Synthesis and mechanical and thermal properties of multiblock terpoly(ester-ether-amide) thermoplastic elastomers with variable mole ratio of ether and amide block

A series of the terpolymers of poly[(trimethylene terephthalate)-block-(oxytetramethylene)-block- laurolactam] with a variable molar ratio of ether and amide block and constant molecular weights of PA12 = 2000 g/mole and PTMO = 1000 g/mole have been obtained. The in ﬂ uence of changes of these molar ratios on the functional properties and the values of phase change temperatures of the products have been determined. The thermal properties and the phase separation of obtained systems were de ﬁ - ned by DSC, DMTA and WAXS methods. The chemical structure of obtained materials was studied by FT-IR and 13C NMR methods. The mechanical and elastic properties of these polymers were evaluated.


INTRODUCTION
Thanks to their excellent mechanical and physical properties thermoplastic elastomers (TPE) have found wide applications in many industrial branches as engineering materials.TPE combines the properties of vulcanized rubber and thermoplastics.These polymers are characterized by high elastic recovery, good fl exibility at low temperatures like vulcanized rubbers, but they have great impact strength and may be processed simply like thermoplastics 1-3 .
The multiblock TPE macromolecule consists of fl exible and rigid blocks with different physical and chemical properties.Flexible blocks are able to form a continuous matrix, most often it is a soft-elastic phase.As a result of aggregation, rigid blocks form a hard phase, usually dispersed.The characteristic feature of TPE is capable of microphase (nanophase) separation 4-6 .As a result of this phenomenon, two (or more) phases are distinguished, differing in the values of physical transformation temperatures.These temperatures determine the broad "plateau" of the modulus of elasticity.A matrix-domain structure is created 2, 3, 6-8 .
TPE properties are the result of combining the features of individual blocks.When designing multi-block systems, the type of blocks should be selected appropriately, paying attention to their chemical structure, chemical and physical properties as well as weight and molar ratios.Due to their properties polyamides (stiffness and excellent thermal and mechanical properties), polyesters (biocompatibility and biodegradability) and polyethers (relatively low price, increasing the elastic properties of the system) are often used as TPE blocks [9][10][11][12][13][14][15][16][17][18][19][20][21][22] .
The aim of the study was to obtain a series of thermoplastic elastomers by reaction of α, ω-dicarboxyl(oli golaurinolactam) (PA12) with oligo(oxytetramethylene) diol (PTMO) and with dimethyl terephthalate and 1,3-propanediol (forming a block of poly(trimethylene terephthalate -during the synthesis) PTT).The effect of changing the molar ratio of ether to amide blocks on the properties of terpolymers obtained was investigated.

Synthesis of multiblock PTT-b-PTMO-b-PA12 polymers
The synthesis of PTT-b-PTMO-b-PA12 terpolymers consisted of the structural modifi cation of PTT macromolecule.Some fragments derived from terephthalic acid have been replaced by a dicarboxylic oligoamide block and some derived from propylene glycol by a diol oligoether block.Synthesis of block terpolymers proceeded as a two-step process.The initial stage was the transesterifi cation reaction of DMT with 1,3-PDO leading to the formation of polyester and the release of methanol (Equation 1) and the esterifi cation (in a separate reactor) of PA12 with PTMO (by-product is water) in the presence of titanate catalyst (Equation 2).From Polish Journal of Chemical Technology, 23, 4, 10-16, 10.2478/pjct-2021-0032 Equation 1. Transesterifi cation reaction of DMT with 1,3-PDO Equation 2. Esterifi cation reaction of PA12 with PTMO humidity) for at least 24 hours.By the methods DSC (TA Instruments, heating-cooling-heating, 10 °C/min), DMTA (Rheovibron DDV-II, profi les for the examinations were made by injection molding) and WAXS (diffractometer Geiger-Flex, 2Θ: 5-38, 2 o C/min) the polymers' physical structure was evaluated.

RESULTS AND DISCUSSION
To study the effect of macromolecule architecture on the properties of the materials obtained, a TPEEA series with a variable molar ratio of PTMO to PA12 block was synthesized.These terpolymers are composed of PTMO and PA12 blocks with a constant molecular weight of 1000 g/mol and 2000 g/mol, respectively, and the PTT block formed during synthesis.The type and molar and weight ratios of the substrates used for synthesis are shown in Table 1.Molar ratios characterize the architecture of terpolymer macromolecules (distribution of blocks) and weight ratios are related to the content of blocks.
The selected properties of TPEEA were presented in Table 2.
TPEEA with [η]>1.25 dL/g have molecular weights ensuring good mechanical properties 23 .All the materials obtained have [η] above this value.The water swelling results indicate the hydrophobic nature of all polymers obtained.The swelling does not exceed 2%, which indicates that water penetrates the amorphous phase of the polymer to a very small extent.Along with the increase in the content of ether blocks in the obtained copolymers, an increase in benzene absorption and a decrease in melting point and hardness is noticeable (Table 2).This proves that PTMO fl exible blocks build the amorphous phase of the TPEEA and create more and better polymer matrix.
All obtained terpolymers have strength curves characteristic of elastomeric materials in which there is no yield point. Figure 1 shows only those samples that differ signifi cantly in the course of the curves.Samples 1 and 2 (PTMO/PA12 = 2 and 2,5 respectively) have similar stress-elongation characteristics for polyesters and the respective amounts of methanol and water, it was concluded that the conversation in the transesterifi cation reaction was 95% and the degree of esterifi cation was 90% (degrees was expressed as the weight ratios of the released methanol or water to the respective stoichiometric amounts of these products).The second stage of the process comprises the proper condensation polymerization of mixed intermediates obtained in the fi rst stage of synthesis.

Methods
The limiting viscosity number [η] of the terpolymers in phenol-tetrachloroethylene mixture (60:40 vol/vol) was determined by an Ubbelohde viscometer II at 30 o C. Optical melting points Tm were determined using a Boetius microscope (HMK type Franz Kustner Nacht KG) at a heating rate of 2 o C/min.Hardness (H) measurements were performed on a Shore A and D apparatus (Zwick, type 3100) according to PN-80/C-04238.Swelling in the benzene and water (pH2O, pb) was performed according to PN-66/C-08932.FT-IR analyses were carried out on a spectrometer Nexus.Spectra were acquired in the scan range 4000-530 cm -1 , with a resolution of 4 cm -1 (4 measurements were made for each material). 13C NMR were performed on spectrometer Bruker Avance 300 MHz.The tensile and elastic data were collected at room temperature with an Instron 4026.These parameters were measured per sample on ten replicates.The speed of the moving clamp was 100 mm/min.Received two types of mechanical hysteresis loops.The fi rst type was obtained by stretching terpolymers at a constant elongation of 100%.Specimens were extended to 100% elongation and then they were allowed to relax and return to the initial gauge length.The cycle was repeated fi ve times.Based on this measurement the per cent elastic recovery and permanent deformation of specimens were calculated.The second one was obtained by stretching a polymer sample from 10% to 100% at elongation growing by 10%.Before testing, all specimens were conditioned without any stress in a standard atmosphere (22 ±2 o C, 65 ±5% In all samples between the fi rst and second cycles, there were large energy losses due to the internal structuring of these copolymers.This behavior suggests that any products made of TPEEA must be pre-stretched before proper use.To better understand the nature of the deformations in the obtained terpolymers, the surface areas between the hysteresis curves and the x axis were calculated and assigned to individual accumulated or released energies during a single stretch-relaxation cycle (Table 3).
polyamides [24][25][26] .Materials 3, 4 and 5 (PTMO/PA12=3; 3,5 and 4) are typical elastomeric materials where the elongation exceeds 200% 27, 28 .The terpolymer with the highest content of fl exible block (over 60%) has low mechanical strength, which is most likely due to the excessive dispersion of the hard domains in the large amorphous phase.To examine in detail the elastic properties of TPEEA, two types of mechanical hysteresis loops characterized by elasticity and stress relaxation in selected polymers were obtained (Figures 2 and 3).
The lowest values of permanent deformation after the fi rst cycle of stretching (elastic returns after 100% elon-  The increase in the amount of PTMO fl exible blocks improves the quality of the amorphous continuous phase, which results in a decrease in dispersed energy (responsible for both elastic and highly fl exible response).Increasing the content of fl exible blocks causes an increase in the volume of the polymer matrix and thus greater dispersion of the crystalline domains.As a result, they can more easily return to their pre-stretching state.This conclusion is confi rmed by the decreasing values of energy accumulated in the fi rst hysteresis cycle.
As the content of the ether block increased, the copolymers showed increasing strain values at break (ε) and decreasing values of breaking stress (σ) and Young's modulus (E).
Analysis of FT-IR indicated the occurrence in the spectrograms of all characteristic bands for esters: symmetrical and asymmetrical stretching vibrations of the CH group in CH2 groups (2), stretching vibrations of the carbonyl group C=O in ester bond (3), asymmetrical (8) and symmetrical (9) stretching vibrations (C-O-C) of ester segments, deformation vibrations of CH groups outside the plane bonds in the aromatic ring (13) and amides: stretching vibrations of NH groups bound by hydrogen bridges (1), I amide band (4), II amide band (5), III amide band (7), IV amide band (13) superimposed on the band corresponding to the vibrations of the CH groups in the aromatic ring.
Analysis of the 13 C NMR spectra showed the presence of all characteristic groups present in the esters, ethers and amides.Signals are noted in the range 26.89-218.53ppm.There are no signals derived from the carbon atoms of the carboxyl group.A 39.5 ppm chemical shift signal corresponds to an aliphatic-aliphatic ester group.Such binding can only be formed by reacting the carboxyl end groups of the oligoamide with an ester or ether block.Analyzing the above, it can therefore be concluded that the oligoamide was incorporated into the polymer macromolecule.
The effect of the PTMO/PA12 molar ratio on the thermal properties and physical structure of the copolymers were examined by WAXS (Fig. 6), DSC (Figs. 7, 8 and Table 4) and DMTA (Fig. 9) methods.
The diffraction pattern of the PA12 homopolymer has one broad diffraction maximum with two extreme points:    For the mixture of these blocks, the well-known phenomenon of Tg depression is observed.As the PTMO/PA12 molar ratio increases, the temperature and melting heat of the crystalline fraction of PTMO blocks increases and the melting endotherms become slender.This demonstrates the improving phase separation of the soft phase.As the content of fl exible blocks increases, the melting enthalpy and the melting point (Tm 1 ) of the PTMO blocks increase.Most likely, an increasingly better-formed polymer matrix is being formed.
There are two melting endotherms in the high temperature area.The thermal effect at a temperature of about 50 o C (Tm 2 ) is characteristic for many melt crystallizing polymers and is responsible for the melting of para-crystalline and microcrystalline formations and defective structures.The second endotherm (Tm 3 ) determines the temperature and heat of fusion of the terpolymer crystalline phase.Endotherms determining the temperature and heat of fusion are very fl attened and reach maxima on average around 100 o C. All melting endotherms in the high temperature area are very wide, which indicates the heterogeneity of the crystalline phase.In addition, they are characterized by a signifi cant decrease in melting point compared to PA12 homopolymer.This indicates that in the synthesized terpolymers the crystalline phase is essentially in decline and is very defective.Confi rmation of this application can be found in the cooling cycle.Cooling DSC curves have broad exotherms of PA12 blocks and increasingly well-developed PTMO exotherms.
Figure 9 shows the effect of temperature on the dynamic, mechanical properties of TPEEA depending on the PTMO/PA12 molar ratio.The temperature spectra obtained are characteristic curves for thermoplastic elastomers.The spectra of the storage modulus have three temperature regions: range from -110 o C to -70 o C (value of the storage modulus above 1 GPa characteristic for the glassy state), range from -70 o C to 15 o C (viscoelastic relaxation processes), range from 15 o C to 70 o C (highly elastic state observed in the form of "Plateau of fl exibility").All tgδ maxima are the superposition of at least two relaxation transitions associated with the glassy transitions of PTMO (α) and interphase (α') (a mixture of blocks occurring at the phase boundary resulting from covalent bonds between blocks).As the PTMO/PA12 molar ratio increases, the tgδ maxima shift towards lower temperatures.Maximum α" is a relaxation transition associated with the amorphous phase of PA12.

CONCLUSIONS
Synthesis parameters (temperature, time, pressure, catalyst concentration) of a new type of terpoly(ester-bether-b-amides) with a variable molar ratio of ether to amide block were selected.The copolymer microstructure was assessed based on DSC, DMTA and WAXS analyses.The synthesized copolymers were found to be typical elastothermoplasts and have a multi-phase (crystallineamorphous) physical structure.The amorphous phases (matrix) are made of PTMO fl exible blocks contaminated with short ester sequences (Tg 1 ) and a mixture of ester and amide blocks (Tg 2 ).The crystal phase (domains) is made of PA12 rigid blocks and is disturbed by inclusions of PTT blocks.With the increasing molar ratio of ether to amide block in TPEEA, the proportion of soft phase increases, while the content of hard phase and interphase decreases.The synthesized multi-block copolymers were characterized by good mechanical properties, including good fl exibility, thermostability, and thermal and chemical resistance.TPEEA can successfully meet the specifi c requirements of many industries.

Figure 3 .
Figure 3. Mechanical hysteresis loops at elongation growing by 10% of the terpolymers with varied molar ratio of ether and amide block and constant molecular weights of PA12 = 2000 g/mole and PTMO = 1000 g/mole

Figure 2 .Figure 1 . 6 Table 3 .
Figure 2. Mechanical hysteresis loops (5 cycles) at constant elongation by 100% of the terpolymers with varied molar ratio of ether and amide block and constant molecular weights of PA12 = 2000 g/mole and PTMO = 1000 g/mole.A -area proportional to elastic dissipated energy, B -area proportional to highly elastic dissipated energy, C -area proportional to accumulated energy, ε ps -irreversible elongation (permanent), ε hs -highly fl exible elongation, ε s -elastic elongation

Table 2 .
Properties of the TPEEA with varied molar ratio PTMO/PA12 and constant molecular weights of PA12=2000 g/mole and PTMO=1000 g/mole gation reached 80%) showed terpolymers with a molar ratio of PTMO/PA12 blocks in the range from 3 to 4.

Table 4 .
Selected thermal properties of the terpolymers produced