Study of the molecular weight dependence of glass transition temperature for amorphous poly(L-lactide) by molecular dynamics simulation Jian Zhang, Yu Liang, Jizhong Yan, Jianzhong Lou * Department of Mechanical and Chemical Engineering, North Carolina A&T State University, 1601 E. Market Street, Greensboro, NC 27411, USA Received 4 May 2007; received in revised form 13 June 2007; accepted 16 June 2007 Available online 21 June 2007 Abstract Molecular dynamics simulation has been used to investigate the molecular weight dependence of glass transition temperature for amorphous poly(L-lactide). Amorphous PLLA systems were created using molecular modeling and NPT ensemble MD simulations were carried out using the modified OPLS-AA force field. The fractal dimension of the PLA systems was 1.62. The molecular weight dependence of glass transition temperature, self-diffusion coefficient and shear viscosity were studied and the good agreement between the simulation results and experiments was obtained. Published by Elsevier Ltd. Keywords: Molecular dynamics simulation; Poly(L-lactide); Glass transition temperature 1. Introduction The disposal problem due to nondegradable petroleum- based plastics has raised the demand for biodegradable poly- mers [1]. Poly(lactide) (PLA) is a biodegradable aliphatic polyester derived from 100% renewable resources, such as corn and sugar beets. Moreover, it has unique physical properties that make it useful in diverse applications including paper coat- ing, fibers, films, and packaging [2]. Currently, PLA is primarily used for medical applications such as drug delivery devices, absorbable sutures, and as a material for medical implants and other related applications [3e13]. PLA, function- alized PLA and PLA block copolymers can be prepared by direct condensation or by the ring-opening polymerization [14e17]. The stereochemical microstructure of polymers can be controlled by the monomer composition in the feed or by the stereochemical preference of the initiating/catalytic system [18e20]. Phase transition behavior, morphology, miscibility and other physical properties of PLA-based nano-scale structures are widely studied recently [21e26]. All these nano-scale re- search and applications make the fundamental understanding of the PLA-based material at the molecular level a necessity. Glass transition temperature T g is of great importance con- cerning drug release, stability of the drug formulation and me- chanical properties [27e30]. But current theories only can describe some aspects of glass transition phenomena correctly. There is no unified description that puts all the phenomena into one coherent framework. Experimentally, it was found that the main effect of varying degree of polymerization N is a shift of the glass transition temperature, which is a sensitive control parameter to check theories [32e33]. Molecular dynamics simulation has been used extensively in the study of different aspects of polymer structures and properties [34e38]. Recently, the resistance of PLA to hydro- lysis based on the PLLA and PDLA blends and compatibility of PLLA and PVA blends were studied by molecular modeling simulations [28e29]. A new PLA force field developed by O’Brien using quantum mechanic calculations demonstrated quantitative improvement in performance compared to exist- ing models [30]. Several approaches to determine the glass transition of polymers by computer simulations are reported. One common approach is to determine the kink in a graph * Corresponding author. Tel.: þ1 336 334 7620; fax: þ1 336 334 7417. E-mail address: lou@ncat.edu (J. Lou). 0032-3861/$ - see front matter Published by Elsevier Ltd. doi:10.1016/j.polymer.2007.06.030 Polymer 48 (2007) 4900e4905 www.elsevier.com/locate/polymer