Crystallization kinetics,morphology, and hydrolytic degradation of novel bio‐based poly(lactic acid)/crystalline silk nano‐discs nanobiocomposites

In this work, novel biodegradable crystalline silk nano‐discs (CSNs) having a disc‐like morphology have been utilized for fabrication of poly(lactic acid) (PLA) nanocomposites by melt‐extrusion. The main focus is to investigate the effect of CSN on isothermal melt crystallization kinetics, spherulitic growth rates, morphology, and hydrolytic degradation of PLA. Spherulitic morphology and growth rates are examined over a wide range of crystallization temperatures (90–120 °C). With incorporation of CSN, the isothermal crystallization kinetics of PLA/CSN increases, however, the crystallization mechanism remains unaltered. The apparent activation energy and surface energy barrier for crystallization process decreases upon addition of CSNs. At lower isothermal crystallization temperatures (T_c) viz. (90–100 °C), reduced growth rates of PLA spherulites is observed. Both PLA and PLA/CSN exhibit highest crystallization rates at around ～107 °C. The hydrolytic degradation rates calculated from molecular weight reduction shows that PLA/CSN nanocomposites' degradation rates are lower as compared to PLA in acidic, neutral, and alkaline media at pH = 2, 7, and 12, respectively, due to hydrophobic nature of CSN. Scanning electron microscopy study demonstrated the surface erosion mechanism of hydrolytic degradation of PLA and PLA/CSN nanocomposites. This work provides valuable insight for the application and reclamation of PLA/CSN bionanocomposites in moist and wet working environments. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46590.     

Thermal stability and degradation of biological terpolyesters over a broad temperature range

Because of high susceptibility to thermal degradation during conventional melt processing of poly(3‐hydroxybutyrate) (P3HB) homopolymer, incorporation of a second or third monomer unit in the polyester backbones is expected to reduce the melting temperature and crystallinity, resulting in a controlled thermal degradation with improved stability. In this work, random poly(3‐hydroxybutyrate‐co‐3‐hydroxyvalerate‐co‐4‐hydroxyvalerate) (P3HB3HV4HV) terpolyesters biologically synthesized by Cupriavidus necator were investigated for the thermal stability and degradation over a broad temperature range (100–300°C) in comparison with P3HB homopolyester. The work revealed that below the complete melting point (around 150°C), the terpolyester exhibited a high thermal stability and became an amorphous semisolid suitable for conventional thermal processing. Size exclusion chromatography plus nuclear magnetic resonance analysis was used to examine the thermal degradation products and the vulnerability of different monomer units at high temperatures (240–290°C). We found that 3HV unit in P3HB3HV4HV copolymers was more vulnerable to thermal degradation than 3HB unit under air. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41715.     

Study of thermomechanical,structural and antibacterial properties of poly(lactic acid) reinforced with graphene oxide nanoparticles via melt mixing

Addition of graphene oxide (GO) to poly(l ‐lactic acid) (PLLA) offers an alternative approach for tuning its crystallinity, improving its mechanical properties and transfusing an antibacterial behavior. GO/PLLA nanocomposites were prepared by melt extrusion, thus avoiding the potentially toxic, for biomedical applications, residue of organic solvents. Fourier transform infrared spectroscopy verified the formation of intermolecular hydrogen bonds. Using differential scanning calorimetry experiments concerning the isothermal crystallization of PLLA and PLLA containing 0.4 wt% GO, a two‐dimensional disc‐like geometry of crystal growth was determined, whereas at 125 and 130 °C the nanocomposite developed three‐dimensional spherulitic growth. Higher crystallization rate constant values suggest that the incorporation of 0.4 wt% GO accelerated the crystallization of PLLA. The lowest crystallization half‐time for PLLA was observed at 115 °C, while at 110 °C GO caused its highest decrease, accompanied by the highest increase in melting enthalpy (ΔH_m), as compared to that of PLLA, after completion of isothermal crystallization. Their ΔH_m values increased with T_ic, whereas multiple melting peaks transited to a single one with increasing T_ic. GO improved the PLLA thermal stability, tensile strength and Young's modulus. Incorporation of 0.8 wt% GO endowed PLLA with another potential application as a biomaterial since the derived composite presented good thermomechanical properties and effective prohibition of Escherichia coli bacteria attachment and proliferation. This effect was more prominent under simulated sunlight exposure than in the dark. The preparation method did not compromise the intrinsic properties of GO. © 2020 Society of Chemical Industry     

Effects of fumed silica on the crystallization behavior and thermal properties of poly(hydroxybutyrate‐co‐hydroxyvalerate)

The effects of fumed silica on the crystallization behavior and thermal properties of poly(hydroxybutyrate‐co‐hydroxyvalerate) (PHBV) were investigated. The PHBV/silica composites were prepared by a melt‐blending method. The nonisothermal crystallization, melting process, and isothermal crystallization kinetics of PHBV and PHBV/silica composites were characterized with differential scanning calorimetry. The spherulite development and morphology were observed by polarized optical microscopy. In addition, the thermal degradation properties were determined via thermogravimetric analysis. The results indicated that the melting and crystallization kinetics of PHBV were greatly affected by fumed silica, and this was due to the effective nucleation function of silica, which enhanced the crystallization process. The thermal onset degradation temperature of PHBV increased with the addition of fumed silica. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Influence of melt processing on biodegradable nisin‐PBAT films intended for active food packaging applications

Biodegradable poly(butylene adipate‐co‐terephthalate) (PBAT) films incorporated with different levels of the antimicrobial peptide nisin were developed by melt processing. Structural, morphological, thermal, mechanical, and antimicrobial properties of the films were determined. The X‐ray diffraction patterns exhibited decreasing levels of intensity at 2θ values as the concentration of nisin increased. Scanning electron microscopy showed a heterogeneous morphology when higher amounts of nisin were incorporated. The antimicrobial films tested presented no significant differences in the melting temperature (123–125°C), and the crystallization temperature ranged from 69 to 75°C. The addition of nisin caused no significant modification in tensile strength values. However, results of Young's modulus and deformation at break differed significantly among samples. Active films demonstrated inhibition against the Gram‐positive bacterium Listeria monocytogenes. These results demonstrated that PBAT/nisin films produced by melt processing present a great potential for use as active food packaging materials aiming enhanced food safety. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43212.     

Influence of Temperature and Time on Isothermal Crystallization of Poly-L-lactide

The thermal behavior of poly-L-lactide (PLLA) isothermal crystallization upon cooling from the melt was investigated using differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD) and polarizing microscope (POM) by changing the crystallization temperature and time. It was indicated that 110°C should be a critical temperature for PLLA melting crystallization. The melting point of crystallized PLLA discontinuously changed with crystallization temperature, increased with temperature, but decreased at about 110°C, and thereafter again increased with higher crystallization temperatures. At 110°C a multiple endothermic peak was observed. PLLA crystals of higher perfection form when crystallized under higher temperature, which reflects the effects of high chain mobility in higher temperatures. During isothermal crystallization, PLLA crystallites become increasingly perfect, and thicken with prolonged time, leading to an increasing melting point.     

Annealing effect on the microstructure of poly(vinyl chloride)

The effect of annealing on the microstructure of commercial grade poly(vinyl chloride) was investigated by calorimetric, X-ray and viscoelastic measurements. The degree of crystallinity increases with increasing annealing temperature from above the glass transition temperature up to 130°C, at which point the degree of crystallinity takes on a maximum value. Also, the crystal melting temperature increases with increasing annealing temperature. Thermal analysis and X-ray study suggest that the crystallite of poly (vinyl chloride) decomposes by thermal degradation when annealed, above 170°C. The isothermal crystallization process is analyzed using Avrami's equation employing the degree of crystallinity as a function of annealing time at various annealing temperatures. The crystallization rate has a maximum value at around 140°C. It is expected that the crystalline texture grows in the shape of a lineal-like habit, judging from the magnitude of Avrami's constant and from a study of the X-ray intensity distribution. The α_f-transition was observed to occur at temperatures 5 to 10°C lower than the crystalline melting temperatures for annealed specimens of poly(vinyl chloride) using a dynamic spring analysis. The α_f-transition may be attributed to thermal molecular motions with a long time scale, resulting from the cross-link points introduced by the small crystallites.     

Effects of crystalline and orientational memory phenomena on the isothermal bulk crystallization and subsequent melting behavior of poly(trimethylene terephthalate)

The effects of crystalline and orientational memory phenomena on the subsequent isothermal crystallization and subsequent melting behavior of poly(trimethylene terephthalate) (PTT) were investigated by studying the effect of prior melt‐annealing temperature, T_f, on the subsequent isothermal crystallization kinetics, crystalline structure and subsequent melting behavior of neat and sheared PTT samples. On partial melting, choices of the T_f used to melt the samples played an important role in determining their bulk crystallization rates, in which the bulk crystallization rate parameters studied were all found to decrease monotonically with increasing T_f. The decrease in the values of these rate parameters with T_f continued up to a critical T_f value (ie ca 275 °C for neat PTT samples and ca 280 °C for PTT samples which were sheared at shear rates of 92.1 and 245.6 s^?1). Choices of the T_f used to melt neat PTT samples had no effect on the crystal structure formed. The subsequent melting behavior suggested that the T_f used to melt both neat and sheared samples had no effect on the peak positions of the melting endotherms observed and that the observed peak values of these endotherms for all sample types studied were almost identical. Copyright © 2004 Society of Chemical Industry     

Thermal study of polyester networks based on renewable monomers citric acid and gluconolactone

A detailed thermal study is presented of the melt polycondensation between the renewable monomers citric acid and d ‐glucono‐δ‐lactone. It was found that the polyester networks formed have glass transition temperature ranges that increase with increasing reaction temperature and time, corresponding to an increase in molecular weight. The minimum reaction temperature was investigated and found to be 130 °C for a 1/1 system. Moreover, the monomers show eutectic melt behaviour, with a eutectic melting temperature of 125 °C. A range of additional co‐monomers were evaluated, revealing that aliphatic and aromatic bifunctional co‐monomers result in lower glass transition temperatures. When polyfunctional co‐monomers were employed it was found that the chain flexibility influenced the resulting thermal properties. Moreover, it is shown that the ring structure of d ‐glucono‐δ‐lactone plays a key role in the thermal properties of the resulting polyesters. © 2016 Society of Chemical Industry     

Isothermal crystallization and melting behaviors of bionanocomposites from poly(lactic acid) and TiO2 nanowires

Two representative poly(lactic acid) (PLA) nanocomposites with 1% TiO₂ nanowires were prepared through in situ melt polycondensation and easy solution‐mixing approaches, respectively. The former was denoted as ISPLANC, and the latter as SMPLANC. The isothermal crystallization kinetics and melting behaviors of pure PLA, ISPLANC, and SMPLANC were comparatively investigated by differential scanning calorimetry in the temperature range of 80–115°C. Maximum crystallization growth rate (G_exp) was observed at 100°C for all three samples. The well dispersed TiO₂ nanowires acted as effective nucleation agents in ISPLANC, which exhibited much higher G_exp in compared to pure PLA and SMPLANC below 110°C. However, much smaller crystallization enthalpy of ISPLANC was obtained because of its restricted chain mobility in forming crystalline lamellar. The crystallization behavior of all three samples fit the Avrami equation quite well, with most of the R² values larger than 0.9990. Double‐melting behaviors were observed after heating the samples after isothermal crystallization at various temperatures, which was explained by the melt recrystallization of the smaller and imperfect crystals formed at lower isothermal crystallization temperatures. We also obtained the equilibrium melting temperatures of the three samples by carrying out Hoffman–Weeks plots. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Synthesis of a novel polyester plasticizer based on glyceryl monooleate and its application in poly(vinyl chloride)

The use of bio‐based polymeric plasticizers could expand the application range of plasticized poly(vinyl chloride) (PVC) materials. In this study, a novel bio‐based polyester plasticizer, poly(glutaric acid‐glyceryl monooleate) (PGAGMO), was synthesized from glutaric acid and glyceryl monooleate via a direct esterification and polycondensation route. The polyester plasticizer was characterized by gel permeation chromatography, ¹H‐nuclear magnetic resonance, and Fourier‐transform infrared spectroscopy. The plasticizing effect of PGAGMO on PVC was investigated. The melting behavior, thermal properties, and mechanical properties of PVC blends were studied. The results showed that the PGAGMO could improve the thermal stability and reduce the glass transition temperature of PVC blends; when phthalates were substituted by PGAGMO in PVC blends, the thermal degradation temperature of PVC blends increased from 251.1°C to 262.7°C, the glass transaction temperature decreased from 49.1°C to 40.2°C, the plasticized PVC blends demonstrated good compatibility, and the decrement of the torque and the melt viscosity of PVC blends were conducive to processing. All results demonstrated that the PGAGMO could partially substitute for phthalates as a potential plasticizer of PVC. J. VINYL ADDIT. TECHNOL., 22:514–519, 2016. © 2015 Society of Plastics Engineers     

Physical and mechanical properties of Al(OH)3/polypropylene composites modified by in situ‐functionalized polypropylene

Al(OH)₃/polypropylene (PP) composites modified by in situ‐functionalized polypropylene (FPP) were prepared by a one‐step melt‐extrusion process. The effect of in situ FPP on the crystallization and melting behavior, melt‐flow index, limiting oxygen index, thermal degradation, mechanical properties, and fracture morphology of Al(OH)₃/PP composites was studied. Formation of in situ FPP resulted in a decreased crystallization temperature and melting point of PP in the composites, an increased melt‐flow index, and improved tensile and flexural strengths of Al(OH)₃/PP composites, whereas the thermal degradation behavior and limiting oxygen index was not been influenced. The impact strength of the Al(OH)₃/PP composites modified by in situ FPP depended upon the content of the initiator, dicumyl peroxide, and the monomer, acrylic acid. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 83: 2850–2857, 2002; DOI 10.1002/app.10269     

Effects of silver nanoparticles on the dynamic crystallization and physical properties of syndiotactic polypropylene

The effects of silver (Ag) nanoparticles on the physical properties of syndiotactic PP (sPP) were investigated concentrating on the isothermal melt crystallization behavior under shear. sPP with 5 wt % Ag nanoparticles presented higher crystallization temperature (T_c) and heat of crystallization (ΔH_c) than pure sPP. At 90°C, the Ag nanoparticles had little effect on the induction time of crystallization but a little increased the half‐time (t_1/2) for the crystallization. At 100°C, however, the induction time was decreased with increasing the Ag content and the t_1/2 was decreased up to the Ag content of 0.5 wt %. DSC melting endotherms exhibited double melting peaks when crystallized at 90°C under shear but a single melting peak when crystallized at 100°C. The WAXD patterns exhibited that the presence of Ag nanoparticles did not produce any change in the crystal structure of sPP. The tensile strength of sPP is little changed up to the Ag content of 0.1 wt % but it was decreased with further addition. In addition, the introduction of less than 0.1 wt % Ag increased the elongation at break, but further addition decreased it abruptly. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Biobased blends of poly(propylene carbonate) and poly(hydroxybutyrate‐co‐hydroxyvalerate): Fabrication and characterization

Poly(propylene carbonate) (PPC), a CO₂‐based bioplastic and poly(hydroxybutyrate‐co‐hydroxyvalerate) (PHBV) were melt blended followed by injection molding. Fourier transform infrared spectroscopy detected an interaction between the macromolecules from the reduction in the OH peak and a shift in the C?O peak. The onset degradation temperature of the polymer blends was improved by 5% and 19% in comparison to PHBV and PPC, respectively. Blending PPC with PHBV reduced the melting and crystallization temperatures and crystallinity of the latter as observed through differential scanning calorimetry. The amorphous nature of PPC affected the thermal properties of PHBV by hindering the spherulitic growth and diluting the crystalline region. Scanning electron micrographs presented a uniform dispersion and morphology of the blends, which lead to balanced mechanical properties. Incorporating PHBV, a stiff semi‐crystalline polymer improved the dimensional stability of PPC by restricting the motion of its polymer chains. © 2016 The Authors Journal of Applied Polymer Science Published by Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134 , 44420.     

Crystallization kinetics and morphology of poly(ethylene suberate)

The crystallization kinetics and morphology of poly(ethylene suberate) (PESub) were studied in detail with differential scanning calorimetry, polarized optical microscopy, and wide‐angle X‐ray diffraction. The Avrami equation could describe the overall isothermal melt crystallization kinetics of PESub at different crystallization temperatures; moreover, the overall crystallization rate of PESub decreased with increasing crystallization temperature. The equilibrium melting point of PESub was determined to be 70.8°C. Ring‐banded spherulites and a crystallization regime II to III transition were found for PESub. The Tobin equation could describe the nonisothermal melt crystallization kinetics of PESub at different cooling rates, while the Ozawa equation failed. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43086.     

Thermal and mechanical evaluation of cyanate ester resin catalyzed by nonylphenol and stannous octoate

Nonylphenol (NP), stannous octoate [Sn(Otc)₂], and a mixture of NP and Sn(Otc)₂ were employed for catalyzing cyanate ester resin. The curing reaction was studied by differential scanning calorimetry. A water‐absorption test at 85 °C was utilized to study the resistance to warm and humid conditions. The thermal properties were evaluated through measuring thermal weight loss and the glass‐transition temperature (T_g), and the mechanical properties were evaluated through three‐point bending tests and tensile tests. The results show that the mixture of NP and Sn(Otc)₂ exhibits the best catalytic efficiency by decreasing the exothermic peak temperature by almost 148 °C. The mixture of NP and Sn(Otc)₂ has unfavorable effects on the thermal stability. Nevertheless, all catalyst systems have good water‐absorption resistance. The mechanical investigation confirms that the tensile properties show a little reduction that is due to the plasticization of the catalyst, while the excellent flexural properties are maintained. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43959.     

Thermoplastic polyvinyl alcohol/multiwalled carbon nanotube composites: Preparation,mechanical properties,thermal properties,and electromagnetic shielding effectiveness

This study uses the solution mixing method to combine plasticized polyvinyl alcohol (PVA) as a matrix, and multiwalled carbon nanotubes (MWCNTs) as reinforcement to form PVA/MWCNTs films. The films are then laminated and hot pressed to create PVA/MWCNTs composites. The control group of PVA/MWCNTs composites is made by incorporating the melt compounding method. Diverse properties of PVA/MWCNTs composites are then evaluated. For the experimental group, the incorporation of MWCNTs improves the glass transition temperature (T_g), crystallization temperature, T_c), and thermal stability of the composites. In addition, the test results indicate that composites containing 1.5 wt % of MWCNTs have the maximum tensile strength of 51.1 MPa, whereas composites containing 2 wt % MWCNTs have the optimal electrical conductivity of 2.4 S/cm, and electromagnetic shielding effectiveness (EMI SE) of ?31.41 dB. This study proves that the solution mixing method outperforms the melt compounding method in terms of mechanical properties, dispersion, melting and crystallization behaviors, thermal stability, and EMI SE. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43474.     

Isothermal crystallization behavior of poly(L‐lactic acid)/organo‐montmorillonite nanocomposites

The isothermal crystallization behavior of poly(L ‐lactic acid)/organo‐montmorillonite nanocomposites (PLLA/OMMT) with different content of OMMT, using a kind of twice‐functionalized organoclay (TFC), prepared by melt intercalation process has been investigated by optical depolarizer. In isothermal crystallization from melt, the induction periods (t_i) and half times for overall PLLA crystallization (100°C ≤ T_c ≤ 120°C) were affected by the temperature and the content of TFC in nanocomposites. The kinetic of isothermal crystallization of PLLA/TFC nanocomposites was studied by Avrami theory. Also, polarized optical photomicrographs supplied a direct way to know the role of TFC in PLLA isothermal crystallization process. Wide angle X‐ray diffraction (WAXD) patterns showed the nanostructure of PLLA/TFC material, and the PLLA crystalline integrality was changed as the presence of TFC. Adding TFC led to the decrease of equilibrium melting point of nanocomposites, indicating that the layered structure of clay restricted the full formation of crystalline structure of polymer. The specific interaction between PLLA and TFC was characterized by the Flory‐Huggins interaction parameter (B), which was determined by the equilibrium melting point depression of nanocomposites. The final values of B showed that PLLA was more compatible with TFC than normal OMMT. POLYM. COMPOS., 2009. © 2008 Society of Plastics Engineers     

Annealing and crystallization kinetics of poly(lactic acid) pieces obtained by additive manufacturing

In this study, we investigated the influence of isothermal treatment of poly(lactic acid) (PLA) 3D printed samples at different crystallization temperatures. In this case, we analyzed the effect of each crystallization temperature on spherulites formation in printed PLA, affecting the final mechanical properties of pieces. For such, the thermomechanical properties, morphological structure, and crystallization kinetics were analyzed before and after thermal treatment. The 3D printed samples were heat treated at 80°C, 90°C, 100°C, 110°C, and 119°C. With annealing, we observed an improvement in the mechanical PLA properties; however, the exothermic crystallization peak was different for the samples. Pieces before annealing were found to have a low crystallinity index (Ic) of 2%–7%, and the pieces after annealing presented a considerable Ic (27%–34%). Annealing temperatures of 100°C, 110°C, and 119°C produced the fastest crystallization kinetics, while annealing temperatures of 80°C and 90°C resulted in the lowest crystallization kinetics for complete crystallization. After annealing, improvement in the flexural strength (34%–47%) and Young's modulus (26%–51%) for all annealed pieces occurred. The appropriate condition was observed at 100°C, which was the onset temperature of crystallization, owing to the combination of the shorter time of crystallization with the increased mechanical properties.     

Effects of plasticizer on structures,non‐isothermal crystallization,and rheological properties of polyarylates

We herein report the effects of plasticizer content (1–5 wt %) on the structure, non‐isothermal crystallization kinetics, thermal stability, and rheological property of a new type of multicomponent polyarylate (PAR). Fourier transform infrared spectra reveal the presence of a specific interaction between plasticizer and PAR chains, indicating the good dispersion of the plasticizer at the molecular level. The plasticizer influences on the non‐isothermal crystallization behavior of the PAR in two different ways: a mobility enhancer of PAR chains and an impurity to the crystallization of PAR. The melt‐crystallization temperature (T_mc) and enthalpy (ΔH_mc) of the plasticized PARs at cooling runs are higher than those of the neat PAR, which is owing to the enhanced mobility of PAR chains by the plasticizer. On the other hand, the non‐isothermal crystallization rates at different cooling rates of 5–40 °C/min are slower for the PARs with higher plasticizer contents, which is due to the impurity effect of the plasticizer on the melt‐crystallization of PARs. Although the PARs with 1–5 wt % plasticizer have lowered thermal decomposition temperatures, compared to the neat PAR, they are thermally stable up to ～400 °C. The complex melt viscosity of PAR with only 1 wt % plasticizer is far lower than that of the neat PAR. Overall, it is found that only 1 wt % plasticizer is quite effective to facilitate the melt‐processibility and to increase the crystallinity of PAR. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 45704.