Molecular Simulations Of Lamellar Crystal Thickness Of Polymers

21st century is the century for polymers.Materials by polymers continuously play a central role on industrial productions and commodities via the renewable properties.For better polymer material performance,it is quite significant to understand the underlying mechanism and thereby modify the crystalline state of polymers.The discovery of polymer single crystal and the subsequent concept of chain-folding act as the milestone for polymer crystallization research.In this field,the limited lamellar crystal thickness attracts wide research interest.The selection mechanism for nano-scale crystal thickness and the kinetics of lamellar crystal growth are the key points remaining in polymer crystallization.Breakthrough is required in new century,therefore we can start our systematic work from the study on growth kinetics,in particular on the influence by lamellar thickness.Various key concepts such as entropic elasticity exist in polymer science.Advanced experimental and theoretical techniques are permanently demanded.Apart from them,molecular simulation is able to provide molecular details within the mesoscopic length scale and time window for polymer crystallization.Since abundant random events root in chain-like polymer systems,dynamic Monte Carlo simulation can supply the reliable statistical results by its high efficiency.Hu et al has developed the methodology on polymer crystallization and phase separation.The whole thesis is based on this technique.The main simulation work includes the research on the selection mechanism of finite crystal lamellar thickness in polymer crystallization and the lamellar thickness influence on the co-crystallization kinetics in binary mixtures of long and short chain polymers.Meanwhile,the author has researched the reversible melting at fold-end surface of polymer lamellar crystals,via the combination of advanced thermal analysis and dynamic Monte Carlo simulation.The implicit mechanism of lamellar thickness fluctuation has been illustrated therein as well.Lamellar thickness acts as the thread throughout the whole context connecting the three subjects.In chapter 1,we introduce the morphology in polymer crystallization and the corresponding models,aiming at providing an intuitive outline.Afterwards we introduce the nucleation concept and its kinetics,and then the metastable states in polymer crystallization.In order to interpret various unique phenomenon in crystallization kinetics,we depict some common and some frontier kinetic models,particularly for the explanation of crystal thickness determination.We end up with the introduction of polymer crystal melting,from the two aspects as irreversible and reversible melting.In chapter 2,we introduce the simulation techniques by dynamic Monte Carlo method.We start by describing the 3-dimensinonal lattice model.Then we provide the details for periodic boundary conditions,the micro-relaxation model including the single site jumping and sliding diffusion.The energetic parameters adopted in simulation have been subsequently introduced,evolving the parallel interaction among bonds,the conformational parameter,the sliding diffusion barrier and the interaction parameter among different components.The main sampling method,i.e.the Metropolis sampling,has been explained as a summary.In chapter 3,we introduce the innovative work on crystal thickness selection mechanism of polymer crystallization.We first introduce the role by lamellar crystal thickness on crystallization thermodynamics and kinetics.Then we describe the concept of minima stable lamellar thickness.We compare our recent research results concerning the crystal growth in polymer thin films with the derived conclusions by Lauritzen-Hoffman model and Sadler-Gilmer model.We rationalize our results and figure out the current doubt on the previous models.Afterwards we introduce the unique technique for measuring the minima thickness.We confirm its existence and prove its temperature dependence same as the tendency in melting line by Strobl.We collect a list of lamellar crystal thicknesses and minima thicknesses at various temperatures.The excess thickness between those decreases with rising temperature.Lauritzen-Hoffman model fails to interpret the temperature dependence of excess thickness,since the deduced opposite trend is in contradictory to the experimental observations.We further investigate the stem sliding probabilities at the lamellar crystal growth front.It appears to be two stages divided by the minima thickness.One is controlled by secondary nucleation and the other is the excess thickness gain process via crystal thickening.We figure out one reasonable kinetic equation.The numerical fitting by this kinetic equation matches well with our simulation data.Additionally,the kinetic equation predicts the upper limit for growth lamellar thickness within the crystallization temperature range.In chapter 4,we introduce the research work on co-crystallization kinetics of binary mixtures of long and short polymer chains dominated by lamellar thickness.The co-crystallization rate shows two linear dependences on the long chain mole fraction.Within each linear stage,co-crystallization rate increases with increasing long chain fraction linearly,due to the higher nucleation efficiency by long chain.Different slopes of two linear regimes correspond to different lamellar crystal thicknesses.From lamellar crystal growth rate and thickness dependence on molecular weight,we find that short chain tends to perform integer folding at the chosen temperatures.It gives rise to thinner crystal thickness compared to that conventionally determined by temperature.As a consequence the co-crystallization rate will slow down.At the concentrated end of short chains,the integer folding by short chains dominates the crystal thickness,thus the co-crystallization rate curve bends toward the slower rate direction.In chapter 5,we introduce the research on the variation of lamellar thickness with the temperature modulation.We reveal the inherent mechanism of reversible melting at fold-end surface of lamellar crystal by combining fast scanning chip calorimetry and dynamic Monte Carlo simulation.Fast scanning chip calorimetry provides wide scanning rate range,thus we can utilize this apparatus for wide frequency temperature modulation.We choose a form and ? form isotactic polypropylene crystals for analysis.The error by different chemical structure can be avoided consequently.We acquire the specific reversing heat capacities by evaluating the sample masses on the chip sensor.It turns out to be that both samples have specific reversing heat capacities beyond the standard value for amorphous state.At the low frequency modulating range,the specific reversing heat capacity of ? form crystal is higher than that of a form.We attribute the higher value to the more loosely packing crystalline lattice in ?form crystal.The reversible melting at the fold-end surface of ? form crystal is more intensive due to the more mobile sliding diffusion.This phenomenon and the underlying mechanism have been further confirmed by our parallel simulation.It proves that the reversible melting does exist at the fold-end surface of crystal.The degree relates with chain sliding diffusion ability.The more easily chain slides inside the crystalline lattice,the more intensive the reversible melting is.Besides,the reversible melting at the fold-end surface of lamellar crystal accompanies the periodic fluctuation of lamellar thickness.In chapter 6,we make an overall summary for the main content.Furthermore we conceive the possible approaches for the future work.