Crystalline PLA

The properties of PLA, as indeed those of other polymers, depend on its molecular characteristics, as well as on the presence of ordered structures, such as crystalline thickness, crystallinity, spherulite size, morphology and degree of chain orientation. The physical properties of polylactide are related to the enantiomeric purity of the lactic acid stereo-copolymers. Homo-PLA is a linear macromolecule with a molecular architecture that is determined by its stereochemical composition. PLA can be produced in a totally amorphous or with up to 40 per cent crystalline. PLA resins containing more than 93 per cent of L-lactic acid are semi-crystalline, but, when it contains 50-93 per cent of it, it is entirely amorphous. Both meso- and D-lactides induce twists in the very regular PLLA architecture. Macromolecular imperfections are responsible for the decrease in both the rate and the extent of PLLA crystallization. In practise, most PLAs are made up of L-and D,L-lactide copolymers, since the reaction media often contain some meso-lactide iir turities. 

Interestingly, annealing is perhaps the most effective treatment to get highly crystalline PLA. Accordingly, the effect of annealing treatment on the toughening... 

The PDLA, PLLA or high d- or L-lactide copolymers have regular structures. The polylactides are either amorphous or semicrystalline at room temperature, depending on the molecular weight and content of l, d or meso-lactide in the main chain. PLA can be totally amorphous or up to 40% crystalline. PLA resins containing more than 93% L-lactic acid can crystallize. However, high molecular weight can reduce the crystallization rate, and therefore the... 

PLA is a biodegradable polymer used in tissue engineering for medical applications. Based on optical rotation, PLA materials include the dextrorotatory (D) and dextro, levo rotatory (D,L) forms. The properties of PLA are given in Table 2.6. The structures of the different forms are identical, but they differ in their crystallinity. PLA (D) has a crystalline structure and is tough, with melting temperature = 180 °C and glass transition temperature Tg = 67 °C, whereas PLA (D,L) has an amorphous structure (with toughness) and Tg = 57 °C [30]. 

As shown in Tables 11.1 and 11.2, the heat distortion temperatures (HDT) for PLA do not change much with molecular weight. Amorphous and crystalline PLLA show HDT values of55-57°C and 60-66°C, respectively, therefore PLA heat deflection temperature seems little influenced by its crystallinity crystalline PLA reaches slightly higher heat resistance than amorphous PLA. This is due to main effect of glass transition temperature onto HDT, effect that is very similar in both cases. Heat distortion temperature value for PDLLA is around 50°C and this difference is easily understandable if we take into consideration the physicochemical properties of this material in fact, PDLLA exhibits the lowest Tg, around 50°C, approximately corresponding to its HDT value. 

Polylactic acid plastics should be dried with standard desiccant drying conditions of 4 hours at 45°C for amorphous PLA and of 2 hours at 60"C for crystalline PLA. Typical injection molding parameters for PLA are listed in Table 4.9 (NatureWorks 2012). 

As usual, the addition of plasticizer results in a pronounced effect on the characteristics of a polymer. The addition of plasticizer can introduce flexibility to a rigid polymer while processability can be significantly improved, with lower Tm and Tg. Although the introduction of an amorphous structure can reduce the Tg using the copolymerization of L- and D-lactide isomer, Kulinski and Piorkowska (2005) noted that there was a decrease of only 1—2 K of Tg for amorphous PLA compared to crystalline PLA. Nevertheless, the Tg of the crystallized PLA is significantly reduced from 59°C to 35—37°C after the addition of only 5% monomethyl ether polyethylene glycol as a... 

A photosensitizer can be added to enhance the photodegradation of PLA. The purpose of this is to increase the rate of degradation when accelerated PLA waste treatment is required. Tsuji et al. (2005) has studied the effect of N,N,N, N -tetramethyl-l,4-phenylenediamine (TMPD) on amorphous and crystalline PLA... 

Amorphous PLA crystalline PLA tensile strength " Young s modulus elongation at break. 

Mihai M, Huneault MA, Favis BD, Li H. Extrusion foaming of semi-crystalline PLA and PLA/ thermoplastic starch blends. Macromol Biosci 2007 7 907-20. 

Among the coordination insertion catalysts, tin 2-ethylhexanoate (tin octoate, or Sn(Oct)2) is the most widely used and studied due to its ability to produce highly crystalline PLA in relatively short periods of time with high conversion and low racemization up to 180°C. It has also been approved by the United States Food and Drug Administration for food contact (16), making it ideal for many packaging applications. 

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