As a result of the level of control over composition and topology attained through CRP, there are now many novel approaches available for the formation of chemically or physically formed networks and gels. Much of the earlier work on the preparation of Structured Macromolecules and Hybrid Materials from multi-functional macroinitiators was conducted under conditions that targeted a low concentration of radicals in the reaction in order to avoid inadvertent preparation of crosslinked networks or gels. However an examination of the properties of crosslinked polymer brushes led to identification of a new class of materials called SuperSoft elastomers. (1) This focused attention on the potential for development of highly functional polymer networks and gels with properties tailored for specific applications that require such unique properties at economical cost.
There are two methods of forming a crosslink network; one indicated above is the formation of inadvertent chemical bonds between multifunctional molecules to form the network, however this can be conducted in a deliberate manner by adding a multifunctional crosslinking agent to a CRP; the other approach is physical links formed by phase separation of a segmented copolymer.
Many workers have employed CRP to prepare segmented copolymers capable of phase separation. In the following discussion two examples of physically crosslinked systems, a hydrogel and a supersoft elastomer, are discussed in addition to chemically crosslinked systems and an example of a degradable crosslinked gel.
An early example of a physically crosslinked gel that targeted a known application was the preparation of a hydrogel. (1)
Well defined vinyl macromonomers of polystyrene were prepared by ATRP using vinyl chloroacetate as an initiator. Because styrene and vinyl chloroacetate do not radically copolymerize, no branching or incorporation of the initiator into the backbone was observed. Macromonomers of several different MW were prepared and were free radically copolymerized with N-vinylpyrrolidinone, in varying feed ratios, in order to produce poly(NVP-g-Sty) graft copolymers. The macromonomers used were of sufficiently high MW to undergo phase separation and form physical crosslinks in solvents, such as water, which favor the hydrophilic NVP, preventing the copolymer from dissolving and allowing it to swell. These recyclable thermoplastic materials formed hydrogels with swell-abilities in water exceeding 95%, depending on the amount of styrene that was incorporated into the copolymer. (1-2)
In a later example, examining the properties of a chemically crosslinked hydrogel, the benefit of a CRP was seen when gels formed by ATRP copolymerization of 2-(N,N-dimethylamino)ethyl methacrylate (DMAEMA) and ethylene glycol dimethacrylate (EGDMA) using ethyl 2-bromoisobutyrate as the initiator were analyzed, via a comparison to Flory's gelation theory. (3) The copolymers were found to be more homogeneous than similar hydrogels prepared by conventional free-radical polymerization methods. (4) This observation is consistent with the work of Fukuda using NMP to form well defined polystyrene gels. (5)
A hybrid between these approaches was developed when PEG-b-PNIPAM block copolymers were synthesized by ATRP of NIPAM using a PEG macro-initiator. At 25 C, the block copolymer is soluble in water but phase-separates to form micelles when the temperature is raised to 50 C, i.e. above the LCST of the PNIPAM block in the PEG-b-PNIPAM block copolymer. To prepare stable hydrogel nanoparticles in water at room temperature a small amount of N,N'-ethylenebisacrylamide can be added as a physical cross-linker to the reaction system. The size of nanoparticles is controlled by the composition of the mixed solvent. (6)
An example of post polymerization photo-crosslinking was examined by the preparation of well-defined copolymers of 2-(dimethylamino)ethyl methacrylate (DMAEMA) and benzophenone methacrylate (BPMA) with different compositions. (8) The MW of the copolymers these copolymers were held close to 30 000 g/mol, while the BPMA content varied from 2.5 to 10 mol %. The copolymers with a low content of BPMA (2.5 and 5 mol %) exhibited a sharp thermal transition at 33-36 C in aqueous solution. A hydrogel was immobilized and patterned on a silicon wafer via UV treatment of the spin-coated polymer layer using a photomask technique. The thermoresponsive behavior of the patterned polymer gel was quantitatively investigated by variable temperature in situ contact mode atom force microscopy, which revealed the presence of two lower critical solution temperature regions. One region was between 25 and 30 C, corresponding to the topmost layer of the hydrogel film, and the other region, around 40 C, corresponded to the bulk of the hydrogel. Concurrent lateral force microscopy measurements revealed that, just above the transition temperature, the bulk region exhibited enhanced friction.
Elastomers typically have a Young's modulus (at small strains) in the order of 106 Pa, with reversible extensibility reaching 1000%. They are therefore approximately five orders of magnitude softer, and three orders of magnitude more deformable than typical solids. Weakly cross-linked rubbers preserve the modulus of the rubbery plateau seen for the melt of linear entangled polymers (I.e. Mc remains similar to the order of Me), whereas, for highly cross-linked systems the modulus increases.
Temperature (a) and frequency (b) dependencies of the storage (G') and loss (G") of a normal linear molecule showing typical values for a lightly crosslinked network.
It is not easy to move in the opposite direction and prepare stable soft rubbers, i.e. rubbers with a modulus lower than the bulk plateau modulus of a given polymer. However the plateau modulus does decrease considerably in polymer solutions, (9, 10) and soft gels can be obtained by swelling weakly cross-linked systems with a good solvent for the matrix material as noted above for hydrogels. One example of an application for such a material is soft contact lenses. However these solvent swollen states are not stable in environments in which the solvent can evaporate and such gels can also appear unstable when external forces are exerted on the gel. In water-swollen networks of hydrophilic polymers (e.g. hydrogels), shear modulus on the order of 103 Pa can be obtained at low cross-link densities and relatively high degrees of swelling. However, higher levels of swelling result in an increase in modulus, because of strong extension of the network chains. (9-11)
These unusual mechanical properties that were initially observed in aqueous gels are now observed for the first time in bulk crosslinked polymeric brushes because the molecular network consisting of a backbone "diluted" by short side chains behaves in a similar manner to a solvent swollen network. When the MW of the side chains are below the critical entanglement MW they do not entangle but, since they are covalently attached to the network, they provide stability against evaporation or deformation, while preventing the networks from collapsing. At a superficial phenomenological level, they behave as a cross-linked network of "thick" molecules, i.e. polymers with a multiplicity of short chains attached to the polymer backbone, or as a stable swollen cross-linked network, where the "solvent" is attached to the backbone. The meanings of "thick" and "attached solvent" will become apparent in the following description.
The first materials recognized to be precursors to SuperSoft elastomers were bottle brush macromolecules with a very long backbone and densely grafted PnBA side chains. They displayed an ultra low modulus plateau in the soft gel range. When they are transformed to a network by chemical cross-linking, the material become a SuperSoft rubber (1) which instead of the expected global flow range display a plateau in G'
extending towards low frequencies. This plateau indicates elastomeric properties for such a cross linked polymer network. In this case the plateau modulus is much lower than that seen for typical polymeric rubbers which can be attributed to the large fraction of the short dangling chains in the system. Such chains provide significant mobility, making the material extremely soft. This behavior has been observed for several materials. (12)
A SuperSoft thermoplastic elastomer is formed when the backbone is selected to be an ABA block copolymer with phase separable A-blocks and a bottle brush B segment. The properties of one such block copolymer is shown below.
Frequency dependencies of the storage (G') and loss (G") modulus for the triblock copolymer with a brush-like middle block (master curves at the reference temperature of 254K). A discontinuity seen in G' corresponds to melting in the pOMA microphase taking place at 292K.
The bulk tactile response can be modified by selecting the composition of the "dangling chains or hairs" and can vary from hydrophilic to hydrophobic and encompass attached oleophilic and oleophobic diluents that respond to environmental pressures. The bulk properties can be further modified, as discussed above, by changing the degree of cross-linking of the backbone matrix to form a material with the desired modulus for the targeted application. Higher levels of cross-linking, or increased stiffness of either the backbone or tethered dangling graft or "hair" will increase the modulus while still providing polymers significantly softer than current elastomers.
For example densely heterografted brush macromolecules with crystallizable grafts were prepared by copolymerizing a poly(ethylene glycol) methyl ether methacrylate macromonomer (PEOMA, MW = 1100 g/mol, DPPEO = 23) and octadecyl methacrylate or acrylate (ODMA, ODA). The one-pot copolymerization via grafting through (macromonomer method) yielded densely heterografted copolymers (DPn = 300-500). (13) The structure of the copolymers strongly depended on the selection of comonomer pairs; i.e., two methacrylates (PEOMA/ODMA) or methacrylate/acrylate (PEOMA/ODA) led to formation of a spontaneous random or gradient copolymer, respectively. The combination of hydrophilic and hydrophobic segments in the same polymer chain caused microphase segregation. X-ray results indicated that frustration in packing the crystallizable segments, helical PEOMA and hexagonal ODMA crystals, can direct to an amorphous fraction instead of a semi-crystalline ODMA in the graft copolymer. Thermomechanical measurements showed the soft rubbery behavior of the copolymers (104 > G' > 103 Pa, G' > G'').
Molecular brushes with poly(dimethylsiloxane) (PDMS) side chains were also prepared via the macromonomer method (grafting through). (14) Well-defined polymers with Mw/Mn = 1.22-1.35 and DP up to 350, for the macromonomers with methacrylate functionality (PDMSMA, MW = 1000 g/mol, DPPDMS = 10), were formed in the presence of CuCl/Me6TREN catalyst. Similar conditions were used for copolymerization with another macromonomer, i.e. poly(ethylene glycol) methyl ether methacrylate (PEOMA, MW = 1100 g/mol, DPPEO = 23). The comparable reactivity of both methacrylates resulted in heterografted brush copolymers with random distribution of the macromonomers along the copolymer backbone and with narrow molecular weight distribution (Mw/Mn = 1.07-1.18). The obtained molecular brushes, consisting of amorphous PDMS fraction and crystallizable PEO segments, after annealing at high temperature had properties characteristic for soft gels (G' 104 Pa).
Degradable gels were initially prepared by the copolymerization of styrene or methyl methacrylate and a disulfide-containing difunctional methacrylate, bis(2-methacryloyloxyethyl) disulfide, using reaction conditions similar to those employed for the formation of a linear polymer with an internal degradable link which employed bis[2-(2-bromoisobutyryloxy)ethyl] disulfide as the difunctional initiator. (15)
Reversible cleavage of the disulfide to a terminal thiol occurred under reducing and oxidative conditions.
When a difunctional methacrylate, bis(2-methacryloyloxyethyl) disulfide was added to the reaction a multifunctional gel was formed. The advantages of using ATRP for this reaction were the simplicity of driving the reaction to high monomer conversion and controllable cross-link density, which ultimately controls the degradation rate. The procedure should be applicable all monomers and has been conducted with styrene, and (meth)acrylates and the resulting products retained a high degree of Br-functionalization (end-group chemistry). Since the materials degrade in a reducing environment they have potential use in cancer treatment as drug delivery media. (16)
In order to show the broad applicability of the reducibility capability of the gels, they were reduced with tributylphosphine to form soluble, low molecular weight linear poly(methyl methacrylate) fragments containing thiol groups at the chain end and along the backbone, each terminal functionality originating from the difunctional disulfide initiator and monomer, respectively. (17)
The disulfide-cross-linked gels were further used as "supermacroinitiators" for the bulk ATRP of styrene at 90 C, employing CuBr/pentamethyldiethylenetriamine as the catalyst, in the same manner used for the preparation of mikto-arm degradable star copolymers. (18) The chain length of the polystyrene pendant chains could be controlled by varying the reaction time. The prepared gels with segmented structure swelled more than the starting polymethacrylate gels in both THF and toluene. Degradation experiments confirmed the high degree of chain-end bromine functionalization of the "supermacroinitiators".
The synthesis can be conducted in an inverse miniemulsion to provide injectable degradable nanobeads. (19) The degradation of the nanoparticles was confirmed by conducting reductive degradation of the latex particles in THF. One can see that the size of the particle increased as the crosslink density decreased and the swelling ratio of the particles increased, then the particles rapidly degraded into soluble polymer fragments as shown by the sharp decrease in the measured hydrodynamic volume.