Thermal properties and crystallization behaviors of polylactide/redwood flour or bamboo fiber composites

A series of polylactide/redwood flour (PLA/RWF) and polylactide/bamboo fiber (PLA/BF) composites were successfully prepared using a solution mixing procedure. Fourier transform infrared spectroscopy (FTIR) and wide-angle X-ray diffraction (XRD) were employed to characterize these composites. Thermal properties and crystallization behaviors of PLA composites were determined by their respective techniques of differential scanning calorimetry (DSC) and polarized optical microscopy (POM). With the increasing content of fibers, the glass transition temperature (T _g ), crystallization temperature (T _c ), and melting temperature (T _m ) of PLA/RWF composites decreased first and then increased, but T _g and T _m of PLA/BF composites increased first and decreased afterwards. It is suggested that fibers could improve the segmental mobility of PLA; meanwhile, the different morphologies, sizes, and densities of RWF and BF have different effects on thermal properties of composites. Under the increasing content of RWF, the crystallization rate of the composite increased first and decreased afterwards. When the content of RWF was 5%, the crystallization rate was at its maximum. It could be possible that the addition of fibers was able to nucleate PLA and increase the degree of crystallinity, but the excess content of fibers easily led to heterogeneous composites and subsequent poor crystallization behaviors. In a word, thermal properties and crystallization behaviors of PLA composites were regularly changing by increasing content of fibers.     

Effect of flax fiber content on polylactic acid (PLA) crystallization in PLA/flax fiber composites

Crystallization of polylactic acid (PLA) has a profound effect on its thermal stability and mechanical properties. However, almost no crystallization occurs in actual injection molding process due to rapid cooling program. In this paper, flax fiber was employed as nucleator to enhance the crystallization capability of PLA. Effects of flax fiber content on cold crystallization, melt crystallization, crystallinity, crystal form, morphologies, and size of spherulites of PLA/flax fiber composites were investigated. Dynamic mechanical analysis was innovatively employed to study cold crystallization temperature of PLA/flax fiber composites under dynamic force, and the relationship between cold crystallization temperature (y) and flax fiber content (x) data was fitted by the function y = 34.1 × exp (?x/5.7) + 78.0. The differential scanning calorimetry results showed that the cold crystallization temperature of composites dropped, the melt crystallization temperature of composites increased, and the crystallinity of composites improved with increasing of flax fiber content. Using polarized optical microscopy, it has been found that the spherocrystal size of composites was much smaller than that of neat PLA, and flax fiber induced transcrystallization on the flax fiber surfaces. Wide-angle X-ray diffraction was applied to reveal that flax fiber significantly enhanced the formation of α-form PLA crystals.     

Rheological study of crystallization behavior of polylactide and its flax fiber composites

In this work, the isothermal crystallization of compounded polylactide (PLA) and PLA-flax fiber composites was investigated by means of rheometry using small amplitude oscillatory shear (SAOS). The rheological measurements were carried out in parallel plate flow geometry at a crystallization temperature (T _c ) ranging from 110 to 140 °C. In addition, the effect of shear on polylactide crystallization was studied at 140 °C. Rheological behavior in the molten state was employed to predict the initial viscosity in the T _c interval by applying time-temperature superposition, and results were found to be in agreement with experimental values at low crystallization rates. A simple empirical model was also used to determine the crystallization induction time in a wide range of supercooling conditions. The evolution of the complex viscosity under quiescent conditions of the PLA-based flax-fiber composite indicated an enhancement of the rate of crystallization due to the presence of cellulosic fibers, while shear flow effectively accelerated the crystallization of neat PLA. This work shows that rheometry is an accurate technique for analyzing the crystallization behavior of polymers in a temperature range which presents low to very low crystallization rates, which is the case for PLA at T ≥ 130 °C.     

Non-isothermal crystallization kinetics assessment of poly(lactic acid)/graphene nanocomposites

In this work, the effects of the presence and modification of graphene nanoplatelets (GNps) on the crystallization of the poly(lactic acid) (PLA) were studied. Functionalization of GNps was accomplished by acid treatment. Nanocomposite samples were prepared by solution method containing pristine and functionalized graphene. In contrast to pristine PLA, crystallization of the samples contains nano filler initiates at higher rates that showed the role of heterogeneous nucleating effects of these particles in crystallization of the PLA. Then, the effect of nano filler functionalization was comprised. Initial slope of the crystallization (S _i) and full width at the half height maximum of crystallization peak are indicative of nucleation rate and spherulite size distribution, respectively; which upon the addition of the functionalized graphene nanoplatelets (FGNps), S _i increased and spherulites gained normal size distribution. Non-isothermal and crystallization kinetics of the samples were studied using differential scanning calorimetry at heating rates of 2, 4, 6 and 10 °C/min. Performed techniques such as furrier transform infrared, dynamic-mechanical thermal analysis and visual observation of sediments confirmed the successful modification of the graphene platelets. Also, non-isothermal analysis pinpointed the fact that crystallization temperature (T _c) of the nanocomposites has increased by 11–21 °C, compared to the neat PLA. Upon verification of Avrami’s theory, it was conducted that dominant mechanism of nucleation of the nanocomposite samples was 2D circular diffusion; wherein, Avrami’s exponent (n) was determined as 2. Moreover, it was deduced from Avrami’s equation that “n” have no discernible changes in nanocomposites containing GNps or FGNps. Electrical devices and shape memories can be the main application of these nanocomposites.     

A coarse-grained model for polylactide: glass transition temperature and conformational properties

In this article, we present a coarse-grained (CG) model of poly(lactic acid) (PLA) developed by the iterative Boltzmann inversion (IBI) method. The coarse-grained potential was derived by matching the structural probability distribution functions to those of reference atomistic simulation. The resulting coarse-grained potential was found to be temperature-dependent when trying to reproduce the thermal expansion behavior of PLA. To satisfactory reproduce this behavior, the potential needs to be modified by a temperature factor of (T/T ₀)^0.3; T ₀ = 327 K is the temperature at which the potential has been derived. The glass transition temperature (T _g) as predicted by the modified CG potential compared favorably with those from experiment and atomistic simulation. Chain conformational properties were also evaluated in terms of a chain length (N)-radius of gyration (R _g) relation and the persistence length. The model we develop was also noted to provide a considerable speed-up of computer time compared to its atomistic counterpart.     

Combined effect of nucleating agent and plasticizer on the crystallization behaviour of polylactide

Different plasticized and nucleated polylactide (PLA) systems were prepared and characterized. Two PLA with different l-lactic acid contents (96 and 99.5%) were plasticized with dioctyl adipate (DOA) and nucleated by talc, ethylene bis(stearamide) (EBS), or d-lactic acid-based PLA (PDLA). Crystallization behaviour was studied by differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD), and polarized optical microscopy (POM). The combination of plasticizer and nucleating agent was proved to be a very effective approach to improve crystallization velocity of different PLA matrices. Within the studied crystallization temperature range, faster crystallization rates were achieved at lower temperatures. WAXS results indicated the coexistence of α and α′ crystals in all studied systems, except those which showed very low crystallization degrees. Avrami exponent remained constant at around n ≈ 3 for all systems, suggesting equivalent three-dimensional spherulitic growth behaviour regardless crystallization temperature, nucleating agent, and the stereochemistry of the matrix used. Usually, injection-moulding process, where molten polymer is under high pressure, is used for PLA polymer processing. To analyze the effect of pressure on the crystallization process, pressure volume temperature (PVT) measurements were carried out on the systems that showed the fastest crystallization process under atmospheric pressure by DSC. Results showed that the crystallization process was considerably accelerated under pressure.     

Preparation and characterization of poly(ethylene carbonate)/poly(lactic acid) blends

Poly(ethylene carbonate)/poly(lactic acid) blends were successfully prepared by means of a solution film-casting method, and their physicochemical properties were investigated. PEC/PLA blends exhibit partial miscibility and are characterized by the interaction of the ester and carbonic ester groups. One such interaction is between partial charges in –C–O– in –O–C=O of PLA and the carbonyl –C=O of PEC. Another is between –C–O– in –O–C=O of PLA and –C–O– in –CH₂–O– of PEC. The value of T_g varies by more than 10 °C across the blends. PEC does not significantly influence the melting temperature of neat PLA, but non-spherical spherulites are formed in PEC-rich blends, whereas the spherulites are spherical with an average size of 30 μm in PLA-rich blends. Crystallization of PLA is influenced by the addition of flexible PEC and by the proportion of PLA in the blends. Interestingly, addition of at least 10 wt% PLA increased T_g, with a crystallinity, X_c of 47% and better thermal degradation properties, with the temperature at 5 wt% weight loss (T_d5) more than 30 °C higher than for neat PEC.     

The influence of lithium salt on the interfacial reactions controlling the thermal stability of graphite anodes

The thermal stability of graphite anodes used in Li-ion batteries has been investigated, with the influence of electrolyte salt under special scrutiny, LiPF₆, LiBF₄, LiCF₃SO₃ and LiN(SO₂CF₃)₂ in an ethylene carbonate (EC)/dimethyl carbonate (DMC) solvent mixture. Differential scanning calorimetry (DSC) showed exothermic reactions in the temperature range 60-200 °C for all electrolyte systems. The reactions were coupled to decomposition of the solid electrolyte interphase (SEI) and reactions involving intercalated lithium. The onset temperature of the exothermic reactions increased with type of salt in the order: LiBF₄<LiPF₆<LiCF₃SO₃<LiN(SO₂CF₃)₂. X-ray photoelectron spectroscopy (XPS) was used to identify surface species formed prior to and after the exothermic reactions, to clarify different thermal behaviour for different salts. The decomposed SEI's in LiCF₃SO₃ and LiN(SO₂CF₃)₂ electrolytes were found to be mainly solvent-based, including lithium alkyl carbonate decomposition to stable Li₂CO₃ and the formation of poly(ethylene oxide) (PEO)-type polymers. In the LiBF₄ and LiPF₆ systems, decomposition was governed by salt reactions, which decomposed the salts and resulted in the main product LiF.     

Change of the thermal behavior of 55Bi<Subscript>2</Subscript>O<Subscript>3</Subscript> ⋅ 45B<Subscript>2</Subscript>O<Subscript>3</Subscript> glass at annealing close to the vitrification temperature

The characteristic properties of relaxation processes for 55Bi₂O₃ ? 45B₂O₃ glass were described. The pattern of changes of the vitrification temperature (T _g ) is found to be quasi-periodic; the value of the endothermic effect near T _g (ΔC’ _p ) and the difference between the softening point and the vitrification temperature (T _m –T _g ) were determined. The temperature-time parameters of the transition from quasi-periodic to the continuous change of the properties were determined and an explanation of such behavior of the system was offered.     

Crystallization mechanism,microstructure and mechanical property of poly (L-lactic acid) modified by introduction of hydrogen bonding

In this work, 4,4′-Thiobis Phenol (TDP) was incorporated into PLLA using melt blending method. Overall crystallization mechanism and chain folding parameters in crystallization process were analyzed based on Avrami eq. and Lauritzen-Hoffman kinetic theory. Avrami exponent n for all samples were between 2.87 and 3.23, which indicated three dimensional crystal growth with heterogeneous nucleation. Meanwhile, overall crystallization rate of PLLA was significantly reduced by incorporation of TDP. The fold surface free energy (σ _e) and work of chain folding (q) increased with TDP addition, implying that crystallization of PLLA was retarded. Small angle X-ray scattering (SAXS) analysis demonstrated that long period of PLLA was increased by TDP introduction, meaning more amorphous component between adjacent lamellae. As a result, the tensile strength decreased with TDP addition, while the elongation at break of PLLA was improved by more than 40 %.     

Effect of nitrogen to niobium atomic ratio on superconducting transition temperature of δ-NbN<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> powders

Powders of cubic niobium nitride δ-NbN _x with a particle size of below 20 μm were prepared by reactive diffusion at T = 1455?1475°C under nitrogen pressures of P ₁(N₂) = 0.1?3 MPa and P ₂(N₂) = 25 MPa. For these powders, the values of the stoichiometric coefficient x, lattice parameter a, and the superconducting transition temperature Tc were measured and the a(x), T _c(x) and T _c(a) functions were analyzed. The T _c values were found to linearly grow with increasing a (decreasing structure imperfection). A maximum value of T _c (15.8 K) corresponded to a maximum value of a (4.3934 Å). Maximain the a(x) and T _c(x) curves were found to correspond to a slightly substoichiometric nitride with x = 0.98. Having synthesized cubic niobium nitrides with 0.892 < x < 1.062, we managed to measure the dependences of a and T _c on x all over the almost entire homogeneity range for δ-NbN _x . Our a(x) and T _c(a) functions were found to reasonably agree with those previously reported for SHS-produced δ-NbN _x powders.     

Synthesis and crystal structure of the low-temperature modification of Li<Subscript>12</Subscript>Zn<Subscript>4</Subscript>(P<Subscript>2</Subscript>O<Subscript>7</Subscript>)<Subscript>5</Subscript>

The crystal structure of a low-temperature modification of the Li₁₂Zn₄(P₂O₇)₅ compound has been determined by full-profile analysis from the X-ray powder diffraction data. The compound crystallizes in the monoclinic crystal system (a = 5.130(1) Å, b = 13.454(1) Å, c = 8.205(1) Å, β = 90.36(1)°, space group P2₁/n, Z = 4) and has a framework structure in which the zinc and lithium atoms statistically occupy equivalent positions.     

Investigation on microstructures,melting and crystallization behaviors,mechanical and processing properties of <Emphasis Type="Italic">β</Emphasis>-isotactic polypropylene /CaCO<Subscript>3</Subscript> toughening masterbatch composites

In this study, β-isotactic polypropylene (β-iPP)/CaCO₃ toughening masterbatch (CTM) composites were compounded in a single screw extruder. The microstructures and properties of the composites were investigated. It was shown that CTM influenced the crystallinity and crystallization temperature of β-iPP. The flexural modulus and storage modulus (E’) at 23 °C increased with the increasing CTM content, implying the increased stiffness of the composites. The improved miscibility between β-iPP and CTM was demonstrated by the decreased glass transition temperatures of the composites. The Izod notched impact strength at 23 °C of the composites was directly related to the CTM content because of the competition between the morphological feature of β-iPP and the content of inorganic rigid toughening particles. The critical ligament thickness (τ _c = 1.47 μm) at 40% CTM content was calculated according to the modified Wu’s equation. The morphologies of impact fractured surfaces were observed and the massive shear deformation was related to the debonding of CaCO₃ particles. The presence of CTM also improved the melt flowability and dimensional stability but it was detrimental to heat deflection temperature (HDT) of the composites.     

Optimal prediction of PKS: RSO modified alkyd resin polycondensation process using discrete-delayed observations,ANN and RSM-GA techniques

Alkyd resins are widely used in the paint industry and although they have a long history about 70–100 years, today the developments in alkyds are still welcome and innovations are still needed. Artificial neural network (ANN) and response surface methodology based on a 2^5?1 fractional factorial design were used as tools for simulation and optimization of the polycondensation process for autooxidative drying alkyd resin from palm kernel stearin: rubber seed oil blend of 70:30 ratio. A feed forward neural network model with Levenberg–Marquardt back propagation training algorithm was adapted to predict the responses (conversion Y ₁, viscosity Y ₂, and molecular weight average Y ₃). The studied input variables were reaction time, temperature, catalyst concentration, oil ratio, and stirring rate. The performance of the RSM and ANN model showed adequate prediction of the responses in terms of the process factors, with MRPD of ±4.47% (Y ₁), ±2.08% (Y ₂), ±8.92% (Y ₃) and ±6.50% (Y ₁), ±3.31% (Y ₂), ±10.20% (Y ₃), respectively. The sensitivity analysis showed that while reaction time is the most effective process parameter, the interaction of the five process variables produced the most significant effect on the studied responses with the overall minimum MSE of 0.079. The optimization task performed using a genetic algorithm linked to the RSM model gave a viable, nondominated optimal response and optimum operating conditions regarding the route to high-quality resin at reduced material and operational costs. Overall, coupled RSM-GA was found to be a better tool for modeling and optimization of the alkyd resin production.     

Local conformation controlled crystallization of isotactic poly(butene-1)

The slow spontaneous solid-solid transition from metastable form II to stable form I in typical isotactic polybutene-1 (iPB-1) homopolymer after melt crystallization may be unavoidable and it is challenging to obtain form I directly from bulk iPB-1 melt. In this work, by DSC and WAXS, form I was observed to directly crystallize from bulk iPB-1 while cooling to T far below the crystallization temperature T_c of form II. It is further proposed that the local formation of 3/1 helix conformation is the determining process of direct formation of form I and there exists a temperature window for 3/1 helix conformation to form at low T due to the competition between the entropy effect and the synergistic effect of enthalpy and the energy barrier. The key of polymorph selection of iPB-1 lies in the T-dependent formation of the crystal-favored helix conformations. The results could shed lights on developing charming industrial approaches to obtain the stable form I directly in bulk iPB-1 as well as on understandings to the physics behind polymer crystallization and polymorph selection processes.     

Effects of the addition of LiCl,LiClO4, and LiCF3SO3 salts on the chemical structure,density, electrical,and mechanical properties of rigid polyurethane foam composite

The effect of the incorporation of several lithium salts on the electrical and mechanical properties of polyurethane rigid (PUR) foams was investigated. Different amounts of lithium chloride (LiCl), lithium perchlorate (LiClO₄), and lithium trifluoromethanesulfonate (LiCF₃SO₃) were added to the polyuretanic precursor. The salts affected the cellular microstructures and consequently the mechanical properties of the composite. Composite foams containing an amount of LiCl greater than 2 wt% showed low‐surface resistivity (～10⁶ Ω), whereas the LiClO₄ and LiCF₃SO₃ composites showed, in all range of filler percentage analyzed, high‐surface resistivity values (～10¹¹ Ω). This behavior was related to the different interactions between PUR and lithium salts, as confirmed by FTIR/Attenuated Total Reflectance analysis. Only the LiCl was able to create a motion of the ions Li⁺ and Cl^? along the polyurethanic chains, because LiCl was completely dissociated. On the contrary, LiClO₄ and LiCF₃SO₃ that affected the macromolecular structure of the polymeric network did not permit the formation of polyurethanic channels, where the ions could move, thus creating a charge motion. POLYM. ENG. SCI., 2011. © 2011 Society of Plastics Engineers     

Photon interaction parameters for some ZnO–Al<Subscript>2</Subscript>O<Subscript>3</Subscript>–Fe<Subscript>2</Subscript>O<Subscript>3</Subscript>–P<Subscript>2</Subscript>O<Subscript>5</Subscript> glasses

Some photon interaction parameters such as mass attenuation coefficient, effective atomic number, half value layer, mean free path and electron density for 15ZnO–(17.5–x)Al₂O₃–xFe₂O₃–67.5P₂O₅ glass system (x = 0, 7.5, 12.5, 17.5) and 15ZnO–(25–x)Al₂O₃–xFe₂O₃–60P₂O₅ glass system (x = 0, 25) have been investigated in the photon energy range of 1 keV to 100 GeV. It has been observed that all the photon interaction parameters for the selected glass systems vary with the photon energy. Among the selected glass systems, the sample 15ZnO–25Fe₂O₃–60P₂O₅ glass system shows maximum values for mass attenuation coefficients, effective atomic numbers, electron densities and minimum values for mean free path and half value layer in the entire energy grid.     

SHS of Y<Subscript>2</Subscript>Ti<Subscript>2</Subscript>O<Subscript>7</Subscript>-based mineral-like ceramics: Influence of green density

Explored was the influence of compacting pressure (P) and green density (ρ) on the properties of Zr-doped mineral-like pyrochlore ceramics Y₂(Ti_{1 – x} Zr _x )₂O₇ (x ≤ 0.3) prepared by SHS method. The optimal ρ values that provide minimal porosity and maximal mechanical strength of synthesized ceramics were found. An increase in ρ was found to decrease combustion temperature and increase pyrochlore lattice parameter a. Green density was also found to affect phase composition of the SHS-produced ceramics under study.     

Multilayer coatings on Ti substrate by SHS method

Explored was the deposition of multilayer TiC _x –Ti _x Si _y -based coatings onto Ti substrate by SHS method. Sandwich-type green multilayer structures were assembled from Ti foils and Ti + 0.5C, Ti + Si, 5Ti + 3Si, and Ti + 0.65C pellets and ignited under 1 atm of Ar and a load (400 g). Burned sandwiches were characterized by SEM, EDS, and XRD. In all cases, we observed good metal–ceramic joining. Prerequisites for such a joining are (i) the presence of the liquid phase in combustion products, (ii) good metal–ceramic wettability, and (iii) closeness of reaction temperature to the melting point of substrate.     

FTIR spectra and elastic properties of Cd-substituted Ni–Zn ferrites

Solid solutions Ni_0.5–x Cd _x Zn_0.5Fe₂O₄ (x = 0, 0.15, 0.30) were prepared by solid-state synthesis and characterized by FTIR spectroscopy. The FTIR spectra of synthesized ferrites showed two absorption bands (ν₁ and ν₂) in the range 400–600 cm^–1 belonging to tetrahedral (A) and octahedral (B) interstitial sites in the spinel lattice. The force constants for tetrahedral (K _t) and octahedral sites (K _o) were determined, as well as Young’s modulus (E), rigidity modulus (G), bulk modulus (B), Debye temperature (Θ_D), and velocity of transverse (V _t) and longitudinal (V _l) elastic waves. The relevant interionic cation–anion, cation–cation distances and bond angles are also reported.