Effect of molecular weight and crystallinity on poly(lactic acid) mechanical properties

Abstract

SYNOPSISSeveral samples of poly(1actic acid) with different molecular weights and tacticity have been prepared, and some PLLA injection moulded specimens have been annealed to promote their crystallization. From the characterization data, poly(L-lactide) showed more interesting mechanical properties than poly(D,L-lactide), and its behavior significantly improves wit.h crystallization. In fact, annealed specimens possess higher values of tensional and flexural modulus of elasticity, Izod impact strength, and heat resistance. The plateau region of flexural strength as a function of molecular weights appears around Mu = 35,000 for PDLLA and amorphous PLLA and at higher molecular weight, around Mu = 55,000, for crystalline PLLA. The study of temperature effect shows that a t 56°C only crystalline PLLA still exhibits useful mechanical properties. 0 1996 John Wiley & Sons, Inc.I NTRODU CTI 0 N Scientific and industrial interest in lactic acid polymers is increasing, not only for biomedical applications, but also for potential large-scale uses, such as packaging and many consumer goods.' In spite of the very good thermoplastic behavior of these materials, the relatively high cost of the intermediate lactide deeply affects the possibility of their commercialization 2-4 : an intensive purification, based on several crystallizations, is in fact generally required for reaching high-molecular-weight polymers and, consequently, useful mechanical properties. In this context, the industrial manufacturing of lactic acid polymers for high volume production should be oriented by evaluating the correlations between molecular weight and characteristics of the material. Moreover, lactic acid crude polymers easily show a thermal degradation during melt processing, 5-7 with relevant reduction of molecular weight. This also affects the properties of these materials with unfavorable consequences in several application fields.The aim of this research was to investigate the * To whom correspondence should be addressed.Journal of Applied Polymer Science, Vol. 59,37-43 (1996) 0 1996 John Wiley & Sons, Inc.CCC 0021-8995/96/010037-07 correlation between molecular weight, of poly (L-lactide) (PLLA) and poly (D,L-lactide) (PDLLA) and their mechanical properties. We decided to characterize medium-and low-molecular-weight samples since they can be obtained from monomers having received less purification steps and because they can derive from the degradation of higher molecular weight materials. The polymers were synthesized by ring opening polymerization of L-and D,L-lactide with S n (11) 2-ethylhexanoate (stannous octoate) , iats described in a previous paper8 and generally in the chemical literature.'-" We decided to prepare lower molecular weight samples by extruding the polymers several times without any purification or stabilization step. I t is well known that in these conditions lactic acid polymers exhibit a fast degradati01-1,~'~ especially if it is associated with hydrolitic phenomena.Besides mechanical testing the injection moulded sp...

NMR analysis and chemical shift calculations of poly(lactic acid) dimer model compounds with different tacticities

“…for 13 C methyl. Before Fourier transformation, the free induction decays were zero-filled twice in the 13 C dimension, and a shifted sine bell function was applied to both dimensions.…”

“…Thus, mechanical properties and heat resistance have been reported to depend on tacticity. 13,14 Recently, it was shown that tacticity could influence thermal stability, T m , T g , crystallinity, solution stability and polymer degradability in star-shaped PLA. 15,16 In addition, poly (L-lactic acid) and poly(D-lactic acid) could form a 1/1 stereocomplex, and the crystalline structure of the stereocomplex was quite different from that of poly(L-lactic acid) or poly(D-lactic acid).…”

Effect of Ethyl Ester L-Lysine Triisocyanate addition to produce reactive PLA/PCL bio-polyester blends for biomedical applications

“…It is known that the remodeling process during wound healing at the suture sites is helped by a moderate strain field on the part [13]. For this reason, a PLLA grade with an Elastic Modulus in the order of GPa could be too rigid for sutures applications [17].…”

“…A grade of entirely laevogyrate Poly(lactide) (PLLA) could also be used since its elastic modulus value is in the range of 4GPa [17].…”

Numerical investigation of the mechanical properties of the additive manufactured bone scaffolds fabricated by FDM: the effect of layer penetration and post-heating

“…In this study, the proposed model does not include post-heating. However, it is suggested that post heating could enhance mechanical properties (Perego et al, 1996). To do so, some scaffolds with the struts' diameter of 700 and pore size of 350 micron are heated up to 3 more than crystallization temperature of the PLA which is about 115 .…”