摘要
利用哈克旋转流变仪在一定的温度和压力条件下对聚乳酸(PLA)熔体施加应变为正弦变化的振荡剪切场,从而对PLA分子链进行高效的降黏解缠.结果表明,当温度为190°C、样品厚度为1.5 mm时,PLA熔体在振动频率和剪切应变分别为3.5 Hz和50%处呈现最佳的降黏效果,其黏度相比于未经处理的熔体黏度下降了3~4个数量级.同时,凝胶渗透色谱(GPC)测试结果表明经过振荡剪切处理的PLA其分子量基本不变,表明熔体黏度的大幅下降是由PLA分子链有效解缠结导致而非分子链降解.示差扫描量热仪(DSC)结果表明,解缠效果最佳的PLA样品相比于未经处理的样品呈现出更低的玻璃化转变温度和较高的结晶度,进一步验证了熔体解缠的效果.不仅如此,我们研究了不同退火时间(1~30 min)和温度(180~200°C)对PLA分子链重新恢复缠结的影响,发现随着退火温度和时间的增加,PLA在120°C下等温结晶的半结晶时间不断增加,并向未经解缠处理的PLA样品的半结晶时间不断靠近,表明振荡剪切导致的解缠能在低温度下保持较长时间,在高温下快速复缠.
The Haake rotational rheometer was employed to disentangle the polylactide melt by well-controlled oscillatory shear stress with sinusoidal strain and to monitor the melt viscosity in real time. Fisrtly, a PLA sample was disentangled with different strains at various frequency ranges, and the results indicated that the PLA melt represented the lowest melt viscosity which was four orders of magnitude lower than that of the PLA without any treatment when the strain was 50% with the frequency at 3.5 Hz. Then, the molecular weights of all these PLA were measured by gel permeation chromatography （GPC） and almost no change was detected after the oscillatory shear. Taken into account the results of the melt viscosity and the molecular weight measurement, it was reasonable that the significant reduction of PLA melt viscosity was attributed to the effective disentanglement of PLA chains, rather than their degradation. Furthermore, the effect of oscillatory shear on glass transition, crystallization and melting behavior was also studied. It was found that the effective disentanglement of PLA chains was achieved by oscillatory shearing, leading to a lower glass transition temperature and a cold- crystallization temperature together with largely improved crystallinity of PLA. Simultaneously, when compared to the PLA melt without any treatment, the isothermal crystallization of PLA at 120 ~C with the lowest melt viscosity also demonstrated that the oscillatory shear could disentangle the PLA melt and thus accelerated the crystallization of PLA. More importantly, the influence of annealing time （1 - 30 min） and temperature （180 - 200 ~C） was investigated as well. The semi-crystallization time at 120 ~C of disentangled PLA constantly increased with the increasing annealing time and temperature, which got gradually closer to that of PLA without any treatment. These results demonstrated that the disentanglement could be maintained at relatively low temperature and re-entangled rapidly at relatively high temperature. In summary, the Haake rotational rheometer, the common test instrument for the rheological properties of polymer melt, can be employed for the investigation of the disentanglement of polymer melt, which is not merely a simple and effective method to disentangle the polymer melt, but also a well-controlled and real-time monitoring approach for systematically investigating the disentanglement of polymer melt.
作者
刘天宇
蒋维娇
杨卫星
张琴
傅强
Tian-yu Liu;Wei-jiao Jiang;Wei-xing Yang;Qin Zhang;Qiang Fu(College of Polymer Science and Engineering,State Key Laboratory of Polymer Materials Engineering,Sichuan University,Chengdu 610065)
出处
《高分子学报》
SCIE
CAS
CSCD
北大核心
2018年第8期1107-1115,共9页
Acta Polymerica Sinica
基金
国家自然科学基金(基金号21627804
51721091)资助项目
关键词
振荡剪切
聚乳酸熔体
解缠结
等温结晶行为
Oscillatory shear
Polylactide melt
Disentanglement
Isothermal crystallization behavior
