Thermally induced phase separation in poly(lactic acid)/dialkyl phthalate systems

Thermally induced phase separation in poly(lactic acid)/dialkyl phthalate systems was investigated. Poly(DL ‐lactic acid) (PDLLA) and poly(L ‐lactic acid) (PLLA) with different molecular weights were used. A series of dialkyl phthalates, with different numbers of carbon atoms in the alkyl chain, were employed as solvents to control the interaction between polymer and solvent. The liquid–liquid phase‐separation temperature of the poly(lactic acid) solutions decreased systematically with a shorter alkyl chain in the phthalate. Based on the interaction between polymer and solvent and the molecular weight of polymer influencing liquid–liquid phase‐separation temperature significantly but crystallization temperature only slightly, proper thermal conditions were employed to investigate competitive phase separation and crystallization in PLLA solutions. Factors that can influence the final morphology of PLLA solutions were examined. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 2224–2232, 2003     

Synthesis and properties of random poly(lactic acid)-based ionomers

A random poly(lactic acid), PLA, based ionomer was synthesized by copolymerizing a methacrylate-terminated PLA macromonomer and methyl methacrylate. The copolymerization kinetics were studied using ¹H NMR spectroscopy and the copolymer composition was characterized by ¹³C NMR spectroscopy. Carboxylic acid groups were introduced into the copolymer by reacting the hydroxyl end groups of the PLA macromonomer with succinic anhydride, and the acid groups were neutralized with metal acetates to produce Na-, Ca-, and Y-PLA ionomers. Significant increases in T_g were observed for the ionomers and thermomechanical analysis indicated that the ionomers were more resistant to penetration by a weighted probe and an apparent rubbery plateau was observed above T_g. The ionomers were more hydrophilic than PLA, but relatively low water absorption could be achieved for the Ca²⁺-salt ionomer.     

Spatial inhomogeneity in poly(acrylic acid) hydrogels

The crosslink density distribution in gels known as the spatial gel inhomogeneity has been investigated in poly(acrylic acid) (PAAc) gels with the static light scattering measurements. PAAc gels were prepared at a fixed chemical crosslink density but at various initial monomer concentrations. The gels were characterized by swelling and elasticity tests as well as by light scattering measurements. PAAc gels exhibit a maximum degree of spatial inhomogeneity at a critical monomer concentration . shifts toward higher concentrations as the gel swells beyond its dilution degree after preparation. Depending on the polymer concentration in gels, swelling reduces or enhances the extent of the spatial inhomogeneities. It was shown that the apparent degree of the spatial gel inhomogeneity is determined by the combination of three effects, namely the effects of the effective crosslink density, charge density and segment density. The relative magnitudes of these effects vary depending on the polymer concentration and lead to the appearance of a maximum degree of spatial inhomogeneity at a critical concentration.     

Synthesis and thermal properties of telechelic poly(lactic acid) ionomers

Telechelic poly(lactic acid) (PLA) ionomers were synthesized using a chemical recycling process. A transesterification reaction between a commercial PLA and 2-hydroxyethyl methacrylate or ethylene glycol was used to produce a hydroxy-terminated PLA. The hydroxy-terminated PLA was then reacted with itaconic anhydride to produce terminal carboxylic acid groups, which were neutralized with appropriate metal acetates to produce Na-, Li-, K-, Zn-, Ca- and Y-ω- and α,ω-telechelic PLA ionomers. ¹H NMR spectroscopy was used to confirm the presence of the itaconic acid end-groups and FTIR spectroscopy was used to quantify the extent of neutralization. The addition of the ionic groups increased the glass transition (T_g), and T_g increased as the strength of the ion-pair increased. The ionic groups suppressed crystallinity, especially when multivalent cations were used.     

The synthesis of poly (lactic acid)-fulvic acid graft polymer and its effect on the crystallization and performance of poly (lactic acid)

ABSTRACT

The poly (lactic acid)-fulvic acid graft polymer (PLA-FA) was synthesized with lactic acid and fulvic acid (FA). The optimum parameters were determined by orthogonal experiment. X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy exhibited that FA was successfully grafted onto PLA. Then, PLA/PLA-FA composites were prepared with PLA-FA as fillers by melt blending. The structure characterization and performance tests demonstrated that PLA-FA effectively enhanced the comprehensive performance of PLA composites. The rheological analysis demonstrated that PLA-FA had plasticization effect. The non-isothermal crystallization kinetics demonstrated that PLA-FA promoted the crystallization rate of PLA composites, improving toughness of PLA composites.     

Effect of nucleation and plasticization on the crystallization of poly(lactic acid)

In this paper, different strategies to promote PLA crystallization were investigated with the objective of increasing the crystalline content under typical polymer processing conditions. The effect of heterogeneous nucleation was assessed by adding talc, sodium stearate and calcium lactate as potential nucleating agents. The PLA chain mobility was increased by adding up to 10 wt% acetyl triethyl citrate and polyethylene glycol as plasticizers. The crystallization kinetics were studied using DSC analysis under both isothermal and non-isothermal conditions. The isothermal data showed that talc is highly effective in nucleating the PLA in the 80-120 °C temperature range. In the non-isothermal DSC experiments, the crystallinity developed upon cooling was systematically studied at cooling rates of 10, 20, 40, and 80 °C/min. The non-isothermal data showed that the combination of nucleant and plasticizer is necessary to develop significant crystallinity at high cooling rates. The nucleated and/or plasticized PLA samples were injection molded and the effect of mold temperature on crystallinity was determined. It was possible to mold the PLA formulations using mold temperatures either below 40 °C or greater than 60 °C. At low temperature, the molded parts were nearly amorphous while at high mold temperatures, the PLA formulation with proper nucleation and plasticization was shown to achieve crystallinity levels up to 40%, close to the maximum crystalline content of the material. Tensile mechanical properties and temperature resistance of these amorphous and semi-crystalline materials were examined.     

Super-tough poly(lactic acid) materials: Reactive blending with ethylene copolymer

Poly(lactic acid) (PLA) is well known as a biocompatible, bioresorbable, and biodegradable polymer superior to petrochemical polymers from the standpoint of total energy consumption and life-cycle CO₂ emission, since it can be obtained from natural sources. However, the brittleness of PLA is a big drawback for its wide application. Although many studies have been carried out modifying PLA, there is very limited work on reactive blending of PLA. This study demonstrates a dramatic improvement in the mechanical characteristics of PLA by its reactive blending with poly(ethylene-glycidyl methacrylate) (EGMA). It is shown that the interfacial reaction between the component polymers contributes to the formation of super-tough PLA materials, superior to benchmark acrylonitrile-butadiene-styrene (ABS) resins. The novel material highlights the importance of interface control in the preparation of multi-component materials.     

Higher-order structures and mechanical properties of stereocomplex-type poly(lactic acid) melt spun fibers

Equimolar blend of poly(l-lactic acid) (PLLA) and poly(d-lactic acid) (PDLA) was melt spun into fibers and the relations among the processing conditions, crystalline structures, thermal properties, and mechanical properties were investigated. Drawing and annealing were performed in order to obtain fiber mainly consisting of the stereocomplex crystal phase. Fibers drawn at various temperatures exhibited either amorphous, highly oriented homo crystal, or the mixture of homo and stereocomplex with a fairly low orientation depending on the drawing temperature. Annealing of the drawn fibers at an elevated temperature higher than the melting temperature of homo crystal increased the stereocomplex content significantly. The fractions of the homo and the stereocomplex crystals strongly depended on the higher-order structure of the drawn fibers and the annealing temperature.     

A novel phosphorus-containing poly(lactic acid) toward its flame retardation

An inherently flame-retardant poly(lactic acid) (PLA) was synthesized via the chain-extending reactions of dihydroxyl terminated pre-poly(lactic acid) (pre-PLA), which was synthesized by direct polycondensation of l-lactic acid using 1,4-butanediol as initiator and stannous chloride (SnCl₂) as catalyst, using ethyl phosphorodichloridate as chain extender. The resulting phosphorus-containing poly(lactic acid) (PPLA) was characterized by gel permeation chromatography (GPC), ¹H and ³¹P nuclear magnetic resonance (¹H, ³¹P NMR) and homonuclear correlation spectroscopy (COSY) and inductively coupled plasma-mass (ICP). A comprehensive flame retardant property of PPLA was evaluated by microscale combustion calorimetry (MCC), limiting oxygen index (LOI), vertical burning test (UL-94) and cone calorimeter test (CCT). PPLA has excellent flame retardancy and also can be used as a flame retardant for commercial PLA. Only 5 wt.% of PPLA added into PLA can obtain good flame retardant properties. As the content of PPLA is further increased to 10 wt.%, PLA can have much better flame retardancy (LOI = 35 and UL-94 V-0 rating), lower peak heat release rate (pHRR) and longer ignition time (TTI) than neat PLA. All those results mean that this novel approach to impart flame retardancy to PLA is very effective.     

Time resolved study of oriented crystallisation of poly(lactic acid) during rapid tensile deformation

Poly(l-lactic acid) with 4% d-lactic acid comonomer has been drawn in the amorphous state at 80, 90, 100, 110 and 120 °C at an extension rate of 4 s⁻¹ while simultaneously recording WAXS and SAXS patterns at intervals of 0.12 s. At 80, 90 and 100 °C, crystallisation is very rapid (1-4 s⁻¹) and follows a first order transformation process to give highly oriented crystals. SAXS patterns were barely detectable at these temperatures despite fractional crystallinity of ∼0.2. At 110 and 120 °C, crystallisation was very slow (∼0.01 s⁻¹) and gave rise to crystals with a lower degree of orientation. After eventual crystallisation at 120 °C, a two-point SAXS pattern develops with narrow lateral spread, suggesting ‘shish kebab’ morphology. When the 80 °C drawn sample was annealed at 120 °C, a strong four point SAXS pattern develops. The change in drawing and crystallisation behaviour at higher draw temperature is attributed to the onset of chain retraction relaxation processes. The WAXS fibre pattern after annealing shows sampling on intermediate layer lines that is consistent with the α crystal form with a 10₃ helix. However, prior to annealing, sampling indicates a different, less defined helical configuration.     

Study on the effect of dispersion phase morphology on porous structure of poly (lactic acid)/poly (ethylene terephthalate glycol-modified) blending foams

A methodology for blending foam of poly (lactic acid) (PLA)/poly (ethylene terephthalate glycol-modified) (PETG) was proposed. PLA/PETG blends were prepared through a melt blending method, using multiple functionality epoxide as reactive compatibilizer. The effects of blending ratio and compatibilizer content on the dispersion morphology, molecular structure, mechanical properties, and rheological behavior of PLA/PETG blends were studied. Then PLA/PETG blends were foamed using supercritical CO₂ as physical blowing agent, and their porous structure, pore size, as well as pore density were investigated. The results showed that the mechanical properties and rheological parameters such as melt strength and melt elasticity, as well as the porous structure of the foams dispersion morphology of PLA/PETG blends were affected strongly. The melt elasticity of PLA/PETG blends increased with increasing compatibilizer content. Dispersion phase morphology of PLA/PETG blends also had a significant effect on the pore density of all the samples. The results indicated that homogeneous and finer porous morphology of PLA/PETG foams with high expansion ratio could be achieved with a proper content of compatibilizer in the blends.     

Raman spectroscopic study of conformational changes in the amorphous phase of poly(lactic acid) during deformation

The conformational distribution change of amorphous poly(lactic acid) (PLA) induced by deformation has been studied using Raman spectroscopy. Spectroscopic features associated with the rotational isomeric states (ttt, ttg′, tg′t, tg′g′) have been established experimentally and supported by normal coordinate analysis. Deformation induced a significant increase in the most favorable tg′t conformation. Based on the relative intensities of Raman-active skeletal modes, a quantitative method has been developed that can be used to elucidate structural changes in a variety of deformed polymer samples. For biaxially oriented PLA films, the overall tg′t conformation increased from the 76% in undeformed sample to a value as high as 92%. The change in conformational distribution in the amorphous phase follows a different trend (76-88%) as compared to the increase in sample crystallinity (0-43%). A large change in amorphous chain conformation occurred at relatively low draw ratios. In contrast, the large change in the degree of crystallinity occurred at higher draw ratios.     

Adherability and weldability of poly(lactic acid) and basalt fibre-reinforced poly(lactic acid)

The adherability and weldability of pure poly(lactic acid) (PLA) and basalt fibre-reinforced PLA were investigated in this research. The joining efficiency rate is introduced as a comparative parameter among different joining processes. In the case of adhesive bonding, 16 different adhesives were used to join specimens together. The highest bond strength and joining efficiency rate for both the pure (16 MPa, 78%) and basalt fiber-reinforced (18 MPa, 44%) adhesive-bonded specimens was achieved with acrylate-based two-component adhesives. The bond strength and joining efficiency rates of bonded specimens manufactured with four welding technologies (hot gas welding, friction stir welding, ultrasonic welding, laser welding) were also investigated. The highest bond strength for both pure PLA and basalt fibre-reinforced PLA specimens (51 and 125 MPa, respectively) was attained by laser welding. The highest joining efficiency rate for pure PLA specimens (85%) was attained by ultrasonic welding, while it was achieved by laser welding for basalt-fibre reinforced PLA specimens (70%).     

Radiation effects on poly(lactic acid)

The effect of γ-irradiation on poly(lactic acid), PLA, synthesized by the solution polymerization of lactic acid in air and N₂ atmosphere, is studied. From the intrinsic viscosity and average molecular weight data of γ-irradiated samples, radiation chemical yields G(s) and G(x) for chain scission and crosslinking respectively were determined. Results indicate that the presence of air causes a decrease in both chain scission and crosslinking. Furthermore, the melting temperature decreased with dose and by using the Flory equation, the value of G (crystalline units) was estimated in air and N₂ atmosphere as 19 and 14 respectively.     

Study of crystalline and amorphous phases of biodegradable poly(lactic acid) by advanced thermal analysis

The qualitative and quantitative thermal analysis of biodegradable poly(lactic acid) PLA is presented. The glass transition, melting process, and heat capacity of a semi-crystalline poly(lactic acid) are studied utilizing the differential scanning calorimetry and temperature-modulated DSC. The mobile amorphous fraction, W_a degree of crystallinity, W_c and rigid-amorphous fraction, W_RAF were estimated depending on the thermal history of semi-crystalline PLA. From qualitative thermal analysis, the glass transition of rigid-amorphous phase was observed as a broadening from the changes of heat-flow-rate between mobile glass transition temperature and melting temperature. The amount of the rigid-amorphous fraction (RAF) was evaluated from W_RAF = 1−W_c − W_a and graphically was presented as the result of a deflection from the linearity of the dependence of the change of degree of mobile amorphous phase (W_a) vs. the degree of crystalline fraction (W_c) for semi-crystalline PLA with different thermal history. The degree of crystallinity of semi-crystalline samples of PLA can be discussed in terms of a two- or three-phase model. In contrast, the quantitative thermal analysis of the experimental apparent heat capacity of semi-crystalline PLA did not show any appearance of RAF in the examples of analyzed samples. The experimental heat capacity of PLA was analyzed in reference to the solid and liquid equilibrium heat capacities of poly(lactic acid) found in the ATHAS Data Bank.     

可降解聚乳酸的合成及改性研究进展

介绍了可降解材料聚乳酸树脂的合成与改性方法。合成聚乳酸的主要方法有开环聚合、直接缩聚合等。通过共聚、共混、嵌段、接枝、纤维复合可以对聚乳酸进行改性,改进聚乳酸的结构与性能,以进一步扩大应用范围。     

A DFT study on poly(lactic acid) polymorphs

Poly(lactic acid) (PLA) can crystallize in α-, β-, γ- and stereocomplex (sc)- forms. It has been shown that the formation of stereocomplex between poly(l-lactic acid) and poly(d-lactic acid) significantly improve thermal stability and mechanical properties. However the mechanisms of enhancements are still unclear. In this study, we investigate the PLA polymorphs from the first-principles theoretical perspective in order to understand the intermolecular interaction in the crystals. Density functional theory at the level of Perdew-Wang generalized-gradient approximation was applied to optimize PLA crystal unit cells. A comparison of energies in the various unit cells reveals that sc-form is the most energetically favorable form among the four PLA polymorphs. The order of thermodynamically relative stability is that sc-form is 0.3, 1.1, and 1.3 kcal/mol more stable than α-form, β-form, and γ-form, respectively (when using the ultrasoft pseudopotential and a plane-wave basis set with an energy cutoff of 380 eV) or 0.4, 1.1, and 1.3 kcal/mol more stable than α-form, β-form, and γ-form, respectively (when employing the density functional semi-core pseudopotentials and the double numerical plus polarization orbital basis set with a global orbital cutoff of 3.7 Å). In addition to the energetic properties, structural and electronic properties were calculated as well. The theoretical predicted stability rank is in agreement with some reported observations. Such as, sc-form has higher melting point and larger heat of fusion than those of α-form. The enhanced thermal stability of the sc-form compared to the other three homopolymer forms may be attributed to the unique intermolecular non-conventional hydrogen bonding C-H?O(C) network in the stereocomplex.     

Tributyl citrate oligomers as plasticizers for poly (lactic acid): thermo-mechanical film properties and aging

Poly (lactic acid), PLA, is a biodegradable thermoplastic that can be produced from renewable resources. The polymer is of interest for production of films for packaging applications. However, plasticization of PLA is required in order to obtain films with sufficient flexibility. PLA was blended with tributyl citrate (TbC) and two oligomers of TbC that were synthesized by transesterification of tributyl citrate (TbC) and diethylene glycol (DEG). Dynamic mechanical analysis (DMA) and differential scanning calorimetry (DSC) were used to investigate the dynamic mechanical and thermal properties of the blends. All the plasticizers investigated decreased the glass transition temperature of PLA, and the reduction was the largest with the plasticizer having the lowest molecular weight. The PLA matrix became saturated with plasticizer at a certain concentration and phase separation occurred; the higher the molecular weight of the plasticizer, the lower the saturation concentration. Aging of the blends at room temperature for several months induced partial phase separation in the material. It was observed that the morphological stability of the blends was enhanced when the plasticizer concentration was reasonably low, i.e. 10-15 wt%.     

Properties of grafted wood flour filled poly (lactic acid) composites by reactive extrusion

Composites of poly(lactic acid) with wood flour which was grafted by melt extruding with methyl acrylate in the presence of benzoyl peroxide (BPO) were investigated. The modification of filler (WF-g-PMA) was carried out to enhance the filler-matrix interactions, while the treated component was characterized by infrared spectrum. Properties of binary (PLA/WF, PLA/WF-g-MA) composites were analyzed as a function of the grafting monomer amount by scanning electron microscopy, differential scanning calorimeter, thermogravimetric analysis, water absorption and mechanical tests. Compared with the untreated system (PLA/WF), all treated composites showed higher interfacial compatibility as a result of chemical bonding between WF and grated monomer. All composites showed higher tensile modulus and lower strength and elongation at break as compared to pure PLA; grafting modification with methyl acrylate led to an increased stiffness and decreased water absorption of the composites because of an enhanced filler-matrix interfacial compatibility.     

聚乳酸成核剂的研究进展

回顾并展望了聚乳酸成核剂的研究与开发进展,比较和评价了不同类型成核剂对聚乳酸结晶行为的影响,在此基础上提出了开发聚乳酸专用成核剂的思考和建议。