Synthesis,stereocomplex crystallization and properties of poly(l‐lactide)/four‐armed star poly(d‐lactide) functionalized carbon nanotubes nanocomposites

Functionalized carbon nanotubes (F‐CNTs) were synthesized through the nucleophilic substitution reaction between four‐armed star poly(d ‐lactide) (4PDLA) and acryl chloride of carbon nanotubes and were characterized using Fourier transform infrared spectroscopy, X‐ray photoelectron spectroscopy and thermogravimetric analysis. The results indicated that the 4PDLA was successfully grafted onto carbon nanotubes, and it contained 45.5 wt% of 4PDLA. Poly(l ‐lactide) (PLLA) nanocomposites with different F‐CNTs content were prepared by solution casting. Optical microscopy and scanning electron microscopy results showed that F‐CNTs were uniformly dispersed in the nanocomposites. Crystallization behavior and crystal structure of PLLA nanocomposites were investigated using differential scanning calorimetry, polarizing microscope and X‐ray diffraction. The results found that poly(lactide) stereocomplex crystal could be formed between PLLA and F‐CNTs. F‐CNTs played different roles in the process of solution casting and melting crystallization. Polarizing microscope also revealed that crystallization temperature had a significant effect on the nucleation and spherulites growth of PLLA. Thermal stability and mechanical properties of the nanocomposites were also investigated by thermogravimetric analysis, dynamic mechanical analysis and tensile testing. These results demonstrated that the addition of F‐CNTs obviously improved thermal stability and tensile strength of PLLA. The results showed that PLLA/F‐CNTs would have potential values in engineering fields. Copyright © 2015 John Wiley & Sons, Ltd.     

Nonisothermal melt crystallization and subsequent melting behavior of biodegradable poly(hydroxybutyrate)/multiwalled carbon nanotubes nanocomposites

In this work, nonisothermal melt crystallization and subsequent melting behavior of poly(hydroxybutyrate) (PHB) and its nanocomposites at different multiwalled carbon nanotubes (MWCNTs) loadings were investigated. Increasing the MWCNTs loadings has enhanced the nonisothermal melt crystallization of PHB significantly in the nanocomposites when compared with that of the neat PHB; furthermore, increasing the cooling rates shift the crystallization exotherms to low temperature range for both neat PHB and its nanocomposites. Double melting behavior is found for both neat PHB and its nanocomposites crystallized nonisothermally from the melt, which is explained by the melting, recrystallization, and remelting model. Effects of the MWCNTs loadings, cooling rates, and heating rates on the subsequent melting behavior of PHB were studied in detail. It is found that increasing the MWCNTs loadings, decreasing the cooling rates, and increasing the heating rates would restrict the occurrence of the recrystallization of PHB in the nanocomposites. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 2238–2246, 2009     

Influence of low multi‐walled carbon nanotubes loadings on the crystallization behavior of biodegradable poly(butylene succinate) nanocomposites

Biodegradable poly(butylene succinate) (PBSU)/carboxyl‐functionalized multi‐walled carbon nanotubes (f‐MWNTs) nanocomposites were prepared via solution casting method at low f‐MWNTs loadings of 0.5 and 1 wt%, respectively, in this work. Scanning and transmission electron microscopic observations reveal a fine dispersion of f‐MWNTs throughout the PBSU matrix. Non‐isothermal melt crystallization at different cooling rates, isothermal melt crystallization at different crystallization temperatures, spherulitic morphology, and crystal structure of neat PBSU and its nanocomposites were investigated with various techniques in detail. The addition of f‐MWNTs is found to enhance the crystallization of PBSU, apparently in the nanocomposites during both nonisothermal and isothermal melt crystallization, due to the heterogeneous nucleation effect; however, the crystallization mechanism and crystal structure of PBSU remain almost unchanged. Effect of the presence of f‐MWNTs and their loadings on the thermodynamic driving force for nucleation and nucleation activity of PBSU was evaluated quantitatively through two methods. Moreover, it is found that incorporating with 1 wt% f‐MWNTs significantly improves the storage modulus of PBSU in the nanocomposites by about 147% at room temperature as compared with that of neat PBSU. Copyright © 2010 John Wiley & Sons, Ltd.     

Poly(L‐lactide)/C60 nanocomposites: Effects of C60 on crystallization of poly(L‐lactide)

The effects of solvent type and concentration of C₆₀ on the crystallization of poly(L ‐lactide) (PLLA) during solvent evaporation, heating from room temperature, and cooling from the melt were investigated by polarized optical microscopy and differential scanning calorimetry. The addition of C₆₀ enhanced the PLLA crystallization during solvent evaporation, during heating of the melt‐quenched films, and during cooling from the melt of As‐cast films, except for heating and cooling of the PLLA film with 1 wt % of C₆₀ prepared with dichloromethane. In the case of solvent evaporation, the difference in crystallinity between the PLLA films with and without C₆₀ became higher for the solvent with a lower boiling point. In the case of heating of melt‐quenched films, the addition of C₆₀ had a small effect on the crystallinity of PLLA, whereas significantly lowered the peak top and ending temperatures of cold crystallization, except for melt‐quenched PLLA film with 1 wt % of C₆₀ prepared with dichloromethane. The crystallinity of PLLA was determined by the solvent type, rather than by the C₆₀ concentration. In the case of cooling from the melt of As‐cast films, the addition of C₆₀ elevated the crystallinity and cold crystallization temperature values of PLLA films, except for PLLA films prepared with dichloromethane. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 2167–2176, 2007     

Comparative study of poly(L‐lactide) nanocomposites with organic montmorillonite and carbon nanotubes

Organic montmorillonite (OMMT) and the one‐dimensional functionalized multiwalled carbon nanotubes (FMWCNTs) were introduced into poly(L ‐lactide) (PLLA) to prepare PLLA/OMMT and PLLA/FMWCNT nanocomposites, respectively. The effects of nanofillers on melt crystallization and cold crystallization of PLLA were comparatively investigated by using polarized optical microcopy, differential scanning calorimetry and wide angle X‐ray diffraction. The results show that FMWCNTs exhibit higher nucleation efficiency for the melt crystallization of PLLA, whereas OMMT is the better one for the cold crystallization of PLLA. Rheological properties show that both OMMT and FMWCNTs at relatively higher concentrations can form the percolated network structure in the PLLA matrix, however, the latter nanocomposites exhibit relatively denser or more compact percolated networks. The difference of the networks between OMMT and FMWCNTs is suggested to be the main reason for the different cold crystallization behaviors observed in the PLLA/OMMT and PLLA/FMWCNT nanocomposites. The dynamic mechanical analysis measurements show that OMMT is the better one to improve the stiffness of the nanocomposites in the present work. The thermogravimetric analysis measurements show that FMWCNTs have higher efficiency in improving the thermal stability of PLLA compared with OMMT. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2013     

Poly(L‐lactide)/layered double hydroxides nanocomposites: Preparation and crystallization behavior

Novel nanocomposites from poly(L ‐lactide) (PLLA) and an organically modified layered double hydroxide (LDH) were prepared using the melt‐mixing technique. The structure and crystallization behavior of these nanocomposites were investigated by means of wide‐angle X‐ray diffraction (WAXD), transmission electron microscopy (TEM), Fourier transform infrared (FTIR) spectroscopy, differential scanning calorimetry (DSC), and polarized optical microscopy (POM). WAXD results indicate that the layer distance of dodecyl sulfate‐modified LDH (LDH‐DS) is increased in the PLLA/LDH composites, compared with the organically modified LDH. TEM analysis suggests that the most LDH‐DS layers disperse homogenously in the PLLA matrix in the nanometer scale with the intercalated or exfoliated structures. It was found that the incorporation of LDH‐DS has little or no discernable effect on the crystalline structure as well as the melting behavior of PLLA. However, the crystallization rate of PLLA increases with the addition of LDH‐DS. With the incorporation of 2.5 wt % LDH‐DS, the PLLA crystallization can be finished during the cooling process at 5 °C/min. With the addition of 5 wt % LDH‐DS, the half‐times of isothermal melt‐crystallization of PLLA at 100 and 120 °C reduce to 44.4% and 57.0% of those of the neat PLLA, respectively. POM observation shows that the nucleation density increases and the spherulite size of PLLA reduces distinctly with the presence of LDH‐DS. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 2222–2233, 2008     

Isothermal crystallization behavior of poly(L‐lactide) in poly(L‐lactide)‐block‐poly(ethylene glycol) diblock copolymers

The crystal unit‐cell structures and the isothermal crystallization kinetics of poly(L ‐lactide) in biodegradable poly(L ‐lactide)‐block‐methoxy poly(ethylene glycol) (PLLA‐b‐MePEG) diblock copolymers have been analyzed by wide‐angle X‐ray diffraction and differential scanning calorimetry. In particular, the effects due to the presence of MePEG that is chemically connected to PLLA as well as the PLLA crystallization temperature T_C are examined. Though we observe no variation of both the PLLA and MePEG crystal unit‐cell structures with the block ratio between PLLA and MePEG and T_C, the isothermal crystallization kinetics of PLLA is greatly influenced by the presence of MePEG that is connected to it. In particular, the equilibrium melting temperature of PLLA, T, significantly decreases in the diblock copolymers. When the T_C is high so that the crystallization is controlled by nucleation, because of the decreasing T and thereafter the nucleation density with decreasing PLLA molecular weight, the crystallinity of PLLA also decreases with a decrease in the PLLA molecular weight. While, for the lower crystallization temperature regime controlled by the growth mechanism, the crystallizability of PLLA in copolymers is greater than that of pure PLLA. This suggests that the activation energy for the PLLA segment diffusing to the crystallization site decreases in the diblocks. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 2438–2448, 2006     

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     

On the kinetics of melting and crystallization of poly(l-lactic acid) by TMDSC

The crystallization and melting process of poly(l-lactic acid), PLLA, is investigated by temperature modulated differential scanning calorimetry, TMDSC. The sample is cooled from the melt to different temperatures and the crystallization process is followed by subjecting the material to a modulated quasi-isothermal stage. From the average component of the heat flow and the application of the Lauritzen–Hoffman theory two crystallization regimes are identified with a transition temperature around 118 °C. Besides, the oscillating heat flow allows calculating the crystal growth rate via the model proposed by Toda et al., what gives, in addition, an independent determination of the transition temperature from modulated experiments. Further, the kinetics of melting is studied by modulated heating scans at different frequencies. A strong frequency dependence is found both in the real and imaginary part of the complex heat capacity in the transition region. The kinetic response of the material to the temperature modulation is analyzed with the model proposed by Toda et al. Finally, step-wise quasi-isothermal TMDSC was used to investigate the reversible surface crystallization and melting both on cooling and heating and a small excess heat capacity is observed.     

Temperature‐modulated differential scanning calorimetry studies on the origin of double melting peaks in isothermally melt‐crystallized poly(L‐lactic acid)

In this work, the melting behaviors of nonisothermally and isothermally melt‐crystallized poly(L ‐lactic acid) (PLLA) from the melt were investigated with differential scanning calorimetry (DSC) and temperature‐modulated differential scanning calorimetry (TMDSC). The isothermal melt crystallizations of PLLA at a temperature in the range of 100–110 °C for 120 min or at 110 °C for a time in the range of 10–180 min appeared to exhibit double melting peaks in the DSC heating curves of 10 °C/min. TMDSC analysis revealed that the melting–recrystallization mechanism dominated the formation of the double melting peaks in PLLA samples following melt crystallizations at 110 °C for a shorter time (≤30 min) or at a lower temperature (100, 103, or 105 °C) for 120 min, whereas the double lamellar thickness model dominated the formation of the double melting peaks in those PLLA samples crystallized at a higher temperature (108 or 110 °C) for 120 min or at 110 °C for a longer time (≥45 min). © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 466–474, 2007     

Crystallization improvement of poly(L‐lactide) induced by functionalized multiwalled carbon nanotubes

In the present work, the crystalline structures and the melting behaviors of poly(L ‐lactide) (PLLA) obtained after being annealed at different conditions have been investigated through differential scanning calorimetry and wide‐angle X‐ray diffraction, respectively. To improve the crystallization of PLLA, functionalized multiwalled carbon nanotubes (f‐MWCNTs) are introduced into PLLA. Our results show that by prolonging the annealing duration or enhancing the annealing temperature, the degree of crystallinity of PLLA gradually increases. Very important, the addition of f‐MWCNTs promotes the cold‐crystallization of PLLA dramatically even at relatively lower annealing temperature or in shorter annealing duration. Further results show that, whether in neat PLLA or in PLLA/f‐MWCNTs nanocomposite, only α form crystal forms during the annealing process. The glass transition temperature shifts to high temperatures because of the increase of crystallinity. F‐MWCNTs exhibit great heterogeneous nucleation effect for PLLA crystallization through enhancing the nucleation density, leading to homogeneous and tiny spherulites formation in a very short time. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 326–339, 2009     

Melt‐crystallization mechanism of poly(ethylene terephthalate)/multi‐walled carbon nanotubes prepared by in situ polymerization

A series of poly(ethylene terephthalate)/multi‐walled carbon nanotubes (PET/MWCNTs) nanocomposites were prepared by in situ polymerization using different amounts of multi‐walled carbon nanotubes (MWCNTs). The polymerization of poly(ethylene terephthalate) (PET) was carried out by the two‐stage melt polycondensation method. The intrinsic viscosity (IV) of the composites is ranged between 0.31 and 0.63 dL/g depending on the concentration of the MWCNTs. A decrease of IV was found by increasing MWCNTs content. This is due to the reactions taking place between the two components leading to branched and crosslinked macromolecules. These reactions are, mainly, responsible for thermal behavior of nanocomposites. The melting point of the nanocomposites was shifted to slightly higher temperatures by the addition till 0.55 wt % of MWCNTs while for higher concentration was reduced. The degree of crystallinity in all nanocomposites was, also, reduced by increasing MWCNTs amount. However, from crystallization temperature, it was found that MWCNTs till 1 wt % can enhance the crystallization rate of PET, whereas at higher content (2 wt %), the trend is the opposite due to the formation of crosslinked macromolecules. From the extended crystallization analysis, it was proved that MWCNTs act as nucleating agents for PET crystallization. Additionally, the crystallization mechanism due to the existence of MWCNT becomes more complicated because two mechanisms with different activation energies are taking place in the different degrees of crystallization, depending on the percentage of MWCNT. The effect of molecular weight also plays an important role. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 1452–1466, 2009     

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     

Study of the crystallization and melting region of PET and PEN and their blends by TMDSC

The cold crystallization and melting of poly(ethylene therephthalate) (PET), poly(ethylene 2,6-naphthalene dicarboxylate) (PEN) and their blends were studied using temperature modulated differential scanning calorimetry (TMDSC) at underlying heating rates of between 1 and 3 K min^-1 and periods ranging from 30 to 90 s. The amplitude of modulation was selected in order to give an instantaneous heating rate β≥0. Heat flow is analyzed by the total heat flow signal o, which is equivalent to the conventional DSC signal, and the reversing heat flow o_REV, which only detects the glass transition and the melting processes. The dependence of the melting region in the reversing heat flow on the frequency of modulation is analyzed. The use of the so-called non-reversing heat flow o_NREV (=o-o_REV)) and the effect of frequency and amplitude on the complex heat capacity are also studied. The results show the complexity of these magnitudes. This revised version was published online in July 2006 with corrections to the Cover Date.     

Highly improved crystallization behavior of poly(L‐lactide) induced by a novel nucleating agent: substituted‐aryl phosphate salts

In this work, a novel nucleating agent (NA) based on substituted‐aryl phosphate salts was introduced into poly(L‐Lactide) (PLLA). The nonisothermal and isothermal crystallization behaviors of nucleated PLLA samples were investigated through differential scanning calorimetry (DSC), wide angle X‐ray diffraction, and polarized optical microscope (POM). Furthermore, the effect of annealing treatment on the cold crystallization behaviors of nucleated samples was also investigated. The results show that the crystallization of PLLA, whether for the melt crystallization (including nonisothermal and isothermal crystallization process) or for the cold crystallization (including the cold crystallization occurring during the DSC heating process and during the annealing process), is greatly dependent upon the content of NA. At relatively lower NA content (≤0.1 wt%), the nucleation effect of NA is inconspicuous, however, at higher NA content (≥0.2 wt%), it exhibits great nucleation effect for the crystallization of PLLA. Further results show that the double endothermic peak of PLLA depends on the temperature applied for the crystallization. Copyright © 2012 John Wiley & Sons, Ltd.     

Effects of carbon nanotubes on polymer physics

A single carbon nanotube has many similarities with an individual polymer chain including the fact that the end‐to‐end length of both are often about the same and the diameter of the chain is about the same (for single‐walled nanotubes) or only ～10 to 20 times larger (for multiwalled nanotubes). The combination of the solid surface and the similarity of the two materials means that polymer physics are altered in manners not seen with any other type of commonly used filler. The purpose of this review is to update a chapter that appears in a recent tome by Grady (2011) and describe how polymer physics is altered in composites that contain carbon nanotubes. Subjects that will be discussed include chain configuration, glass transition, polymer diffusion, unit cells and crystalline superstructure (lamellae, spherulites and shish‐kebabs), and crystallization kinetics. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

Poly(L‐lactide) crystallization induced by multiwall carbon nanotubes at very low loading

Poly(L ‐lactide) (PLLA)/multiwall carbon nanotube (MWNT) composites were prepared by the solvent‐ultrasonic‐casting method. Only very low concentrations of MWNTs (<0.08 wt %) were added in the composites. Isothermal and nonisothermal crystalline measurements were carried out on PLLA/MWNT composites to investigate the effect of MWNTs on PLLA crystalline behavior. DSC results showed that the incorporation of MWNTs significantly shortened the crystalline induction time and increased the final crystallinity of the composite, which indicated MWNTs have a strong nucleation effect on PLLA even at very low concentrations. The nonisothermal crystallization measurements showed that the MWNTs greatly speed up crystallization during cooling. From isothermal crystallization results, both PLLA and PLLA/MWNT composites samples closely followed a relationship based on Lauritzen‐Hoffman theory, with the regime II to III transition shifting to lower temperature with increasing MWNT concentration. A double melting peak appeared in both nonisothermal and isothermal measurements. The conditions under which it appeared were found to closely depend on the regime II‐III transition temperature obtained from Lauritzen‐Hoffman theory. From the magnitude and position of melting peaks, it is proposed that the double melting peak is caused by a disorder‐order crystal phase transition. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 2341–2352, 2009     

Toughening of poly(L‐lactide)/multiwalled carbon nanotubes nanocomposite with ethylene‐co‐vinyl acetate

Poly(L ‐lactide)/multiwalled carbon nanotubes (PLLA/MWCNTs) nanocomposite recently attracts much attention because of its excellent comprehensive properties including improved thermostability, tensile strength, and conductivity. However, the nanocomposite exhibits similar brittleness compared with unmodified PLLA. In this work, a polar elastomer, that is, ethylene‐co‐vinyl acetate (EVA), was introduced into PLLA/MWCNTs nanocomposite. The selective distribution of MWCNTs and the effects of EVA on crystalline structure of PLLA were investigated using scanning electron microscope, transmission electron microscope, differential scanning calorimetry, and wide angle X‐ray diffraction. The results show that the presence of EVA induces the change of the distribution of MWCNTs in the nanocomposites, and consequently, the cold crystallization of PLLA is prevented. With the increase of EVA content, both the ductility and the impact resistance of PLLA/FMWCNTs are improved greatly, indicating the toughening effect of EVA on PLLA/MWCNTs nanocomposite. The decreased tensile strength and modulus can be compensated through annealing treatment. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

Step-scan TMDSC and high rate DSC study of the multiple melting behavior of poly(1,3-propylene terephthalate)

The multiple melting behavior of poly(1,3-propylene terephthalate) (PPT) samples after isothermal crystallization from the melt was studied. The step-scan temperature-modulated differential scanning calorimetry (TMDSC) and high rate DSC were used to investigate this behavior in conjunction with standard DSC, wide-angle X-ray diffraction (WAXD) and polarizing light microscopy (PLM). The effect of PPT average molecular weight on the melting was also examined. In general multiple endotherms after isothermal crystallization of PPT were attributed to a continuous crystal perfection process during the subsequent heating scan via melting-recrystallization-remelting. Multiple melting behavior was more pronounced for the low molecular weight PPT. Step-scan TMDSC showed that extensive recrystallization occurs in PPT samples, especially after rapid isothermal crystallization. In fact two recrystallization exothermic peaks were observed. High rate DSC revealed the initial morphology generated during the isothermal step and showed that the low and middle peaks are associated with melting of primary crystals while the high temperature peak should be attributed to melting of recrystallized material.