Self Assembly And Application Of Amphiphilic Block Copolymers Based On PDMAEMA

Zwitterionic copolymer containing positive and negative ions possesses special properties in the application of drug delivery systems,which enhance the ability of drug carriers to break through cell membranes.However,being lack of the colloidal stability in water,it is hard to prepare nanoparticles of zwitterionic copolymer.In our previous work,it was discovered that the zwitterionic block copolymers（polymethacrylic acid-block-poly（4-vinyl pyridine）,PMAA-b-P4VP）presented properties quite different from the random copolymers（PMAA-co-P4VP）in ethanol,namely that the random copolymers were soluble in ethanol,whereas the block copolymers form particles.Accordingly,in order to prepare zwitterionic particles and by which to elucidate the mechanism of particle formation,in this thesis,the poly（N,N-dimethyl amine ethyl methacrylate）（PDMAEMA）ended with4-cyano-4-（phenyl carbonothioylthio）pentanoic acid（CPDB）was adopted as a reversible addition-fragmental macromolecular chain transfer agent（RAFT）,and2,2’-azo bis（2,4-dimethyl）valeronitrile（V65）as the initiator to carry out the RAFT polymerization of MAA in a variety of alcohols.First of all,the effects of the polarity of alcohol on particle formation were investigated by using PDMAEMA₁₀₂ as a RAFT reagent under the condition of equivalent amount of MAA.Methanol,ethanol,isopropanol and their aqueous solutions were used as the continuous phase.The polymerization degree of PDMAEMA-CTA and PDMAEMA-b-PMAA was determined by 1HNMR,and the particle size was characterized with the scanning electron mirror（SEM）,dynamic light scattering（DLS）and small angle X-ray scattering（SAXS）.As a result,it was found that the polarity of alcohol greatly impacted on the overall conversion of MAA and the diameter of particles.The greater the polarity,the higher the overall conversion of MAA was,while the particle size decreased.In methanol,for example,MAA’s overall conversion and particle size were 84.7%and 285nm（SEM particle size,the same as below）,respectively,whereas in isopropanol,these values were 70.6%and 344nm,respectively.As water（volume ratio to alcohol:1/14）was added,both of the conversion and particle size were increased.For example,in methanol/water,conversion and particle sizes were 87.1%and 301nm,respectively.Meanwhile,the chain length of MAA in the block copolymer decreased with the increase of alcohol polarity.Addition of water did not show significant effect on MAA chain length.For example,in methanol and methanol/water,the block copolymers were PDMAEMA₁₀₂-b-PMAA₅₄ and PDMAEMA₁₀₂-b-PMAA₆₃,respectively,and in isopropyl alcohol and isopropyl alcohol/water,they were PDMAEMA₁₀₂-b-PMAA₇₈and PDMAEMA₁₀₂-b-PMAA₈₀,respectively.Variation of the overall solid content in ethanol did not affect the chain length of PMAA in the block copolymers,but promoted the size of particles and shortened the turbid time.Above 15wt%,the particles were not prepared.However,at the constant concentration of PDMAEMA-CTA,the increase of MAA concentration significantly affected the chain length of PMAA in the block copolymers.For example,with the theoretic composition of block copolymer,PDMAEMA₇₉-b-PMAA₁₀₀,the resultant composition was PDMAEMA₇₉-b-PMAA₅₄ in addition of particles,however,with the theoretic composition of PDMAEMA₇₉-b-PMAA₂₀₀,the coagulation of DMAEMA₇₉-b-PMAA₉₃ was obtained.It indicated that the chain length of PMAA was severely restricted by the chain length of PDMAEMA.Combined the results of SAXS,it was conclude that the polarity of alcohol affected the partition of MAA and V65 between PDMAEMA-CTA/MAA complex minidroplets and continuous phases,in turn to affect the diameter of particles.With the increase of alcohol polarity,MAA and V65 inclined to be partitioned more in the continuous phase,so that more PMAA formed in the continuous phase.These PMAAs were removed before characterization.Most of MAA RAFT polymerization possibly occurred in the vicinity of PDMAEMA-CTA chains.Therefore,PDMAEMA-CTA acted as a template to control the length of PMAA.The p H sensitivity characteristics of microspheres were also dependent to the additional length of PDMAEMA-CTA,i.e.the difference in length between the two segments.When the length of PMAA was too short,such as PDMAEMA₆₀₀-b-PMAA₈₇,or PMAA was closed to the length of PDMAEMA,such as PDMAEMA₇₉-b-PMAA₉₃ and PDMAEMA₁₀₂-b-PMAA₁₃₁,the amphoteric characteristics of microsphere disappeared.The self-assembly behavior of zwitterionic block copolymer in KCl alcohol aqueous solution was also determined by the additional length of PDMAEMA.When PDMAEMA was shorter in the copolymer,the microsphere transformed into nanofiber or nanoband.In contrast,for PDMAEMA₆₀₀-b-PMAA₈₇,the microsphere maintained its original spherical form.These results showed that the charge screening of salt played a key role in the process of self-assembly.In addition,this article also analyzes the role of nanoparticles in the process of crystal growth.Ammonium carbonate diffusion method（ADM）was used to study the crystallization process of zwitterionic block copolymers in calcite.The results show that the block copolymer mainly relies on the chelation of anions and calcium ions to generate co-crystals.The chelating sites on the surface of calcite are limited.When the carboxylate ion is excessive,only a low level of occlusion will occur on the surface of the calcite crystal.