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...

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The mechanisms of charge injection, transport and trapping in low-density, high-density and cross-linked polyethylene (LDPE, HDPE and XLPE) are investigated in this paper through charging-discharging current measurements and space-charge observations. The conductivity of LDPE is much larger than that of XLPE and HDPE. The threshold for space-charge accumulation and that for a space-charge-limited current mechanism, coinciding for the same material, are almost identical for LDPE and HDPE, while the threshold of XLPE is higher. However, HDPE accumulates more charge than the other two materials. The depolarization space-charge curves and the conduction current versus field characteristics indicate that the mobility of LDPE is larger than that of XLPE and HDPE, which supports the significant difference in conductivity. The lower mobility, as well as the nature, depth and density of trap sites, can explain the difference in space-charge accumulation and thresholds.

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