Epitaxial Crystallization Of PLLA,PLLA-b-PCL Block Copolymer And Their Blends Induced By Oriented Polyethylene

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(?-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(?-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(?-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.