正十八烷封端的聚氧化乙烯的受限结晶和熔融行为——束缚点密度效应

CONFINED CRYSTALLIZATION AND MELTING BEHAVIOR OF POLY (ETHYLENE OXIDE) CAPPED WITH n-OCTADECYL GROUPS——THE EFFECT OF TETHERING DENSITY

通过对一系列不同分子量的聚氧化乙烯(PEO)进行端基改性,得到正十八烷基团双封端的聚氧化乙烯(ODPEO).通过改变PEO的分子量来调整片晶折叠表面端基的束缚点密度,采用差示扫描量热仪(DSC),偏光显微镜(POM)和原子力显微镜(AFM)考察了正十八烷端基对PEO结晶和熔融行为的影响.DSC结果表明,引入端基后,不同分子量的PEO比改性前都更容易形成非整数折叠(NIF)晶体,并且熔点都出现了不同程度的降低.对于104分子量的PEO,正十八烷端基的存在显著降低了PEO的成核能力,并导致片晶折叠表面的自由能轻微增加,但对平衡熔点的影响很小;对于4×103和2×103分子量的PEO,引入端基后平衡熔点发生显著的下降,特别是对4×103分子量的PEO,正十八烷端基的存在限制了一次折叠IF(1)晶体向伸直链IF(0)晶体的增厚.对于2×103分子量的PEO,通过AFM原位观察了ODPEO2k片晶的增厚和结晶,发现引入正十八烷端基后,PEO的NIF晶体稳定性显著提高,同时排入IF(0)晶体的伸直链长度较短.总之,PEO晶体的增厚会增大片晶表面端基束缚点的密度,而烷基链要实现构象熵最大化要求束缚点密度尽可能低,正是这两个过程的相互竞争导致了上述结晶和熔融行为的变化.

Poly （ethylene oxide） （PEO） with different molecular weight （ PEO10k, PE04k, PE02k） was modified by octadecyl end groups. The end group tethering density on the folding surface was tuned by changing the molecular weight of PEO. The crystallization and melting behavior of modified PE0 were studied by differential scanning calorimetry （DSC） , polarized optical microscopy （POM） and atomic force microscopy （AFM）. DSC results show that the PEOs tend to form nonintegral-folding （NIF） crystals after end-group modification. For PEO with molecular weight of 104,octadecyl end groups significantly hinder the nucleation of PEO and cause a slight increase of free energy of folding surface. However, it has very little effect on the equilibrium melting temperature. For PEO4k and PEO2k after end-group modification,the equilibrium melting temperatures greatly depressed. The octadeeyl end group hampers the transformation of PEO4k IF（ 1 ） crystal to IF（0） crystal. In situ AFM observation shows the stability of PEO2k NIF crystals increases greatly and the chain length arranged in the IF （0） crystal is also shortened after introducing the octadecyl end groups. The thickening of lamellae will increase the tethering density of octadecyl end groups on the lamella surface,while the tendency of the octadecyl chains to adopt maximum conformation entropy requests the tethering density to be decreased. The competition between these two processes leads to the above changes of crystallization and melting behaviors.