Peroxide induced cross-linking by reactive melt processing of two biopolyesters: Poly(3-hydroxybutyrate) and poly(L-lactic acid) to improve their melting processability Liqing Wei, Armando G. McDonald Renewable Materials Program, Department of Forest, Rangeland and Fire Sciences, University of Idaho, Moscow, Idaho 83844-1132 Correspondence to: A. G. McDonald (E - mail: armandm@uidaho.edu) ABSTRACT: Poly(3-hydroxybutyrate) (PHB) and poly(L-lactic acid) (PLLA) were individually cross-linked with dicumyl peroxide (DCP) (0.25–1 wt %) by reactive melt processing. The cross-linked structures of the polymer gel were investigated by nuclear magnetic reso- nance (NMR) and Fourier transform infrared (FTIR) spectroscopies. The size of the polymer crystal spherulites, glass transition temper- ature (T g ), melting transition temperature (T m ), and crystallinity were all decreased as a result of cross-linking. Cross-linking density (m e ) was shown to increase with DCP concentration. Based on parallel plate rheological study (dynamic and steady shear), elastic and viscous modulus (G 00 and G 0 ), complex viscosity (g*) and steady shear viscosity (g) were all shown to increase with cross-linking. Cross-linked PHB and PLLA showed broader molar mass distribution and formation of long chain branching (LCB) as estimated by RheoMWD. Improvements in melt strength offer bioplastic processors improved material properties and processing options, such as foaming and thermoforming, for new applications. V C 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 41724. KEYWORDS: biopolymers and renewable polymers; cross-linking; rheology; thermal properties Received 23 September 2014; accepted 3 November 2014 DOI: 10.1002/app.41724 INTRODUCTION Recently, biodegradable and renewably derived polymers have attracted much attention due to the environmental awareness and sustainability issues associated with petroleum-based poly- mers. 1 Aliphatic polyesters such as poly(D , L-lactic acid) (PLA) and bacterial poly(3-hydroxyalkanoates) (PHAs) including poly(3-hydroxybutyrate) (PHB) and its copolymer [poly(3- hydroxybutyrate-co-3-hydroxyvalerate (PHBV)] are gaining interest due to their renewability, biocompatibility and biode- gradability. 2,3 Rather, PLA and PHB are linear polymers, lacking in branches which contributes to their poor melt elasticity as evidenced by low die swell and “neck in,” low thermodegrada- tion temperature, and high crystallinity. 4,5 These features, espe- cially the low melt elasticity, limit their processability in cast film extrusion, foaming, blown-film manufacture, thermoform- ing, and fiber spinning etc. 1,4 Long chain branching (LCB) and polymer chain entanglement in PHB and PLAs can improve their processability. 6 Various methods have been used to intro- duce cross-links into linear polymer such as copolymerization/ blend with other biodegradable polymer blocks, 7,8 radiation induced cross-linking, and peroxide induced cross-linking. 6,9–13 Some of these methods have improved the polymer foaming ability and blown-film processability, 14,15 and the resultant products modified by some of these methods are still biodegradable. 15,16 Practical cross-linking involves the use of peroxides, which, when used at very low levels, can result in significant increases in melt elasticity. Furthermore, cross-linking, can alter their physical properties such as the crystallinity and transition tem- peratures. 1 Studies on dicumyl peroxide (DCP) induced cross- linking in carbon black polyethylene and poly(e-caprolactone) were dependent on reaction temperature, peroxide concentra- tion, and extrusion residence time. 12 Recently the effect of type and amount of peroxide with high-, moderate-, and low- decomposition rate on cross-linking of PLLA during extrusion was investigated. 17 For the slowly decomposed peroxide, e.g. DCP, the lifetime is relatively close to the residence time of extrusion. DCP has relatively high H abstraction ability, which makes it ideal as a cross-linking agent for plastics. When DCP is exposed to high temperature, it will decompose into cumy- loxy radicals, of which 60% formed methyl radicals and aceto- phenone by b-scission. 17 These free radicals are capable of abstracting H atoms from any tertiary ACH along the PLLA or PHB backbone. The corresponding H abstraction mechanism of PHB and PLLA resulting in cross-linking and chain extension are simplified in Scheme 1. This cross-linking may be expected V C 2014 Wiley Periodicals, Inc. WWW.MATERIALSVIEWS.COM J. APPL. POLYM. SCI. 2015, DOI: 10.1002/APP.41724 41724 (1 of 15)