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    Transworld Research Network37/661 (2), Fort P.O., Trivandrum-695 023, Kerala, India

    Polymeric Materials, 2009: 29-48 ISBN: 978-81-7895-398-4

    Editor: Aleksandra B. Nastasovi

    2Thermoplastic elastomersbased on poly(butylene

    terephthalate) and various

    siloxane prepolymers

    Vesna V. Anti1, Marija V. Vukovi1, Milica R. Balaban1

    Milutin N. Govedarica1 and Jasna Djonlagic2

    1

    Polymer Department, Center of Chemistry, Institute of ChemistryTechnology and Metallurgy, Studentski trg 12-16, 11000 Belgrade, Serbia2Faculty of Technology and Metallurgy, Karnegijeva 4, 11000 Belgrade, Serbia

    AbstractThermoplastic poly(ester-siloxane)s (TPES)

    and poly(ester-ether-siloxane)s (TPEES), based on

    poly(butylene terephthalate) (PBT) as the hard

    segment and different siloxane-containing prepolymers

    as the soft segments were synthesized by catalyzedtwo-step transesterification/polycondensation reaction.

    Incorporation of disilanol- or dicarboxypropyl-

    terminated poly(dimethylsiloxane)s (PDMS-OH and

    PDMS-CP) into the polar poly(butylene terephthalate)

    Correspondence/Reprint request: Dr. Vesna V. Anti, Polymer Department, Center of Chemistry, Institute of

    Chemistry, Technology and Metallurgy, Studentski trg 12-16, 11000 Belgrade, Serbia

    E-mail: [email protected]

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    Vesna V. Antiet al.30

    chains resulted in rather inhomogeneous TPES copolymers. The inhomogeneity

    was a consequence of a pronounced phase separation of the polar and non-polar

    reactants during the synthesis. In order to avoid or reduce phase separation,

    siloxanecontaining triblock prepolymers with hydrophilic terminal blocks, such

    as ethylene oxide (EO), poly(propylene oxide) (PPO) or poly(caprolactone)(PCL), were investigated in order to improve the compatibility of corresponding

    prepolymer with the polar reactants, dimethyl terephthalate (DMT) and 1,4-

    butanediol (BD). Homogeneity was significantly improved in the case of

    copolymers with poly(caprolactone)-poly(dimethylsiloxane)-poly(caprolactone)

    soft segments, but the melting temperatures and degrees of crystallinity of the

    resulting copolymers were much lower in comparison with all the previous

    synthesized series of copolymers. Thermal stability of the obtained copolymers

    was higher in comparison with PBT homopolymer, with exception of PDMS-CP

    based samples, which were somewhat less stable.

    IntroductionThermoplastic elastomers represent polymeric materials that combine the

    elasticity and flexibility of chemically cross-linked elastomers and the easinessof processability of thermoplastic materials. Combined thermoplastic and

    elastomeric behavior is characteristic for block copolymers, as well as for

    mixtures of elastomers and thermoplasts. Block copolymers belonging to theclass of thermoplastic elastomers consist of two types of chemically incompatible

    segments: hard, glassy or crystalline blocks, which provide for physical

    crosslinking, and soft, flexible blocks with a low glass transition temperature,which provide for elasticity. The chemical incompatibility of the hard and

    soft blocks leads to phase segregation and the formation of a two-phasemicrostructure. Due to the two-phase microstructure, thermoplastic elastomers

    possess excellent mechanical properties, such as low-temperature flexibility,impact strength, toughness and high modules in the rubbery plateau region.

    At elevated temperatures, the physical crosslinking disappears, allowing the

    copolymer to soften and flow like thermoplastic materials thus enabling it to

    be processed in the melt by techniques such as injection molding [1, 2].

    The properties of thermoplastic elastomers depend on the chemical nature of

    the two types of segments, hard and soft, as well as on their mass ratio andrespective lengths [1, 2]. Organic-siloxane block copolymers, due to the presenceof poly(siloxane) chains, show lower glass transition temperatures, higher thermal

    and thermo-oxidative stability, as well as higher resistance to UV-radiation,

    atomic oxygen and ozone, enhanced permeability to many gases, hydrophobicity, biocompatibility and resistance to many solvents in comparison to common

    organic polymers. As a result of the mentioned properties, organic-siloxane

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    Thermoplastic elastomers based on siloxane prepolymers 31

    copolymers have received special attention as elastomers, protective coatings,

    photoresists, biomaterials, gas separation membranes, emulsifiers, etc [3].Reactive, telechelic siloxane oligomers are the most important starting

    material for the synthesis of siloxane-containing copolymers which behave as

    thermoplastic elastomers. Step-growth polymerization is usually used for thesynthesis of such organo-siloxane copolymers, mainly due to the availability

    of a wide variety of well-defined, organofunctionally terminated reactive

    siloxane oligomers (,-telechelic siloxanes).

    The main factors determining the reactivity of telechelic siloxane oligomers

    toward other reactants are the type and nature of the terminal functional groups.Due to the fundamental differences in their structures, chemical reactivities and

    overall properties, it is possible to divide functionally terminated siloxane

    oligomers into two groups. The first group consists of oligomers with Si-Xterminal units and the other with Si-R-X units, where Xand R represent the

    reactive functional group and a short organic moiety, respectively.In most of the published reports concerning siloxane containing

    copolymers, the soft segment was almost exclusively poly(dimethylsiloxane).

    Poly(dimethylsiloxane) has one of the lowest values of glass transition

    temperature (Tg = -123 C) and the largest molar volume (75.5 cm3/mol). Chain

    flexibility and low intermolecular forces are also responsible for its low surface

    tension, low solubility parameters and low dielectric constant. Moreover, theseproperties show only a very small variation over a wide range of temperatures,

    which is also an important characteristic of poly(siloxane)s [3, 4]. Theextremely non-polar nature of PDMS, together with the very low level of inter-

    and intramolecular attractions, leads to the formation of thermodynamically and

    mechanically incompatible blends with virtually all other polymers. This is

    reflected in the very low value of the solubility parameter of PDMS [ =15

    (J/cm3)

    1/2] [3, 5], when compared with other polymers [ =15.4-28.0 (J/cm

    3)

    1/2]

    [6]. This is the most important driving force in the formation of a two-phasemicrostructure in PDMS containing copolymers. Another very important factorwhich makes the morphology and structure-property relationships of siloxane

    copolymers somewhat unusual when compared with conventional organic block copolymers is the fact that at room temperature (20-25 C), at which

    most of polymer experiments are conducted, PDMS is about 150 C above itsglass transition temperature. At these temperatures, due to the absence of inter

    and intramolecular interactions, PDMS behaves like a non-polar viscous liquid,thus providing perfect conditions for the formation of phase separated

    copolymer structures. In many cases, a siloxane molar mass as low as 500-600g/mol (6-8 siloxane repeat units) and an organic segment having only a single

    repeat unit are sufficient to obtain two-phase morphologies [3].

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    Vesna V. Antiet al.32

    Several studies on thermoplastic elastomers based on soft

    poly(organosiloxane)-based segments and hard poly(butylene terephthalate)

    segments have been presented and described in the literature since 1990 [7-

    26]. Poly(organosiloxane)s exhibit many important and interesting properties,

    such as low-temperature flexibility, high thermal and thermo-oxidative

    stability, good biocompatibility, low surface energy, ultraviolet resistance and

    high permeability to many gases [3, 4]. The incorporation of poly(organosiloxane)-containing segments into a PBT-backbone results in

    improved clarity, surface smoothness and non-sticking properties, as well as

    good film, fiber and hydrophobic properties of the resulting copolymers [7-16].

    Our research in the field of siloxane containing copolymers commenced withthe synthesis of poly(estersiloxane)s (TPES) based on poly(butylene terephthalate)

    (PBT) as the hard segment and poly(dimethylsiloxane) prepolymers as the softsegments. The TPES copolymers were synthesized by catalyzed two-step

    transesterification, from dimethyl terephthalate, (DMT), 1,4-butanediol, (BD) andpoly(dimethylsiloxane)s with either disilanol- (PDMS-OH) or dicarboxypropyl-

    (PDMS-CP) terminal groups. Incorporation of PDMS-OH or PDMS-CP

    homopolymers into the polar PBT chains resulted in rather inhomogeneous TPES

    copolymers, because of the pronounced phase separation of the polar and non-

    polar reactants during synthesis [21-24]. All the copolymers prepared werepartially soluble in chloroform, as a consequence of their significant structural

    and compositional inhomogeneity. Siloxane-containing triblock prepolymers

    with hydrophilic terminal blocks (which served as a compatibilizer between the

    extremely non-polar PDMS and the polar DMT and BD), such as ethylene oxide(EO), poly(propylene oxide) (PPO) or poly(caprolactone) (PCL) [25, 26], wereused in further investigations to avoid phase separation during copolymer

    synthesis and to increase the miscibility of the polar and non-polar reactants.The structure, composition and size of the poly(ester-siloxane)s and poly(ester-

    ether-siloxane)s macromolecules were investigated by1H NMR and

    13C NMR

    spectroscopy and the viscosity of dilute solutions. The thermal properties ofsamples from all the synthesized series were investigated by differential

    scanning calorimetry (DSC) and thermogravimetric analysis (TGA).

    Experimental1. Siloxane-containing prepolymers

    ,-Dicarboxypropyl-poly(dimethylsiloxane)s (PDMS-CP, nM =1030

    g/mol) and their dimethyl-esters were prepared as described previously [21,

    27]. ,-Disilanol-poly(dimethylsiloxane) (PDMS-OH, nM =2400 g/mol),

    ,-dihydroxy-poly(ethylene oxide-dimethylsiloxane-ethylene oxide (EO-

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    Thermoplastic elastomers based on siloxane prepolymers 33

    PDMS-EO), ,-dihydroxy-poly(propylene oxide-dimethylsiloxane-propylene

    oxide) (PPO-PDMS-PPO) and ,-dihydroxy-poly(caprolactone-dimethylsiloxane-caprolactone) (PCL-PDMS-PCL) were supplied from

    ABCR (Germany). The number-average molar masses of the EO-PDMS-EO,

    PPO-PDMS-PPO and PCL-PDMS-PCL, were determined by 1H NMR

    spectroscopy. The nM of the prepolymer EO-PDMS-EO was 1200 g/mol,

    the molar mass of central poly(dimethylsiloxane) (PDMS) block was

    1080PDMSM = g/mol, and the terminal ethylene oxide, (EO) consisted of

    one unit. The number-average molar mass of the PPO-PDMS-PPO was 2900g/mol, with a molar mass of the central poly(dimethylsiloxane) (PDMS)

    block of 1100PDMSM = g/mol and a molar mass of the terminal

    poly(propylene oxide) (PPO) blocks of 900PPOM = g/mol. The nM of the

    PCL-PDMS-PCL was 6100 g/mol, the molar mass of the centralpoly(dimethylsiloxane) (PDMS) block was 2000PDMSM = g/mol. The molar

    mass of the terminal poly(caprolactone) (PCL) blocks was 2050PCLM = g/mol.

    2. Poly(ester-siloxane) and poly(ester-ether-siloxane) synthesisThe poly(ester-siloxane)s and poly(ester-ether-siloxane)s were

    synthesized by catalyzed two-step reactions involving transesterification

    and polycondenzation in the melt, under the optimal conditions [21-25].

    The reactants were dimethyl terephthalate (DMT), 1,4-butanediol (BD), and

    the corresponding siloxane prepolymer (PDMS-OH, PDMS-CP, EO-PDMS-EO, PPO-PDMS-PPO or PCL-PDMS-PCL). The catalyst was tetra-n-butyl-titanate (1.0-2.5 mmol / mol DMT), whilst the thermal stabilizer

    was N,N-diphenyl-p-phenylenediamine. The first step, transesterification,was performed from 160 to 230-240 C, at atmospheric pressure, whereby

    the formed methanol was distilled off. The second step, polycondensation,

    was performed for 1.5 to 4.5 h (depending on which prepolymer was used)

    at 230-250 C, under reduced pressure. In this manner, three series of

    TPESs and two of TPEESs, with different soft segments and hard-to-softweight ratios in the range from 90/10 to 40/60, were obtained. The

    synthesized TPESs (except for the copolymers based on PCL-PDMS-PCL)and the TPEESs were extracted with chloroform and the obtained soluble

    and insoluble fractions were analyzed by1H NMR spectroscopy [21-26].

    3. Characterization of the copolymers1H NMR (200 MHz) spectra were obtained on a Varian Gemini-200

    instrument. The TPES samples based on PDMS-OH and PDMS-CP, as well

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    Vesna V. Antiet al.34

    as the TPEES samples, were measured as solutions in CF3COOD. The

    solvent was simultaneously used as the internal standard. The TPES samplesbased on PCL-PDMS-PCL prepolymer were measured as solutions in CDCl3.

    The inherent viscosities (inh) of the TPES and TPEES samples weremeasured in a mixture of phenol/trichloroethylene/toluene (1:1:2 by vol.) at

    30 C, using an Ubbelohde viscometer.Gel permeation chromatography (GPC) was conducted on a Waters 600E

    instrument equipped with a refractive index detector, on three Supelco Pl-Gel

    columns connected in line (cross-linked polystyrene with pore sizes of 10-5

    ,10-6 and 10-7 m) at 30 C. Twenty l of a sample solution in chloroform (10 mass

    %) were injected in all cases. The flow rate was 1.5 m3

    min1. The system was

    calibrated with a number of high and low molecular weight siloxanes [27].Differential scanning calorimetry (DSC) was performed using a Perkin

    Elmer DSC2 thermal analyzer. The DSC scans were recorded under a dynamic

    nitrogen atmosphere (flow rate 25 cm3

    min1

    ) at a heating and cooling rate of 10

    C min1

    (two scans were run for each sample). The weight of the samples

    was approximately 10 mg. The samples were analyzed between 50 and 250 C.Thermogravimetric analysis (TGA) was performed using a TA Instruments

    SDT Q600 analyzer in the temperature range from 30 to 750 C. The heating

    rate was 10 C min1. The TGA scans were recorded under a dynamic nitrogen

    atmosphere with a gas flow rates of 100 cm3

    min1. The sample mass was about

    10 mg.

    Results and discussionStructure and composition of synthesized TPESs and TPEESsOur work on thermoplastic, siloxane-containing copolymers commenced

    with the synthesis and characterization of poly(estersiloxane)s based on

    PBT as the hard segment and PDMS prepolymers with either silanol- or

    carboxypropyl- terminal groups as the soft segment [21]. The series of

    copolymers prepared with PDMS-OH is denoted as TPES-OH, whilst theseries obtained with PDMS-CP is denoted as TPES-CP. The structures of the

    employed prepolymers and the synthesized copolymers are presented in

    Scheme 1 and 2, respectively. Poly(butylene terephthalate) was chosen as the

    hard segment for the thermoplastic elastomers because of its high structuralregularity, rapid crystallization and high degree of the crystallinity [28, 29].

    After the optimal reaction conditions had been established [21], the effect ofthe length of the carboxypropyl-terminated PDMS prepolymers (5502170

    g/mol) on some characteristic properties of the TPES-CP was examined,while the mass ratio of the hard and soft segments was kept constant (57/43)

    [22]. The weight fractions of PBT segments in the poly(ester-siloxane)s were

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    Thermoplastic elastomers based on siloxane prepolymers 35

    Scheme 1. Molecular structure of the prepolymers that were used as the soft segments

    in thermoplastic poly(ester-siloxane)s and poly(ester-ether-siloxane)s

    C

    O

    C

    O

    O CH2 O4

    p

    SOFT SEGMENT

    q n

    Scheme 2. Molecular structure of the synthesized copolymers based on hard PBTsegments and different siloxane-based soft segments

    in the range from 5 9 to 63 % and the average lengths of the PBTsegments lay between 3.8 and 16.9, calculated on 1 mole of the soft

    segment, indicating an increase in the length of the hard segments with

    increasing length of the soft poly(dimethylsiloxane) segments. As a

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    Vesna V. Antiet al.36

    consequence of the increase in the length of the hard PBT segments, the

    melting temperature, the enthalpy of melting and the hardness of the

    poly(ester-siloxane)s increased. The degree of crystallinity calculated from

    DSC data was in the range from 18.5 to 22.6 %, while the degree of

    crystallinity from WAXS data was from 37 to 40 %. The glass transitiontemperatures of the poly(ester-siloxane)s increased with decreasing PDMS

    segment length, which indicates that the amorphous domains are a mixture of

    PDMS and non-crystallized PBT segments [22]. Further investigations wererelated to the investigation of the effect of the mass ratio of hard and soft

    segments, with a constant length of the carboxypropyl-PDMS prepolymers

    (1030 g/mol) on some properties of the obtained thermoplastic poly(ester-

    siloxane)s (Table 1) [23]. The mass fraction of PBT segments in poly(ester-

    siloxane)s, calculated from their1H NMR spectra, were in the range 39 to 76

    %, while the average degree of the polymerization of the PBT segments

    increased from 3.1 to 15.4, with respect to 1 mol of soft segments. Theinherent viscosity of the copolymers increased from 0.35 to 0.45 dl g

    -1, as the

    molar mass of the TPES increased from 11500 to 35000 g mol-1

    . It appeared

    that there was a pronounced molar mass maximum when the PBT content of

    the copolymers was approximately 60 mass %, whereby all the samples wereconsiderably inhomogeneous with respect to the distribution of the lengths of

    the hard segments. Extraction with chloroform confirmed the multiblockstructure of both the soluble and insoluble fractions.

    Analysis of the homogeneity of the synthesized TPESs and

    TPEESsAll the samples of poly(ester-siloxane)s based on PDMS-OH or PDMS-

    CP prepolymers exhibited considerable inhomogeneity with respect to thedistribution of the lengths of the hard segments. Due to the high

    incompatibility of the employed extremely non-polar siloxane prepolymers

    with the polar reactants, DMT and BD, phase separation occurred during thereaction in the melt. It was observed that the PDMS prepolymers floated to

    the surface of the reaction mixtures when the stirring was stopped during the

    first, transesterification step. Hence, it was obvious that the reaction mixturewas not homogeneous. As a consequence, the incorporation of

    dicarboxypropyl- and disilanol-terminated PDMS into the polar poly(butylene

    terephthalate) chains resulted in rather inhomogeneous TPES copolymers.The effectiveness of the incorporation of PDMS prepolymers into the

    copolymer chains was examined by Soxhlet extraction with chloroform. It is

    well known that PBT homopolymer is insoluble, while the PDMS

    prepolymers are soluble in chloroform. The obtained results showed that allTPES-OH and TPES-CP samples were comprised of a soluble, as well as of an

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    Thermoplastic elastomers based on siloxane prepolymers 37

    insoluble fraction (Table 1). The TPES-OH samples lost 6-40 %, while the

    TPES-CP samples lost 1942 % during extraction. The content of the soluble

    fraction increased with increasing mass fraction of soft segments in both the

    TPES-OH and TPES-CP series, however the increase was more pronounced in

    the first series (Fig. 1A). This meant that the amount of soluble fraction washigher in the TPES-OH in comparison to the TPES-CP series, at the same

    hard to soft segments ratio.The composition and structure of the soluble and insoluble fractions were

    investigated by1H NMR spectroscopy. The spectra of both fractions contained

    signals of aromatic protons from the PBT segments, as well as signals of theSiCH3 protons from the PDMS segments. The spectra also showed that the

    extracted and insoluble fractions differed in both their composition and structure,

    containing considerably different amounts of PDMS and PBT segments. Themass fraction of soft segments was significantly larger in the soluble than in

    insoluble fractions in both TPES-OH and TPES-CP series (Table 1). Thecalculated value ofp (the average degree of butylene terephthalate units in the

    hard segments, Scheme 2) was very small in the soluble fractions in both series(psol = 1.02.9). In the insoluble fractions,p was much higher, especially for the

    TPES-OH series (pinsol = 63300 for TPES-OHs and pinsol = 432 for TPES-

    CPs, Table 1). It can be concluded that both the extracted and insoluble fractions

    have a segmented (multiblock) structure, but with much more shorter PBTblocks in the soluble than in the insoluble fractions. Based on these results, it is

    obvious that the TPES-OH samples were more inhomogeneous regarding their

    structure and composition in comparison with the PDMS-CP samples. It seems

    that the PDMS-CP prepolymers had better miscibility with DMT and BD thanthe PDMS-OH ones, due to the presence of polar carboxyl-end groups.

    In order to avoid or reduce phase separation during the synthesis of TPES-OH and TPES-CP, hydroxyl-terminated triblock prepolymers of the ABA type,

    with hydrophilic terminal blocks, i.e., ethylene oxide (EO), poly(propylene

    oxide) (PPO) or poly(caprolactone) (PCL) and with central PDMS block, wereused in further investigations (Scheme 1). It was expected that the terminal EO,

    PPO or PCL blocks would serve as a compatibilizer between the extremely

    non-polar PDMS and the polar DMT and BD. The results of solubility andextraction in chloroform, as well as analysis of the

    1H NMR spectra of the

    obtained fractions showed that the presence of ether chains, either EO or PPO,did not improve the homogeneity of the prepared poly(ester-ether-siloxane)s, in

    comparison with the TPES-OH and TPES-CP copolymers. The TPEESsamples from both series (TPEES-EO and TPEES-PPO) were partially soluble

    in chloroform, as well as the previously synthesized TPESOH and TPESCP

    samples. The compositions of the chloroform soluble and insoluble fractions

    of the TPEES samples are also presented in Table 1.

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    Table1.Inherentviscosities(inh)and1HNMRanalysisofthechloroformsolubleandinsolublefractionsofsomeof

    theTPES

    andTPEESsamples.

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    Thermoplastic elastomers based on siloxane prepolymers 39

    Figure 1. Change in the content of the soluble fraction with changing mass fraction of

    soft segments for the poly(ester-siloxane)s based on PDMS-OH and PDMS-CP prepolymers (A) and the poly(ester-ether-siloxane)s based on EO-PDMS-EO and

    PPO-PDMS-PPO prepolymers (B).

    The TPEES-EO samples lost 356 mass % during extraction. The soluble

    fractions of the TPEES-EO samples contained 8186 mass % of soft

    segments, while the insoluble ones contained 424 mass %. It can beconcluded that the length of the EO block (only 1 repeating unit) was too

    short to be an efficient compatibilizer between PBT and PDMS, in spite of

    the high solubility parameter of poly(ethylene oxide) [PEO = 20.2 (J/cm3)

    1/2]

    [5], which is close to the solubility parameter of PBT[PBT = 23.0 (J/cm3)

    1/2]

    [6]. The TPEES-PPO samples lost 3960 mass % during extraction. The

    sample TPEES-PPO2, with a mass fraction of PBT of 52.8 %, lost 43 %during extraction, while the sample TPEES-EO5, with a very similar

    content of the hard segment (53.4 mass %), but with shorter soft segments,

    lost 34 % during extraction. This result indicates a relationship between thelength of the soft segments and the amount of soluble fraction: the longer

    the soft segments, the higher is the amount of extracted fraction. The

    relationship between the mass fraction of soft segments and amountof thesoluble fraction for the series TPEES-EO and TPEES-PPO is presented in

    Fig. 1B.The soluble fractions of the TPEES-PPO samples contained 8589 mass

    % of soft segments, while the insoluble ones contained 1928 mass %. In thecase of the TPES-PPO copolymers, it can be concluded that compatibilizing

    effect between PBT and PDMS was not achieved, because of the rather low

    solubility parameter of PPO [PPO = 15.4 (J/cm3)

    1/2] [6, 30],

    which is very

    similar to that of PDMS [PDMS = 15 (J/cm3)1/2][3].

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    Vesna V. Antiet al.40

    The calculated values ofpsolfor the samples of both TPEES series were

    very small (psol =1.0-2.4) and similar to thepsol of the previous TPES-OH and

    TPES-CP series. The calculated value ofpinsol was much higher in the

    TPEES-PPO series than in the TPEES-EO series at a similar PBT content

    (pinsol was 62 for TPEES-PPO2 and 18 for TPEES-EO5). This result showsthat the length of the PBT segments in the insoluble fraction is directly

    related to the length of the prepolymer used, at a fixed mass ratio of hard-to-soft segments.

    The relationships between the content of PBT segments in the

    copolymers and the length of the hard segment in the insoluble fraction arepresented in Fig. 2. It is obvious that increasing the content of PBT leads to

    an increase ofpinsol. The longest hard segments were founded in the insoluble

    fractions of the TPES-OH series. This indicates that the prepolymer mostimmiscible with PBT within the prepolymers used in this work was PDMS-

    OH. At a fixed PBT content, the increase of the length ofpinsol in the differentseries was as follows: TPEES-PPO > TPEES-EO > TPES-CP (Fig. 2). The

    differences inpinsol for TPEES-EO and TPES-CP were not very pronounced,probably because of the similar length of the soft segments (1200 and 1030

    g/mol, respectively). The values ofpinsol in the TPEES-PPO series were

    higher, probably as a consequence of the much longer soft segment (2930

    g/mol).The series of poly(ester-siloxane)s based on PBT and poly(caprolactone)-

    poly(dimethylsiloxane)-poly(caprolactone), TPES-PCL, with hard-to-soft

    segment ratios in the range from 74/26 to 51/49, were completely soluble in

    chloroform, which enabled their molar masses to be determined by gel- permeation chromatography. The chromatograms showed the presence of

    only one peak, the shape of which corresponded to a typical high-molecularweight product of step-growth polymerization. The values of the number

    average molar masses, nM , the weight average molar masses, wM , the

    polydispersity index, wM / nM and of the molar masses which corresponded

    to the maximum of the peak,peak

    M are given in Table 2. The values of the

    inherent viscosities of the TPES-PCLs and of the number of repeating units

    within the hard segments are also given in Table 2. The values of nM werein the range from about 20000 to almost 50000 g/mol. It is obvious that the

    presence of the longer (18 repeating units), polar poly(caprolactone) blocks

    [PCL = 20.0 (J/cm3)]

    1/2][31, 32] played a significant role in the improvement

    of the miscibility of the prepolymer with the polar reactants and PBT. The

    reaction mixture was homogeneous, as were the synthesized copolymers with

    respect to their structure and composition.

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    Thermoplastic elastomers based on siloxane prepolymers 41

    Figure 2. Change in the length of the hard segment in the insoluble fraction (pinsol)

    with changing mass fraction of PBT segments for the synthesized poly(ester-siloxane)s and poly(ester-ether-siloxane)s.

    Table 2. Inherent viscosities (inh),1H NMR and GPC analysis of the TPES-PCL

    samples, based on PCL-PDMS-PCL prepolymer ( nPCL PDMS PCLM = 6100 g/mol).

    SamplePBT

    Soft segment

    mass % mass %

    (NMR)

    p

    (NMR)inh

    (dl/g)nM

    (g/mol)

    wM (g/mol)

    w

    n

    M

    M

    peakM

    (g/mol)

    TPES-PCL1 70/30 74/26 80 0.52 22470 43200 1.9 36700

    TPES-PCL2 60/40 68/32 59 0.65 21170 45160 2.1 39480

    TPES-PCL2 55/45 63/37 48 0.62 22540 45830 2.0 39480

    TPES-PCL4 50/50 59/41 41 0.60 26610 49850 1.9 42500

    TPES-PCL5 40/60 51/49 30 0.89 49340 91890 1.9 83950

    DSC analysis of the TPESs and TPEESs

    The synthesized TPES and TPEES were semi-crystalline polymers. Themelting temperatures (Tm) of the hard segments were observed by DSC. The

    DSC measurements were performed between 50 and 250 C to determined

    the melting temperatures (Tm), the enthalpy of melting (Hm) and the degree

    of crystallinity (wc) of the TPES and TPEES samples.The values ofTm of the synthesized copolymers were in the following

    ranges: 224-226 C for TPES-OH; 214-223 C for TPEES-EO; 208-218 C

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    Vesna V. Antiet al.42

    for TPES-CP; 175-181 C for TPEES-PPO and 103-196 C for TPES-PCL. It

    was obvious that the copolymers from the different series, with the same

    mass fraction of PBT segments, have different melting temperatures in the

    previously given order, as is presented in Fig. 3. From Fig. 3, it can also be

    seen that with increasing mass fraction of PBT segments, the meltingtemperatures mostly shifted to higher values. This phenomenon was the most

    pronounced in the TPES-PCL series.The enthalpies of melting were calculated from the corresponding

    thermograms. The total degree of crystallinity, i.e., the mass fraction of

    crystallities in the TPES and TPEES copolymers, were calculated from the

    determined Hm using the equation:

    wc = Hm / Hm

    where Hm

    = 144.5 J/g is the enthalpy of melting of perfectly crystalline

    PBT homopolymer[6].

    The degree of crystallinity of the TPES and TPEES samples increased

    with increasing mass fraction of the hard PBT segments in all theinvestigated series, as can be seen in Fig. 4. Also, the copolymers from the

    TPES-OH, TPES-CP and TPEES-PPO series have the highest degrees ofcrystallinity at a fixed content of PBT (17-28, 11-23 and 10-17 %,

    respectively). The trend of changing wc with mass fraction of PBT in the

    copolymer was almost the same in all of these series. The samples from theTPEES-EO series had a somewhat lowerwc (3-28 %). The samples from the

    TPES-PCL series (3-15 %) had the lowest degrees of crystallinity. This can

    be explained by the high miscibility of PCL-PDMS-PCL prepolymer with

    PBT, which leads to poor phase segregation of soft and hard segments in the

    TPES-PCL copolymer, and also to the obstruction of the crystallization of thePBT segments.

    The ratio of the total degree of crystallinity and the determined mass

    fraction of PBT segments (1H NMR) gave the mass fraction of hard

    segments which were incorporated into the crystallites, wcPBT

    . The degrees ofcrystallinity, w

    c

    PBT, were in the range 28 to 31 % for the synthesized TPES

    and in the range 27 to 30 % for the TPEES-EO and TPEES-PPO. This means

    that less than 30 mass % of the PBT segments in all the TPES and TPEESsamples crystallized. These results show that not all of the hard segments in

    these segmented copolymers could crystallize completely due to their partial

    incorporation into the amorphous phase. The degree of crystallinity of PBT

    homopolymer was wc=wcPBT

    = 35.7 %.

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    Thermoplastic elastomers based on siloxane prepolymers 43

    Figure 3. Change in the melting temperature (Tm) measured by DSC with changingmass fraction of PBT segments for the synthesized poly(ester-siloxane)s andpoly(ester-ether-siloxane)s.

    Figure 4. Change in the degree of crystallinity (wc) measured by DSC with changing

    mass fraction of PBT segments for the synthesized poly(ester-siloxane)s and

    poly(ester-ether-siloxane)s.

    PBT homopolymer is a fast-crystallizing polymer. By comparing the

    supercooling, i.e., the difference between melting and crystallizationtemperature, it is possible to analyze the rate of crystallization of different

    segmented copolymers. PBT has a supercooling of 32oC. Supercooling of the

    hard segments, Th = Tm - Tc , of the poly(ester-siloxane)s and poly(ester-

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    Vesna V. Antiet al.44

    ether-siloxane)s was in the range from 38 to 45oC, respectively, indicating

    that the rate of crystallization of the copolymers TPES and TPEES was lower

    than that of PBT, due to the presence of the soft polysiloxane segments. The

    results show that the crystallization rates of the TPES and TPEES were

    dependent both on structure and composition in the examined range.Poly(ester-ether-siloxane)s, TPEES-PPO, exhibited two glass transition

    temperatures. The glass transition temperatures of the amorphous PDMSphase, Tg

    PDMS, were between -121 and -124

    oC, which are very close to that of

    PDMS homopolymer, i.e., -123oC. The glass transition temperatures of the

    amorphous PPO phase, TgPPO

    , decreased from -21 to -43oC with increasing

    content of soft segments in the copolymers. PPO homopolymer has a glass

    transition temperature of -75oC, which is lower than those of the TPEESs,

    and indicates that the amorphous fraction of the hard PBT segments wasmixed with PPO-PDMS-PPO segments.

    Thermal stability of the TPESs and TPEESsThe results of thermogravimetric analysis under a nitrogen atmosphere

    of the samples from different series, but with very similar content of PBT

    segments (about 60 mass %), are given in Table 3 and shown in Figure 5.

    The thermal stability and degradation behavior of the copolymers wascompared to the behavior of PBT homopolymer, synthesized in the same

    way and stabilized with the same heat stabilizer[21-23]. It can be seen thatall copolymers, as well as PBT homopolymer, with exception of TPES-CP3

    sample, commenced to degrade above 360 C (Table 3). It seems that thethermal stabilities of the TPES-OH, TPEES-EO, TPEES-PPO and TPES-

    PCL copolymers were somewhat higher in comparison to that of the PBThomopolymer. The thermal stability of the TPES-CP3 sample was lower

    than those of all the other copolymers and the PBT homopolymer (Fig. 5A).

    These results are in a agreement with the fact that the thermal stability of

    the poly(ester-siloxane)s TPES-CPs decreased slightly with increasingconcentration of ester-bonds between the hard and soft segments, which are

    the weakest bonds in the copolymer chains [23]. From Fig. 5B, it can beseen that the temperatures of the maximal rate of degradation of the

    copolymers were higher than the corresponding value for PBThomopolymer. This means that when the degradation of the PBT

    homopolymers starts, it proceeds faster than the degradation of the

    copolymers. The residual masses were between 6.0 and 10.9 % at 450 C,

    originating probably from the PBT fraction, since the PDMS chainsdegrade through depolymerization under a nitrogen atmosphere, with the

    formation of cyclosiloxanes [4].

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    Thermoplastic elastomers based on siloxane prepolymers 45

    Figure 5. TGA (A) and DTGA (B) curves for the TPES-CP5, TPEES-EO4, TPEES-

    PPO1 and PBT homopolymerunder a nitrogen atmosphere.

    Table 3. Thermal gravimetric analysis data of some TPESs and TPEESs samples andPBT homopolymerunder a nitrogen atmosphere.

    SampleT for 5 % mass

    loss (oC)

    T for 50 %

    mass loss (oC)

    T for maximal rate of

    degradation (oC)

    Residue

    at 450 oC

    PBT 361 389 389 10.3

    TPES-OH5 364 403 407 6.0

    TPES-CP3 343 389 387/398 6.0

    TPEES-EO4 362 402 385/405/407 9.4

    TPEES-PPO1 365 409 389/400/408 6.8

    TPES-PCL4 367 404 394/407 10.9

    Thermogravimetric analysis of the synthesized TPES and TPEES in

    oxygen gave more pronounced differences between the samples, including

    that of the PBT homopolymer. The thermo-oxidative stability of thecopolymers decreased with decreasing PBT content. Under a thermo-

    oxidative atmosphere, the degradation of the PDMS chains or segments

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    Vesna V. Antiet al.46

    commences with the formation of peroxy structures in the side-chains, which

    leads to the occurrence of different free radical reactions and results in the

    formation of gaseous silicon compounds, CO, CO2, H2, H2O, HCHO and

    HCOOH, leaving as a residue SiO2[4]. Therefore, the residues at 500oC in

    oxygen for TPES and TPEESs were higher compared to PBT, due to theincreased amount of SiO2 formed.

    ConclusionsThermoplastic elastomers with hard segments based on poly(butylene

    terephthalate) (PBT) and soft segment based on: a) ,-dicarboxypropyl-

    poly(dimethylsiloxane)s (PDMS-CP); b) ,-disilanol-

    poly(dimethylsiloxane) (PDMS-OH); c) ,-dihydroxy-ethylene oxide- poly(dimethylsiloxane)-ethylene oxide (EO-PDMS-EO); d),-dihydroxy-

    poly(propylene oxide-dimethylsiloxane-propylene oxide) (PPO-PDMS-PPO)and e) ,-dihydroxy-poly(caprolactone-dimethylsiloxane-caprolactone)(PCL-PDMS-PCL) were synthesized by two-step transesterification in themelt. The obtained poly(estersiloxane)s based on PDMS-OH (TPES-OH)

    and PDMS-CP (TPES-CP) and poly(esterethersiloxane)s based on EO-

    PDMSEO (TPEES-EO) and PPO-PDMSPPO (TPEES-PPO) were rather

    inhomogeneous. After extraction with chloroform, soluble and insolublefractions of different structure and composition were obtained. Both the

    soluble and insoluble fractions had multiblock structures. Due to the presenceof long, polar PCL sequences, the miscibility of the PCL-PDMSPCL

    prepolymer with the polar monomers in the reaction mixture wassignificantly improved, in comparison with all the other examined systems.

    As a result, the obtained PCL-PDMSPCL based copolymers (TPES-PCL)were structurally and compositionally homogeneous and completely soluble

    in chloroform. The semi-crystalline nature of the poly(estersiloxane)s andpoly(esterethersiloxane)s was confirmed by DSC analysis. With increasing

    mass fraction of hard PBT segments, the melting temperature and the degree

    of crystallinity of the TPES and TPEES increased. Copolymers from theTPES-OH, TPES-CP and TPEES-PPO series had the highest degrees of

    crystallinity. Copolymers from TPES-PCL series had the lowest degrees of

    crystallinity. This was a consequence of the increased miscibility of PCL-PDMS-PCL with PBT, which on the other hand led to reduced crystallization

    of the hard segments in the copolymers. The thermal stability of thecopolymers was somewhat higher than that of the PBT homopolymer, with

    exception of the TPES-CP samples, which were less stable due to the

    presence of ester-bonds between the hard and soft segments, which are the

    weakest bonds in copolymer chains.

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    Thermoplastic elastomers based on siloxane prepolymers 47

    AcknowledgementThis work was financially supported by the Ministry of Science of the

    Republic of Serbia (Project No. 142023).

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