Crystallization kinetics of poly(L‐lactide) in contact with pressurized CO₂

Abstract

The effect of CO2 on the isothermal crystallization kinetics of poly(L‐lactide), PLLA, was investigated using a high‐pressure differential scanning calorimeter (DSC), which can perform calorimetric measurements while keeping the sample polymer in contact with pressurized CO2. It was found that the crystallization rate followed the Avrami equation. However, the crystallization kinetic constant was changed depending upon the crystallization temperature and concentration of CO2 dissolved in the PLLA. The crystallization rate was accelerated by CO2 at the temperature in the crystal‐growth rate controlled region (self‐diffusion controlled region), and depressed in the nucleation‐controlled region. CO2 has also decreased the glass transition temperature, Tg, and the melting temperature, Tm. As a result, the CO2‐induced change in the crystallization rate can be predicted from the magnitudes of depression of both Tg and the equilibrium melting temperature. The crystalline structure and crystallinity of polymers crystallized in contact with pressurized CO2 were also investigated using a wide angle X‐ray diffractometer (WAXD). The resulting crystallinity of the sample was increased with the pressure level of CO2, although the presence of CO2 did not change the crystalline structure.

Biodegradable polylactide/hydroxyapatite nanocomposite foam scaffolds for bone tissue engineering applications

“…T sat was therefore decreased to 175°C and 165°C and P sat was varied between 100 and 250 bars. Although these T sat are below the nominal melting point of the as-received PLLA (about 180°C), they remain well above the melting point of PLLA in the presence of CO 2 at comparable pressures (about 130°C [41,42]). T sat and P sat were maintained for 10 min to allow adequate diffusion of the supercritical CO 2 into the molten PLLA.…”

“…The CCT curves were assumed to provide some indication of the crystallization rates during the foaming process, although one should bear in mind: (i) the possible plasticizing effect of residual dissolved CO 2 after depressurization [41,55,56]; (ii) that the temperatures measured inside the autoclave did not necessarily give a precise indication of the local temperatures within the PLA; (iii) the influence of deformation on the crystallization rates [55,57]. For the range of cooling rates investigated, i.e.…”

Carbon dioxide‐induced crystallization in poly(L‐lactic acid) and its effect on foam morphologies

“…Using CO 2 to prepare biodegradable polymer foams has been done for poly(lactic-co-glycolic acid), 24 -26 poly(ε-caprolactone), 27,28 poly(L-lactic acid) (PLLA), 11,26,27 poly(glycolic acid), 26,27 PLA, 29 PLA/silk composites 30 and PLA/layered silicate nanocomposites. 31,32 Studies have also demonstrated that CO 2 is able to depress T g and T m , 33,34 and induce crystallization 11 in PLLA. The type of morphologies produced by CO 2 not only depends on the degree of solubility and temperatures at which instability in the system is established, but also on the physical properties of polymers.…”

Effect of dissolved carbon dioxide on the glass transition and crystallization of poly(lactic acid) as probed by ultrasonic measurements

“…In a previous study on polypropylene (PP), 5 they observed that the dissolved CO 2 reduced the overall crystallization rate within the nucleation-dominated temperature region, that is, in the high T c range. In the case of PLA, Takada et al 6 demonstrated that the crystallization rate was accelerated by the presence of CO 2 molecules in the crystal-growth-controlled region, whereas it was depressed in the nucleation-controlled region. They also demonstrated that the crystallization rate followed the Avrami equation, with Avrami coefficient (n) values varying between 1.63 and 2, depending on the value of T c and the CO 2 pressure.…”