Br. Polym. J. 1975, 7, 329-341

Thermal and Mechanical Properties of Block Copolymers of Poly(Hexamethy1ene Terephthalate) and Poly(Oxyethy1ene) Units Isaac Goodman,a Raymond H. Peters and Victor T. J. Schenkb

Department of Polymer and Fibre Science, University of Manchester Institute of Science and Technology, Sackville Street, Manchester M60 I QD

(Paper received 15 April 1975 and accepted 18 April 1975)

Block copolyetheresters of poly(hexamethy1ene terephthalate) (6GT) with poly- (oxyethylene) terephthalate (POET) units have been prepared by polycondensation and certain of their thermal, morphological and mechanical properties have been determined. The copolymers rich in 6GT or POET units show the crystalline characteristics of the single respective homopolymers, whereas in the middle of the composition range crystalline phases of both types coexist. Increase in the proportion of POET units causes a decrease in the melting and glass transition temperatures and in the crystallisation rates whilst concomitantly the mechanical properties change from fibrous to elastomer in type. The results are interpreted in terms of a polycrystallite model in which the composition-dependent nature of the crystallites and tie bars determines the overall mechanical properties of the copolymers.

1. Introduction

Fibres from block polymers composed of polyester and polyether units were first made in order to modify the properties of the polyester in two main directions, namely to improve the moisture sorption and dyeing properties without substantial deterioration of the mechanical properties and to introduce flexible or soft segments which could impart rubber-like elasticity. Thus Coleman19 2 prepared copolymers of poly(ethy1ene terephthalate) with up to 30 wt- % of poly(oxyethy1ene) units of mole- cular weight 4000. His products retained good mechanical strength as fibres, and dyeability and moisture uptake were much enhanced.

In a more comprehensive examination, Charch and Shivers3 examined such copolymers over the complete range of compositions. Those containing poly- (oxyethylene) units of m.w. 4000 were hard and crystalline in compositions up to 40% of the polyether; from 40-70 % they were elastomeric, and compositions with > 70 % of polyether units gave products which were soft and plastic. Differences in the properties of the products were also found when the molecular weight of the polyether blocks was changed.

"Present address: School of Polymer Science, University of Bradford, Bradford BD7 IDP, W. Yorkshire.

b Present address: Centre of Japanese Studies, University of Sheffield, Yorkshire. 329

330 I. Goodman et al.

Other workers have chosen different molecules for the polyester portion. Thus Haward and Riches4 prepared materials based on polyesters of terephthalic acid with 2,2-bis-(hydroxyphenyl)propane and a mixture of 1,3- and 1,4-dirnethylolcyclohexane together with poly(ethylene oxide). The physical properties of solvent-cast films showed a dependence on the composition and on the DP of the polyester. Similar results were achieved when the polyester was replaced by polycarbonate.596

These workers, however, did not cover a wide range of physical properties or examine in detail the morphology of their products. It was because of this that the present work was undertaken with block polymers of poly(hexamethy1ene tereph- thalate) (6GT) and poly(oxyethy1ene) terephthalate (POET) units. These materials were chosen because of their ease of transformation into fibres whose mechanical properties could be readily measured.

2. Experimental 2.1. Reagents Hexane-l,6-diol was supplied by BDH and showed no significant impurities when analysed by gas-liquid chromatography.

Dimethyl terephthalate was supplied by ICI Ltd. The sample of poly(oxyethy1ene) glycol was “Carbowax” 4000 (Union Carbide Ltd) and the catalyst, tetraisopropyl- ortho-titanate, was supplied by Eastman Chemical Company as a solution in n-butanol.

2.2. Polymerisation The polymers were prepared by standard melt polycondensation procedures from 1 : 2.2 molar ratios of dimethyl terephthalate and diol mixtures comprising varied proportions of hexane-l,6-diol and the poly(oxyethy1ene) glycol by heating the reactants together with the catalyst in an atmosphere of dry nitrogen (“white spot”), (a) at 197”C/1 atm to eliminate methanol, and (b) at 258”C/O.l mmHg to remove surplus hexane diol and effect polymerisation by ester-interchange. The reaction was stopped when the viscosity of the melt, as judged by the rate at which nitrogen bubbled through, was adequate. There was some variance in the molecular weights of the various samples formed, possibly due to decomposition of a fraction of the POE units at the temperatures used for polymerisation.

2.3. Molecular weight determination From end-group determinations made for “Carbowax” by reaction with acetic anhydride pyridine and titration of the unreacted reagent,’ the value found was Mn =2800. The molecular weights of 6GT homopolymer and its block copolymers were determined from the carboxyl end-group contents found in the materials after succinoylation with succinic anhydride in methylnaphthalene at 160 “C/3 h using a modification of the method of Conix.8 The treated polymers were freed of excess reagent by precipitation from solutions in fresh methylnaphthalene with methanol,

Tab

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1

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0.85

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45

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82

13.4

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17.7

1.

78

21.4

2.

24

30.5

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56

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4.

71

40.3

5.38

47

.8

7.15

58

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10.7

69

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91

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1720

0 0.

564

1750

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530

(135

00)

0.50

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1610

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586

1200

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466

12 00

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(21 600)

0.65

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) 0.

560

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630

1890

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620

(269

00)

0.75

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676

146

146

145

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5 14

6.5

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143

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332 I. Goodman et a/.

and after intensive drying, weighed samples were titrated with a solution of NaOH in benzyl alcohol.

The reduced viscosities of the polymers in chloroform solution were determined at 25 "C and correlated with the values calculated from end-group measurements. The molecular weights by end-group analysis when plotted against the relative viscosities gave a linear relationship which enabled molecular weights in some instances to be interpolated from the viscosities (Table 1).

2.4. Compositions of the copolymers The compositions were calculated from the initial weights of the reactants. However, in order to confirm that loss of volatile component had not occurred during the preparation, the compositions were determined by hydrolysis of the copolymers with KOH followed by acidification with HCI and gravimetric determination of the liberated terephthalic acid. The agreement was satisfactory.

Finally, samples of the copolymers were extracted for 24 h in cold water to ensure that no free polyether was present. Only those samples with polyether contents of 70% or more showed any loss in weight, and then only of the order of 410%.

The compositions of the copolymers are shown in Table 1. With two exceptions, they had molecular weights of 14000 or more. Also are given the total DPs of the polyester sequences in the polymers as well as the average number of polyether blocks, the latter being referred to as the DP (polyether).

3. Results 3.1. Morphology Optical examination of films of the copolymers cast from chloroform solutions showed the presence of small birefringent species, believed to be crystalline polyester, up to polyether contents of 59 wt-%. The sample containing 91 wt-% of polyether showed well-developed spherulitic structures of this component which disaggregated when the temperature was raised to 50°C.

Electron microscope examination of the fractured surfaces of samples cooled from the melt showed no well-defined structural features, except for polymers of high polyether content when lamella-like structures of thickness approximately 100 A were present, suggestive of crystalline polyether.

X-ray powder diffraction patterns were obtained from specimens either crystallised from the melt or precipitated from chloroform solution with methanol or n-hexane. From the photographs, the lattice spacings could be calculated and used to demonstrate the presence of crystalline forms of polyester and/or polyether. The ranges of crystallinity detected by the different methods are indicated in Table 2.

The presence of crystalline structures was confirmed by the thermal analysis (with the Perkin-Elmer DSC-1B apparatus) of samples that had been annealed in vucuo for 24 h at temperatures close to the melting point of the polyester and subsequently close to that of the polyether. The results obtained at heating rates of 16"C/min for

Block copolymers

I50

146

142 I

0

0 a rn

0 138-

134 - 2

130

126

I22

333

1.78 3.56 5.38 7.15 10.7 16 I I I / I I

-O.,O,O 0

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0 20 40 60

Table 2. The nature of the crystalline phases in the block copolymers

Polyester Both polyester Polyether Method of detection only (%) and polyether (%, only (%)

Optical microscopy 0-70 709-0 90-100

X-ray 0-50 5074 70-1 00 DSC 0-20 2 0 7 4 70-100

The values refer to wt-% of POET units in the copolymer.

I / e

5 0 7 0 90 POET units ( w l %)

Figure l@). The melting point of the polyether segments as a function of copolymer composition.

334 I. Goodmaa et al.

the polyester blocks and 32"C/min for the polyether blocks are shown in Figure I(a) and l(b).

The melting points of the polyester units are plotted against the average D P of the polyester segments in Figure 2: in this plot, the DP is that calculated from the experi- mentally determined DP of the polyester divided by DP (polyether)+ 1. This factor was used to take some account of the fact that the introduction of x polyether sequences into a chain results in x+ 1 polyester sequences. It is assumed that polyether groups at the ends of the chains are negligible in number compared to those combined within the chain. Nevertheless, the value chosen gives a better measure of the average

20 40 60 00 I 20

DP (polyester) DP( D0lvether)t l

Figure 2. Melting points of polyester units us average DP of polyester segments.

chain lengths of the polyester sequences. The curve shows that the melting point drops catastrophically when the length of the chain sequences becomes quite small (say below the order of 5 units) suggesting that only sequences of at least this length are capable of crystallising. This observation is in line with results obtained with block polymers of poly(tetramethy1ene terephthalate) and poly(oxytetramethy1ene) units12 and indicates that sequences of relatively small polyester units are capable of aggre- gating into crystalline units.

3.2. Crystallisation behaviour from the melt The dynamic crystallisation of 6GT and POE homopolymers and of certain 6GT/ POET copolymers was studied by DSC using an initial temperature of 180°C and a cooling rate of 32 "C/min for the polyester, and 80 "C and 16 "C/min for the polyether components. Polyester crystallisation occurred at a supercooling of ca 25 " below Tm for all samples examined, and crystallisation of the polyether component in the 47.8 and 69.4 wt-% POE copolymers was observed at a similar interval below that of the polyether block melting points.

Block copolymers 335

Tlme (min)

Figure 3. Isothermal crystallisation rates. 1,6GT, m.w.=4000; 2,6GT, m.w.= 6500; 3, 6GT, m.w. =9000;4,6GT,m.w.=ll 500;5,6GT,m.w.=17000;6,6GTPOET(9.7%);7,6GTPOET(21.4%); 8, 6GT POET (37.3%).

Rates of isothermal crystallisation were measured from the heats of crystallisation at 122°C for 6GT homopolymer samples of various m.w.s and for three of the copolymers; the results are shown in Figure 3. The rates for the homopolymer showed a marked dependence on m.w. being at a maximum Bn- 10 000. From Table 1, the average m.w. of the 6GT component in the block polymers examined are as follows:

Copolymer, wt- POE “m.w.” of 6GT component 9.7

21.4 37.3

8800 6696 3472

Comparison of the copolymer crystallisation rates with those for 6GTs of m.w.s comparable with those of the polyester blocks in the copolymers shows a progressive retardation as the proportion of polyether units increases.

From the above observations, it would seem that on cooling the melt, the polyester crystallises readily to be followed at a later stage by the polyether. The ability of the latter to crystallise depends on the extent to which the mobility of the polyether chains is restricted. Similar results have been obtained for block polymers of styrene with poly(oxyethy1ene) units of constant molecular weight, when the melting point of the latter was depressed as the styrene content increased.lO Such a situation is not observed with blends where the Tm of the lower-melting polymer was almost independ- ent of composition.ll

The ability of the polyether to crystallise depends on the size of the segments; with poly(oxyethylene)/polycarbonate block polymers, crystallisation was not observed until the polyether blocks had a m.w. of 3000.6 Alternatively the nature of the hard segment is important; block polymers of various polyesters with 50 wt- % of poly-

336 I. Goodman et al.

l o -

0 -

?- -10-

I

0

k.-

-20

-30

(oxytetramethylene) units of m.w. ca 2000 only showed the presence of crystalline polyether when the polyester component was amorphous.13

% \ 0 . \.

\

*\o - ' t o

- \ 0

I I I I I I I J

3.3. Glass transition temperatures The glass transition temperatures of the copolymers were obtained from the dynamic mechanical loss curves measured with the samples as compression-moulded sheets using a torsion pendulum apparatus of the Nonius type and operating at a frequency of 1 Hz over the temperature range -50" to +6O"C.

For the copolymers of POE content 537 wt-%, plots of logarithmic decrement vs temperature showed only one major loss peak which was assigned to the glass transition process of the (amorphous) polyester component. The T B values are shown as a function of composition in Figure 4. The copolymers with 40-69 wt-% of POE units

showed an additional small but sharp damping peak at temperatures in the range 35-46"C, with maxima corresponding closely to the melting points of the POE blocks determined by DSC; these subsidiary peaks are therefore ascribed to the melting of crystalline polyether units.

3.4. Mechanical properties I n order to test these properties, the block polymers were converted into fibres by rod-spinning after previous drying in vucuo at 80°C/24 h, using a nine hole spinneret with orifices of diameter 9/1000 in. The filaments were allowed to fall freely for about 10 ft before being wound up on the take-up bobbin. The deniers of the fibres were very similar for all fibres, between 280-290. The ability of the polymers to melt-spin decreased when the polyether content became high; indeed those containing 69 wt- % or more of polyether could not be melt spun. Attempts at dry-spinning such polymers from chloroform solutions gave only very weak filaments. Filaments formed by removing a glass rod from a solution were highly elastic but very weak. All the polymers used for spinning possessed m.w.s greater than 14 000.

Block copolymers 337

The yarns (9 filaments) were extended using an Instron tensometer, and stress- strain curves were obtained using gauge lengths of 5 cm and an extension rate of 5 cm/min, ten tests being carried out on each sample. Prior to mechanical measure- ments, the samples were conditioned for 2 days at 20 "C and 65 % RH. The choice of 5 cm/min was made for convenience although it is realised that the values obtained are dependent on the rate of extension.

In addition to load/extension measurements, the instantaneous and delayed re- coveries were determined. The former of these was measured by first extending the yarn to 300% strain at a constant rate of extension when the movement of the cross- head was stopped and the yarn was allowed to relax for 5 min. The stress relaxation was measured. The crosshead was then reversed and the sample allowed to recover

D 0

J

Extension (cm)

Figure 5. Change in shape of load extension curve with copolymer composition. 1, Homopolymer; 2, 30 wt-% polyether; 3, 59 wt-% polyether.

at the same rate as it was extended. The point on the extension axis when the load became zero was taken to be the instantaneously recovered length. The difference between the extended length (300 %) and the recovered length gave the recovery.

The delayed recovery was determined by allowing the crosshead to reverse without stopping beyond the point where the load first fell to zero enabling the yarn to relax at zero load for 5 min. The delayed recovery was that extension which was obtained when the load just increased on further extension.

A summary of the kinds of extension behaviour is shown in Figure 5 . It is interesting to note that the initial modulus, the yield stress, and the draw load all decrease as the polyether content is increased. The iatio yield/draw load steadily decreases (Figure 6) until it is equal to unity at about 50 wt- % of polyether.

Moreover as the polyether content is increased the reinforcement point occurs at lower strains: indeed at the highest concentration of polyether examined there is little or no drawing in the normal sense. Concomitant with these changes is a continued increase in the delayed recovery (Figure 7) and a greater ability of the stress in the fibres to relax under load.

338 1. Goodman et d.

POET units ( w t %)

Figure 6. Ratio of yield load to drawing load us wt-% of POET units.

I 1 1 I I I

POET units ( w t %)

Figure 7. Recovery properties of block copolymers.

10 30 50 70

4. Discussion

From the foregoing, it is possible to construct a simple model of the structure. Since the rate of crystallisation of the polyester is comparatiygly rapid, this component in the block copolymers crystallises first on cooling from the melt. As the temperature drops, the still liquid polyether segments subsequently crystallise to an extent which is determined by the restraints imposed on them by the preformed solid polyester matrix. The solid homopolymer 6GT may itself be regarded as composed of a series of crystalline polyester blocks joined together by polyester tie bars. On introducing polyether segments, the lengths of the 6GT sequences are reduced so that the polyether sequences introduce defects into the organised polyester matrix. The effect will resemble those brought about by reduction of the molecular weight of a homopolymer and hence will cause a marked deterioration of the mechanical properties. However, as the proportion of polyether is increased, a situation will be reached when the

Block copolymers 339

crystalline polyester regions are joined by tie bars now composed essentially of pol yether sequences. In such circumstances, the polyester crystalline regions simply act as reinforcement for a polyether matrix. Thus one may expect a gradual transition from an essentially polyester matrix containing distortions and dislocations to one in which a polyether matrix is reinforced. In part of the composition range, crystallites of both components will coexist (Table 2). However, the above can only be a very simple picture since it must be remembered that both the polyester and polyether portions will possess a distribution of molecular weights (and hence of segment sizes), a fact which will be reflected in the construction of the individual chains of the block copolymer. It may be expected that segments of less than a certain length will not be incorporated into the crystalline regions.

The breakdown of the polyester matrix by the introduction of polyether will cause a considerable decrease in the number of polyester sequences capable of spanning at least two crystalline regions : since the inital modulusa depends on the number of such tie bars, this property must rapidly deteriorate as is shown in Figure 8. In Figure 9

POET units (wt %)

Figure 8. Initial moduli of block copolymers. 1 g/dtex=9.81 x 107 N m-2.

initial modulus is plotted against DP (polyester)/DP (polyether) + 1, emphasising the loss in stiffness as the length of the polyester sequences decreases. As the initial polyester matrix is destroyed, the polyester tie bars are replaced by polyether ones until ultimately the number of stress-carrying tie bars will be determined by the number of crystalline reinforcing regions. These decrease as the lengths of the polyester sequences decrease. The polyether sequences may be expected to show elastic properties above their glass transition temperatures and hence as their number increases the fibre develops improved recovery properties. A model which is essentially the reinforcement of an amorphous polyether (or polyether joined by short sequences of polyester) has

"The initial modulus is expressed in grams per decitex (g/dtex-l): 1 g/dtex-1=9.81 x lo7 p N m-2, where p is the fibre density in grams per cubic centimetre.

340

0.6

0.5 - aJ - & 0.4

x 0.3

.~ "3 3

E e L 0.2

~

0.1

I. Goodman et d.

-

.-

~-

-

-~

-

,X/X -x.X I 1 I I

been postulated to explain the properties of block polymers prepared from poly- (tetramethylene oxide) glycol and 4GT.12*147189 l9

Relations similar to those for initial modulus and composition were also found for the yield stress. As indicated earlier the yield point characteristic of the normal polyester fibre disappears rapidly as the polyether content increases (Figure 5). If the yield point is a condition in which the polyester tie bars are extended and the crystal- line regions are cleaved to be followed by the drawing process, then the reduction in this property is again a reflection on the breakdown of the polyester matrix. By contrast, the fibres composed of a polyester-reinforced polyester matrix are not expected to show this behaviour and display load vs extension curves which are rubber-like in character.

5. Importance of the size of the polyether units In the above discussion, it has been tacitly assumed that the polyether played no role in modifying the polymer structure except insofar as it reduced the length of the polyester sequences or simply acted as tie bars joining polyester sequences together. However, the properties of the polymers are dependent on the molecular weight of the polyether. Thus some workers have shown that in any particular polyether block copolymer, the polyether tends to crystallise as the molecular weight of its segments increases. In terms of initial modulus, increases in molecular weight cause a dramatic drop in the modulus for a constant size of polycarbonate block.6 It may be that this is the point in the concentration range where crystallisation of the polyether begins. Crystallisation of the latter causes significant contraction at the higher concentrations and may be expected to dislocate the structure to some extent.

6. Strength at break With the samples that contain small proportions of polyether, the material can be drawn and presumably in so doing, microfibrillar structures are formed. With those

Block copolymers 341

samples containing high proportions of polyether and whose mechanical properties are considerably weaker, extension is first associated with the extension of the chains of polyether or possibly polyether joined together with single terephthaloyl groups or small sequences of polyester; the ultimate strength would seem therefore to depend on the strength of the crystalline reinforcements and hence to decrease rapidly as the DP of the polyether is reduced. This point is shown in Figure 5 .

Acknowledgement

The award of an SRC Research Studentship to V.T.J.S. is gratefully acknowledged.

References 1. Coleman, D. J. Polym. Sci. 1954, 14, 15. 2. Coleman, D. British Patent 682 866. 3. Charch, W. H.; Shivers, J. C. Text. Res. J . 1959, 29, 536. 4. Haward, R. N.; Riches, K. Polymer 1969,9, 103. 5. Merril, S. J. J. Polym. Sci. 1961,55,343. 6. Merril, S. J.; Petrie. S . E. J. Polym. Sci. 1965, A3,2189. 7. Sorenson, W. R.; Campbell, T. W. Preparative Methods ofPolymer Chemistry Interscience, 1951. 8 . Conix, A. Makromol Chem. 1958,26,228. 9. Shivers, J. C. USA Patent 3 023 192. 10. OMalley, J. Polymer Preprints 1959, 10, 796. 11. Inoue, M. J. Polym. Sci. 1963, Al, 3427. 12. Cella, R. J. J. Polym. Sci. Polymer Symposia No. 42, 1972, 727. 13. Ghaffar, A.; Goodman, 1.; Hall, I. H. Br. Polym. J . 1973,5, 315. 14. Hoesschele, K. G.; Witsiepe, W. K. Angew Makromol. Chem. 1973,29/30,267. 15. British Standard Handbook 188:1957. 16. Billmeyer, F. W. Textbook of Polymer Science, Wiley International Edition, 1962. 17. Iwakura, Y.; Taneda, Y.; Uchida, S. J. App. Polym. Sci. 1961, 5, 108. 18. Ghaffar, A.: Goodman, 1. ; Peters, R. H. In preparation. 19. Ghaffar, A.; Goodman, I.; Segarman, E.; Peters, R. H. In preparation.