北京化工大学：《材料导论》课程阅读材料（高分子材料）Toward_mart nano-objects_by_self-assembly_of_bloc.pdf_P6-P10

696 J.Rodriguez-Herndndez et al.Prog.Polvm.Sci.30 (2005)691-724 n he sie of e aeet出 balance be by a varie ety of parameters system.These force refect:the of between the blocks forming the core (the block will be more or less stretched depending on the solvent).the (b) interaction between chains forming the corona.and the surface energy between the solvent and the core of the micelle.From a theoretical point of view.the ne aggregate structure requir 米尊 he mac cromolecules inside the ags gate forces combined with the interactions between different aggregates(inter-aggregate forces)determine the type of self-assembled structure formed at equilibrium.It is then essential to understand the fundamentals that dependence d sse the rela ati (b)crew-cut micelle. architecture.and the solvents used. 2.2.Theoretical aspects Theories at different levels of refinement have been developed to describe the behavior of block copoly. The micellization process in block mers in solution and its dependence on parameters lescribed above.The theories can be classified into (CMT and the ritical m concentration (CMC).If the CMT or the CMCa wo main groups.The belongs to the reached.self-assembly will not occur.and the block copolymer will behave in the solution as a unimer.On d by No the contrary,if micelle formation is triggered,the ch de cer micelles will be in thermodynamic equilibrium with like the ag ration number or the radius for crew unimers. ype micelles from the block length and interfacial In order to characterize a micellar system,several tension data [19).Daoud and Cotton [20]extended the have to co ding the ange of applicability of this approach to the case of th MT star-like micelles nd CM More detailed studies have out along th lines by Zhulina an 21 aa c Z and its morphology.These variables affect the hydrodynamic radius r..the radius of gvration R the ratio of ru to re (which depends on the micellar for star-like micelles or mor ntly Wu and Gac shape),the core radius Rc,and the thicknessLof the (231 and Shusharina 1241 have made theoretical corona.For more detailed information the reader is contributions to this field,but a description of such referred to general books on block copolymers [14. studies is beyond the scope of this review. 16].In the next few pages,we only outline this well The self-consistent mean field theory was first emphasizing the important They

2.2. Theoretical aspects The micellization process in block copolymers depends mainly on two parameters: the critical micelle temperature (CMT) and the critical micelle concentration (CMC). If the CMT or the CMC are not reached, self-assembly will not occur, and the block copolymer will behave in the solution as a unimer. On the contrary, if micelle formation is triggered, the micelles will be in thermodynamic equilibrium with unimers. In order to characterize a micellar system, several parameters have to be considered, including the equilibrium constant, the quality of the solvent, the previously mentioned CMT and CMC, the overall molar mass Mw of the micelle, its aggregation number Z and its morphology. These variables affect the hydrodynamic radius RH, the radius of gyration RG, the ratio of RH to RG, (which depends on the micellar shape), the core radius RC, and the thickness L of the corona. For more detailed information the reader is referred to general books on block copolymers [14, 16]. In the next few pages, we only outline this welldocumented literature, emphasizing the important concepts needed for this review. The shape and the size of the aggregates are controlled by a variety of parameters that affect the balance between three major forces acting over the system. These forces reflect: the extent of constraints between the blocks forming the core (the block will be more or less stretched depending on the solvent), the interaction between chains forming the corona, and the surface energy between the solvent and the core of the micelle. From a theoretical point of view, the description of the aggregate structure requires that the thermodynamic parameters of self-assembly be accounted for as well as the forces generated between the macromolecules inside the aggregates. These two factors (thermodynamics and intra-aggregate forces) combined with the interactions between different aggregates (inter-aggregate forces) determine the type of self-assembled structure formed at equilibrium. It is then essential to understand the fundamentals that govern the interdependence between morphology and size of the aggregates obtained by self-assembly, including decisive factors such as concentration, temperature, composition, block length, copolymer architecture, and the solvents used. Theories at different levels of refinement have been developed to describe the behavior of block copolymers in solution and its dependence on parameters described above. The theories can be classified into two main groups. The first group belongs to the ‘scaling theory’ of de Gennes [17]. The second one is based on the ‘self-consistent mean field theory’ developed by Noolandi and Hong [18]. In his theoretical approach, de Gennes predicted parameters like the aggregation number or the radius for crew-cut type micelles from the block length and interfacial tension data [19]. Daoud and Cotton [20] extended the range of applicability of this approach to the case of star-like micelles. More detailed studies have been carried out along these lines by Zhulina and Birshtein [21], who proposed a classification of micelles in four main categories based on the nature of the diblock copolymers. Other authors, including Halperin [22] for star-like micelles, or more recently Wu and Gao [23] and Shusharina [24], have made theoretical contributions to this field, but a description of such studies is beyond the scope of this review. The self-consistent mean field theory was first developed by Noolandi and Hong in 1982 [25]. They were able to predict the size of spherical micelles at Fig. 3. Schematic representation of two extreme morphologies of micelles depending on the relative block lengths: (a) star micelle, (b) crew-cut micelle. 696 J. Rodrı´guez-Herna´ndez et al. / Prog. Polym. Sci. 30 (2005) 691–724

J.Rodriguez-Hemdndezet al /Prog.Polym.Sci.3(005)691-724 697 equilibrium.and the variation of the aggregation copolymers.If we take the particular case of number as a function of the degree of polymerization. amphiphilic molecules in aqueous solution (Fig.4). Their model is based on the molecular characteristics the major forces governing the assembly into well of the polymer,its concentration in solution,and an defined structures are,on the one hand,the hydro estimation of the core/corona interfacial tension 1261. attraction between soluble hydrophobi The results of their model are in very good agreement the hy dophi on the othe on be with X-ray and neutron scattering data.Leibler et al. e o le [27]expanded the mean field theory to the case of with th eamphiphili the attractive forc ate the inte facial are per molecule will decrease and if repulsive force e parame predominate,a will increase.The competition cnergy con core and the between these two opposing forces.which strongly g depends on the geometry of both blocks,is mirrored in va ，ofhi s sume such as the ers (see evolution of the cMC with block co olymer structure (triblocks versus diblocks).as studied by Linse [26]. hobic chains.and the ngth le of or the temperature dependence of the hydrodynamic these chains.These parameters are interrelated by radius and aggregation number [28],the transition between spherical and cylindrical micellar systems p= 29,and the influence of polydispersity,which was explained in a similar fashion by Linse [26]and where p is the packing parameter (also called the Eisenberg [30]. shape factor)that determines the final structure. In addition to the above described treatments varying from small values (less than unity)for and rkers [31]developec spherical micelles to approximately unity for bicon- appro ncal nd the tinuous bilayers to greater than unity for inverted structures(Fig.5), More hinhilic this theory can also be applied to block app obtained from 、dif rer Interfacial (hydrophobic) fesctrhaRftinhaiagaroesaatcaanrtoandheadgopepukiootemectanmofmoeeamio

equilibrium, and the variation of the aggregation number as a function of the degree of polymerization. Their model is based on the molecular characteristics of the polymer, its concentration in solution, and an estimation of the core/corona interfacial tension [26]. The results of their model are in very good agreement with X-ray and neutron scattering data. Leibler et al. [27] expanded the mean field theory to the case of spherical micelles obtained from diblock copolymers in a low molar mass solvent. They found that the free energy of a micelle is a function of three parameters: the energy components attributable to the core and to the corona, and the interfacial energy. Accordingly, the size of the micelle, the aggregation number, and the fraction of copolymer chains forming micelles could be calculated. Further development of this theory allowed its use for other aspects such as the evolution of the CMC with block copolymer structure (triblocks versus diblocks), as studied by Linse [26], or the temperature dependence of the hydrodynamic radius and aggregation number [28], the transition between spherical and cylindrical micellar systems [29], and the influence of polydispersity, which was explained in a similar fashion by Linse [26] and Eisenberg [30]. In addition to the above described treatments, Israelachvili and coworkers [31] developed a very accessible approach using geometrical considerations that predicts the micellization phenomenon and the resultant morphologies. Initially developed to address the situation of amphiphilic molecules of low molar mass, this theory can also be applied to block copolymers. If we take the particular case of amphiphilic molecules in aqueous solution (Fig. 4), the major forces governing the assembly into well defined structures are, on the one hand, the hydrophobic attraction between insoluble hydrophobic moieties, and on the other, the repulsion between the hydrophilic head groups due to electrostatic or steric interactions that both force amphiphilic molecules to be in contact with the aqueous solution. If the attractive forces predominate, the interfacial area ao per molecule will decrease; and if repulsive forces predominate, ao will increase. The competition between these two opposing forces, which strongly depends on the geometry of both blocks, is mirrored in a variety of known morphologies. The hypothesis of Israelachvili assumes geometric properties to depend on three parameters (see Fig. 4): the optimal interface ao, the volume v occupied by the hydrophobic chains, and the maximum length lc of these chains. These parameters are interrelated by p Z n aolc where p is the packing parameter (also called the shape factor) that determines the final structure, varying from small values (less than unity) for spherical micelles to approximately unity for bicontinuous bilayers to greater than unity for inverted structures (Fig. 5). More recently, and by analogy with Israelachvili’s approach, Disher and Eisenberg [7a] tried to unify the experimental results obtained from different Fig. 4. Schematic illustration of the contributing forces (interfacial attraction and head-group repulsion) to the mechanism of micelle formation in solution. Adapted from Ref. [31]. J. Rodrı´guez-Herna´ndez et al. / Prog. Polym. Sci. 30 (2005) 691–724 697

amphiphilic block copolymers. Reasoning from a series of examples drawn from the literature, they proposed a unifying rule for the formation of polymersomes (polymer-based vesicles) in water: i.e. a ratio f of the mass of the hydrophilic part to the total mass 35G10%, as in the case of phospholipids [32]. An asymmetric molecule with a cylindrical shape and f!50% presumably reflects a certain balance between its hydrated part and a disproportionately large hydrophobic fraction. Finally, molecules with fO45% are expected to form micelles and those with f!25% are expected to self-assemble into inverted structures. 2.3. Experimental aspects In this paragraph we briefly describe the most relevant experimental studies on the self-assembly of block copolymers in solution. First, an introduction to the preparation and characterization of micellar structures, including the determination of the critical micelle concentration, the micelle shape and the dimensions, is given. Then, some illustrative examples from the abundant literature are discussed. 2.3.1. Preparation of micelles Two main procedures can be followed for the preparation of micelles. The first approach is to introduce the copolymer in a nonselective solvent, i.e. a common solvent for both blocks. In some cases, the desired micellar structure can be obtained through the manipulation of certain solution parameters, e.g. temperature or the use of a cosolvent. In other cases, the subsequent addition of a selective solvent, into a previously prepared solution from a common solvent, may be necessary. In a second step, the common solvent is removed from the solution, usually via dialysis. The second method to prepare micelles consists simply of introducing the dry copolymer powder into a selective solvent. The preference for one or another method depends on the system investigated. For example, the presence of glassy blocks such as polystyrene (PS) induces the formation of very stable micelles with almost no exchange between unimers and micelles (‘frozen micelles’).In this case the first method will be favored, and experimental means such as heating or ultrasonic stirring will be required. 2.3.2. Formation of polymeric micelles: experimental determination of the CMC The use of fluorescent probes is the most used method for the determination of the CMC [33]. Pyrene is the preferred fluorescent probe because of its strong fluorescence in nonpolar domains and its weak radiation in polar media. The shift of the excitation peak can be used to probe the transfer of pyrene molecules into an increasingly nonpolar micellar environment. The ratio of intensities of the excitation maxima at 339 and 333 nm can be plotted as a function of concentration; the crossover value represents the CMC as depicted in Fig. 6. Fig. 5. Dependence of final micelle structure on intrinsic molecular parameters: volume v of the hydrophobic group, and area a0 and length lc of the hydrophobic block. Adapted from Ref. [31]. 698 J. Rodrı´guez-Herna´ndez et al. / Prog. Polym. Sci. 30 (2005) 691–724

UV-absorption spectroscopy has also been reported as a powerful technique for the determination of the CMC. This method is based on the tautomerism of 1-phenyl-1,3-butadione between keto and enol forms that possess different absorption maxima: 312 nm for the enolic form and 250 nm for the keto form, the former appearing in nonpolar solvents like cyclohexane and the latter in polar solvents, where Hbonding is destabilized in favor of the keto configuration [34]. Other methods, mainly scattering methods such as static light scattering (SLS), dynamic light scattering (DLS) or small-angle X-ray scattering (SAXS), extensively used for small surfactants, can in principle be used for block copolymer micelles. However, they have found only limited application because of the very low signal intensity due to the much lower CMC’s in block copolymers in comparison with low molar mass surfactants [14b]. 2.3.3. Characterization of micellar size and shape Several techniques have been utilized for characterization of typical micelle parameters such as shape, size and aggregation number Z. Depending on the method of analysis employed, a variety of information about the system under study can be determined. It is beyond the scope of this review to explain in detail all the techniques that can be used [35], but the most useful ones are worth a brief description in the context of micelle characterization. The most important are the scattering methods. Static light scattering (SLS) [36] is a powerful technique to estimate average molar masses of self-assembled structures (and their CMC). In addition, if scattering from the core and the corona of the micellar system is not very different, RG can be also calculated. Dynamic (or quasi-elastic) light scattering (DLS) [37] can be used to estimate the hydrodynamic radius (RH) of a block copolymer micellar system from the determination of its diffusion coefficient; in addition, the sensitivity and versatility of DLS allow changes in the micelle equilibrium due to variations of temperature, pH, or other parameters to be monitored. Small-angle X-ray scattering (SAXS) has been employed in the analysis of micellar solutions to obtain overall and internal sizes from differences in electron density of the solvent and the solute. Finally, small-angle neutron scattering (SANS) gives information not only about the shape, but also the cross-section. Other nonscattering methods such as transmission electron microscopy (TEM) and atomic force microscopy (AFM) provide images whereby size, shape or internal structure of the micelles can be confirmed. Further methodologies include dilute solution capillary viscometry, membrane osmometry, ultracentrifugation, size exclusion chromatography, and typical spectroscopic methods (e.g. nuclear magnetic resonance) [10]. 2.4. Examples of micellar systems After this brief overview on the strategies to prepare and characterize micellar systems, we turn to illustrative examples of self-assemblies. Micelles can be classified into several types with morphologies varying from spherical to vesicular or other less Fig. 6. Experimental determination of CMC from fluorescence measurements with pyrene as a probe. Increasing intensity corresponds to encapsulation of pyrene in a hydrophobic environment and hence with micelle formation. From Ref. [33]. J. Rodrı´guez-Herna´ndez et al. / Prog. Polym. Sci. 30 (2005) 691–724 699

common structures, such as inverse micelles, bilayers, or cylinders (Fig. 7). Recent reviews analyze in more detail the parameters that afford one or another structure [38]. Since the literature on this topic is abundant and diverse, our discussion is limited to a few selected examples. Because of our interest in biological applications and ‘green’ chemistry we focus on micelles formed from amphiphilic block copolymers in aqueous solution. As reported extensively by Riess [39], the structure of amphiphilic block copolymers in aqueous media can be divided into three classes depending on the nature of the hydrophilic block. There are uncharged blocks such as poly(ethylene oxide) (PEO)—also referred to as poly(ethylene glycol) (PEG)—positively charged blocks such as quaternized poly(2- or 4-vinylpyridine), polypeptides such as poly(L-lysine), or negatively charged ones such as poly(acrylic acid) (PAA), poly(styrene sulfonate) (PSS), or poly(L-glutamic acid) (PGA). As indicated subsequently the characteristics of these systems make them suitable for applications in pharmaceuticals, as vehicles for drug delivery or as separating agents, etc [40]. 2.4.1. Spherical micelles Spherical micelles with the so-called ‘core-shell’ structure have been extensively studied. Formation of spherical micelles via self-assembly of diblock copolymers is directed by an entropically driven association mechanism. In the last decade special attention has been paid to aqueous micellar systems, mainly motivated by their applications as emulsifiers, foam stabilizers or detergents, and in biomedicine (as stabilizing agents in dermatological creams, lotions, etc. PEO is a hydrophilic, biocompatible, nontoxic, thermoresponsive polymer, which has been widely used as the solubilizing block to form the shell in spherical micelles. Hydrophobic blocks include PS, poly(lactic acid) and polyethers like polypropylene oxide (PPO) or poly(butylene oxide) (PBO). PEO-bPPO or PEO-b-PBO are commercially available as diblock- (and triblock) copolymers and as a consequence, have been extensively investigated. Chou and Zhou [41] reviewed in detail the characteristic micellization features of these block copolymers. Fig. 7. Examples of structures obtained from block copolymers: (i) direct micelles, (ii) vesicles, and (iii) other morphologies: (iiia) inverse micelles, (iiib) lamellar structures, and (iiic) cylindrical or tubular micelles. 700 J. Rodrı´guez-Herna´ndez et al. / Prog. Polym. Sci. 30 (2005) 691–724