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.     

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.     

Effect of different structured POSS/GMA-containing copolymers on the morphology of porous coating by breath-figures method

Polyhedral oligomeric silsesquioxane (POSS) and glycidyl methacrylate (GMA) are used to synthesize linear L-(PGMA-b-MA-POSS), dicephalous D-(PGMA-b-MA-POSS)₂, and four-armed T-(PGMA-b-MA-POSS)₄ through atom transfer radical polymerization (ATRP). These different structured POSS/GMA-containing copolymers are applied to build the porous films through breath-figures (BF) method. L-(PGMA-b-MA-POSS) is easy to form the ordered porous BF film, and T-(PGMA-b-MA-POSS)₄ tends to form well-distributed BF film due to its high segment density which is beneficial to stabilize water drop. It is proven that solvent has a great effect on the morphologies of BF film. The perfect BF film is developed in the relatively wet atmosphere as 5–8 μm diameter of pores casted in THF and 2–4 μm diameter of pores casted in CH₂Cl₂ with regular morphology. The developed BF film has superior stability than the film developed in natural conditions due to the shorter stable balancing time. Therefore, it is believed that the obtained porous films show great potential in coating application.     

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.     

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.     

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.     

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.     

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.     

Investigation of the fast relaxation in glass-forming selenium by low-frequency Raman spectroscopy

The low-frequency Raman spectra of liquid and vitreous selenium are investigated. It is demonstrated that the temperature dependence of the intensity of the fast relaxation at the glass transition temperature (T _g = 308 K) exhibits a specific feature. This feature manifests itself in a sharper increase in the intensity at temperatures T > T _g as compared to that observed at lower temperatures. The intensities of the fast relaxation at the critical temperature T _c are evaluated by the extrapolation of the linear dependence to the temperature range T > T _g in the framework of the mode-coupling theory. The new results obtained for selenium are compared with the available data for other glass-forming materials (boron oxide, toluene, arsenic sulfide). It is shown that, for all the glasses under investigation, the parameter describing the contribution of the fast relaxation to the Raman spectrum takes on the same value at the critical temperature T _c and is approximately equal to 0.3.     

Basalt fiber reinforced and elastomer toughened polylactide composites: Mechanical properties,rheology, crystallization,and morphology

A series of the reinforced and toughened polylactide (PLA) composites with different content of basalt fibers (BF) were prepared by twin screw extruder. The toughness of BF/PLA composite s was improved further by the addition of polyoxyethylene grafted with maleic anhydride (POE-g-MAH), ethylene–propylene–diene rubber grafted with maleic anhydride (EPDM-g-MAH), and ethylene-acrylate-glycidyl methacrylate copolymer (EAGMA), relatively. The mechanical properties, rheology, crystallization, and morphology of BF/PLA composites were studied. The results showed that basalt fiber had significant reinforcing and toughening effect in comparsion with glass fiber. EAGMA was more effective in toughening BF/PLA composites than POE-g-MAH and EPDM-g-MAH. When the content of EAGMA achieved to 20 wt %, the impact strength of BF/PLA/EAGMA composite increased to 33.7 KJ/m², meanwhile the value was improved by 71.1% compared with pure PLA. According to dynamic rheometer testing, the use of the three kinds of elastomers increased the melt dynamic viscosity. Differential scanning calorimetry analysis showed that POE-g-MAH and EPDM-g-MAH can decrease the cold crystallization temperature (T_cc) to approximately 20°C and dramatically improve crystallinity (χ_c) of BF/PLA composites. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

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.     

Modifying the thermal and mechanical properties of poly(lactic acid) by adding lithium trifluoromethanesulfonate

The effect of the addition of lithium trifluoromethanesulfonate (LiCF₃SO₃) on the linear viscoelastic properties, crystallization behavior, and mechanical properties of poly(lactic acid) (PLA) was studied. The glass transition temperature (T_g) was enhanced by adding LiCF₃SO₃, without any loss of transparency of the PLA. This was attributed to the ion-dipole interaction between the lithium cation and oxygen atom in the PLA carbonyl group. The interaction weakened at higher temperature. Consequently, the rheological terminal region was clearly detected, which suggested that the system possessed good melt-processability. The Young’s modulus and yield stress at room temperature were also enhanced by the addition of LiCF₃SO₃, although the toughness was reduced due to the brittle failure. Finally, the presence of LiCF₃SO₃ retarded the crystallization of PLA, because the segmental motion of the PLA chains was reduced.     

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.     

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.     

Influence of PLA stereocomplex crystals and thermal treatment temperature on the rheology and crystallization behavior of asymmetric poly(L-Lactide)/poly(D-lactide) blends

Asymmetric poly(L-lactide)/poly(D-Lactide) (PLLA/PDLA) blends were prepared by adding small amounts of PDLA into the PLLA matrix with the formation of stereocomplex crystallites (sc-crystallites). Rheological results indicated that the PLLA/PDLA melt at lower temperatures (<T_m,sc, the melting temperature of the formed stereocomplex crystallites) underwent the transition from liquid-like to solid-like viscoelastic behaviors with increasing of the PDLA concentration, which was related to the sc-crystallites reserved in the melt of asymmetric PLLA/PDLA blends. Dissolution experiment indicated the presence of sc-crystallites network structure in the PLLA/PDLA blends, and the size of the sc-crystallite junction particles network increased with increasing of the PDLA concentration. DSC and POM studies indicated that the PDLA concentration and the thermal treatment temperature had a significant influence on the PLLA crystallizability behavior. At low thermal treatment temperature (<T _m,sc ), reserved sc-crystallites showed an obvious promoting effect for PLLA crystallization. With increasing of the thermal treatment temperature, its promoting effect decreased due to melting of the sc-crystallites. This result suggests the sc-crystallites played two roles: nucleation sites and cross-linking points, and the two roles had a competitive relationship with change of the thermal treatment temperature and the PDLA concentration.     

Optimization of controlled ring-opening polymerization of <Emphasis Type="SmallCaps">l</Emphasis>-lactide to hydroxyl terminated polylactides using zinc acetate catalyst

Hydroxyl terminated polylactide polymers with number of average molecular weights (M _n ) varying from 1625 to 3459 g mol^?1 were synthesized by ring opening bulk polymerization of lactide in the presence of zinc acetate being a potent catalyst. The use of 1,4 butanediol (BDO) initiator leads to hydroxyl terminated polylactides, thus excellent precursors for shape-memory biodegradable polyurethanes. Different reaction conditions employed for the synthesis of hydroxyl terminated polylactide polymers via activated monomer mechanism may result in differences in M _n , percentage mass conversion and percentage degree of crystallinity (%χ _c ) of the product. Influence of process parameters, i.e. catalyst concentration, initiator concentration, reaction temperature and time on characteristics of hydroxyl terminated polylactides was studied. These polymers were characterized by Nuclear Magnetic Resonance (¹H-NMR) spectroscopy, Fourier transforms infrared (FTIR) spectroscopy, gel permeation chromatography (GPC) and X-ray diffraction (XRD) techniques. FTIR and ¹H-NMR confirmed the formation of hydroxyl terminated polylactides. M _n was determined by ¹H-NMR, GPC and end group analysis. %χ _c was calculated from XRD spectra. Maximum mass conversion, M _n and %χ _c were observed at 5 mol% SnOct₂ and 5 mol% BDO concentration. At optimum temperature of 145 °C, these characteristics improved linearly with polymerization time up to 6 h and declined thereafter.     

Sonocrystallization of Interesterified Fats with 20 and 30% of Stearic Acid at the sn-2 Position and Their Physical Blends

Physical blends (PB) of high oleic sunflower oil and tristearin with 20 and 30% stearic acid and their interesterified (IE) products where 20 and 30% of the fatty acids are stearic acid at the sn-2 position crystallized without and with application of high intensity ultrasound (HIU). IE samples were crystallized at supercooling temperatures (ΔT) of 12, 9, 6, and 3 °C while PB were crystallized at ΔT = 12 °C. HIU induced crystallization in PB samples, but not in the IE ones. Induction in crystallization with HIU was also observed at ΔT = 6 and 3 °C for IE C18:0 20 and 30% and at ΔT = 9 °C only for the 30% samples. Smaller crystals were obtained in all sonicated samples. Melting profiles showed that HIU induced crystallization of low melting triacylglycerols (TAGs) and promoted co-crystallization of low and high melting TAGs. In general, HIU significantly changed the viscosity, G′, and G″ of the IE 20% samples except at ΔT = 12 °C. While G′ and G″ of IE 30% did not increase significantly, the viscosity increased significantly at ΔT = 9, 6, and 3 °C from 1526 ± 880 to 6818 ± 901 Pa.s at ΔT = 3 °C. The improved physical properties of the sonicated IE can make them good contenders for trans-fatty acids replacers.     

Sol–gel transition characterization of thermosensitive hydrogels based on water mobility variation provided by low field NMR

Sol–gel transition properties play a key role in various applications of thermosensitive hydrogels, but conventional methods for studying the sol–gel transition have some limitations. For extensive characterization of the water–polymer interaction and microstructure change during sol–gel transition, we propose a rapid and nondestructive method based on monitoring water mobility through low field NMR (LF-NMR), and this was applied to chitosan/β-glycerophosphate (CS/GP) hydrogels. The spin–spin relaxation time (T ₂) that depicted water mobility was measured by LF-NMR within 90 s. The T ₂ component corresponding to water protons trapped in polymer networks (T ₂₁ ) was very sensitive to sol–gel transition. A remarkable decrease of T ₂₁ value indicated obvious variations of water mobility when CS/GP was heated, and a turning point was observed on the T ₂₁–time curve. The gel point associated with this turning point could be easily determined by fitting the T ₂₁–time curves to a bilinear regression model, and the results showed good accuracy and repeatability owing to the nondestructive nature of LF-NMR. Variations in water components and microstructure of CS/GP caused by water migration after solidification were also analyzed by monitoring dynamic changes of T ₂. This rapid, nondestructive method provides a powerful tool for studying the sol–gel transition of hydrogels.     

Problems of compatibility of the values of glass transition temperatures published in the world literature

The influence of different factors (primarily, the temperature-time conditions for preparation and measurement of samples) on the glass transition temperatures determined from the temperature dependences of properties is analyzed using the calculations performed in terms of the relaxation theory of glass transition. The most optimum conditions for measurement of the glass transition temperatures T _g that ensure the compatibility of the values of T _g obtained by different researchers are recommended. The validity of the assertion that the glass transition temperature T _g is a temperature at which the viscosity of glasses is equal to 10¹³ P is considered.     

Effect of Sodium Carbonate on the Cloud Point in Alkyl Ether/Brine Systems: Apparent Relation with Dynamic Interfacial Tension Minimum

The effect of Na₂CO₃ on the cloud point in Na₂CO₃/surfactant/brine was investigated using two series of nonionic surfactants, C₁₃EO _x and C₁₇EO _x . The cloud point, T _cp, was found to decrease linearly with increasing Na₂CO₃ concentration. This was attributed to Na⁺ and particularly to CO₃ ^2?salting-out effect. The slope a = dTcp/d[Na₂CO₃] became more and more negative as the degree of ethoxylation is increased, suggesting that the higher the number of ethylene oxide (EO) groups the stronger is the cloud point depression for a given increment in Na⁺and CO₃ ^2?ions in solution. This was also illustrated by the linear variation of ΔT _cp = T _cp,0 ? T _cp,[Na2CO3] with the surfactant degree of ethoxylation.