropylene’s crystallization morphology during the non-isothermal crystallization process and its electrical properties

Crystallinity, Rheology, and Mechanical Properties of Low-/High-Molecular-Weight PLA Blended Systems.

As semi-crystalline polyester (lactic acid) (PLA) is combined with other reinforcing materials, challenges such as phase separation, environmental pollution, and manufacturing difficulties could hinder the benefits of PLA, including complete biodegradability and strong mechanical properties. In the present investigation, melt blending is utilized to establish a mixture of low- and high-molecular-weight polylactic acids (LPLA and HPLA). The crystallinity, rheology, and mechanical properties of the combination were analyzed using rotational rheometry, differential scanning calorimetry, X-ray diffraction, polarized optical microscopy, scanning electron microscopy, and universal testing equipment. The results demonstrate compatibility between LPLA and HPLA. Moreover, an increase in LPLA concentration leads to a decrease in the crystallization rate, spherulite size, fractional crystallinity, and XRD peak intensity during isothermal crystallization. LPLA acts as a diluent during isothermal crystallization, whereas HPLA functions as a nucleating agent in the non-isothermal crystallization process, promoting the growth of LPLA crystals and leading to co-crystallization. The blended system with a 5% LPLA mass fraction exhibits the highest tensile strength and enhances rheological characteristics. By effectively leveraging the relationship between various molecular weights of PLA's mechanical, rheological, and crystallization behavior, this scrutiny improves the physical and mechanical characteristics of the material, opening up new opportunities.

Non-Isothermal Crystallization Kinetics of Poly (ɛ-Caprolactone) (PCL) and MgO Incorporated PCL Nanofibers.

The study delves into the kinetics of non-isothermal crystallization of Poly (ɛ-caprolactone) (PCL) and MgO-incorporated PCL nanofibers with varying cooling rates. Differential Scanning Calorimetry (DSC-3) was used to acquire crystallization information and investigate the kinetics behavior of the two types of nanofibers under different cooling rates ranging from 0.5-5 K/min. The results show that the crystallization rate decreases at higher crystallization temperatures. Furthermore, the parameters of non-isothermal crystallization kinetics were investigated via several mathematical models, including Jeziorny and Mo's models. Mo's approach was suitable to describe the nanofibers' overall non-isothermal crystallization process. In addition, the Kissinger and Friedman methods were used to calculate the activation energy of bulk-PCL, PCL, and MgO-PCL nanofibers. The result showed that the activation energy of bulk-PCL was comparatively lower than that of nanofibers. The investigation of the kinetics of crystallization plays a crucial role in optimizing manufacturing processes and enhancing the overall performance of nanofibers.

Temperature Effects on the Crystalline Structure of iPP Containing Different Solvent-Treated TMB-5 Nucleating Agents.

TMB-5 nucleating agent (NA) treated by different solvents were used as the β-NA of iPP. The effects of temperature on the crystalline structure of different iPP/TMB-5, as well as the crystallization and melting behaviors were investigated. It was found that strong polar solvent treated TMB-5 (TMB-5DMSO and TMB-5DMF) could induce more β-crystal at high Tc = 140 °C than the other TMB-5 NAs, while the β-crystal inducing efficiency of untreated TMB-5 (TMB-5UT) and non-polar solvent treated TMB-5 (TMB-5LP) is seriously reduced at high Tc = 140 °C. TMB-5DMSO can induce a high and stable content of β-crystal with Kβ = 83-94% within Tc = 90-140 °C, and TMB-5ODCB can induce a high content of β-crystal with Kβ > 91.3% within Tc = 90-130 °C. TMB-5DMF is the most temperature-sensitive one, but can induce a high fraction of β-crystal with Kβ > 92% both at low Tc = 90 °C and high Tc = 140 °C. High temperature pre-crystallization at Tpc = 150 °C tremendously reduces the β-crystal inducing efficiency of all TMB-5 NAs. TMB-5UT and TMB-5LP exhibit higher nucleating efficiency than TMB-5DMSO, TMB-5DMF and TMB-5ODCB. During the non-isothermal crystallization process, TMB-5UT induced β-crystal possesses higher structural perfection and stability, while TMB-5LP is more likely to induce α-crystal with considerable quantity and stability. The structural perfection and stability of TMB-5 induced β-crystal can be enhanced with appropriate increasing of Tc.

Miscibility, crystallization and properties of bio-based polyamide 56/polyamide 6 blends

The miscibility, crystallization behaviors, and morphology of polyamide 56/polyamide 6 (PA56/PA6) blends were investigated. The blends had only one Tg, which confirmed that PA56 and PA6 are miscible in the blends. The melting point, crystallization temperature, crystallinity of the blends depressed due to the interaction of PA56 and PA6 in the blends. The crystalline structure of both PA56 and PA6 in the blends did not change and crystallized separately in both isothermal and non-isothermal crystallization process. PA56 in the blends crystallized in all blending ratio, while PA6 only crystallized with PA6 content φ6 > 0.2. The crystals of PA56 and PA6 grew in the same crystal skeleton but in different layers. Mechanical properties of the blends indicated that the elongation at break could be improved significantly without scarifying the strength in the case of adding a small amount of PA6 in PA56, which provides a promising approach for fabricating polyamide materials with desired properties.

Non-Isothermal Crystallization Kinetics of Polyvinylidene Fluoride (PVDF)/Microcrystalline Graphite (MCG) Composites

The Avrami equation modified by Jeziorny, the Ozawa model and the Mo equation combining the Avrami and Ozawa theories were employed to describe the non-isothermal crystallization kinetics of poly(vinylidene fluoride) (PVDF), and PVDF/microcrystalline graphite (MCG) composites. It was found that the Ozawa model failed to describe the non-isothermal crystallization process for PVDF and its composites, while Mo’s theory provided good fitting for neat PVDF but did not describe the non-isothermal crystallization well for the PVDF/MCG composites. The addition of MCG promoted the crystallization rate of PVDF, which indicates that MCG acted as a nucleating agent. The absolute values of the effective activation energy of the PVDF/MCG composites calculated by the Kissinger equation, were higher than that of neat PVDF, confirming that MCG acted as a nucleating agent and promoted the crystallization rate of PVDF.

Crystallization Behavior and Morphology of Biodegradable Poly(ε-caprolactone)/Reduced Graphene Oxide Scaffolds.

Morphology, thermal properties and the non-isothermal melt crystallization kinetics of biodegradable poly(ε-caprolactone) (PCL)/reduced graphene oxide (rGO) scaffolds are studied with differential scanning calorimetry (DSC) at various cooling rates (5, 10, 15 and 20 °C/min). Thermally induced phase separation was used to manufacture the scaffolds (TIPS). The micrographs show a more homogeneous and defined morphology with larger pores and thicker pore walls. The melting temperature (Tm), melting enthalpy (ΔHm), crystallization enthalpy (ΔHc) and degree of crystallinity (Xc) increased with the addition of rGO, suggesting larger and more perfect crystalline structures. The degree of crystallinity increased with the presence of rGO. The crystallization peak shifted to higher temperatures as the rGO concentration increased independently of the cooling rates. The peak shifted to lower temperatures as the cooling rate increased with the same rGO composition. The values of t1/2 (time needed to reach 50% crystallization) were lower for scaffolds with rGO. The values of the crystallization rate coefficient were higher when the porous support contained rGO, which indicates that their crystallization systems are faster. The activation energy obtained with the Kissinger method decreased with the presence of rGO. The results indicate that reduced graphene oxide acts as a nucleating agent in the non-isothermal melt crystallization process. The addition of small quantities of rGO changes their thermal properties with which they can be modified for application in the field of tissue engineering.

Effects of Dopamine-Modified and Organic Intercalation on the Thermophysical Properties of Octadecane/Expanded Vermiculite Composite Phase Change Materials.

For phase change materials (PCMs) confined in porous structure, the interface interaction between the PCM and the porous skeleton is the key factor to determine the thermal storage performance of PCM. In this study, the modified expanded vermiculite (EVM) was prepared by dopamine modification and DTA+ intercalation method and a series of EVM-based composite phase change materials with form shape (fs-CPCMs) were prepared by physical impregnation using n-octadecane as PCMs. The heat storage performance of the composite phase change materials were improved obviously, and the encapsulation ratio is increased to 84.7% and 83.1%. The nonisothermal crystallization process of prepared composite phase change material were further studied. The effects of dopamine modification and organic intercalation on the encapsulation performance of EVM-based fs-CPCMs were analyzed. The results of XRD, FT-IR, thermal cycle, and TGA tests showed that EVM-based composite phase change materials had good chemical compatibility, thermal reliability, and thermal stability. This study provides a theoretical basis for the preparation of EVM-based fs-CPCMs with high thermal stability and good heat storage performance.

Preparation of porous titanium dioxide foam impregnated with polyethylene glycol as shape-stable composite phase change materials

In this work, a novel shape-stabilized composite phase change material, was prepared by using high porosity and uniform, open, controllable, 3D interconnected porous titanium dioxide foam (PTF) as the package carrier material and polyethylene glycol (PEG) as phase change material. The heat storage performance and non-isothermal crystallization process of prepared composite phase change material were further studied and the effect of 3D fully interconnected pore structure of PTF on the phase change behavior and thermal conductivity of PEG were also further investigated. The DSC results indicated that the enthalpy of melting and solidification of the PEG-PTF were 145.9 and 142.9 J/g, respectively, and the corresponding actual impregnation ratio (φ) reached 70%. Because PTF porous matrix provides abundant crystallization sites during PEG solidification process, the inhibition effect of matrix on PEG phase transformation is significantly alleviated, which is demonstrated by the decrease in DH value. Non-isothermal crystallization results showed that the activation energy of PEG (−2.67 kJ/mol) was higher than that of PEG-PTF (−2.19 kJ/mol) but the half crystallization time of PEG was similar with that of PEG-PTF, which indicated that the 3D fully interconnected pore structure of PTF partially eliminated the inhibition effect of porous carrier on PEG phase transformation behavior. The results of XRD, FT-IR, thermal cycle and TGA tests showed that PEG-PTF composite phase change material had good chemical compatibility, thermal reliability, and thermal stability. This study showed that the PTF was a novel and promising support material for the preparation of PEG composite phase change materials.

Melt and nucleation reinforcement for stereocomplex crystallites in poly(ʟ-lactide)/lignin-grafted-poly(ᴅ-lactide) blend

Lignin-grafted-poly (ᴅ-lactide) (LGPD) with two different poly (ᴅ-lactide) (PDLA) block lengths (1100 and 2000 Da), denoted as LGPDL and LGPDH respectively, were synthesized by ring opening polymerization using dodecylated lignin as initiator. They were then blended with poly(ʟ-lactide) (PLLA) to improve its crystallization, mechanical properties and UV-absorbance properties. It was found that stereocomplex (SC) crystallites could be formed in both PLLA/LGPD blends, and the crystallization rate of PLLA/LGPDH was faster than that of PLLA/LGPDL because of the longer PDLA block length. Both in the non-isothermal and isothermal crystallization process, the crystallization of PLLA in the PLLA/LGPDH blend was the fastest when the ratio of LGPDH was 3%. SC crystallites in the PLLA/LGPDH blends also showed broader melting ranges and higher final melting temperatures comparing to the blends of PLLA and 4-armed PDLA (4a-PDLA), whose number of PDLA arms and molecular weight was similar to LGPDH. These results proved that the lignin structure grafted by the PDLA arms could affect the state of SC crystallites and their acceleration effect on the crystallization of PLLA in the blend. The degradation behavior showed that the degradation rate of the LGPDH-5% blend in the final stage of degradation (about 24 h) was the slowest comparing to other blends. Besides, the PLLA/LGPD blends had better tensile strength and UV-absorbance properties than neat PLLA.

Molecular dynamics simulation of nonisothermal crystallization of a single polyethylene chain and short polyethylene chains based on OPLS force field

Abstract Molecular dynamics simulations on the nonisothermal crystallization of a single polyethylene chain and short polyethylene chains based on the all-atom model and optimized potentials for liquid simulations-all atom (OPLS-AA) force field are conducted in this article. Four all-atom single chain models with different chain lengths (C1000, C2000, C3000, and C4000) and four all-atom short chain models with the same chain length and different number of chains (2C500, 4C500, 6C500, and 8C500) are constructed. The collapse process at a high temperature of 600 K and the nonisothermal crystallization process with different cooling rates at the temperature range of 600–300 K are simulated. Roles of chain length, number of chains, cooling rate on the potential energy, van der Waals (V dw) energy, radius of gyration, root mean square deviation, and crystallinity are explored. By comparing with the existing results obtained by the united atom model, the validity and accuracy of this study are proved. Results show that in the collapse process, the chain length is the major factor, whereas the cooling rate has the greatest influence during the nonisothermal crystallization process. As the cooling rate decreases, a “platform” appeared in the V dw energy curve, which has a profound impact on the crystallization.

The Isothermal and Nonisothermal Crystallization Kinetics and Morphology of Solvent-Precipitated Nylon 66.

Solvent-precipitated nylon 66 (SP PA66) is a key material used to fabricate microfiltration membranes. The crystallization kinetics and behavior of SP PA66 were investigated through differential scanning calorimetry (DSC), polarized optical microscopy (POM) and X-ray diffraction (XRD). The Avrami equation was used to describe the isothermal crystallization of SP PA66. Nonisothermal crystallization behaviors were analyzed using Avrami equations modified by Jeziorny, Ozawa and Mo. The Avrami analysis demonstrated that the k values of SP PA66 were higher than those of neat PA66. The was between 2 and 3 indicating the presence of two- and three-dimensional mode with thermal nucleation. With an increasing cooling rate, the Jeziorny crystallization rate constant increased for SP PA66; however, the Ozawa model was not satisfactory for all SP PA66 samples. The Mo method suggested that SP PA66 had a faster crystallization rate than neat PA66 during the nonisothermal crystallization process. The solvents dissolved nylon 66, rearranged it and formed a regular hydrogen-bonded region. These regions served as nucleation sites and increased the crystallization rate constant in the subsequent melting process. The crystal morphology of the SP PA66 under the POM investigation exhibited Maltese cross spherulites. The sizes of the spherulites of SP PA66 were significantly smaller than those of neat PA66. Wide-angle XRD revealed that SP PA66 had the same crystal structure and a higher crystal perfection than neat PA66.

The Effect of Varying the Amount of Short Hemp Fibers on Mechanical and Thermal Properties of Wood-Plastic Composites from Biobased Polyethylene Processed by Injection Molding.

Biobased HDPE (bioHDPE) was melt-compounded with different percentages (2.5 to 40.0 wt.%) of short hemp fibers (HF) as a natural reinforcement to obtain environmentally friendly wood plastic composites (WPC). These WPC were melt-compounded using a twin-screw extrusion and shaped into standard samples by injection molding. To improve the poor compatibility between the high non-polar BioHDPE matrix and the highly hydrophilic lignocellulosic fibers, a malleated copolymer, namely, polyethylene-graft-maleic anhydride (PE-g-MA), was used. The addition of short hemp fibers provided a remarkable increase in the stiffness that, in combination with PE-g-MA, led to good mechanical performance. In particular, 40 wt.% HF drastically increased the Young’s modulus and impact strength of BioHDPE, reaching values of 5275 MPa and 3.6 kJ/m2, respectively, which are very interesting values compared to neat bioHDPE of 826 MPa and 2.0 kJ/m2. These results were corroborated by dynamic mechanical thermal analysis (DMTA) results, which revealed a clear increasing tendency on stiffness with increasing the fiber loading over the whole temperature range. The crystal structure was not altered by the introduction of the natural fibers as could be seen in the XRD patterns in which mainly the heights of the main peaks changed, and only small peaks associated with the presence of the fiber appeared. Analysis of the thermal properties of the composites showed that no differences in melting temperature occurred and the non-isothermal crystallization process was satisfactorily described from the combined Avrami and Ozawa model. As for the thermal degradation, the introduction of HF resulted in the polymer degradation taking place at a higher temperature. As for the change in color of the injected samples, it was observed that the increase in fiber generated a clear modification in the final shades of the pieces, reaching colors very similar to dark woods for percentages higher than 20% HF. Finally, the incorporation of an increasing percentage of fibers also increased water absorption due to its lignocellulosic nature in a linear way, which drastically improved the polarity of the composite.

The Synthesis of Biodegradable Poly(L-Lactic Acid)-Polyethylene Glycols Copolymer/Montmorillonite Nanocomposites and Analysis of the Crystallization Properties

This study makes use of polycondensation to produce poly (L-lactic acid)-(polyethylene glycols), a biodegradable copolymer, then puts it with organically modified montmorillonite (o-MMT) going through an intercalation process to produce a series of nanocomposites of PLLA-PEG/o-MMT. The exfoliation and intercalation of the montmorillonite-layered structure could be found through X-ray diffraction and transmission electron microscopy. The lower the molecular weight of poly (ethylene glycol), the more obvious the exfoliation and dispersion. The nanocomposites were investigated under non-isothermal crystallization and isothermal crystallization separately via differential scanning calorimetry (DSC). After the adding of o-MMT to PLLA-PEG copolymers, it was found that the PLLA-PEG nanocomposites crystallized slowly and the crystallization peak tended to become broader during the non-isothermal crystallization process. Furthermore, the thermal curve of the non-isothermal melt crystallization process of PLLA-PEG copolymers with different proportions of o-MMT showed that the melting point decreased gradually with the increase of o-MMT content. In the measurement of isothermal crystallization, increasing the o-MMT of the PLLA-PEG copolymers would increase the t1/2 (crystallization half time) for crystallization and decrease the value of ΔHc. However, the present study results suggest that adding o-MMT could affect the crystallization rate of PLLA-PEG copolymers. The o-MMT silicate layer was uniformly dispersed in the PLLA-PEG copolymers, forming a nucleating agent. The crystallization rate and the regularity of the crystals changed with the increase of the o-MMT content, which further affected the crystallization enthalpies.