Synthesis and characterization of ABA type tri‐block copolymers derived from p‐dioxanone,L‐lactide and poly(ethylene glycol)

A series of triblock co‐polymers, consisting of a poly(ethylene glycol) (PEG) central block joined to two blocks of random p‐dioxanone‐co‐L ‐lactide copolymers were synthesized by ring‐opening polymerization of p‐dioxanone (PDO) and L ‐lactide (LLA) initiated by PEG in the presence of stannous 2‐ethylhexanoate catalyst. The resulting copolymers were characterized by various techniques including ¹H and ¹³C NMR and FTIR spectroscopies, gel permeation chromatography, inherent viscosity, wide‐angle X‐ray diffractometry (WAXD) and differential scanning calorimetry (DSC). The conversion of PDO and L ‐lactide into the polymer was studied various mole ratios and at different polymerization temperature from ¹H NMR spectra. Results of WAXD and DSC showed that the crystallinity of PEG macroinitiator was greatly influenced by the composition of PDO and L ‐lactide in the copolymer. The triblock copolymers with low molecular weight were soluble in water at below room temperature. © 2003 Society of Chemical Industry     

Stereocomplex formation between enantiomeric PLA–PEG–PLA triblock copolymers: Characterization and use as protein‐delivery microparticulate carriers

Two enantiomeric triblock ABA copolymers composed of poly(L ‐lactide)–poly(ethylene glycol)–poly(L ‐lactide) (PLLA–PEG–PLLA) and poly(D ‐lactide)–poly(ethylene glycol)–poly(D ‐lactide) (PDLA–PEG–PDLA) were synthesized with two different middle‐block PEG chain lengths by ring‐opening polymerization of L ‐lactide and D ‐lactide in the presence of PEG, respectively. A pair of enantiomeric triblock copolymers were combined to form a stereocomplex by a solvent‐casting method. The triblock copolymers and their stereocomplexes were characterized by ¹H‐ and ¹³C‐NMR spectroscopy and gel permeation chromatography. Their crystalline structures and crystalline melting behaviors were analyzed by the wide‐angle X‐ray diffraction method and differential scanning calorimetry. The stereocomplex formed between a pair of enantiomeric triblock copolymers exhibited a higher crystalline melting temperature with a distinctive 3/1 helical crystalline structure. PLLA–PEG–PLLA and its stereocomplex with PDLA–PEG–PDLA were used to fabricate a series of microspheres encapsulating a model protein drug, bovine serum albumin (BSA). They were prepared by a double‐emulsion solvent‐evaporation method. The morphological aspects of the microspheres were characterized and BSA release profiles from them were investigated. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 75: 1615–1623, 2000     

Investigation of poly(lactide) stereocomplexation between linear poly(L‐lactide) and PDLA‐PEG‐PDLA tri‐block copolymer

In this study, stereocomplexed poly(lactide) (PLA) was investigated by blending linear poly(l ‐lactide) (PLLA) and tri‐block copolymer poly(d ‐lactide) ? (polyethylene glycol) ? poly(d ‐lactide) (PDLA‐PEG‐PDLA). Synthesized PDLA‐PEG‐PDLA tri‐block copolymers with different PEG and PDLA segment lengths were studied and their influences on the degree of sterecomplexation and non‐isothermal crystallization behaviour of the PLLA/PDLA‐PEG‐PDLA blend were examined in detail by DSC, XRD and polarized optical microscopy. A full stereocomplexation between PLLA and PDLA‐PEG_4k‐PDLA200 could be formed when the L/D ratio ranged from 7/3 to 5/5 without the presence of PLA homocrystals. The segmental mobility and length of both PEG and PDLA are the dominating factors in the critical D/L ratio to achieve full stereocomplexation and also for nucleation and spherulite growth during the non‐isothermal crystallization process. For fixed PEG segmental length, the stereocomplexed PLA formed showed first an increasing and then a decreasing melting temperature with increasing PDLA segments due to their intrinsic stiff mobility. Furthermore, the effect of PEG segmental mobility on PLA stereocomplexation was investigated. The results clearly showed that the crystallization temperature and melting temperature of stereocomplexed‐PLA kept increasing with increasing PEG segmental length, which was due to PEG soft mobility in the tri‐block copolymers. However, PEG was not favourable for nucleation but could facilitate the spherulite growth rate. Both the PDLA and PEG segmental lengths in the tri‐block copolymers affect the crystallinity of stereocomplexed‐PLA and the stereocomplexation formation process; they have a different influence on blends prepared by solution casting or the melting method. © 2015 Society of Chemical Industry     

Synthesis and characterization of poly(ϵ‐caprolactone)‐b‐poly(ethylene glycol) block copolymers prepared by a salicylaldimine‐aluminum complex

A series of poly(?‐caprolactone)‐b‐poly(ethylene glycol) (PCL‐b‐PEG) block copolymers with different molecular weights were synthesized with a salicylaldimine‐aluminum complex in the presence of monomethoxy poly(ethylene glycol). The block copolymers were characterized by ¹H NMR, GPC, WAXD, and DSC. The ¹H NMR and GPC results verify the block structure and narrow molecular weight distribution of the block copolymers. WAXD and DSC results show that crystallization behavior of the block copolymers varies with the composition. When the PCL block is extremely short, only the PEG block is crystallizable. With further increase in the length of the PCL block, both blocks can crystallize. The PCL crystallizes prior to the PEG block and has a stronger suppression effect on crystallization of the PEG block, while the PEG block only exerts a relatively weak adverse effect on crystallization of the PCL block. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Influence of the synthesis conditions on the structural and thermal properties of poly(l‐lactide)‐b‐poly(ethylene glycol)‐b‐poly(l‐lactide)

The poly(l ‐lactide)‐b‐poly(ethylene glycol)‐b‐poly(l ‐lactide) block copolymers (PLLA‐b‐PEG‐b‐PLLA) were synthesized in a toluene solution by the ring‐opening polymerization of 3,6‐dimethyl‐1,4‐dioxan‐2,5‐dione (LLA) with PEG as a macroinitiator or by transterification from the homopolymers [polylactide and PEG]. Two polymerization conditions were adopted: method A, which used an equimolar catalyst/initiator molar ratio (1–5 wt %), and method B, which used a catalyst content commonly reported in the literature (<0.05 wt %). Method A was more efficient in producing copolymers with a higher yield and monomer conversion, whereas method B resulted in a mixture of the copolymer and homopolymers. The copolymers achieved high molar masses and even presenting similar global compositions, the molar mass distribution and thermal properties depends on the polymerization method. For instance, the suppression of the PEG block crystallization was more noticeable for copolymer A. An experimental design was used to qualify the influence of the catalyst and homopolymer amounts on the transreactions. The catalyst concentration was shown to be the most important factor. Therefore, the effectiveness of method A to produce copolymers was partly due to the transreactions. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40419.     

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     

Investigations on the morphology and melt crystallization of poly(L‐lactide)‐poly(ethylene glycol) diblock copolymers

Ring opening polymerization of L ‐lactide was realized in the presence of monomethoxy poly(ethylene glycol), using zinc lactate as catalyst. The resulting PLLA‐PEG diblock copolymers were characterized by using ¹H‐NMR, SEC, WAXD, and DSC. All the copolymers were semicrystalline, one or two melting peaks being detected depending on the composition. Equilibrium melting temperature (T_m⁰) of PLLA blocks was determined for three copolymers with different EO/LA molar ratios. T_m⁰ decreased with decreasing PLLA block length. A copolymer with equivalent PLLA and PEG block lengths was selected for melt crystallization studies and the resulting data were analyzed with Avrami equation. The obtained Avrami exponent is equal to 2.6 ± 0.2 in the crystallization temperature range from 80 to 100°C. In addition, the spherulite growth rate of PLLA‐PEG was analyzed by using Lauritzen‐Hoffmann theory in comparison with PLLA homopolymers. The nucleation constant was found to be 2.39 × 10⁵ K² and the free energy of folding equal to 53.8 erg/cm² in the range of 70–94°C, both higher than those of PLLA homopolymers, while the spherulite growth rate of the diblock copolymer was lower. POLYM. ENG. SCI., 2008. © 2007 Society of Plastics Engineers     

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     

Synthesis and characterization of star‐shaped poly(ethylene glycol)‐block‐poly(L‐lactic acid) copolymers by melt polycondensation

Compared with linear diblock or triblock poly(ethylene glycol)‐block‐poly(L ‐lactic acid) copolymer (PEG‐b‐PLLA), star‐shaped PEG‐b‐PLLA (sPEG‐b‐PLLA) copolymers exhibit smaller hydrodynamic radius and lower viscosity and are expected to display peculiar morphologies, thermal properties, and degradation profiles. Compared with the synthesis routine of PEG‐b‐PLLA form lactide and PEG, the traditional synthesis routine from LA and PEG were suffered by the low reaction efficiency, low purity, lower molecular weight, and wide molecular weight distribution. In this article, multiarm sPEG‐b‐PLLA copolymer was prepared from multiarm sPEG and L ‐lactic acid (LLA using an improved method of melt polycondensation, in which two types of sPEG, that is, sPEG₁ (four arm, Mn = 4300) and sPEG₂ (three arm, Mn = 3200) were chosen as the core. It was found the molecular weight of sPEG‐b‐PLLA could be strongly affected by the purity of LLA and sPEGs, and the purification technology of vacuum dewater and vacuum distillation could help to remove most of the impurities in commercial available LLA. The polymers, including sPEG and sPEG‐b‐PLLA with varied core (sPEG₁ and sPEG₂) and LLA/sPEG feeding ratios, were characterized and confirmed by ¹H‐NMR and ¹³C‐NMR spectroscopy, Fourier transform infrared spectroscopy (FT‐IR) and gel permeation chromatography, which showed that the terminal hydroxyl group in each arm of sPEGs had reacted with LLA to form sPEG‐b‐PLLA copolymers with fairly narrow molecular weight distribution. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Synthesis and characterization of poly(ε‐caprolactone)‐b‐poly(ethylene glycol)‐b‐poly(ε‐caprolactone) triblock copolymers with dibutylmagnesium as catalyst

Two series of poly(ε‐caprolactone)‐b‐poly(ethylene glycol)‐b‐poly(ε‐caprolactone) triblock copolymers were prepared by the ring opening polymerization of ε‐caprolactone in the presence of poly(ethylene glycol) and dibutylmagnesium in 1,4‐dioxane solution at 70°C. The triblock structure and molecular weight of the copolymers were analyzed and confirmed by ¹H NMR, ¹³C NMR, FTIR, and gel permeation chromatography. The crystallization and thermal properties of the copolymers were investigated by wide‐angle X‐ray diffraction (WAXD) and differential scanning calorimetry (DSC). The results illustrated that the crystallization and melting behaviors of the copolymers were depended on the copolymer composition and the relative length of each block in copolymers. Crystallization exothermal peaks (T_c) and melting endothermic peaks (T_m) of PEG block were significantly influenced by the relative length of PCL blocks, due to the hindrance of the lateral PCL blocks. With increasing of the length of PCL blocks, the diffraction and the melting peak of PEG block disappeared gradually in the WAXD patterns and DSC curves, respectively. In contrast, the crystallization of PCL blocks was not suppressed by the middle PEG block. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Influence of racemization on stereocomplex‐induced gelation of water‐soluble polylactide–poly(ethylene glycol) block copolymers

Ring‐opening polymerization of L ‐ or D ‐lactide was realized at 140 °C for a period of 7 days in the presence of dihydroxyl poly(ethylene glycol) (PEG), with M?_n = 4000 g mol^?1, using zinc lactate as initiator. The resulting poly(L ‐lactide)–PEG–poly(L ‐lactide) and poly(D ‐lactide)–PEG–poly(D ‐lactide) triblock copolymers are water soluble with polylactide (PLA) block length ranging from 11 to 17 units. Both the tube inverting method and rheological measurements were used to evaluate the gelation properties of aqueous solutions containing single copolymers or L /D copolymer pairs. Stereocomplexation between poly(L ‐lactide) and poly(D ‐lactide) blocks is observed for mixed solutions. Hydrogel formation is detected in the case of relatively long PLA blocks (DP _PLA = 17), but not for copolymers with shorter PLA blocks (DP _PLA = 11–13) due to partial racemization of L ‐lactyl units. Racemization is largely reduced when the reaction time is shortened to 1 day. Under these conditions, DP _PLA of 8 is sufficient for the stereocomplexation of PLA–PEG block copolymers, and DP _PLA above 10 leads to the formation of hydrogels of PLA–PEG block copolymers. On the other hand, racemization appears as a general phenomenon in the (co)polymerization of L ‐lactide with Zn(Lac)₂ as initiator, although it is negligible or undetectable in the case of high molar mass polymers. Therefore, racemization is the limiting factor for the stereocomplexation‐induced gelation of water‐soluble PLA–PEG block copolymers where the PLA block length generally ranges from 10 to 30. Reaction conditions including initiator, time and temperature should be strictly controlled to minimize racemization. Copyright © 2010 Society of Chemical Industry     

Effect of poly(l‐lactide)/poly(d‐lactide) block length ratio on crystallization behavior of star‐shaped asymmetric poly(l(d)‐lactide) stereoblock copolymers

Effect of Poly(l ‐lactide)/Poly(d ‐lactide) (PLLA/PDLA) block length ratio on the crystallization behavior of star‐shaped poly(propylene oxide) block poly(d ‐lactide) block poly (l ‐lactide) (PPO–PDLA–PLLA) stereoblock copolymers with molecular weights (M_n) ranging from 6.2 × 10⁴ to 1.4 × 10⁵ g mol^?1 was investigated. Crystallization behaviors were studied utilizing differential scanning calorimetry (DSC), polarized optical microscopy (POM), and wide‐angle X‐ray diffraction (WAXD). Only stereocomplex crystallites formed in isothermal crystallization at 140 to 156°C for all samples. On one hand, the overall crystallization rate decreased as PLLA/PDLA block length ratio increased. As PLLA/PDLA block length ratio increased from 7:7 to 28:7, the value of half time of crystallization (t_1/2) delayed form 2.85 to 5.31 min at 140°C. On the other hand, according to the Lauritzen–Hoffman theory, the fold‐surface energy (σ_e) was calculated. σ_e decreased from 77.7 to 73.3 erg/cm² with an increase in PLLA/PDLA block length ratio. Correspondingly increase in nucleation density was observed by the polarized optical microscope. Results indicated that the PLLA/PDLA block length ratio had a significant impact on the crystallization behavior of PPO–PDLA–PLLA copolymers. POLYM. ENG. SCI., 55:2534–2541, 2015. © 2015 Society of Plastics Engineers     

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     

Miscibility and phase structure of binary blends of poly(L‐lactide) and poly(vinyl alcohol)

Blend films of poly(L ‐lactide) (PLLA) and poly(vinyl alcohol) (PVA) were obtained by evaporation of hexafluoroisopropanol solutions of both components. The component interaction, crystallization behavior, and miscibility of these blends were studied by solid‐state NMR and other conventional methods, such as Fourier transform infrared (FTIR) spectra, differential scanning calorimetry (DSC), and wide‐angle X‐ray diffraction (WAXD). The existence of two series of isolated and constant glass‐transition temperatures (T_g's) independent of the blend composition indicates that PLLA and PVA are immiscible in the amorphous region. However, the DSC data still demonstrates that some degree of compatibility related to blend composition exists in both PLLA/atactic‐PVA (a‐PVA) and PLLA/syndiotactic‐PVA (s‐PVA) blend systems. Furthermore, the formation of interpolymer hydrogen bonding in the amorphous region, which is regarded as the driving force leading to some degree of component compatibility in these immiscible systems, is confirmed by FTIR and further analyzed by ¹³C solid‐state NMR analyses, especially for the blends with low PLLA contents. Although the crystallization kinetics of one component (especially PVA) were affected by another component, WAXD measurement shows that these blends still possess two isolated crystalline PLLA and PVA phases other than the so‐called cocrystalline phase. ¹³C solid‐state NMR analysis excludes the interpolymer hydrogen bonding in the crystalline region. The mechanical properties (tensile strength and elongation at break) of blend films are consistent with the immiscible but somewhat compatible nature of these blends. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 762–772, 2001     

Synergistic effect of poly(ethylene glycol) and graphene oxides on the crystallization behavior of poly(l‐lactide)

In this article, we report the combined effects of poly(ethylene glycol) (PEG) and/or graphene oxides (GOs) on the crystallization behavior of poly(l ‐lactide) (PLLA) under different crystallization conditions, such as nonisothermal crystallization, isothermal crystallization, and annealing‐induced cold crystallization. Differential scanning calorimetry (DSC), wide‐angle X‐ray diffraction, and polarized optical microscopy were used to study the crystallization kinetics and crystallinity to illustrate the effects of PEG and/or GOs on the crystallization behavior of PLLA. The results show that PEG functioned as a plasticizer and improved the chain mobility of /PLLA during crystallization and the GOs acted as efficient nucleation agents and accelerated the crystallization rate. Finally, both PEG and GOs improved the crystallization ability of PLLA. Importantly, the simultaneous addition of PEG and GOs led to a synergistic effect on the crystallization behavior of PLLA under all conditions. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 3498–3508, 2013     

AB‐ and ABC‐type di‐ and triblock copolymers of poly[styrene‐block‐(ε‐caprolactone)] and poly[styrene‐block‐(ε‐caprolactone)‐block‐lactide]: synthesis,characterization and thermal studies

Polystyrene terminated with benzyl alcohol units was employed as a macroinitiator for ring‐opening polymerization of ε‐caprolactone and L ‐lactide to yield AB‐ and ABC‐type block copolymers. Even though there are many reports on the diblock copolymers of poly(styrene‐block‐lactide) and poly(styrene‐block‐lactone), this is the first report on the poly(styrene‐block‐lactone‐block‐lactide) triblock copolymer consisting of two semicrystalline and degradable segments. The triblock copolymers exhibited twin melting behavior in differential scanning calorimetry (DSC) analysis with thermal transitions corresponding to each of the lactone and lactide blocks. The block derived from ε‐caprolactone also showed crystallization transitions upon cooling from the melt. In the DSC analysis, one of the triblock copolymers showed an exothermic transition well above the melting temperature upon cooling. Thermogravimetric analysis of these block copolymers showed a two‐step degradation curve for the diblock copolymer and a three‐step degradation for the triblock copolymer with each of the degradation steps associated with each segment of the block copolymers. The present study shows that it is possible to make pure triblock copolymers with two semicrystalline segments which also consist of degradable blocks. Copyright © 2009 Society of Chemical Industry     

Crystallization kinetics of poly(1,4‐butylene‐co‐ethylene terephthalate)

The crystallization kinetics of poly(butylene terephthalate) (PBT), poly(ethylene terephthalate) (PET), and their copolymers poly(1,4‐butylene‐co‐ethylene terephthalate) (PBET) containing 70/30, 65/35 and 60/40 molar ratios of 1,4‐butanediol/ethylene glycol were investigated using differential scanning calorimetry (DSC) at crystallization temperatures (T_c) which were 35–90 °C below equilibrium melting temperature . Although these copolymers contain both monomers in high proportion, DSC data revealed for copolymer crystallization behaviour. The reason for such copolymers being able to crystallize could be due to the similar chemical structures of 1,4‐butanediol and ethylene glycol. DSC results for isothermal crystallization revealed that random copolymers had a lower degree of crystallinity and lower crystallite growth rate than those of homopolymers. DSC heating scans, after completion of isothermal crystallization, showed triple melting endotherms for all these polyesters, similar to those of other polymers as reported in the literature. The crystallization isotherms followed the Avrami equation with an exponent n of 2–2.5 for PET and 2.5–3.0 for PBT and PBETs. Analyses of the Lauritzen–Hoffman equation for DSC isothermal crystallization data revealed that PBT and PET had higher growth rate constant G_o, and nucleation constant K_g than those of PBET copolymers. © 2001 Society of Chemical Industry     

ABA triblock copolymers from poly(p‐dioxanone) and poly(ethylene glycol)

Poly(p‐dioxanone)–poly(ethylene glycol)–poly(p‐dioxanone) ABA triblock copolymers (PEDO) were synthesized by ring‐opening polymerization from p‐dioxanone using poly(ethylene glycol) (PEG) with different molecular weights as macroinitiators in N₂ atmosphere. The copolymer was characterized by ¹H NMR spectroscope. The thermal behavior, crystallization, and thermal stability of these copolymers were investigated by differential scanning calorimetry and thermogravimetric measurements. The water absorption of these copolymers was also measured. The results indicated that the content and length of PEG chain have a greater effect on the properties of copolymers. This kind of biodegradable copolymer will find a potential application in biomedical materials. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102:1092–1097, 2006     

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     

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