Novel copolyesters containing naphthalene structure. II. Copolyesters prepared from 2,6-dimethyl naphthalate, 1,4-dimethyl terephthalate, and ethylene glycol

Novel Copolyesters Containing Naphthalene Structure, II. Copolyesters Prepared from 2,6-Dimethyl Naphthalate, 1,4-Dimethyl Terephthalate, and Ethylene Glycol

TSU-SHANG LU,' YIH-MIN SUN? and CHUN-SHAN WANG',*

'Department of Chemical Engineering, National Cheng Kung University, Tainan, Taiwan, Republic of China; 'Department of industrial Safety and Hygiene, China Junior College of Medical Technology, Tainan, Taiwan 701, Republic of China

SYNOPSIS

Copolyesters containing rigid segments (naphthalene and terephthalene) and flexible segments (aliphatic diol) structure were synthesized from DMN/DMT/EG (2,6-dimethyl naphthalate/l,4-dimethyl terephthalate/ethylene glycol) ternary monomers with various mole ratios. Copolyesters having intrinsic viscosities of 0.52-0.65 dL/g were obtained by melt polycondensation in the presence of metallic catalysts. The effect of reaction temperature and time on the formation of the copolyesters was investigated to obtain an optimum condition for copolyester manufacturing. The optimum condition for PNT (polyethylene naphthalate terephthalate) copolyester manufacturing is the transesterification under nitrogen atmosphere for 4 h at a temperature of 185 k 2°C followed by polymerization under 2 mm Hg for 2 h a t a temperature of 280°C. Most copolyesters have better solubilities than poly(ethy1ene naphthalate) (PEN) and poly(ethy1ene terephthalate) (PET) in various solvents. The effect of the starting mole ratio of DMN, DMT, and EG on the thermal properties of the resulted copolyesters was also investigated using differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA). Glass transition temperatures of copolyesters were in the range of 70.7-115.2OC, and 10% weight loss in nitrogen were all above 426°C. 0 1995 John Wiley & Sons, Inc. Keywords: aromatic copolyesters poly(ethy1ene terephthalate) poly(ethy1ene naphthalate) * melt polycondensation * physical properties

INTRODUCTION

Due to its good thermal and mechanical properties, poly(ethy1ene terephthalate) (PET) is one of the most widely used engineering plastics. Structurally related poly(ethy1ene naphthalate) (PEN) has been obtained from 2,6-dimethyl naphthalate and ethylene This newly developed high-perfor- mance polymer containing a rigid naphthalene ring has exhibited superior physical and mechanical properties than widely used P E T s . ~ Much attention has been focused recently on the preparation and applications of PEN. Due to its enhanced physical and mechanical properties, PEN has found many

* To whom all correspondence should be addressed. Journal of Polymer Science: Part A Polymer Chemistry, Vol. 33,2841-2850 (1995) 0 1995 John Wiley & Sons, Inc. CCC OSS7-624X/95/162841-10

applications: Yamamoto et aL4 reported PEN bottles with good gas-barrier property, transparency, and thermal resistance (up to 110°C). High-quality fibers from PEN with flexibility, toughness, and resistance to heat and abrasion have been p r ~ d u c e d . ~ An ori- ented multilayer polyester film for magnetic record- ing tape with good machine direction strength and heat resistance was reported by Tahoda et aL6 PEN film is particularly well-suited for electronic and electrical application^,^ such as flexible printed cir- cuits, class "F" insulation, wiring applications, tough membrane switches, and flexible heaters. Although poly(ethy1ene naphthalate) has superior physical and mechanical properties than PET, however its relative low production volume and high price will limit its applications in the near future.

We have already reported on the preparation and characterization of PEN and copolyesters

2841

2842 LU, SUN, AND WANG

derived from bis(hydroxyethy1)naphthalate with bis[4-(2-hydroxyethoxy)phenyl] compounds.' The physical properties of PEN (such as solubility, mechanical properties, and thermal stability) were successfully improved by the introduction of the aryl ether linkage and the bulky pendant group, while the raw material cost can be reduced by choosing an inexpensive comonomer, such as bis- phenol A.

In the present study, a series of polyethylene naphthalate terephthalate copolymers (PNT) with various compositions were synthesized from DMN/ DMT/EG ternary monomers with the objectives of improving the solubility and processability of PEN,

reducing the cost of PEN, while improving physical and mechanical properties of PET.

Transesterification is generally the preferred process for the manufacture of PEN.1,2,9 There are two steps in the preparation of PNT. The first step is the formation of 2,6-bis(hydroxyethyl)naphthalate (BHEN) and 2,6-bis(hydroxyethyl)terephthalate (BHET), respectively, from the transesterification of 2,6-dimethyl naphthalate (DMN) or 2,6-dimethyl terephthalate (DMT) with ethylene glycol (EG). The second step is the PNT formation from the polycondensation of BHEN and BHET mixture at ele- vated temperature and reduced pressure. The reaction schemes are shown below:

0

&-OCH,

mH,CO-C \ II 0

+ 0

@-OCH,

nH,CO-C \ I1 0

+ 2(m + n ) HOCH,CH,OH c 0

~ - o C H , C H ~ o H rn HOH,CH,CO-C \

II 0

+ 0

@-oCH,cH,OH n HOH,CH,CO-C \

I1 0

+ 2(m + n) CH,OH

Polycondensation:

0 0

~ o C H , c H ~ O H m HOH,CH,COC \

0 II 0

(2) I -(m + n- 2) HOCH,CH,OH

T-T type N-N type N-T type

EXPERIMENTAL

Raw Materials

termination of solubility, N-methyl-2-pyrrolidone (NMP), N,N-dimethylformamide (DMF), and pyridine, were purified by distillation under reduced pressure over calcium hydride and stored over 4 A

2,g-Dimethyl naphthalate (Amoco) is a commercial molecular sieves. Zinc acetate and antimony trioxide product and was used without further purification. were commercial products (guaranteed reagent 1,4-Dimethyl terephthalate (Janssen) and ethylene grade) and were used without further purification. glycol (Ferak) were reagent grade and used without The solvents, phenol (Ferak) and tetrachloroethane further purification. The solvents used for the de- (Merck), used for the determination of solubility and

NOVEL COPOLYESTERS CONTAINING NAPHTHALENE STRUCTURE. I1 2843

Table I. Characterization of BHEN and BHET

Sample Bis(hydroxyethy1)naphthalate Bis(hydroxyethy1)terephthalate

Formula C16H1606

mp ("C) 129-1 3 1 Elemental analysis (%): found (calcd) C: 63.16 (63.76)

H: 5.26 (5.22) 0: 31.58 (31.02) M+(304) 43.12 M - 61'243' 100 M - 88('16' 42.97 M - 105'199' 46.31 M - 133'17'' 28.35 M - 178'126' 13.49 3450 (0-H) 3050 (Ar: C - H) 2900 (Alkyl: C - H) 1720 (C=O) 1600 (Ar: C - C) 1240 (C-0) 1160 (C-OH)

MS (rile): (relative intensity, %)

IR (cm-')

C12HI406 108-109 C: 56.64 (56.70) H: 5.52 (5.51) 0: 37.84 (37.79) M+(254' 48.11 M - 61'193' 100 M - 88"66' 46.27 M - 105"49' 50.29 M - 133'121' 32.13

3450 (0-H) 3045 (Ar: C -H) 2900 (Alkyl: C - H) 1700 (C=) 1600 (Ar: C-C) 1150 (C-OH)

intrinsic viscosity measurement of the polymer were also used without purification.

Instrumentation

Elemental analyses were performed by the Heraeus CHN-0-Rapid elemental analyzer. FTIR spectra were recorded with a Nicolet 5DX-B spectropho- tometer. Mass spectra were recorded by the VG 70- 250s GC/MS. Intrinsic viscosities were obtained using a Ubbelohde capillary viscometer (Schott- AVS310). Melting points of BHEN and BHET were determined in a polarizing microscope (Laboratory Devices MEL-TEMPII). DSC data were obtained from 8-10 mg samples in a nitrogen atmosphere a t a 20°C min-' heating rate using a Du Pont 910 differential scanning calorimeter. Thermal gravimetric analysis (TGA) was measured with a Du Pont 945 a t a heating rate of 20°C min-' in a nitrogen atmosphere. The wide-angle x-ray measurements were performed a t room temperature with powdered specimens with a Rigaku Geiger Flex D-Max/IIIa x-ray diffractograms, using Ni-filtered Cuka radia- tion (40 kV, 15 mA). The scanning rate was 2" min-'.

Syntheses of BHET and BHEN Compounds

BHET [bis(hydroxyethyl) terephthalate] was prepared from dimethyl terephthalate (DMT) and ethylene glycol (EG) by the modified method (modifi- cation in catalyst and reaction condition) of Ba1iga.l' BHEN [bis(hydroxyethyl) naphthalate] was synthesized from corresponding dimethyl naphthalate

(DMN) and ethylene glycol (EG) following the method described in a previous report." The purified BHET, BHEN monomers were identified by elemental analyses, mass spectra, IR spectra, and melting points which are listed in Table I.

Preparation of Copolyesters

Transesterificafion

The reactor for transesterification of DMN/DMT with EG was the same as the one previously reported." To the reaction vessel, 1 mol of DMN/ DMT mixture (various mol % ratios of DMN/DMT were prepared: 100/0, 85/15, 70/30, 50/50, 30/70, 15/85, and 0/100), 2 mol EG and zinc acetate (15 X mol/mol ester group) were introduced. The reaction was carried out with stirring under a nitrogen atmosphere. The temperature of the reaction mixture was measured with a thermocouple detector and was maintained a t 185 * 2°C. The temperature of the distillation column was maintained a t 100 k 3°C. The reaction was considered to have started when the first drop of the methanol formed in the acceptor. The transesterification reaction was followed by measuring the volume of methanol collected in the acceptor.

Polycondensation

The monomers (BHEN and BHET) synthesized above were mixed in various mole ratios for the copolymerization reaction. Besides the esterification between BHEN and BHET, the polycondensation


0

Table 11. Solubilities of PET-PEN Copolyestersa

,'d

Solventb PEN 80 : 20 60 : 40 50 : 50 40 : 60 20 : 80 PET

DMAC DMF DMSO NMP m-cresol Pyridine CHCI, CZHZCI4

- - - -h -h -h - -h

+h - - ++ ++ -h - f h

+h +h -h ++ ++ +h

++ -

+h +h

++ ++ +h

++

-

-

-h - - +h +h -h

+h -

a (++) Soluble a t room temperature, (+h) soluble on heating, (-h) partially soluble on heating, (-) insoluble. DMAC: N,N-dimethylacetamide, DMF: N,N-dimethylformamide, DMSO: dimethyl sulfoxide, NMP: N-methyl-2-pyrolidone.

of BHEN (or BHET) itself could occur a t the same time and a random copolymer would be generated. The reaction equation and product are indicated in eq. (2). A mixture of BHEN/BHET (0.4 mol), zinc acetate, and antimony trioxide (8 X mol) were introduced into a 250 mL four-neck flask fitted with a reflux condenser, a thermometer, a gas inlet, a gas outlet, and a mechanical stirrer. The reaction mixture was heated to 240 * 2°C and maintained at that temperature for 90 min under dry nitrogen. The temperature was raised to 250°C and stirring was continued for 30 min. The pressure of the reaction system was gradually reduced first to 180-200 mm Hg over the course of 20 min. Over the course of another 10 min, the pressure was further reduced to 1-3 mm Hg and the reaction temperature was raised to the final operating temperature (- 280°C). The polymerization was done isothermally a t the final temperature for the required period of time with si- multaneous removal of ethylene glycol and other volatiles by distillation. Finally, the pressure was returned to normal atmospheric pressure using ni-

I00

75

h

@

z 50

8

25

0 0 I00 ?OO 300 400

Time ( m i n )

Figure 1. Time-conversion curve of the transesterification of DMT and DMN with EG a t 1 : 1 : 4 mole ratio.

trogen to prevent degradation by oxidation, and light amber-colored, amorphous copolymers were obtained. In search of the optimum conditions for various compositions for the polycondensation step, various mol 5% ratios of BHEN/BHET were prepared (100/0, 75/25, 50/50, 25/75, 0/100) and the final operation temperatures were varied from 250 to 295°C.

Solubility Test

The solubilities of these polymer were determined by adding polymer (1-2% by weight) to the desired solvent in a test tube. The tube was left to stand for 24 h to observe whether the polymer dissolved. When the polymer did not completely dissolve a t room temperature, the test tube was heated and cooled. The polymer was defined to be soluble when no polymer has precipitated after the cooled.

1 6

1 2

0 x x >.

4

P

tube was


i n n .

0 100 200 300 4 0 0 5 0 0

Time ( m i n i

Figure 3. Time-conversion curve of the transesterification of DMT and DMN with EG at various compositions.

Intrinsic Viscosity Determination

Intrinsic viscosity of the polymer was measured using an Ubbelohde viscometer. The polymer sample (0.06 g) was accurately weighed (+.0.001 g) and dissolved in 25 mL of symmetric tetrachloroethane- phenol (2 : 3 w/w). The solution was maintained a t 120°C for 20-25 min to achieve a complete solution of the polymer in the solvent. The solution was then cooled to room temperature and filtered through a 0.45 pm disposable membrane filter (cellulose acetate). Using the viscometer a t 30°C, the intrinsic viscosity was calculated from the relative viscosity by the Ram Mohan Rao equation.13

Determination of the Moisture Absorption of Copolyesters

Disk samples [3 mm (T) X 20 mm (D)] were dried under vacuum a t 120°C until moisture had been ex-

0 0.2 0.4 0.6 0.8 1

composition DMT/(DMT+DMN)

Figure 4. various compositions.

Relative initial transesterification rate for

BHET/BHEN = 2511.5

0.1 1 0 ,

0 100 200 300 400

REACTION TIME (min)

Figure 5. condensation of BHET-BHEN (25 : 75).

Time-intrinsic viscosity curve for the poly-

pelled. Then, the samples were put inside a dry box for cooling. After being weighed, the samples were placed in the boiling water (100°C) for 24 h and then weighed again. The moisture absorption was calculated as: Percent weight gain = [(W/Wo) - 11 X 10096, where W = weight of copolymer sample after standing a t 100°C water for 24 h, and Wo =weight of copolymer sample after dried under vacuum a t 120°C.

RESULTS AND DISCUSSION

Characterization of BHET and BHEN

Both hydroxyethyl monomers were synthesized under the most preferable conditions, and results are summarized in Table 11. A polarized microscope was used to determine the melting point of BHET and BHEN. The sharp melting point of monomers was indicative tha t the monomers were pure. The synthesized BHET has a melting tem-

0.6

0.5

>. 0.4

$ 0.3

> 0.2

0. I

C

t

z

BHET/BHEN=SO/SO

- 2sooc ........ 0. . ... 2650c

.... 0 .... 2800c

.__- * -.-. 29.50c

0 so 100 IS0 200 2.50 300

REACTION TIME (min)

Figure 6. condensation of BHET-BHEN (50 : 50).



perature of 109-llO°C which is the same as reported value."

The results of elemental analyses of these monomers are shown in Table I1 which agree well with the assigned structures. Electron impact induced fragmentation patterns of these monomers a t 30 eV have been obtained. The common features observed in the mass spectra and the infrared spectra of bis(hydroxyethy1) monomers are also shown in Ta- ble 11. From these results, it can be concluded that the products were in good agreement with the assigned structures.

Searching for the Optimum Conditions for Syntheses of Copolyesters

The ester-exchange polymerization (alcoholysis) of aromatic diester with aliphatic diol using zinc acetate or antimony trioxide as a catalyst used by the authors'' is a convenient method for the preparation of copo-

0

0

lyesters on the laboratory scale. The same technique was applied here to prepare copolyesters from DMN/ DMT/EG ternary monomers in various mole ratios. The extent of the transesterification reaction was followed by measuring the quantity of methanol collected. Figure 1 shows how the transesterification reaction proceeds with zinc acetate as catalyst. Using the DMN/DMT mole ratio of 1 : 1 and the catalyst concentration of 15 X mol/mol ester group, the time required for 90% completion of the reaction was ca. 5 h. Application of previously derived kinetic equation12 to these data, and plotting the Y values" thus obtained against time, the points in Figure 2 were obtained. It is obvious that the copolymerization reaction does not fit into the reaction model as previously reported." An explanation for the deviation may be attributed to the different reactivity of the naphthalate ester and the terephthalate ester. The reaction is a competition reaction for ethylene glycol between DMN and DMT. This competition reaction can be represented as:

The rates of reactions are represented by:

cco - cc and p2 = CL30 cco

CBO - CB P1 =

where p1 and p2 are the extents of reaction (3) and (4); CA, C,, and Cc are the concentration terms for reactant A , B, and C; CAo, CBo, and Cco are the initial concentration terms for reactant A , B, and C; and

BHET/BHEN =75/25 0.7 I 1

0 100 200 300 400

REACTION TIME (min)

Figure 7 . condensation of BHET-BHEN (75 : 25) .



28OOC

0.6-

0.5-

8 0.4-

>

G G

0.3-

a BHET:BHEN + 1M):o

........ +..... 7y25

----0--- 5o:so

0.2 #fm

t I I I I 0 100 200 300 400

TIME (min)

Figure 8. condensation of P N T copolymers at 280°C.


kl and k2 are apparent rate constants for reactions (5) and (6).

As discussed above, the reactivity difference between benzene ring and naphthalene ring makes their reaction rate constants ( k , and k2, respectively) different. To explain the effects of DMN/DMT mol % ratio on the transesterification reaction, the initial rate was simply used in the following discussions.

The dependence of transesterification rates on the composition of reactants in the region 0/100- 100/0 (DMN/DMT) have been investigated. Figure 3 is a representation of the results as plots of the extent of the reaction against time at various mol % ratios of DMN/DMT. It can be seen that when the DMT amount was increased, the rate of the transesterification was increased, especially in the initial stage of the reaction. Figure 4 shows the initial rates of transesterification reaction versus composition of reactants. As shown in Figure 4, the reaction rate increases with increasing amounts of DMT.

Thus, it requires different lengths of reaction time for various mole ratios of reactants (DMT/DMN) to achieve the same conversion. For example, at 100/ 0 or 85/15 ratio (DMT/DMN), the time required for 95% conversion was about 4 h, whereas for 70/ 30 or 50/50 ratio, it required 5 h for 90% conversion. When the DMN became dominant component (DMT/DMN = 30/70, 15/85, 0/100), the time required for 85% conversion became 5.5 h. Since transesterification has to be close to completion (> 80%) before the polycondensation can be started to guarantee an expected composition of copolymer, the reaction temperature at the final stage of transesterification can be slightly increased to increase the reaction rate and shorten the operation time.

Because of the very low solubility of these ho- mopolymers and copolymers in common solvents for

the determination of their MW, an assumption was made that the copolymers all have the same hydro- dynamic volume and using intrinsic viscosity as a criteria in comparing the growth of molecular weight. Figures 5-7 show the plots of the intrinsic viscosities of copolymers vs time at various reaction temperatures. Three BHET/BHEN ratios were prepared 25/75, 50/50, and 75/25. The results indicated that an increase in intrinsic viscosity with an increase in reaction temperature at the beginning. We also found that when the BHET component was dominant, the reaction rate increased faster with the increase in the reaction temperature. With a mixture of BHET and BHEN, three type of con- densations, namely BHET and BHET (T-T), BHET and BHEN (T-N), BHEN and BHEN (N- N), could take place simultaneously and a random polymer would be formed. The relative rates of the three condensation models can be obtained from the initial rates in Figure 8 and approximated as T-T > T-N > N-N. So the polycondensation of T-T model was mostly affected by temperature. The rea- son for difference in polycondensation rates (T-T > T-N > N-N) may be attributed to that the steric hindrance of benzene ring is less than that of naphthalene ring.

Another interesting phenomenon is observed from these figures, the degradation reactions depend greatly on the proportion of BHEN in the reactants. This may be explained as follows: the heat resistance of naphthalene ring is much better than that of benzene ring, so the polymerization with excess of BHET monomers degrade faster than copolymers with excess of BHEN monomers at high reaction temperature. It was also observed that the high ratio of BHET always ended in the formation of deeply colored products at high reaction temperatures.

Table 111. Time-Dependent Dissolution of Poly(ethy1ene Naphthalate/terephthalate) Random Copolymers

Polymer Approximate Time Need for Solutionsa (h)

Feed Ratio v DMN:DMT (dL/g) 0.5% 1% 5%

100 : 0 0.528 > 6 > 6 > 6 80 : 20 0.523 5 > 6 > 6 60 : 40 0.567 1 2 4 50 : 50 0.547 0.5 0.6 1 40 : 60 0.583 0.6 1 2 20 : 80 0.657 3 5 > 6 0 : 100 0.574 > 6 > 6 > 6

a Measured at 50°C in m-cresol.


I t may be concluded that the polycondensation temperature should not exceed 300°C and each composition has its own optimum reaction condition. For BHET(%) > 50, the optimum operation condition was 265-270°C and 3 h. For BHET(%) < 50, the optimum condition was 275-280°C and 2- 2.5 h. Therefore, a general operating condition for the polycondensation step in P N T process was cho- sen to be 28O"C, 2 h and the resulted copolyesters had intrinsic viscosities of more than 0.52 dL/g.

Properties of Copolyesters

Structures of the resultant copolymers were ana- lyzed by FTIR spectra: two strong aromatic absorp- tions appeared at 1600 and 1500 cm-' due to the naphthalene and benzene ring; prominent absorp- tions owing to ester carbonyl group (C =0) a t 1680- 1700 cm-l and methylene group at 2950 cm-' were also present. A strong hydroxy (-OH) absorption a t 3450 cm-' for the starting monomers (BHEN and BHET) weakened as the reaction proceeded. Un- fortunately, the nearly complete overlap of the characteristic peaks for aromatic PET/PEN systems in the FTIR spectra disqualified the use of this an- alytical tool for compositional analyses of these copolymers. So, the initial feed ratios were used to approximate the product composition of copolymers.

The solubilities of copolymers were determined using powdery specimens in various solvents a t am- bient temperature, and the results are summarized in Table 11. The homopolyesters (PEN and PET) had the poorest solubility as they dissolved only partially in l,l,Z,Z-tetrachloroethane on heating while the solubilities of the copolyesters improved dramatically as expected in various solvents, such as, NMP, m-cresol, pyridine, and tetrachloroethane. Another interesting phenomenon observed from Table I1 is that the enhanced solubility character of

2

_u_?

I

5.0 r0 15 20 25 30 35 40 45 2 8

Figure 9. WAXS diffractograms from poly(ethy1ene naphthalate): (1) product of' polycondensation; (2) melt annealing a t 200°C, 10 min.

the resultant copolyesters depend greatly on the compositions. Therefore, to prove the phenomenon, two different methods were used to study the solubility of the copolymers. First, the time required for dissolution at 80°C and 1 atm pressure was measured (Table 111). Next, solubility limits were approxi- mately determined (Table IV). Judgment of whether complete dissolution had occurred was based on the optical clarity of the system. Generally, it was found that the composition of PEN/PET in the copolymer affected both the rate of dissolution and the amount of polymer that could be dissolved. From these data the 50 : 50 copolymer had the best solubility in all solvents tested. The enhanced solubilities of the copolyesters may be attributed to the random copo- 1ymeri~ation.l~ The random blocks in the copolyester chain could make it more difficult to form a crys-

Table IV. Concentration-Dependent Dissolution of Poly(ethy1ene naphthalate/terephthalate) Random Copolymers

Polymer

Feed Ratio 9

Approximate Concentration of Solutions" (%)

DMN : DMT (dL/g) 0.5 1 5 15 20 25

100 : 0 0.528 80 : 20 0.523 X X 60 : 40 0.567 X X X X 50 : 50 0.547 X X X X X X 40 : 60 0.583 X X X X X X 20 : 80 0.657 X X X X X 0 : 100 0.574 X X

a Time was held constant at 12 h in each case. solvent is rn-cresol.


-fB- Tg

Table V. Thermal Properties of Poly(ethyle1e naphthalate/terephthalate) Copolyesters

PEN 100 : 0 267.16 115.24 453.35 33 0.528 80 : 20 102.17 450.45 30 0.523 60 : 40 94.29 444.64 27 0.567

PNT 50 : 50 91.89 444.64 27 0.547 40 : 60 86.75 444.64 24 0.583 20 : 80 83.65 428.68 20 0.657

PET 0 : 100 253.38 70.69 426.10 18 0.574

a A 10% weight-loss temperature observed by TGA at a 20"C/min heating rate in nitrogen. Residual weight at 5.70°C in nitrogen.

talline lattice structure and thus enhanced the solubility of the copolyesters. The x-ray diffraction patterns of the copolymers verified that all copolyesters synthesized are amorphus except for very low copolymerization (< 8%). Another phenomenon was observed in x-ray diffraction patterns that none of the copolyesters could be induced to crystallize from the melt by annealing, however both homo- polymers gave crystalline x-ray patterns on melt annealing (see Fig. 9).

The TGA curves of all polymers exhibited a 10% weight loss (Td) a t 426-454°C and residual weight at 530°C (RW) of 18-33% in nitrogen. The TGA data are listed in Table V. Their thermal stability increased with the increase in DMN content in reactant monomers (Fig. 10).

The Tg of polymers evaluated by DSC are also tabulated in Table V. Tg was 70.7"C for benzene- based homopolymer (PET). The Tg of naphthalene- based homopolymer (PEN) was 115.2"C, which is 445°C higher than that of PET. All copolymers showed single Tg between those of the two homo- polymers, which increased monotonously with the

5 0- 3 8 0 0 2 0 4 0 6 0 8 0 I r ' o

DMN content in mol%

Figure 10. Glass transition temperature (T,) and a 10% weight-loss temperature (Td) versus DMN content in reactants.

increase in naphthalene content of polymers (Fig. 10). The higher Tgs and better thermal stability of copolyesters over those of PET, should be ascribed to the existence of bulky, thermally, and thermoox- idatively stable naphthalene ring in the main chain.15 Figure 11 is a plot of Tg and T,,, of the copolymers against DMN content in reactants. It can be found that the copolyesters still have melting points when DMN or DMT component is smaller than 8 mol %. However, in 20-80 mol % range, random copolymers were formed and they were amorphous with single Tg and no T,.

Disk samples [3 mm (T) X 20 mm (D)] were fab- ricated from copolyesters and placed into 100°C boiling water for 24 h. The weight gains from this moisture absorption tests are shown in Figure 12. Their moisture absorption increased monotonously with the increase in DMT content in reactants.

CONCLUSIONS

A series of P N T copolyesters were synthesized through melt polycondensation of DMN/DMT/EG ternary monomers:

2 8 0

2 6 0 - E.' v

2 4 0

2 2 0

5 0 - 1 2 0 0 0 2 0 1 0 6 0 8 0 I 0 0

D M N Conlcnt in md%

Figure 1 1. Glass transition temperature (T,) and crystalline melt point (T,) versus DMN content in reactants.


0 20 40 SO 60 80 100

Composition (DMT/(DMT+DMN))

Figure 12. tions (lOO"C, 24 h in water).

Moisture absorption for various composi-

1. Each composition has its own optimum copolymerization condition. Generally, a reaction condition of 4 h a t 185°C for the transesterification followed by 2 mm Hg for 2 h a t 280°C for polycondensation is sufficient to yield good copolymer.

2. The copolymers have higher solubility, less moisture absorption, higher Tgs, and are thermally more stable than PET and may cost less than PEN. Thus, these copolymers would be expected to find various commercial applications.

Financial support of this work by the National Science Council of Republic of China is gratefully appreciated (NSC83-0405-E006-145).

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Received April 26, 1995 Accepted June 7, 1995

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Novel copolyesters containing naphthalene structure. II. Copolyesters prepared from 2,6-dimethyl naphthalate, 1,4-dimethyl terephthalate, and ethylene glycol