Structural Regularization in the Crystallization Process from the Glass or Melt of Poly(l-lactic Acid) Viewed from the Temperature-Dependent and Time-Resolved Measurements of FTIR and Wide-Angle/Small-Angle X-ray Scatterings

Abstract

INTRODUCTIONPoly(L-lactic acid) (PLLA) is now attracting many attentions because of its possibility for the practical usages as one of the general multipurposes polymer materials. However, there are many problems to be clarified including the relatively poor mechanical property, the relatively low melting point and so on. 1,2 Among these problems, the slow crystallization rate in the melt processing is one of the most serious problems in the industrial point of view. 1,2 As one important factor, the existence of D-lactic acid units in PLLA chains as a contaminant is said to reduce the melt-crystallization rate by one order or higher. 3 It might be easy to speculate that the inclusion of different type of comonomer units disturbs the formation of regular helical sequences along the chain axis, resulting in the poor crystallization. However, there have been quite limited number of reports which describe concretely and in detail the structural evolution process itself in the crystallization of PLLA. Most of these papers investigated the crystallization behaviors from the melt or glass by measuring the thermal behavior or by observing the optical microscopic images of the spherulites. 4À31 At first let us review these previous studies about the cold crystallization from the glassy sample, which is prepared by quenching the melt into iceÀwater temperature. 13À15,17,21,24À26,28 The thus-obtained glassy sample is not necessarily in the perfectly amorphous state but some parts are more or less ordered in the chain aggregation mode. 32À37 This sample is called the mesomorphic phase or mesophase. 36 Stoclet et al. measured the X-ray diffraction pattern of the stretched PLLA glass containing 4 mol % D component and proposed the partially ordered liquid-crystal-like structure for the mesophase of PLLA. 32,33 But the detailed structure ABSTRACT: Structural evolution has been traced in the crystallization process of poly-(L-lactic acid) (PLLA) from the melt or from the glass by measuring the FTIR spectra, wide-angle X-ray diffraction (WAXD), and small-angle X-ray scattering (SAXS) patterns as functions of temperature or time. Using the infrared bands characteristic of the melt, mesomorphic phase (mesophase) and crystalline phase as well as the WAXD 200/110 and 0010 reflections and the SAXS long-period peak, the regularization processes were analyzed systematically from the different points of view. In the case of cold-crystallization from the glass, the mesophase was found to regularize into the disordered α (α 0 or δ) form in the temperature region from just above the glass transition point (Tg, 70°C) to 120°C, whereas it changed to the regular α form in the higher temperature region. On the other hand, in the melt-crystallization process, the crystallization to the α or δ form occurred not directly from the melt but via the mesophase from the melt. In this way the isothermal crystallizations from the glass and from the melt were found to occur always with the first appearance of the mesophase from the amorphous phase foll...

Structural evolution of poly(l-lactide) block upon heating of the glassy ABA triblock copolymers containing poly(l-lactide) A blocks

“…Tashiro and co-workers reported that the isothermal crystallization of PLLA both from glassy and melt state occur through the mesophase [40]. They also reported that meltquenched PLLA is not perfectly amorphous phase, but it contains the mesophase [40].…”

“…Zhang et al showed that physically aging of glassy PLLA below the glass transition temperature (T g ) resulted in the formation of mesophase [24]. Tashiro and co-workers reported that the isothermal crystallization of PLLA both from glassy and melt state occur through the mesophase [40]. They also reported that meltquenched PLLA is not perfectly amorphous phase, but it contains the mesophase [40].…”

Multi-scale analysis of the impact of polylactide morphology on gas barrier properties

“…However, there was as difference in behaviour occasioned by the crystalline polymorph of PLA [14]. In case of crystallization in a'-form (also named d-form by some authors), which is a structurally less ordered crystalline form of PLA [15] obtained at crystallization temperatures lower than 100 C [16,17], no reduction of the gas permeability was obtained, even if the crystallization was carried out up to the maximum crystallinity degree [14]. When the PLA crystallization was carried out at high temperatures (>100 C) to obtain the a-polymorph, the oxygen permeability could be decreased by a factor of approximately 4 [13,14].…”

Deformation and structure evolution of glassy poly(lactic acid) below the glass transition temperature

“…Recently, it was reported that melt crystallization of polymers does not occur directly from the melt to the crystalline phase, but via a pre-ordered amorphous phase named mesophase. 28,29 Mano et al 30,31 and Iannace et al 32 speculated about the existence of a rigid amorphous phase apart from the mobile amorphous phase in crystalline PLA, in which the former could be the mesophase. 28 On the other hand, melt-quenched PLA is not in a perfectly amorphous state but it forms the mesophase containing the disordered aggregation of IJ10/3) helical chain segments, which can rearrange into crystalline phase upon annealing.…”

“…The mesophase appearing in the crystallization process is formed by unoriented amorphous chains and refers to the necessary "transition state" from the amorphous to the crystalline phase, [24][25][26][27][28][29] whereas that formed in the stretching process is the strain-induced intermediate phase between the amorphous and crystalline phases, composed of highly oriented amorphous chains. The mesophase appearing in the crystallization process is formed by unoriented amorphous chains and refers to the necessary "transition state" from the amorphous to the crystalline phase, [24][25][26][27][28][29] whereas that formed in the stretching process is the strain-induced intermediate phase between the amorphous and crystalline phases, composed of highly oriented amorphous chains.…”