12 /department of mechanical engineering PO Box 513, 5600 MB Eindhoven, the Netherlands Looking at Polymer Crystallization with SALS D.G.Hristova, G.W.M.Peters and H.E.H.Meijer Eindhoven University of Technology, Department of Mechanical Engineering Introduction Crystallization of polymers during processing affects the properties of the final products. Recently, nucleation phe- nomena is subjected to an increasing interest. One of the promising techniques to study the evolution of the early stages of crystallization is Small Angle Light Scattering (SALS) [1, 2]. Method and analysis Samples of isotactic polypropylen iPP HD120MO (Borealis) with thickness of 40μm are inserted in a LINKAM hot-stage at a certain temperature T cr (125 0 C - 145 0 C) and the crystal development during isothermal crystallization at T cr is ob- served by SALS. P sample He-Ne laser A screen CCD camera q m l=632.8nm parallel mode Vv: P ||A crossed mode Hv: P ⊥A Debye-Bueche approach: I (q)=1/(A + Bq 2 ) 2 (1) I - scattering intensity q = (4π/λ)sinθ/2 - scattering vector For crystallization kinetics [1] : Q δ = +∞ 0 IHvq 2 dq (2) Qη = +∞ 0 (IVv - 4 3 IHv)q 2 dq (3) density fluctuations <η 2 > ∝Qη orientation fluctuations <δ 2 > ∝Q δ Figure 1. Small Angle Light Scattering set-up. Parameters to obtain: ⇒ time evolution of Qη and Q δ ⇒ R sph - from θ at which IHv has a max ⇒ correlation length ξ = (B/A) - from eq.(1) Experimental observations and Results 50 100 150 200 250 300 350 50 100 150 200 250 300 350 100 150 200 250 300 350 400 100 150 200 250 300 350 400 Vv mode: t=180s pixels 1 2 3 4 5 *10E4 Intensity pixels 50 100 150 200 250 300 350 50 100 150 200 250 300 350 100 150 200 250 300 350 400 100 150 200 250 300 350 400 Vv mode: t=660s pixels 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 *10E4 Intensity pixels 50 100 150 200 250 300 350 50 100 150 200 250 300 350 100 150 200 250 300 350 400 100 150 200 250 300 350 400 Hv mode: t=180s pixels 500 1000 1500 2000 2500 3000 3500 Intensity pixels 50 100 150 200 250 300 350 50 100 150 200 250 300 350 100 150 200 250 300 350 400 100 150 200 250 300 350 400 Hv mode: t=660s pixels 1 2 3 4 5 *10E4 Intensity pixels Figure 2. Scattering patterns obtained by P||A (top) and by P ⊥A mode (bottom) at Tcr = 135 o C; q-scale:100pixels =1.5μm -1 . 0 200 400 600 800 1000 1200 1400 1600 0 1000 2000 3000 4000 5000 6000 time [s] Q η , Q δ T=135 0 C Q η Q δ Φ d = 0.5 secondary crystallization Figure 3. Time evolution of Q δ (for μ = 45 o ) and Qη (for μ =0 o ) invariants during crystallization of iPP at Tcr = 135 o C.Φ d is the volume fraction of growing crystals. 0 0.5 1 1.5 0 0.01 0.02 0.03 0.04 I Vv -1/2 t c = 120s t c = 60s Debye-Bueche plot ξ=2.59 μm q 2 [ μm -2 ] ξ=2.83 μm Figure 4. Debay-Bueche plots for parallel mode Vv at different times for Tcr = 135 o C, and calculated values of the correlation length ξ . Table 1. Spherulite radius R sph of iPP obtained by two techniques - SALS and hot-stage microscopy at different temperatures Tcr for a crystallization time of 360sec. R sph , [μm] T cr , [ ◦ C] SALS hot-stage microscopy 125 43.1 44. 4 135 27.9 31.6 140 12.5 - Conclusions ⋄ The SALS patterns show the expected 2-fold (for paral- lel Vv mode) and 4-fold (for crossed Hv mode) symme- try. ⋄ Density fluctuations <η 2 > appear prior to crystal de- velopment and reach a maximum when volume filling Φ d is 0.5. ⋄ Secondary crystallization is observed after spherulite impingement. ⋄ The good approximation of I -1/2 (q 2 ) with a linear fit confirms the validity of Debye-Bueche approach to evaluate the data from SALS. ⋄ SALS technique could be a very usefull method for future investigations of flow-induced crystallization, where enhancement in crystal nucleation is expected. References: [1] STEIN, R.S., CRONAUER, J., ZACHMANN, H.G.: Journal of Molecular Struc- ture, 1996, 383, 19 [2] KUMARASWAMY, G., KORNFIELD, J.A., Y EH, F., HSIAO, B.S.: Macro- molecules, 2002, 35, 1762