Flash DSC Study On The Kinetics Of Polymer Crystallization

The physical properties of semi-crystalline polymers and the application performance of their products depend largely on the crystalline structure,including the crystalline forms,crystal morphology and crystallinity.The crystallization process of polymers is controlled by thermodynamic factors;however,it is affected significantly by kinetic factors in the meantime.Flash DSC,with characteristics of its less sample consumption,super-fast scanning rate and super-high time resolution,is considered as a novel method for studying the crystallization and annealing kinetics of semi-crystalline polymers.First,in Chapter 1,we have reviewed the basic concepts of nucleation and lamellar growth kinetics of semicrystalline polymers,as well as the application of Avrami equation in describing the crystallization kinetics of semicrystalline polymers.Next,in Chapter 2,we have surveyed comprehensively the principle,history and application development of Flash DSC.Taking advantage of its unique performance,Flash DSC can be used to control the thermal history of samples accurately and suppress effectively the structural reorganization of initial crystals during the following heating and cooling process.Flash DSC is expected to make greater breakthroughs in polymer crystallization kinetics.In Chapter 3,we have utilized Flash DSC to link the fine differences in crystallization kinetics of UHMWPE with its molecular weight successfully.The results have shown that UHMWPE exhibits a faster total crystallization rate in heterogeneous nucleation region when comparing with HDPE counterpart.The relatively slower relaxation rate of UHMWPE,due to its serious entanglement effect from its long molecular chain and long branching,results in the strong memory effect and contributes to the faster crystallization.In Chapter 4,the crystallization kinetics of PLLA with nucleating agent in the global temperature range have been studied by means of ultra-fast cooling rate(-4000 K s-1).Compared with pure PLLA,the crystallization rate of nucleated PLLA with TMC306 is faster.This is mainly due to the fact that self-assembled crystals of TMC306 can enhance greatly the primary nucleation and crystal growth when the content of TMC306 is high.Self-nucleation experiments also confirm that lower self-nucleation temperature increases the crystallization rate of PLLA at high undercooling.The higher self-nucleation the temperature,the less nucleation effect can be detected in the low temperature zone.In Chapter 5,we have studied the polymorphism of PLLA.Under the conditions of normal melt-and cold-crystallization,PLLA is easy to form a' and a crystals,and the a' forms transform into the a forms irreversibly.However,the transformation processes and related mechanism are still controversial.Flash DSC can inhibit the reorganization of a' crystal by means of its ultra-high heating and cooling rate.The annealing behavior of a' crystal prepared at low temperature is studied at a higher temperature.The annealing transformation spectra of ?' crystal in a wide temperature range(132-152?)have been obtained.It is found that the transformation of a'-a crystals is achieved by the adjustment of chain segment in the local domain in the lower annealing temperature range(below 140 C),which is attributed as so-called solid phase transformation mechanism.At relatively high temperatures(above 148?),the transformation of a'-a crystals is achieved by melt recrystallization.In the middle temperature range(140-146 C),there are both solid phase transformation and melt recrystallization.No essential difference between ?' and ? crystals is claimed,although the regularity of molecular chain stacking is different.In Chapter 6,the host-guest complex with thiourea and PEO has been studied.Since the long chain PEO is confined in the cavity composed by thiourea,there exists obvious competition between pure PEO crystals and the complex crystals.The isothermal crystallization studied by Flash DSC has shown that the formation of the complex crystals form is obviously time-dependent,and the complex crystals occur earlier than the pure PEO ones.During the heating process,metastable complex crystals(Form ?)transform into more perfect complex ones with the higher melting point via melting-recrystallization at low heating rate.With the completion of complex crystallization,pure PEO begins to crystallize and results in a sharp decrease in the glass transition temperature of the system.Complex crystals are quite stable and can coexist with pure PEO crystals.Finally,we have summarized the results and made a perspective on the further development of polymer crystallization.Investigations on the crystallization and annealing kinetics by aid of Flash DSC pave a simpler and more intuitive way for the theoretical and practical development of semi-crystalline polymers.