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     

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 structure control of l‐lactic acid–glycolic acid copolymer by homo‐copolymerization

A two‐step direct melt copolymerization process of l ‐lactic acid (L ‐LA)/glycolic acid (GA) was developed: poly(l ‐lactic acid) (PLLA) and poly(glycolic acid) (PGA) with different molecular weight was first synthesized respectively by binary catalyst (tin chloride/p‐toluenesulfonic or tin chloride); and then poly(l ‐lactic‐co‐glycolic acid) (b‐PLGA) was produced by melt polymerization of the as‐prepared PLLA and PGA, wherein the composition and chain structure of b‐PLGA copolymers could be controlled by the molecular weight of PLLA. The chain structure and thermal properties of copolymers were studied by Wide‐angle X‐ray diffraction, nuclear magnetic resonance, differential scanning calorimetry, and thermogravimetric analysis. In comparison with the random PLGA (r‐PLGA) synthesized by one‐step direct melt polymerization, the average l ‐lactic blocks length (L_LA) in b‐PLGA was longer while the average glycolic blocks length (L_GA) in b‐PLGA was shorter which further resulted in the improved crystallinity and thermostability. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41566.     

Preparation of high‐molecular‐weight poly(L‐lactic acid)‐based polymers through direct condensation polymerization in bulk state

Poly(L ‐lactic acid‐co‐succinic acid‐co‐1,4‐butanediol) (PLASB) was synthesized by a direct condensation copolymerization of L ‐lactic acid, succinic acid (SA), and 1,4‐butanediol (BD) in bulk state using titanium(IV) butoxide (TNBT) as a catalyst. Weight average molecular weight (M_w) of PLASB increased from 3.5 × 10⁴ to 2.1 × 10⁵ as the content of SA and BD went up from 0.01 to 0.5 mol/100 mol of L ‐lactic acid (LA). PLASB having M_w in the range from 1.8 × 10⁵ to 2.1 × 10⁵ showed tensile properties comparable to those of commercially available poly(L ‐lactic acid) (PLLA). In sharp contrast, homopolymerization of LA in bulk state produced PLLA with M_w as low as 4.1 × 10⁴, and it was too brittle to prepare specimens for the tensile tests. M_w of PLASB synthesized by using titanium(IV)‐2‐ethyl(hexoxide), indium acetate, indium hydroxide, antimony acetate, antimony trioxide, dibutyl tin oxide, and stannous‐2‐ethyl 1‐hexanoate was compared with that of PLASB obtained by TNBT. Ethylene glycol oligomers with different chain length were added to LA/SA in place of BD to investigate effect of chain length of ethylene glycol oligomers on the M_w of the resulting copolymers. Biodegradability of PLASB was analyzed by using the modified Sturm test. Toxicity of PLASB was evaluated by counting viable cell number of mouse fibroblast cells that had been in contact with PLASB discs. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 466–472, 2006     

Deterioration of metal–organic framework crystal structure during fabrication of poly(l‐lactic acid) mixed‐matrix membranes

Poly(l ‐lactic acid) (PLLA) and metal–organic framework (MOF) mixed‐matrix membranes were prepared by melt extrusion of PLLA with 5% (w/w) of either activated or water‐saturated Cu₃(BTC)₂ (Cu₃(C₉H₃O₆)₂(H₂O)₃·xH₂O, HKUST‐1). The morphology and the stability of injection‐molded samples were evaluated using thermogravimetric analysis, differential scanning calorimetry, gel permeation chromatography, X‐ray diffraction (XRD) and scanning electron microscopy (SEM). The presence of activated and saturated MOF crystals increased the cold crystallization onset temperature as compared to neat PLLA. This can be attributed to the MOF crystals incorporated in the PLLA matrix, which decreased the mobility of PLLA and thus impeded the crystallization process. According to the XRD results, the activated MOF crystals were successfully incorporated into the PLLA matrix without altering the crystal structure of the MOF. Moreover, the findings from permeability and tensile tests as well as SEM imaging indicated good interfacial interactions between PLLA and activated MOF. However, during melt extrusion of PLLA with saturated MOF, water molecules from the saturated MOF altered the MOF crystal structure and contributed to the degradation of the PLLA polymer by reducing its molecular weight by around 21%. © 2013 Society of Chemical Industry     

Synthesis and properties of crystalline dicarboxylated poly(L‐lactic acid) prepolymers

Crystalline dicarboxylated poly(L ‐lactic acid)s (dcPLLAs) with number‐average molecular weights (M_n's) of 10³ to 10⁴ g/mol were synthesized via the melt polycondensation of L ‐lactic acid (LLA) in the presence of succinic anhydride (SAD), with tin(II) chloride and toluene‐4‐sulfonic acid as binary catalysts. They were characterized by end‐group titration, ¹H‐NMR, differential scanning calorimetry, and wide‐angle X‐ray diffraction. The terminal COOH percentage reached over 98%, and the molecular weight could be controlled by the molar ratio of LLA to SAD. The thermal behaviors depended on the molecular weight. The poly(L ‐lactic acid)s (PLLAs) crystallized slowly for M_n ≤ 2000 but quickly for M_n ≥ 4000. The crystallinity increased from 27 to 40% when M_n grew from 4000 to 10,000. With comparison to ordinary PLLA, the dcPLLA had the same crystallization structure but a slightly lower crystallizability. The glass‐transition temperature was clearly higher than that of amorphous dcPLLAs. With a controllable molecular weight, high COOH percentage, and crystallinity, the dcPLLA with M_n ≥ 4000 appeared to be a suitable prepolymer for the preparation of high‐molecular‐weight crystalline PLLA via chain extension. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Melt polycondensation of L‐lactic acid to poly(L‐lactic acid) with Sn(II) catalysts combined with various metal alkoxides

Melt polycondensation of L ‐lactic acid (LA) was examined in the presence of binary catalyst systems consisting of SnCl₂·2H₂O and metal alkoxides as co‐catalysts. Among the co‐catalysts examined, viz (Al(O ⁱPr)₃,Ti(O ⁱPr)₄,Y(O ⁱPr)₃,Si(OEt)₄ and Ge(OEt)₄), Ge(OEt)₄ was found to be the most effective in enhancing the catalytic activity of Sn(II). With an optimized composition of SnCl₂·2H₂O–Ge(OEt)₄, the molecular weight (M_n) of PLLA reached 40 000 Da in a short reaction time (<15 h) at the optimum reaction conditions of 180 °C and 10 Torr. This catalyst system was also superior to the conventional single metal ion catalysts such as Sn(II) in terms of racemization and discolouration of the resultant polymer. The metal alkoxides, added as co‐catalysts, should work as oxo acids that can effectively control the catalytic activity of Sn(II) ion in the direct polycondensation of LA, in a manner similar to that of proton acids. © 2003 Society of Chemical Industry     

Synthesis and thermal properties of poly(methyl methacrylate)‐poly(L‐lactic acid)‐poly(methyl methacrylate) tri‐block copolymer

Poly(methyl methacrylate)‐poly(L ‐lactic acid)‐poly(methyl methacrylate) tri‐block copolymer was prepared using atom transfer radical polymerization (ATRP). The structure and properties of the copolymer were analyzed using infrared spectroscopy, gel permeation chromatography, nuclear magnetic resonance (¹H‐NMR, ¹³C‐NMR), thermogravimetry, and differential scanning calorimetry. The kinetic plot for the ATRP of methyl methacrylate using poly(L ‐lactic acid) (PLLA) as the initiator shows that the reaction time increases linearly with ln[M]₀/[M]. The results indicate that it is possible to achieve grafted chains with well‐defined molecular weights, and block copolymers with narrowed molecular weight distributions. The thermal stability of PLLA is improved by copolymerization. A new wash‐extraction method for removing copper from the ATRP has also exhibits satisfactory results. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Preparation and characterization of high‐molecular‐weight poly(l‐lactic acid) by chain‐extending reaction with phosphites

Poly(l ‐lactic acid) (PLLA) of high molecular weight was prepared by a chain‐extending reaction in a microcompounder. Phosphites were used as chain extenders to increase the molecular weight of the PLLA prepolymer, which was prepared by the bulk polycondensation of l‐lactic acid. The effects of the amount of phosphite, the temperature, and the screw speed on the torque of the PLLA melt were studied. Under the optimal conditions, the molecular weight of PLLA increased from 62,100 to 126,000 g/mol. The chemical structure and crystallinity of PLLA were characterized by Fourier transform infrared spectroscopy, ¹H‐NMR and ¹³C‐NMR, differential scanning calorimetry, and X‐ray diffraction. The mechanical properties of PLLA were measured. The results indicate that triphenyl phosphite (TPPi) was an effective chain extender for PLLA. The role of the TPPi in chain extending is suggested to be an esterification‐promotion agent. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Highly enhanced accelerating effect of melt‐recrystallized stereocomplex crystallites on poly(L‐lactic acid) crystallization: effects of molecular weight of poly(D‐lactic acid)

The effects of the molecular weight of poly(D ‐lactic acid) (PDLA), which forms stereocomplex (SC) crystallites with poly(L ‐lactic acid) (PLLA), and those of processing temperature T_p on the acceleration (or nucleation) of PLLA homocrystallization were investigated using PLLA films containing 10 wt% PDLA with number‐average molecular weight (M_n) values of 5.47 × 10⁵, 9.67 × 10⁴ and 3.67 × 10⁴ g mol^–1 (PDLA‐H, PDLA‐M and PDLA‐L, respectively). For the PLLA/PDLA‐H and PLLA/PDLA‐M films, the SC crystallites that were ‘non’‐melted and those that were ‘completely’ melted at T_p values just above their endset melting temperature and recrystallized during cooling were found to act as effective accelerating (or nucleation) agents for PLLA homocrystallization. In contrast, SC crystallites formed from PDLA‐L, having the lowest M_n, were effective accelerating agents without any restrictions on T_p. In this case, the accelerating effects can be attributed to the plasticizer effect of PDLA‐L with the lowest M_n. The accelerating effects of SC crystallites in the PLLA/PDLA‐H and PLLA/PDLA‐M films was dependent on crystalline thickness for T_p values below the melting peak temperature of SC crystallites, whereas for T_p values above the melting peak temperature the accelerating effects are suggested to be affected by the interaction between the SC crystalline regions and PLLA amorphous regions.     

Part 7. Effects of poly(L‐lactide‐co‐ϵ‐caprolactone) on morphology,structure, crystallization,and physical properties of blends of poly(L‐lactide) and poly(ϵ‐caprolactone)

Blended films of poly(L ‐lactide) [ie poly(L ‐lactic acid)] (PLLA) and poly(?‐caprolactone) (PCL) without or mixed with 10 wt% poly(L ‐lactide‐co‐?‐caprolactone) (PLLA‐CL) were prepared by solution‐casting. The effects of PLLA‐CL on the morphology, phase structure, crystallization, and mechanical properties of films have been investigated using polarization optical microscopy, scanning electron microscopy, differential scanning calorimetry and tensile testing. Addition of PLLA‐CL decreased number densities of spherulites in PLLA and PCL films, and improved the observability of spherulites and the smoothness of cross‐section of the PLLA/PCL blend film. The melting temperatures (T_m) of PLLA and PCL in the films remained unchanged upon addition of PLLA‐CL, while the crystallinities of PLLA and PCL increased at PLLA contents [X_PLLA = weight of PLLA/(weight of PLLA and PCL)] of 0.4–0.7 and at most of the X_PLLA values, respectively. The addition of PLLA‐CL improved the tensile strength and the Young modulus of the films at X_PLLA of 0.5–0.8 and of 0–0.1 and 0.5–0.8, respectively, and the elongation at break of the films at all the X_PLLA values. These findings strongly suggest that PLLA‐CL was miscible with PLLA and PCL, and that the dissolved PLLA‐CL in PLLA‐rich and PCL‐rich phases increased the compatibility between these two phases. © 2003 Society of Chemical Industry     

Poly(L‐lactic acid)/silicone dioxide nanocomposites prepared via in situ melt polycondensation of L‐lactic acid in the presence of acidic silica sol: Dispersion stability of nanoparticles during dehydration/oligomerization

In this work, the stability of nanoparticles during the dehydration/oligomerization (D/O) stage in situ melt polycondensation of L ‐lactic acid to prepare poly(L ‐lactic acid) (PLLA)/SiO₂ nanocomposite was studied. The change in the appearance of the reaction mixture was tracked, and the resultant oligo(L ‐lactic acid) (OLLA)/SiO₂ and PLLA/SiO₂ nanocomposites were characterized by transmission electron microscopy, ¹H‐NMR, and light transmittance. The electric double layer and the grafted OLLA chains provided static and steric stabilities during the early and late phases, respectively. However, there existed an intermediate transitional phase with weak stability when the static stability was weakened, but sufficient steric stability had not yet been established; this led to soft or hard aggregation, depending on the SiO₂ loading and agitation conditions. At low or moderate SiO₂ loading (<5–10%), the soft aggregation could be depressed with appropriate agitation conditions and redispersed with the aid of gradually established steric interaction energy. Consequently, well‐dispersed PLLA/SiO₂ nanocomposites with SiO₂ loadings of 5 and 10% were successfully prepared by in situ melt polycondensation with an arc stirrer at 400 and 600 rpm, respectively, during the D/O stage. © 2011 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     

Enhanced crystallization rate and mechanical properties of poly(l‐lactic acid) by stereocomplexation with four‐armed poly(ϵ‐caprolactone)‐block‐poly(d‐lactic acid) diblock copolymer

Poly(l ‐lactic acid) (PLLA) was blended with a series of four‐armed poly(? ‐caprolactone)‐block ‐poly(d ‐lactic acid) (4a‐PCL‐b ‐PDLA) copolymers in order to improve its crystallization rate and mechanical properties. It is found that a higher content of 4a‐PCL‐b ‐PDLA copolymer or longer PDLA block in the copolymer lead to faster crystallization of the blend, which is attributed to the formation of stereocomplex crystallites between PLLA matrix and PDLA blocks of the 4a‐PCL‐b ‐PDLA copolymers. Meanwhile, the PDLA block can improve the miscibility between flexible PCL phase and PLLA phase, which is beneficial for improving mechanical properties. The tensile results indicate that the 10% 4a‐PCL_5k‐b ‐PDLA_5k/PLLA blend has the largest elongation at break of about 72% because of the synergistic effects of stereocomplexation between enantiomeric PLAs, multi‐arm structure and plasticization of PCL blocks. It is concluded that well‐controlled composition and content of 4a‐PCL‐b ‐PDLA copolymer in PLLA blends can significantly improve the crystallization rate and mechanical properties of the PLLA matrix. © 2017 Society of Chemical Industry     

Stereocomplexation and morphology of enantiomeric poly(lactic acid)s with moderate‐molecular‐weight

The thermal behavior and spherulitic morphologies of poly(L ‐lactic acid) (PLLA)/poly(D ‐lactic acid) (PDLA) 1/1 blend with weight‐molecular‐weight of 10⁵ order, together with those of pure PLLA and PDLA, were investigated using differential scanning calorimetry and polarized optical microscopy. It was found that in the blend, stereocomplex crystallites could be formed exclusively or coexisted with homocrystallites depending on thermal history. Banded to nonbanded spherulitic morphological transition occurred for melt‐crystallized PLLA and PDLA, while the blend presented exclusively nonbanded spherulitic morphologies in the temperature range investigated. The spherulite growth of the blend occurred within a wider temperature range (≤180°C) compared with that of homopolymers (≤150°C), while the spherulite growth rates were comparable for both the blend and homopolymers. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Super‐Tough and Highly‐Ductile Poly(l‐lactic acid)/Thermoplastic Polyurethane/Epoxide‐Containing Ethylene Copolymer Blends Prepared by Reactive Blending

Ethylene‐methyl acrylate‐glycidyl methacrylate copolymer (E‐MA‐GMA) is employed to improve the impact toughness of poly(l ‐lactic acid) (PLLA)/thermoplastic polyurethane (TPU) blends by reactive melt‐blending. The reaction and miscibility between the components are confirmed by Fourier transform infrared spectroscopy, dynamic mechanical analysis, and differential scanning calorimetry. A super‐tough PLLA/TPU/E‐MA‐GMA multiphase blend (75/10/15) exhibits a significantly improved impact strength of 77.77 kJ m^?2, which is more than 17 times higher than that of PLLA/TPU (90/10) blend. A co‐continuous‐like TPU phase structure involving E‐MA‐GMA phase at the etched cryo‐fractured surface and the high‐orientated matrix deformation at the impact‐fractured surface are observed by scanning electron microscopy. The high‐orientated matrix deformation induced by the co‐continuous TPU phase structure is responsible for the super toughness of PLLA/TPU/E‐MA‐GMA blends.     

Non‐Isothermal Crystallization Behavior of Poly(L‐lactic acid) in the Presence of Various Additives

Summary: The effects of various additives: poly(D ‐lactic acid) (PDLA), talc, fullerene C₆₀, montmorillonite, and various polysaccharides, on the non‐isothermal crystallization behavior of poly(L ‐lactic acid) (PLLA), during both the heating of melt‐quenched films from room temperature, and the cooling of as‐cast films from the melt, was investigated. When the melt‐quenched PLLA films were heated from room temperature, the overall PLLA crystallization was accelerated upon addition of PDLA or the stereocomplex crystallites formed between PDLA and PLLA, the mixtures containing PDLA, and the mixture of talc and montmorillonite. No significant effects on the overall PLLA crystallization were observed for talc, C₆₀, montmorillonite, and the mixtures containing C₆₀. Such rapid completion of the overall PLLA crystallization upon addition of the aforementioned additives can be ascribed to the increased density (number per unit volume or area) of PLLA spherulites. When the as‐cast PLLA films were cooled from the melt, the overall PLLA crystallization completed rapidly, upon addition of PDLA, talc, C₆₀, montmorillonite, and their mixtures. Such rapid overall PLLA crystallization is attributable to the increased density of the PLLA spherulites and the higher nucleation temperature for PLLA crystallization. In contrast, the addition of various polysaccharides has no significant effect, or only a very small effect, on the overall PLLA crystallization during heating from room temperature or during cooling from the melt. This finding means that the polysaccharides can be utilized as low‐cost fillers for PLLA‐based materials, without disturbing the crystallization of the PLLA. The effect of additives in accelerating the overall PLLA crystallization during cooling from the melt, decreased in the following order: PDLA > talc > C₆₀ > montmorillonite > polysaccharides.

Polarization optical photomicrographs of pure PLLA, and the PLLA‐F film, with the fullerene additive, during cooling from the melt (Process IIB). Both of the photomicrographs were taken at 120 °C.     

Synthesis and characterization of poly(L‐lactic acid)‐b‐poly(ethylene terephthalate‐co‐sebacate)‐b‐poly(L‐lactic acid) copolyesters

Random copolyester namely, poly(ethylene terephthalate‐co‐sebacate) (PETS), with relatively lower molecular weight was first synthesized, and then it was used as a macromonomer to initiate ring‐opening polymerization of l ‐lactide. ¹H NMR quantified composition and structure of triblock copolyesters [poly(l ‐lactic acid)‐b‐poly(ethylene terephthalate‐co‐sebacate)‐b‐poly(l ‐lactic acid)] (PLLA‐PETS‐PLLA). Molecular weights of copolyesters were also estimated from NMR spectra, and confirmed by GPC. Copolyesters exhibited different solubilities according to the actual content of PLLA units in the main chain. Copolymerization effected melting behaviors significantly because of the incorporation of PETS and PLLA blocks. Crystalline morphology showed a special pattern for specimen with certain composition. It was obvious that copolyesters with more content of aromatic units of PET exhibited increased values in both of stress and modulus in tensile test. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

A comprehensive single‐particle model for solid‐state polymerization of poly(L‐lactic acid)

We propose here, a comprehensive model for the solid‐state polymerization (SSP) of a low to moderate molecular weight (MW) prepolymer of lactic acid, to produce high MW poly(L ‐lactic acid) (PLLA). The reactions are rationally assumed to occur only in the amorphous region, and effective concentrations of end groups, vary with crystalinity, X_c, during SSP. We estimate byproduct diffusivities, D, using free volume theory. The effects of various parameters on the SSP of PLLA prepolymer have been examined with respect to the optimum MW, X_c and D. We introduce self‐consistently, scaling factors of ～ 0.27, in the experimental procedure, to determine via ¹⁹F‐NMR, concentrations of the end groups, after converting them to fluorinated ester groups. The relevant reaction rate constants are obtained by fitting to early time data from representative SSP experiments at 150°C, under high vacuum, on PLLA prepolymer powder (i.e., spherical geometry) of number average MW, M_n0 ～ 10,200 Da, which attains M_n ～ 150,000 Da, via SSP. The subsequent successful comparison of the model predictions with experimental data throughout the entire SSP duration indicates that the model is comprehensive and accounts for all the relevant phenomena occurring during the SSP to synthesize high MW PLLA. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Peroxide induced cross‐linking by reactive melt processing of two biopolyesters: Poly(3‐hydroxybutyrate) and poly(l‐lactic acid) to improve their melting processability

Poly(3‐hydroxybutyrate) (PHB) and poly(l ‐lactic acid) (PLLA) were individually cross‐linked with dicumyl peroxide (DCP) (0.25–1 wt %) by reactive melt processing. The cross‐linked structures of the polymer gel were investigated by nuclear magnetic resonance (NMR) and Fourier transform infrared (FTIR) spectroscopies. The size of the polymer crystal spherulites, glass transition temperature (T_g), melting transition temperature (T_m), and crystallinity were all decreased as a result of cross‐linking. Cross‐linking density (ν_e) was shown to increase with DCP concentration. Based on parallel plate rheological study (dynamic and steady shear), elastic and viscous modulus (G″ and G′), complex viscosity (η^*) and steady shear viscosity (η) were all shown to increase with cross‐linking. Cross‐linked PHB and PLLA showed broader molar mass distribution and formation of long chain branching (LCB) as estimated by RheoMWD. Improvements in melt strength offer bioplastic processors improved material properties and processing options, such as foaming and thermoforming, for new applications. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41724.