Isothermal Crystallization and Melting Behavior of Composites Composed of Poly(L-lactic Acid) and Poly(glycolic Acid) Fibers

Several composites of poly (L-lactic acid) (PLLA) with poly (glycolic acid) (PGA) fibers were prepared. The isothermal crystallization kinetics and melting behavior of PLLA and all of the composites were characterized by using differential scanning calorimetry. The experimental data were processed by using the Avrami equation. The relative parameters, such as the Avrami exponent and half-time crystallization, revealed that PGA fibers had positive effects on the crystallization of PLLA, but these effects had only a minimal dependence on the PGA fiber content. Moreover, at low isothermal crystallization temperatures (85°C～110°C), recrystallization during the heating scan was observed, which could lower the melting point of the samples to a certain extent.     

The Effect of Crystallization on the Solid State Polycondensation of Poly(L‐lactic Acid)

Melt solid polycondensation is an approach to increase the molecular weight of poly (L‐lactic acid) (PLLA). For this report, the effect of crystallization time of PLLA prepolymer on the molecular weight of the biomaterial was studied. In this process, PLLA prepolymer with a molecular weight of 18,000 was first prepared by the ordinary melt‐polycondensation process. The prepolymer was crystallized at 105°C for various times, and then heated at 135°C for 15–50 h for further solid state polycondensation (SSP). The differential scanning calorimetry (DSC) and viscosity measurements were used to characterize the crystalline properties and molecular weight of the resulting PLLA polymers, respectively. The results showed that the molecular weight of PLLA reached a maximum value under the condition of a crystallization time of 30 min and SSP of 35 h.     

Influence of Blend Composition on Crystallization of Poly(L-Lactic Acid)/Poly (Ethylene Oxide) Crystalline/Crystalline Blends

The effect of blend composition on crystallization morphology and behavior of a crystalline/crystalline blend, poly(l-lactic acid) (PLLA)/poly(ethylene oxide) (PEO), during slow, non-isothermal crystallization was studied by polarized light microscopy (PLM) connected with a hot-stage and differential scanning calorimetry (DSC). The results showed that all of the PLLA/PEO blends produced spherulites which gradually became bigger and looser, as well as coarser, with the increment of the PEO content, indicating that the PEO crystals was resided in the interlamellar or interfibrillar (between clusters of commonly oriented lamellae) regions of the PLLA spherulites. In the (25/75) and (10/90) blends, the nucleation and growth processes of the PEO spherulites could be clearly observed in the pre-existing PLLA spherulites. The onset crystallization temperature and the melting point of one component decreased with increasing the content of the other one owing to the good miscibility of the two components in the non-crystalline state and the interaction between their macromolecules, indicating that the crystallization of each component was influenced by the other one.     

Synthesis of Co(II) Complex with Hexamethylenetetramine Using Jet Milling and Its Influence on the Thermal Performance of Poly(l-lactic acid)

Cobalt(II)-hexamethylenetetramine (Co(II)-HMTA) complex was prepared using jet milling. Elemental analysis and thermogravimetric analysis confirmed that the structure of the Co(II)-HMTA complex was Co(HMTA)₂Cl₂·6H₂O (LG). The influence of LG on the thermal performance of poly(l-lactic acid) (PLLA) was investigated. Isothermal crystallization behavior and X-ray diffraction analysis (XRD) results of PLLA/LG showed that LG could improve the crystallization performance of PLLA; 1% LG caused the half time of overall crystallization (t_1/2) of PLLA to decrease from 96.5 min to a minimum value 3.8 min at 100°C. However, the isothermal crystallization kinetics of PLLA/LG described using the Avrami equation and XRD analysis indicated that the isothermal crystallization temperature and the LG concentration significantly affected the isothermal crystallization process of PLLA. In particular, 0.3% LG caused the intensity of the X-ray crystal diffraction peaks of PLLA to decrease with an increase of isothermal crystallization time after increasing for the first 5 min. The thermal decomposition analysis of PLLA/LG showed that the onset decomposition temperature of PLLA with a small amount of LG was higher than that of the neat PLLA and PLLA with a high concentration LG.     

Water‐Responsive Shape Memory Polyurethane Block Copolymer Modified with Polyhedral Oligomeric Silsesquioxane

Nanostructured polyurethane block copolymers with shape memory were synthesized by using polyhedral oligomeric silsesquioxane (POSS) molecules and poly(ethylene glycol) (PEG), which acted as the hydrophobic and hydrophilic groups, respectively. The hard segment domains appeared to be dominantly governed by POSS molecules according to X‐ray diffraction and Fourier transform‐infrared (FT‐IR) measurements. As the POSS content increased, the phase separation between hard and soft segments gradually developed, followed by the crystallization of POSS molecules, and the melting temperature and heat of fusion for POSS crystals increased. The water‐responsive shape memory behavior which resulted from the dissolution of soft segments was demonstrated at 30°C. More than 70% shape recovery was achieved in the water‐responsive shape recovery test, which depended on the POSS content.     

Influence of Bovine Bone Content on In Vitro Degradation of Poly-L-lactic Acid and Bovine Bone Composite Materials

In vitro degradation experiments of poly-L-lactic acid (PLLA) and bovine bone (BB) composites were carried out in a phosphate-buffered solution (PBS) at 37°C with a pH of 7.4. The influence of BB content on pH value of PBS, water uptake, molecular weights, molecular weight distributions, weight losses, mechanical strengths, and morphologies of PLLA/BB was investigated with degradation times. The results indicated that the presence of the BB modified the degradation of the PLLA matrix. The degradation rate of PLLA in the PLLA/BB composite was slower than the degradation rate of the sole PLLA material. Furthermore, the degradation rate of the composites became slower with the increasing content of BB in PLLA/BB composites.     

The Effect of Poly(Ethylene Succinate) on Mechanical Properties of PLLA/PES Blend Prepared by Melt-Blending

Fully biodegradable poly(L-lactide) and poly(ethylene succinate) (PLLA/PES) blends were prepared via melt-blending using PLLA and PES as reactants in a stainless steel chamber. The prepared PLLA/PES blend, as well as neat PLLA and PES, was characterized by Fourier transform infrared spectra (FTIR) and X-ray diffraction (XRD) to confirm the structure and the crystallization of PLLA in the blend. The mechanical properties of PLLA/PES blends were determined by bending and tensile tests and the effects of PES content on the mechanical properties of PLLA/PES blends were investigated. It was found that blending some amount of PES could significantly improve the elongation at break while still keeping considerably high strength and modulus. With increasing PES content, both strength and modulus gradually decreased; however the elongation at break significantly increased. SEM was used to examine the morphology of fracture surfaces of PLLA/PES blends.     

Thermal and Mechanical Properties of Poly(lactic acid)/Modified Carbon Black Composite

Poly(ethylene glycol) (PEG) was added as a plasticizer to the composite of poly(lactic acid) (PLA) and a modified carbon black (MCB). Among the three different molecular weight (M_n = 1000, 2000, 6000) PEGs used, PEG2000 promoted crystallization of PLA and enhanced the nucleation activity of MCB more efficiently than the other two. The crystallization rate of PLA/PEG2000/3 wt% MCB composite was three times that of PLA. Although a small decrease in tensile strength and modulus of elasticity of the composite was found as the PEG content increased, the elongation at break of the PLA/PEG/MCB composites significantly improved. When the PEG2000 content was 15 wt%, the elongation at break of the blend was 90%, 4.5 times that of the neat PLA.     

ISOTHERMAL CRYSTALLIZATION: THERMAL AND OPTICAL STUDY OF PEG

In this work, the solidification process and the crystal growth kinetics of polyethylene glycol 4000 (PEG4000) and polyethylene glycol 6000 (PEG6000) (molecular average weight 4000 and 6000, respectively) were characterized by thermal optical microscopy (OM) and differential scanning calorimetry (DSC). The growth rate decreased with increasing crystallization temperature. The kinetic rate constant was lower for PEG6000 than for PEG4000 (29,300 and 32,900 K², respectively). This behavior is related to the different number of folds (folding index n) of the crystal structures. From comparison of microscopy and calorimetry measurements, the kinetic rate constant values were higher by microscopy due to different melt histories, self-nucleation, and degree of entanglement.     

Effect of Metallic Salts of Phenylmalonic Acid on the Crystallization of Poly(L-lactide)

The effect of the metallic salts of phenylmalonic acid (PMA), as novel nucleating agents, on the melt and crystallization behaviors, spherulitic morphologies, and crystal structures of poly(L-lactide) (PLLA) was studied by means of differential scanning calorimetry, polarized light microscopy, and wide angle X-ray diffraction (WAXD). The results showed that calcium and cadmium salts of PMA are good nucleating agents for PLLA. Lithium, sodium, magnesium, strontium, and zinc salts of PMA are moderate nucleating agents, barium and aluminum salts of PMA are weak nucleating agents, while potassium phenylmalonate is not a nucleating agent for PLLA. The presence of nucleating agents significantly increased the number and decreased the size of the spherulites, but the crystal structures of the nucleated PLLA samples were not changed.     

Thermal Performance of a Blend System Based on Poly(l-lactic acid) and an Aliphatic Multiamide Derivative of 1H-Benzotriazole

An aliphatic multiamide derivative derived from 1H-benzotriazole, N, N'-bis(1H-benzotriazole) sebacic acid acethydrazide (SA), was synthesized to evaluate its effect on the thermal performance, including non-isothermal crystallization and melting behavior as well as thermal stability, of poly(l-lactic acid) (PLLA). The comparative study, by means of DSC measurements, showed that the incorporation of SA caused a non-isothermal crystallization peak to appear and become sharp, showing its advanced crystallization promoting effect for PLLA. The non-isothermal crystallization results further indicated that 2 wt% SA was the saturation concentration for PLLA crystallization, and that the cooling rate was also a crucial determinant for PLLA crystallization. Considering the melting behavior, the difference between the virgin PLLA and PLLA/2%SA further confirmed the crystallization accelerative effect of SA for PLLA, with the increase of crystallization temperature in the temperature zone from 90 to 130°C being beneficial to the crystallization of PLLA during processing. Compared to the virgin PLLA, the trends of thermal decomposition curves were similar, suggesting that the introduction of SA of 0.5–3 wt% did not significantly change the thermal decomposition behavior of PLLA.     

Crowding-Induced Crystallization of Poly(Ethylene Terephthalate)

Samples of poly(ethylene terephthalate) (PET) extracted from three-component systems with different ratios among PET, phenol, and poly(ethylene glycol) (PEG) were prepared. As a crowding agent, PEG can greatly increase PET crystallinity. The crystal and thermal behaviors were characterized by wide-angle x-ray scattering and differential scanning calorimetry. There were two endothermic maxima of the crowding-induced crystallization process as molecular weight and concentration of PEG increased. The theory of crowding can interpret the phenomena well.     

Nanocomposites of poly(l-lactide) and surface modified magnesia nanoparticles: Fabrication, mechanical property and biodegradability

A novel nanocomposite based on biodegradable poly(l-lactide) (PLLA) was prepared by the incorporation of surface modified magnesia (g-MgO) nanoparticles using a solution casting method. The mechanical properties, biodegradable properties and protein adhesion behavior of the g-MgO/PLLA nanocomposites were investigated. Scanning electron microscopy (SEM) results showed that g-MgO nanoparticles could comparatively uniformly disperse in PLLA matrix. The addition of g-MgO nanoparticles to PLLA matrix improved the tensile strength and elastic modulus, whereas reduced the elongation at break. The mass loss results showed that the nanocomposites with higher g-MgO content had faster degradation rates. The in vitro pH value determination results indicated that the g-MgO nanoparticles could neutralize effectively the lactic acid resulting from the degradation of PLLA. The g-MgO/PLLA nanocomposites exhibited enhanced protein adsorption, i.e., with the increase of g-MgO content, the amount of protein adsorption increased. The (5 wt%)g-MgO/PLLA nanocomposites adsorbed 33% more protein than the pure PLLA.     

Miscibility and Isothermal Crystallization Behavior of Poly (Butylene Succinate-co-Adipate) (PBSA)/Poly (Trimethylene Carbonate) (PTMC) Blends

Poly(butylene succinate-co-adipate) (PBSA)/poly (trimethylene carbonate) (PTMC) blend samples with different weight ratios were prepared by solution blending. The morphologies after isothermal crystallization and in the melt were observed by optical microscopy (OM). Differential scanning calorimetry (DSC) was used to characterize the isothermal crystallization kinetics and melting behaviors. According to the OM image before and after melting, it was found that the blends formed heterogenous morphologies. When the PTMC content was low (20%), PBSA formed the continuous phase, while when the PTMC contents was high (40%), PBSA formed the dispersed phase. The glass transition temperatures (T_g) of the blends were determined by DSC and the differences of the T_g values were smaller than the difference between those of pure PBSA and PTMC. In addition, the equilibrium melting points were depressed in the blends. According to these results, the PBSA/PTMC blends were determined as being partially miscible blends. The crystallization kinetics was investigated according to the Avrami equation. It was found that the incorporation of PTMC did not change the crystallization mechanism of PBSA. However, the crystallization rate decreased with the increase of PTMC contents. The change of crystallization kinetics is related with the existences of amorphous PTMC, the partial miscibility between PLLA and PTMC, and the changes of phase structures.     

Mechanical Properties and Crystallization Behavior of Polycarbonate/Polypropylene Blends

The mechanical properties, morphology, and crystallization behavior of polycarbonate (PC)/polypropylene (PP) blends, with and without compatibilizer, were studied by tensile and impact tests, scanning electron microscopy (SEM), X-ray diffraction (XRD), and differential scanning calorimetry (DSC). The tensile and impact strengths of PC/PP blends decreased with increasing the PP content due to poor compatibility between the two phases. But the addition of compatibilizer improved the mechanical properties of the PC/PP blends, and the maximum value of the mechanical properties, such as tensile and impact strengths of PC/PP (80/20 wt%) blends, were obtained when the compatibilizer was used at the amount of 4 phr. The SEM indicated that the compatibility and interfacial adhesion between PC and PP phases were enhanced. DSC results that showed the crystallization and melting peak temperatures of PP increased with the increase of the PP content, which indicated that the amorphous PC affected the crystallization behavior. However, both the PC and compatibilizer had little effect on the crystallinity of PP in PC/PP blends based on both the DSC and XRD patterns.     

Effects of poly(ethylene glycol) on electrical conductivity of poly(3,4-ethylenedioxythiophene)–poly(styrenesulfonic acid) film

A series of poly(3,4-ethylenedioxythiophene)–poly(styrenesulfonic acid) composite thin films with prescribed concentrations of poly(ethylene glycol) were prepared. The PEDOT–PSS pristine film and PEDOT–PSS/PEG films were studied using four-probe method, photoelectron spectroscopy and atomic force microscopy. The electrical conductivity of PEDOT–PSS/PEG hybrid films was found to be enhanced compared to the PEDOT–PSS pristine film, depending on the PEG concentration and molecular weight. XPS analysis and AFM results showed that PEG induces the phase separation between the PEDOT–PSS conducting particles and the excessive PSSNa shell. Simultaneously PEG may form hydrogen bond with sulfonic groups of PSSH, and hence weaken the electrostatic interactions between PEDOT cationic chains and PSS anionic chains. These resulted in the creation of a better conduction pathway among PEDOT–PSS particles, attributed to the improvement of conductivity.     

Nonisothermal Crystallization Kinetics of Poly(Vinylidene Fluoride) in a Poly(Vinylidene Fluoride)/Poly(Methyl Methacrylate)/Dipropylene Glycol Dibenzoate System via Thermally Induced Phase Separation

The nonisothermal crystallization kinetics of poly(vinylidene fluoride) (PVDF) in PVDF/polymethyl methacrylate (PMMA)/dipropylene glycol dibenzoate (DPGDB) blends, where DPGDB served as a diluent, via solid–liquid (S-L) phase separation during a thermally induced phase separation process was investigated through differential scanning calorimetry (DSC) measurements. It was found that the Ozawa model could only describe the nonisothermal crystallization behavior of PVDF/PMMA/DPGDB system to some extent. The influence of the cooling rate and PMMA/PVDF weight ratio in the PVDF/PMMA/DPGDB system on the crystallization mechanism was also examined based on the Avrami–Jeziorny method and Mo method. Primary crystallization and secondary crystallization were observed in the Avrami–Jeziorny analysis. The analysis by the Avrami–Jeziorny and Mo models indicated that the increase of PMMA/PVDF weight ratio decreased the crystallization rate during the primary crystallization stage. The results showed that the Mo method could well explain the kinetics of the primary PVDF crystallization. The Avrami–Jeziorny method, however, could not well describe the nonisothermal crystallization process of PVDF in the primary crystallization stage. The activation energy, determined by the Kissinger method, was not suitable to reflect the PVDF crystallization process in the PVDF/PMMA/DPGDB system.     

Steady–shear‐induced Isothermal Crystallization of Poly(L‐lactide) (PLLA)

The isothermal crystallization of poly(L‐lactide) (PLLA) under steady‐shear flow was investigated in situ using an optical polarizing microscope with a hot shear stage. The steady–shear‐induced crystalline morphology of PLLA, to a great degree, depends on the crystallization temperature. There is a critical temperature, 120°C, below which shear‐induced row nuclei enhance nucleation ability, leading to the improvement of crystallinity, and above which cylindrite structure is generated. Their numbers increase and size reduces with temperature owing to the better movement and relaxation behavior of chains in the presence of shear flow. The results of 2D wide‐angle x‐ray diffraction (WAXD), showing the oriented structure at high T _c , and differential scanning calorimetry (DSC), detecting the rising of T _m with increasing T _c , well confirm the effect of T _c on the crystallization of PLLA under shear flow.     

Nonisothermal Crystallization Nucleation of In‐Situ Fibrillar and Spherical Inclusions in Poly (Phenylene Sulfide)/Isotactic Polypropylene Blends

Nonisothermal crystallization nucleation and its kinetics of in‐situ fibrillar and spherical dispersed phases in poly (phenylene sulfide) (PPS)/isotactic polypropylene (iPP) blends are discussed. The PPS/iPP in‐situ microfibrillar reinforced blend (MRB) was obtained via a slit‐die extrusion, hot stretching, and quenching process, while PPS/iPP common blend with spherical PPS particles was prepared by extrusion without hot stretching. Morphological observation indicated that the well‐defined PPS microfibrils were in situ generated. The diameter of most microfibrils was surprisingly larger than or equal to the spherical particles in the common blend (15/85 PPS/iPP by weight). The nonisothermal crystallization kinetics of three samples (microfibrillar, common blends, and neat iPP) were investigated with differential scanning calorimetry (DSC). The PPS microfibrils and spherical particles could both act as heterogeneous nucleating agents during the nonisothermal crystallization, thus increasing the onset and maximum crystallization temperature of iPP, but the effect of PPS spherical particles was more evident. For the same material, crystallization peaks became wider and shifted to lower temperature when the cooling rate increased. Applying the theories proposed by Ozawa and Jeziorny to analyze the crystallization kinetics of neat iPP, and microfibrillar and common PPS/iPP blends, both of them could agree with the experimental results.     

Nonisothermal Crystallization Kinetics of Poly(lactic acid)/Nanosilica Composites

Poly(lactic acid) (PLA)/nanosilica composites were prepared by blending the PLA and nanosilica in chloroform and then evaporating the solvent to form the composite films in a dish. The Ozawa and Mo equations were used to characterize the nonisothermal cold crystallization kinetics of the PLA/nanosilica composites. The results indicated that the Ozawa equation was not successful while the Mo equation was successful to describe the nonisothermal crystallization kinetics of PLA/nanosilica composites. The values of crystallization activation energy (E _c) of the samples were calculated by the Kissinger method. Although the sample crystallization rates were enhanced with the increase of nanosilica content, the samples exhibited increased E _c in the presence of nanosilica. The results showed that nanosilica had an effect on both the nucleation and the crystal growth of PLA, promoting the nucleation but interfering with the molecular motion of PLA in the crystallization process.