【作者】 李丽；

【导师】 黄启谷；

【作者基本信息】 北京化工大学 ， 化学， 2020， 博士

【摘要】 材料的结构决定了材料的性能,材料的性能又影响了材料的用途,因此,对材料的结构进行调控一直是材料界的研究热点。其中,附生结晶是调控材料结构的重要手段之一,能够控制材料的晶型和取向结构。因此,通过附生结晶控制结构从而调控材料的性能备受关注,得到了广泛研究。本论文以生物可降解聚酯左旋聚乳酸(PLLA)、左旋聚乳酸-聚己内酯嵌段共聚物(PLLA-b-PCL)及其共混物为研究对象,系统研究了其在取向聚乙烯(PE)薄膜基底上的结晶行为,探讨了 PLLA自身化学结构、PLLA-b-PCL嵌段共聚、以及PLLA-b-PCL/PLLA共混等对PLLA在取向PE基底上附生结晶行为的影响。实验结果表明,PE基底可以诱导PLLA附生结晶,生成两组不同取向的侧立(Edge-on)片晶。其中一组片晶中的PLLA分子链与PE分子链方向垂直取向,即PLLA片晶平行于PE分子链方向(简称平行片晶),另一组PLLA片晶与PE分子链约呈64°夹角(简称倾斜片晶)。由于PLLA在取向聚丙烯基底上也可以观察到同样的取向结构,这种特殊的取向行为难以用晶格匹配加以解释,因此我们从取向PE基底诱导PLLA拟态附生结晶形成平行片晶和拟态附生结晶产生的PLLA平行晶体诱导玻璃态PLLA均相附生结晶形成倾斜片晶的角度进行了解释,并通过附生接触面内和自由表面处PLLA片晶的不同取向行为验证了这种解释。文中进一步研究了 PLLA自身化学结构和结晶温度对PLLA在取向PE基底上附生结晶时的影响,结果发现,PLLA的均相附生结晶受等规度和冷结晶温度的影响,PLLA的均相附生结晶能力随等规度和冷结晶温度的降低而降低。差式扫描量热仪(DSC)的结果表明:在低温冷结晶时,PLLA的半结晶时间比PLLA/PE的长很多,此时有利于PLLA的异相附生结晶,可以生成更多的平行片晶;在高温冷结晶时,PLLA与PLLA/PE的半结晶时间的差距缩短,因此异相附生后,均相附生随即发生,生成的倾斜片晶的数量比低温时多。在同样的冷结晶温度下,PLLA倾斜方向片晶的出现频率随着PLLA等规度的降低而降低。DSC、傅里叶变换红外光谱仪(FTIR)的实验结果证明,高等规度PLLA的均相附生能力强,而且均相附生结晶开始的时间与异相附生结晶接近,因此附生时会形成明显的交叉片晶结构。而低等规度PLLA的均相附生能力差,异相附生结晶开始很久后,均相附生才开始发生,因此主要形成edge-on的平行片晶结构。基于PCL/PE的平行链附生结晶行为,文中设计合成了不同分子量的PLLA-b-PCL嵌段共聚物,考察PLLA链、PCL链通过化学键相互牵制受限时,在PE基底上的附生结晶情况,并分析了 PLLA、PCL链的长度对其在取向PE基底上附生结晶的影响。研究发现,不同分子量的PLLA-b-PCL嵌段共聚物在PE基底上结晶时,PCL组分的附生结晶效果都较好,而PLLA组分的附生效果较差,但随着PLLA的分子量的增大,嵌段共聚物中PLLA组分的附生结晶效果有所改善。这主要是因为在附生结晶过程中,PLLA组分是冷结晶,且PLLA与PCL通过化学键连接,PLLA组分的运动能力会受到PCL组分的抑制,导致其附生效果差。但是随着PLLA分子量的提高,PLLA链长度增加,受到PCL链的影响变小,所以其附生结晶能力随着PLLA分子量的提高而提高。而本来柔性就高的PCL组分是从熔体降温结晶的,分子链的运动能力强,受到PLLA链的影响小,其附生结晶效果较好。通过将PLLA-b-PCL与PLLA共混,探讨了 PLLA-b-PCL对PLLA在取向PE基底上附生结晶行为的影响。研究发现,PLLA-b-PCL/PLLA共混物在PE基底上结晶时受到共混比影响:当共混比大于30/70时,共混物中的PLLA在PE基底上结晶时不发生取向;当共混比小于30/70时,共混物中的PLLA可以在PE基底上定向附生,生成交叉片晶结构,且随着共混比例的降低,PLLA沿PE分子链方向排列的片晶逐渐增加。更多还原

【Abstract】 It is well known that the properties of polymeric materials depend on their multiscale structures,which determine the application fields of the materials.Thus,many techniques have been developed to control the multiscale structures of the materials.Among them,epitaxial crystallization is an effective method to control the crystal and orientation structures of polymers.Therefore,epitaxy of polymers has been widely studied.In this thesis,the epitaxial crystallization of poly(L-lactide)(PLLA),a biodegradable polyester attracted great attention but with limited application,and its poly(L-lactide)-b-poly(ε-caprolactone)diblock copolymers(PLLA-b-PCL)as well as the PLLA-b-PCL/PLLA blends on highly oriented polyethylene(PE)substrates was studied.The results indicated that crystallization of PLLA on oriented PE substrate results in the formation of two populations of oriented PLLA lamellae.One population of the PLLA lamellae is oriented parallel to the PE chain axis direction,i.e.,the molecular chains of both polymers are perpendicularly oriented.The other set of PLLA lamellae is tilted ca.64° to the PE chain axis.This kind orientation of PLLA on oriented PE substrate cannot be explained simply by lattice match,since similar orientation has also been observed when crystallizing PLLA on oriented polypropylene substrate.We proposed that the parallel lamellae result from soft heteroepitaxy,in which the grooves on the highly oriented PE film surface guide the orientation of the PLLA chains,while the tilted PLLA lamellae result from the homoepitaxy of PLLA on parallel aligned PLLA with parallelism of helical paths.This has been successfully confirmed by the different tilting behavior of PLLA lamellae in top free surface and in the contact plane with PE.Furthermore,the influence of PLLA tacticity and crystallization temperature on the soft and homoepitaxy of PLLA on PE substrate has also been studied.It was found that the homoepitaxial crystallization ability decreases with decreasing tacticity of PLLA and crystallization temperature This is related to the crystallization kinetics of PLLA.Differential scanning calorimetry(DSC)results showed that the half crystallization times(t1/2)of PLLA is longer than the t1/2 of PLLA on the PE sustrate at low crystallization temperature.Therefore,the heteroepitaxy PLLA in contact with PE is predominate,which results in the formation of almost only parallel aligned lamellae.The t1/2 difference between the bulk PLLA and PLLA/PE gets smaller and smaller with increasing crystallization temperature.As a result,the homoepitaxy of PLLA takes place immediately after the begining of heteroepitaxy,leading to the formation of biaxial orientation of PLLA.This indicated that the homoepitaxial crystallization of PLLA is more favored at higher temperatures.Besides,the frequency of the tilted lamellae decreases with decreasing the tacticity of PLLA when the PLLA crystallized at the same temperature.The DSC results indicated the crystallization of PLLA with high tacticity is kinetically favored,and the high tacticity provides a favorable circumstance for the homoepitaxy of PLLA.And the fourier transform infrared spectrometer(FTIR)showed the time-lag of the onset crystallization time between PLLA and PLLA/PE decreased with increasing the tacticity.Thus,the homoepitaxy of PLLA will be prohibited when the tacticity is low,and the frequency of tilted lamellae decreases accordingly.Taking into account that the poly(ε-caprolactone)(PCL)can crystallize epitaxially on oriented PE substrate with both molecular chains aligned parallel to each other,we have synthesized the poly(L-lactide)-b-poly(ε-caprolactone)diblock copolymers(PLLA-b-PCL)with different molecular weight and studied their epitaxial ability on PE substrate.It was found that the epitaxy of PCL rarely affected by PLLA.The PCL chain axis is always parallel to the chain axis of PE when PLLA-b-PCL crystallized on PE substrate.However,only part of PLLA chains is perpendicular or tilted 64° to the chain axis of PE.The epitaxial capability of PLLA on PE is improved by increasing the molecular weight of PLLA component.The epitaxial crystallization of PLLA in PLLA-b-PCL/PE is realized by cold crystallization,and the PLLA and PCL are connected by chemical bond.Thus,the chain mobility of PLLA were inhibited,resulting in poor epitaxial effect.This hindering effect decreases as the molecular weight of PLLA increase.For the epitaxial behavior of PLLA-b-PCL/PLLA blends on oriented PE substrate,it was found that the epitaxial ability of PLLA is composition dependent.When the blend ratio of PLLA-b-PCL/PLLA is less than 30/70,the oriented cross-hatched lamellae can be observed.Meanwhile,the frequency of parallel lamellae decrease with increasing the blend ratio.When the blend ratio of PLLA-b-PCL/PLLA is greater than 30/70,the PLLA lamellae are then randomly oriented.In other words,the PLLA loses its epitaxial ability on the PE substrate.This may be attributed to the different interchain interaction of PE with PCL and PLLA,respectively.更多还原