| Poly(lactic acid) (PLA), has attracted many interests as it possesses excellent biodegradability and good mechanical properties. Furthermore, PLA has a broad market prospects as many properties of PLA are similar to those of general plastics (polyethylene, polypropylene and polystyrene, et al). However, one of the drawbacks of processing PLA in the molten state is its tendency to undergo thermal degradation. These processing lead to low molecular weight, low intrinsic viscosity and high melt flow rate, which hinders the applications of PLA in practice of extrusion and injection molding. Fortunately, PLA has two end groups, hydroxyl and carboxyl, which can be crosslinked by chain extenders, thus increase the molecular weight and intrinsic viscosity, reduce the MFR. The introduction of chain extenders expands PLA application area. At present, most of the industrialized PLA chain extenders are high-molecular compounds and they have two limitations:the preparation process is complex and the residue of chain extension reaction is not environmental friendly. Thus, developing an effective and environmental friendly chain extender has very important practical significance. In this paper, a series of polysilsesquioxane microspheres(PSQs) with six different functional group:methyl, thiopropyl, epoxy, vinyl, aminopropyl and methacryloxypropyl, were synthesized and then mixed with Poly(L-lactic acid)(PLLA) as a concentration of 2 wt% through melt processing. FTIR Spectra was used to characterize PSQs structures and determine the functional groups. The melt flow properties were analysed by rotational rheometer, melt index instrument and gel permeation chromatograph (SEC). The results showed that the PSQs can improve the melt flow behavior of PLLA effectively. The dispersibility of PSQs in PLLA, particle size of PSQs and the influence of different functional groups of PSQs were also investigated. As a result, poly(epoxypropoxy)silsesquioxane (PESQ) has the highest reactivity with PLLA in these PSQs, which can decrease the MFR of processed PLLA from 14.1 g·10min-1 to 6.0 g·10min-1, less than a half.Then PESQ modified PLLA was studied as a chain extender to improve PLA’s thermal property. Its structure was characterized by Si Nuclear Magnetic Resonance Spectrometer (29Si NMR). The result showed that PESQ was fully condensed. Thermal Gravimetric Characterization (TGA) showed that the thermal stability of PESQ was good as the 1% weight loss temperature was 250 ℃. much higher than the processing temperature of PLLA (around 200 ℃). PESQ was grafted onto commercial PLLA via functional group reactions and employed as the chain extenders to improve the thermal degradation during melt processing. Based on the rheological, Mw, and MFR results, the reaction time, temperature, and the additive concentration of PLA-PESQ were optimized to 10 min,200 ℃ and 2 wt%, respectively. Results showed that the shear viscosity of PLA-PESQ in low-frequency range increased from 750 to 5500 Pa-s, approximately six times larger than that of neat PLLA. The Mw of PLA-PESQ was 250.4 kg·mol-1, nearly tripled compared to the Mw of neat PLLA. The MFR was 3.9 g-10min-1, nearly half of neat PLLA. TGA revealed that the addition of PESQ into PLLA increased the onset temperature of thermal degradation. The 5% weight loss temperature was 8℃ higher after addition of P-PESQ. Moreover, during the whole reaction time 35 min, the torques of P-2PESQ was 60% higher than that of Processed PLLA. Furthermore, after 35 min processing, MFR of P-PESQ was increased slightly compared to PLLA raw material, while that of processing PLLA without any additive increased twice. It means that PESQ can inhibit PLA degradation in processing of 35 min. Finally, the impact strength of P-PESQ was 52 J·m-1, increased to twice of neat PLLA, indicating that the toughness was significantly enhanced.PEASQs with different ratio of epoxy group and amino group were synthesized by base catalyzed sol-gel process in aqueous medium. It was confirmed by FTIR that the amino in PEASQ increases with increasing of amino group in reactants. Results of 29Si NMR showed that the coprecipitation of PEASQs is quite completed. TGA showed that PEASQs have good thermal stability as the 5% weight loss temperature are 265 ℃, much higher than the processing temperature of PLLA (around 200 ℃). With the increase of ratio of amino, the dynamic shear viscosity in low frequency increase first then decrease. When ratio of epoxy group and amino group was 7:3, P-PEASQ showed the highest dynamic shear viscosity, which was the best rheological property in all our PSQ particles. The shear viscosity of PLA-PEASQ was 33% higher than that of PLA-PESQ. The reason is that amino group has higher reactive activity with PLA, although it can also accelerate degradation of PLA. Based on the rheological, Mw and MFR results, the reaction time, temperature, and the additive concentration of PLA-PEASQ were optimized to 10 min,200 ℃ and 1 wt%, respectively. Results showed that the Mw of PLA-PESQ was 211.7 kg-mol-1, more than twice compared to the Mw of neat PLLA. The MFR was 3.6 g-10min-1, smaller than half of neat PLLA. TGA revealed that PEASQ was a more efficient chain extender, the thermal decomposition temperatures of P-PEASQ at 5%weight loss was 334 ℃, about 16 ℃ higher than that of neat PLLA, while that of P-PESQ was 8 ℃ higher than that of neat PLLA. Moreover, during the whole reaction time of 35 min, the torques of P-PESQ and P-PEASQ were 60% higher than that of Processed PLLA. After 35 min processing, the MFR of P-PESQ and P-PEASQ was increased slightly compared to PLLA raw material, while processed PLLA without any additive increased twice, which means PESQ and PEASQ can inhibit PLA degradation in processing of 35 min. Finally, the impact strength of P-PESQ and P-PESQ were 52 J-m"1 and 46 J·m-1, increased to twice of that of neat PLLA, indicating that the toughness was significantly enhanced.Influence of functional polysilsesquioxane microspheres, PESQ and PEASQ, on the morphology and physical properties of PLLA foaming by supercritical carbon dioxide was investigated. The influence of foaming temperature, foaming pressure and foaming time to foaming property was studied. The results showed that the foaming window of modified PLLA was broadened. Foaming pressure broaden from 14-19 MPa to 12-19 MPa, while foaming temperature broaden from 75-95℃ to 65-95 ℃ The volume expansion rate of foamed material was also increased obviously. The volume expansion rate of P-PESQ and P-PEASQ were 30.40 and 32.44, respectively, while the rate of neat PLLA was 25.44. Moreover, the density of P-PEASQ was 0.03 g·cm-3, while that of neat PLLA was 0.05 g-cm’3 Finally, a simple preparation of multi-branched poly(L-lactic acid)s (m-PLLAs) and their performance as nucleating agents for PLLA were investigated. By a simple polycondensation of L-lactic acid (L-LA) on the multi-functional core of PESQ using stannous octoate (SnOct2) as a catalyst, the m-PLLA can be obtained easily. Its structure was characterized by FTIR,1H NMR and SEC. The m-PLLAs were added into PLLA as nucleating agent to study the promotion for crystallization. The crystallization rate, crystal morphology of PLLA and the influence of nucleating agent to PLLA melting behavior were confirmed by differential scanning calorimeter (DSC) and polarized optical microscopy (POM). The results showed that m-PLLAs can promote PLLA crystallization, which can reduce the cold crystallization temperature by 7 ℃ with 1 wt% content. M-PLLA 10, whose functional group molar ratio of PESQ and L-LA was 1:10, performed the best nucleating property in PLLA with 1 wt% content. Additive concentration of 0.2-5 wt% was investigated, and 1 wt% was the optimum adding content. Non-isothermal crystallization studies of PLLA with 1 wt% m-PLLA 10 indicated that cold crystallization temperature (Tcc) decrased from 121.2 ℃ to113.4 ℃, degree of crystallinity (&) increased from 15.7% to 31.1%, compared to Neat PLLA. Those data of PLLA/Talc were 115.8 ℃ and 27.8%. The crystallization growth rate of PLLA/m-PLLA10 also faster than Neat PLLA and PLLA/Talc as observed from POM. At the same time, the number of spherocrystal increase and the size of spherocrystal decrease significantly. The average size of spherocrystal of PLLA with 1 wt%m-PLLA 10 was decreased from 100 μm to 15 μm. while the size of PLLA with 1 wt% Talc is 45 μm. Results showed that m-PLLA 10 is a PLA nucleating agent with high potential application. |
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