Poly(L-lactide). IX. Hydrolysis in acid media (original) (raw)
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Properties and morphology of poly(L-lactide). II. hydrolysis in alkaline solution
Journal of Polymer Science Part A: Polymer Chemistry, 1998
Hydrolysis of poly(L-lactide) (PLLA) films in 0.01N NaOH at 37ЊC was investigated by gel permeation chromatography, differential scanning calorimetry, scanning electron microscopy, and polarizing optical microscopy. The change in molecular weight distribution and surface morphology of PLLA films during hydrolysis revealed that PLLA film hydrolysis in dilute alkaline solution proceeded mainly via the surface erosion mechanism. An insignificant dependence of the rate of weight loss per unit surface area on the PLLA film thickness also supported this conclusion. Etching of the outside of PLLA spherulites resulted in preferred hydrolysis of PLLA chains in the amorphous region. The disorientation of lamella and inhomogeneous erosion in the spherulites implied that hydrolysis of PLLA chains occurred predominantly in the amorphous region between the crystalline regions in the spherulites. The rate of weight loss per unit surface area decreased linearly with the increase in the initial crystallinity of PLLA film, while the radius of spherulites had practically no significant effect on the hydrolysis of PLLA film. The specific low molecular weight of PLLA chains produced by hydrolysis increased with the rise in annealing temperature of the PLLA film, suggesting that the PLLA chains released were the component of one fold in the crystalline region.
Polymer, 2001
Isothermal and non-isothermal crystallization kinetics of poly-l-lactide (PLLA) were studied in order to analyse the effect of crystalline and amorphous morphology on hydrolytic degradation. A modi®cation of the nucleation/growing mechanism was observed in both isothermal and non-isothermal analysis. Samples isothermally crystallized at T c , 1108C showed a higher fraction of amorphous phase which does not relax at T g . Lower and decreasing values of this 'rigid-amorphous phase' fraction were detected at higher crystallization temperatures T c . 1108C: The effect of hydrolytic degradation on the amorphous and crystalline phases depends upon the initial morphology developed during the isothermal crystallization. The larger crystals developed at T c 1308C are more resistant to erosion compared to the less perfect and smaller crystals developed at higher levels of undercooling T c 908C: q
Journal of Applied Polymer Science, 2003
Attempts were carried out to enhance the surface hydrophilicity of poly(l-lactide), that is, poly(l-lactic acid) (PLLA) film, utilizing enzymatic, alkaline, and autocatalytic hydrolyses in a proteinase K/Tris-HCL buffered solution system (37°C), in a 0.01N NaOH solution (37°C), and in a phosphate-buffered solution (100°C), respectively. Moreover, its chain-scission mechanisms in these different media were studied. The advancing contact-angle ( a ) value of the amorphous-made PLLA film decreased monotonically with the hydrolysis time from 100°to 75°and 80°without a significant molecular weight decrease, when enzymatic and alkaline hydrolyses were continued for 60 min and 8 h, respectively. In contrast, a negligible change in the a value was observed for the PLLA films even after the autocatalytic hydrolysis was continured for 16 h, when their bulk M n decreased from 1.2 ϫ 10 5 to 2.2 ϫ 10 4 g mol Ϫ1 or the number of hydrophilic terminal groups per unit weight increased from 1.7 ϫ 10 Ϫ5 to 9.1 ϫ 10 Ϫ5 mol g Ϫ1 . These findings, together with the result of gravimetry, revealed that the enzymatic and alkaline hydrolyses are powerful enough to enhance the practical surface hydrophilicity of the PLLA films because of their surface-erosion mechanisms and that its practical surface hydrophilicity is controllable by varying the hydrolysis time. Moreover, autocatalytic hydrolysis is inappropriate to enhance the surface hydrophilicity, because of its bulk-erosion mechanism. Alkaline hydrolysis is the best to enhance the hydrophilicity of the PLLA films without hydrolysis of the film cores, while the enzymatic hydrolysis is appropriate and inappropriate to enhance the surface hydrophilicity of bulky and thin PLLA materials, respectively, because a significant weight loss occurs before saturation of a value. The changes in the weight loss and a values during hydrolysis showed that exo chain scission as well as endo chain scission occurs in the presence of proteinase K, while in the alkaline and phosphate-buffered solutions, hydrolysis proceeds via endo chain scission.
Polymer Degradation and Stability, 2000
Hydrolysis of poly(l-lactide) (PLLA) ®lms with dierent crystallinities (x c 's), crystalline thicknesses (L c 's), and spherulite sizes, was studied in phosphate-buered solution at 37 C. The ®lms were prepared by altering the annealing or crystallization temperatures (T a 's) from the melt without or with quenching before annealing. The initial x c and L c of PLLA ®lms increased as they were prepared at higher T a . The induction period until the start of a decrease of mass and tensile strength by hydrolysis became shorter as the initial x c of the PLLA ®lms was higher, when compared at the same hydrolysis time. This indicates that PLLA ®lms with higher initial x c underwent faster hydrolysis. This result could be explained in terms of the density of eective tie chains which should decrease with an increase in x c . It also seemed likely that higher x c and hence higher L c produced more defects in the amorphous region, which promoted hydrolysis by enhancing water diusion. The radius of spherulites had an insigni®cant eect on the hydrolysis of PLLA ®lms in phosphate-buered solution. A speci®c peak appeared at a low molecular weight around 1Â10 4 in gel permeation chromatography spectra of hydrolyzed PLLA ®lm, irrespective of the thermal history of the PLLA ®lm. This suggested that the observed speci®c peak was ascribed to the component of one-fold in the crystalline region. The relationship between the melting temperature (T m ) and L c was found to be T m u 4711 À 1X59aL nm for PLLA ®lm hydrolyzed for 3 years. #
Thermally induced crystallization and enzymatic degradation studies of poly ( L -lactic acid) films
Journal of Applied Polymer Science, 2013
Poly(L-lactic acid) (PLLA) films with different crystallinities were prepared by solvent casting and subsequently annealed at various temperatures (T a) (80-110 C). The effects of crystallinity on enzymatic degradation of PLLA films were examined in the presence of proteinase K at 37 C by means of weight loss, DSC, FTIR spectroscopy, and optical microscopy. DSC and the absorbance ratio of 921 and 956 cm À1 (A 921 /A 956) were used to evaluate crystallinity changes during thermally induced crystallization and enzymatic hydrolysis. The highest percentage of weight loss was observed for the film with the lowest initial crystallinity and the lowest percentage of weight loss was observed for the film with highest crystallinity. FTIR investigation of degraded films showed a band at 922 cm À1 and no band at 908 cm À1 suggested that all degraded samples form a crystals. The rate of degradation was found to depend on the initial crystallinity of PLLA film and shown that enzymatic degradation kinetics followed first-order kinetics for a given enzyme concentration. DSC crystallinity and IR absorbance ratio, A 921 /A 956 ratio, showed no significant changes with degradation time for annealed PLLA films whereas as-cast PLLA film showed an increase in crystallinity with degradation; this revealed that degradation takes place predominantly in the free amorphous region of annealed PLLA films without changing long range and short range order V
Polymer, 2002
Poly(dl-lactide), i.e., poly(dl-lactic acid) (PDLLA), poly(l-lactide), i.e. poly(l-lactic acid) (PLLA), and poly(d-lactide), i.e., poly(d-lactic acid) (PDLA) were synthesized to have similar molecular weights. The non-blended PDLLA, PLLA, and PDLA films and PLLA/PDLA(1/1) blend film were prepared to be amorphous state, and the effects of l-lactide unit content, tacticity, and enantiomeric polymer blending on their autocatalytic hydrolysis were investigated in phosphate-buffered solution (pH7.4) at 37 °C for up to 24 months. The results of gravimetry, gel permeation chromatography (GPC), and tensile testing showed that the autocatalytic hydrolyzabilities of polylactides, i.e. poly(lactic acid)s (PLAs) in the amorphous state decreased in the following order: nonblended PDLLA>nonblended PLLA, nonblended PDLA>PLLA/PDLA(1/1) blend. The high hydrolyzability of the nonblended PDLLA film compared with those of the nonblended PLLA and PDLA films was ascribed to the lower tacticity of PDLLA chains, which decreases their intramolecular interaction and therefore the PDLLA chains are susceptible to the attack from water molecules. In contrast, the retarded hydrolysis of PLLA/PDLA(1/1) blend film compared with those of the nonblended PLLA and PDLA films was attributable to the peculiar strong interaction between PLLA and PDLA chains in the blend film, resulting in the disturbed interaction of PLLA or PDLA chains and water molecules. The X-ray diffractometry and differential scanning calorimetry (DSC) elucidated that all the initially amorphous PLA films remained amorphous even after the autocatalytic hydrolysis for 16 (PDLLA film) and 24 [nonblended PLLA and PDLA films, PLLA/PDLA(1/1) blend film] months and that the melting peaks observed at around 170 and 220 °C for the PLLA/PDLA(1/1) blend film after the hydrolysis for 24 months were ascribed to those of homo- and stereocomplex crystallites, respectively, formed during heating at around 100 and 200 °C but not during the autocatalytic hydrolysis.
Polymer, 2001
Poly(l-lactide) (PLLA) ®lms having different initial crystallinities (x c ) (0±57%) and a ®xed crystalline thickness were prepared by annealing the melt at a ®xed temperature for different times. Their enzymatic hydrolysis was investigated in the presence of Proteinase K w . The rate of weight loss decreased rapidly and slowly with an increase in the initial x c for x c below and above 33%, respectively, where the free and the restricted amorphous regions, respectively, are the major amorphous components in the PLLA ®lms. This is ascribed to the higher hydrolysis-resistance of the PLLA chains in the restricted amorphous region than that in the free amorphous region. Gel permeation chromatography (GPC) results revealed that in the restricted amorphous region the folding chains are much more hydrolysis-resistant than the tie chains and the chains with free ends. The increased x c during the enzymatic hydrolysis is due to the preferential hydrolysis and removal of the amorphous regions, but not to the crystallization of the amorphous regions. q
Journal of Applied Polymer Science, 2012
Amorphous and crystallized poly(L-lactic acid) (PLLA) films were prepared and the hydrolytic degradation of the ultraviolet (UV)-treated and UV-nontreated films was investigated. This study reveals that the combination of UV and thermal treatments can produce the PLLA materials having different hydrolytic degradation profiles and that the UV-irradiation in the environment will affect the design of recycling process for PLLA articles. In an early stage, the degrees of hydrolytic degradation monitored by weight loss (W loss ), number-average molecular weight (M n ), and melting temperature (T m ) were higher for the UV-treated films than for the UV-nontreated films. In a late stage, the trend traced by W loss was reversed, and the difference in the degrees of hydrolytic degradation between the UV-treated and UV-nontreated films monitored by M n and T m became smaller, with the exception of the degrees of hydrolytic degradation of the amorphous films traced by T m . Also, in the early stage, the degrees of hydrolytic degradation monitored by W loss and M n were higher for the crystallized films than for the amorphous films. In the late stage, this trend was reversed, with the exception of the degrees of hydrolytic degradation of the UV-treated films monitored by M n . The main factors that determined the W loss and T m were the molecular weight and initial crystallinty but not the molecular structures such as terminal C¼ ¼C double bonds and crosslinks.
Polymer, 2002
Poly(dl-lactide), i.e., poly(dl-lactic acid) (PDLLA), poly(l-lactide), i.e. poly(l-lactic acid) (PLLA), and poly(d-lactide), i.e., poly(d-lactic acid) (PDLA) were synthesized to have similar molecular weights. The non-blended PDLLA, PLLA, and PDLA ®lms and PLLA/PDLA(1/1) blend ®lm were prepared to be amorphous state, and the effects of l-lactide unit content, tacticity, and enantiomeric polymer blending on their autocatalytic hydrolysis were investigated in phosphate-buffered solution (pH7.4) at 37 8C for up to 24 months. The results of gravimetry, gel permeation chromatography (GPC), and tensile testing showed that the autocatalytic hydrolyzabilities of polylactides, i.e. poly(lactic acid)s (PLAs) in the amorphous state decreased in the following order: nonblended PDLLA . nonblended PLLA, nonblended PDLA . PLLA/PDLA(1/1) blend. The high hydrolyzability of the nonblended PDLLA ®lm compared with those of the nonblended PLLA and PDLA ®lms was ascribed to the lower tacticity of PDLLA chains, which decreases their intramolecular interaction and therefore the PDLLA chains are susceptible to the attack from water molecules. In contrast, the retarded hydrolysis of PLLA/PDLA(1/1) blend ®lm compared with those of the nonblended PLLA and PDLA ®lms was attributable to the peculiar strong interaction between PLLA and PDLA chains in the blend ®lm, resulting in the disturbed interaction of PLLA or PDLA chains and water molecules. The X-ray diffractometry and differential scanning calorimetry (DSC) elucidated that all the initially amorphous PLA ®lms remained amorphous even after the autocatalytic hydrolysis for 16 (PDLLA ®lm) and 24 [nonblended PLLA and PDLA ®lms, PLLA/PDLA(1/1) blend ®lm] months and that the melting peaks observed at around 170 and 220 8C for the PLLA/PDLA(1/1) blend ®lm after the hydrolysis for 24 months were ascribed to those of homo-and stereocomplex crystallites, respectively, formed during heating at around 100 and 200 8C but not during the autocatalytic hydrolysis. q