Study Of The Confined Nucleation And Crystallization Kinetics And Melting Reorganization Behavior Of Crystalline-amorphous Diblock Copolymer By Ultrafast Differential Scanning Calorimetry

During the past few decades,many efforts have been devoted to explore the crystallization and melting behavior of polymers under confinements.Block copolymers are widely used as templates to study the confinement effect because their self-assembled microdomains provide a convenient way to achieve the confinement under nanosacles.When the crystallizable phase is confined in a large number of microdomains,the crystallization and melting behavior is dramatically different with that of the homopolymer,which has been widely reported.However,there are still some basic issues not clear,such as the nucleation and melting reorganization processes under confinements.Recently,the development of ultrafast differential scanning calorimetry(UFDSC)with temperature scanning rates up to 105 K/s,gives us excellent opportunity to separately study both nucleation and crystallization at a certain crystallization temperature over a wide temperature range on submillisecond time scale,and to capture the metastable state and follow their reorganization process.Herein,we systematically studied the nucleation and crystallization kinetics and melting reorganization behavior of poly(?-caprolactone)-b-poly(4-vinylpyridine)(PCL-P4VP)diblock copolymer by UFDSC,and compared with that of PCL homopolymer with similar molecular weight.Firstly,PCL homopolymers and PCL-P4VP diblock copolymers were synthesized via living ring opening polymerization(ROP)and atom transfer radical polymerization(ATRP)in sequence.The composition and morphology of PCL-P4VP copolymers were characterized by 1H NMR,GPC,FTIR,SAXS,TGA and DSC.The results show that the thermal stability is improved in the PCL-P4VP copolymer.The copolymer has a lower melting temperature than the homopolymer because the constraint of the glassy P4VP block results in less stable crystals in the copolymer.The morphology of the copolymer is preserved after crystallization of the PCL block.Due to ToDT>the glass transition of amorphous phase(Tg(P4VP))>the crystallization temperature of crystallizable temperature(Tc(PCL))and the phase separation strength xN>10.5,the PCL-P4VP bock copolymer is a strong segregated hard confinement system.Secondly,we investigated the nucleation and crystallization kinetics of lamellar crystalline-amorphous diblock copolymer PCL-P4VP by UFDSC.The critical cooling rate(ccr)to get the fully amorphous PCL is one order of magnitude slower than that for the PCL homopolymer with the similar molecular weight.Isothermal nucleation and crystallization of PCL block in the PCL-P4VP copolymer and the PCL homopolymer were studied covering times from 10-2 s to 103 s and temperatures from 200 K(10 K below the glass transition temperature of PCL)to 300 K(about 40 K below the equilibrium melt temperature of PCL).It was found that the PCL block in the PCL-P4VP copolymer experienced a slower homogeneous nucleation rate as well as crystallization rate than the PCL homopolymer,indicating that even the local nucleation events of PCL chains is affected by the long-range glassy P4VP in copolymer.The confinement also hinders the long-range diffusion of PCL chains and becomes more effective once the chains get the mobility from the glassy state at crystallization temperatures above the Tg.Another effect of the confinement is the lower Avrami index in copolymer than that in homopolymer attributed to the restricted growth dimension under confinement.At last,we studied the melting and reorganization behavior of the lamellar PCL-P4VP by UFDSC.Recrystallization is so fast that the heating rate needs to be as high as 20,000 K/s to prevent the recrystallization of PCL homopolymer crystals isothermally formed at 260 K and for PCL-P4VP crystals,6,000 K/s is fast enough,indicating the recrystallization process is hindered in nanoscale confined copolymer.Without the influence of recrystallization on heating,annealing of these initial crystals were performed in the temperature range from 270 K(10 K below the onset of melting peak of initial crystals)to 310 K(melting peak temperature)on time scale starting from 10-4 s.Two different reorganization pathways were observed in this annealing temperature range.We found the higher melting peak temperatures of crystals formed in the melting-recrystallization of the copolymer than that of the homopolymer.The slower recrystallization rate in the copolymer was discussed from the viewpoint of the nanoscale spatial constraint and the intermediate state prior to recrystallization.