Dynamics of Polyether Polyols and Polyether ... - ACS Publications

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Dynamics of Polyether Polyols and Polyether Carbonate Polyols M. Pohl,† E. Danieli,‡ M. Leven,† W. Leitner,§ B. Blümich,‡ and T. E. Müller*,§ †

CAT Catalytic Center, ‡Lehrstuhl für Makromolekulare Chemie, and §Lehrstuhl für Technische Chemie und Petrolchemie, RWTH Aachen University, 52074 Aachen, Germany S Supporting Information *

ABSTRACT: The presence of carbonate groups along the polymer backbone of polyether carbonate polyols leads to distinctly different physicochemical properties compared to structurally related polyether polyols. For instance, polyether carbonate polyols are characterized by their substantially higher viscosity. Targeting at understanding the underlying reasons for the different properties, we carried out detailed diffusion and viscosity studies on a series of diols with molecular weights in the oligomer range. We found that conventional polyether diols are subject to few topological constraints and show classic diffusiveness over a broad molecular weight range. In comparison, polyether carbonate diols exhibit much stronger topological constraints and form temporarily fixed networks on the same time scale. This results in timedependent diffusion coefficients, a distinctive phenomenon, which is analogous to subdiffusion in porous media. Accordingly, the dynamics of polyether carbonate diols in melt is very different from the dynamics of polyether diols. Such detailed understanding of the properties of polyols, which are widely used bulk chemicals in polyurethane manufacture, seems essential for introducing CO2-derived polyether carbonate polyols as sustainable building blocks for well-defined polyurethane materials.

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INTRODUCTION Polyurethanes, the reaction product of polyols and isocyanates, are an example of a ubiquitous high-performance material in our daily life. Polyurethanes find applications as thermoplastic materials, in coatings, and as flexible foams for mattresses and rigid foams for insulation materials. They are synthesized by mixing two components, a liquid polyol and a liquid isocyanate, which, in a chemical reaction, are connected to linear polymer chains or a three-dimensional network. The connectivity in the polyurethane materials depends on the terminal functionality of the building blocks1 and determines the properties of the material.2 In the manufacture of polyurethane materials the viscosity of the polyol is of particular importance. A low viscosity ensures ready processing of the polyol, rapid and efficient mixing with the isocyanate, and uniform curing to homogeneous materials. A low viscosity also avoids mass transport limitations during the initial stage of network formation and results in a more uniform, well-defined product. Low viscosities of the neat polyol component are therefore essential. Similar to other low molecular weight polymers,3 the viscosity of polyols is influenced by many factors: the chemical nature of the building blocks, the molecular weight and molecular weight distribution, as well as the internal structure and molecular architecture of the polymer chains. An understanding of how these factors relate to each other enables tailoring the viscosity of the polyol to the boundary conditions encountered during polyurethane material manufacture. Conventionally, polyether polyols, the homopolymers of epoxides, are employed as major constituent of polyurethanes. At present, epoxides are produced in highly energy demanding processes from mineral-oil-derived feedstock. Consequently, © XXXX American Chemical Society

the manufacture of polyether polyols is associated with a large CO2 footprint.4 Recently, the use of CO2 as ubiquitous and renewable comonomer5 has been reported for the production of oligomeric polyether carbonate polyols6 and high molecular weight polycarbonates7 with a more beneficial eco-balance and enhanced sustainability.4 However, the incorporation of CO2 into the polyol backbone leads to substantial increase in viscosity, which becomes more pronounced the more CO2 is incorporated.6a Targeted at gaining deeper understanding of the relationship between the molecular structure, the microdynamics and the complex viscoelastic properties of polyols this study provides an in-depth analysis of diffusion data measured with a nuclear magnetic resonance−mobile universal surface explorer (NMRMOUSE) and viscosity data measured by rheometry. Focusing on linear polymers, the properties of polyether diols and polyether carbonate diols are compared. The diols examined by us in this study had molecular weights in the oligomer range between 400 and 22 000 g mol−1. The samples were low viscous to honey-like liquids at room temperature. All measurements were performed in substance. The glass transition temperature of the oligomeric polyether diols (poly(propylene oxide): Tg = −68 °C) and polyether carbonate diols (Tg = −60 to −30 °C6a) was well below room temperature, and the physical state during the measurement corresponded to a polymer melt. Received: August 9, 2016 Revised: November 8, 2016

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DOI: 10.1021/acs.macromol.6b01601 Macromolecules XXXX, XXX, XXX−XXX

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To determine the self-diffusion coefficients, we used a NMRMOUSE as unilateral NMR sensor. The special magnet geometry (anti-Helmholtz assembly), consisting of four permanent magnet blocks with reverse polarity on an iron block, induces a strong, flat, and uniform static magnetic field gradient (Figure 1A).8 For the measurement, the polymer

To answer the question concerning the origin of the higher viscosity of polyether carbonates, we carried out an in-depth study of the trends in a series of polyether diols and polyether carbonate diols in the low molecular weight range of 435− 22 000 g mol−1. By understanding the polymer dynamics and the factors that have the most significant impact on the dynamics, we formulate guidelines for the use of polyether carbonate polyols in applications, where viscosity is a critical factor.

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RESULTS Dynamics of Polyether Diols. To obtain a fundamental understanding of the dynamics of polyether diols in melt, we investigated the self-diffusion coefficient of polypropylene glycol homopolymers. The samples were derived by elongating propylene glycol as low molecular weight starter with propylene oxide using a double-metal cyanide (DMC) catalyst (Scheme 1).6a,c,d Samples were obtained with molecular weight in the

Figure 1. (A) Magnet geometry of the unilateral NMR sensor used for the diffusion measurements. The polarity direction of the magnets is represented by the intensity of the shades of gray. The sensitive volume is adjusted by the distance between the magnet blocks. (B) SGSTE pulse sequence for spin encoding followed by a CPMG sequence to enhance sensitivity;8,9 Δ = τ1 + τ2; δ = τ1 (see ref 9, Figure 3.25d).

Scheme 1. Reaction Scheme for the Synthesis of Polyether Diols and Polyether Carbonate Diols with Statistical (stat.) Configuration of the Two Terminal Chain Segmentsa

sample was placed within the sensitive volume, and the proton spins were encoded with a stimulated echo sequence (SGSTE) in a constant gradient field.8,9 By diffusion of the polymer chains within the magnetic gradient field, the echo amplitude showed an irreversible decrease proportional to the average change in procession movement due to the translational progress of the molecules. The diffusion time τ2 is the time defined as the interval between the second and the third radiofrequency (RF) pulse of the SGSTE sequence (Figure 1B). The Carr−Purcell−Meiboom−Gillman (CPMG) pulse sequence was applied after the main diffusion-encoding period to increase the sensitivity of the experiments.8 Note that diffusion of magnetization along the polymer chain via dipole−dipole coupling sets a limit for the minimum diffusion coefficient, which can be determined. The induction decay was measured for a series of polyether diols in the molecular weight range of 435 to 18 000 g mol−1 (E1−E7) allowing for a diffusion time of Δ = 5 ms. For each of the polyether diols, the signal intensity of the induction decay decreased exponentially. The normalized logarithmic amplitudes of the free induction decays revealed the corresponding linear relationship (Figure 2). To investigate the relation between the induction-decay amplitudes and the diffusion time Δ, the measurements were repeated, whereby Δ was increased stepwise to 100 ms. For polyether diols E1−E6, the observed decay was identical to the decay at Δ = 5 ms. Thus, in this range of molecular weights the diffusion coefficient DNMR did not depend on Δ. Clearly, for the polyether diols the meansquare displacement of the polymer chains, being proportional to the self-diffusion coefficient, is directly proportional to the diffusion time. For polyether diol E7 the decays drifted slightly apart upon increasing Δ, indicating that the limit of the linear regime is encountered (vide inf ra). To obtain deeper insight into the dynamics of the polyether diols, we plotted the mean-square displacement (MSD) ⟨r2⟩ against the diffusion time Δ (Figure 2). Polyether diols E1−E6 showed a direct proportionality of ⟨r2⟩ to Δ. This linear trend according to the law ⟨Δx2⟩(t) ∝ tα (α = 1)9 is typical for the

a

Polyol starter: propylene glycol for polyether diols and poly(propylene glycol) with Mn = 1000 g mol−1 for polyether carbonate diols.

range from 435 to 22 000 g mol−1 (Table 1). The polyether diols had a narrow molecular weight distribution (Mn/Mw = 1.1). The viscosity increased steadily with the molecular weight from the liquid state (η0 = 69 mPa·s) to the consistency of honey (η0 = 39 075 mPa·s). Table 1. Chemical and Physicochemical Data of the Polyether Diols Explored in This Study sample

MWa [g mol−1]

Δ [ms]

E1 E2 E3 E4 E5 E6

435 1000 2000 4000 8000 12000

E7

18000

E8

22000

5−100 5−100 5−100 5−100 5−100 5−100 20 50 100 n.d.

DNMR b [10−9 m2 s−1] 9.91 3.81 1.40 5.50 1.98 1.04 7.12 5.25 3.68 n.d.

× × × × × × × × ×

10−3 10−3 10−3 10−4 10−4 10−4 10−5 10−5 10−5

η0 [mPa·s] 69 167 352 1037 2719 5724 24219 39075

a

Calculated on the basis of end-group titration with an accuracy of MW ± 5%. bFor polyether diol E1 to E6 the average diffusion coefficient determined at diffusion times Δ = 5−100 ms is stated; n.d. = not determined. B


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Figure 2. Mean-squared displacement (MSD) vs diffusion time Δ for E5−E7. Polyether diols E5 and E6 show direct proportionality of ⟨r2⟩ to Δ consistent with normal diffusion in ideal viscous liquids (α = 1). The MSD of E7 exhibits a dependency on diffusion time with exponent αE7 = 0.56 indicative of subdiffusion in viscoelastic liquids (α < 1).

Figure 3. Double-logarithmic plot of the diffusion coefficient DNMR (Δ = 100 ms) vs molecular weight for polyether diols E1−E7 and polyether carbonate diols C1−C5.

Table 2. Chemical and Physicochemical Data of the Polyether Carbonate Diols Explored in This Study

normal diffusivity of ideal viscous liquids (Brownian motion/ Rouse regime). Thus, in the molecular weight range up to 12 000 g mol−1 topological constraints are insignificant. This is in full agreement with observations for other homopolymers such as poly(ethylene oxide)10 or polystyrene.11 For polyether diol E7, the MSD exhibited a dependency on the diffusion time12 to the power of α = 0.56. The value below one is indicative of a subdiffusive chain motion. This suggests that a regime is entered, where the polymer chains are entangled and motion is confined to the space defined by the backbone of the polymer chain (reptation motion with preferential movement along a fictional tube). The transition from rouse dynamics to reptation motion occurs when the molecular weight exceeds 12 000 g mol−1. This is in full agreement with melts of poly(ethylene oxide)s with molecular weights of 438 000− 5 000 000 g mol−1, for which the reptation/tube model provides an adequate description of the experimental data.13 Self-diffusion coefficients (DNMR, Δ = 100 ms) are related to the molecular weight in the double-logarithmic plot shown in Figure 3. Noteworthy, two regimes with distinct power law relation DNMR ∝ MW−α between the two quantities were observed. For E1−E5, the exponent α1 was 1.36. The dependence on molecular weight, reflected in the value for the exponent slightly above 1, is explained readily by free volume effects of the chain ends in the effective available space within the polyol melt. For the relation to the dynamics and macroscopic properties we refer the reader to the theories of Fox and Flory; Williams, Landel, and Ferry; Simha and Boyer; and Cohen and Turnbull.14 For E5−E7 the exponent changed to α2 = 2.16. This is consistent with transition from the Rouse regime to the entanglement or reptation regime. Dynamics of Polyether Carbonate Diols. To study the dynamics of polyether carbonate diols in melt the self-diffusion coefficient of epoxide−CO2 copolymers was investigated. The samples were obtained by elongating poly(propylene glycol), Mn 1000 g mol−1, with propylene oxide in the presence of CO2 and DMC catalyst (Scheme 1). Samples were prepared with a molecular weight in the range from 948 to 7634 g mol−1 (Table 2). With increasing molecular weight, the amount of CO2 incorporated into the polyol chain increased steadily to 16.7 wt

sample

MWa [g mol−1]

CO2 [wt %]

C1 C2 C3 C4

948 1861 3499 6001

5.2 11.5 12.4 16.3

C5

7634

16.7

Δ [ms] 20−100 20−100 20−100 20 50 100 20 50 100

DNMR b [10−9 m2 s−1] 3.29 6.71 1.05 6.30 5.48 4.39 1.10 6.71 4.34

× × × × × × × × ×

−3

10 10−4 10−4 10−5 10−5 10−5 10−4 10−5 10−5

η0 [mPa·s] 221 1168 6380 49458

279320

a

Calculated on the basis of end-group titration with an accuracy of MW ± 5%. bFor polyether carbonate diol C1 to C3 the average diffusion coefficient determined at diffusion times Δ = 20−100 ms is stated.

%. The oligomeric ether segments between adjacent carbonate groups comprised between 2 and 15 units. The polyether carbonate diols had a slightly higher molecular weight distribution (Mn/Mw = 1.3−1.6) compared to the polyether diols. The viscosity was noticeably higher than that of the corresponding polyether diols and increased steadily with the molecular weight from the liquid state (η0 = 221 mPa·s) to highly viscous (η0 = 279 320 mPa·s). To investigate the relation between the self-diffusion coefficient and the molecular weight, we measured the induction decays for polyether carbonate diols C1 to C5 in an analogous manner to the polyether diols. The normalized logarithmic induction decays are plotted for polyether carbonate diols C2 and C5 in Figure 5. The measurements were repeated at different diffusion times Δ = 20−100 ms. While the observed decays were independent of diffusion time Δ for C1 to C3, the induction decays for polyether carbonate diols C4 and C5 were strongly dependent on the diffusion time Δ and deviated from the monoexponential correlation. This pronounced dependence on diffusion time is quite different than the polyether diols. The presence of at least two components with different diffusivity could be related to variations in the carbonate content along the polymer chain during the statistical incorporation of CO2. Another plausible C


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may also be restrained to pure reptation motion. Alternatively, the short, though flexible, polyether segments may be confined to a temporarily fixed network of highly entangled species (analogues to the diffusion of a tracer through a matrix). To investigate the relation between D and the diffusion time Δ, the measurements were repeated, whereby Δ was increased stepwise to 100 ms. In the case of C1−C3 the slope of the observed induction decays was equal to the induction decay found at Δ = 20 ms. In contrast, for the samples C4 and C5 the slope of the induction decays decreased with increasing Δ. The self-diffusion coefficients decreased from 6.30 × 10−5 10−9 m2 s−1 for Δ = 20 ms to 4.39 × 10−5 10−9 m2 s−1 for Δ = 100 ms (C4a−c). This trend was more distinct for C5. A comparison of the dependence of DNMR on the molecular weight for polyether diols and polyether carbonate diols in the range of low molecular weights revealed similar values for E2 (3.81 × 10−3 10−9 m2 s−1) and C1 (3.29 × 10−3 10−9 m2 s−1). For C2 (6.71 × 10−4 10−9 m2 s−1) and C3 (1.05 × 10−4 10−9 m2 s−1), DNMR decreased more than that of the corresponding polyether diols E3 (1.40 × 10−3 10−9 m2 s−1) and E4 (5.50 × 10−4 10−9 m2 s−1). In the range of higher molecular weights C4 and C5 showed similar values for the diffusion coefficient DNMR. Closer inspection of the echo attenuation curves revealed that the average slope decreased with increasing diffusion time Δ (see Figure 5 for C5; Δ = 20, 50, and 100 ms;

explanation is the presence of fast and slow diffusing components in the samples caused by the higher molecular weight distribution.15 Linear functions were fitted to the logarithmic representation of the data series and the average diffusion coefficient determined from the slope of the linear equations (Figure 5). For polyether carbonate diols C4 and C5, only the slow diffusing component was considered. The selfdiffusion coefficient ranged from 3.29 × 10−3 10−9 m2 s−1 for C1 with a molecular weight of 948 g mol−1 to 4.34 × 10−5 10−9 m2 s−1 for C5 with 7634 g mol−1 (Table 2). For all polyether carbonate diols, the self-diffusion coefficients (Table 2) were significantly lower than the values for the corresponding polyether diols (Table 1). A likely explanation is that the reduced diffusion rates are triggered by pronounced interactions between the carbonate moieties (vide inf ra).

Figure 4. Normalized logarithmic induction decays of polyether carbonate diol C2 with 1861 g mol−1 and C5 with 6001 g mol−1 in dependence on the diffusion time Δ (measurement at 25 °C). The diffusion times Δ were 20−100 ms for C2 (a−g) and 20, 50, and 100 ms for C5 (a, b, and c, respectively). The slope represents the selfdiffusion coefficient of the respective polyether carbonate diol.

Self-diffusion coefficients (DNMR, Δ = 100 ms) for C1−C5 related to the molecular weight in a double-logarithmic plot are included in Figure 3. Analogous to the polyether diols, two regimes were observed in the time scale of the measurement. For C1−C3, the exponent according to the power law DNMR ∝ MW−α scaled with α1 = 2.68. For C4−C5, the exponent changed to α2 = 0.05. Thus, a transition from a reptative to confined diffusion regime with almost constant D value was observed. The first regime implies that strong topological constraints and/or strong chain interactions are present already at low molecular weights. Rouse kinetics (expected scaling with α = 1) in the untangled or freely diffusing regime was not observed. The underlying reason may that an activation energy is associated with the diffusion process.16 An alternative explanation can be related to the stiffness of the chain,17 local excluded volume interactions described by the free volume theory,17,18 and/or hydrodynamic interactions.19 The second regime with constant diffusion rate is entered once the critical molecular weight Mξ,C of 5200 g mol−1 (derived on the basis of zero-shear-rate viscosity data; vide inf ra, Figure 7) is exceeded. A likely explanation is that additional interactions, which result from the presence of carbonate moieties, dominate over the influence of the molecular weight. Shorter polyether carbonate chains (reflected in the broader polydispersity of the sample)

Figure 5. Mean-squared displacement (MSD) vs diffusion time Δ for C2−C5. Polyether carbonate diols C2 and C3 showed direct proportionality of ⟨r2⟩ with Δ (α = 1) characteristic for normal diffusion. For polyether carbonate diols C4 and C5 the MSD exhibited a dependency on diffusion time with exponents αC4 = 0.77 and αC5 = 0.42. The value below one is characteristic for subdiffusional or confined chain motion.

a, b, and c, respectively). Thus, the diffusion coefficients are no longer directly proportional to the diffusion time and follow the relation ⟨Δx2⟩(t) ∝ tα with α < 1. The time dependence was determined from the relation of ⟨r2⟩ to Δ (Figure 6). A value of αC4 = 0.77 reveals the transition from isotropic diffusion in subdiffusive motion for C4. The lower exponent αC5 = 0.42 for C5 indicates that the motion of the polymer chains is restricted by increasingly strong topological constraints, resulting in a distinct subdiffusive or confined diffusion similar to the diffusion of a molecule through porous media.20 To investigate the macroscopic dynamic behavior of polyether diols and polyether carbonate diols, we recorded the zero-shear-viscosity (η0) on a rheometer. The viscosities of D


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molecular weight of effective entanglement and α = 3.4 above the critical chain length Mξ of entangled linear polymer chains.15,21 The exponents derived for polyether carbonate diols (α1,C, C1−C3 and α2,C, C4−C5) were about twice as high as those of the corresponding polyether diols (α1,E, E1−E5 and α2,E, E6−E8). This is consistent with the more polar character of the polyether carbonate chains compared to the polyether chains. For both polyether and polyether carbonate diols α assumed a 3-fold higher value, when the molecular weight exceeded Mξ. Interaction Energies and Preferential Conformations Calculated by DFT. To gain further insight into the underlying forces on a molecular level, the intra- and intermolecular interactions of the polyether and polyether carbonate chains were investigated. Structures with six repetition units were optimized by means of DFT calculations, and the interactions were analyzed for the limiting cases of two interacting polyether chains (without incorporated CO2) and the corresponding alternating polycarbonate chains (maximum CO2 content). For interacting polyether chains, the structure optimization resulted in separated coils with a low average intermolecular interaction energy of −2.8 kcal mol−1 per repetition unit (Figure 7, left). In comparison, the interaction between two adjoining ether units of the same chain was calculated to be 3 times higher (−7.6 kcal mol −1 ). Consequentially, the chains adopt separate coils with globular conformation and relatively small hydrodynamic diameter. This is in good agreement with published reports that entropy favors polymer coiling.22 Optimization of the structure of two interacting polycarbonate chains provided a helix-like conformation with much higher average intermolecular interaction energy of −8.8 kcal mol−1 per repetition unit (Figure 7, right). Neither a coiled conformation nor an ensemble of loosely interacting chains was stable. Thus, the polycarbonate chains energetically favor a lengthened conformation, which allows maximizing attractive forces between the oxygen atom of the carbonate groups and positively polarized methylene groups of a neighboring chain. This type of interaction corresponds to an unconventional hydrogen bond.23 The presence of trigonal planar carbonate moieties, where rotational degrees of freedom are hindered, imparts a significant rigidity to the polymer backbone. The preferred conformation of statistical polyether carbonates will be in between the two limiting cases. In the case of the longer polymer chains of polyether carbonate diols with polyether segments comprising 2−15 ether units between adjacent carbonate groups, the strong interchain interactions imparted by the presence of the carbonate groups strongly favor the effective entanglement of the chains.

Figure 6. Double-logarithmic plot of zero shear-rate viscosity η0 vs molecular weight. Mξ marks the critical molecular weight, where the two types of diols become effectively entangled. The approximate scaling factors are stated in gray (see text).

the representative samples E2 (167 mPa·s) and C1 (221 mPa· s) were similar at low molecular weights, while at higher molecular weights the values measured for the viscosities diverged (Figure 6). The trends were analogous to the trends derived for the respective diffusion constants DNMR. The empirical eq 1 describes the relation between the melt viscosity η0 and the average molecular weight MW η0 = K ·MW α

(1)

where K is a temperature and compound-specific constant and α a characteristic exponent.21 Relating η0 and the molecular weight in a double-logarithmic plot (Figure 6), the exponents α were calculated from the slopes of the fitted linear equations. For the polyether diols E1−E5, the zero shear-rate viscosity increased proportional to the increasing molecular weight by the power of α = 1.27. For E6−E8 the viscosity increased faster with respect to the increase in the molecular weight, and the exponent was derived to be α = 3.71. Both straight lines intersect at a molecular weight of 10 600 g mol−1, representing the critical chain length Mξ,E (Figure 6 and Table 3). For the Table 3. Proportionalities of Zero Shear-Rate Viscosity η0 and Molecular Weight for Polyether Diols and Polyether Carbonate Diols polyol

MW ≤ Mξ

Mξ< [g mol−1]

MW ≥ Mξ

polyether diol polyether carbonate diol

η0 ∝ MW η0 ∝ MW3.71

10600 5200

η0 ∝ MW2.57 η0 ∝ MW7.19

1.27

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DISCUSSION There are a number of diverging theories on the dynamic behavior of untangled and entangled polymer melts. The Rouse model24 and the tube reptation model of de Gennes25 and Doi and Edwards26 are established theories, applied to describe the rheology of polymer melts above and below a certain crossover chain length Nc or critical molecular weight Mc. A general empirical relationship between the zero-shear viscosity η0 and the molecular weight is the power law of Fox and Flory. The direct proportionality η = K × MW1 in untangled regimes changes to a dependence on molecular weight by a power of 3.4 in entangled regimes for linear polymers. The Rouse model24 for polymers below the critical entanglement molecular weight and the reptation model25 or tube model26a,b for polymers

polyether carbonate diols, the zero shear-rate viscosity generally increased faster. For the polyether carbonate diols C1−C3 the increase in η0 was proportional to the molecular weight by the power of 2.57. In the case of C4 and C5 the exponent increased to 7.19. The critical chain length Mξ,C was calculated to be 5200 g mol−1 (Figure 6 and Table 3). Thus, the critical molecular weight Mξ,C for polyether carbonate diols was about half of Mξ,E for polyether diols. The critical molecular weight Mξ is a polymer-specific constant, which marks the transition of untangled to entangled polymer chains. Mξ is expected to be lower the more polar the polymer is. The values derived for the exponent α in the rheometer measurements for the polyether diols correspond well with the typical value of α = 1 below the E


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Figure 7. Two interacting polyether diol chains preferentially assume coiled conformations (left), while two interacting polycarbonate chains preferentially adopt a helix-like elongated conformation (right).

above Mξ describe the rheology of polymer melts below and above the crossover chain length. Traditionally, the selfdiffusion coefficient also scales according to a power law. Below the critical entanglement molecular weight, a directly proportional scaling (D ∝ MW−1), referred to as Rouse regime, is usually observed. Thereby the polymer chains are seen as discretized series of virtual beads connected by harmonic springs or rigid rods. For long chains with observable entanglement the Rouse model holds only up to a crossover time τe. For polymer chains equal to or above the critical entanglement molecular weight the chain can move only in a fictional tube formed by the surrounding chains. These topological constraints restrict the lateral motion of the chains, described as a curvilinear reptation motion scaling D ∝ MW−2. Polyether diols are close to the anticipated relationship of zero shear-rate viscosity and molecular weight with an exponent α = 1.27 for the untangled regime. Effective entanglement becomes noticeable above a molecular weight of 10 500 g mol−1 (corresponding to 181 propylene oxide repeating units), where η0 starts to scale with molecular weight to the power of α = 3.71 in an entangled regime. Analogously, in the diffusion measurements two regimes were identified: (a) For molecular weights below the critical molecular weight the polyols follow a normal or center-of-mass diffusion regime, where the exponent of 1.36 is slightly above the value of 1 anticipated from the Rouse model. The theory of excluded volume effects considers the competing interactions, when two segments of the same polymer chain spatially get close to each other. The result of this long-range interaction between segments of the same polymer chain is an end-to-end radius of the polymer about 20% larger than the one considered in the Rouse model (see ref 9, section 5.2.5). (b) For molecular weights around and above the critical molecular weight stronger interactions between polyol chains were evident corresponding to an entangled regime. The power law establishing the relation between the diffusion coefficient D and the molecular weight gives an exponent of α = 2.16, which is close to the value 2 anticipated for polymer diffusion, where D typically scales with MW−2 independent of the molecular weight.27 A double-logarithmic plot of DNMR and η0 shows a directly proportional relation between these measurement parameters (Figure 8), in good agreement with the Einstein−Sutherland equation, where D is proportional to the friction coefficient of the growing chain. When Mξ is exceeded, the direct proportionality disappears and

Figure 8. Double-logarithmic plot of the normalized diffusion coefficient D vs zero viscosity η0. The diffusion coefficient was normalized to the respective diffusion coefficient of E1 and C1.

fluidity is controlled by the dynamics of constraints due to the entanglement of the polymer chains. Polyether carbonate diols deviate with regard to viscosity from the expected scaling relationship with the molecular weight. Already in the low molecular weight range direct proportionality is not noticeable and η0 scales with η0 ∝ MW2.57. This suggests that intermolecular interactions dominate already for short chains. The critical molecular weight Mξ,C, is encountered at 5200 g mol−1, which is about half of that of the corresponding polyether diols. When Mξ,C is exceeded, the viscosity increases rapidly with η0 ∝ MW7.19. A reasonable explanation is the polar character of the incorporated carbonate moieties and discrete intermolecular interactions. Conversely, the incorporated carbonate units are quite stiff. The polyether segments between neighboring carbonate groups of the polymer backbone provide the necessary flexibility for effective entanglement of the polymer chains. The strong intermolecular interactions with penetrating chains in the polymer melt are reflected by the observed subdiffusion, by which the surrounding chains form a temporary stable network around the diffusing polymer chain. The relation of the diffusion coefficients with the molecular weight indicates that the scaling law for the Rouse regime does not apply to polyether carbonate polyols. Even at low molecular weight, pure reptation motion seems to occur, showing that topological constraints restrict the movement of the chains and that only curvilinear movement along the axis of the chain is possible. F


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varied from 5 ms to a maximum value of 90 ms. The detection of the NMR signal was performed with a CPMG multiecho train with an acquisition time tacq = 4 μs, and the time in between echoes τ′ was set to τ′ = 50 μs. The typical pulse length used was 3 μs. The number of scans employed ranged from 32 for samples with low MW to 256 for the largest MW using a repetition time in between scans of 2 s. The signal-to-noise-ratio for the CPMG multiecho train was calculated as the ratio of the amplitude of the real component of the first detected echo and the standard deviation of the imaginary component of the whole multiecho train. Typical values of the signal-to-noise ratio of the CPMG multiecho train for the samples with low MW ranged from 120 to 78, corresponding to τ2 = 5 ms and τ2 = 40 ms, respectively. For the samples with largest MW, typical values for the signal-to-noise ratio ranged from 200 to 42 for τ2 = 10 ms to τ2 = 90 ms, respectively. The longitudinal relaxation times for polyether carbonate diols (52 ms < T1 < 68 ms) showed similar values as the ones observed in the case of polyether diols when comparing samples with similar molecular weight. Correspondingly, the apparent transverse relaxation times tended to be smaller when the same molecular weight was compared, reaching a limiting value of 7 ms for the sample C5, while for C1 the value was 54 ms. In this situation, and in order to generate an effective diffusion encoding process at a short encoding time, a second singlesided sensor was used. The geometric characteristics of the sensitive volume are similar to the sensor described above, but it worked at a higher field strength of 0.69 T (29.3 MHz 1H Larmor frequency), and the strength of the field gradient was G0 = 44 ± 1 T m−1. The encoding time value ranged from 0.01 ms to a maximum value of 1.1 ms, adjusted in 32 discrete steps. Diffusion times τ2 were varied from 5 ms up to a maximum value of 90 ms. The NMR signal was detected with a CPMG multiecho train with an acquisition time tacq = 4 μs, and the time in between echoes τ′ was set to τ′ = 32 μs. The typical pulse length used was 2.8 μs. The number of scans employed ranged from 32 for samples with low molecular weight to 256 for samples with the largest molecular weight using a repetition time in between scans of 2 s. The calculated values for the signal-to-noise ratio for the CPMG multiecho train corresponding to samples with low molecular weight ranged from 58 to 32 for τ2 = 10 ms to τ2 = 40 ms, respectively. Accordingly, for samples with the highest MW the signalto-noise ratio ranged from 84 (τ2 = 10 ms) to 17 (τ2 = 90 ms). Rheometer Measurements. The zero viscosity η 0 was determined using a Physica MCR 501 rheometer (Anton Paar GmbH) with a conical plate configuration with a diameter of 50 mm (tool system CP50-1/TG) and a gap of 0.101 mm. The polyol (0.8 mL) was applied to the plate system, and η0 was measured at 25 °C by means of an oscillation method with an initial deformation angle of 10% and gamma amplitude of 5%. Computational Details. Details of the DFT calculations are described in the Supporting Information.

EXPERIMENTAL SECTION

Materials and Methods. A sample of the DMC catalyst, synthesized according to WO2001080994A1, was provided by Covestro Germany AG. The polyol starters, α,ω-dihydroxypoly(propylene oxide) ARCOL POLYOL 1004 (OH number = 250−270 mgKOH/g) and ARCOL PPG-1000 (OH number = 107.4−115.4 mgKOH/g), were supplied by Covestro Germany AG and used without further purification. Propylene oxide (purity 99.9%, water content ≤200 mg/kg, propionaldehyde ≤100 mg/kg) was purchased from CHEMOGAS and used without further purification. Synthesis of Polyether Diols and Polyether Carbonate Diols. The high pressure reactor was supplied by Parr Instruments. The stainless steel vessel (300 mL) was equipped with a U-shaped cooling line, gas entrainment stirrer, and a 3 mm in situ FT-IR spectroscopy diamond probe. Polyether diols and polyether carbonate diols were synthesized according to the published procedure.6a Details are given in the Supporting Information. Diffusion Measurements by NMR-MOUSE. The diffusion28 of samples of polyether diols was measured with a single-sided magnet generating a sensitive volume of lateral dimensions of 10 × 10 mm2 located at 5 mm of the magnet surface. Experiments were performed at room temperature, which was maintained constant at 25 ± 1 °C. Reference experiments between 22 and 25 °C showed that the relative error in the determination of the diffusion coefficient caused by temperature fluctuations was ≤2% per °C. A drop of the sample was placed on a microscope glass plate located within the sensitive volume separated by an air gap from the RF coil to minimize heat effects. The magnetic field strength over this volume was 0.40 T (17.2 MHz 1H Larmor frequency), and the strength of the field gradient was G0 = 20.0 ± 0.5 T m−1. For the procedure used for gradient calibration, refer to the Supporting Information. The longitudinal relaxation time T1, measured with a saturation recovery sequence, ranged from 79 to 51 ms for the samples with the lowest MW to the highest MW, respectively. Apparent transverse relaxation times, measured with a CPMG sequence, ranged from 64 to 21 ms, respectively. A stimulated echo sequence (SGSTE) in the presence of this constant field gradient was used to measure the self-diffusion coefficients D of the different samples (see Figure 1B). The normalized recorded signal amplitude can be expressed as ln(I /I0) = − (gG0t1)2 (t 2 + 2/3t1)D − 2t1/T2 − t 2/T1

(2)

where γ stands for the gyromagnetic ratio, and τ1 and τ2 represent the encoding and diffusion time, respectively, defined according to Figure 1B.25 The normalization factor I0 corresponds to the signal amplitude recorded in the limit τ1 → 0, and T1, T2 are the longitudinal and transverse relaxation times, respectively. The encoding times τ1 was set to satisfy τ1 ≪ T1, T2, which was feasible for most samples. Under this condition, the last two terms in eq 2 can be neglected, and the coefficient D is obtained by a linear fit. To increase the detection sensitivity of the stimulated echo, it was refocused multiple times in a CPMG multiecho train following the SGSTE module (see Figure 1B). By adding the first echoes (of the order of T2/τ′) of the train, the signal-to-noise ratio was enhanced. Signal decay during the CPMG module run in a stray magnetic field,29 or filtering of a particular spectral component of the self-diffusion coefficient (generally defined via the Fourier transformation of the autocorrelation function of a spin-bearing particle velocity along the gradient direction)30 through the set value of τ′, is independent of the variables τ1 and τ2 and consequently plays no role (apart from a modification of the signal-to-noise ratio) in the values of the selfdiffusion coefficients obtained through eq 2. For the samples with molecular weight comparable or larger than Mc, for which τ1 < T1, T2, a 0.7 T Halbach magnet31 was used to obtain the relaxation rates under homogeneous field conditions. The selfdiffusion coefficient was obtained with a linear fit after relaxation correction (see ref 9, section 3.2.2.5). Depending on the sample, the encoding time value ranged from 0.05 ms to a maximum value of 1 or 3 ms and was adjusted in 16 discrete steps. Diffusion times τ2 were

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CONCLUSIONS

The goal of this study was to explore the underlying reasons for the higher viscosity of polyether carbonate polyols compared to the corresponding polyether polyols. Comparing the trends in viscosity with the diffusion dynamics determined by NMRMOUSE experiments and intra- and intermolecular interactions investigated by DFT calculations revealed essential differences for polyether diols and polyether carbonate diols: For polyether diols the diffusion measurements (i) revealed Rouse-like dynamics with a scaling of D ∝ MW−1.36 for samples with low molecular weight, which changed to D ∝ MW−2.16 above the critical molecular weight of 10 500 g mol−1, consistent with reptation motion in the entangled regime. (ii) In parallel, polyether diols showed close relations to the scaling law of viscosity and molecular weight with η0 ∝ MW1.27 for the untangled regime and η0 ∝ MW3.71 for the entangled regime. (iii) Because of their flexible backbone structure and relatively G


Macromolecules

■

weak intermolecular interactions, polyether chains form entropically favored compact coils. For polyether carbonate diols the diffusion measurements (i) disclosed that already at low molecular weights a pure reptation motion occurred with D ∝ MW−2.68. Above the critical molecular weight, the diffusion constant D was almost constant. (ii) Likewise, the viscosity was, already at low molecular weight, distinctly higher compared to polyether diols with comparable molecular weights. The scaling of η0 ∝ MW2.57 is between the untangled and the entangled regime. After exceeding the critical molecular weight at 5200 g mol−1, about half of that of the polyether diols, the viscosity increased very fast with increasing molecular weight (η0 ∝ MW7.19). (iii) The DFT calculations exposed that the presence of carbonate moieties in the backbone of the polyether carbonate diols leads to strong intermolecular interactions. Effective entanglement of the chains is achieved through spacing of the rigid carbonate moieties with flexible polyether segments. The strong intermolecular interactions result in a temporarily fixed network. Consequentially, the diffusing polymer chain is restricted mostly to motion along the contour of the chain leading to diffusion similar to that in porous media and referred to as subdiffusion. Thus, it is the rigid and more polar character of the polyether carbonate polyol chains and the stronger intermolecular interactions invoked by the presence of carbonate groups that lead to strong entanglement of the chains and a temporarily fixed network within of the time frame of the measurement. Both factors translate into higher viscosities compared to the corresponding polyether polyols. Already at a relatively low molecular weight, polyether carbonate diols change from untangled to entangled state. Consequently, the critical molecular weight is a particularly important property, when using polyether carbonate polyols in material manufacture. Already at present, polyols are widely used bulk chemicals in polyurethane manufacture. CO2-based polyether carbonate polyols are currently introduced in the industry as more sustainable building blocks.6a If low viscosities are necessary to e.g. ensure ready processing during polyurethane manufacture, polyols with a molecular weight well below the critical molecular weight could be used. The viscosities can also be reduced by the use of suitable comonomers that may ease the intermolecular interactions.32 The molecular level understanding of the physicochemical properties is also at the base of introducing polyether carbonates to further application fields, such as elastomers and soft-touch coatings.2 Thus, polyether carbonates with their specific dynamic properties are fascinating CO2-based building blocks for sustainable polymeric materials.

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Article

AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Tel +49 241 80 26455; Mobile +49 175 30 30560; Fax +49 241 80 22593 (T.E.M.). Present Address

T.E.M.: Institut für Technische and Makromolekulare Chemie, RWTH Aachen University, Worringerweg 2, 52074 Aachen, Germany. Funding

Covestro Germany AG is acknowledged for financial support. E.D. and B.B. acknowledge financial support of Deutsche Forschungsgemeinschaft (DFG Gerätezentrum Pro2NMR). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Volker Marker is highly acknowledged for his support. ABBREVIATIONS MW, molecular weight; NMR-MOUSE, nuclear magnetic resonance−mobile universal surface explorer; E, polyether diol; C, polyether carbonate diol; DFT, density functional theory.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b01601. Experimental details; supplementary experimental data; chemical characterization and physicochemical properties of polyether polyols and polyether carbonate polyols; atomic coordinates (PDF) H


Article

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