Crystallization, Morphology, and Mechanical Behavior of Polylactide/Poly(-caprolactone) Blends N. Lo ´ pez-Rodrı ´guez, A. Lo ´ pez-Arraiza, E. Meaurio, J.R. Sarasua School of Engineering, University of Basque Country (EHU-UPV) Alameda de Urquijo s/n. 48013 Bilbao, Spain Optically pure polylactides, poly(L-lactide) (PLLA) and poly(D-lactide) (PDLA), were blended across the range of compositions with poly(-caprolactone) (PCL) to study their crystallization, morphology, and mechanical be- havior. Differential scanning calorimetry and dynamic mechanical analysis (DMA) of the PLA/PCL blends showed two T g s at positions close to the pure compo- nents revealing phase separation. However, a shift in the tan  peak position by DMA from 64 to 57°C suggests a partial solubility of PCL in the PLA-rich phase. Scanning electron microscopy reveals phase separation and a transition in the phase morphology from spherical to interconnected domains as the equimolar blend ap- proaches from the outermost compositions. The spherulitic growth of both PLA and PCL in the blends was followed by polarized optical microscopy at 140 and 37°C. From tensile tests at speed of 50 mm/min Young’s modulus values between 5.2 and 0.4 GPa, strength val- ues between 56 and 12 MPa, and strain at break values between 1 and 400% were obtained varying the blend composition. The viscoelastic properties (E and tan ) obtained at frequency of 1 Hz by DMA are discussed and are found consistent with composition, phase separa- tion, and crystallization behavior of the blends. POLYM. ENG. SCI., 46:1299 –1308, 2006. © 2006 Society of Plastics En- gineers INTRODUCTION Polylactides are gaining attention in the medical field for application areas such as sutures, antibiotic release, and macroscopic temporary implants [1, 2]. Polylactide bioresorb- able implants have advantages in repair and regeneration of healing tissues, provided they are biodegradable, biocom- patible, and have proper mechanical performance. How- ever, one of the drawbacks of polylactides for biomedical applications is their brittleness. The backbone chain of polylactide is stiff [3] as confirmed by the value of the characteristic ratio (C  = 11.8) obtained by light scattering experiments [4]. Polylactides are linear polyesters in which the presence of an asymmetrical carbon atom leads to the existence of different stereoisomers. Because of their stereoregular chain microstruc- ture, optically pure polylactides, poly(L-lactide) (PLLA) and poly(D-lactide) (PDLA), are semicrystalline. The crystalliza- tion ability of polylactides decreases with chain stereoregular- ity [5, 6] and below 43% optical purity crystallization is no longer possible [7]. However, regardless of the different lactyl structural unit chain arrangement tried for tuning the properties of polylactides by diverse polymerization methods, both amor- phous and crystalline polylactides use to show brittle behavior at room and body temperatures [8, 9]. Crystallization of poly- lactides in the form of stereocomplex also leads to a brittle mechanical behavior [8]. One of the most practical strategies for tuning the prop- erties of polymers is blending with another polymer. Al- though chemical modification of a polymer, for instance copolymerization, can provide better performance than physical blending, a good knowledge on reaction and con- trol of polymerization during manufacturing is necessary. Therefore, polymer blending can be envisaged as a less expensive and practical alternative [1]. Poly(-caprolac- tone) (PCL) is a biodegradable, semicrystalline, flexible polymer, which has low glass transition and melting tem- peratures (T g =-50°C, T m = 60°C). An approach often successful to strengthen or toughen brittle or stiff polymers is by incorporating a soft or elastomeric second component by blending. This conventional approach may also be ap- plied to biodegradable polymers for biomedical applica- tions; when the softer component forms a second phase within the more brittle continuous phase, it may act as a stress concentrator, enabling ductile yield mechanism and preventing brittle failure [2]. However, application of this method requires detailed knowledge of the phase behavior and morphology of polymer blends. A priori, PCL is a good candidate to toughen PLA. At room and body temperatures it shows low tensile modulus (E = 400 MPa) and high elongation and break ( b  400%). In contrast, PLA at room and body temperatures shows high modulus and low elongation at break (E = 3– 4 GPa,  b = Correspondence to: J.R. Sarasua; e-mail: jr.sarasua@ehu.es Contract grant sponsors: Department of Industry, Trade and Tourism, Basque Government; University of Basque Country EHU-UPV. DOI 10.1002/pen.20609 Published online in Wiley InterScience (www.interscience.wiley. com). © 2006 Society of Plastics Engineers POLYMER ENGINEERING AND SCIENCE—2006