Characterization of Poly(lactic acid) Multifilament Yarns. I. The Structure and Thermal Behavior M. Radjabian, 1 M. H. Kish, 1 N. Mohammadi 2 1 Department of Textile Engineering, Amirkabir University of Technology (Tehran Polytechnic), Tehran 15875-4413, Iran 2 Department of Polymer Engineering, Amirkabir University of Technology (Tehran Polytechnic), Tehran 15875-4413, Iran Received 30 September 2009; accepted 20 December 2009 DOI 10.1002/app.32046 Published online 29 March 2010 in Wiley InterScience (www.interscience.wiley.com). ABSTRACT: The structure and thermal behavior of pol- y(lactic acid) (PLA) multifilament yarns were studied by complementary techniques of differential scanning calo- rimetry (DSC), Fourier transform infrared (FTIR) spectros- copy, and wide angle X-ray diffraction (WAXD). As for PLA filaments, notable differences in the WAXD patterns, DSC curves, and FTIR spectra were observed. The combi- nation of the WAXD and FTIR results showed that PLA samples with different crystallinity contain a-form crystal structure. The FTIR spectra of the filaments were analyzed to study their crystallinity and crystal structure. The total crystallinity of the PLA filaments was obtained from the percent area loss of the skeletal amorphous band at 955 cm 1 . Crystalline fraction from FTIR and DSC were com- parable with each other. The C¼¼O stretching region, which is sensitive to crystallization and dipole–dipole interactions, was evaluated to provide information about chain conformers and crystallinity of the samples. Depend- ing on the processing conditions, double melting peaks were observed in the DSC curves of the samples. This exhibited the structural reorganization of the crystal phase during heating affected by heating and cooling rate. In the DSC curves of the nearly amorphous multifilament yarn, the exothermic peak observed right above the glass transi- tion temperature (T g ) indicated two relaxed and deformed amorphous regions. However, the multifilament yarn with higher crystallinity showed just endothermic melting peak after its glass transition. V C 2010 Wiley Periodicals, Inc. J Appl Polym Sci 117: 1516–1525, 2010 Key words: poly(lactic acid); structure; thermal behavior; differential scanning calorimetry; FTIR INTRODUCTION Poly(L-lactic acid) (PLLA) (A[CH(CH3)COO]nA) is a well-known biodegradable and biocompatible semi- crystalline aliphatic polyester. It has attracted enor- mous attention in recent years due to its ease of pro- duction from renewable resources (mainly starch and sugar) and degradation to nontoxic materials. PLLA is produced via two conventional ways: Poly- condensation of lactic acid and ring-opening poly- merization of lactide, a chiral monomer that is pro- duced by bacterial fermentation. 1,2 The polymer is relatively hard, with the glass transition and melting temperatures of 60–70  C and 150–170  C, respec- tively. 2 High mechanical strength, high modulus, and proper degradation time of PLLA make it appropriate thermoplastic polymer for either medi- cal or textile fibers with industrial applications. Until the last decade, the applications of PLLA were mainly limited to biomedical such as tissue engi- neering, surgical suture, bone fixation devices, and controlled drug delivery systems, due to its high cost, low availability, and limited molecular weight. However, in the last few years, the discovery of new polymerization routes, allowed for the advancement of economical production of high molecular weight PLLA, which caused new potential applications as a commodity plastic in packaging, disposable, and other consumer materials. 2 PLLA crystallizes at least in three forms of a, b, and c depending on the crystallization and process- ing conditions. 3–6 In the late 1960s, De Santis and Kovacs 3 published the a form and proposed a pseudo-orthorhombic structure with two left-handed 10 3 helical conformation chains in antiparallel orien- tation passing through the unit cell. The a modifica- tion grows from the melt or solution spinning pro- cess at low temperatures or low draw ratio under low drawing temperatures. Eling et al. 4 reported the b polymorph as the second crystal form, which is characterized by a frustrated structure. The b crystal form is produced by stretching the a form or solu- tion spinning at high temperatures or high draw ra- tio under high drawing temperatures. 4,5 In 1990, an orthorhombic unit cell for the b form containing six Correspondence to: M. H. Kish (mhkish@aut.ac.ir). Journal of Applied Polymer Science, Vol. 117, 1516–1525 (2010) V C 2010 Wiley Periodicals, Inc.