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.     

Crystallization kinetics and morphology of biodegradable poly(lactic acid) with a hydrazide nucleating agent

The effect of tetramethylenedicarboxylic dibenzoylhydrazide (designated here as TMC) on the nonisothermal and isothermal crystallization behavior of PLA was investigated by differential scanning calorimetry (DSC), polarized optical microscopy (POM) and wide angle X-ray diffraction (WAXD). TMC shows excellent nucleating effect on PLA. With the addition of 0.05 wt% TMC, the crystallization half-time of PLA decreases from 26.06 to 6.13 min at 130 °C. The isothermal crystallization data were further analyzed by the Avrami model. The values of the Avrami exponent of the blends are comparable to that of neat PLA, indicating that the presence of TMC does not change the crystallization mechanism of the matrix. The observation from POM and WAXD measurements showed that the presence of TMC increases significantly the nuclei density of PLA but has no discernible effect on its crystalline structure.     

Crystallization kinetics and morphology of poly(lactic acid) with polysaccharide as nucleating agent

Journal of Thermal Analysis and Calorimetry - An ever-increasing concern about environmental pollution has spearheaded research into alternative biodegradable polymers. Currently, poly(lactic acid)...     

Effect of phthalimide as an efficient nucleating agent on the crystallization kinetics of poly(lactic acid)

The effect of phthalimide compound on the nonisothermal and isothermal crystallization behavior of poly(lactic acid) (PLA) was investigated by differential scanning calorimetry (DSC) and polarized optical microscopy. It was found that the incorporation of a small amount of phthalimide promoted the crystallization of PLA significantly. The Avrami model was applied to analyze the isothermal crystallization kinetics. It was found that the Avrami exponent was higher for PLA/phthalimide blends than for neat PLA, indicating a heterogeneous nucleation mechanism. These results indicate that phthalimide may act as an efficient nucleating agent to improve the crystallization of PLA and expand its applications.     

Cooperativity length evolution during crystallization of poly(lactic acid)

Effects of stereoregularity and crystallization mode on the amorphous phase dynamics are investigated for poly(lactic acid) PLA. An isothermal crystallization from the melt and a cold crystallization are imposed. For each PLA, the cold crystallization leads to the appearance of a less perfect crystalline phase and to an important rigid amorphous fraction RAF content (35%), although only 10% of RAF is generated after crystallization from the melt. Temperature Modulated Differential Scanning Calorimetry is used to determine the Cooperative Rearranging Regions (CRR) size at the glass transition temperature in the mobile amorphous phase MAP. It is shown that the CRR size in the MAP is not modified by the appearance and the spherulite growth. For the intra-spherulite MAP, a confining effect is evidenced, causing an amorphous phase thickness decrease during crystallization, and inducing a drastic CRR size reduction.     

Crystallization kinetics study of poly(L‐lactic acid)/carbon nanotubes nanocomposites

Effects of carbon nanotubes (CNT) on the isothermal crystallization kinetics of poly(L ‐lactic acid) (PLLA) were quantitatively investigated using the Avrami equation and the secondary nucleation theory of Lauritzen and Hoffman. CNT via grafting modification with PLLA could well disperse in the PLLA matrix and give significantly enhanced crystallization rate and crystallinity of PLLA as analyzed by differential scanning calorimetry and polarized optical microscopy. Analysis of isothermal crystallization kinetics using the Avrami equation demonstrated that CNT significantly enhanced the bulk crystallization of PLLA. Analysis of spherulite growth kinetics using the secondary nucleation theory of Lauritzen and Hoffman found that CNT could expand the temperature range of the crystallization regime III of PLLA. Values of the nucleation constant (K_g) in crystallization regimes III and II of PLLA both increased with increasing CNT contents. The K_g III/K_g II ratios were found to be close to the theoretical value 2 but were not clearly found to depend on the CNT contents. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 983–989, 2010     

Thermal properties and cold crystallization kinetics of surface-treated banana fiber (BF)-reinforced poly(lactic acid) (PLA) nanocomposites

Nucleation capacity of organically modified natural montmorillonite within the surface-treated banana fiber (BF)-reinforced PLA biocomposites has been studied using DSC analysis in the present investigation. Both the surface treatments and nanoclays play vital roles in the variation in nucleation process of PLA during cold crystallization process. Biocomposite made up of silane-treated BF and its bionanocomposite prepared using cloisite 30B (C30B) were showed superior nucleation parameters, n and K values, in the Avrami plots. Enhanced equilibrium melting point and lower E _a suggests the reinforcing effect imparted by the BF surface treatments and C30B within the PLA matrix. Even though, Louritzen–Hoffmann theory was revealed that no change in crystallization regimes of PLA even after the biocomposite and bionanocomposite preparation. TG analysis revealed better heat barrier capacity for all the biocomposites and bionanocomposites in comparison with virgin PLA (V-PLA). Increased storage modulus values for biocomposites and bionanocomposites also confirm the reinforcing effects of the fillers. Heat deflection temperature and the flammability studies concluded better application window for newly developed materials than that V-PLA.     

Free poly(l‐lactic acid) spherulites grown from thermally induced phase separation and crystallization kinetics

Free poly(L‐lactic acid) (PLLA) sheaves and spherulites were prepared by thermally induced phase separation method from its tetrahydrofuran solution without the assistance of other additives. The effects of variables such as polymer concentration, quenching temperature and time on the morphology of PLLA spherulites were studied. The morphology, size, degree of crystallinity, and crystal structure of spherulites were characterized by SEM, DSC and XRD, and so forth. No obvious sheaves or spherulites were observed at quenching temperature of 8 and 0 °C, whereas sheaves composed of fluffy nanofibers with diameter of about 250 nm were formed at quenching temperature range of ?10 to ?40 °C. With increasing quenching time, the PLLA morphology changed from small sheaves to big sheaves (cauliflower‐like) to spherulites. Low concentration (3 and 5 wt %) solutions were favorable for the formation of sheaves, whereas high concentration (7 wt %) solution as good for the formation of spherulites. The mechanism for the formation of PLLA sheaves or spherulites was examined by the isothermal and nonisothermal crystallization of PLLA/tetrahydrofuran solutions using DSC. The Avrami equation was used to analyze the data and good linear double‐logarithmic plots were obtained. The Avrami exponent n and rate constant K indicated the crystal growth mechanism was intermediate between completely instantaneous and completely sporadic types of nucleation and growth, and the spherulites were there dimensional. Compared to the spherulites embedded in the bulky film obtained from the melt processing, this study provided a feasible technique for the fabrication of free PLLA spherulites. The PLLA spherulites composed of fluffy nanofibers with a high porosity (≥90%) may be potentially applied as functional materials such as catalyst support, adsorption and biomedical materials, and so forth. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014 , 52, 1476–1489     

Influence of the combination of nucleating agent and plasticizer on the non-isothermal crystallization kinetics and activation energies of poly(lactic acid)

Journal of Thermal Analysis and Calorimetry - The influence of the combination of the nucleating agent (talc, N, N′-ethylene bis-stearamide (EBS) and a commercial nucleating agent NT-20) and...     

Crystallization kinetics of poly(hexamethylene succinate)

Isothermal and nonisothermal crystallization kinetics of polyester 64 have been investigated by means of differential scanning calorimetry and optical microscopy. The Avrami analysis has been performed to obtain the kinetic parameters of primary crystallization. These indicate a three-dimensional spherulitic growth on heterogeneous nuclei for the isothermal crystallization, whereas an sporadic nucleation becomes dominant in the nonisothermal crystallization. The maximum crystallization rate of polyester 64 was deduced to take place at a temperature close to −3 °C. Polarizing light microscopy showed that spherulites with a negative birefringence are formed during isothermal crystallization, whereas transmission electron microscopy indicates that the b crystallographic axis is aligned parallel to the spherulitic radius.     

Crystallization kinetics of poly(trimethylene terephthalate)

The isothermal crystallization kinetics of poly(trimethylene terephthalate) (PTT) have been investigated using differential scanning calorimetry (DSC) and polarized light microscopy (PLM). Enthalpy data of exotherm from isothermal crystallization were analyzed using the Avrami theory. The average value of the Avrami exponent, n, is about 2.8. From the melt, PTT crystallizes according to a spherulite morphology. The spherulite growth rate and the overall crystallization rate depend on crystallization temperature. The increase in the spherulitic radius was examined by polarized light microscopy. Using values of transport parameters common to many polymers (U* = 1500 cal/mol, T_∞= T_g − 30 °C) together with experimentally determined values of T (248 °C) and T_g (44 °C), the nucleation parameter, k_g, for PTT was determined. On the basis of secondary nucleation analyses, a transition between regimes III and II was found in the vicinity of 194 °C (ΔT ≅ 54 K). The ratio of k_g of these two regimes is 2.1, which is very close to 2.0 as predicted by the Lauritzen–Hoffman theory. The lateral surface‐free energy, σ = 10.89 erg/cm² and the fold surface‐free energy, σ_e = 56.64 erg/cm² were determined. The latter leads to a work of chain‐folding, q = 4.80 kcal/mol folds, which is comparable to PET and PBT previously reported. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 934–941, 2000     

Crystallization measurements via ultrasonic velocity: Study of poly(lactic acid) parts

Crystallization has significant effects on the physical and mechanical properties of polymer products; therefore, crystallization measurements are important for understanding and predicting polymer products' properties. However, traditional crystallization measurement methods have disadvantages in practical applications because they can be destructive, offline, unsafe, and expensive. Recently, ultrasonic technology has shown great potential as a nondestructive, online, real‐time, and environmentally friendly measurement method for polymer characterization. In this study, a novel measurement method based on ultrasonic technology was proposed to study the crystallization characteristics of poly(lactic acid) (PLA) parts. An annealing process was employed to produce PLA parts with different degrees of crystallinity. A new ultrasonic water immersion method was used to measure the ultrasonic velocities of these annealed PLA parts. It has been found that the plot of the inverse ultrasonic velocity versus the degree of crystallinity shows good linearity over the whole crystallinity range for all three annealing temperatures. The linear relationship between the inverse of the ultrasonic velocity and the crystallinity observed in this study could provide a nondestructive method for investigating the degree of crystallinity of polymers, which can be implemented both offline and online. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 700–708     

Crystallization kinetics of poly(glycolic acid-alt-6-aminohexanoic acid)

The crystallization behavior of a new regular poly(ester amide) constituted by glycolic acid and 6-aminohexanoic acid units under both isothermal and non-isothermal conditions is studied. Differential scanning calorimetry (DSC) is used to monitor bulk crystallization, and subsequently Avrami and Ozawa analyses are applied. A three-dimensional spherulitic growth from heterogeneous nuclei is deduced for isothermal crystallization, whereas higher exponents are obtained for non-isothermal crystallization when an Avrami equation is applied. However, modifications of the Ozawa methodology indicate a crystallization mechanism similar to that of the isothermal process.The maximum crystallization rate is deduced to take place at a temperature close to 91 °C by considering experimental data and theoretical equations with adjusted parameters. The equilibrium melting temperature is determined to be 168 °C by the characteristic Hoffman-Weeks plot. One crystallization regime is detected by using the Lauritzen-Hoffman kinetic theory for isothermal crystallization and also with an isoconversional method applied for non-isothermal crystallization. Activation energy of molecular transport and nucleation constant are close to 1500 cal/mol and 1.81 × 10⁵ K², respectively. Crystal morphology, nucleation, and spherulitic growth rates are also investigated with hot-stage optical microscopy (HSOM).     

Crystallization behavior and mechanical properties of poly(lactic acid)/poly(ethylene oxide) blends nucleated by a self-assembly nucleator

Poly(lactic acid) (PLA)/poly(ethylene oxide) (PEO) blends nucleated by a self-assembly nucleating agent, N,N′,N″-tricyclohexyl-1,3,5-benzenetricarboxylamide (BTCA), were prepared by melt blending. The crystallization behavior and mechanical properties of the materials were investigated by differential scanning calorimetry, polarized optical microscopy, wide-angle X-ray diffraction, dynamic mechanical analyzer and tensile testing. It was found that PEO had a synergistic effect together with BTCA on promoting PLA crystallization, besides its toughening effect on the material. Moreover, BTCA revealed prominent reinforcement effect on both neat PLA and PLA/PEO blends in the glass transition region and above, indicating the improvement on the heat resistance of the materials.

     

Influence of crystallization temperature,molecular weight,and additives on the crystallization kinetics of poly(ethylene terephthalate)

The spherulite growth rate, the maximum spherulite radius, and the overall rate of crystallization of poly(ethylene terephthalate) (PETP) were measured by means of scattering and transmission of depolarized light. The influence of crystallization temperature, molecular weight, and additives on the above-mentioned quantities was investigated. An expression has been derived for the spherulite growth rate of PETP as a function of crystallization temperature and the number-average molecular weight for M?_n in the range of 19,000 to 39,000.     

Design,synthesis and characterization of novel poly(urethane-urea) based on a macrodiol from poly(lactic acid) and poly(p-dioxanone)

A series of novel poly(urethane-urea)(PUU) was synthesized from poly(lactide-co-p-dioxanone) macrodiol(HO-P(LA-co-PDO)-OH), hexamethylene diisocyanate(HDI) and butanediamine(BDA).The obtained PUU,which is recorded as P(LA-co-PDO)-PUU here,may demonstrate enhanced phase separation and thus improved shape memory property.FTIR was employed to characterize the copolymers,and the effects of NCO/OH molar ratios on T_g of PUU was investigated by means of differential scanning calorimetry (DSC).The results revealed the successful synthesis of P(LA-co-PDO)-PUU.In addition,the T_g of P(LA-co-PDO)-PUU increased from 37.9℃to 44.2℃with the increase NCO/OH ratios from 1.1 to 1.2.The P(LA-co-PDO)-PUU with T_g close to body temperature will have potential applications as shape memory polymers in biomedical fields,especially in minimally invasive surgery.     

Non-isothermal crystallization behaviors and spherulitic morphology of poly(lactic acid) nucleated by a novel nucleating agent

Journal of Thermal Analysis and Calorimetry - The effect of cadmium phenylmalonate (PMA-Cd) as a novel nucleating agent on the crystallization behaviors and spherulitic morphology of poly(lactic...