The Aggregation Structure And Crystallization Mechanism Of Poly(n-alkyl Methacrylate)s

Bottlebrushes with crystallizable side chains are a special kind of graft copolymers.The side chains will crystallize when they are long enough.Therefore,they possess special aggregation structure compared to other graft copolymers.They have been widely used in nanotemplate,photonic crystal,biological medicine and so on due to their special aggregation structure.The study of aggregation structure and crystallization mechanism is of great significance to the development of new bottlebrushes with special functions.Poly(n-alkyl methacrylate)s(PMA(n)s)are widely studied as typical bottlebrushes.In this thesis,the aggregation structure,crystallization behavior and crystallization mechanism of PMA(n)s have been investigated via SAXS,WAXD and flash DSC techniques.The results are as follows:1.Microstructure of PMA(n)s bottlebrushesBased on the detailed analysis of SAXS and WAXD data,a multi-stage aggregation structure model of PMA(n)s was proposed.The multi-stage aggregation structure consists of ordered region and amorphous region.The amorphous region is embedded in the ordered region.In the ordered region,main chains and side chains arranged regularly to form layered structure.High entanglement of the main chain and the inability to provide enough space for crystallization of side chains is the reason of the formation of multi-stage aggregation structure.2.A new way for crystallinity determination in PMA(n)sThe study of the aggregation structure of PMA(n)s indicates that some side chains cannot participate in crystallization.Therefore,we introduce the fraction of crystallizable side chain(fc)to study the crystallization behavior of side chains of bottlebrushes.When the chemical structure,grafting degree and way of linkage between main chains and side chains are the same,the bottlebrushes with different side chain length have the same fc.The slope of the straight line obtained by the melting enthalpy to the number of carbon atoms in the side chain is equal to?HCH2×fc,where ?HCH2 stands for the melting enthalpy of one mole alkyl group packed into the crystal.For PMA(n)s,the minimum value of fc can be estimated to be about 0.67 from SAXS data.Therefore,it can be estimated that the maximum value of ?HCH2 is 5.74 kJ/mol when the alkyl side chain packed into hexagonal crystal.On this basis,the crystallinity of bottlebrushes can be calculated according to Xc=fc×Nc/N where N and Nc stands for the number of carbon atoms in a side chain and the number of carbon atoms packed in the crystal.3.Effect of graft degree on the crystallization behavior of PMA(n)sFirstly,a series of(P(MA(18)-co-MMA)copolymers with different graft degree(G<1)were prepared by changing the molar ratio of SMA and MMA.When the chemical structure,graft degree and way of linkage between main chains and side chains are the same,the fc of copolymers(G<1)will be smaller than homopolymer.When the graft degree is higher than or equal to 1,fc will be not affected by the graft degree.The melting temperature of PMA(n)has a high dependence on the thickness of the crystal along the side chains.With the decrease of graft degree,the melting temperature,dc and Nc all decrease.When the graft degree decreases,the aggregation structure also changes obviously:Bottlebrushes still have multi-stage ordered structure,and the layered structure formed by alternate main chains and side chains in the ordered region is still retained.On the other hand,the continuous phase in the multi-stage ordered structure changes from ordered region to amorphous region.The structure model is still consistent with the structure of PMA(n)s(G=1),but the continuous phase and the dispersed phase exchange.4.Crystallization mechanism of PMA(n)sThe crystallization mechanism and crystallization kinetics of PMA(18)were studied in detail by flash DSC and synchrotron X-ray scattering techniques.A two stage crystallization mechanism of PMA(18)is proposed.The first stage is the appearance and fusion of a large number of small crystals.In this stage,the crystal growth proceeds dominantly along the main chain direction,so the adjustment of main chains mainly occurs in the beginning of this stage.The crystallization in the second stage is dominated by the growth along the side chain direction.In the first stage,the melting enthalpy of PMA(18)increases rapidly due to the appearance of a large number of small crystals.At the same time,the fusion between adjacent crystals interfaces reduces the lateral surface free energy of crystals,thus the melting temperature increases.At the same time,the adjustment of the distance between the main chains occurred in the initial stage of crystallization,which indicates that the fusion process is mainly the growth of the crystals along the main chain direction.In the second stage,the growth of crystals along the side chain direction can further stabilize the crystals making a less dependency of melting enthalpy to the melting point than that in the first stage.In addition,the crystal will melt simultaneously along both main chain direction and side chain direction when annealing at a temperature higher than the crystallization temperature.The melting along the side chain direction will stop first.Long time annealing at high temperature is beneficial to the crystal thickening along the side chain direction.But the crystallinity and fc will decrease when recrystallization complete.Finally,the results of pre-crystallization experiments indicate that in the initial incubation period,the molecular chains are ordered in small range,low temperature pre-crystallization is conducive to nucleation and thus increase the crystallinity of the sample.