Stretch‐induced molecular ordering in amorphous/crystalline polylactide blends

The effect of amorphous poly(D ,L ‐lactide) (PDLLA) on the molecular orientation and crystallization of crystalline poly(L ‐lactide) (PLLA) on stretching is reported in this study. It is indicated that the presence of PDLLA in its miscible blends with PLLA is not favorable for either cohesive mesophase formation below T_g or strain‐induced crystallization above T_g at strains beyond the segmental extensibility limit. Because of lack of constraints imposed by cohesive mesophase or crystals, oriented segments are liable to slide each other or recoil, responsible for low‐molecular orientation in the stretched blends. POLYM. ENG. SCI., 2013. © 2012 Society of Plastics Engineers     

Mesomorphic phase formation of plasticized poly(l‐lactic acid)

The effects of the crystallization temperature, T_c, on the crystal structure as well as its thermal behavior of plasticized poly(l ‐lactic acid) were investigated by means of wide‐angle X‐ray diffraction (WAXD), Fourier‐transform infrared spectroscopy (FTIR), and differential scanning calorimetry (DSC). PLLA blended with succinic acid‐bis[2‐[2‐(2‐methoxyethoxy)ethoxy]ethyl] ester (SAE) showed clear difference in T_c dependence of crystalline form compared to PLLA homopolymer. PLLA with 26 wt % SAE crystallized into orthorhombic α form for T_c above 80°C, while a peculiar disordered structure (mesophase) was obtained for T_c at 40°C. A detailed FTIR analysis of the mesophase of PLLA, focusing on the intra‐ and inter‐chain interaction in the structure, indicated that mesophase had a large degree of disorder in 10/3 helical conformation as well as its packing manner of disordered 10/3 helical chain. Upon heating, mesophase showed a steep exothermic peak at 80°C in DSC thermogram, indicating the phase transformation from mesophase to a form crystal. FTIR results showed that the degree of interchain interaction of C=O in PLLA started to decrease above 60°C, followed by steep increase at 80°C due to the recrystallization into a form. Melt‐recrystallization process in mesophase‐α transformation was clarified. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 39762.     

Glass transition and mechanical properties of PLLA and PDLLA‐PGA copolymer blends

The change of the glass transition temperatures (T_g) in the blend of poly(L ‐lactic acid) (PLLA) and the copolymers of poly(D,L ‐lactic acid) and poly(glycolic acid) (PDLLA‐PGA) with different D,L ‐lactic acid and glycolic acid composition ratio (50 : 50, 65 : 35, and 75 : 25) was studied by DSC. Dynamic mechanical measurement and tensile testing were performed at various temperatures around T_g of the blend. In the blend of PLLA and PDLLA‐PGA50 (composition ratio of PDLLA and PGA 50 : 50), T_g decreased from that of PLLA (about 58°C) to that of PDLLA‐PGA50 (about 30°C). A single step decrease was observed in the DSC curve around T_g between the weight fraction of PLLA (W(PLLA)) 1.0 and 0.7 (about 52°C) but two‐step changes in the curve are observed between W(PLLA) = 0.6 and 0.3. The T_g change between that of PLLA and that of PDLLA‐PGA and the appearance of two T_gs suggest the existence of PLLA rich amorphous region and PDLLA‐PGA copolymer rich amorphous region in the blend. A single step decrease of E′ occurs at around T_g of the pure PLLA but the two‐step decrease was observed at W(PLLA) = 0.6 and 0.4, supporting the existence of the PLLA rich region and PDLLA‐PGA rich region. Tensile testing for various blends at elevated temperature showed that the extension without yielding occurred above T_g of the blend. Partial miscibility is suggested for PLLA and PDLLA‐PGA copolymer blends. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 2164–2173, 2004     

Photodegradation of Poly(L‐lactic acid): Effects of Photosensitizer

Summary: Amorphous and crystallized poly(L ‐lactic acid) (PLLA‐A and PLLA‐C, respectively) films with different contents of N,N,N′,N′‐tetramethyl‐1,4‐phenylenediamine (TMPD) as a photosensitizer were prepared, and the effects of the addition of TMPD on the photodegradation of PLLA films were investigated. It was found that the addition of TMPD effectively enhanced the photodegradation of PLLA films and thereby decreased their molecular weight of PLLA films regardless of their crystallinity, and that PLLA films with different molecular weights can be prepared by the addition of different amounts of TMPD and subsequent UV irradiation. Too high contents of TMPD however caused the brittleness of PLLA films due to a large decrease in molecular weight. The PLLA chains in crystalline regions as well as those in amorphous regions are photodegradable even at an early stage, in marked contrast to their hydrolytic degradation, where the chains in the amorphous regions are selectively degraded. The basic changes in glass transition, cold crystallization, and melting temperatures (T_g, T_cc, and T_m, respectively) of PLLA films during UV irradiation can be ascribed to low‐temperature annealing effects; i.e., annealing‐induced stabilization in chain packing should have elevated T_g, and annealing‐induced formation of crystallite nuclei should have lowered T_cc and increased T_m. The exceptional large decreases in T_cc and T_m of UV‐irradiated PLLA‐A films and in T_g of UV‐irradiated PLLA‐C films at high TMPD contents are attributable to the large decrease in molecular weight, whereas the exceptional decrease in T_m of PLLA‐C films at high TMPD contents can be due to the folding surface structural change of crystalline regions or to the lattice disorder caused by molecular structural changes.

of PLLA‐A films before UV irradiation and after UV irradiation for 60 h as a function of TMPD content.     

Development of novel biopolymer blends based on poly(l‐lactic acid), poly((R)‐3‐hydroxybutyrate), and plasticizer

Poly(l ‐lactic acid) (PLLA), a biopolymer that can be derived from renewable resources, is known for its brittleness as a result of slow crystallization rates under supercooling conditions, which is associated with the formation of large spherulites. In addition, the glass transition temperature (T_g) of PLLA is 60°C, such that the polymer chain is immobile at room temperature. These disadvantages make PLLA unsuitable for use in the food packaging sector. In this research, biopolymer blends based on PLLA and poly((R)‐3‐hydroxybutyrate) (PHB), together with tributyl citrate (TBC) as a plasticizer, were developed. TBC was added to increase polymer chain mobility, and to decrease PLLA T_g from 60 to 10°C in blends. Furthermore, the addition of PHB as a nucleating agent to PLLA resulted in an increased proportion of smaller spherulites. Fourier‐transform infrared (FT‐IR) spectroscopy indicated that the carbonyl group and several other characteristic peaks in blends are shifted to lower wavenumber. In addition, polarized optical microscopy experiments confirmed the results of differential scanning calorimetry, FT‐IR, and wide‐angle X‐ray diffraction, showing that PHB enhances the crystallization behavior by acting as a bionucleation. POLYM. ENG. SCI., 54:1394–1402, 2014. © 2013 Society of Plastics Engineers     

Poly(L‐Lactide), 8. High‐Temperature Hydrolysis of Poly(L‐Lactide) Films with Different Crystallinities and Crystalline Thicknesses in Phosphate‐Buffered Solution

Poly(L ‐lactide) (PLLA) films having different crystallinities (X_c's) and crystalline thicknesses (L_c's) were prepared by annealing at different temperatures (T_a's) from the melt and their high‐temperature hydrolysis was investigated at 97°C in phosphate‐buffered solution. The changes in remaining weight, molecular weight distribution, and surface morphology of the PLLA films during hydrolysis revealed that their hydrolysis at the high temperature in phosphate‐buffered solution proceeds homogeneously along the film cross‐section mainly via the bulk erosion mechanism and that the hydrolysis takes place predominantly and randomly at the chains in the amorphous region. The remaining weight was higher for the PLLA films having high initial X_c when compared at the same hydrolysis time above 30 h. However, the difference in the hydrolysis rate between the initially amorphous and crystallized PLLA films at 97°C was smaller than that at 37°C, due to rapid crystallization of the initially amorphous PLLA film by exposure to crystallizable high temperature in phosphate‐buffered solution. The hydrolysis constant (k) values of the films at 97°C for the period of 0–8 h, 0.059–0.085 h^–1 (1.4–2.0 d^–1), were three orders of magnitude higher than those at 37°C for the period of 0–12 months, 2.2–3.4×10^–3 d^–1. The melting temperature (T_m) and X_c of the PLLA films decreased and increased, respectively, monotonously with hydrolysis time, excluding the initial increase in T_m for the PLLA films prepared at T_a = 100, 120, and 140°C in the first 8, 16, and 16 h, respectively. A specific peak that appeared at a low molecular weight around 1×10⁴ in the GPC spectra was ascribed to the component of one fold in the crystalline region. The relationship between T_m and L_c was found to be T_m (K) = 467·[1–1.61/L_c (nm)] for the PLLA films hydrolyzed at 97°C for 40 h.     

Synergistic effects of N,N′‐bis (benzoyl) sebacic acid dihydrazide and talc on the physical and mechanical behaviors of poly(l‐latic acid)

The important practical problem of poor heat stability of poly(l ‐lactic acid) (PLLA) is addressed by the addition of N, N′‐bis (benzoyl) sebacic acid dihydrazide (BSAD) and talc as a nucleating agent system. The idea of incorporating talc into the PLLA/BSAD composites is that talc can provide supplementary nucleation effect with very small amount of BSAD (0.2 wt %) and therefore can improve the heat deflection resistance of PLLA materials. Effects of BSAD/talc on morphology, crystallization behavior, heat resistance, and mechanical properties of PLLA/BSAD/talc were investigated after annealing processes. The results indicated that the BSAD/talc system increased the crystallinity from 6.0% of pure PLLA to a maximum 42.9% by the synergistic effects of BSAD and talc increasing the growth of spherulites and nucleation density, respectively. After annealing at different temperatures, the heat deflection temperature (HDT) of PLLA was improved dramatically due to synergistic effects of BSAD/talc between restricted chain movement and acceleration of crystallization. At high temperature (above T_g), the thermo‐mechanical properties of PLLA is mainly determined by the crystallinity and the reinforcement effect of talc acted as a filler. Moreover, effects of BSAD/talc on mechanical properties were discussed. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41454.     

Plasticization of Poly‐L‐lactide with L‐lactide,D‐lactide,and D,L‐lactide monomers

Poly‐L ‐lactide (PLLA) is being widely considered for repair of damaged tissues, for controlled antibiotic release, and also as scaffolds for cultured cells. PLLA was blended with the lactide monomer in its two enantiomeric forms: D ‐lactide (D ‐la) and L ‐lactide (L ‐la) and with the cyclic dimmer D ,L ‐la, in order to enhance its flexibility and thereby overcome its inherent problem of brittleness. In this work, the crystallization, phase structure, and tensile properties of PLLA and PLLA plasticized with 5, 10, 15, and 20 wt% of D ‐la, L ‐la, and D ,L ‐la are explored. The three plasticizers used were effective in lowering the glass transition temperature (T_g) and the melting temperature (T_m) of PLLA, around 20°C for a plasticizer content of 20 wt%. The tensile strength and modulus of the blends decreased following the increasing content of plasticizers from approximately 58 MPa to values below 20 MPa, and from 1667 to 200 MPa, respectively. Aging the blends at storage ambient temperature revealed that the enhanced flexibility as well as the morphological stability was lost over time due to the migration of the plasticizer to the surface, this being less marked in the case of D ‐la as a result of interactions between the polymer and its enantiomeric monomer of complementary configuration. POLYM. ENG. SCI., 53:2073–2080, 2013. © 2013 Society of Plastics Engineers     

The glass‐forming ability and thermal stability of novolac–phenolic resin

Glass‐forming ability and thermal stability of novolac–phenolic resin with different contents of curing agent hexamethylenetramine (HMTA) are investigated. Melt fragility (M_g) and T_gd (T_gd = T_g/T_d) are proposed to characterize glass‐forming ability and thermal stability of the novolac–phenolic resin, respectively. It is found that M_g has a negative linear relationship with T_gd. The lower M_g represents the better glass‐forming ability of liquid phenolic resin while the higher T_gd represents the better thermal stability of the fully cured phenolic resin. Further more, M_g establishes a kinetic relationship between glass‐forming ability of liquid uncured and thermal stability of fully cured novolac–phenolic resin in terms of comparing with T_gd. POLYM. COMPOS., 2012. © 2011 Society of Plastics Engineers     

Compatibility of poly(butadiene‐co‐acrylonitrile) with poly(L‐lactide) and poly(3‐hydroxybutyrate‐co‐3‐hydroxyvalerate)

Poly(L ‐lactide) (PLLA) and poly(3‐hydrobutyrate‐co‐3‐hydroxyvalerate) (PHBV) were blended with poly(butadiene‐co‐acrylonitrile) (NBR). Both PLLA/NBR and PHBV/NBR blends exhibited higher tensile properties as the content of acrylonitrile unit (AN) of NBR increased from 22 to 50 wt %. However, two separate glass transition temperatures (T_g) appeared in PLLA/NBR blends irrespective of the content of NBR, revealing that PLLA was incompatible with NBR. In contrast, a single T_g, which shifted along with the blend composition, was observed for PHBV/NBR50 blends. Moreover NBR50 suppressed the crystallization of PHBV, indicating that PHBV was compatible with NBR50. Decrease of both elongation modulus and stress at maximum load was less significant and increase of elongation at break was more pronounced in PHBV/NBR50 blends than in PLLA/NBR50 blends. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 3508–3513, 2004     

Effect of low‐temperature annealing on dimensional stability of poly(ethylene 2,6‐naphthalene dicarboxylate) fibers melt‐spun at high speed

To investigate structural factors, necessary to obtain a valuable industrial fiber possessing excellent thermomechanical properties, poly(ethylene 2,6‐naphthalene dicarboxylate) (PEN) fibers were produced by high‐speed melt‐spinning to a take‐up speed of 8 km/min, followed by low‐temperature annealing between the glass‐transition temperature (T_g) and exothermic cold crystallization temperature (T_{c cold}), where little transition of crystalline phase, as well as little thermal degradation, takes place. Their thermomechanical behavior, as well as structural variations, were investigated through differential scanning calorimetry, Rheovibron, thermomechanical analysis (TMA), and tensile testing. Two types of the α‐ and α′‐dispersions were observed at near T_g and at a temperature 50–60°C higher than T_g, respectively. The dispersions were affected by rearranged structures, which are generated by developing an inhomogeneous taut structure with rigidity of aromatic segment and aliphatic segment. The α‐dispersion seemed to reflect an inhomogeneous taut structure by the less nearly arranged segments. Consequently, at intermediate take‐up speeds between 2 and 6 km/min the inhomogeneous taut structure may be partially formed, but the homogeneously ordered structure may be enlarged as the take‐up speed and annealing temperature increased. Thermal shrinkage increased above the α‐dispersion temperature, which suggested that the onset point of dimensional change in PEN fibers was attributed to α‐dispersion. In the case of annealed fibers, the start of length change coincided with the respective annealing temperatures, which indicated that dimensional stability could be gained from restraining the inhomogeneous taut structure in the amorphous region without the transition of crystalline phase by annealing between T_g and T_{c cold}. Therefore, to obtain dimensional stability in PEN fibers, it is supposed that the inhomogeneous taut structure exhibited by the α‐dispersion should be controlled. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 95: 212–218, 2005     

The role of polycarbonate molecular weight in the poly(L‐lactide) blends compatibilized with poly(butylene succinate‐co‐L‐lactate)

Poly(butylene succinate‐co‐L ‐lactate) (PBSL)–compatibilized poly(L ‐lactide) (PLLA) polymer blends with two commercial grades of polycarbonate (PC) were investigated. The capillary tests showed that the steady shear viscosity of high molecular weight PC (PC‐L) was 10 times higher than that of low molecular weight PC (PC‐AD) throughout the shear rate range under investigation. Morphologic examination revealed that the shape of the dispersed PC‐L phase in the as‐extruded blends was largely spherical, but the PC‐AD phase was more like a rod and elongated further during injection molding. Notched Izod impact strength (IS) of the unmodified PLLA/PC‐L blend was higher than that of PC‐AD blend. The IS of modified ternary blends increased with PBSL content because of enhanced phase interaction indicated from thermal and morphologic analysis. The PBSL modification also enhanced IS more significantly in PLLA/PC‐L than in PLLA/PC‐AD blends. On the contrary, the heat deflection temperature (HDT) of PLLA/PC‐L binary system was much lower than that of PLLA/PC‐AD. HDT of PBSL‐modified PLLA/PC‐AD blends dropped with increasing PBSL content, which is a ductile polymer. Thermal and dynamic mechanical analysis of the ternary blends showed that individual components were immiscible with distinct T_gs for PC and PLLA and distinct T_ms for PBSL and PLLA. POLYM. ENG. SCI., 2013. © 2012 Society of Plastics Engineers     

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.     

Accelerated crystallization of poly(L‐lactide) by physical aging

The low‐temperature physical aging of amorphous poly(L ‐lactide) (PLLA) at 25–50°C below glass transition temperature (T_g) was carried out for 90 days. The physical aging significantly increased the T_g and glass transition enthalpy, but did not cause crystallization, regardless of aging temperature. The nonisothermal crystallization of PLLA during heating was accelerated only by physical aging at 50°C. These results indicate that the structure formed by physical aging only at 50°C induced the accelerated crystallization of PLLA during heating, whereas the structure formed by physical aging at 25 and 37°C had a negligible effect on the crystallization of PLLA during heating, except when the physical aging at 37°C was continued for the period as long as 90 days. The mechanism for the accelerated crystallization of PLLA by physical aging is discussed. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

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     

Improvement of heat stability of poly(l-lactic acid) by radiation-induced crosslinking

Poly(l-lactic acid) (PLLA) has poor heat stability above its glass transition temperature (T_g∼60 °C). To improve its softing above T_g, PLLA was mixed with small amount of crosslinking agents and irradiated with various irradiation doses to introduce crosslinking between polymer chains. The most effective agent for radiation crosslinking was triallyl isocyanurate (TAIC). For melt-quenched PLLA, it was found that the most optimal conditions to introduce crosslinking were around 3% of TAIC and the irradiation dose of 50 kGy. The typically crosslinked PLLA showed very low crystallinity because of wide formation of molecular chain network that inhibited molecular motion for crystallization. Notable heat stability above T_g was given by annealing of PLLA samples. Enzymatic degradation of PLLA was retarded with introduction of crosslinks.     

Flexible all‐aromatic polyesterimide films with high glass transition temperatures

A series all‐aromatic poly(esterimide)s with different molar ratios of N‐(3′‐hydroxyphenyl)‐trimellitimide (IM) and 4‐hydroxybenzoic acid (HBA) (IM/HBA = 0.3/0.7 and 0.7/0.3) was prepared with the aim to design flexible high T_g films. Melt‐pressed films, either from high molecular weight polymer or cured phenylethynyl precursor oligomers, exhibit T_gs in the range of 200 °C to 242 °C and are brittle. After a thermal stretching procedure, the films became remarkably flexible and very easy to handle. In addition, the thermally stretched 3‐IM/7‐HBA and 7‐IM/3‐HBA films show tensile strengths of 108 MPa and 169 MPa, respectively. Thermal treatment increased the T_g of 3‐IM/7‐HBA from 205 °C to 248 °C, whereas the T_g of 7‐IM/3‐HBA increased from 230 °C to 260 °C. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 133, 44774.     

The crystallization of a copoly(chlorotrifluorethylene-vinylidene fluoride) from the amorphous rubbery state

A random copoly(chlorotrifluorethylene-vinylidene fluoride) in the ratio of 3 : 1 was annealed at the temperature range of T_g < T < T_m. The copolymer slowly crystallizes, attaining a rather low ultimate degree of crystallinity, depending on the annealing temperature, in the form of randomly distributed ribbonlike lamellae. The crystallites' melting temperatures are much lower than those of the corresponding homopolymers, increasing with annealing temperature and time. The crystallization kinetics, analyzed using the Avrami equation, indicates the formation of small, low-ordered crystallites. The crystallization process results in a dramatic increase in the elastic modulus at T > T_g. Annealing of stretched samples results in oriented crystallization at much higher rates than in the unstretched material, without markedly affecting the ultimate degree of crystallinity. The oriented crystallites, distributed in an isotropic amorphous matrix, exhibit lower thermal stability than the corresponding unoriented crystals. Their melting temperatures increase with annealing time; however, they decrease with the extent of stretching, suggesting a strong kinetic effect on the crystallites' degree of order.     

Poly(L‐lactide): v. effects of storage in swelling solvents on physical properties and structure of poly(L‐lactide)

The effects of storage at 25°C in swelling solvents having different solubility parameter (δ_s) values of 16.8–26.0 J^0.5 cm^−1.5 on the physical properties and structure of as‐cast poly(L ‐lactide) (PLLA) films was investigated by the degree of swelling (DS), differential scanning calorimetry (DSC), and tensile tests. It was found that PLLA film shows durabity to swelling solvents having δ_s values much lower or higher than the value range of 19–20.5 J^0.5 cm^−1.5 and that the polymer solubility parameter (δ_p) for PLLA is in the value range of 19–20.5 J^0.5 cm^−1.5. The decrease in the glass transition temperature (T_g) and tensile properties and the increase in melting temperature (T_m) and crystallinity (x_c) were larger for PLLA films swollen in solvents having a high DS at 7 days (DS_7days). The slight increase in T_m and x_c for PLLA films after swelling in solvents with high DS_7days values was due to the crystallization of PLLA that occurred during swelling, while the small increase in T_g and elongation at break (ε_B) for PLLA films after immersion in the solvents having low DS_7days values was ascribed to stabilized chain packing in the amorphous region. The T_g, ε_B, and Young's modulus of the PLLA films after swelling in the solvents varied in the ranges of 47–57°C, 4–8%, and 55–77 kg/mm², depending on their DS_7days or δ_s values. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 79: 1582–1589, 2001     

Thermal behaviour of poly(lactic acid) in contact with compressed carbon dioxide

BACKGROUND: The thermal behaviour of poly(lactic acid) (PLA) in contact with compressed CO₂ was studied using high‐pressure differential scanning calorimetry. In particular, the effect of annealing below and above the glass transition temperature (T_g) on the glass transition, cold crystallization and melting temperatures was studied systematically as a function of annealing time and pressure. RESULTS: The effect of compressed CO₂ on the thermal properties of PLA is time dependent. Annealing below T_g decreases the temperature and enthalpy of cold crystallization. Similar, but more evident, behaviours are observed when annealing above T_g. Crystallization temperature and enthalpy during cooling decrease with increasing pressure, and the peak is narrower. CONCLUSION: Annealing PLA in the presence of compressed CO₂ accelerates cold crystallization, but retards crystallization during cooling. Copyright © 2009 Society of Chemical Industry