Influence of Chain Topology (Cyclic versus Linear) on the Nucleation and Isothermal Crystallization of Poly(l-lactide) and Poly(d-lactide)

Abstract

In this paper, ring closure click chemistry methods have been used to produce cyclic c-PLLA and c-PDLA of a number average molecular weight close to 10 kg/mol. The effects of stereochemistry of the polymer chains and their topology on their structure, nucleation and crystallization were studied in detail employing Wide Angle X-ray Scattering (WAXS), Small Angle X-ray Scattering (SAXS), Polarized Light Optical Microscopy (PLOM) and standard and advanced Differential Scanning Calorimetry (DSC). The crystal structures of linear and cyclic PLAs are identical to each other and no differences in superstructural morphology could be detected. Cyclic PLA chains are able to nucleate much faster and to produce a higher number of nuclei in comparison to linear analogues, either upon cooling from the melt or upon heating from the glassy state. In the samples prepared in this work, a small fraction of linear or higher molecular weight cycles was detected (according to SEC analyses). The presence of such "impurities" retards spherulitic growth rates of c-PLAs making them nearly the same as those of l-PLAs. On the other hand, the overall crystallization rate determined by DSC was much larger for c-PLAs, as a consequence of the enhanced nucleation that occurs in cyclic chains. The equilibrium melting temperatures of cyclic chains were determined and found to be 5 ºC higher in comparison with values for l-PLAs. This result is a consequence of the lower entropy of cyclic chains in the melt. Self-nucleation studies demonstrated that c-PLAs have a shorter crystalline memory than linear analogues, as a result of their lower entanglement density. Successive selfnucleation and annealing (SSA) experiments reveal the remarkable ability of cyclic molecules to thicken, even to the point of crystallization with extended collapsed ring conformations. In general terms, stereochemistry had less influence on the results obtained in comparison with the dominating effect of chain topology.

About the transformation of low Tm into high Tm poly(l-lactide)s by annealing under the influence of transesterification catalysts

“…Again, a guest-host copolymer was prepared by the Müller group via an end-to-end cycloaddition reaction of a linear precursor. 18 A T m of 148.6 C and DH m of À24.3 J g À1 were found, similar to the values reported by the Tezuka group. An equilibrium T m0 of 164 C was calculated for a perfect crystal of cyclic poly(L-lactide) and a T m0 of 159 C for a linear polymer of similar low molecular weight (M w < 17 000 g mol À1 ).…”

“…linear chains can adopt the same high melting morphology with T m values in the range of 193-196 C in contrast to reports of other research groups. 17,18,21 Yet another consequence of this nding concerns the calculation of the equilibrium T m0 of perfect crystals (215 C) as described by Kalb and Pennings. 19 The present and previous 20 experimental T m values suggest that the extrapolated 215 C are close to reality, whereas T m0 values below 205 C are certainly too low.…”

High Tm Poly(l‐lactide)s by Means of Bismuth Catalysts?

“…T m values below 180 °C are either a result of kinetically controlled morphologies [2,[18][19][20][21] or a result of structural defects such as D-units [33] or guest comonomers. [32,34] Fifth, a few cocatalysts stimulate an enthalpy-driven alteration of the molecular weight distribution possibly via formation of crystallites with a certain thermodynamically favorable size and lamellar thickness.…”

Cyclic polymers: Advances in their synthesis, properties, and biomedical applications

“…[174,175] The crystallization from the melt is also affected by the ring topology. First, nucleation kinetics have been investigated for polymers of different natures, namely PTHF, [170,176] poly(ethylene) (PE), [166] PCL, [165,173] PLA, [177] and PCL-b-PLA. [178] They all concluded on an increased nucleation rate and a higher density of nuclei for the cyclic polymers with respect to their linear counterpart, which can be easily observed using polarized light optical microscopy ( Figure 3).…”