Copoly(ether ester)s are multiblock copolymers consisting of polyester hard segments and low molecular weight polyether soft segments. The crystalline hard segments typically consist of poly(butylene terephthalate) (PBT) or poly(ethylene terephthalate) (PET), sometimes also poly(butylene isophthalate) (PBI) is used.115 The soft segment comprises different hydroxy telechelic polyethers, like poly(ethylene oxide) (PEO), poly(propylene oxide) (PPO), poly(tetramethylene oxide) (PTMO), and poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) (PEO-b-PPO-b-PEO) triblock copolymers.116 Copoly(ether ester)s were discovered independently in the 1950s by Imperial Chemical Industries and Du Pont by the incorporation of PEO into PET.117,118 The synthesis of PBT based copoly(ether ester)s has been studied intensively by Hoeschele and co-workers (Du Pont).116,119,120 In analogy to the preparation of PBT121,122, the synthesis is accomplished by a 2 step melt polycondensation of a mixture of dimethyl terephthalate, 1,4-butandiol, and a low molecular weight polyether in the presence of a suitable catalyst (Scheme 1.1).
In the first step transesterification between dimethyl terephthalate and the diol components occurs at ca. 200 °C under formation of a prepolymer. Usually an excess of 1,4-butandiol is used to accelerate the formation of the pre-polymer. The released methanol from the transesterification reaction is removed by distillation. Polycondensation proceeds in the second step under release of 1,4-butandiol. Here the temperature is increased to ca. 250 °C and vacuum is applied in order to distill of surplus 1,4-butandiol. The degree of polymerization strongly depends on the complete removal of the formed 1,4-butandiol during the second step, since the polycondensation reflects an equilibrium reaction. Usually tetrabutyl orthotitanate is used as catalyst. More recent investigations show that mixtures of tetrabutyl orthotitanate with lanthanide- and hafnium-acetylacetonate catalysts exhibit a higher activity compared to pure tetrabutyl orthotitanate.123,124 In addition, the polymerization time can be significantly reduced using this novel catalyst system.
1. Step (transesterification)
Scheme 1.1: Preparation of copoly(ether ester)s by a 2 step melt polycondensation.
Copoly(ether ester)s are multiblock copolymers with alternating hard and soft segments along the polymer chain (Figure 1.8). In these materials the soft polyether chains act as network chains, while the polyester hard segments form crystalline domains acting as physical (thermoreversible) crosslinks. The high melting point of the polyester hard segment (PBT, Tm = 220 °C) in combination with the low glass transition temperature of the polyether soft segment (Tg ca. –60 °C) results in a rubber like behavior over a wide temperature range.
hard soft hard soft hard PBT
Tm220°C
Polyether Tg-60°C hard soft hard soft hard PBT
Tm220°C
Polyether Tg-60°C hard soft hard soft hard PBT
Tm220°C
Polyether Tg-60°C
Figure 1.8: Schematic representation of the multiblock structure of copoly(ether ester)s.
Commercially important are copoly(ether ester)s based on PBT hard segments. The two main commercial grades are Hytrel® (Du Pont) and Arnitel® (DSM). The mechanical properties can be adjusted by variation of the amount and block length of hard and soft segments, which in turn creates a wide range of properties. PBT based copoly(ether ester)s show good tear, fatigue, high abrasion and solvent resistance as well as very good low- and high-temperature properties. Thus, these materials are used in applications where severe requirements are demanded towards stiffness and strength at high and low temperatures. For Arnitel grades the main market segments are: automotive (constant velocity joints, air bag covers), hose and tube (hydraulic tubing, cover jackets for fire hoses), wire and cable (fiber optic applications, steel cable sheaths, retractable coil cords for telephones), and film (breathable films for sportswear, shoes, rainwear, etc.).
The morphology of PBT-PTMO based copoly(ether ester)s has been studied intensively.116,119,125-129 It is generally assumed that the structure can be described by a two-phase model consisting of a crystalline PBT hard two-phase and a mixed PBT-PTMO soft two-phase, both being co-continuous (Figure 1.9).119,125,128,129 Because of the miscibility of PBT and PTMO segments in the melt structure formation upon cooling is induced by crystallization, resulting in the formation of the characteristic two-phase structure consisting of interconnected PBT crystallites embedded in an amorphous matrix of mixed PBT and PTMO segments.126,130 However, more recent studies utilizing solid-state NMR131 and thermomechanical analysis132 demonstrate that the amorphous phase is not homogeneous, but consists of a PTMO-rich phase and a PBT/PTMO mixed phase.
The structure of the crystalline polyester hard segment phase strongly depends on the crystallization conditions. Different structures have been reported: next to lamellar128,133-135, spherulitic125,127,136,137, dendritic125,136, and even shish kebab structures125.
PBTc
amorphous PBT/PTMO
Figure 1.9: Schematic representation of the two-phase structure of PBT-PTMO-based copoly(ether ester)s (PBTc corresponds to crystalline PBT domains).128
The presence of a co-continuous PBT hard phase in PBT-PTMO based copoly(ether ester)s causes a significant plastic deformation and hence minor elastic properties of these materials especially upon relatively large elongations.138 Orientation studies revealed that upon elongation, the soft segments orient parallel to the direction of the applied stress139, whereas the crystalline hard segments orient transverse to the stress direction for small strain values. Upon higher elongations the crystalline PBT segments orient parallel to the direction of stress, which is connected with an irreversible disruption of the continuous crystalline hard segment phase.140 This in turn results in the observed high plastic deformations especially at high strains. Finally, after complete reorientation of the crystalline PBT phase the stress is submitted through the continuous soft segment phase, until it breaks.
The general idea is that the elasticity of copoly(ether ester)s can be improved by changing the continuous PBT hard phase to a dispersed phase (Figure 1.10). This can be achieved by increasing the incompatibility of the hard and soft segments, as was demonstrated in thermoplastic polyurethanes141,142, and in strongly phase separated copoly(ether ester aramides)143.
In this work (cooperation with DSM Research, Geleen) the incorporation of hydroxy telechelic hydrogenated polybutadiene soft segments (HO-PEB-OH, KRATON® liquid Polymer HPVM-2203 (Shell)) into PBT based copoly(ether ester)s in order to improve the elasticity of common PBT-PTMO based systems has been investigated. The high incompatibility of the nonpolar PEB segments should result in an extreme phase separation between the PEB and the PBT segments in the melt, and thus in a dispersed PBT hard phase,
copoly(ether ester)s is limited due to macrophase-separation during the melt polycondensation process. This was shown for poly(butylene terephthalate)-block-polyisobutylene segmented block copolymers with polyisobutylene soft segments.144,145 Due to the high incompatibility of polyisobutylene with the polar reactants dimethyl terephthalate and 1,4-butandiol phase-separation occurs during the melt polycondensation, resulting in a very poor incorporation of the soft segment. The macrophase-separation can be reduced to some extent by using high boiling solvents like m-cresol and 1,2,4-trichlorobezene, which are good solvents for PBT and polyisobutylene. The solvent is removed together with surplus 1,4-butandiol in the polycondensation step by applying vacuum during polymerization. Nevertheless, incorporation of polyisobutylene is incomplete, which in turn results in poor mechanical properties.
A B
Figure 1.10: Schematic representation of a continuous (A) and a dispersed (B) crystalline hard phase.
The approach used in this work to avoid macrophase-separation implies the chain extension of HO-PEB-OH (Mn = 3600 g/mol) with ethylene oxide by means of anionic ring-opening polymerization to yield the corresponding PEO-b-PEB-b-PEO triblock copolymers.
The polar PEO blocks are expected to act as compatibilizer between the nonpolar PEB block and the polar PBT segments, thus resulting in a homogeneous reaction mixture during melt polycondensation. Several PEO-b-PEB-b-PEO triblock copolymers with varying PEO block length have been synthesized and successfully incorporated into PBT-based copoly(ether ester)s.146 Copoly(ether ester)s with PBT contents below 45 wt-% and PEO-b-PEB-b-PEO triblock copolymers exhibiting a PEO block length < 1400 g/mol show a clear melt during melt polycondensation. This demonstrates, that the PEO blocks efficiently act as compatibilizer between the nonpolar PEB blocks and the polar PBT segments. The
comparatively high molecular weight of the PEO-b-PEB-b-PEO soft segments (Mn = 5300 – 8600 g/mol) results in an increased average PBT hard segment length, compared to the case of conventional PBT-PTMO-based copoly(ether ester)s with an average Mn of the PTMO soft segment between 1000 and 2000 g/mol, assuming similar PBT contents. This in turn results in a comparatively higher melting point of the PBT hard segments (Tm = 190 – 220 °C) in PEO-b-PEB-b-PEO based copoly(ether ester)s.
Dynamic shear experiments in combination with small-angle X-ray scattering (SAXS) reveal that crystallization of the PBT hard segments occurs from a microphase-separated melt.147 This in turn results in the formation of a dispersed PBT hard phase, as is demonstrated by transmission electron microscopy (TEM) and scanning force microscopy (SFM). As an example the TEM micrograph of PBT30-1380 is shown in Figure 1.11.
Because of the used staining technique (RuO4), the crystalline PBT domains remain unstained and appear as bright regions, which are clearly dispersed within the matrix of the PEO-b-PEB-b-PEO soft segment.
0.5 µm
Figure 1.11: TEM micrograph of PBT30-1380 (30 wt-% PBT. Mn(PEO) = 1380 g/mol) stained with RuO4 vapor, showing dispersed crystalline PBT domains.
The microphase structure has been investigated in more detail applying differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA).147 The results indicate a pronounced microphase separation in the soft segment phase, exhibiting a pure microphase separated PEB phase. This is reflected by the observation of a glass transition temperature at ca. –60 °C, which is independent of composition. In addition, glass transition temperatures attributable to a mixed amorphous PEO/PBT phase and a pure amorphous PBT phase are
endotherm is observed for the PEO blocks, indicating the presence of a PEO rich phase, enabling crystallization of PEO. Thus, from the combination of results obtained by DSC and DMA a structure model can be proposed as depicted in Figure 1.12. The copoly(ether ester)s with PEO-b-PEB-b-PEO triblock copolymer soft segments consist of a crystalline PBT phase and an amorphous phase, which can be divided into a pure PEB phase, a PEO-rich phase besides a mixed PEO/PBT phase, and a pure amorphous PBT phase. However, the existence of a pure amorphous PBT phase and a mixed amorphous PEO/PBT phase cannot be proven from the performed DMA experiments. To provide more evidence for the proposed different phases, the PEB containing copoly(ether ester)s have been studied in more detail at DSM Research using solid-state NMR.
Figure 1.12: Schematic representation of the proposed structure of copoly(ether ester)s with PEO-b-PEB-b-PEO soft segments.
Solid-state NMR is a powerful tool to study the microphase structure of polymers.148 NMR relaxation experiments are of special interest, since relaxation times are highly sensitive towards differences in chain mobility, and thus provide information about morphological changes. A combination of 13C inversion recovery cross-polarization measurements (IRCP), proton-T1ρ relaxation experiments, and investigations on PEB based copoly(ether ester)s with selectively deuterated PBT segments using 2H-solid-state echo and inversion recovery-T1
techniques has been applied to confirm the structure model proposed from DSC and DMA investigations.149
The IRCP experiment distinguishes between carbons with high and low mobility. This enables the study of the molecular mobility of the hard and soft segments within PEB-based copoly(ether ester)s. The experiment is composed of two contiguous parts. The first part is a classical cross-polarization step, during which magnetization is transferred from protons to carbons for a contact time τ1 in order to enhance the 13C signal. In the subsequent step (τ2) the carbon magnetization is inverted. The rate of this inversion is determined by the cross-polarization dynamics. The cross-cross-polarization rate depends on the strength of the magnetic dipole-dipole coupling between 13C and 1H spins, which in turn is affected by molecular motions. For rigid segments showing slow motions, the cross-polarization is relatively fast.
On the contrary, in the case of fast motions the cross-polarization is a relatively slow process.
Therefore, it can be expected that the magnetization of the crystalline PBT hard segments inverts faster than that of the PEO and PEB segments in the soft phase (Figure 1.13).
τ
1τ
2Figure 1.13: Magnetization build-up and decay during an IRCP experiment.
As an example, the results from IRCP investigations will be described for the PEO segments in the following.149 The IRCP measurements show that the PEO resonance is actually composed of two parts, exhibiting different inversion times. This is contributed to PEO segments showing different mobility. The resonance that inverts faster is attributed to an amorphous PEO-rich phase exhibiting a higher mobility. The resonance with a higher inversion time corresponds to a mixed amorphous PEO/PBT phase, reflecting a restricted mobility due to partial mixing with the more rigid PBT segments (Tg ca. 50 °C).
In summary, the IRCP results indicate that the amorphous phase is composed of a highly mobile PEO-rich phase, a PEO/PBT mixed phase, and a pure PEB phase. This assignment is in agreement with the DSC and DMA results and has been further underlined by 1H-τ1ρ relaxation experiments and 2H-solid-state echo measurements on copoly(ether ester)s with selectively deuterated PBT segments.149 However, from these experiments it is not possible to prove the existence of a pure amorphous PBT phase, as was concluded from the observation of a glass transition temperature at ca. 50 °C.147 Therefore, additional inversion-recovery solid state deuterium NMR investigations on deuterated PBT and copoly(ether ester)s with selectively deuterated PBT segments have been performed, and confirm the presence of a pure amorphous PBT phase in PEB-based copoly(ether ester)s with
Morphological investigations show that the nonpolar PEB segments in copoly(ether ester)s with PEO-b-PEB-b-PEO triblock copolymer soft segments induce a pronounced microphase-separation within the soft segment phase. This results in the formation of a dispersed PBT hard segment. This in turn is expected to improve the elasticity of these materials compared to the case of conventional PBT-PTMO-based copoly(ether ester)s exhibiting a continuous PBT hard phase. Mechanical testing reveals a significantly improved elastic recovery for the copoly(ether ester)s based on PEO-b-PEB-b-PEO soft segments.147 As an example, the stress-strain traces obtained for a PEB-based copoly(ether ester) with 20 wt-% PBT (PBT20-1000) and a PTMO-based copoly(ether ester) (PBT1000/50) are compared in Figure 1.14. It can be clearly deduced, that the elastic recovery is significantly improved by changing the continuous PBT hard phase in PBT1000/50 to a dispersed hard phase in PBT20-1000.
0 10 20 30 40 50 60 70 80 90 100 0
2 4 6 8 10 12
εplast PBT20-1000 PBT1000/50
Stress [MPa]
Strain [%]
Figure 1.14: Comparison of hysteresis measurements for PBT20-1000 (20 wt-% PBT, Mn(PEO) = 1000 g/mol) and PBT1000/50 (50 wt-% PBT, Mn(PTMO) = 1000 g/mol), a PBT-PTMO-based copoly(ether ester) exhibiting a continuous PBT hard phase.