Indian Journal of chemistryVol. 31A, April 1992, pp. 219-226

Thermal properties of ABA and BAB triblock copolymers

Sukumar Maiti* & Subhas Chandra Shit

Polymer Division, Materials Science Centre, Indian Institute of Technology, Kharagpur 721302

Received 30 August 1991; revised and accepted 16 December 1991

Thermal studies (TGA and DSC) of various ABA and BAB copolymers have been made and thecrosslinking and swelling behavior of the polymers have been reported in order to establish the suitabil-ity of these polymers for their application as thermoplastic elastomers. From the thermal recycling andoutdoor stability studies it is concluded that the ABA copolymers having polyester as the hard block andbutadiene based rubber as the soft block would be suitable for use as thermoplastic elastomers. Additionof appropriate antioxidants increases the outdoor stability and thermal recycle ability of these block co-polymers.

ABA triblock copolymers 1 represent an import-ant class of materials, known commercially asthermoplastic rubbers or elastomers" The uniquebehavior of these polymers mostly depends on the

difference in the response to thermal energy of thetwo block segments present in ABA copolymers.The thermal performance depends on the micros-tructure and crystallizability of the two blocks pres-ent in such copolymers". Thermal properties ofABA copolymers are, therefore, important both forunderstanding their nature and for assessment oftheir commercial applications. In this paper we re-port the results of investigation on the glass trans-ition behavior, thermal stability, crosslinking andthermal recycling properties of a few triblock copo-lymers, synthesis of which was reported earlier" - H.

Materials and MethodsABA or BAB type copolymers were synthesized

from two types of the block segments, viz., (1) a hardsegment (A) consisting of polyester or polyestera-mide or polyether sulfone, and (2) a soft segment (B)consisting of butadiene-acrylonitrile copolymer orpolybutadiene or butadiene-polyethylene glycol-acrylonitrile copolymer. The synthesis and charac-terization of these triblock copolymers were report-ed earlier" - x.

Solvents such as N-methylpyrrolidone (NMP),hexamethyl phosphoric triamide (HMPA), and 1,4-dioxane were purified by the standard procedure.

Thermal measurementsSample preparation+ The polymers were pow-

dered, washed with either alcohol or n-hexane: hen-zene mixture and dried at ahout 4()OC for 24 h undervacuum.

Thermogravimetric analysis (TGA) of copolymersamples was made with a DuPont thermal analyzer,Model 990, in air at a heating rate of 1(tc Imin.

Differential scanning calorimetry (DSC) of thepowdered dry samples was run by Mettler Tc lOAsystem at a heating rate of 20°C/min in air.

Crosslin king reactionCrosslinking of the polymers was studied by heat-

ing the polymer samples at different temperaturesfor different time intervals. The sample (2.0g) wasplaced on a glass plate and kept in a closed contain-er under NJ and heated at the specified temperaturefor a fixed-period. The percentage of crosslinkingwas determined by Sol-Gel analysis.

Analysis of sol-gel fractionThe crosslinked polymer sample was suhjected to

soxhlet extraction hy NMP or HMPA or IA-diox-ane solvent (whichever was found suitable to dis-solve the uncrosslinkcd fraction of the sample) for9() h. The insoluble product was dried to constantweight.

Swelling behavior of crosslinked copolymersSwelling hehavior of a sample was determined

gravimetrically by the quantity of solvent taken hy acrosslinked polymer hy the following relation

M -MSw= S I) X 100

Mil

where M, is the weight of the wet swollen cross-linked sample after 7 days' immersion in the solventat 3(tC (washed with dicthyl ether and blotted byfilter paper and weighed quickly), and Mil the corre-sponding weight of the dry sample.

220 INDIAN J CHEM, SEe. A, APRIL 1992

IR spectroscopy of copolymersIR spectra of the copolymers were recorded with

a Perkin-Elmer, Model 237 spectrophotometer onKB. discs.

Thermal recycling behavior of copolymersThe copolymers without antioxidant were repea-

tedly heated upto their respective softening temper-ature and cooled down to room temperatureand heated again for a total period of 6 h. Similarly,samples containing an antioxidant were treated asabove. The heat treated samples were dissolved inrespective solvents to judge whether crosslinkingoccurs or not.

Outdoor stabilityCopolymer samples were kept at room tempera-

ture in the outdoor environment for about 2months.

Results and DiscussionThe structures of the repeat unit of various copo-

lymers (ABA or BAB) investigated in this study areshown in Table 1.

Four types of hard segments i.e., polyethyleneisophthalate (PEl), polyethylene terephthalate(PET), polyether sulfone (PES) and polyesteramide(PEA), and three types of soft rubbery blocks i.e.,poly(butadiene-co-acrylonitrile) (CTBN), polybuta-diene (CTB), and CTBN-polyethylene glycol-CTBN (CTBN-PEG-CTBN) have been used in thesynthesis of these ABA or BAB copolymers. Thenumber average molecular weight. Mn, of the hardblocks ranges from as low as about 1000 to 9000while the corresponding values for the three softblocks are fixed: 3200,4200 and 6840 for CTBN.CTB and CTBN-PEG-CTBN, respectively. The de-tails of synthesis and characterization of these copo-lymers and the prepolymer blocks were reportedelsewhere" - II.

Table 1- The structure and molecular weight of ABA andBAB copolymers

Mn(Theoret.)

PEI-CTBN-PEI 5,196 XV PES-CTBN-PES(998-3,200-998) (2,870-3,200-2,870)

II PEI-CTBN-PEI 5,888 XVI PES-CTB-PES(1,344-3,200-1,344) (2,870-4,200-2,870)

III PEI-CTBN-PEI 7,200 XVII PES-CTBN/PES/CTBN-PES(2,000-3,200-2,000) (3,870-6,864-3,870)

IV PEI-CTB-PEI 8.200 XVIII PEA-CTBN-PEA(2.000-4.20Q-2,OOO) (999-3.200-999)

V PEI-CTBNIPEG/CTBN-PEI 10,840 XIX PEA-CTBN-PEA(2,000-6,840-2,000) (1,477-3.200-1.477)

VI PET-CTBN-PET 5,696 XX PEA-CTBN-PEA(1,248-3,200-1,248) (4.106-3,200-4,106)

VII PET-CTBN-PET 5,884 XXI PEA-CTBN-PEA(1,342-3,200-1,342) (8,408-3,200-8,408)

VIII PET-CTBN-PET 7,424 XXII PEA-CTB-PEA(2,112-3,2Q0-2,112) (6,900-4,200-6,900)

IX PET-CTBN-PET 8,960 XXIII PEA-CTBN/PEG/CTBN-PEA(2,880-3,200-2,880) (9,000-6.840-9.000)

X PET-CTB-PET 9,960 XXIV CTBN-PEI-CTBN(2,880-4,200-2,880) (3,200-972-3,200)

XI PET-CTBNIPEG/CTBN-PET 12,600 XXV CTBN-PEI-CTBN,(2,880-6,840-2,880) (3,20G-I,216-3,200)

XII PES-CTBN-PES 6,736 XXVI CTBN-PEI-CTBN(1,768-3,200-1,768) (3,200-1,940-3,200)

XIII PES-CTBN-PES 6,736 XXVII CTB-PEI-CTB

( 1 I 760) (4,200-3,94.0-4,200)1.768-.. 200-, 0

XXVIII CTBN/PEG ICTBN-PEI-CTBN/PEGXIV PES-CTBN-PES 7,440 CTBN

(2,120-3,200-2,120) (6,840-1,940-6,840)

'The figures in the parentheses indicate M. of the respective block segments of the copolymer. (PEl - polyethylene isophthalateblock, PET = polyethylene terephthalate block, PES - polyether sulfone block, PEA - polyesteramide block, CTBN =carboxy ter-minated butadiene-acrylonitrile rubber block, CTB - carboxy terminated butadiene-rubber block and PEG =polyethylene glycol)

Polymer ABA Structure' Polymer ABA Structure' ~(Theor et.)

8,940

9,940

12,580

5,198

6,154

11,412

20,016

18,000

24,840

7,372

7,616

8,340

10,340

15,620

MAm et aL: TIlERMAL PROPERTIES OF ABA & BAB TRIBWCK COPOLYMERS

90

(0)

221

100 300 400 500 600 100

TEMPERATURE °c300 400 500

(e) (d)

200

XXII

600

Fig. 1- TGA curves of ABA and BAB copolymers(a) ABA copolymers m-v. (b) ABA copolymers IX-XI; (c) ABA copolymers XV-XVII; and (d) BAB copolymers XXI-XXm

loot-·---__ ~

80

70

UJ;:)cu;~loot-----_~l-x\!)

iAj 90~

60 v

so

30

20

80

70

60

XVII50

40

30

20

10

200

222 INDIAN J CHEM, SEe. A, APRIL 1992

Thermal behavior of copolymersTGA curves of copolymers 1Il, IV and V are

shown in Fig. 1(a), those of copolymers IX, X andXI in Fig. 1(b), those of copolymers XV, XVI andXVII in Fig. 1(c) and those of copolymers XXI,XXII and XXIII in Fig. 1(d). Thus, Fig. 1 offers acomparative evaluation of the contribution ofCTBN, CTB and CTBN-PEG-CTBN soft (B) seg-ments to the thermal stability of the copolymers vis-a-vis those of the different hard (A) terminal blocks:PEl, PET, PES and PEA, respectively. It is clear thatthe ABA copolymers having CTBN central blocksdegrade faster than those with CTB block segments,which in turn are found to be less thermostable thanthose with CTBN-PEG-CTBN as the middle block(B).

Polymers having PEl and PET hard terminal seg-ments degrade by one step (curves V and XI) whilethose with PES (curve XVII) and PEA (curves XXI-XXIII) segments degrade by two stages. This is per-haps due to the fact that in polyarylether sulfone andpolyesteramide based copolymers the initial weightloss step indicates the degradation of its rubber con-tent?'!". The improved thermal stability of PESblock segment is due to its higher aromatic ring con-tent in the polymer structure, while that of PEA seg-ment is due to its high crystalline structure 1 1 and thepresence of strong sites for interchain interaction.Such sites for strong interchain interaction are notpresent in the polyester (PEl and PET) segments al-though both of them are also crystalline, and conse-quently the latter are not so thermostable as the for-mer.

However, the initial weight loss occurs in bothPES and PEA segments containing block copolym-ers earlier than those containing polyester blocksegments probably because PES and PEA do notget mixed well with the rubbery B blocks of the re-spective chain as it happens in PET or PEl based

axw

~ JI~ __ ~I~ ~ ~ __ ~~~

50 100 150 200 250 300

TEMPERATURE ·C

>-<J

~ aoozw

Fig. 2-~C curves of the p~polymers: a: PET (fin = 2,0(0); b:PET(Mn = 2,!lOOi;c:PEI(Mn = 2,OOO);d:PES(Mn = 2,870).

copolymers. It has been found that copolymers hav-ing almost same percentage of rubber (soft) seg-ments but of different molecular weights differ inthermal behavior; the higher the molecular weightthe higher is the thermal stability (i.e., V is morethermostable than II). The thermal stability of thecopolymers increases with increase in chain lengthof either segments.

For all the samples the maximum weight loss oc-curs in the range of 300-500°C. The BAB copolym-ers having soft rubbery segments as the endblocksand PEl as the central block are less thermostablethan the corresponding ABA copolymers whererubber segments are present as the central block.

DSC curves of the prepolymers constituting thehard segments of the ABA or BAB copolymers areshown in Fig. 2. It was found that both Tgand Tmin-crease with increase in molecular weight. Both theseendothermic transitions become sharper and moredistinct with increase in Mn. No Tmwas observed inthe case of polyarylether sulfone possibly due to itsamorphous nature.

DSC curves of the copolymers (Fig. 3) show thetwo phase nature of the block copolymers 13. In cop-olymers II, VI and VII the Tg of the A segment, i.e.,polyester segments (PEl and PET) is somewhat lessdistinct and difficult to assign (not shown in Fig. 3).On the other hand, in copolymers XV and XVII theTg (near 160°C) of the polyarylether sulfone seg-ment is prominent. In all the copolymers studied theTgof the soft segments, i.e., CTBN or CTB comes al-most at the same position. Since increase in length ofA segment, i.e., PEA block does not change the Tgofthe soft segment (curve XX, M, of A block 4,106

oxwt>-<l

g 'Tv of c r azw

o 50 100 150 200 250

TEMPERATURE ·C

Fig. 3-DSC curves of ABA copolymers. The curve numberscorrespond to the number of Table I.

MAITI et al.: THERMAL PROPERTIES OF ABA & BAB TRIBLOCK COPOLYMERS 223

versus curve XXI. Mn of A block 8,408), it is reason- because the difference between Tgof A and B hlocksable to assume that anchoring effect by the hard seg- ranges from about 100-ISO°C. Two Tgs for hothment!' 14 is nil in XXI copolymer. Further, the dif- hlock segments were also observed in the case of

~ fcrcnce between Tgof A and B blocks in copolymers BAB copolymers (not shown in Fig. 3).XX and XXI is very large ( - 330°C); thus one meltshefore the other starts melting. This is also observed Crosslinking behavior of ABA copolymersin copolymers XV and XVII containing PES hard Crosslinking reaction of the copolymers was car-segments. Due to increase in the length of soft seg- ried out at 100, 1SOand 200°C for time intervals ofmerit (B) in copolymer XVII its Tg decreases". The 2 h, 4 hand 6 h (Table 2). The extent of crosslinkingTg of the soft block in CTBN based copolymer XV reaction depends on the nature and percentage of

"' is decreased by two degrees when CTBN is substi- the hard and soft segments of the copolymers as welltuted by CTBN-PEG-CTBN in copolymer XVII. as on the temperature and time of heating. For exam-

:)Since the difference between Tg of A and B seg- pie, copolymers (I-V) at ISO°C for 6 h heating the

ments is very large in PES (e.g, curves XV and XVII) percentage of crosslinking increases from 27-40and PEA (e.g. curves XX and XXI) based copolym- and from 6S-80 at 200°C for 6 h heating. In PEl (1-crs, their processing becomes difficult, because at V) and PET (VI-XI) based copolymers the degree ofthe processing temperature above Tg of their hard cross linking increases gradually with increasing(A) hlocks the rubber (B) blocks get crosslinked and percentage of the rubber segment. But in the case of

\ thereby a thermoplastic material is converted to a polyarylether sulfone (XII-XVII) and polyestera-I thermoset. On the other hand, although there is a mide (XVIII-XXIII) based copolymers the extent of

phase separation (i.e., two phase behavior) observed crosslinking is almost quantitative at 20(}OCfor 6 hfrom the DSC studies in the case of copolymers III, heating. Since the Tgof PET and PEl are not as highIX and X, there would he some processing advan- as that of PSU (polyarylether sulfone) and PEAtage at temperatures ahove Tg of the hard segment (polyesteramide), their segmental motion is not res-

Table2-CrosslinkingofABA copolymersCopolymer Percentageofcrosslinkedcopolymersafterheatingat Solvent %ofA

used segment100°C 150°C 200°C

2h 4h 6h 2h 4h 6h 2h 4h 6hI 10 20 29 40 45 58 80 NMP 38,- II 6 15 27 38 40 52 70 NMP 45

III 5 10 17 27 30 50 65 NMP 54IV 10 20 25 30 40 45 56 78 NMP 49V 10 25 30 38 42 54 78 NMP 40VI 10 15 25 36 42 50 70 NMP 42VII 10 15 22 32 40 45 67 NMP 45

VIII 5 10 15 25 36 39 60 NMP 56IX 4 10 15 25 32 40 60 NMP 64X 6 12 15 25 38 42 52 72 NMP 58XI 10 15 25 36 40 60 72 NMP 42XII 5 10 15 30 35 46 58 67 98 NMP 51XIII 5 8 12 30 36 42 58 68 95 NMP 50XIV 4 6 10 25 30 38 48 65 90 NMP 54

-',\ XV 3 5 20 25 30 36 45 92 88 NMP 66XVI 10 20 26 30 35 45 50 70 98 NMP 46XVII 5 10 16 35 40 48 53 70 96 NMP 48XVIII 10 20 30 35 40 50 56 70 98 1,4-Dioxane 34XIX 8 14 26 30 34 48 55 68 92 NMP 47XX 5 10 16 30 36 44 52 62 82 HMPA 72XXI 4 8 12 25 32 42 48 60 80 HMPA 92

XXVI" 10 20 25 50 55 80 88 90 98 1,4-Dioxane 21'BAB copolymer

224 INDIAN J CHEM, SEe. A, APRIL 1992

tricted and thus good phase mixing is possible inpolyester based copolymers. On the other hand, inPEA or PSU based copolymers the soft segments arewithin the soft domain even at above 150°C becauseof the high Tg of the PEA or PSU segment. This al-lows easy crosslinking of the rubber segments in thecopolymer having PSU or PEA hard segments. ThePET based copolymers are not easily crosslinkedand 80% crosslinking occurs at 200°C after 8 h. Thepercentage of crosslinked product with BAB copo-lymer is more than that with ABA of nearly samemolecular weight, since the former contains higherpercentage of rubber.

Swelling behavior of the crosslinked copolymersThe virgin copolymers containing unsaturation in

the soft block are soluble in highly polar solventssuch as NMP, DMF, DMAC. The swelling is possibledue to the fact that irreversible covalent crosslinksin polymers are much stronger force than polarity orother secondary valence forces, which make .cross-

linked polymers insoluble even in highly polar sol-vents 16 - 18. The percentage swelling of various copo-lymers (I-XXIll) depends on the nature of the sol-vent. For example, the % swelling in n-hexane, ben-zene, 1,4-dioxane, MEK, DMF, DMAC and NMPranges from 98-110, 120-150, 130-160, 130-180,130-200, 140-200 and 140-200, respectively. Withthe increase in the percentage of PEl in the PEI-based copolymers (I-V) swelling in nonpolar sol-vents such as n-hexane and benzene decreases. Onthe other hand, in polar solvents e.g., l,4-dioxane,MEK, DMF, DMAC and NMP swelling increasesexcept in copolymers (IV) containing CTB as themiddle segment. When the percentage of PET is in-creased in PET based copolymers (VI-XI) thechange in swelling is not much significant except inthe case of X. Rather when the percentage of polyar-ylether sulfone (PSU) is increased in polyarylethersulfone based copolymers swelling in polar solventsis prominent and it increases gradually as the solventpolarity increases. It has been found that swelling of

Copolymers

Table 3- Thermal recycling behavior of ABA copolymers

T~ Softening Recyclable upto temperaturerarge(°C)

A segment Bsegment

I

IIIIIIVV

VIVllVlIJIXXXIxn-XIII'XIV'XV'XVI'XVII'XVIII"XIX'XX'XXI'XXII'XXIll"XXIV"

-78-78-78-82-80-78-78-78-78-82-80-78-78-78-78-82-80-78-78-78-78-82-80-78

44484848565858686868

155155157159159159

48"Infusible melt was observed'BA8 copolymer

Without Withantioxidant antioxidant

90-100 100 12090-110 110 13090-110 110 13090-110 110 13090-100 100 120

100-110 llO 120100-110 110 125105-115 115 125110-125 120 1~0110-125 110 125100-110 100 120

80-90 9590

MAITI et al.:THERMAL PROPERTIES OF ABA & BAB TRIBLOCK COPOLYMERS 225

the copolymer is maximum in those solvents whosesolubility parameter values coincide with the copo-lymer's solubility parameter value'", The extent ofswelling is minimum in the PEA-based copolym-ers, and as the percentage of polyesteramide blockis increased in the copolymers, the swelling dec-reases significantly. This is due to the highly crystal-line nature of PEA homopolymer'".

IR spectroscopy of the copolymersThe IR spectra of the copolymers showed theab-

sorption peaks characteristic of the groups present,viz. 1740 cm-I (ester group), 3400 cm-I (OH),1600 cm -I (aromatic double bond), 1620 cm-I(CH=CH), 2240-2250 cm-I (C=N), 1315 cm "!

(S02' asymmetric), 1152 cm-I (S02 symmetric),1248 cm-I (Ar-O-Ar), 3300-3580 cm-I (phenolicOH and/or = NH), 1150 cm-I (CH2-O-CH2)and 1710 em - 1 ( - COOH). But in the crosslinkedcopolymer samples, absorption peaks other thanthe peak for aliphatic double bonds for the rubberysoft segment at 1620 em -I are present. This indi-cates that the double bonds of the rubber segmentsare involved in crosslinking reaction of the copo-lymers.

Thermal recycling behavior of the ABA copolymersTo become a successful thermoplastic elastomer

the ABA copolymers should exhibit a certain de-gree of phase separation i.e., a significant degree ofimmiscibility between the two types of block seg-ments (hard and soft blocks) should exist. Sincethese copolymers contain segments having higherglass transition (plastic block) as well as lower glasstransition (elastomeric block), the thermoplasticelastomeric behavior of these copolymers is expect-ed-, provided that the blocks do not interact witheach other. But it has been found both from lGAand crosslinking studies that there is a lack of prop-er phase mixing (thermally homogeneous" i.e. asingle-phase material results due to the mobility ofboth the segments) at temperatures above Tg of thehard segment of polyesteramide and polyarylethersulfone based copolymers. When these copolymersare heated upto 140°C which is below the Tg of thehard block, the soft block which is molten at thattemperature cannot diffuse into the hard segmentas the latter is still infusible at 140°C22;and thereforea non-uniform physical mixture results. And whenthe temperature is raised to 170°C i.e., above the Tgof PES, the crosslinking reaction occurs in the rub-bery phase converting the soft block into a hard in-fusible mass. So, at higher temperature also an in-homogeneous product will be obtained as the molt-en hard segment fails to diffuse into the crosslinked

infusible rubber block. Thus at the temperatureabove the Tgof the hard segment, the PES and PEAbased ABA copolymers could not be processed.

Polyester (PEl and PET) based copolymers be-come fluid at a temperature just above the Tgof PElor PET which are comparatively lower than the Tgof PEA or PES segments. And since the tempera-ture is lower, the crosslinking of the rubber blockdoes not occur and consequently a thermally homo-geneous material will result. This has been corrobo-rated by the present study. The important criteria ofthermoplastic elastomers should be such that A andB blocks should not be miscible in the normal statebut miscible in the rnelt/",

It has been found (Table 3) that the copolymers (1-XI) can be thermally recycled with or without an-tioxidant. However, the presence of antioxidant inthe copolymers exhibits better service temperaturethan those without it. The copolymers having higherpercentage of polyester (PEl or PET) retain therm-oplasticity upto higher temperature than that con-taining lower amount of polyester segment.

BAB copolymers (PEl based) can be recycled up-to 90°C and above this temperature they start cross-linking after five cycles of heating.

Outdoor stabilityWhen the ABA copolymers (I-XI) were kept

without antioxidant in air they easily undergo cross-linking or oxidative degradation after 15 days. Butwhen they are left with antioxidant (N-isopropyl-N-phenyl phenylenediamine) the copolymers do notundergo crosslinking even after 45 days. It is alsoobserved that copolymers containing higher percen-tage of polybutadiene ( - CH2 - CH = CH - CH2 - )

block exhibit more facile cross linking reaction thanothers.

AcknowledgementAuthors thank the Department of Science and

Technology, New Delhi for partial support of thisinvestigation and the B.F. Goodrich Company, USAfor a gift of CTBN and CTB reactive rubber sam-ples.

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