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Self-Nucleation and Enhanced Nucleation of Polymers. Definition of a Convenient Calorimetric “Efficiency Scale” and Evaluation of Nucleating Additives in lsotactic Polypropylene (a! Phase)

B. FILLON,” B. LOTZ, A. THIERRY, and J. C. WITTMANN

lnstitut Charles Sadron (CRM-EAHP), CNRS-ULP Strasbourg 6, rue Boussingault 67083 Strasbourg Cbdex, France

SYNOPSIS

A simple, convenient and reliable calorimetric efficiency scale is proposed for the evaluation of nucleating additives for polymers. The scale is based on conventional differential scanning calorimetry cooling runs and makes use of a crystallization range determined in self-nucleation experiments. It can be correlated with spherulite sizes, and indicates the potential range of improvement of nucleating additives. Typical nucleating agents for isotactic polypropylene are evaluated; at best they rate at 60 to ca. 70% on this efficiency scale. 0 1993 John Wiley & Sons, Inc. Keywords: crystallization nucleation nucleating additives isotactic polypropylene

DSC

INTRODUCTION

Nucleating additives are used routinely in industrial practice to shorten injection-molding cycles and/or to impart improved optical and mechanical prop- erties to crystalline polymers by reducing spherulite sizes.’

The efficiency of nucleating additives usually is measured by their impact on global crystallization kinetics. Two different variables may be considered (1) the reduction of the crystallization half-time at some fixed temperature T, (the only variable is the nuclei concentration N since the growth rate G is constant) ( 2 ) the shift to higher temperatures of the crystallization exotherm when cooling the sample in a differential scanning calorimetry (DSC) experiment ( N and G are variables). In both cases, comparison is made with a “reference,” taken to be the “blank polymer, ” the polymer with no additives

* Present address: Centre de Recherches de Voreppe SA Groupe GM/ Interfaces-BP 27-38340 Voreppe, France. Journal of Polymer Science: Part B: Polymer Physics, Vol. 31,1395-1405 (1993) 0 1993 John Wiley & Sons, Inc. CCC 0887-6266/93/101395-11

but submitted to the same processing conditions as used to incorporate the nucleating agents (e.g., melt blending, etc.) .

These evaluations have, however, an intrinsic weakness as they use only a single reference, the blank polymer, which happens to be the lower limit (i.e., the worst one). A more satisfactory and telling evaluation would require to put the observed mod- ification imparted by the nucleation additive on a scale which requires to define also an upper limit, a goal that would correspond to the ideal improvement. Knowing the possible range of variation of the property ( T,, t1/2) the observed improvements (and potential further ones!) can be clearly defined.

Several authors have addressed the question of maximum limit of nucleation efficiency. In one of the earliest attempts, Beck’ “saturated” a polymer with the “best” nucleating agent he had found by using up to 25% nucleating agent concentration. He thus recorded an increase in T, which he took as the maximum achievable one. Whereas this procedure presumably ensures both high concentration and good dispersion of the additive in the polymer, it is still plagued by uncertainties as to the “quality” of interactions between polymer and additive.

1395

1396 FILLON ET AL.

Experiments on self-nucleation described pre- viously3 have shown that appropriate thermal treatments can create two extreme states in self- nucleated isotactic polypropylene (iPP) :

A blank polymer melt with only foreign particles left, among which the “active” ones induce heterogeneous nucleation. This state corresponds to the lower reference considered above.

A polymer melt wholly self-nucleated with polymer crystal fragments (potential crystallization nuclei) a t the highest achievable concentrations. The crystal fragments produced in the self-nucleation procedure satisfy the three criteria which characterize an ideal nucleating additive: ( i ) ideal (i.e., highest achievable) concentration; ( i i) ideal dispersion in the molten polymer, as they result from the break-up of the original polymer spherulite lamellae; ( iii ) ideal polymer-“substrate” interactions since, unlike the standard of reference used by Beck, the “substrates” (the nuclei) have the chemical constitution and crystal lattice of the polymer.

Within this framework and using isotactic polypropylene ( iPP) as a test material the purpose of this paper is threefold

1. To introduce an “efficiency coefficient” for nucleating additives based on the Tc scale determined in self-nucleation experiments

2. To establish the correspondence between the observed Tc’s and spherulite size; in other words, to translate the efficiency coefficients and scale into physically more telling variables: spherulite size or nuclei concentrations. This correspondence is in particular critical for high nuclei concentrations, where direct determination by optical means becomes in- operative. For this purpose, we will analyze the kinetics of anisothermal crystallization as performed in a classical DSC run and establish a correspondence with isothermal crystallization. Respective merits of nucleation efficiency evaluations based on isothermal and nonisothermal methods are then compared, and the validity of the proposed efficiency scale established

3. Finally, to evaluate several additives known to promote the crystallization of iPP.

EXPERIMENTAL

Materials

All experiments are performed with the same iPP sample of high (iso) tacticity as used in previous studies; it is produced by SNEA (PI France, ref. 3030 BN1 with Mw = 315 X lo3 and polydispersity 5.5.

Nucleating additives are mostly of commercial origin. Selection is based on several criteria: ( 1 ) ac- tual use in industry, ( 2 ) reported efficiency in pub- lished literature or patents, (3) anticipated efficiency based on their conformity with the selection rules introduced by Beck,2 (4) anticipated epitaxial interactions with the polymer, based on their crystal structure (note that only physical polymer-substrate interactions are considered here as opposed to possible chemical reactions between agent and some reactive polymers).

Sample Preparation

Concentration and dispersion are crucial issues in testing of nucleating agents. In general, a 1% concentration, common in industrial practice was used, since impact on crystallization kinetics, which is largest at low concentration, already levels off a t and beyond 1%.

Dispersion of the nucleating additive may be achieved in various ways. Since the number of spherulites produced is in the 109-10’2/cm3 range with 1% nucleating agent, the particle volume should be small ca. 0-0.01 pm3, respectively. Best dispersion is achieved only in rare cases: for DBS, the agent dissolves in the molten polymer and recrystallizes on cooling in the form of thin, twisted filaments4 on which the polymer subsequently crystallizes. For solid, infusible agents, the often used soaking of polymer powder is inappropriate. Indeed polymer powders have usually particle dimensions in the 100 pm range, whereas the final spherulite size is down to 1 pm. Soaking would thus result in gross local fluctuations, the nucleating agent being concen- trated at the polymer particles surfaces.

In the present study, dispersion of nucleating additives in the polymer is achieved by codissolution in p -xylene and lyophilization. Polymer concentration inp-xylene is 5%, dissolution is made by heating for 3 h at 120°C. When cooled, the solution produces a gel, which is lyophilized for 24 h; the resulting dry product is pressed in a vacuum mold at 200°C. This procedure is quite lengthy when compared with melt mixing (e.g., in a Brabender). However, it can be

ENHANCED NUCLEATION OF POLYMERS 1397

used to achieve good dispersion with modest amounts of material. In all cases, the final agent particule size is < 1 pm, as checked by phase contrast optical microscopy.

The blank polymer used as reference in the present study is virgin iPP submitted to the above dissolution, lyophilization, and molding procedure. This processing increases slightly the crystallization and self-nucleation domains defined by the limits Tcl and Tc2, and Tsl and Ts2 of domain 11, respectively.

Techniques

Experimental techniques are essentially as described previ~usly.~ Crystallization experiments are performed either under anisothermal or isothermal conditions, the latter by using an unconventional mode of the TADS temperature program of the DSC.

Optical microscopy was performed using both crossed polars and phase contrast illumination; the latter proved more appropriate for evaluation of particle size and dispersion of nucleation additives.

Determination of spherulite diameters in the range 1 to 10 pm usually can be made by small- angle light scattering ( SALS ). SALS is however ill adapted for a iPP spherulites since lamellar branching reduces considerably An, the difference between radial and tangential refractive indices, and therefore the SALS signal. For this reason, alter- native methods have been used to determine small spherulite sizes, namely optical microscopy (crossed polars ) and electron microscopy of etched spherulites. Figure 1 illustrates four different states of nucleation reached in our experiments.

RESULTS

This section is organized as follows: ( a ) the efficiency scale and efficiency coefficient for nucleating additives is introduced; ( b ) it is translated into a physically more telling variable, namely nuclei concentrations using a development of the Avrami equations due to Eder et al.5 Next, the correlation with isothermal experiments is established, and the most significant advantages of the efficiency coefficient are highlighted; ( c ) finally, the efficiencies of several nucleating additives of iPP are evaluated.

The Efficiency Scale and Efficiency Coefficient for Nucleating Additives

The blank melt and the melt self-nucleated to the saturation limit provide the two extreme situations

Figure 1. Spherulite sizes reached for various states of artificially enhanced nucleation or self-nucleation in iPP: ( a ) no nucleating agent, only heterogeneous nuclei active, T, = 114.5"C (blank polymer). This is the lower reference of the efficiency scale; ( b ) 1% of paratertiobutylbenzoic acid (efficiency: 20%); ( c ) 1% of 2-naphtoic acid (efficiency: 62%); ( d ) self-nucleatedpolymer, T, = 168OC; ( e ) self-nucleated polymer, T, = 166°C (upper reference of the efficiency scale). a-c: Optical micrographs, crossed polars. Scale bars: 100 pm. d and e: electron micrographs, replicas of sections treated with permanganic etchant, Pt- C shadowing. Scale bar: 5 pm.

needed to define an efficiency scale for any given polymer. The most straightforward variable char- acterizing the different nucleation states in the two samples is their crystallization temperature Tc2 on

1398 FILLON ET AL.

cooling at some fixed cooling rate (e.g., 10 K/min) in a DSC run. As shown in the preceding r e p ~ r t , ~ the blank (i.e., not self-nucleated) sample ( T, above Tsl) crystallizes at some low temperature T,, whereas a polymer self-nucleated to saturation crystallizes at some high temperature TC2,,,. The range of experimentally achievable possible nuclei concentrations in iPP can be characterized by and translated into the range of Tcz, (i.e., AT = T c ~ m a x

- TCz,i,) [obviously TcZmin = T,, (blank sample)]. Any polymer melt that incorporates “foreign”

nucleating agents (NA) crystallizes at a temperature T c N A located within the temperature range TcPmax to T,,: T c N A is higher than T,, if the nucleating agent has any efficiency at all; it is also most probably lower than TcSmax since, even if the concentration is at saturation and dispersion is ideal, the polymer- agent epitaxial interactions are usually less favorable than crystallographic ones implicit in self-nucleated samples. As a consequence, the improvement introduced by the nucleating agent may be expressed as AT,, = T,NA - T,, . Comparing AT,, to the maximum achievable improvement TcPmax - T c l , we define the nucleating efficiency NE as the following ratio:

The NE is here expressed as a percentage where 0 stands for no nucleating action and 100 for optimum efficiency. Compared with earlier methods of evaluation, based on the determination of T,NA and the temperature difference AT,, alone,’ the most significant improvement of the present scale rests in the introduction of an upper limit, expressed

In practice TcgmaX and T,, must be determined on the polymer used to test the nucleating agents, and processed in a similar manner (blank polymer). Processing may slightly modify the range of AT, and affect the lower and upper limits TcPmax and T,, determined for the “unprocessed” polymer (cf., the preceding paper). For example, lyophilization of the original iPP sample shifts its T,, and Tczmax from 112.5 and 137.4 to 114.5 and 136.1°C, respectively: the crystallization interval is thus 21.6”C. Once these polymer crystallization characteristics are established, T c N A is determined for nucleated polymers in the usual manner; namely by recording the crystallization peak in a simple DSC cooling run from T > T,,, at the standard 10 K/min cooling rate.

by Tczmax *

Evaluation of Nuclei Concentration

Spherulite sizes measured by optical microscopy help determine nuclei concentrations up to ca. lo8/ cm3. For spherulite sizes smaller than a few microns, analysis of the crystallization kinetics with the Avrami equation is useful. We recall here the essen- tials of this analysis for isothermal and anisothermal conditions.

1. The Avrami equation. The Avrami equation describes the overall progress of isothermal crystallization:

( 2 ) 1 - X t = exp(-Kt”)

with xt = crystalline fraction at time t , K = rate constant, n = Avrami exponent, which characterizes the nucleation process and geometry of growth.

The rate constant K can be deduced from the crystallization half time ( x = 4 ) :

(3)

K is proportional to the concentration of nuclei N. Assuming three dimensional growth and simulta- neous nucleation events (cf. later), it is:

4a K = - N G 3

3

and

1 4a N 3

- R 3

( 4 )

where G is the linear growth rate at T,, and R is the radius of spherulites.

This analysis applies for isothermal conditions, whereas the efficiency coefficients defined above are obtained under anisothermal conditions at 10 K / min cooling rate. Such anisothermal conditions have been considered by several authors. The analysis of Eder et aL5 is used extensively in what follows.

2. Nuclei concentration under anisother- ma1 conditions. Eder et aL5 define the thermal variation of growth rate G and nuclei concentration N b as:


and

where T, is a temperature of reference chosen in the temperature range where the analysis is applicable (ca. 105-150°C for iPP) .5

They further show that for a typical DSC run, the degree of crystallinity at the maximum of the crystallization peak (T,,,) is N 0.63. Since most of the material crystallizes near this temperature, they consider anisothermal crystallization to be well ap- proximated by isothermal crystallization at T,,, . Assuming again spherulitic, i.e. three dimensional growth, nuclei concentration N at the temperature of the DSC peak is given by:

where G ( T,,,! is the growth rate a t the peak temperature, and T the cooling rate. In our experiments, T = 10 K/min or 0.1667 K/s. 0 and p, the temperature coefficients for growth

rates and nucleation [cf. eqs. ( 6 ) and ( 7 ) ] are deduced from experiments made under isothermal conditions to be described later. Note however that, as pointed out by Eder et al.,5 N is determined mainly by the experimental value of T,, through the strong temperature dependence of G ( T ) .

The formulae derived by Eder et al. make it possible to correlate the peak temperatures T,, with the concentration of nuclei N. Results are presented in

Table I (and Fig. 5, cf. later) : in the T, interval of our anisothermal experiments (ca. 114 to ca. 138°C) N varies from %lo6 to more than 10l2; in other words, spherulite diameters decrease from several tens to less than one micron.

Note that the is0 and anisothermal crystallization analyses are based on predetermined nucleation and three dimensional growth. Predetermined nucleation is applicable in the self-nucleated and artificially nucleated samples. Analysis of crystallization peaks on cooling in the DSC by the method of Ozawa' yields indeed Avrami coefficients of 3 when the peak T, 2 115°C. Also, uniform distribution of nuclei in the sample is assumed. Whereas generally valid, this hypothesis may not apply for high T,, when the "memory effect" sets in. If, for example, the memory effect were to reflect the existence of elongated fragments of lamellae which act as strings of nuclei, growth would rapidly turn from three to two dimensional (to form cylinders with the string of nuclei a t their center). This would result in slow- down of overall crystallization rate and shift of the T, peak to lower temperatures, as is indeed observed at the onset of the memory effect (cf. Fig. 3b). Thus, use of eq. (8) amounts (but only in this small, high temperature crystallization range) to calculate an "equivalent concentration of nuclei" if growth were three dimensional and nuclei uniformly distributed.

Correlation of calculated spherulite dimensions with observed ones can be made over the whole T, range by optical and electron microscopy on sections of the sample crystallized in the DSC pan (Fig. 1 ) . The overall agreement is satisfactory considering

Table I. Nuclei Concentrations N and Average Spherulite Diameter @ as a Function of T, (Cooling Rate: 10 K/min)

N (nu~ le i . cm-~)~ ; N (nu~lei.cm-~)"; N (nu~le i .cm-~)~; T c 2 lo3 G" @ (Irm) @ (w-4 @ (w4

113 118 123 128 133 138 143e 148"

4.7 9.1 3.3 1.23 0.45 0.166 0.06 0.028

7.2.105; 138 1.4.107; 51 3.10'; 18 5.8.10'; 7 1.2.10"; 2.5 2.3.10"; 0.9 4.8.1013; 0.34 4.9.1014; 0.15

1.85.106; 101 3.6.107; 37 7.8.10'; 13.5 1.5.10"; 5 3.10"; 1.8 6.10"; 0.7 1.23.1014; 0.25 1.27.1015; 0.11

6.63.105; 142 1.3.107; 52 2.8.1OS; 19 5.4.10'; 7 1.1.10''; 2.6 2.1.10'*; 0.95 4.4.1013; 0.35 4.5.1014; 0.16

* Growth rates are suitably represented by: In G ( T ) = 10.116-0.20 (T-113) (G in cm/s). N and 9 calculated using eqs. (8) and (5). Constants given by Eder et al?: p = 0.196 K-' (measurements from Linz University,

cf. ref. 5) = 0.030 K;' (value taken from work by Van Krevelen, cf. ref. 5). ' Constants p and p determined in the presentwork p = 0.2083 K-l (cf. Fig. 2) = 0.1439 K-' (cf. Fig. 4, curve a).

p = 0.2083 K-', as determined from Fig. 2, p set to be = 0. (Its determination is a lengthy procedure (cf. Fig. 4) and does not

Crystallization peaks not recorded, located outside the presently accessible range. significantly affect the results.)

1400 FILLON ET AL.

that we deal with orders of magnitude in N o r factors of 2 in spherulite dimensions. In particular, crystallization at the highest T, range yields spherulite diameters smaller than 1 pm, possibly 0.5 pm, and are therefore compatible with nuclei concentrations in the 10'2-10'3/cm3 range.

3. Nuclei concentration and efficiency scale under isothermal conditions. The above analysis under anisothermal conditions makes use of several results obtained under isothermal conditions. The present section presents these results, points out some of their limitations and correlates the efficiency scale with more classical evaluations.

The variation of growth rate G with temperature is measured by optical microscopy on unnucleated samples. The resulting curve Log G = f ( T,) (Fig. 2) displays the by now familiar, characteristic change in slope at 138OC (arrowed) which has been associated by several authors with a change in growth regimes, 7,8 although this notion has recently been ~hallenged.~ The value of p [ eqs. (6) and ( 8 ) ] used in Table I refers to the temperature range below 138°C since our anisothermal experiments yield crystallization peak temperatures below the break of the log G = f ( T ) curve (Fig. 2 ) .

The nuclei concentrations N are determined for a number of nucleated, blank and self-nucleated samples by analysis of isothermal crystallization kinetics performed in the DSC. Figure 3a shows the schematic form of the typical output of such isothermal crystallization runs. Integration of the curves yields the familiar crystallinity (or percentage of conversion) versus log t curves. Six such curves are displayed in Figures 3b and c respectively. Note that two different T;s need be used ( 132 and 138°C) in order to keep the whole crystallization process within reasonable time limits given the very different nucleation activities involved. Experimental con- straints are set a t short times by time lag needed to achieve isothermal conditions in the DSC and at long times by prohibitive immobilization of the DSC apparatus.

Figure 4 gathers the most significant results of this study. It represents the variation of log N with T, for a blank iPP melted at T, > 170°C (curve a ) , for several samples with nucleating agents (curves b to g ) and for two self-nucleated samples after partial melting at T, = 166.5 and 166"C, that is the second best and best self-nucleated samples (curves h and i, respectively).

Analysis of Figure 4 reveals a number of features, as it can be read according to different criteria:

Figure 2. Variation of Ln of growth rate G with temperature as determined in isothermal experiments. Break in the curve arrowed. The lower range ( T < 138°C) only needs be considered in anisothermal experiments.

For self-nucleated samples, the range of N ex- tends from 1: l o6 when T, > 170"C, the blank sample (this is the concentration of heterogeneous nuclei that survive on melting) to lo1' for the wholly self-nucleated sample (T , = 166°C). Self-nucleation thus increases the concentration of nuclei by a factor of 106

In the isothermal T, interval explored for any one sample, a small but perceptible decrease of N is noted with temperature. Curve a (for T, 2 170°C) has been used to determine p [eqs. (7) and (8) ; Table I] . Note that similar variations are observed for the various nucleating additives. However, p is larger for self-nucleated samples which display the memory effect (0.43 and 0.55 K-' for curves h and i, T, = 166.5 and 166OC, respectively)

Most and least efficient nucleating agents are close to the upper right and lower left corner of the graph, respectively. The comparison becomes more telling when efficiencies are compared at fixed N , and fixed T,:

For a fixed concentration of active nuclei, crystallization temperatures may differ by over 10°C : 10' nuclei initiate crystallization at 142 and 132°C for curves d and c, respectively. Comparison of different nucleating agents is however quite restricted given the small variation of N with temperature

For a fixed (isothermal) crystallization temperature, comparison is again limited to a small number of samples and nucleating agents: only samples c and b can be compared to the blank (nonnucleated) one at T, = 132°C (cf. Fig. 3b), three nucleating agents can be compared at T, = 138°C (cf. Fig. 3c).

When using the criterion of either is0 N or is0 T,, the representation of log N = f ( T,) illustrated


b

a

C Figure 3. (a) Schematic representation of the output of an isothermal crystallization experiment performed in the DSC. At corresponds to the time needed to reach thermal equilibrium; t lIz is the crystallization half time used to determine K and N [ eqs. ( 4 ) and (5)]. (b, c ) Isothermal crystallization kinetics at 132°C (b) and 138°C ( c ) of a blank (curve a ) and of various artificially nucleated iPPs (curves b to f, cf. legend to Fig. 4 for the nature of the additives).

in Figure 4 for isothermal crystallization appears ill adapted for a meaningful comparison of different nucleating agents, given the very small range of overlap. It is however possible to link, on this dia- gram, all experiments which resulted in equal crystallization half times. The corresponding broken lines have been drawn for is0 tl12 = 4, 6, and (although with less accuracy based on two experimental points only) 15 min. Since these “is0 t l Iz7’ lines cross all the curves of Figure 4, it becomes possible to define “is0 t l lz” efficiency factors for all samples. For is0 tl12 = 4 minutes, maximum and minimum efficiencies are represented by points A and C in Figure 4. Efficiency of the nucleating agent giving

rise to curve g would thus be (if expressed as a percentage) 100 AB/AC.

The criterion of is0 t l l z used to draw the broken lines in Figure 4 and to determine the “isothermal” efficiency coefficient (ratio ABIAC) is an “So K” or “is0 NG3” criterion [cf. eqs. ( 3 ) and (4)]. As such it covers the whole T, range for any tl12 or K since increase of N is compensated by decrease of linear growth rate G (i.e., crystallization at higher temperature ) and reciprocally.

4. Correlation of anisothermal and isothermal crystallization. As indicated, anisothermal crystallization conditions can be approxi-

1402 FILLON ET AL.

I ,- I

1 I

150 TC(OC) 125 130 135 140 145

Figure 4. Variation of nuclei concentration (per cm3) as a function of isothermal crystallization temperature T,. Each data point has been determined from an experiment as in Figure 3. Curves a through i refer to self- nucleated and artificially nucleated samples: “blank” iPP (curve a, no nucleating additive, high temperature melting prior to crystallization); with various additives at 1% concentration: ( b ) paratertiobutylbenzoic acid, ( c ) a mixture of dibenzylidene-sorbitol (DBS) and benzoic acid, ( d ) diphenylglycine (DPG) , ( e ) M888 ( a propriatory nucleating agent), ( f ) DBS-benzoic acid-DPG mixture, (g) 4-biphenyl carboxylic acid; iPP after self-nucleation at T, = 166.5”C (h ) , and a t 166°C. ( i ) Oblique broken lines link experimental points corresponding to crystallization times of 4, 6, and 15 min. Nuclei concentrations are determined using eqs. (31, (4 1, and ( 5) . Note the slope of curve ( a ) used to determine f l in eq. ( 7 ) , its near constancy through curve g and decrease for high T, range (curves h and i ) . “Isothermal efficiency coefficient” for nucleating agent giving rise to curve g is given by the ratio AB/AC (see text).

mated with isothermal crystallization at the peak temperature. Given the cooling rate used (10 K / min) and the width of the peak (ca. 10°C), the anisothermal efficiency coefficient amounts to an “is0 t,I2 = 30 seconds” criterion performed under isothermal crystallization conditions, if such fast isothermal crystallizations were permitted by the thermal inertia of the DSC.

The correspondence between isothermal and anisothermal crystallization is illustrated in Figure 5 which includes, in schematic form, the data of Figure 4: curves a, b bo g, and h and i correspond to blank, nucleated with different nucleating agents, and self- nucleated iPP, respectively. The peak temperatures ( shaded squares determined for the same samples in the DSC at a 10 K/min cooling rate from T > 170°C (i.e., under anisothermal conditions) have been drawn on the extrapolations of lines a to i although neither N nor $ were determined directly. This procedure is questionable for the two upper points, as the linear extrapolation used assumes that the sharp high temperature variation of nuclei concentration (i.e., large negative slopes of curves h and i ) also applies at lower temperature, whereas curves a-g indicate a smaller variation.

The bold line represents the calculated nuclei concentrations using eq. (8). It is parallel to curves of iso-4, 6 and 15 minutes (bold dotted lines) and fits with anisothermal peak temperatures (squares) (except the two upper points for reasons just dis- cussed). Except for small variations which appear

1 1 5 1 2 0 1 2 5 1 3 0 1 3 5 1 4 0 1 4 5 1 5 0

Tc (“C)

Figure 5. Correspondence between isothermal and anisothermal crystallization conditions. The figure shows (in schematic form) data of Fig. 4 and includes crystallization maxima for DSC cooling runs at 10 K/min (anisothermal t l I z = 30 s, squares). The corresponding data points have been drawn on linear extrapolations of curves a to i. Nuclei concentrations (bold curve) calculated from eq. (8) with parameters from Table I, and translated into average spherulite diameters using eq. ( 5 ) .


to be beyond experimental scatter (e.g., curve c ) , anisothermal and isothermal efficiency scales are therefore identical.

From the above analysis, it appears that determination of nucleating efficiency with a scale based on straightforward cooling runs in DSC is simple, fast, reliable and has a clear physical meaning. It is therefore possible to use these anisothermal conditions to evaluate the nucleating additives of iPP.

Evaluation of Nucleating Additives of lsotactic Polypropylene

A wide variety of chemical compounds have been tested as potential nucleating additives for isotactic polypropylene: Beck and Ledbetter’ report having tested “over 240 compounds representing many different classes of both organic and inorganic compounds” by measuring shifts in peak crystallization temperatures in DSC runs (in our notation AT = TcNA - T c l ) ; Binsbergen” has investigated more than 2000 compounds as nucleating additives for PE and iPP.

Out of a number of T;s and efficiency coefficients determined in our investigation, we present in Table I1 some additives, listed in order of decreasing efficiency. The T;s are in good agreement with those reported for the various agents, when available. The usefulness and generality of the proposed scale are immediately apparent since only percentages and no absolute values of T, or AT are needed. Further, the margin of improvement for any additive relative to the best additive, and of the latter relative to the “ideal” nucleating agent is easily evaluated: for example, the best nucleating agents commonly used in industrial practice are only rated at 40 to 60% on the present scale. Dibenzylidene sorbitol, which im-

parts significant clarity to iPP, is in the 40% range, a figure which changes only slightly with concentration. Conversely, an unexpected outcome of the present analysis rests in the very poor efficiencies of some additives that have been specifically pat- ented as nucleating additives for isotactic polypropylene. These poor efficiencies are not related, for infusible agents, to possible coarseness of particule size, which from optical microscopy is in the 1 pm range.

DISCUSSION

Table 11. Efficiencies of Various Nucleating Additives of iPP

As a result of partial melting and recrystallization experiments in the DSC, the present study makes it possible to reach the highest achievable concentration of crystallographically ideal nuclei in the polymer melt via self-nucleation. Our experiments set the upper, practical limit of N and of T,, and define an upper boundary to the crystallization domain. They make it possible to express modifications to crystallization kinetics induced by nucleating additives as a fraction or a percentage of a scale.

Although the proposed efficiency scale has several practical advantages it is not an absolute one. It requires standardization of certain experimental variables, in particular the cooling rate, and must be calibrated for a given polymer sample since the crystallization range depends on experimental procedures and sample characteristics which affect nucleation and growth. In particular the lower T,, limit is clearly a material characteristic which depends on the presence in most iPPs, of N l o6 nucleating “im- purities” per cubic centimeter.”

The high temperature “saturation” limit of Tc2 is also well defined for the 10K/min cooling rate

Additive and concentration T,,, additive T,iPP

(“C) (“0 % Efficiency

4-Biphenyl carboxylic acid, 2% 2 Naphtoic acid, 1% Nicotinic acid, 1% Thymin, 1% DBS (dibenzylidene sorbitol), 0.4% 9-Anthracene carboxylic acid, 1% Talc, 1% Sodium benzoate, 1% 9-Fluorene carboxylic acid, 1% 2-Hydroxy 3-naphtoic acid, 1% 2,4-(bis Buty1amine)G-hydroxy, 1,3,5 triazine, 1%

225 185 239 317

214

>300 230 220

210-220

-

-

128.8 127.8 125.7 125.3 123.2 122.7 121.4 121.2 118.4 116 115.5

66 62 52 50 41 38 32 31 18

7 5

1404 FILLON ET AL.

used. Reduction of T would increase T, and the measurable concentration of nuclei. As established by Eder et al.,5 variation in T, and active nuclei are:

n m Y 1 ‘2 TmaX(-Pl) - Tmax( -T2) = -In T and (9)

3 P + 6 T1

Thus, use of an unrealistic 1K/min cooling rate in- duces a +9”C shift in T, but makes it possible to measure only a 4.5-fold increase in nuclei concentration.

At the selected Tof 10 K/min and for T, = 138OC, N is of the order of 10 13. If N were to reach T, would be N 145”C, but the volume of the nuclei would become significant. If nuclei are N (40 nm) in size, 1015 nuclei would represent ca. 6.4% of the material and become detectable on remelting in the form of a small additional high temperature melting peak. Such a sample would be outside our self-nucleation range, which was precisely defined as not showing such additional peaks due to annealing ef- fects.

From an experimental standpoint, observation of this additional melting peak is a convenient criterion. Samples with T,2 of 139.2 rather than 138.5”C display a prominent high temperature melting peak: they are clearly in domain I11 or annealing range defined in ref. 3.

Both high and low temperature boundaries of the crystallization range used to define the efficiency scale are thus rather well defined. We note however that:

If T;s were observed above TcZmaxr the influence of other processes, such as chemical modifi- cation of the polymer and/or chain scission as occur in chemical nucleation l2 would be likely

Crystallization peaks below the TcPmin = TCl limit would be indicative of “antinucleant” activity, in particular desactivation of the heterogeneous nuclei present in the sample. Such antinucleant activity is well documented in the case of for example aliphatic polyesters and polyamides doped with small amounts ( a few % ) of p01yvinylbutyral.l~

CONCLUSION

Investigations on self-nucleation of crystalline polymers3 make it possible to determine the behav-

iour on crystallization of a polymer that is either nonnucleated or “ideally” nucleated and thus to determine the two limits of the experimentally accessible range of crystallization behavior. Therefore, they provide the necessary framework to evaluate artificially enhanced nucleation (i.e., the impact of nucleating additives on the kinetics of crystallization). The major improvement over previous evaluation methods rests in the definition of the ideally nucleated polymer, a reference state that was not considered so far.

The experimental variable used to characterize the crystallization behavior of self- or artificially nucleated polymers is the position of the crystallization peak on cooling at 10 K/min in the DSC. This is approximately an “is0 NG3” or ‘ ‘ i ~ o - t ~ / ~ ” criterion with the crystallization half-time t l / 2 set at 30 s. This evaluation method is preferred over all other ones (notably over more tedious isothermal crystallization) because it covers the whole range of nuclei concentrations.

Improvement in overall crystallization rate, ma- terialized by the increased crystallization temperature T c ~ A recorded for an artificially nucleated polymer over T,, or TcZmin of the blank (non-nucleated) polymer can then be compared to the accessible range, and expressed as a percentage of that range: the efficiency coefficients of widely different nucleating agents can be established and the margin for further improvements quantified. As an illustration, the best nucleating additives available for iPP have been shown to have efficiencies in the 50-60% range, i.e. they are only half way from ideality.

Further analysis of the data taking into account the variation of growth rate G with T, makes it possible to translate the efficiency scale into physically more telling variables: nuclei concentrations and spherulite sizes. The DSC evaluation of nuclei concentration saturates in the range 10’3-10’4 ~ m - ~ , near 100% efficiency, which is well beyond presently recorded efficiencies for nucleating additives.

The proposed calorimetric efficiency scale for nucleating additives appears therefore as a simple, convenient and reliable tool: efficiencies are expressed as percentages, correspondence with nuclei concentrations or spherulite sizes is easy, experimental procedures are standard (but were used so far only to determine A T = T c ~ ~ - Tcl) , conventional DSC cooling runs are used to calibrate the efficiency scale (Tcl to TcPmax range) which once established, becomes a new and permanent sample characteristic. The efficiency scale is ideally suited for fast crys- tallizing polymers and has already been applied, be- sides iPP, to polyethylene, poly (vinylidene fluoride) and aliphatic p01yamides.l~


We thank C. Naud, A. Schierer and C. StraupC. for tech- nical help. We acknowledge the constant interest of our industrial partners, Dr. G. Meynier and the late Dr. D. Ranceze (GRL, Lacq, France) and generous financial sup- port of ELF-ATOCHEM.

REFERENCES AND NOTES

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Received August 26, 1992 Revised March 9, 1993 Accepted March 17, 1993

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