Crystallization and Thermal Properties of PLLA–PEG 600/Clay Nanocomposites

This study explored the crystallization and thermal properties of copolymers of polylactic acid–polyethylene glycol (Mn: 600) (PLLA–PEG 600 copolymer) nanocomposites with added montmorillonite (clay). The d-spacing of the hydrophilic montmorillonite was increased by using a quaternary ammonium salt. The salt was used to modify montmorillonite via ion exchange. The modified montmorillonite was then evenly dispersed in the matrix of the PLLA–PEG 600 copolymer to synthesize the nanoscale polymer via solution casting method. Thermogravimetric analysis and differential scanning calorimetry were used to measure the thermal properties of the PLLA–PEG 600 copolymers with various proportions of added clay. Wide-angle X-ray diffraction was used to identify the crystal structure and lattice displacements of PLLA–PEG 600 copolymer and the copolymer with added clay. Using polarized optical microscopy and transmission electron microscopy, the crystal growth and size of PLLA–PEG 600 copolymer with various proportions of added clay, as well as the dispersion of the clay, were observed. Adding clay to PLLA–PEG 600 copolymer improved its thermal properties and crystallinity without changing its original crystal structure.     

Catalyst free synthesis of poly(l‐lactic acid)–poly(propylene glycol) multiblock copolymers and their properties

A simple, green, and economical method for the synthesis of poly(l ‐lactic acid)–poly(propylene glycol) (PLLA–PPG) copolymers is put forward and a series of multiblock PLLA–PPG are synthesized with 1,6‐hexamethylene diisocyanate as chain extender of the melt polymerization. The effect of PPG content on the properties of PLLA–PPG copolymers is also investigated. The elongation at break of the resulting copolymer film with only 5% weight content PPG is 280%, and the tensile strength is 20 MPa. Dynamic mechanical analysis results demonstrated the existence of the shape memory effect for all the copolymers films and the shape recovery ratio of 101% is achieved for PLLA–PPG copolymer film with 5% weight PPG. The process for the synthesis of PLLA–PPG copolymers in the total absence of potentially toxic solvents and catalysts is analyzed, and the films of PLLA–PPG exhibit toughness and shape memory effect. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45299.     

Influence of clay content on the melting behavior and crystal structure of nonisothermal crystallized poly(L‐lactic acid)/nanocomposites

To study the effect of organophilic clay concentration on nonisothermal crystallization, poly(L ‐lactic acid) (PLLA)/montmorillonite (MMT) nanocomposites were prepared by mixing various amounts of commercial MMT (Cloisite^® 30B) and PLLA. The effect of MMT content on melting behavior and crystal structure of nonisothermal crystallized PLLA/MMT nanocomposites was investigated by differential scanning calorimetry (DSC), small‐angle X‐ray scattering, and wide‐angle X‐ray diffraction (XRD) analyses. The study was focused on the effect of the filler concentration on thermal and structural properties of the nonisothermally crystallized nanocomposite PLLA/MMT. The results obtained have shown that at filler loadings higher than 3 wt %, intercalation of the clay is observed. At lower clay concentrations (1–3 wt %), exfoliation predominates. DSC and XRD analysis data show that the crystallinity of PLLA/MMT composites increases drastically at high clay loadings (5–9 wt %). In these nanocomposites, PLLA crystallizes nonisothermally in an orthorhombic crystal structure, assigned to the α form of PLLA. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Crystallization behavior and mechanical properties of poly(ethylene oxide)/poly(L‐lactide)/poly(vinyl acetate) blends

Poly(vinyl acetate) (PVAc) was added to the crystalline blends of poly(ethylene oxide) (PEO) and poly(L ‐lactide) (PLLA) (40/60) of higher molecular weights, whereas diblock and triblock poly(ethylene glycol)–poly(L ‐lactide) copolymers were added to the same blend of moderate molecular weights. The crystallization rate of PLLA of the blend containing PVAc was reduced, as evidenced by X‐ray diffraction measurement. A ringed spherulite morphology of PLLA was observed in the PEO/PLLA/PVAc blend, attributed to the presence of twisted lamellae, and the morphology was affected by the amount of PVAc. A steady increase in the elongation at break in the solution blend with an increase in the PVAc content was observed. The melting behavior of PLLA and PEO in the PEO/PLLA/block copolymer blends was not greatly affected by the block copolymer, and the average size of the dispersed PEO domain was not significantly changed by the block copolymer. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 3618–3626, 2001     

Fully biobased biodegradable poly(l‐lactide)‐b‐poly(ethylene brassylate)‐b‐poly(l‐lactide) triblock copolymers: synthesis and investigation of relationship between crystallization morphology and thermal properties

Well‐defined poly(l ‐lactide‐b‐ethylene brassylate‐b‐l ‐lactide) (PLLA‐b‐PEB‐b‐PLLA) triblock copolymer was synthesized by using double hydroxyl‐terminated PEBs with different molecular weights. Gel permeation chromatography and NMR characterization were employed to confirm the structure and composition of the triblock copolymers. DSC, wide‐angle X‐ray diffraction, TGA and polarized optical microscopy were also employed to demonstrate the relationship between the composition and properties. According to the DSC curves, the cold crystallization peak vanished gradually with decrease of the PLLA block, illustrating that the relatively smaller content of PLLA may lead to the formation of a deficient PLLA type crystal, leading to a decrease of melting enthalpy and melting temperature. Multi‐step thermal decompositions were determined by TGA, and the PEB unit exhibited much better thermal stability than the PLLA unit. Polarized optical microscopy images of all the triblock samples showed that spherulites which develop radially and with an extinction pattern in the form of a Maltese cross exhibit no ring bond. The growth rate of the spherulites of all triblock samples was investigated. The crystallization capacity of PLLA improved with incorporation of PLLA, which accords with the DSC and wide‐angle X‐ray diffraction results. © 2019 Society of Chemical Industry     

Synthesis and characterization of poly(ethylene glycol)-b-poly(l-histidine)-b-poly(l-lactide) with pH-sensitivity

In this paper, amphiphilic biodegradable methoxy-poly(ethylene glycol)-poly(l-histidine)-poly(l-lactide) (mPEG–PH–PLLA) triblock copolymers with pH sensitivity were synthesized. The properties of mPEG–PH–PLLA triblock copolymers were investigated by GPC, ¹H NMR, DSC, TGA, XRD and polarized optical microscopy. The results showed that the thermal properties of the triblock copolymers varied with the chain length of PH blocks. The glass transition temperatures (Tg) of the triblock copolymers increased with increasing poly(l-histidine) chain length. The morphologies of PLLA crystals changed from spherulite to dendritic crystal. Moreover, the crystallization rate of triblock copolymers was faster than that of PLLA homopolymer. The pH sensitivity of the self-assembled mPEG–PH–PLLA nanoparticles was investigated. The mean diameter and morphology of the nanoparticles were characterized by DLS, AFM and TEM. The results showed that the mean diameter of mPEG₄₅–PH₃₀–PLLA₈₂ nanoparticles in pH = 5.0 was smaller than that in pH = 7.4.     

Confined crystallization morphology of poly(ϵ‐caprolactone) block within poly(ϵ‐caprolactone)–poly(l‐lactide) copolymers

The confined crystallization of poly(?‐caprolactone) (PCL) block in poly(?‐caprolactone)–poly(l ‐lactide) (PCL‐PLLA) copolymers was investigated using differential scanning calorimetry, polarized optical microscopy, scanning electronic microscopy and atomic force microscopy. To study the effect of crystallization and molecular chain motion state of PLLA blocks in PCL‐PLLA copolymers on PCL crystallization morphology, high‐temperature annealing (180 °C) and low‐temperature annealing (80 °C) were applied to treat the samples. It was found that the crystallization morphology of PCL block in PCL‐PLLA copolymers is not only related to the ratio of block components, but also related to the thermal history. After annealing PCL‐PLLA copolymers at 180 °C, the molten PCL blocks are rejected from the front of PLLA crystal growth into the amorphous regions, which will lead to PCL and PLLA blocks exhibiting obvious fractionated crystallization and forming various morphologies depending on the length of PLLA segment. On the contrary, PCL blocks more easily form banded spherulites after PCL‐PLLA copolymers are annealed at 80 °C because the preexisting PLLA crystal template and the dangling amorphous PLLA chains on PCL segments more easily cause unequal stresses at opposite fold surfaces of PCL lamellae during the growth process. Also, it was found that the growth rate of banded spherulites is less than that of classical spherulites and the growth rate of banded spherulites decreases with decreasing band spacing. © 2019 Society of Chemical Industry     

Crystallization behavior of biodegradable amphiphilic poly(ethylene glycol)‐poly(L‐lactide) block copolymers

Poly(ethylene glycol)‐poly(L ‐lactide) diblock and triblock copolymers were prepared by ring‐opening polymerization of L ‐lactide with poly(ethylene glycol) methyl ether or with poly(ethylene glycol) in the presence of stannous octoate. Molecular weight, thermal properties, and crystalline structure of block copolymers were analyzed by ¹H‐NMR, FTIR, GPC, DSC, and wide‐angle X‐ray diffraction (WAXD). The composition of the block copolymer was found to be comparable to those of the reactants. Each block of the PEG–PLLA copolymer was phase separated at room temperature, as determined by DSC and WAXD. For the asymmetric block copolymers, the crystallization of one block influenced much the crystalline structure of the other block that was chemically connected to it. Time‐resolved WAXD analyses also showed the crystallization of the PLLA block became retarded due to the presence of the PEG block. According to the biodegradability test using the activated sludge, PEG–PLLA block copolymer degraded much faster than PLLA homopolymers of the same molecular weight. © 1999 John Wiley amp; Sons, Inc. J Appl Polym Sci 72: 341–348, 1999     

Investigation of phase behavior and water binding in poly(alkylene oxide) solutions

The aqueous solution properties of alkylene oxide polymers and copolymers are related to their interaction with water. In an attempt to better understand this behavior, differential scanning calorimetry has been employed to measure phase changes and water binding in solutions of polyethylene glycol (PEG), polypropylene glycol (PPG), and a 50/50 random copolymer of ethylene oxide and propylene oxide. PEG (M _n = 3510) forms a crystalline eutectic with water at 0.48 weight fraction of polymer. The liquidus curve for water can be fit accurately using the Flory–Huggins expression for solute activity with an interaction parameter of 0.05. PPG and the random copolymer do not crystallize and thus do not form a crystalline eutectic. Based on decreases in the heat of fusion of free water with added polymer, PEG binds more water than the copolymer which binds more water than PPG. The estimated hydration numbers per polymer segment are 1.5 for PPG, 2.3 for the copolymer, and 2.7 for PEG.     

Microwave-assisted preparation of poly(2-EHA-co-ST) copolymer and poly(2-EHA-co-ST)/MMT nanocomposite

Copolymerization of 2-ethylhexylacrylate and styrene was performed in presence of benzoyl peroxide as initiator at varying concentrations of the comonomers in a microwave oven. Montmorillonite (MMT) clay was added with a view to prepare nanocomposites, which actually enhanced the water absorption capacity and pressure sensitive adhesive properties. The copolymer and its nanocomposite were characterized by Fourier Transform Infrared, ¹H- and ¹³C-NMR, thermogravimetric-differential thermal analysis (TG-DTA), differential scanning calorimeter, scanning electron microscopy, X-ray diffraction (XRD), and transmission electron microscopy (TEM). The MMT layers were partially exfoliated/intercalated during the polymerization process as evident from the XRD and TEM observations. Their adhesive properties, water absorbancy, and biodegradability in different conditions were studied for their future applications. The monomer reactivity ratios were determined using Finemann–Ross and Kelen–Tüdos method. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Reactive blending of poly(L‐lactic acid) with poly(ethylene‐co‐vinyl alcohol)

Poly(L ‐lactic acid) (PLLA) was blended with poly(ethylene‐co‐vinyl alcohol) (EVOH) in the presence of an esterification catalyst to induce reaction between the hydroxyl groups of EVOH and the terminal carboxylic group of PLLA. Nascent low‐molecular‐weight PLLA, obtained from a direct condensation polymerization of L ‐lactic acid in bulk state, was used for the blending. Domain size of the PLLA phase in the graft copolymer was much smaller than that corresponding to a PLLA/EVOH simple blend. The mechanical properties of the graft copolymer were far superior to those of the simple blend, and the graft copolymer exhibited excellent mechanical properties even though the biodegradable fraction substantially exceeded the percolation level. The grafted PLLA reduced the crystallization rate of the EVOH moiety. Melting peak temperature (T_m) of the PLLA phase was not observed until the content of PLLA in the graft reaction medium went over 60 wt %. The modified Sturm test results demonstrated that biodegradation of EVOH‐g‐PLLA took place more slowly than that of an EVOH/PLLA simple blend, indicating that the chemically bound PLLA moiety was less susceptible to microbial attack than PLLA in the simple blend. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 886–890, 2005     

Shape memory effects of poly(L-lactide) and its copolymer with poly(ε-caprolactone)

Summary The thermal properties, crystalline structure and shape memory effects of poly(L-lactide) (PLLA) and its copolymer with poly(ε-caprolactone) (PCL) are systematically investigated by differential scanning calorimetry (DSC), X-ray diffraction (XRD) and tensile tests. The influences of composition and intrinsic viscosity on structure and shape memory effects are also revealed. It is found that the PLLA homopolymer and poly(L-lactide-co-ε-caprolactone) (PCLA) copolymers exhibit good shape memory effects. The existence of PLLA crystal and amorphous phase play very important roles for shape memory effects. The intrinsic viscosity obviously affects the crystallinity of polymers and further affects the shape memory effects. The shape recovery rate decreases with increasing deformation strain, which is relate to the deformation of PLLA crystal. The recovery stress increase with the increase of the ε-CL content and maximum recovery stress is 3.54MPa obtained in the PCLA804 (20wt% ε-CL content, M_w=304,400). With the increase of cyclic testing number, the shape recovery rates decrease and the shape retention rates increase at the beginning and then approach to a steady value.     

Influence of dispersed organophilic montmorillonite at nanometer‐scale on crystallization of poly(L‐lactide)

We investigated the effects of surface‐treated organophilic clay on the crystallization of poly(L ‐lactide) (PLLA) in their hybrids. The natural nano‐clay in PLLA/clay hybrids acts as a heterogeneous nucleating agent to facilitate crystallization. On the contrary, extensive distributions of induction periods for nucleation are observed in the individual spherulites of neat PLLA and PLLA/organophilic clay hybrids. Therefore, it is suggested that nucleation type of neat PLLA and PLLA/organophilic clay hybrids implies nearly growth geometry as a homogeneous one. Further, under the presence of nano‐clay in their composites, PLLA matrix form the orthorhombic lattice structure corresponded to the α‐form crystal. Since this experimental fact implies little effect of the clay particles on polymorphism of PLLA crystal, the nucleating effect of the organophilic clay seems weaker than the natural clay itself. However, an increase in clay content enhances the growth rates of spherulite for hybrids. Consequently, most of hybrids exhibit an increase in overall crystallization rates at any crystallization temperature in spite of relatively lower nucleation rate of PLLA crystallites itself. In addition, the Avrami exponents (n) obtained by relatively low crystallization temperature ranged from 4 to 6, implying that the growth geometry was dominated sheaf‐like structure in early stage of isothermal crystallization. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers     

Poly(L‐lactide)/Poly(D‐lactide)/clay nanocomposites: Enhanced dispersion,crystallization, mechanical properties,and hydrolytic degradation

Poly(L ‐lactide) (PLLA)/poly(D ‐lactide) (PDLA)/clay nanocomposites are prepared via simple melt blending method at PDLA loadings from 5 to 20 wt%. Formation of the stereocomplex crystals in the nanocomposites is confirmed by differential scanning calorimetry and wide‐angle X‐ray diffraction (WAXD). The internal structure of the nanocomposites has been established by using WAXD and transmission electron microscope analyses. The dispersion of clay in the PLLA/PDLA/clay nanocomposites can be improved as a result of increased intensity of shear during melt blending. The overall crystallization rates are faster in the PLLA/PDLA/clay nanocomposites than in PLLA/clay nanocomposite and increase with an increase in the PDLA loading up to 10 wt%; however, the crystallization mechanism and crystal structure of these nanocomposites remain unchanged despite the presence of PDLA. The storage modulus has been apparently improved in the PLLA/PDLA/clay nanocomposites with respect to PLLA/clay nanocomposite. Moreover, it is found that the hydrolytic degradation rates have been enhanced obviously in the PLLA/PDLA/clay nanocomposites than in PLLA/clay nanocomposite. POLYM. ENG. SCI., 54:914–924, 2014. © 2013 Society of Plastics Engineers     

Increasing the compatibility of poly(l‐lactide)/poly(para‐dioxanone) blends through the addition of poly(para‐dioxanone‐co‐l‐lactide)

To modify the mechanical properties of a poly(l ‐lactide) (PLLA)/poly(para‐dioxanone) (PPDO) 85/15 blend, poly(para‐dioxanone‐co‐l ‐lactide) (PDOLLA) was used as a compatibilizer. The 85/15 PLLA/PPDO blends containing 1–5 wt % of the random copolymer PDOLLA were prepared by solution coprecipitation. Then, the thermal, morphological, and mechanical properties of the blends with different contents of PDOLLA were studied via differential scanning calorimetry (DSC), scanning electron microscopy (SEM), and tensile testing, respectively. The DSC result revealed that the addition of PDOLLA into the blends only slightly changed the thermal properties by inhibiting the crystallization degree of the poly(l ‐lactide) in the polymer blends. The SEM photos indicated that the addition of 3 wt % PDOLLA into the blend was ideal for making the interface between the PLLA and PPDO phases unclear. The tensile testing result demonstrated that the mechanical properties of the blends containing 3 wt % PDOLLA were much improved with a tensile strength of 48 MPa and a breaking elongation of 214%. Therefore, we concluded that the morphological and mechanical properties of the PLLA/PPDO 85/15 blends could be tailored by the addition of the PDOLLA as a compatibilizer and that the blend containing a proper content of PDOLLA had the potential to be used as a medical implant material. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41323.     

The role of the ratio (PEG:PPG) of a triblock copolymer (PPG‐b‐PEG‐b‐PPG) in the cure kinetics,miscibility and thermal and mechanical properties in an epoxy matrix

The aim of this study was to evaluate the role of different poly(ethylene glycol):poly(propylene glycol) (PEG:PPG) molar ratios in a triblock copolymer in the cure kinetics, miscibility and thermal and mechanical properties in an epoxy matrix. The poly(propylene glycol)‐block‐poly(ethylene glycol)‐block‐poly(propylene glycol) (PPG‐b‐PEG‐b‐PPG) triblock copolymers used had two different molecular masses: 3300 and 2000 g mol^?1. The mass concentration of PEG in the copolymer structure played a key role in the miscibility and cure kinetics of the blend as well as in the thermal–mechanical properties. Phase separation was observed only for blends formed with the 3300 g mol^?1 triblock copolymer at 20 wt%. Concerning thermal properties, the miscibility of the copolymer in the epoxy matrix reduced the T_g value by 13 °C, although a 62% increase in fracture toughness (K_IC) was observed. After the addition of PPG‐b‐PEG‐b‐PPG with 3300 g mol^?1 there was a reduction in the modulus of elasticity by 8% compared to the neat matrix; no significant changes were observed in T_g values for the immiscible system. The use of PPG‐b‐PEG‐b‐PPG with 2000 g mol^?1 reduced the modulus of elasticity by approximately 47% and increased toughness (K_IC) up to 43%. Finally, for the curing kinetics of all materials, the incorporation of the triblock copolymer PPG‐b‐PEG‐b‐PPG delayed the cure reaction of the DGEBA/DDM (DGEBA, diglycidyl ether of bisphenol A; DDM, Q3‐4,4′‐Diaminodiphenylmethane) system when there is miscibility and accelerated the cure reaction when it is immiscible. All experimental curing reactions could be fitted to the Kamal autocatalytic model presenting an excellent agreement with experimental data. This model was able to capture some interesting features of the addition of triblock copolymers in an epoxy resin. © 2018 Society of Chemical Industry     

Compatibilization-like effect of reactive organoclay on the poly(l-lactide)/poly(butylene succinate) blends

The morphology of an incompatible polymer blend composed of poly(l-lactide) (PLLA) and poly(butylene succinate) (PBS) was examined by scanning and transmission electron microscopy, X-ray scattering, and X-ray photoelectron spectroscopy before and after the incorporation of an organoclay containing reactive functional groups, namely twice functionalized organoclay (TFC). TFC was prepared by treating Cloisite^® 25A with (glycidoxypropyl)trimethoxy silane. When a small amount of TFC was incorporated into the PLLA/PBS blend, the clay layers became fully exfoliated and were located mainly in the PLLA phase. At the low clay content, the dispersed phase had an almost constant domain size comparing with the PLLA/PBS blend, which decreased sharply as the clay content was further increased. When the clay content became high, the clay layers were dispersed not only in the PLLA phase but also in the PBS phase with intercalated/exfoliated coexisting morphology. The reactive TFC was found to play an important role in the blend similar to the in situ reactive compatibilizer. The specific interaction between the TFC and the polymer matrix was quantified by the Flory-Huggins interaction parameter, B, which was determined by combining the melting point depression and the binary interaction model. The morphology of the PLLA/PBS/clay composites was analyzed by considering the interaction parameter.     

Melt intercalation of poly(l-lactide) chains into clay galleries

The melt intercalation of poly(l-lactide) (PLLA) chains into silicate galleries has been investigated via a melting process without any shearing force at elevated temperature. Under the melting process, the incorporation of various types of organo-modified montmorillonites into PLLA matrix lead to the increase in the basal spacing of clay particles in different manner without delamination into individual layers. The changes in layer-stacked structures of the clay particles in the PLLA matrix were examined by use of wide-angle X-ray diffraction and transmission electron microscopy. The effects of clay content in PLLA matrix and clay surfactants on the melt intercalation of PLLA were discussed in terms of chain mobility.     

Less‐ordered lamellar structure of intercalated poly(L‐lactide)/organo‐modified montmorillonite hybrids

Hybrids of poly(L ‐lactide)/organophilic clay (PLACHs) have been prepared via a melt‐compounding process using poly(L ‐lactide) (PLLA) and different contents of surface‐treated montmorillonite modified with dimethyl dioctadecyl ammonium‐salt. The crystalline structures of PLLA and dispersion states of clay particles in those PLACHs were investigated by use of wide‐angle X‐ray diffraction, small‐angle X‐ray scattering, transmission electron microscopy (both cross section and replication modes), and polarized optical microscopy. Those structures are viewed from the conformational changes of PLLA chains in the space of a few nanometer widths between silicate galleries to crystalline lamellae of several nanometer thicknesses, and spherulitic textures more than micrometer sizes. After annealing treatments at 115°C for 1 hr, the PLACHs formed coarse‐grained spherulitic textures with 40 μm diameter composed of less‐ordered and fragmented lamellae, caused by the reduced mobility of the PLLA chain due to the dispersed clay particles in the PLLA matrix and the intercalation of the PLLA chains in the silicate galleries. The formation of the interfibril structure accompanied by the fragmented lamellae among the dispersed clay particles was examined. POLYM. ENG. SCI., 46:703–711, 2006. © 2006 Society of Plastics Engineers     

Crystalline structure and morphology of poly(L‐lactide) formed under high pressure

The poly(l ‐lactide) (PLLA) samples were prepared by the annealing under 100 MPa at 75–145^°C and 200 MPa at 105–145^°C for 6 h, respectively. The crystalline structures, thermal properties and morphology were investigated using differential scanning calorimetry (DSC), wide‐angle X‐ray diffraction (WAXD), and scanning electron microscopy (SEM). On the basis of the DSC and WAXD results, it can be seen that the α′ form was formed by the annealing under 100 MPa at 85–95°C but not found under 200 MPa at 105–145°C. A phase diagram of PLLA crystal form under high pressure was constructed under the given experimental conditions, which displayed the α′ form was formed at limited temperature and pressure range. Besides, SEM suggested that the PLLA samples annealed under 100 MPa crystallize to form lamellar‐like crystals due to the low growth rate and the confined crystallization behavior under high pressure. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40637.