Miscibility of polyester/nitrocellulose blends: A DSC and FTIR study

Miscibility of Polyester / Nitrocellulose Blends: A DSC and FTIR Study

JEAN-JACQUES JUTIER, EVEN LEMIEUX and ROBERT E. PRUD'HOMME, Centre de Recherche en Sciences et Inginierie des

Macronwldcules, Diparternent de chi&, Universith Laval, Qdbec, Canada GlK 7P4

Synopsis

The miscibility of polyester/nitrocellulose blends was investigated by differential scanning calorimetry and Fourier-transform infrared (FTIR) spectroscopy. Two nitrocelluloses (NC) de- rived from wood and having different nitrogen contents (12.62 and 13.42%) were used. On the basis of the glass transition temperature criterion, poly( r-caprolactone) (PCL), poly(valerolactone), poly(ethy1ene adipate), and poly(buty1ene adipate) are miscible with nitrocellulose, whereas poly( a-methyl a-propyl 8-propiolactone) and poly( a-methyl a-ethyl P-propiolactone) are immiscible. The Tg versus composition curves of PCL/NC blends do not follow a monotone function but exhibit a singular point a t a critical PCL volume fraction of 0.51 for NC-1342 and 0.45 for NC-1262 in agreement with Kovacs' theory. A shift of 17 em-' of the carbonyl stretching band was observed with PCL/NC blends and is taken as evidence for hydrogen bonding interaction between the PCL carbonyl group and NC hydroxyl group. The frequency difference between the free hydroxyl absorbance and the absorbances of the hydrogen-bonded species was found to be 85 cn-' in pure NC and 125 cm-' in PCL/NC blends; it indicates that the average strength of this interaction is stronger than the corresponding self-associated hydrogen bonding in pure NC. The presence of a dipole-dipole interaction between the nitrate-ester groups of NC and the carbonyl groups of the polyesters is reported. The relative strength of the hydrogen bonding and dipole-dipole interactions is discussed and correlated with polymer miscibility.

INTRODUCTION

Miscibility between polymers is often ascertained through measurements of glass transition temperatures (Tg): miscible systems exhibit a single Tg intermediate between the Tg of the individual components, whereas immiscible blends show two Tgs at a given composition. Glass transition temperatures can be measured by a variety of methods, including thermal analysis, dynamic mechanical testing, refractive index variation, and nuclear magnetic resonance spectro~copy.'-~

Fourier-transform infrared (FTIR) spectroscopy has recently been em- ployed to study a large number of miscible polymer blend^.^-'^ The FTIR method has been shown to give useful information concerning specific interactions between polymer segments in polyester/poly(vinyl chloride) (PVC) b1ends.l0-l2 Miscible blends exhibit a shift of the carbonyl band with composition. This observation has been associated with a hydrogen bonding interaction between the carbonyl groups of the polyester and the &-hydrogens of the chlorinated polymer and/or to a dipole-dipole interaction between the carbonyl groups of the polyester and the C-C1 groups of the chlorinated polymer.

Journal of Polymer Science: Part B: Polymer Physics, Vol. 26, 1313-1329 (1988) 0 1988 John Wiley & Sons, Inc. CCC ooS8-1273/88/061313-17$04.00

1314 JUTIER E T AL.

With miscible polyester/hydroxylated polymer blends, hydrogen bonding leads to a second carbonyl group absorption peak, or a shoulder, overlapping the original peak. The relative intensity of this second band increases a t the expense of the first one as the concentration of hydroxyl groups becomes larger. Significant changes in the hydroxyl stretching region of the spectrum are also observed as a function of the hydroxyl concentration. It was also proposed16. l7 to relate the shift of the carbonyl absorption band of a polyester, in a miscible blend, to the strength of the interaction between polymer segments.

Koleske et a1.l'. l9 have blended poly(capro1actone) (PCL), and other polyesters, with a wide variety of polymers. PVC and poly(hydroxy ether of bisphenol A) have been shown to be miscible with PCL over the full range of composition, whereas blends of poly(epichlorohydrin) and PCL lead to phase separation at high PCL concentrations, and PCL/nitrocellulose (NC) system are miscible at PCL blend compositions larger than 50%.

Using various methods such as differential scanning calorimetry (DSC) and infrared (IR) spectroscopy, Hubbell and Cooper20y21 studied the miscibility of PCL blended with a NC containing 12% of nitrogen. In the IR spectra, a shift of the carbonyl band was observed and associated with the miscibility of PCL/NC blends in the 0-50% PCL composition range, although a similar shift was not seen in the hydroxyl stretching region, and phase separation was reported by DSC measurements in this same composition range. These authors also found by DSC that these blends are miscible in the 50-100% PCL range, whereas the frequency of the PCL carbonyl absorption band is constant a t PCL blend compositions greater than 65%.

In order to clarify the miscibility of NC with PCL, several polyester/NC blends were studied by DSC and FTIR spectroscopy. Two nitrocelluloses having nitrogen contents of 12.62 and 13.42% and various linear polyesters were used. The influence of the concentration of nitrate-ester groups (0-NOz) on miscibility is discussed in terms of the relative strengths of intermolecular and intramolecular interactions.

EXPERIMENTAL

Table I gives a list of the polymers used in the present study with their weight-average molecular weight M,, number-average molecular weight M,, glass transition temperature Tg, and melting temperature T,.

Three polyesters (PCL, PBA, and PEA) were purchased from Aldrich Chemicals, whereas PVL, PMPPL, and PMEPL were synthesized in our L a b o r a t ~ r y . ~ ~ - ~ ~ The nitrocelluloses (NC) used (Expro Chemical Products Inc.) were prepared from wood and contained 13.42 and 12.62% of nitrogen, which corresponds to 0.27 and 0.55 OH groups per glucose unit, respectively. NC is assumed to be mostly amorphous.

Sample Preparation

NC and polyester solutions were prepared separately using freshly distilled tetrahydrofuran (THF), NC dry fibers, and polyester granules. After dissolu- tion, the polyester solution was poured into the NC solution, stirred 15 min, and films were cast thereof. In all cases, solvent evaporation was conducted at

TAB

LE I

Cha

ract

eriz

atio

n of

the

Pol

ymer

s

cd 8 s

Poly

mer

A

cron

ym

Stru

ctur

e M

* M

" (K

) (K

) m

Mol

ecul

ar w

eigh

t (G

PC)

(kg

mol

- ')

Tg Tm

4

Poly

( c-c

apro

lact

one)

PC

L -(

CH

2)5-

C00

- 36

20

8 33

0 M

21

5 33

2 \#

Poly

(val

ero1

acto

ne)

PVL

-(CH

2)4-

COO

- 61

CH

3 3

88

268

360

s Po

ly( a

-met

hyl a

-eth

yl P

-pro

piol

acto

ne)

PME

PL

- C

H 2-

C-

COO

- 80

25

5 40

6 I I

0 R

CH

3 F

I

a I C3

H7

E

C2H

5

131

Poly

( a-m

ethy

l a-p

ropy

l P-p

ropi

olac

tone

) PM

PPL

- CH

,- C-

CO

O-

M

a Po

ly(b

uty1

ene

adip

ate)

PB

A

-(CH

2

)4 -C

OO

-(CH

2

)4 -0

CO

- 9

212

327

2

Poly

(eth

y1en

e ad

ipat

e)

PEA

-(C

H2)

4-CO

O-(C

H2)

Z-O

CO-

2.6

220

320

Nitr

ocel

lulo

se d

eriv

ed from

woo

d

Nitr

ocel

lulo

se d

eriv

ed from

woo

d

U

cont

aini

ng 1

3.42

% N

N

C-1

342

409

100

m

cont

aini

ng 1

2.62

% N

N

C-1

262

405

117

1316 JUTIER ET AL.

room temperature. The resulting films were removed from Petri dishes and dried in a vacuum oven until they reached constant weight.

Thermal Analysis

Thermal analysis was conducted with a Perkin-Elmer differential scanning calorimeter (DSC-4) equipped with a TADS microcomputer. The DSC was calibrated with ultrapure indium. Reported melting points T, were recorded at the end of the melting curve, and Tg was taken a t the half-height of the corresponding heat-capacity jump.

In the DSC apparatus, the samples were first cooled to 173 K and main- tained at that temperature for 10 min. A first scan was made a t a heating rate of 20 K/min, up to 463 K, yielding T, and AHm. Samples were then quenched to 173 K, kept 10 min a t this temperature, and reheated under the same thermal regime. The Tgs reported in this paper were always recorded during the second scan. Every Tg, T,, and AH, reported here corresponds to a mean value calculated from three different samples a t each composition.

FTIR Spectroscopy

FTIR results were obtained with a BOMEM DA3.02 spectrometer. Thin films were cast unto a KBr window from 1% (w/v) solutions in freshly distilled THF. After evaporation of most of the solvent, the films were transferred to a vacuum dessicator and kept a t room temperature for more than 12 h to remove residual solvent. The samples were stored under vacuum to minimize water absorption. Each spectrum was obtained using a high-temperature cell connected to an Omega D-921 thermoregulator mounted in the spectrometer. At selected temperatures, spectra were recorded a t a resolution of 2 cm-’ after 100 scans, signal-averaged, and stored on a magnetic disk system. The frequency of the FTIR bands was determined by the method of the center of gravity.25 The films used in this study were sufficiently thin to obey the Beer-Lambert law.26

RESULTS AND DISCUSSION

Calorimetric Analysis

Calorimetric measurements were conducted with PCL/NC-1262 and PCL/NC-1342 samples of various compositions, and the results obtained are summarized in Table 11. For illustrative purposes, the phase diagram of these mixtures is shown in Figure 1. The two systems are clearly miscible according to the Tg criterion. Indeed, a single Tg is observed at each composition, and i t is found a t a temperature intermediate between those of PCL and nitrocellulose. However, the observation of a single Tg does not necessarily mean that the two polymers are dispersed at the molecular although it certainly indicates extensive mixing between the segments of the two polymers. Figure 1 shows that Tg increases regularly but not linearly with composition and that the T, of PCL does not vary significantly with the NC content. At compositions greater than 40% in NC, the melting peak of PCL no longer appears.

Several theoretical and empirical equations are u ~ e d ~ ~ 9 ” to describe the Tg-composition dependence of polymer blends. These equations are mostly

POLYESTER/ NITROCELLULOSE BLENDS 1317

TABLE I1 Glass Transition Temperature (Tg), Melting Temperature (Tm),

and Enthalpy of Fusion (AH,) of PCL/NC-1342 and PCL/NC-1262 Blends

NC-1342

0 10 15 21 30 40 50 60 70 80 90

100

NC-1262 0

10 20 29.6 40.1 49.2 59.3 70.2 70.5 88.9

100

0.00 0.17 0.23 0.26 0.32 0.37 0.40 0.51 0.61 0.73 0.86 1 .00

0.00 0.17 0.26 0.35 0.40 0.40 0.50 0.62 0.72 0.85 1.00

208 215 219 223 232 242 250 260 283 304 328 353b

208 215 226 239 255 270 285 302 316 330 353b

342 341 341 340 340 340

342 341 340 340 339

99 88 84 79 54 33 0 0 0 0 0 0

92 88 71 67 42 0 0 0 0 0 0

"Volume fraction calculated using a PCL density of 1.094 and a NC density of 1.610 g/cm3. bExtrapolated from Figure 1.

associated to a monotonic variation of Tg with composition. However, a theoretical approach by KovacsN predicts that Tg does not vary monotoni- cally as a function of volume fraction but exhibits a singular point at a critical volume fraction +,, where Tg is equal to a critical temperature T, corresponding to an iso-free volume equal to zero:

Above T,, Tg can be calculated as

whereas below T,, Tg is given by

where Tgi is the glass transition temperature, Aai is the difference in the

1318 JUTIER ET AL.

A PCL/NC-1262

0 PCL/MC-1342 200

QO ai 0.2 0.3 0.4 05 0.6 0.7 0.0 0.9

NC VOLUME FRACTION

Fig. 1. Phase diagram of poly( 6-caprolactone)/nitrocellulose blends.

volume expansion coefficients between the glassy and the liquid states, and +i is the volume fraction of polymer i (corrected for crystallinity in the case of PCL composition larger than 0.6, using the enthalpies of fusion given in Table 11); f' is the free volume fraction at Tpz which is equal to 0.025 according to the iso-free-volume theory. The critical volume fraction += for component 1 can be calculated using eq. (2) assuming Tgl = T,. Nandi et al.31 have already noticed this peculiar behavior of polymer blends, which can be predicted from the relevant Aa data.

In order to apply those equations to PCL/NC data, the following parameters must be known: Tgl, T', ha,, and Aaz. Tgl (for PCL) is given in Figure 1 as equal to 208 K. From data on PCL/PVC and PCL/chlorinated PVC blends, Aubin and Prud'homme3' recently calculated a value of ha, for PCL of 4.1 x K-'. Then, the Tgs of PCL/NC blends measured at several compositions above +c were used to extrapolate a Tpz value of 353 K for nitrocellulose (the Tg of pure NC cannot be directly estimated experi- mentally33). Finally, with eq. (2), the slope of (2'' - Tg) plotted as a function of [(+,/I#B~)(T~ - Tgl)] gives a value of ha, equal to 5.2 X K-' for NC-1262 and 3.0 X K-' for NC-1342 (with a correlation coefficient equal to 0.998).

By inserting these parameters into eqs. (2) and (3), one can plot a Tg-composition diagram as illustrated in Figure 1 where the theoretical curve is represented by the full line. Equation (3) is common to either PCL/NC-1262 or PCL/NC-1342 blends, at low volume fractions and low temperatures, up to a volume fraction of 0.51 for NC-1342 and 0.45 for NC-1262. At larger volume fractions (and higher temperatures), two different theoretical lines are gener- ated for the two series of experiments.

In this latter region (r& > 0.5), i t is interesting to note a variation in the slope of the T,-composition plot. For any given NC volume fraction, the Tg


I 200 220 2 4 0 260 2 b 360 320 340 580

TEMPERATURE (KI

Fig. 2. Representative calorimetric curves in the Tg region of ply( c-caprolactone)/nitrocellulose blends; percentiles are those of the NC composition in blends.

value of the blend is lower with NC-1342 than with NC-1262, which implies greater mobility of the polymer segments in the former blend. Therefore, the plasticization by PCL is more important with NC-1342 than with NC-1262. Even if the two series of blends are miscible according to the single Tg criterion, it is tempting to suggest that NC-1262 has a greater affhity and stronger interactions towards PCL than NC-1342.

Figure 2 shows specific examples, at several compositions, of the heat capacity curves of PCL/NC blends in the Tg area. I t can be seen that the width of the transition is broader for the blends than for pure PCL. In fact, pure PCL exhibits a 10-12 K width at Tg, but PCL/NC-1262 and PCL/NC- 1342 blends give rise to a significantly broader transition. This is particularly true for compositions larger than 50% in NC. This observation can be ascribed to the presence of microheterogeneities where local composition fluctuations are in excess of normal density and temperature fluctuation^.^^-^^ Similar broadening effects have been found in several miscible

DSC measurements were carried out with other polyester/NC blends. Table I11 summarizes the T'. values measured for polyester/NC-1342 blends of various compositions: a single Tg is observed at each composition and at a temperature intermediate between those of pure components with PVL, PEA, and PBA. In contrast, the values of Tg as a function of composition for

1320 JUTIER ET AL.

TABLE111 Glass Transition Temperature (T,) of Polyester/NC-1342 Blends

T, (K) NC-1342 (wt W ) PVL PEA PBA PMEPL PMPPL

0 215 220 212 255 268 27 242 237 234 257 276 53 266 257 257 256 275 78 336 280 330 258 274

loo" 353 353 353 353 353

"Extrapolated from Figure 1.

PMEPL/NC and PMPPL/NC blends are always close to those of the polyester. Similar results were obtained with the lower nitrogen content nitrocellulose (NC-1262). According to the 2'' criterion, PVL/NC, PEA/NC, and PBA/NC blends are miscible, whereas PMEPL/NC and PMPPL/NC blends are not. This latter behavior can be explained by steric hindrance between NC and PMEPL or PMPPL: the bulky side groups of these polyesters may either prevent the approach of the hydroxyl groups of nitrocellulose from the carbonyl groups of the polyesters or the approach of the nitrate-ester groups of NC from the carbonyl groups of the polyesters. These blends form a two-phase structure with domains composed almost uniquely of PMEPL or PMPPL and domains composed almost uniquely of nitrocellulose.

Spectroscopic Analysis

In order to determine the nature of the specific interactions of polyester/NC blends, FTIR spectroscopy measurements were performed a t various temperatures as a function of blend composition and nitrogen content. As noticed above, the carbonyl stretching and the hydroxyl stretching regions of hydroxylated polymer/polyester blends are particularly sensitive to specific interactions involving specific groups of each component. More specifically, the analysis of the FTIR spectra of polyester/NC blends showed that the vibrational bands that are significantly perturbed as a function of the composition of the mixture are the carbonyl stretching vibration band at about 1734 cm-', the hydroxyl stretching region (3000-3700 cm-I), and the nitrate-ester stretching vibration at about 1650 cm-'. It should be pointed out that the spectrum of pure nitrocellulosea is free from absorption in the 1700-1800 cm-' and that the spectra of the polyesters used here are also free from absorption in the 3000-3700 cm-l region.

Figure 3 shows the infrared spectra in the carbonyl stretching region (1690-1770 cm- ') of PCL/NC-1262 blends of varying compositions recorded a t 353 K (above the melting point of PCL). The spectrum of pure PCL exhibits a rather broad band a t 1734 cm-' that is assigned to self-associated amorphous PCL.40 The most striking feature of these spectra is the ap- pearance of a shoulder, centered a t 1717 cm-', which increases in intensity with the NC content in the blend. This band may be reasonably assigned to a hydrogen-bonded carbonyl group. In other words, this band indicates an


Fig. 3. FTIR spectra of PCL/NC-1262 blends in the 1690-1770 cm-' region at 353 K; numbers correspond to the PCL percentage in blends.

intermolecular interaction involving the PCL carbonyl group and likely the NC hydroxyl group (i.e. -C=O---H-0-).

The broadening and shift of the carbonyl stretching band in a miscible blend may result in part from the superposition of more than one absorption band and not by a uniform perturbation of the original absorpti~n.~' The carbonyl stretching band can be decomposed into two components that are related to the absorption of the noninteracting carbonyl groups (1734 cm-') and the absorption of the interacting carbonyl groups (1717 cm-'). Curve fitting, using Gaussian and Lorentzian bands, reveals that only two bands are necessary to satisfactorily match the spectra of the blends in the carbonyl region and that these two components are, within experimental error, identi- cal in position, shape, and half-width ( A v ~ , ~ ) as a function of blend composition. The 1717 cm-' band is significantly broader than the 1734 cm-' band, which reflects a relatively wide distribution of hydrogen bond distances and geometries in the blend. Curve fitting shows a decrease in the intensity of the absorption band corresponding to the noninteracting carbonyl groups and an increase in the intensity of the absorption band related to the interacting carbonyl groups as a function of composition.

The evolution of the spectra shown in Figure 3 with the NC composition of PCL/NC blends can be understood by considering that, in blends with NC

1322 JUTIER ET AL.

composition smaller than 608, the ratio of the number of hydroxyl groups to the number of carbonyl groups is rather small, whereas in blends rich in nitrocellulose (70% and over), this ratio is considerably higher. For example, a 40 : 60 PCL/NC blend contains 0.36 OH group per carbonyl group, a 20 : 80 PCL/NC blend contains 0.95 OH group per carbonyl group, and a 10:90 PCL/NC blend contains 2.14 OH group per carbonyl group. The carbonyl groups that are not involved in hydrogen bonds are numerous in PCL/NC blends of NC composition less than 608, whereas the probability of finding a noninteracting carbonyl group is small at high NC concentrations. In all cases, the fraction of PCL carbonyl groups that can form hydrogen bonds with NC hydroxyl groups is limited by the dimensions of the individual chemical repeating units and the physical problem of intimately mixing macromolecules.

Figure 4 shows the carbonyl stretching region of PCL/NC-1262 blends at various compositions, recorded at 298 K. In the carbonyl region these spectra exhibit additional features, as compared with those taken at 353 K, which can be assigned to crystalline PCL. The spectrum of pure PCL exhibits two bands: a relatively sharp one at 1724 cm-’ attributed to PCL in its “crystalline” conformation and a second one, which appears as a shoulder at 1734 cm-’ and is associated with amorphous PCL.40 As the concentration of PCL decreases in

1717 17241

1760 I740 I720 li00

FREQUENCY (crn-l) Fig. 4. FTIR spectra of PCL/NC-1262 blends in the 1690-1770 cm-’ region, at 298 K;

numbers correspond to the PCL percentage in blends.


the blend, a third band centered at 1717 cm-' is observed, and its relative intensity increases with the NC-1342 content. Although the 1724 cm-' band is still visible in the spectrum of the 60 : 40 PCL/NC-1342 blend, it disappears a t a 50:50 composition, which implies that the crystalline PCL is no longer present in such a system.

A shift in frequency of about 17 cm-' is then observed for the carbonyl stretching band at 353 and 298 K as a function of composition (Figs. 3 and 4). The value of this shift is in agreement with data from recent publications related to polyester/hydroxylated polymer blends: Coleman et a l . ' 4 7 4 2 observed a shift of the carbonyl band of about 15 cm-' in PCL/phenoxy blends and of about 25 cm-' in PCL/poly(p-vinylphenol) systems. If the shift in frequency of the carbonyl stretching band of the polyester in a miscible blend is related to the strength of the interaction between polymer segments,16* l7 we can reasonably suggest that the interaction between the hydroxyl group of nitrocellulose and the accessible carbonyl group of PCL is slightly stronger than the interaction in PCL/phenoxy blends but weaker than that in the PCL/poly( p-vinylphenol) blends. Similar spectral features were also obtained using NC-1342.

Figure 5 shows spectra in the hydroxyl stretching region from 3000-3700 cm-' of PCL/NC-1262 blends a t 353 K. The bottom spectrum corresponds to NC-1262 and exhibits a narrow band at 3575 cm-', attributed to the "free" or nonhydrogen-bonded hydroxyl stretching mode, and a broad band centered a t 3490 cm-l, which is assigned to a wide distribution of the hydrogen-bonded hydroxyl stretching modes (self-association). With an increase of the PCL concentration in the blend, the spectrum in the hydroxyl region shows a third band centered a t 3450 cm-', which is related to hydroxyl groups that are involved with hydrogen-bonded PCL carbonyl groups. The relative intensity of these three bands vary with the PCL concentration in the blend: both hydroxyl stretching bands of pure NC (3575 and 3490 cm-') decrease, whereas the 3450 cm-' band increases with an increase of the PCL concentration. Furthermore, there is no evidence for the presence of free hydroxyls (3575 cm - '), and the concentration in hydroxyl-hydroxyl interactions appears minimal at a PCL concentration larger than 40%.

Purcell and drag^^^ have shown that the frequency difference ( Av) between the free hydroxyl absorption and those of the hydrogen-bonded species yields a measure of the average strength of the intermolecular interaction. In pure NC-1262, Av is equal to about 85 cm-', whereas in PCL/NC-1262 blends the frequency difference Av increases to about 125 cm-'. The above results suggest that the average strength of the hydrogen bond between the PCL carbonyl groups and the NC hydroxyl groups is stronger than the one occurring between the hydroxyl groups in pure NC. The possibility of having two hydroxyl groups sufficiently close to one another so as to create a hydrogen bond between them is rather low; and, as the concentration in PCL increases in the blends, this probability decreases even further. In such blends, a significant fraction of the NC hydroxyl groups associates with the polyester carbonyl groups through hydrogen bonds. Increasing the concentration in carbonyl groups favors the interaction of carbonyl and hydroxyl groups, leading to a disappearance in the FTIR spectra of the high frequency band at 3575 cm-' and an increase in the frequency difference Av.

1324 JUTIER ET AL.

3600 3400 3200 3000 FREQUENCY (cm-'1

Fig. 5. FTIR spectra of PCL/NC-1262 blends in the 3000-3700 cm-' region at 353 K; percentiles are those of the NC composition in blends.

By considering the hydroxyl stretching region of the spectra a t 298 K, one observes only slight differences arising as a function of composition. For compositions larger than 60% in PCL, these slight differences can be related to a fluctuation in the amorphous phase due to the crystallization of a portion of the PCL a t low temperatures. The essential features recorded are the same as those described above (Fig. 5).

Table IV gives a list of frequency differences that were reported in the hydroxyl region of various polymer blends. The frequency differences observed in the hydroxyl region of the PCL/NC systems are in good agreement, but smaller, than those measured with phenoxy/poly(ethylene oxide),14 poly( p- vinylphenol)/poly(vinyl pyrrolidone), and poly( p-vinylphenol)/poly(ethylene

blends. In all of the above systems, there is an hydroxylated polymer, and, in every case, the intermolecular interactions are more important than the intramolecular interactions. However, these frequency differences are in marked contrast with data from poly( p-vinylphenol)/PCL blends42 and phenoxy/PCL systems,14 where the average strength of the hydroxyl bond between the PCL carbonyl group and the poly(p-vinylphenol) or phenoxy hydroxyl group is smaller than that occurring between hydroxyl groups in pure poly(p-vinylphenol) or phenoxy. This implies a greater affinity of the

POLYESTER / NITROCELLULOSE BLENDS 1325

TABLE IV Frequency Difference A u Reported in the Hydroxyl Region of Various Polymer Blends

System

NC/PCL Phenoxy/

poly(ethy1ene oxide) Poly( p-vinylphenol)/

poly(viny1 pyrrolidone) Poly( p-vinylphenol)/

poly(ethy1ene oxide) Poly( p-vinylphenOl)/PCL Phenoxy/PCL

Intermolecular A v Intramolecular A v (a-1) (cm-') Reference

125 85 This study

270 160 14

295 165 42

325 105 65

165 165 120

42 42 14

PCL carbonyl group to hydrogen bond with the hydroxyl group of NC-1262 as compared with poly( p-vinylphenol) or phenoxy polymers.

The asymmetric stretching mode of the 0-NO, groups of NC has also been carefully analyzed at 298 and 353 K, since it enables one to investigate the interactions between PCL and nitrocellulose. As shown in Figure 6A, there is a broad band centered a t 1659 cm-', which can be assigned to the asymmetric stretching mode of the nitrate-ester units in pure nitrocellulose. Adding PCL to NC results in a splitting of this band into two new bands a t 1648 and 1664 cm-'. The relative intensity of the 1648 cm-' band decreases,

A 1664

I 1648

FREQUENCY (cm-')

3 1663

FREQUENCY (cm-'I Fig. 6. FTIR spectra of PCL/NC-1262 blends in the 1600-1700 cm-' region, recorded at 298

K (A) and 353 K (B); numbers correspond to the NC percentage in blends.

1326 JUTIER ET AL.

whereas the one relating to the 1664 cm-' band remains constant as the concentration of PCL increases in the blend. This behavior is in good agreement with data published by Hubbell and Cooper20 on the same system. Similar spectral behavior is observed a t 353 K (Fig. 6B), which indicates a comparable phase behavior in the molten state.

In fact, there are two types of nitrate-ester groups on nitrocellulose: these groups can be attached to secondary or primary carbon atoms. In this study, the high frequency band at 1664 cm-' is assigned to the primary carbon asymmetric stretching mode of the nitrate-ester group and the shoulder, a t a lower frequency (1648 cm-'), to the secondary carbon asymmetric stretching mode of the 0-NO2 groups, this frequency difference reflecting the increased electron attracting power of the glucose ring. Increasing the PCL concentration in the blends favors dipole-dipole interactions (C=O ___ 02N-0-) with nitrate-ester groups that are probably attached to a secondary carbon. This might explain the decrease in the relative intensity of the low-frequency shoulder (1648 em-'). The primary carbon nitrate-ester group does not benefit from the electron attracting power of the glucose ring: dipole-dipole interactions are not favored, and the stability of the nitrate-ester environment leads to no significant displacement of the high-frequency band a t 1664 cm-'.

I780 1760 1740 1720 1700 FREQUENCY (cm")

Fig. 7. FTIR spectra of polyester/NC-1262 blends (30:70) in the 1690-1790 cm-' region, recorded at about 25OC above the melting point of the polyester.


Figure 7 shows the carbonyl stretching region corresponding to various 30 : 70 polyester/NC blends recorded a t about 25°C above the melting temperature of the polyester. FTIR spectra of NC blended with PVL, PEA, and PBA exhibit a shift of the carbonyl stretching band in the amorphous state combined with a shift of the stretching band assigned to hydrogen-bonded hydroxyl groups. These shifts imply intermolecular interactions between, on the one hand, PVL, PEA, or PBA and, on the other hand, NC that are comparable to those found with PCL/NC blends. In contrast, spectra of NC blended with PMPPL and PMEPL show no significant changes in both the hydroxyl and carbonyl stretching regions as a function of PMPPL or PMEPL concentration.

CONCLUSIONS

PCL/NC blends were shown to be miscible over their full composition range according to the Tg criterion. The iso-free-volume theory of Kovacs leads to a Tg versus composition diagram exhibiting a singular point a t a critical volume fraction t&. At volume fractions larger than 0.5, the Tg value of a blend of a given composition is smaller with NC-1342 than with NC-1262, which implies a greater mobility of the polymer segments. These results suggest that NC-1262 has a greater affinity and stronger interactions towards PCL than NC-1342. Similar results were obtained using PVL/NC, PEA/NC, and PBA/NC blends, whereas values of Tg as a function of composition for PMEPL/NC and PMPPL/NC blends are always close to those of the polyesters, which is indicative of immiscible systems.

The analysis in the molten state of the FTIR spectra of polyester/NC blends shows that the vibrational bands that are significantly perturbed as a function of composition are the carbonyl stretching band (about 1734 cm- '), the hydroxyl stretching region (3000-3700 cm- l), and the nitrate-ester stretching band (about 1650 cm-'). The carbonyl stretching band of PCL/NC blends broadens and shifts as a function of composition, indicating that the carbonyl groups of PCL are involved in an interaction with NC. Furthermore, spectra in the hydroxyl-stretching region of PCL/NC blends show the ex- istence of a band centered at 3450 cm-' that is related to hydroxyl groups hydrogen-bonded with PCL carbonyl groups. These infrared spectral observations in the hydroxyl and carbonyl stretching regions are entirely consistent with a miscible blend system, a t any composition, in agreement with DSC measurements. finally, adding PCL to NC results in the splitting of the asymmetric stretching mode of the nitrate-ester units, which is related to dipole-dipole interactions. Similar trends were obtained using NC- 1262 and NC-1342 blended with PVL, PEA, or PBA.

In contrast, a constant half-width of the carbonyl stretching band and free hydroxyl band (3575 cm-') is observed as a function of composition with PMEPL/NC and PMPPL/NC blends whatever their composition. These two complementary observations are interpreted as being indicative of a two-phase immiscible system in the melt, in agreement with the DSC Tg measurements reported above, and are common to all hydroxylated polymer blends reported immiscible. The immiscibility of PMEPL/NC and PMPPL/NC mixtures may be explained by steric hindrance between NC and PMEPL or PMPPL.

1328 JUTIER ET AL.

We have shown the presence of intermolecular interaction between the PCL carbonyl group and the NC hydroxyl group in PCL/NC blends because there is a shift of 17 cm-’ in the carbonyl stretching band and because the free hydroxyl absorption band is no longer visible in the blend. The average strength of this intermolecular interaction (Av = 125 cm-’) is stronger than the intramolecular interaction occurring between the hydroxyl groups in pure nitrocellulose (Av = 85 cm-’). In addition, the data indicate the presence of dipole-dipole interactions between the nitrate-ester groups of the nitrocellulose and the carbonyl groups of the polyester as illustrated by the splitting of the asymmetric stretching mode of the nitrate-ester units, which is correlated to a modification of the environment of the 0-NO, groups that are bonded to secondary carbon atoms. Similar results were obtained using PVL/NC, PEA/NC, and PBA/NC blends.

This study was supported by the Department of National Defense (Canada) (Contract No. 8SD82-00150). All nitrocellulose samples were provided by the Defense Research Establishment of Valcartier. The authors are grateful to the late Dr. Michel Asselin from the Defense Research Establishment of Valcartier for his collaboration throughout this project.

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Received August 7, 1987 Accepted November 4, 1987

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Miscibility of polyester/nitrocellulose blends: A DSC and FTIR study