Study On The Relationship Between Branched Structure And Glass Transition Behavior Of Hyperbranched Polymers

Hyperbranched polymer（HBP）is a new kind of polymers first reported in 1990’s.Much attentions has been paid to HBP by scientific and industrial researchers because of its unique properties,such as highly branched molecular structure,multi-functional end-groups,lack of chain entanglement,difficulty to crystallize and good solubility etc.Up to now,there are a great number of papers about the synthesis,characteriztion and applications of HBPs.In general,there are only two kinds of structure units（i.e.,linear unit and terminal units）in a linear polymer,whereas there are three different kinds of structure units（i.e.,dendritic units,linear unit and terminal units）in a hyperbranched polymer.The great structure difference of linear and hyperbranched polymers must result in completely different physical properties.Glass transition behavior is one of basic problems which is chronically concerned by scientists in the field of polymer physics.So what is the specific characteristic of glass transition behavior of HBPs?What is the relationship between the different branched structure and glass transition behavior?…….In order to answer above important questions,we designed and synthesized a series of typical polymers（including crystalline,amorphous or strong polar polymers）with similar molecular weight and various DB to study the relationship between the branched structure and glass transition behavior by GPC、NMR、FTIR、DSC and DMA etc.Furthermore,the method of all-atom molecular dynamics simulation was adopted to simulate and predict glass transition behaviors of various polymeric systems.Then the transition mechanism of uncrystallizable liquid-solid of branched polymer systems and the formation law of uncrystallizable structure was described from the point of view on experiments and finally the basic theory was supplied to control the uncrystallizable structure of branched polymers.The main content and the corresponding results are shown as follows.1 The relationship between DB and T_g of branched polyethylenesA series of branched polyethylene（HBPE）samples with similar molecular weight but DB were prepared by the coordination polymerization of ethylene with amine-imine nickel orα-diimine palladium as the catalyst respectively.The number-averaged molecular weights of the resulting HBPEs were in the range from 100-110 kg/mol with PDI from 1.03 to 1.64.According to the equation proposed by Hawker et al and the corresponding data of ¹H NMR,the calculated DB of HBPEs was between 0.11 and 0.35.Then the HBPEs was characterized by differential scanning calorimetry（DSC）and dynamic mechanical analyzer（DMA）,respectivly.The results displayed that HBPEs were still easy to crystallize,but their crystallized ability was decreased with the increased DB.Furthermore,the crystallization and the glass transition behavior of HBPE samples are relevant with each other,and with increasing the DB,the melting temperature（T_m）and the glass transition temperature（T_g）are both decreased.In addition,ultra-fast differential scanning calorimetry（UF-DSC）was adopted to study the crystallization property and glass transition behavior of HBPEs.The results indicated that the crystallization of HBPE samples with high DB could be inhibited when the cooling rate was high enough and only glass transition was observed.Finally the relationship between T_g and DB of HBPE samples have been simulated by all-atom molecular dynamics（MD）.The values of MD-determined T_g are consistented with the experimental ones.Besides,the crystallization behavior of HBPE samples was also studied by XRD.The kinetics results of isothermal and nonisothermal crystallization verify the peculiar effects of DB on the crystallization process of HBPEs.In detail,a little of branch structure could accelerate the crystallization process of HBPEs,however a large number of branch could inhibit it.2 The relationship between DB and T_g of branched copolystyrenesSeveral branched copolystyrene（HBCPS）samples with the similar molecular weight but various DB were synthesized by atom transfer radical self-condensing vinyl copolymerization（ATR-SCVCP）of p-chloromethylstyrene（CMS）and styrene with the complex of CuBr/2,2’-bipyridyl as catalyst.The DB of the resulting HBCPS were contolled by adjusting the feed ratio of CMS and styrene and the molecular weights were determined by the polymerization time.The weight-averaged molecular weights of HBCPS samples were in the range from 46.35 to 90.69 kg/mol with the PDI from 1.73 to2.50.According to the equation to calculate DB proposed by Muller and the data of ¹H NMR,the DB of HBCPS was from 0.02 to 0.38.The glass transition behavior of HBCPS samples was first studyed by DSC and DMA measurements.The results exhibited that all HBCPS samples were amorphous polymers and their T_g decreased with increasing the DB.The T_g measured by DMA was higher about 20 ^oC than that by DSC.Furthermore,the spin-lattice relaxation times(¹H T_1r)of protons were measured by solid NMR and displayed the same downtrend with increasing DB.In addition,the relationship between the T_g and the DB of HBCPS samples were well built up by all-atom MD simulations,and the dependent trend of T_g on DB by the simulation was consistent with that by the experiment well.3 The relationship between DB and T_g of branched poly（amidoamine）sBranched Poly（amidoamine）（HBPAA）samples were synthesized from1-（2-aminoethyl）piperazine（AP）and N,N-methylenebisacrylamide（MBA）by Michael adition reaction.Due to the polymerization rate of AP and MBA in H₂O is very higher than that in DMF,the DB of HBPAA samples can be controlled by using a mixed solvent with the different volume ratio of H₂O and DMF.Then a series of HBPAA samples with different DB and similar molecular weight were prepared.The weight-averaged molecular weights of HBPAA samples were in the range of 35.6-70.9 kg/mol with PDI from 1.22 to2.99 by GPC measurement.The DB of HBPAA samples determined by the equation proposed by Hawker and ¹³C NMR data was in the range of 0.08-0.41.The T_g of HBPAA samples were studied by DMA and DSC measurements.The results showed that T_g first decreased with increasing DB when the DB was lower than 0.27,and then increased with increasing DB when the DB was higher than 0.27.The results of variable-temperature FTIR spectroscopy measurement displayed that there were strong intramolecular hydrogen bonds existing in HBPAA samples.In order to eliminate the effect of hydrogen bonds on T_g,all HBPAA samples were acidified by hydrochloric acid to destroy the interaction of hydrogen bonds and then measured by DSC again.The results showed that the T_gs of acidified HBPAA samples were decreased with increasing DB.Furthermore,HBPAA6with the highest DB was acidified by the different amount of hydrochloric acid and then characterezed by DSC measurement again.The results showed that T_g was decreased gradually with increasing the degree of acidification.In summary,the strong inerraction of hyrogen bonds have a great effects on T_g of HBPAA.In addition,the relationship between the T_g and the DB of HBPAA samples before and after acidified was built up by all-atom MD simulations,and the dependent trend of T_g on DB by the simulation was consistent with that by the experiment well.But the T_g obtained by the simulation was higher than the cprresponding one by the experiment due to the high heating rate in the simulation.4 The relationship between DB and T_g of hyperbranched polyethersA series of hyperbranched polyethers with similar molecular weights but different DBs were synthesized by the oxyanionic polymerization of 1,4-butanediol diglycidyl ether and trimethylolethane in an efficient one-step approach.The DB can be controlle by adjudting the molar ratio of 1,4-butanediol diglycidyl ether and trimethylolethane.The chemical structure of the resulting hyperbranched polyethers was confirmed by ¹H NMR,¹³C NMR,and FTIR analysis.The glass transition temperature of hyperbranched polyehter samples were studied by DSC measurement.The DSC results showed that T_g first increased with increasing DB,then passed through a maximum（DB=0.61）,and finally decreased with further increasing DB.The variable-temperature FTIR spectroscopy and variable-temperature ¹H NMR spectroscopy measurements showed that there were interaction of hydrogen bonds in hyperbranched polyethers.In order to remove the influence of hydrogen bonds on T_g,the hyperbranched polyether samples were end-capped by benzoyl chloride,and then measured by DSC again.The results showed that the T_g of the end-capped hyperbranched polyether samples is a little higher than that of the unend-capped,but the relationship of T_g and DB was not changed.This implied that the effect of hydrogen bonds on the T_g of hyperbranched polyethers was limited.