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Sequence determination of acrylonitrile-(ethyl acrylate) copolymers by 13C-NMR spectroscopy and correlation with glass transition temperature
Eur. Polym. J. Vol. 28, No. 7, pp. 803-808, 1992 Printed in Great Britain. All rights reserved
0014-3057/92 $5.00 + 0.00 Copyright © 1992 Pergamon Press Ltd
SEQUENCE DETERMINATION OF ACRYLONITRILE-(ETHYL ACRYLATE) COPOLYMERS BY 13C-NMR SPECTROSCOPY AND CORRELATION WITH GLASS TRANSITION TEMPERATURE A. S. BRAR* and SUNITA Department of Chemistry, Indian Institute of Technology, Hauz Khas, New Delhi 110016, India (Received 22 November 1991)
Abstraet--Acrylonitrile-(ethyl acrylate) copolymers of various compositions were prepared by bulk polymerization using benzoyl peroxide as initiator. Copolymer compositions were determined by elemental analyses; monomer reactivity ratios were determined using a nonlinear least square errors in variables model which gave more reliable value of r~ and r 2. Terminal and penultimate reactivity ratios have been calculated using the observed monomer triad sequence distribution determined from ~3C{tH}NMR spectra. The triad sequence distribution was used to calculate diad concentrations, conditional probability parameters and number--average sequence lengths in the copolymers. The observed triad sequence concentrations determined from 13C{IH}-NMR spectra agreed well with those calculated from reactivity ratios. Glass transition temperatures of various copolymers determined from DSC agreed well with those obtained from diad concentrations.
INTRODUCTION
~3C-NMR spectroscopy has become an important technique for the investigation of the structures of copolymers. The significant effects on chemical shifts of c o m o n o m e r sequence arrangement have enabled important information on these parameters to be obtained [1]. Several techniques are used for the estimation of reactivity ratios. The use of nonlinear techniques is preferred over the traditional linear graphical methods [2, 3]. The nonlinear techniques require the derivatives of copolymer composition equation with respect to the parameters. The use of EVM program for estimating reactivity ratios and the derivation of the pertinent equations for its use are well documented [4, 5]. It is well known that the sequence length distribution of comonomers has a large influence on the chemical and physical properties of copolymers and may influence the practical uses of the copolymers. The effect of copolymer composition and m o n o m e r sequence distribution on the glass transition temperature (Tg) has been investigated for some copolymers [6-9]. The relationships between sequence distribution and Tg for several copolymers have been reported [10, 11]. In our earlier work, we reported the microstructure of acrylonitrile-(methyl methacrylate) [12], acryionitrile-(ethyl methacrylate) [13], acrylonitrile-(butyl methacrylate) [14] and acrylonitrile-(vinyl acetate) copolymers [15] by ~3C-NMR spectroscopy. Perusal of the literature showed that there has been no publication on sequence determination for acrylonitrile-(ethyl acrylate) (A/E) copolymers. We now report the reactivity ratios for the A / E system using EVM through the use of a computer program written by O'Driscoll et al. [16]. The primary quantities such as m o n o m e r corn-
position, diad and triad sequence distribution, conditional probabilities and n u m b e r - - a v e r a g e sequence lengths of A / E copolymers have been determined on the basis of ~3C{IH}-NMR analyses and compared with those calculated from reactivity ratios as determined from the EVM program. The diad sequence distribution determined from terminal model reactivity ratios using Harwood's program [17] has been related with Tg of A / E copolymers. EXPERIMENTAL PROCEDURES
Monomers were vacuum distilled and stored below 5°C. A monomer mixture was taken in a flask and 0.5 (w/v) % benzoyl peroxide was added. A slow stream of purified N 2 was purged in and the polymerization was allowed to proceed at 70°C. The reaction was stopped at 5-10% conversion by precipitating the copolymers in excess methanol after 40 min. Elemental analyses The C, H and N analyses was done on Perkin-Elmer 240 C Elemental Analyser. From the nitrogen content, the copolymer composition was calculated. N M R analyses IH-NMR and J3C{IH}-NMR spectra were recorded on a Bruker WH 270 NMR Spectrometer operating at 270 and 67.89MHz respectively. The conditions of operation were:--temperature of probe, 25°C; reference, centre peak of CDCI 3 assigned as 77.0 ppm; spectral width, 13,500 Hz; pulse repetition time, 5 sec. The details of recording the spectra and Lorentzian shape curve fitting have been described elsewhere [12-14]. The copolymer samples were examined as 5 wt% solutions for ~H-NMR and 15-20 wt% for ~3C{~H}-NMR spectra. The Z2 for various computer fitted resonance signals was always taken as less than unity. Molecular weight determination The molecular weights of copolymers were determined by viscometry, using an Ubbelohde viscometer, at 25 + 0.1°C in dimethylformamide as solvent.
*To whom all correspondence should be addressed. 803
804
A.S. BRAgand SUNITA
Glass transition temperature (Tg) determination Tgs of A/E copolymers were determined on Perkin-Elmer Differential Scanning Calorimeter 7 series under following conditions: atmosphere, N2; heating rate, 5°C/min; temperature range, 20-120°C.
1.0
O
0.8
0.6
RESULTS AND DISCUSSION Reactivity ratios determination Table 1 shows the composition of feeds, copolymers and the nitrogen contents of the copolymers. The measurements of molecular weights of A / E copolymers from intrinsic viscosity data showed that these copolymers have molecular weights of the order of 105; the molecular weights are all approximate since they have been calculated using K and a values which apply only to polyacrylonitrile and not to the copolymers. The variation of copolymer composition (FA) with composition of feed (fA.) is shown in Fig. 1. The copolymer composition data were used to calculate the terminal model reactivity ratios using a nonlinear least square errors in variables method (EVM). The EVM program requires knowledge of the measurement errors for both dependent and independent variables. It was estimated that the error in the composition of copolymer and feed was 1%. The initial estimate of reactivity ratios was made by the method of Kelen-Tfid6s [2], giving r A = 1.15 and rE = 1.16. These values are then fed in the computer program to calculate the exact values. The values obtained from the EVM program are rA = 0.92 + 0.08 and r E = 1.20 ___0.07. The 95% posterior probability contour (PPC) for A/E system is shown in Fig. 2. The values of r A, r E and r A • r E are ca 1.0, indicating that nature of the chain-end, whether A or E, has little influence on the relative rates of m o n o m e r additions. Such a copolymerization is termed as ideal. The reported values [18] of reactivity ratios are rA= 1.12, rE=0.93, for copolymers prepared by suspension polymerization; in the present case, copolymers were prepared by bulk polymerization so that the reactivity ratios appear to depend on the method of polymerization. I H - N M R studies The various resonance signals in ~H-NMR spectra of A / E copolymers have been assigned by comparing the spectra with those of homopolymers. In the case of polyacrylonitrile (PAN), the methylene (---CH2) proton signal appeared around 6 2.10ppm and methine ( - - C H ) proton signal appeared around 3 3.40 ppm. In the case of poly(ethyl acrylate) (PEA), methyl (--CH3) protons appeared around 6 1.08-1.48 ppm. - - C H 2, - - C H and - - O C H 2 proton Table 1. Copolymer composition for A/E copolymers Sample Percent Percent No. fA .]E Nit. FA FE Conv. 1 0.30 0.70 4.53 0.28 0.72 5.4 2 0.40 0.60 6.79 0.39 0.61 8.8 3 0.50 0.50 6.85 0.41 0.59 6.9 4 0.60 0.40 9.66 0.52 0.48 7.0 5 0.70 0.30 15.58 0.73 0.27 7.8 6 0.80 0.20 19.84 0.85 0.15 8.0 fs are the mole fractions of monomers in feed. Fs are the mole fraction of monomers in copolymers.
--
FA
o
o
0.4
0.2 I 0
0.2
0.4
0.6
0.8
1.0
fA Fig. I. The variation of copolymer composition (FA) as a function of monomer composition in feed (fA) for A/E copolymerization. signals appeared around 6 1.50ol.84, 2.12-2.60 and 3.80-4.40 ppm respectively. In the case of A / E copolymer, (----CH3)E and (----OCH 2)E protons appeared around 6 1.11-1.48 and 3.70--4.44 ppm respectively. The two broad signals (6 1.5002.28ppm and 6 2.3003.50 ppm) can be attributed to - - C H 2 and ---CH protons of both m o n o m e r units. The ~H-NMR spectrum of A / E copolymer is very complex and various signals from different monomefic units overlap. Therefore I H - N M R spectra could not be used for determination of the compositions of copolymers. 13C-NMR studies The 13C{lH}-NMR spectrum of A / E copolymer (A = 39.0 mol%), recorded in a mixture of CDCI3 and DMSO-d6 at r o o m temperature, is shown in Fig. 3. The various resonance signals have been assigned by comparison with homopolymers. In the case of PAN, - - C H 2 and ---CH carbons, appeared around 6 33.4 ppm and 27.8-28.1 ppm respectively. The nitrile (---CN) carbon in P A N appeared as a muitiplet in the region 6 118.2-120.8 ppm, indicating that the - - C N carbon of P A N is sensitive to tacticity. Kamide et al. [19] have evaluated the pentad tacticity 1.30
-
1.28
-
1.26
-
/
/
1.24 1.22
7E 1.20 1.18 1.16 1.14 1.12 0.84
]
I 0.88
I
L 0.92
I
I 0.96
I
I 1.00
I
"~A Fig. 2. 95% Posterior probability contour (PPC) for A/E comonomer pair. The + point represents the best estimate o f r A and r E.
Sequence determination of A/E copolymers
n
'
4
805
~-c'2-°'h /
l/-'c"2'E
,,'8 ,I,
,,08 (pprn)
H 0Mso- 6
( X-=O)F_ -(CN) A
--
I 220
I 200
I 180
I 160
I lt..O
I 120
100
I 80
I 60
[ ~0
I 20
I 0
S (ppm)
Fig. 3. ~3C{IH}-NMR spectrum ofA/E copo]ymer (A = 39.0 tool%) along with expanded carbony] carbon (I_-C--O) and nitrile carbon ( ~ N ) in mixture of CDCI 3 and DMSO-d 6 at room temperature.
of R-PAN using high-field NMR. In the case of PEA, --CH3, ---CH 2 and - - C H carbon signals appeared around 6 14.59, 35.0-36.0 and 41.3 ppm, respectively. The resonance signals around 6 60.6 and 173.7 ppm can be assigned respectively to --OCH2 and ~ C-----O carbons in PEA. In A/E copolymer, the signals around 6 13.2 ppm, 26.4-27.5 ppm, 33.1-34.2 ppm and 59.0~0.3ppm can be assigned to (--CH3)E, (--CH)A, (--'CH2)A and (--OCH;)E carbons respectively; they could not be used for the sequence analysis because of poor resolution. The (--CH2) E and (--CH)E carbons of E unit overlapped with solvent DMSO-d6 signals (6 37.5-39.8 ppm), therefore could not be used for analysis of E-centred sequences. The carbonyl carbon in A/E copolymer appeared as a multiplet around 3 171.0-173.7ppm and indicates that the splitting of > C----O signal is due to sensitivity to the compositional sequences. The nitrile carbon of A unit appeared as a well resolved multiplet from 3 118.5 to 120.5 ppm, showing its sensitivity towards different monomer placements. In the case of A/E copolymer, there is shift in the position of various functional groups of A and E units as compared with homopolymers; this effect is due to the change in the nature of adjacent monomeric units in the copolymer which alters the chemical shifts of A and E centred triads. The carbonyl carbon (>C----O) and nitrile carbon (--CN) expansions of A/E copolymer (A = 39.0 mol%) are shown in Fig. 3. PEA shows a singlet centred around 6 173.7 ppm. As the concentration of A in the copolymer increases, the signals characteristics of homopolymer PEA decrease, whereas a set of signals centred around ! 72.2 ppm is appearing. These signals, with further increase in the A content, increase to a maximum and then decrease, whereas a third new set of signals appears around 6 171.7 ppm. The three sets of signals, with intensities changing with copolymer composition, can be assigned to the carbonyl carbon of the central E unit in EEE, AEE (EEA) and AEA triad sequences from low- to high-
field. The assignment of signals was made on the basis of electronic interaction. In the case of E-centred triads, addition of an A unit to the EEE triad causes upfield shift in the position of EEA (by ~0.50 ppm) and AEA (by ~0.50 ppm). This effect is due to the increase in electron density at the carbonyl carbon of the central E unit which can be due to the diamagnetic shielding from the anisotropy of immediate - - C N neighbours. Concentrations of various Ecentred triads can be calculated from the relative areas of resonance signals. These triad concentrations are the normalized areas of respective resonance signals. The relaxation times for the carbon functionalities in different comonomeric units have been found to be the same within the experimental errors of T~ measurements. Spin-lattice relaxation (TI) measurements for acrylonitrile-(methyl methacrylate) copolymers has been reported by Gerken et al. [20]. The quantitative calculations were done from the areas of respective signals in 13C{~H}-NMR of A/E copolymers. Assuming the Alfrey-Mayo (A-M) model (=first order Markov terminal model) [21] to be valid at any moment of the reaction for these low conversion copolymers, the number fraction (F) of acrylonitrile (A) and ethyl acrylate (E) centred triads can be predicted. The relation between intramolecular structure (number fraction of triads) and reaction kinetics is given by the following equations: FEE E = [l - - P(A/E)] 2 FEEA = FAEE =
2[P(A/E)( 1 - -
(la) P(A/E))]
(lb)
FAEA = [P(A/E)]2
(1 C)
FAA A = [1 - - P(E,A)] 2
(ld)
FAA E = FEA A =
2[P(E/A)(I -- P~E/A))]
FEAE = [PIE A)]2
(le)
(If)
806
A.S. BRARand SUNITA Table 2. Calculated and observed fractions o f A- and E-centred triads in A/E copolymers Triad concentrations*
Obs.
Calculated A - M model
Calculated from Harwood Pen. model
0.12 0.42 0.46 0.51 0.38 0. l I
0.08 0.41 0.51 0.54 0.39 0.07
0.10 0.41 0.49 0.57 0.36 0.07
rAA = rsA = rEE = rAE =
1.33 1.06 1.17 0.74
AAA AAE EAE EEE EEA AEA
0.22 0.45 0.33 0.42 0.44 0.14
0.15 0.47 0.38 0.41 0.46 0.13
0.18 0.47 0.35 0.44 0.43 0.13
rAA = rEA = rEE = rAE =
1.46 1.02 1.27 1.05
0.52
AAA AAE EAE EEE EEA AEA
0.34 0.50 0.16 0.20 0.51 0.29
0.33 0.49 0.18 0.20 0.49 0.31
0.38 0.46 0.16 0.21 0.46 0.33
rAA = rEA = rEE = rAE =
0.91 1.04 1.18 1.32
4
0.73
AAA AAE EAE EEE EEA AEA
0.51 0.40 0.09 0.08 0.45 0.47
0.47 0.43 0.10 0.11 0.45 0.44
0.52 0.40 0.08 0.12 0.42 0.46
rAA = rEA = rrE = rAE =
1.09 0.95 0.83 1.12
5
0.85
AAA AAE EAE EEE EEA AEA
0.59 0.36 0.05 0.06 0.33 0.61
0.62 0.34 0.04 0.05 0.36 0.59
0.66 0.30 0.04 0.05 0.33 0.62
rAA = rEA = tEE = rAE =
0.82 0.90 1.45 1.08
Sample No.
A Mole fraction (in topoi.)
Triads
1
0.28
AAA AAE EAE EEE EEA AEA
2
0.39
3
Penultimate reactivity ratios
*Standard deviation = +0.04.
where P A/E =
PE/A= l + r A ' q and q = [A]/[E] is the feed ratio. F represents the number fraction of triads normalized to unity. Table 2 contains the compositional information concerning the various E centred triads along with the calculated values obtained from the A - M model using the terminal model reactivity ratios (r A ~- 0.92 + 0.08 and rE----1.20+ 0.07). Penultimate reactivity ratios (rE and rAE) were evaluated (Table 2) from experimentally obtained triad distributions from carbonyl carbon resonance of ~3C{IH}-NMR spectra using the following equations: rnE
2[EEE] [EEA + A E E ] [Af]/[Ef]
(2a)
rAE =
[AEE + EEA] [Ar]/[Ef] 2[AEA]
(2b)
where [Af] and [Er] are the feed composition in mole fractions of A and E monomers. The values of rEE and rAE are listed in Table 2. Penultimate reactivity ratios change with change in copolymer composition. Similar compositional information on A-centred triads can be made using the - - C N carbon resonance region. A multiplet around 6 118.5-120.5 ppm shows
splitting into three envelopes. The chemical shift difference within the resonance signals and splitting pattern shows its sensitivity on monomer sequences and cotacticity in the nitrile resonance. Increase in concentration of A units in the copolymer increases the most upfield signal of the nitrile group centred around 6 l l 8 . 8 p p m , while the downfield signal around 6 119.8 ppm decreases. Because of this compositional variation in the intensities of A centred triads, the most upfield resonance signal is assigned to the A A A triad and the most downfield to the EAE triad. The introduction of ethyl acrylate to A A A centred triad causes a downfield shift in the position of AAE (by ~,0.40 ppm) and EAE (by ~0.60 ppm) due to the deshielding effect of the carbonyl group in ethyl acrylate. A-centred triads along with those calculated using the A - M model are given in Table 2. On the basis of the triad resonance assignments as described earlier, penultimate reactivity ratios were evaluated for A/E copolymer from the triad fractions determined from nitrile resonance of t3C{JH}-NMR spectra using the following equations:
2[AAA]
rAA = [AAE + EAA] [Ef]/[Af]
(2c)
[AAE + EAA] [Ef]/[Af]. 2[EAE]
(2d)
rEA =
The values of rAA and rEA are listed for individual copolymers in Table 2. Using the average values of
Sequence determination of A/E copolymers
807
Table 3. Copolymerization parameters of A/E copolymers determined by ~3C-NMR spectroscopy Sample No.
A mole fraction (in copol.)
q
PE/A
PA/E
]~A
NE
rA
rE
I 2 3 4 5
0.28 0.39 0.52 0.73 0.85
0.92 0.92 0.91 0.92 0.94
0.67 0.55 0.41 0.29 0.23
0.30 0.36 0.54 0.69 0.77
1.49 1.82 2.44 3.45 4.35
3.33 2.78 1.85 1.45 1.30
1.15 1.20 0.96 1.05 0.84
1.00 1.18 1.25 1.02 1.16
penultimate reactivity ratios, the triad sequence distribution was obtained by Harwood's program [17] and is given in Table 2 the uncertainty in the values of triad compositional sequences obtained from N M R spectra and reactivity ratios is ___0.04. In earlier publications, the sequence and cotacticity have been reported for acrylonitrile-(methyl methacrylate) [12], acrylonitrile--(ethyl methacrylate) [13], acrylonitrile--(butyl methacrylate)[14] copolymers. In the present case of A/E copolymer, the cotacticity could not be observed in the carbonyl carbon and nitrile carbon signals even at 67.89 MHz frequency. The replacement of the ~t-methyi group in alkyl methacrylate by a hydrogen atom causes drastic change in the 13C-NMR spectrum with respect to the signals of carbonyl and nitrile resonance, which are cotacticity sensitive in alkyl methacrylate copolymers. From the results of composition with respect to various A and E centred triads, the conditional p r o b a b i l i t i e s PE/A and PAlE have been calculated using the equations: [EAE] + [EAA]/2 PE/A = (3a) [AAA] + [AAE] + [EAE] [AEA] + [EEA]/2 (3b) [EEE] + [EEA] + [AEA] where PE/A is the probability that A-E unit comes as a result of an A growing chain end adding E and PAlE is the probability that the E-A unit comes about as a result of an E growing chain-end adding A. The value of PE/Adecreases linearly from 0.67 to 0.23 with decrease in the content of ethyl acrylate in the copolymer while the value of PA/E increases linearly from 0.30 to 0.77 with increase in concentration of acrylonitrile in the copolymer. Using these equations, the terminal model reactivity ratios (r A and rE) have been calculated for individual copolymers using the following equations: PA/E =
rA =
__[El] [Af]"
[I/PE/A --
l]
[Af] [1/PA/E- 1].
rE "~" [--~f] •
(4a)
The block character, (r/) is a measure of the departure from random character [22]. It is given by the equation: [A-E] (5) r/ 2[A] [E] where [A-E] is the mole fraction of A-E diads and [A] and [E] are the mole fractions of A and E units in the copolymer. 0<,7 < 1 reflects more block character and 1 < q < 2 means more alternating tendency of the copolymer than expected from the random distribution. The value of ~/for A/E copolymers lie in the range 0.91-0.94 (0 < ,7 < 1), showing that the A/E copolymer has more tendency towards block character. Table 3 contains the copolymerization parameters of A/E for various monomer feed ratios.
Glass transition temperature (TQ measurements The dependence of Tg on copolymer composition cannot be expressed by simple additive relations such as the Fox equation [10]. The curvatures observed in the dependence of T~ of random copolymers on composition have been explained by Barton [11]. He described the effect of sequence distribution of monomeric units in linear copolymers on Tg and considered the contribution of diads. Suzuki et aL [23] modified the Barton equation and applied it to some published data obtaining good agreement. Barton [11] expressed Tg on the basis of the following equation: Tg = nAA Tg,AA + nEE Tg, EE q" nAE Tg, AE
160
u.I LU
o)
(4b)
The reactivity ratios calculated' from the EVM program are r a = 0.92 _+0.08, r E = 1.20 _+0.07 and from triad composition are r A = 1.04, r E = 1.12, these values are in reasonable agreement. The numberaverage sequence lengths (NA and NE), the reciprocal of conditional probabilities, is also obtained. The value of "~Aincreases linearly from 1.49 to 4.35 as the content of acrylonitrile in the copolymer increases while the value of ~E decreases linearly from 3.33 to 1.30 with decrease in the content of ethyl acrylate in the copolymer.
(6)
¢J I
~ bo9
140
12o 100
so 60
I
~ I-.
40 2O
0
I 0.1
I 0.2
I 0.3
I 0.4
I 0.5
I 0.6
I 0.7
(nAE) * (nEA) Fig. 4. The plot o f ( T , - r/AATg.AA -- nEE T,.EE ) VS (nAE -q- nEA).
A. S. BRARand SUNIT,~
808
Table 4. Glass transition temperature (TB) and diad composition data for A/E copolymers Sample No.
A mole fraction (in copol.)
Diads fraction
Tg (K) Calc. from Barton eqn
Tg (K) from DSC (Expt.)
1
0.28
[AAI = 0.08
278.7
--
289.8
--
314.9
315
328.5
330
343.5
346
2
0.39
3
0.52
4
0.73
6
0.85
IEEI= 0.54 [AE] = 0.38 [AA] = 0.14 IEEI = 0.41 [AEI = 0.45 [AA] = 0.33 lEE] = 0.19 [AE] = 0.48 IAAI= 0.46 [EEl = 0. I 1 [AE] = 0.43 [AA]= 0.62 lEE] = 0.05 [AE] = 0.33
Acknowledgements--The authors thank IIT Delhi for providing research facilities and Professor C. L. Khetrapal, l.I.Sc. Bangalore, for recording NMR spectra. They also thank Professor K. F. O'Driscoll for providing the EVM computer program. REFERENCES
TS,AE= 301 K.
where Tg is the glass transition temperature of the copolymer and Tg,AA and Ts, EE refer to polyacrylonitrile and poly(ethyl acrylate) respectively. Tg.AE represents Tg of alternating copolymers. The plot of (Tg -- nAA Tg, AA -- nEE Tg, EE) VS (nAE d- nEA ) is linear passing through the origin (Fig. 4). The value of Tg,AE obtained from the slope is 301 K. Experimental values are in good agreement with those calculated from diad concentrations. Table 4 contains the diad composition (obtained from the terminal model H a r w o o d program) and the Tg values of various A / E copolymers. In order to use any sequence distribution-T~ relationship, it is necessary to have the Tg,AE value, The value of Ta,AE for A/E copolymer is 301 K; using this value, the sequence-Tg relation can be established. The value of Tg can be predicted for any sequence of this copolymer. The diad model has been called the terminal model in the kinetics of copolymerization. Only the terminal unit of a growing polymer chain is regarded as affecting the probability of m o n o m e r addition. Han [24] extended the diad model of Barton for copolymer TgS to the triad model:
T, = nAAATs,AAA + nEEETg,EEE +nAAE Ts,AAE + nEAA Tg,EAA +nA~E Tg,AEE + nEE/,Tg,EEA q-nAEATs, AEAq- nEAETg,EAE
assigned to Tg,AAE and Tg,AE. However the grouping and equality seem to be unreasonable. Urematsu and Honda [25] tried to reduce eight different triad sequences to three groups and relating them to TsAA, TgEE and TgAE. Tg predicted for a random copolymer is formally written as equation (7) in the penultimate model. It is difficult to trace the Tg of eight different triads of this copolymer. We have used diad sequence-Tg relation for this polymer and obtained good agreement between predicted and experimental values.
(7)
here nx and Tg,x are respectively the mole fractions and Ts related to the sequence specified by the suffix X. H a m reduced the eight triad sequence to the four sequences, viz. two homopolymer Tg values, Tg,AAE (which is assumed to be equal to Tg,EAA) and the fourth coded as Tg,AE (assuming equality between Tg, AEA, Tg, EAE , Tg, AEE to Tg, EEA). Applying this extended equation to some published data on (methyl methacrylate)-acrylonitrile copolymers, he obtained improved agreement, when particular values were
1. P. F. Barton, D. J. T. Hill, J. H. O'Donnell and P. W. O'Sullivan. Macromolecules 16, 1967 (1984). 2. T. Kelen and F. Tiid6s. J. macromolec. Sci. Chem. Ag, 1 (1975). 3. M. Finemann and S. D. Ross. J. Polym. Sci. 5, 259 (1950). 4. H. Patino-Leal, P. M. Reilly and K. F. O'Driscoll. J. Polym. Sci.; Polym. Left. 18, 219 (1980). 5. K. K. Chee and S. C. Ng. Macromolecules 19, 2779 (1986). 6. M. Sanchez-Chaves, F. Arranz and M. Montes. Polymer 29, 2244 (1988). 7. F. Arranz, M. Sanchez-Chaves and A. Molinero. Macromolec. Chem. 185, 2153 (1984). 8. S. H. Harris and R. D. Gilbert. J. Polym. Sci.: Polym. Chem. Edn 20, 1653 (1982). 9. H. A. Schneider and H. N. Neto. Polym. Bull. 9, 457 (1983). 10. T. G. Fox. Bull. Am. Phys. Soc. 1, 123 (1956). 11. J. M. Barton. J. Polym. Sci. C 30, 573 (1970). 12. G. S. Kapur and A. S. Brar. Polymer 32, 1112 (1991). 13. G. S. Kapur and A. S. Brar. J. Polym. Sci. A: Polym. Chem. 29, 479 (1991). 14. G. S. Kapur and A. S. Brar. Makromolek. Chem. 192, 2733 (1991). 15. A. S. Brar and Sunita. Polymer Science, Contemporary Themes, Vol. II, Symposium Proceeding of Polymers" 91, Pune, p. 582 (1991). 16. M. Dube, R. Amin Sanayel, A. Penlidis, K. F. O'Driscoll and P. M. Reilly. J. Polym. Sci.: Polym. Chem. 29, 703 (1991). 17. H. J. Harwood. J. Polym. Sci. C. 25, 37 (1968). 18. W. M. Ritchey and L. E. Ball. Polym. Lett. 4, 557 (1966). 19. K. Kamide, H. Yamazaki, K. Okajima and K. Hikichi. Polym. J. 17, 1233 (1985). 20. T. A. Gerken and W. M. Ritchey. J. appl. Polym. Sci.: Appl. polym. Symp. 34, 17 (1978). 21. J. L. Koenig. Chemical Microstructure of Polymer Chain, Chap. 3, p. 39. Wiley Interscience, New York (1980). 22. K. Ito and Y. Yamashita. J. Polym. Sci. A. 3, 2153 (•965). 23. H. Suzuki and V. B. F. Mathot. Macromolecules 22, 1380 (1989). 24. G. E. Ham. J. macromolec. Sci. Chem. A 9, 461 (1975). 25. L. Uematsu and K. Honda. Rep. Prog. Polym. Phys. J. 8, 111 (1965).
0014-3057/92 $5.00 + 0.00 Copyright © 1992 Pergamon Press Ltd
SEQUENCE DETERMINATION OF ACRYLONITRILE-(ETHYL ACRYLATE) COPOLYMERS BY 13C-NMR SPECTROSCOPY AND CORRELATION WITH GLASS TRANSITION TEMPERATURE A. S. BRAR* and SUNITA Department of Chemistry, Indian Institute of Technology, Hauz Khas, New Delhi 110016, India (Received 22 November 1991)
Abstraet--Acrylonitrile-(ethyl acrylate) copolymers of various compositions were prepared by bulk polymerization using benzoyl peroxide as initiator. Copolymer compositions were determined by elemental analyses; monomer reactivity ratios were determined using a nonlinear least square errors in variables model which gave more reliable value of r~ and r 2. Terminal and penultimate reactivity ratios have been calculated using the observed monomer triad sequence distribution determined from ~3C{tH}NMR spectra. The triad sequence distribution was used to calculate diad concentrations, conditional probability parameters and number--average sequence lengths in the copolymers. The observed triad sequence concentrations determined from 13C{IH}-NMR spectra agreed well with those calculated from reactivity ratios. Glass transition temperatures of various copolymers determined from DSC agreed well with those obtained from diad concentrations.
INTRODUCTION
~3C-NMR spectroscopy has become an important technique for the investigation of the structures of copolymers. The significant effects on chemical shifts of c o m o n o m e r sequence arrangement have enabled important information on these parameters to be obtained [1]. Several techniques are used for the estimation of reactivity ratios. The use of nonlinear techniques is preferred over the traditional linear graphical methods [2, 3]. The nonlinear techniques require the derivatives of copolymer composition equation with respect to the parameters. The use of EVM program for estimating reactivity ratios and the derivation of the pertinent equations for its use are well documented [4, 5]. It is well known that the sequence length distribution of comonomers has a large influence on the chemical and physical properties of copolymers and may influence the practical uses of the copolymers. The effect of copolymer composition and m o n o m e r sequence distribution on the glass transition temperature (Tg) has been investigated for some copolymers [6-9]. The relationships between sequence distribution and Tg for several copolymers have been reported [10, 11]. In our earlier work, we reported the microstructure of acrylonitrile-(methyl methacrylate) [12], acryionitrile-(ethyl methacrylate) [13], acrylonitrile-(butyl methacrylate) [14] and acrylonitrile-(vinyl acetate) copolymers [15] by ~3C-NMR spectroscopy. Perusal of the literature showed that there has been no publication on sequence determination for acrylonitrile-(ethyl acrylate) (A/E) copolymers. We now report the reactivity ratios for the A / E system using EVM through the use of a computer program written by O'Driscoll et al. [16]. The primary quantities such as m o n o m e r corn-
position, diad and triad sequence distribution, conditional probabilities and n u m b e r - - a v e r a g e sequence lengths of A / E copolymers have been determined on the basis of ~3C{IH}-NMR analyses and compared with those calculated from reactivity ratios as determined from the EVM program. The diad sequence distribution determined from terminal model reactivity ratios using Harwood's program [17] has been related with Tg of A / E copolymers. EXPERIMENTAL PROCEDURES
Monomers were vacuum distilled and stored below 5°C. A monomer mixture was taken in a flask and 0.5 (w/v) % benzoyl peroxide was added. A slow stream of purified N 2 was purged in and the polymerization was allowed to proceed at 70°C. The reaction was stopped at 5-10% conversion by precipitating the copolymers in excess methanol after 40 min. Elemental analyses The C, H and N analyses was done on Perkin-Elmer 240 C Elemental Analyser. From the nitrogen content, the copolymer composition was calculated. N M R analyses IH-NMR and J3C{IH}-NMR spectra were recorded on a Bruker WH 270 NMR Spectrometer operating at 270 and 67.89MHz respectively. The conditions of operation were:--temperature of probe, 25°C; reference, centre peak of CDCI 3 assigned as 77.0 ppm; spectral width, 13,500 Hz; pulse repetition time, 5 sec. The details of recording the spectra and Lorentzian shape curve fitting have been described elsewhere [12-14]. The copolymer samples were examined as 5 wt% solutions for ~H-NMR and 15-20 wt% for ~3C{~H}-NMR spectra. The Z2 for various computer fitted resonance signals was always taken as less than unity. Molecular weight determination The molecular weights of copolymers were determined by viscometry, using an Ubbelohde viscometer, at 25 + 0.1°C in dimethylformamide as solvent.
*To whom all correspondence should be addressed. 803
804
A.S. BRAgand SUNITA
Glass transition temperature (Tg) determination Tgs of A/E copolymers were determined on Perkin-Elmer Differential Scanning Calorimeter 7 series under following conditions: atmosphere, N2; heating rate, 5°C/min; temperature range, 20-120°C.
1.0
O
0.8
0.6
RESULTS AND DISCUSSION Reactivity ratios determination Table 1 shows the composition of feeds, copolymers and the nitrogen contents of the copolymers. The measurements of molecular weights of A / E copolymers from intrinsic viscosity data showed that these copolymers have molecular weights of the order of 105; the molecular weights are all approximate since they have been calculated using K and a values which apply only to polyacrylonitrile and not to the copolymers. The variation of copolymer composition (FA) with composition of feed (fA.) is shown in Fig. 1. The copolymer composition data were used to calculate the terminal model reactivity ratios using a nonlinear least square errors in variables method (EVM). The EVM program requires knowledge of the measurement errors for both dependent and independent variables. It was estimated that the error in the composition of copolymer and feed was 1%. The initial estimate of reactivity ratios was made by the method of Kelen-Tfid6s [2], giving r A = 1.15 and rE = 1.16. These values are then fed in the computer program to calculate the exact values. The values obtained from the EVM program are rA = 0.92 + 0.08 and r E = 1.20 ___0.07. The 95% posterior probability contour (PPC) for A/E system is shown in Fig. 2. The values of r A, r E and r A • r E are ca 1.0, indicating that nature of the chain-end, whether A or E, has little influence on the relative rates of m o n o m e r additions. Such a copolymerization is termed as ideal. The reported values [18] of reactivity ratios are rA= 1.12, rE=0.93, for copolymers prepared by suspension polymerization; in the present case, copolymers were prepared by bulk polymerization so that the reactivity ratios appear to depend on the method of polymerization. I H - N M R studies The various resonance signals in ~H-NMR spectra of A / E copolymers have been assigned by comparing the spectra with those of homopolymers. In the case of polyacrylonitrile (PAN), the methylene (---CH2) proton signal appeared around 6 2.10ppm and methine ( - - C H ) proton signal appeared around 3 3.40 ppm. In the case of poly(ethyl acrylate) (PEA), methyl (--CH3) protons appeared around 6 1.08-1.48 ppm. - - C H 2, - - C H and - - O C H 2 proton Table 1. Copolymer composition for A/E copolymers Sample Percent Percent No. fA .]E Nit. FA FE Conv. 1 0.30 0.70 4.53 0.28 0.72 5.4 2 0.40 0.60 6.79 0.39 0.61 8.8 3 0.50 0.50 6.85 0.41 0.59 6.9 4 0.60 0.40 9.66 0.52 0.48 7.0 5 0.70 0.30 15.58 0.73 0.27 7.8 6 0.80 0.20 19.84 0.85 0.15 8.0 fs are the mole fractions of monomers in feed. Fs are the mole fraction of monomers in copolymers.
--
FA
o
o
0.4
0.2 I 0
0.2
0.4
0.6
0.8
1.0
fA Fig. I. The variation of copolymer composition (FA) as a function of monomer composition in feed (fA) for A/E copolymerization. signals appeared around 6 1.50ol.84, 2.12-2.60 and 3.80-4.40 ppm respectively. In the case of A / E copolymer, (----CH3)E and (----OCH 2)E protons appeared around 6 1.11-1.48 and 3.70--4.44 ppm respectively. The two broad signals (6 1.5002.28ppm and 6 2.3003.50 ppm) can be attributed to - - C H 2 and ---CH protons of both m o n o m e r units. The ~H-NMR spectrum of A / E copolymer is very complex and various signals from different monomefic units overlap. Therefore I H - N M R spectra could not be used for determination of the compositions of copolymers. 13C-NMR studies The 13C{lH}-NMR spectrum of A / E copolymer (A = 39.0 mol%), recorded in a mixture of CDCI3 and DMSO-d6 at r o o m temperature, is shown in Fig. 3. The various resonance signals have been assigned by comparison with homopolymers. In the case of PAN, - - C H 2 and ---CH carbons, appeared around 6 33.4 ppm and 27.8-28.1 ppm respectively. The nitrile (---CN) carbon in P A N appeared as a muitiplet in the region 6 118.2-120.8 ppm, indicating that the - - C N carbon of P A N is sensitive to tacticity. Kamide et al. [19] have evaluated the pentad tacticity 1.30
-
1.28
-
1.26
-
/
/
1.24 1.22
7E 1.20 1.18 1.16 1.14 1.12 0.84
]
I 0.88
I
L 0.92
I
I 0.96
I
I 1.00
I
"~A Fig. 2. 95% Posterior probability contour (PPC) for A/E comonomer pair. The + point represents the best estimate o f r A and r E.
Sequence determination of A/E copolymers
n
'
4
805
~-c'2-°'h /
l/-'c"2'E
,,'8 ,I,
,,08 (pprn)
H 0Mso- 6
( X-=O)F_ -(CN) A
--
I 220
I 200
I 180
I 160
I lt..O
I 120
100
I 80
I 60
[ ~0
I 20
I 0
S (ppm)
Fig. 3. ~3C{IH}-NMR spectrum ofA/E copo]ymer (A = 39.0 tool%) along with expanded carbony] carbon (I_-C--O) and nitrile carbon ( ~ N ) in mixture of CDCI 3 and DMSO-d 6 at room temperature.
of R-PAN using high-field NMR. In the case of PEA, --CH3, ---CH 2 and - - C H carbon signals appeared around 6 14.59, 35.0-36.0 and 41.3 ppm, respectively. The resonance signals around 6 60.6 and 173.7 ppm can be assigned respectively to --OCH2 and ~ C-----O carbons in PEA. In A/E copolymer, the signals around 6 13.2 ppm, 26.4-27.5 ppm, 33.1-34.2 ppm and 59.0~0.3ppm can be assigned to (--CH3)E, (--CH)A, (--'CH2)A and (--OCH;)E carbons respectively; they could not be used for the sequence analysis because of poor resolution. The (--CH2) E and (--CH)E carbons of E unit overlapped with solvent DMSO-d6 signals (6 37.5-39.8 ppm), therefore could not be used for analysis of E-centred sequences. The carbonyl carbon in A/E copolymer appeared as a multiplet around 3 171.0-173.7ppm and indicates that the splitting of > C----O signal is due to sensitivity to the compositional sequences. The nitrile carbon of A unit appeared as a well resolved multiplet from 3 118.5 to 120.5 ppm, showing its sensitivity towards different monomer placements. In the case of A/E copolymer, there is shift in the position of various functional groups of A and E units as compared with homopolymers; this effect is due to the change in the nature of adjacent monomeric units in the copolymer which alters the chemical shifts of A and E centred triads. The carbonyl carbon (>C----O) and nitrile carbon (--CN) expansions of A/E copolymer (A = 39.0 mol%) are shown in Fig. 3. PEA shows a singlet centred around 6 173.7 ppm. As the concentration of A in the copolymer increases, the signals characteristics of homopolymer PEA decrease, whereas a set of signals centred around ! 72.2 ppm is appearing. These signals, with further increase in the A content, increase to a maximum and then decrease, whereas a third new set of signals appears around 6 171.7 ppm. The three sets of signals, with intensities changing with copolymer composition, can be assigned to the carbonyl carbon of the central E unit in EEE, AEE (EEA) and AEA triad sequences from low- to high-
field. The assignment of signals was made on the basis of electronic interaction. In the case of E-centred triads, addition of an A unit to the EEE triad causes upfield shift in the position of EEA (by ~0.50 ppm) and AEA (by ~0.50 ppm). This effect is due to the increase in electron density at the carbonyl carbon of the central E unit which can be due to the diamagnetic shielding from the anisotropy of immediate - - C N neighbours. Concentrations of various Ecentred triads can be calculated from the relative areas of resonance signals. These triad concentrations are the normalized areas of respective resonance signals. The relaxation times for the carbon functionalities in different comonomeric units have been found to be the same within the experimental errors of T~ measurements. Spin-lattice relaxation (TI) measurements for acrylonitrile-(methyl methacrylate) copolymers has been reported by Gerken et al. [20]. The quantitative calculations were done from the areas of respective signals in 13C{~H}-NMR of A/E copolymers. Assuming the Alfrey-Mayo (A-M) model (=first order Markov terminal model) [21] to be valid at any moment of the reaction for these low conversion copolymers, the number fraction (F) of acrylonitrile (A) and ethyl acrylate (E) centred triads can be predicted. The relation between intramolecular structure (number fraction of triads) and reaction kinetics is given by the following equations: FEE E = [l - - P(A/E)] 2 FEEA = FAEE =
2[P(A/E)( 1 - -
(la) P(A/E))]
(lb)
FAEA = [P(A/E)]2
(1 C)
FAA A = [1 - - P(E,A)] 2
(ld)
FAA E = FEA A =
2[P(E/A)(I -- P~E/A))]
FEAE = [PIE A)]2
(le)
(If)
806
A.S. BRARand SUNITA Table 2. Calculated and observed fractions o f A- and E-centred triads in A/E copolymers Triad concentrations*
Obs.
Calculated A - M model
Calculated from Harwood Pen. model
0.12 0.42 0.46 0.51 0.38 0. l I
0.08 0.41 0.51 0.54 0.39 0.07
0.10 0.41 0.49 0.57 0.36 0.07
rAA = rsA = rEE = rAE =
1.33 1.06 1.17 0.74
AAA AAE EAE EEE EEA AEA
0.22 0.45 0.33 0.42 0.44 0.14
0.15 0.47 0.38 0.41 0.46 0.13
0.18 0.47 0.35 0.44 0.43 0.13
rAA = rEA = rEE = rAE =
1.46 1.02 1.27 1.05
0.52
AAA AAE EAE EEE EEA AEA
0.34 0.50 0.16 0.20 0.51 0.29
0.33 0.49 0.18 0.20 0.49 0.31
0.38 0.46 0.16 0.21 0.46 0.33
rAA = rEA = rEE = rAE =
0.91 1.04 1.18 1.32
4
0.73
AAA AAE EAE EEE EEA AEA
0.51 0.40 0.09 0.08 0.45 0.47
0.47 0.43 0.10 0.11 0.45 0.44
0.52 0.40 0.08 0.12 0.42 0.46
rAA = rEA = rrE = rAE =
1.09 0.95 0.83 1.12
5
0.85
AAA AAE EAE EEE EEA AEA
0.59 0.36 0.05 0.06 0.33 0.61
0.62 0.34 0.04 0.05 0.36 0.59
0.66 0.30 0.04 0.05 0.33 0.62
rAA = rEA = tEE = rAE =
0.82 0.90 1.45 1.08
Sample No.
A Mole fraction (in topoi.)
Triads
1
0.28
AAA AAE EAE EEE EEA AEA
2
0.39
3
Penultimate reactivity ratios
*Standard deviation = +0.04.
where P A/E =
PE/A= l + r A ' q and q = [A]/[E] is the feed ratio. F represents the number fraction of triads normalized to unity. Table 2 contains the compositional information concerning the various E centred triads along with the calculated values obtained from the A - M model using the terminal model reactivity ratios (r A ~- 0.92 + 0.08 and rE----1.20+ 0.07). Penultimate reactivity ratios (rE and rAE) were evaluated (Table 2) from experimentally obtained triad distributions from carbonyl carbon resonance of ~3C{IH}-NMR spectra using the following equations: rnE
2[EEE] [EEA + A E E ] [Af]/[Ef]
(2a)
rAE =
[AEE + EEA] [Ar]/[Ef] 2[AEA]
(2b)
where [Af] and [Er] are the feed composition in mole fractions of A and E monomers. The values of rEE and rAE are listed in Table 2. Penultimate reactivity ratios change with change in copolymer composition. Similar compositional information on A-centred triads can be made using the - - C N carbon resonance region. A multiplet around 6 118.5-120.5 ppm shows
splitting into three envelopes. The chemical shift difference within the resonance signals and splitting pattern shows its sensitivity on monomer sequences and cotacticity in the nitrile resonance. Increase in concentration of A units in the copolymer increases the most upfield signal of the nitrile group centred around 6 l l 8 . 8 p p m , while the downfield signal around 6 119.8 ppm decreases. Because of this compositional variation in the intensities of A centred triads, the most upfield resonance signal is assigned to the A A A triad and the most downfield to the EAE triad. The introduction of ethyl acrylate to A A A centred triad causes a downfield shift in the position of AAE (by ~,0.40 ppm) and EAE (by ~0.60 ppm) due to the deshielding effect of the carbonyl group in ethyl acrylate. A-centred triads along with those calculated using the A - M model are given in Table 2. On the basis of the triad resonance assignments as described earlier, penultimate reactivity ratios were evaluated for A/E copolymer from the triad fractions determined from nitrile resonance of t3C{JH}-NMR spectra using the following equations:
2[AAA]
rAA = [AAE + EAA] [Ef]/[Af]
(2c)
[AAE + EAA] [Ef]/[Af]. 2[EAE]
(2d)
rEA =
The values of rAA and rEA are listed for individual copolymers in Table 2. Using the average values of
Sequence determination of A/E copolymers
807
Table 3. Copolymerization parameters of A/E copolymers determined by ~3C-NMR spectroscopy Sample No.
A mole fraction (in copol.)
q
PE/A
PA/E
]~A
NE
rA
rE
I 2 3 4 5
0.28 0.39 0.52 0.73 0.85
0.92 0.92 0.91 0.92 0.94
0.67 0.55 0.41 0.29 0.23
0.30 0.36 0.54 0.69 0.77
1.49 1.82 2.44 3.45 4.35
3.33 2.78 1.85 1.45 1.30
1.15 1.20 0.96 1.05 0.84
1.00 1.18 1.25 1.02 1.16
penultimate reactivity ratios, the triad sequence distribution was obtained by Harwood's program [17] and is given in Table 2 the uncertainty in the values of triad compositional sequences obtained from N M R spectra and reactivity ratios is ___0.04. In earlier publications, the sequence and cotacticity have been reported for acrylonitrile-(methyl methacrylate) [12], acrylonitrile--(ethyl methacrylate) [13], acrylonitrile--(butyl methacrylate)[14] copolymers. In the present case of A/E copolymer, the cotacticity could not be observed in the carbonyl carbon and nitrile carbon signals even at 67.89 MHz frequency. The replacement of the ~t-methyi group in alkyl methacrylate by a hydrogen atom causes drastic change in the 13C-NMR spectrum with respect to the signals of carbonyl and nitrile resonance, which are cotacticity sensitive in alkyl methacrylate copolymers. From the results of composition with respect to various A and E centred triads, the conditional p r o b a b i l i t i e s PE/A and PAlE have been calculated using the equations: [EAE] + [EAA]/2 PE/A = (3a) [AAA] + [AAE] + [EAE] [AEA] + [EEA]/2 (3b) [EEE] + [EEA] + [AEA] where PE/A is the probability that A-E unit comes as a result of an A growing chain end adding E and PAlE is the probability that the E-A unit comes about as a result of an E growing chain-end adding A. The value of PE/Adecreases linearly from 0.67 to 0.23 with decrease in the content of ethyl acrylate in the copolymer while the value of PA/E increases linearly from 0.30 to 0.77 with increase in concentration of acrylonitrile in the copolymer. Using these equations, the terminal model reactivity ratios (r A and rE) have been calculated for individual copolymers using the following equations: PA/E =
rA =
__[El] [Af]"
[I/PE/A --
l]
[Af] [1/PA/E- 1].
rE "~" [--~f] •
(4a)
The block character, (r/) is a measure of the departure from random character [22]. It is given by the equation: [A-E] (5) r/ 2[A] [E] where [A-E] is the mole fraction of A-E diads and [A] and [E] are the mole fractions of A and E units in the copolymer. 0<,7 < 1 reflects more block character and 1 < q < 2 means more alternating tendency of the copolymer than expected from the random distribution. The value of ~/for A/E copolymers lie in the range 0.91-0.94 (0 < ,7 < 1), showing that the A/E copolymer has more tendency towards block character. Table 3 contains the copolymerization parameters of A/E for various monomer feed ratios.
Glass transition temperature (TQ measurements The dependence of Tg on copolymer composition cannot be expressed by simple additive relations such as the Fox equation [10]. The curvatures observed in the dependence of T~ of random copolymers on composition have been explained by Barton [11]. He described the effect of sequence distribution of monomeric units in linear copolymers on Tg and considered the contribution of diads. Suzuki et aL [23] modified the Barton equation and applied it to some published data obtaining good agreement. Barton [11] expressed Tg on the basis of the following equation: Tg = nAA Tg,AA + nEE Tg, EE q" nAE Tg, AE
160
u.I LU
o)
(4b)
The reactivity ratios calculated' from the EVM program are r a = 0.92 _+0.08, r E = 1.20 _+0.07 and from triad composition are r A = 1.04, r E = 1.12, these values are in reasonable agreement. The numberaverage sequence lengths (NA and NE), the reciprocal of conditional probabilities, is also obtained. The value of "~Aincreases linearly from 1.49 to 4.35 as the content of acrylonitrile in the copolymer increases while the value of ~E decreases linearly from 3.33 to 1.30 with decrease in the content of ethyl acrylate in the copolymer.
(6)
¢J I
~ bo9
140
12o 100
so 60
I
~ I-.
40 2O
0
I 0.1
I 0.2
I 0.3
I 0.4
I 0.5
I 0.6
I 0.7
(nAE) * (nEA) Fig. 4. The plot o f ( T , - r/AATg.AA -- nEE T,.EE ) VS (nAE -q- nEA).
A. S. BRARand SUNIT,~
808
Table 4. Glass transition temperature (TB) and diad composition data for A/E copolymers Sample No.
A mole fraction (in copol.)
Diads fraction
Tg (K) Calc. from Barton eqn
Tg (K) from DSC (Expt.)
1
0.28
[AAI = 0.08
278.7
--
289.8
--
314.9
315
328.5
330
343.5
346
2
0.39
3
0.52
4
0.73
6
0.85
IEEI= 0.54 [AE] = 0.38 [AA] = 0.14 IEEI = 0.41 [AEI = 0.45 [AA] = 0.33 lEE] = 0.19 [AE] = 0.48 IAAI= 0.46 [EEl = 0. I 1 [AE] = 0.43 [AA]= 0.62 lEE] = 0.05 [AE] = 0.33
Acknowledgements--The authors thank IIT Delhi for providing research facilities and Professor C. L. Khetrapal, l.I.Sc. Bangalore, for recording NMR spectra. They also thank Professor K. F. O'Driscoll for providing the EVM computer program. REFERENCES
TS,AE= 301 K.
where Tg is the glass transition temperature of the copolymer and Tg,AA and Ts, EE refer to polyacrylonitrile and poly(ethyl acrylate) respectively. Tg.AE represents Tg of alternating copolymers. The plot of (Tg -- nAA Tg, AA -- nEE Tg, EE) VS (nAE d- nEA ) is linear passing through the origin (Fig. 4). The value of Tg,AE obtained from the slope is 301 K. Experimental values are in good agreement with those calculated from diad concentrations. Table 4 contains the diad composition (obtained from the terminal model H a r w o o d program) and the Tg values of various A / E copolymers. In order to use any sequence distribution-T~ relationship, it is necessary to have the Tg,AE value, The value of Ta,AE for A/E copolymer is 301 K; using this value, the sequence-Tg relation can be established. The value of Tg can be predicted for any sequence of this copolymer. The diad model has been called the terminal model in the kinetics of copolymerization. Only the terminal unit of a growing polymer chain is regarded as affecting the probability of m o n o m e r addition. Han [24] extended the diad model of Barton for copolymer TgS to the triad model:
T, = nAAATs,AAA + nEEETg,EEE +nAAE Ts,AAE + nEAA Tg,EAA +nA~E Tg,AEE + nEE/,Tg,EEA q-nAEATs, AEAq- nEAETg,EAE
assigned to Tg,AAE and Tg,AE. However the grouping and equality seem to be unreasonable. Urematsu and Honda [25] tried to reduce eight different triad sequences to three groups and relating them to TsAA, TgEE and TgAE. Tg predicted for a random copolymer is formally written as equation (7) in the penultimate model. It is difficult to trace the Tg of eight different triads of this copolymer. We have used diad sequence-Tg relation for this polymer and obtained good agreement between predicted and experimental values.
(7)
here nx and Tg,x are respectively the mole fractions and Ts related to the sequence specified by the suffix X. H a m reduced the eight triad sequence to the four sequences, viz. two homopolymer Tg values, Tg,AAE (which is assumed to be equal to Tg,EAA) and the fourth coded as Tg,AE (assuming equality between Tg, AEA, Tg, EAE , Tg, AEE to Tg, EEA). Applying this extended equation to some published data on (methyl methacrylate)-acrylonitrile copolymers, he obtained improved agreement, when particular values were
1. P. F. Barton, D. J. T. Hill, J. H. O'Donnell and P. W. O'Sullivan. Macromolecules 16, 1967 (1984). 2. T. Kelen and F. Tiid6s. J. macromolec. Sci. Chem. Ag, 1 (1975). 3. M. Finemann and S. D. Ross. J. Polym. Sci. 5, 259 (1950). 4. H. Patino-Leal, P. M. Reilly and K. F. O'Driscoll. J. Polym. Sci.; Polym. Left. 18, 219 (1980). 5. K. K. Chee and S. C. Ng. Macromolecules 19, 2779 (1986). 6. M. Sanchez-Chaves, F. Arranz and M. Montes. Polymer 29, 2244 (1988). 7. F. Arranz, M. Sanchez-Chaves and A. Molinero. Macromolec. Chem. 185, 2153 (1984). 8. S. H. Harris and R. D. Gilbert. J. Polym. Sci.: Polym. Chem. Edn 20, 1653 (1982). 9. H. A. Schneider and H. N. Neto. Polym. Bull. 9, 457 (1983). 10. T. G. Fox. Bull. Am. Phys. Soc. 1, 123 (1956). 11. J. M. Barton. J. Polym. Sci. C 30, 573 (1970). 12. G. S. Kapur and A. S. Brar. Polymer 32, 1112 (1991). 13. G. S. Kapur and A. S. Brar. J. Polym. Sci. A: Polym. Chem. 29, 479 (1991). 14. G. S. Kapur and A. S. Brar. Makromolek. Chem. 192, 2733 (1991). 15. A. S. Brar and Sunita. Polymer Science, Contemporary Themes, Vol. II, Symposium Proceeding of Polymers" 91, Pune, p. 582 (1991). 16. M. Dube, R. Amin Sanayel, A. Penlidis, K. F. O'Driscoll and P. M. Reilly. J. Polym. Sci.: Polym. Chem. 29, 703 (1991). 17. H. J. Harwood. J. Polym. Sci. C. 25, 37 (1968). 18. W. M. Ritchey and L. E. Ball. Polym. Lett. 4, 557 (1966). 19. K. Kamide, H. Yamazaki, K. Okajima and K. Hikichi. Polym. J. 17, 1233 (1985). 20. T. A. Gerken and W. M. Ritchey. J. appl. Polym. Sci.: Appl. polym. Symp. 34, 17 (1978). 21. J. L. Koenig. Chemical Microstructure of Polymer Chain, Chap. 3, p. 39. Wiley Interscience, New York (1980). 22. K. Ito and Y. Yamashita. J. Polym. Sci. A. 3, 2153 (•965). 23. H. Suzuki and V. B. F. Mathot. Macromolecules 22, 1380 (1989). 24. G. E. Ham. J. macromolec. Sci. Chem. A 9, 461 (1975). 25. L. Uematsu and K. Honda. Rep. Prog. Polym. Phys. J. 8, 111 (1965).