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Crystalline phases are extremely relevant for properties and applications of many polymeric materials. In fact, their amount, structure, and morphology constitute the main factors controlling physical properties of fibers, films, and thermoplastics1, 2 and can be also relevant for properties of rubbers3, 4 and gels.5, 6
It is also well known that processing and physical properties of polymer-based materials are strongly affected by the occurrence of “polymorphism” (i.e., the possibility for a given polymer to crystallize in different crystalline forms)1, 7 and “mesomorphism” (i.e., the occurrence of “disordered” crystalline phases, characterized by a degree of structural organization that is intermediate between those identifying crystalline and amorphous phases).7, 8
Different has been the destiny of polymeric “cocrystalline forms,” that is, structures were a polymeric host and a low-molecular-mass guest are cocrystallized. In fact, although they were early recognized, and in many cases also well characterized by thorough X-ray diffraction studies (Cocrystalline forms Section), cocrystalline polymeric phases have been for many decades ignored in material science.
Only in the last two decades, few polymeric cocrystalline forms have received attention in material science due to their ability to produce, as a consequence of guest removal, nanoporous-crystalline forms exhibiting a density lower with respect to the density of the corresponding amorphous phases (Nanoporous-crystalline forms Section). For these nanoporous-crystalline forms, many applications in the fields of molecular separation and sensors have been proposed (Nanoporous-crystalline polymeric materials Section).
In the last decade, it has been also discovered that for these systems, by choosing suitable guest molecules and processing, it is possible to control the orientation of the host polymer crystalline phase (axial, uniplanar, or uniplanar-axial) and, as a consequence, also the orientation of active-guest molecules, not only in the crystalline phase but also in macroscopic films (Orientations of cocrystalline and nanoporous-crystalline polymer phases Section). In addition, on the basis of this orientation control, in the last few years, films where active molecules are present as guests of polymer cocrystalline phases have been proposed for advanced applications. In fact, the formation of cocrystalline phases allows to reduce diffusivity of active molecules in solid polymers and to prevent their self-aggregation, more easily than with the classical methods based on polymerization of suitable monomeric units or on grafting of the active species onto preformed polymers. In particular, films with relevant fluorescent, photoreactive, magnetic, and ferroelectric properties have been prepared (Functional polymeric materials based on cocrystalline forms Section).
In the last few years, it has been also discovered that polymer cocrystallization induced by nonracemic guest molecules can produce large and extremely stable circular dichroism (CD) phenomena. This allows an easy production of optically active transparent films, whose CD peaks can be controlled by the choice of (also achiral) chromophore guest molecules (Chiral optical polymer films by cocrystallization with nonracemic guests Section).
Systems composed of solid polymers and of low molecular mass molecules find several practical applications, including advanced applications.9–20 In several cases, additives (often improperly referred as guest molecules) are simply dispersed at molecular level in polymeric amorphous phases, although frequently, to reduce their diffusivity, the active molecules are covalently attached to the polymer backbone, either by polymerization of suitable monomeric units or by grafting the active species onto preformed polymers.9–20
A more simple alternative method to reduce diffusivity of active molecules in solid polymers and to prevent their self-aggregation consists in the formation of cocrystals with suitable polymer hosts.
Polymeric cocrystalline forms are quite common for several regular and stereoregular polymers, such as, for example, isotactic21–23 and syndiotactic polystyrene (s-PS),24–40 syndiotactic poly-p-methyl-styrene,41–47 syndiotactic poly-m-methyl-styrene,48, 49 syndiotactic poly-p-chloro-styrene,50 syndiotactic poly-p-fluoro-styrene,51 polyethyleneoxide,52–55 poly(muconic acid),56, 57 polyoxacyclobutane,58 poly(vinylidene fluoride),59, 60 syndiotactic polymethylmethacrylate,61–63 isotactic poly-4-methyl-1-pentene,64 or poly(2,6-dimethyl-1,4-phenylene ether) (PPO).65–68
As for PPO, since the end of the sixties of the last century, it is well known that it can cocrystallize with many guest molecules. Some of these cocrystalline forms, including volatile guest molecules (such as methylene chloride, methylene bromide or bromochloromethane) are unstable.65 For instance, the methylene chloride guest is completely removed after few hours of air drying at room temperature. PPO cocrystalline forms with less volatile guests, such as decahydronaphthalene,68 tetrahydronaphthalene,68 or α-pinene,66–68 are instead extremely stable and can be also obtained as single crystals.66, 67
The only crystalline structure defined for PPO is the cocrystalline structure with α-pinene,69 whose unit-cell has been defined many years ago by electron diffraction of single crystals.67 The packing model of the cocrystalline form of PPO with (1S)-(−)-α-pinene in a tetragonal unit cell (a = b = 1.19 nm and c = 1.71 nm) and according to the P43 space group is shown in Figure 1.
The X-ray diffraction pattern of a PPO film as obtained by casting and including the cocrystalline form with α-pinene is compared in Figure 2 with the X-ray diffraction patterns of semicrystalline PPO samples (further discussed in Nanoporous-crystalline forms Section). The narrower diffractions of the cocrystalline sample clearly show the occurrence of a higher crystalline order, associated with interactions of the host polymer with the guest molecules.
As far as s-PS cocrystalline forms, all those described up to now exhibit as a common feature the s(2/1)2 helical polymer conformation, with a repetition period of nearly 0.78 nm.24–40 However, the packing of the host helices and of the guest molecules can largely change, mainly depending not only on the molecular structure of the guest molecules but also on the preparation procedure.
The large number of s-PS cocrystalline forms can be divided in three classes: δ-clathrates, intercalates (also named δ-intercalates) and ε-clathrates.
δ-Clathrates present isolated centrosymmetric guest locations, cooperatively generated by two enantiomorphous helices of two adjacent ac polymer layers, formed by closely packed enantiomorphous polymer helices [Fig. 3(A–C)].
δ-Clathrates can be divided in two different classes: monoclinic [Fig. 3(A-A′)] and triclinic [Fig. 3(B-B′, C-C′)].36 In particular, monoclinic δ-clathrates are obtained with guests whose molecular volume is significantly lower than 0.12 nm3 and, typically, as the bulkiness of the guest increases, there is an increase of the spacing (d010 = bsinγ) between the ac layers (in the range 1.06–1.2 nm).36 In these monoclinic δ-clathrates, the cocrystalline structures are characterized by P21/a symmetry and the cavity shape imposes a guest orientation with their main molecular plane nearly perpendicular to the helical axes.25–29, 34 Because in most cases a single guest molecule is present in each cavity, the maximum molar ratio between guest molecules and styrenic units is 1/4. On the other hand, triclinic δ-clathrates are generally obtained with guests whose molecular volume is higher than 0.12 nm3.36 For triclinic δ-clathrates with guests having a volume close to 0.12 nm3, such as p-nitro-aniline (NA, Vmol ≈ 0.122 nm3), these are accommodated by shifting along the chain axis the ac layers, keeping a d010 almost equal to that one of the δ form, thus leading to a triclinic unit-cell and to guest molecular planes inclined with respect to the helical chain axes [Fig. 3(B-B′)].34 In addition, in this case, the maximum molar ratio between guest molecules and styrenic units is 1/4. For triclinic δ-clathrates obtained with guests having higher molecular volume, such as, for example, 1,4-dinitrobenzene (DNB) (Vmol ≈ 0.134 nm3) or dibenzofuran (Vmol ≈ 0.159 nm3), the close packing of the polymer chains along in the ac layers is partially lost and one half of the cavities initially present in the δ phase is lost while the other one half increases its volume and becomes suitable to host bulky guest molecules [Fig. 3(C-C′)].36 This of course halves the maximum guest/monomer-unit molar ratio that becomes 1/8.
A second class of s-PS δ cocrystals, defined as δ intercalate (or simply intercalates), has been suggested on the basis of qualitative interpretation of X-ray diffraction data since 199670–72 and proved by crystalline structure resolutions only in recent years.30, 31 In addition, these cocrystals are characterized by the same layers of alternated enantiomorphous s(2/1)2 polymer helices [Fig. 3(A,B,D)], but the spacing between the ac layers (d010), for all the known s-PS intercalates is larger than 1.3 nm and values as high as 1.75 nm have been observed. In fact, in these cocrystals, the guest molecules are not isolated into host cavities but contiguous inside layers intercalated with the polymer layers. Of course, these intercalate structures present a higher guest content with respect to the clathrate structures and the guest/monomer-unit molar ratio generally is 1/2 rather than 1/4 [Fig. 3(D-D′)]. For bulky guest molecules, like a dimer of styrene32 or azobenzene,75 the occurrence of a different intercalate structure, which is characterized by guest/monomer-unit molar ratio of 1/4, has been suggested. Presently known s-PS intercalate phases exhibit guest molecular volume in the range 0.15–0.36 nm3.31, 32
Recently, the occurrence of a third class of s-PS cocrystals has been established.33, 35, 73 In this third class of s-PS cocrystals, guest molecules are imprisoned into channels formed between enantiomorphous s(2/1)2 polymer helices.33, 35, 38 These s-PS cocrystals have been named ε clathrates [Fig. 3(E-E′)]. The most relevant structural features of this new class of s-PS cocrystals is that suitable guests are not only small molecules35, 73–75 but also long molecules such as, for example, 4-(dimethyl-amino)-cinnamaldehyde76 (Vmol ≈ 0.183 nm3), which are unable to be enclosed as guest into the isolated cavities of the δ-clathrates. Moreover, for the ε-clathrates, guest molecules tend to assume orientations with their main molecular axis roughly parallel to the crystalline chain axis.35, 38, 75
Finally, it is worth adding that, for several guests, cocrystalline forms belonging to two or three of these classes can be obtained (e.g., δ34 and ε35 clathrates for p-nitroaniline and δ and ε clathrates as well as intercalate for 2,2,6,6-tetramethyl-piperidinyl-N-oxyl (TEMPO)74 and for azobenzene75).
The removal of the low-molecular-mass guest molecules from cocrystals can generate nanoporous-crystalline phases. In this respect, it is worth noting that nanoporous crystalline structures can be achieved for a large variety of chemical compounds: inorganic (e.g., zeolites),77, 78 metal-organic,79–82 and organic.83–86 These materials, often referred as inorganic, metal-organic, and organic “frameworks” are relevant for molecular storage, recognition, and separation techniques.
The removal of the low-molecular-mass guest molecules from polymer cocrystalline forms generates host chain rearrangements, generally leading to crystalline forms that, as usual for polymers, exhibit a density higher than that one of the corresponding amorphous phase. However, in few cases (to our knowledge, up to now only for s-PS and PPO), by using suitable guest removal conditions,87, 88 nanoporous crystalline forms, exhibiting a density definitely lower than that of the corresponding amorphous phases are obtained.
The first polymeric nanoporous-crystalline form, the δ form of s-PS, was discovered and patented in 1994.89 The crystal structure of the δ form has been determined by the analysis of X-ray fiber diffraction patterns and packing energy calculations. Chains in the helical s(2/1)2 conformation are packed in a monoclinic unit cell with axes a = 1.74 nm, b = 1.185 nm, c = 0.77 nm, and γ = 117°, according to the space group P21/a (Fig. 4, upper part).90 The calculated density is of 0.98 g cm−3, that is, definitely smaller than that one of the amorphous phase (1.05 g cm−3). The structure is similar to the model proposed for the δ-clathrate cocrystals but as a consequence of the removal of the guest molecules, the b axis is shorter and the distance bsinγ between ac layers of macromolecules is shortened to 1.06 nm.90
Shape and volume of empty space of the δ nanoporous phase of s-PS have been evaluated by considering the space available to probe spheres of given radii into crystalline structures of different polymorphic forms. These analyses have shown the presence of isolated cavities having a volume close to 0.125 nm3,37, 91, 92 and their number is equal to the number of chains while the maximum number of guest molecules per cavity is a well-defined integer (generally one or two, depending on guest molecular volume).25–29 It is worth adding that the cavity is rather flat, that is, presents its maximum dimension (nearly 0.8 nm) nearly perpendicular to the polymer chain axis, while its minimum dimension (nearly 0.3 nm) essentially along the c axis.91 This allows to understand the typical orientation of molecular planes of guests of δ-clathrates, being nearly perpendicular to the polymer chain axes.93
The nanoporous ε phase of s-PS, discovered only in 2007,73 presents an orthorhombic unit cell with axes a = 1.61 nm, b = 2.18 nm, and c = 0.79 nm.33 Four chains of s-PS in the s(2/1)2 helical conformation are included in the unit cell, whose calculated density is 0.98 g cm−3, that is, very close to the density established for the δ phase. The space group proposed is Pbcn.33 The crystal structure (Fig. 4, lower part) is characterized by channel-shaped cavities crossing the unit cells along the c axis and delimited, along b axis, by two enantiomorphic helical chains. In these channels, guest molecules are generally hosted with their longer molecular axis roughly parallel to the polymer chain axis. Moreover, the presence of channels easily allows to rationalize the formation of polymer cocrystals with guest molecules presenting a molecular axis much longer than the s-PS chain axis periodicity, such as, for example, 4-(dimethyl-amino)-cinnamaldehyde), which are not able to form polymer cocrystals with the δ host phase.76
In addition, for the case of PPO, the guest-induced cocrystallization followed by removal of the low-molecular-mass guest molecules can lead to nanoporous crystalline phases, exhibiting density definitely lower than that of its amorphous phases. In fact, semicrystalline samples present a density of 1.009 ± 0.002 g cm−3 while the fully amorphous samples present a density of 1.016 ± 0.004 g cm−3.94 Correspondingly, these low-density semicrystalline PPO samples present a much higher solubility of many guests (e.g., benzene, CCl4), with respect to fully amorphous samples. Sorption experiments, as well as density measurements and classical BET experiments, have clearly shown that these PPO crystalline forms are nanoporous.94 It is worth adding that only few of many papers devoted to the transport properties of PPO have earlier recognized that its crystalline phases can play a role in gas sorption and diffusion processes.95–97
X-ray diffraction patterns on PPO samples, as crystallized in the presence of many different solvents, indicate the formation of many (if not a continuum of) nanoporous-crystalline modifications between two limit ones, exhibiting diffraction peaks at lowest or highest angles [Fig. 2(C,B), respectively].94 Complete structural analyses, possibly based on X-ray diffraction data of oriented nanoporous crystalline samples, are still needed.
ORIENTATIONS OF COCRYSTALLINE AND NANOPOROUS-CRYSTALLINE POLYMER PHASES
A relevant feature of the s-PS cocrystalline phases is the possibility to be obtained with different kinds of uniplanar orientations, by using suitable processes of film production.98–107 The degree and the kind of uniplanar orientation depend on the selected technique (solution crystallization procedures,99, 100 solvent-induced crystallization in amorphous samples,98, 101, 102 or solvent-induced recrystallizations of γ and α unoriented samples104) as well as on the chemical nature of the guest.
It has been recently suggested that for s-PS the structural feature determining three different kinds of uniplanar orientations is the layer of close-packed alternated enantiomorphous helices105 that characterizes the δ phase (upper part of Fig. 4), as well as all related clathrate [Figs. 3(A,B)] and intercalate [Fig. 3(D)] cocrystalline phases. The three observed uniplanar orientations correspond to the three simplest orientations of the high planar-density ac layers (i.e., of close-packed alternated enantiomorphous s-PS helices) with respect to the film plane. In particular, it has been proposed that the three uniplanar orientations of s-PS should be named a∥c∥, a∥c⟂, and a⟂ c∥, indicating crystalline phase orientations presenting the a and c axes parallel (∥) or perpendicular (⟂) to the film plane.105
By using suitable procedures, the three uniplanar orientations can be maintained, without substantial loss of degree of orientation, not only for the δ phase but also for γ99, 101, 102 and ε106 phases. The three uniplanar orientations of s-PS can be also maintained102 after guest-exchange procedures transforming a cocrystalline phase with a given guest into a cocrystalline phase with a different guest.108–110
Recently, it has been also described the possibility to achieve for s-PS two different kinds of uniplanar-axial orientations,111 that is, exhibiting the polymer chain axis (c-axis) parallel to the main draw direction and the ac plane parallel or perpendicular with respect to the film plane (Fig. 5). These uniplanar-axial orientations can in principle lead to a complete 3D orientational order among the crystallites and as a consequence maximize the anisotropy of the physical properties.111
The availability of s-PS films with different kinds of orientations not only allows establishing fine structural features of crystalline and cocrystalline phases (e.g., experimental evaluation of the orientation of transition moment vectors of host and guest vibrational modes, with respect to the host chain axes)112, 113 but also can be relevant for practical purposes (see Films Section).
NANOPOROUS-CRYSTALLINE POLYMERIC MATERIALS
Sorption studies from liquid and gas phases have shown that nanoporous crystalline phases of polymers are able to absorb suitable guest molecules, even when present in traces.94, 119–125
As usual for thermoplastic materials, polymers with nanoporous crystalline phases can be easily processed to obtain suitable morphologies. In fact, melt and solution processes are available to obtain films, fibers, sheets, thick articles, foams, membranes, and aerogels.
Particularly suitable for many applications are films and aerogels, which are discussed in two specific subsections (Films and Aerogels). The other four subsections (Molecular separations, Gas sorption, Active packaging, and Molecular sensors) briefly present the main applications that have been reported for nanoporous-crystalline polymeric materials.
Films and Control of Guest Diffusivity by Orientation of the Nanoporous Crystalline Phase
Films of any thickness (from few nm up to tenths of μm) exhibiting nanoporous crystalline phases can be obtained by typical solution processes, such as casting or spinning, for both s-PS and PPO. Films with nanoporous crystalline phases can be also obtained from amorphous samples (as obtained by usual polymer melt processing, such as, e.g., extrusion), by suitable guest sorption–desorption procedures.
Guest sorption studies from dilute aqueous solutions and from gas phases as well as desorption studies have been conducted for s-PS films presenting the three different kinds of uniplanar orientation of the nanoporous δ phase (a∥c∥, a∥c⟂, and a⟂ c∥).116–118 These investigations have been mainly effected by FTIR measurements calibrated by gravimetric measurements.
Many studies116–118 have shown that, at low guest activities, the sorption occurs nearly only by the nanoporous crystalline phase and that the guest transport behavior is dependent on the kind of uniplanar orientation of the host crystalline phase. In particular, in agreement with predictions based on molecular simulations,115 the lowest diffusivity has been measured for films with a∥c∥ uniplanar orientation while the highest diffusivity has been measured for films with a⟂ c∥ uniplanar orientation.116–118
The possibility to control the guest diffusivity by the crystalline phase orientation could be useful for some applications. For instance, for sensing elements of molecular sensors,126–130 the high diffusivity a⟂c∥ uniplanar orientation of the nanoporous crystalline phase would be most suitable, because it maximizes the sensor response rates. On the other hand, for applications requiring a long-term stability of the cocrystals, like for instance for films including active guests (e.g., fluorescent, photoreactive, etc.)131–138 low diffusivity a∥c∥ uniplanar orientation should be most suitable.
Gels consist of three-dimensional network structures swollen by a liquid. For chemical gels, the crosslinks that give rise to this network are covalent bonds while for physical gels the connectedness between polymer chains is achieved by intermolecular physical bonding forming junction zones that can be created and removed by cooling and heating, respectively.5, 6, 139, 140
X-ray diffraction, neutron diffraction, and differential scanning calorimetry characterizations have allowed clarifying that the junction zones of s-PS gels,70, 141–143 as well of many other physical gels,5, 6 are consisting of cocrystalline phases.
Monolithic high porosity s-PS aerogels exhibiting three-dimensional networks of highly crystalline nanofibers can be easily obtained by supercritical CO2 extraction of the solvent present in physical gels.144–146 Similar polymeric aerogels have been also obtained for polysaccarides,147–149 poly(vinylidenefluoride-co-hexafluoropropylene),150 polyvinylidenefluoride,151 and isotactic poly-4-methyl-pentene-1.64
Particularly interesting are s-PS-based aerogels exhibiting nanoporous crystalline phases, because they present, beside the amorphous porosity typical of other polymeric aerogels, also the crystalline nanoporosity typical of the δ144–146 (Fig. 6) or ε76 phases.
Sorption measurements of volatile organic compounds (VOC), both from gas phase at low pressures144 and from dilute aqueous solutions,146 have shown that δ-form aerogels present the high sorption capacity characteristic of s-PS δ-form samples (due to the sorption of molecules as isolated guests of the host nanoporous crystalline phase) associated with the high sorption kinetics typical for areogels (due to the high porosity and hence high surface area).144, 146 In particular, sorption and desorption kinetics measurements have shown that the use of δaerogels allows to increase the apparent guest diffusivity of several orders of magnitude (up to 7!), with respect to δ-form films.146
s-PS nanoporous crystalline aerogels, due to their fast kinetics and high sorption capacity of VOCs and due to their good handling and stability, are particularly suitable for sorption media to remove traces of pollutants from water and air.76, 144–146
Particularly relevant molecular separations are those implying the removal of VOC from water and moist air. To this purpose, the nanoporous-crystalline polymeric phases are extremely suitable because they are able to include apolar molecules, as guest of their crystalline cavities, and to exclude the highly polar water molecules.
In particular, several experiments have been conducted relative to the uptake from aqueous solutions of 1,2-dichloroethane (DCE). The choice of DCE was motivated by the additional information, which comes from its conformational equilibrium. In fact because essentially only its trans conformer is included into the s-PS clathrate phase while both trans and gauche conformers are included in the amorphous phase, quantitative evaluations of vibrational peaks associated with these conformers allow to evaluate the amounts of DCE confined as guest in the clathrate phase or simply absorbed in the amorphous phase.116, 120, 152, 153 The choice of DCE was also motivated by its presence in contaminated aquifers and by its resistance to remediation techniques based on reactive barriers containing Fe0.154, 155
DCE equilibrium uptakes from diluted aqueous solutions, as obtained by FTIR measurements, for s-PS (aerogel and powders, circles) and PPO (film, squares) samples are for instance compared in Figure 7. For the sake of comparison, equilibrium sorption capacity of DCE from activated carbon is also shown.156 It is clearly apparent that samples including nanoporous crystalline phases (δ for s-PS and crystallized by benzene for PPO) present higher guest solubility than samples absorbing DCE only in the amorphous phase (γ for s-PS and amorphous for PPO).
As for s-PS, for low pollutant concentration, the DCE sorption occurs essentially only in the crystalline phase. In particular, for the most diluted aqueous solution (1 ppm), the sorption capacity is larger than 5 gDCE/100gpolymer, that is, leads to a concentration increase of 50,000 times. As for PPO, the DCE uptake from the amorphous sample is remarkable, as expected on the basis of its well recognized high free volume.95–97 However, the sorption from the semicrystalline sample is definitely higher than for the corresponding amorphous phase and, in particular for 50 ppm solutions, the sorption capacity of the semicrystalline sample is more than double than for the amorphous sample.
It is worth adding that the sorption ability of the nanoporous crystalline phases, of course, depends on the guest. For instance, unlike DCE uptake, benzene uptake is much higher for the nanoporous crystalline samples of PPO than for those of s-PS.94
The improvement of gas storage, recognition, and separation techniques represents a strategic industrial and environmental objective. Some techniques are based on gas absorption on high surface amorphous materials, such as, for example, activated carbons, crosslinked polymers, or carbon nanotubes. More selective techniques are based on the inclusion of gas-molecules into cavities of crystalline materials leading to cocrystalline phases (generally clathrate phases).
Nanoporous materials have been the focus of attention to solve the challenge of hydrogen storage in an efficient, cheap, and safe way. Among the most common materials considered to adsorb hydrogen143, 144 are porous carbons, nanotubes, zeolites, porous polymers, and metal-organic frameworks. Recently, a new mechanism of H2 uptake from polymeric materials based on adsorption in the ordered cavities of the δ nanoporous crystalline phase of s-PS, rather than on disordered amorphous surfaces, has been reported.159, 160
The H2 uptake from s-PS samples exhibiting different crystalline phases and different morphologies has been studied by gravimetric measurements at 77 K, in the hydrogen pressure range from 0 to 1.7 MPa [Fig. 8(A)]. Gravimetric experiments show that the molecular hydrogen sorption is strongly dependent on the sample morphology, that is, negligible for films, poor for powders, higher and increasing with porosity for aerogels.160 This clearly indicates that, as usual for carbon based adsorbents, the H2 uptake increases with the sample surface area.
However, for a given morphology, the H2 uptake is also strongly dependent on the nature of the crystalline phase.159, 160 In particular, the H2 uptake is minimum for the dense β and γ crystalline phases, intermediate for the channel-shaped nanoporous ε phase (not shown here) and highest for the cavity-shaped nanoporous δ phase [Fig. 8(A)].146 The occurrence of molecular hydrogen absorption in the crystalline cavities is clearly confirmed by FTIR spectra, as collected at 20 K [Fig. 8(B)], showing narrow components of the H2 band for aerogels exhibiting the δ nanoporous-crystalline phase and only broad and very weak signals for aerogels exhibiting dense β crystalline phase.
Monte Carlo simulations clearly indicate that the hydrogen molecules are absorbed into the crystalline cavities and channels of the δ and ε crystalline phases, respectively. In particular, large uptakes are already reached for low H2 pressures and limit sorption values correspond to average contents of nearly three molecules per cavity and of 3.5 molecules per unit-height, for the δ and ε phases, respectively.160
The studies on H2 sorption from nanoporous-crystalline aerogels have also suggested that, to further increase the H2 uptake from carbon based materials, crystalline cavities smaller than those of the δ phase (≈0.12 nm3)37, 91, 92 would be needed.160
As for gas sorption, of particular economic relevance could be the removal of ethylene118 and carbon dioxide117 from the packaging environment of fruit and vegetables. Indeed, it is well known that the postharvest life and quality of many fruits, vegetables and flowers are negatively affected by the presence of carbon dioxide and seriously shortened by exposure to trace amounts (also few ppb) of ethylene. For this purpose, active packaging have been realized by adding zeolites, silica, or activated carbon to commercial polypropylene or polyethylene packaging films.161–163
s-PS films presenting its nanoporous host δ phase exhibit a large ethylene uptake, which is associated with the formation of a cocrystalline phase with ethylene guest molecules, oriented nearly perpendicular to the crystalline polymer helices.118 Figure 9 compares the ethylene desorption behavior from different 85-μm films, after equilibrium absorption of ethylene at one atmosphere. It is apparent that the ethylene uptake from the nanoporous-crystalline film is much higher than for the amorphous film and also much higher than for an industrial isotactic-polypropylene film containing large amounts (up to 25 wt %) of silica.118
As already observed for other guest molecules of s-PS, the ethylene diffusivity in the host nanoporous crystalline phase is markedly reduced with respect to the diffusivity in the corresponding glassy amorphous phases. In addition, ethylene diffusivity can be further reduced by suitable selection of the uniplanar orientation (a∥c∥ or a∥c⟂) of the host crystalline phase.118
Hence, the δ-nanoporous crystalline phase of s-PS presents high ethylene solubility and low ethylene diffusivity (which can be also controlled by the orientation of the crystalline phase), associated with negligible water uptake. These features, combined with good chemical and mechanical properties, make polymeric films presenting the δ-nanoporous crystalline phase suitable candidates for produce packaging.118
Chemical sensors have an important and growing role in diverse fields including environmental (ground water and air pollution) monitoring, toxic chemical agent detection, and medical diagnosis.
In recent years, great efforts have been dedicated to realizing chemical sensors based on the integration of proper sensitive layers and suitable high-sensitivity transducing mechanisms. The sensitive element should optimize specific interactions with a target analyte, provide a fast and reversible diffusion of the penetrants and small recovery times, and also maintain the physical state as well as the geometry over several cycles of use, in order to avoid hysteresis effects, thus ensuring the reproducibility.
Several studies have shown that polymer films, presenting the nanoporous crystalline phases, are suitable sensing elements for detection of organic pollutants, being effective with most VOCs (mainly chlorinated and aromatic) that are present in industrial wastes, such as benzene, toluene, chloroform, methylene chloride, DCE, tetrachloroethylene, and trichloroethylene.126–130
In particular, s-PS films have been tested as sensing elements of resonant sensors for vapors of VOCs.126 For instance, the response to chloroform vapor of quartz crystal microbalance (QCM) sensors, coated with δ form s-PS films have been analyzed and compared to analogous systems coated with films of amorphous polymers. The sensitivity of sensors based on nanoporous crystalline s-PS films was found markedly higher, particularly for low chloroform pressures. Of course, the higher sensitivity of the nanoporous crystalline s-PS films is associated with the peculiar sorption mechanism: in fact, the organic compound, rather than being dissolved only into the amorphous phase, is mainly absorbed into the nanoporous crystalline phase, each molecule being confined into regularly spaced crystalline nanocavities.126
Optical transduction techniques are also very attractive in chemical sensing applications due to some unique features such as immunity to electromagnetic interference, small size, lightweight, low cost, and the possibility to use them in a harsh environment. As for sensors based on nanoporous-crystalline polymer films, methods based on optical fiber technology are particularly relevant, because they allow to detect organic pollutants in water (even in deep water). Optical sensors based on the integration of s-PS δ-form films with optical fiber technology has demonstrated an in-water parts-per-million detection capability of chloroform and toluene.127
An also more efficient optochemical sensor using long-period fiber gratings coated with sensitive overlays, based on nanoporous crystalline s-PS, has also been proposed.128–130
FUNCTIONAL MATERIALS BASED ON COCRYSTALLINE POLYMER PHASES
Systems composed of solid polymers and low molecular mass active compounds find several practical and advanced applications. Recent studies have suggested that a simple method to reduce diffusivity of active molecules in the solid state and to prevent their self-aggregation consists in the formation of cocrystals with suitable polymer hosts.
In particular, studies of guest desorption kinetics116, 152 and of gas transport124, 125 on s-PS films have shown that the guest solubility can be much higher in the crystalline phase (mainly for low solute activities) while the solute diffusivity is generally higher in the amorphous phase. This offers the opportunity to prepare samples including low-molecular-mass molecules essentially only as guests of the cocrystalline phase.
As discussed above, X-ray diffraction and linear dichroism infrared studies have clearly shown that the guest molecules present well defined average locations and orientations into cocrystalline phases. Moreover, solid-state 2H NMR studies have shown that the mobility of the solute molecules is heavily reduced when they are guests of the cocrystalline phase, rather than simply absorbed in the amorphous phase.164–166
As for possible applications of cocrystalline films, particularly relevant is the possibility to achieve three different kinds of uniplanar orientation (see Orientations of cocrystalline and nanoporous-crystalline polymer phases section), which allow controlling the orientation of the guest molecules not only in the microscopic crystalline phase but also in macroscopic films.
On these bases, films presenting s-PS/active-guest cocrystalline phases have been proposed as advanced materials, mainly as optical (chromophore, fluorescent, photoreactive), ferroelectric, and paramagnetic materials.
This constitutes an innovative approach in the area of functional polymeric materials, which are instead generally characterized by a disordered distribution of active groups into amorphous phases.
Chromophore and Fluorescent
Transparent films exhibiting polymer/chromophore cocrystalline phases can be easily prepared for s-PS.131, 132, 167, 168
Particularly interesting appear the results relative to films exhibiting the cocrystalline phase between s-PS and 1,3,5-trimethyl-benzene (TMB).132 X-ray diffraction measurements have proved the formation of two different kinds of polymer/chromophore host/guest cocrystals: “clathrate,” including isolated TMB molecules into the cavities137 and “intercalate,”31 including layers of TMB molecules intercalated with layers of polymer helices.
UV absorption and emission studies on s-PS films have shown that fluorescence phenomena are essentially additive when the chromophore is simply absorbed in the polymeric amorphous phase or isolated guest of the clathrate cocrystal. On the other hand, the fluorescence of the intercalate s-PS/TMB cocrystal, when excited at its absorbance maximum, is red-shifted with respect to both host and guest emissions (Fig. 10). This phenomenon has been attributed to a fluorescence bleaching, which is related to the three-dimensional order of the intercalate s-PS/TMB cocrystals.132
The achieved fluorescence enhancement and red-shift could be relevant for optical and optoelectronic applications. Particularly relevant seems the ability to emit at longer wavelengths, which could bring the benefit of minimum losses due to reabsorption of the host phase.
Photoisomerizations of organic dyes have been widely used as a means to record optical data. In particular, photoisomerization of norbornadiene and its derivatives (N, see scheme 1) leading to quadricyclane and its derivatives (Q, see scheme 1) has been deeply studied169–172 and polymeric materials containing N derivatives (both as covalently bonded pendant groups or simply added to transparent amorphous phases) have been investigated for optical waveguides (utilizing photoinduced refractive index changes) and for data storage.173–177 However, in all these optical materials there is a complete disorder in the spatial disposition of the photoisomerizing molecules.
It has been recently suggested that s-PS films presenting cocrystalline phases with N and its derivatives could be suitable for data storage systems with molecular size marks.133, 134 In particular, it has been shown that in these films photoisomerization of N to Q can be easily achieved and that both reactant N and product Q present positional and orientational order with respect to the polymeric host crystallographic axes.133
Infrared linear dichroism associated with gravimetric experiments on uniaxially stretched films have shown that it is possible to obtain samples with stable s-PS/N intercalate cocrystalline phases and with negligible N content in the amorphous phases. Gravimetric and thermogravimetric measurements have shown that irradiation experiments, leading to N→Q photoisomerization, can be conducted without any significant loss of guest molecules in the cocrystalline phases. Moreover, N→Q photoisomerization reactions allow the preparation of patterns of micrometric size, by using suitable opaque masks, and a FTIR imaging technique has allowed characterizing the obtained N and Q patterns (Fig. 11).134
Macroscopic polymer films hosting iso-oriented magnetic molecules would be the dream of many researchers, allowing to easily measure the anisotropy of isolated molecules.178 A preliminary study of cocrystalline phases of s-PS presenting as guest a simple and well characterized nitroxide radical compound, 2,2,6,6-tetramethyl-piperidinyl-N-oxyl (TEMPO) was reported by Kaneko et al.,135 with only preliminary characterization of the cocrystalline structure. More recently, clathrate and intercalate cocrystalline phases of s-PS with TEMPO have been more deeply characterized,74 and it has been shown that TEMPO can be ordered in s-PS films, opening perspectives for ordered organizations of magnetic molecules.
Magnetic properties have been deeply studied for axially oriented films including the s-PS/TEMPO intercalate cocrystalline phase, which exhibit a long-range contiguity between guest molecules along the c axis [Fig. 12(A)]. The TEMPO content in the film, as evaluated by thermogravimetric and magnetometric analysis, is close to 27 wt % while the degree of crystallinity, as evaluated by FTIR measurements, is close to 35%. A clear mark of magnetic anisotropy is shown by the EPR linewidths, whose angular dependence is presented in the lower part of Figure 12.136
Simple and low-cost polymers can hence act as host for magnetic molecules, keeping the features of a crystal for high guest concentrations. This is the first time that the anisotropic behavior typical of low dimensional magnetic materials can be directly probed for paramagnetic molecules in a porous host.
As for magnetic properties another exciting feature of s-PS cocrystals, is the opportunity of using films with uniplanar-axial orientations,111, 168 which should give in principle the opportunity to achieve for the magnetic guest molecules not only the axial order with respect to the polymer stretching direction but also a 3D orientational order in the whole macroscopic film.
After nearly twenty years from the discovery of the s-PS cocrystals, although several dozen of s-PS cocrystals were already described, all the presented guests were apolar or poorly polar. In this respect, it is worth citing the relevant ability of the nanoporous δ-and ε phases of s-PS in separating molecules of different polarity (like, e.g., removing traces of organic pollutants from aqueous solutions, Molecular separations section).
Only in 2007, it has been shown that by sorption in nanoporous or in cocrystalline s-PS samples of guests dissolved in suitable solvent-carriers, cocrystalline phases can be easily obtained with molecules of very high polarity.137 Solvents suitable as carriers are in general volatile guests of s-PS cocrystals, such as, for example, acetone or acetonitrile. These cocrystalline films present stable and three-dimensionally ordered disposition of polar guests, being characterized by high first order hyperpolarizability, as for instance trans-4-methoxy-β-nitrostyrene (β = 17 × 10−30 esu) or 4-(dimethyl-amino)-cinnamaldehyde (β = 30 × 10−30 esu) and, hence, can be in principle used for nonlinear optical applications.137
Cocrystalline phases of s-PS with polar molecules have been also proposed as a basis of a new class of ferroelectric polymeric materials.138 Commercially available ferroelectric polymers179, 180 are semicrystalline materials based on a specific crystalline phase of polyvinylidenefluoride (β phase), which exhibits a polar unit cell.181, 182 The ferroelectric hybrid materials, based on polymer cocrystalline phases with polar guest molecules also exhibit a polar unit cell. However, the unit cell polarity is given by the orientation of polar guest molecules into cocrystalline phases rather than by the orientation of polar polymer chains.
The ferroelectric behavior can be shown by nanoimprinting processes based on the Piezoresponse Force Microscopy (PFM) technique. The typical piezoresponse phase-voltage hysteresis of a s-PS film, including p-nitro-aniline molecules (nearly 5 wt %) as guests of the δ clathrate phase34 is shown in the central part of Figure 13. In the right and left of the figure, the a∥c∥ orientation of the host polymer helices (as determined by the polymer processing) and the orientation of the guest polar molecules (as determined by the applied electrical field) are schematically shown. This ferroelectric response is only observed when the polar molecules are guests of polymer cocrystalline phases, while it is not observed when the same molecules are simply dispersed in amorphous polymer phases.138
CHIRAL OPTICAL POLYMER FILMS BY COCRYSTALLIZATION WITH NONRACEMIC GUESTS
Cocrystallization phenomena, as induced by nonracemic guests, can lead to films with relevant and stable chiral optical properties both for s-PS183–188 and PPO.69
In particular, robust chiral optical films have been obtained by cocrystallization of amorphous s-PS films as induced by sorption of many volatile nonracemic molecules.183–186 For s-PS, the induced CD (ICD) presents a major Cotton band at 200 nm and a minor Cotton band of opposite sign at 223 nm (Fig. 14). The obtained chiral optical response of the polymer films occurs also for the infrared spectral region185–187 and remains essentially unaltered as a consequence of the nonracemic guest removal leading to the nanoporous crystalline δ phase (Fig. 14, green-solid lines) as well as after thermal treatments (up to 140 °C) leading to the dense helical γ phase99 (Fig. 14, red-dashed lines) or after thermal treatments (up to 240 °C) leading to the trans-planar α phase189 (Fig. 14, black solid-lines).184–186 The memory of the volatile nonracemic guest molecules can be erased only by thermal treatments at temperatures higher than the s-PS melting temperature (≈270 °C) or by long-term treatments with strong s-PS solvents.184–186
These results indicate that the ICD phenomena observed for s-PS are not associated with nonracemic molecular structures but with the formation of nonracemic supramolecular (possibly crystalline) morphologies.184, 185
Recently, intense CD and VCD phenomena have been also induced to achiral chromophores by their sorption as guest of the above described chiral crystalline host phase of s-PS. In particular, these unexpected CD and VCD phenomena are observed provided that (i) the initial crystallization of s-PS has been induced by a nonracemic guest from an amorphous phase; (ii) the chromophore molecules are suitable as guest of a s-PS cocrystalline phase.187
For instance, the CD behavior, as achieved in the visible region, for azulene molecules being guest of a cocrystalline phase with s-PS, whose initial crystallization has been induced by (−)-(R)-carvone or (+)-(S)-carvone, are shown in Figure 15(A). X-ray diffraction characterizations have shown, for the case of the azulene guest, the formation of a δ clathrate monoclinic structure, including both R and L helical polymer chains [Fig. 15(B)]. This confirms the hypothesis184, 185 that CD phenomena induced in amorphous s-PS films by the temporary sorption of nonracemic guests are not due to the formation of nonracemic unit cells but to the formation of nonracemic morphologies of cocrystalline phases, which are maintained also after solvent or thermal treatments leading to different crystal-to-crystal transformations.
These results open the possibility to achieve s-PS based films with chiral optical response at many different wavelengths. In this respect, it is worth adding that s-PS can be easily melt processed (also in industrial plants) leading not only to films but also to solid samples of any shape (e.g., by injection molding), which can be made fully amorphous by simple quenching procedures.189, 190 As a consequence, materials and devices exhibiting chiral optical responses in selected wavelengths, based on s-PS, can be easily designed and produced.
Several exciting new materials based on cocrystalline and nanoporous-crystalline polymer phases have been achieved. In particular, the unprecedented achievement of polymeric nanoporous crystalline phases has given very interesting results in the fields of molecular separations, water/air purification and sensorics. Moreover, several kinds of polymer cocrystalline phases have been prepared, mainly for s-PS, and polymer cocrystals with active guest molecules show unusual physical properties, which makes them promising for several kinds of advanced materials.
In this respect, it is worth citing the relevant advantages of polymeric materials with respect to molecular or covalent solids. In fact, beside simple processing conditions (low pressure and temperatures) and automated processing techniques (both from melt and solutions), polymers also present highly desirable bulk and surface properties. It is also worth adding that both s-PS190 and PPO192, 193 are commercially available engineering thermoplastics exhibiting several relevant properties, such as a high melting point (>250 °C), excellent chemical resistance, dimensional stability, and low moisture sorption.
As for perspectives of the polymeric nanoporous-crystalline phases, further applications are expected in the field of controlled release of drugs and pesticides. As for nanoporous-crystalline films and aerogels, a relevant objective will also be the modification, with different kinds of functional groups, of the amorphous phase. The functionalization of the sole amorphous phase could bring several advantages, like for instance increase of rates of guest sorption from the nanoporous crystalline phases. Relevant results have been already achieved for semicrystalline s-PS samples with sulfonated amorphous phases.194–196
As for perspectives of applications of cocrystalline phases, studies will be mainly devoted to films, also trying to exploit the unique availability of different kinds of uniplanar orientations, which allow also macroscopic control of the guest orientation. Particular attention will be devoted to possible applications of the recently discovered ε-clathrates of s-PS, mainly due the possibility to control the orientation of very long guest molecules, which could give relevant nonlinear-optical properties.
Relevant new materials could also be obtained by chemical reactions (e.g., polymerization) between guest molecules, for polymer cocrystals exhibiting guest–guest proximity (ε-clathrates and intercalates of s-PS). Polymerization of benzylmethacrylate guest monomers has been already achieved,197, 198 but the formation of the guest-polymer also leads to the destruction of the host cocrystalline phase.
The field of cocrystalline and nanoporous polymer materials is expected to be widely expanded. In fact, till now, only materials based on s-PS and, more recently, on PPO have been considered for possible applications. Relevant new cocrystalline and nanoporous materials are expected in the future for other regular and stereoregular polymers.
The authors thank V. Venditto, G. Milano, A. Albunia, and C. D. Aniello of University of Salerno, G. Mensitieri and V. Petraccone of University of Naples, P. Musto of the ICTP-CNR of Naples and M. Giordano of the IMCB-CNR of Naples for the contributions to the reviewed research work as well as for useful discussions. Financial support of the “Ministero dell'Istruzione, dell'Università e della Ricerca” (PRIN2007); and of “Regione Campania” (CdCR) is gratefully acknowledged.
Gaetano Guerra, Professor of Macromolecular Chemistry at the University of Salerno, received his doctor degree in Chemistry at Federico II Naples University, under the direction of Paolo Corradini. All his scientific activity, documented by more than 250 scientific papers on international journals, has been devoted to semicrystalline polymeric materials. He has received many awards in his career and his research group has received in 2010, from the Italian Republic President, the “Premio Nazionale Innovazione” (for the best business ideas of high technological content).
Christophe Daniel received his Ph.D in Physics from the University of Strasbourg and was a postdoc at Ethyl Petroleum Additives, Inc. (Richmond, VA) and at the University of Leeds. Currently, he is an Assistant Professor in the Dipartimento di Chimica e Biologia at the Università degli Studi di Salerno. He has coauthored around 50 peer-reviewed publications and book chapters.
Paola Rizzo, Assistant Professor at the University of Salerno, received her doctor degree in Chemistry at Federico II Naples University.
Her research interests are prevalently concerned to structural characterization of polymeric materials focusing on structure-properties correlations. Her current interests are devoted to study the chiral amplification phenomena and memory effect of nonracemic molecules by a racemic host polymer film. Her scientific activity is documented by more than 40 scientific papers on international journals.
Oreste Tarallo received his Ph.D in Chemical Sciences from the Federico II University of Naples (Italy) in 2003, where, at present, he holds a permanent position as researcher. His research interests are focused on the study of the relationships between chemical microstructure, crystal structure and physical properties of semicrystalline materials based on stereoregular polymers and, in particular, on the study of new crystalline forms of syndiotactic polymers of styrene and of its derivates, with particular attention to the cocrystalline and nanoporous phases that these polymers are able to form.