(517g) Control of Thermal Degradation of Poly(lactic acid) Using Poly(ã-glycidyloxypropylsiloxane) Microspheres As Chain Extenders | AIChE

(517g) Control of Thermal Degradation of Poly(lactic acid) Using Poly(ã-glycidyloxypropylsiloxane) Microspheres As Chain Extenders

Authors 

Han, T. - Presenter, East China University of Science and Technology
Xin, Z. - Presenter, East China University of Science and Technology
Shi, Y., East China University of Science and Technology



Control of thermal degradation of poly(lactic acid) using poly(γ-glycidyloxypropylsiloxane) microspheres as chain extenders

Ting Han, Zhong Xin*, Yaoqi Shi

State Key Laboratory of Chemical Engineering, School of Chemical Engineering, East China University of Science and Technology, P.O. Box 545, Meilong Road 130, Shanghai 200237,People’s Republic of China

Corresponding author. Tel.: +86 2164252972; fax: +86 2164240862.

E-mail address: xzh@ecust.edu.cn (Z. Xin).

Poly(lactic acid) (PLA) is a linear, aliphatic thermoplastic polyester that can either be semi-crystalline or amorphous depending on the stereopurity of the polymer backbone. PLA can be synthesized from 100% renewable resources such as fermentation products of corn and sugar beets. Due to its good biodegradability, biocompatibility, high mechanical strength, and excellent shaping and moulding properties,PLA should have been widely applied and is on the expectation to substitute petroleumbased polymers.

However, the processing instability of PLA (i.e.,thermal, oxidative, and hydrolytic degradations),  hinders its development in practice, These degradations will cause the cleavage of polymer chains and a decrease in molecular weight, resulting in a deterioration of the rheological properties. Moreover, the low melt viscosity of PLA can also destroy its performance in  the blow molding and foaming  process. To overcome such shortcomings, attempts to control the melt rheology of PLA by means of increasing molecular weight to compensate for the molecular weight decrease caused by processing degradation, have motivated considerable research efforts. Copolymerization, blending and filling techniques can be used, among of which, the incorporation of chain extension agents into PLA to produce extended PLA is more attractive due to its lower cost .

Chain extension reactions not only offer the opportunity for enhancement of physical and chemical properties by increasing the molecular weight but also introduce new functional groups onto the PLA backbone paving the way for preparation of composites, laminates, coated items, and blends/alloys with improved properties and cost effectiveness. In the meantime they are economically advantageous because they can be carried out in the melt, with only low amounts of chain extending agents, and separate purification steps are not required. In industry, chain extension reactions have been developed to generate branching structures to increase shrinkage and extension of melt-spun PLA fibers.

For the purpose to eliminate degeration during the processing of PLA, the functional polysilsesquioxane microspheres(PSQ), synthesized by one-step process with high content of epoxy groups, were introduced as a novel chain extension agent to graft onto the commercial PLA with relatively controlled structures via functional group reactions through melt processing. The characterization of the product was, investigated by rheometer, FTIR, SEC and melt index meter.

Firstly, the effects of reaction time on the properties of  branched poly(lactic acid) grafted by Poly(γ-glycidyloxypropylsiloxane) microspheres (PSQ-g-PLA) was fully investigated. With the increase of reaction time, there was only a slight change in molecular weight, between 200.0kg·mol-1 and 220.0kg·mol-1(PDI=1.7), which was indicative of inhibited degeration of PLA. During the process of thermal degradation, PSQ could work as a chain extender to covalently bond the segments of PLA through functional group reaction, which dynamically balanced the molecular weight. With the increase of reaction time, melt index of PSQ-g-PLA showed a increasing trend, but the change is not obvious. The melt index showed a minimum of 3.793g·10min-1 at the reaction time of 5min. The rheological properties of PSQ-g-PLA, dynamic shear viscosity, storage modulus, loss modulus were greatly improved compared with the neat PLA. From the plot of viscosity (η) vs frenquecy (γ) in low frequency,the existence of branched structure was confirmed and 10min-40min was enough for increasing molecular weight compensate for the molecular weight decrease caused by processing degradation and increase the melt viscosity. With the increase of reaction time, the mechanical properties showed the same trend with rheological properties. The stiffness and toughness of PSQ-g-PLA were simultaneously enhanced when the processing time was 10min and 20min. The impact strength reached to a maximum value of  60 J m-1 at the reaction of 30min, which was two times more than that of the neat PLA.

Secondly, the effects of processing temperature to PSQ-g-PLA have been studied. With the increase of the reaction temperature, the molecular weight showed the maximum of 250.4kg·mol-1 at 200 °C, which can be attribute to the balance of the reaction rate and the thermal degradation of polylactic acid. The melt index showed the minimum of 3.938g·10min-1 at 190 °C. The rheological properties showed the same trend as the molecular weight, and the dynamic shear viscosity showed the maximum value at 190 °C,which indicated the optimized temperature for higher degree of branching.

Lastly, the influence of the additive concentration to the grafting reaction have been researched. With the content of PSQ increased, the molecular weight showed a maximum of 196.7kg·mol-1 at 2%, which provided us convincing evidence to hypothesize that a small amount or excess PSQ will aggravate the degradation of polylactic acid, while melt index showed a minimum of 3.624g·10min-1 at 4%. Compared with neat PLA, shear viscosity of each PSQ-g-PLA sample increased which implied the formation of branched structure. A positive correlation between viscosity and additive concentration was found below the content of 2%, while a plain was found after 2%.

According to the values of shear viscosity, the reaction time, temperature and the additve concentration were optimized to 10mins, 190 ?C and 2%, respectively. The coresponding shear viscosity of PSQ-g-PLA was 4500 Pa.s, approximately six times that of neat PLA, while the increment by peroxide initiated branching was just 25%. The Mw of PSQ-g-PLA was 250.4kg mol-1, about three times more than that of neat PLA, and the melt index was 3.624 g 10min-1, reduced to almost the half of the neat PLA. More importantly, the stiffness and toughness of PSQ-g-PLA were simultaneously enhanced with the addition of  functional polysilsesquioxane microspheres. the impact strength was increased by more than two-fold, up to 60 J m-1. Finally, this structural information was correlated with its foaming ability, with guidance on its potential applications in polymeric foam or film.

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