Rheology and thermal stability of polylactide/clay nanocomposites

Polylactide/clay nanocomposites (PLACNs) were prepared by melt intercalation. The intercalated structure of PLACNs was investigated using XRD and TEM. Both the linear and nonlinear rheological properties of PLACNs were measured by parallel plate rheometer. The results reveal that percolation threshold of the PLACNs is about 4 wt%, and the network structure is very sensitive to both the quiescent and the large amplitude oscillatory shear (LAOS) deformation. The stress overshoots in the reverse flow experiments were strongly dependent on the rest time and shear rate but shows a strain-scaling response to the startup of steady shear flow, indicating that the formation of the long-range structure in PLACNs may be the major driving force for the reorganization of the clay network. The thermal behavior of PLACNs was also characterized. However, the results show that with the addition of clay, the thermal stability of PLACNs decreases in contrast to that of pure PLA.     

Enhanced water vapor barrier properties for biopolymer films by polyelectrolyte multilayer and atomic layer deposited Al2O3 double-coating

Commercial polylactide (PLA) films are coated with a thin (20 nm) non-toxic polyelectrolyte multilayer (PEM) film made from sodium alginate and chitosan and additionally with a 25-nm thick atomic layer deposited (ALD) Al₂O₃ layer. The double-coating of PEM + Al₂O₃ is found to significantly enhance the water vapor barrier properties of the PLA film. The improvement is essentially larger compared with the case the PLA film being just coated with an ALD-grown Al₂O₃ layer. The enhanced water vapor barrier characteristics of the PEM + Al₂O₃ double-coated PLA films are attributed to the increased hydrophobicity of the surface of these films.     

Effects of ultrasonic vibration on the micro-molding processing of polylactide

A new ultrasonic micro-molding system was used to process polylactide (PLA) and fabricate reduced dimension specimens. Plasticization and molding of PLA were achieved by applying ultrasonic waves after feeding the polymer into a plasticizing chamber. Chemical and physical characteristics of processed PLA varied depending on the processing window (i.e. changes in ultrasonic wave amplitude between 14.2 and 48.1 μm and molding pressure between 0.5 in 6 bars). In terms of chemical effects, the application of ultrasound can lead to lower molecular weights (e.g. decreases of more than 45% in the weight average molecular weight), revealing partial degradation of the material. Also, the processed materials exhibited slightly higher thermal degradability than pure PLA because ultrasonic vibrations break molecular linkages and worsen the polymer structure. Finally, the processing conditions for the preparation of PLA specimens could be optimized without causing degradation and preserving structural characteristics and mechanical properties. Specifically, the use of an amplitude of 48.1 μm and a pressure of 3 bars gave samples with the same molecular weight as the raw material (i.e. 117,500 g/mol as opposed to 117,300 g/mol for M_w).     

UV initiated oxidation and chemiluminescence from aromatic–aliphatic co-polyesters and polylactic acid

The chemiluminescence of UV irradiated aromatic–aliphatic polyesters (Ecoflex and PBAT) and polylactic acid (PLA) was examined in nitrogen and oxygen atmospheres during the non-isothermal temperature runs in the interval from 40 to 250 °C and isothermally from 150 to 190 °C. The non-isothermal thermogravimetry and FTIR spectroscopy have been used for the characterization of differences in original samples. The temperature maxima of peroxidic peaks in nitrogen scaling their stability were compared and found to be in the order PBAT < Ecoflex < PLA. Their positions on the temperature scale that correlate well with the melting temperature of the respective polymer have been discussed. With prolongation of the UV irradiation period, the intensity of peroxidic peaks increased. On the other hand, the effect of pre-irradiation on non-isothermal runs of chemiluminescence measured in oxygen was not as distinct as for measurements in nitrogen. The deconvolution of the non-isothermal chemiluminescence runs in nitrogen revealed two kinds of peroxides in irradiated samples.     

Preparation of Propargyl-terminated Polylactide by the Bulk Ring-opening Polymerization

Propargyl-terminated polylactide was prepared by bulk ring-opening polymerization of L-lactide (LLA) at 105°C in the presence of 3-methyl-1-pentyn-3-ol as the initiator and Sn(Oct)₂ as the catalyst. A significant decline of the alkynes chain-end functionality was observed by ¹H NMR even at the early stage of the polymerization. The most probable reason is the intermolecular oxidative coupling of the propargyl end groups. Propargyl-terminated polylactide having higher chain-end functionality (f = 86%) and low polydispersity (PDI = 1.22) was prepared with the addition of N,N,N′,N″,N″-pentamethyldiethylenetriamine, whose huge steric hindrance provides the protective effect of propargyl groups.     

Thermal,mechanical and electroactive shape memory properties of polyurethane (PU)/poly (lactic acid) (PLA)/CNT nanocomposites

Polymer blend nanocomposites based on thermoplastic polyurethane (PU) elastomer, polylactide (PLA) and surface modified carbon nanotubes were prepared via simple melt mixing process and investigated for its mechanical, dynamic mechanical and electroactive shape memory properties. Chemical and structural characterization of the polymer blend nanocomposites were investigated by Fourier Transform infrared (FT-IR) and wide angle X-ray diffraction (WAXD). Loading of the surface modified carbon nanotube in the PU/PLA polymer blends resulted in the significant improvement on the mechanical properties such as tensile strength, when compared to the pure and pristine CNT loaded polymer blends. Dynamic mechanical analysis showed that the glass transition temperature (T_g) of the PU/PLA blend slightly increases on loading of pristine CNT and this effect is more pronounced on loading surface modified CNTs. Thermal and electrical properties of the polymer blend composites increases significantly on loading pristine or surface modified CNTs. Finally, shape memory studies of the PU/PLA/modified CNT composites exhibit a remarkable recoverability of its shape at lower applied dc voltages, when compared to pure or pristine CNT loaded system.     

Viscoelasticity and thermal stability of polylactide composites with various functionalized carbon nanotubes

Polylactide (PLA) nanocomposites containing various functionalized multi-walled carbon nanotubes (MWCNTs) were prepared directly by melt compounding. The linear rheology and thermal stability of the PLA nanocomposites were, respectively, investigated by the parallel plate rheometer and TGA, aiming at examining the effect of surface functionalization on the dispersion of MWCNTs by using viscoelastic and thermal properties. Among three MWCNTs used in this work, the carboxylic MWCNTs present better dispersion in PLA matrix than the hydroxy and purified MWCNTs because the corresponding composite shows the lowest rheological percolation threshold, which is further confirmed by the TEM and solution experiments. The presence of all these three MWCNTs, however, nearly cannot improve the thermal stability effectively at the initial stage of degradation and the temperature corresponding to a weight loss of 5 wt% (T_5 wt%) only shows slight increase in contrast to that of the neat PLA while with increase of decomposition level, the presence of carboxylic and purified MWCNTs retards the depolymerization of PLA evidently, showing remarkable increase in the temperature corresponding to maximum rate of decomposition (T_max). Both the dispersion state and the surface functionalization of MWCNTs are very important to the thermal stability of PLA matrix.     

新型含悬挂羧基聚乳酸的制备及其血液相容性和细胞粘附性

制备了乳酸-苹果酸共聚物(PLMA), 并在前期工作的基础上制备了悬挂羧基聚乳酸(PLMACA), 考察了手臂长度及端功能基团对改善聚乳酸的血液相容性及细胞粘附性的影响. 结果表明, PLMACA同时具有良好的血液相容性和细胞亲和性, 极有可能成为新一代血管(修复)材料.     

接枝咪唑阳离子对聚乳酸结晶行为影响研究

聚乳酸(PLA)受到伽马射线辐照发生降解,加入1-烯丙基-3-甲基咪唑氯盐(AMIMCl)与PLA发生接枝反应,可缓解其降解,得到咪唑阳离子接枝聚乳酸(PLA-IM).核磁共振(NMR)和凝胶渗透色谱(GPC)测试结果表明和直接辐照样品(PLA-S)相比,PLA-IM端羟基含量显著下降,分子量为PLA-S的3倍.利用偏光显微镜(POM)和示差扫描量热仪(DSC)研究PLA-IM等温结晶行为,发现其结晶时仍呈现球晶形貌,晶体生长速度变化相对较小,主要仍受分子量的影响,在相同结晶温度时,PLA-S的晶体生长速度最快,其次是PLAIM,而PLA-L最慢.咪唑阳离子接枝使PLA-IM成核密度显著提高,致使结晶动力学加快,当结晶温度≥125℃时,PLA-IM的半结晶时间小于低分子量的PLA-S.成核能力的提高是导致PLA-IM结晶动力学加速的主要原因.与共混物中离子液体降低PLA成核作用不同,PLA链上接枝阳离子后,离子相互作用有利于成核时高分子链段发生局部有序排列,促进晶核数目的提高.     

Enhanced crystallization,thermal properties,and hydrolysis resistance of poly(l-lactic acid) and its stereocomplex by incorporation of graphene nanoplatelets

Poly(d-lactic acid) (PDLA) and graphene nanoplatelets were used as nucleating agents for poly(l-lactic acid) (PLLA). The graphene (1 wt%) shows a more pronounced effect than PDLA in facilitating PLLA crystallization. Graphene effect on crystallization of stereocomplex (SC) polylactide is also demonstrated. Although medium molecular weight PLLA was blended with a limited content (1 wt%) of low molecular weight PDLA in the presence of graphene (0.5 phr), SC melting temperature is slightly increased without the use of high molecular weight polylactide pair. Also, optimal graphene content (0.5 phr–1.5 phr) promotes crystallization of PLLA homocrystals in the three-component system (PLLA/PDLA/graphene). Graphene additionally enhances Young's modulus and barrier property to thermal degradation of both PLLA and SC systems. Furthermore, PLLA/graphene is more resistant to hydrolysis than PLLA. Likewise, PLLA/PDLA/graphene is more stable than PLLA/PDLA during hydrolysis.