Anionic copolymers of caprolactam with laurolactam (nylon 612 copolymers) II. Crystallisation, glass transitions and tensile properties

Eur. Polym. J. Vol. 19, No. 4, pp. 321 325. 1983

0014-3057 83/040321-05503.00/0 Copyright © 1983 Pergamon Press Ltd

Printed in Great Britain. All rights reserved

ANIONIC COPOLYMERS OF CAPROLACTAM WITH L A U R O L A C T A M ( N Y L O N 6/12 COPOLYMERS)--II CRYSTALLISATION, GLASS TRANSITIONS AND TENSILE PROPERTIES I. GOODMAN School of Polymer Science, University of Bradford, Bradford BD7 1DP, U.K. A. H. KEHAYOGLOU Laboratory of Organic Chemical Technology, Aristotelian University of Thessaloniki, Thessaloniki, Greece (Received 7 September 1982)

Abstract The crystalline of PCL/PLL random copolymers, prepared by anionic polymerisation in the melt and freed of residual monomer(s), vary with composition and are at a minimum for approx. 65.,"35 mole°,o composition. The Tg's of the copolymers show a minimum with composition; this effect is discussed in relation to the different co-unit compositions of the crystalline and the amorphous phases. The copolymers are tough thermoplastics with values of yield stress, stress at 100~i~ elongation, and modulus all tending to a minimum for the compositions of lowest crystallinity and highest birefringence. The tensile strengths at break are independent of crystallinity and vary linearly with NH(CH2)sCO unit content.

INTRODUCTION

tions showed that the products were random copolymers, crystalline over the whole range of compositions but with a minimum of crystallinily and melting point at ca. 50/50 mole~,,i (36.5 wt°/o CL units) composition which corresponded with the transition from a-nylon 6 to 7-nylon 12 structure of the crystalline phase. All the copolymers were substantially fully hydrogen-bonded in the solid state.

Few systematic investigations of structure-property relationships have been reported for copolymers resulting from the copolymerisation of lactams. For e-caprolactam/~o-laurolactam (nylon 6/12) copolymers, composed of NH(CH2)sCO (CL) and NH(CH2)11CO (LL) units and prepared by anionic copolymeristion, mechanical properties have been described only for compositions ~<30 mole0/o LL unit content [1, 2]. In both studies, the properties were determined with materials obtained directly from casting polymerisation and not freed of residual monomer(s). Observations on crystalline character and thermal transitions have been published by Russian workers [3,4] for copolymers with a wider range of C L / L L ratios; these too were prepared anionically, but without detailed descriptions of polymerisation procedure or sample history. A single value of tensile strength has been given for a 33.3 mole~,~ LL copolymer prepared by hydrolytic copolymerisation [5]. The purpose of the present investigation was to extend these studies to nylon 6/12 copolymers of defined compositions and histories. The materials were synthesised by anionic copolymerisation in the melt and extracted intensively with hot toluene to remove residual monomers; preparative details have been given [6]. The individual copolymers will be denoted in this paper by the molar ratios of caprolactam:laurolactam in the monomer mixtures used: these ratios were generally close to the actual C L : L L unit contents in the products. Preliminary investiga-

EXPERIMENTAL

Crystallisation and li~sion

The measurements reported here were made using a Perkin Elmer Differential Scanning Calorimeter (DSC-21 operating at heating or cooling rates of 10 C rain ~. calibrated against pure indium, and with the samples in an atmosphere of Nz. Each specimen was taken to 20 C above the melting temperature then cooled to 40C, equilibrated, and reheated to observe the crystallisation and remelting behaviour. Heats of fusion (AH,,, AH~,) for the two melting cycles, and heats of crystallisation (AHc) were obtained from the areas of the appropriate peaks measured by planimeter, using four samples of each copolymer and of the related homopolymers. The mean and extreme values are shown in Table 1. Glass transition temperatures

These were determined by DTA using a du Pont Differential Thermal Analyser Model 900 with glass beads as reference. The measurements were made in N z, first heating each sample to 2 0 C above the crystalline melting point and then quenching in liquid nitrogen to maximise the amorphous content. Heating rates for the To determinations were 10 or 15°C rain 1, and several scans were 321

322

I. GOODMANand A. H. KEHAYOGLOU Table 1. Heats of melting (first cycle, AH,,: remelting, A H ' ) and crystallisation (AHc), and glass transition temperatures (To) Data from DSC (cal g-~) (mean and extreme values) Copolymer (CL : LL ratio) 10/90 20/80 40/60 50/50 60/40 80/20 90/t0 PCL PLL

AH,. 13.6 (13.5-13.7) t2.7 (11.3-13.0) 10.5 (10.3 10.7) 10.0 and 12.8 (9.8-10.3) (12.5 13.1) 7.3 (7.2 7.4) 9.8 (9.4-10.2) 12.4 (11.4-13.5) 16.6 (16.1 1 6 . 9 ) 13.9 (13.6-14.1)

AHc

To (°C)

AH~.

1l.l (9.8 1 2 . 7 ) 9.3 (9.1-9.5) (a) (a) (a) 7.2 (7.2-7.3) 11.0 (10.7 1 1 . 4 ) 14.7 (14.~14.9) 13.2 (12.8-13.6)

11.2 (10.3-12.0) 9.6 (8.9 10.4) 7.2 (7.1-7.3) 5.0 (5.0-5.1)

33 27 22 20

3.8 (3.7-4.0) 7.7 (7.3-7.9) 11.5 (11.1-11.9) 16.2 (15.7-16.7) 13.5 (12.8 13.9)

29 35 36

(a) No crystallisation curve observed in the conditions of scanning.

made with each copolymer. The mean values obtained are given in Table 1.

Tensile mechanical properties Thin sheets of the copolymers were made by compression-moulding at T,, + 20°C and 248 kgf cm-2 followed by cooling in the mould. Dumb-bell shaped tensile-test specimens (central portions approx. 3.6 mm wide x 1.6 mm thick ; gauge length 20.0 mm) were cut from the sheets and conditioned at 23 +_ 2°C and 50+ 5% R.H. for 48 hr prior to testing with an Instron tensometer, Model TM/SM (I 102), maintained in the same conditions and operated at an extension rate of 5 mm min ~. Values of yield stress, modulus and tenacity and elongation at break were determined in the usual way.

RelY'active indices and bireffingence Oriented specimens recovered from the tensometric experiments were examined with a polarising microscope (Becke line method) using standard immersion liquids of known refractive index. The specimens were also examined conoscopically with a ( + ) Nicol prism. A parabolic scheme was observed by inserting a Bertrand's lens (biaxial crystals), with appearance of a yellow line along the inner margin when the microscope compensator was used (positive crystals).

central zone of the composition range (Godovskii et al. [4] give the slowest rate for their copolymers at 50/50), and indeed the 40/60, 50/50 and 60/40 copolymers did not crystallise under the scanning conditions used though partial crystallisation did occur during heating to the remelting temperature. Hybart and coworkers [7, 8] have noted a progressive reduction in crystallisation rates with extents of copolymerisation in nylon 6/11 and nylon 6/6.6 copolymers. The crystallisation exotherms for the CL-rich copolymers 80/20 and 90/10 were each recorded as broad but single peaks; those for 10/90 and 20/80 both showed dual peaks. The observed heats of fusion (AH m) were used to calculate the crystallinities of the copolymers (K) from the relationship K = AHm/AHu where AH, is the heat of fusion of the crystalline phase of each copolymer (PCL or P L L as appropriate, with values of 45.6 and

I o % K from 40 -4 % K from

the first melting the remelting

/

,,

~

a RESULTS

AND

o

DISCUSSION

Crystallisation behaviour and crystallinity Whilst no quantitative measurements of crystallisation rates were made, the following relative order was inferred from the DSC behaviour on dynamic cooling from the melt: 10/90 > 20/80 > 90/10 > 80/20 > 40/60 > 50/50 > 60/40 The slowest apparent rates thus occurred in the

~

20

to

I

I0

I 20

I 30

I 40

I 50

% Caprola~am~

I 60

I 70

I 80

I 90

I

I00

tool

Fig. 1. Copolymer crystallinities from heats of fusion.

Anionic copolymers of caprolactam with laurolactam--II

323

Table 2. Mechanical properties

Copolymer (CL:LL ratio)

Tensile strength at break (kgf/cm2)

Elongation at break (~,)

Yield stress at break (kgf/cm2)

Stress at 100% elongation (kgf/cm2)

Young's modulus (kgf/cmz)

Relative energy to break

10/90 20/80 40/60 60/40 80/20 90/10

513 532 543 563 600* 585}

304 337 358 380 375t 307§

313 313 148 100 163 385

303 289 155 108 174 421

ll,000 8500 4000 2700 4500 14,000

1.22 1.31 1.07 1.00 1.30 1.60

PCL

62211 830¶ 548¶ 400~650tt

--

24,600¶ 1Z75011 11,960¶

--

PLL

2501! 285** 200-300¶ 200-380tt

45911 400-450tt

* - 464 kgf/cm2 [1]. + = 130 [1]. ++= 534 kgf/cm-' [1]. § - 63 [1]. I = Ref. 15. ¶ = Ref. 16. ** = Ref. 17. t+ = Ref. 18.

53.7 cal g 1, respectively). The results for the first and for the remelting cycles are shown in Fig. 1. For both series of measurements, the crystallinity minimum occurs near to 65/35 moleSo composition which corresponds to about equal weight fractions of CL and LL units. Whilst the absolute values of ~ K derived from DSC are lower than those inferred from the densities (Part I), particularly for the LL-rich copolymers, both methods indicate the same minimum in the composition vs crystallinity relationship, in reasonable agreement with earlier conclusions [3, 4]. Glass transitions temperatures

The structural interpretation of observed glass transition temperatures in nylon polymers, and their variation with sample history, conditions and method of measurement have been discussed extensively [9-14]. The factors which give rise to widely different values even for a given homopolyamide include the effects of cold crystallisation, the extent of hydrogen-bonding, plasticisation by moisture, and whether the polymer has odd- or even-numbered polymethylene sequences in the repeating unit. The values obtained by DTA for the present copolymers (Table 2) are in fair agreement with those reported by Godovskii et al. [4] for quenched samples, though lower than those for crystallised samples. Both the present and the previously published results show that the T~'s of nylon 6/12 copolymers fall progressively from the values for the related homopolymers to a minimum at about 50:50 (molar) CL/EL unit composition. The depression of glass transition temperature (AT,) from the value for nylon 6 to that of the copolymer with 30 mole% LL units was approx. 13°C whereas the depressions reported by Kubota and Nowell [1] using DTA, and by Simfinkovfi et al. from tan 6 peak values on dynamic mechanical testing [2], were respectively 36 ° and 47 C. As was noted in Part I, these workers employed non-extracted samples containing from 6 to 9.8% of

residual low molecular weight components which may influence Ty. The occurrence of a minimum in T, with composition is an unusual feature since the values for copolymers generally follow a monotonic pathway between those for the extremes, related to the weight fractions of the components as predicted by the Fox or the G o r d o ~ T a y l o r equations [9]. These findings for nylon 6/12 copolymers can be understood on the basis of the marked changes in the extent and type of crystallinity on traversing the range of compositions. As the volume fraction of reinforcing ordered network is reduced, molecular motion in the disordered phase becomes easier. Moreover, since the crystalline phase present at any CL:LL ratio of composition does not contribute to the glass transition phenomenon, it follows that the amorphous component (in which the relaxation occurs) must have a different chemical composition from that of the whole copolymer. By assuming that each copolymer is composed of discrete crystalline and amorphous phases, and subtracting the proportion of crystalline component (taken as PCL or PLL as appropriate, and using the crystallinity values of Fig. 1 since those in the conditions of DTA measurement were unknown), the compositions of the amorphous phases can be calculated and are plotted in Fig. 2 in relation to the weight percentages of the components in the whole copolymers. Comparison of these relationships with the 77q vs weight~o composition (Fig. 3) leads to the conclusion that the copolymers fall into two separate groups determined by the prevailing crystalline entities present, and with To's falling in each as the crystallinities diminish. It also shows that, in the region of change-over of crystalline type, there is an abrupt change of composition of the amorphous phase from one where the weight fraction of each component in turn is greater than its proportion in the whole copolymer to one where (as a result of partial segregation of the component in crystalline form) the weight

324

I. GOODMANand A. H. KEHAYOGLOU /

I00

(A)

/

/

90

4O

/ /

80

/e

/ /

s

~ 7o

10/90 9o/Io

/ / / .8o/2o

g o

50

.¢ w "~

40

U

2O

0/80/////

10

/,

40.0.4;

30

"60/40

I

I

I

I

I

I

I

20

30

40

50

60

70

80

90

~ 7o

60/40,

/

/ , ' / ///

5oi

/

I

I

I

I

I

60

70

80

90

IOO

Mouldings of the 40/60, 50/50 and 60/40 copolymers were transparent and flexible, whilst the others were opaque and stiffer. The principal tensile properties are given in Table 2; stress-strain curves are shown in Fig. 4. It is evident that the samples of highest CL or LL unit contents showed distinct yield points and drawing behaviour characteristic of crystalline thermoplastic polymers whereas these properties were attenuated in the less crystalline copolymers whose To's approached the temperature of tensile measurement. As a group, the nylon 6/12 copolymers behave as tough materials with high tensile strengths and high elongations to break. As compared with the previously described polymers made by casting polymerisation and containing up to 30 mole% of LL units [1, 2], the present materials have significantly lower values of yield stress, slightly higher tensile strengths, and notably higher extensibilities. The differences probably reflect the effects of different morphologies established in the former cases by crystallisation co-

• 40/60

"50/50

/

I

I

I

L

I

I

I

I

I

IO

20

30

40

50

60

70

80

90

I I00

Wt % LL units in whole copolymer

Fig. 2. Weight percentages of CL units (A) and LL units (B) in the amorphous phase of the copolymers in relation to their weight percentages in the whole copolymers. (Note: The experimental points (t) in this Figure and in Fig. 3 show the real compositions of the whole copolymers as determined from % N-contents).

I |

600 In 500

-

Copolymer ( C L / L L ) + 10/90 zx 20/80 e 40/60 x 60/40 • 80/20

,,

~ - ~'~'-

9o/Io

NE o %.

I 50

Mechanical properties

9o/,o/," / /

I 40

fraction of the component is less than in the whole copolymer.

80120,/////

/

I 30

I00

~o/9o~ / l/ //..20180

80

20

I 20

I

/

(B) 90

40

l I0

Fig. 3. T o vs composition for the copolymers.

I

I00

g 3o

40/60 ~ . 5.~ 0/50

io

Wt % CL unite in whole copolymer

_/ -J

60/40

20/;0~

Wt % CL units in copolymer

10/90 I0

em

k"

0

I

E

J 8 0 / 2 0

20

50/50

%...."'~90/10

oo 30 ~ X , ~

• ,o, / ~

...

ff/

.

s+

400

X'

300 (n 200 I00

~ p-..X.X~

X/ X~

X~ x /

I I

I 2

I 3

Stroin

Fig. 4. Stress strain curves for the copolymers.

" x

Anionic copolymers of caprolactam with laurolactam

1540

o

24 -

+

II

325

ri

N6 ~ 530

22

~og

e ._,...__.-- • ~ e

I1=

•

1520

"t-

20 I 5~0

~E u

18~-

~

1(3

--

'E 1500

NI2/

2018/

x

12

-a o E

8

~"

4

40/60780/20

z

o/go/

10/90/

o I0

i

I

20

30

I

I0

20

l

I

I

40

,'50

60

% Caprolactarn,

J

I

BO

90

o I00

mol

Fig. 6. Refractive indices and birefringence vs composition for the copolymers.

J 40

Fig. 5. Modu]lus vs crystallinity relationship for the homopolymers (N6, N I2) and copolymers.

incident with the polymerisation process and in the present work by crystallisation in moulds from temperatures above the melting points. Considering the whole range of compositions examined, the tensile strengths at break increase almost linearly with CL unit content and are independent of crystallinity, whereas the values of yield stress, stress at 100% elongation and modulus all fall on curves similar to those for T,,, To and % crystallinity, with minima at about 60 mole% CL unit content. A plot of modulus vs crystallinity (K) determined from AH,, data (Fig. 5), is linear. This again emphasises the importance of crystalline order with regard to the properties of the materials.

Refi'active index and birc[binoence The values of refractive indices parallel (t/,) and perpendicular (t/x) to the elasticity axes of oriented specimens are shown in Fig. 6, together with those of birefringence (At/ = t / , - rh). There is a direct relationship of t/, with copolymer composition, corresponding with the vibration of light along the axes of molecular orientation, whereas t/l shows a dependence on the amorphous content leading to a higher birefringence towards the middle of the composition range. The conoscopic examination indicated that the crystalline entities were biaxial positive.

])

001

i

Acknowledgements We are indebted to the Department of Polymer and Fibre Science, University of Manchester Institute of Science and Technology for the provision of facilities for part of this work.

% Crystallinity

FPJ. 19/4

--002

A,7

REFERENCES

1. H. Kubota and J. B. Nowell, J, appL Polym. Sci. 19, 1521 (1975). 2. E. Sim0nkovfi. J. Zelinger, V. Kubanek and J. Krfili6ek, J. appl. Polym. Sci. 21, 65 (1977). 3. T. M. Frunze, A. Sh. Cherdabayev, R. B. Shleifman, V. V. Kurashev and D. Ya. Tsvankin, Polym. Sci., U.S.S.R. 18, 793 (1976); Vysokomolek. Soedin. AI8, 696 (1976). 4. Yu. K. Godovskii, I. I. Dubovik, S. L. Ivanova, V. V. Kurashev. T. Frunze and G. L. Slonimskii, Polvm. Sci., U.S.S.R. 19, 453 (1977); Vysokomolek. Soedin. AI9, 392 (1977). 5. J. G. Dolden, Polymer 17, 875 (1976). 6. A. H. Kehayoglou, Eur. Polym. J. 19, 183 (1983). 7. F. J. Hybart and B. Pepper, J. appL Polym. Sci. 13, 2643 (1969). 8. E. D. Harvey and F. J. Hybart, J. appl. Polym. Sci. 14, 2133 (1970). 9. D. C. Prevorsek, R. H. Butler and H. K. Reimschuessel, J. Polym. Sci. A-2, 9, 867 (1971). 10. G. A. Gordon, J. Polym. Sci. A-2, 9, 1693 (1971). 11. G. Goldbach, Angew. Makromolek. Chem. 32, 37 (1973). 12. F. W. Lord, Polymer 15, 42 (1974). 13. R. Greco, L. Nicodemo and L. Nicolais, Macromolecules 9, 686 (1976). 14. K.-H. lllers, Polymer 18, 551 (1977). 15. D. W. van Krevelen, Properties of Polymers, p. 181 Elsevier Publishing Co., Amsterdam (1972). 16. E. C. Schule Encyclopedia q[Polymer Science and Technology, (Edited by H. Mark, N. Gaylord and N. Bikales),, Vol. 10, p. 468. lnterscience Publishers, New York (1969). 17. W. Sweeny and J. Zimmerman, ibid 10, 562. 18. M. I. Kohan (Ed.), Nylon Plastics, p. 481. WileyInterscience, New York (1973).