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Back | Show only these items: | Urethane cross-linked poly(oxyethylene)/siliceous nanohybrids doped with Eu3+ ionsPart 1. Coordinating ability of the host matrix

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Urethane cross-linked poly(oxyethylene)/siliceous nanohybrids doped with Eu³⁺ ionsPart 1. Coordinating ability of the host matrix

Type: Object | Advantage: None | Novelty: New | ConceptID: Obj1

Fourier transform mid-IR and Raman spectroscopies were extensively used to examine cation/polymer and cation/cross-link interactions, as well as hydrogen bonding, in Eu³⁺-doped sol–gel derived organic/inorganic materials (monourethanesils).

Type: Object | Advantage: None | Novelty: New | ConceptID: Obj1

The hybrid framework of these xerogels contains short methyl end capped polyether segments covalently bonded to a siliceous backbone through urethane groups.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res1

The cations were incorporated as europium triflate, Eu(CF₃SO₃)₃.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res2

Samples with compositions ∞ > n ≥ 10 (where n indicates the ratio of (OCH₂CH₂) moieties per lanthanide ion) were investigated.

Type: Object | Advantage: None | Novelty: New | ConceptID: Obj2

The spectral data obtained provided unequivocal evidence that the cations coordinate exclusively to the urethane carbonyl oxygen atoms in compounds with 400 ≥ n ≥ 50.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res3

The participation of the ether oxygen atoms of the polymer chains in the Eu³⁺ complexation process is initiated at n = 30.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res4

The presence of a crystalline phase was detected at n = 10.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res5

The variation of the glass transition temperature of the monourethanesils studied with salt concentration is in perfect agreement with these claims.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res6

The beginning of the Eu³⁺–polyether interaction is accompanied by a breakdown of the hydrogen bonded aggregates of the matrix, as confirmed by photoluminescence spectroscopy.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res7

Introduction

The development of advanced nanoscale organic/inorganic frameworks with potential application in many fields has attracted considerable attention in the last few years.¹

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac1

The combination of the hybrid strategy, that has inherited the advantages of the sol–gel chemistry, with the host–guest concept of polymer electrolytes^2–4 has aroused special interest in the domain of optics.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac2

In this context, a family of intensely luminescent nanohybrid xerogels with outstanding features was introduced.^5–7

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac3

The hybrid host employed contains poly(oxyethylene), POE, chains of variable length grafted at both ends to a siliceous backbone through urea linkages (–NHC(O)NH–).

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac3

This framework (diureasil)⁸ was represented by U(Y) (Y = 2000, 900 and 600), where U indicates the urea group and Y denotes the average molecular weight of the organic precursor used.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac3

The active constituents are in general Eu³⁺ ions, added as a triflate salt, Eu(CF₃SO₃)₃, whose concentration may be taken to higher levels than those normally used in conventional composite materials.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac4

The Eu³⁺-doped long chain diureasils, processed as highly transparent monoliths,^6a,6b,9,10 are acceptable ion conductors^6a,b,10b and behave like white-light phosphors.^6a,b,9,11 Their most remarkable feature is the possibility of tuning their color along the Commission Internationale d'Éclairage chromaticity diagram by simply varying either the salt content or the excitation wavelength.^9c

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac4

The multifunctionality of the diureasils containing Eu³⁺ ions derives from the coordinating ability of the hybrid matrix itself.^6d,e,9,10 The activation of the coordinating sites of this POE/siloxane structure (the oxygen atoms of the urea carbonyl groups and the oxygen atoms of the polyether chains) can be easily controlled by changing either the salt content at constant chain length or the length of the organic segments at constant salt concentration.^6d,e,10

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac4

In the present work urethane cross-linked xerogels doped with Eu(CF₃SO₃)₃–a family of close analogs of the diureasils–will be thoroughly investigated with the primary goal of elucidating the structure/properties relationship.

Type: Object | Advantage: None | Novelty: New | ConceptID: Obj3

The organic/inorganic host of these compounds (monourethanesils)^12,13 is composed of short methyl end capped POE chains covalently bonded to a siliceous network through urethane moieties (–NH(CO)O–).

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met1

This matrix was designated as m-Ut(Y′) (Y′ = 2000, 750 and 350, corresponding to about 45, 17 and 7 (OCH₂CH₂) repeat units, respectively),¹² where m denotes mono, Ut represents the urethane group and Y′ indicates the average molecular weight of the starting organic material.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met1

The creation of monohybrids, through the reduction of the number of cross-links/polymer segment from two to one, allows to simplify the complicated structure of the diureasils.

Type: Method | Advantage: Yes | Novelty: Old | ConceptID: Met1

Preliminary studies of the luminescence features of the Eu³⁺-doped monourethanesils suggested that the local chemical environment of the cation is similar to that found in the corresponding diureasil materials.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met2

Thus, despite having poorer mechanical properties, the monourethanesils may be viewed as good model compounds of the diureasils in terms of cationic coordination.

Type: Method | Advantage: Yes | Novelty: Old | ConceptID: Met2

Therefore, an in-depth characterization of the monourethanesil system will help to enlighten the complex behavior of the diureasils and clarify many questions that remain unanswered regarding the cationic and anionic environments in the salt-doped diurea cross-linked hybrids.

Type: Motivation | Advantage: None | Novelty: None | ConceptID: Mot1

The systematic analysis that we have decided to perform on a series of m-Ut(750)-based monourethanesil samples containing a wide concentration range of Eu(CF₃SO₃)₃ will be divided into two parts.

Type: Method | Advantage: None | Novelty: New | ConceptID: Met3

Part 1 will be devoted to the study of cation/polymer and cation/cross-link interactions.

Type: Method | Advantage: None | Novelty: New | ConceptID: Met3

Fourier Transform mid-IR (FT-IR) and Raman (FT-Raman) spectroscopies will be employed extensively to check if the methyl-terminated POE chains of m-Ut(750) participate in the complexation of the Eu³⁺ ions, since rich information regarding the cationic environment is provided by cation-induced changes in vibrational motions of conformation sensitive polymer bonds^14–23 and in metal ion–oxygen stretching motions.^18–23

Type: Goal | Advantage: None | Novelty: None | ConceptID: Goa1

The “amide I” and “amide II” regions²⁴ will be also inspected to estimate the degree of hydrogen bonding in the monourethanesil materials^25,26 and to determine the role of the urethane carbonyl oxygen atoms in the Eu³⁺ coordination mechanism.²⁷

Type: Goal | Advantage: None | Novelty: None | ConceptID: Goa2

This work will be complemented with data retrieved from photoluminescence spectroscopy and differencial scanning calorimetry.

Type: Method | Advantage: None | Novelty: New | ConceptID: Met3

The scope of Part 2 will be to examine ionic association in the monourethanesil hybrids in order to gain additional insight into the local chemical environment of the lanthanide ions.

Type: Method | Advantage: None | Novelty: New | ConceptID: Met3

Cation–anion interactions will be evaluated through the analysis of characteristic modes of the triflate ion in the FT-IR and FT-Raman spectra of the same set of samples considered in Part 1.

Type: Goal | Advantage: None | Novelty: None | ConceptID: Goa3

Results and discussion

Vibrational spectroscopy

Cation/polymer interactions

Several vibrational modes of POE are extremely sensitive to the interaction of the polymer backbone with cations and can be thus employed as diagnostic tools to monitor the changes undergone by the polyether chains upon the addition of the guest salt.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac5

Classically, two spectral regions are widely used in the field of polymer electrolytes to probe the coordination of the cations to the host polyether:^4,14–23 (1) Skeleton CO stretching, ν(CO), region.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac6

In this frequency interval (1200–1000 cm^–1) the solvation of the cations by the heteroatoms of the polyether chains produces a distinct shift of the intense ν(CO) band to lower wavenumbers.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac6

The magnitude of this shift depends on the strength of the interaction.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac6

(2) CH₂ rocking, ρ_r(CH₂), region.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac7

Between 1000 and 800 cm^–1 the bands ascribed to a mixture of CH₂ rocking modes and CC stretching vibrations (from 1000 to 900 cm^–1)²⁸ and the bands originated from CH₂ rocking modes coupled with CO stretching vibrations (from 900 to 800 cm^–1)²⁸ are found.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac7

Their frequency is strongly dependent on the –O–C–C–O– angle and hence on the polymer local conformation.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac7

The room-temperature mid-IR spectra of the m-Ut(750)_nEu(CF₃SO₃)₃ family of materials analyzed in the present work are collected in Fig. 1.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs1

The assignment of the main bands produced by the polymer chains of the same hybrid samples is given in Table 1.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res8

Fig. 2 shows that in the ν(CO) region the IR spectrum of the non-doped monourethanesil matrix displays a peak around 1113 cm^–1 and a shoulder at about 1143 cm^–1 ascribed, respectively, to the CO stretching vibration mode and to the coupled vibration of the CO stretching and CH₂ rocking modes (Table 1).

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs2

In the spectra of the m-Ut(750)_nEu(CF₃SO₃)₃ xerogels with 400 ≥ n ≥ 30, both the 1113 cm^–1 feature, characteristic of non-coordinated oxyethylene moieties, and the 1143 cm^–1 shoulder persist practically unchanged as increasing amounts of Eu(CF₃SO₃)₃ are incorporated into the m-Ut(750) host (Fig. 2).

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs3

In the low-frequency side of the ν(CO) envelope, the only change worth noting in the spectra of the same Eu³⁺-doped compounds is the growth of a shoulder near 1068 cm^–1 at n = 30 (Fig. 2, Table 1).

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs4

The further addition of Eu(CF₃SO₃)₃ (i.e., n = 10) has, however, tremendous implications.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs5

Fig. 2 and Table 1 clearly illustrate that the m-Ut(750)₁₀Eu(CF₃SO₃)₃ material gives rise to a series of extraordinary changes: (1) the very strong band centered at approximately 1113 cm^–1 disappears; (2) the shoulder seen at about 1068 cm^–1 in the spectrum of m-Ut(750)₃₀Eu(CF₃SO₃)₃ is transformed into a strong event at 1072 cm^–1; (3) two new intense features are distinctly observed at 1115 and 1105 cm^–1; (4) two new strong components appear at 1097 and 1057 cm^–1, together with two shoulders located at 1123 and 1063 cm^–1.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs5

Bernson et al¹⁴. reported that in the case of the electrolytes formed with La(CF₃SO₃)₃ and POE all the (OCH₂CH₂) moieties are virtually bonded to the cations at n = .9¹⁴

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met4

The characteristic band attributed to La³⁺-coordinated ether oxygen atoms was a single event at 1073 cm^–1.¹⁴

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met4

It is noteworthy that in the case of a POE-based system doped with lithium triflate, LiCF₃SO₃, a crystalline complex was formed for composition n = 3,^17ai.e., a composition that in terms of anionic concentration corresponds to n = 9 for a trivalent cation.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met5

Moreover, the frequency of a characteristic spectral feature of the Li⁺-based compound appeared at 1092 cm^–1.^17a In addition, in the spectra of the NH₄CF₃SO₃-based polymer electrolytes with n < 8^17b and in that of the POE₁KCF₃SO₃ crystalline compound,^17c the broad ν(CO) band at 1102 cm^–1 was replaced by two peaks at 1116 and 1107 cm^–1.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met4

Considering the remarkable coincidence between the spectral features just indicated and those of m-Ut(750)₁₀Eu(CF₃SO₃)₃, it seems logical to associate the new bands appearing in the ν(CO) region of the latter material to a salt-rich crystalline complex.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res9

Further evidences of the formation of a crystalline phase in the m-Ut(750)₁₀Eu(CF₃SO₃)₃ compound may be found in the ρ_r(CH₂) region.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res10

Fig. 3 shows that the main feature of the IR spectra of the monourethanesil samples with ∞ > n ≥ 30 within the 980–900 cm^–1 region is a peak at approximately 955 cm^–1 and two shoulders at lower frequency (Table 1).

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs6

In this range of compositions the incorporation of increasing amounts of lanthanide salt to the hybrid matrix does not modify the intensity and position of these three spectral events.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs7

In contrast, in the spectrum of m-Ut(750)₁₀Eu(CF₃SO₃)₃ the band envelope becomes considerably stronger and a new, intense sharp feature develops at 937 cm^–1 (Fig. 3, Table 1), characteristic of crystalline high molecular weight poly(ethylene glycol)dimethyl ether, PEGDME.^28c

Type: Result | Advantage: None | Novelty: None | ConceptID: Res11

Fig. 3 also reveals that the dominant feature of the compounds with ∞ > n ≥ 30 in the 900–800 cm^–1 interval is a band centered at approximately 847 cm^–1.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs8

Several shoulders are also observed (Table 1).

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs9

Within this range of salt concentration the intensity and position of the bands remain essentially unaffected.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs10

Again, the contour of the envelope is drastically modified in the spectrum of the xerogel with n = 10, the most remarkable change being the presence of a new sharp, medium intensity band located at 843 cm^–1 (Fig. 3, Table 1).

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs11

This event was also assigned to crystalline PEGDME.^28c Interestingly, the crystalline complexes POE₁NH₄CF₃SO₃ and POE₁KCF₃SO₃ exhibit bands around 933 and 841 cm^–1^17b and 936 and 843 cm^–1,^17c respectively.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res12

Although less sensitive to alterations in the backbone conformation, the CH₂ scissoring and CH₃ deformation modes, which appear in the 1500–1400 cm^–1 frequency interval, and the CH₂ wagging modes, situated in the 1400–1300 cm^–1 spectral range, can be also used to complement the conclusions drawn from the two spectral regions discussed above.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res13

A close examination of Fig. 4 and Table 1 allow us to state that, between 1500 and 1300 cm^–1, the spectra of the monourethanesil compounds with n > 10 closely resemble the spectrum of m-Ut(750).

Type: Result | Advantage: None | Novelty: None | ConceptID: Res14

It may be also immediately inferred that, in the same region, the spectral signature of the host hybrid is deeply modified at n = 10.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs12

The most significant change detected between 1500 and 1400 cm^–1 in the spectrum of the latter sample is the presence of a new band of medium intensity at 1467 cm^–1 and several new shoulders (Fig. 4, Table 1).

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs12

In the CH₂ wagging region of m-Ut(750)₁₀Eu(CF₃SO₃)₃ a significant enhancement of the shoulder located at 1360 cm^–1 occurs (Fig. 4, Table 1).

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs13

The features at 1467 and 1360 cm^–1 are also produced by crystalline PEGDME,^28c thus corroborating the fact that in the most concentrated sample the disordered polyether chains of m-Ut(750) adopt regularly ordered conformations to bond to the Eu³⁺ ions.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res15

The intense band situated at approximately 872 cm^–1 in the Raman spectrum of the most concentrated xerogel (Fig. 5) represents another spectroscopic proof of the interaction between the Eu³⁺ ions and the ether oxygen atoms of the oligopolymer chains at this salt concentration.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res16

The 872 cm^–1 event is visible already in the Raman spectrum of m-Ut(750)₃₀Eu(CF₃SO₃)₃ as a very weak shoulder (Fig. 5).

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs14

The absence of this feature in the Raman spectra of the xerogels with lower salt content (Fig. 5) suggests that the participation of the oxyethylene moieties in the cation complexation process is initiated near the composition at which the saturation of the urethane cross-links is attained, i.e., at n = 17.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res17

In analogy with previous authors,^18–23 we will associate the 872 cm^–1 band to a stretching vibration mode involving wrapping of the pendant oligopolyether segments of m-Ut(750) around the Eu³⁺ ion.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res18

This mode, represented by ν(M–O_n) and usually termed as “oxygen breathing” mode, is typically seen in the Raman spectra of complexes of crown ethers and metal cations.²⁹

Type: Result | Advantage: None | Novelty: None | ConceptID: Res18

On the basis of the model proposed by Papke et al.,¹⁸ it is accepted that the formation of the cation/ether oxygen complex favors a gauche conformation of the C–C bonds.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac8

In this context it is fundamental to point out that a band indicative of the trans conformations of the –O–C–C–O– sequences^30,31 and centered at about 810–805 cm^–1 is distinctly discerned in the spectra of the doped xerogels with 400 ≥ n ≥ 30 (Fig. 5).

Type: Result | Advantage: None | Novelty: None | ConceptID: Res19

The drastic reduction of the intensity of this feature in the Raman spectrum of the m-Ut(750)₁₀Eu(CF₃SO₃)₃ material (Fig. 5) is consistent with the marked decrease in the relative amount of trans conformations that occurs at the expense of the increase of the proportion of gauche ones.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res19

It should be stressed that these spectral results do not provide any information regarding the number of pendant oligopolyether chains of the monourethanesil structure that are used to coordinate each cation.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res20

Powder diffraction studies demonstrated that in the crystalline complexes POE₃LiX (where X = CF₃SO₃^32a,b and N(SO₂CF₃)₂, bis(trifluoromethanesulfonate)imide^32c), POE₃NaClO₄^32b and POE₄MSCN (where M = K, Rb and NH₄)^32b each polymer chain forms an isolated one-dimensional coordination complex.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met6

In all these structures the monovalent cations are located within the POE helix.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met6

No ionic cross-linking was observed in any case.^32a,b The structures were preserved on going from the crystalline to the amorphous state.^32a,b There are no diffraction data available in the literature regarding POE-based complexes doped with Eu(CF₃SO₃)₃.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met6

However, considering that the ionic radius of Eu³⁺ lies between those of Li⁺ and Na⁺, it seems logical to assume henceforth that “POE–Eu³⁺ coordination” corresponds to the interaction of the lanthanide ion with several ether oxygen atoms of a single polymer chain.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met6

A third pronounced effect discerned in the Raman spectra reproduced in Fig. 5 helps to substantiate the formation of cation/polyether bonds in the most concentrated monourethanesil.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res21

It involves the growth of a series of bands between 925 and 885 cm^–1 in the two most concentrated samples as the content of guest lanthanide salt is increased (Fig. 5).

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs15

The curve-fitting performed on this region (not shown) allowed us to conclude that these new events are situated at approximately 918, 911 (shoulder), 907, 902, 896 and 893 cm^–1.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res22

Furlani et al²⁰. also found a band centered at about 906 cm^–1 in the Raman spectra of PEG(400)-based electrolytes doped with Eu(N(SO₂CF₃)₂)₃.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met7

We therefore conclude that the significant transformation of the spectral profiles in m-Ut(750)₁₀Eu(CF₃SO₃)₃ are caused by an increase in the local order of the polymer segments arising from their interaction with the Eu³⁺ ions and the formation of crystalline complexes.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con1

It must be stressed that we cannot determine the extent of crystallization occurring in the system at n = 10.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con2

However the changes observed in the above discussed spectra unambiguously prove the presence of crystalline domains.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con2

Therefore we tentatively describe the m-Ut(750)₁₀Eu(CF₃SO₃)₃ xerogel as a material containing a significant fraction of crystalline phase(s).

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con2

Cation/cross-link interactions

We provided above strong arguments that support the claim that the cation coordination process that occurs in the m-Ut(750)-based xerogels containing Eu(CF₃SO₃)₃ closely resembles that reported for the long chain diureasil parent analogs U(2000)_nEu(CF₃SO₃)₃.^10,11a Similarly to what happens in the latter compounds, in the monourethanesils examined in the present study the pendant methyl end capped polyether segments start to solvate the guest lanthanide ions near the cross-link saturation composition.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con3

Another noteworthy similarity found between both hybrid systems is the formation of a crystalline compound at high salt content.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res23

It is important to note that the number of hydrogen donor sites present in the urethane and urea groups is not the same: the urethane linkage contains just one N–H group, whereas the urea moiety is composed of two N–H groups.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac9

This implies that, while the CO moiety of a urethane group may form a single hydrogen bond with the N–H moiety of another urethane group, urea groups of neighboring molecules may be linked by means of planar bifurcated hydrogen bonds.³³

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac9

As a consequence, the hydrogen bonding geometry of the urethane groups might be substantially different from that of the urea groups.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac10

Such situation resulted for instance in the tighter packing of adjacent chains observed in bis-urea compounds with respect to the bis-urethane analogues.^33a The distance between the carbonyl carbon atoms of two adjacent urea molecules and two adjacent urethane molecules in these materials was found to be 4.63 and 5.10 Å, respectively.^33a The hydrogen bonding behavior in the essentially amorphous diureasils and monourethanesils should be, however, quite different from that occurring in the compounds reported in ^{ref. 33a}, which are highly crystalline.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac10

Indeed, the IR spectra demonstrate the presence of a large number of “free” CO groups in the matrices based on U(2000) and on the investigated m-Ut(750), as will be shown below.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res24

Hence, we do not expect a significant difference in hydrogen bond strengths in both hybrid systems.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con4

To complete the description of the chemical environment around the lanthanide ions in the m-Ut(750)_nEu(CF₃SO₃)₃ hybrids, we will investigate in this section the spectral signature of these composites in the “amide I” and “amide II” regions, as it seems logical to presume that the Eu³⁺ ions will very likely bond to the carbonyl oxygen atom of the urethane cross-links in the monourethanesil xerogels with 400 ≥ n ≥ 50.

Type: Object | Advantage: None | Novelty: New | ConceptID: Obj4

This study will enable us to assess in parallel the extent of hydrogen bonding in this family of xerogels.

Type: Goal | Advantage: None | Novelty: None | ConceptID: Goa4

Our analysis of the “amide I” and “amide II” bands of the Eu³⁺-doped monourethanesil compounds will take into account numerous spectroscopic studies of simple and segmented poly(urethane)s,²⁵ of poly(urethane urea) copolymers²⁶ and of lithium-doped polymer electrolytes based on a thermoplastic polyurethane^27a–c or a segmented polyether poly(urethane urea)^27d host.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met8

“Amide I” region

The “amide I” region of poly(urethane)s²⁵ corresponds to the amide I region of polyamides.³⁴

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac11

100

The amide I mode (or simply, the carbonyl stretching mode) is a highly complex vibration that is comprised of a major contribution from the CO stretching vibration and less important contributions from the C–N stretching and C–C–N deformation vibrations.³⁴

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac12

101

As the CO stretching vibration is sensitive to the specificity and magnitude of hydrogen bonding, the amide I envelope consists of several distinct components reflecting different environments of CO groups, usually termed as associations, aggregates or structures.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac12

102

Hence quantitative analysis is feasible in the amide I region only if the difference in the absorption coefficients of the non-bonded and bonded carbonyl bands is taken into account.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac12

103

Band area determinations may be thus considered an adequate reflection of functional group concentration.^25d,34

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac12

104

The “amide I” envelope of the salt-free m-Ut(750) matrix (Fig. 7) contains three distinct components centered at about 1750, 1720 and 1700 cm^–1 (Fig. 8).

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs16

105

The event observed near 1750 cm^–1 is inherent to urethane cross-links free from hydrogen bonding interactions, i.e., urethane linkages whose N–H and CO groups remain non-bonded (Scheme 1, A).²⁵

Type: Result | Advantage: None | Novelty: None | ConceptID: Res25

106

On the basis of previous works,²⁵ we propose that the 1720 cm^–1 feature is related to the absorption of hydrogen-bonded CO groups in disordered aggregates (urethane–polyether association of Scheme 1, B) and that the component at 1700 cm^–1 corresponds to the absorption of CO groups included in a much more ordered hydrogen-bonded aggregate (urethane–urethane association illustrated in Scheme 1, C).

Type: Result | Advantage: None | Novelty: None | ConceptID: Res26

107

Fig. 7 provides unequivocal evidence that the addition of the lanthanide salt to the m-Ut(750) hybrid matrix produces a series of pronounced changes in the “amide I” region.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs17

108

In general terms, we may state that, as the amount of guest salt rises, the center of gravity of the whole “amide I” profile shifts toward lower frequencies, suggesting that the strength of the hydrogen bonds globally increases.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res27

109

In the spectra of the monourethanesils with high salt concentration a new component located at 1665 cm^–1 emerges.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs18

110

With the increase of Eu(CF₃SO₃)₃ content, while the intensity of this feature is markedly enhanced, that of the 1750 cm^–1 mode, originated from free CO groups, is significantly reduced.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs19

111

In the spectrum of the rich-salt material with n = 10 the 1665 cm^–1 band becomes the most prominent event and the 1750 cm^–1 peak disappears.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs20

112

The spectral findings just described sustain the hypothesis that the Eu³⁺ ions coordinate to the carbonyl oxygen atoms of the urethane cross-links over the whole range of composition studied.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con5

113

Another conclusion that may be immediately drawn from the spectra reproduced in Fig. 7 is that the band position of the three components of m-Ut(750) situated at about 1750, 1720 and 1700 cm^–1 is totally unaffected by salt doping.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res28

114

Van Heumen et al.^27b also observed this effect in lithium triflate-based thermoplastic polyurethanes.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac13

115

Because of the lability of hydrogen bonds, it was fundamental to study the evolution of the spectra of the monourethanesils as a function of temperature.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met9

116

As the spectra recorded at three different temperatures (see Experimental section) are coincident in the “amide I” region, we may deduce that below 85 °C the disordered and ordered hydrogen-bonded aggregates of the m-Ut(750)_nEu(CF₃SO₃)₃ hybrids are totally insensitive to temperature, a proof of their stability.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res29

117

To gather additional information of the modifications undergone by the characteristic modes of the “amide I” region of the monourethanesil xerogels upon inclusion of increasing amounts of the triflate salt, we performed curve-fitting in the 1800–1630 cm^–1 frequency envelope.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met10

118

The complex spectral profiles were decomposed into Gaussian bands.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met10

119

The results obtained with three representative doped samples are illustrated in Fig. 8.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs21

120

The composition dependence of the area of the bands at 1750, 1720, 1700 and 1665 cm^–1 is shown in Fig. 9.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res30

121

Fig. 9 allows to infer that the presence of a growing amount of Eu³⁺ ions in the monourethanesil system has several important consequences:

Type: Result | Advantage: None | Novelty: None | ConceptID: Res31

122

(1) From n = 400 to 30 the fraction of non-bonded CO groups is progressively reduced.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res32

123

At n = 10 the 1750 cm^–1 event vanishes completely, indicating that no CO groups are left free.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res33

124

(2) In the 400 ≥ n ≥ 70 composition interval, the amount of hydrogen-bonded CO groups belonging to disordered regions increases.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res34

125

At higher salt concentration, a considerable amount of these aggregates are, however, disrupted.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res35

126

In the most concentrated xerogel only weak traces of the hydrogen-bonded associations B (Scheme 1) are found.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res35

127

The drastic destruction of these urethane–polyether hydrogen-bonded structures at n = 10 derives most probably from the fact that, at this salt concentration, the polymer chains are being extensively required to interact with the cations and can no longer bond to the N–H groups of urethane cross-links via hydrogen-bonding.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con6

128

(3) The area of the feature at 1700 cm^–1 suffers an increase in hybrids with 400 ≥ n ≥ 90, but decreases rapidly upon further addition of Eu(CF₃SO₃)₃.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res36

129

This abrupt loss of band area may be interpreted as a solid indication that the introduction of the guest salt deeply perturbs the ordered aggregates C (Scheme 1) of the monourethanesil samples, presumably leading to their breakdown.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con7

130

A minor proportion of such hydrogen-bonded structures remains at n = 10.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res37

131

(4) In compounds with 70 ≥ n ≥ 30 a new component, that gradually grows with the increase of salt content, appears at approximately 1665 cm^–1.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs22

132

In the “amide I” spectral region of m-Ut(750)₁₀Eu(CF₃SO₃)₃ this band becomes dominant.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs22

133

The trend demonstrated by the various components of the “amide I” envelope of the monourethanesils confirms that the most concentrated xerogel studied is subject to a tremendous structural transition as a result of the formation of the crystalline polyether/lanthanide salt complex.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con8

134

The nature of the species that give rise to the intense 1665 cm^–1 feature is unknown at this stage.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con9

135

Because of its low frequency, we feel tempted to speculate that it is associated with highly ordered, strongly hydrogen-bonded aggregates, formed at the expense of the breakdown of practically all the previously existent ones (associations B and C of Scheme 2).

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con9

136

The regular increase of this band with the increase of salt content reveals that the new hydrogen-bonded aggregates are strongly coordinated to the Eu³⁺ ions.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res38

137

It is instructive to comment that the interpretation proposed for the “amide I” region of the monourethanesil materials relies essentially on the fact that the host hybrid m-Ut(750) matrix is only slightly hygroscopic, a result of the utmost relevance, since it indicates that in the doped samples the main species responsible for the presence of water is the guest lanthanide salt, owing to the typical great water affinity of the Eu³⁺ species.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con10

138

This explains why the almost complete removal of water from the salt-containing compounds could be easily accomplished through classical procedures, such as drying under vacuum at room temperature (see Experimental section), as confirmed by the low intensity of the characteristic broad OH stretching envelope in the high-frequency region of the FT-IR spectra reproduced in Fig. 1.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con10

139

Thus any contribution of the water bending mode, which appears typically near 1640 cm^–1 in pure water, to the 1665 cm^–1 feature found in the “amide I” envelope of the m-Ut(750)_nEu(CF₃SO₃)₃ xerogels with n lower than 70 is considered to be negligible.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res39

“Amide II” region

140

The “amide II” mode of poly(urethane)s is akin to the amide II feature of polyamides.³⁴

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac14

141

The amide II band (which is often simply designated as the N–H in-plane bending, δ_i.p. NH, band) is a mixed mode containing a major contribution from the N–H in-plane bending vibration and minor contributions from the C–N stretching and C–C stretching vibrations.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac15

142

It is sensitive to both chain conformation and intermolecular hydrogen bonding and adequately reflects the distribution of hydrogen bond strenghts.³⁴

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac15

143

In the spectra of the m-Ut(750)_nEu(CF₃SO₃)₃ xerogels with ∞ > n ≥ 30 the “amide II” mode appears as a single broad, medium to weak band centered at approximately 1534 cm^–1 (Fig. 7).

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs23

144

In the spectrum of m-Ut(750)₁₀Eu(CF₃SO₃)₃ the same feature undergoes a dramatic shift to 1568 cm^–1 (Fig. 7).

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs24

145

In the whole range of salt concentration analyzed we do not detect any indication of the presence of separate contributions presumably originated from hydrogen-bonded associations of various degrees of order (Fig. 7).

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs25

146

Moreover, there is no evidence for a distinct free component.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs26

147

The marked upshift of the “amide II” feature observed at n = 10 confirms that in this monourethanesil sample hydrogen bonding is much stronger than in the more dilute ones.^11a

Type: Result | Advantage: None | Novelty: None | ConceptID: Res40

148

The coincidence of the “amide II” region of the spectra recorded at three different temperatures for each composition (see Experimental section) leads us to state that at temperatures lower than 85 °C the “amide II” mode of the monourethanesil is a temperature independent event.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res41

149

This finding is consistent with the behavior of the materials in the “amide I” region.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res41

Differential scanning calorimetry

150

Rich information regarding cation/polyether complexation may be obtained from DSC measurements.⁴

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met11

151

In polymer electrolytes the interaction between the cations and the ether oxygen atoms of the polymer chains is known to be accompanied by the formation of transient ionic cross-links that partially arrest the local motion of the adjacent solvating segments.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met11

152

As a consequence the glass transition temperature, T_g, of the host polymer is shifted to higher temperatures.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met11

153

This effect may be quite dramatic in the case of rich-salt compounds.

Type: Method | Advantage: Yes | Novelty: Old | ConceptID: Met11

154

The dependence of the T_g of the m-Ut(750)_nEu(CF₃SO₃)₃ monourethanesils with composition is in accordance with the spectroscopic data presented above.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res42

155

Fig. 6 clearly shows that, as expected, the T_g of the hybrid matrix (–48 °C) remains nearly constant in samples with n > 30.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs27

156

This result confirms once more that the pendant oligopoly(oxyethylene) chains of m-Ut(750) are not involved in the coordination of the lanthanide ions in this range of composition.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res43

157

In addition, the same plot provides conclusive evidence that in xerogels with n = 30 and 10, Eu³⁺/polymer bonding occurs, leading to a marked increase of the T_g of the polyether segments (17 and 76 °C, respectively) (Fig. 6).

Type: Result | Advantage: None | Novelty: None | ConceptID: Res44

Photoluminescence spectroscopy

158

The photoluminescence studies carried out give reliable support to the spectroscopic results discussed in Section 1.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res45

159

The 14 K emission spectra obtained for the m-Ut(750)_nEu(CF₃SO₃)₃ monourethanesils with n = 200, 60 and 30 at the excitation wavelength that maximizes the cation luminescence intensity are reproduced in Fig. 10.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs28

160

These spectra are composed of a series of yellow–red sharp lines-assigned to intra-4f transitions between the ⁵D₀ and the ⁷F_0–4 levels^6a,9a,e-that overlap a large broad band in the purple–blue–green region.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res46

161

In the case of the most dilute sample examined here (n = 200), an intra-4f self-absorption, attributed to the ⁷F₀ → ⁵D₂ transition, is also observed (marked with an asterisk in Fig. 10).

Type: Result | Advantage: None | Novelty: None | ConceptID: Res47

162

It was recently demonstrated that the broad emission which already appears in the emission spectra of the non-doped diureasil and diurethanesil host frameworks^9d,11b may be expressed by the convolution of a long-lived emission from the NH groups of the urea and urethane bridges, respectively, with a short-lived component originated from electron–hole recombinations occurring in the siliceous domains.^9d The maximum intensity energetic position of the hybrid host emission and its relative intensity with respect to the Eu³⁺ lines strongly depend on the amount of guest salt incorporated.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met12

163

Fig. 10 shows, not only that the energy band of the m-Ut(750)₂₀₀Eu(CF₃SO₃)₃ xerogel is red-shifted with respect to those of the more concentrated compounds, but also that its intensity is greater than that of the ion lines.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs29

164

We may also infer from Fig. 10 that an increase in the salt concentration from n = 200 to 30 induces an inversion in the relative intensity of the emission of the hybrid host and that of the Eu³⁺ ions, since the yellow–red lines become clearly more intense in the spectrum of the salt-rich material.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res48

165

Nevertheless, variations on salt content do not affect the Eu³⁺ intra-4f lines:^6a,9a,b,e the energy, shape or relative intensity of the observed transitions remain unchanged (Fig. 10).

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs30

166

These observations substantiate the presence of a similar cation local coordination site within the entire concentration range considered.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con11

167

Furthermore, as the ⁵D₀ → ⁷F₂ forced electric-dipole transition is stronger than the ⁵D₀ → ⁷F₁ magnetic-dipole one, we are led to conclude that the point symmetry group of the Eu³⁺ ions in the m-Ut(750)-based monourethanesils does not have an inversion center,³⁵ a finding that completely discards the occurrence of a first coordination sphere for the lanthanide ions exclusively composed of water molecules.³⁶

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con11

168

This extremely important result is in perfect agreement with our claim that the local chemical environment of the cations in the m-Ut(750)_nEu(CF₃SO₃)₃ composites includes, apart from a minor number of oxygen atoms of water molecules, oxygen atoms from three different species: the urethane carbonyl moieties (in the whole range of salt composition analyzed here), the polymer oxyethylene units (in samples with n ≤ 30) and the triflate ions (see Part 2 of this series of papers).

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con11

169

The decrease in the relative intensity of the host broad band with respect to the Eu³⁺ lines observed as the amount of Eu³⁺ is increased from n = 200 to 30, already reported for similar organic/inorganic hybrids,^9a,b,e,10a may be explained in terms of energy transfer between the emitting centers of the matrix (donors) and the lanthanide ions (acceptors).^9e The energy transfer mechanism associated with the host emission component resulting from the photoinduced proton-transfer between urethane groups is enhanced as a consequence of the progressive suppression of the hydrogen bonds that occurs with the increase of salt content (as demonstrated in section 1), which leads to a delocalization of the proton, rendering the induced proton transfer between NH groups easier.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con12

170

This effect is manifested in the emission spectra through a decrease in the relative host broad band intensity.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con12

Conclusions

171

Monourethane cross-linked poly(oxyethylene)/siloxane hybrids incorporating a wide range of europium triflate, Eu(CF₃SO₃)₃, concentration (∞ ≥ n ≥ 10, where n is the molar ratio of (OCH₂CH₂) repeat units per Eu³⁺ ion) were investigated by means of several techniques to study the cation/polymer, cation/cross-link and hydrogen bonding interactions.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con13

172

The analysis of the specific vibrational modes of the POE chains, as well as of the “amide I” and “amide II” frequency envelopes, characteristic of the urethane cross-links, provided strong evidences that in these materials the coordination of the lanthanide ions takes place in the following way: (1) at n > 30 the cations interact exclusively with the carbonyl oxygen atoms of the urethane linkages; (2) at n = 30 the cations start to bond to the oxygen atoms of the polyether chains; (3) at n < 30 a dramatic breakdown of the matrix hydrogen-bonded aggregates occurs caused by cation entrapment; (4) at salt composition n = 10 a crystalline polymer/salt complex is formed.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con14

173

Photoluminescence spectroscopy clearly confirmed conclusion (3).

Type: Result | Advantage: None | Novelty: None | ConceptID: Res49

174

The composition dependence of the glass transition temperature of the xerogels studied corroborated conclusion (4).

Type: Result | Advantage: None | Novelty: None | ConceptID: Res50

175

The role of the anion in the coordinating behavior of the monourethanesils will be examined in Part 2 of this series of papers.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con15

Experimental

Synthesis

176

3-Isocyanatepropyltriethoxysilane (ICPTES, Fluka) and poly(ethylene glycol) methyl ether (PEGME, Aldrich, average molecular weight = 750) were used as received. tetrahydrofuran (THF, Merck) and ethanol (CH₃CH₂OH, Merck) were stored over molecular sieves.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met13

177

Europium(iii) trifluoromethanesulfonate (Eu(CF₃SO₃)₃, Aldrich) was dried under vacuum at 90 °C for several days prior to being used.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met13

178

High purity distilled water was used in all experiments

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met13

179

The preliminary stage of the preparation of the Eu³⁺-doped monourethanesils involved the formation of a covalent urethane linkage between the terminal hydroxyl group of a poly(ethylene glycol) methyl ether molecule containing about 17 oxyethylene units, PEGME(750), and the isocyanate group of an alkoxysilane precursor, ICPTES (Scheme 2).

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met14

180

The second stage of the synthesis implied the addition of water and ethanol, to start to hydrolysis and condensation reaction characteristic of the sol–gel chemistry (Scheme 2).

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met15

181

The nomenclature m-Ut(750)_nEu(CF₃SO₃)₃ was adopted to identify the final materials.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met16

182

Xerogels with n = 400, 200, 90, 70, 60, 50, 30 and 10 were prepared.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met17

183

An undoped sample, designated as m-Ut(750), was also obtained.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met17

Step 1: synthesis of the monourethanesil precursor, m-UtPTES(750)

184

PEGME(750) (2.5 g, 3.3 mmol) was dissolved in THF (10 mL) by stirring.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met18

185

ICPTES (0.823 mL, 3.3 mmol) was added to this solution in a fume cupboard.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met18

186

The flask was then sealed and the solution stirred for 24 h at moderate temperature (≈70 °C).

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met18

187

The structure of the transparent polydisperse oil obtained in such conditions, designated as monourethanepropyltriethoxysilane (m-UtPTES(750)), was confirmed by ¹H NMR, IR and HR-MS (FAB).¹H NMR (400.13 MHz, CDCl₃, 25 °C, TMS): δ 3.79 (q, ³J(H,H) = 7 Hz, 6H, H^d), 3.72–3.52 (m, 68H, H^g), 3.36 (s, 3H, H^f), 3.18–3.12 (m, 2H, H^c), 1.62–1.57 (m, 2H, H^b), 1.22–1.18 (m, 9H, H^e), 0.62–0.58 (m, 2H, H^a); IR (KBr): 1716, 1660 cm^–1 (ν(CO)), 1533 cm^–1 (δ_i.p.(NH)); HR-MS (FAB) M + H⁺: found 984.570282, calc.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res51

188

984.577463 for C₄₃H₉₀NO₂₁Si (H₃C(OCH₂CH₂)₁₆O(CO)NH(CH₂)₃Si(OCH₂CH₃)₃).

Type: Result | Advantage: None | Novelty: None | ConceptID: Res51

Step 2: synthesis of the monourethanesils, m-Ut(750)_nEu(CF₃SO₃)₃

189

Ethanol (0.776 ml, 13.3 mmol), an appropriate mass of EU(CF₃SO₃)₃ (Table 2) and water (90 µl, 5 mmol) were added to the m-UtPTES(750) solution prepared in the first stage of the synthesis procedure (molar proportion 1 ICPTES∶4 CH₃CH₂OH∶1.5 H₂O).

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met19

190

The mixture was stirred in a sealed flask for 30 min and then cast into a Teflon mould which was covered with Parafilm and left in a fume cupboard for 24 h.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met19

191

After a few hours gelation was already visible.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met19

192

The mould was transferred to an oven at 60 °C and the sample was aged for a period of 4 weeks.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met19

193

Transparent monolithic xerogels with a yellowish coloration were formed.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs31

Differential scanning calorimetry

194

A disk section with a mass of approximately 40 mg was removed from the monourethanesil film, placed in a 40 µL aluminium can and introduced in a dessicator over phosphorus pentaoxide for at least 12 h at room temperature.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp1

195

After this drying treatment the can was hermetically sealed and the thermogram of the sample was recorded with a DSC131 Setaram Differential Scanning Calorimeter equipped with a liquid-nitrogen cooling accessory.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp1

196

The purge gas used was high purity nitrogen supplied at a constant 35 cm³ min^–1 flow rate.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp1

197

The sample was first cooled to –110 °C at 10 °C min^–1 and then heated up to 150 °C at the same heating rate.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp1

198

It was then quenched to –110 °C at 20 °C min^–1 and finally heated to 150 °C at 10 °C min^–1.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp1

Fourier transform IR spectroscopy

199

FT-IR spectra were acquired under vacuum at room temperature and 40 and 85 °C using a Bruker IFS-66V Fourier transform spectrometer connected to an Oxford cryostat system.