Temperature-responsive diblock copolymers

Formation and disintegration of self-assembled nanostructures in response to external stimuli are important phenomena that have been widely explored for a variety of biomedical applications. In this contribution, we report the thermally triggered assembly of block copolymer molecules in aqueous solution to form vesicles (polymersomes) and their disassembly on reduction of temperature. A new thermo-responsive diblock copolymer of poly(fV-isopropylactylamide) poly((3-methactylamidopropyl)trimethylammonium chloride) (PNIPA-A-PMAPTAC)... 

The PPO block is temperature-sensitive, with an LCST (cloud point) of around 15 °C, whereas the DEA block is pH-responsive. At pH 6.5 and 5°C, both blocks are solvated and the copolymer is molecularly dissolved in aqueous solution. On adjusting the solution pH to pH 8.5, the DEA block becomes deprotonated and hence hydrophobic, leading to the formation of DEA-core micelles at 5 °C. On the other hand, if the pH is held constant at pH 6.5 and the temperature is raised above the LCST of the PPO block, PPO-core micelles are obtained with the weakly cationic DEA chains forming the micelle coronas. Since these new diblock copolymers can exist in two micellar states we have christened them schizophrenic copolymers. ... 

It is an important aspect that block-copolymer micelles are characterized by much longer relaxation times than compared to low molecular surfactants. Non-equilibrium morphologies can easily be obtained in a vitrified state due to the efficient suppression of structural reorganization, because of the corresponding very slow response of the micelles to changes of temperature, solvent and concentration. In the case of a block-ionomer, i.e. a diblock copolymer where one block consists of ionic units, it was observed that micelles which formed in non-polar solution needed weeks to re-equilibrate after dilution of the solvent [226-228]. 

LI1 Liu, X., Ni, P., He, J., and Zhang, M., Synthesis and mieellization of pH/ temperature-responsive double-hydrophilic diblock copolymers polyphosphoester-/)/oc -poly[2-(dimethylamino)ethyl methacrylate] prepared via ROP and ATRP,... 

Feijen et al. have reported the thermogelling system using SC formation of star-shaped diblock copolymers, PEG- -(PLLA)8 and PEG- -(PDLA)8 [89,90]. They have investigated the effects of arm number of the branched architecture on their temperature-responsive gelation by comparing the PEG- -(PLLA)8/PEG-h-(PDLA)8 system with the... 

Group transfer polymerization has proven to be an extremely usefiil technique for the synthesis of such AB diblock copolymers. For example, the ssmthe-sis of AB diblock copolymers in which the A block was 2-(dimethylamino)ethyl methacrylate (1C in Fig. 38) and the B block was either 2C or 3C in Figure 33 have been reported. 1C is temperature responsive whereas 2C and 3C are both pH responsive. At low pH when both the tertiary amine blocks (1C -I- 2C or 1C + 30 are protonated the block copolymers are molecularly dissolved. Raising the solution pH above the pifa of tertiary amine residues for 2C and 3C renders these blocks hydrophobic and as such the block copolymers self-assemble to form micelles with the hydrophobic 2C (or 3C) residues in the core, stabilized by coronal chains of 1. Supramolecular self-assembly is completely reversible— lowering of the pH back below the respective plea s results in molecular redissolution (183). 

Although most polymers tend to accumulate at the fluid interface, reports involving the transfer of polymeric micelles (micellar shuttle) between two immiscible phases have been pubHshed. Poly(N-isopropylacrylamide) (PNIPAM), a thermally responsive polymer, is insoluble and can undergo a conformation change above its lower critical solution temperature of 32 ° C. The thermo reversible miceUization—demicellization process and micellar shuttle of PNIPAM-PEO diblock copolymer at a water-IL interface were investigated by dissipative particle dynamics (DPD) simulations (Soto-Figueroa et al, 2012). Simulation results confirm that the phase transfer behavior of polymeric micelles is controlled by the temperature effect that changes the diblock copolymer from hydrophilic to hydrophobic (as shown in Fig. 33). 

Another interesting design of mixed polymer brushes was obtained by sequential grafting of asymmetric nonsticky/sticky diblock copolymers of poly(styrene-b-3-(trimethoxysilyl)propylmethacrylate) and monomethoxypoly(ethylene glycol)-trimethoxysilane onto silicon wafers (Han et al., 2013). The nanoscopic morphology of these mixed polymer brushes exhibited stimuli responsive behavior to various temperatures and solvents. 

Combination of two stimuli-responsive polymers with low polydispersity as grafted diblock copolymer brushes in cylindrical pores leading to four different effective pore diameters as a function of the combination of the two stimuli (temperature change, AT, around the lower critical solution temperature of the first polymer block/here PNlPAAm pH change, ApH, around the pKa of the second polymer block/here PAA) (From Friebe, A., and Ulbricht, M., Macromolecules, 42,1838-1848,2009.)... 

A cleavable, temperature-responsive polymeric cross-linker was utilized by Xu and cowoikers [111] to stabilize micelles from PEO-b-PAPMA-b-poly((Af,Af-diisopropylamino)ethyl methacrylate) triblock copolymer. The PNIPAm cross-linker contained activated ester end groups that were reacted with the primary amines on the PAPMA middle block. The trithiocarbonate moiety located at the middle of PNIPAm cross-linker could then be degraded by aminolysis to break the cross-links. Temperature-responsive micelles and vesicles from diblock and triblock copolymers were shell cross-linked via interpolyelectrolyte complexation [108, 112]. The cross-links formed by the electrostatic interactions of oppositely charged polyelectrolytes could be disrupted by the addition of SME. 

Controlling the spontaneous formation of ordered domains in soft materials such as block copolymers [189] may lead to the development of stimuli-responsive materials for applications such as actuators [190] and photonics [191] due to the reversible nature of order formation. However, the stimuli that are typically used to control the morphology of block copolymers are e.g., temperature, pressure, solvent type and concentration... Pioneering work by Abbott and co-workers used the chemical oxidation approach to control the self-assembly of small-molecule amphiphiles containing ferrocene [192]. Rabin and co-workers have shown that the introduction of dissociated charges on one of the blocks of a diblock copolymer leads to stabilization of the disordered phase [193]. They also quantified the increase in x at the order-disorder transition (ODT), xodt, due to the entropic contribution of the dissociated counterions. The Flory-Huggins parameter,x, that is used to quantify interactions between polymer chains is assumed to be proportional to the difference in the polarizibility of the blocks [194]. The polarizibility of polyferrocenyldimethylsilane, which is larger than that of either polystyrene or polyisoprene [195], must increase upon oxidation due to the presence of the NO ions. 

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