Effects Of Nanoconfinement On The Crystallization And Melting Behavior Of Polymers

With the development of composite materials,biomedical materials and electronic devices,people have higher and higher requirements on nano-polymer materials.The analysis and control of nano-polymer material structure plays a decisive role in material properties and applications.Most polymers are semi-crystalline and the crystalline structure is an important part of the regulation of polymer structure.The work of this paper is mainly focused on the regulation of crystallization behavior of polymers under nano-constrained conditions.To systematic study the crystallization behavior of nanotubes and nanorods,a large number of a single pore size distribution of PVDF nanotubes and PHB nanorods were prepared by using anodic aluminum oxide(AAO)template.PHB ultrathin films were also prepared.We use scanning electron microscope(SEM),transmission electron microscopy(TEM)and atomic force microscope(AFM)to observe the morphology,differential scanning calorimeter(DSC)to figure out the crystallization and melting behavior.The crystal structure was characterized by X-ray diffraction(XRD)and infrared spectroscopy(FTIR).The rule and nature of the influence of nano-restricted conditions on the crystallization and melting behavior of polymer nanotubes and nanorods were discussed.The main results are as follows:1.Poly(vinylidene fluoride)(PVDF)nanotubes were fabricated by melt-wetting into porous anodic aluminum oxide(AAO)templates with two different interfacial properties:one is pristine AAO,and the other is modified by FOTS(AAO-F).Their crystallization and melting behaviors are compared with those of a bulk sample.For the PVDF in AAO-F,the nonisothermal crystallization temperature is slightly lower than that of bulk,and the melting temperature is similar to that of bulk.For the PVDF in pristine AAO,when the pore diameter is 200 nm,the crystallization is induced by two kinds of nucleation:heterogeneous nucleation and interface-induced nucleation.On the contrary,in the AAO template with pore diameter smaller than 200 nm,only interface-induced nucleation occurs.The melting temperature of PVDF crystals in the pristine AAO is much higher than that of bulk which can be attributed to the presence of an interfacial layer of PVDF on the template inner surface.The interaction between PVDF and AAO template produces the interfacial layer.Such an interfacial layer plays an important role in enhancing the melting temperature of PVDF crystals.The higher melting peak is always observed when the PVDF is nonisothermally crystallized in the AAO template irrespective of the thermal erasing temperature suggesting the interfacial layer is very stable on the AAO template surface.If the PVDF nanostructures are released from AAO template,the higher melting peak disappears with the enhancement of thermal erasing temperature.2.It was found that the PVDF nanotubes crystallized in the AAO templates exhibit only a small amount of a-PVDF crystals.After releasing from the AAO template and aging at room temperature,the crystallinity of PVDF nanotubes increases obviously with time,indicating the occurrence of secondary crystallization and/or structure reorganization of the PVDF nanotubes.During this process,except for the formation of β-PVDF crystals at very short time and y crystals at relatively longer aging time(e.g.,10 days),some new crystals with very low melting temperature of around 120℃ are detected.By combining with FTIR results,after releasing from the AAO template,PVDF nanotubes soon form molecular chain segments with TTTGTTTG’ conformation(the same as the molecular chains in the γ-PVDF crystal form)which gradually induce the related y-PVDF crystals with ultraslow kinetics(after aging for 10 days).Moreover,a large amounts ofγ’-PVDF crystals form in the heating process at around 130℃.Theseγ’-PVDF crystals may grow directly from amorphous phase.The large amount of γ and γ-PVDF fabricated by aging α-PVDF nanostructures have never be observed before.This provides a new way for developing PVDF nanomaterials with high content of β and γ phase.3.We have also identified two different relaxations of PHB nanorods corresponding to the interfacial effect and spatial confinement,respectively.While the relaxation correlated to the interfacial effect with a slower relaxation time strengthens,the relaxation corresponding to spatial confinement with a faster relaxation time(bulklike α-relaxation)weakens with decreased pore size.The reduced crystallization kinetics of thin film is attributed to the reduction of long-range chain mobility.By contrast,the inhibited crystallization of PHB nanorods in AAO is attributed to the reduced segmental dynamics and the reduced chain mobility of PHB layer at interface.Stable intermediate structures of PHB confined in narrow pores(diameter<100 nm),which have never been observed in bulk,are obtained for the first time.In larger pores(pore diameter>100 nm),crystallization occurs both in the center and interfacial regions in contact with AAO inner wall.This implies that the strength of interfacial layer weakens with decreased curvature and has been further proved.Besides,there is a gradient in the crystallinity,and the inner is higher and the outer is lower.4.Poly(3-hydroxybutyrate)(PHB)is taken as an example to explore whether the interfacial effect of curve surface is the same as flat surface.The crystallization behavior and morphology of poly(3-hydroxybutyrate)(PHB)ultrathin films sandwiched between Si wafers and amorphous thin polymer layers were studied by using grazing incident X-ray diffraction(GIXD)technology.Two kinds of amorphous polymers,i.e.,atactic polystyrene(aPS)and poly(vinyl phenol)(PVPh),were used to check the influence of interaction between the PHB and capping layer on the crystallization behavior of PHB.The results show that the crystallization behavior of the PHB thin film depends on the film thickness,crystallization temperature and the capping layers.The PHB ultrathin films of 16 nm in thickness cannot crystallize and an amorphous layer is always obtained within the experimental time scale irrespective of crystallization temperature and capping layer properties.When the PHB layer is 60 nm thick,flat-on lamellae of PHB form under the PS layer at any crystallization temperature.This implies that the PS layer favors the formation of flat-on PHB lamellae in thin films,while the PVPh layer encourages the formation of edge-on lamellae.For PHB films of 110 nm,edge-on lamellae show up at a lower crystallization temperature under the PS layer.In contrast,flat-on and edge-on lamellae were identified under the PVPh layer at lower crystallization temperatures.