An easy procedure was applied to prepare high-melt-strength polylactide (PLA) that involves γ-radiation-induced free-radical reactions to introduce a long-chain branched structure onto a linear PLA precursor with addition of a trifunctional monomer, trimethylolpropane triacrylate (TMPTA). The results from size-exclusion chromatography coupled with multiangle laser light scattering (SEC-MALLS) detection indicate that the resultant long-chain branched PLA (LCB PLA) samples have an increased molecular mass and an elevated branching degree with increasing amount of TMPTA incorporated during the irradiation process. Various rheological plots including viscosity, storage modulus, loss tangent, Cole−Cole plots, and weighted relaxation spectra were used to distinguish the improved melt strength for LCB PLA samples. The effect of LCB structure on elongational rheological properties was further investigated. The LCB PLA samples exhibited an enhancement of strainhardening under elongational flow. The enhanced melt strength substantially improved the foaming performance of the LCB PLA samples.
The influence of a molten liquid polymer layer on the crystallization of the beneath semicrystalline polymer has been seldom considered. In the study, the nucleation and growth of spherulites for the beneath polylactide (PLA) layer in poly(ethylene oxide)/polylactide (PEO/PLA) double-layer films during isothermal crystallization at various temperatures above the melting point of PEO have been investigated by using polarized optical microscopy, with the particular results compared with that for neat PLA and PLA/PEO blend films. It is interesting to find that the top covering molten PEO layer can greatly accelerate the spherulitic growth rate (G) of the beneath PLA layer. Another significant result is that the temperature for the measurable nucleation and spherulitic growth of PLA in the double-layer films can be eventually pushed down close to the glass transition temperature of neat PLA. The changes of glass transition temperature, T g , for PEO/PLA multilayer films have been measured by using modulated differential scanning calorimetry and dynamic mechanical analysis, which reveal slight decreases of T g for PLA layer due to the influence of PEO layer. The layer structures of fractured surface of the double-layer films are analyzed on the basis of the observation from scanning electron microscopy, and the existence of interdiffusion areas with irregular boundary between PEO and PLA layers is the key clue to understanding the significant acceleration of G for PLA. The layer-by-layer film method infers promising applications, which might be considered to well replace the blending method.
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