polymer hybrid structures
2 Overview of the thesis
The research presented within this thesis, including five publications, focuses on the synthesis and the utilization of a novel, double stimuli-responsive ABC triblock terpolymer sequence for the construction of “smart” hydrogels being responsive to several stimuli, including pH, temperature and magnetic fields.
The synthesis of the desired triblock terpolymers first of all required the development of a new synthetic route based on sequential anionic polymerization. The success of this route was demonstrated by synthesizing exemplary triblock terpolymers, as shown in Chapter 3.
In a next step, a variety of P2VP-b-PEO-b-P(GME-co-EGE) triblock terpolymers for hydrogel formation was synthesized. Systematic investigations on the pH- and temperature- dependent solubility and gelation properties of these polymers, including DLS, rheology, and SANS, are presented in Chapter 4.
The initial SANS studies revealed, that gels formed at pH > 5 and room temperature are composed of a cubic packing of core-shell-corona (CSC) micelles. A determination of the exact crystal lattice of these gels was achieved by performing additional SANS measurements under steady shear. The different states of alignment correlated with rheological features and the exact type of crystal structure of the sample (body centered cubic) are discussed in Chapter 5.
After a successful proof of our “smart” hydrogel concept we further wanted to extend it to ABC triblock terpolymers with more versatile C blocks, in particular POEGMA und PDMAEMA. This purpose required the establishment of an alternative synthesis route combining anionic polymerization and ATRP via “click” chemistry. Challenges occurring during this procedure and rheological features of aqueous solutions of the finally obtained polymers are described in Chapter 6.
In a last part of the thesis, which is described in Chapter 7, the “smart” hydrogel concept was extended by introducing sensitivity to magnetic fields. Slightly modified P2VP-b-PEO-b-P(GME-co-EGE) triblock terpolymers were complexed with maghemite nanoparticles. The resulting hybrid micelles formed hydrogels under suitable conditions, which can be manipulated by remote heating via external AC magnetic fields.
In the following, a brief summary of all results is presented.
Overview
46
2.1 One-pot synthesis of polyglycidol-containing block copolymers with alkyllithium initiators using the phosphazene base t-BuP
4The one-pot synthesis of a variety of triblock terpolymers including a polystyrene-block-poly(ethylene oxide)-block-poly(ethoxyethyl glycidyl ether) (PS-b-PEO-b-PEEGE), a poly(2-vinylpyridine)-block-poly(ethylene oxide)-block-poly(ethoxyethyl glycidyl ether) (P2VP-b-PEO-b-PEEGE) and a poly(ethylene oxide)-block-poly(ethoxyethyl glycidyl ether) (PEO-b-PEEGE) diblock copolymer, according to Figure 2.1 is presented. The crucial step within this synthetic procedure is the addition of the phosphazene base t-BuP4 right after ethylene oxide is added to the reaction mixture.
Figure 2.1. One-pot synthesis of polystyrene-block-poly(ethylene oxide)-block-poly(ethoxyethyl glycidylether) (PS-b-PEO-b-PEEGE) triblock terpolymers via sequential monomer addition using sec-BuLi as initiator and subsequent deprotection of the PEEGE block to yield the corresponding polystyrene-block-poly(ethylene oxide)-block-polyglycidol (PS-b-PEO-b-PG) triblock terpolymer.
The initial polymerization of the vinyl monomers styrene and 2-vinylpyridine is initiated with commercially available sec-butyllithium, respectively. Consequently, the “living” carbanion
47 chain ends carry a Li+-counterion. After the addition of ethylene oxide, the chains are endcapped with an EO unit, i.e. the “living” carbanions are transferred into oxyanions. In combination with Li+, these anions form aggregates preventing further propagation. The phosphazene base t-BuP4, however, acting as a complexing agent, helps to break up the aggregates by an effective complexation of Li+ and thus promotes the polymerization of EO even in the presence of Li+.
In consequence, the block sequence could be built up in a one-pot procedure just by adding each monomer right after the previous block was finished. The SEC-traces in Figure 2.2 demonstrate the success of this procedure. The resulting product was free from precursor species and had a PDI of 1.02. In a final step, the resulting polymers were further treated with formic acid and KOH to deprotect the PEEGE block yielding polyglycidol (PG). The SEC-trace of the resulting polymer is shown in the inset of Figure 2.2.
Figure 2.2. SEC (THF) traces of the synthesized PS58-b-PEO282-b-PEEGE27 triblock terpolymer (dotted) including the PS (solid) and PS-b-PEO (dashed) precursors. The inset in shows the SEC trace of the PS58 -b-PEO282-b-PG27 triblock terpolymer obtained after deprotection of the PEEGE block, using NMP as eluent.
The polymerization of the last monomer, EEGE, was monitored by online FT-NIR spectroscopy. The first-order kinetic plot showed a slow initiation but appeared to be linear upon further progression, pointing to the absence of termination reactions.
It is emphasized, that the synthetic route presented here strongly simplifies the synthesis of block copolymers containing ethylene oxide and/ or glycidyl derivatives, since it avoids a change of counterions when switching from a vinyl to an epoxide monomer. Alternatively, potassium based initiators might be used throughout the whole synthesis, however, lithium based initiators are commercially available and provide for instance high 1,4-contents in polydienes.
Overview
48
2.2 Smart hydrogels based on double responsive triblock terpolymers
After the development of a suitable synthetic route (see section 2.1), a series of poly(2-vinyl pyridine)-block-poly(ethylene oxide)-block-poly(glycidyl methyl ether-co-ethyl glycidyl ether) (P2VP-b-PEO-b-P(GME-co-EGE)) triblock terpolymers was synthesized in order to realize our concept towards double stimuli-responsive hydrogels. The synthesized polymers contain a pH-sensitive P2VP and a thermo-sensitive P(GME-co-EGE) block. They are supposed to form core-shell-corona (CSC) micelles under conditions, where only one outer block is insoluble (Scheme 2.1), potentially leading to a gelation via a regular packing of the micelles. By switching the respective second block insoluble, too, these micelles will crosslink through their corona and form an open association, which will lead to gel formation as well, however with altered gel properties.
Scheme 2.1. Scheme of the formation of double responsive hydrogels based on P2VP-b-PEO-b-P(GME-co-EGE) triblock terpolymers.
Prior to the synthesis of the triblock terpolymers, a series of P(GME-co-EGE) copolymers with varying comonomer ratio was synthesized by anionic polymerization. The incorporation of GME was slightly preferred, yielding copolymers with a weak compositional gradient along the chain. It turned out, that the cloud point of P(GME-co-EGE) depends linearly on the GME/EGE molar ratio (Fig. 2.3). The coil-to-globule transition was very sharp in each case and showed almost no hysteresis. Within the P2VP-b-PEO-b-P(GME-co-EGE) triblock terpolymer, the respective coil-to-globule transition is shifted to higher temperatures by 5-10 °C at pH = 3, which marks the influence of the two hydrophilic blocks, PEO and P2VP.
49
Figure 2.3. Cloud points of P(GME-co-EGE) copolymers and their dependence on the GME content (heating rate 1 K/min, 2.5 g/L in water, quality factor of the linear fit: 0.99); the open square corresponds to the cloud point of a homo-PGME taken from literature.
Concentrated aqueous solutions of P2VP57-b-PEO477-b-P(GME22-co-EGE22) and P2VP62 -b-PEO452-b-P(GME36-co-EGE36) (subscripts denote the degree of polymerization of the corresponding block) at pH = 7 showed a gel-sol-gel transition upon temperature increase, which was accompanied with a unique gel strengthening (Fig. 2.4A).
It was further shown, that below certain threshold concentrations, only gel-sol transitions or no gelation at all were detectable in the case of both polymers. On the other hand, sol-gel transitions were observed at pH < 5. In this case, regular packings of CSC micelles with inverse structure, i.e. a P(GME-co-EGE) core and a P2VP corona were formed.
Figure 2.4. A) Temperature dependent G′ and G″ of an 18 wt% aqueous solution of P2VP57-b-PEO477 -b-P(GME22-co-EGE22) at pH = 7 solutions (1 Hz, 0.7 % strain, 0.1 K/min); B) SANS data for a 16.6 wt% solution of P2VP62-b-PEO452-b-P(GME36-co-EGE36) in D2O at pH = 7, top x-axis normalized to 1st order reflection.
Small angle neutron scattering (SANS) experiments of diluted solutions at pH = 7 (not shown here) confirmed the presence of spherical micelles with the proposed core-shell-corona structure. The micellar dimensions were extracted from SANS and DLS experiments.
Overview
50
The low temperature gel (Fig. 2.4A) could be identified as a cubic packing of the CSC micelles. The relative peak positions of 1:21/2:31/2 in the corresponding SANS profile (Fig.
2.4B)point to the presence of a simple cubic or a body centered cubic packing. It is noted, that it was not possible to make a decision about the exact type of the formed cubic lattice, due to the lack of higher order reflections. An improvement of the long-range order within the sample by applying steady shear should help to derive more information about the crystal structure.