Investigation of joncryl chain extender reactivity with amorphous and semicrystalline polylactide

Abstract

Plastic pollution has become one of the major concerns of the millennia and if proper precautions are not taken today, it would be much harder to address tomorrow. Plastics have become an integral part of modern world but the same care has not been given on their sustainability over their production and consumption. Biopolymers can fill an important niche in helping overcome this problem since these polymers can easily fulfill the requirements of commodity plastics and replace them without needing to sacrifice quality or performance. Polylactide or poly(lactic acid), PLA for short, is one such biodegradable polymer that has comparable qualities to PET and PS which make up a good portion of the commodity plastics, with an added benefit of being sustainable and environmentally friendly. Unfortunately, PLA has some drawbacks that has to be addressed before it can see wider use than it has today. These shortcomings can be summarized as slow crystallization kinetics, low melt strength, low thermal stability and brittleness which makes the processing and application of PLA a challenge. From the increasing number of studies being done on PLA, it was revealed that these properties can be improved with the usage of chain extenders. One such chain extender with an exceptional track record among others is Joncryl ADR, which is a commercially available oligomeric styrene-acrylic-epoxy based multifunctional chain extender. Studies on Joncryl ADR have shown that incorporation of small amounts of this chain extender improves melt strength and thermal stability considerably without compromising its biodegradability and food-safety. Since its introduction, the application of Joncryl ADR has found usage not only on improving melt properties but also acting as compatibilizer in blends and nanocomposites which opens up the usage are of Joncryl ADR even more since blending and nanocompositing also improves the lacking qualities of PLA without requiring expensive processes. Fundamental studies of Joncryl ADR have shown that the epoxy groups on the oligomer is very reactive with the end groups of PLA. During epoxy ring opening reaction, the carboxyl end groups of PLA combines with Joncryl to form long chain branches and since Joncryl has multiple epoxy groups, the resulting structure becomes highly branched. Linear PLA has low melt strength because of weak and limited amount of physical entanglements linear molecules possess. Moreover, entanglements of linear PLA are directly related to its molecular weight, which is further reduced during processing because of thermal degradation. Introduction of highly branched structures by Joncryl allows more physical entanglements to be formed through the long chain branching while same branching reactions also surpass the thermal degradation and increase the molecular weight of PLA. Highly branched structures also impart strain hardening behavior to PLA and improve the melt strength even further. However, highly branched structures also reduce crystal perfection and total crystallinity of semicrystalline PLA but can also improve crystallization kinetics to reduce the total amount of cold crystallization. In this scientific work, two distinct epoxide-based oligomeric chain extenders, Joncryl ADR 4468 (high functionality) and Joncryl ADR 4400 (low functionality), were processed using an internal melt mixer with amorphous PLA (aPLA) and semicrystalline PLA (scPLA). Small amplitude oscillatory shear (SAOS) and elongational rheological tests were employed along with differential scanning calorimetry (DSC) and Fourier transform infrared spectroscopy to investigate the samples that were prepared with and internal melt mixer. When compared to their unmodified counterparts, Joncryl 4468 and 4400 both increased the melt viscosity and thermal stability of PLAs, according to rheological measurements. Since higher functionality allowed for more branching and physical entanglement, the addition of Joncryl 4468 led to a greater improvement in the melt behaviors of both PLAs when compared to lower functionality of Joncryl 4400. When compared to those of scPLA, the melt characteristics of aPLA improved even more with both Joncryl. This might be the result of aPLA's greater optical isomer content, which imparts lower chain mobility and boosts physical entanglements. Increasing the processing temperatures also increased the reactivity of Joncryl, as higher temperatures imparted both higher mobility to PLA and lowered the activation energy of epoxy ring opening reactions. The elongational rheology showed that unmodified aPLA, which had linear geometry, had the lowest melt strength that was also further reduced with increasing processing temperatures. On the other hand, Joncryl modified aPLA samples had higher melt strength as processing temperatures increased, with Joncryl 4400 imparting strain hardening behavior at the highest processing temperature and Joncryl 4468 in all of the temperatures. According to DSC analysis, scPLA samples displayed enthalpy relaxation and cold crystallization during first heating, which was followed by slow cooling (2oC/min) and fast second heating (10oC/min) programmes. Chain extender addition was shown to shift the crystallization temperature (Tc) to lower temperatures while broadening the crystallization peak and reducing its intensity, suggesting that perfect crystal ordering was made more challenging. Since Joncryl 4468 introduced higher branching than Joncryl 4400, this effect was more pronounced for the former. It was established that aPLA exhibited no crystallization behavior and that the addition of chain extenders had not imparted any crystallization behavior either. Glass transition temperature (Tg) of both PLA were impacted with Joncryl addition, however for aPLA it was the reason was dense branching while for scPLA it was changing crystallinity. FTIR spectroscopy also revealed that Joncryl 4400 modified scPLA samples had distinct peaks at 1640 and 1538 cm-1 that signified Joncryl reactivity but were not detected in unmodified or Joncryl 4468 modified samples. Furthermore, aPLA samples had a broad peak between 3700-3100 cm-1 that corresponded with degradation and epoxy ring opening reactions.