Effect of wood flour as nucleating agent on the isothermal crystallization of poly(lactic acid)

The effect of wood flour (WF) as an efficient nucleating agent on the isothermal melt crystallization and isothermal cold crystallization behavior of poly(lactic acid) (PLA) was investigated by differential scanning calorimeter and polarized optical microscopy. It was found that the incorporation of 4 wt% WF promoted the crystallization of PLA about 4.2%. Polarized optical microscopy results showed the Maltese cross of the samples. The presence of the 4 wt% WF may increase the nucleation density, leading to the increase of the spherulites; however, the size of the spherulites decreased, and the structure became incomplete. The Avrami model was applied to analyze the isothermal crystallization kinetics. It is concluded that the addition of WF modified the crystallization process of PLA (the value of Avrami exponent changed). Various parameters, such as the crystallization half time and crystallization rate constant, reflect that 4 wt% WF significantly improves the crystallization process. The observations in this article indicate that WF is an efficient nucleating agent of PLA. Copyright © 2016 John Wiley & Sons, Ltd.     

Effect of silica nanofillers on isothermal crystallization of poly(vinyl alcohol): In-situ ATR-FTIR study

Isothermal crystallization behavior of poly(vinyl alcohol) (PVA) in the presence and absence of silica nanoparticles was systematically investigated using in-situ attenuated total reflectance Fourier-transform infrared (ATR-FTIR) spectroscopy. The content, size, and surface characteristics of silica nanoparticles were considered as main factors affecting the crystallization behavior, and the effect of annealing time and temperature was also examined. First, very low concentrations of silica nanoparticles (less than 0.5 wt%) could accelerate the crystallization process, whereas higher silica loadings reduced the degree of crystallization. In the PVA/silica (0.5 wt%) nanocomposites, 22-nm silica nanoparticles provided the most suitable interparticle space for nucleation and crystal growth. Compared with hydrophobic silica nanoparticles, hydrophilic silica nanoparticles are favorable to achieve higher crystallinity due to the increased chemical affinity in the nanocomposites. The degree of crystallization became higher with increasing annealing time and it was also enhanced in a high-temperature region. When 0.5 wt% of 22-nm silica nanoparticles was used as a nucleating agent for the crystallization of PVA, the crystallinity of nanocomposites was ca. 20% higher than that of pristine PVA.     

Prediction of poly(oxymethylene) non-isothermal crystallization behaviour from isothermal data

The spherulite growth, nucleation-related,K _g, parameter values obtained from isothermal data (by DSC or optical microscopy) and two other adjustable parameters (the spherulite growth rate preexponential factor and the Avrami's or Tobin's exponent,n) have been used with Nakamura's and Tobin's modified non-isothermal equations to model the kinetics of polymer non-isothermal crystallization. Malkin's model was also tested, for comparison. It is shown that, for polymers that crystallize on cooling almost entirely at temperatures higher than the maximum growth rate temperature, this Tobin's-like non-isothermal model accurately describes the experimental behaviour with only 2 parameters.     

Morphology, isothermal and non-isothermal crystallization kinetics of poly(methylene terephthalate)

The morphology of crystals, isothermal and non-isothermal crystallization of poly(methylene terephthalate) (PMT) have been investigated by using polarized optical microscopy and differential scanning calorimeter (DSC). The POM photographs displayed only several Maltese cross at the beginning short time of crystallization indicating that some spherulites had been formed. The crystal cell belonged to the Triclinic crystal systems and the cell dimensions were calculated from the WAXD pattern. The commonly used Avrami equation and that modified by Jeziorny were used, respectively, to fit the primary stage of isothermal and non-isothermal crystallization. The Ozawa theory was also used to analyze the primary stage of non-isothermal crystallization. The Avrami exponents n were evaluated to be in the range of 2-3 for isothermal crystallization, and 3-4 for non-isothermal crystallization. The Ozawa exponents m were evaluated to be in the range of 1-3 for non-isothermal crystallization in the range of 135-155 °C. The crystallization activation energy was calculated to be −78.8 kJ/mol and −94.5 kJ/mol, respectively, for the isothermal and non-isothermal crystallization processes by the Arrhenius’ formula and the Kissinger’s methods.     

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...     

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.     

Shear-induced non-isothermal crystallization of poly(butylene adipate-co-terephthalate)

The influence of shear on non-isothermal crystallization of commercial poly(butylene adipate-co-terephthalate) (PBAT) was investigated. PBAT melt was sheared at 130 and 150 °C at rates of 10–100/s, and then cooled. The crystallization was followed by a light depolarization technique, whereas the crystallized specimens were analyzed by DSC, 2D-SAXS, 2D-WAXS, PLM and SALS. Shear flow shifted crystallization to higher temperature, and the effect was augmented by lower temperature of shearing as well as by higher shear rate and strain. Crystallization peak rate temperature of PBAT, sheared at 130 °C for 5 min at 100/s, increased by up to 12 °C. However, no evidence of crystal orientation due to shear was found, indicating that the shear induced the point-like nucleation. Only a small increase of melting peak temperatures, by up to 2–5 °C, was observed for the specimens sheared at the highest rates (≥50/s).     

Effect of molecular weight and temperature on the isothermal crystallization of poly(oxetane)

Poly(oxetane) fractions ranging in number-average molecular weights from 7800 to 157000 have been isothermally crystallized in the temperature range from –50 to 19 C, using dilatometric and calorimetric techniques. In both cases, reproducible isotherms were obtained with an Avrami exponent equal to three. The crystallization rate against crystallization temperature presents a maximum at –30 C. The level of crystallinity changes with molecular weight and the influence of this parameter on the rate of crystallization is pronounced. The crystallization temperature coefficient was studied using nucleation theory and it was found an slight increase in the basal interfacial free energy for the lowest molecular weight fraction. For the analysis of the temperature coefficient at the higher undercoolings, different approximations for the free energy of fusion and the transport term have been considered. The conclusion of this analysis is that, independently of these approximations, the obtained temperature coefficients are the same.     

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.     

Crystallization kinetics of a commercial poly(lactic acid) based on characteristic crystallization time and optimal crystallization temperature

Journal of Thermal Analysis and Calorimetry - A model is proposed to fit differential scanning calorimetry (DSC) isothermal crystallization curves obtained from the molten state at different...     

Oriented crystallization of poly(L‐lactic acid) in uniaxially oriented blends with poly(vinylidene fluoride)

This article describes the oriented crystallization of poly(L ‐lactic acid) (PLLA) in uniaxially oriented blends with poly(vinylidene fluoride) (PVDF). Uniaxially drawn films of PLLA/PVDF blend with fixed ends were heat‐treated in two ways to crystallize PLLA in oriented blend films. The crystal orientation of PLLA depended upon the heat‐treatment process. The crystal c‐axis of the α form crystal of PLLA was highly oriented in the drawing direction in a sample cold‐crystallized at T_c = 120 °C, whereas the tilt‐orientation of the [200]/ [110] axes of PLLA was induced in the sample crystallized at T_c = 120 °C after preheating at T_p = 164.5–168.5 °C. Detailed analysis of the wide‐angle X‐ray diffraction (WAXD) indicated that the [020]/ [310] crystal axes were oriented parallel to the drawing direction, which causes the tilt‐orientation of the [200]/ [110] axes and other crystal axes. Scanning electron microscopy (SEM) suggested that oriented crystallization occurs in the stretched domains of PLLA with diameters of 0.5–2.0 μm in the uniaxially drawn films of PVDF/PLLA = 90/10 blend. Although the mechanism for the oriented crystallization of PLLA was not clear, a possibility was heteroepitaxy of the [200]/[110] axes of the α form crystal of PLLA along the [201]/[111] axes of the β form crystal of PVDF that is induced by lattice matching of d₁₀₀(PLLA) ≈ 5d₂₀₁(PVDF). © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1376–1389, 2008     

Miscibility and isothermal crystallization of poly(L-lactide) and poly(trimethylene carbonate) blends

Miscibility, isothermal crystallization kinetics, and morphology of poly(L-lactide)/poly(trimethylene carbonate)(PLLA/PTMC) crystalline/amorphous blends were studied by differential scanning calorimetry(DSC) and optical microscopy(OM). The heterogeneity of OM images and an unchanged glass transition temperature showed that PLLA was immiscible with PTMC. During isothermal crystallization, the crystallization rate of PLLA improved when the PTMC content was low(≤ 20%). However, when the PTMC content was high(≥ 30%), the crystallization rate decreased significantly. The reason of these nonlinear changes in crystal kinetics was analyzed according to the nucleation and growth process by virtue of a microscope heating stage. The isothermal crystallization morphologies of the blends were also studied by polarized optical microscopy and the results confirmed the conclusions obtained from crystallization kinetics.     

Effect of small amount of poly(ethylene naphthalate) on isothermal crystallization and spherulitic morphology of poly(trimethylene terephthalate)

The effect of a small amount of poly(ethylene naphthalate) (PEN) in its blends with poly(trimethylene terephthalate) (PTT) on isothermal melt-crystallization kinetics and spherulitic morphology of the blends was thoroughly investigated. The maximum PEN content in the blends was 9 wt%. Due to the single composition-dependent glass transition temperature (T_g) that was observed for each blend, these blends appeared to be miscible in the amorphous state. After isothermal crystallization from the melt state, the neat PTT and its blends with PEN exhibited either double or triple melting endotherms. The triple endothermic peaks were observed in both the neat PTT and the blends when being crystallized at crystallization temperatures (T_c) of less than or equal to 195 °C. The equilibrium melting temperature () for the neat PTT was determined based on the linear Hoffman–Weeks extrapolative method to be 248 °C. Such values for the blends were found to decrease with the addition and increasing amount of PEN. Both the neat PTT and the blends were isothermally crystallized over the T_c range of 190–205 °C. At a given T_c, the 97PTT/3PEN blend exhibited a half-time of crystallization (t_0.5) value that was lower, while it exhibited reciprocal half-time (), Avrami rate constant (K_A), and spherulitic growth rate (G) values that were greater, than those of the neat PTT. With further increase in the PEN content, the t_0.5 value increased, while the , K_A, and G values decreased. Analysis of the G values based on the Lauritzen–Hoffman's (LH) secondary nucleation theory showed that the neat PTT and the 91PTT/9PEN blend exhibited a regime II→III transition at 194 °C (467.2 K), while no regime transition was observed for the other two blends. The lateral and the fold surface free energies (σ and σ_e) and the work of chain folding (q) for the neat PTT and the blends were 19.4, 30.2–46.3 erg cm⁻², and 2.4–3.6 kcal mol⁻¹, respectively. Lastly, the effect of both the T_c and the PEN content on morphology and texture of the PTT spherulites was also investigated and the results showed that the texture of the spherulites became coarser with increasing T_c and PEN content.     

Unified isothermal and non-isothermal modelling of neat PEEK crystallization

A differential generalized Avrami’s law is used to model crystallization kinetic of PEEK in considering that PEEK crystallization results from the contribution of two distinct mechanisms. The form of this equation allows to predict with good accuracy both isothermal and non-isothermal crystallization kinetics. Nevertheless, isothermal model parameters are not entirely satisfactory for predicting non-isothermal crystallization and the identification of kinetic parameters is needed for both isothermal and non-isothermal cases. The results show that the Avrami exponents and Arrhenius activation energies remain constant for both conditions and therefore suggest that these parameters are only material dependent. On the other hand, the other kinetic parameters depend on the crystallization condition and vary with temperature and/or cooling rate.     

Effect of different inorganic filler over isothermal and non-isothermal crystallization of polypropylene homopolymer

In this study, the effect of several inorganic fillers: silicon oxide (SiO₂), nanoclay (C20A), alumina (Al₂O₃), and calcium carbonate (CaCO₃) on the crystallization behavior of polypropylene were analyzed for composites with fixed filler content (5 mass%) prepared by intensive mixing following by compression molding. In addition, for calcium carbonate, which produces the highest increase on toughness, PP grafted with maleic anhydride (PP-g-MA) was added to enhance the compatibility. In that case, different content of particles was used (from 5 to 20 mass%) and the synergic effect of both incorporations was demonstrated. For this purpose, isothermal and non-isothermal crystallization tests were carried out in the bulk (by differential scanning calorimetry). In addition, the spherulitic growth was studied (by optical microscopy). Different models were used to predict the relative degree of crystallinity and several parameters were analyzed. All results indicate that whereas alumina and calcium carbonate acted as nucleating agents, silica and nanoclay displayed an opposite behavior. The full models that take into account the different parameters during cooling under isothermal and non-isothermal conditions were used to construct continuous cooling transformation and time temperature transformation diagrams. Both kind of diagrams provide a fundamental tool to understand the crystallization behavior of studied composites and are useful to determine the processing conditions.     

Effect of sepiolite on the biodegradation of poly(lactic acid) and polycaprolactone

PLA and PCL nanocomposites prepared by adding 5 wt% of a sepiolite (SEPS9) were degraded in compost, leading to effective degradation for all samples.PLA and PLA/SEPS9 seem to be mainly degraded by a bulk mechanism, showing a significant level of polymer degradation, however the presence of SEPS9 particles partially delays the degradation probably due to a preventing effect of these particles on polymer chain mobility and/or PLA/enzymes miscibility. PCL and PCL/SEPS9 showed a preferential surface mechanism of degradation; and in contrast to PLA, sepiolite does not present a considerable barrier effect on the degradation of PCL.     

Hoffman-Lauritzen parameters for non-isothermal crystallization of poly(ethylene terephthalate) and poly(ethylene oxide) melts

Summary By applying an advanced isoconversional method to DSC data one can evaluate a dependence of the effective activation energy (the temperature coefficient of the growth rate) on the relative extent of melt crystallization. The conversion dependence can further be converted into a temperature dependence and parameterized in terms of the Hoffman-Lauritzen equation. For poly(ethylene terephthalate) (PET) we observe a transition from regime I to II. Poly(ethylene oxide) (PEO) crystallization appears to begin in regime II and then undergoes 2 consecutive changes that however cannot be clearly interpreted as regime III. The K_g and _e parameters obtained for regime I and II (PET) and regime II (PEO) are consistent with the respective parameters reported for isothermal crystallization.     

Effects of top confinement and diluents on morphology in crystallization of poly(l‐lactic acid) interacting with poly(ethylene oxide)

Effects of top confinement and diluent poly(ethylene oxide) (PEO) on poly(l ‐lactic acid) (PLLA) crystal morphology have been investigated. When crystallized at 120 °C, uncovered neat PLLA sample exhibits higher growth rate ringless spherulites; while the covered sample exhibits lower growth rate ring‐banded spherulites. As PEO is introduced into PLLA, the morphology also undergoes significant changes. For the same T_c,PLLA = 120 °C, the PEO/PLLA blend with PEO composition greater than 25% exhibits ring‐banded patterns even in uncovered sample. However, in much greater PEO composition (>80 wt %), uncovered samples exhibit ring bands diverging into dendritic patterns, while top covered samples tend to maintain the spiral ring‐band patterns. Both PEO inclusion in PLLA and top cover on films impose growth kinetic alterations. Additionally, the top glass cover tends to prevent the lower surface tension PLLA to be accumulated on the surface, resulting in the formation of ring‐band pattern. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 1160–1170     

XPS and wettability characterization of modified poly(lactic acid) and poly(lactic/glycolic acid) films

Poly(lactic acid) (PLA) and poly(lactic/glycolic acid) copolymers (PLGA) are biodegradable drug carriers of great importance, although successful pharmaceutical application requires adjustment of the surface properties of the polymeric drug delivery system to be compatible with the biological environment. For that reason, reduction of the original hydrophobicity of the PLA or PLGA surfaces was performed by applying a hydrophilic polymer poly(ethylene oxide) (PEO) with the aim to improve biocompatibility of the original polymer. PEO-containing surfaces were prepared by incorporation of block copolymeric surfactants, poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (Pluronic), into the hydrophobic surface. Films of polymer blends from PLA or PLGA (with lactic/glycolic acid ratios of 75/25 and 50/50) and from Pluronics (PE6800, PE6400, and PE6100) were obtained by the solvent casting method, applying the Pluronics at different concentrations between 1 and 9.1% w/w. Wettability was measured to monitor the change in surface hydrophobicity, while X-ray photoelectron spectroscopy (XPS) was applied to determine the composition and chemical structure of the polymer surface and its change with surface modification. Substantial reduction of surface hydrophobicity was achieved on both the PLA homopolymer and the PLGA copolymers by applying the Pluronics at various concentrations. In accordance with the wettability changes the accumulation of Pluronics in the surface layer was greatly affected by the initial hydrophobicity of the polymer, namely, by the lactide content of the copolymer. The extent of surface modification was also found to be dependent on the type of blended Pluronics. Surface activity of the modifying Pluronic component was interpreted by using the solubility parameters.