Reactive Blending of Aromatic Polyesters: Thermal Behaviour of Co-precipitated Mixtures PTT/PET

Reactive Blending of Aromatic Polyesters: Thermal

Behaviour of Co-precipitated Mixtures PTT/PET

Maila Castellano,1 Antonio Turturro,1 Barbara Valenti,*1,2 Alessandra Avagliano,2 Giovanna Costa2

1Dipartimento di Chimica e Chimica Industriale, Via Dodecaneso 31, 16143 Genova, Italy2 Istituto per lo Studio delle Macromolecole, ISMAC-CNR Sezione di Genova, Via De Marini 6, 16149 Genova, ItalyFax: (þ39) 10 6475880; E-mail: [email protected]

Received: June 28, 2005; Revised: November 21, 2005; Accepted: November 23, 2005; DOI: 10.1002/macp.200500276

Keywords: blends; crystallization; poly(trimethylene terephthalate); transesterification

Introduction

The introduction of poly(trimethylene terephthalate) (PTT)

into the market by Shell Chemicals in 1995 drew attention

to its unique combination of properties such as atmospheric

dyeability, high breaking strain and elastic recovery, which

allow its use in specific fields. In particular, PTT exhibits

better elastic recovery than the other polyesters made from

the homologous series of n-methylene glycols poly(ethyl-

ene terephthalate) (PET) and poly(butylene terephthalate)

(PBT).[1] Moreover, it shows an unusual combination of the

outstanding properties of PET and of the processing char-

acteristics of PBT; owing to these aspects it has been widely

investigated recently.[2–6] While PTT is being used in fibre

applications for carpets, conventional and non-woven texti-

les, this polyester is equally promising in film and engineer-

ing thermoplastic applications, both as neat polymer and in

formulated products, since it combines many advantages of

nylons and other polyesters.[7,8]

To bring down the cost and yet keep some of the advan-

tageous properties of PTT, it seems interesting to explore

the possibility of using this new polyester in blends with

Summary:Due to its unique combination of properties, PTTis a promising material for textile fibre and engineeringthermoplastic applications, both as neat polymer and in for-mulated products. To bring down the cost yet keeping some ofits advantageous properties it seems interesting to apply PTTin blends and particularly with PET. We consider here theeffect of composition and reaction time on co-precipitatedmixtures PTT/PET, in terms of melting and crystallizationbehaviour, solution properties and morphology. Blends ofvarious compositions were prepared from mixed solutions,kept in theDSC at 280 8C for different times (0–120min) andanalysed during a successive cooling-heating cycle. A singleTg was detected for each composition, indicative of misci-bility in the glassy state. Melting temperatures, at fixed com-position, decrease with increasing treatment time. Mixturesrich in one component show a single Tm, whereas two distinctpeaks were observed for the 50/50 blend; on increasing thetreatment time some transesterification takes place, evi-denced by themelting peak broadening andmerging together.The overall crystallinity developed when transesterificationplays a role strongly depends on composition: it is inhibitedfor the 50/50 mixture on cooling, though it can be induced tosome extent by reheating or holding during cooling, whereasin one component-rich blends themain effect is a reduction ofthe crystallization temperature. Isothermal crystallizations

were also carried out as a function of the composition and ofthe residence time in the molten state; crystallization ratedecreases with increasing holding time in the melt. Morpho-logy of PTT and PTT-rich mixtures under the adoptedcrystallization conditions exhibits typical banded spherulites,whose growth rate and band spacing vary with melt holdingtime; moreover, less regular concentric circles develop onincreasing residence time at 280 8C.

Spherulitic texture developed at 198 8C after 60 min storageat 280 8C by a PTT/PET 90/10 mixture.

Macromol. Chem. Phys. 2006, 207, 242–251 � 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

242 DOI: 10.1002/macp.200500276 Full Paper

other polymers and particularly with PET. Indeed, polymer

blending is an attractive alternative to produce new poly-

meric materials with desirable properties without having to

undertake the synthesis of totally new compounds. Binary

blends of both crystallizable polymers are usually believed

to be immiscible; however, partialmiscibility can arise if the

chemical structure of the components is very similar[9–11]

and if they exhibit polar or specific intermolecular inter-

actions. Ester-interchange reactions often take place during

melt processing of polyester-polyester blends, thus improv-

ing miscibility or compatibility.[12–15] For example, the

transesterification reactions occurring in poly(ethylene

naphthalate) (PEN)/PET blends during melt processing

were recently studied by several authors,[15–19] since these

blends have been attracting increasing interest because they

may combine the superior properties of PEN with the

economy of PET. PEN and PETare intrinsically immiscible

for most compositions, however the copolymers formed

during melt mixing act as compatibilizers for the starting

materials.

Poly(ethylene terephthalate) and PTT, as well as PBT,

crystallize in a triclinic crystal structure; for PTT

periodicity along the c-axis contains two repeating units,

the methylene groups being arranged in a highly contracted

gauche-gauche conformation.[20–22] Crystallization

kinetics of PTTwere investigated and compared with those

of PET and PBT.[21–27] The three –(CH2)– units of the

chemical structure account for the chain flexibility of PTT;

hence, at the same undercooling, PTT displays a greater

crystallization rate than does PET.[26,27] Whereas blends of

PET and PBT have been quite widely considered in the

past,[28–31] blends involving PTT have been explored only

recently.[32–37] Physical intimate miscibility on a fine

segmental scale was found in the quenched state of binary

PBT/PTTand PET/PTTand ternary PET/PBT/PTT blends,

prepared under the conditions of no chemical transester-

ification, whereas during crystallization, the components

form their own crystals; unit cells of the crystalline regions

remain individually different and co-exist in common

spherulites.[32,33]Miscibility arising from the occurrence of

transesterification reactions during melt blending was also

reported.[34,35]

The aim of this work is to investigate fundamental

aspects of the reactive blending of PTT and PET by

performing thermal analysis on mechanical mixtures of

various compositions. The effect of blending on crystalliz-

ability of PTT was also explored by isothermal crystal-

lization kinetics.

Experimental Part

Materials

Poly(trimethylene terephthalate) and PET used in this studywere commercially manufactured granulate supplied ascourtesy research materials by Shell Research s.a.

Poly(trimethylene terephthalate) is the Corterra 509200fibre grade, with a nominal melting temperature of 228 8C and[Z]¼ 0.92 dL � g�1 (determined at 30 8C in a 60/40 w/w mix-ture of phenol/1,1,2,2-tetrachloroethane Ph/TCE); its averagedmolecular weight isMv ¼ 54 000 Da.[38]

Poly(ethylene terephthalate) is the Cleartuf P82 bottle resin,with a nominal melting temperature of 250 8C (indicating adegree of modification with isophthalic acid) and [Z]¼0.71 dL � g�1 [measured in trifluoroacetic acid (TFA) at30 8C], corresponding to an averaged molecular weightMv ¼ 53 000 Da.[39]

Preparation of the Mixtures

Mechanical mixtures PTT/PET 90/10, 50/50 and 10/90 w/wwere prepared by mixing solutions of the two polyesters inCHCl3/TFA (80/20 and 50/50 v/v for PTT and PET,respectively) and co-precipitating in methanol. The amountof polymer recovered corresponds to the mass sum of the twocomponents; thus, the original composition is guaranteed(weight loss no more than 2%). After drying under vacuum at130 8C for 48 h samples of themixtureswere kept in theDSC at280 8C for different times (0–120 min) and analysed during asuccessive cooling-heating cycle (scanning rate 20 8C �min�1)or by performing isothermal crystallizations. Neat PTT andPET obtained by precipitation and processed under the sameconditions were also investigated as reference materials.

Characterization Techniques

Some samples of the mixtures and of neat polyesters wereanalysed in terms of thermogravimetric stability by means of aPerkin Elmer TGS 2 analyser on heating from 50 to 700 8C at20 8C �min�1 or from 50 to 280 8C at 10 8C �min�1 and, afteran isothermal treatment of 60 or 120min at 280 8C, from 280 to700 8C at 20 8C �min�1.

Calorimetric analysis was performed up to 280 8C at a scanrate of 20 8C �min�1 on dried samples of 5–10mg using aDSC821 (Mettler Toledo).

Isothermal melt crystallization kinetics were carried out in aDSC 2920 (TA Instruments) at various Tc, in the range of 195–201 8C. It has not been possible to obtain reliable data by DSCin a wider range of Tc: in fact, below 195 8C crystallization istoo fast to evaluate properly the initial variation of heat flow asa function of time and above 200 8C it is too slow to appreciatecorrectly the completion of the crystallization process. Datahere reported refer to an intermediate temperature, Tc¼198.5 8C, since the results in the explored range indicated onlya moderate effect of temperature on the rate of crystallization.

Samples of PTT/PETwith the following composition, 100/0,90/10, 50/50 and 0/100 w/w, were analysed; aluminium pansfilled with the same quantity of 7� 1 mg (in order to avoiduneven thermal conduction through the samples) were kept at280 8C for 5–120 min and then quickly cooled to the crys-tallization temperature, where they were held until thecompletion of the crystallization process, achieved when nofurther significant variation of the heat flow as a function oftime was further noticed. The development of crystallinityversus time was derived from the enthalpy of the exothermic

Reactive Blending of Aromatic Polyesters: Thermal Behaviour of Co-precipitated Mixtures PTT/PET 243

Macromol. Chem. Phys. 2006, 207, 242–251 www.mcp-journal.de � 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

peak of the DSC isothermal analysis at each crystallizationtime t (dHc is the crystallization enthalpy related to aninfinitesimal time interval dt) by normalising to the meltingenthalpy of 100% crystals of neat polyesters (DHm

0 ¼ 125 and146 J � g�1 for PET and PTT, respectively[2,40]) or of mixtures(weight average of DHm

0 s).

Crystallinity ð%Þ ¼ðt

0

ðdHc=dtÞdt=DH0m

0@

1A� 100 ð1Þ

Moreover, the analysis of the kinetic data based on theAvrami model was carried out as a function of compositionand, on the PTT/PET100/0 and 90/10mixtures, as a function ofthe residence time at 280 8C.

Viscometric analysis of dilute solutions of the mixtures wascarried out with an Ubbelhode viscometer at 30 8C in Ph/TCE60/40 w/w.

Morphology of some samples was investigated by using aLeica Stereoscan 440 scanning electron microscope at 20 KVaccelerating voltage and a Reichert-Jung Polivar-Pol polariz-ing microscope, equipped with a Mettler FP82HT hot stageconnected to an FP80 central processor. SEM analyses werecarried out on samples fractured in liquid nitrogen and coatedwith gold in a sputtering unit mod. Agar Aid PS3. Opticaltextures were developed on rapidly cooling polymer samplesinserted between glass slides from 280 8C to the selectedcrystallization temperature (ranging between 196 and 200 8C);the formation and growth of selected spherulites was analysedbetween crossed polars and recorded at appropriate timeintervals up to impingement by a video camera connected witha computer for image acquisition. The growth rate wasobtained from the slope of the plot crystal radius (measuredafter calibration with a micrometric reticule) versus time.

Results and Discussion

Thermal profiles of the mixtures obtained by co-precipita-

tion show the melting endotherms of both PTT and PET at

the expected temperatures, preceded by a low enthalpy

endotherm at about 150 8C (Tendo), ascribable to themelting

of small and imperfect crystals formed during the drying

procedure of both components; PET glass transition emer-

ges better than the PTTone, probably due to the easier crys-

tallisability of PTT arising from its higher chain flexibility.

The melting profiles of the mixtures are given in Figure 1.

Their DSC parameters are compared with those of neat

polyesters obtained by precipitation in Table 1. For all the

mixtures, the total melting enthalpy is fairly consistent with

composition, the difference between the calculated and

experimental values being around 5%; high Tg values are

due to solidification from solution.

The DSC plots of the cooling and the second heating

cycle after different times of reaction are reported in

Figure (2)–(4) for the three blends investigated. The melt-

ing temperatures, at fixed composition, decrease on incre-

asing the treatment time in DSC. The two mixtures rich in

one component showessentially a singleTm [seeFigure 2(b)

and 3(b)], whereas two distinct peaks at Tm1 and Tm2 are

evident for the 50/50 mixture [Figure 4(b)] which, on

increasing treatment time, shift to lower temperatures and

merge together; after 60 min of reaction the 50/50 blend

exhibits a single Tm (Table 2) which, in principle, seems to

be ascribable to PTT sequences. The results qualitatively

agree with those referring to composition and reaction time

effects given in the literature.[34–36]

The presence of two crystalline polymers in the blends

explains the two melting peaks at intermediate composi-

tions; however, each blend exhibits only one crystallization

peak, whose temperature varies with both reaction time and

composition. Probably the two expected crystallization

peaks are partially overlapped into an apparent single one.

Figure 1. DSC traces of PTT/PET co-precipitated mixtures:(a) 90/10; (b) 50/50; (c) 10/90.

Table 1. Thermal behaviour of neat polyesters and co-precipitated mixtures.

PTT/PET(w/w)

TgPTT TgPET Tendo DHendo TmPTT TmPET DHmtot

8C 8C 8C J � g�1 8C 8C J � g�1

100/0 65 – 149.8 4.8 230.5 – 64.90/100 – 96 142.8 5.2 – 251.0 43.350/50 60 93 153.8 5.3 228.5 249.0 58.190/10 77 – 147.8 5.2 229.8 243.1 61.510/90 – 95 149.1 7.7 226.6 250.7 48.1

244 M. Castellano, A. Turturro, B. Valenti, A. Avagliano, G. Costa


Alternatively, simultaneous crystallization upon cooling

from the melt might suggest full miscibility; however, as

reported elsewhere,[35,36]WAXDpatterns of the 50/50mix-

ture processed for short reaction times show the presence of

the crystalline peaks of both PETand PTT. Furthermore, the

overall crystallinity developed depends strongly on com-

position. It is greatly inhibited for the 50/50 mixture, where

crystallization on cooling at Tc decreases and disappears in

about 30 min of reaction [Figure 4(a)]; at the same time, the

cold-crystallization phenomenon emerges and develops

showing a single peak at Tcc [Figure 4(b)]. Moreover the

amount of overall crystallinity (DHcþDHcc) decreases on

increasing the reaction time and the melting enthalpy

(DHm) is reduced from about 40 to 25 J � g�1 (Table 2). This

effect is less evident for the blends rich in one of the two

components;moreprecisely, thePTT-richblend[Figure2(a)

and Table 3] shows a broadening of the crystallization

profile and a vanishing reduction of Tc, whereas the PET-

rich mixture exhibits a decrease of both Tc and DHc

[Figure 3(a) and Table 4]. Furthermore, in both cases, no

evidence of cold-crystallization appears in the successive

heating run [Figure 2(b) and 3(b)], except for the small

enthalpy exotherm (4–7 J � g�1) preceding themelting peak

Figure 2. DSC traces of the PTT/PET 90/10 co-precipitatedmixture kept in DSC at 280 8C for 0–120 min: (a) coolingprofiles; (b) second heating profiles.

Figure 3. DSC traces of the PTT/PET 10/90 co-precipitatedmixture kept in DSC at 280 8C for 0–120 min: (a) coolingprofiles; (b) second heating profiles.

Figure 4. DSC traces of the 50/50 co-precipitated mixture keptin DSC at 280 8C for 0–120min: (a) cooling profiles; (b) secondheating profiles.



in the PTT-rich mixture profile, typical of PTT melting

behaviour.

In order to justify the observed effects the occurrence of

some transesterification when the blends are kept in the

DSC at 280 8C must be taken into account; the compatibi-

lization reaction starts to take place for treatment time in the

order of 15 min and it is notably enhanced after 45 min. It

must be pointed out that, as given in Table 5, no significant

effect is shown on the melting temperatures of the neat

polyesters, on increasing residence time at 280 8C, whereascrystallization phenomena areweakly affected. Indeed, it is

reported that degradation and transesterification processes,

occurring at high temperatures in the melt, influence the

crystallization behaviour of PTT.[41]

A single-glass transition temperature, Tg, was detected

for eachmixture composition; in particular about 55, 65 and

75 8Cwere measured for PTT/PET 90/10, 50/50 and 10/90,

respectively, in the second heating run, independently of the

treatment time at 280 8C [see Table (2)–(4)]. Even though it

might be difficult to single out the glass transition of the two

components, this result seems to indicate miscibility of the

polyesters in the glassy state, as already pointed out by

several authors.[32,35,36]

TGA experiments on neat PTTand PETand on the 50/50

mixture revealed onset temperatures of 366, 424 and

395 8C, respectively. Isothermal treatments of the mixture

at 280 8C for 60 and 120min inducedweight loss lower than

1.5%; onset temperature is not affected by the thermal

history.

Morphological analysis performed by SEM on DSC

specimens of the 50/50 mixture, after different times of

reaction at 280 8C followed by a cooling run to room tem-

perature, shows for all the samples similar homogeneous

fracture surfaces, lacking any discernible separated

domains for all the samples.

Poly(trimethylene terephthalate) and PETand their phys-

ical mixtures were characterized by viscosity measurements

Table 2. Effect of treatment in DSC on the PTT/PET 50/50 w/w co-precipitated mixture.

Reaction time Cooling Second heating

min Tc DHc Tg Tcc DHcc Tm1 Tm2 DHm

8C J � g�1 8C 8C J � g�1 8C 8C J � g�1

0 157.2 45.1 66 – – 224.0 245.7 41.415 140.5 35.6 70 125.7 1.5 216.8 234.9 38.630 122.4 8.2 64 130.5 23.9 206.4 223.3 35.345 – – 63 134.0 26.8 201.6 217.9 25.960 – – 65 138.6 26.7 198.2 214.6 30.375 – – 65 140.2 27.6 196.0 26.090 – – 65 142.3 24.4 194.2 24.0120 – – 61 137.6 23.0 192.1 26.3

Table 3. Effect of treatment in DSC on the PTT/PET 90/10 w/w co-precipitated mixture.

Reaction time Cooling Second heating

min Tc DHc Tg Tcc DHcc Tm1 Tm2 DHm

8C J � g�1 8C 8C J � g�1 8C 8C J � g�1

0 179.9 46.6 54 212.3 3.8 226.8 240.2 47.815 169.6 46.1 57 204.5 4.1 222.9 – 46.930 151.2 46.0 57 193.0 7.0 219.8 – 50.260 141.8 48.6 56 189.4 6.8 218.6 – 51.190 137.7 47.0 52 187.2 5.1 217.8 – 53.7120 141.4 46.4 50 184.1 6.1 214.2 – 53.7

Table 4. Effect of treatment in DSC on the PTT/PET 10/90 w/wco-precipitated mixture.

Reactiontime

Cooling Second heating

min Tc DHc Tg Tm1 Tm2 DHm

8C J � g�1 8C 8C 8C J � g�1

0 190.4 43.6 78 236.0 249.2 40.415 190.1 45.7 75 236.0 247.5 45.830 177.3 25.1 76 – 243.1 37.160 175.6 37.2 75 – 241.2 37.390 162.6 27.7 76 – 238.9 33.2120 163.6 32.0 75 – 237.5 34.4



at 30 8C in dilute solution of Ph/TCE 60/40 w/w; the results

indicate viscosity values for the mixtures intermediate

between those of pure materials (0.92 dL � g�1 for PTT and

0.63 dL � g�1 for PET): Z values of 0.88, 0.78 and 0.65

dL � g�1 have been found for PTT/PET 90/10, 50/50 and 10/

90, respectively.

The development of per cent crystallinity versus time at

198.5 8C, after 15 min of reaction at 280 8C, is given in

Figure 5(a) for different compositions. The time needed to

reach the maximum attainable degree of crystallinity

increases from neat PET to neat PTT because, due to the

higher melting temperature of PET, the degree of under-

cooling for this polymer is higher. The behaviour of the

PTT/PET 90/10 mixture is intermediate between those of

the two polyesters, whereas for the 50/50 mixture crystal-

lization appears to be significantly inhibited. The time-

dependent relative crystallinity functionsX(t) [0�X(t)� 1]

are given in Figure 5(b); they have been obtained, for each

sample, after subtraction of the induction period ti, by

XðtÞ ¼ðt

0

ðdHc=dtÞdt=ð1

0

ðdHc=dtÞdt ð2Þ

where t and 1 are the elapsed times during the course of

crystallization and at the end of the process, respectively.

Crystallization and kinetic data are summarized in Table 6,

in terms of induction time ti and half-time of crystallization

t0.5, defined as the elapsed time from the onset of

crystallization to the point where crystallization is half-

completed [X(t)¼ 0.5]. The apparent degree of under-

cooling DT¼ Tm� 198.5 8C, derived from the melting

temperatures of the mixtures given in Table 2 and 3, and per

cent crystallinity are also reported. The apparent melting

point Tm instead of Tm0 has been used because it is extremely

difficult to determine reliable data of Tm0 for reactive blends.

In spite of the low degree of crystallinity attained by the 50/

50 mixture under the adopted conditions, its crystallization

rate, in terms of t0.5, appears to be comparable with that of

the 90/10 mixture and faster than that of neat PTT.

The results obtained can be justified taking into account

that interchange reactions occurring in themelt undermode-

rate mixing conditions (15 min), in excess of one of the

components (90% of PTT), should lead to the formation of

copolymers bearing long sequences of the predominant unit

TT. These sequences are still able to segregate and crystal-

lize; indeed, their crystallization behaviour resembles that

of a lower molecular weight sample and implies higher

crystallization rate and lower half-time of crystallization

t0.5. Moreover, a nucleation action of unreacted PET chains

might also be taken into account. Generally speaking, the

ability of the system to crystallize is affected by the block

Table 5. Effect of treatment in DSC at 280 8C on neat polyesters.

Polyester Treatment time Cooling Second heating

min Tc DHc Tg Tcc DHcc Tm DHm

8C J � g�1 8C 8C J � g�1 8C J � g�1

PTT 0 153.0 50.0 55 202.0 7.7 227.0 52.030 141.1 57.1 54 197.6 9.0 226.6 51.260 136.0 56.0 54 194.6 9.9 225.6 52.0

PET 0 154.0 13.8 82 156.0 20.4 246.0 33.730 149.5 13.7 82 156.6 20.3 245.3 31.260 150.9 12.8 81 158.6 23.5 244.5 28.3

0 20 40 60 800

5

10

15

20

25

30a)

time (min)

Cry

stal

linit

y, %

0 20 40 60 800,0

0,2

0,4

0,6

0,8

1,0b)

time (min)

X(t

)

Figure 5. Development of per cent crystallinity vs. time (a) andtime-dependent relative crystallinity function X(t) (b) at 198.5 8C,after 15 min of reaction at 280 8C: (&) PET; (&) PTT; (~) PTT/PET 90/10; (*) PTT/PET 50/50.



length and by their distributions along the copolymer

chains. As the PET content in the mixture increases (50%),

the high crystallization rate could be still related to the

persistence of TT sequences still able to segregate and

crystallize; in order to justify the low degree of crystallinity

the melting behaviour of the isothermally crystallized

samples has been taken into account (see Table 7). Both

PTTand themixtures exhibit a doublemelting profile;DHm

refers to the totalmelting enthalpy, and per cent crystallinity

relates to the individual contributions. Whereas for the 90/

10 mixture both peaks are due to the melting of TT se-

quences, in the case of the 50/50 sample the ET unit

contribution is also evident and comparablewith that of TT.

As a consequence, the degree of crystallinity shown in

Figure 5(a) is underestimated, the contribution of ET blocks

to the overall phenomenon being too fast to be detected by

DSC, under the adopted experimental conditions used.

The role played on per cent crystallinity and on the X(t)

function by the residence time at 280 8C for the 90/10

mixture is shown in Figure 6(a) and (b). Evidently, the time

needed to reach ultimate crystallinity at 198.5 8C increases

with increasing residence time above Tm; the crystallization

kinetics and the degree of crystallinity are also affected. It is

well known that, if the crystalline memory of a polymer is

not completely destroyed during melting, the persisting

crystalline tracesmay act as nucleation agents and affect the

crystallization rate. The memory effect can be erased by

melting the polymer at a sufficiently high temperature

(close to Tm0 ); however, as suggested by Ziabicki and

Alfonso,[42,43] at a given value of the processing temper-

ature, the number of nuclei decreases with increasing the

holding time, the total concentration of predetermined

nuclei being an exponential decay function of the residence

time in the melt. In order to detach the role played by

Table 6. Crystallization and kinetic data.

PTT/PET (w/w) Time at 280 8C ti t0.5 DT Crystallinity k n r2

min min min 8C % min�n

100/0 5 2.7 11 28.5 25.2 2.5� 10�3 2.3 0.9997100/0 15 6.3 24 28.3 27.2 2.3� 10�4 2.4 1.0000100/0 60 9.0 26 27.1 22.7 4.5� 10�4 2.3 0.9999100/0 120 5.9 28 25.5 23.3 4.2� 10�4 2.2 0.99990/100 15 0.3 9 47.5 23.6 2.2� 10�2 2.0 0.999650/50 15 4.2 18 18.3–36.4 9.4 2.1� 10�3 2.0 0.999890/10 15 3.4 17 24.4 25.0 4.5� 10�4 2.6 0.996590/10 30 10.2 26 21.3 24.3 3.7� 10�4 2.3 0.998890/10 60 14.3 56 20.1 23.3 6.7� 10�5 2.3 0.999490/10 90 13.4 62 19.3 21.7 7.8� 10�5 2.2 0.998890/10 120 25.0 74 15.7 18.4 7.0� 10�5 2.1 0.9995

Table 7. Melting behaviour of polyesters and mixturesisothermally crystallized at 198.5 8C after a 15 min treatment at280 8C.

PTT/PET(w/w)

Tm DHm Crystallinity

8C J � g�1 %

100/0 220.6–225.8 54.8 21.3–16.00/100 241.2 30.7 24.390/10 217.6–223.4 47.3 17.7–15.150/50 220.6–236.5 37.9 15.0–13.0

0 50 100 150 200 250 3000

5

10

15

20

25

30

time (min)

a)

Cry

stal

linit

y, %

0 50 100 150 200 2500,0

0,2

0,4

0,6

0,8

1,0b)

X(t

)

time (min)

Figure 6. Development of per cent crystallinity vs. time (a) andtime-dependent relative crystallinity function X(t) (b) at 198.5 8Cfor the PTT/PET 90/10 mixture, after different reaction timesat 280 8C: (}) 15 min; (~) 30 min; (~) 60 min; (þ) 90 min;(*) 120 min.



transreactions in the 90/10 mixture from memory effects of

the predominant component, per cent crystallinity devel-

oped by neat PTT at 198.5 8C has been determined for

different residence times at 280 8C. The results, given in

Figure 7(a) and (b), show that memory effects vanish after

quite short times, in the order of 15 min (see also Table 6).

As a consequence, residence time effects of Figure 6 should

be ascribed to the occurrence of transesterification; the

tendency of the distribution of TT block lengths toward

equilibrium values could justify the progressively reduced

difference between kinetic plots related to longer isother-

mal treatment at 280 8C.The analysis of the results in terms of the Avrami

model[44–46] was done by relating the X(t) functions to the

crystallization time t according to

XðtÞ ¼ 1� expð�ktnÞ ð3Þ

where k and n are the Avrami crystallization rate constant

and exponent, respectively; they can be directly obtained

from the slope and the intercept of the best-fit lines of the

double logarithmic form of Equation (3) for different

residence times at 280 8C and compositions. Table 6 sum-

marizes these kinetic parameters also. Non-integral values

and values of n lower than 3 may hint at crystal branching

and/or two-stage crystal growth and/or mixed growth and

nucleation mechanisms.[24,47,48] Similar n values for

PTT[26] and PET[27] at 198 8C are given in the literature,

whereas much faster rates of crystallization, in terms of

both t0.5 and Avrami rate constant k, are reported for both

polyesters melted at 280 8C for a shorter time.[6,26]

Poly(trimethylene terephthalate) bulk samples form

banded spherulites when crystallized from the melt be-

tween 195 and 215 8C (regime II).[24] Figure 8(a)–(c) show

spherulitic textures developed at 198 8C after different

isothermal treatment at 280 8C (30–90 min); ring patterns

are also evident for the PTT/PET 90/10 mixture

[Figure 8(d)–(f)]. Radial growth rate (G) for both systems

are given in Table 8; on increasing residence time in themelt

G of PTTis only slightly affected (as expected on the base of

kinetics shown in Figure 7), whereas a definite decrease

appears for themixture between 30 and 60min of treatment.

This result, justified by the texture shown in Figure 8(d) and

in agreement with data of Figure 6, can be ascribed to a

persisting nucleation action of PET after moderate resi-

dence time at 280 8C. On increasing crystallization temper-

ature (at fixed reaction time in themelt)G of PTTdecreases,

as expected; data of Table 8 are in good agreement with

those found in the literature for the same Tc range.[23,49] For

themixture invariance ofG is observed: the expected lower-

ing of G due to higher crystallization temperatures is

probably balanced by a higher growth rate caused by the

shortening of TT sequences through interchange reactions.

For both systems less regular concentric circles develop on

increasing residence time at 280 8C; it is difficult to interpretthis phenomenon as well as to relate band spacing with

melting and crystallization parameters. The reasons may be

the fundamental effect played by film thickness, not pro-

perly controlled during our experiments, and the role

exerted by amorphous segment distribution along the co-

polymeric chains, which hardens regular lamellar twisting

along the direction of radial growth.

Concluding Remarks

Poly(trimethylene terephthalate)/PET mechanical mixtu-

res of various compositions (90/10, 50/50 and 10/90 by

weight) were prepared by precipitation from mixed

solutions, kept in DSC at 280 8C for different times

(0–120 min) and analysed during a successive cooling-

heating cycle or under isothermal crystallization condi-

tions. Melting temperatures and crystallization ability, at

fixed composition, decrease on increasing treatment time in

DSC; the overall crystallinity developed depends on

composition. All the observed effects can be justified

taking into account interchange reactions occurring in the

melt between the polyesters.

Transesterification is also evidenced by the isothermal

crystallization behaviour of the mixtures, in terms of

0 20 40 60 80 1000

5

10

15

20

25

30a)

Cri

stal

linit

y, %

time (min)

0 20 40 60 80 1000,0

0,2

0,4

0,6

0,8

1,0b)

X(t

)

time (min)

Figure 7. Development of per cent crystallinity vs. time (a) andtime-dependent relative crystallinity function X(t) (b) at 198.5 8Cfor the neat PTT, after different reaction times at 280 8C:(*) 5 min; (~) 15 min; (*) 60 min; (~) 120 min.



variation of the degree of crystallinity and of the crystal-

lization rate with composition and reaction time. The

compositional microstructure after the occurrence of trans-

esterification is currently under investigation by different

techniques, in particular X-ray analysis,MALDI TOFmass

spectrometry and NMR spectroscopy and results will be

given in a forthcoming paper.

Acknowledgements: The authors express their gratitude toDr. Ian Carson for profitable discussions of the results andto Mr. Vincenzo Trefiletti for help in performing thermalcharacterizations. The authors also acknowledge the kind supplyof polymeric materials from Shell Research s.a. Partial financialsupport by the University of Genova and by the Italian NationalResearch Council is gratefully acknowledged.

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Figure 8. Spherulitic textures developed at 198 8C after different isothermal treatmentat 280 8C by PTT [(a) 30, (b) 60 and (c) 90 min] and PTT/PET 90/10 [(d) 30, (e) 60 and(f) 90 min].

Table 8. Radial growth rate of PTTand PTT/PET 90/10 mixtureas a function of the crystallization temperature Tc and the time ofisothermal treatment at 280 8C.

Tc Time at 280 8C G PTT G mix

8C min mm � s�1 mm � s�1

198 30 0.46 0.27198 60 0.43 0.17198 90 0.41 0.14196 60 0.57 0.13200 60 0.42 0.13



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Reactive Blending of Aromatic Polyesters: Thermal Behaviour of Co-precipitated Mixtures PTT/PET

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