Understanding at molecular level of nanoporous and co-crystalline materials based on syndiotactic polystyrene

Progress in Materials Science 54 (2009) 68–88

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Progress in Materials Science journal homepage: www.elsevier.com/locate/pmatsci

Understanding at molecular level of nanoporous and co-crystalline materials based on syndiotactic polystyrene Giuseppe Milano *, Gaetano Guerra Department of Chemistry, University of Salerno and INSTM Research Unit, Modeling Lab for Nanostructure and Catalysis and Centro Interdipartimentale NanomateS, via Ponte Don Melillo, Fisciano (SA), Italy

a r t i c l e

i n f o

Article history: Received 13 June 2008 Accepted 5 July 2008

a b s t r a c t New polymeric materials based on syndiotactic polystyrene (s-PS) are presented. These materials are able to absorb rapidly and efficiently volatile organic compounds from water and air, also when present at very low concentrations and can be considered as the first example of polymeric molecular sieves, as they display a high sorption selectivity similar to zeolites. To our knowledge, this is the first case of polymeric semicrystalline material whose sorption ability is higher for the crystalline phase than for the amorphous phase. Moreover, these new molecular sieves are hydrophobic and hence seem particularly suitable for water and moist air purification. Research work focused on experiments and modeling aimed to understand the properties of these materials in term of chemical structure will be reviewed. In particular, different aspects relative to the structure, absorption properties, host–guest interactions, penetrant diffusion and molecular guests mobility of these new polymeric materials will be covered. Ó 2008 Elsevier Ltd. All rights reserved.

Contents 1. 2. 3. 4. 5.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Relevant structural features of the nanoporous d and related co-crystalline phases. Absorption properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nature of the host–guest interactions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Diffusion mechanisms in the d crystalline phase . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

* Corresponding author. E-mail address: [email protected] (G. Milano). 0079-6425/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.pmatsci.2008.07.001

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6. 7.

Guest molecule orientation and dynamics . Conclusions and perspectives . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . .

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1. Introduction Syndiotactic polystyrene (s-PS), whose synthesis was reported about two decades ago [1,2], is a semicrystalline polymer (typical degrees of crystallinity in the range 30–50%) which is easily crystallizable and high melting (270 °C) stereoregular polymer presenting a very complex polymorphic behavior [3–7]. In particular s-PS can form two crystalline phases with trans-planar chains (a [8–10] and b [11,12]), by melt-processing as well as by thermal treatments on glassy samples, as shown in Scheme 1. The other three crystalline phases (c [3,5,13,14], d [15–19], and e [20–22]) exhibit helical s(2/1)2 chains and are obtained by solution processing. In most cases, crystallization procedures from solutions lead to the formation of co-crystalline phases of s-PS with low molecular mass guest molecules, which also present helical s-PS chains [3,4]. Conditions suitable to prepare these helical crystalline and co-crystalline phases are collected in Scheme 2. It is worth adding that two mesomorphic phases presenting trans-planar [23–25] and helical [18,26,27] chains (not shown in Schemes 1 and 2) have also been found and characterized. Two of the helical crystalline phases (d and e), which can be only obtained by guest removal from co-crystalline phases, are nanoporous, i.e. are able to absorb, also from diluted solutions, low-molecular-mass guest molecules in cavities of their crystalline structures. The d phase is known since 1994 [15] and its crystalline structure as well as its physical properties has been thoroughly characterized. It presents two identical cavities and eight styrene monomeric units per unit cell [16] and rapidly and selectively absorbs low-molecular-mass guest molecules even at very low activities, producing clathrate [28–33] and intercalate [34–37] co-crystals. Several recent studies have shown that the d crystalline phase is promising for applications in chemical separations [38–41] and air/water purification [42–50] as well as in sensorics [51–56]. It has been also found that the self-assembling of this polymeric framework and active guest molecules into co-crystals reduce guest diffusivity and prevent guest self-aggregation (without recurring to chemical reactions). On this basis, films presenting

Polymorphism and melt-processing quenching MELT

s-PS

high T slow cooling

low T fast cooling

AMORPHOUS T>140˚C

α

CO2, T, P

α form with most solvents

Co-crystalline phases

β

β form never transformed in Co-crystalline phase

Scheme 1. Schematic representation of the main crystallization and interconversion conditions for the polymorphic crystalline phases of syndiotactic polystyrene, as obtained by melt-processing.

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Polymorphism and solution-processing

s-PS

SOLUTIONS

most solvents

high T low T

β

HOST-GUEST CO-CRYSTALS

γ + CHCl3

guest removal

S-PS/CHCl3 CO-CRYSTAL − CHCl3

ε

δ

Nanoporous crystalline phases Scheme 2. Schematic representation of the main crystallization and interconversion conditions for the polymorphic crystalline phases of syndiotactic polystyrene, as obtained by solution-processing.

s-PS/active guest co-crystals have been proposed as advanced materials, mainly for optical applications (e.g. fluorescent, photo-reactive, chromophore and nonlinear optical materials) [57–63]. The nanoporous e crystalline phase of s-PS has been discovered only recently and much less information is presently available [6,20–22]. It has been shown that, in co-crystals obtained from the e phase, the orientation of the guest molecular planes is generally parallel to the polymer host chain axes [20,21], rather than perpendicular, as generally observed [28,30,32,62,64,65] for co-crystals from the d phase. Moreover, the e ‘‘polymeric framework”, is able to host (to form stable co-crystals) with low-molecular-mass polar molecules, being definitely longer than those hosted by the d ‘‘polymeric framework” [20,21]. These results have been rationalized by the e-form crystalline structure which is going to appear in the literature [22], showing that the empty space is not distributed as isolated cavities, as occurs for the d phase, but as connected cavities (roughly channels). To our knowledge, materials based on the nanoporous crystalline phases of s-PS are the first polymeric materials whose sorption ability is higher for the crystalline phase than for the amorphous phase. This review is centered on the nanoporous d phase and on the corresponding co-crystalline phases with low-molecular-mass guest molecules. Emphasis will be given to the contribution of molecular simulations to the understanding of structure–property relationships of these new and rather unique polymer materials. 2. Relevant structural features of the nanoporous d and related co-crystalline phases The monoclinic structure of the d form of s-PS (space group P21/a, a = 1.75 nm, b = 1.19 nm, c = 0.77 nm, and c = 117°) has per unit cell two identical cavities centered on the center of symmetry (Fig. 1) [16,17]. Crystal structure data contain all the information necessary to evaluate the empty volume fraction as well as the shapes and sizes of possible empty spaces (cavities and/or channels) [17]. The region of empty space calculated for the d form, by assuming r = 1.8 Å, that is a typical van der Waals radius of chlorine atoms or methyl groups is shown as a dotted region for two different views of the unit cell, along c and perpendicular to the ac plane, in Fig. 1. The figure shows that this empty space corresponds to finite cavities (two per unit cell) centered on the center of symmetry of the crystal structure, whose boundary is essentially defined by 10 phenyl rings. By considering, for instance, the view along the c axis, there are four phenyl rings below the cavity (average height 0), four phenyl

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L

R

½

0,1

0,1

d 010

0,1

0,1

½

L

R

a c

Fig. 1. Crystal structure of d form (two views along c and perpendicular to the ac plane), the empty space corresponding to the cavity is in gray. Each crystalline cavity is confined by 10 host phenyl rings (whose approximate center of mass z fractional coordinates are indicated). R, right-handed; L, left-handed helices.

Table 1 Crystal structure parameters, guest volume fraction, guest volume, and volume of the cavity for the d form and three different clathrates of s-PS Crystal structure d-Form s-PS/DCE s-PS/iodine s-PS/toluene a b c d e f

a (Å)

b (Å)

c

c

17.5 17.1d 17.3e 17.6f

11.8 12.1d 12.9e 13.3f

c (Å) c

7.7 7.7d 7.7e 7.7f

c

Guest volume fractiona

Guest volume (Å3)a

Cavity volume (Å3)b

0.22 0.29 0.30

91 124 132

115 125 151 161

c

117 120d 120e 121f

Guest volume fraction = Vguest/(4  Vstyrenic unit). Volume calculations are based on the van der Waal radii. Calculated with the assumption of r = 1.8 Å. From Ref. [16]. From Ref. [30]. From Ref. [12]. From Ref. [28].

rings above the cavity (average height c = 7.7 Å), and two phenyl rings whose average height is equal to the average height of the cavity (c/2) (bolded rings in Fig. 1). For a probe sphere with r = 1.8 Å, the cavity has a volume of nearly 115 Å3 and its maximum dimension is nearly 8.1 Å (essentially along the  direction) while its minimum dimension is nearly 3.4 Å (essentially along the c axis). Following b a the same procedure, the shapes and volumes of the hypothetical cavities generated by ignoring the presence of the guest molecules have been evaluated for some clathrate structures [17] and the relative cavity volumes are reported in the last column of Table 1. Following standard procedures for porosity evaluation of powders [66], by nitrogen sorption experiments at low temperature (77 K) on amorphous and semicrystalline s-PS samples, it is possible to give an experimental evaluation of the cavities size. Sorption and desorption isotherms of d and c (not nanoporous) form samples are compared in Fig. 2, where the sorption is expressed as cm3 of nitrogen in normal conditions (1 atm, 0 C) per gram of polymer. Sorption of the nanoporous form is already higher for low nitrogen activities: for instance, at p/p0 = 0.4, the sorption is close to 15 cm3/g for the delta form while it is only 2 cm3/g for the gamma form. Moreover, starting from p/p0 0.6, there is a steeper increase of sorption of the d form up to p/p0

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80

60

3

N2 sorption volume (cm /g)

70

50

δ

40 30 20

γ

10 0 0.0

0.2

0.4

0.6

P/P°

0.8

1.0

Fig. 2. Sorption (continuous curves) and desorption (dashed curves) isotherms of nitrogen at 77 K into s-PS powder samples, exhibiting d and c crystalline phases.

0.9. This makes the sorption difference between the d form and the other semicrystalline samples particularly large (nearly 70 cm3/g) at p/p0 = 0.98. It is also worth noting that for the d form powder there is a remarkable hysteresis in the sorption–desorption phenomenon. Because the two samples present similar crystallinity and morphology, it has been assumed that the sorption increase observed for the d form sample is essentially associated with condensation of nitrogen molecules into the cavities of the nanoporous crystalline phase and that the observed hysteresis is, probably, due to the formation of polymer-gas clathrates [67,68]. In the assumption of formation of s-PS/nitrogen clathrates, the number of N2 molecules per crystalline cavity can be approximately evaluated. In fact, if nN2 and ncavity are moles of nitrogen and cavities, respectively, for a given mass of polymer Mpol, then:

nN2 ðPV N2 M pol Þ=RT 4PV N2 M styr ¼ ¼ ¼ 0:0186V N2 =X c ncavity ðM pol X c Þ=4Mstyr RTX c

ð1Þ

where V N2 is the volume of nitrogen (for P = 1 atm and T = 273 K) sorbed in the crystalline phase per gram of polymer, Mstyr is the molecular mass of the styrenic unit, Xc is the crystalline fraction of the d form polymer sample, and four is the number of styrene units per cavity in the d crystalline phase. On the basis of Eq. (1), by taking V N2 = 70 cm3/g (from Fig. 2), and Xc = 0.43 (determined by X-ray diffraction), the number of nitrogen molecules per crystalline cavity is calculated to be close to 3. Two possible limit evaluations of the space occupied by the molecule of N2 can be obtained considering the space occupied by three molecules in the two possible nitrogen crystalline structures (cubic and hexagonal [69]): 137 and 163 Å3, respectively. In our assumptions these volumes can be taken as a rough evaluation of the volume of the cavity of the crystalline phase. This independent evaluation is in satisfactory agreement with the calculated volumes of column 8 in Table 1. It is worth adding that the d crystalline form (Fig. 1) can be considered as formed by the packing of layers of close-packed enantiomorphous s(2/1)2 polymer helices, parallel to the ac crystalline planes (Fig. 1). These structural layers remain essentially unchanged after sorption of guest molecules, leading to formation of several different co-crystalline phases [28–32,34–37]. For most s-PS co-crystals, isolated low-molecular-mass compounds are imprisoned as guest into the cavities of the nanoporous host d phase, only producing minor changes to the distance between the ac layers [28–32]. These s-PS co-crystals have been defined as clathrate phases and are generally characterized by a guest/monomeric-unit molar ratio 1/4. More recently, the occurrence of a second class of s-PS co-crystals, defined as intercalate, has been established [34,35]. For these co-crystals

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18 17

dmax (Å)

16 15

intercalates

14 13 12

clathrates 11

δ

10 0

50

100

150

200

250

300

3

guest Volume (Å ) Fig. 3. Maximum Bragg distance (dmax), associated with the crystalline peak at lowest diffraction angle, for several s-PS cocrystals versus the guest molecular volume. Squares indicate clathrates while circles indicate intercalates. s-PS molecular complexes with unknown crystal structures are indicated with empty symbols while crossed circles indicate crystalline phases in s-PS physical gels. For co-crystals with established clathrate (full squares) and intercalate (full circles) structures, dmax is equal to the distance between ac layers of helices (d010).

the guest molecules are not isolated into host cavities but contiguous inside layers intercalated with mono-layers of enantiomorphous polymer helices. Of course, these intercalate structures present larger distances between the ac layers as well as higher guest content (the guest/monomer-unit molar ratio generally is 1/2 rather than 1/4), with respect to the clathrate structures [34,35]. To compare different s-PS co-crystalline phases, particularly informative can be the distance between the ac layers, i.e. the spacing of the Bragg planes 010 (d010, see Fig. 1). For the co-crystals, whose crystal structure has been determined (starting from X-ray diffraction patterns of axially oriented samples) [28–32,34,35], the d010 spacing corresponds to the maximum Bragg distance (dmax), i.e. it is associated with the crystalline peak at lowest diffraction angle. Hence, for a large number of s-PS co-crystals, the dmax distance (Table 7 of Ref. [35]) has been plotted versus the molecular volume of the guest in Fig. 3. The distance between these ac layers, being 10.5 Å for the nanoporous d phase, remains essentially unaltered as a consequence of inclusion of small molecules, having a volume smaller the cavity volume. This distance increases only slightly (up to 12 Å) for clathrate co-crystals, where isolated guest molecules exhibiting a volume lower than 250 Å3 are located into the cavities of the d phase (squared symbols in Fig. 3). The distance between these ac layers is instead much larger (above 13 Å) for intercalate co-crystals, where these layers of polymer helices are alternated to layers of guest molecules. Intercalate co-crystals are formed with guest molecules exhibiting a volume higher than 150 Å3 (circle symbols in Fig. 3). In synthesis, as a simple criterion, we suggest that clathrate and intercalate structures can be anticipated for s-PS co-crystals, as obtained from the nanoporous d phase, with dmax < 12 Å and dmax > 13 Å, respectively [35]. 3. Absorption properties By using suitable solvent carriers, it is possible to prepare highly stable co-crystalline phases with several guest molecules also presenting high polarity [21,62]. However, several s-PS co-crystals can be formed only by using high guest concentrations in the carriers. Completely different is the sorption behavior of low-polarity volatile organic compounds (VOC), like chlorinated and/or aromatic hydrocarbons, which are readily absorbed by the nanoporous crystalline forms of s-PS, also when present in traces in water and in air.

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Fig. 4. Sorption kinetics of DCE from saturated (8100 ppm; solid lines) or 100 ppm (dashed lines) aqueous solutions by semicrystalline powder samples: (N) d form and (d) b form.

Just as an example, sorption kinetics of 1,2-dichloroethane (DCE) from saturated (8100 ppm) or diluted (100 ppm) aqueous solutions by powder s-PS samples [45], as obtained by measurements of FTIR absorbances of conformationally sensitive guest peaks (calibrated by thermogravinetry), are shown in Fig. 4. In addition to powders in d form, powders in the b crystalline form, which absorb low-molecular-mass compounds only into the amorphous phase [4,70,71], are also considered. It is apparent that for d form powders, few minutes are sufficient to obtain substantial sorption of DCE from water solutions, whereas b form powders absorb DCE more slowly and to a lower extent. Sorption from s-PS samples, which are amorphous or in crystalline forms other than d, is negligible for low DCE activities as occurs for b form samples. The recent preparation of aerogels exhibiting the nanoporous d form [48] has allowed to increase the guest diffusivity of several order of magnitude and has allowed to establish that the equilibrium sorption of DCE from 1 ppm aqueous solutions is close to 5 wt% [50]. These results hence indicate the occurrence of high partition coefficients between the polymeric phase and the aqueous phase. The obtained results indicate that nanoporous crystalline samples of s-PS can be suitable for water purification from chlorinated and aromatic hydrocarbons. In this respect, particularly relevant could be the ability to absorb DCE, which is present in contaminated aquifers and is resistant to remediation techniques based on reactive barriers containing Fe0 [72,73]. As will be discussed in the next sections, the occurrence of different conformational equilibria of some guest molecules, into amorphous and clathrate phases of s-PS, allows evaluation of the guest contents into both phases. On this basis, it has been established that sorption involves preferentially the crystalline phase [43,45,48] and that desorption from co-crystalline phases is much slower than desorption from amorphous phase [44]. 4. Nature of the host–guest interactions Several FTIR studies [43,46,74,75] have given information about the nature of host–guest interactions in s-PS co-crystals. Particularly informative are conformational studies relative to chlorinated guest molecules [1,2dichloroethane (DCE), 1,2-dichloropropane (DCP) and 1-chloropropane (CP)], which present similar and simple conformational equilibria. In fact, the FTIR spectra of these chlorinated compounds in

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the wavenumber range 1500–450 cm1, present a number of well-resolved conformationally sensitive peaks. Moreover, due to the relative simplicity of these molecules a complete normal vibrational analysis is feasible, which has allowed unambiguous assignment of the various absorptions to the normal modes of the different conformers [76–78]. The FTIR studies relative to chlorinated hydrocarbons have shown that the trans conformation of 1,2-dichloroethane (DCE) and 1,2-dichloropropane (DCP) is largely prevailing in the clathrate phase, while the trans and gauche conformations are nearly equally populated when both chlorinated compounds are sorbed in the amorphous phase [43,45]. Completely different is the chloropropane (CP) behavior, for which the population of the trans conformer is not increased by clathration into s-PS. The prevailing presence of the trans DCE conformer into the clathrate phase has been confirmed by X-ray diffraction determination of its crystal structure [30]. The occurrence for DCE and DCP (but not for CP) of conformational selectivity, favoring the trans conformers in the cavities of the d form of s-PS, indicates the presence of specific attractive, rather than van der Waals repulsive, interactions involving the trans chlorine atoms. In fact, since chlorine atoms and the methyl groups present similar van der Waals radii (close to 1.8 Å), if the conformational selectivity had been due to repulsive nonbonded interactions, it would have been observed also for CP (see Chart 1). It is reasonable to assume that the specific locations of the phenyl rings delimiting the cavities of the nanoporous structure would influence the location of the two chlorine atoms of DCE and DCP, thus determining the observed conformational selectivities. A molecular mechanics approach has been used to rationalize the previously described experimental conformational results [45,79]. This approach contributed to elucidate the nature of the host–guest interactions into s-PS clathrate co-crystals. As for the polymeric host, an electrostatic potential map, relative to the cavity region of the unit cell of the s-PS/DCE co-crystalline phase, at a quote of 1/2c, as obtained by ignoring the guest molecule, is shown in Fig. 5A. The map of Fig. 5A shows that the electrostatic potential is essentially negative in the central part of the cavity while it is prevailingly positive in some regions which are external to the cavity. As for the chlorinated guests, trans and gauche conformers of DCE, positioned with respect to the cavity in the way suitable to minimize overall potential energy, are shown in Fig. 5, parts B and C, respectively. For comparison with electrostatic potential of the cavity (Fig. 5A), the electrostatic potential of both conformers of DCE is also shown. It is apparent on inspection that there is a good electrostatic fit between the quadrupolar trans conformer and the substantially quadrupolar cavity. In particular, the positive electrostatic region of the DCE trans conformer, corresponding to the carbon and hydrogen atoms (Fig. 5B), presents a good superposition with the negative electrostatic nature of the cavity (Fig. 5A). The electrostatic fit between the cavity and the substantially dipolar gauche conformer (cf. Fig. 5C with Fig. 5A) is instead poor. Strictly analogous considerations hold for DCP. On the contrary, CP is dipolar and its electrostatic field is essentially independent of its conformation, as a consequence its electrostatic interaction with the cavity is similar for all conformers. Results of minimizations of the host–guest interaction energies for the different guest conformers, into the cavity of Fig. 5, are reported in Table 2. For the sake of simplicity, only the absolute energy minimum situations of both conformers are listed. It is apparent that for all the considered chlorinated guests the main attractive contribution (up to 16 kcal/mol) is of van der Waals type, being however poorly dependent on the guest conformation. The electrostatic attractive contribution is smaller for

Chart 1. trans Conformers of some chlorinated guest molecules.

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A X (Å) 4 2 0 -2 -4 -4

B X (Å)

-2

2

0 Y(Å)

4

C X (Å) 4

4 H

2 0 -2

H H

C

|

0

C

H

C

Cl

C

H

|

H H

2

|

Cl

H

-2

Cl

-4

Cl

-4

-4

-2

0

2

-4

4 Y(Å)

-2

0

2

4 Y(Å)

Fig. 5. Electrostatic potential maps: (A) relative to the cavity region of the unit cell of the s-PS/DCE clathrate, at a quote of 1/2c, when the guest molecule is ignored; relative to the trans (B) and gauche (C) conformers of DCE positioned with respect to the cavity in a way suitable to minimize the overall potential energy. The zero energy curves are dotted while positive and negative are dashed and continuous, respectively.

Table 2 Results of minimization of the interaction energies (kcal/mol) with the cavity of the host crystalline phase (Figs. 1 and 5A) for trans and gauche conformers of DCE DCE

T

G

Van der Waals Electrostatic Total

12.8 1.1 13.9

12.8 0.0 12.8

all the considered chlorinated compounds; however, it is nearly 1 kcal/mol larger for the trans conformers, with respect to the gauche conformers, for both DCE and DCP. On the other hand, the electrostatic contribution to the minimum energy values is poorly dependent on the CP conformation. Hence, the calculations of Fig. 5 and Table 2 are able to rationalize the absence of conformational selectivity for CP and the preference toward trans conformers for DCE and DCP, in the cavities of the d form of s-PS. 5. Diffusion mechanisms in the d crystalline phase Most attention of computational and experimental studies on diffusion and molecular mobility studies have been devoted to amorphous polymers. In fact, also for semicrystalline polymers, the diffusivity is largely determined by that of their amorphous phases [80]. This behavior is a consequence of the lower density, associated with the less efficient packing of polymer chains generally observed in

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the amorphous domains, which leads to more space being available to the penetrant. In most cases, the sorption and diffusion behavior of small molecules in semicrystalline polymeric samples can be rationalized by assuming negligible contributions from the crystalline phase. For isotactic poly(4-methyl-1-pentene) (PMP), however, the density of the crystalline phase is close to that of the amorphous phase. In that case, as pointed out by Paul and co-workers [81] and studied by means of molecular dynamics (MD) simulations by Muller-Plathe [82], penetrant solubility and diffusivity in the crystalline phase are still lower with respect to those of the amorphous phase but not negligible. MD simulations techniques have been used to understand the diffusive behavior of helium and CO2 in the crystalline phase of s-PS and particularly its anisotropy, also as a function of temperature [83,84]. The diffusion of CO2 is particularly relevant because, at present, extraction procedures by CO2 represent the most efficient way to remove guest molecules from s-PS co-crystalline phases and regenerate the nanoporous d and e phases [21,85,86]. The main advantage of the simulations is that a well-defined and purely crystalline system can be studied, whereas the experiment has to cope with semicrystallinity (i.e. fraction of amorphous material generally larger than 0.5), incomplete orientation of the crystallites, and the general difficulty of measuring anisotropic diffusion coefficients. The polymer starting structure was taken from the experimental X-ray structure [16] and simulation box is shown in Fig. 6. Periodic boundary conditions to the unit cell were applied having and infinite crystal. MD was run at constant temperature T and constant pressure P (pressure bath of 101.3 kPa.) For both systems helium/polymer and CO2/polymer, equilibrations of 200 ps followed by production runs of 4 ns have been performed.

Fig. 6. Simulation box considered in our calculations for the d form of s-PS. The periodic system, shown for two different views, along the c direction (A) and perpendicular to the ac plane (B), consists of four cells stacked in the c direction, two along the a direction, and three along the b direction. Gray regions indicate the cavities characterizing the nanoporous structure. Dashed lines show the orthorhombic box adopted in our calculations. The crystallographic directions connecting neighboring cavities are indicated in part B.

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In this conditions the force-field used well reproduce the polymer structure and no restrictions were applied to the motion of penetrant molecules. The diffusion coefficient D was calculated from center-of-mass mean-square displacement curves of the penetrants according to the Einstein relation between the mean-square displacement and diffusivity. Helium atom pathways obtained from simulation trajectories at 25 and 80° C are shown in Fig. 7 for ab and ac projections of the crystal unit cells. Diffusion proceeds by hopping between different crystalline cavities. For some period of time, the penetrant stays in a cavity region. During such a quasi-stationary period, it explores this region but does not move beyond the cavity confines. The quasistationary periods are interrupted by quick leaps from one cavity into a neighboring one. Similar motion patterns have been found in all MD studies of the diffusion of small penetrants in amorphous polymers. However, because for the nanoporous form of s-PS all cavities are of the same size and occupy well-defined crystallographic positions, jumps between different cavities generally occur along well-defined directions. For both temperatures, the hopping between cavities generally occurs along directions which are parallel to the crystalline ac planes (Fig. 7). This behavior can be easily rationalized by the presence in these planes of rows of parallel alternated enantiomorphic helices with min directions imum interchain distances (0.87 nm; Fig. 8). In fact, most jumps occur along [101] and ½101 (that is, along the family of directions <1 0 1>) or along the c axis (or the [0 0 1] direction). The large diffusional anisotropy [83] cannot be explained by the standard diffusion mechanism for small penetrants in amorphous polymers. Although, as usual, hopping events are facilitated by transient channels of free volume between existing cavities and the channels are formed by thermal

T=80°C

T=25°C

A

C

b

b a

a

B

D

c a

c a

Fig. 7. Trajectory (trace) of one helium atom in crystalline s-PS in its nanoporous d form during 2 ns of simulations at 25 (blue) and 80 C (red). Similarly to Fig. 1, different views along the c direction (A and C) and perpendicular to the ac plane (B and D) are shown. At room temperature (A and B), a hopping motion mechanism between different crystalline cavities, prevailing along <1 0 1> directions, is apparent. Pathways at 80 C (C and D) could be interpreted as penetrant motion mainly along the [0 0 1] and <1 0 1> directions.

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L

79

R

½

0,1

0,1

0,1

0,1

L

½

R

8.7 Å Fig. 8. Along c view of two adjacent ac layers of close-packed alternated enantiomorphous helices, which characterize the host d form of s-PS and the corresponding co-crystalline phases. Three different kinds of films exhibiting different orientations of these ac layers, with respect to the film surface, have been obtained. In particular, both a and c can be preferentially parallel to the film surface (ak ck [13,87]) or a perpendicular and c parallel (a\ ck [90]) or a parallel and c perpendicular (ak c\ [88,89]).

motions of the host polymer atoms [83–86], for a nanoporous crystal phase average sizes and locations of the cavities, as well as crystallographic directions of the channels, are all identical. Several recent experimental studies have shown the additional ability of s-PS, possibly unique for polymer films, to be formed with three different kinds of orientation of the helical crystalline phase [13,87–90]. In particular, films exhibiting three different orientations of the ac layers of close-packed alternated enantiomorphous polymer helices (Fig. 8) with respect to the film plane have been recently achieved by casting [87,88] or by solvent induced crystallization [89,90] procedures. These three kinds of orientation, according to the terminology by Heffelfinger and Burton [91], can be defined as ‘‘uniplanar”, and a nomenclature suitable for both c and d phases has been recently defined (ak ck [13,87] or a\ ck [90] or ak c\ [88,89,92]). Of course, ideal ak ck and a\ ck orientations present the chain axes of the crystalline phase parallel to the film plane while the ideal ak c\ orientation presents these chain axes perpendicular to the film plane. The present availability of semicrystalline films with three different kinds of uniplanar orientation of the d crystalline phase gives the opportunity to study the possible influence of the orientation of the host crystalline phase on guest diffusion kinetics. This can be more easily observed, for diffusion experiments conducted at low activities, which assure guest sorption essentially only by the nanoporous crystalline phase [67,68,93]. Transport kinetic studies have conducted by using 1,2-dichloroethane (DCE) [67], carbon dioxide [93], and ethylene [68], as low-molecular-mass guests. Particularly suitable for this kind of study are s-PS/CO2 clathrate films. In fact, as proved by the guest peak dichroism being high and nearly independent on desorption time (see above), the experimentally observed diffusivities can be considered a good approximation of the diffusivities into the crystalline nanoporous phase. CO2 desorption kinetics from 0.05 mm thick s-PS films, at room temperature and atmospheric pressure, are reported in Fig. 9. The kinetic curves, presenting the absorbance variations of the 658 cm1 p peak (A0  At)/A0 versus the square root of desorption time divided by film thickness ( t/L), are linear for a large absorbance range and clearly indicate a Fickian behavior. The results of Fig. 9 clearly indicate that the CO2 diffusivity is larger in the amorphous phases of atactic PS as well as of a-form s-PS films (D  8.0  108 cm2/s) while it is reduced of more than an order of magnitude, as a consequence of clathration into the host d phase (D [d, unoriented]  0.7108 cm2/s). Moreover the guest diffusivity is largely different for films with different uniplanar orientations, in particular D [d, akck]  0.2  108 6 D [d, akc\]  0.3  108 < D [d, unoriented] < D [d, a\ck]  1.6  108 cm2/s. In agreement with the previously described molecular dynamics simulations [83] and less precise desorption kinetics of 1,2-dichloroethane [93], the lowest diffusivity has been measured for films with

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1.0

(A0-At)/A0

0.8

aPS

0.6

α

0.4

(210) unoriented (002) (010)

δ 0.2 0.0 0

3000

6000

9000

12000

15000

18000

1/2

t /L (sec/cm) Fig. 9. Desorption isotherms of CO2 at room temperature from polystyrene films: (empty symbols) from amorphous phases of atactic-PS films and of s-PS films presenting the a-form; (filled symbols) from the nanoporous d phase of s-PS films, being  unoriented or presenting the three different kinds of uniplanar orientation sketched in Fig. 8, i.e. (0 1 0), ð210Þ or (0 0 2).

ak ck uniplanar orientation, presenting layers of close-packed enantiomorphous s(2/1)2 helices nearly parallel to the film surface. Moreover, the highest diffusivity has been measured for films with a\ ck uniplanar orientation, in agreement with the prediction, based on molecular simulation [83], that the a axis of the d form is the direction of maximum guest diffusivity. 6. Guest molecule orientation and dynamics Mobility of guest molecules confined in the crystalline cavity and resultant effects on the reorientational dynamics of the guests have been investigated in detail by means of different experimental [94–96] and molecular modeling techniques [83,97–99]. Particularly informative are solid state 2H NMR spectra of deuterated molecules being guest of cocrystalline s-PS phases [94,96], which have been compared (over a broad temperature range, 233– 333 K) to the spectra of the same molecules absorbed in the amorphous s-PS phase. Motional models, suggested also on the basis of 2H NMR line-shape simulation procedures, indicate a generally more restricted mobility for the considered aromatic molecules in the co-crystalline phases as opposed to the amorphous phase. Molecular reorientations on a micro second time scale, which are accessible to guest molecules in the co-crystalline phases, are highly restricted and essentially temperature independent for the examined temperature range. In particular, the spectra of the benzene-d6 guest molecules indicate that the motion of the benzene guest molecule is restricted to rotation about its C6 symmetry axis [94]. The spectra of toluene-d8 guest molecules can be described in terms of superposition of two Pake patterns indicating that the motion of the toluene guest molecule is restricted to rotation of the methyl group about its C3 symmetry axis [94,96]. Also the spectra of chlorobenzene-d5 guest molecules can be described in terms of superposition of two Pake patterns indicating that about 10% of the rings undergo an ill-defined 180° flip motions about the C–Cl axis [96]. The spectra of naphthalene-d8 guest molecules present a static Pake pattern with a splitting of 142 Hz, indicating the absence of motion of the naphthalene guest molecule, at least on a time scale for which the NMR line-shape is sensitive (s << 10 ls) [96]. The reorientational relaxation of benzene, toluene, and p-xylene, which are clathrated in the d form of crystalline syndiotactic polystyrene, has also been investigated using a molecular dynamics simulation [97]. For low temperatures (usually below 240 K), the 2H NMR spectra of aromatic molecules absorbed in the amorphous s-PS phase are similar to those of the corresponding guest molecules in the clathrate phase. However, as the temperature increases, substantial spectral changes are produced by a fraction of the molecules, which becomes involved in fast pseudo-isotropic reorientations, for temperatures

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81

well below the polymer glass transition. For samples containing guest molecules in both the amorphous and the clathrate phase, the quantification of the fractions of such molecules in each phase is feasible, based on the large differences in the respective 2H NMR spectra. 2 H NMR spectral changes also confirm that desorption from the amorphous phase is generally much more rapid than desorption from the clathrate phase [96]. This result is in agreement with the more restricted molecular mobility in the clathrate phase, as clearly established by the 2H NMR line-shapes. The average location and orientation of the guest molecules with respect to the crystalline axes of the host polymer phase, when the degree of occupancy of the crystalline cavities is high, can be generally determined by X-ray diffraction measurements, mainly on axially oriented films and fibers [28– 30,34]. When the degree of occupancy of the crystalline cavities is low, due to the related crystalline disorder, the X-ray diffraction method is not able to give reliable information relative to the guest location and orientation. However, infrared linear dichroism measurements on uniaxially oriented films easily allow establishing the orientation of the guest molecules with respect to the host chain axis [57,62,64]. In particular, accurate information relative to the orientation of the guest with respect to the chain axis of the polymeric crystalline host can be obtained from infrared dichroism measurements on films uniaxially stretched at different draw ratios. This procedure is described, just as an example, for the nitrobenzene guest [62]. The infrared order parameters of guest peaks S ¼ ðAk =A?  1Þ=ðAk =A? þ 2Þ, where Ak and A? are the measured absorbance for electric vectors parallel and perpendicular to the draw direction, are reported versus the orientation factor relative to the helical chains of the crystalline phase, as shown in Fig. 10. Oriented films were obtained by monoaxial stretching of the extruded ones, at different draw ratios up to 3.8 at constant deformation rate of 0.1 s1, in the temperature range 105–110 °C. The stretched films are still essentially amorphous and are crystallized into the nanoporous form by exposure for 3 days to CS2 vapors, followed by treatment under vacuum. The transition moment vector of in-plane vibrational modes at 1346 cm1 (NO2 s. stretching) and 851 cm1 (NO2 s. bending) are parallel to the C–N direction (x-axis in Fig. 11) while the transition moment vector of the vibrational mode at 1528 cm1 (NO2 as. stretching) is perpendicular to the C–N direction and in the ring plane (y-axis in Fig. 11). The vibration mode at 792 cm1 (CH bending out of plane) has the transition moment vector perpendicular to the nitrobenzene ring (z-axis in Fig. 11). In the plot of Fig. 10 positive and negative slopes correspond respectively to out-of-plane and in-plane vibrational modes, as generally observed for s-PS planar guest molecules [65]. By applying the equation

F c;IR

ðR  1Þ ð2cot 2 a þ 2Þ ðR þ 2Þ ð2cot 2 a þ 2Þ

ð2Þ

0.8 0.6

0.8 792

0.6 0.4

0.2

0.2

0.0

0.0

S

0.4

-0.2 -0.4 -0.6 0.00

-0.2

851 1346 1528

-0.4

0.20

0.40

0.60

0.80

-0.6 1.00

fc, IR Fig. 10. Order parameter S of infrared peaks of nitrobenzene guest molecules versus the axial orientation factor of the host polymer phase (fc,IR): (j) 792 cm1 (out-of-plane, along z of Fig. 11); (s) 851 cm1 (D) 1346 cm1 (in-plane, along x of Fig. 11); (h) 1528 cm1 (in-plane, along y of Fig. 11).

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Z αz=25°

c αx=77°

X

αy=72°

Y Fig. 11. Nitrobenzene orientation with respect to the c axis of the host d crystalline phase of s-PS.

to the data of Fig. 10, the a angle between the transition moment vectors and the c axis of the host crystalline phase have been calculated: a1346 = a851 = ax = 77°; a1528 = ay = 72°; a792 = az = 25° and the corresponding orientation is shown in Fig. 11. As for polymer host, due to the more complex molecular architecture, the mode assignment is more complicated. To this purpose a full vibrational spectra of alpha and beta crystalline phases of syndiotactic polystyrene, that is, phases presenting the trans-planar conformation, have been recently experimentally determined and compared with that calculated at the B3LYP/6-31G(d,p) level of theory for an infinite trans-planar chain [100]. As for the assignment of full vibrational spectra of gamma, delta and epsilon crystalline phases, due to the higher difficulty in the modeling of a helical conformation, this result has been finally achieved very recently [101]. The unique ability of the nanoporous d form of s-PS to host several guest molecules, imposing to them a high orientational order has also allowed to propose a new method for an easy and clearcut discrimination between in-plane and out-of-plane guest transition moment vectors [65]. Energy (light frequency) and intensity of electronic and vibrational transitions are widely used in structural chemistry. On the other hand, polarization, i.e. the direction in the molecular framework of the transition moment vectors, is much less studied. This is mainly due to the intrinsic difficulties in making polarization measurements, which need a molecular orientation control. The usual techniques to get molecular alignment are based on the use of anisotropic solvents, mainly of stretched polymers [102]. The organic molecule is absorbed in the amorphous phases of semicrystalline films of polyethylene or of polyvinylalchool, uniaxially stretched at high draw ratios. This method has been greatly successful and has been applied in several dozens of papers to hundreds of organic molecules [103–106]. The orientation control of the solute molecules is generally poor and the molecules are assumed to be oriented with their smallest cross section nearly perpendicular to the stretching direction (as schematically shown for indole in Fig. 12A). As a consequence, linear dichroism (LD) measurements do not allow an easy discrimination between in-plane and out-of-plane transition moment vectors, mainly for low-symmetry molecules. Organic molecules can be more efficiently oriented by absorbing them as guest of the crystalline nanoporous phase of uniaxially stretched syndiotactic polystyrene (s-PS) films. As usual for semicrystalline polymeric films, high degrees of crystalline phase orientation can be easily reached (e.g. orientation factors higher than 0.9 for draw ratios, final length/initial length ratio larger than 3) [64,65,107]. The new molecular alignment method not only ensures higher degrees of guest orientation but also a new kind of guest orientation. In fact, planar molecules are oriented with their smallest cross section nearly parallel to the stretching direction [28–30,32,65], as schematically shown for indole in Fig. 12B. As a consequence, in-plane and out-of-plane transition moment vectors maximize their absorption intensities for light polarization nearly perpendicular and parallel to the stretching direction, respectively. Hence, simple LD ¼ ðAk  A? Þ measurements by polarized spectra of uniaxially stretched s-PS

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Fig. 12. Schematic representation of the preferential orientation of a planar molecule (indole): (A) in the amorphous phase of uniaxially stretched polymeric films (B) in the cavity of the s-PS host d crystalline phase (along b projection; for clarity of presentation, only the guest molecule is represented as stick and balls). The arrows close to the draw direction remind that the stretched samples present a cylindrical symmetry.

films, including planar guest molecules into the nanoporous crystalline phase, can allow an easy discrimination between in-plane and out-of-plane transition moment vectors of the guest. The FTIR spectrum of indole molecules being guest (4.1 wt%) of the nanoporous phase of a uniaxially stretched s-PS films taken with polarization plane parallel (thick lines) and perpendicular (thin lines) to the draw direction are shown in Fig. 13. The spectra have been obtained by subtracting out the polarized spectra of the polymer host. The reported spectral ranges are those with most relevant indole peaks and with negligible disturbance from the s-PS peak subtraction. Due to the guest plane orientation nearly perpendicular to the draw direction (Fig. 12B), in-plane (i) and out-of-plane (o) vibrational modes can be immediately identified, since they maximize the absorption intensities for light polarization nearly perpendicular (A\) and parallel (Ak) to the stretching direction, respectively (see labeled peaks in Fig. 13). A detailed comparison between the FTIR peak positions (m, cm1) and their relative dichroism (LDr = LD/Aisotropic=3(Ak  A\)/(Ak + 2A\)) for indole molecules being absorbed in the amorphous phase of stretched polyethylene [108] or in the crystalline d phase of s-PS, is presented in Fig. 14. It is clearly apparent by histograms of the number of IR peaks (nm) versus LDr showing a poor (Fig. 14A) or a clearcut separation (Fig. 14B) between in-plane and out-of-plane vibrations, for indole molecules absorbed in the polyethylene amorphous phase [108] or in the s-PS d crystalline phase, respectively.

0.08

i

0.08

i

0.08

Absorbance

0.06 0.06

0.06

i i

0.04

o

o

i i

0.10

0.04

0.04

0.05 0.02

0.02

i 0.00

0.00 3500

3400

1400

1300

0.02

0.00 1200

0.00 600

400

cm-1 Fig. 13. FTIR spectra of indole being guest of the nanoporous d phase of uniaxially stretched s-PS films, taken with polarization plane perpendicular (thin lines) and parallel (thick line) to the draw direction. The spectra have been obtained by subtracting the polarized spectra of the polymer host. Due to the guest plane orientation nearly perpendicular to the draw direction (Fig. 11B) in-plane (i) and out-of-plane (o) vibrational modes can be immediately identified.

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A

nν

B

8

6

7

5

i

6 5

o

nν

i

4

4 3

3

o

2

2 1

1 -0.5

0.0

0.5

LD

r

1.0

1.5

-0.5

0.0

0.5

LD

1.0

1.5

r

Fig. 14. Histograms of the number of IR peaks (nm) versus linear dichroism (LDr) for indole molecules absorbed in the polyethylene amorphous phase (A) or in the s-PS d crystalline phase (B).

7. Conclusions and perspectives The structural studies on syndiotactic polystyrene have allowed achieving several exciting new materials. In particular, the unprecedented achievement of polymeric nanoporous crystalline phases (d and e) has given very interesting results in the fields of molecular separations, water/air purification and sensorics. Moreover, several kinds of polymer co-crystalline host–guest phases have been prepared, belonging to three different classes: clathrates with guest molecular planes perpendicular and parallel with respect to the polymer chain axis (obtained by guest sorption in d and e s-PS phases, respectively) and intercalates. Polymer co-crystals with active guest molecules have also been prepared, which show unusual physical properties and hence are promising for several kinds of advanced optical materials. As for the perspectives, studies that are possibly relevant to the understanding at molecular level of the behavior of nanoporous and co-crystalline phases are firstly discussed. Molecular dynamics simulations will be useful to rationalize and possibly predict orientation and mobility of guest molecules in the channels of the new nanoporous e phase. These studies should help to rationalize differences between guest sorption kinetics, observed for the two nanoporous crystalline phases, and also to possibly predict proximity between guest molecules which could favor their intermolecular reactivity. In fact, reactions between guest molecules in the channels of the polymer host e phase could open the possibility to achieve completely new classes of hybrid materials. Interactions of gaseous guest molecules with both nanoporous crystalline phases of s-PS will be also studied, both experimentally and by molecular simulation methods. Most studies will be devoted to hydrogen sorption, also due to the interesting preliminary results of hydrogen absorption in crosslinked microporous amorphous polystyrene [109]. Modeling studies will be conducted also on ethylene sorption and dynamics, also in the attempt to rationalize recent results which have shown that d form s-PS films are suitable for (also repeated use) food packaging requiring ethylene (well known fruit ripening hormone) removal [68]. Molecular modeling is also needed to try to understand recently published results relative to the fluorescence of polymer co-crystalline systems, which could be relevant for optical and optoelectronic applications [63]. For instance, for s-PS/tri-methyl-benzene films, fluorescence phenomena are essentially additive when the chromophore is simply absorbed in the polymeric amorphous phase or isolated guest of the clathrate co-crystal while the fluorescence of the intercalate s-PS/TMB co-crystal is enhanced and red-shifted with respect to both host and guest emissions [63]. As for possible application perspectives, a relevant objective will be the modification, with different kinds of functional groups, of the amorphous phase of semicrystalline s-PS films presenting the nanoporous crystalline phases. This material modification by functionalization of only the amorphous phase could bring several advantages, like for instance increase of rates of guest sorption from the nanoporous phases.

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Very recent studies have also shown that suitable d form s-PS films are suitable for detection and memory of nonracemic molecules [56]. These studies will be extended trying to establish if these kinds of films could be suitable as chiro-optical memories. As for perspectives of applications of s-PS based co-crystalline phases, studies will be mainly devoted to films, also trying to exploit the unique availability of three different kinds of uniplanar orientations, which allow also macroscopic control of the guest orientation. In recent years, advanced materials with special optical properties were mainly developed while for the future, by using co-crystalline phases with highly-polar or paramagnetic guest molecules, the achievement of relevant materials with special electric and magnetic properties are expected. The field of co-crystalline and nanoporous polymer materials is expected to be widely expanded. 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