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Molecular composites for molecular reinforcement: a promising concept between success and failure.

INTRODUCTION

The words "molecular composite" can be ubiquitously found in the literature. Remarkable achievements and new questions have arisen in this area during the last decade, although some of the sources tend to use the idea of molecular composites only to motivate their studies on blends but not to perform homogeneous composites at a molecular level. In our opinion there is a demand for this review article on molecular reinforcement, in which we try to clarify the state of the art and inform readers what coming developments they could expect in this field. So far, only a few contributions have been devoted to reviews. Giving a general overview [1-4] or creating a deeper understanding [5-12], most of these enlightening contributions remain restricted in the kinds of technological or synthetic approaches. We hope to present the main concepts and promising efforts but not the synthetic realization. It is the aim of this contribution to give a systematic review of the concepts in the field limited to molecular composites for molecular reinforcement, which is, to our knowledge, currently not available.

Since the concept of molecular composites was clearly articulated by the Air Force Materials Laboratory [13-19] and by Takayanagi et al. [20-29] at the end of the 1970s and in the early 1980s, many efforts were made to design successful systems of this so-called "next generation" of polymeric materials. Indeed, the prime mover is that molecular composites promise the distinct synergistic properties of flexible polymeric matrices and rigid rod segments, so we might expect superior materials in the field of structural, as well as functional, polymeric materials. The fulfillment of this concept is expected to improve environmental resistance [1-5, 30, 31], thermal stability [1-5, 30-37] and toughness [1-5, 7, 11, 13-29, 38-47], as well as electronic and optical properties [48-61]. Given that motivation one has to wonder why the crucial stage in development, a striking success in an application, has not yet been reached, and why the rigid rod molecular composite technology is still in its infancy. But, neither the failed attempts nor the quite academic successes should obscure the fact that we try to unravel a Gordian knot. The problem is to get molecularly dispersed rigid rods in a flexible polymeric matrix, which is the fundamental presupposition for a molecular composite. It is even worse than with a polymeric blend of flexible structures, where the poor entropic force allows only a minority of systems to build a homogeneous mixture. The molecular anisotropy of rigid rod leads to accumulating entropic forces accompanied by a demixing process [62, 63].

II MOTIVATION

There have been increasing demands on structural materials during the past decades. We can identify three main points:

* the property combination of advanced mechanical properties (tensile, shear and compressive properties, toughness, resistance to fatigue) and low specific weight or/and costs;

* extraordinary high strength and high stiffness materials;

* high performance property combinations of mechanical properties and thermal stability, processability, low moisture absorption, hydrolytic stability, etc.

Consequently it is not surprising that non-metallic materials have continued to penetrate the structure materials market because of their main characteristic advantages, namely low weight and costs. They have further proved their adaptability to advanced demands through adjustable material design by polymer synthesis, as well as processing. Most notable here should be the concept of blending to obtain new, improved products.

One of the most successful approaches for producing improved structural materials is reinforced polymer composite technology. Unfortunately, the limitations of fiber reinforced materials, for instance, have also produced drawbacks and necessitated the consideration of an improved materials concept, Indeed, molecular reinforcement seemed to be an innovative concept for structural materials and, what is more, one of the most promising because it relies on the well-established techniques of composite technology, and it is subject to market demands.

Molecular reinforcement is effected by the consequent translation of fiber reinforcement to the molecular level. The motivation for it can be divided into the following main reasons:

* There are two main characteristics of fibers, which can be optimized to increase the reinforcement effect. On the one hand there is the aspect ratio of the fiber and on the other hand the mechanical properties - specific modulus, strength, and toughness. Analysis of the mechanical data, as well as the superior reachable aspect ratio of single stiff molecules identifies single molecule fibers as the limit of materials aimed at fiber reinforcement. Lindenmeyer [64] explained the concept of molecular reinforcement by arguing that increasing the ratio by decreasing the diameter will lead to a single molecule fiber as the ultimate reinforcing material.

* The processing of fiber reinforced materials is complicated, expensive and restricted. In practical terms, fewer processing steps, easier access to isotropically reinforced materials and the use of a variety of processing techniques associated with thermoplastics are demanded. Therefore homogeneous compounds containing single molecule fibers seem to be an innovative method for creating a fundamental change in the processing costs and possibilities based on their improved thermoplastic flow and tribologic properties.

* Fiber reinforced materials are restricted in their application because of their inhomogeneity. Their principal characteristics are two coefficients of thermal expansion, lack of transparency, and rough surfaces. Efforts to control and adjust these properties have failed or led to complicated and restricted solutions. Major efforts were made especially to optimize the fibermatrix interface, which often determines mechanical coupling and failure. In fact, a particular branch of study has grown that deals with improvement of adhesion and avoiding delamination. The main characteristic of molecular composites is their homogeneity, and therefore we might expect crucial advantages.

* Single molecule fibers can be called a advanced route to attain perfect fibers. The failure of macroscopic fibers is often substantially determined by their defects. There is no reason to doubt that the synthetical control of the chemical structure of single molecules will be easier than the minimization of fiber defects during the processing.

The points mentioned illustrate the great potential of molecularly reinforced materials for industrial applications. The advantages of molecular composites and single molecule fibers lead us to expect that these materials will not only replace fiber reinforced components but in addition will open a variety of new applications for reinforced polymeric materials.

III FROM THE CONCEPT TO THE PREDICTION OF MECHANICAL DATA

III. 1 Rigid Rods: Chemical Structure, Persistence Length, Mechanical Characteristics

Rigid rods as single molecule fibers hold the key to exchange Van der Waals interactions by covalent bonds as the molecular origin for strength and stiffness. The bond energies for C-C, C=C, C-O, C-N, C=N, C-S, C=S are given by 347, 611, 360, 306, 615, 272, 536 kJ/mol, respectively. Stiffness and strength values, which can be obtained by covalent bonds, may be increased by several orders of magnitude in relation to properties based on Van der Waals interaction. In principle. every polymeric structure offers this possibility, but we do not expect outstanding mechanical behavior for flexible coil conformations, because a single chain deformation can be mainly attributed to rotational movements. A different picture begins to emerge for rigid rods and flexible structures hand in hand with extreme stretching, when the chain conformation equals a rigid rod. Then the strain must be attributed to a deformation of angles and lengths of covalent bonds. A factor ten marks each step from bond rotation, to bond bending to bond stretching. So it is not surprising that values up to 50-400 GPa for the stiffness and 10-40 GPa for strength can be expected [65, 66]. Note that the persistence length of rigid polymeric structures indicates how much strain is passed on no-rotational changes of covalent bonds. From this fact, molecular reinforcement demands intrinsically rigid rods. In practical terms, structures are demanded with a persistence length above 8 nm. In Fig. 1, some suitable rigid segments that most sources tend to use are displayed. In Table I a brief overview of persistence length analysis [67-101] is given, based on computational methods and experimental data for some corresponding polymeric structures. The discrepancies are caused by the different methods and should not obscure the fact that the data convincingly present a homogeneous picture of the chains' stiffness.

[TABULAR DATA FOR TABLE 1 OMITTED]

The strength and stiffness of a single molecule are surely not properties one would expect to be determined directly by an experiment on a macroscopic sample. But certain computational calculation and simulation methods, like semiempiric quantum mechanics or optimized force field evaluations, that have been developed over the last decades are likely to provide a good qualitative overview and reliable data [102]. The results obtained by these methods are often significantly higher than the experimental data for fibers or results evaluated by x-ray studies on the crystalline phase - especially if they are limited to single molecule systems or to perfect alignment and structure.

Furthermore, the semiempiric approaches were originally evaluated to describe bond stability and electronic excitations rather than mechanical properties. In addition, some of them concentrate essentially on the intramolecular system and take no account of the role of intermolecular interactions, which significantly influence the bulk properties. In fact it is possible to rule out neither a drastic decrease of the mechanical properties nor an increase for some exceptions. Even if calculations and simulations are determined using artificial systems, they give an accurate representation of the ultimate limits and demonstrate the potential of molecular reinforcement. The data evaluated on existing systems might be viewed as a representation of the available potential. In Table 2 a brief overview is outlined for the Young's modulus [2, 65, 66, 103-116] without providing a fair comparison, because it is arranged regardless of the distinct methods, assumptions used, details of processing, differences of the morphology and small deviations in the chemical structures.

III.2 Composites: From the Macroscopic to the Molecular Level

We mentioned above that the concept of molecular reinforcement is the translation of fiber reinforcement to the molecular level. Consequently on this basis the different sources tend to describe the molecular reinforcement analogously to the well-established descriptions of macroscopic fiber reinforcement, if the reinforcement effect itself did not already satisfy the full extent of one's desire. The use of macroscopic mechanical models for the molecular level is based mainly on the unambiguity of the concept. It resolves conclusively neither the specification of an internal stress and strain field for the molecular level nor the role of inter- and intramolecular interactions. At an atomistic level we could be confronted with strong inhomogeneous anisotropic materials. Consequently, we cannot expect a consistent description based on an equivalent homogeneous body. A suitable model on how to evaluate the mechanical properties based on the knowledge of the molecular structure does not exist. Experimental studies that try to illuminate the molecular deformation or stress are extremely rare. Nevertheless, the selective axial stretching of molecules in [TABULAR DATA FOR TABLE 2 OMITTED] the stretch direction of an isotropic molecular composite was proved by Raman spectroscopy [117]. Beyond the accordance between the macroscopic and molecular stress field directions, the vibration frequencies of Raman-active bands were also found to be sensitive to the level of applied strain. Therefore this study pointed to the validity of the concept of molecular reinforcement. Furthermore a consistent mathematical description of the mechanical data of a macroscopic sample has a direct bearing on gathering evidence for the concept of molecular reinforcement.

If we want to use well-established descriptions of macroscopic fiber reinforcement for molecularly reinforced systems, we first have to grade suitable approaches. Continuous fibers can be characterized by their aspect ratio l/d [approaches] [infinity] (l = fiber length, d = fiber diameter), their perfectly aligned orientation and their orthotropic and transversely isotropic symmetry. They show simple rules of mixtures. For the longitudinal properties (here index 33) we can assume uniform strain, attributed to the Voigt average [118-120] based on a parallel model. The transverse response (here Index [perpendicular]) is crudely given by the serial model with uniform stress, which is only true for a perfect interface, called Reuss average [118, 119, 121].

Voigt average

[E.sub.33] = [E.sub.f][V.sub.f] + [E.sub.m][V.sub.m] (1)

Reuss average

[E.sub.perpendicular]] = [E.sub.f][E.sub.m]/[E.sub.m][V.sub.f] + [E.sub.f][V.sub.m] (2)

where E is the Young's modulus in fiber orientation direction [E.sub.33] and transversal to the fiber orientation [E.sub.[perpendicular]. The indexes f and m refer to the fiber and matrix, respectively. The sum of the volume fractions [V.sub.t] is normalized to 1. But to be fair to realistic systems, the Voigt and Reuss averages are upper and lower limits. Consequently it is not surprising that in most sources the semiempiric Halpin-Tsai (H.-T.) [118, 119, 122, 123] equation is preferred. It is well established for the description of fiber reinforcement [124], and its central feature is that the properties of the composites must lie somewhere between the Voigt and Reuss average. The dimensionless quantity [Eta] acts as reduced property and [Xi] serves to adjust the effective properties. Then the parameter [Xi] can be used to take account of the imperfect mechanical coupling of the interface, for instance.

Halpin-Tsai equation (H.-T.)

[Mathematical Expression Omitted] (3)

Most notable is the dependence of E (index c refers to the composite) on the moduli ratio and on the parameter [Xi]. But, the H.-T. equation is not only used to describe the mixing behavior between the upper and lower limits based on an empirical parameter [Xi]. If we set [Xi] = 2l/d we get the dependence of [E.sub.33] on the aspect ratio of fibers with finite lengths. Only for l/d [approaches] [infinity] we get the simple mixing rule of uniform strain and the reinforcement decreases drastically for decreasing finite aspect ratios. For l/d [approaches] 0 we get the Reuss average as the lower limit. Figure 2 is a schematic illustration of the reinforcement effect in dependence on the aspect ratio we expect, based on the H.-T. description. A modulus of 200 GPa for the single molecule fiber and 2.5 GPa for the matrix was used.

[TABULAR DATA FOR TABLE 3 OMITTED]

The outcome of the following estimations for three different molecular reinforcement scenarios of an orientated system may be noted and, it is hoped, illustrate the potential of molecular reinforcement:

* For use as a reinforcing additive in polymeric matrices, the volume fraction of the rigid molecules is surely limited to 10%. With a fiber modulus of 200 GPa, a matrix modulus of 2.5 GPa and an aspect ratio of 50 we obtain a reinforcement factor [E.sub.33]/[E.sub.m] of 5.64 or a composite modulus of 14.1 GPa.

* To get a reasonable chance to substitute materials, we require a reinforcement factor of [greater than] 10 caused by a rigid molecule volume fraction of 20%. With a matrix modulus of 2.5 GPa and a fiber modulus of 250 GPa, an aspect ratio [greater than]33 is necessary;

* or with a matrix modulus of 2.5 GPa and an aspect ratio of 50, a fiber modulus [greater than] 178 GPa is needed.

Even these modest estimations illustrate the potential of molecular reinforcement. Realistic assumptions for single molecule modulus and the aspect ratios of polymeric rigid structures are likely to produce interesting composites for many applications.

The proof of the concept of molecular reinforcement based on the conformity of predicted moduli and experimental data was shown using PBT as rigid and poly-2,5(6)benzimidazole (ABPBI) as the flexible matrix for the first time [15]. The homogeneous blends were processed into molecularly reinforced fibers, so we are still in the framework of an anisotropic reinforcement. In Table 3 a comparison between the predicted modulus based on H.-T. and the experimental data is displayed. The experimental data and the used aspect ratio are according to [15]. But we used 300 GPa for the fiber modulus and the H.-T. equation for the prediction.

As mentioned above, perhaps isotropically reinforced materials should be the main target. Consequently, the isotropic composite modulus [E.sub.c] is the interesting property. The modulus of a long fiber composite characterized by the angle 8 between fiber orientation and draw direction is given by:

[Mathematical Expression Omitted] (4)

Fiber reinforcement does not follow a linear dependence on [Delta] but is strongly sensitive to it. A rigorous approach based on the laminate matrix transformation equations [125], a three-dimensionally random fiber orientation distribution and the simple mixing rules lead to [125, 126]:

[Mathematical Expression Omitted] (5)

respectively, if [Mathematical Expression Omitted].

Wiff et al. [38] used instead

[Mathematical Expression Omitted], (6)

which is proposed commonly for random in plane orientation [127, 128]. To get a better description we could replace [E.sub.33] and [E.sub.[perpendicular]] by corresponding H.-T. terms and take account of aspect ratio, for instance.

III.3 Composites: Simulation of the Molecular Level

Certain systems are believed to show molecular reinforcement by proving the validity of equations, which are well established for fiber reinforcement, down to the molecular level. However, descriptions of the stress and strain fields are strongly lacking at the atomistic level. Without deeper understanding, it is difficult to avoid the conclusion that the accordance of experimental and predictable data is only coincidental. Especially, the local stress and strain fields and the characteristics of the coupling mechanism between rigid and flexible molecules influence the mechanical data. In this context it is relevant to note that also for successful macroscopic fiber reinforcement, the interface between fiber and matrix must provide an excellent mechanical coupling.

In recent papers [38, 129] a description of molecular reinforcement on atomistic approaches was tried by computational simulations. The work of Wendling et al [103, 129] seems to be the first accurate atomistic description of molecular reinforcement and is summarized here as follows. Based on a force field approach, different systems of rigid and flexible molecules were simulated by means of amorphous cells. Most notable is that the simulated characteristics of the amorphous matrix system (PAR) were compared with experimental data, showing excellent agreement with density, cohesive energy, solubility parameter and mechanical data. The variations of stiffness, length, concentration and chemical structure of the rigid rods were realized and the influence on the mechanical characteristics described. Based on the fluctuation theory, the mechanical data were evaluated from molecular dynamic simulations [130, 131]. The elastic stiffness constants [Mathematical Expression Omitted] can be attributed to the ensemble, averaged value of the cell axes fluctuations < [[Epsilon].sub.i][[Epsilon].sub.j] according to the Parrinello-Rahman fluctuation formula

[Mathematical Expression Omitted] (7)

with T = temperature and V = volume of consideration. Indeed, current opinion largely favors and establishes it as excellent procedure for disordered glassy systems [132]. In Fig. 3, the fluctuations of the axes of the amorphous cells are displayed, representing a molecular composite of a polyarylate (PAR) and a PP chain in the z-direction. The fluctuation of the cell axes (x, y) in the transverse direction of the fiber corresponds to the three axes (x, y, z) of the isotropic matrix, whereas the fluctuation of the axis in the PP chain direction is strongly decreased. In Table 4 the reported elastic stiffness matrices for the pure matrix and two molecular composites with different rigid rods are displayed. Table 5 summarizes the results for the Young's moduli of the different chemical structures simulated.

The simulation data were compared with an H.-T. description regarding the aspect ratios of the rigid rods. For the moduli of the pure rigid rods, very high [TABULAR DATA FOR TABLE 4 OMITTED] [TABULAR DATA FOR TABLE 5 OMITTED] values of 356 GPa and 260 GPa were used for PP and PA, respectively. Also, the concentrations of the rigid rods were varied and the almost linear increase with increasing concentrations of rigid rods is described. The main results can be thus summarized: The presented atomistic simulation approach was able to illustrate molecular reinforcement effects and provided an insight into the relevant molecular mechanisms. An excellent accuracy has been achieved for the simulated and measured data of the matrix. This enabled us to step beyond a general proof of the concept. First, the reinforcement effect was shown to depend linearly on the concentration of rigid rods. The aspect ratio of the rigid rods plays the same role as for macroscopic short fiber reinforcement and can be well described by the H.-T. equation. Furthermore, the influence of molecular interactions between rigid and flexible segments applied equally to the interfaces for fiber reinforcement. Strong intermolecular interactions between matrix and rigid rods were proposed as a necessary requirement for a successful reinforcement.

IV THE MAJOR OBSTACLE: HOMOGENEOUS MISCIBILITY

IV.1 Description of the Thermodynamics: Lattice Calculations

As mentioned above, the major obstacle is the realization of blends with molecularly dispersed rigid rods, in which the superior properties as a high molecular axis ratio and high strength based on dealing with single molecule fibers as well as one coefficient of expansion based on the homogeneity of the material could be used, for instance. Unfortunately, rigid rods seem to be born to build anisotropic phases with short range order. Without wishing to go into the topic of liquid crystallinity, the findings of Flory [62, 63, 133-143] for the occurrence of the nematic state in terms of lattice model especially [62], are some of the most cited in the literature of molecular composites. Even though the aim of this and related works were to illuminate the interrelationship between anisotropic molecular shape and liquid crystalline phases, the description of the thermodynamics explained the experimentally established incompatibility [15, 144-150] of rodlike and coiled polymers. The success of descriptions [62, 63, 140, 141, 151-160] based on a lattice model is given by the fact that they are well suited to describe configuration partition functions dominated by volume exclusion. Indeed, the accordance of experimental data and the prediction is better than anything reached by alternative approaches such as molecular dynamic simulation [161, 162], which will not be discussed in such detail in the following. Lattice approaches were found to be less precise for increasing importance of enthalpy contributions for the mixing behavior as for systems with strong specific interactions, for instance. The considerations based on simple lattice models give us a good understanding of thermodynamics of systems consisting of rigid rods and flexible coils whether in solution or not. The major mixing behavior trends are well illustrated.

According to Flory et al. [62, 63, 133] the complete partition function [Z.sub.M] for a binary lyotropic system consisting of rigid rods and a diluent can be written as the product of the combinatory ([Z.sub.comb]) and the orientational ([Z.sub.orient]) part:

[Mathematical Expression Omitted] (8)

[n.sub.p] = total number of lattice sites filled with rigid

[n.sub.py] = number of rods disorientated from the preferred axis marked by y:

[v.sub.j] = number of situations accessible to molecule

J with orientation [y.sub.i];

[[Omega].sub.y] = fraction of solid angle associated with y

where [Mathematical Expression Omitted]

and [Mathematical Expression Omitted] (9)

[n.sub.s] = number of vacant sites, respectively filled with solution molecules;

x = axis ratio;

[Mathematical Expression Omitted] = average value of the parameter of orientation

So, whereas Zcomb increases with orientation, Zorient decreases. Zorient dominates for small axis ratios or concentrations leading to the state of complete disorder []. With increasing axis ratio and concentration, a maximum for [Z.sub.M] occurs with [] where [] marks the limit of perfect orientation. It was possible to approximate the minimum axial ratio for stable nematic order to 6.42. Replacing factorials by their Stirling's approximations we can write:

[Mathematical Expression Omitted] (10)

[v.sub.p] = volume fraction of rodlike solute;

[v.sub.s] = volume fraction of solvent

Consequently, with the contour length [x.sub.c], total number of size [n.sub.c] and internal configuration partition function [z.sub.c] describing the coil component for the ternary system rigid rods/flexible coils/solvent,

[Mathematical Expression Omitted] (11)

where [n.sub.o] = [n.sub.s + [n.sub.p]x + [n.sub.c][x.sub.c] is the total number of lattice sites. The resulting ternary phase diagram for [x.sub.c] = x = 100 is shown in Fig. 40. In this case, only for highly diluted systems, one could expect homogeneous mixtures. The whole homogeneous binary system flexible coil/rigid rod is far from a thermodynamic stable state. One expects the ternary system to demix into an ordered liquid crystalline phase and an isotropic phase mainly holding the flexible component. It does so by rejecting the flexible molecules with a high selectivity. The demixing is favored with increasing axis ratio and concentration of the rigid rods. Corresponding results were obtained for rods connected by flexible Joints [138] and underlined by experimental data [163]; that is why they could not be considered a means to increase the miscibility.

Ballauff [151, 152, 155, 164] evaluated the compatibility of coils and rods bearing flexible side chains (hairy rod) based on lattice calculations. The results mark a possible crucial improvement of the miscibility. Figure 4b is a phase diagram of such a ternary system with [x.sub.c] = x = 100 and zm = 100 (m is the number of segments for each side chain and z is the number of side chains of each rigid rod). Since this means of producing molecular composites was suggested, different systems have been synthesized that clearly underline this concept, owing to experimental results for their solubility [79, 151, 152, 165-182].

Rigid multipodes (or rigid star molecules) were Indicated to be a promising means of molecular reinforcement based on lattice calculations by Wendorff et al. [154, 183]. The calculations showed that the combinatory entropy of mixing for rigid stars equals one of flexible chain molecules, if the arms pointing in the three orthogonal directions are of the same length. With nonlinear rigid structures, the strong anisotropy of rigid rods could be avoided, diminishing or eliminating the tendency to build up an orientational order. An increase of compatibility extending even to the binary blends of rigid and flexible components is apparent. The lattice theory predicted, further, that molecules with planar variations of this shape such as cross-shaped molecules should show better solubility than the ones exhibited by a corresponding single linear segment. Figure 4c is a phase diagram of the system rigid star/flexible coil/solvent for x = [x.sub.c] = 100 and [x.sub.2k] = 100, where [x.sub.2k] the length of the rigid segments in the two directions orthogonal to the rod axes.

So far, we have considered only entropy and have disregarded the enthalpic contributions to the mixing behavior caused by molecular interactions. Nonetheless, we have been able to illustrate the major problem and suggest two solutions. However, the description of experimental data remains incomplete without taking enthalpy effects into account. In addition to introducing [Chi] [multiplied by] [v.sub.i] [v.sub.j] in the ln(z) formulation of the free energy to deal with isotropic long range interactions, an anisotropic potential provides one with the necessary instrument for a suitable description of systems with the tendency to build up lyotropic phases. Even these procedures were based on a simple mean field approach; they were extremely successful for describing and predicting many systems dominated by the usual interactions such as dipole-dipole forces [184, 185]. If the attractive forces between the segments of different kinds are larger than those between the same kind, solubility could of course be increased.

In comparison with mixtures of conventional polymers, where the potency of entropy to mix is already lost, the starting point is even worse for molecular composites. Therefore, only systems with strong specific interactions between coils and rods such as hydrogen and ionic bondings are candidates for homogeneous mixtures caused by the mixing enthalpy. Given that mean field assumptions of the lattice calculations are not suitable to deal with strong specific interactions we might anticipate that they provide a principal understanding but no longer an accurate description, even though Coleman and Painter [186] managed to do so for hydrogen bonds.

Alternative or recently improved theoretical descriptions [187-197] hold the key to even better records of the phase behavior of systems constituted of rigid and flexible components. Of lattice approaches were recognized as extremely superior if volume effects influence the mixing behavior or the representations of the molecular shape or interactions are unsatisfactory on a lattice. Brostow and Walasek [187], for instance, convincingly described the formation of various phases - isotropic, nematic or smectic and cholesteric - for polymeric liquid crystals as competition between energetic and entropic effects.

IV.2 Description of Realistic Blends: Phase Separation and Mixing Condition

Besides systems in which the thermodynamic equilibrium state is characterized by a homogeneous mixtures of flexible coils and rigid rods, we expect a variety of systems in which it is possible to freeze a homogeneous mixture in a glassy state by suitable technologies, where a phase separation could not occur. Studies on the phase separation in molecular composites [198-204] rapidly prepared from homogeneous solutions, for instance, have a direct bearing on how to produce molecularly reinforced materials by controlling the processing. These results largely relate to the well-known characteristics of spinodal decomposition found in polymers [205]. The concentration fluctuations in the single phase state, which rule the beginning of spinodal decomposition, are characterized by a distinct temporal scale. The characteristic time [t.sub.c] for polymers is longer than for small molecules by a factor of 107. It should nevertheless be mentioned that it is still nearly impossible to obtain blends without any hint of phase separation and particularly to come close to processing techniques associated with thermoplastics. Furthermore, a beginning of phase separation means an accumulation of rigid molecules that could strongly influence the properties determining the reinforcement effect. Parallel aligned rigid rods will deliver an extremely small axis ratio, and the mechanical data could be dominated by slipping processes. The sizes of the growing patterns during phase separation in polymers are greater than those of small molecules by a factor of [square root of N] (N = degree of polymerization), so that light scattering becomes an important technique to use in addition to small angle X-ray and neutron scattering. The knowledge of and suitable methods to study phase behavior and phase separation are key challenges in the field of molecular composites.

Suitable methods are needed to obtain an accurate description of the mixing state. The key questions are: composition of the blend, orientation and dispersion of the rigid molecules. Unfortunately, the common methods are sensitive to inhomogeneity but do not rule out an inhomogeneous dispersion below their resolution. Finally, a method with molecular resolution must display the inhomogeneity given by the different molecular structures. The definition of the term "homogeneous mixture" is for that reason blurred compared with that of the term "molecularly dispersed."

Optical microscopy and light scattering are well-established methods supplying information about the morphology down to 0.5 [[micro]meter]. Besides, inspecting the clarity of prepared films is perhaps the easiest and most effective first method of examination. Electronoptical techniques, like TEM and SEM, cover the region from about 0.2 nm to 2 [[micro]meter]. Both optical and electronoptical methods were used to study the morphology of molecular composites since their introduction [15, 24]. Also, the fundamental characterization techniques of wide and small angle X-ray and neutron scattering are suitable tools to get information about dispersion and orientation. If a homogeneous mixture will lead to distinct changes in the molecular arrangements, spectroscopic methods like FTIR [206] could hold the key to prove homogeneous mixtures.

Side by side with these direct studies of the morphology, analysis of properties depending on the mixing state are used - first of all, the occurrence of one glass transition temperature depending on composition according to an established mixing rule. This behavior was used to prove homogeneous mixture for a number of molecular composites and is commonly investigated by DSC, but results obtained by dielectric, mechanical, methods, etc. could also be used. Transition temperatures deviating from mixing rule or a broadening of the transition region are related to partial miscibility or immiscibility.

Last but not least solid, state MR experiments can provide valuable insights into the composition, dispersion and orientation in polymeric systems with the molecular resolution demanded. Indeed. NMR investigations often lead to important leaps in our understanding of properties at the molecular level. The technique of solid MR 1H spin diffusion, for example, was found to be an especially powerful tool in determining domain size [207, 208]. Unfortunately, NMR techniques are almost never performed to prove molecular dispersion of rigid rods, though there have been some exceptions [209, 210].

We can try to give only a brief overview of the more common methods to investigate the state of mixing. In fact, more unconventional methods not mentioned above can also often provide valuable insights into the state of mixing. For example, photothermal and ultrasonic-imaging were reported to evaluate the demixing of a molecular composite [211].

The discrepancy between the variety and number of possible methods and given facts regarding the mixing state is striking, at least from some sources. The problem often lies in finding the suitable method for the specific system regarding the unchangeable restraints. Almost all the methods mentioned require distinct actions on or knowledge of the sample. Hence, most efforts to determine the mixing state often result only in an estimation.

V POSSIBLE ROUTES TO REACHING HOMOGENEOUS MIXTURES

In the previous sections, distinct routes to obtaining molecular composites were suggested. We now wish to define six topics and discuss them in V.1-V.6. They are classified by different basic approaches. But to be fair to successful systems. we must mention that perhaps the most promising ones can be sorted under more than one heading. Indeed, already, some of the examples mentioned for molecular reinforcement combine several routes to obtain molecular composites.

It is not the aim of this paper to arrange the different studies by materials. The state of the art for special classes of materials can be found elsewhere. One could get the impression that most of the available studies are based on systems related to PPBT or PPTA. This illustrates the industrial potential of these materials as well as the activities of the leading groups for molecular reinforcement over the years. We could consequently find that the most extensive screening is done for PPTA and PPBT blends. The influences of different parameters are evaluated by systematic studies. In Ueta et al., [45] an example showing the effects of molecular weight of nylon-6 on the structure and properties of the molecular composites with PPTA was reported. The detailed knowledge of conformation and properties of molecular composites should help us not only to optimize miscibility but also to establish suitable processing technologies.

V.1 Rapid Preparation Technologies to Exclude a Phase Separation

The phase separation kinetic of polymers can be controlled by temperature, concentration, etc. to achieve time scales longer than typical ones for quick preparation methods. For polymer blends that are characterized by lower critical solution temperatures in the melt, for instance, it is well established to freeze the homogeneous morphology by rapid cooling in the glassy state. Also for molecular composites that display stable homogeneous phases only in high diluted solutions, a quick coagulation [146, 199, 200] or evaporation can lead to homogeneous binary glassy systems. This obviously implies a process from solutions starting at lower than critical concentrations, which in practical term could restrict the use of solutions up to 2-4 wt%. In the literature, spinning processes, especially dry-jet wet-spinning [5, 9, 11, 16], are preferred for obtaining homogeneous but uniaxially orientated molecular composite fibers. Also, analogous techniques could lead to homogeneous films.

Such an extrusion apparatus consists essentially of the solution delivery system, a spinnerette or a coat-hanger die, a coagulation hath and the take-up device. The jets of the spinning solution leave the spinnerette and come into contact with air (or a stream of hot gas), where the solvent is evaporated. In contrast to a pure dry-spinning method the region related to the gas contact is only realized as a gap before the coagulation hath, where the polymer is precipitated from the solution and solid gel filaments are formed. Usually a final heat treatment is carried out to improve the mechanical properties.

The spinning process from concentrations above the critical concentration has for years been established for polymeric liquid crystalline self reinforcing materials. The process is used to obtain high strength/high modulus fibers and can be connected with trade names such as Kevlar (DuPont), Twalon (Akzo), and Vectra (Hoechst Celanese). Also, PPBT and PPBO are often used for spinning from the lyotropic liquid crystalline phase. For these materials it is advantageous that the procedure introduces strong forces by shear or elongational stress to orientate [115] the molecules of the fiber, but it is not suitable to obtain homogeneous mixtures. A comprehensive and direct comparison of a blend system spun from solution above and below the critical concentrations is reported by Krause et al. [40, 41], where the different mixing states of the solution led to phase separated or homogeneous materials, respectively. The homogeneous samples were found to provide moduli and strength up to two orders of magnitude larger than the separated materials. Furthermore, the spin processing itself could clearly induce liquid crystalline phases, crystallization or demixing. However, a precise understanding of these participated molecular processes is lacking. Furthermore, it was found that coagulation [212, 213] and/or additional thermal treatment, of course, could lead to a demixing [199, 200, 214, 215].

First, the dry-jet wet-spinning procedure was worked out for high temperature molecular composites with PPBT and PPBO [15, 16, 40, 41]. For such materials the preparation from solution is the usual approach because the lack of transition temperatures below the onset of degradation rules out melting processes. PPBT reinforced molecular composites were evaluated, and many efforts were made to optimize the procedures to develop a suitable technology [15, 16, 39, 40, 146]. The mechanical data improved typically by a factor of 3, e.g., from 35.9 GPa to 122 GPa for the tensile modulus using 30% rigid rod fraction. The same procedure was also suggested for thermoplastics [17] and successfully adopted for the system between PPBT and aromatic copolyamide consisting of hard segments units of p-phenylene-terephthalamide and soft segment units of 3,4-oxidiphenylene-terephthalamide (PPOT-50) [42]. Molecular composites were dry-jet wet processed for spinning and tape extrusion. The concentration of materials in solution and the molecular weight of PPBT were varied. A maximum tensile modulus of 143 GPa for a fiber (PPBT/PPOT-50 (30/70)) was found, which corresponded to the linear rule of mixtures of almost perfectly orientated PPBT and matrix molecules. Using PPBT with lower molecular weight results in moduli that complied with the Halpin-Tsai relationship, taking the aspect ratio into account. The tensile properties of tapes obtained by the phase transition process and samples attained by stacking and compression molding were compared. Only with PPOT-50 coated tapes to improve the adhesion could worthwhile results be achieved. Since that is an encouraging but not a very satisfactory start, different attempts to produce moldable molecular composites have been carried out [199, 216-218]. Unfortunately, most of these systems phase separated during heat treatment because the molecular dispersion obtainable is far from thermodynamically stable. Even worse, for some systems it seemed to be impossible to maintain the homogeneous mixture, for example, molecular composites between rigid rods and bisbenzocyclobutene [219]. Nevertheless some results promised moldable molecularly reinforced materials.

Apart from these two points, preparation from solution of unmoldable materials and quick evaporation or coagulation for quenching in the mixed state, the third main stream demanding spinning processes is the applications of molecular composites in the field of polymeric fibers. Therefore, it is not surprising that beyond the PPBO and PPBT systems, more common fiber-materials such as PPTA/amorphous nylon [203, 220-223] were investigated. The aim of these studies was to reinforce polymeric fibers by a rigid component. It was possible to manufacture homogeneous thin films, fibers or flakes by rapid coagulation in distilled water. A molecular reinforcement was achieved by threefold increases of the tensile modulus relative to that of the neat amorphous nylon using 30% PPTA, but thermal treatment induced phase segregation. In this context it is relevant to note that also a necessary improvement of the ductility of fibers could be the reason to use a molecular composite [40]. Fibers spun of PPBO and 2,6-bis-4-benzocyclobutene benzo[1,2-d:5,4d[prime]]bisoxasole were found to be homogeneous molecular composites. Low contents of flexible segments improved the compressive strength of the PPBO fibers up to 18%, keeping the modulus of 210 GPa with only minor variations [224].

Apart from the strongly fiber-technology-oriented approaches for composites consisting of PPTA or derivatives with different flexible compounds like aliphatic polyamides. for example, homogeneous blends are described as being attained only by coagulation [21, 24, 26-29, 43, 44]. The PPTA compounds were metalated to obtain homogeneous solutions. The blends show single shifting glass transition temperature but microfibrils of 10-30 nm size. Improved mechanical properties were found. Especially studies to optimize the properties of PPTA containing molecular composites point at interesting materials. Using high molecular weight nylon-6 as matrix, a 10 wt% PPTA content in a molecular composite produces a strength of 170 MPa and a modulus of 12 GPa [45]. Also, recent works on aryl-aliphatic polyamides for molecular composites use only a simple coagulation to prepare molecular composites successfully [225-227]. At least partial miscibility was found in the blend systems with 10 wt% rigid component. The mechanical data of nylon-6 were dearly enhanced. The modulus could be increased by up to a factor of 3, and the tensile strength by up to a factor of 2. Also, the occurrence of molecular composites was reported for PPTA and poly(p-phenylene 1, 3, 4-oxadiazole) (in dependence of PPTA content that determined the morphology of the spun or coagulated samples). The reinforcement effect for the homogeneous blends could be discussed in terms of the Halpin-Tsai description [228].

Beyond the studies based on PPTA and PPBT obtained by spinning processing, molecular composite studies were also published based on coagulation and PPBT. Systematic coagulation studies of blend systems with poly(etherketone) and nylons were reported [229] as well as the preparation of shear flow aligned materials based on coagulated PPBT/polyphosphoric acid solution [230].

Higher critical concentrations and hence an improved processability could be achieved by molecular composites from soluble complexes [53]. This was shown for rigid-rod PPBT and flexible polyamides, nylon 66 and poly((trimethylhexamethylene) terephthalamide). The system was prepared from ternary solutions of their coordination complexes in organic solvents up to critical concentration of 8-10 wt%, using Lewis acid (AlC13 or GaC13). The films and coatings obtained were homogeneous.

Processing methods to quench the homogeneous state of a solution in the binary solid state of the rigid and flexible components may be combined with approaches improving the miscibility described in the following parts. Copolymers, for example, are well known to provide an enhanced miscibility and a diminished demixing, because the rigid segments are difficult to aggregate. Consequently, the critical concentrations were found to be higher for block copolymers by about 3% and the occurring domains were smaller in size [5, 46].

V.2 Advanced Synthesis to Obtain a Homogeneous Blend

In-situ and precursor techniques for the polymer synthesis share one feature in common with the advanced preparation methods: They are suitable procedures for a combination with a variation of the chemical structure to enhance miscibility. It is the principal idea of in-situ or precursor approaches to produce final product features, like homogeneous dispersion of rigid segments, before finishing the synthesis. These procedures are superior if the task was easier to achieve by the precursor compounds based on their different properties. In the case of molecular composites, two scenarios can be found in the literature. First a selective transformation of flexible segments into rigid ones of a homogeneous blend consisting of flexible compounds, and second the polymerization of homogeneously mixed rigid and flexible monomers.

By thermally induced isomerization of an isoimide to an imide, a series of molecular composites with polyarylsulfone, polysulfone and an acetylene terminated isoimide thermosetting resin were obtained [231]. After two flexible polymeric compounds were blended in solution, films were cast and the isomerization process was carried out. The thermally treated films were insoluble and showed no sign of phase separation.

Another approach using a flexible polymeric precursor for rigid polyimide (polybiphenyltetracarboximides with rigid p-phenylene) [232] was described leading to molecular reinforcement [233]. Again the transformation into a stiff structure was induced by thermal treatment. After this imidization of cold drawn precursor films the samples remained transparent and showed one shifting glass transition temperature. The tensile modulus and tensile strength of the uniaxially orientated molecular composite increased in proportion to the rigid component content. For samples containing 70% rigid polyimide molecules, a modulus of up to 35 GPa was attained.

Low-temperature solution polycondensations of a series of soluble aromatic polyamides, subjected to thermal cyclization to convert them to the corresponding polybenzothiazoles, were reported starting with 2,5-bis[[(methoxycarbonyl) ethyl]thio]- 1,4-phenylenediamine with aromatic diacid chlorides [56, 234, 235]. The introduction of bulky and polar pendant [(methoxycarbonyl)ethyl]thio groups in the polyamides improved the solubility of the precursor in organic solvents. GC-MS analysis, X-ray diffraction and dynamic mechanical studies were performed to investigate the mechanism of the thermal cyclization, the precursor polyamides and the glass transition temperature, respectively. The described polybenzothiazole was comparable to those synthesized by usual syntheses. The blend systems PPBT/PAI 40/60 reached a tensile modulus of 7.6 GPa and a tensile strength of 126 MPa, which is more than 2.6 times and 1.5 times than those of PAI.

As an alternative to poly(amic acid)s, flexible and therefore soluble poly(amic diethyl ester) precursors of rodlike poly(p-phenylene biphenyltetracarboximide) and poly(4[prime]4-oxydiphenylene biphenyl-tetracarboximide) were proposed [236]. Again, the dried precursor blend films were thermally imidized. Optically transparent films, regardless of compositions and process conditions, were attained and showed single glass transition behavior. In addition, film properties of composites were characterized.

Recently, in-situ polymerization of rigid monomers in matrix polymer solutions have been proposed as an approach for polyimide molecular composites [10, 237]. Homogeneously mixed blends of monomers were found in contrast to the corresponding polymeric systems. The in-situ method was found to be much better than the simple solution method, in terms of compatibility of both polymers. Thus high-performance composites of acrylonitrile-butadiene copolymer and polyamic acid as well as vinylpyridine-styrene copolymers and polyamic acid were prepared. Furthermore, solvent casting of clear transparent films and a very effective improvement of the mechanical properties of commodity polymers were reported, especially an increase of the tensile modulus and strength in comparison to the flexible matrix and rigid compound respectively. Larger reinforcement was attained when a strong interaction between the matrix polymer and the in-situ formed polyimide took place, which is in good accordance with the principal results of [129]. Earlier, Ogata et al. [10, 238, 239] described in-situ polycondensation of terephthaloyl chloride and p-phenylenediamine. Novel composite polymers of rubbers and poly(p-phenyleneterephthalamide) were obtained; the synthesis was performed by solution or interfacial polycondensation methods in the presence of rubbers such as styrene-butadiene triblock copolymers and acrylonitrile-butadiene random copolymers. The materials displayed enhanced mechanical properties. Another in-situ approach to molecular reinforcement was also presented. N-carboxy methyl-L-glutamate was polymerized in solutions of rubbers. Again, mechanical properties of rubbers were greatly improved, while solution blending of poly[L-glutamate) did not result in enhanced mechanical properties.

For poly-azomethine blends with nylon 6 and epoxy resin, molecularly reinforced blends by an in-situ process were evaluated [38, 240]. The rigid rod polymer precursor was polymerized in a reactive matrix precursor, which was later cured in the mold. Large sheets of homogeneous material were obtained. Furthermore, a 71% increase in tensile modulus in comparison with that of the neat epoxy resin was found for 20% poly-azomethine, which is comparable to the predictions of using micromechanics equations for chopped fiber composites. Molecular modeling simulations were performed to understand these findings, but they also fail in respect of quantitative prediction.

Interpenetrating networks of cellulose acetate as a rodlike component and a variety of monomeric vinyl solvents that were photopolymerized to flexible chains were reported by Kozakiewicz and Maginess [241]. A homogeneous mixture and molecular reinforcing effects were found. Also; molecular composites were attaIned between a series of wholly aromatic rodlike polyamides, polyesters and polyesteramides with vinylpolymers by in situ photopolymerization, which provides a rapidly increasing viscosity by its network forming character [242, 243]. By screening different compounds, two different routes to prevent the phase separation by rapid conformation of a solid molecular composite were compared in these studies: a) rapid solvent evaporation and b) network-forming in situ polymerization of the flexible components that occur more rapidly than aggregation. Both routes are based on a rapid change of viscosity to hinder sufficiently the diffusion of the rigid segments. Only limited combinations of compounds could be prepared to a molecular mixed material by rapid evaporation whereas strong evidence of several true molecular composites by netforming photopolymerization was found. So the in situ network-forming procedure proved to be a more promising process even though the additional contribution of hydrogen bonding to achieve the molecular composites in these materials played a substantial role. Novel high-performance clear coats based on molecular composites utilizing arylsilane aramids were evaluated [244]. The aramids were incorporated in an elastomeric epoxy coating. These composites also exhibited dramatic improvements of their mechanical data.

V.3 Homogeneous Mixtures by Increased Enthalpy: Strong Dipole-Dipole Interaction and Hydrogen Bonding

Similar to polymer blends thermodynamically stable molecular composites could be achieved by favorable interactions balancing the unfavorable entropic contributions to the free energy of mixing. As mentioned before, the starting point to choose suitable favorable interactions given by the entropic contribution is even worse than for a system of two flexible polymeric compounds caused by the high tendency of serf alignment of the rigid polymer. Indeed, it is unlikely to overcome the immiscibility by dipole-dipole interactions. On this basis, it has been suggested to introduce hydrogen bonding.

However, for some systems of polyimide/poly(etherimide) homogeneous mixtures (6, 8, 30, 31, 245) with interesting molecular reinforcement effects especially for drawn samples were reported, even the polyimide could be called a rigid rod. The mechanical properties and intermolecular interactions of homogeneous blends composed of wholly aromatic polyamide, poly(p-phenylene-3,4[prime]-oxydiphenyleneterephthalamide), and poly(amideimide), poly(4,4[prime]-oxydiphenylene-4-carbonamidephthalimide-N-yl) were also reported (246), where H bonds were found to enhance miscibility. The elastic modulus of the composite was always higher than the modulus predicted on the basis of the additivity caused by an increase of the packing density.

In general one must be careful not to mistake viscosity for molecular stiffness especially in the field of polyimidic structures. Polyimide derivatives often show high glass transition temperatures caused by molecular interactions, steric effects and weighty segments without being built up by a rigid single chain structure at all. While some evidence indicates that some so-called rigid polyimides are flexible in fact, this uncertainty should not obscure the fact that these compounds are available with various chain rigidity levels, all having very similar molecular interactions. Therefore, they are the perfect materials to evaluate the mixing problem. Additionally, they are interesting materials for applications because of their high performance properties.

Homogeneous mixtures caused by strong dipole-dipole interactions are reported for trifluoromethyl-substituted polyaramide with aliphatic polyamides (247). The approach was encouraged by the earlier published solution properties of trifluoromethyl-substituted polyaramides (248-250), The molecular dispersion of the rigid molecules was proved by the existence of a single composition dependent glass transition temperature and the melting temperature behavior. The molecular composites could be found for different nylons, and it was proposed that the change in the polar nature modified the linearity of the rigid molecules.

Certain systems are believed to build molecular composites by hydrogen bonding like synthetic polypeptides, namely different poly(glutamates) with poly(vinylphenol) (206). In these studies, infrared spectroscopy was used as a powerful tool to detect hydrogen bonding and prove the miscibility and maintenance of the helical structure of the glutamates. Molecular composites caused by hydrogen bonding and specific acid-base interactions were also evaluated for blend systems between poly(benzyl-L-glutamate) as rigid rodlike polypeptide and lightly sulfonated polystyrene as flexible coil ionomer (251). The acid-base and the hydrogen bondings could be detected by FTIR. A reinforcement effect up to a factor of 1.5 for the Young's modulus was reported. Molecular composites of certain side chain functionalized polyisocyanates that form stiff helical macromolecules and different flexible matrices were also reported (252). The molecular miscibility, hydrogen bonding and the helical structure could be proved by glass transition, infrared, microscopic and light scattering investigations.

Recently. molecular composites between N-phenyl substituted aromatic poly(amide ester)/poly(styrenran-4-hydroxystyrene) (6) based on hydrogen bonding were reported.

Using sulfonated PPTA, different systems with poly-(4-vinylpyridine), poly(vinylpyrrolidone) and polyvinylalcohol were found to be homogeneously mixed at a molecular level as a result of hydrogen bonding. Furthermore, molecular composites based on the matrices poly(4-vinylpyridine) and poly-(vinylpyrrolidone) were prepared with the cupric or zinc salts of PPTA and poly (3-alkylthtophenes) where the alkyl group is octyl or dodecyl (199, 200). The hydrogen bond nature of the specific interaction between the rigid component and the matrix polymers was identified by NMR chemical shifts and infrared spectroscopy. The miscibility was proved by DSC determination of one shifting glass transition temperatures and by NMR experiments.

V.4 Homogeneous Mixtures by Increased Enthalpy: Ionic Interactions

Different chemical binding forces are nominated to enhance the miscibility of the component flexible polymers. Among these, intermolecular forces (hydrogen bonding, charge-transfer complexation, etc.), ionic interactions are known to be the strongest. An ion-dipole interaction could be found to lead to molecular composites for the system ionic Kevlar poly(p-phenylene terephthalamide propane sulfonate)(PPTA-PS)/poly(4-vinylpyridine) by Hara et al. (253). Since then, analysis of three types of ionic PPTA blended with various polar polymers (PVP, S-AN, PVC, PEO) (7, 254256) and sodium salt of Kevlar and poly(4-vinylpyridine) (6) [ILLUSTRATION FOR FIGURE 5 OMITTED] were evaluated. Good dispersion of the rigid rods was proven by different methods and the transparent samples showed no phase separation upon heating. Therefore, they are melt-processable. The modulus and strength reinforcement effects occurring in these molecular composites is extremely significant. The addition of only 2% of rigid rod to PVP led to an increase by 50% of the tensile modulus, for instance. Furthermore, not only was the molecular composite stiffer and stronger, but also an improvement of the ductile properties was noticed. Poly(para-phenyleneterephthalamido)propanesulfonate was proposed as a new polyelectrolyte for application in conductive molecular composites (36). Molecular composites were found with pyrrole, which show an improved thermal stability. As mentioned above, specific acid-base interactions were found, enhancing molecular composites for poly(benzyl-L-glutamate) as rigid rodlike polypeptide and lightly sulfonated polystyrene as flexible coil ionomer (251).

Molecular composites by ion-ion interaction enhanced miscibility were described in the early 1990s for poly(2-acrylamido-2-methylpropanesulfonic acid)/PPBT (257) [ILLUSTRATION FOR FIGURE 5 OMITTED]. The flexible polymer was formed in situ from its sodium salt form and a blend was prepared by co-dissolving the components in methanesulfonic acid. The strong coulomb interactions between the protonated thiazole rings and pedant sulfonated groups enhance miscibility. In the resulting films, no phase-separation could be detected by scanning electron microscopy.

For systems of rigid polydiacetylenes with functional or ionic side groups and carboxylated or sulfonated polystyrene or sulfonated polyester-urea urethanes, molecular composites could be achieved by ionic interactions (258-260). The blends exhibited no microphase separation and the miscibility on a molecular length scale was proven by infrared spectroscopic, dynamic mechanical and differential scanning calorimetry analysis. The molecular reinforcement amounted to up to 1 order of magnitude in compliance with a Halpin-Tsai description and was achieved by only a few weight percent of the rigid compound.

V.5 Advanced Molecule Structure Consisting of Rigid and Flexible Segments

The predicted incompatibility of rods and coils has its origin in the entropic part of the mixing partition function but can be modified in various ways. Most sources tend to refer to copolymeric structures between rigid and flexible segments. The different structure may be divided in block copolymers and graft copolymers, that could consist of rigid side chains and flexible backbone or vice versa [ILLUSTRATION FOR FIGURE 6 OMITTED]. Using a rigid backbone corresponds to the concept presented above (151, 152). We mentioned already that entropic considerations do not result in an enhanced miscibility of block copolymers. Here, the major advantages seem to be found in the changed dynamic of phase separation. The typical separation time is increased and the domain sizes are decreased. The concept to use covalent bonds between flexible and rigid components to hinder or even to rule out the molecular mobility, which determines a phase separation, leads also to the idea to use network structures [ILLUSTRATION FOR FIGURE 6 OMITTED]. Indeed, in the case of rigid and flexible segments fixed in a network structure, the chance to rule out the phase separation totally seems to be quite realistic.

Alternative views argue convincingly that copolymers could be regarded as suitable materials to attain molecular composites. The covalent bond between rigid and flexible structures presents a strong enough interaction to overcome the immiscibility.

Furthermore, molecular dynamic simulation (161, 162) leads to the assumption that copolymeric structures strongly reduce the correlation of the rigid rods' orientation.

If we are correct to assume that increasing the strength of interactions between rigid and flexible components enhances the translation of stress or strain, we should also expect better reinforcement effects in copolymeric structures. Apart from the topological entanglement and intermolecular interactions, a chemical bond between rigid segment and flexible coil matrix is created in copolymeric compounds. Hence, the translation of stress or strain from the reinforcing rigid-rod molecule in the matrix would be more efficient than in the physical blend. Indeed, a comparison of copolymerically and physically blended molecular composite fibers (3) establishes the increased tensile strength of the copolymeric materials and equal moduli even though the incorporated rigid rod lengths are shorter.

Liquid Crystalline Polymers

For copolymeric structures it becomes clear how closely the concepts of self-reinforcement of liquid crystalline polymers and molecular reinforcement are related. Most materials called liquid crystalline polymers can be interpreted as copolymers consisting of rigid and flexible structures and every such copolymer that builds up homogeneous phases could be called a molecular composite. However, in comparison to the original concept of molecular composites based on homogeneous isotropic physical blends, each chain of a liquid crystalline material bears rigid and flexible segments and the anisotropic characteristics of molecular shape and interactions are central to their properties. It is well known, for instance, that flexible segments of liquid crystalline molecules could be extremely aligned and stretched by a nematic field.

While accepting that similar or even the same procedures, principles, materials and questions are pronounced for these two fields it must be pointed out that most of the liquid crystalline materials are not suitable for molecular composites. In general their rigid segments define unsuitable minor lengths for effective reinforcement in an isotropically dispersed state. Therefore, in the field of self-reinforcing liquid crystalline polymers some of the largest reinforcing effects found are those which are induced by stretching the materials or which are achieved by the agglomeration of the rigid compounds into microfibrils. A large overlap of the two fields is given for lyotropic systems, as mentioned for fiber spinning processes. Perhaps the self-reinforcement concept for liquid crystalline materials, which is fixed to the broad class of materials and not to a principle of mechanism, is the more pragmatic approach to reach mechanical reinforcement. However, like molecular composites, the adjustment of advanced mechanical properties is the key question for self-reinforcing liquid crystalline polymers by controlling the morphology and interfacial adhesion. So it is not surprising that we deal with a similar choice of materials and demands on processing technology. Apart from the literature on liquid crystalline polymers, excellent reviews on the field of self-reinforcement in liquid crystalline polymers and their blends are also available (115, 261-266).

Block Copolymers

Molecular reinforcement attained for block copolymers was reported (3, 46) dealing with carboxy-terminated poly(benzo[1,2d:4,5[prime]]bisthiazole-2,6-diyl)-1,4-phenylene as rigid rod copolymerization with 3,4-diaminobenzoic acid and 4-amino-3-mercaptobenzoic acid. With 30% rigid rod content a Young's modulus of up to 115.8 GPa, tensile strength of up to 1.7 GPa tensile strength and a 2.3% breaking strain could be achieved. The physical blend was also prepared and showed 116.5 GPa, 1.27 GPa and 1.4%. The mechanical properties of the different compounds followed the linear rule of mixture.

Perhaps one of the most enlightening papers deals with systems of 30% PPBT and 70% semi-flexible coil poly(2,5(6)benzimidazole (ABPBI) (41). A comparison of the mechanical data of the pure materials, physical blends and copolymers was made, as well as of the processing techniques based on a solution below and above critical concentration conditions (Table 6). Copolymeric and physically blended molecular composites were only obtained by processing from solution below the critical concentration and showed far superior mechanical properties. The modulus of the physical blend (120 GPa) was given by linear mixing rules, whereas the copolymeric system resulted in a minor modulus of 100 GPa, that was still higher in comparison to 36 GPa of the semiflexible matrix. Nevertheless, the copolymeric structure exhibited the more interesting strength and maximal elongation. The tensile strength was even given by a linear rule of mixture, which could not be reached by the physical blend.

Similarly promising results were obtained by block copolymers based on PPBO rigid segments (5). As expected for a molecular composite, the tensile strength and modulus are given by a linear rule of mixture, while the elongation-to-break is inversely proportional. Hence it is possible to adjust the tensile modulus above 100 GPa with rigid rod contents of around 40%. Unfortunately the bulk viscosity of the block copolymers was extremely high and could be seen as a key drawback.

Molecular composite between lyotropic polyamide amorphous polyimides by block copolymerization were reported (9, 267). The materials composed of terephthaloyl dichloride and 2,2[prime]-dimethyl-4,4[prime]-di-amino-biphenyl [TABULAR DATA FOR TABLE 6 OMITTED] and amorphous polyimides were processed via dry-jet wet-spinning. The blends showed no phase separation. The mechanical properties were found to be linearly located between those of the fibers of the pure compounds. Therefore a tensile strength between 24 and 498 MPa and modulus between 1.6 to 19.2 GPa were attained.

Single component molecular composites using copolymers consisting of nylon-6 and polyimide were reported (30, 31). The thermal stability, chemical resistance and the mechanical data were substantially improved. The concept of block copolymeric structures was also applied to polyimide structures based on rigid and semiflexible segments (268). An improvement of the mechanical data was found.

Graft Copolymers

Graft copolymers based on PPBT were synthesized and studied as molecular composites in the last decade (33, 269-273). The flexible side chains were realized by aromatic poly(ether ketone) and bulk rigid-rod molecular composites that show a significant increase in glass transition temperature and were successfully obtained at a lower content of the rigid component. The blends show three-dimensionally isotropic properties and a tensile modulus predicted by the Halpin-Tsai equation taking into account the aspect ratio. It was observed that the flexible side chains could be able to suppress the thermally induced rod aggregation. The frequency of the graft sites along the backbone was found to be the key structural influence ruling the rod aggregation and not the side-chain length. Significant phase separation was found for the copolymer of higher rod content. It was attempted to describe the influence of molecular structure on mechanical data and processing properties.

Other authors dealt with rigid-rods like PPBO connected to polysiloxane side chains (274, 275). Furthermore, it was proposed to prepare molecular composites by in-situ polymerization of the rigid-rod graft-copolymer in the presence of polydimethylsiloxane as matrix. Even though some reinforcement was observed, unbound materials resulted in plasticizing effects at the same time.

By an in-situ synthesis, the anion of the rigid rod PPTA initiated the anionic polymerization of acrylamide molecular composites of PPTA-graft and homonylon 3 (276). The films showed greatly improved mechanical properties while phase separation into PPTA fibrils did not occur for weight fractions below 30% rigid rod. Hence in comparison with the matrix material the molecular composite consisting of 30% PPTA delivered an increase in tensile strength and modulus from 28 to 191 MPa and 1.06 to 7.55 GPa, respectively. The mechanical data were fitted convincingly by a Halpin-Tsai description.

The articulation of molecular composites based on grafted PPTA was also described (277). The copolymer was synthesized from PPTA and epoxy via metalation of PPTA in a solution of sodium methylsulfinylcarbanion in dimethylsulfoxide. The bending modulus (3.0 GPa) and bending strength (164 MPa) were found reinforced with regard to those of epoxy resin by factors of 1.66 and 1.36, respectively, whereas no reinforcing effect could be found for the Young's modulus. The reasons for low reinforcing effects must be discussed in terms of unsatisfactory molecular dispersion.

Recently, several graft copolyamides have been presented based on connecting chains of polycaproamide (nylon 6) onto the stiff aromatic polyamide (278, 279). in contrast to early approaches this was done directly to the aromatic rings, and not to the amide groups of the stiff chain. Molecular composites between graft copolymers and nylon 6 could be easily prepared and several methods proved that the rigid segments were molecularly dispersed, A reinforcement of up to a factor of 2 were attained by 15% stiff polyamide, so that the tensile properties of the graft copolymers and their mixtures were far superior to those of the ungrafted blends. The reinforcement effect by the stiff polyamide fits very well in the terms of a Halpin-Tsai formulation.

Networks

We have described some network-based molecular composites if a phase separation was ruled out by a quicker in situ polymerization into a network structure. Apart from this facet, we wish to mention networks as promising molecular structures compared to a coil conformation based on the possible additional immobilization effects, which are clearly illustrated if we consider a network given by a copolymeric structure of rigid and flexible segments. Recently, molecular composites by semi-interpenetrating polymer networks have been reported (280). To increase the solubility of PA/PI block copolymers, and to produce a three-dimensional isotropically reinforced molecular composite, a lyotropic amine-terminated polyamide prepolymer was copolymerized with an amine-terminated polyimide via a coupling reaction using terephthaloyl dichloride. The resultant block copolymers were characterized, and no large phase separation was observed. The preparation of a network consisting of rigid and flexible segments so that the rigid component was fixed by covalent bonding were reported for partially hydrogenated cis-1,4-polybutadiene and a rigid component synthesized from bis(1,2,4-triazoline-3,5d-dione)s, for example (281). The different influences on the thermal and mechanical behavior were described. Molecular composites based on thermosetting matrix polymers were prepared for the more common system poly(p-phenyleneterephthalamide)-epoxy (37). The preparation was performed by in situ polymerization and the successful grafting was shown by NMR spectroscopy. The properties of the molecular composite were compared with those of Kevlar pulp-filled composites and exhibited a better tensile strength, modulus, and heat resistance compared with the conventional fiber-filled system. The epoxy molecular composites showed a tendency to phase separation, ruled by crystallization. The study indicated the possibilities inherent in the in situ network polymerization for building up molecular composites: On the one hand the physical prevention of diffusion controlled phase separation and on the other the fixing of a homogeneous state by covalent bonding.

V.6 Advanced Molecule Structure: Rigid Star Molecules or Multipodes

Since the end of the 1980s, Wendorff et al. (47, 103, 129, 154, 183, 282-286) have proposed complex rigid molecules to achieve homogeneous mixtures between rigid and flexible structures. The approach was largely based on the idea that isotropic structures although being rigid are dismissing the properties caused by the anisotropy of rigid rods. Indeed, lattice calculations for binary and ternary systems with solvent and flexible coils result in a substantial increase of miscibility. The expectations based on these theoretical considerations were found to be justified because multipodes display an enhanced solubility both in low molar mass solvents and in polymer matrices. Furthermore, the rigid and complex structures seem to be able to prevent aggregation into an ordered phase caused by the strongly hindered molecular dynamic. Therefore even the investigated low molecular mass model compounds are glass forming materials with high transition temperatures. Most notable is that the prediction is not restricted to the variety of rigid starlike molecules based on one central core but it does also apply to more dendrimeric or so called fractal rigid structures [ILLUSTRATION FOR FIGURE 7 OMITTED]. Therefore the term "rigid multipodes" was introduced.

Nevertheless, the concept requires a fixed isotropic shape that rules out nearly all of the known star and dendritic structures that contain flexible segments. We could find the preparation of interesting nonlinear rigid structures in the literature. Different approaches to build up planar structures based on a benzene as central core (282, 285, 287, 288) or a spirocompound (289) were published. Three-dimensional, tetrahedral rigid star molecules could be obtained based on an adamantane core (284, 288, 290-294) or a tetraphenylmethane core (295-298). Also, dendritic structures (299-306) and hyper branched macromolecules (306312) consisting of rigid segments are known. Recently the preparation and characterization of fractal polyamides built up by rigid segments were presented (313, 314), which could perhaps open an approach for realizing materials suitable for molecular reinforcement by nonlinear rigid structures. These molecular structures were highly porous, which distinguishes them from classical dendritic ones. This fact resulted in the possibility of producing a mixture between the flexible segments and rigid ones at a molecular level. Already, a variety of interesting blend systems with nylon-6 matrices with a description of transition and mechanical properties have been investigated. One can be eager for the results if such materials are suggested for molecular reinforcement in the future.

The increases of solubility of rigid structures with increasing isotropy were proved for p-phenylen, p-benzamide (288) and p-aromatic ester (293) by comparing different rigid star molecules. The number (one, two, three and four) and geometry as well as the lengths of the arms were varied. As an example, the solubilities in NMP of model aramidic rigid multipodes based on a central core are displayed in Fig. 8.

As one could expect from the improved solubility in low molecular weight solvents, the rigid multipodes also showed an improved miscibility with polymeric matrices. Different homogeneous mixtures between rigid structures as lambda shaped, cross and star molecules and flexible matrices (polyarylate, polycarbonate, polyetherketones, polyetherimides among others) were described up to 10 to 30 wt% rigid component. Unfortunately, rigid multipodes consisting of larger rigid segments, which could result in a convincing reinforcing effect, are still missing. Only for lambda shaped molecules could molecular reinforcement clearly be described for arm lengths of around 2.3 and 2.7 nm based on a rigid aromatic polyester structure. Homogeneous blends consisting of 10 wt% rigid molecule and polyarylate were cast from solution and show isotropic molecular reinforcement of the Young's modulus nearly up to a factor of 3. Therefore the isotropic reinforcement was comparable to values one would expect for an anisotropic sample, which is very surprising. Again, the outstanding possibilities of molecular reinforcement and the validity of the concept were apparent, even though the rigid molecules used were still far too small to prevent a drastic decrease of strength resulting from the increase of molecular inhomogeneities due to the increase of chain ends per volume.

VI CONCLUSION

Molecular reinforcement of flexible matrix polymers by using homogeneous mixtures with intrinsic rigid-rod structures is an exciting, emerging concept in the field of structural materials. The expected range of reinforcement is outstanding. Many results were obtained in support of this concept. For some systems the validity of macroscopic equations downscaled to a molecular level could be proved. A few approaches were begun to study the stress and strain fields at a molecular level.

The main obstacle to overcome is achieving sufficiently homogeneous blends between rigid and flexible molecular structures without phase separation. The problem was well discussed in terms of a lattice model. Different approaches were chosen to deal with this problem. Some of them presuppose the development of an advanced technology of preparation or synthesis. Others are based on extraordinary chemical structures. Most of the suggested processes could be pushed into molecularly reinforced systems, but until now no system has shown a breakthrough to industrial success. The practical application of molecularly reinforced blends seems to be restricted, because of complex preparation processes, poor processability, high material costs and minor stability against demixing. The results recorded so far have involved a minimal screening of possible compounds, so it is too soon to predict success. On the other hand, a further optimization of some systems is promising. So far it is too early to assume failure of this next generation of structural materials. Certain approaches are still in their infancy. The already available systems, which have a convincing molecular reinforcement, are disappointing in other properties, which are not necessarily connected with the mechanical data. It becomes clear that in certain fields the main challenge will be the optimization of the whole range of properties to obtain materials suitable for industrial application. It remains to be seen ff such activities will be met with success.

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