Synthesis, structure and properties of poly(L-lactide-co-ε-caprolactone) statistical copolymers

Abstract

Four poly(L-lactide-co-ε-caprolactone) (PLCL) copolymers were synthesized at 120, 130, 140 and 150 °C by ring opening polymerization using stannous octoate catalyst at a 2000:1 comonomer:catalyst ratio. Gel permeation chromatography (GPC) and (1)H NMR measurements were performed to determine the molecular weight, composition and chain microstructure of copolymers of L-lactide(LA):ε-caprolactone(CL) synthesized using 90:10, 80:20, 75:25 and 70:30 feed ratios. The overall conversion of these PLCL copolymers was in the range of 80%-90% leading to weight average molecular weights (M(w)) between 98,500 and 226,000 g mol(-1) depending on feed composition and polymerization temperature. At temperatures lower than 140 °C, the incorporation of CL units into polymer chains was incomplete because of the low reactivity of CL, thus at 120 °C the copolymer composition was difficult to control obtaining more LA in the copolymer than the desired, hence the blocky character of PLCL copolymers also increased. At 150 °C the catalyst was less effective and the molecular weights of the copolymers took lower values. A temperature of 140 °C was established as optimal to obtain highest yields and molecular weight. The number average crystallizable lactide sequence lengths (l(LA)) shifted from 6.5 to 16.7 LA repeat units for PLCL polymerized at 140 °C while the randomness character (R) value shifted from 0.4 for polymerization at 130 °C to 0.6, at 150 °C. Increasing the LA content in the copolymers the glass transition temperature and the crystallizability and melting temperature of PLCLs approached to that of PLLA homopolymer. The aging sensitivity of PLCLs increased with CL content. A double T(g) behavior due to phase separation associated to crystallizing LA blocks was observed during aging. The mechanical properties, however, evolved toward the PLLA character when the molar content of LA in PLCL was increased from 66% to 90%, observing a shift from an elastomeric thermoplastic behavior to that of a glassy plastic, reflected by an increase in tensile modulus (from 12.0 to 1343.1 MPa) and a decrease in strain recovery after break (from 93.5% to 25.0%). Small amounts of CL content in the copolymers produced large improvements in their deformability with regard to PLLA. In addition, thermogravimetric analysis demonstrated that PLCLs are more stable to thermal degradation than PLLA and they undergo a more complex degradation mechanism than those of the corresponding homopolymers.