Living Radical Polymerization - American

Chapter 26

Downloaded by NORTH CAROLINA STATE UNIV on August 8, 2012 | http://pubs.acs.org Publication Date: June 26, 2003 | doi: 10.1021/bk-2003-0854.ch026

Polymers, Particles, and Surfaces with Hairy Coatings: Synthesis, Structure, Dynamics, and Resulting Properties 1

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Tadeusz Pakula , Piotr Minkin , and Krzysztof Matyjaszewski 1

Max-Planck-Institute for Polymer Research, Postfach 3148, 55021 Mainz, Germany Center for Macromolecular Engineering, Department of Chemistry, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh, PA 15213 2

Summary: In this chapter, we discuss problems related to the synthesis, structure and dynamic behavior of macromolecular systems, in which many linear chains are anchored to other macromolecules, to nanoparticles or, in a general sense, to surfaces forming "hairy" coatings at these objects. Computer simulated and real systems are considered. In the preparation of such systems, both by means of adsorption and by means of synthesis, strongly competitive situations on the molecular level take place, which result in various, not necessary desired effects, such as high polydispersity of surface layers or backbone extension in molecular brushes. It is demonstrated how the considered molecular structures can influence the system dynamics and consequently the mechanical properties. Examples of new super soil materials based on a specific class of macromolecular architectures are presented.

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© 2003 American Chemical Society

In Advances in Controlled/Living Radical Polymerization; Matyjaszewski, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.

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Introduction Examples of systems we will discuss in this chapter are illustrated in Fig. 1. A l l of these Macromolecular, colloidal and macroscopic objects contain a coating layer consisting of end-grafted Macromolecular chains. Such a coating can strongly influence interactions of the considered objects with the surrounding because of a large variety of effects related to the new volume requirements, new chemical composition and new dynamics at the object peripheries. The hairy new interesting properties.

multiarm stars

polymer brush

copolymer micelles

surface brush

Figure 1. Examples of systems consisting of Macromolecules, particles and surfaces with coating layers formed by end-grafted polymers

There is a considerable interest in controlled preparation of thin polymer layers with chains covalently bound to surfaces. Traditional methods consisting of adsorption of end-functionalized polymers and bounding them to the substrate show disadvantages, because an adsorption of the first fraction of chains is hindering access of additional Macromolecules, which results in limited density of the layers. A n alternative method has been proposed, in which surface initiated chain growth was expected to impose less restrictions on the formation of dense brushes (1,2). The latter method has been used for growth both from planar surfaces and from small particles with diameters down to the nanometer range. Similar technique has been Appl.ied to the synthesis of molecular brushes, in which a polymer chain is used as a macroinitiator, from which a large number of side chains grows (3). Various kinds of brushes have been extensively studied theoretically as model systems, however, the most detailed information about the structure of brushes and the conformation of chains has been obtained from computer simulations (4). The "cooperative motion algorithm" (CMA), which is particularly suitable for simulation of dense polymer systems, has been very successful for simulation of both dry and wet layers of polymers consisting of


Mac


368 chains grafted by one end at a surface. Various specific features of the layers such as orientation and extension of chains, concentration profiles and distributions of free ends have been established to primarily depend on the length of chains, the density of grafting and on the interaction with the surrounding medium. In this chapter, we discuss some examples of recent advances in possibilities of control of synthetic procedures leading to these structures as well as the advances in characterization and understanding the properties of materials based on the complex constituents having their internal structures and dynamics. To a large extent this discussion is based on the results of computer simulation, which is considered as a guide indicating reasonable directions for experimental studies.

Methods Simulation: The method used was the cooperative motion algorithm ( C M A ) , in which Macromolecules are approximated as beads connected by non breakable bonds in order to represent structures corresponding to cooperative rearrangements taking place on a lattice system with all lattice sites occupied by the Macromolecular elements or by single beads representing a solvent. The method has been described in the literature (4-7), nevertheless, it is useful to stress here some unique properties, which made it particularly effective for the studies of molecular and Macromolecular systems within the size scale corresponding to the nanometer range. The cooperative rearrangements involve displacements of beads, which lead to changes of Macromolecular conformations, however, in such a way that identities of Macromolecules given by the specific architecture of the bond skeletons as well as by the sequences of beads within the skeletons remain preserved. A large variety of rearrangements are possible and therefore, it is not possible to specify all of them. The C M A is suitable for the studies of static properties of Macromolecular models as well as for characterization of their dynamic behavior in a broad time range (5). Simulations are usually performed for three dimensional systems on the fee lattice. 5

Motion o f simulated molecules allows generating a large number of states, which can be averaged to get representative information about the structure of molecules in equilibrium B y monitoring displacements and orientations of the model molecules and of their elements in time, one gets information about the dynamics. The properties of the method important for simulation of complex various kinds of complex Macromolecules with variable degree of simplification,


M

Macr

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(ii) the model for the dynamics is the same for both small simple molecules and large Macromolecules and does not require adjustments to particular structures, (iii) systems can be generated according to various polymerization processes taking place in space, which are good representations of reality with distributions of Macromolecular masses, forms and constitutions and (iv) the simulation is very fast, which allows to consider large systems in a broad time range on small computers (i.e. PC). Synthesis of brush polymers: The brush polymers have been obtained by grafting-from copolymerization using A T R P (8,9). The macroinitiator (poly(2(2-bromopropionyl-oxy)ethyl methacrylate), pBPEM) was dissolved in chlorobenzene followed by three cycles of freeze-pump-thaw to exclude any oxygen from the reaction. Separately, a measured amount of catalyst (CuBr) and dNbpy were place in a Schlenk flask followed by degassing under vacuum and backfilling three times with N . Into the Schlenk flask was added a measured amount of w-butyl acrylate monomer (nBA) after degassing through bubbling of N for 30 min. The catalyst and monomer solutions were transferred to the solution at room temperature under N . The polymerization reactor was immersed in an oil bath, which was preset to the specified reaction temperature. At timed intervals, samples were taken out from the flask via syringe. 2

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Characterization: The structure of the bottle-brush polymer melts has been anode X-ray source (Rigaku) with the position sensitive detector (Brucker). In order to characterize the dynamics, mechanical measurements have been performed using the mechanical spectrometer R M S 800 (Rheometric Scientific). Frequency dependencies of the complex shear modulus at a reference temperature (master curves) have been determined form frequency sweeps measured at various temperatures with a small amplitude sinusoidal deformation.

Results

Generation of systems: Off-space models for description of kinetics of polymerization processes can not account for a number of effects such as diffusion control of reaction rates, cyclization and steric or topological barriers. A l l of these effects can become very important steric or topological barriers. A l l of these effects can become very important when bulk polymerization of complex observed when polymerization is taking place in situations of a strong competition between growing sites, we consider here three cases with different



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Figure 2. Illustration of various polymerizations initiating structures and resulti

initiator structures: (1) a planar surface, (2) a flexible linear chain and (3) a dendrimer from, which after polymerization surface brushes (Fig 2a), brush-like polymers (Fig. 2b) and multiarm stars (Fig. 2c) are obtained, respectively. The first example describes the case of bulk polymerization initiated from a planar surface. Figure 3 shows concentration profiles o f the growing polymers at various times after activation of the initiator fixed at the wall. Increase of the polymer layer thickness and layer density with time is well seen. With the progress of the reaction, the chains grow but not necessarily uniformly, which can lead to changes of chain length distributions, as illustrated in Figure 4. The highlighted stages correspond to D P of η = 12, 54 and 92. In the case of relatively low initiation density (σ=0.25), the respective polydispersities of chains grown are M /M„ = 1.11, 1.06 and 1.06 and are remarkably larger than for unperturbed chains grown in solution (1.08, 1.02 and 1.01, correspondingly). Nevertheless, under such conditions, an almost linear dependence of the dry layer thickness on time has been observed, as in the reported experimental observations (70). For other conditions of the layer growth, e.g. with a higher density of the initiator at the surface (p> 0.5), much broader chain length distributions and a w



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χ - distance from the wall

Figure 3. Evolution of polymer concentration distributions with time in a simu system of polymers growing from the initiator attached to the wall. Initiation σ=0.25.

Ν - chain length

Figure 4. Effect of initiation density (σ) on distributions of length of linear polym grown from the planar surface. The evolution of chain length distributions with shown for two initiation densities.


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non-linear conversion versus time dependences have been observed, as illustrated in Figures 4 and 5, respectively. The example shown illustrates that the films obtained by the surface initiated polymerization can exhibit a distribution of chain lengths, which can considerably change the layer structure in comparison with the structure of brushes consisting of uniform polymers, as usually considered. Distributions may become very broad when the initiation density is high and for systems with additional bimolecular activation-deactivation equilibria, which require diffusion of the activator and the deactivator to the chain end, as in A T R P systems.

0

1000

2000

3000

time

Figure 5. Effect of initiation density (o) on dependencies of conversion and polydispersity vs. time for linear polymers grown from the planar surface. Bul polymerization in a layer offixedthickness is considered in this example. In the case of polymerization initiated from a linear macroinitiator, brushlike Macromolecules are obtained. Both the initiation density and the polymerization mechanism can influence the structure of such a complex growth of side chains involves extension of the backbone. This is an important effect, which can strongly influence properties of materials containing such distance with the reaction progress is illustrated in Figure 6.


Macromol


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side chain length

time

Figure 6. (a) Variation of side chain length distribution with the reaction progress system obtained by polymerization from a linear macroinitiator. (b) Extension of backbone accompanying the side chain growth in the same system. When the growing chains can fill the whole space around a small multifunctional initiator, as for example in the case of multiarm stars generated by polymerization initiated from end functionalized dendrimers, there is no effect on the arm size and arm polydispersity as long as the generation of the initiator is not higher than g=5 (64 arms). A n example of star dimensions (mean squared radius-of-gyration of stars) vs. total mass distributed to various numbers of arms and various arm lengths is shown in Figure 7. The effect is observed in the scaling of star sizes with their arm length. Up to generation of g=5, the scaling exponents remain the same as for linear chains, whereas at higher initiator sizes the conditions of crowded growth are created and the arms can become increasingly polydisperse with the decrease of the initiator outer curvature, which shifts the system towards the situation closer to the case of the initiation from the wall. The effects of the limited accessibility of space and limited accessibility of monomers to the reacting sites have been demonstrated here on the basis of simulated systems. There are limited possibilities for experimental verification of these observations in real systems. Attempts have been made to determine the distributions of chain lengths for the surface initiated polymerization. In the known cases, however, the conditions of polymerization were not yet corresponding to situations, which should be considered as the crowded growth conditions. Therefore, in the known experimental results no enhanced heterogeneities of the chain lengths were observed.



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2.0

2.5

3.0

3.5

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Figure 7. Mean squared radius of gyration ofpolymer stars with dendrimeric cente various generations and with various arm lengths. The lines are plotted to guide the e along dependencies with constant number ofarms or constant arm length. Structure and dynamics: Creating new Macromolecular architectures can constitute a challenge for synthetic chemists but can additionally be justified i f it can result in new properties of materials. We discuss here mechanical properties, which are related to the dynamics of systems with various Macromolecular structures. Joining monomelic units into linear polymer chains results in a dramatic change of properties. Whereas, a monomer in bulk can usually be only liquid-like or solid (e.g. glassy), the polymer can additionally exhibit a rubbery state with properties, which make these materials extraordinary, in a large number of chains in comparison with the fast motion of the monomers, especially when the chains become so long that they can entangle in a bulk melt. A mechanical manifestation of the relaxation, taking place in a polymer melt in comparison with the behavior of a low molecular system is illustrated in Figure 8. The dynamic mechanical characteristics of the two materials indicate a single relaxation in the monomer system in contrast to the two characteristic relaxations in the polymer. The characteristic rubbery state of a polymer extends between the segmental (monomer) and the chain relaxation frequencies and is controlled by a number of parameters related to the polymer structure. The most important among these parameters is the chain length, determining the ratio of the two relaxation rates. In the rubbery state the material is much softer than in the solid



375

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Figure 8. (a) Real (G') and imaginary (G") part of the complex shear modulus vs. frequency for an isobutylene oligomer and (b) the same dependencies for a melt of linear p(n-butyl acrylate), p(nBA), chains.


376 state. If expressed by the real part of the modulus, the typical solid state elasticity is on the order of 10 Pa and higher, whereas the rubber like elasticity in bulk polymers is on the order 10 -10 Pa. The effect of polymerization on the relaxation behavior in the simulated systems can be simulated in the similar way. A single relaxation process can be detected in a non-polymeric simulated liquid and two dominant relaxations, the segmental and the chain relaxation, can be distinguished in the dynamic behavior of dense system of linear polymers. A n example of such results is shown in Figure 9. A very good agreement in the behavior of simulated and experimental systems with the linear polymers has been documented elsewhere (6). The next question, which arises, is related to possibilities of influence on the properties of materials by changing the polymer architecture to the one suitable for particular Appl.ications. We will demonstrate here that the highly branched different properties than those obtained by linking monomeric units into linear chains. 9


5

-2

6

0

2

'°9

Macrom

4

1

chain

log t

Figure 9. Dynamic behavior of (a) simulated liquid and (b) simulated melt of linear polymers as characterized by segmental and end-to-end vector correlation functio giving information about relaxation rates of the corresponding units. In the case polymers, systems with various chain lengths are considered



377 The first example, which has been reported already earlier (J), concerns structures and behavior of multiarm stars in the melt. As illustrated in Figure 7, the stars have smaller dimensions than linear chains of the same molecular mass and the difference increases with increasing number of arms in the star. In the stars with a large number of arms, the space around the star center is predominantly occupied by the elements of considered Macromolecule. This leads to ordering of the stars, which manifests itself in the X-ray scattering results. In Figure 10a, an example is shown of the scattered intensity distribution for the melt of multiarm polybutadiene stars synthesized by Roovers (J). The clear intensity peak indicates an ordering of star cores in such a system. The lower part of Figure 10 (b) shows the viscoelastic characteristics of this system, which indicates that besides the usual relaxation processes related to the segmental motion and to the motion of polymeric fragments (arms), there exists the third relaxation process with the longest relaxation time, which is interpreted as related to slow cooperative rearrangements in the structured system of stars.

3 CO

c Φ

b U) ο CD O)

ο

Figure 10. (a) Small angle X-ray scatteringfrom the melt ofpolybutadiene stars with arms of M =7000 and (b) the viscoelastic characteristics of this material by means of the frequency dependencies of the storage (G') and loss (G") shear moduli (maste curves at Τ^218Κ). w



378

Figure 11. Dynamics in simulated multiarm star melt (5). The three correlation fun correspond to segmental ps, arm pR and global pc relaxation. Effect of the num arms on these relaxations is illustrated (arm length Na=20).



379 Simulated systems of the multiarm stars indicated the same relaxation processes as the real systems. The slowest relaxation has been identified as a translational motion of stars and has been observed as strongly dependent on star number, in a good agreement with the observations made for real systems. It was possible to observe in the simulation that the stars with arm numbers higher than 25 reach in the center the intramolecular density equal to the mean density of the system, which means that an impenetrable core of the star is created. A kind of ordering and new slow relaxation processes can be observed also in melts of brush-like Macromolecules. Figure 12 shows X-ray scattering results determined at small angles (SAXS), which indicate correlation distances between the backbones of these Macromolecules. These results show that the correlation length can be controlled by the length of side chains. Figure 12 also presents an example of the viscoelastic properties determined for the melt of such brush-like temperature of 254K. The results indicate again a presence of three relaxation

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10"

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10

2

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ω [rad/s.] Figure 12. Viscoelastic characteristics of the p(nBA) brush polymer melt by means offrequency dependencies of the storage (G') and loss (G") shear moduli (master curves at T $=254K)> The insert: Small angle X-ray scattering from melts of p(nBA) brush like polymer with various side chain length but the same backbone i.e. the same number of side chains per Macromolecule. re


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380 ranges: the high frequency relaxation corresponds to segmental motion the intermediate relaxation is attributed to the reorientation rates of the side chains and the slowest process is the global Macromolecular relaxation in this system, which controls the zero shear flow and the corresponding viscosity. The rate and nature of this relaxation must be dependent on the length of the backbone. For short backbone chains the Macromolecules can behave similarly to stars, for which translational motion dominates the slow dynamics but for longer backbone the reorientation possibilities should become slower than translation. This was observed clearly in the simulated systems as a crossover between translational and rotational relaxation times on the scale of backbone length. It is important to notice that the described polymer architectures lead to relaxation spectra more complex than those of linear polymers. Depending on structural parameters of the systems, the proportions between relaxation rates corresponding to die three distinguished processes can be influenced. This should lead to various properties of materials in frequency ranges corresponding to modulus plateaus related to various unrelaxed states i.e. glassy, rubbery or the super soft state. For example, extension of side chain length to the range where they can entangle should extend the frequency range of the modulus plateau at the level typical for polymer rubber elasticity i.e. at 10 -10 Pa (in detail depending on the chemical nature of the monomer). On the other hand, extension of the backbone or a slight crosslinking of the system with not yet entangled side chains, can lead to an extension of the frequency range with the super soft elastomeric plateau with the modulus level below 10 Pa. A n example of this case is shown in Figure 13 where the mechanical properties of the lightly crosslinked system of brush polymers are characterized. We have recently observed properties of this kind in a number of similar systems including brush copolymers, which can be considered as super soft thermoplastic elastomers. In the permanently crosslinked system illustrated in Figure 13, the modulus plateau related to the soft elastomeric state extends over a very broad temperature range (tested from room temperature up to 180°C) when detected isochronously. 5

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Conclusions Investigation of various systems modified by hairy coatings was presented. Modifications of Macromolecular architectures can generate a new spectrum of relaxation, which leads to interesting mechanical properties of bulk systems. The examples described indicate that a broad variation of properties is possible by changes of the following molecular parameters: side chain length, grafting density, backbone length and also the crosslinking methodology and density. These parameters influence the extent and levels of the elastic plateaus in the complex viscoelastic spectrum and consequently should allow to generate materials with extremely different properties ranging between the hard glassy and super soft elastic, taken as examples.



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Figure 13. Viscoelastic behavior of a lightly cross-linked brush-like nBA polymer characterized by means of the frequency dependencies of the storage (G ') and loss (G ' ') shear moduli (master curves at T j=254K). re

Acknowledgments: Kathryn L . Beers and Shuhui Qin are gratefully acknowledged for their contribution to this work. The reported research has been partially supported by the Marie Curie Training Site at MPIP and by the National Science Foundation (DMR-0090409).

References

1. Halperin, Α.; Tirrell, M.; Lodge, T. P. Adv. Polym. Sci. 1991, 100, 31. 2. Pakula, T. Macromol. Symp. 1999, 139, 49 3. Beers, K . L . ; Gaynor, S. G.; Matyjaszewski, K . ; Sheiko, S. S.; Moller, M. Macromolecules 1998, 31, 9413.



382 4. Pakula, T. J. Chem. Phys. 1991, 95, 4685; Pakula, T.; Zhulina, Ε. B . J. Chem. Phys. 1991, 95, 4691 5. Pakula, T; Vlassopoulos, D . ; Fytas, G.; Roovers, J.; Macromolecules 1998,31,8931 6. Pakula, T.; Geyler, S.; Edling, T.; Boese, D.; Rheol Acta 1996,33,631 7. Pakula, T.; Comput. Theor. Polym. Sci. 1998, 8,21 8. Wang, J. S.; Matyjaszewski, K . J. Am. Chem. Soc. 1995, 117, 5614. 9. Matyjaszewski, K . ; Xia, J. Chem. Rev. 2001, 101, 2921. 10. Matyjaszewski, K . ; Miller, P. J.; Shukla, N.; Immaraporn, B . ; Gelman, Α.; Luokala, Β. B . ; Siclovan, T. M.; Kickelbick, G . ; Valiant, T.; Hoffmann, H.; Pakula, T. Macromolecules 1999, 32, 8716.


Living Radical Polymerization - American

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