Solid‐state linear viscoelastic properties of intercalated poly(L‐lactide)/organo‐modified montmorillonite hybrids

Hybrids of poly(L ‐lactide)/organophilic clay (PLACHs) have been prepared by a melt‐compounding process using poly(L ‐lactide) (PLLA) and different contents of surface‐treated montmorillonite modified with a dimethyl dioctadecyl ammonium salt. The dispersion structures of clay particles in PLACHs were investigated using wide‐angle X‐ray diffraction and transmission electron microscopy. The solid‐state linear viscoelastic properties for these PLACHs were examined as functions of temperature and frequency. The incorporation of organo‐modified silicate into PLLA matrix enhanced significantly both storage moduli (E′) and loss moduli (E″). The strong enhancement observed in dynamic moduli of PLACHs could be attributed to uniformly dispersed state of the clay particles with high aspect ratio (= length/thickness of clay) and the intercalation of the PLLA chains between silicate layers. Copyright © 2006 Society of Chemical Industry     

Miscibility,morphology and thermal properties of poly(para‐dioxanone)/poly(D,L‐lactide) blends

BACKGROUND: Poly(para‐dioxanone) (PPDO) is a biodegradable polyester with excellent biodegradability, bioabsorbability, biocompatibility and mechanical flexibility. However, its high cost and relatively fast degradation rate have hindered the development of commercial applications. Blending with other polymers is a simple and convenient way of modifying the properties of aliphatic polyesters. Poly(D ,L ‐lactide) (PDLLA) is another polyester that has been extensively studied for biomedical applications due to its biocompatibility and suitable degradation rate. However, to our knowledge, blends of PPDO/PDLLA have not been reported in the literature. RESULTS: A series of biodegradable polymers were blended by solution co‐precipitation of PPDO and PDLLA in various blend ratios. The miscibility, morphology and thermal properties of the materials were investigated. DSC curves for all blends revealed two discrete glass transition temperatures which matched the values for pure PPDO and PDLLA. SEM images of fracture surfaces displayed evidence of phase separation consistent with the DSC results. The contact angles increased with the addition of PDLLA. CONCLUSION: PPDO/PDLLA blends exhibit two distinct glass transition temperatures that remain nearly constant and correspond to the glass transition temperatures of the homopolymers for all blend compositions, indicating that blends of PPDO and PDLLA are immiscible. Images of the surface obtained using SEM were also suggestive of a two‐phase material. The crystallinity of the PPDO phase in the blends was affected by the PDLLA content. The mechanical properties of the blends changed dramatically with composition. Adding PDLLA makes the blends less hydrophilic than PPDO. Copyright © 2008 Society of Chemical Industry     

Crystallization kinetics of poly(L‐lactide)/carbonated hydroxyapatite nanocomposite microspheres

Microspheres consisting of carbonated hydroxyapatite (CHAp) nanoparticles and poly(L ‐lactide) (PLLA) have been fabricated for use in the construction of osetoconductive bone tissue engineering scaffolds by selective laser sintering (SLS). In SLS, PLLA polymer melts and crystallizes. It is therefore necessary to study the crystallization kinetics of PLLA/CHAp nanocomposites. The effects of 10 wt% CHAp nanoparticles on the isothermal and nonisothermal crystallization behavior of PLLA matrix were studied, using neat PLLA for comparisons. The Avrami equation was successfully applied for the analysis of isothermal crystallization kinetics. Using the Lauritzen‐Hoffman theory, the transition temperature from crystallization Regime II to Regime III was found to be around 120°C for both neat PLLA and PLLA/CHAp nanocomposite. The combined Avrami‐Ozawa equation was used to analyze the nonisothermal crystallization process, and it was found that the Ozawa exponent was equal to the Avrami exponent for neat PLLA and PLLA/CHAp nanocomposite, respectively. The effective activation energy as a function of the relative crystallinity and temperature for neat PLLA and PLLA/CHAp nanocomposite under the nonisothermal crystallization condition was obtained by using the Friedman differential isoconversion method. The Lauritzen‐Hoffman parameters were also determined from the nonisothermal crystallization data by using the Vyazovkin‐Sbirrazzuoli equation. CHAp nanoparticles in the composite acted as an efficient nucleating agent, enhancing the nucleation rate but at the same time reducing the spherulite growth rate. This investigation has provided significant insights into the crystallization behavior of PLLA/CHAp nanocomposites, and the results obtained are very useful for making good quality PLLA/CHAp scaffolds through SLS. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Study of the thermal stabilization mechanism of biodegradable poly(L‐lactide)/silica nanocomposites

Biodegradable poly(L ‐lactide) (PLA)/silica (SiO₂) nanocomposites were prepared by melt compounding to investigate the effect of spherical nanofillers on the thermal stability of PLA. The nanocomposites displayed improved thermal stability both under nitrogen and in air. The stabilization mechanism was attributed mainly to the barrier effect of the network formed, which was demonstrated by the improved barrier properties and rheological performance. The dispersion of nanofiller and matrix‐nanoparticle interactions were investigated to evaluate the dependence of the network on SiO₂ loadings. Fourier transform infrared spectroscopy and thermogravimetric analysis indicated that hydroxyl groups on SiO₂ surfaces and PLA chain‐ends reacted during melt processing. The resulting grafted SiO₂ and entangled PLA chains formed a dense network, which hindered the diffusion of oxygen and volatile decomposition products. Furthermore, the improvement in thermal stability resulted from the restraining effect on the mobility of active hydroxyl end‐groups, so that some related thermal decomposition reactions were inhibited, which was confirmed from gel permeation chromatography measurements. Copyright © 2010 Society of Chemical Industry     

Synthesis and characterization of poly(L‐lactide)‐poly(ethylene glycol) multiblock copolymers

Poly(L‐lactide)‐poly(ethylene glycol) multiblock copolymers with predetermined block lengths were synthesized by polycondensation of PLA diols and PEG diacids. The reaction was carried out under mild conditions, using dicyclohexylcarbodiimide as the coupling agent and dimethylaminopyridine as the catalyst. The resulting copolymers were characterized by various analytical techniques, such as GPC, viscometry, ¹H‐NMR, FTIR, DSC, X‐ray diffractometry, and contact angle measurement. The results indicated that these copolymers presented outstanding properties pertinent to biomedical use, including better miscibility between the two components, low crystallinity, and hydrophilicity. Moreover, the properties of the copolymers can be modulated by adjusting the block length of the two components or the reaction conditions. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 84: 1729–1736, 2002; DOI 10.1002/app.10580     

Poly(para‐dioxanone)/poly(D,L‐lactide) blends compatibilized with poly(D,L‐lactide‐co‐para‐dioxanone)

The effect of poly(D ,L ‐lactide‐co‐para‐dioxanone) (PLADO) as the compatibilizer on the properties of the blend of poly(para‐dioxanone) (PPDO) and poly(D ,L ‐lactide) (PDLLA) has been investigated. The 80/20 PPDO/PDLLA blends containing from 1% to 10% of random copolymer PLADO were prepared by solution coprecipitation. The PLADO component played a very important role in determining morphology, thermal, mechanical, and hydrophilic properties of the blends. Addition of PLADO into the blends could enhance the compatibility between dispersed PDLLA phase and PPDO matrix; the boundary between the two phases became unclear and even the smallest holes were not detected. On the other hand, the position of the T_g was composition dependent; when 5% PLADO was added into blend, the T_g distance between PPDO and PDLLA was shortened. The blends with various contents of compatibilizer had better mechanical properties compared with simple PPDO/PDLLA binary polymer blend, and such characteristics further improved as adding 5% random copolymers. The maximum observed tensile strength was 29.05 MPa for the compatibilized PPDO/PDLLA blend with 5% PLADO, whereas tensile strength of the uncompatibilized PPDO/PDLLA blend was 14.03 MPa, which was the lowest tensile strength. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Properties and morphology of bioceramics/poly(D,L‐lactide) composites modified by in situ compatibilizing extrusion

Composites of poly(D ,L ‐lactide) (PDLLA) with hydroxylapatite (HA) and PDLLA with tertiary calcium phosphate (TCP) were prepared by in situ modification with methylenediphenyl diisocyanate (MDI) and molded by piston extrusion at temperature between T_g and T_m of PDLLA. Mechanical properties of the composites increased obviously when compared with the unmodified bioactive ceramic particles/PDLLA composites. The effect of MDI contents on mechanical properties of the composites was studied. At the optimum conditions of 1.0/1.0molar ratios of ? NCO groups in MDI to ? OH groups in PDLLA, bending strength 68.4 MPa and bending modulus 2281.5 MPa, were achieved in composite HA/PDLLA/MDI with 15 wt % HA. Both increased by nearly 30% when compared with that of solution cast HA/PDLLA composites. Interfacial adhesion and compatibility between PDLLA and bioactive ceramic particles (HA and TCP) were investigated. Scanning electron microscopy (SEM) indicated that the interface between HA particles and PDLLA was blurred and HA particles were closely surrounded by PDLLA matrix in HA/PDLLA/MDI composites. Oriented fibrils along with longitudinal direction of extrusion die were also observed on the surfaces of HA/PDLLA/MDI composite. It is confirmed that MDI has improved interfacial adhesion and compatibility between HA particles and PDLLA phase. Fibril structures formed in the extrusion, and it contributed a great deal in enhancing the mechanical properties. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 4085–4091, 2006     

Preparation of poly(L‐lactide) film having high elastic recovery

Triacetin (TAC) is known as a lubricant of poly(L ‐lactide) (PLLA), and it is used to improve the mechanical properties of PLLA. However, the compatibility of TAC with PLLA and the morphological changes induced by the addition of TAC to PLLA have not been clarified. This study investigates the effects of the addition of TAC on the morphological changes and physical properties of PLLA. We prepared the PLLA films containing a given amount of TAC by solvent‐cast blending with chloroform under a low temperature. From the investigation of the mechanical properties of the blends, it is found that the glass‐transition temperature of PLLA is remarkably decreased with an increasing amount of TAC, the blend films exhibit high elastic recovery, and the degree of the recovery increases linearly with the amount of TAC. The morphological changes exhibiting high elastic recovery are discussed. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 474–480, 2003     

Crystallization characteristics of intercalated poly(L‐lactide)/organo‐modified montmorillonite hybrids

The crystallization behavior of intercalated Poly(L ‐lactide) (PLLA)/organo‐modified montmorillonite hybrids has been investigated in term of growth of spherulites and bulk crystallization. The PLLA hybrids were prepared via melt‐compounding between PLLA and montmorillonite organo‐modified (organoclay) with two different types of ammonium salts: trimethyl octadecyl‐ or bis(4‐hydroxy butyl) methyl octadecyl ammonium salts. The nucleation rate of PLLA crystallites is slightly enhanced by natural clay, while reduced by organoclay due to the shielding effect and/or miscibility between surfactants of clays and PLLA chains. The incorporation of small amount of organoclay in PLLA matrix shows a tendency to reduce slightly the growth rate and the overall crystallization rate of PLLA crystallites. The secondary nucleation theory was utilized to discuss the crystallization behavior of these hybrids. POLYM. ENG. SCI., 46:39–46, 2006. © 2005 Society of Plastics Engineers     

Synthesis,physical properties,and crystallization of optically active poly(L‐phenyllactic acid) and poly(L‐phenyllactic acid‐co‐L‐lactic acid)

Optically active poly(L ‐phenyllactic acid) (Ph‐PLLA), poly(L ‐lactic acid) (PLLA), and poly(L ‐phenyllactic acid‐co‐L ‐lactic acid) with weight‐average molecular weight exceeding 6 × 10³ g mol^?1 were successfully synthesized by acid catalyzed direct polycondensation of L ‐phenyllactic acid and/or L ‐lactic acid in the presence of 2.5–10 wt % of p‐toluenesulfonic acid. Their physical properties and crystallization behavior were investigated by differential scanning calorimetry, thermogravimetry, and polarimetry. The absolute value of specific optical rotation ([α]) for Ph‐PLLA (?38 deg dm^?1 g^?1 cm³) was much lower than that of [α] for PLLA (?150 deg dm^?1 g^?1 cm³), suggesting that the helical nature was reduced by incorporation of bulky phenyl group. PLLA was crystallizable during solvent evaporation, heating from room temperature, and cooling from the melt. Incorporation of a very low content of bulky phenyllactyl units even at 4 mol % suppressed the crystallization of L ‐lactyl unit sequences during heating and cooling, though the copolymers were crystallizable for L ‐phenylactyl units up to 6 mol % during solvent evaporation. The activation energy of thermal degradation (ΔE_td) for Ph‐PLLA (200 kJ mol^?1) was higher than that for PLLA (158 kJ mol^?1). The ΔE_td for the copolymers increased with an increase in L ‐phenyllactyl unit content. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Compatibility of liquid deproteinized natural rubber having epoxy group (LEDPNR)/poly (L‐lactide) blend

Reaction after mixing of liquid epoxidized natural rubber/poly(L ‐lactide) blend was performed to enhance the compatibility of the blend. The liquid epoxidized natural rubber was prepared by epoxidation of deproteinized natural rubber with peracetic acid in latex stage followed by depolymerization with peroxide and propanal. The resulting liquid deproteinized natural rubber having epoxy group (LEDPNR) was mixed with poly(L ‐lactide) (PLLA) to investigate the compatibility of the blend through differential scanning calorimetry, optical light microscopy, and NMR spectroscopy. After heating the blend at 473 K for 20 min, glass transition temperature (T_g) of LEDPNR in LEDPNR/PLLA blend increased from 251 to 259 K, while T_g and melting temperature (T_m) of PLLA decreased from 337 to 332 K and 450 to 445 K, respectively, suggesting that the compatibility of LEDPNR/ PLLA blend was enhanced by a reaction between the epoxy group of LEDPNR and the ester group of PLLA. The reaction was proved by high‐resolution solid‐state ¹³C NMR spectroscopy. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Nanohybrid shish kebab structure and its effect on mechanical properties in poly(L‐lactide)/carbon nanotube nanocomposite fibers

In our previous work, the formation of a nanohybrid shish kebab (NHSK) structure was successfully achieved in helical polymer systems promoted by using single‐walled carbon nanotube (CNT) bundles with a unique ‘groove structure’, which is of great crystallographic interest. To further investigate the effect of surface groove structure of CNT bundles on the formation of NHSK structure in helical polymer systems, in the work reported here double‐walled carbon nanotube (DWNT) fibers with bundle structure were used as nucleating agents and orientation templates for poly(L ‐lactide) (PLLA) crystallization. A fine NHSK structure with controlled lateral size and period of kebabs was successfully obtained under various experimental conditions by using DWNT bundles. This could be due to the geometric confinement effect of the surface groove structure of the DWNT bundles, which could facilitate the orientation of PLLA chains along the DWNT axis and the lateral formation of a stable nucleus. Our work suggests an efficient method for the functionalization of CNTs with biocompatible PLLA, which may have some potential applications in biomedical areas. In addition, it is demonstrated that the formation of NHSK structure can effectively improve the physical bonding between PLLA and nanotubes, thus significantly improving the mechanical properties of PLLA/CNT nanocomposite fibers. Copyright © 2012 Society of Chemical Industry     

Preparation and in vitro release behavior of urapidil hydrochloride loading into microspheres based on poly(L‐lactide)

Microencapsulation of the antihypertensive drug urapidil hydrochloride was investigated as a means of controlling drug release and minimizing or eliminating local side effects. Poly(L ‐lactide) (PLLA) microspheres were prepared using an alternative oil‐in‐water (O/W) solvent‐evaporation method such as the O/W cosolvent solvent‐evaporation method and O/W with various electrolytes added to the aqueous phase method. The surface morphology and the size of the microspheres were observed by scanning electron microscope. Meanwhile, the drug loading efficiency of microspheres and the in vitro release of urapidil hydrochloride from microspheres were performed. The release study indicated that the urapidil hydrochloride‐PLLA microspheres exhibited better sustained release capacity, and the kinetics of urapidil hydrochloride‐PLLA microspheres in vitro release could be described by the Higuchi equation. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

The plasticizer‐assisted formation of a percolating multiwalled carbon nanotube network in biodegradable poly(L‐lactide)

A plasticizer, polyethylene glycol (PEG), was used to disperse multiwalled carbon nanotubes (MWCNTs) in biodegradable poly(L ‐lactide) during one‐step melt mixing. Incorporation of PEG significantly promotes the formation of a percolating MWCNT network in the plasticized composites, as revealed by rheological measurements and morphological observations. It, in turn, results in the retarded glass transition and cold crystallization behaviors in these plasticized composites as a result of constrained chain relaxation. It is believed that noncovalent wrapping of MWCNTs by PEG is responsible for the improved affinity between MWCNTs and the poly(L ‐lactide) matrix. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

An investigation of the effect of silane water‐crosslinking on the properties of poly(L‐lactide)

A silane‐grafting water‐crosslinking approach was developed to crosslink poly(L ‐lactide) (PLLA) by grafting vinylalkoxysilane onto PLLA using dicumyl peroxide, followed by silane hydrolysis to form siloxane linkages between PLLA chains. The degree of silane grafting onto PLLA was qualitatively characterized using Fourier transform infrared spectroscopy and quantitatively determined using inductively coupled plasma mass spectrometry. Crosslinked PLLA films were obtained by curing of silane‐grafted PLLA in hot water. Gel fractions were evaluated in order to calculate the crosslinking reaction kinetics and crosslinking density. Various techniques were used to investigate the effect of silane water‐crosslinking on the thermomechanical properties, hydrolysis resistance and biodegradation of PLLA. In addition to an improvement in thermal stability and mechanical properties, hydrolysis resistance was significantly enhanced as a result of silane water‐crosslinking of PLLA. Moreover, the biodegradation of silane water‐crosslinked PLLA was retarded compared with neat PLLA. Copyright © 2010 Society of Chemical Industry     

Biodegradable poly(L‐lactide) and polycaprolactone block copolymer networks

New biodegradable block copolymer networks were synthesized from methacrylate‐terminated poly(L ‐lactide) (mLA) and polycaprolactone (mCL) macromers. This allowed the realization of a series of materials in which the macromer ratio can be used to tailor the physical and mechanical properties of the materials. The synthesis of the macromers was characterized using Fourier transform infrared (FTIR) spectroscopy and ¹H NMR spectroscopy. Poly(mCL) and poly(mLA) networks were prepared by photopolymerization of the macromers, and copolymers were also prepared from the two macromers in various proportions. The phase microstructure of the new systems and the network architecture were investigated using differential scanning calorimetry, FTIR spectroscopy, dynamic mechanical analysis and thermogravimetry studies. Copyright © 2010 Society of Chemical Industry     

Effect of gamma‐irradiation on the electret properties of poly(L‐lactide)

Electret stability of poly(L ‐lactide) (PLA) films, gamma‐irradiated up to 100 kGy has been investigated by measuring the surface potential during the storage period. PLA samples—40‐μm thick films—were prepared by the casting method and then irradiated in a ⁶⁰Co radiation facility at a dose rate of 0.25 kGy/h. The structural changes during the irradiation were estimated by viscometric, differential scanning calorimetry and scanning electron microscope measurements. Random chain scission and appearance of end radicals are the most probable results of the irradiation process. After irradiation, the samples were charged in a corona discharge system and surface potential was measured by the method of the vibrating electrode with compensation. The values of the surface potential of the irradiated samples were higher in comparison with the non‐irradiated samples. This effect could be related to the degradation of the macromolecules and changes in the crystal state of PLA during the irradiation. Both of the mentioned factors lead to structural defects that increase the number of discrete trapping levels. The effect of low pressure on the surface potential drop was also investigated. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Characterization of biaxial strain of poly(l‐lactide) tubes

Poly(l ‐lactide) in its l ‐form has promising mechanical properties. Being a semicrystalline polymer, it can be subjected to strain‐induced crystallization at temperatures above T_g and can thereby become oriented. Following a simultaneous (SIM) biaxial strain process or a sequential (SEQ) biaxial strain process, the mechanical properties of biaxially strained tubes can be further improved. This study investigated these properties in relation to their morphology and crystal orientation. Both processes yield the same mechanical strength and modulus, yet exhibit different crystal orientation. Through further wide angle X‐ray scattering analysis it was found that the SEQ biaxial strain yields larger interplanar spacing and distorted crystals and looser packing of chains. However, this does not influence the mechanical properties negatively. A loss of orientation in SEQ biaxially strained samples at high degrees of strain was detected, but was not seen for SIM biaxial strain and did not correlate with mechanical performance in either case. However, post‐annealing reduced the orientation to the same level in both cases, and the modulus and strength decreased for both SIM and SEQ biaxial strain. It is therefore concluded that mechanical properties after biaxial strain are related to strain‐induced amorphous orientation and the packing of crystals, rather than strain‐induced crystallinity. © 2015 Society of Chemical Industry     

PY‐GC/MS an effective technique to characterizing of degradation mechanism of poly (L‐lactide) in the different environment

The biotic and abiotic degradation of poly (L‐lactide) (PLLA) has been studied with pyrolysis gas chromatography mass spectrometry (Py‐GC‐MS). A mixed culture of compost micro‐organisms was used as the biotic medium. Size‐exclusion chromatography (SEC), gas chromatography‐mass spectrometry (GC‐MS), Fourier transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM) were utilized to monitor the degradation and degradation mechanism. Differences in pH, molecular weight, surface structure, and degradation mechanisms were noted between sample aged in biotic and abiotic medium. Using fractionated Py‐GC‐MS at 400 and 500°C, acetaldehyde, acrylic acid, lactoyl acrylic acid, two lactide isomers, and cyclic oligomers up to the pentamer were identified as thermal decomposition products of PLA as well as some other not completely identified products. The ratio of meso‐lactide to L‐lactide was lower in the sample aged in the biotic media than the abiotic media. This is a result of the preference of the micro‐organisms for L‐form of lactic acid and lactoyl lactic acid rather than the D‐form that in turn influences the formation and the amounts of meso and D,L‐lactide during the pyrolysis. Based on SEM micrographs, it was shown that degradation in the biotic medium proceeded mainly via a surface erosion mechanism, whereas bulk erosion was the predominant degradation mechanism in the abiotic medium. The SEC and Py‐GC‐MS data indicate that degradation was faster in the biotic than in the abiotic sample. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 2369–2378, 2000     

Quantitative analysis for reaction between epoxidized natural rubber and poly (L‐lactide) through 1H‐NMR spectroscopy

Reaction between epoxidized natural rubber and poly(L ‐lactide) (PLLA) was investigated quantitatively in terms of conversion of the epoxidized natural rubber. The epoxidized natural rubber was prepared by epoxidation of high ammonia natural rubber (HA‐NR) or deproteinized natural rubber (DPNR) with peracetic acid followed by depolymerization with ammonium persulfate. The resulting liquid HA‐NR having epoxy group (LENR) or liquid DPNR having epoxy group (LEDPNR) were subjected to heating at 473 K for 20 min, after blending with PLLA. The products were characterized through morphology observation, DSC measurement, and ¹H‐NMR spectroscopy. The conversions of the rubbers were estimated from intensity ratio of signals in ¹H‐NMR spectrum for the products after removing unreacted rubber with toluene. Difference in the estimated conversion between the LENR/PLLA and LEDPNR/PLLA blends was interpreted in relation to proteins present in the rubber. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010