PB-b-PI-b-PEO and PE-b-PEP-b-PEO Triblock Copolymers

The PE-b-PEP-b-PEO triblock copolymers have been synthesized by homogeneous catalytic hydrogenation of the corresponding poly(1,4-butadiene)-block-poly(1,4-isoprene)-block-poly(ethylene oxide) (PB-b-PI-b-PEO) triblock copolymers, the latter being synthesized by sequential anionic polymerization.^78,176,177 The anionic polymerization has been performed in benzene using sec-BuLi as initiator in order to achieve a high degree of 1,4-addition for the PB block, which in turn is a prerequisite to obtain the corresponding “pseudo polyethylene”

structure after hydrogenation. Usually polymerization of ethylene oxide in the presence of lithium counterions is not possible due to the formation of strong lithium alkoxide aggregates.^178,179 However, polymerization of ethylene oxide can be achieved by using the strong phosphazene base t-BuP4, as has been recently shown by Möller and co-workers.^180-182 In this case the phosphazene base t-BuP4 forms a strong complex with Li⁺, resulting in a break up of the strong lithium alkoxide aggregates, and thus enabling the polymerization of ethylene oxide. Using this method, a one-pot synthesis of PB-b-PI-b-PEO triblock copolymers is possible using sec-BuLi as initiator. In addition, the phosphazene base t-BuP4 was successfully applied by other groups in the synthesis of PEO containing block copolymers

using organolithium initiators.180,183-185 In contrast to the kinetic results given in the literature^181,182, we find an unexpected induction period involved in the synthesis of the PEO blocks. Kinetic investigations, applying online FT-NIR spectroscopy, of ethylene oxide homopolymerization with organolithium initiators reveals that the induction period depends on reaction temperature, amount of added phosphazene base t-BuP4, type of organolithium initiator, and the sequence of reactant addition (Chapter 3.4).^186,187 An induction period is also present in the synthesis of PS-b-PEO diblock copolymers, and has been additionally proven by MALDI-ToF mass spectroscopy on samples taken during the polymerization of the PEO block in a low molecular weight PS-b-PEO diblock copolymer (Chapter 3.4). The induction period decreases with increasing reaction temperature and amount of added phosphazene base. This points to an association-dissociation pre-equilibrium, which might be responsible for the observed induction period, since the phosphazene base t-BuP4 has first to break up the strong lithium alkoxide aggregates in order to enable ethylene oxide polymerization.

However, experiments using an altered sequence of reactant addition and sequential ethylene oxide addition reveals that dissociation of the strong lithium alkoxide aggregates by complexation of Li⁺ with t-BuP4 is not the only factor which contributes to the observed induction period. Chain length effects arising from the complexation properties of PEO and/or a contribution of ethylene oxide itself in the formation of the active center might also be responsible for the induction period.

Thermal analysis utilizing differential scanning calorimetry (DSC) reveals a strong influence of the confinement active during crystallization on the crystallization and self-nucleation behavior of the PEO and PE blocks within PE-b-PEP-b-PEO triblock copolymers.^78,177 Applying the self-nucleation technique developed by Fillon et al.,¹⁸⁸ more detailed information on the crystallization behavior of the two crystalline end blocks and the influence of confinements can be obtained. Self-nucleation consists of the partial melting of an initial crystalline “standard” state of the polymer at a given self-nucleation temperature (Ts). Upon subsequent cooling recrystallization takes place, using as nuclei the crystallographically “ideal” nuclei which are produced during partial melting, i. e. self-nuclei or crystal fragments of the same polymer under considerations. In a crystallizable homopolymer usually three different domains of self-nucleation can be defined. In domain I, or complete melting domain, crystallization always takes place at the same temperature.

Domain II (self-nucleation domain) represents a Ts range, where the concentration of remaining crystal fragments varies dramatically with T. Small variations in T result in

significant shifts of the crystallization peak to higher temperatures. In domain IIISA (SA stands for self-nucleation and annealing) incomplete melting takes place, which is directly linked to the occurrence of considerable annealing of the remaining crystalline material. However, for block copolymers the situation might be different, especially for systems where the crystallizable block is confined into small isolated microdomains. It has been observed, that domain II vanishes completely in polystyrene-block-polybutadiene-block-poly(ε-caprolactone) (PS-b-PB-b-PCL) and PS-b-PE-b-PCL triblock copolymers exhibiting low PE and PCL contents.^83,189,190 This results directly from the confinement of the crystallizable blocks within small isolated microdomains.

Because of the strong incompatibility of the polar PEO segments with respect to the other block components, the crystallization of PEO is confined into isolated microdomains in PB-b-PI-b-PEO and PE-b-PEP-b-PEO triblock copolymers. The morphology of the synthesized PE-b-PEP-b-PEO triblock copolymers has been investigated using transmission electron microscopy (TEM) and scanning force microscopy (SFM).^78,177 As an example the TEM micrograph of E11EP71EO18123, obtained by catalytic homogeneous hydrogenation of the corresponding precursor B11I70EO19120 using Wilkinson catalyst, is shown in Figure 1.16.

250 nm RuO₄

Figure 1.16: TEM micrograph of E11EP71EO18123 stained with RuO4.

The use of RuO4 results in a preferential staining of the amorphous PEP and PEO segments. Thus, the crystalline PEO domains (thin sections were cut at –130 °C) appear bright and exhibit a distorted spherical structure, which clearly shows the confinement of the PEO blocks within isolated PEO domains. The crystalline PE domains, which are expected to be located in between the amorphous PEP phase, cannot be visualized using RuO4 staining.

As a result of confinement, large supercoolings are necessary to induce crystallization of PEO in PB-b-PI-b-PEO and PE-b-PEP-b-PEO triblock copolymers with PEO contents < 30 wt-%

and < 20 wt-%, respectively. The observed crystallization temperatures of -20 to -25 °C are significantly lower compared to the crystallization temperature observed in PEO homopolymer (Tc ca. 40 °C).⁸³ This is a direct result from the huge number density of PEO microdomains (≈ 10¹⁶ spheres/cm³ or ≈ 10¹⁴ cylinders/cm³ assuming a spherical or cylindrical PEO microdomain)⁷⁸ compared to the number density of heterogeneous nuclei usually present in PEO homopolymer (≈ 10⁵ nuclei/cm³, for a spherulite radius of 100 µm)⁷⁹. Consequently, crystallization of PEO cannot proceed via heterogeneous nucleation. The observed large supercoolings necessary for crystallization of PEO within isolated microdomains might arise from weakly nucleating heterogeneities within the PEO phase, surface nucleation of the interphase, or homogeneous nucleation. The absence of domain II (self nucleation domain) in self-nucleation experiments¹⁹¹ combined with the fact that the crystallization temperatures observed for homogeneous nucleation in PEO containing block copolymers (Tc ≈ -40 °C)⁸² are significantly lower compared to the detected values (Tc ca. – 20 °C) point to a nucleation of the interphase. This absence of domain II is a direct result of the confined PEO crystallization within isolated microdomains. To induce self-nucleation of the confined PEO segments a high concentration of self-seeding nuclei is necessary.

Therefore, Ts has to be lowered well into domain IIISA, where already annealing takes place, in order to provide a sufficiently high concentration of self-seeds. In addition, a strong influence of Wilkinson catalyst ((Ph3P)3Rh(I)Cl) residues in the non-purified triblock copolymers, arising from the hydrogenation reaction, has been observed. In PE-b-PEP-b-PEO triblock copolymers (PEO content < 20 wt-%) the crystallization temperature of the strongly confined PEO blocks exhibits a shift to higher temperatures (Tc ca. 20 °C), which can be attributed to a nucleating property of the Wilkinson catalyst residues. This is also connected to a change in the self-nucleation behavior, since in the non-purified triblock copolymers all three self-nucleation domains are visible.¹⁹¹ Increasing the PEO content to approximately 40 wt-% in B I EO ¹³⁵ and E EP EO ¹³⁸ results in a fractionated crystallization, whereby

homopolymers (Tc ≈ 20/40 °C, double exotherm), thus resembling a heterogeneous nucleation mechanism. Because of the increasing PEO content, most PEO blocks are no longer confined into small isolated PEO microdomains, as is revealed by the lamellar and cylindrical PEO microdomains observed in B19I39EO42135 and E19EP40EO41138, respectively.⁷⁸ However, a minor PEO fraction still crystallizes at comparatively low temperatures (Tc ≈ -20 °C). This might be attributed to the fact, that still small isolated PEO microdomains are present in the system, as the samples were not subjected to annealing prior to the DSC investigations, i. e. the morphologies are not perfectly ordered. Self-nucleation experiments reveal that for the PEO fraction crystallizing in the high temperature exotherm all three self-nucleation domains can be detected. In contrast, the PEO fraction crystallizing in the low temperature exotherm exhibits a similar behavior compared to the PEO blocks in PE-b-PEB-b-PEO with PEO contents < 20 wt-%, i. e. domain II vanishes.

For the PE blocks the situation is different. Due to the low segmental interaction parameter between PEP and PE segments of χ = 0.007¹⁶⁸ at 120 °C crystallization of PE is expected to occur from a homogeneous mixture of PE and PEP segments. This in turn should result in a continuous crystalline PE domain, consisting of interconnected PE crystallites.

Figure 1.17 shows the TEM micrograph of E19EP40EO41138, which was obtained by hydrogenation of the corresponding B19I39EO42135 precursor with p-toluenesulfonyl hydrazide.

125 nm OsO₄

Figure 1.17: TEM micrograph of E19EP40EO41138. The triblock copolymer contains 30%

residual double bonds within the PEP block, which were selectively stained with OsO4.

Because of the used hydrogenation method, the PEP block contains ca. 30% residual olefinic double bonds, which can be selectively stained using OsO4. As a consequence, the crystalline PE and PEO domains appear bright (Figure 1.17). The PE block forms a hexagonal array of interconnected PE crystallites, surrounding the crystalline PEO cylinders, both embedded in a matrix of the selectively stained PEP block. This phase assignment has been derived by comparison of TEM images showing different projections with respect to the PEO cylinder long axis in combination with TEM investigations of the completely hydrogenated E19EP40EO41138 triblock copolymer.⁷⁸ The hexagonal array of PE crystallites show strong distortions, but interconnections between several PE crystallites are still clearly visible.

In conclusion, crystallization of PE is not confined into small isolated microdomains, since it occurs from a homogeneous mixture of PEP and PE segments in the melt, resulting in the formation of a continuous crystalline PE phase. In addition, a continuous crystalline PE phase is also observed in SFM investigations on thin films, prepared from toluene solutions.⁷⁸ However, in this case the formation of a continuous PE phase is found to be partially induced by gelation of the polymer solution upon film preparation. The lack of confinement is directly reflected in the crystallization behavior of the PE blocks within PE-b-PEP-b-PEO triblock copolymers. Triblock copolymers with ca. 20 wt-% PE exhibit crystallization temperatures at about 65 to 72 °C⁷⁸ reflecting a heterogeneous nucleation mechanism, since the observed values are very close to the crystallization temperature of ca. 73 °C⁸³ detected in a hydrogenated polybutadiene of similar branching content. In addition, regardless of the low PE content in the investigated PE-b-PEP-b-PEO triblock copolymers (10 - 25 wt-%) all three self-nucleation domains can be located for the PE blocks.¹⁹¹

Mechanical testing reveals poor mechanical properties for the PE-b-PEP-b-PEO triblock copolymers, exhibiting elongations at break below 100%. This might be attributed to the hindered crystallization of PEO in systems with PEO contents below 20 wt-% (Tc ca.

-20 °C). However, cooling of the sample to –30 °C over night in order to induce PEO crystallization results only in an increased Young’s modulus and shows no improvement with respect to the elongation at break. Also E19EP40EO41138 shows a comparatively low elongation at break, despite the fact that here PEO crystallization can take place well above room temperature. In addition, the continuous crystalline PE phase, observed by TEM and SFM investigations, might also contribute to the poor mechanical properties, as it is expected to be easily disrupted upon elongation. From these results, it might be concluded that two different crystalline end blocks, here PE and PEO, are not favorable with respect to good mechanical

One problem encountered in PE-b-PEP-b-PEO triblock copolymers is the hindered crystallization of the strongly confined PEO blocks for PEO contents below 20 wt-%. As a result, large supercoolings (Tc ca. –20 °C) are necessary in order to induce PEO crystalli-zation. It is well known that PEO homopolymers can form well-defined complexes with low molecular weight components like p-nitrophenol (PNP) and resorcinol (RES), resulting in an increase of both melting and crystallization temperatures.^192-195 Investigations on PEO/PNP complexes with a molar ratio of ethylene oxide (EO) units to PNP units of 3/2 (Mn(PEO) ca.

6000 g/mol) showed that these complexes can be isothermally crystallized at temperatures well above room temperature and exhibit a melting temperature range of 75 – 95 °C, depending on the crystallization temperatures employed.¹⁹⁵ To check the applicability of these molecular complexes for increasing the melting and crystallization temperature of the PEO block within PE-b-PEP-b-PEO triblock copolymers, a complex between the PEO end block in E24EP57EO1969 and PNP (molar ratio EO/PNP = 3/2) has been prepared from toluene solution.¹⁹⁶ DSC investigations show that upon cooling at 10 °C/min only the PE blocks are able to crystallize, whereas no crystallization exotherm attributable to the PEO-block/PNP complex can be detected. Upon subsequent heating at about 20 °C a cold crystallization exotherm is observed for the block/PNP complex. The melting transition of the PEO-block/PNP complex shows a shift of approximately 30 °C to higher temperatures compared to that of the neat PEO block within the copolymers, as is extracted from self-nucleation experiments. In addition, an increased capability for self-nucleation of the PEO block is produced by the complexation with PNP. In contrast to the PEO block in the neat triblock copolymer, where domain II vanishes completely¹⁹¹, all three self-nucleation domains are clearly observed for the PEO-block within the E24EP57EO1969/PNP complex.¹⁹⁶ Similar results are obtained by complexation of the same PE-b-PEP-b-PEO triblock copolymer with resorcinol.