Fibre Chemistry, Vol. 28. No. 4. 1996

MELTING

OF BINARY MIXTURES

OF LOW-DENSITY

POLYETHYLENE AND ALKYLBENZENES

UDC 678.742.23:536. 421.1:678.042

L. N. Mizerovsldi, V. V. Afanas'eva, and N. I. Lytkina

Regardless of the dependence on the LDPE-alkylbenzene binar>' s)'stem molecular-weight component of up systems at temperatures below their of thermodynamic affinity. The heat

thermal histor 3' and structure of the all,3'lbenzene, the melting point of the is a linear hmction of the composition for a concentration of the lowto O. 9-0.95 mass fraction. Annealing of LDPE -- all,3'lbenzene binar3' melting point leads to differentiation of solvents with respect to the level of fusion of LDPE cr)'stals is 293-+15 J/g.

For several years, the Laboratory of Physical Chemistry of Polymer Solutions at the Institute of Chemistry of Nonaqueous Solutions, Russian Academy of Sciences, and the Department of Chemical Fibre Technology at the lvanovo State Academy of Chemical Engineering have been concerned with the problem of formation of the porous fibrillar structure of synthetic leather (SL) on a nonwoven base of bicomponent fibres of the matrix-fibril type by the method of selective extraction. One successful version of the bicomponent fibre developed for these purposes at the All-Russian Scientific-Research Institute of Synthetic Fibres (VNIISV) is a fibre spun from a mixture of low-density polyethylene (LDPE) and polyethylene terephthalate, where LDPE plays the role of the matrix component and is relatively easily extracted from the semifinished SL with alkylbenzenes. In the plan for developing the physicochemical principles of extraction of LDPE and regeneration of the extraction solutions in the Laboratory and Department, systematic studies of phase equilibrium in L D P E - s o l v e n t systems were begun, and the first results were published very recently [1, 2]. In particular, DSC showed [2] that LDPE planned for use as the fibre matrix component forms one or more calorimetrically different crystal structures as a function of the crystallization conditions, and the presence of a thermodynamically active liquid (ethylbenzene) in a LDPE melt can both cause and prevent their formation [1]. The results of a study to determine the effect of the structure of alkylbenzene and the conditions of crystallization of the LDPE -- alkylbenzene binary system on its melting point are reported here. As previously in [1, 2], brand 12203-320 LDPE with a melt index of 26.5 +3.0 g/10 min at 463 K, density at 298 K of 911.2 g/cm 3, and degree of branching of - 2 . 6 CH 3 groups per 100 carbon atoms in the main chain was used. A crystalline phase whose thermogram has one broad asymmetric peak of melting with a maximum at 381.0+0.8 K, transition of the melting peak to the base line at 385.7+0.7 K, and specific (on conversion to I g of polymer) heat of fusion of 59.0_+2.0 J/g, is formed in rapid cooling of a melt of this polymer. After prolonged step annealing [2], the polymer is characterized by a thermogram with three symmetric melting peaks with maxima at 343, 368, and 385 K and a specific heat of fusion of 10.7+_2.4, 25.3+1.0, and 54.3_+1.5 J/g, respectively. The high-temperature peak goes to the base line at 387.0-+0.7 K. The degree of purity of the alkylbenzenes used corresponded to cp and analytically pure grades. The measurements were performed on a DSM-2M differential scanning microcalorimeter with the method described in previous communications [1, 21.

Ivanovo State Academy of Chemical Engineering, Institute of Chemistry of Nonaqueous Solutions, Russian Academy of Sciences. Translated from Khimicheskie Volokna, No. 4, pp. 18-21, July-August, 1996. 228

0015-0541/96/2804-0228515.00

9

Plenum Publishing Corporation

T m, K

Tm-1"103, K-I

;q"' 0,1

0,3

0,5

0,7

0,90A,

Fig. I. Melting ( o ) and crystallization ( ' ) curves of LDPE in the presence of toluene, obtained by the cloud point method in the coordinates of Eqs. (1) and (2).

The cloud point method with a temperature elevation (reduction) rate of - 5 K-h- 1 and visual control of the turbidity of the system were used for plotting the melting and crystallization curves of the binary systems (samples with a volume of - 4 0 cm3). The temperature of the appearance of opalescence in the system was used as the crystallization temperature (Tcr), and the temperature of the disappearance of opalescence was used as the melting point (Tin)According to [I], the melting curves plotted with the temperatures of the maximum and end of the melting peak of the LDPE-ethylbenzene system obey the following equation in a wide (from 0.05 to 0.95 mass fraction of the alkylbenzene) range of ratios of components Tm= T~ - acol,

(1)

where w I is the mass fraction of the low-molecular-weight component; c~ is an empirical constant. Further studies showed that this dependence is satisfied (correlation coefficient r = 0.997-0.999) for other alkylbenzenes as well, and this concerns both the melting curves and the crystallization curves. The difference only consists of the fact that the linearity is preserved in the latter case in a slightly narrower range of compositions of the system, as Fig. 1, which indicates the results of studying the L D P E - t o l u e n e system, shows. The melting and crystallization curves of these systems are also rigorously (r --- 0.999) described (Fig. 1) by the, equation T-'m = (~m) -I + [~1,

(2)

which is a special case of the well-known Flory relation [3, p. 49] T-,m=(Zo

)_,+

RV2 ( ~ t - X ~ ) . ~3/-/f V,

(3)

where V1 and V2 are the molar volumes of liquid and repeating unit of the polymer macromolecule, respectively; ,SHf is the heat of fusion of one mole of crystallized units of the polymer macromolecule; ~1 is the volume fraction of liquid in the system; Xl is the F l o r y - H u g g i n s parameter of interaction of the polymer and liquid. It is evidently possible to go from Eq. (3) to Eq. (2) if Xl = 0, and m, = mlp2pT',

(4)

where Pl and ,o2 are the density of the liquid and polymer, respectively.*

*If the values of Ol and P2 are close, then Eq. (4) is satisfied with good precision. 229

TABLE I. Results of Statistical Processing of Melting and Crystallization Carves of L D P E - A l k y l b e n z e n e Systems Obtained with the Cloud Point Method Melting curve

Alkylbenzene

Crystallization curve

T~m, K

~'I05, K-'

Toluene

383, I +0,4

28,9:t:1,2

312.t: 12

381,0-i0,4

29,8+ 1,2

303:1:12

m-Xylene

383,6:t:0,4 381,3:1:0,4

27,2.t: 1,5 25,6:t: 1,9

288+ 15 306:1:21

381,0-1:0,4 379,2._+0,4

29, 3:t:1,8 29,5:t: I, 3

267:t: 15 265:t:1 I

382, I :t0,4 382,3:t0,4

26,2:t: 1,3 23,0:t 1,3

299t: 14 340-t:18

379,2i-0,4 381,0i-0,4

29,5:1: I, 3 27,2.-t 1.5

265+ 12 288+15

o-Xylene p-Xylene Ethylbenzene

[

A H I . J_/g

I

J'~m,K

[

~3.105,K -I

t

~Hnn, J/g

T,H

F

_j-

,~eo t-

JSO~ &'~

j

0,2

~ 0,~

,

, 0,8 6J2

0,6

Fig. 2. Melting curve of LDPE in the presence of tert-butylbenzene obtained with the cloud point method (") and DSC ( o )

O,7f

0,07

•

0,~5 O, 30 J

~ i , i0'07~ J23 J4J J6J JSJ R JJJ JNJ JTJ Fig. 3. Thermograms of melting of LDPE--tert-butylbenzene systems annealed according to the third regime. Figure on the curves: mass fraction of solvent in the system.

The following relation is obtained in the same condition 7_ I

m = ( ~ m ) -I +

RM2

AHfMt

"1 =(7"0.0 -~ + Ro3t a ~ M, )

(5)

where A H * m has the dimensionality of J per gram of crystallized repeating units; M I and M 2 are the molar mass of liquid and polymer repeating unit. However, it is necessary to note that the condition Xl = 0, from our point of view, does not average equality of the thermodynamic affinity of the liquids for the polymer, in whose presence the polymer is crystallized according to Eq. (5), since not the repeating unit of the macromolecule, but its kinetic segment, whose size is a function of the thermodynamic quality

230

TABLE

2.

Results

of

Statistical

Processing

of

Generalized Melting Curves of LDPE--Alkylbenzene Systems with Eq. (5) Alkylbenzene

i /~m,K

10S'K-I i

I;

I

r

AH'f .

........ i J/g

"riethylenebenzene Toluene Ethylbenzene m-Xylene tert-Butylbenzene

384,0+ 1,0

18, 1:1:3,I

0,968 283_+40

384,3:t0,6 384,2:t0,8 385,8+1,3 385,0:1:0,2

33.2+1,9 29,4:1:1,8 31,0:t:4,0 25,9:t0,8

0,991 272+15 0,992 266_+15 0,907 253_+20 0,997 239_+7

p-Xylene p-Butyltoluene

387,0_+0,9 33,8+_2,2 0,988 232_+14 ,i 387,1_+0,7 24.2:1:2,0 0,989 232-+18

o-Xylene

388,5-+1,4 35,4_+3,5 .0'970. _ 221_+20 !

']

T~I.IOJ,~ "'

0,2

0, ~

0,6

o,8

0t

Fig. 4. Generalized melting curve of LDPE in the presence of p-xylene after annealing according to the first (.), second (A), and third ( o ) regime.

of the low-molecular-weight component, is actually the kinetic unit that participates in thermal motion in the binary homogeneous polymer-liquid system, and for this reason, the AH*f calculated from the slope of the melting or crystallization curves should in the general case be a function of the structure of the liquid added to the polymer melt. The comparison of Eqs. (1 and (5) leads to the expression Tm = 7"~

a

TOmTmCO,,

(6)

zSH'f MI which reveals the sense of coefficient o~ in Eq. (1) and predicts the possibility of linearization of the melting curve in coordinates of Tm = f(Tm~ol). The results of statistical processing of the melting and crystallization curves of LDPE in the presence of five alkylbenzenes obtained by the cloud point method showed that within the limits of the experimental precision, Eqs. (5) and (6) lead to the same values of parameter/3, and the T~ and T~ obtained by extrapolation to ~l = 0 are virtually independent of the type of equation used. The average values of these quantities and the results of calculation of AH*f are summarized in Table 1. It is not difficult to see that although the differences between the values of T~ and T~ obtained by extrapolation of the melting and crystallization curves of LDPE in the presence of these alkylbenzenes are greater than the error of their calculation in many cases, the lack of a system in these differences suggests the practical independence of T~ and Tocr obtained with the cloud point method of the structure of the alkylbenzene added to the LDPE melt and allows setting them respectively equal to 382.5+0.7 and 380.3+0.8 K. The correctness of this approximation is particularly confirmed by the fact that statistical processing of the experimental data with consideration of these values of T~ and T~ results in values of AH*f equal to those reported in Table 1 within the limits of the precision of determination.

231

The average value of AH*f is equal to 293+ 17 Jig and is very close to the published [3, p. 53] value of Tm of polyethylene crystals of 286+ 12 Jig, determined with the dependence of its melting point on the concentration of ethyl benzoate, o-nitrotoluene, Tetralin, and t~-chloronaphthalene. Since the melting point of the binary mixtures determined by the cloud point method and the temperature corresponding to the end of the melting peak in the DSC method have a similar physical meaning, reflecting the melting point of the last crystal, it was possible to predict that the melting curves plotted with these data would also coincide. In Fig. 2, which shows the results of studying the LDPE--tert-butylbenzene system, note that this hypothesis is actually fulfilled. This forms the basis for suggesting that at least in our experimental conditions, a 100-fold (from 5 K'h -1 in the cloud point method to 8 K.min -1 in the DSC method) in~:rease in the temperature elevation rate has almost no effect on the melting regime of LDPE crystals in the presence of an alkylbenzene. To determine how much the temperature regime of crystallization of LDPE in the presence of a solvent affects the position of the melting curve, eight binary LDPE--alkylbenzene systems containing from 0.05 to 0.90 mass fractions of lowmolecular-weight component and crystallized in the following three regimes were investigated by DSC: 1) cooling of the melt from 383 to 353 K at the rate of - 5 K.h - I , annealing at 353 K for 30 days, and rapid cooling to 298 K; 2) cooling of the melt from 383 to 333 K at the rate of - 5 K.h - l , annealing at 333 K for 30 days, and rapid cooling to 298 K; 3) cooling of the melt from 383 to 298 K at the rate of - 10 K.min - t , annealing at 353 K for 10 days, decreasing the temperature to 333 K, holding at this temperature for 10 days, and rapid cooling to 298 K (optimum annealing regime for pure LDPE [2]). In total agreement with the previously published data [1], the thermograms of these samples differed by the number and position of the melting peaks on the temperature axis. The thermograms of the LDPE--tert-butylbenzene binary system crystallized according to the third regime are shown in Fig. 3 as an example. The generalized melting curve of another (LDPE--p-xylene) system in the coordinates of Eq. (5) plotted with the temperatures of transition of the last melting peak to the base line in different crystallization regimes is shown in Fig. 4, and the results of statistical processing of the melting curves of all eight systems studied are summarized in Table 2. Note that the Tm of mixtures undergoing crystallization in the different regimes are relatively closely clustered around one line, and the correlation coefficients, although lower than in processing of the melting curves obtained in samples prepared in the same regime, have values suggesting the presence of one functional dependence between the melting point of the last crystal and the composition of the LDPE--alkylbenzene system outside of the dependence on its crystallization regime. The results reported m Table 2 also indicate that annealing of the crystallized binary polymer-liquid system per se differentiates the solvents by their thermodynamic activity. In particular, considering the values of AH*fobtained for the annealed samples, the series of alkylbenzenes studied can probably be ranked in the order of decreasing thermodynamic activity as follows: triethylbenzene = toluene = ethylbenzene > m-xylene > tert-butylbenzene = p-xylene = p-butyltoluene > o-xylene.* The alkylbenzenes are in almost the same order in comparing the values of TOm. The comparison of these values with the temperature of the end of the melting peak of the starting (385.7 + 0.7 K) and annealed (387.0+ 0.7) pure LDPE suggests that the first five alkylbenzenes indicated in Table 2 prevent formation of perfect crystals of this polymer; p-xylene and pbutyltoluene do not affect crystallization of LDPE in this sense, while o-xylene favors the formation of such crystals. In comparing the T~ in Table 1 from the same angle of view, we can conclude that in relatively rapid cooling of the binary systems, the alkylbenzenes studied prevent the formation of perfect crystals of LDPE.

REFERENCES 1.

2. 3.

L. N. Mizerovsky and V. V. Afanasyeva, J. Therm. Anal., 45, 1217-1222 (1995). L. N. Mizerovskii and V. V. Afanas'eva, Zh. Fiz. Khim., 70, No. 1, 169-170 (1996). L. Mandelkern, Crystallization of Polymers, McGraw-Hill, New York (1964).

*In the given case, it is assumed that with an increase in the thermodynamic affinity of a nonpolar liquid for a nonpolar polymer, the kinetic segment of the polymer will decrease, and the AH* t. calculated from the slope of the melting curve will on the contrary increase. 232