Synthesis of thermoplastic polyesters
The syntheses of thermoplastic polyesters: poly(ethylene terephthalate) and poly(trimetylene terephthalate) (Fig. 21) were performed in steel reactor (Autoclave Engineers, Pennsylvania,
48 USA) with a capacity of 1000 cm3. Due to the similarities between the synthesis processes of both polymers their scheme for PET can be used describes both.
1 step – Transestrification between dimethyl terephtalate (DMT) and ethanediol to bis(2-hydroxyethyl) terephthalate.
2 step- Polycondensation of bis(2-hydroxyethyl) terephthalate to poly(ethylene terephthalate).
In case of synthesis of PTT transesterification reactions took place between dimethyl terephtalate (DMT) and 1,3-propanediol resulting in formation bis(3-hydroxypropyl) terephthalate (BHTP) that then undergo a condensation polymerization reaction, leading to the formation of poly (trimethylene terephthalate) with the effluence of 1,3-propanediol as a by- product.
The substantial differences in obtaining thermoplastic polyesters included the use of different catalysts. In case of PET, zinc acetate and antimony dioxide were used as catalyst in transestryfication and polycondensation reactions respectively. Whereas in case of poly(trimetylene terephthalate) and in both steps of in situ polymerization, tetrabutyl
49 orthotitaniate was used as catalyst. The second fundamental difference accounted for the values of polycondensation process temperatures. PTT was synthesized at the temperature of 260 oC, while PET at about 15oC higher temperature, i.e. 275oC.
Fig. 21 Molecular formula of synthesized polyesters: x= 2 - PET, x=3 – PTT.
The amounts of reactants used in the synthesis were selected depending on the assumed amount of the final product, according to the calculations (Table 5).
The polymerization process was conducted in two stages. In the first stage, the dispersion of SWCNTs/EG or hybrid system (EG+SWCNTs) in ED, DMT and zinc acetate catalyst was charged into1L steel reactor (Autoclave Engineers Inc, USA) equipped with a vacuum pump, condenser, and cold trap for collecting the by-products. Transesterification reaction took place between dimethyl terephtalate (DMT) and ethanediol under nitrogen flow at atmospheric pressure and in a temperature range of 160÷180oC. The ethanediol was used in 50 mol % excess over the dimethyl ester. The methanol formed during the transestrification was distilled off and collected. The first stage of the polymerization process was finished when the amount of formed methanol was close to the theoretical one. Then the second stage was begun: antimony dioxide as catalyst was added, the pressure was gradually lowered to about 0.1 hPa and the polycondensation was carried out at temperature of 275oC and under continuous stirring (stirrer speed 40 min-1). The progress of the polymerization was monitored by measuring the changes of viscosity of the polymerization mixture, i.e. an increase in torque stirrer values during the polycondensation. The reaction was considered complete when the viscosity of the system increased to 14Pa.s. The obtained polymer/nanocomposite was extruded from the reactor under nitrogen flow in the form of polymer wire.
Table 5The amount of raw materials used to obtain 100g of PET and PTT
Estimated weight of the obtained polymer (PET) 100 g
DMT mass: mDMT=MDMT(mPET/MPET) 101.04 g
ED mass: mED=MED(mPET/MPET) 48.44 g
OC mass: 0.25 wt % mDMT 0.25 g
TA mass: 0.25 wt % mDMT 0.25 g
Amount of effluent CH3OH obtained during transestrification (theoretical) 33.3 g MDMT= 194 g/mol, MPET= 192 g/mol (molar weight of mer); MED=1.5 x 62= 193 g/mol; MCH3OH= 2 x 32= 64 g/mol; (used multipliers are due to the reaction stechiometry)
Estimated weight of the obtained polymer (PTT) 100 g
DMT mass: mDMT=MDMT(mPTT/MPTT) 94.17 g
PDO mass: mPDO=MED(mPTT/MPTT) 73.78 g
TiBu mass: 0.25 wt % mDMT 0.24 g
Amount of effluent CH3OH obtained during transestrification (theoretical) 31.07 g MDMT= 194 g/mol, MPTT= 206 g/mol (molar weight of mer); MPDO=2 x 76= 152 g/mol; MCH3OH=2 x 32= 64 g/mol; (used multipliers are due to the reaction stechiometry)
50 Synthesis of block copoly(ether-ester) (PTT-block-PTMO)
PTT-PTMO block copolymer was synthesized according to the same procedure as previously described for PET and PTT.The only difference was that when the first step of reaction was completed, poly(tetramethylene oxide) glycol with molecular weight of 1000 g/mol was added. The chemical formula of the synthesized copolymer is shown in Figure 22.
Multiblock poly(ether–ester) (PEE) based on poly(butylene terephthalate) (PBT) as rigid segments and poly(tetramethylene oxide) (PTMO) as soft segments have been intensively studied [314] [315]. Due to their excellent mechanical properties, like strength and elastic properties in a wide temperature range they are of special interest. The PBT-block-PTMO copolymers are available as commercial products (ElitelTM, Arnitel, Hytrel®, DSM etc.).
Recently a study on a novel family of polyester thermoplastic elastomers based on PTT has been conducted [316] [317].
C C
O O
O
CH2 CH2
O 3 O
4 C C
O O
O
x
PTT rigid segment PTMO flexible segment
n y
TT x PTMO
T y
Fig. 22 Chemical structure of block copoly(ether ester) (PTT-block-PTMO).
1step – Transestrification between dimethyl terephtalate (DMT) and 1,3-propanediol to bis(2-hydroxypropyl) terephthalate (BHPT). During this stage, except of BHPT its low molecular weight oligomers can be formed.
2 step- Polycondensation of bis(3-hydroxypropyl) terephthalate and its oligomers with poly(tetramethylene ether glycol terephthalate) (PTMO-T) unit to PTT-block-PTMO copolymer. The obtained block copolymer is presented as a random copolyester (TT units and PTMO-T units).
51
C C
O O
O
CH2 CH2
O
3 O
4 C C
O O
O
x
PTT rigid segment PTMO flexible segment
n y
where x, y- degree of polymerization of rigid and flexible segments respectively [316].
The amounts of reactants used in the synthesis were selected depending on the assumed amount of the final product, according to the calculations (Table 6). Nanocomposites were synthesised by melt transesterification and subsequently polycondensation as follows. In the first stage, an appropriate amount of nanofiller (Graphene Ang, SWCNT) was dispersed in 250 ml of PDO using ultra-high speed stirrer high (Ultra-Turrax® T25) and ultrasonic homogenizer (Sonopuls HD 2200, Bandelin). Completed time of dispersing was 30 min.
Additionally, in case of Graphene Ang, ultra-low power sonic bath was used for 8h. Then the dispersion of nanofillers in PDO, DMT and TBT catalyst was charged into1L steel reactor (Autoclave Engineers Inc, USA) equipped with a vacuum pump, condenser, and cold trap for collecting the by-products. The transesterification reaction was carried out under a constant flow of nitrogen at 165°C in the presence of catalyst (TBT, 0.15 wt% in relation to DMT) for one and half hour. During the reaction, methanol was distilled off. The conversion of the transesterification reaction was calculated by monitoring the amount of effluent methanol.
When the reaction of DMT with PDO reach conversion level of 90 %, PTMG was introduced in the reactor, together with Irganox 1010 (0.5 wt% of total comonomers mass) and second portion of catalyst (TBT, 0.10 wt% in relation to DMT), and in subsequent step the reaction mixture was heated slowly under reduced pressure. The second step, melt polycondensation was carried out at 250 °C and under reduced pressure of 15-20 Pa. The stirring torque change was monitored in order to estimate the melt viscosity of the product. Synthesis was finished when melt rich a fixed value of melt viscosity corresponding to high molecular weight copolymer. The obtained nanocomposite was extruded from reactor under nitrogen, cooled to room temperature in water bath and granulated. The neat PTT-PTMO copolymer was synthesized following the same procedure, without nanofillers. The content of rigid segments based on PTT was 50 wt% and the content of soft segments based on PTMO was 50 wt%.
52
Table 6The amount of raw materials used to obtain 100g of PTT-block-PTMO
Estimated weight of the obtained polymer (PTT-block-PTMO) 100 g
DMT mass 58.16 g
PDO mass 28.9 g
PTMO mass 48g
Catalyst mass: TiBu 0,25 wt % mDMT 0.15 g
Stabilizer mass: 0,5 wt % mPEE 0.5 g
Calculations performed for a copolymer having a weight ratio of soft segments to the rigid of: PTT/PTMO 50:50 wt/wt;
Substrates molar mass was calculated from the stoichiometric reaction of the polymers for PTT-b-PTMOT – 1881.9 g/mol; x=5.48 x – amount (in moles) of PTT on y=1 soft segment of PTT-b-PTMOT);
MDMT=194 g/mol, MPTMO=1000 g/mol; MPTT=206 g/mol; MPDO – 76 g/mol;