| Lamellar crystal-dominated(LCD)polymer surfaces hold great superiority and broad prospects in polymer surface engineering.Usually,the amorphous region is inevitable when polymer melt crystallizes.How to effectively remove the amorphous phase is the key to realize the LCD surface of polymer material.In the past,many attempts have been done to remove the amorphous phase by polymer dilute solution crystallization or chemical etching.However,these processes are complex and inefficient.It is of great scientific value and engineering significance to explore an efficient method to achieve LCD surface by controlling the crystallization of polymer melt.Here,we give a first report of achieving LCD surfaces of polymer films via melt stretching-induced free surface crystallization,which is established on the difference of chain dynamics between free surface and bulk polymer melt.Also the underlying mechanism has been well analyzed,which provides a new principle and method for preparing the LCD surface of polymer materials.The specific research contents are as follows:The free surface crystallization behavior of polyethylene(PE)induced by melt stretching was studied by means of X-ray scattering,SEM and AFM.It was confirmed that imposing the stretching flow on supercooled PE melt could lead to the formation of LCD surface.The resultant surface was covered directly by orientated and edge-on lamellae within a surface depth of tens to hundreds of nanometers,while the normally existing amorphous phase was almost avoided.Such crystallization-driven formation of LCD surface has been ascribed to the heterogeneous chain dynamics of melt free surface,that is,high chains mobility,low viscosity and loose chains entanglement,which facilitates the complete chains disentanglement during crystallization.In addition,it was shown that the surface morphology was controllable with respect to lamellar orientation,spacing and depth by changing the melt stretching strain,quenching the deformed melt or designing the crosslinked PE.Meanwhile,owing to a possible kinetics competition between crystallization and chains disentanglement,the structural spacing(285～763 nm)of surface lamellae hold a positive correlation with the lamellar depth(110～478 nm).Because the chain dynamics effect of free surface is inherent for polymer materials,the melt stretching approach to construct LCD surface has been demonstrated to be applicable to other semicrystalline polymer systems,such as polybutene-1(PB-1)and polylactic acid(PLA).For PLA material,the affecting factors on melt stretching induced free surface crystallization were studied,including stretching strain(0～4ε),temperature(125～145~oC),molecular weight(1.4～4.0×10~5 g/mol)and crystallization time(0～20 min).The results showed that increasing the melt stretching strain could lead to a change of PLA surface morphology from spherulite to oriented lamellae.When the temperature was reduced or the supercooling degree was increased,the surface lamellae displayed a higher orientation and distribution density due to the acceleration of nucleation and inhibition of chains relaxation at low temperature.In addition,high molecular weight PLA(4.0×10~5 g/mol)had a strong chains entanglement constraints on its surface,which was not conducive to the chains disentanglement during crystallization and also caused a pulling effect on lamellae.As a result,the average orientation of surface lamellae was weaker than that of medium molecular weight samples,similar to the phenomenon of cross-linking.Therefore,the conditions of high strain,high supercooling degree and medium molecular weight(2.1×10~5 g/mol)are more conducive to the formation of highly oriented LCD surface.Finally,by liquid nitrogen quenching of PLA after crystallization with different times,the structural evolution on free surface was frozen at different stages.It was confirmed that,after melt stretching,the free surface had experienced a crystallization process of lamellar protruding,touching and shape distortion.This reveals the formation law of melt stretching induced LCD surface. |
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