Phase transitions during membrane formation of polylactides. I. A morphological study of membranes obtained from the system polylactide-chloroform-methanol

Abstract

The influence of solid-liquid demixing, liquid-liquid demixing and vitrification on the morphology of polylactide membranes has been investigated. To study the effects of crystallization of polylactides on the membrane formation and morphology, polylactides of varying stereoregularity were used. The polymers applied were poly-L-lactide (PLLA) and copolymers with different molar ratios of L-lactide and I>lactide [poly-L95/D5-1actide (PLA95), poly-L80/D20-1actide (PLA80) and poly-L50/D50-1actide (PDLLA) ]. Solutions of polylactides in chloroform cast on a glass plate were immersed in methanol. From solutions containing the slowly crystallizing PLA80 or uncrystallizable PDLLA porous membranes were obtained if the phase separated system was removed from the nonsolvent bath within a few hours after immersion. After longer equilibration times in methanol the structure collapsed. The swelling in the nonsolvent methanol was too high to allow stabilization of the liquidliquid demixed structure by vitrification. Stable membranes were easily obtained with more rapidly crystallizing polymers like PLLA. Casting solutions with low PLLA concentrations gave membranes with a cellular morphology due to liquid-liquid demixing by nucleation and growth of a polymer poor phase. Crystallization only played a role in the fixation of the liquidliquid demixed structure. At increasing PLLA concentrations the demixing sequence gradually reversed to crystallization followed by liquid-liquid demixing. In these cases membranes with porous spherulites or spherulites surrounded with a cellular layer were obtained.

Generation of microparticles using CO2 and CO2‐philic antisolvents

“…This is consistent with observation that, although crystallization may be thermodynamically favorable, the kinetically favored liquid᎐liquid demixing process often dominates the resulting polymer morphology, even for rapidly crystallizing polymers such as L-PLA Ž . van de Witte et al, 1996 . However, the precipitation mechanism is difficult to evaluate directly from crystallinity because of the potential for solvent or CO -induced crystalliza-2 Ž .…”

Polylactide films formed by immersion precipitation: Effects of additives, nonsolvent, and temperature

“…If neither of these transitions occurs, liquid-liquid demixing will proceed until two completely separated layers are obtained. 11 Since PDLLA cannot crystallize, 11,22 and its glass transition line does not cross the binodal for either methanol or water (this Table I. is illustrated in Fig. 4), it will not be solidified but will collapse given sufficient time.…”

“…For a film made out of 5% w/w PDLLA/chloroform, which was immersed and kept in methanol for 1 day or longer, Van de Witte et al 11 found that no structure was preserved. Comparison with our results shows that increasing the polymer concentration reduces the time required for phase separation, and results in faster loss of structure.…”

“…11 This can take place via solid-liquid demixing (i.e., crystallization), or via glassification. If neither of these transitions occurs, liquid-liquid demixing will proceed until two completely separated layers are obtained.…”

“…PDLLA is a random copolymer that cannot crystallize and thus is either in the rubbery or in the glassy state, while PLLA is in optically pure form and crystallizes readily. 11,22,23 The effects of nonsolvent quality and PLA crystallinity on the resulting film morphology were studied separately. Unlike in most studies, in which additives are used, which are soluble in the nonsolvent, 9,12,18 we have used dodecane as an additive, which is not soluble in the nonsolvent.…”