Standard Test Method for Transparency of Plastic

Polylactides (PLAs) have been known for several decades but only recently have these polymers gained commercial significance as a leading environmentally benign plastic available from renewable resources. Therefore, it is highly desirable to understand the optical properties of these materials so that they can be manipulated to develop materials of desirable characteristics for a specific objective. Optical properties such as color, clarity, and refractive index are important in dyeing operations for textile and in various packaging applications [1, 2]. Many of the physical properties of PLA are influenced by the amount and distribution of the Rand S-lactic acid stereocenters in the polymer chain, which reflect its history including the stereochemistry (RR (D-lactide), SS (L-lactide), RS (meso-lactide), or a mixture of equal amounts of Dand L-lactide referred to as racemic or DL-lactide) of the feed composition, polymerization kinetics, and extent of transesterification and racemization (see Chapters 1–4). As this rich variety of structures leads to materials with different characteristics, from fully amorphous to semicrystalline, a discussion on the PLAoptical properties across a broad range of stereooptical compositions is valuable. Since most applications of PLA-based materials are in the solid state, the detailed knowledge of the composition, bulk structure, and conformation of these materials is crucial. For example, it is well known that poly(L-lactide) (PLLA) obtained from LL-lactide is usually molded at 100–120 C in industrial melt processing because of the higher crystallization rate. It has been reported that when it is crystallized within this temperature region a mixture of crystals, aand b-forms, is formed [2]. Although infrared and Raman spectroscopy is a very promising analytical technique, as will be shown below,many aspects of the optical properties for solidstate PLA remain unexplored. In this chapter, the main optical techniques used to characterize PLA-based polymers are discussed in four sections: (1) absorption and transmission of UV–Vis radiation, (2) index of refraction, (3) specific optical rotation, and (4) infrared and Raman spectroscopy and NMR.