Effect Of End Groups On Confined Crystallization Of Long Chain Alkanes And Polymers In AAO

In order to explore the effect of end groups on the confined crystallization of long chain alkanes and polymers.AAO-PDP,AAO-C15,AAO-C₂₀,AAO-eicosanol,AAO-eicosane acid,AAO-PBSA,AAO-PDP/PBSA samples were sequentially prepared by melting infiltration method.Using DSC,WAXD,ATR-IR and other test methods to study the melting and crystallization behaviors of the samples.Firstly,the influence of end groups on the confined melting and crystallization behaviors of odd-numbered alkyl chains were investigated.The space-time equivalence was fully reflected in the PDP crystallization process,i.e.the solid-solid phase transition could be realized by reducing the cooling rate or confining the PDP in the nano space.The confinement effect promoted the complete transition of metastable phase to low-temperature ordered phase in PDP.Compared with C₁₅,the end groups phenol ring led to the disappearance of the solid-solid phase transition of alkyl chain at the high cooling rates,and the overlaping of the melting double peaks in the bulk PDP.In the confined environment,the introduction of the end groups phenol ring caused the melting double peaks of the alkyl chain to become a single melting peak,and it also caused the disappearance of the surface freezing monolayer for alkyl chains.WAXD found that solid-solid phase transition of PDP occurred under confined condition,and the absence of(000)reflections indicated that the stacking of the end groups of the alkyl chain layered structure was seriously disturbed.Thermal analysis(TG)showed that the thermal stability of AAO-PDP was greatly decreased.ATR-IR results revealed that the strong confinement effect of the end groups phenol ring greatly weakened the mobility of the alkyl chain.Secondly,the influence of end groups on the confined crystalline melting behaviors of even-numbered alkyl chains were investigated.With the introduction of end group and the increase of end group volume,the crystallization temperature and melting temperature of bulk C₂₀,eicosanol and eicosanoic acid increased in turn.C₂₀,eicosanol and eicosanic acid all appeared new rotating phases after being confined,and the larger terminal carboxyl groups had a greater influence on the movement of the alkyl chain in the confined space.The thermal decomposition temperature of bulk C₂₀,eicosanol,and arachidic acid increased sequentially,and the thermal decomposition process showed one-step weight loss.The confined C₂₀,eicosanol,and arachidic acid were mixed crystals of different crystal forms,and the thermal stability of each crystal form was different,so the confined sample showed the step-by-step weight loss.Both SEM and DSC confirmed that the excess PBSA on the surface of AAO had been completely removed.The Avrami index of bulk PBSA and AAO-PBSA was between 2.3 and 2.7,indicating that the crystals of PBSA and AAO-PBSA were two-dimensional to three-dimensional crystal growth,with heterogeneous nucleation and homogeneous nucleation co-existed.In the process of isothermal crystallization the crystallization rate of PBSA became slower with the enhancement of the confinement effect.There was no diffraction peak corresponding to(012)crystal planes in the WAXD spectrum of AAO-PBSA,indicating that the crystallization behaviors of PBSA in nanospace were partially confined.ATR-IR found that the absorption peak of AAO-PBSA had barely shifted,indicating that the confinement effect of PBSA in the AAO nanopore was not obvious.The introduction of end groups changed the nucleation mechanism of PBSA in confined space to a certain extent,and PDP/PBSA was surface induced nucleation in confined space.The introduction of end groups or confinement effect could confine the crystallization of PBSA,slow down the crystal growth and make the crystal growth more uniform.For bulk PBSA,the introduction of end groups did not change its thermal stability,but in the nano confined space,the end groups seriously affected the molecular chain arrangement and stacking of PBSA,which greatly reduced its thermal stability.WAXD and ATR-IR measurements showed that the confinement effect of PBSA in nano space was enhanced by the introduction of end groups.