1099 Research Article Received: 3 September 2009 Revised: 18 November 2009 Accepted: 8 December 2009 Published online in Wiley Interscience: 26 April 2010 (www.interscience.wiley.com) DOI 10.1002/pi.2834 Mechanical and thermal properties, morphology and relaxation characteristics of poly(lactic acid) and soy flour/wood flour blends Yonghui Li, Karthik Venkateshan and Xiuzhi Susan Sun * Abstract Poly(lactic acid) (PLA) is a biodegradable polymer derived from sugar-based materials, and its applications are varied. PLA blends are commonly employed to overcome certain disadvantages such as poor impact strength, low heat distortion temperature, poor processability and relatively high cost. In this study, blending PLA with soy flour (SF), wood flour (WF) and sodium bisulfite-modified SF was used to improve the adhesion to PLA. In all cases, 0.5 wt% methylenediphenyl diisocyanate (MDI) was used as a coupling agent. Mechanical and thermal properties, morphology and relaxation characteristics of the blends were investigated. The results showed that MDI was an effective coupling agent for the WF/PLA system in improving tensile strength and elongation. Differential scanning calorimetry results indicated that SF and modified SF act as nucleation agents and facilitate the crystallization behavior of PLA by increasing the percentage crystallinity. From mechanical relaxation of the temperature-variant system, we determined how the mechanical relaxation time evolves during the course of heating and obtained the Kohlrausch–Williams–Watts parameter and activation energy (E). PLA and its blends exhibited highly homogeneous relaxational dynamics in their transition from glass to liquid, and E of PLA and its blends is mainly affected by their densities and compositions. c  2010 Society of Chemical Industry Keywords: poly(lactic acid); PLA blends with soy flour/wood flour; thermomechanical properties; dynamic heterogeneity; mechanical relaxation time; activation energy INTRODUCTION The development of synthetic polymers from petroleum-based products has been of benefit for decades, but has also led to severe problems including overexploitation of fossil resources and environmental pollution. Therefore, there is a great in- terest in the development of alternative and biodegradable polymers. Several bio-based polymers such as poly(lactic acid) (PLA), polyhydroxyalkanoates and functionalized vegetable/plant starch- and protein-based resins have been created from renew- able resources. 1,2 PLA is a biodegradable polymer derived from sugar-based materials. It exhibits mechanical properties similar to those of synthetic polymers such as polyethylene, polypropy- lene, polystyrene and poly(ethylene terephthalate). 1,3 However, the application of PLA has been limited by its relatively high cost. In past decades, blending PLA resin with low-cost, natu- ral biopolymers, such as starch, 4–7 protein, 8 wood flour (WF) and fibers, 9–11 received much attention. These biopolymers are abundant, relatively inexpensive, renewable and biodegradable. Wang et al. 12,13 successfully prepared PLA/starch blends using methylenediphenyl diisocyanate (MDI) as a coupling agent. The blends showed enhanced mechanical properties, with an im- provement of storage modulus at temperatures above the glass transition temperature when the ratio of starch to PLA was up to 45 : 55 (w/w) with 0.5 wt% MDI. Compared with starch (ca $0.2–$0.4 per kg), which is also consumed in the production of fuel ethanol, WF is more attractive, available and affordable (ca $0.2 per kg). Studies involving WF, polymers and MDI have been reported extensively and include WF–polyurethane–MDI prepolymer 14–16 and WF–high-density polyethylene–MDI func- tionalized polyethylene prepolymer 17 systems. An improvement in thermomechanical and interfacial properties with the exception of impact strength was reported for both systems. Furthermore, Pilla et al. 18 investigated the properties of PLA/pine wood flour (PWF) composites by varying the percentage of PWF with and without a silane coupling agent. They concluded that the addition of PWF (up to 40 wt%) increased the modulus but decreased toughness and elongation; tensile strength remained the same (about 51–57 MPa), and the effects of silane on the mechanical properties were not significant. Lee et al. 19 conducted a simi- lar study of a WF/talc mixture-filled PLA and found that tensile strength decreased slightly with the addition of talc but increased with the addition of 1 wt% silane. In our study, we hypothesize that MDI can enhance the mechanical and interfacial properties of WF/PLA and soy flour (SF)/PLA blends and be an effective coupling agent based on the high reactivity of the isocyanate groups of MDI ∗ Correspondence to: Xiuzhi Susan Sun, Bio-Materials and Technology Lab, Department of Grain Science and Industry, Kansas State University, Manhattan, KS 66506, USA. E-mail: xss@ksu.edu Bio-Materials and Technology Lab, Department of Grain Science and Industry, Kansas State University, Manhattan, KS 66506, USA Polym Int 2010; 59: 1099–1109 www.soci.org c  2010 Society of Chemical Industry