self-nucleation and enhanced nucleation of polymers

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Self-Nucleation and Enhanced Nucleation of Polymers

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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-nu- cleation experiments. It can be correlated with spherulite sizes, and indicates the potential range of improvement of nucleating additives. Typical nucleating agents for isotactic poly- propylene 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 sam- ple in a differential scanning calorimetry (DSC) ex- periment ( 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 improve- ment. 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 corre- sponds to the lower reference considered above.

    A polymer melt wholly self-nucleated with poly- mer crystal fragments (potential crystalliza- tion nuclei) a t the highest achievable concen- trations. The crystal fragments produced in the self-nucleation procedure satisfy the three cri- teria which characterize an ideal nucleating additive: ( i ) ideal (i.e., highest achievable) concentration; ( i i) ideal dispersion in the mol- ten 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 poly- propylene ( 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 Tcs and spherulite size; in other words, to translate the efficiency coefficients and scale into physically more telling vari- ables: 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 estab- lish a correspondence with isothermal crys- tallization. Respective merits of nucleation efficiency evaluations based on isothermal and nonisothermal methods are then com- pared, and the validity of the proposed effi- ciency 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 in- teractions with the polymer, based on their crystal structure (note that only physical polymer-substrate interactions are considered here as opposed to pos- sible 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% con- centration, 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 109-102/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 ad- ditives in the polymer is achieved by codissolution in p -xylene and lyophilization. Polymer concentra- tion inp-xylene is 5%, dissolution is made by heating for 3 h at 120C. When cooled, the solution produces a gel, which is lyophilized for 24 h; the resulting dry product is pressed in a vacuum mold at 200C. 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 pres- ent study is virgin iPP submitted to the above dis- solution, 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, respec- tively.

    Techniques

    Experimental techniques are essentially as described previ~usly.~ Crystallization experiments are per- formed 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 spheru- lites. Figure 1 illustrates four different states of nu- cleation reached in our experiments.

    RESULTS

    This section is organized as follows: ( a ) the effi- ciency scale and efficiency coefficient for nucleating additives is introduced; ( b ) it is translated into a physically more telling variable, namely nuclei con- centrations using a development of the Avrami equations due to Eder et al.5 Next, the

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